Formation of tetrahalogenated dibenzo-p-dioxins (TXDDs) by pyrolysis of a mixture of 2,4,6-trichlorophenol and 2,4,6-tribromophenol

The microscale pyrolysis of a mixture of 2,4,6-tribromophenol (2,4,6-TBP) and 2,4,6-trichlorophenol (2,4,6-TCP) was investigated. Several pyrolyzed products resulting from the gas phase reactions were analyzed by gas chromatography/mass spectrometry (GC/MS). Some products, including tetrabromo- and tetrachloro-dibenzo-p-dioxins, were identified by comparison of their retention times and mass spectra with those of products obtained from the pyrolysis of the corresponding 2,4,6-TBP and 2,4,6-TCP. 1,3,6,8- and 1,3,7,9-halogen substituted dioxin isomers were produced as major products via the condensation of phenoxyphenol radicals. The condensation of 2,4,6-TBP led to the more rapid production of tetrabromodibenzo-p-dioxins (TBDDs) than of tetrachlorodibenzo-p-dioxins (TCDDs) from 2,3,6-TCP due to the weaker C-Br than C-Cl bond strength. In the study of a mixture of 2,4,6-TCP and 2,4,6-TBP, the most abundant congeners were dibromo-dichlorodibenzo-p-dioxins. On the basis of the pyro-products analysis, the thermal decay of 2,4,6-TCP and 2,4,6-TBP, and the formation of halogenated dioxins as a function of the pyrolysis temperature were also investigated. The increased reactivity can be explained by the hypothesis that “dibromophenoxy radical are formed more easily by debromination of 2,4,6-TBP than that of 2,4,6-TCP, which rapidly reacts with 2,4,6-TCP and 2,4,6-TBP at low temperatures”. The formation distribution of tetra-bromochlorodibenzo-p-dioxins (tetra-BCDDs) indicated that more chlorinated dioxins were produced at low temperatures, but more brominated dioxins were formed at high temperatures.