Oxidative power loss control in ozonation: Nanobubble and ultrasonic cavitation

Nanobubble and ultrasonic cavitation were applied to support and prolong oxidation reactions of ozonation. Nanobubbles increased ozone dissolution by a factor of 16 due to low buoyancy, high surface area, and stability in water. Hydroxyl radicals generated by ultrasonic cavitation produced hydrogen peroxide rather than recombining due to additional oxygen atoms supplied by the nanobubbles. The generated hydrogen peroxide formed hydroperoxyl ions that reacted with ozone to generate hydroxyl radicals. The process achieved improvements in both the loss of emitted ozone and radical recombination. Rhodamine B decomposition was used to gauge the effectiveness of the process, with the highest rhodamine B decomposition evident at a high initial pH and power and a frequency of 132 kHz as revealed in ultrasonic experiments. The process achieved more than 99% of the rhodamine B decomposition in 20 min under the most efficient conditions. The generation of hydrogen peroxide exhibited tendencies similar to those of rhodamine B decomposition, supporting the proposed mechanism. An ozonation process combined with nanobubble and ultrasonic cavitation can therefore sustain oxidizing power using continuous dissolution by nanobubbles and successive radical generation caused by hydrogen peroxide generated by cavitation.