Pollution Mechanisms
Retention
Non-adsorptive Retention
Infiltration, diffusion and transport by soil solutions
Alteration, transformation, and initiation of chemical changes within the
soil
The behavior and interaction of pollutants with soil comprise of various physical,
chemical, and biological processes that take place in all three components (solid, gas and
liquid) of the soil medium. They generally include three main groups of processes:
1. Retention
2. Infiltration, diffusion and transport by soil solutions
3. Alteration, transformation, and initiation of chemical changes within the soil
While the first two groups include mainly physical processes, by which pollutants are
transported and distributed in the soil, the third group comprise of only chemical and
biological processes, by which pollutants are transformed or stored as residues in the
interstitial space.
Physical processes of soil/pollutant interactions include transport and retention.
They depend mainly upon the physical parameters of the medium (temperature, grain size,
electric charges, etc.). Chemical processes depend largely on the type of pollutants and their
chemical nature. As for biological, or biologically controlled, soil pollution processes, we
may include all processes of biotransformation and biodegradation, each depending on the
microbial ecology, the depth, and the oxygen availability at the site of pollution.
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Figure 1 the overview of the soil-pollutant interactions
Figure 2 a summary of the mechanisms involved in soil pollution
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4.1 Retention
Pollutants will either be retained by adsorption on the surfaces or accumulated in their
inter-granular space, where they may form concentrations retaining their original chemical
composition, or substances that have been altered by various chemical reactions. Pollutants
retained thus on the soil surface, or in its interstitial space, may be organic, inorganic, or a
mixture or complexes of both. The mechanisms of their interaction with the soil will thus
depend upon physical parameters prevailing in the soil medium, such as temperature,
moisture content or salinity of the soil water, as well as upon their own physical and chemical
properties.
4.1.1 Adsorptive Retention
Adsorption and its accompanying phenomena are considered as the most important
physical chemical mechanisms of pollutant retention on the surface of soil grains. Molecules
of pollutants can be retained on the surfaces of soil grains in two ways.
In physiosorption, or physical adsorption, molecules of pollutants will be
attached to the surfaces of soil grains by Van der Waal forces, which are
known to be weak long range forces. The amounts of energies involved in
such attachments are normally of low magnitudes and are not sufficient for
bond breaking. Thus, pollutant molecules sticking to the soil surface will
retain their chemical identities, although they might be stretched or bent on
account of their proximity to the surface.
In chemisorption, or chemical adsorption, the pollutants attach themselves to
the grain surfaces as a result of the formation of a chemical (usually covalent)
bond. In this case, the energy of attachment is very much greater than in
physical adsorption. Thus, a molecule undergoing chemisorption arising from
bond formation with the surface atoms. Although it is very difficult to
differentiate between physical and chemical adsorption, one can generally say
that the amount of physically adsorbed material decreases with increasing
temperature, while this relation for chemically adsorbed material is reversed.
Normally various adsorbents exist in the soil medium; some examples of these are
given by clay minerals, zeolites, iron and manganese hydrated oxides, aluminium hydroxide,
humic substances, bacterial mucous substances, and plant debris. Many rock forming
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minerals such as micas, feldspars, some pyroxenes, and some amphiboles are also considered
as good adsorbents of pollutant molecules.
Figure 3 Adsorption
Source: http://www.intechopen.com/books/nuclear-power-practical-aspects/geologicaldisposal-of-nuclear-waste-fate-and-transport-of-radioactive-materials
The Extent of Adsorption
The extent of adsorption depends upon the exposed surface area of the adsorbent, as
well as upon the concentration of the sorbate in the soil solution (partial pressure in case of
gases), and the temperature of the medium. If measured, then the adsorption data are plotted
against the concentration values of the adsorbate in the surrounding medium; a graph known
as the adsorption isotherm can be obtained.
Adsorption of Ionic Pollutants
The surface of an individual clay particle or organic colloid is negatively (-) charged.
As a consequence their surfaces attract and adsorb positively charged ions (cations). When
water is added to soil, cations can move into solution, attracted to the clay particle or organic
colloid surface .Cation exchange is therefore defined as the interchange between a cation on
the surface of any negatively charged particle (i.e. clay mineral or organic colloid) and the
soil solution.
