The purpose of this research was to investigate the stability of metals and metalloids immobilized through in-situ bioremediation, specifically dissimilatory metal reduction processes. The stability of arsenic and selenium immobilized through microbial reduction by the organisms Desulfovibrio desulfuricans , a sulfur reducing bacteria (SRB), and Shewanella putrefaciens , an iron reducing bacteria (FeRB), were investigated through the use of up-flow, anaerobic reactors using a sand matrix and metals-containing simulated groundwater to approximate aquifer and reactive barrier conditions. Analytical electron microscopy (SEM/EDX) and geochemical modeling (PHREEQC) were used to speciate immobilized metals.; This investigation demonstrated simultaneous microbial reduction and subsequent precipitation of arsenic and selenium by a pure culture of SRB, Desulfovibrio desulfuricans, on silica sand in a simulated groundwater environment.; Additionally, this research has shown that arsenic and selenium precipitated under these conditions remains relatively stable under unchanging as well as changing aquifer conditions. In-column leaching tests were performed to simulate conditions that might occur in a subsurface environment where contaminants had been stabilized by SRB. Results suggest that arsenic immobilized by sulfate reducing bacteria will remain stable, even if environmental conditions change from anoxic to oxic, or if necessary substrate for bacterial growth ceases to be present. This is due in part to the reducing conditions maintained as sulfide mineral consume O2. This research further suggests that arsenic immobilized in this manner will remain at concentrations below the current drinking water standard of 10 ug/L, although this level would be strongly affected by total arsenic present. This level of dissolved arsenic remained consistent under both reducing and oxidized conditions in the influent water. This suggests that arsenic immobilized by SRB in an in-situ remediation scenario such as a reactive barrier could be left in place. This is further reinforced by the results obtained in batch leaching tests, which showed that the sand matrix containing the immobilized arsenic sulfides would pass the TCLP procedure, which is the criteria established by RCRA to identify whether a waste is hazardous. (Abstract shortened by UMI.)
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