Sulfate reduction coupled to U(VI) immobilization: Insights from optimization, mineralogical linkage, and sulfur isotope fractionation

Optimized SRB-ethanol synergy removes 91.8% U(VI) and 86.3% sulfate. • Sequential mineralization ensures long-term U stability via reduction and encapsulation. • SRB dominance and sulfur respiration upregulation drive alkalinity, low Eh, and multi-metal co-precipitation. • δ 34 S used as a potential tracer for evaluating the restoration of underground mining environments. The efficacy of sulfate-reducing bacteria (SRB) in remediating uranium-contaminated groundwater hinges on the coupling mechanism between sulfate reduction and U(VI) immobilization, which remains poorly understood. Herein, we confirmed that under optimal conditions (pH 6.0, 0.4% ethanol, 10% inoculation rate), the combined treatment of an acid-tolerant sulfate-reducing bacterial consortium (previously isolated from the uranium mining area and laboratory-enriched) with ethanol achieved 91.8% removal of dissolved U and 86.3% reduction of sulfate, along with significant co-precipitation of dissolved Fe and Al. Mineralogical characterization reveals a sequentially linked mineralization network of UO 2 , FeS, CaCO 3 , and Al(OH) 3 , sustained by the SRB-mediated alkaline effect and the continuous decline in redox potential (Eh). High-throughput sequencing revealed enrichment of taxa affiliated with known sulfate-reducing lineages and upregulation of sulfur metabolism pathways, suggesting enhanced sulfate-reducing potential within the microbial community. Notably, sulfur isotope analysis (δ 34 S) revealed distinctly different apparent fractionation factors (ε) between low and high sulfate removal conditions (ε ≈ 2.15‰ vs. 0.22‰). This contrast tentatively suggests a potential shift in the rate-limiting step of sulfate reduction, although further time-series isotope measurements are required to rigorously validate this hypothesis. This work demonstrates the feasibility of SRB-enhanced remediation for uranium-contaminated groundwater and establishes δ 34 S as a potential geochemical tracer for assessing the effectiveness and long-term stability of remediation measures in underground mining environments. Download: Download high-res image (187KB) Download: Download full-size image