Mechanism of OH radical production from ozone bubbles in water after stopping cavitation.

It has been experimentally reported that OH radicals are produced from ozone microbubbles even after stopping cavitation. Microbubbles are mostly produced using acoustic or hydrodynamic cavitation. In the present paper, numerical simulations of chemical reactions inside an ozone bubble and an oxygen bubble are performed during and after acoustic cavitation in order to study the mechanism of OH radical production. The results have indicated that less than one molecule of OH radicals is produced from a dissolving ozone bubble after stopping cavitation when local pH near the bubble wall is <8. On the other hand, more than 108 or 106 molecules of H2O2 are produced inside an ozone or oxygen microbubble, respectively, in water during acoustic cavitation per violent collapse. It suggests that OH radicals are possibly produced by the chemical reaction of H2O2 with O3 in the liquid phase even after stopping cavitation. Furthermore, in strongly acidic conditions (pH < 5), the reaction of H2O2 with O3 in the liquid phase is relatively slow, and OH production considerably continues after stopping cavitation. This may be the reason for the enhanced OH signals in strongly acidic conditions observed experimentally after stopping cavitation.