Influence of environmental factors on reductive bioprecipitation of uranium by sulfate reducing bacteria

Abstract

Microbial reduction of soluble uranyl [U (VI)] to insoluble uraninite by sulfate reducing bacteria (SRB) is a promising remediation strategy for uranium-contaminated groundwater. Effects of environmental factors, including pH and coexisting ions, on U (VI) bioreduction processes (UBP) remain unknown. Anaerobic batch experiments were performed to evaluate impact on UBP. Kinetic investigations with varied pH demonstrated that U (VI) was reduced mostly within 48 h. The bioprecipitation yields depended strongly on pH, increasing from 12.9% to 99.4% at pH 2.0 and 6.0, respectively. Sulfate concentration ⩽4000 mg l −1 did not affect UBP; however, sulfate concentration ⩾5000 mg l −1 significantly slowed UBP. Biogenic H 2S produced during sulfate reduction was not directly involved in UBP. At 20 mg l −1 Zn or 10 mg l −1 Cu, no UBP inhibition was observed and uraninite was detected in metal sulfide precipitate. However, 25 mg l −1 Zn or 15 mg l −1 Cu stopped UBP completely. Cu toxicity mechanism probably differed from Zn. The ability to reduce U (VI) was lost permanently with exposure to ⩾15 mg l −1 Cu, but not for Zn ⩾25 mg l −1. No uraninite could be detected before nitrate removal, suggesting nitrate strongly inhibited UBP, which may possibly be related to denitrification intermediates controlling the solution redox potential.