Degradation of Methylene Blue by Pulsed Nanosecond Discharge in Water with Ar-O2 Gaseous Bubbles

The rise of water effluents containing emerging contaminants that resist conventional chemical and physical treatments makes the treatment of wastewater more complex. Plasma-based treatment methods have great potential to degrade many of the emerging contaminants, including dyes. In this study, using pulsed nanosecond discharges, we investigate the degradation of methylene blue (MB) dye in water by generating plasma in Ar-O₂ gas bubbles in water. The scalability of the setup is studied by producing discharges in a one electrode setup (a needle-to-plate configuration) and in a four electrodes setup (four needles-to-wire configuration). The discharge was characterized by electrical measurements (current and voltage waveforms) and optical emission spectroscopy. We find that the discharge properties are stable during the 30 min of processing, with and without the presence of MB in solution at low electrical conductivity. The production rate of H₂O₂ in the one electrode setup was measured in 0% and 70% O₂, and it was found to be ∼2.3 and 2.9 mg/Lmin, respectively. In the four electrodes setup, H₂O₂ production rate was lower: ∼1.2 and 1.9 mg/Lmin in 0% and 100% O₂. Degradation of MB was assessed in both setups for (i) different % of O₂ in the gas mixture, (ii) different MB initial concentration, and (iii) different initial water conductivity. In the one electrode setup, a high MB degradation (> 85%) was generally achieved in all conditions, but a better performance is noted in high O₂ percentage (> 50%) at low initial water conductivity. At low MB concentration and low electrical conductivity, the performance of the four electrodes setup was better than the one electrode setup.