Microbially induced reduction of Uranium in contaminated mine water for bioremediation purposes: A multidisciplinary approach study

The legacy of the former uranium (U) mining in Saxony and Thuringia (Germany) still shows uranium concentrations, e.g., in the mine water of some mines. The present study describes the biostimulation of the native U reducing microbial community in a U contaminated mine water as an efficient and eco-friendly strategy for in situ bioremediation to prospectively support or outperform chemical water treatments.

The microbial community was characterized by 16S and ITS1 rRNA gene analyses, showing a relative abundance of native microbial groups with the ability to alter the speciation and redox state of soluble U (e.g., _{Desulfovibrio}, Gallionella, Penicillium and Aspergillus). Additionally, Inductively Coupled Plasma-Mass spectrometry (ICP-MS) and Ionic Chromatography (IC) were used to determine geochemical profile of the mine water, exhibiting a notable concentration of U (1.01mg/L), SO₄ ^2- (335mg/L), Fe (0.99mg/L) and Mn (1.44mg/L). Cryo-Time-Resolved Laser Fluorescence spectroscopy (cryo-TRLFS) and Parallel Factor Analysis (PARAFAC) determined the aqueous species Ca₂UO₂(CO₃)₃ ^4- as the main U species in mine water. A set of anerobic microcosms, supplemented with glycerol (10mM) as electron donor to stimulate U reducing bacteria, were designed as basis of an in situ bioremediation strategy for uranium contaminated waters. A thermodynamic Eh-pH dominance diagram calculated using Geochemist's Workbench predicted the reduction of U(VI) and the formation of the solid U-ore (uraninite). At the end of the experiment, ICP-MS and Ion-Chromatography analysis from the microcosms revealed a decrease of U (≈98%), Fe (≈91%) and SO₄ ^2- (≈88%). Furthermore, the black precipitate formed at the bottom of the microcosm was analyzed by High Energy Resolution Fluorescence Detected Near-edge X-ray absorption fine structure (HERFD-XANES) and Extended X-ray absorption fine structure (EXAFS) identifying mainly U(IV) (≈80%) and U(V) (≈20%).

The results obtained revealed that microbial cycling processes have a significant impact on the complete enzymatic reduction of soluble U(VI) to U(IV) and U(V) by the addition of an electron donor in low U concentration contaminated mine water. Therefore, this methodology could be an efficient bioremediation approach for the management of U contaminated mine water, as well as low U contaminated mine water scenarios, through the biostimulation of its indigenous microbial community.