OPTION E: ENVIRONMENTAL CHEMISTRY
E 1. Air pollution
E 1.1 Describe the main sources of carbon monoxide (CO), oxides of nitrogen (NOx), oxides of sulfur (SOx),
particulates and volatile organic compounds (VOCs) in the atmosphere.
E 1.2 Evaluate current methods for the reduction of air pollution.
Air pollutant = any chemical in such concentration (greater than its natural concentration) in the air
that it produces an harmful effect on the environment i.e. humans, animals, vegetation or materials.
Primary air pollutant = an air pollutant which is a waste product from a human activity and which is
added directly into the air (as opposed to a secondary air pollutant which is formed in the air as a
product of a chemical reaction in the air).
Their sources, effects on health (no need to remember) and current methods for their reduction
See separate fact sheet “Air pollutants”. Remember health effects are not part of the assessment.
Additional notes

The concentration of most pollutants is measured in ppm (=parts per million) and you should be able to
convert this into % which is done by dividing the ppm value by 10,000.
It is important to appreciate the health effect of these air pollutants depends on the amount present, their
toxicity and length of time that they are in the atmosphere.

 catalytic converter: a device in which hot exhaust gases carbon monoxide, nitrogen oxides and
unburnt hydrocarbons are passed over a catalyst such as platinum during which carbon monoxide is
oxidized with oxygen which comes from the reduction of nitrogen oxides. Chemistry points involved:
 equations:
 carbon monoxide: 2CO (g) + O2 (g)  2CO2 (g)
 overall equation 2NO (g) + 2CO (g)  2CO2 (g) + N2 (g)
 nitrogen oxides are reduced to nitrogen to oxidize carbon monoxide to carbon dioxide
 oxidation of VOCs (unburnt fuel): 2C8H18 (g) + 25O2 (g)  16CO2 (g) + 18H2O (g)

to achieve high rate of reaction for this redox process:
 exhaust gases are passed over platinum which acts as a heterogeneous catalyst;
 the converter has a honey comb structure to increase surface area
 the very high temperature of the exhaust gases also increase the rate of reaction dramatically
E 2. Acid deposition
E 2.1 State what is meant by the term acid deposition and outline its origins.
E 2.2 Discuss the environmental effects of acid deposition and possible methods to counteract them.
Acid deposition refers to the process by which acidic particles, gases and precipitation leave the atmosphere
and are brought back down to earth e.g. on the ground, on trees, buildings, inside plants or animals (e.g.
your lungs).
There are two types of acid deposition:
 wet deposition: acid rain, fog and snow
 dry deposition: acidic gases such as SO2 and salts (when acidic gas reacts with an alkali)
Origin of acid deposition
Rain is naturally slightly acidic because of the dissolved carbon dioxide that forms carbonic acid that is a
weak acid:
CO2 (g) + H2O (l)  H2CO3 (aq)
Acidic rain is rain with a pH of less than 5.6 as it contains nitric acid and sulphuric acid.
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The main chemicals responsible for acid rain: nitrogen oxides and sulfur oxides.
NOx: formed as a result of high temperatures in internal combustion engines, i.e. cars and jet engines.
1. production of nitrogen oxides: N2 (g) +
O2 (g) 
2NO (g) +
2NO (g)
O2 (g) 
2NO2 (g)
2. the following equations show two ways in which nitric acid is formed in the atmosphere
2 NO2 (g) + H2O (l) 
HNO3 (aq) + HNO2 (aq) (=nitrous acid)
or
4 NO2 (g) + 2 H2O (l) + O2 (g)
4 HNO3 (aq)
SOx: from burning of coal which contains sulphur/smelting plants
1. production of oxides:
S (g) + O2 (g)  SO2 (g)
2SO2 (g) +
O2 (g) 
2SO3 (g) (oxidation)
2. formation of atmospheric sulphuric acid and sulfurous acid
SO3 (g) + H2O (l) 
H2SO4 (aq)
SO2 (g) + H2O (l) 
H2SO3(aq)
Environmental effects of acid deposition on:

