Advances in Electron Microscopy for Studying the Microorganism- mediated Reduction of Radionuclides in the Subsurface

As a result of the Manhattan Project and the Cold War, it has been estimated that over 1 billion m of groundwater and >75 million m of soil have been contaminated with radionuclides in sites across the United States. In addition to direct disposal to soils, trenches, cribs, etc., large volumes of waste remain in storage at several Department of Energy (DOE) sites. Due to their long half-lives and relatively high environmental mobility, U and Tc are the two primary risk-driving contaminants at these locations. In their most oxidized forms, U(VI) and Tc(VII) exhibit a high degree of environmental solubility, while they are considerably less soluble and precipitate as oxides at their lowest oxidation state, +4. Under anoxic conditions, the bioreduction of U(VI) and Tc(VII) to insoluble oxides, UO2(s) (uraninite) and TcO2(s)·nH2O, respectively, by dissimilatory metal-reducing bacteria (DMRB) can significantly alter the mobility of these radionuclides in contaminated groundwaters.