Effect of dissolved gases on sonochemical oxidation in a 20 kHz probe system: Continuous monitoring of dissolved oxygen concentration and sonochemical oxidation activity

Dissolved gases have a substantial influence on acoustic cavitation and sonochemical oxidation reactions. Little research on the changes in dissolved gases and the resultant changes in sonochemical oxidation has been reported, and most studies have focused only on the initial dissolved gas conditions. In this study, the dissolved oxygen (DO) concentration was measured continuously during ultrasonic irradiation using an optical sensor in different gas modes (saturation/open, saturation/closed, and sparging/closed modes). Simultaneously, the resulting changes in sonochemical oxidation were quantified using KI dosimetry. In the saturation/open mode using five gas conditions of Ar and O(2), the DO concentration decreased rapidly when O(2) was present because of active gas exchange with the atmosphere, and the DO concentration increased when 100% Ar was used. As a result, the order of the zero-order reaction constant for the first 10 min (k(0-10)) decreased in the order Ar:O(2) (75:25) > 100% Ar ≈ Ar:O(2) (50:50) > Ar:O(2) (25:75) > 100% O(2), whereas that during the last 10 min (k(20-30)) when the DO concentration was relatively stable, decreased in the order 100% Ar > Ar:O(2) (75:25) > Ar:O(2) (50:50) ≈ Ar:O(2) (20:75) > 100% O(2). In the saturation/closed mode, the DO concentration decreased to approximately 70–80% of the initial level because of ultrasonic degassing, and there was no influence of gases other than Ar and O(2). Consequently, k(0-10) and k(20-30) decreased in the order Ar:O(2) (75:25) > Ar:O(2) (50:50) > Ar:O(2) (25:75) > 100% Ar > 100% O(2). In the sparging/closed mode, the DO concentration was maintained at approximately 90% of the initial level because of the more active gas adsorption induced by gas sparging, and the values of k(0-10) and k(20-30) were almost the same as those in the saturation/closed mode. In the saturation/open and sparging/closed modes, the Ar:O(2) (75:25) condition was most favorable for enhancing sonochemical oxidation. However, a comparison of k(0-10) and k(20-30) indicated that there would be an optimal dissolved gas condition that was different from the initial gas condition. In addition, the mass-transfer and ultrasonic-degassing coefficients were calculated using changes in the DO concentration in the three modes.