Effects of typical groundwater ions on U(VI) removal by Leifsonia sp.

Coexisting ions in uranium-contaminated groundwater can alter aqueous speciation and interfacial interactions, thereby affecting microbial U(VI) removal and immobilisation. Here, we evaluated how representative ions (Ca²⁺, Mg²⁺, CO₃²⁻, SO₄²⁻, and NO₃⁻) influence U(VI) removal by an indigenous Leifsonia sp. in batch experiments and 45-day simulated-groundwater microcosms. The results showed that Mg²⁺ and Ca²⁺ significantly delayed early-stage U(VI) removal, reducing the removal efficiency by 17.64% and 15.59%, respectively, at 6 mM and pH 6, and the inhibitory effect became more pronounced under higher U loading or lower biomass dosage (≈25%). In contrast, CO₃²⁻, SO₄²⁻, and NO₃⁻ exerted comparatively weaker effects within the tested ranges. FT-IR, SEM-EDS, and XPS analyses showed Mg/Ca enrichment on the cell surface, accompanied by lower surface-associated U and a reduced apparent U(IV) fraction, supporting the interpretation that ionic-strength-related electrostatic shielding, together with additional divalent-cation interfacial interactions, contributed to the inhibition. In the inoculated microcosms, dissolved U(VI) decreased to ∼1.0 mg L⁻¹ and did not rebound over 45 days, suggesting limited remobilisation under the tested conditions. These findings clarify ion-dependent constraints on microbial U(VI) removal and provide guidance for optimising microbial U(VI) immobilisation in Ca/Mg-rich groundwater.