This presentation will provide you knowledge on physical transport of chemicals. Overall cycling of pollutants are well discussed with adequate details.
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Physical transport of chemical.pptx
1. Physical transport of chemical
in
soil surface, water & sub surface
V.Vijitha
Lectrurer (Prob.)
Dept. of Biosystems Technology
FoT, UoJ
2. โข Fate and transport refers to,
โข How the nature of contaminants might change and
โข Where they go as they move through the environment
โข Different forms of information are taken into account for the analysis of
fate and transport of toxicant.
โข Those are,
๏Possible transport processes
๏Physical, chemical and biological factors
๏Site-specific environmental conditions
2
Introduction
4. โข This model considers
๏ผThe type of pollutant
๏ผThe source of pollution
๏ผThe transport medium (air, water, and soil)
๏ผThe target (aquatic ecosystems, atmosphere, living organisms, or
soil ecosystem)
โข Therefore, soil can act as a source or a sink of pollutants.
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5. โข Two main groups:
โข Organic pollutants (OPs)
โข Inorganic pollutants (IPs): Eg-Potentially toxic elements (PTEs)
โข PTEs: Group of metals & metalloids with an atomic density
> 4๐๐๐โ๐
.
โข Both PTEs and OPs can have natural or anthropogenic
origins.
โข These two types of pollutants highly differ in their
behaviour in soils. 5
Types of soil pollutants
6. โข PTEs are non-degradable and persistence in environment
for long periods.
โข Therefore, it allows their transfer from the contamination
sources to other locations.
โข PTEs can occur in the soil solution under different forms.
โข ionic
โข Molecular
โข chelated
โข colloidal forms
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7. โข Mobility of PTEs in soil is always depend on aqueous phase and
intrinsic characteristics of each element.
โข Both will determine the ease of release and the sorption onto
surfaces of soil components.
โข 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
โข OPs can occur naturally from,
โข Volcanic emission
โข Forest fires
โข Fossil fuels
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9. โข Soil, surface water, groundwater, and atmosphere can become as
an exposure pathway.
โข Due to that:
โข Pollution will affect seriously soil functions
โข Decreasing the number and diversity of species that live or depend on the
soil
โข Decrease in the microbial activity of soil which will reduce the cycling of
organic matter
โข The soil structure will be affected
โข Man can be also a target of soil pollution by,
โข Direct ingestion
โข Inhalation
โข Dermal exposure
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10. Factors affecting the distribution, transport
and fate of soil pollutants
โข Physical and chemical properties of soils
โข Soil texture
โข Soil structure
โข Solid, liquid, and gaseous phases
โข pH
โข Redox potential
โข Cation exchange capacity
โข Soil organic matter
โข Clay minerals
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12. Physical and chemical properties of soils
Soil texture
โข It is determined by the particle size distribution of the
solid fraction.
โข It influences the ability to retain water.
โข Coarse soils: Water retention is low and drainage is rapid
โข Fine soils : Water retention is high and drainage is poor
โข Fine particles (<100 ยตm) have higher surface area.
โข So they are more reactive in pollutants retention.
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13. Soil structure
โข It is related with shape and arrangement of the particles
in soil profile.
โข It influences the behaviour of pollutants by controlling
the degree of contact between them and water/soil
solution.
โข Particle density determines the physical conditions of
the soil
โข High density may indicate the possibility of pollution
cases by metal(loid)s.
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14. Solid, liquid, and gaseous phases
โข Water has a greater importance than air concerning the,
โข Pollutants transport and
โข Reactions in soil
โข Liquid phase of soil comprises not only water but also
contains solutes and dissolved gases
โข This mixture is crucial for the transfer of pollutants to the
plants, groundwater, and atmosphere.
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15. pH
โข It expresses the concentration of H+ of a solution.
โข Anthropogenic activities contribute to acidification of
soil.
Eg: Intensive use of fertilizer
โข Soil pH greatly influences the mobility and availability of
metal(loids) in soil.
โข The acidic conditions favour the increase on mobility.
