Limitations of physicochemical property-based explanations for bulk nanobubble effects

Bulk nanobubbles have been proposed for a variety of applications e.g. wastewater treatment, mineral recovery, and aeration, but the underlying mechanisms remain poorly understood. Few studies have systematically linked experimental observations to physicochemical properties. A necessary first step is therefore the characterization of these properties, which is currently feasible mainly in clean water systems. We compiled experimental data on nanobubble size distributions, ζ-potentials, and number concentrations and use these to derive additional physicochemical properties. Although nanobubbles are commonly generated using cavitation-based or membrane-based methods, data show no clear dependence of nanobubble characteristics on the production method. Nanobubble diameters range from 100 to 300 nm, with concentrations below 10⁹ nanobubbles mL^-1, i.e. levels that do not affect bulk viscosity or density. The amount of gas encapsulated in nanobubbles is <10 mg m^-3 water, while their gas-liquid interfacial area ranges from 1 to 100 m² m^-3 water which is low compared with solid adsorbents. The potential energy associated with nanobubble collapse range between 0.1 and 10 J m^-3 liquid. The calculated physicochemical properties do not fully explain the reported experimental observations, indicating that a comprehensive understanding will require improved experimental methods to quantify nanobubble concentrations and properties in complex media.