Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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flocculation
FIguRE 2.16
Interpretation of presence of microstructures from ranges of average zeta potential \u3b6 measure-
ments from dispersion stability study of clays. (Interpreted from data reported by Yong, R.N., 
and Sethi, A.J., 1977, Turbidity and zeta potential measurements of clay dispersibility, ASTM-
STP 623, pp. 419\u2013431.)
65Nature of Soils
interparticle action. The second group of forces and bonds, which are those 
that are developed between particles with mediating forces and bonds from 
the solutes in water and the water molecules themselves, will be discussed 
in greater detail when we deal with clay\u2013water interactions in Chapter 3.
2.7.1 Forces and bonding
The forces and bonds associated with interparticle action consist of
\u2022	 Primary valence bonds between particles: Except for cementation bonds, 
primary valence bonds are the strongest interparticle attraction 
forces; the activation energy for breakage normally exceeds 125.5 
kJ/mol. They are dominant in heavily consolidated clays where the 
crystal lattices of adjacent particles are in contact.
\u2022	 London-van der Waals forces: These forces are important because they 
operate in the range from 0.2 nm to more than 10 nm regardless of 
whether the particles are charged or uncharged. The bond strength 
is less than 4.184 kJ/mol.
\u2022	 Hydrogen bonds: Hydrogen bonds are weak (4.184\u201312.55 kJ/mol) but their 
numbers can be large, and the net attraction force will thus be high.
\u2022	 Bonds by sorbed cations: These are Coulombic bonds. The integrated 
attraction force between neighbouring but not contacting particles 
is significant. The approximate activation energy is intermediate to 
that of hydrogen bonds and primary valence bonds, that is, on the 
order of 41.84\u201362.76 kJ/mol.
\u2022	 Dipole-type attraction between particles with different charge: Depending 
on the equilibrium or near-equilibrium pH of the immediate micro-
environment, the edges of clay particles can be positively or nega-
tively charged.
\u2022	 Bonds by organic matter: The attraction forces are primarily due to 
hydrogen bonds, and purely physical coupling is obtained by 
embracing hyphae and flagellae. The bonds are flexible and can sus-
tain large strain. However, their strength is very much dependent on 
the environment and can be short-lived.
\u2022	 Cementation bonds: Precipitated matter binding particles together 
can develop bonds with strengths that can approach that of primary 
valence bonds (>125.5 kJ/mol). Their practical importance can be 
substantial, depending on the amount and nature of the precipita-
tion as shown, for example, by the difference between plastic clay 
and claystone.
Short-range forces such as those developed as a function of ion\u2013dipole inter-
action, dipole\u2013dipole interaction, and dipole\u2013particle site interaction are of 
66 Environmental Soil Properties and Behaviour
considerable importance in arrangement of particles in a microstructure. A 
more complete discussion of these will be found in Chapter 3.
2.8 Microorganisms in Soils
Microorganisms in soils include bacteria, protozoa, fungi, algae, and viruses. 
They are smaller than plant and animal cells and are divided into two groups 
depending on their cell structures:
\u2022	 Prokaryotes (simple, single cells less than 5 µm): Prokaryotes have a 
nuclear region not encompassed in a membrane, with only a single 
strand of deoxyribonucleic acid (DNA).
\u2022	 Eukaryotes (single or more complex multicells that are greater than 20 µm): 
Eukaryotes have a nucleus surrounded by a membrane containing 
DNA molecules and are subdivided into unicellular organisms that 
have multipurpose cells and multicellular organisms (plants and 
animals) with special-purpose cells.
2.8.1 Types of Microorganisms
The various types of organisms and microorganisms in the soil environ-
ment fall within the Whittaker (1969) five-kingdom classification as shown in 
Figure 2.17. Whilst all of these organisms and microorganisms are important 
in the geoenvironment, our concern is with the effect of these bioorganisms 
on the soil engineering quality of a soil. In that respect, we consider bacteria 
and fungi to be the microorganisms of importance because of their role in (a) 
bioclogging, that is, changing the nature of pore spaces as a result of accu-
mulation of living bodies and hyphae, resulting in a decrease in the infiltra-
tion rate of water, hydraulic conductivity, flow path of fluid in soils, and so 
forth. (Seki et al., 2006), and (b) alteration and decomposition of organic mat-
ter, transformation and bioweathering of clay minerals, resulting in changes 
in the properties and behaviour of the clay minerals. The discussion for bac-
teria will be conducted in the next subsection. Although viruses that are 
acellular are not included in the classification scheme shown in the figure, 
they are classified as microorganisms and are included in the short summa-
ries of the kinds of organisms.
2.8.1.1 Protozoa
The protozoas, which include pseudopods, flagellates, amoebas, ciliates, 
and parasitic protozoa, have sizes that can vary from 1 to 2,000 mm. They 
67Nature of Soils
are aerobic, single-celled chemoheterotrophs, and are eukaryotes with no 
cell walls. They are divided into four main groups: (a) Mastigophora, which 
are flagellate protozoans; (b) Sarcodina, which are amoeboid; (c) Ciliophone, 
which are ciliated; and (d) Sporozoa, which are parasites of vertebrates and 
invertebrates. Protozoa scavenge particles such as bacteria, yeasts, fungal 
spores, and other protozoa. They are found in water and soil. Water is an 
absolute requirement for their survival.
2.8.1.2 Fungi
Fungi live mainly in the soil or on dead plants and are sometimes found 
in fresh water. Filamentous fungi (mould) are especially important because 
of their role in soil in clogging of pores from the production of hypha and 
mycelium, resulting in a decrease in hydraulic conductivity and gas dif-
fusion coefficient in soils (Seki et al., 1998). Fungi are aerobic multicellular 
eukaryotes and chemoheterotrophs that require organic compounds for 
energy and carbon. Reproduction is by formation of asexual spores, and in 
comparison to bacteria, they (a) do not require as much nitrogen, (b) are more 
sensitive to changes in moisture levels, (c) are larger, (d) grow more slowly, 
and (e) can grow in a more acidic pH range (less than pH 5).
Under the general classification of fungi are slime moulds, yeasts, and 
mushrooms that form mycelium for adsorbing nutrients from a substrate. 
AlgaeProtozoa
Protista
(Undifferentiated Eukayrotes) 
Monera
Bacteria Cyanobacteria
Differentiated Eukaryotes
Plants Fungi Animalia
FIguRE 2.17
Organisms and microorganisms grouped according to the Whittaker 5-kingdom hierarchical 
system.
68 Environmental Soil Properties and Behaviour
Note that in the context of biological activities, the term substrate means food 
for microorganisms; that is, substrate serves as a nutrient source and a source 
of carbon, or energy for microorganisms. Yeasts are unicellular organisms 
that are larger than bacteria. They are shaped like eggs, spheres, or ellipsoids 
and reproduce by formation of buds or fission. The two main components of 
mushrooms are (a) the basidia, the fruiting body above the ground, and (b) 
the mycelium, which is below the ground.
2.8.1.3 Algae
Algae are single-celled and multicellular microorganisms that are green, 
greenish tan to golden brown, yellow to golden brown (marine), or red 
(marine). They grow in the soil and on trees or in fresh or salt water. Those 
that grow with fungi are called lichens. Seaweeds and kelps are examples 
of algae. Since they are photosynthetic, they can produce oxygen, new cells 
from carbon dioxide or bicarbonate (HCO3¯), and dissolved nutrients includ-
ing nitrogen and phosphorus. They