Photochemistry, PCBs

Abstract This article updates the one on the same topic published in this series in 1999. The photochemistry of PAHs and PCBs in liquid water and on ice and other solids such as silica, soil and titanium dioxide continues to be actively studied. The photochemistry of PAHs in all phases continues to be dominated by oxidation by O2, with superoxide (O2- ), excited singlet oxygen (102), and hydroxyl radical ( OH) being the active agents. The recent photochemistry of PCBs has been dominated by practical considerations, i.e. how to use photochemistry to clean up environmental problems involving PCBs. The use of surfactants, the semiconductor TiC>2, and various sources of the powerful oxidant, the hydroxyl radical, in this regard has received considerable attention. 

The photochemistry of polycyclic aromatic hydrocarbons (PAHs) and poly-chlorobiphenyls (PCBs) in water and on solids was reviewed in this series in 1999 [1]. The large interest in this field was due to the fact that many PAHs and mixtures of PCBs are on the United States Environmental Protection Agency s fist of priority pollutants (see [1] for the fist of compounds). Photochemists wanted to know what the fate of these materials in the environment is when exposed to sunlight and if one could use photochemistry to remediate contaminated sites. This interest has continued and will be the focus of the present review. The review covers the recent literature through the middle of 2003. 

No mechanistic studies have appeared on the photochemistry of PCB mixtures or their congeners in water, the environmental liquid, during the reporting period. Several studies on the photochemistry of chlorobiphenyls in... 

Ghosh and co-workers have shown in several studies that Brij 35 and Pol 10 in water provide suitable environments for the photochemically induced reductive dechlorination of chlorobiphenyls [75,77,82,83]. Ghosh and Sayler have demonstrated that photochemistry and microbes work symbiotically to degrade PCB mixtures [76,80]. Photochemistry in a surfactant/water solution converts highly chlorinated biphenyls into much less chlorinated ones. The microbes, which do not oxidize the highly chlorinated biphenyls, then oxidize the less chlorinated biphenyls. For a related study on the combined degradation of a PCB mixture and microbes see [88]. 

In the last of the six papers Hong and Huang have examined how added surfactants such as FC-143, ammonium perfluorooctanoate [CF3(CF2)6 CO2NH4], influenced the Ti02-photocatalyzed decomposition of a large number of PCB congeners adsorbed on soils [102]. The authors concluded that the perfluorinated surfactants are photostable and do not get directly involved in the photochemistry of the chlorobiphenyls (unlike the surfactants mentioned earlier), and the surfactants extract the PCBs from the soil which are then expeditiously photoxidized on TiC>2. This represents a novel two-stage procedure to remove PCBs from soil and then destroy them. 

Pagni RM, Sigman ME (1999) The Photochemistry of PAHs and PCBs in water and on solids. In Boule P (ed) The Handbook of Environmental Chemistry, vol 2, part 2. Springer, Berlin Heidelberg New York, p 139... 

Fig. 10.6 Chemical structures of the brhverdin IXa (BV), phycocyanobrhn (PCB) and phytochro-mobdin (Pd>B) chromophores shown in the ZsZsZa configuration. Each chromophore forms (at C3) a thioether linkage to a cysteine residue of the protein. Reproduced from Strambi A, Durbeej B (2011) Initial excited-state relaxation of the bdin chromophores of phytochromes a computational study, Photochem Photobiol Sci 10 569-579, with permission of the Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC...

If phytochromobilin (P />B) is replaced with phycocyanobilin (PCB) the photochemistry remains the same as described above. However if the D ring of the chromophore is modified then differences in the time-course of the photoreaction are observed [68],... 

The principal direction of the published photochemical research to date has been to elucidate pathways which might be responsible for the degradation of these refractory substances in nature. This basic research has not been extensively applied either to on-or off site treatment of hazardous substances. In this chapter we extend the existing PCB photochemistry literature by considering this potential. The development and fabrication of a prototype surface photoreactor, and its preliminary evaluation at a PCB-contaminated site are described. Photochemical treatments may be applicable to many other classes of hazardous chemicals as well including the polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF) about which this monograph is chiefly concerned. 

The Photochemistry of PCB. The light-mediated reactions of PCB and their analogs have received considerable attention and several reviews of PCB photochemistry are available (i,17,18). Rappe (19) has reviewed the photochemistry of halogenated aromatics in general. 

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). 

Lepine FL, Milot SM, Vincent NM et al (1991) Photochemistry of higher chlorinated PCBs in cyclohexane. J Agric Food Chem 39 2053-2056 Li A, Schoonover TM, Zou Q et al (2005) Polycyclic aromatic hydrocarbons in residential air of ten Chicago area homes concentrations and influencing factors. Atmos Environ 39 3491-3501... 

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... 

Pagni, R. M. and Sigman, M. E., The photochemistry of PAHs and PCBs in water and on solids, in The Handbook of Environmental Photochemistry, Yol. 2, Part L, Environmental Photochemistry, P. Boule, Ed., Springer-Verlag, Berlin, 1999,139. 

Yao, Y, Kato, Y, Kadokami, K., and Shinohara, R., Photochemistry of non-ortho-substituted PCBs by UV irradiation in alkaline 2-propanol, Organohalogen Comp., 36, 381-384, 1998. 

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