Co-Pyrolysis of Fenton Sludge and Pomelo Peel for Heavy Metal Stabilization: Speciation Mechanism and Risk Evaluation

The safe disposal and resource utilization of Fenton sludge (FS) are challenges due to the presence of heavy metals (HMs). Co-pyrolysis with biomass waste can effectively increase biochar quality and immobilize HMs, but research focusing on heavy metal stabilization from Fenton sludge using the co-pyrolysis approach is scattered. In this study, the co-pyrolysis of FS and pomelo peel (PP) was developed as a strategy to reduce the environmental risk of HMs. The results showed that co-pyrolysis greatly increased the pH and aromaticity of biochars, and the maximum specific surface area was 6.5 times higher than the corresponding FS-based biochar due to the sponge-like structure of PP, which was likely conducive to adsorbing HMs during pyrolysis. Meanwhile, the addition of PP promoted the enrichment of HMs in co-pyrolyzed biochars as well as induced the transformation of bio-available HM fractions to stable forms, especially at high temperatures. Finally, the presence of PP led to the decline in HM leachability in biochars; thus, the potential ecological risks of HMs decreased from considerable pollution levels to moderate and even clean levels. This study demonstrated that co-pyrolysis with PP is a promising approach to reduce the toxicity of HMs and improve the functionality of biochar for industrial sludge management.