Pore-Scale Visualization of Colloid Transport and Retention in Partly Saturated Porous Media

Abstract

Wan and Wilson (1994a), colloid transport experiments with porous media have been based on mass balance In unsaturated porous media, sorption of colloids at the air-water of breakthrough colloid concentrations in packed-sand (AW) interface is accepted as a mechanism for controlling colloid retention and mobilization. However, limited actual pore-scale obser-columns (Wan and Wilson, 1994b;Schafer et al., 1998; vations of colloid attachment to the AW interface have been made. To Jewett et al., 1999; Jin et al., 2000; Lenhart and Saiers, further investigate these processes, a real-time pore-scale visualization 2002). On the basis of analyses of outflow concentrations method was developed. The method builds on the light transmission of colloid particles, these authors found that more partitechnique for fingered flow studies in packed-sand infiltration chamcles were retained in the column at lower water contents bers and combines it with high-resolution, electro-optical hardware (or under conditions where there were higher percentand public domain imaging software. Infiltration and drainage of ages of trapped air). Retention, measured as reduced suspensions of hydrophilic negatively charged carboxylated latex micolloid concentrations in the column outflow, was attribcrospheres provides compelling visual evidence that colloid retentionuted to sorption at the AW interface and film straining. in sandy porous media occurs via trapping in the thin film of waterIn one case retention of bacteriophages in a batch syswhere the AW interface and the solid interface meet, the air-watersolid (AWS) interface. With this modified theory of trapped colloids tem was ascribed to the presence of a dynamic AWS at the AWS interface, we are able to explain the apparent discrepancy interface (Thompson et al., 1998; between previous experimental evidence of hydrophilic colloids seem-1999). New data suggest, however, that conceptual modingly partitioning to the AW interface and more recent findings that els of colloid transport in unsaturated media need to be suggest this type of colloid does not adsorb at the AW interface.reexamined. It has been generally assumed, based on visualization studies using etched-glass micromodels (Wan and Wilson, 1994a), that retention at the AW interface sity,

Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments

“…In addition, for unsaturated systems colloid retention at the stationary, Yao (1968) , Infl uence of suspended particle size on the transport aspect of water fi ltration, University of North Carolina, Chapel Hill, Dissertation. ) mobile or transitional gas -water interface has to be taken into account (Crist et al , 2004 ;Lenhart and Saiers, 2002 ;Ouyang et al , 1996 ;Wan and Tokunaga, 1997 ). While the maximum size of mobile colloids will be limited by straining fi ltration and pore velocity, concentration and size distribution of smaller colloids are controlled by physico -chemical fi ltration.…”

“…There, the liquid -gas interface can act as barrier for colloid movement if the water fi lm in the unsaturated media is thinner than the diameter of the colloids. Colloids can also be retained at the menisci of pendular rings of the fl uid -gas collector interface instead of the liquid -gas interface (Crist et al , 2004 ). Consequently, calculations using equations from the CFT for an unsaturated soil system should be regarded with care and can serve, if at all, only as a fi rst rough estimation.…”

Transport of Nanomaterials in Unsaturated Porous Media

“…Water films surrounding solid grains may be as thin as tens of nanometers (36,45), close to or smaller than the size of most of colloids and nanomaterials. As such, film straining is believed to be at least as important as air-water interface attachment in unsaturated transport (27,32,34,35,(45)(46)(47)(48)(49)(50). Studies with bacteria (34,35), silica colloids (27), and latex microspheres (47)(48)(49)(50)(51) have suggested that film straining may be the dominant mechanism in immobilizing colloids during unsaturated transport in many systems.…”

“…As such, film straining is believed to be at least as important as air-water interface attachment in unsaturated transport (27,32,34,35,(45)(46)(47)(48)(49)(50). Studies with bacteria (34,35), silica colloids (27), and latex microspheres (47)(48)(49)(50)(51) have suggested that film straining may be the dominant mechanism in immobilizing colloids during unsaturated transport in many systems. Through microscope visualization in thin sand chambers, negatively charged hydrophilic latex has been reported to be mainly retained by the air-water-solid contact line, but not by thin water films or the air-water interface (47)(48)(49)(50).…”

“…Studies with bacteria (34,35), silica colloids (27), and latex microspheres (47)(48)(49)(50)(51) have suggested that film straining may be the dominant mechanism in immobilizing colloids during unsaturated transport in many systems. Through microscope visualization in thin sand chambers, negatively charged hydrophilic latex has been reported to be mainly retained by the air-water-solid contact line, but not by thin water films or the air-water interface (47)(48)(49)(50). For hydrophobic latex, in addition to the primary retention at the contact line, attachment to the air-water interface and the solid-water interface has also been found to be significant (47,50).…”