This presentation will provide you knowledge on physical transport of chemicals. Overall cycling of pollutants are well discussed with adequate details.

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
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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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