Site-specific competition at mineral interfaces controls heavy-metal release from coal gangue and long-term...

Heavy metals released during the utilization of coal gangue can become a long-term source of soil and water contamination. However, the links among initial occurrence characteristics, long-term release behavior, mineral interfacial processes, and environmental risks remain unclear. In this study, three representative coal gangue samples from western China were investigated by combining sequential extraction, long-term pH-controlled leaching, soil column experiments, SEM/XRD characterization, and density functional theory (DFT) calculations to establish the relationship between heavy-metal release behavior and mineral interfacial mechanisms. The results show that although most heavy metals are present in residual fractions, their release exhibits pronounced elemental differentiation and strong pH dependence. Acidic conditions enhance the release of Cu, Ni, Pb, Zn, and Cd through mineral dissolution and exposure of reactive surfaces, whereas alkaline conditions promote the migration of As by altering surface states. Dynamic leaching experiments reveal stage-dependent release and secondary activation processes, indicating that short-term leaching tests may underestimate long-term migration risks. DFT calculations show that Si-O sites preferentially stabilize As, Cu, Ni, and Pb, whereas Al-OH sites exhibit stronger affinity for Zn and Cd. Combined with multi-metal adsorption experiments, these results indicate that site-dependent interfacial retention contributes to element-specific heavy-metal release. These findings indicate that site-dependent adsorption stability at mineral interfaces provides a mechanistic basis for interpreting the element-specific release behavior of heavy metals from coal gangue, and the divergence of long-term environmental risks in coal gangue. Based on this mechanism, a "condition-mechanism-classification-strategy" framework is proposed for risk control, providing a scientific basis for prioritized utilization, long-term monitoring, and targeted stabilization.