Technetium reduction and precipitation by sulfate-reducing bacteria

Resting cells of the sulfate-reducing bacterium Desulfovibrio desulfuricans ATCC 29577 were able to precipitate the radionuclide technetium, supplied as the pertechnetate anion (TcO4-), under anaerobic conditions by two discrete mechanisms. Sulfidogenic cultures, supplied with sulfate and lactate as an electron acceptor and donor, respectively, precipitated the radionuclide as an insoluble sulfhide. Using electron microscopy in combination with energy-dispersive x-ray analysis (EDAX), the precipitate was shown to be extracellular, and contained S as the major element at a fivefold stoichiometric excess to Tc as quantified by proton-induced x-ray emission analysis (PIXE). With hydrogen supplied as the electron donor, the pertechnetate anion was utilized as an alternative electron acceptor in the absence of sulfate. The radionuclide was removed from solution, but in these cultures the precipitate was cell associated, with Tc as the major element detected by PIXE (Tc:S ratio of 2:1). Reduction of the radionuclide in lieu of sulfate was confirmed using XAS. Hydrogen uptake, couple to metal reduction, was also monitored manometrically, with 1 mol H-2 used to reduce 1 mol TcO4-. The implications of these results on Tc mobility in the environment are discussed, and possible biotechnological uses of sulfate-reducing bacteria in bioremediation programs to treat Tc-contaminated waters are highlighted.