15
16. Redox potential (Eh)
โข The Eh, together with pH, are the main factors controlling
the behaviour and mobility of pollutants in soil.
โข Oxidation : Loss of electrons
โข Reduction : Gain of electrons
โข Eh provides an indirect information about the soil
aeration status.
โข Oxidizing and reduction conditions have great influence in
the release of pollutants
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17. โข Aerobic conditions promote the degradation of organic
compounds.
โข From this OPs may be released and/or transformed in
others species.
โข Oxidation of some metal(loid)s support mineral phases
(e.g., sulfides).
โข It may enhance the release of these pollutants from the
crystalline lattice
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18. Cation exchange capacity (CEC)
โข Defn: The potential quantity of readily exchangeable
cations that are able to neutralize negative charges from
soil surfaces.
Eg: Ca2+, Mg2+, K+, Na+
โข Degree of CEC in soil - sandy soils < clay soils < organic soil
โข Two types of surface charges
โข Permanent charges
These are not affected by changes in soil-solution conditions &
nature of free ionic species
โข Variable charges
These are affected by changes in soil-solution conditions
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19. Soil organic matter (SOM)
โข SOM is composed by,
โข Plant debris and
โข Colloidal material resulting from the action of
microorganisms
โข It influences the retention of water in soil.
โข Thus providing conditions suitable for soil microbiota to
promote degradation processes of OPs.
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20. Clay minerals
โข Clay minerals are a ubiquitous group of hydrous aluminium
phyllosilicates.
โข It has a significant contribution in the sequestration of ionic
pollutants of soil and soil solution.
โข Iron and Aluminum are two of the most abundant elements
in the Earthโs crust
โข Al- or Fe-(oxy)hydroxides are important agents in the
sequestration of electrical charged particles present in soil
solution.
โข It occurs by both,
โข Specific adsorption reactions or
โข Precipitation processes 20
21. Soil-pollutant interactions processes
โข Interaction will allow pollutants migration or retention in
soil.
Release processes
โข The chemical and biotic processes like transformation or
degradation are crucial in the release of pollutants.
โข 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,
โข Hydrolysis
โข Redox reactions
โข Photodegradation
โข Biodegradation
โข Dissolution
โข Volatilization
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23. Hydrolysis
โข It is the chemical breakdown reactions involving water.
โข Split and/or modification of the compound occurs by
contact with water.
โข Part of the reacting compound is replaced by H+ or OH-
groups.
โข The hydrolysis reactions also affect OPs, particularly those
containing hydrolysable functional groups.
Eg: Esters
23
24. Redox reactions
โข The oxidation processes are very relevant regarding PTEs.
โข In OPs, these processes also affect several compounds in
different ways and typically biotic mediated.
โข Because, the abiotic oxidation is very limited.
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25. Photo-degradation
โข It refers to the decomposition induced by radiant energy
on pollutants.
โข This is very relevant for OPs, while for PTEs, it does not
occur.
โข Photochemical reactions affect a variety of organic
compounds.
โข 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
โข It facilitates the transformation of OPs by microorganisms.
โข It is the most important mechanism for the removal of OPs in
the soil
โข Biodegradation rate depends on
o Moisture
o Temperature
o pH
o Nutrients
o oxygen content
o Type and concentrations of OPs
26
27. Dissolution
โข It is related the solubility of pollutants in water.
โข It is highly dependent on the physicochemical
conditions of soil-solution interface.
โข As the pollutants dissolved in water, they can move
readily from soils to other environmental
compartments.
โข Highly soluble compounds are less likely to volatilize
from water, due to the easy leaching to deeper sites.
โข Anyhow they are more likely to biodegrade
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28. Volatilization
โข In this process, the pollutant will be released from a
liquid or a solid phase into a gaseous phase into the
atmosphere.
โข Lower solubility of OPs are more easily volatilized.
โข In natural conditions, volatilization is more likely to occur
for OPs than for Ips
โข Because metal(loid)s need higher temperatures to be
volatilized
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29. Retention processes
โข After a pollutant has been released from their, the
retention processes will limit their migration until it
reaches a certain target.
