Ion Exchange and Solvent Extraction: A Series of Advances, Volume 17 (Ion Exchange and Solvent Extraction Series)

Ion Exchange and Solvent Extraction

Copyright © 2004 by Marcel Dekker, Inc.

Ion Exchange and Solvent Extraction A Series of Advances Volume 17 edited by

Yizhak Marcus

The Hebrew University of Jerusalem Jerusalem, Israel

Arup K.SenGupta

Lehigh University Bethlehem, Pennsylvania, U.S.A.

Jacob A.Marinsky Founding Editor

M ARCEL DEKKER, INC.

Copyright © 2004 by Marcel Dekker, Inc.

NEW YORK • BASEL

Transferred to Digital Printing 2005 Although great care has been taken to provide accurate and current information, neither the author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book. The material contained herein is not intended to provide specific advice or recommendations for any specific situation. Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress. ISBN: 0-8247-5492-1 Headquarters Marcel Dekker, Inc., 270 Madison Avenue, New York, NY 10016, U.S.A. tel: 212–696–9000; fax: 212–685–4540 Distribution and Customer Service Marcel Dekker, Inc., Cimarron Road, Monticello, New York 12701, U.S.A. tel: 800–228–1160; fax: 845–796–1772 Eastern Hemisphere Distribution Marcel Dekker AG, Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41–61–260–6300; fax: 41–61–260–6333 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright © 2004 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1

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Preface

The seventeenth volume of Ion Exchange and Solvent Extraction is concerned with advances in solvent extraction, a field that was also covered in Volume 15. The present book was conceived at ISEC ’02, the International Solvent Extraction Conference held in Capetown, South Africa. Several authors responded to the invitation to contribute comprehensive review papers on subjects showing recent advances and in which they are experts and have published extensively, and other authors were subsequently solicited to do the same. The advances made in such industrial fields as pharmaceutical chemistry and treatment of radioactive wastes— as well as hydrometallurgy—are based on those made in the basic research into new extractants, solvents, and processes. They are also based on the understanding of the mechanisms and kinetics of the extraction and the ability to model them adequately. Finally, advances must also rely heavily on knowledge gained in seemingly unrelated fields, such as solution chemistry, chemical thermodynamics, and chemical engineering. It is expected that some of the advances described in this book will be elaborated on in the forthcoming International Solvent Extraction Conference ISEC ’05, to be held in Beijing, China. Chapter 1, by Perrut, deals with the extraction of mainly pharmaceutical products by means of supercritical fluids, predominantly supercritical carbon dioxide. This “green” solvent extraction technique is very versatile and obviates a stripping operation in order to collect the extracted product. It can be applied on any desired scale, and plant-size applications are described along with advances concerning laboratory-scale extractions. In this context, the application of supercritical fluids in the pharmaceutical industry is described, not only in supercritical fluid extraction (SFE) but also in supercritical fluid fractionation (SFF) and supercritical fluid chromatography (SFC), employed for the purification of natural or synthetic biologically active products. Supercritical fluids are also attractive as reaction media iii Copyright © 2004 by Marcel Dekker, Inc.

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and can be employed with advantage to prepare particles of finely tailored sizes, for both pure compounds and composites, which is useful in drug formulations. Chapter 2, by Bart and Stevens, describes the chemistry and chemical engineering aspects of reactive solvents that include, but are not confined to, what are also called “liquid ion exchangers.” The extraction of zinc ions by di-2-ethylhexyl phosphoric acid has become a recognized industrial test system, and many of the examples given in this chapter deal with this system. Criteria for the choice of solvents and diluents for the processes are described. An extensive discussion follows of the equilibria, distribution constants, and activity coefficients, due to nonidealities in both the aqueous and the organic phases. Definitive instructions or references to the literature on how to apply these activity coefficients are presented and illustrated by the zinc di-2-ethylhexyl phosphoric acid extraction system. The kinetics involved in reactive extraction are then discussed in cases in which only diffusion limits the rate or the controlling step is the rate of the chemical reaction. Experimental set-ups for measuring the kinetics and the fate of the drops in twophase dispersed systems, and how these affect the operation of various types of extraction columns, are described. Chapter 3, by Chiarizia and Herlinger, describes the first of three classes of extractants that have become mature and extensively studied and applied in recent years: symmetrical diesters of alkylene diphosphonic acid. This type of extractant is an example of a liquid cation exchanger that is able to extract selectively many metal ions from acidic solutions. These include alkaline earth metal ions, iron(III), lanthanides, and Am(III), as well as thorium and uranium and tetravalent transuranium element cations. The synthesis of the extractants belonging to this class is described in detail as well as their properties—in particular, their aggregation in diluting solvents, which leads mainly to dimers and hexamers, depending on the extractants and conditions. An important aspect of the use of this class of extractants is the synergism exhibited when suitable compounds and cosolvents are added, which is well exploited in order to effect difficult separations. Finally, returning to the subject of Chapter 1, the use of supercritical fluid extraction—the possibility of employing the diesters of alkylene diphosphonic acid in supercritical carbon dioxide as a “green” diluent—is explored. Chapter 4, by Kolarik, deals with representatives of another class of extractants, namely, solvating extractants: dialkylsulfoxides. These compounds have many properties in common with the well-known organophosphorus extractants, but group with the one. The synthesis and properties of replace the basic these extractants are briefly described, followed by a survey of metal ion extraction systems. These are based mainly on extraction from nitric acid [in which uranium(VI) features prominently as the extracted metal ion], and to a lesser extent on extraction from hydrochloric acid and other media. The dependence of the distribution ratios and rates of extraction on the structure of the extractant is emphasized, and the nature of the extracted species is described in detail for all the systems studied. The selectivities achieved with dialkylsulfoxide extractants are discussed as well as the effects of interfering processes, such as third-phase

Copyright © 2004 by Marcel Dekker, Inc.

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formation and radiation damage to the extractant. Finally, enhancement of the utility of the extractants by means of synergism with various kinds of reagents is dealt with in detail. Chapter 5, by Rais and Grüner, concerns a class of extremely hydrophobic and highly acidic extractants: metal bis(dicarbollide)s. The most prominent among them are cobalt bis(dicarbollide) and its chloro-substituted analogs, which are particularly suitable for the extraction of cesium and, when augmented by polyethylene glycols, of strontium. These two elements are among the more troublesome fission products present in nuclear wastes, and efforts to remove them from highly acidic or highly alkaline wastes have previously been stymied by the lack of suitable extractants. The history of the development of the metal bis(dicarbollide)s up to plant-scale use is presented as well as methods for the synthesis of many potentially useful extractants of this class. The selective extraction of cesium and strontium by means of these reagents, alone or in combination with synergists, is described in detail, as are some other applications of these unusual materials. Finally, Chapter 6, by Rais, presents the background for extraction by means of the metal bis(dicarbollide)s and similar hydrophobic anions in terms of general expressions that are valid when entire electrolytes are extracted, as is the case in particular for the alkali metal cations. Extraction curves with polar solvating solvents, such as nitrobenzene or even the dialkylsulfoxides of Chapter 4—where cations are extracted as ion pairs with inorganic anions—often exhibit maxima that are shown to depend on the degree of ionic dissociation of the electrolytes in the organic phase. An algorithm useful for the modeling of such systems is presented. The electrolyte distribution ratios between mutually saturated but waterimmiscible solvents are compared with the standard molar Gibbs energies of transfer of electrolytes between water and the neat solvents and the electrochemical potentials measured with ion-selective electrodes (ISEs) at the interface of two immiscible electrolyte solutions (ITIES), showing the relationships between these useful quantities. This survey of the book’s contents demonstrates the viability of solvent extraction as a separation method used in laboratories and in industry and the continued advances made in finding new extractants and methods for their utilization. Also shown are some advances of the general concepts concerning the mechanisms and rates of extraction that underlie its applications. It is expected that readers of this volume, in conjunction with previous volumes and foreseen future ones, will obtain a stimulating view of the research and application possibilities of solvent extraction. The present volume is the last one edited by Y.M. Since my interest in solvent extraction has waned, I do not feel competent to judge what future significant advances in the field will be so I leave this task to younger persons. Yizhak Marcus Arup K.SenGupta

Copyright © 2004 by Marcel Dekker, Inc.

Contributors to Volume 17

Hans-Jörg Bart Lehrstuhl für Thermische Verfahrenstechnik, Technische Universität Kaiserslautern, Kaiserslautern, Germany R.Chiarizia Chemistry Division, Argonne National Laboratory, Argonne, Illinois, U.S.A. Bohumír Grüner Institute of Inorganic Chemistry, Czech Academy of Sciences, Rez, Czech Republic A.W.Herlinger Department of Chemistry, Loyola University Chicago, Chicago, Illinois, U.S.A. Zdenek Kolarik Consultant, Karlsruhe, Germany Michel Perrut Separex, Champigneulles, France Jirí Rais Nuclear Research Institute Rez plc, Rez Czech Republic Geoffrey W.Stevens Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria, Australia

vii Copyright © 2004 by Marcel Dekker, Inc.

Contents

Preface Contributors to Volume 17 Contents of Other Volumes 1.

Applications of Supercritical Fluid Solvents in the Pharmaceutical Industry Michel Perrut I. II. III. IV. V. VI. VII. VIII. IX.

2.

Scope Supercritical Fluid Solvent Properties Applications of SCF as Extraction/Fractionation Solvents Applications of SCF as Chromatography Eluents Applications of SCF as Reaction Media Pollution Abatement Applications of SCF for Particle Design and Drug Formulation Biological Applications Future Trends References

Reactive Solvent Extraction Hans-Jörg Bart and Geoffrey W.Stevens I. Introduction II. Reactive Solvent Extraction Equilibria III. Reactive Mass Transfer

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1 1 2 7 12 13 14 15 26 27 28 37 37 48 58 ix

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Contents IV. Solvent Extraction Equipment V. Concluding Remarks References

3.

Symmetrical P,P’-Disubstituted Esters of Alkylenediphosphonic Acids as Reagents for Metal Solvent Extraction 85 R.Chiarizia and A.W.Herlinger I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII.

4.

5.

68 76 77

Introduction Synthesis Spectroscopic Studies Aggregation Studies Solvent Extraction of Metal Ions at Low Loading Solvent Extraction of Metal Ions at High Loading Enthalpy and Entropy Changes in Metal Solvent Extraction Intra-Lanthanide Ion Separations Synergistic Extraction of Metal Ions Extraction Chromatographic Applications Reagents for Use in Supercritical Fluid Extraction Conclusions Nomenclature References

85 87 94 103 110 120 126 132 138 145 149 155 157 158

Sulfoxide Extractants and Synergists Zdenek Kolarik

165

I. II. III. IV. V. VI. VII.

165 169 195 209 213 217 221 233 234 236

Introduction Extraction from Nitrate Media Extraction from Chloride Media Extraction from Other Media Selectivity of the Extraction Interfering Phenomena Synergism Nomenclature Appendix: Physical Properties of Sulfoxides References

Extraction with Metal Bis(dicarbollide) Anions: Metal Bis(dicarbollide) Extractants and Their Applications in Separation Chemistry 243 Jirí Rais and Bohumír Grüner I. Introduction 243 II. Synthesis and Properties of Metal Bis(dicarbollide)s and Other Cluster Boron Compounds Aimed for Extraction Purposes 247

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Contents III. Extraction with Basic and Halogenated Cobalt Bis(dicarbollide)s IV. Extraction with Cobalt Bis(dicarbollide)s and Synergists V. Extraction with Functionalized Metal Bis(dicarbollide)s and Other Boron Extractants VI. Chloro-Protected Bis(dicarbollide) Technologies for Extraction of Fission Products and Actinide Cations from Radioactive Wastes VII. Analytical and Other Applications of Extraction Systems with Metal Bis(dicarbollide)s VIII. Conclusions Supplementary Material Symbols References 6.

Principles of Extraction of Electrolytes Jiri Rais I. II. III. IV

Introduction Aims and Scope of the Review Description of the Systems Characteristic Examples of Equilibria in Extraction of Electrolytes V Gibbs Energies of Transfer and Some Semiempirical Models VI. Conclusions Symbols References

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282 283 299

306 312 322 322 323 324 335 335 338 338 343 357 378 378 381

Contents of Other Volumes

Volumes 1–4, 6 out of print Volume 5 NEW INORGANIC ION EXCHANGERS A.Clearfield, G.H.Nancollas, and R.H.Blessing APPLICATION OF ION EXCHANGE TO ELEMENT SEPARATION AND ANALYSIS F.W.E.Strelow PELLICULAR ION EXCHANGE RESINS IN CHROMATOGRAPHY Csaba Horvath Volume 7 INTERPHASE MASS TRANSFER RATES OF CHEMICAL REACTIONS WITH CROSSLINKED POLYSTYRENE Gabriella Schmuckler and Shimon Goldstein INFLUENCE OF POLYMERIC MATRIX STRUCTURE ON PERFORMANCE OF ION-EXCHANGE RESINS V.A.Davankov, S.V.Rogozhin, and M.P.Tsyurupa xiii Copyright © 2004 by Marcel Dekker, Inc.

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Contents of Other Volumes

SPECTROSCOPIC STUDIES OF ION EXCHANGERS Carla Heitner-Wirguin ION-EXCHANGE MATERIALS IN NATURAL WATER SYSTEMS Michael M.Reddy THE THERMAL REGENERATION OF ION-EXCHANGE RESINS B.A.Bolto and D.E.Weiss Volume 8 METAL EXTRACTION WITH HYDROXYOXIMES Richard J.Whewell and Carl Hanson ELECTRICAL PHENOMENA IN SOLVENT EXTRACTION Giancarlo Scibona, Pier Roberto Dansei, and Claudio Fabiani EXTRACTION WITH SOLVENT-IMPREGNATED RESINS Abraham Warshawsky SOLVENT EXTRACTION OF ELEMENTS OF THE PLATINUM GROUP Lev M.Gindin SOLVENT EXTRACTION FROM AQUEOUS-ORGANIC MEDIA Jiri Hala Volume 9 ION-EXCHANGE PROCESSES USED IN THE PRODUCTION OF ULTRAPURE WATER REQUIRED IN FOSSIL FUEL POWER PLANTS Calvin Calmon A SYSTEMATIC APPROACH TO REACTIVE ION EXCHANGE Gilbert E.Janauer, Robert E.Gibbons, Jr., and William E.Bernier ION-EXCHANGE KINETICS IN SELECTIVE SYSTEMS Lorenzo Liberti and Roberto Passino SORPTION AND CHROMATOGRAPHY OF ORGANIC IONS G.V.Samsonov and G.E.Elkin

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THERMODYNAMICS OF WATER SORPTION OF DOWEX 1 OF DIFFERENT CROSSLINKING AND IONIC FORM Zoya I.Sosinovich, Larissa V.Novitskaya, Vladimir S.Soldatov, and Erik Högfeldt DOUBLE-LAYER IONIC ADSORPTION AND EXCHANGE ON POROUS POLYMERS Frederick F.Cantwell HUMIC-TRACE METAL ION EQUILIBRIA IN NATURAL WATERS Donald S.Gamble, Jacob A.Marinsky, and Cooper H.Langford Volume 10 SOLVENT EXTRACTION OF INDUSTRIAL ORGANIC SUBSTANCES FROM AQUEOUS STREAMS C.Judson King and John J.Senetar LIQUID MEMBRANES Richard D.Noble, J.Douglas Way, and Annett L.Bunge MIXED SOLVENTS IN GAS EXTRACTION AND RELATED PROCESSES Gerd Brunner INTERFACIAL PHENOMENA IN SOLVENT EXTRACTION Valery V.Tarasov and Gennady A.Yagodin SYNERGIC EXTRACTIONS OF ZIRCONIUM (IV) AND HAFNIUM (IV) Jiri Hala Volume 11 CHEMICAL THERMODYNAMICS OF CATION EXCHANGE REACTIONS: THEORETICAL AND PRACTICAL CONSIDERATIONS Steven A.Grant and Philip Fletcher A THREE-PARAMETER MODEL FOR SUMMARIZING DATA IN ION EXCHANGE Erik Högfeldt DESCRIPTION OF ION-EXCHANGE EQUILIBRIA BY MEANS OF THE SURFACE COMPLEXATION THEORY Wolfgang H.Höll, Matthias Franzreb, Jürgen Horst, and Siefried H.Eberle

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Contents of Other Volumes

SURFACE COMPLEXATION OF METALS BY NATURAL COLLOIDS Garrison Sposito A GIBBS-DONNAN-BASED ANALYSIS OF ION-EXCHANGE AND RELATED PHENOMENA Jacob A.Marinsky INFLUENCE OF HUMIC SUBSTANCES ON THE UPTAKE OF METAL IONS BY NATURALLY OCCURING MATERIALS James H.Ephraim and Bert Allard Volume 12 HIGH-PRESSURE ION-EXCHANGE SEPARATION IN RARE EARTHS Liquan Chen, Wenda Xin, Changfa Dong, Wangsuo Wu, and Sujun Yue ION EXCHANGE IN COUNTERCURRENT COLUMNS Vladimir I. Gorshkov RECOVERY OF VALUABLE MINERAL COMPONENTS FROM SEAWATER BY ION-EXCHANGE AND SORPTION METHODS Ruslan Khamizov, Dmitri N.Muraviev, and Abraham Warshawsky INVESTIGATION OF INTRAPARTICLE ION-EXCHANGE KINETICS IN SELECTIVE SYSTEMS A.I.Kalinitchev EQUILIBRIUM ANALYSIS OF COMPLEXATION IN ION EXCHANGERS USING SPECTROSCOPIC AND DISTRIBUTION METHODS Hirohiko Waki ION-EXCHANGE KINETICS IN HETEROGENEOUS SYSTEMS K.Bunzl EVALUATION OF THE ELECTROSTATIC EFFECT ON METAL ION-BINDING EQUILIBRIA IN NEGATIVELY CHARGED POLYION SYSTEMS Tohru Miyajima ION-EXCHANGE EQUILIBRIA OF AMINO ACIDS Zuyi Tao ION-EXCHANGE SELECTIVITIES OF INORGANIC ION EXCHANGERS Mitsuo Abe

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Volume 13 EXTRACTION OF SALTS BY MIXED LIQUID ION EXCHANGERS Gabriella Schmuckler and Gideon Harel ACID EXTRACTION BY ACID-BASE-COUPLED EXTRACTANTS Aharon M.Eyal HOST-GUEST COMPLEXATION AS A TOOL FOR SOLVENT EXTRACTION AND MEMBRANE TRANSPORT OF (BIO)ORGANIC COMPOUNDS Igor V.Pletnev and Yuri A.Zolotov NEW TECHNOLOGIES FOR METAL ION SEPARATIONS: POLYETHYLENE GLYCOL BASED-AQUEOUS BIPHASIC SYSTEMS AND AQUEOUS BIPHASIC EXTRACTION CHROMATOGRAPHY Robin D.Rogers and Jianhua Zhang DEVELOPMENTS IN SOLID-LIQUID EXTRACTION BY SOLVENTIMPREGNATED RESINS José Luis Cortina and Abraham Warshawsky PRINCIPLES OF SOLVENT EXTRACTION OF ALKALI METAL IONS: UNDERSTANDING FACTORS LEADING TO CESIUM SELECTIVITY IN EXTRACTION BY SOLVATION Bruce A.Moyer and Yunfu Sun

Volume 14 POLYMER-SUPPORTED REAGENTS: THE ROLE OF BIFUNCTIONALITY IN THE DESIGN OF ION-SELECTIVE COMPLEXANTS Spiro D.Alexandratos RECOVERY OF VALUABLE SPECIES FROM DISSOLVING SOLIDS USING ION EXCHANGE Jannie S.J.van Deventer, P.G.R.de Villiers, and L.Lorenzen POLYMERIC LIGAND-BASED FUNCTIONALIZED MATERIALS AND MEMBRANES FOR ION EXCHANGE Stephen M.C Ritchie and Dibakar Bhattacharyya BIOSORPTION OF METAL CATIONS AND ANIONS Bohumil Volesky, Jinbai Yang, and Hui Niu

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Contents of Other Volumes

SYNTHESIS AND APPLICATION OF FUNCTIONALIZED ORGANOCERAMIC SELECTIVE ADSORBENTS Lawrence L.Tavlarides and J.S.Lee ENVIRONMENTAL SEPARATION THROUGH POLYMERIC LIGAND EXCHANGE Arup K.SenGupta IMPRINTED METAL-SELECTIVE ION EXCHANGER Masahiro Goto SYNTHESIS AND CHARACTERIZATION OF A NEW CLASS OF HYBRID INORGANIC SORBENTS FOR HEAVY METALS REMOVAL Arthur D.Kney and Arup K.SenGupta

Volume 15 AN INTEGRATED METHOD FOR DEVELOPMENT AND SCALING UP OF EXTRACTION PROCESSES Baruch Grinbaum DESIGN OF PULSED EXTRACTION COLUMNS Alfons Vogelpohl and Hartmut Haverland PURIFICATION OF NICKEL BY SOLVENT EXTRACTION Kathryn C.Sole and Peter M.Cole TREATMENT OF SOILS AND SLUDGES BY SOLVENT EXTRACTION IN THE UNITED STATES Richard J.Ayen and James D.Navratil THE DESIGN OF SOLVENTS FOR LIQUID-LIQUID EXTRACTION Braam van Dyk and Izak Nieuwoudt EXTRACTION TECHNOLOGY FOR THE SEPARATION OF OPTICAL ISOMERS André B.de Haan and Béla Simándi REGULARITIES OF EXTRACTION IN SYSTEMS ON THE BASIS OF POLAR ORGANIC SOLVENTS AND USE OF SUCH SYSTEMS FOR SEPARATION OF IMPORTANT HYDROPHOBIC SUBSTANCES Sergey M.Leschev DEVELOPMENTS IN DISPERSION-FREE MEMBRANE-BASED EXTRACTION-SEPARATION PROCESSES Anil Kumar Pabby and Ana-Maria Sastre

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Volume 16 ADSORPTION AND ION-EXCHANGE PROPERTIES OF ENGINEERED ACTIVATED CARBONS AND CARBONACEOUS MATERIALS Michael Streat, Danish J.Malik, and Basudeb Saha ENTROPY-DRIVEN SELECTIVE ION EXCHANGE FOR HYDROPHOBIC IONIZABLE ORGANIC COMPOUNDS (HIOCs) Ping Li and Arup K.SenGupta ION-EXCHANGE ISOTHERMAL SUPERSATURATION: CONCEPT, PROBLEMS, AND APPLICATIONS Dmitri N.Muraviev and Ruslan Khamizov METAL SEPARATION BY pH-DRIVEN PARAMETRIC PUMPING Wolfgang H.Höll, Randolf Kiefer, Cornelia Stöhr, and Christian Bartosch SELECTIVITY CONSIDERATIONS IN MODELING THE TREATMENT OF PERCHLORATE USING ION-EXCHANGE PROCESSES Anthony R.Tripp and Dennis A.Clifford ION-EXCHANGE KINETICS FOR ULTRAPURE WATER Dennis F.Hussey and Gary L.Foutch

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1 Applications of Supercritical Fluid Solvents in the Pharmaceutical Industry Michel Perrut Separex, Champigneulles, France

I. SCOPE At the beginning of this new millennium, the pharmaceutical industry is facing many challenges in relation to the rapid and endless growth and aging of the world population, and a growing gap between health care systems in industrialized and developing countries. While higher and higher quality standards are required by authorities and consumers, and massive low-cost supply of drugs is strongly demanded by developing countries, it appears more and more difficult to introduce innovative new drugs and to improve the therapeutic efficacy against numerous pathologies. Furthermore, the industry must also make a continuous effort to move to environmentally friendly processes. Regarding the environmental protection, one of the first requirements of the industry is to move to “green chemistry” and to avoid potentially harmful solvents. As we will try to demonstrate in this chapter, the use of supercritical fluid (SCF) solvents is a promising route to both reducing pollutant release and improving the final drug quality and efficacy through innovative processes for active substance preparation and drug formulation. In fact, most applications of supercritical fluid solvents are based on the use of carbon dioxide, pure or with ethanol added, as it presents the definitive advantages, being a “green,” abundant, and cheap solvent perfectly adequate to process food 1 Copyright © 2004 by Marcel Dekker, Inc.

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or pharmaceutical products at a temperature near to ambient. Many processes are now under development: • • •

Supercritical fluid extraction (SFE), supercritical fluid fractionation (SFF), and supercritical fluid chromatography (SFC), for extraction and purification of natural or synthetic active products. Supercritical fluids as reaction media. Supercritical fluid drug formulation by manufacturing innovative therapeutic particles, either of pure active compounds or composites of excipient and active compounds.

As thousands of publications can be found in the domain, this review does not present an exhaustive survey, but aims to guide the scientists to consider supercritical fluid solvents as a new tool to be envisaged to open innovative routes to solve their problems.

II. SUPERCRITICAL FLUID SOLVENT PROPERTIES Pure compounds can be found in three states: solid, liquid, and vapor or gas. On the (pressure, temperature) diagram as presented in Fig. 1, the three regions corresponding to these three states are separated by curves that meet at the triple point. Surprisingly, the vaporization/liquefaction curve presents an end point called critical point (Pc, Tc). Beyond this point (P>Pc and T>Tc), only one phase exists, called supercritical fluid (SCF). At the critical point itself, the fluid compressibility

Figure 1 General pressure-temperature diagram for pure compounds.

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SCF Solvents in the Pharmaceutical Industry

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becomes infinite, meaning that the fluid density rapidly varies with a slight change in pressure at constant temperature. Moreover, even out of the critical region itself, an SCF exhibits large changes in density—and consequently in solvent power— and other physicochemical properties with pressure or temperature. These may be roughly considered as intermediate between those of a low-viscosity liquid and a compressed gas (see Table 1). Similarly, when the compound is maintained at a pressure above its critical pressure and at a temperature below its critical temperature, it is called subcritical liquid (P>Pc and TPc and T>Tc) and to a liquefied gas (P
Very cheap and abundant in pure form (food grade) worldwide. Nonflammable and not toxic. Environmentally friendly, as nonpolluting gas and as most of the CO2 is manufactured from waste streams (mainly gaseous effluents from fertilizer plants).

Table 1 Comparison of Average Properties of Gases, Liquids, and SCF

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Table 2 Critical Conditions of SCF Solvents

a

Currently used compounds shown boldfaced. Source: Ref. 1, except CF3–CH2F (F134a) from Ref. 2.

• •

Critical temperature at 31°C, permitting operations at near-ambient temperature, avoiding product alteration. Critical pressure at 74 bar, leading to “acceptable” operation pressure, generally between 100 and 350 bar.

Carbon dioxide always behaves as a rather weak “nonpolar” solvent. It selectively dissolves the lipids like vegetal oils, butter, fats, hydrocarbons, essential oils, but has a weak affinity to oxygenated or hydroxylated molecules and does not dissolve any hydrophilic compounds like sugars and proteins, and mineral species like salts, metals, etc. However, the CO2 solvent power and polarity can be significantly increased by adding a polar cosolvent that is generally chosen among short-chain alcohols, esters, or ketones. For obvious reasons, ethanol is often preferred as it is abundant and cheap in pure forms (food grade, pharmacopoeia grade), not environ-mentally hazardous, and not very toxic. Moreover, it is to be

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noticed that although water is only slightly soluble in SCF carbon dioxide (1–3 g kg-1), it plays a very important role as “cosolvent” for many polar molecules; in fact, water is present in most applications, especially when natural products are processed. Other types of fluids are considered for specific applications, although they do not present the definitive advantages of carbon dioxide listed above: • • • •

•

Light hydrocarbons, especially liquefied propane, which appears to be a much stronger solvent than carbon dioxide vis-à-vis lipids. Hydrofluoro carbons (HFC), environmentally friendly but very expensive. Dimethyl ether, used as liquefied gas, that behaves as a “polar” solvent able to dissolve a very wide range of compounds including many polymers, in contrast to carbon dioxide [3]. Water exhibits very attractive properties at subcritical or supercritical conditions that are completely different from those commonly found. There is great variation of the dielectric constant and resulting polarity and solvent properties varying from the exceptionally high polarity of the common liquid water to a nonpolar fluid dissolving organics and precipitating salts at supercritical conditions. Promising routes based on micelles or emulsions [4–6] in an SCF phase (generally carbon dioxide) were also subjected to intense investigation as media for extraction, reaction, and particle design.

Regarding biological properties of SCF solvent, carbon dioxide exhibits biocidal properties and is very active on fungi, bacteria, and viruses, so that all processed materials are decontaminated, or even sterilized, depending on the process conditions and raw material contaminants (see Section VIII); CO2 has only a very low toxicity toward humans, although it requires strong safety precautions [7] as asphyxia may happen when it accumulates in nonventilated areas, especially in lower parts of buildings (cellars, etc.). N2O has different biological properties and it is commonly used for anesthesia; however, it should be processed with great care as it must be considered as a comburant that may lead to explosion when contacted with flammable solutes. Light hydrocarbons and dimethyl ether are not toxic, but present a very important explosion hazard that requires enforcement of drastic safety rules. HFCs are neither toxic nor flammable, but may decompose to highly toxic gases when submitted to a flame. Fluid phase equilibria of mixtures are very complex, and many types of phase diagrams can be found. In the literature [8–19], thousands of articles deal with high-pressure fluid phase equilibria covering a very wide range of compounds and operating conditions. Recent progress in thermodynamic modeling permits prediction of the behavior of many mixtures. However, some measurements— that are difficult to perform!—are still required to set some interaction parameters that cannot be calculated yet, especially for polar liquids (for example, in the

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case of strong hydrogen bonding). But happily, it is not always necessary to handle detailed thermodynamic data of the processed mixtures to design the SCF processes! For “simple” systems and relatively low solubility, the empirical correlation proposed by Chrastil [20] can be used to interpret experimental results with a good reliability without any complicated calculations: (1) where C is the solute concentration, and a, b, and k are empirical constants. Moreover, this correlation was very recently used as basis of a similar but more complex equation that satisfactorily fitted solubility data of 104 different organic compounds [21]. In fact, this simple equation (1) shows the dependence of solubility on fluid density ρ, and the following trends: • • •

Solubility is strongly dependent on the fluid density as k is always positive and in the range of several units. Consequently, solubility increases with pressure at constant temperature. Solubility may increase or decrease when temperature is raised at constant pressure.

In all cases, the solubility dramatically decreases (by several orders of magnitude) when the fluid is depressurized at constant temperature below its critical pressure, with solubility variation of several orders of magnitude. Moreover, it has to be emphasized that one of the main advantages of SCFs is related to the ability to set very precisely their solvent power vis-à-vis different compounds by tuning pressure, temperature, and cosolvent content. This permits to perform very selective fractionation of complex mixtures that cannot be resolved with classical organic solvents or by any other process. This is used either for sorting compounds belonging to the same chemical family but differing by their carbon numbers (i.e., fatty acids or oligomers/polymers) or compounds having similar molecular mass but with slightly different polarities (i.e., neutral and polar lipids). Finally, it is also to be noticed that, due to its nonpolar character, SCF carbon dioxide is also used as an “antisolvent” when added to polar organic solvents in which it readily dissolves, leading to a significant decrease of their polar character and causing precipitation of compounds previously dissolved in these solvents as detailed in Section VII.B. Another important property of SCF solvents is related to the drastic viscosity reduction of a liquid phase contacted with a SCF solvent that partly dissolves in the liquid, even at a pressure below critical. This is of great practical importance for fluidizing viscous oils and waxes in order to facilitate processing (filtering, reaction, extraction). Similarly, many polymers are “swollen” and “plasticized” by compressed carbon dioxide, with a decrease of the glass transition temperature by several tens of degrees, permitting an easy processing (forming and atomizing, mixing, grafting, foaming).

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SCF transport properties [1, 8] are very attractive as they are as dense as liquids but mobile like gas (very low viscosity, intermediate diffusivity), as shown in Table 1. Thus, mass transfer (and similarly heat transfer) is fast in an SCF in comparison with liquid organic solvents or water. SCFs rapidly diffuse into porous media, easing either extraction from solid materials or impregnation of solutes into porous media.

III.

APPLICATIONS OF SCF AS EXTRACTION/ FRACTIONATION SOLVENTS

As nature is an almost unlimited source of active substances, a great effort is paid to concentrate them or to remove undesired compounds from them, using mainly extraction with organic solvents or water or ethanol/water mixtures, depending on the polarity of the targeted molecules. In order to improve the product quality and to reduce the use of potentially harmful and polluting organic solvents, most industrial applications of supercritical fluid solvents have been developed during the last two decades for extraction/fractionation of natural products, both for nutritional and pharmaceutical products, as detailed in many books and symposium proceedings [8–19]. At the present time, these applications are still continuing to spread worldwide as requirements for high-quality products and concerns for the environment and health are growing [22].

A. Extraction (SFE) Extraction (SFE) from solid materials, especially natural materials, is by far the most developed application. This includes food products (coffee, tea, low-fat cholesterol-free egg yolk powder, etc.), food ingredients and supplements (hops and beverage aromas, flavors and fragrances, colorants and carotenoids, vitaminrich extracts, high-grade oils and lipids, etc.), natural insecticides (Neem, Pyrethrum) and many nutra- and phytopharmaceuticals. I estimate at about 100 the number of industrial-scale SFE units now under operation with a growth of about 10% per year. The general flow-sheet of industrial SFE plants is presented in Fig. 2. It generally comprises at least two extraction autoclaves with fast-opening systems, a series of separators (gravity and/or cyclonic), heat exchangers to supply enthalpy (the first one to reach the desired temperature for extraction and the second one to vaporize the fluid after depressurization to permit separation of the extract from the fluid) and to condense the recycled fluid, a liquefied fluid buffer tank, and a high-pressure volumetric pump (see Fig. 3). For very large-scale units, the fluid condensation, liquid pumping, and reheating are replaced by gas recompression trough a compressor followed by cooling to reach the desired pressure and temperature conditions. Among the drugs presently registered in Europe, the most important business is related to Saw palmetto (Serenoa repens) extraction by large-scale SFE; Pygeum

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Figure 2 General flow sheet of an industrial-scale SFE plant.

africanum can also be extracted without chlorinated solvents, among many other active principles of natural origin (Kava-kava, Tanacetum parthelium, bee pollen, Sea buckthorn, Gingko biloba, etc.). Residual organic solvents and pesticides are also removed from final active compounds (natural, like ginseng, or synthetic) on a large scale. Delipidation of protein extracts or plasma fractions [23] can be operated under mild conditions so as not to alter the biomolecular structure and destroy their bioactivity. Moreover, some “niches” applications concern high-added-value biomedical products, like bone delipidation for allografts [24–26], or specialty polymer stripping (biomedical implants).

B. Fractionation (SFF) Fractionation (SFF) of liquid mixtures is very promising as it combines the very high selectivity of supercritical fluids with attractive costs related to continuous operation. For completing difficult separations, SFF takes advantage of the “tunable” properties of SCF when operated with high-performance equipment (Fig. 4) incorporating some or all of the following: •

High-performance multistage countercurrent columns are preferred to ensure a good contact between the phases—packed for low viscosity feeds and stirred for either viscous or nonviscous feeds. Moreover, to increase

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Figure 3 Commercial-scale SFE plant: two extractors of 500 1, fluid flow rate 3000 kg h-1 cosolvent addition. (Courtesy HITEX & NOVELECT. Photo by Philippe Baudet.)

• • •

selectivity, a reflux of extract is performed either by operating a temperature gradient along the contactor on pilot-scale equipment (causing solvent power decrease and consequently precipitation of the less-soluble compounds that reflux in the liquid phase) or by an external reflux on large-scale equipment. Multistage separation of the fluid-solute mixture through separators in series operated at decreasing pressures in order to fractionate the solute components according to their affinity to the fluid. Combination of fractionation with selective adsorption of the solute mixture dissolved in the depressurized fluid onto a selective adsorbent. Adsorption of the most volatile compounds of the solute in order to avoid recycling with the fluid and important losses of such compounds or decrease of selectivity.

Nevertheless, few industrial units are now in operation, mainly for aroma production from fermented and distilled beverages (rum, cognac, whisky), and some niches

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Figure 4 General flow sheet of a SFF plant.

products (perfluoro-polymers as computer hard-disk lubricants). Presently, the more promising pharmaceutical applications seem to be: •

•

•

Fractionation of lipids [11]: Purification of specialty oils (deodorization, decoloration), separation of mono-, di- and triglycerides, separation of polyunsaturated fatty acid esters like EPA/DHA, concentration/purification of sterols and tocopherols [8], separation of ceramides, glycolipids (monoand di-galactosyl-diglycerides) and phospholipids from wheat gluten oil [27], etc. Fractionation of specialty polymers: It is possible to obtain very “narrow” fractions as the SFF process is extremely selective if operated under adequate conditions. For example, we successfully processed clinical lots of a pharmaceutical poloxamer on a large-scale SFF unit in order to eliminate the shortest chains that present toxicity. Recovery of active metabolites from fermentation broths.

C. Membrane Processes Membrane Processes have been investigated for a long time, but membrane material selection was difficult due to irreversible alteration of many porous polymers. At

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the present time, several processes take advantage of the unique properties of SCF solvents [28], especially high diffusivity, low viscosity, viscosity-reduction of liquids saturated with SCF and selective solvent power. They are using long-lasting membranes, either inorganic membranes, derived from gas diffusion membranes, or polymer membranes, made of various porous polymers (like polypropylene) that are not “swollen” by the SCF, or composite asymetric membranes, made by polymer deposition on a porous inorganic material used as a support. Among the numerous applications now under consideration, some can be specifically applied in the pharmaceutical industry: • • •

Nanofiltration, especially for highly viscous liquids. Coupling SCF extraction and a membrane downward of the SCF extractor for solvent-solute separation, extract fractionation by nanofiltration, SCF-assisted ultrafiltration. A membrane contactor using a series of polypropylene hollow-fiber modules (Porocrit) shown in Fig. 5 in order to supply a high contact surface between the liquid and SCF phases inside a small-volume contactor (typically a cartridge module of 10 cm diameter and 71 cm length that supplies a contact surface of 19 m2). This was proposed for the extraction of valuable compounds from aqueous suspensions or viscous solutions [29]. This technique appears of special interest for extracting metabolites from fermentation broths as it does not demand a significant density difference between the two phases and permits

Figure 5 Porocrit membrane module supplied by Membrana-Charlotte, a Division of Celgard, Inc. (Courtesy of Porocrit LLC.)

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• •

IV.

Perrut to treat highly foaming liquids without flooding, contrary to the classical supercritical fluid fractionation. Enzymatic reactors, using an enzyme immobilized onto the membrane as catalyst [30]. A contactor for pasteurization of aqueous liquids, as also proposed by the manufacturer of Porocrit, the rapid permeation of carbon dioxide leading to saturation of the liquid phase. The process was demonstrated on a small scale under conditions that are very attractive and open the way to scale up to commercial scale for many pumpable food products. A high killing efficiency (6–8 powers of 10 reduction) is obtained at a relatively low pressure (~75 bar), near room temperature and for short residence times inside the contactor (10–15 s) and a holding tube downward of the contactor (1–2 min) [31].

APPLICATIONS OF SCF AS CHROMATOGRAPHY ELUENTS

Numerous analytical applications were developed where supercritical fluid eluents have been shown to lead to very rapid and selective analysis both on capillary and packed columns with the possibility of an easy coupling with many types of detectors. Subsequently, preparative-scale supercritical fluid chromatography (PSFC) was developed to achieve the ultimate fractionation of very similar compounds, including enantiomers [32–34]. A few large plants are now on stream, mainly for purification of lipids like polyunsaturated fatty acids, as shown in Fig. 6. Recent development of simulated-moving bed chromatography with an SCF eluent (SF-SMB) proved that the variability of the elution power of an SCF is a key advantage over liquid solvents, leading to a significant increase in fractionation performance in comparison with classical SMB [35]. This opens an attractive separation route, especially for enantiomer resolution that constitutes one of the main issues in pharmaceutical synthesis [36]. The concept of SCF extrography (combination of extraction and chromatography) is also very attractive as it leads to very high selectivity: It consists of adsorbing the mixture to be fractionated onto a porous stationary phase and extracting the different components with an SCF with a gradient or stepwise increase of its solvent power obtained by adjusting the pressure at constant temperature, or more seldom, by adjusting both pressure and temperature. This process is well adapted to resolve mixtures of very similar compounds at a reasonable cost, much lower than with the preparative chromatography one, even if it cannot afford so high a selectivity. For example, we obtained excellent results in fractionation of terpenes, oxygenated terpenes, sesquiterpenes, and heavies (mainly waxes and psoralens) from citrus oils, whereas this fractionation is impossible by distillation and hardly possible by supercritical fluid fractionation.

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Figure 6 Large-scale supercritical fluid chromatography unit with a 150-mm-diameter column. (Courtesy of NOVASEP.)

V. APPLICATIONS OF SCF AS REACTION MEDIA Due to their properties and tunability, supercritical fluid solvents are widely investigated as reaction media as reported in several recent books and reviews [13–19, 37, 38]. Very promising processes are being developed for fine highly selective synthesis. In fact, the exceptionally wide property range of SCF solvents opens new routes of synthesis with easy postreactor processing for recovery of the

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products and nonreacted substrates. For organic synthesis, carbon dioxide is the most widely used SCF in spite of its limited solvent power vis-à-vis polar substrates, requiring cosolvents. Light alkanes are also used, especially propane which is a strong solvent of lipids. Recently, more polar SCF solvents were investigated, like dimethyl ether. Moreover, water and ammonia are widely used for high temperature reactions. Promising routes based on micelles or emulsions [5, 6] in a SCF phase (generally carbon dioxide) are also subjected to intense investigation as media for reaction, especially polymerization. Regarding the reaction itself, among numerous works already published, I would cite some examples describing the significant advantages of SCF media over classical liquid solvents: • • • • •

Reaction rates increase due to the high diffusivity in SCF solvents, both for homogeneous and heterogeneous reactions. The number of phases is reduced and mass transfer limitations are eliminated in heterogeneous catalytic gas-liquid reactions, with resulting considerable increase of reaction rates and selectivity [39–42]. Strong diminution of coke deposition on the catalyst [43]. Selective photoisomerization due to narrow transmission “windows” of SCF carbon dioxide [44]. Special properties of SCF water for hydrothermal reactions (material synthesis [45], destructive oxidation of wastes, etc.).

One of the most impressive examples is catalytic hydrogenation in a liquid phase that is always strongly limited by hydrogen mass transfer from the gas phase through the liquid phase toward the catalytic sites, leading to slow kinetics and reactants degradation (e.g., cracking, isomerization, oligomerization). On the contrary, hydrogenation in an SCF phase where both reactants and hydrogen are dissolved is much faster (by up to 3 orders of magnitude in the cited case of the hydrogenation of unsaturated fatty acid derivatives in SCF propane!) and side reactions are considerably reduced as hydrogen is always present in excess (isomerization of cis- to trans-fatty acids in the cited case) [39, 40]. These advantages are leading to many investigations and to industrial development in the pharmaceutical industry in Switzerland and with the recent commissioning of a toll-processing SCF hydrogenation plant for fine chemical synthesis in the UK [41, 42]. Enzymatic reactions operated in carbon dioxide have also received a great deal of attention, although no major developments have happened yet.

VI. POLLUTION ABATEMENT SCFs, and especially carbon dioxide, lead to environmentally friendly processes through substitution of organic solvents. Moreover, water streams polluted with

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organic compounds can be treated with CO2 for pollutants elimination. On the other hand, supercritical water appears as a unique medium for safe destruction of dangerous wastes by total oxidation due to its special physicochemical properties, especially for highly hazardous wastes, as proven in a few demonstration plants, including one for recycling precious metals from spent catalysts [46]. Moreover, pollutant destruction in subcritical water is also used in pharmaceutical companies, even if the oxidation rate is lower than in supercritical water.

VII. APPLICATIONS OF SCF FOR PARTICLE DESIGN AND DRUG FORMULATION Particle formation processes using supercritical fluids [47–50] are now the subject of an increasing interest, especially in the pharmaceutical industry, with three aims: Increasing bioavailability of poorly soluble molecules, designing sustained-release formulations and facilitating drug delivery less invasive than parenteral (oral, pulmonary, transdermal). The most complex challenge is related to therapeutic proteins as it is extremely difficult to process and deliver biomolecules due to their instability and very short half-life in vivo. In fact, SCF technology comprises several processes that offer various possibilities to address the different issues to be solved and to prepare various forms or formulations of the drug, e.g, dry inhalable powder, nanoparticle suspension, microspheres or microcapsules of drug embedded in a carrier, and drug-impregnated excipient or matrix. Moreover, although most previous works dealt with water-insoluble (or poorly soluble) molecules, recent development also permits the processing of very hydrophilic molecules, including fragile biomolecules.

A. Rapid Expansion of Supercritical Solutions (RESS) This process consists in atomizing a solution of the product in a supercritical fluid into a low-pressure vessel [51–53]. It could find valuable applications on a commercial scale only when the product solubility in the supercritical fluid is not too small (≥10-3 kg/kg). This limits the application to nonpolar or low-polarity compounds when CO2 is used as solvent. However, recent works demonstrated that a much wider range of molecules can be processed by RESS when a polar SCF like dimethyl ether was used. In fact, the particle morphology (shape, size, crystalline pattern) can be tuned by playing on the process and equipment parameters [54, 55], as shown in two examples: •

Micronization of lovastatin, an anticholesterol drug, by RESS with CO2 (Fig. 7). From large and irregular particles (Fig. 7a), we obtained either highly porous agglomerates of nanoparticles (Fig. 7b), or microparticles in form of rod crystals (Fig. 7c) or spheres (Fig. 7d) depending on the type of nozzle.

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Figure 7 Micronization of lovastin by RESS. (a) Raw lovastatin; (b) micronized lovastatin (agglomerates of nanoparticles); (c) micronized lovastatin (capillary “long” nozzle); (d) micronized lovastatin (laser-drilled “short” nozzle). (From Ref. 55, courtesy of Separex.)

•

Micronization of Celecoxib, a COX-2 inhibitor registered for arthritis cure. Rapid depressurization of a Celecoxib solution in SCF CO2 (50°C, 29 MPa) led to fluffy agglomerates of elementary nanoparticles. The XRD patterns of these particles (lower two curves) are compared on Fig. 8 with the starting material one (upper curve), the former two curves being presented with an offset of 2000 and 3000 counts per second, respectively, for ease of interpretation. It clearly appears that the starting material is highly crystalline and the generated particles are completely amorphous when the temperature in the atomization vessel is kept low (sample b), and mostly amorphous (sample a) when this temperature is near ambient. We consider that the very short RESS nucleation leads to amorphous material that immediately tends to recrystallize during the particle residence time in the atomization vessel and on the collection filter, if the temperature is not kept lower than a recrystallization temperature that is much below (at least 30°C according to some estimations) the solid glass transition temperature.

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Figure 8 XRD profiles for Celecoxib raw material and RESS-CO2 samples. (From Ref. 55.)

B. Supercritical Antisolvent (SAS) This process can be applied to most molecules that can be dissolved in a very wide range of organic solvents, as reported in numerous articles among which I would cite [56–63] as representative. Recent development opens a bright future for tailor-making new types of particles of different morphologies (Fig. 9), leading to nanoparticles (50–500 nm) or microparticles (0.5–5 µm) or empty “balloons” (5–50 µm) made of nanoparticles. Such particles permit a very significant increase in bioavailability of poorly water-soluble drugs, or the preparation of a drug with a narrow particle size distribution dedicated to pulmonary delivery. It has been shown by numerous examples that the particle morphology can be tuned, including the generation of one or another crystal polymorph in case of polymorphism [55, 63]. As a significant example of antisolvent precipitation of therapeutic drugs, the preparation of insulin particles for inhalation delivery has been subjected to a great effort since the pioneering work of Debenedetti and coworkers [64]. This biomolecule exhibits a good stability during processing as demonstrated by in vivo tests on animals. The first results [65] confirm that the bioactivity of the micronized particles is similar to the activity of the starting material. However, a recent disclosure [66] is rather puzzling as the authors report that the micronized insulin, obtained by an antisolvent on the basis of the work of Foster and coworkers [62, 67, 68], exhibits an “enhanced hypoglyceamia effect” that they suspect to be linked to a different behavior of this form when redissolved in water. If this were

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Figure 9 Micronization by antisolvent. (a) Atorvastatin nanoparticles; (b) atorvastatin microparticles; (c) pigment crystals; (d) pristinamycin needles. (From Ref. 55, courtesy of Separex.)

confirmed, this would be of major interest in both human and economic terms, especially if this could also be applied to other biomolecules. Moreover, microspheres of drug embedded in an excipient for sustained-release delivery can be prepared by this process as detailed below (Section VII.D).

C. Supercritical Fluid Drying This permits the preparation of dry powders from aqueous solutions. The main target is to obtain stabilized dry powder of proteins or other biomolecules that may be denatured by the classical drying processes like spray drying. Contrary to lyophilization, it is also possible to control the particle size and particle size distribution. Several processes can be used to remove water: •

Supercritical Antisolvent: This process is used for obtaining particles from aqueous solutions using a CO2-ethanol mixture as fluid, the alcohol serving to entrain the water into the fluid. However, this process requires huge amounts

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of fluid (the fluid-to-solid mass ratio is in the range of 10,000) since water is only very slightly soluble in the fluid mixture and the solvent residue adsorbed on the particles must be eliminated by a final stripping with pure carbon dioxide [69]. Moreover, it was found that the protein bioactivity may be significantly altered as shown for trypsin, but not for lysozyme [63], depending on pH and temperature stability of the molecule. However, in spite of these strong limitations, this process might still find some applications for drying and formulating various biomolecules like antibodies [70] or DNA fractions [71]. Emulsion Extraction: The solution of an active material in an aqueous medium is emulsified into a polar organic solvent, often in the presence of a surfactant. This emulsion is then pulverized into a supercritical fluid stream that extracts the solvent and water, leading to a dry powder of particles consisting of the active material mixed with other compounds dissolved in the aqueous medium (salts, sugars, etc.)[72]. According to our recent experience [73], it is possible to prepare dry particles (moisture less than 5% wt) of controlled size from aqueous solutions of very hydrophilic compounds emulsified in n-pentanol. Such particles include sugars (sorbitol), amino acids (valine), and proteins (BSA, insulin, various enzymes), as presented in Fig. 10. It is noteworthy to notice that the particle size distribution can be tuned in order to fit the specifications for inhalation (Fig. 11). Stabilized formulation of proteins incorporating buffer salts, sugars, and possibly surfactants can be obtained and bioactivity is preserved as shown by several enzymes (catalase, trypsin, lactase) and by insulin particles injected into diabetic rats after conservation during 1 week and 8 weeks (Fig. 12).

Figure 10 Protein particles of (a) BSA and (b) insulin. (From Ref. 73, courtesy of Separex.)

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Figure 11 Insulin particle size distribution. (From Ref. 73.)

Figure 12 Glycemia (ratio to origin value) after subcutaneous injection of 10 units/kg to diabetic rats: no injection , starting insulin (∆), SCF particles after 1 week , SCF particles after 8 weeks (Φ). (From Ref. 73.)

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Polar SCF Extraction: Instead of using carbon dioxide that requires a polar cosolvent for extracting water, it is possible to use a polar fluid that does dissolve water, like ammonia for insulin drying [68]. CO2-assisted Nebulization and Bubble Drying. Based on the high solubility of carbon dioxide in water, this process (CAN-BD) consists in mixing the pressurized fluid with the aqueous solution of the drug, and expanding the mixture to atmospheric pressure through a nozzle that generates an aerosol of very fine droplets of liquid carrying microbubbles of gas. This plume is further dried by contact with a stream of inert gas (nitrogen) warmed at temperatures between 25 and 75°C, much lower than those currently used in spray drying [74, 75]. Solid or hollow dry particles are obtained in the range of 0.5–5 µm adequate for pulmonary delivery. This technique was demonstrated by drying model proteins (trypsinogen and lactate deshydrogenase) in the presence of stabilization agents (sugars, buffers and surfactants) added in the starting aqueous solution with a satisfactory recovery of the initial enzymatic bioactivity [76].

D. Microencapsulation This process consists in preparing particles in which the active substance is embedded inside a carrier either with a core-shell structure (microcapsules) or with a matrix structure (microspheres). Many processes have been developed, according to the solubilities of the active substance and carrier in the SCF solvent: •

•

Some processes can be applied when the coating is soluble in the supercritical fluid, such as waxes, glycerides, alcohols, fatty acids and esters, and certain polymers. The RESS process is applicable when both the active and the carrier are soluble in the SCF solvent, as firstly demonstrated by Debenedetti et al. [77]. Benoit et al. [78, 79] are developing a deposition process consisting of dissolution of the coating agent into supercritical carbon dioxide and, by changing the pressure and the temperature, precipitating the coating agent onto the active substance particles dispersed into the supercritical solution inside a stirred vessel, leading to microcapsules that are collected after depressurization. In most cases, the coating is not soluble in the supercritical fluid and a significant number of works [58, 80–84] are based on the antisolvent process after the pioneering patent of Fischer and Muller [80], in spite of the strong limitations of this process, as detailed earlier. In another implementation, the active substance particles are in suspension in a solution of a slightly polar polymer in an organic solvent. This suspension is contacted with supercritical carbon dioxide causing coacervation of the coating polymer onto the particles by the antisolvent effect [84].

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22 •

•

Perrut Collection and encapsulation of nano- or microparticles suspended in a stream of supercritical fluid are effected by scrubbing this fluid with a liquid saturated solution of the coating agent in an organic solvent: Extraction of part of this solvent by the fluid causes supersaturation and nucleation of the coating agent preferably onto the particles [85]. Several processes are based on the property of pressurized CO2 to dissolve in various substances that can be used as drug carriers (lipids, polymers, etc.) up to 30% wt, and causing a significant decrease (10–50°C) of the carrier melting temperature or of the carrier viscosity when liquid. This behavior, widely exploited for powder coating production or paint spraying (viscosity reduction), is also used for the preparation of thermally sensitive bioactive materials that can be processed at a much lower temperature than without the presence of an SCF solvent. Bone-replacement material (calcium hydroxyapatite+PLGA) [86, 87], drug composite particles, like micro-spheres of vaccine [88], are examples for such processes.

But the most attractive implementation of microencapsulation is the formation of particles according to the concept known as particle generation from supercritical solutions or suspensions (PGSS). This consists of atomizing a solution of compressed gas or supercritical fluid inside a substance in the liquid state. The rapid fluid demixing induces solidification of the substance in the form of fine particles [89, 90]. In the so called fluid assisted microencapsulation process, particles of the active material are dispersed in the form of a slurry inside the liquefied carrier that leads to very small core-shell microcapsules of the active material inside the carrier (Fig. 13) [91, 92]. According to this process, microcapsules of proteins can be easily prepared under mild conditions that do not lead to protein denaturation and loss of bioactivity as demonstrated by lactase and more recently by a therapeutic peptide. It is to be noticed that this process is very

Figure 13 (a) Ovalbumin and (b) lactase microencapsulated in a lipid (hydrogenated palm oil GV-60). (From Ref. 92, courtesy of Separex.)

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easy to scale up and to be operated in compliance with GMP rules, possibly in a sterile environment when required. Moreover, in contrast with antisolvent processes, the CO2 consumption is rather negligible. As shown in Fig. 14, various release curves can be obtained depending on the coating agent: For most excipients, the “burst” effect is very limited, proving the quality of the active particle coverage by the coating. Similarly, a variant was recently disclosed [93], consisting of a combination of milling the active substance inside a carrier liquefied by pressurized fluid (typically a lipid like lecithin) and further atomizing it to form microcapsules. A process derived from the same concept was recently patented [94] for tablet coating. It consists of pulverizing the suspension of the coating agent(s) into a supercritical fluid onto the tablets processed in an established coating equipment. •

Filardo et al. [95, 96] proposed to polymerize (or copolymerize) monomers onto particles of a substrate suspended in supercritical carbon dioxide, in the presence of a surfactant and a polymerization initiator, in order to obtain microcapsules.

E. Complexation Cyclodextrins and derivatives (CDs), prepared from whey, have a very specific cage structure with an external surface that is highly hydrophilic—explaining their

Figure 14 Release curves of ovalbumin microencapsulated in various excipients in a buffer solution at 37°C (From Ref. 92.)

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high solubility in water—and an internal surface that is hydrophobic, permitting them to host water-insoluble molecules in the form of a labile complex. Therefore, CDs are very promising carriers for small molecules in order to increase their bioavailability. At present, a few drugs including CDs are marketed in various countries, although CDs cannot be considered as a neutral excipient as they do interact within the biological fluids. SCF processing of CDs or CD-complexes have not been widely investigated until recently, where it was shown that an SCF can be used either as a solvent for drug extraction [97] or as a vector for drug inclusion [98]. We just disclosed a new process [99] consisting of atomizing a solution of the active drug and a CD derivative in an organic solvent, into a stream of SCF solvent similarly to the antisolvent process. This procedure permits both to obtain a high degree of complexation of the drug into the CD cage and to control the particle size distribution (Fig. 15), leading to a drastic increase of apparent solubility of the drug in water (about 30 times for the case of Celecoxib!) although bioavailability improvement has not yet been proven.

F. Impregnation The high diffusivity and tunable solvent power of SCF are the basis of supercritical impregnation. Supercritical fluid-soluble substrates can be easily impregnated inside porous media as demonstrated by many investigators using various matrixes like polymers, wood, and paper. This can be used to prepare controlled drug delivery systems [100], food-grade carrier microparticles impregnated with flavors or colorants, etc. It is of special interest to combine online supercritical fluid extraction

Figure 15 Particles of complex (methyl-ß-cyclodextrin/Celecoxib). (Courtesy of Separex.)

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and impregnation, especially for nutraceuticals production as illustrated by kavakava extraction with online impregnation of the kavalactone-rich extract into maltodextrine [101]. However, these impregnation processes are only feasible when the active compound is soluble in the supercritical fluid. That is not the case for the so called concentrated powder form (CPF) process [102] through which powdery agglomerates with unusual high liquid concentrations of up to 90 wt % can be obtained by spraying gas-saturated solutions and admixing a solid carrier material with the spray. The gas, which must be at least partially soluble in the liquid, generates small droplets that infiltrate the porous carrier particles or agglomerate the nonporous ones. More surprisingly, although these molecules are not at all soluble in supercritical carbon dioxide, microspheres were obtained by infusion of a protein or a peptide into a polymer swollen by the fluid [103], as later exemplified by insulin infusion into polymer beads [104, 105], or of a fluorescent protein into a PMMA disk [106]. This infusion looks very slow (impregnation takes about 24 h) and is not yet

Table 3 Formation of Neat or Composite Microparticles

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understood, but it is so simple that it could raise interest for preparation of drug delivery systems incorporating biomolecules.

G. Process Choice Table 3 summarizes the different cases in order to guide the reader in his choice through these various formulation processes.

VIII. BIOLOGICAL APPLICATIONS While the biotechnological synthesis of therapeutic products is in progress, cell lysis by SCF is the more interesting because this process does not lead to very small membrane fragments in contrast with classical homogenization, preserving fragile molecules and facilitating downward processing [107]. Regarding sterilization, it is known for long that CO2 has a biocide effect on most bacteria [108], and it has been demonstrated that processing with SCF carbon dioxide at a pressure near 200 bar leads to results similar to those obtained by the classical high-pressure treatment currently used in the food industry (4000–8000 bar) [109]. However, whereas excellent bacteria destruction is completed even at very moderate pressures and temperatures, spore inactivation is not as easy and conflicting results were published in the literature. It seems that combination of pressure cycling and/or ethanol addition and/or short thermal treatment at around 75°C greatly improves spore collapse and final decontamination [109, 110]. Nevertheless, large-scale orange juice pasteurization with pressurized carbon dioxide is very promising either by direct contact [111], or permeation through a polymeric membrane [31], as discussed in Section III.C. The inactivation mechanism is not definitely cleared at the present time and several conflicting hypothesis have been proposed; e.g, probably, several actions do contribute to the cell death. Membrane alteration due to the strong interaction between carbon dioxide and phospholipids, irreversible inhibition of key enzymes by the pH decrease inside the cytoplasm, and precipitation of carbonate ions, may contribute to this effect. It was also proven that virus inactivation occurs during CO2 delipidation of bone implants [25, 26]. On the other hand, virus inactivation was also demonstrated for plasma fractions [23, 112] with N2O or CO2 under mild conditions to avoid denaturation of the very fragile proteins. The preliminary results presently available show that inactivation is very acute for enveloped viruses, but is too variable for nonenveloped viruses to permit the use of this process for clinical product treatment. Finally, I would stress on the fact that this biocidal effect is of key importance for pharmaceutical and biomedical applications, as SCF processing does not at all increase the bioburden, but contributes to maintain or reach sterility.

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IX FUTURE TRENDS Even if supercritical fluid technology is not yet widespread in the pharmaceutical industry, except for extraction of active compounds from vegetal sources (phytopharma- or nutraceuticals), many promising applications are now under development, especially for new drug formulations for which SCF processes propose innovative routes adapted to each case. The pipeline is now rich in several formulations to be shortly introduced for registration, especially for manufacturing inhalable particles. Among the advantages of SCF solvents, intrinsic sterility and low operating temperature should be emphasized, especially when processing biomolecules. Ironically, the intense R&D work is leading to many attractive results, but also to many patents, and is rendering the “intellectual property” situation rather complex. This may temporarily cause pharmaceutical companies to refrain from entering this promising and “green” technology into their formulation “toolbox” on the short term. Regarding scale-up, most SCF processes are now operated at a scale corresponding to commercial needs in the pharmaceutical industry [113, 114], and compliance to GMP seems accessible at present. For particle design and formulation, equipment for the preparation of clinical lots is already available (RESS, antisolvent, drying, microencapsulation) and some commercial-scale demonstration are now ongoing without any major issue, except micro- or

Figure 16 Lab-scale SCF particle design unit. (Courtesy of Separex.)

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Perrut

Figure 17 SCF particle design pilot unit in compliance with GMP. (Courtesy of Separex.)

nanoparticle collection and harvesting as for any process leading to such particles. We recently designed equipment of several sizes: laboratory-scale for screening new chemical entities or biomolecules available in very limited amounts (Fig. 16), pilot-scale equipment for preparing gram samples, and four semi-industrial plants for manufacturing clinical lots that are designed, built, and operated under strict quality assurance and documentation according to GMP rules (Fig. 17).

ACKNOWLEDGMENTS The author thanks Dr. Jennifer Jung and Dr. Fabrice Leboeuf for fruitful discussions and text reviewing.

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2 Reactive Solvent Extraction Hans-Jörg Bart Technische Universität Kaiserslautern, Kaiserslautern, Germany

Geoffrey W.Stevens The University of Melbourne, Victoria, Australia

I. INTRODUCTION Reactive solvent extraction makes use of specific chemical reactions in order to promote or achieve a separation task. Commercially available liquid ion exchangers are widely used in that respect. The nature of the chemistry starts with Van der Waals’ interactions, covalent or ionic binding which may be supported by steric effects. Additional effects can result from solvents and cosolvents (modifiers) and surfactants present in the system. After a short introduction into the chemistry the focus of this chapter will be on reactive equilibria and mass transfer and its impact on the performance of extraction columns and their current applications. The first extraction processes were with solid extraction in the extraction of perfumes, waxes, pharmaceutical active oils in an operation quite similar to a modern Soxhlet apparatus. An extraction pot with an age of about 3500 BC was found 250 km north of Baghdad (see Fig. 1) and extraction instructions were documented by a Sumerian text of 2100 BC [1]. The next major improvements were in the medieval age with new solvents like ethanol, mineral acids, and amalgams used to extract and purify metals. The first extraction of a metal was reported by Peligot [2] who used diethylether to extract uranyl nitrate which gave a basis to uranium extraction within the “Manhattan” project in the 1940s [3]. Reactive solvent extraction was then a niche for pyrometallurgically difficult-to-separate metals (Nb/Ta, Zr/Hf) until the 1960s when there was a breakthrough with copper extraction. LIX (liquid 37 Copyright © 2004 by Marcel Dekker, Inc.

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Figure 1 Extraction pot: 1=cooling cap, 2=vapor, 3=extraction pot, 4=heating, 5= condensate droplet, 6=condensate film, 7=feed material, 8=solvent (oil or water). (From Ref. 10.)

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ion exchanger) chemicals [4] were size-selective extractants for separation of copper from iron which allowed copper recovery from low-grade ores after a sulfateleaching process. Meanwhile the use of liquid ion exchangers has expanded to a large array of ions and neutral solutes in hydrometallurgical, environmental, petrochemical, chemical, and biochemical applications [5–9]. Solvent extraction systems can be categorized according to single solvent, reactive solvent, and amphoteric, respectively, polymeric solvent systems.

A. Single Solvent Systems Single solvent systems are traditionally used in physical extraction when bulk organic chemicals (toluene, xylene, butanol, etc.) are used to extract a solute. Nowadays a new type is based on room temperature ionic liquids, which is a new class of solvents comprised of low melting organic salts. Much of the interest has centered on their unusual properties in reaction engineering [11, 12] and liquidliquid separations [13]. They are composed of large organic cations (see Fig. 2) which—when appended with alkyl groups with various anions—results in low melting salts with virtual no vapor pressure and varying water miscibility due to the anions. Typical anions promoting water miscibility are Cl , and and immiscibility is with Due to their ionic nature there is a considerable interest in environmental or hydrometallurgical fields and immobilizing transition metal catalysts. Their use is still limited because of lack of stability in the presence of water and replacing organic extraction phases with ionic liquids for recovery of metals in high cost [14]. However, a price level twice as high as the solvent N-methyl-2pyrolidone is expected for industrial-scale applications in the near future [H. Schoenmakers (BASF), personal communication, 2003]. One of the well-known ionic liquids used in reactive solvent extraction is methyltrioctylammonium chloride, known as Aliquat 336. This is one of the liquid ion exchangers which also belongs to the class of reactive solvent systems which shows there is no strict borderline in the above classification scheme.

Figure 2 Representative ionic liquid cations.

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B. Reactive Solvent Systems The characteristic of reactive solvent systems is that the organic phase consists of a mixture of liquid ion exchangers and a diluent. The latter is used to adjust the rheological and transport properties of the high viscous or sometimes solid ion exchanger. For practical purposes, the liquid ion exchanger is diluted usually in a nonaromatic, high boiling (b.p.≈500 K) organic diluent, which is immiscible with water. This prevents solvent loss and toxic problems and gives the organic phase the required physical properties (high interfacial tension, low viscosity, low density), since most liquid ion exchanges are highly viscous or even solid. In some cases a modifier, usually a long chain alcohol (e.g., isododecane) is added to help in the solubilization of the solute-ion exchanger complex. At very high solute loadings the organic phase may split into a solvent-rich and a solventless fraction, especially when using aliphatic diluents. Such a liquid three-phase system can be inhibited by the modifier. As can be seen, the organic phase in reactive extraction is usually a mixture in contrast to physical extraction systems. The practical handling and design of a reactive solvent extraction process is given in appropriate handbooks (e.g., Refs. 5–9), but a short review on the principles involved are given. Liquid ion exchangers are available as either anion, cation, or solvating exchanges. An example of an anion exchange is (1) The quarternary R4-alkyl-substituted ammonium chlorides are commercial available and can be stripped with a surplus of chloride, hydroxide, etc. and thus the solute is regenerated in the reextraction or stripping step. The quarternary compound has the advantage of being able to be used in alkaline media compared to the frequently used ternary amines. Primary, secondary (both are water soluble, less used), and tertiary amines are only stable in acidic aqueous media, since hydroxide destroys the ammonium complex: (2) Volatile anions like acetate, formate, etc. can also be removed and stripped by temperature swing which yields the free tertiary amine, R3N, similar to Eq. (2). The change in counter ion concentration and temperature gives rise to a reversible extraction process according to Eq. (1). Generally, the selectivity of anion exchangers is not always good and there are many developments of new hostguest-ligands which take advantage of the different sizes of the solutes [15]. The cation exchange mechanism is as follows: (3) Here zinc is extracted with a di(2-ethylhexyl) phosphoric acid in its H form (HDEHP) and two protons are set free. This causes a pH shift during extraction,

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which can be avoided if the ion exchanger is, for example, in the Na form. Reextraction is usually with strong mineral acids (preferably H2SO4). Besides the alkylated phosphoric compounds, also available are phosphonic and phosphinic acids and their thio forms. The latter ones are strong extractants and dithiophosphoric acids are difficult to strip, but can be used at a feed pH value lower than 1. Carboxylic acid-based ion exchangers are seldom used due to their high water solubility and aryl-substituted compounds have also limited applications for steric reasons. Most liquid ion exchangers have branched alkyl substituents since n-alkyls tend to crystallize and are not liquid. As mentioned above, the breakthrough with reactive solvent systems was with chelating ion exchangers for copper recovery. A size-specific host-guest complexation in addition to ion exchange separates copper from impurities such as iron. As can be seen in Fig. 3, the nitrogen chelates the copper ion, developing a new six-ring structure only stable with copper as solute. An example of nonstoichiometric extraction with solvating agents is shown in Eq. (4): (4) The difference with physical extraction is that the capacity and extraction power of that liquid neutral ion exchanger is much higher than with any bulk organic solvent (toluene, xylene, butanol, etc.) used in physical extraction. Alkyl-substituted phosphates, phosphonates, and phosphine oxids (e.g., trioctylphosphine oxid) are widely used.

Figure 3 Copper chelate structure.

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Tributylphosphate has a very high solvating power for neutral substances; e.g., undissociated acids, salts, and will extract them. High temperatures and low ionic strength (pure water) will achieve reextraction. Most of the solvating exchangers can be used undiluted due to appropriate physical properties. Carbon-based compounds (ketones, ethers, etc.) suffer from lower capacity and higher water solubility and the chemistry is thus very similar to physical extraction systems. They can be regenerated by distillation which is otherwise limited due to high boiling points. Liquid ion exchangers can be mixed together in order to generate synergistic effects. This means that the effect of a mixture gives a nonlinear improvement in regard to the single systems. Basically, the effect results from an improvement to solvate the new ion exchanger solute complex. Such synergistic behavior was also reported with extraction kinetics when Henkel improved their first commercial copper extraction reagent LIX64 with a mixture of a small amount of LIX63. It markedly enhanced the kinetics of the new reagent LIX64N [16] which was a milestone and the commercial start to extract base metals on a large scale. In general, one can find numerous references to synergistic effects reviewed in all the solvent extraction textbooks. However, an equimolar mixture of cation and anion exchanger gives a “mixed” extraction system [17] which can extract salts or acids according to Eq. (5). Reextraction is then either by shift of temperature, aqueous ionic strength or acidity/basicity. (5) The selection of the right reactive solvent phase is the key to a successful separation process. Below is a list of the various solvent-selection criteria. Some of these are essential for the separation while others are desirable properties which will improve the separation and/or make it more economical. The solvent selectivity, recoverability, and a large density difference with the raffinate are essential. Some of the requirements on physical or reactive solvent phases will conflict and a compromise may be necessary. •

•

Selectivity. A high separation factor enables fewer stages to be used. Extract or raffinate purities can be improved by using more solvent. If the feed is a complex mixture where multiple components need to be extracted, group selectivities become important. Capacity. A high value of the distribution ratio indicates a high solvent capacity for solute and permits lower solvent/feed ratios. Since it is a function of the temperature, a temperature gradient over the extractor might be used to increase the capacity (higher temperatures tend to increase the capacity but lower selectivity). Also, addition of other components—antisolvents—might influence the distribution. Often there must be a compromise between selectivity and capacity. The use of extract or raffinate reflux can also improve the purity of either product. A more striking factor on capacity is the pH value

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•

•

• •

•

•

• •

43

which is used for extraction/stripping purposes [see Eqs. (2), (3)]. For difficult separations a pH gradient in columns is sometimes used in the enriching section, optimizing the reflux conditions [18]. Recoverability of solvent. Recovery of the solvent phase should be easy. In physical extraction systems it is preferably by means of a simple distillation or a distillation followed by a stripping column. In reactive solvent extraction systems the regeneration is either by pH shift or change in ionic strength which alters complexation, as has been discussed above. Density. The density difference must be large enough (e.g., 150 kg m-3) to ease the settling of the liquid phases. Higher density differences permit higher equipment capacities. Systems with very low density differences require expensive centrifugal extractors. Viscosity and melting point. High viscosities reduce the mass transfer efficiency and lead to difficulties with pumping and dispersion. Low viscosities also benefit rapid settling and capacity (the more viscous phase is usually dispersed). Column temperatures are usually determined by the viscosities. The melting temperature of the solvent should preferably be lower than ambient for ease of handling. Insolubility of solvent. The mutual solubilities of nonsolute and solvent should be low. If this is not the case, an additional separation step is needed to recover solvent from the raffinate. Interfacial tension. High interfacial tension permits a rapid settling due to an easier coalescence, permitting higher capacities. Low interfacial tension facilitates the phase dispersion; large interfacial areas are easily achieved and thus higher separation efficiencies, but may require large volumes for phase separation. A too-low interfacial tension leads to emulsification. Which phase dispersed. Column internals should not be wetted by the dispersed phase which reduces column capacity. Often physical mass transfer from the continuous to the dispersed phase enhances droplet coalescence and bigger droplets have lower mass transfer efficiency Toxicity and flammability. For food processing only nontoxic solvents will be taken into consideration. In general, any hazard associated with the solvent will require extra safety measures. As to this, aliphatic diluents are the preferred ones. Corrosivity. Corrosive solvents increase equipment cost but might also require expensive pre- and posttreatment of streams. Such a problem can be alleviated somewhat by dispersing the corrosive solvent. Thermal and chemical stability. It is important that the solvent should be thermally and chemically stable as it is recycled. Especially, it should resist breakdown during the solvent recovery in, for example, stripping columns. Sometimes special measures are required to prevent solvent degradation (for example, for furfural, a temperature limitation, nitrogen blanketing, and feed deaeration are required).

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Bart and Stevens Availability and costs. Solvent should be ready available. It is not the price of the solvent that is important, but the annual costs due to the inevitable operation losses. Environmental impact. The solvent should not only be compatible with upstream and downstream process steps, but also with the environment (minimal losses due to evaporation, solubility, and entrainment).

In general, the requirements for reactive solvent extraction phases are similar to that of physical extraction ones. The viscosity should be lower than 2 mPa s-1, boiling range in the region from 420 to 520 K, and densities from 750 to 900 kg m-3. The flash point should be at least 25 K higher than working temperature and a value higher than 330 K is recommended. Aromatic diluents with equivalent molecular weight as aliphatic ones are more polar and thus more water soluble. The higher price and higher toxicity of aromatic diluents lead to a preference of aliphatic diluents in industrial practice. The degradation of the diluent is usually negligible in comparison to that of the ion exchanger. The latter can be chemically, thermally, and radiation-chemically degraded and also can be poisoned by an irreversibly extracted compound. “Crud” (Chalk River unidentified deposit) (G.M.Ritcey, personal communication, 1998) is the term describing the pollutant phase containing mineral or biological solids that tends to build up at the phase interfaces in the solvent extraction plant. Colloidal and dissolved substances (especially silica) precipitate at high shear rates and humic acids promote this behavior as reported in hydrometallurgical applications [19].

C. Amphoteric Surfactant or Polymeric Solvent Systems Polar or apolar polymeric extraction systems can be characterized by their solubility and form either aqueous biphasic systems, reverse micelles, respectively, liquid membranes, or are insoluble polymers which act as porous carriers.

1. Aqueous Biphasic Systems A wide variety of techniques use neutral or ionic surfactants in aqueous biphasic systems. Depending on the kind of surfactant used a range of different types of extraction have been developed [20] including cloud-point extraction [21], micellar extraction [22], thermo-separating polymers [23], and microemulsions [24]. Micoemulsions are thermodynamically stable and may exhibit a density between a pure aqueous and pure organic phase. They are classified according to the Winsor types and as all aqueous biphasic systems are very temperature sensitive [25, 26]. All the aqueous two-phase systems depend on poly(oxyalkylenes) like poly (ethylene)glycol, PEG, and similar polymers to be effective. The shift between

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extraction and reextraction is achieved either by change in temperature or ionic strength. Classic aqueous biphasic systems are formed of either mixtures of polymer 1 (e.g., PEG) and polymer 2 (e.g., dextran) or polymer 1 and salt (e.g., K3PO4) [27, 28] above their critical concentrations. A split in two aqueous phases with decreasing temperature is related to the upper critical solution temperature with PEG polymer systems [29, 30] and with increasing temperature with PEG/salt systems [31]. While the applications to biomolecule partitioning are particularly well known [32], metal ions can also be separated [33]. The distribution of charged species depends on the pH value and the distribution of uncharged species follows the order of partitioning in 1-octanol/water systems [34]. Cloud point extraction systems and thermo-separating polymers, in contrast to aqueous biphasic systems, are also known as micellar. An increase in temperature or change in salt concentration will lower the solubility of the nonionic surfactant and at its lower consolute point (cloud point) two phases will appear: a micellefree solution of surfactant close to its critical micelle concentration (cmc) and a less dense upper phase depleted in surfactants. Thermo-separating polymers (PEO– PPO copolymers) form micelles at lower concentrations and therefore have lower cmc’s as cloud point extraction systems [alkylethoxylates, poly(oxyethylene) ethers], which reduce the amount of polymer required to achieve phase separation. The use of cloud point extraction for nickel with Triton X-100 as surfactant and 1(2-thia-zolylazo)-2-naphthol as extractant was first reported by Ishii et al. [35]. The solventlike properties of polymeric solubilizing systems may vary considerably. On a certain polarity scale [36] Triton X-114 micelles at 5°C are equivalent to the solvent 1,2-dichloroethane and at 25°C it increases its polarity equivalent to the solvent 1,5-pentanediol. In order to reduce losses, the process may be performed in hollow fiber membranes [37, 38]. The membrane separates the contaminated feed stream from the receiving polymer solution and only the solute, not the polymer, can pass the membrane and be separated [39]. Alternatively, such polymers can be covalently bound to a supporting chromatographic stationary phase, and thus losses of the polymer in operation can be avoided [40].

2. Inverted Micelles and Liquid Membranes The primary focus of aqueous soluble polymers is to allow extraction of biomolecules (proteins, amino acids, etc.) into a new aqueous environment, avoiding denaturation/unfolding of the solute. An alternative to this is inverted micellar extraction which is used for the same solutes. The aqueous environment in the inverted micelle avoids denaturation and the micelle is extracted into the organic phase, and reextraction is by change in temperature or ionic strengths [38]. For low-molecular-weight solutes (organic acids, metal ions) the liquid membrane process is an alternative to solvent extraction especially with dilute feeds (less than 1 g L-1 solute). Surfactants like SPAN 80 (sorbitanmonooleate) or ECA 4360 (n-oligo(ethylenimino)succinimido-polyisobutylene) are dissolved in the organic

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phase II, which is recycled, and makeup in a process circuit is therefore at a minimum. The aqueous stripping phase I is emulsified in the organic phase. In an emulsion liquid membrane process (see Fig. 4) this emulsion is dispersed in the aqueous feed solution III, and after mass transfer it is split in an electric field and the organic phase is reused. The mass transfer or pertraction is performed in mixer settlers or in extraction columns as long as there is a proper density difference for dispersed emulsion phase buoyancy and phase separation in a settler. An alternative to this are dispersion-free membrane processes [41] with all the advantages/disadvantages of membranes with extraction processes as discussed in Section IV. A detailed discussion of industrial applications is given by Marr & Draxler [42]. An advantage of this technique is the use of the facilitated transport [43, 44] which greatly helps with very diluted feed streams. As can be seen in Fig. 5, the facilitated transport concentrates the dilute solute in the feed solution into the stripping solution depending on the phase ratio of strip to feed solution.

3. Extraction Chromatography An alternative to ion exchange resins are polymeric porous carrier matrices used in extraction chromatography. The recovery of substances from highly diluted aqueous phases with liquid-liquid extraction is difficult because the amount extracted in one equilibrium stage is usually rather small. The use of solvent-impregnated

Figure 4 Liquid membrane circuit (static mixer, permeation column and electrostatic emulsion splitter, I=receiving phase, II=organic phase, III=aqueous feed resp. raffinate phase).

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Figure 5 Facilitated transport for zinc with D2EHPA (I=receiving phase, II=membrane phase, III=feed phase).

resins combines the advantages of reactive liquid-liquid extraction and ion exchange in fixed beds [45, 46]: • • • • •

Liquid ion exchanger is adsorbed undiluted which results in a higher capacity. Liquid ion exchangers are cheap and easily available, which generates wide flexibility in solute extraction. Even mixtures of liquid ion exchangers might be used. Operation of a fixed bed is simple and the feed might be discontinuous. Bleeding loss of the ion exchanger is less than entrainment losses in liquidliquid systems.

However, there is a necessity to install two columns, one for extraction and one for stripping/regeneration, in order to achieve a continuous operation. In general, the mass transfer is slower compared to a liquid-liquid extraction process, since the solute has to diffuse into the porous support material. Inert cheap macroporous materials are available (e.g., DELOXAN AP II, Degussa Hanau, or XAD, Rohm & Haas) and the impregnation [47] is easily performed in situ in fixed bed columns [48]. The support materials are either gel-analogous (e.g., Amberlite XE-305) or macroporous inert (e.g., XAD2, XAD4, XAD16) or functionalised resins (e.g., DELOXAN AP II). In the first alternative the ion exchanger is only on the surface, which is not very efficient. The porous materials start to adsorb the ion exchanger in the smallest pores and also in the bigger ones with high loadings [49]. An additional functional group [50] may interact with the ion exchanger and thus reduce bleeding [51] or may actively extract a solute [52]. Alternatively, the ion exchanger can be mixed during the synthesis of the resin [53] and such resins are

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known as Levextrel resins (Bayer Leverkusen). This procedure affects the resulting porous structure and gives different results when different ion exchangers are used.

