Degradation of methylene blue by pulsed nanosecond discharge in Ar, O₂, and N₂ gaseous bubbles in water: Evaluation of direct and postprocessing modes

Dye-based water pollution is a subject of great concern as it has been linked to many health hazards. Considering their stable structures, some dyes, such as methylene blue (MB), cannot be easily removed from water. However, recent studies show that plasma-assisted processing has great potential for eliminating these dyes from water bodies. Among the different configurations of plasmas, discharges in gaseous bubbles in water are particularly promising in terms of water processing. The bubble-liquid discontinuity in such plasmas significantly facilitates the occurrence of discharge, and the modification of gas composition allows for controlling induced chemical reactions. In this study, we investigate the degradation of MB using pulsed discharges (amplitude of 20 kV, pulse duration of 500 ns, and repetition rate of 1 kHz) in Ar, O₂, and N₂ gaseous bubbles dispersed in water. The degradation of MB is evaluated in the direct mode, i.e., MB is present in the water during discharge as well as in the postprocessing mode, i.e., MB is added to water after plasma processing. Based on the obtained results, the most and least efficient degradation rates measured in the direct mode are achieved with O₂ and N₂ bubbles, respectively. Meanwhile, in the case of the postprocessing mode, degradation with N₂ bubbles is initially more efficient than that with O₂ or Ar. However, after hundreds of hours, higher degradation efficiency (up to 100%) is observed with O₂ and Ar gas bubbles and not with N₂. The findings reported herein are of great importance, particularly considering that plasma processing is complementary to the conventional techniques used in water depollution applications.