Utilizing Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> and calcium superphosphate fertilizer for the passage of heavy metals in coal gangue: Mechanical insights and efficiency evaluation

Coal gangue is a typical industrial waste, which will bring environmental challenges and resource depletion when piled up in large quantities, and heavy metal pollution is a prominent problem. In this study, Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> and calcium superphosphate fertilizer (CSF) were utilized to passivate coal gangue at different pollution levels, and its passivation mechanism was discussed. The results show that both Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> and CSF can effectively passivate coal gangue and effectively reduce moderate or high levels of coal gangue pollution to a slight or clean level. The passivation effect of CSF is slightly better than that of Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>. Through characterization of coal gangue before and after passivation and simulation with Visual MINTEQ and PHREEQC software, it is found that Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> can provide PO3<sup>2-</sup>, while CSF releases HPO3 –, which is ionized to generate more PO3<sup>2-</sup>, and finally forms insoluble or slightly soluble phosphate with heavy metals. Leaching experiments show that the treated samples contain obviously more stable heavy metal components, and the leaching risk is lower than that of untreated coal gangue. In summary, this method proves its effectiveness in fixing heavy metals in coal gangue and provides an effective method for the harmless treatment of coal gangue.