Dynamic and static migration of heavy metals and mineral-regulated immobilization mechanisms during co-combustion of sewage sludge and high-ash coal

Heavy metals (HMs) pollution limits large-scale utilization of sewage sludge ( SS )-high ash coal (HC) co-combustion as an efficient solid waste treatment strategy. This study employed ICP-MS, BCR sequential extraction method, and time-resolved combustion experiments (0–540 s) to elucidate migration behavior of Pb, Cd, Zn, Cu, Cr, and Ni during co-combustion, revealing that synergistic interactions regulate HM migration through mineral-mediated pathways, and clarifying dynamic coupling between HMs and targeted minerals and organic components. Results showed that co-combustion suppressed HMs volatilization and promoted conversion to more stable forms, with 50% SS blend at 900 °C achieving the highest stabilization efficiency, accompanied by a decline in ecological risk. HMs stabilization was attributed to the synergistic effects of physical encapsulation and chemical bonding: SS promoted high-temperature fuel fusion and the formation of a glassy phase that encapsulates HMs, while mineral components chemically adsorbed and stabilized HMs in bottom ash. CaO exhibited a three-stage effect on HMs release: suppression at low temperature, promotion at intermediate-high temperature, and renewed suppression at higher temperature due to core-shell structure formation. FeS 2 enhanced Pb enrichment by 22.96% through suppressing sulfate decomposition. HMs release followed distinct stages: Pb and Cd volatilized rapidly in the initial phase (0–60 s) with KCl decomposition, whereas Cu and Zn peaked later (120–240 s), concurrent with the combustion of Na, Mg, and fixed carbon. The migration of HMs resulted from the dynamic coupling of gasification and molten-phase encapsulation under chlorination, sulfidation, silico-alumination, calcification, and ferritization effects. These findings provide a theoretical basis for optimizing HMs solidification in clean waste incineration processes.