Harmonization of Leaching Extraction Tests (Studies in Environmental Science)

Harmonization of Leaching/ Extraction Tests

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Studies in Environmental Science 70

Harmonization of Leaching/ Extraction tests H.A. van der Sleet

Netherlands Energy Research Foundation ECN Soil & Waste Research Petten, The Netherlands

L. Heasman

M.J. Carters Associates Ltd Warwickshire, United Kingdom

Ph. Quevauviller

DGXll Measurements and Testing, CEC, Brussels, Belgium with contribution from: M.J.A. van den Berg A. Gomez K.H.Karstensen G. Rauret

ELSEVIER Amsterdam

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Lausanne-

M. Boonstra I. Hohberg M. Kersten P. Schiessl

New

York

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M. Brener O. Hjelmar J. M6hu N. West

Oxford

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Shannon

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Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211,1000 AE Amsterdam, The Netherlands

First printing: 1997 Second impression: 1998

ISBN 0-444-82808-7 91997 ELSEVIER SCIENCE B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive Danvers, Ma 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the publisher. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

Studies in Environmental Science Other volumes in this series 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Atmospheric Pollution 1978 edited by M.M. Benarie Air Pollution Reference Measurement Methods and Systems edited by T. Schneider, H.W. de Koning and L.J. Brasser Biogeochemical Cycling of Mineral-Forming Elements edited by P.A. Trudinger and D.J. Swaine Potential Industrial Carcinogens and Mutagens by L. Fishbein Industrial Waste Management by S.E. Jergensen Trade and Environment: A Theoretical Enquiry by H. Siebert, J. Eichberger, R. Gronych and R. Pethig Field Worker Exposure during Pesticide Application edited by W.F. Tordoir and E.A.H. van Heemstra-Lequin Atmospheric Pollution1980 edited by M.M. Benarie Energetics and Technology of Biological Elimination of Wastes edited by G. Milazzo Bioengineering,Thermal Physiology and Comfort edited by K. Cena and J.A. Clark Atmospheric Chemistry. Fundamental Aspects by E. Mesz&ros Water Supply and Health edited by H. van Lelyveld and B.C.J. Zoeteman Man under Vibration. Suffering and Protection edited by G. Bianchi, K.V. Frolov and A. Oledzki Principles of Environmental Science and Technology by S.E. Jorgensen and I. Johnsen Disposal of Radioactive Wastes by Z. Dlouh~, Mankind and Energy edited by A. Blanc-Lapierre Quality of Groundwater edited by W. van Duijvenbooden, P. Glasbergen and H. van Lelyveld Education and Safe Handling in Pesticide Application edited by E.A.H. van Heemstra-Lequin and W.F. Tordoir Physicochemical Methods for Water and Wastewater Treatment edited by L.. Pawlowski Atmospheric Pollution 1982 edited by M.M. Benarie Air Pollution by Nitrogen Oxides edited by T. Schneider and L. Grant Environmental Radioanalysis by H.A. Das, A. Faanhof and H.A. van der Sloot Chemistry for Protection of the Environment edited by L. Pawlowski, A.J. Verdier and W.J. Lacy Determination and Assessment of Pesticide Exposure edited by M. Siewierski The Biosphere: Problems and Solutions edited by T.N. Veziro~lu Chemical Events in the Atmosphere and their Impact on the Environment edited by G.B. Marini-Bett61o Fluoride Research 1985 edited by H. Tsunoda and Ming-Ho Yu Algal Biofouling edited by L.V. Evans and K.D. Hoagland Chemistry for Protection of the Environment 1985 edited by L. Pawlowski, G. Alaerts and W.J. Lacy Acidification and its Policy Implications edited by T. Schneider Teratogens: Chemicals which Cause Birth Defects edited by V. Kolb Meyers Pesticide Chemistry by G. Matolcsy, M. Nadasy and Y. Andriska Principles of Environmental Science and Technology (second revised edition) by S.E. Jergensen and I. Johnsen Chemistry for Protection of the Environment 1987 edited by L. Pawlowski, E. Mentasti, W.J. Lacy and C. Sarzanini

35 36 37 38 39

40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69

Atmospheric Ozone Research and its Policy Implications edited by T. Schneider, S.D. Lee, (3.J.R. Wolters and L.D. Grant Valuation Methods and Policy Making in Environmental Economics edited by H. Folmer and E. van lerland Asbestos in Natural Environment by H. Schreier How to Conquer Air Pollution. A Japanese Experience edited by H. Nishimura Aquatic Bioenvironmental Studies: The Hanford Experience, 1944-1984 by C.D. Becker Radon in the Environment by M. Wilkening Evaluation of Environmental Data for Regulatory and Impact Assessment by S. Ramamoorthy and E. Baddaloo Environmental Biotechnology edited by A. Blazej and V. Privarov~t Applied Isotope Hydrogeology by F.J. Pearson Jr., W. Balderer, H.H. Loosli, B.E. Lehmann, A. Matter, Tj. Peters, H. Schmassmann and A. (3autschi Highway Pollution edited by R.S. Hamilton and R.M. Harrison Freight Transport and the Environment edited by M. Kroon, R. Smit and J.van Ham Acidification Research in The Netherlands edited by (3.J. Heij and T. Schneider Handbook of Radioactive Contamination and Decontamination by J. Severa and J. B~r Waste Materials in Construction edited by J.J.J.M. (3oumans, H.A. van der Sloot and Th.(3. Aalbers Statistical Methods in Water Resources by D.R. Helsel and R.M. Hirsch Acidification Research: Evaluation and Policy Applications edited by T.Schneider Biotechniques for Air Pollution Abatement and Odour Control Policies edited by A.J. Dragt and J. van Ham Environmental Science Theory. Concepts and Methods in a One-World, Problem-Oriented Paradigm by W.T. de (3root Chemistry and Biology of Water, Air and Soil. Environmental Aspects edited by J. T~lgyessy The Removal of Nitrogen Compounds from Wastewater by B. Halling-SQrensen and S.E. JQrgensen Environmental Contamination edited by J.-P. Vernet The Reclamation of Former Coal Mines and Steelworks by I.(3. Richards, J.P. Palmer and P.A. Barratt Natural Analogue Studies in the Geological Disposal of Radioactive Wastes by W. Miller, R. Alexander, N. Chapman, I. McKinley and J. Smellie Water and Peace in the Middle East edited by J. Isaac and H. Shuval Environmental Oriented Electrochemistry edited by C.A.C. Sequeira Environmental Aspects of Construction with Waste Materials edited by J.J.J.M. (3oumans, H.A. van der Sloot and Th. (3. Aalbers. Caracterization and Control of Odours and VOC in the Process Industries edited by S. Vigneron, J. Hermia, J. Chaouki Nordic Radioecology. The Transfer of Radionuclides through Nordic Ecosystems to Man edited by H. Dahlgaard Atmospheric Deposition in Relation to Acidification and Eutrophication by J.W. Erisman and (3.P.J. Draaijers Acid Rain Research: do we have enough answers? edited by (3.J. Heij and J.W. Erisman Climate Change Research: Evaluation and Policy Implications (in two volumes) edited by S. Zwerver, R.S.A.R. van Rompaey, M.T.J. Kok and M.M. Berk Global Environmental Biotechnology edited by D.L. Wise Municipal Solid Waste Incinerator Residues by H. van der Sloot, J. Chandler, T. Eighmy, J. Hartlen, O. Hjelmar, D. Kosson, S. Sawell and J. Vehlow Freshwater and Estuarine Radioecology edited by (3. Desmet, R.J. Blust, R.N. Comans, J.A. Fernandez, J. Hilton and A. deBetencourt Acid Atmospheric Deposition and its Effects on Terrestrial Ecosystems in The Netherlands: The Third and Final Phase (1991-1995) edited by (3.J. Heij and J.W. Erisman

Foreword This book has been produced as a result of the collective effort of a number of individuals. The project was initiated and organised by Dr Hans van der Sloot of ECN m the Netherlands within the framework of the European Commission Standards, Measurement and Testing Programme. The project partners working with Dr van der Sloot in this E U funded RTD project are Drs Marjoos van den Berg of NNI m the Netherlands, Leslie Heasman of M J Carter Associates in the UK, Ole Hjelmar of VKI m Denmark, Prof Dr Gemma Rauret of Universitat de Barcelona in Spain, Prof Dr Ing Peter Schiessl of Institut fur Bauforschung in Germany and Nicole West of AFNOR in France. The Project Manager for the Standards, Measurement and Testing Programme of the European Commission was Phillipe Quevauviller of DGXII. Contact details for the group members are listed at Annex 1. Major contributions were made to each of the chapters as listed m Annex 2. The editing work was done by Leslie Heasman, Hans van der Sloot and Philippe Quevauviller with contributions from other members of the group m order to minimise dup#cation and style changes however the book remains as a collection of contributions and each chapter shouM be readable on its own. Many of the issues and concepts which are discussed in the book were developed during or as a result of the two network expert meetings. Those who attended and contributed to the meetings are listed at Annex 3. The project group wouM like to acknowledge the input from all those involved and hope that the collective document will assist in the objective of improving the understanding of leaching from a variety of matrices and will, where appropriate, help to bring together the approaches used in different technical fields and in different countries.

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CONTENTS

Foreword

.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Annex 1 Annex 2 Annex 3

Introduction General principles for the leaching and extraction of materials Soils Contaminated soil Sediments Sewage sludges Composts Granular waste and industrial sludges Waste stabilized/solidified with hydraulic binders Construction materials Preservative treated wood Standardization of leaching/extraction tests Concluding observations and discussion of potential for harmonization Summary

1

13 41 57 75 101 123 131 171 187 209 227 239 263

Contact details for the group members The main chapter contributors Those who attended and contributed to the Network meetings

267 269

Glossary of terms

275

Index

279

271

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CHAPTER 1 CHAPTER

1

1: I N T R O D U C T I O N

Introduction and background information The use of leaching test methods is increasing in different areas - eg waste treatment and disposal, incineration of waste, burning of waste fuels, soil clean-up and the reuse of cleaned soil, sludge treatment, use of compost from different sources, use of secondary materials in construction. These leaching and extraction procedures may be operationally defined i.e. each test applied to a specific matrix may correspond to a well defined chemical treatment, which may or may not be standardized (e.g. by CEN or ISO). In many cases different types of tests are applied to similar types of matrices which limit the comparability of results. In view of these developments it was established that effort was needed to harmonize the leaching procedures that could be adapted for different matrices and to validate the use of existing tests in other technical fields. The further development of an even wider variety of leaching test methods for different matrices is undesirable from a regulatory point of view and undesirable for industry owing to risks of lack of mutual recognition of data from country to country. Clarity in testing is crucial in producer-consumer relations. In regulations on environmental impact for disposal, reuse or transport of materials, the boundary lines between fields are not always clear which can be complicated further by different testing requirements. A material may be a waste in one circumstance and a useful secondary material in another. In national programmes leaching tests are developed in relation to national legislation. In CEN and ISO technical committees leaching tests are standardized. Different groups of scientists (soil, sediment, waste and construction) may develop leaching assessment schemes with similar aims without cross-information leading to a multiplication of slightly different protocols. The operationally defined nature of most of the existing methods hampers comparison of analytical data, minimizes the chances of data interpretation and creates problems when standardization of all these methods is needed at a European level. An important aspect to be addressed, therefore, is to identify those properties assessed by specific methods and identify how existing leaching test procedures can be correlated. In the framework of the current European Commission Measurements & Testing Programme different projects are in progress or under discussion in the area of extraction and leaching methods for different matrices with the aim of standardizing leaching/extraction tests for the analysis of soil, sediments and wastes [Ure 1993, van der Sloot 1995] and produce reference materials certified for their extractable trace metal content, e.g. for soil and sediment [Ure 1993, Quevauviller 1996 and 1995]. The potential problem of a proliferation of leaching tests in various sectors has been recognized. It has been the subject of discussion at a meeting in Brussels organized by DG XII M&T on January 14th, 1994 on the Harmonization of Leaching / Extraction Procedures for Environmental Risk Assessment, where the scientific aspects of harmonisation of leaching tests were highlighted [BCR 1994]. A meeting on the Coordination of leaching tests in ISO and CEN committees was organized by CEN TC 292 on February 4th, 1994 in London [CEN TC/292 1994]. In this meeting the emphasis was placed on coordination and cooperation at standardization level. In both meetings the need for coordination was stressed. Some concerns were expressed about perception issues. As long as the questions to be resolved are discussed on a scientific basis, solutions to deal with fundamental aspects can be found. Political aspects tend to confuse the technical discussions. It is important that politicians recognise the technical rationale behind testing protocols, whereas scientists should be sensitive to the practical and legal aspects of regulatory work. A workshop held after the WASCON 1993 conference was a first step towards the harmonization of leaching/extraction tests. The results of the workshop have been published in a special issue of Science of the Total Environment [ 1996]

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CHAPTER 1

To facilitate the development of a generic approach to leaching, extensive consultation of experts working in different fields was needed to exchange information and define the specific problems in a specialized field. From there a common strategy for the use/validation and interpretation of leaching tests could be established to assess and optimize environmental properties of materials in a variety of applications. To this end a network of experts in the fields of waste treatment and disposal, soil clean-up, soil use, materials in construction has been set up in the frame of the European Commission's M&T Programme to begin this exchange of information and to formulate a common approach or at least to link the developments together so that results from one field can be linked to those in other fields. Meetings between key specialists in different fields have been organized within the framework of this activity. For specific aspects of leaching specialized meetings with selected experts have been convened to discuss the topic and to formulate recommendations for the implementation and evaluation of leaching and define the research needed to solve specific issues. The use of leaching tests can differ in different areas. A clear distinction in strategy is needed to establish how different tests relate to one another, recognizing that for regulatory control and quality control shorter procedures are needed. For understanding mechanisms and leaching processes, which are used to define and optimize these shorter procedures for specific purposes, more fundamental and elaborate tests are required. Aims of the Network for the Harmonization of Leaching/Extraction Tests

The aims of the network for the harmonization of leaching/extraction tests are: to harmonize the approaches in existing leaching tests and tests to be developed in the fields of soil, sediments, sludges, waste, stabilized waste and construction materials; to exchange information among different fields and define the problems in specific fields; to facilitate the development of a generic approach to leaching by intensive consultation of experts working in different fields. From the evaluation of the needs in specific fields to attempt to build a common strategy for the use/ validation and interpretation of leaching/extraction tests to assess and optimize environmental properties of materials in a variety of applications. This will at least provide a link between the leaching/extraction tests in different fields such that results from one field can be related/linked to that in other fields; to form a network of experts in the fields of waste treatment and disposal, soil cleanup, soil use, and materials in construction to disseminate information; to formulate recommendations for the implementation of more generally applicable approaches in the evaluation of leaching results in different fields and to define the research needed to resolve specific issues. Meetings between key specialists in different fields have been be held within the framework of this project. The use of leaching tests can differ in different areas. Experts for the meetings organized within the framework of the Network for the Harmonization of Leaching Tests were

CHAPTER 1

3

selected according to the following criteria: 9

Adequate representation of the different fields of expertise

9

Expertise in the development of tests

9

Balanced representation of countries from the European Union

9

Broad coverage of leaching/extraction tests

9

Participation in standardization work.

It is recognized that there is a strong interest in the topic of harmonization among representatives from industry, standardization technical committees, the research community, governmental and municipal authorities. Therefore, a database of interested parties has been generated to facilitate dissemination of information through the publication of a newsletter. This newsletter is a means of reaching such interested parties. It also gives interested people the opportunity to provide the coordination group with relevant information on the topic of harmonization as well as new names for distribution of the newsletter. Exchange of information and experience between the network and standardization committees will be assured hence mutually benefit from the discussions. Although sampling and the analysis of leachates are important in connection with leaching, these aspects have not been given a high priority in the present project. The emphasis of the work reported here is on inorganic constituents. Although it is realized that organic contaminants are important in the different fields, as the development of tests of the leaching of organic contaminants with aqueous extractants has been limited, this topic will be addressed at a later date. Leaching of radionuclides resulting from fall-out or spills are not covered by this work although it is recognised that the underlying physico-chemical principles of transport and reaction are the same. State of the art

In the Measurement and Testing programme of DGXII, projects in the field of extraction of soils [Ure 1993, Quevauviller 1995], leaching of stabilized waste [van der Sloot 1995] and leaching of construction materials [M&T in progress] are finished or in progress. The recent developments in DGXII are discussed by Quevauviller and Maier [ 1994]. As a direct result of the meetings in Brussels and London [BCR 1994, CEN TC/292 1994] a subsequent meeting was held immediately after the second WASCON Conference in Maastricht on June 3rd 1994 [Science of the Total Environment 1996], and the Network for the Harmonization of Leaching/Extraction Tests was established [-Network newsletters 1, 2 & 3]. This Network now has more than 350 participants from different technical fields - soil, contaminated soil, sediments, sludge, compost, waste, stabilized waste, construction materials, drinking water pipes and preservative treated wood. Regulatory bodies, provincial and national governments, industry, standardization bodies and research organizations are represented in the Network. Discussions during the expert meeting held in Paris on the 1st and 2nd June 1995 has led to some potentially far reaching conclusions on similarities across all the fields represented [Network newsletters 1 & 2]. Worldwide a variety of leaching tests has been developed, of which a few are used for regulatory control purposes [Wallis 1992]. In the different jurisdictions this is a major cause for confusion as different testing methods lead to different results. When these results are not placed in the proper perspective the risk that misjudgments will be made is considerable.

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For an understanding of leaching behaviour of materials in the long term and to assess improvements of material properties, single extraction tests are generally of limited use. Therefore, CEN TC 292 has proposed three levels of tests: characterization tests focused on understanding the long term leaching behaviour and parameters influencing leaching behaviour, compliance tests for regulatory control once the characteristics of an evaluated material have been established and on-site verification tests that are applied at a gate as a quick control to verify that the material delivered meets the specifications. Studies of leaching behaviour of materials show that many factors influence release. In spite of this complexity many similarities exist in material properties as well as similarities in leaching of constituents from different matrices [van der Sloot 1995 & 1991]. In the WASCON Conference series the issue of the type of testing is addressed extensively. In a recent study of Municipal Solid Waste Incinerator residues through the International Ash Working Group_ which is linked to the International Energy Agency (lEA) an extensive evaluation was made of different test procedures [Chandler 1989-1994]. At the national level activities have begun to develop standardized test methods (e.g. AFNOR- France, NNI - the Netherlands, Nordtest Nordic countries, etc). A recent development is the modelling of leaching behaviour through chemical speciation modelling [Comans 1993] and the modelling of release rates to facilitate the prediction of long term release of contaminants in a given scenario relating to the utilization or disposal of materials [van der Sloot 1995 (Leeds)]. The new Working Group 6 in CEN TC 292 will address long term leaching behaviour by evaluating well-defined scenarios. The proposed work will provide useful input to such modelling work. The Network for the Harmonization of Leaching/Extraction Tests provides the forum for the dissemination information to the numerous laboratories in Europe concerned with leaching/extraction testing. This will achieve a more consistent choice of test(s) for specific purposes, harmonization of tests and test use, more uniform reporting of data leading to better comparability of results, better use of resources and standardization. In view of the need for test methods to evaluate the environmental impacts of materials in different fields, leaching/extraction procedures are modified and developed. The concern is that a multiplicity of tests creates more confusion than it solves. This necessitates the establishment of relationships and similarities between tests used in different fields which will facilitate the choice of the most appropriate tests for specific purposes. Such comparisons have been made on a limited scale which show promise for harmonization [van der Sloot 1991 & 1995]. The technical work reported here is unique in its approach, as a comparison of this nature across a wide range of different fields using a variety of test methods more or less commonly used in the respective fields has never been carried out before.

Relevance for society In the European Community consistent and reliable is needed on the environmental impact of material is needed so that decisions can be made on the use, treatment or disposal of contaminated soil, sewage sludge, contaminated dredgings, compost, industrial by-products (formerly considered wastes), and construction materials. In all of these cases leaching/extraction tests will play a decisive role in determining the environmental impacts as the total contaminant content is meaningless for this purpose. For an evaluation of the wide variety of treatment methods and utilization scenarios

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5

comparability of leaching test data is essential to facilitate conclusions on the improvements in environmental performance achieved after treatment. Possibilities for recycling/reuse and possible treatment options are increasing in the European Community, e.g. coal fly ash is utilized in concrete to a large extent. In addition municipal solid waste incinerator residues are applied in increasing quantities in road base construction. Industrial slags are applied in increasing quantities in coastal protection. The EC supports studies by metallurgical industries to improve the quality of residues so that slags from the production processes can be utilized. The economic benefit of such utilization of secondary raw materials is very promising as it preserves resources of natural aggregates and limits the increase in disposal costs. The use of different tests may cause confusion and uncertainty, which can delay the possible reuse of materials. Such delays have negative economic effects as residues with potential for use have to be stored for as long as the confusion persists. In overlapping or closely related fields the confusion can result from a material being considered for example as an agricultural soil in one scenario and as a waste/contaminated soil in another. The question arises as to what tests are necessary to assess the material for re-use, ie. soil tests or waste tests. If the relationship between the tests used in the different areas is clarified, this type of confusion can be minimized or eliminated. Consistency between different regulatory domains can lead to a more consistent regulatory framework. The market for immobilization of contaminants in waste and waste-derived products through solidification/stabilization to reduce detrimental environmental effects is expected to increase substantially over the next decade. Clean-up of contaminated soil has raised questions on the properties of the cleaned soil and its possible uses. More sophisticated evaluation procedures are necessary prior to acceptance of wastes for disposal to minimize long term adverse environmental impacts. In agricultural applications information is needed to assess the relationship between soil treatment procedures used to control plant growth. A proper evaluation of these procedures and the resultant environmental effects is necessary together with an assessment of cost benefit. Test methods are therefore needed to evaluate the risks and benefits and to guide industry and regulators in the application of appropriate utilization and disposal methods. The development of such test methods will facilitate a more rigorous assessment of long-term environmental impacts and better informed decision making on the utilization and reuse of materials and the most appropriate conditions for ultimate disposal. The development of a wide range of unrelated test methods for different classes of materials could lead to a poorly manageable situation and high related cost. The development of test methods and their international standardization which involves many people at the national and international level will be costly. The standardization of tests will be costly where there is lack of comparability in overlapping or adjacent fields hence conflicts at ill-defined boundaries between, for example, soil and contaminated soil and between construction materials and stabilized waste products. The interaction between scientists active in the field of test development will prevent such confusion and lead to a unified approach to leaching with a focus on specific aspects of leaching that may have been given more emphasis in one field than in another. It is difficult to quantify at this stage the savings in time and money that will be achieved when persistent confusion in the interpretation of leach test data is avoided, but it is indisputable that real and substantial savings will be made. The work reported here is relevant to EC directorates responsible for drafting regulations in the different fields. For industry it provides a better basis for decision-making through an improved understanding of the factors controlling chemical release, hence control measures

6

CHAPTER 1

through process control, input control and ultimately treatment of residues can be developed. For the general public clarity in presenting the assessment of long term risks will help solve the problem of public perception and acceptance in contrast to public suspicion created by confusion in the interpretation of test results. Fields of work

The fields of work relevant to the Network for the Harmonization of Leaching~xtraction tests are given below. Waste Bulk wastes Stabilized wastes Chemical wastes Inert wastes Vitrified wastes

Soil Natural soil Contaminated soil Compost

Sludge Industrial sludges Sewage sludge Water treatment sludge

Sediments Natural sediments Dredge spoils

Construction materials Concrete Aggregates Bricks Composites Tiles Preservative treated wood Drinking water pipes

Figure 1.1"

I soil I Contaminated soil

Compost I

Construction materials concrete bricks

Sludge

Sediments

l WasteI Stabilized waste

NETWORK HARMONIZATION OF LEACHING EXTRACTION TESTS

Construction materials aggregates

Wood conservation

Drinking water pipes ,,

CHAPTER 1

7

Initiative to start the network

The work reported here is a result of discussions on the harmonization of leaching/extraction tests which started by identifying key specialists active in the different technical fields on the development and use of leaching/extraction methods. These specialists were recruited from current Measurement and Testing projects, CEN TCs and national specialists. The experts were invited to meetings to discuss the main questions posed in different technical fields that lead to a need for a leaching/extraction tests and to identify potential overlap in testing needs between fields. In preparation for these meetings information was collected on existing tests and tests under development in different technical fields. Working documents were prepared to facilitate discussion at the specialist meetings. The scientific background of leaching was addressed to find common ground in the different fields. The specifies of different leaching/extraction tests, their limitations and the interpretation of test results were addressed. Some specialized topics were addressed such as the role of pH, redox conditions, complexation, organic matter, sorption etc to identify the approaches followed and the steps necessary to define these aspects. A main aim of the work is to disseminate information through the channels of the Network to interested parties at the regulatory level, including national provincial and municipal level, in industry and in research organizations. Three newsletters covering the meetings held by the Network had been issued by the end of 1996. Responses to newsletters

The appeal to register interest in the Network for the Harmonisation of Leaching~xtraction Tests has led to many responses. By mid-1996 more than 400 people had responded positively and been listed for distribution of further mailings of the newsletter. The respondents originate from 28 countries, several of which are outside the EU. The current distribution between countries is shown in Figure 1.2. The distribution between the different technical fields, the types of organizations responding and the uses to which the tests are put as specified by the respondents to the questionnaire are listed in Table 1.1. Contents of this publication

The fundamentals of leaching are addressed in Chapter 2. In Chapters 3 to 11 the specific leaching/extraction characteristics for each material are discussed and the current status of test development in each technical area is discussed. In Chapter 12 current standardization issues are highlighted. In Chapter 13 a summary of findings is presented, the potential for harmonization is indicated,, recommendations are made and initiatives for further work are discussed. Chapter 14 summarises the key issues discussed throughout the book.

8

CHAPTER 1 Figure 1.2:

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Network members

Table I.I" Membership of the network

Fields of expertise: Soil Contaminated soil Sediment Sludge Compost Waste Stabilized waste Construction material aggregates Construction material monolithic Drinking water pipes Preservative treated wood

% 10 17 7 8 4 18 12 10 8 4 2

The fields of expertise are reasonably well distributed in the network.

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CHAPTER 1

Organization type: Industry Standardization body Research Commercial laboratory Government

9

% 13 2 48 13 24

With 48% of the total response research organizations are more strongly represented than any other type of organization, which is not surprising given the topic of the network. Type of use/development of tests: Test development (research) Application for regulatory purposes Evaluation/interpretation of test data Regulatory (limit values)

% 31 27 13 29

It is apparent that the use of tests is strongly linked to regulation. More than 30% of the respondents are involved in test development.

10

R E F E R E N C E S TO C H A P T E R 1

REFERENCES A.J.Chandler, T.T.Eighmy, J.Hartlen, O.Hjelmar, D.S.Kosson, S.E.Sawell, H.A. van der Sloot, J.Vehlow. International Ash Working Group: Treatise on Municipal Solid Waste Incinerator Residues. 1989 - 1994. R.N.J.Comans, H.A. van der Sloot, P.Bonouvrie. Geochemical Reactions Controlling the Solubility of Major and Trace Elements During Leaching of Municipal Solid Waste Incinerator Residues. Proceedings Municipal Waste Combustion. VIP 32. Air & Waste Management Association Pittsburg, Pennsylvania. 1993. 667 -679. M&T-DGXII Project on Extractable Trace Metals in Sediment and Soil : "Preparation of candidate certified reference materials for the quality control of EDTA - and acetic acid-extractable trace metal determinations in sewage sludge-amended soil and Terra Rossa soil" Ph. Quevauviller, G. Rauret, A. Ure, R. Rubio, J.F. Lopez-Sanchez, H. Fiedler and H. Muntau, Mikrochim. Acta, 120, 289-300 (1995) M&T-DGXII Project on Development of Leaching Standard for the Determination of the Environmental Quality of Concrete (Final report in 1997). M&T-DGXII Project on Intercomparison of Leaching Tests for Stabilized Waste : H.A van der Sloot, G.J.L. van der Wegen, D. Hoede and G.J de Groot, Ph. Quevauviller. Intercomparison of leaching tests for stabilized waste. Commission of the European Communities, EUR 16133 EN, 1995. Minutes of meeting on Harmonisation of leaching/extraction tests at BCR in Brussels January, 14, 1994. Minutes of Workshop on Leaching organized by CEN TC 292 in London on February, 4th 1994. Newsletter Network of Harmonization of Leaching/Extraction Tests No 1 and 2. 1995 Newsletter Network of Harmonization of Leaching~xtraction Tests No 3. 1995 Ph. Quevauviller and E. Maier. Research trends in the field of environmental analysis. EU Environment and quality of life. EU report 16000 EN. 1994. Ph. Quevauviller, M. Lachica, E. Barahona, G. Rauret, A. Ure, A. Gomez, H. Muntau. Interlaboratory comparison of EDTA and DTPA procedures prior to certification of extractable trace elements in calcareous soil. The Science of the Total Environment, 178 (1996) 127-132. H.A. van der Sloot, D. Hoede and P. Bonouvrie, 1991. Comparison of different regulatory leaching test procedures for waste materials and construction materials. ECNC-91-082. H.A. van der Sloot, R.N.J. Comans and O. Hjelmar. 1995. Similarities in the leaching behaviour of trace contaminants from waste, stabilized waste, construction materials and

REFERENCES TO CHAPTER 1

11

soil. The Science of the Total Environment, 178 (1996) 111-126. H.A. van der Sloot. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. International Symposium on Bulk "Inert" Wastes: An Opportunity For Use. September 1995, Leeds, UK ; Waste Management 16 (1996) 65 - 81. Special issue leaching/extraction tests for environmental risk assessment. The Science of the Total Environment, volume 178, 1996. A. Ure, Ph. Quevauviller, H. Muntau and B. Griepink, Report EUR 14763 EN, CEC, Brussels (1993). A. Ure, Ph. Quevauviller, H. Muntau, and B. Griepink. Int J. Environ. Anal. Chem., 51 (1993) 135. S.M. Wallis, P.E. Scott and S. Waring. Review of leaching Test Protocols with a view to developing an Accelerated Anaerobic leaching Test. AEA-EE-0392. AEA Environment & Energy, Harwell, Didcot. 1992.

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CHAPTER 2 CHAPTER

2: G E N E R A L P R I N C I P L E S F O R T H E L E A C H I N G EXTRACTION OF MATERIALS

13 AND

Introduction

When solid materials come into contact with a liquid some constituents will dissolve to a greater or lesser extent. The degree of dissolution of individual constituents in the contacting liquid leads to a leachate/percolate or extract composition that is of interest for different purposes. Leaching/percolation of materials can occur in the field by exposure of materials to natural infiltration or precipitation or in the laboratory during column tests, batch leaching/extraction tests. Leaching/extraction tests are often designed to reflect a field exposure situation. Leaching/extraction tests are often designed to reflect the field exposure situation. There are a number of factors that can influence the rate at which constituents are dissolved from the material matrix.. These can be divided into physical, chemical and biological factors. The latter can generally be translated into chemical factors such as pH effects, the generation of dissolved matter or the development of reducing conditions. Typical physical factors that influence leaching include: particle size as leaching is in part related to the surface exposed to leaching homogeneity or heterogeneity of the solid matrix in terms of mineral phases 9

the time frame of interest

9

flow rate of the leachant

9

the temperature during leaching

9

the porosity of the solid matrix the geometrical shape and size of materials from which leaching is controlled predominantly by diffusion processes permeability of the matrix during testing or under field conditions

9

hydrogeological conditions

Typical chemical factors that influence leaching include: 9

equilibrium or kinetic control of release

9

potential leachability of constituents pH of the material or that imposed by the surroundings (e.g. CO2 effects)

9

complexation with inorganic or organic compounds

14

CHAPTER 2

9

redox condition of the material or that imposed by the surroundings

9

sorption processes biologically generated factors capable of affecting pH, redox and complexation with organic matter.

The biological aspects are not discussed separately but are integrated into the discussion on chemical factors which control release.

Physical factors influencing leaching The physical factors which influence leaching relate strongly to the manner of contact between the liquid and the solid material. This cannot be considered separately from the conditions in which the material is subjected to leaching either in the field or in a laboratory test. Examples of different leaching conditions include a natural soil exposed to rainwater infiltration, a concrete wall exposed to sea water, the release from a sediment to the overlying water column or a waste exposed to percolating rainwater. The ultimate goal may be predicting long term field leaching behaviour of soils, sediments, disposed, treated or utilized residues. Such data cannot be obtained by experimenting in realistic time frames. The combination of modeling and accelerating certain aspects of leaching in batch or column leaching tests can help to simulate such long term scenarios. This can be accomplished for instance by increasing the throughput of liquid through columns or increasing the volume of liquid used in batch tests. Both procedures simulate exposure to naturally occurring precipitation over a longer time frame. Batch tests can be used to address specific influences such as temperature, gas partial pressures, pH changes, complexing ligands, oxidation/reduction potential changes and so on.. In batch experiments acceleration of leaching is achieved by the mode of agitation chosen which promotes mass transfer from the solid to the liquid. Generally batch experiments are carried out to attain some state of equilibrium or quasi-equilibrium Column tests usually simulate field conditions such as fluid flow, mass transfer and dissolution mechanisms in a more realistic manner. One way to accelerate the process of leaching compared with that achieved in field conditions is to increase the rate of percolation. For materials that have a monolithic character and due to their form feature release mechanisms quite different from those discussed above other approaches of testing in which the relevant release controlling processes are taken into account are needed. For monolithic materials test procedures such as a tank leach test can facilitate the assessment of intrinsic parameters that allow translation/extrapolation to longer time scales provided that the release mechanisms are understood sufficiently. The specific situation or scenario being simulated can be far more complex when other factors such as temperature differences, wet/dry cycling and hydrogeological properties of a site need to be taken into account. For materials with a monolithic character release is controlled by either surface processes or diffusion processes within the matrix. In the latter case release parameters can be derived to make predictions over longer time scales.

CHAPTER 2

15

Particle properties The particle surface area to volume ratio, the average particle size and internal pore structures in the material all control the surface area where dissolution from the solid to the liquid can occur. Larger surface areas per mass or volume can allow more rapid dissolution at the surface. In materials of different nature particles exhibit widely different surface area to volume ratios, grain size distributions and internal pore structures. The chemical and mineralogical properties of individual particles constituting a soil, a sediment or a waste may vary substantially. Some major and minor minerals are more soluble than others. Leachate pH and oxidation/reduction potential are governed largely by the major elements in such more soluble major minerals. In a bulk sample consisting of a wide range of heterogeneous particles the leachability as observed in laboratory tests as well in the field reflects the sum of all interactions. The high leachability of some minerals may be controlled by the retention of released constituents in others.

Flow past particles In many field scenarios the liquid flow past the particles is facilitated typically by gravity. This often implies that kinetics are involved. The porosity of the material and the hydraulic head above the material govern the velocity of the leachant past and through the particles. Materials with low values of interparticle porosity and low permeability will not transmit water hence the velocity of flow will be quite low. In an extreme case a solidified, impermeable mass will not conduct water. Instead water would be forced to flow around such a monolith. The occurrence of such flow conditions is correlated strongly with the permeability difference between the matrix under consideration and its surroundings. A fine grained material can behave under given circumstances as a monolith for example, a clay lense in a coarse sandy soil. Such prevalent mechanisms are shown in Figure 2.1. In an agitated batch leaching test the relative rate of flow of a liquid past a particle is a function of the energy put into the system and the rate of fluid shear between adjacent parcels of water. Agitated systems where the particles remain in suspension and do not settle usually result in very high degrees of mixing and mass transfer. Kinetics in a physical sense comes into play when fluid flow conditions are such that the concentrations of dissolved contaminants in the percolating liquids have not yet reached equilibrium with the dissolving solid. The rate of mass transfer from the particle to the bulk solution is critical in such cases. In designing a test to simulate field conditions where flow may be rather slow the rate at which fresh leachant moves through the system needs to be optimized between obtaining an answer in a reasonable time and approaching (semi)equilibrium. For conditions of slow fluid flow and a highly soluble solid phase the rate of dissolution can be faster than the rate that the dissolved constituents are advected or carried away and equilibrium between the liquid and solid can be achieved. Conversely for conditions of fast fluid flow and relatively insoluble solid phases the rate of advection is greater than that of dissolution and equilibrium will not be achieved.

16

CHAPTER 2 Figure 2.1: Modes of contaminant transport as controlled by advective flow and diffusion controlled conditions for granular wastes and monolithic materials.

In completely static systems where diffusion of constituents of dissolving solids are carried into the bulk solution by aqueous diffusive fluxes the rate of diffusion will be the limiting process for release. Depending on the degree of confinement of the system equilibrium may or may never be reached in this case.

Degree of saturation Leaching tests generally are carried out under saturated test conditions as the amount of liquid required to carry out chemical analysis of constituents is in the order of several to hundreds of millilitres. Important major aspects are that the liquid/solid ratio in the field (unit volume of leachant passed through each unit mass of solid) is much lower in the field than in most leaching tests and that under unsaturated conditions the materials are exposed to neutralization

CHAPTER 2

17

by CO2 and oxidation by 02. This implies that concentrations in pore waters are much higher than obtained in most batch tests. A column test can simulate this condition reasonably well. The unsaturated conditions in the field can lead to a much lower pH and higher redox potential than may be found in laboratory test conditions. If release conditions are diffusion controlled unsaturated conditions will reduce further transport of contaminants [Schaeffer 1995]. These aspects are important in the evaluation of long term release based on laboratory test data.

Physical changes in materials due to ageing~weathering/mineralization Physical changes that may occur at time scales generally considerably longer than common testing times include changes in surface mineralogy of a material resulting from exposure to processes such as carbonation and oxidation. For example in cement-stabilized materials exposed to sea water precipitation of new mineral phases led to surface sealing which resulted in a significant reduction in uptake of sea salts in the matrix and in the release of mobile contaminants from the stabilized matrix. The precipitation largely consisted of calcite and brucite formed by the reaction of lime from the cementitious product with magnesium and sulphate present in high concentrations in sea water [Hockley 1991 ]. In municipal solid waste incinerator bottom ash weathering of the material has been observed that leads to the formation of new clay minerals [Zevenbergen 1994]. This enhances the cation exchange capacity and thereby the potential for metal retention. In field measurements the occurrence of such long term changes have been cited as an explanation for observed discrepancies between leaching studies and observations of material weathered for 10 years [Schreurs 1995]. The formation of reaction rims on the surface of particles can provide a resistance to the reaction of components within the particles. Modelling work has been carried out to describe the influence of reaction rims on the leaching process [Kersten 1996].

The local equilibrium assumption For all of the identified leaching scenarios the relative rates of dissolution and fluid flow or advection must be understood before determining whether there is equilibrium at the local level between a constituent dissolving from a particle and the fluid in which the particle is situated. In many leaching scenarios and in many tests equilibrium can be approached for the majority of the principal phases. This is important for the identification of the chemical factors controlling leaching. On the other hand several kinetically controlled release mechanisms can be described mathematically. Such modeling capabilities are important for prediction of long term release [WASCON 1994, IAWG 1997, Hockley 1992].

Chemical factors influencing leaching The chemical factors influencing leaching relate to the fundamental processes controlling the solubility of solids, such as: 9

the influence of pH on controlling solubility of specific chemical phases, the influence of soluble inorganic and organic complexing agents mobilizing otherwise insuble constituents,

9

the role of oxidation-reduction potential in changing constituent solubility,

18

CHAPTER 2 reprecipitation processes or sorption processes whereby initially dissolved constituents return to the solid phase.

Several of these factors are linked as complexation with either inorganic or organic complexing agents is often strongly pH dependent [WASCON 1991, WASCON 1994, Gomez 1986, van der Sloot 1996, IAWG 1997]. In addition redox and pH are often related as a change in redox will in several cases result in a change in pH [van der Sloot 1994]. Finally reprecipitation and sorption are to a large extent a function of pH. This leads to the conclusion that release as a function of pH is a very common leaching characteristic with which many aspects of leaching can be correlated.

Equilibrium versus chemical kinetics Many of the chemical dissolution reactions that occur in solids are relatively quick. This allows the use of equilibrium based reactions and equilibrium reaction constants to describe the leaching system.. Some reactions are relatively slow or extremely slow. Examples include some sorption reactions and remineralization reactions. Reaction kinetics are usually described as the rate of appearance of a solute in solution from a slowly dissolving solid which is a function of system parameters such as temperature, pH and reaction stoichiometry. Another type of change that is dynamic rather than kinetic is related to changes in pH or redox during an experiment. Such changes can lead to momentary adjustment of the leaching equilibrium, so kinetics are not really a problem. However the changes can affect dramatically the release of constituents. When a system is at equilibrium it is possible to quantify the mass of a constituent that is in solution in relation to the mass remaining at equilibrium in the solid phase. As the system approaches equilibrium the transfer of mass from the solid phase to the solution phase slows down. The final equilibrated mass distribution between the two phases describes the equilibrium condition. Under such conditions geochemical modeling of leaching behaviour is often possible [Kersten 1996].

hTfluence of pH on dissoh~tion Many metals exhibit a marked increase in solubility at both low and high pH values for example lead and zinc. Other constituents may exhibit maximum solubility at neutral pH values for example oxyanions such as vanadate, molybdate or show no dependence on pH for example sodium and chloride. In Figure 2.2 some typical examples of the pH dependence of leaching are given for calcium, sulphur, silica, barium, molybdinum, cadmium, copper, lead and zinc in cement-stabilized municipal solid waste incinerator (MSWI) fly ash [van der Sloot 1995]. The pH in leaching/extraction experiments aimed at (semi-)equilibrium is usually governed by the dissolution of the major mineral phases in the solid or by atmospheric CO2. The initial pH of the leachant and the equilibrium pH may differ widely particularly if the liquid to solid ratio (LS) or the ratio of mass of dry material being leached to the mass of leachant is low and the solid phase dominates the system. At high LS ratios the solution may become more important.

CHAPTER 2

19

Figure 2.2: pH controlled leaching test data for a number of elements as obtained in the intercomparison study for stabilized waste [van de Sloot 1995] illustrating the different leaching characteristics of elements.

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In terms of pH control a distinction must be made between systems closed from the atmosphere and systems open to the atmosphere. In the latter case the pH can be strongly affected by uptake of CO2 from the air. This is particularly important in percolates collected from laboratory scale column experiments, lysimeter tests and field collected percolates. The effect is largest in the latter case as the exposure time to the atmosphere is generally longest. Due to carbonation the evaluation of long term conditions of alkaline materials must always take neutralization by carbonation into account. The degree of wetting is important for the rate of carbonation as illustrated in Figure 2.3. Partially filled pores lead to a faster carbonation due to the 10,000 times higher diffusion of carbon dioxide in air than in water. The acid

20

CHAPTER 2

neutralization capacity of the material under study is a crucial parameter in this context as it dictates how long a material can maintain alkaline properties. The pH can also be influenced by biological factors. This pH effect can be caused indirectly by formation of carbon dioxide through biological degradation of organic matter. The concentration of gaseous carbon dioxide in the subsoil due to this process is generally much higher than atmospheric carbon dioxide. Biologically generated carbon dioxide must be considered with regard to the neutralizing effect on alkaline materials brought in or in contact with soil. Another biologically mediated process is the oxidation of sulphide to sulphate. This process can lead to the generation of acidic solutions (for example acid mine drainage water). Figure 2.3: Illustration of porous matrix featuring the highest rate of carbonation under partial saturation due to the much faster diffusivity of gases in air than in water.

Influence of complexation on dissolution In the presence of specific complexing agents constituents that would otherwise not be soluble under the conditions in the leachant can be mobilized and reach concentrations far exceeding the equilibrium concentration of mineral phases present in the system. A common example of such inorganic complexation is the mobilization of cadmium by the formation of mobile anionic CdCh 2" complexes.

CHAPTER 2

21

The stability of the sequestered or complexed state ensures that the bound solute is not as accessible to participate in solid phase dissolution/precipitation equilibria as in the absence of the complexant. In the case of soluble complexants the hydrology of the system under consideration is important as soluble complexes can percolate from the surroundings into a material and mobilize constituents or be washed out of a matrix thereby loosing its mobilizing potential. In some specific cases a potentially critical mobilizing constituent (chloride) may be leached before the complexing capacity can be activated and constituents are mobilized (cadmium). In a highly alkaline matrix the chloride complexation of cadmium is not active and before the pH drops to a level where chloride mobilization becomes important the chloride has been washed out to a level that is far less critical for cadmium complexation (Figure 2.4). In the evaluation of test results in particular in batch leaching tests these consecutive reactions can be easily overlooked. Figure 2.4: Leachability of Cu from municipal solid waste incinerator bottom ash controlled by formation of Cu-DOC complexes. Top solid line reflects the total composition of Cu in bottom ash, the broken line represents the potential leachability, the drawn line with the black squares reflects the actual Cu release. The dotted line reflects the leaching behaviour of inorganic copper.

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In systems containing degradable organic matter the complexation of metals with dissolved organic carbon (DOC) is also well known in several matrices [Belevi 1993, McCarty 1989]. In Figure 2.5 the role of DOC in copper complexation in municipal solid waste incinerator bottom ash is given. The formation of DOC can occur through biological degradation as well as through chemical degradation. An example of the chemical route is the release of DOC from material containing organic matter when exposed to an alkaline environment. In terms of mechanisms of mobilization a distinction can be made between metals in solution complexed by DOC in competition with other binding sites and metals already bound to fragments of

22

CHAPTER 2

organic matter which are liberated by either biological or chemical degradation. The latter process is not as well recognised as the former. The two mechanisms are written as reactions with P O M representing particulate organic matter and DOC as dissolved organic carbon. Mechanisml"

POM-Me P O M - Me

+ OH ~ POM + DOC-Me + biological activity--~ POM + D O C - Me

Mechanism 2: P O M + O H ~ P O M + DOC P O M + biological activity ~ POM + DOC D O C + M e 2+ ~ D O C - M e

Figure 2.5: Leaching behaviour of Cd from municipal solid waste incinerator fly ash to illustrate the influence of an initial low pH in a column experiment on the different cumulative release at LS=I0 for a column and a single batch test. Circle:individual column test data. Triangle: batch test data.

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12

CHAPTER 2

23

Recent measurements of copper in agricultural soil and forest soil has shown significant complexation of copper by DOC, as evidenced by the activity of copper, which shows a decreasing trend with increasing pH [Japenga 1995]. From pH 4 to pH 7 the copper-DOC fraction is 2 to 3 orders of magnitude higher than the free copper concentration, which implies that effectively all of the copper in solution is in the DOC complexed form. The leachability of zinc from forest soil in particular increases significantly as the pH drops below 4.5. (Figure 2.6) From these observations it was concluded that the acidification of forest soils may proceed too far and neutralization could be needed. In this context the addition of specific pH buffering minerals to forest soil is considered a better alternative than lime addition [Del Castilho 1996]. Figure 2.6" Leaching of zinc from soils of different pH

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Influence of oxidation-reduction potential on dissolution The redox potential in a system is important as the absence of oxygen leads to formation of different chemical phases with significantly different solubilities compared to oxidized conditions. The formation of very insoluble metal sulphides is a clear example of such reactions. The reducing conditions can be a property of the material studied, e.g. industrial slags and sediments or a property that is imposed on an otherwise oxidized material. This condition may develop when material contains, gets mixed with or gets in contact with degradable organic matter. In Figure 2.7 A and B relevant pH - EH domains are indicated for a range of conditions that may occur in practice. The normal pH conditions in the field may range from pH 3 to pH 9 whereas in waste and cement - bound materials the pH may rise to 12 and 13. In leaching and extraction tests the role of redox changes is often neglected [van

24

CHAPTER 2

der Sloot 1994]. Since changes in leachability of orders of magnitude may occur it is important to at least be aware of when such conditions may play a role.

h~uence of sorption on leaching Many solid mineral phases in materials have sorptive properties and are capable of binding dissolved constituents onto the surface via a number of sorption reactions. Sorption reactions can involve the formation of bonds that are relatively weak to those that are quite strong. Strong binding implies that the chances for the sorbed species to be desorbed again become small unless the conditions in the leachate or extract in terms of pH, redox or complexation change significantly. Many sorption processes are strongly pH dependent. The surface charge of the sorption sites is important and determines whether anions or cations are retained, most prominently The surface charge is a function of pH. The zero point of charge located at a specific pH may form an adsorption/desorption edge. Sorption generally is subject to hysteresis. The pH condition at which the sorption becomes effective is different from the pH at Figure 2.7: A: Approximate position of some natural environments as characterized by Eh and pH.

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which the desorption starts. Generally more effort is needed to desorb constituents once they are adsorbed. A typical example of sorption onto ferrichydroxide is given in Figure 2.8 for

CHAPTER 2

25

vanadium sorption. In particular for soils sorption onto iron and manganese oxides and organic matter controls the leachability or release to porewater. The iron content, the cation exchange capacity and the organic matter content are therefore key parameters in soil characterization.

Influence of secondary weatheringproducts on leaching The formation of secondary reaction products on the surface of particles has an influence on leaching that cannot be assessed by short term leaching experiments on relatively fresh samples. An example of such a condition is the aging of MSWI bottom ash [Zevenbergen 1994]. Fresh MSWI bottom ash is a material that takes time to stabilize. A major relevant factor is the Figure 2.7 B: Distribution of Eh-pH measurements in natural aqueous environments. The shaded section indicates the area covered by waste materials, particularly those derived from high temperature processes.

combined neutralization/carbonation by atmospheric and biogenic carbon dioxide. Relatively fresh material tested features a relatively high pH due to unreacted lime. Upon aging and weathering the material reacts as more neutral, which has direct consequences for the leachability of metals. Upon exposure to field conditions for several years such changes have been recorded [Schreurs 1995]. Similar observations have been made by Kersten et al. [1995] who stated that dissolution reactions that are relevant initially are replaced by desorption processes once new mineral phases have been formed. The role of ferric hydroxides is crucial in this context. The formation of interface precipitates as described earlier can also lead to transport resistence. These phenomena are still poorly documented. Based on laboratory

26

CHAPTER 2

studies, situations in which such reactions occur can be predicted [Hockley 1992] Another example is related to changes due to oxidation of reducing materials containing significant amounts of Fe (II). These may form a protective coating of ferrichydroxide that changes the leaching behaviour of vanadium. The difference in leaching behaviour of vanadium from freshly obtained reducing steel slag and from aged/weathered surface-oxidized steel slag is quite significant [CROW 1996]. Figure 2.8: Sorption characteristic of V on ferrichydroxide as a function of pH. Above pH 9 the role of sorption on ferrichydroxide is limited.

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0 o...q

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1.0 ._= 05 -

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In Figure 2.9 the conceptual situation regarding many physical and chemical aspects of release from granular materials is shown. As depicted in the figure the solid phase is a structurally and mineralogically complex heterogeneous material. Basic reactions such as acid-base chemistry and redox chemistry are governed by many interactive equilibrium-based reactions that are sometimes kinetically dissimilar. Precipitation, dissolution and sorption phenomena are also kinetically controlled and interactive. Kinetics can govern the appearance of solutes in the leaching solution. Both slow kinetic mechanisms and mass-transfer constraints can prevent equilibria from being attained. The internal porosity, tortuous path lengths and internal reaction mechanisms constitute an internal resistance to the diffusion of solutes or solvents into and out of the particle or monolith. The fluid boundary layer external to the particle constitutes an external resistance to the diffusion of solutes or solvents between the particle surface and the bulk liquid.

CHAPTER 2

27

Figure 2.9: Processes and reactions in heterogeneous systems subject to percolation.

In field and laboratory column leaching scenarios, the solid granular particle is stationary and a leachant flows through or around the solid particles and carries away dissolved constituents. In certain batch leaching scenarios agitation is used to cause fluid to flow past particles and accelerate the dissolution of constituents in the material. In other batch leaching scenarios fluid flow or agitation is absent permitting only strict molecular diffusion and Brownian motion to carry away dissolving constituents. The rate at which constituents are carried away (via advection) plays a fundamental role in influencing chemical reactions associated with leaching. For most continuously agitated systems kinematic eddy viscosity and shear is high. This means that fluid and chemical boundary layers are very compressed and not likely to constitute a resistance to diffusion or limit fast reactions at the particle surface. By increasing mixing energy external diffusional resistances can be virtually abolished.

28

CHAPTER 2

Leaching tests Leaching/extraction tests are employed to simulate field leaching scenarios or to assess specific properties of a material, such as the release of constituents that are relevant for the fertility of soil (nutrient availability) or the release as a result of contamination or spillage (environmental impact).

Classification of test methods Worldwide the number of available leaching/extraction tests is very large [Environment Canada 1990, Wallis 1992, CEN TC 292/NNI 1994, Quevauviller in press, Ure 1993]. In Table 2.1 an updated overview of test methods is provided. Many tests however are a variation on the same basic principle with small modifications in the specific testing conditions. All existing tests can be grouped according to their main characteristics such as equilibrium or semi-equilibrium leaching tests single batch extractions with and without pH control single batch extractions with some form of complexation by organic constituents single batch extractions aimed at low liquid/solid ratios dynamic leaching tests flow through type tests or multiple batch procedures tank leaching tests compacted granular leaching tests specific tests focused on chemical speciation issues pH static test sequential chemical extraction These observations form the basis of the need to harmonize leaching/extraction tests. Each of the variety of test procedures available to characterize materials with respect to their leaching/extraction behaviour addresses certain aspects of leaching. The question of which test(s) is (are) adequate for which purpose(s) arises. In this respect it is important to distinguish between regulatory requirements, specific quality criteria, impact assessment, scientific evaluation of leaching and management tools for daily practice. For regulatory purposes, the protection of the environment (quality of air, water and soil) and of human health is the prime concern. This requires an evaluation of potential environmental impact in both the short and the long term. 9

Specific quality criteria may be relevant to soil fertility requirements For environmental impact assessment site specific conditions, waste/soil interaction, transport and long term changes in the utilization/disposal conditions need to be addressed. A more rigorous approach is needed to cope with a wide range of technical, physical, chemical and economic aspects. Such an assessment should be the basis for the development of regulation.

CHAPTER 2

29

Table 2.1 Examples of leaching tests performed throughout the world. LEACHING TESTS FOR GRANULAR MATERIALS SINGLE BATCH LEACHING TESTS (Equilibrium based) pH Domain 4 - 5

pH 5-6

Material dictated

Complexation

Low LS

TCLP EPtox Availability test (NEN 7341) California WET

Swiss TVA

DIN 38414 $4 AFNOR X-31-210 O-norm $2072 CEN TC 292 test Canada EE MCC-3C (Canada) ASTM D 3987 Soil - NaNO3 Soil - CaC12

MBLP (synth) (California WET test)

MBLP CEN TC 292 compliance test Wisconsin SLT

Ontario LEP Quebec QRsQ Soil HAc

Soil-EDTA

MULTIPLE BATCH AND PERCOLATION TESTS (Mostly based on Local Equilibrium -LEA) Serial Batch (low LS)

Serial Batch LS> 10

UHHamburg WRU

NEN 7343 (NVN2508) Column up NF-X31-210 ASTM Column up WRU Column Germany (pH static) ASTM D4793-88 NEN 7349 (NVN 2508) MEP method 1320 Sweden ENA MWEP

STA TIC METHODS

Percolation or flow throu[gh tests

SPECIA TION METHODS

Sequential Chemical Extraction MCC-1 pH static test procedures MCC-2 Compacted granular tank leaching test (Rutgers/ECN)

LEACHING TESTS FOR MONOLITHIC MATERIALS

DYNAMIC LEA CHING TESTS ANS 16.1 Tank leach test NEN 7345 Spray Test (impregnated wood) Swedish MULP

30

CHAPTER 2 In scientific studies, a detailed knowledge of phenomena and modeling of processes under controlled conditions is needed. This may involve testing under conditions that are not likely to occur under any conceived environmental condition. This type of testing is undertaken solely to understand how the process of leaching is affected by specific controlling parameters. For daily practice in waste management a relatively simple fast screening test or compliance procedure is needed allowing reliable judgement of treatability, reuse, acceptance at landfill and so on.

It has been stated before [van der Sloot 1990] that assessing a wide variety of materials in an even broader range of applications and disposal situations cannot be addressed adequately by one single extraction test. Consequently the aim of a test has to be clearly identified before a decision can be made in respect of which test is most appropriate in a given situation. The situation represented by a test result should reflect as closely as possible the situation under assessment. In the framework of developing test methods for the characterization of waste [CEN Technical Committee 292 1994] three levels of testing have been distinguished: "Basic characterization" tests that are used to obtain information on the short and long term leaching behaviour and characteristic properties of waste materials. Liquid/solid (L/S) ratios, leachant composition, factors controlling leachability such as pH, redox potential, complexing capacity and physical parameters are addressed in these tests. "Compliance" tests are used to determine whether the waste complies with specific reference values. The tests focus on key variables and leaching behaviour identified by basic characterization tests. "Onsite verification" tests are used as a rapid check to confirm that the waste is the same as that which has been subjected to the compliance test(s). This classification provides another cross-section of leaching tests more related to the use of tests in a management framework.

Identification of solubi#ty versus availabi#ty control Leaching test results can be expressed either as leachate concentration (mg/l) or as constituent release (mg/kg of solid material). The basis selected for expressing leaching results should be based on the type of data comparison which is desired. Regulatory test results are expressed most often as leachate concentrations for comparison with limit values but do not always take into account the underlying basis for the release phenomena which are observed. Results expressed as leachate concentrations permit a comparison of contaminant solubility which reflects the chemical speciation of the elements and leaching solution conditions (e.g. pH). Transformation of measured concentrations into mass release is necessary for comparison of the data obtained at different liquid-to-solid (L/S) ratios and for determination of availability. Release is defined as the mass of a contaminant dissolved divided by the mass of material subjected to leaching. In Figure 2.10 the leaching of chloride represents leachability of an availability controlled species. Data from tests at different L/S expressed in mg/l lead to apparent differences in leaching behaviour while data presented in mg/kg show that in all cases the fraction available for leaching is released. The element silica represents a solubility

CHAPTER 2

31

controlled constituent. Here presentation of the results in mg/kg demonstrates apparent differences whereas data represented in mg/l show the importance of solubility control in the pH region 3 to 8 [IAWG 1997, van der Sloot 1996]. At this point it is relevant to distinguish materials in terms of their degree of equilibrium with their surroundings. Soils and sediments taken from the field are generally in their most stable condition thermodynamically, whereas waste materials sampled directly from a high temperature process are by definition thermodynamically out of equilibrium with their surroundings. The process of change that sets in on contact with the atmosphere and with water results often in major changes in solubility. Testing such materials during their process of attaining "equilibrium" with their surroundings is likely to lead to significant differences as a function of this aging process. Oxidation and neutralization are important processes in this context. In evaluating such unstable materials it is important to keep this aspect in mind as the materials may behave quite differently after longer exposure to ambient conditions.

Identification of leaching mechanisms from leaching tests From the results of multiple step leaching tests conclusions regarding the mechanism controlling or dominating release can be made.

Percolation controlled system In a percolating system the release of constituents depends to a large extent on the solubility of the constituent in the porewater under the chemical conditions dictated by the material under evaluation. A distinction in release can be made between constituents that are entirely soluble in the porewater right from the beginning, for example, soluble salts such as sodium, potassium, chloride and nitrate and constituents that undergo interaction with the matrix in varying degrees, ie moderately retained constituents and strongly retained constituents. In Figure 2.11 this is reflected by the leaching behaviour as a function of the liquid/solid ratio (1/kg) relative to the potential leachability as defined by the release from the materials at extreme dilution (minimizing solubility control) and low pH (pH=4 as an extreme condition for field situations). When the difference between the potential and the actual leaching behaviour is extreme the matrix retention is very high. When the pH and redox conditions do not vary significantly and the flow rate is kept low such constituents can often be regarded as solubility controlled. As a first approach a continuous stirred tank reactor (CSTR) model can then be used which is based on the fact that changes in leaching behaviour are not related as much to the process of percolation as to significant changes in chemical conditions at time scales that are long relative to the rate of percolation. If elements are present in different dissolved chemical forms only those that feature limited or slow chemical exchange will generally be observed as separate species in leaching test data (e.g. Cr III and Cr VI, Cu 2§ and DOC bound Cu). In such cases the more mobile form may be depleted before any significant release of the other species occurs.

32

CHAPTER 2

Figure 2.10: Illustration of solubility versus availability control of leaching. Data expressed in m g ~ g reflect availability control, when data for different liquid to solid (LS) ratios coincide (C! here). Data expressed in mg/! reflect solubility control when data for different solid to liquid ratios coincide (Si here). 1000

9L S = 1 0

--

1000

9L S = 5 o

E~

LS=2

r.r

07

O

100

-

-

t--

[]

LS=2

[]

lOO-

Q

O

,(]> _.J

9L S = 1 0 9L S = 5

o

I

E~

E

I

-

[]

[]

10--

-I--, r" (D r

9

[]

~

00A

0

10-

n

Si '

1

!

