1. Air Nanobubbles
- Flotation: Air NBs are extensively used in mineral processing (e.g., coal, phosphate, quartz) to enhance the recovery of fine particles. They act as a secondary collector, bridging particles to larger bubbles and increasing the contact angle (hydrophobicity) of mineral surfaces Fan et al., 2022; Azevedo et al., 2019.
- General Aeration & Cleaning: Air NBs are used for general water aeration and have been shown to reduce fluid friction (drag reduction) in pipes, as well as facilitate detergent-free cleaning of surfaces Agarwal et al., 2011; Nirmalkar et al., 2018.
2. Oxygen Nanobubbles (ONBs)
- Agriculture: ONBs significantly promote seed germination (e.g., barley, vegetables) and plant growth. The mechanism involves the supply of exogenous reactive oxygen species (ROS) and enhanced oxygen availability for root respiration Liu et al., 2016; Ahmed et al., 2018.
- Aquaculture: ONBs maintain high Dissolved Oxygen (DO) levels for extended periods due to their low buoyancy and high mass transfer efficiency. This leads to improved survival rates and metabolic acceleration in fish and shellfish Ebina et al., 2013; Agarawal et al., 2011.
3. Ozone Nanobubbles
- Disinfection and Wastewater Treatment: Ozone NBs are potent for decomposing organic pollutants and sterilizing pathogens. Upon collapse, they generate high concentrations of hydroxyl radicals (^\• OH), which are critical for Advanced Oxidation Processes (AOPs) Sakr et al., 2022; Agarwal et al., 2011.
- Stability Enhancement: Compared to macro-bubbles, ozone NBs have a much higher internal pressure and surface area, leading to higher solubility. They persist in water for days rather than minutes, prolonging the oxidative reactivity of ozone within the water matrix Xia and Hu, 2018; Hu and Xia, 2018.
4. CO2 Nanobubbles
- Applications: CO2 NBs are utilized for pH control (neutralization of alkaline solutions) and have shown potential in promoting plant growth and food processing applications Ahmed et al., 2018; Phan et al., 2021. Additionally, they are investigated for mineral carbonation and geological storage applications due to their high mass transfer efficiency into liquid phases Li et al., 2023.