Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 243

arrival time for each BTC, Cecil soil exhibited higher affinity for V than P.
This result was consistent for all three consecutive pulses. The location of
the BTC maximum was the same for both P and V. This suggests that the
retention mechanisms for P as well as V on Cecil soil were similar. However,
a major difference is that the amount that appears to be irreversibly sorbed
(or slowly released) is larger for V than P. We infer the extent of irreversible
sorption (or slow release) based on the area under the BTCs. Moreover, the
extensive tailing during leaching or desorption also suggests nonlinear and/
or kinetic retention mechanisms for both V and P (Mansell et al., 1992).
Competition between V and P for the same retention sites on soil surfaces
is evident when one compares the BTCs shown in Figure 7.37 with those in
Figure 7.38. The absence of P in the pulse solution resulted in a much higher
retention of V, with a much lower peak concentration when compared to the case
when both P and V were present in the input pulse solution. The extent of reten-
tion can be estimated from the area under the BTC for V in the first input pulse
where four times the amount of V was found in the effluent solution when P was
present. In addition, a subsequent P pulse displaced a significant portion of V
already retained by the Cecil soil during the first pulse. However, the opposite
was not as apparent when a P pulse followed a V pulse, as shown in Figure 7.39.
Phosphate-soil interactions exhibit various reactions, including kinetic,
reversible, and nonlinear sorption, often with some degree of irreversibility
(Mansell et al., 1992; Chen et a1., 1996). Nonlinear sorption occurs primar-
ily due to reactions with Fe and Al oxides in the soil. The term sorption is
used in a general sense here to include P adsorption by mineral surfaces

1.0

Cecil, Column 3
Pulse: P --- V --- ( V+P )

0.8

0.6

0.4

Flow
Interuption

0.2

0.0

0 20 40 60 80

Pore Volume (V/V o )

FIGURE 7.39
Breakthrough curve results of V and P from Cecil soil (column 3). The column received three
pulses of a mixed solution of P and V.