PRECIPITATION OF ARSENIC DURING BACTERIAL SULFATE REDUCTION

Contaminated sediments from the Milltown Reservoir in western Montana release arsenic and various heavy metals (e.g., Cu, Cd, Pb, Zn, Mn) in to an underlying alluvial aquifer as redox conditions in the sediments change with seasonally fluctuating water levels. Porewater analyses i ndicate that sulfate is depleted with depth. In this study, the feasib ility of inducing As(III) precipitation through bacterial reduction of sulfate; was evaluated in laboratory microcosms established under str ictly anaerobic conditions. As(III), Fe(II), and sulfate concentration s were routinely monitored in the aqueous phase as sulfate was reduced to sulfide. Both As(III) and Fe(II) concentrations in the sediment mi crocosms decreased as sulfide was made available. Energy-dispersive x- ray (EDS) analysis indicated that some of the arsenic was precipitated as an iron-arsenic-sulfide solid phase. The precipitation of arsenic observed in this laboratory study suggests that bacterial sulfate redu ction may be a process by which heavy metals are immobilized in sedime nts; however, even though the Milltown sediments contained sulfate-red ucing bacteria, their activity appears to be both sulfate and carbon l imited.