Immobilization of Uranium at Nanoscale by Bacillus cereus 12-2 at Different U(VI) Concentration

Uranium can be immobilized as nanoscale minerals by biomineralization under aerobic conditions. Current researches on nonreductive biomineralization of U(VI) mainly focus on revealing the mechanisms associated with functional groups and enzymes. However, studies on the effect of initial uranium concentration on the uranium bio-immobilization are relatively rare. This paper researched the immobilization of U(VI) at different concentrations by Bacillus cereus 12-2. Adsorption experiments showed that the adsorption equilibrium was rapidly reached within 10 min when U(VI) concentration was 25 mg/L while over 2 h when U(VI) concentration was 200 mg/L. The biological adsorption capacity improved gradually from 24.68 to 163.17 mg/g (dry weight) as U(VI) concentration increased, but the removal rate of uranium decreased from about 100% to 80%. X-ray diffraction (XRD) showed that nanoscale uramphite ((NH4)(UO₂)PO₄·3H₂O) formation time was 4, 4, 12 and 24 h, respectively when initial U(VI) concentration was 25, 50, 75 and 100 mg/L. No mineral was formed after reaction for 24 h when U(VI) concentration was 200 mg/L. Uramphite had better peaks in spectrum after 50 mg/L U(VI) had interacted with Bacillus cereus 12-2 for 4 h. XRD, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) indicated that a large number of amorphous uranium-containing particles appeared extracellularly after 10 min and nanoscale uramphite formed intracellularly after 4 h when uranium concentration was 50 mg/L. This study showed that the time required for adsorption equilibrium and mineralization by Bacillus cereus 12-2 was shorter when the initial U(VI) concentration was lower within a certain range. This phenomenon could be attributed to the increase of relative content of functional groups and the decrease of cytotoxicity of uranium at low concentrations. This work had certain guiding significance for further understanding the mechanism of uranium biomineralization and the application of Bacillus cereus 12-2 under actual uranium-contaminated environments.