Immobilization of heavy metals from MSWI fly ash using a novel cementitious material: Long-term leaching behavior and mechanism

Little attention has been paid to the long-term leaching hazard of heavy metals in municipal solid waste incineration (MSWI) fly ashes solidified by phosphate-containing cementitious material. Herein, long-term leaching behaviors of Pb, Zn, and Cd in MSWI fly ashes solidified by a novel cementitious material (MSCM) containing phosphoric acid sludge were investigated. During 120-days acid rain leaching test, cumulative leaching concentrations of Pb, Zn, and Cd in MSCM-solidified fly ash increased to 50.4, 3694.0, and 24.0 μg/L, achieving reductions of 51.5, 27.4, and 23.1%, respectively, compared to those in ordinary Portland cement (OPC) solidified fly ash. During 300-years leaching simulation, leaching concentrations of Pb, Zn, and Cd continuously increased, yet remained lower in MSCM-solidified fly ash compared to OPC-solidified fly ash. Mechanism studies revealed that the MSCM promoted significant formation of R₁₀(PO₄)₆(OH)₂ (R²⁺: Pb²⁺, Zn²⁺, and Cd²⁺) and resulted in high contents of C–S–H, AFt, and Friedel’s salt in fly ashes. In contrast, transformation from R–O–Al–O–R to R–O–Al–O–Si bonds (R: Pb and Zn) resulted in higher release of Pb²⁺ and Zn²⁺ in OPC-solidified fly ash. This study demonstrated that the MSWI fly ashes solidified by phosphate-containing material could achieve superior long-term environmental safety owing to its complex leaching control processes.