Arsenical pesticides: A symposium sponsored by the Division of Pesticide Chemistry at the 168th meeting of the American Chemical Society, Atlantic City, N.J., Sept. 9, 1974 (ACS symposium series ; 7)

Arsenical Pesticides Ε. A . Woolson, Editor A symposium sponsored by the D i v i s i o n of Pesticide Chemistry at the ...

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Arsenical Pesticides Ε. A . Woolson, Editor

A symposium sponsored by the D i v i s i o n of Pesticide Chemistry at the 168th M e e t i n g of the American Chemical Society, Atlantic City, N. J., Sept. 9, 1 9 7 4 .

ACS

SYMPOSIUM

SERIES

7

AMERICAN CHEMICAL SOCIETY WASHINGTON,

D. C.

1975

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Library of Congress CIP Data Arsenical pesticides. ( A C S symposium series; 7) Includes bibliographical references and index. 1. Pesticides—Environmental aspects—Congresses. 2. Arsenic compounds—Environmental aspects—Congresses. I. Woolson, Ε. Α., 1 9 4 1 - ed. II. American Chemical Society. III. American Chemical Society. Division of Pesticide Chemistry. I V . Series: American Chemical Society. A C S symposium series; 7. [DNLM: 1. Arsenicals—Congresses. 2. Pesticides—Congresses. QV294 A 5 1 2 1974] QH545.P4A77 I S B N 0-8412-0243-5

Copyright ©

574.5'222 74-31378 A C S M C 8 7 1-176 (1975)

1975

American Chemical Society All Rights Reserved

PRINTED IN THE UNITED STATES OF AMERICA

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

A C S Symposium Series Robert F. Gould,

Series Editor

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

FOREWORD The

A C S S Y M P O S I U M SERIE

a m e d i u m for publishing symposia q u i c k l y i n book form.

The

f o r m a t of the S E R I E S parallels that of its predecessor, A D V A N C E S I N C H E M I S T R Y S E R I E S , except that i n order to save t i m e the papers are not typeset b u t are r e p r o d u c e d as they are subm i t t e d b y the authors i n c a m e r a - r e a d y

form.

As a further

means of s a v i n g t i m e , the papers are not e d i t e d or r e v i e w e d except b y the s y m p o s i u m c h a i r m a n , w h o becomes e d i t o r of the book.

Papers p u b l i s h e d i n the A C S S Y M P O S I U M

SERIES

are o r i g i n a l c o n t r i b u t i o n s not p u b l i s h e d elsewhere i n w h o l e or major p a r t a n d i n c l u d e reports of research as w e l l as r e v i e w s since s y m p o s i a m a y e m b r a c e b o t h types of presentation.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

PREFACE A r s e n i c is a n e n v i r o n m e n t a l l y u b i q u i t o u s G r o u p V a element. A l t h o u g h it has a m e t a l l i c a l l o t r o p i c f o r m l i k e the t w o h i g h e r G r o u p V a elements,

a n t i m o n y a n d b i s m u t h , arsenic's

s i m i l a r to p h o s p h o r u s .

c h e m i c a l b e h a v i o r is q u i t e

It has f o u r valence states, —3, 0, + 3 ,

and

+5.

A r s i n e a n d methylarsines are characteristic of A s i n the —3 valence state a n d are generally unstable i n air. A r s e n i c m e t a l is f o r m e d b y r e d u c t i o n of arsenic oxides a n d is a glossy b l a c k m a t e r i a l .

A r s e n i c t r i o x i d e is a

p r o d u c t of s m e l t i n g operations, a n d it is the starting m a t e r i a l for synt h e s i z i n g most arsenica b a c t e r i a to arsenic p e n t o x i d e or orthoarsenic a c i d ( H A s 0 ) . 3

4

Arsines

a n d arsenites are generally m o r e toxic t h a n arsenates. I n the n a t u r a l e n v i r o n m e n t , most arsenicals d e g r a d e or w e a t h e r to f o r m arsenate, a l t h o u g h u n d e r anaerobic c o n d i t i o n s arsenite m a y f o r m . B i o t r a n s f o r m a t i o n s m a y o c c u r w h i c h result i n v o l a t i l e arsenicals.

These,

i n t u r n , are r e t u r n e d to the l a n d w h e r e soil a d s o r p t i o n , p l a n t u p t a k e , erosion, l e a c h i n g , r e d u c t i o n to arsines, or other processes occur.

This

n a t u r a l arsenic c y c l e reflects a constant s h i f t i n g of arsenic into the various environmental compartments. T h i s s y m p o s i u m was a r r a n g e d because of current c o n c e r n a n d interest i n the q u a l i t y of o u r e n v i r o n m e n t , p a r t i c u l a r l y as it pertains to trace h e a v y metals. W h i l e arsenic is not a h e a v y m e t a l , it is a m e t a l l o i d w h i c h has a l w a y s generated m u c h interest a n d controversy because m a n y of its c o m p o u n d s have toxic properties.

T h i s s y m p o s i u m , then, was i n t e n d e d

to examine the analysis, soil b e h a v i o r , biotransformations, a c c u m u l a t i o n s , c y c l i n g , a n d m o d e l i n g of arsenate a n d arsenical pesticides i n nature. E.

Beltsville,

A.

WOOLSON

Md.

N o v e m b e r 1,

1974

vii

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1 Review of Arsenical Pesticides S. A. PEOPLES School of Veterinary Medicine, University of California, Davis, Calif. 95616

It is not the purpos tive usefulness of various arsenic compounds as herbicides and insecticides. It is rather to present information which is needed to form the basis for evaluating the effects of such usage on the ecosphere and the p o s s i b i l i t y of adverse effects on animals and man. Arsenic has had such a bad name as to be nearly synonymous with the word "poison" and I think it would be worth putting it in i t s proper place. It is a relatively common element, present in a i r , water, s o i l , plants and animals, and the pharmacology of chemical compounds depends on the dose given; no action, useful and toxic. Considering that arsenic compounds have been known since 250C B.C. and used in medicine since the time of Hippocrates since 400 B.C. (1), one would expect that information would be available as to the chemical nature of arsenic compounds in nature and their biochemical effects and metabolic fate in plants and animals. Such is not the case, however, due in large part to the failure to develop analytical procedures for specific compounds. A l l samples were wet or dry ashed to inorganic arsenic, reduced to arsine with zinc and hydrochloric acid which was then quantitated by some modification of the Gutzeit method. The results were usually expressed as arsenic trioxide, a practice which has lead many to the conclusion that the arsenic in the sample was indeed trivalent arsenic and worse yet, toxicologists frequently use the term "toxicity of arsenic for animals" as synonymous with the toxicity of arsenic trioxide for animals (2). Recently, an analytical method has been developed in my laboratory which w i l l separately quantitate the inorganic and methylated arsenic in water, grass and urine (3) and is now being modified for s o i l samples. A method using a helium plasma has been introduced by Braman which w i l l separate trivalent and pentavalent inorganic arsenic and methyl and dimethyl arsonic acids (4). The use of these methods in combination with such isolation technics such as thin layer and column chromatography w i l l hopefully yield as much information about the biochemistry 1

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2 of

ARSENICAL

a r s e n i c as i s now

PESTICIDES

known about phosphorus.

Chemistry A r s e n i c occurs i n nature as s u l f i d e s such as orpiment and as complex s u l f i d e s or i r o n , n i c k e l and c o b a l t . I t has valences of 3 , 5 and i n o r g a n i c and o r g a n i c compounds o f both are known. A valence o f -3 i s a l s o known where a r s e n i c i s combined with hydrogen i n compounds known as the a r s i n e s . Since the s y n t h e s i s o f s a l v a r s a n by E h r l i c h i n 1905, l i t e r a l l y hundreds o f organic a r s e n i c a l s have been synthesized f o r use i n medicine and i n d u s t r y . Except f o r the treatment o f such p a r a s i t i c diseases such as trypanosomiasis, amebiasis and f i l a r i a s i s , the advent o f the a n t i b i o t i c s rendered most o f the m e d i c i n a l products obsolete but i t i s i n t e r e s t i n have found new uses unde tive. The information i n Table 1, taken from pharmacology textbooks used i n the 1930 s shows some o f these compounds and the therapeut i c dose l e v e l s should be o f p a r t i c u l a r i n t e r e s t to those concerned with t h e i r r e s i d u e l e v e l s i n food (5) (6). For example, i t would be necessary t o eat 10 kg o f food c o n t a i n i n g 5 ppm of c a c o d y l i c a c i d t o o b t a i n a t h e r a p e u t i c dose. +

+

f

D i s t r i b u t i o n of A r s e n i c i n Nature A r s e n i c i s a ubiquitous element present i n a i r , water, s o i l and a l l l i v i n g t i s s u e s . The f i n d i n g o f a r s e n i c i n any sample i s t h e r e f o r e not s i g n i f i c a n t unless compared t o the c o n c e n t r a t i o n normally expected. The range o f these values i s given i n Table 2 (7). I t must be p o i n t e d out that these values are given as t o t a l a r s e n i c and t h e i r chemical nature i s unknown. That high values occur i n sea food has been known s i n c e 1935 when Coulson reported i t i n shrimp and found i t to be nontoxic t o r a t s (18). I t i s c l e a r that animals and man r e c e i v e a d a i l y i n t a k e o f a r s e n i c which v a r i e s with geographical l o c a t i o n and type o f d i e t . Once again i t should be noted that the chemical nature o f the a r s e n i c i n these sources i s l a r g e l y unknown and t h e r e f o r e the f a t e and e f f e c t i n animals and man cannot be p r e d i c t e d . For example Lakso (3) found that most o f the background a r s e n i c i n Johnson grass was a methylated a r s o n i c a c i d , probably MSMA, a compound o f low t o x i c i t y . Absorption, D i s t r i b u t i o n and E x c r e t i o n A r s e n i c compounds can be absorbed by any route although the usual entry i s by i n g e s t i o n . There i s a common b e l i e f that a r s e n i c i s a cumulative poison which i s l a r g e l y based on i t s tendency t o accumulate i n high concentrations i n the h a i r and n a i I s , which are a c t u a l l y e x c r e t o r y products and not i n e q u i l i b rium with the l i v i n g body. A second b a s i s f o r t h i s b e l i e f i s the

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

PEOPLES

Review

TABLE 1 Arsenical Drugs Used in Treating Human Disease Drug Fowler's Solution

Dose grams

Use

Formula

.005 CH

Pearson's Solution

,005

Arrhenal

.05

3

NaOAs = 0 CH CH Na OAs = 0 CH / H Na OAs^O CH As = As

Tonic

X

3

3

Tonic

X

Sodium Cacodylate Arsphenamine

3

3

.05

Tonic

3

.3-6

Syphilis

ONH

ONHO

2

OH 0 Atoxyl

. 0 2 - 2 0 Trypanosomiasis

OH

Na OAs

(~}NH

2

OH 0 Tryparsamide

2.0

Trypanosomiasis

Carbarsone

.75

Amebiasis

Melarsoprol

.18

Trypanosomiasis

0

Na OAs ( )NH-CH -C-NH OH 0 0 II = , li NaOAs { >NH-C — N H OH 2

2

/

2

NH >~ 2

N

\ s — CH — C H 0 H

Pharmacotherapeutics A Manual o f Pharmacology

2

(5) (6)

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

4

ARSENICAL

PESTICIDES

T a b l e II Concentration

Substance

o f A r s e n i c i n Nature

Concentration ppm

Water Soil

1.0

- 500.0

Grass

.1

1.6

Ferns

.2

3.64

Vegetables

.00 -

2.9

Grains

.11

Corn o i l

.00

Fish

o i l (ocean)

.16

-

1.0

5.0

Fish

2.0

9.0

Shellfish

1.6

-

2.9

Shrimp,

1.5

-

100.0

lobster

Meat

.06

1.07

Milk

.01

.05

J . Chronic Diseases (7)

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.

PEOPLES

Review

5

use o f the r a t as an experimental animal i n a r s e n i c s t u d i e s , an animal which i s unique i n i t s a b i l i t y to s t o r e a r s e n i c i n i t s red blood c e l l s . Table 3 (9) shows the normal values i n the t i s s u e s o f common experimental animals and t h e i r response to the feeding o f a r s e n i c . I f man were i n c l u d e d i n the t a b l e he would have values s i m i l a r to those o f the r a b b i t with values o f a l l t i s s u e s v a r y i n g between .05 and .30 ppm. Milk i s e x c e p t i o n a l l y low i n a l l animals, being .01 - .03 ppm. H a i r and n a i l s normally have 1.0 - 5.0 ppm but due to the f a c t that they r e a d i l y p i c k up a r s e n i c from the environment, such values are o f u n c e r t a i n value i n r e f l e c t i n g p o i s o n i n g . Blood values are u s e f u l i n measuring the body burden and r a t e o f change i n the body as shown i F i g 1 (10) Th clearanc rate clearl i l l u s t r a t e the d i f f e r e n c e l y slow r a t e i n the r a t The route o f e x c r e t i o n i s i n the u r i n e and feces and the shape o f the e x c r e t i o n curve i n the r a t e as shown i n F i g . 2 (11), showing that there i s a f a s t phase followed by a slow one, i n d i c a t i n g that there are two storage p o o l s . The a r s e n i c l e v e l i n the milk o f cows does not increase with the blood concentration when fed i n o r g a n i c a r s e n i c , methyl a r s o n i c a c i d or c a c o d y l i c a c i d as i l l u s t r a t e d i n F i g . 3 (12). This may mean that there i s an a c t i v e t r a n s f e r o f a r s e n i c i n the mammary gland which i s saturated at a l l times. From a residue p o i n t o f view, the presence o f even t o x i c amounts o f a r s e n i c i n animal feed w i l l not e f f e c t the s a f e t y o f milk. The u r i n a r y e x c r e t i o n o f a r s e n i c i n u r i n e has been suggested as a measure o f exposure but i t i s of no value unless the d i e t i s known. Schrenk (13) found that the e a t i n g o f sea food, p a r t i c u l a r l y Crustacea, r a i s e d the u r i n a r y a r s e n i c l e v e l s by as much as 10-fold, r e t u r n i n g to normal i n 20 - 40 hours as shown i n F i g . 4. T h i s i s due t o the compound known as Coulsons "Shrimp A s e n i c " and i l l u s t r a t e s i t s r a p i d abs o r p t i o n and e x c r e t i o n . Metabolism of A r s e n i c The preceeding studies on the d i s t r i b u t i o n and e x c r e t i o n o f a r s e n i c are based on t o t a l a r s e n i c determinations and say nothing of the chemical changes that might occur. Gosio (14) noted i n 1893 that c e r t a i n molds growing on wallpaper c o n t a i n i n g a r s e n i c a l pigments produced a t o x i c gas which was i d e n t i f i e d by Challenger (15) i n 1931 as t r i m e t h y l a r s i n e and could a l s o be produced from sodium a r s e n i t e and c a c o d y l i c a c i d . Recently McBride (16) r e ported that a methanobacterium could methylate i n o r g a n i c asenic to form dimethyl a r s o n i c a c i d and dimethyl a r s i n e under anaeorbic c o n d i t i o n s . T h i s f i n d i n g suggested t o us that the methyl a r s o n i c a c i d we had found i n bovine u r i n e (17) need not e n t i r e l y come from grass which contains methylated a r s e n i c (3) but could have been produced by the anaerobic b a c t e r i a i n the rumen. Preliminary

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.0

0.0

Fed

Control

15.0

0.0

Control

Fed

1.0

0.0

20.0

0.0

Fed

Control

Fed

Control

Liver

* F o u r in each group.

Hamster

Rabbit

Pig

Guinea

Rat

Animal*

7.0

0.0

0.2

0.0

20.0

0.0

43.0

3.3

Heart

5.0

0.0

1.5

0.0

1.0

0.0

25.0

1.5

2.0

0.0

0.2

0.0

15.0

0.0

60.0

0.67

Kidney Spleen

0.7

0.0

0.2

0.0

0.8

0.0

12.0

0.6

Fat

2.5

1.8

0.2

0.0

2.0

0.0

3.0

0.7

Muscle

30.0

1.5

1.5

0.1

2.0

1.0

15.0

0.6

38.0

0.0

2.5

0.0

—

0.0

27.0

0.6

Skin

2.5

0.0

1.5

0.1

4.0

0.0

125.0

15.0

Blood

Ann. N. Y. Acad. Science (9)

1.0

0.0

0.0

0.0

0.0

0.0

3.8

0.5

Brain

Tissue concentration of arsenic ppm G.I. tract

D A Y S OF F E E D I N G A D I E T

CONTAINING 50 PPM ARSENIC TRIOXIDE

T H E ARSENIC CONCENTRATION IN TISSUES A F T E R 21

TABLE 3

1.

PEOPLES

Review

7

Concentration of arsenic per gram of blood at a particular time is expressed as per cent of the administered dose per gram of body weight (10). 76

University of California Publications in Pharmacology

TIME

IN

HOURS

Figure 2. Total fecal and urinary elimination of carrier-free As by rats (11) 74

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL

8

5

10

15

PESTICIDES

20

DAY OF FEEDING TRIAL - HOLSTEIN COW Figure 3.

2.o r

T i m e , H o u r s , After Eating American Industrial Hygiene Association Journal

Figure

4.

Concentration

of arsenic in urine following

the eating of

lobster

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.

PEOPLES

9

Review

experiments have shown that the feeding of e i t h e r sodium a r s i n a t e or sodium a r s i n i t e r e s u l t s i n a great increase i n methyl a r s o n i c acids i n the u r i n e , a f i n d i n g which seemed t o confirm t h i s hypothe s i s . To show that the rumen was necessary f o r t h i s methylation the same experiments have been repeated i n dogs, and i f confirmed by f u r t h e r experiments, the c a r n i v o r e can methylate i n o r g a n i c a r s e n i c as w e l l as the ruminant. Experiments are now under way to f i n d the r o l e , i f any, o f the rumen, and the biochemical mechanism of the methylation o f a r s e n i c i n the dog. The o r i g i n o f methyl a r s o n i c a c i d i n normal p l a n t s i s s t i l l to be explored, but one could p o s t u l a t e that i t could be o f animal o r i g i n . The work o f Braman (4) i n d i c a t e d that methylated a r s e n i c compounds are probably q u i t e common i n man and nature and only a small beginning has bee I t should be pointe causes a great r e d u c t i o n i n t o x i c i t y and i s a true d e t o x i c a t i o n process. Toxic E f f e c t s o f A r s e n i c The t o x i c i t y o f t r i v a l e n t i n o r g a n i c a r s e n i c f o r l i v i n g organisms v a r i e s widely as shown i n Table 4. Table 4 Item Bacteria Algae Yeast Daphnia

Arsenite 290

magna

Flatworms Minnows Minnows Minnows Rats and mice, 96 hr LD50 o r a l MLD, i n t r a p e r i t o n e a l

Arsenate > 10,000 > 1,000 300

5.2

12.5

40 20 17.8 11.6 11.2 5.8 J . Chronic

361 250 234 60 112 21 Diseases

(7)

The t o x i c dose f o r r a t s o f a r s e n i c a l h e r b i c i d e s i s shown i n Table 5, which are average values from a v a r i e t y o f sources. The mechanism o f t o x i c i t y i s considered to be due to the binding o f - SH groups o f l i p o i c a c i d by t r i v a l e n t a r s e n i c . However, pentavalent a r s e n i c compounds behave l i k e phosphate i n biochemical r e a c t i o n s yet symptoms o f poisoning are s i m i l a r f o r a l l a r s e n i c compounds. I t may be found that they can be reduced t o t r i v a l e n t a r s e n i c by mechanisms p r e s e n t l y unknown. The f a c t

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10

ARSENICAL

PESTICIDES

Table 5 T o x i c i t y o f A r s e n i c a l H e r b i c i d e s f o r Rats Compounds Sodium A r s e n i t e Sodium Arsenate Sodium Methyl Arsenate (MSMA) Sodium Cacodylate

T o x i c Dose mg/kg 20 80 1200

60 120 1600

1200

1600

that BAL, 2,3 dimercaptopropanol, i s used with some success i n a l l forms o f a r s e n i c p o i s o n i n Acute t o x i c i t y i s the usua ment o f acute e n t e r i t i , there i s extensive l i v e r and kidney damage as w e l l . The p o s s i b i l i t y o f c h r o n i c a r s e n i c p o i s o n i n g from continuous i n g e s t i o n o f small doses i s r a r e , due t o the d e t o x i c a t i o n and e x c r e t i o n being f a i r l y rapid. A r s i n e i s a gas formed i n the presence o f a c i d and hydrogen and i s t o x i c i n very low cencentrations i n i n h a l e d a i r . I t causes hemolysis o f the r e d c e l l s , r e s u l t i n g i n anemia, hemog l o b i n u r i a with r e s u l t i n g r e n a l damage. I t i s r a p i d l y f a t a l and there i s no known a n t i d o t e . As p r e v i o u s l y noted, McBride (16) has shown that dimethyl a r s i n e can be produced i n the b a c t e r i a found i n the mud o f waterways and suggested i t could cause t o x i c i t y . T h i s p o s s i b i l i t y seems remote since dimethyl a r s i n e bums spontaneously i n a i r . The q u e s t i o n o f a r s e n i c as a cause o f cancer has been s t u d i e d f o r a century without a c l e a r d e c i s i o n . When a r s e n i c t r i o x i d e i s a p p l i e d t o the s k i n o r taken o r a l l y i n l a r g e doses h y p e r k e r a t o t i c l e s i o n s appear on the s k i n with a delay o f many years i n some cases. These l e s i o n s are precancerous but they can a l s o be caused by s u n l i g h t , x-ray and thermal burns. Cancer has not been produced i n experimental animals by any a r s e n i c compound. The incidence o f cancer i n a r s e n i c i n d u s t r i e s i s e q u i v o c a l i s s p i t e of c o n t i n u i n g s t u d i e s (18). Conclusion The use o f new a n a l y t i c a l methods w i l l h o p e f u l l y e l u c i d a t e the r o l e o f a r s e n i c i n the ecosphere and may even demonstrate that i t i s e s s e n t i a l i n l i v i n g t i s s u e . The high l e v e l s i n sea animals which are constant i n value and not r e l a t e d t o i n d u s t r i a l o r a g r i c u l t u r a l p o l l u t i o n , suggest t h i s p o s s i b i l i t y . While much research needs t o be done, enough i s known about the u s e f u l and t o x i c e f f e c t s o f a r s e n i c compounds so that t h e i r use as p e s t i c i d e s and feed a d d i t i v e s can be c a r r i e d out with s a f e t y .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.

PEOPLES

Review

Literature Cited 1.

Vallee, B . L . , Ulmer, D.D., and Wacker, W.E.C. "Arch. of Industrial Health." Arsenic Toxicology and Biochemistry 21:132-151, (1960).

2.

Luh, M.D., Baker, R.A., and Henley, D.E. "Arsenic Analysis and Toxicity - A Review." Science of the Total Environ. 2:1-12, (1973).

3.

Lakso, J . U . , Peoples, S.A., and Bayer, D.E. "The Simultaneous Determinations of MSMA and Arsenic Acid i n Plants." Weed Science 21:166-169, (1973).

4.

Braham, R.S., and Arsenic in the Environment."

5.

S. Soles-Cohen and T.S. Githens, Pharmacotherapeutics, D. Appleton, New York, (1928).

6.

T. Sollman, A manual of Pharmacology, 7th E d . , W.B. Saunders, Philadelphia, (1948).

7.

Schroeder, H . A . , and Balassa, J.B. "Abnormal Trace Elements in Man:Arsenic." J . Chronic Diseases 19:85-106, (1966).

8.

Coulson, E . V . , Remington, R . E . , and Lynch, K.M. "Metabolism i n the Rat of the Naturally Occurring Arsenic of Shrimp as Compared with Arsenic Trioxide." J . Nutr. 10:255-270, (1935).

9.

Peoples, S.A. "Arsenic Toxicity i n C a t t l e . " Acad. Science 111:644-649, (1964).

10.

Ducoff, H.S., Neal, W.B., Straub, R . L . , Jackson, L . O . , and Brues, A.M. "Biological Studies with Arsenic II Excretion and Tissue Localization." Proc. Soc. Exper. Biol. Med. 69:548-554, (1948).

11.

Lanz, H . , Jr., Wallace, P.W., and Hamilton G. "The Metabolism of Arsenic in Laboratory Animals Using 74 As as a Tracer. Univ. of C a l i f . Pub. in Pharmacol. 2:263-282, (1950).

12.

Peoples, S.A., Unpublished.

Science 182:1247-1249, (1973).

Ann. N. Y.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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ARSENICAL PESTICIDES

13.

Schrenk, H.H., and Schreibeis, L., Jr. "Urinary Arsenic Levels as an Index of Industrial Exposure." Am. Ind. Hyg. Assoc. J. 19:225-228, (1958).

14.

Gosio, B.

15.

Challenger, F. Biological Methylation. 315-361, (1945).

16.

McBride, R.C., and Wolfe, R.S. "Biosynthesis of Dimethyl Arsine by Methanobacterium." Biochem. 10:4313-4317, (1971).

17.

Peoples, S.A., Lakso, J., and Lais, T. "The Simultaneous Determination of Methylarsonic Acid and Inorganic Arsenic in the Urine." Proc (1971).

18.

E. Browning, Toxicity of Industrial Metals, AppletonCentury-Crofts, New York, (1969).

Arch Ital. Biol.

18:253-298, (1893). Chem. Rev. 36:

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2

T h e Determination of Traces of Arsenic: A Review* YAIR TALMI and CYRUS FELDMAN Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830**

This review is organize characterizing a l l trac analysis sampl pretreatmen solution, solution stability, preconcentration, isolation and determination. The options now existing in each step have been presented, but no attempt is made to mention every investigation in which each has been used. It is hoped that this treatment will make i t easier for the analyst to assemble a procedure suited to the needs of a particular case. Pretreatment and Dissolution of the Sample If total arsenic is to be determined, the sample must be mineralized at least to the degree necessary to convert a l l arsenic present to inorganic forms. The treatment chosen should be the mildest treatment which will accomplish this conversion, so that losses and contamination will be minimized. Several approaches are possible:*** 1.Wet Ashing. According to Portmann and Riley1, prolonged digestion with nitric acid, followed by evaporation to dryness (hot-plate surface temperature ~180 ) will give no losses of arsenic, even i f several mg of chloride were originally present. This type of attack could be used for materials such as vegetation * Research supported by the National Science Foundation—RANN (Environmental Aspects of Trace Contaminants) Program under NSF interagency agreement No.389 with the U.S. Atomic Energy Commission. **Operated for the U.S. Atomic Energy Commission by Union Carbide Corporation under Contract No. W-7405-Cong.-26. *** A general and more detailed treatment of this subject is given in T.T. Gorsuch's book "The Destruction of Organic Matter", Pergamon, NYC (1970). º

13 In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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ARSENICAL PESTICIDES

and animal muscle. Stronger treatment i s u s u a l l y advocated f o r other organic m a t e r i a l s , e s p e c i a l l y those high i n l i p i d s ; however, Kingsley and S c h a f f e r t ^ s t a t e that a l l of the a r s e n i c present i n l i v e r , kidney o r muscle can be recovered merely by d i g e s t i n g the b l e n d e r i z e d t i s s u e f o r 30 minutes i n ^4N HC1. The d i g e s t i o n must be open to the atmosphere, so that s u l f i d e s and mercaptans (which may i n t e r f e r e w i t h subsequent determination of As) w i l l be e l i m i nated. S i m i l a r l y , R.F. Abernethy and F.H. Gibson** s t a t e that e s s e n t i a l l y 100% o f the a r s e n i c a l species i n c o a l can be e x t r a c t e d by b o i l i n g the powdered c o a l gently i n a 1:7 mixture of HNO3 and H 0. The HNO3 may then be e l i m i n a t e d , i f d e s i r e d , by adding 1:1 H2SO4 and evaporating the s o l u t i o n to fumes. According to R.B. B a i r d , S. Pourian and S.M. G a b r i e l i a n , raw sewage and primary and secondary e f f l u e n t s can be m i n e r a l i z e d by r e f l u x i n g with 4% HNO3 + 1% H2O2, followed by evaporatio Cacodylates sprayed on shaking w i t h a mixture o f strong H2SO4 and HC1 p l u s d e c o l o r i z i n g carbon. Hamme, Young and Hunter-* s t a t e that recovery by t h i s r a p i d method averages 93%. Other treatments have a l s o been advocated which do not necess a r i l y m i n e r a l i z e b i o l o g i c a l samples completely, but l i b e r a t e their metals to a degree s u f f i c i e n t f o r some types of a n a l y s i s . W. J . A d r i a n , f o l l o w i n g G.W. Gordon, l e f t the sample overnight, w i t h HNO3 and HCIO4, i n a t i g h t l y sealed polyethylene b o t t l e . The b o t t l e was then placed i n hot running water f o r 2-3 hours, cooled and opened. Rapid d i s s o l u t i o n of t i s s u e s was a l s o achieved by A. Bouc h a r d using coned. H2SO4, Cr03 and red fuming HNO3 ( p o s s i b l e contamination from reagents must be considered i n t h i s case). Many t i s s u e s can a l s o be transformed i n t o c l e a r s o l u t i o n s with the a i d of tetramethylammonium hydroxide, e i t h e r i n s o l i d o r aqueous s o l u t i o n form** or as a s o l u t i o n i n toluene^. Heating a t 60°C may be r e q u i r e d . Many o f these milder procedures do not a t t a c k f a t t y and h i g h - l i p i d t i s s u e s or bone and tooth specimens. When stronger treatment i s needed, some combination of n i t r i c , s u l f u r i c and p e r c h l o r i c acids i s u s u a l l y used. In a t y p i c a l p r o cedure of t h i s type, Chu, Barron, and Baumgarner^ a l t e r n a t e l y heat the sample i n a mixture of s u l f u r i c and n i t r i c acids u n t i l the s o l u t i o n darkens, then add more HNO3, and repeat the process u n t i l darkening no longer occurs; they complete the o x i d a t i o n by heating with HCIO4. S a n d e l l recommends the use of r e f l u x i n g with such procedures i n order to prevent the l o s s of AS2O3 and/or ASCI3. I f the a r s e n i c i s e v e n t u a l l y to be reduced to A s 3 f o r determination, the excess HNO3 remaining a f t e r wet-ashing can be destroyed by t r e a t i n g the mixture with a few ml o f saturated ammonium oxalate and warming. G.I. S p i e l h o l t z , G.C. T o r a l b a l l a , and R.J. S t e i n b e r g - ^ r a p i d l y (and s a f e l y ) m i n e r a l i z e d powdered c o a l by r e f l u x i n g i t w i t h a mixture of 68-70% HCIO4 and para-HI04. P. Schramel-^*-^ showed that reagent blanks i n the d i s s o l u t i o n of 150 mg t i s s u e samples can be minimized by warming the samp l e with 100 y l coned. H S0^ and adding 50% H 02 s o l u t i o n dropwise. 2

6

7

1 1

+

2

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2. TALMI AND FELDMAN

15

Traces of Arsenic

1

T h i s i s a refinement of W. Migault's "Caro's a c i d " procedure -*. * Dry Ashing. Various procedures i n v o l v i n g MgO/Mg(1*03)2 a d d i t i o n have been recommended f o r dry-ashing vegetable and animal t i s s u e s . One r e c e n t , widely a p p l i c a b l e procedure suggested by George, Frahn and McDonald ** i n v o l v e s p r e l i m i n a r y b l e n d e r i z i n g o f the t i s s u e , treatment with MgO and c e l l u l o s e powder and an i n i t i a l cautious c h a r r i n g i n a p o r c e l a i n c r u c i b l e . When c o o l , the crucible i s t r e a t e d w i t h Mg(N03)2 • 6H2O and placed i n a c o l d muffle f u r nace. The temperature o f the furnace i s slowly r a i s e d t o 555°, and kept there f o r 2 hours. Good recovery was obtained f o r 1-2 ppm spikes added to samples. R.F. Abernethy and F.H. Gibson** obtained q u a n t i t a t i v e recovery o f As from c o a l by i g n i t i n g i t w i t h MgO a t 650°C; the residue was d i s s o l v e d i n 7N R^SO,. 3. Oxygen Combustion The Schoniger combustion method * (Ignition o f a sample i bing s o l u t i o n ) was use They obtained q u a n t i t a t i v e recovery o f As on 0.5 g d r i e d specimens with a 750 ml f l a s k c o n t a i n i n g 0 and 5 ml o f 3N HC1. I g n i t i o n i n a metal bomb c o n t a i n i n g Oo was a l s o found s a t i s f a c t o r y by H.S. S a t e r l e e and G. B l o d g e t t * . According to C.E. G l e i t and W.D. H o l l a n d , b i o l o g i c a l t i s s u e s , as w e l l as other substances can be m i n e r a l i z e d a t a low temperature by using e l e c t r i c a l l y e x c i t e d oxygen. Complete recovery o f As i s obtained from blood t r e a t e d with HAs0 . 4. Fusion. Minerals are u s u a l l y fused with NaOH i n a s i l v e r or n i c k e l c r u c i b l e ; e.g., by H. O n i s h i and E.B. S a n d e l l . These authors s t a t e that l o s s e s of As are 0.5%; e s s e n t i a l l y a l l of the As i s recovered i n the l e a c h liquid*«even when a r e s i d u e i s p r e sent. J.A. James and D.H. Richards have a l s o a p p l i e d t h i s f u s i o n to the determination of As i n elemental S i . 2

1

1

7

l

g

2

2

2 1

2

2 2

S t a b i l i t y o f Sample S o l u t i o n s During

Storage

T h i s d i s c u s s i o n p e r t a i n s mainly to n a t u r a l water samples and standards, s i n c e the sample s o l u t i o n s produced by any o f the above methods are normally analyzed soon a f t e r p r e p a r a t i o n . To t e s t the s t a b i l i t y of As a t low concentrations i n sea water, J.E. Portmann and J.P. R i l e y f i l t e r e d each sample through a 0.5u M i l l i p o r e f i l t e r , and t r e a t e d the f i l t r a t e with 5 C i o f c a r r i e r - f r e e ^As. Samples i n soda-glass b o t t l e s l e v e l l e d o f f a t 16% l o s s a f t e r 16 days; samples i n polyethylene and b o r o s i l i c a t e g l a s s l e v e l l e d o f f a t 6% l o s s a f t e r 10 days. These authors advise storage o f such samples i n frozen form i n polyethylene c o n t a i n e r s . G.C. Whitnack and R.G. Brophy ^ made small a d d i t i o n s of Na3As03 s o l u t i o n t o well-water samples, and s t o r e d them i n 25 ml p o l y s t y rene v i a l s with polyethylene caps. No l o s s of A s ( I I I ) from these s o l u t i o n s was d e t e c t a b l e a f t e r one week. A.S. A l - S i b b a i and A.G. F o g g ^ measured the s t a b i l i t y o f s o l u t i o n s o f both As (III) and As (V) i n v a r i o u s containers a t the 4-20 ug/ml l e v e l . Samples of AS2O3 were d i s s o l v e d i n NaOH s o l u t i o n and the s o l u t i o n n e u t r a l i z e d . T h i s s o l u t i o n kept i t s f u l l t i t e r f o r 56 days i n b o r o s i l i c a t e glass, soda g l a s s and polyethylene c o n t a i n e r s , i n both l i g h t and dark 1

7

2

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

16

storage areas. S o l u t i o n s of NaHAsC>4 • 7H2O i n water d i d the same for 100 days under the same c o n d i t i o n s . R.S. Braman notes that very low concentrations of As species tend to disappear r a p i d l y from n a t u r a l water samples. In our l a b o r a t o r y , however, no l o s s e s of ^As(V) were experienced i n d i s t i l l e d water (polyethylene and s o f t g l a s s containers) 15% HNO3 or 5% HCIO4 s o l u t i o n (soft g l a s s containers) over a p e r i o d of 3 weeks. 26

2 7

7

Preconcentration and I s o l a t i o n of As Species The methods described i n t h i s s e c t i o n are o r i e n t e d toward the eventual determination of t o t a l (inorganic) As, r e g a r d l e s s of the o r i g i n a l molecular form i n which the As occurs. The p r i n c i p a l methods which have been used to preconcentrate As species are cop r e c i p i t a t i o n and adsorption, v o l a t i l i z a t i o n and l i q u i d - l i q u i d extraction. 1. C o p r e c i p i t a t i o Adsorption Fe(0H) for some time to be an e f f i c i e n t c o l l e c t o r of arsenate i o n . For example, R. P i e r u c c i n i c o l l e c t e d 100 yg of As(V) from 24 l i t e r s of water (= 4 ng/ml) by using 150 mg o f Fe(0H)3 (the As was then determined s p e c t r o g r a p h i c a l l y ) . J.E. Portmann and J.P. R i l e y , p r e c i p i t a t i n g Fe(0H)~ at pH 7, recovered 99% o f the 2 ug of As(V) present i n a l i t e r or water (2 ng/ml). T h i s procedure has a l s o been used to c o l l e c t As f o r determination by the G u t z e i t ^ and xanthate-^0 e x t r a c t i o n procedures. V . I . P l o t n i k o v and L.P.Usatova31 c a r r i e d out numerous c o p r e c i p i t a t i o n experiments i n 50 ml of s o l u t i o n . They found that at pH 7, As(V) i s q u a n t i t a t i v e l y c a r r i e d down by the hydroxides of Ce, Z r , In, Fe, T i and A l . The y i e l d decreases, e s p e c i a l l y f o r A l a t pH >8. In a l l cases, c o p r e c i p i t a t i o n was more e f f i c i e n t than the a d d i t i o n to the As(V) s o l u t i o n of a prepared hydroxide s l u r r y . For A s ( I I I ) , e f f i c i e n t (-95%) coprec i p i t a t i o n was obtained only with In and Zr hydroxides at pH 8.5. W. R e i c h e l and B. G. B l e a k l e y obtained complete recovery of 0.23.0 mg of As(V) (as w e l l as s i m i l a r amounts of Se, Te, Sb, Sn, B i , Pb and Fe) from 20g o f Cu by c o p r e c i p i t a t i n g with La(OH)3 a t pH 910. P.M. S a n t o l i q u i d o removed c a r r i e r - f r e e ^As(V) q u a n t i t a t i v e l y from a 7N HNO3 s o l u t i o n by passing i t through a 7 x 40 mm c o l umn of hydrated Mn02» The c a p a c i t y of t h i s column f o r As(V) i s greater than 272 yg As(V)/g MnOo. Z.G. H a n n a used Mg(0H) as a c o p r e c i p i t a n t : 0.3 yg As(V) i n 10 ml was treated with MgC^ + NB^Cl, and then with NH4OH. The p r e c i p i t a t e obtained was d r i e d , and i g n i t e d a t 600°C. No As was l o s t . 2 8

1

2

3 2

3 3

7c

34

2

T h i o n a l i d e i s s o l u b l e i n acetone, but i n s o l u b l e i n water. J.E. Portmann and J.P. R i l e y , making use of t h i s f a c t , added an acetone s o l u t i o n of t h i o n a l i d e to a 0.5N H2SO4 s o l u t i o n of 50 ng A s ( I I I ) + t r a c e r i n 1 l i t e r of sea water. The s o l u t i o n was s t i r r e d , b o i l e d 30 min. to remove acetone, and allowed to stand overnight. The p r e c i p i t a t e , c o n s i s t i n g almost e n t i r e l y of t h i o n a l i d e i t s e l f , was wet-ashed with concentrated HNO3. Recovery of As was 95%. Talmi, et a l . found that the high s a l t content of the sample sol u t i o n seems to be necessary to insure complete p r e c i p i t a t i o n ; 1

2 7

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TALMI AND FELDMAN

17

Traces of Arsenic

however, overnight storage i s unnecessary i f the p r e c i p i t a t i o n i s performed a t M)°C. Small amounts of A s ( I I I ) (reduced from As(V) i f necessary with KI) were c o l l e c t e d by a s u l f i d e procedure by V.V. Sergeeva, I.S. L e v i n , L . I . Tishchenko and V.S. Dankova ^. I n t e r f e r i n g elements were f i r s t removed by cupferron p r e c i p i t a t i o n and f i l t r a t i o n . Thioacetamide was added to the f i l t r a t e , and h y d r o l i z e d by heating the s o l u t i o n . The r e s u l t i n g AS2S3 p r e c i p i t a t e was c o l l e c t e d by c e n t r i f u g a t i o n ; the y i e l d was 85%. 3

3 6 , 3 7

2. L i q u i d - L i q u i d E x t r a c t i o n . A.K. K l e i n and F.A. V o r k e s were among the f i r s t to use xanthates to e x t r a c t A s ( I I I ) from food and biochemical t i s s u e s . In t h i s case, the f i r s t c o l l e c t i o n was done w i t h F e ^ H ) ^ ; the f i n a l determination, with arsenomolybdate. P.F. Wyatt 8,39 j diethylammoniu CHCI3 t o i s o l a t e As. Sinc s e v e r a l p o t e n t i a l l y i n t e r f e r i n g metals were removed i n i t i a l l y by f i r s t performing t h i s e x t r a c t i o n with As i n the pentavalent s t a t e . As was then reduced to A s ( I I I ) , and e x t r a c t e d from 1-10N ^ S O ^ s o lutions. Some p o t e n t i a l i n t e r f e r e n c e s not e l i m i n a t e d i n t h i s way [Cu, B i , S b ( I I I ) ] can be removed by a p r e l i m i n a r y e x t r a c t i o n w i t h cupferron. According to T.J. V e l e k e r ^ , t h i s reagent can be used to e x t r a c t A s ( I I I ) away from Ge, Sb and B i i n 6N HC1. H. M a l i s s a and E. Schoffmanual found that A s ( I I I ) could be p r e c i p i t a t e d by ammonium p y r r o l i d i n e dithiocarbamate (APDC), as w e l l as by other dithiocarbamates a t pH 2-6. C.E. M u l f o r d used a methyl i s o b u t y l ketone s o l u t i o n o f APDC to e x t r a c t A s ( I I I ) f o r determination by atomic absorption. V.V. Sergeeva, I.S. L e v i n , L . I . Tishchenko and V.S. D a n k o v a , using an 0.5N s o l u t i o n of d i - 2 - e t h y l h e x y l d i t h i o phosphoric a c i d i n decane and organic/aqueous volume r a t i o s of 1:1 to 1:30, e x t r a c t e d t r a c e s o f A s ( I I I ) from 0.5N a c i d aqueous s o l u t i o n s . The As was recovered from the organic phase by shaking with bromine water. A . I . Busev and M.I. I v a n i u t i n found that a s i m i l a r reagent, d i e t h y l d i t h i o p h o s p h o r i c a c i d , used s i m i l a r l y , e x t r a c ted A s ( I I I ) from e i t h e r weakly or s t r o n g l y a c i d i c aqueous solutions. Perhaps the simplest of the procedures f o r segregating As i s E. G a g l i a r d i and H.P. W o s s ' s ^ e x t r a c t i o n of A s C l from 6-7N HC1 i n t o a mixture of 2 volumes of CCI4 w i t h 3 volumes of 2-butanone, 2-pentanone or 2-heptanone. As with other e x t r a c t a n t s , p o t e n t i a l l y i n t e r f e r i n g elements can be e l i m i n a t e d by conducting a p r e l i m i n a r y e x t r a c t i o n w i t h the As i n the pentavalent s t a t e . According to A.R. B y r n e ^ , ASI3 can be e x t r a c t e d with toluene from a s o l u t i o n 12N i n H2SO4 and 0.05 M i n KI. I t can be s t r i p p e d from the toluene with 6N H S 0 + 0 . 0 5 M KI. 3

u s e (

4 2

35

4 3

3

2

4

3. V o l a t i l i z a t i o n . The p r i n c i p a l forms i n which As i s v o l a t i l i z e d f o r a n a l y t i c a l purposes are as a t r i h a l i d e and as a simple or s u b s t i t u t e d t r i h y d r i d e ( a r s i n e ) . A f t e r b i o l o g i c a l m a t e r i a l was decomposed with H S0^ + HNO3, E.B. S a n d e l l d i s t i l l e d AsBr^, using a special s t i l l . Some P and Sn were a l s o d i s t i l l e d . G.R. Kings ley and R.R. S c h a f f e r t noted that a r s e n i c compounds can be leached from t i s s u e homogenates w i t h 1+2 HC1, but that the operation must 1

2

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

18

ARSENICAL PESTICIDES

be performed i n the open, r a t h e r than under t o t a l r e f l u x i f arseni c i s then to be d i s t i l l e d o f f as A s C l ^ . This p r e c a u t i o n i s taken i n order to expel R^S and mercaptans which would otherwise tend to prevent the d i s t i l l a t i o n of ASCI3. Recent years have seen a r a p i d growth i n the use of a r s i n e e v o l u t i o n as a method of separ a t i n g As from i t s o r i g i n a l matrix. Most authors who have used Zn° i n a c i d to generate AsH^ (e.g. R.E. Madsen , F . J . Fernandez and D.C. M a n n i n g ) have f i r s t reduced the As to A s ( I I I ) w i t h Sn&2 and/or KI. E.N. P o l l o c k and S.J. W e s t used T i C l 3 f o r the p r e l i m i n a r y r e d u c t i o n and Mg° i n a c i d f o r generating AsH^. F.E. Lichte and R.K. Skogerboe ^, however, i n j e c t a small volume of a c i d samp l e s o l u t i o n i n t o a columm of granular Zn°; the sample i s forced through the column by a stream of argon. The AsH^ thus generated passes w i t h the argon i n t o the d e t e c t i o n system R.S Braman L.L. Justen and C.C. F o r e b a c k to ASH3. They f i n d tha A s ( I I I ) by c o n t r o l l i n g the pH of the sample s o l u t i o n : As(V) i s r e duced only i f pH ^1.5. The NaBH^ can be i n j e c t e d as a s o l u t i o n , or by dropping a p e l l e t of the s o l i d reagent i n t o the sample s o l u t i o n by means of an e x t e r n a l l y operated hopper as suggested by F.J. Fernandez . R.N. S a n d e l l , as w e l l as R.S. Braman, L.L. Justen and C.C. Foreback ^ caution that e a s i l y reduced ions such as Cu(II) may i n t e r f e r e w i t h the production of ASH3. A f t e r generation, the ASH3 (b.p.-55°C) may e i t h e r be passed d i r e c t l y i n t o the d e t e c t i o n device or accumulated i n a l i q u i d n i t r o g e n - c o o l e d trap and released to the d e t e c t o r over a very short p e r i o d of time by warming the trap (see below). 47

4 8

4

51

1 1

5

Methods of Determination 1. Molecular Absorption—Spectrophotometry. There are a few reagents which produce i n t e n s e - c o l o r d e r i v a t i v e s w i t h a r s e n i c . However, two of them, s i l v e r - d i e t h y l d i t h i o c a r b a m a t e (Ag-DDC) and ammonium molybdate are u n i v e r s a l l y accepted as most s u i t a b l e f o r spectrophotometric measurements. The Ag-DDC reagent i s u s u a l l y used i n c o n j u n c t i o n with the a r s i n e generation m e t h o d * . A r s i n e i s passed through 0.5% Ag-DDC s o l u t i o n i n p y r i d i n e and the i n t e n s i t y of the red c o l o r i s measured at 533 nm. Beer's law i s obeyed over the 1-20 yg As range and the l i m i t of d e t e c t i o n i s below 0.1 ppm. The arseno-molybdate complex i s considered more s u i t a b l e by many because of i t s s e n s i t i v i t y , r e l i a b i l i t y and general freedom from i n t e r f e r e n c e s . Arseno-molybdic a c i d , formed by the reaction of arsenate with a c i d i f i e d molybdate, i s reduced to the blue comp l e x , the a b s o r p t i o n of which i s measured s p e c t r o p h o t o m e t r i c a l l y . SnCl2 i s used by a few workers f o r r e d u c t i o n ^ " , but produces an unstable c o l o r . Others use hydrazine sulfate 9»60 but r e d u c t i o n i s slow. Portmann and R i l e y found that a s o l u t i o n 0.4N i n H2SO4 and 0.12% i n ammonium molybdate w i l l produce a very s t a b l e complex at room temperature w i t h i n 30 minutes. Absorption i s measured at 866 nm. No i n t e r f e r e n c e s were observed i n the a n a l y s i s of sea water, s i l i c a t e rocks or marine organisms. The a r s e n i c can be separated from the matrix v i a a r s i n e g e n e r a t i o n ^ > ^ , solvent ex52

5 3

5 4

5 5

5

5 8

5

1

1

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2.

TALMI AND FELDMAN

19

Traces of Arsenic

2 8

traction63,28,64,65 or c o p r e c i p i t a t i o n with f e r r i c h y d r o x i d e or t h i o n a l i d e ^ . Stara and Stary^b e x t r a c t A s ( I I I ) from s u l f u r i c a c i d s o l u t i o n s f o l l o w i n g conversion to A S I 3 . The compound develops an intense yellow c o l o r when treated w i t h 8-mercaptoquinoline. Procedures were developed to prevent i n t e r f e r e n c e s from the o x i d a t i o n of I to I or from the formation of i n s o l u b l e i o d i d e s . Mankova and Maksiraenko described a method based upon the r e d u c t i o n of AgNO-j to Ag° by AsH . The Ag° exerts a c a t a l y t i c e f f e c t on the f u r t h e r r e d u c t i o n (by F e ) of AgN0 . There i s a r e l a t i o n s h i p between the r e a c t i o n r a t e , measured p h o t o m e t r i c a l l y , and the concent r a t i o n of the c a t a l y s t and thus the c o n c e n t r a t i o n of a r s e n i c . The d e t e c t i o n l i m i t of the r e a c t i o n i s 20 pg/ml. A few workers have developed methods f o r the separate d e t e r mination of a r s e n i t e and arsenate species** »68,62 Spectrophotomeric methods have bee c l u d i n g u r i n e * ^ , blood and atmospheric d u s t . At the present time, spectrophotometry i s s t i l l the most widespread technique f o r the determination of arseni c , mainly because of i t s inherent methodical and t e c h n i c a l simp l i c i t y and i t s low c o s t . 2

67

3

+ 2

3

3

7 2

2.

Radiochemical

Techniques.

