chemistry and the environment

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Chapter 46
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New Way Chemistry for Hong Kong A-Level Book 4
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46.1 Introduction (SB p.220)
Pollution is the introduction of waste matter or energy into the environment by humans. It causes damages or deterioration to living systems and/or the environment
A pollutant refers to any substance or energy produced through human activities which, at an unacceptable level, causes damages to the environment or is harmful to living systems
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46.2 Air Pollution (SB p.221)
Major air pollutants in the modern city are CO(g), SO2(g), NOx(g) and small particulates (e.g. soot and lead)
Produced by burning fossil fuels in car engines, industrial plants and power plants
Major Air Pollutants and their Sources
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produced by incomplete combustion of petrol, coal, and wood
e.g. C8H18(g) + 9O2(g)
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2. Sulphur dioxide
produced by combustion of sulphur-containing fuels
large proportion of SO2 are generated at power stations, metal smelters in which sulphur-containing fuels are used
Power station
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3. Nitrogen oxide
produced when fuels are burnt at high temperatures
the level of NOx is noticeably high because of high-proportions of diesel-driven vehicles running
diesel engines produce less CO but a lot more NOx
4. Hydrocarbons
formed by evaporation of organic materials, e.g. petrol, diesel, etc
found in car exhaust as unburnt hydrocarbons
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5. Ozone
produced by the reaction of oxygen gas with free oxygen atoms which are formed from the reaction between NOx and hydrocarbons in sunlight
O2(g) + O • O3(g)
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6. Particulates
include black smoke (fine carbon particles), dust and soot (coarse carbon particles), so small that remain suspend in air for a long period of time
produced during operations in incinerators, factories, diesel vehicles, construction sites and coal or charcoal burners
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Source of CO
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Source of SO2
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Source of NOx
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Source of particulates
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High levels of pollutants can cause headaches, nausea, irritation of the eyes, nose and throat and ill-feeling generally
Some pollutants may lead to respiratory diseases
Effects of Polluted Air on the Environment
Some plants like lichens will not grow in cities because of high levels of pollutants as they are more sensitive to O3 and SO2 than humans.
Lichens cannot grow in seriously polluted air
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1. Carbon monoxide
poisonous, colourless, tasteless and odourless gas
when one inhales it, it will enter the blood stream and bind preferentially to haemoglobin in the red blood cells replacing O2
reduction of O2 supply to organs and tissues
causes death even in relatively low concentrations
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2. Sulphur dioxide
colourless, toxic gas
form acid rain
damage buildings and forests
The plant on the left (from Germany) is damaged by acid rain
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3. Nitrogen dioxide
irritate lungs, lower the resistance to respiratory infections
gives rise to photochemical smog
impair lung development in young children
form acid rain
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4. Hydrocarbons
5. Ozone
irritate respiratory system causing cough, chest pain and throat and eyes irritation and increase susceptibility to respiratory infections
damage plants
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6. Particulates
environmental impact depends on size and nature
very fine particles (diameter less than 1 m) are the most hazardous
cannot be filtered by hairs and mucus in the nose and the respiratory track.
drawn deep into the lungs
impair lung functions, hinder the gas exchange, damage the lung tissue and cause respiratory illness
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The sources and effects of some air pollutants
Air pollutant
Incomplete combustion of carbon-containing fuels
Reduce the amount of oxygen delivered to body organs and tissues, and can cause death
Sulphur dioxide
Nitrogen oxides
From motor car exhaust, electric power stations, incinerators and industrial plants
Irritate the lungs and lower the resistance to respiratory infections, and cause the formation of photochemical smog and acid rain
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The sources and effects of some air pollutants (cont’d)
Air pollutant
Cause photochemical smog; some are carcinogenic
Ozone
Particulates
From refuse incineration, factories, diesel vehicles, construction sites and coal or charcoal burners
Impair the lung function by hindering gas exchange and damaging lung tissues, and cause respiratory illness
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The harmful effects of pollutants depend on:
1. Concentration &
2. The duration of exposure
Take CO as an example:
If the concentration of CO is 600 – 700 ppm, inhaling it for an hour brings about barely detectable effects
The concentration of CO of 4000 ppm or higher can cause death in less than one hour
If the concentration of CO is 1000 ppm, the people will die from CO poisoning after inhaling for 4 hours
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Parts per million (ppm) is one way to describe the concentration of pollutants
The number of molecules of pollutants per million (106) molecules in air. (It is a ratio with no unit)
It is equal to the number of volumes of pollutants per million volumes of air based on the Avogadro’s law.
