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) into an underlying alluvial aquifer as redox conditions in the sediments change with seasonally fluctuating water levels. Porewater analyses indicate that sulfate is depleted with depth. In this study, the feasibility of inducing As(III) precipitation through bacterial reduction of sulfate; was evaluated in laboratory microcosms established under strictly anaerobic conditions. As(III), Fe(II), and sulfate concentrations were routinely monitored in the aqueous phase as sulfate was reduced to sulfide. Both As(III) and Fe(II) concentrations in the sediment microcosms 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 reduction may be a process by which heavy metals are immobilized in sediments; however, even though the Milltown sediments contained sulfate-reducing bacteria, their activity appears to be both sulfate and carbon limited.