The homogeneous, gas-phase formation of chlorinated and brominated dibenzo-p-dioxin from 2,4,6-trichloro- and 2,4,6-tribromophenols

Abstract The homogeneous, gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observed from the high-temperature thermal decomposition of 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4,6-tribromophenol (2,4,6-TBP), respectively. Experiments were conducted in a 1.0-cm-i.d. flow reactor over a temperature range of 300°–800°C with reactant concentrations of ∼ 3.0 × 10−7 mol/L in a reaction atmosphere of dry air. The 1,3,6,8- and 1,3,7,9-tetra chlorinated isomers were the dominant PCDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yields of 0.05% each. The corresponding tetrabrominated isomers were observed from the thermal oxidation of 2,4,6-TBP; however, the maximum yields were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for using a modified form of the original gas-phase formation model of Shaub and Tsang, if the activation energy for the formation of diphenyl ether by displacement of Cl Br from halophenol by phenoxy is decreased from 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated systems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.