Gas-phase formation pathways and mechanisms of polychlorinated dibenzo-p-dioxins and dibenzofurans

Polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) are a class of chlorinated organic compounds that are very toxic and present in emissions of municipal solid waste and medical waste incineration processes. Of the compounds that have been identified as the precursors to PCDD and PCDF byproducts, chlorinated phenols are believed to be the most important. Chlorinated phenols are present in incineration post-combustion gas as a result of incomplete combustion of phenol constituents of the waste as well as formation of phenol byproducts in flames.; In the present study, the gas-phase formation of PCDD and PCDF compounds from chlorinated phenols was explored. Formation pathways from four dichlorophenols (2,6-, 2,4-, 2,3- and 3,4-DCP) in the temperature range 600-950{dollar}spcirc{dollar}C have been identified; isomer-specific chemical mechanisms have been proposed based on product distributions. Structural features, such as the chlorine substitution of the starting dichlorophenol, had a marked influence on the PCDD and PCDF product distribution.; Two distinctly different mechanisms of PCDD and PCDF product formation were identified. PCDD formation is facilitated by the presence of ortho chlorine substituents on the phenol whereas PCDF formation is facilitated by the presence of ortho hydrogen substituents. Both PCDD and PCDF products are formed by addition of a chlorophenoxy radical to a phenol. While PCDD formation occurs by carbon-oxygen coupling via the formation of a 2-phenoxyphenol intermediate, PCDF formation occurs by carbon-carbon coupling via the formation of a 2,2{dollar}spprime{dollar}-dihydroxybiphenyl intermediate. Aryl addition pathways to other products, such as naphthalenes and benzonaphthofurans, are also discussed.; PCDD and PCDF product isomer distributions obtained experimentally are in good agreement with the computed thermodynamic distributions. Semiempirical molecular orbital modeling was used to estimate thermodynamic quantities of PCDD and PCDF products. The close agreement between the predicted and observed PCDD and PCDF product distribution indicates that statistical factors and steric effects associated with chlorine substitution control the PCDD and PCDF product distribution. These experimental and computational results not only provide a better understanding of the dominant PCDD and PCDF formation pathways in the gas-phase but also contribute to the development of a theoretical framework that may be used as a tool for predicting PCDD and PCDF distributions formed in incinerator post-combustion gas streams.