Comparative Analysis of Volumetric Mass Transfer Coefficients for Oxygen Uptake and Desorption with Nanobubbles

The volumetric mass transfer coefficients (kLa) of oxygen during sorption and desorption were analyzed using nanobubbles (NBs) of air and pure oxygen under various experimental conditions. The results showed that oxygen NBs achieved an increase in dissolved oxygen (DO) levels during absorption, reaching peaks of 30–34 mg∙L−1 and stabilizing at 31.3 ± 0.2 mg∙L−1, with a volumetric mass transfer coefficient of 0.105 ± 0.002 min−1. In comparison, air NBs showed a lower efficiency, with peak DOs of 8∙10 mg∙L−1 and kLa of 0.048 ± 0.001 min−1. In desorption studies, oxygen NBs had higher DO retention, reducing from 30.0 mg∙L−1 to 15.0 mg∙L−1 in 300 min, with a kLa of 0.042 ± 0.003 min−1, while air NBs decreased more rapidly, with a kLa of 0.028 ± 0.002 min−1. When oxygen was used, kLa outperformed air in both absorption and desorption, with a higher kLa during absorption, a lower kLa during desorption, and higher stability. In addition, the results show that the residence time has an important impact on the performance of NBs, showing that the direct influence of the flow dynamics and surface/to/volume ratio influences the value of kLa. The results highlight the superior performance of oxygen NBs versus air NBs in terms of mass transfer efficiency and stability and highlight the effect of residence time and NB composition in applications requiring efficient oxygen transfer, given the promising prospects for the development of advanced aeration technologies in industrial and environmental contexts.