Patterns of isomers of chlorinated dibenzo-p-dioxins as tool for elucidation of thermal formation mechanisms

This study gives a statistically derived proof for the existence of typical patterns for isomers of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) formed as trace byproducts of incomplete combustion. A large number and variety of samples related to combustion was analyzed for the concentrations of the PCDD/F congeners. The resulting data set was subjected to Principal Component Analysis to show similarities in isomeric patterns of either homologue group. The first principal component (PRIN1) gave a good (83-91%) description of the tetra- to heptachlorodibenzo-p-dioxins and -furans for most samples. It provided a tool to compare patterns of isomers of PCDD/F formation from laboratory scale combustion of precursors on annealed fly ash, calculated thermodynamic stability and calculated reactivity. The aim of the combined statistical and experimental studies was to find relevant pathways for thermal PCDD/F formation. Therefore, it was necessary to prove which precursors were relevant and if the observed patterns were controlled either by thermodynamic stability of the compounds or kinetic processes. The investigated precursors led to thermal formation of PCDD/F at 350 degrees C and nearly all resulting isomeric patterns corresponded to the statistically derived combustion patterns of PCDD/F. Relative abundance calculated from thermodynamic stability of the PCDD isomers showed similarities, but also distinct differences with the combustion patterns. So did isomer distributions according to calculated HOMO-LUMO energies, which are quantitative measures for reactivity of congeners. A model, however, which was derived from a superposition of thermodynamic stability and reactivity of PCDD isomers, gave a qualitative description of the typical PCDD combustion patterns. (C) 1998 Elsevier Science Ltd. All rights reserved.