A. Among the v a r i o u s radiochemical techniques, neutron a c t i v a t i o n a n a l y s i s (NAA) i s unique i n i t s widespread a p p l i c a b i l i t y to the determination of a r s e n i c . Although, i n p r i n c i p l e , NAA i s a non-destructive a n a l y t i c a l technique, radiochemical s e p a r a t i o n schemes are almost always r e q u i r e d to avoid overlapping of v a r i o u s photo-peaks. Various such schemes "" are based on a combination of two or more s e p a r a t i o n techniques, such as d i s t i l l a t i o n , p r e c i p i t a t i o n , s o l v e n t e x t r a c t i o n or i o n exchange. With the advent of high r e s o l u t i o n s o l i d s t a t e d e t e c t o r s , the d i r e c t instrumental NAA approach has been attempted. Unfortunately, the high a c t i v i t y of N a induced i n many environmentally based samples prevents the determination of a r s e n i c at concentrations below a few ppm, s i n c e the y - a c t i v i t y of these samples w i l l have to decay f o r 4-5 days before measurement. The induced y - a c t i v i t y of i s measured by monit o r i n g the 559 KeV photopeak. T h i s peak w i l l appear, i n many samp l e s , as the middle peak of a t r i p l e t composed of B r ( t / = 35.3h), A s ( t y = 26.5h) and S b ( t , / = 67.2h) r e q u i r i n g the r e s o l u t i o n performance obtained by Ge(Li) d e t e c t o r s . A l s o , s i n c e A s has the s h o r t e s t h a l f - l i f e i n the t r i p l e t , the counting should be done as soon as p o s s i b l e a f t e r i r r a d i a t i o n . NAA i s one of the most s e n s i t i v e techniques with a d e t e c t i o n l i m i t of 0.1 ng using a thermal neutron f l u x of 1 0 ~ neut.-cnT-^-sec" . The method i s u s e f u l at the sub-ppm concentration l e v e l with sample s i z e s s u b s t a n t i a l l y smaller than those r e q u i r e d by the c o l o r i m e t r i c methods. Through the years, NAA has been s u c c e s s f u l l y a p p l i e d to a l a r g e v a r i e t y of samples; b i o l o g i c a l and marine samples' *" , p l a n t t i s s u e s * ^ , water s a m p l e s ^ ^ l ^ i - b o r n e p a r t i c u l a t e m a t t e r ^ , p e s t i c i d e d i s t r i b u t i o n ^ , heavy o i l s p i l l s ^ , g e o l o g i c a l samples >95,96 oal a s h , and many others. P r e c i s i o n and accuracy f o r these samples 73

7

2 4

8 2

1

7 6

2

1 2 2

x

2

2

7<3

12

1

8

87

8 8

8

2

a

3

r

4

75

s

3 1

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

C

ARSENICAL PESTICIDES

20 are g e n e r a l l y at the 3-10%

range

B. Substoichiometric i i i

isotope-dilution. •

In 1964, Q7

. . A

a

sub-

Qft yo

s t o i c h i o m e t r i c isotope d i l u t i o n technique was r e p o r t e d * ' > for the determination of traces of a r s e n i c f o l l o w i n g a c t i v a t i o n of the sample. The method o f f e r s a simple, r a p i d and rather s e l e c t i v e radiochemical separation of a r s e n i c . Detection l i m i t s were at the O . O l " and 0 . 0 4 ppm l e v e l . The method was s i m p l i f i e d to allow the determination of microgram amounts of a r s e n i c without sample p r e - i r r a d i a t i o n ^ . The a r s e n i c sample i s wet-ashed, radio-labelled with A s standard s o l u t i o n , complexed with l i m i t e d amounts of zincd i e thy ldithiocarbamate and extracted by CHCl^. Arsenic contents of organic and i n o r g a n i c compounds were determined with average recovery b e t t e r than 98.5%. However t h i method t b easil p l i e d to trace a n a l y s i l o g i c a l materials. 3. Electrochemical Techniques. 1 0 0

7 4

A. Amperometric t i t r a t i o n s . Various potentiometric methods are a v a i l a b l e f o r the determination of a r s e n i c with C e , Mn04~, I 0 " 101,102 or with D i c h l o r a m i n e - T . A r s e n i c i n organic m a t e r i a l s can a l s o be determined by potentiometric t i t r a t i o n " * . E r r o r s are t y p i c a l l y 1% and concentration ranges of 0.020-100 mg are t y p i c a l . The technique i s not u s e f u l f o r trace l e v e l s of arsenic + 4

103

3

1

4

1 0 5

10

B. Polarography. Arnold and Johnson ** have studied and reviewed the behavior of a r s e n i c i n various media and the mechanisms of the e l e c t r o d e r e a c t i o n s using DC and square wave polarography. They describe the determination of a r s e n i c at the 5% l e v e l i n i r o n ore. Copper was determined at the 0.02-1 mg l e v e l of a r senic with a r s d of i 3 % . Arsenic i n lead was determined at the 30-5000 ppm l e v e l , and i n z i n c at the 10-1000 ppm l e v e l with r s d of ±2%. In b i o l o g i c a l m a t e r i a l s , the p r a c t i c a l As l e v e l i s 1-1000 Ug. The d e t e c t i o n l i m i t of the technique i s thus g e n e r a l l y 1-10 ppm, but below 1 ppm accuracies and p r e c i s i o n s are much poorer. Whitnack and B r o p h y described a very s e n s i t i v e and r a p i d method f o r the determination of a r s e n i c i n d r i n k i n g water. The method u t i l i z e s a single-sweep polarographic technique with d e t e c t i o n l i m i t of 5 ppb and r s d of 5-10%. No s p e c i a l pretreatment of the sample i s necessary. However, the requirement of an o s c i l l o s c o p i c readout or a s o p h i s t i c a t e d data a c q u i s i t i o n system seems to have prevented the widespread use of the technique. In a d d i t i o n to DC and square wave polarography , a r s e n i c was a l s o determined by anodic s t r i p p i n g from a Pt e l e c t r o d e and by d i f f e r e n t i a l pulse polarography, DPP ^. More r e c e n t l y , Meyers and Osteryoung have f u r t h e r improved the a p p l i c a t i o n of DPP to a r s e n i c determination . The d e t e c t i o n l i m i t under optimized conditions i s 0.3 yg/1 and the response i s l i n e a r to 500 mg/1. The r s d i s 1.4% f o r 19.3 yg/1 and 16.4% f o r 1.93 yg/1. A procedure (applied a f t e r the a r s e n i c polarogram i s obtained) was devised to c o r r e c t f o r i n t e r f e r e n c e s from Pb, S n , T l , or T 1 which was s a t i s f a c t o r y down to 20 yg/1. No 23

1 0 8

10

110

I V

1

I V

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2.

TALMI AND FELDMAN

Traces of Arsenic

21

a p p l i c a t i o n s were d e s c r i b e d , although the method was claimed to have been a p p l i c a b l e to b i o l o g i c a l samples, e.g. u n f i l t e r e d a c t i vated sludge. Coulometry. The determination of a r s e n i c by coulometry was described by Bruckenstein and Johnson and by Simon, C h r i s t i a n and P u r d y who have a p p l i e d coulometric t i t r a t i o n to arsenic determination i n u r i n e . T h e i r method was l a t e r adopted by Woolson, Axley and Kearney** f o r the determination of a r s e n i c i n s o i l . It was found that v a r i o u s s a l t s present i n s o i l do not i n t e r f e r e with the a n a l y s i s . Comparison between c o l o r i m e t r i c and coulometric methods i n d i c a t e d that there i s no apparent d i f f e r e n c e between t h e i r accuracy, although the coulometric method was more p r e c i s e . 1 1 2

1

4. X-ray Fluorescence Spectroscopy The x-ray f l u o r e s c e n c e spectrometric method i simple p r e p a r a t i o n method technique has been used f o r the determination of a r s e n i c In rocks , organically rich s o i l s , r i v e r and sea sediments -*, air f i l t e r s , and many other complex m a t r i c e s . Although t h i s method has seldom been a p p l i e d to water a n a l y s i s because of i t s inadequate s e n s i t i v i t y , a few attempts have been made to improve i t s r e l a t i v e s e n s i t i v i t y by employing i s o l a t i o n and p r e c o n c e n t r a t i o n techniques. These t e c h n i q u e s ' " i n c l u d e evaporation, p r e c i p i t a t i o n , i o n exchange, and s o l v e n t e x t r a c t i o n . M a r c i e ' has developed a quant i t a t i v e microdetermination method f o r a r s e n i c , as w e l l as some other t r a c e elements, where the element r e a c t s with ammonium p y r r o l l i d i n e dithiocarbamate and i s e x t r a c t e d i n t o CHCI3. The CHCI3 e x t r a c t i s evaporated onto a f i l t e r paper d i s k and detected by x-ray f l u o r e s c e n c e down to 0.005 ppm, with p r e c i s i o n and accuracy of ±10%. Watanabe, Berman and R u s s e l l determined a r s e n i c i n water v i a i t s p r e c i p i t a t i o n , with a c a r r i e r ( N i ) , by d i e t h y l d i thiocarbamate and f i l t r a t i o n through a M i l l i p o r e f i l t e r . The p r e c i p i t a t e accumulated on the f i l t e r i s subjected to d i r e c t x-ray f l u o r e s c e n c e using the 48*78° (29) l i n e . Spectral interference was reported only from the L l i n e of l e a d . 1

1 1 4

1 1

11

1 1

1 2 4

1 1

1 2 4

+ 2

a l

5. Atomic Spectroscopy. Spectrometric determinations of a r s e n i c are troubled by s c a t t e r i n g and molecular absorption of the l i g h t (below 200 nm) by a i r , matrix species present i n the combust i o n c e l l and by o p t i c a l components of the instrument. Thus, improvement i n these techniques can g e n e r a l l y be accomplished by e i t h e r operating i n an i n e r t atmosphere (e.g. vacuum, N2, Ar, He), or using more e f f i c i e n t spectrometric e x c i t a t i o n sources such as e l e c t r o d e and e l e c t r o d e l e s s discharge plasmas. The f o l l o w i n g i s a s h o r t summary of v a r i o u s spectrometric sources a p p l i e d to the determination of a r s e n i c : 1 2 5

A. Atomic emission spectrometry. H a l l and L o r e l l determined the a r s e n i c content of c o a l samples, u t i l i z i n g the c a l enrichment process developed f o r the a n a l y s i s of m i n e r a l s The working c o n c e n t r a t i o n range was 25-400 ppm, which i s not adeJ

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

22

quate f o r most environmental samples, and accuracy and p r e c i s i o n were 5-10%, Flame emission techniques are not s e n s i t i v e enough e i t h e r , due to the inadequate e x c i t a t i o n energies provided by flames. On the other hand, plasma discharge spectrometric sources are much more e f f i c i e n t e x c i t a t i o n sources and are t h e r e f o r e more s e n s i t i v e . Dickinson and F a s s e l have s t u d i e d the d e t e c t i o n of a r s e n i c at i t s 228.8 nm s p e c t r a l l i n e with an RF plasma source. The system employs an u l t r a s o n i c n e b u l i z e r and d e s o l v a t i o n before the s o l u t e enters the plasma, and can detect a r s e n i c as low as 0.1 ppm. K i r k b r i g h t , Ward and W e s t ^ have a l s o s t u d i e d a r s e n i c emiss i o n w i t h an RF plasma, but s i m p l i f i e d the sample i n t r o d u c t i o n technique by u s i n g an i n d i r e c t n e b u l i z e r and expansion chamber which allowed the i n t r o d u c t i o n of aqueous samples. The o p t i c a l system was flushed with n i t r o g e n which made p o s s i b l e the monitoring of emission s p e c t r a S e n s i t i v i t y was i n the 0.1-0. p h y s i c a l matrix i n t e r f e r e n c e s were found. Simpler plasma sources were a l s o used i n conjunction with h i g h l y v o l a t i l e a r s e n i c compounds preseparated from any complex matrix. L i c h t e and Skogerboe ^ using 2450 MHz microwave plasma determined generated a r s i n e samples v i a t h e i r decomposition to f r e e a r s e n i c whose emission i n t e n s i t y was monitored. D e t e c t i o n l i m i t s b e t t e r than 1 ng and an r s d of ±5% were obtained with good accuracy. Braman, Justen and F o r e b a c k ^ have s i m i l a r l y detected a r s i n e i n a d-c helium discharge with comparable s e n s i t i v i t y . Following i t s conversion to t r i p h e n y l a r s i n e , a r s e n i c was q u a n t i t a t i v e l y determined with a d e t e c t i o n l i m i t of 15 pg, using the 2450 MHz microwave plasma i n t e r f a c e d to a g c . Thus, i t appears that plasma sources can serve as e f f i c i e n t u l t r a s e n s i t i v e d e t e c t o r s f o r a r s e n i c determination, using the atomic emission spectrometric mode. 1 2 8

12

4

0

2 7

B. Atomic absorption spectrometry. A r s e n i c has been determined by flame atomic absorption spectrometry i n argon-hydrogen * , a i r a c e t y l e n e " , and separated n i t r o u s o x i d e acetylene f l a m e s . The d e t e c t i o n l i m i t s reported are i n the range of 0.5-1 ppm at the 193.7 nm As l i n e . Extensive chemical i n t e r f e r e n c e s are observed from common ions when c o o l flames are used. The use of the separated N2O-C2H2 flame overcomes most of these i n t e r f e r e n c e s . Thus flames, although most convenient sources, only p a r t i a l l y f u l f i l l the requirements of an i d e a l atomizer, namely (1) t o t a l sample u t i l i z a t i o n , (2) e f f i c i e n t and reproducible atomization, (3) low degree of i o n i z a t i o n , (4) i n e r t atmosphere, (5) long residence time to prolong observation time, (6) capability of atomizing s o l i d s , l i q u i d s and gases, (7) f a s t r o u t i n e a n a l y s i s , (8) low cost and simple o p e r a t i o n . To i n c r e a s e the a p p l i c a b i l i t y range of a r s e n i c determination by AA, other atomizers are used which attempt to b e t t e r meet the above c r i t e r i a . Ando, Suzuki, Fuwa and V a l e e ^ s t u d i e d the a p p l i c a b i l i t y of the long tube (Vycor) absorption c e l l w i t h argon, helium or n i t r o g e n air-hydrogen flames. With t h i s system, a long o p t i c a l path was achieved with an almost i n e r t atmosphere. As a r e s u l t , s e n s i t i v i t i e s are improved while 1 3 0

1 3 1

1 3 2

1 3 4

1 3 5

1 3

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

2. TALMI AND FELDMAN

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absorption by a i r was d r a s t i c a l l y reduced. Although s e n s i t i v i t y was 6 ng/ml, the technique s u f f e r s from m u l t i p l e i n t e r f e r e n c e s ; a c i d s t r e n g t h and concentration of s a l t s and elements such as N i , Co, A l are important parameters, and hence, c a r e f u l c o n t r o l i s r e quired i n the p r e p a r a t i o n of both standards and sample s o l u t i o n s , Chu, Barron and Baumgarner-^ determined the evolved a r s i n e by sweeping i t i n t o an e l e c t r i c a l l y heated absorption tube with argon as c a r r i e r gas. Since no flame was employed, the background abs o r p t i o n was l e s s than that of the argon/hydrogen e n t r a i n e d a i r flame method, and the s e n s i t i v i t y was doubled. The r s d f o r a 0.4 yg a r s e n i c standard was 0.36% compared t o 4.8% f o r the argon-hydrogen f l a m e ^ . Gandrud and Skogerboe-*-^ demonstrated the p o t e n t i a l c a p a b i l i t y of a hollow cathode discharge as a means f o r v a p o r i z i n g and atomizing samples f o r AA measurement. A d e t e c t i o n l i m i t of 10 ng was obtained, r s d value nique, however, was no T o r a l b a l l a and S t e i n b e r g ^ compared the tantalum sampling boat and a p p l i c a t i o n flame AA techniques. The sampling boat r e q u i r e s only a f r a c t i o n of a ml sample, whereas the a s p i r a t i o n method r e q u i r e s a minimum of 10 ml. The s e n s i t i v i t y obtained with the boat, 0.1 yg, was a t l e a s t 5 times b e t t e r than with the a s p i r a t i o n . A r s e n i c i n c o a l and i n s e c t i c i d e s was determined by both methods. Among the h i g h l y p u b l i c i z e d AA techniques i n recent years are the g r a p h i t e and tantalum r e s i s t o r heated atomizers. These devices use very small samples (1-50 y l ) and g e n e r a l l y provide e x c e l l e n t s e n s i t i v i t i e s f o r many elements. I t i s r a r e l y , though, i n AA spectroscopy that one gets something f o r nothing, and i t w i l l be worthwhile t o pay a t t e n t i o n to some of the shortcomings c h a r a c t e r i s t i c of these d e v i c e s . F i r s t , the s p e c t a c u l a r absolute s e n s i t i v i t i e s obtained become a l i t t l e l e s s a t t r a c t i v e when they are expressed i n terms of r e l a t i v e s e n s i t i v i t y , s i n c e only 1-50 y l a l i q u o t s can be analyzed. Second, the s e n s i t i v i t y and r e p r o d u c i b i l i t y depend s t r o n g l y on the volume of the samples and the v a r i a t i o n i n t h e i r l o c a t i o n i n s i d e the atomization chamber. A l s o , the performance of the atomizer tube u s u a l l y d e t e r i o r a t e s continuously, e s p e c i a l l y when h i g h l y s a l t e d samples are analyzed. F i n a l l y , the very s m a l l volume to which the vaporized sample i s confined (which i s the main cause f o r the e x c e l l e n t s e n s i t i v i t i e s obtained) r e s u l t s i n very high n o n - s p e c i f i c s p e c t r a l i n t e r f e r e n c e s compared to flame. Nevertheless, these techniques, i f used c a u t i o u s l y and with thorough pretreatment of the sample, can be rather u s e f u l , as demonstrated by B a i r d , Pourian and G a b r i e l i a n * who a p p l i e d a carbon rod atomizer to the determination of a r s e n i c i n waste water. T h e i r procedure r e q u i r e s the d i g e s t i o n of the sample and i t s preconcent r a t i o n (8:1). A l s o , l a r g e q u a n t i t i e s of C I " had to be removed by p r e c i p i t a t i o n with A g and f i l t r a t i o n , and v a r i o u s c a t i o n s , by passage of the s o l u t i o n through a c a t i o n exchange column. R e l a t i v e s e n s i t i v i t i e s of 10 yg/1 were thus obtained with an r s d of 8%. Knudson and C h r i s t i a n ^ S compared the d e t e c t i o n of a r s i n e by an argon/air-H2 flame to that by a graphite r e s i s t a n c e furnace. +

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Detection l i m i t s w i t h the furnace were improved approximately by one order of magnitude. S e n s i t i v i t y was 1 ng and r e l a t i v e s e n s i t i v i t y 0.005 ppb. In t h i s a p p l i c a t i o n , the furnace i s used only as an atomizer, and no i n t e r f e r i n g matrix i s present. Chromatographic Methods 139 T o t a l A r s e n i c . Von Endt, Kearney and Kaufman separated a r s e n i t e , arsenate and monomethyl a r s o n i c a c i d (MMAA) by t h i n l a y er chromatography. McBride and W o l f e ^ i d e n t i f i e d MMAA by t h i n l a y e r e l e c t r o p h o r e s i s and chromatography with radioautographic detection. Sachs, Michael, Anastasia and Wells-*-^ separated a r s e n i t e , arsenate, MMAA at d i m e t h y l a r s i n i c a c i d (DMAA) by paper chromatography. The d e t e c t i o n o f inorgani a r s e n i b chromatograph (gc) v i a i t s transformation t has been studied by a fe , Kontoyannakos and P a r i s s a k i s have described the separation of a r s e n i c t r i - c h l o r i d e , and Juvet and F i s h e r that o f a r s e n i c t r i f l u o r i d e . Neither demonstrated any a p p l i c a t i o n o f t h e i r method. Butts and R a i n e y ^ ^ u t i l i z e d t h e i r gc-ms system f o r the separation and d e t e c t i o n of a r s e n i t e and arsenate, f o l l o w i n g t h e i r conversion to v o l a t i l e t r i m e t h y l s i l y l d e r i v a t i v e s . Schwedt and R U s s e l ^ * ^ determined a r s e n i c by gc f o l l o w i n g i t s conversion to t r i p h e n y l a r s i n e . The a n a l y t i c a l procedure involved combustion of the sample i n a Schoniger f l a s k and absorption of the a r s e n i c i n h y d r o c h l o r i c a c i d s o l u t i o n , e x t r a c t i o n o f the a r s e n i c as dithiocarbamate, and i t s phenylation with diphenylmagnesiun a f t e r solvent evaporation. The t r i p h e n y l a r s i n e formed was separated by gc with d e t e c t i o n l i m i t o f 0.4 ng and r e l a t i v e s e n s i t i v i t y ( i n sample) o f 2 ppm. ^ A m o d i f i c a t i o n o f t h i s method was developed i n our laboratory e l i m i n a t i n g the need f o r the tedious and u n c e r t a i n v a p o r i z a t i o n step, and extending the r e l a t i v e s e n s i t i v i t y to 30 ppb f o r s o l i d samples and 0*05 ppb f o r water samples. The a n a l y t i c a l procedure i n v o l v e s the wet HN03-HC10^ d i g e s t i o n of the sample, c o p r e c i p i t a t i o n of a r s e n i c with t h i o n a l i d e and f i l t r a t i o n on a 0.8 um M i l l i pore f i l t e r , the t r a n s f e r o f the p r e c i p i t a t e to a 20 ml v i a l , and i t s r e a c t i o n with 1.5 M phenylmagnesium bromide-ether s o l u t i o n f o l l o w i n g the d i s s o l u t i o n of the p r e c i p i t a t e i n benzene. The phenylation process i s complete a f t e r 40 minutes, and excess phenylmagnesium bromide i s destroyed with 2% t h i o g l y c o l i c acid-water s o l u t i o n . A l i q u o t s o f the organic l a y e r are i n j e c t e d i n t o the gc column, and the separated t r i p h e n y l a r s i n e i s detected with a microwave emission spectrometric detector (MES) ^»148-151 ± monitoring the emission i n t e n s i t y a t the a r s e n i c 228.8 nm s p e c t r a l l i n e . Sharp, w e l l resolved peaks are obtained with a r e t e n t i o n time i n the 70-80 second range. Due to the e x c e l l e n t spectroscopic s e l e c t i v i t y o f the detector, and s i n c e the plasma i s i g n i t e d a f t e r the solvent peak i s e l u t e d , only t r i p h e n y l a r s i n e i s detected and no cleanup procedures are required f o r the e x t r a c t . The gc column i s made of g r a p h i t i z e d carbon beads (produced i n our laboratory) coated with 1.5% FFAP. The c l o s e - t o - p e r f e c t 1

0

1

2

v

a

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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s p h e r i c a l shape of the beads and t h e i r i n e r t n e s s eliminated adsorpt i o n and t a i l i n g problems that were found f o r a r s e n i c compounds with many other columns. Using the above method, c o a l , f l y ash, orchard leaves,bovine l i v e r , sea and r i v e r water were analyzed with accuracy and p r e c i s i o n of 5-10%. Figures 1 and 2 show some t y p i c a l chromatograms obtained. Speciation-Organo-Arsenic. Determination o f monomethylarseni c a c i d (MMAA) and d i m e t h y l a r s i n i c a c i d (DMAA) i s u s u a l l y accomp l i s h e d through the a n a l y s i s o f t h e i r t o t a l a r s e n i c content. Among t o t a l a r s e n i c methods used f o r t h i s purpose are colorimetry-*- 2-155 ion exchange chromatography* , c o u l o m e t r y * * * ? , NAA* **, t i t r i m e t r y ^ , and d i s t i l l a t i o n ^ . Radioactive t r a c e r s have been used to study the t r a n s p o r t a t i o n , p e r s i s t e n c e and degradation of h e r b i cides in s o i l s ^ l * * * a n a l y t i c a l methods capabl mining the v a r i o u s organoarsenic compounds introduced i n t o or pro duced by the environment. Braman and Foreback*62 reduce a r s e n i c compounds to t h e i r c o r responding a r s i n e d e r i v a t i v e s with NaBIty, accumulate them i n a cold trap and s e l e c t i v e l y v o l a t i l i z e them i n t o a dc helium plasma where they are s p e c t r o m e t r i c a l l y detected down to 1 ng. The a r s i n e s are v o l a t i l i z e d i n the order of t h e i r b o i l i n g p o i n t s , thus achieving a readout f o r m a l l y analogous to a thermal v o l a t i l i z a t i o n curve. U s i n g ' t h i s method, A s ( I I I ) , As(V), MMAA and DMAA were q u a n t i t a t i v e l y determined i n water, u r i n e , b i r d e g g s h e l l s , s e a s h e l l s and l i m e stone a t sub-ppb concentrations. Cox and Alexander**^ accumulated a i r samples above s o l u t i o n s of methylating micro-organisms and separated the generated t r i m e t h y l a r s i n e by gas chromatography. Johnson, Gerhardt and Aue*^4 described two methods f o r the s e p a r a t i o n of MMAA by gc. The f i r s t involved the synthesis o f the t r i m e t h y l s i l y l a t e d e r i v a t i v e o f MMAA, which was h i g h l y v o l a t i l e and chromatographed w e l l . Unfortunately, the method was n o n s e l e c t i v e , s i n c e s e v e r a l a r s e n i c compounds, i n c l u d i n g AS2O3, have y i e l d e d the same product. In the second method, MMAA r e a c t s w i t h ethylene g l y c o l to form a s e l e c t i v e d e r i v a t i v e which chromatographs w e l l . The der i v a t i v e was detected e i t h e r by a microwave plasma detector (1.5ng, d e t e c t i o n l i m i t ) or by an a l k a l i flame i o n i z a t i o n detector (1 ng, detection l i m i t ) . Lodmell reacted MMAA and DMAA w i t h a l l y l t h i o 1 6 7 i aqueous s o l u t i o n a t 70°C f o r 5 minutes. A l i q u o t s were separated by gc and determined simultaneously by flame i o n i z a t i o n detector (FID) and a N2O/C2H2 flame spectrometric detector (FSD). Although the FSD i s h i g h l y s u p e r i o r i n s e l e c t i v i t y (Figure 3), i t s s e n s i t i v i t y i s i n f e r i o r to that o f the FID. We b e l i e v e that the microwave emission spectrometric detector (MES) w i l l f u r n i s h both the s e l e c t i v i t y and s e n s i t i v i t y required f o r t h i s a n a l y s i s . The gc-MES system described e a r l i e r ^ ? has a l s o been a p p l i e d to the determination of v a r i o u s a r s e n i c a c i d s ^ . Although the study i s i n complete, the p r e l i m i n a r y r e s u l t s are very promising. The method i s based on the r e d u c t i o n of the a c i d s with NaBH^ to t h e i r c o r r e sponding a r s i n e s and t h e i r separation by a gc column. The a r s i n e s 5

55

15

56

5

5

0

1

2

1 6 5

1 6 6

u r e a

n

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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SOLUTION III III

Figure 1. Chromatogram of triphenylarsine benzene solutions. I: 0.1 fig/ml; II: 0.2 fig/ml; III: 0.3 fig/ml (all injections are 3 fd).

AS -SPIKED C O A L (NBS) + 5

As -SPIKED F L Y A S H (NBS) + 5

FLY ASH (NBS)

ORCHARD LEAVES (NBS)

COAL (NBS)

[

[

T I M E , » — i 2 minutes

Figure 2. Determination of Total Arsenic as Triphenylarsine in Fly Ash, Orchard Leaves, Coal and Sb-Spiked Sea Water

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Figure 3. Chromatogram of Cacodylic Acid and Monomethylarsonic Acid (MA A) Ally thiourea derivatives

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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28

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generated are extracted i n t o a benzene l a y e r which i s i n d i r e c t contact with the aqueous r e a c t i o n s o l u t i o n . In the f u t u r e , the a r s i n e s w i l l be accumulated on a s o l i d adsorbant or a c o l d trap p r i o r to t h e i r i n t r o d u c t i o n i n t o the column. The column i s a 6% carbowax 20 M on 60/80 mesh chromosorb 101. As with t r i p h e n y l a r s i n e , the a r s i n e s are q u a n t i t a t i v e l y detected v i a monitoring the a r s e n i c emission i n t e n s i t y i n the microwave plasma. To ensure the p o s i t i v e i d e n t i f i c a t i o n of v a r i o u s peaks i n the chromatogram, the column was a l s o i n t e r f a c e d to an organic mass spectrometer. Figure 4 shows the chromatogram obtained from a reduced mixture of a r senous oxide, monomethylarsonic, d i m e t h y l a r s i n i c , e t h y l a r s o n i c and p r o p y l a r s o n i c a c i d s . A l l a c i d s were reduced to the expected c o r responding a r s i n e s , i n a d d i t i o n to small amounts of AsHg, probably due to AS2O3 i m p u r i t i e s i n the reagent. The reduction of cacodylic a c i d was l e s s s t r a i g h t f o r w a r d (CH3>2AsH was formed; a (CH3>AsH2, (CH^^AsH and (CH^^As were a l l shown i n the chromatogram. Under these c o n d i t i o n s , there seems to be some ligand-exchange s i d e - r e a c t i o n s . The p o t e n t i a l advantages of t h i s method over the s e l e c t i v e v o l a t i l i z a t i o n scheme^ are the f o l l o w i n g : 1. gc s e p a r a t i o n i s l e s s ambiguous, b e t t e r c o n t r o l l e d and more r e p r o d u c i b l e than a thermal v o l a t i l i z a t i o n procedure. 2. Sharp, w e l l resolved peaks are obtained and thus peak h e i g h t s , r a t h e r than i n t e g r a t e d areas are used f o r measurements. 3. The MES detector i s more s e n s i t i v e , d e t e c t i o n l i m i t approximately 30 pg, and much more r e p r o d u c i b l e than the dc d i s charge . 4. In c o n t r a s t to the v o l a t i l i z a t i o n - d c plasma system, where severe d e p o s i t i o n problems may occur i n the spectrometric c e l l , no such problems occur with the gc-MES system. Figure 5 shows the chromatograms obtained f o r 0.1 ppm benzene s o l u t i o n s of (CH3>3As. The l i n e a r response of the c a l i b r a t i o n curve was observed f o r at l e a s t the 0.1-10 ng range.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Traces of Arsenic

O.i ^g/ml (CH ) As 3

BENZENE

3

0.6 ng

SOLUTION

0.5 ng

0.4 ng -Q w

t CO

Ld

> t

CO

z

0.3 ng

CO CO

Ld

z o CO CO

TIMEFigure

4.

Chromatogram arsines

TIME of alkyl-

( 20 seconds )

Figure 5. Chromatograms of trimethylarsine benzene solution

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

30

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Ratliff, C.R., Hall, F.F. and Culp, T.W., Clin. Chem. (1972) 18, 717. Morozova, S.F. and Mezhueva, N.S., Gidroliz. Lesokhim. Prom. (1972), 7, 9. Lahrman, E . , Nietruch, F. and Drews, M., Gesundh. Ing. (1973) 94, 258. Smales, A.A., Mapper, D. and Fouché, K.F., Rep. U.K. Atom. Energy Auth., AERE R-5254, 1967. Kiesl, W., "Modern Trends in Activation Analysis," NBS special Publications, Dept. of Commerce (1969), No.312, Vol.I, 302. Morrison, G.H., Gerard, J.T., Travesi, A . , Cunie, R.L., Peterson, S.F. and Potter, N.M., Anal. Chem. (1969), 41, 1633. Laul, J.C., Wechten, D.R., Schmidt-Bleck, F. and Lipschutz, M.E., J. Radioanal Steinnes, E . , "Activation Analysis in Geochemistry and Cosmochemistry", 113, A.D. Brunfett and E. Steinnes, Universitetsforlagt, Oslo (1971). Heydron, K. and Damsgaard, E . , Talanta (1973), 20, 1. Lunde, G., Acta Chim. Scand. (1973) 15, 377. Lunde, G., J. Amer. Oil Chem. Soc. (1972) 49(1), 44. Kroon, J.J. and Oas, A.A., Reactor Centrum Nederland, Petten (RCN-124) Aug. 1970. Robertson, D.E. and Carpenter, R., Report D8RPT7, 1972. Eckhoff, N.D., Clark, R.W., Anthony, H.D. and Gray, A.P., Toxicol. Appl. Pharmacol. (1973) 24(2), 178. Lacroix, J.P., Steinier, J. and Dreze, P., Anal.Lett. (1973) 6(6), 565. Lee, C.L., Kim, N.B., and Park, E . , J. Korean Nuclear Soc. (1973) 5, 279. Lunde, G., J. Sci. Fd. Agric. (1968) 19, 432. Becker, D.A., LaFleur, P.D., Proceedings 5th Univ. Missouri Conf. on Trace Subst. in Environm. Health, Univ. Missouri, Columbia, Missouri, 1971, p.559. Takeo, T. and Shibuya, M., J. Food Sci. Technol. (1972) 19(2), 91. Maruyama, Y. and Komiya, K., Radioisotope (1973) 5, 279. Ray, B.J. and Johnson, D.L., Anal. Chim. Acta (1972) 62(1), 196. Gohda, S., Bull, Chem. Soc. Japan (1972) 45(6), 1704. Geisman, J.R., Carey, W.E., Gould, W.A. and Alban, E.K., J. Food Sci. (1969) 34(3), 295. Tanner, J.T., Friedman, M.H. and Holloway, G.E., Anal.Chim. Acta (1973) 66(3), 456. Guinn, V.P., Bellanca, S.C., U.S. Natl. Bur. Stds. Special Publ. No. 312(1), (1969), 93. Steinnes, E . , Analyst (1972) 97, 241. Onishi, H . , "Handbook of Geochemistry," pp.33-E-5 and 51-E-3, K.H. Wedepohl, Springer Verlag, Berlin, 1969. Zeman, A . , Ružička, J., Starý, J. and Klečková, E . , Talanta (1964) 11, 1143.

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98. Ružička, J. and Starý, J., Inst. Nucl. Res., Report 992/63, Rez., 1963. 99. Arnold, A . , Davis, S. and Jordan, A . L . , Analyst (1969) 94, 664. 100. Zeman, A . , Stary, J. and Kratzer, K., Radiochem. Radioanal. Letters (1970) 4, 1. 101. Akhmedov. G., Zhdanov, A.K., Uzbekh. Khim. Zhur.(1971) 2, 9. 102. Issa, I.M., Hamdy, M.H. and Misbah, A.S., Mikrochem. J. (1972) 17, 480. 103. Jacob, T.J. and Nair, C.G.R., Talanta (1972) 19, 347. 104. Griepink, B.F.A. and Krijgsmann, W., Mikrochim.Acta (1968) 3, 574. 105. Sandhu, S.J., Pahil, S.S. and Sharma, K.D., Talanta (1973) 20, 329. 106. Arnold, J.P. and Johnson, R.M., Talanta (1969) 16, 1191. 107. Hagiwara, K. and Murase 108. Trushina, L.F. and Kaplin, A.A., Zhur.Anal.Khim. (1970) 25, 1616. 109. Melabs Scientific Instruments, Palo Alto, Calif. Application Note 2A. 110. Meyers, D.J. and Osteryoung, J., Anal. Chem.(1973) 45, 267. 111. Bruckenstein, S. and Johnson, D.C., Anal.Chem. (1964) 36, 2186. 112. Simon, R.K., Christian, G.D., and Purdy, W.C., Amer. J. Clin. Path., (1968), 49, 207. 113. Fabbi, B.P. and Espos, L . F . , U.S. Geol. Surv. Prof. Pap. No. 800-B, P.147 (1972). 114. Leake, R.C. and Peachey, D., Inst. Mining Met. Trans., Sect.B (1973) 82, 25. 115. Tanaka, H . , Moriguchi, Y . , Yamamoto T. and Hashizume, G., Bunseki Kagaku (1972) 21, 1456. 116. Cate, J . L . , Univ. Calif. Radiation Lab. Report UCRL 51038 (Rev. 1) Nov. 15, 1971. 117. Marcie, F.J., Envir. Sci. Tech. (1967) 1, 164. 118. Morris, A.W., Anal. Chim. Acta (1968) 42, 397. 119. Haberer, K., Vom. Wasser (1965) 17, 128. 120. Natelson, S., Leighton, D.R. and Galas, O., Mikrochem. J. (1962) 6, 539. 121. Tackett, S.L. and Brocious, M.A., Anal. Lett. (1969) 2, 649. 122. Riley, J.P. and Taylor, D., Anal.Chim.Acta (1968) 40, 479. 123. Watanabe, H . , Berman, S. and Russel, D.S., Talanta (1972) 19, 1363. 124. Taylor, H. and Beamish, F . E . , Talanta (1968) 15, 1497. 125. Hall, R.H. and Lovell, H.L. Anal. Chem. (1958) 30, 1665. 126. Shaw, D.M. Jvensu, O.L., and Ahrens, L.H., Spectrochim. Acta (1950) 4, 233. 127. Wedepohl, K.H., Geochim, et Cosmochim. Acta (1953) 3, 93. 128. Dickinson, G.W. and Fassel, V.A., Anal.Chem. (1969) 41, 21. 129. Kirkbright, G.F., Ward, A.F. and West, T.S., Anal. Chim. Acta (1973) 64, 353. 130. Kahn, H.J. and Schallis, J.E., Atom.Absorption Newsletter (1968) 7, 5. 131. Johns, P., Spectrovision (1970) 24, 6.

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Allen, J . E . , Spectrochim. Acta (1962) 18, 259. Slavin, W. and Sprague, S., Atom.Abs.Newslett. (1964) 18, 1. Chakrabarti, C.L., Anal.Chim.Acta (1968) 42, 379. Kirkbright, G.F. and Ranson, L . , Anal.Chem. (1971) 43, 1238. Ando, A . , Suzuki, M., Fuwa, K. and Valee, B.L., Anal. Chem. (1969) 41, 1974. Gandrud, B.W. and Skogerboe, R.K., Appl. Spec. (1971) 25, 243. Knudson, E.J. and Christian, G.D., Anal.Lett (1973), 6(12), 1039. Von Endt, D.W., Kearney, P.C., and Kaufman, A.A., Agricultural and Food Chem. (1968) 16, 17. McBride, B.C., Wolfe, R.S., Biochemistry (1971) 10, 4312. Sachs, R.M., Michael, J.L., Anastasia, F.B. and Wells, R.M., Weed Sci. (1971) 19, 412. Tadmor, J., J. Gas Chromatog. (1964) 2, 385. Vranti-Piscou, D. J. Chromatogr. Sci Juvet, R.S.Jr. and Fisher, R.L., Anal.Chem. (1965) 37, 1752. Butts, W.C. and Rainey, W.T. Jr., Anal.Chem. (1971) 43, 1538. Schwedt, G. and Russel, H.A., Chromatographia (1972) 5, 242. Schwedt, G. and Russel, H.A., Z.Anal.Chem. (1973) 264, 301. McCormack, A.J., Tong, S.C. and Cooke, W.D., Anal.Chem. (1965) 37, 1470. Bache, C.A. and Lisk, D.J., Anal. Chem. (1966) 38, 1757. Bache, C.A. and Lisk, D.J., Anal. Chem. (1971) 43, 1950. Talmi, Y . , In press, Anal. Chim. Acta. Chaney, A.L. and Magnuseon, H.J., Ind. Eng. Chem., Anal.Edn. (1940) 12, 691. Saito, J., Nagaku Seisan Gigutsu (1962) 7, 8. Small, H.G. Jr. and McCants, C.B., Soil Sci. Soc. Amer. Proc. (1961) 25, 1346. Bogliano, G., Grassini, G., Ledevy, M. and Ossincini, L . , J. Chromatog. (1964) 14, 238. Bruckenstein, S., Johnson, D.C., Anal. Chem. (1964) 36, 2186. Simon, R.K., Christian, G.D., Pardy, W.C., Am.J.Clin.Path. (1968) 49, 207. Lunde, G., Int. Rev. Gesamten Hydrobiologie (1967) 52, 265. Bassett, J., Analyst (1963) 88, 238. Stone, R.L., J. Assoc. Off. Anal. Chem. (1967) 50, 1361. Woolson,E.A. and Kearney, P.C., Environ.Sci.& Tech. (1973) 7(1), 47. Braman, R.S. and Foreback, C.C., Science (1973) 182, 1247. Cox, D.P. and Alexander, M., Bull. Envir. Contam. and Toxicol. (1973) 9(2) 84. Johnson, L.D., Gerhardt, K.L. and Aue, W.A., Sci. Total Environ. (1972) 1, 108. Salmi, E.J., Merivuori and Laaksonen, E . , Suom. Kemistilehti (1946) B19, 102. Lodmell, J.D., Ph.D. Dissertation, Univ. of Tenn. 1973. Raiziss, G.W., and Gavron, J.L., "Organic Arsenical Compounds" Chemical Catalog Co., p.81 (1923).

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

3 Behavior and Phytotoxicity of Inorganic Arsenicals in Soils L. M. WALSH and D. R. KEENEY Department of Soil Science, University of Wisconsin, Madison, Wisc. 53706 Introduction Inorganic arsenicals have been used in agriculture as a pesticide or plant defoliant for many years. In localized areas oxides also contaminate soils by fallout from smelting ore, especially sulfide ores, and from burning coal. Past use of arsenicals has not resulted in serious problems with human or animal poisoning but severely reduced plant growth due to As phytotoxicity has been observed, especially on former orchards and cotton fields. Once toxicity is observed, i t persists for several years even though no additional As is added. Most attempts to ameliorate As toxicity by the addition of salts which might reduce the level of available As in soils have not been very successful. Historically, inorganic arsenicals (lead arsenate, calcium arsenate) were used as insecticides beginning in the latter part of the 17th century (1). Paris Green (which contains cuprous arsenite) was successfully used to control the Colorado Potato Beetle in eastern United States by 1867 (2). Lead arsenate has been used for insect control on a variety of fruit trees and, for many years, was the only insecticide that would successfully control codling moth in apple orchards and horn worm on tobacco. Calcium arsenates have been applied to cotton and tobacco fields to protect these crops from boll weevils, beetles and other insects. Arsenic trioxide has been widely used as a soil sterilant (3) and sodium arsenite has been used for aquatic weed control (4) and as a defoliant to kill potato vines prior to tuber harvest (5). In recent years use of inorganic arsenicals has decreased markedly. Sodium arsenite has been banned and can no longer be used as a vine-killer or defoliant (6). Organic arsenicals have largely replaced inorganic As salts as herbicides and since they are applied at a lower rate of application, this has reduced the amount of As applied to the soil. Also, much lower quantities of lead arsenate are now used in orchards because fruit growers rely primarily on carbamates and organic phosphates to control 35

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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36

i n s e c t s . However, where long-term r e s i d u a l i n s e c t i c i d a l a c t i v i t y i s needed t o p r o t e c t orchards from chewing i n s e c t s , l e a d arsenate i s s t i l l being used t o a l i m i t e d extent by some growers. A l s o , i n o r g a n i c a r s e n i c a l s continue t o be used on g o l f greens and f a i r ways i n some areas o f the United States t o c o n t r o l annual b l u e grass. The primary o b j e c t i v e s o f t h i s paper are: (a) t o review the r e l a t i o n s h i p between a r s e n i c s o r p t i o n and s o i l p r o p e r t i e s and the p o s s i b l e mechanisms o f As s o r p t i o n i n s o i l s , (b) t o review v a r i ous s o i l t e s t methods i n terms o f t h e i r a b i l i t y t o p r e d i c t As uptake and p h y t o t o x i c i t y , ( c ) t o review r e l a t i o n s h i p s between s o i l As and p l a n t uptake which r e s u l t i n p h y t o t o x i c i t y and p o t e n t i a l hazards t o humans or animals consuming p l a n t s grown on s o i l s cont a i n i n g high l e v e l s o f As d (d) t revie method f amelio r a t i n g As t o x i c i t y . Occurrence and D i s t r i b u t i o n o f As i n S o i l s A r s e n i c i s ubiquitous, i n nature and i s found i n d e t e c t a b l e amounts i n n e a r l y a l l s o i l s and i n many rocks and minerals. Surveys o f s o i l s i n the United States show t h a t As l e v e l s range from 0.2 t o 40 ppm f o r uncontaminated s o i l s , and up t o 550 ppm f o r Ast r e a t e d s o i l s (7., 8.)· Comparisons o f As-treated vs. untreated s o i l s were made by s e v e r a l r e s e a r c h e r s and are r e p o r t e d i n Table 1. These data i n d i c a t e t h a t untreated s o i l s seldom c o n t a i n more than 10 ppm As. In f a c t , F l e i s c h e r (9) r e p o r t s t h a t the average As content o f untreated s o i l s i s only 5 ppm. Even though s o i l s can vary g r e a t l y i n t h e i r n a t i v e As content, the l e v e l s o f As i n noncontaminated s o i l s g e n e r a l l y have not been high enough t o cause p h y t o t o x i c i t y and they do not represent a s i g n i f i c a n t h e a l t h hazard (10). The As content o f most rocks and minerals i s s i m i l a r to t h a t found i n s o i l s , except f o r s u l f i d e o r e s , sediment a r y Fe and Mn o r e s , and rock phosphate which o c c a s i o n a l l y cont a i n as much as 2000 ppm As ( 9 ) . Coal u s u a l l y contains about 25 ppm o f As (11). Even though s o i l s and s o i l parent m a t e r i a l s vary c o n s i d e r a b l y i n t h e i r n a t i v e content o f As, no c l e a r l y d e f i n e d r e l a t i o n s h i p has been noted between the As content o f s o i l s and the c l i m a t i c c o n d i t i o n s or g e o l o g i c a l formations i n which the s o i l s were developed. Under f i e l d c o n d i t i o n s high l e v e l s o f t o t a l s o i l As have been observed more o f t e n where crops have been t r e a t e d with an a r s e n i c a l i n s e c t i c i d e as compared to a d e f o l i a n t . T h i s i s due to the d i f f e r e n c e i n the t o t a l amount of As a p p l i e d . For i n s t a n c e , s o i l a n a l y s i s i n d i c a t e s t h a t some orchards have r e c e i v e d over 1000 kg As/ha (12) as compared t o about 10 kg As/ha where sodium a r s e n i t e was used as a potato v i n e d e f o l i a n t (13). Fate o f A r s e n i c i n S o i l s A r s e n i c Chemistry. While As chemistry i s i n many ways s i m i -

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Inorganic Arsenicals in Soil

37

l a r t o the other Group V elements, i t i s more m e t a l l i c ( i . e . more l a b i l e i n behavior) than P, which i s e s s e n t i a l l y c o v a l e n t . In Table 1. A comparison o f As l e v e l s i n A s - t r e a t e d and uncontaminated s o i l s i n North America Sampling site

T o t a l As Content Uncontaminated Treated soil soil* ppm As 1.3-2.3 Colorado 13-69 8 Florida 18-28 Idaho 0-10 138-204 Indiana 2-4 56-250 Maine 9 19-41 Maryland New Jersey 10.0 92-270 New York 3-12 90-625 North C a r o l i n a 4 1-5 10-124 Nova S c o t i a 0-7.9 Ontario 1.1-8.6 10-121 Oregon 2.9-14.0 17-439 3-32 4-103 6-13 Washington 106-830 106-2553 8-80 4-13 48 Wisconsin 2.2 6-26

Crop

Source o f data

orchard potato orchard orchard

(61) (12) (12) (12)

orchard orchard tobacco orchard orchard orchard orchard orchard orchard orchard potato

(12) (12) (30) (28) (62) (50) (12) (34) (12) (44) (54)

* These are r e s u l t s from s o i l s t h a t had been r e p e a t e d l y t r e a t e d with an As p e s t i c i d e o r d e f o l i a n t . S o i l s treated experimentally are not i n c l u d e d . s t r o n g l y reducing environments, elemental As and a r s i n e (-3) can e x i s t , but arsenate (+5) i s the s t a b l e o x i d a t i o n s t a t e i n oxygenated environments. Under moderately reduced c o n d i t i o n s such' as f l o o d e d s o i l s , a r s e n i t e (+3) may be the dominant form (14). E v i dence e x i s t s (15) t h a t d i m e t h y l a r s i n i c a c i d may be an important and u b i q u i t o u s ~ s compound which has not h e r e t o f o r e been i d e n t i f i e d due t o l a c k o f methods f o r d e t e c t i o n and q u a n t i f i c a t i o n . B i o l o g i c a l Transformations. The -3, +3, and +5 valence s t a t e s can form compounds c o n t a i n i n g the C-As bond and a r e r e a d i l y i n t e r c o n v e r t e d by microbes, although there i s no e v i dence the energy produced by o x i d a t i o n i s used f o r m i c r o b i a l growth (16_, 17_). F l e i s h e r (9) and Wood (16) have suggested that m i c r o b i a l formation o f v o l a t i l e a r s i n e o r other v o l a t i l e reduced As compounds might p l a y a r o l e i n the discharge o f As t o the a t mosphere. Reed and S t u r g i s (18) i n d i c a t e d t h a t a r s i n e may be l o s t from f l o o d e d s o i l s . Wood (16) pointed out the marked s i m i l a r i t i e s between the Hg and As c y c l e s . According t o Wood, i n

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

38

ARSENICAL PESTICIDES

reduced environments such as sediments, arsenate i s reduced t o a r s e n i t e and methylated t o methylarsenic a c i d or d i m e t h y l a r s i n i c a c i d . These compounds, which are extremely t o x i c , may be f u r t h e r methylated ( t r i m e t h y l a r s i n e ) o r reduced (dimethylarsine) and may v o l a t i l i z e t o the atmosphere where o x i d a t i o n r e a c t i o n s w i l l r e s u l t i n reformation o f d i m e t h y l a r s i n i c a c i d . Thus s t u d i e s which do not monitor the v a r i o u s p o s s i b l e forms o f As can r e s u l t i n misleading information. While p r e c i s e information o f the o x i d a t i o n - r e d u c t i o n potent i a l a t which a r s i n e , a r s e n i t e , and arsenate dominate i s not a v a i l a b l e , Turner (17_) c a l c u l a t e d the E' f o r the arsenate-arsen i t e r e a c t i o n to be +77 t o +167 mV (pH 7.0). He suggested t h a t b a c t e r i a l o x i d a t i o n o f a r s e n i t e i n v o l v e d an adaptive a r s e n i t e dehydrogenase system. I t i s p o s s i b l e t h a t t h i s enzyme complex evolves as a p r o t e c t i v was between 6.0 and 6.7 capable o f c a r r y i n g out t h i s r e a c t i o n were r e a d i l y i s o l a t e d . Deuel and Swoboda (14) obtained experimental evidence t h a t a r s e n i t e o x i d a t i o n occurs a t about +100 mV. This work shows t h a t a r s e n i t e w i l l be o x i d i z e d i n aerated s o i l s , where the o x i d a t i o n r e d u c t i o n p o t e n t i a l commonly i s i n the range o f +400 t o 600 mV. However, the r a t e o f a r s e n i t e o x i d a t i o n i n s o i l s i s a l s o o f importance i n e s t i m a t i n g the r e s i d u a l a c t i v i t y o f a r s e n i t e . While r a t e data have not been obtained, the p e r f u s i o n experiments o f Quastel and S c h o l e f i e l d (19_) showed complete o x i d a t i o n o f .0025M sodium a r s e n i t e i n 8 days i n an unenriched s o i l . When the s o i l was enriched by p e r f u s i o n with sodium a r s e n i t e u n t i l constant o x i d a t i o n r a t e s were obtained, complete o x i d a t i o n occurred i n about 4 days. Thus a r s e n i t e w i l l not p e r s i s t f o r long p e r i o d s i n w e l l aerated s o i l s which have s i g n i f i c a n t b i o l o g i c a l a c t i v i t y . Sorption o f A r s e n i c i n S o i l . In most s o i l systems, the chemi s t r y o f As becomes the chemistry o f arsenate. This i o n has p r o p e r t i e s s i m i l a r t o phosphate i n c l u d i n g formation o f i n s o l u b l e s a l t s with a number o f c a t i o n s , and s o r p t i o n by s o i l c o n s t i t u e n t s . For a comprehensive d i s c u s s i o n o f the behavior o f s o i l and f e r t i l i z e r phosphorus, see Black (20) and r e f e r e n c e s c i t e d therein.One major d i f f e r e n c e between P and As appears t o be t h a t while s o i l s c o n t a i n a p p r e c i a b l e organic-combined phosphate, they do not cont a i n measurable l e v e l s o f organic As (21). S o l u b i l i t y r e a c t i o n s may p l a y a r o l e i n As r e t e n t i o n by s o i l s . For example, i r o n arsenate i s extremely i n s o l u b l e , maint a i n i n g an arsenate c o n c e n t r a t i o n o f only about 10' i n solut i o n , compared t o about 10~5Af f o r the Ca or Mg arsenates (22). The modified Chang and Jackson (23) procedure f o r f r a c t i o n a t i o n of i n o r g a n i c s o i l P was f i r s t a p p l i e d to As-contaminated s a i l by Johnson and H i l t b o l d (21). They found t h a t about 90% of the As was a s s o c i a t e d with the c l a y f r a c t i o n o f a C h e s t e r f i e l d sandy loam p r e v i o u s l y t r e a t e d with organic a r s e n i c a l s , and that most of t h i s As was a s s o c i a t e d with s o i l A l . However, t h i s approach

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

3.

WALSH AND KEENEY

Inorganic Arsenicals in Soil

39

assumes that As extracted by ammonium f l u o r i d e i s Al-bound and t h a t removed by sodium hydroxide i s Fe-bound. T h i s i s not necess a r i l y the case as i t has been shown that some o f the P r e l e a s e d by ammonium f l u o r i d e i s subsequently resorbed by other s o i l cons t i t u e n t s and then removed by the sodium hydroxide e x t r a c t i o n (2J+). This was confirmed f o r As by Jacobs et a l . (25) who a l s o a p p l i e d the Williams et a l . procedure (which c o r r e c t s f o r P r e s o r p t i o n ) to As-contaminated s o i l s . They obtained r e s u l t s sugg e s t i n g t h a t most of the As was sorbed by amorphous Fe and A l components i n s o i l s . Woolson e t _ a l . (12) conducted an extensive survey of As forms i n As-contaminated s o i l s from throughout the United S t a t e s . They found t h a t , as would be p r e d i c t e d , s o i l s with a high r e a c t i v e Fe (oxalate e x t r a c t a b l e ) content had predominately Fe-bound As, but that when the r e a c t i v As was c o n t r o l l e d by th r e a c t i v e A l (see a l s o r e f e r e n c e 26). A number o f r e p o r t s have i n d i c a t e d that As i s more t o x i c on coarse-textured ( i . e . , sandy) than on f i n e - t e x t u r e d ( i . e . , high i n c l a y ) s o i l s (3_, 27^, 28_, 29_, 3£, 31_, :32, 33, 34^ 315). This i s exp l a i n e d i n p a r t by the marked increase i n surface area as the c l a y content i n c r e a s e s . A l s o , as would be p r e d i c t e d from the As f r a c t i o n a t i o n work discussed p r e v i o u s l y , s o r p t i o n of arsenate i n creases with the amount o f " a c t i v e " ( e x t r a c t a b l e ) Fe or A l cont e n t o f s o i l s (12_, 25, 2£, 30, 36, 37_, 38). Sorption o f As by s o i l s i s time dependent. I n d i c a t i o n s o f r e v e r s i o n to l e s s s o l u b l e (and hence l e s s t o x i c ) forms o f As have been obtained i n the f i e l d (39), greenhouse (35), and l a b o r a t o r y (26, 40). T h i s i s i l l u s t r a t e d by F i g u r e 1 (from reference 25) which shows the amount of As extracted from a sand ( P l a i n f i e l d ) and a s i l t y c l a y loam (Waupun) a f t e r e q u i l i b r a t i o n f o r up to 12 months. More As was extracted by the Bray P - l (0.025$ hydroc h l o r i c a c i d + 0.3N ammonium f l u o r i d e ) than by 1.0N ammonium acet a t e (pH 7.0) and more As was removed from the sand than the s i l t y c l a y loam. From 3 t o 6 months e q u i l i b r a t i o n was r e q u i r e d before As reached an apparent e q u i l i b r i u m with these s o i l s . Simil a r l y , Woolson ejt a l . (26) found that s o l u b l e (ammonium acetate o r ammonium c h l o r i d e e x t r a c t a b l e ) As decreased t o a constant value a f t e r about 4 months, and that the r a t e o f decrease d i f f e r e d among s o i l s . Iron-bound As continued to form even a f t e r the A l bound f r a c t i o n had reached a maximum l e v e l and begun to d e c l i n e . D e c l i n e i n As t o x i c i t y with time i n f i e l d and greenhouse s t u d i e s have been reported (3_, 27, 35). However, i n a f i e l d study, Steevens et ad. (13) found l i t t l e d e c l i n e i n As p h y t o t o x i c i t y to peas or potatoes with time. Arsenic Movement and P e r s i s t e n c e

in Soil

While downward movement ( l e a c h i n g ) o f phosphate i n s o i l s i s b e l i e v e d t o be very l i m i t e d (20) s e v e r a l workers have i n d i c a t e d

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL

40

PKSTICiDES

that arsenate l e a c h i n g may be a s i g n i f i c a n t f a c t o r i n reducing As t o x i c i t y o f surface s o i l s (3_, 13_, 2£, 33_, 4 J J . The p r o f i l e d i s t r i b u t i o n o f As i n a P l a i n f i e l d sand which had r e c e i v e d up t o 720 kg/ha o f As i n 1967 was i n v e s t i g a t e d by Steevens e_t aJL. (13). These r e s u l t s , along with 1974 p r o f i l e analyses by the authors, are presented i n Table 2. Table 2. P r o f i l e d i s t r i b u t i o n with time o f t o t a l s o i l As i n a P l a i n f i e l d sand a f t e r s u r f a c e a p p l i c a t i o n of sodium a r s e n i t e i n 1967. [ J . Environ Qual. (13)] LSD As Applied Depth , cm .05 68-83 Year 23-38 kg/ha 0-23 38-53 53-68 A o

-ppm

0 45 90 180 720

3. 11. 23.0 73.0 250.0

— — — — --

— — — —

1968

0 45 90 180 720

3.0 19.0 26.0 63.0 150.0

— — — — —

— — — — —

1970

0 45 90 180 720

3.6 14.1 27.0 45.0 100.0

1967

— —

—

— — — — — — — — —

— — — — — — — — — —

1.3 1.9 1.2 1.7 3.3

1.2 1.6 1.4 1.1 2.0

0.8 1.1 3.2 4.5 4.2

1.3 .5 1.6 1.0 0 1.4 8.1 1.2 .7 45 14.4 1.8 1.6 1.3 90 180 2.8 1.5 1.1 21.6 50.6 19.6 82.8 2.6 720 * Walsh and Keeney, unpublished d a t a , 1974.

