The oxidation mechanism of polychlorinated dibenzo-p-dioxins under the atmospheric conditions – A theoretical study

The atmospheric polychlorinated dibenzo-p-dioxins (PCDDs) partition appreciably in the gas phase, where they undergo rapid oxidation. The atmospheric oxidation mechanisms of a few PCDDs, initiated by OH radical, are studied using density functional theory calculations. The oxidations start with OH-addition to the aromatic rings, dominantly at γ-sites, followed by the non-chlorinated β-sites; while additions to the α-sites or chlorinated sites are negligible. For PCDDs with all β-sites being chlorinated, formation of PCDD-γ-OH adducts become virtually the only reaction path. Under the atmospheric conditions, the PCDD-β/γ-OH adducts combine with O2 slowly at rates <1 s−1. Instead, the PCDD-β-OH adducts will react with O2 through hydrogen abstraction at rates <50 s−1, forming PCDD-β-ol, and the PCDD-γ-OH adducts will decompose to the substituted phenoxy radicals by fused-ring C–O bond cleavage at rates of 103 ∼ 105 s−1. The reaction mechanisms of PCDDs are drastically different from the peroxy mechanism for the atmospheric oxidations of benzene and dibenzofuran.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The atmospheric oxidation mechanisms of PCDDs are different from those of benzene. ► Initial OH addition to PCDDs occur mainly on the γ-sites. ► Predicted rate constants for OH addition to PCDDs agree reasonably with experiment. ► PCDD-γ-OH adducts undergo fused-ring C–O cleavage. ► 2,3,7,8-TCDDs will be converted to substituted phenoxy radicals.