Geoscience Reference
In-Depth Information
10.4.2 Chemical Properties
In a medium where clay minerals are almost absent and organic
matter abundant, the latter obviously plays a big role in soil properties.
Retention of water and basic cations is correlated with organic-matter
content.
Role of organic matter
In some clay minerals, substitution in the structure, for example the
replacement of Si 4+ by Al 3+ , gives rise to a charge deficiency that is
compensated for by adsorption of a cationic charge. This appears as
exchange capacity and corresponds to the permanent charge of the
crystal structure and is located on the flat surfaces of the mineral layers.
At the same time, there are charges at the edge of the layer, where
the continuity of the crystal structure is interrupted. These charges
are variable charges and correspond to dissociations of the following
types:
Concept of fixed and variable charges
a: Lattice Si-O - H + releases H + (dissociation increases with pH),
b: Lattice Al-O - H + releases H + (increases with pH),
c: Org. matter-COO - H + releases H + (increases with pH),
d: Lattice Al + - OH - releases OH - (decreases if pH rises),
e: Lattice Fe + - OH - releases OH - (decreases if pH rises).
The last two reactions determine the capacity of the layer to retain
anions. But this property rapidly diminishes when the pH rises. The
same type of variable charges exists in allophanes, imogolite and oxides
and hydroxides. They generate a substantial anion exchange capacity,
at least in acid conditions, considering the weak structure of these
minerals, which presents many chain endings of the Al-OH type.
The PZNC, point of zero net charge can be determined. It is the pH at
which the positive charges balance the negative charges on the materials
studied. At a lower pH, the materials release OH - and thus have a
higher anion exchange capacity than cation exchange capacity. Above
the PZNC, it is the reverse. Figure 10.10 illustrates these concepts. The
curves can be experimentally obtained (Espiau 1987).
PZNC
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