Free-Radical Generation from Collapsing Microbubbles in the Absence of a Dynamic Stimulus

Abstract

Free radicals are generated by the collapse of ultrasound-induced cavitation bubbles when they are forcefully compressed by dynamic stimuli. Radical generation occurs as a result of the extremely high temperatures induced by adiabatic compression during the violent collapse process. It is generally believed that extreme conditions are required for this type of radical generation. However, we have demonstrated free-radical generation from the collapse of microbubbles (diameter = <50 microm) in the absence of a harsh dynamic stimulus. In contrast to ultrasound-induced cavitation bubbles, which collapse violently after microseconds, the microbubbles collapsed softly under water after several minutes. Electron spin-resonance spectroscopy confirmed free-radical generation by the collapsing microbubbles. The increase of the surface charges (zeta potentials) of the microbubbles, which were measured during their collapse, supported the hypothesis that the significant increase in ion concentration around the shrinking gas-water interface provided the mechanism for radical generation. This technique of radical generation from collapsing microbubbles could be employed in numerous engineering applications, including wastewater treatment.

Microbubble enhanced ozonation process for advanced treatment of wastewater produced in acrylic fiber manufacturing industry

“…Fig. 6 shows that the fluorescence signal of HTA was observed in air-microbubble water but not in air-macrobubble water, which once again supported the conclusion of Takahashi et al [22] and Li et al [26]. Compared with macrobubble-ozonation, the fluorescence intensity of the samples in ozone microbubble water was significantly higher than that in ozone macrobubble water, indicating a greater amount of hydroxyl radicals generated during microbubble-ozonation.…”

“…Furthermore, the collapse of microbubbles could also generate hydroxyl radicals, which is related to ionic accumulation around the collapsing micro-bubbles [22]. Therefore, microbubble-ozonation process had high dissolved ozone concentration and hydroxyl radical production.…”

“…A great amount of chemical energy is released in a few microseconds due to the accumulation of high concentrations of ions on the gas-liquid interface of the collapsing microbubble, which causes the generation of hydroxyl radicals under high temperature and pressure [22]. Takahashi et al [22] and Li et al [26] tested the generation of hydroxyl radicals from collapsing micro-bubbles by electron spin-resonance (ESR). Chu et al [10] proved that the existence of hydroxyl radicals in ozone microbubble water with fluorescence detection.…”

Removal of residual pesticides in vegetables using ozone microbubbles

“…Therefore, the solubility of the gas in water is very low. On the other hand, MB rise in water slowly and the interior gas is completely dissolved in water (Takahashi et al, 2007a).…”

“…Ozone (O3) is the natural substance in the atmosphere and one of the most potent sanitizers against a wide spectrum of microorganisms (Khadre et al, 2001). O3 is generated by the passage of air or oxygen gas through a high voltage electrical discharge or by ultraviolet light irradiation (Mahapatra et al, 2005), then has a strong oxidative power, and is used for sterilization, virus inactivation, deodorization, bleaching (decoloration), decomposition of organic matter, mycotoxin degradation and others (Cataldo, 2008;Karaca and Velioglu, 2009;Karaca et al, 2010;Takahashi et al, 2007a). In addition, O3 is changed to oxygen by autolysis and does not harm the flavor of vegetables and fruits (Li and Tsuge, 2006).…”