THE HOMOGENEOUS, GAS-PHASE FORMATION OF CHLORINATED AND BROMINATED DIBENZO-P-DIOXIN FROM 2,4,6-TRICHLOROPHENOLS AND 2,4,6-TRIBROMOPHENOLS

The homogeneous, gas-phase formation of polychlorinated dibenzo-p-diox ins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observ ed from the high-temperature thermal decomposition of 2,4,6-trichlorop henol (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 temper ature range of 300 degrees-800 degrees C with reactant concentrations of similar to 3.0 x 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 P CDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yie lds of 0.05% each. The corresponding tetrabrominated isomers were obse rved from the thermal oxidation of 2,4,6-TBP; however, the maximum yie lds were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for u sing a modified form of the original gas-phase formation model of Shau b and Tsang, if the activation energy for the formation of diphenyl et her by displacement of Cl/Br from halophenol by phenoxy is decreased f rom 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated sy stems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.