Isomer distributions of polychlorinated dibenzo-p-dioxins/dibenzofurans formed during de novo synthesis on incinerator fly ash

Polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) emitted from municipal waste incinerators appear to have a chlorination pattern that is quite constant across various samples and conditions. This suggested that these patterns may he controlled by thermodynamic properties of the individual PCDD/F congeners, such as the free Gibbs energy of formation (Delta G degrees(f,T)) This would make prediction of the isomer composition of a particular sample (and hence its TEQ value) possible, based on values of Delta G degrees(f.T). A laboratory scale study was carried out with activated carbon on fly ash as the source of PCDD/F formation. Although it was found that the isomer distributions within homologues were independent of the reaction time (proof of thermodynamic control), other observations (lack of equilibrium/isomerization between isomers and lack of similarity between isomer distributions measured and predicted by Delta G degrees(f,T)) contradicted the possibility of thermodynamic control. Hence, this study could not confirm that de novo formation of PCDD/F could explain thermodynamically controlled isomer distributions in incinerators. Some recommendations for further work-time-based studies with precursors, isomerization studies with single congeners, and more data on Delta G degrees(f,T) values of PCDD/F-were made.