Distribution of Colloid Particles onto Interfaces in Partially Saturated Sand

Colloids have long been known to facilitate the transport
of contaminants in soils, but few direct observations have
been made of transport and retention in unsaturated
porous media. Studies have typically been limited to evaluation
of column breakthrough curves, resulting in differing and
sometimes conflicting proposed retention mechanisms. We
carried out pore scale visualization studies of colloid
transport in unsaturated quartz sand to directly observe
and characterize colloid retention phenomena. Synthetic
hydrophilic (0.8, 2.6, and 4.8 μm carboxylated polystyrene
latex) and relatively hydrophobic (5.2 μm polystyrene latex)
colloidal microspheres were added to steady-state
water flow (0.15 mm min-1) applied to an inclined infiltration
chamber. Bright field microscopy was used to determine
the positions and movement of water and colloids. Confocal
laser scanning microscopy was used to determine water
film geometry in an unsaturated horizontal chamber.
We determined mechanisms of hydrophilic colloid retention
at what is generally termed the air/water/solid (AWS)
interface. Based on our observations, the AWS interface
is here more accurately termed the air/water meniscus/solid
(AWmS) interface, denoting the region where between-grain water meniscii diminish to thin water films on the grain
surfaces. Colloids were retained at the AWmS interface
where the film thickness approximately equaled colloid
diameters. The greater retention for hydrophilic colloids
at this interface (compared to elsewhere in the solid/water
interface) can be explained by the additional surface
tension capillary potentials exerted on colloids at the AWmS
interface. While some 0.8-μm colloids were observed in
thin water films, film straining played no significant role in
the retention of larger colloids. Mechanisms for slightly
hydrophobic colloids differed slightly. In addition to primary
retention at the AWmS interface, hydrophobic colloids
attached to others already present at that interface resulting
in apparent retention at the air/water (AW) interface.
Attachment of hydrophobic colloids was also observed at
water-solid interfaces, as hydrophobicity impelled the
colloids to avoid water. Factors contributing to retention
of slightly hydrophobic colloids were sand grain roughness
and possibly a tendency for these colloids to flow near
surfaces and interfaces, consonant with the enhanced
retention of hydrophobic colloids (relative to hydrophilic
colloids) observed in the literature.