Stability of Oxygen Nanobubbles under Freshwater Conditions

The use of nanobubbles (NBs) has gained significant attention in various applications (e.g., aeration in biological water treatment, water disinfection, membrane defouling, and ground water and sediment remediation) in recent decades because of their superior characteristics such as the improved mass transfer at the gas-liquid interfaces, their lifetime up to a couple of weeks, the formation of reactive oxygen species (ROS) with high oxidative potential. However, there is a lack of information about the effect of various factors on the stability of NBs for a long storage period under freshwater conditions. In this study, a comprehensive investigation was conducted to systematically examine the stability of oxygen NBs in water under various conditions which are closely related to a typical freshwater or the drinking water treatment. The oxygen NB stability in water was evaluated by monitoring the change in the bubble concentrations, size distribution, average diameter, and zeta potential for 60 days of storage time under different pH, hardness, ionic strength, natural organic matter (NOM), chlorine, and temperature conditions. In addition, the formation of hydroxyl radical (•OH) was investigated using disodium terephthalate which form fluorescent adducts with •OH in the presence of oxygen NBs. Among the parameters investigated, the impacts of cations, low pH, and high SUVA₂₅₄ NOM on the stability of oxygen NBs were more significant than other conditions. The half-lives of oxygen NBs under various conditions follow the order Ca²⁺ < Na⁺ < pH 3 < high SUVA₂₅₄ NOM < pH 5 < 30 °C. Oxygen NBs were more stable in softwater than hardwater. Oxygen NBs were relatively stable for 3 days regardless of pH. For a longer storage period, oxygen NBs disappeared faster at pH 3 than at high pH. High SUVA₂₅₄ NOM destabilized NBs more than low SUVA₂₅₄ NOM, indicating the impact of hydrophobicity on the NB stability. The temperature effect on the NB stability was negligible for a short storage time, while higher temperature destabilized oxygen NBs for a longer storage time. One of the main disappearance pathway of oxygen NBs in water was found to be coalescing, rising, and leaving the container, which would be promoted greatly by cations, low pH and NOM with high aromaticity. The formation of hydroxyl radical in NB solutions was detected at pH 3 by a florescent probe molecule. When oxygen NBs are released in water bodies, high calcium, high SUVA₂₅₄ NOM, and low pH would significantly reduce the availability of NBs and their residence time in freshwater.