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 be controlled by thermodynamic properties of the individual PCDD/F congeners, such as the free Gibbs energy of formation (Δ G ° f,T ). This would make prediction of the isomer composition of a particular sample (and hence its TEQ value) possible, based on values of Δ G ° 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 Δ G ° 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 Δ G ° f,T values of PCDD/F were made.