download pdf version of PhD book - Universiteit Utrecht

7.1 Introduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Under unsaturated conditions, various mechanisms may account for (non-ideal)
behavior of colloids under different flow and transport conditions. Under low
flow rates, diffusion may act as the rate-limiting process. Most of experimental
studies which support this idea (e.g., Jacobsen et al. [1997], Laegdsmand
et al. [1999], Schelde et al. [2002]) suggest a linear relationship between the
cumulative mass of mobilized colloids and the square root of time. During
transient conditions, such as drainage or imbibitions, colloid scavenging by airwater
interfaces may be a dominant process [Jacobsen et al., 1997, Laegdsmand
et al., 1999, Bradford and Torkzaban, 2008]. At high flow rates, however, hydrodynamic
shear [O’neill, 1968] may be the dominant process [Kaplan et al.,
1993, Laegdsmand et al., 1999, Weisbrod et al., 2002]. The induced shear force
is opposed by an attractive force due to Derjaguin-Landau-Verwey-Overbeek
(DLVO) interactions [Bradford and Torkzaban, 2008]. Under very low saturations,
due to the discontinuity of water films between grains, film-straining
may be the dominant process [Lenhart and Saiers, 2002, DeNovio et al., 2004,
Bradford et al., 2006, Torkzaban et al., 2008], and transport of suspended colloids
can be retarded due to physical restrictions imposed by thin water films
[Wan and Tokunaga, 1997].
7.1.2 Experimental studies
Experimental studies are a major source of information for adsorptive transport
in porous media. They are also instrumental in guiding the development
of mathematical models for colloid transport and deposition. Studies of reactive/adsorptive
transport in porous media can be categorized into three groups:
pore-scale studies, column-scale studies of ideal systems (mostly under uniform
saturation and constant pore water velocity), and studies conducted on nonideal
systems (such as natural vadose zone environments). Laboratory studies
of colloid and colloid-facilitated transport have focused primarily on the interpretation
of breakthrough of colloids (e.g, using latex microspheres, clays,
oxides, or microorganisms, with or without other tracers) in sand or glass bead
systems. While many studies have been carried out under saturated flow conditions
[Weisbrod et al., 2003, Toran and Palumbo, 1992, Noell et al., 1998,
Elimelech et al., 2000, Bradford and Bettahar, 2005, Bradford et al., 2007],
some have examined unsaturated conditions [Wan and Wilson, 1994b,a, Wan
and Tokunaga, 1997, 1998, Schafer et al., 1998, Thompson et al., 1998, Thompson
and Yates, 1999, Jewett et al., 1999, Sim and Chrysikopoulos, 2000, Jin
et al., 2000, Gamerdinger and Kaplan, 2001, Saiers and Lenhart, 2003b,a, Wan
159