The role of sulfate‐reducing bacteria in the deposition of sedimentary uranium ores

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Abstract
The formation of many important sediment‐hosted uranium ore deposits is thought to have resulted from the reduction of relatively soluble uranyl ion—U(VI)—to insoluble uranium (IV) oxides and silicates by aqueous sulfide species. This study focused on the influence that the sulfate‐reducing bacteria Desulfovibrio desulfuricans (ATCC 7757) has on this process. Preliminary studies showed that bacterial growth was not inhibited by concentrations of uranyl ion up to 100 mg U per liter. More detailed studies showed that sulfate‐reducing bacteria have an influence on uranyl ion removal beyond the simple production of the aqueous sulfide reductant. Comparative studies of bacterial cultures containing high densities of the sulfate reducers with bacterial cell‐free but otherwise identical media showed that the bacteria themselves enhance uranium removal from solution. At pH 8.0, no reaction was observed in H2S‐bearing cell‐free media, whereas at the same H2S concentration, the uranyl ion decreased markedly in the presence of the bacteria. At pH 7.0, some uranium removal occurred in the absence of bacteria, but it was much more rapid in their presence. We postulate that these effects are due to the ability of bacterial cell walls to adsorb uranium. Adsorption to surfaces is known from independent studies to enhance uranium reduction, and evidently this two‐step adsorption‐reduction mechanism is occurring in our experiments. We conclude that sulfate‐reducing and other bacteria may play a significant role in the geochemical cycling of uranium.