'

I

6

10000

8

'

Si

I 10

pH

CI

'

,_-,,

I

6

10000

--

I

O

pH

8

10

9L S = 1 0

--

C1

E~

9L S = 5 []

LS=2

E

E~ E~

E

6

[]

"O

[]

El

O -%

0

1000

8

(13 (D _J

1000

O

-

0

[]

O0

0

0

[]

O

9

|

0

9L S = 1 0 9L S = 5

o. LS=2 100

'

I

6

I

I

8

'

I

10

I 12

pH

100

'

I 6

'

I

'

10

pH

I 12

CHAPTER 2

33

Figure 2.11: Release in a column or serial batch experiment expressed as a function of liquid to solid ratio showing the degree of retention in the matrix relative to the fraction of the total available for leaching.

Surface wash-off Slope = 1

effect

r.r

;>

/~

Slope

/.

--.

0.5

Diffusion controlled release Dissolution of

Slope = 0

material from the surface

0

log (Time) The release ELSField at a given exposure time of a waste material in the field expressed in mg/kg leached, is given by: ELsField -- Availability. (1-e "LsField/K)

with Availability as a measure for potential leachability [NEN 7341, NNI 1994]which is the asymptote to which leaching approaches in the long term unless separate chemical species can be identified requiring adjustment of this parameter, K is a factor expressing the retention of the element of concern in the matrix relative to a mobile constituent such as sodium which is obtained by curve fitting from the test results obtained in a column experiment according to NEN 7343[NNI 1995]. LSFio~d is the actual LS reached a~er t years of exposure to field conditions and is calculated from: LSfield = Ninf* t/h*d

in l.kg "1

with Ni~fthe net infiltration at the given location as obtained from meteorological data in mm'2.yr1, t is the exposure time in the field in year, h is the height of the application in m and d is the bulk density of the material concerned in kg.m 3 . The emission to the underlying soil is then obtained from 9 Imax = db*h*ELSField

in mg.m 2

34

CHAPTER 2

The retention value K is not a constant when the conditions in terms of pH, redox or complexation change. Through the K value adjustments for chemical speciation or externally imposed changes can be made. This aspect needs to be explored further.

Release controlled by internal diffusion Where the permeability between the material to be evaluated and its surroundings is relatively large mass transfer within the material matrix determines the rate of release to the surroundings. This can be described in many cases by a diffusional process in which the physical restriction caused by the pore structure of the material and the chemical interaction of constituents with the matrix are the controlling factors. On the basis of Fick's second law of diffusion a simple relationship can be derived [Crank 1975] between the flux of ions through a unit surface area per unit time (J), the lapsed time t (s) and the effective diffusion coefficient Dc (m2/s): J = Sa q(DdTtt) mmol.s.m"2 with the property S, as the quantity of the constituent present in the matrix that is potentially leachable or available for transport (mmol/m3). The boundary conditions for this simplification are that the chemical conditions remain the same and that no significant depletion of constituent occurs during the testing time. This can in part be resolved by the size of the material subjected to testing. When the flux is plotted against time the slope of the line can be used to identify the prevailing mechanism of release (Figure 2.1). A slope of 0 indicates continued dissolution of materials from the surface of the specimen. A slope of- 1 indicates surface wash-off A slope of - 0.5 can be indicative of diffusion controlled release. For a constituent that can be considered inert (non-reactive towards the matrix) the effective diffusion relates to the molecular diffusion (Dm) through the porosity (e) and the tortuosity factor (x) which is a measure for the tortuous path a diffusing ion has to follow through the matrix.. Dc = Dm *8 / 1: In some systems sodium can be used for this purpose .In others chloride gives a good estimate of the tortuosity. Changes in pore structure due to pore filling, as in the case of self-sealing of cement stabilized waste in the marine environment or changes in chemical conditions in the surface of the materials due to interaction/neutralization with atmospheric carbon dioxide lead to discontinuities in the flux-time plot which in many cases can be explained but are more difficult to quantify. A relatively complex chemical interaction-transport model is needed to solve such problems.

CHAPTER 2

35

Figure 2.12: Mechanisms of release derived from a tank leaching test with regular liquid renewal.

50 Potentially available for leaching -_-_ . . . . . . ........ K=2.5 No retention

:=:-, -~

~_. . . . . . . . . . . . . .

- ~ _ _ _ _ ~ ~

E

_

E

0.1 High retention ....

K=20000

0.01

i

i

i

i

i

i

i

i

1

I

10

20

Liquid to solid ratio (L/S, l/kg)

Chemical speciation (geochemical modeling)

When leaching data are obtained from a leaching test there is no direct indication of the chemical form controlling the release of the element of interest. The chemical form of elements as well as the presence of specific complexing agents and sorbing phases is crucial for release, as discussed previously. Based on stability constants determined for the wide variety of chemical phases in which elements can occur it is possible through geochemical speciation modeling to determine the most likely solubility controlling phases in a given system [Felmy 1984]. The databases for such modeling contain many potential phases. The models have been improved to include sorptive processes and complexation with organic substances. The required input parameters for each combination of constituent and sorbing phase limit the general use of these options. Prediction o f release through modefing

The ultimate goal for the selection of an appropriate leaching test or the evaluation of field leaching data is the ability to interpret mechanistically the observed leaching behaviour. By understanding field leaching behaviour predictions can be made and modeled for how leaching behaviour will change over time and under various management scenarios. This ultimately provides the researcher or regulator with the ability to develop management options based on predicted leaching behaviour.

36

CHAPTER 2

Test parameters Leaching tests employ a number of reactor configurations and control measures for stabilizing pH, pE (or EH), temperature, and so on. The tests are conducted for specific periods of time. A number of initial leaching solutions can be used. Tests need to be selected that describe the leaching system to be studied and are preferably amenable to modeling of leaching. The main focus in addressing leaching tests in different fields is related to the choices made for these parameters to provide an answer to a specific question. In the following chapters the justification and rationale behind the choices of test conditions will be addressed.

Practical LS values The most commonly used LS ratios are around 10. This choice is more related to practical considerations than the question being asked or the situation being evaluated. These practical considerations include the fact that the liquid can be separated more easily from the solid at a higher LS and it is easier to obtain a sufficient amount of liquid for analysis. When the aim is to gain information on concentrated systems (e.g. pore water simulation) the LS ratio should be as low as practically feasible. In some cases the solid is renewed instead of the liquid [Environment Canada 1990]. This leads to lower and lower cumulative LS values at each renewal.

p/-/ In many leaching tests the pH is not controlled and therefore is dictated by the material subjected to testing. However, the pH of a leaching system can be affected in an uncontrolled manner when the system is exposed to the atmosphere. Due to uptake of carbon dioxide, which is even enhanced by stirring, the pH may change during a leaching experiment. Such changes are seldom very reproducible, unless the exposure is carefully controlled. Materials with a limited buffering capacity are most sensitive to such changes. This is why testing in closed vessels is generally more reproducible.

Temperature Leaching tests are generally carried out at room temperature. In some tests elevated temperatures are applied which makes translation to practical conditions more difficult. It is important to note that temperature under field conditions may on average be lower than during test conditions. Since solubility and in particular diffusion are sensitive to temperature this factor needs to be taken into account in carrying out tests in cold or warm climatic conditions and in the translation of test results to practice.

Leachant Demineralized water is the most common leachant used. In soil studies mild salt solutions are used to assess mobilization of labile bound species and more agressive leachants are applied such as EDTA and acetic acid [Quevauviller in press]. In specific situations special leachants are applied, for example seawater to assess release under marine exposure conditions.

REFERENCES TO CHAPTER 2

37

REFERENCES

H. Belevi, N. Agustoni-phan and P. Baccini, Influence of organic carbon on the long term behaviour of bottom ash monofills. Proceedings Sardinia 1993. Fourth International Landfill Symposium. S. Marguarita di Pula, Cagliari, Italy, 11-15 October 1993, 2165- 2173. Characterization of Waste in Europe. State of the art report for CEN TC 292.STB/94/28, 1994. Compliance test for leaching of granular materials, CEN TC 292 Characterization of Waste, Working Group 2 Draft European Standard PrEN12457. June 1994. J.Crank. The Mathematics of Diffusion. Clarendon Press, Oxford U.K., 1975. CROW. Handboek Uitloog Karakterisering. CROW, Ede, the Netherlands, 1996. P. Del Castilho. Chemisch Weekblad, 41,1996, 2. Environment Canada, 1990. Compendium of waste leaching tests. Environmental Protection Series. Report EPS 3/HA/7. A. R. Felmy, D.C. Girvin, and E.A. Jenne, MINTEQ--A computer program for calculating aqueous geochemical equilibria, EPA-600/3-84-032, U.S. Environmental Protection Agency, Athens, 1984. A. Gomez and C. Lejeune. Comparison of the physical and chemical properties of humic acids extracted from a podzolic soil and a mature city refuse compost. In Compost : Production, Quality and Use. Edited by M. de Bertoldi, M. P. Feranti. P. L'Hermite and F. Zucconi.. 1986. D. Hockley and H.A. van der Sloot, Long-term processes in a stabilized waste block exposed to seawater. Environ. Sci. & Technol., 25, 1408 - 1414. 1991. D. E. Hockley, H. A. van der Sloot and J. Wijkstra. Waste - Soil Interfaces. ECN-R-92-003. 1992 IAWG, A.J.Chandler, T.T.Eighmy, J.Hartlen, O.Hjelmar, D.S.Kosson, S.E.Sawell, H.A.van der Sloot, J.Vehlow. International Ash Working Group: Treatise on Municipal Solid Waste Incinerator Residues. Elsevier, Studies in Environmental Science 67, Amsterdam, 1997. J. Japenga, J. Dolfing, P.F.A.M. ROmkens. Annual report 1994. Research Institute for Agrobiology and Soil Fertility (AB-DLO), 1995. M. Kersten. Aqueous solubility diagrams for cementitious waste stabilization systems. 1. The C-S-H solid solution system. Env. Sci. Techn., 30 (7), 1996, 2286 - 2293. M. Kersten, C. Moor and C.A. Johnson. Emissionspotential einer Mullverbrennungsschlacken Monodeponie fur Schwermetalle, Mull und Abfall, 11 (1995) 748 - 758.

38

REFERENCES TO CHAPTER 2

M. Kersten. 1997. Personal Communication. J.F.M. McCarty and J. Zachara. Environ. Sci & Technol.23(5) 496-502. 1989. NEN 7341 Leaching characteristics of solid (earthy and stony) building and waste materials. Leaching tests. Determination of the aavailability of inorganic components for leaching. First edition. March 1995. Netherlands Normalization Institute, Delft. NEN 7343. Leaching characteristics of solid (earth and stony) building and waste materials. Leaching tests. Determination of the leaching of inorganic constituents from granular materials with the column test. First edition. February 1995. Netherlands Normalization Institute. Ph. Quevauviller, G. Rauret, A. Ure, R. Rubio, J.-F. Lopez-Sanchez, H. Fiedler and H. Muntau. Certified reference materials for the quality control of EDTA and Acetic acid extractable trace metals in soil. Microchimica Acta (in press). C. E. Schaeffer, R. R. Arands, H. A. van der Sloot and D. S. Kosson. Prediction and experimental validation of liquid phase diffusion resistance in unsaturated soils. J. Contaminant Hydrology, 20, 1995. 145 - 166. J.P.G.M. Schreurs, H.A. van der Sloot, L.G. Wesselink. Relatie uitlooggedrag laboratorium praktijk bij wegenbouwkundige projecten. Intron rapport nr. 95146. 1995. H.A. van der Sloot, R.N.J. Comans and O. Hjelmar. Similarities in the leaching behaviour of trace contaminants from waste, stabilized waste, construction materials and soil. The Science of the Total Environment, 178 (1996) 111 - 126. H.A van der Sloot, G.J.L. van der Wegen, D. Hoede, G.J de Groot and Ph. Quevauviller. Intercomparison of leaching tests for stabilized waste. Commission of the European Communities, EUR 16133 EN, 1995. H.A. van der Sloot, D. Hoede and R.N.J Comans. The influence of reducing properties on leaching of elements from waste materials. In: WASCON 1994: Environmental aspects of construction with waste materials Eds. J.J.J.M. Goumans, H.A. van der Sloot and Th. G Aalbers, Elsevier, Amsterdam, 1994, 483 -490. H. A. van der Sloot. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. International Symposium on Bulk 'Inert' Wastes: An Opportunity for Use. September 1995, Leeds, UK. Special Issue Waste Management, 16(1 - 3), 1996, 65 - 81. H. A. van der Sloot. Leaching behaviour of waste and stabilized waste materials; characterization for environmental aassessment purposes. Waste Management and Research,8, 1990, 215-228. A. Ure, Ph. Quevauviller, H. Muntau and B. Griepink, Report EUR 14763 EN, CEC, Brussels, 1993.

REFERENCES TO CHAPTER 2

39

S.M. Wallis, P.E Scott and S. Waring. Review of leaching test protocols with a view to developing an accelerated anearobic leaching test. AEA-EE-0392. Environment Safety Centre. 1992. WASCON 1991: Waste materials in construction. Eds J. J. J. M. Goumans, H. A. van der Sloot and Th. G. Albers. Elsevier, Amsterdam, 1991. WASCON 1994: Environmental aspects of construction with waste materials. Eds. J. J. J. M. Goumans, H. A. van der Sloot and Th. G. Albers. Elsevier, Amsterdam, 1994. C. Zevenbergen, T. van der Wood, J.P. Bradley, P.F.C.W. van der Broeck, A.J. Orbons, and L.P. van Reeuwijk. Morphological and chemical properties of MSWI bottom ash with respect to the glassy constituents. Hazard. Waste Mater., 11 (1994) 371-383.

This Page Intentionally Left Blank

CHAPTER 3 CHAPTER

41

3: S O I L S

Introduction

Leaching procedures are used widely in soil science. They are designed to dissolve a phase of the soil whose elemental content may be correlated with their bioavailability. In this respect one element, essential or toxic, is considered bioavailable in a soil if it is in a chemical form that plants can absorb readily and if, once absorbed, it affects the life cycle of the plant. The uptake of one element depends on various soil and plant factors and it is accepted that the concentration of one element in the soil solution rather than its total content in the soil determines its short term bioavailability. Leaching procedures for soils are well established for major nutrients and they are commonly applied in studies on fertility and quality of crops for predicting the uptake of essential elements and for diagnosis of deficiency or excess of one element in a soil and consequently for taking remedial actions. Leaching tests are applied also to elements considered as pollutants and their application is generally restricted to polluted soils either in industrial and agricultural land or in semi-natural environments. For these elements leaching procedures are used not only to predict the possible mobilisation of trace elements from polluted soils and their transfer to plants, but also for clarifying their uptake pathways or in predicting migration of a pollutant through the soil profile to groundwater. In semi-natural environments, forests and meadows, the migration of a pollutant through the soil is highly relevant because the root uptake by a plant, the dominant pathway in soil to plant transfer, is highly dependent on the depth of the roots. In contrast the pollutant content in the arable soil layer can be roughly homogeneous with depth. Leaching tests are applied also in geochemical research to assess the distribution of pollutants among the geochemical soil phases and to asses the environmental impact and possible remediation actions. In a regulatory context, two applications for leaching tests can be recognised: the assessment or prediction of the environmental effects of a pollutant concentration in the environment and the promulgation of guidelines or objectives for soil quality for example for land application of sewage sludge or dredged sediments. Each leaching test in the field of soil science is always restricted to a small group of pollutants. Since many of these compounds, for example heavy metals, may be involved in a large number of chemical reactions in the soil such as adsorption-desorption, dissolution-precipitation and complexation-decomplexation the total content determination or the information obtained from the application of one extraction procedure is not sufficient to predict their uptake by plants or the mobility in the soil profile. Extraction with more specific extraction agents or by a set of extractants facilitates the examination of the distribution of an element in different fractions which may be better related to pollutant mobility if geochemical conditions change. General characteristics of soils Most relevant constituents

Soils are porous media of variable depth formed at the surface of the earth which are undergoing change as a consequence of chemical, physical and biological processes. Soils are stratified into soil horizons produced by the continuous influence of percolating water and living organisms. From a chemical point of view soils are multicomponent, open, biogeochemical systems containing solids, liquids and gases. Approximately one-half to two

42

CHAPTER 3

thirds of the soil volume is made up of solid matter. Of this matter generally more than 90% consists of inorganic compounds except for peat and manure in which organic matter accounts for >50% of the solid matter. The most abundant compounds are silicates namely quartz, feldspar, mica, amphibole, piroxene and olivine, which are known as primary materials because they come from the parent rock and they are very resistant to weathering. These components are abundant in sandy soils. Other common minerals in soils are the so called secondary minerals because they result from weathering. These secondary minerals form the colloidal inorganic particles. Silicate secondary minerals are abundant in clays (kaolinite, smectite, vermiculite, chlorite). Other secondary minerals include iron, aluminium and manganese oxides (gibbsite, goethite, hematite ferrihydrite, birnessite) and calcium carbonate or sulphate (calcite, gypsum). The extent of weathering and the type of minerals present determine the type of soil formed and its subsequent properties. Organic matter with a high structural complexity is an important constituent of the solid phase. Organic matter is present mainly in the top soil horizons and consists of dead and living substances such as plant litter, decaying plants and numerous organisms. The microbiologically transformed organic matter is known as humic substances the most studied substances of which are humic and fulvic acids. The coating of soil minerals by humus plays a major role in the cycling of chemical elements and in the formation of soil aggregates. Humic substances influence the water-holding capacity of a soil, its ion exchange capacity and its ability to bind to metal ions. The fluid phases of the soil are soil air and soil water. The first one has generally a similar composition to atmospheric air. Soil water is a repository for dissolved solids generally called soil solution with the compounds dissociated in ions being the most important ones in this phase. The soil solution is in equilibrium with atmospheric oxygen and so metals are in their highest naturally occurring oxidation state. In some cases when no oxygen is present, such as in flooded conditions or in the presence of high amounts of reducing components such as organic matter, or when reducing bacteria are present, the oxidation state of some metals may be changed and their solubility characteristics may change significantly. In soil profile studies, changes in redox potential are relevant. In the liquid phase of the soil dissolved organic matter plays an important role. It has a different chemical composition according to its origin from for example natural humic substances or synthetic organic substances originating from industrial or agricultural activities.. Each group of compounds is highly heterogeneous in composition and has a different interaction with trace elements. The pH and the macroelement concentration in the soil solution, mainly the sodium content, play an important role in the solubilization of metals from the solid phase.

Soil properties Several properties of soil are relevant in understanding the retention-release processes of inorganic compounds from soils. The most relevant properties include mineral solubility, particle size distribution, soil particle surface reactivity, soil adsorption phenomena, ion exchange capacity and soil pH.

Mineral solubility When water enters a dry soil it begins at once to hydrate the surface of the solid phase present. The water molecules are attracted to ionic constituents of the minerals and begin to form

CHAPTER 3

43

solvation complexes with them. The solvated ions will detach readily from the mineral structure and diffuse into the soil solution. These ions may form soluble complexes with other solutes.

Particle size distribution Many of the soil properties are related to the surface area exposed to interaction with the soil solution. The smallest size fraction of a soil is known as the clay fraction which contains clay minerals and other particles less than 21.tm in diameter. The particle size distribution of minerals in soils is of considerable relevance to the cationic exchange capacity and drainage characteristics.

Soil particle surface Both organic and inorganic soils have surface functional groups such as carboxyl groups in organic soil particles and the hydroxyl groups aluminol and silanol in inorganic soil particles. These functional groups are responsible for the reactivity of the soil particles such as complex formation between particle surface and constituents of the soil solution.

Adsorption phenomena Adsorption of inorganic pollutants on soil particle surfaces can take place via three mechanisms: inner sphere complex, outer sphere complex and diffuse ion. The inner sphere surface complex includes compounds formed between an element and the functional groups of the weathered materials. This type of interaction involves both ionic and covalent bonds and produces a specific adsorption. The two other mechanisms involve ions which are fully dissociated from surface functional groups and involve almost exclusively electrostatic bonding producing non specific adsorption. Adsorption properties play a definitive role in the interactions between trace elements and inorganic soils.

Ion exchange capacity The ion exchange capacity of a soil is its ability to hold and exchange ions. It is the number of moles of adsorbed ion charge per unit mass of soil that can be desorbed under given conditions. Organic matter and clays are effective ion exchangers. Exchangeable ions in soils are those than can be replaced easily by leaching with an electrolyte solution of prescribed composition, concentration and pH. Only fully solvated ions adsorbed on soils, that is ions electrostatically bonded to soil surface, are exchangeable ions. Soil pH

Soil pH affects the degree of surface charge on colloidal sized soil particles, high pH is generally associated with negatively charged surfaces whereas low pH is associated with positively charged surfaces although some soils may have a lot of negative charges at low pH. The tendency for adsorption of anions or cations is thus dependent on the pH of the soil solution. The chemical processes that influence naturally the pH of the soil solution are carbonic acid, acid-base reactions of soil humus, aluminium hydroxy polymers and mineral weathering reactions. Moreover acidity can be introduced in a soil as an inorganic pollutant from anthropogenic origin. The ion uptake or release by plant roots are important biological processes in soil acidity. Plants often take up more cations from soil than anions, with the result that protons are excreted to maintain charge balance. As a consequence soil pH may

44

CHAPTER 3

change in the short term.

Typical composition of soils The most abundant elements in soils are: oxygen, silicon, aluminium, iron, carbon, calcium, potassium, sodium, magnesium and titanium. The elements in soils are often classified as macroelements or microelements according to their concentration in the soil solution. The macroelements comprise carbon as bicarbonate, nitrogen as nitrate, silica as silicic acid, sulphur as sulphate, chlorine as chloride, sodium, potassium, calcium, and magnesium as ions and oxygen. The elements whose concentration in uncontaminated soil solutions are below lmmol m "3 are termed microelements [Sposito, 1989]. Examples of microelements include phosphorous, aluminium and manganese. A trace element is any element whose concentration in a solid phase is less than or equal to 100 mg kg "l. In unpolluted soils trace metals exist mainly as relatively immobile species in silicates and primary minerals but as the result of weathering the trace element content is gradually mobilised to forms accessible to plants. In polluted soils metals are mainly in non silicate bound forms and contribute to the pool of potentially available metals. The most relevant trace elements are :boron, vanadium, titanium, cadmium, chromium, cobalt, nickel, copper, zinc, molybdenum, arsenic, selenium, lead. Some of these trace elements such as copper, chromium, nickel and zinc are essential for plant growth but they are toxic at high levels. Others such as cadmium and lead are considered non essential and are potentially toxic. Leaching tests are applied to obtain information in respect of both types of constituents: nutrients/major elements and microelements. As far as nutrients and major elements are concerned, nitrogen as nitrates, phosphorous as phosphates and boron are the compounds considered most relevant and whose extractability is frequently determined. As pollutants many elements may be considered relevant although fifteen elements comprising aluminium, arsenic, cadmium, cobalt, chromium, copper, fluorine, mercury, lithium, manganese, nickel, lead, selenium, thallium, and zinc are the most frequently monitored. In some circumstances other elements or compounds are considered as pollutants. These are barium, chlorine, caesium, cyanide, sodium, antimony, tin, strontium, vanadium and to a lesser extent platinum, silver and iodine. The reasons for the relevance of the elements monitored are: their potential toxicity to biological life, directly or indirectly, their impact on the quality of the vegetation and on animals and their impact on the quality of water and on the quality of air. In the case of atmospheric impact this is due to the presence of volatile compounds.

Release controlling mechanisms The scenario in which the pollutants are mobilised from soils is governed by : the solubility of the soil solid phase which is governed by soil chemical processes. As a first approximation it may be assumed that there is an equilibrium between the total element content in the solid phase, the fraction of the element participating in the solid/liquid equilibrium and the solid-liquid distribution coefficient of the element in each specific scenario. This equilibrium may fail in certain circumstances such as when a large input of an element or compound occurs in a short time, for example in an

CHAPTER 3

45

accidental situation or in conditions which are favourable for soil weathering. uncontrolled infiltration of water through the material, such as in raining or flooding events as shown in Figures 3.1 and 3.2. In the field the mechanisms controlling the transport of pollutants through the soil to the groundwater are percolation and particle transport, commonly called run-off, although diffusion from soil particles plays a role. The hydraulic, physical and chemical properties of a soil can affect the quantity of inorganic contaminants in runoff waters.

Figure 3.1. Mechanisms controlling the mobility of pollutants through the soil

rain wofer i V

V

V

SOIL III .

|

.

im

.

.

m

.

.

m

.

4 , 4 , 4 , ~CO~TiON grounoter II

i---i!

__

III

I IIIII

IIII

__

i ii

ii

The objective of leaching procedures carried out in the laboratory is to simulate field scenarios. To establish the mechanisms controlling element leaching in laboratory tests two different types of leaching tests need to be considered, namely those carried out in batches and those carried out in columns. In the first type of test only solubility is considered relevant in controlling release processes whereas in column tests percolation and to some extent diffusion are dominant.

Relevant release controlling parameters Soil leaching in the field is highly dependent on three aspects: the properties of the solid phase, the characteristics of the liquid phase and the environmental conditions. As far as the properties of the solid phase are concerned parameters such as granulometry, drainage, porosity and structure together with the mineralogical and chemical composition and the soil properties described previously are fundamental. In relation to the liquid phase in contact with the soil solid phase parameters such as pH, redox potential, dissolved organic carbon and complexation capacity are the most relevant. Other general parameters which affect mobility of pollutants in the field are water regime, rainfall and evaporation and biological aspects.

46

CHAPTER 3

Figure 3.2. Mechanisms controlling the mobility of pollutants during flooding events

water

tt II

---

-

-

~OIL

groun~vater In the laboratory, the results obtained in the tests carried out depend on the experimental conditions and some parameters need to be strictly controlled. For batch experiments it is generally agreed that the most relevant controlling parameters are the composition, concentration and pH of the extractant solution, its ionic strength, the ratio mass of soil sample/volume of extractant, the shaking time, shaking intensity and the type of shaking device used, the temperature, the separation procedure for liquid/solid phases (centrifugation or filtration), the atmosphere in which the procedure is carried out (air or inert gas) and the volume of air in the shaking bottle. For column experiments some of the relevant parameters are identical to those for batch procedures such as the type, concentration and pH of the eluant solution, its ionic strength, the temperature, and the atmosphere in which the procedure is carded out (air or inert gas), but others are specific to this approach such as the elution rate, the percolation time, the drainage of the column, the filling procedure, the particle-eluate separation and particually the type of column used that is the material, the design and the dimensions.

State of the art in the leaching of soils

Commonly used methods During the last decades analytical methods based mainly on leaching procedures directed at the assessment of metal mobility and bioavailability have been developed and modified. In this respect two groups of tests must be considered: the single reagent leachate test which comprises one extraction solution and one soil sample and sequential extraction procedures which comprises the sequential use of several extraction solutions with the same soil sample. Both types of extraction are applied using not only different extracting agents but also different laboratory conditions. This leads to the use of a large number of extraction procedures. In Table 3.1 a summary of the most common leaching tests used in soil science is shown. As shown in the table the single extraction tests used include a large spectra of extractants. These range from very strong acids, such as aqua regia, nitric acid or hydrochloric acid, to neutral unbuffered salt solutions, mainly CaCI2 or NaNO3.. Other extractants such as buffered salt solutions or complexing agents, because of their ability to form very stable water soluble complexes with a wide range of cations, are frequently applied. Hot water is also used for the extraction of boron. Basic extraction by using sodium hydroxide is used to assess the influence

CHAPTER 3

47

of dissolved organic carbon in the release of heavy metals from soils. A large number of extractants are reviewed by Pickering [ 1986] and Lebourg [ 1996]. Some of these methods have been adopted officially in different countries with different objectives. An summary of the adoption of these methods is provided in Table 3.2. The increasing improvement in the analytical techniques used for element determination in an extract together with the increasing evidence that exchangeable metals correlate better with plant uptake has led to the evolution of extraction methods towards the use of less and less aggressive solutions [Gupta 1993]. These solutions are sometimes called sott extractants and are based on non buffered salt solutions although diluted acids and complexant agents are also included. Neutral salts dissolve mainly the cation exchangeable fraction although in some cases the complexing ability of the anion can play a certain role. Diluted acids solubilize partially trace elements associated to different fractions such as exchangeable, carbonates, iron and manganese oxides and organic matter. Complexing agents solubilize not only the exchangeable element fraction but also the element fraction forming organic matter complexes and the element fraction fixed on the soil hydroxides. Owing to the need to establish common schemes for a single extraction using sott extractants, the EC Standards, Measurement and Testing Program, formerly BCR (Bureau Community of Reference) has sponsored a project, the first step of which was to adopt common procedures in Europe for the single extraction of trace metals from soils. As a result of this project single extraction procedures using acetic acid, 0.43 mol.11, and EDTA, 0.005 mol 1"1 for inorganic soils and EDTA, 0.005 mol 11, and DTPA, 0.005mol 11 diethylenetriamine pentaacetic acid, 0.01mol 1~ CaCI2 and 0.1mol 1~ triethanolamine were adopted for extractable cadmium, chromium, copper, nickel, lead and zinc determination in calcareous soils [Ure 1993 and Quevauviller 1995]. Ammonium acetate has been recommended also as a third extractant for inorganic soils [Ure 1993 ii]. In order to improve the quality of the determination of extractable metal content in different types of soil using the procedures adopted previously the extraction procedures were validated by means of intercomparison exercises [Quevauviller 1995]. The lack of suitable certified reference material for this type of study did not enable the quality of the measurements to be controlled. To overcome this problem three standard reference materials : a terra rosa soil, a sewage amended soil and a calcareous soil were prepared and their extractable trace metal content certified. (CRM 483, CRM 484 and CRM 600). In relation to the quantification of extractable trace elements the same importance should be given to sampling and sample pre-treatment as is usually accorded to the subsequent analytical steps. To determine the leaching behaviour of one element it is essential to preserve the integrity of the extractable forms in sampling, storage and pre-treatment of the sample. As topsoils are typically oxygenated to some extent handling procedures are not mandatory. These aspects are more dramatic if the sample is an anoxic soil. In this case there is a risk of errors owing to irreversible changes of the matrix. The possibility of change depends on the type of pre-treatment applied to the sample. The errors associated with this step, according to the type

48

CHAPTER 3 Table 3.1. The most c o m m o n leaching tests used in soil science Group

Acid extraction

Extractant type and solution strength

HNO3 0.43 -2 mol. 1-1 Aqua regia HC1 0.1-1 mol. 1-1

[Novozamski 1993] [Ure 1993]

CH3COOH 0.1 mol. 1-1 Melich 1 : HC1 0.05 mol. 1-1 + H2SO4 0.0125 mol. 1-1 Chelating agents

Reference

[Novozamski 1993] [Colinet 1983]

EDTA 0.01-0.05 mol. 1"1 at different pH DTPA 0.005 mol. 1-1

[Mulchi 1992] [Novozamski 1993] [Lindsay 1978]

+TEA 0.1 mol. 1-1 CaC12 0.01 mol. 1"1 Melich 3 :

[Melich 1984]

CH3COOH 0.02 mol. 1-1 NH4F 0.015 mol. 1-1 HNO3 0.013 mol. 1-1 EDTA 0.001 mol. 1"1 Buffered salt solution

NH4-acetate, acetic acid buffer pH =7

[Ure 1993]

1 mol 1"1

NH4-acetate, acetic acid buffer pH = 4.8

[Novozamski 1993]

1 mol 1"1

Unbuffered salt solution

CaCI20.1 mol 1-1

[Novozamski 1993]

CaCI2 0.05 mol 1-1 CaCI2 0.01 mol 1-1 NaNO3 0.1 mol I" 1 NH4NO3 1 mol 1-1

[Gupta 1993] [Novozamski 1993]

AICI3 0.3 mol 1-1

[Hughes 1991]

BaCl2 0.1 mol l" 1

[Juste 1988]

CHAPTER 3

49

of pre-treatment, decrease in the following order : rapid freezing of the sample and storage under liquid nitrogen, storage in ice and drying of the sample. Because of the heterogeneity of many soils it is advisable to apply sample quality control procedures during sampling. More detailed information on the precautions to be taken to ensure good quality assurance from sampling to sample pre-treatment for trace metals determination in soils is provided in the literature [Rubio 1995]. Table 3.2. Soil extraction methods standardised or proposed for standardisation in some European countries Country

Method

Objective

Reference

[DIN 19931

Germany

1 mol.l "1NH4NO 3

mobile trace element determination

France

0,01 mol.1-1 Na2-EDTA + 1 mol.1-1 CH3COONH 4 at pH=7

available Cu, Zn and Mn evaluation for fertilisation purposes

[AFNOR 1994]

available Cu,Zn,Fe and Mn evaluation in acidic soils

[UNICHEM 1991 ]

availability and mobility of heavy metals in polluted soils evaluation

[Houba 1990]

DTPA 0,005 mol.1-1 + TEA 0,1 mol.l "1 + CaC12 0,01 mol.1-1 at pH=7,3 Italy

0,02 mol.1-1 EDTA + 0,5 mol.1-1 CH3COONH4 at pH=4,6 DTPA 0,005 mol.1-1 + TEA 0,1 mol.1-1 + CaCI2 0,0 lmol.1-1 at pH=7,3

Netherlands

I CaC12 0.1 mol 1-1

Switzerland

NaNO 3 0.1 mol 11

soluble heavy metal (Cu, Zn, Cd, Pb and Ni) determination and ecotoxicity risk evaluation

[VSBO 1986]

United Kingdom

EDTA 0.05 mol 1-1 at pH=4

Cu availability evaluation

[MAFF 1981 ]

Methods currently in development Sequential extraction schemes are now under development for soils. These schemes are designed in relation to the problems arising from disposal of solid wastes and are focused on differentiating between the different association forms of metals in the soil phases. Generally the fractions obtained are exchangeable, carbonates, reducible, oxidisable and residual fractions. The extractants more commonly used in these tests are applied generally according

50

CHAPTER 3

to the following order: unbuffered salts, weak acids, reducing agents, oxidising agents and strong acids. One of the limitations of the methodology is the lack of reproducibility of the extracted amount of metals when diluted reducing or oxidising agents are used. A new EC Standard, Measurement and Testing project including a feasibility study on the adoption and validation of a sequential extraction scheme for soil samples is being undertaken currently. This scheme includes three steps: acetic acid, hydroxylamine hydrochloride or a reducing reagent and hydrogen peroxide or an oxidising reagent. The causes of non reproducibility, the performance of reagents as well as the validation of the procedure for a soil sample in order to prepare certified reference materials applying the validated protocol for soils are now under development. The validation of the extraction procedures applied currently to organic soils and feasibility studies on the preparation of a certified reference material for extractable heavy metal in these soils are also the subject of current research. Another EC Standard, Measurement and Testing project under development is focused on the comparative evaluation of methods for sampling and sample preparation in soils. The main objectives are to test and improve the comparability and reproducibility of soil sampling methods as well as to develop the scientific basis for laboratory accreditation for sampling. In relation to the determination of extractable heavy metals in soils using aqua regia as the extractant the application of microwave heating systems is now under development for use as a potential alternative extraction procedure. Test d a t a interpretation - use a n d limitations

The information derived from leaching methods is dependent on the scenario in which the test is applied that is the element of interest, the type of soil and the type of test. There is no test which is able to predict the mobility of a group of elements in a given scenario although much work has been done in this field. In all cases data need to be critically interpreted and tests need to be chosen taking into account the aim of the test. Ure [ 1993], after consultations with expert opinion in Europe, states that the most generally acceptable extractant, in relation to the extractable metal content and plant available forms, is EDTA 0.05 mol 11 or DTPA 0.005 mol. 11, both having similar roles. Generally EDTA is preferred as it extracts greater amounts of metal and is simple to prepare and use. DTPA solutions are applied only to evaluate the available metal from calcareous soils. In acidic soils the results obtained by chelating agents need to be corrected by other soil parameters such as pH or organic matter content to be related to plant availability. Complexing solutions are not well adapted for use in heavily polluted soils as it seems that the use of this type of extractant overestimates the metal available to plants. The use of diluted acids for trace metal availability evaluation seems to be unsatisfactory except for some acidic soils. The great variability of experimental conditions applied when neutral salt solutions are used as the extractant makes it difficult to define their scope. Due to the low extraction ability of these extractants they seem to be more appropriate for polluted soil. When choosing an extraction method consideration must be given to the potential of obtaining reliable and validated results. One limitation in the current test is its analytical performance. The errors derived from misinterpretetation of the protocal or from not using validated

CHAPTER 3

51

analytical methods for element determination after extraction may by one of the causes for the lack of reproducibility of the results obtained b different laboratories when a very low level of analyte is determined in the extractant solution. In a workshop held in 1992 in Sitges (Spain) the main analytical limitations in single and sequential extraction of trace metals in soils were thoroughly discussed and practical recommendations were given [Quevauviller 1993 and Griepink 1993]. These recommendations deal with sampling and sample pre-treatment, practical experience with reagents and matrices and analytical problems after extraction In respect of soil, drying samples in air at less than 40~ and not grinding but breaking down the aggregates was recommended. In respect of practical experience it was considered relevant to obtaining reproducible results to centrifuge after extraction and separate the solid and the liquid phase immediately to avoid adsorption, to use well defined and controlled shaking conditions maintaining the solid in suspension and to control carefully the final pH of the extract. In respect of final element determination in the extracts, the maintenance of the sample integrity and the use of matrix matched calibrant solutions is highly recommended. Despite the limitations described above the data obtained in leaching tests are relevant and they are used for agricultural recommendations, for decision makers in areas such as soil land use or in countermeasures application, for inventory purposes, in the prediction of long term effects and in studies related to processes occurring in soils.

Level of modelling associated with testing The model generally used is a simple linear regression model between metal concentration in the extraction media and the metal concentration in indicator plants or a log linear regression model. Gupta [ 1993 ] developed a simple model to predict the biorelevant metal concentration in anthropogenically or artificially contaminated soils and proposed NaNO3 solution as the extractant. The model proposed can be written as follows: log Mp = a + b log

(MNaNO3)

where Mp is the metal concentration in the test plant and (MNaNO3)is the metal concentration in NaNO3-soil extract (mg/kg soil) and a and b the intercept and slope of the linear regression line. This model was tested in laboratory, in greenhouse and in field experiments. Rye grass and lettuce were used as indicator plants. It was concluded that copper, zinc and cadmium in the soil extract were correlated significantly with the concentration of these metals in both indicator plants, in spite of the fact that NaNO3 solution extracts comparatively less than other extractants. Other authors [del Castilho 1993] apply ammonium acetate solution, at pH 7, to extract "exchangeable" ions and state that this solution has advantages over combining extracts (e.g. salt with pH buffer and strong complexant) because only a single salt is introduced, so the test results should be more easily modelled. Verification of test data with available field information

The applicability of a soil test is proven empirically by greenhouse and field experiments. The criteria of acceptance is a significant correlation between soil tests and plant response. From

52

CHAPTER 3

the experiments carried out many tests have been proved to be acceptable for plant uptake prediction. According to Ure [ 1993] a large number of single extractants for soils have been validated by field experiments. These include: hot water for boron, EDTA and DTPA for copper and zinc, acetic acid for cobalt and nickel, mixed ammonium acetate/EDTA for copper and zinc, ammonium acetate for molybdenum, weak neutral salts for cadmium and lead, zinc, and copper. Juste and Solda [1988] and Dider et al [1992] claimed that diluted acids are too aggressive and scarcely discriminatory for plant uptake prediction. According to Gupta [ 1993] the best relationship between the extractable amount of heavy metals from soil and the plant metal content, independent of soil characteristics, seems to be found when unbuffered electrolyte solutions are used such as sodium nitrate or calcium chloride in various concentrations. These solutions extract the heavy metal present in the soil solution and in the easily exchangeable forms in soils although calcium gives higher results than the monovalent ions. According to Novozamski et al [1993] the best relationship between the extractable amount and plant content of heavy metals independent of soil characteristics, seems until now to have been found when neutral and unbuffered electrolyte solutions are used. It has been shown that extraction with CaCI2 in various concentrations from 0.1 mol 11 to 0.01 mol 1"1 gives a good indication of the availability of different heavy metals. It has been shown that extraction with CaCI2 in various concentrations from 0.1 mol 11 to 0.01 mol 1"1 gives a good indication of the availability of different heavy metals [Novozamski 1993 ]. In this context Hogg [ 1993] found that EDTA gives an overestimate in the prediction of copper in polluted soils. In relation to the correlation between extractability from soils and plant content, it must be pointed out that not only the type of soil plays an important role in metal availability but also the type of plant used for the validation study. Therefore it is necessary both to adopt good extraction methods which are universally applicable and to establish a reference type of plant to be used in these studies.

Relationship between different tests In Table 3.3 the data obtained with a sewage amended soil applying different extractant solutions (EDTA, acetic acid and CaCI2,) together with the coefficient of variation of the mean value obtained by different laboratories are given [Quevauviller 1997]. The table shows that EDTA and acetic acid extract similar amounts of cadmium, chromium, nickel and zinc; EDTA solubilizes six times more copper than acetic acid and two orders of magnitude more lead. The ratio between the extracted amount of cadmium, chromium, nickel and zinc with EDTA or acetic acid versus calcium chloride ranges from 20 to 80 times more, whereas for copper the ratio EDTA/CaC12 is 179, data which is in agreement with the statement that EDTA overestimates copper availability. The ratio of lead extractable with EDTA versus extractable with CaC12 is 38.167 as opposed to only 1.5 for acetic acid versus calcium chloride. This different behaviour for the different metals when using different extractants must be taken into account during the interpretation of test data. Lebourg [ 1996] has compared the extractable amounts of cadmium, copper, zinc and nickel in different soils using the CaC12, NaNO3 and NH4NO3 and the levels measured in different plants. The results obtained show a similar range and variability of the log of the regression coefficient for the three extractants.

CHAPTER 3

53

Table 3.3. Extractable amounts of metals using different extractant solutions MEAN VALUE EDTA, mg/kg uncertainty (mg/kg)

MEAN VALUE acetic acid, mg/kg uncertainty (mg/kg)

MEAN VALUE CaCI2, mg/kg uncertainty (mg~g)

Cd

24.3 (1.3)

18.3 (0.6)

0.45 (0.05)

Cr

28.6 (2.6)

18.7 (1.0)

0.35 (0.09)

Cu

215 (11)

(1.6)

(0.4)

Ni

28.7 (1.7)

25.8 (1.0)

1.4 (0.2)

Pb

229

2.10 (0.25)

<0.06

(8) 612 (19)

620 (24)

8.3 (0.7)

ELEMENT

Zn

33.5

1.2

(-)

Relationship with other technical fields In the environment it is sometimes difficult to establish the borderline between some materials such as soil, polluted soil and soils amended with compost or sewage sludge. Moreover the scenario in which the pollutants are mobilised from soils - clean, polluted and amended with compost or sludge - is governed and controlled by the same mechanisms and parameters. So many leaching tests may be applied similarly to all of these materials and it seems reasonable to make an effort to harmonise these tests. The addition of compost, sewage sludge, domestic refuse derived compost or manure to soil enhances the physical and chemical characteristics of organic matter and as a consequence there is an increase in the production of soluble complexing substances by organic matter decomposition. Some of these complexes are anionic or neutral and so they are mobile in soils. The addition of stabilised waste for landfill application may introduce relevant amounts of pollutants on a long term basis due to a process similar to weathering. Outlook for the future According to Forstner [1993] despite the advantages of a differential analysis over investigations of total metal concentrations and the fact that sequential chemical extraction is a more useful tool for predicting long-term adverse effects from polluted solid material, it has became obvious that there are still many questions to be answered with these procedures for example:

1)

reactions are not selective and are influenced by the duration of the experiment and by the experimental conditions - mainly the ratio of mass of solid matter to volume of

54

CHAPTER 3 extractants. An excessive solid content, together with an increased buffering capacity may cause a system overload; such an effect is reflected, for example, by changes in pH-values in time-dependent tests.

2)

labile fractions could be transformed during sample preparation and during the application of sequential extraction schemes.

3)

analytical problems due to the low level of analyte.

As a conclusion it can be said that there is a need to have available data not only on the extensive properties of soil such as organic matter content, clay content, texture or ion exchange capacity but also on intensive properties that may be related to metal retention/interaction with soils since these properties are of key importance in predicting metal behaviour in soils. Moreover there is a need for harmonisation and validation of the methods used and once the methods are validated there is a need for the preparation of new certified reference materials.

REFERENCES TO CHAPTER 3

55

REFERENCES

AFNOR (Association Francaise de Normalization). 1994 AFNOR Paris 250p E. Colinet, H. Gonska, B. Griepink and H. Muntau. EUR Report 8833 EN 1983 ,pp 57 P del Castilho and I. Rix. Inter. J. Environ. Anal. Chem. 51, 59-64 (1993) V. Didier, A. Gomez, M. Mench, P. Manson and C. Lambrot. Convention de Recherche 90231 INRA/Minist6re de l'Environnement, INRA Centre de Bordeaux (1992). U. F6rstner. Intern. J. Environ. Anal.Chem, 51, 5-23 (1993) DIN (Deutches Institut fi~r Normung) 1993 Bodenbeschaffenheit. Vornorm DIN V 19730. in Boden - Chemische Bodenuntersuchungsverfahren, ed DIN Berlin, 4p. B. Griepink. Inter. J. Environ. Anal. Chem. 51, 123-128 (1993) S. K. Gupta, C Aten. Inter. J. Environ. Anal. Chem. 51, 25-46 (1993) D. Hogg, R.G. Mc Laren, R.S. Swift. Soil Sci. Soc. Am. J. 57, 361-366 (1993) V.J.G. Houba, I. Novozamski, T.X. Lexmon and J.J. Van der Lee Common. Soil Sci. Plant Anal.21,2281-2291. (1990) J.C. Hughes and A.D. Noble. Common. Soil Sci. Plant Anal. 22 1753-1766 (1991) C. Juste and P. Solda. Agronomie 8,10, 897-904 (1988) A. Lebourg, T. Sterckeman, H. Cielsielki, N. Proix. Agronomie, 16, 201-215 (1996) W. L. Lindsay and W.A. Norvell. Soil Sci. Soc. Am. J. 42, 421-428 (1978) MAFF(Ministry of Agriculture, Fisheries and Food) 1981. Reference Book 427 MAFF. London. A. Melich. Common. Soil Sci. Plant Anal. 15, 1409-1416 (1984) C.L. Mulchi, C.A. Adamu, P.F. Bell and R.L. Chaney. Common. Soil Sci. Plant Anal 23, 1053-1059(1992) I. Novozamski, Th.M. Lexmon and V.J.G. Houba. Inter. J. Environ. Anal. Chem. 51, 47-58 (1993) W. P. Pickering. Ore Geological Reviewa, 1, 83-146 (1986) Ph. Quevauviller, G. Rauret and B. Griepink. (1993)

Inter. J. Environ. Anal. Chem. 51, 231-235

56

REFERENCES TO CHAPTER 3

Ph. Quevauviller, G. Rauret, A. Ure, R. Rubio, J-F L6pez-Shnchez, H. Fiedler and H. Muntau. Mikrochimica Acta 120, 289-300 (1995). Ph. Quevauviller, G. Rauret, A. Ure, J Bacon and H. Muntau. Eurpoean Commission. Brussels. 1997

Report EUR 17/27 EN.

Ph. Quevauviller, M. Lachica, E. Barahona, G. Rauret, A. Ure, A Gomez, H. Muntau. The Science of the Total Environment. 178 (1996) 127-132 R. Rubio and M. Vidal. Quality Assurance in Environmental Monitoring. Verlaggesellshatt mbH. (1995), Ph. Quevauviller ed pag 157-178. G. Sposito. The chemistry of soils. Oxford University Press Inc 1989. A.M. Ure, R. Thomas and D. Litlejohn. Inter. J. Environ. Anal. Chem. 51, 65-84 (1993) [ii] A.M. Ure, Ph. Quevauviller, H. Muntau and B. Griepink. Inter. J. Environ. Anal. Chem. 51, 135-151 (1993) [i] UNICHIM (Ente Nazionale Italiano di Unificazione) 1991. UNICHIM Milan VSBo (Veordnung tiber Schadstoffgehalt im Boden) 1986 Nr. 814.12, Publ. eidg.Drucksachen und Materialzentrale, Bern, 1-4.

CHAPTER 4 CHAPTER

4: C O N T A M I N A T E D

57 SOIL

Introduction

The degree of attenuation of a pollutant by soil depends to a large degree upon the water content of the soil. Above the water table there is an unsaturated zone of soil in which attenuation is enhanced relative to the saturated zone. Normally soil has a greater surface area at liquid-solid interface in this zone so that sorption and ion-exchange processes are favoured. Aerobic degradation is possible in the unsaturated zone, enabling more rapid and complete degradation of biodegradable substances. Heavy metals are particularly damaging to groundwater and their movement through the soil is of considerable concern. Heavy metal ions may be sorbed by the soil, held by ion-exchange processes, interact with organic matter in the soil, undergo oxidation-reduction processes leading to mobilization or immobilization or be volatilized as organometallic compounds formed by methylating bacteria. A large number of factors affect heavy metal mobility and attenuation in soil. These include pH, pE, temperature, cation exchange capacity, the nature of soil mineral matter and the types of soil organic matter present [Manahan 1994]. Assessment of leaching from contaminated soil

The application of leaching procedures to polluted or naturally contaminated soils is mainly focused to ascertain the potential availability and mobility of pollutants, pollutant migration in order to assess pollution levels in contaminated areas and in survey programmes. Leaching tests are used for predicting deficiency of essential elements, for assessment of potential toxicity, to ascertain the probability of disease generation and in studies about the physicochemical behaviour of elements in soils. The data obtained when applying these tests are used for decision makers in topics such as land use of a soil or in the application of countermeasures. They are also used for inventory purposes, in the prediction of long term effects and in studies related to processes occurring in soils. The scenario in which the pollutants are mobilized from soils is governed primarily by an uncontrolled infiltration of water through the material. Characteristics of contaminated soil

The general principles that determine the total content and the occurrence of trace elements in soils are related primarily to the geochemical character of the rocks from which they are derived. For some elements the total content may vary by a hundred or even a thousandfold from soil to soil depending upon the origin. The total contents of the major constituents (oxygen, silica, aluminium, iron, calcium, sodium, potassium, magnesium, titanium and phosphorus), however, which together constitute over 99% of the total elemental content of the earth' s crust, seldom vary from soil to soil by more than five fold [Ure 1982]. Soil contamination is usually caused by industrial and human activities but can also be due to a variety of natural phenomena. The definition of contaminated soil will usually differ widely from country to country, but in this context contaminated soil is defined as a soil which

58

CHAPTER 4

contains contaminants in concentrations which exceed certain threshold values defined by regulatory authorities or based on risk assessments. The inorganic contaminants in soils arsenic, cadmium, chlorine, cobalt, chromium, copper, cyanide, fluorine, mercury, nickel, lead, antimony, selenium, tin, thalllium, zinc are considered relevant in connection with the potential toxicity of each substance to biological life directly or indirectly, the impact on the health of vegetation and animals, the impact on the quality of water and on air quality. In the case of air quality the presence of volatile compounds is the key factor.

Soil polluting processes The process of soil pollution comprises the introduction of polluting substances into the soil. Pollution may take place in one or more different phases and will result in the formation of one or more static incidents of soil contamination. The final spatial distribution of the polluting substances in the soil profile depends on many different and location-specific factors, for example soil layering, texture and structure, the presence of permeable or impermeable layers, water saturation, grain size distribution, the presence of permafrost or specific alteration products, etc. Pollutant distribution can be grouped into two main classes namely homogeneous or heterogeneous distribution [ISO 1994]. In the first place it is essential to determine whether it is reasonable to expect whether a site is contaminated or not. At this stage judgement should be made on whether the site should be categorised as "probably uncontaminated" or as "potentially contaminated". This is done by establishing whether potentially contaminating activities have been present, or could have been present, on the site. For example signs of past or present industrial activities, any evidence of the presence of storage tanks, of waste disposal or of intensive use of pesticides etc., may lead to the conclusion that the site is "potentially contaminated" [ISO 1994]. In the case of a "potentially contaminated" site the information should be defined in more detail with regard to the type of pollution, its spatial distribution throughout the site, possible spreading and migration, homogeneity and the likely nature number of contaminating substances. In cases where more than one potentially contaminating substance is anticipated, a separate assessment should be carried out for each individual substance, and the following factors should be taken into account: the nature of the source and the manner in which the pollution has entered the soil (diffuse or spot contamination); where in the soil or groundwater the pollution is to be found on the basis of the expected spreading processes (in both a horizontal and a vertical direction) depending on:

the nature of the pollutants: solubility in water, interaction with clay (mainly by metals) and interaction with organic matter in the soil (mainly by organic compounds); the soil stratification (highly pervious sandy soil or highly impervious clay or peat);

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59

the period over which the pollution has been in the soil; the depth of the groundwater table. In a highly permeable sandy soil which has been the subject of pollution for a long period of time, a mobile substance that is loosely combined with the sandy soil will have migrated over a wide area. In this case it is likely that a more homogenous distribution of the polluting substances will have occurred [ISO 1994]. In a highly impermeable clay soil, or alternatively in a more pervious peat that has only recently been subjected to pollution with a substance that combines strongly with the soil (adsorption), the contaminating substance or substances will have migrated over a limited area and depth and often preferentially into certain portions of the available space. In the latter case a more heterogeneous distribution of the pollution will normally have occurred while the spatial extent of the contaminated area will normally be limited [ISO 1994]. Soil-contaminant interaction

Evaluation and analysis of contaminant-soil interaction provides us with an insight into the various processes which control the release, accumulation, transport and fate of the contaminants. Studies of these processes have been conducted by various scientific disciplines e.g. soil science, aquatic chemistry, clay science, geochemistry, colloid science, surface chemistry, electrochemistry etc. [Yong 1992]. Water is the primary carrier or transport agent for contaminants. The liquid phase of a soilwater system consists of water and dissolved substances such as free salts, solutes, colloidal material, and/or organic solutes. All dissolved ions and probably all dissolved molecules are surrounded by water molecules. The principal constituents to be considered in the basic interactions include solutes, e.g. ions, molecules, substances in the pore fluid, the aqueous phase, e.g. pore fluid considered essentially as a solvent and solid surfaces e.g. soil solids, minerals, amorphous materials, soil organics, etc. The processes of transfer from the aqueous phase of soil to the soil solid particle surfaces are grouped as processes of sorption, complexation or precipitation. Sorption can be further divided into physical adsorption, occurring principally as a result of ion-exchange reactions, and chemical adsorption, which involves short-range chemical bonds. Complexation involves association with organic or inorganic ligands. Precipitation involves the formation of new solid phases. The mechanisms of interactions between contaminants and soil constituents are greatly influenced by the chemistry of the soil constituents, the contaminants and the pH of the system. Clay minerals differ in cation exchange capacity and in their capacity to adsorb ions. For example large organic cations are adsorbed more strongly than inorganic cations by clays. The general term sorption is used to indicate the process in which the solutes are partitioned between the liquid phase and the soil particle interface. Of the various phenomena that can contribute to sorption, chemical interactions constitute the major subject of interest in contaminant-soil interactions. When it is difficult to distinguish between the mechanisms of physical adsorption, chemical adsorption and precipitation, the term sorption is used to indicate the general transfer of material to the interfaces [Yong 1992].

60

CHAPTER 4

With regard to contaminant-soil interaction, the adsorption reactions which occur are processes by which solutes of contaminants in solution become attached to the surface of soil particles through mechanisms which seek to satisfy the forces of attraction from the particle surfaces. These processes are governed by the surface properties of the soil solids (inorganic and organic) and the chemistry and physical chemistry of the contaminant leachate and its constituents, e.g. cations, anions and non-ionic molecules. The net energy of interaction due to the adsorption of a solute ion or molecule onto soil constituent surfaces is the result of both short range chemical forces such as covalent bonding and long range forces such as electrostatic forces [Yong 1992]. Physical adsorption occurs when the contaminants in the aqueous soil solution are attracted to the surface of the soil constituents because of the attractive forces of the soil particles. Cations and anions are specifically or non-specifically adsorbed by the soil solids. In nonspecific cation adsorption the ions are held primarily by electrostatic forces for example the adsorption of most of the alkali and alkaline earth cations by the clay minerals. The replacement of exchangeable cations involves those cations associated with the negative charge sites on clay soil solids through largely electrostatic forces. Ion exchange reactions occur in the various soil constituents, i.e. clay minerals and non-clay soil fractions. The solutes in the pore water that will react with the "charged" soil particle surfaces include cations, anions and non-ionic molecules. In general, we can consider non-specific adsorption as adsorption occurring as a result of electrostatic attraction [Yong 1992]. Polymeric hydroxyl cations are adsorbed in preference to monomeric species because they have a higher positive charge than monomeric ions and because the electrostatic forces between them and clay mineral particle surfaces are much stronger. In addition, they have lower hydration energies than monomeric ions and they are larger. This type of adsorption is called specific adsorption and the exchange of a number of monomeric species for one polymeric cation involves an increase in number of ions together with several adsorbed water molecules, thus increasing the entropy of the system [Yariv 1979]. Chemical adsorption refers to high affinity, specific adsorption which generally occurs through covalent bonding. The three principal types of chemical bonds between atoms are ionic bonding, where electron transfer between atoms results in an electrostatic attraction between the resulting oppositely charged ions, covalent bonding, where there is more or less equal sharing of electrons, and co-ordinate-covalent bonding, where the shared electrons originate only from one partner. Chemical adsorption reactions can be either endothermic or exothermic and usually involve activation energies in the process of adsorption, that is the energy barrier between the molecule being adsorbed and the soil solid surface must be surmounted if a reaction is to occur. Strong chemical bond formation is often associated with high exothermic heat of reaction [Yong 1992]. Adsorption of anions by soil particles can occur as specific adsorption for example a ligand exchange reaction where anion displacement of O H or H20 occurs and becomes a ligand in the co-ordination of the structural cations. The hydrous oxides and hydroxides of iron and aluminium are typical examples of soil materials which allow specific adsorption of anions. The adsorption of anions can also be non-specific e.g. held by electrostatic forces [Yong 1992].

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61

Complexation occurs when a metallic cation reacts with a ligand. The inorganic ligands which will complex with the metallic ions include most of the common anions, e.g. OH', CI', SO 42, CO 32", PO 33", CN', etc. The complexes formed between the metal ions and inorganic ligands are much weaker than the complexes formed with organic ligands [Stumm 1981 ]. As might be expected, the organic component of soil constituents has a high affinity for heavy metal cations because of the presence of ligands or groups that can form chelates with the metals. The order for the stability of heavy metals complexes is as follows [Jones 1981]: Cu 2+ > Fe 2+ > pb 2+ > Ni 2§ > Co 2§ > Mn 2§ > Zn 2§ Precipitation is the converse of dissolution. The process occurs in two stages: nucleation and particle growth. Precipitation can occur on the surfaces of the solids or in the pore water. Since both adsorption and precipitation are concerned with the removal of substances from the aqueous phase a distinction between the two processes is not always easy to obtain. The pH of both the soil and the soil pore water and the concentration of the solutes are important factors which control precipitation. Precipitation is regarded as a major factor in the retention of heavy metals in soils.

Buffering capacity Leachates containing inorganic and organic contaminants interact with the soil solid constituents through processes of sorption, precipitation and/or complexation of the contaminants resulting in the accumulation of contaminants in the soil. For the soil substrate to act as a proper buffer against the transport of the contaminants it is important to understand how the contaminants are "held" by the soil since this will tell us how strongly or how "permanently" they are "fixed" to the soil. A useful way of determining the retention capacity of the soil is to determine its buffering capacity. A soil does not completely sorb all the contaminants from the solution. There is an equilibrium between solvent and solution phases. The amount lett in solution gradually increases as the buffering capacity of soils is approached. The term is used both from the viewpoint of a chemical buffering system which describes the capability of the system to act as chemical barrier against the transport of contaminants and also as a physical system where pore constrictions and pore blockage can play major roles, especially in regard to the transport of large organic molecules or polymers [Yong 1992]. The buffering capacity is usually expressed in terms of moles per litre of strong base or strong acid which when added to solution causes a unit change in pH. The buffering capacity of a soil determines the potential of a soil for effective interaction with leachate contaminants and is more appropriate for inorganic soils and inorganic contaminant leachates. When the pH of the soil system falls rapidly upon addition of acid, we can interpret this to mean that the soil system shows poor ability to adsorb acidic contaminant leachates. The same kind of titration procedure can be buffering capability for alkaline leachates. The resistance of a soil solution to a pH change depends on the equilibrium established between the adsorbed H § ions by the soil solids and the H § ions in the pore water. The pH values between 1 and 4 are not generally used for evaluation of the buffering capacity since dissolution effects are dominant. Silicate minerals, carbonates and gibbsite exhibit strong buffeting upon addition of strong acid under slightly alkaline to slightly acid conditions [Yong 1992].

