Micro and nanobubbles-assisted advanced oxidation processes for water decontamination: The importance of interface reactions

Micro and nanobubbles (MNBs), as an efficient and convenient method, have been widely used in water treatment. Composed of gas and water, MNBs avoid directly introducing potential secondary pollutants. Notably, MNBs exhibit significant advantages through interface reactions in assisting AOPs. They overcome barriers like low mass transfer coefficients and limited reactive sites, and shorten the distance between pollutants and oxidants, achieving higher pollutant removal efficiency. However, there is a lack of systematic summary and in-depth discussion on the fundamental mechanisms of MNBs-assisted AOPs. In this critical review, the characteristics of MNBs related to water treatment are outlined first. Subsequently, the recent applications, performance, and mechanisms of MNBs-assisted AOPs including ozone, plasma, photocatalytic, and Fenton oxidation are overviewed. We conclude that MNBs can improve pollutant removal mainly by enhancing the utilization of reactive oxygen species (ROS) generated by AOPs due to the effective interface reactions. Furthermore, we calculated the electrical energy per order of reaction (EE/O) parameter of different MNBs-assisted AOPs, suggesting that MNBs can reduce the total energy consumption in most of the tested cases. Finally, future research needs/opportunities are proposed. The fundamental insights in this review are anticipated to further facilitate an in-depth understanding of the mechanisms of MNBs-assisted AOPs and supply critical guidance on developing MNBs-based technologies for water treatment.