Visualization of the role of the gas‐water interface on the fate and transport of colloids in porous media

This paper exposes the significant role played by the gas‐water interface in the fate and transport of colloids in porous media and also introduces a micromodel method to allow direct visualization of colloid behavior in pore networks. The gas‐water interface was created by trapping air in the pore space. Various types of latex and clay particles, as well as bacteria, were studied. The results suggest that the gas‐water interface sorbs not only hydrophobic but also hydrophilic particles. The degree of sorption is controlled by particle surface hydrophobicity, solution ionic strength, and particle charge sign. Sorption increases with increasing particle hydrophobicity and solution ionic strength, while positively charged particles have a very strong affinity for the gas‐water interface. The sorption on the gas‐water interface is essentially irreversible, in that the capillary free energy provides a large attractive force to hold particles on the gas‐water interface after its rupture. These findings reveal a mechanism of vadose zone transport: A static gas‐water interface behaves as a sorbent phase retarding the transport of particulate contaminants. The visualization method developed in this research is very useful for the investigation of particulate contaminant behavior and interface‐related transport, especially in the context of bioremediation.