Modeling the removal of uranium U(VI) from aqueous solutions in the presence of sulfate reducing bacteria

The reduction kinetics of soluble hexavalent uranium (U(VI)) to insoluble t etravalent U(IV) by both a mixed culture of wild-type sulfate-reducing bact eria (SRB) and a pure culture of Desulfovibrio desulfuricans (ATCC 7757) we re studied at variable cell concentrations. The SRB were grown in chemostat s, and the uranium reduction kinetics were evaluated from batch experiments at 21 degrees C. The initial U(VI) concentration was 1 mM, while the initi al cell concentration varied from 0.18 to 1.27 mg dry wt/mL. A modified non growth Monod model best fit the data across all cell concentrations. For th e mixed culture, average values for the maximum specific reaction rate, k(U ), and for the half saturation constant, K-U, are 0.023 mM U(VI)/min.mg cel ls and 0.25 mM U(VI); for D. desulfuricans, k(U) is 0.030 mM U(VI)I min mg SRB and K-U is 0.50 mM U(VI). A lag-time before enzymatic U(VI) reduction o ccurred was present for systems tested and was inversely correlated to cell concentration, Transmission electron microscopy (TEM) of the cell/precipit ate suspension after U(VI) reduction indicated an extracellular amorphous m ass of electron dense material. Examination of the cell/precipitate suspens ion 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.