.6 .7 1.3 1.3 3.7

.9 4.5 5.1 9.9 24.9

1974*

1.2 2.3 3.4 4.8 65.0

1.8 1.4 1.5 1.9 8.6

Through 1970, these p l o t s r e c e i v e d about 75 cm o f p r e c i p i t a t i o n and 50 cm o f i r r i g a t i o n per year. A l f a l f a has been grown on the experimental area during the past 3 y e a r s , but i t was not harv e s t e d . The p l o t s were not i r r i g a t e d s i n c e 1970. Phytotoxicity p e r s i s t e d at the 180 and 720 kg/ha As p l o t s from 1967 to 1970, and the 720 kg/ha p l o t s are s t i l l e s s e n t i a l l y barren i n 1974, 7 years a f t e r As a p p l i c a t i o n on a coarse textured s o i l . However, there has been a s i g n i f i c a n t d e c l i n e i n t o t a l As with time i n the surface s o i l o f a l l As-treated p l o t s . Part of t h i s decrease can be explained by As l e a c h i n g ; by 1970 As had d e f i n i t e l y been leached t o the 38-53 cm depth i n the 720 kg/ha p l o t s , and l e a c h ing r a t e was r e l a t e d t o the amount a p p l i e d i n agreement with the work o f Tamines and de L i n t (33). By 1974, the surface s o i l of

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

3. WALSH AND KEENEY

Inorganic Arsenicals in Soil

41

the high As-treated p l o t s had decreased f u r t h e r , but the r a t e o f decrease was much l e s s than i n previous years. S i g n i f i c a n t amounts o f As had accumulated i n the s u b s o i l , and i n the 720 kg/ha p l o t s , t o t a l As was above background t o 83 cm. These r e s u l t s show that As w i l l l e a c h i n a sandy p r o f i l e , and t h a t i n time s u f f i c i e n t As w i l l be leached t o s i g n i f i c a n t l y reduce p h y t o t o x i c i t y . However, the r a t e o f As l o s s from the surf a c e s o i l decreased markedly with time, i n d i c a t i n g t h a t As i s p e r s i s t e n t even i n a s o i l with low s o r p t i o n c a p a c i t y . When t h e p r o f i l e r e s u l t s a r e summed, a l l o f the As a p p l i e d i n 1967 cannot be accounted f o r as t o t a l As i n the p r o f i l e . One p o s s i b i l i t y i s that s i g n i f i c a n t amounts o f As a r e l o s t from t h i s s o i l by wind e r o s i o n (40). Tammes and de L i n t (33) c a l c u l a t e d an average h a l f l i f e o f 6.5 ± 0.4 years f o r As p e r s i s t e n c e on two Netherland s o i l s . Sinc p l o t s reported i n Tabl c u l a t i o n s with these data a r e not p o s s i b l e . However, i n t h e 720 kg/ha p l o t s , t o t a l As i n the surface s o i l had d e c l i n e d to 40% o f the o r i g i n a l value, i n only 3 y e a r s , and t o 33% by 7 years. Arsenic P h y t o t o x i c i t y S o i l "unproductiveness" due to a r s e n i c a l poisoning r e s u l t i n g from heavy a p p l i c a t i o n o f a r s e n i c a l p e s t i c i d e s has been noted by many r e s e a r c h e r s (3_, 28_, 42_, 43_, 44_). In reviewing the l i t e r a t u r e , i t i s apparent t h a t the most s e r i o u s problems occur i n f i e l d s where orchards a r e removed and then r e p l a n t e d t o an agronomic o r h o r t i c u l t u r a l crop (28_, 44_). S u b s t a n t i a l amounts o f a r s e n i c a l h e r b i c i d e s have been used on g o l f greens but As t o x i c i t y has not been reported on bent grass o r c o a s t a l bermuda g r a s s . A r s e n i c i s not an e s s e n t i a l p l a n t n u t r i e n t but o c c a s i o n a l l y s m a l l y i e l d i n c r e a s e s have been observed a t low l e v e l s o f As, esp e c i a l l y f o r t o l e r a n t crops such as potatoes, corn» r y e and wheat (40, 45, 46). L i e b i g et_ al_. (47_) r e p o r t e d that r o o t growth o f lemon p l a n t s i n s o l u t i o n c u l t u r e was enhanced by 1 ppm As as a r senate or a r s e n i t e ; 5 ppm o f e i t h e r form o f As was t o x i c and adv e r s e l y a f f e c t e d both t o p and r o o t growth. Woolson (46) p o s t u l a ted t h a t As response was due t o s t i m u l a t i o n o f p l a n t systems by small amounts o f As s i n c e other p e s t i c i d e s (e.g. 2, 4-D) stimul a t e p l a n t growth a t s u b l e t h a l dose l e v e l s . Another reason f o r the y i e l d i n c r e a s e might be that arsenate would d i s p l a c e phosphate from the s o i l with a r e s u l t a n t increase i n phosphate a v a i l a b i l i t y (40). Plant S e n s i t i v i t y . P l a n t s vary c o n s i d e r a b l y i n t h e i r t o l e r ance t o high l e v e l s o f s o i l As. In h i s review on As, L i e b i g (43) included the f o l l o w i n g c l a s s i f i c a t i o n o f vegetables and small f r u i t s i n terms o f t h e i r t o l e r a n c e t o water-soluble As: Very t o l e r a n t : Asparagus, potato, tomato, c a r r o t , tobacco, dewberry, grape, and r e d r a s p b e r r y .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

42

ARSENICAL PESTICIDES

F a i r l y t o l e r a n t : Strawberry and sweet corn, beet and squash. Low or no t o l e r a n c e : Snap bean, l i m a bean, onion, pea, cucumber, a l f a l f a and other legumes. L i e b i g (43) a l s o i n d i c a t e d t h a t r y e and Sudan grass were v e r y t o l e r a n t to s o l u b l e As. Recently, Deuel and Swoboda (29) r e ported t h a t soybeans were more s e n s i t i v e t o As t o x i c i t y than c o t ton. Jacobs e t _ a l . (40) found that crop t o l e r a n c e t o As was i n the f o l l o w i n g order: potatoes > peas > sweet corn > snap beans. Woolson et a l . (32) reported the As t o l e r a n c e o f s i x vegetable crops t o be as f o l l o w s : Cabbage > tomato > r a d i s h > spinach > lima beans > green beans. R e l a t i o n s h i p o f E x t r a c t a b l e As to P l a n t Growth Many r e searchers have attempte with p l a n t growth. As good p r e d i c t o r o f water s o l u b l e As or As p h y t o t o x i c i t y when these r e l a t i o n s h i p s are compared among s o i l s with widely d i f f e r i n g chara c t e r i s t i c s (12, 40, 48). However, t o t a l As a c c u r a t e l y p r e d i c t s p h y t o t o x i c i t y when s t u d i e s have been l i m i t e d t o a narrow range o f experimental t r e a t e d s o i l s (40, 49). In g e n e r a l , a h i g h l y s i g n i f i cant r e l a t i o n s h i p has been observed between some form o f e x t r a c t a b l e As and p l a n t growth. E x t r a c t a n t s commonly used t o measure a v a i l a b l e As i n c l u d e hot water, Bray P - l , mixed a c i d (0.05/17 hydroc h l o r i c a c i d + 0.025# s u l f u r i c a c i d ) , 0.5N h y d r o c h l o r i c a c i d , IN ammonium c h l o r i d e , IN ammonium acetate (pH 7.0) and 0.5N sodium bicarbonate. The r e s u l t s o f a few r e c e n t s t u d i e s , summarized i n Table 3, show t h a t s o i l type and p l a n t specie i n f l u e n c e the " c r i t i c a l " l e v e l of s o i l As a t which a y i e l d depression might be expected. The " c r i t i c a l " l e v e l ranged from 25 to 85 ppm As and from 3 t o 28 ppm As f o r t o t a l s o i l As and f o r water s o l u b l e As, r e s p e c t i v e l y . The c r i t i c a l l e v e l s f o r a v a i l a b l e As as measured by the Bray P - l , the mixed a c i d or sodium bicarbonate ranged from 10 to 22 ppm As. Greenhouse work by Woolson e t _ a l . (46) c o n c l u s i v e l y shows that p l a n t growth on As-contaminated s o i l s i s r e l a t e d to t o t a l As and a v a i l a b l e As. A l l c o r r e l a t i o n s presented i n F i g u r e 2 are h i g h l y s i g n i f i c a n t , however, use o f the a v a i l a b l e t e s t s more e f f e c t i v e l y p r e d i c t e d p l a n t response as compared to t o t a l As. The three a v a i l a b l e As t e s t s compared i n F i g u r e 2 were e q u a l l y e f f e c t i v e i n p r e d i c t i n g response to As. In a r e l a t e d study Woolson (32) found the growth o f s i x vegetable crops were reduced by 50% at l e v e l s o f 6.2 t o 48.3 ppm o f a v a i l a b l e As as measured with the mixed a c i d procedure. These data presented i n Table 4 a l s o show that the a v a i l a b l e As l e v e l s account f o r 64 t o 83% o f the v a r i a t i o n i n y i e l d f o r s i x greenhouse-grown vegetable crops. Greenhouse experiments were a l s o conducted by Deuel and Swoboda (29) i n which experimentally t r e a t e d s o i l s were used to r e l a t e l e v e l s o f As extracted from the s o i l s with y i e l d s o f c o t ton or soybeans. These r e s u l t s , presented i n Table 5, show t h a t

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

WALSH AND KEENEY

Inorganic Arsenicals in Soil

24QE

200

PLAINFIELD--BRAY PI

160-

120

80 •

WAUPUN—BRAY PI PLA INFIELO —NH4OAC

40 WAUPUN—NH4OAC

EQUILIBRATION TIME, MONTHS Soil Science Society of America, Proceedings

Figure 1. Extractability of added As (320 pg/g) from Plainfield sand and Waupun silty clay loam by Bray P-l and NH OAc as related to equilibration time (25) /f

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' *

#

*',.

As Soil Science Society of America, Proceedings

Figure 2. A comparison of four methods for the estimation of As availability by a correlation between growth reduction (4-week-old corn) and the log of As concentration in soils (46)

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

44

ARSENICAL PESTICIDES

hot water, IN ammonium c h l o r i d e and 0.5/1? h y d r o c h l o r i c a c i d e x t r a c ted As l e v e l s that were s i g n i f i c a n t l y c o r r e l a t e d with p l a n t growth. Table 3. Levels o f s o i l As a t which s i g n i f i c a n t y i e l d depressions occur. L e v e l o f As a t which Source Crop Soil s i g n i f i c a n t y i e l d deof name type p r e s s i o n s occurred data T o t a l Water A v a i l a b l e As soluble As As - ppm As Colton Blueberry loamy sand Cotton Amarillo f i n —— — _ sandy loam 8 (29) Cotton Houston Black (29) clay 28 Soybean Amarillo f i n e (29) sandy c l a y 3 Soybean Houston Black clay (29) 12 Potatoes, Plainfield — (to) 68 sweet corn loamy sand 22t Snap beans, P l a i n f i e l d — peas loamy sand lot 25 Corn (Avg. o f 13 — 85 soils) 10* 0*6) t E x t r a c t e d with Bray P - l (0.025/7 HCL + 0.3i^ NH F ) # E x t r a c t e d with e i t h e r O.SN NaHC0 o r 0.05/7 HC1 + 0.025/7 ^ S O ^ 3

Table 4. Regression o f a v a i l a b l e a r s e n i c * on t o t a l d r y - p l a n t weight f o r s i x vegetable crops. [Weed Science (32)3 A v a i l a b l e As Regression Correlation at G R * Crop equationt coefficient 50

(r) Green beans y = 77-34 l o g x§ 0.89 0.83 Lima geans y = 107-55 l o g x Spinach y = 88-37 l o g x 0.91 y = 114-38 l o g x 0.80 Cabbage 0.87 Tomato y = 109-42 l o g x 0.81 Radish y = 96-36 l o g x * A v a i l a b l e A s - s o i l extracted with 0.05/7

(ppm) 6.2 10.9 10.6 48.3 25.4 19.0 H SO^ and 0.025/17 HC1

t C a l c u l a t e d by l e a s t squares method 4 GR = a v a i l a b l e s o i l As content necessary t o reduce p l a n t growth t o 50% o f t h a t grown on non-arsenate-treated s o i l § x = a v a i l a b l e s o i l As s

50

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3. WALSH AND KEENEY

Inorganic Arsenicals in Soil

45

Table 5. C o r r e l a t i o n c o e f f i c i e n t s f o r y i e l d vs. a r s e n i c . [ J o u r n a l o f Environmental Q u a l i t y (29)] Soil

Crop

Amarillo Houston Amarillo Houston

Soybeans Soybeans Cotton Cotton

H0 2

-.943 -.968 -.951 -.954

extractable

Extractant* HCl NH^Cl -.914 -.938 -.960 -.918

-.915 -.931 -.830 -.895

* A l l o f the c o r r e l a t i o n c o e f f i c i e n t s a r e s i g n i f i c a n t a t t h e 0.05 level. Jacobs et_ ajU (40 and the y i e l d s o f s e v e r a P l a i n f i e l d Sand. A l l extractants were n e g a t i v e l y c o r r e l a t e d with crop y i e l d s a t the 0.01 p r o b a b i l i t y l e v e l (Table 6). Total As and ammonium acetate o r Bray P - l e x t r a c t a b l e As were e q u a l l y e f f e c t i v e i n p r e d i c t i n g reduced y i e l d s as l e v e l s o f s o i l As i n creased. Table 6. R e l a t i o n s h i p s between t o t a l o r e x t r a c t a b l e y i e l d o f vegetable crops. [Agronomy J o u r n a l (40)] As fraction

Potatoes

Peas

Crop* Snap beans

s o i l As and

Sweet corn

NH^OAc

-0.91

-0.85

-0.73

-0.91

Bray P - l

-0.91

-0.88

-0.77

-0.93

Total

-0.92

-0.87

-0.76

-0.03

" A l l c o r r e l a t i o n c o e f f i c i e n t s s i g n i f i c a n t a t the 0.01 p r o b a b i l ity level. P r i o r work c l e a r l y i n d i c a t e s that s e v e r a l s o i l t e s t s can be used t o p r e d i c t As p h y t o t o x i c i t y . Even though water s o l u b l e and t o t a l As s a t i s f a c t o r i l y p r e d i c t As p h y t o t o x i c i t y , s o i l t e s t i n g l a b o r a t o r i e s w i l l f i n d i t more convenient t o use Bray P - l , sodium bicarbonate or the mixed a c i d e x t r a c t a n t because these e x t r a c t a n t s a r e now r o u t i n e l y used f o r a v a i l a b l e P. Furthermore, they e x t r a c t more As than water, and e l i m i n a t e the d i g e s t i o n step needed f o r t o t a l As. P l a n t Uptake o f A r s e n i c . A p p l i c a t i o n o f many chemical e l e ments r e s u l t s i n s u b s t a n t i a l increases i n crop a s s i m i l a t i o n o f those elements. In the case o f As such bioaccumulation would be hazardous t o human beings o r animals because o f t o x i c i t y o f As and i t s p o s s i b l e r e l a t i o n s h i p t o cancer, a r t e r e o s c l e r o s i s and c h r o n i c l i v e r diseases (10). F o r t u n a t e l y , the e d i b l e p o r t i o n o f p l a n t s seldom accumulates a hazardous l e v e l o f As, p r i m a r i l y

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

46

ARSENICAL PESTICIDES

because most p l a n t s a r e s e n s i t i v e t o As t o x i c i t y and growth i s u s u a l l y s e v e r e l y reduced b e f o r e a l e v e l o f As hazardous t o man or animals accumulates i n t h e p l a n t . When As poisoning does occur, i t i s u s u a l l y due t o d i r e c t i n g e s t i o n o f a s u r f a c e r e s i d u e o f As or t h e i n g e s t i o n o f s p r i n g water o r muds c o n t a i n i n g abnormally high l e v e l s o f As (50, 51). The e d i b l e p o r t i o n o f most f r u i t s and vegetables grown i n Ast r e a t e d s o i l s c o n t a i n l e s s than the t o l e r a n c e o f 2.6 ppm as a l lowed by the U.S. P u b l i c Health s e r v i c e f o r A s - t r e a t e d f r u i t s and v e g e t a b l e s . The h i g h e s t l e v e l s o f As a r e found i n p l a n t r o o t s , the v e g e t a t i v e top growth i s intermediate, and e d i b l e seeds and f r u i t s c o n t a i n the lowest l e v e l o f As. Jones and Hatch (50) anal y z e d vegetable p l a n t s growing on A s - t r e a t e d s o i l s on s e v e r a l experimental farms i n Oregon and found t h e r o o t s , tops (stems and l e a v e s ) and e d i b l e p o r t i o 1.2 ppm As, r e s p e c t i v e l y (53) observed t h a t vegetables grown on s o i l s t r e a t e d with h i g h l e v e l s o f l e a d arsenate seldom contained more than 1 ppm As i n the e d i b l e p o r t i o n s . A wide range o f forage and vegetable crops were grown by Jones and Hatch (50) on A s - t r e a t e d and adjacent untreated s o i l s i n Oregon. They found the As content i n the e d i b l e p l a n t p a r t s t o be only 1.2 and .41 ppm As i n the t r e a t e d and untreated areas, r e s p e c t i v e l y . On t h e other hand, Small and McCants (3£) found t h a t the c o n c e n t r a t i o n o f As i n f l u e - c u r e d t o bacco v a r i e d from 2 ppm As where no As was a p p l i e d t o 14.3 ppm where 54 kg As/ha or l e a d arsenate had been a p p l i e d t o the s o i l . A r s e n i c l e v e l s o f l e s s than 1.0 ppm were found by Jacobs e t a l . (40) i n potato tubers even where severe As t o x i c i t y occurred. The p e e l i n g , however, contained up t o 48 ppm As where the s o i l had been experimentally t r e a t e d with 720 kg As/ha. The authors suggested the high l e v e l s o f As i n t h e p e e l i n g s was due t o minute q u a n t i t i e s o f As-contaminated s o i l adhering t o the s u r f a c e o f the potato t u b e r , r a t h e r than a s s i m i l a t i o n o f As i n t o the p e e l i n g itself. A survey o f potato grower f i e l d s i n C e n t r a l Wisconsin where 8 t o 65 kg As/ha as sodium a r s e n i t e had been a p p l i e d over a p e r i o d o f s e v e r a l years r e v e a l e d t h a t the potato p e e l i n g s d i d not c o n t a i n more than 2-3 ppm As (54). I t was concluded, thus, t h a t past usage o f sodium a r s e n i t e i n Wisconsin had not caused harmful l e v e l s o f As t o accumulate i n potato tubers. High As l e v e l s have a l s o been observed i n washed, unpeeled r a d i s h e s (32). As with potatoes, p a r t o f the As may have been adsorbed on the s u r f a c e o f the r a d i s h r o o t . Woolson (32) found a s i g n i f i c a n t p o s i t i v e r e l a t i o n s h i p between a v a i l a b l e s o i l As and the c o n c e n t r a t i o n o f As i n the whole p l a n t f o r s i x greenhouse grown vegetable crops (Table 7 ) . Correl a t i o n s between a v a i l a b l e s o i l As and the c o n c e n t r a t i o n o f As i n the e d i b l e p l a n t p a r t s were g e n e r a l l y poorer, e s p e c i a l l y f o r p l a n t s i n which the seed or f r u i t was consumed (32). As p r e v i ously noted, p l a n t s tend t o exclude As from the seeds and f r u i t s . Hence, s o i l t e s t s f o r a v a i l a b l e As would not be a r e l i a b l e

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47

i n d i c a t o r o f the amount o f As i n the e d i b l e p l a n t t i s s u e . Woolson (32) has pointed out t h a t where the e d i b l e p a r t o f a crop i s not the r o o t or whole p l a n t , i t i s u n l i k e l y t h a t the t o l e r a n c e l e v e l f o r As (2.6 ppm) would be exceeded even where As t o x i c i t y reduced growth by 50%. Table 7. Regression o f a v a i l a b l e s o i l As on the As content i n whole dry p l a n t o r e d i b l e d r y - p l a n t p a r t [Weed S c i . (32] Regression equation

Crop Green bean Lima bean Spinach Cabbage Tomato Radish

y y log y y y log y

= = = = =

Correlation coefficient

0.4 +4.2 l o g x* 0.5 + 1.2 l o -0.12 0. g -0.1 + 3.3 l o g x -0.147 + 1.4 l o g x

(r) 0.93 0.49

(ppm) 3.7t 1.7

0.80 0.88

4.5 43.8

* x = a v a i l a b l e s o i l As t A v a i l a b l e As a t GR_ was used t o c a l c u l a t e As content table 4). n

b

Arsenic a t GR

(see

0

Wherever s o i l s have been t r e a t e d with s u b s t a n t i a l q u a n t i t i e s of As, the surface o f the a e r i a l p o r t i o n o f the p l a n t may be contaminated with dust. The work o f Jacobs e t a l . (40) showed t h a t the concentration o f As i n potato l e a v e s , stems and p e t i o l e s was r e l a t e d t o the surface area o f the p l a n t p a r t s and the p r e v a i l i n g wind i n r e l a t i o n t o p l o t s t r e a t e d with high r a t e s o f As. Plant As c o n c e n t r a t i o n bore l i t t l e r e l a t i o n s h i p t o As treatment; thus, these workers concluded t h a t s u b s t a n t i a l e x t e r n a l contamination of the p l a n t t i s s u e i n the f i e l d had occurred. In l i g h t o f t h i s work, i t appears t h a t some o f the data i n the l i t e r a t u r e on p l a n t uptake o f As would be questionable unless the researcher very c a r e f u l l y washed the p l a n t t i s s u e t o remove adsorbed s o i l p a r t i c l e s . For i n s t a n c e , Jones and Hatch (50) reported t h a t the top growth o f vegetable p l a n t s growing on untreated s o i l adjacent t o As-treated s o i l contained 3.1 ppm As while the r o o t s from these p l a n t s contained only 1.1 ppm As. Since r o o t s normally accumul a t e more As than the top growth, i t appears t h a t the top growth must have been contaminated with s o i l p a r t i c l e s from the adjacent orchard s o i l which had been t r e a t e d with As f o r s e v e r a l years. A l l e v i a t i o n o f As T o x i c i t y Since r e g i s t r a t i o n o f inorganic a r s e n i c a l s f o r use on n e a r l y a l l vegetable and agronomic crops was c a n c e l l e d i n 1968 ( 6 ) , a high p r i o r i t y problem a t the present time i s t o f i n d ways t o r e s t o r e As-contaminated s o i l s t o t h e i r optimal l e v e l o f production (55). Several approaches have been attempted.

American Chemical Society Library In Arsenical Pesticides; Woolson, E.; 1155 16thChemical st. N. Society: w. Washington, DC, 1975. ACS Symposium Series; American l»f_ _L

v.

.

—

_

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ARSENICAL PESTICIDES

Molar P/As Ratio. One approach i s t o add s u f f i c i e n t phosphate t o the system t o depress the uptake o f arsenate by the plant. Hurd-Karrer (37) showed i n s o l u t i o n c u l t u r e work t h a t t h i s hypothesis was v a l i d and that a molar P/As r a t i o o f a t l e a s t 5 was needed t o p r o t e c t against As t o x i c i t y t o wheat. Rumberg et a l . (56) a l s o reported that phosphate w i l l improve growth i n n u t r i e n t s o l u t i o n s containing s u f f i c i e n t arsenate t o be t o x i c . The r e s u l t s with s o i l s systems, however, have been l e s s c l e a r , due i n part t o the d i f f i c u l t y o f evaluating " a v a i l a b l e " P and As. For example, Jacobs and Keeney (35) found that P a d d i t i o n s d i d not i n f l u e n c e As t o x i c i t y on a s i l t loam s o i l . T h i s s o i l had a high P f i x a t i o n c a p a c i t y and a v a i l a b l e P probably d i d not i n crease g r e a t l y . However, with a sandy s o i l , P a c t u a l l y enhanced As t o x i c i t y . The hypothesized t h a t with the sand phosphate may have d i s p l a c e hanced t o x i c i t y . Simila (26) with a sandy loam s o i l . When s u f f i c i e n t phosphate was added to maintain an a v a i l a b l e P/As r a t i o o f about 7, improved y i e l d s r e s u l t e d . T h i s e f f e c t was not c o n s i s t e n t , however, and a t very high l e v e l s o f added As (1,000 ppm), phosphate d i d not overcome As t o x i c i t y even a t a P/As r a t i o o f 10. A d d i t i o n o f Fe o r A l Compounds. Since As i s sorbed by Fe and A l components o f s o i l s , another obvious approach i s t o ammend the s o i l with Fe or A l s a l t s . Large amounts ( 5 t o 10 metric tons/ha) o f f e r r o u s s u l f a t e or f e r r i c s u l f a t e have occas i o n a l l y reduced As t o x i c i t y (57_, 58, 59). Steevens et_ a l . (13) attempted to a l l e v i a t e As t o x i c i t y on a sandy s o i l by a p p l i c a t i o n of 4 metric tons/ha o f f e r r i c s u l f a t e or aluminum s u l f a t e . The Fe treatment had a s l i g h t b e n e f i c i a l e f f e c t while the A l t r e a t ment a c t u a l l y depressed y i e l d s f u r t h e r . Both treatments decreased As uptake by potatoes. C u l t u r a l P r a c t i c e s . Deep plowing t o d i l u t e the As concentrat i o n o f the surface s o i l and expose As t o more s i t e s f o r f i x a t i o n would seem t o be one o f the most economical methods o f decreasing t o x i c i t y . T h i s approach was suggested by McLean e t a l . (53) and A l b e r t (31). Vincent (27) a l s o suggested growing t o l e r a n t cover crops such as r y e or Sudan grass. When plowed under, these crops reduced subsequent As t o x i c i t y . Apparently As t o x i c i t y t o f r u i t t r e e s i s involved with induced Zn d e f i c i e n c y , as f o l i a r a p p l i c a t i o n o f zinc s u l f a t e or Zn chelates w i l l overcome As t o x i c i t y to peach t r e e s (42, 60). Leaching. Woolson et_ a l . (26) pointed out t h a t i n s o i l s i n which added phosphate desorbs arsenate, d e l i b e r a t e leaching o f the s o i l a f t e r a d d i t i o n o f phosphate may be a v i a b l e approach t o removing As from the r o o t zone. This would seem f e a s i b l e s i n c e the l i m i t e d data (13, 26, 35) i n d i c a t e s that phosphate most l i k e l y w i l l desorb arsenate on sandy s o i l s , which a l s o are e a s i e s t

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Fleming, W. F., F. E. Baker, and L. Koblitsky. J. Econ. Ent. (1943) 36, 231-233. McLean, H. C., A. L. Weber and J. S. Joffe. J. Econ. Ent. (1944) 37, 315-316.

53.

J. Environ. Qual. (1973)

Soil Sci. (1945) 60, 277-288.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

52

AFSENICAL PESTICIDES

54.

Steevens, D. R., L. M. Walsh, and D. R. Keeney. Monitoring Journal (1972) 6, 89-90.

55.

Allaway, W. H. In A. G. Norman (ed.) "Advances in Agronomy," (1968) 20, 235-274.

56.

Rumberg, C. G., R. E. Engel, and W. F. Meggitt. Agron. J. (1960) 52, 452-453. Kardos, L. T., S. C. Vandecaveye, and N. Benson. Wash. Agr. Exp. Sta., Bull. 410 (1941). Overly, F. L. Wash. Agr. Exp. Sta. Bull 514, 1950, p. 14.

57. 58.

Pesticides

59.

Vandecaveye, S. C., C. M. Keaton, and L. T. Kardos. Wash. State Hort. Assoc. (1938) 34, 150-158.

Proc.

60.

Batjer, L. P. and N. R. Benson. (1958) 72, 74-78.

61.

Headden, W. P.

62.

Miles, J. R. W. J. Food Agr. Chem. (1968) 16, 620-622.

Proc. Amer. Soc. Hort. Sci.

Colo. Agr. Exp. Sta. Bull. 131, 1908.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

4 Behavior of Organoarsenicals in Plants and Soils A. E. HILTBOLD Agronomy and Soils Department, Agricultural Experiment Station, Auburn University, Auburn, Ala. 36830

During the past 20 have come into extensive are sodium or ammonium salts of MAA/1 (methanearsonic acid), MSMA (monosodium methanearsonate), DSMA (disodium methanearsonate), and MAMA (monoammonium methanearsonate). The methanearsonates differ from inorganic orthoarsenate in having a methyl substitution of one of the hydroxyl groups linked to the arsenic atom. Replacement of another hydroxyl by a second methyl group produces dimethylarsinic or cacodylic acid (hydroxydimethylarsine oxide). OH OH 0-Na+ i i i 0=As-OH 0=As-CH3 0=As-CH3 &h3 bn3 CH3 MAA Cacodylic acid Na cacodylate OH 0~Na+ OH 0=As-0~Na+ 0=As-0~Na+ 0=As-0"NH4 CH3 CH3 CH3 MSMA DSMA MAMA Figure 1. Formulae of organic arsenical herbicides. Methanearsonic acid is a dibasic acid with pKa values of 3.61 and 8.24 at 18C for dissociation of the first and second OH groups (1). The relative proportion of ionic and molecular forms of methanearsonate is determined by solution pH; Figure 2. At pH 5.93 essentially a l l of the methanearsonate exists as the univalent ion, or in terms of herbicide formulation as MSMA. The undissociated acid (MAA) increases in solution as pH falls below 5.93 and the proportion of univalent ion decreases. Solutions of MAA are acid, about pH 2. With increasing pH from 5.93 the divalent ion (DSMA) increases in proportion. DSMA solutions are aklaline, about pH 10.5. .1/Abbreviations used in the text are the common names of these herbicides accepted by the Weed Science Society of America. 53

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

54

Cacodylic a c i d has a s i n g l e a c i d i c OH; pK =6.19 a t 25C (I). The p r o p o r t i o n o f i o n i c form increases from near zero a t pH 4.0 to e s s e n t i a l l y 100% a t pH 8.5, Figure 3. Cacodylic a c i d s o l u t i o n s are m i l d l y a c i d i c while Na cacodylate s o l u t i o n s are m i l d l y a l k a l i n e . While v a r i o u s a c i d or s a l t formulations o f these organoars e n i c a l s may be a p p l i e d , t h e i r chemical form i n s o i l s o l u t i o n i s determined by the e x i s t i n g pH. In most a g r i c u l t u r a l s o i l s with pH values i n the range pH 5.0 t o 7.0 the u n i v a l e n t i o n of methanearsonate predominates almost to the e x c l u s i o n o f the u n d i s s o c i a t e d or d i v a l e n t forms. Both cacodylate i o n and a c i d may occur i n s o i l s , with the a c i d form predominant a t pH<6.2 and the i o n i c form predominant a t pH>6.2. a

H e r b i c i d a l P r o p e r t i e s and Use Patterns MSMA, DSMA, and MAM marketed a l s o as a white, c r y s t a l l i n e powder. Cacodylic a c i d may be obtained i n c r y s t a l l i n e form o r i n mixture with Na cacodylate as a l i q u i d . A l l of these products are h i g h l y s o l u b l e i n water. They are a p p l i e d with s u r f a c t a n t as sprays to the f o l i a g e of weeds. Theyhave no preemergence a c t i v i t y a t r a t e s used f o r weed c o n t r o l . Methanearsonates are used most e x t e n s i v e l y f o r postemergence c o n t r o l o f annual grass weeds i n c o t t o n (Gossypium hirsutum L . ) . A c t i v i t y of the methanearsonates on johnsongrass [(Sorghum h a l e pens e (L.) Pers.)] purple nutsedge (Cyperus rotundus L . ) , and yellow nutsedge (Cyperus esculentus L.) makes these h e r b i c i d e s an important part of the weed c o n t r o l program i n cotton. Substantial c o n t r o l o f these weeds i s provided by the methanearsonates (2), but repeated a p p l i c a t i o n s are u s u a l l y r e q u i r e d . MSMA i s r e g i s tered f o r use i n cotton a t 2.2 to 2.8 kg/ha and DSMA at 2.2 t o 3.4 kg/ha. In order to avoid i n j u r y o f cotton and a r s e n i c (As) r e s i dues i n cottonseed, use i s r e s t r i c t e d to the stage between the time when p l a n t s are about 10 cm t a l l and when they f i r s t bloom (3). Sprays must be d i r e c t e d to cover small weeds i n the d r i l l row but minimize contact with cotton leaves. S e l e c t i v e c o n t r o l of crabgrass ( D i g i t a r i a s a n g u i n a l i s (L.) Scop, and D. ischaemum (Schreb.) Muhl.) and d a l l i s g r a s s (Paspalum d i l a t a t u m P o i r . ) i n e s t a b l i s h e d t u r f i s provided by the methanearsonates with minimal i n j u r y of most t u r f g r a s s s p e c i e s . Nutsedge i s more p e r s i s t e n t but may be e f f e c t i v e l y c o n t r o l l e d with repeated a p p l i c a t i o n s o f 4 kg/ha (4). Methanearsonates are a l s o widely used i n non-crop areas f o r c o n t r o l of johnsongrass and v a r i o u s weeds on rights-of-way, along d i t c h banks, e t c . Cacodylic a c i d and Na cacodylate are n o n - s e l e c t i v e , f o l i a r contact-type h e r b i c i d e s (_5). Use i s r e s t r i c t e d t o non-crop areas where they are u s e f u l i n c o n t r o l l i n g many herbaceous and woody s p e c i e s . Rates r e q u i r e d f o r e f f e c t i v e c o n t r o l range from 3 t o 5 kg/ha and may be repeated as necessary. Tree i n j e c t i o n with cacod y l i c a c i d provides s e l e c t i v e e l i m i n a t i o n of undesirable s p e c i e s i n e s t a b l i s h e d stands. Cacodylic a c i d i s a l s o used i n t u r f g r a s s

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

HILTBOLD

Figure 2.

Organoarsenicals in Plants and Soils

Per cent total methanearsonate as the univalent ion

'4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

CH AsO(OH)0~ 3

8.0 8.5

PH Figure 3.

Per cent total cacodylate as the univalent ion (CH. ) AsO {

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

O'

ARSENICAL PESTICIDES

56

renovation f o r complete k i l l of e x i s t i n g vegetation p r i o r t o r e seeding the d e s i r e d species. The i n a c t i v a t i o n of c a c o d y l i c a c i d i n s o i l and i t s l a c k of r e s i d u a l p h y t o t o x i c i t y are advantages. Behavior of Organoarsenicals

i n Plants

Organic a r s e n i c a l h e r b i c i d e s are intended f o r i n t e r c e p t i o n by weed f o l i a g e r a t h e r than by s o i l as would be the case with a preemergence-type h e r b i c i d e . The f r a c t i o n of the a p p l i e d h e r b i c i d e i n t e r c e p t e d by f o l i a g e v a r i e s widely with weed density and spraying c o n d i t i o n s . The extent to which the i n t e r c e p t e d f r a c t i o n i s absorbed, t r a n s l o c a t e d , and metabolized may i n f l u e n c e the amount and chemical form o f the m a t e r i a l that u l t i m a t e l y reaches the s o i l An understanding o f the f a t e of an h e r b i c i d e i n p l a n t s may provide u s e f u l background f o r i t Absorption. While organoarsenicals may be absorbed by roots of johnsongrass (6), Coastal bermudagrass [Cynodon dactylon (L.) Pers.] (7), and bean (Phaseolus v u l g a r i s L.) (8) i n s o l u t i o n c u l ture, root uptake from s o i l i s d r a s t i c a l l y reduced (7). The p r i mary pathway o f entry i n t o p l a n t s i s through the leaves and stems. Duble, H o l t , and McBee (7) compared uptake of As by Coastal bermudagrass from s e v e r a l sources of a p p l i c a t i o n of DSMA, Figure 4. A p p l i c a t i o n to f o l i a g e was at 4.5 kg/ha, to s o i l a t 17.9 kg/ha, and to roots i n n u t r i e n t s o l u t i o n a t 12 ppm. Absorption u s u a l l y occurs w i t h i n s e v e r a l hours a f t e r a p p l i c a t i o n (9). Keeley and T h u l l e n (10) found that exposure of yellow nutsedge leaves to MSMA and DSMA f o r as l i t t l e as 5 and 15 minutes, r e s p e c t i v e l y , c o n t r o l l e d t h i s weed. However, 24- and 48-hr exposures were required f o r c o n t r o l of purple nutsedge. Surfactants are r o u t i n e l y included i n spray s o l u t i o n s to increase coverage and enhance absorption. McWhorter (11) found that increases i n p h y t o t o x i c i t y of DSMA to johnsongrass r e s u l t i n g from added s u r f a c t a n t were greatest a t low r a t e s o f DSMA near th~ margin f o r weed c o n t r o l . S i m i l a r l y , Keeley and T h u l l e n (10) observed improved c o n t r o l of regrowth of purple nutsedge f o l l o w i n g f o l i a r a p p l i c a t i o n of DSMA with s u r f a c t a n t , Table I. The l a r g e r responses to s u r f a c t a n t were obtained a t the low r a t e of DSMA and at the high temperature. Apparently s u r f a c t a n t s f a c i l i t a t e entry of methanearsonate i n t o the t r e a t e d l e a f ; then, with temperature favorable f o r growth, the h e r b i c i d e i s t r a n s l o c a t e d to c r i t i c a l sites. In cotton, however, severe i n j u r y from t o p i c a l a p p l i c a t i o n of MSMA occurred a t low (13C) temperature but not a t 31C, a more favorable temperature f o r growth. This i n j u r y was aggravated by s u r f a c t a n t (12). There i s evidence of greater absorption of MSMA than o f DSMA. When equal amounts of MAA, MSMA, and DSMA were a p p l i e d to c o t y l e dons of young cotton, Keeley and T h u l l e n (12) found that absorpt i o n of MAA and MSMA was s e v e r a l f o l d greater than that of DSMA. In the f i e l d , t o p i c a l a p p l i c a t i o n s of MSMA were more i n j u r i o u s to

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.68 1.68 3.36 3.36 1.68 1.68 3.36 3.36

DSMA DSMA DSMA DSMA MSMA MSMA MSMA MSMA

Herbicide

Table I .

QO)

none 0.5% none 0.5% none 0.5% none 0.5%

Surfactant

100 99 89 54 12 1 7 0

14 0 0 0 0 0 0 0

66 77 40 21 48 17 6 0

27 16 12 0 4 5 0 0

84 12 77 20 31 32 5 1

Weed Science

0 0 0 0 0 0 0 0

Temperature 13C 20C 29C Purple Yellow Purple Yellow Purple Yellow (Fresh weight as % o f c o n t r o l )

Following a F o l i a r A p p l i c a t i o n of DSMA o r MSMA t o P l a n t s Grown a t Various Temperatures

Regrowth o f Purple and Yellow Nutsedge (% Fresh Shoot Weight o f the C o n t r o l ) 3 Weeks

ARSENICAL PESTICIDES

58

FOLIAGE

REGROWTH

SOLUTION

SOURCE

OF DSMA

SOIL

J o u r n a l of A g r i c u l t u r a l and Food Chemistry Figure 4. D i s t r i b u t i o n o f As i n Coastal bermudagrass 7 days a f t e r f o l i a r , n u t r i e n t s o l u t i o n , and s o i l a p p l i c a t i o n s o f DSMA. Regrowth from a f o l i a r a p p l i c a t i o n represented 3 weeks growth a f t e r c l i p p i n g to ground l e v e l 7 days a f t e r treatment (!>• cotton than equal r a t e s o f DSMA, and the d i f f e r e n c e became more pronounced with higher a p p l i c a t i o n r a t e s and l a t e r stages o f growth (13). Temperature may a l s o i n f l u e n c e the r e l a t i v e absorpt i o n of MSMA and DSMA. In purple nutsedge growing at29C, absorption of MSMA and DSMA d i d not d i f f e r , but a t 13 and 20C absorption o f MSMA exceeded that o f DSMA (10). An explanation o f the d i f f e r e n t i a l absorption o f the methanearsonates may l i e i n the pH o f t h e i r s o l u t i o n s , t h e i r predominant i o n i c or molecular s p e c i e s , and the p e r m e a b i l i t y of l e a f membranes t o these s p e c i e s . The evidence suggests that the u n d i s s o c i a t e d a c i d or u n i v a l e n t i o n penetrates more r e a d i l y than the d i v a l e n t i o n . P l a n t species d i f f e r i n t h e i r s e n s i t i v i t y t o methanearsonates, probably due i n p a r t to d i f f e r e n c e s i n p e r m e a b i l i t y o f l e a f surf a c e s . Leaves of yellow nutsedge have been found to be more permeable t o these h e r b i c i d e s than those o f purple nutsedge (10). The t o l e r a n c e shown by cotton and many t u r f g r a s s e s f o r methanearsonates i s fundamental t o the use of these h e r b i c i d e s f o r s e l e c t i v e weed c o n t r o l . The b a s i s f o r t h i s t o l e r a n c e i s not y e t c l e a r . Tolerance among plant species i s r e l a t i v e , however, as evidenced by severe

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

4.

HILTBOLD

Organoarsenicals in Plants and

Soils

i n j u r y of cotton from f o l i a r a p p l i c a t i o n s at bloom stage

59 (3).

T r a n s l o c a t i o n . H e r b i c i d e movement out of the t r e a t e d t i s s u e and i n t o more c r i t i c a l s i t e s of a c t i v i t y i s e s s e n t i a l i f the h e r b i c i d e i s to f u n c t i o n more than as a desiccant. The s u p e r i o r i t y of methanearsonates over contact type h e r b i c i d e s such as Na a r s e n i t e i s due i n part to t h e i r more extensive t r a n s l o c a t i o n (8, 9). Compared to amounts that may be present i n t r e a t e d t i s s u e , however, r e l a t i v e l y small amounts are t r a n s l o c a t e d to other p l a n t p a r t s . In soybean [Glycine max (L.) Merr.] and crabgrass, Rumber& Engel, and Meggitt (9) found that l e s s than 2% of the DSMA recovered 6 hr a f t e r a p p l i c a t i o n to the leaves was t r a n s l o c a t e d i n t o other p l a n t p a r t s . Sachs and Michael (8) reported that 6% of the MSMA absorbed i n t r e a t e d leaves of bean was t r a n s l o c a t e d during a 3-day p e r i o d . In purpl DSMA was moved out of th C o a s t a l bermudagrass, 24% of the a p p l i e d DSMA was t r a n s l o c a t e d from t r e a t e d leaves to other plant parts i n a 5-day p e r i o d (7). D i f f i c u l t y of o b t a i n i n g adequate t r a n s l o c a t i o n of methanearsonates to johiBongrass rhizomes (11) and nutsedge tubers (14, 15) makes repeated a p p l i c a t i o n s necessary. Increasing temperature w i t h i n the range favorable f o r growth g e n e r a l l y a c c e l e r a t e s t r a n s l o c a t i o n of methanearsonate. Rumberg et a l . (9) reported more r a p i d t r a n s l o c a t i o n of DSMA at 30C than at 15C i n soybean and crabgrass. The increased t r a n s l o c a t i o n was a l s o a s s o c i a t e d with increased p h y t o t o x i c i t y of DSMA to crabgrass. S i m i l a r l y , movement of methanearsonate from t r e a t e d nutsedge p l a n t s i n t o daughter p l a n t s was found to be greater at 29C than at 13C (10). In cotton, contact i n j u r y of cotyledons by MSMA may preclude t r a n s l o c a t i o n to the leaves and terminal bud, sparing the plant severe i n j u r y . At low temperature (13C), however, cont a c t i n j u r y was n e g l i g i b l e and MSMA t r a n s l o c a t i o n exceeded that at 31C, r e s u l t i n g i n s t u n t i n g or death of the p l a n t (12). Both a c r o p e t a l and b a s i p e t a l t r a n s l o c a t i o n moves methanearsonate out of t r e a t e d leaves of johnsongrass (6), nutsedge (10, 14), bean (8), Coastal bermudagrass (7), soybean and crabgrass (9). S c k e r l and Frans (6) observed very l i t t l e b a s i p e t a l movement of methanearsonate i n cotton, although f o l i a r and stem a p p l i c a t i o n moved a c r o p e t a l l y . B a s i p e t a l movement of methanearsonate to rhizomes and tubers i s c r i t i c a l to the c o n t r o l of hardy p e r e n n i a l s such as johnsongrass and nutsedge. Methanearsonate a p p l i e d to nutsedge shoots i s t r a n s l o c a t e d to newly developing rhizomes, shoots, and tubers as i n d i c a t e d by phytotoxic r e a c t i o n and As determination (14, 16). Accumulation of As i n these a c t i v e growth s i t e s suggests that meristematic t i s s u e s are the major p o i n t s of herbicidal activity. Metabolism. There i s no evidence of s u b s t a n t i a l degradation of rupture of the C-As bond of organoarsenicals i n p l a n t s . MSMAderived CO2 i n the r e s p i r a t o r y CO2 of t r e a t e d Coastal bermudagrass

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

60

and purple nutsedge leaves amounts to a small f r a c t i o n o f the h e r b i c i d e i n the t i s s u e (7, 14)• Cacodylic a c i d i s apparently a very s t a b l e compound i n bean p l a n t s ; chromatographic a n a l y s i s showed l i t t l e i f any As a s s o c i a t e d with any f r a c t i o n other than c a c o d y l i c a c i d i t s e l f (8). E x t r a c t s o f MSMA-treated bean, however, revealed two As-containing f r a c t i o n s : parent MSMA and an u n i d e n t i f i e d complex of MSMA with some plant component (8). Simil a r complexes have been observed i n johnsongrass a f t e r treatment with MAA (6) and C o a s t a l bermudagrass a f t e r a p p l i c a t i o n o f DSMA (7). Radioassay and As a n a l y s i s o f the DSMA complex v e r i f i e d the p e r s i s t e n c e of the C-As bond (7). The mechanism o f p h y t o t o x i c i t y of the organoarsenicals i s not known. C r i t i c a l l e v e l s o f As r e q u i r e d f o r c o n t r o l o f regenerative p a r t s such as nutsedge tubers have not been e s t a b l i s h e d (16). Behavior o f Organoarsenical Much of the spray t;hat i s not i n t e r c e p t e d by f o l i a g e i s deposited d i r e c t l y on the s o i l . In a d d i t i o n , much o f that which i s i n t e r c e p t e d by weeds reaches the s o i l i n d i r e c t l y when r a i n washes l e a f surfaces o r k i l l e d v e g e t a t i o n decays. While methanearsonate or cacodylate may be a p p l i e d i n a c i d o r s a l t form, once i n the s o i l the predominant i o n i c o r molecular form i s determined by s o i l s o l u t i o n pH. In most s o i l s (pH 5 to 7) the u n i v a l e n t i o n of methanearsonate i s the dominant form. Cacodylate e x i s t s i n approximately equal concentrations o f u n d i s s o c i a t e d a c i d and cacod y l a t e i o n i n moderately a c i d s o i l s . Adsorption. Much o f the r e d u c t i o n i n a c t i v i t y o f organoarsen i c a l s upon reaching the s o i l i s a t t r i b u t a b l e to adsorption by s o i l c o l l o i d s . When s o l u t i o n s o f DSMA, Na cacodylate, arsenate, and phosphate were e q u i l i b r a t e d with 16 s o i l s , Wauchope (17) found that adsorption by i n d i v i d u a l s o i l s increased i n the order: phosphate
In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

4.

HILTBOLE

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61

e q u i l i b r i u m w i t h 3 ppm As i n s o l u t i o n . Results c i t e d by Wauchope (17) i n d i c a t e adsorption of 2270 yg As per g of c l a y s o i l i n e q u i l i b r i u m w i t h DSMA s o l u t i o n c o n t a i n i n g 75 ppm As. These r e s u l t s show great a d s o r p t i v e c a p a c i t y i n s o i l s , y e t , from the standpoint o f p o t e n t i a l i n j u r y to p l a n t s , the primary concern must be with the concentration i n e q u i l i b r i u m s o l u t i o n . While i n i t i a l adsorption of a r s e n i c a l s i s r a p i d (17), l o n g term changes r e s u l t i n r e d i s t r i b u t i o n o f As i n t o l e s s s o l u b l e forms (20). During an 8-week p e r i o d a f t e r a p p l i c a t i o n o f cacodyl i c a c i d t o three s o i l s , Woolson and Kearney (20) observed dec l i n i n g amounts o f water-soluble c a c o d y l i c a c i d and i n c r e a s i n g amounts i n a l e s s s o l u b l e f r a c t i o n a s s o c i a t e d w i t h aluminum. Inorganic arsenate i n s o i l has been shown by Jacobs, Syers, and Keeney (21) to become p r o g r e s s i v e l y l e s s e x t r a c t a b l e w i t h as much as 1 t o 6 months r e q u i r e of arsenate was c o r r e l a t e soils. Removal o f i r o n oxides and alumina by p r i o r treatment markedly reduced the a d s o r p t i v e capacity o f s o i l f o r arsenate (21} P l a n t growth i n s o i l s c o n t a i n i n g added organoarsenicals prov i d e s another view of the p r o g r e s s i v e i n a c t i v a t i o n . Schweizer (22) added DSMA to a s i l t loam s o i l a t r a t e s extending from 2.5 to 480 ppm. Cotton planted immediately a f t e r a p p l i c a t i o n was i n j u r e d a t DSMA r a t e s o f 80 ppm o r g r e a t e r . I n r e p l a n t i n g s 16 weeks a f t e r a p p l i c a t i o n , r a t e s o f 120 ppm or more were r e q u i r e d for i n j u r y . Continued a m e l i o r a t i o n o f t o x i c i t y was evident i n c o t t o n planted 24 and 32 weeks a f t e r a p p l i c a t i o n . A d d i t i o n o f phosphate t o DSMA-containing s o i l aggravated the i n j u r y of cotton (22). Apparently phosphate d i s p l a c e s methanearsonate from adsorption s i t e s , i n c r e a s i n g i t s a c t i v i t y i n the s o i l s o l u t i o n at the l e v e l s o f phosphorus a p p l i e d . Table I I .

