Fabrication of the Active Microscale Zero-Valent Iron-CaO Synergy System for the Treatment of Phosphorus-Containing Wastewater

Phosphogypsum stockpiles are significant sources of phosphorus pollution, especially during the flood season, generating phosphorus-rich acidic wastewater that exceeds the China GB 3838–2002, Class III standard (P ≤ 0.2 mg/L, pH = 6–9, applicable to China). This study developed a microscale zerovalent iron–calcium oxide synergy system (mZVI/Fe(II)/CaO) to treat on-site wastewater (P = 144.0 mg/L, pH = 2.43). Laboratory results showed that adding 0.39 g/L CaO and 10.0 g/L mZVI/Fe(II) reduced phosphorus to below 0.2 mg/L in 40 s, while maintaining a pH between 8 and 9. Mechanistically, the system removed phosphate by forming insoluble precipitates, including Fe₃(PO₄)₂·8H₂O, FePO₄, and CaHPO₄·2H₂O. Additionally, CaO created an alkaline environment that altered the chemical form of the phosphate, thereby enhancing the adsorption capacity of mZVI/Fe(II) for phosphate. Leachate analysis after phosphorus removal by mZVI/Fe(II)/CaO showed compliance with the surface water Class III standard, indicating that its treatment would not cause secondary pollution to surface water bodies. The field experiment (wastewater flow rate = 150 m³/h) confirmed that the mZVI/Fe(II)/CaO ensured that both phosphorus concentration and pH reached the surface water Class III standard within 40 s and maintained stable phosphorus removal for 1–2 h. This study provides a feasible technical solution for the emergency treatment of phosphorus-containing wastewater.