New insights into the role of calcium in the bioreduction of uranium(VI) under varying pH conditions

The effect of calcium in the uranium-contaminated groundwater on U(VI)aq bioreduction remains uncertain. Some studies indicated that the presence of calcium may inhibit the bioreduction. However, our calculations show the negative standard molar Gibbs free energy of reduction. The bioreduction of the ternary uranylcarbonate-calcium complexes seems thermodynamically favorable at specific pH. Sorption and reduction experiments were conducted to gain new insights of calcium into the bioreduction. The results show that the complexes were greatly reduced by Shewanella putrefaciens in the slightly acidic pH -6.0 and alkaline pH -7.9 solutions with the relatively high CaCl2 (1.0?6.0 mmol/L) although the reduction was difficult at the nearly neutral pH -6.9. At pH -6.9, the removal percentage of U(VI)aq decreased from 97.0% to 24.4% with increasing CaCl2 from 0 to 6.0 mmol/L, in contrast to the increasing percentage from 50.9% to 89.7% at pH -7.9. The obvious removal of U(VI)aq was ascribed to the bioreduction instead of the biosorption, as evidenced by XPS, HRTEM and UV?vis spectra. The calculations such as Eoh(CaUO2(CO3)2-3 /UO2) = -0.5801 V and Eoh(Ca2UO2(CO3)3(aq)/UO2) = -0.6797 V partially accounted for the reduction mechanisms. Accordingly, the U (VI)aq bioreduction is a promising method to remediate the groundwater even rich in calcium and carbonate.