Environmental Engineering Reference
In-Depth Information
- Soils wet up and swell when given access to water;
-Temperatures above 100°C are required to drive off all water from soils.
Much of the water in soil is adsorbed i.e. the electrical charge of the water dipoles and the
electrical charge associated with the surface of the clay minerals attract each other.
There are several explanations for this as shown in
Figure 7.6
:
-
hydrogen bonding,
where the positive H side of the water dipole is attracted to surface
oxygens, or the negative O side of the water dipole is attracted to surface hydroxyls;
-
ion hydration
, where water dipoles are attracted with cations, which are in turn
attracted by the negative charge of the surface of the clay mineral;
-
osmosis
: as discussed below, there is a greater concentration of cations close to the sur-
face of the clay than further away. Water molecules tend to diffuse towards the surface
in an attempt to equalise concentrations;
-
dipole attraction
- water dipoles orienting themselves as shown in Figure 7.6(d), with
a central cation to equalise charges.
Some clay minerals, particularly those of the smectite, or montmorillonite group, have
undergone significant isomorphous substitution giving a large net negative charge on the
surface. They also tend to have particles which are very thin giving a large exposed surface
area. As a result, these clays tend to attract water more than those which do not have a large
negative surface charge and are thicker.
The surface area of clay minerals is measured by the specific surface, where
surface area
volume
specific surface
(7.2)
surface area
mass
or
(7.3)
Figure 7.7
shows the typical dimensions and specific surface for four common clay
minerals.
7.3.2
Cation exchange
As outlined above, one of the bonding mechanisms present in the montmorillonite (or
smectite) group of clay minerals is exchangeable cation bonding. The cations, which are
held in the space between sheets of clay minerals, are able to be exchanged for other
cations, and, in the process, the properties of the clay can be altered. This process is
known as cation exchange.
This source of cation exchange is the most important one in clay minerals except for
kaolinite. Other sources are:
-Broken bonds: electrostatic charges on the edges of particles and particle surfaces. This
is the major cause of exchange in kaolinite;
-
Replacement of the hydrogen of an exposed hydroxyl.
The most commonly present cations are Ca
, Mg
, Na
and K
. The ease with
which the cations replace each other is dependent on:
-Valence - divalent (
) substitute readily for univalent (
), e.g. Ca
for Na
;
-
Relative abundance or concentration of the ion type;
-
Ion size, or charge density - smaller high charge cations will displace large, low charge
cations.
