Washington University, St. Louis
Biogeochemical processes influencing the mobility of uranium in the environment
Past activities associated with the mining, processing, and disposal of nuclear materials have left a legacy of uranium-contaminated soil and groundwater. Remediation of contaminated environments can be achieved by removal and off-site disposal of contaminated media, but in situ immobilization of the uranium can also provide long-term management of contaminated sites. Two potential strategies for in situ remediation are (1) stimulation of natural microbial communities to mediate the reduction of the more mobile U(VI) state to less mobile U(IV) species such as UO2 and (2) addition of phosphate to promote immobilization of U(VI) in precipitated and strongly adsorbed species. The structure and stability of biologically produced UO2 as well as a chemically-synthesized analog have been investigated using an array of techniques over a wide range of conditions. The long-term stability of bioremediation strategies can be influenced by the structure of the solid and the specific site conditions. Dissolution rates of UO2 have been examined as a function of chemical conditions that include dissolved oxygen, pH, dissolved inorganic carbon, and common cation concentrations. The impact of diffusive transport limitations on the longevity of UO2 in oxic environments has also been examined. The addition of phosphate can induce U(VI) immobilization through both enhanced adsorption and precipitation. Experiments, equilibrium speciation modeling, and direct solid phase characterization have identified regimes over which different U(VI)-phosphate interactions control the mobility of U(VI).