Modeling the Removal of Uranium U(VI) from Aqueous Solutions in the Presence of Sulfate Reducing Bacteria

Abstract

The reduction kinetics of soluble hexavalent uranium (U(VI)) to insoluble tetravalent U(IV) by both a mixed culture of wild-type sulfate-reducing bacteria (SRB) and a pure culture of Desulfovibrio desulfuricans (ATCC 7757) were studied at variable cell concentrations. The SRB were grown in chemostats, and the uranium reduction kinetics were evaluated from batch experiments at 21 °C. The initial U(VI) concentration was 1 mM, while the initial cell concentration varied from 0.18 to 1.27 mg dry wt/mL. A modified nongrowth Monod model best fit the data across all cell concentrations. For the mixed culture, average values for the maximum specific reaction rate, kU, and for the half saturation constant, KU, are 0.023 mM U(VI)/min·mg cells and 0.25 mM U(VI); for D. desulfuricans, kU is 0.030 mM U(VI)/min·mg SRB and KU is 0.50 mM U(VI). A lag-time before enzymatic U(VI) reduction occurred was present for systems tested and was inversely correlated to cell concentration. Transmission electron microscopy (TEM) of the cell/precipitate suspension after U(VI) reduction indicated an extracellular amorphous mass of electron dense material. Examination of the cell/precipitate suspension with X-ray Photoelectron Spectroscopy (XPS) indicated the presence of U(IV). The determined rate constants can be used to design an SRB dominated bioreactor for U removal from aqueous solutions.