Abstract
Nanobubbles have many potential applications depending on their types. The long-term stability of different gas nanobubbles is necessary to be studied considering their applications. In the present study, five kinds of nanobubbles (N2, O2, Ar + 8%H2, air and CO2) in deionized water and a salt aqueous solution were prepared by the hydrodynamic cavitation method. The mean size and zeta potential of the nanobubbles were measured by a light scattering system, while the pH and Eh of the nanobubble suspensions were measured as a function of time. The nanobubble stability was predicted and discussed by the total potential energies between two bubbles by the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The nanobubbles, except CO2, in deionized water showed a long-term stability for 60 days, while they were not stable in the 1 mM (milli mol/L) salt aqueous solution. During the 60 days, the bubble size gradually increased and decreased in deionized water. This size change was discussed by the Ostwald ripening effect coupled with the bubble interaction evaluated by the extended DLVO theory. On the other hand, CO2 nanobubbles in deionized water were not stable and disappeared after 5 days, while the CO2 nanobubbles in 1 mM of NaCl and CaCl2 aqueous solution became stable for 2 weeks. The floating and disappearing phenomena of nanobubbles were estimated and discussed by calculating the relationship between the terminal velocity of the floating bubble and bubble size.
Highlights
Nanobubbles have some unique properties, unlike conventional milli- to micro-bubbles, such as high mass transfer [1], long-term stability [2,3,4,5,6,7,8,9,10], high zeta potential, high surface to volume ratio, and generating free radicals when collapsing [11,12]
Nanobubbles can be divided into surface nanobubbles absorbed on solid surfaces and bulk nanobubbles dispersed in aqueous solutions, experiencing Brownian motion
Bulk nanobubbles have diameters of less than 1 micrometer [13]. Because of their unique physico-chemical properties, nanobubbles can be used in various application fields, e.g., improvement of plant growth and productivity [14], membranes cleaning [15,16,17], waste-water treatment [1,18,19,20,21,22], visualization improvement as the ultrasound contrast agent [23], froth flotation [24,25], improvements of methane production in the anaerobic digestion [26,27], applications in food processing [28,29] and reactions with concrete using CO2 nanobubbles [30,31]
Summary
Nanobubbles have some unique properties, unlike conventional milli- to micro-bubbles, such as high mass transfer [1], long-term stability [2,3,4,5,6,7,8,9,10], high zeta potential, high surface to volume ratio, and generating free radicals when collapsing [11,12]. Bulk nanobubbles have diameters of less than 1 micrometer [13] Because of their unique physico-chemical properties, nanobubbles can be used in various application fields, e.g., improvement of plant growth and productivity [14], membranes cleaning [15,16,17], waste-water treatment [1,18,19,20,21,22], visualization improvement as the ultrasound contrast agent [23], froth flotation [24,25], improvements of methane production in the anaerobic digestion [26,27], applications in food processing [28,29] and reactions with concrete using CO2 nanobubbles [30,31]. O2 nanobubbles produce the methane in anaerobic digestion of cellulose [33] and CO2 bulk nanobubbles can be used in food processing [29]
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