Biological U(VI) reduction in a fixed-bed bioreactor using radiotoxic tolerant mixed-culture of bacteria

Abstract

Nuclear energy has been proposed as an alternative energy source in the bid to reduce carbon emissions that results from fossil fuel. Hexavalent uranium [U(VI)] is the most abandoned radioactive waste discharged from nuclear fuel processing, electricity generation power plants, radioisotope manufacturing plants. Improper disposal or storage of U(VI) containing waste may poses a threat to aquatic systems and related ecological systems. Treatment of U(VI) containing effluents from these industrial activities requires the reduction of highly mobile and radiotoxic U(VI) to tetravalent U(IV) which readily forms the hydroxide precipitate [U(OH)4(s)] under neutral pH conditions. Processes employing biofilm and fixed-film systems for treatment of wastewater are considered more robust than planktonic culture systems in the presence of high toxicity, and therefore, are preferred in the treatment of toxic liquid waste. In this study U(VI) reduction is investigated using a fixed-film reactor inoculated with U(VI) reducing bacteria isolated from tailing dump soil collected from an abandoned uranium mine in South Africa. The fixed-film bioreactor was operated as a continuous flow reactor under oxygen stressed conditions without the addition of external organic carbon source. The results from this study showed that the fixed-bed bioreactor was able to achieve >90 % U(VI) removal efficiency at U(VI) concentration of up to 85 mg/L, concentration which is higher than that observed at the study site. This demonstrates the feasibility of the fixed-bed bioreactor in continuously removing U(VI) in aqueous solutions without the need of introducing external organic carbon source and without re-seeding the bioreactor. The bioremediation technology proposed in this study significantly demonstrated the effectiveness of the biofilm processes in treating process effluent streams and U(VI) contaminated sites as part of the pump-and treat. Fundamental understanding of uranium-bacteria interactions taking place in a complex biofilm structure under such conditions would be effective in developing an appropriate radioactive waste treatment system for the subsurface bioremediation process.