Page 89 - Soil and water contamination, 2nd edition

76 Soil and Water Contamination

a. Kaolinite b. Illite c. Montmorillonite
Tetrahedral sheet
Octahedral sheet
H O
2
K +
+
2+
Na , Ca , Mg 2+

0.7 nm 1.0 nm 1.5 nm
6642 6642 6642

Figure 4.4 Lattice structures of a) kaolinite (1:1 clay mineral), b) illite (2:1 clay mineral), and c) montmorillonite
(2:1 clay mineral).
+
The layers of illite are bonded with dehydrated potassium (K ) ions (no surrounding water
molecules) partly sunken into the hexagonal holes of the tetrahedral layers. The layers of
+
montmorillonite are separated by water molecules containing cations such as sodium (Na ),
2+
2+
calcium (Ca ), and magnesium (Mg ).
Aluminium and silicon atoms are similar in size to each other and to other metal ions.
3+
4+
It is therefore possible for the Si or Al in the basic crystal structure to be replaced by
2+
2+
2+
3+
Al , Fe , Mg , or Ca without disturbances to the crystal structure. This replacement
is called isomorphous substitution and results in a charge imbalance that leaves most
of the clay particles negatively charged. The process of isomorphous substitution is rather
slow and, therefore, the resulting negative charge is fairly stable and is neutralised by
cation adsorption at the clay mineral surface. Like sesquioxides , clay minerals also have an
additional pH-dependent charge due to the protonation as described by Equation (4.1).
These protonation reactions occur wherever charge imbalances at ‘broken bonds’ of the
tetrahedral and octahedral layers occur, i.e. at the edges of the clay minerals. The faces of
the clay minerals do not generally have any free hydroxyl groups for protonation reactions,
so the pH-dependent surface charge occurs primarily on the clay edges. The Si-OH groups
at the edges of the tetrahedral layer have a very low PZC and, therefore, they are negatively
charged or near neutral, even at low pH values. In contrast, the behaviour of the edges of the
octahedral layer is to some extent like gibbsite , which has a relatively high PZC value (about
7.0). Consequently, the edges of the octahedral layer are likely to have a positive charge at
low pH values.
Kaolinite and other 1:1 clay minerals are barely susceptible to isomorphous substitution
and develop only a pH-dependent charge. Kaolinite has a PZC of 4.6, thus at low pH it
has a near-zero surface or slightly positive charge, but the surface becomes slightly negative
with increasing pH as protons dissociate from surface hydroxyls. The fixed charge on 2:1 clay
minerals always contributes more to total surface charge than the variable charge. Illite has a
moderately large fixed negative charge. The negative charge of montmorillonite is larger than
that of illite .
Because clay minerals are primarily negatively charged, they will basically be sorbents for
cations . Cation adsorption can occur via various mechanisms, which results in a different
degree of bonding between the negatively charged mineral surface and the cations. These
sorption mechanisms include the formation of an inner-sphere complex, formation of an
outer-sphere complex, adsorption in the so-called diffuse layer , and specific adsorption .

The formation of an inner-sphere surface complex implies a surface complex formed

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