Los Alamos THE OZONIZER DISCHARGE AS A GAS - PHASE ADVANCED OXIDATION PROCESS ( Paper for Proceedings

Ozonizers have been used for over a century in water treatment and for about two decades in advanced oxidation. Ozonizers are fundamentally based on non-thermal plasmas, which are useful for generating reactive species (free radicals) in gas streams. Because radical-attack reaction rate constants are very large for many chemical species, entrained pollutants are readily decomposed by these radicals. Non-thermal plasmas can generate both oxidative and reductive radicals; therefore, they show promise for treating a wide variety of pollutants. However, this application is only about a decade old, so more work is needed for optimizing and commercializing the process. Introduction In the past several years, there has been increased interest in gas-phase pollution control arising from a larger body of environmental regulations and a greater respect for the environment. One promising class of pollution-control technologies is that called advanced oxidation processes (AOPs). Historically, AOPs first used highly reactive hydroxyl radicals (OH), generated from the photolysis of ozone (03) or hydrogen peroxide (H202), or the direct combination of 03 and H202, to treat recalcitrant water pollutants [1]. Therefore, ozone played a seminal role in the invention of AOPs. Recently, the definition of an AOP has been expanded to include processes which involve other free radicals, some of which are reductive rather than oxidative, and to the treatment of gaseous as well as aqueous-based effluents. AOPs show particular promise for the treatment of hazardous and toxic pollutants (e.g., volatile hydrocarbons and halocarbons and oxides of sulfur and nitrogen) because the reaction rates of free radicals with organic compounds can be orders of magnitude larger than a strong oxidizer like 03. In the gas phase, free radicals and other highly reactive species can be generated with plasmas. A plasma (in electrical terminology) is an ionized state of matter (sometimes called the fourth state of matter) containing electrons and ions. A plasma behaves much like an electrical gas, consisting of charged particles. Plasmas can be created thermally by heating ordinary matter to a temperature greater than about 10,000 C. In such a thermal plasma, all the species electrons, ions, neutral atoms and molecules are all in thermal equilibrium (i.e., at the same temperature). In contrast to a thermal plasma, a non-thermal plasma (or non-equilibrium plasma) is characterized by electrons which are not in thermal equilibrium with the other gas species. The electrons are hot (few to tens of eV temperature), while the ions and neutral gas species are cold (near-ambient temperature). The key idea in non-thermal plasma processing is to direct electrical energy into favorable gas chemistry through energetic electrons, rather than using the energy to heat the gas. Two common ways of creating a non-thermal plasma are by an electrical discharge in a gas or the injection of an energetic electron beam (e.g., 100 keV 1 MeV) into a gas. Both processes create secondary plasma electrons, with a distribution of electron energies defined by an average electron energy (or electron temperature). An excellent non-thermal plasma source for generating atmospheric pressure, gas-phase, pollutant-decomposing species is the ozonizer discharge [2, 3]. This paper is intended to serve as an introduction to the subject of pollutant decomposition with the nonthermal plasmas generated by ozonizer discharges. Basic plasma and decomposition chemistry, laboratory experiments, and example applications are discussed. Ozonizer Discharges Plasma Chemistry and Gaseous Electronics For over a century, ozonizers have been used for the large-scale generation of ozone for water treatment and bleaching applications. Ozonizers are frequently described in terms of corona discharges; however, they are more properly called dielectric-barrier discharges or silent electrical discharges [4]. A dielectricbarrier electrical discharge is produced when one or both electrodes bounding a thin gas space are covered with a dielectric (see Figure 1). Dielectric 3arrier Metal lectrode