Structural Chemistry of Cation-Doped Bacteriogenic UO2

The chemical stability of bacteriogenic UO2 is the seminal issue governing its success as an "in-situ waste form" in the subsurface. An intriguing hypothesis we are investigating is that ground water cations, such as Ca, may incorporate into bacteriogenic UO2 and alter its solubility and dissolution kinetics. Such behavior is possible because the open fluorite structure of UO2 permits extensive nonstoichiometry and cation substitution. We are using in-situ synchrotron-based EXAFS, WAXS and SAXS to characterize the atomic- and nano-scale structures of bacteriogenic UO2 systematically as a function of Ca, Mg, Fe and Mn content and reaction time. Preliminary analyses indicate that bio UO2 synthesized in this fashion is nanoparticulate, and particle size and vacancy defects are present in abundance. Alongside the characterization of the structural chemistry of cation-substituted bacteriogenic U(IV), this multi-institution collaborative team will measure the corresponding solubility and oxidation kinetics of the products. It is anticipated this method will provide a powerful engineering tool to increase the stability of bacteriogenic UO2 in groundwater