humans



buildings
aquatic life
vegetation
irritation of the mucus membranes and lung tissue when breathing in fine droplets of
acid rain;
increase in risk of respiratory illnesses like asthma and bronchitis;
acidic water also dissolves and leaches poisonous ions like Al3+ (linked with Alzheimer
disease) and Pb2+.
corrosion of materials such as marble and dolomite (CaCO3.MgCO3): equation:
CaCO3 (s) + H2SO4 (aq)  CaSO4 (s) + H2O (l) + CO2 (g)
 faster corrosion of iron and steel structures in buildings or bridges.
increased levels of aluminum ions dissolved from the soil kills fish as it effects the function
of the gills; a lot of fish, algae, insect larvae even plankton cannot survive in water below a
certain pH.
 increased soil acidity leaches important nutrients (e.g. Ca2+/K+/ Mg2+) out of the top soil
 Mg2+is necessary to make chlorophyll so removal of this ion results in lowering rate of
photosynthesis and reducing growth of plants and crop yields.
 Increased concentration of Al3+ which damages roots
 stunted growth,
 thinning of tree tops,
 yellowing and loss of leaves.
Methods to counteract the environmental effects of acid rain
Fundamentally, any measure that controls the emissions of nitrogen and sulphur oxides will reduce the
occurrence of acid rain. You have studied these already under air pollutants:
 cutting down emissions of nitrogen oxides by using catalytic converters; thermal exhaust systems;
 use of low sulphur fuels; use of these fuels still release sulphur dioxide
 removal of sulphur oxides from exhaust fumes (Flue Gas Desulphurization);
 use of alternative sources of energy which do not involve fossil fuels and the production of very high
temperatures.
 dealing with acid rain itself: neutralization of lakes using calcium carbonate or calcium oxide,
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e.g. CaO +
H2SO4 (aq)  CaSO4 (s) + H2O (l)
E 3. Greenhouse effect
E 3.1 Describe the greenhouse effect.
E 3.2 List the main greenhouse gases and their sources, and discuss their relative effects.
E 3.3 Discuss the influence of increasing amounts of greenhouse gases on the atmosphere.
Whenever solar radiation hits a surface or a gas molecule it is converted into radiation of a longer
wavelength.
Greenhouse effect
After passing through the atmosphere, the visible light and UV radiation (short wavelength radiation) coming
from the Sun hits the surface of the Earth and is converted and reradiated by the Earth as radiation with
longer wavelength such infrared radiation, IR (longer wavelength/lower frequency than visible light and UV).
Some of the infrared radiation is prevented from returning to space by gases (called greenhouse gases) in
the atmosphere which absorb the infrared radiation to increase their bond vibrations; these vibrations cause
the atmosphere to heat up which then radiates heat back to Earth. This effect is called the greenhouse effect.
Natural greenhouse effect.
This greenhouse effect has always been there – ‘natural’ greenhouse effect - and is one of the factors that
has allowed live to exist on Earth; but there has always been an equilibrium between the energy reaching the
Earth from the Sun and the energy reflected back into space which is partly controlled by the greenhouse
effect. This equilibrium controls the Earth’s temperature.
Recently, the greenhouse effect has increased upsetting the equilibrium and this is resulting in global
warming.
Global warming = a gradual increase in planet-wide temperatures.
Evidence for global warming (or the significance of human contribution)

When considering recent temperature recordings i.e. last 100-130 years:
 An almost 1.0 C warming over the last century (is this rise due to human activity or is it as the
result of some natural temperature variation cycle?);
 3 hottest years of the 20th century occurred in the last decade of that century i.e. 1990’s.
The increase is small compared to variations observed in the past but do we wait for more years for a
more pronounced temperature increase after which it could be too late to act. Consider the
limitations of the currently available data! Solution: more frequent data i.e. weather data needs to be
collected.

Variations in temperature over a long period of time (thousands of years): the temperature can be
tracked back using the ratios of oxygen isotopes trapped in the ice (the ratios are an indicator of the
temperature at that time) as well as fossil records. Also the concentration changes of carbon dioxide and
methane during the same period of time have been measured and a strong correlation between the
variations in temperature and the chemicals can be observed although this is still questioned by some
scientists.
At present there is an increase in concentration of both carbon dioxide and methane (as a result of
human activity) which suggests a more drastic future increase in temperature.
Measurement of carbon dioxide in the atmosphere needs to taken away from industrial activity e.g.
top of volcano.
Greenhouse gases: CO2, H2O, CH4, N2O and CFCs: their sources and relative effects
Greenhouse gases are gases which allow visible light and UV radiation (short-wavelength/high
frequency) to pass through them but (because of the nature of the covalent bonds in their molecules)
absorb the infrared radiation (longer-wavelength radiation) of the same frequency as the one the Earth
converts the energy from the Sun into and reradiates this infrared radiation back to the Earth.
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As each molecule has its own type of bond, they each absorb at slightly different IR wavelengths.
The combined absorption bands of carbon dioxide and water can be seen to block most of the infrared
radiation radiated from the earth. There is, however, a region within the infrared spectrum through which
radiation can escape; this region is referred to as the “atmospheric window”.
This window can be closed by greenhouse gases other than water and carbon dioxide such as CFCs and
methane.
The fact that methane and CFC’s absorb in the atmospheric window region is one of the factors why both
methane and CFCs have a greater greenhouse factor than water and carbon dioxide.
The greenhouse factor of a greenhouse gas (or its greenhouse effect) is determined by:

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their ability to absorb heat or infrared radiation in the atmospheric window or not
the number of different ranges of wavelength they absorb is caused by the molecules having different
types of bonds; each type absorbs at its own frequency;
the intensity of the absorption e.g. methane has 4 C-H bonds which means it can absorb more at a
certain frequency!
the abundance of the chemical
the lifespan of the chemical
Greenhouse
gas

Sources
Relative effects/significance
Relative effect: 1 (= standard)
Most important greenhouse gas (50%
contribution) because of its great
abundance (second largest after water
vapour) and the large range over which it
absorbs IR.
Relative effect (compared to CO2): 0.1
Least effective in trapping radiation but it
is the most abundant.
Relative effect: 30
Low abundance in atmosphere but it is
effective in absorbing infrared radiation,
however, its atmospheric lifespan in the
atmosphere is short.
CO2

Human: burning fossil fuels and wood,
forest fires, burning waste
Natural: respiration, decay of organic
matter, natural forest fires
H2O