Representation of Concentration of Pollutants
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e.g. 50 ppm of CO means 50 cm3 of pollutant for every
1 000 000 cm3 of air
If the concentration is represented by percentage by volume, the unit is readily converted to ppm by multiplying with a factor of 10 000
e.g. 0.1 % by volume = 1 000 ppm
The concentrations of pollutants are commonly expressed in g m–3 nowadays
Using ideal gas equation, PV = nRT, g m–3 and ppm can be interconverted
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Example 46-1
Express the concentration of 0.05 ppm carbon monoxide measured at 298 K (25°C) and 101 kPa (1 atm) in g m–3.
(R.a.m. : C = 12.0, O = 16.0;
ideal gas constant = 8.314 J K–1 mol–1)
Answer
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Solution:
In 1 m3 of air, there is m3 carbon monoxide.
Using the ideal gas equation,
PV = nRT
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Solution: (cont’d)
Mass of CO
∴ The concentration of carbon monoxide is 57.1 g m–3
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(a) (i) State the major air pollutants.
(ii) For each kind of pollutants, state one of the harmful effects.
Answer
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Air pollutant
Harmful effect
Carbon monoxide
Reduce the amount of oxygen delivered to body organs and tissues, and can also cause death
Sulphur dioxide
Nitrogen oxides
Irritate the lungs and lower the resistance to respiratory infections, and cause the formation of photochemical smog and acid rain
Hydrocarbons
Cause the formation of photochemical smog and some are carcinogenic
Ozone
Particulates
Impair the lung function by constricting respiratory passage and damaging lung tissues, and cause respiratory illnesses
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Check Point 46-1 (cont’d)
(b) Given that the temperature is 0°C and the pressure is
1 atm (i.e. 101 kPa), express the following concentrations of the gases in the units given.
(i) Express 10 ppm NO2 in % by volume;
(Ideal gas constant = 8.314 J K–1 mol–1; R.a.m.: N = 14.0,
S = 32.0, O = 16.0, C = 12.0)
Answer
= 0.001%
of NO2 by volume.
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(ii) Express 0.3 ppm SO2 in g m–3;
S = 32.0, O = 16.0, C = 12.0)
Answer
(b) (ii) In 1 m3 of air mass of SO2
= 854.4 g
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(iii) Express 10 000 g m–3 CO in ppm;
S = 32.0, O = 16.0, C = 12.0)
Answer
(b) (iii) Let the volume of CO in 1 m3 of air be V.
V = 8.03 10–6 m3
=
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Check Point 46-1
(iv) Express 600 g m–3 SO2 in % by volume.
S = 32.0, O = 16.0, C = 12.0)
Answer
(b) (iv) Let the volume of SO2 in 1 m3 of air be V.
V = 2.11 10–7 m3
% by volume =
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Unpolluted rainwater is slightly acidic, with pH value of about 5.7
CO2(g) + H2O(l) H2CO3(aq) pH 5.7
Rainwater reacts with CO2 to form carbonic acid
Acid Rain
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Rainwater with pH values lower than 5.7 is called acid rain
Acidic gaseous pollutants : SO2 and NO2
In the atmosphere, SO2 is oxidized to SO3 slowly and then dissolves in rainwater readily
2SO2(g) + O2(g) 2SO3(g)
SO3(g) + H2O(l) H2SO4(aq)
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SO2(g) also dissolves in rainwater to form sulphuric(IV) acid
2H2SO3(aq) + O2(g) 2H2SO4(aq)
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NO(g) is formed when burning fossil fuels in automobiles and power stations.
