Insights into the Structural Evolution and Heavy Metal Immobilization in a CaO–Fe₂O₃–SiO₂ System during Multi-Source Solid Waste Co-Vitrification: Integrated Experimental and Theoretical Study

A novel pathway for the co-disposal of municipal solid waste incineration fly ash (MFA) and bottom slag (BS) with waste glass (WG) was proposed. Combined melting experiments and theoretical calculation were employed, which revealed the structural evolution of the CaO-Fe₂O₃-SiO₂ system and the underlying mechanisms of heavy metal immobilization. It was demonstrated that stable glassy products were obtained within a CaO/(Fe₂O₃+SiO₂) mass ratio ( M C / ( SF ) $M_{C/(\mathit{SF})}$ ) range of 0.13~0.41. The leaching concentrations of heavy metals in these vitrified products were suppressed to the relevant regulatory standards. With an increasing M C / ( SF ) $M_{C/(\mathit{SF})}$ , the glass network became more depolymerized, as evidenced by a higher population of low-connectivity Qⁿ units in the structure. However, under Fe₂O₃-enriched system, the Fe coordination shifted from solely tetrahedral [FeO₄] towards a mixed tetrahedral/octahedral environment. Moreover, Pb and Cd were incorporated into the glass matrix via interstitial doping. The depolymerized network dominated by CaO favored the Pb enrichment. Cd tended to be stabilized in the Fe-rich structure, facilitated by the formation of highly localized Cd-O bonds. This study elucidated the coupling effects of oxide composition on glass structure and heavy metal immobilization behavior, providing a fundamental theoretical basis for understanding and optimizing the vitrification process for solid waste treatment.