Human: combustion of hydrocarbons
Natural: evaporation of ocean/lakes water

Human: cattle farming, rice paddies,
petroleum and natural gas production,
Natural: in digestive tracts of ruminants,
cattle, bogs or marshes, bacterial
fermentation – methane is produced when
organic matter is decomposed
anaerobically
Human: use of nitrogen based fertilizers,
decomposition of biomass
Natural: bacterial action
Human: refrigerators, air- conditioning,
aerosols in spraying cans, foaming agents
Natural: none!!!!
CH4


N2O


CFCs

Relative effect: 150
Very effective in absorbing radiation,
fairly long atmospheric life.
Relative effect: 10 000 – 25 000
Very effective in absorbing radiation,
very long atmospheric life.
Why are oxygen, nitrogen and argon not greenhouse gases?
For IR to be absorbed the following requirements need to be met:

IR photons can only cause bond vibrations to move up to higher energy levels; noble gases cannot
absorb IR as they are monatomic molecules with no covalent bonds;

IR absorption requires that the radiation causes changes to the electric field or the dipole moment of the
molecule which eliminates non-polar molecules e.g. oxygen and nitrogen; all greenhouse gases are
polyatomic;
*Positive feedback: global warming increases evaporation which result in more water vapour in the
atmosphere which increases the greenhouse effect further.
SF6 is also a greenhouse gas.
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Influence of increasing amounts of greenhouse gases on global warming

An increase in the earth’s average temperature by almost 1.0 degree over last 100 years.

The 10 warmest years have all occurred since 1998

Increase in precipitation in some areas of the world

World wide rise in sea levels resulting from partial melting of glaciers and polar ice caps and thermal
expansion of water (as a result of heating).

Changes in crop yields: some crops will grow better, other worse.

Changes in distribution of commercial crops, animals, insects,..

Changes in the distribution of pests and disease-carrying organisms e.g. malaria.

More floods in particular of coastal areas; more severe storms e.g. 2010 monsoon floods in Pakistan

More severe droughts e.g. 2010 worst drought in Amazon

Increase in size of deserts
Seasonal fluctuations for carbon dioxide
The use (photosynthesis) and release (respiration) of carbon dioxide in the northern hemisphere has a much
greater impact on the global carbon dioxide concentration than the southern hemisphere because there is
more land and therefore plants.
Peaks: Coincides with winter in northern hemisphere when most trees have lost leaves, less photosynthesis
activity so less carbon dioxide removed resulting in a relative increase in the global annual carbon dioxide
concentration
Troughs: Coincide with summer in northern hemisphere and winter in southern. Northern hemisphere has
more land mass/trees so more photosynthesis occurs during summer resulting in a net decrease of carbon
dioxide in the atmosphere
Effect of particulates on global warming: cooling effect/counteract greenhouse effect



particulates scatters and reflect the incoming sunlight (visible and UV) so that less solar radiation enters
the atmosphere;
particulates also cause a lowering of the temperature as they provide condensation nuclei around which
water particles condense to form clouds reducing solar heating;
volcanic eruptions and large forest fires greatly increase the amount of particulates.
E 4. Ozone depletion in stratosphere
E 4.1 Describe the formation and depletion of ozone in the stratosphere by natural processes.
E 4.2 List the ozone-depleting pollutants and their sources.
E 4.3 Discuss the alternatives to CFCs in terms of their properties.
Ozone = very pale bluish gas, pungent smelling odor, powerful oxidizing agent (easily reduced to O 2).
Detection of ozone: [O3] in an air sample can be measured using UV spectroscopy: the sample of air is
radiated with UV light and its absorption measured; the more UV is absorbed the greater [03].
Ozone in the upper atmosphere (15 to 45 km) has two functions which are beneficial to us:
Absorbs UV-B which has a wavelength of 290-320 nm; this type of radiation:
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


