Soil Colloids
Chapter 8
Ion exchange
Cation exchange capacity
Anion exchange capacity
Types of soil colloids
Emphasis on layer silicate minerals
Types and properties of layer silicates
1:1
2:1
2:1:1
Types of electrostatic charge
Permanent (isomorphic substitution)
pH-dependent
Acidic and basic cations and soil acidity
Small so
BIG SURFACE AREA / mass
Electrostatic charges (- / +) so
Adsorb ions
Usually more
Adsorbed cations and anions
Not stuck permanently, irreversibly
and for ever and ever on colloids
Can trade places with other cations
and anions in solution
Cation Exchange
Equilibrium between cations in solution
and adsorbed on colloids
2+
+ +
█Ca2+
+2K
Ca2+2+++ 2K
█2K++
Ca
+ 2K
 Ca
- charged sites of a colloid
These equilibria are complex, involving all exchangeable species. The above is
an example binary exchange reaction for which an equilibrium constant can be
written as KK-Ca = [Ca2+][K+ad]2 / [K+]2[Ca2+ad]. If you’ve had 2nd semester chemistry
or remember high school chemistry it should make sense.
Anion Exchange
Like cation exchange
- 
- █SO
SO442-2- ++█2Cl

SO42-42-+2Cl
+ 2Cl
 SO
2Cl
+ charged sites
Since a decrease in solution concentration of a nutrient cation or anion by plant
uptake or leaching tends to cause release of the same type ion into solution from
colloids (this is accompanied by replacement on the colloid by a different type cation
or anion), the adsorbed ions are a reservoir of nutrients. Much greater quantity so
adsorbed than in the soil solution.
Are adsorbed cations and anions
important to plant nutrition?
Does ion exchange replenish the soil
solution with nutrient ions?
Does ion exchange slow the
movement of charged contaminants?
If a portion of a substance in the soil is distributed between solution and solid
(adsorbed to) phases, its mobility must be less than if it were all in solution.
Is a soil with a lot of adsorbed ions
more fertile than a soil with very few
adsorbed ions?
Does a charged contaminant move
more slowly in a soil with a high
capcity to adsorb ions than in a soil
with a low capacity to adsorb ions?
Yes to the first, assuming these were nutrient ions and certainly yes to the second.
Cation Exchange Capacity (CEC)
These are the units in which the concentration of exchangeable cations in a
soil are expressed, particularly cmol(+) kg-1. The others are probably archaic
but notice that they are numerically the same.
Milli-moles (+) charge / 100 g soil
Milli-equivalents (meq) + / 100 g soil
An equivalent is a mole of reactive units, in this case, charge.
Is a centimole + charge / kg the same?
cmol (+) / kg
How do you measure CEC? Or AEC?
In principal, this is a straightforward matter but there are complications in practice.
The basic idea is that you use cation exchange to force all initially adsorbed cations
into solution, separate solution from soil (like filter) and measure the concentration
of all cations in solution. This requires use of a cation in solution that is not very
common in the soil and it requires a high concentration of it. Look back at the
example cation exchange reaction and notice that if the concentration of K+ was
very high, the extent to which Ca2+ would be displaced into solution would be
greater than if the concentration of K+ were modest.
However, there is a problem with determining acidic cations like H+ and Al3+ in this
way. A portion of these cations is very strongly held by adsorption onto colloids
so that even a very high concentration of displacing cations won’t drive the
exchange reaction to completion. However, alternatives exist to deal with this.
For base cations, ammonium, NH4+, is the typically cation used to displace them.
AEC is done the same way but with a displacing anion, of course.
CEC is sum
Acidic + basic cation charges / kg
cmol (+) / kg
Depends on
Types of colloids
Amounts of colloids
pH
It should make sense that different
colloids likely have different CECs
(and AECs). Thus, the relative
amounts of different colloids determine
the CEC. However, the charges on
colloidal particles partly depend on
the concentration of H+ in solution
(i.e., pH, which is –log[H+])
Types of colloids
Sources of charge
permanent
pH-dependent
Types
Layer aluminosilicates
Amorphous aluminosilicates
Al and Fe oxides
Organic (humic)
These are the general types of soil colloids. The layer aluminosilicates are
crystalline, however, amorphous ones have limited and interrupted crystal
structure. Strictly, besides oxides there are related non-siliceous minerals, like
hydroxides and oxy-hydroxides, including ones besides just Al and Fe forms.
Layer Aluminosilicates
Alternating sheets of
Si tetrahedra
and
Al (or Mg) octahedra
Carry electrostatic charges due to
Isomorphic substitution
pH-dependent ionization or protonation
Isomorphic substitution
3+
4+
Al for Si in tetrahedral layer
2+
3+
Mg for Al the octahedral layer
What charge (+ or -) does the crystal carry?
