Doctor of Philosophy (PhD)
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) are toxic compounds produced in combustion systems such as incinerators, and their emissions are an important environmental issue. Chlorinated phenols and benzenes form PCDD/F through heterogeneous reactions following adsorption onto fly-ash surfaces. Current research implicates transition metals in promoting formation of phenoxyl radicals that react to form PCDD/F, although this has not been studied extensively. This objective of this work is to better understand PCDD/F formation from chlorinated aromatic precursors. Using 2 chlorophenol as a model PCDD/F precursor and silica as a fly-ash surrogate, time-dependent chemisorption of 2 chlorophenol onto silica was studied in situ from 200-500 °C with Fourier transform infrared spectroscopy. Chlorophenolate adsorption products and partial oxidation products were detected and characterized. Rates of chemisorption were also determined. Since copper species accelerate the formation of PCDD/F, further infrared spectroscopic studies were conducted to investigate the reactions of 2-chlorophenol, 1,2 dichlorobenzene and chlorobenzene with a copper oxide containing silica surface. Chemisorption mechanisms were determined and kinetics of the adsorption were measured. It was discovered that chlorinated benzenes react to produce the same phenolate species as phenols and this may place much greater emphasis on chlorinated benzenes as PCDD/F precursors. An X ray adsorption near-edge structure (XANES) spectroscopic study was conducted to examine the role of copper oxide in the formation of phenoxyl radicals. The time-dependent reduction of Cu(II) in a CuO/silica mixture was characterized in the presence of the three above mentioned PCDD/F precursors from 275-375 °C. The rate of reduction and Arrhenius parameters are given for each system, as well as copper speciation throughout the reaction. The results obtained support the contention the redox reactions of copper species promote the formation of phenoxyl radicals. Additionally, a predictive model was developed to correlate PCDD/F emissions with gas phase chlorophenol concentrations. The primary motivation for this is the desire to continuously monitor PCDD/F emission by measuring chlorophenol levels, since the gas-phase concentration of chlorinated phenols in combustion systems is much higher that those of PCDD/F. The results of the model give new insight into the formation of PCDD/F from aromatic precursors.
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Alderman, Steven L., "Infrared and x-ray spectroscopic studies of the copper(II) oxide mediated reactions of chlorinated aromatic precursors to PCDD/F" (2005). LSU Doctoral Dissertations. 800.