Nanobubble Retention in Saturated Porous Media under Repulsive van der Waals and Electrostatic Conditions

An understanding of nanobubble (NB) migration in porous media is needed for potential environmental applications. The solution chemistry is well known to be a critical factor in determining interactions of other colloids and nanoparticles with surfaces. However, little quantitative research has examined the influence of solution chemistry on NB transport. One-dimensional column experiments were therefore conducted to investigate the transport, retention, and release of NBs in glass beads under different solution chemistry conditions. NB concentrations in the effluent were reduced with an increase in ionic strength (IS) or a decrease in pH due to a reduction in the repulsive force between the glass surface and NBs, especially when the solution contained Ca²⁺ as compared to Na⁺ and for larger NBs. This result was somewhat surprising because electrostatic and van der Waals interactions for NBs were both repulsive on a homogeneous glass bead surface. NB retention on the surface was explained by ubiquitous nanoscale roughness on the glass beads that significantly lowered the energy barrier and localized attractive charge heterogeneity and/or hydrophobic interactions. In contrast to Na⁺, adsorbed Ca²⁺ ions produced charge heterogeneity that enhanced NB retention and inhibited release with IS reduction.