Co-pyrolysis of monobasic potassium phosphate and plastic processing sludge: Characteristics and environmental risks of potentially toxic elements

A high concentration of potentially toxic elements (PTEs) can be frequently observed in the plastic processing sludge (PPS), thereby restricting its environmental applications. The main objective of this study was to investigate the effects of the co-pyrolysis of PPS and KH₂PO₄ (0, 5, 10 and 20 wt%) on the characteristics and environmental risks associated with the PTEs in PPS and derived chars. General characteristic analysis revealed that the char yield, ash content, pH, and particle size of the chars prepared with KH₂PO₄ were greater than those of the char prepared without KH₂PO₄ by 3.13-4.89 wt%, 2.95-4.4 wt%, 0.77-0.93, and 9.64-30.07 µm, respectively. The results of sequential extraction indicated that co-pyrolysis with KH₂PO₄ could considerably increase the distribution of PTEs in the F4 fraction (non-bioavailable) in PPS by 1.30-65.90% when compared with that obtained via co-pyrolysis with 5 wt% of KH₂PO₄. The toxic leaching tests indicated that the leaching concentrations of Cr, Ni, Cu, Zn, Cd, and Pb in the char prepared without KH₂PO₄ decreased to different extents when PPS was subjected to co-pyrolysis with KH₂PO₄, especially in case of co-pyrolysis with 5 wt% of KH₂PO₄. The range of decrease was 26.40-88.34%. However, in case of Cu, Zn, and Pb, the leaching concentration of the chars prepared with more than 10 wt% of KH₂PO₄ increased owing to the decomposition of (Cu Zn)PbVO₄(OH) in an acidic environment. The results obtained using Hakanson’s equations revealed that the potential ecological risk associated with the PTEs in chars obtained by co-pyrolysis with KH₂PO₄ decreased, with a minimum decrease of 38.17%. In addition, the risk level associated with PPS reduced from considerable to low after co-pyrolysis with KH₂PO₄. The observations of this study imply that the co-pyrolysis of PPS with KH₂PO₄ can be a promising treatment for PTE immobilization.