Download Persistent Organic Pollutants

The Turtle
by Ogden Nash
The turtle lives 'twixt plated decks
Which practically conceal its sex.
I think it clever of the turtle
In such a fix to be so fertile.
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Uptake & accumation of POPs in
living organisms
Some terms describing the uptake of chemicals by organisms:
Bioconcentration – the increase in concentration of a chemical in
an organism compared to that in the medium in which it lives
Bioconcentration factor (BCF) – the ratio of the concentration of a
chemical in an organism compared to that in the ambient medium
Bioaccumulation – the processes by which an organism takes up
and retains a contaminant through multiple exposure routes
Biomagnification – a series of processes that results in a chemical
becoming increasely concentrated at successively higher trophic
levels of a food chain or food web.
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Examples of bioconcentration
and biomagnification of DDT
in the Lake Kariba ecosystem
The extent of bioconcentration
is quantified with the BCF
E.g.
DDT in mussels of the
Firth of Clyde (1989) = 300 μg kg-1
DDT in sea water = 1 ng L-1
BCF
= 300×10-6 g (1 kg mussels)
1×10-9 g (1 kg water)
= 300 000
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Partitioning of organic solutes
between water and soil or sediment
In the aqueous environment, molecules such as POPs distribute
themselves between water and solids, whether sediment and
suspended solids.
For organic solutes at low concentrations, a reasonable description
of this partitioning is given by a modified Freundlich relation:
Cs = KdCaq
where Kd is the distribution coefficient (Kd = Cs / Caq)
Kd depends on: (a) solute, (b) chemical & physical nature of the
solid phase, (c) environmental factors such as temperature and
ionic strength
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Partitioning of organic solutes
between water and soil or sediment
Since Kd depends on multiple, partially defined variables, its actual
value is difficult to predict with any accuracy:
Other distribution coefficients are therefore commonly used to
quantify how much particulate surfaces will take up an organic
solute:
KOW – the octanol-water partition coefficient
KOM – the organic matter-water partition coefficient
These partition coefficients can be related to the distribution
coefficient, Kd
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Sorption of organic species by
solids in the environment
The distribution of organic species between the solid phase and
water depends on some properties of the solid:
1) Mineral components:
Exposed surfaces may contain functional groups (such as
hydroxyl groups from hydrous oxides and clays) that may
form specific interactions with organic solutes
Interactions include dipole-dipole and hydrogen bonding
Interactions with organic solutes are usually weak because
adsorption necessarily displaces water molecules
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Sorption of organic species by
solids in the environment
2) Organic components:
Organic solutes generally interact more strongly with organic
matter on solids
These include humic material from plant and microbial sources
Organic matter has some polar functional groups but also
hydrocarbon regions
Organic solutes have little competition with water for these
hydrocarbon sites
Organic solutes can adsorb onto or absorb into these organic
areas – hence the term sorption
Even small concentrations of organic components on solids
typically dominate partitioning of organic solutes from water
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Sorption of organic species by
solids in the environment
The equilibrium concentration of sorbed solutes, Cs, is then:
Cs = fOM × COM + fMM × CMM
where fOM and fMM are the fractions of organic matter and mineral
matter in the soil or sediment, and COM and CMM are the
concentrations of the organic solute in each phase
Since organic solutes interact weakly with mineral surfaces in
sediment, CMM is usually small and we can write:
Cs ≈ fOM × COM
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Octanol-water partition coefficient
n-Octanol serves as a useful model for humic-like substances:
- it is an amphiphilic solvent with both hydrophilic and
hydrophobic constituents
- it has a similar ability to associate with polar and nonpolar
compounds
The octanol-water partition coefficient, KOW, therefore provides a
convenient measure of the extent to which a hydrophobic solute
will dissolve in water or be sorbed by organic matter.
KOW = CO/Caq
where CO is the equilibrium molar solubility of the solute in noctanol and Caq is the corresponding solubility in water
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Octanol-water partition coefficient
Large values of KOW indicate that
the solute is strongly
associated with the organic
matter; small KOW values that
the solute favours the aqueous
phase
How does KOW relate to
bioconcentration?
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KOW and BCF
As we would expect, there is a correlation between KOW and BCF for a
range of persistent organic pollutants:
KOW can therefore be
used to predict the
BCF of a particular
solute
Bioconcentration of a molecule becomes a concern for BCF values
above about 10 000
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The Ant
by Ogden Nash
The ant has made himself illustrious
Through constant industry industrious.
So what?
Would you be calm and placid
If you were full of formic acid?
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