Heavy Metal Speciation and Ecological Risk Assessment of Biochar from Co-pyrolysis of Livestock Sludge and Calcium Carbide Slag

• • At 600 °C and a sludge:CCS ratio of 2:1, co-pyrolysis increased the residual fraction of Cu and Zn by up to 45% and 38%, respectively, compared to sludge-only pyrolysis, significantly reducing their leachability and ecological risk. • • The addition of CCS promoted the formation of apatite phosphorus (Ca5(PO4)3OH and Ca3(PO4)2), enhancing the bioavailability of phosphorus in the biochar, which is critical for agricultural application as a slow-release fertilizer. • • Calcium compounds and SiO2 in CCS act as stabilizers via crystal solid solution and complexation, effectively immobilizing heavy metals at elevated temperatures, thereby reducing the potential for secondary pollution. • • The co-pyrolysis process achieves a synergistic waste management solution: it not only stabilizes heavy metals but also converts alkaline waste (CCS) and livestock sludge into a value-added product, addressing both environmental and economic concerns.