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.