Agriculture Reference
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0.6
Column 105
Olivier - AsO
3
K
e = 0.0583 L kg
-1
k
1
= 0.00278 h
-1
k
2
= 0.00727 h
-1
k
i
= 0.0575 h
-1
Mass Recovery = 46.5%
0.5
0.4
0.3
0.2
OBS
SOM
MIM-SOM
0.1
4 days
no †ow
0.0
0
10
20
30
40
50
60
70
Pore Volumes (V/V
o
)
FIGURE 8.20
Measured and predicted arsenite for Olivier soil column. Predictions are based on the second-
order mobile-immobile model (MIM-SOM) and the original second-order model (SOM).
parameters (
D
,
f
, and α ) were based on the estimated values from tritium
BTCs, whereas kinetic retention parameters (
K
e
,
k
1
,
k
2
,
k
i
) were obtained
from inverse modeling. The incorporation of the physical nonequilibrium
approach resulted in a slightly sharper front, reflecting the effect of preferen-
tial flow path. The coupled model successfully predicted the concentration
drop during flow interruption on the adsorption side. Based on the results
of model predictions, we concluded that kinetics of arsenite reaction on soil
surface is the dominant process of arsenite transport in soils. The utiliza-
tion of the physical nonequilibrium parameters in the combined model is
expected to increase for modeling contaminant transport in heterogeneous
natural porous media compared with columns repacked in the laboratory
with homogenized soils.
Although the two-region model concept has been shown to successfully
describe the appearance of lack of equilibrium behavior and tailing for
a wide range of conditions, this approach has several drawbacks. First,
the value of α is difficult to determine for soils because of the irregular
geometric distribution of immobile water pockets, and second, the frac-
tion of mobile and immobile water within the system can only be esti-
mated. Thus, two parameters are needed (for nonreacting solute) and
they can only be found by curve fitting of the flow equations to effluent
data. Another drawback of the mobile-immobile approach is the inabil-
ity to identify unique retention reactions associated with the dynamic and
stagnant soil regions separately. Due to this difficulty, a general assump-
tion implicitly made is that similar processes and associated parameters
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