Transport of colloids in unsaturated porous media: A pore‐scale observation of processes during the dissolution of air‐water interface

Abstract

[1] We present results from pore-scale observations of colloid transport in an unsaturated physical micromodel. The experiments were conducted separately using three different sizes of carboxylate polystyrene latex spheres and Bacteriophage MS2 virus. The main focus was to investigate the pore-scale transport processes of colloids as they interact with the air-water interface (AWI) of trapped air bubbles in unsaturated porous media, as well as the release of colloids during imbibition. The colloids travel through the water phase but are attracted to the AWI by either collision or attractive forces and are accumulated at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids can be transported by advective liquid flow, as colloidal clusters. The clusters can then attach to other AWI down-gradient or be trapped in pore throats that would have allowed them to pass through individually. We also observed small air bubbles with attached colloids that traveled through the porous medium during the gas dissolution process. We used Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to help explain the observed results. The strength of the force that holds the colloids at the AWI was estimated, assuming that the capillary force is the major force that holds the colloids at the AWI. Our calculations indicate that the forces that hold the colloids at the AWI are larger than the energy barrier between the colloids. Therefore it is quite likely that the clusters of colloids are formed by the colloids attached at the AWI as they move closer at the end of the bubble dissolution process. Coagulation at the AWI may increase the overall filtration for colloids transported through the vadose zone. Just as important, colloids trapped in the AWI might be quite mobile when the air bubbles are released at the end of the dissolution process, resulting in increased breakthrough. These pore-scale mechanisms are likely to play a significant role in the macroscopic transport of colloids in unsaturated porous media.INDEX TERMS: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport; 5139 Physical Properties of Rocks: Transport properties; KEYWORDS: air-water interface, DLVO, hydrophobic, colloid, unsaturated, micromodel Citation: Sirivithayapakorn, S., and A. Keller, Transport of colloids in unsaturated porous media: A pore -scale observation of processes during the dissolution of air-water interface, Water Resour.

Influences of Flow Transients and Porous Medium Heterogeneity on Colloid-Associated Contaminant Transport in the Vadose Zone

“…Insular air bubbles scavenged colloids, which is consistent with results of experiments conducted with etched micromodels [Wan and Wilson, 1994;Sirivithayapakorn and Keller, 2003]. Colloids also were retained within thin films of water that stretched between pendular rings of unsaturated pores (Figure 3a) results that confirm previous speculation of the significance of film straining in influencing colloid mobility [Wan and Tokunaga, 1997;Lenhart and Saiers, 2002;Saiers and Lenhart, 2003].…”

Transport of colloids in unsaturated porous media: Explaining large‐scale behavior based on pore‐scale mechanisms

“…The transport of colloids in the unsaturated zone is thus greatly influenced by water content [Vilker and Burge, 1980;Lance and Gerba, 1984;Powelson et al, 1990;Poletika et al, 1995]. The AWI has high affinity for both hydrophobic and hydrophilic colloids and the sorption of colloids onto the AWI is often considered almost irreversible under unsaturated conditions, due to the capillary forces that hold the colloids at the interface [Ducker et al, 1994;Wan et al, 1994;Wan and Wilson, 1994b;Wan and Tokunaga, 2002;Costanza-Robinson and Brusseau, 2002;Sirivithayapakorn and Keller, 2003]. Therefore the AWI is believed to play an important role in retaining colloids in the vadose zone [Powelson et al, 1990;Poletika et al, 1995].…”

“…From pore-scale observations, this could happen as trapped air bubbles mobilize or dissolve during rewetting. Colloids sorbed on the AWI can be transported with the mobile air bubbles or released as the AWI disappears [Wan et al, 1994;Sirivithayapakorn and Keller, 2003]. Colloids that are retained by film straining or attached to the SWI can also be released into the water phase due to the increase in moisture content following the flow events [El-Farhan et al, 2000].…”

Geological Disposal of Nuclear Waste: Fate and Transport of Radioactive Materials

“…For the transport and mobilization of radioactive materials in colloidal size or its attachment with colloidal particle, the balance among electrostatic, hydrodynamic, and capillary forces are responsible for attraction of particle towards the liquid-gas interface [29,67,68,70,72,79]. If capillary force dominates then the colloidal particles attracted towards the liquid-gas interface and if electrostatic dominates then the colloidal particles remain stay over the grain surface.…”