| Uranium (U) processing has resulted in widespread environmental contamination, particularly at U.S. Department of Energy (DOE) sites, as well as in various contaminated sites worldwide. Oxidized hexavalent U, U(VI), is generally soluble and mobile, but forms sparingly insoluble uraninite precipitates upon reduction to tetravalent U, U(IV). Therefore, bioreduction of U(VI) to U(IV) is a promising approach to immobilize U in subsurface environments. The mobility and long-term stability of U are influenced by the form of the reduced U product; therefore, identifying and characterizing microbial U(VI) reduction products are vital. In this study, a multicomponent reaction model is developed, to evaluate the thermodynamic and kinetic constraints impacting the U(VI) reduction and its subsequent precipitation as U(IV) nanoparticles by the model SRB, Desulfovibrio desulfuricans G20, under different environmental conditions. The different environmental conditions refer to the (i) the use of different electron donors (lactate or pyruvate) and electron acceptors (sulfate, thiosulfate or fumarate), (ii) the effect of different buffers (PIPES or bicarbonate) present in the medium that impact the reduction and immobilization of soluble hexavalent U (VI) as biogenic U(IV) particles by G20 under growth and non-growth conditions. The study also investigates the impact of pH, as well as the effect of biogenic U(IV) particle size on its solubility impacting the overall bioreduction of U(VI) in the presence of these conditions. The experimental profiles of biomass growth and U(VI) reduction are well captured by the numerical model for all cases. Our model results showed a good match with the experimental data considering U(IV) precipitate sizes to be between 200-3 nm when sulfate and thiosulfate were used as the electron acceptor. Below 3 nm, the model deviated from the experimental data, due to the sensitivity of the abiotic U(VI) reduction by HS- reaction, to the thermodynamic solubility of uraninite under growth conditions. However, when fumarate was used as the electron acceptor, the modeling results matched the experimental data regardless of the U(IV) particle sizes. The model results showed that the reductive precipitation of U(VI) was more sensitive to the choice of the electron acceptor, rather than the electron donor tested in this study. The impact of pH on U(VI) reduction was also simulated. The results showed higher U(VI) reduction at lower pH values. No U(VI) reduction was observed at pH above 8. This research would be useful in assessing the key thermodynamic and kinetic factors affecting the overall reductive precipitation of U(VI) under different environmental conditions, which would provide insight towards the U(VI) bioremediation efforts. |
