Environmental
Chemistry Option
E1.1 - Pollution
•Pollution refers to changes in the equilibrium (or
balance) of biological and non-biological systems,
as a result of human activity
•Although many so-called pollutants are substances that
occur naturally, such as ozone or carbon dioxide, human
activity has led to an increase in the concentrations of
such substances, which upsets the delicate balance of
natural cycles
•The atmosphere consists of a relatively thin layer
of gas surrounding the Earth. (to 100km)
•By comparison, the earth is 6400km in radius
E1.1 – Layers of the Atmosphere
•The atmosphere consists of four layers, separated by a
change in temperature gradient
•Troposphere
-Most of human activity takes place here
-Up to 10-12 km
-90% of matter in the atmosphere
•Stratosphere
•Mesosphere
•Thermosphere
E1.1 - Troposphere
•In the troposphere temperature falls with increasing
height, so that at a height of 12 km the temperature
is about -55oC
•This temperature gradient allows convection currents
(warm gases rise, cool gases sink) causing mixing in
the atmosphere pollutants.
•Pollutants at ground level quickly spread throughout
the troposphere (not true in stratosphere).
•Also, horizontal movement of air masses (wind)
causes lateral dispersion of pollutants as well.
E1.1 – Atmospheric Pollutants
•As discussed, many gases we consider
pollutants, also occur naturally (CO, CO2, etc)
•Anthropogenic, or man-made, output is
often not even a large proportion of the
worldwide total but can cause issues due
to high concentrations in localized areas.
•Many atmospheric pollutants arise from the
combustion of fossil fuels in motor vehicles or in
power stations. This effect has the potential to
be minimized
E1.1 – Sources of Pollutants
E1.2 – Reduction of Pollution
•In the internal combustion engine, hydrocarbon fuels
are mixed with air, injected into a cylinder and ignited
with a spark
•The resultant explosion forces the piston to move
and is converted to the rotation of the crankshaft,
which in turn drives the wheels of the vehicle
•We can have complete, partial, and incomplete
combustion of the hydrocarbon, ideally it would be
complete as follows:
2C8H18 + 25O2 → 16CO2 + 18H2O
Ratio of air/fuel (mass) is approx 15:1
E1.2 – Incomplete Combustion
•When the ratio of air/fuel (mass) is less than 15:1, the
mixture is said to be ‘rich’ and incomplete combustion
ensues
•This results in the formation of carbon monoxide
C8H18 + 10O2 → 3CO2 + 5CO + 9H2O
•These unburnt hydrocarbon molecules (CO) are called
volatile organic compounds (VOC’s) from exhaust
•A very poorly designed (or maintained) engine may emit
solid particles (soot) from the exhaust (rich)
C3H8 + 2O2 → 3C + 4H2O
E1.2 – Reaction with Nitrogen
•When the air/fuel ratio is increased the mixture is said
to be ‘lean’. A lean mixture will not produce
carbon monoxide
•When a mixture becomes ‘lean’ (excess O2) the
likelihood that oxygen will react with nitrogen in the air
(78%) increases.
•Under extreme conditions (like in an engine) the two
elements can combine to form nitrogen oxides NOx
which leads to ‘misfire’ in the engine (known as
knocking) because the fuel ignites before the spark
E1.2 – Rich vs Lean
•An engine that runs ‘rich’ will produce a lot of power,
but with poor fuel consumption and high emissions of
CO and VOC’s
•An engine that runs ‘lean’ will produce less power, less
CO and VOC’s, consume less fuel, but may produce
more nitrogen oxides. When mixture is too lean,
misfiring causes rise in VOC production
N2 + O2 → 2NO• (remember the radical?)
2NO• + O2 → 2NO2
Localized in urban areas, causing health effects and
contribute to the formation of acid rain
E1.2 – Catalytic Converter
•Three way catalytic converter is placed in exhaust
systems of cars to treat the exhaust gases
•Consists of a fine mesh or honeycomb of ceramic
material or metal, coated with a thin layer of finely
divided platinum, rhodium, or palladium (these T-metals
act as heterogeneous catalysts)
•Three way refers to:
1.Reduction of nitrogen monoxide to nitrogen
2.Oxidation of unburnt hydrocarbons to carbon dioxide
and water vapor
3.Oxidation of carbon monoxide to carbon dioxide
E1.2 – Catalytic
Converter
•First, gases pass over
metal catalyst, reacting
NO and CO
2NO + 2CO → N2 + 2CO2
•Then, pass over
oxidation catalyst
(rhodium). If a proper
ratio of air/fuel exists,
there is proper O2
remaining for this:
2CO + O2 → 2CO2
E1.2 – Sulfur as Pollutant
•SO2 (sulfur dioxide) is an important primary
pollutant.
- It’s a pungent smelling toxic gas
- damages the respiratory system and may lead
to asthma attacks.
- Highly soluble in water
- Contributes to formation of acid rain
E1.2 – Primary/Secondary Pollutants
•Primary pollutants are emitted directly from
the sources and remain unchanged once they
enter the environment (particulate matter,
inorganic gases, SO2, etc)
•Secondary pollutants are formed in the
atmosphere by chemical reactions involving
primary pollutants and gases normally present
in the air.
•Most man-made sulfur dioxide emissions arise from
the sulfur that exists as an impurity in coal which is
burned extensively in many power plants
E1.2 – Sulfur reactions
•Sulfur is oxidized during the combustion process
S + O2 → SO2
•Sulfur dioxide dissolves and reacts with water to produce
sulfurous acid, H2SO3
SO2 + H2O → H2SO3
•Sulfur dioxide also undergoes photochemical oxidation in the
atmosphere. This occurs in water droplets in which SO2 is
dissolved, and is catalyzed by particulates (soot, etc)
2SO2 + O2 → 2SO3
•Sulfur trioxide dissolves and reacts with water to produce
sulfuric acid
SO3 + H2O → H2SO4
E1.2 – Reducing SO2 Emissions
•There are three methods by which sulfur dioxide
emissions from power stations can be limited
The coal or oil can be refined to remove sulfur before
combustion
Fluidized bed combustion (FBC) reduces the
amount of sulfur oxides resulting from
combustion
Flue gas desulfurization (FGD) removes sulfur
dioxide from the exhaust gases before they
leave the power station flue (chimney)
E1.2 – Fluidized Bed Combustion
•This process suspends the solid coal on an upward
flowing jet of air during combustion
•Coal dust is mixed with limestone powder (CaCO3)
and blasted into the furnace with a jet of air. The jet
of air suspends the solid particle so they flow like a
fluid
CaCO3 → CaO + CO2
2CaO + 2SO2 + O2 → 2CaSO4
•The Calcium sulfate can then be removed by
electrostatic precipitation
E1.2 – Flue Gas Desulfurization
•Sulfur dioxide emissions can be removed from the flue
gases by passing the gases through a suspension of
calcium carbonate and calcium oxide in water.
Product is calcium sulfite
CaCO3 + SO2 → CaSO3 + CO2
CaO + SO2 → CaSO3
•Calcium sulfite is then further oxidized, producing
calcium sulfate
2CaSO3 + O2 → 2CaSO4
E1.2 – Particulate Emissions
•Particulate emission refers to the generation of
small particles of solid or liquid. Some such
pollutants are visible to the naked eye, but most
are too small to be seen.
Metal particles
Metal oxide particles
Fly ash
Asbestos dust
Organic particles
Aerosol mist
E1.2 – Fly Ash
•Fly ash is a combination of very fine carbon,
hydrocarbon and metal oxide particles
released during the combustion of fossil fuels
•Most fly ash is filtered out in flues, but some
very fine particles escape into the atmosphere
E1.2 – Removal of Particulates
•Via sedimentation which relies on letting heavy particles
settle out under gravity, or by filtration, in which simple
fabric filters capture particles.
•Most effective method is electrostatic precipitation – which
has two sections
Ionization section, consists of a mesh with thin wires, carrying an
electrical charge. Flue gas passes through this mesh, causing any
solid or liquid particles to acquire charge
Collection section, consists of metal plates carrying the opposite
charge. Particulates are attracted to the plates, and stick. Plates are
shaken at intervals to dislodge the build-up layer of particles, then
they are removed.
Enviro Chemsitry Part 2 – Acid Deposition
Acid Deposition refers to the
process by which acidic particles
leave the atmosphere. The most
well known example is acid rain
• but acidic substances may also be
removed by snow and fog, as well as by
dry processes involving gases and solid
particles.
