The synergistic pyrolysis effects of polyethylene terephthalate with the additive of sewage sludge

Pyrolysis could be one of the promising ways for polyethylene terephthalate (PET) waste-based char production, while the adherence environmental persistent free radicals (EPFRs) brought the risks for the wide application. Co-pyrolysis was employed as the potential way to reduce the EPFRs and improve the char quality simultaneously with the introduction of sewage sludge (SS), and the optimization reaction conditions and the relative mechanisms were proposed in this work. EPFRs in PET-based char were found to decrease from 1.07 × 1018–4.84 × 1018 spins·g−1 to 2.17 × 1017–4.90 × 1017 spins·g−1, with 38.86–48.55% reduction at the mass ratio of 1:1 (PET:SS). The corresponding specific surface area of biochar increased 43.62 times at 400 °C, with 30% higher yield rate. The activation energy decreased around 74.84%, and the terminal pyrolysis temperature reduced around 16 °C due to the synergistic effects of co-pyrolysis. Co-pyrolysis facilitated the decomposition of benzoic acid and its derivatives by enhancing the cleavage of oxygen-containing functional groups such as CO according to the products characterization results, which contributed to reducing the precursors for oxygen/oxygenated-carbon centered EPFRs, more hydrogen transfer and cross-linking of EPFRs, and increasing the production of ketones, amines and polycyclic aromatic compounds. Nitrogen atoms and inorganic substance in SS transformed the oxygen centered radicals to carbon centered radicals and promoted more defect structures in the biochar. This work provides a potential approach to reduce EPFRs from the plastic waste-char and to deal with multiple waste streams in venous industry park under the implementation of zero waste cities construction.