8.6.2 Influence of Ultraviolet Radiation on the 

 FateofPCB's 



SERGEI M. CHERNYAK 



Institute of Global Climate and Ecology. State Committee for Hydrometeorology and Academy of Sciences, Moscow. USSR 



Introduction 



Chloroorganic compounds are considered to be the most 

 environmentally resistent of organic compounds. The only 

 abiotic process resulting in PCB destruction is photochemical 

 decomposition (Brown et al.. 1984), which leads to 

 dechlorination of the higher halogenated molecules. Studies 

 on the photochemical degradation of PCB"s showed that the 

 most active destruction of PCB" s at low concentrations occurs 

 at the level of 1 ng/1 and that the PCB half-life values were 

 about 1 to 2 years, depending on solar radiation rates (Bunce 

 etai. 1978) 



It is important to note that PCB transformation by sunlight 

 occurs both as a result of direct light absorption by these 

 compounds and also due to the interaction between reagents 

 formed by the sunlight and subsequent reaction with the PCB 

 molecules — for example, by irradiation of an organic solute 

 that the PCB's might be associated with (Van Noort et al., 

 1988). 



New data on photochemical PCB decomposition processes 

 were obtained during the Third Joint US-USSR Bering & 

 Chukchi Seas Expedition on board the research vesselAkademik 

 Korolev in 1988. 



Materials and Methods 



In order to make an assessment of the impact of 

 photochemical processes on the behavior of chlorinated 

 hydrocarbons in marine ecosystems, scale-model experiments 

 on the decomposition of standard Aroclor 1232 were carried 

 out in the aquatic environment of the Chukchi Sea and the 

 Bering Sea by exposing this Aroclor to natural sunlight. 



The experiments were conducted in 5-1 reactors with a 

 surface area of 400 cm-. The side walls of the reactors were 

 screened with dark foil. The Aroclor 1232 mixture was added 

 in acetone to give a concentration of 100 ng/1 in the sterilized 

 seawater that was placed in the reactors. Control flasks were 

 prepared identically and screened from the sunlight. Every 

 hour, three 0.5-1 samples were removed from each reactor. The 

 samples were extracted by shaking with n-hexane (twice with 

 50 ml). The extracts were concentrated using a rotary evaporator 

 to a volume of 2 ml, whereupon they were purified by shaking 

 them with concentrated sulfuric acid. Tests for PCB microbial 

 decomposition were carried out at the same time as the 

 photochemical tests and under similar conditions. These 

 results are described in Subchapter 4.4 of this volume. 



The PCB content in the solutions was determined using a 

 Hewlett-Packard model 5840A gas chromatograph. The 

 chromatography conditions were as follows: a 30-m fused 

 quartz capillary column with an internal diameter of 0.32 mm 

 and coated with a0.25-m layer of DB- 1 chromatography phase. 

 The analysis was carried out using column thermostat 

 temperature programming as follows; the initial temperature 

 was 120°C. and the temperature was raised at 5°C/min up to 

 250°C and held there for 14 min; thus, the chromatography 

 time was 40 min. The injector temperature was 225°C, the 

 electron capture detector temperature was 300°C. 



Results and Discussion 



Table 1 shows the findings of the photochemical PCB 

 decomposition in Bering Sea water. The data demonstrates 

 that after 6 h of exposure, up to 50% of 

 2,2',3,4-tetrachlorobiphenyl component (BZ# 4 1 - Ballschmitter 

 & Zell, 1980) and 30 to 40% of certain tri- and 

 tetrachlorobiphenyls were decomposed. In the microbial tests, 

 however, 75% of the added 2,2',3,4-tetrachlorobiphenyl and up 

 to 50-60% of some of the low-chlorinated components 

 underwent a reaction of oxidation in 12 h (Fig. 1). 



In the photolysis tests, only seven components of 

 pentachlorobiphenyls exhibited some minor degradation. 

 Hexachlorobiphenyls and higher-chlorinated compounds did 

 not undergo any reaction under the experimental conditions. 



Therefore, only 16 main components of the Aroclor 1232 

 technical mixture were significantly altered in the photochemical 

 degradation tests. 



The experimental data showed that in the seawater under 

 the influence of photochemical PCB degradation, only direct 

 dechlorination proceeded, and this was accompanied by 

 isomerization and condensation. The rate of the reaction 

 appeared to depend more on the molecular configuration than 

 the numberof chlorine substituents. The availability of chlorine 

 atoms in locations 2,2' or 4,4' of biphenyl molecules appears to 

 be a necessary requirement for PCB's to undergo a reaction of 

 photochemical degradation. 



Schematically, the process of photochemical PCB 

 degradation ( hexachlorobipheny 1 ) in seawater can be shown in 

 the schematic (Fig. 2) 



As the reaction scheme demonstrates, the photochemical 

 PCB degradation in the seawater proceeds in a very regular 

 pattern. Direct dechlorination takes place without the rupture 

 of the bonds between the benzene rings and without biphenyl 



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