Kinetics of Arsenate Reduction by Dissolved Sulfide

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 (H x As3S6 x -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 μM 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.