Uranium (VI) reduction in a fixed-film reactor by a bacterial consortium isolated from uranium mining tailing heaps

Biological uranium (VI) reduction was investigated using a mixed-culture of U(VI) reducing bacteria isolated from tailing dumps at an abandoned uranium mine in Pharaborwa (Limpopo Province, South Africa). A fixed film reactor was used in the investigation, whereby the reactor was operated in the up-flow mode under fully submerged conditions at a recirculation ratio of, Q(in)/Q(R) = 20. The performance of the bioreactor was evaluated over a range of influent U(VI) concentrations [75-100 mg U(VI)/L] and 24 h hydraulic retention time [HRT]. Complete U(VI) removal was observed in phases with 30-85 mg/L influent U(VI). When influent U(VI) was increased to 100 mg/L, approximately 60% U(VI) removal was achieved. The oxidation states of reduced uranium species were determined by Scanning and Transmission Electron Microscopy followed by X-ray Diffractometer (SEM/TEM-XRD). Earlier studies in batch systems showed that U(VI) was non-toxic to U(VI) reducing organisms at concentrations up to 400 mg/L. The decrease in U(VI) removal efficiency observed in the fixed-film reactor after 42 days was therefore attributed to the accumulation of U(IV) hydroxide precipitates in the reactor. Genetic identification using the 16S rRNA gene sequence analysis showed that the species Kocuria turfanensis, Arthrobacter creatinolyticus, Bacillus lichenifonnis, and Microbacterium aerolatum survived from the original cultures. The feasibility of continuous removal of U(VI) in an inoculated indigenous culture system was thus demonstrated.