• Production of SO2 (as discussed in Part
1) aids in this process
E2.1 – Carbonic Acid
•Natural rain water is acidic, with a pH around 5.6
•The acidity of rain is a result of CO2 naturally present
in the atmosphere
•When CO2 is dissolved in water it’s referred to as
carbonic acid (H2CO3) but only a very small
amount actually exists as a solution
H2O + CO2
→
H2CO3
•Carbonic acid molecules immediately dissociate in water
to form hydrogencarbonate ions, HCO3-, and
hydronium ions, H3O+
H2CO3 + H2O
→
HCO3- + H3O+
E2.1 – Wet Deposition
•The most important sources of acid rain are the sulfur oxides
produced in power stations
•When sulfur oxides dissolve and react in rain water, solutions
of sulfuric acids are formed (as discussed in E.1)
SO2 + H2O → H2SO3
SO3 + H2O → H2SO4
Another route to sulfuric acid is a gas-phase reaction of a
sulfur dioxide molecule with a hydroxyl radical, OH•, to give
sulfuric acid
SO2 + OH• → HSO3• + • OH → H2SO4
E2.1 – Wet Deposition
•Nitrogen Oxides also contribute to acid rain. Formed in
vehicle engines:
HO• + NO• + M → HNO2 + M
M represents the ‘third body’ which is an inert
molecule which absorbs some of the excess energy of
the reaction (in the atmosphere M is generally N2).
•NO2 is formed by the oxidation of NO in the
atmosphere and reacts with HO•
HO• + • NO2 + M → HNO3 + M
E2.1 – Wet Deposition
•These acids may be deposited in places other than
water, such as snow and fog.
•Fog is a particular problem for high-altitude forests
•The lower temperature at high altitudes causes water
vapor to condense out of the atmosphere, forming a
moist ‘blanket’ of acidic fog which surrounds trees.
E2.1 – Dry Deposition
•Dry deposition refers to acidic substances such
as gases and particulates leaving the
atmosphere in the absence of precipitation
(without rain or fog)
•Heavy particulate particles may settle out of the
atmosphere under gravity.
•Acidic gases such as sulfur dioxide may have directly
harmful effects on the environment without first being
dissolved in rain water.
E2.2 – Environmental Effects
•Acid deposition effects the environment in 5
ways
1.It affects the pH of lakes/rivers, which impacts
organisms living there
2.It affects the availability of metal ions in soil,
which goes on to affect nearby plant life and
surface water
3.It directly affects plants
4.It affects buildings and other materials
5.It directly affects human health
E2.2 – Impact(1): Lakes/Rivers
•Below a pH of 5.5
-Some species of fish (salmon) are killed
-Algae, zooplankton, which are food for larger
organisms
-Prevents hatching of fish eggs
•Fish are also killed when aluminum, leached from
the soil by acid rain, enters lakes and rivers.
•The function of fish gills is affected by Al, leaving
the fish unable to extract oxygen from the water
E2.2 – Impact(2): Soil
•The pH of soil is a key factor which species of plants
will grow
•Aluminum (naturally present in soil) forms insoluble
hydroxide (Al(OH3) at high pH values.
•When pH falls due to acid rain, Al becomes soluble
and is released into soil.
•Other ions (Mg, Ca, etc) which are essential for plant
growth are washed away in the same fashion.
2Al(OH)3 + 2H2SO4 → Al2(SO4)3 + 3H2O
E2.2 – Impact(3): Plants
•Beyond damaging soil, and lowering
available nutrients, acid rain can also
damage plants directly
•Acid deposition can damage leaf
chlorophyll, turning leaves brown and
reducing the photosynthetic ability of the
plant
E2.2 – Impact(4): Buildings
•Limestone and marble are forms of CaCO3 which can
be eroded by acid rain:
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
•Metallic structures (mainly steel, Fe, Al) are readily
attacked. The sulfur dioxide gas may attack directly as
follows:
Fe + SO2 + O2 → FeSO4
•Sulfuric acid may attack Fe as well:
Fe + H2SO4 → FeSO4 + H2
Fe + 2H+ → Fe2+ + H2
E2.2 – Counteract Acid Dep.
•Limit (lower) the amount of acidic substances
released to the atmosphere
NOx are removed from vehicle emissions with a
catalytic converter
SO2 emissions from coal power plants can be
decreased in several ways (scrubbers, etc)
•Addition of compounds that will aid in neutralizing
acidic effects
Addition of limestone (CaCO3)
Addition of calcium hydroxide (Ca(OH)2)
Topic E – Enviro Chemistry Part
3 – Greenhouse Effect
E3.1 – Greenhouse Effect
•Greenhouse gases allow the passage of
incoming solar short-wavelength radiation but
absorb the longer-wavelength radiation from the
Earth. Some of the absorbed radiation is reradiated back to Earth.
•TOK: Some people question the reality of climate change and
question the motives of scientists who have “exaggerated” the
problem. How do we assess the evidence collected and the models
used to predict the impact of human activities?
•The greenhouse effect is the cause of the
phenomenon of global warming in which the
average temperature of Earth rises, causing
various environmental disasters
•The greenhouse effect itself is absolutely necessary for
the Earth to regulate its temperature at a habitable
level.
•Humans are thought to impact this delicate balance by
disrupting the natural equilibrium in the atmosphere,
causing the planet to become warmer
Earths “Average” Temp.
• Average temperature in the troposphere 14-15oC
• Maintained bc the energy incident on Earth (from sun),
is balanced by the energy leaving Earth (to space)
• Most radiation from the sun is in the visible region, also
along with “near UV” and near “IR radiation”
• Only 47% of energy directed at the earth is absorbed,
remainder is reflected back to space
• The peak radiation is 500nm and is not absorbed by
atmospheric gases so is absorbed and radiated by the
earth to the rest of the atmosphere
•When
Earth absorbs energy, surface temperature rises,
and energy flows from hot (earth) to atmosphere (cold),
etc
•Once energy is re-radiated from the earth, it’s no longer
in the visible region, it’s wavelength is much longer and in
the infrared region
• If all this energy released into space instead, our
average atmospheric temp would be -20oC
• Gases such as H2O and CO2 help to re-radiate energy in
the atmosphere and toward the earth as well.
It can be seen that an increase in [IR absorbing gases]
such as CO2 and H2O results in a decreased amount of
energy escaping from the Earth by moving toward the
surface
E3.2 – Factors for GH Gases
•A contribution of a greenhouse gas to the warming of
the atmosphere depends on three factors:
1.The abundance of the gas in the atmosphere
2.The ability of the gas to absorb infrared radiation
3.The lifetime of the gas molecules in the atmosphere,
before being removed by chemical processes
The 2nd and 3rd factors are often combined to give a
figure called the Global Warming Potential (GWP)
E3.2 – Greenhouse Gases
•Major Contributors as Greenhouse Gases
Water vapor, H2O
Carbon dioxide, CO2
Methane, CH4
Nitrous Oxide, N2O
Chlorofluorocarbons, CFC’s
Ozone, O3
GH Gases – H2O
•Most important GH Gas, has a GWP of 0.1
•Percentage of H2O(g) in atmosphere 1-4%, ranges
•Absorbs IR over a broad range of frequencies
•Increased atmospheric temperatures lead to more rapid
evaporation of the oceans, and larger capacity of the air
to carry water vapor (humidity)
•Estimates of H2O’s contribution to Global Warming is
36%-75%
GH Gases – CO2
•Percentage of CO2 in atmosphere is 0.035%
•CO2 has a GWP of 1.
