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 ty pical patterns for isomers of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) formed as trace byproducts of incomplete com bustion. A large number and variety of samples related to combustion w as analyzed for the concentrations of the PCDD/F congeners. The result ing data set was subjected to Principal Component Analysis to show sim ilarities in isomeric patterns of either homologue group. The first pr incipal component (PRIN1) gave a good (83-91%) description of the tetr a- to heptachlorodibenzo-p-dioxins and -furans for most samples. It pr ovided a tool to compare patterns of isomers of PCDD/F formation from laboratory scale combustion of precursors on annealed fly ash, calcula ted thermodynamic stability and calculated reactivity. The aim of the combined statistical and experimental studies was to find relevant pat hways for thermal PCDD/F formation. Therefore, it was necessary to pro ve which precursors were relevant and if the observed patterns were co ntrolled either by thermodynamic stability of the compounds or kinetic processes. The investigated precursors led to thermal formation of PC DD/F at 350 degrees C and nearly all resulting isomeric patterns corre sponded to the statistically derived combustion patterns of PCDD/F. Re lative abundance calculated from thermodynamic stability of the PCDD i somers showed similarities, but also distinct differences with the com bustion 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 qualit ative description of the typical PCDD combustion patterns. (C) 1998 El sevier Science Ltd. All rights reserved.