Background The rapid development of industry has resulted in a yearly increase in wastewater discharge, which induces a gradually rising fluorine concentration in water. In this context, removing fluoride using natural materials and industrial waste has emerged as a popular research topic. Coal gangue, one of the largest contributors to industrial waste in China, contains substantial clay minerals, and its pore structure can be effectively improved through modification. This helps significantly enhance its adsorption performance and thus achieve efficient fluoride removal from water.

Methods This study investigated coal gangue from a typical mining area of the Henan Energy and Chemical Industry Group Co., Ltd., accompanied by wastewater with a high fluoride concentration of 200 mg/L. The modification conditions of coal gangue were initially determined through single factor experiments. In combination with the characterization and analysis using scanning electron microscopy (SEM) and organic element analysis, this study comprehensively investigated the adsorption patterns of fluorides by modified coal gangue and underlying defluorination mechanisms.

Results and Conclusions  Original coal gangue and magnesium chloride (MgCl₂) were evenly mixed at a mass ratio of 1∶0.25 and were then roasted at 450 ℃ for 1.5 h, yielding coal gangue-based defluorination materials. Subsequently, oscillatory adsorption experi-ments were carried out under a fluoride solution-to-coal gangue ratio of 20∶1, a temperature of 25 ℃, and a rotation speed of 120 r/min, yielding a defluorination rate of up to 94.451%. These experiments, as well as characterization and analysis, reveal that primary adsorption processes of modified coal gangue for fluoride ions include electrostatic adsorption, hy-drogen bonding, ion exchange, and precipitation combined with complexation. Brucite peaks were observed within the modified coal gangue. Meanwhile, mica-bearing materials and partial kaolinites transitioned into amorphous sili-con-aluminum compounds, while some kaolinites transformed into illites. All these enhanced the electrostatic adsorption and ion exchange within the coal gangue. Beryllium fluoride was observed in the coal gangue post-adsorption, corrobo-rating the adsorption of the modified coal gangue for fluoride ions. Furthermore, the post-adsorption coal gangue exhib-ited a looser overall morphology, elevated specific surface areas, and a slight increase in the relative content of metal ox-ides. These create favorable conditions for metal oxides to remove fluoride ions from water through hydrogen bonding and precipitation combined with complexation. The results of this study provide an important theoretical basis for pre-paring coal-gangue-based agents to treat wastewater.