Do Ultrafine Bubbles Work as Oxygen Carriers?

Fine bubbles (FBs) are bubbles with sizes less than 100 μm and are divided into ultrafine bubbles (UFBs, < 1 μm) and microbubbles (MBs, 1-100 μm) depending on their size. Although FB aeration is known as a more efficient way than macrobubble aeration to increase the oxygen level in unoxygenated water, few reports have demonstrated whether dispersed UFBs work as oxygen carriers or not. Furthermore, oxygen supersaturation is one of the attractive characteristics of FB dispersion, but the reason is yet to be revealed. In this study, we evaluated the relationship between the FBs, especially UFB concentration, and oxygen content in several situations to reveal the two questions. The FB concentration and oxygen content were examined using particle analyzers and our developed oxygen measurement method, which can measure the oxygen content in FB dispersion, respectively. First, in the evaluations of the oxygen dispersion from UFBs with respect to the surrounding oxygen level, UFBs did become neither small nor diminish even in degassed water. Second, the changes in UFBs and oxygen content upon storage temperature and the existence of a lid during storage were evaluated, and there was no correlation between them. It means UFBs contribute little to the oxygen content in UFB dispersion. Furthermore, the oxygen content in the UFB dispersion decreased over time identically as that of the oxygen-supersaturated water with little UFBs. Third, we evaluated the relationship between FB concentration and oxygen content during FB generation by measuring them simultaneously. The results showed that dispersed MB and UFB concentrations did not account for the supersaturation of the FB dispersion. From the result, it was revealed that 100-200 nm of UFBs themselves did not work as oxygen carriers, and the oxygen supersaturation in FB dispersions was due to the supersaturated state of dissolved oxygen that was prepared during the FB generation process.