Polychlorinated biphenyls photolysis

S. Safe and O. Hutzinger, Polychlorinated Biphenyls Photolysis of 2,4,6,2, 6 Hexachlorobiphenyl, Nature232 (1971) 641-42 O. Hutzinger et al., Polychlorinated Biphenyls Metabolic Behavior of Pure Isomers in Pigeons, Rats, and Brook Trout, Science 178 (1972) 512-14 O. Hutzinger et al., Identification of Metabolic Dechlorination of Highly Chlorinated Biphenyl in Rabbits, Nature 252 (1974) 698-99. 

IT Corporation (IT) developed a two-stage photolytic and biological soil detoxification process to treat soils contaminated with polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-/7-dioxin (TCDD). The photolysis/biodegradation process has been evaluated under the U.S. 

Polychlorinated diphenyl ethers (PCDE) are common impurities in chlorophenol formulations, which were earlier used as fungicides, slimicides, and as wood preservatives. PCDEs are structurally and by physical properties similar to polychlorinated biphenyls (PCB). They have low water solubility and are lipophilic. PCDEs are quite resistant to degradation and are persistent in the environment. In the aquatic environment, PCDEs bioaccumulate. These compounds are found in sediment, mussel, fish, bird, and seal. PCDEs show biomagnification potential, since levels of PCDEs increase in species at higher trophic levels. PCDEs are also detected in human tissue. Despite the persistence and bio accumulation, the significance of PCDEs as environmental contaminants is uncertain. The acute toxicity and Ah-receptor-me-diated (aryl hydrocarbon) activity of PCDEs is low compared to those of polychlorinated di-benzo-p-dioxins (PCDD) and dibenzofurans (PCDF). Due to structural similarity to thyroid hormone, PCDEs could bind to thyroid hormone receptor and alter thyroid function. Furthermore, PCDEs might be metabolized to toxic metabolites. In the environment, it is possible that photolysis converts PCDEs to toxic PCDDs and PCDFs. 

The reactivity, fate, and distribution of bound solutes are certainly changed by association with stream humic substances. The rate of photolysis of certain organic compounds (Zepp et al., 1981a,b), the rate of volatilization of polychlorinated biphenyls (Griffin and Chian, 1980), the bioaccumulation of polynuclear aromatic hydrocarbons in fish (Leversee, 1981), the rate of humic acid induced acid-base catalysis (Perdue, 1983), and the rate of microbiological decomposition are some specific examples. The octyl ester of 2,4-D (2,4 DOE) was predicted by theoretical and mathematical models and found by experimentation to be resistant to base hydrolysis when bound to humic substances (Perdue, 1983). The same model predicted the humic acid catalyzed hydrolysis of atrazine as demonstrated by Li and Felbeck 11972). 

Effects of microlayer constituents or other surface-active agents on photochemical reactions in aqueous media have not been widely studied. Zadelis and Simmons (1983) demonstrated that the photodecomposition of naphthalene in Lake Michigan microlayer material was slower by a factor of about 2 relative to that in lake water. Larson and Rounds (1987) also showed that a 30 mM concentration of a surface-active material (sodium dodecyl sulfate) significantly decreased the photolysis rate of 1-naphthol at pH 7. In contrast, Epling et al. (1988) showed that borohydride-promoted photodechlorination of polychlorinated biphenyls was significantly increased in the presence of ca. 100 mM concentrations of the surface-active agents Brij-58 (a polyethoxyethanol) and sodium dioctyl succinate. The mechanisms for these observed rate effects are unknown. 

Chesta et al. (1986, 1988) have shown that the photolysis of N,N-dimethylaniline can be coupled with uptake of the electron (or with direct electron transfer from the excited aniline singlet or triplet) by chlorinated benzenes and fragmentation of the resulting radical anion to chloride ion. Hydrogen-substituted benzene derivatives appear to be the other products. Bunce and Gallagher (1982) have shown that a similar aniline-sensitized dechlorination occurs with polychlorinated biphenyls. 

Attempts to couple electrochemical ozonisers with UV light have been investigated [75]. When dissolved ozone is irradiated with 254 nm wavelength UV light, a photo-enhanced oxidation process occurs due to the increased reactivity caused by the hydroxyl radical (which is formed by photolysis of the ozone). This method totally oxidises a wide range of persistent organics (pesticides, polychlorinated-biphenyls) and pyrogens. 

An initial photolysis reaction can result in the generation of reactive intermediates that participate in chain reactions that lead to the destruction of a compound. One of the most important reactive intermediates is the hydroxyl radical, HO-. In some cases, sensitizers are added to the reaction mixture to absorb radiation and generate reactive species that destroy wastes. Hazardous waste substances other than 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) that have been destroyed by photolysis are herbicides (atrazine) 2,4,6-trinitrotoluene (TNT) and polychlorinated biphenyls (PCBs). The addition of a chemical oxidant, such as potassium peroxydisulfate, K2S20g, enhances destruction by oxidizing active photolytic products. 

Aryl halides tend to be chemically unreactive and include persistent environmental pollutants such as dichloro-diphenyl-trichloroethane (DDT), polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and polybrominated diphenyl ethers (PBDEs). Many studies of the photochemistry of halogenated aromatic compounds have been stimulated by environmental concerns, the goal often being to understand whether photolysis is an important sink for these compounds in natural waters - or in the atmosphere.The photochemistry of aryl halides causes problems in this context because many aryl halides have minimal absorption in the region of the tropospheric solar spectrum (>295 nm), and experiments at environmentally irrelevant wavelengths such as 254 nm are... 

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