Abstract
BACKGROUND Carbon dioxide (CO2) is a gas that has a distinguished effect on the sonochemical process. Dissolving pure CO2 in solutions prevented sonochemical action in all laboratory-scale experiments, reported until know. The mechanism underlying the pure-CO2 nullifying sonochemical treatment is until now under debate. RESULTS Herein, thanks to confronting detailed numerical simulation results for single bubble sonochemistry with literature experimental observations, the mechanism of pure CO2-quenching sonochemical reaction was clarified. The acoustic generation of free radicals under CO2 atmosphere was simulated for different conditions of frequency (20-1100 kHz), acoustic power (0.5-1 W cm(-2)), liquid temperature (20-50 degrees C) and external pressure (0.6-1.6 atm) and compared with that generated for air-saturation, for the same conditions. Depending on the sonochemical parameters, it was found that CO2 may enhance, decrease or completely suppress the acoustic generation of hydroxyl radicals. The effect of CO2 was strongly operating conditions-dependent. CONCLUSION Given that CO2 nullified all sonolytic actions in aqueous solution, it was concluded that owing to its very high solubility in water (46-fold much higher than that of air), CO2 could suppress the inertial cavitation bubbles responsible of all chemical actions. Gases of too higher solubility could favor the bubble-bubble coalescence rather than the production of inertial cavitation bubbles. The bubble-bubble coalescence in this case could be a suppressor of inertial cavitation. A high extent of bubbles coalescence could take place under CO2 saturation provoking total disappearance of the chemical activity. (c) 2019 Society of Chemical Industry