Agriculture Reference
In-Depth Information
Box 2.6
Cation Adsorption on Clay Surfaces
Negative charge located at a tetrahedral site (in a silica sheet) is much closer
to an adsorbed cation than is a charge at an octohedral site (in an alumina sheet).
This means its polarizing effect on the cation is greater and can overcome the force
of attraction between the cation and water molecules in its hydration shell. If the
cation loses its hydration shell, it can approach the surface very closely to become
strongly adsorbed. The ease with which a cation can be stripped of its water
molecules and become strongly bound depends on its hydration energy. For the
common exchangeable cations, the energy of hydration per mole of cation falls in
the order
Al
3
Mg
2
Ca
2
Na
K
NH
4
K
, which has a relatively low hydration energy, is incorporated as an unhy-
drated cation in the interlayers of mica-type clays, where the lattice charge is pre-
dominantly in the tetrahedral sheet. The interlayer K
pulls the adjacent mica lay-
ers close together. However, when the lattice charge is buried more deeply in the
crystal layers, as in montmorillonite, there is less tendency for adsorbed cations to
dehydrate and draw adjacent mineral layers close together. Adsorbed cations such as
Ca
2
remain partially hydrated and form complexes with the surface, as in the case
of Ca-saturated montmorillonite. Adsorbed cations such as Na
and Li
remain fully
hydrated and do not complex with the surface.
2:1 mineral types,
illite
,
vermiculite
, and
smectite
, whose chemical formulas are
summarized in table 2.2.
A true soil mica such as
illite
has isomorphous substitution predominantly in
the tetrahedral sheet. The permanent negative charge is neutralized by unhydrated
K
ions that fit snugly in the interlayer spaces between adjacent crystal surfaces.
The interlayer bonding is therefore very strong, and the basal spacing is between
0.96 and 1.01 nm. But as illite weathers, the interlayer K
is gradually replaced
by cations such as Ca
2
and Mg
2
. These have higher hydration energies and re-
main partially hydrated. The basal spacing expands to 1.4-1.5 nm, which is equiv-
alent to a double layer of water molecules between the mineral layers. Conse-
quently, the interlayer bonding becomes weaker, and the stacking of the layers to
form crystals is much less regular. Minerals of this type are sometimes called
hy-
drous micas
.
Table 2.2
Examples of the Chemical Composition and Moles of Charge for 2:1 Clay Minerals
Moles of
Mineral
Typical Unit Cell Formula
a
Charge/Unit Cell
[(Si
7.1
A1
0.9
)
IV
(Al
3.3
Mg
0.7
)
VI
O
20
(OH)
4
]
y
yK
Illite (hydrous mica)
y
1.6
[(Si
7.0
Al
1.0
)
IV
(Al
3
Mg
0.5
Fe
0.5
)
VI
O
20
(OH)
4
]
y
0.5yCa
2
Vermiculite
y
1.5 to
2.0
[Si
8
IV
(Al
3.2
Mg
0.6
Fe
0.2
)
VI
O
20
(OH)
4
]
y
0.5yCa
2
Smectite (montmorillonite)
y
0.6 to
0.8
a
The superscripts IV and VI indicate, respectively, whether the atoms are in the tetrahedral sheet (4 O around each Si) or
octahedral sheet (6 O or 6 OH around each Al); the Fe is shown as Fe
2
, but can also be Fe
3
.
Source:
Data from Sposito (1989) and White (1997)
