Ca-UO₂-CO₃ complexation - Implications for bioremediation of U(VI)

An attractive remediation strategy for the removal of highly soluble U(VI) from groundwater is microbial reduction of soluble U(VI) to U(IV) as insoluble uraninite. The feasibility of this remediation strategy depends on the rate and extent of microbial reduction of U(VI) in the presence of ions commonly found in groundwater that can form complexes with uranyl. Recently, Ca in solutions with U(VI) has been shown to inhibit microbial reduction of U(VI). Indirect evidence supports the formation of a Ca-UO₂-CO₃ complex that may be responsible for the limited microbial reduction of U(VI) in the presence of Ca, but direct evidence for the formation of a Ca-UO₂-CO₃ complex is needed for verification. X-ray absorption fine-structure measurements of solutions containing 50 μM U(VI) and 30 mM bicarbonate, with and without 5 mM Ca, were made to determine whether a Ca-UO₂-CO₃ complex was present. These results are discussed.