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1.0
Cd - C
a
isotherm
0.8
0.6
0.4
K
CdCa
= 1
K
CdCa
= 0.5
K
CdCa
= 2
Windsor soil
Eustis soil
0.2
0.0
1.0
0
0.2
0.4
0.6
0.8
Relative Concentration (C/C
o
)
FIGURE 7.3
Cadmium-calcium exchange isotherm for Windsor and Eustis soils. Solid and dashed curves
are simulations using different selectivities (
K
CdCa
). (From H. M. Selim and M. C. Amacher. 1997.
Reactivity and Transport of Heavy Metals in Soils
. Boca Raton, FL: CRC Press. With permission.)
cation-exchange selectivities, however, typically requires development of
exchange isotherms. The experimental methods are laborious. If exchange
selectivities for reference materials could be used to obtain fairly accurate
predictions of cation movement in field soils, then it might be feasible to
1.0
Ca
0.8
Ca—experim.
Calculated
0.6
Mg—experim.
Calculated
0.4
Na—experim.
Calculated
0.2
0.0
0
5
10
15 20
Pore Volume (V/V
o
)
25
30
35
40
FIGURE 7.4
Breakthrough results for a ternary system (Na, Ca, and Mg) in a Sharkey clay soil. Solid curves
are predictions based on the equilibrium ion exchange transport model.
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