II. REACTIVE SOLVENT EXTRACTION EQUILIBRIA A. Introduction The basis of any early process design and feasibility study is in the knowledge of the equilibrium conditions. There are several sources for liquid-liquid equilibria (LLE), as in the DECHEMA databank and others [54–56]. They are implemented in commercial flowsheet simulators, as are ASPEN, ChemSep, HYSIM, ChemCad, etc. and usergenerated experimental data bases may be appended. In contrast to LLE for physical extraction, the reactive solvent extraction data base is much more limited and only available in a few texts [6, 7, 57, 58] and compiled also in the Proceedings of the International Solvent Extraction Conferences [59–70]. This has not been implemented in commercial flow-sheet simulators or data banks yet. Equilibria, physical, and nonequilibria data for zinc extraction with the cation exchanger bis(2etylheyxl) phosphoric acid are given, available in the web (http://www.dechema.de/Extraction/ or http://www.icheme.org/ learning) as this system is recommended by the EFCE (European Federation of Chemical Engineering) as standard test system for reactive extraction studies, considering the nonideality of mixtures [57, 71]. The nonideality in the aqueous phase can be described with an activity coefficient model (Pitzer, etc.) and for the organic phase are two alternatives: • •

The organic phase can be considered to be ideal and all deviations are compensated when allowing different ion exchanger-solute complexes to exist. There is only one complex in the organic phase and deviations are calculated by an activity coefficient related model which is discussed in the text below.

In a liquid-liquid system the conditions for the thermodynamic equilibrium requires thermal and mechanical equilibria (i.e., the same temperature and pressure in all phases), as well as chemical and reaction equilibria. The last conditions requires the same chemical potential µ1 for each component in each phase (α and β) and (6) In an n-component system with k reactions vi,R is the stoichiometric coefficient of compound i in the reaction R. An example of such a reaction equilibrium is with the complexation of zinc chloride with hydrochloric acid in aqueous solution: (7) with k=1, n=2, and

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The fugacity (activity) in both phases can be obtained from an equation of state (EOS) when the system is at high pressures or when any of the components are close to or above the critical point. This is of interest with high-pressure extraction using for instance liquid CO2 as solvent in applications with food and pharmaceutical industry [72]. At moderate or low pressures, where the liquid phase is incompressible, the use of excess Gibbs energy models (GE models) and activity coefficients is handier. Recall that the chemical potential of a component in the solution is (8) where a is the activity, x the mole fraction, and f the (rational) activity coefficient, and is the chemical potential of pure component i (the reference state), for which fi=1. Since both phases (α and β) at equilibrium are at the same temperature, the reference states are the same and the equilibrium condition reduces to [73]: (9) The distribution ratio remains constant if the ratio of activity coefficients is independent of the species concentration i. This is valid in dilute systems and known as Nernst’s distribution law Di (usually in molar units): (10) The Gibbs-Duhem equation relates the chemical potentials to the composition at constant p and T: (11) and with Eq. (8): (12) Thus any empirical or theoretical relations representing the composition dependence of the activity coefficient must be a solution of this equation.

B. Aqueous Nonideality Electrolyte systems play a vital part in many solvent extraction processes, especially when ionic solutes are extracted by liquid ion exchangers. Anions, cations, and even nondissociated compounds will be extracted and the extractability mainly depends on the dissociation equilibria of complexes, salts, and acids.

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In regard to the reaction equilibrium the law of mass action for a single reaction R is:

(13) Expressed in molal units this becomes (14) Additionally, as a summation condition, electroneutrality is required in electrolyte systems: (15) which means that the charges z of cations and anions must be balanced. In ionic mol kg-1 water) and solutions the reference state is usually on a molal scale ( thus also the activity coefficient: Henceforth, the superscript (m) for molal will be omitted and a conversion from molal to molar (c) or mole fraction (x) is given elsewhere [71]. An extended review on thermodynamics of electrolyte systems is in the textbooks of Horvath [74] or Zermaitis et al. [75]. A summary of electrolyte activity coefficient models, examples, and data are given by Zermaitis et al. [75]. Beyond extremely dilute solutions, where the Debye and Hückel expression [76] is valid, the Pitzer model [77, 78], which is a virial type equation for the excess Gibbs energy and is an improvement upon an earlier model proposed by Guggenheim [79, 80], is applicable up to 6 molal ionic strength. In order to tackle weak electrolyte systems [81] with molecular solutes, the second and third virial coefficients in the basic Pitzer equation for molecule-ion and molecule-molecule interactions must be considered. However, binary and ternary parameters between species of equal charge are usually set to zero. As an alternative to the modified Pitzer model the NRTL model is also used. It is based on two fundamental assumptions: • •

The local composition n of cations (anions) around a central cation (anion) is zero. The distribution of anions and cations around a central molecule is such that the net local charge is zero.

The Pitzer model reduces to the Debye-Hückel formula at infinitely diluted electrolyte solution and in the absence of electrolytes to the NRTL model [82]. It

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has been also extended to model-mixed solvent electrolyte systems and can even be used for nonaqueous solvents (MNRTL) [83]. Commercially available flow-sheeting simulators (e.g., ASPENPLUS, CHEMCAD, etc.) give access to the electrolyte activity models with an appropriate parameter data base. For solutions of common industrial interest, special inserts can be loaded (e.g., with ASPEN BRINE (H2O, CO2, H2S, NaCl), CAUST (H2O, NaCl, Na2SO4, NaOH),…), where all the necessary reaction equilibria parameters are included.

C. Organic Nonidèality 1. Regular Solution Theory A predictive concept is the regular solution theory first developed by Van Laar [84] deriving activity concepts from critical data using the van der Waals equation of state. However, the results show strong dependence on the mixing rules applied. This was improved by Hildebrand and Scott [85] who replaced the two van der Waals’ parameters with two new ones, the molar volume Vt and the solubility parameter δt of a solute i. The regular solution is one for which the excess entropy of mixing is zero. The solubility parameter δ is defined as (16) where ∆Ui is the internal energy of vaporization and at normal pressures is related to the heat of vaporization, ∆vap Hi as shown. For multicomponent mixtures the Hildebrand and Scott model relates the activity coefficient to the solubility parameters δ >)2 RT ln fi=Vi(δl-<δ

(17)

with the mean solubility parameter of the organic phase <δ>=Σϕkδk

(18)

where ϕ is the volume fraction of the component k in the mixture which can be calculated from the molar volumes Vi of all solutes and the solvent. A Flory-Huggins correction considers effects of different molar volumes: (19) These effects exceed 10% if the molar volumes of the solutes differ more than 50% with respect to the solvent, and the use of this correction is recommended. The reference state is infinite dilution where the mole fraction of the solvent xs approaches unity, and Vs can replace Σxt Vr

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The regular solution model is suitable only for nonpolar mixtures of molecules that are not too different in size, the activity coefficients always exceed unity, and the excess enthalpy is always positive. Nevertheless, even in the absence of any other information, the predictive model is quite useful in certain cases. A compilation of solubility parameters is given by Barton [86].

2. Excess Thermodynamic Functions There are two main causes of nonideality in a mixture: 1. 2.

The molecules in a mixture differ in size or shape. The interactions between the surface of the molecules differ.

Abrams and Prausnitz [87] have combined the local composition concept with the quasi-chemical lattice model of Guggenheim [88] to a truly two-parameter model: UNIQUAC (universal quasi-chemical theory). Here the Gibbs excess energy results from two additive parts, a combinatorial term gC accounting for molecular size and shape differences, and a residual term gR accounting for the molecular interaction energies. The UNIQUAC equation is of approximately equal accuracy to the simpler Wilson equation in respect to VLE, whereas only UNIQUAC can handle a liquid phase split and the enthalpy of mixing hmix [89]. The two-liquid theory of Scott [90] combined with the local composition concept is the basis of the NRTL (nonrandom two-liquid) theory of Renon and Prausnitz [82] and Abrams and Prausnitz [87]. The local composition on a molecular level differs from the macroscopic composition. The molecules do not mix randomly because of interactions with their surroundings (polarity effects, hydrogen bonding, etc.). However, the local mole fraction cannot be measured easily but must be related to the overall composition. A Boltzmann factor from statistical thermodynamics relates the local mole fractions xij to the overall ones xj; (20) The three-model parameters are the residual Gibbs energies with RT and gij and gij an empirical “nonrandomness” parameter αij which characterizes a binary system. Both gij and αij are inherently symmetric (gij=gij and αij=αij ). The general expression for a multicomponent mixture is then (with γi written instead of fi) (21) The nonrandomness parameter is approximated by the inverse of the number of the nearest neighbours of a molecule. The values αij of are usually between 0.2 and

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0.47 and when a liquid phase split takes place, These limits have been found empirically and in liquid-liquid extraction a value of 0.2 is often assigned [91, 92]. The binary parameters of the NRTL equation can be determined by fitting activity coefficients obtained from VLE data and is often found to vary linearly with temperature. Besides VLE and LLE, one can calculate limiting activity coefficients and hmix in multicomponent mixtures from the binary coefficients.

3. Solvatochromic Scales Deviations from ideality may be caused by differences in size (small, large, spheric) or energy (polarity, functional groups). Properties like polarizability, acidity, basicity, etc. are usually considered only indirectly in that respect. However, a solvent is often characterized by the latter terms. On a molecular scale a solvation process will take place in several stages, although only the overall process is measurable. First, a cavity must be created in the solvent to accommodate the solute. Donor-acceptor bonds between the solute and solvent will develop and dipole orientations will be induced in nonpolar but polarizable solvent molecules by ions and dipolar solutes. Once this new aggregate is formed, the solvated solute may further interact with its surrounding (hydrogen bonding etc.). The Gibbs energy change for the process of transfer of a solute i from phase α to β is zero at equilibrium: (22) At infinite dilution the activity coefficients approach unity hence from the definition of the distribution coefficient: (23) The standard molar Gibbs energy of solvation can be derived from pure component data using spectroscopic information for determining solvatochromic parameters. A generalized equation for with a linear dependence on solvatochromic parameters is (24) Here is the cohesive energy density of the solvent (the square of the solubility parameter); αs and βs characterize, respectively, acidity and basicity which in general represent the ability to form hydrogen bonds; defines the polarity and/or polarizability of the solvent. These three parameters are the solvatochromic parameters, obtained spectroscopically by the use of suitable indicators. These LSERs (linear solvation energy relationships) not only correlate spectroscopic positions of indicators in different solvents, they can also be used to correlate the influence of the solvent on reaction equilibria and kinetics.

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There exists a number of such linear solvatochromic scales. One of the most widely used is that of Kamlet and Taft [93, 94]. The application of LSER can be found in different fields, e.g., distribution coefficients (retention times) in chromatography [95], solubility of HCl in different solvents [96], and liquidliquid distribution coefficients [97]. The Nernst distribution of solute i according to Kamlet is (25) Here the cohesive energy density in Eq. (24) is replaced by the molar volume Vi of the solute (as a measure of the size of the cavity to accommodate the solute i in the solvent); ∂t is an empirical parameter which takes also account for polarisability π*; and the C’s are solvent characteristics independent of the solute. Meyer and Maurer [98] used this equation for 30 systems (371 substances, 947 experimental distribution coefficients) to evaluate generalized solvent Cj parameters. This equation is quite accurate in comparison with group-contributing methods [99] or other predictive LSER methods [100]. For compounds where the solvatochromic parameters are known, the mean absolute error in log is about 0.16. It is usually less than 0.3 if solvatochromic parameters of the solute and solvent have to be estimated according to empirical rules [97]. In contrast to the prediction of gas-liquid distribution coefficients, which is usually easier, the LSER method allows a robust estimation of liquid-liquid distribution coefficients. However, such equations always involve empirical terms, despite of the physicochemically founded thermodynamic models. This is due to the fundamental character of the solvatochromic scales. As an alternative to the LSER method the MOSCED (modified separation of cohesive energy density model) has been developed in order to correlate and predict activity coefficients at infinity dilution [101, 102]. An advantage especially with liquid-liquid systems is that experimental distribution coefficients and limiting activity coefficients are known at ambient temperatures. Thus it is very convenient to develop correlations on the basis of solvatochromic parameters.

D. Case Example 1. Nonideal Organic Phase The application of the Pitzer and the Hildebrand-Scott model will be discussed in the following with the EFCE Zn/D2EHPA system. A discussion of mixed solutes or anion exchange can be found elsewhere [71]. The cation exchanger di(2ethylhexyl)phosphoric acid (D2EHPA) is a well-known extractant, which usually forms dimers in aliphatic diluents and is mainly monomeric in aromatic ones [103]. The overall extraction of zinc with dimeric D2EHPA (R2 H2) is as follows: (26)

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At higher concentrations polynuclear Zn-D2EHPA complexes are known to occur. These complexes can have up to eight zinc-ions, i.e., a is in the range from 1 to 8 [104]. However, at lower zinc concentrations, there are only mononuclear zinc complexes to consider [105–108]: a=1 in Eq. (26). The law of mass action on the basis of concentrations is then (27) where DZn denotes the distribution coefficient of zinc between the organic and water phases. In logarithmic form, log DZn=b log[R2H2]+2pH+log K1,2b ≈b log[R2H2]0+2pH+log K1,2b

(28)

where at low solute concentrations the uncomplexed D2EHPA concentration can be approximated by the initial concentration [R2H2]0. Additionally, at a low initial pH value the concentration of H+ during the extraction does not change significantly, keeping the pH value almost constant. As a result, the distribution coefficient DZn is not a function of [Zn2+] and the pH but is a linear function of log [R2H2]0 with the slope b. The slope represents the stoichiometric ratio of R2 H2/Zn of the organic zinc complex and thus b in Eq. (28) can be derived from Fig. 6. The slope at this low zinc concentration (0.1 mM) is approximately 1.5, which is equivalent to three D2EHPA per one zinc molecule in the organic complex: (29) However, the amount of coextracted water bound in the solute-ion exchanger complex has to be considered as well. At different temperature levels the physical and the chemical extracted water can be determined, e.g., with thermally enhanced Karl-Fischer titration [109]. Fortunately, in the system zinc/D2EHPA dissolved in alkanes, the water content is negligible. There is a minor increase of the water content with rising D2EHPA concentration, which gives a ratio of 1 molecule of water per 20 molecules ion exchanger. This can be attributed to a solvation effect with an increasing polar organic phase and no additional water is observed in the Zn-D2EHPA complex, as can be verified by FTIR-measurements [110, 111]. This minor amount of water coextracted is in contrast to other metals as, for example, nickel extraction with D2EHPA [112–114]. The FTIR-spectrum of the zinc complex in n-heptane shows two concentration dependent v(P=0) bands at 1234 cm-1 and 1206 cm-1. In Fig. 7 it can be seen that the band of 1234 cm-1 decreases with increasing zinc concentration and the contrary occurs at 1206 cm-1. The first one represents the free D2EHPA, and the latter one the ion exchanger bound in the metal complex [115].

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Figure 6 Slope analysis of the Zn-D2EHPA complex.

Figure 7 FTIR spectrum of the Zn/D2EHPA complex at different organic zinc concentrations.

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For a quantification and analysis of the complex structure the bond of the free D2EHPA at 1234 cm-1 should be used [71]. The result is a linear calibration curve in the range from 0.02 to 0.09 M D2EHPA. From a simple balance the concentration of the D2EHPA in the complex can be calculated [RH]bound=[RH]0-[RH]free. As can be seen in Fig. 8, the slope is 3, which gives the same result as the slope analysis according to Eq. (28) and Fig. 6. The mass action law according to Eq. (27) in terms of activities reads (30) Here ␥ denotes the individual activity coefficient which can be calculated in the aqueous phase according to the Pitzer model [77, 78, 116–118]. The Masson parameters for the conversion of molar concentrations into molalities are documented [71, 86]. The nonideality in the organic phase can be considered with the Hildebrand-Scott solubility parameters [85], which are tabulated or can be regressed by using the software from Baes [119] (http://www.ornl.gov/divisions/ casd/csg/ sxlsqi/). Figure 9 gives the relative error when using concentrations instead of activities. As can be shown [120], a consistent parameter set obtained with the above models allows the prediction of phase equilibria in a changed environment (e.g. another diluent, chloride or sulphate system) without major problems.

2. Ideal Organic Phase Wenzel and Maurer assumed the organic phase to be ideal and that there exists a Zn-(D2EHPA)2 and a Zn-(D2EHPA)4 complex simultaneously in the organic phase

Figure 8 FTIR analysis of the complex stoichiometry.

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Figure 9 Relative deviation δ with respect to the experimental data, computed with concentration (triangles) and activities (circles).

when deriving the same slope results from FTIR analysis as in Fig. 8. The complex dissociation constants (K12, K14) are estimated according to the following equations: (31a) (31b) The values of the resulting constants are ln with a12=1650, b12= 0.67, and a14=1447, b14=4.09 [121] when considering the aqueous nonidealities with the Pitzer equation. This approach of considering the organic phase as ideal with all the nonidealities in the number of reactions, respectively, of the complexes is quite often used in reactive solvent extraction modeling, even neglecting the aqueous phase nonideality. The disadvantage resides in the specific parameters of the system, which make it difficult to transfer the thermodynamic data to slightly different systems (different aqueous electrolyte matrix or organic diluents). However, the method is simple and fast and of sufficient accuracy as far as moderate concentrations are involved [122]. At very high concentrations additional reactions have to be taken into account [105]. A compilation of equilibrium constants can be found elsewhere [123–127].

III. REACTIVE MASS TRANSFER In reactive solvent extraction, transfer is accomplished through diffusion processes and reaction kinetics.

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Diffusional processes are sometimes the slowest and therefore the controlling step in reaction systems involving interfacial transfer. For these cases the effect of reaction rate on the interfacial transfer rate can be ignored. For processes that are controlled by diffusion, the overall mass transfer coefficient k0 is usually made up of the two individual phase mass transfer coefficients kα and kβ where Di is the distribution coefficient [see Eq. (27)]. However, if an interfacial or heterogeneous reaction occurs, then the interfacial concentrations are no longer in equilibrium and an extra resistance is included as follows: (32) where k+ is the forward reaction rate constant and n is the order of the reaction [8]. If this last term is large with respect to the first two terms, the process is reaction controlled. In many cases in metal ion liquid extraction processes, this has been shown to be the case. If the reaction is homogeneous, then as the diffusion species reacts its concentration is lower than if no reaction occurred, leading to a higher concentration gradient from the interface to the bulk and so enhancing the transfer rate. Analysis of this was presented by a range of authors and resulted in the overall mass transfer coefficient being determined from Eq. (33) as follows: (33) where E is the enhancement factor calculated from the kinetic parameters [128]. The appropriate overall mass transfer coefficient is then use in the equipment model to predict performance as described in the next section. From a kinetic study an overall stoichiometric equation can be derived, indicating the relation between the number of moles of each of the products and reactants. For a typical metal ion (Mn+) extraction using an acidic ligand (HL), the overall reaction is (34) Thus the rate is usually expressed as (35) The rate of extraction is a function of the composition of the system, the temperature, and the nature of the solvent. Unfortunately, it is not possible to determine the form of the rate law, except by inference from experimental data. In general, the stoichiometry of the overall reaction is not directly related to the rate law as this is controlled by one or more elementary steps. During reactive extraction,

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the reactions steps occur between the solute and the complexing ligand. These can either take place in the bulk aqueous phase (homogenous reaction) or at the interface (heterogenous reaction), or a combination of both. The general mechanism proposed for extraction when the reaction occurs in the aqueous phase is the diffusion of the extractant (HL) into the aqueous phase where it reacts with the metal ions (Mn+) in that phase in an n-step process, for the formation of the neutral complex (MLn) which then transfers into the organic phase. A general summary of this mechanism is given by Flett et al. [129]: HL(org)

HL(aq)

(36) (37) (38) (39)

or together, (40) and finally MLn(aq)

MLn(org)

(41)

For a reaction at the interface, the species, which have the (aq) subscripts in Eqs. (36)–(41), would be replaced with interfacial species, subscript (i). In the discussion over the site of the rate-determining step in the complexation reaction, increasing consideration is being given to the importance of the role of the interface. Several researchers, including Watarai and Satoh [130], Nitsch [131], Perera et al. [132], McCulloch et al. [133, 134], Hokura et al. [135], and Yamada et al. [136] have proposed the liquid-liquid interface as the site of the reaction in many systems, whereas previously the mechanism of the reaction was considered to be a bulk aqueous reaction between the ligand (either ionised or neutral) and the metal ion, and the extraction of the neutral complex [Eq. (41)] the final step in the complexation reaction. A number of factors, including the extremely low aqueous solubility and the known interfacial activity of some reagents, lend support to the hypothesis of an interfacial mechanism. The kinetic parameters can be measured using a range of techniques and one example is in a stirred Lewis-type cell (Fig. 10) in the plateau region, where the mass transfer is independent of stirring and thus diffusion [137]. Above 160 rpm there is a disruption of the planar interface and droplet formation starts which enlarges the mass transfer area and thus the transfer rate. Below 80 rpm diffusional resistances limit the mass transfer (see Fig. 11).

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Figure 10 Experimental setup of a stirring cell: 1=organic phase supply, 2=aqueous phase supply, 3+4=temperature control, 5=stirrer, 6+7 sampling, 8=waste.

Figure 11 Kinetics regime in a stirring cell [71].

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Considering again the Zn/D2EHPA system, assuming that the reaction takes place at the interface and according to the model by Klocker et al. [138], the overall extraction mechanism consists of two rate-determining steps: (42a) (42b) Here ad denotes the species adsorbed at the interface. The rate equations are then (43a)

(43b)

The quasi-stationary interfacially adsorbed species are related to the bulk concentrations using the Gibbs adsorption law: (44) The constants α and ␥L can be determined from interfacial measurements. A combination of Eqs. (43) and (44) gives the rate determining law:

(45)

with C2=␥L/K0,1· A complete derivation of Eq. (45) can be found elsewhere [71]. For the EFCErecommended zinc extraction system the parameters (http://www.dechema/ extraction) C1=1.238 mol-1/2 m3/2, C2=0.5962 mol1/2 m-3/2, and ␬f=2.38×10-4 s-1 are regressed from experimental data when using isododecane as solvent. The two constants C1, C2 and the overall forward kinetic constant ␬f have to be determined from experimental data. With the value of ␬f known, ␬r can be calculated using the

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equilibrium constant K1,3=10-1.1863 mol1/2 L-1/2=2.059 mol1/2 m-3/2 [see Eq. (30)] to be ␬r=␬f/K1,3=1.156×10-4 mol-1/2 m3/2 s-1. A typical time/ concentration diagram according to Eq. (45) is given in Fig. 12. Rising droplet or Venturi tube experiments allow analysis of overall mass transfer in a droplet or droplet swarm [139]. In a rising droplet apparatus (see Fig. 13) a mono-dispersed droplet swarm is generated at a dual-flow nozzle (3) when feeding the organic phase (1) with a pump (2). The flow of the continuous phase is adjusted with a gear pump (4) so that the droplet from the tip of the nozzle is lifted at a certain diameter. Depending on the nozzle diameter, metering, and gear pump flow, distinct mono-dispersed droplet swarms can be produced. A sampling device (7) allows analysis (8). Different concentrationtime profiles can be obtained by either varying the sampling height in the rising droplet apparatus or residence time in the Venturi tube. This is essential to obtain reliable data [140]. An overall experimental mass transfer coefficient can be obtained: (46)

that on integration yields C(t)=C* · (1-exp[-ky · a · (t-t0)]

(47)

Figure 12 Comparison between simulation and experiment [0.01 mol D2EHPA, experiments 1, 3, and 4 initial pH=2.3, experiment 5 initial pH=5, initial concentration of zinc sulfate in mmol/L: experiment 1:0.05, experiment 5:0.01, experiment 3:0.02, experiment 4:0.04, T=298 K].

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Figure 13 Experimental setup of the droplet rising apparatus (1=storage tank, 2=metering pump, 3=nozzle, 4=gear pump, 6=heat exchanger, 7=sampling funnel, 8=analysis).

assuming the concentration in the continuous phase (C*) and the mass transfer coefficients ky are constants. Here t0 presents the time residuals during droplet formation and sampling. The residence time in a rising droplet apparatus is limited to less than 30 s, so the Venturi tube is the more appropriate apparatus in reactive extraction. With proper adjustment (9) of the circulating pump (5) the droplets can be kept in the Venturi-tube for the necessary length of time (Fig. 14). Figure 15 compares experimental and calculated mass transfer coefficients for rigid droplets. The model combines molecular Maxwell-Stefan diffusion within the droplet and chemical reaction at the interface. The data scatter is large and in order to achieve a better agreement, the molecular diffusion coefficient has to be increased three to twentyfold, which cannot be justified physically. It is clear that diffusion inside the droplet is influenced by the convection of the continuous phase as reported by Newman [141, 142] with rigid droplets and Kronig and Brink [143] for laminarily circulating droplets. With their correlations the transfer rate can be enhanced maximally 2.5 times by convection compared with the pure diffusion. Handlos and Baron [144] assume complete mixing within the droplet. (Note: diffusion vanishes in this model and only terminal velocity of the droplet and viscosity have any influence.)

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Figure 14 Experimental setup of the Venturi tube (1=storage tank, 2=metering pump, 3 =nozzle, 4=gear pump, 5=circulating pump, 6=heat exchanger, 7=sampling funnel, 8=analysis).

Figure 15 Comparison between experimental and predicted mass transfer for the rigid sphere model.

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In more recent models the simplified analytical approaches are corrected with empirical terms. Steiner [145] gives an empirical correlation for mass transfer for circulating and oscillating droplets: (48) The correlation is valid for the correction term being less than 10 and a similar relation is given for droplet swarms. Slater [146] developed a “stagnant cap” model where impurities enrich the lower part of the droplet during the droplet rise. The droplet is divided into a circulating part fz and a rigid part (1-fz) (see Fig. 16), where fz is as follows: (49) The parameter kz must be derived from experimental data. The mass transfer in the circulating part of the droplet is analogues to the Handlos-Baron model which gives (50) Henschke and Pfennig [147] describe a model which is not based on hydrodynamic effects enhancing mass transfer but on interfacial instabilities induced by mass transfer. They define a transport coefficient which combines the hydrodynamic coefficient εm and a mass transfer coefficient εσ which yields (51)

Figure 16 Stagnant cap model.

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CIP is the so called instability coefficient which is system specific and must be determined experimentally. A modified Fourier number (52) can be used to determine the mass transfer: (53) This equation can be approximated as follows: (54)

with a deviation of up to 0.1%. A comparison of experimental and simulated mass transfer coefficients for the zinc extraction is given in Fig. 17 when combining the microkinetics model (eq. (45)) with the mass transfer model according to Henschke and Pfennig using the CIP value of 9.1. At higher concentrations of zinc there is a

Figure 17 Comparison between experimental and predicted mass transfer coefficients for the zinc extraction for 1, 2, and 3 mm droplets.

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systematic deviation, which may be attributed to droplet swarm effects (wake effects, etc.), not considered by the model. As has been shown, the macrokinetics modeling comprises a microkinetics model derived from Lewis cell experiments combined with effective mass transfer rate resulting in at least one further adjustable parameter. A shortcut method is to use the overall experimental mass transfer coefficient of Eq. (46) to describe reactive mass transfer with a linear driving force model. Here only one parameter has to be determined for further use in the hydrodynamics models described in the next chapter. The only disadvantage is its limited predictive use. However, it correlates the effects of a number of chemical systems, impurities and nonidealities (wake and Marangoni effects, surface covering/blockage, etc.) which are commonly found in solvent extraction systems.

IV. SOLVENT EXTRACTION EQUIPMENT A. Introduction Reactive solvent extraction processes are usually carried out in equipment originally designed for physical solvent extraction processes. The majority of the process installations use mixer settler cascades which allow for individual adjustment of pH, which phase is to be dispersed and residence times in the mixers and settlers are as dictated by the mass transfer kinetics and hydrodynamics. The disadvantage of mixer settler cascades is the uneven shear in the mixer unit causing a large drop size distribution causing carry over of the small drops in the raffinate, resulting in loss of organic and loss of efficiency. Mixer settlers also suffer from a low turndown ratio and large organic inventory when compared to other types of contactors. In cases where only one or two stages are required and the flow rates are high, such as for copper extraction, mixer settlers are still preferred. However, increasingly in the minerals industry, for cases where more than one stage is required, column contactors are preferred. For example, a recent installation of pulsed plate columns to treat 800 m3/h of uranium solutions of Western Mining Corporation’s Olympic Dam operations in South Australia has shown that columns have the ability to reduce evaporative losses, entrainment, and inventory when compared to mixer settlers. The Alamine 336/kerosene extractant in this case has relatively fast extraction kinetics and the extraction is controlled by diffusion processes. Other examples where large-scale columns are being considered are in INCO’s Ni/Co extraction process being built at Goro in New Caledonia. Outside of the metallurgical industry, the pharmaceutical industry has used columns for the extraction of a range of pharmaceuticals in preference to mixer settlers for many years. The advantage of using columns is the reduced solvent inventory, reduced solvent losses, better turndown and smaller footprint. The residence time of the dispersed

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phase is however limited and so they are only applicable to relatively fast kinetics, that is, where equilibrium is reached in less than 1 min. There are many types of columns available. The following is a brief overview of column types; for a more detailed discussion on column types and their performance, see Ref. 148. Columns can be subdivided broadly into nonagitated and agitated types. Nonagitated columns are the simplest, ranging from those with no internals (spray columns) to more complex arrangements with packing or trays as shown in Fig. 18. The spray column although the simplest and cheapest is not very efficient because of the large axial mixing or dispersion that occurs in the continuous phase as a direct result of the droplet rising through the continuous phase causing a large-scale circulation or axial mixing. In order to overcome this and improve the efficiency, various types of internals have been used in columns. The internals also promote breakage and coalescence of the drops as they move through the column and enhance mass transfer. The packing used in many columns is the same as that used in distillation, although the hydrodynamic parameters are different for solvent extraction. For example, the packing is usually wet by the continuous phase and the dispersed phase rises as discrete droplets in solvent extraction compared to distillation where the liquid wets the packing and falls as a film through the packing. Nevertheless, high surface area and low voidage packing enhances the contactor performance significantly and both random and more recently structured packings have been used in a range of applications. In nonagitated columns the hydrodynamics of the dispersion is controlled by the physical properties and the phase flow rates. In order to decouple the phase flow rates from the hydrodynamics and allow larger flexibility in operation, a

Figure 18 Nonagitated extractors [left to right: spray, packed, sieve tray (light), sieve tray (heavy) column].

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number of mechanically agitated columns are used. Examples of these are shown in Fig. 19. The agitation increases the interfacial area and increases droplet break up and coalescence, increasing column mass transfer efficiency. The tradeoff is usually a slight loss in total throughput and a more sophisticated design and scaleup procedure. Most of the new installations in reactive extraction in the minerals area are with mechanically agitated columns because of their greater flexibility and ability to control drop size independently of flow rates, so that losses of solvent can be minimized while maintaining efficiency. An alternative is nondispersive contacting in hollow fire membrane modules (e.g., Liqui-cel [149] or TNO fibre modules [150]). They are good for extreme phase ratios and low-solute concentration. As a result, most applications have been for recovery of heavy metals from wastewater. There is essentially no entrainment and scaleup is not difficult but compatibility of membrane material with organic solvents can be a problem (swelling and degradation), also loss of extractant through solubility in the aqueous phase and suspended solids can be a problem and needs to be monitored. The largest application reported so far is up to 15 m3 m-2 h-1 in a wastewater application [151]. Centrifugal extractors are seldom used in high volume applications due to maintenance costs. They offer better handling of small solids (i.e., fermentation broths), highly foaming systems, and systems that require very short contact times because of degradation of the product. An example of the latter is the extraction of penicillin with n-octylamine at low pH where the penicillin is not stable. Their major advantage is the good phase separation even at low density difference and interfacial tension. In modern counter current centrifuges [152, 153] several equilibrium stages can be achieved in one device as a result of the internal configuration.

Figure 19 Agitated extractors (left to right: RDC, Karr, Oldshue-Rushton, Kühni, Scheibel extraction column).

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B. Contactor Hydrodynamics and Mass Transfer Performance In predicting the performance of solvent extraction equipment, two key parameters are required. The number of stages or the height and number of transfer units in order to determine the mass transfer efficiency and the hydrodynamics which control the throughput of the device. The hydrodynamics and mass transfer performance of mixer settlers, centrifugal extractors and nondispersive liquid extraction models are relatively straightforward and the reader is referred to a range of standard texts for design methods [148, 149, 154]. In comparison, design procedures for column contractors are more complex but the methods developed for nonreactive extraction can in general be used for the design of columns that are controlled by reactive extraction processes, provided that allowance is made for the effect of reaction on mass transfer process as described in the previous section. The hydrodynamics of column contactors are usually characterized by the dispersed phase hold up, the flood point and the average drop size. Correlations and models are available for these for a range of column types and are usually not affected by reactions providing the physical properties of the system are constant throughout the column [6]. The prediction of the height of a column required for a given separation can be obtained from either a staged approach or a transfer unit approach. The plug flow models for determining the height of an extraction column are of limited value because of the effect of axial dispersion which is caused by: • • •

Molecular and turbulent diffusion in axial and radial directions Carrying of continuous phase in the wake of a droplet Entrainment of smaller droplets

The simplest axial dispersion model based on the differential model or transfer unit approach results in the following equations describing the concentration profiles in continuous and dispersed phases of the column.

(55)

where Ec and Ed are the axial dispersion coefficients, Uc and Ud are the phase flow rates, cc and cd are the concentrations in each phase, a is the interfacial area and koc is the overall mass transfer coefficient based on the continuous phase and z refers to the distance along the column, the subscripts c and d referring to the continuous and dispersed phases respectively.

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With appropriate boundary conditions, numerical solutions to these equations can be found and used to predict the height of a column required for a given separation [8]. Correlations and models exist for the prediction of the axial dispersion parameters Ec and Ed for a range of columns [6, 8]. Reactive extraction influences the value of ko in these equations as shown in the previous section. A similar model based on the stage approach exists where the amount of axial dispersion is characterised by the back flow ratio α. Both of these models lead to the same solutions in the limit of a large number of stages. The model that is used depends on the type of contactor that is being considered [6, 8]. Two dispersion coefficients representing the nonideal hydrodynamics cannot fully reflect the physical behavior of droplet swarms in commercial columns. In this respect current research is focused on drop population balance models which account for the different rising velocities of the different size droplets and their interactions, like droplet breakup and coalescence [155–162]. The back-mixing model assumes the dispersed phase to be semicontinuous. The different age of the droplets is neglected. This is in contrast to population balance models where coalescence and breakage of droplets are considered. Equation (56) gives the droplet density distribution P(t, z, d) as a function of convection wd, back-mixing Dd,ax, breakage SB,P and coalescence SC,P of the dispersed phase. For the concentration profile of the solute in Eq. (57), a mass transfer term is included. For the profile of the continuous phase the mass transfer is integrated with respect to all droplet classes [see Eq. (58)].

(56)

(57)

(58)

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The local holdup is obtained from an integration of all droplet classes: (59) The source term for breakage is according to (60) where g(t, z, d) is the breakage frequency and β(d0, d) is the daughter droplet distribution. The same parameters are then valid to describe the concentration distribution within the droplet swarm: (61) The positive part in the integral is due to the increase in the number in a given class gained from a higher class of bigger droplets and the negative part is due to the loss into a lower class of smaller droplets. The integral terms for coalescence can be developed in analogy. Here it is assumed that only two droplets with diameter d1, respectively, d2 will react (no multilateral effects). The coalescence probability ω(d1, d2) is the determining parameter for the probability density functions of droplets

(62)

and of solutes

(63)

The breakage frequency and daughter droplet distribution have to be determined in the geometry under consideration [163] as is depicted in Fig. 20 for an

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Figure 20 Experimental setup of the droplet breakage apparatus (1=storage tank continuous phase, 2=level control, 3=sample taking, 4=stirred RDC compartment, 5=two phase nozzle, 6=metering pump, 7=pump organic phase, 8=circulating pump).

RDC-geo metry. The daughter droplet distribution is usually according to a decay function which is a β distribution [164] starting with a maximum value which is the mother droplet d0. In a similar manner the coalescence rates have to be estimated which can be done in a Venturi tube (see Fig. 21). Here the coalescence of a monodispersed droplet swarm ω (d1, d1) or with other droplets ω (d1, d2) can be estimated up till holdup values of 15% [161]. This gives basic information when developing correlations to evaluate the influence of the system parameters (density, viscosity, interfacial tension, droplet size, holdup, etc.). The influence of the column geometry has to be determined in a setup similar to Fig. 20 in a coalescence dominated regime. The local aspects of liquid-liquid two-phase flow in reactive extraction has been the focus of CFD analysis by different research groups [165–168]. In principle, all aspects of single-phase flow phenomena (residence time distribution, impeller discharge flow rate, etc.) can be tackled even with complex geometries. Figure 22 depicts the velocity profile in a compartment of a Kühni column. The two vortices

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Figure 21 Experimental setup of the Venturi tube (1=storage tank dispersed phase, 2= metering pump, 3=two-phase nozzle, 4=aqueous phase pump, 5=circulating pump, 6=heat exchanger, 7=photoelectrical suction probe, 8=sampling, 9=flow control).

Figure 22 Velocity profile in a compartment of a Kühni column.

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visible below and above the impeller yield an impact on the residence time behavior [169]. However, the two phase CFD is still a challenge, and the droplet interactions (breakup and coalescence) and mass transfer are not implemented in commercially available codes. Thus these issues constitute an open area for further research and development.

V. CONCLUDING REMARKS The chemistry of reactive solvent extraction systems is relevant in many liquidliquid extraction systems and in extraction chromatography. Thus there is a need for design procedures for large-scale applications and for small-scale niche applications. Reactive solvent extraction is a mature technique when applied to metal recovery in hydrometallurgy using mixer settlers. Countercurrent centrifugal and hollow fiber extractors have niche applications due to either high maintenance costs or membrane stability limits. The challenge is the use of countercurrent columns which offer greater flexibility, lower costs, and reduced space and organic inventory, but more complex design and scaleup. This is particularly true for the chemical and pharmaceutical industry where processes often demand a high number of theoretical stages. The modeling and design of reactive solvent extraction columns based on the equilibrium-stage model neglects reaction chemistry and mass transfer. A concept of the height equivalent to a theoretical stage (HETS) is only useful for very fast reactions and then nonideal flow phenomena are not considered. More sophisticated models like dispersion, back-mixing or drop-population-balance models are necessary. The latter ones take the age of droplets into account and the parameters: daughter droplet distribution, breakage and coalescence frequency, have to be estimated for a certain system and geometry. A more realistic rate-based approach implies that the actual rates of multicomponent mass transfer and chemical reactions are determined. This involves the inclusion of eddy diffusivity concepts using adjustable parameters and prediction of mass transfer so far has been shown to be difficult. Since chemical reactions are involved, pilot-scale measurements, where an effective overall mass transfer coefficient can be correlated from experimental data, are necessary for process scaleup. A more general approach is to match micro-kinetics with any appropriate mass transfer model, which allows an extrapolation of the behaviour of the system. In that respect it is also advised to include a more robust thermodynamic equilibrium model to predict the distribution of species between the two phases. As has been discussed, the Pitzer model handles quite reasonably ion-ion interactions even at high ionic strength. The nonideality in the organic phase results from solute-solvent interactions and the complexes formed. Here the complex stoichiometry, solvation and hydration can be tackled, considering the organic phase as ideal with all the nonidealities related to the number of chemical reactions. Alternatively it may be

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assumed that only one reaction occurs and nonideality is considered by appropriate models (solubility parameters, NRTL, UNIQUAC, etc.) However, in that respect it is helpful to determine the complex stoichiometry with spectroscopic methods. Summing up: Depending on the accuracy of the prediction required, there is the option to use overall parameters or very detailed ones. The latter can be determined with a small sample volume from lab scale experiments. This allows process design in an early stage in order to evaluate alternatives or to optimize operating conditions in any design task rapidly and economically.

ACKNOWLEDGMENTS The authors wish to thank for the continuous financial support of the national science foundations (DFG, German Research Foundation, and AiF, Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” c.V.) and BMWi (Bundesministerium für Wirtschaft und Technologie), the Particulate Fluids Processing Centre of the Australian Research Council and the Australian Reserach Council for financial support.

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3 Symmetrical P,P’-Disubstituted Esters of Alkylenediphosphonic Acids as Reagents for Metal Solvent Extraction R.Chiarizia Argonne National Laboratory, Argonne, Illinois, U.S.A. A.W.Herlinger Loyola University Chicago, Chicago, Illinois, U.S.A.

I. INTRODUCTION In the course of the studies leading to the development of the TRUEX (transuranium extraction) process in the chemistry division of Argonne National Laboratory (ANL) [1], it became apparent that the need existed for powerful water-soluble complexing agents of actinide ions that could be used as holding-back reagents during the extraction stages or as stripping agents. An important requirement for these ligands was that they must be easily destroyed by heating and/or mild oxidation, to avoid the problems associated with the presence in nuclear wastes of powerful but extremely stable complexing agents, e.g., polyaminocarboxylic acids [2]. One family of thermally unstable complexants (TUCS) identified and investigated was the alkylenediphosphonic acids [3]. The strong acidity of the diphosphonic acid group, its tendency to chelate actinides through either ionized phosphonic acid or groups, as well as the tendency to form protonated complexes, allow the formation of very stable metal complexes under highly acidic conditions [4]. 85 Copyright © 2004 by Marcel Dekker, Inc.

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Studies of diphosphonic acids as aqueous complexing agents led to the development of the chelating ion exchange resin Diphonix, which contains geminally substituted diphosphonic acids chemically bonded to a styrene-based polymeric matrix [5]. The Diphonix resin exhibits extraordinarily strong affinity for actinide ions, especially in the tetra- and hexavalent oxidation states, and for Fe(III). This resin has found applications in TRU and mixed waste treatment, in procedures for rapid actinide preconcentration and separation from environmental and biologic samples, and in hydrometallurgical processes where efficient separation of Fe(III) from other transition metals is required [5]. As the next step in our attempt to exploit the potential of diphosphonic acids for metal ion separations, we elected to investigate in detail the properties of the symmetrical P,P’-dialkyl esters of alkylenediphosphonic acids. The diesters retain the diphosphonic acid functionality with two of the acidic hydrogen ions replaced by alkyl groups making the molecule lipophilic and soluble in water-immiscible organic diluents. Such compounds can be regarded as liquid analogs of the Diphonix resin suitable for use in solvent extraction procedures. In developing the diesters as solvent extraction reagents, our objective was to provide the separation chemist with another powerful tool for applications in the field of actinide separations. In their 1969 classic book on solvent extraction [6], Marcus and Kertes gave only cursory mention of the acidic diphosphoryl reagents. The two groups in these reagents are separated by either an oxygen atom (an alkyl-substituted pyrophosphoric acid) or a methylene group (an alkyl-substituted methylenediphosphonic acid). The information on diesters of methylenediphosphonic acid available at that time was essentially limited to the works published by scientists of the Rudjer Boškovic Institute of Zagreb, Croatia. These scientists had synthesized and investigated several compounds of this type (with alkyl groups ranging from ethyl to n-octyl), in their quest for uranium extracting reagents more resistant to hydrolysis [7–15]. The methylenediphosphonic acids were expected to be stronger complexing agents than pyrophosphoric acids since the basicity of the phosphoryl donor groups should be increased by replacing the oxygen atom of the pyrophosphoric acid with a methylene bridge. The di-n-octyl ester of methylenediphosphonic acid received the most attention as a reagent for solvent extraction of metal species. Studies were reported for the extraction of niobium and tantalum [12–14], germanium [10], titanium [11], and zirconium and hafnium [12, 15]. The extraction of other metal ions, however, was only briefly mentioned. Among other properties of dialkyl methylene-diphosphonic acids, the authors noted that the reagents are dimeric in petroleum ether and chloroform [10, 11], and the niobium, tantalum, zirconium, and hafnium complexes exhibit a strong tendency to polymerize [12, 13, 15]. During the past several years, the scientific collaboration between members of the chemistry division of ANL and the chemistry department of Loyola University Chicago has resulted in investigation of two novel types of alkylenediphosphonic acids, i.e., dialkyl and disilyl esters (Structure I).

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Structure I

Table 1 reports schematically the compounds synthesized and investigated as well as their trivial nomenclature. These abbreviations indicate the presence of two ionizable hydrogen atoms in the molecule, the nature of the alkyl or silyl groups (DEH or DTMSP, respectively) and the value of n in Structure I (the first letters in the square brackets identify methylene (M), ethylene (E), propylene (Pr), butylene (Bu), pentylene (P), and hexylene (H) diphosphonic acids, respectively). The metal extraction chemistry of these reagents has been investigated using a variety of techniques ranging from liquid-liquid distribution methods and infrared spectroscopy, to vapor pressure osmometry (VPO) and small-angle neutron scattering (SANS). This chapter reviews the results we have obtained on the diphosphonic acid diesters as solvent extraction reagents for metal separations. Although we do not claim that all aspects of metal extraction by these reagents have been explored and/or completely understood, we believe that the information available provides a coherent picture of these extractants in terms of their aggregation, inductive effects, and coordination behavior.

II. SYNTHESIS Utilization of partial esters as solvent extraction reagents has been limited to some extent by the lack of efficient synthetic methods for their preparation. The main Table 1 Investigated Diesters of Alkylenediphosphonic Acids

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difficulty in preparing partial esters is the selectivity of the reactions used to obtain a product with the desired number and location of ester substituents. The separation of partial ester mixtures generally requires laborious chromatographic or fractional crystallization methods [16] and linking two partial monoesters together is often difficult by literature procedures [17]. Consequently, our synthetic efforts have focused on selective addition of ester groups to diphosphonic acids, selective dealkylation of diphosphonate tetraesters, and partial alcoholysis of alkylenebis(phosphonic dichlorides).

A.

Alkylenediphosphonic Acids and Tetraalkyl Alkylenediphosphonates

Alkylenediphosphonic acids can be prepared in nearly quantitative yield by hydrolysis of tetraethyl alkylenediphosphonates with concentrated acid, Eq. (1) [18, 19]. A very pure acid product is usually obtained upon recrystallization from water.

(1) The Michaelis-Becker and Michaelis-Arbuzov reactions are used to prepare tetraalkyl alkylenediphosphonates [20, 21]. The Michaelis-Becker method involves displacement of a halide ion from a haloalkane by a dialkyl phosphite anion. The anion is usually generated from the neutral phosphite with base or via direct metallation [22]. Dialkyl alkylphosphonates are formed via the Michaelis-Arbuzov reaction when a trialkyl phosphite reacts with a haloalkane [23]. The MichaelisBecker reaction is usually carried at room temperature, whereas the MichaelisArbuzov reaction requires elevated temperatures. The Michaelis-Becker reaction is conducted in solution and solvent selection can be problematic due to the limited solubility of the dialkyl phosphite salts. Alcoholysis of bis(phosphonic dichlorides) is a widely used method for the synthesis of identically substituted tetraalkyl alkylenediphosphonates. Alcoholysis of the second chlorine atom of an alkylphosphonic dichloride is slower than that for the first due to the presence of the electron-donating alkoxy group after the initial alcoholysis [24]. As discussed below, the reduced reactivity of the second chlorine is used to good advantage in the direct preparation of P,P’-disubstituted alkylenediphosphonic acids and mixed methyl diphosphonate tetraesters [25–27].

1. The Michaelis-Becker Reaction Nylen and Cade independently discovered that reaction of sodium diethyl phosphite and diiodomethane yields disodium diethyl methylenediphosphonate rather than

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the expected tetraester [28, 29]. Hormi and coworkers subsequently synthesized tetraethyl methylenediphosphonate in good yield via the Michaelis-Becker reaction using sodium diethyl phosphite and methylene chloride, but reaction time is long [30]. Moedritzer and Irani prepared a homologous series of alkylenediphosphonic acids, (HO)2OP(CH2)nPO(OH)2, (n=1–6 and 10) by hydrolysis of the corresponding tetraethyl esters [18]. They observed that increasing alkylene chain length decreases the influence of one phosphoryl group upon the other, becoming very small for n>3. Esters with n≥3 were obtained by slow addition of the dibromoalkane to sodium diethyl phosphite in ether:

(2) Tetraethyl methylene- and ethylene- diphosphonates were not prepared via this route, but rather a stepwise approach was used [18]. Diethyl haloalkylphosphonates were prepared by a Michaelis-Arbuzov reaction and subsequently reacted with diethyl phosphite salts in a Michaelis-Becker type reaction to produce the tetraethyl alkylene-diphosphonates with n=1 or 2 in 50–60% yield:

(3)

2. The Michaelis-Arbuzov Reaction Tetraalkyl alkylenediphosphonates are formed by the Michaelis-Arbuzov reaction when a dihaloalkane replaces the haloalkane in the reaction with a trialkyl phosphite: (4) The distribution of substitution products depends upon the mole ratio of the reactants employed in the reaction [31, 32]. Monosubstituted products predominate with excess dihaloalkane while disubstituted products are favored by high phosphite to dihaloalkane mole ratios. We demonstrated that the yields of some tetraethyl alkylenediphosphonates are significantly improved (~90%) following the procedure of Ford-Moore and Williams [33] using a 4:1 triethyl phosphite to dihaloalkane mole ratio [19]. The yield of tetraethyl methylenediphosphonate (~25%), however, showed no improvement over the yield obtained using the reagents in stoichiometric amounts.

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Symmetrically Substituted Esters of Alkylenediphosphonic Acids

Several methods exist for the complete esterification of an alkylenediphosphonic acid or an alkylenebis(phophonic dichloride), but few methods are available for the direct synthesis of symmetrical disubstituted esters. Nylen [9], Cade [28], and Gorican [29] independently synthesized salts of dialkyl methylenediphosphonic acids via the Michaelis-Becker reaction. However, the method requires conversion of the salt to the free acid and it is plagued by low yields.

1.

Carbodiimide-Activated Esterification of Alkylenediphosphonic Acids

A selective method for the preparation of P,P’-dialkyl partial esters is the carbodiimide-promoted coupling of methylenediphosphonic acid with a primary alcohol [34]. Khorana and Todd were the first to use N,N’-dicyclohexylcarbodiimide (DCC) for the esterification of organophosphorus compounds, reporting a high yield synthesis of tetra- and symmetrical diesters of pyrophosphoric acid [35]. Burger and Anderson then used DCC methodology to prepare monoesters of aromatic phosphonic acids [36]. We subsequently applied their method to the synthesis of H2DO[MDP] and H2DEH[MDP]: (5) where DCC = dicyclohexylcarbodiimide DCU = dicyclohexylurea R = n-octyl or 2-ethylhexyl The yield, typically ~80%, is a significant improvement over the yields obtained by Gorican using Nylen’s method [9], but product purification requires a laborious acid-base extractive workup [34]. H2DEH[EDP] and H2DEH[BuDP] were also prepared using this approach [37, 38]. Potentially, the DCC-activated esterification could have general applicability for the synthesis of heteroatom-containing disubstituted esters of alkylenediphosphonic acids. Partial esters with silicon- or fluorine- containing groups are of particular interest since these moieties are expected to provide enhanced solubility in nontraditional solvents such as supercritical carbon dioxide (SCCO2) [25]. The position of the silicon must be considered when synthesizing a siliconcontaining diphosphonate extractant [39]. Attachment to oxygen adjacent to phosphorus or an α carbon of a linker creates a bond sensitive to hydrolysis

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while attachment at a β carbon produces a bond susceptible to elimination. Thus, separating silicon from a phosphonate group by at least three carbon atoms is optimal for achieving chemical stability [39]. A homologous series of siliconcontaining alkylenediphosphonic acids (n =1–6) was prepared in good yield (~60%) via the DCC-activated reaction using 3-trimethylsilyl-1-propanol [40–42]. N,N’diisopropylcarbodiimide (DIC) was investigated as a possible alternative to DCC in these preparations, but DIC is difficult to remove from the crude ester product, making purification difficult [25]. Fluorinated compounds and poly(dimethylsiloxane) polymers are particularly soluble in SCCO2 [43, 44]. In spite of this, the dimethylsiloxy group has received relatively little attention as a ligand-solubilizing substituent. This prompted us to investigate the DCC-activated esterification of methylenediphosphonic acid with 3-(1,1,1,3,5,5,5-heptamediylsiloxy)-1-propanol [25]. DCC methodology using this alcohol, however, failed to yield the desired diester product. Although carbodiimidepromoted coupling is highly selective, the reaction lacks general synthetic applicability and suffers from several major disadvantages [26].

2. Alcoholysis of Phosphonic Dichlorides A simple and convenient method for the synthesis of identically substituted tetraalkyl alkylenediphosphonates is the reaction of an alkylenediphosphonic dichloride with an unhindered alcohol in the presence of a catalyst and tertiary base, Eq. (6) [26, 45]. The alcoholysis is quite facile and essentially quantitative. Pyridine, triethylamine, and diisopropylethylamine are frequently employed in the reaction as acid scavengers. Purification of the tetraester is much simpler than for the partial esters since the former are amenable to separation by flash chromatography [26]. CH 2 [POCl 2 ] 2 is available commercially and alkylenebis(phosphonic dichlorides) are readily prepared [46].

(6)

where R

= butyl, hexyl, cyclohexyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl, phenyl, 3-trimethylsilylpropyl, fluoroalkyl DIEA = diisopropylethylamine Nitrogen heterocyclic bases catalyze the alcoholysis reaction [47]. 1H-tetrazole may be used for this purpose since it is a good leaving group and enhances the susceptibility of acid chlorides to alcoholysis [48]. Tetrazole selectively catalyzes the alcoholysis reaction so that syntheses of symmetrical partial esters as well as mixed tetraesters of methylenediphosphonic acid are possible via this route.

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Partial Dealkylation of Tetraalkyl Alkylenediphosphonates. Partial dealkylation of tetraalkyl alkylenediphosphonates is an attractive method for preparing symmetrical P,P’-disubstituted esters. The dealkylation, however, must remove a single substituent from each phosphonate group to yield a symmetrical disubstituted ester. Dealkylation of phosphonates may be achieved with mineral acids, bases, and silylating reagents, but the methods frequently are not selective and produce mixtures that require chromatography or efficient fractional crystallization for separation [16]. A few selective methods for partial cleavage of identically substituted phosphonate esters have been reported [49, 50]. Krawczyk reported that partial dealkylation of diethyl phosphonates and tetraethyl diphosphonates may be selectively achieved using lithium bromide or lithium chloride [51]. The lithium salts are obtained in nearly quantitative yields, but conversion of the salts to the acids are problematic since acidification or ion exchange chromatography leads to partial hydrolysis of the remaining ester groups [52]. Vepsäläinen and coworkers have developed a synthetic strategy to make symmetrical P,P’-disubstituted and asymmetrical P,P’-disubstituted methylenediphosphonates that are halogenated at the central carbon. They reported that secondary amines such as piperidine and morpholine selectively cleave tetrasubstituted methylenediphosphonates to the symmetrical partial esters when the bridging methylene group bears electron-withdrawing halogen substituents [16, 53–55]. Although the presence of an electron-withdrawing group was reported to be required to instigate the cleavage, we have found that a variety of methylenediphosphonate tetraesters undergo selective cleavage in refluxing morpholine to give the P,P’-disubstituted methylenediphosphonic acid in high yield [56]. We have used this method to selectively dealkylate fluoroalkyl-substituted methylenediphosphonates as well as tetraethyl methylene- and ethylenediphosphonates. The morpholinium salts are readily converted to the acids upon treatment with a cation exchange resin in acid form [52]. Trimethylsilyl halides are widely used for the removal of phosphonate ester groups. McKenna and coworkers reported that trimethylsilyl bromide (TMSBr) may be used for the complete hydrolysis of tetrasubstituted esters or it may be used selectively [57, 58]. Selective deprotection using TMSBr methodology exploits the greater reactivity of methyl esters and the significantly higher hydrolysis rate of silyl esters compared to other ester groups. We have achieved selective dealkylation of a variety of P,P’-substituted dialkyl dimethyl esters of methylenediphosphonic acid using TMSBr as the deprotecting agent [26]. TMSBr dealkylations of mixed methyl fluoroalkyl esters, however, are not selective. Direct Synthesis of P,P’-Disubstituted Alkylenediphosphonic Acids. We recently developed a facile highly efficient method for the synthesis of P,P’-disubstituted methylenediphosphonic acids by direct alcoholysis of methylene bis(phosphonic dichloride) [27]. The method, which does not require chromatographic or acidbase extractive purification, offers substantial advantages over the previously used

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DCC coupling route. 1H-tetrazole is used to selectively catalyze the coupling of CH2[P(O)Cl2]2 with two equivalents of alcohol under mild, anhydrous conditions to initially form a P,P′-disubstituted diphosphonate partial ester acid chloride [26]. The symmetrical partial ester is formed when the unreacted P-Cl groups of the diester acid chloride intermediate react with water:

(7)

where R

= hexyl, cyclohexyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl, 3-trimethylsilylpropyl DIEA = diisopropylethylamine A hindered base, diisopropylethylamine (DIEA), is used to drive the reaction to completion by acting as an acid scavenger. If the ester is water immiscible, it remains in the organic phase while tetrazole and DIEA partition into the aqueous phase as the salts. The yield of high-purity product (typically 85–95%) is higher than that previously obtained using DCC coupling procedures. Advantages of the method over existing preparative methods include high yields, short reaction times, mild conditions, small secondary waste streams, and simple isolation and purification procedures. Although 1H-tetrazole is an effective catalyst, it suffers from several disadvantages [59]. Its thermal instability motivated us to search for less hazardous bases to use as catalysts [47]. The compounds investigated were structurally similar to the heterocyclic catalysts used for acyl chloride esterification [60]. The size, presence of oxygen or sulfur donor atoms, as well as the number and position of nitrogen atoms in the ring, were systematically varied to determine the effect of these structural modifications on catalyst selectivity. Five-member aromatic nitrogen bases with two or more nitrogen atoms in the ring were shown to provide the greatest selectivity. Sulfur or oxygen atoms in the ring diminish the selectivity. Two compounds studied, 1H-1,2,3-triazole and 1-methylimidazole, yield symmetrical partial esters exclusively and are viable alternatives to 1H-tetrazole in CH2[P(O)Cl2]2 coupling procedures [47]. Synthesis of Mixed P,P’-Dialkyl Dimethyl Alkylenediphosphonates. The procedure developed for the direct synthesis of P,P’-disubstituted diphosphonic acids is not applicable for the preparation of water-soluble partial esters or siloxy chain-containing esters, nor does it work well with hindered alcohols, phenol, or fluoroalkyl containing alcohols. The applicability of the method to a wider range

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of alcohols, however, may be expanded through the synthesis of mixed P,P’dimethyl tetraesters [26]:

(8)

where R

= butyl, hexyl, cyclohexyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl, phenyl, 3-trimethylsilylpropyl, 2,2,3,3,4,4,4-heptafluorobutyl, 3-poly(dimethylsiloxy)-1-propyl DIEA = diisopropylethylamine In the procedure shown in Eq. (8), excess methanol is used in the second step to selectively form P,P’-dialkyl dimethyl methylenediphosphonates. The tetraesters, which are amenable to separation and purification by flash chromatography, are viable precursors to symmetric partial esters. In certain cases, this method provides an attractive alternate route to partial esters when the first alcohol reacts vigorously with the acid chloride to produce a mixture of all possible phosphonate esters, e.g, phenol [26]. The tetrazole catalyzed coupling of CH2[P(O)Cl2]2 with siloxy chaincontaining alcohols was successful. Three P,P’-dimethyl di-[3poly(dimethylsiloxy)-1-propylene] methylenediphosphonates were prepared via this route and converted to the symmetric partial esters by selective dealkylation [25]. Unfortunately, the phosphonic acid functionality promotes degradation of the dimethylsiloxy group making P,P’-di-[3-poly(dimethylsiloxy)-1-propylene] methylenediphosphonic acids unsuitable for use as solvent extraction reagents or studies in supercritical CO2.

III. SPECTROSCOPIC STUDIES Our synthetic efforts rely on several spectroscopic techniques for characterizing products, identifying reaction intermediates, and determining their stereochemistry. We also utilize instrumental methods for characterizing extracted species and diphosphonate complexes as well as ligand aggregation and solvent extraction chemistry. Nuclear magnetic resonance and infrared spectroscopy are tools frequently employed in our studies. Many reference texts on the spectroscopy of organophosphorus compounds are available, including two by L.C.Thomas that are quite useful [61, 62].

A. Nuclear Magnetic Resonance Spectroscopy P and 1H NMR spectroscopy are very effective tools for characterizing diphosphonic acids and their symmetrically substituted esters.

31

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1. Symmetrically Substituted Alkylenediphosphonic Acids The 31P NMR spectra of symmetrical P,P’-disubstituted partial esters, identically substituted tetraesters and the parent diphosphonic acids consist of a single sharp resonance [18, 26]. The length of the alkylene bridge and the nature of the ester group influence the position of the 31P resonance [18, 41]. The 1H NMR spectra of these esters and acids exhibit signals with the expected intensity and splitting pattern in the appropriate spectral regions [26, 27, 41]. The 31P NMR spectra of the mixed methyl tetraesters Me2R2[MDP] consist of two singlets of approximately equal intensity indicating that diastereomers, a dl enantiomeric pair and a meso form, are present in solution [26, 35]. This is confirmed by the 1H NMR behavior of the P–CH2–P, OCH3, and OCH2 functionalities, which show additional signals of expected intensities for the presence of equal amounts of the diastereomers [26]. The 31P and 1H NMR spectra of the partial esters do not show additional signals due to the presence of diastereomers because the acidic hydrogen atoms undergo rapid exchange on the NMR time scale [27].

2. Acid Chloride Intermediates The selective esterification of CH 2[P(O)Cl2] 2 with 2-ethyl-1-hexanol was investigated using 1H-tetrazole as the catalyst and base [26]. The reaction was followed by 31P and 1H NMR spectroscopy and two principal acid chloride reaction intermediates were identified on the basis of their chemical shifts and splitting patterns. During the early stages of the reaction, the 31P NMR spectrum showed the disappearance of CH2[P(O)Cl2]2 and the appearance of two doublets of equal intensity due to 2-ethylhexyl methylenediphosphonic trichloride. During the later stages of the reaction, two singlets of nearly equal intensity appeared due to the formation of methylenedi(2-ethylhexyl phosphonic chloride) that exists in solution as a mixture of diastereomers. The 1H NMR spectrum confirmed the presence of these partial ester acid chloride intermediates [26]. The signals associated with the P–CH2–P groups of the intermediates appear ~ 0.3 ppm downfield from the corresponding signal in the symmetric partial ester, consistent with the presence of electron-withdrawing chlorine atoms on the phosphorus atoms of the diphosphonate intermediates.

3.

Inductive Influence of Alkylene Bridge Length and Substituent Ester Groups

Table 2 lists the positions of the 31P NMR chemical shifts for a number of diphosphonates. As can be seen, the same general trends previously observed for the parent diphosphonic acids and their tetraethyl esters [18] hold for the silyl-substituted partial esters and their di(2-ethylhexyl) analogues [41]. Specifically, the 31P chemical shift becomes more positive as the number of methylene groups in the bridge increases. The chemical shift behavior is

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Table 2 31P Chemical Shifts for Diphosphonates (ppm vs. 85% H3PO4)

analogous to that for monophosphonates in which one phosphonate group of the diphosphonate has been replaced by an electronegative atom like chlorine, i.e., Cl(CH2)nP(O)(OR)(OH) [18]. As noted previously, the basicity and POH acidity vary in opposite directions with the electronegativity of the substituents on the phosphorus [63, 64]. In the case of diphosphonic acids, as the number of methylene groups in the bridge increases, the basicity increases and the POH acidity decreases. Based on the position of the 31P NMR chemical shifts observed for the parent acids, this trend continues until four methylene groups separate the phosphorus atoms [18]. After this, additional methylene groups have no effect on the 31P chemical shift, and the δ values are equivalent to a phosphonate with an alkyl substituent, i.e., CH3(CH2)n P(O)(OR) (OH). Based on these considerations, PO basicity should follow the order with POH acidities following the opposite order. The largest differences should be observed between [MDP] and [EDP], since the chemical shift difference is the largest in this case. The 31P chemical shifts of the silyl-substituted partial esters are slightly larger than in the analogous di(2-ethylhexyl) compounds, suggesting that the groups of the silyl-substituted ligands are more basic [41, 42]. The difference could arise from the higher electron donating nature of the trimethylsilyl (TMS) group relative to the 2-ethylhexyl group, but this difference might be small since the TMS group is separated from the rest of the molecule by a propylene linker.

B. Infrared Spectroscopy Infrared spectroscopy has proven to be a useful tool for characterizing alkylenediphosphonic acids, their metal complexes, and ester derivatives. The infrared spectra of the symmetric partial esters have three broad bands of medium intensity at approximately 2715, 2325, and 1675 cm-1 that are characteristic of the

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P(O)(OH) group. The presence of these bands is indicative of a strongly hydrogenbonded phosphonic acid [62]. These features have been discussed in detail for H2DEH[MDP] and H2DTMSP[MDP] [40, 65, 66]. The P(O)(OH) bands are not present in the spectra of the tetraesters and the phosphoryl band in the partial esters is lower in energy than in the tetraesters. The position of the band is influenced by the length of the alkylene bridge, the nature of the ester group, and hydrogen bonding.

1. Inductive Influence of Alkylene Bridge Length and Ester Groups Table 3 lists the positions of the phosphoryl bands for a number of diphosphonates. As shown, the band for a disubstituted acid is generally 30–50 cm-1 lower than for the analogous tetraethyl ester due to the formation of strong hydrogen bonds. Nevertheless, in both cases, the band appears at lower energy as the number of bridging methylene groups increases. The observed shift to lower energy cannot be attributed solely to changes in basicity in the acids because of possible complications due to hydrogen bonding. The tetraesters, however, do not form hydrogen bonded aggregates [26]. In this case, the frequency shifts are solely dependent upon electronic effects and therefore indicative of the increasing basicity with increasing bridge length. The electron-rich TMSP group potentially could increase the electron density on a phosphoryl group via an inductive influence which enhances the electron donating properties of the phosphoryl oxygen, i.e., resonance form II is more favorable:

Three identically tetrasubstituted diphosphonic acids were investigated to assess the importance of inductive effects from remotely positioned ester groups. Table 3

Infrared Stretching Frequencies for Selected Diphosphonates (cm-1)

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Specifically, Am(III) and nitric acid extraction, which was followed by FTIR, were measured for the 2-ethylhexyl, 3-trimethylsilyl-1-propyl, and 3-perfluorohexyl-1propyl tetrasubstituted methylenediphosphonates, EH4[MDP], TMSP4[MDP], and PFHP4[MDP], respectively [70]:

Table 4 summarizes the infrared data in the phosphoryl region for the free and HNO3-coordinated 2-ethylhexyl-, silyl- and fluoroalkyl-functionalized tetraesters. In agreement with 31P NMR data, the phosphoryl bands of TMSP-substituted diphosphonates appear at lower energy than in analogous di(2-ethylhexyl) compounds, suggesting that the groups in the silyl-substituted extractants are more basic. Formation of a HNO3-hydrogen bonded adduct weakens the bond causing a pronounced shift of the absorption band to lower energy. Upon contact with concentrated aqueous nitric acid, the spectra of the 2-ethylhexyl and 3-trimethylsilyl-1-propyl esters show only the presence of strongly hydrogenbonded phosphoryl groups. The less efficient fluoroalkyl-substituted extractant shows the presence of both free and hydrogen-bonded groups. The frequencies associated with the free groups follow the order fluoroalkyl→alkyl→trimethylsilyl-substituted diphosphonate. This order is consistent with the relative HNO3 and Am(III) extraction efficiency observed for these compounds [67]. Relative to the 2-ethylhexyl-substituted ester, the lower frequency of the silyl-substituted extractant suggests that the electron-donating 3Table 4 FTIR Frequencies of the Phosphoryl Region for Free and HNO3-Coordinated Diphosphonate Tetraesters ([Extractant]=0.015 M, [HNO3]=8 M)

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trimethylsilyl-propyl groups decrease the bond strength by inductively increasing electron density on the phosphoryl oxygen; i.e., resonance form II is more favorable. Increased electron density on the phosphoryl oxygen enhances the basicity and extraction efficiency of the silyl-substituted compounds. Similarly, the higher stretching frequency of the fluoroalkyl extractant suggests that electron-withdrawing 3-perfluoro-hexylpropyl groups increase the bond strength by inductively removing electron density from the phosphoryl oxygen; i.e., resonance form I is more favorable. Decreased electron density on the phosphoryl oxygen decreases group basicity thus lowering Am(III) extraction by the fluoroalkyl-substituted ligand. The nitric acid and Am(III) extraction studies, complemented by FTIR data for EH4[MDP], TMSP4[MDP], and PFHP4[MDP], demonstrate that the electron-donating properties of the phosphoryl oxygen in diphosphonates can be significantly influenced by the nature of remotely positioned ester groups, despite the relatively large separation of a propylene linker [67].

2. Metal-Diphosphonate Complexation Spectroscopic studies of metal complexes of P,P’-disubstituted alkylenediphosphonic acids are limited by the poor water solubility of the ligands and their salts as well as a lack of X-ray crystallographic data for model compounds that could serve as reference structures. Nevertheless, attempts have been made to identify diphosphonate coordination modes and the nature of metal-diphosphonate interactions [65, 68, 69]. Phosphonate Coordination Modes. Four limiting types of bonding modes are possible for organophosphorus acids that have two oxygen atoms available for metal coordination (Fig. 1). The figure also shows a methylenediphosphonic acid which exhibits both chelate and bridging coordination modes. Proceeding from an ionic environment to a monodentate, bidentate, or bridging structure, symmetry and bonding changes occur that result in differences in the P–O stretching region. A vibrational analysis by Kumamoto indicates that the P–O bond order plays a major role in determining the frequency of phosphorus-oxygen stretching vibrations [70]. In the case of ionic phosphonates, the P–OH and bonds of the acid are replaced by two equivalent phosphorus-oxygen bonds of intermediate bond order which give rise to two infrared active POO- stretching modes, i.e., the asymmetric and symmetric vibrations, νasym(POO-) and νsym(POO-), respectively. The frequency difference between the two modes ∆ν reflects differences in P–O bond orders. Coordination of a single oxygen of group results in considerably different P–O bond orders (analogous to the free acid), increasing the frequency of the asymmetric stretching mode and ∆ν, i.e., [νasym(POO-)-νsym(POO-)]. Symmetrical coordination of group through a chelate or bridging interaction reduces the P–O bond order and decreases the frequencies of both stretching modes as well as the value of ∆ν. Copyright © 2004 by Marcel Dekker, Inc.

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Figure 1 Types of bonding for organophosphorus acids and for a methylenediphosphonic acid. (Adapted in part from Ref. 68.)

The infrared spectra of metal compounds of H2DEH[MDP], H2DEH [EDP], H2DEH[BuDP], and H2DTMSPMDP] have two strong absorption bands in the 1000–1300 cm-1 region [37, 38, 65, 69, 71]. The higher frequency band is assigned as νasym(POO-) and the lower frequency band as νsym(POO-). The asymmetric and symmetric POO- stretching modes in the Ca(II), Ba(II), Cu(II), La(III), Eu(III), U(VI), Th(IV), and Fe(III) compounds appear at lower energy than in the corresponding ionic sodium salts. The shift of the νasym(POO-) mode to lower

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frequencies relative to the values in the sodium salts rules out monodentate coordination. The decrease in frequency of both stretching modes indicates a symmetrical interaction of the diphosphonate ligands through chelate and/or bridging coordination modes [71]. The similarities in the spectra of H2DTMSP[MDP] and H2DEH[MDP] complexes indicate that replacement of the 2-ethylhexyl group by the 3-trimethylsilylpropyl group does not change the coordination mode of the diphosphonate ligand attached to the metal. Figure 2 shows that ∆ν, the energy difference between the POO stretching bands, varies linearly with the ionic potential Φ of the metal ion (Φ=e/r, where e=the charge and r=the Shannon ionic radius) [37, 65, 69, 71]. The ∆ν values for metal-diphosphonate compounds increase as the chain length of the alkylene bridge between the phosphonate groups increases. Based on the ∆ν values, the strength of the metal-diphosphonate interaction is expected to vary in the order H2DEH[MDP]>H2DEH[EDP]>H2DEH[BuDP]. Solvent extraction and aggregation studies confirm this order. Stripping of metal ions from the extractant, for example, is difficult with H2DEH[MDP], whereas for H2DEH[EDP] and H2DEH[BuDP] with weaker binding metal stripping is more readily accomplished. In the systems studied, the frequency of νasym(POO-) is especially sensitive to the nature of the metal ion while the frequency of νsym(POO-) remains relatively

Figure 2 Ionic potential Φ of the metal ion vs. ∆ν=[νasym(POO-)-νsym(POO-)] for 2-ethylhexyl diesters of alkylenediphosphonic acids (circles: H2DEH[MDP], squares: H2DEH[EDP], triangles: H2DEH[BuDP]). (Adapted in part from Ref. 68.)

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constant. The highest νasym(POO-) values are observed in the sodium salts while the lowest energy values occur in the iron compounds. The decrease in νasym (POO-) and consequent decrease in ∆ν with the charge to radius ratio of the metal ion are a result of the increased strength of the metal-phosphonate interaction. The interaction is weakest in the sodium salts and becomes progressively stronger in the other studied compounds. Proceeding from the sodium salts to compounds of metal ions with very high ionic potentials, increased P–O bond polarization is expected to occur. In compounds containing very highly charged metal ions, such as Th(IV) and Fe(III), bond polarization might occur to such a great extent that the resulting metal-oxygen bond has substantial covalent character. Nature of the Metal-Diphosphonate Interaction. Evidence of the covalent character of the Fe(III)-diphosphonate interaction is provided by the far-infrared spectra for the sodium, copper, selected lanthanide, and iron(III) salts of H2DEH[MDP] [69]. In the region below 600 cm-1 where the POO- and POC deformation modes appear, the Fe(III)-H2DEH[MDP] complex has an additional strong absorption band at 256 cm-1 that is sensitive to the mass of the Fe isotope used and is absent in the spectrum of the free acid and the other compounds investigated. This band and another band at 551 cm-1, which is also sensitive to the mass of the Fe isotope, were assigned as Fe–O stretching vibrations based on their frequency and 54Fe isotopic shift compared with related iron(III) compounds [69]. The appearance of Fe–O stretching bands in the anhydrous iron-H2DEH[MDP] complex indicates that the Fe-diphosphonate interaction has a substantial covalent component. A metal-oxygen (M–O) stretching mode will be infrared active only if the metal-oxygen bond is sufficiently covalent. The absence of M–O stretching bands in the lanthanide compounds indicates that in these salts the binding is predominantly ionic. Based on these findings, it is evident that the changes observed in the P–O bond order in metal-diphosphonate compounds arise from a combination of covalent and bond polarization effects. Far-infrared results for the H2DEH[EDP]- and H2DEH[BuDP]-iron complexes show that alkylene bridge length affects the frequency of the iron-oxygen stretching modes [68]. The frequencies of the Fe–O bands decrease as the length of the alkylene bridge increases, with the lower energy band appearing at 254 and 249 cm-1 for H2DEH[EDP] and H2DEH[BuDP], respectively. The lower Fe–O stretching frequencies indicate weaker iron-oxygen binding, consistent with the conclusions based on the ∆ν values discussed above. Since the value of ∆ν depends upon the ionic potential of the metal ion, it may be used to obtain qualitative information about the nature of the M–O interaction. When both oxygen atoms of the anion of an organophosphorus extractant interact with the metal ion, high ∆ν values are indicative of high ionic character in M–O bonds while low ∆ν values indicate a high degree of covalent character [68].

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IV. AGGREGATION STUDIES When alkylenediphosphonic acid diesters are dissolved in nonpolar diluents, selfassembly into more complex structures is suggested by the very structure of this class of compounds. In fact, the presence of both hydrogen bond donors (–OH) and acceptors(=O) in structure I clearly indicates that hydrogen bonding is the primary factor in driving the aggregation process. This is confirmed in the infrared spectra by the frequency shifts observed for the phosphoryl bands as compared to those of the tetraesters. Band shifts to lower energy are spectral features characteristic of systems with strong intermolecular hydrogen bonding. Recent osmometric measurements on tetracyclohexyl and other tetraalkyl esters of methylenediphosphonic acid have unequivocally demonstrated that when all the -OH groups in Structure I are esterified, no aggregation takes place and the tetraesters exist in solution as monomers [26], confirming that strong hydrogen bonding is key to aggregate stability. Information about the specific types of aggregates formed through hydrogen bonding (i.e., dimers vs. higher aggregates) cannot be provided by the infrared spectra. Useful indications, however, can be deduced from the behavior of monofunctional organophosphorous acidic extractants, such as di(2-ethylhexyl) phosphoric acid (HDEHP) and mono(2-ethylhexyl) phosphoric acid (H2MEHP), which are known to strongly aggregate in nonpolar diluents [72, 73]. Monoprotic acids usually dimerize to form an ring, analogous to that formed in the familiar dimerization of carboxylic acids. , in the Etter hydrogen bond assembly classification [74], denotes an 8-membered ring structure containing two hydrogen bond donors and two hydrogen bond acceptors. Hydrogen bonding in organophosphorus acid dimers, however, is known to be stronger than in carboxylic acid dimers [75, 76]. The aggregation behavior of diprotic acids such as H2MEHP is more complicated. When two monomers hydrogen bond to form a dimeric species containing one ring, the resulting aggregate has additional –OH groups that can serve as sites for further aggregation [77]. This is shown schematically in Structure II:

Structure II

This scheme rationalizes why HDEHP is dimeric in benzene [73, 78], while H2MEHP can have an aggregation number as high as 12 in the same diluent [78, 79]. Since HDEHP and H2MEHP can be regarded as monofunctional analogues of

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the diphosphonic acids shown in Structure I, an interesting question arose in our investigation. That is, would the aggregation behavior of the diphosphonic acid diesters be similar to that of diprotic acids, or would the aggregation be limited to the formation of dimers as is the case for monoprotic acids? Our physicochemical data revealed that both behaviors are possible, with the value of n in Structure I, i.e., the number of carbon atoms in the alkylene bridge connecting the phosphorus atoms, as the determining factor [37, 38, 41, 65, 80–83].

A. Vapor Pressure Osmometry (VPO) Measurements Vapor pressure osmometry (VPO) can be used to measure the aggregation of an extractant in an organic solution by comparing its behavior to that of a monomeric standard [84]. In our studies, experimental data were used to obtain plots of osmometer response (the electrical potential µV needed to equilibrate an electric circuit comprising two thermistors, one wet by the pure solvent, the other by the test solution) vs. solute concentration. The experimental details can be found in the original publications [37, 38, 41, 65, 82, 83]. Figure 3 collects VPO data obtained for diphosphonic acid diesters in toluene, a convenient diluent for this type of investigations. Panel A shows the data for the 2-ethylhexyl diesters, while the data for the 3-trimethylsilylpropyl diesters are presented in panel B. At each solute concentration, the electrical potential values for the diphosphonic acid diesters are considerably lower than the value for the monomeric standard. Since vapor pressure is a colligative property, its lowering is proportional to the number of particles in solution and thus, consequently, related to the aggregation of the solute. In the simplest case, when the electrical potential vs. concentration data are linear, the number-average aggregation number nav is given directly by the ratio of the slopes of the monomeric standard and the extractant solution. As can be seen in Figure 3 for H 2 DEH[MDP], H 2 DTMSP[MDP], H2DTMSP[PrDP] and H2DTMSP[PDP], compounds for which n in Structure 1 is odd (1, 3, or 5), the value of nav is very close to 2, indicating that these extractants exist in toluene as dimers. Thus, in terms of aggregation behavior, independent of the esterifying groups, the methylene-, propylene- and pentylenediphosphonic acid diesters behave similar to the monoprotic monofunctional analogues in Structure II. Since the VPO data are linear, values for the dimerization constants cannot be estimated. However, it is evident that the dimerization constants are sufficiently large that the compounds are completely dimerized over the entire concentration range investigated. The compounds for which n in Structure 1 is an even number (2, 4, or 6), form larger aggregates, analogous to the diprotic monofunctional analogues in Structure II. For example, the slopes of the VPO data for H2DEH[BuDP] and H2DEH[EDP] (panel a) indicate that these diesters form primarily trimeric and hexameric aggregates, respectively. Similar behavior is exhibited by the silyl-containing diesters (panel b). The nav values for H2DEH[BuDP] and H2DTMSP[EDP] shown

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Figure 3 VPO data for diphosphonic acid diesters in toluene at 25°C. (a) Monomeric standard: bibenzyl (squares); data for H2DEH[MDP] (circles), H2DEH[EDP] (diamonds) and H2DEH[BuDP] (triangles). (b) Monomeric standard: sucrose octaacetate (full squares); data for H2DTMSP[MDP] (full circle), H2DTMSP[EDP] (empty circles), H2DTMSP[PrDP] (up triangles), H 2DTMSP[BuDP] (diamonds), H 2 DTMSP [PDP](empty squares), and H2DTMSP[HDP] (down triangles).

