Study On Mechanism Of Reactions Of Biphenyl And 4-Chlorobiphenyl With NO₃ Radical

The fate of polychlorinated biphenyls in the environment constituted one of today's public environmental concerns in respect that they were recognized as one kind of persistent organic pollutants. The study on the reactions between biphenyl/4-chlorobiphenyl and NO₃, a dominant radical in the night atmosphere, would facilitate the thorough understanding of the atmospheric sink of polychlorinated biphenyls due to NO₃ radical. Nanosecond laser photolysis/transient absorption spectroscopic technique was employed to investigate 266 nm laser flash photolysis of biphenyl/4-chlorobiphenyl in acetonitrile solution, as well as the reactions of biphenyl/4-chlorobiphenyl with NO₃ radical generated by 355 nm laser flash photolysis of cerium (IV) ammonium nitrate (CAN) in acetonitrile solution. According to the assignment of transient species combined with the analysis of products distributions, the mechanism of reactions of NO3 with biphenyl/4-chlorobiphenyl was proposed. Meanwhile, the feasibility of biphenyl/4-chlorobiphenyl abatement by means of UV irradiation and strategy in the photo abatement progress were discussed in this paper. Based on the experimental results, the following points could be drawn.(1) Neither biphenyl nor 4-chlorobiphenyl was detected to undergo photo-dissociation under the combined conditions performed. Addition of reductive nucleophilic reagent was identified as a promising method to the photo-degradation of biphenyl/ 4-chlorobiphenyl.(2) Fumaronitrile, which was used as quencher of excitation triplet, attacked excited biphenyl/4-chlorobiphenyl and produced considerable amount of biphenyl /4-chlorobiphenyl cation. The absorption peaks around 400 nm and 700 nm were attributed to 4-chlorobiphenyl cation with A_400nm/A(700nm) equal to 2.8. It has not been reported in literature. 4-chlorobiphenyl cation decayed slower than biphenyl cation with a first-order rate constant of 1.25×10⁵ s^-1, and 2.96×10⁵ s^-1 for biphenyl.(3) The initial reaction of NO3 radical with biphenyl or 4-chlorobiphenyl was a charge-transfer process to yield charge-transfer complex. The reaction was much faster than that with acetonitrile or Ce^â…¢(NO₃)₅²⁺, the other photolytic piece of CAN. The second-order-reaction rate constants for the reactions of NO₃ with biphenyl and 4-chlorobiphenyl were detected as (8.24 ±0.03) ×10⁹ L-mol^-1·s^-1 and (6.90 ± 0.03) × 10⁹ L·mol^-1·s^-1, respectively.(4) The primary product of the reaction between biphenyl and NO3 transformed to o- and p- nitro-biphenyls via elimination. The rate constant k_obs was (2.30 ± 0.01) x 10⁵ s^-1.(5) The charge-transfer complex, also as main transient intermediate from the reaction between 4-chlorobiphenyl and NO₃, was observed to decay in first order, and the rate constant was determined to be (1.47 ± 0.05) ×10⁵ s^-1. It will transform to chloro-nitro-biphenyl via elimination. More complicated ring-cleavage reactions were found to generate smaller pieces which contributed to cross-link products such as dioxin-like substances.(6) It was showed that analyzing light could also excite the species produced by laser flash photolysis. Some excited species absorbed light, while others emitted light. Both of them could disturb the determination and analysis of the transient absorption spectrum. In this paper, cutters-off were adopted to discriminate the different absorption peaks by their real source.These conclusions will be not only helpful to the study on photochemical transformation of organic pollutants like biphenyl and 4-chlorobiphenyl in atmosphere, but will provide meaningful information for UV photo-degradation of biphenyl and 4-chlorobiphenyl as well.