Effect of Mn(II) on the Oxidative Dissolution of Biogenic UO2

Bioremediation of U(VI) contaminated sites is predicated on the stability and long-term immobilization of the bioremediation product: biogenic uraninite (UO2). Preventing re-oxidation of UO2 is an important aspect governing the success of bioremediation. The goal of the study was to evaluate the effect of divalent cations on the structure and reactivity of biogenic UO2. Thus, UO2 was produced biologically in the presence of Mn(II), a divalent cation ubiquitous in the groundwater. U(VI) reduction was carried out at two different pH values (6.3 and 8.0). The presence of 1mM or 5mM Mn(II) did not significantly affect U(VI) reduction. A method involving alkaline treatment followed by organic phase separation was developed to separate UO2 from the biomass in order to perform spectroscopic and reactivity characterization. Characterization techniques included imaging with conventional TEM, BET determination of surface area, adsorption and dissolution experiments, and synchrotronbased analyses (XANES, EXAFS, WAXS spectroscopy). Sorption experiments showed uptake of Mn(II) by UO2 at near neutral and alkaline pH, but rule out significant sorption at and below pH 5. Thus, Mn(II) sorbed onto biogenic UO2 during its synthesis was removed by multiple washes using water buffered with MES at pH 5 until no further Mn(II) could be desorbed. A mass balance study (involving acid digestion) and EXAFS measurements indicated that, prior to the pH 5 wash, the majority (~92%) of Mn(II) associated with UO2 was adsorbed. However, after the wash, Mn(II) was found to be predominantly incorporated into the UO2 crystal structure. To date, oxidative dissolution experiments using air as an oxidant indicate that the presence of Mn(II) does not significantly affect biogenic UO2 dissolution rates at pH 6.3, but that at alkaline pH values, Mn(II) accelerates oxidative dissolution significantly