Adsorption o f DSMA from 25 ml o f 5.0 ppm As S o l u t i o n s by 1-g Samples o f Augusta S o i l Separates and by Cert a i n Reference M a t e r i a l s (18)

Material

Augusta s i l t loam Sand Silt Clay Limonite MontmorilloniteKaolinite Vermiculite

Equilibrium solution ppm As

4.40 4.43 0 0 4.39 1.20 3.71

DSMA adsorbed

% yg As per g

t o t a l applied

12 11 100 100 12 76 26

15.0 14.3 125.0 125.0 15.3 95.1 32.3

Weeds

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ARSENICAL PESTICIDES

Leaching. Packed columns o f s o i l have been used i n l e a c h i n g s t u d i e s i n the l a b o r a t o r y . Ehman (23) reported r e s u l t s from a p p l i c a t i o n s o f DSMA and c a c o d y l i c a c i d to s o i l columns 30 cm i n depth. Passage o f 152 cm of water through the columns removed 9% of the a r s e n i c a l from a sand s o i l and 6% from a c l a y s o i l . Dickens and H i l t b o l d (18) a p p l i e d DSMA t o the surfaces of a loamy sand and a c l a y loam i n 23-cm columns a t 112 kg/ha on an area b a s i s . Approximately 52% o f the a p p l i e d DSMA was leached from the loamy sand during passage o f 76 cm o f water. None was leached from the c l a y loam. A n a l y s i s of the c l a y loam columns revealed nearly h a l f o f the a p p l i e d DSMA i n the 0 to 2.5 cm depth w i t h none deeper than 15 cm (18). V a r i a t i o n o f s o i l pH i n the range pH 5.5 to 6.5 d i d not a f f e c t the l e a c h i n g o f methanearsonate i n N o r f o l k loamy sand (18) This i s not s u r p r i s i n g form o f methanearsonat as 20% o f the a p p l i e d methanearsonate to e x i s t as u n d i s s o c i a t e d a c i d o r as d i v a l e n t i o n the s o i l pH would have to be 4.2 o r 7.6, r e s p e c t i v e l y . Therefore, pH changes i n the range encountered i n a g r i c u l t u r a l s o i l s should not appreciably i n f l u e n c e the chemical form o f methanearsonate. S o i l pH i n f l u e n c e s the a c t i v i t y o f a l u minum i n c l a y and hydrous oxides o f the s o i l c o l l o i d a l system, but apparently the adsorptive capacity o f s o i l i s determined more by the amount o f these c o l l o i d s than by pH e f f e c t s on t h e i r a c t i vity. Leaching o f a r s e n i c a p p l i e d as MSMA, DSMA, and MAMA i n the f i e l d was reported by Johnson and H i l t b o l d (24). These h e r b i c i d e s were a p p l i e d repeatedly over a 4-year p e r i o d f o r c o n t r o l o f nutsedge i n t u r f on a sandy loam s o i l . At the end o f t h i s p e r i o d the s o i l was analyzed f o r As. Greatest concentrations o f As were found i n the upper 5 cm and decreasing concentrations with depth to 30cm, Figure 5. The s o i l was not s t i r r e d during the a p p l i c a t i o n p e r i o d ; thus, downward movement was a t t r i b u t e d to l e a c h i n g . There were no d i f f e r e n c e s among methanearsonates as t o t h e i r r a t e of l e a c h i n g . Rate o f a p p l i c a t i o n a f f e c t e d the quantity o f As recovered a t the s e v e r a l depths but d i d not appear to i n f l u e n c e the r a t e of downward movement. Results i n t h i s s o i l i n d i c a t e d a slow but p e r c e p t i b l e movement o f As i n t o subsurface horizons t o at l e a s t 30 cm depth (24). Further f i e l d experiments o f H i l t b o l d , Hajek, and Buchanan (19) showed l e s s m o b i l i t y o f As i n s o i l profiles. A f t e r 6 years o f MSMA a p p l i c a t i o n s to s o i l s growing c o t ton, As d i s t r i b u t i o n i n the p r o f i l e s showed l i t t l e i f any e v i dence of l e a c h i n g below the plow depth, 23 cm. The chemical form of As i n these s o i l s at the time of t h e i r a n a l y s i s was not.determined. Metabolism. I n c o n t r a s t to the s t a b i l i t y o f the C-As bond observed i n methanearsonates i n p l a n t s , m i c r o b i o l o g i c a l o x i d a t i o n occurs i n s o i l s . Dickens and H i l t b o l d (18) found methanearsonatederived CO2 evolved from 4 s o i l s i n amounts ranging from 0.7 to

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Figure 5. Distribution of As in soil after applying methanearsonates at three rates for 4 years. Total As applied equivalent to 16, 32, and 64 kg As/ha. Shaded portions represent recovery of applied As, averaged across MSMA, DSMA, and MAMA treatments (24).

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

64

ARSENICAL PESTICIDES

5.5% of the added MAA during a 30-day i n c u b a t i o n . Von Endt, Kearney, and Kaufman (25) reported o x i d a t i o n of the methyl carbon of MSMA i n 4 s o i l s amounting to 1.7 to 10% during a 3-week p e r i o d . Methanearsonate o x i d a t i o n i n these s o i l s was p r o p o r t i o n a l t o t h e i r organic matter contents (25) or t o t h e i r t o t a l CO2 e v o l u t i o n (18). These observations, along w i t h the dependency o f m i c r o b i a l i s o l a t e s on energy sources other than methanearsonate (25), i n d i cate a predominantly passive metabolism of methanearsonate i n soil,, with l i t t l e response on the part of the m i c r o b i a l population. A r senate has been i d e n t i f i e d as the product of methanearsonate o x i d a t i o n i n s o i l s (25). Woolson and Kearney (20) reported two l o s s processes f o r cacod y l i c a c i d i n s o i l s . Under aerobic c o n d i t i o n s , approximately 41% of the a p p l i e d c a c o d y l i c a c i d was o x i d i z e d t o C02 and arsenate, while 35% was l o s t fro compound during a 24-wee degradation v i a CO2 was n i l , y e t 61% of the a p p l i e d c a c o d y l i c a c i d was v o l a t i l i z e d , p o s s i b l y as dimethyl a r s i n e . A gaseous l o s s mechanism f o r As was proposed by Thorn and Raper (26) i n 1932. They observed i n laboratory s t u d i e s the mic r o b i o l o g i c a l r e d u c t i o n of arsenate with e v o l u t i o n o f a r s e n i c a l gases. A c t i v e f u n g i were i s o l a t e d from a r s e n i c - t o x i c f i e l d s o i l s . Challenger (27) and coworkers l a t e r defined the methylation o f As by S c o p u l a r i o p s i s , A s p e r g i l l u s , and other f u n g i . T h e i r studies showed the reduction and methylation o f a r s e n i c a l s i n c l u d i n g 2 ° 3 > Na2HAs04, DSMA, Na cacodylate, and Na propylarsonate. Trimethylarsine [(CH3)3As] was the most common gaseous product. I n a l l cases, As was reduced to the t r i v a l e n t s t a t e and a l l s u b s t i tuent oxygen atoms replaced by methyl groups. With methanearsonate, cacodylate, and propylarsonate, the evolved a r s i n e contained the o r i g i n a l a l k y l i n t a c t : t r i m e t h y l a r s i n e and dimethy1-n-prop y l a r s i n e . The mechanism proposed by Challenger (27) was a stepwise r e d u c t i o n and methylation of arsenate through a r s e n i t e , methanearsonate, and cacodylate to the completely reduced and methylated t r i m e t h y l a r s i n e . Recently, Cox and Alexander (28) i s o l a t e d species o f Candida, Gliocladium, and P e n i c i l l i u m producing t r i m e t h y l a r s i n e from a r s e n i c a l s i n c l u d i n g MAA and c a c o d y l i c a c i d . The only evidence of b a c t e r i a methylating As i s that o f McBride and Wolfe (29), r e p o r t i n g the synthesis o f dimethylarsine by the anaerobe Methanobacterium. I n t h i s mechanism the i n t e r mediate c a c o d y l i c a c i d was reduced d i r e c t l y to dimethylarsine without the f i n a l methylation. The extensive v o l a t i l i z a t i o n of As from c a c o d y l i c a c i d - t r e a t e d s o i l s (20) when incubated anaerobic a l l y would n e c e s s a r i l y i m p l i c a t e b a c t e r i a such as Methanobacterium. Under aerobic c o n d i t i o n s , f u n g i appear to be the a c t i v e microorganisms. As

Persistence. Loss o f p h y t o t o x i c i t y with time a f t e r a p p l i c a t i o n o f organoarsenicals to s o i l may be viewed as a measure of p e r s i s t e n c e . Results of Schweizer (22) and Ehman (23) i n d i c a t e

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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65

extensive i n a c t i v a t i o n and d i m i n i s h i n g e f f e c t s on subsequent crops. Y i e l d s o f a v a r i e t y o f f i e l d crops were u n a f f e c t e d by r e s i d u e s o f methanearsonate equivalent t o 68 kg As/ha a p p l i e d during the previous 4 years (24). S i m i l a r l y , y i e l d s o f cotton were u n a f f e c t e d by 6 annual p r e p l a n t a p p l i c a t i o n s o f MSMA at 10, 20, and 40 kg/ha (19). On the other hand, Singh and Campbell (30) found that both DSMA and MAMA, when a p p l i e d to t u r f a t r a t e s o f 5.0 and 4.5 kg/ha, r e s p e c t i v e l y , twice annually f o r 2 years, pers i s t e d i n the upper 5 cm of a s i l t loam s o i l i n amounts s u f f i c i e n t to i n j u r e oats (Avena s a t i v a L.) and soybeans, as shown by b i o assay o f s o i l 9 months a f t e r the l a s t a p p l i c a t i o n . Estimates of the r a t e of methanearsonate o x i d a t i o n t o CO2 and i n o r g a n i c arsenate are provided by s o i l incubations with 1Rel a b e l e d methanearsonate (JJS, 25) I n l i g h t t e x t u r e s o i l s of low organic matter content estimate of o x i d a t i o n and l a r g e r m i c r o b i a l p o p u l a t i o n s , about 10% per month may be o x i d i z e d . I f these r a t e s remain constant, as would be the case i n a f i r s t - o r d e r r e a c t i o n , the amount o f r e s i d u a l methanearsonate at any time a f t e r a p p l i c a t i o n may be c a l c u l a t e d . A f i r s t - o r d e r f u n c t i o n appears to be the simplest and most r e a l i s t i c representat i o n o f methanearsonate metabolism i n s o i l . T h i s i s supported by the o b s e r v a t i o n o f equal percentage l o s s e s among widely d i f f e r i n g r a t e s o f a p p l i c a t i o n (20, 25). Moreover, the general l a c k o f m i c r o b i a l response to added methanearsonate (18, 25) suggests a p a s s i v e degradation dependent p r i m a r i l y upon c o n c e n t r a t i o n . T h i s c o n t r a s t s with the degradation o f the phenoxyalkanoic acid h e r b i c i d e s , f o r example. With these, m i c r o b i o l o g i c a l adaptation occurs, r e s u l t i n g i n r a p i d l y a c c e l e r a t e d decomposition, e l i m i n a t i o n o f the l a g p e r i o d , and development o f the enrichment e f f e c t (31). The f i r s t - o r d e r r a t e law may be expressed k

=

^

l

o

g

as f o l l o w s :

^ and ^ = ^ 9 3

where:

k = r a t e constant t = time C = i n i t i a l concentration C = c o n c e n t r a t i o n a t time t tjg = h a l f - l i f e o f decomposition S u b s t i t u t i n g and s o l v i n g f o r 2 and 10% decomposition per month y i e l d s values o f 0.7847 and 0.2827, r e s p e c t i v e l y , f o r conc e n t r a t i o n s remaining 1 year a f t e r a p p l i c a t i o n o f u n i t concentrat i o n of methanearsonate. H a l f - l i v e s corresponding t o 2 and 10% per month l o s s r a t e s are 34.31 and 6.58 months, r e s p e c t i v e l y . The r e s i d u e o f N annual a d d i t i o n s i s given by the equation (32): Q

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

66

ARSENICAL PESTICIDES

r = 1+ f

+ fj 2 + f 3 +

l

x

Co \C J°>

l -

0

f

l

where:

f i = f r a c t i o n l e f t a f t e r one year r * accumulated r e s i d u e , immediately a f t e r a d d i t i o n of annual increment/ % The l i m i t of the maximum amount of r e s i d u e s — J i s given by the expression J _ . The r e s i d u e p a t t e r n f o r annual a d d i t i o n s o f i 1 ppm MSMA-As i s given i n Figure 6, assuming f i r s t - o r d e r decompos i t i o n to i n o r g a n i c arsenate a t 10% per month (above) and 2% per month (below). No l o s s e s o f As from the s o i l a r e considered Annual a p p l i c a t i o n o f mately that provided b at 2.5 kg/ha each. At the higher decomposition r a t e the maximum amount of r e s i d u a l MSMA i s 1.39 ppm and i s approached a f t e r sever a l a p p l i c a t i o n s . Residual MSMA i n t h i s s i t u a t i o n remains a t very low l e v e l s and the p r i n c i p a l product accumulating i s a r s e nate. At the low r a t e of decomposition the maximum amount of r e s i d u a l MSMA i s 4.64 ppm and i s approached a f t e r 30 annual a d d i t i o n s . At t h i s time the s o i l residues would be 15% as MSMA and 85% as arsenate. The foregoing estimates o f residue accumulation do not take i n t o account l o s s e s of As from the s o i l . F i e l d experiments (19, 24) i n d i c a t e l o s s o f As from s o i l during periods o f repeated a p p l i c a t i o n o f methanearsonate. In s i t u a t i o n s where l o s s of As i n s o i l e r o s i o n and crop removal were n i l , approximately 40 to 60% of the As a p p l i e d as methanearsonate was unaccounted f o r i n t o t a l As determinations of the s o i l . These r e s u l t s suggest a gaseous l o s s process of considerable important with regard to r e s i d u e accumulation. P r e f e r e n t i a l methylation and v o l a t i l i z a t i o n of methanearsonate would f u r t h e r reduce r e s i d u a l concentrat i o n s and a l t e r the p r o p o r t i o n of methanearsonate and arsenate. 00

f

l

Uptake o f As by Crops. The l i t e r a t u r e d e a l i n g with i n o r g a n i c As i n s o i l s and crops shows that p l a n t s g e n e r a l l y absorb and t r a n s l o c a t e r e l a t i v e l y s m a l l amounts of As. L i t t l e i n f o r m a t i o n i s a v a i l a b l e on the appearance of As residues i n crops r e s u l t i n g from root a b s o r p t i o n of methanearsonate or methanearsonate-der i v e d As. Ehman (23) reported s i g n i f i c a n t l e v e l s o f As i n s e v e r a l f i e l d crops planted immediately a f t e r a p p l i c a t i o n of DSMA to s o i l , but r a t e s of 35 kg/ha o r more were r e q u i r e d . Johnson and H i l t b o l d (24) reported As residues i n f i e l d crops grown on p l o t s r e c e i v i n g methanearsonate a p p l i c a t i o n s during the previous 4 years. Cottonseed and soybeans contained higher l e v e l s of As than corn (Zea mays L.) g r a i n o r v e g e t a t i v e m a t e r i a l o f sorghumsudan h y b r i d (Sorghum vulgare Pers. X Sudanense (Piper) Stapf.). Cool-season forages accumulated l e s s As than summer crops.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Figure 6. Residue pattern for annual additions of 1 ppm MSMA-As first-order half-life of 6.58 mo (top) and 34.31 mo (bottom)

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

and

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ARSENICAL PESTICIDES

Later experiments (19) with soil applications of MSMA at elevated rates preplant to cotton over a 6-year period failed to show As residues in cottonseed grown on 3 soil types. Sustained used of methanearsonates at rates recommended for weed control does not appear to result in hazardous accumulation of As in harvested crops. Methanearsonates are extensively adsorbed in soil and subject to l i t t l e movement in leaching. Loss from treated fields may occur where erosion is severe. Volatilization of methylated As appears to be a significant loss process that limits As accumulation in soil and probably accounts for the ubiquitous occurrence of As in soils generally. The organoarsenicals are oxidized microbiologically producing arsenate. While arsenate is subject to methylation and volatile loss, much of the residual As in soil reverts to progressively less soluble forms with aluminum and iron compounds. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Sillen, L. G., Martell, A. E . , "Stability Constants of Metal-Ion Complexes" Second Edition, P. 206. Chemical Society, London, 1964. Widiger, R. E., Proc. S. Weed Conf. (1966) 19, 51-56. Baker, R. S., Arle, H. F., Miller, J. H., Holstun, J. T. Jr., Weed Sci. (1969) 17, 37-40. Long, J. A., Allen, W. W., Holt, E. C., Weeds (1962) 10, 285-287. Stevens, G. D., Proc. S. Weed Conf. (1966) 19, 545-549. Sckerl, M. M., Frans, R. E . , Weed Sci. (1969) 17, 421-427. Duble, R. L., Holt, E. C., McBee, G. G., J. Agr. Food Chem. (1969) 17, 1247-1250. Sachs, R. M., Michael, J. L., Weed Sci. (1971) 19, 558564. Rumburg, C. B., Engel, R. E . , Meggitt, W. F., Weeds (1960) 8, 582-588. Keeley, P. E . , Thullen, R. J., Weed Sci. (1971) 19, 601606. McWhorter, C. G., Weeds (1966) 14, 191-194. Keeley, P. E . , Thullen, R. J., Weed Sci. (1971) 19, 297300. Arle, H. F., Hamilton, K. C., Weed Sci. (1971) 19,545-547. Duble, R. L . , Holt, E. C., McBee, G. G., Weed Sci. (1968) 16, 421-424. Hamilton, K. C., Weed Sci. (1971) 19, 675-677. Holt, E. C., Faubion, J. L . , Allen, W. W., McBee, G. G., Weeds (1967) 15, 13-15. Wauchope, R. D., Proc. S. Weed Sci. Soc. (1974) 27, 389. Dickens, Ray, Hiltbold, A. E . , Weeds (1967) 15, 299-304. Hiltbold, A. E., Hajek, B. F., Buchanan, G. A . , Weed Sci. (1974) 22, 272-275. Woolson, E. A . , Kearney, P. C., Environ. Sci. Technol.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

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(1973) 7, 47-50. Jacobs, L. W., Syers, J. K., Keeney, D. R., Soil Sci. Soc. Amer. Proc. (1970) 34, 750-754. Schweizer, E. E . , Weeds (1967) 15, 72-76. Ehman, P. J., Proc. S. Weed Conf. (1965) 18, 685-687. Johnson, L. R., Hiltbold, A. E. Soil Sci. Soc. Amer. Proc. (1969) 33, 279-282. Von Endt, D. W., Kearney, P. C., Kaufman, D. D., J. Agr. Food Chem. (1968) 16,17-20. Thom, C., Raper, K. B., Science (1932) 76, 548-550. Challenger, F., Chem. Rev. (1945) 36, 315-361. Cox, D. P., Alexander M., Bull. Env. Cont. Tox. (1973) 9, 84-88. McBride, B., Wolfe, R., Biochem. (1971) 10, 4312-4317. Singh, R. K. N . 171. Audus, L. J., Plant and Soil (1951) 3, 170-192. Hamaker, J. W., Advan. Chem. Ser. (1966) 60, 122-131.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

5 Arsenicals in Animal Feeds and Wastes C. C. CALVERT Biological Waste Management Laboratory, Agricultural Environmental Quality Institute, Agricultural Research Service, Department of Agriculture, Beltsville, Md. 20705 Since arsenic trioxid ing over 4000 years ago of medicinal purposes (1). The modern era of arsenic use in medicine began with the characterization of arsanilic acid by Erlich and Bertheim in 1907 (2) and was extended more recently by the discovery in 1945 by Morehouse and Mayfield (3) that an organic arsenical, 3-nitro-4-hydroxyphenylarsonic acid commonly called (3-nitro), could be used to control coccidiosis and promote growth in chicks. Since that time, other organic arsenicals such as arsenosobenzene, arsanilic acid, 4-nitrophenylarsonic acid and p-ureidobenzenearsonic acid have been shown to have both therapeutic and growth-promotant properties as feed additives for poultry and swine. At the present time, a l l of these compounds, with the exception of arsenosobenzene, are approved by the U.S. Food and Drug Administration for use in poultry and swine feeds but only at levels low enough to preclude residues in edible animal tissue which would be a hazard to human health. The objective of this discussion will be to review briefly the efficacy of these arsenic compounds with respect to animal production, the absorption and excretion of these arsenicals when fed to animals and, finally, the fate of arsenic in animal excreta. Arsenicals in Swine and Poultry Feeds Table 1 shows the chemical structure of 4 arsenicals currently approved for use as growth promotants or therapeutic agents, or both, in poultry and swine feeds. The arsenic in a l l these compounds is pentavalent. Changing the substituents on the ring changes the growth-promoting and anti-parasite effects of the compound. Note that arsanilic acid and 3-nitro are approved for both poultry and swine, whereas 4-nitrophenylarsonic acid and p-ureidobenzenearsonic acid are approved only for turkeys. The use of these compounds in turkeys is limited to feeding for therapeutic purposes and may not be used as growth promotants (4). 70

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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71

Animal Feeds and Wastes

Table 1.

Chemical s t r u c t u r e of a r s e n i c a l s used i n animal feeding As 0 (OH), As 0 (OH),

0

N0 NH.

OH 3-nitro-4-hydroxyphenylarsonic A c i d (Poultry & Swine)

A r s a n i l i c Acid (Poultry & Swine) As 0 ( 0 H )

o

n

As 0 ( 0 H )

o

o

NO,

4-nitrophenylarsonic (Turkeys)

Acid

p-ureidobenzenearsonic A c i d (Turkeys)

The FDA has e s t a b l i s h e d l e v e l s a t which these compounds may be used i n animal feeds, and i t has a l s o determined the maximum l e v e l s of a r s e n i c that may be present i n marketed p o u l t r y and swine. These maximum use l e v e l s and maximum t i s s u e l e v e l s are shown i n Table 2. In order that these t i s s u e l e v e l s may be met, the user of a r s e n i c a l - c o n t a i n i n g feed i s r e q u i r e d by FDA regul a t i o n s to withdraw the a r s e n i c - c o n t a i n i n g feed f o r a t l e a s t 5 days before slaughter. Growth Response from the Use o f A r s e n i c a l s Many experiments have expanded on the o r i g i n a l demonstration of the e f f i c a c y of 3 - n i t r o and other a r s e n i c a l s i n both p o u l t r y and swine r a t i o n s . No attempt w i l l be made i n t h i s d i s c u s s i o n to cover a l l of the research that has been conducted with the a r s e n i c a l s i n feed, but a few examples of the types o f responses obtained w i l l be presented. A number of reviews on t h i s subject are i n the l i t e r a t u r e and should be consulted f o r more d e t a i l e d information (5-9.). Baron (9), a t a symposium i n London i n 1969, presented examples of the type of responses that may be obtained by feeding a r s e n i c a l s to growing chickens. Results of a study i n which 3- n i t r o was f e d i n 15 t r i a l s to a t o t a l of 1148 b i r d s f o r a 4-week p e r i o d are shown i n Table 3. As can be seen i n t h i s t a b l e , the average response i n weight increase due to a r s e n i c i n these s t u d i e s was 4.1%. The response i n feed to gain r a t i o was 1.6%. The r e s u l t s shown i n Table 4 show the average responses from 5 t r i a l s i n v o l v i n g more than 2500 chickens f o r an 8-week feeding p e r i o d . This p e r i o d represents the time needed to b r i n g a

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

InArsenicalPesticides;Woolson,E.; ACSSymposiumSeries;AmericanChemicalSociety:Washington,DC,1975.

21

Turkeys

PoultrySwine

Swine

Poultry-

170g/ton(187 mg/kg)

45g/ton(50 mg/kg) 68g/ton(75 mg/kg)

90g/ton(100 mg/kg)

90g/ton(100 mg/kg)

p-ureidobenzenearsonic acid Turkeys 340g/ton(375 mg/kg) 1/ Source: (4) . 2] B r o i l e r s , l a y i n g hens and turkeys.

4-n i tropheny1arsonic acid

3- n i t r o - 4 hydroxyphenylarsonic acid

Arsanilic acid

~2T

Same as a r s a n i l i c a c i d

Same as a r s a n i l i c a c i d

Same as a r s a n i l i c a c i d Same as a r s a n i l i c a c i d

0.5 mg/kg f r e s h , uncooked muscle 2.0 mg/kg f r e s h , uncooked by-products 0.5 mg/kg f r e s h muscle and by-products other than kidney & l i v e r 2.0 mg/kg f r e s h , uncooked kidney & l i v e r

^, Maximum p e r m i s s i b l e l e v e l s of a r s e n i c a l s i n animal feeds and maximum D e r m i s s i b l e l e v e l s o f a r s e n i c i n animal t i s s u e . — Maximum Maximum Species feed l e v e l tissue arsenic l e v e l Compound

Table 2.

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chicken to market s i z e . In t h i s t e s t , the i n c r e a s e i n weight was not s i g n i f i c a n t , but feed conversion w i t h a r s e n i c supplement was s i g n i f i c a n t l y improved - 1.99 g of feed to produce 1 g of l i v e weight gain with the a r s e n i c a l compared w i t h 2.04 g of feed f o r each gram of g a i n with the c o n t r o l s . Table 3.

Treatment

Summary of t e s t s to show the e f f e c t of feeding 3-nitro-4-hydroxyphenylarsonic a c i d (0.005%) on the weight g a i n and feed e f f i c i e n c y of b r o i l e r c h i c k s during the f i r s t 4 weeks of l i f e . — Average Average feed Average feed conversion Average weight T r i a l s Bird conversio weight (No.) (No. weight)

3-Nitro 15 Nonmedicated controls 15 1/ Source: (9). Table 4.

1148

530

1148

510

4.1

1.70

1.6

1.73

Summary of t e s t s to show the e f f e c t of feeding 3 - n i t r o (0.005%) on the growth and feed e f f i c i e n c y , of b r o i l e r s during the f i r s t 8 weeks of l i f e . Average feed Average Trials conversion weight Birds (No..) (No.) (g feed/g weight) (g) 7

Treatment

3-Nitro 5 Nonmedicated controls 5 11 Source: ( 9 ) .

2568

1448

1.99

2568

1430

2.04

Results of an i n v e s t i g a t i o n by Hansen (10) on the response of growing swine to 3-nitro are shown i n Table 5. The a r s e n i c a l was f e d i n t h i s study f o r the f u l l growing p e r i o d and r e s u l t e d i n a s i g n i f i c a n t improvement i n both average d a i l y g a i n and feed conversion.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

74 Table 5.

E f f e c t of 3 - n i t r o on growth r a t e and feed conversion of growing sheep.— Average d a i l y g a i n from weaning Feed conversion to 90.9 kg (kg feed/kg weight) (kg)

3-nitro (40 mg/kg of feed) Nonmedicated c o n t r o l s 1/ Source: (10).

0.79 0.75

3.50 3.56

Reporting these r e s u l t s i s not intended as an endorsement f o r 3 - n i t r o as the a r s e n i c a l of choice i n animal feeding. S i m i l a r responses have been shown w i t h other a r s e n i c a l s such as a r s a n i l i c a c i d f o r chickens experiments reviewed f o a r s e n i c a l a d d i t i v e s were obtained i f supplementation d i d not exceed recommended l e v e l s . There were a few instances reported i n which no improvements i n growth and feed e f f i c e n c y were obtained from the a d d i t i o n of a r s e n i c a l s to swine and p o u l t r y d i e t s , but the high frequency of p o s i t i v e e f f e c t s from a r s e n i c a l s gives the producer that small edge that may w e l l be the d i f f e r e n c e between a p r o f i t and l o s s i n a modern swine and p o u l t r y feeding enterprise. A r s e n i c a l s are a l s o approved f o r feeding i n combination with other feed a d d i t i v e s such as c o c c i d i o s t a t s and a n t i b i o t i c s . There have been many experiments reported i n the l i t e r a t u r e t e s t i n g combinations of the a r s e n i c a l s with the wide v a r i e t y of a n t i b i o t i c s and c o c c i d i o s t a t s used i n p o u l t r y and swine d i e t s . Two reviews of these experiments (5,9) should be consulted f o r more detailed information. In many i n s t a n c e s , a combination of a r s e n i c a l and a n t i b i o t i c has improved growth that i s greater than that obtained w i t h e i t h e r of the a d d i t i v e s fed alone. There i s a t present no explanation as to why one a n t i b i o t i c w i l l respond one way when combined w i t h an a r s e n i c a l and another w i l l respond another way. Mechanism of. A c t i o n A number of t h e o r i e s have been proposed f o r the mechanism of a c t i o n of a r s e n i c a l s i n i n c r e a s i n g growth i n swine and p o u l t r y . Peoples (8) has suggested that the a r s e n i c a l may i n h i b i t those organisms that cause t h i c k e n i n g of the gut w a l l and therefore r e s u l t i n more e f f i c i e n t absorption of n u t r i e n t s . Another theory i s that these compounds a c t l i k e a n t i b i o t i c s and i n h i b i t harmful b a c t e r i a . However, s t u d i e s of the b a c t e r i o s t a t i c value of a r s e n i c a l s by F r o s t and Spruth (6) i n d i c a t e that with c e r t a i n b a c t e r i a , E s c h e r i c h i a c o l i and C l o s t r i d i u m p e r f r i n g e n s , f o r example, the a r s e n i c a l s are much l e s s e f f e c t i v e than conventional a n t i b i o t i c s . Another theory, proposed by Pope and Schaible (11),

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suggests that a r s e n i c a l s may exert a sparing e f f e c t on p r o t e i n . In t h i s study, egg production i n hens f e d a 13% p r o t e i n d i e t was increased by the a d d i t i o n of a r s a n i l i c a c i d to the d i e t to a l e v e l comparable with the egg production i n hens f e d a 16.5% p r o t e i n r a t i o n . Russo e t a l . (12) a l s o found that a r s a n i l i c a c i d reduced u r i n a r y n i t r o g e n e x c r e t i o n i n swine, and they suggested that a r s e n i c a l s exert a sparing e f f e c t on p r o t e i n by reducing p r o t e i n catabolism. Thus, no one mechanism can l i k e l y e x p l a i n the a c t i o n of an a r s e n i c a l , and probably a combination of f a c t o r s produces the observed responses. Uptake and D e p l e t i o n of A r s e n i c from Tissue A r s e n i c a l s i n an animal' feed w i l l r e s u l t i a r s e n i residues i n t i s s u e s . Dat a r s a n i l i c a c i d a t 0.01 3-nitro-4-hydroxy phenylarsonic a c i d a t 0.005% r e s u l t s i n low l e v e l s of a r s e n i c i n l i v e r t i s s u e . Feeding 10 times the approved l e v e l s of e i t h e r of these a r s e n i c a l s d i d not r e s u l t i n 10 times the l e v e l of a r s e n i c i n the t i s s u e s . The data suggest that i n chicken l i v e r t i s s u e , there i s an upper l i m i t f o r a r s e n i c . These same authors a l s o presented evidence that a r s e n i c i n l i v e r and muscle of c h i c k s q u i c k l y reached a p l a t e a u l e v e l and d i d not i n c r e a s e f u r t h e r when a r s a n i l i c a c i d was f e d continuously a t 0.005% o f the r a t i o n f o r a 9-week p e r i o d . Table 6. Compound and

A r s e n i c found i n l i v e r s of chickens f e d v a r i o u s arsenicals.— A r s e n i c (As^O^) i n -

feed l e v e l

Arsanilic acid: 0.01% 0.1% 3-nitro-4-hydroxyphenylarsonic acid: 0.005% 0.05% Dodecylamine p-chlorphenylarsonate: 0.01% 1/ Source: ( 6 ) .

Feed —:

Fresh ppm

liver

45.4 455

1.2 6.4

18.7 187

2.4 7.5

23^3

2 ^

Evidence f o r the r a p i d i t y with which a r s e n i c i s taken up and e l i m i n a t e d from chicken t i s s u e s i s shown i n Table 7. These data by Baron (9) show that i n the kidney, l i v e r , muscle and s k i n of chicks f e d 0.005% 3 - n i t r o , there i s a l o w - l e v e l uptake of a r s e n i c that plateaus a f t e r 7 days of feeding and i s w e l l below tolerance l e v e l s a f t e r only 1 day of withdrawal o f the a r s e n i c a l .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

76 Table 7. Days On Test On Med. Off Med.

Summation of a r s e n i c l e v e l s (3-nJtro, of the d i e t ) i n chicken t i s s u e s . — 1 1

-

Arsenic Kidney .93 Liver 1.31 Muscle .03 Skin .05 1/ Source: (9).

7 7

-

56 56

70 70

-

-

.005%

71

75

80

84

-1

-5

-10

-14

NonMed.

.22 .69 .03 .08

.10 .43 .01 .02

.09 .32 .02 .03

.08 .19 .02 .03

.05 .08 .02 .02

PPM

.76 2.43 .07 .11

.52 1.26 .05 .06

.64 1.26 .04 .05

Feeding a r s a n i l i c a c i a r s e n i c i n l i v e r , kidne muscle and blood. The method of feeding, and i n p a r t i c u l a r the water i n t a k e , may a l s o i n f l u e n c e the d e p o s i t i o n of t i s s u e a r s e n i c . Vorhies e t a l . (13) reported that a r s e n i c i n l i v e r increased from 1.8 ppm to 3.3 ppm when water intake i n swine was reduced. An e a r l i e r study by Bridges et a l . (14) a l s o showed t h i s e f f e c t . None of the a r s e n i c a l s have been approved by FDA f o r use i n feed f o r ruminant animals (beef c a t t l e , d a i r y cows or sheep). However, the c u r r e n t i n t e r e s t i n the use of animal wastes, p a r t i c u l a r l y d r i e d p o u l t r y excreta, as animal feed supplements has generated i n t e r e s t i n the a r s e n i c content of these wastes and i n the e f f e c t of a r s e n i c on the ruminant. Studies on the uptake of a r s e n i c from a r s e n i c a l s i n sheep have been conducted a t our l a b o r a t o r i e s at B e l t s v i l l e . In these i n v e s t i g a t i o n s , a r s e n i c , as a r s a n i l i c a c i d , has been fed to mature wethers at l e v e l s up to 273 mg/kg of d i e t f o r a p e r i o d of 28 days. The l e v e l s of a r s e n i c found i n various t i s s u e s i s shown i n Table 8. A l l t i s s u e s showed i n c r e a s i n g l e v e l s of a r s e n i c as the d i e t a r y a r s e n i c l e v e l s increased. Table 8. Arsenic fed (mg/kg of diet) 0.0 26.8 144.4 273.3

A r s e n i c l e v e l s i n t i s s u e s of wethers fed a r s a n i l i c a c i d f o r 28 days. Whole blood Liver Kidney Muscle mg/kg dry t i s s u e <0.01 0.063 0.270 0.536

<0.01 3.1 26.8 29.2

<0.01 3.2 12.2 23.6

<0.01 0.2 1.1 1.2

At the end of the 28-day feeding p e r i o d , a l l sheep were placed on an a r s e n i c - f r e e d i e t , and every 2 days an animal from each treatment group was k i l l e d f o r sample c o l l e c t i o n . Table 9

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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shows the r e s u l t s of a 6-day d e p l e t i o n p e r i o d i n l i v e r At a l l l e v e l s of a r s e n i c a l f e e d i n g , there was a r a p i d of a r s e n i c from l i v e r t i s s u e to l e v e l s that are below t o l e r a n c e l e v e l s e s t a b l i s h e d by FDA f o r e d i b l e t i s s u e p o u l t r y and swine.

arsenic. depletion the from

Table 9.

D e p l e t i o n of a r s e n i c i n l i v e r of wethers fed a r s a n i l i c a c i d f o r 28 days. Arsenic Withdrawal time (days) Fed 0 2 4 6 (mg/kg of d i e t ) mg/kg D.M. 0.0 <0.01 <0.01 <0.01 <0.01 26.8 3.1 4.9 2.9 1.9 144.4 26. 273.3 29. As described i n t h i s s e c t i o n , a r s e n i c i s deposited i n animal t i s s u e a f t e r l o w - l e v e l feeding, but i n a l l species s t u d i e d , there i s a r a p i d e x c r e t i o n of t h i s element a f t e r withdrawal of the a r s e n i c a l from the feed. Metabolism of

Arsenic

The metabolism of ingested a r s e n i c from the a r s e n i c a l s has been i n v e s t i g a t e d by a number of r e s e a r c h e r s . Moody and Williams (15-17) i n a s e r i e s of experiments showed that chickens excreted a r s a n i l i c a c i d l a r g e l y unchanged without evidence that i t was converted i n t o any other organic a r s e n i c compound or to i n o r g a n i c a r s e n i c . F o u r - n i t r o p h e n y l a r s o n i c a c i d , however, was p a r t l y converted to a r s a n i l i c a c i d and 3 - n i t r o was p a r t l y converted to 3-amino-4-hydroxyphenylarsonic a c i d . There was no i n d i c a t i o n that any of the 3 - n i t r o was converted to i n o r g a n i c arsenic. F r o s t (18), Hansen e t al.(19) and Overby and Frederickson (20) a l l showed that i n p o u l t r y and swine, a high percentage of ingested a r s e n i c i s excreted and that t h i s e x c r e t i o n i s very r a p i d . In our own work w i t h growing sheep i n a 5-day balance t r i a l , about 87% of a l l ingested a r s e n i c was excreted. Table 10 presents the r e s u l t s of t h i s study. Results from t h i s study a l s o show that about 76% of the 3 - n i t r o i n p o u l t r y waste fed to sheep was excreted i n f e c e s .

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Table 10.

Ration

A r s e n i c e x c r e t i o n i n wethers fed driecjl,broiler 2i manure during a 5 day balance t r i a l . — DriedTotal Total Total poultry Arsenic arsenic fecal urinary Arsenic manure i n feed intake arsenic arsenic excreted (%) (mg/kg DM) (mg) (%)

1 2 3

<0.01 <0.01 0 <0.01 <0.01 <0.01 83.2 3.4 7 3.42 8.5 14.3 90.8 14 4.4 18.4 4.95 25.1 1/ Feed intake measured and t o t a l feces and u r i n e c o l l e c t e d during the l a s t 5 days of a 15 day feeding p e r i o d . 2/ Contained a r s e n i c from 3-nitro-4-hydroxyphenylarsonic a c i d i n dehydrated b r o i l e Fate of Excreted

Arseni

There i s concern over the extent to which a r s e n i c i n animal excreta used as a f e r t i l i z e r or as a n u t r i e n t source i n animal feeds presents a hazard to crops or the food c h a i n . One study conducted by Morrison (21) showed no increase i n a r s e n i c i n s o i l , water or forage a f t e r p o u l t r y l i t t e r c o n t a i n i n g from 15 to 30 ppm a r s e n i c had been a p p l i e d to land a t a r a t e of 4 to 6 tons per acre per year f o r 20 years. The r a t e of a r s e n i c a p p l i c a t i o n was approximately 100 g per acre per year, and the amount found i n the s o i l was approximately 1.8 ppm. This l e v e l was comparable with that found i n s o i l s with no p o u l t r y l i t t e r added. V o l a t i l i z a t i o n and l e a c h i n g undoubtedly would account f o r the disappearance of the 2000 g of a r s e n i c deposited over the 20-year p e r i o d . Undoubtedly, higher l e v e l s of manure w i l l be used, which could r e s u l t i n higher l e v e l s of the a r s e n i c a l residues being deposited on v a r i o u s kinds of s o i l s . However, l e v e l s of a r s e n i c a p p l i e d under these c o n d i t i o n s w i l l not l i k e l y begin to approach the l e v e l s reported when various h e r b i c i d e s and p e s t i c i d e s have been used on crops and s o i l s f o r long periods of time. To produce a l e v e l of a r s e n i c i n s o i l comparable to the 550 ppm found by Williams and Whetstone (22), 2200 tons of p o u l t r y l i t t e r per acre per year c o n t a i n i n g 15 to 30 ppm a r s e n i c would have to be a p p l i e d . In a recent experiment by Woolson (23), the p e r s i s t e n c e and chemical d i s t r i b u t i o n of a r s a n i l i c a c i d i n Lakeland sandy loam, Hagerstown s i l t y loam and C h r i s t i a n a c l a y loam were examined. Two a p p l i c a t i o n r a t e s (158 and 790 ppm a r s a n i l i c a c i d ) and two moisture l e v e l s (75 percent of s o i l c a p a c i t y and f l o o d i n g to e s t a b l i s h a e r o b i c and anaerobic c o n d i t i o n s r e s p e c t i v e l y ) were imposed on the three s o i l types. A f t e r a 32-week i n c u b a t i o n p e r i o d l e s s than 10 percent of a p p l i e d a r s a n i l i c a c i d could be extracted from any s o i l type. T o t a l a r s e n i c remained constant i n the Lakeland and Hagerstown s o i l s and decreased somewhat i n the C h r i s t i a n a s o i l sample. The disappearance of a r s a n i l i c a c i d

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occurred more r a p i d l y under anaerobic c o n d i t i o n s i n a l l s o i l types. I t appears that under the c o n d i t i o n s of t h i s experiment a r s a n i l i c a c i d i s r a p i d l y degraded to arsenate and an organic f r a c t i o n and t h i s degradation proceeds more r a p i d l y i n anaerobic soils. There seems to be some r e d u c t i o n of a r s a n i l i c a c i d to a v o l a t i l e organic a r s e n i c compound i n C h r i s t i a n a s o i l . Processed animal wastes are being considered f o r use as a f e e d s t u f f f o r c e r t a i n c l a s s e s of l i v e s t o c k and p o u l t r y ; consequently there has been some concern with respect to feed a d d i t i v e r e s i d u e s , i n c l u d i n g a r s e n i c , i n processed animal wastes. The r e g u l a t i o n s regarding waste r e c y c l i n g w i l l l i k e l y s p e c i f y maximum residue l e v e l s that w i l l not be harmful to animals or present a hazard to human h e a l t h . The f i n a l proposal has not been prepared by the FD e x a c t l y what w i l l be r e q u i r e feed a d d i t i v e s . Summary In summary, a r s e n i c a l s are used as feed a d d i t i v e s to promote growth i n p o u l t r y and swine as w e l l as to t r e a t s p e c i f i c diseases of these species and appear to have an e s t a b l i s h e d r o l e i n modern l i v e s t o c k and p o u l t r y production. The responses i n general are small but may represent the d i f f e r e n c e between a p r o f i t and l o s s to the animal producer. A r s e n i c can be detected a t low l e v e l s i n t i s s u e s of animals r e c e i v i n g a r s e n i c a l s . However, t h i s a r s e n i c i s r a p i d l y e l i m i n a t e d when a r s e n i c a l s are removed from the feed f o r the required 5-day p e r i o d . This process appears to be w e l l regulated, and there does not appear to be a hazard to the consumer from a r s e n i c i n e d i b l e animal t i s s u e . The a r s e n i c a l s appear to be l a r g e l y excreted unchanged i n chemical s t r u c t u r e w i t h no t o x i c metabolites produced and with no evidence of the conversion to i n o r g a n i c forms of a r s e n i c i n the animal. Because a r s e n i c a l s are excreted f o r the most part i n f e c e s , no evidence i s a v a i l a b l e to i n d i c a t e that the amounts of a r s e n i c that might be incorporated i n t o s o i l s by the d i s p o s a l of animal wastes would c o n s t i t u t e any problem with regard to p l a n t growth or u l t i m a t e l y to a r s e n i c contamination of consumer products.

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ARSENICAL PESTICIDES

Literature Cited 1. 2. 3.

Frost, D. V. Federation Proc. (1967), 26, 194. Erlich, P., and Bertheim. Berichte (1907), 40, 3292. Morehouse, N. F . , and Mayfield, O. J. J. Parasitol., (1946), 32, 20. 4. Anonymous. "Feed Additive Compendium", The Miller Publishing Company, Minneapolis, Minnesota (1973). 5. Frost, D. V . , Overby, L. R., and Spruth, H. C. J. Agr. and Food Chem. (1955), 3, 235. 6. Frost, D. V . , and Spruth, H. C. Symposium on medicated feeds. Medical Encyclopedia, New York (1956). 7. Smith, L. W., Calvert, C. C., Frobish, L. T . , Dinius, D. A . , and Miller, R. W. USDA,ARS 44-224 (1971). 8. Peoples, S. A. Proc (1969), 77. 9. Baron, R. R. Proc. Seminar at the Criterion, Lower Regent Street, London, S.W. 1, Salsbury Laboratories, London (1969). 10. Hansen, L. E., H i l l , E. G., and Ferrin, E. F. J. Animal Sci. (1956), 15, 280. 11. Pope, C. W., and Schaible, P. J. Poultry Sci. (1959), 38, 350. 12. Russo, J. M., Hanson, L. E . , and Jeseski, J. J. J. Animal Sci. (1954), 13, 998. 13. Vorhies, M. W., Sleight, S. D., and Whitehair, C. K. Cornell Vet. (1969), 59, 3. 14. Bridges, J. H., Hale, F., Kunkel, H. D., and Tyman, C. M. J. Animal Sci. (1954), 13, 912. 15. Moody, J. P., and Williams, R. T. Food Cosmet. Toxicol. (1964), 2, 687. 16. Moody, J. P., and Williams, R. T. Food Cosmet. Toxicol. (1964), 2, 695. 17. Moody, J. P., and Williams, R. T. Food Cosmet. Toxicol. (1964), 2, 707. 18. Frost, D. V. Poultry Sci. (1953), 32, 217. 19. Hansen, L. E . , Carpenter, L. E . , Aunan, W. J., and Ferrin, F. E. J. Animal Sci. (1955), 14, 513. 20. Overby, L. R., and Frederickson, R. L. J. Agr. and Food Chem. (1963), 11, 378. 21. Morrison, J. L. J. Agr. and Food Chem. (1969), 17, 1288. 22. Williams, K. R., and Whetstone, R. R. USDA Tech. Bull. 732, (1940). 23. Woolson, E. A. Paper No. 58, Pesticide Chemical Division Abstract. American Chem. Soc., Los Angeles, California (1974).

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

6 Microbiological Methylation of Arsenic D O N A L D P. C O X

1

Department of Agronomy, Cornell University, Ithaca, N.Y. 14850

The b i o l o g i c a l methylation of arsenic was f i r s t recognized during severa early nineteenth century (1). Several i n d i v i d u a l s succumbed i n t h e i r sleep to arsenic poisoning, the cause of which was not immediately recognized. Initially i t was attributed to the presence of p a r t i c l e s of arsenic i n the room a i r o r i g i n a t i n g from wallpaper pigments. Others thought b i o l o g i c a l a c t i v i t y might have reduced these arsenic pigments to arsine which was the toxic agent. An experimental approach ultimately determined the cause of gas formation when an I t a l i a n s c i e n t i s t , Gosio, i s o l a t e d and characterized some v o l a t i l e arsenic-producing fungi. He exposed potato pulp cont a i n i n g arsenic t r i o x i d e to the a i r and soon detected growth of molds accompanied by a g a r l i c - t y p e odor. From these cultures he i s o l a t e d a mold P e n i c i l l i u m brevicaule (Scopulariopsis b r e v i c a u l i s ) which produced copious quantities of gas with a g a r l i c - l i k e odor. Gosio trapped samples of the gas from t h i s c u l t u r e and converted it to arsenic proving that the metal had t r u l y been v o l a t i l i z e d by the fungus (2). An associate, B i g n e l l i , performed an elemental analysis on the gaseous material and i n c o r r e c t l y deduced the structure of d i e t h y l a r s i n e (3). It wasn't for another t h i r t y years that the structure of "Gosio gas" was determined by Challenger's group i n England. A great deal of research on b i o l o g i c a l methylation of metals was performed by Challenger and associates over twenty odd years beginning with the methylation of arsenic compounds (1,4). These researchers cultured several s t r a i n s of S. b r e v i c a u l i s on bread crumbs containing arsenic t r i o x i d e and the gas produced was p r e c i p i t a t e d i n aqueous solutions as e i t h e r the mercuric c h l o r i d e , 1 Present address: Union Carbide Corp., Chemicals and Plastics, P. O. Box 8361, South Charleston, W. Va. 25303 81

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ARSENICAL PESTICIDES

p i c r a t e , or n i t r a t e d e r i v a t i v e s . These s a l t s were tediously compared with d e r i v a t i v e s made from authentic methylated arsenic compounds by melting point a n a l y s i s . Challenger indisputedly proved the structure of "Gosio gas" to be that of t r i m e t h y l arsine. From these studies, Challenger proposed a metabolic pathway i n S. b r e v i c a u l i s f o r the production of trimethylarsine from arsenite (Figure 1). To elucidate the mechanism of the methylation reaction, Challenger demonstrated that alkylated arsenicals supplied to the mold were methylated i n a v a i l a b l e p o s i t i o n s on the metal and were not f i r s t reduced to the inorganic state. This was shown by incubating ethylarsonic acid with the mold and i s o l a t i n g the v o l a t i l e product, ethyldimethylarsine experiment establishe not be formed by dismutation or the movement of a l k y l groups from one a r s e n i c a l to another. A dismutation reaction would have resulted i n a mixture of alkylated a r s e n i c a l s (l,j>). Following Challenger's e f f o r t , l i t t l e other experimentation concerning b i o l o g i c a l arsenic v o l a t i l i z a t i o n by pure cultures was performed for some time except f o r several notable observations. Thorn and Raper (6) detected g a r l i c odors produced by f i v e species of A s p e r g i l l u s , one of P e n i c i l l i u m and one species of Fusarium i n the presence of 1500 iig/ml AS2O3, however, the gas was not i d e n t i f i e d . Zussman et a l . (7) observed v o l a t i l i z a t i o n of arsenate but not arsenite by Trichophyton rubrum. The use of a r s e n i c a l s as wood preservative agents spawned another study which revealed the b i o l o g i c a l methylation phenomenon (8). In t h i s study a t o t a l of s i x t y - f i v e species of "wood r o t t i n g " fungi were tested f o r i n h i b i t i o n by or metabolism of arsenic t r i o x i d e . Of these, two species of Lenzites provided evidence for v o l a t i l e arsenic formation. The gaseous product presumed to be trimethylarsine was not i d e n t i f i e d . The study did not reveal whether arsenic-containing wood preservat i v e s would promote a s i m i l a r production of the arsenic gas by these two species. Studies of arsenic methylation i n s o i l have been also somewhat l i m i t e d . The formation of v o l a t i l e arsenic compounds by s o i l microorganisms was observed by Epps and Sturgis (9). When passing a i r through flooded s o i l containing arsenate, they were able to trap arsenic compounds i n an acid s o l u t i o n r e c e i v i n g the exhaust a i r stream. Recent studies with an arsenic herbicide (cacodylic acid) further

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supported the hypothesis that a r s e n i c - v o l a t i l i z i n g organisms are ubiquitous i n nature. Three types of s o i l demonstrated l o s s of t o t a l arsenic over several months of incubation. These samples containing high l e v e l s (100 jig/ml) of cacodylic acid emitted detectable g a r l i c odors (10). Arsenic metabolism i n the s o i l i s discussed i n greater d e t a i l i n another portion of t h i s symposium. With the advent of e c o l o g i c a l concern over d i s posal of waste arsenic compounds and the p o t e n t i a l contribution of arsenic residues from p e s t i c i d e a p p l i c a t i o n , new studies were i n i t i a t e d concerning b i o l o g i c a l methylation of arsenic. The biomass of waste treatment plants are a p o t e n t i a l source of microorganisms whic compounds. Under anaerobi , specie Methanobacterium has already been observed to v o l a t i l i z e arsenic. C e l l free extracts and/or whole c e l l preparations of t h i s organism produced dimethylarsine i n presence of arsenate or arsenite which were shown to obstruct the methane-forming mechanism (11). Under these conditions, methane formation was not only shown to be i n h i b i t e d by arsenic compounds but also by selenium and t e l l u r i u m compounds and previously by mercury (12). Recognizing the p o t e n t i a l f o r v o l a t i l i z a t i o n of arsenic v i a metabolism of p e s t i c i d e residues, studies i n our laboratory were begun to f i n d organisms capable of methylating arsenic compounds. Aerobic enrichment cultures were i n i t i a t e d using domestic sewage and four d i f f e r e n t arsenic compounds. Sodium arsenite, sodium arsenate and s a l t s of methylarsonic and dimethylarsinic acids were chosen on the basis of t h e i r use as p e s t i c i d e s and t h e i r being intermediates i n Challenger's proposed pathway. A f t e r a short incubat i o n period, g a r l i c odors were noted i n enrichment cultures containing e i t h e r dimethylarsinic acid at pH 4, 5, and 7, monomethylarsonic acid at pH 5 or sodium arsenate at pH 4. Three d i f f e r e n t species of fungi were then i s o l a t e d and i d e n t i f i e d as: 1) Candida humicola (Dazewska) Diddens and Lodder; 2) Gliocladium roseum Bain and 3) a P e n i c i l l i u m species (13). Each of these species was then examined for i t s a b i l i t y to produce trimethylarsine from four d i f f e r e n t arsenic-containing compounds. The production of trimethylarsine by each c u l t u r e appears i n Tables I, II, and I I I . As i s evident a l l three species formed the gas expeditiously on the methylated arsenic substrates at neutral and acid pH. Only the Candida species was able to demonstrate trimethylarsine

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ARSENICAL PESTICIDES

84

production on inorganic arsenicals and only i n acid conditions. TABLE I (13) Trimethylarsine Production by Candida humicola nmole TMA Compound

pH 5

DMA MMA As04= As02"

87 9 6 8

Culture age:

i n headspace pH 6

pH 7

41 6 0 6

2 0 0 11

1 week

TABLE II (13) Trimethylarsine Production by"TTlioc1adium roseum Compound DMA MMA ASO4S ASO2" Culture age:

nmole TMA piT5

i n headspace piTg

piTT

10 2970 0 0

52 3700 0 0

253 2970 0 0

1 week, time a f t e r capping:

6 days.