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CHAPTER 4

Retention of heavy metals Studies of the retention of heavy metal ions using pure clay mineral soil solutions (kaolinite, illite and montmorillonite) indicate a higher retention capacity by various clay suspensions with increasing pH. The retention capacity is markedly increased when the soil solution pH exceeds the value required for precipitation or formation of metal hydroxy species. The retention capacity for the clay minerals shows that for the same mineral solids concentration, kaolinite retains less than illite and illite in turn retains less than montmorillonite consistent with the cation exchange capacity and the surface area of the clays [Yong 1992]. The retention mechanisms of heavy metals in soils are different at different pH values. The ability of soil to retain heavy metals depends on the resistance of the soil to a change in pH i.e. the buffering capacity. For example, heavy metals may be retained in soils in the form of oxides, hydroxides, carbonates, exchangeable cations, and/or bound to organic matter in the soil depending on the conditions of the local environment and on the kinds of soil constituents present in the soil-water system. The use of sequential extraction as a procedure for the determination of the mechanism for the retention of heavy metals in a soil is still a valuable tool since the results obtained can help identify the retention mechanisms for heavy metals at different buffering conditions of the soils [Tessier 1979]. Selective adsorption of heavy metals, sometimes referred to as affinity or selectivity, refers to the situation where one heavy metal is preferred over another species in adsorption by a soil. Selectivity of heavy metal retention shows for the case of an illite soil, which contains some carbonates and organics, the following: Pb > Cu > Zn = Cd. For montmorillonite, two different patterns are obtained: when the soil solution pH is <3, then Pb > Cd > Zn > Cu; and when the soil solution pH is > 3, then Pb > Cu > Zn > Cd [Yong 1992]. The differences in selective adsorption are due to the differences in soil and heavy metal properties. The ionic size of the heavy metals exerts a dominant influence on the selectivity order. The strength with which cations of equal charge are held to the soil particle surfaces is in general inversely proportional to the hydrated radii [Yong 1992]. Since sorption of heavy metals is not exclusively via physical adsorption mechanisms the sequence or order of selectivity will depend on other factors besides the unhydrated radii and the softness of the metal ion [Stumm 1981 ]. At high pH levels, the aqueous metal cations hydrolyse resulting in precipitation and in a suite of soluble metal complexes which can be represented by the generalised expression for divalent cations as : M 2+(aq) + nH20 r

M(OH)n 2-n + nH+

Chloride ions, sulphates and organics may form complexes with heavy metals and interfere with their adsorption by soil particles. Donor [1978] shows that the transport of nickel, copper and cadmium in leaching cell experiments was 1.1 to 4 times greater in the presence of

CHAPTER 4

63

CI than in the presence of CIO4". Studies on cadmium adsorption by soil particles indicate that the cadmium which was not adsorbed by the soil was in the form of CdCI2~ CdC13 and CdCI42". The mononuclear complexes formed between a central metal ion and a number of ligands may be positive, negative or neutral. Cadmium combines with chloride ions to form the following complexes: CdCI § CdCI2 o, CdC13 and CdCI42". As the concentration or activity of free chloride ions increases, the concentration of the Cd-CI complex increases. Research evidence shows that Cd 2+ is adsorbed more readily than CdCI +. There is also evidence to indicate that ion pairs such as CdCI § are adsorbed onto clay mineral surfaces and that neutral and negative ion pairs, such as CdCI2~ and CdC31"are not measurably adsorbed by kaolinite. As the pH increases, there is a tendency for cadmium to be removed from solution as hydroxides, or to precipitate onto the clay surface [Yong 1992]. Release processes in leaching

Another aspect which is important to identify are the relevant processes which control release from contaminated soils. Two types of approaches can be considered in this context, the processes controlling leaching of elements to water or water bodies in the fieM and the processes controlling the leaching process in the tests performed in the laboratory. The main parameters controlling the release of elements in the field are percolation, diffusion and panicle or colloidal transport. For soils percolation and particle transport, commonly called run-off, is considered of paramount importance although the role of diffusion is not negligible. In Table 4.1 the most relevant controlling processes in laboratory experiments are summarised. To establish the parameters controlling element leaching in the laboratory tests, two different types of leaching tests are considered, those carried out in batches and those carried out in columns. In batch tests only solubility is considered as relevant in controlling the release process, whereas in column tests it is considered that percolation dominates although diffusion must be considered relevant as well. With regard to relevant release controlling parameters two approaches can be considered as well, the parameters which control release in the field and those which control release when leaching tests are applied in the laboratory. Several parameters can be considered for leaching in the field. Some of them are related to the material, such as physical properties (granulometry, drainage, porosity and structure), composition, sorption properties and cation exchange capacity. Other parameters are related to the environment or to the liquid phase in contact with the material, such as pH, redox potential, dissolved organic carbon and complexation capacity. Other general parameters which can be considered are the water regime, rainfall and evaporation and biological factors. For soils pH, redox potential and rainfall or evaporation are the most relevant to leaching. Other parameters which can be considered relevant are the physical and sorption properties, the chemical composition and the cation exchange capacity of the soil. Less relevant but not negligible are dissolved organic carbon, complexation properties of the liquid phase and biological factors.

64

CHAPTER 4 Table 4.1 The most relevant processes controlling leaching in the laboratory Batch tests

Time Soil water content Ratio of liquid/solid Shaking intensity Leachant composition Ionic strength Type of shaking Separation liquid/solid Air of inert atmosphere Volume of air in the shaking bottle

Column tests

Elution rate Soil water content Time Filling procedure Particle eluate separation Ionic strength Column material Column design Column dimension

There is no doubt that the results obtained in laboratory tests depend on the experimental conditions some of which must be more strictly controlled than others. For batch experiments the most relevant controlling parameters are pH, temperature, time, the ratio of mass/volume, shaking intensity, type of shaking, concentration and nature of the leachant, ionic strength, separation procedure for liquid/solid phases, atmosphere (air or inert) and the volume of air in the shaking bottle. For column experiments the most relevant parameters are pH, elution rate, temperature, time, filling procedure, particle eluate separation, ionic strength, column material, column design and column dimensions. Examples of leaching methods used in the assessment of contaminated soil

Many different leaching tests are applied using not only different extracting agents but also different laboratory conditions. This leads to the use of many extraction procedures. In Table 4.2 a summary of the most common leaching tests together with information on the rationale and the use of the data is presented. For polluted and naturally contaminated soils the extractants most widely used in simple extractions are very similar to the ones used for unpolluted soils (see Table 4.3) ie. strong acids including mixtures such as aqua regia or nitric acid which result in an extract which is related to the total metal content, complexing agents such as EDTA or DTPA, neutral salt solutions such as CaCI2 or NaNO3, weak acid solutions such as acetic acid or ammonium acetate which result in an extract related to the total exchangeable metal. The use of oxidising agents such as hydrogen peroxide is related to metals bonded to organic matter and sulphides whereas reducing agents such as dithionite or hydroxilamine are generally related to the metal fraction bonded to iron and manganese oxides. Alkaline solutions are also used. When applying sequential extraction the same options are used as for unpolluted soils. The following sections give examples from the literature of leaching methods used in the assessment of contaminated soil. Normally the leachable fraction of contaminants from contaminated soil is less than and not directly related to the total contaminant concentration.

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65

Leachingfrom stabilized contaminated soil In 1991 Wahlstrom et al. investigated leaching from stabilized contaminated soil in Finland. The goal of the project was to develop an optimal binding system for soils contaminated with wood preserving chemicals, i.e. arsenic, chromium, copper and for soils contaminated with lead from an old lead smelter. The soils from the wood preserving plant consisted of humus, sawdust, sand, clay and arsenic in the concentration range of 2400-9300 mg/kg, chromium in the range of 1600-7000 mg/kg and copper in the range of 1400-7300 mg/kg. The soil from the old lead smelter consisted of slag, peat, sand, clay and lead in the range of 43000-24000 mg/kg. Table 4.2 The most common leaching tests used for contaminated soil Field of application

Polluted soil and naturally contaminated

Objective of assessment

9Potential availability and mobility -Migration 9Assess pollution levels and survey purposes 9Prediction of deficiency, toxicity, disease .Physical-chemical behaviour

Use of data (scenario)

9Land use decision 9Counter measures application 9Inventory and long term effects 9Process studies

Frequently used leaching tests

9Strong acids (mixture) (aqua regia), HNO3, etc.) 9NEN 7343 Column test 9Complexing agents (EDTA, DTPA, etc.) 9Neutral salt (CaCI2, NaNO3, etc.) 9Weak acids (HAc, NH4Ac, etc.) *Oxidizing-reducing agents (dithionite, hydroxilamine, H202) 9(Alkaline solutions) 9(Sequential extraction)

Soil samples from both sites where homogenized and blended with a mixture (20-50% by weight) of Portland cement, coal fly ash, blast furnace slag and silica. Mortar specimens larger than 40 mm were stored for 28 days and tested for leaching using the Dutch tank test. The water used in the tests was adjusted to pH 4 with sulphuric acid. The eluates from the tests were alkaline or neutral and the test results showed that the contaminants had low to intermediate mobility with diffusion coefficients from 10"12 to 10"11 m2/s. The cumulative leached amounts per square metre during the 64 day test was 14 mg arsenic, 34 mg chromium, 73 mg copper and 75 mg lead. On the basis of these findings the stabilized soil was recommended for use in the construction of roads in municipal solid waste landfills.

Leaching from lead-contaminated urban soil In 1991 van der Sloot et al. investigated the potential for reuse of lead-contaminated urban soil in the Netherlands. Soils in the major cities contaminated with lead from paint production and traffic emissions should in accordance with Dutch regulations be treated and cleaned before reuse. For construction in urban areas this results in high cost and serious delay. The aim of the study was therefore to predict the potential for long term lead mobility in urban soil and to issue recommendations with regard to soil utilization based on the results of the study.

66

CHAPTER 4

The soil samples which were investigated had lead concentrations of between 3mg/kg to 3250 mg/kg. The availability for leaching based on test NVN 2508 was higher in sandy soils (413%) than in clay soils (1-4%). A direct correlation between the leachable quantity and the organic content was observed in both soil types i.e. an increase of organic content from 0 to 20% reduced the availability of lead from 10% to around 1%. No correlation was found between the fraction of clay (mass < 2 ktm) and the leachability of lead. The percentages of lead leached after extraction in a serial batch procedure without pH adjustment was less than 0.2% at liquid to solid ratio (LS) 100 in all the samples. The effective diffusion coefficient of lead was found to be 2.5x10 ~4 m2/s after a contact time of 327 days. Measurement of the lead mobility in soil samples spiked with Pb210 isotopes showed very low mobility under low flow conditions, even after percolating the soil with humic and fulvic substances. The conclusion of the study was therefore to recommend the reuse of moderately lead-contaminated urban soils without further isolation. Overlaying the contaminated soil with a layer of clean soil was considered sufficient protection from direct (ingestion) or indirect (skin contact) contact.

Leaching from soil exposed to contaminated waste materials In a study by Wesselink et a1.(1994), the chemical processes controlling the mobility of contaminants from waste materials in soil was investigated. Synthetic rain water at pH 4.5 and a flow rate of 0.2 LS per day was percolated through steel and phosphorous slag in large columns. The eluate from the slag columns was percolated through four different columns each holding 1 kg of soil containing 0.3%-2.2% organic carbon, at a flow rate of 0.2 LS per day. The experiments indicated that exchange processes of hydrogen ions, calcium, cadmium, lead and zinc on organic matter control the solubility of these components in the soil samples.

Leaching from natural uncontaminated soil a reference study In a study by Keijzer et al. in 1991 the content and the leachability of arsenic, barium, calcium, cadmium, cobalt, chromium, copper, mercury, potassium, magnesium, molybdenum, sodium, nickel, lead, antimony, selenium, tin, vanadium and zinc from natural, uncontaminated soil was investigated. The aim of the study was to obtain a reference or background level of metal leachability in a representative range of Dutch soils and to asses the soil parameters which control the leaching process. Topsoil samples (from depths between 0-10 cm) were obtained from 19 natural locations and were dried and sieved prior to chemical analysis and determination of leaching behaviour using a column test and an availability test. Most of the soils demonstrated elevated levels of trace metals in the first percolates from the column test. This release was accompanied with an extremely low pH of 3 to 5. However, for some elements the release showed a different pattern with delayed or constant release. The maximum available quantity of most elements was low related to the total composition, i.e. < 10%. The elemental content of calcium, cadmium, cobalt, lead, magnesium, and sodium correlated well with the release from the column test at LS 10. The release of cobalt, copper and mercury did not correlate with the organic content or the cation exchange capacity and no correlation was identified between the release at LS 10 and the clay content or pH for any element. The release of calcium, magnesium and sodium from the column test correlated well with the release from the availability test.

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67

To be able to decide on the possible reuse of contaminated soil after treatment soil has to be tested, in some cases in accordance with procedures described in regulations. For example the leaching test to be carried out in the Netherlands is the column test according to the Dutch Standard NEN 7343. In view of the duration of the test in NEN 7343 faster tests are needed for quality control. For a better understanding of factors controlling release modelling work using a geochemical transport/reaction model is in progress in which sorption onto mineral surfaces and interaction with dissolved organic carbon (DOC) is incorporated. The soil solution pH is identified as an important factor as surface reactions and interaction with DOC are strongly influenced by pH. Soils derived from soil clean-up projects carried out under the control of SCG have been subjected to column leach tests and the CEN draft compliance leaching test. Several soil parameters have been measured in these soils including the total composition (Heyen et al., in press). The results of the work are in part illustrated by the relationship between test results obtained from the column leach test and from the CEN procedure (Figure 4.1). For copper the correlation between the column and CEN methods is generally good. At low copper concentrations the variability increases, which not surprising and acceptable in view of probable regulatory limits. The first indications of correlation between the column and CEN compliance test methods are hopeful. Further work is in progress to increase the number of datapoints to enable more statistically sound conclusions to be drawn. For soils within the pH domain pH 6.5 8, leachability is generally low for all constituents of concern. An exception is cyanide, which often shows a relatively high leachability. The anionic character of this constituent is partly responsible for this. This conclusion is valid for several assessments of contaminated soils. -

Figure 4.1" Comparison of copper released from a column test and CEN draft compliance leaching test U1

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68

CHAPTER 4

Field conditions The fate of chemicals in soil can be compared with the fate of chemicals in aquatic environments. The difference is only in the degree of leaching. The terrestrial environment, like the aquatic one, can be viewed as a four-compartment system comprising air, water, solids and biota. However, whereas the terrestrial environment consists primarily of solid particles covered by thin films of water, the aquatic environment consists mainly of water in contact with small amounts of suspended solids and a large contiguous envelope of modified soil or bottom sediments. Water buffers the aquatic system against dessication and temperature fluctuations. Because terrestrial environments lack this buffer they experience large variations in temperature and moisture. Terrestrial systems have a larger reactive surface area and biotic loading and less variability in these factors than aquatic systems [Laskowski 1982]. Treatment of soils to reduce the total composition of contaminants by thermal processes or by physical/chemical extraction processes generally leads to lower leachability of metals. This is particularly true for thermally treated soil. In Figure 4.2 the leachability of zinc is plotted against the total composition. In general the trend shows decreased leachability aider treatment. It is also clear that there is no direct relationship between composition and leachability. Other factors such as soil pH and DOC are important controlling factors [Heynen et al, in press].

Figure 4.2: Comparision of total zinc composition of soil with amount leached

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CHAPTER 4

69

coated with a layer of organic matter. Because they contain a charge, they are also surrounded by clusters of positively or negatively charged ions that decline rapidly in concentration with increasing distance from the colloidal surface. The complex structural arrangement of the colloidal surfaces must be carefully considered. The outer surface of large stable aggregates is composed of groupings of smaller aggregates and discrete particles. The inner surfaces comprise smaller aggregates and individual particles buried deep within the host aggregate. Water filled pathways leading to the interior are smaller and more tortuous than those connecting the exterior aggregate surfaces. There is therefore a critical time requirement for equilibration and true expression of the properties of a chemical in relation to the total surface area of the aggregate. Such a system has the following implications: the results of extraction and leaching tests are not valid unless proven against aged samples; mobility is not totally what it appears if judged only by experiments using fresh, unaged samples; and studies that involve leaching require knowledge of the rate of water movement in the soil column or soil profile. Relationship with other fields of study Leaching from contaminated soil is similar to leaching from unpolluted soil. In Table 4.3 a summary of the most common leaching tests used for unpolluted soil together with information on the rationale and the use of the data are given. Leaching procedures using a single extractant are widely used in general soil science. These procedures are designed to dissolve a phase whose element content is correlated with the availability of the element to the plants. This approach is well established for major elements and nutrients and it is commonly applied in studies on fertility and quality of crops, for predicting the uptake of essential elements, for diagnosis of deficiency or excess of an element in a soil, in studies on the physico-chemical behaviour of elements in soils and for survey purposes. They are applied to a lesser extent to elements considered as pollutants. These tests are always restricted to a small group of elements and are designed as element or plant specific and restricted to a particular type of soil. The data obtained in such leaching tests are used for agricultural recommendations, in studies about the processes occurring in the soil, for inventory purposes and for predicting long term effects. Single extraction tests are widely used including a large spectrum of extractants from very strong acids, such as aqua regia or nitric acid, to neutral salt solutions, mainly CaCI2 or NaNO3. Other extractants such as complexing agents, EDTA or DTPA, and weak acid solutions, acetic acid, ammonium acetate are frequently applied. Hot water is used for the extraction of boron. Sequential extractions are mainly used in research activities and the extractants commonly used in these tests are applied in the following order: unbuffered salts, weak acids, reducing agents, oxidizing agents and strong acids. The procedure proposed under the Standards, Measurement and Testing Programme in the EU is a short method including three steps: acetic acid, hydroxylamine hydrochloride and hydrogen peroxide.

Practical considerations There are practical considerations which are of paramount importance in assessing leaching from contaminated soil. The first is connected with sampling and sample pre-treatment, the second to permeability testing.

70

CHAPTER 4

Table 4.3 The most common leaching tests used for unpolluted soil Field of application

Soil

Objective of assessment

9Fertility and quality of crops (diagnosis of deficiency or excess) .Physical-chemical behaviour

Use of data (scenario)

.Agricultural recommendations 9Process studies 9Survey purposes 9Inventory and long term effects

Frequently used leaching tests

9Strong acids (mixture) (aqua regia, HNO3, etc.) 9Complexing agents (EDTA, DTPA, etc.) 9Neutral salt solutions (CaCI2, NaNO3, etc.) 9Weak acid solutions (total exchangeable) (HAc, NH4Ac, etc.) 9Hot water 9Sequential extraction

Samp#ng and sample pretreatment When land is contaminated with chemicals and other substances that are potentially harmful to human health and safety or to the environment, it may be necessary to carry out an investigation as a part of a hazard and/or risk assessment i.e. to determine the nature and extent of contamination, to identify hazards associated with the contamination, to identify potential targets and routes of exposure, and to evaluate the risks relating to current and future use of the site and neighbouring land. A sampling programme for risk assessment may have to comply with legal or regulatory requirements therefore careful attention to sample integrity is needed. Sampling strategies should be developed on a site-specific basis. Determination of the extent of a contaminated area or the assessment of human and environmental risks caused by contamination may be rather complex. Because of the complexity, a soil investigation should be carried out in several consecutive steps, each step leading to a specific objective. If a risk analysis is to be carried out, the spatial distribution of the pollutants in the soil will be significant. Depending on the principal objective(s), it will usually be necessary to determine for all or part of the soil mass: 9

the nature, concentrations and distribution of naturally occurring substances

9

the nature, concentrations and distribution of contaminants (extraneous substances)

9

the physical properties and variability the presence and distribution of biological species of interest.

CHAPTER 4

71

It will often be necessary to take into account changes in the above parameters with time caused by contaminant migration, atmospheric conditions and land/soil use. Information should be collected on soil stratification and the hydrogeological situation. The scale at which this information should be collected and the degree of detail that is required can only be determined subjectively and should be in related to the objective. Considerable importance should be attributed to sampling and to sample pretreatment. It is considered essential to preserve the integrity of the extractable forms of contaminants present during sampling, storage and pretreatment of the sample. This aspect is more important if the sample is an anoxic soil or sediment. With regard to sampling, the inhomogenity of many soils or sediments has a significant influence on the number of samples to be collected in a studied area. It is recommended that quality control procedures are applied during sampling.

Laboratory methodsfor permeabi#ty testing The permeability values measured in the laboratory are not usually consistent with permeability values obtained in field tests. The ratio between field and laboratory permeability values may be as large as 1000 [Yong 1992]. The differences in values are due to several causes. In addition to the general problem of obtaining proper representative samples from the test site for laboratory testing and the problems associated with duplicating fluid flow direction and gradient, the two major points that need to be assessed to reconcile laboratory and field permeability values are given below. Laboratory testing is generally confined to small volume samples. These tend to be homogeneous and uniform (unless one deliberately seeks samples that are not) hence are not representative of actual in-situ conditions. In the field, soils tend to be heterogeneous with cracks, fissures, roots and animal burrows. These factors will all affect overall soil permeability. Different techniques for the measurement of soil permeability will produce different results, i.e. the test results are a direct function of the test boundary conditions imposed by the test apparatus or the methodology. The permeability as a test property is not an unique property. For these reasons care is needed when testing soils for permeability both in the laboratory and in the field. The methods, technology and protocol for field testing of permeability are presented in the standard textbooks dealing with hydrogeology and/or contaminant hydrogeology. With regard to laboratory tests for soil permeability, there are several methods for the determination of the transmissibility of fluid through a soil sample. The reasons for adopting any one method as the standard method relate to reliability of test results, repeatability and reproducibility, how well the tests mirror the field situation, test simplicity and economics. Research needs

Extensive work has been and is being done to identify and remediate landfills and hazardous "waste" contaminated soils, the so called "past sins", in many countries. When facing the large costs of remediating "past sins", it is necessary for the chemical analyses that provide the foundations for remediation efforts to be comparable, reproducible and reliable [Karstensen 1996].

72

CHAPTER 4

Sample treatment and preparation along with chemical analysis is often performed differently at different laboratories and therefore provides different results. The most crucial step in attaining a comparable analytical method, apart from the sampling, is the pretreatment (i.e. crushing and sieving) and the preparation step (i.e. extraction and digestion). If there is no detailed guideline available on how to perform the sample pretreatment and preparation it is meaningless to assess the data against given risk based values. The comparison with risk based values or background data will be more or less random. As has been explained earlier in this chapter, inorganic compounds will often be attenuated in the soil, i.e. have a low mobility. A leaching test is valuable as an supplementary tool to chemical analysis for the assessment of the mobility of inorganic compounds in contaminated soil. Depending on the afteruse of the contaminated soil it may be satisfactory to leave the contamination as it is as long as the mobility is low and predictable and does not present an unacceptable risk of harm to the environment or to health. A prerequisite to the assessment is the availability of a scientifically recognised leaching test which is reproducible and reliable and which is validated for a broad range of soils and contaminants. Accordingly the principle research issue is to be able to predict leaching behaviour in the longer term when leaching conditions may change from those used under test conditions in the laboratory.

R E F E R E N C E S TO C H A P T E R 4

73

REFERENCES Donor, H.E., "Chloride as a factor in mobility's of Ni, Cu and Cd in soil". Journal of Soil Science, 42 (1978) 882. ISO 10381, Soil quality - Sampling - Part 5: "Guidance on the procedure for the investigation of urban and industrial sites with regard to soil contamination.", 1994. Jones, L.H.P. and Jarvis, S.C., "The fate of heavy metals". In Greenland, D.J. and Hayes, M.H.B., The chemistry of soilprocesses. New York: John Wiley, 1981. Heynen, J.J.M; Comans, R.N.J.; Zevenbergen, C.; Honders, A. (in press) Evaluation of the effect of soil clean up on the leachability of soil. Karstensen, K.H. (editor), "Nordic guidelines for chemical analysis of contaminated soil samples". Oslo. SINTEF/NORDTEST Report STF 27 A95040, 1996. Keijzer, J., Zevenbergen, C., de Wilde, P.G.M. and Aalbers, Th.G., "A reference study on leachability of metals from natural soils". In Goumans, J.J.J.R., van der Sloot, H.A. and Aalbers, Th.G., Waste Materials in Construction. Amsterdam: Elsevier, 1991. Laskowski, D.A., Goring, C.A.I, McCall, P.J. and Swann, R.L., "Terrestrial environment". In Conway, R.A., Environmental risk analysis for chemicals. New York: van Nostrand, 1982. Manahan, S.E., "Environmental Chemistry". Boca Raton: Lewis, 1994. van der Sloot, H.A., Wijkstra, J. and van Leeuwen, J., "Potential for reuse of leadcontaminated urban soils". In Goumans, J.J.J.R., van der Sloot, H.A. and Aalbers, Th.G., Waste Materials in Construction. Amsterdam: Elsevier, 1991. Sposito, G., "The surface chemistry of soils". New York: Oxford University Press, 1984. Stumm, W. and Morgan, J.J., "Aquatic chemistry". New York: John Wiley, 1981. Tessier, A., Campbell, P.G.C. and Bisson, M., "Sequential extraction procedure for the speciation of particulate trace metals". Analytical Chemistry, 51 (1979), 844. Ure, A.M. and Berrow, M.L., "The elemental constituents of soil". In Bowen, H.J.M., Environmental chemistry, vol. 2. London: Royal Society of Chemistry, 1982. Wahlstr~m, M., Tailing, B., Paatero, J., Makela, E. and Keppo, M., "Utilization and disposal of solidified and stabilized contaminated soils". In Goumans, J.J.J.R., van der Sloot, H.A. and Aalbers, Th.G., Waste Materials in Construction. Amsterdam: Elsevier, 1991. Wesselink, L.G., Dekker, P.M. and Aalbers, Th.G., "Chemical processes controlling the mobility of waste material contaminants in soil". In Goumans, J.J.J.R., van der

74

REFERENCES TO CHAPTER 4 Sloot, H.A. and Aalbers, Th.G., Environmental Aspects of Construction with Waste Materials. Amsterdam: Elsevier, 1994.

Yariv, S. and Cross, H., "Geochemistry of colloid systems". New York: Springer Verlag, 1979. Yong, R.N., Mohamed, A.M.O. and Warkentin, B.P., "Principles of contaminant transport in soils". Amsterdam: Elsevier, 1992.

CHAPTER 5 CHAPTER

75

5: S E D I M E N T S

Introduction Natural sediments are usually a sink for trace metals but may also become a source under certain conditions especially in heavily contaminated or drastically changing environments. Almost all the problems associated with understanding the release processes that control the availability of trace metals concern particle-water interfaces. Solid surfaces govern the dissolved levels of these elements via coordinative chemical interactions in particular sorptiondesorption and dissolution-precipitation reactions [Stumm 1996]. Thus, trace metal mobility experiments tend to be far more instructive than any total element concentrations. The tendency of a trace element to be accumulated by aquatic organisms depends in particular upon the capacity of the solids to resupply in solution trace elements removed from solution by biotic and abiotic processes [Chapman 1996]. Numerous reviews of factors affecting the biological availability of metals in sediments have been published among them one which provided extended lists of both sequential extraction results and concentrations in pore waters for a number of various trace metals [Campbell 1988]. There are a variety of well established methods to assess the environmental impact of a given contaminant in sediment-water systems. They range from pore water gradient measurements, in-situ or laboratory incubation experiments up to leaching approaches like sequential chemical extraction procedures all of which are addressed here.

Mechanisms of metal mobility after deposition Diffusive transport and diagenetic flux data reported in the early paper of Elderfield and Hepworth [1975] and in many other later papers indicate that a complex set of processes affect the distribution of trace metals both in pore water and the solid phase of the sediment column. The microbial, chemical and physical interactions of dissolved, colloidal and particulate organic carbon and Fe/Mn-oxihydroxides play a crucial role in the early diagenetic reactions at the sediment-water interface which affect partitioning of trace metals [Santschi 1990]. These reactions are fueled by supply rates of organic carbon and can cause rapid backdiffusion of released trace metals to the overlying water column or their removal onto secondary mineral phases within the sediments. The processes that lead to the release and/or removal of metals at and below the sediment-water interface can be discussed in conjunction with the summary scheme shown on Figure 5.1. The feature common to many of the reported pore water metal data is that peaks indicating metal release occur in three distinct redox zones: copper, cadmium, zinc and vanadium are significantly enriched at the sediment-water interface; chromium, molybdenum, vanadium, and to a lesser extent lead and copper show sharp peaks at the oxic/suboxic transition boundary; and manganese, nickel, cobalt, molybdenum, arsenic, lead, uranium, and vanadium show broad peaks in the Mn/Fe reduction zone. Below this zone most of these metals (copper to a minor extent) are precipitated as insoluble sulphides. Of importance to the extent of these zones are the hydrographic conditions and sediment dynamics. An increase in productivity in the overlying water column will increase the detrital flux of metals which are associated with biogenic material, while compressing sediment redox zones. Moderate changes in bottom water oxygen concentration in the absence of any change in productivity can also compress redox boundaries, while this trend may be in turn reversed by extensive bioturbation. Release from biogenic material at the interface may then not be

76

CHAPTER 5

distinguishable from oxic release in productive shallow water sediments, often leading to overlap of the dissolved metal peaks. In those sediments evidence of the manganese reduction zone may also be eliminated while the iron reduction zone becomes much more pronounced. From Figure 5.1 it can be concluded that copper and cadmium show significant release at the sediment-water interface into the overlying water under most common conditions. This remobilization flux is usually a small percentage of current inputs in shallow water environments due to higher sedimentation rates as shown by detailed trace metal budgets [e.g., Paulson 1988]. The fluxes are controlled by diffusion across the benthic boundary sublayer adjacent to the sediment-water interface due to the fast remobilization kinetics [Petersen 1995].

Figure 5.1. Schematic representation of typical pore water profiles showing characteristic zones of release and removal of trace metals along redox boundaries [Shaw 1990]. Se0tment-Water Imer~ace

.

"~

Release Irorn Biogen~ Malenal

Cu. v. anO C,d

Release at II~eoxic/suboxic front

Cr. V and Mo

Removal in Suboxic Zone

end Cu

Rmlem in M m ~ m m ~ -9

Release In Iron Refuel,on Zone

A

Removal inlo Sediments

C;, V. Mo0

Zone

Mn, Hi, Co, V, and Mo

Fe, Ni and V

Ni, Co, Cu

and V

This release may account for a loss of copper (sediment-sea water flux estimated from pore water data and compared to the accumulation rate in sediments) from 3 % in the Kalix River estuary [Widerlund, 1996], 10 % in an Alaskan fjord [Heggie, 1983] or the Bay of Monaco [Fernex 1986], 15 % in the Elbe estuary [Petersen 1995] up to 90 % in pelagic deep sea sediments at four equatorial Pacific sites [Klinghammer, 1983]. The extent of the remobilized fraction also depends however on the extent of anthropogenic contamination. Baeyens et al. [1986] have found that the excess copper load (defined somewhat unspecifically by the ratio of copper to aluminium relative to a soil reference value) may be lost by the epibenthic remineralization flux to 100 % even in sediments of a shallow coastal area. Hunt and Smith [1983] have shown that diffusive loss of copper may be

CHAPTER 5

77

sufficient to return the upper 1 cm of contaminated sediment collected in Narrangansett Bay to background levels in 44 years when the sediments were isolated from metal and organic carbon sources. Most copper that is remobilized in the surface layer is returned to bottom waters and little (< 3 %) is transferred to deeper layers by subsequent diagenetic reprecipitation in the buried sediments [Heggie 1983]. Much higher remobilization rates during biogenic detritus remineralization have been reported for cadmium. Gendron et al. [1986] have found a 3 to 5 fold drop in Cd/AI ratios between sedimenting particles and the sediments of the Laurentian Trough indicating an intense depletion of excess cadmium between the sediment and the source material. Up to 40 % of the deposited cadmium can be released from surficial sediments in the Bay of Monaco [Fernex 1986] or Elbe estuary [Petersen 1995] by this process. In the relatively shallow Kiel Bay sediments, the effective pore water release out of the sediment is 25 % of the particulate cadmium accumulation rate [Lapp 1993]. Sharp peaks of dissolved cadmium in the pore water were observed just below the sediment-water interface (0.8cm to'l.6 cm) in sediments of the St. Lawrence Estuary where dissolved iron and manganese concentrations are low leading to fluxes both to the sediment-water interface where the concentration is one order of magnitude lower and deeper into the sediments [Gobeil 1987]. Downward fluxes ranged between 2.5 and 7.2 nmol m-2 d-1. Precipitation in the sulphate reduction zone below the M l ~ e reduction zone as insoluble sulphide phases leads to a post-depositional doubling in particulate cadmium concentrations at these depths compared to the concentration at the sediment surface. Where relatively low sedimentation rates occur in conjunction with high sulphate reduction rate, such early-diagenetic redistribution processes may lead to a significant natural enrichment of trace metals which can thus be misinterpreted as an anthropogenic signal [Gaillard 1986; Wang 1986; Pedersen 1989; Santschi 1990]. The precipitation effect may be diminished in favour of the remobilization effect by the bioturbation-enhanced oxic sediment zone. [Emerson 1984]. The benthic flux of other metals may contribute also to the dissolved metal pool in the overlying water column (e.g. the contaminated sediments of San Francisco Bay appear to be a primary source of dissolved lead in the estuary [Rivera-Duarte 1994]) but there are no indications of a significant loss of these metals at the oxidized sediment-water interface due mostly to the relatively high KD-values (e.g. for lead: log K D > 4, even in acidified environments: [Davis 1993]). Removal of chromium and molybdenum directly above the oxic/suboxic boundary and removal of manganese, nickel, arsenic, lead and cobalt above the manganese reduction zone indicate little diffusion out of the sediments. The main source of metals such as molybdenum and uranium is the dissolved pool in the overlying water column from where they diffuse into the manganese oxidation zone [Shimmield 1986; Pedersen 1989; Klinkhammer 1991]. The soluble oxyanion species are reduced on the manganese oxihydroxides and coprecipitated at the suboxic redox boundary leading to a substantial accumulation. Mobility of arsenic is similarly associated with iron diagenesis [Petersen 1986; Brannon 1987; Sullivan 1996]. A significant reduction of trapping efficiency and concomittant remobilization of both these metals could occur only with anoxic (but non-sulphidic) conditions in the overlying water column [Sundby 1986]. Reduced arsenic(III) is not efficiently readsorbed at ambient pH values [Pierce 1982]. Special attention is required with regard to mercury which may be released by methylation in suboxic sediment layers as discussed in an earlier review [Kersten 1988]. The early diagenetic behaviour of methylmercury has been studied recently by high-resolution pore water profiles extracted under rigourous oxygen-flee conditions from organic rich contaminated marine sediments [Gagnon 1996]. These data suggest that the surficial oxic sediment layer serves as a geochemical barrier to the diffusion of this toxic mercury species to the overlying water column.

78

CHAPTER 5

Methodology for measuring release rates

Pore water profiles Ample literature exists on the methodology of sampling pore waters for trace metal analysis. Recent reviews have been provided by for example, Bufflap and Allen [1994]. Most commonly pore waters are sampled by either centrifugation and squeezing of sliced sediment cores or by in-situ dialysis samplers known as "peepers". An in-situ probe has the advantage that sampling and handling artefacts are minimized but the dialysis sampler has to be prepared adequately especially for exposure into anoxic sediment layers [Carignan 1994]. While these conventional sampling approaches were successful for determination of nutrient release rates (e.g. Sundby et al. [1992]) a principal problem with trace metals is that a high spatial resolution on a millimeter depth-scale is necessary to measure the steep gradients at the sediment-water interface. A precise measurement of these gradients is the basis of any reliable flux calculations. Moreover in dynamic shallow water environments sediments may be disturbed at a time scale much shorter than the time necessary for equilibration of the dialysis samplers (1-2 weeks). A promising recent development for pore water sampling at high spatial and temporal resolution is the technique of diffusive equilibration in thin gel probes (DET) provided by Krom et al. [ 1994]. The gel probe relies on a similar equilibration principle as the peeper but rather than confining the solution to compartments it uses a thin film (_< 1 mm) of polyacrylamide gel known from chromatography gel plates in biochemistry. Moreover the DET technique can be extended to measure diffusion gradients (DGT) by backing the gel layer with Chelex resin [Zhang 1995]. DGT measures labile metal species by immobilizing them in the Chelex layer after diffusion through the gel layer. The measured mass per unit area which accumulates in a known time can be used to calculate an in-situ flux from the pore waters to the resin. This flux to the resin can be interpreted to provide pore water metal concentrations or even in-situ fluxes of metal from sediments to pore waters. Exposue times of both DET and DGT are reduced to about one hour. First measurements of a zinc pore water profile on a millimetre resolution with DGT showed tightly defined (<1.5 mm) maxima at the sediment-water interface located 1.5mm to 3.0 mm above the broader iron and manganese maxima. Scanning of the resin bed with a laser ablation ICP-MS may even provide two-dimensional spatial element mapping at sub-mm resolution. Another benefit of the DGT technique is its ability for chemical in-situ speciation which may help in geochemical modelling of the release processes [Furrer 1996]. hl-situ benthic flux chambers Clearly benthic flux chambers represent the most sophisticated approach to study trace metal release rates at the sediment-water interface. The available data for trace metals from this approach are limited to only a few field experiments mostly undertaken in the first half of the eighties. A review of data gained by both diver-operated and lander-connected benthic chambers was provided by Santschi et al. [1990]. Even though different chamber designs are in use, all are modifications of the same basic prototype, that is a stirred bell-jar with sampling ports. Fluxes of cadmium, copper, nickel, zinc and lead have been determined by this approach in a coastal marine environment by Westerlund et al. [ 1986]. The regulated chamber was equipped to control dissolved oxygen and pH near ambient seawater values. Fluxes (in nmol m -2 d"l) of 13 (Cd), 118 (Cu), 209 (Ni), and 1400 (Zn) were measured. Neither release nor uptake by the sediment could be demonstrated for lead. On the other hand the pore water in a diver-collected sediment core was depleted in cadmium, copper, and zinc and slightly

CHAPTER 5

79

enriched in nickel and lead, relative to the ambient seawater. There was thus no correspondence between fluxes calculated from porewater profiles (at least at the probably insufficient 1 cm-resolution scale) and fluxes measured by the benthic chamber approach; nor could the fluxes be directly related to the degradation rate of organic matter. Manipulation of oxygen concentration in the benthic chamber showed that the critical factor which controls iron, manganese, phosphorus and cobalt fluxes at the sediment-water interface is the flux of oxygen from the water column into the sediment [Balzer 1982; Sundby 1986]. The flux could be artificially enhanced for these elements by the rapid depletion of oxygen in an unregulated benthic chamber. The interest in trace metal flux measurements by this approach has ceased in the last years although improved models have been provided to interpret the data [Rabouille 1994]. The recent release of a commercially available regulated benthic chamber of 100 1 capacity may renovate the interest in this approach [Martinotti 1997]. Ex-situ incubation experiments An alternative to measure trace metal release rates is to incubate an intact topmost sediment layer with overlying water in flow-through laboratory devices. This microcosm approach was developed initially to measure benthic phosphate fluxes (e.g. Twinch and Ashton, [1984]; Fowler et al. [ 1987]; Sondergaard, [ 1989]; Sweerts et al., [ 1989]) but was later adapted also for trace metal studies (e.g. Riedel et al., [ 1987]; Slauenwhite and Wangersky [ 1991 ]; Kersten and Anagnostou, [1994]; Kerner and Geisler, [1995]). While exposing the sediment to a stirred and continuously refreshed water column under laboratory controlled conditions diffusive exchange of solutes occurs between the pore water and the water reservoir. Because this reservoir is generally finite, solute concentrations may change with time to an extent which is determined by the magnitude of the flux and the height of the water column. Flushing of the overlying water during incubation experiments through an adsorber column prevents the build-up of excess solute concentrations in the supernatant. This is especially important with trace metals where rapid equilibration and back-reactions with sediment occur [Lu 1977]. Incubation without substantial build-up of solutes may then provide the most accurate method for estimating release rates [Aller 1989]. The existence of diffusive boundary layer resistance above the sediment-water interface requires natural hydrodynamic conditions to be simulated in the incubation chamber [Santschi 1990]. The fast chemical reactions which occur at the sediment-water interface besides being regulated by biological processes such as the supply of organic matter to the interface can become controlled by hydrodynamic forces in the overlying waters that is, the turbulence in the benthic boundary layer. Simple rotating paddles cannot be used however because they would induce an advective pore water flow. Instead a special suction rotating disk for the outflow can be used to adjust flow velocity in the incubation chamber [Booij 1991, 1992]. This is again very important for the measurement of metal remobilization processes that peak within the upper few millimeters of sediments. Particulate and dissolved metals are removed by passage through a precleaned membrane filter mounted into a PTFE housing and a PTFE column filled with the scavenger. A polystyrene-immobilized cation exchanger (8hydroxyquinoline bonded to XE-305 resin) has sufficient specificity for most relevant trace metals at a maximum flow rate in the order of 100 ml min-1 [Hofstraat 1991 ]. Alternatively a XAD or C18 column can be used to collect hydrophobic organic or organo-metallic compounds [Slauenwhite1991; Helmstetter 1994]. Monitoring of the physicochemical parameters such as oxygen saturation and pH/pE by appropriate electrodes is essential with any such incubation approach.

80

CHAPTER 5

Figure 5.2: Scheme of the laboratory incubation arrangement for pollutant flux measurements. Peristaltic Pump r

I I I I

I ! I II

! ! I j

! II I II.

Adsorber Column Post

Kersten and Anagnostou [1994] have used this approach to study trace metal release rates from a slag dump site in Greece. Trace metal concentrations in the ferrous converter slag from a iron/nickel smelter are two orders of magnitude higher compared to the natural sediment (e.g. cadmium > 10 mg/g; nickel > 1000 mg/g). Sea water analyses at the coastal slag dump site revealed a 10-fold increase in nickel concentrations while cadmium concentrations were still in the range known for unpolluted mediterranean coastal environments. Continuous-flow incubation experiments identified the slag as the primary source of the former but not of the latter element. The cadmium concentration in the outflow of the incubation chamber was increased only on the first day probably due to a wash-off effect (Figure 5.3), which suggests that no significant cadmium contamination will be caused by the slag after settling at the dump site. On the other hand after the initial washout effect nickel showed constant release fluxes throughout the entire experiment of up to 100 mmol m -2 d 1. Nickel release was dissolution rather than availability controlled which indicates that the slag is a significant long-term Nickel contamination source. A major benefit of this type of incubation experiment is that factors controlling the trace metal release such as pH, redox potential, salinity, complexing capacity etc. can be varied to match changing environmental conditions. Moreover the effects of sediment perturbation can be

CHAPTER 5

81

studied. While bioturbation seems to have only an indirect effect on trace metal release rates by removal of sulphide from pore water and extending the surficial remineralization zone [Emerson 1984], sediment resuspension may have a dramatic effect on metal release. Skei and Naes [1989] observed an increase in the flux of dissolved mercury from 10 nmol m -2 d-1 to 2.5 mmoI m-2 d-1 when the turbidity of the water column was raised by a factor of 110. Another example is a study of mobilization kinetics of trace metals following resuspension of anoxic sediments into an oxic water column where the order of total release from sediments in fiver water was found by Calmano et al. [1994] to be cadmium (5%) > zinc (1.5%) > copper (1%) > lead (0.7%). It is worth mentioning also that the first reliable flux measurements for organic pollutants (PAHs and chlorinated hydrocarbons) were made by this laboratory flux chamber approach [Booij 1992; Helmstetter 1994]. The magnitude of the fluxes could be related to the octanol-water partition coefficient and suggests that contaminated sediment may act as a weak but persistent source of the pollutants to the water column. A problem which cannot yet be addressed by this approach is the release by colloidal (DOC) material which is known from groundwater studies to be an important transport pathway for organic pollutants.

Summary of issues relating to the measurement of release rates Ample evidence exists to show that sediments are not only a sink for particulate trace metals that are deposited at the sediment-water interface but can also be a source for dissolved metals in the water column. Basically there are three different approaches to the determination of trace metal fluxes at the sediment-water interface: (i) calculation from pore water concentration gradients by the Fickian diffusion equation, (ii) direct in-situ measurement by benthic flux chambers, and (iii) measurement in a laboratory with flow-through sediment core incubation systems. Flux data from all three approaches are summarized in Table 5.1. From Table 5.1 no trend towards lower or higher flux values produced with any of the three techniques can be deduced. When taking the original literature into consideration it seems probable that polluted sediments gave much higher trace metal fluxes than sediments from relatively pristine areas. The pollution status of the sediment may thus have a much more significant effect than the choice of a certain method. There are no studies yet which compare all three methods on the same sediment with the exception of one for manganese. Thamdrup et al. [ 1994b] measured manganese release rates with a free operating benthic flux-chamber in Aarhus Bay (Denmark). Constant but seasonally changing effiuxes (330-420 mmol m-2 d-1) were observed during short (3 hour) deployments. Similar fluxes were observed from sediment cores incubated in the laboratory under in-situ conditions (200-600 mmol m-2 d-l). Manganese reduction in the upper 1 cm of sediment supported steep pore water gradients of manganese towards the surface. However, calculated Fikian diffusive manganese fluxes were 3-16 times higher than the benthic effiuxes. This demonstrated high rates of manganese reoxidation in the topmost l mm to 2mm thin oxic surface layer which could not be resolved by the measured porewater gradients though a relatively thin core slicing of 2.5 mm. Similarly, Sakata [1985] found 7 times higher diffusive manganese fluxes calculated from porewater profiles than with submerged chamber experiments. Petersen et al. [1995] found that under oxic conditions, a flux of copper, cadmium, and zinc into the water column was observed as a result of the mineralization of organic matter in the thin (3 mm 02 penetration depth) oxic layer of the sediment. The diffusive trace metal fluxes calculated from both pore water profiles were 5 to 100 times higher than those determined from laboratory incubation. In both cases the resistivity of a thin benthic boundary layer may contribute also to the difference between pore water and benthic fluxes which are not accounted for with pore water gradients [Santschi 1990]. Conventional pore water sampling methods like squeezing, centrifugation or

82

CHAPTER 5

peeper techniques may thus overestimate the trace metal release rates. The promising new DET and DGT techniques capable of estimating in-situ pore water solute gradients on a submillimetre depth-scale have been introduced recently and await more testing before they can be recommended for common use. The direct measurement using regulated in-situ or ex-situ microcosm approaches provide the most reliable data on trace metal fluxes even under varying environmental conditions. Figure 5.3: Daily cadmium and nickel release from the slag per square metre as determined by the continuous-flow incubation arrangement (Cadmium - blank bars, Nickel - black bars). Note that there are three orders of magnitude difference between both metal fluxes.

1400 ~'E 1200

T

0 E.~.IO00

,,,.,,..,.

.

u

Z

800

"~ r-.

x

600 400

200

2

3

4

day

5

6

7

CHAPTER 5

83

Table 5.1: Diffusive dissolved trace metal fluxes (in mmol m -2 d -1) from sediments of lake, river, estuarine and coastal marine environments. Metal Flux

Fe

Mn

Zn

Cd

Ni

820 490 31-720 1.5-728 1-500 20 300 1710 236 230-2600 40-420 170-780

Environment (method)

lake (pore water) marine (pore water) marine (pore water) lake (pore water) marine (pore water) marine (pore water) marine (benthic chamber) lake (benthic chamber) lake (pore water) marine (pore water) marine (pore water) marine (pore water) 180-280 lake (pore water) 93 marine (pore water) 65 marine (pore water) 66 marine (benthic chamber) 13-860 marine (laboratory incubation) 150 fiver (pore water) 28 river (laboratory incubation) 26-72 marine (pore water) 0.75 estuarine ( pore water) 1.2 river (laboratory incubation) 2-9 river (laboratory incubation) 0.3-2.3 marine (laboratory incubation) 2.5-12.7 marine (laboratory incubation) 5.5-45 marine (pore water) 1.12 estuarine (pore water) 0.88 marine (pore water) 0.21 lake (pore water) 0.08 estuarine (pore water) 0.20 marine (pore water) 0.08 marine (pore water) 0.12 marine (benthic chamber) 9.5 river (pore water) 0.11 river (laboratory incubation) 0.18-3.1 marine (pore water) 0.003 marine ( pore water) 0.09-2.5 marine (laboratory incubation) 0.013 marine (benthic chamber) 23.8 lake (pore water) 9.9 river (pore water) 1.7 river (laboratory incubation) 2.57 estuarine (pore water) 0.21 marine (benthic chamber)

Reference

Sakata, 1985 Trefry and Presley, 1982 Thamdrup et al., 1994a Ullman and Aller, 1989 Aller, 1980 Sundby et al., 1986 Sundby et al., 1986 Sakata, 1985 Sakata, 1985 Aller, 1980 Trefry and Presley, 1982 Thamdrup et al., 1994 Ullman and Aller, 1989 Sundby et al., 1986 Femex et al., 1984 Sundby et al., 1986 Skei and Naes, 1989 Petersen et al., 1995 Petersen et al., 1995 Baeyens et al., 1986 Petersen et al., 1995 Petersen et al., 1995 Kerner and Geisler, 1995 Hunt and Smith, 1983 Skei and Naes, 1989 Baeyens et al., 1986 Elderfield and Hepworth, 1975 Heggie, 1983 Carignan and Nriagu, 1985 Widerlund, 1996 Gaillard et al., 1986 Fernex et al., 1984 Westerlund et al., 1986 Petersen et al., 1995 Petersen et al., 1995 Baeyens et al., 1986 Fernex et al., 1984 Skei and Naes, 1989 Westerlund et al., 1986 Carignan and Nriagu, 1985 Petersen et al., 1995 Petersen et al., 1995 Elderfield and Hepworth, 1975 Westerlund et al., 1986

84

CHAPTER 5

Table 5.1 continued: Diffusive dissolved trace metal fluxes (in mmol m -2 d -1) from sediments of lake, river, estuarine and coastal marine environments.

Metal Flux

Environment (method)

Reference

Pb

estuarine (pore water) marine (pore water) estuarine (pore water) marine (laboratory incubation) marine (laboratory incubation) marine (pore water) marine (pore water) estuarine (laboratory incubation) marine (pore water) marine (laboratory incubation)

Elderfield and Hepworth, 1975 Baeyens et al., 1986 Gobeil and Silverberg, 1989 Skei and Naes, 1989 Hunt and Smith, 1983 Sundby et al., 1986 Gaillard et al., 1986 Riedel et al., 1987 Sullivan and Aller, 1996 Skei and Naes, 1989

Co Cr As Hg

0.49 0.37 0.05-0.26 0.29-25.5 0.06 0.001 0.02 5.1-22.3 0.008-2.5 0.005-0.02

Leaching Basic rationale

While the aforementioned approaches are aimed at studying actual trace metal release potential, one may also be interested in predicting the long-term emission potential of sediments. Wet chemical leaching experiments have been shown to provide a convenient means of determining the metals associated with source constituents in sedimentary deposits most relevant for diagenetic transformation and release processes on changing environmental conditions [Kersten 1989]. A general aim of all studies involving selective chemical leaching is the accurate determination of partitioning of elements of environmental concern among different discrete phases of a sediment sample. Sorbent phases considered important in controlling metal concentrations in pore waters are oxides, sulphides and organic matter [Horowitz, 1991]. Fractionation is usually performed by a sequence of "selective" chemical leaching techniques which include the successive removal of these phases and their associated metals. The concept of chemical leaching is based on the idea that a particular chemical solvent is either phase or mechanistic specific (e.g. buffered acetic acid will attack and dissolve only carbonates, neutral magnesium chloride will only displace adsorbates). There is no general agreement in the literature on the solutions preferred for the various sediment components to be leached due mostly to the "matrix effects" [Martin 1987] involved in the heterogeneous chemical processes. The most appropriate extractants are determined by the aim of the study, by the type of sediment and by the elements of interest. A vast literature on specific research areas exists in which appropriate extractant formulations may be found for a selected problem [Kersten 1989, Picketing 1986; Hall 1996]. Despite the clear advantages of a differentiated analysis over investigations of the bulk chemistry of sediments, verification studies conducted in recent years indicate that there are many problems associated with operational speciation experiments involving partial leaching techniques. It is common for studies in wet chemical leaching to point out that the various solutions used are not as selective as expected.

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85

The term "speciation" encompasses three aspects: (i) the actual distribution among molecular level entities in a given matrix, (ii) the processes responsible for an observable distribution (species distribution) and (iii) the analytical methods used. The first aspect is confined largely to aqueous solutions and to particle-water interfaces [Stumm 1996]. The second aspect is now seldom used in environmental chemistry and this usage, conveying the idea of speciation as the process of transformation from one species to another, is confined largely to biological science. The third aspect is the most relevant with regard to leaching experiments with sediments which the present chapter deals with. Most element-specific bulk and surface analysis methods applied to date rarely yield information at the molecular level in the solid phases for trace elements (except, maybe, the EXAFS method introduced recently in environmental geochemistry: Manceau et al. [ 1996]). Partial leaching, on the other hand may incorporate reagents which are sensitive to only one of the various sediment components significant in trace metal binding, or representative for a particular release controlling mechanism. In sequential multiple leachings chemical solutions of various types are applied successively to the sample of sediment, each follow-up treatment being more aggressive in chemical action than the previous one. Although "selectivity" for a specific phase or binding form, in the strictly thermodynamic sense of speciation, cannot be expected for these operational procedures there are also differences in the specificity between the various extractants and methods used. In practice three major factors may influence success in leaching of sediment components namely (i) the chemical properties of an extractant chosen, (ii) its extraction efficiency and (iii) experimental parameter effects. In applying sequential extraction schemes another three factors may be involved (iv) the sequence of the individual steps, (v) specific "matrix effects" such as crosscontamination and readsorption and (vi) heterogeneity as well as physical associations (e.g., coatings) of the various solid fractions [Kersten 1989]. In evaluating the suitability of an extractant chosen for a specific investigation all these factors have to be considered critically. Single leaching stages are usually not selective for a constituent but may be chosen to represent a particular release controlling mechanism such as desorption by increasing salinity or competing organic complexing agents. While the first "exchangeable metal binding form" is selectively displaceable by weak extractants, the reagents used for the components considered subsequently are all "non-selective" in that they are co-extracting more or less extensively the more readily soluble components [Tack 1996]. A careful combination of such non-selective extractants in a sequence may then turn these "overlaps" in leaching efficiency to good use in an efficient scheme of selective extraction steps. While the least aggressive reagents should be applied first there has been little uniformity with regard to the reagents used or the order of leaching. Most of the recommended schemes seek to displace first the exchangeable fraction as a separate entity using MgC12 or NH4OAc (pH 7) treatments. Most sequential extraction schemes call for removal of carbonates present as the next step (using HOAc, with or without buffering by NaOAc to pH 5). In subsequent steps proposals made after 1977 show more obvious similarities because they are modifications of the protocols introduced by Engler et al. [ 1977] and Tessier et al. [ 1979]. Most schemes seek to use extractants in decreasing order of pH values but the percentage released by subsequent steps of the sequential extraction procedure is always higher than that released at the same pH by progressive acidification [Rauret 1991 ]. Most of the variations in the schemes arise from the initial problem definition that is differences in experimental design and sample characteristics. The degree of interaction between solid phases and extractant

86

CHAPTER 5

solutions can also be altered by changes in experimental parameters such as reagent concentration, final suspension pH, solid-to-solution ratio, temperature, contact time and intensity and especially if a second extraction with fresh solution in each of the leaching steps is being applied [Hall 1996]. The most recent development is sequential extraction in a microwave oven [Real 1994; Ginepro 1996]. The absence of standardized conditions makes it difficult to compare data derived from studies in which such parameters are significantly different or not even listed. During recent years investigators have tended to use similar leaching sequences by adapting or modifying the most popular Tessier scheme. A number of leading European experts have started an initiative to harmonize their leaching studies and agreed to use a reduced scheme consisting of four steps [Quevauviller 1994] which, however, has not yet been widely accepted. Whatever the leaching procedure chosen the validity of the leaching results will be primarily dependent on the ways in which the samples are collected and preserved prior to analysis.

Sediment samp#ng, storage and preparation Fresh deposited sediments are fairly reactive due to their organic matter content which may render them rapidly anoxic if enclosed in a sample vial or otherwise changing in geobiochemical (dis-)equilibria. Moreover leaching techniques are handicapped by disruptive preparation techniques which alter the chemical speciation of inorganic components or lead to loss of analyte before analysis (e.g., freezing, lyophilization, evaporation, oxidation, changes in pH, light catalyzed reactions, reactions with the sample container, time delays before analysis with biologically active samples and so on.). Care is required to minimize changes in metal speciation due to changes in the environmental conditions of the system during sampling and preparation if we want to study the effect of such changes only in carefully designed experiments. Clearly sediment sampling must avoid mixing or alteration of natural system biogeochemical zones and processes which would lead to results unrepresentative of the original equilibria. Common measures to consider the heterogeneity of the deposit by methods such as batch homogenization cannot be recommended. It is just the thin surficial oxidized sediment layer that controls the exchange of trace elements between sediment and overlying water in aquatic environments and provides the pool of pollutants to which the benthic ecosphere is exposed. On the other hand a number of efficient natural immobilization process for metals take place in the underlying anoxic layer [Casas 1994]. The effects of various preservation techniques (wet storage, freezing, freeze- and oven drying) on metal speciation in anoxic sediments is reported by Kersten and F6rstner [ 1987]. Drying causes instant and major speciation changes in anoxic sediments but also in oxic sediments. Drying of the latter was reported to reduce the quantity of iron extracted by techniques which remove amorphous iron oxides (acetic acid - hydroxylamine) suggesting an increase in the oxide crystallinity [Thomson 1980]. Changes in the extractability of trace metals were found to be mostly consistent with their partitioning between iron and manganese oxides and organic matter. Extractability of copper by oxalic acid, pyrophosphate and DTPA was found to be enhanced to more than twice that of the control by oxic sediment drying, reflecting the predominant binding of this metal by organic matter [Thomson 1980]. Freezedrying of oxic samples are adequate methods of storage if the sediments are to be leached with diluted mineral acids. This preparation procedure is thus adequate for pollution reconnaissance studies since "non-residual" concentrations of trace metals are usually determined using a single leach by 0.5 M HCI [Chester 1985]. A remoistened sample may at best require a fairly long incubation time before it approaches the original chemical characteristics defined by chemical leaching of this sample immediately following field

CHAPTER 5

87

sampling [Bartlett 1980]. Wet storage of oxidized sediments and soils is inadequate because of a rapid microbially induced shift from oxidizing to reducing conditions in the stored sediments. Extractability of copper with the most insoluble sulphide phase was reported to decline rapidly during wet storage [Thomson 1980]. Refrigeration should delay or inhibit these effects although extractability of copper and iron by DTPA was found to be halved and doubled respectively within 15 days of storage relative to that of immediately leached subsamples [Thomson 1980]. Freezing is usually a suitable method to minimize microbial activity. However it may lyse cells and thereby free organic excudates and any associated trace metals. Freezing was thus found to enhance water solubility of metals in the order manganese (8 to 17%) > copper (7 to 15%) > zinc (6 to 12%) > iron (3 to 7%), and storage subsequent to freezing significantly affected extractability of these metals by weak agents [Thomson 1980]. Storage of anoxic sediments by freezing was found to cause the least change in the fractionation pattern of the various metals studied however one should be aware that air is able to penetrate the walls of plastic vials. Success was demonstrated with a double wall sealing concept that is an inner plastic vial with the frozen sediment contained under argon in an outer glass vial. In general, however, it seems to be impossible to totally avoid changes in the delicately-poised in situ chemical speciation of trace elements found in nature definable by extractants, unless the sediment samples are leached immediately upon collection. One should be aware also that the high concentration in dissolved organic substances found in the first leaching steps of fresh or frozen anoxic sediments tends to suppress cadmium and other metal peaks in atomic absorption spectroscopy or voltammetry analysis which is not found with dried samples. These interferences can be reduced by carefully selecting the ashing and atomization temperatures of GFAAS [Pai 1993]. For most trace elements ICP-AES can be used as an alternative [Li 1995; Hall 1996]. Sieving in order to achieve sediment heterogeneity cannot be recommended although some investigations have pointed to a relationship between specific surface, grain size fraction and speciation of trace elements in sediments. Amorphous iron-oxide coatings appear to be the most significant in affecting both surface area and sediment-trace metal levels in oxic sediments. This relationship has been demonstrated in a study of interrelations between specific surface area and trace element geochemistry in fluvial sediments [Horowitz 1987]. The results supported the view that external surface area as determined by the one-point BET-method is a function of both grain size and of composition (geochemical phase). Removal by a sequential extraction procedure of such geochemical sorbents as carbonates, oxyhydrate coatings and organic matter decreased the surface area. Results indicated that the same trace metal adsorbates (e.g. carbonate-extractable or acid-reducible coatings) may act as either a surface-area inhibitor (its removal produces an increase in active surface area) or a contributor (its removal produces a decrease in active surface area) depending on the median grain size of the sample. Although coatings may make a contribution to sample surface area this contribution is significant only in the case of coarse-grained material which characteristically has a low surface area [Horowitz 1987]. For fine-grained material with a higher surface area the effect is indirect in that the coatings cement fine grains together forming water-stable aggregates. Removal of this cement breaks down the agglomerates to their original, smaller component particles which have larger surface areas than their coatings. It is apparent that the breakdown of these aggregates by either physical separation or chemical leaching will increase the surface area of the sediment (three to five-fold) causing matrix effects by enhanced re-adsorption of either the major and/or the associated trace elements [Horowitz 1987].

