Inhibited nanobubble transport in a saturated porous medium: Effects of deposited colloidal particles

Nanobubbles have recently attracted much interest for their practical use and potential applications in environmental issues. The pre-existence of deposited colloidal particles in porous media commonly occurs when nanobubbles applied to porous media interact with deposited colloidal particles. However, the current understanding of the effect of the interactions with pre-deposited colloidal particles on nanobubble transport in saturated porous media remains incomplete, and the effects are often overlooked. Therefore, we performed 1D column experiments with sequential injections of colloidal and nanobubble suspensions to study the effect of pre-deposited materials on the retention and release of colloids and nanobubbles in packed glass beads. In this study, we used resonant mass measurements to measure the number concentrations of colloids and nanobubbles during transport experiments for the first time to distinguish between coexisting solid colloidal particles and nanobubbles with different buoyancies in mixed effluent during transport. The nanobubble retention increased because of the pre-existence of deposited colloidal particles, indicating that the deposited colloidal particles act as additional deposition sites and physical obstacles for nanobubbles through physicochemical (including hydrophobic) interactions. This study also provides a future reference for the applicability of resonant mass measurement to cotransport experiments of different buoyant particles, including colloids, nanobubbles, and oil droplets.