Transcript Slide 1

Organic Chemicals in the
Environment
COURSE CONTENT
1. Polycyclic aromatic hydrocarbons (PAHs)
2. Chemical and physical properties and their influence on
environmental fate of pollutants
3. Persistent Organic Pollutants (POPs)
1.
The “Dirty Dozen” and DDT
2.
Polychlorinated biphenyls (PCBs)
3.
Dioxins
4.
Degradation mechanisms
5.
Regulation and monitoring
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Focus of the course
Molecule type and structure
How physical and chemical characteristics influence
distribution and fate in the environment
Sources and uses
Toxicity
References:
Van Loon & Duffy, Environmental Chemistry
Baird, Environmental Chemistry
Finlayson-Pitts & Pitts, Atmospheric Chemistry: Fundamentals & Experimental
Techniques
Alloway & Ayres, Chemical Principles of Environmental Pollution
Hester & Harrison, Chlorinated Organic Micropollutants
Schwartzenbach, Gschwend, & Imboden, Environmental Organic Chemistry
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Polycyclic Aromatic Hydrocarbons‡
(PAHs)
• Group of more than 100 different chemicals containing 3 or
more fused aromatic rings:
E.g.,
anthracene
naphthacene
coronene
• Health hazard – many PAHs are known carcinogens
• Formed mainly as a result of incomplete combustion –
widespread and strongly associated with human activity
• Associate with particulate matter, soils, and sediments
‡ Also
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known as polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons
PAH structure
PAHs have multiple, fused 3 – 7 member rings:
– Benzene and naphthalene are not formally PAHs
– PAHs do not include heteroatoms such as N or S
– Not all PAHs are fully conjugated aromatic molecules
(cf., Hückel Rule of (4n + 2) π electrons)
Fries Rule:
Most stable form of a polynuclear hydrocarbon is the
one with the maximum number of rings with a
benzenoid arrangement of 3 double bonds
E.g.,
Naphthelene (but NOT a PAH!)
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History of PAHs
1775: Sir Percival Pott reported high rates of scrotum cancer in
London chimney sweeps. Attributed to a carcinogenic
component in fireplace soot
1880’s: High rates of skin cancer reported for workers in paraffin
refinery, shale oil, and coal tar industries
1915-8: Japanese scientists showed that repeated painting ears of
rabbits with coal tar induced tumors
1922: Organic extracts of soot are carcinogenic
1933: Kennaway et al. – isolation of the “coal tar carcinogen”,
Benzo(a)pyrene; first example of a pure chemical
compound demonstrating carcinogenic activity
1942: Extracts of ambient particulate matter are carcinogenic
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History of PAHs
1949: Benzo(a)pyrene identified in domestic soot
1952: B(a)P found in ambient particles in the UK
1954: Extracts of respirable ambient particulates from L.A.
photochemical smog are carcinogenic
1970s: Carcinogenic activity of organic extracts of ambient and
primary combustion particles was higher than sum of
known carcinogenic PAHs – “excess carcinogenicity”
- carcinogenic activity could be 100 – 1000 times higher
than that of B(a)P content → many unknown chemicals of
high biological activity must exist in organic extracts of
ambient particles and particulate organic matter (POM)
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History of PAHs
1970s (cont.):
- testing of compound carcinogenicity greatly speeded up
with introduction of the Ames Salmonella mutagenicity
assay, a sensitive bioassay for bacterial mutagens
- organic extracts of fine particles contained not only
promutagens, such as B(a)P, but also direct-acting
mutagens
- some PAHs react with environmental NO2, HNO3, or O3
to form directly mutagenic nitro-PAH and oxy-PAH
Ames test:
direct mutagen (-S9) requires no metabolic activation
promutagen (+S9) requires mammalian enzymes
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Sources of PAHs
PAHs are predominantly anthropogenic and are formed by:
- incomplete combustion of organic matter such as coal, wood,
oil, petrol and diesel
- coke and Al production, bitumen production, vehicle and
aircraft exhaust
- smoking cigarettes
- charbroiled meats
PAHs are also found in natural fuel deposits
A few PAHs are used to produce medicine, dyes, plastics, &
pesticides
Natural sources of PAHs include volcanoes and natural fires
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Sources of PAHs
PAHs can be found in water also as a direct pollution from
industries or from road runoff
They can settle in the sediment, remain in the water or be taken
up by organisms like plankton, mollusks and fish, thereby
entering the food chain
E.g., In the USA, residential wood and coal combustion produces
about 700 tons/yr of PAHs compared to 1 ton/yr by coal power
stations
Source
%
B(a)P in foodstuffs
μg/kg
Heating, power production
Industrial producers
Incineration & open burning
Vehicles
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20
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Charcoal broiled steak
Margarine
Sausages
Roasted coffee
Toast
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1-36
4-50
1-13
0.5
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Sources of PAHs
Mechanism of formation during combustion:
- radicals formed by pyrolysis of hydrocarbons between 500
and 800ºC in zone of flame with insufficient O2
- C1 and C2 fragments combine in reducing atmosphere to
form condensed aromatics
- on cooling, PAHs condense onto existing particles – their
distribution reflects their differing thermodynamic stability in
O2 deficient flame
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Physical and chemical properties
Vapour pressure:
- VPs vary enormously between PAHs, up to 107 difference
- Larger PAHs have much lower VPs
E.g., naphthalene mainly found in gas phase; larger PAHs tend
to adsorb onto particles
Solubility:
- PAH solubility low in water (ng/L to mg/L)
- Smaller PAHs are more soluble
- Oxidation to more polar species greatly increases solubility
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Physical and chemical properties
Spectra:
- All PAHs have highly structured absorption in the ultraviolet
& actinic UV radiation
- Strong absorption arises from aromatic structure
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Environmental Fate
Usually enter air when released to the environment, often
attaching to particles in air. Can be transported far from their
sources.
