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4.0
Zn - Windsor
v
= 27.23 cm/d
3.0
2.0
Equal a�nity
RK
1.0
0.0
0
5
10
15
20
25
30
6.0
Ca - Windsor
5.0
v
= 27.23 cm/d
4.0
3.0
Equal a�nity
RK
2.0
1.0
0.0
0
5
10
15
20
25
30
V/V
o
FIGURE 7.11
Zn and Ca breakthrough curves in a Windsor soil column at variable ionic strength. Predictions
were based on equal affinity (
K
12
= 1) and the Rothmund-Kornfeld approach. (From H. M.
Selim and M. C. Amacher. 1997.
Reactivity and Transport of Heavy Metals in Soils
. Boca Raton, FL:
CRC Press. With permission.)
of the Zn and Cd input pulse solutions were much lower than that of the
displacing Ca solution, chromatographic peaks were observed after 13 pore
volumes. The BTCs were somewhat similar in shape except for earlier Zn
arrival at high flow velocity. Early appearance of Zn was well described by
the predicted BTC (dashed curves) where equal Ca-Zn exchange affinity was
assumed. In fact, the chromatographic effect for Ca and Zn was adequately
described by the equal affinity BTCs, for both flow velocities. However, the
tailing was not well predicted. Contrary to our expectations, the Rothmund-
Kornfeld equation predictions (solid curves), for both data sets, were disap-
pointing. The extent of Zn retardation was overestimated by two to three
pore volumes (right shift). The opposite trend was observed for Ca, where
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