causes sunburn, skin cancer and eye cataracts in humans;
reduces plant growth; it reduces the rate of photosynthesis because the ozone destroys the apparatus for
photosynthesis;
can cause genetic mutations as it can damage biological molecules like amino acids and nucleic acids;
causes loss of plankton.
Ozone production releases energy (UV radiation is converted into kinetic energy) which produces an
increase in temperature with increase in altitude in the stratosphere; this gives stability to the stratosphere as
it prevents large convections of air in the atmosphere.
Depletion of the ozone would result in an increase of sunburn, skin cancer, cataracts and even blindness. It
would reduce crop yields, cause loss of plankton which would upset food chains in aquatic environments and
might increase mutations.
Evidence for ozone depletion
Monitoring of stratosphere above Antarctica since 1981 using UV spectroscopy from satellites, high-altitude
planes and balloons: in 1985 ozone column was reduced by 40% in September and October. Also ozone
concentrations have fallen over Northern Europe.
Natural formation and depletion maintaining a steady-state concentration
Stratospheric ozone is in a dynamic equilibrium with oxygen and is continually being formed and decomposed as
shown by the following processes.
Formation of ozone:
1. The whole cycle starts with the formation of free radical atoms; the most important free radical (= a
particle with an unpaired electron), as it is present at the highest concentration, is the oxygen free
radical which is formed as the result of the photodissociation (=decomposition by light) of oxygen
molecules;
O2 + uv
 O
+
O
It is important to realize that the UV radiation absorbed in this process has higher frequency/lower
wavelength radiation than the UV radiation we need protecting from. The frequency needed is high
so it can break the strong double bonds in the oxygen molecules. (hv = ultraviolet photon with a
wavelength of maximum 242 nm). Any UV radiation not absorbed by the above process does not
reach the earth’s surface anyway.
2. ozone is formed when such an oxygen free radical then reacts with a diatomic oxygen molecule:
O2 + O
 O3
[03] fluctuates during night and day and seasons as the production involves UV light.
Depletion of ozone:
Ozone is depleted in two ways: (ozone is unstable but is depleted slowly)
1. by reacting with a free radical e.g. oxygen in which case it forms two diatomic oxygen molecules;
O3 + O
 2O2
2. photodissociation by absorbing harmful UV radiation (the photon of UV radiation has less energy/lower
frequency/higher wavelength than the one needed to break the double bonds in the oxygen molecule as
the oxygen bonds in ozone are less strong) producing a diatomic oxygen molecule and an oxygen free
radical which then can be used to form a new ozone molecule or decompose an existing molecule; the
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oxygen free radical is then used to make ozone again; the rate of reaction usually depends on season,
time of day, latitude and solar emissions
O3 + uv  O2 + O
The ultraviolet photon necessary for this reaction has the same wavelength (or required amount of
energy) to the radiation which causes sunburn and skin cancer;
Condition: production of ozone needs shorter wavelength/higher frequency radiation as more energy
is needed than the destruction of ozone.
The rate at which ozone is formed is equal to the rate at which it is depleted.
Up in the stratosphere, the gas-phase reactions that have been described above occur at a low rate.
However, there are other free radicals which cause natural depletion of ozone but which occur at far smaller
concentrations in the atmosphere, these chemicals are methane, nitrogen monoxide and hydroxyl radical.
Just like with oxygen the process starts with the photodissociation of these molecules to form radicals which
react with ozone molecules; the free radicals are regenerated in another step which makes the destruction of
ozone by these chemicals a catalytic chain reaction as the regenerated free radical can destroy further ozone
molecules. Each free radical can decompose many ozone molecules.
These catalytic destructions also occur at higher rates than the depletion caused by the oxygen free radical;
this is also why this depletion is called a catalytic chain reaction.
General mechanism for these depletion reactions:
X + O3  XO + O2
XO + O  X + O2
O + O3 
X = any radical other than oxygen
2O2
Man-made depletion: CFC’s and nitrogen oxides which catalyse the depletion
Caused by the release of chemicals which produce radicals which can catalyse/speed up the depletion of
ozone by the mechanism shown above.
Anthropogenic sources of these chemicals:
1. CFC’s: refrigerants, air conditioners, blowing/foaming agents, aerosol propellant, solvents eg dry cleaning
agents
2. Nitrogen oxides: nuclear explosions, Shuttle spacecraft, combustion (car engines/power stations),
nitrogenous fertilizers, high-flying commercial and military aircraft
Reasons why CFCs are still used widely despite being banned in many countries:

CFC’s are industrially useful compounds as they are or have

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





chemically stable: strong bonds and little polarity; (unfortunately this also means long atmospheric
life-time);
low toxicity;
low cost to manufacture;
volatile liquids but can also easily be liquified
good solvent characteristics;
insulating;
fire suppressive.
CFC’s photodissociate (UV) in the stratosphere (they get up there because they are stable) to form
chlorine free radicals (it is the C-Cl bond which breaks at it is weaker) which catalyse the depletion of
ozone as they are also regenerated in a subsequent step; a CFC like trichlorofluoromethane can release
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3 chlorine free radicals which can each destroy 100000 ozone molecules each before they collide with
another particle that changes them chemically; (the concentration of other particles will be a lot less than
the concentration of ozone at that level in the atmosphere) (catalytic depletion);

Equation
photodissociation of CFC’s:
CCl2F2 + uv   CClF + Cl
Catalytic depletion: Cl free radical atom catalyses the decomposition of ozone and is not used up
Cl + O3  ClO + O2
ClO + O  Cl + O2
O + O3 
O2 + O2

This reaction occurs at a faster rate than the natural depletion of the ozone as the reaction between the
chlorine and the ozone has a lower activation energy; the formation of ozone cannot keep up with the
rate of depletion; (it must be appreciated that the rates are slow as the concentrations are low at that
height);

As the ozone is depleted (thinner layer = ‘hole’) by the radicals there are less ozone molecules to absorb
the harmful UV-B radiation;

