Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 887

PCDDs

polychlorinated phenoxy radical → non toxic com-

bustion product

PCDDs → non-toxic com-

bustion product.

Increasing substitution by chlorine by chlorophenols would

be expected to result in slower rates of attack by hydroxyl radi-

cal and therefore the more highly chlorinated dioxins are likely

to be less reactive towards bimolecular decomposition with

hydroxyl radicals than the lesser chlorinated congeners. One

would therefore expect that the dioxin isomers distribution

would tend to be skewed toward higher chlorinated species.

The unimolecular rupture of the weakest bond yields a

polychlorinated phenoxy radical (Figure 25).

The phenoxy radical is also produced when hydroxyls rad-

icals cause hydrogen atom abstraction. If, alternatively, •OH

addition occurs, then the adduct may undergo further reaction

with oxygen, or some other species, to give ring opening and

thereby reduce the likelihood of dioxin formation.

When the phenoxy radical decomposes by eliminating

CO, it forms non-dioxin products.

Polychlorinated 2-phenoxy-phenols are termed prediox-

ins because, in this configuration, the molecule is stereo-

chemically able to cyclize into the dioxin. The bimolecular

reaction can also occur in other positions around the ring

which would then not produce pre-dioxins.

The unimolecular elimination of HCl from the predioxin

yields dioxin.

The reaction is actually two reactions in one. The hydro-

gen abstraction reaction is similar to reaction (2) and will

ultimately depend upon the equilibrium concentration of

•OH radicals. The polychlorinated 2-phenoxy-phenoxy radi-

cal produced reacts very fast unimolecularly to yield a dioxin

and eliminated either H or Cl.

R represents a gaseous organic fuel molecule. It is assumed

to allow hydrogen abstraction to form a fuel free radical spe-

cies which is more stable than the original phen oxy radical.

Both species are free radicals and therefore are likely

to react very fast. The products indicated are not meant to

FIGURE 23 Mechanism of dioxin formation in flames [Schaub and Tsang, 1983].

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