Effects of Citrate, NTA, and EDTA on the Reduction of U(VI) by Shewanella putrefaciens

Reduction of U(VI) by Shewanella putrefaciens in the presence of citrate, NTA, and EDTA was examined to elucidate the effects of strong complexation agents on biological reduction. Uranium(VI) and lactic acid were supplied as the sole electron acceptor and donor, respectively, under anaerobic conditions at 30°C at initial pH 7.0. In the control medium containing no citrate, NTA, or EDTA, the concentration of dissolved U decreased with time and precipitates appeared. X-ray absorption near-edge structure analysis showed that the precipitates contained U(IV). In the presence of citrate, NTA, or EDTA, the concentrations of dissolved U scarcely changed. The UV-VIS spectra of the media showed that concentrations of U(VI) species decreased and those of U(IV) species increased with time. These results indicate that the presence of strong complexation agents inhibits biological reductive UO2 precipitation by forming soluble U(IV)-organic complexes. The initial reduction rate of U(VI) in the citrate medium in which polynuclear U(VI)-citrate complexes were formed was much slower than those in the NTA and EDTA media. Formation of the polynuclear complexes may be one of the reasons for retardation of the U(VI) reduction by S. putrefaciens. Our results indicate that the presence of strong complexation agents affects the chemical species of U(IV) and the reduction rate of U(VI).