Mechanisms of Dioxin Formation from the High-Temperature Pyrolysis of 2-Bromophenol

Abstract

Brominated hydrocarbons are the most commonly used flame retardants. Materials containing brominated hydrocarbons are frequently disposed in municipal and hazardous waste incinerators as well as being subjected to thermal reaction in accidental fires. This results in the potential for formation of brominated dioxins and other hazardous combustion byproducts. In contrast to chlorinated hydrocarbons, the reactions of brominated hydrocarbons have been studied only minimally. As a model brominated hydrocarbon that may form brominated dioxins, we studied the homogeneous, gas-phase pyrolytic thermal degradation of 2-bromophenol in a 1-cm i.d., fused-silica flow reactor at a concentration of 90 ppm, with a reaction time of 2.0 s, and over a temperature range of 300 to 1000 degrees C. Observed products included dibenzo-p-dioxin (DD), 1-monobromodibenzo-p-dioxin (1-MBDD), 4-monobromodibenzofuran (4-MBDF), dibenzofuran (DF), naphthalene, bromonaphthalene, 2,4- and 2,6-dibromophenol, phenol, bromobenzene, and benzene. These results are compared and contrasted with previous results reported for 2-chlorophenol. At temperatures lower than 700 degrees C, formation of 2-bromophenoxyl radical, which decomposes through CO elimination to form a bromocyclopentadienyl radical, forms naphthalene and 2-bromonaphthalene through radical recombination/rearrangement reactions. However, unlike the results for 2-chlorophenol, where naphthalene is the major product, DD becomes the major product for the pyrolysis of 2-bromophenol. The formation of DD and 1-MBDD are attributed to radical-radical reactions involving 2-bromophenoxyl radical with the carbon- (bromine) centered radical and the carbon- (hydrogen) centered radical mesomers of 2-bromophenoxyl radical, respectively. The potential product, 4,6-dibromodibenzofuran (4,6-DBDF) for which the analogous product, 4,6-dichlorodibenzofuran (4,6 DCDF), was observed in the oxidation of 2-chlorophenol, was not detected. This is attributed to the pyrolytic conditions of our experiments (e.g., shorter reaction times and higher temperatures) that favor reaction intermediates that form DD and 1-MBDD.

Pollutant emissions during pyrolysis and combustion of waste printed circuit boards, before and after metal removal

“…• by H • (Evans and Dellinger, 2003;Evans and Dellinger, 2005). As observed in the present study, they also reported that the formation of benzene occurred at lower yields and higher temperatures than the formation of phenol, and that this was consistent with the more endothermic displacement of OH • by H • from phenol to form benzene, in comparison to the previous mechanism proposed for the formation of phenol.…”

Kinetic Study of the Gas-Phase Reaction of OH with Br2

“…Equation IV was obtained by simplification of a formula derived by Mozurkewich and Benson 37 (equation 12 of reference 37) for reactions proceeding over shallow potential energy wells. The simplification resulted from the above mentioned assumption of predominant dissociation of the OH···Br 2 complexes to OH + Br 2 and from an approximate treatment of the effects of angular momentum (J) conservation where the rate coefficient value obtained for J = 0 is multiplied by the ratio of the partition functions of the two-dimensional rotational (inactive) degrees of freedom of the transition state and the active molecule. The latter ratio (1.2) is close to unity, which justifies the use of the approximation.…”

Biodegradation of 2,4,6-Tribromophenol byOchrobactrumsp. Strain TB01

“…High-temperature pyrolysis of bromophenols leads to dioxin formation. 14) Hence the development of a biological purification system is one alternative to reduce levels of exposure to 2,4,6-TBP.…”