• More efficient than water in absorbing IR radiation
•Absorbs IR in a “window” that H2O does not
•[CO2] rise due to the following human activities:
• Combustion of fossil fuels
• Manufacture of cement (CaCO3 → CaO + CO2)
• Deforestation in tropics, lower rate of photosynthesis,
meaning CO2 is entering atmosphere more quickly than
removed
GH Gases – CH4
•Percentage in atmosphere CH4 = 1.7x10-4
•It’s GWP is 25 (compare to H2O=0.1 and CO2=1)
•Estimates say 4%-9% contribution to Global Warmíng
•It is removed from the atmosphere relatively quickly
•Formed when cellulose (plant fiber) decomposes anaerobically via
bacteria (CH2O)n → CH4 + CO2
•Occurs on large scale as a result of human actions:
- Rice cultivation (paddy fields)
- Fermentation of grass in cows, and rotting manure
- Leaking gas pipelines
- Fermentation of organic materials in covered landfills
GH Gases – N2O
•GWP of 296. It’s less efficient at absorbing IR than CO2
but it’s high number comes from a long residence in the
atmosphere
•Percentage in atmosphere 0.031%
•Accounts for 5% of Global Warming effects
•Human activity only accounts for 10-12% of it’s production, but
anthropogenic NO2 from:
- Industrialized agriculture, N fertilizers
- Industrialized livestock farming, poor handling of animal waste
- Chemical industry, HNO3 and nylon production
GH Gases – CFC’s
•Chlorofluorocarbons (CFC’s) have largely been replaced
in aerosols, propellants, and refrigerants by
hydrochlorofluorocarbons (HCFC’s) and
hydrofluorocarbons (HFC’s)
• These gases are less damaging to the ozone layer but
still have GWP values much higher that CO2 and are
important contributors to global warming
E3.3 – Influence of Greenhouse Gases
Rising Sea Levels
•As atmospheric temperature increases, sea levels will
rise for two reasons:
• The increased atmospheric temperature causes
accelerated melting
• This does not include floating ice in the arctic as it
already displaces water while it floats
• As oceans warm up, the water in them will expand,
occupying more volume (even minor amounts could be
significant due to the quantity of water in the ocean!)
Glacier Retreat
•Glaciers undergo a seasonal melting and freezing as
temperatures vary throughout the year.
•In the Himalayas glacial melt water is an important
source of fresh water, feeing the rivers of South Asia
•Increased melting increases erosion and risk of
flooding downriver, a particular problem in low-lying
countries
Changing Patterns of Agriculture
•In temperate regions (such as Europe) yields of grain will
most likely increase due to higher temperature, longer
growing season and increased [CO2] available for
photosynthesis
•But, increased humidity and rainfall could lead to
increased incidence of fungal crop diseases, and migration
of tropical insects to higher altitudes.
•At higher latitudes, more workable land may become
available due to thawing and temperature changes.
• Worldwide, the possibility of extreme weather increases
the likelihood of ruined harvest
CO2 Increases
• Atmospheric CO2
levels have been
recorded with increases
shown at right
• A comparison was also
studied from data
collected from the
analysis of air bubbles
trapped in Arctic ice.
• Shows an increase in
CO2 content over the
last 150 years or more
E3.3 – Greenhouse Gas Correlation to Temperature Change
• Analysis of polar ice in the Greenland Ice Core Project, showed evidence
for long-term climate changes (correlated to CO2 and CH4)
Enviro Chemistry Part 4 – Ozone Depletion
E4.1 – Ozone Formation and Depletion
•Formation:
O2 + uv → 2O•
O2 + O• → O3
•Depletion:
O3 + uv → O2 + O •
O3 + O• → 2O2
E4.1 – Ozone Formation and Depletion
•The processes of formation and depletion of ozone
in the stratosphere is normal.
•The rates of formation and depletion are equal in
“normal” circumstances
•This delicate balance must be maintained for the
ozone to be effective
•Ozone Formation
From O2 and UV energy
•Ozone Depletion
From O3 and UV energy
E4.1 – Natural Ozone Formation
•Diatomic oxygen absorbs a photon with λ < 242nm
which is in the UVc region
O2 (+UV 242nm) → O• + O•
O2 + O• + M → O3 + M
M is used as a catalyst (third body) for the reaction.
It carries away excess energy.
E4.1 – Natural Ozone Depletion
•The UV photon can cause an ozone molecule to
dissociate into a diatomic oxygen as well.
•This process requires a photon λ = 290-320nm which
is the UVb region
O3 (+ UV 290-320nm) → O2 + O•
O3 + O• → 2O2
•These two processes (formation, depletion) lead to a
steady concentration of ozone and UVc and UVb are
largely filtered out in this process if the ideal!
E4.1 – UV Radiation
•UV region of the spectrum is
divided into three regions:
•UVa: 320-400nm
-Not harmful
-Not blocked
•UVb: 280-320nm
-Less Harmful
-Less blocked
•UVc: 200-280nm
-HARMFUL
-blocked
Small changes in Ozone can lead to a
significant rise in UVb , resulting in
increased incidence of skin cancer
E4.1 – Ozone Layer
• Ozone is a powerful oxidizing agent
• When formed in the air, it’s considered a pollutant since it can
reach concentrations harmful to humans and exist outside of the
ozone layer
• Ozone is formed by the photochemical reactants that lead to
smog
• The Ozone layer is considered “good” ozone as it absorbs
harmful UV radiation which leads to skin cancer. Without
the ozone layer, we could not have evolved
• Major impacts to the ozone layer due to human activity can be
seen at the polar regions
E4.1 – Stratospheric Ozone
•The stratosphere is the layer lying above the
troposphere at altitudes around 12km(~39,000ft)-52
km.
•Over 90% of all ozone (O3) found in stratosphere
•Exists even in ozone layer in [O3] of <5ppm
•In the stratosphere, the level of ozone is maintained
by a cyclic series of processes, in which ozone is
continuously created and destroyed
• These processes involve the absorption of UV
E4.2 – Ozone-depleting Pollutants
• Examples include chlorofluorocarbons (CFCs) and
oxides of nitrogen (NOx).
• In 1985, the British Antarctic Survey published
research showing that the ozone concentration over
Antarctica was lower than expected.
• This unexpected lowering of the ozone
concentration became known as the “hole”
• Suspected cause of the hole in the ozone was the
use of CFC’s
E4.2 – What are CFC’s
•Chlorofluorocarbons (CFC’s) were designed as
propellants for aerosol sprays and as refrigerants
•These chemicals are volatile and chemically inert in
the troposphere which is why they were used in
aerosols as they didn’t contaminate the liquid
•When released at ground level, they slowly mix
throughout the troposphere. They remain unreactive
in the upper troposphere since UV light that can illicit
a reaction is absorbed by ozone
•Once the CFC’s reach the stratosphere, they react
with large amounts of radiation from the sun and
produce chlorine radicals Cl•
E4.2 – Ozone Depletion (CFC’s)
•When the CFC molecules eventually reach the
stratosphere, they dissociate by UV radiation:
CFCl3 + UV → CFCl2• + Cl•
CF2Cl2 + UV → CF2Cl• + Cl•
•A chain reaction is initiated in which Cl•
destroy an ozone molecule, and are then
regenerated in another reaction, allowing
further depletion
E4.2 – Ozone Depletion (N2O)
• Most nitrogen oxides formed in the troposphere are
sufficiently reactive that are used up in chemical reactions,
such as those which produce smog, or acid rain.
• BUT, N2O (nitrous oxide), is relatively unreactive allowing
it to rise to the atmosphere where it can combine with a
O•
O• + N2O → 2NO•
• NO participates in similar chain reactions to chlorine
atoms, and so is regarded as a serious ozone-depleting
pollutant as well
E4.3 – Alternatives to CFC’s
•Alternatives include hydrocarbons (CxHy) ,
fluorocarbons (FC’s) and
hydrofluorocarbons (HFCs).
•Include toxicity, flammability, the relative
weakness of the C–Cl bond and the ability to
absorb infrared radiation.
E4.3 – CFC Alternative (HCFC’s)
•The presence of weaker C-H bonds in the
molecule affects the reactivity of the molecule.
- CFC’s exists as stable molecules until the
stratosphere
- HCFC’s react in troposphere by attack from
OH•
- The H-atom combines with OH• to form H2O
and the remainder of the molecule (CFCl2•) is
able to break down further and release watersoluble compounds into the atmosphere which
can be removed by rain
E4.3 – CFC Alternative (HFC’s)
•Hydrofluorocarbons (HFC’s) are preferable to
HCFC because they contain no chlorine.
•The C-F bond is much shorter than the C-Cl
bond and is thus much stronger so UV photons
carry insufficient energy to break the C-F bond.
•HFC’s are broken down in the troposphere in a
similar way to HCFC’s, but the absence of
chlorine means that HFC’s have no tendency to
deplete the ozone
E4.3 – CFC Alternative (CxHy’s)
•Mixtures of propane and butane have been
marketed as alternatives to CFC’s, especially in
refrigeration and air conditioning applications
•Appliances must be carefully designed to
minimize the possibility of the hydrocarbon being
ignited, for example by an electrical spark.