Balanced by cations?
Is this source of charge permanent?
Substitution of a lower valence cation for a higher valence cation during the
formation of the crystal results in a deficit of positive charge relative to negative
charge carried by the O and OH in the structure. Thus, the charge is – and it
is permanent to the crystal structure.
\
\
Al – OH  Al – O- + H+
/
/
\
\
Al – OH + H+ Al – OH2+
/
/
pH dependent charges
Besides permanent charge
there are functional groups
on the surfaces of colloids
that can ionize or protonate
to give rise to - / + charge.
Here is a common example,
surface Al–OH groups. Under
conditions of higher soil pH
(i.e., low concentration of H+),
they tend to dissociate as in
the top reaction.
But when the pH is low, the
O tends to be protonated by
H+ from solution, giving a +
site.
There are lots of functional
groups, both on mineral and
organic colloids that do this.
Tetrahedral sheet
Octahedral sheet
Three types
of layer silicates
1:1
2:1
2:1:1
Having said a bit about electrostatic
charges, let’s look at the common
layer aluminosilicate minerals. These
are they.
2:1 layer silicates
Unit consists 1 octahedral sheet between 2
Si tetrahedral sheets
d
Certain types expan
2:1 Types
This is a cutaway
showing interlayer
space between
two units of a
2:1 type mineral.
In this case, the
stack of crystal
units are shown
to be able to
expand, imbibing
water in between
adjacent crystals.
Some 2:1 do this,
others don’t.
Those can are
responsible for
macroscopic
shrinking and
swelling behavior.
Three types of 2:1 minerals
Smectite
Vermiculite
Illite
2:1 Types
Smectite
Units weakly
held together
by cations
Expand when
adsorb water
between units
2:1 Types, Smectite
Big
CEC
Highly plastic and swelling
Does this soil have a lot
of smectite in it?
2:1 Types
Vermiculite
Even bigger
CEC
More isomorphic substitution
Most of the isomorphic substitution in
smectite is in the octahedral layer and
these expand.
The CEC of vermiculite is bigger and a
lot of it is due to substitution in the
tetrahedral layers.
Does vermiculite expand as much as
smectite?
Very little, in fact. Apparently, the higher density of negative charge located very
near the surface of the crystal face (tetrahedral sheet) leads to higher electrostatic
attraction for cations in the interlayer space. The mutually strong attraction by two
adjacent crystals for these cations greatly limits the extent to which water enters
the interlayer space and causes expansion. Make sense?
2:1 Types, Vermiculite
No because
Strong affinity for cations that bridge
tetrahedral layers
Limited-expansion
Illite
Isomorphic substitution in Si
tetrahedral sheet
Geometry favors
+
adsorption of K at
interlayer positions
Holds units
tightly together
This is much the same thing as with
vermiculite, however, the presence of
K+ leads to especially strong bridging
of adjacent crystals. See next slide.
These are different representations of the silica tetrahedral sheet. Notice
the hole-like features that some call siloxane cavities. K+ has just the
right ionic radius to fit into these. Thus, electrostatic attraction between
it and the isomorphic negative charge (much of it in the tetrahedral sheet)
leads to very strong bridging between one crystal unit of illite and its
neighbor. Thus, illite does not expand.
2:1 Types, Illite
By the way, surface area is measured
from gas adsorption.
Nonexpanding
Whereas smectite has open interlayer
space, illite does not. Thus, much of the
planar area of the tetrahedral layers in illite
is not exposed to the gas.
Smaller surface area than smectite or
vermiculite
CEC much less than other 2:1 minerals
Further, the K+ in the interlayer space is not
exchangeable. Thus, the high amount of
negative charge (high extent of isomorphic
substitution) cannot be measured by
summing the charge of cations released
by CEC determination.
1:1 layer silicates
1 Si tetrahedral sheet
1 Al octahedral sheet
Adjacent units
H-bonded
together
Os from the tetrahedral sheet of
one crystal H-bond with the –OHs
of the octahedral sheet of the
neighboring crystal.
If adjacent crystal units are H-bonded
together, do 1:1 minerals expand?
Little plasticity or swelling
Small CEC
Little isomorphic substitution
And since there is little isomorphic substitution, most of the CEC is
due to pH-dependent charge that arises from ionization of edge –OHs.
2:1:1 minerals
Additional octahedral
sheet (2:1:1) contains Mg
Nonexpanding
and fairly low CEC
Less common than the others.
