Geoscience Reference
In-Depth Information
11.2 Colloids and Sorption on Colloids
In previous chapters, we discussed the role of colloids and colloid-like materials,
as both carriers of adsorbed chemicals and contaminants in their own right. It is
well recognized that contaminants farther can interact with and adsorb onto
migrating colloids, and thus advance much farther or deeper into the subsurface
than would be expected if the role of colloids were ignored (e.g., Saiers and
Hornberger
1996
). For example, an organic colloid particle can act as a sorbent for
a neutral organic molecule, thus facilitating advective transport of the neutral
molecule.
In this situation, transport equations similar to those discussed previously can
be applied. For example, by assuming sorption to be essentially instantaneous, the
advective-dispersion equation with a reaction term (Saiers and Hornberger
1996
)
can be considered. Alternatively, CTRW transport equations with a single w(s, t)
can be applied or two different time spectra (for the dispersive transport and for the
distribution of transfer times between mobile and immobile—diffusion, sorption—
states can be treated; Berkowitz et al.
2008
; Bijeljic et al.
2011
).
The transport of colloids in porous media is usually considered to be controlled
by four mechanisms: advection, dispersion, straining, and physicochemical parti-
cle-surface interactions (McDowell-Boyer et al.
1986
; Ryan and Elimelech
1996
);
see also
Sect. 5.7.3
.
Straining refers to the permanent physical trapping of colloids
in narrow pore throats and is a key mechanism, particularly in porous media with
grain diameters smaller than 20-50 times the colloid diameter (McDowell-Boyer
et al.
1986
). Physicochemical interactions with the porous medium occur when
colloids approach grain surfaces due to pore-scale diffusion, sedimentation, or
inertial forces. These interactions lead to permanent or temporary attachment to
the porous medium solid phase and are controlled by electrostatic forces (including
London-van der Waals forces). Such electrostatic forces are explained in terms of
electric double-layer theory (Israelachvili
1991
). Particles deposit in the secondary
energy minimum of the electric double layer at the grain surface. Moreover,
deeper secondary minima closer to the solid surfaces occur at higher ionic strength
(Redman et al.
2004
). Thus, changes in solution chemistry promote colloid
mobilization (deposition) by altering the double-layer potential energy.
The transport behavior of colloids commonly is modeled by colloid filtration
theory (CFT) (Yao et al.
1971
), which is based on extension of the common
advection-dispersion equation. The one-dimensional advection-dispersion-filtra-
tion equation is written
kmc
o
c
ot
¼
v
o
c
ox
þ
o
D
h
o
c
ox
ð
11
:
5
Þ
ox
where k is known as the filtration coefficient, which is assumed to be constant in
time and space. The CFT allows determination of k from the physical properties of
the colloid and the porous medium:
