Environmental Engineering Reference
In-Depth Information
Typical
thickness
(nm)
Typical
diameter
(nm)
Specific
surface
(km
2
/kg)
Edge view
Montmorillonite
3
100-1000
0.8
IIIite
30
10,000
0.08
Chlorite
30
10,000
0.08
kaolinite
50-2000
300-4000
0.015
Figure 7.7.
Average values of the relative sizes, thicknesses and specific surfaces of the common clay
minerals (Holtz and Kovacs, 1981, reproduced with permission of Pearson Education).
A typical replacement series is:
Na
Li
K
Mg
Ca
Al
Fe
(7.4)
However, Ca
can be replaced by Na
if the concentration of Na
is sufficiently high.
The rate at which cation exchange occurs depends on:
- Clay type - for example, exchange will be almost instantaneous in kaolinite, a few
hours in illite, and longer in montmorillonite;
- Concentration of cations in the soil water;
-Temperature.
The quantity of exchangeable cations required to balance the charge deficiency of a clay
is known as the cation exchange capacity (CEC) and is usually expressed as milliequiva-
lents per 100 grams of dry clay.
7.3.3
Formation of diffuse double layer
In a dry soil, the exchangeable cations are held tightly to the negatively charged clay sur-
face. Excess cations and their associated anions precipitate as salts.
When placed in water, the salts go into solution and because of the high concentration
of adsorbed cations near the clay surface these diffuse from the clay surface as they repel
each other. However, this is counteracted by the negative charge on the clay surface. The
anions in solution are repelled by the negative charge surface of the clay (but attracted to
the cations). The overall effect is to result in a distribution of ions shown in
Figure 7.8
.
The negative surface and the distributed ions adjacent are together known as the diffuse
double layer. Details are discussed in Mitchell (1976, 1993).
