Direct VUV photolysis of chlorinated methanes and their mixtures in an oxygen stream using an ozone producing low-pressure mercury vapour lamp [An article from: Chemosphere]

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Description:
The gas-phase photooxidations of CCl"4, CHCl"3, CH"2Cl"2 and their binary mixtures in an O"2 stream were studied in a flow reactor under various experimental conditions using a low-pressure mercury lamp as light source covered with a high-purity silica sleeve being used. The 184.9nm VUV irradiation emitted is responsible for the Cl-C bond rupture in the chlorinated methanes and for the formation of O"3 from O"2. The rate of degradation of H-containing chlorinated methanes increased sharply on increase of their initial concentrations, most probably of a ^?Cl chain reaction, as indicated by the increase in the molar ratio of the amount of HCl formed to the amount of H-containing target substance decomposed. The experimental results suggested that the further transformations of the radicals and products formed play an important role as ^?Cl sources, causing a considerably higher rate of decomposition of the H-containing target substances. In a humidified O"2 stream, the ^?OH formed opens up another route for oxidation of the target substances. Thus, the rates of degradation of CH"2Cl"2 and CHCl"3 increased on increase of the relative humidity, whereas the water vapour had no effect at all on the decomposition of CCl"4. At the same time, competition occurs between ^?Cl or ^?OH for reactions with the target substance. The photooxidation of binary mixtures was investigated too. The addition of CCl"4 or CHCl"3 to CH"2Cl"2 strongly increased its degradation rate. The addition of CH"2Cl"2 did not have a considerable effect on the rate of degradation of CHCl"3.