The document discusses the fate and transport of chemical pollutants in soil and their effects on the environment, highlighting the differences between organic and inorganic pollutants. It outlines various factors affecting the mobility and retention of pollutants in soils, including pH, soil structure, and chemical properties. Additionally, it details the transport pathways of pollutants through different environmental compartments such as air, water, and soil.

1. Physical transport of chemical
in
soil surface, water & sub surface
V.Vijitha
Lectrurer (Prob.)
Dept. of Biosystems Technology
FoT, UoJ

2. • Fate and transport refers to,
• How the nature of contaminants might change and
• Where they go as they move through the environment
• Different forms of information are taken into account for the analysis of
fate and transport of toxicant.
• Those are,
Possible transport processes
Physical, chemical and biological factors
Site-specific environmental conditions
2
Introduction

3. Overall cycling of a pollutant in soil system
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4. • This model considers
The type of pollutant
The source of pollution
The transport medium (air, water, and soil)
The target (aquatic ecosystems, atmosphere, living organisms, or
soil ecosystem)
• Therefore, soil can act as a source or a sink of pollutants.
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5. • Two main groups:
• Organic pollutants (OPs)
• Inorganic pollutants (IPs): Eg-Potentially toxic elements (PTEs)
• PTEs: Group of metals & metalloids with an atomic density
> 4𝒈𝒄𝒎−𝟑
.
• Both PTEs and OPs can have natural or anthropogenic
origins.
• These two types of pollutants highly differ in their
behaviour in soils. 5
Types of soil pollutants

6. • PTEs are non-degradable and persistence in environment
for long periods.
• Therefore, it allows their transfer from the contamination
sources to other locations.
• PTEs can occur in the soil solution under different forms.
• ionic
• Molecular
• chelated
• colloidal forms
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7. • Mobility of PTEs in soil is always depend on aqueous phase and
intrinsic characteristics of each element.
• Both will determine the ease of release and the sorption onto
surfaces of soil components.
• In addition, the mobility of PTEs are highly dependent on the
source.
PTEs from anthropogenic inputs are more mobile than from
pedogenic or geogenic ones
• OPs can occur naturally from,
• Volcanic emission
• Forest fires
• Fossil fuels
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8. Main soil pollutants exposure pathways
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9. • Soil, surface water, groundwater, and atmosphere can become as
an exposure pathway.
• Due to that:
• Pollution will affect seriously soil functions
• Decreasing the number and diversity of species that live or depend on the
soil
• Decrease in the microbial activity of soil which will reduce the cycling of
organic matter
• The soil structure will be affected
• Man can be also a target of soil pollution by,
• Direct ingestion
• Inhalation
• Dermal exposure
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10. Factors affecting the distribution, transport
and fate of soil pollutants
• Physical and chemical properties of soils
• Soil texture
• Soil structure
• Solid, liquid, and gaseous phases
• pH
• Redox potential
• Cation exchange capacity
• Soil organic matter
• Clay minerals
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11. Factors cont…
• Soil-pollutant interactions processes
• Release processes
• Hydrolysis
• Redox reactions
• Photo-degradation
• Biodegradation
• Dissolution
• Volatilization
• Retention processes
• Adsorption
• Incorporation into biological agents
• Precipitation/surface precipitation
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12. Physical and chemical properties of soils
Soil texture
• It is determined by the particle size distribution of the
solid fraction.
• It influences the ability to retain water.
• Coarse soils: Water retention is low and drainage is rapid
• Fine soils : Water retention is high and drainage is poor
• Fine particles (<100 µm) have higher surface area.
• So they are more reactive in pollutants retention.
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13. Soil structure
• It is related with shape and arrangement of the particles
in soil profile.
• It influences the behaviour of pollutants by controlling
the degree of contact between them and water/soil
solution.
• Particle density determines the physical conditions of
the soil
• High density may indicate the possibility of pollution
cases by metal(loid)s.
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14. Solid, liquid, and gaseous phases
• Water has a greater importance than air concerning the,
• Pollutants transport and
• Reactions in soil
• Liquid phase of soil comprises not only water but also
contains solutes and dissolved gases
• This mixture is crucial for the transfer of pollutants to the
plants, groundwater, and atmosphere.
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15. pH
• It expresses the concentration of H+ of a solution.
• Anthropogenic activities contribute to acidification of
soil.
Eg: Intensive use of fertilizer
• Soil pH greatly influences the mobility and availability of
metal(loids) in soil.
• The acidic conditions favour the increase on mobility.
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16. Redox potential (Eh)
• The Eh, together with pH, are the main factors controlling
the behaviour and mobility of pollutants in soil.
• Oxidation : Loss of electrons
• Reduction : Gain of electrons
• Eh provides an indirect information about the soil
aeration status.
• Oxidizing and reduction conditions have great influence in
the release of pollutants
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17. • Aerobic conditions promote the degradation of organic
compounds.
• From this OPs may be released and/or transformed in
others species.
• Oxidation of some metal(loid)s support mineral phases
(e.g., sulfides).
• It may enhance the release of these pollutants from the
crystalline lattice
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18. Cation exchange capacity (CEC)
• Defn: The potential quantity of readily exchangeable
cations that are able to neutralize negative charges from
soil surfaces.
Eg: Ca2+, Mg2+, K+, Na+
• Degree of CEC in soil - sandy soils < clay soils < organic soil
• Two types of surface charges
• Permanent charges
These are not affected by changes in soil-solution conditions &
nature of free ionic species
• Variable charges
These are affected by changes in soil-solution conditions
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19. Soil organic matter (SOM)
• SOM is composed by,
• Plant debris and
• Colloidal material resulting from the action of
microorganisms
• It influences the retention of water in soil.
• Thus providing conditions suitable for soil microbiota to
promote degradation processes of OPs.
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20. Clay minerals
• Clay minerals are a ubiquitous group of hydrous aluminium
phyllosilicates.
• It has a significant contribution in the sequestration of ionic
pollutants of soil and soil solution.
• Iron and Aluminum are two of the most abundant elements
in the Earth’s crust
• Al- or Fe-(oxy)hydroxides are important agents in the
sequestration of electrical charged particles present in soil
solution.
• It occurs by both,
• Specific adsorption reactions or
• Precipitation processes 20

