Transport and retention of nanobubbles in saturated porous media: Overlooked role of grain size and shape

Nanobubbles (NBs) are widely used in groundwater pollution treatment due to their unique properties such as small size and high gas mass transfer rate. Therefore, the transport behavior of NBs in the groundwater environment has attracted more attention. Due to the diversity and complexity of aquifer media in the natural environment, grain sands with different sizes and shapes are ubiquitous. Thus, consideration of grain size and shape in porous media is crucial to predict and evaluate the transport and fate of NBs in soil and groundwater systems. This work investigated the stability of NBs in pure water. The transport behavior and mechanism of NBs under different grain sizes and shapes were demonstrated by the column experiment, microscopic observation, and numerical simulation. The results showed that NBs had excellent stability, remaining in solution for more than 10 days. The retention of NBs in coarse, medium and fine sands was 23.5 %, 43.8 % and 50.8 %, respectively. The retention in sphere sands, oval sands and irregular sands was 22.9 %, 24.0 % and 43.8 %, respectively. The smaller grain size and roundness limited the NBs transport in the saturated porous media, which was mainly due to the complexity of the flow channel and the narrowing of the pore throat of fine sands and angular shape of irregular sand that would hinder the NBs transport. This was proved by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and ultra-depth-of-field microscope. The BTCs of NBs were fitted by the one − site deposition model (only attachment) and two − site deposition model (attachment and straining), and it was found that two − site deposition model showed a better fit. The straining rate (kstr) was greater than the attachment rate (katt) in different grain sizes and shapes media, indicating that the retention mechanism of NBs in porous media was a synergistic mechanism dominated by pore straining and supplemented by electrostatic attraction.