Photo-induced Transformation Mechanism Of Polychlorinated Biphenyls In Environmental Condensed Water

The fate of polychlorinated biphenyls in the environment constituted one oftoday's public environmental concerns in respect that they are recognized as one kindof persistent organic pollutants.Condensed water,eg.river,lake,sea,clouds,rain,fog,ice,and snow,is abundant in environment.The study on the reactions ofbiphenyl/4-chlorobiphenyl in environmental condensed water would facilitate thethorough understanding of the sink of polychlorinated biphenyls in environment.Nanosecond laser photolysis/transient absorption spectroscopic technique,togetherwith HPLC-UV and GC-MS, are employed to investigate photo-inducedtransformation mechanism of biphenyl/4-chlorobiphenyl.N(â…¢) and H₂O₂ are selectedas ^·OH precusor in liquid phase and in ice phase respectively.According to theassignment of transient species combined with the analysis of products distributions,the biphenyl/4-chlorobiphenyl reaction mechanism is proposed.Based on the experiment on the photochemical reaction between biphenyl(4-chlorobiphenyl) and N(â…¢),the following points could be drawn:(1) Both biphenyl and 4-chlorobiphenyl would undergo photo-induced reactionin the presence of N(â…¢) in aqueous phase.The reactions are initiated by the additionof OH radical,produced from N(â…¢) photolysis,to benzene ring.The second-orderreaction rate constants for the reactions of OH radical with biphenyl and4-chlorobiphenyl were determined to be (9.6±0.1)×10⁹ L mol^-1 s^-1 and (8.0±0.5)×10⁹ L mol^-1 s^-1 at 298 K,respectively.(2) It is found that pH value would exert a significant impact on thephotochemical reaction.The decomposition rate of biphenyl and 4-chlorobiphenylslows down with pH.(3) The decay of Bp…OH adduct and 4-PCB…OH adduct is fairly complicated.H⁺,HONO,H₂ONO⁺ are found to be capable of reacting with Bp…OH adduct and4-PCB…OH adduct.In addition,4-PCB…OH adduct can also undergo self decay.The reactions of Bp…OH adduct and 4-PCB…OH adduct with H⁺,H₂ONO⁺ are morelikely to produce hydroxylated compounds,while the reactions with HONO toproduce nitrate ones.Based on the experiment on the photochemical reaction between biphenyl(4-chlorobiphenyl) and H₂O₂ in ice,following conclusions could be drawn: (1) The photochemical reaction between biphenyl and H₂O₂ could also take placein ice phase.Since biphenyl is mainly located on grain boundary and H₂O₂ are locatedin bulk ice,the reaction rate is fairly slow as compared with that in liquid phase andsupercooled water.(2) Co-exist organic pollutants are concentrated in quasi-liquid layer.Therefore,the scavenge of OH radical in ice phase is not significant.On the contrary,becauseorganic pollutants could not only tranfer energy with biphenyl but also increase theQLL fraction in ice,the degradation of biphenyl is acclerated in ice in the presence ofco-exist organic pollutants.These conclusions will be helpful to the study on photo-induced transformationof organic pollutants like biphenyl and 4-chlorobiphenyl in environment.