Medium-temperature co-sintering with clay for the immobilization of heavy metals in MSWI fly ash and associated environmental risk assessment

Municipal solid waste incineration (MSWI) fly ash is a hazardous waste, and traditional landfill disposal lacks sustainability. Resource utilization offers a viable pathway for its future management. Heavy metals are key hazardous components in fly ash, and their stabilization is essential for resource utilization. However, traditional high-temperature treatments are energy-intensive and costly, limiting large-scale application. This study proposed an energy-efficient, medium-temperature treatment method for fly ash and evaluated its environmental risks. Molecular dynamics simulations were conducted to elucidate the underlying mechanisms of heavy metal stabilization. The study revealed that co-sintering fly ash with clay at 750°C and 950°C led to a significant reduction in heavy metal leachability, with Pb and Zn concentrations decreasing by 97.4% and 61.7%, respectively. The sintered products developed new fibrous mineral phases, predominantly wollastonite and rankinite, within which heavy metal ions were incorporated through isomorphic substitution for Ca²⁺ in the crystal lattice, leading to stable immobilization. Sequential extraction analysis showed that the chemical forms of heavy metals shifted from acid-soluble to more stable reducible and oxidizable fractions after treatment. Consequently, the environmental risk levels of Zn and Pb decreased from moderate to negligible, while that of Cd was reduced from high to negligible. Long-term leaching tests under simulated acid rain conditions confirmed that the sintered products maintain high stability during prolonged environmental exposure.