PHOTOLYSIS OF CHLORINATED DIOXINS IN ORGANIC-SOLVENTS AND ON SOILS

Photoreduction of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) in isooctane was found to account for approximately 10% of the loss of TCDD and is thus a minor photolysis pathway. The remainder undergoes conversion by other pathways, which may involve carbon-oxygen cleavage. One new photoproduct of TCDD, formed by a reductive rearrangement, is 4,4;5,5′-tetrachloro-2, 2′-dihydroxybiphenyl, as demonstrated by mass spectrometry. Photolysis of TCDD on soils is slow relative to solution photolysis. Organic solvent added to the soil enhances the extent of photolysis. Evidence is presented that transport of TCDD to the surface in the organic solvent film is primarily responsible for the increased photolysis, rather than an effect from addition of a reducing hydrogen source. In the unamended soils, photochemical loss of TCDD was observed only for the first 5 days of a 15-day irradiation. In soils containing hexadecane, more than twice the amount of TCDD was lost, and photochemical loss continued at both 10 and 15 days of irradiation. This observation suggests that transport of the light-exposed soil/air interface is occurring. Octachlorodibenzo-p-dioxin (OCDD) undergoes photoreduction on soil surfaces to the lower chlorinated congeners. For the pentachloro and tetrachloro congeners, the toxic 2,3,7,8-tetrachlorinated isomers were observed in greater yield than would be expected on the basis of the number of potential isomers. © 1990, American Chemical Society. All rights reserved.