Although nitrogen oxides are natural reagents in the depletion of ozone, human activity greatly increases
the concentration and therefore also speeds up the depletion using that reagent.
NO + O3  NO2 + O2
Alternatives to CFC’s: must have similar positive properties but without the production of Cl radicals
or any other radical than can cause it.
Advantages of these alternatives: reduced production of Cl free radical.
These alternatives should be/have: low boiling points, non-toxic, non-reactive, non-flammable and not acts
as greenhouse gases.
However, there are also some disadvantages using these alternatives:
1. Hydrocarbons: Use of propane and 2-methylpropane hydrocarbons as refrigerant coolants: no halogens
so do not release chloride ions!!! But they are greenhouse gases (i.e. absorb infrared radiation) + they
are more flammable than CFCs!!!;
2. Fluorocarbons (e.g. tetrafluoromethane) and hydrofluorocarbons: not toxic or flammable, strong C-F bond
makes them more stable to UV (also greenhouse gases).
3. Hydrochlorofluorocarbons: HCFC’s, the hydrogen in the molecule makes it a little more stable but still
causes some depletion as it still has C – Cl bond but bond is relatively stronger than in normal CFC.
But…….also greenhouse gas.
E 5. Dissolved oxygen in water
E 5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen- demanding wastes in water.
E 5.2 Distinguish between aerobic and anaerobic decomposition of organic material in water.
E 5.3 Describe the process of eutrophication and its effects.
E 5.4 Describe the source and effects of thermal pollution in water.
Importance of dissolved oxygen in water
Dissolved oxygen is necessary for the aerobic respiration of aquatic plants and animals. Fish need on
average 3 ppm or 0.003 g dm-3 or 0.003% to survive although to have a very large biodiversity 6 ppm is
needed.
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Solubility of oxygen at 1 atm and 20 C is 9 ppm. Solubility of any gas decreases as temperature rises.
Biochemical oxygen demand (BOD) as a measure of the amount of oxygen-demanding wastes in
water as a result it is also a measure of the purity of a water sample.
The quality of a water supply depends on the amount of oxygen-demanding wastes it contains. The amount
of oxygen-demanding wastes can be determined by measuring the BOD of the water supply. Impure water
has more oxygen-demanding waste and therefore a greater BOD than pure water.
The BOD = the amount of oxygen (in ppm) used up by aerobic bacteria to decompose the organic matter in a
fixed volume of water over a period of usually 5 days at 20 C.
Any organic waste (e.g. faeces, dead organisms, urea, sewage, wastes from food producing plants and so
on) and ammonia that is added to water is initially decomposed by aerobic bacteria, who oxidise the waste
into water, carbon dioxide (from the carbon in the waste) and inorganic ions like nitrates (from the nitrogen in
the waste), nitrites, phosphates (from the P in the waste), sulphates, hydrogen carbonates and sulphates, in
order to obtain energy.
This decomposition of the organic waste uses up the oxygen which is dissolved in the water; so the more
organic matter a water sample contains the less oxygen will be dissolved in it.
This amount of oxygen that is used up for this purpose can be measured using the following procedure:
1. water sample is collected and added to oxygen saturated water (initial concentration of DO –
dissolved oxygen - is known);
2. water is left for 5 days at 20 C.
3. lower oxygen concentration is measured using an oxygen electrode or titration involving MnO 2.
4. the difference between the original and final oxygen concentration is the BOD.
The greater the BOD, the greater the amount of oxygen-demanding waste there was in the water.
Almost pure water has a BOD of less than 1ppm; anything higher than 5 ppm is considered as polluted
water.
Eutrophication
Under normal natural conditions there is little natural organic waste in rivers and lakes and there will be
sufficient oxygen dissolved in the water to decay any organic waste.
However, the inorganic ions formed as a result of the decomposition of the organic matter are nutrients to
some aquatic organisms like algae, when they grow fast we call it an algal blooms or ‘red tides’. Under
normal conditions the amount of nutrients flowing into lakes increases very slowly - taking thousands of
years; the process by which a lake becomes richer in nutrients is called natural eutrophication.
If the organic matter (=oxygen-demanding waste e.g. untreated sewerage) is great the situation could arise
where there is not enough oxygen in the water to allow the aerobic bacteria to complete the decomposition of
the organic waste.
In that case the decomposition of organic matter is taken over by anaerobic bacteria which oxidise the waste
products using other oxidising agents and producing different products, usually foul-smelling, as shown in the
table below.
Element
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Aerobic respiration
Anaerobic respiration
9
C
N
S
P
CO2
NO3SO42PO43-
CH4
NH3
H2S
PH3
x
The initial reduction in oxygen caused by the algae bloom also causes fish and other aquatic organisms to
die which increases the amount of dead organic matter further increasing the oxygen-demanding waste even
more
The algae bloom referred to earlier usually occurs as the result of a rapid eutrophication (this time a fast
increase in the amount of nutrients in the river or lake) which can be caused by:
 large concentrations of nitrates (from fertilizers) and phosphates (from detergents) in the water;
 large amounts of organic waste e.g. from sewerage, meat processing, food packing, ..
These chemicals are excellent nutrients for algae which multiply and grow rapidly in a short period of time
after which they all die as they consume too much oxygen; when they all die they dramatically increase the
amount of organic waste.
Eutrophication is the result of:
 rapid growth of algae (caused by excessive use of e.g. fertilizers or dumping of sewage)
 reduces amount of dissolved oxygen
 aerobic decomposition when these algae die.
Effect of heat on dissolved oxygen and metabolism
The higher the temperature, the lower the solubility of gases. There is an equilibrium between the dissolved
and undissolved oxygen in the air; the dissolving of oxygen is exothermic; increasing the temperature of the
water invokes Le Chateliers Principle causing the equilibrium to shift towards the undissolved oxygen.
The problem of a reduced amount of dissolved oxygen in warm water is made even worse as the higher
temperature also increases the rate of metabolism of aquatic animals causing them to need more oxygen of
which less is available to them. In addition some species are intolerant to higher temperatures although
algae seem to thrive on higher temperatures.
E 6. Water treatment
E 6.1 List the primary pollutants found in waste water and identify their sources.
E 6.2 Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is
removed during each stage.
E 6.3 Evaluate the process to obtain fresh water from sea water using multi- stage distillation and reverse osmosis.
Reasons for treating water: high demand for water + plenty of pathogens





