Release behavior of uranium in waste rock from abandoned stone coal mine: Evidence from static leaching and sequential chemical extraction

The oxidation of sulfide minerals in stone coal waste rock generates uranium-rich acid mine drainage (AMD), leading to severe uranium contamination in the surrounding ecosystem. However, the release behavior of uranium from waste rock remains poorly understood. In this study, static leaching experiments combined with sequential chemical extraction were conducted to investigate the release behavior of uranium and reveal underlying mechanisms. The results demonstrated that the initial pH of the leaching solution and the particle size of the waste rock significantly influenced uranium release. Specifically, a lower initial pH and finer particle size were associated with higher uranium leaching ratios (the proportion of leached uranium based on total U content in the sample), with a maximum leaching ratio of 34.6%. AMD generated from pyrite oxidation in the waste rock was identified as a key driver of uranium release, rather than atmospheric precipitation. The release of uranium was also closely linked to its mode of occurrence. In the raw waste rock, uranium was primarily present in the oxidizable fraction (47.3%), followed by the reducible fraction (27.3%) and the residual fraction (21.4%), while the weak acid-extractable and water-soluble fractions accounted for only 4.0% combined. Scanning electron microscopy coupled with energy-dispersive spectroscopy analysis revealed a strong association between uranium and phosphorus-bearing minerals such as apatite. During the leaching process, uranium was predominantly released from its oxidizable and reducible fractions, while the residual fraction remained relatively stable and resistant to release. The pH level played a critical role in determining both the extent of uranium release and its chemical speciation. PHREEQC simulations indicated that uranium in acidic systems primarily existed in highly soluble and mobile speciation, such as UO2SO4, UO2F+ and UO22+. Fitting results from Fick's model suggested that uranium release were more consistent with surface desorption reaction. According to the shrinking core model, the chemical reaction model showed a stronger correlation with uranium release. These findings provide valuable insights for developing effective management strategies to mitigate the risk of uranium release and contamination from stone coal waste rock. • U release is influenced by environmental factors (e.g., pH, particle size) and its occurrence. • AMD from pyrite oxidation is the primary driver of uranium release. • The release process of uranium were consistent with surface desorption or chemical reaction.