Page 91 - Soil and water contamination, 2nd edition

78 Soil and Water Contamination

Table 4.2 Typical values of the cation exchange capacity for some common clay minerals .
Clay mineral CEC (meq/100 g)
Kaolinite 1–10
Illite 20–40
Montmorillonite 80–120
Vermiculite 120–150

Furthermore, the CEC of soils is determined by the content of sesquioxides (see Section
4.2.2) and organic matter (see Section 4.3), both of which are also able to sorb cations.
Section 4.4 gives more details on the CEC of soils.

Besides the interactions between cations at and near the clay mineral surface and in

solution, there are some specific adsorption phenomena at the edges of clay minerals . Illitic
+
clay minerals contain considerable amounts of potassium (K ) which bonds the octahedral
and tetrahedral lattices. The amount of K may correspond to twice to four times the CEC
+
associated with the illite surfaces. These K ions are held extremely preferentially and are
+
largely non-exchangeable under natural conditions. However, at the illite edges some K
ions may be exchanged, causing the interlattice space to open partially, especially if the
+
illite is brought into contact with a solution containing very small concentrations of K .
This opening of the interlattice space results in frayed edge sites : open lattice structure at
+
+
the clay mineral edges. If the K concentration in solution increases, K ions move rapidly
into the open edges, subsequently causing the open structure to collapse. The K adsorbed
in this manner is trapped irreversibly inside the clay lattice structure. This phenomenon is
+
+
+
called K fixation and is specific to K ions and also to ammonium (NH ), caesium (Cs ),
4
+
and rubidium (Rb ) ions, which are of similar size .
A second sorption phenomenon at clay mineral edges involves the adsorption of anions .
As mentioned above, depending on the pH of the solution due to protonation reactions at
the broken bonds of the octahedral and tetrahedral layers, the clay mineral edges may be
slightly positively charged. These positively charged sites give rise to the adsorption of
anions to neutralise the positive charge. Because anion adsorption is only limited to the clay
mineral edges which usually have a limited surface area, the anion exchange capacity (AEC )
is considerably smaller that the cation exchange capacity. The sorption of anions is very
selective, and depends on their valence and size. A larger ion size results in a lower degree
of hydration, which in turn favours sorption. For common anions, experimental evidence
indicates the following order of preference for anion adsorption at clay mineral edges (Bolt
and Bruggenwert, 1978):
4-
3-
-
SiO > PO >> SO 2 – > NO ≈ Cl -
4 4 4 3
3-
Accordingly, if only very small concentrations of phosphate (PO ) anions are present,
4
-
2-
sulphate (SO ) and chloride (Cl ) anions are not adsorbed.
4
The capacity of clay minerals to adsorb ions is a significant feature for the retention and
transport of substances (pollutants) through soil and water. In addition, clay minerals have
the ability to stick to each other, which can also be attributed to the charge imbalances at the
clay mineral surfaces. This property of clay minerals is crucial for the settling characteristics
of clay minerals suspended in surface waters and, therefore, for the transport behaviour of
clay minerals and their associated pollutants. The formation of flocs or aggregates is called
flocculation or coagulation; it results in apparently larger particles, which settle much faster
than smaller particles, even though the specific density of the flocs is usually less than that of

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