The Effect of Discharge Frequency of a Gas–Liquid Plasma Reactor on Bulk Liquid Transport and Removal of Organic Contaminants

The relationship between the operational parameters and the performance of gas–liquid plasma reactors for water treatment and other applications is complex and often affects the plasma and the bulk liquid physicochemical processes simultaneously. This work investigates the effects of discharge frequency between 40 and 120 Hz on degradation of six organic compounds (solutes): phenol, pyrazole, rhodamine B, perfluorooctanoic acid (PFOA), 2-(2-Aminoethoxy) ethanol (DGA), and caffeine in a rail-plane gas–liquid plasma reactor without external mixing. To explain the differences in the measured degradation rates, the bulk liquid mass transport (i.e., flow fields) was visualized and quantified using particle image velocimetry (PIV) and subsequently coupled with interfacial reaction kinetics. Results show that the discharge frequency controls the compound degradation via multiple mechanisms that include variation in reactive species production and hydrodynamic effects based on charged particle transport (ionic wind) and surface tension (Marangoni flow). Moreover, two qualitatively different degradation regimes are identified: the mass transport limited regime that is characterized by an exponential decay of the solute concentration with time and is independent of the discharge frequency and the reactive species production limited regime that is characterized by a linear decay of the solute concentration with respect to both time and discharge frequency. A model has been developed that predicts the shapes of the experimentally measured concentration profiles by assessing the relative magnitudes of transport and reaction (kinetic) fluxes. The results of this study in which a solute’s degradation kinetic profile identifies the mechanism(s) inhibiting its degradation (i.e., transport vs. kinetics) have significant implications for the design of chemical reactors in water treatment applications.