Photodegradation dechlorination

Among the organochlorine insecticides studied, lindane was the least affected chemical. This somewhat surprising, since it has been known that lindane is susceptible to dechlorination as well as dehydrochlorination just like DDT. Such differences in substrate susceptibility to this flavoprotein-stimulated photodegradation process indicate some degree of specificity and point to the need for future studies. 

Bell (1956) reported that the composition of photodegradation products formed were dependent upon the initial 2,4-D concentration and pH of the solutions. 2,4-D undergoes reductive dechlorination when various polar solvents (methanol, butanol, isobutyl alcohol, ferf-butyl alcohol, octanol, ethylene glycol) are irradiated at wavelengths between 254 to 420 nm. Photoproducts formed included 2,4-dichlorophenol, 2,4-dichloroanisole, 4-chlorophenol, 2- and 4-chlorophenoxy-acetic acid (Que Hee and Sutherland, 1981). 

Photolytic. The UV photolysis (7, = 300 nm) of bifenox in various solvents was studied by Ruzo et al. (1980). In water, 2,4-dichloro-3 -(carboxymethyl)-4 -hydroxydiphenyl ether and 2,4-di-chloro-3 -(carboxymethyl)-4 -aminodiphenyl ether were identifled. In cyclohexane, 2,4-dichloro-4 -nitrodiphenyl ether and methyl formate were the major products. In methanol, a dichloro-methoxy phenol was identified. Photodegradation occurred via reductive dechlorination, de-carboxymethylation, nitro group reduction, and cleavage of the ether linkage (Ruzo et al., 1980). 

Just as the interiors of micelles in water provide a hydrophobic environment to solubilize chlorobiphenyls and then photodegrade them by reductive dechlorination, so do octadecyl-functionalized silica gel [89], normally used as reverse phase packing in HPLC and for solid-phase extraction, and the hquid-semisohd polydimethylsiloxane [-OSi(CH3)2 -] [90,91], also used for solid-phase extraction. In both media, reductive dechlorination is the primary photochemical pathway. 

UV irradiation at 260-320 nm in a Rayonette reactor in methanol of Chloralkylene-12 led to formation of reductive dechlorination products at a significant rate. This result indicated a greater overall ease of photodegradation of the alkyl substituted PCBs relative to the parent compound [66]. By irradiation at over 290 nm wavelength UV both in water and on a wet silica gel column in air flow, Chloralkylene-9 was transferred mainly to hydroxylation and reductive dechlorination products by similar mechanisms to PCBs [67]. 

PCDEs can be dechlorinated or they can form chlorinated dibenzofurans by photodegradation [71]. The conversion of PCDEs into PCDFs can happen under environmental conditions via natural photolytic reactions. The photochemical ring closure yielding PCDFs happens to PCDEs which have at least one ortho-chlorine. In this reaction ortho-chlorine is lost. Photochemical reactions of some PCDEs have been presented in Fig. 3. 

The majority of PCB photodegradation studies have been carried out in solution with organic or combined solvents preferred due to their low water solubility. Reductive dechlorination is the predominant photodegradation pathway in hydrocarbon solvents as well as alcohols. In aqueous media and alcohols photonucleophi 1 ic displacement reactions yielding phenols or alkoxy derivatives, respectively, also are detected. In the gas phase PCB are photochemically converted to hydroxylated derivatives (26). Photochemical reduction also is the major reaction pathway in thin films (7), solid phase (8) and on silica gel surfaces (9). 

Chlorinated terphenyls are reductlvely dechIor1nated, but also undergo substantial photosubstitution In alcohol solvents ( ) In contrast to the low yield of substitution products from PCB. In the terphenyl series photosubstitution is believed to take place by attack of the nucleophile directly on the excited state with concomitant chloride ejection. Alkylated PCB molecules (chloroalkylenes), candidate PCB replacement compounds, exhibit UV electronic spectra nearly Identical to those of PCB yet photodegrade more rapidly (28). Another possible PCB substitute consists of a mixture of bis-(chlorophenyl)ethanes (Iralec) that also undergoes dechlorination and substitution reactions on Irradiation (32). 

Enhancement of PCB photodegradation also is observed in the presence of t r i f luoroace t i c acid (40), presumably via the protonated intermediates (ArClH ). Dienes, such as 1,3-cyclohexadiene, accelerate haloaromatic photoreactions (41.) and modify chloronaphthalene photochemistry by enhancing reductive dechlorination and suppressing dimerization (42). The mechanism is believed to involve exciplex formation with the diene or protonation by the olefin (43). 

Chlorinated dioxins can undergo photodegradation, losing their chlorine atoms. Such dechlorination takes place in the presence of a hydrogen donor such as alcohols and hydrocarbons and UV light (Crosby et al. 1971 Liberti et al. 1978) and occurring preferentially at the 2,3,7, and 8 positions (Nestrick et al. 1980). 

Other routes of breakdown of PCBs are photolysis and catalytic processes. High-energy UV radiation and sunlight can cause photodegradation of PCBs, which absorb energy within the range 280-300 nm. The process is accelerated by amines and Ti02. Photochemical dechlorination is slow when the chlorine content of PCBs is low and does not produce fully dechlorinated biphenyls... 

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