TABLE III (13) Trimethylarsine Production by a P e n i c i l l i u m Species Compound DMA MMA As04= ASO2" Culture age:

pH 5 22 429 0 0

nmole TMA p H

i n headspace 6 p H 14 62 0 0

4 days, time a f t e r capping:

7 6 254 0 0

3 days.

The Candida species was unique i n i t s a b i l i t y to methylate four a r s e n i c a l s and t h i s organism was selected f o r more thorough studies of i t s control mechanisms (14). The greatest quantity of trimethylarsine per c e l l was produced when C. humicola was cultured i n presence of e i t h e r arsenate or dimethyl-

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Methylation of Arsenic

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arsinate (Figure 2). Smaller amounts were produced from c e l l s grown i n presence of a r s e n i t e or methylarsonate even though, i n t h i s case, t h e i r arsenic concentrations were about twice as high as i n the arsenate or dimethylarsinate grown c u l t u r e s . The s p e c i f i c i t y of the methylation r e a c t i o n of t h i s organism f o r arsenic compounds was next tested by growing c u l t u r e s with other anions of Group 5 elements which were of s i m i l a r chemical nature to arsenate. The phosphate ion i s very s i m i l a r i n s t r u c t u r e and chemical properties to the arsenate ion although s t r i c t e r reducing conditions would be necessary to allow the formation of methylated phosphines. Not s u r p r i s i n g l y , our attempts at viewing methylated phosphorus compound were unsuccessful Similarly ga productio be detected i n presence of n i t r a t e , n i t r i t e , ammonia or antimonate ions. The presence of some of these ions i n a c u l t u r e of C. humicola methylating arsenic was not, however, without e f f e c t . We did observe a d e f i n i t e i n h i b i t i o n by the presence of phosphate on t r i m e t h y l a r s i n e production from arsenate, monomethylarsonate or arsenite (Figure 3). The i n h i b i t i o n by phosphate of trimethylarsine formation from arsenate was e f f e c t i v e at phosphate l e v e l s as low as 10 jig/ml or one onehundredth the concentration of arsenate. Phosphate was completely i n h i b i t o r y at 1000 jig/ml (0.1%) (Figure 4). The addition of phosphate to suspensions producing t r i m e t h y l a r s i n e from dimethylarsinate caused no i n h i b i t i o n (Figure 3) even at concentrations as high as eight times the arsenate concentration. The observation that phosphate does not i n h i b i t the conversion of dimethylarsinate to t r i m e t h y l a r s i n e by C. humicola i s of s p e c i a l i n t e r e s t i n l i g h t of the report by Da Costa (15) that phosphate d i d not r e l i e v e dimethylarsinate t o x i c i t y to a r s e n i c - s e n s i t i v e microorganisms. Phosphate addition d i d r e l i e v e arsenate and a r s e n i t e t o x i c i t y . These two phenomena, arsenic t o x i c i t y and v o l a t i l i z a t i o n , might conceivably be coupled i n mold metabolism. The t o x i c i t y mechanisms of a r s e n i c a l s are not completely understood but one postulated mechanism involves t h e i r r e a c t i o n with s u l f h y d r y l groups i n c e l l u l a r protein. Another may be arsenate interference with o x i d a t i v e phosphorylation, s p e c i f i c a l l y i n h i b i t i n g adenosine triphosphate synthesis. Possibly, a t h i r d mechanism of arsenic t o x i c i t y e x i s t s i n which a s i n g l e toxicant i s formed from metabolism of the various a r s e n i c a l s . Our r e s u l t s

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

0

HO-As-OH

CH+

o

ii

CH*

HO-As-CH,

I

l

OH

OH

ARSENKXJS ACO

1+

O

II

^

H C-As-CH CH 3

3

3

TRIMETHYLARSINE OXIDE Figure 1.

Challengers

HO-As-CH, i

CH

METHYL ARSONIC ADD

12

ii

o

3

DIMETHYLARSINIC ACID

•

H3C-AS-CH3 CH 3

TRIMETHYLARSINE proposed pathway

OPTICAL DENSITY OF CULTURE Figure 2. Formation of trimethylarsine from arsenate (240 fig As/ml), arsenite (544 fig As/ml), monomethylarsonate (577 fig As/ml), and dimethylarsinate (272 fig As/ml) in relation to cell density in growing cultures of Candida humicola

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Methylation of Arsenic

6. cox

ARSENITE

ARSENATE Ay»

400-

/

300-

100-

o

150-

/ /

100

200-

<

200-

J

-o-

2 DIMETHYL ARSINATE 800-

600-

400-

200-

-i

l.

HOURS Figure 3. Effect of high phosphate levels on the formation of trimethylarsine by Candida humicola grown in media with arsenate, arsenite, monomethylarsonate, and dimethylarsinate. The arrow indicates the time that supplemental KH PO, was added. 2

A. No additions; B. 0.1% KH ?O 2

t

h

added

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ARSENICAL PESTICIDES

indicated that phosphate suppresses gas evolution by preventing trimethylarsine formation i n the pathway between monomethylarsonic and dimethylarsinic acids. The hypothetical i n h i b i t o r might then be an i n t e r mediate i n the pathway of trimethylarsine production. The conversion of arsenate, arsenite or monomethylarsonic acid to the toxicant may conceivably be blocked by phosphate while dimethylarsinate conversion ( i f indeed, i t i s not the toxicant) to the hypothetic a l compound i s not. Much further study i s needed to c l a r i f y t h i s point. The i n h i b i t i o n of trimethylarsine formation i s catalyzed with anions of other Group 5 elements. S u r p r i s i n g l y enough sodium hypophosphite did not i n h i b i t trimethylarsin (Figure 5). Antimonat trimethylarsin production at equimolar concentrations to arsenate but n i t r a t e at t h i s l e v e l was not. N i t r i t e stopped trimethylarsine production but the s i t e of i n h i b i t i o n could be unrelated to the methylation process since i t i s known to be t o x i c to c e l l s at s l i g h t l y higher concentrations. It i s well known that S. b r e v i c a u l i s and c e r t a i n other cultures can form methylated d e r i v a t i v e s of selenium and t e l l u r i u m compounds (1,11>1§.)« Our s t r a i n of C. humicola was also shown to produce dimethylselenide from s e l e n i t e and selenate and a v o l a t i l e product from t e l l u r a t e (17). It was determined i f the presence of these anions and those of other Group 6 elements would a f f e c t trimethylarsine formation. In fact, sodium s a l t s of s e l e n i t e and selenate strongly i n h i b i t e d trimethylarsine synthesis and t e l l u r a t e did moderately so (Figures 6 and 7). The addition of as l i t t l e as 10 pg/ml, a r a t i o of one part s e l e n i t e to 100 parts of arsenate, i n h i b i t e d trimethylarsine production (Figure 8). Selenate also i n h i b i t e d trimethylarsine but a ten times higher concentration than s e l e n i t e was required to be as e f f e c t i v e . T e l l u r a t e i n h i b i t e d at equimolar concent r a t i o n s to arsenate. Interestingly enough, the s u l f a t e anion d i d not i n h i b i t trimethylarsine production from arsenate at equimolar concentrations of s u l f a t e to arsenate (Figure 6). It was noted, however, that dimethylselenide production from s e l e n i t e by C. humicola was p a r t i a l l y i n h i b i t e d by equimolar amounts of s u l f a t e but not arsenate (Figure 9). These previous r e s u l t s indicated that p a r t i a l l y separate methylation systems i n C. humicola might function f o r selenium and arsenic. To aid i n studying t h i s phenomenon, C. humicola was grown i n presence of

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Methylation of Arsenic

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89

NO • PHOSPHATE/

4 6 HOURS Figure 4. Inhibition of trimethylarsine formation from arsenate (240 fig As/ml) by resting cells in the presence of various KH PO,, concentrations 2

Figure 5.

0.025 0.050 0.075 0.10 CONCENTRATION OF P, N, OR Sb SALT, % Influence of orthophosphate hypophosphite, nitrate, and antimonate on the production of trimethylarsine by resting cells y

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ARSENICAL PESTICIDES

100

Figure 6.

500

1000

Trimethylarsine evolution from 1000 fig NaH>AsO, /ml by resting cells incubated with various concentrations of selenate, selenite, and sulfate t

Figure 7. Trimethylarsine production by a growing culture of Candida humicola in the presence of either 100 fig Na Se0 , 10 mg Na,SeO or 1.0 mg Na TeO,,/ml 2

h

3

2

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91

Methylation of Arsenic

Figure 8. NaH AsOt 2

Selenite inhibition of trimethylarsine formation from by resting cells of Candida humicola. The concentrations shown are the levels of Na SeO . 2

a

% 5

I

I

250

Figure 9.

,

500 CONCENTRATION,

I

750

-

-L.

1000

>JG/ML

The effect of concentration of several compounds on the rate of dimethylselenide production by Candida humicola

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ARSENICAL PESTICIDES

e i t h e r s e l e n i t e , arsenate or i n absence of both. The arsenate-grown c e l l s were then incubated i n presence of e i t h e r arsenate or s e l e n i t e and the time was measured f o r the formation of the appropriate gas. The same was done with the selenite-grown and the un-induced c e l l s . The l a g periods f o r trimethylarsine production from selenite-grown, arsenate-grown and un-induced c e l l s were measured and compared. As i s evident i n Figure 10, the arsenate-induced c e l l s produced trimethylarsine sooner than the s e l e n i t e induced c e l l s which i n turn were more rapid than the un-induced c e l l s . Likewise, the selenite-induced c e l l s produced dimethylselenide sooner than the arsenate-induced or the un-induced c e l l s . One might conclude that the methylatio separate enzymatic needed. As stated e a r l i e r , trimethylarsine production i s i n h i b i t e d by selenate and s e l e n i t e as well as phosphate however, the addition of these compounds did not only i n h i b i t trimethylarsine production but a c t u a l l y appeared to remove trimethylarsine from a growing c u l t u r e i n a closed system as indicated i n Figure 11. This was supported by demonstrating a l o s s of a known amount of authentic trimethylarsine placed i n a closed system with s e l e n i t e (17). A l o s s of the gas did not occur i n presence of phosphate or arsenate or i n absence of c e l l s (Figure 12). Previous work describing a chemical complex which can form between selenium metal (18) and trimethylarsine may explain the phenomemon since C. humicola suspensions containing s e l e n i t e produce an orange c o l l o i d a l material, i d e n t i f i e d as selenium metal (17), i n addition to dimethylselenide. No chemical evidence f o r such a trimethylarsine-selenium compound was found but i f formed, could account f o r the severe i n h i b i t i o n of the arsenic-methylating system. From an e c o l o g i c a l standpoint, studies are needed to determine i f v o l a t i l e forms can be assimilated and stored by higher animals. An arsenic analysis on a species of marine kelp found that t h i s plant was concentrating the metal one-thousand f o l d (19). In l i g h t of past evidence that methylated metals, namely mercury, can be bio-magnified to toxic l e v e l s by passing through natural food chains (20) studies are needed to determine i f t h i s phenomenon occurs with arsenic. To date one study has been made i n d i c a t i n g that some a r s e n i c a l s may not be as ominous as mercury. A model ecosystem containing four aquatic organisms with dimethylarsinic acid and dimethylarsine did not

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Methylation of Arsenic

Figure 10. The formation of trimethylarsine (top) and dimethylselenide (bottom) by cell suspensions of Candida humicola grown in the presence of 1.0 mg Na SeO /ml (A), 1.0 mg NaH AsO /ml (B) or neither (C) 2

i

2

h

}

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

94

no additions • IOOO jjg/ml KH P0 added 2

4

Figure 11. The effect of the addition of sodium selenate or potassium phosphate on trimethylarsine production by a growing culture of C . humicola

no additions 4CM 3020'

\

20 • 10'

0.5

1.0

1.5

no cells

1

Figure 12. Rate of disappearance of trimethylarsine incubated with and without cell suspensions of Candida humicola and 100 ^g of Na>SeO>, 1.0 mg of KH PO or NaH AsO,,/ml 2

2

h

20

4

3 o \

V

2 10-

^ 40a> 1 30 C (

As0 = 1000 pg/ml

0

05

1.0

1.5

o-o cells plus effector

Se0 =

pq=

100 >ig/ml

1000 jjg / ml

4

\

3Q |,

V

20 10

10 1.0

1.5 O HOURS

0.5

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

1.0

1.5

6. cox

Methylation of Arsenic

95

show s i g n i f i c a n t bio-magnification (21). In summary, evidence for the methylations of arsenic compounds by three mold species from sewage has been presented. The frequent use of arsenic compounds as pesticides provides a p o t e n t i a l hazard i f closed b i o l o g i c a l systems containing populations capable of methylating are exposed to a r s e n i c a l s . One of these three species, a yeast, has been shown to methylate selenium and probably t e l l u r i u m anions as well as arsenic compounds. The methylation mechanisms for selenium and arsenic i n t h i s organism appear to be p a r t i a l l y found i n a common pathway. Despite the widespread usage of arsenic compounds, l i t t l e i s known of the behavior of t h i s element i n natural ecosystems an hazard of arsenic poisoning is self-evident. L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Challenger, F. Chem. Rev. (1945) 36:315-361. Gosio, B. Arch. I t a l . B i o l . (1893) 18:253-265. Gosio, B. Arch. I t a l . B i o l . (1901) 35:201-213. Challenger, F. " B i o l o g i c a l M e t h y l a t i o n , " Advances i n Enzymology, pp. 429-491, Interscience Publishers, New York, 1951. Challenger, F . , Higginbottom, C., and Ellis, L. J . Chem. Soc. (1933) 1933:95-101. Thom, C. and Raper, K. Science (1932) 76:548-550. Zussman, R., Vicher, E. and Lyon, I. J . B a c t e r i o l . (1961) 81:157. M e r r i l l , W. and French, D. W. Proc. Minn. Acad. S c i . (1964) 31:105-106. Epps, E. A. and Sturgis, M. B. Proc. S o i l S c i . Soc. Amer. (1939) 4:215-218. Woolson, E. A. and Kearney, P. C. Environ. S c i . Tech. (1973) 7:47-50. McBride, B. C. and Wolfe, R. S. Biochemistry (1971) 10:4312-4317. Wood, J . M . , Kennedy, F. S., and Rosen, C. G. Nature (London) (1968) 220:173-174. Cox, D. P. and Alexander, M. Bull. Environ. Contam. T o x i c o l . (1973) 9:84-88. Cox, D. P. and Alexander, M. Appl. M i c r o b i o l . (1973) 25:408-413. Da Costa, E. W. B. Appl. M i c r o b i o l . (1972) 23:4653. Fleming, R. W. and Alexander, M. Appl. M i c r o b i o l . (1973) 24:424-429.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

96

ARSENICAL PESTICIDES

17. Cox, D. P. and Alexander, M. Microbial Ecology (1974) 1 ( i n press). 18. Renshaw, R. R. and Holm, G. E . J. Amer. Chem. Soc. (1920) 42:1468-1471. 19. Gorgy, S., Rakestraw, N. W., and Fox, D. L. J. Marine Res. (1948) 7:22-32. 20. Katz, A. Crit. Rev. Environ. Centr. (1972) 2:517534. 21. Isensee, A. R., Kearney, P. C., Woolson, E. A., Jones, G. E . and Williams, V. P. Environ. S c i . Tech. (1973) 7:841-845.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

7 Bioaccumulation of Arsenicals E. A. WOOLSON Pesticide Degradation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Md. 20705

Arsenic, as a natura accumulated by aquatic organisms for arsenic, as used i n this paper, i s defined as the ratio of arsenic concentration in the organism divided by the concentration of arsenic in water. An arsenic content of 2 μg/liter or 2 ppb, an approximate average for sea water, w i l l be assumed for data supplied by authors who did not measure the concentration of arsenic in sea water when analyzing arsenic in marine organisms. Salt water (marine) organisms accumulate much more arsenic than do most fresh water organisms. Furthermore, aquatic plants accumulate much more arsenic than do higher members in the aquatic food chains. In marine plants, BR values of 71,000 have been observed. A.

Arsenic in fresh waters

Arsenic concentrations measured in fresh water bodies are l i s t e d in Table 1. The concentrations reported in a survey of U. S. lakes and rivers ranged from <10-1100 μg/liter (1). The high value of 1100 μg/liter was taken downstream from a sewage treatment plant on the Irwin Creek in North Carolina. The con

tamination source was never i d e n t i f i e d although a company p r o ducing a r s a n i l i c a c i d had been i n operation r e c e n t l y . At the

next monitoring station downstream, the arsenic level returned

to 10 μg/liter. A r s e n i c i s p r e c i p i t a t e d and complexed i n water and appears i n the sediments o f both lakes and r i v e r s . Searles Lake, C a l i f o r n i a , a h i g h l y s a l i n e lake, had the highest values o f

any water reported.

Other high saline lakes, however, contain

only t r a c e s o f a r s e n i c . The Waikato River i n New Zealand has a r s e n i c present from geothermal waters, the outflow from Lake Taupo, the Wairakei thermal area, and the thermal region at Orakei-Korako. Spring waters (2) which were high i n bicarbonate and passed through thermal l a y e r s f r e q u e n t l y contain elevated

arsenic levels. 97

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

98 Table 1.

A r s e n i c content o f n a t u r a l f r e s h waters

Source

CorTcenTr^Hon" (yg As/1)

Reference

4-117 U. S. Lakes, Wisconsin 0.5-2.4 " , Michigan 0.1-1.6 " , Superior 198,000 , Searles Lake 243,000 Calif. 0-100 " , C a l i f . ; <2000 ppm dissc )lved s o l i d s " , C a l i f . ; >2000 ppm d i s s o l v e d s o l i d s 0-2000 <10-1100 lakes and r i v e r s 1.6 Columbia R i v e r Japan, lakes , rivers Germany, Elbe R i v e r 20-25 1.1-54.5 Greece, lakes Sweden, r i v e r s 0.2-0.4 5-100 N. Zealand, Waikato R i v e r * / 130-1000 Spring Waters]?/; C a l i f o r n i a , Kamchatka, USSR, N. Zealand O i l and gas f i e l d waters; C a l i f o r n i a , L o u i s i a n a , Hungary 0.0-5800 Thermal waters; Wyoming, Nevada, C a l i f o r n i a , Alaska, Iceland 20-3800 Spring waters£/; USSR, Wyoming, A l g e r i a , Iceland 30-500 11

11

11

(3) (4) (4)

11

11

11

11

11

(5) (2) (2) (1) (6)

11

W Si

(6) (6) (6) (8) (2)

(2) (2) (2)

High i n b i c a r b o n a t e , geothermal o r i g i n s High i n bicarbonate and boron Deposit t r a v e r t i n e

High a r s e n i c l e v e l s have been found i n some d r i n k i n g waters (Table 2 ) . Contamination a r i s e s from o b t a i n i n g water that passes through layers o f a r s e n i c a l p y r i t e s , l e a c h i n g o f mine waste p i l e s (9), o r d r i l l i n g w e l l s through l a y e r s o f contaminated s o i l s (10). The last-named source o f contamination r e s u l t e d from improper storage o f an a r s e n i c a l grasshopper b a i t , and those d r i n k i n g the water became i l l . Of the 18,204 community water s u p p l i e s s t u d i e d , fewer than 1% exceeded the recommended 10 y g / l i t e r l e v e l f o r a r s e n i c (14). The average content o f a r s e n i c i n f r e s h water i s below the recommended d r i n k i n g water q u a l i t y standards f o r the U. S. and probably i s about 1.5 to 2 y g / l i t e r .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

WOOLSON

7.

Table 2.

99

Bioaccumulation

A r s e n i c content o f d r i n k i n g waters

Source

Concentration (yg As/1)

Argentina Argentina, Cordoba Province Canada U. S. , Wenatchee, Wash. " , Minnesota Taiwan

Reference

tr-300 300-1000*/ 300-7500 tr-100V 5-6£/ 11,800-2i,oooi/ 250-850£/

(11) (12) (13) (1£> (15) (10) (16)

a/ W SJ _V —f B.

Contains vanadium a l s o Less than 1% exceeded 10 yg/1 Watershed which r e c e i v e d lead arsenate Well dug through area c o n t a i n i n g grasshopper b a i t N a t u r a l l y contaminated a r t e s i a n w e l l water A r s e n i c i n marine waters

Concentrations o f a r s e n i c i n the marine environment g e n e r a l l y range from 0.15 t o 6 y g / l i t e r , with an average value o f ca. 2 yg/1 (Table 3). Areas near the r i v e r outflows tend t o have higher a r s e n i c l e v e l s than the oceans as a whole. The data o f Chapman (17) are 100-fold higher than those reported by other authors. No apparent reason f o r t h i s discrepancy e x i s t s . S c i e n t i s t s disagree as t o the o x i d a t i o n s t a t e o f a r s e n i c i n sea water. Many b e l i e v e that a r s e n i c e x i s t s i n sea water i n the +3 s t a t e ; others b e l i e v e that i t i s predominately i n the +5 s t a t e . Sugawara and Kanamori (18) showed that the +5 A s / t o t a l As r a t i o was c l o s e t o 0.8 i n ocean water. Diagrams o f vs pH f o r sea water i n d i c a t e that a r s e n i c would be i n the +5 s t a t e i n a l l o x i d i z i n g l a y e r s o f the ocean (19). Chemical reduction may take place at lower depths where o x i d a t i o n i s not great o r may be mediated by microorganisms. In any event, an e q u i l i b r i u m between arsenate and a r s e n i t e e x i s t s i n sea water. The same type o f e q u i l i b r i u m probably e x i s t s i n any s t r a t i f i e d lake (19) where p r e c i p i t a t i o n , o x i d a t i o n and r e d u c t i o n , and adsorption, reduction and methylation occur w i t h i n a s i n g l e lake. In the aerobic aqueous l a y e r , reduced forms o f a r s e n i c w i l l be o x i d i z e d to arsenate and both may c o p r e c i p i t a t e with f e r r i c hydroxide. Turbulence and convection t r a n s p o r t arsenate to the oxygen-depleted hypolimnion where i t may be reduced and form a r s e n i t e o r A s S . These reactions depend on s u l f u r concentration and the Eft. C o p r e c i p i t a t i o n , absorption, adsorption, and c r y s t a l 2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

100

Table 3.

A r s e n i c content o f marine waters

Source

Concentration (ug/As/1)

England England E n g l i s h Channel P a c i f i c coast N. W. P a c i f i c Ocean Indian Ocean S. W. Indian Ocean

106- 760 2> 2- 5 3- 6 0.15- •2.5 1.3- •2.2 1.4-5.0

Reference

(17) (20) (21, 22) ~ ( 6 r (6) (6) (6)

growth may cause arsenat reduction o f f e r r i c arsenat s o l u b i l i z a t i o n as a r s i n e s , s t a b i l i z a t i o n as i n s o l u b l e s u l f i d e s , or r e d u c t i o n t o a r s e n i c metal. M i c r o b i a l methylation and reduct i o n s o l u b i l i z e a r s e n i c , a l s o . Further, d i f f u s i o n through the sediments, or mixing by currents o r burrowing organisms, can cause the a r s e n i c to r e e n t e r the water phase. A r s e n i c i n the water phase i s adsorbed o r i n c o r p o r a t e d i n t o benthic organisms, algae, zooplankton and phytoplankton. T h i s a r s e n i c i s converted to organo-arsenical compounds as w e l l as water-soluble compounds w i t h i n these organisms (23, 24, 25). F i s h consume the algae o r microscopic organisms and f u r t h e r t r a n s form the a r s e n i c a l t o a more complex compound (26). Crustacea and f i l t e r - f e e d i n g s h e l l f i s h may absorb a r s e n i c from the water d i r e c t l y o r from microscopic organisms. C.

A r s e n i c i n f r e s h water organisms

In model ecosystem studies (Table 4 ) , c a t f i s h and Gambusia concentrated a r s e n i c from <1 t o 130 times the cone, i n water r e s p e c t i v e l y . Daphnids and c r a y f i s h concentrated arsenic by f a c t o r s o f 4 and 5, r e s p e c t i v e l y , i n water, while algae concent r a t e d i t to 34 times the water c o n c e n t r a t i o n . Concentrations o f c a c o d y l i c a c i d and arsenate i n f r e s h water organisms do not appear to c o r r e l a t e w e l l with one another. Algae and daphnids accumulated more c a c o d y l i c a c i d than they d i d arsenate, but Gambusia f i s h accumulated much l e s s . MSMA (monosodium methanearsonate) accumulated to give BR values s i m i l a r t o those f o r arsenate, except the value was n i n e f o l d l e s s f o r algae. 14

1 4

The C-BR values f o r C - c a c o d y l i c a c i d (CA) i n the aquatic system were much h i g h e r than those f o r a r s e n i c , p o s s i b l y because i t degraded. C-BR values reached 163 t o 27,000 i n algae, 89 t o 35,000 i n duckweed, 4 to 1,000 i n s n a i l s , 2 to 275 i n c a t f i s h , and 3 t o 14 i n c r a y f i s h (27). Corresponding BR values f o r 14

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

As0

4

C-CA

45

17

7

0.1

1.0

10.0

4

34

-

<3-17

-

algae

-1

1

1

Water cone. ppm As

-

-

1-3

ND

ND-l/

duckweed

c/ ^ . — Organism not i n the experiment

—/ Not Determined

25

42

39

ND

8

4

Daphnia

1-7

2-68

9-20

ND

ND

<1

<1

<1

108

130

Species snail Gambusia

-

-

-

<1

ND

ND

catfish

-

-

<1-16

1

5

crayfish

Bioaccumulation Ratio (BR)—' values f o r As obtained i n model ecosystem s t u d i e s

a/ — BR = cone, i n tissue/cone, i n water

14

C-MSMA

14

7 4

Compound tested

Table 4.

(28)

(28)

(28)

(27)

(26)

(26)

Reference

§

|

£

I*

03

§

ARSENICAL PESTICIDES

102

a r s e n i c i n t h i s same experiment were: algae, 3 t o 17; duckweed, 0 t o 3; s n a i l s , 2 t o 2 1 ; c a t f i s h , <1; c r a y f i s h <1 to 16 (28). Presumably, the C i s s p l i t from the As atom and i n c o r p o r a t e d as l a b e l e d * C 0 2 o r some other metabolite, p o s s i b l y CH3. 4

4

D.

A r s e n i c i n aquatic animals

Concentrations o f a r s e n i c are g e n e r a l l y much lower i n f r e s h water f i s h than i n marine f i s h (Table 5). A BR value o f 3 t o 30 f o r various f r e s h water f i s h was reported by P r a t t e t a l . (29) and 10 t o 20 by Gilderhus (30). Marine organisms concentrated a r s e n i c t o give BR values ranging from 9 f o r Crustacea and s h e l l f i s h (17) to 64,100 f o r shrimp (31). The l a t t e r value was on a dry weight b a s i s . Dry weigh d th r e s u l t i n ratio are always higher than thos Table 5. Specie

Bioaccumulation Ratio values f o r As i n aquatic organisms!!/ ppm As i n BR values^/ Reference tissue

Haddock Kingfish Crustacea and shellfish Assorted f i s h Assorted f i s h Shrimp Mackrel Cod Assorted f r e s h water f i s h

2-10.8 8.86 1.5- 3.1 0.018-1.06 0.076-2.27
1000-5400£/ 4430 750-1550 9-530 38-1135 <500-3,200£/ 1836-64,100£/ 2350-4600£/ 12,150£/ 10-20 3-30£'

(5)

(25)

@ (5) (17) (32) (33) (31) (25) (25) (25) (30) (29)

a/ Concentration i n t i s s u e / c o n c e n t r a t i o n i n water A marine concentration o f 2 yg As/1 i s used i n a l l c a l c u l a t i o n s i j / Dry weight b a s i s _/ Water c o n c e n t r a t i o n assumed t o be 10 yg A s / l i t e r In general, f o r various a s s o r t e d s a l t water f i s h , the concent r a t i o n r a t i o s are 10 to a 100 times higher than those reported f o r f i s h and Crustacea i n f r e s h water. T h i s d i f f e r e n c e may be a f u n c t i o n o f a r s e n i c concentration to which the organism i s exposed on a c o n t i n u i n g b a s i s . A r s e n i c concentrations are g e n e r a l l y lower i n f r e s h water than i n the marine environment. E.

A r s e n i c i n aquatic p l a n t s B i o c o n c e n t r a t i o n r a t i o values f o r a r s e n i c i n aquatic f r e s h

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

7. WOOLSON

Bioaccumulation

103

water p l a n t s ranged from 1 f o r duckweed i n a model ecosystem study (27) t o 20,000 (7) f o r submerged weeds i n the Waikato River i n Ngw Zealand (Table 6 ) . The a r s e n i c i n the Waikato River i s from geothermal o r i g i n s and at places i s q u i t e h i g h . Emerged weeds from the same r i v e r concentrated a r s e n i c only about 100 times. Lakeweeds concentrated a r s e n i c t o give BR values o f 11014,500 (_8). C a c o d y l i c a c i d was accumulated by algae t o a much higher degree than arsenate (28), whereas MSMA BR values were about the same (26) as f o r arsenate. Table 6.

BR values f o r a r s e n i c i n f r e s h water plants.?/

Specie

ppm As i n tissu

Algae Algae Algae Algae Algae Algae Submerged weeds Emerged weeds Lakeweeds Duckweed

2-1 27 4 4.5-9.8 550 20-971 8-12 11-1450 1-3

BR values

34 4^ 7-1635£' 590-4600 7,000 800-20,000 100 110-14,500 1-3

Reference

(26) (26) (28) (24) (7) (7) (7) (8) (27)

a/ —• BR = concentration i n p l a n t / c o n c e n t r a t i o n i n water b/ Treated with MSMA Si Treated with c a c o d y l i c a c i d Marine p l a n t s accumulated o r bioconcentrated a r s e n i c t o a much higher l e v e l than f r e s h water p l a n t s (Table 7 ) . Seaweed contained 350 t o 71,000 times more a r s e n i c than sea water (34, 25). Algae contained between 50 and 47,500 times the concentration i n water, depending on the s p e c i e s . Table 7.

BR values f o r a r s e n i c i n marine p l a n t s — '

Specie

ppm As i n tissue

BR values

Algae Algae Algae Algae Seaweed Seaweed

4.8-94 0.1-95 1.67

2,400-47,000 50-47,500 835 200-3000 350-6000 30,000-71,000

—

0.7-12 60-142

Reference

(36) (37) (20) (24, 35) (5) (25, 34)

a/ i n t i s s u e / c o n c e n t r a t i o n i n water —' Concentration

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ARSENICAL PESTICIDES

104

Since a r s e n i c a l s do bioaccumulate i n the aquatic environment, the question o f p o s s i b l e b i o m a g n i f i c a t i o n e x i s t s . Biomagnificat i o n i s the process whereby succeeding organisms i n the food chain have higher l e v e l s o f a contaminant than previous members. With a r s e n i c , t h i s does not appear t o be t r u e . Algae, which are low i n a food chain, contain much higher a r s e n i c l e v e l s than do f i s h and consumers o f f i s h . Since the BR value i s derived by d i v i d i n g the a r s e n i c concentration i n t i s s u e by the a r s e n i c content i n water, one reason f o r high BR values i n algae i s the i n h e r e n t l y low value f o r a r s e n i c both i n sea water and i n f r e s h water. Algae does concentrate a r s e n i c against a concentration gradient. The a c t u a l l e v e l s o f a r s e n i c i n any organism seldom exceed 100 ppm. F.

Form and t o x i c i t y o

Two f u r t h e r questions about a r s e n i c bioaccumulation are the i d e n t i t y o f the a r s e n i c compound a f t e r i n c o r p o r a t i o n and i t s r e s u l t i n g t o x i c i t y . Lunde (23, 24, 25, 35) and Woolson e t a l . (26) have shown that a r s e n i c i s present i n algae and f i s h as both water-soluble and l i p i d - s o l u b l e arseno-organic compounds. Lunde concluded that "algae seem t o be an important source o f the arseno-organic compounds found i n higher marine organisms. In s t u d i e s with f r e s h water model ecosystems (26), small amounts o f arsenate were present i n a l l organisms f r a c t i o n a t e d on a Sephadex column ( F i g . 1). However, algae contained an a d d i t i o n a l unknown, X , and c r a y f i s h contained another unknown X , i n addition to X MSMA ( F i g . 2) i n the same ecosystem was found i n a l l three organisms and was apparently transformed t o another unknown, X 3 , only i n Daphnia. About 30% o f the MSMA was t r a n s formed, based on C contents o f the e x t r a c t s . These studies by Lunde and Woolson i n d i c a t e that arsenate i s not a major form o f a r s e n i c i n aquatic organisms. 11

1

2

1 #

1 4

The t o x i c i t y t o sheep o f a r s e n i c i n lakeweeds has been examined by Lancaster e t a l . (8). The weed, Lagarosiphon major, contained 288 ppm As, and was fed as 20% o f the sheep d i e t f o r 3 weeks with no i l l e f f e c t s on the animals' h e a l t h . Residues increased during feeding, but d e c l i n e d when the lakeweed was removed from the d i e t . Coulson (31) fed shrimp, c o n t a i n i n g 128 ppm As, t o r a t s a t a d i e t a r y l e v e l o f 13.3 ppm As. He a l s o included A s 0 i n h i s feeding studies at the same d i e t a r y l e v e l . The r a t l i v e r s cont a i n e d 20-fold l e s s a r s e n i c with the shrimp d i e t than with AS2O3 i n the d i e t . More than 98% o f the a r s e n i c fed i n the shrimp was excreted w i t h i n 4 days o f feeding, whereas only 21% o f the A s 0 fed was excreted. When humans ate shrimp, a l l o f the a r s e n i c was excreted w i t h i n 4 days a f t e r consumption. The a r s e n i c contained i n shrimp obviously i s not as t o x i c o r as e a s i l y absorbed as As^Og. 2

3

2

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

7. WOOLSON

Bioaccumulation

105

SEPARATION OF M S M A FRACTIONS MSMA

40

80 100 120 140 TUBE N U M B E R Figure 1. Separation of incorporated arsenic compounds from aquatic organisms treated with As in a model ecosystem. X,—tubes 75-80; X —tubes 90-110 74

60

h

2

SEPARATION OF At04 FRACTIONS

40

60

80

100

120

TUBE NUMBER

140

Figure 2. Separation of incorporated arsenic compounds from aquatic organisms treated with C-MSMA in a model ecosystem. X . , — tubes 120-135 14

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

106 G.

ARSENICAL PESTICIDES

Conclusions

In summary, arsenic is bioconcentrated by aquatic organisms but not biomagnified. Plants usually accumulate more arsenic than fish, and Crustacea accumulate intermediate amounts. Marine organisms normally contain more arsenic than their fresh water counterparts. However, the arsenic contained in the organisms is apparently not toxic to animals or humans, and is readily excreted. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Durum, W. H., Hem, J. D., Heidel, S. G., Geo. Survey Circ., (1971), 643. Fleisher, M., "Data of Geochemistry", 6th edit. Geo. Survey Prof Office, Washington Chamberlain, W., Shapiro, J., Limnol. Oceanogr., (1969), 14:921-7. Seydel, Use S., Archiv fur Hydrobiologie (1973), 71(1): 17-30. Schroeder, H. A., Balassa, J. J., J. of Chronic Diseases (1966), 19:85-106. Wedepohl, K. H., "Handbook of Geochemistry", (1969), Vol. II, Ch. 33, Springer-Verlag, Berlin. Reay, P. F., J. Appl. Ecol., (1972), 9(2):557-65. Lancaster, R. J., Coup, M. R., Hughes, J. W., N. Z. Vet. J. (1971), 19:141-45. Neubauer, O., Br. J. Cancer (1947), 1:192-251. Feinglass, E. J., N. Eng. J. Med. (1973), 288:828-30. Bado, A. A . , J. Am. Water Works Assn., (1937), 31:19751977. Arguello, R. A., Cenget, D. D., Tello, E. E . , Rev. Argent, Dermatosif., (1938), 22:461. Wyllie, J., Can. J. Publ. Health (1937), 28:128. McCabe, L. J., Symons, J. M., Lee, R. D., Robeck, G. G., J. Am. Water Works Assn. (1970), 62:670-87. Fairhall, L. T . , New Eng. Water Works Assn. (1941), 55: 400-10. Tseng, W. P., Chu, H. M., How, S. W., J. Nat'l. Cancer Inst. (1968), 40:453-63. Chapman, A. C., Analyst (1926), 51:548-63. Sugawara, K., Kanamori, S., Bull. Chem. Soc. Japan (1964), 37(9):1358-63. Ferguson, J., Gavis, J., Water Research (1972), 6:125974. Portmann, J. E . , Riley, J. P., Anal. Chim. Acta (1964), 31:509-19. Smales, A. A . , Pate, B. D., Anal. Chem. (1952), 24:717. Smales, A. A . , Pate, B. D., Analyst (1952), 77:188. Lunde, G., Nature (London) (1969), 224(5215):186-7.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

7. WOOLSON

24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Bioaccumulation

107

Lunde, G., Acta Chemica. Scandinavica (1973), 27:158694. Lunde, G., J. Sci. Fd. Agric. (1973), 24:1021-27. Woolson, E. A . , Kearney, P. C., Isensee, A. R., McShane, W. G., Irgolic, K. J . , Zingaro, R. A . , 168th ACS Meeting (1974), Paper No. 8, Pest. Chem. Div. Schuth, C. K., Isensee, A. R., Woolson, E. A . , Kearney, P. C., J. Ag. Food Chem. (1974) in press. Isensee, A. R., Kearney, P. C., Woolson, E. A . , Jones, G. E . , Williams, V. P., Environ. Sci. & Tech. (1973), 7(9):841-5. Pratt, D. R., Bradshaw, J. S., West, B., Utah Academy Proceedings (1972), 49(l):23-26. Gilderhus, P. A . , Trans. Am. Fisheries Soc. (1966), 95 (3):289-96. Coulson, E. J . , Remington, R. E . , Lynch, K. M., J. Nutrition (1935), 10:255-70. Cox, H. E . , Analyst (1925), 50:3-13. Windom, H., Stickney, R., Smith R., White, D., Taylor, F., J. Fish Res. Board Can. (1973), 30:275-9. Wilson, S. H., Fieldes, M., N. Z. J. of Sci. & Tech. (1941), 23B:47-48. Lunde, G., Acta Chemica Scandinavica (1972), 26:2642-44. Jones, A. J . , Chemist and Druggist (1922), 97:166-7. Vinogradov, A. P., Sears Found. Mar. Res. (1953), Memoir 2.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8 Arsenic in the Environment R O B E R T S. B R A M A N University of South Florida, Tampa, Fla. 33620

Introduction Environmental studies of arsenic are intriguing exercises largely because (1) the chemistry of this element provides for several molecular forms to exist in both air and water; (2) interactions with the biosphere can occur; and (3) in most instances analytical chemistry is required involving very small sample sizes. A brief h i s t o r i c a l excursion into the subject of arsenic in the environment would start with the ancient, known toxicity of compounds of this element, touch on the toxicity of arsenic i f used i n wallpaper and be composed thereafter of laboratory studies on the biomethylation of arsenic i n cultures, analyses for total arsenic, analyses of industrial locations for arsine i n a i r , and radiochemical tracer studies of arsenic in s o i l experiments. Much of this work has gone a long way i n helping to define the analytical chemistry problem, a logical starting point i n studying the environmental chemistry of arsenic. Because of the likelihood that methylated forms and inorganic forms of arsenic would exist, it became obvious early that analytical methods were needed to speciate arsenic, that i s , to determine the chemical forms present and at concentrations ordinarily found in the environment. Except i n instances of high arsenic pollution, total arsenic in water is i n the 1-10 ppb range, and i n a i r i n the 0-10 ng/m3 range. Prior to work i n our laboratories adequate methods for air and water arsenic speciation analyses at these levels had not been developed. The problem of analyzing water, air and to some extent biological samples for methylarsenic compounds and inorganic As(III) and As(V) at environmental concentrations usua l l y encountered has now to a considerable extent been solved. By using these methods and the laboratory work of others i t has been possible to better substantiate the environmental chemistry of arsenic.

108

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8.

BRAMAN

Arsenic in the

Environment

109

A n a l y t i c a l Methods Many methods have been developed f o r the determination of t o t a l a r s e n i c and f o r the s e v e r a l a r s e n i c compounds of i n t e r e s t i n environmental s t u d i e s . The s i l v e r d i e t h y l d i t h i o c a r b a m a t e method (1) has a l i m i t of d e t e c t i o n of 0.2 yg As or 2 ppb, too i n s e n s i t i v e f o r much work. In a d d i t i o n , the methylarsenic compounds produce a much d i f f e r e n t molar a b s o r p t i v i t y than does i n o r g a n i c a r s e n i c (2). Thus the method i s i n a c c u r a t e . Peoples, Lakso and L a i s (3) made use of t h i s i n developing a d i f f e r e n t i a l spectrophotometric method f o r p a r t i a l l y d i s t i n g u i s h i n g between methylarsenic and i n o r g a n i c a r s e n i c compounds down to 0.02 ppM. N e u t r o n - a c t i v a t i o n and atomic absorption methods have been widely used f o r t o t a l a r s e n i c Talmi and Feldman (4) are develop ing gas chromatographic-emissio ported t h e i r progress i Johnson and P i l s o n (5) have developed a c o l o r i m e t r i c method f o r d i s t i n g u i s h i n g between a r s e n i c ( I I I ) and a r s e n i c (V) and app l i e d i t to sea water analyses. The method i s not s u i t a b l e f o r organic analyses. Based upon work i n our l a b o r a t o r i e s (2, 6, 7) methods have been developed and a p p l i e d to water analyses f o r the s p e c i a t i o n of a r s e n i c i n aqueous samples. Fundamentally, the method i n v o l v e s r e d u c t i o n of methylarsenic a c i d s or i n o r g a n i c a r s e n i c to the c o r responding a r s i n e s which are then separated and detected i n an emission type d e t e c t i o n system. Observation of the a r s e n i c emiss i o n l i n e gives a l i m i t of d e t e c t i o n w e l l below 1 ng and a p o s i t i v e means of i d e n t i f y i n g a r s e n i c . I d e n t i f i c a t i o n of methylars e n i c compounds i n samples has been confirmed by comparison to standards and by a m o d i f i c a t i o n of the c o l o r i m e t r i c method d e v e l oped by Peoples, Lakso and L a i s (3). I d e n t i f i c a t i o n i s a l s o supported by the r e d u c t i o n pH s e l e c t i v i t y of the s e v e r a l a r s e n i c compounds. The method has been e x t e n s i v e l y studied (2) and a l s o succ e s s f u l l y used by others (8) i n environmental analyses. I t has been adapted f o r use to a n a l y s i s of b i o l o g i c a l m a t e r i a l s (9)• A i r analyses f o r a r s e n i c have proved d i f f i c u l t . Arsenic i s present as f i n e p a r t i c u l a t e (8). Gordon and Z o l l e r (10) have found that the element i s enriched i n p a r t i c u l a t e f a r above that expected f o r average c o a s t a l m a t e r i a l s . T h i s again i m p l i e d i n e f f i c i e n t c o l l e c t i o n of very f i n e p a r t i c u l a t e or the presence of vapor forms. Because of known environmental methylation of a r s e n i c from our e a r l i e r work on water a n a l y s i s i t was decided to attack a i r a n a l y s i s from the p o i n t of view that t r i m e t h y l a r s i n e , dimethy1a r s i n e , methylarsine, a r s i n e and t h e i r o x i d a t i o n products as w e l l as i n o r g a n i c , n o n - v o l a t i l e As (III) and As(V) could be present i n a i r and had to be s p e c i f i c a l l y i d e n t i f i e d . The a n a l y t i c a l method f o r water a n a l y s i s provided a good procedure to apply so long as sample c o l l e c t i o n and p r o c e s s i n g could be s u i t a b l y adapted. E a r l y work w i t h s o l u t i o n a i r scrubbers and M i l l i p o r e f i l t e r s showed that

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

110

ARSENICAL PESTICIDES

the v o l a t i l e forms of a r s e n i c are not trapped q u a n t i t a t i v e l y ( i f at a l l ) and t h a t , moreover, glassware was an i n t o l e r a b l e c o n t r i b u t o r to i n o r g a n i c a r s e n i c background. Studies were then i n i t i a t e d on c o l l e c t i o n of the v o l a t i l e a r s i n e s on gold-coated and s i l v e r coated g l a s s tubes of the same type used i n e a r l i e r , s u c c e s s f u l a i r analyses f o r v o l a t i l e forms of mercury (11). A r s i n e s were q u a n t i t a t i v e l y but i r r e v e r s i b l y absorbed onto gold-coated g l a s s beads. S i l v e r - c o a t e d glass beads d i d , neverthel e s s , prove to be a s a t i s f a c t o r y c o l l e c t i n g medium. A l l of the v o l a t i l e a r s i n e s were found to be q u a n t i t a t i v e l y c o l l e c t e d on 4 cm long columns at a i r flow r a t e s up to at l e a s t a face v e l o c i t y of 32 cm/second from 22°C to 35°C and under c o n d i t i o n s of up to 100% r e l a t i v e humidity. Q u a n t i t a t i v e removal f th a r s i n e without d e a l k y l a t i o d i s p r o p o r t i o n a t i o n was obtaine lowed by a hot water wash probably i n the washing process s i n c e a n a l y s i s of the a i r sample c o l l e c t i n g tubes r e q u i r e d r e d u c t i o n by sodium borohydride to detect the methy l a r s i n e s . A t y p i c a l a n a l y s i s i s shown i n Figure 1. Identificat i o n of peaks as t r i m e t h y l a r s i n e or dimethylarsine again was done by r e t e n t i o n time comparison w i t h known standards. The l i m i t of d e t e c t i o n f o r a i r a n a l y s i s was approximately 0.3-0.5 ng per samp l e . Concentration l i m i t s of d e t e c t i o n obviously depend upon sample size. These methods b r i e f l y described above have been used to c a r r y out environmental s t u d i e s on a r s e n i c . Results of t h i s together with the referenced works of others has helped to d e f i n i t e l y establ i s h the ambient environmental chemical forms of a r s e n i c and to b e t t e r d e f i n e i t s chemical transformations. Environmental Forms of A r s e n i c Table I summarizes the molecular forms reported as found i n the environment. Table I I l i s t s the a n a l y t i c a l r e s u l t s on a number of samples. Not included are commercial phenylarsenic type compounds, obviously present where used, but not yet i d e n t i f i e d remote from such l o c a t i o n s . Methyl and d i m e t h y l a r s i n i c a c i d or s a l t s are not excluded from the t a b l e because they are produced by biomethylation and are not e x c l u s i v e l y commercial products. It i s suspected that t r i m e t h y l a r s i n e and p o s s i b l y dimethylarsine may e x i s t to an unknown extent as a complexed or oxide form i n a i r or i n water but t h i s has not been d e f i n i t e l y e s t a b l i s h e d . The l i s t i n g of the methylarsenic compounds does not imply that they are completely ubiquitous. For example, many water samp l e s have been analyzed g i v i n g l e s s than the l i m i t of d e t e c t i o n of methylarsenic compounds. Sea water i s a case i n p o i n t . In work done at s e v e r a l l o c a t i o n s i n the Sargasso Sea (9), methylarsenic a c i d s were not detected i n open ocean water. T h i s means that i f present they were l e s s than 1% of the t o t a l sea water a r s e n i c which i s approximately 2-3 ppb.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. 6, 9 5 6, 9 6, 8 *

Sea water Fresh water ponds, r i v e r s , lakes Sea water Fresh water Fresh water

CH AsO(OH)

2

3

3

Reported i n t h i s a r t i c l e

soil

Over-treated

(CH ) As

2

soil

3

(CH ) AsH

* 14, 15, 17

* 14, 15, 17

Over A s - t r e a t e d s o i l Over-treated

2

CH AsH

3

8

*

Particulate

3

As (III) and As(V)

3

( C H ) A s (or the oxide)

3

(CH ) AsO(OH)

AIR

6, 8

Fresh water ponds, r i v e r s , lakes

As(V), arsenate i o n

2

5, 6, 8, 9

Sea water

As ( I I I ) , a r s e n i t e i o n and

3

Reference

Source

Compound

WATER

TABLE I . ENVIRONMENTAL FORMS OF ARSENIC

3

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Reported i n t h i s

3

2

3

2

(CH ) AsO(OH)

CH AsO(OH) 3

(CH ) As

(Fungi and mixed)

2

2

Aerobic c u l t u r e s

3

3

(CH ) AsO(OH)

*

* 14

13, 14

6, 3

6

2

CH AsO(OH) 3

9, 8

3

As ( I I I ) , As(V)

3

(CH ) AsH

article

9, 8

(CH ) As

2

9, 8

3

Methanobacterium Cultures

Urine

9, 8

(CH ) AsO(OH)

2

CH AsO(OH) 3

9, 8

A s ( I I I ) , As(V)

Reference

Sea weed and epiphytes

(CONT.)

Forms

BIOLOGICAL SAMPLES

ENVIRONMENTAL FORMS OF ARSENIC

Type

TABLE I .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. 0.32

< 0.02

Remote pond (Withlacooehee F o r e s t )

1.29

0.62 0.06

Tidal Flat

McKay Bay

4.3 3.0

0.91 6.4

Shrimp (Sargassum community)

17.7

3.9

Sargassum weed (Gulf of Mexico)

1.9 1.8

(4 sample average)

BIOLOGICAL SAMPLES (wet wt.)

Sargassum weed (Bermuda)

Human u r i n e

1.45

0.12

S a l i n e Bay Water

0.35

0.49

0.41

2.74 0.89

R e s i d e n t i a l Lake B - Tampa

R e s i d e n t i a l Lake C

(sum)

0.27

< 0.02

W e l l - near above r i v e r

0.76

0.16

< 0.02

Withlacooehee R i v e r

R e s i d e n t i a l Lake A - Tampa

As(V)

As(III)

WATER

tr

0.005

0.01

1.8

0.07

0.08

0. 072

0. 064

0. 185

15

1.00

0.29

0.20

0.15

< 0.02

0.32

0.11

0.16 +0.14 TMA

0.62

0.30

0.20

CA

0.22

0.07

0.12

0.11

0.06

MAA

SELECTED ENVIRONMENTAL ANALYSES FOR ARSENIC

Sample

TABLE I I .

ppb

ppb

ppb

9.5

5.8

19.5

22.6

1.48

2.28

1.77

1.75

3.58

ppM

ppM

ppM

ppb

1.13 ppb

0.06

0.68

0.42

Total

| >

114

ARSENICAL PESTICIDES

Methylarsenic compounds were found, nevertheless, i n l a r g e amounts a s s o c i a t e d w i t h the p a l e g i c Sargassum weed biocommunity on t h i s same research c r u i s e . Using the same a n a l y t i c a l methods as r e ported by us, Carpenter and C r e c e l i u s (8) have a l s o found methyla r s e n i c compounds i n c l u d i n g a l s o p o s s i b l y t r i m e t h y l a r s i n e assoc i a t e d w i t h Kelp plankton and shrimp t i s s u e from the North P a c i f i c . Methylarsenic compounds are a l s o a s s o c i a t e d w i t h lakes and ponds, and are e s p e c i a l l y high where the pond i s surrounded by f e r t i l i z e d lawns as i n r e s i d e n t i a l areas. P o l l u t e d seawater bays a l s o contain the a l k y l a r s e n i c compounds. As i s shown i n Table I I , the amount of i n o r g a n i c a r s e n i c g e n e r a l l y exceeds the amount of organic a r s e n i c present. Organic a r s e n i c i n b i o l o g i c a l specimens could exceed the amount of i n o r g a n i c a r s e n i case i n human u r i n e and L a i s (3) i n t h e i r work w i t h animals. A i r analyses f o r v o l a t i l e a r s e n i c compounds have been only r e c e n t l y underway and i t i s not p o s s i b l e to be completely g e n e r a l . Dimethylarsine and t r i m e t h y l a r s i n e have been detected i n a i r , but only i n a r s e n i c t r e a t e d areas. Methylarsenic compounds were w e l l below 1 ng/m^ i n other ambient a i r samples. A few analyses of a i r from houses have shown <0.5 ng/nr* methylarsenic compounds. Trimethylarsine has been detected i n a i r over a lake c o n t a i n i n g methy l a r s e n i c compounds. A r s i n e i t s e l f may be produced i n s o i l t r e a t e d w i t h a r s e n i c a l s but more work i s needed to confirm t h i s . Environmental Transformations Much work has been done w i t h c u l t u r e s . E a r l y evidence f o r biomethylation of i n o r g a n i c a r s e n i c to the methylarsenic a c i d s and v o l a t i l e forms i s i n the work of Challenger (12). McBride and Wolfe (13) l a t e r showed that anaerobic c u l t u r e s of methanebacterium could produce what they i d e n t i f i e d as dimethylarsine. It i s a l s o p o s s i b l e to o b t a i n t r i m e t h y l a r s i n e from c u l t u r e s of aerobic f u n g i from sewage p l a n t s as reported by Cox i n t h i s symposium (14X We have found that a pond water sample (mixed c u l t u r e ) having a r s e n i t e added and a n u t r i e n t media added, a f t e r a p e r i o d of time w i l l produce the a l k y l - a r s e n i c a c i d s and t r i m e t h y l a r s i n e under a e r o b i c c o n d i t i o n s . In these experiments i t was found that methy l a r s o n i c a c i d appeared f i r s t , followed by dimethylarsine and f i n a l l y followed by t r i m e t h y l a r s i n e or t r i m e t h y l a r s i n e oxide. The presence of bottom sediments and anaerobic c o n d i t i o n s were not necessary to achieve the biomethylation. Thus, methanobacterium are not the s o l e source of environmental biomethylation. Core samples from a lake and a pond e x h i b i t i n g methylarsenic a c i d s were taken and analyzed by s e c t i o n f o r a l k y l a r s e n i c compounds. Res u l t s i n d i c a t e d a maximum percentage of methylarsenic a c i d s i n the top l a y e r but by no means i n high concentrations. The absence of methylarsenic compounds below the top l a y e r of bottom core i n d i cates that the methylarsenic a c i d s may remain i n the aqueous phase

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8.

