Comparison and mechanism analysis of MgO, CaO, and Portland cement for immobilization of heavy metals in MSWI fly ash

The significant production of municipal solid waste incineration fly ash (MSWI FA) underscores the importance of developing efficient solidification materials. This study employed MgO and CaO for immobilizing MSWI FA (with a 70% fly ash incorporation), and the immobilization effect was compared with that of Portland cement (PC). Experimental findings revealed that MgO exhibited the most effective stabilization for heavy metals (Cd, Cu, Pb, and Zn) compared to CaO and PC. XRD, FTIR, TG, and SEM analysis indicated that the principal hydration products in MSWI FA binders solidified with MgO, CaO, and PC were Mg(OH)₂, CaCO₃, and C-S-H gel, respectively. Mg(OH)₂ efficiently immobilized heavy metals through chemical complexation and surface adsorption mechanisms. The MgO-treated MSWI FA demonstrated the highest residual fractions and the lowest easily leachable fractions. Moreover, the leaching characteristics of heavy metals were significantly influenced by the pH level, so MgO-treated MSWI FA with a leachate pH of 9.18 achieved the precipitation and stabilization of most heavy metals. In summary, this study provided an effective material selection for MSWI FA immobilization and presented a novel strategy for MSWI FA management.