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88

Separation of suspended particulate matter (SPM) is most frequently performed by membrane-filtration which can be used in single leaching studies [Tillekeratne 1984] but leads to limited sample quantities for sequential extraction studies. The continuous-flow separation procedure is simpler to carry out especially in open sea where SPM concentrations are low. With this technique enough material could be sampled to analyze the grain size distribution, the specific (BET) surface, the bulk concentration and partitioning between five sequential extraction steps of trace elements in SPM from the open North Sea [Kersten 1991]. The results indicated that the amorphous iron-oxyhydroxide reducing extraction is suitable to explain the grain size or surface area effect for most trace elements.

Evaluation of sequential extraction techniques The most important application of the partial leaching approach is usually to assess the mobility of trace metals in contaminated anoxic sediments on disposal in a terrestrial oxic environment that is on substantial changes of redox and pH conditions. Evaluation of this approach is particularly important for anoxic harbour sludges with relatively low acid neutralizing capacity [Kersten 1991a]. However, it was with such anoxic sediments where most evaluation experiments failed to produce unbiased results due to (i) improper sample handling and (ii) the use of improper leaching agents. A particularly instructive example is the study of anoxic harbour sediment with the electron beam microprobe to obtain direct evidence of the partitioning of the high cadmium contents [Lee 1984]. In this study cadmium was found to be most frequently associated with sulphur (in about 90% of the traced cadmium-beating particles). Thermodynamic calculations also suggested the formation of cadmium sulphides. In contrast, when studied by sequential extraction, exchangeable (34%), carbonate (36%), and reducible oxide-bound (22%) cadmium represented the most important fractions from these samples (Table 5.2). It is quite probable however that this disagreement is due to improper sample handling because the authors dried their sediment samples prior to applying the leaching procedures. In experiments on anoxic harbour sludges performed with fresh samples under oxygen-free conditions non-residual trace metals have indeed been found predominantly in the oxidizable (organic + sulphide) fraction, while the previous four leaching steps did not affect significantly the release of cadmium [Kersten 1986, 1987; Rule 1992; Wallmann 1993a]. Proper sampling, preparation and experimental conditions are thus the main prerequisite for attaining reliable data from such evaluations. Another approach is to use artificially prepared sediments spiked with model sulphide phases. Table 5.2: Sequential extraction results for cadmium and zinc for dried but originally anoxir harbour sediment samples compared with the probability of association of both metals with major elements as determined by direct electron microprobe study of the same sediments, in percentages (95% confidence level, data from Lee and Kittrick, 119841). Sequential Extraction Results Fraction Cd

Zn

Exchangeable "Carbonate" Reducible Oxidizable Residual

3.8 • 0.1 50.3 + 1.9 38.6 + 0.2 0.6 + 0.1 6.8 + 0.3

34.0 • 0.8 36.2 + 0.9 21.9 + 1.6 0.5 • 0.0 7.4 + 0.1

Microprobe Results Element Zn

Cd

CI Ca Fe, Mn S

11 (1-34) 6 (0-27) 7 (1-22) 89 (66-99)

10 (2-27) 0 17 (6-35) 83 (65-94)

Kheboian and Bauer [1987] used zinc-doped iron sulphide (mackinawite) and found that a

CHAPTER 5

89

significant portion of zinc was extracted in steps 2 and 3 of the Tessier procedure and that a strong odour indicated the generation of hydrogen sulphide. As the major acid-volatile sulphide (AVS) component however, iron sulphide is fairly soluble in acidic media [Rapin 1986a]. The solvent used in step 3 of the procedure (25 % acetic acid) apparently solubilized much of the mackinawite. The results and interpretations of this paper were debated intensely in the literature but the question "to what extent do such artificial reactions affect the leaching results?" was not answered satisfactorily. An examination was made of trace metal binding forms in a fine-grained, organic-rich sediment which was incubated under anaerobic conditions until the dissolved sulphide concentration in the suspension reached a steady-state and a significant concentration of acid volatile sulphide had built up in the sediment [WaUmann 1993a]. Thermodynamic equilibrium modeling was performed to (i) evaluate the selectivity of the extraction steps and redistribution processes among phases during leaching and (ii) to evaluate the possibility of concurrent precipitation of sulphide minerals and adsorption of trace metals on iron oxyhydroxides. The overall good correlation between the calculated solubility of trace metal sulphides and the respective concentrations in the various sequential extraction steps is striking and suggests that the extractability of all considered trace metals is controlled by dissolution-precipitation processes. Both experimental results and equilibrium calculations show that the trace metal sulphide minerals are not extracted together in the same step of the sequential extraction procedure but in all fractions to an amount depending on their solubility. Iron, nickel, cobalt, zinc and lead sulphides in anoxic sediments are more or less soluble in acidified extractants (pH < 5). Figure 5.4 demonstrates the effect of increasing acidification of the acetate agent on the solubility of different metal sulphides. Only copper, lead and mercury sulphides were stable enough to "survive" the initial leaching steps down to pH 2. The consequence of this effect is that when using the Tessier procedure the proportions of trace metals extracted in steps 1 to 3 are determined by the sulphide equilibrium adjusting during each step and the degree of contamination of the anoxic sediments. The higher the particulate metal concentrations the higher the proportions of metals transferred down the extraction sequence as sulphide precipitates in spite of the increasing acid-induced sulphide dissolution. However, the attendant generation of dissolved sulphide anions, especially due to solubilization of iron sulphide and zinc sulphide, might have precipitated copper and cadmium originally bound in non-sulphidic (e.g. organic) sedimentary phases as sulphide minerals during the extraction. This process is the probable explanation for the observation that lead and copper were removed later than expected in the experiment of Kheboian and Bauer [ 1987] rather than an insufficient reactivity of the reducing reagent or competitive adsorption effects. In fact it has been shown by recent studies that post-extraction readsorption in sediments may not significantly bias the results obtained by the Tessier procedure for both anoxic and oxic sediments unless large amounts of strong sorbents such as organic matter are present [Belzile 1989; Kim 1991; Howard 1996]. The solid/solution ratio was set to 1:100 and the extraction temperature at 25~ The total dissolved sulphide concentration is controlled by the solubility of iron sulphide and the dissolved ferrous iron concentration. The latter was set as constant at 10 mmol/l due to concurrent dissolution of other ferrous phases [Wallmann 1993a]. Solubility products and stability constants for the sulphide phases, bisulphide and acetate complexes were taken from critical compilations [Dyrssen 1990; Morel 1993]. Correction for ionic strength was made using the Davies equation.

90

CHAPTER 5

Figure 5.4: Solubility of metal sulphide phases in 0.1 M acetate buffer in the pH range 2 to 6. For the calculation of the data presented in Figure 5.4 using the geochemical program MICROQL [Miiller 1993] the following sulphide concentrations were assumed for the model sediment: FeS 1 mmoi g-l; ZnS and CuS 10 mmoi g-l; PbS and NiS 1 mmol g-l; CdS 0.1 mmol g-l, HgS 10 nmol g-l.

100

80 "0

~ 0

60

~

4O 20 0 ~EIEIEi 2

3

4

5

6

pH FeS .-o- ZnS - - ~ NiS - . a - C d S PbS

-- CuS

---HgS

The partial sulphide pre extraction effect implies also that model sediments spiked with high amounts of sulphide phases may not adequately show this dissolution effect. Shannon and White [ 1991 ] spiked a natural lake sediment with amorphous FeOOH, FeS and FeS2 and tried to evaluate selectivity of the Tessier procedure for the added solid phases by determining the difference in the mass of iron and sulphide extracted from treated and control sediments. The procedure was moderately selective for iron added as FeOOH and FeS; a mean of 77 % of the iron added as FeOOH was extracted in the step designed to reduce Fe/Mn oxyhydrates while 69 % of iron added as FeS was extracted in the fraction designed to oxidize sulphides and organic matter. Approximately 25% of the iron added as FeS was extracted prematurely which agrees with the findings of other studies [Rapin 1986]. The procedure was however, highly selective for FeS2:92 % of the iron added as pyrite was extracted in the sulphide extraction step. The portion of sulphidic bound iron extracted prematurely therefore may depend also on the degree of pyritization in anoxic sediments

CHAPTER 5

91

which can be determined in turn also by sequential extraction [Huerta-Diaz 1992]. The amount and speciation of reactive iron, sulphide, and calcium in anoxic sediment as determined by a single-step HCI leaching followed by a voltammetric determination of the oxidation state of the iron extracted [Wallmann 1993b] or by a four-step sequential extraction procedure are basic parameters for an assessment of the acidification potential of dredged material disposed of on land [Kersten 1991b].

Summary of the issues related to leaching Leaching techniques provide data for use in an assessment of the potential for long-term release rather than information on actual release. The limitations reported here and elsewhere lead to the conclusion that results given by sequential sediment extraction experiments can be used for an assessment of specific release scenarios particularly related to changing pH and redox environments rather than for metal speciation in sediments. This conclusion also highlights the limitations of some of the terminology often used (such as the "organically bound fraction" and "oxyhydroxide fraction"). Clearly a link exists between the reagent chosen and the sediment component most strongly attacked but secondary reactions reduce the validity of these convenient labels. Of equal (and possibly better) value would be groupings such as water soluble, acid soluble, salt-displaceable, acetate extractable, acidreducible or-oxidizable and so on, that is, the labels defined by the extractants choosen. Moreover sequential extractions should be regarded as a procedure in which the natural sediment is transferred in a well defined artificial environment and induced shitts in reaction equilibria are registered. They can be most efficiently used in combination with thermodynamic modeling because the leaching solutions are usually better defined than in the natural aquatic environment. In this way sequential extraction experiments can be used as an effective confirmational tool to reduce the complexity of the natural sediment-water system and to gain a more complete understanding of the solid trace metal speciation and long-term fate in certain scenarios.

92

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REFERENCES TO C H A P T E R 5

97

sequential extractions of heavy matels in estuarine sediments. Sci. Total Environ. 152, 135. Ridgway I.M. and Price N.B. (1987): Geochemical associations and post-depositional mobility of heavy metals in coastal sediments: Loch Etive, Scotland. Mar. Chem. 21,229-248 Riedel G.F., Sanders J.G. and Osman R.W. (1987): The effect of biological and physical disturbances on the transport of arsenic from contaminated estuarine sediments.Estuar. Coast. Shelf Sci. 25, 693-706. Rivera-Duarte I. and Flegal A.R. (1994): Benthic lead fluxes in San Francisco Bay, California, USA. Geochim. Cosmochim. Acta 58, 3307-3313. Rule J.H. and Alden R.W. (1992): Partitioning of Cd in geochemical fractions of anaerobic estuarine sediments. Estuar. Coastal Shelf Sci. 34, 487. Sakata M. (1985): Diagenetic remobilization of manganese, iron, copper, and lead in anoxic sediment of a freshwater pond. Water Res. 19, 1033-1038. Santschi P., HOhener P., Benoit G. and Buchholtz-ten Brink M. (1990): Chemical processes at the sediment-water interface. Mar. Chem. 30, 269-315. Shannon R.D. and White J.R. (1991): The selectivity of a sequential extraction procedure for the determination of iron oxyhydroxides and iron sulphides in lake sediments. Biogeochem.. 14, 193. Shaw T.J., Gieskes J.M. and Jahnke R.A. (1990): Early diagenesis in differing depositional environments: The response of transition metals in pore water. Geochim. Cosmochim. Acta 54, 1233-1246. Shimmield G.B. and Price N.B. (1986): The behaviour of molybdenum and manganese during early sediment diagenesis - offshore Baja California, Mexico. Mar. Chem. 19, 261-280. Skei J.M. and Naes K. (1989): Experimental work on polluted sediments. In: Proc.7th Int. Conf. Heavy Metals in the Environment, Geneva, September 1989, Vol. 1. CEP Consultants, Edinburgh, p. 508-511. Slauenwhite D.E. and Wangersky P.J. (1991): Behaviour of copper and cadmium during a phytoplankton bloom: a mesocosm experiment. Mar. Chem. 32, 37-50. Sondergaard M. (1989): Phosphorus release from a hypertrophic lake sediment: Experiments with intact sediment cores in a continuous flow system. Arch. Hydrobiol. 116, 45-59. Stumm W. and Morgan J.J. (1996): Aquatic Chemistry. Wiley, New York, 1022 pp. Sullivan K.A. and Aller R.C. (1996): Diagenetic cycling of arsenic in Amazon shelf sediments. Geochim. Cosmochim. Acta 60, 1465-1477. Sundby B., Anderson L.G., Hall P.O., Iverfeld A., Rutgers van der Loeff M.M. and Westerlund S.F.G. (1986): The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface. Geochim. Cosmochim. Acta 50, 12811288.

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REFERENCES TO CHAPTER 5

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REFERENCES TO CHAPTER 5

99

Widerlund A. (1996): Early diagenetic remobilization of copper in near-shore marine sediments: a quantitative pore-water model. Mar. Chem. 54, 41-53. Zhang H., Davison W., Miller S. and Tych W. (1995): In situ high resolution measurements of fluxes of Ni, Cu, Fe, and Mn and concentrations of Zn and Cd in pore waters by DGT. Geochim. Cosmochim. Acta 59, 4181-4192.

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CHAPTER 6 CHAPTER

6: S E W A G E

101 SLUDGES

Introduction Any sludge from a sewage treatment plant can be considered as a complex waste specific to the source which comprises a solid matrix finely associated with an organic and biological matrix. The currently available leaching tests are directed towards the evaluation of the pollutant mass contained in the waste and the assessment of the release of contaminants to the aqueous phase by physico-chemical transfer. In case of release from sewage sludges, the solubilization of the organic or mineral element can be viewed as the result of a number of factors which include: the heterogeneity of the sludge matrix the biodegradation of the organic fraction contained in the sludge the extraction by leaching of the soluble elements in the degraded fraction the impact of these released metabolites on the sludge degradation process by diffusion within the biomass. The composition of sewage sludges Sludge is derived from a variety of sources and is of differing composition and quality. It is produced at all points in the water cycle:

Drinking water production plants The characteristics of sludge from drinking water plants depends on the origin of the water and on the water and sludge treatment method: suspended matter, nature of the flocculant (iron or aluminium salts), flocculated colloids from the clarifiers, filter washing (sand or coal) sludge production method, sludge stabilization method (for example with the addition of lime) precipitates from the iron, manganese and carbon removal units.

Sewage treatment plants The sludges generated in sewage treatment plants include:

primary sludge created by sediment deposition at the base of the clarifier comprises: sands, suspended matter, colloids, oils and greases which bypassed the sedimentation or trapping process in the wastewater network. biological sludge (bacteria and protozoans) produced from biodegradable matter in the course of the secondary and tertiary purification treatment. physico-chemicai sludge separated during coagulation, clarification or flotation treatment. This sludge is rich in iron or aluminium salts.

102

CHAPTER 6

The composition of the sludge, particularly the mineral and organic matter content, depends upon the treatment process. Similarly the effect of the sludge treatment method on the physical and chemical structure of the sludge must be taken into account, particularly the influence on the stabilization phases by biological (aerobic, anaerobic) or physico-chemical processes (see Tables 6.1 and 6.2) [AGHTM 1988]. Table 6.1" Sludge composition related to the water treatment process

Parameters % dried matter

Primary clarification

Biological Cm>0.1

Extended aeration

Lagoon

PhysicalChemical Treatment

Or~;anics

55-65

70-85

55-70

45-60

35-55

Total Nitro~;en

2.5-3

4-6

4-5

2-3

1.5-2

1-1.5

2.5-3

2-2.5

1.5-2.5

1.5-3

K

0.2-0.3

0.2-0.3

0.2-0.3

0.2-0.3

0.1-0.2

Carbon

33-40

38-50

33-40

25 35

20-30

Calcium

5-15

5-15

5-15

5-15

5-30

Masnesium

0.4-0.8

0.4-0.8

0.4-0.8

0.4-0.8

1.7-4.5

Iron

1-3

1-3

1-3

1-3

3-15

1-3

0.1-0.3

0.1-0.3

0.1-0.3

0.1-0.3

0.1-15

N-NH4

0.2-0.5

0.2-0.5

0.2-0.5

0.2-0.5

0.1

Si02

10-20

5-10

5-10

5-10

10-20

Putrid value

++

++

(+ +) (-)

Sludge production Sludge concentration outlet

++

60-120g/I

20-60~,/1

CHAPTER 6

103

Composition of the mineral element of the sludge

The mineral element present in the sludge is distributed in different chemical forms, either absorbed or in equilibrium between the solid phase and the liquid phase. These forms depend on the origin of the mineral elements. They may be present in the effluent discharging into the plant in dissolved, colloidal or finely divided form. The form of the mineral element depends also on the chemical nature (ionic or complexed). The mobility of the mineral element contained in a sludge is associated with the sludge structure, its pH, redox potential, and conditioning. However mobility also depends on the state of degradation of the sludge. Table 6.2- The impact of sludge stabilization on the quality of the sludge

Sludge stabilization

aerobic

anaerobic

chemical (lime)

Organic matter

constant

constant

augmentation from 10% to 50 %

N

low loss on dried matter- but loss on liquid

40% of organic nitrogen been changed to NH4

NI-I3 been stripped from liquid and solubilization of a fraction of organic nitrogen

constant

constant

precipitate

reduction: 0% to 10 %

reduction: 15% to 30%

augmentation from 10% to 30 %

Sludge mass

Physico-chemical analysis of sludge The characteristics of a sludge may be determined by its origin, its structure, and its chemical composition. Characteristic sludge parameters

Dry matter concentration (in g/l) determined by drying at 105~ Loss on ignition (in g/l) determined after oven treatment at 550 ~

This value is

indicative of the organic matter content of the sludge.

Organic carbon (C), total nitrogen (NK), total phosphorus (P), the values of which enable the agricultural value of the sludge to be assessed.

104

CHAPTER 6

The total heavy metals (extraction by aqua regia) are analysed in order to determine the route for the use or disposal of the sludge (agricultural use, incineration, discharge to a landfill).

Sludge rheology, which characterises the physical structure of the sludge. Water content of the sludge. The water in the sludge is present partly as free water which is readily removable and partly in a form linked to the colloids, to the cells or to mineral or organic chemical compounds which requires more energy for removal. PermeabUityfactor (K) (in m/s); this factor is defined by Darcy's Law. It relates to the resistance of the medium to water flow within the sludge.

Distribution of the mineral elements in the sludge The mineral elements (essentially metals) are distributed between the different size fractions present in the sludge. In anaerobically digested sludge more than 90% of the metals are fixed to the larger sized fractions comprising agglomerates of organic and mineral fractions [Gould 1978] as shown in Table 6.3.

Table 6.3: Distribution of metals within the granular fraction of sludge.

Metal

Particulate (> 100 ~tm) ~

Supra colloidal (0.6 to 100 ~tm) %

cadmium

90.4

8.2

chromium

92.9

7.1

l

Colloidal (0.002 to 0.6 ~tm) %

Dissolved (< 0.002 ~tm) %

1.4

-

-

-

0.1

0.1

0.1

-

0.1

1.0

-

0

0.3

0.3

0.04

0.06

i

;

I |

cobalt

92.8

7.2

copper

92.9

6.9

|

|

iron

95.5

4.4 |

manganese

95.5

3.4 |

nickel

95

5.0 |

lead

92.1

7.3 |

zinc

91.5

8.4

The metals are physically fixed by the biological floe. The adhesion of the metals results from the production of extracellular polymers by the bacteria which complex with the metals.

CHAPTER 6

105

In addition, the majority of the metals are distributed between the insoluble (mineral) phase and the cellular phases for example as shown in Table 6.4.

Table 6.4: Distribution of metals in the solid phase of sludge [Hayes 1978].

Metal

Total mg/i

Soluble %

Insoluble %

Extracellular %

Intracellular %

chromium

63.2

0.1

16.6

0.2

75.8

chromium (III)

76.5

0.1

31.5

0.15

69.0

copper

18.6

0.15

53.0

0.7

48.7

nickel

6.38

4.4

54.8

1.1

34.8

(vi)

The distribution of metals in the solid phase was assessed by submitting the sludge to an elutriation test and passing the suspended matter through a sieve. The majority of the metals are distributed between the insoluble (mineral phase) and the intracellular (organic) phase. The affinity of metals to each solid phase depends on the sludge treatment method. The mobility of the metallic elements is closely linked to the sludge stabilization treatment as well as the sludge conditioning and dewatering processes [Lester- in press].

Table 6.5: Mobility of metals in sludge

Metal

Exchangeable

Adsorbed

cadmium

Fixed with (%)

Organic

Carbonate

Sulphide

14.8

48.8

17.5

copper

6.4

10.4

10.4

22.5

35.1

nickel

13.9

8.3

14.2

32.4

6.8

8.8

29.1

61.4

4.4

0.4

50.3

18.2

9.3

lead zinc

0.3

106

CHAPTER 6

The anaerobic digestion of sludge has a strong effect on the immobilization of metals due to the formation of metallic sulphides which have a low solubility. Tests on the extraction of metals from anaerobic digested sludge using pure water have demonstrated that the extractable metal fraction was very low, representing in average 10 % of the total elements [Siddle 1977].

lnfluence of the pH on mineral element solubilization The pH of the environment influences the solubilization of the mineral elements fixed in a sludge. Figure 6.1 shows that calcium, iron and zinc should be rapidly mobilized and solubilized at pH = 7 to pH = 5, whereas other elements such as phosphorus are only significantly mobilized at pH values of less than 3.5 [Scott 1975]. It appears that for all sludges, zinc, cadmium, and nickel are the most soluble metals in acid environments as opposed to lead and chromium. It should be noted that the quantity of solubilized metals depends not only on the final pH value but also on the quantity of acid and the quantity of dry matter (including organic and mineral compounds) involved. Leachate formation process

In the case of a sewage treatment plant sludge deposited on a landfill the transfer of pollutants to the percolate involves a number of phenomena [Barres 1986]. At a physico-chemical level, the transfer of mineral elements into the solution which passes through the sludge will depend on the rate of water flow, the changes with time in the sludge permeability (atter a conditioning treatment sludge can have a permeability of 10.6 m/s which is comparable with mixtures of fine sands and clays), the pH value, the redox potential, and the buffeting capacity of the solution and the sludge. At a biological level, the analysis of leachates and gases released by landfills differentiates between two processes which occur namely sludge degradation in an aerobic and an anaerobic phase. The metabolites produced (amino acids, volatile fatty acids, aldehydes, ammonia, nitrates, carbonic anhydride, hydrogen and sulphides) form the source of new reactions within the biomass including: -

mineral dissolution in the acid environment

-

stabilization of dissolved mineral ions by complexation with organic acids

-

metal fixation as sulphides or carbonates.

CHAPTER 6

107

Figure 6.1" The influence of pH on the solubilization of mineral elements fixed in the sludge.

f--START I-0 0-90.8

z

00.T 0-6 0.5 ,.J

,:I: 0-4 0.3

0.2 O-I 2.0

-

3<)

4.0

"

5-0

6.0

7.0

Aerobic sludge degradation The presence of oxygen in the sludge mass generates encourages intensive microbial activity. The biological reactions and the biological degradation of the organic matter involves exothermic reactions. The released heat results in the temperature of buried sludge reaching up to 60 ~ The degradation products are mainly carbon dioxide, ammonia and phosphate. For fresh, untreated sludge significant release of nitrogen is observed in the form of ammonia and nitrate. Anaerobic sludge degradation Following the initial aerobic degradation of the sludge further degradation takes place in anaerobic environment. In this type of environment the highly complex microbial ecosystem gradually transforms the organic matter in the sludge into methane and carbon dioxide. The development of this microbial ecosystem depends on the origin of the sludge, its physical state, pH and water content. Figure 6.2 summarises the biological degradation pathways.

108

CHAPTER 6 Figure 6.2: The anaerobic degradation of organic matter.

COMPLEX ORGANIC MATTER Hydrolytic Bacteria HYDROLYSIS SOLUBILIZED COMPOUND

ACID PRODUCING Bacteria VOLATILE FATTY ACIDS AND SOLVENTS Acetate Producing Bacteria ACETATE PRODUCING CH3COO-

i

[,

CO2,H2

Sulfate reducing bacteria METHANOGENESIS

Acetate producing

CH4, CO2 H2S

Phases in the degradation of biomass Hydrolysis phase During this phase long chain organic molecules (lipids, sugars, proteins, ...) are degraded into shorter chain molecules (sugars, amino acids, ...) which form the substrate for the next phase.

CHAPTER 6

109

Acidogenesis phase The products of hydrolysis are utilised by other bacteria which transform them into volatile fatty acids (butanoic acid, propanoic acid, acetic acid etc) and ethanol with the production of hydrogen andcarbon dioxide. At this stage hydrogen sulphide is formed through the action of sulphate-reducing bacteria. In the presence of sulphate, the released hydrogen is immediately consumed to form hydrogen sulphide. Acetogenesis phase This phase involves only anaerobic bacteria. The production and accumulation of volatile fatty acids and ethanol produced in the course of the previous phase are transformed into acetate, carbon dioxide and hydrogen. Methanogenic phase During this phase which occurs only in strictly anaerobic environment the acetic acids and the alcohols are transformed into carbon dioxide and methane. The biogas formed contains between 17% and 25 % carbon dioxide and 21% and 30 % methane as well as hydrogen sulphide in varying concentrations. The rate of development of anaerobic degradation depends on the age of the site where sludge is deposited.

Diffusion of the leached organic and mineral substances within the biomass This diffusion of the leached organic and mineral substances in the biomass is one of the limiting factors for the release from the deposited sludge thus affects the composition of the final leachate. The Wilke and Chang relationship [1955] permits the diffusion factor value (D) to be approximated for a given molecule in water. Williamson's works [1976] have permitted an empirical relationship to be determined to obtain the molecule diffusion factor in the biofilm: Dbiof =0.8 Dwate r with D in m'/s where Dbiof = molecule diffusion in the biofilm, Dwater = molecule diffusion in the water. The molecular diffusion is expressed by Fick's second law with the molecule consumption term by the biofilm, given by the Monod kinetics. The consumption diffusion phenomenon can be expressed as: OS(x,t) = Ds 0 2 S ~ Vmax.S(x,t) X Ot Ox~ S(x,t)+Ks Y Where, S(x,t) = substrate concentration Ks = half-reaction constant X = bacterial density

Ds = diffusion factor Vmax = maximum consumption rate (speed) Y = efficiency term.

Metabolite generation from organic substances In an aerobic environment, the biofilm breaks down into two zones (Figure 6.3). The first zone which is rich in oxygen is favourable to biological growth. It is particularly in this zone that the

110

CHAPTER 6

large molecules degrade into smaller, more soluble molecules. These soluble compounds may be dissolved and serve as substrate for other microorganisms, or complex with certain metals. The released ammonia can be transformed into nitrate by subsequent microbial processes if there is sufficient oxygen present (nitrification). In parallel, the excess solubilized ammonia can be complexed with copper ions. If bacterial degradation proceeds, phosphate and sulphate ions are produced together with carbon dioxide all of which play a significant role in the buffering capacity of the water. The second zone which does not receive any substrate is in the endogenous respiration phase.

Figure 6.3 Influence of mass transfer resistance on the substrate concentration gradient in the bulk water compartment. The concentration continues to decrease in the biofilm as a result of diffusion in the biofllm and removal of the substrate by micro-organisms.

In an anaerobic environment, the diffusion rates are slow. A reaction area and a substrateless area where the biofilm is degraded and detached are also observed. The by-products of the intermediate degradation processes have significant effects: on the pH value, the acetic acid release can lower the pH to values between 5 and 4, which r6sults in the decomplexing and solubilization of certain metals; the bacterial release of hydrogen sulphide favours the complexing of certain heavy metals and their immobilization as insoluble sulphide forms during its transfer to the neutral or alkaline phases; in the anaerobic phase, with a low redox potential the nitrates in the presence of high carbon concentrations are reduced to nitrogen; in the final anaerobic fermentation stage, acetic acid is transformed into carbon dioxide and methane.

Leaching and extraction tests

In 15 countries of the European Community, about 50% of the sludge produced by waste water treatment plants is dumped as non-hazardous waste and the conditions governing the disposal of sludge are specific to the legislation in each country [Brunner 1989].

CHAPTER 6

111

Assessment of leachability The leaching tests used for sludges are based on national standardization procedures [Tukker 1994]. The tests used most frequently can be described as procedures for placing a mass of sludge (S) in static contact with a volume of water (L) and defined by the ratio of L over S, the generally deionised nature of the water and the water-sludge contact time. These tests are carried out in order to obtain a preliminary approximation of the content of mineral or organic components of the soluble sludge fraction, expressed in mg/l. Since the sludge from waste water treatment plants contains a large fraction of water, the mass of sludge introduced in the test is expressed in terms of the dry residue obtained at 105~ Consequently, it is accepted practice to express the dissolved elements not in mg/l but in mg/kg of dry residue. Leaching tests performed on sludge from waste water treatment plants highlight the impact of the treatment processes used in these plants on the mobility of such elements as phosphorus, copper, zinc, nickel, lead and organic matter, in relation to the pH and the redox potential of the sludge. The leaching tests were carried out on sludge samples from Denmark, France and the Netherlands. The sludges were characterized by the sewage treatment and final sludge treatment processes. Sludge from Denmark (samples 1 - 14). The samples were from sewage plants with primary biological treatment followed by chemical and physical treatment for the removal of phosphate. The sludge volume is reduced by dewatering in a filter press (VKI, 1996). Sludge from France. Two types of sludge were subjected to the leaching tests. Samples 18 - 19 had been subjected to biological treatment for removal of carbon and nitrogen from the sewage liquor. The sludges were not stabilized. For samples 15 - 17 and 20 - 24 the fermenting capacity had been reduced by a lime chemical stabilization treatment. The pH of the sludges increased to 11 to 12. For samples 20 - 24 the pH of the sludges decreased after 6 months to one year following the development of bacteria in the sludge. Sludge from the Netherlands (sample 25). The sewage sludge was obtained from a local treatment plant with phosphate removal and no chemical additives such as iron [ECN, 1996]. The sewage amended soil studied was the BCR reference material SRM 483 [van der Sloot 1996]. The total concentration of elements contained in a sludge are given in Table 6.6. A mean pH value of 7 was assumed for all 14 Danish plants as this is the value normally observed in waste water. It is however recommended that pH is always measured in leachates. The pH, metals and nutrients released during the leaching tests are listed in Table 6.7. The leached concentrations of phosphorus, copper, zinc, nickel, lead and organic carbon are shown on Figures 6.4, 6.5, 6.6, 6.7, 6.8 and 6.9. It is not easy to interpret these figures based only on the dissolved concentrations. Reference must be made to the soluble fraction of each element, expressed in % of the total content of each element present in the sludge (Table 6.8).

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5!"3

CHAPTER 6

115

Total phosphorus (Figure 6.4) The leaching tests show that the dissolution of phosphorus occurs via three mechanisms which are not directly related to the pH of the sludge: 1. Sludge of biological or physico-chemical origin where iron or aluminium salt has been added for physico-chemical phosphate removal. The pH of these sludges is close to 7. Phosphate mobility remains less than 5% (samples 1 - 14). 2. For sludges treated with lime large quantities of lime are added (up to almost 20% of dry extract). All the phosphate is isolated in the form of calcium phosphate which is not very soluble. Less than 1% of the phosphate migrates to the leachate (samples 15 - 17 and 20 - 24). 3. Biological phosphate removal. Figure 6.4: Leachability of phosphorous from a range of sludges as a function of pH

1000

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100

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5

6

7

8

9

10

11

12

pH

o Danish samples (1-14)

13

116

CHAPTER 6

Sludge sample 19 indicates a very high level &phosphorus mobility, 33% of the phosphorus in the sludge migrates to the water. This non-stabilized fresh sludge is fermented very quickly with elimination of nitrates and a rapid drop in redox potential of between -150 and -250 mV. Under these conditions, the phosphates stored in the biological matrix are released into the water by the bacteria. Sludge sample 18 came from a medium-load treatment plant which eliminates only organic carbon. Under these conditions, a large quantity of volatile fatty acids is released during fermentation of this sludge, lowering the pH to around 5. The release of organic compounds of low molecular weight allows the reabsorption of phosphorus in the biological matrix. No salting out of phosphates was observed on this type of sludge. Figure 6.5: Leachability of copper from a range of sludges as a function of pH.

,0o I 10

E

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r

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o Danish samples (1-14) Copper (Figure 6.5) Copper is found in all sludges in the form of copper carbonate or copper hydroxide, both depending on the pH value and on the sludge treatment process. For sludges treated with lime, the copper is solubilized with the lime to form Cu(OH)2 and CuCO3. The percentage of

CHAPTER 6

117

dissolved copper released varies for this type of sludge between 7% and 21%. For sludges with a mean pH value of 7, the copper release rate in the form of CuCO3 remains less than 3%. For most sludges, the copper content does not exceed 1%. The concentration of copper found is very close to the solubility limits for copper hydroxide specified by the EPA [USEPA 1987]. Figure 6.6: Leachability of zinc from a range of sludges as a function of pH.

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o Danish samples (1-14) Zinc ff'igure 6.6) The leaching tests carried out on the various sludges indicate zinc solubility in the form of zinc carbonate with a pH value close to 7, and in the form of zinc hydroxide for pH values greater than 10. For sludge (19) zinc mobility is controlled by the salting out of organic carbon under a redox potential close to -250 mV and by released phosphorus.

118

CHAPTER 6

Lead (Figure 6.7) Less than 1% of the lead present in the sludges is released into solution. These tests, performed on a wide variety of sludges, confirm that lead is a metal that fixes on the surface or within the biomass and is thus difficult to mobilize. For sludges with pH value close to 7, the mean lead solubility is 20 lag/l. Lead solubility remains less than 100 lag/l for lime-treated sludges. Figure 6.7: Leachability of lead from a range of sludges as a function of pH.

10

%

Pb

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12

13

CHAPTER 6

119

Nickel (Figure 6.8] The mobility of nickel in a sludge depends on the pH of the sludge. At a pH of 7 its mobility may exceed 5% and reach 8% to 10% at a pH value of 12 in the presence of lime. Figure 6.8: Leachability of nickel from a range of sludges as function of pH.

,~ I % E o o~ (t3

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Organic Carbon (Fig,ure 6.9) v

The measurement of dissolved organic carbon (DOC) is not in itself sufficient to explain the role played by DOC in the mechanism of metal transfer during a leaching test. For

120

CHAPTER 6

mechanically dried sludges at a pH of 7, very wide variations in DOC are observed in the leachate and this would seem to indicate the existence of bacterial activity in these sludges. For sludges treated with lime, a small proportion of the DOC is dissolved in aqueous phase and the sludge is found to remain stable for a period of several months. The DOC and the redox potential of the sludge, which are indicative of bacterial activity, are highly important parameters which must be taken into account in determining the transfer of metallic elements and nutrients from the sludge to the leachate and to the ground. Figure 6.9: Leachability of DOC from a range of sludges as a function of pH.

10000

Sewage a m e n d e d soil 19

8 O

1000

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+

15 V ~___...__

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8

9

10

11

12

pH

o Danish samples (1-14)

13

CHAPTER 6

121

Relationship with other fields In France water cycle sludge is used in agricultural or mountain soil improvement. In the field of soil science distilled water is not used in laboratory tests to evaluate the mobility of metals and organic compounds in soils due to the very low solubility of these parameters in water. To assess the fraction which can be leached by water in the short and medium term in a soil, extraction reagents offering variable aggressiveness are used. European Directive 86/278 which defines the utilization conditions for sludge in agriculture, determines the limit values of metallic elements and fertilisers based on maximum allowable concentrations in the dry residue. The total heavy metal concentrations are therefore determined by aqua regia extraction.

Future developments The first step towards the harmonization of the technical fields which involve leaching or extraction tests in an aqueous solution consists of promoting the knowledge of the physicochemical and biological processes which govern the transfer of constituents present in the complex solid matrix to the water phase. The second step is the identification of the driving factors which control this constituent transfer, as a function of the nature of the sludge and the scenario being considered. In addition to these first two steps, there must be a will to harmonize the analytical methods while acknowledging the influence of the matrix on leaching.

122

REFERENCES TO CHAPTER 6 REFERENCES

AGHTM (General Association of Municipal and Sanitary Engineers). Produire des boues utilisables en agriculture. Techniques Sciences et Methodes. 1988. M.Barres., C.Bouster., F.Colin., A.Navarro., G.Pillay., P.Revin., C.Rouph., J.Roussy., Etude bibliographique sur les lixiviats produits par la mise en d6charge de d6chets industriels. Minist&e de l'Environnement, Service de recherche, des 6tudes et du traitement de l'information sur l'environnement (A bibliographic study on leachates produced by industrial waste deposits on landfills. French Ministry of the Environment, Environment Research, Study and Information Processing Department) 4 volumes, 321. 1986. P.H.Brunner.,T.H.Lichtensteiger.,Treatment of sewage sludge: Thermophilic aerobic Digestion and processing requirements for landfilling .COS 61 ,52-57 ,Elsevier Applied Science 1989. i

ECN, Petten, The Netherlands. Unpublished results. 1996 M . S Gould, E.J Genetelli. The effect of methylation and hydrogen ion concentration on heavy metals binding by anaerobically digested sludge.Water Research, 12,889. 1978. T, D. Hayes, T.L.Theis, The distribution of heavy metals in anaerobic digestion .Journal Water Pollution Control Fed, 50, 01, 61-72. 1978.

J, N. Lester. Heavy metals in wastewater and sludge treatment process. Vol I . Sources, analysis and legislation (ISBN 0-8493-4668-1,4468-1,183 p), CRC. In press. D.S. Scott., H.Horlings., Removal of phosphate and metals from sewage sludges. Environmental Science and Technologic., 09, (09), 849-855. 1975. R.C. Siddle, L.T Kardos. Aqueous release of heavy metals from two sewage sludges. Water Air, and Soil Pollution, 08, 453 - 459. 1977. A.Tukker, M. van den Berg.,H.A. van der Sloot., State-of-the-Art Document Waste Characterization CEN/TC 292.,Final Draft May31 1994. H. A. van der Sloot, R. N. J. Comans and O. Hjelmar. Similarities in the leaching behaviour of trace contaminants from waste, stabilized waste, construction materials and soil. The Science of the Total Environment, 178, 111-126. 1996. VKI, H~rsholm, Denmark (unpublished results, 1996) U.S.EPA. Federal Register. 52(155). 29999(Aug. 12,1987) C.R.Wilke.,P.C.Chang.,AICH J., 1,264-270, 1955. K.J.Willamson.,P.L.J.McCarty.,Water Pollution Control.Feder.,48,281-296, 1976.

CHAPTER 7

CHAPTER

123

7: C O M P O S T S

Introduction

Compost has traditionally been produced from plant material. Compost is not considered as waste but as an organic soil improver. The addition of organic substances from industrial or municipal wastes, for instance, can lead to the consideration of composts as secondary wastes. The reasons for the use of wastes in composting processes are many and include: recycling elements with agronomic interest (e.g. phosphorus, nitrogen, organic matter), reduction of the initial volume of the waste, degradation of toxic organic substances, production of energy (fermentation followed by composting processes) and decrease of the heavy metal content of a waste by dilution with other organic substances to allow agricultural use. Due to the origin of the organic component material composts may contain heavy metals, toxic organic substances and inert components. Usually, composts are used on agricultural land. Accordingly it is important to evaluate the quality of the final product to protect the quality of the crops and/or quality of the environment. Different analytical methods may be used to determine the composition of composts. Methods for the analysis of total or extractable components are similar to those used for soil analysis. An overview is provided below of the different processes and the main organic component materials used for compost production. Following the overview the main extraction processes used currently to evaluate compost quality are discussed together with the problems associated with their use. Compost production processes

Composting is a microbial reaction of mineralization and partial humification of organic substances which under optimum conditions take place within a month. Composting requires mostly aerobic conditions so that organic matter is partially mineralized and humified. To make composting suitable for use by the waste disposal industry three fundamental requirements must be met :

brevity of the process and low energy consumption; to guarantee a standard end-product which is not only safe for agricultural use but also of a satisfactory fertilising value; hygienic safety of plant and end-products. [De Bertoldi 1984]

For these reasons composting must be controlled in order to obtain a high quality end-product. Several systems are used to control the microbial activity.

Open systems Turned pile This is one of the most classical processes, it is simple but it has some disadvantages. This

124

CHAPTER 7

process needs a large open area and the control over the level of oxygen is poor. Maturation of composts using this process is not very consistent. Static pile A static composting pile is aerated by forced air. This process allows control of the exact amount of oxygen but also the control of moisture and temperature of the pile.

Closed systems A vertical reactor can be continuous where mass is introduced at one time or discontinuous where mass is introduced at different times. The main disadvantage is the extreme difficulty of controlling the process mainly in respect of the oxygenation of the mass. The lower part is over-ventilated, dried and cooled whereas the upper part is insufficiently aerated, air is warm, enriched in carbon dioxide and has a low level of oxygen. Horizontal reactor In this type of equipment it is possible to control oxygenation, humidity and temperature of the composting process effectively by blowing in air from the bottom of the reactor. It is possible to turn the pile inside the reactor and to produce a uniform compost.

Main organic component materials In the traditional compost production process agricultural plant material is used. This class of compost has a very low content of trace metals and when compost is correctly matured only agronomic analysis is needed to evaluate the fertilising value.

Today, composts are produced from other organic sources including:

Wastes from the food and agricultural industries. These wastes are composed of organic matter are not mixed with other impurities. They are generally free from contamination. Town horticultural wastes from parks and gardens (bushes, leaves of trees, wood chippings, grass). These wastes can be polluted with lead from the combustion of gasoline and with other metals produced by town activities (e.g. zinc and cadmium from tyre wear). Tree bark, sawdust and wood chip. Because of their high ratio of carbon/nitrogen these materials need the addition of a nitrogen rich material. In some cases manure or pig slurry is used, in many cases sewage sludge is used. Input of pig slurry or sewage sludge increases significantly the level of heavy metals in the end product. Sewage sludge is frequently composted with wood wastes. It is a means of diluting heavy metal concentrations to obtain levels of metals sufficiently low for the compost to be used in agriculture. Household refuse. The process of composting household refuse is well known and the quality of the product is dependent on the quality of the primary material.

CHAPTER 7

125

Table 7.1: Concentration of total trace metals in French sewage sludge (mg/kg of dry matter) [Wiart 1994] Element Mean Median

Cd 5.3 4.5

Cr 80 64

Cu 334 286

Hg 2.7 2.1

Ni 39 35

Pb 133 107

Se 7.4 3.2

Zn 921 761

It is usually considered that quality of composts is related to their inert content: domestic refuse compost (inert< 6%of D.M.), good quality town refuse compost (inert<15-20% of D.M.), average quality town refuse compost (inert<25-30% of D.M.). Table 7.2 shows that the amount of inert components is related to the enrichment of compost with metals. It is possible to produce composts from a mixture of different wastes. Where this is the case the pollution of the end product is increased because it is the sum of the contamination in the initial material. During the composting process some organic contaminants disappear and the product is sterilized during the pasteurization period but trace metals remain in the end product and this constitutes a very important problem from an agricultural and environmental point of view. Based on the fact that the composting process leads to a loss of about 50% of the initial mass the level of metals is increased by a factor two compared with the concentrations in the original material [Morvan 1995].

Table 7.2: The average composition of household refuse in France [Morvan 1995] component putrescible paper glazed paper cardboard composite textiles sanitary textiles polyester leaf polyvinyl Ch. polyethylene P.E.&PP bottles polystyrene other plastics combustible materials glass ferrous metals aluminium non combustible materials battery 8 to 20 mm < 8 mm

% D.M. 11 11 6 9 2 3 4 6' 2 0.6 1 1

Cd 9

Cr |

|

J

|

|

|

|

|

0

|

Hg

Ni |

0.47 0.77 2.8~ 0.4 2.3 3.7 4.3 ~ 1.2 i 0.77 2.6 36.3 1.9 25.1 41 7.9 i 10 0.49 4 0.23 1 12.2 2 0.88 0.8, 10.4, 12 0.98 8 4.28 .

Cu

|

|

6 10 14 13 10 6 18 58 128 77 27 28 38 266

|

|

|

i

|

|

i

|

i

289 225 47 2 5 30

17 36 52 41 41 115 31 99 56 174 19 95 803 152 6.8 200 265 11 461 33 346

Pb |

0 0i 0.15 0 0' 1.04 0.12 0.15 0.12 0 0 0.18 0.12 0 0 0.96 0 0 3064 0 0.12 |

.

.

|

|

.

|

.

|

|

|

|

|

Zn |

9.1 170 83 6.9 22. 50 6.1 33 59 5.2 22 93 19' 24 ~ 65 14i 32 158 23 198 323 76 132 276 160 1280 85 87 85 143 27 37 124 41 2890 136 40 380 287 40 300 1208 4.8 120 7 227 158 135 80 173 121 59 31 61 72 175 198000 11 41 299 24 380! 674 ,

j

.

|

|

i

|

.

.

|

|

.

.

.

.

|

|

.

|

|

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|

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i

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126

CHAPTER 7

Quality of composts

Effect of organic starting material Depending on the origin of the organic components the composition of composts may change significantly. In Table 7.3 the metal composition of different composts is shown.

Table 7.3: Trace element content of different MSW composts [De Feyter 1994]. Trace elements MSW compost without separation MSW compost with mechanical separation MSW compost with selective source separation

As 7.3 -

Cd 7.3 2.1

Cr 164 75

Cu 512 290

Hg 3.3

Ni 112 40

Pb 850 740

Zn 1640 870

2.9

0.74

19.7

33

0.14

9.5

83

160

In Table 7.4, the composition and properties of Dutch composts derived from source separated municipal putrescible and garden waste are given.

Table 7.4.: Composition and properties of GFT compost. [Quality control GFT compost.Vereniging van Afvalverwerkers -VVAV,1996] Parameter Density k~m 3 Dry matter content Percent water Percent or~;anic matter Cd mg/kg d.m. Cr Cu HB Ni Pb Zn As N-tot ~/k~ d.m. P K M8 C1

Average (n = 82) 650 70 35 38 0.5 14 25 0.12 7 45 160 4 17 2.7 7.5 2.5 3.4

Standard deviation 90 8 6 5 0.1 7 6 0.05 3 15 30 1 2 0.57 1.5 0.7 1.2

Maturation effect Maturation of compost has a considerable influence on solubility/extraction of metal, organic carbon and nitrogen. As far as there is a relationship between solubility, transportation of trace elements by organic soluble carbon and bioavailability, it is possible to suppose that maturation is an important factor in heavy metal plant uptake from composts. Table 7.5 shows

CHAPTER 7

127

the evolution of solubility of metals at two steps in the maturation process. Table 7.5: Trace metals, organic carbon and nitrogen extracted with water or potassium pyrophosphate in fresh or mature composts. [Prudent 1993].

H20 Cd lag/g D.M. Cu ~g/g D.M. Pb ~tg/g D.M. C.O.T. m8/l N m8/l

Fresh compost K4P207 0.1M 0.9 0.3 3.7 8.8 2.1 110 270 1200 5.6 2.8

Mature compost H20 K4P207 0.1M 0.2 0.7 8 7.4 1 130 330 2000 8.4 8.4

Comparison of humic substancesfrom soil and compost. Comparing the chemical composition of humic substances extracted from soil (sandy podzolic soil) and compost (MSW), it is easy to see important differences (Table 7.6). Ash content is lower in the case of compost which means that humic acids are weakly bounded with the mineral part of the waste This point is verified by studying infra red spectra at 1000 and 1100 -1 cm [Gomez 1986]. Nitrogen and sulphur contents are high in composts resulting in a strong capacity to bind trace elements. The infra red spectra points out the presence of N-H stretching associated to the O-H stretching by H bounds. This indicates clearly the presence of proteinic groups in the humic acids confirming the ability to bond with metals [Gomez 1986]. A study of biological stability with respirometric techniques shows that humic compounds from composts are easily biodegradable [Gomez 1986]. Table 7.6: Organic and mineral composition of humic acids extracted from soil and compost [Gomez 1986]

Soil Ashes % C% N% C/N S% P205 ~tg/g Ca lag/g Ve lag/g Zn lag/g Pb ~tg/g C.E.C. Ph 6

24.8 48.7 2.15 18.6 0.51 4134 116 23969 48 31 332

MSW Compost (4 month maturation) 10.1 48.2 5.41 8.9 1.07 5348 109 2026 207 152 160

Parameters relevant to leaching Clearly organic matter is one of the most relevant parameters in leaching and extraction processes. It facilitates metal transportation by means of soluble metal complexes, pH is of great importance in this case. On the other hand manganese and iron oxides may reduce the mobility by sorption processes and for this reason are also important factors. In this case redox

128

CHAPTER 7

conditions are important. It must be assumed that soluble salts are present in all composts. Percolation therefore predominates in the control of release of metals from composts.

Leaching/extraction tests All the extracting reagents for soils are frequently used for composts: for example neutral salts such as ammonium acetate, EDTA, acid solutions and oxidising/acid reagents. Because of the relatively high level of trace elements it is possible to use water for extraction of the soluble components or with weak extractants such as CaCI2, MgCI2, NaNO3. These extractants provide a good indication of nutrient availability for crops and of the risk of release of harmful substances. If prediction in the medium term is needed of the mobility or the availability of trace elements it is useful to work with alkaline reagents. As such reagents dissolve a greater quantity of organic matter they provide a good indication of the risk to crops and groundwater. Many reagents may be used for this purpose and data are available for potassium pyrophosphate [Prudent 1993].

REFERENCES TO CHAPTER 7

129

REFERENCES de Bertoldi M., Vallini G., Pera A. Technological aspects of composting including modelling and microbiology. In Composting of agricultural and other wastes. Edited by J. K. R. Gasser, 1984. Elsevier. p. 2 7-4. de Feyter W., 1994. Biological treatment of garden, fruit and vegetable waste (biowaste) in The Nederlands. In Symposium international sur le traitement des d6chets. Pollutec 94, 18-20 Octobre 1994, Lyon 199-211. Gomez A., Lejeune C., 1986. Comparison of the physical and chemical properties of humic acids extracted from a podzolic soil and a mature city refuse compost. In Compost: production, quality and use. Edited by Bertoldi M., Ferranti M. P., L'Hermite P. and Zucconi F. Morvan B., Carre J., 1995. Oligo-616mentset micropollutants dans les composts. number 2. P. 13 8-140.

TSM

Prudent P., Origine et sp6ciation des m6raux en traces dans les d6chets m6nagers, leur 6volution au cours du compostage et de la valorisation agricole. Th6se Universit6 de Marseille, France, 1993. VVAV, Data on compost composition from quality control (1996). Wiart J., Verdier M., 1994. Etude de la teneur en d6ments traces m6talliques des boues de stations d'~puration urbaines frangaises. Rapport AGHTM et FNDAE. 60 p.

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CHAPTER 8 CHAPTER

8: G R A N U L A R

WASTE AND INDUSTRIAL

131 SLUDGES

Introduction

Leaching tests play an important role in the characterization of granular waste materials and industrial sludges, particularly in relation to assessments of their actual or potential environmental impacts when utilized or deposited. As pointed out elsewhere, different types of leaching tests are needed to address different aspects of leaching. However the existence of a multitude of leaching procedures for waste characterization, many of which differ only slightly from each other, not only reflects a real need to assess a number of fundamentally different leaching situations but also is the result of different and sometimes uncoordinated approaches to a relatively limited number of basic leaching problems which are similar across geographical borders and industrial and technical fields. There is therefore a need for harmonization of leaching tests between this field and others and within the field of application of leaching tests to the characterization of granular waste and industrial sludges. Leaching tests are applied to waste materials to provide information about the release of specific components under reference conditions, or under conditions that may approximate more closely or simulate the actual field situation under consideration. This information may be used subsequently in mathematical models to predict the results of short and long term leaching. Whenever the use of a leaching test is required to investigate the behaviour of waste products, the relevant scenario (i.e. the conditions, properties and time scale related to disposal or utilization) must be described sufficiently well to facilitate identification of the leaching regime and the mechanisms likely to control the release of contaminants by leaching. This will facilitate an appropriate choice of test method(s) and test conditions and will provide some of the information needed for the evaluation and interpretation of the test results. Almost invariably leaching/extraction tests on granular waste and industrial sludges are undertaken either directly or indirectly to facilitate an assessment of the actual or potential impacts on the environment or human health caused by the management of these waste materials. For a specific waste disposal or utilization scenario it may be argued that if the ultimate question: "What is rate of leaching of all contaminants of interest from the waste as a function of time and/or the liquid/solid ratio (L/S)?" has been answered, then most other relevant questions could be derived from the answer. Although this argument is valid to a certain degree, arriving at the full answer to the above question may require comprehensive and complicated testing. Even then the results may not always account for potential changes in the leaching conditions caused by unanticipated external influences. In many cases the questions are less comprehensive and the objectives of the leaching procedures necessary to provide the answers may be simpler and more specific. A number of common objectives of leaching tests applied to waste materials are presented in table 8.1. The transformation of a general objective into one or more specific question(s) and then the selection of appropriate leaching procedures is illustrated in table 8.2 for three of the objectives shown in table 8.1, ie environmental impact assessment, quality control in waste treatment and waste classification. For example the general purpose of an environmental impact assessment could be to determine the short term and/or long term leaching of potential contaminants from landfilled granular waste which could lead to the more precisely formulated question: "What is the rate of leaching of a given component as a function of time and/or

132

CHAPTER 8

L/S?" Depending on the scenario under consideration, the answer could be provided from the results of column, lysimeter or multiple batch leaching tests. If the rate of leaching of a component is known as a function of time and/or L/S, another question could be: "What is the influence of various factors (e.g. pH, redox potential, ionic strength, DOC) on the rate of leaching of various contaminants?" which, in turn, could be investigated and answered from the results of pH-static batch leaching tests or from batch leaching tests under special conditions (forced values Of EH, I, DOC).

Table 8.1: Typical objectives of leaching tests applied to waste materials. Adapted from Chandler et al. [1995].

Objective

Description

Environmental impact assessment

Estimate potential impact of waste disposal or utilization on the environment.

Quality control in waste treatment

Verify the efficiency of a treatment pro tess using a simple pass/fail criterion.

Waste classification

Compare wastes against performance cri teria for classification e.g. as hazardous or non-hazardous.

Identification of leachable constituents

Determine which constituents of a waste are subject to dissolution upon contact with a liquid.

Evaluation of process modifications

Determine if modifications to a waste-generating process result in less leachable waste.

Design of leachate treatment systems

Obtain a typical leachate to use for treatability experiments.

Field concentration estimates

Express leaching over time (e.g. to be used as a source in transport modelling).

Parameter quantification for modelling

Quantify partition coefficients and kinetic parameters for use in transport modelling.

Most users of leaching tests share the general objective of obtaining results which are as "true" or realistic as possible. The importance placed on various test requirements may vary between different groups of users. Administrators and regulators who want to use a leaching test for regulatory purposes will oi~en favour such properties as reproducibility, versatility, simplicity, adaptability into a regulatory framework, expediency and low costs over scientific stringency.

Table 8.2 Examples of relationships between general objectives and specific questions

General field of applicaUon

Environmental impact assessment

General objecUve

9 Determination of the short and/or long term leaching of contaminants from utilized/deposited waste.

Examples of specific questions

Typical examples of leaching tests which may provide Ihe answer

9 VVhat is the rate of leaching of various contaminants as a function of time and/or L/S?

9Column or lysimeter leaching test 9Multiple batch leaching tests

9 What is the influence of various factors (pH, redox potential, DOC, etc.) on the rate of leaching of contaminants?

9pH-static batch leaching tests 9Batch leaching tests under special conditions

9 Determination of the total amount of any contaminant which may potentially be leached from utilized/deposited waste in the long term.

9 What is the total leachable amount of any contaminant under conditions which favour release (small particle size, high L/S, pH in the range of high solubility of the contaminants in question)?

9Availability test

Quality control

9 Determination of whether the leaching properties of a waste material or product which has undergone treatment comply with a given norm.

9 Does the amount of specified components leached under specified conditions comply with the appropriate limit values?

9Single or multiple batch leaching test

Waste classification

9 Grouping of various types of waste into different classes in accordance with their leaching properties (e.g. in relation to placement in different types of landfills).

9 Does the amount of specified components leached under specified conditions comply with the appropriate limit values?

9Single or multiple batch leaching test

:=

).

,H r~

oo

134

CHAPTER 8

Many waste producers who must comply with regulations may have similar priorities. However, for waste producers and managers faced with long term liability issues in relation to disposal or utilization, the ability of a test to provide scientifically accurate and reliable results may be more important than cost, expediency and simplicity. The same is true for those studying the release mechanisms controlling the leaching of various components from waste materials or those seeking to predict the short and/or long term leaching behaviour of deposited or utilized granular wastes and industrial sludges. Their main interest will be the ability of the leaching tests to simulate field conditions as closely as possible or to study specific leaching phenomena under varying well defined conditions. All users are, of course, interested in good reproducibility of the leaching procedures used. The predominance of administrative concerns over scientific interests have sometimes led to the implementation of inadequate leaching procedures in waste regulations. In regulatory frameworks it is not uncommon to see leaching tests used for purposes for which they were not developed under conditions for which they are not scientifically valid. The interpretation of the results from these tests and the limit values applied in relation to waste classification do not always reflect the existing knowledge of the underlying release mechanisms and may therefore, under critical conditions, lead to erroneous conclusions and decisions. One such example is the application of test methods developed for granular waste and industrial sludges under equilibrium conditions to monolithic waste materials under non-equilibrium conditions. When used in a regulatory framework the result of a leaching test is normally evaluated by comparing it to a set of limit values. This comparison is only meaningful in a scientific sense if both the test and the limit values are related to a scenario consistent with the leaching situation and the risk against which the limit values are meant to protect. The development of rationally motivated limit values for regulatory leaching procedures, e.g. based on landfilling or utilization scenarios, has often been neglected. New international developments may gradually improve the situation described above. Within the framework of the Technical Committee 292 "Characterization of waste" of the European standardization organization CEN, two important steps have been taken in this direction. The first step is the identification and general acceptance of three logistic levels of testing: Basic characterization, Compliance testing and On-site verification (see Chapter 2). Regulatory tests belonging to all three categories exist, although most of the regulatory leaching procedures currently used are Compliance tests. Most of the tests that are useful to researchers will belong to the Basic characterization group Compliance testing should by definition be based on and related to an understanding of the basic leaching behaviour of a material in a given environment. This requirement is seldom fulfilled by current regulatory use of Compliance leaching tests but it may gradually lead to more appropriate future regulations. The other step forward is the development of a Methodology Guideline for the Determination of the Leaching Behaviour of Waste under Specified Conditions [CEN 1996a]. This guideline emphasizes the need to formulate precise questions concerning the leaching behaviour of waste before choosing a test method, and it prescribes the use of scenarios as part of this process. The guideline provides a stepwise methodology which includes consideration of the following issues:

CHAPTER 8 1) 2) 3) 4)

5) 6) 7)

135

Formulation of the question(s) to be answered Description of the scenario considered Description of the waste Determination of the leaching behaviour and the influence of various parameters on the leaching behaviour Modelling of the leaching behaviour Model validation Conclusions

Following this sequence of steps will not in itself ensure a good result, but it will help in creating a rational and scientific background for the selection and performance of leaching tests for characterization of waste materials and for the subsequent interpretation of the results. General characteristics of granular waste and industrial sludges

Definitions The most important defining characteristic common to both granular wastes and industrial sludges is that they consist of relatively small particles (granular means made of granules, i.e. small particles like fine grains). In practice neither is defined very precisely. CEN TC 292 [ 1996] defines granular waste as "solid waste that is not mono#thic" and industrial sludge as a "material obtained by precipitation, sedimentation or filtration". Flotation should perhaps be added to the processes producing sludge. Consequently the term granular waste may cover any waste stream consisting of particles ranging in size from fine powder to characteristic diameters of 1 - 4 cm, while the term industrial sludge will apply to fine-grained industrial wastes formed by precipitation, sedimentation or filtration. Sludges will often have a tendency to retain substantial amounts of water. In this context, only granular waste streams and industrial sludges which are predominantly of inorganic or mineral composition will be considered.

