Chlorine-bromine coexistence redirects hazardous dioxin formation from phenolic precursors under post-combustion conditions.

Chlorine and bromine coexist in industrial thermal processes and can alter the formation of hazardous halogenated dioxins. This study investigated the formation of chlorinated, brominated and mixed-halogenated dioxins from phenolic and naphtholic precursors under different Cl/Br molar ratios. Tube furnace experiments, in situ electron paramagnetic resonance and density functional theory (DFT) calculations were combined to evaluate the effects of halogen identity, precursor structure and radical-mediated reactions. In single-halogen systems, polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were formed in higher yields and dominated by highly chlorinated congeners. In contrast, polybrominated dibenzo-p-dioxins/dibenzofurans (PBDD/Fs) favored lower-brominated products, reflecting stronger steric constraints and less favorable bromination. In mixed systems, brominated precursors suppressed PCDD/Fs formation, whereas chlorinated precursors promoted selected PBDD/Fs congeners and mixed-halogenated dioxins (PXDD/Fs). Br2Cl2-DD was the dominant PXDD congener group, indicating that cross-coupling between chlorinated and brominated precursor-derived radicals contributes to mixed-halogenated product formation. DFT calculations supported the greater thermodynamic favorability of chlorination over bromination and the feasibility of radical-coupling pathways, although product selectivity should be interpreted together with precursor availability, radical abundance and kinetic constraints. These findings show that Cl-Br coexistence redirects hazardous dioxin formation and highlights the need to include PBDD/Fs and PXDD/Fs in risk assessment and control strategies for mixed-halogen thermal processes.