Mechanism of Cu migration and transformation during pyrolysis of Copper-Bearing sludge regulated by...

Effective stabilization of heavy metals during thermal conversion is critical for the safe disposal of solid wastes. In this study, two types of Cu-enriched sludge containing CuCl₂ or CuSO₄ were examined. A multidimensional experimental framework was established that coupled pyrolysis temperature (600-800 ℃) with organic (cellulose) and inorganic (kaolinite) additives. This approach systematically elucidated the mechanisms governing Cu migration, transformation, and stabilization during sludge pyrolysis. The results indicated that pyrolysis temperature was the dominant factor controlling Cu immobilization. At low to intermediate temperatures, Cu immobilization primarily occurred through complexation with oxygen- and nitrogen-containing functional groups and Cu-π interactions with aromatic structures in the char matrix. At higher temperatures, Cu stabilization relied on mineralization reactions forming stable crystalline phases (e.g., CuS and Cu₂S), accompanied by lattice incorporation, resulting in durable immobilization. The additives exhibited distinct regulatory effects, with cellulose enhancing Cu stabilization at intermediate temperatures by complexing with pyrolytic fragments. In contrast, kaolinite markedly promoted inorganic mineral formation at elevated temperatures, resulting in stronger immobilization. The initial chemical speciation of Cu also influenced its thermal stability. CuSO₄-bearing sludge more readily formed stable sulfides, exhibiting superior high-temperature immobilization compared with CuCl₂ and showing enhanced mineralization-based stabilization when combined with kaolinite. This study elucidates the mechanisms for Cu stabilization during sludge pyrolysis under the combined influence of temperature and additives, clarifies distinct solidification pathways regulated by organic and inorganic components, and provides a theoretical basis and process guidance for the safe thermal treatment, leaching-risk reduction, and resource-oriented utilization of Cu-bearing solid wastes.