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

Abstract

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 activation energy decreased by 78.83% with 30.93% higher biochar yield rate due to the synergistic effects. Co-pyrolysis facilitated the decomposition of benzoic acid and its derivatives by enhancing the cleavage of oxygen-containing functional groups and led to the binding of nitrogen with more pyrolysis intermediates, resulting in a reduction of precursors for oxygen/oxygenated-carbon centered radicals. More phenyl radicals were polymerized into carbon-centered radicals through hydride transfer and crosslinking in the mixed system. The additive of SS promoted the aromatization degree, enhanced the defect and pore structure of the biochar, and led to a 43.62-fold increase in the surface area of PET-based char at 400 °C. This work provides a potential approach to reduce EPFRs from the pyrolysis of plastic waste and to deal with multiple waste streams in venous industry park under the implementation of zero waste cities construction.