Impact of microbes on U reduction in mine water of former U mines – a remediation perspective

Environmental pollution by heavy metals and radionuclides is one of the biggest challenges which have to be solved globally. In the former U mine of the Wismut GmbH Schlema-Alberoda (Saxony, Germany), mine water is pumped to the surface, where it is treated by a conventional water treatment plant since certain amounts of U (1 mg/L) and other water pollutants can still be found despite remediation by flooding. Microbiological studies of the mine water, using 16S rRNA gene sequencing, revealed a microbial community, which is characterized by a relative abundance of indigenous microbial groups with U(VI)-reduction ability (e.g., sulfur- and iron-oxidizing bacteria and sulfate-reducing bacteria). From previous studies it is known that microbial cycling processes have a significant impact on the complete enzymatic reduction of soluble U(VI) to U(V) and U(IV) by the addition of an electron donor in low U contaminated mine water. In our experiments, a set of anoxic microcosms with mine water were supplemented with glycerol (10mM) as electron donor. The monitoring of the redox potential, pH and the concentration of U, Fe, As and SO₄2^- revealed a substantial decrease of the U(VI) concentration of up to 98%, as well as Fe with up to 91% and SO₄2^- up to 88% after four months, accompanied by a significant change of the redox potential. A thermodynamic Eh-pH dominance diagram calculated using Geochemist's Workbench predicted the reduction of U(VI) and the formation of a solid U(IV)-mineral. The black precipitates, which were formed during the experiments, were analyzed spectroscopically. Extended X-ray absorption fine structure (EXAFS) indicates the formation of immobile uraninite as a U(IV) solid phase. By means of high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) it was even possible to identify U(V), a highly unstable intermediate in the reduction of U(VI) to U(IV) and poorly reported in environmental samples. In electron microscopy and energy-dispersive X-ray studies (SEM/EDXS) U(IV) nanoparticles were probably detected on the surface of calcite crystals, which have formed during the reduction experiment.

The results reveal that the indigenous microbial communities in mine waters are able to modify the speciation and redox state of the soluble U(VI) to insoluble U(V) and U(IV). The in-situ biostimulation of microorganisms could thus offer an eco-friendly water remediation strategy for the management of U contaminated mine water through bioreduction and could support chemical on-site mine water treatments.