water is an excellent solvent (you should be able to explain why: highly polar solvent, capable of
hydrogen bonding !!!!) and therefore any water used for drinking purposes will very likely contain many
solutes which could be harmful to humans; solutes like heavy metals, nitrates and carcinogenic organic
compounds. Most of these solutes end up in the water as a result of human activity;
water also forms the habitat to many micro-organisms some of which could be pathogenic ( a lot of
pathogens enter the water via human faces) and need to be killed;
a lot of water also contains suspended particles (makes it look cloudy) as it is a moving medium!
to remove unpleasant odours and colour;
to reduce BOD.
Primary pollutants in water
pollutants
pesticides
dioxins
sources
agriculture
formed as part of large-scale processes like combustion of waste (waste incineration),
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PCB’s:
polychlorinated
biphenyls;
organic matter
nitrates and
phosphates
manufacture of herbicides and paper pulp bleaching with chlorine; extremely toxic.
 electrical insulators, circuit breakers, transformers, capacitors
 overexposure result in a disease called chloracne which is a skin condition which
produces cysts containing a straw coloured liquid; other symptoms are loss of
appetite, nausea, vomiting and weakness
 most PCB’s have low toxicity for humans
Sewage, agricultural run off e.g. cleaning out of stables, cow sheds, food industry
nitrates
Over-use of fertilisers results in leaching of nitrates into rivers/runoff of fertilizers, animal and human waste.
phosphates
cadmium
heavy metals
lead
mercury
Phosphate-containing detergents
 by-product of the extraction of zinc and lead; cadmium is always
found near zinc
 used in pigments in ceramics;
 industrial discharge from the following processes:
 zinc-plating always contains some cadmium;
 rechargeable batteries
 lead paints
 lead based solder in water pipes and fittings;
 car exhausts;
 car batteries
 mercury cell e.g. used in the electrolysis of brine ; greatest
dischargers;
 seed dressing; reduces mould formation;
 batteries;
 mercury salts used as fungicides
Primary, secondary and tertiary treatment of waste water
The purpose of waste water treatment is to produce an effluent (=treated water) that can safely be
discharged into a river system.
Waste water (can) contain:
solids
soluble
insoluble
liquids
soluble/miscible
insoluble/immiscible
organic compounds, phosphates, nitrates, heavy metals,
organic matter
* floating solids;
* suspended solid:
* settleable solids
* non-settleable solids (e.g. organic waste)
alcohols
oils, lubricants
micro-organisms
Primary treatment: removal of the floating objects and most of the suspended solids (colloids)
Processes involved (physical processes):
*
filtration: removes the solid floating objects by using screens or grids or sand beds; also immiscible
liquids like oil and grease are removed;
*
flocculation: involves the addition of alum (Al2(SO4)3 which contains aluminium ions (flocculating agent)
which causes coagulation (= coming together) of the non-settleable (do not settle at the bottom)
suspended solids; the Al3+ ions (also Ca2+ can be used) react with alkali substances to produce an
Al(OH)3 precipitate which traps the suspended solid particles and causes them to settle at the bottom.
IB option E SL notes
11
*
sedimentation: settleable solids sink to the bottom when the water is kept stationary in the sedimentation
tank; the sludge (all the sediment) is removed daily as anaerobic decomposition starts occurring.
The primary treatment removes approx 50 % of suspended solids as primary sludge but does not remove the
finer suspended organic particles or any of the dissolved particles.
Effectiveness of primary treatment:

removes: insoluble solids/liquids and suspended particles e.g. sand approx 50% of solid removed and
some oxygen-demanding wastes.

water still contains:
 dissolved and some suspended organic matter (large BOD);
 micro-organisms
 dissolved inorganic ions and heavy metals.
Secondary treatment: use of oxygen and bacteria to remove oxygen demanding wastes
Processes involved (biological processes):
Activated sludge: involves the agitation of the water with bacteria in a reactor.
Reduces the BOD greatly by removing a lot of mainly suspended organic waste by using natural biological
processes; respiration by bacteria which oxidise the organic waste into water, carbon dioxide, nitrates and
phosphates (remember that the last two cause eutrophication and should really be removed before the water
is released in the river).
The waste water is added into a settling tank in which the bacteria act; this tank is constantly aerated with
oxygen and this causes flocs to form; a floc is a large porous clump (=bubble of air) which contains the
bacteria and around which the suspended organic waste adsorbs and coagulates. The bacteria decompose
the waste and turn it in into a gelatinous slime (=sludge), which sinks to the bottom of another sedimentation
tank, and other products like water, carbon dioxide, nitrates and phosphates.
As the slime contains also the bacteria, most of the sludge is recycled and pumped back into the aeration
tank; pollutants in the incoming sewage are adsorbed onto the recycled sludge flocs which bubble up as the
tank is aerated. Occasionally excess sludge is removed and can be sued as fertilizer.
The aeration also provides enough oxygen for the bacterial decomposition.
There are two main reasons why the sludge is recycled:
*
*
to maintain the bacteria populations as they are still active and can be recycled;
addition of the sludge to the new incoming waste water makes the adsorbance of the incoming pollutants
onto the flocs more efficient as the sludge makes the formation of flocs more efficient.
Spraying of wastewater over a bed of gravel or sand/trickle filter.
Rotating pipes sprinkle the water over a bed of stones onto which bacteria and algae are allowed to grow;
these organisms consume the organic molecules as well as some nitrates
Effectiveness:


removes: 90% of organic waste is removed reducing BOD;
water still contains: some dissolved organic compounds (only traces are they are only very slightly
soluble) and inorganic ions like nitrates, phosphates and heavy metal ions;
Pathogens are removed using chlorine or ozone before water is released in river or lakes.
Tertiary treatment: to remove soluble particles: heavy metal ions, nitrates and phosphates
IB option E SL notes
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Processes (more chemical processes but also biological): one or more of the following are used:

activated carbon-bed (chemical): activated carbon can easily adsorb organic waste, dioxins and PCBs
oxidize them; it is activated by heating it to high temperatures.

