MESOSCALE BIOTRANSFORMATIONS OF URANIUM IN SEDIMENTS AND SOILS (Program Element: Biogeochemistry)

In-situ bioreduction is being considered as a remediation strategy for uranium (U) contaminated sediments because of its potentially low cost, and because short-term studies support its feasibility. However, any in-situ approach for immobilizing U will require assurance of either permanent fixation, or of very low release rates into the biosphere. Our long-term laboratory studies have shown that reoxidation of bioreduced UO{sub 2} can occur even under reducing (methanogenic) conditions sustained by continuous infusion of lactate. The biogeochemical processes underlying this finding need to be understood. Our current research is designed to identify mechanisms responsible for anaerobic U oxidation, and identify effects of key factors controlling long-term stability of bioreduced U. These include: (1) effects of organic carbon (OC) concentrations and supply rates on stability of bioreduced U, (2) influences of pH on U(IV)/U(VI) redox equilibrium, (3) the roles of Fe- and Mn-oxides as potential U oxidants in sediments, and (4) the role of microorganisms in U reoxidation. Findings from some of these studies are summarized here.