Degradation of aqueous organic pollutants in hybrid gas-liquid electrical discharge reactors

Abstract

Summary form only given. Different types of electrical discharges generated either directly in the liquid phase or in the gas phase close proximity to the liquid surface have been studied as possible methods for water treatment. It has been demonstrated that non-thermal plasma produced by such discharges produce various reactive chemical species such as radicals (H/spl middot/, O/spl middot/, OH/spl middot/) and molecular species (H/sub 2/O/sub 2/, H/sub 2/, O/sub 2/, O/sub 3/). In addition, depending upon the solution conductivity and the magnitude of the discharge energy, pulsed high voltage discharges generated directly in water may also initiate shock waves and ultraviolet radiation. These reactive species and physical conditions in turn have been shown to be effective at degrading a variety of organic compounds and also in the destruction and inactivation of microorganisms in water. We have recently developed hybrid electrical discharge reactors, which combine gas phase non-thermal plasma formed above the water surface and direct liquid phase corona-like discharge in the water. Two types of configurations have been constructed: the hybrid-parallel and hybrid-series. In the series configuration the high voltage needle-point electrode is submerged and the ground electrode is placed in the gas phase above the water surface. The parallel configuration employs a high voltage electrode in the gas phase and a high voltage needle-point electrode in the liquid phase with the ground electrode placed at the gas-liquid interface. Previous experiments have demonstrated the simultaneous formation of ozone in the gas phase and hydrogen peroxide and hydroxyl radicals in the liquid phase of hybrid electrical discharge reactors. These highly reactive oxidative species can degrade many organic pollutants and transform some inorganic pollutants to less toxic products. It is expected that utilization of hybrid reactors will increase the overall efficiency of the corona discharge process for the removal of pollutants from the water as well as to provide for simultaneous treatment of the contaminant in both the gas and liquid phases. In the present study we report on the degradation of several organic pollutants such as phenol, chlorophenol and nitrophenol in hybrid electrical discharge reactors. The mechanism of the degradation process induced by the pulsed high voltage discharges in gas and liquid phases as well as at the interface between a gas and liquid phase will be discussed.