Photodegradation, 2,3,7,8-TCDD

CjTCDD on a silica plate was exposed to summer sunlight at Beltsville, MD for 20 h. A polar product was formed which was not identified. An identical experiment using soil demonstrated that photodegradation occurred but to a smaller extent. No photoproducts were identified (Plimmer, 1978). TCDD on a silica plate was exposed to a GE Model RS sunlamp at a distance of 1.0 m. The estimated photolytic half-life for this reaction is 140 h (Nestrlck et al., 1980). 

Liberti et al. (1978) reported that TCDD will photodegrade provided a hydrogen donor is added in the presence of sunlight or high-pressure mercury lamps (X = 280-320 nm). The photodegradation rate was found to be related to the intensity of the irradition, temperature, and the medium in which TCDD is dispersed. The primary photodecompositions product of 2,3,7,8-TCDD were 2,3,7-trichlorodibenzo[Ae][l,4]dioxin and hydrogen chloride. 

Determine the mechanistic features of the photodegradation of 2,3,7,8-TCDD in mice... 

Dougherty et al. (1993) conducted a theoretical analysis of a proposed in situ method for decontaminating soil by photodegradation. Up to 86% of TCDD in the soil can be degraded by this process (Zhong et al. 1993). Because of its extremely low water solubility and volatility, TCDD is a very persistent soil contaminant. With the method, based on the physical properties that facilitate photolysis... 

Zhong Y, Overcash MR, McPeters AL. 1993. Near sunlight zone model for photodegradation of TCDD in soils containing organic solvents. Chemosphere 26(7) 1263-1272. 

Photodegradation. Of all the possible natural degradation mechanisms, photodegradation has been suggested to be the most significant (3,9,10,27). 2,3,7,8-TCDD undergoes rapid degradation when... 

Molecular orbital calculations of 2,3,7,8-TCDD and dibenzo- -dioxin indicate that excitation of either molecule results in promotion of an electron into an anti-bonding orbital with electron density primarily located on the 2,3,7,8 positions, with only a small percentage on the 1,4,6,9 positions (37). The primary photoproducts of 2,3,7,8-TCDD in organic solvent has been suggested to be the 2,3,7-trichloro Isomer and the dlchloro isomers (38). These photoproducts also appear to photodegrade faster than 2,3,7,8-TCDD. No other photoproducts have been identified from the photolysis of... 

Vaporization and/or photodegradation at the surface of a contaminated soil is a loss mechanism for TCDD. 

If vaporization and/or photodegradation of TCDD at the soil surface occurs, the soil near the surface should have lower TCDD concentrations than the soil layers below the surface. Previous calculations by the authors indicated that the the lowering would only be detectable in the top 1 cm of soil after a normal hot Missouri Summer. 

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