Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


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The higher 
the valence of the replacing ion, the greater its replacing power. Also, the 
higher the valence of the surface exchangeable ion, the more difficult it will 
be to replace the ion. For ions of the same valence, increasing ion size gives 
greater replacing power. The relationship used to determine replacement of 
exchangeable cations with the same positive charge and similar geometries 
as the replacing cations is given as follows: Ms/Ns = Mo/No = 1, where M and 
N represent the cation species, and the subscripts s and o represent the par-
ticle surface and the bulk solution.
Cations can be arranged in a series on the basis of the replacing power. The 
position in this series will depend on the kind of clay and on the ion being 
replaced, with the replacement positions being largely dependent on the size 
of the hydrated cation. The replacing power of some typical ions is shown as 
a lyotropic series as follows (Yong, 2001):
 Na+ < Li+ < K+ < Rb+ < Cs+ < Mg2+< Ca2+ < Ba2+ < Cu2+ < Al3+ < Fe3+ < Th4+
Changes in the relative positions of the lyotropic series depend on the kind of 
clay and the ion being replaced. The number of exchangeable cations replaced 
53Nature of Soils
depends on the concentration of ions in the replacing solution. In heterova-
lent exchange, the selective preference for monovalent and divalent cations is 
dependent on the magnitude of the electric potential in the region where the 
greatest amount of cations is located. When the outside concentration var-
ies, the proportion of each exchangeable cation to the total cation exchange 
capacity is determined by exchange-equilibrium equations. Perhaps the most 
commonly used relationship is the Gapon equation (Yong, 2001):
 
M+me
e
n
o
m
o
nN
K
mM
nN
+
+
+
= (2.2)
where the superscripts m and n refer to the valence of the cations, the sub-
scripts e and o refer to the exchangeable and bulk solution ions, and the 
constant K is a function of specific cation adsorption and nature of the clay 
surface. K decreases in value as the surface density of charges increases. Na+ 
versus Ca2+ represents a particularly important case of exchange competi-
tion. Figure 2.11 shows the effect of the proportions of Na+ and Ca2+ in three 
clays\u2014kaolinite, illite, and montmorillonite\u2014on the permeability of these 
clays. Note that the illustration shows a normalized format for the perme-
ability ordinate.
0 100
100 0
Pe
rm
ea
bi
lit
y, 
(lo
g s
ca
le)
Exchangeable Cation
Percent Na+
Percent Ca2+
Kaolinite
Illite
Montmorillonite
FIguRE 2.11
Schematic illustration showing the influence of exchangeable cations on the permeability of 
three different clays. Note that the ordinate shows a normalized permeability property.
54 Environmental Soil Properties and Behaviour
When the amount of exchangeable calcium on the clay mineral is 
decreased, its release becomes more difficult. On the other hand, when the 
degree of saturation with sodium ions is decreased, they become easier to 
release. Potassium is an exception because its effective ionic diameter of 
0.274 nm is about the same as the diameter of the cavity in the oxygen layer. 
This allows the potassium ion to just fit into one of these cavities, making it 
very difficult to replace. For other cations, it is the size of the hydrated ions 
rather than the size of the nonhydrated ones that controls their replaceabil-
ity. For ions of equal valence, those that are least hydrated have the greatest 
energy of replacement and are the most difficult to displace. Although Li+ 
is a very small ion, it is considered to be strongly hydrated and therefore to 
have a very large hydrated size. The low replacing power of Li+ and its ready 
replaceability can be taken as a consequence of the large hydrated size, but 
there are in fact indications that Li+ and Na+ are only weakly hydrated in 
interlamellar (i.e., interlayer) positions.
2.5.4 Anion Sorption and Exchange
Anion attraction to clay solids is mainly associated with solids containing 
oxide surfaces. Clay mineral particles with negatively charged reactive sur-
faces will attract anions at the edges of the particles because of the presence 
of broken bonds, as has been discussed in Section 2.5.2. For example, Al\u2219(OH)
H2O anion attraction is similar to those associated with anion attraction to 
oxide surfaces. The larger proportion of edge surfaces to planar surfaces in 
1:1 layer silicates (kaolinites for example), in comparison to the 2:1 layer sili-
cates (e.g., smectites), gives them a higher potential for anion attraction and 
sorption. Anion sorption capacity for illites appears to be attributable to their 
hydrous mineral characteristic. The three types of anion exchange in smec-
tites and kaolinite include
\u2022	 Replacement of OH\u2212 ions of clay-mineral surfaces. The extent of the 
exchange depends on the accessibility of the OH\u2212 ions; those within 
the lattices are naturally not involved.
\u2022	 Anions that fit the geometry of the clay lattice, such as phosphate 
and arsenate, may be adsorbed by fitting onto the edges of the silica 
tetrahedral sheets and growing as extensions of these sheets (Pusch 
and Karnland, 1988; Pusch, 1993). Other anions, such as sulphate and 
chloride, do not fit that of the silica tetrahedral sheets because of 
their geometry and do not become adsorbed.
\u2022	 Local charge deficiencies may form anion-exchange spots on basal 
plane surfaces.
The last mechanism contributes to the net anion exchange capacity of smec-
tites. The other two may be important in kaolinite but are assumed to be 
55Nature of Soils
relatively unimportant in smectite clays. The latter minerals commonly 
have an anion exchange capacity of 5\u201310 mEq/100 g but can be considerably 
higher for very fine-grained kaolinite and palygorskite.
Clay minerals such as allophane also attract anion owing to variable 
charge depending on pH. The surface positive charge increases with 
an increase in hydrogen ions in porewater because of the reaction of the 
hydroxyls attached to Al with hydrogen ions and, consequently, the amount 
of anion adsorption increases with a decrease in pH as shown in Figure 2.12.
2.5.5 Specific Surface Area (SSA)
Because of the many interparticle actions and interacting phenomena 
between clay particles and porewater involving the reactive surfaces of these 
particles\u2014as will be discussed in the next chapter\u2014the specific surface area 
(surface area per unit weight) in a clay is an important property. Many of 
the differences between clay minerals in respect to physical-chemical and 
mechanical properties such as water retention, plasticity, or cohesion can be 
explained by the different amounts of surface area between these minerals. 
This explains the characteristic high swelling performance of montmorillon-
ites with their high specific surface areas. In general, one can obtain a good 
4 5 6 7 8 9 10 11
0
300
200
400
100
Ad
so
rp
tio
n,
 m
g/
kg
pH
60 ppm-KIO3 solution, 3 ml
60 ppm-KIO3 solution, 1 ml
30 ppm-KI solution, 1 ml
FIguRE 2.12
Adsorption of IO3\u2212 and I\u2212 on allophane depending on pH owing to pH-dependant charge 
because of the reaction between hydroxyl of Al and the hydrogen ion when KIO3 and KI solu-
tion of 1 or 3 mL are applied to volcanic ash soil. (Data from Nakano 2010 \u2014personal commu-
nication to Yong.).
56 Environmental Soil Properties and Behaviour
appreciation of the nature of the surface properties of a clay from knowledge 
of the specific surface area (SSA) of the clay.
Clay mineral particles are plate shaped or tabular in shape because the 
layer-lattice structure results in strong bonding along two axes but weak 
bonding between unit layers. The thickness of a clay particle depends on the 
magnitude of the forces of attraction between