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Many soil components (e.g. clay minerals) have a marked tendency of replacing ions
with similar species from the ambient medium (solution). The major source of cations in soil
solution are from mineral weathering (i.e. primary minerals), mineralization of organic matter
and addition of soil ameliorants (i.e. lime, gypsum, etc).
When the species lost or gained cations, the phenomenon will be described as cation
exchange, otherwise as anion exchange. Cation exchangers do not attract all ions with the
same intensity. This preference, or selectivity, depends principally on the cationic
concentration in the solution, the cationic dimensions, as well as on the structural properties
of the exchange surface. Below is the cation strength ranged from stronger to weaker.
Al3+
> Ca 2+ > Mg 2+ > K
+
> Na +
> H
+
It indicates, from left to right, the decreasing strength of adsorption of the various
cations. As such, the less tightly held cations are located furthest from the surface of colloids
and are most likely to be leached away or further down the profile most quickly. Conversely,
the most strongly adsorbed cations will tend to move the slowest down through the profile.
The proportion and kinds of cations adsorbed on soil mineral particles and organic colloids is
also a function of the concentration of cations in the soil solution. If the concentration of a
cation in soil solution is high, there is an increased chance or tendency for that cation to be
adsorbed.
Factors Affecting Adsorption
The intensity of adsorption depends upon several factors, including physical and
chemical properties of the pollutants themselves, as well as the soil matrix, composition, and
surface properties. It is generally possible to summaries all these factors as follows:
Mineralogical composition of the soil; Clay minerals are the most important
adsorbents in the soil environment, followed by some silicates and organic components.
Accordingly, the intensity of adsorbance in soils will largely depend on the clay content of
the soil, as well as on the proportion of other silicates in the mineralogical composition.
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Grain size distribution in the soil; the rate of adsorption is higher on finer sediments
than on coarser ones.
The content and distribution of humic substances in soil; the presence of active
functional groups (e.g. carboxyl, hydroxyl, carbonyl, methoxy and amino groups) is
thought to be a positive influence on the CEC of a soil.
Chemical and physical properties of the soil solution; the presence of clays, water
molecules are adsorbed on their surfaces to form hydration shells; these provide
adsorption sites for pollutant molecules.
CEC of organic and mineral components;
The pollutants, their nature, and chemical constitution; The composition and nature of
contaminants control to a considerable extent not only the solution and diffusion
processes, but also adsorption on the soil grains. Such control may be explained by
the fact that ion exchange and hydrolysis reactions are particularly sensitive to the
parameters (pH, Eh) of the chemical environment, created by the contaminants in
their direct vicinity.
4.1.2 Non-adsorptive Retention
Trapping
The entrapment of solid particles and large dissolved molecules in the pore space of
the soil forms one of the major mechanisms in the retention of pollutants in the soil. This type
of retention occurs following the three mechanisms
1. Caking. This may occur physically when the pollutant particles are larger than the
soil pores. In this case, the entrapped particles form a layer (cake) on the surface where the
pore sizes become too small.
2. Straining. Straining occurs when pollutant particles are about the size of the soil
pores. They move down the pores until they are entrapped at the entrance to a pore which is
too small to pass through.
3. Physical-chemical trapping. The limitation of the flow, through the clogging of the
pore space, may occur because of physical or chemical transformation, such as the
production, by chemical reactions, of new products having molecular sizes that exceed that of
the soil pores.
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Figure 4 Trapping mechanism
Precipitation
The retention of contaminants in the soil may often occur through the passing of
contaminants from a dissolved form to an insoluble form. Precipitation reactions are
controlled by acidbase equilibria and redox conditions. They are reversible and may lead to
the dissolution of formerly precipitated compounds, if conditions are changed.
4.2 Infiltration, diffusion and transport by soil solutions
4.2.1 Infiltration/rainfall
It is the most common mechanism of contamination of soil solutions in the vadose
zone, as well as, deeper regions of the saturated zones of the groundwater. As fluids move
downward under the influence of gravity, they dissolve materials to form leachates that
contain inorganic and