Study On The Degradation Mechanism Of Bromobenzene Induced By The Photolysis Of HNO₂(NO₂^-) Under Ultraviolet Light

As one kind of the aromatic hydrocarbons exists in the environment with strongtoxicity and low degradation, the transformation of bromobenzene has been paid moreand more attention. Nitrous acid is an important pollutant widely distributed in naturalwaters and the liquid phase of atmosphere (precipitation droplets such as cloud, frog,rain and the surface layer of water around the aerosol particles). Not only it canabsorbs ultraviolet radiation of 300-400 nm in the sunlight to reduce·OH radical, butalso it can make cross reaction with certain pollutants in the environment. In thisstudy, bromobenzene and 1,4-dibromobenzene were selected as two main subjects tobe investigated on the photoreaction under the laser irradiation at 355 nm andultraviolet light at 365nm from macroscopic and microscopic points, respectively.The cross-reaction mechanism under ultraviolet light at 365nm was investigatedabout C₆H₅Br-HNO₂(NO₂^-) aqueous phase. The photolysis and attenuation behaviourof bromobenzene in some certain aqueous phases was investigated systematically.Various factors which have effects on the photolysis of bromobenzene were probedinto and the following points could be drawn:(1) The reaction rate constant of C₆H₅Br photolysis which induced by the photolyzeof NO₂^- was found to be independent with the initial concentration of bromobenzenebut quickened linearly with the increase of the initial concentration of sodium nitrate.(2) The photolyzing rate of C₆H₅Br photolysis which induced by the photolyze ofNO₂^- varied with different pH value of the solution. The process of C₆H₅Br photolysisquickened when the solution pH was 4 and slowed down at alkaline condition.(3) Through researches on the five chosen anions, it was found that Cl^-, PO₄^3- andCO₃^2- had relatively more negative effects on the degradation process ofbromobenzene than that of SO₄^2- and NO₃^-. The negative influencing degree of thefive chosen anions from high to low was CO₃^2-＞PO₄^3-＞Cl＞SO₄^2-≈NO₃^-.(4) Br^- was detected in the photolysis products when the aqueous phase of C₆H₅Br-HNO₂(NO₂^-)exposed to ultraviolet light after a certain period of time. The GC-MSanalysis suggested the formation of bromophenol and nitrophenol.Also, the ultraviolet photoreaction mechanism of C₆H₅Br-HNO₂(NO₂^-) and1,4-dibromobenzene-HNO₂(NO₂^-) aqueous phases was studied by laser flashphotolysis transient absorption spectrum technique. The correlated reaction rate constants were obtained by kinetic analysis of the transient absorption spectrum. Themicro reaction mechanism was revealed from the micro kinetic view. The followingpoints could be drawn:(1) The OH radical was produced by the irradiation at 355nm ultraviolet light onHNO₂. C₆H₅Br…OH was created by the reaction of OH radical with bromobenzene bya rate constant of (8.1±0.7)×10⁹ L·mol^-1·s^-1, whose absorption peak centered at320nm. C₆H₅Br…OH can be oxidized by HNO₂ by a rate constant of (3.0±0.5)×10⁷L·mol^-1·s^-1. The oxidation product was nitro compound.(2) When encountered with oxygen in the oxygen enrichment water, C₆H₅Br…OHwould react quickly with it to produce C₆H₅Br…OHO₂ at a rate constant of (4.0±0.6)×10⁸ L·mol^-1·s^-1. C₆H₅Br…OHO₂ would decay at a rate constant of (2.4±0.1 )×10⁴s^-1 rather than existing stably in the water and resulted in quinines.(3) The OH radical could react with dibromobenzene to produce BrC₆H₄Br…OH byproduced by a rate constant of (5.3±0.3)×10⁹ L·mol^-1·s^-1. BrC₆H₄Br…OH could beoxidized by HNO₂ at a rate constant (5.2±0.3)×10⁷ L·mol^-1·s^-1 with the absence ofO₂ or react with O₂ to produce BrC₆H₄Br…OHO₂ in oxygen enrichment condition.(4) The degradation kinetics process of bromobenzene induced by the irradiation of355nm ultraviolet of some certain inorganic negative anions, such as Cl^-, PO₄^3- andCO₃^2- were probed into by microscopic view. The complex reaction between·OH andCl^- was discussed and some reasonable constructions were brought forward to themacroscopical experiment phenomenon.(5) The research shows that bromobenzene could be converted into nitro compoundthrough series of photochemical reactions in the natural environment. Part of the nitrocompounds in the natural environment does not mainly originate from directdischarge, but most possibly from the conversion of derivation of benzene throughsome photochemical reactions of HNO₂ or NO₂^- which are widely distributed innatural waters and the liquid phase of atmosphere.