Anaerobic batch experiments were conducted to evaluate the reduction and precipitation of U6+ complexed to acetate, oxalate, citrate and tiron using a pure culture of sulfate reducing bacteria (Desulfovibrio desulfuricans). These ligands were chosen to evaluate the nature of complex formation on uranium bioreduction. Experiments in which uranium was initially complexed to the ligand (>92%) indicated that bacteria reduce U6+ to U4+ in the presence of each ligand tested. The rate of acetate complexed uranium reduction was fastest and that of citrate complexed uranium was slowest. Precipitation of reduced uranium was evaluated by filtering the reduced complex through 0.2 μm filters. The U4+ formed after biotransformation in acetate solutions was retained on the filter whereas U4+ in the oxalate, citrate and tiron solutions passed through the filter. When ligand concentrations were lowered (44–50% of the uranium initially complexed) a greater amount of UO2 precipitate from oxalate samples was collected on the filter indicating that ligand concentration can impact uranium removal via precipitation. Little or no precipitate was collected from citrate or tiron containing solutions. The preferred reaction of U4+ appeared to be recomplexation with these ligands rather then precipitation as UO2.

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