Formation of PCDF from photodegradation of solutions of polychlorinated phenoxyacetic acid herbicides
Rahim, R.A.A., Dlugogorski, B.Z., Kennedy, E.M. and Mackie, J.C. (2013) Formation of PCDF from photodegradation of solutions of polychlorinated phenoxyacetic acid herbicides. Organohalogen Compounds, 75 . pp. 910-914.
First introduced in 1940s, phenoxy herbicides are inexpensive and selective in targeting broad leave weeds, and are harmless to commercial crops. Unfortunately, the most common industrial process for production of one of the herbicides, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), involves a side-reaction forming highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). In spite of its attractive herbicidal properties, by late 1980s, difficulties in manufacturing 2,4,5-T without the 2,3,7,8-TCDD contamination resulted in the banning and phase-out of this herbicide. Since then, other phenoxy herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), have replaced 2,4,5-T.
Photodegradation, biodegradation and combustion, through accidental fires control the fate of phenoxy herbicides in the environment. In particular, photodegradation of these herbicides has been reported to induce the production of polychlorinated phenols (PCP). The polychlorinated phenoxy species that form PCP might also produce polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in solutions of phenoxy herbicides. A large body of literature exists on condensation of chlorophenols into various congeners of PCDD/F, via side reactions in thermal oxidation processes, as reviewed by Altarawneh et al. The absorbance of high energy from UV radiation could also initiate ring activation, dechlorination and radical condensation reactions in aqueous phase that could result in the formation of PCDD/F. However, until present, the UV-induced generation of PCDD/F from solutions of phenoxy herbicides via PCP has not been reported in the literature. Likewise, no measurements exist on the formation of PCDD/F from photodegradation of chlorophenols, except for pentachlorophenol.
This contribution reports new measurements of PCDD/F forming in solutions of phenoxy herbicides as a consequence of their photodegradation, indicating a new source of PCDD/F in the rural environment. We selected 2,4,5-T and 2,4-D, the former because of its high chlorine content and the latter due to its widespread use, and applied methanol as solvent to achieve higher initial concentration of the herbicides. The results demonstrate formation of several PCDF congeners, especially 2,3,7,8-TCDF, by UV irradiation of solutions of phenoxy herbicides.
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