Cumulative effects of pyrolysis temperature and process on properties, chemical speciation, and environmental risks of heavy metals in magnetic biochar derived from coagulation-flocculation sludge of swine wastewater

Abstract

Coagulation-flocculation sludge (CFS) of swine wastewater was pyrolyzed at 400 °C – 700 °C to produce magnetic biochar (MBC) under anaerobic (MBCN) and hypoxic (MBCA) environments in order to investigate the characteristics and risks of heavy metals (HMs) in the MBC. Magnetite dominated the crystal phase of the MBCs obtained at 400 °C to 600 °C. Two forms of zero valent Fe were observed in the MBCs obtained at 700 °C, while γ-Fe was dominant in MBCA700 and α-Fe was dominant in MBCN700. Owing to the high concentration of chloride in the sludge, metal chlorides generated by chlorination lowered the pH of the MBCs compared to that of the CFS, except for MBCN700 because of Fe-C micro-electrolysis. The unstable fractions of some HMs, such as Zn, Cu, Ni, and Mn, increased when the increasing temperature from 400 °C to 700 °C as metal chlorides formed. The percentages of the acid-soluble and exchangeable became zero in MBC700 because HMs (except for Mn) could be adsorbed by ferric hydroxide colloid during the extraction. The lowest risk assessment code values were obtained in MBC700, except for the case of Mn. Compared with the CFS, the potential risk of HMs in the MBCs was significantly reduced, and MBCA600 presented the lowest environmental risk. This work demonstrates that pyrolysis is a promising and cost-effective method for the safe disposal of HM-polluted CFS and production of MBCs.