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 gamma-sites, followed by the non-chlorinated beta-sites; while additions to the alpha-sites or chlorinated sites are negligible. For PCDDs with all beta-sites being chlorinated, formation of PCDD-gamma-OH adducts become virtually the only reaction path. Under the atmospheric conditions, the PCDD-beta/gamma-OH adducts combine with O-2 slowly at rates <1 s(-1). Instead, the PCDD-beta-OH adducts will react with O-2 through hydrogen abstraction at rates <50 s(-1), forming PCDD-beta-ol, and the PCDD-gamma-OH adducts will decompose to the substituted phenoxy radicals by fused-ring C-O bond cleavage at rates of 10(3) similar to 10(5) s(-1). The reaction mechanisms of PCDDs are drastically different from the peroxy mechanism for the atmospheric oxidations of benzene and dibenzofuran. (C) 2012 Elsevier Ltd. All rights reserved.