Enviro Chemistry Part 5 –
Dissolved Oxygen in Water
E5.1 – Biochemical Oxygen Demand
•Biochemical Oxygen Demand
(BOD) describes the quantity of
oxygen used when the organic
material in the water is
decomposed by microorganisms.
E5.1 - [O2]
•The solubility of O2(g) in water is quite low
8.3 ppm (~0.009g dm-3) at 25oC
Solubility can be altered by changes in
temperature, concentrations of dissolved
materials, and quantitates of biological waste
If the [O2] falls below 5 ppm fish will start to
die
If the [O2] falls below 3 ppm fish cannot
survive
E5.1 – Biological Wastes
•Anthropogenic Biological wastes
Human and animal waste (sewage/manure)
Food processing factory waste
Slaughterhouses
Paper mills
•Organic material will gradually decay by the action of
microorganisms. It consumes O2 as:
(CH2O)n(aq) + nO2(g) → nCO2(g) + nH2O(l)
(CH2O)n is an empirical formula representing
carbohydrates such as cellulose which form the structure
of plants
E5.1 – BOD Values
•The previous process,
(CH2O)n(aq) + nO2(g) → nCO2(g) + nH2O(l),
is called aerobic decomposition, as the microorganisms
involved require O2. As decay occurs, the O2 available for
other organisms such as fish is decreased
•Biochemical Oxygen Demand (BOD) describes the
quantity of O2 used when the organic material in the water
is decomposed by microorganisms.
- Small BOD = small amount of organic matter present,
water is quite pure
- Large BOD = impure water, much of the O2 present is
used up in decomposition, less for fish
E5.1 – BOD Values
•BOD [pure water] ≈ 1 ppm
•BOD [polluted water] > 5 ppm
•The [H2O] at 25oC ≈ 8 ppm, if the BOD is 5
ppm it lowers the [O2] ≈ 3 ppm where it can
no longer sustain fish
•BOD values can be measured using a
dissolved oxygen probe or via a redox titration
E5.2/3 - Eutrophication
•Eutrophication is defined as ‘an increase in the
level of chemical nutrients in an ecosystem.’
However the terms if often used to mean the
resultant increase in plant growth, lowering of
[O2] and decline of fish populations
•Lake environments typically favor fish over plant
life
- Because levels of P and N are low, which results in
limited plant growth and little decaying vegetation
- Lake water has a low BOD, therefore fish populations
are able to reach a high level!
E5.3 – Algal Bloom
•Human activities can lead to increases of nutrients in
rivers and streams.
- Use of fertilizers on farmland
- Release of sewage onto rivers
•Both result in a large increase in [nutrient], and
phosphorus in particular in the water they feed
•This excess of nutrients leads to excess growth of
primitive plants called algae, which float on the surface
of the water, a ‘green scum’ known as an algal bloom
- The turbidity (cloudy) in the water increases
Eutrophic lakes can be recognized by a green
tint to the water
Algae blooms result in unpleasant tasting water and may
release harmful toxins even
•The color, taste, and toxicity of the water affect human
activities such as fishing and boating
•Water clogged with algae is more difficult to treat for
drinking
•Biggest problem is AFTER the algae die
- Dead algae is consumed by aerobic bacteria which use
up the dissolved O2 in the water
- [O2] falls below necessary level, only material like
sludgeworms can survive. Lake is nearly useless for
fishing and has an effect on local communities
After the algal bloom, [O2] falls so low
that aerobic bacteria cannot survive
• Instead, anaerobic bacteria decompose
the remaining dead algae, forming foulsmelling products such as ammonia,
hydrogen sulfide, methane and
thioalcohols
• Bacteria may release dangerous toxins
lethal to animals
E5.4 – Thermal Pollution
•The temperature of water highly affects
the solubility of gases.
• At 25oC the solubility is 8.3 ppm
• At 30oC this falls to 5 ppm when fish
begin to die
Human activities can lead to an increase in water
temperature known as thermal pollution
•This occurs near power stations where water is drawn
to cool the steam from the turbines in devices called heat
exchangers. When water leaves it could be 20oC warmer
• [O2] is decreased when temperature rises because
• Solubility of O2 is decreased with lower Temps
• Microorganisms respire more quickly
High temperatures also cause enzymes in
microorganisms to denature and as a result they
cannot digest their food molecules
• Changing temperatures may also alter the cycle of
fish such as:
• Spawn (lay eggs) earlier
• Eggs hatch earlier
• At this time in the cycle the necessary nutrients may
not be available in the system
• Thermal pollution may also be cooling due to cold
water at the bottom of reservoirs released into
streams below
Enviro Chemistry Part 6 – Water Treatment
E6.1 – Water Cycle
E6.1 – Water and its Pollutants
• Water is an excellent solvent and dissolves many
different chemical substances
• As water falls through the atmosphere it dissolves
gases, such as CO2, and pollutants such as SO2 and NOx.
• Chemical fertilizers washed off the farmland will add
NO3- and PO43- ions
• River water may contain pesticides, bacteria and oil
• All of these impurities must be removed before it can be
used for drinking
E6.1 – Types of Water Pollutants
•Water pollution falls into two broad categories:
Point sources – occurs when the source of
pollution is clearly identifiable at one point
For example: a chemical factory releasing toxic substances
into a river
Non-point sources – describes situations in which
water collects pollutants over a larger area, and
thus cannot be attributed to a single source
Examples: Acid rain polluting a lake, fertilizer run-off
which accumulates N and P in compounds in rivers
The primary pollutants found in waste water are:
Heavy metals (toxic metals)
Pesticides (insecticides and herbicides)
Chemical wastes (dioxins, PCB’s)
Organic Wastes (sewage)
Fertilizers
E6.2 – Waste Water Treatment
•The treatment of waste water before it can be safely re-introduced to
rivers takes place in three stages:
Primary treatment – screening to remove solids, followed by
sedimentation of sand, grit and sludge
Secondary treatment – use of oxygen and bacteria to remove
organic matter
Tertiary treatment – chemical precipitation of remaining
organic compounds, heavy metals, nitrates and phosphates
E6.2 – Primary Treatment
•Water is sent through coarse mesh screens:
- Raw waste water contains floating and suspended solid
material. This may include paper, rags, wood and plastic rubbish
that may cause blockage elsewhere in the treatment plant.
•Water is passed through grit channel
- The speed of the water is slowed so sand and grit settle out. A
spinning centrifuge could also be used.
•Water is then passed into large holding tanks to settle
- Fine solid particles form sludge at bottom, insoluble grease
floats to top and is skimmed off
• Sometimes this treatment alone is enough
E6.2 – Secondary Treatment
•Secondary treatment focuses on lowering BOD by
removing organic matter
•The principal secondary treatment involves allowing
aerobic bacteria to oxidize the organic matter
- One method involves pouring of the waste over a
bed of small stones covered in microorganisms
- More effective method called the activated sludge
process. Bacteria and sewage are mixed and
blasted with air allowing for the bacteria to
multiply rapidly and feed on the organic material.
This process removes 90% of the BOD from
waste water
B6.2 – Tertiary Treatment
• Tertiary treatment is sometimes called advanced
water treatment and removes the remaining
inorganic pollutants from the water (PO43-,
N- complexes, heavy metal ions).
• This process is very important where industrial point
sources such as metal works or chemical plants have led
to high [pollutants]
• Nitrogen compounds are removed so they do not
contribute to eutrophication of rivers and lakes. There
are two common types (ammonium ions, and nitrate
ions.)
E6.2 – Tertiary, NH4+ removal
•In the tertiary treatment, water is first treated with
nitrifying bacteria, which oxidize the NH4+ ions to
nitrate ions
•This is a two-step process:
NH4+(aq) + 1½ O2(g) → NO2-(aq) + 2H+(aq) + H2O(l)
NO2-(aq) + ½ O2(g) → NO3-(aq)
E6.2 – Tertiary, NO3- removal
•The nitrate ions originally present in waste water, along
with those formed in the treatment of NH4+, are then
reduced (by different strains of bacteria) to
nitrogen gas, which is released into the
atmosphere
•2NO3−(aq) + 10e− + 12H+(aq) → N2(g) + 6H2O(l)
E6.2 – Tertiary, PO43- removal
•Phosphates are removed by either biological or
chemical processes.