Review
Cation Exchange
Equilibrium between cations in solution
and adsorbed on colloids
2+
2+ + █2K++
2++2K+ + Ca2+
█Ca
Ca + 2K  Ca + 2K
- charged sites of a colloid
CEC is sum
Acidic + basic cation charges / kg
cmol (+) / kg
Depends on
Types of colloids
Amounts of colloids
pH
Types
Layer aluminosilicates
Amorphous aluminosilicates
Al and Fe oxides
Organic (humic)
Tetrahedral sheet
Octahedral sheet
Three types
of layer silicates
1:1
2:1
2:1:1
Three types of 2:1 minerals
Smectite
expanding, high CEC
Vermiculite
limited expansion, higher CEC
Illite
not expanding, trapped K+
1:1 layer silicates
1 Si tetrahedral sheet
1 Al octahedral sheet
Adjacent units
H-bonded
together
2:1:1 minerals
Additional octahedral
sheet (2:1:1) contains Mg
Nonexpanding
and fairly low CEC
Formation and stability of mineral colloids
Primary minerals
2:1 clays
1:1 clays
Oxides
weather to
which weather to
which weather to
Thus, soils in mildly weathering climates tend to have minerals towards
the top of this sequence, and soils in harshly weathering climates (lots
of water and high temperatures), tend to have minerals towards the bottom.
For edification, check out Jackson-Sherman weathering sequence.
More on Electrostatic Charges
Permanent
You know negative charges come from
3+
4+
isomorphic substitution, like Al for Si
or Mg2+ for Al3+
But what if Al3+ substitutes for Mg2+?
What do you get?
Refer to next slide. There are types of octahedral sheets that contain Mg2+ as
the central cation. These are called trioctahedral and those with Al3+ are called
dioctahedral. Basically, the ideally electro-neutral structure in a trioctahedral
sheet requires 1½ times as many Mg2+ as there are Al3+ in a dioctahedral sheet.
Thus, isomorphic
substitution of the
higher valence Al3+
for Mg2+ results in
an excess of + charge
in the crystal lattice,
which must be
balanced by adsorption
of anions from solution.
pH-dependent
Negative charge
Ionization of H from –OH on surface of
oxides and edges of silicate clays
Al—OH → Al—O- + H+
Ionization of –OH and –COOH on
humic colloids
O
O
║
║
--C—OH → --C—O- + H+
Positive charge
Protonation of –OH to give OH2+
Oxide surfaces and silicate clay edges
Al—OH + H+ → Al—OH2+
Does CEC increase or decrease as pH
increases?
What about AEC?
Think of it this way –permanent charge is unaffected, right, but as the concentration
of H+ in solution decreases (i.e., pH increases), whatever ionizable H there is on
colloidal surfaces tends to ionize, creating negative sites and making the colloid
more negative. So, the capacity of the colloid to adsorb cations increases, i.e.,
the CEC increases. The AEC is opposite. As the concentration of H+ in solution
increases, more and more sites become protonated, increasing the positive charge
on the colloid and its capacity to adsorb anions from solution.
Charge at pH 7
Type
Perm
Humus
20
Vermiculite 140
Smectite
95
lllite
20
1:1 minerals 0.4
Oxides
0
pH-dep
Total
180
10
5
6
7.6
4
200
150
100
30
8
4
This is somebody’s breakdown of CEC into permanent and pH-dependent
components. The notion that organic colloids (humus) have permanent charge
makes no sense since isomorphic substitution is not applicable. What, however,
makes sense is that even at very low pHs (not to be encountered except in some
drained wetlands or contaminated sites) some of the acidic functional groups
on soil organic matter are sufficiently acidic to be ionized.
More on CEC
2+
2+
+
+
Ca , Mg , K and Na are basic cations
+
3+
H and Al are acidic cations
Percentage of CEC that is made up of basic
cations is called
Percentage base saturation
%BS
Here’s an example calculation:
Extractable cations
Ca2+
Mg2+
K+
H+
----------- cmol(+) kg-1----------2
1
1
1
For this case, CEC = 5 cmol(+) kg-1 and there are 4 cmol(+) kg-1 due to the bases
so the %BS = 4 / 5 x 100% = 80%
True or False
As pH ↑ %BS ↑
True. If the pH increases, there is less acidity in the system (H+ and other
acidic cations, e.g., Al3+, both in solution and adsorbed on colloidal surfaces).
Thus, since the negative charge on colloids must be satisfied by adsorbed
cations, decreased concentration of acidic cations means increased
concentration of basic cations. Also, with increasing pH the negative charge
on colloids increases, compounding the effect of increased concentration
of adsorbed bases.
Soil A Soil B
cmol (+) / kg
Basic cations
Acidic cations
90
10
5
5
Which soil has the lower pH?
Which soil is more fertile?
Let’s just say likely lower pH. A has a %BS = 90 and B, 50. Thus, B likely
has the lower pH. The matter of fertility is clearer since most basic cations
are nutrients –A has 18x as many.