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in Fig. 3 (3.3 and 5.5, respectively) are those obtained from a best straight-line fit of the data. Closer inspection reveals that these data are not linear, indicating an equilibrium between two or more species with different aggregation numbers. This behavior is particularly evident for the H2DTMSP[BuDP] and H2DTMSP[HDP] diesters (panel b). In this case, the VPO data can be used to calculate values for the aggregation constants following the procedure described earlier [85]. The values of the aggregations constants of the various diesters investigated are summarized in Table 5. It appears from Table 5 that the aggregation of the diphosphonic acid diesters exhibits an even-odd effect as the number of the methylene bridging groups varies. This effect is likely to be due to the “zig-zag” pattern adopted by the alkylene chain separating the phosphorus atoms. This pattern controls the orientation of the and POH groups and changes the geometry of the hydrogen bonded aggregates that can be formed. This even-odd effect also manifests itself in the melting points of the partially esterified diphosphonic acids [40] and the parent acids [18], with the molecules containing an even number of bridging methylene groups exhibiting higher than expected melting points. Table 5 shows a significant difference between the 2-ethyhexyl (EH) and corresponding 3-trimethylsilyl-1-propyl (TMSP) diesters, the latter being less strongly aggregated than the former, due to larger steric hindrance by the TMSP groups. Also, within the series of TMSP diesters, the stability of the hexameric aggregate decreases with an increase in the number of bridging methylene groups, indicating that self-assembly into hexamers is more difficult for molecules with longer alkylene chains. This phenomenon has practical consequences in metal ion extraction studies. Speciation diagrams [37, 41, 83] have shown that, under the Table 5 Aggregation Constants of Alkylenediphosphonic Acids Diesters in Toluene

a b

Equilibrium constant too large to be calculated from VPO data; Calculated from metal distribution data (see Section V).

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conditions typically used for solvent extraction studies, the presence of monomeric species cannot be ignored for weakly aggregated diesters. The presence of monomeric extractants must be taken into account to explain some solvent extraction data.

B. SANS Measurements Light, X-ray, and neutron scattering are powerful techniques in structural studies of polymers, micellar aggregates, and other materials [86]. The nanoscale size of the aggregates and the large amount of hydrogen atoms in organic materials make small-angle neutron scattering (SANS) particularly well suited for investigating extractant aggregates in organic diluents [87]. The SANS technique is based on the large difference in the neutron scattering properties of hydrogen and deuterium atoms. Dissolution of an extractant in a deuterated diluent provides the neutron scattering contrast required to make the solute particles “visible” against the solvent background. SANS measurements of deuterated toluene solutions of H 2DEH[MDP], H2DEH[EDP], H2DEH[BuDP] and their metal complexes were made using the time-of-flight small-angle neutron diffractometers (SAD and SAND) at the Intense Pulsed Neutron Source (IPNS) at ANL. The characteristics of the diffractometers, the background corrections and the procedure for placing the data on an absolute scale can be found in the original publications and references therein [80, 81, 88]. The SANS data were obtained as plots of scattering intensity I (cm-1) vs. momentum transfer, Q(Å-1) (Q=(4π/␭) sin θ, where θ is half the scattering angle and λ is the wavelength of the probing neutrons). The SANS scattering signals were interpreted using the Guinier analysis {ln[I(Q)] vs. Q2} [89]. The Guinier fit was used to determine the molecular weight of the extractant aggregates and, hence, the weightaverage aggregation number nw. The SANS results obtained with metal loaded extractants are discussed later in this chapter. Here we summarize the results obtained from SANS measurements on deuterotoluene solutions of the extractants alone. Figure 4 shows the SANS data for 0.1 M solutions of H2DEH[MDP], H2DEH[EDP] and H2DEH[BuDP] (panel A), and the Guinier fits of the data (panel B). The extrapolation to Q2=0 of the Guinier straight lines shown in panel B provided the I(0) values which were used to calculate the aggregation number of the extractants [80, 81]. The results of these calculations confirmed that H2DEH[MDP] exists in solution as a dimer, H2DEH[EDP] as a hexamer, and H2DEH[BuDP] predominantly as a trimer.

C. Structure of the Aggregates The MDP, PrDP, and PDP dimers could have a variety of different structures. Some possibilities, based on various combinations of intra- and intermolecular hydrogen bonds, are shown in Structures III–VI below for the MDP dimer.

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Figure 4 SANS behavior for 0.1 M H2DEH[MDP], H2DEH[EDP], and H2DEH [BuDP] in deuterotoluene. (a) Log-log plots of scattering data; (b) Guinier fits of data.

The results from continuous variation infrared spectroscopic studies, in which the diluent in a solution-containing H2DEH[MDP] was progressively replaced with the depolymerizing diluent 1-decanol, were consistent with Structures V and VI [66]. To fully elucidate the relative stabilities of Structures III–VI, molecular mechanics calculations were performed. A variety of starting structures containing two (such as III and IV) or four (such as V and VI) intermolecular hydrogen bonds were generated and geometrically optimized. For ease of computation, the 2-ethylhexyl groups in

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H2DEH[MDP] were replaced by methyl groups. Two highly hydrogen-bonded dimer structures were found to be the most stable. They exhibited the hydrogenbonding patterns of Structures V and VI. The lowest energy conformation found for Structure VI was slightly higher in energy than the lowest energy conformation found for Structure V [66].

Structure III

Structure V

Structure IV

Structure VI

The predominantly trimeric aggregates of the BuDP diesters, and the hexameric aggregates of the EDP and HDP diesters quite possibly have a cyclic structure, based on spectroscopic evidence indicating that all the hydrogen bonds in these aggregates are equivalent [37, 38]. In our attempts to obtain more detailed information about the three-dimensional shape of the H2DEH[EDP] aggregate, we were able to fit the SANS data for H2DEH[EDP] using the equation for a homogeneous sphere [81, 90]. The best fit provided a radius R of the spherical aggregate equal to 11.8±0.2 Å. The hexamer probably assumes a spherical shape best illustrated by the seams on a tennis ball. A representation of this shape for the H2DEH[EDP] hexamer is shown in Structure VII. In this aggregate, the alkyl groups are likely oriented outward (toward the lipophilic solvent), and a large hydrophilic internal cavity is available to accommodate metal cations and/or water molecules. Thus, Structure VII strongly resembles a reverse micelle. Although no SANS measurements have been performed on the TMSP diesters, it is very likely that the structures of their aggregates parallel those of the 2-ethylhexyl analogues.

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Structure VII

V.

SOLVENT EXTRACTION OF METAL IONS AT LOW LOADING

The preceding sections were propaedeutic for the interpretation of data on metal ion extraction by alkylenediphosphonic acid diesters dissolved in water-immiscible diluents. Spectroscopic and aggregation studies have shown that a number of effects arise when the distance between the phosphorus atoms of a diester is increased by increasing n in Structure I. All these effects, in turn, have an impact on metal extraction behavior. First, the basicity of the group increases and is expected to follow the order with the acidity of the most acidic POH proton following the opposite order (see Section III). The increase in basicity of the donor group increases the strength of the interaction with positively charged ions and thus enhances metal ion extraction. On the other hand, the concomitant decrease in acid strength increases the competition between the proton and the metal ion for the ligand, thus opposing metal extraction.

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The second effect concerns the cooperative bonding of the metal ion by both phosphonate groups of the ligand, which strongly enhances metal extraction. While this behavior is expected for the methylenediphosphonic acid diesters (n=1 in Structure I), cooperative binding becomes less likely for ligands with n>1. When the separation between the phosphonate groups is sufficiently large, the solvent extraction behavior of the diesters should parallel that of the analogous monofunctional organophosphorus extractants. The third effect is due to the size of the chelate ring formed when both phosphonate groups bind to the same metal cation. The methylenediphosphonic acid diesters can form highly stable six-membered rings, as can be seen in Structure I. For ligands with n≥2, larger and progressively less stable chelate rings would be formed, resulting in greatly diminished extraction efficiency. Finally, the effect of extractant aggregation on metal extraction chemistry manifests itself in several ways. Aggregation shifts the partition equilibrium of the extractant toward the organic phase reducing its concentration in the aqueous phase. Further, aggregation of the metal-extractant complexes shifts the overall extraction equilibria to the right. Both effects increase the metal distribution ratio. The presence of large extractant aggregates in the organic phase also affects the stoichiometry of the extraction reaction. This can be easily understood by considering the following simplified equilibria describing the extraction of a metal ion Mz+ by a highly aggregated acidic extractant HA: (9) where the bar denotes organic phase species. The metal distribution ratio, defined as the ratio of the equilibrium metal concentrations in the organic and aqueous phase, can be written as (10) where K is the equilibrium constant for the extraction reaction. Assuming that the concentration of monomeric extractant is negligible, it follows that (11) The analytical concentration of the extractant, C HA, is proportional to the concentration of the extractant aggregate with the proportionality constant equal to the aggregation number n. Substituting Eq. (11) in Eq. (10), one obtains (12) which demonstrates that in a log-log plot of D vs. CHA, the slope of the extractant dependency will be unity regardless of the charge on the cation. This type of behavior has been reported for the extraction of actinide and lanthanide ions by H2MEHP in aromatic diluents [77, 79, 91]. With highly

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aggregated extractants, such as H2MEHP, metal ions are buried within the extractant aggregate and held in place by forces analogous to those which operate in solid ion exchange resins [77]. Similar results have been reported for other highly aggregated extractants, e.g., quaternary alkylammonium salts [92] and dinonylnaphthalene sulfonic acid in nonpolar diluents [93]. All of these effects have been observed to some extent in the metal extraction chemistry of alkylenediphosphonic acid diesters at trace metal concentration level. Several examples are provided in the following with emphasis on the behavior of alkaline earth cations and Am(III). Information on the behavior of other metal ions can be found in the original publications [34, 38, 41, 83, 94–96].

A. Alkaline Earth Cations Solvent extraction data for alkaline earth cations are important for several reasons. Efficient and selective removal of Sr2+ from high level nuclear waste, and the separation of 226Ra and 228Ra from the environment are difficult problems facing the separations community [97, 98]. Also, the coordination chemistry of alkaline earth cations is usually less complicated than transition metal chemistry since the

Figure 5 Acid dependencies for the extraction of selected alkaline earth cations by 0.1 M solutions of H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] in o-xylene.

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former is controlled almost exclusively by electrostatic interactions. Therefore, these cations may be used as a convenient tool to probe the solvent extraction chemistry of novel reagents. Figures 5 and 6 show acid and extractant dependencies for the extraction of selected alkaline earth cations by o-xylene solutions of H2DEH[MDP], H2DEH [EDP] and H2DEH[BuDP], respectively In the following discussion it is assumed that the aggregation behavior of the extractants in o-xylene is the same as in toluene, the diluent used for the aggregation studies. The acid dependencies for all extractants exhibit a slope of -2, which is consistent with the displacement of two protons by a divalent cation upon extraction in the organic phase. The extractant dependencies for H2DEH[MDP] and H2DEH [BuDP] are close to 2. Since H2DEH[MDP] is dimeric, each cation is extracted by two H2DEH[MDP] dimers according to the equation: (13)

where H2A represents the fully protonated H2DEH[MDP] extractant. A similar equilibrium can be written for H2DEH[BuDP], with two extractant trimers involved in the extraction equilibrium.

Figure 6 Extractant dependencies for the extraction of selected alkaline earth cations by H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] in o-xylene from 0.01 M HNO3 in the aqueous phase.

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Structure VIII has been proposed for the organic phase complexes between alkaline earth cations and H2DEH[MDP]:

Structure VIII

In this structure, where for simplicity only one of the monodeprotonated dimers is shown, two protons are displaced from two extractant dimers, as required by the slope analysis. The six-member chelate rings shown in Structure VIII arise from the interaction of the metal ion with the phosphoryl oxygens of the extractant. The extractant dependencies for H2DEH[EDP] have a limiting slope of one in the 0.01–0.1 M concentration range. The slope, however, increases at lower extractant concentrations. As previously shown through Eqs. (9)–(12), a slope of 1 in the log-log plot of metal distribution ratio vs. the analytical concentration of the extractant is consistent with extraction by a highly aggregated species in which the aggregation is not disrupted by the metal ion. This is in agreement with the results from aggregation studies, which indicated that H2DEH[EDP] is primarily hexameric over the concentration range studied. Based on the slope analysis, the extraction of alkaline earth cations by the H2DEH[EDP] hexamer can be expressed as (14)

where (H2A)6 is the H2DEH[EDP] hexamer and MH10A6 is the metal-hexamer complex in the organic phase. In this complex, an alkaline earth cation lies in the hydrophilic cavity formed by the H2DEH[EDP] aggregate, which resembles a reverse micelle. The higher slope values in the 0.001–0.01 M concentration range can be attributed to incomplete aggregation of the extractant. At low extractant concentrations, the measured D values include contributions from extraction by the monomeric extractant. The deviations of the extractant dependency data from a slope one observed in Fig. 6 for the extraction of Ca2+, Sr2-, and Ra2+ were used to calculate the aggregation constant for H2DEH[EDP] reported in Table 5 [94]. A comparison of H2DEH[MDP] and H2DEH[EDP] in Figs. 5 and 6 indicates that the introduction of an additional CH2 group into the alkylene bridge of the diphosphonate profoundly affects alkaline earth extraction. H2DEH[MDP] exhibits no selectivity over the alkaline earth series, while H2DEH[EDP] behaves in a manner

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similar to a monofunctional extractant [79, 99], preferentially extracting Ca. The selectivity exhibited by H2DEH[EDP], however, is accompanied by lower D values than observed with H2DEH[MDP]. The lower D values exhibited by H2DEH[EDP] are consistent with its expected lower acidity and the formation of larger rings upon metal extraction. The silyl-substituted partial esters behave similarly to the dialkyl esters, with the extraction of alkaline earth cations being slightly more efficient. This aspect of the solvent extraction chemistry of the silyl-substituted partial esters will be discussed in more detail for Am(III) extraction.

B. Am(III) Solvent extraction studies with the symmetrical P,P’-disubstituted partial esters were extended to tri-, hexa-, and tetravalent actinides [34, 38, 41, 83, 94, 96]. The behavior of Am(III) is particularly important in view of the need to remove trivalent actinides from high level nuclear wastes. Trivalent actinides are typically left in the waste from the PUREX process which is based on the use of tri-n-butyl phosphate (TBP) as the extractant [100]. Figure 7 shows the acid dependencies for Am(III) extraction by the dialkyland disilyl-substituted esters. Figures 8 and 9 show the extractant dependencies

Figure 7 Acid dependencies for the extraction of Am(III) by (a) 0.1 M solutions of H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] and (b) 0.01 M solutions of H2DTMSP[MDP], H2DTMSP[EDP], H2DTMSP[PrDP], H2DTMSP[BuDP], H2DTMSP[PDP], and H2DTMSP[HDP] in o-xylene.

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for these solvent extraction systems. Panel C in Figure 9 compares the extractant dependencies for Am(III) extraction measured with the dialkyl and disilyl partial esters of methylenediphosphonic ([MDP]) and ethylenediphosphonic ([EDP]) acids, respectively. The data in Figure 7 indicate that the efficiency of metal ion extraction by the diesters follows the series [MDP]>[EDP]>[HDP]>[BuDP]>[PrDP]>[PDP]. This order does not correspond to the expected trends in basicity, POH acidity, or size of chelating rings possible upon metal ion complexation. This suggests that these factors, as well as the aggregation state of the extractants, have varying degrees of importance and combine in a complex way as the length of the alkylene bridge of the diphosphonate is increased. The acid dependency data for the extraction of Am(III) by H2DEH[MDP] and H2DTMSP[MDP] exhibit a maximum at 0.2–0.3 M HNO3. This feature of the data, which differs from the expected acid dependency for a trivalent metal cation (i.e., a straight line with a slope of -3), probably arises from the formation of species having different stoichiometries at different acid concentrations. In the 0.05–0.3 M HNO3 concentration range, a positively charged species (1:1 ligand to metal complex, for example) can form which requires coextraction of nitrate ions. At higher acidities, a neutral species (2:1 ligand to metal complex) that preferentially reports to the organic phase could be formed, leading to the expected acid dependency slope of -3. The acid dependency data for the disilyl-substituted esters in Fig. 7 exhibit slopes of close to -3 over at least a part of the HNO3 concentration range, consistent with the displacement of three protons upon the extraction of a trivalent metal ion. At higher acid concentrations, however, the extractants with more than three bridging methylene groups tend to exhibit acid dependency plots with a less negative or even positive slope. This suggests an increased importance of extraction by the neutral (fully protonated) extractants via a solvating mechanism, where extraction into the organic phase is dependent on the coextraction of nitrate ions for charge neutralization. This provides additional evidence that the ligands become less acidic as more methylene groups separate the phosphorus atoms. As the bridge length increases, POH acidity decreases and extraction via the solvating mechanism becomes more important at high aqueous acidities. The acidities at which the solvation mechanism manifests itself are generally lower for the ligands with more basic groups. The extractant dependencies for Am(III) extraction by the dialkyl-substituted esters (Fig. 8) show a behavior similar to that discussed previously for the alkaline earth cations. For H2DEH[MDP] and H2DEH[BuDP], the slope values of 2 suggest extraction equilibria of the type: (15) The Am(III) complex with H2DEH[MDP], a ligand for which it can be safely assumed that both phosphonate groups of the molecule cooperatively bind to the same cation (bifunctional behavior), should have a structure similar to that shown

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Figure 8 Extractant dependencies for the extraction of Am(III) by H2DEH[MDP], H2DEH[EDP] and H2DEH[BuDP] in o-xylene from various concentrations of HNO3 in the aqueous phase.

for the alkaline earth cations in Structure VIII, with three protons displaced from two extractant dimers, as required by the slope analysis. The value of unity for the extractant dependency in the extraction of Am(III) by H2DEH[EDP] (Fig. 8), as discussed earlier for the alkaline earth cations, can be explained by the highly aggregated state of the extractant. The extraction equilibrium can be written as (16)

and it has been postulated that Am(III) lies in the hydrophilic cavity formed by the H2DEH[EDP] hexamer. Figure 9 shows the extractant dependencies for Am(III) extraction by the series of disilyl-substituted esters. The plots for the extractants with an odd number of bridging methylene groups (dimers) exhibit extractant dependencies slopes of 2, suggesting that two dimeric aggregates participate in Am(III) extraction. The extraction efficiency decreases as the number of bridging groups increases from 1 to 5, despite the increased basicity of the phosphoryl oxygen along the series. This suggests that the chelate effect may be the dominant factor in determining Am(III) extraction efficiency, with the six-membered rings formed upon complexation by

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Figure 9 Extractant dependencies for the extraction of Am(III) by several diphosphonic acid diesters in o-xylene. (a) Data for H2DTMSP[MDP] (1 M HNO3), H2DTMSP[PrDP], and H2DTMSP[PDP] (0.1 M HNO3). (b) Data for H2DTMSP [EDP], H2DTMSP[BuDP], and H2DTMSP[HDP] (0.1 M HNO3). (c) Comparison between H2DTMSP[MDP] and H2DTMSP[EDP] (full symbols) and H2DEH[MDP] and H2DEH[EDP] (empty symbols) (1 M HNO3).

H2DTMSP[MDP] expected to be considerably more stable than the larger rings possible upon Am(III) complexation by H2DTMSP[PrDP] and H2DTMSP[PDP]. However, it should be noted that the acidity of the ligands is expected to decrease over this same series and this may also play a significant role in determining extraction efficiency. The ligands with an even number of bridging methylene groups exhibit Am(III) extractant dependency plots with slopes less than 2, suggesting the importance of extraction by a single, highly aggregated species. H2DTMSP[EDP] exhibits an extractant dependency of 1 and the same considerations made for the H2DEH[EDP] apply. For H2DTMSP[BuDP] and H2DTMSP[HDP], on the other hand, the fractional values for the extractant dependencies arise from simultaneous equilibria involving metal extraction by the monomeric extractant. At the low extractant concentrations used, the concentration of monomeric H2DTMSP[BuDP] and H2DTMSP[HDP] is not negligible and cannot be ignored. Figure 9 (panel C) compares the extractant dependency data for the extraction of Am(III) by H 2 DEH[MDP], H 2 DTMSP[MDP], H 2 DEH[EDP], and

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H2DTMSP[EDP]. While the disilyl esters exhibit nearly identical aggregation behavior as their dialkyl analogues, the disilyl esters extract metal ions two to three times more efficiently. As described earlier, the increased extraction efficiency is due to the slightly higher basicity in the disilyl esters due to the increased electron donating nature of the TMS group. Because of the interplay between extractant structure and diluent properties, the extraction behavior of diphosphonic acid diesters is expected to change significantly if a more polar diluent such as 1-decanol is used. Due to hydrogen bonding between these extractants and the diluent, only monomeric solute species are observed in 1decanol [66]. An investigation of the extraction of alkaline earth and actinide cations from aqueous nitric acid into 1-decanol solutions of H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] was initiated to compare the solvent extraction chemistry of these reagents in the absence of aggregation effects [95, 96]. The extractant dependencies observed for Am(III) and the other cations with H2DEH[EDP] in 1-decanol are not unity. In this diluent, metal ions bind to a number of monomeric extractant molecules depending upon cationic charge (up to three with Am(III)), analogous to monomeric monofunctional extractants [91]. This confirms that the extractant dependencies of unity observed for the extraction of metal ions by H2DEH[EDP] in nonpolar diluents arise from the highly aggregated state of the extractant. Extraction of Am(III) by H2DEH[MDP], H2DEH[EDP], and H2DEH [BuDP], in the aqueous acidity region where an acid dependency of -3 applies, can be described by the simultaneous equilibria (17) and (18), which are analogous to those describing the extraction of actinide ions by HDEHP [101]: (17) (18) Equilibrium (18) is stoichiometrically indistinguishable from equilibrium (19): (19) Equilibria (18) and (19) imply different structures for the metal species in the organic phase [96]. Similar equilibria can be written for the extraction of alkaline earth cations by the three ethylhexyl-substituted extractants in 1-decanol [95]. The most striking feature of the data is the extent to which metal ion extraction is suppressed by changing the diluent from 0-xylene to 1-decanol. In most cases, metal extraction is reduced by several orders of magnitude [96]. Extraction of metal ions by 1-decanol solutions of the ligands is suppressed relative to that in oxylene because the alcohol is strongly hydrogen-bonded to the phosphoryl groups of the acids. This makes the groups of the ligand unavailable for extractant aggregation and provides competition with the metal ion for ligand binding sites.

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The reduced extraction efficiency in 1-decanol can provide a facile route for actinide stripping. Recovery of actinide ions from an o-xylene phase containing H2DEH[MDP] is very difficult because of the extremely strong affinity of the extractant for these metal ions. Stripping could be accomplished much more readily by diluting the loaded organic phase with 1-decanol or a similar alcohol. Although the extraction properties of diphosphonic acid esters in 1-decanol/o-xylene mixtures have not been investigated, it is reasonable to assume that conditions for substantially reducing actinides distribution ratios at conveniently low aqueous acidities could be found, thus permitting facile stripping based solely on diluent effects.

VI. SOLVENT EXTRACTION OF METAL IONS AT HIGH LOADING The solvent extraction data discussed in Section V were obtained at tracer metal concentration levels. Although distribution measurements at very low metal concentrations are essential for establishing the stoichiometry of metal extraction through graphical slope analysis and other methods, the information provided by these studies generally cannot be extrapolated to pratical solvent extraction conditions where the metal concentrations are much higher. Very little information exists in the literature on the organic phase speciation at high metal loading. When the concentrations of extracted metal approach those corresponding to saturation of the extractant in the organic phase, the discrete complexes familiar in solution coordination chemistry tend to disappear in favor of self-assembled structures. The driving force for the aggregation of the electroneutral metalextractant complexes is generally provided by van der Waals attraction between polar solutes in low-polarity diluents. As mentioned earlier, the technique of smallangle neutron scattering (SANS) is well suited for studies on the morphology of these aggregates. For example, self-assembly of the metal-extractant complexes in large cylindrical structures having lengths of hundreds of Å’s have been observed through SANS measurements for HDEHP and other monofunctional organophosphorus acidic extractants after extraction of Co(II) at high concentrations [87]. Besides being a fascinating problem in physical and structural chemistry, such species are of considerable technological importance. Recent studies have correlated the formation of aggregates in organic solutions of extractants with the formation of a third organic phase, a phenomenon still largely unexplained from a structural standpoint [102, 103]. Bifunctional extractants such as diphosphonic acid partial esters are expected to exhibit a strong tendency to aggregate or polymerize under the influence of high metal concentrations, as metal ions can promote the formation of large polynuclear species by bridging functional groups of different extractant molecules. This behavior was suggested by some peculiarities of the solvent extraction data at

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low metal loading, and confirmed by VPO measurements on H2DEH[MDP] solutions in toluene containing high concentrations of various metals. Numberaverage aggregation numbers as high as 10, 14, and 33 were obtained for U(VI), Th(IV), and Fe(III) solutions, respectively [65]. To gain more information on the morphology of the species present in organic phases containing diphosphonic acid diesters after extraction of a variety of metal ions at high concentrations, SANS measurements were performed using deuterated toluene as the diluent [80, 81, 88]. Representative results are reported in Table 6 for H2DEH[MDP], H2DEH [EDP] and H2DEH[BuDP]. For each system investigated, the table reports the HNO3 concentration in the aqueous phase and the concentrations of diester and metal in the organic phase. Typically, these metal concentrations are the highest attainable under the conditions used for metal extraction. The values of the diester to metal concentration ratios in the organic phase give an indication of how close each system is to extractant saturation. The radius of gyration Rg, provided by the Table 6 Radius of Gyration Rg and Weight-Average Aggregation Number nw for Solutions of H2DEH[MDP], H2DEH[EDP] and H2DEH[BuDP] After Extraction of Metal Ions at High Concentrations

a b

Maximum value Solution obtained by dissolving 33.0 mg Fe2(DEH[MDP])3 in 1 mL deuterotoluene.

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Guinier analysis of the data, is a measure of the spatial extension of the aggregate and is given by the root-mean-squared distance of all the atoms from the center of gravity of the scattering particle. The weight-average aggregation number of the extractant nW, calculated from the SANS data, gives the number of diester molecules contained in each aggregate.

A. Metal Species Formed by H2DEH[MDP] The results in Table 6 show that very large aggregates are formed in the extraction of Th(IV) and Fe(III) at high concentrations. The Th(IV)–H2DEH[MDP] system is polydisperse with a number of aggregates of different size contributing to the overall scattering. The largest particles that can be observed under the experimental limitations of the SANS measurements [88] exhibit Rg and nw values of 87 Å and 190. The modified Guinier analysis for rod-shaped particles was performed on the Th(IV)–H2DEH[MDP] data. This analysis allows an estimation of the radius of the rodlike structure from the slope of the straight line describing the SANS data in the form {ln[I(Q)·Q] vs. Q2} [104]. The modified Guinier analysis revealed the existence in solution of cylindrical aggregates about 300 Å long with a radius of about 10 Å. The analysis of the data, however, also revealed the existence of larger aggregates which grow simultaneously in all directions. It is likely that crosslinking occurs to form particles analogous to Structure IX (where the double bonds have been omitted for simplicity):

Structure IX

A structure similar to Structure IX was hypothesized previously for the polymers formed by tetravalent uranium with dialkylpyrophosphates [105], ligands that are structurally similar to H 2DEH[MDP], but have the two phosphorus atoms bridged by an oxygen atom instead of a methylene group. A

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remarkable feature of the Th(IV)–H2DEH[MDP] system is that large aggregates form only at the highest possible metal concentration in the organic phase. Attempts to further increase the concentration of Th(IV) result in the precipitation of the Th(DEH[MDP])2 salt. Structure IX probably represents the formation in solution of large structural units having the same threedimensional structure as the Th(DEH[MDP])2 salt. The Fe(III)–H2DEH[MDP] system was investigated in great detail [80]. Table 6 summarizes the results obtained for the solutions closest to extractant saturation. Figure 10 shows the SANS data in the form of Guinier plots for the samples obtained by extracting Fe(III) from 0.1 M HNO3. The progressive increase of scattering for increasing Fe(III) concentrations is clearly visible. Figure 11 shows the nw aggregation number of the aggregates as a function of the organic phase Fe(III) concentration. As more Fe(III) is transferred into the organic phase and the extractant saturation is approached, the nw values grow dramatically independently of the aqueous phase acidity. In all cases, the scattering particles for solutions close to extractant saturation contain from about 70 to about 110 H2DEH[MDP] molecules. Modified Guinier analysis for rod-shaped particles demonstrated that these aggregates are cylindrical with lengths up to about 300 A. However, unlike the largest Th(IV)–H2DEH[MDP] aggregates, the radius of the Fe(III)–H2DEH[MDP]

Figure 10 Guinier plots of the SANS data for 0.1 M H2DEH[MDP] in deuterated toluene after extraction of Fe(III) from 0.1 M HNO3. (From Ref. 80.)

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Figure 11 Weight-average aggregation number, nw, and ratio of H2DEH[MDP] to Fe(III) concentration in the organic phase as a function of organic Fe(III) concentration and aqueous phase acidity. (From Ref. 80.)

aggre gates is always constant at about 9 Å, suggesting that particle growth takes place only along the long axis of the cylinder. To obtain more information on these “wormlike” aggregates, SANS measurements were performed on a sample prepared by direct dissolution of the Fe2(DEH[MDP])3 salt in deuterated toluene. The SANS data revealed the presence of cylindrical particles which are similar to those observed for samples prepared through Fe(III) extraction. In these particles, the hydrocarbon chains of the diester should be oriented toward the exterior of the cylindrical aggregates, while the metal ions interact with the polar groups of the extractant which are oriented toward the interior of the cylinder. The metal ions thus are located along a channel in the center of the cylinder. In this case, a likely structure of the metal-extractant polymeric species is shown in Structure X. This structure is consistent with the 3:2 stoichiometric ligand to metal ratio in the salt, and the preferred octahedral coordination environment of Fe(III). Similar structures have been reported for solid-state, eight- and seven-coordinate, crystalline lanthanide complexes of 1-hydroxyethane-1,1-diphosphonic acid (HEDPA), an aqueous soluble, unsubstituted analog of H2DEH[MDP] [106]. From the ligand to

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metal stoichiometric ratio and the information provided by the Guinier analysis, the Fe-Fe distance in Structure X was calculated to be 3.7±0.3 Å.

Structure X

Structure X and a Fe-Fe distance of 3.7 Å suggest formation of covalent bonds between Fe(III) and the ligand molecules. Covalent binding of Fe(III) to H2DEH[MDP] was independently demonstrated in an investigation of the farinfrared spectrum of Fe2(DEH[MDP])3 [69]. Based on the information provided by IR and SANS measurements, it seems reasonable to conclude that, at least for the Fe(III)-H2DEH[MDP] system, the aggregation is more likely to be an actual polymerization process than the formation of aggregates where the individual molecular units are held together by a purely physical interaction.

B. Metal Species Formed by H2DEH[EDP] and H2DEH[BuDP] The nw values measured for H2DEH[EDP] solutions after extraction of Ca2+, La3+, and are not significantly different from that of the extractant alone. These results confirm that metal extraction occurs through cation transfer into the hydrophilic cavity of a hexameric aggregate with little, if any, disruption of the solution structure of the extractant. Infrared spectra indicate that extraction of Th(IV) and Fe(III) profoundly alter the solution structure of H2DEH[EDP] [37]. The nav values in Table 6 are consistent with the hypothesis that high concentrations of Th(IV) and Fe(III) in the organic phase disrupt the hexameric extractant aggregation and result in the formation of different metal extractant species. Formation of large aggregates was observed for H2DEH[BuDP] only upon Fe(III) extraction [81]. When the organic phase is fully loaded with Fe(III), these aggregates contain up to 85 H2DEH[BuDP] molecules. The modified Guinier analysis for rod-shaped particles confirmed that the Fe(III)-H2DEH[BuDP] aggregates are cylindrical. However, a fundamental difference exists between the aggregates formed by Fe(III) with H 2DEH[BuDP] and those formed with H2DEH[MDP]. The Fe(III)H2DEH[MDP] aggregates described earlier have a constant radius and grow length-wise with increasing Fe(III) concentration. In contrast, the Fe(III)-H2DEH[BuDP] aggregates have an approximately constant

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length (~60 Å), but a radius that increases significantly with the concentration of Fe(III) in the organic phase (up to ~15 Å). We conclude this section by emphasizing that also from the standpoint of self-assembly of metal-extractant complexes in the organic phase, the diphosphonic acid diesters exhibit a variety of behaviors that depend on the charge and coordination chemistry of the metal ion, as well as on the aggregation behavior of the diester. For example, the behavior of heavily metal loaded H2DEH[EDP] solutions is dominated by the presence of spherical hexameric extractant aggregates that in many cases retain their morphology even in the presence of high metal concentrations. The behavior of H2DEH[MDP] and H2DEH[BuDP] solutions close to the point of saturation, on the other hand, is characterized by the tendency to form long cylindrical polymeric species (especially with Fe(III) and Th(IV)), whose thickness depends on the specific metal ion-extractant system. While it is difficult to explain the differences in the behavior of Fe(III) with the three extractants, especially the absence of large aggregates with H2DEH[EDP], it is remarkable that small changes in the distance between the phosphorus atoms of the ligand can lead to such profound differences in the aggregation of the extracted complexes.

VII. ENTHALPY AND ENTROPY CHANGES IN METAL SOLVENT EXTRACTION When a cation is transferred from an aqueous medium into an organic phase through an ion exchange complexation mechanism, the net enthalpy and entropy changes associated with the extraction mainly result from four opposing processes: (1) the dehydration of the extracted cation and the hydration of the exchanged protons; (2) the metal coordination by the organic ligand and the deprotonation of the ligand [107]. For ligands aggregated through hydrogen bonds, as typical for organophosphorus acids, this second process also involves breaking hydrogen bonds. Dehydration of the cation generally involves a positive enthalpy change (∆H> 0) as a result of breaking ion-water bonds, and a positive entropy change (∆S>0) due to the increased disorder of the system. The opposite changes will occur for the hydration of the proton. Metal coordination by the organic ligand will result in a negative enthalpy change (∆H<0), as a consequence of replacing relatively weak hydrogen bonds with stronger metal-ligand coordination bonds, and in a decreased entropy (∆S<0) due to the increased order caused by the new bonds. The deprotonation of the extractant will of course produce similar changes but with opposite signs. In Section V it was shown that metal extraction processes and the types of metal-extractant complexes formed in the organic phase are different for H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP]. With H2DEH[MDP], Am3+ ions

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transfer from the aqueous to the organic phase to become part of a complex involving two extractant dimers and a number of six-membered and eight-membered rings. For this type of bonding to occur, it is likely that complete dehydration of the cation is required. In the case of H2DEH[EDP], where the extraction process is reminiscent of a micellar extraction, the metal ion can be transferred into the organic phase hexameric aggregates retaining some of its water of hydration. Furthermore, the rings formed through coordination of the metal to the and POO groups of the extractant should be seven- and eight-membered, respectively, and therefore considerably less stable than in the H2DEH[MDP] case. With H2DEH[BuDP], interaction of the cation with the extractant trimers included in the complex should only involve formation of very large and unstable rings. Given the pronounced differences in extraction processes and metal species formed in the organic phase, it is reasonable to expect that the extraction of metal ions by H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] should be accompanied by different enthalpic and entropic contributions to the overall stability of the species formed. The different stabilities of the chelate rings formed should be reflected in different enthalpy changes. Also, since the diesters form highly ordered structures in the organic phase, the extracted metal ions, becoming part of these structures, add little order to the system. Consequently, the entropy changes should be mainly determined by hydration-dehydration processes. To shed further light on metal extraction by the di(2-ethylhexyl) diphosphonic acid partial esters, the enthalpy and entropy changes that occur upon extraction of Am3+, Sr2+, and UO 22+ were determined by using the temperature dependence of the metal distribution ratio [108–110]. The results obtained for Am3+, as a representative case, are summarized here. A discussion of the results for Sr2+ and UO 22+ can be found in the original works [109, 110]. To evaluate the role of preorganization of ligand binding groups on both the aggregation and cation extraction performance, the structurally hindered compound, P,P’-di(2-ethylhexyl) benzene-1,2-diphosphonic acid, H 2DEH[1,2-BzDP], Structure XI, was included in this study [111, 112]:

Structure XI

The orientation of the phosphonate groups in Structure XI should lead to an optimum bonding environment for metal ions of appropriate size. Although the phosphorus atoms in H2DEH[1,2-BzDP] are separated by a two-carbon linker, as

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in H2DEH[EDP], the aggregation behavior of H2DEH[1,2-BzDP] was expected to be different from that of H2DEH[EDP] because of the rigidity introduced into the molecule by the benzene ring. Using the temperature dependence of the distribution ratio to determine the thermodynamic parameters associated with a chemical process requires that the same equilibria are operative over the entire temperature range investigated. Therefore, both the extractant aggregation equilibria and the stoichiometries of the extraction reactions had to be determined for each system at several temperatures in the 25–60°C range.

A. Aggregation and Extraction Equilibria The VPO results showed that the aggregation state of the extractants does not significantly change over the temperature range investigated, i.e., H2DEH[MDP] remains dimeric, H2DEH[EDP] hexameric, and H2DEH[BuDP] primarily trimeric. The VPO measurements for H2DEH[1,2-BzDP] indicate that the ligand is dimeric in toluene, confirming that rigidity makes its aggregation different from that of H2DEH[EDP]. H2DEH[1,2-BzDP], however, is unstable with respect to acid hydrolysis in toluene and o-xylene solution and slowly degrades to the insoluble benzene-1,2-diphosphonic acid. In the presence of significant amounts of the hydrolysis by-product 2-ethylhexanol, the residual H 2DEH[1,2-BzDP] is predominantly monomeric in the temperature range investigated, as a result of the monomerizing effect of the alcohol. A set of Am3+ extraction data at constant aqueous acidities and various extractant concentrations as a function of temperature is shown in Fig. 12. The data clearly show the different behavior of the extractants. The extraction is exothemic (∆H <0) for H2DEH[MDP], since the D values decrease with an increase of temperature, while the opposite is observed for H2DEH[BuDP], although to a less pronounced extent. For H2DEH[EDP], on the other hand, Am3+ extraction is temperature independent. Slope analysis performed on extractant and acid dependencies for the partial esters at various temperatures confirmed that the extraction equilibria were the same at all temperatures. For each system, based on the measured acid and extractant dependencies, extraction equilibria can be written and equilibrium constants calculated. The extraction of Am3+ by H2DEH[MDP] is expressed as (see Section V) (20) where H2A stands for the diprotic extractant. The thermodynamic equilibrium constant is: (21)

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Figure 12 Effect of temperature on extraction of Am3+ from aqueous HNO3 by H2DEH[MDP] (2.2 M HNO3), H2DEH[EDP] (1.0 M HNO3), and H2DEH[BuDP] (0.10 M HNO3), at various extractant concentrations in o-xylene. (From Ref. 108.)

where γ±,HN and γ±,AN are the mean molal ionic activity coefficients of HNO3 and Am(NO3)3, respectively, is the analytical concentration of the extractant, and ideal behavior is assumed for the organic phase species. (At low metal ion concentrations, if the monomer concentration is negligible, the analytical concentration of the ligand can be expressed as The formation of Am3+ nitrate complexes in the aqueous phase can be written as (only the first complex is reported in the literature [113]): (22) and the total concentration of Am(III) in the aqueous phase, [Am3+]tot, can be shown to be equal to (23)

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where β1 is the formation constant of the nitrate complex. Recalling that the distribution ratio D is the ratio of the concentration of metal in the organic phase to the total metal concentration in the aqueous phase, and introducing mean molal activity coefficients for the species in Eq. (23), Eq. (21) becomes (24) Similar expressions can be derived for the other extractants by properly taking into account their different state of aggregation. From these equations, the equilibrium constants of the extraction reactions were calculated at different temperatures. In the calculations, distribution ratios on the molal scale were used, ß1 was obtained from the literature [113], and the mean ionic molal activity coefficients of Am3+ (europium was used as a stand-in for americium) at tracer concentration level in solutions of HNO3 at the relevant ionic strengths were calculated according to the method of Kusik and Meissner [114]. The variation of the activity coefficients with temperature was neglected.

B. Free Energy, Enthalpy, and Entropy Changes The Gibbs-Helmholtz equation can be written in the form (25) where R is the gas constant. By assuming that ∆H° is constant over the temperature range investigated, the enthalpy and entropy changes of the extraction processes can be calculated from the slope (-∆H°/2.303R) and intercept (∆S°/2.303R), respectively, of a plot of log Kvs. 1/T. The plots of log Kvs. 1/T for the extraction of Am3+ by the investigated diesters are shown in Fig. 13. The ∆H° and ∆S° values calculated from these plots, along with the ∆G° values calculated from the relation ∆G°=-2.303 RT log K are reported in Table 7. The free energies of extraction indicate that the relative efficiency of Am3+ extraction varies in the order H2DEH[1,2-BzDP]>H2DEH[MDP]>H2DEH[EDP]>H2DEH [BuDP]. Although the thermodynamic parameters measured for H2DEH[1,2-BzDP] cannot be directly compared with those for the other partial esters due to the presence of significant amounts of 2-ethylhexanol in solution, the superior efficiency of H2DEH[1,2-BzDP] compared to H2DEH[MDP] reflects the effect of preorganized ligand binding groups. This superiority is somewhat surprising since a comparison with carboxylate ligands of the same geometry shows that malonic acid is a stronger complexant of Eu3+ than phthalic acid [115]. The ∆H° values for Am3+ extraction increase along the H2DEH[MDP]< H2DEH[EDP]
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Figure 13 Equilibrium constants as a function of reciprocal temperature for the extraction of Am3+ from aqueous HNO3 by H2DEH[1,2-BzDP], H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] in o-xylene.

interpreted as arising from the formation of progressively less stable rings as the length of the alkyl chain separating the two phosphorus atoms of the molecule increases. A favorable enthalpic term is observed only for Am3+ extraction by H2DEH[MDP], a ligand with which the metal can form several highly stable sixmembered chelate rings. In all cases, Am3+ extraction is facilitated by a favorable entropic term, as typically observed for the formation of chelate complexes [107]. The nearly zero or positive enthalpy change with a concomitant favorable entropy change is consistent with a micellarlike type of extraction process for H2DEH[EDP] and possibly for H2DEH [BuDP]as well[116].

Table 7 Thermodynamic Parameters for the Extraction of Am3+ by H2DEH[1,2-Bz DP], H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] in o-Xylene

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The interpretation of the results obtained for Sr2+ and UO 22+ is less straightforward than for Am3+. In the UO 22+ case, for example, the data suggest that a solvation mechanism is operating in conjunction with an ion exchange mechanism. The simultaneous operation of two alternative mechanistic pathways complicates the interpretation of the thermodynamic data [109]. Overall, however, the thermodynamics of metal solvent extraction by the alkylenediphosphonic acid diesters demonstrates the rich variety of solvent extraction behaviors brought about by small changes in extractant structure.

VIII. INTRA-LANTHANIDE ION SEPARATIONS The rare earth elements are increasingly important for the production of modern technological materials. Besides their use as neutron poisons in the nuclear industry, they find application in superconductors, optoelectronic materials, special alloys, catalysts, and radiotherapeutic reagents [117]. The high level of lanthanide purity required for many applications justifies the interest in efficient preconcentration and separation procedures both from an analytical and industrial standpoint [118]. Organophosphorus extractants are widely used in solvent extraction procedures for rare earth separations. In these procedures, the separation factors for adjacent lanthanides are typically in the 1.5–2.5 range, and efficient separations among the various elements is achieved using multistage countercurrent systems [119]. HDEHP, with an average separation factor of 2.5 for adjacent members of the lanthanide series, is one of the most effective separation reagents for these elements [120]. In lanthanide separation studies using HDEHP and analogous acidic organophosphorus extractants, periodic variations across the lanthanide series have been reported. Typically, in a plot of the logarithm of the metal distribution ratio (or the extraction equilibrium constant) vs. Z, the lanthanide atomic number, the data points are grouped in four sets of four elements, with a pronounced minimum for gadolinium and two secondary minima in the 60–61 and 67–68 Z region. This periodic behavior is called the “tetrad effect” [121] or “double-double effect” [122]. Possible explanations for this effect, mostly based on variations of the nephelauxetic parameter, have been summarized by various authors [123, 124]. Given the importance of organophosphorus reagents in lanthanide separations, it is of interest to evaluate the ability of the diesters of alkylenediphosphonic acids to effect separations across the lanthanide series. For this purpose, the extraction of all 15 members of the lanthanide series plus yttrium and selected trivalent actinides (Am, Cm, and Cf) by o-xylene solutions of H 2DEH[MDP] and H2DEH[EDP] was measured under identical experimental conditions [125]. To reduce the number of experiments needed to characterize the behavior of all 15 lanthanides, the elements were divided in three groups. Each group contained a number of lanthanides that could be analyzed simultaneously by inductively coupled

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plasma atomic emission spectroscopy (ICP/AES) or mass spectroscopy (ICP/MS). Pr, Sm, Eu, Er, Tm, and Y were in group I; Ce, Eu, Gd, Tb, Yb, and Lu were in group II; while La, Nd, Eu, Dy, and Ho were in group III. Eu was present in all groups so that its behavior could be used to monitor the internal consistency of the extraction data. Extraction data for prometheum and actinides were obtained using group I solutions spiked with the radioisotopes 147Pm, 241Am, 244Cm, and 249Cf, whose distribution between organic and aqueous phases was measured radiometrically. Acid dependencies at constant extractant concentration and extractant dependencies at constant aqueous acidity were measured for all cations keeping the ionic strength in the aqueous phase constant at 1 Since in these experiments the total metal ion concentration was not much lower than that of the extractant, HNO3 and extractant concentrations at equilibrium were obtained by subtracting the concentration of extractant bound to the cations from the total extractant concentration. This correction implies knowledge of the extraction stoichiometry. For each system the extraction reaction was identified by selecting the stoichiometry that provided slopes of the acid dependency that best approximate the expected value of -3. Figure 14 shows the extractant dependencies for lanthanide extraction by H2DEH[MDP]. For all cations, the extraction stoichiometry that provides acid dependecies of -3 involves organic complexes where one metal atom is bound to two extractant dimers. The slopes of the extractant dependencies in Fig. 14, however, are higher than the value of 2 implied by the extraction stoichiometry. The deviations from a slope 2 value are particularly high for the lighter lanthanides. A likely explanation for this behavior is that at relatively high concentrations of the metal in the organic phase, the reaction of a cation with three H2DEH[MDP] monomers also takes place. This reaction, which causes extractant dependencies values higher than 2, is more important for the lighter lanthanides which can more easily accommodate three extractant monomers in their coordination sphere. By considering, as an approximation, only the reaction of a cation with two extractant dimers, the extraction equilibrium can be written as (see Section V) (26) (where H2A represents H2DEH[MDP]), with the equilibrium constant: (27) where represents the metal in the organic phase, ideal behavior is assumed for the species in the organic phase, and the aqueous phase activity coefficient at constant ionic strength are included in the equilibrium constant. By introducing in Eq. 27 the relations (28)

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Figure 14 Extractant dependencies for lanthanide extraction by H2DEH[MDP] in o-xylene at 1 M HNO3, obtained assuming that each cation in the organic phase binds two extractant dimers.

(29) (30) where is equal to 1 and ß1 is the formation constant of aqueous phase nitrate complexes, one obtains (31) By further expressing the H2DEH[MDP] dimer concentration as where is the H2DEH[MDP] equilibrium concentration, the expression for the equilibrium constant reduces to (32)

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Figure 15 shows the extractant dependencies obtained for the lanthanide extraction by H2DEH[EDP]. The extraction stoichiometry that provides acid dependecies of –3 for all cations involves organic complexes where each metal ion is bound to one extractant hexamer. The values of the extractant dependencies in Fig. 15 are all close to 1. Therefore, the extraction of trivalent lanthanides and actinides by H2DEH[EDP] can be written as (see Section V) (33) Following the reasoning used for H2DEH[MDP], the following expression for the equilibrium constant can be derived: (34) where, now, is the H2DEH[EDP] equilibrium concentration. The β1 values for nitrate complexation of lanthanides and other cations to be used in Eqs. (32) and (34) were obtained from available sources [115]. The values

Figure 15 Extractant dependencies for lanthanide extraction by H2DEH[EDP] in o-xylene at 1 M HNO3, obtained assuming that each cation in the organic phase binds one extractant hexamer.

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of the equilibrium constants calculated for o-xylene solutions of H2DEH[MDP] and H2DEH[EDP] are plotted vs. the atomic number of lanthanides and actinides in Figs. 16 and 17. In the figures, Y has been positioned at Z=68, since it is well known that the chemical properties of this cation most closely resemble those of holmium and erbium [121]. The log K values in Fig. 16 have been arbitrarily grouped in four tetrads to emphasize that a tetrad effect may be present in the data, although obscured by the experimental uncertainties in the values of the equilibrium constants. However, the striking feature in Fig. 16 is the near complete lack of selectivity across the lanthanide series (and the limited actinide series) exhibited by H2DEH[MDP]. This behavior is similar to that discussed in Section V for the alkaline earth cations. For both series of cations, trivalent and divalent, respectively, the highly stable chelate complexes formed with the extractant are not sensitive to cation size.

Figure 16 Logarithmic values of the equilibrium constants for the extraction of trivalent lanthanides and actinides by H2DEH[MDP] in o-xylene plotted vs. the atomic number Z.

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Figure 17 Logarithmic values of the equilibrium constants for the extraction of trivalent lanthanides and actinides by H2DEH[EDP] in o-xylene plotted vs. the atomic number Z.

The log K values in Fig. 17, on the other hand, demonstrate that H2DEH[EDP], although a less efficient rare earth extractant, exhibits a strong selectivity across the lanthanide series with the selectivity being especially high for the heaviest members of the series. The limited data available for the trivalent actinides parallel those for the lanthanides, with Cf3+ being significantly more extracted than Cm3+ and Am3+. Furthermore, a tetrad effect is clearly visible in the data. The K values in Figure 17 span nearly 3 orders of magnitude. This makes H2DEH[EDP] less intralanthanide selective than HDEHP, for which the equilibrium constants measured across the lanthanide series span about 5 orders of magnitude [126]. However, the behavior of H2DEH[EDP] is fully comparable to that of diprotic monofunctional organophosphorus acids, such as, for example, the 2-ethylhexyl

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phosphonic acid (H2[EHP]) [127]. It is interesting that the behavior of H2DEH[EDP] with the lanthanide elements parallels that observed with alkaline earth cations. In both cases H2DEH[EDP], unlike H2DEH[MDP], exhibits a stronger affinity for cations with a smaller ionic radius, a feature that confirms the essentially electrostatic nature of the ion-ligand interaction.

IX. SYNERGISTIC EXTRACTION OF METAL IONS In synergistic systems, the extraction of a metal ion by an acidic extractant, typically of the carboxylic, β-diketone, or organophosphorus type, is enhanced by the addition of a third component, usually a neutral organophosphorus compound or a crown ether, to the organic phase. This effect is generally attributed to the ability of the synergist to form an adduct with the metal-extractant complex, where the synergist contributes to saturating the coordination sphere of the cation [128]. To date, numerous types of synergistic solvent extraction systems have been investigated [129–132]. Since no information was available on synergistic effects involving alkylenediphosphonic acid diesters, we have measured potentially synergistic extraction of a variety of metal cations by mixtures of H2DEH[MDP], H2DEH[EDP] or H2DEH [BuDP] and crown ethers or neutral organophosphorus compounds. The general objective of the investigation was to exploit synergistic effects to find suitable conditions for more selective metal ion separations [133, 134].

A. Crown Ethers as Synergists Our previous work, directed primarily at the development of improved methods for the separation and preconcentration of alkaline earth cations (particularly Ra2+) for subsequent determination, has focused largely on combinations of crown ethers such as dicyclohexano-18-crown-6 (DCH18C6) and dialkylphosphoric acids (e.g., di(2-ethylhexyl)phosphoric acid, HDEHP) [98, 135–138]. From these studies an understanding emerged that the effectiveness of a given crown ether as a synergist depends upon its stereoisomeric form. For combinations of DCH18C6 and dialkylphosphoric acids, for example, it was found that only systems involving the cis-syn-cis and cis-anticis isomers provided synergistic effects large enough to be of practical significance. Therefore, our investigation of synergistic effects involving alkylenediphosphonic acid diesters and DCH18C6 was limited to the cis-syn-cis and cis-anti-cis stereoisomers. Elucidation of extraction equilibria in synergistic systems is greatly simplified in the absence of interactions between the extractants. The possible presence of interactions between DCH18C6 and the diesters in toluene was investigated by following the position of the stretching frequency of the diphosphonic acid diesters upon the introduction of a stoichiometric excess of crown ether in the

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organic solution. In all cases the stretching frequency remained constant, indicating no significant interaction between the acidic and neutral extractants. This conclusion was confirmed by VPO measurements [133, 134]. Synergistic effects are best seen in extractant dependency plots, where the metal distribution ratios are plotted as a function of the synergist concentration at constant concentrations of the other extractant and aqueous acid. Figure 18 shows such a plot for H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] as a function of the cis-syn-cisDCH18C6 concentration at 0.05 M HNO3. (The acid dependencies for the same systems were always -2, indicating that whether or not the crown ether is present, alkaline earth cation extraction always displaces two hydrogen ions from the diester extractants [133]). A number of features in the data of Fig. 18 stand out. First, no synergism is observed in the extraction of Ca2+, a feature that the diphosphonic acid diesters share with the previously investigated dialkylphosphoric acids [136]. This feature undoubtedly results from the poor size match between the crown cavity and the small Ca2+ cation [139]. The second outstanding feature is the lack of alkaline earth extraction enhancement for H2DEH[MDP]. The lack of synergism in H2DEH[MDP]-based systems can be understood by recalling that this extractant exhibits little, if any, selectivity for Ca2+ over the heavier alkaline earth cations (see Section V). All alkaline earth cations are extracted by H2DEH[MDP] to the maximum possible extent, leaving no room for synergistic extraction enhancement.

Figure 18 Crown ether extractant dependencies for the extraction of alkaline earth cations by mixtures of 0.01 M H2DEH[MDP], 0.01 M H2DEH[EDP], or 0.1 M H2DEH[BuDP] and cis-syn-cisDCH18C6 in 0-xylene ([HNO3]=0.05 M, [DCH18C6] variable).

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The alkaline earth cations form complexes with H2DEH[MDP] involving a number of highly stable chelate rings. In these complexes, the metal ions are completely dehydrated and all metal coordination sites are occupied by the donor groups of the extractant. Under these conditions, the formation of extraction enhancing adducts with the synergists becomes highly unlikely. The enhanced extraction of Sr2+, Ba2+, and Ra2+ by mixtures of DCH18C6 and H2DEH[EDP] or H2DEH[BuDP] is the third noteworthy feature in Fig. 18. The extent of synergism, as measured by the increase of the D values, is approximately the same for both diesters and slightly higher for Ba2+ than for Ra2+ and Sr2+. Larger synergistic effects for Ba2+ than for Sr2+ have been reported previously for DCH18C6 and dialkylphosphoric acid mixtures [139]. Data similar to those in Fig. 18, but with the cis-anti-cis isomer of DCH18C6 as the synergist, showed that the two systems behave almost identically. However, the synergistic effects measured with cis-anti-cisDCH18C6, in agreement with previous findings [136–138], are less pronounced than those observed with the cis-syn-cis isomer. For the determination of the composition of the synergistic complexes, Sr2+ was used as the metal ion probe, and a variety of techniques such as infrared spectroscopy, vapor pressure osmometry, and various types of distribution methods were employed, following the guidelines reported for investigating synergistic systems [132, 140]. Distribution experiments included metal concentration dependencies (both at low and high loading, up to saturation of the organic phase), single and double extractant dependencies, continuous variation plots, and mole ratio plots [134]. A comparative analysis of the results from these experiments, some of which are summarized below, was used to determine the composition of the extracted synergistic complexes. Figure 19a, b shows continuous variation plots of the strontium D values vs. the crown ether mole fraction in mixtures of H 2DEH[EDP] and cis-syncisDCH18C6 or cis-anti-cisDCH18C6, with the sum of the concentrations of the two extractants remaining constant. The position of the maximum in the data, obtained from the extrapolation of the linear regions of the plots, lies at a crown ether mole fraction of about 0.15. This is consistent with the formation of a synergistic complex containing six H2DEH[EDP] molecules (one hexamer) for each DCH18C6 molecule (1/(1+6)=0.14). The experimental maximum, however, corresponds to an abscissa of about 0.25, which would imply a complex involving two crown ether molecules and one H2DEH[EDP] hexamer. Based on experimental evidence discounting the formation of polynuclear complexes [134], we believe that the extrapolation procedure provides the correct extraction stoichiometry. Similar data are shown in Fig. 19c, d for H2DEH[BuDP], Again in this case, the maximum lies at a crown ether mole fraction of about 0.15. However, the 1:6 DCH18C6 to H2DEH[BuDP] molar ratio in the complex is interpreted as the involvement of two H2DEH[BuDP] trimers in the formation of the synergistic complex. Extraction experiments were also performed using organic phases with the concentration of one extractant held constant, while the concentration of the

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Figure 19 Continuous variation plots for the extraction of Sr2+ by mixtures of H2DEH [EDP] and either (a) cis-syn-cisDCH18C6 or (b) cis-anti-cisDCH18C6 at a total extractant concentration of 0.05 M; and by mixtures of H2DEH[BuDP] and either (c) cis-syn-cisDCH18C6 or (d) cis-anti-cisDCH18C6 at a total extractant concentration of 0.1 M. Diluent: o-xylene; aqueous phase: 0.0113 M HNO3. (From Ref. 134.)

other extractant was varied. In this case, a plot of the D values vs. the molar ratio of the two extractants exhibits a break at the molar ratio value corresponding to the composition of the synergistic complex. Such plots are shown in Fig. 20 for mixtures of H2DEH[EDP] or H2DEH[BuDP] and cis-syncisDCH18C6. The data in Figure 20 were obtained at constant concentration of H2D EH[EDP] hexamer (panel A) or H2DEH[BuDP] trimer (panel B) and variable concentration of DCH18C6. The position of the breaks was determined by extrap olation of the

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Figure 20 Mole ratio plots for the extraction of Sr2+ by (a) 0.01 M H2DEH[EDP] plus variable cis-syn-cisDCH18C6; (b) 0.05 M H2DEH[BuDP] plus variable cis-syn-cisDCH18C6; diluent: o-xylene; aqueous phase: 0.0113 M HNO3. (From Ref. 134.)

linear portions of the plots. In panel A, a barely visible discontinuity is located at concentration ratios of one, consistent with the formation of a synergistic complex containing six H2DEH[EDP] molecules (one hexamer) for each DCH18C6 molecule. In panel B, the break at a molar ratios of 0.5 for H2DEH[BuDP] unequivocally confirms the formation of complexes containing two H2DEH[BuDP] trimers and one DCH18C6 molecule. In conclusion, for the H2DEH[BuDP] system, the results of all measurements agree in providing a composition for the synergistic complex of one metal ion, one DCH18C6 molecule, and two H2DEH[BuDP] trimers. In this complex, it is very likely that the metal ion is surrounded by the crown ether and the cation charge is neutralized by two monoionized H2DEH[BuDP] trimers located at either side of the macrocyclic ring. The interpretation of the data for the H2DEH[EDP] system is less straightforward. Most of the data indicate that a 1:1:1 metal ion:DCH18C6:H2DEH[EDP] hexamer complex is formed. Other data, however, provide a 1.5:1 H2DEH[EDP] hexamer to DCH1 8C6 ratio in the synergistic complex [134]. This has been interpreted as resulting from the simultaneous formation of two complexes, containing one or two H2DEH[EDP] hexamers, respectively. For the latter complex, a structure similar to that for the H2DEH[BuDP] complex can probably

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be assumed, with two monoionized H2DEH[EDP] hexamers, located at either side of the crown ether ring, neutralizing the cationic charge. For the complex containing only one H2DEH[EDP] hexamer, however, it is possible that the metalcrown ether complex lies within the hydrophilic cavity of the H2DEH[EDP] hexamer.

B. Neutral Organophosphorus Esters as Synergists Synergistic effects in the extraction of metal cations by mixtures of dialkylphosphoric acids and neutral organophosphorus esters have been known for a long time and have been discussed in a number of works [128–130]. Little, however, is known on synergistic phenomena in metal solvent extraction with the diprotic monoalkylphosphoric acids of the type (OH) 2(RO)PO (e.g., H2MEHP). Similarly to H2DEH[EDP], these compounds form large aggregates in nonpolar diluents [79]. It was recognized early that the addition of neutral coextractants such as tri-n-butyl phosphate (TBP) to monoalkylphosphoric acids generally leads to a strong decline in extraction efficiency (antagonism) [91, 141], an effect similar to that observed when a depolymerizing diluent such as decanol is used. The experimental observations were explained by assuming that the neutral compound interacts with the acidic extractant through strong hydrogen bonding, leading to depolymerization of the latter and to formation of an association product which effectively prevents both extractants from participating in metal extraction. To compare the behavior of the alkylenediphosphonic acid diesters with that of the di- and monoalkylphosphoric acids, data on Am(III) extraction by mixtures of H2DEH[MDP], H2DEH[EDP], and H2DEH[BuDP] with neutral organophosphorus synergists were obtained [133]. In these experiments, TBP, diamyl amylphosphonate (DA[AP]), and trioctylphosphine oxide (TOPO) were used as neutral synergists. These compounds are characterized by an increasing basicity of the phosphoryl group. Figure 21 summarizes the results obtained in continuous variation experiments as plots of DAm versus the mole fraction of neutral ester present in the extractant mixture. As shown, addition of any of the neutral esters to the diphosphonic acids results in D values that decrease with an increase in the mole fraction of the neutral organophosphorus compound, indicating the absence of synergistic effects. The data demonstrate a significant antagonistic effect in the presence of the neutral organophosphorus compounds. The plots in Fig. 21 are not linear but exhibit a deviation caused by a decrease of the distribution ratios which is sharper than that expected from simple replacement of the diesters by the neutral extractants. This phenomenon, likely due to the interaction of the diphosphonic acids with the synergists, is particularly evident as the neutral extractant becomes more basic. To determine the origin of this effect, mixtures of the various neutral esters and diphosphonic acid diesters were examined by infrared spectroscopy. Interactions

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Figure 21 Continuous variation plots for the extraction of Am(III) by mixtures of diphosphonic acid diesters and TBP, DA[DP], or TOPO in o-xylene ([diester]+[synergist]= 0.01 M; [HNO3]=1, 0.5, and 0.1 M for H2DEH[MDP], H2DEH[EDP] and H2DEH [BuDP], respectively). (From Ref. 133.)

between the acidic and neutral extractants should be manifested through shifts of the stretching bands to higher and lower energy, respectively, as the acid-toacid hydrogen bonds are broken and acid-to-neutral extractant hydrogen bonds are formed. These measurements unequivocally demonstrated that neutral organophosphorus esters strongly interact with diphosphonic acid esters, with the strength of the interaction increasing with the basicity of the phosphoryl group of the neutral extractant [133]. The presence of these compounds in mixtures with diphosphonic acid diesters would thus be expected to lead to disruption of the aggregation of the latter and, more importantly, to a reduction in the effectiveness of the extractant that increases with the basicity of the neutral ester, exactly as observed (Fig. 21). In this respect, the behavior of the diesters in mixtures with neutral organophosphorus compounds parallels that of diprotic monoalkylphosphoric acids [91, 141]. The results discussed above suggest that the length of the alkylene bridge separating the two phosphorus atoms of the acid is the determining factor that ultimately governs the magnitude of synergistic effects in extraction systems based on P,P’-di(2-ethylhexyl) alkylenediphosphonic acids. By controlling the aggregation state of the extractant, the mechanism of metal ion extraction (strong chelation vs. micellarlike extraction), the affinity of the extractant for a particular

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metal ion, and the selectivity exhibited by the extractant, the alkylene bridge length determines the susceptibility of the extraction system to synergistic enhancement. For synergistic effects to be actually observed, however, other requirements must also be met. The coordination requirements of the cation employed, for example, must make adduct formation possible. In addition, the interactions between the diphosphonic acid and the neutral synergist must not be too strong. Unfortunately, although the extraction of the heavier alkaline earth cations by combinations of DCH18C6 and either H2DEH[EDP] or H2DEH[BuDP] clearly satisfies these requirements, the magnitude of the synergistic effects observed is not large enough to be of practical significance.

X. EXTRACTION CHROMATOGRAPHIC APPLICATIONS Extraction chromatography is a type of liquid-liquid chromatography in which the stationary phase consists of an extractant or a solution of an extractant in an appropriate diluent supported on an inert substrate. The technique couples the selectivity of solvent extraction with the multistage character of a chromatographic process and the ease of handling an ion exchange resin. Since its introduction in the late fifties, extraction chromatography has been extensively studied, and it is now widely recognized as a simple and effective means by which the separation and preconcentration of a variety of metal ions can be accomplished [142]. In the determination of radionuclides in environmental or biological samples, the low levels of the nuclides typically encountered and the complexity of the sample matrix often preclude a direct determination of the species of interest. Because of the presence of a number of common constituents of such samples, among them silica, various metal ions [e.g., Al(III), Fe(III), Bi(III), Ti(IV)], and certain anions, the separation of actinides, lanthanides and other species of environmental interest (e.g, Ra) from these samples involves time-consuming and expensive procedures. One approach to avoid the troublesome and often inexplicable problems collectively referred to as “matrix effects” is to preconcentrate the analytes into a common form that then behaves uniformly and predictably during a subsequent separation scheme. For this purpose a number of extraction chromatographic resins have been developed in recent years at ANL for the separation and preconcentration of actinides and radio-strontium for subsequent determination [97, 143–147]. These materials, however, although extremely useful for the selective recovery of actinides from complex sample matrices, suffer from certain limitations. Typically, actinide retention is not particularly strong when high concentrations of complexing anions (e.g, fluoride, phosphate, or oxalate) are present in the sample [145]. This poses a significant problem in attempts to isolate actinides from soil samples, as hydrofluoric acid is generally added to dissolve any silica present. In addition, americium retention is inadequate for certain applications (e.g., large volume natural water

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samples). The alternative use of the chelating ion exchange resin Diphonix [5], containing geminally substituted diphosphonic acid groups chemically bonded to a styrene-based polymer matrix, for separation and preconcentration of actinides from complex samples also suffers from limitations, especially with respect to Am(III) uptake and ease of stripping [148]. The problems of inadequate retention of trivalent actinides and difficult actinide stripping/recovery can be alleviated by using an extraction chromatographic resin, hereafter referred to as the Actinide Resin, consisting of H2DEH[MDP] supported on an inert polymeric substrate. The details of the resin preparation and its complete characterization can be found in the original works [148, 149]. Some of the more interesting results are summarized here. Figure 22a depicts the acid dependency in the form of capacity factor k’ (i.e., the number of free column volumes to peak maximum) vs. aqueous acid concentration, for the uptake of several representative actinides from 0.01 to 10 M HCl, while Fig. 22b presents analogous results for various nonactinide metal ions commonly encountered in analytical procedures involving environmental or bioassay samples. [Bi(III) and Ti(IV), for example, are common and troublesome constituents of fecal and soil samples.] As can be seen, retention of actinides by the resin is extraordinarily high. Especially noteworthy is the strong retention of Am(III). Although the k’ values

Figure 22 Capacity factor k’ vs. HCl concentration for the uptake of (a) selected actinide and (b) nonactinide species by the Actinide Resin. (From Ref. 148.)

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for Am(III) fall off rapidly at higher acidity, they remain above 1000 over nearly the entire range of acidities examined. Of the nonactinides, Eu(III) and Fe(III) exhibit the highest k’ values, as expected from the known affinity of the diphosphonic acid ligand for trivalent lanthanides and iron. The behavior of alkaline earth cations on the Actinide Resin is consistent with the results discussed previously (see Section V). Specifically, little selectivity is observed among the various alkaline earths. Figure 23 shows a comparison of the acid dependencies of Am(III), Ac(III), and Al(III) measured with the Diphonix and Actinide resins. The data are reported as Vmax, the number of free column volumes to peak maximum (for the Actinide Resin, Vmax=k’). The figure shows that Al(III) is significantly less strongly retained and Am(III) dramatically more strongly retained by the Actinide Resin. A likely explanation for this behavior is that in the Actinide Resin the diphosphonic acid ligands are not attached to a rigid polymer network and thus can more easily rearrange to satisfy the coordination requirements of a metal ion. As a result, the extraordinary complexing power of the diphosphonic acid ligand can be better exploited with an extraction chromatographic resin than with a polymeric chelating ion exchange resin. The data for the Actinide Resin in Fig. 23 also demonstrate that the uptake of Am(III) should not be affected by large concentrations of Al(III)

Figure 23 Comparison of the acid dependencies for the uptake of Am(III), Ac(III), and Al(III) by the Diphonix and Actinide Resins (Vmax=number of free column volumes to peak maximum). (From Ref. 148.)

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in the sample. Furthermore, the high retention of Ac(III) over Ra2+ suggests that the Actinide Resin could find application for the determination of 226Ra and 228Ra in environmental samples [148]. Figure 24a shows how the presence of Fe(III) leads to a strong suppression of the retention of Am(III), as expected from the data in Fig. 22. This interference is the single most serious matrix effect with the Actinide Resin. This effect can be easily minimized, however, by the addition of a reducing agent capable of converting Fe(III) to Fe(II) (for example, ascorbic acid). The effectiveness of this approach is clearly demonstrated in Fig. 24a, which shows that high concentrations of Fe(II) have little if any effect on Am(III) retention. Figure 24b shows the effect of HF on the retention of representative actinides in the tri-, tetra-, and hexavalent oxidation states on the Actinide Resin. Although the k’ values for U(VI) and Np(IV) fall rapidly at the highest HF concentrations, up to ~2 M HF can be tolerated without serious adverse impact on actinide retention. This is significant because several potential interfering cations, among them Ti(IV), Zr(IV), and Al(III), form strong anionic fluoride complexes; as a result, their sorption on the actinide resin is significantly reduced (and thus, their interference minimized) by addition of fluoride. In addition, these results, together with those of similar studies with other complexing anions (e.g., phosphate) [148], illustrate the relative insensitivity of actinide uptake to matrix composition, a property that considerably

Figure 24 (a) Effect of FeCl3 and FeCl2 concentration on Am(III) uptake by the Actinide Resin at 2 M HCl. (b) Effect of HF concentration on the uptake of Am(III), U(VI), Np(IV), and Ti(IV) by the Actinide Resin at 3 M HCl. (From Ref. 148.)

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simplifies the application of the resin in the analysis of environmental and biological samples, and thus represents a significant advantage of the Actinide Resin over previously described extraction chromatographic materials. One very useful application for the Actinide Resin is for the determination of “gross alpha” activity, one of the most frequently requested radioanalytical analyses in environmental monitoring. In this approach, after uptake of actinides and radium isotopes from the sample, the Actinide Resin is transferred to a scintillation vial containing liquid scintillation cocktail. The cocktail dissolves the extractant (and radionuclide-extractant complexes) from the support, leaving the inert, translucent support beads at the bottom of the vial. Samples are then counted on a liquid scintillation counter with significant improvements in counting efficiency, precision, sample throughput, and detection limits over traditional techniques [149]. In the analysis of soil samples, however, actinides must be recovered from the loaded Actinide Resin column for further processing. In this and other cases, actinide recovery can be achieved simply by stripping the entire stationary phase from the inert support with an alcohol (e.g., propanol). Because the volume of H2DEH[MDP] is only ~15% of the column bed volume, this treatment yields a very small quantity of material which can be readily oxidized [148] to convert the actinides to a form suitable for subsequent chemical separations using one or more of the previously developed actinide selective extraction chromatographic materials [144–147].

XI. REAGENTS FOR USE IN SUPERCRITICAL FLUID EXTRACTION Supercritical fluid extraction (SFE), e.g., using an extractant-supercritical CO2 mixture instead of an extractant-organic solvent mixture, has been a key element in our efforts to devise environmentally benign methods for metal ion separations. Supercritical CO2 (SCCO2) offers numerous benefits over conventional organic solvents: It is nontoxic, nonflammable, and does not contribute to photochemical smog or ozone destruction. Moreover, above its readily accessible critical point of 31°C and 73.8 atm, the solvating power of SCCO2 can be tuned over a wide range by relatively small changes in temperature and pressure [150]. The use of SCCO2 as a medium for metal extraction is well established and reports describing SFE by various complexants from a wide range of matrices have appeared [151–154]. Our work has focused on coupling the unique solvent properties of SCCO2 with the metal complexing power of alkylenediphosphonic acids [25–27, 42]. Unfortunately, neither the acids nor their dialkyl-substituted derivatives are sufficiently soluble in unmodified SCCO2 for practical metal extraction in this medium [155; J. Brennecke, personal communication, 2000]. Although the low solvent power of SCCO2 can be improved through the addition of modifiers (such as methanol), this approach is not always effective or desirable,

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as such measures increase operating costs and render the process less environmentally benign. For this reason, much effort has been directed towards the development of metal complexants incorporating “CO2- philic” groups, such as fluoroalkyl or silicone polymer functionalities [156, 157]. Our efforts recently have concentrated on alkylenediphosphonic acids possessing discrete, well-defined, silicon-containing groups for possible application in SCCO2, e.g., the 3-trimethylsilyl-1-propyl group [40–42, 83]. Such compounds were targeted because they are expected to be considerably less expensive than fluorinated analogues, increasing the likelihood of eventual large-scale application. Also, since current plans for the disposal of high-level radioactive waste include vitrification into borosilicate glass (typically consisting of 30–50% SiO2) [158], the incorporation of silicon functionalities into alkylenediphosphonic acids could increase their compatibility with this waste form. Alkylenediphosphonic acids contain two sites that can be readily functionalized to modify their solubility properties: the alkylene bridge separating the phosphorus atoms and the acidic POH groups. In principle, the presence of these two reactive sites, the ability to adjust the bridge length, and the many structural variations possible in the appended functional groups offer numerous opportunities to enhance the SCCO2 solubility of diphosphonic acids. At the same time, this flexibility poses a significant challenge, namely, the identification of those compounds most likely to exhibit satisfactory SCCO2 solubility from among the many possible candidates for synthesis. Our primary interests are not in predicting SCCO2 solubility from first principles, but rather, once the solubility of a “parent” ligand has been established, in simply correlating changes in this solubility with various structural modifications in the ligand framework. To this end, we examined the SCCO2 solubility of a series of alkylenediphosphonic acids of varying bridge length symmetrically substituted at two of the acidic hydrogen atoms with either a 2-ethylhexyl or 3-trimethylsilyl-1propyl functionality (Structure 1, with n=1–6 and R=EH or TMSP) [42]. We also determined the effect of branching of the ester group for a series of dialkylsubstituted methylenediphosphonic acids on the aggregation and SCCO2 solubility properties of these compounds and demonstrated the utility of molecular connectivity indices in quantifying the influence of branching on SCCO2 solubility [42].

A. Silyl- and 2-Ethylhexyl-Substituted Compounds Solubility data obtained from dynamic transfer experiments [159] for the disubstituted 2-ethylhexyl and 3-trimethylsilyl-1-propyl alkylenediphosphonic acids (Fig. 25) suggest that the SCCO2 solubility of these ligands is determined by several factors, most notably the length of the alkylene chain bridging the phosphorus atoms. Specifically, those ligands with an odd number of bridging methylene groups, which are dimeric in nonpolar diluents, are significantly more soluble in SCCO2 than those with an even number of bridging methylene groups,

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Figure 25 Percent recovery of 3-trimethylsilyl-1-propyl- and 2-ethylhexyl-substituted alkylenediphosphonic acids from glass beads at 250 bar, 60 °C. (From Ref. 159.)

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which tend to form more highly aggregated species [41]. The lower solubility for the more highly aggregated compounds is expected based on the reported greater SCCO2 solubility of the monomeric form of carboxylic acids relative to the corresponding dimers [160]. The results presented in Fig. 25 demonstrate that among the compounds with an odd number of bridging methylene groups, i.e., the ligands that are primarily dimeric in toluene and expected to remain so in SCCO2, the SCCO2 solubility decreases as the length of the bridge increases. Thus, H2DTMSP[MDP] is significantly more soluble in SCCO 2 than either H 2 DTMSP[PrDP] or H2DTMSP[PDP]. Clearly, for these compounds, a factor other than aggregation plays a role in determining this difference in solubility. Increases in solute molecular weight are known to lead to decreases in SCCO2 solubility [161]. Thus, the lower solubility of H2DTMSP[PrDP] and H2DTMSP[PDP] is due, at least in part, to their higher molecular weights compared to H2DTMSP[MDP]. It is also evident that the nature of the ester group has a significant effect on the solubility of the diphosphonic acid. Specifically, TMSP-substituted extractants are generally more soluble than the analogous EH-substituted extractants. To determine if the greater solubility of the TMSP compounds arises from the silicon atoms or from differences in branching between the TMSP and EH groups, the SCCO2 solubility of symmetrical partial esters of methylenediphosphonic acid with selected C7 and C8 groups with various degrees of branching were determined.

B.

Symmetrical C7 and C8 Partial Esters of Methylenediphosphonic Acid

The results of the SCCO2 solubility measurements for the series of selected methylene-diphosphonic acids containing seven- or eight-carbon ester groups are shown in Fig. 26. It is readily apparent from these results that the extent of branching of the ester group [which has no effect on their aggregation; all are dimeric in toluene [42]] has a pronounced effect on the SCCO2 solubility of these compounds, with the straight chain and cyclic esters exhibiting far poorer solubility than the highly branched esters. An increase in SCCO2 solubility is expected to accompany an increase in ester group branching based on reports of the influence of branching on the solubility of hydrocarbons and alcohols in SCCO2 [161], although the magnitude of the effect (most readily evident from Fig. 27) was somewhat surprising. Figure 27 summarizes the results of our efforts to correlate the solubility of the C7 and C8 esters with a topological molecular descriptor, the molecular connectivity. This descriptor can be calculated at a number of different levels to take into account various properties of a molecule [162, 163], and it has been successfully employed in the development of structure-function relationships to predict such properties as boiling points [164], gas chromatographic retention times [162], and toxicity [165]. The details of how the molecular connectivity index of order n, n␹, is calculated for a given ester group and how the best fit of the data using the fewest molecular

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Figure 26 Percent recovery of (a) C7-methylenediphosphonates and (b) C8-methylenediphosphonates from glass beads at 200 bar, 60 °C.

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Figure 27 Supercritical carbon dioxide solubility vs. total␹ for methylenediphosphonic acids with seven and eight carbon ester groups.

connectivity indices, total␹=a1␹+b2␹, is obtained, may be found in the original publications [42, 162, 163, 166]. Figure 27 shows the results obtained when the coefficients a and b were optimized using the data for the C8 esters. As can be seen, the data for the C8 compounds are highly correlated to total␹. Interestingly, the same coefficients yield a straight-line relationship between the solubility and total␹ for the C7 esters, suggesting that the relative SCCO2 solubility of methylenediphosphonates bearing different alkyl ester groups can be predicted through the calculation of molecular connectivity indices. It is important to note (Fig. 27) that H2DTMSP[MDP] is roughly an order of magnitude more soluble in SCCO2 than would be expected from its molecular connectivity index alone. Thus, the greater solubilizing effect of the TMSP group vs. the EH group arises not merely from the introduction of greater branching into the extractant, but also from the presence of the silicon atoms. That a siliconcontaining functionality should increase the solubility of the extractant in SCCO2 is not surprising given previous reports of “CO2-philic” silicon-based polymers and amphiphiles [44, 167]. The magnitude of the effect observed was unanticipated,

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however, given that each TMSP group contains only a single silicon atom. Interestingly, the observed solubility (0.054 M at 60°C and 200 bar) of H2DTMSP[MDP], while not as great as that of TBP (1.2 M under the same conditions of temperature and pressure [167]), is sufficient to render it applicable in the supercritical fluid extraction of metal ions [168].