Do not dissolve in water but stick to soil or sediment to be found
at the bottom of lakes. Some can be transported into
groundwater.
Higher concentrations in urban areas than rural areas
PAHs are quite persistent in the environment and can
bioaccumulate
PAHs break down by photolysis and chemical reaction over days
and weeks
Microorganisms also break down PAHs over time
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Environmental Fate
PAHs can breakdown by reacting with sunlight and other chemicals
(OH radicals) in the air over days to weeks
Besides B(a)P, other PAHs are emitted or formed in the atmosphere
which account for additional mutagenicity. Gas-phase reactions can
convert volatile PAHs to nitro-PAHs and nitro-PAH lactones,
which are strong "direct-acting" mutagens
The presence of nitro-PAH lactones formed in the atmosphere
contributes significantly to the mutagenicity of ambient air
Several reaction products of B(a)P and ozone are strong mutagens
– a major contributor has been identified as benzo[a]pyrene-4,5oxide, an animal metabolite, which is a strong direct mutagen
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Toxicity of PAHs
The toxicity of PAHs varies across this large group of
compounds :
- Some PAHs are carcinogenic and some are even mutagenic
- Some seem to have no toxic effects at all
A large percentage are not even studied
Toxicity depends on whether the compound is inhaled as a gas,
inhaled as a particle or adsorbed onto or absorbed into
preexisting particles
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Health Effects of PAHs
Once released into the atmosphere,
airborne PAHs can be inhaled
into the body on “carrier”
particles
These particles have a diameter <
2.5 m and can be inhaled into
the lungs
The particles are too small to be
removed by the upper
respiratory tract
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Health Effects of PAHs
Not clear if PAHs cause short term effects but may be responsible
for eye irritation, nausea, vomiting, diarrhea and confusion
Long term – cataracts, kidney and liver damage, jaundice,
breakdown of red blood cells
Because certain PAHs are carcinogenic, exposure to high levels
of these PAHs can lead to an increased risk of developing
tumours of the lungs, skin and bladder
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16 EPA “priority PAH pollutants”
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Benzo(a)anthracene
Benzo(a)pyrene
Crysene
Dibenzo(a,h)anthracene
Benzo(b)fluoranthene Benzo(k)fluoranthene
Indeno(1,2,3-cd)pyrene
Benzo(ghi)perylene
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Benzo(a)pyrene (B(a)P)
Benzo(a)pyrene (B(a)P) is one of the more common PAHs and is
also one that is known to have toxic effects
Widely distributed and strongly carcinogenic
- regarded as the most dangerous PAH
Is not produced or used commercially but is a result of incomplete
combustion
Short-term: red blood cell damage, leading to anemia; suppressed
immune system
Long-term: developmental and reproductive effects, cancer
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Benzo(a)pyrene (B(a)P)
1915-1918 Japanese scientists discovered that painting the ears of
rabbits and mice with coal extracts produced tumours, some of
which were malignant
1933 B(a)P and B(e)P were synthesised
Confirmation of carcinogenicity of B(a)P came when all 5
survivors of a group of 10 mice whose backs had been painted
with synthetic B(a)P developed tumours
The isomer B(e)P is not carcinogenic
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Benzo(a)pyrene (B(a)P)
B(a)P concentrations in Fleet Street London have been falling:
1962-1963: 39 ng m-3, 1972-1973: 10 ng m-3, 1987: 2 ng m-3
However a reduction in B(a)P levels does not necessarily mean a
reduction in potential health hazards
There are two dominant removal processes for B(a)P:
1) physical loss processes for the particles on which B(a)P
resides
2) adsorbed phase reactions of B(a)P on the particles
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Benzo(a)pyrene (B(a)P)
Considering only physical removal processes, the lifetime of
B(a)P due to particle dry deposition is about ten days.
However, in the adsorbed phase the chemical reactions include
photolysis and reaction with O3, SO2, NO2, HNO3 and N2O5
It is difficult to estimate an atmospheric lifetime for B(a)P due to
chemical reactions and/or photolysis. Based on available
information, the atmospheric lifetime of B(a)P is a few hours
in polluted urban atmospheres during the summer months.
This may explain the low concentrations of B(a)P measured in
the ambient air during summer
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