Mechanism for OH-Initiated degradation of 2,3,7,8-tetrachlorinated dibenzo-p-dioxins in the presence of O2 and NO/H2O

The atmospheric oxidation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TeCDD) is investigated theoretically by high-accuracymolecular orbital calculation. The study shows that the OH radical can easily be added to the C atom adjacent to the O atom in dioxin ring to form OH radical adduct. The 2,3,7,8-TeCDD OHadduct can immediately react withO2 to formthe 2,3,7,8- TeCDD OH-O2 adduct which can react withNOorH2Oto complete the decomposition process. The degradation mechanism varies with the addition position of O2 and the O-ion by NO. The OH radical can be reproduced through the H-ion of H2O and initiate a new round of degradation. The direct dynamic calculation is performed, and the rate constants is calculated over a temperature range of 200 1200K, using the canonical variational transition state theorywith small- curvature tunneling effect. The four-parameter formula of rate constants with the temperature is fitted and the lifetimes of the reaction species in the troposphere are estimated according to the rate constants, which is helpful for the atmospheric model study on the formation and degradation of dioxin.