Uranium contamination in groundwater is a concern because of the chemical toxicity and the volume of polluted subsurface media at many U.S. Department of Energy sites. One potential option for immobilizing uranium from groundwater is in situ precipitation under enzymatically catalyzed reaction by sulfate reducing bacteria (SRB). This is typically done by amending groundwater with an organic electron donor to stimulate SRB, reducing soluble U(VI) and precipitating it as UO2. However, it has been shown that the precipitated uranium can re-oxidize in the presence of Fe(III)-(hydr)oxide minerals once the electron donor is exhausted. Multi-component biogeochemical simulations (using PHREEQCII and TOUGHREACT) are used to evaluate thermodynamic and kinetic constraints affecting uranium reduction and the subsequent onset of re-oxidation, including biotic and abiotic effects. The effect of nanoscale particle size on the solubility of biogenic UO2 is evaluated and taken into account to determine biogenic UO2 dissolution and chemical fate. For ~3nm particles, the calculated UO2 solubility increases by 3 orders of magnitude compared to bulk uraninite, and agrees closely with reported solubility values for amorphous UO2. It is also shown that Fe(III) oxidizes sulfide preferentially to biogenic UO2. As a result, the relative rates of sulfide and UO2 oxidation by Fe(III) play a key factor on rates and extent of UO2 reoxidizes or not. The amount of Fe(II) in solution is another important factor, with the precipitation of Fe(II) minerals lowering Fe(II) in solution and increasing the potential for UO2 reoxidation. Simulations include formation of U(VI) carbonate and calcium-carbonate complexes and show that the dominance of these species is enhanced by carbonate produced by the degradation of the organic electron donor used for bioreduction.

Uraninite reoxidation by Fe(III)-(Hydr)oxides: Thermodynamic and kinetic considerations

239th National Meeting (2010)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.