Hybrid Gas-Liquid Electrical Discharge Reactors for Organic Compound Degradation David R. Grymonpre ´ , † Wright C. Finney, † Ronald J. Clark, ‡ and Bruce R. Locke* ,† Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University and Florida A & M University, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046, and Department of Chemistry, Florida State University, Tallahassee, Florida 32306-4390 A gas-liquid hybrid pulsed corona discharge reactor that utilizes high voltage needle-point electrodes submerged in the aqueous phase coupled with planar ground electrode suspended in the gas phase above the water surface has been developed and analyzed for the removal of low concentrations of phenol. Two types of ground electrodes were evaluated. One type consisted of a solid disk made of stainless steel, and the second type consisted of a disk made of high porosity reticulated vitreous carbon (RVC). The liquid-phase discharge leads to the formation of hydrogen peroxide and hydroxyl radicals, and the gas-phase discharge leads to the formation of ozone. The reticulated carbon electrode produced a higher number and more uniform distribution of plasma channels in the gas phase above the liquid surface. This case also led to the largest amount of ozone dissolved in the liquid phase. The combined action of the reactive species formed in the gas and the liquid phases on the degradation of phenol, the formation of primary byproducts, and the removal of total organic carbon was evaluated for a variety of system conditions, including the addition of ferrous sulfate (for Fenton’s reactions), activated carbon (for adsorption and reaction), and various electrode gap spacing. A mathematical model, including sensitivity analysis, has been developed to illustrate the major reaction pathways. Introduction Recent interest in the application of advanced oxida- tion technologies for the degradation of organic com- pounds in water treatment has spurred a wide range of studies to develop novel combinations of the various techniques and to investigate the chemical reactor design and analysis of such systems. Advanced oxida- tion, or more recently advanced oxidation/reduction, technologies include a wide range of processes that can broadly be classified into chemical (e.g., direct ozone, direct hydrogen peroxide, and combinations of ozone and hydrogen peroxide), 1-3 photochemical and photocatalytic (e.g., UV/ozone/hydrogen peroxide and TiO 2 ), 4,5 mechan- ical (e.g., ultrasonic), 6 and electrical (e.g., electrohy- draulic discharge, 7 corona discharge, 8 and glow dis- charge electrolysis 9,10 ). A variety of studies have been conducted to investigate novel combinations of such processes (as well as combinations with more conven- tional physical, chemical, and biological water treatment methods 11,12 ) in order to enhance the formation of the highly reactive radicals (e.g., hydroxyl radical) and molecular species (e.g., ozone and hydrogen peroxide) formed in such systems and to optimize the degradation of the target pollutants. Gas-phase electrical discharge reactors (including dielectric barrier discharge, DC, AC, and pulsed corona discharge) have long been used as the most efficient means for the formation of ozone. 13-15 Although ozone is a relatively selective oxidant, the combination of ozone and hydrogen peroxide, along with certain catalysts and/ or ultraviolet light, can lead to hydroxyl radical forma- tion. In addition, direct reactions of ozone with byprod- uct species may enhance hydroxyl radical reactions with the primary species by reducing the competition for radicals. The application of a short high voltage electri- cal pulse (pulsed corona or corona-like discharge) di- rectly in the aqueous phase has been demonstrated to lead to the production of hydroxyl and other hydrogen and oxygen radicals. 16-20 Previous electrical discharge reactors for the treat- ment of liquid-phase organic compounds have utilized two basic types of electrode configurations. The first type of discharge utilized high voltage needle electrodes and stainless steel planar ground electrodes both fully submerged within the liquid phase. 8,16,20-23 In this type of electrode configuration the discharge was formed in only the liquid phase. Another type of corona discharge reactor used point electrodes in the gas phase above the water surface, and the ground electrode was either submerged in the liquid or placed below the liquid phase. 24,25 This reactor electrode configuration leads to only gas-phase discharge. Furthermore, several studies have considered bubbling oxygen through the sub- merged hollow needle electrodes. 21 Although bubbling oxygen through the submerged needle electrodes leads to the formation of ozone, the amount of hydrogen peroxide formed in this case was suppressed in com- parison to the case without gas flow in the same electrode system. 26 Another novel system utilized a high voltage discharge in foam in order to form ozone in the gas and hydrogen peroxide in the liquid. 27 The present study seeks to combine the advantages of gas-phase discharge (i.e., for ozone and oxygen radical * To whom correspondence should be addressed. Tel.: (850) 410-6165. Fax: (850) 410-6150. E-mail: locke@eng.fsu.edu. Corresponding author address: Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310-6046. † Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University and Florida A & M University. ‡ Department of Chemistry, Florida State University. 1975 Ind. Eng. Chem. Res. 2004, 43, 1975-1989 10.1021/ie030620j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004