8 Arsenic in the Environment R O B E R T S. B R A M A N
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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.
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In Arsenical Pesticides; Woolson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
8.
BRAMAN
Arsenic in the
Environment
109
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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 developing gas chromatographic-emission d e t e c t i o n methods and have r e ported t h e i r progress i n t h i s symposium. 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.
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ARSENICAL PESTICIDES
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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 of the a r s i n e s without d e a l k y l a t i o n or d i s p r o p o r t i o n a t i o n was obtained by a m i l d , warm a l k a l i n e wash f o l lowed by a hot water wash. The a r s i n e s are probably a i r o x i d i z e d 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
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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 .
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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 .
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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
| >
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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 c present. This was found to be the case i n human u r i n e and i s a l s o suggested by Peoples, Lakso 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
