pH-dependent microbial reduction of uranium(VI) in carbonate-free solutions: UV-vis, XPS, TEM, and thermodynamic studies.

U(VI)aq bioreduction has an important effect on the fate and transport of uranium isotopes in groundwater at nuclear test sites. In this study, we focus on the pH-dependent bioreduction of U(VI)aq in carbonate-free solutions and give mechanistic insight into the removal kinetics of U(VI)aq. An enhancement in the removal of U(VI)aq with increasing pH was observed within 5 h, e.g., from 19.4% at pH 4.52 to 99.7% at pH 8.30. The removal of U(VI)aq at pH 4.52 was due to the biosorption of U(VI)aq onto the living cells of Shewanella putrefaciens, as evidenced by the almost constant UV-vis absorption intensity of U(VI)aq immediately after contact with S. putrefaciens. Instead, the removal observed at pH 5.97 to 8.30 resulted from the bioreduction of U(VI)aq. The end product of U(VI)aq bioreduction was analyzed using XPS and HRTEM and identified as nanosized UO2. An increasing trend in the biosorption of U(VI)aq onto heat-killed cells was also observed, e.g., ~ 80% at pH 8.38. Evidently, the U(VI)aq that sorbed onto the living cells at pH > 4.52 was further reduced to UO2, although biosorption made a large contribution to the initial removal of U(VI)aq. These results may reveal the removal mechanism, in which the U(VI)aq that was sorbed onto cells rather than the U(VI)aq complexed in solution was reduced. The decreases in the redox potentials of the main complex species of U(VI)aq (e.g., [Formula: see text] and [Formula: see text]) with increasing pH support the proposed removal mechanism.