Comparison of gas phase discharge and gas-liquid discharge for water activation and methylene blue degradation

In this paper, an air gas-liquid discharge operated in contact with water is compared with air gas phase discharge operated in contact with quartz surface for their performance on the generation of aqueous reactive species and degrading methylene blue. The discharge images, electrical characteristics, optical emission spectra detected from discharge region, FTIR spectra detected from effluent gas, and plasma gas temperatures in both discharge regimes are compared. The results show that the gas-liquid discharge has lower efficiency to produce gaseous reactive species due to its higher discharge power and lower emission intensities of N-2 (C-B) and O, and lower FTIR absorbance intensities of O-3, NO2, and N2O, than those of gas phase discharge. Besides, the plasma gas temperature in gas-liquid discharge is much higher. Furthermore, both gas-liquid discharge and gas phase discharge are used for treating deionized water and methylene blue solution. It is found that gas-liquid discharge is more conducive to the activation of deionized water due to its higher production of aqueous H2O2, NO2, and NO3-. However, as for methylene blue degradation, the G(50) of methylene blue degradation in gas-liquid discharge (2.29 g.kWh(-1)) is much lower than that of gas phase discharge (9.14 g.kWh(-1)).