Impact of organic ligands on the bioreductive precipitation of hexavalent uranium

Author

Rajagopalan Ganesh

Date of Award

12-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Kevin G. Robinson

Committee Members

Gary Sayler, Michael Essington, Gregory Reed, Mriganka Ghosh, Chet Francis

Abstract

Microbial reduction of soluble hexavalent uranium (U⁶⁺) to the less soluble tetravalent state (U⁴⁺) has recently been proposed as a treatment option for selective uranium removal from complex waste streams. This research focuses on the impact of various organic ligands present in uranium containing waste streams on the microbial reduction and subsequent precipitation of dissolved uranium. A sulfate reducing bacteria (Desulfovibrio desulfuricans) and an iron reducing bacteria (Shewanella alga (BrY)) were used to study reductive precipitation of uranium. Five organic ligands (acetate, malonate, oxalate, citrate and tiron) were chosen to evaluate their impact on bioreductive precipitation. These ligands varied significantly in their nature as well as affinity of association with hexavalent uranium. Batch anaerobic experiments, under non-growth conditions, were performed to evaluate U⁶⁺ reductive precipitation in the presence of each ligand. A bacterial concentration of ~ 5x10⁸ cells/mL was used. In all the experiments, an initial U⁶⁺ concentration of 100 mg/L was maintained. The test solutions contained H_{2,/sub> gas as sole electron donor, U⁶⁺ as the primary electron acceptor and each ligand as the sole organic complexing agent. All experiments were conducted at constant ionic strength (0.17 M). U⁶⁺ reduction experiments were performed using two concentrations of each target ligand. At the higher concentration of ligands used, ~ 100 % of the U⁶⁺ introduced, initially complexed to the ligands. At the lower ligand concentration, ~ 50% of the U⁶⁺ complexed to the ligands. Microbial reduction of U⁶⁺ was evaluated by measuring the decrease in U⁶⁺ concentration, over time, after cell incubation. In addition, the fate of uranium was determined by monitoring the concentration of bioreduced uranium (U⁴⁺) remaining in solution and the fraction removed via precipitation. Experimental results indicated that, D. desulfiiricans reduced rapidly from organic/inorganic monodentate complexes. In organic free and acetate containing test solutions, the bacteria reduced U⁶⁺ at an initial rate of ~ 1.6 X 10^-3 mg/mg cell-hr. However, U⁶⁺ reduction in multidentate malonate, oxalate and citrate solutions was nearly twenty fold slower (~ 6 X 10^-5 mg/mg cell-hr). Results indicated that, (i) multidentate ligands non-competitively inhibited U⁶⁺ reducing enzymes, or (ii) hexavalent uranium chelated to multidentate ligands was not directly available for reduction for D. desulfuricans. However, > 90 % U⁶⁺ reduction was achieved after 96 hours of incubation in all organic solutions. An exception to the above trend was observed in ternuclear tiron containing solutions. D. desulfuricans reduced U⁶⁺ at a rate (~3.2X10^-3 mg/mg cell-hr) much faster than that observed in binuclear citrate solutions. Differences in the structure (aromatic vs aliphatic) and functional group involved in complexation (hydroxyl vs carboxyl) appeared to have enhanced the U⁶⁺ reduction rate in tiron solutions. Hexavalent uranium reduction in test solutions incubated with Shewanella alga (BrY) followed a different trend. BrY rapidly reduced hexavalent uranium in carbonate solutions. However insignificant U⁶⁺ reduction was observed in organic free (NaNO3) solutions where U⁶⁺ formed uranyl hydroxide species. In acetate containing solutions, U⁶⁺ reduction was slow (~4X10^-5' mg/mg cell-hr) as well as incomplete. In malonate, oxalate and citrate solutions, when ~ 100 % of the U⁶⁺ was initially complexed to the ligands, reduction was very rapid (~2.4X10^-3 mg/mg cell-hr). However, in solutions containing ~ 50 % uranyl hydroxide species and ~ 50% multidentate aliphatic complexes, reduction was slow(~4X10^-5 mg/mg cell-hr ) and incomplete. It appeared that in solutions containing multidentate carboxyl U⁶⁺ complexes as well as uranyl hydroxyl species, BrY reduced U⁶⁺ essentially from the carboxyl complexes. Subsequent reduction appeared to continue through equilibrium redistribution of U⁶⁺ from hydroxyl to carboxyl species. Once U⁶⁺ was bioreduced U⁴⁺ to subsequent precipitation appeared to be impacted by the type and concentration of individual ligand introduced into the test solution. In acetate and malonate solutions, almost all the bioreduced uranium precipitated from the test solutions. However, in oxalate and citrate solutions, an increase in the ligand concentration decreased the amount of U⁴⁺ precipitated. In tiron containing solutions less than 10 % of reduced uranium precipitated at the lowest concentration of ligand used. In most cases the trends in uranium precipitation observed agreed with the prediction made using the thermodynamic constants for the association of tetravalent uranium with each ligand. This indicated that, while trends in bioreduction of hexavalent uranium depended on the metabolism of microorganism used, subsequent precipitation of bioreduced uranium was dictated by its thermodynamic constants for the association with test ligands.}

Recommended Citation

Ganesh, Rajagopalan, "Impact of organic ligands on the bioreductive precipitation of hexavalent uranium. " PhD diss., University of Tennessee, 1995.
https://trace.tennessee.edu/utk_graddiss/9984