Gursharan Singh, S. Sevinç Sengör, Aditya Bhalla, Sudhir Kumar, Jaysankar De, Brandy Stewart, Nicolas Spycher, Timothy M. Ginn, Brent M. Peyton and Rajesh K. Sani
Department of Chemical and Biological Engineering , South Dakota School of Mines and Technology, Rapid City, SD, USA.
Uraninite (UO2) is the most desirable end product of in situ bioreduction because of its low solubility under reducing conditions. For effective long-term immobilization of uranium (U), there should be no biotic or abiotic reoxidation of the insoluble biogenic U(IV). It is therefore critical to understand the long-term stability of U(IV) under oxic- and nutrient-limited conditions at U-contaminated subsurface sites. It has now been established that following in situ bioremediation of U(VI) via nutrient addition in the subsurface, a range of physical, chemical, and biological factors control the rate and extent of long-term stability of U(IV). Some of these factors are tied to site specific conditions including existence of oxidants such as Fe(III)(hydr)oxides, Mn(IV) oxides, oxygen, and nitrate; the presence of organic carbon and the reduced forms of U (e.g., mononuclear U(IV) or nanometer-sized uraninite particles); and the carbonate concentration and pH of groundwater. This review analyzes the contribution of these factors in controlling U(IV)-reoxidation, and highlights the competition among U(IV) and other electron acceptors and possible mechanisms of reoxidation of various forms of U(IV).