Bioremediation of Metals and Radionuclides: Results from Field-scale Experiments and Future Prospects

Over the last several years, a number of field-scale experiments have been conducted at contaminated sites in an effort to immobilize metals and radionuclides as a remediation approach. The underlying processes for immobilization range from changing redox status of metals, to incorporation of radionuclides into precipitated phases, to enhanced sorption on residual biomass. Approaches include both biotically and abiotically catalyzed reactions but all of the immobilization methodologies share the goal of decreasing groundwater contaminant concentrations below applicable standards by shifting the contaminant from groundwater to the in situ geologic media. Such approaches also tend to be relatively low in terms of energy consumption and thus are near the low impact end of the spectrum of remediation technologies that extend from pump-and- treat to monitored natural attenuation. These approaches also face a common challenge, ensuring that reoxidation or mineral dissolution rates are low enough to maintain groundwater contaminant concentrations below applicable standards over regulatory periods of performance. While a number of hydrogeological, geochemical, and microbiological factors control the long-term behavior of immobilized contaminants, the precipitation of new phases, whether by design as part of an abiotic remediation strategy or incidental to a microbial mediated process, appears to favor long- term sequestration of contaminants. This suggests that it may be possible to enhance remediation by manipulating in situ microbial populations and biogeochemical conditions to tailor precipitate abundance and composition. For example, at the U.S. Department of Energy Integrated Field Challenge Site (IFC) in Rifle, Colorado, U(VI) concentrations are decreased in groundwater when acetate is used to stimulate a bloom of Fe-reducers, enzymatically reducing U(VI) to U(IV). A transition from Fe-reduction to sulfate reduction occurs consistently at ~20 to 30 days after the start of in situ acetate amendment and initiates the precipitation of iron monosulfide and calcite. The precipitation of these phases appears to correlate with a decrease in U concentration as much as two years after acetate amendment is stopped. Decreased U concentrations also correlate with post-amendment microbial communities underscoring the importance of understanding the impact of both microbial succession and mineral precipitates on contaminants and their interactions with heterogeneous permeability and geochemical composition.