Development of a Demonstrable Model of Dioxin Formation

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by applicant): We will develop demonstrable models of the formation of polyhalogenated dibenzo-p-dioxins and dibenzofurans (PXDD/F), persistent free radicals, and other toxic halogenated hydrocarbons from the combustion and thermal reactions of hazardous wastes. This includes polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), polybrominated dibenzo-p- dioxin/dibenzofurans (PBDD/F), mixed bromo/chloro dibeno-p-dioxins/dibenzofurans (PBCDD/F), and persistent resonance stabilized, surface-associated, semiquinone type radicals that are key reaction intermediates and pollutants. This will be accomplished through a combination of mechanistic studies using a high-temperature flow reactor coupled to a gas chromatograph/mass spectrometer (GC-MS) analytical system and reaction kinetic studies using laser-photolysis/laser induced fluorescence and laser photolysis/laser ionization time-of-flight mass spectrometry. We will: 1) determine the role of chlorinated benzenes in the formation of PCDD/F, 2) determine how particle size from the nanometer to micron size regime effects formation rates and product distributions, 3) measure elementary radical- molecule reaction rates of key reactions in formation of PXDD/F, 4) use ab initio and semi-empirical modeling techniques to calculate rates of key reactions, and 5) consolidate this information into a detailed, comprehensive model of PXDD/F formation within the CHEMKIN reaction kinetic modeling platform. We suspect that transition-metal, surface-mediated PCDD/F formation pathways are related to formation of persistent, reductive radicals on the surfaces of fine and ultra-fine particles. Thus, in addition to determining the biodelivery of PCDD/F via exposure to ultra-fine particles, we will characterize the formation of persistent free radicals (PFRs) and their role in the toxicity of ultrafine particles. Our experimental-studies of PCDD/F formation and modeling studies (incorporating the experimental results of Project 3) will allow us to better elucidate the relative contributions of gas-phase and surface-mediated processes to the formation of PXDD/F. The models developed in this project will