Modeling Reduction of Uranium U(VI) under Variable Sulfate Concentrations by Sulfate-Reducing Bacteria
ABSTRACT The kinetics for the reduction of sulfate alone and for concurrent uranium [U(VI)] and sulfate reduction, by mixed and pure cultures of sulfate-reducing bacteria (SRB) at 21 � 3�C were studied. The mixed culture contained the SRB Desulfovibrio vulgarisalong with a Clostridium sp. determined via 16S ribosomal DNA analysis. The pure culture was Desulfovibrio desulfuricans (ATCC 7757). A zero-order model best fit the data for the reduction of sulfate from 0.1 to 10 mM. A lag time occurred below cell concentrations of 0.1 mg (dry weight) of cells/ml. For the mixed culture, average values for the maximum specific reaction rate,V max, ranged from 2.4 � 0.2 μmol of sulfate/mg (dry weight) of SRB � h−1) at 0.25 mM sulfate to 5.0 � 1.1 μmol of sulfate/mg (dry weight) of SRB � h−1 at 10 mM sulfate (average cell concentration, 0.52 mg [dry weight]/ml). For the pure culture,V max was 1.6 � 0.2 μmol of sulfate/mg (dry weight) of SRB � h−1 at 1 mM sulfate (0.29 mg [dry weight] of cells/ml). When both electron acceptors were present, sulfate reduction remained zero order for both cultures, while uranium reduction was first order, with rate constants of 0.071 � 0.003 mg (dry weight) of cells/ml � min−1 for the mixed culture and 0.137 � 0.016 mg (dry weight) of cells/ml � min−1 (U0 = 1 mM) for the D. desulfuricans culture. Both cultures exhibited a faster rate of uranium reduction in the presence of sulfate and no lag time until the onset of U reduction in contrast to U alone. This kinetics information can be used to design an SRB-dominated biotreatment scheme for the removal of U(VI) from an aqueous source.