The NO(g) is then oxidized to NO2(g)
2NO(g) + O2(g) 2NO2(g)
In a series of complex reactions, NO2(g) is oxidized to HNO3(aq)
4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)
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Acid rain causes a lot of harmful effects on the environment
In water of pH lower than 4.5, calcium metabolism in freshwater fish will be affected, leading to poor health and stunted growth
diversity and population of some freshwater species will be reduced
In soil of pH lower than 4.5, absorption
of essential nutrients (e.g. K+, Ca2+
and Mg2+) will be affected
results in the death of plants
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Acid rain also corrodes metals and accelerates the rate of deterioration of buildings, rocks and statues
The statue was corroded by acid rain
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A summary of the formation and effects of acid rain
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Photochemical smog is a mixture of pollutants including particulates, NOx, aldehydes, peroxyacetyl nitrate (PAN) and unreacted hydrocarbons, etc
Brownish haze that irritates our eyes is an indicator of photochemical smog
(Note: NO2 is responsible for the brown colour)
Photochemical Smog
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Appearance of a city:
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The formation of photochemical smog is initiated by sunlight and involves hydrocarbons and nitrogen oxides
The free O reacts with unburnt hydrocarbons, O2 and water vapour in a series of complex reaction to produce a variety of lachrymatory and toxic chemicals such as peroxyacetyl nitrate (PAN)
NO2 from automobile exhaust absorbs sunlight and breaks down into NO and free O
NO2(g) NO(g) + O(g)
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Principal reactions involved in the formation of photochemical smog (much simplified)
O(g) + O2(g) O3(g)
O(g) + hydrocarbons aldehydes
O3(g) + hydrocarbons aldehydes
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Formation of photochemical smog
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The reactions of the formation of photochemical smog vary with altitude
wavelength of ultraviolet radiation from the sun determines which bonds are broken in the reaction
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Photochemical smog causes headaches, eye, nose and throat irritation, worsening the lung function, coughing and wheezing
Ozone attacks the C = C linkage in rubber in a process known as ozonolysis. It cause rubbers and fabrics to deteriorate and car tyres to crack
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(a) Describe the chemical reactions involved in the formation of
(i) acid rain;
(a) (i) Acidic gaseous pollutants like sulphur dioxide and
nitrogen dioxide are responsible for the formation of acid rain.
In the atmosphere, sulphur dioxide is slowly oxidized to sulphur trioxide which dissolves readily in water droplets to form sulphuric(VI) acid.
2SO2(g) + O2(g) 2SO3(g)
When nitrogen monoxide is released to the atmosphere, it combines with atmospheric oxygen to form nitrogen dioxide.
2NO(g) + O2(g) 2NO2(g)
In a series of complex reactions, nitrogen dioxide combines with
oxygen and water vapour to form nitric(V) acid
4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)
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(a) Describe the chemical reactions involved in the formation of
(ii) photochemical smog.
(a) (ii) Photochemical smog is a mixture of pollutants including particulates, nitrogen oxides, ozone, aldehydes, peroxyacetyl nitrate (PAN) and unreacted hydrocarbons.
Nitrogen dioxide from automobile exhaust first absorbs sunlight and breaks down into nitrogen monoxide and free oxygen atom which is very reactive.
NO2(g) NO(g) + O(g)
The oxygen atom then reacts with other components of automobile exhaust (e.g. unburnt hydrocarbons) and those of the atmosphere (e.g. oxygen and water vapour) in a series of complex reactions to produce a variety of
lachrymatory substances such as peroxyacetyl nitrate (PAN).
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Very much simplified, some of the principal reactions in the
formation of photochemical smog are as follows:
O(g) + O2(g) O3(g)
O(g) + hydrocarbons aldehydes
O3(g) + hydrocarbons aldehydes
Hydrocarbons + O2(g) + NO2(g)
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(b) Iron bars and marble stones (CaCO3) are two common building materials. Describe how sulphuric (VI) acid in acid rain affects these materials. Explain with the help of equations.
Answer
(b) Iron bars are corroded by sulphuric(VI) acid as shown in the following equations.
Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g)
CaCO3(s) + H2SO4(aq) CaSO4(aq) + CO2(g) + H2O(l)
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(chlorine-like) smell
harmless in low concentrations
cause respiratory problems and headaches when concentration rises above 100 ppm
The Ozone layer and Chlorofluorocarbons
Ozone
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In troposphere, O3 can be formed by:
1. reaction between NOx and hydrocarbons in sunlight
2. electric sparks in car engines and electrical appliances
3. lightning
4. passing a stream of O2 through an electric discharge
O3 is very reactive, and does not stay in the atmosphere for a long time
Sources and Properties
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O3 can be generated (a) by passing a stream of O2 through an electric discharge or (b) by lightning
(a)
(b)
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In stratosphere,
O3 is formed from atmospheric O2 by absorption of UV radiation of the right energy (wavelength 250 nm)
dissociates O2 molecules into free O
free O reacts with other O2 to form O3
UV light
O2(g) 2O(g)
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Formation of ozone
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When O3 absorbs UV radiation of wavelength 215 – 295 nm, it undergoes photodissociation and splits up again
O3(g) O(g) + O2(g)
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This reaction is responsible for vital screening effect of ozone
It screens out harmful UV radiation
O3 is constantly created and destroyed in the stratosphere and its concentration remains constant
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The presence of O3 in the stratosphere is very important for living organisms on the Earth
Although the concentration is very low, it filters out 99% harmful UV radiation
Thinning of the O3 layer may result in increase in amount of UV radiation reaching the Earth’s surface
leads to an increase in the incidence of skin cancer, genetic mutation, cataract and a decrease in crop yield
Desirability of Ozone in the Stratosphere
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Answer
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(b) Explain how the concentration of ozone is kept constant in the stratosphere.
Answer
(b) In the stratosphere, ozone is formed from atmospheric oxygen by absorption of ultraviolet radiation of the right energy (wavelength 250 nm), which dissociates oxygen molecules into free oxygen atoms.
O2(g) 2O(g)
The oxygen atoms produced react with other oxygen molecules to form ozone.
O(g) + O2(g) O3(g)
On the other hand, when ozone absorbs ultraviolet radiation of wavelength 215 – 295 nm, it undergoes photodissociation and splits up again.
O3(g) O2(g) + O(g)
destroyed, its concentration remains constant.
UV light
UV light
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Chlorofluorocarbons (CFCs) are a family of compounds containing chlorine, fluorine and carbon
They are unreactive, low flammability and low toxicity
Due to its properties, CFCs have a wide range of uses
Chlorofluorocarbons
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1. As aerosol propellants
CFCs such as CCl3F and CCl2F2 used as propellants
Upon depressurization when the valve is opened, the propellant vaporizes and expels the materials in the form of aerosol spray
Uses of CFCs
An aerosol canister
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• odourless
• non-flammable
• stable
• non-toxic
Commonly found aerosol canisters in daily life
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2. As solvents in cleaning electronic components and metals
CFCs such as CCl2FCClF2 can dissolve grease and are widely used as solvents in cleaning electronic components and metals
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3. As refrigerants
Freon is a series of CFCs (CCl2F2 may be the most important one)
Freon absorbs heat of vaporization on evaporation, leading to cooling of the surroundings
Widely use as refrigerants in refrigerators and air-conditioning units
Advantages of using CFCs:
1. same thermal properties as traditional refrigerants such as SO2 and NH3
2. non-toxic
3. non-corrosive
CFC refrigerant
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4. Blowing agents in foam plastic manufacturing
Volatile CFCs such as CCl3F are incorporated into the plastics when making foam plastics
The heat evolved during the polymerization reaction vaporizes the CFC which then forms thing bubbles in the plastics
Use of foam plastics:
Foam plastic products
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Some of the CFCs escape into the atmosphere during use
More CFCs enter the environment when refrigerators and air conditioning units are scrapped or when foam plastics are disposed of
CFCs are unreactive and non-biodegradable that they break down in atmosphere very slowly
CFCs are relatively insoluble in water, therefore they cannot be removed by either rainfall or dissolution into the ocean.
Causes for the Accumulation of CFCs in the Stratosphere
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They have long lifetime (e.g. CCl3F about 75 yrs)
give plenty of time to travel up to stratosphere
Once in the stratosphere, the UV radiation from the sun is intense enough to dissociate CFCs molecules
giving Cl free radicals which destroy ozone dramatically through a series of free radical chain reaction
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Check Point 46-4
(a) Explain why an aerosol propellant should be a good solvent, chemically unreactive and has a low boiling point.
Answer
(a) It should be a good solvent as it has to dissolve the solute. It should be chemically unreactive so that it does not react with the solute or the substances it sprays on. It has a low boiling temperature so that the aerosol can vaporize very easily when pressure is released.
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Check Point 46-4
(b) Besides the properties mentioned in (a), what other properties should an aerosol liquid possess?
