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KINETICS OF ARSENATE REDUCTION BY DISSOLVED SULFIDE. (R825399)
Citation:
Rochette, E. A., G. C. Li, B. C. Bostick, AND S. Fendorf. KINETICS OF ARSENATE REDUCTION BY DISSOLVED SULFIDE. (R825399). ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 34:4714-4720, (2000).
Description:
Arsenic toxicity and mobility in soil and aquatic environments depends on its
speciation, with reducing environments generally leading to more hazardous
conditions with respect to this element. Aqueous sulfide (H2S or
HS-) is a strong reductant and often occurs at appreciable
concentrations in reduced systems. Consequently, it may play an integral part in
arsenic redox chemistry. Therefore, reactions between arsenic and sulfide may
strongly influence water quality in arsenic-contaminated systems. To evaluate
this possibility, we investigated the kinetics and reaction pathways of arsenate
with sulfide. Arsenate reduction by hydrogen sulfide is rapid and conforms to a
second-order kinetic model, having a rate constant, k = 3.2 ?
102 M-1 h-1, that is more
than 300 times greater at pH 4 than at pH 7. However, arsenite is not the direct
reaction product. Rather, arsenic-sulfide complexes develop, including the
formation of a trimeric species
(HxAs3S6x-3),
that persist in solution for several days, ultimately dissociating and leading
to the production of dissolved arsenite. The precipitation of orpiment is
dominant only at high (20:1) S:As ratios, considering the reaction conditions
used in this study (133 
M As, pH 4).
Hence, models of arsenic behavior in the environment should consider abiotic
reduction of arsenate by sulfide, at least under moderately acidic conditions,
and the possibility of dissolved arsenic-sulfide complexes.