Isomer Distributions of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans Formed during De Novo Synthesis on Incinerator Fly Ash

Abstract

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 (∆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 ∆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 ∆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 workstime-based studies with precursors, isomerization studies with single congeners, and more data on ∆G°f ,T values of PCDD/ Fswere made.

Mechanisms for formation, chlorination, dechlorination and destruction of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs)

“…The reactivity of each congener from a chemical kinetics perspective was estimated by calculating the difference in energy across the frontier orbitals, namely, between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). Addink et al [229] found that the congeners' distribution produced in the de novo process within each homologue group is not time-dependent (i.e., no kinetic control). However, the final distribution of isomers was not consistent with the Gibbs free energy profiles.…”

The formation of dioxins in combustion systems

“…The hypothesis that the congener patterns are controlled by thermodynamic equilibrium was investigated by Addink et al [106]. The congener distributions from various annealing experiments were compared to those predicted by a procedure, which minimised the Gibbs free energy of formation.…”

“…The Ullmann reaction, which is the biaryl synthesis of chlorinated phenols, is likely at low temperatures (∼300 • C), only if they occur via a C-Cl bond and not a C-H bond. An analysis of the system's energetics has demonstrated the influence of the extent of chlorination on the free energy of reaction [106]. The thermodynamic study by Yuzawa et al [107] predicted the equilibrium concentrations of the various congeners under typical conditions, with no PCDD/F formation likely above 360 • C. The influence of chlorine concentration on product distribution has also been considered from a free energy point of view [108].…”

Novas tendências para o tratamento de resíduos industriais contendo espécies organocloradas

“…No entanto, a sua utilidade é muito duvidosa, porque além de ser muito cara, pode levar à formação de compostos mais tóxicos que o próprio efluente, tipicamente dioxinas e furanos. A formação de dibenzo-p-dioxinas e dibenzofuranos policlorados em processos de incineração de resíduos domésticos é um fato bastante documentado 11,[78][79][80][81][82][83] . Em função da crescente necessidade de procedimentos que apresentem uma maior eficiência no tratamento de efluentes, vá-rias técnicas têm sido testadas nas últimas décadas.…”