Detection of HO₂•/O₂•^- Radicals Formed in Aqueous Solutions Irradiated with Megasonic Waves Using a Cavitation Threshold (CT) Cell Set-Up

Abstract

Acoustic cavitation, especially transient cavitation, in solutions is accompanied by a number of physical and chemical effects. Due to high temperature and pressure conditions inside bubbles at their collapse, excitation of various species as well as formation of radicals occurs in solution [1-4]. Water molecules excited by megasonic irradiation typically dissociate to hydrogen and hydroxyl radicals (H• and OH•) [5]. The hydroxyl radical is a strong oxidant while the hydrogen radical has reducing properties. In presence of O2 in the solution, H• reacts with O2 to form hydroperoxyl (HO2•) radicals, which act as a reducing as well as a (weak) oxidizing agent [6]. Dissociation of hydroperoxyl radicals result in the formation of superoxide anion radicals (O2•-) as follows [6]:

Estimation of the Generation Rate of H· Radicals in a Megasonic Field Using an Electrochemical Technique

“…Sonochemical reactions produce active free radical species such as OH• (hydroxyl), HO2• (hydroperoxyl) and H• (hydrogen). Hydrogen radicals have reducing properties, and in fact, can react with oxygen to form hydroperoxyl (HO2•) radicals that can act both as a reducing and weak oxidizing agent [5,6].…”

“…While many studies on the characterization of hydroxyl and hydroperoxyl radicals have been reported [5][6][7], the detection and quantitation of hydrogen radicals formed in aqueous solution during sonication have not received much attention. In the detection of HO2• and H• radicals, their reaction with cupric (Cu 2+ ) ions has been recognized.…”