Biological uranium (VI) reduction in suspended culture and fixed-media bioreactor systems using indigenous isolates of uranium (VI) reducing bacteria

Abstract

Tailing dumps and process waste stockpiles at uranium mining sites and nuclear power processing facilities contain significant levels of uranium. Uranium in the tailing dumps can exist either as U(VI) or U(IV) depending on the pH and redox conditions within the dump. However, it is desirable to keep uranium in the dump sites in its tetravalent form, U(IV), since the hexavalent form, U(VI), is highly mobile and very toxic to aquatic life forms and humans. Natural attenuation processes such as bacterial reductive/precipitation and immobilization of soluble uranium emerge as viable method for remediating U(VI) contaminated sites. For example, dissimilatory metal-reducing bacteria (DMRB) have been investigated for their capability to remove uranium from aqueous solutions. These bacteria were able to use U(VI) as an electron acceptor thereby reducing U(VI) to U(IV) which is easier to remove from solution by precipitation. In this study, the efficiency of indigenous culture of bacteria from the local contaminated site in reducing U(VI) was evaluated using both batch and continuous flow bioreactor systems. Because the stability of uranium in the tailing dumps and stockpiles of uranium concentrate at uranium mining fields is affected by the pH, redox potential, the presence of complexing anions in the waste rocks, toxic metals, organics, inhibitors, and chelators, the effect of these factors in U(VI) bioremediation process was also evaluated in this study. Batch kinetics studies showed near complete U(VI) removal of up to 400 mg/L. Experiments on suspended culture bioreactor system conducted in 10 L Erlenmeyer’s flask under shock loading conditions also showed U(VI) removal of up 400 mg/L. Higher U(VI) removal rates achieved in a suspended culture system operated without re-inoculation were associated with continuous addition of nutrients and glucose in a bioreactor over time. This demonstrate the effectiveness of carbon source and nutrients in enhancing U(VI) reduction process in bioreactor systems. Further experiments were conducted in a fixed-film, continuous flow bioreactor system to evaluate the capacity of the indigenous mixed culture in reducing U(VI) under oxygen stressed and nutrient deficient conditions. The experiments in the fixed-film bioreactor system were conducted using columns with four equally spaced intermediate sampling ports along the length to facilitate finite difference modelling of the U(VI) concentration profile within the column. Near complete U(VI) removal of up to 85 mg/L was achieved in the fixed-film bioreactor operated without organic carbon source. At higher U(VI) feed concentration of 100 mg/L the bioreactor system was able to achieve the removal efficiency of 60%. A sterile control column on the other hand showed insignificant U(VI) removal over time, indicating U(VI) removal by biochemical processes. The shift in microbial culture was monitored in the fixed-film bioreactor after 99 days of exposure to U(VI) using the 16S rRNA genotype fingerprinting method. The fate of U(VI) within a complex biofilm structure was predicted and evaluated using mathematical modelling. The mathematical model developed in this study for describing the biofilm system incorporated both the mass transport kinetics, microbial growth kinetics, and reduction kinetics, thus the diffusion-reduction equation. The model successfully captured the trends of U(VI) removal within the biofilm for different loading conditions. The validity of the model in predicting U(VI) reduction within the bench-scale biofilm reactor at various U(VI) concentrations demonstrated the feasibility of the model in predicting field scale system and improving design and operation of site for clean-up.