7 Bioaccumulation of Arsenicals
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0007.ch007
E. A. WOOLSON Pesticide Degradation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Md. 20705
Arsenic, as a natural component of aquatic systems, is accumulated by aquatic organisms. The bioaccumulation ratio (BR) 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 (15) (10) (16)
mining
Contains vanadium a l s o ; contains waste products from W Less than 1% exceeded 10 yg/1 SJ Watershed which r e c e i v e d lead arsenate _V Well dug through area c o n t a i n i n g grasshopper b a i t —f N a t u r a l l y contaminated a r t e s i a n w e l l water B.
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
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0007.ch007
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 arsenate t o accumulate i n the sediments, where reduction o f f e r r i c arsenate and a r s e n i t e r e s u l t s i n e i t h e r 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
