Thermal gradient-driven heavy metal speciation and risk evolution in municipal and industrial sludge: a 300–700 °C pyrolysis perspective

Abstract

This work investigated the effect of pyrolysis temperatures (300–700 °C) on the speciation distribution and environmental risks of heavy metals (Cu, Zn, Pb, Cd, Ni, Cr) in biochar derived from municipal sludge (MS) and industrial sludge (IS). Through analyses of characterization, heavy metal content, sequential extraction experiment, leaching experiments, and ecological risk assessment, high-temperature pyrolysis promoted the migration of heavy metals into stable forms. The stabilization mechanism primarily relied on the increased specific surface area and porosity of biochar, leading to surface adsorption and pore-filling effects. In contrast, the stabilization of Cu and Ni under low-temperature pyrolysis was closely associated with complexation involving N/O-containing functional groups. The leaching concentration of heavy metals in pyrolytic biochar exhibited a gradual decrease with increasing pyrolysis temperature, and the potential ecological risk index decreased compared to raw sludge. The results demonstrated that high-temperature pyrolysis technology can effectively achieve the stabilization and resource recovery of heavy metals in sludge.