Photodegradation, PCBs

Surface water half-lives range from t/2 4-11 d in freshwater systems, t/2 = 0.1-10 d in cloud water, t,/2 > 1000 d in oceans for PCBs with as many as 8 chlorines for OH- oxidation (Sedlak Andren 1991) photodegradation t,/2 = (7.1 1.5) h in aqueous solution with the presence of diethylamine after exposure to simulated sunlight (Lin et al. 1995). 

Four papers have appeared on the photodegradation of PCBs on sediments [118-121]. Tang and Myers, for example, have carried out a study to model the behavior of PCBs on sediments confined in disposal facilities [ 118, 120]. In their glass aquariums the authors found PCB levels decreased by 40% over a 5-month span. Unfortunately, they were unable to pin point the source of the loss. Tang and Meyers believed the loss was due to a combination of volatilization, photo degradation and biodegradation. 

Three papers have been published on the photodegradation of PCBs in transformer and insulation oils [122-124]. Photolyses successfully degraded PCBs in oils mixed with ground water in spite of the turbidity of the mixtures. 

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]. 

One of the first examples of the use of fluorous solvents in reactions was their use in the extraction of photodegraded solid and liquid wastes contaminated with polychlorinated biphenyls (PCBs). Fluorinated ligands and scavengers... 

At the present time, insufficient data are available to assess the importance of photolysis and/or chemical reactions of particle phase congeners, although studies of Tysklind and Rappe (1991) and Koester and Hites (1992) on the photodegradation of polychlorinated dioxins and fiirans suggest that photolysis for particulate phase PCBs is not important (Atkinson 1996). 

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). 

Other compounds enhance PCB photodegradation including sodium borohydride (44)/ alkaline alcoholic solutions (45) and aqueous suspensions of titanium dioxide (46), a system also effective in treating pentachlorophenol (47). 

The degree to which PCDF and PCDD accumulate in irradiated PCB mixtures will depend on many factors including the concentration of precursor molecules and, in turn, the photolability of the dibenzo products. In any event, the photochemical removal of PCDD and PCDF is likely to be far more efficient than PCB photodegradation itself. PCDD and PCDF are more photolabile than PCB because of their large absorption cross sections and high reaction quantum efficiencies. Chlorinated dibenzodioxins are extremely unstable in UV light in methanol, diesel oil, liquid phenoxy ester formulations and other organic media (52,53). Fortunately, chlorines in the lateral positions (i.e.,... 

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... 

Hutzinger, 0., S. Safe and V. Zitko, Photodegradation of Chlorobiphenyls, In The Chemistry of PCB s, Hutzinger, Safe and Zitko (eds.), CRC Press, Cleveland, Ohio, 1974, Chapter 6. 

Little information exists on the environmental transport and fate of the dioxin-like-PCBs. However, the available information on the physical/chemical properties of dioxin-like PCBs, coupled with the body of information available on the widespread occurrence and persistence of PCBs in the environment, indicates that these PCBs are likely to be associated primarily with soils and sediments and to be thermally and chemically stable. Soil erosion and sediment transport in water bodies and emissions to the air (via volatilization, dust resuspension, or point source emissions) followed by atmospheric transport and deposition are believed to be the dominant transport mechanisms responsible for the widespread environmental occurrence of PCBs. Photodegradation to less chlorinated congeners followed by slow anaerobic and/or aerobic biodegradation is believed to be the principal path for destruction of PCBs. Similar situations exist for the polybrominated biphenyls (PBBs). 

---