Characterization of groundwater and cores in the decommissioned acid in-situ leach uranium mining area: Enlightenment for uranium contaminated groundwater remediation

Acid In-situ Leach (AISL) is predominant technique for uranium mining in sandstone-type deposits in China. However, its impact on the biogeochemical conditions of sandstone uranium deposits remains poorly understood. In this study, we performed detailed physicochemical analyses of groundwater, as well as mineralogical, chemical and biogeochemical analyses of cores collected from the decommissioned AISL uranium mining area in the Yili Basin, northwestern China. After AISL uranium mining, U(IV) minerals and pyrite were oxidized and dissolved. The groundwater in the AISL-mined uranium deposit (AISLMUD) became highly acidic and oxidizing after mining ceased. Pyrite, which significantly influences the migration, transformation and immobilization of uranium, was minimal in all cores. Kaolinite, chlorite, quartz, K-feldspar, albite and muscovite exhibited poor adsorption capacity of U(VI) under strong acid conditions. The AISLMUD demonstrated limited capacity to retain U(VI) under highly acid conditions. Furthermore, high-throughput sequencing analysis revealed a substantial reduction in both microbial diversity and abundance in the cores. Microorganisms associated with U(VI) reduction were detected in the cores, with the exception of sulfate reduction bacteria. However, the results of microbial incubation showed that U(VI) reduction-related bacteria could not be in-situ activated by the addition of electron donors under the environmental stresses induced by prolonged acid in-situ leaching. These findings cast doubt on the feasibility of remediating uranium-contaminated groundwater with a pH below 3 through insitu activation of U(VI) reduction-related bacteria. This research holds significant implications for evaluating the suitability of existing methods and developing new approaches for the restoration of AISLMUD.