Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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generally result in the production of par-
ticles that are granular in shape.
\u2022	 For granular-sized and granular-shaped particles, packing den-
sity together with particle strength are important factors in the 
47Nature of Soils
development of soil strength and resistance. Granularity and particle 
size distribution are also important in obtaining optimum density.
\u2022	 As we have seen in the discussions in the previous sections, for clay 
particles such as the layer lattice minerals, the particles are platy in 
structure. The importance of size and shape, especially shape, can be 
seen in not only in (a) the packing and arrangement of soil particles 
(macro and microstructure) resulting in anisotropy of permeability, 
strength, and compaction, but also in (b) the amount of surface area 
per unit volume of soil (specific surface area, SSA) as it relates to the 
formation and properties of soil aggregates. These aspects will be 
discussed in greater detail when we discuss the subject of specific 
surface area of soils.
\u2022	 Whether molecular forces will predominate over gravitational forces 
in interparticle action depends on how much control particle surface 
forces have on these actions. These will depend on the types of reac-
tive surfaces presented by the clay particles, as discussed in the pre-
vious section. The greater the specific surface area is, the greater will 
be the potential for interparticle action to be dominated by molecu-
lar forces.
2.5.1.2 Texture
There are at least two important points of note in considering the role of 
surface texture of particles: (a) roughness (or smoothness) and (b) surface 
imperfections. In the case of roughness, this surface feature is more impor-
tant in coarse-grained soils, where soil strength and integrity depends on 
packing and interparticle action involving surface resistance from frictional 
forces between particles. The rougher the particles, the greater the frictional 
resistance between adjacent particles in relative motion. In the case of sur-
face imperfections, we note that these are important considerations in clay 
minerals since these are likely sources for electrical charges, due to unsatis-
fied valence charges. These will be discussed in the next section.
2.5.2 Electrical Charges
2.5.2.1 Sources of Electrical Charges
The second column in Table 2.1 gives the sources of charges associated with 
clay mineral particles. Substituting ions of lower positive valence for higher 
valence ions in the crystal lattice, shown for example in the third column 
in the table, will leave the clay layer lattice structure with a net negative 
charge. Substitution of aluminium for silicon in the silica (tetrahedral) sheet, 
and magnesium, iron, or lithium substituting for aluminium in the alumina 
(octahedral) sheet, account for the main sources of charges in the 1:1 and 2:1 
layer lattice minerals\u2014but only a minor part in the 1:1 kaolinite mineral. In 
48 Environmental Soil Properties and Behaviour
this case, clay minerals have permanent charges regardless of the immediate 
surrounding pH. The amount of silanol groups on the siloxane bounding 
surfaces of these 1:1 and 2:1 layer lattice minerals depends on the crystallin-
ity of the interlinked SiO4 tetrahedra. The top illustration in Figure 2.3 shows 
three variations for the sources of charges associated with siloxane surfaces.
Breakage of edges of phyllosilicates will produce hydrous oxide-type of 
edge surfaces. Unsatisfied valence charges at the edges of particles due to 
the breakage constitute another source of electrical charges. These are gener-
ally referred to as broken-bond charges or pH-dependent variable charges. 
A significant source of charges for the 1:1 layer lattice kaolinites comes from 
these broken bonds; that is, the silanol and aluminol groups at the edges of 
the kaolinite particles. This is also the case for the various kinds of oxides in 
the oxide group.
These attract hydrogen or hydroxyl ions from the porewater, and the ease 
with which the hydrogen ion can be exchanged increases as the pH of the 
porewater increases, that is, as the hydrogen ion concentration in the pore-
water decreases. The Al3+ in the exposed edges of the octahedral sheets 
will complex with both H+ and OH\u2212 in the coordinated OH groups. On the 
other hand, the Si4+ will complex only with OH\u2212. Association of the surface 
hydroxyls with a proton occurs below the point of zero charge (pzc). This will 
produce a surface with a positive charge. The donation or loss of a proton by 
surface hydroxyls above the pzc will produce a negatively charged surface. 
In short, the charge due to broken bonds will increase as the pH of the pore-
water increases.
The surfaces of the hydrous oxides (of iron and aluminium for example) 
show coordination to hydroxyl groups that will protonate or deprotonate 
in accordance with the pH of the surrounding medium. Exposure of the 
Fe3+ and Al3+ on the surfaces provides development of Lewis acid sites when 
single coordination occurs between the Fe3+ with the associated H2O; that 
is, Fe(III)\u2219H2O acts as a Lewis acid site. Charge reversal due to changing pH 
values is a significant characteristic of kaolinites and hydrous oxides, and 
charge reversal at the surfaces of the particles because of pH changes is the 
result of proton transfers at the surfaces. In the case of soil organic matter, the 
chemical structure of the macromolecule exerts a direct role on the nature of 
the surface functional groups and hence on the nature of the charge devel-
oped. The negative charges associated with soil organic matter are due to the 
ionization of hydrogen from carboxyl and phenolic hydroxyl groups.
2.5.2.2 Net Surface Charges and Surface Charge Density
The charge density for any clay mineral particle is the sum of all the charges 
acting on the total surface of the particle, that is, the sum of all the positive 
and negative charges. Strictly speaking, one should use the term net surface 
charge densities in referring to the sum of all charges acting on the total sur-
face. However, since the terms surface charge density and charge density have 
49Nature of Soils
been used in the literature to mean the net surface charge density, these 
terms will be used in this book. Without the presence of potential determin-
ing ions (pdis), the surface charge density of a soil particle \u3c3ts consists of \u3c3s, 
the permanent charge due to the structural characteristics of the clay particle 
(isomorphous substitution), and \u3c3h, the resultant surface charge density due 
to hydroxylation and ionization (net proton surface charge density). This can 
be expressed as
 \u3c3ts = \u3c3s + F(\u393H \u2212 \u393OH) (2.1)
where F refers to the Faraday constant; \u393 is the surface excess concentration, 
that is, surface concentration in excess of the bulk concentration; \u393H and \u393OH 
refer to adsorption densities of H+ and OH\u2212 ions and their complexes, respec-
tively; and F(\u393H \u2212 \u393OH) is the net proton surface charge density \u3c3h.
When |\u393H| = |\u393OH|, the point of zero net proton charge (pznpc) is reached, 
and the pH associated with this is designated as pHpznpc. The point of zero 
net proton charge (pznpc) should not be confused with the point of zero charge 
(pzc) or the isoelectric point (iep). The pHpzc, which represents the pH at which 
titration curves intersect, differs from the pHiep, which represents the pH at 
which the zeta potential \u3c2 is zero. Whilst the separate pH values might be 
very close to each other (Figure 2.9), it is important to realize that the zeta 
2 3 4 5 6 7 8 9 10 11
pH
0
0.5
1
1.5
2
\u20130.5
\u20131
\u20131.5
Titration results
pHpzc \u2248 4.08
\u413 H
+ \u2013
 \u413
O
H
\u2013 
(c
m
ol
/k
g)
0
20
\u201320
\u201340
Ze
ta
 P
ot
en
tia
l (
m
v)
pHiep = 4.1
Zeta potential results
FIguRE 2.9
Titration test results