The effect of U(VI) bioreduction kinetics on subsequent reoxidation of biogenic U(IV)

Microbially mediated in situ reduction of soluble U(VI) to insoluble U(IV) (as UO 2) has been proposed as a means of preventing the migration of that radionuclide with groundwater, but preventing the oxidative resolubilization of U has proven difficult. We hypothesized that relatively slow rates of U(VI) bioreduction would yield larger UO 2 precipitates that would be more resistant to oxidation than those produced by rapid U(VI) bioreduction. We manipulated U(VI) bioreduction rates by varying the density of Shewanella putrefaciens CN32 added to U(VI) containing solutions with lactate as an electron donor. Characterization of biogenic UO 2 particles by extended X-ray absorption fine-structure spectroscopy and transmission electron microscopy revealed that UO 2 nanoparticles formed by relatively slow rates of U(VI) reduction were larger and more highly aggregated than those formed by relatively rapid U(VI) reduction. UO 2 particles formed at various rates were incubated under a variety of abiotically and biologically oxidizing conditions. In all cases, UO 2 that was formed by relatively slow U(VI) reduction was oxidized at a slower rate and to a lesser extent than UO 2 formed by relatively rapid U(VI) bioreduction, suggesting that the stability of UO 2 in situ may be enhanced by stimulation of relatively slow rates of U(VI) reduction.