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

Abstract

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.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…Similar stirring experiments at 486 and 490 kHz have yielded different results with reaction rates and sonochemical efficiency being found to increase [149,192]. At 613 kHz sonochemical production has been found to increase with mechanical stirring up to 370 rpm (and gas flow rate), this was attributed to more homogeneous bubble distribution [99]. For circulation flow, an increase in yield was demonstrated at 376 and 995 kHz at a flow rate of 34 ml min -1 and 76 W L -1 [116].…”

“…Overhead stirring for example has been shown to increase sonochemical activity at frequencies 40, 486, 490, 613 kHz and decrease activity at frequencies 376, 1179 kHz [42,99,149,192]. A large increase (up to fivefold) in sonochemical yield with overhead stirring (600, 900, 1300 rpm) was found at 40 kHz with a general reduction for mid to high frequencies; 376, 995, 1179 kHz [42].…”

“…The reactions are often described as an advanced oxidation process (AOP) based on a method of accelerated oxidation due to generation of oxidative species, particularly •OH and HO ଶ • and H 2 O 2 as a secondary product of radical combination [99]. The number of radicals formed via oxidisation is often referred to as the sonochemical yield, though dosimetric measurements are generally based on the measurement of the hydroxyl radical formation [100].…”

“…There is a limited conical active cavitation zone near the horn tip due to the high energy density delivered at that point as opposed to the plate transducer which delivers the ultrasonic wave more diffusely [43,99,150]. A thin layer of bubbles is formed at the horn surface which impedes the sound field's propagation and causes a reduction in pressure amplitude beyond its formation.…”

“…The utilisation of flow to influence the cavitation environment can be applied via mechanical stirring [42,99,149,192], circulation pumping [116,158] or through frequency-power dependant quasi and acoustic streaming [61,115,128]. Flow can prevent cavitation bubbles from coalescing and clustering by counteracting the primary and secondary Bjerknes forces [158,193].…”