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, 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.

Modeling Reduction of Uranium U(VI) under Variable Sulfate Concentrations by Sulfate-Reducing Bacteria

“…The mixed culture of SRB was obtained as previously described (38). All chemicals utilized for these studies were reagent grade or better and were used without further purification.…”

“…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.…”

Soil Humic Acid Decreases Biological Uranium(VI) Reduction by Shewanella putrefaciens CN32

“…Biological reduction of soluble U(VI) to a sparingly soluble form of U(IV) [e.g., uraninite U IV O 2 (s)] has been proposed as a remediation strategy (Lovley, 1993). A variety of dissimilatory metalreducing bacteria (DMRB) and sulfate-reducing bacteria can catalyze this reaction under anoxic conditions (e.g., Gorby and Lovley, 1992;Truex et al, 1997;Spear et al, 1999;Liu et al, 2002).…”

Ethanol-Fed or Solid-Phase Organic Sulfate Reducing Bioreactors for the National Tunnel Drainage, Clear Creek/Central City Superfund Site?

“…Another milliliter of solution was taken and introduced to 100 µL of 0.1M zinc acetate (Spear et al, 1999;Machemer et al, 1993). The zinc acetate is known to precipitate out dissolved sulfide.…”