Answer
(b) It has to be odourless, non-flammable and non-toxic
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O3 in the stratosphere undergoes photodissociation to give O2 and O atoms by absorbing UV radiation
O3(g) O2(g) + O(g) ………………… (1)
The O atoms formed further react with other O3 molecules to give O2 molecules
O(g) + O3(g) 2O2(g) ………………… (2)
reduce the concentration of O3
O3 is formed at the same time. Their rates are somewhat equivalent
the concentration of O3 remains fairly constant
Depletion of the Ozone Layer
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During the past few decades, the concentration of O3 has fallen over the Antarctica and the Arctic
Evidence shows that O3 depletion is related to release of CFCs to the atmosphere
The destruction of O3 by CFCs is a complex chemical process
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The free Cl • from the above initiation steps readily react with O3 via the following reactions
Cl • (g) + O3(g) ClO • (g) + O2(g)………………… (5)
ClO • (g) + O3(g) Cl • (g) + 2O2(g) ……………… (6)
Take CCl3F and CCl2F2 as examples:
CCl3F(g) CCl2F • (g) + Cl • (g) …………………… (3)
CCl2F2(g) CClF2 • (g) + Cl • (g)………………….(4)
UV light
UV light
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The depletion of ozone by (5) is faster then that by (2)
The worst issue is that the Cl • consumed in the (1) is regenerated in (2)
1 Cl • can destroy many O3 molecules effectively in series of chain reactions
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Since 1975, scientists have detected a continuous and severe drop in the concentration of ozone in the layer over the Antarctica
In 1987, it was found that half of the Antarctica’s ozone had disappeared over a region twice of the size of the United States, creating a huge hole in the ozone layer
At the altitude of 18 km, the concentration of ozone drop as much as 50% of the normal value
The concentration of ozone decreased up to 3 % was observed in the Northern hemisphere
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The maps of the O3 layer over the Antarctica indicated by the Total Ozone Mapping Spectrometer (TOMS) from 1979 to 1992
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Ozone depletion increases the absorption of UV light by the stratosphere which will lead to
adverse consequences on aquatic and terrestrial ecosystems
reduced crop yield
Solution:
minimize the use of CFCs and develop suitable substitutes which do not affect the environment adversely
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In 1987, Montreal Protocol on Substances that Deplete the Ozone Layer was signed in Montreal, Canada.
The Protocol requires the phasing out of the Ozone Depleting Substances (ODS)
Control of the Ozone Depletion Problem
Ozone depleting substances
Chlorofluorocarbons (CFCs)
Tetrachloromethane
1,1,1-Trichloroethane
Hydrochlorofluorocarbons (HCFCs)
Freezing of consumption at base level starting 1st January 1996 Import for local consumption banned by 1st January 2003
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Consumption of ozone depleting substances in Hong Kong
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To fulfill the Hong Kong SAR’s international obligations, the Ozone Layer Protection Ordinance was enacted in July 1989 to control the use of ozone depleting susbstances
The Ordinance prohibits the manufacture of such substances and imposes controls on import and export of these substances through legislation and licensing provisions
Possible solutions:
Installation of recovery and recycling units to recycle CFCs from used refrigeration systems and motor vehicles air conditioners
Using substitutes for CFCs
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Compounds with low ozone depletion potential have been developed to replace CFCs, but they suffer from some disadvantages:
1. HCFCs and HFCs
The presence of H atom makes the compounds less stable and do not reach the stratosphere in high concentration
low % of Cl and low ozone depletion potential
damage the ozone layer if overused
HFCs do not contain Cl and thus are “ozone safe”, but the questions on their toxicity and safety are still unsolved
Possible Alternatives for Chlorofluorocarbons
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2. Butane and propane
not expensive and readily available
but they are flammable and poisonous
3. Water and steam
effective for cleaning applications in some occasions but the applications are rather limited
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Possible alternatives for CFCs and some of their properties
Compound
Toxicity
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Check Point 46-5
Give some methods for reducing the depletion of the ozone layer.
Answer
Firstly, the consumption of CFCs and other ozone depleting substances should be controlled. Secondly, recovery and recycling units to recycle CFCs used refrigeration system and motor vehicle air conditioners could be installed in order to reduce the amount of CFCs released to the atmosphere. Thirdly, substitutes for CFCs such as butane and propane could be used as they do not
contain chlorine, they are “ozone safe”
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46.3 Water Pollution (SB p.242)
Water pollution is the degradation of the quality of water by the introduction of chemical, physical or biological wastes into rivers, streams, lakes and oceans.
