Aeration and dissolution behavior of oxygen nanobubbles in water

Hypothesis: Nanobubbles (NBs) in water elicit unique physicochemical and colloidal properties (e.g., high stability and longevity). Aeration kinetics and dissolution behavior of oxygen (O₂) NBs are assumed to be bubble size dependent. Experiments: As an indicator for aeration efficiency, volumetric mass transfer coefficient (K_L·a) was assessed by measuring the dissolved oxygen (DO) levels during aeration using O₂ NBs with different sizes. Mass transfer coefficient (K_L) was estimated by correlation analysis. Moreover, a modified Epstein-Plesset (EP) model was developed to predict the dissolution behavior by monitoring the DO and size changes during the dissolution of O₂ NBs in water. Findings: A higher rate of DO increase and a higher equilibrium DO level were both observed after aeration with NBs that present higher surface areas for the mass transfer of O₂ and a higher vapor pressure of O₂ to drive the partitioning equilibrium. Dissolution kinetics of O₂ NBs were highly dependent on the initial bubble size as indicated by the changes of bubble size and DO. Smaller NBs raised up DO faster, whereas larger NBs could lead to higher equilibrium DO levels. Moreover, the rate of DO decline and the quasi-steady DO levels both decreased when the dilution ratio increased, confirming that O₂ NBs dictates the DO level in water. Finally, the dissolving NBs may either swell or shrink according to the model prediction.