•Water treated with biomass containing specific
bacteria which absorb the PO43- ions, and the
product can later be used as fertilizer
•Alternatively, PO43- ions can be removed by chemical
precipitation
Fe3+ (aq) + PO43-(aq) → FePO4(s)
Al3+ (aq) + PO43-(aq) → AlPO4(s)
E6.2 – Tertiary, Heavy Metal removal
•Heavy metals can be removed by chemical precipitation or
ion exchange.
•Most transition metal ions have insoluble hydroxides, so
Ca(OH)2 or Na2CO3 are added
Cr3+ (aq) + OH-(aq) → Cr(OH)3(s)
• A coagulant can then be added to clump and collect insoluble
particles together
•Ion exchange resin is a material that binds reversibly to
particular cations or anions as they are more attracted to
heavy metals than original ions
2Na+(resin) + Cd2+ (aq) ⇌ 2Na+(aq) + Cd2+ (resin)
E6.3 – Fresh water from the Sea
•Osmosis is a natural process which serves to equalize
the concentrations of solutions.
•If two solutions (one salt water, one water) are
placed on two sides of a semi-permeable membrane,
the water will tend to flow to the salt solution side until
concentrations are in equilibrium
•If pure water is desired, we want the opposite and
the solution must flow against the osmotic flow.
This process is called reverse osmosis.
Reverse osmosis requires that a pressure be applied to
the salt water, greater than the osmotic pressure.
This high pressure forces the solvent, water, through the
membrane, leaving the solute, salt, behind.
•This process takes energy because you must force the
water through the membranes
E6.3 – Thermal Desalination
•The most commonly used method to date is multi-stage
flash distillation (MSF).
•Sea water is heated under high pressure and then passed
into a chamber at lower pressure :
- Rapid decrease in pressure causes water to ‘flash’
evaporate
- After each ‘flash’ the steam is cooled in another chamber
- The cycle takes advantage of the steam to heat more
water, keeping energy consumption minimized
E6.3 – Advantage of Different Methods of
Attaining Fresh Water
Enviro Chemistry Part 7 – Soil
Soil is composed of organic and inorganic components in
varying proportions
• Organic component is called the humus and
consists of plant material that has been partly
decayed by bacteria and fungi
• Inorganic component includes mineral particles
(gravel, sand, silt, and clay), water, and air.
• Water trapped between soil grains contains ions (from
dissolving of minerals) which can be absorbed by plant
roots.
• This liquid containing dissolved ions and organic
substances, is called the soil solution
Soil supports a variety of living organisms, including
bacteria, insects and worms which by growth and
decay and the circulation of water and air, modify the
soil structure.
•Composition by volume
of typical surface soil:
45% inorganic (rock)
5% organic
25% water
25% air
Soil contains rotting organic matter in various stages of
decomposition
Early states of decomposition:
- Plant tissue (cellulose): broken down into shorter
polysaccharides
- Animal tissue: broken down into their proteins
Further decay:
- Breaks polysaccharides into simpler sugars and proteins into
amino acids
When soil is aerated
- Aerobic bacteria and fungi can decompose most of these
molecules still further, forming CO2 and H2O
Soil Organic Matter (SOM) is beneficial to soil in
the following ways:
Products of partial decomposition replenish the soil by
turning large molecules to smaller ones and soluble
ones which plant roots can take up
Partially decayed organic material helps to hold soil
together. When soil is bound together, water (and
thermal) retention is improved
Dark-colored SOM-rich soil absorbs heat better than
lighter-colored (low SOM) soil
In wet soils, O2 is less able to reach the organic material.
As a result, anaerobic bacteria take over, and form more
complex organic compounds such as phenols and
carboxylic acids
These are ‘humic substances’ and the mixture is called
humus
In cold/wet climates. Up to 95% organic matter known
as ‘peat soils.’
• The phenols and carboxylic acids in the humus (weak
acids) help to buffer the soil pH
Important since pH affects the solubility of metal
cations in the soil
Much of the SOM is insoluble in water but helps to retain water
and water-soluble plant nutrients so that they can be taken up by
plants roots.
Nutrients include inorganic minerals such as phosphorus, nitrogen
and sulfur, which are needed for healthy plant growth.
SOM also has many other important physical functions. There are
spaces or pores that exist between the SOM which act to hold
water (much like a sponge), oxygen and nitrogen.
Some of the SOM along with fungi create aggregates that
allow for a more stable structure of the soil.
This creates a soil layer that is more resistant to erosion; therefore,
soils with greater amounts of SOM have
less degradation.
Soil degradation
Soil degradation takes place when actions result in the soil
being unhealthy or infertile.
This degradation can take place naturally as well as a result of
human activity.
Natural sources of soil degradation include wind and water erosion.
The rates of these types of erosion can be increased by improper
land use
practices.
- For example, when small shrubs are removed to make way for
farming, the roots that once held soil in place are no longer there,
which means that wind and water erosion increase and valuable
soil is lost.
The three main anthropogenic forms of soil degradation are
salinization, nutrient depletion, and soil pollution.
Salinization:
The build-up of salts in the soil often becoming toxic
to plants.
Can arise from :
- Irrigation: water is diverted from waterways to
farmland to support crops where rainfall is limited. The
irrigation water may contain some dissolved salts.
- The natural water table
The water transports salts to the soil as they are
dissolved. As the water evaporates, the salt is left
behind and can build up over time.
Nutrient Depletion
The nitrogen cycle requires that the minerals
taken up during plant growth will re-enter the
soil when the plants die and decay.
If, through cultivation and harvesting, crops are
continuously removed, the nutrients and
minerals go along with them.
Nutrient depletion can be avoided by crop
rotation over a number of years (and some off
years where the crops are plowed back into the
soil for nutrients).
Soil Pollution
Soil nutrient depletion is often addressed by the
use of chemical fertilizers which contain nitrates
(NO3-) and phosphates (PO43-)
•These excess nutrients can lead to
environmental issues on their own
•As well as inorganic pollutants, organics can be
soil pollutants as well.
Soil pollution can also occur when harmful air
pollutants settle onto the topsoil, get turned
under and contaminate the soil food web.
These contaminants can be released into our
water supplies and back into the
atmosphere in the form of particulate matter.
Soil pollution can also result when hazardous
waste from industry is placed in landfills and
leaches into the surrounding soil.
Enviro Chemistry Part 8 – Waste
Anything you throw away in your rubbish bin is called
waste. As the world population and the standard of living
increases, larger and larger amounts of waste are
produced.
Once your garbage is driven away in the garbage truck
there are two main ways in which it is:
Landfill sites (burying the waste)
Incineration (burning the waste)
- Both methods are potentially damaging
Recycling waste is advantageous because it removes the
need to dispose of waste and cuts down on the need for
non-renewable resources
Solid Waste falls into five categories
Food and kitchen waste and plant waste –
biodegradable
Recyclable materials – glass, plastic, paper, etc
Composite wastes – mixtures of materials such as
clothing and packaging materials – difficult to recycle
Inert wastes – rubble, debris, etc
Hazardous wastes – paints, garden chemicals,
batteries, light bulbs, medicines
E8.1 - Landfill
•Landfills are designed to limit surface water (rain, runoff,
etc) from passing through and leaching out harmful
materials.
•Sites chosen are often made of clay – a low permeable
soil so ground water movement is small
•Pits are lined with plastic, gravel, and a drainage system
This ensures that polluted water that accumulates
(leachate) can be collected and disposed of properly
When full, the site is covered with the same layer to
keep rain water out.
When waste is added to site
At first, when O2 is present, aerobic bacteria
work on the organic material producing CO2
When O2 used up, anaerobic decomposition
occurs and generates CH4, H2S, and H2.
- These gases can accumulate below ground so
an ventilation system must be constructed
- Air can be pumped into the landfill in order to
increase the aerobic bacteria decomposition
The liquid leachate can contain heavy metals and
cyanide and must be extracted from the landfill
via the drainage system.
•The liquid must be treated by chemical or biological
means to remove pollutants before re-entering
waterways (discussed previously)
- Heavy metal precipitation (E6)
• The cyanide can be removed by oxidation with
sodium chlorate
NaCN + NaOCl → NaCNO + NaCl
2NaCNO + 3NaOCl + H2O → 3NaCl + N2 + 2NaHCO3
E8.1 - Incineration
•Incineration has many advantages
- Removes bulk from solid waste
- Resultant ash has a uniform composition that can be
more densely packed into landfills
- Can be burned for power generation
•However, the flu gas must be thoroughly cleaned before
being allowed to enter the environment as it contains CO,
HCl, HCN, organics, heavy metals, and particulates
- Two types of incinerators are the Rotary-kiln and
Fluidized-bed.
