Analysis of gas phase intermediates and mineralization during the photocatalytic oxidation of organic pollutants: a critical step towards the performance evaluation

Abstract

This work illustrates the capabilities of two advanced analytical methods, Automated Thermal Desorption/Gas Chromatography/Mass Spectrometry, (ATD/GC/MS) and Gas Chromatography, Pulse Discharge Helium Ionization Detector, (GC/PDHID) for a better understanding of the photocatalytic degradation of organic compounds in air at ppbv levels. For this purpose two compounds, viz. dichlorvos and 2-propanol were chosen and the influence of relative humidity (RH%) on the oxidation efficiency was investigated. According to the RH levels, different mechanisms of degradation as well as rate of mineralization were proposed. For example at 40% RH, the main reaction intermediates coming from dichlorvos degradation was dichloroacetaldehyde while traces of phosgene were observed. In contrast, at 0% RH, trichloroacetaldehyde was found to be the main intermediate and a higher amount of phosgene was observed. When only the conversion rate was followed, it was impossible to assess the photocatalytic efficiency of the air cleaning system since whatever RH values considered, high rates of conversion were always measured. However using PDHID detector it was possible to calculate the rate of mineralization which varies drastically as a function of RH: a decrease in the mineralization rate with increasing RH is shown. In addition, it was shown that during photocatalytic treatment of bilge water in an aerated reactor, volatile organic compounds (VOCs) were emitted which are potential source of pollution for workers in engine room. In this case, an additional treatment of the gaseous phase would be necessary.