Geology Reference
In-Depth Information
The strength of the attractive force between the cation in solution and
the negatively charged surface of the soil component is governed by
Coulomb's law:
F
¼
q
0
:
q
00
ð
constant
Þ
D
:
r
2
ð
5
:
37
Þ
Where
F is the attractive (or repulsive) force
q
0
and q
00
are the charges on the ion and the surface
D the dielectric constant and
r the distance of separation between the two charges.
Thus, more highly charged ions are held at exchange surfaces in
preference to lesser charged ions. Trivalent ions such as Al
31
and Fe
31
are most strongly held, then divalent ions such as Ca
21
and Mg
21
, with
monovalent ions being the least strongly held. For ions of the same
charge, the strength of binding depends on the size of the hydrated ion,
which is the species that exists in soil solution, and hence how close to
the surface it can approach. So for monovalent ions, the relative
strength of binding, or anity for the surface, is Cs
1
4 Rb
1
4 K
1
4 Na
1
4 Li
1
. Ionic concentration must also be taken into account, as
these preferences can be overcome by high concentrations of an ion in
soil solution. For example, sodium ions would dominate the exchange
sites in soil flooded by seawater.
Ions are considered to be held at the charged surface in a diffuse double
layer (the Guoy-Chapman model) in which the concentration of cations
falls, and that of anions increases, with distance from the surface. In a
refinement of this, the Stern model introduces a layer of cations held
directly on the surface of the soil component (Figure 13). Small cations,
or dehydrated ions having lost their water of hydration, can sit at the
surface and form a strong coulombic bond. This allows for the speci-
ficity described above.
The exchange process can be written as a chemical reaction, so, for
example, for K
1
ions in soil solution exchanging with Ca
21
on the
surface (X)
CaX
þ
K
þ
Ð
KX
þ
1
2
Ca
2
þ
ð
5
:
38
Þ
At equilibrium, the ratio of the ions on the surface is related to their
ratio in solution:
ð
K
þ
Þ
Ca
KX
CaX
¼
k
:
ð
5
:
39
Þ
p
2
þ
