Agriculture Reference
In-Depth Information
200
Webster Soil
Mixed Pulses
Cd + P
150
P
100
50
Cd
0
0
20
40
60
80
Pore Volume (V/V
o
)
FIGURE 7.44
Cadmium and P breakthrough curve results for a Webster soil column that received two mixed
pulses of Cd and P solution. The solid and dashed curves are multireaction model predictions.
x-ray diffraction analysis indicates that for Webster soil, smectite is the domi-
nant clay type.
The solid curves shown in Figures 7.43 and 7.44 are multireaction trans-
port model (MRTM) simulations for the two Webster columns. For the
sequential column, the model provided a good description of Cd as well
as P results, with
R
2
of 0.96 for Cd and 0.95 for P. For the mixed column, the
respective values were 0.85 and 0.86. Overall good model description of the
BTCs for Cd and P were achieved for the sequential and mixed columns.
Peak P concentration and the tailing of the BTC were not well described by
the model, however. The model version that provided the best prediction
for Cd was that which assumed only kinetic reactions. Other model ver-
sions that account for equilibrium sorption resulted in the finite-difference
solution of the convection-dispersion transport equation being unstable
and convergence was not attained. This is an indication that all Cd reten-
tion mechanisms for Webster soil are dominated by kinetic or time-depen-
dent reactions. For P, the dashed curves shown in Figures 7.43 and 7.44, the
MRTM versions were fully kinetic which included reversible and irrevers-
ible P retention. The breakthrough results for P indicate that for both the
sequential and mixed columns more than 90% P recovery was obtained.
These results were not expected and a significant amount of P sorption was
anticipated based on the soil properties of this soil. In fact the lack of P sorp-
tion by the soil columns is in contrast to strong sorption of P for this soil
with
K
F
of 134 and
b
of 0.682. These results suggest that the presence of Cd
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