PNNL

Abstract

Reductive bioremediation is currently being explored as a possible strategy for uranium contaminated aquifers such as the Old Rifle site (Colorado, USA). The stability of U(IV) phases under oxidizing conditions is a key to the performance of this procedure. Biogenic uraninite, a bioreduction product for which kinetic and thermodynamic parameters are known, was deployed into wells using a novel membrane-walled cell to observe the rates and mechanisms of oxidative dissolution in situ in aquifers with different dissolved oxygen conditions. Observed in-aquifer dissolution rates were at least 50 to 100 times lower than measured in laboratory continuous flow reactors with artificial ground water. Upon retrieval, uraninite was found to have similar structure and stoichiometry as the parent material and to be free of U(VI), indicating dissolution occurs via oxidation and removal of surface atoms. Reactive transport modeling suggests that molecular diffusion is likely to be an important factor in limiting the rates of in-aquifer oxidation, and the presence of biomass enhances this effect. Interaction of ground water solutes such as Ca2+ or silicate with uraninite surfaces also may retard in-aquifer U loss rates. These results constrain in-aquifer oxidation rates and indicate U(VI) is more stable in oxic ground water than previously expected.