Evaluation of migration of heavy metals and performance of product during co-pyrolysis process of municipal sewage sludge and walnut shell

A feasible and efficient type of biological sludge-activated carbon (BSAC) was produced by co-pyrolysis of municipal sewage sludge (MSS) and walnut shell (4:1, w/w) at 500 degrees C. It was found that BSAC was typical mesoporous material with favorable pore structure and abundant surface functional groups, whose performance was improved compared with conventional sludge-activated carbon (CSAC), combined with walnut shell-activated carbon (WSAC). The migration and transformation behavior of heavy metals (Zn, Cu, Ni, Cd, and Cr) in raw material after co-pyrolysis process were investigated. The results indicated that co-pyrolysis could promote mobile fraction (acid soluble/exchangeable and reducible fractions) of heavy metals to stable fraction (oxidizable and residual fractions). The leaching concentrations Cu, Ni, Cd, Cr, and Zn were lower than restrictive standards in China, and the environmental risk assessment results showed that after co-pyrolysis, the risk levels of Cu, Ni, and Cd were decreased to low risk, especially Cr in product was confirmed to no risk.