โข It include,
๏ผAdsorption
๏ผIncorporation into biological agents
๏ผPrecipitation/surface precipitation
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30. Adsorption
โข It is the most important factor on the transport and fate
of PTEs and Ops.
โข It is usually defined as the reversible binding of a
chemical to a solid.
โข Adsorption corresponds to the accumulation of matter
at the solid/water interface.
โข Sorption can occur primarily through hydrophobic
interactions with organic matter. 30
31. Incorporation into biological agents (bioaccumulation)
โข The accumulation of pollutants in living organisms
results from direct uptake and indirect way via food
chain.
โข This is particularly relevant for PTEs that cannot be
degraded in soil.
โข 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
โข Migration pathways
๏ผWind transport
๏ผErosion and mass wasting
๏ผWater transport
โข Processes involved in pollutants transport
๏ผAdvection, dispersion, and diffusion
๏ผColloid-facilitated transport
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33. Migration pathways
โข Transport of pollutants can also occur between abiotic and
biotic media.
โข Abiotic processes have a greater impact on the pollutants
dispersion in soil.
โข It includes the movement of pollutants dissolved in water or
soil solution, or in particulate form, by the action of,
โข Water
โข Air
โข Soil masses
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34. Wind transport
โข Pollutants contained in or adsorbed to soil can be
transported within and offsite as windblown particles or in
aerosols.
โข Wind is the most selective transport agent.
โข So that only small size particles can be transported by
wind.
โข Exception โ Cyclones (non-selective grain size particles can
be easily transported)
โข The main modes of wind transport are,
o Surface creep
o Saltation
o Suspension
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35. โข Other factors that account for wind transport
oSoil moisture - It promotes particles aggregation
oVegetative cover - Act as barriers in particles dispersion
โข Finer particle size (<2 ยตm) facilitates wind transport
โข It can be dispersed to distances very far away from the
sources
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36. Erosion and mass wasting
โข Erosion is a physical process where, the solid materials
experience wear and breakdown by the action of,
โข Water
โข Air
โข Plants
โข Temperature
โข This increases the surface area exposed and facilitates the
transport by weathering agents.
โข In higher slope areas, mass-wasting processes of the eroded
materials facilitated by the action of gravity.
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37. Water transport
โข The transport of pollutants in water can occur under particulate or
dissolved forms.
โข In surface waters, soil particles can be introduced in streams and
move from upper stream by rolling, sliding, and saltation.
โข Finally those particles will be deposited downstream.
โข This transport depends on,
oFlow velocity
oTurbulence
oSize, shape, and density of grain
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38. โข Pollutants can easily reach other sites or environmental
compartments via leaching.
โข Leaching is a process by which pollutants are released from solid
phase into the aqueous phase.
โข It occurs under the influence of dissolution and desorption of
pollutants from their support-phases.
โข It depends on factors like,
โข Soil pH
โข Redox conditions
โข Biotic action
โข The amount of water percolating the soil
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39. Processes involved in pollutants transport
โข Advection, dispersion, and diffusion
๏ผThe transport of dissolved pollutants can occur due to these three
processes.
๏ผAdvection involves transport with flowing direction.
๏ผIt is associated with the mean velocity of the fluid.
๏ผThis transport may be explained by Darcyโs Law.
๏ผBecause here the chemical transport caused by a hydraulic
gradient.
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40. ๏ผDispersion is the spreading of pollutants due to the changes
of fluid velocity within the porous medium.
๏ผIt happens as pollutants moves with water through soil
particles.
๏ผIt includes two components according to fluid velocity:
๏ผDiffusion : low velocity
๏ผMechanical dispersion : high velocity
๏ผDiffusion is a process where pollutants moving under an
influence of kinetic energy in the direction of the
concentration gradient.
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41. โข Colloid-facilitated transport
๏ผThis corresponds to the movement of pollutants in small size
particles to which pollutants are,
๏ผSorbed or
๏ผIonic exchangeable
๏ผThese particles are transported in the aqueous phase.
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