Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 215

Figure 7.13. Moreover, the shape of these isotherms depicts an L-type curve
as described by Sposito (1984). These n values are within the range of val-
ues of those reported earlier by Buchter et al. (1989); 0.57-0.78 for Cd and
0.65-0.74 for Ni. A comparison of the adsorption isotherms indicates that for
both Ni and Cd sorption affinities follows the sequence; Windsor < Olivier <
Webster soil (Figure 7.11). This is also illustrated by the respective K values
for Cd; 5.62, 24.59, and 26.78 L kg -1 and for Ni; 2.55, 13.30, and 37.57 L kg -1 .
The work of Gomes, Fontes, and da Silva (2001), among others, indicates that
Cd and Ni adsorption by a number of soils are correlated with CEC. Papini,
Saurini, and Bianchi (2004) reported that Cd and Ni adsorption was largely
due to cation-exchange reaction on an Italian red soil.
Results of competitive Ni sorption in the presence of a range of Cd con-
centrations are given in Figure 7.14 for all three soils. Here the amount of
Ni sorbed (mmol per kg soil) is presented versus input concentration of the
competing Cd ion for two initial Ni concentration, 0.025 mM (Figure 7.14,

0.5

Webster

0.4

0.3

Olivier

0.2

Windsor

0.1

Initial Ni = 0.025 mM

0.0

0.0 0.2 0.4 0.6 0.8 1.0

8

Webster

6

Olivier

4

2

Windsor

Initial Ni = 0.766 mM

0

0.0 0.2 0.4 0.6 0.8 1.0

Initial Cd Concentration (mM)

FIGURE 7.14
Competitive sorption of Ni in the presence of Cd for Windsor, Olivier, and Webster soils. Initial
Ni concentrations were 0.025 mM (top) and 0.766 mM (bottom).