Kinetics of arsenate reduction by dissolved sulfide

Arsenic toxicity and mobility in soil and aquatic environments depends on i ts speciation, with reducing environments generally leading to more hazardo us conditions with respect to this element Aqueous sulfide (H2S or HS-) is a strong reductant and often occurs at appreciable concentrations in reduce d systems. Consequently, it may play an integral part in arsenic redox chem istry. Therefore, reactions between arsenic and sulfide may strongly influe nce water quality in arsenic-contaminated systems. To evaluate this possibi lity, we investigated the kinetics and reaction pathways of arsenate with s ulfide. Arsenate reduction by hydrogen sulfide is rapid and conforms to a s econd-order kinetic model, having a rate constant, k = 3.2 x 10(2) M-1 h(-1 ), that is more than 300 times greater at pH 4 than at pH 7. However, arsen ite is not the direct reaction product. Rather, arsenic-sulfide complexes d evelop, including the formation of a trimeric species (HxAs3S6x-3), that pe rsist in solution for several days, ultimately dissociating and leading to the production of dissolved arsenite. The precipitation of orpiment is domi nant only at high (20:1) S:As ratios, considering the reaction conditions u sed in this study (133 muM As, pH 4). Hence, models of arsenic behavior in the environment should consider abiotic reduction of arsenate by sulfide, a t least under moderately acidic conditions, and the possibility of dissolve d arsenic-sulfide complexes.