Study On Mechanism And Kinetics Of Reactions For Hydrated Electron With Halogenated Aromatic Hydrocarbons

Halogenated organic compounds including polychlorinated biphenyls (PCBs), are frequently employed as solvents, pesticides and refrigerants and widely distributed in the environment. Their removal technologies and travel processes in aqueous phase and natural water are becoming hot fields which attract more and more researchers’attention at present. The hydrated electron (eaq-) is a good nucleophile and the reactions of eaq-with halogenated organic compounds are known to liberate halide ions. Nanosecond laser flash photolysis transient absorption spectroscopic technique was employed to investigate the reaction mechanisms and kinetics of halogenated organic compounds (chlorobenzene, bromobenzene, dichlorobenzene, biphenyl and2-chlorobiphenyl) with eaq-. Electron beam irradiation was employed to study the dechlorination and degradation efficiency and the G value were calculated. Based on the spectra and kinetic analysis, the reaction products were attributed, the rate constants were acquired and the reaction mechanisms were derived.(1)The photochemical reaction mechanism of pyrenetetrasulfonate (PyTS) aqueous solution was studied with laser flash photolysis-transient absorption spectrum techniques under irradiation at355nm. The characteristic absorption peak of the excited singlet (PyTS1*) at260nm, the excited triplet (PyTS3*) at300nm, and the anion radical (PyTS-*) at330nm were first confirmed. Self-quenching and reaction with PyTS were regarded as the main decay channels for hydrated electrons and the pseudo-first-order rate constant of hydrated electrons with PyTS was determined as2.7×103s-1.The quantum yield of eaq-generated via two-photon ionization of PyTS was computed as3.2×10-2under this condition.(2)Based on the transient spectrum of eaq-at690nm, the rate constants of second-order reaction between eaq-with bromobenzene, chlorobenzene, m-dichlorobenzene, o-dichlorobenzene, p-dichlorobenzene,biphenyl and2-Chlorobiphenyl were1.7×1010,5.3×l08,1.01x1010、4.76×109、3.29×109, 1.28×1010and1.73×1010L mol-1s-1,respectively.(3) In aqueous phase, reactions of eaq-with halogenated organic compounds yield anion radicals. And the transit absorptions of m-dichlorobenzene, o-dichlorobenzene, p-dichlorobenzene,biphenyl and2-Chlorobiphenyl anion radicals lie in390-440nm,390-420nm,410-440nm,390-460nm and390-430nm according to our experiments, respectively. Because the limitation of nanosecond laser flash photolysis apparatus, we cannot get complete absorption spectrums of these anion radicals. So DFT calculations were performed to inspect the optical properties of radicals and the results were in good agreement with the experimental data.(4) Chlorobenzene, dichlorobenzene and2-Chlorobiphenyl removal and dechlorination efficiencies by electron beam irradiation were investigated and the G (Cl-) valve was also calculated. The results show that dichlorobenzene can be decomposed more efficiently than chlorobenzene, but less than2-chlorobiphenyl.(5) Qualitative and quantitative analyses of reaction products were performed by GC/MS, HPLC and IC. The major products of irradiated samples were chloride ion and lower chlorinated benzenes. However in the laser samples of halogenated benzenes, chlorinated biphenyls and biphenyl were detected. That means the concentration of transit species could affect the final products. And no ring-opening products were detected during experiments.