Reoxidation of bioreduced uranium under reducing conditions

Nuclear weapons and fuel production have left many soils and sediments contaminated with toxic levels of uranium (U). Although previous short-term experiments on microbially mediated U(VI) reduction have supported the prospect of immobilizing the toxic metal through formation of insoluble U(IV) minerals, our longer-term (17 months) laboratory study showed that microbial reduction of U can be transient, even under sustained reducing conditions. Uranium was reduced during the first 80 days, but later(100-500 days) reoxidized and solubilized, even though a microbial community capable of reducing U(Vl) was sustained. Microbial respiration caused increases in (bi) carbonate concentrations and formation of very stable uranyl carbonate complexes, thereby increasing the thermodynamic favorability of U(IV) oxidation. We propose that kinetic limitations including restricted mass transfer allowed Fe(111) and possibly Mn(IV) to persist as terminal electron acceptors (TEAS) for U reoxidation. These results show that in-situ U remediation by organic carbon-based reductive precipitation can be problematic in sediments and groundwaters with neutral to alkaline pH, where uranyl carbonates are most stable.