21. Soil-pollutant interactions processes
• Interaction will allow pollutants migration or retention in
soil.
Release processes
• The chemical and biotic processes like transformation or
degradation are crucial in the release of pollutants.
• The release rates can be relatively fast or extremely slow
depending on the pollutant, solid phase, and solution
properties
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22. Different pollutant release process are,
• Hydrolysis
• Redox reactions
• Photodegradation
• Biodegradation
• Dissolution
• Volatilization
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23. Hydrolysis
• It is the chemical breakdown reactions involving water.
• Split and/or modification of the compound occurs by
contact with water.
• Part of the reacting compound is replaced by H+ or OH-
groups.
• The hydrolysis reactions also affect OPs, particularly those
containing hydrolysable functional groups.
Eg: Esters
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24. Redox reactions
• The oxidation processes are very relevant regarding PTEs.
• In OPs, these processes also affect several compounds in
different ways and typically biotic mediated.
• Because, the abiotic oxidation is very limited.
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25. Photo-degradation
• It refers to the decomposition induced by radiant energy
on pollutants.
• This is very relevant for OPs, while for PTEs, it does not
occur.
• Photochemical reactions affect a variety of organic
compounds.
• The rate of decomposition depends on,
o Molecular structure of the compound
o Intensity of light source
o Presence of other reactant compounds
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26. Biodegradation
• It facilitates the transformation of OPs by microorganisms.
• It is the most important mechanism for the removal of OPs in
the soil
• Biodegradation rate depends on
o Moisture
o Temperature
o pH
o Nutrients
o oxygen content
o Type and concentrations of OPs
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27. Dissolution
• It is related the solubility of pollutants in water.
• It is highly dependent on the physicochemical
conditions of soil-solution interface.
• As the pollutants dissolved in water, they can move
readily from soils to other environmental
compartments.
• Highly soluble compounds are less likely to volatilize
from water, due to the easy leaching to deeper sites.
• Anyhow they are more likely to biodegrade
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28. Volatilization
• In this process, the pollutant will be released from a
liquid or a solid phase into a gaseous phase into the
atmosphere.
• Lower solubility of OPs are more easily volatilized.
• In natural conditions, volatilization is more likely to occur
for OPs than for Ips
• Because metal(loid)s need higher temperatures to be
volatilized
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29. Retention processes
• After a pollutant has been released from their, the
retention processes will limit their migration until it
reaches a certain target.
• It include,
Adsorption
Incorporation into biological agents
Precipitation/surface precipitation
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30. Adsorption
• It is the most important factor on the transport and fate
of PTEs and Ops.
• It is usually defined as the reversible binding of a
chemical to a solid.
• Adsorption corresponds to the accumulation of matter
at the solid/water interface.
• Sorption can occur primarily through hydrophobic
interactions with organic matter. 30

