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Investigations of Photolysis and Photooxidation Reaction Mechanism of Fluorinated Organic Compounds

Author: ZhangRenXi
Tutor: HouHuiQi
School: Fudan University
Course: Environmental Science
Keywords: Fluorinated organic compounds Perfluorooctanoic acid Benzotrifluoride Fluorobenzene p-difluorobenzene Hexafluorobenzene Laser flash photolysis-transient absorption spectrum technique Photolysis Photooxidation Reaction mechanism and kinetics
CLC: X132
Type: PhD thesis
Year: 2007
Downloads: 351
Quote: 0
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Abstract


The distinct physical and chemical properties of fluorinated organic compounds （FOCs） makethem valuable constituents in a wide range of industrial and commercial applications, includingherbicides, surface treatment agents, insecticides, polishes, wetting agents, fire-fighting foams,polymer synthesis and so on. The distribution, fate and degradation of fluorochemicals have beenreceived much attention because they have an impact on human health or the environment due totheir bioaccumulation and toxicological properties. The photolysis and photooxidation reaction mechanism and kinetic parameter of FOCs, such as perfluorooctanoic acid（PFOA）,benzotrifluoride（BTF）,monofluorobenzend（MFB）,p-difluorobenzene（DFB）,hexafluorobenzene（HFB）, was investigated by laser flash photolysis-transient absorption spectrum technique underthe irradiation at 355 nm or 266 nm. The objective of these studies was to determine the reactionprocess of fluorochemicals with reductive radicals (·CH3, I2·-) and oxidative （·OH）, and also toinvestigate the overviews of the photolysis kinetics of them and the cross-reaction mechanism ofaromatic organofluorine compounds with HNO2. Based on the experimental results, theconclusions are drawn as below:The photoreaction mechanism of PFOA-acetone and PFOA-I2-KI aqueous solutions werestudied under the irradiation at 266 nm. The results indicated that PFOA could not react with·CH3and I2·-, although·CH3 is the strongest reactivity alkyl radicals and the photodetachment efficiencyfor I2·-, an important nucleophilic reagent, is larger than for Cl2·- and Br2·-. Therefore, theconclusion is accessibly believed to be that PFOA can not be decomposed to F- by other alkylradicals or common nucleophilic reagents.BTF in the aqueous or hexane solutions have been proved to be rather photoisomerized withformation of i-Ph-CF3*A and i-Ph-CF3*B than photodissociated into fragments under theirradiation at 266 nm （6 mJ/pulse）. Under the pulse energy of 23 mJ, the generally accepted viewis that BTF in aqueous can be photoionized by absorbing two 266nm photons without any furtherphotodissociation.BTF was oxidized by hydroxyl radicals derived from the photolysis of H2O2 irradiated at266nm, with a second-order rate constant of 2.2x1010 L·mol-1·s-1 to form two type adducts,C6H5CF3OH and C6H5（OH）CF3. The former, C6H5CF3OH, yielding in the process of the CF3group attacked by·OH, decayed by elimination of F- ions to form benzoic acid （C6H5COOH）. Andthe latter, C6H5（OH）CF3, which was produced by another reaction route with the·OH attackposition in the aromatic ring of C6H5CF3, yielded o-, m- and p-trifluoromethylphenol by anon-defluorination reaction.The addition of OH radical to C6H5F was followed by a rapid formation of a C6H5F…OH adduct with a second-order rate constant of 5.83×109 L·mol-1·s-1 in the C6H5F-HNO2 aqueoussolution irradiated at 355nm. The adduct, C6H5F…OH, which had the absorption band of260～340nm peaked at 300 nm, mainly decayed by the reaction with HNO2 with an apparentsecond-order rate constant of 8.02×107 L·mol-l·s-1. The elimination reaction of H2O was alsoobserved to give the corresponding intermediate product, monofluorophenyl radical, butdefluorination reaction not observed in the above solution. The final products were identified byGC-MS technique to be monofluorophenol, nitrylmonofluorobenzene, nitrylmonofluorophenoland 4,4’-difluorobiphenyl in the C6H5F-HNO2 aqueous solution under the 355nm irradiation.The reaction of DFB with OH radicals, with a second-order rate constant of 4.7×109 L·mol-1·s-1, have been carried out in the C6H4F2-HNO2 aqueous solution under the 355nm irradiationwith the generation of C6H4F2-OH adduct which had the absorption bands of 260～340nm and340～400nm peaked at 290 nm and 390nm,respectively. C6H4F2-OH adduct decayed not only bythe elimination of HF to form para-fluorophenoxyl radical （p-FC6H4O·） peaked at about 280nm,but also by the reaction with HNO2 with an apparent second-order rate constant of 5.3×107 L·mol-1·s-1. The final products were also analysed by GC-MS apparatus to be 4-fluoro-2-nitylphenol,1,4-difluoro-2-nitylbenzonene, 4-fluorophenol, 2,5-difluorophenol in the C6H4F2-HNO2 aqueoussolution irradiated at 355nm UV light.The reaction mechanism of C6F6-HNO2 aqueous solution was studied by laser flashphotolysis-transient absorption spectrum technique under the irradiation at 355 nm. Thecharacteristic absorption peaks and the kinetic parameters of transient species were alsoinvestigated. Hydroxyl radical derived from the photolysis of HNO2 added to Hexafluorobenzenewith a second-order rate constant of 1.8×109 L·mol-1·s-1 to form an adduct, C6F6…OH, which hadabsorption peaks at 250, 270 and 400 nm. The C6F6…OH adduct decayed by elimination of HF toyield C6F5O·with an apparent first-order rate constant 6.1×105 s-1. In the presence of O2,C6F6…OH underwent a complex reaction with a rate constant of 2.8×109 L·mol-1·s-1 to formC6F6OHO2, which had the same absorption bands as C6F6…OH.The final products of C6F5OH andC6F4O2 were identified by GC-MS technique, but the nitro compounds, which are more harmfulthan hydrocarbon chemicals, were not found in the above reaction solution.This paper has emphasized a deep comprehending on the photochemical conversion reactionsprocess of fluorinated organic compounds. The experiment results may provide the usefulinformation about the fate of the fluorochemicals in environment, and also offer additionalknowledge for treatment and degradation of them.

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