Leaching mechanisms of potentially toxic elements from coal gangue: Thermodynamic, kinetic, and colloidal effects

Coal mining generates substantial quantities of coal gangue (CG) as a by-product. Limited reuse pathways have led to extensive and long-term surface accumulation of CG. Precipitation intensifies the leaching and migration of potentially toxic elements (PTEs) from CG, posing severe environmental risks to mining ecosystems. However, the migration behavior and environmental impacts of PTEs are poorly understood. This study collected representative CG samples from the middle reaches of the Yellow River Basin, China, and systematically investigated the thermodynamic, kinetic, and colloidal processes governing PTE release. Results indicated that pH was the dominant factor influencing PTE concentrations, and PTE release was primarily controlled by pH-dependent thermodynamic dissolution. Geochemical modeling revealed the principal mineral phases responsible for liquid-solid partitioning under different pH conditions. Kinetic analysis showed that the release of most PTEs (As, Cu, Mn, Pb, Sb, and Tl) was best described by the parabolic model, exhibiting rapid release during the initial stage followed by diffusion-controlled behavior at later stages. The release of Zn was best captured by the second-order kinetic model, whereas the release behaviors of Co, Ni, and Se were best described by the Elovich model. Approximately 6.2-33.4 % of total PTEs existed in colloid-bound form. TEM-EDS and TOF-SIMS analyses demonstrated that Al-Si colloids adsorbed Co, Ni, Pb, Tl, and Zn ions through outer- and inner-sphere coordination, thereby amplifying PTE migration. This study identified As, Co, Cu, Mn, Ni, Pb, Sb, Se, Tl, and Zn as the primary environmental risk elements in CG, with maximum leaching concentrations exceeding national water quality standards by factors of 1.1-46.6. These findings provide critical evidence for pollution control and environmental risk assessment of CG stockpiles in mining areas.