Unearthing the fate and transport of nanobubbles in groundwater: Significance of straining in nanobubbles deposition

Knowledge of the transport and fate of nanobubbles (NBs) in groundwater is decisive to assess their remediation scope. In this work, based on the exploration of the stability of NBs in groundwater environmental conditions, the transport behavior and mechanism of NBs under physical (grain size, NBs concentration and flow velocity) and chemical conditions (pH, iron strength (IS) and types and dissolved organic matter (DOM)) were investigated by the column experiment and numerical simulation. The results showed NBs were more stable under alkaline and humic acid (HA) environment. NBs transport increased with the increase of grain size, NBs concentration, pH and HA, respectively. However, NBs transport was inhibited with the increase of acidity, electrolytes and L-Tryptophan (L-Trp) concentration, which was mainly due to these unfavorable factors will lead to an increase in the NBs size and a stronger electrostatic attraction at the interface between NB and sand, resulting in more retention of NBs in porous media. This was demonstrated by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and straining calculations. The two–site deposition model showed a better fit. The straining rate (k_str) was greater than attachment rate (k_att), which proved the main retention mechanism of NBs in groundwater was a synergistic mechanism dominated by pore straining and supplemented by attachment (electrostatic attraction and blocking). These findings provided a theoretical basis for the application of NBs technology in groundwater.