Adaptive Responses and Uranium Immobilization Mechanisms of Indigenous Rhizobacterium Alcaligenes sp. A4 from a Uranium-Contaminated Rhizosphere

Uranium contamination from mining activities poses a long-term risk to soil and water because of its toxicity and environmental mobility. In this study, strain A4 was isolated from the rhizosphere of Miscanthus sinensis growing at a uranium-affected site. Analysis of a 1442 bp 16S rRNA gene sequence assigned the isolate to Alcaligenes sp. Strain A4 remained capable of growth in the presence of up to 800 mg L−1 U(VI). In the single-factor experiments, the highest removal efficiencies were obtained at pH 6.0 (71.14 ± 0.83%), 30 °C (84.49 ± 0.73%), an initial U(VI) concentration of 20 mg L−1 (84.49 ± 0.73%), a contact time of 12 h (90.42 ± 0.03%), and a biomass dosage of 0.5 g L−1 (84.49 ± 0.73%). Adsorption capacity increased with initial U(VI) concentration, reaching 107.59 ± 0.40 mg g−1 at 100 mg L−1. The Langmuir model gave a fitted maximum capacity of 137.73 ± 5.13 mg g−1 and described the equilibrium data well (coefficient of determination (R2) = 0.986). The pseudo-second-order model provided a better fit to the kinetic data than the pseudo-first-order model (R2 = 0.989 vs. 0.969; fitted equilibrium adsorption capacity (qe) = 39.219 ± 0.506 mg g−1 vs. measured 12 h capacity = 36.167 ± 0.013 mg g−1). The quadratic response-surface model accounted for 79.1% of the variation in removal efficiency. Microscopic and spectroscopic analyses revealed clear uranium-induced changes, confirmed uranium association with the biomass surface, and implicated oxygen-containing functional groups in uranium binding. Overall, strain A4 combined high uranium tolerance with effective dissolved-uranium removal, highlighting its potential for further development in uranium-contaminated environments.