Unraveling the role of plastic polymers in PAHs emissions from municipal solid waste incineration: Quantifying contributions, synergy, and source tracers

Global municipal solid waste (MSW) incineration faces escalating environmental risks from polycyclic aromatic hydrocarbons (PAHs) due to increasing plastic content, yet the specific role of plastics remains poorly understood. Hence, this study systematically characterized PAHs EFs and distribution profiles during MSW combustion across a range of temperatures (300 - 900 °C) and plastic mass fractions (M_pc, 15.43% - 63.43%), while the transition in PAHs formation pathways was elucidated using density functional theory (DFT) calculations. Results demonstrated that M_pc significantly increased ∑PAHs EFs (3.41-7.63 g/kg) and altered pPAHs distribution (87.98-93.14%), explaining 45.78% of the variance. This alteration featured the shift in predominant product of six-ring indeno[1,2,3-cd]pyrene (IdP) to five-ring benzo[k]fluorene (BkF). In contrast, combustion temperature (21.02% of variance) affected only the EFs (2.18-5.13 g/kg) with negligible distribution changes (<0.38%). DFT calculations demonstrated that the shift in the dominant reaction pathways during high-plastic combustion is driven by the accumulation of intermediate acetylene, which triggers a mechanistic transition from the hydrogen abstraction-vinylacetylene addition (HAVA) pathway to the hydrogen abstraction-acetylene addition (HACA) pathway. Among seven polymers, PVC showed the strongest promotion of PAHs formation, and this effect was further amplified in mixed-plastic systems, where experimental EFs exceeded theoretical values by up to 34.95-fold, indicating complex radical-chain interactions during co-combustion. The proposed BkF/Perylene ratio performed a robust molecular tracer, stable across thermal fluctuations yet highly sensitive to plastic-to-cellulose ratios. These findings clarify plastics' role in PAHs formation during MSW combustion and support optimized incineration strategies for plastic-containing waste.