In unsaturated porous media, sorption of colloids at the air–water (AW) interface is accepted as a mechanism for controlling colloid retention and mobilization. However, limited actual pore‐scale observations of colloid attachment to the AW interface have been made. To further investigate these processes, a real‐time pore‐scale visualization method was developed. The method builds on the light transmission technique for fingered flow studies in packed‐sand infiltration chambers and combines it with high‐resolution, electro‐optical hardware and public domain imaging software. Infiltration and drainage of suspensions of hydrophilic negatively charged carboxylated latex microspheres provides compelling visual evidence that colloid retention in sandy porous media occurs via trapping in the thin film of water where the AW interface and the solid interface meet, the air–water–solid (AWS) interface. With this modified theory of trapped colloids at the AWS interface, we are able to explain the apparent discrepancy between previous experimental evidence of hydrophilic colloids seemingly partitioning to the AW interface and more recent findings that suggest this type of colloid does not adsorb at the AW interface.

tion and deposition has improved in the past 10 yr, scientific reviews emphasize the need for more research onThe transport, retention, and release of hydrophobic and hydrothe mechanisms controlling transport in the unsaturated philic polystyrene latex microsphere colloids were examined in 0.5cm-thick, 26-cm-long slab chambers filled with either regular (hydro-

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,

The transport, retention, and release of hydrophobic and hydrophilic polystyrene latex microsphere colloids were examined in 0.5‐cm‐thick, 26‐cm‐long slab chambers filled with either regular (hydrophilic) or weakly water‐repellent sand. The water‐repellent sand consisted of a mixture of 0.4% strongly water‐repellent grains with unmodified regular sand for the remainder. The concentration of colloids in the outflow water was measured at the same time as the pore‐scale distribution of colloids was recorded in still and video images. Although the type of sand affected the flow pattern in the top of the chamber, it did not affect the breakthrough for the same type of colloids. More hydrophilic colloids were eluted in the drainage water than hydrophobic colloids. Images showed that there was a greater retention of the hydrophobic colloids due to strongly attractive hydrophobic interaction forces between colloids and subsequent filtering of colloidal aggregates in the narrow passages between grains. Once filtered, these aggregates then served as preferred sites for attachment of other hydrophobic colloids. The hydrophilic colloids were retained primarily in a thin film of water at the edge of the menisci, the air–water–solid (AWS) interface. Centrifugal motion within the pendular rings observed in the videos contributed to movement of the colloids toward the AWS interface, where colloids were retained due to both low laminar flow velocities near the grain surface and straining in the thin water film at the edge of the meniscus. Except near the solid interface, sorption at the air–water (AW) interface was not observed and appeared unimportant to the retention of colloids. The findings form an essential link between colloid retention and transport processes at the interfacial, pore, and Darcy scales.

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