Migration and retention mechanisms of cadmium in coal gangue reclamation soils: Insights from isotopic fractionation

Heavy metals present in coal gangue can migrate to surrounding soils through atmospheric deposition and rainwater leaching, resulting in soil heavy metal contamination. This study measured the Cd content and isotopic composition (δ¹¹⁴/¹¹⁰Cd) of surface and profile soils in a coal gangue reclamation area in eastern China and examined the impact of coal gangue reclamation on soil physicochemical properties, Cd enrichment, and retention. The results indicate that coal gangue reclamation substantially influences Cd enrichment in the soil (mean: 0.47 mg·kg⁻¹), with notable isotopic fractionation observed at different soil depths. The δ¹¹⁴/¹¹⁰Cd values display complex trends with increasing depth across various soil profiles, and the migration behavior of Cd within the soil profile is primarily governed by iron and manganese oxides. Furthermore, the significant correlation between soil pH and Cd speciation suggests that alkaline conditions markedly affect Cd mobility. Source analysis results reveal that coal gangue is the predominant source of Cd in the study area's soil. Cd pollution is not confined to the surface layers of the upper soil profiles but progressively migrates downward with depth, forming stable residual states with coal gangue oxides. These findings provide a scientific basis and theoretical support for the prevention and remediation of soil Cd pollution in coal gangue reclamation areas.