Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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the unit layers, and variations 
in specific surface area are due primarily to the difference in thicknesses of 
the particles, taking into account the participation of surfaces of all unit lay-
ers in those 2:1 minerals (i.e., interlayer surfaces).
Theoretically, we can calculate the surface area of a representative elemen-
tary volume (REV) of a clay if we have information on (a) the shapes and 
sizes of the individual clay particles, and (b) their distribution, as has been 
undertaken by Greenland and Mott (1985) using knowledge of the unit cell 
of a mineral to determine its representative surface area. The a and b dimen-
sions of a unit cell for a dioctahedral 2:1 mineral such as a smectite were used 
together with Avogadro\u2019s number of such unit cells to arrive at a calculated 
specific surface area of 757 m2/g.
It is important to distinguish between surfaces available for interparticle 
action and interaction with water or other fluids. Figure 2.13 shows how the 
Microstructural unit (msu)
Because of aggregation of
particles, available (exposed)
surfaces for interaction are less
than the sum of all the
combined surface areas of
particles constituting the msu
Formation of
macrostructure from
microstructural units further
reduces available (exposed)
surfaces for interaction
Clay
 par
ticle
Macrostructure
4 µ
FIguRE 2.13
Surface areas of mineral particles and reduction of available (exposed) surface areas for inter-
actions due to formation of microstructural units and aggregation into the macrostructure of a 
soil. Picture at bottom right is an SEM of kaolinite showing an aggregation of particles.
57Nature of Soils
formation of microstructural units reduces the available or exposed surfaces 
and their surface areas.
Aggregation of packing of individual particles into packages identified as 
domains, peds, aggregate groups, floccs, clusters, and so forth creates the 
situation where the surfaces of individual particles exposed to fluids can 
be hidden or totally emasculated. As will be discussed in the later section 
and in the other chapters of this book, the formation and presence of these 
aggregated particle groups are factors that need full consideration in evalu-
ation of soil properties and behaviour. Since different disciplines and inter-
est groups use different terminologies to identify these aggregated particle 
groups, we will use the term microstructural unit (msu) for these aggregated 
particle groups. As shown in Figure 2.13, these microstructural units com-
bine to form the macrostructure of a soil, thereby setting the stage for the 
many soil reactions evaluated as soil behaviour.
Knowledge of the type of clay mineral, the microstructural features of 
the clay, and also of the difference between exposed and nonexposed par-
ticle surfaces is most important. This is particularly relevant, for example, 
for smectitic clays, where the planar surfaces of the individual unit layers 
of montmorillonites (Figure 2.14) are active participants in particle interac-
tions. The uncleaved state of the mineral particle exhibits an apparent planar 
shape. The surface area for this particle is defined by the two basal planes 
and the sides of the particle. Cleavage or separation of the dioctahedral 2:1 
layer silicate mineral into n number of individual unit layers or particles with 
smaller numbers of unit layers will result in an increase in exposed particle 
surface areas (bottom portion of Figure 2.14).
In swelling clays, cleavage or separation of the layer silicate into indi-
vidual unit layers will occur as a result of water uptake. The nature and 
amount of separation will depend on the chemistry of the water and also 
on the availability of the water. The footnotes in Table 2.1 highlight this 
phenomenon. The next chapter provides a detailed discussion of this 
important property.
Laboratory measurements are generally used to determine the specific 
surface area of clays since theoretical calculations are not only tedious, but 
also unrealistic if the clays contain different clay and nonclay minerals. In 
the procedure that is commonly used, a gas or liquid is used as the adsorbate 
for the clay solids. The choice of adsorbate is important since the amount of 
adsorbate that forms a monolayer coat on the surfaces of the clay solids (par-
ticles) needs to be determined. One needs to be sure that all the individual 
clay particles\u2019 surfaces are available for interaction with the adsorbate. This 
means that the clay particles must be in a totally dispersed state. The avail-
ability of clay particles in a totally dispersed state and the choice of adsorbate 
are the two most important factors in any laboratory measurement of the 
specific surface area (SSA) of a clay sample. Because of inherent uncertain-
ties, laboratory determinations of the SSA of clay and other types of clays 
will produce operationally defined measurements of SSA.
58 Environmental Soil Properties and Behaviour
Historically, nitrogen gas, krypton, water vapour, and methyl alcohol were 
used as the adsorbates, with nitrogen gas being the more popular adsorbate 
of choice. One key consideration supporting this popular choice is that the 
number of molecules of nitrogen gas sorbed by the clay particles will be 
dependent on both the partial pressure of the gas and on the test tempera-
ture. Furthermore, since nitrogen gas is not able to migrate into the inner 
pores of aggregates and the interlayer of 2:1 layer silicates, this method is 
considered to provide observations pertinent to the outer surface area of 
particles, aggregates, and clay minerals in soils. This method allows one to 
measure a small SSA of soils. The relationship developed by Brunauer et al. 
(1938) is used to determine the amount (volume) of gas equivalent to a sorbed 
monomolecular layer of gas, since more than one layer of gas will be sorbed 
by the clay particles in the actual determination process. This relationship is 
generally known as the BET equation for multilayer sorption.
More recently, polar fluids have been used as the adsorbates. Because of 
their ability to penetrate and migrate into microscopic pores, the techniques 
described by Mortland and Kemper (1965) for ethylene glycol and Carter 
 et al. (1986) for ethylene glycol-monoethyl ether (EGME) are used to observe 
the whole surface area, including the inner surface of aggregates and the 
2:1 dioctahedral mineral particle separated into n number
of unit layers \u2013 all of which have top and bottom planar
surfaces (interlayer surfaces) available for interaction 
As a solid particle,
surface area is considerably
smaller in comparison to 2:1 mineral
particle with available interlayer surfaces 
Kaoliniteparticle
2:1 u
nit la
yer
Montmorillonite
particle
2:1 unit layer
Top planar surface of kaolinite
Bottom planar surface of kaolinite
Top planar surface
of kaolinite
Ed
ge
su
rfa
ce
Edge surface of particle
FIguRE 2.14
Surface areas of two clay mineral particles. Top sketch shows a well-crystallized kaolinite par-
ticle with edge surface and top and bottom planar surfaces contributing to the total surface 
area of the particle. In the bottom sketch, the 2:1 montmorillonite particle is shown with its 
single particle separated into n number of interlayers. The total surface area of the montmoril-
lonite particle includes all the interlayer surfaces.
59Nature of Soils
interlayer of clay minerals. Common to all the techniques used to determine 
the SSA is the implicit requirement for the absorbate to only form a uni-
form monolayer coating on each individual clay particle. The SEM shown in 
Figure 2.13 will tell us why and how difficult this is. In short, the values of 
SSA determined are directly dependent on the procedure used and the ana-
lytical technique used to reduce