Impact of pyrolysis temperature on heavy metals environmental risk in biochar derived from co-pyrolysis of Alternanthera philoxeroides and sludge

Resource utilization of dried livestock sewage sludge (SS) and invasive plants, e.g. Alternanthera philoxeroides (AP) is the focus of cracking the environmental challenges. In this study, AP and sludge were used as raw materials to prepare co-pyrolysis biochar at 300-800 degrees C. The total amount of heavy metals, BCR forms, and leaching toxicity were analyzed to evaluate the environmental risk of heavy metals (HMs) in biochar. The results showed that the percentage of unstable states and leaching toxicity of six (HMs) decreased, and the total amount and ratio of stable states increased with the increasing pyrolysis temperature. Compared with SS and SBCs, the unstable fraction and leaching toxicity of (HMs) in SABCs were significantly reduced, and the values of RAC and RI were lower. The RI value of SS was 1001.40, which belongs to a very high-risk level, while the RI of SABC800 was low risk with a value of 51.69, indicating that the co-pyrolysis of AP and SS at high temperatures could effectively reduce the environmental risk of biochar. In addition, the Er value of Cd in SBCs was significantly higher than that of SS, but it was lower in SABCs than in SS, which suggested that AP addition is beneficial to reducing the ecotoxicity and environmental risk of Cd in livestock sewage sludge. This study demonstrates the superiority of AP addition in reducing the environmental risk of (HMs), especially Cd in SS, and provides a new idea of resource utilization for invasive plants and livestock sewage sludge disposal.