31. Incorporation into biological agents (bioaccumulation)
• The accumulation of pollutants in living organisms
results from direct uptake and indirect way via food
chain.
• This is particularly relevant for PTEs that cannot be
degraded in soil.
• Several PTEs may disturb metabolic function in
organisms.
Eg: Hg and Pb are bio-accumulated in fish
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32. Soil pollutants transport
• Migration pathways
Wind transport
Erosion and mass wasting
Water transport
• Processes involved in pollutants transport
Advection, dispersion, and diffusion
Colloid-facilitated transport
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33. Migration pathways
• Transport of pollutants can also occur between abiotic and
biotic media.
• Abiotic processes have a greater impact on the pollutants
dispersion in soil.
• It includes the movement of pollutants dissolved in water or
soil solution, or in particulate form, by the action of,
• Water
• Air
• Soil masses
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34. Wind transport
• Pollutants contained in or adsorbed to soil can be
transported within and offsite as windblown particles or in
aerosols.
• Wind is the most selective transport agent.
• So that only small size particles can be transported by
wind.
• Exception – Cyclones (non-selective grain size particles can
be easily transported)
• The main modes of wind transport are,
o Surface creep
o Saltation
o Suspension
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35. • Other factors that account for wind transport
oSoil moisture - It promotes particles aggregation
oVegetative cover - Act as barriers in particles dispersion
• Finer particle size (<2 µm) facilitates wind transport
• It can be dispersed to distances very far away from the
sources
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36. Erosion and mass wasting
• Erosion is a physical process where, the solid materials
experience wear and breakdown by the action of,
• Water
• Air
• Plants
• Temperature
• This increases the surface area exposed and facilitates the
transport by weathering agents.
• In higher slope areas, mass-wasting processes of the eroded
materials facilitated by the action of gravity.
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37. Water transport
• The transport of pollutants in water can occur under particulate or
dissolved forms.
• In surface waters, soil particles can be introduced in streams and
move from upper stream by rolling, sliding, and saltation.
• Finally those particles will be deposited downstream.
• This transport depends on,
oFlow velocity
oTurbulence
oSize, shape, and density of grain
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38. • Pollutants can easily reach other sites or environmental
compartments via leaching.
• Leaching is a process by which pollutants are released from solid
phase into the aqueous phase.
• It occurs under the influence of dissolution and desorption of
pollutants from their support-phases.
• It depends on factors like,
• Soil pH
• Redox conditions
• Biotic action
• The amount of water percolating the soil
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39. Processes involved in pollutants transport
• Advection, dispersion, and diffusion
The transport of dissolved pollutants can occur due to these three
processes.
Advection involves transport with flowing direction.
It is associated with the mean velocity of the fluid.
This transport may be explained by Darcy’s Law.
Because here the chemical transport caused by a hydraulic
gradient.
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40. Dispersion is the spreading of pollutants due to the changes
of fluid velocity within the porous medium.
It happens as pollutants moves with water through soil
particles.
It includes two components according to fluid velocity:
Diffusion : low velocity
Mechanical dispersion : high velocity
Diffusion is a process where pollutants moving under an
influence of kinetic energy in the direction of the
concentration gradient.
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41. • Colloid-facilitated transport
This corresponds to the movement of pollutants in small size
particles to which pollutants are,
Sorbed or
Ionic exchangeable
These particles are transported in the aqueous phase.
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42. THANK YOU
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