Characterization of Various Air Plasma Discharge Modes in Contact with Water and Their Effect on the Degradation of Reactive Dyes

Abstract

Plasma in- or in-contact-with liquid is an emerging technology that has high potential for use in liquid treatment applications. Due to the simultaneous production of highly reactive species, plasmas can efficiently eliminate pollutants that are difficult to remove using conventional methods. In this study, we investigate an AC-driven air plasma in contact with water, generated using five discharge modes. Modes 1, 2, and 3 are run at air-gap distance of 1, 3, and 3.5 mm, respectively, at a repetition rate of 10 kHz (sinusoidal waveform). Water polarity alternates between cathode and anode in modes 1 and 2, whereas mode 3 produces anode polarity only. Mode 4 is similar to mode 1, but with a repetition rate of 5 kHz, while mode 5 is similar to mode 3, but without the high-voltage preceding discharge when water is anode. The electrical characteristics of each discharge mode are presented in detail, and variations in water properties (namely water acidity and conductivity) are also discussed. The effect of discharge mode on the degradation rate of methylene blue (MB), a standard pollutant, is also investigated. Modes 1, 2, and 4 exhibit comparable MB degradation efficiencies, indicating insignificant dependence neither on air-gap distance or number of discharges. The difference between the efficiencies obtained using modes 3 and 5 suggests a substantial effect of the electric field applied between two successive discharges.