XII. CONCLUSIONS Organophosphorus complexing agents have figured prominently in the development of nuclear energy and in the production of transuranium elements. Several decades of research have led to the development of a multitude of reagents for separations of actinides, lanthanides, and other metal species, both at the laboratory and industrial scale. In spite of the present trend toward the development of nonphosphorus-based complexing agents to reduce the problems associated with the presence of phosphorus containing materials in nuclear wastes [169], organophosphorus compounds remain uniquely versatile reagents for a variety of separative purposes. No other class of complexing agents, in fact, exhibits the extreme variety of structures, affinities for metal ions, and selectivities that can be achieved by careful control of the many factors involved in determining the properties of these reagents. In recent years, research has largely focused on multifunctional compounds, adding one more dimension to the vast array of properties of traditional organophosphorus compounds. The alkylenediphosphonic acid diesters investigated by us represent our latest contribution to this expanding field. The high strength of the interaction of alkylenediphosphonic acid diesters with many metal ions, together with the possibility to adjust this strength through modifications of the extractant structure, provide a great opportunity for the development of highly selective separations by fine-tuning the bridging backbone of the extractant. For example, the addition of a single methylene group in the alkylene bridge connecting the phosphorus atoms of the methylenediphosphonic acid diesters causes a transition from powerful but unselective extractants to less powerful but very selective ones. The former can be used as general extractants when the separation of an entire class of metal ions, for example, actinides or lanthanides, is desired. The latter, on the other hand, can be considered as “smart reagents” for those cases where a selective separation is required. Several aspects of the chemistry of organic solutions of alkylenediphosphonic acid diesters have been discussed in this chapter, with particular emphasis on their interaction with selected metal ions, the self-assembly of the extractants and their metal complexes in a variety of nanosized aggregates, and their possible application to separations of practical interest. Our studies have revealed that most features of the behavior of these reagents can be rationalized in terms of extractant aggregation properties, metal ion coordination chemistry, and inductive effects of substituents on the basicity of the phosphoryl groups. Many facets of

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the alkylenediphosphonic acid diesters chemistry, however, still remain to be elucidated. Nothing is known, for example, on the interfacial behavior of alkylenediphosphonic acid diesters. These compounds likely exhibit surfactant properties and their interfacial activity should determine the kinetics and mechanism of liquid-liquid mass transfer of metal ions. The interfacial behavior of the diesters should also be investigated in relation to diluent effects, since the type of diluent determines the extent of diester aggregation through intermolecular hydrogen bonding, and, most likely, their interfacial activity and metal extraction kinetics. Similarly, the acid dissociation constants of the diesters in biphasic systems are yet to be determined. Furthermore, slope analysis should be complemented by more sophisticated computer-based analyses to unravel the simultaneous equilibria which take place in the extraction of metal ions leading to the coexistence of several species having different stoichiometries and different degrees of ligand protonation. Finally, the investigation of alkylenediphosphonic acid diesters as reagents for solvent extraction of metal ions should be extended to their sulfur-containing analogues in which some or all of the oxygen atoms of the and POH groups are replaced by sulfur atoms. If synthesized, these compounds should exhibit quite different properties than their oxoanalogues because of the lower basicity and soft donor character of sulfur as compared to oxygen. This should affect the aggregation behavior and all other properties that depend on hydrogen bonding. The softness introduced by sulfur atoms makes these reagents promising for separations of soft metal ions (such as mercury, lead, and cadmium), and possibly, for trivalent actinidelanthanide separations. Future research will address at least some of these issues.

ACKNOWLEDGMENTS Too many undergraduate and graduate students, postdoctoral fellows, visiting scientists, and external collaborators, as well as past and present members of the chemistry division of ANL, have contributed to this research to recognize their contributions individually. We would like, however, to single out the efforts of Dan McAlister, who, as a Loyola University Chicago graduate student, helped us greatly in obtaining much of the data discussed in this work. We must also acknowledge the vision of Phil Horwitz, under whose guiding hands the investigation of alkylenediphosphonic acid diesters was initiated. Finally, this work would not have been possible without the continuing support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (Contract No. W-31-109-ENG-38). We also acknowledge the support of the Environmental Management Sciences Program of the Office of Science and Environmental Management (extractant synthesis) under grant number DE-FG07– 98ER14928.

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The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the government.

NOMENCLATURE DA[AP] DCC DCH18C6 DCU DIC DIEA EH EH4[MDP] HDEHP HEDPA H2A H2DEH[BuDP] H2DEH[EDP] H2DEH[MDP] H2DEH[1,2-BzDP] H2DO[MDP] H2DTMSP[BuDP] H2DTMSP[EDP] H2DTMSP[HDP] H2DTMSP[MDP] H2DTMSP[PDP] H2DTMSP[PrDP] H2[EHP] H2MEHP TBP PFHP PFHP4[MDP] THF TMS TMSP TMSP4[MDP] TOPO

diamyl amylphosphonate N,N’-dicyclohexylcarbodiimide dicyclohexano-18-crown-6 dicyclohexylurea N,N’-diisopropylcarbodiimide diisopropylethylamine 2-ethylhexyl tetra(2-ethylhexyl) methylenediphosphonate di(2-ethylhexyl) phosphoric acid 1-hydroxyethane-1, 1-diphosphonic acid generic alkylenediphosphonic acid P,P’-di(2-ethylhexyl) butylenediphosphonic acid P,P’-di(2-ethylhexyl) ethylenediphosphonic acid P,P’-di(2-ethylhexyl) methylenediphosphonic acid P,P’-di(2-ethylhexyl) benzene-1,2-diphosphonic acid P,P’-di(n-octyl) methylenediphosphonic acid P,P’-di(3-trimethylsilyl-1-propyl) butylenediphosphonic acid P,P’-di(3-trimethylsilyl-1-propyl) ethylenediphosphonic acid P,P’-di(3-trimethylsilyl-1-propyl) hexylenediphosphonic acid P,P’-di(3-trimethylsilyl-1-propyl) methylenediphosphonic acid P,P’-di(3-trimetiiylsnyl-1-propyl) pentylenediphosphonic acid P,P’-di(3-trimethylsilyl-1-propyl) propylenediphosphonic acid 2-ethylhexyl phosphonic acid mono(2-ethylhexyl) phosphoric acid tri-n-butyl phosphate 3-perfluorohexylpropyl tetra(3-perfluorohexyl-1-propyl) methylenediphosphonate tetrahydrofuran trimethylsilyl 3-trimethylsilylpropyl tetra(3-trimethylsilyl-l-propyl) methylenediphosphonate tri-n-octylphosphine oxide

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A. Frequently Used Symbols C D I(Q) K k’ n nav nw Q R Rg Z

analytical concentration distribution ratio neutron scattering intensity equilibrium constant capacity factor aggregation number number-average aggregation number weight-average aggregation number momentum transfer generic substituent group or geometric radius gyration radius atomic number

B. Greek Symbols β γ± δ ␭ ␯ ␪ ␾ ␹

aggregation or complexation constant mean ionic molal activity coefficient NMR chemical shift wavelength of neutrons infrared vibration frequency half scattering angle ionic potential connectivity index

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124. Mioduski, T. Comments Inorg. Chem. 1997, 2, 93–119. 125. Chiarizia, R.; McAlister, D.R.; Herlinger, A.W. Sep. Sci. Technol. 2004. Submitted. 126. Peppard, D.F.; Bloomquist, C.A.A.; Horwitz, E.P.; Lewey, S.; Mason, G.W J. Inorg. Nucl. Chem. 1970, 32, 339–343. 127. Peppard, D.F.; Mason, G.W.; Lewey, S. In Solvent Extraction Research; Kertes, A.S., Marcus, Y, Eds.; Wiley-Interscience: New York, 1969; 49–57. 128. Marcus, Y.; Kertes, S. Ion Exchange and Solvent Extraction of Metal Complexes; John Wiley & Sons: New York, 1969; 815–858. 129. Sekine, T.; Hasegawa, Y. Solvent Extraction Chemistry; Marcel Dekker, Inc: New York, 1977; 200–217. 130. Rydberg, J.; Sekine, T. In Principles and Practices of Solvent Extraction; Rydberg, J., Musikas, C., Choppin, G.R., Eds.; Marcel Dekker, Inc.: New York, 1992; 101. 131. Ramakrishna, V.V.; Patil, S.K. In Structure and Bonding; Dunitz, J.D., Hemmerich, P., Ibers, J.A., Jørgensen, C.K., Neilands, J.B., Reinen, D., Williams, R.J.P., Eds.; Springer-Verlag: Heidelberg, Germany, 1984; Vol. 56, 36–90. 132. Bond, A.H.; Dietz, M.L.; Chiarizia, R. Ind. Eng. Chem. Res. 2000, 39, 3442–3464. 133. McAlister, D.R.; Chiarizia, R.; Dietz, M.L.; Herlinger, A.W.; Zalupski, P.R. Solvent Extr. Ion Exch. 2002, 20, 447–469. 134. Chiarizia, R.; McAlister, D.R.; Herlinger, A.W. Solvent Extr. Ion Exch. 2003, 21, 171–197. 135. Chiarizia, R.; Horwitz, E.P.; Dietz, M.L.; Cheng, Y.D. React Funct. Polym. 1998, 38, 249–257. 136. Bond, A.H.; Chiarizia, R.; Huber, V.J.; Dietz, M.L.; Herlinger, A.W.; Hay, B.P. Anal. Chem. 1999, 71, 2757–2765. 137. Dietz, M.L.; Bond, A.H.; Hay, B.P.; Chiarizia, R.; Huber, V.J.; Herlinger, A.W. Chem. Soc, Chem. Commun. 1999; 1177–1178. 138. Chiarizia, R.; Dietz, M.L.; Bond, A.H.; Huber, V.J.; Herlinger, A.W.; Hay, B.P. In Solvent Extraction for the 21st Century (Proceedings of ISEC ’99); Cox, M., Hidalgo, M., Valiente, M., Eds.; Society of Chemical Industry: London, UK, 1982; Vol. 1, 299–304. 139. Hiraoka, M. Crown Compounds: Their Characteristics and Applications’, Elsevier: New York, 1982. 140. Baes, C.F., Jr.; McDowell, W.J.; Bryan, S.A. Solvent Extr. Ion Exch. 1987, 5, 1–28. 141. Ferraro, J.R.; Peppard, D.F. J. Phys. Chem. 1961, 65, 539–541. 142. Braun, T.; Ghersini, G. Extraction Chromatography; Elsevier: New York, 1975. 143. Horwitz, E.P.; Dietz, M.L.; Chiarizia, R.J. Radioanal. Nucl. Chem. 1992, 161, 575– 583. 144. Horwitz, E.P.; Dietz, M.L.; Chiarizia, R.; Diamond, H.; Essling, A.M.; Graczyk, D. Anal. Chim. Acta. 1992, 266, 25–37. 145. Horwitz, E.P.; Chiarizia, R.; Dietz, M.L.; Diamond, H.; Nelson, D.M. Anal. Chim. Acta. 1993, 281, 361–372. 146. Horwitz, E.P.; Dietz, M.L.; Chiarizia, R.; Diamond, H.; Maxwell, S.L., III; Nelson, M.R. Anal. Chim. Acta. 1995, 310, 63–78. 147. Dietz, M.L.; Horwitz, E.P.; Sajdak, L.R.; Chiarizia, R. Talanta 2001, 54, 1173–1184. 148. Horwitz, E.P.; Chiarizia, R.; Dietz, M.L. React. Funct. Polym. 1997, 33, 25–36.

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149. Burnett, W.C.; Corbett, D.R.; Schultz, M.; Horwitz, E.P.; Chiarizia, R.; Dietz, M.L.; Thakkar, A.; Fern, M. J. Radioanal. Nucl. Chem. 1997, 226, 121–127. 150. Jessop, P.G.; Leitner, W. In Chemical Synthesis Using Supercritical Fluids; Jessop, P.G., Leitner, W., Eds.; Wiley-VCH: New York, 1999; 9–13. 151. Laintz, K.E.; Wai, C.M.; Yonker, C.R.; Smith, R.D. Anal. Chem. 1992, 64, 2875– 2878. 152. Lin, Y.; Brauer, R.D.; Laintz, K.E.; Wai, C.M. Anal. Chem. 1993, 65, 2549–2551. 153. Wai, C.M.; Kulyako, Y.; Yak, H.K.; Chen, X.; Lee, S. J. Chem. Commun. 1999; 2533– 2534. 154. Powell, C.J.; Beckman, E.J. Ind. Eng. Chem. Res. 2001, 40, 2897–2903. 155. Phelps, C.L.; Smart, N.G.; Wai, C.M. J. Chem. Ed. 1996, 73, 1163–1168. 156. Lagalante, A.F.; Hansen, B.N.; Bruno, T.J. Inorg. Chem. 1995, 34, 5781–5785. 157. Ashraf-Khorassani, M.; Combs, M.T.; Taylor, L.T. Talanta 1997, 44, 755–763. 158. Luo, S.; Sheng, J.; Tang, B.J. Nucl. Mater. 2001, 298, 180–183. 159. Herlinger, A.W.; McAlister, D.R.; Chiarizia, R.; Dietz, M.L. Sep. Sci. Technol. 2003, 38, 2741–2762. 160. Yamamoto, M.; Iwai, Y.; Nakajama, T.; Arai, Y J. Phys. Chem. A 1999, 103, 3525– 3529. 161. Dandge, D.K.; Heller, J.P.; Wilson, K.V. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 162–166. 162. Kupchik, J.K. J. Chromatog. 1993, 630, 223–230. 163. Kier, L.B.; Hall, L.H. Molecular Connectivity in Structure-Activity Analysis; John Wiley & Sons: New York, 1986; 1–23 and 43–66. 164. White, C.M. J. Chem. Eng. Data 1986, 31, 198–293. 165. Hong, H.; Shuokui, H.; Xiaorong, W.; Liansheng, W. Envir. Sci. Technol. 1995, 29, 3044–3048. 166. McAlister D.R.; Dietz, M.L.; Stepinski, D.C.; Zalupski, P.R.; Dzielawa, J.A.; Barrons, R.E. Jr.; Herlinger, A.W. Sep. Sci. Technol. 2004, 39. in press. 167. Fink, R.; Beckman, E.J. J. Supercritical Fluids 2000, 18, 101–110. 168. Meguro, Y.; Iso, S.; Sasaki, T.; Yoshida, Z. Anal. Chem. 1998, 70, 774–779. 169. Musikas, C. Sep. Sci. Technol. 1988, 23, 1211–1226.

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4 Sulfoxide Extractants and Synergists Zdenek Kolarik Consultant, Karlsruhe, Germany

I. INTRODUCTION A. General Sulfoxides are rather strong electron donors and act as typical solvating extractants. Thus, they extract metals as salts at moderate to high concentrations of mineral acids, and the efficiency and selectivity of the extraction can largely be controlled by the nature and concentration of electrolytes in the aqueous phase. Extracted metals can simply be stripped with appropriately diluted acid solutions, and presence of complexing agents in the aqueous phase is not typically needed for enhancement of the stripping efficiency. In the extraction of metals by acidic extractants, sulfoxides can be powerful synergists. The ability of sulfoxides to extract uranium(VI) salts and mineral acids was predicted as early as in 1962. Extractant properties of sulfoxides were foreseen to lie between those of TBP and phosphine oxides [1]. The prediction was subsequently corroborated when DOSO (for abbreviations of solvent names, see the Notation section at the end of this chapter) was shown to extract uranyl chloride and nitrate [2] as well as nitric, perchloric, and hydrochloric acids [3]. The action of sulfoxides as synergists was demonstrated in 1967, when DBSO was shown to enhance strongly the extraction of Am(III), Eu(III), Lu(III), and Sc(III) by TTA [4]. Numerous studies have since then appeared, predominantly directed to the extraction by solvation. A review on sulfoxide extractants appeared in 1976 [5], and thus it is appropriate to update the survey of the extensive work made since then. 165 Copyright © 2004 by Marcel Dekker, Inc.

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B. Potential of Sulfoxide Extractants A number of solvating extractants was known and available at the time at which sulfoxides were introduced, ranging from ketones to phosphoryl compounds. Many of them were used on a laboratory scale, e.g., for analytical and radiochemical separations. The most practicable, e.g., MiBK and especially TBP, had been or were being used even in large-scale processes such as reprocessing of irradiated nuclear fuel. Thus, the questions arise as to what was expected of sulfoxides as a new class of extractants, and what properties of them stimulated the subsequent extensive investigations which still are continued. TBP remains the principal competitor of sulfoxides. Thus, the properties of sulfoxides are in many papers compared with those of TBP, and in this review as well. It played an important role that sulfoxides were considered a plausible alternative for TBP in the reprocessing of spent nuclear fuel, both in the U/Pu and U/Th cycles. Not only do sulfoxides possess adequate extraction efficiency and selectivity, but products of their radiation degradation (sulfides, sulfones) do not essentially extract fission products. Hence, the efficiency of sulfoxides in the decontamination of U(VI) and Pu(IV) should deteriorate only minimally with increasing radiation dose. Let us remember that the main degradation products of TBP, i.e., dibutyl hydrogen and butyl dihydrogen phosphates, can form soluble or insoluble complexes with U(VI), Pu(IV), Zr(IV), and other elements. This may lower the effectiveness of stripping and cause hydraulic disturbances. Advantages of sulfoxide extractants might to some extent be counterbalanced by their price, which is by 1 or 2 orders of magnitude higher than that of TBP. However, the price would decrease if application of sulfoxides in large-scale processes increased the demand.

C. Synthesis and Accessibility Few water-immiscible sulfoxides, e.g., DBSO, are commercially available, and laboratory-synthesized compounds have been used in most studies. Symmetrical dialkyl sulfoxides are typically synthesized in two steps, namely, the preparation of a dialkyl sulfide and its subsequent oxidation to sulfoxide. To obtain the sulfide, anhydrous Na2S [6] or Na2S · H2O [7] is reacted in ethanol with the corresponding bromoalkane under prolonged reflux. The sulfide is oxidized at ambient temperature by treatment with ~11 M HNO3 [6] or by hydrogen peroxide in a 3:1 mixture of acetic acid and acetone [7]. The raw product forms a separate layer, directly in the former method and after pouring the reaction mixture into ice water in the latter method. The product is purified by washing the separate layer with an alkaline solution [6, 7] and, eventually, by recrystallization from a benzene/ cyclohexane mixture or petroleum ether [7]. Synthesis of unsymmetrical sulfoxides involves as a first step the preparation of a ion pair involving a methylsulfinyl carbanion, It is formed

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in a reaction of dimethyl sulfoxide with sodium hydride at 60–65°C under the removal of hydrogen by purging with argon. A bromoalkane is added after cooling the reaction mixture to room temperature and allowed to react with the ion pair, maintaining the temperature at 26–35°C by cooling. The product is purified by vacuum distillation or recrystallization [8]. To compare, let us mention that TBP is preferably synthesized on the industrial scale by prolonged reaction of POCl3 with excess 1-butanol at room temperature. Then HCl and other acidic impurities are removed by washing with NaOH or Na2CO3 solutions, unreacted butanol is distilled off, and TBP is purified by vacuum distillation [9]. A mixture of natural sulfoxides, so called petroleum sulfoxides, can be obtained from a diesel fuel fraction (bp 190–360°C) of crude oil. Three methods can be used for the preparation of a sulfoxide concentrate. In one, acidic sulfides are extracted by 80–82% H2SO4, reextracted into gasoline after diluting the acid to 70%, and neutralized. Gasoline is distilled off, and a sulfide fraction with a boiling point of 260–360°C is oxidized to sulfoxides by 27–39% hydrogen peroxide. In another method sulfides are oxidized directly in the diesel fuel fraction by 30% H2O2, catalyzed by concentrated acetic acid. Sulfoxides are then extracted by 62% H2SO4, from which they separate as a light layer after dilution with water. The third method also involves the oxidation of the sulfides in diesel fuel by H2O2, but catalyzed by acetone. The method avoids the use of sulfuric acid and sulfoxides are extracted by a 1:3 (w:w) mixture of water and acetone. They are concentrated by evaporation of acetone and a part of the water, and dried. The molecular weight of the sulfoxides in the concentrate is 200–280. They contain in their molecule 5or 6-membered thiaheterocycles, which are monocyclic (50–60%), bicyclic (20– 30%), and tricyclic or larger (5–10%) [10].

D. Properties in Pure State and in Solutions Compounds with straight chain alkyl substituents, both symmetrical and unsymmetrical, are solids at room temperature. Compounds with branched substituents are fluids. Physical properties of individual sulfoxide extractants are given in the appendix, and it is seen there that mainly melting points have been measured and there are some discrepancies between values reported in various sources. Di-n-alkyl sulfoxides, namely, DPSO to DDSO, are slightly soluble in alkanes and cyclohexane, moderately soluble in CCl4 and aromatic diluents, and well soluble (up to 2 M) in halogenated solvents [24]. The solubility of DOSO is <1% in alkanes and 0.5–0.7 M in benzene, xylene, and CCl4 [7]. Sulfoxides with branched alkyl substituents, such as DEHSO and di(2-octylsulfoxide), are completely miscible with xylene and kerosene [11] and highly soluble in benzene and dodecane [7]. The solubility of di(alkylsulfinyl)methane in benzene and dodecane is <1% with alkyl being n-octyl, 20 and 8%, respectively, with alkyl being 2-ethylhexyl, and higher with alkyl being 2-octyl [7].

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DPSO and DOSO in CCl4 or CHCl3 decompose slowly in contact with 6 M HNO3, and the decomposition is faster at 8–10 HNO3 [12]. DOSO in toluene decomposes in contact with ≥8 M HCl [13]. In assessing the basicity of sulfoxides, the stretching frequency of the S→O bond is not a suitable measure because it is little influenced by the nature of the substituents. Good assessment is possible in measuring the shift of the symmetric stretching frequency of the OH bond of water coordinated to the oxygen atom of sulfoxides and, generally, also other O donors. The basicity increases in the order of extractants (∆vOH in cm-1) DΦSO (160)
E. Structure of the Review and Critical Remarks The extraction systems are ordered according to their importance, judged in conformance with the attention devoted to them in the literature. Therefore, nitrate systems are treated first, followed by chloride systems, and then by systems still less studied. In each system the treatment starts with hexavalent elements and proceeds to elements with lower valences. The reason is that U(VI) belongs to the most studied elements, and Th(IV) and Pu(IV) have also been of considerable interest. A survey of important data is given in a table in the beginning of each section, giving the components of the phases and their concentration ranges. The table includes only data which can be used in direct assessment of the extraction behavior and of the applicability of a sulfoxide to a particular separation. Thus, only those

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data are referred which consist of directly measurable or adjustable variables, and not those which include, for example, thermodynamic activities or terms considering side equilibria. Data on common, frequently studied sulfoxides are referred to in the table, while less commonly studied sulfoxides are appropriately mentioned in the text. Further on, the effect of extraction variables is discussed, such as the sulfoxide structure, salting out, diluent, etc. The composition of the extracted complexes is listed as completely as possible. Besides saturation experiments, it has typically been determined by slope analysis where the most frequent source of errors is negligence or insufficient consideration of activity coefficients. For example, the distribution ratio DM on a concentration scale is sometimes plotted vs. the activity of the acid participating in the formation of the extracted complex. The change of the activity coefficients of the species of the extracted metal in the aqueous phase with the concentration of the acid is neglected, and the slope of the dependence may not correspond to the composition of the extracted complex. Thermodynamic functions of the extraction reactions are gathered in tables comprising detailed information about the respective system. Where a single extractant concentration is given in the first column of a table, the ∆H value was determined from the slope of the log DM vs. 1/T dependency Where a concentration range is given, the ∆H value was determined from the slope of the log Kex vs. 1/T dependence, with the extraction constant Kex being calculated from the DM vs. extractant dependence at a constant HNO3 concentration. It must be kept in mind that the determination of thermodynamic functions from the temperature dependencies comprises unjustified negligence and simplifications [19]. For example, activity coefficients and heats of dilution of system components are not taken into account, extrapolation to infinite dilution is not made, and a possible effect of the acid concentration on the slope of the log Kex vs. 1/T dependence is ignored. In spite of these faults, the thermodynamic functions are given as reported. They often have been measured for various systems under analogous conditions and may at least be sufficiently good for mutual comparison. Data without specifying the concentration of the extracted metal relate to systems in which the loaded fraction of the extractant plays no important role.

II. EXTRACTION FROM NITRATE MEDIA A. Nitric Acid A considerable fraction of a sulfoxide can be converted to a nitric acid solvate, and a sulfoxide to acid ratio of 1:1 is approached or attained. For example, the HNO3 concentration increases •

From 0.036 to 0.097 M in 0.1 M DOSO in benzene after contact with 1–10 M HNO3 [20]

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Kolarik From 0.003 to 0.04 M in 0.04 M DOSO in CCl4 after contact with 1–8.12 M HNO3 [3] From 0.0019 to 0.05 M in 0.05 M DEHSO, from 0.005 to 0.1 M in 0.1 M DEHSO, from 0.014 to 0.2 M in 0.2 M DEHSO, and from 0.037 to 0.4 M in 0.4 M DEHSO, all in dodecane, after repeated contact with 0.5–8 M HNO3 [21]

The efficiency of the HNO3 extraction changes nonmonotonously with the alkyl chain length in the sulfoxide molecule. At >3 M HNO3 it slightly decreases in the order DPSO>DHxSO>DOSO>DNSO>DHpSO (all 1 M in TCE). At lower acid concentrations the chain length has no influence [22]. Distribution data in Ref. 7, although unclearly formulated, allow the assessment of the effect of the branching of the substituents in the sulfoxide molecule on the HNO3 extraction in the presence of 6 M LiNO3. The extraction effectiveness of mono-and bifunctional dioctyl sulfoxides in xylene decreases with increasing branching of alkyl substituents, the order of the monofunctional compounds being DOSO> DEHSO>di(sec-octyl) sulfoxide. The order of the bifunctional compounds is 1,2-di(2-ethylhexylsulfinyl)ethane>1,2-di(sec-octylsulfinyl)ethane Ⰷ di(2ethylhexylsulfinyl)methane Ⰷ di(sec-octylsulfinyl)methane, and it also shows that the ethane derivatives extract HNO3 much more effectively than the methane derivatives [7]. To compare sulfoxides with other extractants, the ability to extract HNO3 as a monosolvate increases in the sequence TBPDEHSO at 7 M HNO3 [28]. Selected data in Fig. 1 allow the comparison of the effect of some extraction variables. Provided that the length of the n-alkane substituents does not influence the extraction ability of sulfoxides appreciably, and neglecting small differences in the extractant concentration, the extraction efficiency changes in the diluent order TCE ~xylene>chloroform (curves 1, 2, and 4 in Fig. 1). Surprising is the difference between curves 3 and 5 in Fig. 1. They were obtained with similar diluents but at different temperatures, and it is shown below that the temperature effect can be expected to be negligible. Let us mention that curve 3 in Fig. 1 is the only one without maximum, indicating that saturation of the solvent phase is not yet attained. U(VI) replaces nitric acid from the organic phase, as shown, for example, for DHpSO in TCE [24]. With rare exceptions, the extraction of nitric acid has been studied without measuring the concentration of water in the organic phase. The composition of the extracted species, strictly speaking the extractant to HNO3 ratio, has in most cases been determined by slope analysis and in few cases by measuring the saturation of

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Figure 1 Distribution ratio of HNO3 as a function of the HNO3 concentration in the aqueous phase: Curve 1–0.7 M DHxSO/xylene at 23°C [27]; 2–1.0 M DHpSO/TCE at 25°C [24]; 3– 0.2 M DEHSO/dodecane at 25°C [17]; 4–1.0 M DOSO/CHCl3 at room temperature [12]; 5– 0.2 M DEHSO/dodecane at 10°C [28, 11].

the organic phase. A monosolvate was the species typically found, namely in the extraction of HNO3 by • • • • • • •

DiPSO in Solvesso 100 from 0.5 and 2 M HNO3 [25] DHxSO in Solvesso 100 from 0.5 and 2 M HNO3 [25, 26], and in xylene from 0.5 to 3.4 M HNO3 [27] DHpSO in TCE from <5 M HNO3 [22] DOSO in CCl4 from 0.6 to 2.2 M HNO3 [12] and from 8.1 M HNO3 [3], in CHCl3 from 0.6 to 2.2 M HNO3 [12], and in Solvesso 100 from 0.5 and 2 M HNO3 [25] DEHSO in kerosene from 3.5 M HNO3 [11, 28] Ethyl dodecyl sulfoxide in xylene from 1 to 8.1 M HNO3 [3] 2-Heptylthiophane S-oxide in xylene from 0.5 to 2.1 M HNO3 [27]

Formation of solvates with a HNO3 to extractant ratio of >1 is indicated in the extraction by DOSO in CCl4 and CHCl3 from 3.74 M HNO3 [12]. The HNO3 monosolvate is anhydrous in chloroform [12]. A very sophisticated evaluation of distribution data in the system 0.05–0.4 M DEHSO in dodecane/0.25–8 M HNO3 has been made, including calculation of activity coefficients. It shows that the monosolvate B · HNO3 is not the only species formed. The species coexists with the species 2B · NO3, 2HNO3 · 5H2O, and 2B· HNO3 · 2H2O. The formation constant on the activity scale,

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is K 1,1,0 =0.308±0.006, K 2,1,0 =1.31±0.06, K1,2,5=(1.05±0.03)×10-4, and K2,1,2=0.82±0.04 [17]. The distribution ratio is temperature independent in the HNO3 extraction by 0.2 M DOSO in CCl4 from 5.2 M HNO3 at 0 and 30°C [3], and by 0.2 M DEHSO in kerosene from 3.5 M HNO3 at 10–50°C [28].

B. Uranium(VI) and Other Hexavalent Metals 1. Distribution and Extraction Rate Data Uranyl nitrate is the most studied extracted salt, due to the initial intention to introduce sulfoxides as potential alternatives for TBP in the reprocessing of nuclear fuel. A survey of selected sources of distribution data is given in Table 1. 1 M DHpSO in TCE extracts Mo(VI) weakly, the distribution ratio increasing from 0.06 at 3 M HNO3 to 0.11 at 8 M HNO3 [34]. Cr(VI) is extracted by 0.1 M DBSO in benzene as H2Cr2O7. Nitric acid displaces it from the organic phase and, as far as the unclear presentation of the data can be understood, DCr(VI) decreases from 0.04 to 0.007 and ~0.0006 at 1, 2, and 5 M HNO3, respectively [50]. The rate of the extraction of U(VI) by DEHSO in toluene from nitrate solutions was studied in an improved stirred cell. The diffusion controlled rate of the forward extraction was with A denoting the ratio of the phase volume to the interfacial area. The rate constant was kf=7.9×10-2 m×L2.5×mol-2.5×s-1, and the apparent activation energy was 0.9 kJ/mol [51]. The extraction by PetrSO in kerosene was investigated by the single drop and stirred cell methods, and it was found to be a pseudo-first-order reaction with respect to U(VI). The rate constant k’ (cm/s) passed a maximum at 3-4 M HNO3 and 0.3–0.4 M PetrSO, and was almost temperature independent. The rate was controlled by the diffusion of the extracted complex, UO2(NO3)2 · 2B, from the interface into the bulk organic phase [52]. To describe the equilibrium organic U(VI) concentration, CU,org, as a function of the U(VI) concentration in the aqueous phase, CU,aq, the equation CU,org=akCU,aq/ (1+kCU,aq) was derived, based on reaction kinetics. With a pair of the parameters a and k the equation was valid for a constant concentration of HNO3. Based on the Langmuir isotherm, the equation CU,org=a’bX/(1+bX) with X=exp(x[HNO3]y) CU,aq was derived for variable acidity. Data on the system UO2(NO3)2/HNO3/ DHpSO/TCE from Ref. 22 are well described with a’=0.606, b=4.666, x=0.385, and y=0.899 [53]. In another approach, the dependence of a variable comprising the activity of uranyl nitrate in the aqueous phase on a variable comprising the organic U(VI) concentration was used. The dependency had a sigmoidal form and it was maintained that values of the variables in the inflexion point can be directly taken

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Table 1 Survey of Data on the Distribution of U(VI) Between Solutions of Common Sulfoxides and Aqueous Nitrate Solutionsa

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Table 1 Continued

Single temperature value applies to the measurement of isothermal concentration dependencies of DM(VI), a temperature range to the measurement of temperature dependency. rt, room temperature. a

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for the calculation of the equilibrium extraction constant [54]. Data on DHpSO from Refs. 22, 24 were described by the model. In a thermodynamic approach the equilibrium extraction constant, 0Keq, was expressed in terms of activities, indeed with different concentration scales in the phases. For the extraction of U(VI) as UO2(NO3)2 · 2B with B=DHpSO into xylene the constant was written as Here xU and xB are mole fractions and fU and fB are activity coefficients of the U(VI) disolvate and free sulfoxide, respectively, in the organic phase. Terms in parentheses are molalities in the aqueous phase, and γ± is the mean ionic activity coefficient of uranyl nitrate. The activity coefficients in the organic phase were determined directly with the aid of the Gibbs-Duhem equation, and they were shown to be well described by the Scatchard-Hildebrand and Guggenheim quasi lattice models [55]. 2. Salting Out The extraction of U(VI) was most frequently studied from nitric acid solutions. Figure 2 shows that the dependency of the distribution ratio of U(VI) on the HNO3 concentration has a maximum, which is typically found with solvating extractants containing a sulfinyl, phosphoryl, or amidocarbonyl group in the molecule. Left of the maximum, salting out is the predominating action of nitric acid, while competition of HNO3 with U(VI) for free extractant prevails right of the maximum.

Figure 2 Effect of HNO3 on the extraction of UO2(NO3)2 by various sulfoxides. Curve 1– 0.25 PetrSO in benzene [43]; 2–0.2 M PetrSO in kerosene [45]; 3–0.4 M DEHSO in dodecane [39]; 4–0.25 M DEHSO in toluene [41]; 5–0.2 M DHxSO in Solvesso 100 [26]. Initially <0.01 M U(VI) in the aqueous phase, 25°C or room temperature.

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It is seen in Fig. 2 that the position of the maximum varies from 2 to 6 M HNO3, obviously influenced by the nature of the extractant and diluent. Data in the figure and other available data are too diversified to reveal a correlation between the position of the maximum and the extractant character. The shift of the maximum with the diluent nature is discussed below in the paragraph dealing with the diluent effect. The maximum disappears, or is shifted to very high nitrate concentrations, if U(VI) is salted out predominantly by a nonextractable salt, such as alkali or ammonium nitrate. An example is given in Table 2. The position of the maximum at ~5 M HNO3 is independent of the solvent loading with U(VI) in the extraction with 1 M DHpSO in TCE [22, 24] and CCl4 [31, 35]. 3. Effect of the Extractant Structure The effect of the n-alkyl length in the molecule of dialkyl sulfoxides is shown in Fig. 3. The curves are given without experimental points in the original source and it cannot be assessed how far they are idealized. Nevertheless, they clearly illustrate the decrease of the extraction ability of sulfoxides for U(VI) with increasing nalkyl length. The same trend has been observed with the same solvents in the absence of nitric acid [22, 24] and in the extractant series DPSO to DLSO in CCl4 [31]. DPSO and pentyl hexyl sulfoxide in xylene extract U(VI) with practically the same efficiency from initially 0.1 M HN03 [30]. Branching of alkyl chains appears to suppress the extraction ability of sulfoxides. Figure 4 shows that DOSO is a more effective extractant for U(VI) than DEHSO in three different diluents. This is in accord with data about a longer series of

Table 2 Extraction of U(VI) by 0.25 M DEHSO and 2-Ethylhexyl p-Tolyl Sulfoxide in Toluene from Different Ionic Mediaa

Initially 0.004 M U(VI) in the Aqueous Phase, 25°C From Ref. 41. c From Ref. 56. a

b

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Figure 3 Extraction of UO2(NO3)2 by 1 M sulfoxides in TCE from 1 M HNO3 [24]. Curve 1–DPSO; 2–DHxSO; 3–DHpSO; 4–DOSO; 5–DNSO, 25°C.

dioctyl sulfoxides (all 0.04–0.2 M in xylene), in which the extraction efficiency decreases in the order DOSO>DEHSO>di(2-octyl) sulfoxide=di(3-octyl) sulfoxide [33]. Another sequence confirms the effect of the alkyl branching and implies a strong suppression of the extraction ability in the replacing of alkyl by aryl substituents. The sequence is DOSO>DEHSO>dodecyl tolyl sulfoxide>2ethylhexyl tolyl sulfoxide>D ΦSO, all 0.25 M in toluene and at the HNO3

Figure 4 Extraction of UO2(NO3)2 by 0.1 M DOSO (open points) and DEHSO (other points) in various diluents. Initially, 1×10-4 M U(VI), 20°C. (From Ref. 40.)

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concentration varied from 1 to 7 M [37]. Elsewhere the extractant structure effect has been found to be dependent on the concentration of HNO3. At 1 M HNO3 the extractant (DU(VI)) order is DEHSO (2.65) >2-ethylhexyl p-tolyl sulfoxide (1.46)>di(2-octyl) sulfoxide (1.12)>di(p-ethylphenyl) sulfoxide (0.24), being in accord with the trend revealed by the preceding sequence. However, at 4 M HNO3 the sequence is 2-ethylhexyl p-tolyl sulfoxide (4.41)>DEHSO (3.16)>di(pethylphenyl) sulfoxide (1.21)>di(2-octyl) sulfoxide (0.20) [11]. Cyclooctyl octyl sulfoxide [33] and cyclohexyl butyl sulfoxide [32] are in xylene less effective than dialkyl sulfoxides, while cyclohexyl hexyl sulfoxide and dicyclohexyl sulfoxide are more effective [15]. Also in xylene, 2-heptylthiophane S-oxide [30, 32, 15], 3-hexylthiophane S-oxide [32], 2-butyl-5-methylthiophane S-oxide [57], 2-propyl-4-isopropylthiophane S-oxide [32], 2(cyclohexylmethyl)thiophane S-oxide [30], 2-butylthiacyclohexane S-oxide [57], 2-pentylthiacyclohexane S-oxide [15], and 2-methyl-1-thiadecalin S-oxide [30] are more effective than dialkyl sulfoxides, while 2-thiadecalin S-oxide [30] and hexyl phenyl sulfoxide [15] are less effective. Petroleum sulfoxides are mixtures of thiaheterocyclic S-oxides and their composition, together with their extracting power, is dependent on their origin. For example, in kerosene, they may be more effective than dialkyl sulfoxides, as indicated in Fig. 2, or they may be comparably or less effective [43, 44, 46, 47]. In the pair DHxSO/2-heptylthiophane S-oxide the higher effectiveness of the latter is ascribed to the entropy factor [32]. The extraction ability of both cyclic and noncyclic sulfoxides is well correlated with their basicity, and that of noncyclic compounds is well correlated with the sum of the electronegativities of the substituent groups [15]. Surprising is the weak extraction ability of bifunctional extractants. One of them is bis(octylsulfinyl)methane, which at a concentration of 0.5 M in 1,1,2,2tetrachloroethane gives DU(VI) values of >1 only at >4 M HNO3. A maximum DU(VI) value of merely 5.5 is attained at 8.5 M HNO3 [58]. The other is phenyl-N,Ndibutylcarbamoylmethyl sulfoxide. In the extraction of initially 0.005 M U(VI) by a 0.3 M solution of the compound in toluene, the DU(VI) value increases from 0.09 at 1 M HNO3 to 1.4 at 6 M HNO3 [59]. Comparison with curve 4 in Fig. 2 shows that the compounds extracts U(VI) much less effectively than DEHSO. More effective is a trifunctional extractant, namely, bis(N,N-dioctylmethylcarbamoyl) sulfoxide. A 0.2 M solution in dodecane yields in the extraction of trace U(VI) the values DU(VI)=14 and 1.4 at 1 and 9.5 M HNO3, respectively [60]. Rough comparison with curve 3 in Fig. 2 indicates that the trifunctional compound is a more powerful extractant for U(VI) than DEHSO. In comparison with other extractant types, the ability to extract uranyl nitrate into benzene or CCl4 as a disolvate at a low concentration of HNO3 increases in the series TBP4 M HNO3 [45]. Similarly, with dodecane [39] and kerosene [28] diluents, TBP extracts U(VI) less

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effectively than DEHSO at <3.5 M HNO3, and more effectively at >3.5 M HNO3. PetrSO in kerosene extracts U(VI) more strongly than TBP at 1–10 M HNO3 [45]. The weakly basic DΦSO in benzene extracts U(VI) from 4.2 M HNO3 very ineffectively in comparison with TBP [66]. 4. Diluent and Modifier Effects The effect of diluent on the extraction of U(VI) is illustrated in Fig. 4 and, in more detail, in Table 3. The highest extraction efficiency is generally reached with benzene as diluent, and is the lowest with chloroform, which is assumed to form a hydrogen bond with the sulfoxide oxygen atom. Figure 4 also shows that in the extraction by DOSO and DEHSO the position of the maximum of the DU(VI) vs. [HNO3] dependence changes with the diluent nature. The phenomenon is further illustrated by data on the extraction of U(VI) by 0.25 M PetrSO. There the maximum appears at 3 M HNO3 with the aromatic diluents benzene and toluene, at 3.5–4 M HNO3 with heptane, cyclohexane and CCl4, at 5 M HNO3 with kerosene, and at 6 M HNO3 with chloroform [43].

Table 3 Diluent Effect in the Extraction of ≤0.004 M U(VI) from HNO3 Solutionsa

1 Columns 1–3:0.25 M PetrSO (from Ref. 43). Columns 4–6 (italics): 0.2 M DEHSO (from Ref. 42). Column 7:0.32 M PetrSO (from Ref. 46, 47). Column 8 (italics): 0.5 M DΦSO (from Ref. 61). Room temperature.

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Contrary to these reports, an insignificant or no influence of the diluent on the position of the maximum on the DU(VI) vs. [HNO3] dependency has been observed in two systems. In the extraction of U(VI) by DΦSO it shifts from ~6.2 M HNO3 with benzene to ~6.8 M HNO3 with chloroform [61]. In the extraction by DEHSO the maximum has been found at 3 M HNO3 with as many as 16 diluents of different nature, even if the DU(VI) vs. [HNO3] dependencies do intersect at lower and higher acid concentrations [42]. The consequence of this intersection is that different diluent orders of decreasing extraction efficiency are obtained at different acid concentrations. This is illustrated in Table 3. There is only a limited series of diluents for which common data for different extractants at a constant composition of the aqueous phases are available. Thus, a short general diluent order of decreasing extraction efficiency can be formulated, namely, benzene>toluene>cyclohexane>CCl >chloroform. Only one 4 doubly published source [46, 47] says that the extraction efficiency is higher with toluene than with benzene (see the last column in Table 3). A correlation between the DU(VI) value and properties of diluents can only be found within certain diluents classes or groups. Separate straight lines are obtained for groups of three alkane and three aromatic diluents, if log DU(VI) is plotted vs. an empirical polarity parameter or vs. with δd and δp being the dispersion and polar components of the solubility parameter δ of the diluent. The log DU(VI) vs. δ plot is parabolic and is fitted by seven diluents as different in the nature as hexane, cyclohexane, benzene, o-dichlorobenzene, nitrobenzene, nitromethane, toluene, and p-xylene. Five diluents (CCl4, benzene, chlorobenzene, toluene, and p-xylene) obey the linear correlation log (DU(VI)/sw) vs. the polarizability index, with sw being the water solubility in the diluent [42]. A polar modifier is sometimes added to the solution of a sulfoxide in an alkane diluent, in order to increase the solubility of extracted complexes and to prevent the formation of a third liquid phase. Alcohol modifiers can form a stable hydrogen bond with the sulfinyl group and suppress markedly the extraction. If 0.4 M DEHSO in dodecane contains 3, 5, and 20 vol% 2-ethylhexanol, the DU(VI) value in the extraction from 2 M HNO3 is lowered from 10.1 without modifier to 4.7, 2.16, and 0.88, respectively. With isodecanol the DU(VI) value is decreased to 4.2, 2.5, and 1.50, respectively [62]. Contrary to other metals, the extraction of U(VI) by DBSO and DiPSO in Solvesso 100 cannot be appreciably improved by water-miscible organic additives. The DU(VI) value is slightly increased by acetone and acetonitrile, and suppressed by dioxane, methanol, ethanol, and 1-propanol [29]. 5. Nature of the Extracted Complexes Based on slope analysis and saturation experiments, it is generally accepted that the principal extracted species is the anhydrous disolvate UO2(NO3)2 · 2B, as is

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clearly indicated in Fig. 3 in attaining high solvent loading. The disolvate has been found to be formed in the extraction of UO2(NO3)2 by • • • • • • • • • • • • • • • • • • •

DPSO to DNSO in TCE in the absence of HNO3 or from 1 M HNO3 [22, 24] and by DPSO to DLSO in CCl4 in the absence of HNO3 [31] DPSO and pentyl hexyl sulfide oxide in xylene from 0.1 M HNO3 [30] DHxSO in Solvessso 100 from 1 M HNO3 [26] DHpSO in CCl4 from 0 to 4 M HNO3 [35] DOSO in benzene from 1 M HNO3 [63] DOSO in xylene from 0.02 M HNO3+2.0 M NaNO3 [33], in CCl4 from 0.1 M HNO3+5 M NaNO3 [2], in toluene from 2 M HNO3 [37] DOSO in Solvesso 100 from 2 M HNO3 [38] DEHSO in dodecane from 1 M HNO3 [39], in heptane and benzene from 3.5 M HNO3 [40], in kerosene from 3.5 M HNO3 [28, 40], in toluene from 2 M HNO3 [37, 41], in xylene from 0.02 M HNO3+2.0 M NaNO3 [33] Di(2-octyl) and di(3-octyl) sulfoxides in xylene from 0.02 M HNO3+2.0 M NaNO3 [33] Hexyl octyl sulfoxide [64] and octyl dodecyl sulfoxide [65] in toluene from 2 M HNO3 Octyl decyl sulfoxide in CCl4 from 2 M HNO3 [48] Dodecyl tolyl sulfoxide [37] and 2-ethylhexyl tolyl sulfoxide [37, 56] from 2 M HNO3 DΦSO in benzene from 4.2 M HNO3 [66] and from 7 M HNO3 [61], in toluene from 2 M HNO3 [37] and from 7 M HNO3 [61], and in CCl4 and chloroform from 7 M HNO3 [61] PetrSO in heptane and cyclohexane from 2 M HNO3 [43] PetrSO in kerosene from 2.1 and 3 M HNO3 [45], from 2 M HNO3 [43, 44], from 2 M HNO3+0.1 M NH4NO3 [46, 47] PetrSO in CCl4 [43, 48, 49] and chloroform [43] from 2 M HNO3 PetrSO in toluene [37, 43] and benzene [43, 67] from 2 M HNO3 Bis(hexylsulfinyl)methane [68] and bis(octylsulfinyl)methane [58] in 1,1,2,2tetrachloromethane from 5 M HNO3 Phenyl-N,N-dibutylcarbamoylmethyl sulfoxide in toluene from 3 M HNO3 [59]

A more sophisticated evaluation of distribution data, including introduction of aqueous activity coefficients, implies that small fractions of U(VI) exist as trisolvates in solutions of DHESO in dodecane. They are the anhydrous species UO2(NO3)2 · 3B and the hydrated species UO2(NO3)2 · 3B · xH2O, in which the value of x cannot be determined and may be 1–5 [17]. In contrast, based on the evaluation of data from Ref. 24, DPSO to DNSO are said to form in TCE a disolvate and a monosolvate, the fraction of the latter being up to 24% [69]. 2-(Cyclohexylmethyl)thiophane S-oxide (0.7 M in xylene) forms a monosolvate of uranyl nitrate. 2-Heptylthiophane S-oxide is indicated by slope analysis to form

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Table 4 Thermodynamic Values of the Extraction of U(VI) and Pu(VI) in the Form of the Complex UO2(NO3)2 · 2B

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Table 4 Continued

a b

With 4 M NH4NO3 also present. With 0.1 M NH4NO3 also present.

a monosolvate and by saturation experiment to form a disolvate at ⱕ0.33 M U(VI) in the organic phase. The discrepancy is explained by dimerization of the extractant and the extracted species is written as UO2(NO3)2 · B2. The species polymerizes up to [UO2(NO3)2–B2]4 when the solvent loading is increased to 0.58 M U(VI). 2Methyl-1-thiadecalin S-oxide forms a mono- or disolvate at <0.3 M U(VI) in the organic phase and large polymers at higher loading [30]. DOSO in benzene was said to be able to extract hydrolysis products of UO2(NO3)2 [70], but no convincing evidence was given. Infrared spectra show that the uranyl ion is bound directly to the oxygen atom of the sulfoxide group. The respective shift of the band of the SO group is from 1030 to 970 cm-1 with DHpSO in benzene and CCl4 [35], and from 1044 to 1030 cm-1 with PetrSO in kerosene [45]. The uranyl ion is bound to the O atoms of both the carbonyl and sulfinyl groups of the bifunctional extractant phenyl-N,Ndibutylcarbamoylmethyl sulfoxide [59]. 6. Thermodynamic Functions Published values are gathered in Table 4. They were determined without considering side equilibria, with the exception of the extraction of U(VI) [72, 73] and Pu(VI)

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[71], where nitrate complexing in the aqueous phase and the extraction of HNO3 together with the extracted metal have been taken into account. Activity coefficients have been introduced only in one work [32]. It is a fortunate circumstance that many values in Table 4 have been measured for U(VI) at 2.0 M HNO3 with various sulfoxides and at 7.0 M HNO3 with DΦSO, so that they can give some idea about the compatibility of information from various sources, about the effect of variables on the thermodynamics of the extraction reaction, and about the effect of temperature on the distribution equilibrium. Data on Pu(VI) are given in Table 4 to the full reported extent for different concentrations of DEHSO and nitric acid, to show that they do not indicate a systematic variation of ∆H and ∆S with the two variables.

C. Tetravalent Metals 1. Distribution Data and Extraction Rate Less attention has been devoted to Th(IV) than to U(VI), and still less to tetravalent actinides, fission products, and Hf(IV). A survey of selected sources of distribution data is given in Table 5. Irregular extraction rate has been observed in the extraction of Th(IV) by 30%(v/ v) DBSO in xylene from 2 M HNO3, indeed at an unspecified mode of shaking. The DTh reaches a maximum value of 14.5 after ~7 min shaking time, then it sinks and attains a time independent value of 12.3 after 25 min [74]. The authors explain it by nitric acid being extracted more slowly than Th(IV), and displacing a part of already extracted Th(IV) from the organic phase. The explanation assumes a too slow extraction of HNO3 and could only be accepted if the extraction rates of Th(IV) and HNO3 are directly measured under the specified conditions. DTh values found after a shaking time of 5 min are taken as relevant in Ref. 74, but use of the time independent values would be more reasonable. 2. Effect of Extraction Variables As in the extraction of U(VI), the DM(IV) vs. [HNO3] dependences pass through a maximum. Examples are given in Fig. 5, where data on U(VI) are shown for comparison. It is seen that the form and the position of the maximum depends on the extracted metal. Influence of the nature of system components cannot be clarified, due to a broad variety of conditions in the work of different authors. The maximum on the curve for Pu(IV) in Fig. 5 is noticeably broad, whereas a narrow maximum at 5-6 M HNO3 has been found in the extraction by 0.2 and 1.1 M DEHSO in dodecane [77, 78]. A maximum at ~11 M HNO3 is indicated in the curve for Zr(IV) in Fig. 5, but no maximum has been observed up to 15 M HNO3 in the extraction by 0.2 M DOSO in CCl4 [80]. The DHf(IV) value passes a maximum at 8 M HNO3 in the extraction by 50% DBSO in cyclohexane [81]. A double

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Table 5 Survey of Data on the Distribution of Th(IV), Np(IV), Pu(IV), Zr(IV), and Hf(IV) Between Solutions of Common Sulfoxides and Aqueous Nitrate Solutionsa

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Table 5 Continued

a

Single temperature value applies to the measurement of isothermal concentration dependencies of DM , a temperature range to the measurement of temperature dependency. rt, room temperature.

(IV)

maximum is perceivable on the DNP(IV) vs. [HNO3] dependencies shown in Fig. 6. It has not been explicitly considered in the original source [25], but the slight deepening between the maxima can be true and it has been followed in this review in the smoothing of the data in Fig. 6. If it is true, it could be plausibly ascribed to partial oxidation of Np(IV) and subsequent disproportionation of Np(V) at higher HNO3 concentrations.

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Figure 5 Effect of HNO3 on the extraction of M(IV) and uranyl nitrates. Initially, 0.086 M Th(IV) by 1 M DHpSO in TCE [22] and trace metals by 0.2 M DHxSO in Solvesso 100 [26].

Contrary to the extraction from HNO3 solutions, the DTh(IV) value increases monotonously with the nitrate ion concentration, if it is partly adjusted by sodium nitrate. In the extraction of Th(IV) by 30% DBSO in xylene the DTh(IV) value decreases from 17.2 at 2 M HNO3 to 9.5 at 8 M HNO3. With 2 M HNO3+variable NaNO3 the DTh(IV) value increases from 17.2 at 2 M HNO3 to 23.0 at 2 M HNO3+ 3.8 M NaNO3 [74].

Figure 6 Effect of HNO3 on the extraction of Np(NO3)4 by 0.2 M sulfoxides in Solvesso 100. Room temperature. (From Ref. 25.)

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The efficiency of the Th(IV) extraction by dialkyl sulfoxides was clearly shown to decrease in the order all in TCE and with no HNO3 added to the system [22]. The effect of alkyl branching is ambiguous, as indicated by the order DEHSO>di(2-octyl) sulfoxide>DOSO~ cyclooctyl octyl sulfoxide, all in xylene and with 0.02 M HNO3+4 M NaNO3 as the aqueous phase [33]. The effect of the sulfoxide structure can be dependent on the HNO3 concentration, as shown by the extractant (DTh) sequences DEHSO (0.67) >di(p-ethylphenyl) sulfoxide (0.44)>di(2-octyl) sulfoxide (0.12)>2-ethylhexyl ptolyl sulfoxide (0.094) at 1 M HNO3 and di(2-octyl) sulfoxide (19.6)>DEHSO (1.31)>2-ethylhexyl p-tolyl sulfoxide (0.59)>di(p-ethylphenyl) sulfoxide (0.1) at 4 M HNO3 (al in xylene) [11]. 2-Heptylthiophane S-oxide is more efficient than dialkyl sulfoxides [75]. Pu(IV) is very effectively extracted by bis(N,N-dioctylcarbamoylmethyl) sulfoxide. A 0.2 M solution of the compound in dodecane yields at 1.5-9 M HNO3 DPu(IV) values as high as 100–220, with a maximum attained at 4 M HNO3. So Pu(IV) is extracted selectively over U(VI), the separation factor ␣Pu(IV)/U(VI) being 7, 19, and 63 at 1, 5, and 9 M HNO3, respectively [60]. A dependence of the structure effect on the acid concentration was found also in the extraction of Zr(IV). DEHSO was more efficient than di(2-octyl) sulfoxide at 1 M HNO3, but it is less efficient at 4 M HNO3 [11]. To compare with other extractant types, the ability to extract Th(IV) nitrate at a low HNO3 concentration into benzene as a trisolvate increases in the series TBP< DOSO<2-nonylpyridine N-oxide [23]. TBP extracts Th(IV) less effectively than DEHSO in kerosene at<2 M HNO3, but more effectively at 2–7 M HNO3 [28]. Little is known about the diluent effect. The efficiency of the extraction of Hf(IV) by 50% DBSO from 8 M HNO3 decreases in the diluent (DHf) order cyclohexane (12.5)>xylene (9.5)~toluene (9.3)>CCl4 (3.4)>benzene (2.2) [81]. The extraction of Pu(IV) is strongly suppressed by alcohol modifiers which may be used to prevent the formation of a third liquid phase. If 0.4 M DEHSO in dodecane contains 3, 5, and 20 vol% 2-ethylhexanol, the DPu(IV) value in the extraction from 2 M HNO3 is lowered from 9.0 without modifier to 4.1, 2.1, and 0.61, respectively. With isodecanol the DPu(IV) value is decreased to 3.8, 2.0, and 0.85, respectively [62]. To be noticed is the effect of water-miscible organic additives to the aqueous phase. As observed in the extraction of trace Pu(IV) by 0.2 M DEHSO in dodecane, the effect of organic additives is the highest at 20 vol%, and such an addition to the aqueous phase essentially changes the form of the DPu(IV) vs. [HNO3] dependence. The round maximum at 5 M HNO3 is changed to a very sharp one at 2.1 M HNO3, and the DPu(IV) value at this acidity is increased by a factor of 3.9 by acetonitrile, 3.2 by acetone, 2.6 by ethanol, 2.3 by methanol, and 1.8 by dioxane. Propanol suppresses the DPu(IV) value [78]. An unlike effect was found in the extraction by DBSO and DiPSO in Solvesso 100 at 20–50% additive. Also at 2 M HNO3, with 0.2 M DBSO the DPu(IV) value is changed by a factor of 1.4–2.2 by acetone, 1.6–2.9 by acetonitrile, 0.73–0.59 by dioxane, 1.0–0.57 by methanol, 1.3–0.52 by ethanol,

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and 0.43–0.21 by 1-propanol. With 0.2 M DiPSO the factors are 3.2–4.2 by acetone, 3.2–3.8 by acetonitrile, 1.7–1.1 by dioxane, 2.5–0.76 by methanol, 2.4–1.2 by ethanol, and 0.82–0.37 by 1-propanol [29]. 3. Nature of Extracted Complexes There is evidence by slope analysis or indication by saturation experiments for the formation of disolvates of the type M(NO3)4 · 2B, with few exceptions for Th(IV) and Zr(IV). In the case of Zr(IV) the disolvates could also possibly be Zr(NO3)2(OH)2 · 2B or Zr(NO3)4 · 2HNO3 · 2B. The disolvates have been found in the extraction of • • • • •

Th(IV) by DBSO in xylene from 2 M HNO3 [74], DPSO in TCE [22] and DOSO in benzene in the absence of HNO3 [76], DEHSO in kerosene from 3.5 M HNO3 [11, 28], and PetrSO in kerosene from 3 M HNO3 [46, 47] Np(IV) by DiPSO, DHxSO, and DOSO in Solvesso 100 from 2 M HNO3 [25] Pu(IV) by DHxSO [26] and DOSO [38] in Solvesso 100, and DEHSO in dodecane [78], all from 2 M HNO3 Zr(IV) by DHxSO in xylene from 4 M HNO3 [75] and in Solvesso 100 from 2 M HNO3 [26], and DOSO in CCl4 from 3.7–7.9 M HNO3 [80] Hf(IV) by DBSO in cyclohexane from 8 M HNO3 [81]

Contrary to these reports, • • •

A trisolvate of the type Th(NO3)4 · 3B was found to be extracted by DOSO, DEHSO, di(2-octyl) sulfoxide, and cyclooctyl octyl sulfoxide from 0.02 M HNO3+4 or 6 M NaNO3 [33]. A trisolvate of the type Hf(NO3)4 · 3B was found to be extracted by DPSO in CCl4 from 9 M HNO3 [82]. A monosolvate of the alleged composition ZrO(NO3)2 · B was found in the extraction of Zr(IV) by DBSO from 7 M HNO3 [79].

A slight effect of the Zr(IV) concentration on DZr values was observed in the extraction of Zr(IV) by 40% DBSO in xylene from 7 M HNO3. The DZr value increased from 2.8 at 5.5×10-4 M Zr(IV) merely to 6.7 at 0.055 M Zr(IV) [79]. The experimental points scattered considerably and the increase was not an unambiguous proof of partial self-association of the extracted Zr(IV) complex. The DHf values decrease with increasing Hf(IV) concentration in the extraction by 1 M DHpSO in TCE from 7–10 M HNO3. At 10 M HNO3 the DHf value is lowered from ~1 to ~0.1 when the initial aqueous Hf(IV) concentration is increased from 5.6×10-4 to 0.017 M [34]. Thus, the suppression cannot be ascribed to a decrease of the free extractant concentration. The phenomenon was not commented on in the original source and, without a more detailed study, it does not suffice for evidencing dimerization of Hf(IV) in the aqueous phase.

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The Zr4+ ion is bound to the O atom of the SO group: The respective band of DOSO in CCl4 is shifted by Zr(IV) from 1032 to 934 cm-1 [80]. 4. Thermodynamic Functions Published values are gathered in Table 6. In Refs. 73, 74 they were determined from the dependence of the concentration equilibrium constant of the extraction on 1/T, in other work from the temperature dependence of the distribution ratio of M(IV). The nitrate complexing of M(IV) in the aqueous phase and the extraction of HNO3 together with M(IV) were taken into account in Refs. 72, 73.

D. Miscellaneous Metals 1. Distribution Data Selected sources of distribution data are given in Table 7. It is seen that little attention has been paid to the extraction of trivalent, pentavalent, and bivalent elements. Nb(V) and Ru, the latter in an unspecified valency state, are weakly extracted by 0.2 M DEHSO in kerosene from 0.7 to 4 M HNO3. The maximum values are DNb(V)~0.004 at ~3 M HNO3 and DRu(?)~0.012 at ~1.5 M HNO3 [11]. The good extractability of Pd(II) by DEHSO and the weak extractability of a series of metals by DBSO is shown in Fig. 7 where the extraction of Th(IV) is shown for comparison. Fe(II) and Al(III) are slightly less extractable than Pb(II), the extractability of Zn(II) and Mn(II) is similar to that of Ni(II), the extractability

Table 6 Thermodynamic Values of the Extraction of M(IV) in the Form of the Complex Th(NO3)4·2B

a

With 4 M NH4NO3 also present.

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Table 7 Survey of Data on the Distribution of Miscellaneous Metals Between Solutions of Sulfoxides and Aqueous Nitrate Solutions

of Mg(II) and Ca(II) is similar to that of Sr(II), and Na(I) and K(I) are still less extractable than Sr(II). Fe(III) is moderately extracted by 1 M DHpSO in TCE, with DFe(III) increasing from 0.017 at 2 M HNO3 to 1.0 at 8 M HNO3 [34]. 2. Effect of Extraction Variables Am(III) and lanthanides(III) are so weakly extractable by monofunctional sulfoxides that relevant extraction efficiency is attained only at high concentrations of salting out agents. These must be salts of nonextractable metals, such as alkali or alkaline earth nitrates. Nitric acid cannot be used as a salting-out agent, because it strongly suppresses the extraction. For example, in the extraction of Am(III) by 0.4 M

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Figure 7 Extraction of various metals (initially <0.001 M) from HNO3 solutions, Pd(II) by 0.2 M DEHSO in toluene [85], other metals by 30 vol% DBSO in xylene [74].

DEHSO in dodecane from 3 M Ca(NO3)2 the DAm(III) value is lowered from 1.1 at 0.01 M HNO3 to ~0.01 at 0.8 M HNO3 [83]. Optimally the elements are extracted in a not too acid pH region; e.g., a maximum DLa value is attained at pH ~2.5 in the extraction by 0.1 M DOSO in benzene from 6 M LiNO3. Self-salting out is inefficient. For example, with 0.5 M DOSO in benzene an organic concentration of 0.155 M La(III) is attained, if 3 M La(NO3)3 is present in the aqueous phase together with 4 M LiNO3 (the organic phase is then practically saturated with La(III); see below). Without LiNO3 present, the organic concentration is only 0.095 M La(III); i.e., the solvent is saturated to 57% only [84]. As seen in Table 7, LiNO3 is most frequently used as a salting-out agent. Another agent, Ca(NO3)2 at rather high concentrations, supports the extraction of Am(III) by 0.4 M DEHSO in dodecane at 0.03 M HNO3 quite markedly indeed. The DAm(III) value is <0.01 in the absence of Ca(NO3)2, and is increased to 0.03, 0.93, 5.34, 40.6, and 163 at 1.5, 3, 4, 5, and 6 M Ca(NO3)2, respectively [83]. The unsymmetrical ethyl octyl sulfoxide in xylene extracts Y(III), La(III), Gd(III), and Lu(III) from 5 M LiNO3 at pH 2 somewhat more effectively than the symmetrical analogue DHxSO. DHxSO extracts the elements slightly less effectively than DOSO, but the difference is too small to indicate a trend. Clearly visible is the effect of branching. As shown in Fig. 8, the effectiveness of the extraction of all lanthanides(III) but Pm(III) (not studied) decreases in the sequence DOSO>DEHSO>di(2-octyl) sulfoxide>di(3-octyl) sulfoxide [16]. A very similar picture was obtained in the extraction by 0.5 M DOSO, DEHSO, and di(2-octyl) sulfoxide in xylene from 0.28 M HNO3+6 M LiNO3 [7]. By the way, data in Fig. 8 show the tetrad effect to a limited extent only, and data given in Ref. 7 do not do it much more visibly.

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Figure 8 Extraction of Y(III) and lanthanides(III) by 0.4 M isomeric dioctyl sulfoxides in xylene. Curve 1—DOSO; 2—DEHSO; 3—di(2-octyl) sulfoxide; 4—di(3-octyl) sulfoxide. Initial aqueous phase: all elements (each 0.005 M), 5.0 M LiNO3, pH 2.0. 20°C. (From Ref. 16.)

Bifunctional bis(octylsulfinyl)methanes and 1,2-bis(octylsulfinyl)ethanes with various octyl isomers are much more effective extractants for lanthanides(III) than the corresponding dioctyl sulfoxides. The ethane derivatives are more efficient than the methane derivatives, and within each extractant group the extraction ability for Eu(III) decreases in the octyl order n-octyl>2-ethylhexyl>2-octyl (0.2 M extractants in chloroform, initially 0.06 M Eu(III)+6 M NH4NO3+0.3 M HNO3). The bi-functional compounds give lower separation within the lanthanide(III) group than dioctyl sulfoxides [7]. High extraction efficiency for Am(III) is exhibited even in the absence of a salting out agent by the trifunctional extractant bis(N,N-dioctylcarbamoylmethyl) sulfoxide. The DAm(III) value increases from 6.3 at 3 M HNO3 to 28 at 9 M HNO3 (0.2 M extractant in dodecane) [60]. The position of sulfoxides among diverse extractants for Tm(III) is exemplified by two sets of data. They imply that not only the basicity of the oxygen donor atom but also the diluent and the ionic medium in the aqueous phase can influence the order of increasing extraction ability. The extraction of Tm(III) nitrate into CCl4 from 5.9 M Al(NO3)3 at pH 1.0 increases in the order trioctylamine N-oxide
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The extraction of Co(II) nitrate in the absence of nitric acid increases in the order TBPnitrobenzene (1.20)>benzene (0.88)>toluene (0.73)>p-xylene (0.56)>o-dichlorobenzene (0.39)>dodecane (0.22)>CCl4 (0.04)>chloroform (0.02). No simple relation can be found between the DAm(III) value and the polarizability or dipole moment of the diluent. To a limited extent it is true that the DAm(III) value is high if both the polarizability and the dipole moment are low [83]. Compared with the extraction of U(VI) by DEHSO (Table 3), it is noteworthy that the highest effectivity of the extraction of Am(III) is attained with nitromethane and nitrobenzene as diluents. When Pd(II) is extracted by 0.2 M DEHSO from 2 M HNO3, the diluent (DPd(II)) order is toluene (13.4)>dodecane (12.0)>Solvesso 100 (10.0)>CCl 4 (9.2)> cyclohexane (8.0)>benzene (7.4) ~o-dichlorobenzene (7.3)>1,2-dichloroethane (5.0)>xylene (3.74)>chloroform (1.96) [85]. The extraction of Am(III) is enhanced by adding a water-miscible organic additive. In the extraction by DEHSO in dodecane and at additive concentrations of 30–40 vol%, the extent of the enhancement decreases in the additive order acetonitrile >acetone>methanol>dioxane>ethanol. Propanol suppresses slightly the extraction [83]. Similar results were obtained with DiPSO and DOSO [86]. 3. Nature of the Extracted Complexes Trisolvates of the type M(NO3)3·3B were found in the extraction of • • • •

Y(III), La(III), Gd(III), and Lu(III) by DHxSO, ethyl octyl sulfoxide, DOSO, DEHSO, di(2-octyl) sulfoxide, di(3-octyl) sulfoxide and cyclooctyl octyl sulfoxide in xylene from 5 M LiNO3 at pH 2 [16] Nd(III) and Tm(III) by DOSO in CCl4 from 6 M LiNO3 at pH 3.0 [84] La(III) by DOSO and DEHP in probably xylene from an unspecified acidic LiNO3 solution [7] Am(III) by DEHSO in dodecane from 0.03 M HNO3+3 M Ca(NO3)2 [83]

Disolvates of the type M(NO3)3·2B or M(NO3)2·2B were found in the extraction of • • •

La(III) by 1,2-bis(2-octylsulfinyl)ethane in xylene from an unspecified solution [7] Pd(II) by DEHSO in dodecane, benzene, and toluene from 2 M HNO3 [85] Co(II) by DOSO in benzene from Co(NO3)2 [87]

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Some fraction of M(NO3)3·4B may be present in the extraction of La(III) by DHxSO, ethyl octyl sulfoxide, DEHSO, di(2-octyl) sulfoxide, and di(3-octyl) sulfoxide, of Gd(III) by di(2-octyl) sulfoxide, and of Y(III) and Lu(III) by di(2octyl) sulfoxide and di(3-octyl) sulfoxide, all in xylene. A nitrate to M(III) ratio has been determined as 3.0±0.2 in the extraction of Nd(III), Er(III), and of Y(III) by DHxSO in xylene [16]. A tetrasolvate of the type Co(NO3)2·4B is extracted by DOSO in heptane, CCl4, and chloroform [87]. The water content of the organic phase loaded with lanthanides(III) was reported in Ref. 7 but, unfortunately, the composition of the phase was not given and no picture of the water to lanthanide(III) or water to extractant ratios can be obtained. The average water content decreases in the order bis(2-octylsulfinyl) methane>DOSO>DEHSO>di(2-octyl) sulfoxide. An intensive water band was also found in the infrared spectrum of the solid complex of 1,2-bis(2-ethylhexylsulfinyl) ethane with Pr(NO3)3, and it was suggested that the band belonged to coordinated water [7]. Ir spectra show that the trisolvate Nd(NO3)3·3B with B=DOSO is anhydrous in CCl4. A shift of the band of the SO group from 1050 to 1002 cm-1 is evidence that the Nd3+ ion is bonded to the O atom of the SO group [84]. Contrarily, the Pd2+ ion is bound to the S atom of DEHSO, as substantiated by a 190 cm-1 shift of the S=O stretch toward higher frequency [85].

III. EXTRACTION FROM CHLORIDE MEDIA A. Hydrochloric Acid HCl is generally weakly extractable by symmetrical sulfoxides. For example, the concentration of HCl in 0.1 M DHxSO in benzene is 8×10-4 and 0.062 M after contact with 4 and 10 M HCl, respectively [20]. A HCl concentration of ~3×10-4 M is attained in 0.02 M ethyl dodecyl sulfoxide in xylene after contact with 6 M HCl. The extraction of HCl is visibly weaker than that of HClO4 and much weaker than that of HNO3 [3]. 0.1 M methyl 4,8-dimethylnonyl sulfoxide in p-xylene extracts HCl rather strongly. An acid to sulfoxide ratio of 1.7 is attained in the organic phase at 9 M HCl. Formation of a sesquisolvate or of a mixture of mono- and disolvate is indicated at 1–8 M HCl, but the picture may be obscured by a high solubility of the extractant in the aqueous phase (the DB value is as low as 4 at 8 M HCl) [8].

B. Hexavalent and Pentavalent Metals 1. Distribution Data U(VI) and Pa(V) belong to often studied metals, even if chloride solutions are not compatible with the anticipated use of sulfoxides in nuclear processes where a

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nitrate medium is preferred. A survey of important sources of distribution data is given in Table 8. 2. Effect of Extraction Variables Hydrochloric acid is the most frequently used salting-out agent. Figures 9 and 10 illustrate different forms of the DM vs. [HCl] dependencies, showing that many of them exhibit a maximum at different HCl concentrations. In the extraction of U(VI)

Table 8 Survey of Data on the Distribution of Hexa- and Pentavalent Metals Between Solutions of Sulfoxides and Aqueous Chloride Solutions

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Figure 9 Effect of HCl on the extraction of trace Pa(V) and U(VI) by various sulfoxides. Curve 1—Pa(V) by 0.25 M DOSO in CCl4; 2—Pa(V) by 0.25 M DPSO in CCl4; 3—U(VI) by 0.25 M DPSO in CCl4; 4—U(VI) by 0.25 M DOSO in CCl4; 5—U(VI) by 0.25 MDDSO in CCl4. Room temperature. (From Ref. 88.)

Figure 10 Effect of HCl on the extraction of Cr(VI), Mo(VI), and W(VI) by various sulfoxides. Curve 1—initially 0.001 M Cr(VI) by 0.1 M DBSO in benzene [50]; 2—trace Mo(VI) by 10% PetrSO in toluene [90]; 3—initially 0.0005 M Mo(VI) by 0.2 M DOSO in benzene [76]; 4—initially 0.0008 M W(VI) by 0.2 M DOSO in benzene [76]; 5—trace W(VI) by 10% PetrSO in toluene [90]. Room temperature.

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by DPSO to DDSO the maximum occurs at 7–9 M HCl. It is slightly shifted toward higher HCl concentrations with increasing alkyl length [88]. The distribution ratios increase continuously with the HCl concentration (0.5–10 M) in the extraction of Nb(V) and Ta(V) by 0.5 M DOSO in benzene. Nb(V) is extracted preferably to Ta(V) at 3–10 M HCl [76]. To view the effect of the extractant structure, DU(VI) and Dpa(v) values were measured with DHxSO to DDSO in CCl4 at 7 M HCl and plotted vs. the molar mass of the five sulfoxides. They lie on a curve with a maximum at DOSO [93]. However, more extensive data on U(VI) are given in Ref. 88 and they show that this is a simplified picture. In fact, the DU(VI) vs. [HCl] dependences for DPSO to DDSO do not reveal any simple trend in the influence of the sulfoxide alkyl chain length. Three of the curves are shown in Fig. 9. The only regularity is that the position of the maximum is shifted to higher HCl concentrations at increasing chain length. However, the height of the maximum decreases in the order DOSO>DPSO>DNSO>DHpSO>DDSO>DHxSO. Also the width of the maxima changes with the chain length and, thus, the dependencies cross to the left and right of the maxima and different relations between the extraction efficiency and the chain length are found at different HCl concentrations. An obvious order can be seen only at 4-6 M HCl, namely DPSO>DOSO>DHxSO ~DHpSO~DNSO~ DDSO. TBP extracts U(VI) from HCl less efficiently than DPSO and DOSO in CCl4 [94], and the order DOSO>DPSO>TBP (all in CCl4) has been reported for Pa(V) [88]. Also, Mo(VI), W(VI), Nb(V), and Ta(V) are extracted by TBP more weakly than by DOSO, both in benzene [76]. The diluent effect has been investigated in the extraction of U(VI) by 0.25 M DPSO from 3 M HCl. The DU(VI) value decreases in the diluent (DU(VI)) order benzene (1.23)>toluene (0.85)>xylene (0.67)>CCl 4 (0.63)>butyl acetate (0.28)> cyclohexanol (0.19)>1-pentanol=benzyl alcohol (0.15). A linear relationship between the logarithms of DU(VI) and the dielectric constant of the diluent is fairly well followed [94]. The same relationship has been found in the extraction of Pa(V) by DPSO from probably 3 M HCl, but for some of the studied diluents only. Here the diluent order is CCl4>CS2>chloroform>chlorobenzene>1-pentanol>cyclohexanol (Dpa(V) values were not given), and xylene, benzene, toluene and 1,2dichloroethane deviate from the line [88]. 3. Nature of the Extracted Complexes and Extraction Enthalpy The slope of the log DU(VI) vs. log aHCl dependence has been found to be 3.2 with DPSO [94] and 2.0 or 2.2 with DOSO [2, 94], both in CCl4. Thus, U(VI) has been suggested to be extracted as a solvated trichloride species by DPSO but as solvated dichloride by DOSO [94]. Trisolvates are the most frequently reported extracted

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species but, in some cases, different extracted species have been found by different authors even in identical systems. To summarize the reports, the species extracted are • • • • • • •

UO2Cl2 · 2HCl · 2B by DPSO in CCl4 from 3 M HCl [94] UO2Cl2 · 3B together with UO2Cl2 · HCl · 3B by DPSO in CCl4 from 3 M HCl [88] UO2Cl2 · 3B by DHxSO and DHpSO in CCl4 from 3 M HCl [88] UO2Cl2 · 3B by DOSO in CCl4 from 3 M HCl [88, 94] UO2Cl2 ·3B by DOSO in CCl4, heptane, and benzene from weakly acidic solutions of UO2Cl2 [87] UO2Cl2 · 2B by DOSO in chloroform from weakly acidic solutions of UO2Cl2 [87] UO2Cl 2 · 3B by DNSO and DDSO in CCl4 from 3 and 5 M HCl [88], respectively

Polymerized complexes at >0.006 M U(VI) in the organic phase are presumed to be extracted by 0.5 M DPSO in CCl4 from 3 M HCl [94]. The Cr(VI) complex formed in the extraction by DBSO in benzene possibly embodies a chloride ion [50]. The log Dpa(V) vs. log aHCl dependence in 5 M HCl+variable LiCl has a slope of ~3 with DPSO in CCl4 and DΦSO in benzene [92]. Probably solvated trichloride species such as PaOCl3 or Pa(OH)2Cl3 are extracted, even if a Dpa(v) value based on the thermodynamic activity of Pa(V) in the aqueous phase should be plotted vs. aHCl. A trisolvate PaOCl3 · 3B is extracted by DOSO [88, 92] and DΦSO [92] in CCl4 from 7 or 8 M HCl. DPSO in CCl4 is reported to extract a mixture of a disolvate and a trisolvate from 8 M HCl [92] or only a trisolvate from 7 M HCl [88]. It has been substantiated by infrared spectra that the uranyl ion is bound to the O atom of DOSO. The S→O frequency is 1051 cm-1 in the spectrum of solid UO2C12 · 2B, but it is 925 cm-1 in pure DOSO [2]. Enthalpy of the extraction of a small amount of U(VI) by 0.7 M DOSO in xylene from 3.0 M HCl is -13.4 kJ mol-1 [94].

C. Tetravalent Metals 1. Distribution Data A survey of important sources of distribution data is given in Table 9. As implied by the table, rather little interest has been paid to the subject. Th(IV) is weakly extractable. Distribution ratios of 0.007–0.037 have been observed in the extraction of Th(IV) by 30 vol% DBSO in xylene from 1–9 M HCl [74]. In the extraction by 0.1 M DOSO in chloroform, the DTh value is <0.01 at 8 M HCl and still lower at 9–11 M HCl [96].

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Table 9 Survey of Selected Data on the Distribution of Tetravalent Metals Between Solutions of Sulfoxides and Aqueous Chloride Solutions

In the extraction of U(IV) by 0.05 M DOSO in toluene the distribution ratio does not change with the contact time between 4 and 30 min at 7 M HCl. However, at 9.87 M HCl it falls from 64 after 4 min contact to 0.18 after 78 min, because DOSO decomposes at >8 M HCl [13]. The rate of the extraction of Pt(IV) by 0.4 M PetrSO in kerosene from 2 to 6 M chloride ions was studied in a stirred cell at 4–50°C. The rate is controlled by the

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diffusion of the complex H2PtCl6 · 2B · 6H2O from the interface into the bulk organic phase [101]. 2. Effect of Extraction Variables There is an appreciable effect of the extractant structure on the extraction of Pt(IV). Branching suppresses the extraction efficiency in the order DOSO>DEHSO>di(2octyl) sulfoxide>di(3-octyl) sulfoxide, and the introduction of a ring substituent suppresses it in the order DHxSO>cyclohexyl hexyl sulfoxide>dicyclohexyl sulfoxideⰇDΦSO, both orders observed at 6 M HCl [100]. The structure effect is further illustrated by extensive results by DHxSO and asymmetrical sulfoxides (see Fig. 11). The effect of branching is not clear with butyl branched-alkyl sulfoxides, where the extraction ability decreases in the order of the varied alkyl group 2-ethylhexyl>3,5,5-trimethylhexyl>octyl. Notice that at the same number of C atoms DHxSO is more effective than butyl octyl sulfoxide. With 2-tolyl as the invariant group, the extraction ability decreases with increasing branching in the varied group order octyl>2-ethylhexyl>3,5,5-trimethylhexyl. Replacement of butyl as the invariant group by 2-tolyl suppresses the extraction ability much more than any variation of an alkyl group. TBP extracts Th(IV) chloride less effectively than DPSO and DOSO, but slightly more effectively than DΦSO (CCl4 diluent) [102]. The efficiency of the Pt(IV) extraction from 1–6 M HCl decreases in the sequence tributylphosphine oxide>Nbutyl octanamide>DHxSO>dibutyl butylphosphonate>N,N-dibutyl octanamide [100].

Figure 11 Extraction of Pt(IV) by 1 M DHxSO and asymmetrical sulfoxides in xylene. Initially, 0.005 M Pt(IV) in the aqueous phase, room temperature. (From Ref. 99.)

