Effect of chemical species and temperature on the stability of air nanobubbles

Abstract

The colloidal stability of air nanobubbles (NBs) was studied at different temperatures (0–30 °C) and in the presence of sulfates, typically found in mining effluents, in a wide range of Na2SO4 concentrations (0.001 to 1 M), along with the effect of surfactants (sodium dodecyl sulfate), chloride salts (NaCl), and acid/base reagents at a pH range from 4 to 9. Using a nanobubble generator based on hydrodynamic cavitation, 1.2 × 108 bubbles/mL with a typical radius of 84.66 ± 7.88 nm were generated in deionized water. Multiple evidence is provided to prove their presence in suspension, including the Tyndall effect, dynamic light scattering, and nanoparticle size analysis. Zeta potential measurements revealed that NBs are negatively charged even after two months (from − 19.48 ± 1.89 to − 10.13 ± 1.71 mV), suggesting that their stability is due to the negative charge on their surface. NBs were found to be more stable in alkaline solutions compared to acidic ones. Further, low amounts of both chloride and sulfate dissolved salts led to a reduction of the size of NBs. However, when high amounts of dissolved salts are present, NBs are more likely to coalesce, and their size to be increased. Finally, the investigation of the stability of air NBs at low temperatures revealed a non-monotonic relationship between temperature and NBs upon considering water self-ionization and ion mobility. This research aims to open a new frontier towards the application of the highly innovative NBs technology on the treatment of mining, mineral, and metal processing effluents, which are challenging aqueous solutions containing chloride and sulfate species.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability

“…Indeed, this is corroborated by previous studies showing that NBs are much less stable under acidic conditions than in neutral to basic solutions. 25,59,63 In our proposed model, the solvent and any ions or additives that may be present induce a dipole on the NB surface by dipole-induced dipole or ion-induced dipole interactions. We also know from the theory of colloidal stability developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO) that the magnitude of the induced surface charge or the zeta potential directly controls the colloidal stability of monodisperse systems.…”

“…Bulk nanobubbles (NBs) are a class of materials that consist of bubbles with dimensions on the nanometer scale that have captured the interest of scientists and engineers from a variety of disciplines. − These materials have found application in an assortment of areas, such as water treatment and disinfection, − agriculture and hydroponics, − and most recently, advanced chemical oxidation processes. − Unlike bubbles of larger diameter (>1 μm), NBs are known to be stable in water for long periods of time. − The standard model currently adopted in the literature to explain this stability involves a balance between the Laplace pressure and the pressure from electrostatic interactions at the NB-solvent interface. In general, we can express this balance using the Young–Laplace equation as follows: P 0 + P YL = P NB + P e where P 0 is the pressure of the atmosphere outside the solution, P YL is the Laplace pressure, P NB is the pressure inside the NB, and P e is the electrostatic pressure.…”

“…However, many of them are very specific to the bubble generation circumstances and lack generalizability, and many of them make imprecise predictions on the properties of NBs. Ljunggren and Eriksson pioneered a theoretical model for the stability of colloidal gas particles that predicted their lifetimes to be 1–100 μs, while NBs in water have been reported to be stable up to a few months. − Attard et al has also been unsuccessful in explaining the stability of NBs without invoking electrostatic effects. , …”

A digital image analysis approach to understand the microscopic and macroscopic phenomena in dissolved air flotation

“…Using micro-imaging technology, Wang et al 9 studied the effect of floc size on the separation efficiency in a flotation cell and indicated that small flocs are more favorable for the high separation efficiency. It was found that the existence of efficient bubble-particle contact is initially controlled by DAF hydrodynamics.…”

“…In the research study conducted by Montazeri et al 9 , the impact of adding chemicals on ultrafine nanobubbles stability was examined, but the contact between oil droplets and bubbles in the presence of chemical materials wasn’t determined. They have found that in alkaline solutions, ultrafine bubbles are more stable compared to the bubbles in acidic solutions.…”