Pulsed nanosecond air discharge in contact with water: influence of voltage polarity, amplitude, pulse width, and gap distance

Abstract

Plasma technology is a highly promising and advantageous technology for liquid processing. In air in-contact with water, plasma produces highly reactive species (ions, electrons, radicals, photons, etc) that diffuse into the water volume and initiate physical and chemical phenomena of interest, e.g. organic and inorganic pollutant degradation. In this study, we investigate the influence of basic parameters, such as voltage polarity, voltage amplitude, plasma lifetime, and air-gap distance, on the properties of a discharge in air in-contact with deionized water. Specifically, we analyze the electrical characteristics, the plasma behaviour at the water surface, water acidity and conductivity, and the decoloration rate of a standard organic dye (methylene blue). The concentration of the main reactive oxygen and nitrogen species produced in water is also reported. Compared to positive polarity conditions, the negative polarity voltages enhance the decolaration rate of methylene blue. For instance, under negative polarity voltages and while applying 4 and 6 kV, the decoloration rate is relatively low (<30%) and reaches 100% after 25 min of processing at −10 kV. The decoloration rate of MB is also strongly influenced by air-gap distance. Under positive polarity conditions, the decoloration rate decreases from ∼80 to ∼0% as the air-gap distance increases from 0.5 to 7.5 mm, whereas, under negative polarity conditions, the decoloration rate is ∼100%, irrespective of the air-gap distance (0.5 and 4.5 mm).

Discharge characteristics and reactive species production of unipolar and bipolar nanosecond pulsed gas–liquid discharge generated in atmospheric N2

“…It is clear that, as for the unipolar pulse mode, the polarity of pulse voltage has little effect on the excited state of active species production, and the emission intensities of those active species in unipolar positive NPG-LD are slightly higher than those in unipolar negative NPG-LD. The results are similar to the work of Hamdan et al [15] who reported that the concentration of gaseous reactive nitrogen and oxygen species produced by both unipolar positive and negative pulse polarities driven discharge in-contact with water is similar. However, under the same gas flow rate and pulse peak voltage, the intensities of N 2 (C), OH(A), and O(3p) emitted from bipolar NPG-LD are much higher than those of unipolar (both the positive and negative) NPG-LDs.…”

“…Thagard et al [14] found that in the unipolar positive pulsed discharge, the production of H 2 O 2 relies on the solution pH, whereas in the unipolar negative pulsed discharge, the production of H 2 O 2 is lower and depends on the liquid conductivity. Hamdan et al [15] found that the change of the solution conductivity and pH, and concentrations of H 2 O 2 , -NO , 3 and -NO 2 are similar in both positive and negative polarity pulsed discharges. However, negative pulsed discharge has higher degradation rate of methylene blue, which is attributed to that the untreated solution at bottom can flow to the plasma-liquid interface at higher flow rate in negative pulsed discharge.…”

Low-temperature plasmas in contact with liquids—a review of recent progress and challenges

“…As is known from the literature [22,96], the shortening of the voltage pulses to submicrosecond range is an efficient way to prevent transition from streamer to spark, which is also valid for discharges with liquid electrodes. That is confirmed by spectroscopic measurements of the electron density in nanosecond pulsed pin-to-liquid discharges where the values of electron density are found to be in the order of 10 15 cm −3 [180]. The increase of the duration of the voltage pulse to the microsecond range allows the transition of the discharge from streamer to transient spark regime with the electron density values approaching 10 17 cm −3 [122,181,182].…”

“…The dominant method for temperature determination is optical emission spectroscopy of OH and N 2 emissions [83,100,115,122,123,173,[180][181][182][183][184][185][232][233][234][235][236]. Besides, rotational Raman spectroscopy [137,189,191,192], broadband UV absorption spectroscopy [231] and pressure broadening [100] were employed for the estimation of gas temperature in discharges in contact with liquids.…”

“…Yuan et al studied needle-to-water AC DBD in oxygen [123]. Depending on the applied voltage they distinguished three operating modes, see figure 12, with different gas temperatures: streamer mode with T g = 2750 K, glow-like mode with T g = 3050 K and abnormal glow/arc mode with T g ⩾ 3700 K. The influence of the liquid conductivity on the gas temperature is studied in discharges with a liquid electrode [180,184]. It is found that the gas temperature rises with the liquid conductivity which enables higher current density.…”