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Arsenic in the

Environment

115

r a t h e r than become a s s o c i a t e d with sediment. D e a l k y l a t i o n could a l s o occur i n the sediment. Methylarsenic a c i d s could be c a r r i e d down w i t h n a t u r a l f e r r i c hydroxide p r e c i p i t a t i o n processes. Radiochemical techniques have been used to show that dimethy l a r s i n i c a c i d and methylarsonic a c i d (or s a l t s ) when a p p l i e d t o s o i l a r e a t l e a s t p a r t l y l o s t by v o l a t i l i z a t i o n (15, 16). By the use o f the a i r a n a l y s i s technique t o analyze a i r trapped under g l a s s (see F i g u r e 2 ) , a number o f a r s e n i c compounds have been found to be converted to v o l a t i l e dimethylarsine and trimethylarsine i n s o i l . Table I I I shows the c o l l e c t i o n of a r sines under the g l a s s jugs as a f u n c t i o n o f time o f day over a two-day p e r i o d . D i m e t h y l a r s i n i c a c i d t r e a t e d s o i l gave the more s t r i k i n g r e s u l t s . Note that methylarsines are immediately observed with dimethylarsine decreasing slowly Trimethylarsine e v o l u t i o n b u i l d s up slowly two-hour sampling p e r i o d a r s e n i t e , methylarsonic a c i d and phenylarsonic a c i d . Sodium a r s e n i t e e x h i b i t e d a several-hour i n d u c t i o n p e r i o d p r i o r to detect i o n and a slow e v o l u t i o n o f t r i m e t h y l a r s i n e . Methylarsonic a c i d e x h i b i t e d a comparatively more r a p i d production o f t r i m e t h y l a r s i n e than a r s e n i c ( I I I ) . S o i l t r e a t e d with s u b s t a n t i a l amounts o f phenylarsonic a c i d d i d not produce any det e c t e d a r s i n e s over the f i r s t 12 hours of the experiment. A f t e r 13 days phenylarsonic a c i d t r e a t e d s o i l d i d evolve v o l a t i l e a r s i n e s , p a r t i c u l a r l y t r i m e t h y l a r s i n e . Aside from the dimethylars i n i c a c i d experiments, t r i m e t h y l a r s i n e was the c h i e f a r s i n e observed i n these experiments. T h i s observation could be a t t r i b u t e d to the pKa values and t h e i r e f f e c t on r e d u c t i o n of the methylars e n i c a c i d s . D i m e t h y l a r s i n i c a c i d has a pKa o f 6.13 while the others have pKa values on the order of 2. A r s e n i c a c i d r e d u c t i o n appears to r e q u i r e a c i d s i n the u n d i s s o c i a t e d form before reduct i o n w i l l proceed. Nevertheless, biomethylation r e a c t i o n s of a l l a r s e n i c compounds appear to proceed through d i m e t h y l a r s i n i c a c i d and i f t h i s i s so, why i s dimethylarsine not observed together with t r i m e t h y l arsine i n every case? Two mechanisms could be f u n c t i o n i n g i n these s o i l experiments. One could be a reducing process which operates to reduce f r e e d i m e t h y l a r s i n i c a c i d t o the a r s i n e ; the other methylates. Biomethylation t o t r i m e t h y l a r s i n e of dimethyla r s i n i c a c i d would then have to proceed i n a manner i n v o l v i n g a complex or s u r f a c e a c t i o n o f some type. Those working with l a b o r a t o r y c u l t u r e biomethylation have mixed i d e n t i f i c a t i o n s . C h a l l e n ger (12) reported t r i m e t h y l a r s i n e i n aerobic c u l t u r e s , McBride and Wolfe (13) reported dimethylarsine i n anaerobic c u l t u r e s w h i l e our work gave mixed products. I t i s p o s s i b l e that i n anaerobic c o n d i t i o n s that r e d u c t i o n o f the d i m e t h y l a r s e n i c intermediate to the f r e e a r s i n e could be more r a p i d than i n the presence of oxygen. Furthermore, the presence of mixed products could r e s u l t from two types of biomethylation o c c u r r i n g i n s o i l a t the same time. C l e a r l y f u r t h e r work i s

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

116

ARSENICAL PESTICIDES

(CH ) AsH 3

2

U— 15 sec-J time Figure 1. Air sample analysis for dimethylarsinic-treated soil Relative emission intensity—228.8 nm As line

Figure

2.

Apparatus for arsenic-treated soil experiments

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

a

St. Augustine grass

3.6

0

3-5 PM

9.8

b not separated ^500 ng

3-5 PM

1-3 PM

7-9 AM

3

5.5

5

1.9

0

1-3 PM

5.5

0

11-1 PM

14

tr

9-1 PM

6.7

0

9-11 AM

tr

0

5-9 AM

2.0

0

1.6

0

3-5 AM

3-7 AM

2.1

4-7 AM

0

tr

0

1-3 AM

11-3 AM

7.5

0

12-4 AM

0

0

11-1 AM

73

7-9 PM e not w e l l separated

70

176

130

460

HIGH

39

62

24

5-7 PM

1-5 PM

11-1 PM

9-11 AM

7-9 AM

9-11 PM

4.4

30

0

5-7 PM 7-9 PM

TMA ng.

b

11.3

3

Time Period

Methylarsonic Acid:

0

10-12 PM

0

0

9-11 PM

1.6

0

B.

3.7

7-10 PM

0

0

7-9 PM

TMA ng.

2

DMA ng.

m

tr

5-7 PM

0

0

5-7 PM

Time Period

TMA ng.

140 mg As/0.056

DMA ng.

Sodium A r s e n i t e :

EVOLUTION OF ARSINES FROM AN ARSENIC-TREATED LAWN GRASS

Time Period

A.

TABLE I I I .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. 5

10-12 PM

12-4 AM 4-7 AM

111

131

71

306

77

36

48

47

41

21

11-1 AM

1-3 AM

3-5 AM

5-7 AM

7-9 AM

a

247

St. Augustine

1-3 PM

11-1 PM

grass

^not

^500 ng

97

82

3-5 PM

separated

126

84

c

472

1-3 PM

11-1 PM

c

288

231

500

179

9-11 AM

11

1.6

ng

ng

2.23 ng

not w e l l

2

detected

separated

TMA - t r i m e t h y l a r s i n e

DMA - dime thy l a r s i n e

126

48 77

TMA:

241

63

39

DMA:

Inorganic As:

A f t e r 13 days - 3 hour p e r i o d

no a r s i n e s

15 mg As/0.056 m

(Continued)

Acid:

3

I n i t i a l 12 hours:

D. Phenylarsonic

22

7-9 AM

572

123

46

7-10 PM

57

19

9-11 PM

- —

85

58

5-7 PM

23

27

7-9 PM

9-11 AM

197

242

3-5 PM

tr

37

5-7 PM

266

TMA ng.

DMA ng.

Time Period

TMA ng.

2

DMA ng.

10.5 mg As/0.056 m

Time Period

Acid:

EVOLUTION OF ARSINES FROM AN ARSENIC-TREATED LAWN GRASS

C. D i m e t h y l a r s i n i c

TABLE I I I .

8.

BRAMAN

Arsenic in the

119

Environment

indicated. The p o s s i b i l i t y of v o l a t i l i z i n g a r s e n i c out of f e r t i l i z e d s o i l was explored using a p e l l e t i z e d phosphate-containing commercial fertilizer. Inorganic a r s e n i c i s a known t r a c e c o n s t i t u e n t of phosphates used i n f e r t i l i z e r . A 20 f t x 20 f t patch of f e r t i l i z e d grass was prepared using the recommended 3 lb/100 f t f e r t i l i z e r l o a d i n g . The f e r t i l i z e r was "watered i n " with a lake water s p r i n k l i n g system. The lake water analyzed 0.76 i n o r g a n i c a r s e n i c , 0.07 ppb methylarsonic a c i d , 0.16 ppb d i m e t h y l a r s i n i c a c i d and 0.14 ppb t r i m e t h y l a r s i n e i n s o l u t i o n . A i r was analyzed i n jugs placed i n the f e r t i l i z e d grass patch, and over an u n f e r t i l i z e d but watered patch of the same g r a s s . An ambient a i r sampler was a l s o set up to sample a i r over the f e r t i l i z e d patch. A f t e r a p e r i o d of a few hours, s m a l l amounts of t r i m e t h y l a r s i n e were noted Data are given i n the p e l l e t i z e d grass f e r t i l i z e a r s i n e e v o l u t i o n . I t seems l i k e l y , i n f a c t , that the t r i m e t h y l a r s i n e produced came from the s o i l biomethylation of the 0.76 ppb i n o r g a n i c a r s e n i c i n s o l u t i o n i n the lake water. 2

Conclusions The presence of methylarsenic compounds i n the environment i s confirmed. Adequate a n a l y t i c a l methods f o r t h e i r study are a v a i l a b l e . E a r l y work i n d i c a t e s t h a t commercially used a r s e n i c a l s are biomethylated to v o l a t i l e forms which must at l e a s t have some l o c a l and p o s s i b l y g l o b a l m i g r a t i o n . Even phenylarsenic type compounds can become converted to p r i m a r i l y , t r i m e t h y l a r s i n e . A number of matters can now be explored. Ambient v o l a t i l e a r s e n i c i n a i r may be w e l l below 1 ng/nr i n the absence of a s o i l contamination w i t h a r s e n i c . Nevertheless, a i r analyses have been l i m i t e d to only a few l o c a t i o n s and much f u r t h e r work i s i n d i c a t e d . A r s e n i c i n a i r i n places of human h a b i t a t i o n needs study. The evol u t i o n o f methylarsenic compounds out of lake water needs study. Mechanisms and r a t e s of s o i l b i o m e t h y l a t i o n and r a t e s of a i r o x i d a t i o n can now be s t u d i e d . Figure 3 summarizes the environmental chemistry of a r s e n i c . Acknowledgement Much of the work reported here i s p a r t of s e v e r a l p r o j e c t s completed or underway i n the l a b o r a t o r i e s of the author. T h i s work i s c a r r i e d out w i t h the a s s i s t a n c e of D. L. Johnson, M. Tompkins and J . L. B r i c k e r . The support of t h i s work by the N a t i o n a l Science Foundation, RANN program under grant number GI-43753 i s g r e a t l y a p p r e c i a t e d . T h i s paper was presented i n p a r t a t the SNF-RANN Second Annual Trace Contaminants Conference, Asilomar, C a l i f o r n i a , August 29, 1974.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

0.8

1.7

7:30-11:30 AM

11:30- 4:30 PM

1.5 ^ 0.4 0.6 ^ 0.4

11:00- 3:00 AM

3:00- 7:00 AM

7:00- 1:00 PM

1:00- 5:00 PM

*Air above f e r t i l i z e d patch, n o t trapped by a g l a s s j u g

A i r trapped under g l a s s jugs

3.4

0

3.5

~ 0.4

3.7

2.7

7:00-11:00 PM

0

3.3

3:30- 7:30 AM

4:30- 7:00 PM

2.9

0

0

9:30-11:30 PM

11:30- 3:30 AM

0

0

< 0.6 ng

0

^ake-Watered and F e r t i l i z e d Grass

7:30- 9:30 PM

Lake-Watered Grass 9:30 PM

11:00- 5:30 PM

7:00-11:00 AM

3:00- 7:00 AM

11:00- 3:00 AM

7:00-11:00 PM

12:30- 7:00 PM

3:30-12:30 PM

9:30- 3:30 AM

5:30

Time P e r i o d

<

^

0.3

0.1

12

2.1

< 0.3 ng/m

3

Ambient Yard A i r Analysis

0.5 0.4 0.3

^ < <

2.3 DMA, 2.5 TMA

EVOLUTION OF TRIMETHYLARSINE FROM A WATERED AND FERTILIZED LAWN

5:30- 7:30 PM

Time P e r i o d

TABLE IV.

8.

BRAMAN

121

Arsenic in the Environment

s>

"6V.

%

i S ε

c ε i

ci 5S

Ε

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

122

ARSENICAL PESTICIDES

Literature Cited 1.

Stratton, G., and Whitehead, H. C., J. Am. Water Works Assoc. (1962), 54, 861.

2.

Foreback, C. C., Ph.D. Dissertation, "Some Studies on the Detection and Determination of Mercury, Arsenic and Antimony in Gas Discharges", University of South Florida (1973).

3.

Peoples, S. A . , Lakso, J., and Lais, T . , Proc. West. Pharmacol. Soc. (1971), 14, 178-182.

4.

Talmi, Y . , and Feldman Amounts of Arsenic", Symposium on Review of Arsenical Pesticides, 168th National Meeting, American Chemical Society, Atlantic City, N. J., (September, 1974).

5.

Johnson, D. L . , and Pilson, M. E. Q., Anal. Chim. Acta (1972), 58, 289.

6.

Braman, R. S., and Foreback, C. C., Science (1973), 182, 1247-1249.

7.

Braman, R. S., Justen, L. L . , and Foreback, C. C., Anal. Chem. (1972), 44, 2195.

8.

Crecelius, E. A . , Ph.D. Dissertation, "The Geochemistry of Arsenic and Antimony in Puget Sound and Lake Washington, Washington", University of Washington (1974).

9.

Johnson, D. L . , and Braman, R. S., "The Speciation of Arsenic and the Content of Germanium and Mercury in Members of the Pelagic Sargassum Community", unpublished report (1974).

10.

Gordon, G. E . , and Zoller, W. H . , Proceedings, First Annual NSF Trace Contaminants Conference, ORNL, (August 1973), p.314.

11.

Braman, R. S. and Johnson, D. L . , Env. Sci. and Technol. (October 1974), in press.

12.

Challenger, F., Chem. Rev. (1945), 36, 315.

13.

McBride, B. C., and Wolfe, R. S., Biochemistry (1971), 10, 4312.

14.

Cox, D. P., "Microbial Methylation of Arsenic", Symposium on the Review of Arsenical Pesticides, 168th Nat'l. Meeting, American Chemical Society, Atlantic, N.J., (September 1974).

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8.

BRAMAN

Arsenic in the Environment

123

15.

Woolson, E. A . , and Kearney, P. C., Environ. Sci. Technol. (1973), 7, 47-50.

16.

Isensee, A. R., Kearney, P. C., Woolson, E. A . , Jones, G. E . , and Williams, V. P., Environ. Sci. Techno. (1973), 7, 841-5.

17.

Hiltbold, A. E . , "Behavior of Organoarsenicals in Soil", This Symposium, 1974.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

9 A Proposed Arsenic Cycle in an Agronomic Ecosystem G A R Y R. S A N D B E R G and I N G R I D K.

ALLEN

The Ansul Co., Weslaco, Texas 78596

A.

Introduction

Arsenical pesticides have been widely used in agriculture for more than a century and they represent the largest single man-made input of arsenic into the environment. Copper acetoarsenite (Paris green) was f i r s t used in 1867 followed by many other inorganic arsenicals in the early 1900's. Inorganic arsenicals, such as arsenates of calcium, copper, lead and sodium, potassium and sodium arsenites, arsenic trioxide and arsenic acid were commonly used as insecticides, herbicides, algicides or desiccants. Organic pesticides have replaced most uses for the inorganic arsenicals in the past few decades, with organic arsenicals replacing the inorganic types for herbicidal applications. The overall use patterns of arsenical pesticides have shifted from general use, broad spectrum pesticides to herbicidal uses only. Organic arsenicals marketed in agriculture today are cacodylic acid (CA) (hydroxydimethylarsine oxide), MSMA (monosodium methanearsonate) and DSMA (disodium methanearsonate). Effects of Inorganic Arsenicals on Crop Yields and Arsenic Residues in S o i l . Attention was focused on inorganic arsenical pesticides when accumulations of arsenic in soils eventually became toxic to several agricultural crops (1) (2) (3) (4) (5) (6) (7). Treatment rates for effective control of agricultural pests were usually quite large compared to pesticide applications today. In the State of Washington, the estimated peak use of lead arsenate insecticides occurred in 1943 when the average annual rate of application was 56.0 kg of elemental arsenic per ha (7). Arsenic trioxide was used as a soil sterilant to control vegetation at rates of 504.0 to 2,520.0 kg/ha (8). The effects of these massive applications on soil residue levels were obvious. Woolson (9.) reported the average arsenic levels in virgin soils at 2.5 to 10.0 ppm As and levels as high as 73 to 335 ppm As in soils with arsenical application histories. In another survey, the average arsenic level measured in 58 con124

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

9.

SANDBERG AND ALLEN

An Agronomic

Ecosystem

125

taminated soils was 165 ppm As and ranged from 106 to 2,553 ppm As. Soils from Washington averaged the highest (627 ppm As), whereas soils from check plots averaged 13.0 ppm As (10). Canadian soils contained up to 121.0 ppm As when lead arsenate insecticides were used in apple orchards Q l ) . Turf soils treated with lead arsenate and arsenic pentoxide for grub control in 1934-37 were analyzed at 130-550 ppm As. Untreated soils were reported at 12-13 ppm As (12.). Sodium arsenite was applied directly to soil at 90, 180 and 720 kg/ha to represent long-term applications for potato vine desiccation. These treatments increased average soil arsenic levels by 20.0, 70.0 and 147.0 ppm As, respectively, the first year and 23.4, 41.4 and 96.4 ppm As three years after application (13). This use in potato fields was apparently not as detrimental to soils as insecticida that were known to hav for potato vine desiccation determined that arsenic levels ranged from 2.2 to 25.7 ppm As (14). Lead arsenate sprays for 20-25 years in Oregon increased arsenic residues in soil to 40-115 ppm As. Untreated soils contained 2.5-4.6 ppm As (15). The persistence of inorganic arsenical applications was emphasized when phytotoxic effects of 2,690 kg/ha sodium arsenite were observed 14 years after treatment (16). It should be noted that phytotoxicity due to arsenic residues in soil are not necessarily correlated with total soil arsenic. Woolson, et al (10) found a growth reduction correlation of 0.74 when compared to total soil arsenic from 58 contaminated soils. A correlation of 0.82 was noted when plant growth was compared to arsenic fractions of Fe, Al or Ca. Soils with high reactive levels of Al were less phytotoxic than soils with low reactive Al levels; young corn was tolerant to treatments of 670 ppm As in soil that was highly reactive in Al. Vandecaveye, et al (3) concluded that poor growth of alfalfa and barley was due to the amount of readily soluble arsenic (3.4-9.5 ppm As) in old orchard soil. Another study noted that the amount of water soluble arsenic in soil was directly related to the rate of sodium arsenate applied and inversely related to time and the amount of reactive Fe, Al or Ca in the soil. Reaction of arsenic in the water soluble form to the Fe-As form was slower than to the Al-As form, but Fe-As was less soluble (17). Soil texture has been related to arsenic fixation (18) but both reactive Fe and Al in soils usually vary directly with clay content. Much larger rates of sodium arsenite were required to sterilize heavy soils than light textured soils (19); red soils also required higher rates of application (4). Toxicity of arsenic trioxide treatments was greater on coarse-textured than finetextured soils (£). Soil acidity influenced arsenic toxicity in certain soils (20) (21). Toxicity was greater in acid soils than alkaline but may be altered by As-P ratios. These results may also reflect

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

126

ARSENICAL PESTICIDES

the role of pH on the availability of reactive Fe, Al and P in soils. Woolson (22) concluded that plant growth in soils contaminated with arsenic was affected by the amount of available phosphorus in the soil solution. When considering water soluble forms of arsenic and water soluble phosphorus, a P/As ratio of less than 4.0 significantly reduced plant growth in Lakeland loamy sand, Hagerstown silty clay loam and Christiana clay loam soils. This ratio response was valid only i f the potential soluble arsenic in a given soil was large enough to be toxic in the first place. Treatments of 100 ppm As as sodium arsenate increased available arsenic levels to 27.9 ppm As in the Lakeland soil and 26.1 ppm As in the Christiana soil but growth of radishes was reduced to 23 and 93% of the controls respectively The P/As ratios were 4.4 in the former soil Organic matter di appea y sorption (23) when sodium arsenite was added to a Superior clay loam, Waupan silty clay loam or a Plainfield sand. Arsenic sorption was increased with increasing amounts of extractable Fe203 and A I 0 O 3 in soil; the greatest sorption of arsenic was observed with the Superior clay loam soil (0.5% organic C and 2.34% free iron oxides). The least arsenic sorption was noted with the Plainfield sand with 0.7% organic C and 0.34% free iron oxides. Excessive soil moisture was found to increase phytotoxicity of AS2O3 to Monterrey pine seedlings by 25% (24). A dose of 8,960 kg/ha AS0O3 required to reduce seedling survival to less than 5% in a Chenango s i l t loam but only 6,720 kg/ha were required under excessive moisture conditions. w a s

Effects of Organic Arsenicals on Crop Yields and Arsenic Residues in Soil. In perspective, organic arsenical herbicides are applied at considerably lower single and annual rates than the inorganic forms and have less impact on phytotoxicity to crops or soil residues. CA and MSMA applications contribute 9.1 and 5.5 kg respectively of elemental arsenic per hectare per year when applied at maximum recommended rates!/. Soil arsenic residues in the plow layer were increased an average of 3.0 and 4.5 ppm As after six annual applications of MSMA or CA respectively at recommended rates (25). Exaggerated doses of MSMA at 110 kg/ha over a six-year period increased arsenic levels in soil from 16.0 to 21.2 ppm in the 0-15 cm depth of soil. Arsenic levels in soil were elevated from 1.6 ta 7.8 ppm with doses of 27 kg/ha of MSMA. None of these treatments reduced cotton yields. Soil types were Decatur s i l t loam, Hartsells fine sandy loam and Dothan loamy sand (26). 1/Based on three annual applications of CA at 5.6 kg ae/ha/treatment and MSMA at 4.5 kg ai/ha/treatment recommended by The Ansul Company.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

9.

SANDBERG AND ALLEN

An Agronomic Ecosystem

127

Organic arsenicals are not active in the soil at normal rates of application. Data from studies to observe accumulative effects of annual CA or MSMA applications indicate that soil residues have not been phytotoxic to a wide variety of crops. Yields of soybeans, sugarbeets and wheat were not reduced after two yearly preplant treatments of CA at 8.4 kg ae/ha or MSMA at 6.7 kg ai/ha. Sugarbeets and wheat yields were tolerant to single applications of MSMA or CA as high as 67.2 or 84.0 kg/ha, respectively; soybean yields were not affected by applications of MSMA at 22.4 kg/ha or CA at 28.0 kg/ha. Soil type was a Hidalgo sandy clay loam (27). Unpublished data from The Ansul Company concluded that corn, cotton and grain sorghum yields have not been adversely affected by six annual preplant applications of MSMA at 6.7 kg ai/ha or CA at 8.4 kg ae/ha to a Hidalg were slightly increase as high as 280 kg/ha. Corn and grain sorghum data, collected the second year but not the first year after treatment,showed that sorghum yields were not reduced by either herbicide at 280 kg/ha and that corn was tolerant to 224 kg/ha MSMA or 280 kg/ha CA. A rate of 224 kg/ha is equivalent to approximately 15 yearly applications of MSMA or CA at maximum recommended rates. Four applications of DSMA and MSMA at rates ranging from 2.23 to 8.95 kg/ha per treatment were made for four successive years on a Chesterfield sandy loam. Yields of cotton, soybeans, sorghumsudan hybrid, corn, oats, vetch and crimson clover were not reduced by any of the treatments. Average soil arsenic residues varied from 10.4 to 26.7 ppm above background in soils treated with the low and high rates of MSMA; DSMA applications increased levels 8.4 to 17.3 ppm As. The total depth of soil sampled was 30 cm. About 90% of the soil arsenic was associated with Al in the clay fraction while phosphorus was identified in the Fe and organic matter fractions (28). DSMA applications at 134.4 kg/ha were not phytotoxic to winter or summer crops grown for two years after treatments in Arizona. Winter crops were barley and safflower; summer crops were cotton and sorghum. Soil texture was 42% sand, 37% s i l t and 21% clay (29). A greenhouse study concluded that DSMA treatments of 50 ppm inhibited rice growth by 75%, soybeans by 39%, oats by 12%, corn and cotton by 10% and wheat by 0%, when incorporated into a Bosket s i l t loam. Incorporations of 100 ppm DSMA were not phytotoxic to rice in Dubbs s i l t loam or Sharkey clay. The 50 and 100 ppm doses were calculated for soil on an oven dried basis so the equivalent rates were greater than 112 and 224 kg/ha, respectively based on a 6 in. furrow slice of soil. Incorporations of DSMA at 120 ppm (268.8 kg/ha) in the Bosket s i l t loam reduced cotton growth i n i tially but did not inhibit growth 32 weeks later. Additions of phosphorus increased arsenic toxicity in both of the loam soils but not the clay soil (30). Apparently the amount of phosphorus

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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added was sufficient to displace DSMA or its breakdown product (probably arsenate) to increase the amount of available arsenic in the soil solution. Woolson (personal conversation) found that low levels of phosphorus added to an arsenic toxic soil will displace arsenic from soil particles to increase toxicity to plants but that large applications of phosphorus will compete with arsenic at the root surface and actually decrease toxicity. Also, high phosphorus additions may increase arsenic uptake by plants without causing toxic symptoms. Other researchers (31) found that the amount of DSMA adsorbed in Norfolk loamy sand, Augusta s i l t loam, Decatur clay loam and Vaiden clay soils was 7.7, 34.3, 27.2, and 50.2% of the equilibrium solutions added for each soil, respectively. These values corresponded to the clay fractions of each soil type Lower application associated with normal organi application distinct advantage over inorganic forms; however, the additional importance and uses of these products in agriculture have reemphasized the need to understand the behavior of arsenicals in the environment and the potential hazards of their continued use. B.

Background Arsenic in Nature

To understand conditions in which arsenicals could accumulate to undesirable levels in nature, it is necessary to examine the distribution of arsenic in nature and the mechanisms which affect the arsenic cycle. In terms of abundance of elements, arsenic ranks 18th in the universe, 20th on the earth's crust, 14th in sea water and 12th in the human body (32). Arsenic levels of 13 ppm in shales but only 1.8 ppm in igneous rock and 1.0 ppm in sandstone and limestone (33) suggests the influence of biological systems on the accumulation of arsenic during early geological times. Relatively high levels of 140 ppm As in coal (34) and 42 ppm As in unweathered shale (12) also support the role of primordial plant life in arsenic distribution. Over sixty soil types which were believed to be free of arsenical applications were surveyed by Williams and Whetstone in 1940 (12). About 30% of the samples contained less than 5 ppm As, 50% contained 5-10 ppm As and about 20% more than 10 ppm As. There was no uniformity in the distribution of arsenic within the profiles and no relationship between the climatic conditions or geographical formations from which the soils were developed. In general, the arsenic level in sandy soils and soils with a high silica :sesquioxide ratio was relatively low, and that of soils of the semi-humid and arid regions was higher. The soil with the lowest level of arsenic observed was a Brassua sandy loam (New Hampshire) at 0.1 ppm; the highest level was 42 ppm As in a Canyon clay loam (Kansas). Unpublished data from The Ansul Company noted that the aver-

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age background arsenic levels in soil ranged from 3.37 ppm As in Georgia to 11.1 ppm As in Texas. California, Arizona and Mississippi soils averaged 5.42, 5.06 and 6.16 ppm As respectively. The variability within a sampling site was as much as five to tenfold. Detectable levels of arsenic have often been reported in freshwater systems and were not attributed to sources of contamination. Durum, et al (35) surveyed freshwater systems in the U.S. and found that 76% of 726 samples were below limits of detection (0.01 ppm As); the remaining 24% of the samples averaged 0.01 ppm As but ranged from 0.01 to 0.14 ppm As. Kopp and Kroner (36) noted that 94% of 1,577 samples collected in another survey were below 0.10 ppm As. The average As level was 0.06 ppm and ranged from 0.01 to 0.34 ppm As The Lake Erie watershed contained the highest level (0.34 ppm the California watershed Hopk (37) average arsenic level in a New York lake was 0.01 and ranged from 0.01 to 0.04 ppm As. Schroeder and Balassa (38) reported arsenic in many common foods from commercial sources. Fish and seafood averaged 4.64 ppm As with shrimp shells being the highest component (15.3 ppm As). Meats ranged from below limits of detection (<0.1 ppm As) to 1.4 ppm As in pork liver and averaged 0.49 ppm As for all meats. Vegetables and grain averaged 0.41 ppm As; corn meal was highest with 0.78 ppm As. A survey of 28 U.S. cities during 1969-70 concluded that arsenic levels in dairy products were less than 0.1 ppm As but ranged from 0.1 to 2.6 ppm As in the meat, fish and poultry food class (39). Virtually all living organisms are exposed to arsenic. The role of arsenic as an essential element has been suggested (40) (41) (42) (43) but not proven. C.

Review of Arsenic Cycles and Related Ecological Studies

Studies to observe the movement of arsenic between given phases in an ecosystem have been complicated by the absence of a stable arsenic isotope and large quantities of analytical data to establish reliable background levels of arsenic. Natural arsenic levels are characterized by large variations within samples and between sampling sites. Frost (43) proposed a closed arsenic cycle in nature that continually transferred arsenic between water, soil, air and animals; volatile arsines were characterized as the form of arsenic in air and contributed to soil and water arsenic levels. Methylarsines are degradation products of organic arsenicals and play a very significant role in the arsenic cycle. More discussion on methylarsines is presented in the proposed cycle. Allaway (44) described the environmental pathways of trace elements to include arsenic, but did not estimate the input by agriculture. Annual uptake of arsenic by vegetation was a major

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transfer mechanism in the arsenic cycle. Isensee, et al (45) studied the distribution of CA and dimethylarsines (DMA) in an aquatic ecosystem. Neither chemical showed a high potential for biomagnification in algae, Daphnia, snails or fish. Treatments at 0.1, 1.0 and 10.0 ppm CA or DMA were not toxic to the organisms. A thesis by Macklin (46) observed the effects of CA tree injections*/ on a forest ecosystem. A large amount of the arsenic applied was transferred to the forest floor with leaf drop (1,050 ppm As), but subsequent movement of arsenic from the litter to soil was relatively small; soil arsenic levels were increased from 15.4 ppm As (before treatment) to 19.2 ppm As 16 weeks after treatment. Final concentration of arsenic in the litter was 96.0 ppm which suggests that a loss other than leaching was occurring probably reduction to volatil Greaves (47) discusse tribution of organic arsenical silvicides in the forest ecosystem. It was concluded that the movement of arsenic with litter fall and decomposition of woody tissue could be controlled with timing and density of treatments. In the forest ecosystem, various parts of the tree could be "sinks" in the model system which would minimize exposure to non-target organisms. Norris (48) found that arsenic was mobile in the forest floor litter after treatments of CA or MSMA for forest thinning operations, but that once the arsenic reached the soil i t was rapidly fixed. The soil represented a large "sink" in this study but i t was suggested that redistribution of arsenic from the litter may occur with methylarsines. Malone (49) (50) studied the effects of CA on a fescue meadow. The responses of microorganisms, plant biomass and community structure were essentially equal to those of the controls. Other ecological studies have been conducted with organic arsenicals to assess effects on community structures. In his thesis, Edwards (51) found that 30 sprayings of MSMA at total doses of 3,000, 30,000 and 300,000 ppm MSMA over a three-month period had relatively l i t t l e effect on a salt marsh ecosystem. The highest rate was toxic to Spartina alterniflora, the major vegetation in southeastern marshes, and Littorina irrorata, a salt marsh mollusc, but not the lower rates. Arsenic levels in two mollusc species were high initially after treatment but decreased appreciably within one month after treatment. Spraying or flooding methods of MSMA applications produced equal results on Spartina. Hunter and Young (52) surveyed the effects of military defoliants on vegetative succession in Florida. CA was applied at total rates of 12.3 and 59.4 kg ae/ha over an 8 year period; 2,4-D, 2,4,5-T and picloram were also used in the 240 acre site. Soils within the treated area were Lakeland, Chipley and Rut!edge 2/A CA formulation is marketed commercially as a silvicide.

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sands and ranged from poor to moderate in drainage. Nine months after the last application, all but two of the 74 dicotyledonous species collected in untreated areas were found in the treated site. Vegetative composition and successional patterns were not affected by defoliant treatments or application patterns but were apparently influenced by soil type and previous mechanical disturbances. In a similar study (53) Pate, et al determined that species diversity among mammals, BTrds, reptiles and amphibians was large in treated and untreated areas. The observed differences in species found could be accounted for in habitat preferences, especially vegetative cover, but not chemical treatments. D.

A Proposed Arsenic Cycle in an Agronomic Ecosystem

The cycle proposed in this paper considered plant, soil, air and water, with special emphasis on the soil since that is assumed to be the "sink" for the system. The inputs of arsenic into the model were primarily CA and MSMA applications, but additions with fertilizers, irrigation water, and oxidation of arsines were also considered. Industrial and municipal contributions were recognized but the amount of contamination from this source was not estimated. The most important transfer mechanisms in the model, or for any arsenic cycle, include arsenical oxidation and reduction reactions, soil erosion, plant uptake and translocation and harvesting of crops for food, feed and fiber. The transfer rates used in this model are often quite general and based on available data at this time. Additional studies are required to identify the effects of specific factors on the arsenic cycle; some factors suspected are soil type, soil chemistry, organic matter, climate, agronomic cultural practices, crops grown, arsenical use history and others. This cycle is based primarily on field crops grown in the Lower Rio Grande Valley of Texas but the basic concepts are applicable to other types of agriculture such as forest, pasture, orchard or even landscape applications. It should be noted that the arsenic cycle will continue to operate without an agricultural input but at a greatly reduced rate. In the absence of any arsenic inputs, the soil depletion time was estimated to be 100 years due to plant uptake and mining alone (44). A aTagram of the proposed cycle is presented in Figure 1. Each pathway is numbered and discussed individually. Transfer 1: Applications of MSMA or CA Herbicides. The primary inputs of arsenic into this system are post-emergent applications of organic arsenical herbicides. The number and rate of applications vary with chemical used and with crops treated. The maximum recommended yearly doses of either chemical were selected

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Figure 1

A proposed model for the arsenic cycle in an agronomic ecosystem

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for this model to demonstrate conditions which most favor a build up of arsenic in the system. MSMA or CA herbicides are applied to certain orchard crops up to three times within a season and at rates not to exceed 4.5 kg ai MSMA/ha or 5.6 kg ae CA/ha per application. The greatest possible seasonal input is 13.5 kg/ha MSMA or 16.8 kg/ha CA. Normally the rates required for most agricultural crops are not this large, especially for weed control in field crops. MSMA and CA contain 40.7 and 54.3% elemental arsenic respectively. The greatest input of arsenic/ha/yr is 5,495 gm (about 2.4 ppm) from MSMA or 9,122 gm (about 4.1 ppm) from CA. Transfer 2: Contamination of Water with Spray Drift or Volatilization. Pentavalent organic arsenicals are not volatile and direct spray contact o tion precautions are observed MSMA and CA are completely water soluble and are rapidly dispersed in water. Eventually MSMA or CA are oxidized to the arsenate form and fixed on sediment particles or are reduced to methyl arsines which migrate out of the treated area. Experimental data from applications of MSMA to irrigation ditchbanks showed that arsenic residue levels in water from treated canals reached a maximum of 0.16 to 0.86 ppm As when the spray solution directly contacted the water surface. These levels dispersed to below 0.05 ppm As (U.S. Public Health Service drinking water standards) within 80 to 100 minutes after application. The higher residue levels were noted in small irrigation laterals with relatively low flow rates (54). The same study found that 28 and 7% of the MSMA added to nonfiltered and filtered water samples respectively could not be recovered after 36 days. This loss may be due to arsenic fixation on soil sediment particles and/or methylarsine movement out of the samples. Isensee, et al (45) found that 92.0% of the CA and 61.2% of dimethylarsine added to an aquatic ecosystem was recovered after 24 hours. The 8% loss was attributed to initial adsorption to aquatic organisms. It was assumed that the spray drift transfer was negligible in this model. Transfer 3: Arsenic Movement from Treated Vegetation to Soi1. The movement of arsenicals to soil from treated foliage takes place with spray runoff during application, direct spray contact with the soil surface and precipitation after treatment (rain, dew, sprinkler irrigation, etc). These transfers occur rapidly and do not allow chemical reactions to other forms of arsenicals. Other plant-to-soil movements are found with the death and decay of treated foliage and with translocation of MSMA and DSMA to the stems and roots; CA does not translocate and is strictly a

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contact herbicide. It has been reported that biochemical complexing of organic arsenicals takes place in plant tissues; also, these transfers take place at relatively slow rates. Translocation of MSMA and DSMA in susceptible weeds and tolerant crops has been investigated by several researchers. Johnsongrass and nutsedge applications were most commonly reported but evidence of MSMA and DSMA translocation in cotton and bermudagrass has also been cited. Duble, et al (55) found that less than 15% of the DSMA or amine methylarsonate (AMA) applied to purple nutsedge moved out of the treated shoots. It was also noted that DSMA was not readily broken down in the plant but that a DSMA complex may have been formed in the plant. Another study (56) concluded that arsenic was translocated in purple nutsedge tuFer the treated shoot. Arseni ber at the opposite end of the chain and in tubers with actively growing points. Arsenic content in the tubers was not correlated with their ability to produce new shoots. Wills (57) found that 12.9% of the Hc-MSMA applied to the foliage of purple nutsedge was absorbed into the leaf and that 11.7% of the amount applied was translocated to the tubers. Keeley and Thullen (58) treated yellow nutsedge with 3.36 kg/ha MSMA or DSMA and found that treated tubers contained 4 to 33 ppm As while untreated tubers were found to have only 1 ppm As. Small tubers contained more arsenic (23 to 33 ppm) than large tubers (4 to 12 ppm); vitality was reduced in small tubers but not in large tubers. In another study (59) Keeley and Thullen found that C-MSMA and C-DSMA treatments caused greater radioactivity in yellow nutsedge than in purple nutsedge. The differential penetration of the leaves probably contributed to poor control of purple nutsedge. Larger amounts of radioactivity in the plant were generally noted with MSMA treatments than with DSMA treatments and at warm temperatures. Approximately 85% or more of the radioactivity remained in the plant but some activity was found in daughter plants. It was suggested that nutsedge did not metabolize C MSMA or C-DSMA after 72 hours of exposure. Nutsedge treated with repeated applications of amine methylarsonate (AMA) increased starch hydrolysis in tubers but had l i t tle effect on fat and protein content. AMA-treated tubers respired at lower rates than untreated tubers (60). Cooley (61_) found that MSMA foliar applications at 2.24 kg/ha to woolly leaf bursage caused severe cell damage to root tissue. It was concluded that MSMA was translocated to the roots in herbicidal quantities since regrowth or initiation of new growth was not observed. Sckerl and Frans (62) showed that methanearsonic acid ( CMAA) applied to the foliage of johnsongrass was translocated to the root tips and terminal leaf within 8 hours. It was suggested 14

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that MAA may complex with either a sugar or organic acid or both. MAA was translocated from treated cotton stems to all parts of the plant but moved very l i t t l e from treated foliage to the terminal leaves, stems, lower leaves and petioles. Ehman (63) reported arsenic translocation to cottonseed to levels as higF as 40 ppm As i f topical applications are made at the closea-boll stage of growth. Approximately 25% of the DSMA applied to coastal bermudagrass foliaae was translocated to the roots and rhizomes within 5 days. Some C DSMA was found in respiratory CO2 but most of the C-As bonds were intact (64)., _ ,„ In studies using C-MSMA, C-DSMA and C-MAA, Keeley and Thullen (65) found l i t t l e translocation from cotyledons to developing leaves of cotton seedlings It was assumed tha CA was transferred to th via this mechanism was the return of crop residues to the soil after harvest; this rate was calculated by subtracting the amount of arsenic transferred with harvest from the total uptake value for this model; 15.8 gms As/ha/yr (harvest loss) was subtracted from 23.7 gms As/ha/yr (total uptake) to arrive at 7.9 gms As/ha being returned to the soil as crop residues each year. More detailed explanations of these figures are presented in each transfer discussion. T4

1 A

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Transfer 4: Uptake of Arsenic by Vegetation. Uptake of arsenic by plants is a continuous process and will occur in the absence of arsenical pesticide applications. The presence of arsenic in vegetation growing in virgin forest soils was observed by Schroeder and Balassa (38). Various fern species contained from 0.0 to 0.73 ppm As ancTplne needles were analyzed at 0.32 ppm As; the forest soil contained 3.64 ppm As. Williams and Whetstone (12) found that native vegetation on natural soil did not exceed 10 ppm As. Liebig (41) reviewed arsenic residue levels reported in several studies on almost 40 agricultural crops. This summary established low, intermediate or high ranges of arsenic levels observed for tissue analysis. Vegetables and other plants contained from 0 to 10 ppm As and usually had higher levels of arsenic when grown in soil contaminated with inorganic arsenical insecticides. It was concluded that arsenic does not normally accumulate to any extent in the above-ground parts, so tissue analysis of the tops of plants is a very poor indication of arsenic toxicity. Arsenic does accumulate in larger amounts in or on roots and causes plasmolysis or rotting of roots in solution cultures (42). Apparently the effect of arsenic toxicity retards plant growth before extensive amounts of arsenic are absorbed and translocated to the top. Data collected by The Ansul Company (27) concluded that two annual preplant applications of MSMA or CA at rates of 6.7 or 8.4

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kg/ha/yr respectively did not significantly increase arsenic residue levels above background in corn seed and fodder, cottonseed, sorghum seed and fodder, wheat, sugarbeets or soybean fodder. Arsenic residue in soybean seed was not affected by MSMA treatments but was significantly increased (0.40 ppm As) after the second annual treatment of CA. In the same report (27), single preplant applications of MSMA or CA were used to approximate the effects of 10 years of normal applications. MSMA at 22.4 and 67.2 kg/ha did not increase arsenic residues in any of the crops mentioned above after the first or second year after treatment except soybeans; the high rate of MSMA increased residues after the first year (1.15 ppm As) and both rates significantly increased residues after the second year (0.36 and 0.58 ppm As) A single preplant treatment of CA at 28.0 kg/ha di the crops after the firs idues in soybean seed (0.46 ppm As) and sugarbeets (0.63 ppm As) two years after treatment. The high rate of CA (84.0 kg/ha) applied as one preplant treatment increased arsenic residues in corn seed (0.16 ppm As), corn fodder (3.53 ppm As), cottonseed (0.32 ppm As), soybean seed (1.60 ppm As) and wheat (0.38 ppm As) after the first year; after the second year, residue levels continued to be significantly greater in corn fodder (0.89 ppm As), soybean seed (0.55 ppm As;, sugarbeets (0.85 ppm As) and wheat (0.21 ppm As). It is of interest to note that crop yields were not reduced with any of these treatments except soybeans treated with 67.2 kg/ha MSMA and 84.0 kg/ha CA the first year but not the second year after treatment. As with arsenic toxicity, uptake of arsenic by plants is not necessarily correlated with the total amount of arsenic in the soil. Woolson (22) found that arsenic uptake by six vegetable crops from soils treated with sodium arsenate was generally correlated with the amount of available soil arsenic required to reduce crop growth by 50% (GR50). Residues were low in crops where the edible portion of the crop was not the root or the entire plant. A P/As ratio has been implicated in arsenic toxicity (22) and probably has the same effect upon arsenic uptake. Woolson feels that high phosphorus additions to arsenic-contaminated soils may increase arsenic uptake by plants without causing toxic symptoms. Schroeder and Balassa (38) found that arsenic levels in vegetables and grain grown in soTT never sprayed with arsenicals were essentially equal to those grown in soil treated with heavy applications of phosphate fertilizer with 4.75 ppm As. The average residues in control and treated grains were 0.93 and 0.47 ppm As respectively; vegetables were 0.25 and 0.28 ppm As. Johnson and Hiltbold (28) found unusually high arsenic residues (1.61 to 5.20 ppm As) in several agricultural crops when grown on soils that had received previous DSMA or MSMA treatments for four successive years. The highest total dose of elemental

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arsenic applied during this period was 66 kg As/ha with MSMA treatments and 58 kg As/ha with DSMA treatments. A light phosphorus application was made before planting but crop yields were not affected. It was not clear what background levels were observed in untreated crops. The variability in arsenic uptake with different crops and soil conditions has been emphasized. Since this model is an attempt to illustrate general transfers in their perspective, no attempt was made to pick one type of crop for this transfer. To arrive at a general value, data from All away (44) was used to calculate that 25.5 x 10 tons of biomass were produced each year from 12 x 10 acres of agricultural land. If the total arsenic concentration in plants was assumed to be 5.0 ppm As, a transfer of 23.7 gms As/ha/yr was calculated The 5.0 ppm value for plants is probably low but wa a soil build-up of arsenic 9

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Transfer 5: Reduction of Organic Arsenicals to Volatile Forms of Arsenic (Plant-AirT^ Although not supported with data, it is suspected that the reduction of organic arsenicals to volatile methylarsines may take place while the dead vegetation is s t i l l standing. This reduction reaction is further discussed in the section on Transfer 7. The form of arsenic remaining in plants was determined to be an MSMA or DSMA complex (see discussion for Transfer 3) and CA may also be complexed within the cellular tissue. The role of microorganisms in the degradation of plant tissue is not questioned and supports this loss mechanism. Malone (50) found that treatments of CA to a fescue meadow actually increased the rate of litter decomposition from standing dead vegetation. He concluded in a similar study (49) that CA temporarily stimulated some soil bacteria and decreased the number of fungi but increased the overall decomposition process. For purposes of modeling this system, i t was assumed that this transfer was negligible. It is possible that this loss is sufficient to further decrease the potential build-up of arsenic in soil. Transfer 6: Arsenic Losses with Crop Harvest. The portion of an agricultural crop that is harvested for food, feed or fiber represents a loss of arsenic from the agronomic ecosystem. It has been noted that arsenic residues are generally lower in seed and fruit crops than in root crops or crops harvested for the entire plant; the amount of biomass removed also follows the same pattern. In the following summary, yield data were taken from the U.S.D.A. 1972 Annual Summary CrPr 2-1(73). Arsenic residue data were collected by The Ansul Company. To calculate a general transfer value for this model, all of the crop losses were averaged and a value of 15.8 gms As/ha/yr was

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Crop Alfalfa hay Barley Bermudagrass Corn seed Corn fodder Cotton lint Cottonseed Oats Sorghum seed Sorghum fodder Soybeans Sugar beets Sugarcane Wheat

Annual Yield

5,040 24,000 450 680 1,600 3,080 20,000 1,500 40,000

Ave. As Level

0.10 0.22 1.27 0.06 0.80 0.17 2.15 0.12 2.83

Est. As Loss (gms/ha/yr) TM 0.45 5.51 0.56 5.96 0.67 0.11 1.46 0.56 48.37 0.22 123.35

the amount lost. Trees (forest and orchard) are actually large temporary "sinks" for arsenic until harvested for lumber or taken out of cultivation. Therefore the amount of arsenic lost would be much greater than the values for field crops. Transfer 7. Reduction of Arsenicals to Volatile Forms of Arsenic (Soil-Air). Reduction of arsenicals to volatile forms of arsenic has been recognized for many years, but the role of this reaction in the arsenic cycle 1s just being realized. It has previously been concluded that once an arsenical came Into contact with soil i t was permanently bound at that site. It now appears that the reduction of organic and, to a lesser extent, Inorganic arsenicals may be a significant loss mechanism from the soil which redistributes arsenicals within the environment. Arsine gas was discovered in 1775 by Scheele and described as a poisonous gas with a persistent garlic-Uke odor. Vallee, et al (66) reviewed the history and toxicology of arsines. Arsine was liberated when hydrogen was generated in the presence of arsenic but also resulted from the reduction of arsenous or arsenic acid, from electrolysis of arsenous solutions, or from the action of water or dilute acid on metallic arsenides. The review (66) pointed out that most of the accidents involving arsines were reported in chemistry laboratories or in industry. But, in 1839, Gmelin found that a garlic odor, common in many damp European homes, was probably due to arsines that were generated from arsenic used as a coloring pigment in wallpaper and carpets. Apparently the damp and moldy environment was conducive to the reductive process. In 1891, Gosio (67) reported the generation of arsenical gas from the action of the mold, Penicillium brevicaule, on potato mash containing arsenious oxide^ He was not able to characterize the gas but later work suggests that i t was probably dimethyl or trimethylarsine. Gosio later reported that Aspergillus glaucus,

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A. virens, Mucor mucedo and M. ramosus also produced this gas TGosio-Gas); the exact nature of this gas was debated but was characterized as being punaent and with a garlic odor. Challenger, et al (68) cultivated four strains of Penicillium brevicaule on bread crumbs containing arsenious oxide to get t r i methylarsine. Later work (69) indicated that certain fungi were responsible for the biological methylation of arsenic, but negative results were noted with bacteria. In contrast, McBride and Wolfe (70) reported the formation of dimethylarsine with Methanosarcina and Methanobacterium species of bacteria, Smith and Cameron (71) studied the use of the mold Scopulariopsis brevicaulfs (previously Penicillium brevicaule) to detect arsenic in many kinds of food. If arsenic was present in amounts of 0.001 mg or less, a characteristic garlic odor was detected in two hours; large to sand produced garlic Zussman, et al (72) noted that arsine generation was reported for several species of saprophytic fungi but not for dermatophytic species. In their work Trichophyton rubrum, the latter type, produced a strong garlic odor from cultures with arsenate but not with arsenite. It was not established i f arsine or a methylated form of arsenic was produced. The gas was generated at concentrations of 0.006 to 0.0015 M arsenate; fungal growth was inhibited at higher concentrations and l i t t l e odor was detected at lower concentrations. Merrill and French (73) found that Lenzites trabea and I. saepiaria, brown-rotting fungi, liberated a strong garlic odor when grown on wood treated with an arsenical preservative or AS2O3. Sixty-three other species of wood-rotting fungi did not cause a garlic odor when grown on a medium containing AS2O3. All attempts to identify the gas were unsuccessful. In a report by Thorn and Raper (67), 10 strains or species of Scopulariopsis (or P. brevicaule) were active arsine gas producers and 14 strains of Aspergillus sydowi were positive; both of these groups are common in soil. Many of the organisms tested were not found to be gas producers at 0.1 and 0.15% arsenic (as arsenious oxide) but may be found to be positive at lower concentrations. A Durham sandy loam soil was surveyed to identify the mold species present. This soil contained enough arsenic from inorganic arsenical applications to be toxic to certain crops. , About two species of Fusarium and two or three unidentified sterile masses were the most abundant molds. Actinomyces and bacteria colonies were noted and eventually Myamoeba and Plasmodium organisms developed. No infusoria or nematodes were seen. Of these organisms surveyed, all but Aspergillus ustus, a Penicilla, and some of the sterile forms were active gas producers. It was concluded from this study that accumulations of arsenic in the soil may be expected to occur only when massive amounts are used or under special conditions unfavorable to the development of a varied microflora. McBride and Wolfe (74) presented a metabolic pathway using

