TABLE 1 



Photochemical degradation of different PCB components in sea water. 



Time of Exposure 

 BZ' CI 



'Numbering corresponds to the convention of Ballschmitter and Zell ( 1 980). 

 h = hours d = days 



c " 

 o c 



b ° 



o ^-' 



B 



o 

 O 



10 

 Exposure time. 



15 



21 davs 



Fig. 1. 



Decomposition of PCB (Dichlorobiphenyls) under microbial and 

 Photochemical transformation. Bering Sea. 53°58' N, I75°28' W. 



microbial decomposition 



photochemical decomposition 



pholochemical dccompostiton in presence if PAH. 



Structure decay. These reactions are accompanied by 

 isomerization condensation processes, resulting in the fonnation 

 of terphenols, tetraphenyls, and dibenzofurans (Bunce et ai, 

 1 978 ). These dechlorination products are now more susceptible 

 to subsequent microbial degradation. 



When comparing the data on the scale-model experiments 

 on photochemical breakdown with microbial PCB degradation 

 in the seawater, it was determined that many of the PCB 

 congeners that underwent photochemical degradation were not 

 the ones undergoing decay from exposure to microflora. Of all 

 the components of the technical Aroclor 1232 product, only 

 2,3"4,4'-tetrachlorobiphenyI and 2,3'.4'.5-tetrachlorobiphenyl 

 responded to both microbial and photochemical degradation. 

 However, the rate of photochemical reactions is 10 to 15 times 

 slower than the rate of microbial degradation (see Subchapter 

 4.4 of this volume). 



It was also determined that othercontaminants (for example, 

 PAH's) inhibited the photochemical degradation of PCB's by 

 10%. Furthermore, it was found that PCB's were inhibitory 

 toward the photochemical oxidation of benzo(a)pyrene by 

 20% (Fig. 3 ). (These experiments were carried out jointly with 

 N. Irha and E. Urbas from the Estonian Academy of Sciences 

 and the final results will be published in the future.) 



Therefore, the processes of photochemical PCB 

 degradation were studied for the first time in the marine 

 environment in the arctic regions of the oceans during the Third 

 Joint US-USSR Bering & Chukchi Seas Expedition. The data 

 allowed definition of photochemical PCB oxidation in seawater, 

 and gave a quantitative assessment of this process, which is 

 very important to know when studying biochemical cycles and 

 predicting ecological situations in marine ecosystems. 



344 