Rotary-kiln Incinerator
•Rotating chambers are used to allow movement of
waste and to ensure that all of the waste is exposed to
air.
•Most of the waste is combusted to form gases
Passed to ‘afterburner’ to ensure that solid particulates
are fully combusted
Gases are then ‘scrubbed’ to remove pollutants
•Remaining solid waste drops out of the kiln and is
cooled with water, recyclable materials can be reclaimed
from this solid waste.
Fluidized-bed Incinerator
•A sand bed is used to allow hot air to be basted in.
Separates the sand grains, allowing air between them
The sand is suspended on the air currents and it
behaves like a fluid, flowing and circulating.
Ground-up waste is introduced to the sand bed where
it’s suspended and the air is mixed throughout ensuring
that maximum surface area of the waste particles is
exposed to air for combustion
Incinerator Air Pollution
•Most of the carbon present in waste (mostly in organic
material and plastics) is converted into CO2 in the
incinerator.
•This is arguably preferable to landfills where methane is
formed.
If the methane is not reclaimed as fuel it enters the
atmosphere as a green house gas with a very high GWP
(higher than CO2)
•Other flu gas pollutants are removed in procedures
described in E1.
Recycling
One of the best ways to minimize the influence waste has
on our environment and to provide a sustainable
environment is to recycle.
Recycling can be an expensive way of dealing with
problematic waste and not all waste can be recycled;
separating the different recyclables can be time
consuming and difficult.
E8.3/4 – Radioactive Wastes
Radioactivity is the release of radiation from the
nucleus of an atom as it changes, or decays, into
a different element .
Nuclei decay in order to stabilize their structure
Reduces neutron:proton ratio in nucleus
Three types: alpha, beta, gamma
Nuclear power plants have many advantages over other sources of
energy. They do not contribute to air pollution or greenhouse gases,
and their fuel source, uranium, is abundant enough to supply
energy for the next 1000 years.
One of the major problems of nuclear power plants is the disposal
of the
spent fuel rods that provided the energy for the nuclear fission
reactor.
These spent fuel rods are sources of high-level waste (HLW)
because they give off large amounts of ionizing radiation for a
long time.
These spent fuel rods must be cooled for several years in deep
pools inside the plant or in special shielded storage facilities at
another site.
It is said that this waste must be stored for tens of thousands of
years before it can be disposed of safely.
A second category of radioactive waste is low-level waste
(LLW),
which refers to waste that gives off small amounts of ionizing
radiation for a short amount of time.
Low-level waste is waste that may have come into contact with
radioactive substances directly and therefore has become
contaminated.
It includes radioactively contaminated industrial or research waste
such as paper, rags, plastic bags, rubber gloves, protective clothing
and packaging material.
Hospitals, medical schools and radiopharmaceutical schools all
produce large amounts of LLW.
This waste can be placed in steel drums and buried in landfills.
Disposal of radioactive waste
There are very different disposal requirements for radioactive waste
that depend on whether it is LLW or HLW.
The difference in disposal methods is primarily based on the amount of
radioactivity remaining in the waste.
Lowlevel waste is the least contaminated and therefore requires less
complicated means of disposal. It can simply be stored on site until it
has sufficiently decayed and then disposed of with the regular waste.
The spent fuel that is used to fuel a nuclear power
plant is initially stored in deep pools of water on the
nuclear power plant site. The water in these pools
acts to absorb the heat energy that is released by the
spent fuel and to protect the workers from the
radiation.
These spent fuel rods are then
reprocessed to recover the unfissioned uranium ore.
This liquid waste is then classified
as HLW and must be disposed of safely.
The US Nuclear Regulatory Commission, for example, has designed a HLW
disposal site in the Yucca Mountains in Nevada.
Nuclear Medical Waste
•Radioactive isotopes are used in numerous medical applications
Medicines or diagnostic tools – short ½ life substances
are used. Technetium is used as a tracer. Iodine-131 is
used to treat thyroid cancers (since the gland absorbs
iodine)
Radiotherapy – long ½ life substances like cesium-137
are used to generate radiation in machines used for
radiotherapy
X-rays for radiography – long ½ life substances such as
cobalt-60 and iridium-192 are used to generate X-rays for
imagine
•Medical gloves and clothing are considered low-level, isotopes
from defunct X-ray machines are considered high-level
Topics E9-E12 are HL material only!
Enviro Chemistry Part 9 –
Further Ozone for HL
Ozone is an allotropic form of oxygen, but its chemistry
is very different from that of diatomic oxygen.
An oxygen molecule has a relatively strong double bond
holding the two oxygen atoms together.
An ozone molecule has a much more unstable bonding
arrangement of oxygen atoms. It consists of two
resonance structures with one double and one single
bond.
The bond between the oxygen atoms in O2 is shorter
(121 pm) than that of
the oxygen–oxygen bond in ozone (128 pm).
The longer oxygen–oxygen bond length indicates that
the oxygen–oxygen bond in ozone is weaker than that in
O2.
Light of shorter wavelength, and thus higher energy, is
needed to break the bond in a molecule of oxygen than
in ozone.
O=O ΔHf = 498 kJ
•O-O ΔHf = 144 kJ
•The O3 molecule contains resonance and
the overall bonds are considered
intermediate between single and double
bonds so
• In O3 for the O O bonds, ΔHf = 445 kJ
Mechanism for O3 depletion
•E.9.2 Describe the mechanism in the catalysis of O3 depletion by
CFCs and NOx. (2) For example: (explanation on following slides)
CCl2F2 → CClF2 + Cl•
Cl• + O3 → ClO•+ O2
ClO• + O• → O2 + Cl•
•AND
NO● + O3 → NO2 + O2
NO2 + O• → NO● + O2
•The net effect in both is:
O3 + O• → 2O2
Chlorine Photodissociation
•Ozone depletion by CFC’s involves the
photodissociation of a CFC molecule by a photon,
CF2Cl2 + UV → CF2Cl• + Cl•
(photodissociation of CFC-12)
•The highly reactive chlorine atom (free radical)
goes on to react with an ozone molecule. The
chlorine atom can later regenerate by reaction with
an oxygen atom:
Cl• + O3 → ClO•+ O2
ClO• + O• → O2 + Cl•
Overall: O3 + O• → 2O2
This is a chain reaction. One photon
initiates the formation of one chlorine
atom, which can then destroy
thousands of ozone molecules, as the
chlorine is regenerated by collision
with O2 atoms, which are abundant in
the stratosphere.
N2O Ozone Depletion
•As discussed earlier (part 04) nitrogen monoxide,
NO (exists as radical), is formed in the stratosphere
by the reaction of nitrous oxide, N2O, with an oxygen
atom
N2O + O• → 2NO•
•Nitrogen monoxide can deplete ozone as follows:
•NO + O3 → 2NO2 + O2
NO2 + O• → NO• + O2
•Again, the nitrogen monoxide has been regenerated
and can continue depleting more ozone molecules.
Polar Ozone
Most CFC’s are released in the tropical and temperate regions, where
most of the Earth’s population live
•However, ozone layer damage has been confined to the polar regions
Long-term studies of [O3] suggest that depletion is fastest in the spring
months
-September-October in the Antarctic
-March-April in the Arctic
•The polar regions are naturally colder than the tropics, and in addition,
air currents in the stratosphere tend to isolate a region of air above the
poles so it does not mix with the tropical air during the polar winter.
•This results in very low temperatures in the polar stratosphere, -80oC.
This is termed the polar vortex
Polar Vortex
Polar Stratospheric Clouds
•These low temps result in high-altitude polar
stratospheric clouds (PSC’s)
• H2O and HNO3 condense on pre-existing microparticles
on sulfur (S) containing compounds. (at low T’s form ice
crystals)
• The existence of these clouds (PSC’s) along with the
isolation of gas at the poles, causes a shift in the chlorine
containing compounds that occurs:
• Instead of HCl and ClONO2, species such as Cl2 appear
• This occurs bc the cloud particles offer a catalytic
surface which speeds up gas phase reactions and
through the winter the PSC reactions build up Cl2
Remember, ozone depletion is caused by UV
radiation.
