Multidisciplinary Characterization of Uranium Mine Waters: A Bioremediation Perspective.

An intensive uranium mining took place for decades in East Germany. These intensive mining activities have left many uranium contaminated areas. For a remediation purpose, the mine water has to be pumped to the surface and treated by a conventional chemical wastewater treatment plant. However, such chemical treatments are time- and cost-intensive. The resulting release of the soluble uranium into the mine water represents a major health risk. Remediation approaches using indigenous microbial communities are an efficient strategy [1,2]. In this study, we have characterized the microbial diversity and geochemistry of water samples from a german former uranium mine to design bioremediation approach based on uranium enzymatic reduction.

Inductively Coupled Plasma-Mass Spectrometry and Ion-Chromatography studies showed that the mine water exhibited a high concentration of uranium (1.01 mg/L), sulfate (335 mg/L), iron (0.99 mg/L) and manganese (144 mg/L). Cryo-Time-resolved Laser-induced Fluorescence Spectroscopy studies identified an aqueous uranyl carbonate species [UO2(CO3)3]. The 16S and ITS1 rRNA gene sequencing revealed an extensive microbial diversity. The total bacterial community composition indicated a high relative abundance of sulfate-reducing-bacteria (e.g., Desulfovibrio) and iron-oxidizing-bacteria (e.g., Gallionella, Sideroxydans). These bacterial groups are reported to be involved in uranium (VI) reduction as a key process in the bioremediation of anoxic uranium contaminated sites [2].

To design a bioremediation strategy for this uranium-contaminated mine water, the original mine water was used directly as a reference to set up anoxic microcosms. Concretely, uranium-reducing-bacteria were stimulated by glycerol (10mM) as electron donor. ICP-MS and Ion-Chromatography analysis from the microcosms revealed a decrease of uranium (≈90%), sulfate (≈60%), iron (≈86%) and manganese (≈88%). In addition, a drop of Eh and pH of the system was detected. A theoretical thermodynamic Eh-pH predominance diagram was calculated by Geochemist ́s Workbench, indicating the formation of uranium (IV) precipitates, probably uraninite, after 3 months at the latest. Finally, uranium (IV) was detected by UV-Visible spectroscopy in the precipitate at the end of the experiment.

These results show that the uranium reduction of soluble uranium (VI) to insoluble uranium (IV) is favoured by the addition of an electron donor (glycerol) in low concentrated uranium contaminated mines water by biostimulating their native microbial community.