Microbial activity associated with uranium mill tailings

The microbial characteristics of groundwater from seven uranium mill tailing sites were investigated using several chemical and microbiological techniques. Microbial investigations included estimations of total cell counts, viable cell counts on different media, metabolic capabilities of some isolates and numbers of total aerobic heterotrophs, total anaerobic heterotrophs, denitrifying, sulfate-reducing and Fe(III) reducing bacteria. Groundwater samples were dominated by gram-negative strains of the genera Pseudomonas and Acinetobacter. In addition, some gram-positive bacteria including Bacillus sp were also found. Identification of 51 isolates using the {dollar}rm Biologspcircler{dollar} system showed a predominance of P. fluorescens, P. fragi and P. putida. This study focuses on the use of Desulfovibrio desulfuricans (ATCC 27774), Desulfovibrio desulfuricans (DSM 642), Desulfovibrio vulgaris (DSM 644), Desulfovibrio gigas (ATCC 19364) and mixed and isolated cultures from uranium mill tailing sites for the bioremoval of uranyl, U(VI), nitrate, sulfate, molybdenum and selenium.; The mobility of uranium in the environment is strongly dependent on its chemical oxidation state. Under oxidizing conditions, the uranyl ion {dollar}rm UOsb2sp{lcub}2+{rcub}{dollar} is highly soluble and mobile. One approach would be to control the distribution of uranium in the groundwater by converting the water-soluble uranium {dollar}rm UOsb2sp{lcub}2+{rcub}{dollar} U(VI) to one that is sparingly soluble, {dollar}rm UOsb2,{dollar} U(IV). About 82% to 95% of U(VI) was removed by a Clostridium sp isolated from the Tuba City site. While in an anaerobic consortium of bacteria obtained by enrichment techniques using groundwater from Tuba City site, approximately 60% of the uranium added to the culture of anaerobic bacteria was removed. In general, the utilization of nitrate by mixed bacterial cultures as well as bacterial isolates was dependent on the carbon source being used. Both cultures utilized nitrate rapidly when acetate was the electron donor. While growth was found to be greater in defined medium, the utilization of nitrate was greater in simulated plume water. Bacteria from the groundwater were capable of reducing selenite to elemental selenium with inorganic phosphate or inositol phosphate additions. It was not known whether Desulfovibrio could grow in environments of high nitrate and other metal contaminants. However, in laboratory studies, cultures of D. vulgaris and D. desulfuricans were inhibited only when uranyl ions exceeded 0.15 mM. With lactate-sulfate media, nitrite inhibition was directly related to nitrite concentration, however, measurable growth was observed with 0.1 M nitrite. Molybdate at 1 mM failed to inhibit growth of a strain of D. desulfuricans that is capable of growing with nitrite respiration. The results indicate that co-culture of sulfate reducers with denitrifiers would seem feasible in certain uranium contaminated environments and can be used for the remediation of sites contaminated with uranyl salts.