Heavy Metal Speciation and Ecological Risk Assessment of Biochar from Co-pyrolysis of Livestock Sludge and Calcium Carbide Slag
The rapid expansion of livestock and poultry farming has intensified the challenge of managing sludge, which contains heavy metals (primarily Cu and Zn), antibiotics, and pathogens. Calcium carbide slag (CCS), an alkaline industrial waste rich in Ca(OH)2, CaCO3, and other minerals, poses environmental risks due to its high alkalinity. This study investigates the speciation transformation of heavy metals in biochar derived from co-pyrolysis of livestock sludge and CCS under varying temperatures (400–700 °C) and mixing ratios (sludge:CCS = 1:1, 2:1, 3:1, 4:1). The results demonstrate that at 600 °C and a 2:1 mixing ratio, calcium-based compounds and SiO2 in CCS effectively immobilize heavy metals through crystal solid solution and complexation, reducing their ecological risk. Sequential extraction indicated a shift from exchangeable and reducible fractions to residual fractions, with the residual fraction of Cu and Zn increasing by up to 45% and 38%, respectively, compared to sludge-only pyrolysis. The formation of apatite phosphorus (Ca5(PO4)3OH and Ca3(PO4)2) enhances the bioavailability of phosphorus in the biochar, making it a potential slow-release fertilizer. The study provides a novel strategy for the synergistic treatment of livestock sludge and CCS, offering environmental and economic benefits by producing stable, nutrient-rich biochar while mitigating heavy metal toxicity.