Waste types and composition Granular waste materials such as bottom ash and air pollution control residues from waste incinerators, coal fired power plants and industrial processes, mining wastes, iron and steel production wastes and residues from numerous other industrial processes are produced in bulk quantities and must be landfilled or utilized in an environmentally safe manner. Some examples of common granular waste materials which have been characterized with respect to leaching characteristics are listed in table 8.3. For a number of the granular waste materials the pH of the accumulated fractions of eluate collected in a column leaching test at L/S = 1 and 10 1/kg, respectively, is shown [Aalbers, 1992]. The composition of granular waste materials is very diverse and cannot be generalized but some examples of typical composition data on some combustion residues are presented in table 8.4.

136

CHAPTER 8

Table 8.3: Examples of granular waste materials which have been subjected to leaching tests [Aalbers 1992, VKI 1996]. The table also shows pH values in eluate fractions from column leaching tests. Types of granular waste materials pH at pH at L/S = 1 L/S = 10

Acid gas cleaning residues from MSW incineration (dry lime injection) Acid gas cleaning residues from MSW incineration (semidry process) Alkaline battery waste Aluminum production ash Ash from incineration of wastewater treatment sludge Blast furnace slag Bottom ash from municipal solid waste incineration Catalytic cracker from catalytic oxidation of RVC Coal fly ash (alkaline) (pH at L/S = 1 otten > 11 for alkaline CFA) Coal fly ash (neutral) Coal fly ash (acidic) Cryolite waste from Zeolite production Dust from primary aluminum production (fluoride containing) Dust from a sand blasting unit Fe-norit waste from pharmaceutical industry Filter dust from ceramic industry Flue gas desulphurization waste from coal firing (semidry process) Flue gas desulphurization waste from coal firing (gypsum process) Fluorescent powder Fly ash from industrial waste incineration Fly ash from incineration of industrial waste and RDF Fly ash from isolation material production Fly ash from municipal solid waste incineration Foundry oven dust Foundry sand (waste) Glass oven rubble from glass production Oven waste from primary aluminum production Phosphogypsum waste Plastic waste material Shredder waste Spent catalyst from oil industry (activated AI) Steel slag Tailings from Zn-Pb mining Zn-Fe salt residue from Zn varnish installation Zn-MnO battery waste

11.6

12.5

11.5 12.4 6.3

12.5 12.5 6.1

11.9 5.8 12.6 9.3 8.9 67 63 89 6.3 9.2

11.5 6.3 12.6 10.1 9.7 7.0 6.1 7.7 5.5 9.5

9.0 4.7 9.3 11.2 12.0 10.1 9.8 7.9 12.4 2.6 9.8

9.9 6.0 10.3 6.8 11.2 8.0 9.2 7.5 11.6

5.3

5.4

6.2 5.0

6.6 5.3i

9.4

Sludges from a multitude of different industrial production, treatment and dewatering processes are also produced in large quantities and must, like the granular waste materials, be managed safely. A list of examples of industrial sludges which have been subjected to leaching tests is shown in table 8.5. This table also shows the pH of eluates from column leaching tests at L/S = 1 and 101/kg [Aalbers, 1992]. The composition of an industrial sludge will depend on the process from which it originates and may vary widely between sludges of different origin.

|!

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137

Some typical composition data on three specific industrial sludge types are shown in table 8.6. The waste and sludge composition data shown in table 8.4 and table 8.6 represent the total concentrations of the constituents shown, i.e. the materials have been analyzed by x-ray fluorescence (major components) and AAS or ICP/ICP-MS atter total digestion with HF+HNO3+HCI (trace elements). It should be noted that in many cases chemical composition data for mineral granular waste materials and industrial sludges are based on analysis atter partial digestion, e.g. with aqua regia or nitric acid which are unable to dissolve silicates. Since many granular waste materials and industrial sludges have a substantial content of silicates, trace elements associated with the silicate matrix may not be fully dissolved by partial digestion methods. In a coal fly ash study [Hjelmar 1991] the relative contents of trace elements determined by AAS atter partial digestion with nitric acid and total digestion with HF+HNO3+HCI, respectively, were 19-32% (Cr), 83-94% (Mo), 72-94% (Se), 41-55% (V), 56-83% (Cd), 33-48% (Pb) and 33-53% (Zn). Similar studies on other waste materials have shown that the percentage of the total amount of a trace element recovered by nitric digestion varies both with the waste material and the trace element. Since true total composition data are much more useful and well-defined than composition data based on partial digestion methods, and since they may also be used for general purposes such as mass balances etc., it is generally recommended that only chemical analytical methods which provide true total contents are used to describe the composition of waste materials.

Leaching of contaminants from granular waste and industrial sludges

Leaching from waste Leaching may be defined as the dissolution of a soluble constituent from a solid phase into a solvent. Accumulated dissolved constituents in the leachant eventually comprise a leachate (or an eluate, as the liquid resulting from a leaching test is called). Leaching of contaminants from granular waste and industrial sludges can be a complex process and many factors influence the release of specific constituents both in the short and long term. A granular leaching system may be at equilibrium or steady state conditions or it may be kinetically controlled. Kinetically controlled release can be associated with relatively slow chemical dissolution or remineralization reactions or with physical diffusion processes.

Release controlling mechanisms For granular waste materials and sludges, many of the relevant leaching scenarios will comprise the percolation of waste by water. In such systems, equilibrium conditions may form if the rate of release of a constituent from the individual particles is fast compared to the advective velocity of the percolating water. Conversely, non-equilibrium or kinetically controlled leaching conditions are likely for a particular constituent in systems where the advective velocity of water through the material is high compared to the release rate of that constituent from the particles.

Table 8.4 Examples of composition of mineral granular waste materials: Combustion residues. (Hjelmar & Thomassen, 1992; Hjelmar, 1996).

Residues from coal fired power plants

Residues from municipal solid waste incineration Parameter

Unit Flue gas desulphurization

Acid gas cleaning Bottom

ash

Fly ash

waste from dry/semid-

C o a l fly ash

waste from semidry process

ry p r o c e s s Si

g/kg

210 - 290

95 - 190

57 - 9 8

150 - 2 6 0

1.4-

190

A1

g/kg

4 7 - 72

4 9 - 78

17-46

97-

1.4-

120

Fe

g/kg

2 7 - 150

18 - 35

3.6-

18

140

35 - 120

1.3 - 43

Ca

g/kg

6 5 - 97

74 - 130

170 - 2 9 0

2 0 - 52

74 - 320

Mg

g/kg

7.7-

19

11 - 19

7.1 - 12

5.9 - 25

4.4 - 15

K

g/kg

9.2 - 22

23 - 4 7

27-40

9.0-

0.8-

Na

g/kg

22 - 41

22 - 57

12 - 19

1.8 - 6.8

2.2 - 14

Ti

g/kg

3.2 - 7.2

7.5 - 12

1.5 - 5.1

5.2 - 8.5

0.3 - 7.1

S

g/kg

1.3 - 8

11 - 32

1.4-4.4

20-

C1

g/kg

1.2 - 3.2

45 - 101

P

g/kg

2.9 - 13

4 . 8 - 9.6

Mn

g/kg

0.8-

As

mg/kg

19 - 8 0

49 - 320

40-260

15-

Ba

mg/kg

900 - 2700

920 - 1800

Cd

mg/kg

1.4 - 4 0

250 - 450

140 - 3 0 0

0 . 4 - 1.5

Co

mg/kg

< 10 - 4 0

29 - 69

4 - 15

53 - 82

<0.7-

1.7

1.7

8-

18

19

10

180

0.01 - 0.09

0 . 2 - 33

1.7-4.6

1.1 - 3 . 4

O.04 - 2.3

0.3 - 0 . 7

0 . 2 - 2.9

0.07 - 0.4

92 - 2 2 0

310-

1400

790-

150

3 - 54

1800 0.07 - 0.43 33 - 110

Cr

mg/kg

230 - 600

140 - 5 3 0

150 - 5 7 0

78 - 2 3 0

Cu

mg/kg

900 - 4800

860 - 1400

4 5 0 - 1100

69 - 320

<1 - 70

Hg

mg/kg

<0.01 - 3

0.8 - 7

9.3 - 4 4

0 . 0 9 - 0.4

<0.1 - 0.4

Mo

mg/kg

2.5 - 4 0

15 - 4 9

9.3 - 2 0

6.4 - 20

1 -14

Ni

mg/kg

60 - 190

92 - 2 4 0

2 0 - 63

58 - 2 1 0

1.8 - 1 5 0

Pb

mg/kg

1300 - 5400

7400 - 19000

4000 - 6500

54 - 170

7.1 - 4 3

Se

mg/kg

0.6 - 8

6.1 - 31

8.2 - 16

2.1 - 22

3.5 - 13

Sn

mg/kg

< 1 O0 - 13 O0

1400 - 1900

620 - 780

3 - 9

26-62

V

mg/kg

36 - 9 0

32 - 150

Zn

mg/kg

1800 - 6200

19000 - 41000

12000 - 19000

TOC

mg/kg

4 . 8 - 13

4.9 - 17

6-9"

pH (1% slurry)

!

160-

340

120 - 450

7.6 - 400 23 - 6 8

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Table 8.5: Examples of industrial sludges which have been subjected to leaching tests [Aalbers 1992, VKI 1996]. The table also shows pH values in eluate fractions from column leaching tests.

Types of industrial sludges

Dewatered, neutralized water treatment sludge Dewatered, neutralized water treatment sludge (filter cake) Dewatered, neutralized water treatment sludge (oil containing) FeOH water treatment sludge (needle factory) Flotation concentrate from sand blasting waste purification process Flotation sludge from sand blasting material Fluorescent sludge Gravitational concentrate of sand blasting waste Glaze/enamel sludge Gypsum sludge from flue gas cleaning at a coal fired power plant Jarosite sludge Metallurgical process sludge (CaO type) Natural gas production sludge Phosphating sludge Phoshogypsum leachate treatment sludge Pigment sludge Purification sludge from textile paint production Sb contaminated water treatment sludge Tannery sludge Water purification sludge from polymer production 1 Water purification sludge from polymer production 2 Water treatment sludge from Br industry Wet scrubber residue from MSW incineration (mixed with fly ash)

pH at L/S-1

pH at L/S-IO

6.8 7.0 9.2 5.5 82 7.7 83 73 92

7.6 8.2 9.4 7.9 7.1 8.2 9.3 7.2 9.6

18 80 82 31 6.4 6.4 7.1

2.6 9.9 7.0 4.5

6.0 9.7 10.0 3.8 9.8

7.2 12.5 12.6 3.0 10.6

8.2 7.2

For granular waste systems under equilibrium conditions the amount of a specific component released at a specific L/S value may be solubility controlled or availability controlled. Solubility controlled release can be described as leachate composition dictated by the partial dissolution of mineral phases that reach equilibrium with the leachant. The flow of liquid past individual particles enhances contact of the leachant with the solid phase, facilitating chemical equilibrium. In some instances very soluble mineral phases (e.g. chlorides) can completely dissolve resulting in a leachate composition controlled by the available amount of the elemental compound present in the solid [Chandler 1994]. Low L/S ratios (e.g. < 0.5 l/kg) generally favour solubility control whereas availability control is more likely at higher L/S values, depending on the solubility of the various constituents. For fine-grained granular waste or industrial sludge systems which are placed in stagnant water, the release rate controlling mechanism may be diffusion through the bulk surface.

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The various release mechanisms and the factors influencing them have already been discussed in Chapter 2. It is, however, very important that the mechanisms which are likely to control the release of a given component from a given type of granular waste or industrial sludge under a given set of conditions are identified and taken into consideration when choosing a leaching procedure to simulate or provide information on the leaching behaviour of the material under these conditions. If this is neglected the information obtained may result in misinterpretation and erroneous conclusions. It is preferable to choose a leaching test which provides the information necessary for an assessment of the leaching mechanisms involved. The results should show, for example, whether the assumptions (such as assumed equilibrium/nonequilibrium conditions, solubility/availability control) upon which the interpretation of the results is based have been fulfilled. However, many simple regulatory tests do not provide this information, and in these cases it is necessary to rely on past experience and general knowledge of leaching fundamentals. This is one of the reasons why it is important to support regulatory Compliance testing of a certain waste with Basic characterization background information on the same type of waste (see Chapter 2 and earlier this chapter).

Factors influencing contaminant release A number of physical and chemical factors which may influence both the total release and the rate of release have been discussed in detail in Chapter. Some of the factors which may influence the leaching of granular waste materials and industrial sludges are listed below 9 9 particle size of the waste 9 homogeneity of the waste 9 mode of contact between leachant and waste 9 leachant/waste contact time or flow rate of percolating water 9 temperature 9 sorption/ion exchange properties of the waste 9 chemical/mineralogical composition of the waste, including the presence of organic material 9 pH in the liquid phase, alkalinity/acidity of the waste, buffering capacity of the liquid 9 degree of contact with the atmosphere (oxidation, uptake of CO2) 9 redox conditions of the leaching system and the surroundings 9 the composition of the leachant, particularly the ionic strength and the presence of complexing agents These and other factors will influence the leaching behaviour of a waste material both under field conditions and when it is subjected to laboratory leaching tests. Any laboratory leaching procedure should be chosen or designed to simulate the field conditions under consideration and to avoid the influence of factors which may disturb the interpretation of the results. In most cases this merely implies the proper selection of test conditions and options for existing test methods. The sensitivity of waste materials to the various controlling factors will vary with the type of waste investigated and the scenario addressed.

Leachable components As may be seen from the description of waste characteristics, the wide variation of types and composition of granular waste and industrial sludges makes it impossible to produce a general list of "typically" leachable components. The leachable components vary between waste types and depend upon composition and leaching conditions. In this context, only leaching of inorganic constituents (mainly inorganic salts and trace elements) will be addressed.

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Table 8.6: Examples of composition of industrial sludges (FGD - Flue Gas Desulphurization)[Hjeimar 1992, Fiyvbjeng 1996].

Gypsum sludge from FGD at coal fired power plants [Hjelmar 1992]

Wet scrubber residue from municipal solid waste incinerators [Flyvbjerg 1996]

Galvanic sludge [AVR Bedrijven 1997]

g/kg g/kg g,/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg

49- 200 19 - 44 18 - 4 0 80- 100 9.0- 18 5.0 - 22 2.4- 2.8 0.7 - 2.2 17 -48

61 19 17 210 20 2.2 2.5

5.1 1.2 180 44 4.1 6.0 24

g/kg g/kg

1.0- 5.1 4.5 - 28

Unit

Parameter

r si

IAI ! ~Fe Ca Mg K Na Ti S C1 P Mn CN (total) As Ba Cd Co Cr Cu Hg Mo Ni Pb Se Sn V Zn TOC

mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg

24 -48

10- 190 56 - 130 83 - 120 13 - 34 <5 - 8 270 -430 38- 120 <3 - 160 95 - 210 540 - 1600

2.8 510 340 620 12 360 2700 1700 12 5O 17000

41000

20 4.7 3.1 32 31 2.8 140 18000 7300 33 150 13000 2200 2600 1400 180 4000

i

Leaching tests for granular waste and industrial sludges

Overview of types of leaching tests All existing leaching tests for granular waste and industrial sludges belong to one of two categories: test procedures for which equilibrium or equilibrium-like conditions are assumed and test procedures performed under non-equilibrium conditions (dynamic tests). The former are by far the most commonly used test methods for granular waste and industrial sludges.

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When the interpretation of the results of the leaching test is based upon the assumption that equilibrium conditions exist it is the requirement that a thermodynamic equilibrium or a steady state condition between the solid and the liquid phases is attained within the liquid/solid contact time allocated for the test. The assumed equilibrium condition may in some cases correspond to a so-called local equilibrium assumption (LEA). This applies for example to the performance of column leaching tests on granular materials provided the rate of percolation is sufficiently slow. Concentration gradients can exist along the length of the column, but at any point within the column equilibrium/steady state conditions must exist between the granular waste and the percolating liquid phase. Failure to fulfil the LEA will result in poor reproducibility of results which depend strongly upon the rate of flow through the column. Such results cannot be subjected to a direct L/S-based comparison with the results of batch leaching tests. For column leaching tests therefore there is a maximum flow rate for any given material and any given constituent. The maximum flow rate should not be exceeded or the LEA will be invalidated. For landfilled or utilized granular waste exposed to infiltration of precipitation under field conditions the rate of percolation is generally slower than the flow rates applied in accelerated column leaching tests, hence it is likely the LEA at full scale system will be fulfilled for many constituents of interest. The equilibrium assumption may sometimes be regarded as fulfilled even when the actual condition is only equilibrium-like. For example this may apply to situations where the waste material undergoes reactions which are very slow compared to the period during which the leaching properties are being evaluated (e.g. the release of Si from various silicate minerals). If the results are extrapolated beyond this time frame, such potential changes should be accounted for, e.g. by modelling. In dynamic leaching tests no equilibrium is reached. In practice maintenance of the leaching system at a condition as far from equilibrium as possible is often attempted in order to maximize the driving force for mass transfer from the solid phase to the liquid. The property measured in this case is the rate of release, often expressed in terms of a flux through a surface (e.g. mg/m2/s), which is comparable to the results of the tests used to measure the rate of diffusion of contaminants from monolithic materials (see Chapter 10). Some of the most common types of leaching tests for granular waste and industrial sludges are shown in table 8.7. The test conditions (equilibrium/non-equilibrium) upon which the interpretation of the results is normally based are also indicated. Column leachi.ng tests In a column leaching test the leachant is typically passed through a vertical column of the waste material in upflow or downflow, collected in fractions and analyzed. To a certain extent the leaching process occurring when rainwater infiltrates and percolates through a granular waste material which has been landfilled or utilized as a filling material is simulated in the test. The flow rate is often accelerated compared to natural conditions but a rate of flow which fulfils the LEA must be used. The duration of column leaching tests is typically several weeks to several months and if sufficiently large columns are used fractions corresponding to L/S = 0.0 - 0.1 l/kg or even lower may be collected. Column leaching tests are well suited to describe the progression of leaching in the range L/S = 0 - 2 1/kg and are in some cases used up to L/S = 10 l/kg.

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Table 8.7: Different types of leaching tests for granular waste and industrial sludges.

Type of test

Test conditions

Remarks

Column leaching tests

Equilibrium (LEA)

May become dynamic at high flow rates and/or large particle sizes

Lysimeter tests

Equilibrium (LEA)

Extraction or batch tests - single batch leaching test - multiple batch leaching tests - pH-static leaching test - availability test - special extractions

Equilibrium

May become dynamic for short contact times and/or large particle sizes

Combined column and batch leaching test

Equilibrium/LEA

May become dynamic for short contact times and/or large particle sizes

Tank leaching test for granular material

Non-equilibrium (dynamic)

Lysimeter leaching tests Lysimeter leaching tests are, in principle, large scale column leaching tests which are often performed outdoors under "natural" conditions and may be used to verify under fieldresembling conditions the results of laboratory leaching tests. The duration of lysimeter leaching tests may be of the order of one year to several years. The tests can provide information on the composition of the initial leachate at very low L/S ratios. Lysimeter leaching tests will not be discussed in further detail in this context. Extraction or batch leaching tests There are a number of different extraction or batch leaching procedures in which granular waste or industrial sludge is brought into contact with a leachant in a closed or open vessel and agitated for a period, normally for sufficent time to attain equilibrium/steady state conditions. Subsequently the liquid and solid phases are separated and the liquid is analyzed for the parameters of interest. Contact times may vary from a few hours to a few days and are often adjusted to accommodate normal working hours. In a single batch leaching test the procedure is run once at a specified L/S ratio. For most

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waste types it is impractical to perform batch leaching tests at L/S ratios lower than approximately 2 l/kg. The tests may be performed at any L/S ratio above 2 l/kg but at high L/S values analytical detection problems may arise. Standardized batch leaching tests are otten performed at L/S ratios between 2 and 20 l/kg. If aliquots of leachant are removed and analyzed at different time intervals during a single batch leaching test, the results may provide an indication of the contact time needed to attain equilibrium-like conditions. In a multiple batch leaching test the procedure described above is repeated a number of times, usually using fresh leachant each time. In a few variants the solid phase is renewed. pH-static leaching tests are single batch leaching tests in which pH is maintained at a constant value through feed-back control and the automated addition of an acid or base. The results of such pH-static leaching tests are otten used as input to hydrogeochemical models. Availability tests are single or multiple batch leaching tests carried out at high L/S ratios (typically 50 to 200 1/kg) on finely ground waste with a typical grain size of < 100 - 200 mm under pH-static conditions at one or two pH values favouring the solubility of the constituents in question or simulating specific conditions (see e.g. Chapter 9). The test conditions are usually designed to minimize physical and chemical resistance to leaching and ensure availability control at the chosen pH value(s). In some regulatory tests availability is pursued merely through the application of aggressive leachants containing acids or complexing agents (e.g. carbonic acid/CO2, acetic acid, ammonium acetate, EDTA, citric acid). Special extractions are sometimes used to investigate specific aspects of waste leaching such as the association of leachable components with various mineral phases. Sequential chemical extraction procedures [Tessier 1979, Environment Canada 1990] such as those used to characterize sediments (see Chapter 5) have been developed for granular waste materials but their practical applicability is questionable [e.g. Wallmann 1993; Gruebel 1988]. Combined column and batch leaching test A combination of a column and a single or multiple batch leaching test may combine the ability of the column leaching test to describe leaching behaviour at low L/S with the ability of the batch leaching test to describe leaching conditions at high L/S values within a relatively short period of time. The test is started as a column leaching test and run until a L/S of 1 or 2 l/kg is reached. The waste material in the column is then removed, mixed thoroughly and subjected to a single or multiple batch leaching test until the desired L/S ratio has been reached [Hjelmar 1995]. In order to ensure compatibility between the results both the column and the batch tests must be carried out under equilibrium/steady state conditions. Tank leaching test for compacted granular materials A tank leaching test is being developed in which diffusion-driven leaching from compacted granular waste materials is investigated under non-equilibrium conditions [NNI 1994]. The results are related to the exposed surface area of the compacted waste and interpreted in a manner similar to the interpretation of the results of the tank leaching test for monolithic waste forms (see Chapter 9 and 10).

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Testing conditions Some of the test conditions which are of importance to the outcome of a leaching test and which may vary from test to test, are listed below: 9 9 9 9 9 9 9 9

Pretreatment of the waste sample to be tested Composition of the leachant Mode and method ofwaste/leachant contact Liquid/solid ratio (L/S) Contact time Temperature Separation of the eluate from the solid phase Analysis of the eluate

In many standardized leaching tests for granular waste materials most of the above mentioned conditions are already fixed or prescribed. In other cases the test conditions must be chosen in accordance with the type and purpose of the leaching test and the scenario in question. The testing conditions will determine to a great exent the influence on the leaching system of the previously listed factors for a given test and a given constituent. Not all items on the list are relevant to all test methods. The following discussion is based on Hjelmar and Traberg [ 1995] and Chandler et al. [ 1995]. Pretreatment Depending on the situation and the leaching test selected, it may be necessary to subject the waste material to pretreatment including liquid/solid separation, drying, subsampling, particle size reduction, compaction and/or conservation. Sludges in particular may contain a free water phase which must be separated from the bulk sample by sedimentation and decanting, vacuum or pressure filtration or centrifugation prior to testing. The separate water phase may be analyzed as an "initial eluate". It may be necessary to dry humid waste samples prior to particle size reduction when there will be a risk of losing volatile components (e.g. Hg and various organics as well as crystal water). Therefore any drying operation should be carried out as gently as possible at the lowest convenient temperature. The subsampling techniques used (coning and quartering or application of riffles or other dividing apparatus) must ensure that the subsamples obtained are truly representative of the bulk sample. Frequently oversize material must be crushed or ground to the prescribed maximum particle size, e.g. by a jaw crusher (down to 2-4 mm) or by a tungsten carbide or agate grinder (down to < 0.125 mm). When crushing or grinding precautions are necessary to avoid cross-contamination of samples with other samples or the apparatus. Different degrees of compaction of granular waste material may be necessary during placement of the waste in columns for column leaching tests and vessels for tank leaching tests. Some waste materials, particularly when humid, may undergo aging, carbonation and oxidation reactions with the atmosphere as well as microbial degradation reactions if stored prior to testing. These reactions will be enhanced for size reduced material. If storage cannot be avoided, the waste should be placed in a dry state in airtight containers in a cool environment until testing can take place. Composition of the leachant In many leaching tests non-aggressive leachants such as demineralized water or water initially adjusted to pH = 4 with HNO3 are prescribed to simulate leaching with rainwater. At low and

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moderate L/S ratios the pH and the composition of the eluate will in most cases be controlled by the solid phase and will be relatively independent of the composition of the leachant. At higher L/S values or if a more aggressive leachant is used, the pH and the leaching conditions may be influenced by the leachant. For tests simulating the leaching conditions of scenarios such as disposal or utilization in a marine environment, disposal at a sanitary landfill, etc., other leachants such as seawater or artificial landfill leachate may be used. Mode and method of waste/leachant contact Since the objective of most leaching tests is to investigate the transfer of components from a solid phase to a liquid phase, it is important to control conditions which may affect the transfer rate and to control the risk of transfer of matter between the leaching system and the surroundings, primarily the atmosphere. If the interpretation is based on the assumption that all of the waste present is in contact with the leachant, this condition must be ensured. For column leaching tests good liquid/solid contact is often achieved by using upflow at a sufficiently slow flow rate. For batch leaching tests good mixing may be achieved by rotating the leaching vessel end-over-end, by using special eccentric rotators or by stirring. The agitation should be sufficiently gentle to avoid extreme size reduction caused by abrasion. If the waste to be tested is thermodynamically unstable under ambient conditions or otherwise easily oxidized or carbonated, it may be necessary to prevent contact with the atmosphere by the use of equipment made from diffusion resistant materials or by performing the tests in a nitrogen atmosphere, e.g. in a glove box. Zero headspace leaching equipment is sometimes used to prevent the escape of volatile components. In column leaching tests where fractions of eluate are collected over a period of time it may be necessary to protect the eluate against oxygen which will oxidize reducing components or carbon dioxide which will lower the pH of alkaline eluates and form carbonates by keeping the eluate under a nitrogen atmosphere. Liquid/solid ratio (L/S) The L/S is defined as the ratio between the amount of leachant which at any given time has been in contact with the amount of waste tested. L is generally a volume (e.g. 1) and S is the dry mass of the waste (e.g. kg) prior to testing. The dimension of L/S is therefore usually 1/kg. The choice of L/S ratio or a range of L/S ratios for a leaching test depends on the objectives and the type of tests in question, the solubility of the components of interest and the analytical detection limits. Expressing the results of leaching tests on granular waste and industrial sludges in terms of eluate composition or accumulated leached amounts of components as a function of L/S facilitates comparison of the results of different types of leaching tests and, under certain circumstances, comparison of laboratory leaching test results with field observations. Therefore in some cases it is possible to use, with considerable caution, laboratory leaching tests for the prediction of various aspects of leaching under full scale field conditions. Most laboratory leaching tests on granular waste and industrial sludges performed under equilibrium-like conditions are accelerated in time compared to the actual duration of leaching under field conditions. Under certain conditions and when the physical design and the hydraulic situation is known, the L/S scale may be converted to a time scale for a specific disposal or utilization scenario. Contact time The amount of time during which a liquid phase is in contact with a solid phase will influence the quantity of waste components leached unless equilibrium-like conditions are achieved. In

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batch extraction tests the contact time equals the duration of the test, whereas the contact time in for example column leaching tests is also a function of the flow rate. In leaching tests that are based on the assumption that thermodynamic equilibrium or local equilibrium is reached attempts should be made to ensure that this condition is reached during the testing period. For test methods of short duration or methods for which small particle sizes are not prescribed or used it may be difficult to attain equilibrium within the allocated contact time. Temperature Temperature affects the results of leaching tests. Both the van't Hoff relationship, which applies to thermodynamic equilibrium constants and solubility products, and the Arrhenius relationship, which applies to kinetic processes such as adsorption and diffusion, indicate that properties or mechanisms relevant to leaching vary exponentially with temperature. For convenience, most leaching tests are performed at room temperature. Although higher temperatures may be used to accelerate the rate of leaching, this may change the properties of the waste hence the practice is not recommended. Separation of eluate from the solid phase Eluates are commonly separated from granular waste and industrial sludge by vacuum or pressure filtration using 0.45 mm membrane filter which is a convention used to define soluble species. However, small colloidal particles can pass through a 0.45 mm filter. A smaller pore size (e.g. 0.2 mm) should be used for the removal of small particles. The use of the smaller filter size should be reported with the data. Glass fibre filters are necessary when hydrophobic, low solubility organic molecules are expected in the eluate since they may have a high affinity for membrane filters made of an organic polymer such as cellulose acetate. In some cases filtration may be preceded by centrifugation. For column leaching tests the filter should be an integral part of the eluate collection system which ensures that the eluate is not exposed to the atmosphere prior to filtration. Pressure filtration is often preferable to vacuum filtration, particularly for eluates containing volatile components. Chemical analysis and characterization of the eluate Leaching tests include the characterization of the eluate(s) produced. Generally the characterization programme consists of chemical analyses of the components of interest. Any eluate produced should be analyzed immediately prior to filteration for pH, conductivity and, if possible, redox potential. Eluates produced from leaching of soluble materials, particularly at low L/S ratios, are often complex, concentrated mixtures of numerous components (e.g. high salt contents) and chemical analysis may be difficult due to interferences (AAS, ICP-MS). Standard analytical methods used for drinking water and wastewater may not be applicable and special precautions may be necessary. It is important therefore that the analytical laboratory is informed about the nature of the eluates prior to analysis. In addition to chemical characterization, eluates from leaching tests may be submitted to ecotoxicological testing.

Existing leachingprocedures A number of selected leaching procedures commonly used in various countries, including for regulatory testing, are presented in table 8.8 and briefly described in the following text. For more thorough descriptions and discussions of these and other leaching tests please refer to

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Environment Canada [ 1990], Fallman [ 1990], van der Sloot et al. [ 1991], Wahlstr6m [ 1992], Wahlstr6m et al. [ 1994], Tukker et al. [ 1994], Chandler et al. [ 1995] and Hjelmar and Traberg [ 1995]. The descriptions below are based on these references.

NEN 7343 Column Test is a Dutch standard column test in which 7 eluate fractions are collected within the range of L/S = 0.1-10 l/kg [NNI 1995a]. The total test duration is approximately 21 days. The leachant is demineralized water (DMW) initially adjusted to pH = 4 with HNO3 but pH is not controlled. The test material must be < 4 mm and upflow is applied through a column waste height of 20 cm and a diameter of 5 cm. The test is used for regulatory purposes to simulate leaching from mineral types of waste in the short and medium term (< 50 years). The L/S is related to time by the infiltration rate and the height of the application. Factors such as aging effects and mineral changes are not accounted for. Nordtest Method: Leaching procedure for granular waste materials. Column test: The method is described and recommended by Nordtest and applies to Denmark, Finland, Iceland, Norway and Sweden [Nordtest 1995a]. The method is in most respects identical to NEN 7347, except that the column dimensions are optional (minimum diameter = 5 cm, minimum waste height = 20 cm, minimum height/diameter = 4), the flow rate is slower (0.03 to 0.1 times L/S per 24 hours as compared with 0.5 times L/S per 24 hours) and only 4 to 5 eluate fractions are collected in the range L/S = 0.1 to 2 l/kg.

Table 8.8: Overview of selected existing leaching procedures for granular waste and industrial sludges. Type of tests

Test procedure

Country/Region

Column leaching tests

NEN 7343 Column Test Nordtest Procedure: Column test. Combined column and batch leaching test

The Netherlands The Nordic Countries Denmark

Batch leaching tests

DIN 38414 $4 AFNOR X31-210 JST-13 Draft European Standard prEN 12457 (Compliance batch leaching test) Nordtest Procedure: Serial batch test NEN 7349 Serial Batch Test ENA Skaktest TVA-Eluattest WRU Batch Extraction EP Tox Method 1310 TCLP Method 1311

Germany France Japan Europe The Nordic Countries The Netherlands Sweden Switzerland United Kingdom USA USA

NEN 7341 Availability test Nordtest Procedure: Availability test

The Netherlands The Nordic Countries

Availability tests

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149

Combined column and batch leaching test: In the Danish combined column and batch leaching test developed by VKI [Hjelmar 1995] granular waste is leached in a relatively large column (diameter = 15 cm, waste height = 60 cm) with DMW initially acidified to pH = 4 with HNO3 in upflow. Other relevant leachants such as seawater may be used depending on the scenario. Four eluate fractions are collected in the range L/S = 0-1 l/kg and optionally one more at L/S = 1-2 l/kg. The column test is then discontinued, all waste is removed from the column and mixed thoroughly and a representative subsample is submitted to a serial batch leaching test with contact times of 24 hours and collection of eluate fractions, for example corresponding to L/S = 1-5 (or 2-5), 5-25, 25-50, 50-100 and 100-200 l/kg. The method is useful when it is necessary to provide information quickly on the leaching properties of granular waste or industrial sludges both at very low and very high L/S ratios. Particle sizes should be < 4 mm and flow rates in the column test are comparable to those of the Nordtest Method above. DIN 38414 $4 is the German standard batch leaching test developed to assess the leaching of sludges and sediments from water and wastewater treatment [DIN 1984] and is widely used for regulatory purposes in Germany and Austria. The method is considered applicable to solids, pastes and sludges and does not simulate field conditions. The test material should be < 10 mm (in German legislation this requirement appears to have been interpreted rather liberally: particle size reduction has generally been avoided except for particles >32mm). The test is run at L/S = 10 1/kg for 24 hours with DMW under shaking or slow rotation. A distinction is made between readily soluble constituents and sparingly soluble components, and the method allows for a second or third extraction of the latter.

AFNOR X 31-210 is the French standard batch leaching test for granular solid mineral waste [AFNOR 1988] which is used for regulatory purposes in France. The test which resembles DIN 38414 $4 is run with DMW at L/S = 10 1/kg under linear shaking at 60 rpm for 24 hours (16 hours in each of two optional additional extractions). The particle size should be < 4 mm. Sampling and pretreatment of the waste is described in detail.

JST-13 is the Japanese standard batch leaching test for granular waste (0.5 to 5 mm) run with DMW at L/S = 10 l/kg and a contact time of 6 hours [Sakai 1995]. The test resembles DIN 38414 $4 and AFNOR X 31-210 but for several components the duration of the test may be too short for the attainment of equilibrium conditions.

Draft European Standard prEN 12457:

This is the proposed European standard batch leaching test for leaching of granular waste materials and sludges at Compliance Level [CEN 1996]. There are two options depending on the waste properties and the purpose of the testing: extraction with DMW in one stage at L/S = 2 1/kg or L/S = 10 1/kg or extraction in two successive stages at L/S = 2 1/kg and L/S = 2-10 1/kg. The contact time is 24 hours for the one-stage option and 6 and 18 hours, respectively, for the two extractions in the two-stage procedure. The particle size must be < 10 mm and 90 % (w/w) of the material must be < 4 mm. The one-stage version at L/S = 10 l/kg closely resembles DIN 38414 and AFNOR X 31210, while the information provided by the two-stage version may improve substantially the potential for monitoring the leaching behaviour.

Nordtest Method: Leaching procedure for granular waste materials. Serial batch compliance leaching test. This test resembles the Dratt European Standard prEN 12457 test with two options comprising one-stage leaching at L/S = 2 1/kg or two-stage leaching at L/S = 2 and

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L/S = 2-10 (with an additional option of a third extraction at L/S = 10-50 l/kg) with DMW initially adjusted to pH = 4 with HNO3 [Nordtest 1997].

NEN 7349 Serial Batch Test is the Dutch standard serial batch leaching test consisting of five successive extractions of granular waste (< 4 mm) with DMW initially adjusted to pH = 4 with HNO3 at L/S = 20 1/kg in each cycle to an accumulated L/S of 100 l/kg [NNI 1995b]. The contact time is 23 hours for each extraction. The pH will normally be controlled by the solid phase. The test is used for regulatory purposes in the Netherlands and is considered to represent leaching at a very long time scale or an immediate high dilution.

ENA skaktest is the Swedish serial batch leaching test used to simulate the leaching from mineral waste under certain hydraulic conditions [Fallman 1990]. The waste is extracted four consecutive times at L/S = 4 l/kg with DMW initially adjusted to pH = 4 with H2SO4. The contact time is 24 hours for each extraction and the waste particles must be < 20 mm. The method includes a series of four extractions in which fresh waste in each stage is added to the extract from the previous stage. The resulting eluate resembles initial porewater from the waste.

TVA-Eluattest is the Swiss standard serial batch leaching test used in Swiss waste management regulation. The test consists of two consecutive extractions at L/S = 10 l/kg with DMW [D6partement F6deral de rlnterieur 1988]. During the extraction CO2 is constantly bubbled through the water from the base of the vessel which usually results in a controlled pH in the range of 5-6. The contact time is 24 hours for each extraction. There are no requirements on particle size and the test is used both for granular and monolithic waste materials. The method which is intended to assess leaching in the long term is in some respects an availability test. WRU leaching test is the British serial batch leaching test developed by Waste Research Unit (WRU) at the former Harwell Laboratory [Young 1982]. The granular waste (< 10 mm) is leached with DMW, simulating disposal in an inorganic environment, or with a solution of acetic acid buffered at pH = 5 which simulates co-disposal with household waste. The necessary contact time (2-80 hours) is determined from a separate batch extraction in which the eluate is sampled and analyzed at intervals. Five extractions are then performed at an L/S corresponding to 1 pore volume followed by an additional extraction at an L/S corresponding to 10 pore volumes. Since 1 pore volume normally corresponds to an L/S ratio significantly lower than 1 l/kg, practical difficulties in mixing and separating the liquid and solid phases should be anticipated. Extraction Procedure Toxicity Test, method 1310 (EP Tox test): This is the standard batch leaching test developed by US Environmental Protection Agency (EPA) for classification of waste [US EPA 1984]. The granular waste (< 9.5 mm) is extracted for 24 hours at L/S = 20 l/kg with DMW and the intermittent addition of acetic acid (HAC) to maintain a constant pH of 5. However the addition of acetic acid is limited to a maximum of 4 ml of 0.5 N HAC per 100 g of waste hence the degree of control of pH depends on the alkalinity of the waste. The method which was replaced by the TCLP (see below) in 1990 in US waste regulation is intended to simulate a "mismanagement scenario" in which the waste is co-disposed with household waste.

Toxicity Characteristic Leaching Procedure, Method 1311 (TCLP): This is a standard batch leaching test developed by US EPA for classification of waste [US EPA 1990]. The test which

CHAPTER 8

151

replaced the EP Tox test in US regulation is designed to determine the mobility of inorganic and organic contaminants in liquid, solid and multiphase waste. Leaching is carried out at L/S = 20 l/kg with a contact time of 24 hours. If volatile components are present "zero headspace" equipment is necessary. TCLP resembles the EP Tox test. The most significant difference is that the TCLP prescribes the choice between two leachants, depending on the alkalinity of the waste material. Very alkaline waste materials are leached with a fixed amount of acetic acid solution (pH = 2.88) and other waste materials are leached with a fixed quantity of acetic acid solution buffered at pH = 4.93 with NaOH. Generally the resulting pH in the eluate will be approximately 5, but for strongly alkaline waste materials the pH may be anywhere between 5 and 12, which may result in very variable test results for components with highly pH dependent solubilities. Both EP Tox and TCLP are partly availability tests.

NEN 7341 Availability Test is the Dutch standard extraction test for the assessment of maximum leachability [NNI 1995c] which is used in Dutch waste management regulations. The waste is ground to < 1251am mm and extracted in two steps of L/S = 50 l/kg each with DMW at pH = 7 (the first extraction) and pH = 4 (the second extraction), pH is kept constant by feed-back control and the addition of HNO3 or NaOH. The contact time in each extraction is 3 hours. The two extracts are combined prior to analysis.

Nordtest Method: Leaching procedure for granular waste materials. Availability test: The method is described and recommended by Nordtest [Nordest 1995b]. The test is identical to NEN 7341 Availability Test except for the prescribed L/S ratio which in the Nordtest Method is 2 x 100 1/kg as compared to 2 x 50 1/kg for NEN 7341. The higher L/S ratio is intended to ensure availability control.

Leaching procedures under development Several new leaching procedures for granular waste and industrial sludges are currently being developed for various specific purposes, including the following tests.

Tank leaching test for compacted granular waste: A tank leaching test has been developed by ECN in the Netherlands in cooperation with Rutgers University in New Jersey, USA [NNI 1994] to determine the leaching characteristics of inorganic (and possibly of organic) constituents from powders and granular construction and waste materials of a mainly organic nature under conditions where transport by diffusion is more important than transport by convection. The diffusion-controlled flux of components from the waste material is assessed by transferring the test material to a beaker, coveting the surface of the material to be exposed to the leachant with a layer of glass beads, subsequently placing the beaker in a larger container which is filled with DMW so as to immerse the beaker completely and replacing the water at regular time intervals. The extracts are analyzed for components of interest. The measurement of an inert species allows the determination of the physical retardation in the granular matrix. For components interacting with the matrix the chemical retardation can be calculated. Serial batch leaching test simulating landfill conditions: A serial batch leaching test designed to assess the leaching behaviour of an industrial waste in the environment of a landfill containing municipal solid waste is currently being developed under the auspices of the UK Environment Agency [Environment Agency, UK, in preparation]. A synthetic landfill leachate has been developed and is used as the eluant in the test. A batch of waste is leached

152

CHAPTER 8

successively with progressively weaker solutions of the synthetic leachate to represent the change with time in the nature of leachate in a municipal waste landfill.

Availability test under oxidizing conditions: A Nordtest method is being developed to determine the potential leachability of inorganic constituents under controlled oxidized conditions in mainly inorganic waste materials, especially ashes and slags [F~.llman 1996]. The oxidized conditions may both directly increase or decrease the solubility of a substance or make it available for leaching through solubilization of the surrounding matrix. The test specimen is oxidized both by hydrogen peroxide and nitric acid. The test is carried out as two serial extractions at 2 x L/S = 100 l/kg and at pH = 7 and pH = 4 respectively, with contact times of 3 hours (first extraction) and 18 hours (second extraction) on material ground to < 1251am. Like the existing availability tests this procedure does not adequately address slow kinetics and solubility limitations. Leaching under reducing conditions: Modifications to existing leaching tests which aim to identify the reducing properties of waste materials, to determine the reducing potential of a waste material and the leaching behaviour under reducing conditions (redox static behaviour) are being developed at ECN in the Netherlands [van der Sloot 1994a]. Interpretation and use of leaching data

Interpretation of test results Examples of leaching test data as a function of L/S In Figure 8.1 the results of different equilibrium-based leaching tests performed on the same granular waste material (municipal solid waste incinerator (MSWI) bottom ash) with DMW or slightly acidified DMW are presented in terms of accumulated leached amounts as a function of L/S for chloride, sulphate, lead and copper. The graphs show that individual tests provide, in varying degrees of detail, different parts of the total description of the progress of the leaching process which differs between components. The figure also shows the total content of each component as well as the results of an availability test which, ideally, should represent the asymptote for the leaching curves described by the results of the column and serial batch tests. Obviously it is difficult to identify the shape of the curve, i.e. the rate of release of the components, from the results of a test which provides only one point on the curve or which provides more than one point but at rel~.tively high L/S ratios. The most detailed and useful information is obtained from tests providing at least two points on the curve, one of which should be at a relatively low L/S ratio. A column test, possibly in combination with a serial batch leaching test, fulfils this requirement. Column tests are, however, relatively slow and expensive to perform (characterization tests). For granular waste materials or industrial sludges for which the general shape of the curve is known from previous testing at Basic Characterization level, a test like the proposed CEN prEN 12457 (a compliance test) which can provide leaching date for L/S = 2 and 10 I/kg, i.e. two points on the curve, may be sufficient for control purposes. For a specific physical and hydraulic disposal or utilization scenario where granular mineral waste is placed above groundwater level the L/S scale may be converted to a time scale using the following equation [Hjelmar 1990]: t - t o = (L/S) x d x H ~

CHAPTER 8

153

Figure 8.1. Accumulated leached amounts of various components as a function of L/S obtained from various leaching tests performed on bottom ash. cr

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Figure 8.2 which shows cumulative leaching data obtained from a column and serial batch test on a neutral coal fly ash [van der Sloot 199 l a and 1994b] illustrates some of the typical release profiles obtained in column studies. For comparison the total composition as well as the availability (dotted line) are included as relative asymptotes. From the graphs it is clear that calcium is being washed away rapidly, followed by a more slowly dissolving phase as reflected by the difference in the K values (K is a measure for matrix retention and is further described

154

CHAPTER 8

below). Figure 8.2. Column and serial batch leaching data for coal fly ash with retention data obtained from modelling release.

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CHAPTER 8

155

In the case of chromium and molybdenum, all of the fraction available for leaching is released. At the end of the experiment (in this case a combination of a column test up to L/S = 10 l/kg and a serial batch procedure of 5 x L/S = 20 l/kg up to L/S = 100 l/kg) the leached amounts of vanadium, barium, lead and zinc start to approach the available fraction indicating a release largely controlled by solubility. In contrast to calcium, barium release is slow at low L/S and increases more strongly at L/S > 5 l/kg. This result is most likely related to the fact that the solubility of barium is controlled by sulphate which is released initially in high quantities. The more leachable calcium fraction may well be the gypsum phase present in fresh coal ash. The release of copper is very slow and, even at the end of the experiment, is nowhere near to the leachable fraction. This result is an example of very strong matrix retention. Similar observations have been made for other coal ashes and for other materials. These examples also illustrate the need to include analysis of major and release controlling species (e.g. calcium and sulphate) during testing and not focus only on elements of regulatory significance. Based on the leaching data shown in Figure 8.2, a few general types of release pattern associated with column and batch leaching tests on granular waste may be identified as presented in Figure 8.3 [van der Sloot 1994b]: fast (A, wash out), intermediate (B) and slow (C, solubility/dissolution control) release; a decrease in availability due to slow mineral formation or sorption reactions (D); an increase in release due to depletion of a solubility controlling phase (barium) or changes in chemical conditions such as pH (oxyanions) or redox potential with time (E); a decrease in release due to changes in chemical conditions or initial release of a different, more mobile species (fluorine, calcium, DOC, complexed metals). When solubility is the main release controlling factor and changes in major element chemistry are limited, a Continuously Stirred Tank Reactor model (CSTR) can be used for an initial estimation of the release in a column test. Subsequent model refinement would include evaluating the column as plug flow reactor with dispersion. Use of a CSTR model leads to a description of release (R, in mg/kg) from the column as follows: R = AVB x (1 - e"(tes)/K) + Co where

AVB = the availability in mg/kg K = a retention factor for the constituent in question in 1/kg Co = aconstant.

In Figure 8.2 this relationship has been used to quantify the matrix retention parameter, K. In a supporting document for the development of the draft European standard two-stage batch leaching test prEN 12457 [van der Sloot 1993] a comparison of column studies with batch leaching on the same materials was carried out for a wide variety of materials to demonstrate that in several cases the proposed two-stage batch procedure matches well with the more elaborate column procedure. In table 8.9 a comparison of K values calculated for zinc, copper and chloride using data from column and serial batch tests, respectively, are presented for a variety of granular waste materials. As can be seen, there is a reasonably good agreement between K values from column and serial batch tests in several cases. Examples of leaching test data as a function of pH Many studies have pointed out the relevance of pH as one of the most important controlling factors in the leaching of inorganic constituents from mineral granular waste materials [van der

156

CHAPTER 8

Sloot 1993, Environment Canada 1990, van der Sloot 1991b, Eighmy 1993, Comans 1993, Kirby 1993].

Figure 8.3: Examples of leaching patterns for cumulative releases shown as a function of L/S. 2

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In Figures 8.4, 8.5 and 8.6 some results of column leaching tests performed on a number of the granular waste materials and industrial sludges shown in tables 8.3 and 8.5 are presented as a function of pH. Figure 8.4 shows the accumulated leached amounts of arsenic and cadmium at L/S = 10 l/kg, Figure 8.5 shows the accumulated leached amounts of copper and molybdenum for L/S = 10 l/kg and figure 8.6 shows the corresponding data for lead and zinc. Two curves are shown on all the figures: one connecting the data points for MSWI bottom ash (open circles) and one connecting the data points for coal fly ash (open triangles). The general picture is the same for all six elements. The two curves indicate strongly pH-dependent, partly solubility-controlled leaching behaviour for both waste types. The total content and availability of most of the elements in question is much higher for MSWI bottom ash than for coal fly ash (see tables 8,4 and 8,9) and the two curves may be taken as representing hish and low availability. The data points for most of the other granular wastes and industrial sludges are seen to fall within the envelope between the two curves. Those waste types showing very high

157

CHAPTER 8

leachability are predominantly wastes with high salt contents and/or little matrix retention (fly ashes, filter dust, salt residue). Particularly for copper, the leachability may also be influenced by the presence of residual organic matter and a corresponding content of DOC in the eluate.

Table 8.9: Comparison of modelling parameters for evaluating matrix retention parameter K from column and batch test data [van der Sloot 1994b].

Element

Zn

Material

AVB (mg/kg)

K (l/kg)

Co (mg/kg)

Test used

MSWI fly ash

10600

10000 9000

3 5

Column CEN draft

18

1000 400

0.01 0.01

Column CEN draft

6 1.5

1200 1500

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250

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0.01 0.2

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5.8

1500 1800

0.002 0.001

Column CEN draft

0.1 0.2

200 100

Column CEN draft

Coal fly ash 21000 Jarosite

Cu

MSWI fly ash

Coal fly ash

CI

Flue gas desulphuriza tion waste

2200

The leachability of copper from some waste types has been shown to increase with increasing content of DOC [van der Sloot 1996].

In Figure 8.7 the leaching patterns of cadmium from nine waste materials of widely different origin are compared, i.e. refuse derived fuel (RDF) ash, MSWI bottom ash, MSWI fly ash, brown coal ash, coal fly ash, shredder waste, jarosite, cement stabilized MSWI fly ash (crushed) and phosphate slag [van der Sloot 1996]. The diamond symbol indicates the original pH of the material with water without external influences or control. At low pH leaching the curves approach the plateau which corresponds to the level obtained by the availability test.

158

CHAPTER 8

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CHAPTER 8

159

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160

CHAPTER 8

Pb 10000

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.

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. 10

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Figure 8.6 Leachability of Lead and Zinc from a number of wastes and sludges as shown in tables 8.3 and 8.5. The data points show the total amounts leached in mg/kg for L/S = 0-10 1/kg in a column test as a function of pH. The upper curve represents data points for MSWI bottom ash, the lower curve data points for coal fly ash. After Aalbers [ 1992].

CHAPTER 8

161

When a material is leached with DMW any point on the curve may be produced depending on the equilibrium without external control. In spite of the widely different nature of the materials the leaching trends show similar characteristics. The major elements that control the composition of the eluates also control, to a large extent, the leachability of the trace contaminants (in this case cadmium). The differences between the behaviour of individual elements in different elements can be attributed to specific factors.Differences in the leaching as a function of pH generally are caused by a limited number of waste-specific chemical parameters which result in differences in chemical speciation and consequently a different leaching behaviour. For example, the influence of chloride on the leachability of cadmium is substantial and can be modelled with hydrogeochemical speciation models such as MINTEQA2 [Allison 1991]. The formation of soluble cadmium-chloro complexes causes the cadmium leachability curve to be shifted to higher pH when the soluble chloride level in the waste material increases. Figure 8.7 shows the sensitivity of this contaminant to the chloride level, since the related materials MSWI bottom ash, MSWI fly ash and RDF ash form an interesting sequence in this respect. The total chloride contents of these ashes are 1500, 18000 and 50000 mg/kg, respectively. The shift in cadmium leachability to higher pH with increasing chloride concentration in the ash eluates is very pronounced. Shredder waste is characterized by a relatively high content of organic matter and the increased leachability of cadmium from shredder waste at alkaline pH values is caused by the formation of complexing dissolved organic components. See also the discussion in Chapter 13. The data were obtained from pH controlled batch leaching tests at L/S = 10 1/kg. The role of leaching tests in waste characterization

Figure 8.8 illustrates some of the contexts of waste characterization in which leaching tests may be used. The figure shows an idealized procedure for testing of predominantly mineral waste products in relation to utilization or disposal. The procedure is relatively comprehensive and includes several aspects which would not be part of a routinely performed characterization programme. The waste description part of the diagram corresponds to a full Basic Characterization programme for a given type of waste. The diagram shows how a sequence of test procedures could be applied to characterize the waste in terms of chemical composition, physical properties, availability of key pollutants for leaching and short and long term leaching behaviour under specified conditions. An availability test may be used as a screening process. The availability of a large number of constituents is determined, but the eluates from the subsequent leaching tests are only analyzed for those constituents showing significant availability. The diagram also indicates the need to know how pH, redox potential, complexing, chelating and adsorption may influence leaching properties. The diagram is general, and the choice of leaching tests depends on whether the transport mechanism is percolation and convection or diffusion. In and conjunction with a description of the hydraulic situation the leaching results may then be used to model, for example, the flux of contaminants as a function of time for the relevant disposal or utilization scenario. The results of the modelling may finally be supplemented with ecotoxicological data, relevant acceptance criteria and other information and used for an assessment of the environmental impact of the scenario(s) investigated. Verification of leaching results

Little information is available on actual field verifications of the results of laboratory leaching

162

CHAPTER 8

tests on granular waste and industrial sludges. Due to the influence of various factors (aging, dilution, exposure to air, etc.) which are generally not simulated in laboratory tests, it is often necessary to apply various types of hydrogeochemical modelling when comparing field data to data produced in the laboratory.

Figure 8.7: Leachability of cadmium from a variety of different granular waste materials as a function of pH.

100 1o

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0.01 0001

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5

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001

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3

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7

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10 11 12 13

pH

CHAPTER 8

163

Another option is to use large scale lysimeter results instead of "real" field data. In Figure 8.9 large scale (3 m x 3 m surface) lysimeter and laboratory column leaching results for an alkaline coal fly ash are compared in terms of leachate composition as a function of L/S. In this comparison, which shows a strong resemblance between laboratory column and lysimeter leaching results, the lysimeter data represent a period of exposure to natural weather conditions of 7.5 years while the column leaching data were obtained over a period of a few months. In the column test the pH decreased from an initial value of 12.4 to 11.1 to 11.5. In the lysimeter test all pH values were within the range of 12.7 to 11.5. Due to the high alkalinity, the carbonation had not progressed sufficiently during the first 7.5 years to lower the pH significantly [Hjelmar 1990, Hjelmar 1991 ]. The results shown in Figure 8.9 indicate that for granular materials which are robust and/or relatively insensitive to changes in pH and redox potential it is possible to base predictions of leaching under field conditions on the results of small scale laboratory leaching tests by transforming the L/S scale into a time scale. It should, however, be noted that less encouraging results were found in the same study for sulphite containing FGD residues which were thermodynamically unstable and subject to continuous oxidation.

Relationship with other fields There is a strong relationship between the testing of leaching from granular waste materials and industrial sludges and the testing of leaching from other materials. From a technical or scientific perspective the similarities between the leaching behaviour of different types of material result from the leaching processes within different fields being governed by the same physical and chemical laws. Although the circumstances may vary, leaching results from different fields can be made comparable if the interpretation is based on knowledge of the fundamental mechanisms controlling the release of contaminants. That is one of the main points of this book and an example has been shown previously for granular waste and soil by van der Sloot et al. [1996]. The similarities between the leaching from different types of granular materials such as waste, granular products and soil/contaminated soil and industrial sludges and sewage sludge are immediately apparent. However, the results of leaching tests on monolithic materials may also be made directly comparable to the results of leaching tests on granular materials through the use of scenarios and modelling which include the hydraulic situation. From a historical perspective leaching tests developed within one field have in several instances been adapted for use in other fields. Examples include various soil extraction tests found useful for assessment of certain properties of granular waste materials and sludges. Conversely, it may be possible to adapt some of the leaching procedures developed during recent years for testing of granular waste and sludges to the testing of the mobility of components in contaminated soil. When methods from one field are adapted to another field or another leaching regime, attention must be paid to the differences in conditions and mechanisms. This does not always happen in practice. The German standard leaching tests DIN 38414 $4 were developed for assessment of the leaching of contaminants from sediments and sludges from water and wastewater treatment, but it is now widely applied to granular waste and industrial sludges as a regulatory test, often without proper adjustment of the testing conditions. As another example, leaching methods applicable to granular waste materials are in some cases used unchanged to assess the leaching from monolithic waste for regulatory purposes. Such results are generally useless and may be misleading. It is hoped that this will change in the future.

164

CHAPTER $

Figure 8.8: Idealized procedure for testing and evaluating granular waste materials [Hjelmar 1995].

I

WASTE PRODUCT

t

9FUNDAMENTAL KNOWLEDGE OF - ALKALINITY - CONTENT OF ORGANIC C - MINERALOGY

9 CHEMICAL COMPOSITION - MAJOR COMPONENTS - TRACE ELEMENTS 9 PHYSICAL PROPERTIES

J

9AVAILABILITY/POTENTIAL

9FUNDAMENTAL KNOWLEDGE OF - BIOGEOCHEMISTRY AND INFLUENCE OF 9 pH 9 REDOXPOTENTIAL 9 COMPLEXING 9 CHELATION 9 SORPTION

FOR LEACHING OF

POLLUTANTS

9 SHORTAND LONG TERM RELEASE RATE DATA

9CONVECTION/ ,,, PERCOLATION

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9CUMMULATIVERELEASE

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9 RATE OF RELEASE AS A FUNCTION OF TIME

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9 LAB TO FIELD TRANSLATION 9 MODELLING

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9MODIFICATION/ TREATMENT OF WASTE PRODUCT 9SELECT ON OF OTHER ALTERNATIVES

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CHAPTER 8

165

Figure 8.9: Comparison of lysimeter and column leaching results for an alkaline coal fly ash.

166

CHAPTER 8

Outlook for the future

During the past 10 to 15 years substantial progress has been made in the development and, not least, the interpretation of leaching procedures for granular waste materials and industrial sludges. Nevertheless, much work is still necessary to satisfy the need for reliable tools for assessment of the leaching behaviour of these waste materials. Among the main tasks of the future are: 9 Development of leaching tests for waste containing leachable organic compounds. 9 Improvements in field verification of the results of laboratory leaching tests. This includes improved modelling and will lead to better environmental impact assessment tools. 9 Validation and adjustments of existing leaching tests for granular waste. 9 Improvements to the concept and practical testing of "availability". 9 Development and implementation of rational relationships between environmentally related limit values for emissions and the results of specific leaching tests in regulatory frameworks. 9 Further development of the fundamental understanding of leaching processes. 9 Investigation of the role of biological processes in leaching of granular waste materials containing residual organic matter. 9 Further development in the use of hydrogeochemical equilibrium models and kinetic models in the interpretation of leaching results.

