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7. Adsorption under Partially-Saturated Conditions
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[Selim et al., 1976, Cameron and Klute, 1977]. Physical and chemical kinetic
models have kinetic terms which contribute to both early breakthrough and
longer tailing in BTCs, comparing to the solution of the Advection-Dispersion
Equation (ADE) with equilibrium adsorption. Both two-region and two-site
kinetic models have four independent parameters which are functions of one
or two of the following parameters: the dispersion coefficient, D, the distribution
coefficient, K d , the equilibrium mobile fraction, f) and the first-order
rate coefficient, ω. Curve fitting has been the most commonly used method to
determine the parameters of these models.
Among other types of models, Choi and Corapcioglu [1997] modeled colloidfacilitated
transport under unsaturated conditions considering four phases: an
aqueous phase; a carrier phase (the colloids); a stationary solid matrix phase;
and the air phase. Colloidal mass transfer between the aqueous and solid
matrix phases and between the aqueous phase and the air-water interface,
and the contaminant mass transfer between aqueous and colloidal phases and
between the aqueous phase and the air-water interface were represented by
kinetic expressions.
Wan and Tokunaga [1997] developed a model based on film-straining in which
transport of suspended colloids can be retarded due to physical restrictions
imposed by thin water films in partially saturated porous media. In their
model, they introduced critical matric potential and a critical saturation, at
which thick film interconnections between pendular rings are broken and film
straining begins to become effective. They observed that the conventional
filtration theory was not sufficient, but, film-straining theory could explain their
results. They found that the magnitude of colloid transport through water films
depended on the ratio of colloid size to film thickness as well as flow velocity.
Additional factors which might influence film straining in more general cases
include distributions in grain size, grain shape and surface roughness, grain
packing and aggregation, and colloid shape.
7.1.4 Objectives
Although there are some studies on pore-network modeling of reactive/adsorptive
solute under saturated conditions [Acharya et al., 2005a, Algive et al., 2007a,
Li et al., 2006a], there are many fewer studies conducted under unsaturated
conditions. In this paper, we present a new pore-scale model to study flow
and transport of adsorptive/reactive solutes under unsaturated conditions. We
calculate the concentration of solutes in each individual pore element using the
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