denitrifying bacteria (biological): these bacteria change nitrates back into nitrogen.

chemical precipitation: toxic heavy metal ions like cadmium, lead and mercury and phosphate ions can
be precipitated by adding ions which form insoluble compounds with the heavy metal or phosphate ions

to precipitate phosphates, aluminium or calcium ions are added which form insoluble phosphates that
precipitate out:
3Ca2+ (aq) + 2 PO43- (aq)  Ca3(PO4)2 (s)

to precipitate out heavy metal ions, hydrogen sulphide gas can be added; the heavy metal ions form
sulphide salts with very low solubilities in water and precipitate:
e.g. Pb2+ (aq) + H2S (g)  PbS (s) + 2H+ (aq)

reverse osmosis and ion exchange (chemical methods) also work for nitrates.

alkaline treatment for nitrates.
There is an increased need for tertiary treatment of water as more nitrates, phosphates and ammonium ions
end up in the waste water (greater use of artificial fertilizers and detergents) which can eutrophy river and
lake water.
Ways to obtain fresh water from sea water
Multi-stage distillation
Sea water is heated in a series of coiled pipes and then introduced into a chamber at lower pressure. Under
this lower pressure some of the sea water boils immediately. The water vapour is condensed by contact with
cold water pipes which carry the sea water. The condensed water now is free of impurities. In this way the
heat released during the condensation is used to preheat some of the water.
The remaining sea water that has not boiled yet is directed to another set of chambers each with a lower
pressure than the preceding chamber. This method maximises the use of energy.
Reverse osmosis
Osmosis = movement of water from dilute to concentrated solution.
Reverse osmosis is a process which goes against water’s natural movement from a high concentration of
water to a low concentration; this natural movement can be stopped by using a high pressure.
A high pressure (greater then osmotic pressure i.e. greater than 70 atm) is applied to sea water (low
concentration of water) side of a partially permeable membrane (only allows water through and no ions or
solute particles). The pure water from the sea water is forced out of the solution through the membrane
leaving the salt behind (salt ions cannot pas through) and into pure water (high concentration of water).
E 7. Soil.
E 7.1.Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation.
E 7.2 Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical
and biological functions
E 7.3 List common organic soil pollutants and their sources.
IB option E SL notes
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Salinization
Salinization is the result of irrigating soils i.e. watering them. Water used for irrigation usually contains
dissolved salts, which are left behind in the soil after the water evaporates.
In poorly drained soils, the salts left behind are not washed away and begin to accumulate in the topsoil.
Plants cannot grow in soil that is too salty.
Another problem caused by irrigation is that over irrigation/water run off can remove vital nutrients such as
Ca2+, K+ , NH4+ and Mg2+ from the topsoil.
Nutrient depletion
Nutrients are substances that plants need to grow. Plants remove these nutrients from the soil; harvesting
crops removes these nutrients permanently.
If the soil is not allowed to recover i.e. allowed time for the removed nutrients to be replaced through natural
processes the soil becomes depleted in nutrients and this will reduce future productivity. Nutrients can also
be replaced by using artificial fertilizers or compost.
Monoculture i.e. growing the same crop time after time increases nutrient depletion. Allowing soils to remain
fallow (no crops are grown and nutrients can be replaced) for a while or use crop rotation i.e. grow different
crops which absorb different nutrients are some solutions.
Soil pollution
This is the consequence of the use of chemicals such as pesticides and fertilizers. These chemicals can
disrupt the soil food web, reduces the soil’s biodiversity and ultimately ruin the soil. The chemicals also run
off the soil into surface waters and move through the soil, polluting groundwater.
Soil organic matter (SOM)
SOM includes:
 mainly decayed plant and animal matter which can be:
o
high-molecular-mass organic materials such as polysaccharides and proteins and
o
simpler substances such as sugars, amino acids and other small molecules
 smaller amounts of humus: organic material fully broken down by the micro-organisms in the soil.
SOM prevents soil degradation because of the following functions that can be broadly classified into two
groups:
 biological: provides nutrients for the plants in particular N as it contains amines and amino acids;
 physical:
o improves structural stability: reduces soil density, allows more air and water in spaces
o influences water-retention properties: the polar OH and NH2 groups on the SOM molecules
allow hydrogen bonding between these molecules and water molecules in the soil
o alters the soil thermal properties i.e. allows sufficient cooling but also maintains heat
Common organic soil pollutants
Pollutants which can harm plants, contaminate groundwater or contribute to air pollution when they escape
such as the volatile compounds.
Many organic compounds end up in soil through direct spills, leaks (e.g. from landfills, underground storage
tanks), dumping or washed from the air through rain.
Most organic pollutants are petroleum hydrocarbons which are compounds which originally come from
crude oil, coal gas, tar, natural gas; either directly through distillation or manufactured.
These petroleum hydrocarbons are:
 volatile organic compounds (VOCs) such as shorter alkanes, petrol, benzene, toluene, xylenes, …or
IB option E SL notes
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
semi-volatile organic compounds (SVOCs) which are organic compounds with boiling temperatures
higher than water but which evaporate at temperatures above room temperature; examples are phenol,
naphthalene, plasticizers, many PAHs and PCBs.
Because these compounds are volatile they end up in the air from where they are washed into the soil.
Pollutants
agrichemicals
organic solvents
polynuclear aromatic
hydrocarbons (PAHs)
polychlorinated biphenyls
(PCBs),
organotin compounds
dioxins
Sources
pesticides (insecticides, fungicides, herbicides), fertilizers, growth hormones
through direct application which is fine if controlled; problem is spills and
overuse.
 spills/leaks/dumping of dry cleaning solvents, paint thinners, nail polish
remover, detergents, …
 many are also VOCs such as dichloromethane, trichloroethane, and
trichloroethylene
 chemicals which do not degrade easily
 synthetic PAHs banned in 1975
 chemicals common in coal tar/creosote which have been used to
preserve wood in buildings, fences, ships.
 PAHs are present in coal and crude oil and are released during
combustion e.g. industrial/power station emissions
 stable compounds, can function as fire retardants and plasticizers
 transformers, circuit breakers, electromagnets, hydraulic oils, paints
 bioaccumulate
 compounds which contain at least one bond between tin and a carbon
part of a hydrocarbon chain
 used in manufacture of:
o antifouling compounds (coatings or paints)
o fungicides
o pesticides / biocidal agents
 incineration plants, wood burning, traffic
 spread trough the air and settle on soil
E 8. Waste
E 8.1 Outline and compare the various methods for waste disposal.
E 8.2 Describe the recycling of metal, glass, plastic and paper products, and outline its benefits.
E 8.3 Describe the characteristics and sources of different types of radioactive waste.
E 8.4 Compare the storage and disposal methods for different types of radioactive waste.
Methods of waste disposal
Method
Landfills: Waste
is buried in the
ground
Incineration:
burning of waste







