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We have examined extracellular iron-bearing precipitates resulting from the growth of Desulfovibrio desuyfuricans in a basal medium with lactate as the carbon source and ferrous sulfate. Black precipitates were obtained when D. desulfuricans was grown with an excess of FeSO4. When D. desulfuricans was grown under conditions with low amounts of FeSO4, brown precipitates were obtained. The precipitates were characterized by iron K-edge XAFS (x-ray absorption fine structure), 57Fe Mössbauer-effect spectroscopy, and powder x-ray diffraction. Both were noncrystalline and nonmagnetic (at room temperature) solids containing high-spin Fe(III). The spectroscopic data for the black precipitates indicate the formation of an iron-sulfur phase with 6 nearest S neighbors about Fe at an average distance of 2.24(1) Å, whereas the brown precipitates are an iron-oxygen-sulfur phase with 6 nearest O neighbors about Fe at an average distance of 1.95(1) Å.
Biotic Np(V) reduction is studied in light of its potential role for the environmental immobilization of this hazardous radionuclide. The speciation of Np in Desulfovibrio desulfuricans cultures is compared with Np speciation in the spent medium and in the uninoculated medium. Precipitates formed in all three samples. Optical spectroscopy, x-ray diffraction, and x-ray absorption near edge structure (XANES) were used to determine the Np speciation. After 5 days of incubation, there was very little Np left in solution, which was present as Np(V). The precipitate that formed in all samples is an amorphous Np(IV) species, establishing that Np(V) is almost quantitatively reduced. These results demonstrate that the reduction of Np is independent of Desulfovibrio desulfuricans. The underlying chemistry associated with these results is discussed.
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