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

• Published in: Journal of hazardous materials 2021-06, Vol.411, p.125140-125140, Article 125140
• Main Authors: Xie, Jinchuan, Wang, Jinlong, Lin, Jianfeng
• Format: Article
• Language: English
• Subjects: Bioreduction reaction; Calcium; Carbonate; S. putrefaciens; U(VI)
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2021.125140

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 uranyl-carbonate-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 Eho(CaUO2(CO3)32−/UO2)=−0.5801V and Eho(Ca2UO2(CO3)3(aq)/UO2)=−0.6797V 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. [Display omitted] •Bioreduction of the uranyl-carbonate-calcium complexes was favorable at specific pH.•At pH ~6.9, the removal of U(VI)aq was unfavorable at the high CaCl2 (2.5–6.0 mM).•At pH ~6.0 and pH ~7.9, the removal of U(VI)aq was favorable even at the high CaCl2.•The high removal of U(VI)aq was due to the bioreduction instead of the biosorption.•The bioreduction was confirmed by the characterization of XPS, HRTEM and UV–vis.