Recycling:
IB option E SL notes




Advantage
rotting material produces methane gas
which cold be collected
filled ground can be reused
low cost
can deal with large volumes
Waste could be used as fuel –source of
energy
Requires little land
Reduces volume as most of waste is
converted into gases
Disadvantage
 Methane gas could escape i.e.
greenhouse gas
 Pollution of ground water
 Adds to greenhouse effect as CO2
produced
 Can produce toxins such as dioxin
 Requires energy as this need to
be done at high temperatures to
destroy most toxic substances
Reduces energy cost
Reduces CO2 emissions
Reduction in volume of waste
Saves Earth’s metal ores reserves
15
 Creates jobs in collecting, sorting and
recycling
Recycling
Material
glass
plastic
paper
metals
Method
 Different glass types must be separated
 Glass is then crushed
 Other substances are removed such as metals, labels
 Glass is melted and remoulded
Difficult to recycle as there are so many different plastics each needed a different type of
recycling. Main stages:
 After collection, manually sorting of different plastics
 Washing of plastic
 Most plastics melted and then moulded into new shape;
 Other first shredded into flakes, then molten and processed into granules or pellets
Future = biodegradable plastics which are decomposed by aerobic bacteria.
 Water is added to waste paper to make ‘pulp’ and separate fibres
 Pulp is centrifuged to remove denser components such as clay, dirt, metals, ..
 Air is pumped through it to remove ink
 Pulp is compressed to make new paper
 Collecting, sorting and recycling
 Separation in the case of alloys
Radioactive waste
Low level waste
characteristics
sources
short half-life, low activity and high volume (approx 90%)
materials such as gloves, coats, paper, tools, towels, syringes, injection needles
which have become radioactive because they have been exposed to radiation in
activities such as sterilizing equipment in hospitals, manufacture of smoke detectors,
irradiating of food, radiotherapy, …
storage/disposal
Different methods:
 stored on site e.g. in steel containers, until radioactive level is at safe level and
then it is disposed off as normal waste buried in land fill sites
 incinerated
Intermediate level waste
characteristics
sources
storage/disposal
releases some heat
It typically comprises resins, chemical sludges and reactor components, as well as
contaminated materials from reactor decommissioning.
waste is first sealed in concrete and then in steel drums which are placed in concrete
casks in concrete trenches. When a trench is completely filled it is covered with a
concrete slab, a layer of compacted clay and a reinforced concrete intrusion shield
and a final layer of clay.
High level waste
characteristics
sources
IB option E SL notes





long half-life, high activity and low volume
very hot
used nuclear fuels from nuclear reactors,
waste products from processing used fuel
nuclear weapons
16
storage/disposal
issues
is vitrified (made into glass) which is sealed in steel containers and then buried
underground in granite rock or in deep mines;
 may still leak into water table;
 remains active for a very long time;
 geological instability (e.g. earthquakes);
 potential weapon for terrorists;
Half-life = the amount of time it takes for a radioisotope to lose half its radioactivity.
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