Degradation mechanism of decabromodiphenyl ether: Reaction with reactive radicals and formation chemistry of polybrominated dibenzo-p-dioxins and dibenzofurans

Abstract

Although decabromodiphenyl ether (BDE-209) has been banned from production and use as a brominated flame retardant, its threat to human and ecological environment cannot still be ignored. Nevertheless, the mechanism of BDE-209 initiated by reactive radicals in the thermochemical decomposition process remains unknown. The density functional theory method is used to study the thermochemical decomposition behavior of BDE-209, with reactive radical reactions, and the formation mechanism of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). The results show that BDE-209 is not prone to self-decomposition due to its high bond dissociation energy (249.6 ∼ 281.4 kJ/mol) and HOMO-LUMO energy gap (7.10 eV). The reaction of BDE-209 with three active free radicals, including •H, Br•, and •OH, usually results in a variety of hypobromated products or important intermediates at low reaction energy barriers (52.9, 54.3, 50.4 kJ/mol). At 600 K, the branching ratio of H-abstraction reaction in the reaction system of •H and BDE-209 is 67.5 %. At 300 K, the reaction rate constant of BDE-209 with Br• to form P2 and IM2 is 4.14 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹. The branching ratio of BDE-209 + •OH → P9 + Br• is 40.3 % at 400 K. The OH-addition and OH-abstraction reactions of BDE-209 form octabromodibenzo-p-dioxin, which is further debrominated to form PBDDs. Ortho-phenyl-type radical generated by eliminating ortho-Br is a crucial precursor radical for the formation of PBDD/Fs. The most favorable reaction pathway for the subsequent degradation of ortho-phenyl-type radical is IM10 → TS38 → IM13. The participation of polymer materials increases the production of toxic PBDD/Fs. This is because the free radicals formed by the chain scission of the polymer can easily extract the ortho-Br atom of BDE-209, thereby promoting the formation of the precursor (ortho-phenyl radical) of PBDD/Fs. Furthermore, the degradation mechanism of BDE-209 induced by active free radicals is explored to help control the formation of toxic substances during thermal treatment.