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The efficiency of the Th(IV) extraction by 0.25 M DPSO from 7 M HCl decreases in the diluent (DTh) order CS2 (0.34)>CCl4=cyclohexane=benzene (all 0.29)>toluene (0.22)~chlorobenzene (0.21)>1,2-dichloroethane (0.086)>1-pentanol (0.033). Log DTh was suggested to be a linear logarithmic function of the diluent dielectric constant [102], but the plot is not very convincing. In the extraction of Zr(IV) by 0.25 M DPSO from 7 M HCl the sequence is benzene (34.0)>TCE (31.3)>chloroform (7.2)>benzonitrile (1.92)>1-pentanol (1.53) [97]. Finally, in the extraction of Hf(IV) by 0.1 M DPSO from 7 M HCl the diluent order is nitrobenzene (DHf=121)>benzene>toluene>xylene>CCl4>chloroform>m-cresol (DHf=0.002) [103]. 3. Nature of Extracted Complexes and Extraction Enthalpy Th(IV) was said to be extracted by DPSO as a pentachloro species, but as a tetrachloro species by DOSO. This was based on a rather ambiguous evaluation of curved log DTh vs. log aHCl dependencies. The limiting slopes have been interpreted as 5.0 with DPSO and as 4.0 with DOSO, both in CCl4, although they could be 4 and 3, respectively, as well. The slopes of the logarithmic dependencies of DTh on the activities of the Cl- and H+ ions (assuming them to be separable) in HCl+NH4Cl mixtures are 5.0 and 1.16, respectively, with DPSO. Thus, a pentachloro and a tetrachloro species have been suggested to be extracted by DPSO and DOSO, respectively [102]. On the other hand, it was concluded that Hf(IV) was extracted as a tetrachloro species by DPSO in CCl4, because the log DHf vs. log aHCl dependence had a slope of 4.0 [103]. Mono- to trisolvates were reported to be extracted, namely, • • • • • • • • •

ThCl4 · HC1 · 2B by DPSO in CCl4 from 7 M HCl [102] ThCl4 · 2B by DOSO in CCl4 from 7 M HCl [102] ThCl4 · 3B by DΦSO in benzene from 7 M HCl [102] UCl4 · HC1 · 3B by DOSO in toluene from 7 M HCl and 1 M HCl+6 M LiCl [13] ZrCl4 · 2B by DBSO in benzene from 7 M HCl [50] ZrCl4 · 2B and ZrCl3(ClO4) · 2B by DPSO in CCl4 from unclearly specified aqueous solutions [104] ZrCl4 · B by DPSO and DOSO in TCE from 7 M HCl [97] HfCl4 · 3B by DPSO and DOSO in CCl4 from 6 M HCl [82] TeCl4 · HCl · 2B by DHxSO in xylene from 4 M HCl [75]

Pt(IV) and Ir(IV) are suggested to be extracted as the ion pairs [B2 · H3O+]2 and The ion-pair nature of the species was implied by the fast extraction rate and the presence of the hexachloro anions is indicated by near uv and visible absorption spectra, but the presence of a water molecule was not proved.

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In the extraction of U(IV) by 0.05 M DOSO in toluene from 7 M HCl the DU(IV) value decreases with increasing temperature at 5–50°C [13]. In the extraction of Th(IV) by DPSO in CCl4 from probably 7 M HCl the log DTh vs.1/T dependence is curved at 10–50°C and a rough estimate of ∆H ~13 kJ mol-1 has been made [102]. In the extraction of Hf(IV) by 0.1 M DPSO or DOSO in CCl4 from 7 M HCl, DHf increases with the temperature and ∆H is 54.2 and 55.3 kJ mol-1, respectively [103].

D. Other Metals 1. Distribution Data and Extraction Kinetics Selected distribution data are surveyed in Table 10. Notice that also extraction of those elements has been studied which prefer to be bound to soft donor atoms. Weakly or negligibly extractable are • • • •

• • •

Eu(III) by 10% PetrSO in toluene from 0.1 to 8 M HCl (DEu(III)≤0.004) [90] and by 0.1 M methyl 4,8-dimethylnonyl sulfoxide in xylene from 7 to 11 M HCl (DEu(III)<0.01) [8] Y(III) and Cr(III) by 10% PetrSO in toluene from 0.1 to 8 M HCl (DM≤ 0.004) [90] Cu(II) and Zn(II) by DBSO in benzene (unspecified concentration) from 1 to 9 M HCl (DM<0.001) [50] Co(II) and Mn(II) by DBSO in benzene (unspecified concentration) from 1 to 9 M HCl (DM<0.001) [50], and by 0.1 M methyl 4,8-dimethylnonyl and methyl 3-ethylheptyl sulfoxides in xylene from 5 to 11 M HCl (DEu(III)< 0.02) [8] Ni(II) and Fe(II) by 1 M DHpSO in TCE from 2 to 10 M HCl [34] Ca(II) and Na(I) by 0.1 M methyl 4,8-dimethylnonyl sulfoxide in xylene from 6 to 11 M HCl (DM<0.01) [8] Ag(I) by 10% PetrSO in toluene from 2 to 8 M HCl (DAg≤0.05) [90]

Different extraction rates were exhibited by various platinum metal elements in the extraction by 0.5 M DHxSO in xylene at 20 °C. The distribution equilibrium is attained after ~3 min with Pd(II) and after ~2 h with Pt(II) when extracted from 2 M HCl, and after ~8 h with Rh(III) and afterⰇ8 h with Ir(III) when extracted from 6 M HCl [100]. Attainment of the equilibrium in the extraction of Pd(II) by PetrSO in kerosene depends on the concentrations of HCl and the extractant. At 1 M PetrSO it takes <1 min at 4 M HCl, ~5 min at 0.1 M HCl, and ~10 min at 1 M HCl. At 1 M HCl, it takes ~40 and ~60 min at 0.6 and 0.2 M extractant [108]. The kinetics of the extraction of Pd(II) by octyl p(tert-butyl)phenyl sulfoxide in toluene from chloride solutions was measured in a stirred cell. The kinetic equation is d[Pd]/dt= kfA[Pd2+][B]1.06[Cl-]0.8[H+]0.15, with A denoting the ratio of the phase volume to the

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Table 10 Survey of Data on the Distribution of Miscellaneous Metals Between Solutions of Common Sulfoxides and Aqueous Nitrate Solutionsa

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Table 10 Continued

a

Single temperature value applies to the measurement of isothermal concentration dependencies of DM(IV), a temperature range to the measurement of temperature dependency. rt, room temperature.

interfacial area. With different sulfoxides the rate constant of the forward extraction k (in parentheses, m×L2×mol-2×s-1) decreases in the order octyl p-(tertbutyl)phenyl f sulfoxide (6.4×10-5)>di(2-octyl) sulfoxide (4.9×10-5)>DOSO (8.5×10-6)>DLSO (4.0×10-6) [109, 110]. 2. Effect of Extraction Variables HCl is again the most frequently used salting-out agent. It is shown in Figs. 12 and 13 that, typically, DM vs. [HCl] dependences have a maximum and that the form and position of the maximum depends on the extracted metals. The DM values of these metals decrease with the HCl concentration from 4 M down to 0.1 M, but Hg(II) exhibits quite a different behavior at <2 M HCl. This is shown in Fig. 14, which also compares the extractability of Hg(II) with that of Zn(II) and Cd(II). The decrease of the Hg(II) extraction up to 2 M HCl can be ascribed to the formation of tri- and tetrachloro complexes of Hg(II) in the aqueous phase [107]. The extraction of Zn(II) by 0.2 M DHxSO in benzene is suppressed, if HCl alone is replaced in the aqueous phase by 0.5 M HCl+0.5 to 6.5 M LiCl. Then the DZn and DCd values are lowered by a factor of ~2 and ~10, respectively [106]. Fe(III) behaves

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Figure 12 Extraction of selected metals by 10% PetrSO in toluene. Trace metals, room temperature. (From Ref. 90.)

in an opposite manner, its extraction by 0.1 M DBSO in benzene is somewhat enhanced if HCl alone is replaced by 4.0 M HCl+0.5 to 1.5 M NaCl [50]. Extensive data on the effect of the extractant structure are available for Au(III) and Pd(II), all obtained in systems involving xylene diluent. The order of changed extraction ability for Au(III) is DOSO>DHxSO and, at a constant total number of C atoms, DHxSO ~butyl octyl sulfoxide [99]. With Pd(II) the orders are DHxSO> DOSO at 0.2–3 M HCl but DOSO>DHxSO at 4–6 M HCl [99], and DHxSO~

Figure 13 Extraction of selected transition metals by 10% PetrSO in toluene. Trace metals, room temperature. (From Ref. 90.)

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Figure 14 Extraction of Hg(II) by 0.5 M DEHSO in xylene (upper curve) [107] and by 0.2 M DHxSO in benzene [20], and extraction of Zn(II) and Cd(II) by 0.2 M DHxSO in benzene [106]. Initially, ≤0.0037 M Hg(II), 0.0073 M Zn(II), and 0.005 M Cd(II), 25°C.

DOSO>DDSO at 2–6 M HCl [100]. At a constant total number of C atoms, DHxSO ~butyl octyl sulfoxide at 0.2–2 M HCl but DHxSO>butyl octyl sulfoxide at 3–6 M HCl [99]. Branching suppresses the extraction ability for Pd(II) in the order DOSO>DEHSO>di(2-octyl) sulfoxide>di(3-octyl) sulfoxide, and the ring effect is illustrated by the order DHxSO>cyclohexyl hexyl sulfoxide>dicyclohexyl sulfoxideⰇDΦSO, both orders at 6 M HCl [100]. The structure effect in the extraction of Au(III) and Pd(II) by asymmetrical sulfoxides is illustrated in Table 11. Notice there that the invariant substituent influences the effect of the alkyl branching and can change the effect of the replacing of butyl by 2-tolyl. 0.1 M xylene solutions of methyl 4,8-dimethylnonyl and methyl 3-ethylheptyl sulfoxides (both at 1–10 M HCl) and of methyl pentadecyl sulfoxide (at 1–4 M HCl) exhibit similar extraction ability for Fe(III) [8]. The extraction efficiency of Hg(II) from 0.1 M HCl decreases in the order DOSO>DEHSOⰇDΦSO, all in xylene [107]. Surprising is the behavior of Pd(II). It is highly extractable by PetrSO from 0.1 to 8 M HCl (Fig. 13), while in the extraction by 0.2 M DEHSO in toluene the Dpd(II) value decreases from 16.4 at 0.01 M HCl to 0.4 at 1–5 M HCl [85]. To compare sulfoxides with other solvating extractants, the ability to extract CoCl2 as a disolvate into heptane, benzene or chloroform increases in the series TBP< DOSO<2-nonylpyridine N-oxide
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Table 11 Structure Effect in the Extraction by Asymmetrical Sulfoxidesa

Xylene Diluent, room temperature. Source: Ref. 99. a

triisopentylphosphine oxide [23]. Triphenylphosphine oxide in benzene extracts Fe(III) chloride more effectively than DΦSO but less effectively than DBSO [50]. The efficiency of the Hg(II) extraction by 0.2 M DEHSO from 0.1 M HCl decreases in the diluent (DHg(II)) order kerosene (48.0)>MiBK (36.2)>cyclohexane (11.0)>xylene (4.84)>toluene (4.59)>benzene (3.09)>chloroform (0.04) [107]. 3. Nature of Extracted Complexes and Extraction Enthalpy The extracted species can be mono- to trisolvates of neutral complexes or complex acids, namely, • • • • • • •

InCl3 · 2B · H2O at <1 M HCl and HInCl4 · 2B by PetrSO in kerosene from >2 M HC1 [105] Trisolvates of In(III) and Ga(III) chlorides by DHxSO in toluene from 4 M HCl [90] Possibly a mixture of di- and trisolvates of Fe(III) chloride by DHxSO in toluene from 4 M HCl [90] HFeCl4 · 2B and HFeCl4 · 2B at >4 M HCl and FeCl3 · nB (with probably n=2 and 3) by methyl 4,8-dimethylnonyl sulfoxide in xylene from ≤1 M HCl [8] MCl2 · 2B, HMCl3 · 3B, and H2MCl4 · 2B with M=Zn, or Cd by DHxSO in benzene from 7 M HCl [106] HgCl2 · B, HgCl2 · 2B, HHgCl3 · 3B, and H2HgCl4 · 2B by DHxSO in benzene from 0.01–1 M HCl [20] HgCl2 · 3B by DOSO, DEHSO, and DΦSO in xylene from 0.1 M HCl [107]

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Chloride bridged [PdCl2 · B]2 by PetrSO in kerosene from 0.1 to 6 M HCl [108] PdCl2 · 2B by DHxSO in xylene from 2 M HCl [100]

It was suggested that DBSO in benzene extracted Fe(III) as the species [50], but this was a speculative claim. The water concentration in the organic phase was not measured, nor was the degree of the chloride complexation of Fe(III) in the aqueous phase assessed. Also speculative is the suggestion that the ion pairs and are extracted by DHxSO in xylene [100]. The presence of the tetra- and pentachloro anions in the species was assumed because characteristics of the hexachloro anions were missing in absorption spectra, and the presence of the protonated water molecule was not at all proven. The S→O stretching vibration of DHxSO is shifted from 1050 to 990 cm-1 when DHxSO is complexed by Hg(II) in benzene [20]. In the extraction by DHxSO in benzene ∆H=-20.3 and -35.2 kJ mol-1 was found for Hg(II) at 0.01 and 6 M HCl respectively [20], and for Zn(II) ∆H=-36.9 kJ mol-1 [106] and for Cd(II) ∆H=-38.4 kJ mol-1 were found at 6 M HCl [106].

IV. EXTRACTION FROM OTHER MEDIA A. Perchloric, Pertechnetic, and Perrhenic Acids Perchloric acid is less extractable than nitric acid, but substantially more extractable than hydrochloric acid. The concentration of HClO4 in 0.02 M ethyl dodecyl sulfoxide in xylene is 0.01 M after contact with 6 M aqueous acid, while the HNO3 and HCl concentrations are 0.02 and ~3×10-4 M, respectively [3]. Although DBSO in benzene appears to extract a monosolvate of HClO4 [14], the suggestion that the extracted species is the ion-pair with a DBSO molecule bound to the H+ ion via a water molecule is speculative. The amount of HClO4 extracted by DBSO cannot be assessed, due to unclear definition of the scales of figures in [14]. Perrhenic acid is well extractable by 0.7 M sulfoxides in xylene from 1 M H2SO4. With DHxSO the organic concentration of HReO4 is 0.07–0.25 M at 0.03–0.40 M Re(VII) in the aqueous phase. With 2-heptylthiophane S-oxide it is 0.11–0.32 M at 0.017–0.68 M Re(VII) in the aqueous phase [27]. Pertechnetic acid is not extracted by DBSO in benzene from 5×10-4 M NH4TcO4 containing an equivalent amount or excess (1 M) HCl [14].

B. Bivalent to Hexavalent Metals 1. Distribution Data A survey of selected distribution data is given in Table 12. U(VI) has been the most studied extracted metal and thiocyanate media have been the preferred ones.

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Table 12 Survey of Data on the Distribution of Miscellaneous Metals Between Solutions of Common Sulfoxides and Aqueous Solutionsa

a

Single temperature value applies to the measurement of isothermal concentration dependencies of DM, a temperature range to the measurement of temperature dependency. rt: room temperature.

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2. Effect of Extraction Variables Perchloric and phosphoric acids can be used as salting-out agents in the extraction of perchlorates and phosphates. U(VI) perchlorate is extracted rather effectively. In the extraction by 0.5 M DOSO in 1,2-dichloroethane the DU(VI) vs. [HClO4] dependence passes through a maximum, rising from DU(VI)=0.9 at 0.05 M HClO4 to DU(VI)=110 at ~1.5 M HClO4 and falling to DU(VI)=9 at 4.5 M HClO4. Still higher efficiency is attained from 0.025 M HClO4+0.05 to 4.2 M LiClO4, with DU(VI)=1400 at 2.0 M LiClO. HClO4 competes even at low concentration. In the extraction from 1.0 M (Li,H)ClO4 the DU(VI) value sinks from 1900 at pH 2–3 to 35 at pH 0 [116]. The extraction of U(VI) phosphate by 0.1 M DPSO [115] and 0.32 M PetrSO [46, 47] in benzene from 1 to 6 M H3PO4 yields DU(VI) values increasing from 0.6 to 3.9 and from 0.4 to 1.8, respectively. Thiocyanates and picrates are not extractable from strongly acidic solutions and, as seen in Table 12, their distribution has been studied at pH 3. The extraction of Tm(III) thiocyanate becomes less effective in the order DOSO>DPSOⰇDΦSO in benzene [119], but that of Yb(III) thiocyanate varies in the order DPSO>DOSO [120]. In the extraction of Hg(II) iodide by DEHSO in benzene the DHg(II) value decreases from 2300 at 1×10-4 M I- to 2.1 at 0.04 M I-. The effect is ascribed to the formation of anionic triiodo and tetraiodo complexes in the aqueous phase [125]. Similar decrease of the DHg(II) value was observed and analogously explained in the extraction of Hg(II) chloride (see Fig. 14). 1,2-Bis(octylsulfinyl)ethane in butyl acetate extracts Pt(IV) and Pd(II) weakly from 2 to 6 M HCl solutions. Addition of potassium iodide strongly enhances the extraction of Pt(IV) (under simultaneous reduction to Pt(II)) [121] and Pd(II) [122]. Dialkyl sulfoxides are weaker extractants than trioctylphosphine oxide with its strongly basic phosphoryl oxygen atom. This applies to the extraction of Tb(III) [113] and Yb(III) [120] thiocyanates by DPSO and DOSO in CCl4, and of Cd(II) and Hg(II) iodides by DEHSO in benzene [125]. The diluent effect was investigated in the extraction by 0.3 M DPSO from 1 M NH4SCN at pH 3. For Yb(III) the diluent order is benzene (DYb=0.72)>xylene> CCl4>benzonitrile>chlorobenzene>m-cresol>chloroform (DYb=0.001) [120]. Tb(III) behaves similarly, and the diluent (DTb(III)) order is benzene (0.43)>xylene (0.36)>CCl4 (0.24)>chlorobenzene (0.17)>benzonitrile (0.15)>m-cresol (0.04) [113]. A somewhat different diluent (DCe(III)) order was reported for Ce(III), namely, CCl4 (0.50)>xylene (0.346)>benzene (0.295)>benzonitrile (0.181)>chlorobenzene (0.102)>m-cresol (0.020)>chloroform (0.0007) [112]. Again a different order of the diluent-(DU(VI)) pairs was found in the extraction of 0.01 M U(VI) by 0.2 M DOSO from 1 M HClO4. It is nitrobenzene (19.0)>TBP (8.1)>1.2-dichloroethane=2-octanone (3.0)>butyronitrile (2.33)>MiBK (1.86)> 1butanol (1.22)>1-octanol=chloroform=chlorobenzene (1.00). A flaky or oily third phase is formed with butyl acetate, diethyl ether, benzene, toluene, xylene, CCl4, cyclohexane, and methyllaurate as diluents [116]. The efficiency of the Am(III)

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extraction by 0.1 M DOSO from 0.005 M picrate at pH 3.0 decreases in the diluent (DAm(III)) order nitrobenzene (284)>toluene (173)>CCl4 (76.7)>dodecane (14.7) ~dichloromethane (14.4)>chloroform (0.23) [117]. 3. Nature of Extracted Complexes and Thermodynamic Functions The solvation number of thiocyanates shows a weak regularity with regard to the charge of the extracted ion. The complexes formed in the organic phase are • • • • • • • • • • •

UO2(SCN)2 · 2B with DPSO and UO2(SCN)2 · 3B with DOSO, both in chloroform, from 1 M NH4SCN at pH~3 [111] Th(SCN)4 · 3B with DPSO and DOSO in chloroform from 1 M NH4SCN at pH~3 [111] Am(SCN)3 · 4B with DPSO, DOSO in benzene from 1 M NaSCN, and Am(SCN)3 · 5B with DΦSO from 2 M NaSCN [123] Ce(SCN)3 · 4B with DPSO and DOSO in CCl4 from 1 M NH4SCN at pH 3 [112] Nd(SCN)3 · 4B with DEHSO and DOSO in benzene from 1 M NH4SCN at pH 3 [124] Eu(SCN)3 · 4B with DEHSO and DOSO in benzene from 1 M NH4SCN at pH 3 [124] Tb(SCN)3 · 4B with DPSO and DOSO in CCl4 from 1 M NH4SCN at pH 3 [113] Predominantly Er(SCN)3 · 3B with DEHSO and DOSO in benzene from 1 M NH4SCN at pH 3 [124] Tm(SCN)3 · 4B with DPSO, DOSO, and DΦSO in benzene from 3 M NaSCN at pH 3 [119] Yb(SCN)3 · 4B with DPSO and DOSO in CCl4 from 1 M NH4SCN at pH 3 [120] Zn(SCN)2 · 2B and Cd(SCN)2·4B with DEHSO in benzene from 0.1 M NH4SCN at pH 3 [114]

No regularity is revealed in other media, due to the diversity of the data. The extracted complexes are • • • • •

UO2(ClO4)2 · 4B with DOSO in 1,2-dichloroethane from 1 M HClO4 [116] Hf(ClO4)4 · 2B with DOSO in CCl4 from 6 M HClO4 [82] HNbOF · 3B with PetrSO, DHxSO, and 2-heptylthiophane in xylene from 5 M HF+5 M H2SO4 [118] HTaF6 · 3B or H2TaF7 · 3B with PetrSO, DHxSO, and 2-heptylthiophane Soxide in xylene from 5 M HF+2.5 M H2SO4 [118] ZnI2 · 2B with DEHSO in benzene from 0.01 M I- at pH 3 [125]

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Table 13 Thermodynamic Values of the Extraction of Trace Metals in Miscellaneous Systems

• • •

HgI2, HgI2 · B, and HgI2 · 2B with DEHSO in benzene from 0.01 M I- at pH 3 [125] UO2A2 · 2B and AmA3 · 2B with A=picrate and B=DOSO in chloroform from 0.01 M picrate at pH 3.0 [117] PtI2 · B [121] and PdI2 · B [122] with 1,2-bis(octylsulfinyl)ethane in butyl acetate from 2 to 6 M HCl+0.1% KI.

It has been concluded from infrared spectra that complexed Pt2+ [121] and Pd2+ [122] ions are bound to sulfur atoms of 1,2-bis(octylsulfinyl)ethane. Thermodynamic functions of the extraction equilibria (see Table 13) were determined in picrate and thiocyanate systems, in each of them with the same diluent and at similar concentration ranges. This makes them satisfactorily comparable.

V. SELECTIVITY OF THE EXTRACTION Of general interest is discrimination within pairs or groups of chemically similar elements, such as lanthanides(III), actinides(III), Zr(IV)–Hf(IV), or Nb(V)-Ta(V). Of special interest is selectivity for particular elements to be separated and purified in a solvent extraction process. For example, selectivity for U(VI) and Pu(IV) over fission products is needed in the reprocessing of nuclear fuel and selectivity for transplutonides(III) over lanthanides(III) would be needed in the partitioning of nuclear wastes. Sequences of the extractability of a series of elements are illustrated in Figs. 5 and 7 for nitrate systems, and in Fig. 10 (curves 3 and 4) and Figs. 12–14 for

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chloride systems. The figures show nitric and hydrochloric acid dependences of DM and can thus accommodate data on a limited number of elements only. Data on as many as 14 elements are shown in Fig. 15, indeed at one or two acid concentrations for each extractant. Not shown are data on 14 elements from Ref. 79 which are inconsistent within the paper itself and with data in Ref. 74. Still more extensive data on a perchlorate system are given in Table 14. It is quite obvious that no general characterization of the selectivity of sulfoxides can be given. Due to the different forms of the D M vs. [acid] dependencies, the discrimination between extracted elements is strongly dependent on the acid concentration (Fig. 16) and extractant structure. Moreover, discrimination between elements must be dependent on the concentration of the extractant if they are extracted in the form of different solvates. Finally, the temperature can also play a role. Striking differences are reported between the U(VI)/Th(IV) separation factors αU(VI)/Th obtained with different sulfoxides at two HNO3 concentrations (1 M extractant in xylene, 0.2 M U(VI) or Th(IV)). At 1 M HNO3 the extractant (αU(VI)/ Th/DU(VI)) order is 2-ethylhexyl p-tolyl sulfoxide (15.5/1.46)>di(2-octyl) sulfoxide

Figure 15 Sequences of the extractability of elements in various nitrate systems. Curve 1— 30 vol% DBSO in xylene/2 M HNO3 [74]; 2—30 vol% DBSO in xylene/7 M HNO3 [74]; 3—50% DBSO in cyclohexane/8 M HNO3 [81]; 4—1 M DHpSO in TCE/6M HNO3 [34]. Initially, ≤0.01 M metals, room temperature (systems 1–3) or 20°C (system 4).

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Table 14 Extraction of Elements by 0.5 M DOSO in 1,2-Dichloroethane from 0.025 M HClO4+1.0 M LiClO4a

Initially 0.01 M elements, room temperature. Slow reduction to Ce(III). c HCl required to keep Sn(IV) in solution. Source: Ref. 116. a

b

Figure 16 Separation factors in the extraction by 1 M DHpSO in TCE. Initially, ⱕ0.01 M metals, 20 °C. (From Ref. 34.)

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Table 15 Separation of U(VI) and Th(IV) in the Extraction by DPSO and DOSO in CCl4a

0.25 M extractants, 0.02 M U(VI) or Th(IV), room temperature. Source: Ref. 102.

a

(9.3/1.12)>dodecyl p-tolyl sulfoxide (4.0/1.08) ~DEHSO (3.96/2.65)>di (pethylphenyl) sulfoxide (0.54/0.24). At 4 M HNO3 the sequence is di(p-ethylphenyl) sulfoxide (12.1/1.21)>2-ethylhexyl p-tolyl sulfoxide (7.47/4.41)>dodecyl p-tolyl sulfoxide (2.4/2.36) ~DEHSO (2.4/3.16)>di(2-octyl) sulfoxide (0.010/0.20) [11]. Again for separation of the U(VI)–Th(IV) pair, another example of the effect of the extractant structure and acid concentrations is given in Table 15. Temperature can also influence the discrimination between two extracted metals. Table 16 shows it for the extraction of Pu(IV) and U(VI) by DHxSO and DOSO. As seen in Fig. 8, the separation potential of sulfoxides within the lanthanide(III) group is limited. Practicable separation factors between two adjacent lanthanides(III) are attained only for the La(III)-Ce(III) and, to a lesser extent, Ce(III)-Pr(III) pairs. This was also observed in the extraction by 0.1 M DOSO in benzene from an unspecified aqueous solution [84].

Table 16 Comparative Extractability of Pu(IV) and U(VI) in the Extraction by DHxSO and DOSO in Solvesso 100a

0.2 M extractants, trace Pu(IV) and U(VI), 2 M HNO3. Source: Ref. 72. a

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No unique value for the Zr(IV)/Hf(IV) separation factor can be determined from the data on the extraction by 1 M DHpSO in TCE from 7 to 10 M HNO3 [34]. The reason is that the DHf(IV) value decreases with increasing Hf(IV) concentration even at very low solvent loading. Typically the ␣Zr/Hf value is >10. The extractability of platinum metals by 0.5 M DHxSO in xylene from 1 to 6 M HCl decreases in the order Pd(II)>Pt(II)>>Ir(IV)>Pt(IV)>Rh(III)>Ir(III) [100]. The experiments were performed at 40°C with Pt(II), Rh(III), and Ir(III) and at 20°C with the other platinoids, but this need not diminish the plausibility of the order. The selectivity of the synergistic extraction by combinations of acidic chelating and sulfoxide extractants is dealt with in Chapter VIIB.

VI. INTERFERING PHENOMENA A. Third-Phase Formation The solubility of some extracted complexes in the organic phase is so limited that, if a high solvent loading is attained, they separate in the form of a third phase. In some cases the third phase is a solid, but in many systems it is a liquid which forms a second, heavy organic phase. It contains a considerable fraction of the extracted metal, and it causes serious disturbances in countercurrent operations and also batch distributions on any scale. For example, a third phase was reported to be formed in the extraction of 0.01 M U(VI) from 1 M HClO4 by 0.1 DOSO in butyl acetate, diethyl ether, benzene, toluene, xylene, CCl 4 , cyclohexane and methyl laurate, but without a characterization of its properties [116]. Obviously the perchlorate system is highly sensitive to the third phase formation. In nitrate systems the splitting of the organic phase into two layers is limited to solutions of sulfoxides in nonpolar, mainly aliphatic diluents. A short mention in Ref. 77 can be understood in the manner that U(VI) forms a third phase if its concentration in 1.1 M DEHSO in dodecane exceeds 0.21 M, when extracted from aqueous 2 M HNO3. The formation of the third phase was studied extensively in the extraction of U(VI) and Pu(IV) by DEHSO in dodecane from HNO3 solutions [62]. The accuracy of otherwise interesting data may be somewhat impaired by insufficient constancy of the temperature (±2°C), because the formation of a third phase is generally a temperature-sensitive phenomenon. The limiting organic concentration (LOC) of the extracted metal, i.e., the maximum attainable concentration at which still no third phase is formed, is a function of the extracted metal and the composition of the phases. Figures 17 and 18 show that the U(VI) complex is much more soluble than the Pu(IV) complex, and the LOC of both metals slightly decreases with increasing HNO3 concentrations. The LOC of Pu(IV) is suppressed by a factor of ~1.3 if 0.4 M DEHSO is at 1–2.5 M HNO3 loaded with U(VI) to a concentration of 0.0084 M.

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Figure 17 Limiting organic concentration of U(VI) as a function of the aqueous HNO3 concentration at (25±2)°C. (From Ref. 62.)

The LOC increase with the DEHSO concentration (Figs. 17 and 18) can be ascribed to an enhancement of the polarity of the organic phase. Also, alcohols increase the LOC. Figure 19 shows this for 2-ethyl-1-hexanol, and isodecanol gives a similar picture. It should be remembered that alcohols suppress the extraction. Inert salts not only support the extraction, they also enhance the LOC. Addition of 2 MLiNO3 or 0.5 M Ca(NO3)2 to 2 M HNO3 increases the LOC of U(VI) and Pu(IV) by a factor of ~1.3 and ~1.2, respectively [62].

Figure 18 Limiting organic concentration of Pu(IV) as a function of the aqueous HNO3 concentration at (25±2)°C (From Ref. 62.)

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Figure 19 Limiting organic concentrations of U(IV) and Pu(IV) as a function of the HNO3 concentration at various concentrations of 2-ethyl-1-hexanol (EHOH). Open points: U(VI), solid points: Pu(IV), 0.4 M DEHSO in dodecane, (25±2)°C. (From Ref. 62.)

Data on Pu(IV), as given in Ref. 62, have been described by the empirical equation

where S is the LOC and S° is the hypothetical solubility of the extracted Pu(IV) complex in 0.4 M BEHSO in dodecane at and 298 K. ϕm is the volume fraction of diluent modifier, S and S° are in mmol/L, and the other concentrations are molarities. The Sechenov parameters were estimated as S°= 10.4 mmol/L, ka=0.14, kU=2.97×10-5, km=121 for 2-ethyl-1-hexanol and km=71.9 for isodecanol, and ks=-0.166 for LiNO3 and ks=-0.427 for Ca(NO3)2 [127].

B. Radiation Damage Knowledge of the radiation stability of sulfoxides is of importance, due to their intended use in nuclear processing. The radiation stability has typically been studied with phases irradiated by gamma rays of a 60Co source. Unfortunately, the conditions of the irradiation are seldom sufficiently described. Often missing is information about temperature and the presence of air, and it can only be assumed that the phases were not stirred during the irradiation. Figure 20 shows the change of distribution ratios caused by radiation damage of DEHSO. The DU(VI) and Dpu(IV) values decrease up to a dose of ~18 MRad, mainly due to the lowering of the concentration of undamaged DEHSO. A sulfone is indicated by infrared spectra to be the main degradation product, and it seems not

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Figure 20 Effect of irradiation on the extraction of U(VI), Pu(IV), Zr(IV), and fission products (F.P.) by DEHSO. Open points: 0.4 M DEHSO (U, Pu) or 0.2 M DEHSO (Zr, F.P.) in dodecane from 2 M HNO3 [129]; solid points: 0.2 M DEHSO in kerosene from 4 M HNO3 [11]. Real mixture of fission products used, room temperature.

to extract U(VI) and Pu(VI) noticeably. The increase of DU(VI) and DPu(IV) at >18 MRad is ascribed to unidentified products of radiolysis and hydrolysis, which possibly act as synergists in the presence of DEHSO [129]. An unlike behavior was observed in systems involving Solvesso 100 diluent and 2 M HNO3. In the extraction by 0.2 M DHxSO or DOSO, DU(VI) decreases continuously from 2.3 to 0.7 when the radiation dose increases from zero to 169 MRad [128]. With 0.2 M DHxSO, Dpu(IV) increases from 1.10 before irradiation to 2.44 at 1.24 MRad and to 3.25 at 8.7 MRad [26]. With 0.2 M DOSO, DPu(IV) changes from 5.1 before irradiation to 6.8 at 8.5 MRad and to 5.7 at 36.7 MRad [38]. The extraction of Zr(IV) from 2 M HNO3 is enhanced by the radiolysis of DEHSO in kerosene [11] and dodecane [129] (Fig. 20) as well as of DHxSO [26, 128] and DOSO [38, 128] in Solvesso 100. DEu(III) is 2.2×10-4, 3.0×10-4, and 5.2× 10-4 at 0, 1.24, and 8.7 MRad, respectively, in the extraction by 0.2 M DHxSO in Solvesso 100 [26]. Data on Ru(III) [11, 26, 128] were given for an unspecified trivalent form, but more useful would be data on nitrosylruthenium(III) which is the typical form in the reprocessing of nuclear fuel. Figure 20 shows the extraction of the sum of real fission products as a function of the DEHSO degradation. A comparison with TBP is appropriate. Only one source [11] compares directly the effect of radiation damage on the extractant properties of a sulfoxide and TBP. Data excerpted from the source are given in Table 17. They clearly show that the U(VI)/ Zr(IV) separation is deteriorated by radiation damage much more with TBP than with

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Table 17 Comparative Effect of Radiation on the Extraction of U(VI) and Zr(IV) by DEHSO and TBP. 0.2 M Extractants in Kerosenea

a Initially 0.05 M U(VI) and trace Zr(IV) in the aqueous phase, probably room temperature. Source: Ref. 11.

DEHSO. It is favorable that the comparison was made with the kerosene diluent which is preferred in nuclear processes.

VII. SYNERGISM A. Solvating/Solvating and Solvating/Basic Combinations of Extractants These extractant combinations have been studied rather extensively although, as experience shows, only moderate synergistic enhancement can be expected. Data in Table 18 corroborate the expectation. Synergistic enhancement (SE) of >10 has been observed only in one case and even strong antagonismus is not unusual. Extensive data on the extraction of U(VI) by the B1–B2 mixtures DPSO-DOSO, DPSO-D ΦSO, and DPSO-TBP show that the SE value depends on the concentrations of the two extractants, attaining a maximum or changing monotonously within the studied limits. Table 18 shows the highest and lowest SE values, found at a constant sum of component concentrations. A variety of synergistic complexes were reported to be extracted, namely, • • • • • • •

UO2(NO3)2 · B1 · B2 by DOSO and di(2-octyl) methylphosphonate in benzene from 1 M HNO3 [63] UO2(NO3)2 · B1 · B2 by DOSO and TBP in CCl4 from 1 M HNO3+1 M NaNO3 [36] UO2(NO3)2 · B1 · B2 by DΦSO and TBP in benzene from 4.2 M HNO3 [66] UO2(NO3)2 · B1 · B2 by PetrSO and octyl decyl sulfoxide in CCl4 from 2 M HNO3 [48] UO2(NO3)2 · B1 · B2 by PetrSO and TBP in kerosene from 2 M HNO3 [44] UO2(NO3)2 · B1 · B2 by PetrSO and TBP in benzene from 2 M HNO3 [67] Ln(SCN)3 · xB1 · (4–x)B2 with Ln=Nd and Eu, by B1=DOSO and B2=DEHSO or TOPO in benzene from 1 M SCN- at pH 3 [124]

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Table 18 Synergistic Enhancement in the Extraction by Solvating/Solvating and a Solvating/ Basic Mixturesa

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Sulfoxide Extractants and Synergists Table 18 Continued

Only the highest or both the highest and the lowest reported SE values are given for each system. Di(2-octyl) methylphosphonate. c Octyl decyl sulfoxide. d D1 and D2 are not given in the original source. e Aliquat 336. f Alamine 336. a

b

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Table 19 Survey of Data on the Synergistic Extraction by an Acidic Extractant and a Sulfoxide Synergista

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Table 19 Continued

BMPPT: 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-thione, Cyanex 272: bis(2,4, 4-trimethylpentyl)phosphinic acid, Cyanex 301: bis(2,4,4-trimethylpentyl)dithiophosphinic acid, DtBSA: 3, 5-di(tert-butyl)salicylic acid, HDEHP: di(2-ethylhexyl)phosphoric acid, HEh[EhP]: 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, PBI: 3-phenyl-4-benzoyl-5-isoxazolone, PMCBP: 1-phenyl-3-methyl-4(2-chlorobenzoyl)-5-pyrazolone. a Single temperature value applies to the measurement of isothermal concentration dependencies of DM(IV), a temperature range to the measurement of temperature dependency rt, room temperature.

• • •

Er(SCN)3 · xB1 · (3–x)B2 by B1=DOSO and B2=DEHSO in benzene from 1 M SCN- at pH 3 [124] Er(SCN)3 · xB1 (3–x)B2 by B1=DOSO and B2=TOPO in benzene from 1 M SCN- at pH 3 [124] Yb(SCN)3 · xB1 · (3–x)B2=DPSO and B2=DOSO or TOPO in CCl4 from 1 M SCN- at pH 3 [120]

For some systems no unambiguous composition of the extracted synergistic complex can be found by the slope analysis, because the slope of the log DM vs. log [B1] dependence changes when measured at various constant concentrations of B2. Such phenomena were observed, e.g., in the extraction of U(VI) by the pairs DPSO-DΦSO and DPSO-TBP in CCl4 from 3 M HCl [94], of Np(IV) by DiPSODOSO and DHxSO-DOSO in Solvesso 100 from 2 M HNO3 [25], of Th(IV) by

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DPSO-DOSO in CCl4 from 7 M HCl [102], and of Ce(III) by DPSO-DOSO in CCl4 from 1 M NH4SCN at pH3 [112]. The enthalpy of the extraction of U(VI) by two extractant combinations was determined. It is ∆H=-26.2 kJ mol-1 in the extraction by PetrSO and octyl decyl sulfoxide in CCl4 from 2 M HNO3 [48], and ∆H=-23.1 kJ mol-1 in the extraction by PetrSO and TBP in kerosene from 2 M HNO3 [44].

B. Acidic/Solvating Extractant Combinations 1. Distribution Data Selected sources of distribution data in synergistic systems are gathered in Table 19. The table shows that the synergistic action of sulfoxides has mostly been studied in combination with TTA and 5-pyrazolone derivatives as acidic extractants. Unlike systems involving two solvating extractants, combinations of an acidic and a solvating extractant can in particular systems give synergistic enhancement as high as several orders of magnitude. Examples are shown in Figs. 21 and 22. Similar synergistic action has been found, e.g., in the extraction of Tm(III) by TTA+DPSO in benzene [119]. Less marked effects (SE≤55) were observed in the extraction of Hf(IV) by TTA+DOSO in CCl4 from 6 M HClO4 [82]. Moderate synergistic effects (SE≤14.0) were found, e.g., in the extraction of Pa(V) by TTA+DPSO in benzene from 6 M HCl and of Zr(IV) by the same solvent from 2 M HCl+2 M NaCl [91].

Figure 21 Synergistic action of DOSO in the extraction by TTA in benzene. The constant sum of the TTA and DOSO concentrations is 0.01 M (Th) and 0.02 M (Np and Pu). Aqueous phase: 1.0 M HClO4, room temperature. (From Ref. 136.)

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Figure 22 Synergistic action of DOSO in the extraction by TTA in benzene. Th(TV)–0.01 M TTA; Np(IV)–0.02 M TTA; Pu(IV)–0.01 M TTA (lower curve); and 0.0125 M TTA (upper curve). Numerals on the points with arrow are the DTh and DNp(IV) values without DOSO. Aqueous phase: 0.1 M HClO4 (Th), and 1.0 M HClO4 (other metals). Room temperature. (From Ref. 136.)

Weak synergistic actions (SE=2-4) are exhibited, e.g., by DOSO in the extraction of Th(IV) by 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, Cyanex 272 [bis (2,4,4-trimethylpentyl)phosphinic acid], and Cyanex 301 [bis (2,4,4trimethylpentyl)dithiophosphinic acid] [137]. DNSO and especially DΦSO enhance very weakly the extraction of U(VI) by naphthenic acid in benzene [142], and there is no synergistic effect in the extraction of Hf(IV) by TTA+DPSO and TTA+DOSO in CCl4 from 6 M HCl [82]. An antagonistic effect is caused by DΦSO in the extraction of Nd(III) [143] and Pd(II) [144] by 1-phenyl-3-methyl-4-dichloroacetyl-5-pyrazolone in chloroform. It was ascribed to the formation of a 1:1 molecular complex of the acidic extractant with DΦSO. A special case is the extraction of Zr(IV) by TTA from a chloride solution, where DPSO not only increases equilibrium distribution ratios but also accelerates the extraction rate [104]. Sulfoxide synergism can improve but also deteriorate the discrimination between two similar elements. The Eu(III)/Nd(III) separation factor is 6.2 with 0.3 M 4,4,4trifluoro-1-phenyl-1,3-butanedione alone in benzene, and is enhanced to 53 by addition of 0.005 M DEHSO [145]. On the other side, αEu(III)/Nd(III)=10.9 obtained with 01.01 M 1-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone alone in chloroform is lowered to 7.8 by addition of 0.005 M DEHSO [146]. It appears that, generally, sulfoxide synergists cause only small enhancements or suppressions of separation factors within the lanthanide(III) series. The values αLu(III)/Eu(III)=2.7

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and 5.1 are obtained with 0.01 M 1-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone alone and its mixture with 0.005 M DEHSO, respectively, in chloroform [146]. With 0.01 M 3-phenyl-4-benzoyl-5-isoxazolone in xylene αTm(III)/Tb(III) is 4.52 and is changed to 5.13 by 1×10-4 M DOSO, to 3.96 by 1×10-4 M DEHSO, and to 2.63 by 0.001 M DΦSO. αTb(III)/Nd(III) is 15.5 and is changed to 18.1 by 1×10-4 M DOSO, to 19.3 by 1×10-4 M DEHSO, and to 16.2 by 0.001 M DΦSO [147]. Am(III) is separated with noticeable efficiency from unspecified lanthanides (III), if it is extracted by synergistic combinations of 0.1 M 4-benzoyl-2,4-dihydro5-methyl-2-phenyl-3H-pyrazol-3-thione in toluene from 0.1 M NaNO3 at pH 3.6. The αAm(III)/Ln(III) value is ≥28 in the presence of 0.01 M DOSO and ≥81 in the presence of 0.03 M PetrSO [139]. The separation factors in the absence of a synergist are not given. 2. Effect of Extraction Variables Available data allow an assessment of the effect of the sulfoxide structure. In the extraction of U(VI) by 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3thione the synergistic action in toluene decreases in the order PetrSO>>DOSO>TBP [133]. In the extraction of Nd(III) by 3,5-di(tert-butyl)salicylic acid, the synergistic action decreases in the order dicyclohexyl sulfoxide~cyclohexyl hexyl sulfoxide~ cyclohexyl octyl sulfoxide~cyclooctyl octyl sulfoxide>DBSO~DHxSO>DOSO >cyclopentyl octyl sulfoxide>DBzSO>DEHSO~DΦSO. Similar orders have been found with 5-hexyl- and 3,5-diisopropylsalicylic acid as the acidic component [140]. With 0.1 M 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-thione in toluene the separation factor αU(VI)/Th(IV) is 760 in the presence of 0.03 M PetrSO and 480 in the presence of 0.01 M DOSO [133]. Also comparison of sulfoxides with phosphoryl extractants is possible. In the extraction of Eu(III) by 1-phenyl-3-methyl-4-(2-chlorobenzoyl)-5-pyrazolone in xylene the synergistic action increases in the order TBPNaClO4>NH4ClO4, and addition of one of these salt up to a concentration of 2 M enhances the DU(VI) value by a factor of 3–5. The phenomenon is ascribed to suppression of the water activity in the system by the salts [148]. 3. Nature of Extracted Complexes and Thermodynamic Functions Besides slope analysis, also dependencies of the type shown in Fig. 21 can be used for the elucidation of the composition of the extracted synergistic complexes. The position of the maximum on the curves can show the ratio of the acidic and sulfoxide extractant molecules participating in the formation of the synergistic complex.

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Examples of slope analysis in Fig. 22 shows that, in the measured range of the synergist concentration, a synergistic complex can predominate in the organic phase or it can be present together with a simple nonsynergistic complex. It is quite evident in Fig. 22 that synergistic complexes ThA 4 · B and NpA 4 · B are predominantly extracted, while the complex PuA4 · B coexists with PuA4. The extracted complexes have typically a simple composition in the extraction of actinides(IV, VI), where no anions of the aqueous medium participate in the formation of the extracted complex. The complexes contain one sulfoxide molecule and are • • • • •

UO2A2 · B with HA=TTA and B=DBzSO or DΦSO in benzene from 0.01 M HCl[130] UO2A2 · B with HA=1-phenyl-3-methyl-4-acyl-5-pyrazolone (acyl=acetyl, trifluoroacetyl, benzoyl, 2-chlorobenzoyl, or 4-nitrobenzoyl) and B=DOSO in chloroform from 0.5 M HCl [131] UO2A2 · B with HA=1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and B= DΦSO in benzene from 0.05 M HNO3 [132] UO2A2 · B, with HA=4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol3-thione and B=PetrSO in toluene at pH 2.04 [133] ThA4 · B, NpA4 · B, and PuA4 · B with HA=TTA and B=DOSO in benzene from 0.1 M HClO4 [136]

Complexes of Zr(IV) and Hf(IV) typically contain anions of the aqueous medium and one to three sulfoxide molecules. They can be written as • • • • •

ZrA2Cl2 · 2B and ZrA2Cl(ClO4) · 2B with HA=TTA and B=DPSO in CCl4 from unclearly specified aqueous solutions [104] ZrA2(OH)2 · B with HA=TTA and B=DPSO in benzene from 2 M HCl+2 M NaCl [91] HfA(NO3)3 · B with HA=TTA and B=DPSO in CCl4 from 9 M HNO3 [82] HfA3(ClO4) · B and HfA2(ClO4)2 · B with HA=TTA and B=DOSO in CCl4 from 6 M HClO4 [82] HfACl3 · 3B, HfA2Cl2 · 2B, and HfA3Cl · B with HA=TTA and B=DPSO in CCl4 from 7 M HCl [103]

Complexes of lanthanides(III), Am(III), and Zn(II) can contain complexing inorganic anions like thiocyanates, together with up to three sulfoxide molecules. They do not contain noncomplexing anions of the aqueous medium, such as perchlorates, and then the number of sulfoxide molecules is one to two. Reported compositions are • •

AmA3 · B, AmA2(SCN) · 2B, and AmA(SCN)2 · 3B with HA=TTA and B= DPSO in benzene from 1 M NaSCN at pH 3 [123] AmA2(NO3) · 2B with HA=4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3Hpyrazol-3-thione and B=DOSO or PetrSO in toluene from 0.1 M NaNO3 at pH 3.6 [139]

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Table 20 Equilibrium Constants of the Formation of Synergistic Complexes in the Organic Phasea

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Table 20 Continued

a

Ks1=[UO2A2 · B][UO2A2]-1[B]-1 and KS2=[UO2A2 · 2B][UO2A2]-1 [B]-2 (italics).

• • • • • • • • •

MA3 · B and MA3 · 2B with M=Am and Eu, HA=TTA, and B=DBSO in CCl4 from 1 M (H,Na)ClO4 [4] MA3 · B and MA3 · 2B with M=Nd and Eu, HA=4,4,4-trifluoro-1-phenyl1,3-butanedione and B=DEHSO in benzene from 0.1 M thiocyanate at pH 3 [145] MA3 · B and MA3 · 2B with M=Nd, Eu, and Lu, HA=1-phenyl-3-methyl-4trifluoroacetyl-5-pyrazolone from 0.01 M chloroacetate buffer at pH 2.7 [146] NdA3 · B and NdA3 · 2B with HA=3-phenyl-4-benzoyl-5-isoxazolone and B =DOSO, DEHSO, or DΦSO in xylene from 0.1 M NaClO4 at pH 3 [147] EuA2Cl · B with HA=1-phenyl-3-methyl-4-(2-cHorobenzoyl)-5-pyrazolone and B=DOSO or dicyclohexyl sulfoxide in xylene from 0.1 M (HCl+NaCl) at pH 1.5 [141] MA3 · B with M=Tb and Tm, HA=3-phenyl-4-benzoyl-5-isoxazolone, and B=DOSO, DEHSO, or DΦSO in xylene from 0.1 M NaClO4 at pH 3 [147] TmA3 · B and TmA3 · 2B with HA=TTA and B=DBSO in benzene from 1 M NaClO4 at pH3 [119] TmA3 · B, TmA3 · 2B, TmA2(SCN) · 2B, and TmA (SCN)2 · 3B with HA=TTA and B=DBSO in benzene from 1 M NaSCN at pH 3 [119] MA3 · B with M=Sc and Lu, HA=TTA, and B=DBSO in CCl4 from 1 M (H,Na)ClO4 [4]

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Kolarik LuA3 · B with HA=4,4,4-trifluoro-1-phenyl-1,3-butanedione and B=DEHSO in benzene from 0.1 M thiocyanate at pH 3 [145] ZnA2 · B with HA=1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and B= DEHSO in benzene from 0.01 M acetate buffer at pH 3.75 [114]

An unusual complex is reported to be formed by the ion with di(2-ethylhexyl)phosphoric acid (HA) and PetrSO in benzene. Its composition is VO2(HA2) (HA)2 · B, where the monoionized dimer forms the usual 8-membered ring and neutralizes the charge of the ion, the two monomeric molecules HA are bound to the ion via the phosphoryl O atoms, and the PetrSO molecule is bonded to one of the HA molecules via a hydrogen bond of its OH group [138]. The extent of the synergistic action of a sulfoxide is characterized by the formation constants of the synergistic complexes in the organic phase. Data are available only for mono- and disolvate complexes, where the constants are KS1= [UO2A2 · B][UO2A2]-l[B]-1 and KS2=[UO2A2 . 2B][UO2A2]-1[B]-2. The synergistic distribution ratio is then DS=Da(1+KS1[B]+Ks2[B]2) with Da being the distribution ratio in the absence of the synergist. Published formation constants are gathered in Table 20. It is seen there that the stability of the complex UO2A2 · B depends not only on the nature of the synergist but also on the anion A- of the acidic component. As can be expected, infrared spectra show that the sulfoxide molecule is bound to the extracted metal ion via the oxygen atom. The S→O stretch frequency in the TTA complexes UO2A2 · 2B is shifted to lower values, by 55 cm-1 with B=DΦSO and by 58 cm-1 with B=DBzSO. In spite of the small difference between the shifts, it is concluded that DBzSO is bound more strongly to the uranyl ion than DΦSO [130]. Thermodynamic functions of the complex formation of U(VI) and Pu(VI) are given in Table 21.

Table 21 Thermodynamic Functions of the Reaction MO2A2+B=MO2A2 · B in the Organic Phase and of the Biphasic Reaction (italics)a

a

Benzene diluent.

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NOMENCLATURE A. Symbols aw, aHX, B DM, DB SE Kex LOC M αM/M’

activity of water, an acid HX, and X- ions, respectively extractant molecule in the formula of a solvate distribution ratio of a metal (the valence state can also be characterized by the subscript) or of a sulfoxide synergistic enhancement, DB1B2(DB1+DB2)-1, with DB1, DB2 and DB1B2 denoting the DM value with extractant 1, extractant 2, and a mixture of the extractants equilibrium constant of an extraction reaction on the molarity scale limiting organic concentration of the metal metal, eventually with valence specified in parentheses separation factor for metals M and M’ defined as DM/DM’

B. Abbreviations of Sulfoxide Extractants DBSO DBzSO DDSO DHpSO DHxSO DiPSO DLSO DNSO DOSO DPSO DΦSO PetrSO

dibutyl sulfoxide dibenzyl sulfoxide didecyl sulfoxide diheptyl sulfoxide dihexyl sulfoxide diisopentyl sulfoxide didodecyl sulfoxide dinonyl sulfoxide dioctyl sulfoxide dipentyl sulfoxide diphenyl sulfoxide petroleum sulfoxides

C. Abbreviations of Other Extractants and a Diluent MiBK PMBP TBP TCE TTA

2-methyl-4-pentanone, frequently called methyl isobutyl ketone 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone tributyl phosphate 1,1,2-trichloroethane 1-(2-thienyl)-4,4,4-trifluoro-1,3-butanedione, frequently called thenoyltrifluoroacetone

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APPENDIX: PHYSICAL PROPERTIES OF SULFOXIDES A. Melting and Boiling Points

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Distillation from a reaction mixture.

B. Densities, Dipole Moments, and Refractive Indices

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In benzene. At 24.3°C

c

C. Physical Properties of a Solution 1.0 M DHpSO in TCE has at 20 °C a density of 1.280 g cm-3 and a viscosity of 1.775 cp, and its interfacial tension with water is 17.5 dynes cm-1 [34].

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Korpak, W. Nukleonika 1962, 7, 715–723. Korpak, W. Nukleonika 1964, 9, 1–10. Korpak, W. Nukleonika 1963, 8, 747–754. Sekine, T.; Dyrssen, D.J. Inorg. Nucl. Chem. 1967, 29, 1481–1487. Nikitin, Yu.E.; Murinov, Yu.I.; Rozen, A.M. Usp. Khim. 1976, 45, 2233–2252. Laurence, G.C.R.Acad. Sci. Paris Ser. C 1969, 269, 352–354. Chengye, Yuan; Haiyan, Long; Enxin, Ma; Wuhua, Chen; Xiaomin, Yan. Scient. Sinica 1984, 27, 887–897. McDowell, W.J.; Harmon, H.D. J.Inorg. Nucl. Chem. 1971, 33, 3107–3117. Siddal, T.H. III; Brown, D.A. Science and Technology of Tributyl Phosphate. In Synthesis, Properties, Reactions and Analysis, Schulz, W.W., Navratil, J.D., Talbot, A.E., Eds.; CRC Press: Boca Raton, FL, 1984; Vol. 1, 15–24. Sharipov, A.Kh. Khim. Prom. 2000; 24–32. Shen, Chaohong; Bao, Borong; Bao, Yizhi; Wang, Gaodong; Qian, Ju; Cao, Zhengbei. J. Radioanal. Nucl. Chem. 1994, 178, 91–98. Mikhailichenko, A.I.; Sokolova, N.P.; Sulaimankulova, S.K.; Teterin, E.G. Radiokhimiya 1973, 15, 693–697; Soviet Radiochem. 1973, 15, 699–703. Zhang, Zuokui; Chen, Zhengkang. He Huaxue Yu Fangshe Huaxue 1983, 5, 309–314.

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5 Extraction with Metal Bis(dicarbollide) Anions

Metal Bis(dicarbollide) Extractants and Their Applications in Separation Chemistry Jirí Rais Nuclear Research Institute Rez plc, Rez, Czech Republic Bohumír Grüner Institute of Inorganic Chemistry, Czech A.cademy of Sciences, Rez, Czech Republic

I.

INTRODUCTION

A.

History of Metal Bis(dicarbollide) Extractants Research

This review resulted from rather broad studies of two Czech institutes, namely Institute of Inorganic Chemistry, Czech Academy of Sciences, Rez, IIC (B.G.) and Nuclear Research Institute, Rez, NRI (J.R.). The collaboration of the two institutes started not only because of the same location at Rez near Prague, Czech Republic, but mainly from the common interests in development of effective extractants for their use in the area of nuclear applications. Although the two areas of the review— synthesis and properties of metal bis(dicarbollide) anions (IIC) and extraction research using these anions (NRI)—differ in their subjects, techniques and even in style of narration, the two must be put together in order to have a complete picture of the subject. Section II, Tables 1–11, Figs. 1 and 2 and the reaction schemes in the review were written and drawn by B.G. The early studies with various hydrophobic anions have their fundamental standing, but the discovery of the excellent extraction properties of metal 243 Copyright © 2004 by Marcel Dekker, Inc.

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bis(dicarbollide) anions marked a qualitatively new step in the research. The first stable metal bis(dicarbollide)s were synthesized and described by Hawthorne in the United States already in 1965 [1], but their use as perfect hydrophobic anions waited until 1976 when their extraction properties were first published [2]. (For the sake of completeness, Schlaikjer already in 1965 studied the extraction of Cs+ and Ba2+ with polyhedral and anions [3].) The cobalt bis(dicarbollide) anion [(1,2-C2B9H11)2-3-Co]- (1) was very soon established as an ideal hydrophobic anion for extractions by the ion pair mechanism [2]. Its advantages are (1) very high hydrophobicity, (2) extreme acidity of the derived acid H+1- that behaves as a superacid and is fully dissociated even in media of ε =10 to 15, (3) high chemical and radiation stability. This is so especially for the chloroprotected cobalt bis(dicarbollide) derivative. The cobalt bis(dicarbollide) anion surpasses by an order of magnitude the until now examined anions usable for the extraction of radioactive cesium and soon after the publication [2] the process of Cs isolation with this ion was patented by Czech scientists [4]. The important supporting finding of polyethylene glycols as strong synergists for simultaneous extraction of strontium with cobalt bis(dicarbollide)s [5, 6] enabled the construction of a conceptual process for the isolation of both cesium and strontium from highly acidic aqueous solutions. Besides the patenting in the former Czechoslovakia, the method of isolation of 137 Cs and 90Sr was patented by the Czech scientists in those early commencements of the research also in the former USSR [7] and soon a long-standing cooperation between the Radium Institute of St. Petersburg (the former Leningrad) and NRI, Rez started. In this cooperation, the chemistry of the developed process was studied in Rez whereas the hot cell experiments were executed in the hot cell facility at Gatchina, Russia. A number of universities from the Czech side joined the program, devoting themselves to the tasks of studying the radiation stabilities of proposed organic mixtures, explosive hazards, extraction of other valuable components, physicochemical parameters of the extractants in order to check their composition, etc. The preparation of the extractant for the planned plant tests [200 kg of Cs salt of chloroprotected cobalt bis(dicarbollide)] and tests of purity were also performed in the former Czechoslovakia. The process was developed in 1984 to the stage enabling its pilot plant testing and this was performed in a 6-month run at the reprocessing plant Mayak in Russia. The process was repatented with Russian scientists [8] and this stage of development was thus successfully accomplished. The cold war with its complete lack of information flows between the two world blocks caused that a new initiative in the United States for studies of cobalt bis(dicarbollide)s appeared only at the Los Alamos National Laboratory (LANL) in 1990 [9]. However, the LANL was not involved in the main stream of the development of cobalt bis(dicarbollide) technology, which on the contrary has continued for many years between the U.S. Department of Energy (DOE), the Idaho National Engineering and Environmental Laboratory (INEEL), and the Radium Institute of St. Petersburg (RI). The research teams formed two leading working groups: one group is that of DOE, INEEL, and RI and the other one is that

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of IIC, NRI, and other European institutes and universities in the frame of Euro projects. It is characteristic for the, in some aspects surreptitious, collaboration between the United States (DOE) and Russia (RI) that inventor of cobalt bis(dicarbollide)s, Hawthorne, has patented these compounds for extraction purposes only in 1997 [10, 11]. The long-standing cooperation of NRI and RI, during which the cobalt bis(dicarbollide) technology was developed up to the stage of plant testing, is as a rule not mentioned in the subsequent Russian and American literature. During the years, several review articles and reports on the extraction with cobalt bis(dicarbollide)s appeared. Among early reviews, a paper of Makrlik et al. [12] should be mentioned. Some properties of cobalt bis(dicarbollide) technological systems during the cold war period appeared in two American reports [9, 13]. Cobalt bis(dicarbollide) technology as developed in Russia until 1992 was summarized in the Russian monograph [14]. The more recent state (1994) was also briefly described by Kyrs [15]. The huge body of material concerning the development of cobalt bis(dicarbollide) extractants due to NRI and RI cooperation was collected and transferred for public attention to DOE in the form of a report of NRI in 1993 [16], but again this report apparently disappeared in the black hole of early post–cold war era. Important contributions to the cobalt bis(dicarbollide) chemistry were made after 1990 by the Japanese Atomic Energy Research Institute (JAERI) and LANL as referenced later in this review. A significant milestone in the development of technologies with cobalt bis(dicarbollide)s was the start of the plant for the fractionation of highly radioactive waste, for the separation of 137Cs and 90Sr from defense waste, in Russia in August 1996 [17, 18]. This is so far the only instance of a current extraction plant scale aimed at retreatment of the radioactive waste running in the world (with the possible exception of the extraction of the above two elements with crown ethers also accomplished in Russia). Intensive research on still more elaborated technological improvements of the process with chloro-protected cobalt bis(dicarbollide), including proposals of new synergists, new solvents, and improved stripping mixtures is documented in many papers originating after the time of the first industrial application from the RI. New metal bis(dicarbollide)s and other boron derivatives are studied in the frame of current EC projects; these will be reported in the present review. The new methods of synthesis, largely employed at IIC and other European universities, are inspired by the success of the cobalt bis(dicarbollide) technology. They generally aim at determination and synthesis of new derivatives with incorporated selective function groups. This approach can ultimately lead to finding new properties of classical extractants, like P苷O based compounds or crown ethers. In fact, binding the selective group to the cobalt bis(dicarbollide) moiety leads to negatively charged ions, in contrast to the neutral reagents, and completely different behavior can be expected. Finally, new ecologically suitable nonpolar solvents, based on combinations with n-dodecane or isopropylbenzene, are now studied at NRI, Rez. The convenient

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mixtures may, contrary to previous expectations, dissolve chloro-protected cobalt bis(dicarbollide) and lead to solvent mixtures of a third generation. Various newly proposed selective extractants for specific purposes in synergetic mixtures with H+12- (see Table 1 for its identity) are tested at NRI. Returning to the most recent reports, cobalt bis(dicarbollide) technology has been proposed in the United States as a part of future planned nuclear fuel retreatment there. The isolation of two major heat sources, 137Cs and 90Sr, from the waste can substantially improve also the economy of the planned nuclear waste repository, since cooling requirements of the huge volumes of repositories should be drastically reduced [19]. Interestingly, in the report [19] no reference is made either to the initial NRI-Russian nor to contemporary DOE-Russian studies. In spite of the omission, this proposal testifies that cobalt bis(dicarbollide) technology is now rooted firmly into the worldwide technical state of art. A detailed review on extractions in systems with ion pairs was recently published by Moyer and Sun [20]. This review is in certain aspects complementary to the present one, as the subject of extraction by hydrophobic anions and by cobalt bis(dicarbollide)s was to some extent treated there. The main body of the review [20] was, however, devoted to the development and testing of the authors’ model devised and aimed at theoretical calculation of the selectivity of the extraction of alkali metal cations, a goal different from the present one.

B. Scope and Aims of the Review In organizing this survey, we tried to include the main new experimental and theoretical knowledge that may be of use for a potential reader. New, nonpublished results are also included here. We do not insist on the completeness of the review, providing space for a reader to search by himself some concrete information of interest that is not considered here as of primary importance. Several areas had to be omitted in the review, like the X-ray and structural data for new boron compounds, or detailed treatment of extractions with other than cobalt bis(dicarbollide) anions. These broad subjects surpass the scope of the review and could be a matter for other review articles. There exist many analytical procedures based on the cobalt bis(dicarbollide) extractants; these are treated here only in a general review style. The studies of radiation stabilities of cobalt bis(dicarbollide)s and solvents used at 1970 and 1980, though they might be important form a theoretical point of view, were practically superseded by newly proposed solvents. Thus, this area is not covered in detail. The aims and scope of this review are as follows: 1. 2.

Detailed description of synthesis of various metal bis(dicarbollide)s and other boron extractants together with their general and extraction properties Chloro-protected cobalt bis(dicarbollide) technology for the treatment of radioactive waste solutions

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Description of other published technological and analytical uses of cobalt bis(dicarbollide)s

The present review is complemented by another review appearing in this volume (Chapter 6) that is devoted to the principles of the extraction of electrolytes [21]. The mechanism of extraction and physicochemical principles described there apply fully to the extractions with metal bis(dicarbollide) anions.

II. SYNTHESIS AND PROPERTIES OF METAL BIS(DICARBOLLIDE)S AND OTHER CLUSTER BORON COMPOUNDS AIMED FOR EXTRACTION PURPOSES A.

General Properties of Metal Bis(dicarbollide)s and Other Cluster Borate Anions

This section is focused, almost exclusively, on a small segment of boron cluster species based on the stable 12-vertex anionic metallaborate, borate, and carbaborate compounds with icosahedral closo structures which have been designed, synthesized, and tested with the aim of their subsequent use in liquid-liquid extraction. This category of anion-forming compounds ranges now from the parent metal bis(dicarbollide)s through their boron and carbon substituted derivatives to mixed sandwich compounds and simpler closo anions such as the substituted [CB11H12]- and [B12H11NH3]- derivatives. For their schematic structures see Fig. 1 and Tables 1–11.

Figure 1 Schematic structures of the univalent anions most studied for extraction purposes, including the cage numbering schemes. (a) Parent cobalt bis(1,2-dicarbollide)(1-), (b) isomeric parent cobalt bis(1,7-dicarbollide)(2-), (c) 1-amine-closo-undecahydro dodecaborate- ion, (d) closo-1-carba-dodecahydro dodecaborate- ion.

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Table 1 B-halogen and B-Alkyl Substituted Cobalt Bis(1,2-dicarbollides)

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Table 1 Continued

# DCs measured at NRI under following conditions: 0.01 M solution in nitrobenzene, 1 M HNO3; approximately the same value of distribution coefficient is obtained for all halogenated derivatives, regardless to the number of substituents. NA=data not available. References containing also extraction results are denoted with *. When no references for extraction data is given, then the results are from Ref. 256.

The general chemistry of these anions has already been the subject of extensive reviews and monographs. Boron-cluster compounds can be defined as threedimensional aggregates of {BH} cluster units interconnected by 2-electron 3-center B–B–B bonds and classical 2-electron 2-center B–B bonds, the former being the result of the electron-deficient nature of boron in borane molecules [22, 23]. The corresponding borane molecules are composed of a defined number of {BH} vertices (n) that are arranged as triangular facets of a deltahedral cage. The geometry of the cluster is dependent on the number of boron vertices and cage electrons [23]. Each of the {BH} vertex units provides 3 orbitals and only 2 electrons to the cluster bonding scheme. This results in electron deficiency, formation of 3-center bonds, and an extensive electron delocalization over the whole cluster area. The most stable 12-vertex series, which will be discussed here, has 26 cage electrons present in the cage orbitals and adopts an icosahedral geometry. This class belongs the wide series of cluster borate [BnHn]c- anions, in which the values n=12 and c=2 fit for the particular icosahedral arrangement. Notional replacement of {BH}- cluster units by the isolobal {CH} group (providing 3 skeletal electrons) then leads to carboranes of the basic structural formulas [CmBnHn]c m (m is 1 or 2 for known carbaboranes of the closo series). Substitution of the {BH}- moiety by other main groups atoms such as S, P, or N leads to heteroboranes or mixed carbaheteroboranes, etc. An insertion of a metal atom or metal moiety (generally regarded as more electron deficient than boron) into a carborane framework generates

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metallacarboranes. The basic geometrical and electron counting considerations for closo cage metallaboranes and metallacarbaboranes reflects those used for carboranes and heteroboranes. Much of the early and present progress in the metallacarborane area emerged from the recognition by M.F.Hawthorne in 1965 [1, 24–26] that the frontier orbitals of the [C2B9H11]2- nido dicarbaborane dianion (“dicarbollide” anion) open face are similar to those of the cyclopentadiene ion [C5R5]-. This similarity has been subsequently experimentally verified by synthesis of a broad range of mixed sandwich and full sandwich dicarbametallaboranes. These have been followed by other classes of cluster boron compounds, such as monocarba-, tricarba-, tetracarba-, and recently also pentacarbametallaboranes and heterocarbametallaboranes possessing closo-, or open cage nido-, and arachno-structures (not discussed here), including structural types lying on the borderline. The carbametallaboranes were described in hundreds of original studies that have been summarized in extensive reviews and monographs [27–32]. This review will cover only a small section of this extensive area. In principle, bis-icosahedral metalla-complexes containing two dicarbollide ligands [(C2B9H11)2M]c- [c=2 for M(II), c=1 for M(III) and c=0 for M(IV), respectively] are analogues of metallocene complexes that are well known from organic chemistry and are widely used as catalysts in polymer synthesis, supramolecular chemistry, medicinal use, etc. With respect to central metal atom complexation, the dicarbollide [C2B9H12]2- anion serves as a 6-electron donor η5 bonding ligand, preferring low spin complexes. Due to their formal divalent negative charge, the dicarbollide ligands are known to stabilize complexes with high metal oxidation states. The principal differences between the chemistry of metallocene and dicarbollide-metal-sandwich species lie in the very stable space filling “peanutlike” structures formed by the later, which can be described in terms of two fused icosahedra units sharing a common metal vertex. The metal bis(dicarbollide)s possess, in the majority of known cases, a negative charge delocalized over the large surface of the molecule. This and the particular character of bonding in the cluster lead to significantly enhanced thermal, chemical, and electrochemical stabilities.

B. Cobalt Bis(dicarbollide)s 1. Nomenclature A unique place in the design of the borate anion extraction agents has been occupied from the early beginning by the [closo-commo-(1,2-C2B9H11)2-3-Co(III)]- anion (1) and its derivatives (see Fig. 1). It is appropriate to make here some remarks on the nomenclature and cage numbering of this and relates species, due to discrepancies often seen in the literature. The descriptor numbering scheme is shown in Fig. 1; the cage is numbered starting from the carbon positions. If the deltahedral ligands are not substituted, the rotation barrier imposed only by partial the δ+ charge on the carbon atoms is low, and both ligands can almost freely rotate

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in solution around the axis connecting the atoms B10-Co-B10. In this case, clockwise spiral deployment has been preferred, according to IUPAC nomenclature. However, special cases may apply: Bulky groups at the rim of the pentagonal ligand planes can sterically hinder free rotation, or an exo-skeletal bridge could stop it definitely. In such cases, mesomeric and racemic forms can arise as has been pointed out many times in the literature; for example, see Refs. 33–37, where deeper discussion appears. If the racemic form is resolved into enantiomers, absolute configuration of the σ-enantiomer corresponds to a dextrorotary spiral whereas the ρ-enantiomer applies to the laevorotary alternative. In this case, both orientations should be considered equal, as has been discussed several times in papers from this group [36, 37]. The ACS [38] and IUPAC [39] nomenclature recommendations for the anion 1:3,3'-commo-cobalta-bis(undecahydro-1,2-dicarba-closo-dodecaborane)-(1-)ate seem too complex and cumbersome for practical purposes. For this reason, semitrivial or trivial names have been proposed and can bee found more frequently in the literature. The semitrivial names bis(1,2-dicarbollido)-cobalt(III) (1-)ate, cobalta(III) bis(1,2-dicarbollide), or cobalt(III) bis(1,2-dicarbollide) were proposed by Hawthorne and associates [26]. Trivial or technical names have often appeared in the research reports, patents, and even in the open literature, such as COSAN (abbreviation from Cobaltacarborane Sandwich Anion) proposed by the Institute of Inorganic Chemistry for technological use, cobalt dicarbollide, CDC or simply dicarbollide. The latter name, however, can be confused with the 11-vertex dicarbaborane anion without metal, and thus is not recommended. The term cobalt bis(dicarbollide) seems the most widely used in the recent literature and will be used also within this review. From geometrical considerations it follows that 45 positional isomers are possible when considering the presence of four carbon atoms in the two dicarbollide ligands of the [(C2B9H11)2-3-Co]- ion. Surprisingly, one sole isomer has been prepared and characterized in this anionic cobalt bis(dicarbollide) series [26]. This is the 2,2'-commo-cobalta-bis(1,7-dicarba-closododecaborane)-(1-)ate ion, or cobalt(III) bis(1,7-dicarbollide) (2). This isomeric alternative is characterized by nonadjacent (meta) positions of the carbon atoms, lying still in the proximity of the metal atom. For its schematic structure and cage numbering scheme, see Fig. 1b. This limited number of anionic isomers is in deep contrast to the mixed-sandwich series of the neutral [3-(␩5-C5H5)CoC2B9H11] complexes, where a large variety of the positional isomers has been reported [40]. 2.

The Parent Cobalt Bis(dicarbollide) Anions: Synthesis and Properties

The chemistry of the [(1,2-C2B9H11)2-3-Co]- (1) and [(l,7-C2B9H11)2-3-Co]- (2) anions [24–26] has been continuously developed over nearly four decades. It has been already reviewed several times [27, 29] most recently and comprehensively in 1999 [41]. The chemistry of the former species is certainly the most studied among the carba metalla carboranes. The fact that the ion 1 is diamagnetic is an

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advantage, which allows for convenient characterization of its derivatives by 11B, 13 C, and 1H NMR techniques. The main properties, including the m.p., 11B, 1H, UV, and IR spectroscopic and electrochemical data are summarized in Ref. 42. Cobalt bis(dicarbollide) belongs to a class of low nucleophilic and low coordinating anions [43, 44]. The X-ray determined molecular structure of Cs1 was published soon after its first synthesis [45]; however, the carbon positions were fully refined later in the structure of Et3NH1 [46]. The molecular size of this ion is relatively large, the mean distances B(10)-B(10') and B(4)-B(7) and C(2)-B(8) cluster positions being 7.820(8), 2.871(6), and 2.787(5) Å, respectively (average values based on structural data of 31 nonbridged compounds from the Cambrige Crystallographic Data Centre). The terminal hydrogen atoms of 1 have a hydridic character. Along with charge delocalization over the surface, this is apparently the main cause of the high degree of dissociation of the very strong free conjugate acids, and the unique hydrophobicity of all cobalt bis(dicarbollide) derivatives. A characteristic feature of the bis-icosahedral cobalt bis(dicarbollide) is good solubility of its free conjugate acids and most of their salts in medium polarity solvents like ethers, nitro-solvents, halogenated solvents, etc, to which they can be extracted from an aqueous phase. The salts with bulky cations are sparingly soluble in water [42, 47]. These properties were recognized by Plesek already 30 years ago [48]. The unique importance of 1 and 2 in the design of extraction agents depends on the extraordinary chemical and thermal stability of 1 and 2 due to their “pseudoaromaticity” and to the completely filled electronic shell of the Co(III) cation which is furthermore sterically shielded by two bulky dicarbollide ligands. The central cobalt atom is chemically inert and stable towards a nucleophilic attack. Furthermore, the oxidation-reduction potentials are high [42, 49]. Reduction of Cs1 with 1 equivalent of Na(Hg) amalgam or Cs metal proceeds on the central atom without cage decomposition; only the cobalt(II) dianion [Co(II)(1,2C2B9H11)2]2- was isolated, which reverts back to 1 upon exposure to 0.5 equivalent of elemental iodine [27]. This was proven by the molecular structure of the solvated salt of this anion Cs2(DME)4[Co(1,2-C2B9H11)]2- determined by X-ray diffraction. In this particular case, according to some observations, agents such as RLi can also reduce the central cobalt atom [50]. In such circumstances, the lithium atom can penetrate between the two dicarbollide ligands close to the central atom, due to its small diameter. On the other hand, other authors have reported that lithiation of carbon vertices only proceeds when anion 1 is treated with BuLi [51]. The parent cobalt bis(1,2dicarbollide) anion 1 in the form of its Cs salt is stable toward HNO3 up to 2 M for several days contact. Beyond this concentration, the hydridic H atoms in positions B(8) and B(8') are slowly oxidized to -OH groups or substituted by the strongest nucleophile present in the system. Cobalt bis(1,2dicarbollide)s are exceptionally stable toward radiation. Experiments have shown that they survived radiation of 106 Gray per 24 h [52]. The cage is slowly degraded upon treatment with concentrated solutions of NaOH or KOH in protic solvents. The first detectable step is the degradation of

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one of the symmetry equivalent boron atom adjacent to carbon sites, B(6,6'). A cobalt dicarbaborane is formed which contains one open 11-vertex nido cage and one 12vertex closo cage [(1,2-C2B9H12)(7',8'-C2B9H11–3,9'-Co]2- sharing the common cobalt atom. The open-cage part of this ion can be further protonated to the [(1,2C2B9H12)(7',8'-C2B9H11–3,9'-Co]2- univalent anion, oxidized using H2O2 to the closo species [(1,2-C2B9H10)(7',8'-C2B9H11–3,9'-Co]-, or selectively degraded by FeCl3 to the a product missing another boron vertex [53]. Complete degradation to boric acid occurs upon a long-term contact. All reported synthetic strategies to the anions 1 and 2 are based on closo-1,2C2B10H12 (and closo-1,7-C2B10H12), (ortho- and meta-carboranes) [54, 55] as the starting materials; see Scheme 1. The icosahedral o-carborane cage undergoes one boron degradation in position B(3) (or symmetrically equivalent B(6) in the parent unsubstituted compound) when treated with a large variety of basic reagents with the formation of the [7,8-C2B9H11]- (dicarbollide) anion [56, 57]. Typically, NaOH or KOH in methanol or ethanol are used [58] for the unsubstituted compound, although numerous other reagents and conditions were designed using organic bases or the F- ion [59–69]. These are, in turn, particularly useful in the synthesis

Scheme 1 General synthetic route leading to cobalt bis(1,2-dicarbollide)- ion or neutral mixed 3-(␩5-C5H5)-3-Co-(1,2-C2B9H11) complexes. Parent as well as substituted compounds were prepared using this approach, under a large variety of particular reaction conditions.

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of substituted and less stable ions. The [7,8-C2B9H12]- anion still contains a hydrogen bridge, which can be is in situ removed using either NaH (which correspond to the original procedure described by Hawthorne [26, 56, 57] similar to the procedure using BuLi [70]), or alternatively by KtBu [49, 71] in ether solvents. The ether solution of the divalent anion [7,8-C2B9H11]2- is then reacted with anhydrous CoCl2 at higher temperatures. The divalent anion [7,8-C2B9H11]2-, serving as the important intermediate in the synthesis, remained for a long time surprisingly poorly characterized until a recent detailed NMR and theoretical study [72] of its Li+, Na+, and K+ salts. In the synthesis of the parent cobalt bis(dicarbollide) and some other unsubstituted metalla bis(dicarbollide)s, aqueous conditions can also be applied using CoCl2 · 6H2O and 10 M KOH [25, 26]. In this case, an equilibrium between dicarbollide monoanion and dianion forms in the solution, which is sufficient for the formation of the metal complex. Typical yields of the cesium salt of the parent anion 1 are usually within 70–80%, regardless if anhydrous or aqueous procedures are applied. On the other hand, a simultaneous in situ degradation of the cage of 1 proceeds under conditions of the aqueous route, which in turn leads to a small yield of the “double-decker” sandwich with a divalent anionic charge [(C2B9H11)Co(C2B8H10)Co(C2B9H11)]2- (3) (see Table 1) as the main side product from the synthesis. Formation of this ion and a larger trimetallic “triple-decker” sandwich anion [(C2B9H11)Co(C2B8H10) Co(C2B8H10) Co(C2B9H11)]3- in strongly basic solutions was revealed and studied already in the early period of the metallaborane chemistry [73, 74]. The molecular structures of these anions were determined by X-ray diffraction studies [75, 76]. Synthetic routes to even higher congeners were investigated more recently [77]. Extraction properties of 3 have also been tested and found similar to that of parent cobalt bis(dicarbollide) 1 [78]. The isomeric cobalt(III) bis(1,7-dicarbollide) (2) (see Fig. 1) arises from base degradation of meta-carborane 1,7-C2B10H12 to the [7,9-C2B9H12]2- ion and metal insertion reaction [26]. Only a limited number of studies describing extraction properties of this otherwise even more stable species appeared in the literature. This has been undoubtedly influenced by the higher price of the m-carborane and its much slower and difficult degradation to the respective [7,9-C2B9H12]2- anion, which is the limiting reaction step in the synthesis. 3.