REFERENCES TO CHAPTER 8

167

REFERENCES

Aalbers, Th.G. (1992): Uitloging van zware metalen en anionen uit afvalstoffen in relatie tot bodem- en grondwater-bescherming; grenswaarden C2, C3 en C4 afvalstoffen. RIVM report no. 771401002, 1992. AFNOR, Association Francaise de Normalisation (1988): Dech6ts: Essai de lixivation X31210, AFNOR T95J, Pris, France. Allison, J.D., D.S. Brown and K.J. Novo-Gradac (1991): MINTEQA2/PRODEFA2, A geochemical assessment model for environmental systems: Version 3.0 User's Manual, EPA/600/3-91/021, US Environmental Protection Agency, Athens, Georgia. AVR Bedrijven Rotterdam (1997)" Personal communication. CEN (1996a): Characterization of waste. Leaching. Compliance test for leaching of granular waste materials. Determination of the leaching of constituents from granular waste materials and sludges. Draft European Standard prEN 12457, CEN/TC 292. CEN (1996b): Characterization of waste - Methodology guideline for the determination of the leaching behaviour of waste under specified conditions. Draft PrENV, CEN/TC 292. Chandler, A.J., T.T. Eighmy, J. Hartlen, O. Hjelmar, D.S. Kosson, S.E. Sawell, H.A. van der Sloot & J. Vehlow (1995): An International Perspective on Characterization and Management of Residues from Municipal Solid Waste Incineration. Final Document, Report prepared by the International Ash Working Group (IAWG). Chandler, A.J., T.T. Eighmy, J. Hartlen, O. Hjelmar, D.S. Kosson, S.E. Sawell, H.A. van der Sloot & J. Vehlow (1994): An International Perspective on Characterization and Management of Residues from Municipal Solid Waste Incineration. Summary Report prepared by the International Ash Working Group (IAWG). D6partement F6deral de rlnterieur (1988): Bericht zum Entwurf ~ r eine technische Verordnung Ober Abf~ille (TVA), Switzerland. DIN 38414 Teil 4 (1984): Deutsche Einheitsverfahren zur Wasser, Abwasser- und Schlammuntersuchung; Schlamm und Sedimente (Gruppe S), Berlin: Beuth-Vertrieb. Environment Agency, UK. Environment Canada (1990): Compendium of Waste Leaching Tests. Report EPS 3/HA/7, Environment Canada, Ottawa, Canada. Flyvbjerg, J., P.E. Holm, O.W. Asmussen and O. Hjelmar (1996): Kemiske og mikrobiologiske faktorers indflydelse ph stofudvaskningen fra TMT-slam fra vfid roggasrensning pfi affaldsforbra~ndingsanla~g. Rapport til Energiministeriets forskningsudvalg for produktion og fordeling af el og varme (EFP). VKI, Horsholm, Denmark. Fallman, A.-M. (1990): International Seminar on Leach Tests, Held at the Swedish Environmental Protection Agency, Solna, Sweden, September 19th, 1990, SGI, LinkOping, Sweden.

168

REFERENCES TO CHAPTER 8

Fiillman, A.-M. (1996): NT ENVIR 00X: Solid Waste, Granular Inorganic Material: Oxidized Availability Test. Draft Method, Nordtest project no. 1231-95, SGI, Link6ping, Sweden. Gruebel, K.A., J.A. Davis and J.O. Leckie (1988): The feasibility of using sequential extraction techniques for arsenic and selenium in soils and sediments. Soil Sci. Soc. Am. J. 51, 390-397. Hjelmar, O. (1990): Leachate from land disposal of coal fly ash. Waste Management and Research, 8, 429-449. Hjelmar (1996): Disposal strategies for municipal solid waste incineration residues (1996): Journal of Hazardous Materials 47, 345-368. Hjelmar, O., E.A. Hansen, F. Larsen & H. Thomassen (1991): Leaching and soil/groundwater transport of contaminants from coal combustion residues. Final report of research project cofunded by the Commission of the European Communities (EN3F-0033-DK), ELKRAFT Power Company and the Danish Ministry of Energy (EFP 1383/86-18, 1323/86-19 & 1323/89-7), VKI Water Quality Institute, Horsholm, Denmark. Hjelmar, O. and H. Thomassen (1992): Notat til Miljostyrelsen vedrorende Rammeprogram til begramsning af miljoskadelige stofrer i afraid: Deponering af restprodukter fra kulfyrede kraftva~rker og afraldsforbra,~ndingsanla,~g. VKI, Water Quality Institute, Horsholm, Denmark. Hjelmar, O. and R. Traberg (1995): Vejledning i valg og fortolkning af udvaskningstests for affaldsmaterialer. NT Techn. Report 272, Nordtest, Esbo, Finland. NNI (1994): Determination of the maximum leachable quantity and the emission of inorganic contaminants from granular construction materials and waste materials - The compacted granular leach test. Concept Dutch pre-standard, preliminary edition, October 1994. NNI (1995a): NEN 7343, Leaching characteristics of building and solid waste material, leaching tests, determination of the leaching of inorganic components from granular materials with the column test, 1st Edition, February 1995. NNI (1995b): NEN 7349, Leaching characteristics of building and solid waste material, leaching tests, determination of the leaching of inorganic components from granular materials with the cascade test, 1st Edition, February 1995. NNI (1995c): NEN 7341, Leaching characteristics of building and solid waste material, leaching tests, determination of the availability of inorganic components for leaching, 1st Edition, February 1995. Nordtest (1995): Solid waste, granular inorganic material: Column test. Nordtest method NT ENVIR 002, Espoo, Finland. Nordtest (1995): Solid waste, granular inorganic material: Availbility test. Nordtest method NT ENVIR 002, Espoo, Finland. Sakai, S., H. Mizutani, H. Takatsuki and T. Kishida (1995): Leaching test of metallic

REFERENCES TO CHAPTER 8

169

compounds in fly ash of solid waste incinerator. Journal of the Japan Society of Waste Management Experts 6 (6), pp. 225-234. Tessier, A., P.G.C. Campbell and M. Bisson (1979): Sequential extraction procedure for the speciation of particulate teace metals. Anal. Chem. 51, 844-851. Tukker, A., M. van den Berg and H.A. van der Sloot (1994): Characterization of waste in Europe. State of the art. CEN/TC 292, NNI, Delft. US EPA (1984): Extraction Procedure (EP) Toxicity Method 1310; Test Method and Structural Test Methods for Evaluation of Solid Wastes, SW846. US EPA (1990): Part 261, Appendix II - Method 1311 Toxicity Characteristic Leaching Procedure (TCLP), Federal register, Vol. 55, No. 61, March 29, 1990, Rules and Regulations, pp. 11863-11877. van der Sloot, H.A. (1996): Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. Waste Management, 6 (1-3), pp. 65-81. van der Sloot, H.A., G.J. de Groot and O. Hjelmar (1991a): EC contract EN3F-0032 NL. ECN-R-91-008. van der Sloot, H.A., D. Hoede & B. Bonouvrie (1991b): Comparison of different regulatory leaching test procedures for waste materials and construction materials. ECN-C-91-082. van der Sloot, H.A., O. Hjelmar, Th.G. Aalbers, M. Wahlstrrm and A.-M. Fiillman (1993): CEN/TC 292 WG2 document: Proposed leaching test for granular solid waste. van der Sloot, H.A., D. Hoede and R.N.J. Comans (1994a): The influence of reducing properties on leaching of elements from waste materials and construction materials. In J.J.J. Goumans, H.A. van der Sloot and Th.G. Aalbers (Eds.): Environmental aspects of construction with waste materials. Elsevier Science, Amsterdam, pp. 483-490. van der Sloot, H.A., D.S. Kosson, T.T. Eighmy, R.N.J. Comans and O. Hjelmar (1994b): Approach towards international standardization: A concise scheme for testing of granular waste leaching. In J.J.J. Goumans, H.A. van der Sloot and Th.G. Aalbers (Eds.): Environmental aspects of construction with waste materials. Elsevier Science, Amsterdam, pp. 453-466. van der Sloot, H.A., R.N.J. Comans and O. Hjelmar (1996): Similarities in the leaching behaviour of trace contaminants from waste, stabilized waste, construction materials and soils. The Science of the Total Environment 178, pp. 111-126. VKI (1996): Database of leaching behaviour, VKI Water Quality Institute, Horsholm, Denmark. WahlstrOm, M. (1992): Utlakningstester for avfall. Nordtest report, NT Techn Report 193.

170

REFERENCES TO CHAPTER 8

Wahlstr6m, M., A-M. F~illman, O. Hjelmar & K.H. Karstensen (1994): Laktest for granulerade avfallsmaterial. Nordtest-report, NT Techn Report 246. Wallmann, K., M. Kersten, J. Gnaber and U. F6rstner (1993): Artifacts in the determination of trace metal binding forms in amoxic sediments by sequential extraction. Intern. J. Environ. Anal. Chem. 53, 187-200. Young, P.J. and D.C. Wilson (1995): Solid waste, granular inorganic material: Availability test. Nordtest mehod, NT ENVIR 002, Espoo, Finland.

CHAPTER 9 CHAPTER

9: W A S T E S T A B I L I Z E D / S O L I D I F I E D BINDERS

171 WITH HYDRAULIC

Industrial and regulatory context

About fifteen years ago, when stabilization/solidification processes were first developed, their application was mainly limited to the treatment of radioactive waste or to the consolidation of industrial sludges. The operating conditions and performances of these processes were directly dictated by the nature of these particular wastes. In the case of radioactive waste, three basic techniques (vitrification, cementing and bituminous coating) were chosen and adapted to the degree of radioactivity in the wastes. As for industrial sludges, the main objective was simply to transform them into a sufficiently solid form to be shovelled. There again, the mixtures (and the costsT) were appropriate to the specific needs : mainly based on wastes of a more or less pozzolanic nature, with slow setting times (sometimes the solidified product could only be shovelled after one month, and then hardened in landfill over one or two years), or sodium silicate based, more efficient for its adsorbant properties than for pollutant retention, either physical or chemical. The new developments in stabilization/solidification processes are a direct result of recent regulations in the following fields : the incineration of Industrial and Municipal Solid Waste which has given rise to a new generation of "secondary" wastes : the APC residues (air pollution control residues) characterized by a high soluble fraction and amphoteric heavy metals (lead, zinc), the new landfill concept (mainly on the French level) which enforces the notion of stabilized waste, 9

contaminated soil treatment.

These new issues (i.e. the nature of materials to be treated and quality requirements, in particular pollutant retention), have led to : 9

the abandonment or relegation of certain inadequate processes the development of new processes designed specifically for these new contexts, the adaptation of processes first developed for nuclear waste management (vitrification, bituminous coating) which are now potentially competitive given the notable increase in treatment costs for industrial waste.

The increasing severity of waste regulations and the new tools for the evaluation of wastes which have resulted from the regulations, research programmes carried out for the authorities, the producers or waste disposers, have highlighted the need for environmental assessment to

172

CHAPTER 9

provide better knowledge and prediction of long term waste behaviour and environmental impact. The assessment of waste stabilization/solidification process performance must answer the following questions" what can be expected from stabilization/solidification? This question requires research on the processes based on the wastes to be treated and the objectives to be attained, how to evaluate the performances in a given regulatory context? This requires research on the procedures, modelling and prediction of long term behaviour, what is the fate of the stabilized/solidified materials? Landfilling or re-use : this requires research on the technical and environmental feasibilty of the different scenarios. None of these three research fields can be considered independently to the two others. The different processes must be designed to comply with a fixed regulatory objective, using given evaluation tools and depending on the environmental level of exposure under the scenario being considered. Leaching tests play a crucial role in this field. General characteristics of waste solidified/stabilized by hydraulic binders

Composition and structure of the materials The hydraulic binders most otten used for waste stabilization/solidification are Portland cement, blast furnace slag cement, cement industry flue dust, coal fly ash and other wastes of pozzolanic nature in the presence of lime, calcium aluminates or often a mixture of several binders. Hydraulic binders consist mainly of oxides of calcium, silicion, aluminium, iron, magnesium, sodium, potassium and so on but also include calcium sulphate (often added to control setting). The binders acquire a solid porous structure by hydration giving rise simultaneously to both solidification and physico-chemical stabilization of the wastes. Hydration of hydraulic binders is complex. Furthermore the different physico-chemical processes involved (dissolution, neutralization, gelification, precipitation and so on) have different kinetics. The principal phase responsible for hardening of the water-cement system is the tobermorite, a gel formed of hydrated calcium silicates, often called CSH due to its approximate stoichiometric composition (mixture of hydrated tricalcium and dicalcium silicates). The hydration of cement gives rise also to the formation of calcium hydroxide (portlandite) which crystallizes in the pores of the CSH. Pouzzolanic binders react with lime to form the same type of compounds. This can lead to a lower pH of the pore water than that generated by the alkaline medium of the cement (pH 12.5 to 13.5). Their setting time is longer but usually leads to a greater compactness. Different products may be added to hydraulic binders in order to attain different objectives : to increase compactness of the solidified material by using a filler or a fluidifier to decrease the water/cement ratio,

CHAPTER 9

173

to improve physico-chemical pollutant retention (absorbants, etc), or to modify setting time, according to technical needs or waste characteristics,

Pollutant retention mechanisms The use of hydraulic binders has several effects on the leaching behaviour of pollutants (mainly metallic species) including: the alkalinity with the formation of slightly soluble hydroxides (except for amphoteric metals), 9

the formation of slightly soluble silicates, the insertion of certain metals in certain phases of the hydrates, in particular by substitution of calcium or sulphate ions in the ettringite.

9

simple physical retention in the porous structure, and

9

absorption at the CSH surface.

The complexity of the system therefore brings into play multiple immobilization mechanisms acting on the metallic ions.

Main waste types treated by stabilization/solidification Important studies have been carried out recently to make a quantitative and qualitative assessment of the total amount of waste on the market likely to be stabilized/solidifed. The rapid development of industrial processes generating primary or secondary waste (incineration residues, waste water treatment sludges) due to increasingly severe environmental regulations in general and to the considerable increases in landfill costs for hazardous waste in particular makes this assessment rather difficult. The main waste types are : Air Pollution Control Residue (fly ash, dry and wet scrubber residue, boiler dust, industrial process dust filtration) from: MSW Incineration (the main actual waste flow) Hazardous Waste Incineration the metallurgy industry; and the power industry (coal/fuel fly ash) -

-

Slags & Refractories from: hazardous waste incineration; the foundry industry; and the non-ferrous metallurgy industry;

-

Waste Water Treatment Sludges from: the chemical industry;

174

CHAPTER 9 the metallic surface industry; and the iron and steel industry

9

spent catalysts:

9

asbestos containing wastes;

9

foundry sands; and

9

contaminated soils and sediments

State of the art of leaching behaviour assessment

The characterization of leaching behaviour in water of solidified/stabilized waste is crucial in most reuse/disposal scenarios. Water plays a multiple role in physico-chemical phenomena in the solid, in pollutant transfer and pollutant dispersion in the environment. The pollutant flux due to leaching is the combined result of: the intrinsic polluting potential of the stabilized/solidified waste, described by the set of physico-chemical, mechanical and structural parameters, the physico-chemical mechanisms which govern mass transfer when solidified/stabilized waste comes into contact with water in the scenario conditions.

Intrinsic polluting potential Among the parameters which charaterize the intrinsic polluting solidified/stabilized waste, the most important factors are :

potential

of the

the porous structure, where water may penetrate by capillarity (a phenomenon assimilated to water diffusion) to allow transfer of soluble species through the pore system; and the elementary chemical composition of the waste, and more importantly the identification of the mineralogical phases, contributes towards a better understanding of leaching behaviour. It is therefore possible to assess the pollutant release potential through the physico-chemical properties of the waste such as solubility according to the local chemical context, which is an essential parameter of release.

Other important parameters for the definition of the polluting potential are physico-chemical homogeneity, humidity, acid neutralization capacity and resistance to mechanical, meteorological or biological constraints.

Liquid~solid contact conditions Contact conditions of solidified/stabilized (S/S) waste with the leachant are described mainly by:

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175

Table 9.1: The predominant nature of the main pollutants and their interaction with hydraulic binders Alkaline or alkaline earth salts Sulphates

Sulphates can lead to the formation of expansive compounds (in particular secondary ettringite) which deteriorate the hydraulic matrices. This does not occur with calcium aluminates where a primary ettringite is formed before complete hardening of the paste.

Chlorides

Chlorides can be integrated by binders only at low concentrations (a few per cent). However, calcium monochloroaluminates may possibly be formed. For the treatment of waste containing a high soluble fraction possible pretreatments are under study or being developed.

Heavy metals

Heavy metals may be involved in three ways in hydraulic binders : 1. Inhibition or delaying of setting e.g. zinc 2. The amphoteric character of certain heavy metals which makes them particularly soluble in the alkaline pore water - including in particular lead 3. The integration in certain hydrates e.g. chromium by substitution of calcium or the sulphate ions for oxyanions in the ettringite.

Oxidizers and reducers

Waste liable to be oxidized within the matrix e.g. sulphur compounds give rise generally to expansive compounds which may damage the matrix.

Complexing agents

Complexation of certain compounds of hydraulic binders can lead to inhibition of setting.

Hydrocarbons

Above a certain concentration hydrocarbons are incompatible with setting of the hydraulic matrix due to coating of the grains before hydration.

Elementary carbon

It may cause problems of swelling or fissuring of the hydraulic matrix but if it is present as active carbon it may contribute towards the retention of organic molecules.

the contact mode of the solidified/stabilized waste with the leachant. Three typical situations may be encountered; percolation (the liquid flows around the granular bed or through very porous material) which is an extremely minor phenomenon for S/S waste taking into account its low water permeability (otten below 10.9 m/s); washing (a phenomenon which only takes into account contact between the liquid and the external surface of the solid); and immersion (the solid, saturated with water is in continuous contact with the liquid);

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the physico-chemical parameters of the leachant (pH, redox potential, conductivity, composition, temperature and so on) and contact time ; the rate of leachant renewal determining the ionic force and chemical affinity of the species ; the geometric conditions : volume, surface, and so on; and the liquid to solid ratio which is dramatically different at the laboratory scale (between 1 and 100 and very often equal to 10) compared with the field situation (far below 1 and sometimes below 0.01).

Release mechanisms

The release of soluble species contained in a porous medium in contact with water is the result of complex and coupled phenomena including: 9

water transfer in the porous medium up to saturation;

9

dissolution of the species in the pore water according to the local chemical context;

9

transport of species in solution due to the effect of concentration gradients; and potential change of species solubility in the pore water (including possible reprecipitation) if the context of the pore water has undergone certain modifications due to the pH profile for example, following the release of pH controlling species (portlandite)

This coupling of physical and chemical phenomena which is particularly sensitive in the case of leaching of porous monolithic materials has to been taken into account in the interpretation of leaching tests and in the prediction of long term leaching behaviour. Commonly used leaching tests - objectives and methods

There are a great number of leaching tests. The experimental conditions and interpretation of results are dependant on the application context and on the scientific objectives.

a)

application context of the tests : verification tests (on-site tests at the gate for well-known wastes); compliance tests (on key variables of previously fully characterized materials); and characterization tests (for full characterization of unknown materials). -

Most of the compliance tests applied to S/S waste are short term tests involving high liquid to solid ratios and which sometimes include crushing of the materials. Such tests are not appropriate to understanding leaching behaviour or for the prediction of release from S/S wastes for the following main reasons:

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177

as stated previously the release mechanism from monolithic S/S wastes is complex and time dependent. A compliance test (taking into account the wash-off and too short to see the release depending of chemical equilibrium and mass-transfer) may induce misunderstanding of the actual leaching behaviour; the release is highly dependent on the (variable) chemical context at the solid/liquid interface. The high liquid to solid ratio and some specific artificial chemical conditions imposed in compliance tests (like Acetic Acid in the US or Italian TCLP or CO2 in the Swiss and another Italian test) are inappropriate. A comparative approach between compliance tests and a characterization test (at a static pH ) has been conducted in the framework of the Intercomparison Study of Leaching Tests for Stabilized Waste [EC 1995].

b)

scientific objectives. The three most common are : determination of the available release potential 9This notion aims to evaluate the total elementary content of pollutant available for leaching. This value measured generally on crushed waste in very agressive chemical conditions (controlled pH) and with high liquid/solid ratios (50 or 100) is used as parameter Co in a diffusional model. This approach is questioned by certain researchers who argue that under these extreme conditions the porous structure itself is destroyed and that the control of release is more dependent on solubility at equilibrium in the local chemical context. study of solubilities at equilibrium : These tests which are conducted on crushed waste are carried out either at controlled pH or by addition of given quantities of acid when measuring the acid neutralizing capacity (ANC). The aim is to determine the variation in the solubility of species in the chemical context of the matrix subjected to increasing levels of neutralization of the alkaline compounds which control the pH in the pore water. This may allow prediction of pollutant behaviour in a matrix subjected to intensive leaching over a long period hence generating a notable decrease in pH at the solid/liquid interface. assessment of the release dynamics 9These tests are carried out on monolithic samples over long periods from 1 to 3 months or more. The aim here is to determine the pollutant flux by integrating the chemical parameters (mineralogy, solubility) and physical parameters (tortuosity, porosity, diffusivity) of the matrix. The interpretation of results is carded out by the concurrent assessment of results obtained from tests of one of the other two categories presented above.

The different parameters to be considered in leaching tests are therefore as follows :

9

leaching time;

9

liquid/solid ratio;

178

CHAPTER 9

sample granulometry 9monolithic solid or crushed powder; leachant (composition, pH, temperature, etc); and renewal conditions of the leachant, Most leaching tests are performed in a closed reactor with or without sequential renewal of the leachant which is considered perfectly stirred. The Soxhlet reactor is a particular kind of reactor which is fed continuously with distilled water and in which there is sequential extraction of the leachate containing the dissolved species. This procedure allows the concentration of the solutions to be analysed and the permanent maintenance of the concentration of the leachant at the interface and may be interesting for construction products containing waste or inert waste with very low rates of release.

Intercomparison study of leaching tests for stabilized waste An important pre-normative study of leaching tests for stabilized wastes has been funded by the European Commission, Standards Measurements and Testing (DG XII) in 1994 (EUR 16133 EN) involving 23 European laboratories. After a preliminary study of the homogeneity and reproducibility of testing within one laboratory an intercomparison of leaching tests for monolithic stabilized wastes was carried out between 24 European laboratories using a tank leaching test procedure as the common method. Other test methods have been applied to put the test results in perspective. The results of the intercomparison indicate that the agreement between the results is good for elements measured with sufficient analytical precision. The continued hardening of the cementious matrix after 28 days of curing causes changes in the release rates due to an increase of the tortuosity (physical restriction) of the matrix. This aspect of leaching of cementitious matrices needs to be taken into account in evaluating test results. This intercomparison was carried out when initial changes in material properties had been largely stabilized. The results of other tests, mainly consisting of regulatory tests for granular materials could be placed in perspective using a pH static leaching procedure. It was shown that the equilibrium pH reached in these tests largely dictates the leached quantity. Based on these observations the Unified Approach for Leaching as developed for granular materials can be extended to monolithic materials by combining dynamic aspects of leaching from the tank leach test with solubility control of leaching from a pH static leach test. This allows an interpretation of changes in release under different exposure conditions (e.g. carbonation) (see Figure 9.1) The tank leach test data provide a means of assessing physical and chemical parameters controlling the different types of release, which are useful in the improvement of stabilization product development. ( Figure 9.2) In the discussion of results with the participants, it was concluded that a tank leaching test is a good characterization tool to assess the release from stabilized wastes for regulatory purposes. Further work on data interpretation is needed to make full use of the information contained in the test. A short version linked closely to the extended test

CHAPTER 9

179

should be developed for quality control purposes. The method was therefore recommended for standardization by CEN.

Figure 9.1- Unified leaching of zinc from stabilized MSWI fly ash. Leaching as a function of time. Leaching as a function of pH under different exposure conditions.

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pH

Comparison of different leaching tests for granular material

Different (regulatory) tests have been applied on the material prepared for the BCR intercomparison of leaching of stabilized waste alter crushing to less than 2 mm in diameter. The test data have been transformed to units of mg/kg leached to allow a proper comparison of different leaching tests. Due to differences in liquid to solid ratio these results can not be compared on the basis of leachate concentrations. In Figure 9.3 the data points obtained by different participants using the USEPA-tox test, the DIN 38414 $4 procedure, the proposed CEN TC 292 procedure and the Swiss TVA method (CO2 flushing) are compared with the pH static test data (in 8 separate extractions the pH is controlled at values ranging from 4 to 12). The data on the "high pH availability test" on crushed material carried out at pH 12.5 is also included for comparison.

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CHAPTER 9

Figure 9.2: Different types of release as observed in tank leaching tests on stabilized materials. Horizontal solid line - total concentration, horizontal dotted line availability. Solid dots represent calculated release based on leached quantity per interval, plus signs represent the cumulative release E.1 Diffusion-controlled release

60000

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.

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,

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........

100

CHAPTER 9

181

Figure 9.3: Leaching of Ca, Cd, Cu and Pb from crushed stabilized MSWI fly ash as a function of pH, obtained by the pH static test (in triplicate) in comparison with data from other batch extraction tests on granular material: EP tox (diamond), DIN 38414 $4 and X31 210 (circle), Swiss TVA (triangle) "low pH availability" test (square).and "high pH availability" test (cross). Availability 100

EP

Tox

Avail.

12.5

+

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[]

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Swiss

10000

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0.1

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pH

The results of the individual tests match well generally for most elements with data from the pH static test method. In the case of exposure to CO2, the leachability of calcium deviates significantly from the general pH dependent behaviour. This is attributed to calcite precipitation. For the metals copper, cadmium and lead the agreement between the different tests and the pH static test procedure is very encouraging. Also for cadmium, a deviation is observed under excessive CO2 exposure. Here the incorporation of cadmium in calcite, which

182

CHAPTER 9

is known to be significant can explain the observed behaviour. The data for the "availability" test at pH 12.5 relate well with the leach data as given in the pH static test curves. The pH static test data are helpful in understanding the chemical processes in the pore system of a monolithic material when the pH changes particularly in the surface due to external influences such as carbonation. The main release controlling mechanisms for monolithic materials are surface related and test results from pH static tests cannot be used without taking these controlling mechnisms into account.

Data interpretation and modelling of pollutant leaching behaviour from monolithic S/S waste Most authors who have published in this field consider that mass transfer during leaching can most ot~en globally be described by an apparent diffusion mechanism. Whereas certain models only take into account Fick's diffusion law, by proposing the "analytical" solution (semi infinite solid) or the numerical one (finite solid volume), other models take into account a coupling effect like diffusion/chemical reaction, absorption and/or dissolution, or transfer at the interface. Further models have also been developed such as the shrinking core model as well as semi empirical models. Numerous research programmes have been conducted on the interpretation of leaching tests to develop tools for the evaluation of long term release of soluble pollutants contained in S/S waste. From the experimental results obtained two cases may be distinguished : A)

Relatively soluble elements whose solubility does not change according to the variable physico-chemical leaching context (example" Na +, CI-,)

B)

Elements whose solubility depends on the variation of the physico-chemical context, pH in particular (example : metals and in particular, amphoteric metals).

Type A elements Numerous experiments in different scenarios have shown that in this case (eg. Na + release) the diffusional model correctly describes the released flux. Two parameters characterize the intensity and the dynamics of release. Co characterizes the initial leachable concentration (available release potential) and Da the apparent diffusion coefficient of the species in the porous medium. The model can be resolved in 3D to take into account depletion of the species in the solid core. This may occur for highly soluble species incorporated in very porous matrices exposed to a permanent contact with water. Long term simulation for any scenario involving solid/leachate contact is then theoretically possible. The application limits are reached when the physical characteristics of the material itself are modified (increased porosity, destruction of the porous structure and so on). Release of the total soluble fraction or total dissolved salts can generally also be described by these diffusional models.

Type B elements Lead precipitated as a hydroxide in the pores of the material is a typical case of such behaviour. Portlandite in excess gives a pH of 12.5 in the pore water. Release of portlandite gives rise to pH variations in the leaching solution and in the pore water. Lead solubility is

CHAPTER 9

183

influenced strongly by pH and its release can be described only by considering the dissolution/diffusion couple. Data on the cumulative release of lead from a mortar developed from Portland cement during sequential leaching of the same monolithic material containing PbCI 2 in contact with different leachants: demineralized water (W), controlled pH 5 and pH 10, alkaline water at pH 12.5 (AW - lg/1 Ca(OH)2,0.45g/1 KOH, 0.2g/l NaOH) are presented in Figure 9.4. Figure 9.4: Release of lead according to the chemical nature of the ieachant

5000 O "L 4500 E ," 4000 U~

3500 3000 2500 q ~ 2000 I 1500 m "-

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E

~

, "A

9 pH5

0

0

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1000 ..t ,

,~ 500

z

oAW

i

0

[]

m

1000

9

2000

m

I

9

3000

4000

9

5000

[]

6000

7000

t cumulated (hr)

Release is sensitive to the chemistry of the leaching solution and evidently cannot be interpreted by the simple diffusional model. A coupled dissolution/diff-usion model has been developed to describe the release of more complex chemical species contained in a stable porous matrix in contact with water. The model was applied to simulate the case of a porous matrix containing only two leachable components namely calcium hydroxide (portlandite) and lead. The model which describes the phenomenon can be divided into several stages 9 release of portlandite, described by the shrinking front model (Figure 9.5 below); calculation of the induced pH profile, assuming that the thermodynamic equilibrium takes place in the porewater. In Figure 9.6 below the simulation of pH evolution at the interface is plotted.

184

CHAPTER 9

Figure 9.5- Release of the Ca from the matrix

0,5 0

"x

-0,5 al

9

Exp. Sim

M

-~ -1,5

"=,~

-2 -2,5 -1

0

1

2

3

4

t cumulated (hr)

Figure: 9.6 pH profile at the interface 12,2 12 11,8 11,6 11,4 11,2

ir //i

, /

/

/

/ /

!" if

~E f

11 10,8

,/

10,6 10,4 0

1000

2000

3000

4000

5000

t cumulated (hr)

determination of local lead solubility (by calculation assuming the main mineral phases or from specific experimental results (e.g. from ANC) shown in Figure 9.7 below; *

description and calculation of lead transport by diffusion in the porewater.

The coupled dissolution/diffusion model facilitates representation of the results of leaching tests and shows the interfacial character of lead release. As long as lead in solid form is present in the matrix zone near the leaching surface of the matrix its release is controlled by a solubilization phenomenon at the solid/liquid interface. In this case, the leaching model can be

CHAPTER 9

185

simplified that is the lead diffusional transport within the matrix can be neglected. Consequently a model based on the shrinking core model for calcium release and taking into account pH evolution at the solid/liquid interface and variable lead solubility according to the pH facilitates effective description of the leaching phenomena of lead.

Figure 9.7: Experimental lead solubility according to pH

-1 ---2

D ii

'IE

3 .i

-4

I--1 D

-5

4,[Dt

.

-6 0

2

4

6

8

10

12

14

pH Leaching data have been used also for simulation of the composition of a solution at equilibrium with the mineral phases using specialized soRware such as the MINTEQA2. The result is the demonstration of the mineralogical phase or phases which seem to control the dissolution and thereby the release of certain species.

Conclusion Simple tests and the diffusional models are generally sufficient to describe leaching of the type A species and the total soluble fraction contained in porous materials such as solidified/stabilized waste with hydraulic binders. To assess the release of species whose solubility varies according to pH (type B species) it is necessary on one hand to determine and to take into account its "chemical sensitivity" experimentally that is the variation in solubility in relation to the pH in the pore water and on the other hand to assess the dynamics of release of species which control pH at the interface.

186

REFERENCES American National Standard. Measurement of the Leachability of Solidified Low-Level Radioactive Waste by a Short-Term Test Procedure, ANSI/ANS-16.1-1986. Barna R Etude de la diffusion des polluants dans les d6chets solidifi6s par liants hydrauliques. Th6se doctorat, INSA Lyon, 1994, 210 p. Barna R., Sanchez F., Mszkowicz P, M6hu, J. Leaching behaviour of pollutants in stabilized/solidified wastes, submitted to Journal of Hazardous Materials,1997, vol. 52. Cote P., Constable T., Moreira A., An evaluation of cement-based waste forms using the results of approximately two years of dynamic leaching. Nuclear and Chemical Waste Management, 1987, vol. 7, p.129-139. De Groot G.J., van Der Sloot H.A., Determination of leaching characteristics of waste materials leading to environmental product certification in Stabilization and Solidification of Hazardous, Radioactive and Mixed Waste, second volume, Gilliam/Wiles Editors, ASTM STP 11/23, 1992. Hinsenveld M., A shrinking core model as a fundamental representation of leaching mechanisms in cement stabilized waste. Ph.D. thesis, University of Cincinnati, 1992, 400 p. Moszkowicz P., Barna R., M6hu J., v a n Der Sloot H.A., Hoede D., Leaching behaviour assessment of the waste solidified with hydraulic binders: critical study of diffusional approach. I n : Environmental Aspects of Construction with Waste Materials. Proceedings of the International Conference, WASCON'94, edited by Goumans, J. J. M., van Der Sloot H. A., Aalbers Th. G., Elsevier, Amsterdam, 1994, p. 421-432. Sanchez F.,Moszkowicz P., Barna R. Lixiviation de d6chets solidifi6s: mod61e de transfert de mati+re couplant dissolution et diffusion, Proc6d6s de solidification et de Stabilization des d6chets, Congr6s International Nancy, 1995.6 p. Sanchez F. Etude de la lixiviation de milieux poreux contenant des esp6ces solubles: Application au cas de la lixiviation des d6chets solidifi6s par liants hydrauliques. Th6se doctorat, INSA Lyon, 1996, 245 p. Tukker A., van Den Berg M., van Der Sloot H.A., Characterization of Waste in Europe, State of the art, CEN/TC 292, final draft, 1994. Van der Sloot H.A., Kosson D. S., Eighmy T.T., Comans R., Hjelmar O. Approach towards international standardization: a concise scheme for testing of granular waste leachability. In : Environmental Aspects of Construction with Waste Materials. Proceeding of the International Conference, WASCON'94, edited by Goumans, J. J. M., van Der Sloot H. A., Aalbers Th. G., Elsevier, Amsterdam, 1994, p. 453-466. Van der Sloot H.A., Hoede D., De Groot, G.J., van Der Wegen G.J.L, Quevauviller, Ph. Intercomparison of leaching tests for stabilized wastes, ECN-C-94-062, 1994.

CHAPTER

CHAPTER

10

10: C O N S T R U C T I O N

187

MATERIALS

Introduction

The utilization of secondary raw materials in construction products is stimulated in most EC countries from the point of view of recycling, conservation of natural resources and energy saving. The recovery of waste materials has priority over disposal. For regulatory purposes the protection of the environment (quality of air, water and soil) and of human health is a prime concern manifested in the Construction Products Directive (89/106/EEC). This requires an assessment of environmental effects. A valid method for the prediction of the emissions from construction materials into the environment and systematically knowledge on this subject are lacking. For construction materials an important consideration in respect of environmental compatibility is the leaching of environmentally relevant compounds [Hohberg 1996, Wruss 1992 and van der Sloot 1984]. Up to now, no uniform and accepted leaching test exists to evaluate the emissions of environmentally relevant compounds from construction materials with and without the application of secondary raw materials. Such a method is necessary with respect to the increasing need to recover waste materials in future. The aim of this chapter is to describe the general characteristics of construction materials with respect to leaching. A systematic approach to the harmonization of leaching tests in the field of construction materials is given. General characteristics of construction materials

In the field of construction materials, a great variety of different materials with different properties exists. Table 10.1 shows the classification of the different construction materials according to their properties. In this chapter we deal mainly with mineral materials (monolithic and granular). Some of the considerations are valid for metallic materials as well. The leaching of impregnated wood is outlined in Chapter 11. Polymers are not dealt with in this book. In order to get an impression of the wide range of construction materials, a short description of the different materials relevant to this chapter follows. Cement based materials Cement based materials are composed of cement, aggregates (see below), additions (e.g. natural pozzolanic materials, silica fume, coal fly ash and pigments) and admixtures (eg. plasticizers, accelerators, inhibitors, stabilizers). Due to their density concretes and mortars are divided into light weight mortars/concretes, normal mortars/concretes and heavy mortars/concretes. Cement based materials are monolithic, porous materials. The porosity is dependent on different parameters, for example the water/cement-ratio, type of cement, type of addition, compaction and curing [Hohberg 1996]. Masonry units Masonry units are for example natural stones (e.g. sandstone), clay bricks, calcium silicate bricks, concrete blocks and light weight concrete blocks. The raw materials for clay bricks and

188

C H A P T E R 10

calcium silicate bricks may be of natural and industrial origin. The masonry units may be porous or non porous [SchieBl 1995].

Aggregates Aggregates are used for the production of cement based materials and for road construction. They are of natural or industrial origin. Examples of natural aggregates include quartz and limestone (normal aggregates) as well as pumice and lava (lightweight aggregates). Examples of artificial/industrial aggregates are granulated blast furnace slag, metal slags, bottom ash and recycled concrete. Aggregates are divided into porous and non porous aggregates.

Table 10.1: Classification of construction materials

inorganic materials ~;ranular monolithic 2 1 metallic 9 aggregates materials steel aluminum zinc mineral materials cement based materials concrete mortar rendering masonry units

organic materials monolithic 3 9 wood (see Chapter 11) 9 polymers

This overview shows the wide range of very different materials in the field of construction materials. The raw materials/components may be of natural or industrial origin. The release controlling processes as well as the applied leaching tests may be quite different. Therefore, a general discussion of leaching processes follows in order to derive a systematic approach to the assessment of leaching from construction materials.

Life Cycle The leaching behaviour of construction materials has to be checked during their overall life cycle: from production through the period of use until demolition and reuse or disposal. In the case of reuse the recycled material is considered as new raw material for production, in the case of disposal the material is considered as a waste.

Relevant Leaching Conditions The relevant leaching conditions for construction materials is the time dependent release of inorganic or organic compounds into the environment under realistic material (dimensions of the monolithic material) and environmental conditions representing the life cycle period considered.

C H A P T E R 10

189

The relevant leaching conditions may be quite different during different life cycle periods for example, leaching of environmentally relevant compounds from aggregates for concrete production if they are exposed to rain during storage; leaching of compounds from concrete elements during use of the structure to water or soil by diffusion from the interior of the concrete to its surface (e.g. concrete foundations in groundwater or concrete water pipes) leaching of compounds from construction demolition waste for example used as road bed material. Relevant Environments

The relevant environments for construction materials during use are: splash rain water (concrete above the ground, e.g. facades); 9

water (water pipes, drinking water tanks); groundwater (foundations, concrete underground); and

9

soil (foundations, concrete underground, above groundwater level).

Release Controlling Processes Monolithic Materials General

The materials covered in the following considerations are: 9 cement based construction materials (e.g. for foundations, drinking water containers); 9 masonry units; 9 metallic materials; and 9 wood (see also chapter 11) The processes to be considered are the leaching of environmentally relevant compounds for example heavy metals, alkalis and organic compounds. Dominant Mechanisms

A classification of monolithic materials according to their properties with respect to internal water transport is shown in Table 10.2. The type of material and the water transport mechanisms inside the material decisively influence the prevailing leaching mechanisms when exposed to leaching conditions. Permeable materials as defined in Table 10.2 can be treated like granular materials and will not be considered further in this chapter. The influence of the pH on the leaching of different mineral construction products is outlined in Figure 10.1.

190

C H A P T E R 10 Table 10.2: Prevailing leaching mechanisms in monolithic materials Prevailing leaching mechanism

Type of water transport within the material

Type of material

-

non porous -

porous

water intake by capillary suction or pressure water transport (if any) by capillary conduction or water vapour diffusion percolation

-

permeable

surface wash-off surface dissolution diffusion Fick's law applies

surface wash off from and diffusion within the solid particles Darcy's law applies

Figure 10.1" pH-domains covered by mineral construction materials

leached amount (mg/kg) 10000

I total content I

1000

Changes due to mineralization

matrix mineralogy

"potentially leachable" ~,

100

( p H = 4) actual /leaching

10 solution chemistry _

0,1

masonry units

i

J

-~'--Concrete aggregates

10

12

14 pH

CHAPTER

10

191

For porous materials, diffusion of the constituents considered within the monolithic material is the process determining rate with respect to leaching. The following relationship can be derived using Fick's diffusion laws [Crank 1975 and Cot6 1987]. J = Sa

J: dN/dt. A:

~ / ~ e t :=~ De = n . t

(~'.I 2

diffusion current density of the ions considered [mmol. s"1. m-2] (number of particles transported per unit area and time)

De:

effective diffusion coefficient of the ions

t:

time [s]

S,:

"potentially leachable fraction", concentrations of the ions considered in the material at t - 0 [mmol 9m3]. Sa totally soluble and potentially leachable.

considered

[ m 2 S"1]

It is obvious from this consideration that the potentially leachable amount S, could only be determined under pH conditions prevailing within the material considered. This means extraction conditions using a leachant at low pH values (e. g. pH = 4) are irrelevant in the determination of potentially leachable amounts of constituents for example cementitious materials. If the potentially leachable amount of constituents in realistic pH regions is unknown, the total amount of constituents SO and an effective transport coefficient Te can be introduced instead of the potentially leachable S a and the effective diffusion coefficient De. This effective transport coefficient Te includes the chemical insolubility of the constituents considered: S a = a 9S O :=~

J=

So . . . . . . ~.t

with T, = Dc 9a 2 Te= r~-t(s-~l 2 :=~

To: effective transport coefficient [m2 9S"1] So: total amount of constituents in the material {mmol 9 m -3] a: availability (leachability) factor [dimensionless] The effective transport coefficient To can be calculated directly from the results of tank leach tests if diffusion is the rate determining process.The results of a tank leach test allow assessment of the prevailing leaching mechanisms if the leaching rate is plotted in a log/logscale (see Figure 10.2).

192

C H A P T E R 10 Figure 10.2" Evaluation of the results of tank leach tests - prevailing leaching mechanisms

logJ ( ~ d i s s o l u t i o n of the

|

material from

m-0

the s u r f a c e

(~ diffusion controlled process

m = -1/2

(~ surface washo f f effect

m---1 v

Iogt

A line with a gradient of-1/2 results if the rate determining process within the material is diffusion. If the material is completely soluble in the leachant without any additional internal diffusion processes a horizontal line will result from a log J/log t-plot. The gradient will be 1 in the case of surface wash-off [Schiel31 1995]

Fi[[ure 10.3: Combined processes - surface wash off and diffusion

logJ

surface -.q

wash off

' ' i

w

m = -1/2 diffusion r

I I

T

logt

C H A P T E R 10

193

At the beginning of a leaching process data indicating surface wash off may superimpose data indicating the diffusion process. The log J/log t-plot will then look like the example shown in Figure 10.3. Since the wash-off-processes are very short compared with service life considerations, the diffusion controlled process is relevant with respect to the environmental compatibility of a material. The linear relationship with an inclination of m =-1/2 in the log J/log t-plot holds true only if the internal condition of the material remains unchanged over the considered period (the considered period may be hundreds of years). This can be assumed for good quality concrete in contact with soil or groundwater. However, the internal conditions of for example stabilized waste may change in time due to carbonation or leaching of alkalis resulting in a reduction in the pH. Additionally the pore structure may change due to the interaction of the environment with the monolithic material. The consequences with respect to the leaching behaviour may be different. For some constituents a reduction in the pH may increase the solubility thus increase the leaching rate (Figure 10.4). In other cases, a reduction in the pH (e.g. carbonation in the outer layers) may lead to salt precipitation (see Figure 10.4).

Generally, cement based materials may be classified as follows: 9

Type A:

No change in internal pH during life time (e. g. concrete);

Leaching behaviour as shown in Figure 10.2 (m 9

=

-1/2); and

Type B: Change in internal pH due to carbonation or leaching of alkalis Leaching behaviour as shown in Figure 10.4.

Figure 10.4: C h a n g e in the leaching behaviour due to an internal change in pH caused by carbonation or leaching of alkalis log J

log J b) decrease of leaching rate due to carbonation (salt precipitation)

a) increase ofleaching rate e.g. due to carbonation

\ x i i

!

change of pH and/or porosity

!

change ofpH and/or porosity v

logt

log t

194

C H A P T E R 10

The following release and transport processes may be of practical interest in the case of monolithic construction materials: Release of environmentally relevant substances (L) into flowing water (W). Example: Concrete in flowing ground water (see Figure 10.5). Release process diffusion controlled. Typical test set up: Tank test. Release of L into non-flowing water. Typical test set up: Tank test. Liquid/solid ratio depending on boundary conditions to be considered (different boundary conditions can be taken into account by applying calculation models using relevant transport coefficients with of L within W).

Figure 10.5: Concrete in flowing groundwater (schematic)

r f j j / / / /

//// //// //// //// / / / //// m

m

/ / / / / / / /

/ / / /

/ / /

/

/ / / / / / / /

/ / / / / / / / J

n

General The materials covered in the following discussions are all types of aggregates (e. g. for road construction and concrete/mortar preparation). The aggregates may be of natural or industrial origin for example recycled concrete, metal slags and incineration slags. The issues discussed in Chapter 8 are in general valid for granular construction materials as well.

Granular materials Dominant leaching mechanisms In the ease of granular materials, each one of the following three mechanisms (see Chapter 2 and Figure 10.2) may become dominant:

CHAPTER

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195

*

surface wash-off;

9

dissolution of the solid as a process occurring at the actual surface; or

9

diffusion of contaminants (L) within the solid (S) towards its surface (to be released into the leachate (water, W) there)

The following release process may be of practical interest: 9

Water percolating through granular material (see Figure 10.6). Typical test set up: Percolation test.

Figure 10.6: Percolation through granular material (schematic)

Drinking water pipes Drinking water must be safe and pleasant to consume. It should not contain contaminants that could lead to acute or chronic adverse health effects or exhibit undesirable aesthetic aspects such as objectionable odour or taste. Such contaminants can arise from chemicals used in the treatment of drinking water and from the materials used for the transport of the drinking water to the consumer or to store it during its distribution. Harmonized test methods to examine the influence of materials on water intended for human consumption are being developed by CEN TC 163/WG3 and its ad hoc groups. Metals and cementitious materials are widely used in water distribution systems. The general considerations of monolithic construction materials (see above) are valid for these materials. A

196

C H A P T E R 10

special property of metallic and cementitious materials is that the leaching behaviour of those products is dependent on water characteristics such as hardness and aggressiveness. In summary, the following problems may occur for cementitious and metallic materials: interaction between the material and drinking water for example dissolution and/or corrosion of the materials; different flow patterns in the various water supply systems; and change in the leaching behaviour with time (e.g. aging processes (carbonation), self sealing processes ) These problems occur also for cement based and metallic materials with applications other than drinking water distribution and storage. The field of drinking water is very sensitive, therefore in the leaching tests for products used in this field the selection of the appropriate test water, the pre-conditioning of the samples and the parameters that need to be measured to evaluate the suitability of materials are more difficult to choose. Prenormative research is being stimulated by CEN TC 164 WG3 for the development of leaching tests for drinking water pipes (see also Chapter 12) on this basis. The development and discussion of leaching tests is carried out in liaison with CEN TC51 WG12 TG 6 (see Chapter 12). Test Methods

There is no uniform test method or test procedure which could be applied for all construction materials. Moreover methods from other fields are adopted in the field of construction materials. Therefore an outline of how to derive a suitable and acceptable leaching test in the different areas is given later in this chapter. The most commonly applied and accepted methods for monolithic and granular materials are described below. For monolithic materials a suitable and commonly applied leaching test is the tank test (also applied in the field of stabilized waste (see Chapter 9), standardized for example in the Dutch NEN 7345/9). On a European level prenormative work is being undertaken to standardize a tank leaching test for concrete (CEN TC51 WG12 TG6) The leachants used in the tank tests are normally demineralized water or acidified demineralized water. For some purposes water enriched with carbon dioxide (e.g. to investigate the influence of aggressive water on the leaching of concrete/mortar [Spring 1984] is used). Natural waters are unsuitable because their compositions are very different depending on the source and therefore do not provide comparable results. In the field of cement based materials a discussion is taking place with regard to whether demineralized or mineralized water should be used as leachant (Figure 10.7). On the one hand demineralized water may dissolve the surface of the materials on the other hand the self sealing of the surface through calcium carbonate cannot take place using demineralized water. Therefore the leached amounts may be too high when using demineralized water (especially if flowing water is used during the test). Otherwise, demineralized water is easier to handle and all parameters can be studied. To decide which leachant is suitable for a special purpose one should always be aware that test methods in the laboratory are always commonly adopted conventions and are sometimes far from reality.

CHAPTER

10

197

Figure 10.7: Comparision of leaching with demineralization and mineralized water

1E+6

o11.,,=1

E 1E+4

CI

Na

/

"0

.N --

1E+2

" J9

"Cr

.E ~ 1E§

1E-2 1E-2

1E+O

1 E+2

1E+4

1E+6

E14-deminerilized [mg/m 2] Sometimes special methods are applied to describe the leaching behaviour under field conditions (e.g. simulation of rain fall for facades [Schiel31 1995]). pH static test methods are performed for research purposes (e.g. to determine the potentially leachable fraction) as well as to characterize the materials. In addition for the investigation of specific questions for example leaching from shotcrete, very special methods are applied [Breitenb0cher 1994]. Different methods are used for granular materials. These are in general batch tests (including pH-static tests and serial batch tests), column tests and tests for compacted granular material. Most of them are also applied in the field of wastes (see Chapter 8). Table 10.3 provides an overview on the different methods applied in the field of construction materials.

Results From Leaching Tests General

The leaching results for the various materials shown below give an overview on ranges of leaching data of monolithic and granular construction materials. The selected materials are typical examples of construction materials with and without addition of industrial by-products. Monolithic materials

Table 10.4 shows results from tank leaching tests carried out on different monolithic materials. The tests performed were similar to NEN 7345. The leachant was renewed 8 times during the leaching time. The tests lasted 56 days.

198

CHAPTER

10

In Figures 10.8 and 10.9 the changes in time-dependent leaching behaviour of zinc and chromium is shown for different mortar samples with different binder and fly ash addition. The water-cement ratio (w/(c+0.5f)) was 0.5 for all mortar samples, f/c=0.25. The figures show that the addition of fly ash (with rather high zinc and chromium content) does not result in higher leaching rates. For most mortars the addition of fly ash lowers the leached amounts due to the change in porosity and tortuosity (see Table 10.5). Table 10.3: Leaching tests applied in the field of construction materials Material 1

property of material 2

cement based monolithic

area of use 3

foundations

i materials I

facades

masonry units

aggregates

granular

Type of method i

Objective

4

5

NEN 7345; tank tests developed by CEN TC51 WG12 TG6 pH-stat.-methods CEN TC 292 LS=2/10 procedure

determination ofthe leaching rate under . practice-related conditions . chemical characterization leaching under extreme conditions; determination of potentially leachable fraction simulation of rainfall

special methods (e.g. Breitenbucher 1994) drinking migration tests (tank tests); determination of timewater pipes methods to be developed dependent leaching by CEN TC51 WG 12 TG6 in liaison with CEN TC 163 facades tank tests determination of time. dependent leaching special methods [Schiel31 simulation of rainfall 1995] road batch test/sequential batch long-term leaching of construction test e. g. NEN 7343 [draft] environmentally relevant substances pH-stat.-methods e. g. chemical characterization; CEN TC 292 LS=2/10 leaching under extreme procedure conditions; determination of potentially leachable fraction column tests e. g. [Goetz determination of the time1994] . dependent leaching compacted granular determination of the leaching test leaching rate under . practice-related conditions .

Tortuosities are determined in order to evaluate the physical retention capacity of different materials (matrixes). The tortuosity, T, is a measure of the prolonged path of the ions in the pores. The higher the tortuosity, the higher physical retention capacity of the material. The tortuosity is determined normally from mobility data for inert constituents (e. g. sodium, potassium) [van der Sloot 1993]. In Table 10.5 tortuosities (calculated from the quotient of

CHAPTER

10

199

free and effective coefficients o f diffusion of sodium) for a variety o f materials are summarized. The results from the examples in Table 10.5 demonstrate the wide range o f tortuosities for monolithic construction materials.

Granular materials Leaching results for different aggregates are summarized in Table 10.6. The results are selected from the literature [ C R O W 1994]. The materials chosen are typical examples o f aggregate used in concrete production and road construction. The applied leaching tests were an availability test according to N V N 2508 and a column test with L/S = 10.

Table 10.4: Results from tank leaching tests of construction materials [Schiefll 1995] material

masonry unit

rendering -

cement

2)

additive/ aggregate number of samples parameter

mortar 1)

CEM 1 32,5 R

concrete 1)

CEM 1 32,5 R

CEM III A 32,5

2)

2)

-

SFA

-

SFA

1

3

1

4

1

2

SFA 1

Arsenic

< 0.2

< 0.2

!<0.2

Cadmium

< 0.2

< 0.2

< 0.2

total amount leached [ms/m2] 3) 5 ]6 I 7 18 < 0.2 < 0.2 < 0.2 < 0.4 J < 0.2 < 0.2 < 0.2 < 0.4

Chromium

1.1

2.1-10.0

0.41

0.40-3.2

1

4

2

I

2

CEM III/A . 32,5 MS 1

1

9 Ilol 11 < < 0.4 [ 0.55 / 0.4;0.52 < 0.4 n.a. < 0.4 |

|

9Copper

'

0.8

' 0.52-0.64 '

1.6

1,6;1.8

0.82

1.1

<0.4

0.43

0.41;0.44 ~ 0.78

0.39

<0.20.63 668519613 < 0.2 8122139 1.4-1.8

0.64

0.58-0.37

< 0.4

10212

7557; 13807 < 0.2 612;1920

n. a.

J ,

,

Lead

< 0.5

Potassium

n.a.

9Nickel Sodium

~

n.a.

,

0.35-0.63

1102015550 24450 . . < 0.2 2915-4906 1572 i

9

,

Zinc

,

2.1

1) 2) 3)

]

1.9-5.6

! 2.6

< 0.2 1021 0.42

0.39; < 0.2

water-cement-ratio w((c+0,5f)=0,5; f/c=0,25) composition u n k n o w n leached amounts after 56 days and 8 leachant renewals SFA: coal fly ash MS: metal slag

3.64; 1.42 0.73" 0.84 0.40; <0.4 7682; 6930

16.3 i

1.8

i

0.94

0.81

< 0.2

0.76 |

1134 2

|

4925 |

n

n

1794

761;746

956

736

2.6

1.8;1.8

9.2

1.42!

200

CHAPTER

10

General

The aim of this section is to outline a systematic approach to the harmonization of leaching tests in the field of construction materials. The leaching tests need to realistically simulate leaching conditions under natural or imposed environmental and/or leachant conditions according to the general considerations set out in Chapter 2. Practical Approach - General

As mentioned earlier, the leaching behaviour of construction materials needs to be considered during their overall life cycle from production until demolition and disposal or reuse. The scheme presented in Figure 10.9 therefore starts with the raw materials used for cement based construction materials. Some of them are relevant with respect to potential leaching (e.g. aggregates, non water-reactive additions like fly ash, recycled materials if they are exposed to rain during storage), others are not (e.g. cement, as cement must always be stored under dry conditions, otherwise it hardens) and therefore need not be tested with regard to leaching behaviour. Table 10.5 Tortuosity of monolithic construction materials [CROW 1994 and van der Sloot 1993] sample

cement

addition/ aggregate

i

|

,,

1

porous sinterbrick sintered clay brick calcium silicate blocks rendering l) mortar 3)

3

4

l)

2)

tortuosity

Dna/I)e,Na

~)

2)

1)

2)

5 30- 70 4 0 - 70 200- 300

1)

3

3 - 22

CEM I 32,5 R

210 i I

1

SFA CEM III/A 32,5

1)

~light weight concrete concrete 3)

asphalt concrete

number of , samples

4 1 2

. 329-855 I 688 . 1776; 1022 '300-600

1

580

SFA MS

2 1 1

1450; 661 764 644

1)

2)

SFA 1)

CEM I 32,5 R

CEM III/A 32,5 1)

.

-

.

5000090000 w/(c+0.5 f) = 0,5 for 'all mortars and concretes SFA: coal fly ash MS: metal slag i

l) 2)

composition unknown unknown

3)

!

CHAPTER

10

201

Table 10.6" Leaching results for selected granular materials and aggregates [CROW 1994] material ....leaching test

sand availability

I column test

p arameters 1 Antimony Arsenic Barium Cadmium Chromium Cobalt Copper Lead Mercury Nickel Potassium Sodium Zinc

0.5 0.1 1

0.008 0.057 0.28 0.002 0.09 0.018 0.03 0.023

-

0.02

-

0.09

bottom ash from granulated blast coal pox eer plant furnace slag availcolumn avail- ] c o l u m n ability test ability [ test total amount leached (mg/kg) 4 1 7m m / < 0.2 <0.1 0.75 6.1 27 0.59 114 57 2O

0.015 0.1 2.8 0.002 0.013 0.022 0.028 0.015 0.003 0.055 5.2 17.4 0.22

recycled concrete avail- ] c o l u m n ability test

_81

9 0.002 0.03 4.2 O.002 0.12 0.03 0.10 0.033

3.4

< 0.02

< 0.02 1.3

<0.01 0.012

1.3 1.2

< 0.03 0.07

1.1 34 O. 14 < 1 < 1 1.1

0.21 750 657 4.7

< 0.02 76 187 <0.18

< 1 480 305 12.5

-

0.07 253 126 0.35

202

CHAPTER

10

Systematic approach for the assessment of leaching from construction materials. Example selected = Cement Based Materials

Different construction materials are produced from the raw materials, some of them needing extensive different consideration of the issues related to their leaching behaviour since they may be exposed to situation specific environments or scenarios. The different periods need to be considered separately during the life cycle since the release mechanisms and the relevant time periods (and therefore the relevant test methods) may be completely different. For example the mechanisms controlling release from fresh concrete (lasting only a few hours) are completely different from those controlling release from hardened concrete (lasting for decades). Finally the areas of use (e.g. the environment to which the construction material is exposed) needs to be differentiated. However, since the rate determining mechanisms remain the same (diffusion control), the same Basic Characterization Test can be used in most of the cases. As long as the relevant material parameters are determined in the Basic Characterization Tests (e.g. effective diffusion coefficients), the time dependent release rates can be calculated for different environmental conditions (flowing water, standing water, etc.). The classification is particularly important with respect to the relevance of the release rate of inorganic or organic compounds. The basic scheme shown in Figure 10.10 is derived for leaching problems. However it can be used for other areas of environmental compatibility as well by adjusting it slightly to the problem (e.g. radiation or gas-emissions).

C H A P T E R 10

203

Figure 10.10:Environmental compatability of cement based materials basic scheme. raw materials

construction materials

life cycle period

(cement) - - - - - ~ [ concrete ] ~-----~ p r o d u c t i o ~

aggrega ~ / ~ ~

shotcrete " N ~N~ -,~ mortar/rendering 9

.

r g~ ~g

[use ]

r demolition ~disposai ( --- waste) "reuse (.-~ aggregates)

Different leaching conditions and leachants possible -* possibly different relevant leaching tests and test procedures

area of use resp. exposure conditions (environment) drinking water groundwater soil outdoor indoor ~" living storing

mainly influencing limit values with respect to environmental compatibility

Test Me thods Basically all test methods must be based on the relevant leaching mechanisms for the field conditions and the special case (life cycle period and environment) considered (see Figure 10.10). Basic Characterization Tests (see Chapter 2) should be established to measure the time dependent leaching behaviour. Most of them will not be appropriate for quality control purposes aider the production of materials. For this purpose simple Compliance Tests with strongly defined test conditions should be set up. The Compliance Test must be derived from the findings in the Basic Characterization Tests. If the Basic Characterization Tests allow the determination of the relevant material characteristics (e.g. an effective diffusion coefficient if diffusion within the monolithic structural element - (e.g. concrete foundation - is the leaching rate determining process) the Compliance Test set up and/or procedure can be completely different from that of the Basic Characterization Test as long as the relevant material parameters (in the case considered this will be the effective diffusion coefficient) are measured (see Figure 10.10).

Example: Concrete During Use As an example, Figure 10.11 shows a possible scheme of the overall procedure in the assessment and evaluation of the environmental compatibility of concrete during use related to leaching.

204

C H A P T E R 10

F i g u r e 10.11" H a r m o n i z a t i o n o f L e a c h i n g T e s t s - c o n s t r u c t i o n m a t e r i a l s field to be c o v e r e d Construction ] Materials ]

vL T i m e d e p e n d e n t l e a c h i n g of inorganic materials to e n v i r o n m e n t d u r i n g

~

life cycle

Life c y c l e p e r i o d s : - production | - use demolition reuse 9

J

Test m e t h o d s

Environments:

---- b a s e d on the relevant mechanisms for the life cycle period considered

- s p l a s h rain - water - groundwater -soil

__

, _.. W a s t e ~ I

I

I B a s i c L e a c h i n g Tests (BLT)

I

I omp,iaoco,osts ,, I The relevant basic leaching test is the "Tank Leach Test" using specimens with realistic dimensions (3 40 ram). The leachants to be used should represent natural conditions (e.g. mineralized water). The aim of the basic leaching test is to determine the relevant rate determining mechanisms of the ion-release from the concrete. If a surface dissolution of the concrete by the leachant (e.g. acid attack) dominates the reaction and consequently the ion release this will cause a horizontal line in the log I-log t-relationship (see Figure 10.1). If the diffusion of ions within the concrete towards its surface is decisive (which is the ease for most of the heavy-metal leaching processes), this is indicated by a declining linear relationship in the log J-log t-plot with a gradient m = -1/2 (see earlier discussion). If the processes and mechanisms are clear, the details of the test procedure (solid/leachant ratio, renewal ofleachant, etc.) are of minor importance since the determination of the relevant material parameters (e.g. effective diffusion coefficient in the case of diffusion controlled processes) are independent from the test procedure and can be calculated from the result of any test procedure provided that the basic physics and chemistry behind the formulae used are relevant to the leaching process. In the end, the definition of limit values to be met by the results of the Basic Leaching Tests or Compliance Tests for specific applications will always be contentious. As absolute or definitively fixed (e.g. by certain "natural laws") limit values related to environmental compatibility do not exist, limit values will always be debatable. One possibility to overcome the problem of discussing absolute values for allowable limits in leachates is to establish a system based on well proven and accepted applications in practice (see Figure 10.13). For example, a scheme can be set up classifying concretes with certain compositions (classification of materials) for certain applications (classification of environments or applications). For these well established, well known and tried materials the relevant material parameters can be determined using the established Basic Leaching Tests and/or Compliance Tests, leading to a

C H A P T E R 10

205

"leaching behaviour classification of materials" and a correlation with an environmental classification. Figure 10.12: Generic approach for assessment and evaluation of the environmental compatibility of concrete durin~ use - example, leaching Basic leaching Leaching of concrete during use I Test (BLT): Tank Leach Test using natural leachants og J J : leaching rate of ions (~

I

(mmol/s-t 9m-2) t : time (s)

Surface

(~)

Relevant ratedetermining mechanisms

-

|

(~) log t

Definition of the coefficienteffective diffusion --" De

~

an be ann'S\ est set-up aslong as ] strong relation] to B LT is proven.~

:

dissolution

Establishment of a simple Compliance Test (CT) l

Diffusion within the concrete to its surface

the effective disso!ution coefficient ~ Se

[ ] ] ]

[ Calculation of [_. the time dependant ]-" release of relevant ions I for all possible '~micro environments"

Definition of limit values for specific areas of use (e.g. following the procedure given in Fig. 10-10) New materials and/or new material compositions (e.g. substituting natural aggregates by waste material) can then be classified using the established test procedures and by comparing those results with the results of already classified materials.

