Agriculture Reference
In-Depth Information
at all values of
C
o
except for
C
o
= 1 mg L
-1
. Therefore, the applicability of the
five-parameter version to a wide range of solute concentrations was directly
related to the magnitude of the fraction of type 1 sites. As
F
increased (Olivier
< Windsor < Cecil), the concentration range over which the five-parameter
version provided a better description of the data than the three-parameter
version increased. The shapes of the experimental curves and model calcula-
tions (Figures 6.12, 6.13, and 6.14) are influenced by
C
o
. At higher
C
o
, retention
of Cr(VI) from solution was far less kinetic than at lower
C
o
. This behavior is
as expected if the concentration of one or more reaction sites limits reaction
rates. At
C
o
= 100 mg L
-1
there were 4 mg of Cr(VI) available for reaction in the
40 mL of solution volume used in the experiment. The maximum possible
amounts of Cr(VI) that could be sorbed by the 4 g of each soil used in the
experiment were 1.9, 2.9, and 4.5 mg (solute weight basis) for Olivier, Windsor,
and Cecil soils, respectively. Thus, maximum possible Cr(VI) retention in the
Cecil soil was about equal to the amount of Cr(VI) available for retention,
but was much less in the Olivier and Windsor soils than the amount of Cr
available. Since the number of type 1 sites was much less than the total, their
contribution to the overall reaction is actually quite negligible at high solute
concentrations. The influence of
S
max
/
C
o
or Ω (Ω = ρ
S
max
/
C
o
Θ) on solute reten-
tion during transport is illustrated in Figure 6.8.
In general the SOM approach well described the data sets shown in
Figures 6.12, 6.13, and 6.14. Moreover, the retention processes responsible
for Cr(VI) retention may include physical adsorption, formation of outer-
or inner-sphere surface complexes, ion exchange, surface precipitation, etc.
Furthermore, subsequent solute transformations on the soil surface or inter-
nal diffusion into soil particles may occur.
6.4 Experimental Data on Transport
Chromium BTCs from the miscible displacement experiments for all three
soils are shown in Figures 6.15, 6.16, and 6.17. For Cecil and Windsor soils
the measured BTCs appear to be highly kinetic, with extensive tailing. For
Olivier soil little tailing was observed and approximately 100% of the applied
Cr(VI) pulse was recovered. These results are consistent with the batch data
where the irreversible reaction parameter (
k
s
) was found to be quite small.
In order to examine the capability of the SOM, the necessary model param-
eters must be provided. Values for the dispersion coefficients (D) were obtained
from BTCs of tracer data for
3
H
2
O and
36
Cl. Other model parameters such as ρ,
Θ, and
q
were measured for each soil column. In addition, values for
S
max
and
F
used in describing Cr (VI) BTCs using the SOM were those derived from adsorp-
tion isotherms shown in Figure 6.11. Direct measurement of these parameters
by other than parameter-optimization techniques is not available. Moreover,
Search WWH ::
Custom Search