Ultrasound-enhanced hydrogen peroxide generation of microwave plasma discharges in conductive aqueous solutions

This study investigated the enhancement of hydrogen peroxide (H₂O₂) generation through the interaction between an underwater pulsed microwave plasma discharge and high-intensity ultrasound in sodium chloride (NaCl) solutions. The objective was to enhance the H₂O₂ yield of the plasma treatment by simultaneously applying sonication, a technique known to influence chemical transport and reactivity. Experimental parameters, including ultrasound amplitude, NaCl concentration, and the distance between the sonotrode and the microwave source (coupler), were systematically varied to assess their impact. The results indicate that ultrasound significantly enhances H₂O₂ yield compared with plasma treatment alone. The maximum production was achieved at the highest ultrasound amplitude, with a sonotrode-coupler distance of 5 mm and plasma operation with 10 Hz microwave bursts (1950 W, duty ratio 50 %). Under these conditions, the H₂O₂ production rate reached 0.20g.h^-1, with a yield of 0.18g.kWh^-1, corresponding to increases of +47% and +26%, respectively, compared with similar conditions without ultrasound. This resulted in an H₂O₂ concentration of approximately 1.5mM in 1.0L after 15 min of treatment, whereas only 0.9mM was obtained without ultrasound. Potential mechanisms for the observed yield enhancement include improved transport of reactive species and ultrasound-induced motion of the plasma-liquid interface. The effects of microwave burst frequency and duty ratio on H₂O₂ production were also examined. Overall, the results suggest that coupling plasma with ultrasound is a promising approach for enhancing reactive species generation in plasma-liquid systems.