206

C H A P T E R 10

Figure 10.13: Environmental compatibility of construction materials Evaluation - Classification Scheme New materials test methods (BLT

+ CT)

111

TT

[

!--- ~I tried'l" materials'.~l ] ' - '~

classes (materials)

-

(e.g. concrete without

t wm / g

additions and admixtures, ~ - J~~~~ using OPC, w/c = 0,5)

(e.g.

relevant material parameters values for De or leaching rate)

classes (area of use)

-

(e.g. drinking water containments, water pipes)

\V (e.g.

concrete using well

defined and tried materials

_ _ _ ~.

including waste) - - ~

(e.g. concrete foundation with-

out

ground water)

Procedure leading to classification Procedure for new materials

Conclusions Leaching of environmentally relevant compounds from construction materials and their impact on the environment have become an increasingly important issue. The use of secondary raw materials/wastes should be stimulated further from the point of view of the conservation of resources and energy. This requires a guarantee on the technical properties and the environmental compatibility of the construction products. The issues discussed have shown the complex nature of the topic of the leaching behaviour of construction materials. A basic test scheme and harmonized test methods for the separate aspects have to be developed in this field. There is a need to act on the development of a material-class related and an application-class related evaluation scheme for construction materials with regard to their environmental compatibility because of the increasing pressure to recover waste materials expected in the future. New waste materials can be assessed or ruled out as possible raw materials for construction materials through such a scheme that is related to the release rates of contaminants from common concretes.

REFERENCES

TO

CHAPTER

10

207

REFERENCES Breitenb0cher, R.: Leachability of concrete- test methods and evaluation of test results. In: concrete and reinforced concrete structures 89 (1994), Nr. 9, S. 237-243 (in German) Cotr, P.L.; Constable, T.W.; Moreira, A.: An evaluation of cement based waste forms using the results of approximately two years of dynamic leaching. Nuclear and waste management 7 (1987)Nr.2, S.129-139 Crank, J.: The mathematics of diffusion. 2. edition, Oxford: Clarendon Press, 1975 C.R.O.W.: Uitlogen op karakter. Handboek Uitloogkarakterisering II: Materialen. Ede, Niederlande : Centrum voor Regelgeving en Onderzoek in de Grond-, Water- en Wegenbouw en de Verkeerstechniek, C.R.O.W., 1994 European commision: Mati CT93-0026: "Development of a leaching standard for the determination of the environmental quality of concrete". Final report, 1997. Goetz, D.; Gl~iseker, W.: Percolation method for the leaching of aggregates. In: streets and highways (1994), No. 10, S. 605-609 (in German) Hohberg, I.; MOiler, Ch.; SchieB1, P.; Volland, G.: State of the art report on the environmental compatibility of cement based building materials. In: Schriftenreihe of the German committee for reinforced concrete (1996), No. 458 (in German) Hohberg, I.; MOiler, Ch.; Schiel31, P.: Environmental compatibility of cement based building materials. In: Concrete 46 (1996) No. 3, S. 156-160 (in German) NEN 7343 (Draft) Leaching characteristics of building and solid waste materials Leaching tests : Determination of the leaching behaviour of inorganic components from powder and granular building and waste materials. NEN 7345 (Draft) 08.92: Leaching characteristics of building and solid waste materials - Leaching tests : Determination of the leaching behaviour of inorganic components from shaped building materials, monolithic and stabilized waste materials SchieBl, P.; Hohberg, I.; Rankers, R.: Environmental compatibility of building materials for exterior facades. Institute for Building Research, Aachen 1995. Research report Nr. F 415 (in German) SchieBl, P. I Hohberg, I.:Environmental compatibility of cement based building materials: Investigation into the leaching behaviour of secondary building materials, Aachen: Institute for Building Research, - research report Nr. F 414. 1995 (in German) van der Sloot, H.A.; Wegen van der, G.L.; Vega, E.: Beoordeling van immobilisaten. Eeen voorsel voor criteria en testmethoden. CUR report 93-6. Civieltechnisch Centrum Uitvoering Research en Regelgevin. Gouda. 1993

208

REFERENCES

TO CHAPTER

10

van der Sloot, H.A.: Systematic leaching behavior of trace elements from construction materials and waste materials. Amsterdam: Elsevier, 1984 - In: Environmental aspects of construction with waste materials. Proceedings, Maastricht, Netherlands, 1-3 June 1994, (Goumans, J.J.J.M. ; (Ed.)), S. 369-386 van der Sloot, H.A.; Hoede, D.; Groot de, G.J; Wegen, van der G.J.L.; Quevauviller, Ph.: Intercomparison of leaching tests for stabilized waste. Petten: Netherlands engergy research foundation (ECN) 1994, restricted distribution ECN-C-94-062. Sprung, S.; Rechenberg, W.; Bachmann, G.: Environmental compatibility of cement and concrete. Amsterdam: Elsevier, 1984 - In: Environmental aspects of construction with waste materials. Proceedings, Maastricht, Netherlands, 1-3 June 1994, (Goumans, J.J.J.M. ; (Ed.)), S. 369-386 Wruss, W.; Rechenberg, W.; Spanka, G.; Hohberg, I.: Method to test the leaching behaviour of cement-bound materials: Leaching behaviour and evaluation of the test results. Vienna: Federal ministry for the environment, youth and family, 1992. - In: Rilem- Workshop: Leaching behaviour of concrete and cement based materials, Vienna, June 1992, S. 21-37 (in German).

C H A P T E R 11

CHAPTER

11: P R E S E R V A T I V E

209

TREATED WOOD

Introduction

Today wood (excluding fuel wood (50% of world wood goes for fuel) and wood for pulp and paper) is above all a construction material. Modem commercial forestry and sawmills mainly are providers of material for the building industry. The relative importance of the different enduse sectors for EU forest products (pulp and paper excluded) has been estimated in log volume terms as about 55-60% used for construction, 15-20% used for furniture and 25% for other uses [Network Eurowood]. The selection of a particular tree species for various end uses is in most cases based on the physical and chemical characteristics of the wood. However, in many situations proximity to the wood source and total wood procurement cost can significantly affect species selection. Wood kept in unfavourable conditions and not preserved properly either as raw material or in the different stages during usage, transport, processing or exploitation of final products can be subject to the action of biotic or abiotic degradation factors which have an adverse effect on its initial traits and properties. Biotic factors which cause wood degradation include bacteria, fungi (brown-rot, white-rot and soft-rot fungi), insects (beetles and termites) and marine borers. The severity of attack of these organisms will depend on factors such as climate and the condition of the wood. The importance of such biological deterioration depends greatly on the end-use of the wood. Physical and chemical degradation relates to the breakdown of wood by processes which do not involve living plants or animals. The physical and chemical degradation of wood is caused by processes including weathering (caused by light, wind, rain, humidity), mechanical wear (caused by abrasion), fire and chemical degradation (caused by strong acids and alkalis). Where it is necessary to protect wood against biological degradation a correctly applied, appropriate protection technology can contribute greatly to the durability of the material hence acceptance of wood and wood derived products as an industrial material and to the conservation of the forest resource. The choice of the protection method depends on the character of the wood, the proposed use of the material and the properties of the protection method. Methods for protection against biological degradation must [UNEP 1994]: have sufficient efficacy against wood-destroying organisms; be able to penetrate the wood; be chemically stable; be able to be handled safely; be economical to use; not weaken the structural strength of the wood; and not cause significant dimensional changes within the wood. Preservative chemicals used to prevent or slow down biological degradation of wood include waterborne, organic solvent and tar oil preservatives. These preservatives differ from each other in their activity spectra, their leaching resistance, their impregnation process and therefore the final use of the treated wood. In 1993 the annual production of preservative treated wood (vacuum/pressure) was estimated as 34.9 x 106m3, of which 26.6 x 106m3 was

210

CHAPTER 11

treated with water based preservatives, 5.0 x 106m3 w a s treated with creosote and 3.4 x 106m3 was treated with oil based preservatives [Nurmi 1994]. Preservatives can be applied to wood using different treatment techniques depending on the nature of the wood and the intended final use of the treated timber. Dipping or immersion and vacuum/pressure treatments are the most commonly used treatment methods in wood preservation. The successful treatment by any timber preserving process requires the preservative to penetrate evenly into the wood to a sufficient depth. The concentration of the preservative product in the treated wood should be high enough to protect it from wooddegrading organisms in the required environment (threshold value). In order to establish an optimal preservative treatment of timber there are three main requirements: the timber has to be in a suitable condition for treatment (e.g. moisture content, occurrence and degree of biological degradation before treatment); an appropriate preservative should be applied taking into consideration the final application (e.g. hazard class, possible wood degradation organisms); and an appropriate treatment process should be applied. In the late 1980s environmental concerns about treated timber increased. Currently there is increasing concern regarding environmental contamination from leaching losses of wood preservatives especially from wood in service and from wood removed from service and placed in landfills. After processing and treatment three major stages can be identified in the life cycle of treated wood, namely: storage at the treatment facility, actual service (see Table 11.1), decommissioning and waste phase. Each of these phases poses its own potential environmental risks. During these phases it is inevitable that at certain points in time the treated wood will be exposed to leaching influences and it is possible that at this times leaching of preservative components from treated timber into the environment (soil, ground water and/or surface water) may occur. In Table 11.1 potential risk of leaching for preservative treated wood is given for typical wood applications. In order to predict the environmental impact of treated wood several methods have been developed to assess the contribution of preservative treated wood to the pollution of ground, water or sewage. The leaching rate of wood preservatives in actual service may be determined by means of a practical field investigation. However such tests are cost inefficient and time consuming and laboratory based tests to determine leaching rates are preferred. As the scope of this book is the leaching of inorganic contaminants this chapter discusses mainly the leaching of inorganic waterborne preservatives from treated wood. CCA (a preservative based on arsenic, copper and chromium) is the most commonly used preservative worldwide and most leaching investigations are based on CCA. For this reason CCA is used as a model compound but where appropriate results of tests with other waterborne preservatives will be referred to.

CHAPTER 11

211

Table 11.1: Leaching hazards for preservative treated wood [Morris 1996]. Degree of wetting

Risk of leaching

Typical applications

Interior, fully protected from liquid water

No leaching

Framing lumber, joists, flooring

Interior, occasional wetting

No or slight leaching

Sillplates

Exterior, intermittent wetting

Periodic moderate leaching

Windows, fascia boards, decks, fence boards

Exterior, permanent wetting

Severe leaching

Wood foundations, utility poles, marine piling, piers, cribs, cooling towers

General characteristics To understand the leaching of waterborne preservatives from treated wood it is necessary to consider the physical and chemical processes which occur in preservative treated wood during and after the preservation process.

Preservative treatment

The most effective method of preservative treatment of wood is the use of pressure impregnation in specially constructed plants where the wood is treated under pressure in a closed steel vessel. The usual pressure applied is between 800kPa and 1400 kPa. After the pressure period a final vacuum is applied to promote drying of the wood surface and/or to recover a proportion of the preservative from the wood cells. The penetration of the preservative into wood depends on several factors, for example, the treatment process applied, wood permeability, heartwood/sapwood ratio, wood moisture content and wood quality. As a result the penetration could be restricted to the surface (e.g. Norway spruce) or the preservative penetrates as far as or even into the heartwood (e.g. Scots pine). After pressure impregnation components of waterborne preservatives will diffuse from the lumen of the wood cells into the cell wall matrix. The distribution of preservative components in the cell wall matrix varies between wood species and this might affect preservative efficacy. According to Cooper [1988] the distribution of the preservative components in the cell wall matrix depends also on cation exchange reactions and the pH of the treating solution.

212

CHAPTER 11

Fixation After the timber is pressure treated with waterborne preservatives a complex chemical reaction occurs in which the components of the preservative and the wood structural components (cellulose, hemicellulose and lignin) are involved. These chemical reactions result in the fixation of the preservative components (mobile metal ions are converted to insoluble complexes and precipitates, making the wood resistant to leaching and maintaining the protection of wood against biological attack [Anderson 1990, Dahlgren 1972, 1974, 1975, Pizzi 1981, 1982, 1984]. The rate of fixation and the fixation degree is dependent on many factors including temperature, relative humidity, wood species, preservative type, formulation of the preservative and preservative retention [Cooper 1994]. As a result the fixation time varies between several days and several weeks (e.g. winter treated wood). Application of accelerated fixation processes (e.g. steam fixation) could reduce significantly the fixation time (1.5 hours to 3 hours). The efficacy of an accelerated fixation process is however related to the dimensions of the treated wood and the thickness of a board or post could affect the rate of fixation. When incompletely fixed wood is subjected to a high risk of leaching, leaching of preservative components occurs and may affect the preservative efficacy and increase the environmental impact.

Leaching Waterborne preservative components are deposited in treated wood as stable low solubility complexes. The main depletion mechanisms are leaching and mechanical loss of surface deposits. Leaching of preservative components depends on a number of physical/chemical processes including [Cooper 1994]: wetting of the treated wood by capillary absorption; diffusion penetration of moisture into the wood; hydrolysis or solution of the components of complexed preservative chemicals; diffiasion of the dissolved components to the surface; and washing away of the dissolved material. The extent of leaching depends on the conditions of exposure to the leaching medium and the physical nature of the treated wood as it affects moisture absorption, hydrolysis and solution of preservative components and diffusion of the dissolved material out of wood. Loss of surface deposits is mainly affected by the preservative and processing variable that affect wood surface cleanliness. Although there is a large range of different preservatives with different formulations the most relevant inorganic constituents involved in leaching are copper, chromium, arsenic, zinc, fluoride and boron. Physical and chemical factors which affect the leaching of preservative components are shown in Table 11.2 together with other relevant factors which affect leaching The physical and chemical factors are discussed in more detail below.

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213

Physicalfactors affecting leaching The physical factors affecting leaching relate strongly to the manner of contact between the liquid and the solid material. The way in which treated wood is exposed to leaching conditions may have a significant effect on the leaching of preservative components as described by Cooper [ 1994]. For example, treated wood continually exposed to water or damp soil will lose more preservative than wood exposed to occasional rainfall; a continuous light rainfall has a greater leaching effect than the equivalent rainfall from a short heavy shower; wood exposed to intermittent rainfall in above-ground applications such as in fence boards and decks will lose less preservative components than wood submerged permanently in water ; vertically exposed wood is subjected mainly to driving rain from one direction at a time and is less exposed than horizontally applied decking; the initial leaching of preservative components in stirred leaching tests is higher than in static tests [van Eetvelde 1995]. It was assumed that stirring increases leaching from the wood surface. Table 11.2: Physical and chemical factors affecting the leaching of preservative components from wood

Other relevant factors

The natural properties of wood (e.g. permeability, pH) Preservative treatment Fixation

Physical factors

Absorption Diffusion Dissolution Temperature

Chemical factors

pH Ionic strength (Organic) acids

As permeability effects the penetration of preservatives in wood it might be expected that permeability effects also the leaching of preservative components out of wood (absorption of moisture, diffusion of moisture and dissolved salts in treated wood). High density and/or low permeability species tend to be more resistant to leaching [Wilson 1971]. However, these species have preservatives concentrated near the surface where they are leached and eroded more easily. As temperature effects diffusion and dissolution processes it will have a significant effect on the leaching of preservative components. Wood exposed to high rainfall under moderate annual temperature conditions will leach more than wood in colder drier climates [Cooper 1994]. Another study showed that the leaching characteristics of treated wood were much more severe under standard laboratory conditions (approximately 20~ than under cooler temperature conditions (8~ [van Eetvelde 1994]. The shape, size volume and proportion of the end grain in wood may effect the leaching potential of preservative components [Cooper 1994]. Small sized samples exaggerate the rate

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of leaching due to the high surface-area-to-volume ratio. The rate of wetting and the relative area exposed directly to the leaching environment are increased and the diffusion distances are decreased. The rate of leaching of sawdust is a magnitude greater than a solid sample of treated timber according to Weis & Weis [ 1992]. Hayes [ 1994] found a considerable reduction in the amount of metal loss with increasing block size. In small sized samples the relative amount of exposed end grain is higher than in normal practice. This might affect leaching losses to some extend since the diffusion and permeation coefficients along the grain are much higher than for the transverse directions [Cooper 1994]. End sealing of the end grain might reduce this effect. Caution is needed in the extrapolation of data from small sized samples to full scale structures.

Chemicalfactors affecting leaching Several studies have indicated that the pH of the surrounding water affects to a significant extent leaching of preservative components [Cooper 1988, 1991, 1992, Murphy, 1990 van Eetvelde 1994, 1995). With increasing acidity of the leaching water an acid environment is created providing hydrogen ions which act in the acid-ion-exchange reactions taking place at the acid adsorption points on the wood cell walls [van Eetvelde 1994]. As the acidity in the cell wall increases the solubility of the insoluble complexes and precipitates increases resulting in an enhancement of leaching. Although pH is an important factor affecting the leaching rate the acid used also contributes to the leaching effect, for example the use of citric acid resulted in much higher leaching than sulphuric acid/nitric acid solution [Cooper 1991]. Organic acids which can interact with treated wood during service conditions might also affect leaching of preservative components, for example peaty organic soils and surface water containing humic or fulvic acids and wood silos containing silage high in formic, lactic, acetic and other silage acids [Cooper 1994]. An alkaline medium (e.g. NaOH) also increases leaching of preservative components from treated wood [Cooper 1994 van Eetvelde 1994]. In addition to the acidity of the surrounding medium the natural pH of wood also may play an important role in the leaching process. The natural pH of wood varies from 4.6 to 5.3 depending on the species with extremes of 3 and 8. Based on the natural pH wood is capable of buffering leaching solutions and reducing the acidity of the leachant [Cooper 1994]. This buffeting effect may be observed when wood is exposed to acid rain. However the effect might not be significant in acidic lakes or other bodies of water. The ionic strength of the surrounding medium also affects the leaching of preservative components. It has been shown that water of high ionic strength results in an increase of leaching of preservative components. Irvine and Dahlgren demonstrated that leaching is greater with higher salinities. Therefore it is expected that treated wood which is in contact with water with a high salt content (sea water, cooling tower water, water contaminated with road salt or fertilizers) leaches to a greater degree than wood in contact with water of a lower salt content. Some typical examples are given in Figure 11.1 and Figure 11.2 of leaching characteristics of CCA treated wood related to pH and temperature of the leaching liquid and to the wood species [van Eetvelde 1994, 1995]. In these studies the maximum cumulative proportional

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leached quantities of copper, chromium and arsenic after a tank leaching test of 64 days were 2.0%, 0.05% and 0.63% respectively [van Eetvelde 1994]. State of the art

Development of leaching tests Originally leaching tests (e.g. NEN-EN 84) were developed to accelerate ageing of treated wood prior to durability tests (e.g. NEN-EN 113) in order to evaluate any loss in effectiveness. As the environmental concerns regarding the leaching of preservatives to ground, groundwater and surface water increased these leaching tests were used also to determine the leaching characteristics of preservative treated wood. During the last 10 to 15 years many studies have been undertaken to evaluate the impact of preservatives on the environment. Attention has been focused on the contamination of ground, groundwater and surface water. To date a range of different leaching test methods have been Figure 11.1: Leaching characteristics of CCA treated wood in relation to pH and wood species [van Eetveide 1995]. Leaching test method used is ENV 1250-2. The leaching characteristics of arsenic are not given, because the arsenic concentration in the leachates were below the detection limits. (CCA treated wood, accelerated fixation (35~ 60% RH, 7 days))

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217

published most of them laboratory scale sized experiments using small wood samples. In general many tests are based on the same principle with small modifications in the specific testing conditions. Most laboratory-based studies are performed to investigate the effect of preservative treatment (e.g. preservative type, retention, fixation, wood species) and test conditions (e.g. pH, organic acids, ionic strength, temperature, stirring) on leaching. Leaching test methods were evaluated in the presence of different types of soils to investigate the effect of soil on leaching. The behaviour of preservative components in soil was considered also in these studies [Behr 1994, Bergholm 1990, Holland 1993, Murphy 1990]. Field studies to monitor the levels of contamination in groundwater or surface water near treated products are rare.

Commonly used leaching tests Commonly used leaching tests for preservative treated wood are described briefly below. The relevant parameters in each of these tests are described in Table 11.3. NEN-EN 84 Wood preservatives - Accelerated ageing of treated wood prior to biological testing. Leaching procedure. This European Standard specifies a leaching procedure applicable to wood samples that have been treated previously with a preservative in order to evaluate any loss in effectiveness when

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the test samples are subjected subsequently to biological tests (e.g. NEN-EN 113) compared with samples which have not undergone any leaching procedure. Prior to testing the wood samples are impregnated under vacuum with demineralized or distilled water. Following impregnation the wood samples are immersed in water to a ratio of approximately 5 volumes of water to 1 volume of wood. For a period of 14 days nine leachate samples are collected and the water is changed following each sample collection. prENV 1250-2 Wood preservatives- Method for measuring losses of active ingredients from treated timber Part 2: Laboratory method for obtaining samples for analysis to measure losses by leaching into water or synthetic sea water. This test method was developed recently to measure leaching losses of active and other preservative ingredients from treated Scots pine sapwood of dimension 50mm x 25mm x 15mm in water or synthetic sea water. Five test samples are immersed in 500g water (complying with grade 3 of ISO 3696) or synthetic sea water for six periods comprising 1, 2, 4, 8, 16 and 48 hours. After each period the leachate is collected and replaced with fresh water. Between the third and fourth period the samples are left in the test vessel without water but are covered to prevent contamination for a period of 16 hours. During the leaching periods the water is stirred with a magnetic stirrer. NEN 7345 Leaching characteristics of building and solid waste materials - Leaching tests - Determination of the leaching behaviour of inorganic components from building materials, monolithic waste and stabilized waste materials. This method utilises a tank test method developed originally to determine the leaching characteristics of granular materials but applied also to preservative treated wood. The wood samples are immersed in acidified water (pH 4) at a ratio of approximately 5 volumes of water to 1 volume of wood. During eight periods the leachate is collected and replaced with fresh water (after 0.25, 1, 2.25, 4, 9, 16, 36 and 64 days). The shower test method - A leaching test for assessing preservative losses from treated timber under simulated open storage conditions (IRG/WP 93-50001, pp. 77-90). This test method aims to determine the leaching characteristics of preservative treated timber under simulated open storage conditions. The test enables the leaching of preservative components from treated wood during storage to be quantified for extreme climatic conditions. The test allows for the direct determination of leachate quality as a function of time (i.e. fixation) and enables the influence of several material and process variables to be assessed [Boonstra 1995]. This test is being used currently under Dutch legislation as an environmental control system for the Dutch preservation industry.

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A stack of treated wood samples (length 100 cm, total volume 0.5 m3) is tested during a five day period. Each day the stack of treated wood samples is showered for one hour using 20 litres of demineralized water. Table 11.3: Overview of commonly used leaching tests for preservative treated wood and relevant test parameters.

Test method

NEN-EN 84

Sample size and species

Leaching method

Leachant

Duration (days)

Ratio water/wood sample

i i14

Not specified

Tank

Demin water

pr ENV 12502

Scots pine sapwood 50 x 25 15 mm

Tank

Demin or sea water

5.3

4

NEN 7345

>40ram

Tank

pH 4

5

64

Shower test method

Length 100 cm Total volume 0.5m3

Sprinkling

Demin water 0.04

Note:

i

5

Simplified on site leaching test methods have also been developed in order to assess the degree of fixation and/or to obtain an indication of the leaching characteristics of preservative treated wood [Cornfield 1991, Cooper 1993, Homan 1994].

Test data interpretation The results of leaching tests are normally expressed as:

concentration of the leached component in the leachate (e.g. mg/l); the quantity of the leached component per wood volume (e.g. mg/m3) or leaching surface (e.g. mg/m 2) and a leaching factor, the quantity of the leached component compared with the retention of this component (e.g. %, ppm).

These data can be calculated per period or cycle and cumulatively over the total test period. The leaching rate (e.g. mg/m2/day) is also a commonly used method of expressing results. The interpretation of results of leaching tests depends more or less on the objectives of the study. A leaching test can be used as a tool, as part of a larger study to determine the relative

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performance of treated wood or it can act as the main aim to determine the contamination of treated wood and/or to predict the performance in service.

Limitations/possibilities of current tests There are several reasons why mainly laboratory based leaching tests have been used in studies to investigate the leaching of preservative components" 9 easy to control, 9 fast results, 9 cost effective. However, care should be used when the results of these tests are extrapolated to predict the performance of treated products in service. Sample size, natural variation within a wood species or between wood species (as wood is no____!at homogeneous material) and test conditions might affect leaching to a significant extent causing differences when compared to field conditions. For this reason field tests are more suitable for determining the leaching of preservative components. Field tests however are time consuming, expensive and even within field tests it is difficult to extrapolate the leaching data (as leaching might be affected by climate, season, location, etc). Laboratory based test methods are therefore a valid approach where relative performance is of interest. If a laboratory based test method is used to predict the performance of treated wood in service it is recommended that natural conditions are simulated as far as possible.

Level of modelling associated with testing A regression analysis of the leaching data has been used in order to model the leaching results. When the leaching time is linked to the flux a highly correlated double logarithmic formula has been found [Berbee 1987; van Eetvelde 1994]. F = at "b (a,b constants; b>0) The main disadvantage of this mathematical approach is that it assumes an infinite release of active ingredients from treated wood which is not likely to occur. Environmental models have been used to predict the effect of leachates from preservative treated wood on surface and ground water quality for a period of five to twenty years [Hoottman 1995]. Results of a laboratory based leaching test (NEN 7345) were used in this model which can be described as a model box containing water and sediment including the different processes which might occur (e.g. contamination, water flow, sedimentation and resuspension of silt particles to which pollutants are attached, exchange of pollutants between water and sediment, etc). Such a model calculates the steady-state concentration (concentration whereby the supply and removal of pollutants are equal) as the concentration gradient with time. However as no field tests have been performed for verification purposes these extrapolations remain speculative. In the leaching tank method NEN 7345 a mathematical model is given to determine the leaching of the component involved over time if leaching is based on diffusion using the following formula: E~,y = 2 p Ub~s (~/tx- ~/ty) ~/O_ee

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Where:

Ex,y Ubes De tx ty p

is the leaching of a component between the times tx and ty in mg/m 2 is the quantity of the component available for leaching according to NEN 7341 in mg per kg dry matter is the effective diffusion coefficient of the component in m2/s is the starting time of the interval concerned in relation to the start of the test, in s is the end time of the interval concerned in relation to the start of the test, in s 3 is the volume mass of the test piece in kg dry matter per m

If this model is applied to the leaching of preservative components it should be noted that the diffusion coefficient used in this model is determined during the leaching period at which a steady state situation (constant leaching rate in time) is reached.

Repeatabifity/reproducibifity of leaching tests In general no specific test programmes have been carried out to determine the repeatability of the test methods for treated wood described above. There are some limitations when determining the repeatability of these test methods: some methods are still in the development or standardization phase (e.g. ENV 1250-2); it is not possible to produce identical material especially when large test samples are used and the amount of sample necessary for the performance of a test, for example the shower test requires 60 to 90 samples. As wood is not a homogeneous material the production of identical material is limited to small samples and to certain wood species for example the sapwood of Scots pine, which is normally used for research and standardization purposes (EN 113, ENV 1250). Larger wood samples which are representative of practical applications show a large natural variation (e.g. permeability, fixation, etc), which makes it difficult or almost impossible to produce identical material. This is especially true for the shower test where a large number of samples which are representative of practical applications are used. In order to demonstrate the variation of leaching results between two 'identical' wood samples two examples are shown in Figure 11.3 of a tank leaching test (NEN 7345) on CCA treated wood with a total wood volume of about 3.9 dm 3 which was performed in duplicate [Boonstra unpublished]. To date no reproducibility test programmes have been performed for leaching tests, for example round robin tests between several research institutes using identical material. Verification of laboratory based data

As described previously laboratory based tests are suitable for prediction of the relative performance of preservative treated wood, to investigate the effect of preservative treatment and test conditions on leaching. It is possible to simulate variations in leaching conditions which occur in practice (e.g. pH, temperature, ionic strength), but extrapolation of the results of laboratory tests to predict the performance of treated wood in service (treated wood in ground, surface and/or ground water contact) remains speculative. To date field tests on

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preservative treated wood are rare and therefore it is not yet possible to verify laboratory test results with field information. Figure 11.3: Repeatability of a tank leaching test (NEN 7345) performed on CCA treated wood with a total wood volume of 3.9 dm 3

The tank leaching tests described earlier are based on the same principles as above with small modifications in the specific leaching conditions. Therefore the mechanisms causing leaching of preservative components are the same for these leaching tests: namely absorption, diffusion and dissolution. The performance principle of the shower test (sprinkling/drying) varies compared with the tank leaching tests, which might affect the leaching mechanisms. It is not clear whether leaching during the shower test is the result of a 'wash off effect or whether absorption/diffusion/dissolution contributes also to the leaching of preservative components. Relationship with other technical fields

Leaching test methods (e.g. tank leaching tests) developed to determine the leaching of preservative treated wood are quite similar to leaching methods of building materials and monolithic and solidified/stabilized wastes. This is due mainly to the similar objectives of the leaching studies: 'What contamination of the environment is caused and how is this contamination caused?' There are however differences between leaching test methods applied to preservative treated wood and other building materials and/or wastes: leaching time (in order to establish a steady state situation the leaching time required for preservative treated wood is long);

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sample size (a sample size representative of likely situations in practice is required when extrapolations to predict performance in service are the subject of the research); material conditions (e.g. different wood species resulting in different properties, wood is not a homogeneous material, relatively low pH) These differences are of importance when leaching tests are standardized for different technical fields and materials (e.g. harmonisation). Interpretation of leaching results based on modelling are the particular subject of discussion as these results might be affected by the test method used (e.g. leaching time, test conditions). Potential for harmonization

The benefits of harmonization are 9 prevent the application of test methods which are not suitable for a certain field of materials; if possible the development of uniform leaching test methods; incorporation in the Construction Products Directive; determination of the relative contribution of building materials to the contamination of ground, surface and groundwater and determine the effect of various test conditions involved in the leaching process on different fields of materials. Areas or fields where more information/research is needed include: development of reliable test methods capable of simulating service conditions; optimisation of test methods; the repeatability and/or reproducibility of leaching test methods; the verification of test data with field information (contamination of ground, surface water and groundwater) and further research to investigate and assess the significance of the various test conditions involved in the leaching process.

224

REFERENCES TO C H A P T E R 11 REFERENCES

Anderson, D.G. 1990. The accelerated fixation of chromated copper preservative treated wood. American Wood-Preservation Association. 1990. Pp. 129-151. Behr, M; Baecker, A. A. W. 1994. Quantification of creosote migration down wooden poles and the prevention of its depletion during flood irrigation. IR6/WP 94-50032. Bergholm, J. 1990. Studies on the mobility of arsenic, copper and chromium in CCAcontaminated soil. IR6/WP 3571. Boonstra, M.J.; Pendlebury, A.J.; Esser, P.M. 1995. A comparison of shower test results from CCF, CCZF, CCB and Cu-quat treated timber. IRG/WP/95-50054. Cooper, P.A. 1988. Diffusion and interaction of components of water-borne preservatives in the wood cell wall. IRG/WP/3474. Cooper, P.A. 1991. Leaching of CCA from treated wood: pH effects. Forest Products Journal 41(1), pp. 30-32. Cooper, P.A. 1994. Leaching of CCA: Is it a problem? In "Environmental considerations in the manufacture, use, and disposal of preservative treated wood". Forest Products Society. USA. pp. 45-57. Cooper, P.A.; Ung, Y.T. 1992. Leaching of CCA-C from jack pine sapwood in compost. Forest Products Journal 41 (1), pp. 57-59. Cooper, P.A.; Ung, Y.T. 1993. A simple quantitative measure of CCA fixation. Products Journal 43(5), pp. 19-20.

Forest

Cornfield, J.A.; Bacon, M.; Lymann, A.; Waldie, C.; Gayles, M.R. 1991. Rapid leaching test. IRG/WP/2367. Dahlgren, S-E; Hartford, W.H. 1972a. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. I. pH behaviour and general aspects on fixation. Holzforschung 26, pp. 6269. Dahlgren, S-E; Hartford, W.H. 1972b. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. II. Fixation of Boliden K33. Holzforschung 26, pp. 105-113. Dahlgren, S-E; Hartford, W.H. 1972c. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. III. Fixation of Tanalith C and comparison of different preservatives. Holzforschung 26, pp. 142-149. Dahlgren, S-E. 1974. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. IV. Conversion reactions during storage. Holzforschung 28, pp. 58-61.

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Dahlgren, S-E. 1975a. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. V. Effect of wood species and preservative composition on the leaching during storage. Holzforschung 29, pp. 84-95. Dahlgren, S-E. 1975b. Kinetics and mechanism of fixation of Cu-Cr-As wood preservatives. Pt. VI. The length of the primary precipitation fixation period. Holzforschung 29, pp. 130-133. van Eetvelde, G.; Orsler, R.; Holland, G.; Stevens, M. 1995. Effect of leaching temperature and water acidity on the loss of metal elements from CCA treated timber in aquatic applications. Part 1" Laboratory scale investigations. IRG/WP/95-50046. van Eetvelde, Homan, W.J., Militz, H.; G.; Stevens, M. 1993. Effect of leaching temperature and water acidity on the loss of metal elements from CCA treated timber in aquatic applications. Part 2" Semi-industrial investigation. IRG/WP 93-50001, pp. 195-208. Hayes, C.; Curran, P.M.T.; Hynes, M.J. 1994. Preservative leaching from softwoods submerged in Irish coastal waters as measured by atomic absorption spectrophotometry. Holzforschung 48, pp. 463-473. Holland, G.E.; Orsler, R.J. 1993. Methods for the assessment of wood preservative movement in soil. IR6/WP 93-50001, pp. 117-145. Hooflman, R.N.; Bakker, D.J.; Boonstra, M.J.; Esser, P.M. 1995. Voorstudie naar te verwachten concentraties van polycyclische aromatische koolwaterstoffen in oppervlaktewater en bodem als gevolg van uitloging uit gecreosoteerd hout. TNO report MW-R95/111. Morris, P.I. 1996. Towards a unified international hazard class system. IRG/WP/96-20081. Murphy, D.R.; Dickinson, D.J. 1990. The effect of acid rain on CCA treated timber. IRG/WP/3579. Network Eurowood. Expanding the European wood-working industry. The role of R & D. Impact on SMEs, rural economy, employment and environment. Nurmi, A. 1994. Improvement in efficiency and environmental characteristics of preservative treatment. Assessment of low-toxic preservatives: Their efficiency and environmental characteristics. EC-Project No. MA2B-CT91-0036. Pizzi, A. 1981. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood. Jnl. Of Polymer Sci., Ed. Polymer Chem., vol. 19, pp. 30933121. Pizzi, A. 1982a. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. II. Fixation of the Cu/Cr system on wood.. Jnl. Of Polymer Sci., Ed. Polymer Chem., vol. 20, pp. 707-724. Pizzi, A. 1982b. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. III. Fixation of Cu/As on wood. Jnl. Of Polymer Sci., Ed. Polymer Chem., vol. 20, pp. 725-738. Pizzi, A. 1982c. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. IV. Fixation of CCA to wood. Jnl. Of Polymer Sci., Ed. Polymer Chem., vol. 20, pp. 739-764.

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REFERENCES TO CHAPTER 11

Pizzi, A. 1982d. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. V. Reactions of CCB with cellulose, lignin and their simple model compounds. Holzforschung und Holzverwertung 34, pp. 75-83. Pizzi, A. 1982e. The chemistry and kinetic behaviour of Cu-Cr-As/B wood preservatives. VI. Fixation of CCB in wood and physical and chemical comparison of CCB and CCA. Holzforschung und Holzverwertung 34, pp. 80-86. Pizzi, A.; Orovan, E.; Singmin, M.; Jansen, A.; Vogel, M.C. 1984. Experimental variations in the distribution of CCA preservative in lignin and holocellulose as a function of treating conditions. (temperature, concentration, pH, Species, and time). Holzforschung und Holzverwertung 36, pp. 67-77. UNEP. 1994. Environmental aspects of industrial wood preservation. A technical guide. United Nations Publication. ISBN 92 807-1403-1. Weis, J.S.; Weis, P. 1992. Transfer of contaminants from CCA-treated lumber to aquatic biota. J. Exp. Mar. Biol. Ecol. 161, pp. 189-199.

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C H A P T E R 12: S T A N D A R D I Z A T I O N O F LEACHING/EXTRACTION TESTS

Introduction The information sought from leaching tests differs in the various fields in which they are used but the phenomenon they simulate is the same - that is a liquid in contact with solid which extracts components from that solid. Without coordination there is a significant danger of divergence in leaching tests, some of which will be for a scientific logical basis, but others will be for no specific reason. The boundaries between the various materials are not clear and are different in the various countries. This implies that the 'vertical' consideration of leaching tests related only to specific types of materials may lead to duplication of work or to discrepancies or contradictions. Therefore there is a real need for coordination and harmonization. This is true not only from the technical, scientific point of view but also for standardization activities since standardization is mostly 'vertically' organised - there are different technical committees for different types of materials. The European Network on harmonization of leaching/extraction tests has neither intention nor ability to standardize leaching tests. This is the responsibility of standardization institutes. However most experts involved in the Network are also members of national, European or international Technical Committees or working groups which draft the standards. It is the task of the Harmonization Network experts to disseminate the findings of the Network into their own national or international networks of experts that use and interpret the results of leaching tests. In this way the results of a harmonized approach may affect the national, European and international standardization of leaching tests. It might even create 'horizontal' links in the standardization field so that more cooperation between the 'vertically' oriented technical committees takes place.

Standardization work Leaching tests have become more important during recent years in characterizing the environmental behaviour of materials in certain circumstances or scenarios and in considering the health-related aspects of materials. This is done by simulating the leaching process in a laboratory. The laboratory tests are used to determine what can be extracted from materials. A number of different types of tests exist. A number of parameters can be varied in each test, for example liquid to solid ratio, nature of the leachant and contact time. The selection of the variables is linked to the objective of the test. There may be a need for information on the leaching of materials into the environment in the short or long term or in a worst case situation. For example with regard to the safety of products (such as toys) leaching behaviour can be studied using mild acid leachants to represent stomach acids. Standardization institutes have mainly organized their work in material or product related domains. In each of these domains standardization takes place in technical committees consisting of all interested parties which includes organisations concerned with the development and application of the tests and also the end users of the results

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such as governmental or industrial organisations. Horizontal standardization, that is the applicability of the same standard to various materials or products is rare. This is why the standardization of leaching tests in different fields can take place independently without the different committees knowing of each others initiatives. The dangers of 'vertical' standardization regarding leaching was noted in 1992 when several technical committees began discussions on the standardization of leaching tests. In February 1993 a seminar on the co-ordination of development and standardization of leaching tests was organized by the British mirror committee of CEN/TC 292. At the seminar representatives of several Technical Committees were present in addition to representatives of the European Commission. It was at this time that the idea of setting up the harmonization network with the help of the European Commission was emerging. It was concluded from the London meeting that at least at a CEN level coordination should take place to prevent duplication and unnecessary divergence of work. The organisation of this coordination was discussed and a coordination point was installed at the secretariat of CEN/TC 292 which is held by the Dutch standardization institute NNI. The aim of this coordination point is threefold : to identify whether other technical committees are dealing with the standardization of leaching/extraction tests; to coordinate for example by means of correspondence and liaison to ensure that the committees become aware of the initiatives of others; to cooperate: to bring experts and convenors in contact with each other to exchange technical information. Currently the following Technical Committees (TCs) are dealing with the standardization of leaching tests. ISO/TC 190 (Soil Quality) CEN/TC 308 (Characterization of Sludges) CEN/TC 51 (Cement and Building Limes) CEN/TC 104 (Concrete) CEN/TC 154 (Aggregates) CEN/TC 164 (Water supply) CEN/TC 38 (Durability of Wood and Derived Materials) CEN/TC 309 (Footwear) In the next sections the initiatives of these TCs are described in more detail.

I S O / T C 190 (Soil quality) :

In the case of soil two very different issues have to be addressed by leaching tests which lead to at least a divergence in the liquids used as leachants. One issue is the maximum availability of nutrients for plant uptake, the other is the availability and actual leaching of environmentally hazardous components from (contaminated) soil into groundwater.

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Currently several leaching tests for availability using different leachants and different test parameters are being investigated with regard to their suitability for standardization. Four documents are in preparation for international standards using the following leachants: CaC12 90.01 mol.l" 1 NaN03 90.1 mol.l- 1 NH 4 N03 1 mol.1-1 DTPA (Lindsay and Norvell method) which has been issued as Committee Draft ISO/CD 14870-1.

CEN/TC 292 (Characterization of waste) :

The scope of this TC includes not only wastes destined for final disposal, for example incineration or landfill, but also waste materials that can be utilized. These materials fall in some countries under the 'waste'-regime while in other countries they are considered as secondary raw materials or by-products. Examples include fly ashes and bottom ashes from coal fired power plants and waste incinerators and different kinds of slags. In CEN/TC 292 a distinction is made between three types of leaching tests which will be applied in different circumstances: obasic characterization tests which will generate extensive knowledge on the materials investigated ocompliance tests to check previously extensively described waste against limit values oon site verification tests to provide a quick tool at the site entrance to check that waste matches earlier descriptions For the first two types of tests two working groups have been set up - one for compliance tests and the other for characterization tests which investigate the leaching behaviour of materials. The aim is to arrive at tests which provide a very thorough investigation into the leaching characteristics of a material which can once fully characterized be checked on a regular basis by the use of shorter, less elaborate compliance tests. A distinction is made in both groups between materials from which leaching is controlled by different processes (percolation, diffusion, dissolution ...) and the test set-up is different depending on the dominant leaching mechanism (shake test or column test versus tank leaching test). The leaching tests currently being standardized are: oCompliance leaching test for granular material (being published as prEN 12457) which describes three different procedures: shake test at L/S = 2, L/S = 10 and a two step shake test at L/S = 2 and 10. The leachant used is demineralized water. It is likely that the standard will be validated. The driving force behind this standard was the draft European Directive on the landfill of waste.

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9Compliance test for monolithic materials (shortened tank leaching test using vacuum to accelerate the process). For the fundamental characterization of waste a methodology guideline is in preparation for publication as a European Prestandard (ENV) in 1997. In the guideline differentiation is made between tests which simulate the actual leaching behaviour of the wastes and tests which investigate the effect of specific parameters on leaching behaviour. Following approval by the technical committee the following basic characterization tests will be developed as standards: 9Basic characterization test for granular materials (percolation test) 9 pH static leach test The tests being standardized are suitable only for the determination of leaching of inorganic components. More research is needed before standards can be developed for leaching tests for organic compounds. CEN/TC 308 (Characterization of sludges)

The scope of CEN/TC 308 covers sludges and products from, essentially : Urban waste water collection systems, urban waste water treatment plants and similar industrial waters (as defined in EC directive 91/271). Sludges from water supply treatment plants and water distribution systems. These sludges are mineral products and are similar to solid granular waste. Their behaviour on extraction or leaching depends on the treatment process and conditioning methods. The structure of urban waste water is organic and biological. Sludges from industrial activities are excluded from the scope of CEN/TC 308, they fall under CEN/TC 292. The technical committee considers that the compliance leaching test standardised by TC 292 (PrEN 12457) may not be easily applicable to the sludges dealt with by TC 308 because of their biological and organic characteristics. CEN/TC 308 established closed links with TC 292 to improve efficiency and prevent the duplication of work. Leaching tests in the field of building materials

The information sought by the leaching test developed by CEN/TC 164 (Water supply) is related to consumer health. The issue is what components will be leached out of concrete drinking water pipes by the contact with drinking water. To ensure the safety of public health the leaching behaviour of materials (such as concrete) are investigated. CEN/TC 164 is discussing a leaching test developed in a pre-normative research programme funded by DG#XII. The leaching test under consideration for concrete products is a rather elaborate tank leaching test using drinking water as a leachant.

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231

CEN/TC 51 (Cement and building limes) and CEN/TC 104 (Concrete) are cooperating in a project related to the pre-normative study funded by DG#XII for the quality of concrete. With regard to the leaching test being developed by CEN/TC 164 there is discussion as to whether demineralized or mineralized water should be used as the leachant. To date it appears that the difference in leaching using the two leachants is small. CEN/TC 154 (Aggregates) will shortly be discussing the development of a compliance leaching test for aggregates. CEN/TC 38 (Durability of wood and derived materials)

This Technical Committee has already published two standards in relation to leaching tests: EN 84 : 1989 "Wood preservatives - Accelerated ageing of treated wood prior to biological testing - Leaching procedure" which describes a method involving leaching of treated timber which is used solely as an accelerated ageing procedure prior to biological testing for efficacy assessment. The purpose is to study the product durability by means of an accelerated artificial ageing of the material. ENV 1250-2 : 1994 "Wood preservatives - Methods for measuring losses of active ingredients and other preservative ingredients from treated timber Part 2 : Laboratory method for obtaining samples for analysis to measure losses by leaching into water or synthetic sea water" which is more linked to the environment aspects. Product related TCs

In some European countries, such as Germany, regulations exists to ensure environmentally sound landfilling. All products which will become wastes must be tested in terms of potential environmental hazards such as the leaching of heavy metals into groundwater. This regulation has led some product related committees to consider the development of tests to measure the total content of contaminants and to assess leaching behaviour from their products. Examples are CEN/TC 99 (Wallcoverings) and CEN/TC 309 (Footwear). The initiative by CEN/TC 309 to standardize its own leaching test for shoe waste has been discussed with CEN/TC 292. CEN/TC 309 has decided to refer to tests developed by CEN/TC 292 instead of standardizing their own as the tests developed by CEN/TC 292 are generic. Although the compliance leaching test for granular waste materials (prEN 12457) is applicable for shoe waste there are concerns regarding the applicability of leaching tests on shoes. Since the product material (leather) is mostly of organic nature it will be biodegraded very shortly after landfilling, which leads to a completely different leaching behaviour. This makes valid the question of why a leaching test is

232

C H A P T E R 12 performed on the product itself and whether the degradation process has to be taken into account.

Future initiatives As materials which can be characterized by leaching tests can be put to different uses and since there is a Europe-wide movement towards the reutilization of waste materials, the following Technical Committees (and some others) may be interested in the standardization of leaching tests : 9 CEN/TC 223 : 9 CEN/TC 227 : 9 CEN/TC 260 :

"Soil improvers and growing media". "Road materials". "Fertilizers and liming materials"

Conclusions There is a need for coordination and the exchange of information in the field of leaching test standardization. The coordination point in CEN is the secretariat of CEN/TC 292. Formal liaison exists between the Network on the Harmonization of Leaching/extraction Tests and CEN/TC 292 . In addition most of the experts in the Network are involved in national or international standardization activities. This means that the harmonization efforts being made by the Network experts may be mirrored by 'horizontal' development of harmonized standards.

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APPENDIX A TO CHAPTER 12

STANDARDIZATION PROCESS Standardization is the process by which unwanted divergence in product specifications, performance requirements of processes and measurement methods is prevented. The results are laid down in standards which may be issued on a national basis or on the European (CEN) or international (ISO) level. All interested parties are involved in the process including: legislators, researchers, laboratories and industry. They take part in Technical Committees drafting standards based on the principle of consensus ~'. The process is being organized and the standards are published by national standardization institutes. On the European level all EC and EFTA countries are official members of the European Standardization Committee (CEN) and vote on European draft standards. A list of the national institutes which are members of CEN is given in Appendix B to this chapter. On the international level (ISO) it depends on the subject under discussion whether countries are involved in the development of standards. If European standards are produced all CEN-members have the obligation to issue them as national standards and to withdraw their own conflicting national standards. This is not the case for international standards. STANDARDIZATION AND LEGISLATION The use of standards is in principle on a voluntary basis but if one claims to work according to a standard the normative text must be followed strictly. If a national government or the European Commission refers to standards, the use of those standards is no longer voluntary. In the case of standards for measurement methods for the environment a reference method is usually prescribed. One can work according to an alternative method if it has been proven that this method gives the same result as the reference method with performance characteristics which are at least as good. On the European level there is a close link between legislation and standardization. For the single market European standards are needed not only for product specifications but mainly for aspects in the field of health, safety and environment. Under the New Approach, European Directives give only the essential requirements and refer to Standards which give the corresponding technical specifications. If the European Commission needs standards for fulfilling the requirements of European Directives it may give a mandate to CEN to draft such standards. In that case all countries are bound to use the European standards and to refer to them in their national legislation. * Consensus : General agreement, characterized by the absence of sustained opposition to substantial issues by any important part of the concerned interests and by a process that involves seeking to take into account the views of all parties concerned and to reconcile any conflicting arguments. NOTE : Consensus need not imply unanimity

This Page Intentionally Left Blank

C H A P T E R 12 APPENDIX B TO C H A P T E R 12 LIST OF ADDRESSES OF E U R O P E A N AND NATIONAL STANDARDIZATION INSTITUTES Austria

Finland

Osterreichisches Normungsinstitut (ON) Postfach 130 Heinestrasse 38 A- 1021 Wein

Suomen Standardisoimisliitto r.y. (SFS) PO Box 116 FIN-00241 Helsinki Finland

Head: Mr G Hartmann

Head: Mr K Kaartama

TP: +43 1 213 00 TFX: +43 1 213 00 650 TG: austrianorm

TP: + 3 5 8 0 1 4 9 9 3 3 1 TFX: +358 0 146 49 25 TG: finnstandard

Greece

Belgium

Hellenic Organization for Standardization (ELOT) 313, Achamon Street GR- 11145 Athens

Instituut Beige de Normalisation/ Belgisch Institut voor Normalisatie (IBN/BIN) Avenue de la Braban9onne 29/ Brabanconnelaan 29 B- 1040 Bruxelles/Brussel Head: Mr P M Croon

Head" Mr N Malagardis TP TX: TFX: TG:

+30 1 228 00 01 (0601) 2196270 elot gr +30 1 202 0776 elotyp athens

TP' +32 3 738 00 90 TFX: +32 2 733 42 64 TG: benor

France

Iceland

Association Fran~aise de Normalisation (AFNOR) Tour Europe F-92049 Paris la D6fense

Icelandic Council for Standardization (STRI) Keldnaholt IS- 112 Reykjavik

Head: Mr B Vaucelle

Head: Mr J Thorsteinsson

TP: +33 1 4 2 9 1 5 5 5 5 TX: (042) 611974 afnor f TFX: +33 1 42 91 56 56 TTX: 933-142915611 = afnor TG: afnor courbevoie

TP: TX: TFX: TG:

+354 587 70 02 (0501) 3020 istech is +354 587 74 09 irnsi

235

236

CHAPTER 12

Denmark

Germany

Dansk Standard (DS) Baunegaardsvej 73 DK-2900 Hellerup

Deutsches Institut mr Normung e.V (DIN) D- 10772 Berlin

Head: Mr J E Holmblad

Head: ProfDr Ing H Reihlen

TP: +45 39 77 01 01 TX: 119203 ds stand TFX: +45 39 77 02 02 TTX: 238- 1119203 =dsstand TG: danskstandard

TP: +49 30 2 6 0 1 0 TX: (041) 184273 din d TFX: +49 30 26 01 12 31 TTX: 2627-308896=din TG: deutschnormen berlin

Ireland

Italy

National Standards Authority of Ireland (NSAI) Glasnevin IRL-Dublin 9

Ente Nazionale Italiano di Unifficazione (UNI) Via Battistotti Sassi, 11b 1-20133 Milano MI

Head: Mr E Paterson

Head Mr P Morelli

TP: TX: TFX: TG:

TP: TX: TFX: TG:

+353 1 807 38 00 (0500) 32501 olas ei +353 1 807 38 38 research dublin

+392700241 (043) 312481 uni i +39 2 70 10 61 06 unificazione

Norway

Sweden

Norges Standardiseringsforbund (NSF) PO Box 353 Skoyen N-0212 Oslo

Standardiseringen I Sverige (SIS) Box 6455 S- 113 82Stockholm

Head: Mr I Jachwitz

Head: Mr S Lundin

TP: TX: TFX: TG:

TP: TX: TFX: TG:

+47 22 04 92 00 (056) 19050 nsfn +47 22 04 92 11 standardisering

+4686103000 (054) 17453 sis s +46 8 30 77 57 standardis

C H A P T E R 12

Luxembourg

Portugal

Inspection du Travail et des Mines (ITM) Boite Postal 27 26, rue Zithe L-2010 Luxembourg

Instituto Portugues da Qualidade (IPQ) Rua C, Av. dos Tres Vales P-2825 Monte da Caparica

Head: Mr P Weber

Head: Mr C Dos Santos

TP: +352 478 61 50 TX: (0402) 2985 mintssju TF: +352 49 14 47

TP- +351 1 294 81 00 TX: (0404) 13042 qualit p TFX: +351 1 294 81 01 +351 1 294 82 22 TG: igpai

Switzerland

Netherlands

Schweizerische NormenVereinigung (SNV) MOhlebachstrasse 54 CH-8008 Zurich

Nederlands NormalisatieInstituut (NNI) Postbus 5059 Kaltjeslaan 2 NL-2600 GB Delft

Head Dr H C ZOrrer Head: Dr C De Visser TP: TX: TFX: TTX: X.400"

+41 1 254 54 54 (045) 755931 snv ch +41 1 254 54 74 228-47935131 =snv c=ch/a=arcom/p=snv/ o=snv/s=post

TP TX: TFX: TG:

+31 1 5 2 6 9 0 1 9 0 (044) 38144 nni nl +31 15 269 03 90 normalisatie

Spain

United Kingdom

Associacion Espafiola de Normalizaci6n y Certificaci6n (AENOR) Calle Fern~indez de la Hoz, 52 E-28010, Madrid

British Standards Institution (BSI) 389 Chiswick High Road GB-London W4 4AL Head Mr B Geraghty

Head: Mr R Naz Pajares TP: TX: TFX: TG:

+34 1 432 60 00 (052) 46545 unor e +34 1 310 49 76 aenor

T P +44 181 996 90 00 TFX: +44 181 996 74 00

237

238

C H A P T E R 12

Commite European de Normalisation (CEN) Rue de Stassart 36 - B- 1050 Bruxelles TP + 3 2 2 5 1 9 6 8 1 1 TFX +32 2 519 68 19

CHAPTER 13

239

C H A P T E R 13: CONCLUDING OBSERVATIONS AND DISCUSSION OF POTENTIAL FOR HARMONIZATION In this chapter the findings resulting from the expert meetings as well as from the individual chapters on specific topics are integrated as much as possible to provide a starting point for harmonization. It is not possible to achieve harmonization directly. The first step is to provide a common understanding of the issues and to identify relationships and relevant factors that are common to all technical fields. The possibilities for integration may then emerge. Two expert meetings have been held within the framework of the project. In the first expert meeting the main focus was directed at the questions which need to be answered in the different technical fields, the identification of the tests which are used for each purpose and an assessment of the extent to which the tests used in the different technical fields address similar aspects of leaching. In the discussions of the second expert meeting, the attention was focused mainly on the factors which control the leaching/extraction of inorganic constituents from the wide range of materials addressed in the Network. The issues considered for each of the technical fields centred on the following main questions: What is the specific question to which an answer is sought by a leaching/extraction test? What is the rationale behind the selection of a leaching test and the test conditions specified in it? 9

Which tests are used most frequently in each specific technical field? Can the current leaching/extraction tests provide the answer required (what are the possibilities and limitations)? What are the most relevant release controlling mechanisms in a specific technical field? What are the most relevant release controlling parameters? Is it possible to identify key constituents of concern in a specific technical field?

Major conclusions regarding leaching behaviour and testing Main issues in relation to leaching

As a general conclusion to the work carried out in each of the technical fields covered here, it can be stated that the release of constituents from the solid phase to the pore solution and subsequently into the surroundings (environmental impact) or uptake by plants and organisms are the key questions to which an answer is sought. The long term aspect of release (years to several hundreds of years) is most pronounced in all cases where environmental impact is the key issue, whereas in the case of soil fertility and plant uptake the emphasis is mainly on short term processes (for example the growing cycle is usually less than a year).

240

CHAPTER 13

Similar levels of aggressiveness in testing Similarities between the different technical fields were observed in the levels of testing where a range from mild to aggressive extractions are used to assess the mobility of constituents and to distinguish between actual leaching behaviour and potential leachability. The more aggressive extractants are considered to reflect the potential release at longer time-scales, whereas it is assumed that milder extractants relate to release in the short term assuming natural exposure conditions. The distinction between the use of more and less aggressive methods is common to most technical fields although the precise methods used to assess a particular property may not yet be fully developed. In table 13.1 a summarized comparison of the methods used currently in the technical fields of soil, sediments, waste and construction is presented. Table 13.1: Methods and ieachants used to assess different degrees of matrix incorporation. Level aggressiveness Total

Potentially leachable

Actual leaching

of

Soil

Sediments

Waste

HF, Aqua regia lstep extraction HAc, EDTA lstep extraction

HF, Aqua regia

HF, Aqua regia

HAc. Sequential extraction step 1-3 Mg or CaCI2

Availability test. pH controlled at pH4 Demineralized water Column test

Mild extractants, CaCI2,

Construction materials I-IF, Aqua regia

Not yet defined

Demineralized water Tank leach test

NH4NO3, NaNO3

The role of pH Relevant pH domains The pH domains covered by the different materials of interest in relation to the harmonization of leaching/extraction tests are illustrated in Figure 13.1. The different levels of leaching are indicated by total (all that is present), potentially leachable (available for leaching under extreme conditions and/or extremely long term) and the actual leaching of a metal showing minimum leachability at pH= 10. The difference between total and potentially leachable may be small as in the case of soluble salts (e.g. Na, Cl, K) or may be quite large (one or more orders of magnitude) as in the case of a metal tied up in insoluble mineral phases. As discussed in chapter 2, the potential leachability may alter due to changes in mineralogy of the matrix upon ageing. The leaching behaviour of the potentially leachable fraction of a constituent is governed by solution chemistry, solubility, sorption reactions, etc. and for metals leads to a curve such as the one marked "actual leaching" The pH domains given for the various materials are indications of the main conditions which are encountered. Specific conditions can be considered, for example when concrete is exposed to natural flowing water. At the concrete surface a neutral pH will be maintained as the supply of alkalinity from the cement matrix by

CHAPTER 13

241

diffusion is not sufficient build up a high pH environment near the concrete surface. The hydroxide that is released is either dispersed or neutralized by the buffering in natural water. As a consequence of the knowledge of the specific scenario test data may be evaluated differently or different test conditions may be selected. The identification of relevant pH domains in the different technical fields may explain why pH has not been considered an important parameter in some technical fields. The relatively narrow range in pH experienced in some technical fields is too narrow for pH to have a significant influence (e.g. soil, sewage sludge). Given the observed leaching behaviour of constituents, sometimes with changes of an order of magnitude in leachability with less than one pH unit change in either direction, it is concluded that pH is a significant parameter which can affect leaching from all matrices. Relationship between pH and other factors controlling leaching An additional aspect of presenting leaching data as a function of pH is the fact that many other factors which control leaching can be recognised from the pH dependent leaching curve. Some parameters which control leaching may only be relevant in specific pH domains. Although this can Figure 13.1" pH domains relevant for the different fields in which leaching tests are applied in perspective to common characteristics relevant to leaching of specific constituents.

