Experimental Study on Adsorption Characteristics of Coal Gangue to Ca2+ in High-Salinity Mine Water

Targeting the purification of high-salinity mine water in ecologically vulnerable mining areas in Western China, this study conducted batch adsorption experiments using coal gangue from goaf areas to investigate the effects of initial Ca2+ concentration, treatment time, pH, temperature, and particle size on Ca2+ removal. The adsorption process was further elucidated through isotherm, kinetic, and thermodynamic modeling. The results demonstrate that the unique slit-shaped/plate-like mesoporous structure of coal gangue provides a favorable physical foundation for adsorption. Batch experiments identified optimal conditions at pH = 8 and 40 °C, achieving a maximum adsorption capacity of 12.4 mg/g. The process followed the Langmuir isotherm model (R2 = 0.994, χ2 = 0.122) and the pseudo-second-order kinetic model (R2 = 0.952, χ2 = 0.057), reaching equilibrium within 120 min. Thermodynamic analysis confirmed a spontaneous, endothermic, and entropy-driven process, with a relatively low heat of adsorption (ΔH < 20 kJ/mol) indicating physical adsorption as the dominant mechanism. Collectively, the adsorption system is characterized as a complex process governed by physical adsorption accompanied by weak chemical interactions and modulated by multiple environmental factors. Three mechanisms (electrostatic interaction, ion exchange, and surface complexation) jointly contribute to Ca2+ adsorption on coal gangue. This study enhances the understanding of the water purification mechanism by coal gangue, provides a theoretical basis for the application of underground coal mine reservoirs, and proposes a novel technical approach to mitigate membrane scaling caused by Ca2+ during mine water treatment.