Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 246

Cu BTCs appear symmetrical in shape with considerable tailing and a peak
concentration of 40 mg L -1 . The Mg BTC shows an initial increase in con-
centration due to slight increase in ionic strength followed by a continued
decrease during leaching. When Cu was introduced in the absence of a back-
ground solution, the total concentration decreased from 0.005 to 0.0015 M. As
shown in Figure 7.41, the Cu BTC showed a sharp increase in concentration
due to the chromatographic (or snow plow) effect (Selim et al., 1992). The
peak Cu concentration was 94 mg L -1 , and the corresponding Mg concen-
tration in the effluent decreased due to depletion of Mg during the intro-
duction of Cu. The Mg concentration increased thereafter to a steady-state
level during subsequent leaching. This snowplow effect is a strong indica-
tion of competitive ion exchange between Mg and Cu cations. The amount
of Cu recovered in the effluent was 53% of that applied in the presence
of MgSO 4 as the background solution, whereas only 38% was recovered
when no background solution was used. Therefore, miscible displacement
experiments indicated that there was strong ion exchange between Cu and
Mg cations, which was also affected by the counterion used. Effluent peak
concentrations were three- to fivefold that of the input Cu pulse, which is
indicative of pronounced chromatographic effect.

7.8.6 Cadmium-Phosphate

The influence of the presence of P on Cd sorption in soils is illustrated by
the family of Cd isotherms shown in Figure 7.42 where different initial P

2500

Cd Isotherms
Webster Soil

2000

1500

1000

Initial P (mg/L)

P = 0
P = 25
P = 50
P = 100
Simulated

500

0

0 2 4 6 8 10 12 14

Cd Concentration (mg/L)

FIGURE 7.42
Cadmium adsorption isotherms for Webster soil after 24 h of sorption in the presence of vari-
ous P concentrations. Solid curves are simulations using the Freundlich equation.