•In the winter, the extreme polar latitudes
receive almost not sunlight so limited
depletion
•In the spring, the Sun reappears and
photolysis the Cl2(g) (favored chlorine
containing compound) that have built
up through the winter to form Cl•,
which enter the ozone depletion cycle
and cause a sudden rapid depletion of
ozone
In summer, the temperatures rise and the polar vortex
is no longer in effect and the PSC’s disperse
• The balance in chlorine compounds shifts back to
more stable (unreactive) compounds.
• The ozone depletion stops and the hole has a chance
to rebuild itself through photochemical reactions
• The hole also is rebuilt by finally mixing with ozonerich air from higher latitudes through the summer.
• Compounds favored are HCl and ClONO2
Enviro Chemistry Part 10 – Smog for HL
Originally smog referred to fog-like
covering that fell over cities due to
large amounts of smoke and SO2
from the burning of coal.
•Today, the term generally refers to
photochemical smog in which
primary pollutants derived from
vehicle traffic undergo a series of
light-driven chemical reactions
forming toxic compounds
Conditions Leading to Photochemical Smog
•The primary pollutants leading to photochemical smog
are NO and VOC’s from vehicle exhausts
- N2 + O2 → 2NO•
• Geographical location such as L.A. and Mexico City both
located in-between mountains for a geological ‘bowl’ or
‘basin’
• These cities also have inversion layers
(temperature inversion) in which the air close to
the ground is colder than the above which
prevents the polluted air from the city from rising
• The build-up of pollutants are then acted upon by
sunlight
Temperature Inversion
Cities that experience intense photochemical
smog include Athens, Hong Kong, Houston, Los
Angeles, Mexico City, Sao Paulo, and Tehran.
•Photochemical Smog
- Primary Pollutants:
-NOx(NO + NO2), VOC’s, CO, particulate matter
- Secondary Pollutants:
- O2, NO2, H2O2, PAN (peroxyacetyl nitrate),
partially oxidized VOC’s, HNO3, particulate matter
Formation of Smog Pollutants
Harmful substances from photochemical smog
include NO2, O3, aldehydes (HCHO),
peroxyethanoyl nitrate (PAN’s), and secondary
aerosol particles (comprised of sulfates, nitrates
and oxidized organic compounds).
• All from free-radical processes driven by the
sun
Formation of NO2 and Ozone
• Formation of NO2
2NO• + O2 → NO2
• Formation of ozone
NO2• + UV (430nm) → NO• + O•
O• + O2 + M → O3 + M
NO• + O3 → NO2• + O2
• If these were the only processes involved, then a steady state
would be reached, with ozone depletion/formation stable. BUT,
further processes allow NO to re-oxidize to NO2 without
depleting ozone. More ozone created than destroyed =
ozone build up!
Formation of Alkyl Radical
•A proposed mechanism for this re-oxidation involves hydroxyl
radicals and hydrocarbons.
• The hydroxyl radical, HO•, is generated by the reaction of excited
oxygen atoms (O•) with water
• These are then formed from the photodissociation of ozone.
O3 + UV (290-320nm) → O2 + O•
O• + H2O → 2HO•
• Hydroxyl radicals combine with hydrocarbons, RH (like VOC’s)
leading to the formation of alkyl radical
RCH3 + OH• → RCH2• + H2O
Formation of NO2 Radical
The organic free-radicals combine with oxygen, O2,
forming organic peroxy radicals, RCH2O2•
•
• These radicals are able to oxidize NO to NO2
RCH2• + O2 → RCH2O2•
RCH2O2• + NO• → RCH2O• + NO2•
The resulting RCH2O• radicals can react with oxygen molecules to
form aldehydes, RCHO :
RCH2O• + O2 → RCHO + HO2•
•HO2 + NO• → HO• + •NO2
•These radicals convert NO to NO2, which then photolyzes to form
ozone
•OH + RCH3 + 2O2 + 2NO• → H2O + RCHO + 2NO2• + •OH
•Combined with
2NO2 + 2O2 → 2NO• + 2O3
•Results in:
•OH + RCH3 (need NO + O2) → H2O + RCHO + 2O3 + •OH
Daily Cycle of Pollutants
•Primary pollutant concentrations in
polluted cities such as London are
high early in the morning as rush
hour → NO + CxHY.
•Later NO2 increases as NO is
oxidized
•Driven by sunlight (most intense at
noon)
•[Ozone] peaks after lunch when
NO2 generates O• leading to ozone
formation.
•In evening NO is replenished by
rush hour depleting ozone more
Harmful Effects of Ozone
•Contained within many synthetic materials, C=C can
often be found.
- These compounds are paints, dyes, and plastics
- The addition of O3 across this double bond can cause
deterioration and color bleaching
- Polymer chains in rubber tires could be broken down by
ozone, causing them to crack and split
- O3 attacks green plants, discoloring their leaves and
resulting in decreased photosynthesis
Formation of PANs
• Remember, PANs are harmful substances that
results from photochemical smog
• As we have seen NO2 shows up in O3
formation equations
• NO2 can be removed from the photochemical
smog chain reaction by reaction with the PAN
(peroxyacyl) radical. The resultant compound,
peroxyacyl nitrate, has many adverse health
effects
PAN Compounds
•RCHO + HO• → RCO• + H2O
•RCO• + O2 → RC(O)O2•
•RC(O)O2• + NO2• → RC(O)O2NO2
This stable PAN compound can
be transported elsewhere in the
troposphere and slowly
dissociate and each
contributing to ozone
elsewhere.
For the peroxyacyl nitrate
molecule, the R group could
represent (-CH3) which results
in a peroxyethanoyl nitrate
which is toxic, irritates the
eyes, and is damaging to
green plants
Enviro Chemistry Part 11 –
Further Acid Deposition HL
Creating OH•
•As
discussed earlier
O• + H2O
→
2HO•
•The HO• is also formed by:
O3 + H2O
→
2HO• + O2
•Aside from their role in formation of
photochemical smog, the HO• are
important in the formation of nitric,
nitrous, and sulfuric acid in the
atmosphere
Acid Deposition by NO and SOx
Formation of hydroxyl radicals:
H2O+O3 → 2HO•+O2
OR… H2O +O• → 2HO•
HO•+ NO2 → HNO3
HO• + NO → HNO2
HO• + SO2 → HOSO2•
HOSO2• + O2 → HO2•+ SO3
(SO3 + H2O → H2SO4)
Formation of N Acids
•N2 + O2 → 2NO (from combustion of fuel)
•Formation of Nitrous Acid (HNO2)
HO• + NO• → HNO2
Moderately soluble in water, so HNO2 dissociates
HNO2 + H2O ⇌ H3O+ + NO2- (weak acid dissociates)
HNO2 (+UV) → HO• + NO (photolyzed back to NO, HO)
•Formation of Nitric Acid (HNO3)
2NO• + O2 → 2NO2•
HO• + •NO2 → HNO3
Strong Acid = more soluble in water
HNO3 + H2O → H3O+ + NO3- (100% dissociation)
Formation of Sulfurous Acid
S + O2 → SO2
SO2 + H2O → H2SO3 (dissolves and reacts in
water)
H2SO3 is considered a weak acid
H2SO3 + H2O ⇌ HSO3- + H3O+ pKa = 1.85
HSO3- + H2O ⇌ SO32- + H3O+ pKa = 7.20
Formation of Sulfuric Acid
S + O2 → SO2
HO• + SO2 → HOSO2•
HOSO2• + O2 → HO2• + SO3
SO3 + H2O → H2SO4
•Sulfuric Acid formation in clouds:
O3 + SO2 + H2O → HSO4- + H+ + O2
O3 + HSO3- → HSO4- + O2
O3 + SO32- → SO42- + O2
•Important source for nuclei (seeds) in cloud forming
H2O2 + HSO3- → HSO4- + H2O
CH3OOH + HSO3- → HSO4- + CH3OH
Ammonia
In the atmosphere, ammonia neutralizes
the acids formed to a large extent, to form
ammonium salts. Slightly acidic ammonium
salts, (NH4)2SO4 and NH4NO3, formed in the
atmosphere sink to the ground or are
washed out of the atmosphere with rain. As
NH4+ is deposited and enters the soil,
nitrification and acidification can occur.