Substituted Cobalt Bis(dicarbollide) Ions: General Synthetic Methods

From the point of view of synthetic chemistry, the main characteristic feature of 1 (and 2) is the ability of these anions to undergo an easy substitution of its B(8,8') (or B(6,6')) terminal hydrogen atoms for a variety of nucleophiles L. This leads to the formation of the respective B(8)-L and B(8,8')-L2 (or B(6)-L and B(6,6')-L2) derivatives (or bridged structures) [41]. This is apparently the reason why literature reports on boron substituted cobalt bis(dicarbollide) compounds prevails over carbon derivatives of 1. This is in a contrast to the chemistry of the neutral closo-carboranes,

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where the situation is reversed [54, 55]. Many examples are reported in the abovementioned review [41] and in citations therein. The reaction proceeds either via an electrophilic substitution reaction path known from organic chemistry, or by a mechanism that has been defined as electrophile induced nucleophilic substitutions (EINS) [33, 79]. The later pathway is practically unknown for organic compounds, but can be considered as the typical reaction scheme within the boron cluster series and, in particular, for the anionic species. The basic principles of such reactions are as follows: an electrophile, eventually generated in situ, abstracts the hydridic terminal hydrogen atom from the most negative B–H vertex of the boron cluster position (B(8)-H and the symmetry-equivalent B(8')-H in 1) and the transient vacant B orbital then becomes attacked by the most nucleophilic or most electron-rich moiety from the environment. An important alternative way to substituted derivatives proceeds via degradation of already substituted carboranes and subsequent metal incorporation (see Scheme 1). It should be noted that this reaction path is rarely used for synthesis of boronsubstituted compounds, an is used only if special conditions apply, e.g., for radiolabeled [80–82] compounds or isomers not accessible by direct substitution reactions. The reason for less frequent use of this way is obvious: Anhydrous conditions are usually required to accomplish the metal incorporation and the reactions give typically lower yields of the resulting substituted dicarbollide anions; for example, see Ref. 80. The opposite is true for the synthesis of carbon substituted derivatives, where this reaction scheme has been considered until now as the main and the most reliable preparative route. Recently, a direct synthesis proceeding via lithiation of C–H bonds of the cobalt bis(dicarbollide) and subsequent reactions with alkyl halides was reported [51]. On the other hand, other authors failed to employ these reactions, at least for some more sophisticated substitutions [83]. 4. Boron Substituted Anions As has been discussed above, the B(8,8') skeletal positions opposite to the carbon atoms, and in the vicinity of the central metal atom, identified as the sites of the highest electron density [84], are prone to the attack by a nucleophile in the presence of the strongest Brõnsted or Lewis acid reagents that are able to abstracts B(8,8') hydrogen atoms. A reverse side of the easy reactivity of the B(8,8') positions is reflected in the eventuality of introduction of a hydroxy or other polar group upon longterm contact of the parent anion 1 with strong oxidizing aqueous acids. Presence of such polar groups increases the solubility of the resulting products in the aqueous phase and decreases the extraction efficiency. This was recognized very soon in the extraction process development, and hence the most reactive vertices were blocked by appropriate substitution. A large number of derivatives of the anion 1 with substituents at the most reactive B(8,8') positions have been prepared and studied for liquid-liquid extraction. The presence of substituent(s) has increased the chemical stability of the molecule substantially, especially in respect to attack by nitric acid and oxidation in the process. The simplest approach applied from the

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beginning involved halogenation of the boron sites B(8,8') and also the vicinal positions B(9,9',12,12'). Later, aryl [85] and alkyl [86] substituents were introduced in these sites. More sophisticated substitutions within the series of compounds yield anions that simultaneously play the role of a selective metal scavenger that is able to tightly complex the target ions. Halogenated Cobalt Bis(dicarbollide)s. Most of the cluster borate and carborate anions are known to undergo halogenation reactions readily when treated by elemental halogens or with a variety of halogenating agents; for examples, see Refs. 43, 44, 87. These reactions were successfully applied for the protection of the most reactive sites of the cage of 1. As was found almost three decades ago, the oxidation of the cage 1 can be effectively prevented by chlorination leading to [(8,8',9,9',12,12'-Cl6-cobalt bis(dicarbollide) and the best extractants developed for the dicarbollide technology before 1995 belonged to the class of halogenprotected cobalt bis(dicarbollide)s. The halogenated derivatives (for schematic structures and basic extraction data, see Table 1) are very stable species, especially in acidic conditions. Most of them survive long-term contact with up to 10 M HNO3 without a noticeable decomposition [48, 42]. The substitution of the two most reactive positions by halogens is sufficient to protect the boron skeleton. Disubstituted derivatives were proven to be stable in 3 M HNO3 for over a 1-month period [88]. Nevertheless, the hexachloro protected anion 1 is the most widely used species due to its better accessibility and higher hydrophobicity compared to that of the parent compound and the less halogenated species. It should be pointed out that the character of B–X (X=Cl, Br, I) bonds in the cluster boron compounds is unique and differs dramatically from that known in organic chemistry or from simple (BX3) compounds and their organic derivatives; for some illustrative data, see Ref. 88. Although the B–X bonds are longer than that in BX3 their stability is considerably higher. Although quantitative data for dissociation energies are not available, experimental evidence proved this on a qualitative level. A good example of this experimental evidence is the low reactivity of the boron-halogen bonds in the halogen derivatives of the basic icosahedral anion. Halogen atoms from perhalogenated derivatives or cannot be abstracted from the cage even by treatment with alkali metals in liquid ammonia or THF [87]. This is even more valid for less halogenated derivatives. Replacement of the halogen by another nucleophile proceeds only exceptionally and under forced conditions. Iodine substituents of the periodinated or bromine atoms from perbrominated compounds can be partially replaced by the CN moiety under intensive UV irradiation, although less halogenated compounds do not react [87]. Similar observations have been made for the chloro and bromo derivatives of the cobalt bis(dicarbollide). Abstraction of the halogen atom was not possible even upon treatment by NaNH2 or RLi reagents [88]. The stability of the B–X bond is considered rather to be a consequence of kinetic and steric factors [88].

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A noticeable drawback arising from the electronic effect induced in the cage by the halogen substituent(s) is that the positions B(6,6') which lie in the vicinity of the carbon atoms become even more sensitive to degradation with strong bases than those of the parent anion 1 (see text above). In the first stage of such degradation reactions, species [(C2B8H7X3)(1,2-C2B9H8X3)-Co]2- are produced that are not sufficiently soluble in the organic phase and thus are not suitable for extraction purposes. From this point of view follows the limited suitability of halogen derivatives for treatment of strongly basic nuclear waste solutions. The first halogenation reaction of the anion 1 to its hexabromoderivative was reported by Hawthorne [26]. Chlorination, bromination, and iodinations were later studied in the 1970s and 1980s in more detail in connection with extraction process development carried out at the IIC and NRI groups [4, 7, 89, 90]. Studies of the physicochemical properties were done in cooperation with the Comenius University at Bratislava [52, 91–97] and later also by other authors [88]. In contrast, the first successful fluorination reactions were reported as late as 2000 [98]. The use of the mild fluorinating agent, 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA) in anhydrous acetone accomplished fluorine substitution of anion 1, yielding the B(8,8') difluoro derivative [(8,8'-F2-(1,2-C2B9H10)23-Co]- (4). The structure of the salt of the B(8,8') derivative 4 was determined by single crystal X-ray crystallography. The chlorination reactions of 1 with elemental chlorine in ethanol-tetrachloromethane, nitrobenzene-tetrachloromethane or acetic acid lead to stepwise chlorination at the boron atoms known to possess the highest electron densities, i.e., in the order B(8), B(8,8') B(8,8',9), B(8,8',9,9'), B(8,8',9,9',12), B(8,8',9,9’12,12') [91]. In spite of the fact that the reaction rate decreases with the number of introduced halogen atoms, the presence of the species containing up to nine chlorine atoms was observed in some technological samples recently studied by Electrospray M.S. methods [99]. According to the explanation given in the older literature [91], these highly chlorinated species result from poor reaction rate control in the early reaction steps. This can lead to a statistical substitution proceeding simultaneously at less favored skeletal sites, followed by the reactions proceeding at the usual positions. The stereochemistry of these derivatives remains unknown and it may be only assumed that positions B(10, 10') would be the probable substitution sites. The monochloro derivative [(8-Cl-1,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (5) was prepared by direct chlorination in a CCl4-ethanol mixture, or by careful halogenation using ␥ irradiation of 1 in chloroform-benzene mixtures [91]. This is a distinguishable step, due to the decrease of the reaction rate upon introduction of the first chlorine atom [91]. Since the reactivity of the positions B(8,8') is substantially higher than that of the B(9,9',12,12') sites, the disubstituted anion [(8,8'Cl 2-(1,2-C 2B 9 H 10 ) 2 -3-Co] - (6) could be prepared in pure form by direct halogenation carried out in the same solvent and controlling carefully the volume of the introduced elemental chlorine [91]. Other authors reported that derivative 6 can be obtained as the single product, from direct halogenation carried out in THF

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in presence of iron powder, or by heterogenous reaction of the tetramethylamonium salt Me4N1 with a NaOCl/HCl mixture in water (whereas the same reaction with other cations led to cage decomposition) [88], or by halogenation in THF using NaOCl/HCl as the halogenation agent. More reliable seems an alternative way, when the derivative 6 was obtained by halogenation of 1 with N-chlorosuccinimide [88]. The molecular structure of the Me4N6 was determined by X-ray crystallography [88]. The isomeric B(9,9') dichloroderivative [(9,9'-Cl2-(1,2-C2B9H8)2-3-Co]- (7) was prepared starting from halogenation of ocarborane to the 9-C2B10H11Cl cage by degradation and metal insertion reactions [91]. On increasing the chlorine content beyond two atoms, mixtures of several species are always obtained from direct halogenation. Especially, if an odd number of halogen atoms is introduced, isomeric and diastereoisomeric mixtures of each derivative would necessarily be produced, further complicating the isolation of pure species. Despite of this, isolation of almost pure trichloro derivative [(8,8',9Cl3-(1,2-C2B9H9)(1',2'-C2B9H10)2-3-Co]- (8) was reported as being done by direct chlorination in nitrobenzene-CCl4 under cooling followed by chromatographic separation of the products on a Sephadex LH-20 column [91]. Similar purification provided the tetrachloro derivative (9) [(8,8',9,9'-Cl4-(1,2-C2B9H9)2-3-Co]-. The isomeric [(9,9',12,12'-Cl4-(1,2-C2B9H9)2-3-Co]- derivative (10) was prepared via degradation of the 9,12-Cl2-1,2-C2B10H10 substituted carborane and subsequent metal insertion reaction [91]. On the other hand, the leading position in the extraction technology is governed by the hexachloro-substituted product [(8,8',9,9',12,12'-Cl6-(1,2-C2B9H8)2-3-Co](11) [89–91]. This derivative could be prepared in an almost pure state as a smallscale laboratory chemical [91]. However, for larger-scale chemicals, used for extraction purposes, only an average composition of six halogen atoms could be obtained. These materials are typically mixtures of derivatives containing 5, 6, and 7 chlorine atoms, including their respective isomers. These would be referred to as chloro-protected cobalt bis(dicarbollide) (12) (a technical abbreviation frequently used in Russian Federation and the United States is ChCoDic). All the above chloro derivatives were also obtained from radiolysis of 1 in chloroform-benzene or CCl4nitrobenzene solvent mixtures, depending on the dose absorbed [91]. The direct chlorination in acetic acid based on the patent [90] and performed under cooling till introduction of approximately two equivalents of chlorine and then at ambient temperature is currently in the use for its production by the main supplier of this compound (Katchem Ltd. Prague). Addressing the above-mentioned possibility of halogenation to higher stages, special precautions should be followed during the production of 12 in order to ensure the average chlorine content to correspond to 6. These should consist of careful application of the analytical and separation methods such as elemental analysis, 11B and 1H NMR techniques [91], isotachophoresis [93, 97], HPLC [100, 101], or preferably LC-MS, during the chlorination stages of the production and careful analysis of the final product.

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Bromination reactions proceed more slowly and the monobrominated anion [8-Br-1,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (13) can be obtained as well as the dibromoderivative [(8,8'-Br2-(1,2-C2B9H10)2-3-Co]- (14) by direct halogenation in methanol [91]. The latter compound 14 was more recently prepared using the reaction with N-bromosuccinimide in THF [88]. Both derivatives 13 and 14 resulted also upon radiolysis of nitrobenzene-tribromomethane or tribromomethanemethanol solutions of 1 [52]. The isomeric derivative [(9,9-Br2-(1,2-C2B9H10)2-3Co]- (15) was prepared from 9-Br-o-carborane, similarly as compound 7 in the respective chlorinated series [91]. Also the pure isomer [(9,9',12',12-Br4-(l,2C2B9H9)2-3-Co]- of the tetrabromo derivative 16 was prepared using an indirect approach from 9,12-Br2-o-carborane [102]. Another isomeric derivative [(4,7,4',7'Br4-(1,2-C2B9H8)2-3-Co]- (17) was obtained recently from the bromo substituted dicarbollide anion [9,11-Br2-7,8-C2B9H10]-, the molecular structure of which was determined by X-ray crystallography [103]. Room or higher temperature bromination reactions with elemental bromine result in the hexabromoderivative [(8,8', 9,9',12,12'-Br6-(1,2-C2B9H8)2-3-Co]- (18) [91]. The cesium salt of this derivative was characterized by X-ray diffraction analysis [94]. Lower reaction rates of bromination generally mean that the technological samples of bromo protected cobalt bis(dicarbollide) (19) are usually better defined than the respective chloro protected species 12. Direct iodination of 1 in methanol or ethanol gives either the iododerivative [(8-I-1,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (20) or the diododerivative [(8,8'-I2-(1,2C2B9H10)2-3-Co]- (21) [91], depending on the initial ratio of iodine to 1. The molecular structures of the Cs+20- and salt [95] and of the disubstituted derivative Cs+21- [96] were determined by single crystal X-ray diffraction. Although the direct iodination does not proceed to higher stages even under heating, the hexaiododerivative [(8,8',9,9',12,12'-I6-(1,2-C2B9H8)2-3-Co]- (22) was successfully prepared by iodination catalyzed with AlCl3 [91], or by reaction with ICl, which seems to be advantageous over the older procedure due to simpler reaction conditions without need of the catalyst and a good yield (92%) [86]. In addition to the above halogen-substituted anions, zwitterionic compounds have also been reported, where both dicarbollide ligands were connected by iodonium and bromonium bridges in the positions B(8,8') [(8,8'-µ-X-(1,2-C2B9H10)23-Co] (X= I, Br) (23, 24) [104]. These arise from AlCl3 catalyzed intramolecular cyclization reaction of the B(8) iodo or bromo derivative. The compounds 23 and 24 deserve to be mentioned here, due to a facile cleavage of the B–X–B ring proceeding with a large variety of bases. This can serve for further syntheses of the B(8,8') protected compounds bearing a metal selective group. One such example is given in text below (see Section II.4.c, compound 40). Cobalt Bis(dicarbollide)s Substituted at Boron Atoms with Alkyl and Aryl Groups. Direct alkylation of boron vertices was reported only recently. Reaction of anion 1 with neat CH3I catalyzed with AlCl3 produced the [8,8'-(CH3)2-(1,2C2B9H10)2-3-Co]- anion (25) [105]. A similar procedure leading to the formation of

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[8,8'-(i-C4H9)2-(1,2-C2B9H10)2-3-Co]- (26a) can be found in the patent literature [10] along with that for the dibutyl analogue derived from the isomeric anion 2 series [6,6'-(i-C4H9)2-(1,7-C2B9H10)2-3-Co]- (26b) [11]. The later two anions were designed for extraction purposes, but no data are available. An indirect method, i.e., (PPh 3) 2PdCl2/CuCl-catalyzed cross-coupling procedure of iododerivatives with alkyl Grignard reagents, has been reported. Synthesis of [8,8'-(n-C10H21)2-(1,2-C2B9H10)2-3-Co]- (27) using the reaction of the diiododerivative 21 with decylmagnesium bromide in THF led to 76% yield of this anion, according to a recent U.S. patent [10]. A palladium-catalyzed crosscoupling reaction of hexaiododerivative 22 with CH3MgBr produced the hexamethyl derivative [(8,8',9,9',12,12'-(CH3)6-(1,2-C2B9H8)2-3-Co]- (28) obtained in 60% yield [86]. The search for more effective compounds with enhanced hydrophobic properties for Cs+ and Sr2+ extraction promoted synthesis of various aryl-substituted derivatives, where the cage positions were bridged with a variety of aromatic organic groups. The first compound of this class [8,8'-µ-1,2-C6H4)-(1,2-C2B9H10)2-3-CO]- (29) was prepared in low yield, already in 1972, by thermally induced reaction of aryldiazonium salts with Me4N1 [106], but without any intention of its further use. The molecular structure as determined by X-ray diffraction was described in a subsequent paper [105]. A more reliable procedure leading to the bridged derivative 29 in better yield, however, consists of an AlCl3-catalyzed reaction of Cs1 with benzene used as neat solvent [107]. Recent substantial revisions of this reaction led to the synthesis of a large series of compounds bridged in positions B(8,8') with various arylene substituents [85, 108]. Compounds with phenylene 29, tolylene [8,8'-µ-(CH3-C6H3)-(1,2-C2B9H10)2-3-Co]- (30), ethylphenylene [8,8'-µ-(CH3CH2C6H4)-(1,2-C2B9H10)2-3-Co]- (31), o, m, p-xylylene [8,8'-µ-(CH3)2-C6H3)-(1,2C2H10)2-3-Co]- (32a, b, c), biphenylene [8,8'-µ-(C6H5-C6H4)-(1,2-C2B9H10)2-3-Co](33) and tetraline (34) bridging moieties were synthesized (see Scheme 2). For schematic structures and extraction data see Table 2. Reaction with naphtalene

Scheme 2 Synthetic procedure leading to arylene bridged and arylene bis-bridged cobalt bis(dicarbollide) anions. A mixture of both respective types always results, which should be subsequently separated.

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Table 2 Arylene-Substituted Cobalt Bis(1,2-dicarbollides)

0.01 M extractant solution in respective solvent indicated in exponent, 1 M HNO3, NPOE—nitrophenyl octyl ether, NB—nitrobenzene. #NRI—unpublished results. References containing extraction results are denoted with *.

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carried out in cyclohexane as the solvent resulted in an interesting rearrangement of the aromatic ring providing compound, in which the dicarbollide ligands were interconnected via an unexpected 3-carbon atom bridge [8,8'-µ-CH2-C9H6)-(1,2C2B9H10)2-3-Co] (35) [109]. Also, the compound containing a nonbridging phenyl substituent at B(8) position [8-(C6H5-1,2-C2B9H10) (1,2-C2B9H11)-3-Co]- (36) was obtained in moderate yield from dimethyl sulfate—induced reaction of 1 with benzene [110]. The molecular structures of 35 and 36 were determined by X-ray diffraction [109, 110]. Revision and extensions of the AlCl3-catalyzed reactions along with careful product separations and characterization by modern NMR, mass spectrometric and crystallographic techniques emerged in the discovery of a novel promising class of 4,8',8,4'-R2-bis(arylene)- bridged cobalt bis(dicarbollide)s [4,8',4’8-µ(C6H4)2-(1,2-C2B9H9)2-3-Co]- (37), [4,8',4’8-µ-(CH3C6H3)2-(1,2-C2B9H9)2-3-Co] (38) and [4,8', 4’8-µ-(CH3CH2C6H3)2-(1,2-C2B9H9)2-3-Co]- (39) [85]. For the reaction path, schematic structures and extraction data see Scheme 2 and Table 3. Reaction conditions have been optimized to give up to ca. 54% yields of these interesting compounds [108]. The compounds with arylene substituents 37 to 39 were the subject of relatively extensive extraction tests. The basic member of the series 37 proved especially to have excellent complexation properties and extraction selectivity for the cesium cation, surpassing markedly the extraction ability of chloro protected cobalt bis(dicarbollide) (12). A significant advantage of this class of extraction agents lies in their reasonably high solubility in aromatic solvents (toluene, xylene, etc.), provided that some aromatic sulpho compounds, designed and used as solubilizers, are added to the organic phase (see Section VII.D) [111]. X-ray studies of the Cs+ complex of the anion 37 proved, that an angle of 72° between planes of phenylene substituents is favorable for a tight Cs+ complexation [108]. The distribution ratio of Cs+ has been found so high that it imposed a consequent problem in the stripping. As in the case of halogen-protected anions, this could be accomplished by nitric acid of high concentration. On the other hand, Table 3 Bis-Arylenelene Bridge Substituted Cobalt Bis(1,2-dicarbollides)

0.005 M extractant solution in nitrobenzene, 0.5 M HNO3. X, 0.005 M extractant in 0.4 M DEPSAM solution in toluene. References also containing extraction results are denoted with *. When no reference for extraction data is given, then the results are from Ref. 256.

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stripping with concentrated acid is not the optimal solution, since these compounds are more sensitive to the oxidation effect of nitric acid than chloro protected cobalt bis(dicarbollide) 12. Boron-Substituted Cobalt Bis(dicarbollide)s Functionalized with Metal Complexing Groups. A typical procedure for lanthanide and actinide Mn+ cation transfer from an aqueous solution into a low polarity organic phase uses tight complex formation with neutral ligands. In the known schemes employing organic ligands, metal is surrounded with electron donor atoms of the uncharged ligands like crown ethers, malonamides, phosphine oxides, CMPO (N,N-dialkyl carbamoyl methyl dialkyl phosphine oxides), or modified calixarenes, able to form strong, hydrophobic, but still n+ charged complex. In this case, hydrated nitrate anions have to be transferred across the interface together with the target cation. A new approach was recently conceived, aimed at the development of effective separation methods for Sr2+, lanthanides, and minor actinides, i.e., long-lived nuclides, from highly acidic solutions of high level radioactive waste (HLW). Several feasible synthetic ways were found that enabled the provision of anion 1 with cation ligating groups from the above series. Selective groups containing electron donor atoms (e.g., oxygen, nitrogen, and phosphorus) were successfully bonded to the 1 cage by covalent bonds. For schematic structures and extraction data of this class of new compounds, see Tables 4–7. Several new compounds of this type proved effective in transferring the target radionuclides from 1 M HNO3. The first attempt in this direction was made in the early 1990s when the [(8(C5H11-(CH2CH2O)2-1,2-C2B9H10)(8'-I-C2B9H10)-3,3'-CO]- anion (40), was prepared from the [(8-(CH2CH2OCH2CH2O)-C2B9H10)(8'-I-C2B9H10)-3,3'-Co] zwitterion (41) and tested for extraction properties [78]. The disubstituted intermediate compound 41 formed upon treatment of the iodonium bridged derivative 23 with dioxane [104]. The dioxane ring of 41 contained an oxonium oxygen and could be opened by NaOC5H11 to produce anion 40. The compound 40 was tested for strontium extraction, however without observation of extraction enhancement in respect to the corresponding synergetic mixtures [78]. A similar synthetic method was applied in the synthesis of a subsequent anionic series with covalently bonded metal ligating groups. This consisted of the ring opening of the 8-dioxane-1 [8-O(CH2CH2)2O)-C2B9H10)(1’2'-C2B9H11)-3,3'-Co] zwitterion 42. This interesting bipolar derivative became available on a larger scale only recently (based on results of the IIC group) [109] and proved to be a versatile synthon. As in the case of 41, the dioxane ring containing an oxonium oxygen atom can be easily cleaved by almost any nucleophile of choice [112–115] producing the species functionalized with various end groups attached to the hydrophobic anion 1 via a diethylene diglycol chain (see Scheme 3). Similar kind of reactions provides also the recently reported compound containing tetrahydropyrane ring [116].

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Table 4 Cobalt Bis(1,2-dicarbollides) with Functional Groups Attached via Diethyleneglycol Chain

0.05 M 43, 44, 45, 47, 48 in toluene, 0.01 M 49, 50 in nitrobenzene, 0.01 M 51 in dichloroethane, 1 M HNO3. X, 0.1 M HNO3. References also containing extraction results are denoted with *.

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Scheme 3 The flexible synthetic route based on the cleave of the dioxane ring of the zwitterionic derivative 42. The reaction proceeds smoothly with a majority of possible nucleophic reagents, as has been already described in several papers (see Refs. 83, 112–115).

A novel series of compounds of the general formula closo-[(8-X-(CH2-CH2O)21,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (X=1-O-2-CH3O-C6H4 (43), 1-O-2-C6H5CH2C6H4 (44), 1-O-4-tC8H17-C6H4 (45), 1-O-3-CF3-C6H4 (46), P(O)(OC4H9)2 (47) and P(O)(OC4H9)(OH) (48) (see Table 4 and Scheme 3 for the schematic structures and the reaction procedure), including several others [83] resulted from this method and were subsequently tested for extraction purposes. The flexible synthetic methodology implied a possibility to modify and study the influence of the character of the end group and the whole substituent, including the steric strain of the chain on the metal bonding properties. Results were described in the recent article [113] and following papers (see below). This study allowed deeper understanding of the effects of the particular selective groups covalently bonded to 1. The molecular structure of the sodium complex of the anion 43 was determined by single crystal X-ray diffraction. The sodium atom in this case is tightly coordinated to five oxygen atoms of the spacer chain and the guaiacolyl terminal group of 43, one water molecule, and from the opposite side of the ligand plane the short B(8')-H-Na contact [2.26(3)Å] was found to be within a bonding distance. The cation is enveloped by a hydrophobic outer sphere composed of hydrophobic CH2 and CH groups of the organic substituent and B–H groups of the cobalta bis(dicarbollide)(1–) cage [113]. The most interesting derivative found in the above series seem to be the anion 48 that contains the terminal monoester functionality The presence of this phosphoryl end group, acting apparently as the second acidic centre in the molecule, is sufficient to increase substantially the transfer of the M3+ cations into an organic phase even from highly acidic waste solutions. It was shown experimentally that this ester group is resistant toward further hydrolysis to the respective end substituent. Neither alkaline nor acidic conditions succeded in giving the expected product in good yield. On the other hand, partial hydrolysis in the extraction system may possibly explain the observed sharp increase of the DEu extraction coefficients after several days contact with 1 M HNO3.

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Anionic crown ether compounds belonging to this family of compounds resulted upon cleavage of the dioxane ring of the zwitterion 42 with various sodium oxymethylcrowns differing in crown ether ring size [83, 114] (see Table 4). Three compounds [(8-(15-crown-5-CH2-O)-(OCH2CH2)2-1,2-C2B9H10)(1',2'-C2B9H11)3,3’Co]- (49), [(8-(18-crown-6-CH2O)-(OCH2CH2)2)-(1,2-C2B9H10)(1’2'-C2B9H11)3,3’Co] -, (50) and [(8–21-crown-7-CH2-O)-(OCH2CH2)2-1,2-C 2B9H10)(1’2'C2B9H11)-3,3’Co]- (51)] were prepared in high yields. These species represent the first examples where regular, oxygen atom-containing crown ether rings were covalently bonded to the cage of the hydrophobic anion 1. A second series of these derivatives, prepared for comparison (see Table 5), resulted upon deprotonation of the [(8,8'-(OH)2-(1,2-C2B9H12)2-3,-Co]- (52) anion by NaH in THF and subsequent reaction with p-toluenesulfonyl esters of the hydroxymethyl crown ethers [115]. Only compounds with single crown ether substituents could be isolated in preparative amounts from the respective reactions. Two compounds of this class [8-(15-crown-5-CH2O)-1,2-C2B9H10)(8'-HO-1',2'C2B9H10)-3,3'-Co]- (54) and [8-(21-crown-7-CH2O)-1,2-C2B9H10)(8'-HO-1',2'C2B9H10)-3,3'-Co]- (55) (Table 5) were prepared and adequately characterized. The molecular structure of the Cs+ complex of the species 54 was determined by single crystal X-ray diffraction. This structure exhibited coordination of two cesium atoms within two crown-cobalta bis(dicarbollide) ligands. Each Cs+ atom is coordinated to five oxygen atoms of one 15-crown-5 ring and to two O atoms of the second unreacted OH moiety in the position B(8') of the two ligands, thus forming a distorted square planar arrangement of two Cs+ cations and two O donor atoms. The hydrophobic core of the resulting complex, composed of the hydrophobic

Table 5 Cobalt Bis(1,2-dicarbollides) with Crown Ether Moieties Bonded via Shorter Chain

0.01 M extractants in nitrobenzene, 1 M HNO3. References also containing extraction results are denoted with *.

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anions and crown ether CH2 groups, is directed outwards of the complexed metal cation. The oxygen atom included in the -CH2-O-B(8) spacer does not participate on the metal complexation but its connection by intramolecular hydrogen bonds to the H–O–B moiety supports further the stability of this arrangement. The bonding of crown ether groups via a longer chain in compounds 49 to 51 has proven more favorable for Cs and Sr2+ extraction as was described in Ref. 114. The distribution coefficients for Cs+ and Sr2+ at higher acidities were found to be comparable to that observed for the synergetic mixtures of 19 and crown ethers, although a slight enhancement in Sr2+/Na+ selectivity was observed for covalently bonded species. A promising family of boron-substituted derivatives of 1 of the general formula [(8-CMPO-CH 2 -CH 2 O) 2 -1,2-C 2 B 9 H 10 )(1’2'-C 2 B 9 H 11 )-3,3'-Co] - [CMPO= Ph2P(O)CH2C(O)-NR, R=C4H9 (56), C12H25 (57), CH2C6H5 (58)] were prepared using a two-step procedure [115] (see Table 6). The first step consisted of the cleavage of the 8-dioxane-cobalt bis(dicarbollide) 42 ring by the respective primary amine. Subsequent reaction of the resulting amino derivatives with the highly reactive nitrophenyl ester of diphenyl phosphoryl acetic acid resulted in the

Table 6 Cobalt Bis(1,2-dicarbollides) with CMPO-like Groups Attached via Diethyleneglycol Chain

0.01 M extractant in toluene, 1 M HNO3. References also containing extraction results are denoted with *.

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anticipated CMPO-substituted products in high yields. The molecular structure of the supramolecular charge-compensated complex Ln(58)3 · 3H2O was determined by single crystal X-ray diffraction. The crystallographic results illustrated the capability of this anionic ligand to completely displace anions from the primary coordination sphere of the lanthanide cation. The Ln3+ cation is tightly coordinated by six oxygen atoms of the CMPO terminal groups (two from each ligand) and by three water molecules completing the metal coordination number of 9. The atoms occupying the primary coordination sphere form a tri-capped trigonal prismatic arrangement (see Fig. 2). Exceptionally high liquid-liquid D Eu distribution coefficients were observed for all three members 56 to 58 of this series.

Figure 2 Schematic drawing based on the molecular structure of Ln3+ complex Ln(58)3. 3H2O (for details see Ref. 115). Three functionalized cobalt bis(dicarbollide) ligands 58 lie in the coordination sphere of the metal ion, being bound to the cation via bidentate CMPO substituents. Hydrophobic groups are directed outward from the metal binding region. This may account for the high extraction efficiency of this class of compounds.

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In connection with the above CMPO-like family of derivatives, another, substantially different derivative [(8,8'-(Ph2P(O)CH2(CO)N)<(1,2-C2B9H10)23,3'-Co]- (59) (see Table 6) should be mentioned. This compound represents the first known example, where organic substituents typically present at the amidic end of the CMPO moiety (e.g., -C8H17) were replaced by the anionic boron cluster. This anion was synthesized using the well-known bridged aminoderivative [(8,8'-(H 2 N)<(1,2-C 2B 9H 10 ) 2 -3-Co] [117] as the starting material. Treatment of this zwitterion with NaH in THF followed by reaction with the nitrophenyl ester of phosphorylacetic acid provided the anion 59 in high yield. This species had a sufficiently high efficiency for lanthanide and actinide extraction [83]. In the above series of anions (i.e., 40, 43–51, 56–58), the B(8) boron substituent blocks directly the most reactive skeletal site B(8) of the molecule. Also, some protection of the second B(8')–H bond by steric hindrance of the bulky substituent can be assumed. This accounts for a reasonable stability observed in strongly acidic HLW solutions. It should be pointed out, that all the above derivatives, in their H+ or Na+ forms, had substantialy enhanced solubility in aromatic solvents (toluene, xylene, cumene, etc.). This fact is a crucial point in the improvement of properties of the cobalt bis(dicarbollide)s extractants. Alternative routes leading to phosphorus containing groups bonded via oxygen atoms to the B(8) or B(8,8') sites of the anion 1 have been developed [118] (see Table 7). These were based on the progress in development of preparative routes leading to a larger scale availability of the already mentioned 8,8-dihydroxy cobalt bis(dicarbollide) [8,8'-(HO)2-(1,2-C2B9H10)2-3-Co]- (52) along with the 8-hydroxy cobalt bis(dicarbollide) [(8-OH-1,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (53), which subsequently served as the reactive intermediates in these syntheses [119]. Reactions of 53 with either Cl3PO or PhCl2PO followed by hydrolysis gave the phosphorylated species [8-(HO)2PO-O-1,2-C2B9H10(1,2-C2B9H11)-3,3'-Co]- (60) and [(8-PhPO(OH)-O)-1,2-C2B9H10)(1,2-C2B9H11)-3,3'-Co]- (61), respectively. Analogous reactions of Cl3PO with 52 provided an interesting bridge-substituted intermediate product [8,8,-µ-CIP(O)(O)2<(1,2-C2B9H10)2-3-Co]- (62) that could be isolated in the pure form due to its unexpectedly high hydrolytic stability. The presence of a chlorine atom on the bridge substitutent of 62 implied its further use as a reactive chemical for reactions with organic reactants containing amine or hydroxy functionalities. The anticipated anion with phosphoric acid as the bridging substituent [8,8'-µ-(HO)(O)P(O))2<(1,2-C2B9H10)2-3-Co]- (63) was obtained only upon additional alkaline hydrolysis of compound 62 (see Scheme 4). The phenylphosphonic acid–bridged derivative [8,8'-µ-Ph(O)P(O)2<(1,2-C2B9H10)2-3Co]- (64) was prepared by a similar reaction of the dihydroxy derivative 60 with PhCl2PO and subsequent hydrolysis. The potential of the use of 62 as the reactive intermediate in the synthesis was illustrated by the preparation of phosphoramide bridged derivative [8,8'-µ,-(Et2N)P(O)(O)2<(1,2-C2B9H10)2-3-Co]- (65). The X-ray crystallographically determined structures of 62 and 65 were reported along with

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Table 7 Cobalt Bis(1,2-dicarbollides) with Crown Ether Moieties Bonded via Shorter Chain

0.01 M extractants in toluene, 1 M HNO3, x, 0.1 M HNO3. References also containing extraction results are denoted with *.

Scheme 4 Synthetic procedure leading to phosphoric acid bridged cobalt bis(1,2-dicarbollide) derivative 63. Remarkable is the high hydrolytic stability of the intermediate 62 which could be isolated and used as the valuable synthon for the synthesis.

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preliminary solvent extraction properties [118] the anions 62 and 63 being the best for M3+. The sulfuric acid-bridged derivative [8,8'-µ-(O)2S(O)2<(1,2-C2B9H10)2-3-Co]- has a similar structure (66) and was obtained as the second main product from the reaction of 1 in neat (CH3O)2SO2 after subsequent hydrolysis. Alternatively, it was also obtained as the main side product of the dimethylsulfate promoted reaction with dioxane. Its molecular structure was determined by X-ray crystallography [109]. This compound was also tested for Cs+ and Sr2+ extraction. 5. Cobalt Bis(dicarbollide)s Substituted at Carbon Atoms Synthetic Methods, Scope, and Limitations. The design of the carbonsubstituted cobalt bis(dicarbollide)s for extraction purposes has appeared relatively late in the literature. The reason was the tedious route to these derivatives starting from the synthesis of the respective substituted o-carboranes, their degradations to the anion [7,8-1R,2R-7,8-C2B9H12]-, followed by metal insertion reactions, as mentioned above (see Scheme 1). Special procedures had frequently to be designed and applied for the latter two steps. The metal insertion is sometimes difficult or even impossible due to the steric hindrance of bulky substituents attached at the dicarbollide anion or to metal binding interference of polar groups. In addition, diastereoisomeric mixtures arise, if mono or asymmetrically disubstituted carboranes are used for the synthesis. This necessarily creates difficulties in the subsequent step, which is compound isolation and characterization. Carbon substitution alone does not provide adequate protection of the most reactive B (8,8') sites of the cobalt bis(dicarbollide) cage. This is a serious drawback for their use under strongly acidic conditions. Additional substitution at boron sites of the already carbon-substituted anions is difficult, and only few such derivatives have been reported so far. The drawbacks relating to the stability can be partly overcome if the more stable derivatives of the isomeric [(1,7-C2B9H11)2-3-Co]- anion 2 are prepared. For extractants from strongly basic waste solutions, on the contrary, the carbon substituent derivatives can serve as a steric and kinetic protection of the B(6,6') skeletal positions, which are otherwise prone to degradation by bases. This is indicated by the remarkably good extraction properties of the C-tetrahexyl derivative of 1. Several series of the carbon-substituted compounds have by now been prepared and tested for their extraction properties, some of them with positive results. Compounds Prepared from the Substituted Carboranes. Synthesis of dialkylsubstituted cobalt bis(dicarbollide) [(1,1'-(R)2-(1,2-C2B9H10)2-3-Co]- [R=CH3 (67), (n-C8H17) (68)] proceeding via reaction of the (CH3)3NH[7-R-7,8-C2B9H11] salt with CoCl2 . 6H2O in 40% aqueous NaOH was described in the patent literature [10]. The former compound was recently prepared also by direct substitution of 1 (see below). The diphenyl derivative [(1,1'-(C6H5)2-(1,2-C2B9H10)2-3-Co]- (69), known already from the early era of the cobalt bis(dicarbollide) chemistry [24, 26], was recently prepared by a modified procedure [108, 124, 125] for the extraction tests.

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Tetramethyl- and tetrahexyl-substituted derivatives [(1,1',2,2'-(R) 4-(1,2C2B9H10)2-3-Co]- (70, 71) were prepared by degradation of the respective 1,2(R)2-1,2-C2B9H10 carboranes with NaOH in ethanol followed by cobalt insertion [120– 122]. The former compound appeared in the literature already in the 1960s [74]. Metal insertion reaction can be carried out with THF/NaH and proceeds in good yield only if a large excess of CoCl2 is used. The resulting alkyl derivatives 70, 71 are soluble in aromatic solvents such as mesitylene and ethylbenzene and are reasonably stable in alkaline solutions, due to steric shielding of the boron positions adjacent to skeletal carbon atoms by the bulky alkyl chains. This has been confirmed by their good extraction properties under alkaline conditions [121, 123]. An increase of the Cs+/Na+ selectivity was reported, even in acidic conditions. Synthesis of carbon substituted tetraphenyl derivatives of the cobalt bis(dicarbollide) anions from the meta isomeric series based on the anion 2 has been reported. These were obtained by cobalt insertion into the respective [7,9(C 6H5)2-7,9-C 2B9H12] - anion in DME, using KOtBu for deprotonation. The derivative [(1,1',7,7'-(C6H5)4-(1,7-C2B9H9)2-3-Co]- (72) was tested along with 69 for use in ion selective electrodes (ISE), for extraction of Cs+, Sr2+, and Eu3+, and for radionuclide recovery using supported liquid membranes (SLM) [108, 124, 125]. The derivative 72 gave DCs=22 from 0.1 M HNO3 (0.01 M 72 in nitrophenyl hexyl ether, NPHE. This compound proved to have a good permeability in the SLM test. Several derivatives of the general formula [(1-RO(CH2)n-2-CH3–1,2-C2B9H9)23-Co]- [R=CH2CH3 (73), CH3O(CH2)2 (74), (CH2)3CH3) (75), n=3] and a compound with a longer chain (R=CH2)3CH3, n=6) (76) have been prepared and tested [71, 108, 124, 126] (for schematic structures see Table 8). A derivative substituted by an alkyl substituent [(CH3(CH2)3-(CH2)3-2-CH3-1,2-C2B9H9)2-3-Co]- (77) was tested for comparison [126]. These compounds contain flexible chains comprising of up to two oxygen atoms and an alkyl end group. The synthetic route to 74 to 76 proceeded via the 1-Cl(CH2)3-2-CH3-1,2-C2B10H10 substituted carborane [127] as the intermediate. This can be degraded by a one-pot reaction with KOH or NaOH in the respective alcohol used as the solvent to form directly the substituted [7RO(CH2)3-8-CH3-7,8-C2B10H10]- dicarbollide anions. Insertion of the cobalt atom using CoCl2 and KOt-Bu in dimethylether led to the respective substituted cobalt bis(dicarbollide)s. In each case, a diastereoisomeric mixture arose, which could not been separated. This flexible procedure allowed for modulation of the metal extraction properties, the best being observed for the anion 75, containing one oxygen atom in the chain: DCs>100 and DEu=4.8 using a 0.01 M solution of this anion in NPHE and extracting from 0.1 M HNO3. The B(8,8') dibromo and B(8,8', 9,9' 12, 12') hexabromo derivatives (78a, b), prepared by halogenation of the anion 75, were described in a recent report [108]. Compounds Substituted at Carbon Atoms Prepared by Direct Methods. A limited number of reports exist in the literature on the direct carbon substitution

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using a lithiation reaction of 1 and subsequent reaction with alkyl halides [51]. Treatment of a THF solutions of 1 with 1.0 or 2.0 equivalents of n-butyllithium resulted in either purple or blue coloration due to presence of Cs[(LiC 2 B 9 H 10 )(C 2 B 9 H 11 )-3,3-Co] or Cs[1,1'-Li 2 (1,2-C 2 B 9 H 10 ) 2 -3-Co], respectively. Both mono- and dilithiated salts of the cobalt bis(dicarbollide) were subsequently reacted with alkyl halides (R-X, R=CH3, n-C6H13) to form the respective alkyl bis(dicarbollide) derivatives [(1-R-1,2-C2B9H10)(1',2'-C2B9H11)3-Co]- (79, 80) and (1,1'-R2-C2B9H10)2-3-Co]- (67, 80, 80, 81). The stereochemical aspect of these substitutions was studied intensively. A mixture of the racemic and meso form of the 1,1'-substituted anion formed in the case of the methyl derivative 67. In contrast, subsequent treatment of 79 with BuLi and methyl iodide produced 30% of [(1,2-(CH3)2-1,2-C2B9H9)(1',2'-C2B9H11)-3-Co]- along with meso- and racemic- forms of [1,1'-(CH3)2-(1,2-C2B9H10)2]- in 70% yield. Reaction of dilithiated 1 with two equivalents of CH3OCH2CH2OCH2Cl produced the structurally characterized racemic mixture of the anion [1,1'(CH3OCH2CH2OCH2)2-(1,2-C2B9H10)2]- (81). Recently, the carbon bridged derivative [1,1'-µ(C6F4)-(1,2-C2B9H10)2-3-Co]- (82) was prepared by direct lithiation and reaction with perfluorobenzene [83]. For schematic structures of the above compounds see Table 8, summarizing the carbon alkylated derivatives. Compounds containing one or two metal selective P(O)(C6H5)2 groups on the carbon atoms [1-P(O)(C6H5)2-1,2-C2B9H10)(1',2'-C2B9H11)-3,3'-Co]- (83a) and [1,1'(P(O)(C6H5)2-1,2-C2B9H10)2-3-Co]- (83b) were synthesized too, using lithiation of the anion 1 in DME, subsequent reaction with ClP(C6H5)2, followed by oxidation with H2O2 [83]. Also, the carbon-bridged derivative [1,1'-µ-(C6H5)2P(O)-1,2C2B9H10)2-3-Co]- (84) was reported to result from such reactions [83]. For schematic structures and extraction data, see Table 8.

C. 1.

Other Metallaboranes and Simple Cluster Borate Ions Used in Radionuclidc Extractions Mixed Metallaboranes

Two examples of new and interesting redox-active mixed sandwich extractants for the redox-recyclable extraction and recovery of the cationic radionuclides Cs+ and Sr2+ were recently reported. For schematic structures, see Table 9. Synthesis of Na[(␩5C5H5)-3-Fe(II)-1,2-(n-C12H25)2-1,2-C2B9H9]- (85) and (86) proceeded via lithiation of the respective substituted [7,8-C2B9H12] followed by reaction with [(␩5-C5H5)Fe[(␩6-C6H6)] in the presence of sodium amalgam. Neutral Fe(III) complexes arose from this procedure, which were reduced with sodium thiosulfate in a subsequent step to give anionic Fe(II) species. Compounds in the anionic form are able to extract Cs+ from basic or acidic solutions of 1 M NaOH+1 M NaNO3 or 1 M HNO3, and/or 1 M NaCl, the DCs values ranged from 5 to 20, the DSr values were low, ca. 0.04. The extractant-containing

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Table 8 Continued

0.5 M HNO3, negligible extraction of Sr and Eu under the same conditions; nitrobenzene, 0.01 M 73, 74, 75, 77, nitrobenzene, 1 M HNO3, 0.01 M 83 and 84. References also containing extraction results are denoted with *.

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Table 9 Other Metallaboranes Used in Extraction

References also containing extraction results are denoted with *.

organic phase, upon separation from the raffinate and exposure to air, released about 88% of its radioactivity in the form of a solid precipitate. This is due to central metal atom oxidation, resulting in the formation of the neutral complex that cannot extract metal cations [128] (for discussion of the extraction properties, see Section V.B.). 2. Ferra(III) and Nickella(III) Bis(dicarbollide)s A comprehensive survey of the chemistry of both ferra(III) and nickella III) bis(dicarbollide) anions appeared recently in the literature [129]. Synthetic methods leading to these compounds closely resemble those used for preparation of the cobalt bis(dicarbollide) anion 1. In principle, the [closo-commo-(1,2-C2B9H11)2-3Fe(III)]- (87) anion can be applied in extraction similarly to the anion 1, due to its surprising resistance toward reduction to the divalent anion [closo-commo-(1,2C2B9H11)2-3-Fe(II)]2-. A disadvantage of 87 lies in its paramagnetism that creates difficulties in product characterization by NMR methods. The nickella analogue [closo-commo-(1,2-C2B9H11)2-3-Ni(III)]- (88) is easily oxidized to the neutral [closocommo-(1,2-C2B9H11)2–3-Ni(IV)] and the reduction back to Ni(III) complex is difficult. Only limited studies examining the anions 87 and 88 for extraction purposes were carried out (see Section V.B.). These anions were mainly used for comparison with the anion 1 in the procedures for isolation of organic cations and bases [42] and in ISE [124]. One can imagine that due to their oxidation-reduction flexibility, these ions can serve similarly as the above mixed-sandwich redox-active

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agents. However, appropriate substitution should be designed and carried out prior to such applications. 3. [H3N+-B12H11]– Derivatives 2The chemistry of the parent closo-dodecahydro dodecabotate(2-) B12H 12 ion forms the foundation stone of borane chemistry and was therefore the subject of several monographs and review articles [87, 130, 131]. Boron compounds suitable for extraction purposes are typically univalent anions. A good choice of how to reduce 2the divalent charge of B12H 12 to obtain a univalent anion is substitution by an amine moiety. Direct bonding of an NH2 group to the cage effects its easy protonation to a group (see Fig. 1c). This proceeds spontaneously even in slightly basic aqueous solutions, resulting in formation of an inner zwitterion and in partial charge compensation. Subsequent introduction of bulky hydrophobic organic groups is a necessary condition to achieve sufficient solubility in organic solvents and efficient extraction properties. It should be taken into account that these derivatives are more polar and less hydrophobic than the cobalt bis(dicarbollide) family, which is a direct consequence of the charge compensation by the exo-skeletal group. This is often reflected in the formation of a third phase in the extraction system or by partial precipitation of the extractant. On the other hand, DCs for some simply substituted derivatives (see below) lie in the applicable range even at higher HNO3 concentrations up to ca. 1 M [108] (see Table 10). Also, the stability without any additional protection (which is easy to reach, e.g., by subsequent halogenation) is relatively good. No limitations for basic solutions apply, considering the chemical stability. Advantage can be seen in the significantly lower price and good accessibility of the [B12H12]2- starting material. Synthesis of the N-alkyl and N-aryl substituted derivatives of the 1-aminocloso-dodecaborate anion(1-) was described [108, 132]. Derivatives of the formula [R 2NHB 12H 11] - [R=C 6H 5CH 2 (89), C 10 H 7CH 2 (90), C 16H 33 (91), etc.] were synthesized, based on the monovalent anion [H3N–B12H11]- known for a long time [133], using conventional alkylation reactions in aqueous propanol. The dibenzyl derivative 89 was subsequently methylated using dimethylsulfate to [1(C6H5CH2)2NCH3B12H11]- (92), obtained in low yield. Synthesis of 89 and some other dialkylamine derivatives under anhydrous conditions, prepared for boron neutron capture therapy (BNCT) studies, was simultaneously reported by other authors, including its crystallographically determined structure [134]. An additional substitution of the boron cage was achieved by palladium-mediated cross-coupling reaction between [1-C 6H 5CH 2) 2NH-7(2)-I-B 12H 10] - and 1BrMgC10H7. This resulted in the production of [1-C6H5CH2)2NH-7(2)-(C10H7)B12H10]- (93) substituted at boron position 7(2) by a bulky naphthalene substituent. A nonseparated mixture of 1,2- (ca. 15%) and 1,7- (ca. 85%) isomers arising from direct iodination of 89 was used; therefore the resulting anion 93 was also not isomerically pure.

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Table 10 Derivatives of the [H3N-B12H11]- Ion

0.01 M extractant in nitrobenzene, 1 M HNO3, X, 0.4 M HNO3; Y, 0.0035 M 93, References also containing extraction results are denoted with *.

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Several closo-hydroborates bearing metal selective groups were also prepared and tested for extraction of Sr2-. Interesting results were obtained for the anion containing two 18-crown-6 rings [1-N,N-(18-crown-6-CH2)2NHB12H11]- (94). This was prepared from a high yield reaction of hydroxymethyl-18-crown-6 p-tosylate with [H3N-B12H11]- in the presence of NaH in THF. The cesium salt of this anion exhibits very low solubility in water and ethanol, significantly lower than the salt of the parent anion Cs2[B12H12]. The observed unusual solution behavior apparently results from the tight sandwiching of the Cs+ cation between the two crown ether rings [132]. Despite remarkably good extraction properties [108], this compound can better be seen as a good candidate for precipitation of Cs+ from acidic or alkaline waste. A direct cyclization reaction of pentaethylene glycol ditosylate with 1 gave [N15-azacrown-5-B12H11]- (95) the first known closo-borate anion with an attached azacrown ring [108, 132]. A continuation of this study was focused on anions substituted with various organic groups known to bind selectively the M3+ cations. The series included phosphite, phosphine oxide, and several CMPO-like groups [135]. The synthetic procedures for most compounds were via new neutral bipolar intermediates: [1(C 6 H 5 CH 2 ) 2 NH-7(2)-(O+(CH 2 ) 4 )HB 12 H 10 ] and [1-(C 6 H 5 CH 2 ) 2 NH-7(2)(O(CH2CH2)2O)-NHB12H10O-(CH2CH2)2O]. The former compound resulted upon autocatalyzed reaction of the conjugate acid H+89 of the derivative with a tetrahydrofuran and a dioxane ring, respectively. Both reactive synthons were obtained as the unseparable mixture of the positional 1,7- and 1,2-isomers. These are closely related to the dioxane-cobalt bis(dicarbollide) (42) and thus provide similar reactions. The tetrahydrofuran or dioxane ring could be cleaved by suitable nucleophic reagents. This led to the synthesis of several anionic compounds tested for extraction properties. Only two compounds of this series had reasonable extraction properties at higher acidities: Na[1-(C 6H5CH 2)2N(H)B12H10-7(2)(OCH2CH2OCH2CH2-P-(O) (OC4H9))2] (95) and Na[[(C6H5CH2)2NHB12H10-7(2)(O(CH2)4 N(C8H17)C(O) CH2P(O)Ph2)] (96). The later anion 96 was found to extract Eu3+ from HNO3 up to 1 M to nitrophenyl octyl ether (NPOE). Also, some Am/Eu selectivity was observed for this derivative [135]. 4. Derivatives of the [CB11H11]- Anion The closo-1-carba-dodecaborate(1-) [1-CB11H12]- anion [136, 137] (Fig. 1) received considerable attention during the last two decades, due to its low nucleophilicity and good potential for chemical modifications. The icosahedral symmetry of this ion is perturbed by the presence of only one {CH} moiety replacing the formally isolobal {BH}- vertex in the symmetric structure of the divalent anion [B12H12]2-. The high symmetry and charge delocalisation over the cluster surface qualifies the anion [1-CB11H12]- and its substitution derivatives to occupy an exclusive place among the least coordinating anions known in contemporary chemistry. Hence, its recent extensive studies have been focused toward making this interesting anion even

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less nucleophilic using halogenation [138–141] or alkylation [142, 143] schemes or combination thereof [144]. This field was reviewed several times [43, 44]. Surprisingly, little attention has been paid to this anion in the extraction studies. Only a few derivatives of the anion have been designed and tested for extraction purposes. Nevertheless, tailored synthetic chemistry of the anion [CB11H12]- could represent a good choice for the synthesis of potential extractants. The two most reactive skeletal positions B(1) and B(12) suggest a relatively easy and stereoselective substitution with a variety of metal selective groups. In general, two different groups can be introduced on these antipodal sites using two subsequent reaction steps made under particular conditions with minimal risk of side reactions and stereochemical complications. Other sites can be subsequently substituted by halogen or alkyl groups or their combinations. This is unnecessary for improvement of the stability, but rather for enhancement of their solubility in low polarity solvents. The chemical stability of the parent monocarbaborate anion and its derivatives is higher than that of the cobalt bis(dicarbollide) anion 1, especially in strongly basic conditions. For example, the parent anion can be boiled for several days in concentrated NaOH/Na2O2 solution without noticeable decomposition. Charge delocalization leads to an easy extraction of even the unsubstituted anion to a polar organic phase from aqueous or acidic media (not possible in the case of the rather hydrophilic [H3N+-B12H11]- species). Two compounds with bulky aromatic naphthalene and anthracene groups were prepared by palladium-catalyzed cross-coupling reaction of the [12-I-CB11H11]anion with 1-bromomagnesium naphthalene [108] or 9-bromomagnesium anthracene [83], similar to the already described procedure [142], providing the respective derivatives 98 and 99. Only the first anion was tested for Cs+ extraction, due to difficulties to prepare the second one in sufficient quantity. Tests showed a good extraction ability of 98 from HNO3 up to 2 M (see Table 11). Several derivatives with groups tailored for Sr2+ and M3+ complexation were prepared. C-substituted derivatives of this ion with metal ligating groups resulted from reaction of the dilithium salt of the closo-carbadodecaborate ion Li[Li CB11H11] with various reagents. The C-crown ether derivative [1-(18-crown-6-CH2)CB11H12](100) was obtained in a small yield from the reaction of the lithiated reagent with hydroxymethyl 18-crown-6 tosylate. Even without the presence of an additional hydrophobic substituent on the cage, this crown ether derivative extracted Sr2+ from 1 M HNO3. Synthesis of a phosphine oxide-substituted derivative [Ph2(O)P-CB11H11](101) proceeded via reaction of Li[LiCB11H11] with Ph2PCl and subsequent oxidation of the resulting by-product [Ph2P-CB11H11]- with atmospheric oxygen. The final product 101 was obtained in moderate yield after several extraction and chromatographic purification steps. Another compound substituted at the boron B(12) atom was prepared starting from recently reported CB11H11-12-dioxane zwitterion [145]. This was reacted with sodium dibuthylphosphite (BuO)2PONa in THF. This compound was then hydrolyzed to the respective monobutylester [12-((OBu) (HO)P(O) (OCH2-CH2)2)-1-CB11H11]-, similar to compound 48 from the cobalt bis(dicarbollide series). The cleavage of the CB11-dioxane by (Ph2P(O)CH2C(O)

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Table 11 Boron and Carbon Substituted Derivatives of the [CB11H12]- Ion

1 M HNO3, 0.01 M 1 in nitrobenzene, 0.01 M 3 in NPHE, 0.01 M 4 in nitrobenzene, 0.05 M 5 in nitrobenzene; X, 0.5 M HNO3. References also containing extraction results are denoted with *.

NCH2C6H5)Na (deprotonated using NaH in THF) was used for the synthesis of the derivative of this anion containing the CMPO moiety (102). In fact, the tested sample was a mixture of two isomeric/tautomeric compounds, with the CMPO moiety bonded via different sites. Neither this compound nor the previous one (101) had sufficiently high extraction ability of M3+ cations from 1 M HNO3 [83]. Apparently, extraction of the multivalent radionuclides would require a completion of the cage substitution with other hydrophobic groups, at least, at the antipodal reactive cage site. We hope that the considerations outlined in this section may help to promote further search in this area.

III. EXTRACTION WITH BASIC AND HALOGENATED COBALT BIS(DICARBOLLIDE)S Much of the information pertaining to the subject is contained in a previous review [16] where the original citations can be found (especially if the original materials

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were in Czech). Practical information concerning selectivities, losses of reagent into the aqueous phase, etc. is contained in two 5-year reports on cooperation between NRI, Rez and RI, St. Petersburg [146, 147]. The salts of the [(1,2-C2B9H11)2-3-Co]- (1) anion are [with exception of the Ag+ salt in which the cation is probably chemically bonded to the cobalt bis(dicarbollide)] fully dissociated in nitrobenzene and its mixtures with some nonpolar solvent with relative permittivities ε >15 [148, 149]. The selectivity Kexch(Cs+/H+) does not significantly change when switching from pure nitrobenzene to its 60 vol% mixture with 40% CCl4 or 80 vol% nitrobenzene+20% CHBr3 [150]. The extraction of Cs+ from nitric acid media into nitrobenzene and the mixture of 60 vol% of nitrobenzene+40% CCl4 is governed by the extraction exchange constant Kexch(Cs+/H+) which is approximately 1000 for media of 0.5 to 1 M HNO3. The distribution ratio is DCs~20 for 0.01 M anion 1 in the organic phase and 0.5 M HNO3 in the aqueous one. This particular value was largely used as a criterion of the purity of the anion and as a check for possible synergetic power of new boron compounds with presumed bonding to Cs+. The selectivity of extraction of cesium vs. some other ions is very high. As seen from Table 1 of Chapter 6, multivalent ions pass only sparingly into the nitrobenzene phase and there is sufficient selectivity toward Li+ and Na+, and only Rb+ is extracted, but 5 times less than Cs+. The practically obtained selectivities for the chloroprotected cobalt bis(dicarbollide) H+12-, 0.01 M in nitrobenzene as solvent, and 0.5 M HNO3 were as follows: DCs/DRb=5.8, DCs/DSr=17,000, DCs/DRu=22,000, and DCs/DZr = 2700 [16]. The same pattern of extraction into nitrobenzene is valid for other cobalt bis(dicarbollide)s having attached halogen groups. This is so, e.g, for the anions 12, 4, 6, 9,11,14,18, and 19 (Table 1). As described in the theoretical part (Chapter 6), these anions have different extractabilities into polar solvents in the form of their M+B- salts, but if extraction is done in an excess of mineral or hydrogen cation, the distribution ratio is given by the Kexch(Cs+/M+) constant, hence, must be the same for various noninteracting anions and dependent only on the nature of the organic solvent. The values of ⌬tG0 in the system water-nitrobenzene for partially halogenated anions are not known, but some estimate can be made from values in Table 2 of Chapter 6. Thus, the values for 6, 9, and 11 ought to lie between -50 and -54 kJ/ mol and the values for 14 and 18 between -50 and -61 kJ/mol.

IV. EXTRACTION WITH COBALT BIS(DICARBOLLIDE)S AND SYNERGISTS A number of synergists were tested with cobalt bis(dicarbollide) ions. Probably the most important ones are polyethylene glycols (PEGs) that are efficient synergists for alkaline earth cations and are required for completing the scheme of extraction

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of both 137Cs and 90Sr with cobalt bis(dicarbollide)s. Complementary to this category are various crown ethers tested in conjunction with cobalt bis(dicarbollide)s. Bidentate phosphonates, malonamides, and the new reagent from JAERI, tetraoctyl-3- oxapentane-1,5-diamide, TODGA, i.e., all compounds initially used in their pure state for the extraction of trivalent lanthanides and actinides from spent nuclear fuel, were tested in the presence of the chloro-protected cobalt bis(dicarbollide) anion. The mixtures in general possess the following properties compared with pure compounds: (1) The distribution ratio of M3+ strongly increases at low aqueous acidities, (2) the original curves of DM vs. aqueous acidity with maxima for pure compounds change to monotonously decreasing curves, (3) extraction of some other elements like Zr, Mo, and Fe is not completely suppressed for the synergetic mixtures. Some nitrogen-bearing derivatives, originally developed for Ln/Am separations, and calixarenes were also tested with cobalt bis(dicarbollide) extractants. Bifunctional phosphorylated PEGS (PPEGs) belong to the category displaying double functionality for alkaline earth and trivalent cations. Finally, mixtures of two synergists, PEGs and bidentate phosphonates, with extraction properties analogous to PPEGs alone, were tried and successfully used.

A.

Cobalt Bis(dicarbollide) and Polyethylene Glycols/Crown Ethers

The discovery of the synergetic extraction of alkaline earth cations with polyethylene glycols was based on the insolubility of the barium tetraphenylborate in the presence of these compounds [151] and was published in 1976 [5]. Of course, the PEGs display a synergetic effect for other hydrophobic anions than cobalt bis(dicarbollide) too [5], but extractions with cobalt bis(dicarbollide)s were the most studied. The influence of various commercial polyethylene glycols on Sr extraction gives an interesting picture shown in Fig. 3. It is seen that the maximum on the curves occurs at the same weight concentration of different PEGs and not at their same molar concentration (PEG 200 is exception, since due to lack of coordinating sites its concentration must be higher for attaining the maximum). In other words, the maximum corresponds to the same molar concentration of CH2CH 2O units (EOUs) in solution and not to the molar concentration of the PEG. It is probable that the central metal cation does not discriminate among the CH2CH2O groups and “borrows” them at random from different molecules of PEG; the macroscopically observed effect is then an average of quick exchanges of donor sites around the ion. This contradicts the common idea that Sr2+ is “wrapped” inside the cavity formed by coiling the PEG chain around the central ion, but both effects might be operative at the same time. An obstacle when dealing with commercially produced PEGs is their not well defined composition. The number after the PEG abbreviation denotes a mean m.w.

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Figure 3 Extraction of microamounts of Sr2+ by the synergetic mixtures of 0.01 M H+1- in nitrobenzene and various polyethylene glycols from 0.5 M HNO3 in the aqueous phase. Curve 1: PEG 300, 2: PEG 400, 3: PEG 200, 4: PEG 600, 5: PEG 1000, 6: Igepal CO 880 [mean relative molar mass (m.w.) 1540, one nonylphenyl substituent], 7: mean value for PEG 4000, 6000 and 20000, 8: dibutyl PEG 400, 9: propylene glycol with m.w. 1025. The concentration of PEGs up to PEG 600 are in vol%, for higher PEGs in wt% (the density of all the compounds is near 1).

of Gaussian distributed mixtures of compounds. Monodisperse PEGs were studied in one paper [152] and no drastic differences between these and commercial PEGs were observed. A number of extraction studies with various crown ethers was done by Vanura and Makrlik. The typical curves with maxima were treated with the LTGW program (for details, see Chapter 6, Section IV.D). The stability constants determined in nitrobenzene saturated with water for H+, alkali metal cations, and alkaline earth cations with several glymes and PEGs are given in Table 12. Most of the data were also collected in [153]. Because two differing entries exist in the compilation by the author of Ref. 153, both values must be tentatively considered as recommended and they are reprinted here in bold letters in Table 12. The data reported in Ref. 153 have, especially for Cs+, an uncertainty of ±0.5 units on the log scale on the average. Some difficulties in the formal treatment by Makrlik and Vanura are to be noted, concerning the reality of their model. For example, at high ligand concentrations sometimes a new increase of the distribution ratio is observed. This can be explained alternatively by formation of a ML2 complex in the organic phase or by complexing of M with L in the aqueous phase. As an

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Table 12 Logarithms of Stability Constants and of Some Ligands with Proton, Alkali Metal, and Alkaline Earth Cations in Nitrobenzene Saturated with Water Determined by Extraction with Cobalt Bis(dicarbollide) at 25°Ca

Data valid for 25 ± 2°C, those from Ref. 153 are printed in bold. The data on the first and second line are and respectively, if not noted otherwise. For the data reported by Makrlik and Vanura, the typical concentrations of reagents were small during the experiments, of the order of 10-3 M of H+1-, 10-4–10-1 M ligand and up to 0.2 M of aqueous acid or electrolyte. The experimental data were processed by the LTGW program (see Chapter 6) if not stated otherwise.b From [154].c From [155].d From [156].e Estimated by Vanura [157].f From [158].g From[159].h From [160].i Unpublished results of Makrlik and From [161].k From [162].1 From [163].m From [164], in the same paper also the value 7.71 is reported.n From [165] and [166].o From [167].p From [168].q From [169].r From [170].s From [171].t From [172].u From [173].v From [174].x From [175] and [176].y From [177], polarography with an electrolyte dropping electrode.z From [178], polarography with an electrolyte dropping electrode. a

example, we used in our EXTRIT calculation (Chapter 6) the second possibility for the complexes of Sr2+ with Slovafol 909 and no constants were used. This variant seems more reasonable, since Slovafol 909 “wraps” around the central ion with an almost perfect fit of 8 EOUs so that a second molecule of Slovafol could hardly enter the vicinity of the cation; still, the final conclusion ought to be borne out by independent evidence. A second complicating factor, appearing in newer publications, e.g., Ref. 179, is the possibility of ion pairing of the complexed cation with the hydrophobic anion in the aqueous phase. Both effects can be encumbered

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with uncertainty, due to the low complexation constants in the aqueous phase and need high concentrations of ligand for obtaining the data. Then the corrections to activities may be large. The reader is directed to a very recent detailed review by Katsuta and Takeda [180], in which the stability constants of various cations with crown ethers in the aqueous phase and ⌬tG0 values into different solvents (not nitrobenzene, however) are reported. Nevertheless, general trends arise from Table 12, validating the underlying physical models. The maximum stability of alkali metal cations in the series of glymes and PEGs is for the Na+ ion. This is in agreement with other studies in which also the maximal stability constant was found for Na+ for monodisperse polyoxyethylene monodecyl ethers with the number of EOUs, nEOU=4, 6, and 8 upon extraction in the presence of picrate anions into 1,2-dichloroethane [181]. The observed behavior is in contrast to crown ethers for which the maxima in the Table 12 appear for K+. Two effects are distinctive for the Sr2+ and Ba2+complexes with PEG derivatives. In the series of short-length PEGs, the hindrance of the terminal alkyl group for complexation is apparent (compare 4-glyme and PEG 200 with nearly the same nEOU). Secondly, a dramatic increase in complexation of bivalent cations with increasing EOU chain length is clearly apparent from the data. Again, the terminal alkyl substituent decreases complexation (compare PEG 400 and Slovafol 909 with nearly the same nEOU). Technological separations of 137Cs and 90Sr with crown ethers and cobalt bis(dicarbollide)s in their mixtures were not attempted. This is apparently due to high distribution ratios of 137Cs with cobalt bis(dicarbollide)s in the absence of any synergist and because cheap PEGs are available for 90Sr extractions. The use of PEGs and crown ethers does not appreciably increase the selectivity in the row of rare earth cations [16, 182]. Separation of the americium/europium pair in the presence of chloro-protected cobalt bis(dicarbollide), H+12- and some polyoxo-compounds was studied in [183]. The highest separation ratio DAm/DEu=3.2 was obtained with 18-C-6 crown but at overall nonextracting conditions (DAm=0.06 from 1 M HNO3, 0.16 M H+12-, 0.06 M ligand, nitrobenzene). Low DAm/DEu values (≤1.4) were obtained with 15-C-5 crown at similar compositions of the phases as above [184]. In another communication [185], it was found that the latter crown ether with chloro-protected cobalt bis(dicarbollide) and with Al(NO3)3 present in the aqueous phase extracts selectively Ce, whereas the extraction of Eu and Am is suppressed.

B.

Chloro-Protected Cobalt Bis(dicarbollide) and Phosphorylated Reagents

The use of phosphorylated reagents as synergists with chloro-protected cobalt bis(dicarbollide) anion for extraction of trivalent lanthanides and actinides started by a NRI patent claiming the use of two bidentate phosphonates for increasing the

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extraction of the two series of elements [186]. Dihexyl diethylcarbamoylmethyl phosphonate, DHDECMP, and dibutyl diethylcarbamoylmethyl phosphonate, DBDECMP, increased the extraction remarkably. The distribution coefficient of Eu3+ was only DEu=0.15 when using 0.04 M DBDECMP alone, or DEu=0.0006 when using 0.04 M H+12- alone, but DEu=32 was attained for their mixture in 60 vol% of nitrobenzene and 40% of CCl4 when extracted from 3 M HNO3. The distribution ratio of Am was similar to that for Eu. Extraction of Cs was suppressed by using the synergetic mixture; D Cs=6 for the chloro-protected cobalt bis(dicarbollide) alone but for the above mixture DCs=0.06 only. The search of various phosphorus compounds as cobalt bis(dicarbollide) synergists was rather intensive and is presented here only briefly. The research was mainly done for the older polar solvents of the nitrosolvent type. The extractions of Eu3+, Cs+, Sr2+, Ba2+, Zr, and Ru with the mixture of H+12and DBDECMP were studied in Ref. 187 in detail. The system was proposed there for practical use under the name TRUDIC, but apparently never had been further tested in the countercurrent mode. Some reagents that were by themselves already well extracting did not give remarkable synergism, as was the case for tri-n-octyl phosphine oxide TOPO [188] and octyl (phenyl)-N,N-diisobutyl-carbamoyl methyl phosphine oxide, CMPO [189]. Several other bidentate phosphororganic compounds tested in NRI for the synergy with H+12- were bis-diphenylphosphine ethylenedioxide, DPPEDO, tetraethyldimethylamino methylendiphosphonate, TEDMAMDP, dimethylacetyl methylenephosphonate, DMAMP, tetraethyl methylenediphoshonate, TEMDP, and tetraiso-propyl methylenediphosphonate, TPMDP [190]. The systems consisted of 1 M HNO3, 0.05 M H+12- in 60 vol% of nitrobenzene+40% CCl4, and 0.001–0.2 M ligand. They displayed various types of selectivity vs. Eu3+, Sr2+, and Cs+ cations. Thus, with TEDMAMDP, DMAMP, and TEMDP none of the elements could be extracted; with TPMDP and DPPEDO both Eu3+ and Sr2+ could be extracted [190]. Cesium extraction was suppressed in all instances. Some other bidentate ligands were studied in Russia, mainly in a double synergetic mixture with Slovafol 909 and m-nitrobenzotrifluoride as a solvent [191]. These were diphenyl-N,N-dibutylcarbamoyl methylene phosphine oxide, DPhDBCMPO, and dioctyl-N,N-dibutyl methylene carbamoyl phosphonate, DODBCMP, both providing efficient extraction of trivalent lanthanides and actinides. Retrospectively, after finding that the combinations of chloro-protected cobalt bis(dicarbollide) with malonamide dissolve well in nonpolar aromatic solvents like isopropylbenzene, some screening tests were done also for one representative of the bidentate phosphonate class. A mixture of 0.2 M DBDECMP with 0.02 M H+12- dissolved in isopropylbenzene was used and DEu=5.4 and 6.1 were obtained upon extraction from 3 and 6 M HNO3. Extraction of Cs+ and Sr2+ was negligible, but some extraction of the latter was achieved with 0.5 M DBDECMP+0.05 M H+12-, DSr= 0.42 from 0.5 M HNO3 [149].

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Chloro-Protected Cobalt Bis(dicarbollide) and Malonamides/TODGA

Malonamides are nontoxic substitutes of phosphorus-containing extractants largely studied at the French CEA. The following compounds were tested as synergists with chloro-protected cobalt bis(dicarbollide), 12: N,N-tetrabutyl malonamide, N,Ndimethyl-N,N-dihexyl malonamide, N,N-dimethyl-N,N-dibutyl(tetradecyl) malonamide, N,N-dimethyl-N,N-dibutyl (dodecyloxyethyl) malonamide, DMDBDDEMA, dimethyl dioctyl hexyl ethoxy malonamide, and the tridentate ligand tetraoctyl-3-oxapentane-1,5-diamide (TODGA) [192, 193]. Isopropylbenzene was used in these studies as the solvent. A typical picture of the change of mechanism on passing from pure malonamide to the synergetic mixture of malonamide with chloro-protected cobalt bis(dicarbollide) is apparent from Fig. 4. The mechanism in this case changes due to the progressive substitution of the 12- anion for nitrate in the extracted complex. The data may be plotted in coordinates

Figure 4 Extraction of radioactive Eu3+ by mixtures of DMDBDDEMA and H+12- into isopropylbenzene. The conditions and concentrations of H+12- pertaining to each curve are shown in the figure.

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of log DEu vs. log c(H+12-) [192] and in this case we obtain straight lines with various slopes for different acidities of the aqueous phase, namely, slope, c(HNO3): 2.93, 0.5; 2.70, 0.91; 1.72, 2.71; and 0.67, 5.41. According to Section IV. F. of Chapter 6, the slopes give the number of nitrate anions bound in the ion pair for a given acidity; it is seen that at about 5 M HNO3, the complex contains two cobalt bis(dicarbollide) anions and one nitrate anion per Eu3+ for complete neutralization of the cationic charge. If the slopes are plotted against the concentration of the nitric acid, a straight line is obtained. This particular finding means that for the extraction in this specific system the concentration rather than the activity is crucial for the description, see Section III.G of Chapter 6. Whereas with the other studied malonamides the nature of the data is similar, the TODGA reagent must be noted in particular. In this case, strong enhancement of DEu was observed and contrary to Fig. 4, the curves do not decrease on the lefthand side, but have a pronounced maximum. This is so even for very low concentrations of the chloro-protected cobalt bis(dicarbollide) in the system. A noticeable increase of DEu starts already at 0.0001 M dicarbollide with 0.01 M TODGA in isopropylbenzene and at ca. 2 M HNO3 and 0.007 M dicarbollide a conspicuous increase of DEu of more than 5 orders of magnitude was observed. Whereas the mechanism was not fully elucidated, this method may lead to total removal of trivalent lanthanides and actinides from nitric acid solutions using very low concentrations of reagents.

D.

Chloro-Protected Cobalt Bis(dicarbollide) and Nitrogen-Bearing Reagents

Several compounds having more than one nitrogen atom in them proved to be powerful agents for separating trivalent actinides from trivalent lanthanides. The effect is usually described in terms of the “softness” of these ligands that form stronger complexes with actinides than with lanthanides. Two studies were done coupling this kind of nitrogen donors with chloro-protected cobalt bis(dicarbollide) anion. Tris-2-pyridyl-1,3,5-triazine, TPTZ, was tested for the separation of Am3+/Eu3+ in Ref. 188. The choice of this reagent was based on the evidence from an older paper of Musikas et al. [194], where successful separation was done with the reagent from 0.125 M HNO3, when dinonyl naphthalene sulfonic acid (HDNNS) was used as the counter-ion. However, substitution of the latter anion by the much more hydrophobic 12- was counterproductive contrary to all expectations. The extraction with HDNNS was explained in Ref. 194 by the probable reversed micelle formation mechanism of HDNNS in CCl4 used by Musikas. Such a mechanism does not function for the dicarbollide in nitrobenzene. In the latter system, as shown in the paper [188], the TPTZ is protonated in acidic media with concomitant loss of selectivity and expulsion of both Am3+ and Eu3+ from the organic phase. A new class of reagents for the Am3+/Ln3+separation, alkyl-triazine-pyridines, were proposed by Kolarik [195]. They function even in 1 M HNO3, extracting

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Am3+ in the form of the nitrate complex into selected solvents like 2-ethyl-1-hexanol [195]. Again, as in the case of TPTZ, in nitrobenzene as a solvent and cobalt bis(dicarbollide) as an anion, probable protonization of the reagent takes place [196]. Separation of Am 3+/Eu3+ by this class of reagents and with cobalt bis(dicarbollide) anions was not carried out. The extraction of alkaline earth cations by 2,6-di(5,6-dipropyl-1,2,4-triazinyl) pyridine, DPTP, was studied in Ref. 197. Besides cobalt bis(dicarbollide)s, thiocyanates as hydrophobic anions were studied. The systems with dicarbollides were selective for extraction of Ca2+ giving a separation factor for Ca2+/Sr2+ of about 20 even from <0.5 M HNO3, which was, however, lower than with thiocyanates. The extraction was effective also from 0.1 M NaOH, but the sequence of extractabilities was partly reversed. As solvents for chloro protected cobalt bis(dicarbollide)+DPTP, butyl acetate, ethyl acetate, and nitrobenzene were tested [197]. Still, one possibility for extraction separation of Am3+/Eu3+ with soft donors and cobalt bis(dicarbollide)s does exist. The effect consists of the double role of ophenantroline in the proposed scheme [188]. This reagent can be simultaneously used as a soft donor and acid neutralization agent. The reagent is effective for separation of Am3+/Eu3+ only at the pH range of 2 to 4. However, when, eg., 0.2 M HNO3 in the aqueous phase is contacted with 0.25 M o-phenantroline and 0.05 M H+12- in nitrobenzene, the acid is neutralized in the organic phase and equilibrium pH during extraction attains quickly a value of about 3. At such conditions, the attained separation factor of Am3+/Eu3+ was around 30 [188]. A weak point of the process is the regeneration of the organic phase after extraction. Returning to the state of nonprotonated o-phenantroline in the organic phase, cannot be done by equilibrating with alkaline solution because of known instability of 12- in alkaline media. Other derivatives of dicarbollide stable in alkaline conditions could be envisaged if the regeneration is to be made by NaOH or particular care should be necessary in the case of chloro protected cobalt bis(dicarbollide). The process was patented [198], but not tested in a countercurrent mode, probably due to intense search for new Am3+/Eu3+ separating ligands and their testing.

E.

Chloro-Protected Cobalt Bis(dicarbollide) and Calixarenes

The calixarene reagents became a fashionable class of compounds studied largely in the last decade. The state of art for various calix compounds was described in several review articles [199, 200]. The thermodynamic data for calixarene complexation are collected, e.g., in Ref. 200 and both thermodynamic and kinetic data in a review [201]. Several substituted calixarenes used alone in a solution are very effective extractants, e.g., for Cs+ (crown calixarenes) or for trivalent lanthanides and actinides (calixarenes with CMPO groups); this is probably a reason why synergetic mixtures with hydrophobic anions have not been extensively studied so far.

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Vanura and Stibor studied the extraction of Na+, Cs+, Ca 2+, Sr 2+, and Ba2+ by four lower rim substituted calix[4]arenes and one calix[6]arene in mixtures with cobalt bis(dicarbollide) H+1- [202]. Among the studied compounds, the derivative 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(N,Ndiethylcarboxamidomethyleneoxy) calix[4]arene (Cal-tetramide) had the most interesting properties. It gave a synergetic factor for the extraction of Sr2+ of the order 3×105 and very high selectivity of Sr2+/Na+ extraction. In fact, the system is the best one for separating Ca2+ from other alkaline earth cations, still 1000 times more efficient than the system with 12-crown-4 system with cobalt bis(dicarbollide) previously reported by the same authors. The extractions of Ca2+ can be performed from 1 M HNO3 and 0.001 M dicarbollide H+1- (with 5×10-4 M Cal-tetramide in nitrobenzene). The extraction curves of the dependences of D(M2+) vs. c(calixarene) show characteristic maxima as known for the systems with crown ethers [202]. In two studies, Kyrs et al. [203, 204] studied in detail extractions of Eu3+, Sr2+, and Cs+ by mixtures of H+12- and selected thiacalixarenes. Unexpected results were obtained throughout the studies indicating the complexity of these systems. When using Cal4 (thiacalix[4]arene substituted by t-butyl groups at the upper rim and CH2COOEt groups at the lower rim: 5,11,17,23-tetra-tert-butyl-25,26,27,28tetrakis [(ethoxycarbonyl)methoxy]-2,8,14,20-tetrathiacalix[4]-arene(cone)), mixed with H+12- at a total concentration of 0.033 M and a ratio of the two compounds 2:3, the distribution ration of Eu3+ from 0.01 M HNO3 was DEu=992 if the solvent was chlorobenzene. However, when using other chlorinated solvents, the DEu was only 4 to 40 and for acetate esters the values were below 0.002. The authors of Ref. 203 described the effect as a “ternary” synergism between the two extractants and the solvent; however, a mechanism of such ternary interaction was not documented by independent evidence. The selectivity of extraction of cations into chlorobenzene with this particular calixarene (total concentration 0.033 M, ratio 3:2, 0.1 M HNO 3, 24 h of shaking) decreased in the D M sequence: Eu3+1441>Sr2+79.3>Cs+35.2>Co2+2.7 >Ba2+0.25. The kinetics of extraction was rather slow, especially when a weighable amount of Eu was not added into system, and equilibrium was established even with nonactive Eu added only after some 24 h of shaking. In a second paper of the same authors [204], the type of conformation of Cal4 (cone, partial cone, 1,3-alternate) on the extraction properties was studied. Moreover, an oxidation product of Cal4 (cone) in which instead of bridging S atoms in calixarene [4] ring the groups were introduced, Cal4 (cone)SO, was also examined. It was concluded that the type of conformer is decisive for the extraction behavior. The cone conformer can be used for extracting europium with adequate selectivity over Sr and Cs; the 1,3-alternate conformer is selective for the extraction of cesium. The other two conformers were found to be less practicable and were studied only perfunctorily. The unusual kinetics of the extraction found in the second paper must be mentioned. The kinetics of extraction was much faster if the components without Eu or Cs were shaken beforehand, and Eu or Cs was added to such a pre-equilibrated

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system. This suggests that some slow reaction proceeds besides the proper extraction process. Surprising is also the fact that maxima in the dependences of DEu on the concentration of one component (dicarbollide or calixarene) were observed, keeping the concentration of second component constant. This might be an expression of the peculiar properties of the systems under question, in which the usual log-log analysis is of less importance. In addition, the chemical stability of the two calixarene conformers differs. Measured by the decrease of the DEu after treating the organic phase with 6 M HNO3 at 50°C, the 1,3-alternate conformer had an apparent half destruction time of 10 min, whereas 30 min was measured for the cone conformer. Thus, the chemistry of the systems with cobalt bis(dicarbollide)s and calixarenes seems to be rather involved and challenging for further studies. It is noteworthy that the authors tackled in their studies also the practical questions of chemical stabilities of the systems with calixarenes, the area that seems to be in many studies not sufficiently investigated. For practical tasks, it seems that extraction of Cs could be performed from aqueous 3 M HNO3; however, this might be accomplished already by the cobalt bis(dicarbollide) anion itself, so it seems that the direction of the studies of these systems should be the theoretical aspects.

F.

