Hydrothermal treatment coupled with pyrolysis and calcination for stabilization of electroplating sludge: Speciation transformation and environmental risk of heavy metals

Electroplating sludge (ES) produced from treatment of electroplating wastewater is a hazardous waste due to its high content of heavy metals (HMs). This study investigates the feasibility of hydrothermal treatment (HT) coupled with pyrolysis and calcination as a method for safe disposal of ES by immobilizing the soluble fractions of target HMs in ES. The HMs before and after thermal processing were characterized to better understand their speciation transformation and environmental risk. Results showed that over 74% of HMs in ES were accumulated in the resulted solid residues and the other HMs were mainly released into the gas phase. The immobilization rates of HMs from the soluble fractions (F1 and F2) to stable fractions (F3 and F4) after the separate HT and HT coupled pyrolysis and calcination were up to 82.4%, 78.0% and 80.5%, respectively. HT coupled with high-temperature calcination outperformed HT in terms of converting low volatile HMs to stable residual speciations, such as Cu and Ni. HT coupled with pyrolysis showed the best effect in reducing the environmental risks of Cr. In terms of ecological risk index, the separate HT demonstrated an ideal immobilization effect and toxicity reduction for soluble fractions of HMs, especially for Zn and Mn.