IMPACT OF ORGANIC-LIGANDS ON URANIUM REMOVAL DURING ANAEROBIC BIOLOGICAL TREATMENT

Anaerobic batch experiments were conducted to evaluate the reduction a nd precipitation of U6+ complexed to acetate, oxalate, citrate and tir on using a pure culture of sulfate reducing bacteria (Desulfovibrio de sulfuricans). These ligands were chosen to evaluate the nature of comp lex formation on uranium bioreduction. Experiments in which uranium wa s initially complexed to the ligand (>92%) indicated that bacteria red uce U6+ to U4+ in the presence of each ligand tested. The rate of acet ate complexed uranium reduction was fastest and that of citrate comple xed uranium was slowest. Precipitation of reduced uranium was evaluate d by filtering the reduced complex through 0.2 mu m filters. The U4+ f ormed 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% o f the uranium initially complexed) a greater amount of UO2 precipitate from oxalate samples was collected on the filter indicating that liga nd concentration can impact uranium removal via precipitation. Little or no precipitate was collected from citrate or tiron containing solut ions. The preferred reaction of U4+ appeared to be recomplexation with these ligands rather then precipitation as UO2. (C) 1998 IAWQ. Publis hed by Elsevier Science Ltd.