What does the “Trojan horse” carry? Effects of perfluorooctanoic acid on the migration and transformation of sulfur during dyeing sludge incineration

Dyeing sludge (DS) serves as a high-capacity carrier for both conventional pollutants (e.g., organic sulfur compounds) and emerging contaminants such as per- and polyfluoroalkyl substances (PFASs). However, how PFASs influence the thermal transformation of organic sulfur during DS incineration remains poorly understood. This study investigated the effects of perfluorooctanoic acid (PFOA), a representative PFASs, on the combustion behavior of model organic sulfur compounds—sulfone (S) and mercaptan (M). The combustion of S/M–PFOA mixtures was primarily governed by the organic sulfur species, with PFOA reducing the heat threshold required for sustained combustion. The presence of PFOA enhanced the mineralization of organic sulfur, leading to significant emissions of SO₂, CH₃SH, and HF. The yields of CH₃SH and SO₂ from the mixtures increased by 1.11–5.58 and 1.30–6.34 times, respectively, compared with the organic sulfur alone, attributable to a “Trojan Horse” effect involving coupled radical and secondary reactions. PFOA lowered the binding energies of the mixtures by 26.55–62.57%, effectively reducing the combustion barrier of organic sulfur. The low formation energies of S–PFOA (–872.21 kcal/mol) and M–PFOA (–729.70 kcal/mol) confirmed the spontaneous and synergistic combustion of PFAS–sulfur mixtures. Overall, PFASs act as a “Trojan horse,” intensifying sulfur-related emissions during DS incineration. Understanding PFAS–organic sulfur interactions across multiphase systems is crucial for mitigating secondary environmental risks during thermal treatment.