Microbially influenced uranium biomineralization and mineralogical changes in a U(VI)/carbon steel/bentonite system for repository safety assessment.

Predicting long-term uranium (U) behavior in engineered barrier systems is vital for Deep Geological Repository (DGR) safety. This study examines the evolution of microbe-uranium-bentonite interactions across two stages: a four-year anoxic stabilization (Phase I) and a simulated disturbance involving carbon steel (C-steel) corrosion and a new U(VI) spike (Phase II). X-ray diffraction (XRD) analysis revealed a transition from calcium- to sodium-saturated smectite, increasing bentonite swelling capacity and maintaining a carbonate-rich environment in both phases. Multi-analytical characterization using XRD, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) identified U(VI) carbonates (sharpite, bayleyite) and uranyl phosphates (phurcalite) in both phases, suggesting that uranium immobilization was driven by biomineralization processes. 16S rRNA analysis showed that U(VI) exposure in Phase II enriched taxa associated with U-sequestration. Specifically, Pseudomonas and Bacillus facilitated phosphate-mediated U-precipitation via glycerol-2-phosphate hydrolysis, while Desulfobacterota and other acetate-oxidizing taxa promoted carbonate biomineralization. Although C-steel maintained reducing conditions and enriched metal-reducers (e.g., Desulfovibrio, Citrifermentans), no microbial U(VI) reduction was detected by XPS and XRD. This suggests that in inorganic phosphate and carbonate-rich DGR environments, U is predominantly associated with biomineralized phosphate and carbonate phases, while microbial U(VI) reduction, if occurring, is below the detection limits of the analytical techniques employed under the studied conditions. These results indicate that chemical perturbations can stimulate a synergistic microbial response that enhances the mineralogical stability of uranium through dual phosphate and carbonate pathways.