REACTION PROCESSES OF ARSENIC IN SULFIDIC SOLUTIONS

The fate of arsenic in the environment is fundamentally linked to its speciation. Arsenic in aerobic environments is predominantly arsenate, however under reducing conditions arsenite species dominate. In anoxic or sulfidic environments thioarsenite ((As(OH)_x(SH)_y^z-) species along with arsenite are known to play key roles in arsenic speciation. Although thioarsenite species are know to be important in sulfidic environments, little is know about their formation, structure, chemistry, and their stability in the environment. In particular, information is sparse relating to the redox behavior of arsenate and arsenite in the presence of aqueous sulfide and at variable pH. In order to address these questions a series of solutions containing arsenate in the presence of sulfide species and at variable pH were analyzed using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) analysis. With these techniques it was possible to obtain kinetic information about the reductive precipitation of arsenic and changes in speciation of arsenate to arsenite and/or thioarsenite with time. In addition, structural data pertaining to thioarsenite in solution and in solid phase were obtained. The results of these experiments will be presented. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.