An integrated experimental and theoretical study on Si/Al-based and Ca-based additives for immobilizing heavy metals during pyrolysis of high-organic solid waste

The high-organic solid waste (HSW), collected in compliance with China’s mandatory waste classification policy, exhibits elevated levels of heavy metals and a low ash composition. Effective management of HSW has become a contemporary focal point, with pyrolysis emerging as a viable method to mitigate secondary pollution and achieve waste-to-resource conversion. To address metallic contamination, the utilization of Si/Al-based and Ca-based additives was investigated to immobilize and stabilize heavy metals in situ during HSW pyrolysis. Experimental results demonstrated that Si/Al-based minerals, notably kaolin, effectively immobilized Cr, Cu, Pb, and Zn, while CaO enriched As and Cd better. The employment of additives enhanced heavy metal stabilization, thereby reducing ecological risks of derived chars. This comparative study could also provide principle for sorbent selection during other solid waste disposal containing heavy metals. Computational simulations explored the active sites and morphological alterations of additives at varying temperatures, establishing a theoretical basis for selecting appropriate pyrolytic parameters. The adsorption processes of kaolin and CaO for heavy metals exhibited opposite electron transfer directions, resulting in the formation of distinct types of covalent bonds. This disparity accounted for the differential adsorption selectivity of Si/Al-based and Ca-based additives towards various heavy metals. This research presented new ideas for developing targeted in-furnace sorbents during pyrolysis.