Ultrasonic treatment effects on hydroxyl radical generation in various solution systems: an iodometric analysis.

Hydroxyl radicals (•OH) are key reactive species in sonochemical processes; however, their generation is strongly dependent on the operating parameters and solution composition. In this study, probe-ultrasonication-induced oxidizing response was evaluated using an iodometric method in a 0.4mol L-1 KI background, where the measured response reflects the accumulated titratable iodine (I3-) signal and is therefore discussed as an apparent •OH yield rather than an instantaneous steady-state [•OH]. The effects of sonication time (5-35 min), ultrasonic power (60-300 W), duty cycle (20%-100%), and probe position (top/middle/bottom) were systematically investigated. The apparent •OH yield increased with time and power, reached a maximum at a duty cycle of 60%, and was highest when the probe was positioned at the center of the liquid column. The influence of solution composition was further evaluated by adding ethanol, acetic acid, NaOH, NaHCO3, and NaCl to the KI background. Increasing the ethanol and NaOH concentrations generally decreased the apparent •OH yield, whereas acetic acid showed a biphasic response. NaHCO3 decreased the apparent •OH yield in a concentration-dependent manner, and NaCl showed a non-monotonic concentration dependence. In phosphotungstic acid systems, the iodometric response increased with concentration at 10 min but the concentration dependence diminished at 30 min, and a no-ultrasound control showed a measurable baseline signal while ultrasound produced a stronger response. Hydrogen peroxide markedly enhanced the response at moderate concentration but reduced the apparent •OH yield at higher levels due to competing reactions, while a t-BuOH scavenger control reduced the signal to about 10% of that without t-BuOH, indicating predominantly •OH-driven response with minor non-specific contributions. These results provide practical guidance for regulating the accumulated oxidative response in probe-ultrasonic systems.