No abstract

Different types of corona discharges, produced by DC of either polarity (+/-DC) and positive pulsed (+pulsed) high voltages, were applied to the removal of toluene via oxidation in air at room temperature and atmospheric pressure. Mechanistic insight was obtained through comparison of the three different corona regimes with regard to process efficiency, products, response to the presence of humidity and, for DC coronas, current/voltage characteristics coupled with ion analysis. Process efficiency increases in the order +DC < -DC < +pulsed, with pulsed processing being remarkably efficient compared to recently reported data for related systems. With -DC, high toluene conversion and product selectivity were achieved, CO(2) and CO accounting for about 90% of all reacted carbon. Ion analysis, performed by APCI-MS (Atmospheric Pressure Chemical Ionization-Mass Spectrometry), provides a powerful rationale for interpreting current/voltage characteristics of DC coronas. All experimental findings are consistent with the proposal that in the case of +DC corona toluene oxidation is initiated by reactions with ions (O(2)(+*), H(3)O(+) and their hydrates, NO(+)) both in dry as well as in humid air. In contrast, with -DC no evidence is found for any significant reaction of toluene with negative ions. It is also concluded that in humid air OH radicals are involved in the initial stage of toluene oxidation induced both by -DC and +pulsed corona.

A large (ca 0.7 L) wire-cylinder benchtop reactor was developed and tested for DC corona processing of VOC (volatile organic compound)-contaminated air at room temperature and pressure. The aim of our research is the identification and rationalization of the chemical reactions responsible for VOC removal. Model hydrocarbons, n-hexane and 2,2,4-trimethylpentane (i-octane), were used to characterize the process and compare the effects of DC corona polarity and of humidity on its energy efficiency and products. n-Hexane and i-octane behave very similarly. For both, the energy efficiency is significantly better with negative than with positive DC corona, especially in humid air. The effect of humidity is most interesting. Thus, while with -DC corona the process efficiency is significantly better in humid air, a slight inhibition is observed with +DC corona. Differences between +DC and -DC corona are also found in the amounts of volatile products formed, which include CO2, CO, and minor quantities of organic byproducts (aldehydes, ketones, alcohols, and lower hydrocarbons). A significant fraction of the carbon originally present as VOC is, however, unaccounted for by the analysis of gaseous and volatile organic products and must, therefore, end up as nonvolatile materials and aerosols.

A novel dielectric barrier discharge reactor was developed for the oxidative degradation of organic pollutants in water. Phenol, chosen as a model compound, is efficiently removed from the aqueous solution according to an exponential decay as a function of treatment time at constant voltage. The effect of different experimental variables was investigated, including the active electrodes material and size and the flow rate of air above the solution. A few intermediates and CO 2 , the final product of phenol decomposition, were detected and identified by LC/ESI-MS and FT/IR analysis. The major reactive species formed upon application of the discharge in air, the OH radical and ozone, were determined by means of specific chemical probes.

A mechanistic study is reported on i‐octane and hexane processing with +DC and −DC corona in air at room temperature and pressure. Current/voltage profiles are matched with the ion analysis data obtained by APCI mass spectrometry. With a −DC corona, the hydrocarbons do not modify the negative ion population with respect to uncontaminated air. In contrast, with a +DC corona many hydrocarbon‐derived positive ions form. O(3P) and •OH were also investigated using chemical probes (ozone formation and CO oxidation, respectively). The results, combined with efficiency and product data, suggest that with −DC corona radical initiation steps occur, whereas with +DC corona ionic reactions prevail.