NH4+ + 2O2 → 2H+ + NO3- + H2O
Ammonia in the atmosphere can neutralize acid
raindrops, forming ammonium salts
2NH3 + H2SO4 → (NH4)2SO4
NH3 + HNO3 → NH4NO3
•They exist in both aqueous and solid forms.
•These salts (formed in atmosphere) are carried to
the ground by rain, where they cause damage to
plants
•They particles also form an aerosol haze in the
atmosphere decreasing visibility
Enviro Chemistry Part 12 – Further Water and Soil HL
Precipitation by Temp
• Remember, from Part 06, the limited solubility of
transition metal hydroxides allows us to remove the
transition metals from waste water by chemical
precipitation. (the metals can inhibit enzymes)
• Unwanted soluble materials precipitate out of solution
when they pass the saturation point at a given
temperature.
• By addition of other substances, this point of solubility
can be lowered even further
• A form of the equilibrium constant (Kc) called the
solubility product (Ksp) is then used.
Given the equilibrium formed by a metal M and
a non-metal X: MX(s)⇌M+(aq) + X-(aq).
•The Keq for this system is given by Ksp =
[M+][X−], and is called the solubility product
constant.
Solubility of Metals
• For a compound such as M3X2:
M3X2(s) ⇌ 3M2+(aq) + 2X3-(aq)
Ksp = [M2+ ]3 [X3-]2
• For aluminum hydroxide, Al(OH)3:
Al(OH)3(s) ⇌ Al3+ (aq) + 3OH- (aq)
Ksp = [Al3+] [OH-]3
• This is a constant at a given temperature, changing
temperature will change Ksp and the amount of
substance that will dissolve in given conditions
Calculating Ksp from solubility
•The solubility of PbBr2 in water is 4.33 g in 1.0 L of water at 25°C.
What is the value of Ksp for PbBr2?
PbBr2(s) ⇌ Pb2+ (aq) + 2Br- (aq)
Ksp = [Pb2+] [Br-]2
You must calculate the molarity [ ] of PbBr2
4.33 g PbBr2 x
1.00 L
1mol PbBr2
= 0.0118 mol∙L-1
367.01 g PbBr2
PbBr2(s)
⇌
Pb2+ (aq)
+
0.0118mol∙L-1
0.0118mol∙L-1
2Br- (aq)
2(0.0118mol∙L-1)
= 0.0236mol∙L-1
Ksp = [0.0118] [0236]2 = 6.57 x 10-6 for PbBr2 @ 25oC
Calculating Solubility from Ksp
•Ksp for Al(OH)3 = 1.0x10-32 , what is the molarity of a saturated
sol’n?
Al(OH)3(s) ⇌ Al3+ (aq) + 3OH- (aq)
x mol∙L-1
x mol∙L-1
3x mol∙L-1
Ksp = [Al3+] [OH-]3
Ksp = [x] [3x]3
1.0x10-32 = [x] [27x3]
1.0x10-32 = 27x4
3.7x10-34 = x4
4.4x10-9 = x
Therefore [Al(OH)3] = 4.4x10-9 mol∙L-1 in saturated sol’n
Common Ion Effect
•The common ion effect results when one of its
constituent ions is already present in solution
and therefore the salt is less soluble
• Example AgCl in solution, then add NaCl
AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
• With the addition of another salt, NaCl, which also
contains Cl-(aq) ions and has it’s own equilibrium
(NaCl(s) ⇌ Na+(aq) + Cl-(aq))
• Le Chatelier’s principal (Topic 07) predicts that the
equilibrium of AlCl3 will shift back left to a lower [Cl-]
ions than before.
AgCl in Water
•Consider the following example:
•For AgCl Ksp=2.0x10-10 at 298K, dissolved in H2O
AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
Ksp = [Ag+(aq)][Cl-(aq)] (where they are equal, and “x”)
Ksp = x2 = 2.0x10-10
x = (2.0x10-10)1/2
x = 1.4x10-5 mol/dm3 (molarity of AgCl in water)
AgCl in NaCl solution
•If the same salt is dissolved in an NaCl solution instead of H2O
AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
NaCl(s) ⇌ Na+(aq) + Cl-(aq) (solution is 0.10 mol/dm3)
• Since there are now two sources of Cl- ions, we will give the molarity of
Ag+ ions (y), so Cl- ions is (0.10 + y).
Ksp = [Ag+(aq)][Cl-(aq)] = y + (0.10 + y)
• The contriubution of Cl- ions from NaCl is much great than that from AgCl
since AgCl is only slightly soluble, where NaCl is very soluble
y<<0.10 hence (0.10 + y) = 0.10
Ksp= [0.10] [y] = 2.0x10-10
y = 2.0x10-9 mol/dm3
•The presence of NaCl, lowered the solubility of Cl- ions from AgCl
in solution considerably
Clay Soil (E12.2)
•Clay soils are able to retain water and nutrient ions
which might otherwise be leached out of the soils by
rain water.
• The minerals in clay carry a negative charge that is
balanced positive metal ions
Cation-exchange capacity (CEC)
•The cations bound to the clay structure are not permanent as
they can be exchanged.
Clay-−Na+(s) + H+(aq) ⇌ Clay-−H+(s) + Na+(aq)
•Cation-exchange capacity (CEC) is defined as the
amount of the single-positive cations that can be
exchanged with the soil solution, per Kg of clay
• If the clay contains Mg2+ or Al3+, the replacement of these ions
increases the negative charge of the soil and CEC increases.
• CEC values range from 0.03 for kaolinite up to 1.5 for
vermiculite clay.
• Most soils only contain small fraction of clay, so CEC’s for soils
range from 0.02 – 0.60
CEC Example
•A typical exchange process:
clay–Mg(s) + 2NH4+(aq) ⇌ clay–2NH4+(s) + Mg2+(aq)
•This equilibrium can shift in response to changing
concentrations in the soil of Mg2+ and NH4+ ions as a
result of chemical additions to the soil such as fertilizers
or the use of ions in plant growth
•Without this ability to exchange and retain ions, soil
would not be able to supply nutrients to its surroundings
clay–Mg(s) + 2H+(aq) ⇌ clay–2H+(s) + Mg2+(aq)
Effect of pH on CEC
• When low pH solutions (acids with plenty of H+) are added to
soils, the equilibrium changes and the H+ take place in the soil,
releasing nutrients retained by the soil into the ground water to be
washed away.
Low pH also affects the availability of anion nutrients such as
phosphate, PO43-, and nitrate, NO3-. Now, the low pH increases the
ions (since they are negative)
clay–OH(s) + H+(aq) + NO3-(aq) ⇌ clay–OH2+NO3-(s)
• In addition, as we talked about, low pH enables the reduction of
NO3- ions to NH4+ ions
NO3-(aq) + 10H+(aq) + 8e- → NH4+(aq) + 3H2O(l)
• Plants take up nitrogen in the form of NO3- ions, so this process
lowers the availability of nitrogen for plants to take up
Read p. 722-724 Brown-Ford for more
equations involving soil pH on CEC and
availability of nutrients.
Chemical functions of SOM
The action of anaerobic bacteria on organic matter in the
soil leads to the formation of phenols, carboxylic acids,
proteins and peptides.
•These substances lend the soil certain chemical
properties:
- Binding metal cations in soil
- Increasing cation-exchange capacity of soil
- Binding organic pollutants
- Increasing the buffering capacity of the soil (resistance
to pH changes)
Contribution to CEC
Presence of carboxylic acids (and phenols – see
p.725 text) in humus allows it to form complexes
with cations in the soil.
2RCOOH (humus) + Mg2+(aq) ⇌ (RCOO)2Mg (humus) + 2H+(aq)
This increases the CEC of the soil and prevents
nutrient loss
Increased Buffer Ability
Presence of weak organic acids and their salts in
SOM allow it to act as a natural buffer.
RCOOH (humus) ⇌ RCOO- (humus) + H+(aq)
Conjugate base will neutralize added acid, weak acid
will neutralize added base.
Other functions of SOM
As well as binding to nutrient cations,
SOM is able to form complexes with toxic
aluminum and heavy-metal cations
preventing them from entering sol’n.
•
• As humic substances are organic in
nature, they are able to absorb other
organic substances such as herbicides and
pesticides , decreasing pollution reaching
and affecting water supply
IB happy ‘cause IB DONE