Analysis of Chemical Reactions in Pulsed Streamer Discharges: An Experimental Study

Abstract

Over the last decade advanced oxidation reduction processes (AORP) have acquired significant importance in degrading minute concentrations (ppb to ppm levels) of recalcitrant contaminants that are difficult to degrade using conventional water and gas treatment technologies. Pulsed streamer discharges, a type of AORP, utilizes an electrical discharge in the liquid and gas phases to produce highly reactive radicals (OH∙, H∙, O∙) and molecular species (H2O2, H2, O3). This work focuses on two critical areas of development of pulsed streamer discharges: application oriented studies to prove the efficacy of pulsed streamer discharges in degrading a wide variety of organics, and the qualitative and quantitative determination of primary reactive species. The study of the chemical processes occurring due to discharge initiation and investigation of how the local and bulk chemical reactions are affected by the discharge parameters have resulted in qualitative determination and quantification of several reactive species (oxidative (such as OH∙) and reductive species (such as O2∙-)) and enhanced knowledge of the impact of various electrical, chemical and physical parameters (such as applied voltage, conductivity of solution, pH, type of salt, salt concentration, and additives) on production of these reactive species. This has been achieved by using chemical probes that react selectively with reactive species to form readily quantifiable products. The determination of hydroxyl radical production rates in various reactor configurations (reference, hybrid-series and hybrid-parallel) has led to enhanced knowledge of the degradation mechanisms occurring in these reactor systems. This study has highlighted the role that chemical probes play in the determination of reactive species and this work leads to the conclusion that hydroxyl radical is the primary precursor of hydrogen peroxide. Production of significant quantities of reductive species has also been shown using specific chemical probes and the production of superoxide radical anion by the discharge has been documented during the course of this study. The efficacy of pulsed streamer discharges has been demonstrated by performing experiments with a wide variety of organic contaminants. The model contaminants chosen were chlorinated organic compounds such as 2,2',4,4'- tetrachlorobiphenyl and 2-chlorobiphenyl, an organosulfurous compound (2-chloroethyl phenyl sulfide), and an organophosphorous compound (diphenyl chlorophosphate) and their degradation kinetics on being subjected to pulse streamer discharge was evaluated. The effects of various reactor configurations, salts in solution, additives such as zeolites and activated carbon particles, and scavengers such as methanol on the degradation kinetics of these organic contaminants were evaluated to develop an understanding of the degradation mechanisms and the parameters leading to maximum degradation at the lowest energy expenditure. These objectives of enhancing knowledge of fundamental reactive species and experimental…