(643c) Ambient Temperature Mercury Oxidation Thermochemistry and Kinetics Via Chlorine and Bromine

Emissions of gaseous mercury from combustion sources are the major source of Hg in environmental waters and in soil. Reduction of these emissions is one of the major environmental concerns facing power generators. One method for removal of Hg involves oxidation with halogens to form HgX2, which can be separated from effluent of incinerators. The conversion (oxidation) of mercury into mercury bromide and chloride has been studied in a number of recent experimental and theoretical (computational chemistry) papers. Several experimental studies have focused on reactions under atmospheric or only slightly elevated temperature conditions, where significant conversions of Hg (inferred reaction to HgX2) is reported (X2 = Cl2, Br2 or ClBr). In this study we use the thermochemistry from our calculations as well as from published theoretical and experimental studies and we use kinetics and kinetic models from a number of different published studies. A mechanism including all of the reactions considered as contributors to the conversion of Hg (oxidation) and formation of HgX2, (X = Cl , Br) under atmospheric temperature and pressure conditions is compiled to determine agreement between experiment and the model. Association, addition and insertion reactions in our model are then treated as chemical activated reactions, where the energized complexes can react to new products or dissociate back to reactants before stabilization. Dissociation reactions are also calculated for fall-off. The chemically activated HgX2* systems are modeled using quantum Rice-Ramsperger-Kassel (QRRK) theory for k(E), with master equation analysis for falloff. Reverse reactions are included for all reactions, with rate constants calculated in the Chemkin code from thermodynamics. The inclusion of reverse reactions is particularly important for the formation of HgBr and HgCl formation reactions, as the bond energies of these important intermediates are only on the order of 22 kcal mol-1. We find that the reactions generally accepted to explain the formation of HgX2 completely fail to explain the experimental data. We further implement a Hg insertion reaction into X2 ( Hg + X2 = HgX2) that has been reported to have a barrier too high to consider for atmospheric conditions. We find that models can explain the experimental data only if the barrier for the insertion reaction is lower than the reported in the literature. While an empirically modified barrier can be used to explain the data with a chemical activation mechanism, there remains an important need to validate this reaction step and its barrier. We feel further experiments are also needed to verify the fundamental, gas phase, nature of the mechanism.