Occurrence, mobility, and potential risk of uranium in an abandoned stone coal mine of Jiangxi Province, China

Metal sulfide oxidation in abandoned exposed stone coal mines leads to the generation of Acid Mine Drainage (AMD), characterized with high uranium concentration, which is a major concern for local public health. This work employed approaches of geochemical analysis and modeling to determined the mode of occurrence of uranium. Additionally, potential environmental risks were evaluated. The results revealed that the primary source of uranium pollutants in the surrounding environmental media was attributed to the weathering of mine wastes. The acidity and concentrations of harmful metals (e.g., U, Fe) and sulfate in water rapidly decreased to background level with increasing distance from the mine. The geochemical distribution characteristics of sediments and water exhibited notable similarities. The species of uranium underwent a transformation as uranium in mine waste rocks migrated to environmental media. In acidic pit water, uranium primarily existed as uranyl sulfate, gradually transitioning downstream to complexes dominated by hydrophosphate and carbonate. This transition was accompanied by the coprecipitation of significant amounts of uranium with phosphate and iron hydroxides. Results from the geoaccumulation index (I_geo) and risk assessment codes (RAC) indicated that uranium in unweathered waste rocks and newly formed pit sediments posed a high environmental risk, with a bioavailable fraction reaching up to 26.44% and 48.0%, respectively. This research holds significant importance in devising remediation and management strategies for abandoned coal mines to mitigate the impact of uranium release and mobility on the surrounding ecological environment.