Influence of bromination arrangement and level on the formation of polybrominated dibenzo-p-dioxins and dibenzofurans from pyrolysis and combustion of polybrominated diphenyl ethers: Mechanisms and kinetics.

Controlling the emission of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) presents a huge challenge in the thermo-chemical conversion of halogenated waste. It is crucial to understand the formation of PBDD/Fs to effectively control their emissions. This paper utilizes density functional theory calculations to comprehensively explore the mechanisms and kinetics of PBDD/Fs formation using 65 polybrominated diphenyl ethers (PBDEs) with varying bromination arrangements and levels. The study uncovers two new pathways for the formation of PBDFs from PBDEs, i.e., two-step H 2 elimination and ether-ketone isomerization at ortho -C(H) sites. These pathways are applicable to PBDEs with specific bromination arrangements. In the case of some PBDEs, their initial energy barriers are comparable to the generally accepted direct HBr elimination. It is found that low-brominated PBDEs have a higher risk of forming PBDD/Fs, while high-brominated PBDEs are more likely to break ether bonds and generate polybromobenzenes. The study also investigates the effects of polymer materials and metals on the formation of PBDD/Fs from PBDEs to explain the high yield of PBDD/Fs in the co-treatment of PBDEs with polymers and metals. The involvement of polymers and metals significantly reduces the energy barriers for the formation of PBDD/F precursors, namely ortho -phenyl-type radicals, accelerating the formation of PBDD/Fs. This study markedly advances the present understanding of the transformation of PBDEs into PBDD/Fs and provides theoretical guidance for reducing pollutant emissions in the thermal treatment and cleaner conversion of PBDE-containing waste. [Display omitted] • 65 PBDE congeners were selected to study the formation mechanism of PBDD/Fs. • A two-step H 2 elimination mechanism for the formation of PBDFs was proposed. • Assess the importance of ether-ketone isomerization reactions at ortho C(H) sites. • Explain the effects of polymers and metals on the formation of PBDD/Fs from PBDEs. [ABSTRACT FROM AUTHOR]