pH-dependent behavior of uranium(VI) removal by indigenous bacteria under deep geological repository conditions

Uranium and indigenous bacteria interactions under relevant deep geological repository (DGR) environments remain unexplored even though microorganisms notably influence uranium speciation and mobility in DGR systems. Thus, this study used indigenous bacteria present in deep granitic groundwater to investigate biogeochemical hexavalent uranium [U(VI)] removal behavior, at a pH range of 5–9, under anaerobic conditions amended with 20 mM of sodium acetate for 24 weeks. The 30% U(VI) removal was primarily driven by U(VI) bioreduction at pH 9, which was supported by biogenic uraninite (UO 2 ) formation and a 10-fold increase in dsrA gene copy numbers than the initial sample. Despite the activity of sulfate-reducing bacteria, the presence of ternary Ca–UO 2 –CO 3 complex resulted in incomplete U(VI) removal efficiency at pH 9. At pH 7, the U(VI) removal efficiency rapidly increased to 83% during the first 4 weeks, and then markedly decreased and recovered to the initial U(VI) concentration. The rebound in U(VI) concentration occurred because of U(VI) sorption/desorption on extracellular polymeric substances secreted by biofilm-forming bacteria. A 37% U(VI) removal efficiency was observed at pH 5, attributed to U(VI) adsorption onto Fe(III) minerals. A weak acidic condition resulted in a significant decrease in bacterial diversity and the presence of a UO 2 -acetate complex. Overall, this study provides key implications for predicting uranium mobility and long-term stability in DGR, emphasizing the need to consider pH-dependent microbial processes in the safety assessment of DGR systems. • Distinct U(VI) removal pathways identified at various pH conditions. • Ternary Ca–UO 2 –CO 3 complex incompletely removed U(VI) at pH 9. • U(VI) removal at pH 9 was mainly driven by bioreduction. • Various U(VI) complexes exist under pH 7 conditions. • U(VI) adsorption onto Fe(III) minerals is a key U(VI) removal mechanism at pH 5.