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Methanobacterium cells or cell extracts to reduce and methylate arsenate under anaerobic conditions. In the pathway, arsenate is first reduced to arsenite which is methylated to form methylarsonic acid. Methylarsonic acid (MAA) is then reduced and methylated to form dimethylarsinic acid (cacodylic acid or CA); CA is further reduced to dimethylarsine gas. Braman and Foreback (75) developed procedures to analyze for arsenate, arsenite, methylarsonic acid (MAA) and dimethylarsinic acid (CA). All four forms of arsenic were found in various samples of natural waters; the latter two were also detected in bird shells, seashells, and human urine. It was probable that all of these forms of arsenic occurred naturally and were representative of the biological reduction levels that are part of an arsenic cycle. CA was a major and ubiquitous form of arsenic involved in biological systems. C MAA which indicates th sequence. It was noted that CA is approximately 25 times less toxic than the arsenate ion which suggests that the methylation reactions are a detoxifying process. The authors did point out that arsenic Inputs may eventually result in increased levels of the methylated arsenicals in water and air due to bacterial mobilization. Three factors that affect the generation of methylarslnes are species of organism, the concentration of arsenic, and the form of arsenic. Cox and Alexander (76) found that pH may also influence the amount of trimethylarsine generated. After incubation of about one month, garlic odors were detected in cultures with dimethyl arsenic acid (CA) at pH 4, 5 and 7 and in cultures with methylarsonic acid (MAA) at pH 5. Sodium arsenate was reduced at pH 4. Trimethylarsine was generated in cultures with three sewer fungi species, Candida humicola, Gliocladium rosenum and Penicillium spp., in the presence of CA and MW. Reduction of sodium arsenite or sodium arsenate was not observed with G. roseum Penicillium but did take place at low rates with C. humicola. Acid conditions may have enhanced arsenic reduction Tn this study. Newton and Greaves (48) studied the effects of levels of CA or MAA and of glucose levels on the loss of arsenic through volatilization. The losses were probably due to an interaction between the two variables; substantial losses were noted at all glucose levels while the amount of arsenic lost after 28-31 days was generally greater with the high concentrations of CA or MAA. Variations in the losses may have been due to a substrate-organism specificity. All losses were recorded at temperatures below 70°F. Woolson and Kearney (77) found that arsenic losses from soil were influenced by soil type, concentration of CA, and moisture levels. Losses attributed to methylarsines were greater at the highest rate of CA applied and under anaerobic conditions. Methylarsine generation was highest in the Lakeland loamy sand and about equal in the Hagerstown silty clay loam and Christiana clay soils. The average losses were 35 and 61% under aerobic and ano r

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aerobic conditions respectively during a 24-week period. It was concluded that the gas may have been dimethylarsine which is extremely unstable and could be oxidized back to CA which is probably returned to plants or soil. The Ansul Company (25) concluded that as much as 17% of the MSMA and 28% of the CA applied to soil over a six-year period could not be accounted for in the soil profile. Total doses for the period were 40.2 kg MSMA/ha and 50.4 kg CA/ha. It was concluded that the losses were greater from plots treated with high rates than those treated with low rates. Arsenic not recovered was attributed to migration of methylarsines out of the test area. It should be pointed out that while arsines are known to be highly toxic to humans, these gases are also volatile and very unstable at high concentrations Vallee et al (66) stated that simple precautions coul fatalities in industry workers have failed to realize the risk of moisture or acid containing impure metal, especially in closed areas. In humans, arsine concentrations of 3-10 ppm may cause toxic symptoms in several hours, 10-60 ppm may be dangerous in 30-60 minutes, and 250 ppm may be lethal in 30 minutes. The maximum average safe concentration for the exposure of workers to arsine is 0.05 ppm as recommended by the American Conference of Governmental Industrial Hygienists. It is reasonable to conclude that the levels of volatile arsenicals generated in biological systems under normal conditions are within the 0.05 ppm concentration limit. For this model, it was assumed that 17-35% of the total input of arsenic to the soil was lost as methylarsines; this included inputs from crop residues, irrigations, and fertilizers.' The loss calculated was 1,638-3,282 gm As/ha/yr for CA applications and 1,018-2,007 gm As/ha/yr for MSMA applications. Transfer 8: Oxidation of Methylarsines. Methylarsines are very unstable in air at concentrations above .05-.10 ppm and are rapidly oxidized to less reduced forms of arsenic. Problems associated with residue analysis for the methylarsines are evidence of their instability. The pathway proposed by McBride and Wolfe (74) would be reversed so that the oxidation pathway would be a methylarsine-CA-arsenate sequence. Conclusions by Braman and Foreback (75) and Woolson and Kearney (77) also support this oxidation pathway. Arsenic losses reported in recovery studies by Woolson and Kearney (77) and The Ansul Company (25) indicate that at low concentrations methylarsines were stable enough to allow migration out of treated areas. The presence of CA and MAA in bird shells, seashells and water in areas remote from agriculture was evidence of migration of naturally occurring arsenic. Other studies have reported losses in which all of the arsenic applied to soil was not accounted for. Epps and Sturgis (78) found that 21.6, 3.8 and 1.3% of the initial arsenic levels in a

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flooded Crowley s i l t loam soil were lost with applications of calcium arsenate + organic matter, arsenic trioxide + organic matter, and arsenic trioxide alone, respectively. Incubation time was eight weeks. Steevens, et al (13) could not account for all of the sodium arsenite applied to a Plainfield sand (0-83 cm prof i l e ) . Loss by wind erosion was suggested but methylarsines may have geen generated. Hiltbold, et al (26) were unable to recover 67, 57 and 39% of the arsenic applied as~MSMA to Hartsells fine sandy loam, Decatur s i l t loam and Dothan loamy sand soils, respectively, after six years of application. The loss values calculated for Transfer 7 were in addition to the oxidation of methylarsines back to the original site of application. It can only be concluded that the net loss of methylarsines was greater tha could be influenced by concentration of methylarsines in air. Transfer 9: Arsenic Input with Irrigation. It has been established that background levels of arsenic in freshwater do exist (35) (36) (3Z) (74). This input would be significant 1n the overall arsenic cycle, while being more important for certain areas and crops. Salman, et al (54) estimated that four irrigations with water containing 0.05 ppm As would contribute 0.002 lbs of elemental As or 9.0 gms As/ha/yr. This calculation was exaggerated to present conditions that most favor soil build up; the average As level in water is well below 0.05 ppm. Transfer 10: Movement of Arsenicals from Treated Soil to Water" The two mechanisms considered for this transfer were 1) leaching of arsenicals through the soil profile to ground water, and 2) wind and water erosion of treated soil particles directly to water sources. The arsenic fixing capacity of soils has been previously discussed in Sections A-I and A-II. It was concluded that the most influential factors were reactive levels of Al and Fe in soil but that phosphorus additions, soil type, type of clay, and pH may affect Al and Fe availability. 1) Leaching. Jones and Hatch (2) surveyed old orchard soils in Oregon that were unproductive due to high levels of arsenic applications as insecticides. A clay adobe soil contained 441 ppm As in the surface 20.3 cm but only 19 ppm As in the next 20.3 cm; similarly, a s i l t loam soil was found with 61 and 7.6 ppm As at each respective depth. Benson (7) found that arsenic in orchard soils treated with lead arsenate was not readily leached below the root zone with percolating water. Vandecaveye, et al (3) sampled old orchard soils to a depth of 121.9 cm and found that the amount of water soluble arsenic was concentrated in the surface 15.2 cm with very l i t t l e arsenic contained in soil below

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61.0 cm. Arnott and Leaf (24) concluded that arsenic trioxide applications at rates ranging from 1,120-8,960 kg/ha to a Chenango s i l t loam soil did not increase arsenic residues in the soil prof i l e below 35.6 cm. The amount of leachate used affected residues in the surface 25.4 cm only. Steevens, et al (13) recovered arsenic that was leached to a depth of 38 cm after treatments of 90 and 180 kg As/ha/yr (as sodium arsenite) to a Plainfield sandy soil; an application of 720 kg As/ha caused leaching of arsenic to 68 cm. This distribution was observed three years after application and in an area with relatively high precipitation (Hancock, Wisconsin). Research by Dickens and Hiltbold (31) revealed that DSMA was not leached below 15.2 cm in a Decatur cTay loam soil and that approximately 52% of th column of Norfolk loamy applie equivalent to 112 kg/ha and leaching was achieved with 50.8 cm of water. Soil pH did not affect leaching in this study. Ehman (79) reported the results of leaching experiments with about 150 cm of water, through columns (about 30 cm) of sandy and sandy loam soils, containing DSMA (31.4 kg/ha) and CA (16.8 kg/ha) applied to the soil surface. The leachate removed about 9% of each product from the sandy soil and about 6% from the sandy loam soil. Studies by Johnson and Hiltbold (28) concluded that all of the arsenic applied with 16 applications of DSMA or MSMA over a four-year period was within the upper 30 cm of a Chesterfield sandy loam soil; the rate of application was 8.92 kg/ha/yr with either treatment. Increased application rates of 17.88 and 35.80 kg/ha/yr of either chemical caused about 25 and 75% of the arsenic applied, respectively, to move below 30 cm in depth. Soil samples at 90 cm indicated that leaching had not occurred to that depth. Approximately 580 cm of rainfall was measured during the study period. MSMA applications at rates as large as 40 kg/ha/yr over a six-year period did not cause leaching of arsenic below 30 cm. In this study, Hiltbold, et al (26) concluded that movement of arsenic was least in a Decatur s i l t loam, greatest in a Hartsells fine sandy loam, and intermediate in a Dothan loamy sand soil. Norris (48) studied the movement of MSMA and CA in Klicker s i l t loam, Astoria silty clay loam and Edds loam soils from forest floors in the Northwest. MSMA was not leached through any of these soils (7.62 cm soil columns) while 86.4 cm of water were required to move 84 and 50% of the CA applied in the Klicker and Edds soils, respectively. It was concluded that neither chemical presented contamination hazards to groundwater. Total doses of MSMA at 40.2 kg/ha and CA at 50.4 kg/ha over a six-year period have not increased arsenic levels below the plow layer in a Hidalgo sandy clay loam soil (25). These studies indicated that the possibility of contamination

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of groundwater with leaching of arsenicals is very insignificant. All of the conditions which favor leaching (high rates of arsenic applied, light soils, and heavy leaching pressures) were tested, but leaching below the 90 cm depth of soil was not observed; in most studies this depth was considerably less. For the agronomic ecosystem model, this part of Transfer 10 was considered negligible. 2) Erosion. Erosion of soils has been conservatively estimated at 2.78 tons per acre per year when corn, wheat and clover were grown in rotation (80). If the average soil arsenic level is assumed to be 10 ppm As, approximately 60.0 gms As/ha/yr are lost with this transfer. Transfer 11: Arseni Schroeder and Balassa (38) noted that phosphate fertilizers contained as much as 4.50 and 4.75 ppm As. This is not unexpected because of the similarities and interactions between As and P (38) (43) (27). Assuming that a soil test for phosphorus is within average values, a rate of about 84 kg/ha phosphorus is recommended to maintain soil fertility (81). A transfer value of 0.4 gms As/ha/yr was calculated for the model. Transfer 12: Arsenic Contamination from Non-Agricultural Sources^ Inputs of arsenic from the burning of fossil fuels (43), industrial wastes (35), and municipal wastes (27) are known to occur, but the amounts contaminating the agricultural ecosystem are not known at this time. The transfer rate for this model is assumed to be zero. E.

Conclusions

The ubiquity of arsenic in the environment is evidence of redistribution processes that have been operating since early geological times. Recent work by Braman and Foreback (75) confirms the presence of a natural arsenic cycle, but long-term effects of man-made arsenic inputs on this cycle remain uncertain. This model was proposed to give some perspective on the impact of organic arsenical herbicides in an agronomic ecosystem; however, many of the transfers involved would certainly be applicable to other ecosystems. The reduction of arsenicals to methylarsines, soil erosion and arsenic uptake were the primary redistribution mechanisms in this model. Since the soil is the ultimate sink for arsenic, a budget for soil was arranged. Values presented are theoretical and reflect conditions which favor a buildup of arsenic in soil. Net input into the soil with the CA model was 9,139.3 gm As, net losses were 1,721.7-3,365.0 gm As/ha/yr, giving a theoretical buildup of 2.6-3.3 ppm As/ha/yr. In the MSMA model, input was 5,512.3 gm,

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losses were 1,107.7-5,512.3 gm As/ha/yr, or a 1.5-1.9 ppm As gain. The 17-35% values used for methylarsine losses may be low. These values were taken from studies in which herbicides were applied directly to bare soil surfaces. In practice, efficient applications of organic arsenical herbicide will distribute most of the residues in or on weedy plant tissue, increasing the possibility of methylarsine losses directly from decomposing vegetation to air and bypassing soil in the transfer sequence. It has been reported that garlic odors were most noticeable when high rates of CA or MSMA were used, so the percent lost may increase with continued applications. It is suspected that an equilibrium will occur in soil at which methylarsine losses equal organic arsenic inputs, but this has not been clearly demonstrated yet. From the data presented in this model, i t was concluded that arsenic is mobile and nonaccumulativ phases of the agronomic ecosystem. Arsenicals do accumulate 1n soil but redistribution mechanisms preclude hazardous accumulations at a given site. F.

Literature Cited

1. Dorman, C., Tucker, F.H., Coleman, R., J. Amer. Soc. Agron. (1939) 31 1020-28. 2. Jones, J.S., Hatch, M.B., Soil Sci. (1937) 44 37-63. 3. Vandecaveye, S.C., Horner, G.M., Keaton, C.M., Soil Sci. (1936) 42 203-213. 4. Crafts, A . S . , Rosenfels, R.S., Hilgardia (1939) 12(3) 177-200. 5. Ball, E.D., Titus, E.G., Greaves, J . E . , J. Econ. Entom. (1910) 3 187-197 6. Bishop, R.F., Chisholm, D., Can. J. Soil Sci. (1961) 42(1) 77-80. 7. Benson, N . R . , Wash. State Hort. Assoc. Proc. (1968) 1E-5E. 8. Leaf, A . L . , Smith, R.E., Jr., Weed Sci. (1960) 8 374-378. 9. Woolson, E.A., PhD thesis, Univ. of Maryland (1969). 10. Woolson, E.A., Axley, J . H . , Kearney, P.C., Soil Sci. Soc. Amer. Proc. (1971) 35(6) 938-943. 11. Miles, J.R.W., J. Agri. Food Chem. (1968) 16(4) 620-622. 12. Williams, K.E., Whetstone, R.R., USDA Tech. Bull. 732 (1940) 20 pp. 13. Steevens, D.R., Walsh, L.M., Keeney, D.R., J. Environ. Quality (1972) 1(3) 301-303. 14. Steevens, D.R., Walsh, L.M., Keeney, D.R., Pest. Monitoring J. (1972) 6(2) 89-90. 15. Jones, J.S., Hatch, M.B., Soil Sci. (1945) 60 277-288. 16. Isensee, A.R., Shaw, W.C., Gentner, W.A., Swanson, C.R., Turner, B.C., Woolson, E.A., Weed Sci. (1973) 21(5) 409-412. 17. Woolson, E.A., Axley, J . H . , Kearney, P.C., Soil Sci. Soc. Amer. Proc. (1973) 37(2) 254-259. 18. Small, H.G., McCants, C.B., Agron. J. (1962) 54 129-133. 19. Rosenfels, R.S., Crafts, A.S., Hilgardia (19397 12(3 ) 203-229.

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Hurd-Karrer, A.M., USDA Tech. Bull. 911 (1946) 31 pp. Benson, N.R., Soil Sci. (1952) 74(3) 215-224.5 Woolson, E.A., Weed Sci. (1973) 21(6) 524-527. Jacobs, L.W., Syers, J.K., Keeney, D.R., Soil Sci. Soc. Amer. Proc. (1970) 34 750-754. Arnott, J.T., Leaf, A . L . , Weed Sci. (1967) 15(2) 121-124. Sandberg, G.R., Allen, I.K., Dietz, E.A., Jr., Ansul Progress Report (1973) 82 pp. Hiltbold, A . E . , Hajek, B.F., Buchanan, G.A., Weed Sci. (1974) 22(3) 272-275. The Ansul Co., Response to FR As and Pb Notice 36 FR 12709 (1971) 55 pp. Johnson, L.R., Hiltbold, A . E . , Soil Sci. Soc. Amer. Proc. (1969) 33 279-282. Hamilton, K.C., Arle Schweizer, E.E., Weed Sci. (1967) 15(1) 72-76. Dickens, R., Hiltbold, A . E . , Weed Sci. (1967) 15(4) 299-304. Frost, D.V., "The Arsenic Cycle in Nature," unpublished. Bowen, H.J.M., "Trace Elements in Biochemistry," Academic Press, New York (1966). Hertzog, E.S., Ind. Eng. Chem. (1935) 7(3) 163-165. Durum, W.H., Hem, J.D., Heidel, S.G., Geo. Survey Circ. 643, 49 pp. Kopp, J.F., Kroner, R.C., "Trace Metals in Waters of the United States," U.S. Dept. of Interior (1967). Lis, S.A., Hopke, P.K., Environ. Letters (1973) 5(1) 45-51. Schroeder, H.A., Balassa, J.J., J. Chron. Dis. (1966) 19 85-106. Corneliussen, P.E., Pest. Monit. J. (1972) 5(4) 313-330. LeBlanc, P.J., Jackson, A . L . , Marine Pollution Bull. (1973) 4(6) 88-90. Liebig, G.F., Jr., "Diagnostic Criteria for Plants and Soils," 13-23, Univ. of Calif. (1966). Liebig, G.F., Jr., Bradford, G.R., Vanselow, L.A.P., Soil Sci. (1959) 88(6) 342-348. Frost, D.V., Federation Proc. (1967) 26(1) 194-207. Allaway, W.H., Adv. Agron. (1968) 20 235-274. Isensee, A.R., Kearney, P.C., Woolson, E.A., Jones, G.E., Williams, V.P., Environ. Sci. Tech. (1973) 7(9) 841-845. Macklin, C.E., Witkamp, M., MS thesis, Emory Univ., Atlanta (1973) 35 pp. Greaves, R.D., MS thesis, Oregon State Univ. (1974) 94 pp. Norris, L.A., USDA Forest Service report, Corvallis, Oregon (1974) 98 pp. Malone, C.R., Soil Biol. Biochem. (1971) 3 127-131. Malone, C.R., Ecology (1972) 53(3) 507-512. Edwards, A . C . , MS thesis, Auburn Univ. (1973) 90 pp. Hunter, J.H., Young, A . L . , USAF Tech. Rep. AFATL-TR-72-31 (1972) 25 pp.

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53. Pate, B.D., USAF Tech. Rep. AFATL-TR-72-72 (1972) 13 pp. 54. Salman, H.A., Bartley, T.R., Summers, A.D., Bur. of Reclam. REC-ERC-72-37 (1972) 8 pp. 55. Duble, R.L., Holt, E.C., McBee, G.G., Weed Sci. (1968) 16(4) 421-424. 56. Holt, E.C., Faubian, J . L . , Allen, W.W., McBee, G.G., Weed Sci. (1967) 15(1) 13-15. 57. Wills, G.D., Proc. So. Weed Sci. Soc. (1970) 23 334-337. 58. Keeley, P.E., Thullen, R.J., Weed Sci. (1970) 18(4) 437-439. 59. Keeley, P.E., Thullen, R.J., Weed Sci. (1971) 19(5) 601-606. 60. Duble, R.L., Holt, E.C., Weed Sci. (1970) 18(1) 174-179. 61. Cooley, A.W., MS thesis, Texas Tech. Univ. (l971) 63 pp. 62. Sckerl, M.M., Frans, R.E., Weed Sci. (1969) 17(4) 421-427. 63. Ehman, P.J., Proc. 19th Ann. Meeting SWC (1966) 540-541. 64. Duble, R.L., Holt, E.C. 17(6) 1247-1250. 65. Keeley, P.E., Thullen, R.J., Weed Sci. (1971) 19(3) 297-300. 66. Vallee, B.L., Ulmer, D.D., Wacker, W.E., AMA Arch. Ind. Health (1960) 21 132-151. 67. Thom, C., Raper, K.B., Science (1932) 76(1980) 548-550. 68. Challenger, F., Higginbottom, C., E l l i s , L . , J. Chem. Soc. (1933) Trans. 95-101. 69. Challenger, F., Higginbottom, C., Biochem. J. (1935) 29 1757-1778. 70. McBride, B.C., Wolfe, R.S., Advan. Chem. Ser. (1971) 105 11-22. 71. Smith, H.R., Cameron, E.J., Ind. Eng. Chem. (1933) 5 400-401. 72. Zussman, R.A., Vicher, E . E . , Lyon, I., J. Bacteriol. (1961) 81 157. 73. Merrill, W., French, D.W., Proc. Minn. Acad. Sci. (1964) 31(2) 105-106. 74. McBride, B.C. Wolfe, R.S., Biochem. (1971) 10(23) 4312-4317. 75. Braman, R.S., Foreback, C.C., Science (1973) 182 1247-1249. 76. Cox, D.P., Alexander, M., Bull. Envir. Contam. Toxic. (1973) 9(2) 84-88. 77. Woolson, E.A., Kearney, P.C., Envir. Sci. Tech. (1973) 7(1) 47-50 78. Epps, E.A., Sturgis, M.B., Soil Sci. Soc. Amer. Proc. (1939) 4 215-218. 79. Ehman, P.J., Proc. 18th Ann. Meeting SWC (1965) 685-687. 80. Buckman, H.O., Brady, N.C., "The Nature and Properties of Soil," 6th ed., p. 230, The Macmillan Co., New York (1967). 81. Tisdale, S.L., Nelson, W.L., "Soil Fertility and Fertilizers," 2nd ed., The Macmillan Co., New York (1968).

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10 Simulation of the Mobility of Arsenic Compounds in the Environment: A Southern Texas Case Study JOHN E. STOLZENBERG Institute for Environmental Studies and Department of Chemical Engineering, University of Wisconsin, Madison, Wisc. 53706

In studying the behavio or a family of economic the behavior of the material in natural systems, that i s , in the f i e l d . Unfortunately, due to the complexities of such behavior the researcher i s often forced to perform either laboratory experiments or highly controlled f i e l d experiments i n order to hold his experimental design and observations to manageable proportions. Common practice then calls for experts to extrapolate these experimental results to predict the expected behavior of the chemical for much larger and more r e a l i s t i c natural systems. When many interacting parameters are involved, the experts often either consciously or unconsciously simplify their evaluation to be able to arrive at a conclusion. In this paper, the development i s reported of a computer simulation model designed to integrate d i verse information on the mobility of a selected chemical i n a regional environment. Since it i s impossible to derive all the mass transport equations describing such a complex, multidimensional system, the model has been formulated probabilistically rather than deterministically. A flow path is constructed for an i n d i v i s i b l e portion of the chemical. Then flow path construction is repeated many times, and the distribution i s averaged in order to predict the representative behavior of the chemical. To cope with the large data requirements, often on unknown aspects of the mat e r i a l ' s behavior, the model can incorporate either objective or subjective quantifications of the processes influencing the chemi c a l being simulated. Thus, the model can be viewed as a tool which augments experts' understanding of the behavior of specific compounds in a selected system. In the next section an overview of the model w i l l be presented. This w i l l be followed by a discussion of the case study that the model i s being applied to. This case study involves predicting the fate of disodium methanearsonate (DSMA) and i t s derivatives after i t has been applied by cotton farmers i n Southeastern Hidalgo County, Texas.

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Overview o f the Model B a s i c Model L o g i c . The model i s based on the simple observ a t i o n t h a t f o r a p o r t i o n o f the chemical to g e t t o a s p e c i f i e d p o s i t i o n and form (the s t a t e o f the chemical) w i t h i n the r e g i o n a l environment, c e r t a i n r e c o g n i z a b l e t r a n s i t i o n s i n a s p e c i f i c s e quence had t o have happened t o t h i s p o r t i o n . Furthermore, once i n a g i v e n s t a t e , only a l i m i t e d number o f things can then happen. For example, f o r a p e s t i c i d e t o be adsorbed onto s o i l p a r t i c l e s ten centimeters beneath the s o i l s u r f a c e , f i r s t the sprayed chemi c a l had to be c a r r i e d i n t o the s o i l , p o s s i b l y by l e a c h i n g . Once adsorbed, t h i s p o r t i o n o f the chemical c o u l d o n l y be i n f l u e n c e d by a l i m i t e d number o f processes such as d e s o r p t i o n o r uptake by microorganisms. These observations l e a d t o the b a s i c a l g o r i t h m or r e c i p e , f o r the model through the r e g i o n a l environmen processes t h a t may a f f e c t the chemical f o r any given s e t o f cond i t i o n s . C o n s i d e r i n g the a v a i l a b i l i t y o f i n f o r m a t i o n and the r e s o l u t i o n o f the model, "competing processes" a r e those processes which (a) cannot be f u r t h e r d i v i d e d i n t o two o r more processes, and (b) represent the s e t o f probable processes which can i n f l u ence the chemical i n a g i v e n s t a t e . In other words, by examining the competing processes f o r a g i v e n s t a t e , choosing one o f them to operate on the chemical, and then performing the process w i t h i n the model, a subsequent s t a t e can be i d e n t i f i e d . I f these steps are repeated f o r each new s t a t e u n t i l a p p r o p r i a t e t e r m i n a t i o n i s reached, a flow path f o r the chemical i s c o n s t r u c t e d . As expressed i n Table I , these operations form the b a s i s f o r the model logic. Table I . B a s i c Model L o g i c Governing T r a n s i t i o n s Between States 1.

Determine R e a l i s t i c Environmental Conditions

2.

Determine P o s s i b l e Competing Processes

3.

S e l e c t a Process

4.

Perform P r e s c r i b e d Process

The environmental c o n d i t i o n s o f i n t e r e s t are determined by the c u r r e n t s t a t e o f the chemical. F o r example, i f the flow path has followed the m a t e r i a l t o i t being attached t o dust p a r t i c l e s f l o a t i n g through the a i r , the model would not have t o r e t r i e v e i n formation on s o i l pH. Instead, i n f o r m a t i o n such as wind speed and wind d i r e c t i o n would be estimated based on h i s t o r i c a l r e c o r d s . The i n f o r m a t i o n on environmental c o n d i t i o n s i s then used i n Step 3, " S e l e c t a Process."

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The inherent problem w i t h Step 2, "Determine P o s s i b l e Comp e t i n g Processes," i s t h a t a computer can't generate these p r o cesses. The l i s t o f a l l p o s s i b l e processes has t o be s t r u c t u r e d so t h a t f o r any s t a t e , the s e t o f competing processes i s immedi a t e l y apparent, and once a process has been chosen (thus d e f i n ing a new state) the new s e t o f competing processes i s apparent. The graph t o s t r u c t u r e the processes p r o p e r l y has been designed, and an example o f i t i s presented below i n the d i s c u s s i o n o f the case study. The non-numeric computer programming needed t o cons t r u c t the process graph i s too lengthy t o d i s c u s s here, but i t i s presented elsewhere (1) . " S e l e c t a Process" r e f l e c t s the q u a n t i f i c a t i o n o f the d e c i s i o n r u l e which determines how t o p a r t i t i o n the chemical among the competing processes U n f o r t u n a t e l y the understanding o f the m o b i l i t y o f t r a c e chemical few mechanisms which c o n t r o t i v e l y . To circumvent t h i s u n c e r t a i n t y , the model has been formu l a t e d p r o b a b i l i s t i c a l l y with the four steps i n Table I a p p l y i n g to an i n d i v i s i b l e p o r t i o n o f the chemical. T h i s i m p l i e s t h a t a t any g i v e n s t a t e , o n l y one o f the competing processes w i l l a c t upon the chemical. To q u a n t i f y t h i s s e l e c t i o n , each process must be d e s c r i b e d i n terms o f a p r o b a b i l i t y f u n c t i o n where the probab i l i t y o f the process o c c u r r i n g i s a f u n c t i o n o f the p r o p e r t i e s of the chemical and environmental parameters i n f l u e n c i n g the process (as determined i n Step 1 ) . Then, the process t o f o l l o w i s chosen randomly. The l i k e l i h o o d o f p i c k i n g a process i s weighted according t o the s i z e o f the p r o b a b i l i t y o f t h a t process occurr i n g . T h i s random s e l e c t i o n i s demonstrated i n F i g u r e 1 f o r a s e t o f three competing processes. The p r o b a b i l i s t i c computat i o n a l scheme u s i n g random numbers t o choose outcomes j u s t desc r i b e d c o n s t i t u t e s a Monte C a r l o s i m u l a t i o n (2). The f o r m u l a t i o n of the p r o b a b i l i t y f u n c t i o n s d e s c r i b i n g the processes w i l l be presented l a t e r i n t h i s paper. The f i n a l step i n c o n s t r u c t i n g one t r a n s i t i o n i n a flow path i s t o perform the p r e s c r i b e d process w i t h i n the model. T h i s i s e s s e n t i a l l y a bookkeeping s t e p . The time t h a t the process takes must be accounted f o r and the new s t a t e o f the chemical must be i d e n t i f i e d . A l s o , as mentioned e a r l i e r , whether o r not the flow path should be terminated must be determined i n t h i s step. A path can be terminated f o r any one o f the f o l l o w i n g f o u r reasons: (1) the model p r e d i c t s t h a t the chemical i s t r a n s p o r t e d beyond the boundaries o f the r e g i o n a l environment being modeled; (2) the end o f the modeling p e r i o d i s reached; (3) the model p r e d i c t s t h a t the chemical i s i n a " s i n k , " such as being t i g h t l y adsorbed onto c l a y p a r t i c l e s ; and (4) no f o l l o w i n g processes are s p e c i f i e d f o r a g i v e n s t a t e , r e f l e c t i n g a l a c k o f knowledge o f what c o u l d happen next. In order t o program t h i s l o g i c f o r a d i g i t a l computer, the r e p r e s e n t a t i o n o f the r e g i o n a l system must be d i s c r e t i z e d i n t o a three dimensional s p a t i a l g r i d . The g r i d c e l l boundaries

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10. STOLZENBERG

Step 1.

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151

Evaluate p r o b a b i l i t y f u n c t i o n s .

Probability environmental parameters) = 0.25 Probability

(Deposit onto S o i l ) = f ( c h e m i c a l and environmental parameters) - 0.40

P r o b a b i l i t y (Deposit onto P l a n t Leaves) = f(chemic a l and environmental parameters) = 0.35

Step 2.

Use a random number t o p i c k a p r o c e s s .

Let the random number =0.35 Spray D r i f t

Deposit onto S o i l

Deposit onto P l a n t Leaves

0 Random Number Therefore choose "Deposit onto S o i l " f o r t h i s Flow Path

Figure 1.

Selecting a process

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correspond t o p h y s i c a l a t t r i b u t e s o f the system such as a water course bank o r the edge o f a s o i l s e r i e s . Superimposed on t h i s g r i d are f u n c t i o n a l pools and the v a r i o u s forms the chemical can be transformed t o . The p o o l s are where the t r a n s i t i o n s o r changes t o the chemical take p l a c e . An example o f a pool f o r a h e r b i c i d e i s "Inside the Target P l a n t s . " Thus, the chemical can be t r a n s p o r t e d among p o o l s , o r i t can be transformed to a d e r i v a tive within a pool. Process P r o b a b i l i t y F u n c t i o n s . As d e s c r i b e d above, the model i s a mechanistic s i m u l a t i o n depending on both how the p r o cesses are ordered and q u a n t i f i e d . In q u a n t i f y i n g the processes, a balanced y e t f l e x i b l e treatment i s c a l l e d f o r so t h a t n e i t h e r t r a n s p o r t nor transformation processes are b i a s e d . Three steps are followed i n d e r i v i n g a process p r o b a b i l i t y f u n c t i o n To be i n c l u d e d , each process through a b r i e f review F i n a l l y , the process i s q u a n t i f i e d i n a p r o b a b i l i t y f u n c t i o n . Since i t would be n e a r l y impossible t o i n c o r p o r a t e every conceivable process t h a t c o u l d a f f e c t the chemical(s) under study, only " s i g n i f i c a n t " processes are i n c l u d e d . A s i g n i f i c a n t process i s d e f i n e d as a process t h a t i s proven o r i s s t r o n g l y suspected by analogy with the behavior o f other chemicals t o be a major l i n k i n the flow path o f the chemical i n the case study s i t u a t i o n . A l i n k i s major because e i t h e r the amounts o f materi a l i n v o l v e d a r e r e l a t i v e l y l a r g e o r the r e s u l t a n t b i o l o g i c a l e f f e c t s are important. The second step i s t o review the known, p e r t i n e n t c h a r a c t e r i s t i c s o f the process. T h i s i n c l u d e s i d e n t i f y i n g the f o l l o w i n g : the major v a r i a b l e s i n f l u e n c i n g the process, i n c l u d i n g p r o p e r t i e s of both the chemical and o f the environment; important papers on the process i n the l i t e r a t u r e ; mathematical d e s c r i p t i o n s o f the process, i f any e x i s t ; those processes t h a t must proceed and those processes t h a t can f o l l o w the g i v e n process; those p r o cesses t h a t c o u l d be competing with the given process; and the c o n d i t i o n s i n the case study s i t u a t i o n under which the process would take p l a c e . Q u a n t i f i c a t i o n o f the process can be e i t h e r o b j e c t i v e o r s u b j e c t i v e . I f the i n f o r m a t i o n i d e n t i f i e d above i s s u f f i c i e n t to d e s c r i b e the behavior o f the chemical under a range o f c o n d i t i o n s f o r the r e g i o n a l environment o f i n t e r e s t , then an o b j e c t i v e p r o b a b i l i t y d i s t r i b u t i o n o f the process can be formulated. But f o r most processes i t i s a n t i c i p a t e d t h a t l i t t l e , i f any, o f the d e s i r e d f i e l d i n f o r m a t i o n e x i s t s . In t h i s case, experts' understanding o f the process f o r the g i v e n s i t u a t i o n w i l l be q u a n t i f i e d using techniques developed mainly by management s c i e n t i s t s and p s y c h o l o g i s t s (3_) . Here the concern i s not so much how humans process i n f o r m a t i o n i n reaching a d e c i s i o n as t h a t subject i v e human estimates can be r e s o r t e d t o when o b j e c t i v e p r o b a b i l i t i e s cannot be formulated. The format followed i n i n t e r v i e w i n g an expert i s o u t l i n e d i n Table I I and i s based on the review by

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

STOLZENBERG

Table I I .

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153

Steps i n an Interview Designed t o S o l i c i t an Expert's S u b j e c t i v e P r o b a b i l i t y o f a Given Process O c c u r r i n g

1.

I n i t i a l Contact - The i n t e r v i e w e r s t a t e s h i s o b j e c t i v e s and reviews the modeling technique and case study with emphasis on the r o l e o f probabilities.

2.

V a r i a b l e S e l e c t i o n - The i n t e r v i e w e r and expert agree upon the major v a r i a b l e s needed t o d e s c r i b e the process and upon the competin w i t h th , , ; ranges o f the v a r i a b l e s i n the case study s i t u a t i o n are i d e n t i f i e d . The expert ranks the v a r i a b l e s i n order of importance. I n t e r a c t i o n s among the independent v a r i a b l e s a r e i d e n t i f i e d as a r e mass dependencies.

3.

S o l i c i t P r o b a b i l i t i e s - The i n t e r v i e w e r s o l i c i t s f o r specified sets of variables, the p r o b a b i l i t i e s o f the process occurring, or their equivalent, given the presence o f the competing processes. The p r o b a b i l i t i e s are a s s o c i a t e d with the same time i n t e r v a l f o r a complete s e t o f competing processes. Experimental design techniques a r e used t o i n sure adequate coverage o f the ranges o f the v a r i a b l e s .

4.

Consistency Checks - The i n t e r v i e w e r spot-checks the exp e r t ' s p r o b a b i l i t i e s d u r i n g Step 3 by p r o v i d i n g feedback o f the expert's previous r e s u l t s .

5.

Follow-Up Questions - The expert i s asked, f o r the c o n d i t i o n s covered, how d e t a i l e d (accuracy and c o l l e c t i o n periods) data on the v a r i a b l e s should be and how app l i c a b l e h i s process p r o b a b i l i t y f u n c t i o n i s t o other chemicals and other f i e l d c o n d i t i o n s .

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1

Huber (4). Once the e x p e r t s p r o b a b i l i t y responses are obtained, they are formulated i n t o a mathematical f u n c t i o n u s i n g standard m u l t i p l e r e g r e s s i o n a n a l y s i s techniques. In a d d i t i o n to y i e l d i n g a p r o b a b i l i s t i c d e s c r i p t i o n o f the process, the i n f o r m a t i o n obt a i n e d from the experts w i l l compliment t h a t obtained i n the c h a r a c t e r i z a t i o n o f the process. To s i m p l i f y the o v e r a l l modeling e f f o r t , the key assumption i s made t h a t the p r o b a b i l i t y of a process o c c u r r i n g i s independent of the c o n c e n t r a t i o n o f the chemical present i n any g i v e n s t a t e being operated on. T h i s assumption i s based i n p a r t on the judgment t h a t the occurrence o f processes i n the case study w i l l be c o n t r o l l e d more by environmental parameters such as s o i l moist u r e and pH than by the exact amount o f the chemical present. Furthermore, the assumption seems warranted because the model w i l l be more a f f e c t e d by whethe s i m u l a t i o n than by i t s a t l e a s t f o r the case study, a v a i l a b l e h i s t o r i c a l records w i l l be depended upon f o r the values o f many o f the environmental parameters. Since these records are of v a r y i n g q u a l i t y , a h i g h l y s o p h i s t i c a t e d s i m u l a t i o n i s not j u s t i f i e d . Under t h i s assumption, the p r o b a b i l i t y o f a process o c c u r r i n g equals the f r a c t i o n o f the chemical i n the i n i t i a l s t a t e t h a t w i l l be a f f e c t e d by the p r o c e s s . T h i s assumption a l s o i m p l i e s t h a t f o r any g i v e n s t a t e what happens t o the i n d i v i s i b l e p o r t i o n o f the chemical i s independent of the p a s t s t a t e s t h a t the chemical has been i n . T h i s i s the "Markov Property," and i t means t h a t the computational scheme can a l s o be considered a Markov Chain (1). Being a Markov Chain a i d s the i n t e r p r e t a t i o n o f the model output. Now the p r o p o r t i o n o f flow paths ending i n a g i v e n s t a t e a f t e r a s p e c i f i e d time i n the Monte C a r l o s i m u l a t i o n i s e q u i v a l e n t to the f r a c t i o n o f the i n i t i a l l y a p p l i e d chemical i n t h i s s t a t e a f t e r the same time i n t e r v a l . A r s e n i c Case Study As introduced above, the general framework o f the model i s a way to c o n c e p t u a l i z e the behavior of a chemical. To proceed f u r ther w i t h q u a n t i f i c a t i o n , the model must be molded about a s p e c i f i c chemical or f a m i l y o f chemicals a c t i n g i n a s p e c i f i c s e t t i n g . The case study chosen f o r examination i n v o l v e s p r e d i c t i n g the f a t e of the a r s e n i c a l h e r b i c i d e disodium methanearsonate (DSMA) as used by c o t t o n farmers i n Southeastern Hidalgo County, Texas. More p r e c i s e l y , the s i m u l a t i o n o b j e c t i v e i s t o p r e d i c t where the DSMA a p p l i e d d u r i n g a r e p r e s e n t a t i v e c o t t o n growing season w i l l be and i n what form a f t e r t h i s c o t t o n has been harvested. T h i s r e g i o n i s about 50 k i l o m e t e r s (31 miles) up the Rio Grande R i v e r from B r o w n s v i l l e , Texas and i s approximately 500 square k i l o m e t e r s (193 square miles) i n s i z e . Other c h a r a c t e r i s t i c s o f the case study r e g i o n are summarized i n Table I I I . The processes f e l t to be s i g n i f i c a n t have been i d e n t i f i e d , and now they are being c h a r a c t e r i z e d and q u a n t i f i e d . An o r d e r i n g

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

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Environment

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Table I I I . Summary o f the Major Features of the Case Study Area, Southeastern Hidalgo County, Texas

Hydrology - R i o Grand R i v e i th souther boundary Ri Grande Floodwa are used y irrigatio ; centimeters (20 t o 30 inches) o f r a i n f a l l year (coming mainly i n high i n t e n s i t y t r o p i c a l storms). Topography - The area i s f a i r l y f l a t , having an e l e v a t i o n change o f 9 t o 12 meters (30 t o 40 feet) between the Northwest and Southeast c o r n e r s . Meteorology - Predominant winds are from the east; mean annual temperature i s about 23°C (74°F). B i o t a - Area i s predominantly a g r i c u l t u r e ; main crops are c o t t o n and sorghum, grown i n s i m i l a r p r o p o r t i o n s ; g r a p e f r u i t orchards and some truck farms are a l s o present. S o i l - Types vary over approximately 20 s e r i e s , i n c l u d i n g : "1) l e v e l , moderately and slowly permeable, loamy s o i l s o f f l o o d p l a i n s and low t e r r a c e s , 2) l e v e l , moderately and slowly permeable, loamy and c l a y e y s o i l s o f uplands, 3) g e n t l e s l o p i n g , moderately permeable, loamy s o i l s o f uplands, and 4) l e v e l , very slowly permeable, h i g h s h r i n k - s w e l l c l a y e y s o i l s o f low t e r r a c e s and upland." (5); s o i l pH ranges from 6.6 t o 8.4.

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of these processes i s under review. To understand these processes b e t t e r , i t i s u s e f u l t o f i r s t consider the pools where the behavi o r o f the chemicals i n the model i s aggregated and the d e r i v a t i v e s o f the DSMA t h a t are i n c l u d e d i n the s i m u l a t i o n . Pool I d e n t i f i c a t i o n . As d e f i n e d above, the pools are aggregates o f f u n c t i o n a l l o c a t i o n s o f a l l the p o i n t phenomena a f f e c t i n g the chemicals. The use o f pools helps i n i d e n t i f y i n g processes and data requirements. By connecting a block diagram o f the pools with arrows corresponding t o processes, the flow c h a r t o f a l l considered paths i s d e f i n e d . (Such a c h a r t would have t o be a t l e a s t two f e e t by three f e e t t o be l e g i b l e and comprehensive.) The seventeen pools d e f i n e d f o r the case study are given i n Table IV. They r e f l e c t a balance among manageability o f the computat i o n a l aspects o f the model r e a l i s t i c p o r t r a y a l o f the a c t u a l behavior o f the a r s e n i data. Table IV.

Pools Used i n Representing the Case Study Environment

Spray Equipment 1.

Spray S o l u t i o n

S o i l Medium 2.

Ground Surface

3.

Bulk S o i l Water

4.

Soil Particle

A i r Medium 10. A i r T e r r e s t r i a l Biota 11.

Target P l a n t Leaf Surface

12.

Non-Target P l a n t Leaf Surface

Surface 5.

Water Medium 6. Open Water Surface 7.

8.

13.

Inside Target P l a n t

14. 15.

Inside Non-Target Plant Harvested Crop

16.

T e r r e s t r i a l Food

Groundwater

Bulk Channel and Lake Water Channel and Lake Bottom

Web Man-Made S t r u c t u r e s 17.

Cities

Aquatic Food Web

pool.

The s t a r t i n g p o i n t f o r a l l flow paths i s the Spray S o l u t i o n T h i s represents a l l o f the DSMA a p p l i e d t o f i e l d s by

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spraying and r e l a t e d operations such as s p i l l a g e onto f i e l d s . A r s e n i c compounds from o t h e r sources such as t h a t t r a n s p o r t e d by a i r i n t o the r e g i o n o r h e r b i c i d e s sprayed on right-of-ways a r e not considered i n the present v e r s i o n o f the model. From the Spray S o l u t i o n p o o l , the DSMA goes i n t o one o f the f o u r s u r f a c e p o o l s o r i n t o the a i r . The s u r f a c e pools are q u i t e important i n the model because they represent i n t e r f a c i a l areas where c r i t i c a l p a r t i t i o n i n g i n the flow paths o f the a r s e n i c m a t e r i a l s takes p l a c e . The DSMA i s deposited onto the ground and p l a n t leaves by the sprayer. The non-target p l a n t i s mainly c o t t o n while weeds such as Johnson grass a r e the t a r g e t p l a n t s . Spray d r i f t c a r r i e s the h e r b i c i d e to e i t h e r nearby water s u r f a c e s o r i n t o the a i r long enough f o r the DSMA t o be converte s i t a t e the aggregation o a i r i n t o one p o o l . Bulk S o i l Water represents water i n the s o i l t h a t i s n o t t i g h t l y bound, t h a t i s , moisture content above the w i l t i n g p o i n t . Here, v e r t i c a l t r a n s p o r t i n the water column takes p l a c e . When i r r i g a t i o n i s s u f f i c i e n t i n the case study r e g i o n t o c r e a t e a seasonal water t a b l e and the p o s s i b i l i t y o f h o r i z o n t a l water movement, t h i s behavior i s represented i n the Groundwater p o o l . The S o i l P a r t i c l e Surface p o o l i s a combination o f the microorganisms i n the s o i l , o f the s u r f a c e s o f a l l the i n d i v i d u a l s o i l p a r t i c l e s , and o f the t i g h t l y bound i n t e r s t i t i a l s o i l water. These are lumped together because o f the d i f f i c u l t i e s i n experimentally s e p a r a t i n g them. Within t h i s p o o l , gross phenomena such as ads o r p t i o n and m i c r o b i a l o x i d a t i o n take p l a c e . The space i n the expanding l a t t i c e s o f c e r t a i n c l a y minerals can be an important p o o l f o r some m a t e r i a l . But, s i n c e the a r s e n i c i o n s considered i n the model are a n i o n i c i n nature, t h i s p o o l i s n o t i n c l u d e d . In a d d i t i o n t o t h e i r s u r f a c e s , the i n s i d e s o f both t a r g e t and non-target p l a n t s a r e t r e a t e d as p o o l s . Here the concern i s more with what a r s e n i c goes i n t o the p l a n t s and what comes o u t r a t h e r than d w e l l i n g on the biochemical r e a c t i o n s t a k i n g p l a c e w i t h i n the p l a n t s . Harvested Crop i s an important p o o l because i t represents a pathway f o r the a r s e n i c chemicals t o humans. Cotton f i b e r and c o t t o n seeds are p a r t s o f t h i s p o o l . The f i n a l T e r r e s t r i a l B i o t a p o o l i s the conglomerate, T e r r e s t r i a l Food Web (or C h a i n ) . T h i s p o o l represents the uptake o f a r s e n i c compounds by animals and i n c l u d e s t r a n s f e r o f the chemicals by predator r e l a t i o n s up a food c h a i n . I t i s i n c l u d e d even though there i s l i t t l e documentation o f the a r s e n i c h e r b i c i d e s so f u n c t i o n i n g . The C i t i e s p o o l i s another route o f exposure t o man. The DSMA o r i t s d e r i v a t i v e c o u l d c o n c e i v a b l y reach a c i t y v i a a i r t r a n s p o r t processes. The case study r e g i o n has a s t r i n g o f s m a l l c i t i e s i n i t , which due t o t h e i r l o c a t i o n r e l a t i v e t o the p r e v a i l i n g winds, c o u l d be contaminated by the a r s e n i c compounds. I f a flow path does indeed l e a d t o a c i t y , the model w i l l not p r e d i c t what happens t o the chemical once i t gets t o the c i t y .

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The pools r e p r e s e n t i n g the Water Medium i n a d d i t i o n t o the water surface are s t r a i g h t f o r w a r d . Bulk Channel and Lake Water i n c l u d e , besides r i v e r s and lakes, i r r i g a t i o n d i t c h e s and f l o o d ways. The bottom o r sediments o f these waterways c o n s t i t u t e another p o o l . F i n a l l y , the b i o l o g i c a l uptake i s lumped i n t o an Aquatic Food Web p o o l . Forms o f A r s e n i c . To completely understand the behavior o f DSMA, i t i s necessary f o r the model t o incorporate transformat i o n s o f DSMA t o other forms o f a r s e n i c as w e l l as the t r a n s p o r t of DSMA w i t h i n the case study r e g i o n . These transformations a r e assumed t o occur e n t i r e l y w i t h i n p o o l s . Transformations may a l s o occur among the d e r i v a t i v e s o f DSMA, implying t h a t the model i s capable o f s i m u l a t i n g c y c l e s among a r s e n i c compounds. The model i s programmed t o consider the f o l l o w i n g forms i n a d d i t i o n t o DSMA: monosodium methanearsonat a r s i n i c a c i d (cacodyli i n o r g a n i c arsenates. The s a l t s are used because o f the a l k a l i n e nature o f the case study area s o i l . Inorganic a r s e n i t e s are n o t included. MSMA i s i n c l u d e d i n the model due t o a combination o f two reasons. F i r s t , measured by acute t o x i c i t y MSMA i s l e s s t o x i c than DSMA (6), implying d i f f e r e n c e s i n behavior between the two. Secondly, MSMA i s more than l i k e l y being formed i n the case r e gion's s o i l . Since the s o i l pH ranges from 6.6 t o 8.4 while the d i s s o c i a t i o n constant, pK / between MSMA and DSMA has been r e ported t o be 7.82 (7), 8.24 (8), and 8.7 (9), appreciable proport i o n s o f DSMA could be converted t o MSMA, depending mainly on the s p e c i f i c s o i l pH. Evidence continues t o mount t h a t under proper c o n d i t i o n s , v a r i o u s forms o f a r s e n i c can be b i o l o g i c a l l y reduced t o e i t h e r a r s i n e , methylarsine o r dimethylarsine (10-12). Due t o the analyt i c a l d i f f i c u l t i e s i n d i s t i n g u i s h i n g among these forms o f a r s i n e s , a l l three w i l l be considered together i n the model. Based on the author's l i m i t e d review, there are no r e p o r t s i n the l i t e r a t u r e o f DSMA o r MSMA being methylated t o c a c o d y l i c a c i d i n f i e l d c o n d i t i o n s . However, Wood proposed t h a t t h i s r e a c t i o n does occur as p a r t o f the b i o l o g i c a l c y c l e f o r a r s e n i c (13). A recent study a n a l y z i n g environmental samples f o r methylated forms o f a r s e n i c concluded, " D i m e t h y l a r s i n i c a c i d i s a major and ubiquitous form o f a r s e n i c i n the environment," (14). In e a r l y l a b s t u d i e s , Dehn and Wilcox showed t h a t dimethyl a r s i n e s are slowly o x i d i z e d t o c a c o d y l i c a c i d (15). F i n a l l y , f u t u r e v e r sions o f the model may a l s o use c a c o d y l i c a c i d as a s t a r t i n g mat e r i a l . Therefore, f o r these reasons, i t i s f e l t t h a t c a c o d y l i c a c i d i s p o t e n t i a l l y an important form o f a r s e n i c i n the case study and should be i n c l u d e d i n the modeling e f f o r t . Under aerobic c o n d i t i o n s , DSMA (10), MSMA (16) and C a c o d y l i c A c i d (11) have been shown t o u l t i m a t e l y o x i d i z e and demethylate to i n o r g a n i c arsenates. Thus, i t i s necessary t o include the arsenates as a s i g n i f i c a n t a r s e n i c form i n the model. 2

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One form o f a r s e n i c not i n c l u d e d i s i n o r g a n i c a r s e n i t e . A t the l e v e l o f d e t a i l o f the proposed model, a r s e n i t e appears t o be an i n s i g n i f i c a n t form o f a r s e n i c . Quastel and S c h o l e f i e l d found t h a t a r s e n i t e i s r e a d i l y o x i d i z e d b i o l o g i c a l l y t o arsenate (17). Furthermore, Von Endt, e t . a l . , detected o n l y MSMA and arsenate and no a r s e n i t e i n t h e i r work on the degradation o f MSMA (16). Proposed S i g n i f i c a n t Processes. The processes d i s c u s s e d i n t h i s s e c t i o n are matched t o the case study c o n d i t i o n s . I n other s e t t i n g s , a d d i t i o n a l processes may be c a l l e d f o r i n d e s c r i b i n g the behavior o f a r s e n i c compounds. Some o f the processes are i n cluded f o r the sake o f completeness and w i l l probably have t o be compromised and aggregated with other processes due t o data l i m i t a t i o n s . Yet i t i s u s e f u l t o s t a r t with a f a i r l y complete l i s t o f processes because i t f o r c e s one t o examine what he r e a l l y knows about the t o t a l behavio c i t a t i o n s are not exhaustive be s u f f i c i e n t t o i d e n t i f y a process as s i g n i f i c a n t . When poss i b l e , the c i t a t i o n r e f e r s t o a process a f f e c t i n g one o f the f i v e forms o f a r s e n i c i n c l u d e d i n the s i m u l a t i o n . Transport processes imply t r a n s i t i o n s between two pools o r c y c l i n g w i t h i n one p o o l f o r one form o f a r s e n i c . For example, Spray D r i f t i s between the Spray S o l u t i o n p o o l and the A i r p o o l , while Leaching can occur e n t i r e l y w i t h i n a p o o l . Transformation processes r e f e r both t o changes i n p h y s i c a l s t a t e s and t o a l t e r a t i o n t o another a r s e n i c form. P h y s i c a l Adsorption i l l u s t r a t e s the former; B i o l o g i c a l Reduction the l a t t e r . Table V, which contains a summary o f the proposed s i g n i f i c a n t processes, serves as an o u t l i n e t o the i n t r o d u c t i o n t o the model processes which f o l l o w s . The Introductory Transport processes are the routes by which the DSMA enters the r e g i o n a l environment from the spray equipment. Recommended techniques and r a t e s o f a p p l y i n g DSMA are a v a i l a b l e from manufacturers (18). Depending upon the weather c o n d i t i o n s a t the time o f spraying, d r i f t o f the l i q u i d spray drops can occur (19^, 20) . Changes i n the p h y s i c a l s t a t e s o f a given a r s e n i c s p e c i e s occur by p h y s i c a l transformations. P h y s i c a l Adsorption and Des o r p t i o n i n v o l v i n g Van der Waals f o r c e s are one such process. The f a c t o r s i n f l u e n c i n g a d s o r p t i o n have been reviewed by B a i l e y and White (21_) . Various s t u d i e s , where the a d s o r p t i o n i s c o r r e l ated t o the c l a y contents and thus the s o i l s u r f a c e area, i n d i c a t e that t h i s process does i n f l u e n c e a r s e n i c compounds (10, 22). Des o r p t i o n can o f t e n be i n f e r r e d from l e a c h i n g s t u d i e s . Apparently only the a r s i n e s undergo V a p o r i z a t i o n (23). What happens i n f i e l d s o i l s once the a r s i n e s are produced has not been reported even though V a p o r i z a t i o n i s p o t e n t i a l l y a c r i t i c a l l i n k i n c y c l i n g a r s e n i c out o f a f i x e d l o c a t i o n i n the s o i l . Work on measuri n g the V a p o r i z a t i o n o f other p e s t i c i d e s should be o f help i n the modeling e f f o r t (24-27). The P r e c i p i t a t i o n o f i n s o l u b l e a r s e n i c compounds a f t e r Ion Exchange i s the mechanism f o r another type o f s o r p t i o n phenomena, chemisorption. P i e t s c h has reported a t what

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Table V.