Chloro-Protected Cobalt Bis(dicarbollide) and Bifunctional Synergists

Various phosphorus-substituted polyethylene glycols, PPEGS, were synthesized and their extraction capacity for extracting Eu3+ and Am3+ into o-nitroethylbenzene was studied by Smirnov et al. [205]. These reagents did not prove very effective in extraction of europium and americium, but as expected after addition of chloro protected cobalt bis(dicarbollide), the compounds are quite effective [206]. What is important for mixtures of these compounds with H+12- is a double selectivity. The central ethylene oxide chain, at some minimum length as shown above, is a Sr-selective synergist whereas the P苷O group interacts selectively with M3+ cations. On the other hand, the presence of the strongly basic P苷O group in these compounds leads to the formation of protonated hydrophobic PPEG particle which competes with the extraction of Cs+. Thus, generally systems with PPEGs extract Cs less than systems with PEGs [207]. The extraction behavior of the compounds of the general formula (R)2PO(CH2)mO(CH2CH2O)n(CH2)mPO(R)2 is shown in Table 13. The extraction of Sr is practicable with the ligands containing 6–7 ethylene oxide units, whereas the extraction of Eu is inhibited by a long ethylene oxide chain. PPEG chains longer than with n=7 led to higher losses of the substituted

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Table 13 Extraction of Certain Cations by Synergetic Mixtures of Chloro-Protected Cobalt Bis(dicarbollide) and Phosphorus-Substituted Polyethylene Glycolsa

a

b c

Data from Ref. 206 for extraction from 3 M HNO3 into nitrobenzene containing 0.15 M H+12- with added synergist at concentration c. See text for general formula. Without added chloro-protected cobalt bis(dicarbollide), extraction into o-nitroethylbenzene from 1 M HNO3; data from Ref. 205.

PPEGs into the aqueous phase and a ratio of concentration 2:1 (dicarbollide: PPEG) was found to give the highest distribution ratios of Sr [206]. The system with PPEG with n=6 was chosen for further study by the authors of Ref. 207, since also extraction of Sr was required. The advantage of the systems with PPEGs and cobalt bis(dicarbollide)s, i.e. simultaneous extraction of Cs, Sr, Eu, Am, Pu(IV), Np(V) and U(VI), is accompanied with partial loss of selectivity of the system. Rather high distribution of zirconium, molybdenum, and iron was observed [207]. This is a common feature of P苷O containing extractants and may be eliminated by other means, such as scrub with oxalic acid, thus not diminishing the importance of PPEGs as rather universal synergists.

G. Chloro-Protected Cobalt Bis(dicarbollide) and Two Synergists The use of two synergist, the first selective for Sr and the second for trivalent metal cations, is a natural idea for how to accommodate the extraction of many valuable components with one solvent mixture. The first study of this kind was undertaken at NRI in 1994 [208]. In it, a mixture of two synergist, dibutyl diethylcarbamoylmethyl phosphonate, DBDECMP, and nonylphenol nonaethyleneoxide glycol, Slovafol 909, in conjunction with H+12dissolved in a mixture of nitrobenzene with CCl4 was used. Extraction of Cs, Sr, and Eu was examined at constant concentration of one synergist while varying the concentration of other. Decrease of DSr with increasing concentration of DBDECMP and decrease of DEu with increasing concentration of Slovafol 909 were observed.

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This is a natural effect of electrolyte extraction systems, both synergists forming in aqueous acidic media hydrophobic protonated forms that compete with the extraction. Since Sr2+ does not bind with DBDECMP, increasing concentration of the latter must lead to decrease of DSr due to competition with DBDECMP, H+ and the same applies for Eu3+ in the presence of competing PEG, H+ particles. It was therefore deemed in Ref. 208 that combinations of the two synergists are not a viable way to proceed. Still, useful combinations of two synergists were found by Russian scientists [191] and form the basis of the UNEX process that is described below.

H.

Search for New Solvents for Metal Bis(dicarbollide)s and Synergists

The search of new more environmentally benign solvents than nitrosolvents and nitrobenzene in particular became an important point in the development of the chloro protected cobalt bis(dicarbollide) technology. The originally proposed nitrobenzene, in spite of its high selectivity of extraction, is not ecologically suitable due to its high toxicity. The proposed new solvents are described in the following in the chronological order of their proposals. 1.

Solvents of the First Generation: Nitrosolvents

There is not a big choice of nontoxic solvents in the category of nitrosolvents to be used for the cobalt bis(dicarbollide) anions. Only o-nitrophenyl octyl ether, NPOE, and o-nitrophenyl hexyl ether, NPHE, are nontoxic solvent as exemplified by the proposed use of NPOE as a plasticizer for ion selective electrodes for medical purposes. These two solvents are not convenient because of their high commercial price and high viscosity; thus they must be used in practice in mixtures with some less viscous solvent. Fluorinated nitrosolvents are subject of uncertainties concerning their toxicity, although some are claimed to have a lower level of toxicity (“harmful” rather than “toxic”) [209]. Still, m-nitrobenzotrifluoride, MNBTF, proposed and used in Russia [210], increases the safety of the process. This is due to its lower solubility than that of nitrobenzene in the nitric acid solutions. For example, in 5 M HNO3 the solubility of nitrobenzene is 7.3 g/L, whereas that of MNBTF is only 2.0 g/L [210]. Thus, the losses of the toxic diluent into the outgoing strip phase are more than three times lower. 2.

Solvents of the Second Generation: Fluorinated Polar Solvents

A number of fluorinated solvents were prepared and tested at the RI in order to find out a polar solvent but not of the nitrosolvent type [191], all of them being of the fluorinated type. Since then new compounds having possible use in other areas

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than solvent extraction were prepared and tested. The main representatives are shown in Figs. 5 and 6 and Table 14. For each solvent a structure is given, its chemical name as well as the technical term used by the authors (e.g., F8), and a standard value of the distribution coefficient of cesium are shown. The DCs value was determined at 0.06 M of H+12- in the solvent with an aqueous phase of 3 M HNO3+0.001 M CsNO3. For comparison, m-nitrobenzotrifluoride, MNBTF, is also shown as the solvent FS1. The success of fluorinated solvents, i.e., higher Cs+/H+ selectivity, can be explained by their lower basicity, noted also in Ref. 191, as compared to nonfluorinated derivatives. The electron-withdrawing character of the F group is

Figure 5 New fluorinated solvents for H+12- according to Ref. 191. The DCs value refers to 0.06 M H+ H+12- in the solvent and 3 M HNO3+0.001 M CsNO3. a The solubility H+12- was lower than 0.01 M.

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Figure 6 New fluorinated solvents for H+12- .

well known. No data on donor numbers or other basicity indexes of the studied compounds were found in the literature. Another factor responsible for the selectivity of extraction of Cs is the water solubility in the solvent. Quantitative data are not available, but one fact might be indicative. The compound FS20 in Fig. 6 displays the highest DCs just after MNBTF, and due to the long octyl chain the solubility of water in this compound might be very low. Fluorinated ethers FS2 to 5 give generally low DCs’s as well as fluorinated esters FS6 to 12, but the solvent FS12 and particularly FS10 (DCs=2.2) might be technically exploitable. Values that are more favorable were obtained with fluorinated ketones, especially solvent FS14 (DCs=5.5). Fluorinated sulfones gave DCs values ranging from 0.8 to 9.4 (the last one not shown in the figures). Phenyl trifluoromethyl sulfone FS17 was later chosen as a technological solvent for the chloro protected cobalt bis(dicarbollide) process (denoted by the authors as FS-13). Measured under the same conditions, the distribution ratio of Cs is four times lower than for MNBTF,

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Table 14 Properties of New Fluorinated Solvents According to Ref. 191a

a b c d

At 20°C. DCs at conditions defined in Figs. 5 and 6. Solubility of solvent in 3 M HNO3 in g/1. Solubility of H+12- lower than 0.01 M.

but its density and viscosity are suitable for extractions in mixer-settlers. The solvent is allegedly exceptionally chemically stable and does not react with 14 M HNO3 at temperatures up to 120°C. It greatly surpasses TBP solvent in explosion and fire safety. Exposition to an absorbed dose of 20 W · h/L had no effect on its extraction and hydrodynamic properties [191]. Some concerns about the use of the fluorinated diluents for technological use were raised by Horwitz and Schulz [211]. These were (1) incompatibility of the solvent with existing PUREX facilities, a paraffinic hydrocarbon being used in the PUREX process, (2) radiolytic stability of the solvent (however, high chemical and radiation stability of fluorinated solvents was claimed in Ref. 191; see above), (3) difficult cleanup techniques for such a solvent, and (4) environmental issues. From the enumerated points, (4) might indeed be an obstacle for the use of solvents of this type in certain countries such as France. The CHON principle (reagents which contain only C, H, O, and N atoms, upon burning forming more or less nontoxic gaseous products) seems to be strictly observed there. Quite recently, alkylphenoxy fluorinated alcohols were proposed as effective modifiers for extraction of cesium with calix-crowns [212]. 3.

Solvents of the Third Generation: Nonpolar Solvents

In spite of the former conviction that cobalt bis(dicarbollide)s are essentially not soluble in nonpolar solvents, at present many instances of their effective use are known.

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In Section IV.C, the cases of various malonamide mixtures M+H+12- soluble in isopropylbenzene (ε=2.383 [213]) were treated. The solubility of mixtures of this kind in essentially nonpolar solvent signifies that the lack of electrostatic interaction (ion-dipole) in the organic phase is overcome by the high volume of the formed HM+12- ion pair and its effective expulsion from the water structure and/or by the dispersion forces in the organic phase. Still, the extraction ability vs. trivalent cations remains unchanged, thus once more corroborating our opinion that a real ion pair, in which the cationic and anionic parts play their own role, is extracted. In addition, also the bidentate phosphonates+H+12- can be extracted into nonpolar solvents of the isopropylbenzene type. To the same class of systems belong also systems of chloroprotected cobalt bis(dicarbollide)+calixarene mixtures. Recently, we tested solvent compositions enabling dissolution of H+12- without any additional synergist or with the addition of a PEG into the system. These studies were based on a previous finding that H+12- in dioctyl sebacate, DOS, provided low, but not entirely insignificant, DCs values. Thus, hoping to increase the selectivity, we undertook a search of mixtures of DOS with various nonpolar solvents. The research culminated in finding certain combinations providing reasonable extraction of Cs+ and Sr2+ from media of about 1 M HNO3 [214]. As an example Cs+ and Sr2+ can be extracted from 1 M HNO3 by 0.12 M H+12- +0.4 vol% of Slovafol 909 in a mixture of 1 M DOS in the paraffinic PUREX solvent hydrogenated tetrapropylene, THP, used in France, with DCs=2.6 and DSr=9.4 [214]. Mixtures of dioctyl sebacate, DOS, and of dibutyl sebacate, DBS, with isopropylbenzene were studied for extraction. These combinations are characterized by third organic phase formation and the research must be directed into finding the conditions at which the third phase does not form. The effect strongly depends also on the type and concentration of the PEG used. As an example, the extraction of Sr2+ is shown in Fig. 7 [215]. A further successful way of how to solubilize dicarbollide extractants in nonpolar solvents is based on favorable substitutions of the basic dicarbollide skeleton. The most effective examples are BISPHECOSAN (37) and tetralkyl cobalt bis(dicarbollide) (71) treated in detail in the next section. Many other functionalized derivatives of dicarbollides are soluble in nonpolar solvents as shown below.

V.

EXTRACTION WITH FUNCTIONALIZED METAL BIS(DICARBOLLIDE)S AND OTHER BORON EXTRACTANTS

Substituted dicarbollides have already proved and may yet prove to be useful in several aspects. First, more hydrophobic anions than 1 can be prepared that enable better solubility in nonpolar solvents (Section V.A) and also provide also better stability in alkaline media (Section V.C). Successful substitution may lead also to using the valence state of the central atom of the dicarbollide skeleton and thus

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Figure 7 Extraction of Sr2+ from 1 M HNO3 by 0.12 M H+12- and 1.5 vol% Triton X-100 into an organic phase composed from DBS and isopropylbenzene. The relative volume of the third phase is shown on the right-hand ordinate. (From Ref. 215.)

enabling redox control of the extraction process (Section V.B). New complexing properties might be found with new derivatives. This is exemplified by compounds 37 and 39 (Section V.D). The general strategy in the synthesis of functionalized metal bis(dicarbollide)s lies in two aspects. In the first, molecular architecture is used and the bis(dicarbollide) skeleton plays the role of a scaffold to which the substituents are attached at definite positions and orientations. This approach is well demonstrated by the success of derivatives 37 and 39. The preorganized structures of these derivatives enable very selective boding of Cs+. Molecular modeling should improve still further the design of new derivatives. Secondly, since dicarbollides are mostly univalent anions, attachment of classical complexing groups like CMPO, crown ether, etc. should lend the compounds new properties. Such modification of the charge of these ligands cannot be performed by other means because of the low acidity of the common organic acid ligands that could be attached to neutral synergists. The character of the bis(dicarbollide) superacid is crucial for the studies of this kind and neutral synergists in their anionic form can be studied. The properties of various derivatives synthesized to date, together with their structures are given in Tables 2–11. Some more intensely studied cases of functionalized dicarbollides are given below. The bromo-protected dicarbollide is included, since this derivative was the first that enabled to study extraction into other solvents than nitrobenzene.

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No specific properties as regards the extraction could be expected from simple arylene substituted derivatives shown in Table 2. Still their low extraction power is peculiar. The measured DCs values were more than 10 times lower for 34 than the expected value of DCs=10. The reason for this is presently unknown; perhaps some effect of ion pairing in the organic phase may be operative. The ⌬tG0 of the anions are given in the Table 2 of Chapter 6. Interestingly, none of the anions was more hydrophobic than bromo-protected dicarbollide. The high hydrophobicity of the latter permitted its use for practical extractions into the less polar 1,2-dichloroethane; see below.

A.

Bromo-Protected Bis(dicarbollide)

Because of the low solubility of basic cobalt bis(dicarbollide) and H+12- in the low polarity solvent 1,2-dichloroethane (ε =10.45), the more hydrophobic hexabromo derivative H+19- was used for the extraction studies [216]. The solubility of the derived acid in the solvent is higher than that of the cesium salt, but by subsequent shaking the suspension of the supplied cesium salt in the twophase system 5 M HNO3 +1,2-dichloroethane, a 0.06 M solution of the H form of the reagent in the solvent was prepared. The extraction and back-extraction results in absence and presence of polyethylene glycol were similar to those obtained with nitrobenzene as a solvent. This observation was claimed in Ref. 216 as contradicting our previous conclusions that nitrobenzene as a solvent was responsible for the selectivity in the system. However, the generalized scheme of extraction, irrespective of ion association or dissociation, presented in Chapter 6, seems to be supported by these experimental findings. Also, the existence of maxima in curves of log DSr, DCe(III), and DEU vs. log c(Slovafol 909) in the system was quite analogous to the more polar nitrobenzene. This is in good agreement of the description of the systems given in Chapter 6, Section IV.F, in which it is reasoned that nonlinear dependences of this sort are to be found also for cases of full association in the organic phase.

B.

Redox-Recyclable Extractants

In our previous studies we unsuccessfully tried to utilize the redox properties of the dicarbollides of Fe(III)/Fe(IV) (87) and Ni(III)/Ni(IV) (88) for devising systems in which the extraction/stripping could be accomplished by the change of the valence state of the central metal atom and consequently the charge of the anion. This elegant possibility could not be accomplished, because of chemical decomposition of the compounds in the nitric acid medium. The extraction ability of the two dicarbollides diminished very quickly to nearly 1/50 or 1/100 of its initial value when in contact with 2 M HNO3 [217].

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The problem was solved by Chamberlin et al. [128] by devising mixed boron (X=Br) and carbon (R=C 12H25) substituted cyclopentadiene/dicarbollide Fe sandwich complex 86 (Table 9). In the paper [128], the authors describe the extraction results for 86 with and without addition of PEG 400. Reasonable DCs and DSr were obtained for various compositions of the aqueous phase and organic solvents toluene, xylene and diethylbenzene. Stripping is feasible upon oxidation of the complex. A possible problem is that the oxidation in some cases proceeds already in the presence of air, so that the extraction step in such cases should be done under a nitrogen atmosphere. The authors claim that the new extractants are stable; however, no results of experiments of chemical stability upon repeated reduction/oxidation cycles were reported in the paper [128].

C.

Tetra-C-Alkyl Derivatives of Cobalt Bis(dicarbollide)

As a variant of the basic or halogenated dicarbollides, new alkyl derivatives substituted at the C atom of the boron cage were proposed by Miller and Abney et al. at Los Alamos National Laboratory [120, 121, 218, 219]. Two main features of these compounds that differ from previous dicarbollides are their high lipophilicity, and solubility in some nontoxic solvents less polar than nitrobenzene. This discovery opened the possibility that dicarbollide-based extractants can be dissolved in nonpolar solvents like mesitylene or diethylbenzene. Among the proposed tetraalkyl substituted derivatives, tetrahexyl-dicarbollide (71) (Table 8) appeared to be the best and its extraction characteristics compared well with the classical dicarbollide 1 [120]. The extraction behavior of 71 was further studied in Ref. 123. As expected, because of the absence of protecting halogen atoms in the positions 8,8' of the dicarbollide skeleton, the reagent is not sufficiently stable in acidic media. However, in variance to the basic dicarbollide anion 1, the reagent was not stable even in the form of solid H+71-. The latter compound was prepared according to standard procedures by shaking the diethyl ether solution of the supplied Cs+71- with 20% H2SO4 and after evaporation a dark violet colored solid resulted. After 1 month, without any apparent change of color, the solid compound decomposed (distribution ratios of Cs+ from solutions of the latter in isopropyl benzene dropped significantly [123]). Hence, 71 was used only for the extraction from alkaline media, in which it is stable. The reagent enabled extraction of both 137Cs and 90Sr from media of 1 M NaOH+4 M NaNO3, if a mixture of 0.06 M Na71 with added 1 vol% of PEG 400 in isopropyl benzene was used as the organic phase. Thus, the reagent would be in agreement with the previous findings of the discoverers, suitable for technological testing. However, because of insufficient stability in acidic media, the stripping of the loaded organic phase by acid is not practicable. In Ref. 123 some other modes of stripping were studied, but a definite solution before implementing the technological tests would need further studies. The process with 71 may be

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particularly suited for the treatment of high quantities of “sodium-bearing wastes” existing after the cold war in the form of so called “defense wastes” in the United States.

D.

Bis-Arylene-Substituted Cobalt Bis(dicarbollide)s

A new dicarbollide based and highly selective reagent for the extraction of Cs+ was synthesized at the IIC, Rez [85, 220]. The compound contains two o-phenylene bridges between the two deltahedral ligands that are prismatically arranged. They are mutually inclined by 72° and intersect in the original symmetry axis crossing the central Co atom of the anion. Thus, selective binding of Cs+ between two phenylene rings was expected and proved by X-ray data [85]. The derivative 37 was also named BISPHECOSAN. H+37- is practically insoluble in nonpolar solvents like n-heptane or toluene. However, its solubility largely increases when some polar solubilizers, diethylpropane sulphamide, DEPSAM, and dibutylmethane sulphamide [221], are used. At 0.4 M concentration of DEPSAM in toluene a 5×10-3 M solution H+37can be prepared and the solubility increases to 1×10-2 M for 0.8 M DEPSAM [111]. The selectivity of Cs+ extraction with this reagent is outstanding. For example, for 5×10-3 M H+37- in toluene containing 0.4 M DEPSAM or 3.5 vol% of dioctyl adipate as solubilizers, the measured DCs values on extraction from the aqueous 0.5 M HNO3+4 M NaNO3 were log DCs=1.90 or 2.33, respectively. When H+37was dissolved in nitrobenzene, the excess selectivity was lost and the systems behaved similarly as if basic H+[(C2B9H9(8)Cl2(3))2Co]- were used. Thus, the selectivity is narrowly connected with the choice of the particular non-polar solvent and solubilizer. The reagent is not sufficiently chemically stable in contact with concentrated nitric acid solutions; however, stabilization can be achieved by adding urea to the system [111]. Some other commercially available solubilizers as dioctyl sebacate and dioctyl adipate were also used in the study, and screening tests were done with the ethyl derivative 39 [111].

E.

Crown Ether-Substituted Cobalt Bis(dicarbollide)s

Crown-substituted dicarbollide anions 49–51, 54, and 55 were prepared and tested for extraction of Na+, Cs+, and Sr2- in Ref. 114. Their extraction efficacy was examined by comparing the extraction results alternatively with pure crown ethers of the same ring size and with extraction by mixtures of the crown ether and hexabromo substituted dicarbollide 19. Although the results with crown substituted dicarbollides were better than with pure crown ethers, the results for mixtures of crowns +19 were of comparable magnitude to the covalently bound

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crown dicarbollides. For example, the distribution ratios for 18-C-6 crown and its dicarbollide derivative for extraction of Cs+ were, when using 0.01 M concentrations of the compounds in NPHE and 1 M HNO3 as the aqueous phase: DCs=0.003 for crown ether alone, DCs=0.56 for an equimolar mixture of 18-C-6 and Na+19-, and DCs=1.24 for the sodium salt of the corresponding dicarbollide18-C-6 anion 50. Anions with functional groups that withstand strongly acidic media probably need to be dicarbollide superacid derivatives. No theory is at present at hand for the underlying mechanism of extraction in this case, so that only a qualitative speculation is possible, and the extent of intermolecular charge redistribution might be operative. The ion pair in the mixture of crown ether and nonfunctionalized dicarbollide anion forms schematically according to reaction The reaction for the functionalized dicarbollide is written as If both (ML+, B-) and (MLB) are extracted nearly equally, the outcome is that in the latter case the charge distribution in the extracted particle is also similar to that in the former one, i.e., (ML+, B-). Thus, it is inferred that the charge on dicarbollide anion was not transferred to the crown part of the substituted anion, and could not therefore promote the intrinsic binding force of the crown moiety to the cation. The extraction with a particular crown ether substituted derivative of the anion [H3N-B12H11]- (95) given in Table 10 provided upon extraction from 1 M HNO3 with 0.01 M Na+95- in nitrobenzene DCs=2.55, but a much higher value was obtained with the double crown substituted anion (94), DCs=27. This indicates a partial involvement of both crown substituents in complexing Cs+ by 94.

F.

Phosphor Group-Bearing Cobalt Bis(dicarbollide)s

In the Ref. 113 the synthesis and extraction of five functionalized anions 43, 44, 45, 47, and 48, Table 4, was described. These derivatives are characteristic of binding the functional group to the cobalt bis(dicarbollide) anion through a diethyleneglycol chain. Thus, any overlap of electronic density of bis(dicarbollide) anion to the functional group is of minor importance and the main effect of introducing the bis(dicarbollide) anion may lie in increasing the hydrophobicity of the ligand and its charge. Because of absence of a P苷O group in the first three derivatives, no valuable extraction of Eu3+ could be expected, and the results confirm this. Only the derivative 48 was very efficient and was studied in more detail. This extractant gave a maximum in the curve of DEu vs. acidity at 0.07 M HNO3 (DEu=11 520 at 0.05 M Ca(48)2 in toluene, which dropped steeply with further increase of acidity to DEu~0.046 for 3 M HNO3). An unexpected result was observed when the chemical stability was studied. The distribution ratios significantly increased with the time of standing of the organic phase in contact with 1 M HNO3. This effect was tentatively explained by the hydrolysis of the ending ester groups.

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The behavior of the derivatives was studied by standard electrochemical method with ITIES. The respective stability constants of the all derivatives in 1,2-dichloroethane were obtained from facilitated transfer cyclic voltammograms. These constants are particularly high for the 48 derivative and Eu+ion as might be expected (log β°=35.7), but rather surprisingly, the constants for Cs+ and Na+ ions, which were virtually not extracted, are also rather high (log β°=4.8 and 9.4, respectively). Bridged derivatives with P atom containing group were studied in Ref. 118. These comprised the derivatives 62–65; see Table 7. Promising results were obtained with the first three derivatives, of the phosphorus bridged type, especially for 63. At low concentration of 63 in xylene reasonable DEu were reached (DEu=4.79 at 0.01 M Na+ 63-). Hydrolysis of 62 was observed upon contact with nitric acid, again probably connected to hydrolysis of the derivative. The most audacious project in the series was the introduction of a CMPO moiety into the cobalt bis(dicarbollide) anion. The three-step synthesis path from 1 is described in Section II. The interest in preparing CMPO cobalt bis(dicarbollide)s is understandable, in view of the success of pure CMPO reagents for attaining high distribution ratios of trivalent lanthanides and actinides from HLW radioactive waste. The results of the synthetic work and extraction results were described in Ref. 135. Three derivatives were prepared, differing in their alkyl or aryl substituent at the nitrogen atom: 56–58, Table 6. As in the case of crown ether cobalt bis(dicarbollide)s, a double comparison of the extraction results was made: (1) with the extraction by the corresponding pure CMPO derivative, and (2) with the extraction by pure CMPO derivative mixed with Na+19-, in o-nitrophenyl hexyl ether, NPHE, as the solvent. Practical tests were done also with other solvents, especially with toluene, isopropylbenzene, nitrobenzene, and 1,2-dichlorethane. The comparative tests at a given concentration of reagent (0.01 M pure CMPO, or an equimolar mixture of CMPO and Na+19-, or 0.01 M Na+56-, Na+57-, and Na+58in NPHE) showed that Eu3+ was extracted more when using covalently bonded CMPO cobalt bis(dicarbollide)s. Namely, on extraction from 1 M HNO3, DEu=0.19 for pure octyl phenyl carbamoyl methylene phosphine oxide, OcPhCMPO, DEu=55 for the synergetic mixture of the former reagent with bromo-protected cobalt bis(dicarbollide), but>100 for all three CMPO substituted cobalt bis(dicarbollide)s. The shape of the dependence of DEu on the aqueous acidity was in all cases the classical one for the synergetic mixtures of CMPO compounds with simple cobalt bis(dicarbollide)s, i.e. a monotonous decrease with acidity. Hence, the classical behavior of the pure CMPO with a maximum does not exist in this case. The extracted particle extracts with the derivatives in a form similar to the ion pair extracted from the synergetic mixture. From the extraction results it followed that practically two 56- anions bind to one Eu3+ cation, or in other words that sterical hindrance does not allow all three anions of 56- to enter the vicinity of the cation in the solution. This is in contrast to the solid phase measurements; see Section II.

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The ligand 56 is a very powerful extractant of Eu3+ and stripping into an aqueous phase may be difficult. Thus, from 13 possibilities of back-extraction only some have led to an acceptable result, namely a mixture of 0.1 M ammonium oxalate or a tertiary ammonium phosphate with 5% pentasodium salt of diethylene triamine pentaacetic acid, DTPA [135].

VI. CHLORO-PROTECTED BIS(DICARBOLLIDE) TECHNOLOGIES FOR EXTRACTION OF FISSION PRODUCTS AND ACTINIDE CATIONS FROM R ADIOACTIVE WASTES A.

Early Development of the Process

The chloro-protected cobalt bis(dicarbollide) process of extraction of 137Cs and 90Sr was developed to the stage of plant testing as a result of the mutual cooperation between the RI, St. Petersburg, and NRI, Rez, during the years 1975–1985. This early development demanded considerable effort from both sides since a quite new type of process was tested with a new and exotic reagent. From the Russian side, complete installation of hot cells in Gatchina and subsequently a commercial line at the enterprise Mayak were provided for the purpose. From the Czechoslovak side, a full study of the chemistry of the process was performed. Several Czechoslovak universities were engaged in the research: the Universities of Bratislava and Brno and the Technical University of Prague. The voluminous reports of the activities are cited in a preceding review [16] and contained in detail in two 5-year collaborative reports [146, 147]. The studies concerned the choice of suitable diluent for the chloro protected cobalt bis(dicarbollide), evaluation of the best PEG for the purpose, chemical and radiation stabilities of the reagents and solvents, fire and explosion hazard assessments, influence of possible contamination of the extractant by TBP on the performance, modeling of the process—including the cascade calculations, determination of extraction isotherms at various combinations of components, developing the criteria for checking the purity of the supplied H+12- and testing the individual batches of the product, methods of analysis of radionuclides (90Sr) and of the composition of the extractant, and several other questions. For the calculation of the equilibria, the respective constants determined from the distribution of microamounts of radioactive tracers of Na-, Rb+, Cs+, Sr2+, and Ba2+ were calculated by the program EXTRIT (See Chapter 6). From the values determined for 1, 4, and 3 M HNO3, only the last set is reported here. The solvent was 60 vol% of nitrobenzene +40% CCl4 with 1 vol% of Slovafol 909 added, 0.06 M H+12- was used, all at 25±2°C. The logarithms of the calculated constants are given in parentheses after each cation in the following order: Kex(Mz+, zB-)/Kex(MLz+, zB-)/β°1(MLz+)/(βa1(MLz+) (see notation in Chapter 6): H+(1.60/5.48/3.88/0.63), Na+(1.24/6.30/5.06/2.10), Rb+(3.78/7.00/3.22/1.60), Cs+(4.32/6.70/2.38/1.58),

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Sr2+(3.70/11.85/8.15/~1.71), Ba2+(4.91/13.04/8.13/~1.97) [222]. Contrary to data in Table 12, no double complexes of the type ML2 were detected, but complexation in the aqueous phase was taken into account. The above values were used for the calculation of extraction isotherms for modeling the extraction cascade. The calculated data agreed reasonably with experiment for such cases as, e.g., extraction of micro-amounts of Cs+ in the presence of macroamounts of Ba2+, etc. The experimental tables of recorded data were used by Sraier [223] for the modeling of the cascade. These cooperative efforts provided a firm basis for further testing. Present technological processes of the kind are still based on the previous findings of the Czech scientists concerning the nearly “ideal” properties of cobalt bis(dicarbollide) regarding its properties of high hydrophobicity and superacidity [2]. Added to this, the discovery of the complexing properties of PEGs toward Sr2+ made it possible to envisage a viable technological process based on this reagent. Two findings related to the chloro-protected cobalt bis(dicarbollide) are of general interest. One is that it is extremely stable on long-term contact with even concentrated nitric acid. The other is that during radiolysis of the 1 anion in organic solvents containing as a component a bromo- or chloro-substituted alkane radiational synthesis of a halogenated cobalt bis(dicarbollide) anions takes place instead of decomposition of the anion, e.g, Ref. 224. In these aspects of stability, the anion diametrically differs from usual carbon based organic reagents and this property was the main reason of its broad application. In the development of the process, particular attention was paid to the choice and testing of a proper stripping agent for both Cs+ and Sr2+. The original choices included n-propanol in nitric acid, diisopropylether, urea, and ammono-complexes of zinc, ammonium salts, and other reagents, all tested at NRI. The final choice of using concentrated (~10–12 M) nitric acid was decided during the cooperation. Hydrazine nitrate in nitric acid for stripping Sr2+ and Ba2+ was proposed at NRI [146]. Such stripping agents permitted to devise a process in which no solids will remain after evaporation and chemical decomposition of the hydrazine into gaseous products. Because of lack of information in contemporary literature on the technological testing at those early stages (see Section I.A), abbreviated information is provided here. In the two following tests to be reported, 0.06 M H+12- solution in 60 vol% of nitrobenzene+40% of CCl4 was used as an extractant (1 vol% Slovafol 909 was added for the flowsheet with mutual extraction of 137Cs+90Sr). Stripping of 137Cs was done by ~12 M HNO3, stripping of 90Sr by 0.5 M N2H4 in 2 M HNO3, stripping of nonradioactive Ba by 2 M N2H4 in 4 M HNO3, and reconditioning of the extractant by 3 M HNO3. The stripping of Ba is essential for the process and subsequent modifications. The Ba2+ cation, being better extracted than Sr2+ by PEGs, would otherwise accumulate in the organic phase with loss of extraction efficacy. The first hot cell testing of the chloro-protected bis(dicarbollide) process was done at Gatchina in the early 1980s [146]. The extraction line consisted of 47 mixer-settlers. A two stage flowsheet was used: in the first stage 137Cs was extracted

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by the H+12- without PEG and in the second stage 90Sr form the raffinate of the first stage was extracted by the H+12-+Slovafol 909 combination. The feed was spiked with a real raffinate of the PUREX process. The extractant returned six times and good efficiencies for the separations of 137Cs and 90Sr from other nuclides as well as good mutual separation of two were reached. Plant testing [147] at Mayak, Russia was performed in the 6-month period of September 1984 to March 1985. Altogether 39 mixer-settlers with a volume capacity of 10 L each were used. The combined flowsheet, with extraction of 137Cs and 90Sr together in the first stage and their consecutive stripping into separate fractions, was used. During the experiments, 15 m3 of model PUREX HLW solution and about 100 m3 of real raffinate of PUREX HLW were used. For experiments with the model solutions 0.5 m3 of the extractant and for experiments with real PUREX solution 3 m3 of extractant were used. This corresponded to about 33 cycles of return of the extractant with real PUREX HLW solution. Generally, stable operation was attained with no changes in performance or hydrodynamic parameters. The efficacy of separation of both isotopes was higher than 98%. From other variants of the process, the possibility of extraction of trivalent lanthanides and actinides with the chloro protected cobalt bis(dicarbollide) extractant must be noted. A variant in which a more concentrated nitrobenzene solution of H+12- is used (~0.3 M) with Slovafol 909 and extraction is done from diluted nitric acid (~0.5 M) was proposed in Ref. 225. This process unit must be inserted in the complete flowsheet after the step in which 137Cs and 90Sr are already separated. A high concentration of the chloro-protected cobalt bis(dicarbollide) is needed in the extraction of trivalent cations and the stripping of 137Cs and 90Sr might be difficult from >0.2 M H+12-. The use of stronger stripping agents, amminocomplexes of Zn, was proposed by NRI and tested in the hot line at Gatchina [147]. Although the obtained data were fully successful, an unexpected obstacle occurred. This was a formation of a solid NH4NO3 deposits on the apparatus surfaces in the hot cell, originating from the contact of evaporated gaseous HNO3 and NH3 in storage vessels. This obstacle could be possibly eliminated by some other means, but no other experiments were done with this system. The process was tested in Russia in a new version [226]. For it 0.3 M H+12- in MNBTF containing 6% of PEG (of undefined type) was used. Fractional stripping provided separate lanthanide and actinide portions. The composition of the stripping solutions was not given in the paper.

B. 1.

New Processes with Chloro-Protected Cobalt Bis(dicarbollide) Process for Extraction of m-Nitrobenzotrifluoride

137

Cs and 90Sr with

The detrimental toxic properties of the mixture of solvents used in the processes described above were partly relieved by a proposed new solvent—m-nitrobenzotrifluoride,

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MNBTF. Moreover, its use does not lead to the formation of corrosive Cl- ions arising from the CCl4 in the previously used compositions and the losses of the solvent into the aqueous effluent are lower than for nitrobenzene. Potentially carcinogenic CCl4 could be avoided, since the density of MNBTF without any additive is sufficiently high (1.467 g/ml, Table 14) to comply with the demands for mixer-settler operation. Besides the new solvent, the process patented in Ref. 210 does not differ from the previously tested process in the Russian-Czech cooperation [8], including the use of H+12- and PEG for extraction and hydrazine for Sr stripping. Still, the new solvent was important for implementing the process on a plant scale operation in Russia. 2.

Process for Extraction of 137Cs and 90Sr with Phenyl Trifluoromethyl Sulfone Diluent

Fluorinated polar solvents, developed originally in Russia and further in the RussianAmerican DOE cooperation, became a new variant of the chloro protected cobalt bis(dicarbollide) process. One variant concerns the first part of a two-stage process in which 137Cs and 90Sr are extracted by the H+12- and PEG combination in fluorinated ethylene glycol. In the second-stage rare earths, technetium and the actinides (especially uranium, plutonium and americium) are extracted from the aqueous phase using a phosphine oxide in a hydrocarbon diluent. The process was patented in the United States in 2001 and in Russia in 2002 [227, 228]. Several reports on the cooperative activities of the Khlopin Radium Institute (RI), St. Petersburg, and Idaho National Engineering and Environmental Laboratory (INEEL) appeared during late 1990s, e.g., Refs. 229–232. These reports did not specify the compositions of extractants and stripping agents; hence they have little informative value. After patenting, full information at last emerged, as given below. A new process for the extraction of 137Cs and 90Sr was developed conjointly in the cooperation between INEEL and RI. The process uses for the extraction a solution of 0.08 M H+12- +0.6 vol% PEG 400 in phenyl trifluoromethyl sulfone (denoted as FS17 in Fig. 6) [233]. The process was tested with simulated evaporated INEEL acidic waste, containing 1.96 M acid, 1.59 M NaNO3, 0.61 M Al, 0.18 M K, 0.006 M Ca, 0.014 M Zr, 0.099 M F-, and 5.64 M NO3- as macrocomponents. Various stripping agents were tested. Particularly low distribution coefficients of Cs and Sr were obtained with the following mixtures: 1.5 M dimethylformamide in 2 M HNO3 (0.037 and 0.049, respectively), 1 M guanidine nitrate in 1 M HNO3 (0.22 and 0.064), and 3 M methylamine in 4.5 M HNO3 (0.053 and 0.033). It was argued that both guanidine and dimethylformamide might not be easily washed from the organic phase. Because methylamine could be easily washed by the acid, this reagent was used in further tests of the study. However, it should be noted that both methylamine and dimethylformamide may form explosive azides during the washing step, which might be prohibitive for their technological use.

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According to Tables 1 and 3 of Chapter 6, the affinity of CH3NH3+ toward the nitrobenzene phase is comparable to that of K-, so this particular cation is an efficient stripping agent for Cs+ and as seen also for Sr2+. It remains to be resolved whether the stripping proceeds by interchange of CH3NH3+ with Sr2+ cations inside the organic phase complex (the mechanism valid for the much more hydrophobic Slovafol 909 and exchange of doubly protonated hydrazine with Sr2+) or whether all the PEG 400: Sr2+ complex passes into the aqueous phase. If the latter is valid, corresponding changes of the PEG 400 concentration will occur during the Sr2+ stripping, perhaps leading to more difficult process control but not lower economy because of the low price of PEG 400. It may be noted that methylammonium ion as a striping agent was already proposed before in a paper from NRI [234]. 3.

Process for Extraction of 137Cs and 90Sr, Rare Earths, and Actinides with Two Synergists and Phenyl Trifluoromethyl Sulfone Diluent: The UNEX Process

An extensive study was performed within the cooperation between the RI and INEEL, in order to find a system with H+12- and two synergists, which would separate all the title elements from the radioactive waste for “decategorizing” of the waste. This successful research was published in articles [191, 235, 236] reports [237], conference papers (see preceding section), and patents [238, 239]. The process is named UNEX, coined from the words Universal Extraction. In the process, a solution of 0.08 M H + 12 - , 0.02 M diphenyl-N,Ndibutylcarbamoyl- methylene phosphine oxide, DPhDBCMPO, and 0.5 vol% PEG 400 in phenyl trifluoro-methyl sulfone, FS-13 (FS17 in Fig. 5) is used as an extractant. The feed was simulated evaporated INEEL acidic waste, described in the previous section. The title nuclides extracted into the organic phase, were stripped together by a solution of a single composition, namely aqueous 10 g/L diethylene triamine pentaacetic acid and 1 M guanidine carbonate [191, 235, 236]. Protonated guanidinium is a very efficient competing agent for Cs+, according to Tables 1 and 3 of Chapter 6, comparable in its Gibbs energy of transfer to Rb+. Thus, the stripping of Cs+ is probably due to guanidinium+ and the stripping of trivalent lanthanides and actinides is due to the complexing ability of the aqueous complexant at the pH determined by the guanidinium carbonate. This is presumably also the mechanisms of Sr stripping. 4.

Extraction of 137Cs and 90Sr as a Front-End Process Before the PUREX Process

The high radiation stability of H+12- has lead to an audacious project to insert the chloro protected cobalt bis(dicarbollide) process for the separation of 137Cs a 90Sr before the PUREX extraction reprocessing of the irradiated nuclear fuel [240].

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According to the authors of Ref. 240 this concept could have advantages “in essential (in two and more times) reduction of radiation load on tri-n-butylphosphate and a diluent and as a result: reduction of cost of biological protection from ␥-radiation, reduction of explosion and fire risks of PUREX process and its simplification.” Due to the high concentration of uranium in the dissolved fuel, higher concentrations of H+12- and of PEG than in the process for isolation of Cs and 90Sr from PUREX raffinate have to be used, but the process is feasible. For example, with an extractant of the composition of 0.3 M H+ H+12-+6% OP-10 (di-isobutylphenol decaethyleneoxide glycol) in nitrobenzene, the measured distribution ratios were DCs=4.2 and DSr=16.3 for a solution of 250 g/L of U in 4.6 M HNO3. The process was tested under dynamic conditions with 0.3 M H+12-+6% OP-10 in m-nitrobenzotrifluoride, with a wash of the extractant by 3 M HNO3 and consecutive strips with 2 M and 6 M HNO3 (aimed for disposal) and with 4 M HNO3+“organic base” for common strip of 137Cs and 90Sr [240].

C. Plant Scale Operation The main practical result of the application of chloro protected cobalt bis(dicarbollide) extraction chemistry is the plant operation of the process at Mayak, Russia, designed for the fractionation of high level radioactive waste, HLW, as the only plant of the kind in the world. The chloro-protected cobalt bis(dicarbollide) part of the extraction line at Mayak worked in campaign mode, but full information on the work done at the plant has not been fully published from comprehensible reasons. At the end of the 1980s the plant started its operation with the highly toxic and corrosive solvent o-chloronitrobenzene in hexachlorobutadiene, but soon the solvent was changed for m-nitrobenzotrifluoride [18]. The used extractant was 0.06–0.15 M H+12- with 2–3% of PEG in the solvent. During the operations, 65 m3 of nonevaporated and evaporated raffinates of the first extraction the PUREX cycle were treated with the effectiveness of 137Cs and 90Sr uptake of 97 and 99%, respectively. The operation line was constructed in 1995 [18] with total volume of extractors of 2.9 m3 and flowthrough maximal output of 700 L/h. As the extractant, 0.1 M H+12- in 98% of m-nitrobenzotrifluoride and 2% OP-10 was chosen. During only a 3-month campaign in 1996, 210 m3 of highly radioactive defense waste were reprocessed with total β activity of 7 MCi. A practically salt-free concentrate of 137Cs and 90Sr, resulting from the operation, could then be added to the solutions for vitrification. The specific radioactivity of the glass could thus be increased twice, resulting in a proportional economical effect. It was expected that under continuous operation of the line, all of about 4 000 m3 of HLW might be treated in a period of some 5 years [18]. In the Russian report of Romanovskii from 2001, it is stated that by the end of 2000, 600 m3 of HLW was reprocessed and about 25 MCi of 137Cs and 90Sr were recovered [226].

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VII. ANALYTICAL AND OTHER APPLICATIONS OF EXTRACTION SYSTEMS WITH METAL BIS(DICARBOLLIDE)S A.

Extraction of Metal Cations for Preparative Purposes

1.

Selective Separations

In the systems with cobalt bis(dicarbollide)s and polyethylene glycols, the selectivity of extraction increases as Ca2+<Sr2+
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certain radiopharmaceutically important isotopes such as 99mTc (from 99Mo) or 90 Y (from 90Sr). A generator for milking 137Ba from 137Cs based on cobalt bis(dicarbollide) in a nitrobenzene solution held on polytetrafluoroethylene or Poropack P as the solid support was devised by Koprda and Scasnar [247, 248]. Barium, due to its feeble extraction by cobalt bis(dicarbollide) alone without PEG, could be milked with dilute nitric acid with a yield of 45 to 70% and less than 0.1% contamination by 137 Cs. A method of milking of 224Ra from aged thorium was proposed in Ref. 245. Powdered metallic thorium (130 g) was dissolved on heating in 500 ml of concentrated HCl with added 0.03 M HF yielding approximately 1 M aqueous ThCl4 and 5 M HCl solution. From this volume, 224Ra was extracted by 50 ml of 0.01 M H+1-+0.2 vol% Slovafol 909 in nitrobenzene. From the resulting organic phase, a needed aliquot was withdrawn, washed with an equal volume of 2 M HCl and 224Ra was stripped after diluting the organic phase with the same volume of diiso-propylether into aqueous 0.5 M HCl. The resulting solution was radiochemically pure, except for lead formed by the decay of 224Ra, and of specific activity of about 105 counts min-1 mL-1. Thus, the resulting solution can be used for tracing purposes. The organic phase above the stock aqueous solution of thorium may be left until the next milking [245]. A radioisotope generator for milking 90Y from its mother 90Sr was proposed by Vanura and Makrlik in several modifications. The systems with cobalt bis(dicarbollide) and PEG 400, 18-crown-6, or 15-crown-5 [249] gave relatively low selectivity so that aqueous complexing of 90Y had to be used. On the other hand, with a more selective synergist, benzo-15-crown-5, the milking can be done directly into an aqueous phase without complexing agents [250, 251]. Conditions could be found by the above authors using the LTGW program under which DSr was higher than 105 and DY lower than 0.1. This enabled to get purified Y in two steps for nuclear medical purposes (<10-6 % 90Sr in the Y preparation is obligatory) or in one step for less demanding applications [153]. Milking was performed simply by shaking the organic phase with an equal volume of 0.2 M HCl. New results for this type of generator were recently published [252]. A certain drawback is the presence of nitrobenzene as a toxic solvent, and of course recently proposed new procedures with nonpolar nontoxic solvents for cobalt bis(dicarbollide)s are awaited.

B. Analysis of Radioactive Metal Cations The main interest for analytical applications of cobalt bis(dicarbollide) anions is connected with the analysis of radioactive 90Sr in nuclear waste solutions. For this purpose, it is necessary to extract first the cesium by a cobalt bis(dicarbollide) solution without addition of PEG, and only in the second step to extract Sr after addition of PEG. These studies were started for the purpose of the determination

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of Sr contents in the extraction line in Gatchina [253]. The procedure worked perfectly at NRI, but was not quite reliable at the extraction line in Russia. Looking back for reasons of its failure there, it seems that the presence of Ba in the spiking solution of the nuclear fuel was not taken into account and of course Ba would interfere with the determination. The subject became quite popular and the reader is directed to original references for various applications like the determination of Sr in technological samples, fallout, and drinking waters [254, 255]. Special procedures for the determination of the specific radioactivity of radioactive source materials based on the substoichiometric principle were developed by Vanura and Jedinakova-Krizova. These comprise the determination of radioactive cesium [cobalt bis(dicarbollide)+DB-18-C-6] [256], strontium [cobalt bis(dicarbollide)+15-C-5] [257], and barium [cobalt bis(dicarbollide)+15-C-5] [258].

C.

Analysis of Biological Materials

Special types of analysis necessitate the determination of 137Cs and 90Sr in biological samples and tissues such as urine, faeces, lungs, etc. This determination is important both under normal conditions but especially as a preventive method for checking people and animals in case of a sudden contamination by nuclear fallout. The subject was studied by Koprda and Scasnar [259] in detail. Again, the standard two-step procedure for extraction of 137Cs and 90Sr must be used, but the procedure had to be modified according to the type of material. Because of the great salinity of urine, the samples were diluted 10 to 20 fold with water before determination [260]. The interference by the high content of Ca in milk was eliminated by tenfold dilution of the milk by water. The high content of potassium in the mineralizates of muscle tissue was reduced by previous precipitation with HClO4 [261–263].

D.

Drug Analysis

The broad subject of pharmacological research of characterization, determination, and prediction of the activities of many drugs that behave as protonatable bases, BS, deals with determination of their lipophilicity beside other properties. The lipophilicity of neutral molecules is given by their well-known partition indices, determined as the partition constants between water and n-octanol, ␦BS (w→noctanol). The theoretical treatment of the values is beyond the scope of this review; see, e.g., Ref. 264 for introductory information. However, the value ␦BS does not contain full information related to possible pharmacokinetics effects and the transfer of an ionized base may be more informative than the transfer of a molecule. Thus the new area of the studies of ITIES developed, aimed at the determination of the properties of protonated forms

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of bases in form of their values [or in other words, individual extraction constants, Eq. (24) of Chapter 6] in a chosen system water+1,2-dichloroethane [264]. As only one example of these studies, a paper of Girault on the determination of such values for 18 quaternary ammonium bases in the above solvent system may be noted [265]. Only studies of precipitations and extractions with cobalt bis(dicarbollide)s are discussed here. In these studies, the properties of the ion associate BS+1- or their aqueous solubilities were determined. Some approximate solubilities of BS+1- (B= quaternary bases and amino acids), obtained by the spectrophotometric measurements of the anion concentration in the supernatant, are collected in Ref. 42. Extraction with 60Co labeled 1- anion proved to be a useful method for the determination of the hydrophobicity of more than 50 nitrogen bases and drugs [81, 82, 266]. In the standard technique the base was converted into its protonated form by dissolving it in 0.1 M HCl. The distribution ratio of the 1- anion into chloroform was measured for 14–66 µmol/L of Cs+ 1- and 10-6 to 10-3 M of BS+ in 0.1 M HCl. The dependences of log D(1-) on log [BS+]aq were straight lines with regression coefficients >0.99, thus proving the proposed mechanism. This is governed by simple extraction of the ion pair according to the constant Kex(BS, B) [Eq. (7) of Chapter 6]. The majority of studied compounds had log Kex(BS, B) in the range of 1.0 to 8.2 (e.g, morphine, ephedrine, scopolamine, codeine, strychnine, brucine, LSD), but for six bases with log Kex(BS, B) >8.2 [266] the exact value could not be determined. The inverse technique was used by Scasnar, e.g, in Ref. 267, where determination of stobadine, a cardioprotective drug, in the serum of experimental animals was done by extraction of radioactively labeled drug from the serum into a benzene solution of cobalt bis(dicarbollide).

E.

Analysis of Neutral Detergents: Polyethylene Glycols

The high affinity of polyethylene glycols for the alkaline earth cations, especially for Ba, was the basis for the development of a radiometrical procedure for the quantitative determination of nonionic detergents of the polyethylene glycol type. The complex is extracted at a concentration that is proportional to the PEG content in the aqueous sample. For this purpose 0.02 M H+1- in a mixture of 75 vol% nitrobenzene with 25% CHCl3 was used and the distribution ratio of radioactive 133Ba was measured in dependence of the PEG content. The obtained straight-line calibration graphs permitted determination of, e.g., PEG 1000 with a detection limit of 1 µg of the detergent in 10 mL of solution [268, 269]. The extraction yield of 133Ba was the highest for the PEGs with the longest ethylene oxide chain and depended linearly on the number of ethylene oxide units, n, in the broad range of n=7 to 140. The interference of anionic and cationic detergents, possibly present in the samples, was reasonably small.

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New Polymeric Materials with Incorporated Metal Bis(dicarbollide)s

Steckle et al. from LANL grafted cobalt bis(dicarbollide) directly onto cross-linked polystyrene beads [270]. Because such a material did not absorb cesium sufficiently, probably due to the hydrophobic nature of the sorbent, the authors performed a partial sulphonation of the sorbent, but this apparently led to decrease of selectivity due to the presence of sulpho-groups [270]. In another study of LANL, lithiated carbollide was reacted with chloromethyl polystyrene or polybenzimidazole (PBI) onto which epichlorhydrin was grafted to provide a chlorine leaving group. Grafting of epichlorhydrin onto PBI involved opening of the epoxide with formation of a hydroxyl group, which was protected by using hexamethyldisilazane prior to reaction with lithiated carbollide [271]. In a detailed study of Man from MIT, a number of polyelectrolytes of the type of polyanions with pendant cobalt bis (dicarbollide) and carborane were synthesized via ring-opening metathesis polymer (ROMP). The polyelectrolytes were found to work as hydrophilic ion exchangers, binding selectively cesium over sodium. In order to reduce hydrophilicity and solubility in the stripping medium of 8 M HNO3, copolymerization and cross-linking were used [272]. Pyrrolyl/dicarbollide complexes were studied by Teixidor et al. [273] and the cobalt bis(dicarbollide) anion was used as a dopant of a pyrrole-based polymer [274]. Recently, supramolecular polymer materials with cobalt bis(dicarbollide) were synthesized. For this purpose, the nickel(II) macrocycle (5,7,12,14tetramethyldibenzo [b,i]1,4,8,11-tetraazacyclotetradecine)nickel(II) [Ni(TMTAA)] was used [275]. This is a versatile receptor for neutral globular type molecules, including C60, o-carborane, and the phosphorus chalcogenide molecules P4S3 or P4Se3, as well as the disc-shaped 18-crown-6, etc. A mixture of a Ni(TMTAA) grafted crown ether and Cs1 in toluene+CH2Cl2 afforded a 1:1 complex, comprised of layers of infinite two-dimensional polymeric arrays separated by layers of the cobalt bis(dicarbollide) anion which themselves were said to form two B–H…Cs+ hydrogen-bonded interactions [275]. Structural studies of the behavior of the complex in solution by 1H NMR spectra proved that also in solution cesium was bound to the hexaethyleneoxy crown ether oxygen atoms. An analogous supramolecular material was prepared according to [276]. Treatment of M+ 1-, M + =Na + or K + , with the [2.2.2]cryptand and Ni(TMTAA) resulted in a 1:1:1complex, assembled with the cobalt bis(dicarbollide) anion snugly residing in the phenyl-lined face of the Ni(II) macrocycle through C–H…N and C–H…␲ interactions, with the ensuing [M[2.2.2]cryptand]+supercation residing in the smaller methyl faced cavity of the macrocycle. UV–Vis experiments were conducted for Ni(TMTAA) with [Na[2.2.2]cryptand]+1- in dichloromethane, nitro-methane and chloro-benzene, albeit without any evidence of complex formation in solution. Similarly, no multicomponent species in the gaseous phase were detected by the electrospray mass spectra technique.

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G.

317

Precipitates and Chromatography on Inert Support Materials

The simplest method of how to change the extraction systems into their chromatographic analogs is to anchor the organic extractant on a convenient solid support. With cobalt bis(dicarbollide), the early studies of this kind were performed by Koprda and Scasnar, who used a solution of H+12- in nitrobenzene on KEL-F (polytrifluoroethylene) and Teflon 40 supports and isolated 137Cs or 137Cs+90Sr [261]. The rule that the salts of hydrophobic anions which form an aqueous precipitate with a given cation are also well extracted and vice versa (see Chapter 6) has lead to several proposals of new sorption materials based on such precipitates. The precipitates of protonated crown ethers with phosphomolybdic acid proved to be sorbents for cesium [277], protonated polyethylene glycols with the same anion are suitable for the uptake of Ra2+ [278, 279], and analogous precipitates with phosphorus containing reagents such as DBDECMP are selective for the uptake of trivalent lanthanides and actinides [280]. An interesting feature of these precipitates is the way to prepare them; in the case of crown ethers the two starting components are not soluble in the same solvent. Thus, the crown ether was dissolved in a chlorinated organic solvent and phosphomolybdic acid in water, the two phases were brought into contact and a voluminous precipitate formed at the interface, which was not soluble in either solvent [277]. It is only natural that analogous precipitates will form with cobalt bis(dicarbollide) anions; however, at this time they were not studied due to the deemed high price of cobalt bis(dicarbollide) compounds at the beginning of the investigations. The mechanism of sorption on the first material mentioned above was later studied by Fernando [281] and it was ascertained that sorption primarily takes place only at the surface of the solid phase. This seems to be connected with the low diffusivity of ions in this type of precipitates. The low solubility of Cs1 in water (0.62 mmol/L) has lead to an application, in which the formed cesium cobalt bis(dicarbollide) precipitate is sorbed on activated charcoal [282]. The capacity of ordinary activated charcoal (AC), was found to be 0.7–1 mol of 1 anion/kg AC, which corresponds to a 29.2–32.4% solid solution of 1-. The thus prepared material behaved as a pseudo ion exchanger with a capacity of 0.64–0.71 mequiv/g and had the highest affinity for Cs+, followed by K+ and Na+ [282]. The proposed method apparently enables the separation of cesium even at conditions when no visible precipitate is formed, i.e. at low concentrations of Cs and cobalt bis(dicarbollide). For example, when to 1600 ml of an aqueous solution containing 1 mmol of cesium, 1.2 ml of 1 mM of H12 were added and the solution was passed through an AC column, more than 99% of the cesium was retained on the column [282]. Various derivatives of cobalt bis(dicarbollide) were tested and the solubilities of their cesium salts in water were determined. These were found to be 620 µmol/L for Cs+1-, 26 µmol/L for Cs+12-, 5 µmol/L for Cs+36-, and 1 µmol/ L for Cs+33- at 25°C [283]. These values, compared with the classical precipitant

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tetraphenylborate (10 µmol/L for ), show that several cobalt bis(dicarbollide) derivatives may exceed the performance of the latter reagent. Kyrs et al. used the synergetic mixtures of phosphonates with chloro protected cobalt bis(dicarbollide) on the extraction chromatographic support XAD-7 [284– 286]. In the latter of these studies, it was found that in fact no solvent was necessary for the uptake of europium from nitric acid media. At that time, avoiding the toxic nitrobenzene was considered as particularly advantageous. A mixture of H+12with either DBDECMP or CMPO was dissolved in methanol, the support was dipped into the solution, and the methanol was then evaporated at room temperature. The mechanism of Eu uptake in this case seems to be that common for precipitate formation, as supported by the fact that the kinetics of sorption was quite slow [285], possibly proceeding at first only at the surface of the solid [281]. In addition, it is not surprising that at conditions in which the organophosphorus compound was preliminarily sorbed on the support and a solution of the chloro protected cobalt bis(dicarbollide) with Eu added afterwards [285], the “sorption” dramatically increased. This is plausible considering that the process of sorption was in fact substituted by coprecipitation. Finally, a technique that does not necessitate at all the solid support, the organic liquid phase itself working as the support, but belongs to the category of chromatographic methods should be mentioned. Cs and Sr were separated by means of partition counter-current chromatography with a planetary centrifuge, using 0.01 M H+1- in nitrobenzene as a stationary phase [287]. Successful separation, with Cs contained in the first volume fractions and Sr coming consecutively, was achieved by using as a mobile phase (1) 0.25% polyethylene glycol 300 in 0.5 M HNO3 and (2) 0.001 M Ba(NO3)2 in 0.1 M HNO3. A reverse order of elution was obtained upon using (1) 0.001 M Ba(NO3)2 in 0.1 M HNO3 and (2) 0.0005 M Ba(NO3)2+0.25% polyethylene glycol 300 in 0.5 M HNO3. Interestingly, the higher affinity to Ba than to Sr was used for the elution of the latter.

H.

PVC Membranes as Ion Selective Electrodes and for Separation Purposes

The anions [(1,2-C2B9H11)2-3-M]- (M=Co3+, Fe3+, Ni3-, 1-, 87-, 88-) have been implanted in PVC membranes to study their performance as Cs+ sensors in ion selective electrodes (ISE) [124]. The composition of the membrane was: 61 wt% dioctyl phthalate (plasticizer), 30% poly(vinyl chloride) and 9% of one of the above ligands in the form of their Cs salts. The sensors were stable at least for three weeks with nearly Nernstian response (52, 51, 50 mV/decade, respectively). No particular differences were observed among the three sensors, that with Ni displayed the least stability, but also the quickest response (<1 s, compared to <10 s for two others) and a low detection limit of about 1×10-5 M Cs+ (same as for the Fe derivative and two times lower than for the Co derivative). The selectivity

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coefficients toward univalent interfering ions were generally low; KpotCs/M was 24×10-2 for Na+, 2.3-2.7×10-1 for K+, and 2.4×10-1 for Rb+. This low selectivity is apparently connected with the kind of the solvent (plasticizer) used; as is well known, the selectivities of substituted phthalates, adipates, and sebacates are lower than those of nitrosolvents. Inspired by the construction and function of ion selective electrodes, a new type of sorbents containing H+12- embedded in a PVC matrix were prepared and tested for their sorption properties for Cs+ and Sr2+ [209]. The conceptual similarities of the systems used for the construction of the ISE and extraction systems are indeed remarkable, and common notions are described with different terminology. Thus, the notions of “plasticizer,” “ionophore,” and “anion blocker” in ISE have their counterparts “solvent,” “synergist,” and “extraction reagent” in extraction science, respectively (anion blocker in ISE corresponds to the hydrophobic anion in extraction; the latter “blocks” the extraction of simple inorganic anions into the organic phase) [209]. The successful preparation of sorbent materials as reported in Ref. 209 proved that the above concepts of similarity are justified. It has been shown [209] that the distribution ratios DCs and DSr in the extraction systems with H+12- and polyethylene glycol for the solvents (o-nitrophenyl octyl ether, NPOE, 4-chloro-3-nitrotoluene, 2-fluoro-2-nitrobenzene, 1-fluoro-3nitrobenzene were used as polar solvents) are the same as for systems in which the extractant is embedded in the PVC matrix. This analogy indicates that the PVC in the matrix functions as a mere “diluent” of the system, hence having no distinct chemical effect on the properties of the system. The losses of the solvent from the PVC membrane due to evaporation were low, especially for o-NPOE, reaching a value of some 30% after 80 days of standing in open ventilated air. Thus, these materials can find their application in nuclear technologies, mainly from the point of view of similar efficacy as analogous solvent systems, but with the advantage of avoiding any dangers of spills of radioactivity [209, 288].

I. 1.

Potential-Controlled Separations Potential-Controlled Extractions

Since in the majority of the systems discussed the transfer of ions is involved, the question whether the extraction could be affected by changing an applied external voltage to the system was examined in several papers mainly of Japanese origin. The method might be particularly suited for practical separations, since many cumbersome procedures with various types of extraction and stripping agents could be avoided of. The distribution ratio of an ion of interest might ideally depend only on the external applied voltage. Although no concrete application with boron anions was found, the switch from other hydrophobic anions to dicarbollides should be straightforward. Hence, a short description of the method is in place in the context of this review.

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The most recent development and limits of the method may be illustrated by a paper of Kitatsuji et al. [289]. The authors used a simple electrolytic cell with one working electrode (a platinum mesh electrode, 20×50 mm) in the aqueous phase layered over the nitrobenzene, one calomel electrode, and two reference electrodes, one in each layer. The choice of mixing was crucial for the kinetics of the process; the best proved to be a rod with wheels placed at the interface of both phases and stirring at a speed of 350 rpm. For the following experimental conditions: 0.05 M MgCl2+0.001 M Cs+ in the aqueous phase of pH=3.0 and 0.05 M THepA+ TFPBin the nitrobenzene phase, TFPB- being tetrakis [3,5-bis(trifluoromethyl)phenyl] borate-, more than 98% of radioactive cesium transferred into the nitrobenzene phase during 20 min of stirring at an applied external potential of +0.275 V referred to the tetraheptylammonium, THepA+, ion. What is important and justifying the idea of potential control of distribution ratios is the linearity of log DCs vs. potential. Namely, for cesium the log DCs changed by unity with approximately 60 mV difference of applied voltage. Thus, at the applied voltage of 0.1 V (against the THepA+ ion) the measured DCs was about 0.1, whereas at 0.275V it is already about 200. In addition, the separation was quite good; under the described conditions, the separation factor from U and Am was about 105 and 107, respectively. On the other hand, the latter elements could not be transferred into the organic phase by merely changing the voltage due to their high lipophilicity. Hence, bis(diphenylphosphoryl)methane had to be added to the system as a complexing and hydrophobicity increasing synergist [289] for extracting the latter two elements. It can be concluded that potential-controlled extractions might become a powerful tool in extraction technologies, eliminating largely secondary wastes formed by using stripping agents during classical extractions. Their main drawback, as it appears now, is the slower kinetics of transfer than in classical extraction. In certain situations, this might not be a large obstacle for practical use if demands on the rate of throughput are not of primary concern. 2.

Potential-Controlled Sorption

The sorption electrochemical cell and materials used for the preparation of the surface of the working electrode comprising different derivatives of cobalt bis(dicarbollide) were recently patented by Tinker et al. [290–292]. The principle of this method of “electrochemical ion exchange” (EIX) consists of a simple flowthrough electrochemical cell in which the working electrode is coated with a suitable dicarbollide compound and an external voltage is applied to it. It is claimed that Cs and Sr can be sorbed and desorbed on the working electrode by the variation of the potential, but the details, such as the value of the potential and the Kd attained, are not given in the patents. The methods of preparation of various boron compounds are described in the patents as well as the methods of the coating of the metal electrodes. The latter consisted of the following procedures. Thiol-substituted cobalt bis(dicarbollide)s were dissolved in a suitable solvent, e.g. ethanol, and gold or

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platinum electrodes were dipped into the solution. The reaction time ranged from several hours to several days, depending on the thiol, and no heat was necessary for completing the reaction. In the case of carboxyl-substituted cobalt bis (dicarbollide)s, the metal surface of titanium or stainless steel electrodes was preliminarily activated in 6 M HCl. The rest of procedure was as for the previous metals, again no heat was necessary, and depending on the type of carboxylic acid the reaction time ranged from several hours to several days. The binding onto polymer materials followed the method described by Steckle (see Section VII.F) [271]. Leaching tests of hexachloro cobalt (bis)dicarbollide absorbed on a polymer resin on a steel wire have shown that only a negligible amount of the reagent was leached after 7 days of contact with water for 6 M HNO3, showing that the systems with dicarbollides may be effectively used in electrochemical ion exchange cells [290].

J.

Boron Neutron Absorbers Usable in the Reprocessing of Nuclear Fuel

Boron compounds, due to their high content of boron atoms having a very large cross-section for neutron absorption, are ideal tools for applications requiring neutron capture. One extensive area, not treated in this review, is neutron boron therapy, in which the specified boron compounds bind selectively to cancer tissues. The other use, initially proposed at NRI and IIC, Rez, is their application as neutron poisons when dissolved in an organic solvent of the Purex type [293]. For this purpose, a neutral boron cage compound, m-carborane or a neutral nickel (IV) bis(dicarbollide) were proposed. Their solubility in the Purex solvent was high; the losses into the aqueous phase were lower than 0.11% for m-carborane (for 2% m-carborane in the solvent) and their presence apparently did not influence the distribution of uranium. The subject was further studied and developed in JAERI, Japan where m-carborane was used and tested with 239Pu solutions [294]. Eventually, a new process design was patented, comprising the construction of an industrial cylindrical extractor with a 5 cm thick inner layer of m-carborane and the use of 20g/l of dissolved m-carborane in the organic Purex solvent [295]. The neutron multiplication factor could be made less than unity for more than 15 g/L of mcarborane in the organic solvent and a solution of 120 g/L of 239Pu. The total weight of the extractor was considerably smaller compared with the extractor layered with B concrete and the output increased due to the enlargement of the diameter of the working compartment of the extractor. Less than 10 ppm of boric acid formed by decomposition of the solvent under gamma irradiation of 1 MGy [295]. It should be noted that during the extractions of waste solutions with H+12- the same effect as in above application must be expected. Usually, only small amounts of 239Pu are present in defense wastes, nevertheless, this function of the boron extractant adds to a double-security of the mentioned processes.

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VIII. CONCLUSIONS The success of chloro-protected cobalt bis(dicarbollide) technologies for a plant aimed at fractionation of nuclear waste had as its associated effect the support of scientific studies proceeding now for over thirty years. The subject of the present review was, therefore, pertinent information on the scientific background of bis(dicarbollide) extractions. The relatively large size of the present chapter is caused by the fact that the subject from its start in the early 1970s was not fully reviewed and some emerging applications appeared only in recent years. Two different main areas—the synthesis and properties of metal bis(dicarbollide)s and the extraction and other separation methods with metal bis(dicarbollide)s—were deliberately placed into one review. The authors considered both of these subjects important in their connections, and a reader who is an expert in one field can hopefully find useful information also from the other domain. New routes of dicarbollide synthesis may lead to new compounds with specific properties, possibly useful also in other areas than extractions. Newly emerging separation methods may find practical applications of metal bis(dicarbollide)s in the near future. By bringing up new concepts in both areas, the authors hope to stimulate further research in related scientific disciplines.

ACKNOWLEDGMENTS We thank to Dr. J.Plesek (IIC) for valuable discussions. Dr. Selucky (NRI) helped with organizing the extraction data for functionalized dicarbollides. Prof. Y.Marcus kindly corrected the text and had several valuable suggestions to it. The authors wish to express their gratitude for supply of valuable information to Dr. I.Smirnov and Dr. V. Babain from the Radium Institute, St. Petersburg, as well as to Dr. Vanura from the Institute of Chemical Technology, Prague, for providing the text of his Dr. Science dissertation and other materials before publication. Financial support for studies of new nonpolar solvents of the third generation for cobalt bis(dicarbollide)s, provided by the Czech Grant Agency (Grant No. 104/ 01/ 0142) and Ministry of Education of Czech Republic, contract ME 485 is appreciated (J.R.). Synthesis of new compounds was supported by EEC Grant (FIKW-CT 2000 0088 NAS-Calixpart) by Ministry of Education of Czech Republic (LN 00A028) (B.G.).

SUPPLEMENTARY MATERIAL Report 16, review of early stages of chloro-protected cobalt bis(dicarbollide) science and technology until the year 1993 (228 pages of text including 46 figures), can be supplied on request from the author (J.R.) at NRI.

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SYMBOLS Abbreviations and structures of bis(dicarbollide)s and other boron derivatives used for extraction are given in Tables 1–11 and denoted in the text by bold Arabic numbers. New fluorinated solvents, denoted as FS#, are identified in Table 14. 12BISPHECOSAN Cal4

Cal-tetramide CMPO DBDECMP DBS DEPSAM DHDECMP DMAMP DMDBDDEMA DN DODBCMP DOS DPhDBCMPO DPPEDO DPTP EOU FS-13 Glyme HLW ISE ITIES

MNBTF NPHE NPOE n EOU OcPhCMPO OP-10 PEG

Anion of chloro-protected cobalt bis (dicarbollide) Derivative 37 in Table 8 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis [(ethoxycarbonyl) methoxy]-2,8,14,20-tetrathiacalix[4]arene (cone) 5,11,17,23-Tetra-tert-butyl-25,26,27,28- tetrakis(N,Ndiethylcarboxamidomethyleneoxy) calix[4]arene Octyl (phenyl)-N,N-diisobutyl-carbamoyl methyl phosphine oxide Dibutyl diethylcarbamoylmethyl phosphonate Dibutyl sebacate Diethylpropane sulphamide Dihexyl diethylcarbamoylmethyl phosphonate Dimethylacetyl methylenephosphonate N,N-dimethyl-N,N-dibutyl (dodecyloxyethyl) malonamide donor number of the solvent Dioctyl-N,N-dibutyl methylene carbamoyl phosphonate Dioctyl sebacate Diphenyl-N,N-dibutylcarbamoyl methylene phosphine oxide Bis-diphenylphosphine ethylenedioxide 2,6-Di(5,6-dipropyl-1,2,4-triazin-yl) pyridine Ethylene oxide unit, -CH2CH2OPhenyl trifluoromethyl sulfone, Russian solvent for UNEX process 2-Glyme, 3-Glyme, 4-Glyme=CH3O(CH2CH2O)nCH3 with n=2, 3, or 4 High (radioactivity) Level Waste Ion selective electrode Interface of Two Immiscible Electrolyte Solutions; refers to electrochemical studies performed at the actual interface of the two solutions in equilibrium or very near to equilibrium m-Nitrobenzotrifluoride o-Nitrophenyl hexyl ether o-Nitrophenyl octyl ether Number of ethylene oxide units in polyethylene glycol chain Octyl phenyl carbamoyl methylene phosphine oxide Di-iso-butylphenol decaethyleneoxide glycol Polyethylene glycol, PEG 400 is PEG with mean m.w. 400

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324 PPEG ri Slovafol 909 TEDMAMDP TEMDP TFPBTHF THP TODGA TOPO TPMDP TPTZ TRITON X-100 ␦BS (w→n-octanol)

Rais and Grüner Phosphorylated PEG Ionic radius Nonylphenol nonaehtyleneoxide glycol Tetraethyldimethylamino methylendiphosphonate Tetraethyl methylenediphoshonate Tetrakis [3,5-bis (trifluoromethyl) phenyl borate]Tetrahydrofuran Hydrogenated tetrapropene, aliphatic solvent used in radiochemical technologies in France Tetraoctyl-3-oxapentane-1,5-diamide Tri-n-octyl phosphine oxide Tetra-iso-propyl methylenediphosphonate Tris-2-pyridyl-1,3,5-triazine Isooctylphenyl decaethyleneoxide glycol Partition constant of neutral solute between water and n-octanol used as a lipophilic/lipophobic parameter especially in drug research

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Rais, J.; Selucky, P. Czechoslovak Patent 165751, 15th July 1974. Sebesta, F.; Jirasek, V.; Rais, J.; Selucky, P.J. Radioanal. Chem. 1980, 59, 91–97. Sedlacek, J.; Sebesta, F.; Benes, P.J. Radioanal. Chem. 1980, 59, 45. Selucky, P.; Rais, J. Czech. Patent 222731, 15th March 1986. Fernando, L.A., et al. Anal. Chem. 1980, 52, 1115. Plesek, J.; Hermanek, S.; Selucky, P.; Williams, R.E. United States Patent US 5,540,843, 30 July 1996. Plesek, J.; Hermanek, S. In Second International Symposium on Environmental Contamination in Central and Eastern Europe, Budapest, Hungary, September 20–23, 1994; Proceedings; 1047–1049. Kyrs, M.; Navratil, J.D.; Kuca, L.; Vrbova, L.; Svoboda, K. Nucleon (NRI, Rez) No. 2:20, 1995. Svoboda, K.; Kyrs, M.; Vanura, P.J. Radioanal. Nucl. Chem. 1997, 220, 47–54. Svoboda, K.; Kyrs, M.J. Radioanal. Nucl. Chem. 1997, 220, 245–247. Zolotov, YuA.; Spivakov, B.A.; Maryutina, T.A.; Bashlov, L.A.; Pavlenko, I.V.; Fresenius, Z. Anal. Chem. 1989, 335, 938–9444. Abney, K.D.; Kinkead, S.A.; Mason, C.F.V.; Rais, J. United States Patent US 5, 666, 641, September 9, 1997. Kitatsuji, Y.; Yoshida, Z.; Kudo, H.; Kihara, S.J. Electroanal. Chem. 2002, 520, 133–144. Tinker, N.D.; McKinney, J.D.; Richards, S.J. International Patent, WO 99/03582, January 28, 1999. Tinker, N.D.; McKinney, J.D.; Richards, S.J. Great Britain Patent, GB 2,345,652, July 19, 2000. Tinker, N.D.; McKinney, J.D.; Richards, S.J. United States Patent US 6,423,199, July 23, 2002. Kyrs, M.; Plesek, J.; Rais, J.; Makrlik, E. Czech Pat CS 211,942, November 12, 1982. Naito, Y. In Proceedings of the First NUCEF International Symposium NUCEF’95, Hitachi-Naka, Japan, October 16–17, 1995; 71–78. Satoshi, S.; Tachimori, S.; Naito, Y.; Arakawa, T. Japanese Patent, JP 9,080,192, March 28, 1997.

283. 284. 285. 286. 287. 288. 289. 290. 291. 292. 293. 294. 295.

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6 Principles of Extraction of Electrolytes Jirí Rais Nuclear Research Institute Rez plc, Rez, Czech Republic

I. INTRODUCTION From the early 1960s, the methods for separating nuclear fission products were studied at NRI, Rez. For this purpose, the reagents formerly proposed for the precipitation of cesium—like dipicrylaminate and tetraphenylborate—were successfully applied as extractants for this fission product in the pioneering studies of Kyrs et al. [1] and Krtil et al. [2] and the development of the extraction research followed. It was a challenging task at those times to find new extractants for cesium and various hydrophobic anions were proposed. To enumerate just several of them, extraction with phosphomolybdic acid anion was studied [3, 4] and the mechanism of the extraction with picrate and dinitrophenolate anions was elucidated [5]. As new hydrophobic anions, complexes of As(V) with catechol or substituted catechols were proposed for extraction separation of cesium [6–8]. The previously studied hydrophobic anions were subsequently largely superseded by cobalt bis(dicarbollide) anions. The extraction with them was at first reported in 1976 [9] and from that time a large body of material accumulated which is reviewed in Chapter 5 of this volume [10]. It was realized already in the very outset of the studies that the systems are particular in the sense that if a polar organic solvent (nitrobenzene) was used, no interaction between the cation and anion in the organic phase occurred and, in fact, an electrolyte is transferred from one phase to the other without any further chemical interaction. Thus, the extraction results ought to be independent of the type of counteranion and only the properties of the solvent were deemed the leading parameter. In our early studies we checked the selectivity of the exchange constant of the reaction 335 Copyright © 2004 by Marcel Dekker, Inc.

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for a few anions [11, 12]. The constancy of the value for the water—nitrobenzene system supported the underlying idea of the sole role of the solvent in determining the selectivity. The extraction of ion pairs or electrolytes developed into a large branch of extraction systems. The importance of the subject was demonstrated at first by Diamond in his several studies [13-15]. Diamond noted the peculiarities of these systems as opposed to classical extractions with formation of chelates. He argued that in these instances, the compounds are extracted into the organic solvents as ions not forming a neutral molecule, even if in the case of low polarity solvents the ions undergo ionic association [15]. Diamond coined for the systems under consideration a name “coordinatively nonsolvated salts” in order to stress nonchemical bonding in these cases. Some other proposals of classification used the notion of a “mechanism of loss of affinity to water” [16]. Certain hesitation in the exact definition of these systems in the early beginnings of the development of extraction science arose from the fact that typical Bjerrum-type ion pairs as a rule do not extract into organic solvents [17]. Thus, the systems were at the beginnings also characterized by so called “structure enforced ion pairing,” which in distinction to classical ion pairing would proceed in water by the bulky ions disturbing the structure of water [17]. The extraction in these systems is always connected with high hydrophobicity of at least one ion composing the ion pair, this being the essential feature of the systems. Hence, the terms “extraction of ion pairs” or still better “extraction of electrolytes” are believed to be simpler and more exactly defining the systems. (“Extraction of ion pairs” is a more historical and less precise notion, since if a sufficiently polar solvent is used, ironically no ion pairs need to exist at all [17], and in that case also “extraction of dissociated ion pairs” was used.) The classification of the systems belonging to the class of “extraction of electrolytes” is important for developments in this area. Such classifications have been presented long ago by Morrison and Freiser [18] and later by Stary et al. [17] and comprise as expected (1) simple ion pairs/electrolytes like tetraphenylborates, etc., (2) complex ion pairs/electrolytes. The latter group can be further divided into (a) metal cations (b) metal in anionic form (e.g., ), and (c) chelated ion pairs. For the purpose of this review, the case of the extraction of a cationic complex with a neutral ligand as a part of the cation and the anion (e.g., where L is phosphonate, malonamide, or another neutral ligand) is particularly important. The particle might be viewed as one particle, in which no distinction is made between the cationic and anionic counterparts and its extraction is described by the reaction: (1) and this is the most often encountered expression in the literature. However, viewing the reaction as (2)

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leads to different mathematical expressions for the dependences of DEu on as will be shown below. Because on the basis of the latter mechanism the very often encountered maxima on the dependences of DEu on can be accounted for, this expression ought to be given preference (see Section IV.F). With development of the knowledge of the extraction mechanisms of this particular type of extraction, the physical chemistry of the systems developed. The crucial question is: Which driving force is responsible for the transfer of simple inorganic cations accompanied with bulky anions from the aqueous to the polar organic phase with full dissociation in it? Because, as seen from the basic relation for the extraction constant of a uni-univalent salt expressed by activities of ions in both phases: (3) no term is connected with any kind of chemical reaction, the transfer must be determined solely by the hydration and solvation of M+, B- in both phases. Consequently, the underlying physical chemistry of this extraction process is determined exclusively by the physical chemistry of hydration and solvation of ions. The elucidation of the systems therefore reverts into the broadly studied discipline of physical chemistry of solutions, solvation, and resolvation. The introduction of the concept and determination of individual extraction constants of 22 ions in the system water—nitrobenzene [19] presented a link between the purely “extraction” and “physical chemist’s” points of view of the process of transfer into the organic phase. The individual extraction constants are a direct measure of the standard molar Gibbs energy of transfer of an ion from water saturated with a polar solvent into a polar solvent saturated with water, ∆tG°(X, aq(or)→or(aq)). This concept promoted and was largely reflected in another area of research, namely, the electrochemistry at the interface of two immiscible electrolyte solutions, ITIES. It is an achievement of the Czech electrochemical school that this interdisciplinary discipline was established by Koryta and continued by others such as Samec and Marecek. The topic is beyond the scope of the present review and we refer here only to data that are common for the disciplines of extraction of electrolytes and electrochemistry of ITIES. These are the numerical values of the Gibbs energies of transfer of ions, determined in each discipline by a very different technique, but necessarily leading to the same values of ∆tG°(X, aq(or)→or(aq)). During the years a great number of the individual ∆tG°(X, aq(or)→or(aq)) values has accumulated and a comparatively large database is kept and refreshed in web form by Girault [20]. Another critical review to be mentioned is that of Osakai [21, 22]. The physical chemistry of transfer upon extraction is of course complemented by another large area of research—physical chemistry of transfer of ions between two pure polar solvents. The subject is well described in recent monographs by Marcus [23–25] and series of journal publications by Gritzner, e.g., Ref. 26.

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Several theoretical models were presented for the purposes of elucidation and prediction of the relative solvation in different solvents. Among recent treatments, the statistical analysis of the experimental data, based on a number of parameters with subsequent assigning the weight to the most important ones, that of Marcus [27] is one method of how to tackle the problem. A modern treatment based on the modified Born equation was given by Moyer [28] and that based on quantum mechanical calculations with Uhlig’s equation for transfer of voluminous ions was presented by Osakai [22]. The contemporary molecular dynamic simulations and ab initio calculations deal with the calculation of total energies of hydration or solvation of a particular ion in a particular solvent. The much subtler calculations of the energetics of transfer cannot as yet be carried out and new developments are awaited.

