[Immobilization of Heavy Metals in Municipal Sludge by Co-pyrolysis of Shaddock Peel and Sludge].

Abstract

Co-pyrolysis with other biomass is a promising method for municipal sludge treatment and has attracted great attention. However, the dominant mechanism by which the heavy metals in municipal sludge are immobilized during the co-pyrolysis remains unknown. In this study, municipal-sludge biochar was prepared by pyrolysis and co-pyrolysis, and the effects of pyrolysis temperature （400-800 ℃） and the addition of shaddock peel on the properties of biochar, the contents of heavy metals （Cu, Zn, Pb, Cd, Ni, and Cr）, and their environmental risks were investigated. Based on the analysis of characterization results and heavy metal contents in the biochar, it was observed that co-pyrolysis promoted the formation of stabilized crystalline minerals （e.g., CdPbO₃, Pb₅（PO₄）₃OH, CuCl, and ZnS）, which reduced the potential risk of heavy metals in biochar. Furthermore, aromatic groups were detected and could interact with heavy metals through cation-π interaction. Further analysis revealed that the immobilization was enhanced by the complexation between heavy metals and the functional groups in biochar, such as -OH and -CO-NH-, which played the main role in the stabilization of Cu and Ni at low pyrolysis temperatures. However, surface sorption and pore filling, due to the increase in specific surface area and porosity, dominated the immobilization of Cd, Cr, Pb, and Zn. The leaching concentrations of heavy metals in co-pyrolysis biochar were much lower than the limit values of "Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity" （GB 5085.2-2007） and those by US EPA 1311, 1990. Additionally, the potential ecological risk index （RI value） of heavy metals in biochar was significantly reduced by co-pyrolysis compared to that of sludge or biochar without the co-pyrolysis. This study reveals the dominant immobilization mechanism for specific heavy metals during co-pyrolysis of municipal sludge with shaddock peel and provides an alternative practical strategy for the safe disposal of municipal sludge and biomass wastes.