Proposed S i g n i f i c a n t Processes

Introductory Transport Processes

P l a n t Transform Process

Deposit onto S o i l and P l a n t s Spray D r i f t i n t o A i r o r onto Water

Metabolism by P l a n t s Water-Soil Transport Processes

P h y s i c a l Transform Processes Physical Adsorption P h y s i c a l Desorption Vaporization Precipitation Chemical Transform Processes Ion Exchange Dissociation Oxidation

(Complexation)

Water E r o s i o n Water Transport Processes Bulk Flow Transport Mixing by Turbulence Sedimentation Scour Pick-Up A i r Transport Processes

M i c r o b i a l Transform Processes Reduction (and Methylation) O x i d a t i o n (and Demethylation) P l a n t Transport Processes Absorption i n t o Root Uptake P l a n t Decay

P a r t i c l e Transport by Wind and Turbulence Sedimentation P r e c i p i t a t i o n Scavenging Impaction Wind E r o s i o n

Leaves

Food Web Transport Processes Aquatic Organism Uptake T e r r e s t r i a l Organism Uptake Human I n t e r v e n t i o n Transport Processes Harvest Crop Plow i n P l a n t s Plow i n and up S o i l Pump Water from F i e l d s

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pH's v a r i o u s metal ions w i l l form a p r e c i p i t a t e with methylarsonic a c i d and d i m e t h y l a r s i n i c a c i d (28), and Chukhlantsev has reported the s o l u b i l i t y products f o r a s e r i e s o f arsenates (29_, 30) . In chemical t r a n s f o r m a t i o n s , the s t r u c t u r e o f the compound i s a l t e r e d . The arsenates (31, 32), MSMA (33), DSMA (10), and c a c o d y l i c a c i d (11) a l l have been r e p o r t e d t o exchange o r complex w i t h i r o n , aluminum o r calcium ions and o x i d e s . With the b a s i c pH's and h i g h calcium l e v e l s i n the case study s o i l s , chemisorpt i o n w i t h calcium complexes becomes one o f the more important processes i n the s i m u l a t i o n . The D i s s o c i a t i o n o f methanearsonic a c i d between i t s monosodium and disodium s a l t s was introduced i n the d i s c u s s i o n o f the a r s e n i c forms i n c l u d e d i n the model. A t t h i s time, the r e s o l u t i o n o f the model does not warrant d i s t i n g u i s h i n g between the mono and disodium s a l t s o f arsenate The e q u i l i b r i u m constant o t h a t e s s e n t i a l l y most o Chemical O x i d a t i o n i s apparently a r e l a t i v e l y r a p i d decomposition mechanism o f the a r s i n e s (34). On the other hand, c a c o d y l i c a c i d has been r e p o r t e d t o be very r e s i s t a n t t o Chemical O x i d a t i o n (14). Wood r e c e n t l y p o i n t e d out the importance o f m i c r o b i a l conv e r s i o n s o f metals i n the environment (13) . He a l s o proposed a b i o l o g i c a l c y c l e among arsenate, a r s i n i t e , methylarsonic a c i d , d i m e t h y l a r s i n i c a c i d and the a r s i n e s based i n p a r t on the work o f McBride and Wolfe (12). Various other workers have demonstrated the importance o f M i c r o b i a l O x i d a t i o n o r Reduction with a p p r o p r i ate Demethylation o r M e t h y l a t i o n f o r DSMA (1£, 22), MSMA (16), and c a c o d y l i c a c i d (11). The process o f the microbes t a k i n g up a chemical i s lumped i n the model with the a p p r o p r i a t e transformation that follows. The P l a n t T r a n s p o r t processes d e s c r i b e mechanisms f o r moving the a r s e n i c compounds e i t h e r i n t o o r out o f the two p o o l s , Inside Target P l a n t s and I n s i d e Non-Target P l a n t s . Even though the e f f e c t s o f the Absorption o f sprayed a r s e n i c h e r b i c i d e s i n t o p l a n t leaves has been s t u d i e d H , 35-37), few r e p o r t s were found on the a c t u a l A b s o r p t i o n p r o c e s s . Since DSMA i s sprayed w i t h a s u r f a c t a n t added t o the spray s o l u t i o n , Foy and Smith's review on the r o l e o f s u r f a c t a n t s w i l l be h e l p f u l (38) as w i l l Ashton and C r a f t s i n t r o d u c t i o n t o the Absorption o f a r s e n i c a l s (39). Root Uptake o f r e s i d u a l a r s e n i c a l s i s important because o f the i m p l i c a t i o n s f o r understanding the u l t i m a t e l o a d i n g , i f any e x i s t , o f t r e a t e d areas (31, 3_3# 40^ 41) . P l a n t Decay i s the process whereby a r s e n i c w i t h i n a p l a n t i s c a r r i e d t o the s o i l by the dying and subsequent decaying o f the p l a n t . T r a n s l o c a t i o n w i t h i n p l a n t s i s lumped together w i t h Metabolism by P l a n t s i n the model. A t l e a s t i n some s p e c i e s DSMA i s not r e a d i l y broken down (42), while i t i s s t r o n g l y suspected t h a t organic a r s e n i c a l s are reduced t o a r s i n e s w i t h i n some p l a n t s (34). I f p l a n t s do indeed produce a r s i n e s , t h i s would provide an a d d i t i o n a l route f o r m o b i l i z i n g a r s e n i c a l s out o f s o i l and p l a n t pools.

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The Water-Soil Transport processes occur when s u f f i c i e n t water i n t e r a c t s w i t h p l a n t s u r f a c e s or the s o i l i n c a r r y i n g an a r s e n i c compound to or through a s o i l medium p o o l . Even though the a r s e n i c a l s are u s u a l l y s t r o n g l y bound by chemisorption, the short d i s t a n c e s Leaching c a r r i e s the m a t e r i a l can be important i n i n t r o d u c i n g i t to new r e a c t i v e c o n d i t i o n s (1£, 2£, 33.) . In s p i t e of the case study r e g i o n being f a i r l y f l a t , the h i g h i n t e n s i t y r a i n storms i n the area can remove s u r f a c e s o i l by Water E r o s i o n . Therefore, any a r s e n i c a l sorbed to t h i s s o i l w i l l a l s o be t r a n s ported. Work on water e r o s i o n and the removal o f other m a t e r i a l s w i l l be u s e f u l i n c h a r a c t e r i z i n g Water E r o s i o n f o r t h i s model (43, 44). I f an a r s e n i c compound reaches a watercourse or l a k e , the Water Transport processe with the water and t o an imation, the a r s e n i c a l i s assumed to be bound t o a p a r t i c l e i n the water so a standard treatment o f i n l a n d hydrology should be s u f f i c i e n t to c h a r a c t e r i z e these processes (45). Mixing by Turbulence r e f e r s t o mixing w i t h i n the water column while Sedimentat i o n i s a s e t t i n g process c a r r y i n g p a r t i c l e s by g r a v i t y t o the waterway bottom. Scour Pick-up i s the removal o f bottom p a r t i c l e s by the bed load of sediments moving along the channel bottom. In a d d i t i o n to Spray D r i f t , a r s e n i c a l s can enter the a i r p o o l by Wind E r o s i o n or p o s s i b l y by V a p o r i z a t i o n o f a r s i n e s . I f the l a t t e r process does take p l a c e as the evidence on b i o l o g i c a l r e d u c t i o n o f a r s e n i c compounds i n d i c a t e s , then A i r Transport p r o cesses w i l l be e s p e c i a l l y important i n r e d i s t r i b u t i n g a r s e n i c i n the environment. No work on the a i r t r a n s p o r t o f a r s e n i c a l s has been found i n the l i t e r a t u r e , but a review of the concepts with r e s p e c t t o p e s t i c i d e s i n general i s a v a i l a b l e (46_) . Work by the United S t a t e s Atomic Energy Commission a l s o provides a f i r m i n t r o d u c t i o n to these processes (47, 48). As with water, Sedimenta t i o n i s the f a l l out o f p a r t i c l e s due to g r a v i t y . Precipitation Scavenging d e s c r i b e s washing p a r t i c l e s from the a i r both i n and below clouds by r a i n (49), and i t c o n s t i t u t e s an important way o f r e t u r n i n g m a t e r i a l from the lower atmosphere. Impaction i s a d e p o s i t i o n mechanism (50). I t i s o f concern i n the model as a contaminating mechanism o f c i t i e s . Apparently a small amount o f Wind E r o s i o n does occur i n Southeast Hidalgo County as evidenced by the appearance o f small dust clouds even a f t e r crops have s t a r t e d t o emerge from the s o i l (9). Wind E r o s i o n can be chara c t e r i z e d u s i n g f a c t o r s i n a " U n i v e r s a l Equation f o r Measuring Wind E r o s i o n " (51). The Food Web Transport processes d e s c r i b e both the uptake from the environment by organisms and the consumption of a prey c o n t a i n i n g an a r s e n i c a l i n i t s t i s s u e s by a predator. Here the concern i s more with the organisms p h y s i c a l l y t r a n s p o r t i n g the chemical t o another l o c a t i o n , l e a v i n g the i n t e r p r e t a t i o n o f e f fects to b i o l o g i s t s . In a model aquatic ecosystem, c a c o d y l i c a c i d was found t o not magnify t o any g r e a t degree through food

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c h a i n feeding r e l a t i o n s (52). In another s e t t i n g , c e r t a i n species o f earthworms were found to p l a y an important r o l e i n the v e r t i c a l r e d i s t r i b u t i o n of cadium from the l i t t e r i n the s o i l p r o f i l e (53) . The i n f l u e n c e of s o i l i n s e c t s and earthworms i n the case study i s unknown, but i f present i t would probably be important. The f i n a l s e t of processes considered i n the model d e s c r i b e t r a n s p o r t mechanisms immediately r e s u l t i n g from the i n t e r v e n t i o n o f man. I f residues of the a r s e n i c a l s are present i n the c o t t o n f i b e r or seed when the c o t t o n i s harvested, then t h i s suggests an important pathway o f the m a t e r i a l to man. By law, the cotton farmers have to plow i n the cotton p l a n t remaining i n the f i e l d a f t e r h a r v e s t i n g by September 1st each year. Thus, any a r s e n i c i n the p l a n t o r on or nea to 23 to 38 centimeter exposes the a r s e n i c plowed i n (as w e l l as t h a t plowed up) t o a new s e t of r e a c t i v e c o n d i t i o n s . Such a c t i o n more than l i k e l y g r e a t l y a f f e c t s the long term behavior of a r s e n i c r e s i d u a l s i n the s o i l . During intense t r o p i c a l storms, p a r t s o f the case study r e g i o n are so f l a t t h a t the r a i n water doesn't r e a d i l y d r a i n . To p r o t e c t t h e i r crops from the e f f e c t s of ponding, the l o c a l farmers o f t e n pump t h e i r f i e l d s dry, p l a c i n g t h i s water i n i r r i g a t i o n d i t c h e s . I f a r s e n i c a l s are sorbed to s o i l p a r t i c l e s near the s o i l s u r f a c e of such flooded areas, i t i s conceivable t h a t pumping could t r a n s p o r t the chemicals to an aquatic system. Process Graph Example. To demonstrate how the processes, pools and forms d i s c u s s e d above w i l l be s t r u c t u r e d i n the case study s i m u l a t i o n , the process graph showing a l l the combinations of p o s s i b l e flow paths f o r DSMA i s presented i n t h i s s e c t i o n . T h i s graph i n d i c a t e s a transformation from DSMA to one o f the four other forms o f a r s e n i c considered i n the model, but i t doesn't f o l l o w the flow path f o r the new form. When a l l forms of a r s e n i c are i n c l u d e d i n the graph, i t w i l l be considerably longer. A l s o , the process graph f o r DSMA as given i n Table VI w i l l more than l i k e l y be modified as experts are consulted. The number f o r each entry i s a unique i d e n t i f i e r used i n the computer programming o f the graph. Next i s a v e r b a l d e s c r i p t i o n of the process. Following the s l a s h i s the form of the i n d i v i s i b l e port i o n of a r s e n i c present a f t e r the process has occurred. DSMA, MSMA, and CA are the standard a b b r e v i a t i o n s while ASO4 stands f o r arsenate. Since t h i s o r d e r i n g of processes f o r c e s one to be q u i t e s p e c i f i c about the behavior of DSMA i n c l u d i n g unknown aspects, many assumptions are necessary. For example, i t i s assumed t h a t microbes only take up DSMA from s o l u t i o n and t h a t plowing only a f f e c t s bound m a t e r i a l s . Adsorption mechanisms are lumped t o gether i n t o one process. What happens when t e r r e s t r i a l or aquatic organisms take up DSMA i s not s p e c i f i e d , r e f l e c t i n g a l a c k of knowledge about these processes.

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Figure 2 i s presented to h e l p the reader i n t e r p r e t Table V I . The process graph o n l y g i v e s processes (arrows i n F i g u r e 2) as the s t a t e s between processes (boxes i n F i g u r e 2) are i m p l i e d . The process graph can be viewed as an o u t l i n e . Those processes with the same u n i n t e r r u p t e d i n d e n t a t i o n and i n a given sequence of process numbers are competing process. (Processes numbered 06 through 09 i n F i g u r e 2 are competing processes.) As one moves down and i n one i n d e n t a t i o n from any g i v e n process, the subsequent s e t o f competing processes i s g i v e n . (Assuming 01 Deposit onto S o i l i s the f i r s t process chosen, then the s e t o f competing p r o cesses f o l l o w i n g process 01 are processes 06 to 09.) The process d e s c r i b e d as " S i t " i n the graph i s added f o r bookkeeping purposes. When S i t i s chosen as a process to f o l l o w , t h i s i m p l i e s t h a t the chemical remains i n i t s present s t a t e f o r a p r e s c r i b e d amount o f time. Processes l i s t e path loops to another p a r competing processes t h a t can occur a f t e r process 09 are processes 06 to 09). In t h i s way,, p o s s i b l e r e p e a t i n g of processes f o r more than one time step, feedback loops and c y c l i n g can e a s i l y be handled by the model. I f no s e t of competing processes i s l i s t e d beneath a given process, then t h i s process i s the t e r m i n a l p r o cess f o r t h a t p a r t i c u l a r flow path. To i l l u s t r a t e how the model would use t h i s process o r d e r i n g a conceivable flow path f o r an i n d i v i s i b l e p o r t i o n o f DSMA, a s suming a p p r o p r i a t e environmental c o n d i t i o n s , i s : (Spray Solution) 01 Deposit onto S o i l (Ground Surface) 07 A d s o r p t i o n (Ground Surface) 10 Water E r o s i o n (Open Water Surface) 40 Mix i n t o Bulk Chann e l (Bulk Channel Water) 47 Bulk Flow Transport (Bulk Channel Water) 46 Sedimentation (Channel Bottom) 52 S i t on Channel Bottom f o r one time step (Channel Bottom) 50 M i c r o b i a l O x i d a t i o n (Chann e l Bottom). The processes are l i s t e d i n the p r e d i c t e d order of occurrence. The p o o l t h a t the chemical would be i n a f t e r a p r o cess has occurred f o l l o w s i t i n p a r e n t h e s i s . E i g h t random s e l e c t i o n s o f processes among s e t s o f competing processes using the Monte C a r l o a l g o r i t h m would have been used to c r e a t e t h i s flow path. I t terminates because no processes are s p e c i f i e d a f t e r 50 Microbial Oxidation. By s t r u c t u r i n g the processes a f f e c t i n g the a r s e n i c compounds i n the case study environment i n such a f a s h i o n , one e x p l i c i t l y d e f i n e s a l l o f the p o s s i b l e flow paths which the chemicals c o u l d f o l l o w . The q u a n t i t a t i v e s i m u l a t i o n which f o l l o w s the c o n s t r u c t i o n of the process graph takes these p o s s i b l e flow paths and g i v e s the probable flow paths f o r any g i v e n s e t o f environmental c o n d i t i o n s . Furthermore, the process graph f o r c e s one to view each process i n proper p e r s p e c t i v e , n e s t l e d between the process t h a t must come b e f o r e i t and those t h a t can f o l l o w i t .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

H-»

S. Figure 2.

Elaboration of the beginning of the process graph given in Table VI

s c ο

S-

B*

Ci*

ο

"-Κ

g

s

ο

ζ

g

ρ

ce

w ο

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166

ARSENICAL PESTICIDES

Table V I .

Example Process Graph f o r DSMA

01 Deposit onto Soil/DSMA 06 Dissociation/MSMA 07 Adsorption/DSMA 10 Water Erosion/DSMA (40 Mix i n t o Bulk Channel/DSMA) 11 Wind Erosion/DSMA (26 Sedimentation onto Top o f Soil/DSMA) 12 Plow i n Soil/DSMA (21 T e r r e s t r i a l Organism Uptake/DSMA) 13 T e r r e s t r i a l Organism Uptake/DSMA 14 Pump Water from Fields/DSMA (40 Mix 15 Sit/DSMA (10 Water Erosion/DSMA) 08 Leach In/DSMA 16 Dissociation/MSMA 17 Leach In/DSMA (16 Dissociation/MSMA) 18 Nontarget P l a n t Root Uptake/DSMA (62 Nontarget P l a n t Metabolism/?) 19 Target P l a n t Root Uptake/DSMA (56 Target P l a n t Metabolism/?) 20 Adsorption/DSMA 21 T e r r e s t r i a l Organism Uptake/DSMA 22 Plow Up Soil/DSMA (10 Water Erosion/DSMA) 23 Sit/DSMA (23 T e r r e s t r i a l Organism Uptake/DSMA) 24 M i c r o b i a l Reduction/Arsine 25 M i c r o b i a l Oxidation/AS0 09 Sit/DSMA (06 Dissociation/MSMA) 02 Spray D r i f t i n A i r and P a r t i c l e Formation/DSMA 26 Sedimentation onto Top o f Soil/DSMA (10 Water Erosion/DSMA) 27 Sedimentation onto Target Plant/DSMA (53 Absorption i n t o Leaves/DSMA) 28 Sedimentation onto Nontarget Plant/DSMA (59 A b s o r p t i o n i n t o Leaves/DSMA) 29 Sedimentation onto Open Waters/DSMA (40 Mix i n t o Bulk Channel/DSMA) 30 Sedimentation onto Cities/DSMA 31 P r e c i p i t a t i o n Scavenging onto Soil/DSMA (10 Water Erosion/DSMA) 32 P r e c i p i t a t i o n Scavenging onto Open Water/DSMA (40 Mix i n t o Bulk Channel/DSMA) 33 P r e c i p i t a t i o n Scavenging onto Cities/DSMA 4

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10.

STOLZENBERG

Mobility of Arsenic in the

Environment

34 Impaction onto Cities/DSMA 35 Wind Transport/DSMA (26 Sedimentation onto Top of Soil/DSMA) 03 Spray D r i f t onto Nearby Open Water/DSMA 36 Dissociation/MSMA 37 Adsorption/DSMA 40 Mix i n t o Bulk Channel/DSMA (46 Sedimentation/DSMA) 41 Flow Transport a t Surface/DSMA (40 Mix i n t o Bulk Channel/DSMA) 38 Mix i n t o Bulk Channel/DSMA 42 Dissociation/MSMA 43 Bulk Flow Transport/DSMA (42 Dissociation/MSMA 44 Adsorption/DSM 46 Sedimentation/DSMA 49 M i c r o b i a l Reduction/Arsines 50 M i c r o b i a l Oxidation/ASO^ 51 Scour Pick-Up/DSMA (46 Sedimentation/DSMA) 52 Sit/DSMA (49 M i c r o b i a l Reduction/Arsines) 47 Bulk Flow Transport/DSMA (46 Sedimentation/DSMA) 48 A q u a t i c Organism Uptake/DSMA 45 Aquatic Organism Uptake/DSMA 04 Deposit onto Target Plant/DSMA 53 A b s o r p t i o n i n t o Leaves/DSMA 56 Target P l a n t Metabolism/? 57 Target P l a n t Decay/DSMA (10 Water Erosion/DSMA) 58 Plow i n Target Plant/DSMA (21 T e r r e s t r i a l Organism Uptake/DSMA) 54 Leach O f f Target Plant/DSMA (06 Dissociation/MSMA) 55 Sit/DSMA (53 A b s o r p t i o n i n t o Leaves/DSMA) 05 Deposit onto Nontarget Plant/DSMA 59 A b s o r p t i o n i n t o Leaves/DSMA 62 Nontarget P l a n t Metabolism/? 63 Nontarget P l a n t Decay/DSMA (10 Water Erosion/DSMA) 64 Plow i n Nontarget Plant/DSMA (21 T e r r e s t r i a l Organism Uptake/DSMA) 65 Harvest Crop/DSMA 60 Leach O f f Nontarget Plant/DSMA (06 Dissociation/MSMA) 61 Sit/DSMA (59 A b s o r p t i o n i n t o Leaves/DSMA)

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ARSENICAL PESTICIDES

Interim Assessment o f the Model The modeling e f f o r t t o date already has proven t o be a s y s tematic way o f determining the i n f o r m a t i o n r e q u i r e d t o adequately q u a n t i f y the r e g i o n a l behavior o f one f a m i l y of chemicals. Obv i o u s l y , i n f o r m a t i o n i s l a c k i n g to adequately d e s c r i b e many o f the processes i d e n t i f i e d above. At t h i s time, the f o l l o w i n g three areas e s p e c i a l l y need to be i n v e s t i g a t e d t o complete even a b a s i c understanding o f the t o t a l behavior of the a r s e n i c a l s i n the s e l e c t e d system: 1.

The c o n d i t i o n s o f a r s i n e p r o d u c t i o n i n p l a n t s and microorganisms and, once formed, the behavior o f the a r s i n e s must be f u r t h e r c l a r i f i e d

2.

The behavior o known though p o t e n t i a l l y q u i t e important.

3.

The r o l e o f human i n t e r v e n t i o n such as plowing on " n a t u r a l " processes i s i n need o f study.

Once the model i s b u i l t and t e s t e d , a s e n s i t i v i t y a n a l y s i s o f s e l e c t e d processes and environmental parameters should provide f u r t h e r i n s i g h t i n t o the behavior o f the a r s e n i c a l s . Other uses of the model i n c l u d e i n d i c a t i n g sampling s i t e s f o r monitoring programs and w i t h proper i n t e r p r e t a t i o n h e l p i n g to p r e d i c t the l i k e l y b i o l o g i c a l e f f e c t s r e s u l t i n g from a p p l y i n g the h e r b i c i d e s i n a s p e c i f i c manner. Most important though, the model i s f e l t to be a p l a u s i b l e approach to t r e a t i n g the complex problem o f d e s c r i b i n g the m o b i l i t y o f s e l e c t e d t r a c e chemicals. The p r o b a b i l i s t i c treatment allows f o r m u l a t i o n of the s i m u l a t i o n , and the use o f s u b j e c t i v e p r o b a b i l i t i e s circumvents many o f the data l i m i t a t i o n s . In t h i s way, the model h o p e f u l l y w i l l become a t o o l to compliment the work of s p e c i a l i s t s i n a n a l y z i n g and p r e d i c t i n g the m o b i l i t y of a r s e n i c compounds i n the environment. Acknowledgmen t The author i s g r a t e f u l f o r the t h o u g h t f u l a s s i s t a n c e o f T. W. Chapman, G. P. Huber, T. G. Kurtz, D. R. Keeney, E. F. Joeres and 0 . L. Loucks o f the U n i v e r s i t y o f Wisconsin - Madison, and I*. 0 . Moore, R. W. Olson, G. Sandberg, and P. J . Ehman o f the Ansul Company, Marinette, Wisconsin. T h i s research was supported by funds from the N a t i o n a l Science Foundation's Research A p p l i e d to N a t i o n a l Needs program as administered by the I n s t i t u t e f o r Environmental Studies a t the U n i v e r s i t y o f Wisconsin. The a s s i s t a n c e o f the Center f o r Resource P o l i c y Studies and Programs and N. E r i c k s o n ' s h e l p i n p r e p a r i n g the manuscript i s a l s o a p p r e c i a t e d .

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10. STOLZENBERG

Mobility of Arsenic in the Environment

169

Literature Cited 1.

Stolzenberg, J.E., "Computer Aided Synthesis of the Environmental Pathways for Disodium Methanearsonate," Preliminary Research Proposal Submitted in Partial Fulfillment of the Degree of Doctor of Philosophy (Environmental Studies) at the University of Wisconsin-Madison, February, 1974. (Available from the author) 2. Naylor, T . H . , et. a l . , "Computer Simulation Techniques," pp. 1-4, 333-335, John Wiley and Sons, New York, 1966. 3. Slovic, P. and S. Lichtenstein, Organizational Behavior and Human Performance (1971) 6, 649-744. 4. Huber, G.P., Decision Sciences (1974) 5, 430-458. 5. Soil Conservation Service, U. S. Department of Agriculture, General Soil Map #4-R-32792 6. Christensen, H.E. (ed.), "Toxic Substances Annual List 1971," 512 pp., National Institute for Occupational Safety and Health, U. S. Department of Health, Education, and Welfare, Rockville, Maryland, 1971. (Available from the Superintendent of Documents, #1716-0004) 7. Dub, M. (ed.), "Organometallic Compounds - Methods of Synthesis, Physical Constants and Chemical Reactions; Volume III, Compounds of Arsenic, Antimony and Bismuth," 2nd Edition, Springer-Verlog, New York, 1968. 8. Doak, G.O. and C. D. Freedman, "Organometallic Compounds of Arsenic, Antimony, and Bismuth," pp. 27, 31, Wiley-Interscience, New York, 1970. 9. Moore, L.O., Personal Communication, June 6, 1974. 10. Akins, M.B., M.S. Thesis, 107 pp., University of Tennessee, Knoxville, 1973. 11. Woolson, E.A. and P.C. Kearney, Environmental Science and Technology (1973) 7, 47-50. 12. McBride, B.C. and R.S. Wolfe, Biochemistry (1971) 10, 43124317. 13. Wood, J.M., Science (1974) 183, 1040-1052. 14. Braman, R.S. and C.C. Foreback, Science (1973) 182, 12471249. 15. Dehn, W.M. and B.B. Wilcox, American Chemical Journal (1906) 35, 1-41. 16. Von Endt, D.W., P.C. Kearney, and D.D. Kaufman, Agricultural and Food Chemistry (1968) 16, 17-20. 17. Quastel, J.H. and P.G. Scholefield, Soil Science (1953) 75, 279-285. 18. The Ansul Company, Specimen Label for Ansar Brand DSMA Liquid, 2 pp., Marinette, Wisconsin. 19. Seymour, K.G., Advances in Chemistry Series (1969) 86, 135154. 20. Maybank, J. and K. Yoshida, in J.T. Bergsteinsson and W. Baier (eds.), "Meteorological Aspects of Pollution in Relation to Agricultural Pesticides," pp. 13-24, Canada Committee

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on Agricultural Meteorology, Canada Department of Agriculture; Ottawa, 1971. 21. Bailey, G.W. and J.L. White, Residue Reviews (1970) 32, 3080. 22. Dickens, R. and A.E. Hiltbold, Weeds (1967) 15, 299-304. 23. Weed Science Society of America, "Herbicides Handbook," 2nd ed., pp. 23, 252, 261, W. F. Humphrey Press, Geneva, New York, 1970. 24. Parmele, L.H., E.R. Lemon and A.W. Taylor, Water, Air and Soil Pollution (1972) 1, 435-451. 25. Hartley, G.S., Advances in Chemistry Series (1969) 86, 115134. 26. Hamaker, J.W. and H.O. Kerlinger, Advances in Chemistry Series (1969) 86, 39-54. 27. Spencer, W.F. in Pesticid the Soil: Ecology, Degradation and Movement," pp. 120-128, Michigan State University, East Lansing, 1970. 28. Pietsch, R., Midrochim Acta (1958) 2, 220-224. 29. Chukhlantsev, V.G., Chemical Abstracts (1957) 51, 7107a. 30. Chukhlantsev, V.G., Journal of Inorganic Chemistry, U.S.S.R. (1956) 1, 1975-1982. 31. Woolson, E.A., J.H. Axley and P.C. Kearney, Soil Science Society of America Proceedings (1973) 37, 254-259. 32. Jacobs, L.W., J.F. Syers and D.R. Keeney, Soil Science Society of America Proceedings (1970) 34, 750-754. 33. Johnson, L.R. and A.E. Hiltbold, Soil Science Society of America Proceedings (1969) 33, 279-282. 34. Sandberg, G., "The Fate of Organic Arsenicals in an Agronomic Ecosystem: A Proposed Cycle," Weed Science Society of America Meeting Paper, Las Vegas, February 12-14, 1974. 35. Millhollon, R.W., Weed Science (1969) 17, 370-373. 36. Wiese, A.F. and E.G. Hudspeth, Jr., "Effects of DSMA and MSMA on Cotton Yield and Arsenic Content of Cottonseed," Texas Agricultural Experimental Station, Bulletin MP-877, College Station, 1968. 37. Baker, R.S., et. a l . , Weed Science (1969) 17, 37-40. 38. Foy, C.L. and C.W. Smith, Advances in Chemistry Series (1969) 86, 55-69. 39. Ashton, F.M., A.S. Crafts, "Mode of Action of Herbicides," 504 pp., Wiley-Interscience, New York, 1973. 40. Schweizer, E.E., Weeds (1967) 15, 72-76. 41. Woolson, E.A., Weed Science (1973) 21, 524-527. 42. Duble, R.L., E.C. Holt and G.G. McBee, Weed Science (1968) 16, 421-424. 43. Haan, C.T., Transactions of the American Society of Agricultural Engineers (1971) 14, 445-449. 44. Wischmeier, W.H., Agricultural Engineering (1962) 43, 212215.

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

10. STOLZENBERG 45. 46.

47.

48.

49.

50.

51.

52. 53.

Mobility of Arsenic in the Environment

Linsley, R.J., Jr., M.A. Kohler and J.L.H. Paulhus, "Hydrology for Engineers," 340 pp., McGraw Hill, New York, 1958. Bergsteinsson, J.T. and W. Baier (eds.), "Meteorological Aspects of Pollution in Relation to Agricultural Pesticides" 70 pp., Canada Committee on Agricultural Meteorology, Canada Department of Agriculture, Ottawa, 1971. Slade, D.H. (ed.), "Meteorology and Atomic Energy 1968," 445 pp., Atomic Energy Commission Office of Information Services, 1968. (Available as TID-24190 from the National Technical Information Service) Reiter, E.R., "Atmospheric Transport Processes, Part 2 Chemical Tracers," 382 pp., Atomic Energy Commission Division of Technical Information, 1971. (Available as TID-25314 from the National Technical Information Service) Engelmann, R.J. an Scavenging (1970)," 499 pp., Atomic Energy Commission Division of Technical Information, 1970. (Available as CONF700601 from the National Technical Information Service) Mukammal, E.I. and G.A. McKay in J.T. Bergsteinsson and W. Baier (eds.), "Meteorological Aspects of Pollution in Relation to Agricultural Pesticides," pp. 33-47, Canada Committee on Agricultural Meteorology, Canada Department of Agriculture, Ottawa, 1971. Agricultural Research Service, "A Universal Equation for Measuring Wind Erosion - An Aid to Conservation Farming in the Great Plains, ARS 22-69," U. S. Department of Agriculture, 1961. Isensee, A.R., et. al., Environmental Science and Technology (1973) 7, 841-845. Fulkerson, W., W.D. Schults, and R.I. Van Hook (eds.), "Ecology and Analysis of Trace Contaminants Progress Report, January 1973 - September, 1973," p. 115, Oak Ridge National Laboratory Report ORNL-NSF-EATC-6, Oak Ridge, 1974.

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171

INDEX

A Absorption of organoarsenicals Absorption spectrophotometry, molecular Abundance of arsenic in nature Acid, methylated arsonic Adsorption of arsenicals by soil colloids Agronomic ecosystem, arsenic cycle in an Amperometric titrations Analyses for arsenic, environmental Analytical methods for determining total arsenic in the environment Analytical procedures for total arseni adsorption coprecipitation liquid-liquid extraction volatilization Animal (s) arsenic in aquatic feeds and animal tissue, maximum permissible levels of arsenicals in feeds and wastes, arsenicals in feeding, chemical structure of arsenicals used in toxicity of arsenic for Aquatic animals, arsenic in Aquatic plants, arsenic in Arsenates, calcium Arsenic analytical procedures for total for animals, toxicity of in aquatic animals in aquatic plants available soil biomethylation reactions of case study chemistry compounds, disposal of waste content of drinking waters natural fresh waters marine waters critical level of soil by crops, uptake of cycles in an agronomic ecosystem determination of traces of electrochemical techniques for determining in the environment analytical methods for determining total mobility of environmental analyses for environmental forms of 110, fate of excreted forms of forms in contaminated soils

56 18 128 2 60 124 20 113 109 1 16 17 17 102 72 70 71 1 102 102 35 1 16 1 102 102 136 115 154 36 83 99 98 100 42 66 219 124 13 20 108 109 148 113 111 78 158 39

Arsenic (continued) in fresh waters 97 in fresh water organisms 100 in fresh water plants, BR values for . . 103 input 142 leaching of 62 losses from crop harvest 137 in marine plants, BR values for 103 metabolism of 5, 62, 77 methods of determination of 18 microbiological methylation of 81 movement and persistence in soil 39 concentration of 4 distribution of 2 occurrence and distribution in soils .... 36 phytotoxicity 41 vs. plant growth, extractable 42 plant sensitivity to 41 plant uptake of 52 reduction of organic arsenicals to volatile 137 residues in soil 125 phytotoxicity of 125 route of excretion for 5 in soil, sorption 38 speciation-organo25 studies, use of the rat in 5 from tissue, uptake and depletion of 75 toxicity of incorporated 104 toxicity of trivalent inorganic 9 trioxide 1,35,70 by vegetation, uptake of 135 volatization of 83 Arsenical (s) action, mechanism of 74 in animal feeds and animal tissue, maximum permissible levels of 72 in animal feeds and wastes 70 bioaccumulation of 97 on crop yields, effects of organic 125 drugs used in treating human disease 3 as growth promotants 70 growth response from 71 herbicides, formulae of organic 53 herbicides, toxicity of 10 insecticide 36 pesticides 1 poisoning of soils 41 in soils, behavior and phytotoxicity of inorganic 35 by soil colloids, adsorption of 60 in swine and poultry feeds 70 from treated soil to water, movement of 143 used in animal feeding, chemical structure of 71

173

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

174

ARSENICAL PESTICIDES

Arsine generation Arsonic acid, methylated Atomic absorption spectrometry Atomic emission spectrometry Available soil arsenic

139 2 109 22 21 136

B Behavior and phytotoxicity of inorganic arsenicals in soils Bioaccumulation of arsenicals Bioaccumulation ratio (BR) Biomethylation reactions of arsenic (BR), bioaccumulation ratio BR values for arsenic in fresh water plants BR values for arsenic in marine plants

35 97 97 115 97 103 103

C Cacodylates Cacodylic acid (CA) 54, 82,100 Calcium arsenates Chemical structure of arsenicals used in animal feeding Chemistry, arsenic Coccidiostats Colloids, adsorption of arsenicals by soil Contaminated soils, arsenic forms in Contents of natural fresh waters, arsenic Coulmetry Critical level of soil arsenic Crop harvest, arsenic losses from Crop yields, effects of organic arsenicals on Cycles, arsenic

1 35 71 36 74 60 39 98 21 42 137 125 129

D Desiccants Differential absorption of methanearsonates Dilution, substoichiometric isotope Dimethyl arsine Dimethylarsinic acid Disease, arsenical drugs used in treating human Disposal of waste arsenic compounds Distribution of arsenic in soils, occurrence and Drinking waters, arsenic content of Drugs used in treating human disease, arsenical Disodium methanearsonate (DSMA) DSMA, translocation of MSMA and

124 58 20 10 158 3 83 36 99 3 124 134

E Ecosphere Ecosystem, arsenic cycle in an agronomic Electrochemical techniques for determining arsenic Environment, arsenic in the Environment, mobility of arsenic in the Environmental analyses for arsenic Environmental forms of arsenic 110, Erosion Excreted arsenic, fate of Excretion for arsenic, route of Extractable arsenic vs. plant growth

1 124 20 108 148 113 111 144 78 5 42

F Feeds and animal tissue, maximum permissible levels of arsenicals in animal arsenicals in swine and poultry and wastes, arsenicals in animal Feeding, chemical structure of arsenicals used in animal Foliage to soil, movement of arsenicals from Food web transport processes Formulae of organic arsenical herbicides Fresh water(s) arsenic in arsenic contents of natural organisms, arsenic in plants, BR values for arsenic in

72 70 70 71 133 162 53 97 98 100 103

G methods "Gosio gas" Growth extractable arsenic vs. plant promotants, arsenicals as response from arsenicals

109 81 42 70 71

H Harvest, arsenic losses from crop Hemolysis Herbicidal properties and use patterns Herbicides, formulae of organic arsenical Herbicides, toxicity of arsenical Human disease, arsenical drugs used in treating Hydrolysis, increased starch

137 10 54 53 10 3 134

I Inorganic arsenic, the toxicity of trivalent Insecticide, arsenical Isolation of arsenic species, method for preconcentration Isotope dilution, substoichiometric

9 36 16 20

L Leaching of arsenic Levels of arsenicals in animal feeds and animal tissue, maximum permissible

143 62 72

M Marine plants, BR values for arsenic in 103 Marine waters, arsenic content in 100 Markov chain 154 Maximum permissible levels of arsenicals in animal feeds and animal tissue 72 Mechanism of arsenical action 74 Metabolic pathway for producing trimethylarsine 82 Metabolism of arsenic 5, 62, 77 Metabolism of organoarsenicals 59 Metals, microbial conversions of 161 Methanearsonates, differential absorption of 58 Methanearsonate, translocation of 59

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175

INDEX Methylated arsonic acid Methylation of arsenic, microbiological Microbial conversions of metals Microbiological methylation of arsenic .... Mineralization of arsenic dry ashing fusion oxygen combustion wet ashing Mobility of arsenic in the environment . . Molds, trimethylarsine production in Molds, trimethylarsine production by different Molecular absorption spectrophotometry Monosodium methanearsonate (MSMA) MSMA and DSMA, translocation of

2 81 161 81 13 15 15 13 148 86 84 18 124 134

N Nature abundance of arsenic in arsenic in concentration of arsenic in distribution of arsenic in Neutron activation

12 126 4 2 109

O Occurrence and distribution of arsenic in soils 36 Organisms, arsenic in fresh water 100 Organoarsenical (s) absorption of 56, 60 on crop yields, effects of 125 herbicides, formulae of 53 metabolism of 59 in plants and soils, behavior of 53 to volatile arsenic, reduction of 137 Oxidation of methylarsines 141 Oxidation-reduction potential 38

P Patterns, herbicidal properties and use . Persistence, loss of phytotoxicity as a measure of Pesticides, arsenical Phytotoxicity arsenic of arsenic residues in soil of inorganic arsenicals in soils, behavior and as a measure of persistence, loss of Plant (s) arsenic in aquatic behavior of organoarsenicals in BR values for arsenic in fresh water BR values for arsenic in marine growth, extractable arsenic vs sensitivity to arsenic and soils, behavior of organoarsenicals in translocation in uptake of arsenic Polarography Pool identification Poultry feeds, arsenicals in swine and

54 64 1 41 125 35 64 102 56 103 103 42 41 53 161 45 20 156 70

Preconcentration and isolation of arsenic species, method for Properties and use patterns, herbicidal . . .

16 54

R Radiochemical techniques Rat in arsenic studies, use of Reduction of organic arsenicals to volatile arsenic Residues in soil, arsenic

19 5 137 125

S Sensitivity to arsenic, plant Sesquioxide ratio, silica: Silica :sesquioxide ratio Silver diethyldithiocarbamate method ... Sodium

41 128 128 109

arsinite Soil(s) alleviation of arsenic toxicity in arsenic available critical level of forms in contaminated movement and persistence in residues in arsenical poisoning of behavior of organoarsenicals in plants and behavior and phytotoxicity of inorganic arsenicals in colloids, adsorption of arsenicals by .... pH related to arsenic toxicity phytotoxicity of arsenic residues in sorption of arsenic transport processes, waterto water, movement of arsenicals from treated Solutions during storage, stability of sample Sorption of arsenic in soil Speciation-organo-arsenic Spectrometry, atomic absorption Spectrometry, atomic emission Spectrophotometry, molecular absorption Spectroscopy, atomic Spectroscopy, x-ray fluorescence Spray drift Stability of sample solutions during storage Starch hydrolysis, increased Storage, stability of sample solutions during Structure of arsenicals used in animal feeding, chemical Substoichiometric isotope dilution Swine and poultry feeds, arsenicals in

9 47 136 42 39 39 125 41 60 53 35 60 125 125 38 162 143 15 38 25 22 21 18 21 21 162 15 134 15 71 20 70

T Target plants Tissue, maximum permissible levels of arsenicals in animal feeds and animal

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157 72

176

ARSENICAL PESTICIDES

Tissue, uptake and depletion of arsenic from 75 Titrations, amperometric 20 Total arsenic in the environment, analytical methods for determining . . . 109 Toxicity of arsenic for animals 1 of arsenical herbicides 10 of incorporated arsenic 104 soil pH related to arsenic 125 of trivalent inorganic arsenic 9 Traces of arsenic, determination of 13 Translocation of methanearsonate 59 of MSMA and DSMA 134 in plants 161 Transport processes, food web 162 Transport processes, water-soil 162 Trimethylarsine, metabolic pathwa producing 8 Trimethylarsine production in molds 84, 86 Trivalent inorganic arsenic, the toxicity of 9

U Uptake of arsenic, plant 42, 66, 135 Uptake and depletion of arsenic from tissue 75

V Volatilization of arsenic W Wastes, arsenicals in animal feeds and . Water (s) arsenic content of drinking arsenic content of marine arsenic contents of natural fresh arsenic in fresh contamination movement of arsenicals from treated soil to organisms, arsenic in fresh

X-ray fluorescence spectroscopy

In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

83 70 99 100 98 97 133 143 100

21

Hydrocracking and Hydrotreating: A Symposium Sponsored by the Division of Petroleum Chemistry, Inc., at the 169th Meeting of the American Chemical Society, ... Penn., April (Acs Symposium Series, 20.)

Mechanism of pesticide action: A symposium sponsored by the Division of Pesticide Chemistry at the 167th meeting of the American Chemical Society, Los ... April 2-3, 1974 (ACS symposium series ; 2)

Van't Hoff - Le Bel centennial: A symposium sponsored by the Division of the History of Chemistry at the 168th meeting of the American Chemical Society, ... 11-12, 1974 (ACS symposium series ; no. 12)

Air Pollution Effects on Plant Growth: A Symposium Sponsored by the Division of Agricultural and Food Chemistry at the 167th Meeting of the American (Acs Symposium Series, 3.)

Platinum Group Metals and Compounds;: A symposium sponsored by the Division of Inorganic Chemistry at the 158th meeting of the American Chemical Society (Advances in chemistry series 098)

Light-Activated Pesticides (ACS Symposium Series 339)

Computer networking and chemistry: A symposium sponsored by the Division of Computers in chemistry at the 170th meeting of the American Chemical Society, ... Aug. 26-27, 1975 (ACS symposium series ; 19)

Light-Activated Pesticides (ACS Symposium Series 339)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Pesticide Chemistry in the 20th Century: A Symposium sponsored by the Division of Pesticide Chemistry at the 171st Meeting of the American Chemical Society, New York, April 6 - 8, 1976

Approaches to automotive emissions control: A symposium co-sponsored by the Division of Fuel Chemistry and the Division of Petroleum Chemistry at the 167th ... April 1-2, 1974 (ACS symposium series ; 1)

Marine chemistry in the coastal environment: A special symposium sponsored by the Middle Atlantic Region at the 169th meeting of the American Chemical ... April 8-10, 1975 (ACS symposium series ; 18)

Hydrocracking and Hydrotreating.. A Symposium Sponsored by the Division of Petroleum Chemistry

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Adsorption at interfaces : papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry at the 167th meeting of the American Chemical Society, Los Angeles, Calif., April 2-5, 1974

Refining of Synthetic Crudes: Based on a symposium sponsored by the Division of Petroleum Chemistry at the 174th Meeting of the American Chemical Society (Advances in Chemistry Series 179)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Extended interactions between metal ions in transition metal complexes: A symposium sponsored by the Division of Inorganic Chemistry at the 167th meeting ... April 3-5, 1974 (ACS symposium series ; 5)

Physiological effects of food carbohydrates: A symposium co-sponsored by the Division of Carbohydrate Chemistry and the Division of Agricultural and Food ... 11-12, 1974 (ACS symposium series ; 15)

Carbohydrate Antigens (Acs Symposium Series)

Adsorption at interfaces: Papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry ... 2-5, 1974 (ACS symposium series ; no. 8)

Arsenical pesticides: A symposium sponsored by the Division of Pesticide Chemistry at the 168th meeting of the American Chemical Society, Atlantic City, N.J., Sept. 9, 1974 (ACS symposium series ; 7)

Hydrocracking and Hydrotreating: A Symposium Sponsored by the Division of Petroleum Chemistry, Inc., at the 169th Meeting of the American Chemical Society, ... Penn., April (Acs Symposium Series, 20.)

Mechanism of pesticide action: A symposium sponsored by the Division of Pesticide Chemistry at the 167th meeting of the American Chemical Society, Los ... April 2-3, 1974 (ACS symposium series ; 2)

Van't Hoff - Le Bel centennial: A symposium sponsored by the Division of the History of Chemistry at the 168th meeting of the American Chemical Society, ... 11-12, 1974 (ACS symposium series ; no. 12)

Air Pollution Effects on Plant Growth: A Symposium Sponsored by the Division of Agricultural and Food Chemistry at the 167th Meeting of the American (Acs Symposium Series, 3.)

Platinum Group Metals and Compounds;: A symposium sponsored by the Division of Inorganic Chemistry at the 158th meeting of the American Chemical Society (Advances in chemistry series 098)

Light-Activated Pesticides (ACS Symposium Series 339)

Computer networking and chemistry: A symposium sponsored by the Division of Computers in chemistry at the 170th meeting of the American Chemical Society, ... Aug. 26-27, 1975 (ACS symposium series ; 19)

Light-Activated Pesticides (ACS Symposium Series 339)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Pesticide Chemistry in the 20th Century: A Symposium sponsored by the Division of Pesticide Chemistry at the 171st Meeting of the American Chemical Society, New York, April 6 - 8, 1976

Approaches to automotive emissions control: A symposium co-sponsored by the Division of Fuel Chemistry and the Division of Petroleum Chemistry at the 167th ... April 1-2, 1974 (ACS symposium series ; 1)

Marine chemistry in the coastal environment: A special symposium sponsored by the Middle Atlantic Region at the 169th meeting of the American Chemical ... April 8-10, 1975 (ACS symposium series ; 18)

Hydrocracking and Hydrotreating.. A Symposium Sponsored by the Division of Petroleum Chemistry

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Adsorption at interfaces : papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry at the 167th meeting of the American Chemical Society, Los Angeles, Calif., April 2-5, 1974

Refining of Synthetic Crudes: Based on a symposium sponsored by the Division of Petroleum Chemistry at the 174th Meeting of the American Chemical Society (Advances in Chemistry Series 179)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Extended interactions between metal ions in transition metal complexes: A symposium sponsored by the Division of Inorganic Chemistry at the 167th meeting ... April 3-5, 1974 (ACS symposium series ; 5)

Physiological effects of food carbohydrates: A symposium co-sponsored by the Division of Carbohydrate Chemistry and the Division of Agricultural and Food ... 11-12, 1974 (ACS symposium series ; 15)

Carbohydrate Antigens (Acs Symposium Series)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Analytical Chemistry of Bacillus thuringiensis (Acs Symposium Series)

Chemistry of Wine Flavor (ACS Symposium Series, No. 714)

Pesticide Formulations: Innovations and Developments (Acs Symposium Series)

Adsorption at interfaces: Papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry ... 2-5, 1974 (ACS symposium series ; no. 8)

Wood Technology: Chemical Aspects (Acs Symposium Series ; Number 43))

Hemicelluloses: Science and Technology (Acs Symposium Series)

Arsenical pesticides: A symposium sponsored by the Division of Pesticide Chemistry at the 168th meeting of the American Chemical Society, Atlantic City, N.J., Sept. 9, 1974 (ACS symposium series ; 7)

Hydrocracking and Hydrotreating: A Symposium Sponsored by the Division of Petroleum Chemistry, Inc., at the 169th Meeting of the American Chemical Society, ... Penn., April (Acs Symposium Series, 20.)

Mechanism of pesticide action: A symposium sponsored by the Division of Pesticide Chemistry at the 167th meeting of the American Chemical Society, Los ... April 2-3, 1974 (ACS symposium series ; 2)

Van't Hoff - Le Bel centennial: A symposium sponsored by the Division of the History of Chemistry at the 168th meeting of the American Chemical Society, ... 11-12, 1974 (ACS symposium series ; no. 12)

Air Pollution Effects on Plant Growth: A Symposium Sponsored by the Division of Agricultural and Food Chemistry at the 167th Meeting of the American (Acs Symposium Series, 3.)

Platinum Group Metals and Compounds;: A symposium sponsored by the Division of Inorganic Chemistry at the 158th meeting of the American Chemical Society (Advances in chemistry series 098)

Light-Activated Pesticides (ACS Symposium Series 339)

Computer networking and chemistry: A symposium sponsored by the Division of Computers in chemistry at the 170th meeting of the American Chemical Society, ... Aug. 26-27, 1975 (ACS symposium series ; 19)

Light-Activated Pesticides (ACS Symposium Series 339)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Pesticide Chemistry in the 20th Century: A Symposium sponsored by the Division of Pesticide Chemistry at the 171st Meeting of the American Chemical Society, New York, April 6 - 8, 1976

Approaches to automotive emissions control: A symposium co-sponsored by the Division of Fuel Chemistry and the Division of Petroleum Chemistry at the 167th ... April 1-2, 1974 (ACS symposium series ; 1)

Marine chemistry in the coastal environment: A special symposium sponsored by the Middle Atlantic Region at the 169th meeting of the American Chemical ... April 8-10, 1975 (ACS symposium series ; 18)

Hydrocracking and Hydrotreating.. A Symposium Sponsored by the Division of Petroleum Chemistry

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Adsorption at interfaces : papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry at the 167th meeting of the American Chemical Society, Los Angeles, Calif., April 2-5, 1974

Refining of Synthetic Crudes: Based on a symposium sponsored by the Division of Petroleum Chemistry at the 174th Meeting of the American Chemical Society (Advances in Chemistry Series 179)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Extended interactions between metal ions in transition metal complexes: A symposium sponsored by the Division of Inorganic Chemistry at the 167th meeting ... April 3-5, 1974 (ACS symposium series ; 5)

Physiological effects of food carbohydrates: A symposium co-sponsored by the Division of Carbohydrate Chemistry and the Division of Agricultural and Food ... 11-12, 1974 (ACS symposium series ; 15)

Carbohydrate Antigens (Acs Symposium Series)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Analytical Chemistry of Bacillus thuringiensis (Acs Symposium Series)

Chemistry of Wine Flavor (ACS Symposium Series, No. 714)

Pesticide Formulations: Innovations and Developments (Acs Symposium Series)

Adsorption at interfaces: Papers from a symposium honoring Robert D. Vold and Marjorie J. Vold sponsored by the Division of Colloid and Surface Chemistry ... 2-5, 1974 (ACS symposium series ; no. 8)

Wood Technology: Chemical Aspects (Acs Symposium Series ; Number 43))

Hemicelluloses: Science and Technology (Acs Symposium Series)

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