II. AIMS AND SCOPE OF THE REVIEW This review deals with the mechanism and physicochemical basis of the extraction of electrolytes. In this sense, it is complementary to the other contribution in this volume (Chapter 5), in which extractions of metal bis(dicarbollides) are treated. Although the latter anions are by far the most practically used in the considered type of extractions, the extraction of electrolytes goes beyond the use of metal bis(dicarbollides). It is shown in this review that rather ubiquitous maxima experimentally observed for many extractions with neutral ligands and mineral acid anions can be reasonably accounted for with the mechanism of extraction of electrolytes. The subject is divided into (1) description of the systems by the sets of pertinent equilibrium reactions, (2) treatment of characteristic cases encountered in praxis, (3) semiempirical models based on simple normalizing functions applicable to the physical chemistry of transfers. Standard molar Gibbs energies of individual ionic transfers between water saturated by nitrobenzene into nitrobenzene saturated by water and hydration numbers of ions in the nitrobenzene phase are collected and critically reassessed. Other numerical data of interest, mainly constants pertinent to crown ethers systems, are contained in Chapter 5.

III. DESCRIPTION OF THE SYSTEMS The main equilibria involved in the extraction mechanism are given below. Their definition is confined to the most common cases encountered in the practice of extraction of electrolytes. This is a set consisting of z-valent cation (z=1, e.g., for Cs+, Na+; z=2 for Sr2+, Ba2+; z=3 for Eu3+, Am3+) and the rather ubiquitous H+ ion in separation techniques (described with separate constants), in the presence of a univalent hydrophobic anion B- and possibly a neutral synergist L. Only general

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reactions are shown in the following text, with self-explanatory definitions of the respective equilibrium constants according to the law of mass action or based on the equality of the chemical potential of ions and molecules in the two phases at equilibrium. Subscripts aq and or denote the aqueous and organic phases, respectively. The principal constant for extraction into polar solvents is an extraction constant of an electrolyte dissociated in both phases Kex(Mz+, zB-) characterized by the reaction (4) Which, for H+ is written as Kex(H+, B-). The cation may associate in any of the phases, as characterized by the association constant in the aqueous or organic phase,

(or)

(5)

The complementary extraction constant is therefore the constant of the distribution of the ion pair (6) However, since usually the association in the aqueous phase is negligible, another constant is often practically used, namely, the extraction constant of the dissociated Mz+, zB- in the aqueous phase forming an associate in the organic phase Kex(M, zB): (7) To complete the description of simple systems without the added synergist L, a very important constant describes the selectivity of extraction and in fact is the decisive constant if extraction is done from acidic media. It is the extraction exchange constant Kexch(Mz+/zH+): (8) but of course, the reaction may be written for any two ions present in the systems, e.g, in the form Kexch(Mz+/zNa+) or otherwise. In more complicated, but often encountered cases, a neutral ligand L is added into system. The latter generally reacts with both the hydrogen ion and the metal cation, yielding a complex cation of the same charge as the bare one. The important value is the distribution of the ligand between the aqueous and organic phases ␦L: Laq=Lor ␦L

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(9)

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The most often used extraction constant refers to simultaneous chemical bonding of a cation with the ligand and successive transfer of the complex into the organic phase, extraction constant with bonding, Kex(MLz+, zB-): (10) This constant may be written of course also with Laq on the left-hand side of the equation. An analogous constant applies also for the hydrogen ion, i.e., Kex(HL+, B-). The strength of the bond of L with the cation is given by the stability constant β in either the aqueous or organic phase, or or

(11)

and the complex cation may again associate with the anion in both the aqueous and organic phases with respective association constants: association constant of the complex, or

or

(12)

The same relations are valid for the hydrogen ion with constants Kex (HL+, B-), and or these are self-explanatory again. Extraction exchange applies for the complex, either in its exchange for a noncomplexed ion, for the hydrogen ion or for another complex according to the extraction exchange constant defined above, e.g.: (13) Another common case is that the ligand L forms higher complexes with the cation. In this case analogous constants as given by Eqs. (10) to (13) apply; e.g, for the 1:n complex it is the extraction constant with bonding, (14) The latter constant might be expressed in different form, according to need, e.g., as pertaining to the reaction: (15) or (16)

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but for the sake of simplicity it is always denoted by the same symbol, keeping in mind that the reaction is in each case specified. The last expression, because the concentrations of in the expression for the constant cancel, is in fact a mixed constant of the type shown just below. In certain cases, constants are used for which the balance of components does not conform to the condition of electroneutrality of both phases. Still, these are important in some treatments to be dealt with below. These are called “mixed constants,” being half way from ionic constants Kex(i), see below, and normal constants expressing the full equilibrium. Two frequently used constants of this type correspond to the equilibria: (17) and (18) The distribution is usually expressed in the form of the distribution coefficient of M, DM: (19) where Σ denotes the sum of concentrations of all possible forms of M existing in the organic (or) and aqueous (aq) phases. In contrast to classical extraction chemistry, for the more precise definitions of the systems under scrutiny, another distribution coefficient must be defined. This is the distribution ratio of identical ionic species, Didion [29], (20) The value of Didion gives the distribution of just one type of charged particle. For example, if in the aqueous and organic phases there exist the particles M+, ML+, then DM={[M+]or+[ML+]or}/{[M+]aq+[ML+]aq}, whereas Didion(M+)=[M+]or/[M+]aq and Didion(ML+)=[ML+]or/[ML+]aq. Assignment of the individual properties to cations and anions of the constant Kex(Mz+, zB-) is made on the basis of the relation: log Kex(Mz+, zB-)=log Kex(Mz+)+zlog Kex(B-)

(21)

where Kex(Mz+) and Kex(B-) are individual extraction constants of the ions Mz+ and B- [19]. The ionic individual constants are not thermodynamically defined and must be estimated on the basis on some extrathermodynamic assumption. Usually the assumption is used that the two large ions of tetraphenylarsonium+ and

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tetraphenylborate- have identical values of Kex. We find this assumption denoted in the literature as the Ph4AsBPh4 or TATB assumption. Experimentally, only Kex(Mz+, zB-) or Kexch(M+/N+) are available. If Kex(Mz+, zB-) for two cations, let us say Mz+ and H+, are determined, the extraction exchange constant Kexch(Mz+/zH+) can be calculated as (22) Thus, the differences of log K(X) for ions of the same sign are experimentally determinable and these are very important quantities describing the selectivity of the system. The concept of individual ionic extraction constants belongs into the sphere of physical chemistry of ion transfer between two solvents. The appropriate standard thermodynamic values, characterizing the transfer of an ion (i.e., referred to an ideal state of infinite dilution and hence noted with the superscript o) from an aqueous phase (aq) to an organic solvent (or) are the respective standard Gibbs energies of transfer of an ion X, ∆tG°(X, aq→or): ∆tG° (X, aq→or)=µ0,or(X)-µ0,aq(X)

(23)

where µ0,or(X) and µ0,aq(X) are respective standard chemical potentials of X in the phases aq and or. In electrochemistry, the basic condition for the calculation of equilibrium situation is the equality of the electrochemical potentials of each charged particle in both phases at equilibrium, instead of the equality of the chemical potentials [29]. The equilibria are then characterized by the additional quantity, namely “distribution potential” and the notion of the standard distribution potential of an ion X, is introduced. In expressions of both extraction and extractive exchange constants, the terms involving this potential cancel, so that the derivation and validity of the constants given in this chapter is ensured [29]. Nevertheless, values have their primary importance in determining ∆tG°(X, aq(or)→or(aq)) values by electrochemical methods as dealt with below. Since log Kex(M+, B-) values and ionic Kex(X) values refer to a transfer between two liquid phases at equilibrium, the relation between ∆tG°(X, aq(or)→or(aq)) and Kex(X) is (24) where aq(or) and or(aq) denote the mutually saturated phases as before, the last term refers to the electrochemical properties of the system under question, F is Faraday’s constant and zx is the charge of the ion. It must be noted that often simplified notations ∆tG°(X, aq→or) and are used even for systems with mutually saturated phases, the saturation being tacitly understood but ignored.

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Equation (24) reads at 25 °C (∆G in kJ/mol and ∆ϕ in volts) as: (25) It can be readily shown that for extraction systems in equilibrium applies [29]: (26a)

or at 25 °C and with ∆ϕ in volts: (26b)

Thus, the magnitude of in a given extraction system can be simply calculated if we know the individual extraction constant of any one ion and its equilibrium distribution between the two phases; see Fig. 2 as an example (note that only one value of is established in the system, irrespective of number of ions involved).

IV. CHARACTERISTIC EXAMPLES OF EQUILIBRIA IN EXTRACTION OF ELECTROLYTES The systems of ion pair extraction with full or partial dissociation in the organic phase are characterized by special types of equilibria that usually cannot be described by the classical log-log analysis. Some important features of the extraction mechanisms involved and some techniques used are described in the following sections.

A. Nonstoichiometric Decrease of the Distribution Ratio The nonstoichiometric decrease of the distribution ratio was already described by Diamond [15] and studied with reagents of the dipicrylaminate type (DPA) in Refs. 30, 31. The salts M+DPA- were extracted into a polar solvent (nitrobenzene) from the aqueous medium of some electrolyte of M+, e.g., M+OH-. In this case, the OH- anion does not extract, being many orders of magnitude less hydrophobic than DPA- and can be omitted from the balance. If concentrations are substituted for activities,

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permissible for low ionic strengths of both organic and aqueous phases [30, 31], and for the condition [M+]or=[DPA-]or the distribution ratio of the anion is

(27)

or for the case of total dissociation: (28) Hence, at constant concentration of M+ in the aqueous phase and full dissociation in the organic phase, log DDPAdecreases as a straight line with a slope of -½ when plotted against the equilibrium aqueous concentration of the DPA- anion. There is no other reason for such a decrease than the mechanism itself. If there is an association in the organic phase with formation of (MDPA)or, the association constant may be evaluated from the above plots [30, 31]. This decrease has its application for the practical control of losses of the extractant into the aqueous phase. For example, when cobalt bis(dicarbollide) in its H form is used for extraction from the aqueous acid, the relative losses of the extractant will increase with the overall concentration of the extractant in the system. An important example of this effect for cation extraction, having a large impact, was found recently by Moyer et al. [32]. In studying the stripping of cesium from a calixarene crown extractant they found that the distribution ratio of cesium decreases strongly with the total concentration in the system. This may impair the stripping of cesium from the organic phase in a technological process proposed by the authors. The effect was, for example, observed for the system with aqucous 5×10-4 MHNO3 and 0.01 M calixarene in the organic solvent [32]. The distribution ratios DCs for 10-7 M Cs in the system largely exceeded unity, but DCs dropped strongly with increasing Cs concentrations and reached 0.1 already at a cesium concentration of about 10-5 M. Thus, even if the concentration of Cs did not attain by far the values of any other component, its distribution ratio dropped, hence “nonstoichiometric decrease of the distribution ratio” as given in the title of the section. If a constant Kex(ML+, B-) is defined, where L is the calixcrown ligand, it was shown that for the case of full dissociation in the organic phase [32]: (29) An important facet of the given mechanisms with dissociation in the organic phase is that they may function even in media with low dielectric constants. The extent

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of dissociation is of course given by the dissociation constant and by the concentration of the salt. For the system just described [32], the value when half of the cesium dissociated was reached when its total concentration in the organic phase was 10-6 M, if the dissociation constant has this order of magnitude (10-6) in a medium of ε=3 to 5. Hence, contrary to expectation, the dissociation may be important in low polarity solvents if the concentration of the salt is sufficiently low.

B.

Nonstoichiometric Increase of the Distribution Ratio

The nonstoichiometric increase of the distribution ratio is a contrary case to the above one, described for the extraction of picric acid and alkali metal picrates, M+Pic- [5]. Experimentally, it was found that microamounts of radioactive cesium passed into the organic phase from, e.g., sodium picrate solution much more readily than from a solution of cesium picrate. For example, the DCs of microamounts of cesium from 0.02 M Na+Pic- into nitrobenzene was 3.5, whereas the value for the distribution of cesium picrate is characterized by DCs=0.11 only [5]. The distribution in the presence of two cations, even if one is in microamounts, is given by two extraction constants Kex(M+, X-) and Kex(N+, X-). The distribution of the macrocomponent, let us say N+, X-, is given solely by its own concentration: Since it can be deduced [5] that (30) The value of coefficient M+ [5].

under such conditions was termed the “limiting distribution ” since Eq. (30) applies only for microconcentrations of

C. Determination of Selectivity Selectivity of the water–organic solvent systems is an important quantity both form theoretical point of view (concerning the physicochemical mechanism of solvation of ions) and for practical purposes (the Cs+/H+ selectivity determines in practical applications how strongly Cs+ will be extracted from acidic media). The selectivity relative to hydrogen ions in systems with full dissociation in both phases is given by the constant Kexch(Mz+/zH+). There exists an elegant radiochemical method for how to determine the selectivity in the alkali metal cations row relative to Cs+ [9]. The standard procedure for obtaining the data consisted in using 0.005 M Cs+ cobalt bis(dicarbollide)

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dissolved in the respective solvent and shaking the organic phase till equilibrium with aqueous solution of the pertinent nitrate, sulfate, and chloride of the cation under study was reached. The aqueous phase was spiked with 137Cs and its distribution was measured. From the obtained DCs values, the respective constants were evaluated according to Eq. (31) below and the data were plotted against the square root of the aqueous ionic strength, Iaq. The reported values were those extrapolated to zero Iaq and the means from the data for the three inorganic anions used; however the data were nearly identical for all three anions. If the volume ratio of the phases is vaq/vor=1, then and from material balances and A relation for Kexch(Cs+/M+) is thus easily derived for the case that B- is quantitatively extracted into organic phase [9]: (31) In a similar way, e.g. the extraction exchange constants of trivalent rare earth cations Kexch(Ln3+/3H+) may be determined [33], upon measurement of the equilibrium concentrations of lanthanides and extraction of the lanthanide cation by H+B- in a polar solvent from an aqueous mineral acid. Using the material balance of hydrogen ions in both phases, a relation was derived for this case [33]: (32)

D.

Maxima in the Curves of DM vs. Concentration of Ligand (Dissociated Electrolytes)

The maxima in the curves of DM plotted against the concentration of the ligand are a rather typical case for extraction with all kinds of neutral ligands. Still, their existence was not quantitatively explained until the first publication emerging from NRI in Ref. 34. Under the simplifying assumption that the cobalt bis(dicarbollide) (or another hydrophobic) anion is fully extracted, the equation accounting for the maxima on the curves for the distribution of microamounts of divalent cations could be derived analytically by Kyrs [34]. The resulting equation for with cL and cB being the total concentrations of the ligand L and the hydrophobic anion B- in the system, referred to one of the phases is [35]: (33)

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where (34) (35) (36) (37) This treatment forms a fundamental block in computer (minimization) determination of the respective constants mainly in the systems cation-crown ethercobalt bis(dicarbollide), as done by Vanura [36] in his numerous studies. For this purpose, the LETAGROP program [37] was largely improved for the present task and transformed into Windows and Delphi platforms, called LTGW [36]. The minimization is performed by seeking the minimum for the function [36]. When analyzing the equilibrium data for the extraction of Eu3+ in the presence of polyethylene glycol 400 (PEG), a reaction additional to those defined in Section III had to be introduced [38]: The process in which the PEG molecule deprotonizes upon complexation, or (38) This reaction, however, must be considered as tentative since the deprotonized PEG complex was not independently identified. Mathematical analysis of the dependence of DM on cL yielded a condition for the position of maxima both for the extraction of bivalent and trivalent cations. The relations derived by Vanura [34, 38] compared well with experimentally obtained positions. The calculated constants for the stability constants of several simple crown ethers with alkali metal and alkaline earth cations, determined in papers of Vanura and Makrlik, are reported in Chapter 5, Section IV.A.

E. Algorithm for Full Calculation of Equilibria in the Systems More complicated cases than given in the paragraph above are not analytically solvable and iteration procedures must be used. A rather general iteration procedure was proposed by Rais [39], resulting from a demand to express the distribution ratios in the cases when several nuclides at macroconcentrations are present in a complex fission product mixture. The procedure, called EXTRIT (extractive iteration), has the advantage that it is written in a modular form and new ions, differing volumes of aqueous and organic phases, higher complexes, complexing in the aqueous phase, etc., can be easily added. In the algorithm, noncomplete

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distribution of the hydrophobic anion to the organic phase is accounted for and the only assumption used is that the extraction of the mineral anion is much lower, in fact negligible, compared to extraction of the hydrophobic anion. For the sake of simplicity, the equilibrium concentrations in the aqueous phase are denoted by small letters and those in the organic phase by capital letters, e.g., etc. The respective constants are written also in a simplified form, e.g., K ex(H +, B - )=[H + ] or[B -] or/([H +] aq [B -] aq ) is written as K H= HB(hb)-1, as Ks=MB2(mb2)-1. The coordinating neutral ligand is called P (to avoid confusion due to the similarity of the number 1 and the letter l in shorthand visual basic script if the letter L were used). The constant of the type are in simplified notation given as Ls=SPB2(sPb2)-1. In this notation, the main balance given by electroneutrality condition for vaq/vor=1 is: B=H+HP+C+CP+2S+2SP+…(39) and starting equation (40) can be rewritten with terms expressed from the respective extraction constants as (40) On multiplication of the above equation by B, extraction of the square root and division by b, we obtain the main iterative equation (I) for DB of the form:

(41)

The mass balance of added synergist P can be written as the second main iteration equation (II): (42) where cp is total (initial) concentration of P in the system referred to one of the phases and δP is the distribution constant of the ligand P. Transformation relations, which may be easily derived, are used: (43)

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and b=cB(1+DB)-1 B=cB.DB(1+DB)-1

(44)

where cX is the total (initial) concentration of X referred to one of the phases. The course of the double iteration is as follows. At first, some arbitrary value of P and DB are chosen. A new value of P is iterated by Eq. II, the resulting value P is placed in Eq. I and a new value of DB is iterated by Eq. I. This procedure of double iteration is repeated until the required precision is obtained. The iteration in all tested instances converged relatively quickly and is thus suitable for solving cases of one hydrophobic anion and a multitude of cations in various forms and concentrations. The program written in Excel Visual Basic for the variant with only H+, Sr2+ cations, and ␸=vaq/vor=1 is as follows: DefDbl A-Z, Sub sestrontium(), Do While Cells(11+R, 1).Value>0, KH=Cells(2, 1), LH=Cells(2, 2), KS=Cells(2, 3), LS=Cells(2, 4), delta=Cells(2, 5), cH=Cells(4, 1), cB=Cells(4, 2), cP=Cells(11+R, 1), Dbdef=Cells(2, 6), Pdef=Cells(2, 7), precisionP=Cells(2, 8), precisionD=Cells (2, 9), LogKiB=Cells(4, 5), Db=Dbdef, P=Pdef, cSr=Cells(4, 4), If cSr=0 Then cSr=0.0000001, I=0, J=0, If cP=0 Then GoTo AgainDb, AgainP:, denumerator=1+1/delta+LH*cH/(Db+KH +LH*P)+LS*cSr/((Db)^2+KS+LS*P), Pnew=cP/denumerator, Pref= Pnew, differenceP=Abs (Log(Pref)/Log(10)-Log(P)/Log(10)), P=Pref, I=I+1, If differenceP>precisionP Then GoTo AgainP:, AgainDb:, Baq=cB/(1+Db), Borg=cB*Db/(1+Db), h=Db*cH/(Db+KH+LH*P), s=(Db)^2*cSr/(Db+KS+LS*P), TermH=KH*h/Baq, TermSr=2*KS*s/Borg, TermHL=LH*h*P/Baq, TermSrL=2*LS*s*P/Borg, Dnew=Sqr(TermH +TermSr+TermHL+TermSrL), Dref=Dnew, differenceD=Abs(Log(Dref)/Log(10)-Log(Db)/Log(10)), Db=Dref, J=J+1, If differenceD>precisionD Then GoTo AgainP:, Ds=(KS+LS*P)/Db^2, LogDs=Log(Ds)/Log(10), Cells(11+R, 3)=LogDs, deltaf=59.17*(Log(Db)/ Log(10)-LogKiB), Cells(11+R, 4)=deltaf, Cells(4, 6)=I, Cells(4, 7)=J, logDb=Log(Db)/Log(10), Cells(11+R, 5)=logDb, logP=Log(P)/Log(10), R=R+1, Loop, End Sub Numerical values of the constants KH, LH, Ks (Sr2+), Ls, and ␦P (=delta in the program), and the initial concentrations cH, cB, cp, and cS are placed in the indicated cells into an Excel sheet with a button for starting the macro to which the above macro is assigned (Fig. 1). The calculation fills the data into the columns below the row in which the headings cp, log cp, log Dsr, etc, appear. The value of distribution potential is calculated according to Eq. (25), appearing in the fourth column in Fig. 1. The main characteristics of the systems with maxima are apparent from Fig. 2. As described above, only the value DSr=([Sr2+]or+[SrL2+]or)/[Sr2+]aq) passes through a maximum, whereas all Didion show a monotonic change. Thus, the maximum in the DSr curve is not reflected in the variation of the distribution potential. Some of the curves for different stabilities of M2+ with P are given in Fig. 3. It is seen that a

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Figure 1 Layout of the Excel sheet for use with program EXTRIT. The program after filling the constants, concentrations, assumed DB and P values (terms with “def”) and demanded precisions of calculation (terms with “prec.”) starts by the button “calculate.” Besides the results, also the numbers of two iterations are registered (No.it.). These upon terminating the calculation show the results for the last entry of cp.

Figure 2 Calculated distribution of hydrophobic anion B-, bivalent cation Sr2+ (as overall DSr and distribution ratio of individual particle Sr2+), and distribution potential of the system upon extraction from 1 M mineral acid with 0.06 M H+B- in a polar organic solvent and variable concentration of ligand L forming HL+ and SrL2+ particles in the organic phase. The following constants were used: Kex(H+, B-)=10, Kex(Sr2+, 2B-)=5000, Kex(HL+, B-)=1× 107, Kex(Sr2+, 2B-)=1×1012, δL=0.001, log Kex(B-)=8.8, cH=1.06 M, cB=0.06 M, cSr= 0.01 M. Fully dissociated case, calculated by EXTRIT.

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Figure 3 Shapes of maxima for M2+ cations of different extractabilities. The constants employed are shown in the figure. Fully dissociated case, calculated by EXTRIT.

maximum appears only for sufficiently high extraction constants of metal with ligand. For sufficiently high extraction constant LS, the system “mimics” (at the lhs of the two uppermost curves) the classical behavior, namely a plot of log Dsr vs. log cP is a straight line with a slope of 1 as expected for the formation of a 1:1 complex. Notice also that the curves on the right hand side beyond the maximum have the shape of a titration curve, displaying an inflexion point where cP=cB. Other variables tested for convergence were: (1) any number of M+, M2+ and 3+ M ions at macroconcentrations [add respective terms to Eqs. (40) and (41)], (2) vaq/ vor⫽1 [terms with vaq/vor appear in Eqs. (41) and (42)], and (3) formation of aqueous complexes of M with P [respective terms added to the balance of P, changed Eq. (43)].

F. Nonpolar Solvents and Extractions with Mineral Acid Anions For extractions into nonpolar solvents, only the constants with full association in the organic phase do apply. At first sight, and for a long time taken for granted also at the NRI laboratory, the systems with full association are simple and maxima in

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the curves ought not to be observed. This, however, is not the case, and it is shown that the existence of maxima is a natural consequence of the mechanism involved in systems where the electrolyte is dissociated at least in the aqueous phase, as also for systems with hydrophilic mineral acid anions. At first, consider the simple case of the determination of selectivity, analogous to the case with full dissociation in the organic phase. In analogy with extraction exchange constant for fully dissociated electrolytes Kexch(Cs+/M+), an extraction exchange constant with full association in the organic phase Kexch(CsB/M+) is defined as (45) It is trivial to show that same relation as Eq. (31) above is valid [40]. The influence of changing the ratio of mineral acid and hydrophobic anions on the extraction was first investigated by Tachimori et al. [41]. Their relationships [41] pertain to the case of full association in the organic phase and are reproduced in a slightly modified form here. For the often encountered case of the extraction of a trivalent cation M3+ in the presence of a neutral ligand L, forming respectively particles (M+, nL, 3B-) and (M3+, nL, 3X-) in the organic phase—where X- and Bare mineral acid and hydrophobic anions—a composite extraction exchange constant of ion pairs with X and B, Kex(M, nL, iB, (3-i)X) corresponding to the equilibrium: (46) applies, where Bor denotes all forms of B in the organic phase and as a rule is represented by cB since the strongly hydrophobic anion B fully extracts into the organic phase in the presence of L. Under the condition where Kex (M, nL, iB, (3i)X), Lor, and are all constant, the equation for DM becomes log DM=ilog [B]or+const

(47)

If plotting the dependence (47) for various series with constant concentrations of X in the series, we can determine for each instance the composition of the organic phase complex, namely the coefficient i. The gradual change of nitrate for the chloro-protected bis(dicarbollide) anion in the organic phase is described in more detail in Chapter 5, Section IV.C. Noninteger values of i found for different aqueous concentrations of nitric acid apparently show a statistical mean of different compositions of the ion pairs formed. Finally, a general relation for the distribution of microamounts of an M3+ cation either in the presence of a hydrophobic anion or a mineral acid anion can be written

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[42]. For the extraction from acid media, there are two extraction constants with bonding Kex(HL, X) and Kex(ML3, 3X), assuming that the hydrogen ion and trivalent cation form 1:1 and 1:3 complexes with L, respectively. These are: Kex(HL, X)=[HLX]or ([H+]aq [L]aq [X-]aq)-1

(48)

and (49) From the last relation, the distribution of microamounts of M is but the distribution of L must be calculated. The latter is not influenced by microamounts of M3+. Taking into account the electroneutrality of the aqueous phase [H+]aq=[X-]aq, and the mass balance relations cL=[HLX]or+[L]or+[L]aq and CH=[HLX]or+[H+]aq and distribution constant of the ligand L, ␦L, we arrive at the cubic relation:

(50)

from which [H+]aq, [L]aq, and DM can be calculated. Thus, the maxima in the curves of DM vs. cL do occur generally, both for hydrophobic or simple mineral acid anions and for the cases of nonpolar solvents as well as for polar ones. The ubiquitous occurrence of maxima observed in extraction of M3+ ions with organophosphorus reagents and malonamides can be explained by Eqs. (49) and (50). The character of the phenomenon is apparent from Fig. 4. The experimental results were obtained with one particular malonamide and extractions of Eu3+ from HCl, HNO3, and HClO 4 media, with B=H+[(1,2-C2B9H11)2-3-Co]-, H+1-, into isopropylbenzene. The experimental curves were compared with those calculated by Eqs. (49) and (50), and the positions of the maxima could be relatively well correlated (with a less precise result for the H+ 1- acid, for which also a larger spread of experimental results was observed). The results are further discussed in Section V.C.

G. Activity Coefficients The extractions are generally performed at concentrations not permitting to neglect the changes of activity coefficients in both the aqueous and organic phases. This subject was tackled by different authors in various forms. The main obstacle is the general uncertainty of how to express the individual ionic activity coefficients especially in the organic phase.

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Figure 4 Extraction of Eu3+ from media of mineral acids and H+ [(1,2-C2B9 H11)2-3-Co]-, H+1, by N,N-dimethyl-N’, N’-dibutyl(dodecyloxyethyl) malonamide, DMDBDDEMA, into isopropylbenzene from Ref. 42. Ligand concentration and acid type are shown in the figure. Theoretical curves: dashed lines were calculated using δL=200, and Kex(HL, X), Kex(ML3, 3X) equal to 1×100, 8×104 (HCl); 3×100, 1.4×106 (HNO3); 2×101, 1.6×1010 (HClO4); 1×106; 1×1023 (H+1-). The logs of Kex(ML33+, 3X-) vs. logs Kex(H+, X-) give straight lines with a slope of 2.99 as expected.

The classical approach relies on the use of the extended Debye-Hückel equation [43]. Thus, the ion size parameter a in this equation was taken to be same in water and the organic solvent (1,2-dichloroethane) since “very hydrophilic ions such as Na+ might exist as hydrated ions in water-saturated organic solvent and the influence on a of hydrophobic ions might not be significant” [43]. Kielland’s values of a [44] were used for the simple ions like Na+ and picrate-, whereas for large ions Bu4N+ and BPh4- the a values were assumed to be identical to the crystallographic ion diameters (0.82 and 0.84 nm, respectively) [43]. A sophisticated program for the modeling of the extraction of ion pairs was written and several times improved at the Oak Ridge National Laboratory. In its present form, the program is denoted as SXLSQI [28]. An apparent advantage of the program is its flexibility, it permits the user to test and during the calculation to modify the underlying principles used in the calculation. These are as follows [28]: (1) use of the extended Debye-Hückel equation for low concentrations of salts, (2) use of Pitzer’s treatment of aqueous activity coefficients for higher concentrations of different ions present [45], and (3) use of Hildebrand’s treatment of the activity coefficients in the organic phase, based on the regular solution theory [46]. The published verifications of the model, in spite of its apparent complexity,

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show only the simplest examples of extraction equilibria, such as distribution of one electrolyte between two phases as a dependence on its concentration [28]. Thus, it is difficult to asses the limits of validity of the model. From another paper of the authors from ORNL [47], the impression can be obtained that the maxima in the curves log DM vs. log cL could be explained and treated by the model. On an examination of the proposed fit, it is apparent that the experimental curve with a maximum (e.g., in a concrete example of extraction of cesium by calixarene from nitrate media of various concentrations of NO3–) was explained by the fitting of the experimental curve with chosen (aqueous) activity coefficients on the right hand branch of the curve where DCs decreased [47]. However, for different neutral ligands the maximum occurs at different aqueous acidities, whereas the change of activity coefficients would imply always the same position of the maximum. For the model presented in this review, the maximum is intrinsic to it. Thus, the treatment presented in [47], can be criticized on account of both the used extraction model, and the activity coefficients used at high concentrations of In the cases of distribution of some fully dissociated electrolytes between water and nitrobenzene (Me4N+Pic-, Et4N+Pic-, Ph4As+Cl-), the distribution ratio of the salt was constant over the entire examined range, approximately up to 8×10-2 M initial concentration of the salt [19]. Similarly, the distribution potential of electrolytes between water and nitrobenzene and between water and 1,2dichloroethane [48] was constant in all instances up to nearly 0.1 M concentrations. These observations suggest that upon the distribution in simple systems, the effect of changing of activity coefficients in both phases cancels out or is negligible. A simplified method of treatment of activity coefficients in the systems, used frequently at the NRI laboratory and partly also by Vanura [36], consists in the application of “concentration constants,” CC, in which concentrations are used instead of activities. These, as found in several studies [9, 40], often obey the rule: log CC=log CC0+b(Iaq)0.5

(51)

where CC0 is the value of at zero ionic strength of the aqueous phase Iaq. If the concentration of electrolyte in the organic phases is sufficiently low and extrapolation to Iaq→0 is done, the extrapolated value CC0 may be identified with the standard thermodynamical constant. As shown in the Section IV.C of Chapter 5, in certain cases of extraction into nonpolar solvents, the effect of the change of the activity coefficient of the aqueous HNO3 up to high concentrations of it (ca. 8 M) is either small or some compensation occurs, so that concentrations instead of activities can be used. This finding was based on the fact that the compositions of the ion pair in the organic phase determined from Eq. (47) varied linearly with the concentration but not the H+ activity of the nitric acid in the aqueous phase.

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Concluding Remarks on the Mechanism of Extraction of Electrolytes

The peculiar nature of the mechanism of the extraction of electrolytes does not generally conform to the classical log-log analysis applicable to other areas of extraction chemistry. Some intriguing examples of the underlying phenomena were given in this chapter. From them, the existence of maxima of DM, which are often encountered in the extraction science, seems to be the most important. The existence of maxima may be explained also on the basis of competitive reactions in the systems. For the maximum of DM in the dependence on the neutral ligand content cL in the system, at a constant excess concentration of mineral acid in the aqueous phase, the following reactions are to be envisaged. On the ascending part of the curve, the metal cation extraction increases due to the increase of cL. Beyond the maximum, DM decreases because of competition of (MLn+)or for the increasingly formed (HL+) particle. Analogously, in the system with a maximum in the curve of D M vs. aqueous acidity c H at constant concentration of ligand L, two reactions can also be conceived. In the ascending part of curve, the protonation of L applies, forming a HL+ particle, which brings an anion X- into the organic phase according to the electro-neutrality condition. Since the neutral ligand itself in the L form cannot extract Mn+, this process increases the concentration of the extracting species in the system. After reaching the maximum, on the other hand, excess HL+ would cause a decrease of the extraction of Mn+ by competition. However, the above reasoning are not sufficient for explaining the maxima, since the effects would classically lead again into monotonous curves in which two effects simply add. Looking at Fig. 3, the reasoning of this kind may even not be proper (maximum appears only for certain numerical values of constants). The proper ground for the existence of maxima is in the ionic character of the extracted compounds. This is so beyond doubt for fully dissociated electrolytes in the organic phase. However, even if the electrolyte is associated in the organic phase, the ionic particles do exist in the aqueous phase. The systems with dissociation in the aqueous or in both aqueous and organic phases may be compared with solubility relations of salts, which contain also always terms not amenable to log-log analysis. This property is for a pure inorganic salt already contained in the solubility product and in the extractions, either in or The square terms do not exist in the systems conforming to log-log analysis. Attempts to explain the descending part of the curves beyond the maximum by the effect of activity coefficients in the aqueous phase seem to be of less value. For example, we recently studied the extraction of radioactive 137Cs with several crown ethers and calyx-crown compounds. The maxima for extractions from HNO3 (DCs vs. cH at constant cL) were observed in the region of high concentrations of the acid, above 1 M. However, when extracted from HClO4,

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the maxima appeared at acid concentrations around 0.01 M, thus in a very different range of concentrations. The extraction with CO4– anion is higher than with NO3– from reasons discussed in Section V.C.

V.

GIBBS ENERGIES OF TRANSFER AND SOME SEMIEMPIRICAL MODELS

The ionic values of ∆tG° cannot be determined experimentally, but, based on a reasonable extrathermodynamic assumption, they can be evaluated, especially if an independent indirect measure of their validity can be found. During the years large number of Gibbs energy of transfer data for various ions between two solvents accumulated. The reader is directed to a comprehensive review of Kalidas, Hefter, and Marcus for the reassessment of experimental techniques used for the task as well as for the various kinds of extrathermodynamic assumption [49]. The extensively used extrathermodynamic assumptions may suggest that some “correction” to classical thermodynamics, that not only does not define ionic values, but also prohibits dealing with them (unless they appear in sums or differences with balanced charges), is merely introduced. The scientist faces a dilemma: either forget about using single ion thermodynamics, which, in view of all the gained knowledge about ion-solvent interactions does not make sense, or to employ a “new thermodynamics” in which the ionic values have their own meaning. The subject lies far beyond the scope of this review, but the second possibility is considered here as the more useful one [50], and the use of individual ionic thermodynamic values is self-consistent and unambiguous in it. The used experimental techniques leading to the determination of ∆tG° may be divided generally into methods aiming to determine the values involving two pure solvents, ∆tG°(X, aq→or), and those using experimental arrangements in which the two solvents are in contact and equilibrium and ∆tG°(X, aq(or)→or(aq)) values are determined. Since in this review we are primarily interested in the extraction methods, i.e., transfers between mutually saturated solvents, mainly the latter data will be treated here. Further, because of the intensive research with nitrobenzene as a solvent both in extraction as well as in electrochemical studies, the main solvent of interest is nitrobenzene. The choice of this solvent is based not only on the great number of data collected for it, but because the data may be used also for discussing transfers into other even less polar or nonpolar solvents. The data for nitrobenzene as a solvent were gathered by two independent and largely differing techniques, i.e., by extraction methods and by electrochemical methods, mainly cyclic voltammetry at ITIES. If both techniques lead to the same result, the data may be considered as reliable. Selectivity coefficients determined with ion selective membranes serve for further establishment of the consistency of the data.

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The extraction methods of the determination of ∆tG°(X, aq(or)→or(aq)) have already been discussed above. They rely on the determination of the proper extraction constants and the division into ionic contributions that was made almost exclusively with the use of the TATB assumption. Electrochemical methods deserve a short introduction as given below.

A.

Electrochemical Methods of Determination of ∆tG°(X, aq(or)→or(aq))

With the development of the studies at ITIES (interface of two immiscible electrolyte solutions), a number of the ionic ∆tG°(X, aq(or)→or(aq)) values was determined by electrochemical methods. In the past, several studies were performed in which the values were determined of a static electrochemical cell, with beforehand mutually saturated aqueous and organic phases and an appropriate salt bridge between the compositionally different phases [48]. In more modern studies an imposed outer potential is usually applied to the cell and the half-wave potential of an ion X of cyclic voltammograms or polarograms is measured. Reversibility of the wave is a prerequisite for obtaining the correct value of ∆tG°(X, aq(or)→or(aq)). Even if in recent studies employing ITIES the two phases are not as a rule preliminary equilibrated, the systems are considered to be in equilibrium and the measured values are ∆tG° (X, aq(or)→or(aq)) and not ∆tG° (X, aq→or). This is because in the very thin interfacial layer of ITIES the mutual saturation of the two solvents proceeds very quickly and completely, especially if the necessary check of reversibility is done [51]. The electrochemical cell usually consists from two liquid phases brought into contact, one organic and one aqueous with appropriate electrolytes in them (a “supporting electrolyte” in the organic phase) in order to ensure small resistivity of the cell. Two reference and two working electrodes are used, with Luggin’s capillaries for the reference electrodes in order to have their openings near the interface of the liquid phases. The organic reference electrode is made of an aqueous solution near the metal electrode and a salt bridge which usually also ensures the elimination of the potential drop in the reference electrode or permits its control. The experimental techniques advanced largely after Samec et al. [52] introduced the four-electrode potentiostat with IR drop compensation by means of a positive feedback loop and the method of choice is cyclic voltammetry. Nowadays, sophisticated techniques are used and for the purpose of this review are described briefly in notes to the Tables 1–3. The half wave potential determined by cyclic voltammetry relates to the standard distribution potential as follows [53]: (52)

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where Dor(X), Daq(X), ␥or(X), ␥aq, ␥aq(X) are the diffusion and activity coefficients of the ion X in the organic and aqueous phases. (Note: In electrochemical techniques D denotes a diffusion or mobility coefficient, not a distribution ratio as in extraction.) The factor A is connected to the actual geometry of the arrangement, e.g., A=[(4d/␲r)+1] for a microinterface located in an organic solvent-filled microhole of radius r and depth d [53], and the other symbols have their usual meaning. The diffusion coefficients are not always known and two methods are used for their estimation: (1) the Nernst-Einstein relation gives |zi| FDi=RT␭i, where ␭t is limiting ionic conductivity of an ion i, and (2) Walden’s rule states that ␩␣D␣=␩ßDß of two solvents ␣ and ß, where ␩ is the viscosity [53]. For two solvents of similar structure as are, e.g, nitrobenzene and o-nitrophenyl octyl ether, methods (1) and (2) usually lead to identical result, but exceptions were also noted [53]. In the presence of a complexing agent an effect, called “synergy” in extraction, occurs, i.e., a shift of termed “facilitated transfer” in electrochemistry. The stability of the complex in the organic phase can be calculated from the shift of Of course, the stability constants of the complex ought to be identical with that obtained from extraction experiments. The selectivity constants may be determined also from potentiometric measurements when an organic solution of a suitable electrolyte is used as an ion selective electrode, ISE. The measurements in this case provide upon extrapolation of the measured apparent selectivity coefficients to zero ionic strength a “theoretical selectivity coefficient” This quantity is independent of the ion-exchange site, its concentration in the membrane or of the activities of the ions in the test solution, but depends only on the properties of the membrane solvent used [54]. The is related to the extraction exchange constant Kexch(A/B)=Kex(A)/Kex(B), where Kex(A) and Kex(B) are individual extraction constants of the ions A and B, as reported in Refs. 54–56: (53) where DL is a mobility of free ligand in the ISE phase. The relation is often simplified as an approximation to [54, 56]: (54) having practical significance. In fact, for much more complicated conditions, i.e., with the ISE solvent p-nitrocumene embedded into a PVC membrane, Scholer [56] found a strikingly linear correlation of for nine alkali metal and tetraalkylammonium cations with the K ex(A)/K ex(B) measured in waternitrobenzene [19] with a correlation coefficient R2=0.9958. One reason for the apparent success of such a correlation is that PVC in the membrane behaves as an

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inert diluent, thus not influencing the selectivity of the system; see Ref. 57 for cation loading experiments and Ref. 58 for electrochemical measurements with 0nitrophenyl octyl ether+PVC membranes. Thus values may well serve as an auxiliary set of values for ⌬tG°(X, aq(or)→ or(aq)).

B.

Ionic ∆tG°(X, aq(or)→ or(aq)) Values for the Water-Nitrobenzene System

A large data set for the individual ∆tG°(X, aq(or)→or(aq)) is now available. The initial data on the transfer of 22 ions obtained by extraction [19] were thoroughly checked and the set was enlarged both by extraction and electrochemical methods. These data are collected in the well-maintained web database of Girault [20] providing values for seven polar solvents and for nitrobenzene and containing almost 100 entries (sometimes with multiple entries for one ion). Another comprehensive source of recommended values is that of Osakai [21, 22] supplying values for 37 different ions. The data for nitrobenzene as a solvent, due to Senkyr et al. [59, 60], were recalculated into the form of individual transfer values based on the TATB assumption by Makrlik [54, 61]. There are 41 entries for cations [54] and 46 entries for anions [61]. A further set for the purpose of comparison is that of Scholer for p-nitrocumene [56], containing 54 different cation entries. Several important sets of data had to be omitted here for the sake of brevity. These are, e.g., seven substituted ethylenediamine derivatives studied by the electrochemical method by Baruzzi and Wendt [62] or ten aromatic amine derivatives studied by the same method by Marecek and Samec [63]. Takeda et al. studied the transfers of amino acids in the system [64]. The width of contemporary determinations of ion transfers data may be illustrated by a recent publication [65], in which the transfers of alkali metal cations between water and nitrobenzene with monoaza-18-crown-6 modified fullerene were studied. Smaller sets of anionic data for various cobalt bis(dicarbollide) derivatives published in [66, 75] are also important. This review is limited to reporting the most important data for small inorganic ions, tetraalkylammonium cations, some complex ions, and the most important hydrophobic anions. When several data existed for one ion, the preferred values are printed in the tables in bold characters. The data are organized into the three following tables: Table 1 in which the data for small inorganic cations are collected, Table 2 for inorganic and dicarbollide anions, and Table 3 for tetraalkylammonium cations plus some cations which are important in extraction chemistry. The Gibbs energies of transfer for alkali metal cations were determined from selectivities of different extraction systems and extraction data are thus preferred. All electrochemical data for the alkali metal cations series are slightly less positive, but Osakai in his recommended values [21, 22] also preferred the extraction values. The opposite is true for H+, and because of the stability of cobalt bis(dicarbollide), we prefer the second entry for H+, which is based on the extraction data with this

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Table 1 Transfer Gibbs Energies of Simple Cations Between Mutually Saturated Phases of Water and Nitrobenzene (NB), ∆tG° (X, aq(NB)→NB(aq)), kJ/mol, 25°C

Data obtained from extraction experiments in which the selectivity of transfer of two ions was determined and the absolute value was obtained on the basis of the TATB assumption (the same extractability of Ph4As+ and BPh4– from water into nitrobenzene). b Recommended values from extraction and electrochemical experiments by Osakai [21, 22]. c Electrochemical database of Girault (containing also extraction results) [20], mainly with the TATB assumption, but see [20] for details; in the column “Electrochemical data” also other electrochemical data are given. d Data calculated by Makrlik [54] on the basis of experimental data of Sustacek and Senkyr [59]. Nitrobenzene contained 10-4 M solution of tetrakis(4-fluorophenyl)borate with a suitable cation and measurement was done with vigorous stirring of the phases. e From [36]. f From selectivity constants Kexch(Cs+/M+) determined by extraction, the reported values being mean values from several other publications [19]. g Two different values of Kexch(Cs+/H+) were determined; the value 33.7 was obtained with cobalt bis(dicarbollide) [9], which permitted better extrapolation to zero aqueous ionic strength and this value is preferred. h From [67], based on measurements on a liquid/liquid microinterface with no electrolyte in the aqueous phase. i Extraction results from [68]. J From cyclic voltammetry [69]. k From [70]. l From solubility measurements, the data referring to pure solvents are consequently put into brackets [71], m Cyclic voltammetry [72]. n AC cyclic voltammetry [73]. o Personal communication of Hundhammer and T.Zerihun to Wilke quoted in [67]. p Extraction data from [74]. qElectrochemical data [75].r From [76], the value for Cs+ was evaluated by the authors of the paper as the mean best value for this cation with a standard deviation of ±0.2 kJ/mol. s From [77]. t Extraction results [33]. u Cyclic voltammetry at the microinterface of a hole of 12 µm in a polymer foil [78]. v Extraction data from [79]. w Extraction data from [80] [i]. a

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Table 2 Transfer Gibbs Energies of Anions Between Mutually Saturated Phases of Water and Nitrobenzene (NB), ∆tG°(X, aq(NB)→NB(aq)), kJ/mol, 25°C*a

*Cobalt bis(dicarbollide) anions, numbered as in Ref. 10. a,b,c See Table 1. d Data calculated by Makrlik [54] on the basis of experimental data of Senkyr and Kouril [60]. Nitrobenzene contained 1–2×10-4 M crystal violet cation with the suitable anion and measurement was done with stirring of the phases. e Extraction data from [66]. f Cyclic voltammetry at three phase junction [81]. g Extraction study [19], all data also collected in [20] with exception of the value for I–3 the value in the present table being correct. h Flow-injection amperometry with CIO4– as an internal standard [82]. i From solubility measurements, the data refering to pure solvents are consequently put into brackets [71]. j Cyclic voltammetry [72]. k AC cyclic voltammetry [73]. l Square-wave voltammetry with internal standard CIO4– [83]. m Flow-injection amperometry with CIO4– as an internal standard [82]. n Cyclic voltammetry [72]. o from [82]. p From [72]. q From [82]. r From [72]. s From [72]. t From AC and DC cyclic voltammetry [83]. u From [73]. v Cyclic voltammetry in [84], w The anion of phosphomolybdic acid behaves in acidic media upon transfer as an relatively hydrophobic univalent anion PMo–, see [4] for details. The determined extraction constants of H+ PMo- for various acidities was extrapolated to zero aqueous ionic strength and the value in table was calculated using the individual extraction constant of hydrogen ion. Osakai in his papers gives several values for 3-to 6-charged heteropolyacid anions, see original papers [21, 22]. x From [84]. y Cyclic voltammetry at a microinterface of a hole of 12 µm in a polymer foil [78]. z Tentative value for OH- ion from [85], ref. 11 in the paper to unpublished results of the author. 2,6-DNPh- and 2,4-DNPh- denote 2,6dinitrophenolate and 2,4-dinitrophenolate, respectively, ␣-hexylate is 2,4-dinitro-N-picryl-1naphtylaminate, and the data for substituted cobalt bis(dicarbollide) anions are given in the last column of the table with abbreviations given in the respective tables found in Chapter 5 of this volume [10].

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anion. The y=∆tG°(X, aq(NB)→NB(aq)) values of simple inorganic M+, M2+, and M3+ cations from Table 1 can be correlated with the respective values from [94] by the equation: y=-6×10-20x6–2×10-15x5–3×10-11x4–1×10-7x3 -0.0002x2–0.2344x-29.349

(55)

with a correlation coefficient R2=0.9962 (35 ions from the Table 1, with data for Ag+ and UO22+ omitted since they are outliers to the given correlation). Alternatively, the data can be correlated as separate straight lines when plotted against for the series of M+, M2+, and M3- cations, the slope of these lines decreasing considerably in this series. The lower selectivities of extraction for M2+, and especially M3+ are apparently connected with the extensive hydrations of the cations in the organic phase; see data in Section V.C.5. The ⌬ tG°(X, aq (or)→or (aq) ) data for transfer of Cs + ion from water to nitrobenzene+CCl4 mixtures are important for the previously studied transfers into the mixtures of nitrobenzene with non-polar solvents. The data obtained by cyclic voltammetry are as follows (vol% of nitrobenzene in the mixture/∆tG° in kJ/mol): 100/13.2, 95/14.3, 90/15.3, 85/16.3, 80/16.8, 70/17.9, and 60/19.4 [95]. The potentiometric selectivity coefficients yield results for small hydrophilic anions different from extraction and other electrochemical data, but this is not important, since this set is only an auxiliary set. The agreement for extraction and electrochemical data of anions is acceptable with exception of Cl- ion. The gap of nearly 10 kJ/mol for Cl- anion cannot be reasonably explained. Both values are apparently reliable, but the extraction value is given more weight in view of the correlation given in Section V.C.5. The value for Br- was not checked by extraction, recommended value by Osakai is quoted. The behavior of was determined only at the NRI laboratory and was not confirmed by others. The tetraalkylammonium ions give practically same results in all columns with exception of the data for Et4N+. However, its value was twice determined by Osakai [21, 22, 87], hence his newer and by him recommended value [21, 22] is deemed to be more reliable. The data for complexes of alkali metal cations with two crown ethers, although not substantiated by other measurements, seem to be correct. The ions become substantially more hydrophobic if enveloped by an organic hydrophobic moiety.

C. Physical Chemistry of Transfer: Empirical Models Several theoretical approaches have been used in order to interpret the Gibbs energies of transfer between two solvents. While the reader may find the respective theories in the literature, e.g., Refs. 22, 27, 28, the aim of this review is different. We try to find some simple empirical relations and indices that can serve as a practical tool for predicting the properties of the systems. Normalized functions may be used for checking whether a particular data point does or does not conform

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Table 3 Ionic Values of Transfer Gibbs Energies of Alkylammonium and Some Ions Used in Extractions from Water to Nitrobenzene ∆tG°(X, aq(NB)→nitrobenzene(aq)) Between Mutually Saturated Phases, kJ/mol, 25 °C

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Symmetrical compounds noted by s after the compound number. a From [86], extrapolated Kexch(Cs+/M+) constants to zero ionic strength, 0.06 M H+ 1- in nitrobenzene. b From [19]. c Data calculated by Makrlik [54] on the basis of experimental data of Sustacek and Senkyr [59]. Nitrobenzene contained 10-4 M solution of tetrakis(4-fluorophenyl) borate with suitable cation and measurement was done at vigorous stirring of the phases. d From [87], by cyclic voltammetry. e Recommended values from extraction and electrochemical experiments by Osakai [21, 22]. f Recalculated from standard distribution potentials determined from AC polarography at water-NB interface from [88] as referred in [89]. g Database of Girault [20]. h From [84]. i From solubility measurements, the data refer to pure solvents and are consequently put into brackets [71]. From [72]. k From solubility data [90]. L Cyclic voltammetry at a microinterface of a hole of 12 µm in a polymer foil [78]. m From [91]. n From [92]. o Approximate value from aqueous medium 1 M HNO3, calculated by EXTRIT. The data were obtained for nitrobenzene+CCl4 mixture (60 vol.%:40%) and under justified assumption of same selectivity for this mixture and pure nitrobenzene are used here [93].

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to the trend displayed by the majority of other data. The section deals with the transfers of small hydrophilic ions, large hydrophobic ions, data for other polar solvents, the correlation of solubility and extraction, hydration of ions in the organic phase, role of dissolved solvent on the selectivity, and general affinity scale for organic solvents. 1.

of Inorganic Ions in the Water-Nitrobenzene System

The subject of ∆tG° values between pure solvents was recently discussed by Rais and Okada [96]. It was shown in the paper that the choice of the entered experimental data is important. If the data of Gritzner, based on the measurements of the potential of reduction of alkali metal amalgams and performed at one laboratory [26], were used, a relatively simple picture of solvation could be conceived. The values of ∆tG° in the chosen coordinates were well expressed by their linear dependences on the Gibbs energies of hydration of the respective ions. The enigmatic, but often noted property of such systems, i.e., that the behavior of the first complex between the cation and solvent molecule frequently mirrors that of the overall ∆tG°, could be tentatively explained by the formation of energetically quantized clusters of the ion with the first four solvent molecules [96]. An analogous behavior is displayed also for the mutually saturated solvents. The selectivities of the transfers of alkali metal cations from water obtained with 137Cs spiked solutions according to the method described in the Sections IV.C and IV.F. [40] were determined. These selectivities: Kexch(Cs+/M+) for polar solvents or Kexch(CsB/ M+) for essentially non-polar solvents, supplemented with older ones and expressed as relative values, are shown in Fig. 5 and Table 4, respectively, for seven new solvents or solvent mixtures. In Table 4, the data for transfers between pure solvents, recalculated from those by Gritzner [26] to the values with water as a reference solvent, are also included. The reasonableness of the both sets of values is corroborated by the fairly straight line correlations of DtG° or with The data for Na+, K+, + + + Rb , and Cs correlate as straight lines with y=[∆tG°(M )-∆tG°(Cs+)], R= correlation coefficient: dry nitromethane y=-0.229x-58.076, R2=0.996; o-nitrophenylether saturated with water y=-0.1098x-27.529, R2=0.9994; nitromethane saturated with water y=-0.1129x-27.997, R2=0.9972; nitrobenzene saturated with water y=-0.1648x-41.124, R2=0.999. The data of standard molar Gibbs energies of hydration are from Ref. 94. The hydration effects in the organic phase mostly concern the Li+ ion and its transfers are relatively increased into water saturated solvents compared to pure solvents, see the dependences for pure and water saturated nitromethane. As concerns the steepness and sign of the dependence, a general rule that the slope of the straight line correlations of with is primarily determined by the donor number, DN, of the solvent [96] is obeyed. This is exemplified by the

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Figure 5 Transfers of alkali metal cations relative to Cs+ for various water/mutually saturated organic phase extraction systems (except “dry NM”) at 25 °C (From Refs. 40 and 97; data for nitromethane are from Ref. 99.)

most negative slope at Fig. 5 displayed by nitromethane (lowest DN) and the most positive by tri-n-butylphosphate (highest DN). The points for four alkali metal cations give a straight line with R2>0.996 also for a mixture of 70% vol of NB+30% CCl4, thus justifying the choice of as a normalizing function. For PC, DOS and DOA, the data point for Rb+ lies outside the correlation, but the reasons for this particular deviation remain unclear. Consequently, the recommended values for transfers of small cations and anions from water to nitrobenzene from the Tables 1 and 2 are plotted against their respective as shown in Fig. 6. The values for simple inorganic cations and anions fall on a single straight line. The only exception is Li+, probably because of its excessive hydration. This finding says that the standard molar Gibbs energies of transfer of small cations and anions from water to nitrobenzene are same for a given value of irrespective of ionic charge. Thus, for the reverse transfer of both cations and anions

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Table 4 Ion Transfer Values of Alkali Metal Cations: ∆tG°(M+, aq→or) Between Pure Water and Pure Solventsa and ∆tG°(M+, aq(or)→or(aq), Csrel+) Between Mutually Saturated Solventsb, kJ/mol, 25°C

Abbreviation of the solvents: methanol MeOH, ethanol EtOH, 1-propanol PrOH, 1-butanol-BuOH, 1,2ethanediol EG, propylene carbonate PC, trimethylphosphate TMP, N-methylformamide MF, N,Ndimethylformamide DMF, N-methyl-2-pyrrolidinone NMP, hexamethylphosphoric triamide HMP, dimethyl sulfoxide DMSO, acetonitrile AN, propanenitrile PRN, benzonitrile BN, pyridine PY, pyrrole PL, N,N-dimethylthioformamide DMTF, N-methyl-2-thiopyrrolidinone NMTP, nitromethane NM, nitrobenzene NB, o-nitrophenylether NPOE, 1,2-dichloroethane 1,2DClE, Dioctylsebacate DOS, Dioctyladipate DOA, Tri-n-butylphoshate TBP, Toluene TO. a Recalculated from the tabulated values ∆tG° for acetonitrile→solvent transfer from [26]. b Results of new studies, ∆tG°(M+, Csrel+) [40, 97, 98]. See Sections IV.C and IV.F for experimental conditions for “ms” solvents. In some cases (1,2-dichloroethane), a lower concentration of a more hydrophobic cobalt bis(dicarbollide) derivative had to be used because of solubility problems [40, 98]. c p=pure solvents, ms=mutually saturated solvents, ass=associated in the solvent. d From [94]. e From [99].

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Figure 6 Correlation of cationic and anionic data for transfer from water to nitrobenzene. ∆tG°(X, aq(or)→or(aq)) values are the recommended ones in Tables 1 and 2 (X denotes the – value proposed by Osakai for transfer of Cl ). Standard molar Gibbs energies of hydration are from Ref. 94. Hydration numbers of ions in nitrobenzene are those from Table 5 and Refs. 22 and 111.

from nitrobenzene into water the hydration in the aqueous is the main driving force. This reasoning is corroborated by the independence on charge of the hydration numbers in nitrobenzene. A self-consistent picture emerges from Fig. 6: (1) individual ∆tG°(X, aq(or)→or(aq)) values are to be considered as real and appropriate into its numbers, and (2) the values and the method of division of total ionic values [94] are validated. Although in the present paper the data for small inorganic anions and all available solvents were not analyzed, the situation should be similar to that of the cations as concerns the normalizing function. This is based on the finding that the gaseous cluster Gibbs energies of formation for the n=1 to 6 clusters for alkali metal cations and halide anions yield straight lines when plotted against the pertinent hydration energies [100]. The total hydration energetic is expressed already in the simplest 1:1 ion: molecule clusters and all subsequent ones as well, and provides a plausible explanation of the applicability of the chosen normalizing function. The extraction technique used for the determination did not distinguish between dissociation and association in the organic phase. In low polarity DOS, DOA, and toluene the predominance of ion pairs is proved by conductivity measurements. In these cases, the selectivities given in Table 4 pertain to the exchange of the aqueous ion with the organic associate. However, the similarity of the absolute values and

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shapes of the curves in Fig. 5 indicate that ion pairing in the organic phase should not be of importance as far as the selectivity of the system is concerned. Very high selectivities, which were found for toluene as the solvent, appeared also in our previous studies with mixtures of nitrobenzene with some nonpolar diluent. The data for a mixture of 30% of nitrobenzene with 70% of CCl4 are given in Fig. 5 and Table 4. Another conspicuous example of a dramatic increase of selectivity of extraction of strontium is provided by the data plotted in Fig. 7 [101]. The same effect occurred for extraction of Cs+ in the presence of Rb+ and the selectivity apparently increases in all the row of alkali metal cations. A method for how to attain an increased separation factor of the Cs+/Rb+ pair in extraction with cobalt bis(dicarbollide)s was described in Ref. 102. Whereas for the system water/ nitrobenzene the separation factor DCs/DRb is 5, for a mixture of 30 vol% of nitrobenzene+ 70% of benzene DCs/DRb was 16. A similar effect occurred when carbon tetrachloride replaced benzene [102]. The effect of the addition of a nonpolar solvent can be explained as follows. The basicities of nonpolar solvents such as toluene or benzene are not known (no DN data) but presumably are low. The high selectivity may thus be due to a decrease of the basicity of the solvent upon addition of the nonpolar solvents. On the other hand, electrostatic interactions connected with ion-dipole terms decrease with decreasing ε, and this may also be a leading factor. The data plotted in Figs. 5 and 7 express the upper limit of selectivity attainable in the practical systems, since for still lower ε, the solvation of the hydrophobic anion will also drop, and the extractant will pass into the aqueous phase. This is actually the case according to our measurements.

Figure 7 Extraction of Sr2+ from 0.5 M HNO3 in the aqueous phase by 0.001 M bromoprotected cobalt bis(dicarbollide), H+[(l,2-C2B9H8Br3)2–3–Co]-, in the presence of PEG 400 for various compositions of the organic phase. Curve 1:10 vol% of nitrobenzene (NB)+ 90% benzene, 2:20% NB, 3:30% NB, 4:50% NB, and 5:100% NB.

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of Tetraalkylammonium Ions in the Water-Nitrobenzene System

A large amount of attention was paid to the transfer of voluminous organic ions and was treated by several empirical approaches besides other well-known methods [27]. For large nonhydrated ions of this type it may be reasonably supposed that ∆tG°= ∆tG°(X, aq(or)→or(aq)) and this shortened notation is used here. One method uses Uhlig’s formula for expressing the nonelectrostatic part of ∆tG°, ∆tG° (ne); see e.g, Osakai’s theory [21, 22]. In the expression, ri is the radius of the ion and σo,aq is the interfacial tension at water— organic phase interface. The ∆tG° values for transfer from water to nitrobenzene show a constant increment of -2.5 kJ/mol per methylene group [87]. The total ∆tG° is then given by the nonelectrostatic contribution, or, at least, the relative values in a series are independent of the charge of the ion or any electrostatic interaction. Thus, we can expect some simple function of the radius of an ion for the magnitudes of ∆tG°. The van der Waals radii (Me4N+ 280, Et4N+ 337, Pr4N+ 379, Bu4N+ 413, Ph4As+ 425 pm [49], Ph4B- 421 pm [22]) are most often used for the voluminous ions. The difference of ∆tG° for the pair Bu4N+/Ph4As+ is contrary to expectation if the van der Waals radii are used. Before assigning a special property to this pair, we should check whether some other scale of radii might be more appropriate. One such set, closer also to other proposals of crystallographic radii of the ions under question, is that of Krumgalz [103] (216, 267, 335, 385, and 428 pm, respectively, for the cations in the previous paragraph), the difference of ∆tG° for the Bu4N+/Ph4As+ pair falling in line with the tetraalkylammonium cations. A further set of ion radii can be independently calculated, based on the so called “covalent surface area” empirical method [50]. The surfaces of all atoms contained in the ion (using tabulated covalent radii) are summed and the effective radius rCSA is back recalculated. Although the method is based on employing additive contributions and avoiding atom overlaps that appear in the van der Waals radii, its agreement with Krumgalz’s radii is relatively good (200, 267, 321, 367, and 441 pm, respectively). Because of its simplicity, this method may be used for the correlation of the ∆tG° of tetraalkylammonium and related ions. However, the importance of using a particular set of radii or deciding whether a plot against r or r2 is to be used should not be overestimated. In fact, when one tries to express the ∆tG° data contained in the set [87] with the above choices, the best correlation coefficient of the straight line is obtained when ∆tG° values are plotted against the number of carbon atoms in the ion. A similar linear correlation for transfers of alkylammonium ions from water into NPOE solvent (embedded in a PVC membrane) on the number of carbon atoms was reported recently in Ref. 104. Because of noted constant increment per CH2 group, the values may be as well correlated with the partition constants ␦BS (aq→n-octanol). A relatively good similar correlation was obtained for cobalt bis(dicarbollide) anions in Ref. 105.

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For more complicated ions that are not the subject of this review, the correlation of their ∆tG° values based on topological indices, describing in more detail the size and also space orientations of the substituents, was used in Ref. 106. Although this method should be preferred for more complicated ions, the reported agreement for simple tetraalkylammonium ions from Ref. 87 was poorer than when a constant increment per alkyl group was used. 3. Other Polar Solvents In conclusion to the data on ion transfer for the water—nitrobenzene system it can be said that the two methods, extraction and electrochemical, although completely different in concept, furnish very near results, thus mutually corroborating one another. As concerns the transfer to other solvents, the data, not as extensive as for nitrobenzene, cannot be treated here in detail. It is useful to note that Wilke [53] correlated transfer data from water into o-nitrophenyl octyl ether with transfers from water into three other organic solvents: nitrobenzene, 1,2-dichlorethane and o-dichlorbenzene (Fig 8). The linearity for small and large ions is surprising,

Figure 8 Correlation of transfer data among o-nitrophenyloctylether and nitrobenzene, odichlorbenzene, and 1,2-dichloroethane solvents according to Ref. 53. (Reprinted from S. Wilke, T.Zerihun, J. Electroanal. Chem. 515:52, 2001, with permission from Elsevier.) The abbreviations are self-explanatory; DS denotes n-dodecyl sulphate anion. The correlations in [53] equals to ∆tG° by the authors are written in the upper left corner of the figure. here, in ␦J/mol units.

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and could not be explained by the Born equation or other electrostatic theories [53]. This correlation serves here for three purposes: (1) Instead of full tables, it shows the data for transfer to polar solvents other than nitrobenzene, (2) the linearity of the ∆tG° on for simple cations and anions is corroborated by the figure, and (3) a tentative scale of anion hydrophobicity based on the data for transfers in the water—nitrobenzene system can be constructed also for transfers into relatively nonpolar solvents (represented in Fig. 8 by o-dichlorobenzene with ε=10.36). 4. Extraction and Solubility An empirical rule, that salts of cesium with anions like dipicrylaminate or tetraphenylborate form insoluble precipitates in water are also extracted efficiently into nitro-benzene, served for finding new extraction systems for this cation [1, 2]. The analytical method for the determination of polyethylene glycols (PEGs) by their precipitation with tetraphenylborate in the presence of Ba2+ [107] suggested the synergetic effect displayed by PEGs for the extraction of alkaline earth cations [108]. In a reverse manner, the precipitation of protonized crown ethers and some phosphororganic reagents with phosphomolybdate or other hydrophobic anions were devised. The dependence of the logarithm of distribution ratio of potassium salts of several hydrophobic anions between water and nitrobenzene on the logarithm of their aqueous solubilities was reported by Iwachido [109]. For 16 anions a straight-line correlation with a slope of -1.11 resulted with R2=0.8713 which is significant, considering that a range of 5 orders of magnitude of the distribution ratios and solubilities was covered. Although the crystal energies of the precipitates formed by the hydrophobic anions are not known, the effect is tentatively ascribed to a loss of affinity toward water. The total standard molar Gibbs energies of water—nitrobenzene transfers of the salts under consideration are slightly to strongly negative (e.g., -24 kJ/mol for Cs+ DPA- compared with -1.1 for the water soluble Li+ DPA-). Hence, the low solubilities are explained by an assumed positive or nearly positive Gibbs hydration energy of the ion combination, thus not enabling its dissolution in water. 5. Hydration of Ionic Species in the Organic Phase From the outset of the studies, the number of coextracted water molecules with a particular ion in nitrobenzene was studied. For this purpose, usually a Karl Fisher titration of the water in the organic extracts is done. The water content of the organic phase, c(H2O)org, generally increases linearly with the concentration of the electrolyte according to the relation where is the water content of the solvent with no added electrolyte. If an

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electrolyte N+Y- can be found for which throughout its entire concentration range , then nhyd(N+)=nhyd(Y-)=0. For any other salt M+Y-, the hydration is due to M+ only and the ionic values can be determined. Full dissociation of N+Y- and M+Y- in the organic phase is assumed or incomplete dissociation must be corrected for. Hydration numbers, nhyd of several examined cations with use of the cobalt bis(dicarbollide) anion 1- were determined by the Karl Fisher method in [110]. These were based on the experimentally verified n hyd =0 for the cobalt bis(dicarbollide) anion 1- anion and are nhyd(Li+)=6.5, 6.0, 5.8±0.6; nhyd(H3O+)= 4.5; nhyd(Na+)=3.9, 3.8, 3.7±0.2; nhyd(K+)=1.5, 1.0, 1.4±0.1; nhyd(Rb+)=0.8, 0.7, 0.77±0.06; nhyd(Cs+)= 0.5, 0.4, 0.4±0.3; and nhyd(Et4N+)=0.0, 0.0. The first entries in this list are from Ref. 110, the second entries are the revised values of Osakai [111], and the third entries are the mean values from Ref. 110. All tetraalkylammonium cations and the studied organic anions are nonhydrated in the organic phase, whereas the following nhyd were reported [22, 111]: for Cl- 4.0, for Br- 2.1, for I- 0.9, for SCN- 1.1, and for NO3– 1.7. A trend of more positive ∆tG° values with increasing nhyd is observed for alkali metal cations, and most anions (except SCN- and I-). As an auxiliary graph, the dependence of ∆tG° on nH gave a smooth curve for nine univalent cations and anions and even for uni-, bi- and trivalent cations [80], showing the importance of hydration in the organic phase for the magnitude of ∆tG°. The hydration numbers of Ca2+ and Ba2+ in nitrobenzene are nhyd=14 and 11, respectively [22], the latter value being in agreement with a previously published one (11.5±1) [79]. For trivalent Ce3+, nhyd=16.2±2 was measured in [80]. For complexed alkali metal cations, as expected, the value of nH decreases. Thus, e.g., for complexes of M+ with dibenzo-18-crown-6, DB-18-C-6, the hydration numbers are shown in Table 5. These numbers show that the largest decrease of hydration between a bare and a complexed ion is for Li+, whereas the hydration numbers of Cs+ and Rb+ are low and nearly the same for uncomplexed and complexed ions. Furthermore, due to relative independence of nhyd on the length of PEG complexants, it can be inferred that the coordination sites of alkali metal cations are saturated to the same degree for all alkali metal cations by these compounds. For primary, secondary, and tertiary ammonium ions in nitrobenzene, the nhyd values are 1.64, 1.04, and 0.66, respectively, with the water molecules probably directly bound to the central nitrogen atom [114]. The effect of dissolved the water in nitrobenzene and some doubts concerning the physical sense of the hydration numbers determined by Karl Fisher titration were discussed in a previous review [28]. Hydration of Cl-, Br-, I-, NO3–, NO4–, and SCN- anions in deuterated nitrobenzene saturated with water was studied recently, using 1H NMR spectrometry [115]. The study indicated that the hydration proceeds in a stepwise manner in nitrobenzene, i.e. according to the reaction X-(H2O)m-1+H2O=X-(H2O)m (m=1,2, 3,…). Thus, nonintegral values of the hydration numbers in nitrobenzene found in various papers

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Table 5 Hydration Numbers nhyd of Bare and Complexed Cations in Nitrobenzene Saturated with Water

Mean values from [110]. From[112]. c From [35]. d From [113]. e From [79]. a

b

were the average of the of various integral hydrates existing at the concentration of water in nitrobenzene saturated by water (0.168 mol/L at 25°C). The hydrates were characterized by their formation constants Km=[X-(H2O)m]/([X-(H2O)m-1][H2O]). In interesting studies of Stoyanov et al., the composition of the organic extracts in 1,2-dichloroethane and the bound water were studied by IR-spectroscopy [116, 117]. The extraction was studied of H+ cobalt bis(dicarbollide) 1- and of the strontium cation with this anion in the absence and presence of three Sr synergists: PEG 400, 15-crown-5 and 18-crown-6. It was concluded that the hydrogen ion is hydrated in the organic phase, forming there a (H5O2.4H2O)+ particle. This contrasts with other extracts, e.g, in tri-n-butylphosphate, where enters the water core in the form of reverse nanomicelles [118]. In the presence of PEG 400, the hydrating water molecules are completely replaced by the six COC and two COH groups, exactly fitting to the coordination number of Sr2+ (n=8). Because of involvement of the ending COH groups of the polyethylene glycol, the lower efficacy of alkyl substituted PEGs in extraction of Sr2+ could be explained. The behavior of the two studied crown ethers, according to the authors of Ref. 116 differs considerably. In the complex the central cation is coordinated equally by all 10 COC groups of the two crown molecules. In however, the central ion coordinates with eight COC groups of two crown molecules, while one or two molecules are additionally coordinated to Sr2+, thus increasing the coordination number to 9 or 10. Moreover, it was found that H3O+ interacts more fully with 18–C–6 (by 6 oxygen atoms) than with 15–C–5 (with 5 oxygen atoms, due to one linear H bond and two bifurcated H bonds). These two effects led to much higher extractability of Sr2+ with 15–C–5 [116].

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The hydration numbers found for nitrobenzene as a solvent were plotted against their respective ∆hydrG° values as shown in Fig. 6. Good correlation is – observed both for cationic and anionic data. The only exception of Cl might be an inaccurate value in [111], since previously Kenjo and Diamond found for this ion a hydration number nhyd=3.3 [119]. The latter value would lie on the straight line in Fig. 6. Generally, the hydration of ions in the organic phase seems to be one of the decisive factors, which determine the selectivity, especially of ions which are moderately to strongly hydrated in the organic phase. The information on hydration numbers should be viewed as subsidiary to actual ∆tG° values in concrete systems. 6.

Role of Dissolved Water in the Organic Solvent on Alkali Metal Cation Selectivity

For structurally similar solvents with comparable basicities, as are the nitrosolvents, the selectivity of extraction of alkali metal cations could be correlated as smooth monotonous curves if log K(Cs+/M+) were plotted against the solubility of water in the respective solvent [120]. This correlation applied for pure nitromethane, and water-saturated nitrobenzene, 2-nitropropane, nitroethane, and nitromethane solvents, in order of the increasing water solubility. With the newly obtained results plotted in Fig.5, considered importance of water solubility on the selectivity becomes more complicated. The curves for the transfers into nitromethane and o-nitro phenyl octyl ether are practically the same as in the Fig. 5. However, the water solubility in nitro-methane (~1.2 mol/L [120]) is much higher than for o-nitrophenyl octyl ether (0.046 mol/L [90]) and considering simply water solubility as the criterion, the selectivity for the latter solvent ought to be higher even than that for nitrobenzene. This may be explained by supposing that solvation in NPOE differs from that of simple nitrosolvents by letting the ether oxygen of NPOE partially enter the solvation sphere of the cation. Due to the higher basicity of ethers than nitrosolvents, the slope of the dependence for NPOE is then expected to be lower than for simple nitrosolvents as is the case. The dependences for dry NM and water-saturated NM, NB, and NPOE depicted in Fig. 5 display quite characteristic features; namely, they obey the normalizing linearity of ∆tG° (X, aq(or)→or(aq)) on ∆hydr G° for alkali metal ions with the exception of Li+ (see legend to Fig. 5). From this point of view, only the Li+ cation is considerably affected, which is reasonable due to its extensive hydration. The shift of the data point for Li+ against the linear functions appears to be regular in the series of selectivity, i.e., increasing with decreasing slope of the straight-line dependences. The hydration in other solvents than nitrobenzene was not systematically studied, but the data point for Li + and nitromethane gives n hyd=18 [110]. Excessive hydration leads to a decrease of selectivity as discussed above for M+, M2+, and M3+ cations (Section V.B).

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7. General Affinity Scale for Organic Solvents The scale of affinity of anions toward the nitrobenzene, i.e., picrate - >2,6-dinitrophenolate>2,4-dinitrophenolate [125], i.e., exactly in the order predicted by Table 2. It is noted that often the extraction of metal calixarene complexes with perchlorate anion is higher or equal to that with thiocyanate [126] in agreement with the data in Table 2. The use of the hydrophobicity/lipophilicity scale based on ∆tG°(X, aq→or) or ∆tG°(X, aq(or)→or(aq)) for dealing with the extraction

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Rais

constants of calixarene-metal complexes was proposed [126], but no reference set of the values was suggested there.

VI. CONCLUSIONS The extraction of electrolytes is a particular type of extraction because the mechanism is not generally amenable to the usual log-log analysis. The dependences on the ligand concentration or on acidity of the aqueous phase often display maxima characteristic for this mechanism of extraction. In this review, the most common cases encountered in praxis were discussed and a simple procedure for the calculation of the curves with maxima was given. It is shown that, contrary to common belief, the mechanism of extraction of electrolytes applies in many situations in which a neutral ligand is used. This is so, even if no particular hydrophobic anion is used, the role of counter-ion being in this case played by a mineral acid anion present in the system. The experimental results of the extraction studies as well as electrochemical studies of transfers across ITIES deal very often with nitrobenzene as a solvent. Critically examined data of standard molar transfer Gibbs energies and hydration numbers for this solvent were collected here. The former data may be used more generally for predicting the behavior of new systems and ions, based on the proposed correlations in the review: (1) straight-line correlations of ∆tG°(X, aq(or)→or(aq)) values on the respective standard molar Gibbs energies of hydration for small inorganic ions, (2) linear dependences of ∆tG°(X, aq(or)→or(aq)) on the number of carbon atoms or properly chosen ionic radius of the ion for tetraalkylammonium and tetraphenyl derivatives, and (3) a general scale of hydrophobicity of anions based on ∆tG°(X, aq(NB)→NB(aq)). These enable the prediction of particular behaviors in extraction systems both with complete ion dissociation or association. The transfer data for nitrobenzene and small ions permitted an independent check of the underlying principles in the determination of both individual standard molar Gibbs energies of hydration and energies of transfer, justifying dealing with and using the tabulated values. This positive check consists in finding out that the experimentally not accessible values of ∆tG° of cations and anions display the same pattern of behavior as experimentally accessible values of hydration numbers of cations and anions in nitrobenzene namely straight line dependence on respective Gibbs energies of hydration of the ions.

SYMBOLS 1-

cobalt bis(dicarbollide) anion, [(1,2-C2B9H11)2-3-Co]Standard distribution potential of an ion X in the system of mutually saturated aqueous

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Principles of Extraction of Electrolytes and organic phases, [Eq. (24) and (25)],

[M+]aq, [M+]or cM

Didion DM DN EXTRIT

Iaq ISE ITIES

Kex(M, zB). Kex(MLz+, zB

the inner potentials of the denoted phases Equilibrium concentration of M+ in aqueous or organic phase Total (initial) concentration of M in the system referred to one of the phases when va/vo=1; e.g., if 0.06 M H+B- is dissolved in the organic phase and 3 M HNO3 is used in the aqueous phase, then cH=3+0.06= 3.06; used in the program EXTRIT Total (initial) concentration of M in the aqueous or organic phase Didion(X)=[Xj]or/[Xj]aq, distribution coefficient of a individual ion, [Eq. (20)] Distribution coefficient [Eq. (19)] Donor number of the solvent Simple computer program with double iteration, the second immersed in the first one, for calculating especially the curves with maxima Ionic strength of the aqueous phase Ion selective electrode Interface of Two Immiscible Electrolyte Solutions; refers to electrochemical studies performed at the actual interface of the two solutions in equilibrium or very near to equilibrium Association constant of M, zB in aqueous or organic phase [Eq. (5)] Association constant of ML, zB in aqueous or organic phase, [Eq. (12)] Extraction constant of Mz+, zB-, dissociated in the aqueous phase and associated M, zB ion pair in theorganic phase, [Eq. (7)] Extraction constant with bonding of cation by ligand L, [Eq. (10)] Extraction constant with bonding of cation by n molecules of ligand L, similar to Kex(MLz+, zB- [Eq. (14)]

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379

380 Kex(Mz+), Kex(B-), Kex(Mz+, zB-) Kexch(MLz+/zH+). Kexch(Mz+/zH+)

kpotA/B LTGW r1 vaq/vor ∆tG°(X, aq→or) ∆tG°(X, aq(or)→or(aq))

∆tG°(Cs+]rel) ∆tG°(ne)

␦BS (w→n-octanol) ␦L ␦M, zB. ␴or,aq

Rais Individual extraction constant of ion Mz+ or B-, [Eq. (21)] Extraction constant of fully dissociated Mz+, zB-, [Eq. (4)] Extraction exchange constant of exchange of complexed cation MLz+ for zH+, [Eq. (13)] Extraction exchange constant of exchange of Mz+ for zH+, [Eq. (8)] Mixed constant of reaction defined by Eq. (18) Mixed constant of reaction defined by Eq. (17) “Theoretical selectivity coefficient” at ISE, dependent only on the properties of the solvent Program for analysis of the extraction curves developed by Vanura [36] and based originally on the LETAGROP program Ionic radius Volume ratio of aqueous to organic phases Half-wave potential at cyclic voltammetry at ITIES Standard molar Gibbs energy of transfer of an ion X from the aqueous phase into the organic phase, kJ/mol [Eq. (23)] Standard molar Gibbs energy of transfer on an ion X from the aqueous phase saturated with organic phase into the organic phase saturated with aqueous phase, kJ/mol Relative value of ∆tG° referred to Cs+ ion Non-electrostatic component of the overall ∆tG° Stability constant of formation of MLz+ cation in aqueous or organic phase, [Eq. (11)] Partition constant of neutral solute between water and n-octanol used as a parameter in drug research Distribution constant of the neutral ligand L between the aqueous and organic phases, [Eq. (9)] Distribution constant of rion-dissociated ion pair [Eq. (6)] Interfacial tension at the water/organic phase interface

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ACKNOWLEDGMENTS The author thanks Prof. Y.Marcus for editing the text and for his valuable comments. Dr. S.Wilke (former Martin-Luther-Universität Halle-Wittenberg, BRD) supplied some of his data before publication. New results pertaining to the ion transfers between various two solvents in equilibrium and interrelation of cluster with solvation energetics were obtained in collaboration with Dr. T.Okada from NIAST, Tsukuba, Japan and were financially supported by Ministry of Education of Czech Republic, contract ME 485. Financial support for studies of new nonpolar solvents for cobalt bis(dicarbollide)s and for compilation of the newest results provided by Czech Grant Agency (Grant No. 104/ 01/0142) is appreciated. The studies of 137Cs extraction by crown ethers and calix crowns were supported by the Japanese Ministry of Education MEXT through the project ARTIST and the author wishes to express his thanks for this support.

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