Effect of operational conditions on sonoluminescence and kinetics of H2O2 formation during the sonolysis of water in the presence of Ar/O2 gas mixture.

Ultrasonic frequency is a key parameter determining multibubble sonoluminescence (MBSL) spectra of water saturated with Ar/O2 gas mixtures. At 20 kHz, the MBSL is quenched by oxygen. By contrast, at high-frequency ultrasound the maximal MBSL intensity is observed in the presence of Ar/20%O2 gas mixture. Nevertheless, oxygen has no influence on the shape of MBSL spectra. The effect of oxygen on MBSL is explained by oxygen dissociation inside the collapsing bubble which is much more effective at high ultrasonic frequency compared to 20 kHz ultrasound. In contrast to MBSL, a higher yield of H2O2 is observed in Ar/20%O2 gas mixture whatever the ultrasonic frequency. At 20°C and 20% of oxygen the maximal yield of H2O2 is observed at 204-362 kHz. The maximal yield of H2O2 is shifted to 613kHz when the bulk temperature is raised up to 40°C. Coupling of high-frequency ultrasound with mechanical stirring and intensive Ar/O2 bubbling improves H2O2 production. Comparison of MBSL and sonochemistry allowed to conclude that H2O2 is formed from non-excited OH (X(2)Π) and HO2 radicals. Finally, it was shown that at the studied conditions the efficiency of ultrasonic degassing is hardly influenced by frequency.