Reduction of U(VI) to U(IV) by indigenous bacteria in contaminated ground water

We report on bio-catalyzed reduction and immobilization of U(VI) species (0.25 mg/l to 235 mg/l) in ground water in the presence of high concentrations of nitrate, sulfate and carbonate. We studied ground water from the uranium mill tailings site near Tuba City, Arizona (USA). Experiments with the ground water were conducted in the presence of the Navajo sandstone host rock. Uranium in solution is complexed by carbonate. Two indigenous denitrifying bacteria were identified Pseudomonas aeruginosa and P. stutzeri, and one sulfate reducing bacterium, Shewanella putrefaciens, also known as Fe(III)-reducer. S. putrefaciens can use U(VI) as an electron acceptor, instead of Fe(III). Ethanol was used as the organic carbon source. Microbially mediated reactions are sequential in the order of decreasing redox intensity. Metabolic reduction of nitrate to gaseous species (N2, N2O) was complete within 1 week at 16°C. The sulfate concentration remained constant. Some of the U(VI) coprecipitated with aragonite/calcite or was adsorbed on biomass during denitrification. Subsequently, the enzymatically catalyzed reduction of U(VI) to U(IV) was complete within 3 weeks but was accompanied by reduction of sulfate to sulfide. U(IV) precipitated as a uraninite solid solution (U, Ca)O2, adhering to the bacteria. The final concentration in solution was ≤1 μg/l. U(VI) was not reduced by sulfide. Complexation of U(VI) by carbonate made its reduction by sulfide even slower than in pure water. The bio-catalyzed reaction is the faster process under the conditions given by the composition of the ground water.