Molecular Insights into a Precursor–Chlorine Trade-Off Governing Divergent Formation Pathways of PCDD/Fs and Emerging Cl-PAHs in Waste Cocombustion of Cellulose, PET, and PVC

Co-combustion of industrial and municipal solid waste (ISW/MSW) is widely practiced, yet PCDD/Fs and emerging chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) formation under mixed-feedstock conditions remains poorly understood. Here, cellulose, polyethylene terephthalate (PET), and polyvinyl chloride (PVC) were combusted individually and cocombusted with MSW to elucidate how feedstock chemistry regulates chlorinated pollutant formation. Comprehensive mass-spectrometric profiling of target pollutants, aromatic precursors, and the top 50 combustion intermediates, combined with DFT calculations, revealed a precursor–chlorine trade-off governing pollutant formation. PET preferentially generated oxygenated aromatic intermediates, particularly dibenzofuran; PVC supplied the most PAHs and reactive chlorine, and cellulose primarily contributed carbon skeletons. Accordingly, two distinct pathways were proposed: dibenzofuran-centered, chlorination-limited formation of PCDD/Fs and stepwise aromatic growth followed by site-selective chlorination of Cl-PAHs. The higher HOMO energies of two representative PAHs, anthracene and phenanthrene, relative to dibenzofuran indicated their greater susceptibility to electrophilic chlorination. Consequently, Cl-PAHs exceeded PCDD/Fs by 1–3 orders of magnitude and accounted for 83.4% of estimated TEQ. Increasing chlorine availability alleviated the kinetic constraint on dibenzofuran chlorination, causing an approximately 42-fold increase in PCDD/F emissions at 20% PVC blending. These findings establish a molecular basis for feedstock-chemistry-guided management and broaden emission-control priorities from regulated PCDD/Fs to emerging Cl-PAHs.