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(20,21,22,23,26,39). Models for the SRB reduction of these metals and radionuclides can be used to develop and design treatment systems employing SRB for bioremediation. For example, bioreduction of soluble hexavalent uranium [U(VI)] to insoluble tetravalent uranium [U(IV)] can be utilized to remove soluble uranium from groundwaters, mine waters, or secondary waste streams.In addition to SRB, a number of bacterial species have been described as capable of reducing U(VI) to U(IV), including Geobacter metallireducens (25) (previously reported as GS-15 [11]), Shewanella putrefaciens (25), and Shewanella alga strain BrY (previously reported as BrY, or Shewanella halotolerans strain BrY [3]). A kinetic study for the reduction of U(VI) has been described for the iron-reducing S. alga strain BrY (40) but not for SRB. Before a biotreatment scheme can be designed and implemented, studies must go beyond the identification of novel processes, to quantifying the kinetics of such a process. Because of the metabolic diversity of the SRB, it is important to identify the species involved in the processes studied. Since sulfate reduction rates depend upon the species used and experimental conditions, it is important to characterize sulfate reduction for the mixed culture used in previous uranium reduction experiments (38) that were performed under repeatable, uniform conditions.The goals of this study were to characterize the mixed SRB culture previously isolated (38) and to develop models to describe sulfate reduction alone and sulfate reduction with concurrent uranium reduction. Utilizing a mixed cell culture was viewed as an advantage in that in an operational biotreatment scheme, purity of culture in treating large volumes of water will be difficult to maintain. Understanding U(VI) reduction by SRB in a consortium is more likely to be relevant for an operational condition. The nature of the mixed SRB culture was examined via 16S ribosomal DNA (rDNA) analysis. Models for separate sulfate reduction and concurrent sulfate and uranium reduction were fit to data generated by both a mixed and a pure culture of SRB. Sulfate and cell concentrations were varied during these experiments, which were conducted at room temperature, 21 Ϯ 3°C. This is at the upper end of temperatures expected in natural waters that could be treated. Sulfate concentration is expected to be a variable in the treatment of uranium-contaminated waters, and cell concentration is an operational variable for treatment design. These kinetic determinations were made under conditions of insignificant growth (in the absence of nutrients other than a lactate carbon source; the sulfate concentrations present could facilitate growth, but experimental time courses were too short), and under anaerobic batch conditions, to isolate the enzymatic reductive processes of both electron acceptors from those of cellular growth processes. Quantification with modeling of the sulfate reduction rate alone and the uranium reduction rate in the presence of sulfate is important for the ...

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, k U, and for the half saturation constant, K U, are 0.023 mM U(VI)/min·mg cells and 0.25 mM U(VI); for D. desulfuricans, k U is 0.030 mM U(VI)/min·mg SRB and K U 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.

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