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242

CHAPTER 13

not be generalised for all constituents Figure 13.2 illustrates some of the main trends caused by factors controlling metal leachability in matrices with e.g. reducing properties, high organic matter content (TOC) or high sorptive phase content such as Fe - and Mn - oxides. For example for many wastes an inflection point is observed in the leaching curve at pH values around 4, which implies that with a further lowering of pH the increase in leachability is limited. This has formed the basis for a choice of pH = 4 for assessing potential leachability by some workers [NNI 1994]. However in some matrices this inflection point is shifted to lower pH values due to matrix interactions such as with organic matter or with iron and manganese oxides. This has been observed for soils in particular [Comans 1997] where a pH of 2 was needed to reach an inflection point. Figure 13.2: Relevant factors controlling leaching/extraction from a wide range of materials placed in perspective to pH as one of the key controlling factors

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In Figure 13.3 the role of sorptive phases and complexation with organic carbon (DOC) is illustrated for zinc and copper in sewage amended soil in comparison with an inorganic waste matrix with the same content of copper and zinc. On the left of the figure, binding with particulate organic matter and iron and manganese oxides leads to a lower leachability from sewage amended soil than observed for the largely inorganic matrix, whereas in the right of the figure the situation is reversed due to the effects of metal complexation with or incorporation into dissolved organic matter. This illustrates how, when subjected to the same type of testing,

CHAPTER 13

243

significantly different types of leaching behaviour from very different materials can be used in the interpretation of leaching behaviour from new not so well characterised materials. Understanding the controlling factors also provides the opportunity of modifying undesired leaching behaviour. Where material modification is carried out the same type of testing can be used to monitor material improvement with regard to leaching behaviour.

Figure 13.3: Leachability of Cu and Zn from sewage amanded soil as a function of pH (solid line). Dotted lines reflect the predicted leachability based on solubility control by specific mineral phases (Minteqa2). The discrepancy of high pH is attributed to mobilization of dissolved organic carbon (DOC).

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In many cases, additional information on specific characteristics of materials is helpful in identifying relevant factors which control leaching. As an example the leaching behaviour of cadmium as discussed in chapter 8 with regard to granular wastes can be expanded to relate to a wider range of materials. In Figure 13.4 leaching behaviour is presented as obtained based on the pH static leaching test at 8 fixed pH values ranging from 4 to 12 using LS=10 and a 24 hour contact time. For each material specific patterns of behaviour can be identified which are helpful in making decisions. Coal fly ash, cement-stabilized MSWI fly ash and MSWI bottom ash reflect the leaching behaviour of inorganic cadmium from a largely inorganic matrix. The leaching curves as a function of pH are very similar for these matrices. For MSWI fly ash and refuse derived fuel (RDF) ash the cadmium leachability reflects the increase in cadmium leachability caused by complexation of cadmium as CdC14z . The chloride levels in bottom ash, fly ash and RDF ash are 1500mg/kg, 18000mg/kg and 50000 mg/kg respectively. The leaching behaviour of cadmium from vitrified MSWI fly ash demonstrates behaviour which is a result of

244

CHAPTER 13

the treatment process - a substantial part of the cadmium originally present in MSWI fly ash is volatilized thus no longer available for leaching, the remaining cadmium is largely incorporated in the glassy matrix hence not available for leaching and finally almost all of the chloride is removed from the final product by volatilization. These combined effects result in the low level of cadmium leached from this matrix. The leaching behaviour of cadmium from phosphate slag is quite similar to that from vitrified MSWI fly ash. This may be attributable to similarities in the treatment temperature and process of slag formation. The leaching behaviour of the cement-stabilized MSWI fly ash is in part a result of dilution by other constituents in the product (sand and cement) and at the same time the chloride concentration is reduced to a level that is less critical for the complexation of cadmium. In addition, the cadmium is bound in cement mineral phases. Brown coal ash shows the lowest cadmium leachability of all the matrices studied. A characteristic property of this material is the high iron oxide content. It appears that cadmium is effectively bound on the iron oxide surface and therefore virtually non-leachable. Jarosite is a very acidic waste product from the zinc industry. Its leaching behaviour has similar characteristics as that of inorganic cadmium from an inorganic matrix, but the point of inflection in the leaching curve appears at a higher pH than in other materials. The reason for this has not yet been identified. It is clear that a major reduction in cadmium leachability can be achieved by a simple neutralization to a pH around 8. Shredder waste, sewage amended soil and sewage sludge are materials that all contain a significant amount of organic matter. This is reflected in the leaching behaviour of cadmium at pH values higher than 7 , which is elevated due to complexation with DOC. The increase in DOC with pH can be observed visually by the colour of the leachate which changes from yellowish to light brown or dark brown. Given these types of relationships in leaching behaviour across different technical fields and different materials, conclusions can be derived regarding the properties which are necessary to minimise the environmental impact of materials. The consistent leaching behaviour within one matrix type, such as a definite relationship between leaching and pH, is remarkably systematic and can be identified in different matrices as can be seen for coal ash, where data on leaching from 50 coal ashes from around the world are included (Figure 13.4). Similar observations have been made for MSWI bottom ash [IAWG 1997]. One of the limitations in the use of single step extraction tests for regulatory purposes is that the differences observed in the concentrations leached do not necessarily reflect differences in leaching behaviour, but are often a result of a slightly different final pH in the leaching test. By considering leaching data obtained from single step regulatory tests in the context of the pH dependent leaching behaviour of that or a similar material the sensitivity of the leaching behaviour of that material to pH changes will become much clearer. As an example the leaching behaviour demonstrated in a single step regulatory test with no pH control, for example the DIN or AFNOR test, are included in the figure for the different matrices. In the case of RDF ash, the low leachability obtained at the material pH of 12.2 gives a false sense of material acceptability, as even a limited degree of neutralization leads to an order of magnitude increase in leachability. The long term stability of the material is apparently critical here and measures are needed to ensure that in the disposal or utilization scenario of this material cadmium is not released at unacceptable levels at the short or the long term.

CHAP'I'ER 13

245

Figure 13.4: Leachability of Cd from a wide range of materials as a function of pH illustrating common characteristics in the leachability of a specific element in different fields.

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246

C H A P T E R 13

leaching data can be used to indicate and to a large extent quantify the changes in release due to changes in exposure conditions. In most disposal and utilization scenarios there is a tendency to ultimately reach a neutral to mildly acidic pH condition, which can be far from the condition imposed when the material itself dictates the leachate pH in a leaching test. The degree to which such changes occur in time depends on the buffering capacity of the material. This information can be derived from the pH dependent leaching curve by recording the acid or base consumption to reach a fixed pH. The acid- or base- neutralization capacity thus obtained can be related to external influences such as the presence of atmospheric or biologically derived carbon dioxide. This allows the estimation of the time scale over which significant changes in exposure conditions are likely to take place. pH dependence as a basis of reference The pH, which has been identified as a crucial parameter in all cases, can be used as a basis of reference for leaching/extraction data, as has been shown for leaching data obtained with different regulatory leaching tests [IAWG 1997, van der Sloot 1991 ]. It is also very useful as a point of reference against which to compare data from quality control testing [van der Sloot 1996]. The evaluation of materials by means of single or two step tests is possible when the data points obtained can be shown to be consistent with the previously determined basic leaching characteristics of a material. Only where such information is available can the interpretation and evaluation of material on the basis of a relatively simple test be justified. In all chapters reference is made to the relevance of pH as an important controlling factor in leaching/extraction.

Role of redox in leaching~extraction In many testing protocols today the influence of the reducing properties of materials such as a number of industrial slags on the outcome of a leaching/extraction test is not addressed. One should be aware of the reducing properties of a material in order to take measures to ensure that a material possessing such properties is tested under well defined conditions. This may require additional measures in sample pre-treatment to ensure exclusion of oxygen when the test is neededn to reflect an environment where the material will stay in a reduced condition. Reducing conditions can be a material property caused during the generation process, such as many industrial slags, or as a result of oxygen consumption by biodegradation which ultimately leads to reducing conditions. The significance of testing under either reduced or oxidized conditions can be illustrated by the leaching behaviour of barium and chromium from steel slag ( Figure 13.5 [van der Sloot 1995]), which shows that outcome of the test can be very different and uncontrolled if nonrepresentative conditions develop by partial oxidation of the material. For chromium the difference between oxidized and reducing conditions amounts to more than a factor of 10. For barium a difference of almost three orders of magnitude may occur between fully oxidized conditions (solubility control by barite) and reducing conditions. To carry out leaching/extraction tests under strictly oxygen free conditions is not easy and can not be regarded as standard practice. In testing, it is recommended that the redox potential of the leachate or extract is measured even though redox measurement is not straightforward. As a rough indicator, it will identify possible critical conditions that need to be taken into account in the interpretation of the test data.

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247

Figure 13.5: Difference in leachability of Ba and Cr from steel slag under oxidized or reducing conditions. A: Ba leaching from steel slag; dot: controlled laboratory conditions; diamond- field scale data B; Ba leaching as a function of the redoxpotential. C: Cr leaching from steel slag. Dot: controlled laboratory conditions; square: after oxidation by prolonged exposure to the atmosphere.

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Mobilization by dissolved organic carbon The importance of DOC in the leaching of different constituents is recognised in different technical fields (soil, contaminated soil, sediments, sewage sludge, compost and certain wastes), as DOC complexed elements show a release behaviour which is governed by the transport behaviour of the DOC rather than that of the element. In several studies the use of total DOC as an indicative parameter has been shown to be too coarse an approach and a further fractionation of DOC is necessary to explain the release behaviour [Belevi 1996]. In the case of copper leachability from MSWI bottom ash the copper is partly bound to a relatively labile dissolved organic matter fraction and partly to a rather stable dissolved organic matter fraction. The copper in the labile complex is relatively easy exchanged with more stable binding sites, whereas the stable complex is transported relatively unhindered. The degradability of the different DOC sub-fractions is not the same as canbe seen in Figure 13.6. Further characterization of DOC from different matrices is important as has been shown in studies of DOC leached from compost (see chapter 7).

Figure 13.6: Subdivision of dissolved organic carbon (DOC) in light and heavy fractions and the preferential degradation of the light fraction of DOC in Municipal Solid Waste Incinerator bottom ash after incubation for 300 hours at ambient temperature.

Fundamental leaching processes As a general conclusion, it can be stated that the release controlling factors and the basic phenomena leading to a certain leaching behaviour are essentially the same in all fields (Figure 13.7). It is the relative importance of the individual processes that determine to what extent a specific parameter or particular factor plays a dominant role in the leaching behaviour of a specific material. Only a limited number of parameters and factors control release of a particular constituent from a certain material. The controlling factors are not necessarily the same for all relevant elements in a specific material. Thus controlling factors can be tied to element - material combinations. Examples are :

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cadmium- MSWI fly ash: pH, CI cadmium- Shredder waste: pH, DOC cadmium- Soil: pH, Particulate Organic Matter, FeO(OH) cadmium - Sediment: pH, redox, DOC cadmium- Concrete: pH cadmium - Compost: pH, Particulate Organic Matter, DOC

Relationship between test methods developed in different fields In the different technical fields the use of different tests methods may lead to confusion. This may occur particularly in the soil and waste fields, when materials can be judged as soil or as a waste depending on the contaminant level or the use to which the material will be put. Tests used in the soil field have been placed in perspective to pH static leaching data (Figure 13.8 [van der Sloot 1996]). The data from the different soil tests are consistent with the pH static leach test results for several elements. For zinc the EDTA extraction and acetic acid extraction lead to comparable

Figure 13.7: Basic processes controlling leaching/extraction in all matrices.

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leaching levels which are consistent with the pH controlled leaching at a pH of about 3. The extractions with mild extractants lead to quite comparable results with a plain water extraction. It is necessary to establish the error margins of the different procedures to assess the need for three similar extraction procedures. According to Aten and Gupta [1996], the difference between the three methods is not very significant from a plant uptake point of view. The soil pH appears to be a stronger controlling factor than the distinction between the three extractants, as one unit of pH difference can lead to almost an order of magnitude change in

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leachability. As shown before [Del Castilho 1996] acid forest soils can reach pH values of 3.3 in some cases. The data at low pH are directly relevant in these cases. From plant uptake studies [Aten and Gupta 1996] the role of pH appears to be a stronger controlling factor with regard to leaching than the differences between mild extractants.

Generic leaching behaviour demonstrated by individual elements across all technical fields The basic chemical principles governing the leaching/extraction of elements from the various solid matrices relevant to this work are the same. However, the relevance of specific factors can be different for the various matrices. A number of generic aspects can be identified: For halogens the leaching behaviour is mostly independent of pH and, except for fluorine, is generally not controlled by solubility limitations. The complete fraction that is available for leaching will be leached out fairly readily. The alkali elements lithium, sodium and potassium are very similar to each other in their leaching behaviour. They do not generally react with the matrix and there has been no observedrelationship between pH and leaching. The various metals have very characteristic properties, of which some generic properties of the most common elements will be addressed briefly. The total composition is mostly irrelevant for leaching. The chemical speciation of the metals and the chemical conditions in the leachate, dictated by the major elements in the material, govern the release. Aluminum - The solubility of aluminum is in many matrices controlled by some form of AI(OH)3, which shows a minimum leachability around pH 5. Cadmium- The leachability of cadmium usually decreases with increasing pH. The leachability is most strongly affected by the presence of chloride, which leads to the formation of very soluble chloride complexes Copper- The leachability of copper decreases with increase of pH and shows a minimum around pH 9 - 10. Copper is most sensitive to complexation with dissolved organic carbon. Nickel - The leaching behaviour of nickel resembles that of cadmium. However, nickel is not very sensitive to chloride complexation. As for copper nickel is more susceptible to complexation by DOC. Lead - The leaching behaviour of lead is characterized by a minimum leachability at pH 9 - 10. At a high pH lead is highly leachable due to formation of O H complexes (amphoteric character). Zinc - The leachability of zinc is similar to that of lead, as it also has amphoteric properties. The leachability at high pH is, however, not as pronounced as that of lead. The leaching behaviour of oxyanions (e.g. arsenic, chromium, molybdenum, selenium, antimony, vanadium) is markedly different from that of the metals. The oxyanions show otten a maximum leachability around neutral pH, where metal leachability is at a minimum. This is important for the treatment of materials containing a wide range of elements with different leaching behaviour to ensure that the material as a whole becomes more environmentally acceptable or can even be used beneficially.

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Figure 13.8: Comparison of single step soil extraction methods of different agressiveness with pH dependent leaching behaviour of a sewage amended soil (SRM 483) and a natural soil (SRM 484).

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Classification of materials The classification of materials in certain categories is difficult, as the criteria for doing so may not be related to the environment in which the material will be placed. A soil may be quite acceptable in an environment of pH 6 - 7, but when exposed to an acidic situation may release unacceptable concentrations of metals resulting in the consideration of the material as a contaminated soil or as a waste. This example illustrates that a label soil, contaminated soil or waste is not relevant to the leaching behaviour of the material..It is the conditions in the proposed utilization or disposal scenario which will influence leaching behaviour hence affect whether the material will present an environmental risk.. The leaching behaviour of a material

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is not changed, when it changes from a 'soil' to a 'waste'. This leads to the need to consider whether a single step extraction test is sufficient to judge the potential hazardous, nonhazardous or fertile nature of a material. It also raises the query as to whether it is really necessary to develop different procedures to characterize leaching behaviour in each of the different technical fields. Given the general applicability of the tests described above the development of new procedures does not appear to be justified.

Testing monolithic materials The approach in testing solidified/stabilized waste, construction materials and impregnated wood shows similarities as the release is controlled to a large extent by physical resistance resulting from the internal structure of the material tested. This can be attributed to the tortuous path ions have to follow to leave the monolithic specimen. In concrete the pore volume is quite low and therefore the tortuosity is high, which implies a high resistance to release. In stabilized/solidified waste the physical effects are generally much less pronounced and the chemical binding becomes more important. In both concrete and solidified/stabilized waste the pore structure can be considered to be homogeneous (i.e. there is no predominant orientation of the pore structure). This is in contrast to impregnated wood where the release is also controlled by pore transport within the special matrix property of wood. Due to the cell structure of wood diffusion in the direction of the wood nerve is much faster than that perpendicular to it. The leaching test methods used are generally carried out at room temperature. The leaching consists of a number of subsequent extractions of the same material in a tank by refreshing the liquid. The test specimens generally have a minimum thickness of 4 cm. One of the main variables is the total test duration and the liquid to area, liquid to solid or liquid to volume ratio. When data are expressed in release per unit of surface area, the data become largely independent of the size, shape and liquid volume applied in testing. By expressing release in relation to surface area, the surface related release from monolithic specimens is acknowledged. The same ratio can be used to describe solubility control and surface wash-off as well as diffusion from monolithic materials. As release from monolithic materials is predominantly controlled by diffusion processes the tests are carried out in 'real time' and cannot be accelerated to any great extent. Long term leaching behaviour (years) can be extrapolated from the results of relatively short term tests (days or weeks).

Testing of granular or fine grained materials The leaching conditions used for the different granular or fine grained materials show many similarities. Most are carried out at room temperature. The leaching/extraction time is normally between a few hours and 24 hours per step. The liquid to solid ratio (L/S) is mostly between 10 and 20. The mode of agitation is generally not standardized, but the aim is generally to allow sufficient mixing to approach equilibrium or steady state conditions. The major difference between all leaching procedures is the composition of the leachant, in which either availability type considerations are aimed at or specific degrees of binding in the matrix are addressed. In many cases, the leaching behaviour of elements is controlled by solubility or desorption, the kinetics of which can be relatively fast when the material tested is very finegrained.

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Testing approach A good approach in evaluating materials for different purposes is to generate basic characterization information for a specific class of materials as a reference for future measurements by simpler test procedures. This is the background of the distinction in different levels of leaching tests as proposed by CEN/TC 292 [CEN TC 292, 1995]. By assessing the data from simpler procedures against the background of more detailed characterization data the value of the single measurements will become much more meaningful. The costs of providing basic characterization data as a reference base takes an initial effort, of which already a substantial part has been achieved [IAWG, 1997, CROW, 1995, van der Sloot, 1996]. The advantage of having the basic characterization data as a reference base will outweigh the initial costs by the savings made afterwards in being able to concentrate on the most important factors, by allowing better judgements of measures to be taken and by avoiding analysis of constituents that are totally irrelevant for the assessment of the environmental impact of the material. When more basic characterization data are available for different materials, that information can be used to datermine which aspects are most likely to be crucial for a new material to be tested. A few simple material characteristics are helpful to place the material into a general category in terms of leaching behaviour.

Testing & modelling The possibilities of predicting leaching behaviour in the long term based on more detailed characterization data has been shown in recent years for several applications [Kosson 1996, CROW 1996, Schreurs 1996]. Although this field still needs a fair amount of work, the potential is promising. The combination of testing which provides material characteristics, leaching parameters and influencing factors with modelling is crucial to provide answers on the best option for utilization, treatment or disposal of specific materials. Understanding the mechanisms of release is crucial in order to predict long term release. Such mechanisms are different for granular materials and for monolithic materials, as the latter show release control by transport from or through the geometric surface area of the specimen. Models have been developed to describe the release from granular materials by percolation and from monolithic materials by diffusion. This aspect will not be covered here in more detail as it has been addressed in chapters 9 and 10. The transport processes are common to all materials e.g. rainwater percolating though soil and carrying away nutrients necessary for the plants, contaminants diffusing out of sediments at the bottom of waterways and lakes into the overlying water column, contaminants diffusing out of construction materials and stabilized waste materials, contaminants dissolving in water percolating through a landfill of waste.

Major element chemistry The role of major elements in leaching from the wide range of materials is insufficiently addressed when the leaching behaviour of materials is assessed. This is largely caused by the regulatory requirements which only specify the analysis of potentially hazardous elements. The elements which must be analysed under regulatory requirements may not be leachable from the material which is undergoing evaluation thus will not be detected inthe leachate. The major element chemistry largely dictates the leachate composition and controlling conditions such as

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pH, redox and DOC content and as such control the trace and minor element leachability. Measurement of some major elements would prove useful to understand trace contaminant behaviour. For geochemical modelling of leaching behaviour information on the major elements is vital. The potential for the harmonization of tests

The fields studied in the context of the harmonization of leaching/extraction tests show overlap in regulatory domains and local uses throughout the European Union. Materials can be resources in one country or context and wastes in another. As harmonized test methods are not available in specific technical fields, different tests are used frequently in the different countries. This leads to confusion in the case of trans-border activities or in internationally operating companies where apparently different answers to the same question lead to confusion and delays in decision making. In many cases materials can be used for different purposes which require different tests to assess their environmental impact in each context. If the test results can not be related to one another, the judgement of the best option for the use or disposal of the material may not be easy to make. Examples are: soils which may be considered as contaminated soil thus may fall under waste regulations or as a growing medium, in which case the uptake of plants is the crucial aspect to be addressed; wastes which can be used beneficially in construction applications or wastes which need to be disposed of in landfill; 9

sewage sludge which can be used as a fertilizer can become a waste.

An important achievement reached already is that different existing test methods can be related to each other by putting the data in perspective to pH and/or liquid/solid ratio for granular materials where leaching is predominantly a result of percolation or leaching time for monolithic materials where leaching is in many cases controlled by diffusion. Building on this observation leads to the question of whether all the leaching tests developed in the last decades are really necessary or can be reduced considerably when the key factors that they address are covered by other existing and more commonly applied tests. This calls for an evaluation of the background of a test and to what extent it addresses the answers sought. Tests that have a basis for further data interpretation will be favoured over arbitrarily defined tests. Standardized test conditions.

In harmonization the same tests should be applied whenever possible for different matrices unless it can be justified that the conditions do not reflect the properties aimed for. This relates to the use of specific test conditions in terms of temperature, LS, agitation, pH control or not, leachant, etc. However the conditions specified in standards should not be followed regardless of the environmental situation which is being considered. For example, in the case of a tank leach test, it may be important to get information on release in a seawater environment. In that

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case the leachant which is normally demineralized water should be modified to seawater to better mimic exposure conditions. Where exposure to a natural pH imposed by the surrounding is of concern, it may be relevant to maintain neutral conditions in the leachant by bubbling through it air containing carbon dioxide. This leads to a closer match to many field exposure conditions than the use of demineralized water. All the other parameters of the test can remain the same which implies that a single standard can be made to allow for such modifications for predefined situations. A deviation from the standard or the reason for making a new one should be well documented. The outcome of the test should be significantly different and the reason for the modification should be supported by information that indicates why the modification is better than the standard condition. In too many cases, the choice of test conditions is conventional rather than based on condideration of the question to be answered. The first step towards harmonization of existing leaching tests is to increase the understanding of what the test result means and what the consequences - possibilities as well as limitations of a given test are. In any case it is important to have a basic understanding of the leaching characteristics of a material. It is important to not just rely on short procedures when the leaching behaviour is not understood or can not be assumed from similar materials. The information generated in different fields appears to be complementary as specific aspects in one area such as the focus on redox conditions in sediments is directly relevant in other areas (soil, sewage sludge, waste). Similarly issues relating to biological activity in soils is relevant also in compost and sewage sludge as well as in wastes containing degradable organic matter. Where there is similarity in the results of tests using different leachants it suggests that similar fractions of the element of concern are released by the leachants or that the solubility may be governed by the same mineral phases. Generally, this type of comparison is not made.

Sampling issues in relation to leaching tests Since leaching from many materials is controlled by solubility, this has far reaching consequences for some of the aspects of leaching. An important consequence is that the sensitivity to sampling errors is less than will be observed for a material property such as total composition. The major elements controlling the pore water conditions that are responsible for the solubility control of trace constituents are hardly affected by sampling errors. Since sampling errors are considered as one of the major sources of error in the overall evaluation of materials this is an important consideration. A more critical aspect of leaching leading to sometimes considerable errors is the control over the final pH in the leachate. A minor change in final pH can mean an error of 50% to 100 % when the final pH is in a critical pH domain where there is a sharp change in release as a result of a minor change in pH. This aspect can not be identified or assessed in a single extraction test, but can be assessed when data are placed in context using data obtained from previous characterization of similar materials and contaminants.

Sample pre-treatment In the chapters on soil and sediment the sample pretreatment issue is addressed extensively. In soil studies the soil is normally dried before testing. This has definite consequences for the leaching behaviour. When a soil is in a reducing condition the drying step will change that condition and therefore the test conditions will not mimic the field situation. For sediments this aspect is crucial as only a thin surface layer of sediments is usually oxidized and most of the sediment layer is in reducing atmosphere. The consequences for leaching behaviour are

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substantial. In the waste field this aspect has also been recognised. As a consequence wastes are tested without drying in most cases and a separate sub-sample is used to determine the water content. The issue of reducing conditions in wastes has been identified but has not yet been implemented in sample pretreatment prescriptions.

Minimum requirements in testing and reporting Around the world many leaching tests are carried out on many different materials. In trying to compare results, it has been observed frequently that crucial information is missing. In some instances even a crucial parameter such as pH is not reported. To enhance comparability of leaching/extraction test results an agreement on minimum requirements for testing should be defined to ensure that results obtained at great cost are not invalidated by the lack of crucial information on test conditions (e.g liquid solid ratio, type of leachant, contact time and particle size distribution) and on leachate composition. By means of relatively simple measurements in all leachates, such as pH, redox state, DOC (unless the material is strictly inorganic) and conductivity, a number of pitfalls in data interpretation can be eliminated.

Concise testing Based on the knowledge gained and based on the assumption that there is a certain level of background information available, concise tests can be derived covering a broad range of leachability controlling aspects with a minimum of experimental work. An example of a concise test for granular material that can be performed within 2 days with the analysis of 4 extracts has been presented [van der Sloot 1994]. The specifications for the example test are given in Table 13.2

Table 13.2: Example concise test protocol for granular materials pH controlled test

2-Step serial batch test LS = 2 LS = 2 - 10 Size reduction Closed vessels No pH control

: 6 hours : 18 hours 995 % < 4 mm

Measurement of: pH, EH, TDS, DOC, Conductivity Relevant major, minor or trace elements ,

LS = 10 : pH = 8 for 24 hours LS = 50 : pH = 4 for 6 hours Size reduction 9 95 % < 300 mm

Measurement of: Acid or base consumption, DOC, EH Relevant major, minor or trace elements

,

From this combination of 4 extractions the following conclusions can be drawn: Extractions at two L/S values: distinction between solubility control and availability control (see chapter 2), measure of retention in the matrix relative to very mobile species (e.g. soluble salts) by comparing release at pH 4 to that at the material's own pH. EH measurement (relative to pH) gives indication of possible reducing properties of the material under consideration. DOC measurement gives potential role of mobilization of metals

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TDS gives the fraction of soluble salts

Extractions at two pH values: leachability changes in crucial pH domains (pH 4 , 8 , own pH) estimate of availability for leaching/potential for leaching acid/base neutralization capacity derived from acid/base consumption relevant to determine how long a material can impose its own pH conditions on a leachate. DOC measurement gives an indication of the nature of the mobilized type of DOC The combination of leachability/pH data from the four extractions shows the potential sensitivity of changes in leaching conditions in the pH domain relevant in practice. It also allows an indication of the likely changes in leachability when leaching conditions changes in the long term (e.g. pH through carbonation of biologically generated carbon dioxide, remineralization, acidification). For monolithic material similar schemes are in development, but have not progressed as far as the development for granular materials. Development of a harmonized approach

Scenario approach and the value of modelling Over recent years a change has been taking place in the treatment of leaching test data. The usefulness of leaching data is increased when the data can be used for modelling and thereby receive a further interpretation than the straight comparison against regulatory thresholds [CEN TC 292 WG 6 1996]. The advantage of such modelling and interpretation activities is that results from tests need to be consistent to be useful. Outliers in single extraction tests are not easily recognised, whereas data that are obtained to show a certain coherence can reflect anomalous behaviour more easily. Data are now being generated to describe processes and to model release rather than use data for a simple pass/fail evaluation. This development, which covers both chemical speciation modelling as well as release modelling, has led to the development of scenarios describing release which use site specific information. Basic elements are used in combination to arrive at an answer to a specific question. Verification between predictions on the basis of comparing laboratory test results with field observations is important in this context to improve the predictions by adjusting relevant parameters and test conditions.

European database Already much information is available on leaching/extraction procedures on a wide variety of materials throughout Europe and at a world-wide scale. Bringing together that information, which is now hidden in sometimes unknown or inaccessible reports, can help to define the common characteristics of several bulk materials used or disposed in Europe in large quantities. Coolation work has been carried out for data related to MSWI residues and has shown coherence between data generated in Canada, USA, Sweden, Denmark, Germany, The Netherlands and Japan [IAWG, 1997]. In the framework of a Brite Euram project, a

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comparison of the leaching behaviour of lead and zinc slag produced in French, Italian, German and British plants has shown good consistency [Mandin 1996]. A limited comparison of steel slag leaching data has shown consistent behaviour between slags produced in the Netherlands and in Sweden [F~.llman 1996, van der Sloot 1995]. It can be assumed that such consistencies between large scale industrial processes are pertinent for other materials with similar production characteristics. For other materials such as compost, sewage sludges, sediments and soils similarities in leaching behaviour may be expected. This does not imply that the concentrations of contaminants in leachates from leaching tests are the same as liquid/solid ratios may differ and test conditions may differ. However by converting the data to a common unit direct comparison is possible. The advantage of a database is that the key issues with regard to leaching from a specific material in a given circumstance can be identified more easily. In addition where extensive characterization data is made generally available an informed selection can be made of relevant elements for testing. Technical work in support o f the Network

The project "Technical Work in Support of the Network on Harmonization" was approved by the European Commission in June 1996 and started in October 1996. The work proposed covers a wide range of leaching tests and a wide range of materials in order to quantify the relationship between tests used in different technical fields. The project will run for three years and allows a more quantitative demonstration of: applicability of selected tests in different fields; similarities in the leaching/extraction behaviour of elements of concern for the widely different materials covered in the Network; assessment of similar and complementary aspects of other tests used in a specific field; statistically validated comparability/exchangeability of tests; possibilities for unification of data reporting in different fields to facilitate comparison of data regarding issues such as release in the long term and regulatory control and specific properties such as solubility control and diffusion control. This work will ultimately lead to recommendations for leaching/extraction procedures to be used in areas where tests are still lacking. These goals will be achieved by testing a selection of materials from the different fields - soil, contaminated soil, sediment, sludge, compost, waste, stabilized waste, construction materials, drinking water pipes and impregnated wood - by different leaching/extraction tests covering different aspects of leaching. A range of inorganic constituents of environmental concern will be analysed to ensure that general conclusions can be drawn on the validity of the comparisons. The information generated will be discussed with representatives from the different fields at expert meetings organised by the Network on the Harmonization of Leaching/Extraction Tests and disseminated through this channel and through regulatory bodies. Continuation o f the work as a Thematic Network

The pilot project Network Harmonization that started in January 1995 has proven very successful, in that about 400 experts from 30 countries are now listed as members of the

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Network. The experts that participated in the meeting in January 1996 endorsed the need to continue the Network of Harmonization of Leaching/Extraction Tests. The following items of interest could be beneficially pursued: 9

extension of the work on harmonising leaching tests for inorganic contaminants;

9

correlation of laboratory test results with field data;

9

evaluation of organic contaminant leaching (also organic content in general);

9

evaluation of sampling/pre-treatment in relation to leaching;

9

assessment of the role of leachate-soil interactions/interface phenomena;

9

evaluation of the use and applicability of models in relation to leaching;

9

evaluation of the potentially leachable fraction of materials;

9

assessment of the role of micro-biology in relation to biological activity;

9

assessment of chemical speciation (oxidation states, organic binding);

9

consideration of analytical problems in relation to leaching.

260

REFERENCES TO CHAPTER 13

REFERENCES Aten C.F. and S.K. Gupta. On heavy metals in soil; rationalization of extractions by dilute salt solutions, compariosn of extracted concentrations with uptake by rye grass and lettuce, and the possible influence of pyrophosphate on plant uptake. The Science of the Total Environment, 178, 45-53. Belevi, H., N. Agustoni-plan and P. Baccini, Influence of organic carbon on the long term behavior of bottom ash monofills. Proceedings Sardinia 1993. Fourth International Landfill Symposium. S. Marguarita di Pula, Cagliari, Italy, 11-15 October 1993, 2165-2173. Del Castilho, Chemisch Weekblad, 41, 1996, 2. Compliance test for leaching of granular materials, CEN TC 292 Characterization of Waste, Working Group 2 Draft European Standard. June 1994. IAWG- A.J.Chandler, T.T.Eighmy, J.Hartlen, O.Hjelmar, D.S.Kosson, S.E.SaweU, H.A.van der Sloot, J.Vehlow. Municipal Solid Waste Incinerator Residues, Elsevier, Studies in Environmental Science series 67, Amsterdam 1997. Comans, R.N.J. and P.A. Geelhoed. Speciatie onderzoek aan verontreinigde en gereinigde grond en baggerspecie. ECN-C- 96-084. 1996. F~.llman A.-M., B. Aurell, 1996. Leaching tests for environmental assessment of inorganic substances in wastes, Sweden. The Science of the Total Environment, 178, 71-84. Handboek Uitloog Karakterisering Deel I en II. CROW, Ede, 1995. Handboek Uitloog Karakterisering Deel III. CROW, Ede, 1996. Kosson, D.S., H.A. van der Sloot and T.T. Eighmy, An approach for estimation of contaminant release during utilization and disposal of municipal waste combustion residues. J. Hazard. Mat., 47 (1996) 43-75. Mandin, D. 1996, Valorization of Pb and Zn slag. EU Workshop. Lisbon, 1996. NEN 7341 Determination of the availability for leaching from granular and monolithic contruction materials and waste materials. NNI, 1994. Schreurs, J.P.G.M., H.A. van der Sloot en Ch.F. Hendriks. Uitlooggedrag in de wegenbouw: de praktijk getoetst aan de laboratoriumproef. Wegen, 70, 1996, 32-35. van der Sloot, H.A., D. Hoede and P. Bonouvrie. 1991. Comparison of different regulatory leaching test procedures for waste materials and construction materials. ECN- C-91-082. van der Sloot, H.A. Developments in evaluating environmental impact from utilization of bulk inert wastes using laboratory leaching tests and field verification. International Symposium on Bulk "Inert" Wastes: An Opportunity For Use. September 1995, Leeds, UK.

REFERENCES TO CHAPTER 13

261

van der Sloot, H.A., R.N.J. Comans and O. Hjelmar. 1995. Similarities in the leaching behaviour of trace contaminants from waste, stabilized waste, construction materials and soil. The Science of the Total Environment, 178 (1996) 111-126. van der Sloot, H.A., H.L.A. Sonneveldt, N.M. de Rooij, D. Hoede, M. Geusebroek. 1995. Staalslak uitloging bij toepassing als oeverbescherming. ECN-C-95-118. van der Sloot, H.A. and D. Hoede. Relatie kolom- en cascade test resultaten. ECN , 1996

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CHAPTER

14

263

C H A P T E R 14: S U M M A R Y

The scheme below leaching/extraction

summarises the various relations between

d i f f e r e n t a s p e c t s in r e l a t i o n t o

of materials

Leaching Tests Aim/Clas sific a tio n

Influ en ces/P r 9cesses Type of

Environments 9 soil 9 groundwater, rain water, sea w a t e r 9 pH-regions .......

material 9 granular 9 monolithic 9 porous 9 non aorous 9

.......

Technical point of view , equilibrium conditions - batch tests - pH-stat, methods

9 protection of environment 9 environmental

-. ...... 9 dynamic aspects diffusion tests column tests

compatibility assessment 9 scientific studies 9 daily practice

-.

Mechanisms[ +Processes [ | T * dissolution 9 diffusion 9 surface reactions

9 standing water 9 flowing water 9 percolation

Standardisation

Purpose

......

9

9 basic characterisation tests 9 compliance tests 9 on-site verification tests

.......

I Suitable test method

Parameters

sampling/sample pre-treatment 9 total composition 9 potentially leachable 9 actual leachability

9 batch test 9 columntest * tanktest * test under field conditions 9 .......

9 pH 9 redox potential 9 dissolved salts 9 heavy metals 9 TOC .......

9 grain size 9 surface area 9 LS 9 composition of leachate 9 temperature 9 ageing effects 9

,

Data interpretation/crosssections [

.......

264

CHAPTER 14

Relevance of leaching/extraction of materials The leaching behaviour of a particular constituent in a material is not related to the total amount of that component in the material. Since uptake by organisms and transport into the environment is largely controlled by release in the water phase, knowledge regarding the leaching/extraction behaviour of materials is crucial in order to assess environmental release or environmental impact. Levels of testing Based on the degree of available knowledge regarding the material to be evaluated by leaching/extraction tests, different levels of testing comprising characterization testing, compliance testing and on-site verification testing have been proposed. This approach will prove useful as less rigorous test data such as that from compliance tests can be interpreted in perspective to a background of information obtained from characterization tests. Testing related to release control A distinction is made between the testing of granular materials, where in most cases percolation is the prevailing constituent transport mode and the testing of monolithic materials where the release of constituents is predominantly related to the external surface of the monolith. Level of aggressiveness in testing In all fields, three main levels of aggressiveness in testing can be distinguished: total composition (total destruction), a potentially leachable amount (using relatively aggressive agents or conditions in testing) and actual leachability or mobility under normal exposure conditions (using a mild method of extraction). Sampling in relation to leaching Sampling for the purposes of performing a leaching test appears to be less critical than obtaining a sample in order to determine the chemical composition of materials. In leaching/extraction the major element composition will dictate the main leaching controlling parameters. Since the leachability of major elements and many critical elements is solubility controlled, the influence of compositional homegeneity on leachate composition is limited. The focus in sampling should shift toward an assessment of its potential to change solubility controlling factors such as oxidation state and pH. Sample pre-treatment Pre-treatment or the controlled storage of samples prior to leaching is crucial. Size reduction of material may expose fresh lime surfaces leading to a higher equilibrium pH in the sizereduced material than is present in the original material. This leads to differences in results from leaching tests although not necessarily to a different leaching behaviour. A reduced material exposed to oxidation will undergo changes that affect its leaching behaviour. Drying of material that is normally wet may result in changes of leaching behaviour as drying may involve oxidation. This is particularly true for sediments, but also for many soils.

CHAPTER 14

265

pH as common basis of reference The work presented supports the conclusion that pH can be used as a common basis of reference for the leaching behaviour of inorganic constituents. Redox conditions The role of reducing conditions imposed by a material or by the conditions in which a material is ultimately used or disposed is important for an evaluation of the release properties of the material. This aspect is undervalued at the moment. Time dependent leaching as basis of reference for release From leaching tests designed to provide information on release as a function of time or a parameter that can be related to time (e.g. L/S) mechanisms of release can otten be derived that allow conclusions on the long term leaching behaviour of materials. As such time or L/S is a relevant basis for the comparison of leaching data. Leaching test data interpretation Leaching test data have for too long been regarded as numbers generated by a 'black box' process. In recent years the understanding of leaching processes has increased significantly therefore this knowledge should be used in decisions regarding the acceptable or unacceptable uses of materials. To facilitate the assessment of the impact of released constituents leaching data can now be modelled (geochemical and transport modelling) and used to predict release over given time periods. Further developments are needed in the future but the possibilities available now are already providing the means to manage better the selection of the preferred option. Relationship between laboratory generated data and field information There is a need to expand on the available verification of modelled predictions based on laboratory data against field observations in disposal and utilization scenarios. In the soil field this type of study into such relationships have been developed further than in other areas. In the construction and waste utilization field this area of study is just developing. Behavioural similarities In relation to the evaluation of the environmental properties of a material complementary approaches can be taken focusing on: m.

Similarities m leaching behaviour between different materials. This will result from similarities in major element chemistry of the materials. An example is the behaviour of cement-based materials which all exhibit a high matrix pH. Another category of materials with similar properties are slags with reducing properties, such as blast furnace slag, phosphate slag and steel slag.

266

C H A P T E R 14

B.

Similarities m leaching behaviour for a constituent or groups of constituents in different materials. The leaching behaviour of particular constituents is governed by a limited number of main solubility controlling factors. As has been shown for several constituents in widely different materials similarities in behaviour can be observed. Constituents which exhibit similar behaviour can be grouped together (e.g. alkali elements, halogens, metals, oxyanions). Organic contaminants are not addressed here but similar groupings are likely to be present for organics.

C~

Similarities in the evaluation of disposal or utilization scenario descriptions. For different applications or disposal scenarios several factors that will be similar can be taken into account in assessing leaching/release. Assessments can thus be classified into types of assessments allowing a similar genetic approach in evaluating the release of potentially harmful constituents.

Harmonization of tests

Through a common reference base the first step which can be achieved is to be able to compare leaching/extraction test data in a meaningful manner. The development of a common reference base has already helped some industries to eliminate confusion in the use of regulatory test data between countries. Based on further interaction between different technical fields and by carrying out intercomparison studies across different fields further significant harmonization can be achieved.

ANNEX 1

ANNEX 1 C O N T A C T D E T A I L S FOR THE G R O U P M E M B E R S H A van der Sioot (Co-ordinator) ECN (Netherlands Energy Research Foundation) PO Box 1 1755 ZG PETTEN The Netherlands Tel: +31 224 564249 Fax: +31 224 563163

G Rauret Universitat de Barcelona Departament de Quimica Analitica Diagonal 647 8028 Barcelona Spain Tel: +34 3 4021276 Fax: +34 3 4021233

P Schiessl Instittit ~ r Bauforschung Schinkelstr 3 D-52056 Aachen Germany Tel: +49 241 805100 Fax: +49 241 8888139

N West AFNOR- France Tour Europe - Cedex 7 F-92049 Paris La D6fense France Tel: +33 1 42915799 Fax: § 1 42915656

O Hjelmar VKI Vandkvalitet sinstituttet (Water Quality Institute) 11 Agern Alle DK-2970 Horsholm Denmark Tel: § 42 865211 Fax: +45 42 867273

L Heasman M J Carter Associates Long Street Atherstone Warwickshire CV9 1BH United Kingdom Tel: +44 1827 717891 Fax: +44 1827 718507

M J A van den Berg NNI (Netherlands Standardization Institute) PO Box 5059 2600 GB Delft The Netherlands Tel: § 15 2690166 Fax: § 15 2690190

Ph Quevauviller EC DGXII, Standards, Measurement and Testing Programme (Commission of the European Communities) Rue de la Loi 200 Brussels B-1047 Belgium Tel: +32 2 2963351 Fax: +32 2 2958072

267

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ANNEX 2

269

ANNEX 2 THE MAIN CHAPTER CONTRIBUTORS

Chapter

1.

Introduction

H A van der Sioot, L Heasman, Ph Quevauviller

Chapter

2.

General principles

H A van der Sioot

Chapter

3.

Soil

G Rauret

Chapter

4.

Contaminated soil

K H Karstensen

Chapter

5.

Sediments

M Kersten

Chapter

6.

Sewage sludges

L Brener

Chapter

7.

Compost

A Gomez

Chapter

8.

Granular waste/industrial sludge

O Hjeimar, H A van der Sloot

Chapter

9.

Stabilized/solidified waste

J M6hu, P Moszkowicz, R Barna, F Sanchez

Chapter

10.

Construction materials

P Schiessl, I Hohberg

Chapter

11.

Preservative treated wood

M Boonstra

Chapter

12.

Standardisation

M J A van den Berg, N West

Chapter

13.

Discussion and conclusions

H A van der Sloot, L Heasman,

Ph Quevauviiler Chapter

14.

Summary

H A van der Sioot, L Heasman,

Ph Quevauviiler

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ANNEX 3

271

ANNEX 3 THOSE W H O A T T E N D E D AND C O N T R I B U T E D TO THE N E T W O R K M E E T I N G S

The first expert meeting of the Network Harmonization of Leaching~Extraction tests was organised at AFNOR in Paris (June 1995). Thirty experts from different disciplines participated in the discussions. Apart from the co-ordination team the following participated in the first meeting:

E Barahona

N C Blakey

CSIC Estaction Experimental del Zaidin Granada Spain

Water Research Centre plc Medmenham Buckinghamshire United Kingdom

M Boonstra

L Brener

TNO-Bouw Centrum voor Houttechnologie Delft The Netherlands

Lyonnaise des Eaux DUMEZ Division Qualit6 Analyse Le Pecq France

A M Fiillman

A Gomez

Swedish Geotechnical Institute LinkOping Sweden

INRA Bordeaux Villenave d'Ornon France

P Henschel

V J G Houba

Umweltbundesamt Berlin Germany

Landbouw Universiteit Wageningen Wageningen The Netherlands

K H Karstensen

M Kersten

SINTEF SI Environmental Technology and Analysis Oslo Norway

Institute for Baltic Sea Research Geology Section Rostock-WarnemOnde BRD

272

ANNEX 3

J M~hu

J Obiols

Polden- INSA Lyon Villeurbanne Cedex France

IQS - Institut Quimic de Sarria Dpto. Quimica Analitica Barcelona Spain

M Petrangeli

C Rohrer

University of Rome Department of Chemistry Rome Italy

Technische Universit~.t Wien (as before) Insitut for Analytische Chemie Wien Austria

J F Santos Oliveira

G Thielen

University de Lisboa Faculdade Ciencias e Technologia Monte de Caparica Portugal

Forschungsinstitiat der Zementindustrie DOsseldorf Germany

A M Ure

K Vallom

Aberdeen United Kingdom

Forbairt Dublin Ireland

In the discussions the attention was focused on leaching of inorganic constituents as the information of leaching of organic contaminants, although important, was not part of the scope of the present work.

The second expert meeting of the Network Harmonization of Leaching~Extraction tests was organized at Estacion-Experimental Del Ziadin (CISC) in Granada, Spain. Twenty-six experts from different disciplines participated in the discussions. Apart from the coordination team the following experts participated in the second meeting:

E Barahona

R Barna

CSIC Estaction Experimental del Zaidin Granada Spain

Polden- INSA Lyon Villeurbanne Cedex France

ANNEX 3

H Belevi

N C Blakey

EAWAG Atmospheric Chemistry Group Dtibendorf-Z0rich Switzerland

Water Research Centre plc Medmenham Buckinghamshire United Kingdom(as before)

M Boonstra

L Brener

TNO-Bouw Centrum voor Houttechnologie Delft The Netherlands

Lyonnaise des Eaux DUMEZ Division Qualit6 Analyse Le Pecq France

C Cheeseman

A M Fiillman

Centre for Env Control and Waste Management Imperial College of Science Technology and Medicine London United Kingdom

Swedish Geotechnical Institute LinkOping Sweden

A Fernando

A Gomez

University de Lisboa Faculdade Ciencias e Technologia Monte de Caparica Portugal

INRA Bordeaux Villenave d'Ornon France

M Grasserbauer

P Henschel

Technische Universit~it Wien Insitut for Analytische Chemie Wien Austria

Umweltbundesamt Berlin Germany

K H Karstensen

M Kersten

SINTEF SI Environmental Technology and Analysis Oslo Norway

Institute for Baltic Sea Research Geology Section Postock-Warnem0nde BRD

M Lachica

R Morabito

CSIC Estaction Expermental del Zaidin Granada Spain

ENEA CRE Casaccia Divisione Chimica Ambientale Roma Italy

273

274

It Muntau CEC Joint Research Centre Ispra Italy

ANNEX 3

J Obiols

IQS - Institut Quimic de Sarria Dpto. Quimica Analitica Barcelona Spain

E Pottkamp

G Thielen

Preussag-Boliden Blei Lab fiir Umweltschutz Nordenham BRD

Forschungsinstit0t der Zementindustrie DOsseldorf Germany

A M Ure Aberdeen United Kingdom

M Verloo Universiteit Gent Lab voor Analytische Chemie en Toegepaste Ecochemie Gent Belgium

M Wahlstriim

VTT Chemical Technology Espoo Finland

G L O S S A R Y OF T E R M S

GLOSSARY

adsorption anoxic attenuation

anion batch tests

benthic ecosphere buffer

cation cation exchange

complexation

desorption diagenesis diffusion

dissolution EH eluate emission equilibrium

275

OF TERMS

Adherence of the atoms, ions, or molecules of a gas or liquid to the surface of another substance, called the adsorbent. In the absence of oxygen. The reduction of the concentrations of chemical species in a solution by means of physical, chemical and biological reactions as it misrates throush a solid medium. Any ion with a negative chars;e. Leaching tests which are carried out on a single portion of material using a single portion of leachant i.e. there is no renewal of leachant durin8 the test. The physical environment occupied by the bottom dwelling life of an ocean or freshwater system. A solution containing both a weak acid and its conjugate weak base whose pH changes only slightly on addition of acid or alkali. An ion with a positive charge. A reversible chemical reaction between a solid (cation exchanger) and a fluid (usually a water solution) by means of which cations may be interchanged from one substance to another. The formation of an ion into a molecular structure consisting of a central atom bonded to other atoms by coordinate covalent bonds The process of removing an adsorbed material from the solid on which it is adsorbed The set of processes, including solution, that alter sediments at low temperatures after burial The spontaneous mixing of one substance with another when in contact or separated by a permeable membrane or microporous barrier Molecular dispersion of a solid in a liquid A measure of the oxidation reduction potential. See

oxidation/reduction. As leachate but usually in the context of a laboratory test. Release of substances from one environment, medium or phase to another. Chemical equilibrium is a condition in which a reaction and its opposite or reverse reaction occur at the same rate resulting in a constant concentration of reactants. Physical equilibrium is exhibited when t w o o r more phases of a system are changing at the same rate so the net change in the system is zero.

276

extraction

heterogeneous homogeneous hydraulic head hydraulic conductivity hysteresis infiltration inorganic

ionic strength kinetic

labile

leachant leachate leaching

ligands

G L O S S A R Y OF T E R M S

A separation operation that may involve three types of mixture: (1) a mixture composed of two or more solids (2) a mixture composed of a solid and a liquid - as in this context (3) a mixture of two or more liquids. One or more components of such a mixture are removed (extracted) by exposing the mixture to the action of a solution or solvent in which the component to be removed is soluble. Any mixture or solution comprising two or more substances which are not uniformly dispersed Any mixture or solution comprising two or more substances which are uniformly dispersed. The pressure exerted by a fluid expressed as metres above a reference point. The permeability of a material to water. A retardation of the effect, as if from viscosity, when the forces acting upon the body are chan~;in 8. The movement of water (usually rainwater) into and through a solid material. Chemicals that are generally considered to include all substances except hydrocarbons and their derivatives or all substances which are not compounds of carbon with the exception of carbon oxides and carbon disulphide. A measure of the concentration of ions in solution. Chemical phenomena can be studied from two fundamental approaches: (1) thermodynamics, a rigorous and exact method concerned with equilibrium conditions of initial and final states of chemical changes and (2) kinetics, which is less rigorous and deals with the rate of change from initial to final states under non equilibrium conditions. The two methods are related. Thermodynamics, which yields the driving potential - a measure of the tendency of a system to change from one state to another - is the foundation on which kinetics is built. Descriptive of a substance that unstable and is readily inactivated for example by high temperature or radiation Liquid in contact with or which will be brought in contact with a solid which extracts soluble components of the solid. Liquid containing soluble components extracted from a solid. The process by which the soluble components of one phase (usually a solid) are transferred to another phase (usually a liquid). A molecule, ion or atom that is attached to the central atom of a coordination compound, a chelate or other complex. LiBands are also called complexin8 asents.

GLOSSARY OF TERMS

lyophilization organic oxidation/reduction potential

partitioning pE

percolation permeability pH porosity precipitation redox sequential extraction solubility

sorption speciation standard thermodynamic tortuosity validation

277

A method of dehydration or of separating water from biolosical materials Chemicals that are generally considered to include all compounds of carbon except carbon oxides and sulphides. A measure of the ability of a system to cause oxidation or reduction reactions. Oxidation and reduction are reactions in which electrons are transferred. Oxidation and reduction always occur simultaneously (redox reactions). The substance that gains the electrons is termed the oxidizing agent and the substance that loses the electrons is termed the reducin 8 agent. The distribution of molecules in different states or phases in a system for example as solid, liquid or gas. A measure of the redox potential. The movement of a liquid through a solid. A measure of the ability of a material to transmit fluid under a hydraulic gradient. pH is a value taken to represent the acidity or alkalinity of an aqueous solution. The relative volume of void space to the total volume occupied by a material. The settlement of small particles out of a liquid or gaseous suspension by Bravity or as the result of a chemical reaction.

See oxidation/reduction potential The extraction of a single portion of (usually solid) material with more than one portion of liquid in a sequence. The ability or tendency of one substance to blend uniformly with another e.g. solid in liquid, liquid in liquid, gas in liquid, gas in gas. Solids vary from 0% to 100% in their degree of solubility in liquids depending on the chemical nature of the substances. A surface phenomenon that may be either absorption, or a combination of the two. The term is often used when the specific mechanism is not known Determination of the precise chemical form of a substance present in a material. A documented method or specification to which activities should conform. See kinetic. A measure of the actual pathway taken by a fluid relative to the distance between two points. Confirmation of soundness and defensibility (of a method or procedure).

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SUBJECT INDEX Adsorption phenomena (soil) 43 Adsorption (contaminated soil) 60 Ageing 17 Aggregates 188 Alkali metals (generic leaching behaviour) 250 Aluminium (genetic leaching behaviour) 250 Analysis of eluate (granular wastes and sludges) 147 Antimony (generic leaching behaviour) 250 Arsenic (generic leaching behaviour) 250 Batch leaching tests (granular wastes and sludges) 143 Benthic flux chambers 78 Buffering capacity (contaminated soil) 61 Cadmium (generic leaching behaviour) 250 Cement based materials 187 CEN/TC 164 (Water supply) 206 CEN/TC 292 (Characterizatofwaste) 229 CEN/TC 308 (Characterizatof sludges) 230 CEN/TC 38 (Durability of wood and derived materials) 231 CEN/TC 51 ( Cement and building limes) 231 CEN/TC 223 (Soil improvers and growing media) 232 CEN/TC 227 (Road materials) 232 CEN/TC 260 (Fertilizers and liming materials) 232 CEN/TC 309 (Footwear) 231 CEN/TC 99 (Wallcoverings) 231 CEN/TC/104 (Concrete) 231 CEN/TC/154 (Aggregates) 231 Characterizatioofmaterials (general) 253 Chemicals (generic leaching behaviour) 250 Chromium (generic leaching behaviour) 250 Classification of materials 251 Clean-up testing (contaminated soil) 67 Collection of eluate (granular wastes and sludges) 147 Column leaching tests (granular waste and sludges) 142 143 148 Complexation 20 Complexation (contaminated soil) 61

279

Compost production 123 Compost composition 126 Composts 123 Construction material composition 167 Construction materials 187 Contact time (granular wastes and sludges) 146 Contact conditions (stabilized / solidified waste) 174 Contaminated soil 57 Contaminated soil characteristics 57 Contaminated soil (stabilised) 65 Copper (generic leaching behaviour) 250 Data interpretation - see interpretatof data Database 257 Degradation (sewage sludge) 107 Diffusion (general) 34 Diffusion (sewage sludge) 109 Diffusion (stabilized and solidified waste) 177 Diffusion (construction materials) 191 Dissolved organic matter 248 257 Drinking water pipes 195 Equilibrium assumption 17 Extractants (general) 36 240 Extractants (soil) 48 49 Extractants (contaminated soil) 65 Extractants (stabilized and solidified waste) 65 Extractants (sediments) 84 Extractants (sewage sludge) 111 Extractants (compost) 127 Extractants (granular wastes and sludges) 145 Extractants (construction materials) 196 Extractants (preservative treated wood) 219 Extraction leaching tests (granular wastes and sludges) 147 Field conditions (contaminated soil) 68 Fixation (of preservatives in wood) 212 Flow (past particles) 15 Granular waste 131 Granular materials (construction materials) 194 199 Granular materials (general) 252 Granular materials (example concise leaching test protocol) 256 Halogens (generic leaching behaviour) 250

28O

SUBJECT INDEX

Harmonization (of leaching tests) 239 Harmonization (potemial for) 254 266 Hydraulic binders 172 Incubation experiments (sediments) 79 Interpretation of data (granular wastes and sludges) 152 Interpretation of data (stabilized / solidified waste) 182 Interpretation of data (construction materials) 205 Interpretation of data (preservative treated wood) 219 Interpretation of data (general) 265 Ion exchange capacity (soil) 43 Ion exchange (contaminated soil) 60 ISO/TC 190(Soil quality) 228 Kinetics 15 Leachant - see extractant Leachants (general) 13 240 Leaching conditions 26 Leaching tests (soil) 46 48 49 Leaching tests (contaminated soil) 64 Leaching tests (sediments) 84 Leaching (sewage sludge) 111 Leaching tests (sewage sludge) 111 Leaching tests (compost) 128 Leaching test objectives (wastes) 132 Leaching tests (granular waste and sludges) 134 Leaching (granular waste and sludges) 141 Leaching (stabilized / solidified waste) 174 Leaching test objectives (stabilized / solidified waste) 174 Leaching tests (stabilized / solidified waste) 176 Leaching conditions (construction materials) 188 Leaching tests (construction materials) 196 198 Leaching (preservative treated wood) 213 Leaching conditions (preservative treated wood) 219 Leaching tests (preservative treated wood) 219 Leaching tests (harmonization) 239 Leaching tests (minimum requirements) 256 Leaching tests (general) 264 Lead (generic leaching behaviour) 250

Liquid/solid ratio (granular wastes and sludges) 146 Lithium (genetic leaching behaviour) 250 Lysimeter leaching tests (granular wastes and sludges) 143 Masonry units 187 Metal mobility (sediments) 75 83 Metals (sewage sludge) 104 Mineral solubility (soil) 42 Mineralization 17 Modelling(general) 4253 Modelling (geochemical) 35 Modelling(soil) 51 Modelling (sewage sludge) 110 Modelling (stabilized / solidified waste) 182 Modelling (preservative treated wood) 220 Molybdenum (generic leaching behaviour) 250 Monolithic materials (construction materials) 189 Monolithic materials (general) 252 Network (on the harmonization of leaching /extractitests) 2 Network (continuation) 258 Nickel (generic leaching behaviour) 250 Organic matter (general) 248 Oxyanions (generic leaching behaviour) 250 Particle properties 15 Particle size distribution (soil) 43 Particle surfaces (soil) 43 Percolation control 31 pH (general) 18 241 246 240 pH (soil) 43 pH (sediments) 85 pH (sewage sludge) 106 pH (granular wastes and sludges) 155 162 pH (stabilized / solidified waste) 178 pH (construction materials) 190 pH (preservative treated wood) 214 pH as basis of reference 265 Polluting processes (soil) 58 Pore water profiles (sediments) 78 Potassium (generic leaching behaviour) 250 Preservative treated wood 209 Redox (general) 23 242 246 Redox (sediments) 75

SUBJECT INDEX Release mechanisms (general) 33 249 Release mechanisms (contaminated soil) 63 64 Release rates (sediments) 78 Release mechanisms (granular waste and sludges) 137 140 Release mechanisms (stabilized / solidified waste) 180 177 Release mechanisms (construction materials) 189 Reporting (minimum requirements) 256 Sample pretreatment (soil) 47 Sample pretreatment (contaminated soil) 70 Sample pretreatment (sediments) 86 Sample pretreatment (granular wastes and sludges) 145 Sample pretreatment (general) 255 Sampling(soil) 47 Sampling (contaminated soil) 70 Sampling (sediments) 87 Sampling (general) 255 264 Saturation 16 Sediments 75 Selenium (generic leaching behaviour) 250 Sequential extraction (soil) 49 Sequential extraction (sediments) 88 Sewage sludge 101 Sewage sludge composition 101 Sludges (industrial waste) 131 Sodium (generic leaching behaviour) 250 Soil (leaching from) 41 53 Soil characteristics 41 Soil composition 44 Soil(contaminated) 57 Soil / contaminant interaction 59 Sorption (general) 24 242 Stabilized / solidified waste 171 Stabilized / solidified waste composition 172 Standardization 227 Standardizationprocess 233 Standardizationlnstitutes 235 Standards (soil) 49 Tank leaching test (granular wastes and sludges) 127 Temperature (general) 36 Temperature (granular wastes and sludges) 147

251

Testing (minimum requirements) 256 Tortuosity (construction materials) 200 Vanadium (generic leaching behaviour) 250 Verification with field data (granular wastes and sludges) 161 Verification of laboratory data (preservatitreated wood) 221 Verification of test data (soil) 51 Verification with field data (general) 265 Waste (granular) 131 Waste types and composition 135 Waste characterization (granular wastes and sludges) 161 Waste (stabilized / solidified) 171 Weathering 17 25 Wood (preservative treated) 211 Zinc (genetic leaching behaviour) 250

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