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1. Livestock waste
• When there is an excessive amount of organic matter is discharged into water, the existing micro-organisms will grow and multiply rapidly
oxygen is rapidly depleted
Due to lack of oxygen, anaerobic micro-organisms begin to partially break down the organic matter, releasing noxious gases like methane, ammonia and hydrogen sulphide
aquatic organisms are endangered
The Causes of Water Pollution and its Effects on the Environment
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The river is seriously polluted by livestock waste, causing the death of aquatic organisms
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2. Oil spillages
Oil clogs the feathers of sea birds and prevent them from flying or swimming
It also interferes with the insulation provided by the feathers, so affected birds may die of cold or pneumonia
When birds try to clean their feathers with their beaks, they may be poisoned through the ingestion of oil
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Oil deposited on beaches
may drive away holiday-
Oil spillages are difficult to clean up
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3. Residues of pesticides
Pesticides are synthetic organic compounds used to eliminate flying and crawling pests
When pesticides enter water, it cause immediate toxic effects on aquatic life
Pesticides are non-biodegradable and may accumulate along the food chain until the toxic level is reached
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(a) it resists degradation and remains in the
environment
(b) it is very soluble in fat, including the fat in
milk and the body fat of animals, with the potential for damaging internal organs
(c) it affects the reproduction of birds, fish, etc. and weakens the shells of eggs
(d) many species of insects can develop resistance to DDT
The use of DDT has been banned in many parts of the world including Hong Kong
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Synthetic detergents are the major pollutant present in sewage
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Discharge of detergents causes eutrophication (massive growth of algae in water bodies resulting in deaths of other plants and animals due to depletion of oxygen, space and food) brought about by phosphates found in some detergents
Eutrophication produces a mat of algae on the surface of water
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Ingestion of detergents by aquatic organisms can cause death
Eutrophication causes fish to suffocate and die
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5. Industrial effluents
Toxic materials present in industrial effluents vary with the types of industries involved
They may contain heavy metals (e.g. Cd, Cr, Cu, Hg, Ni, Pb, Zn), cyanides, polychlorinated hydrocarbons, etc.
Bioaccumulation of heavy metals is a process by which the heavy metals become more concentrated as they move up food chains
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One of the worst known case of toxicity is the bioaccumulation of mercury found in Minamata Bay, Japan, in 1950s.
It was caused by consumption of fish that has been contaminated by mercury let off from a chemical manufacturing plant
121 people is poisoned after eating contaminated fish, and 46 of them died
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The causes and effects of water pollution
Cause
Harmful effects
Livestock waste
Release noxious gases, and leads to the death of aquatic organisms
Oil spillages
Clog the feathers of sea birds and prevent them from flying or swimming Interfere with the insulation of the feathers of birds, and cause the death of birds from cold or pneumonia Poison birds by ingestion Prevent dissolution of oxygen, and cause the death of aquatic organisms Spoil recreational resorts
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The causes and effects of water pollution (cont’d)
Cause
Residues of pesticides
Accumulate along food chain Have the potential for damaging internal organs Exert harmful effects on reproduction of animals
Detergents in sewage
Cause eutrophication, and lead to the death of aquatic organisms
Industrial effluents
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Dissolved oxygen (DO) can be used as an indicator of oxygen content in water
Water Quality
Dissolved Oxygen
It is measured in mg of oxygen per dm3 of water or percentage saturation of dissolved oxygen
The maximum concentration of dissolved oxygen in water at 16°C is 10 mg dm–3
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Normally, fish require 4 – 6 mg dm–3 of oxygen for survival
A minimum DO is also needed for reducing the odour caused by anaerobic degradation of organic matter
If a water sample contains 6 mg dm–3 of dissolved O2 at 16°C, the percentage saturation of DO is:
The sample is said to be 60% saturated with oxygen
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DO in lakes and rivers is rapidly depleted if organic waste is released into the water
Sources of organic waste: human and livestock sewage and industrial waste from paper mills, tanneries and food processing plant
Organic waste in aquatic ecosystems is decomposed by aerobic decomposers, primarily bacteria and fungi
If the water is overloaded with organic waste, aerobic decomposers proliferate and DO will be consumed more rapidly than it can be replaced from the atmosphere
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If the level of DO falls below 5 mg dm–3, fish start to die
If the concentration of DO continues to fall, other marine animals and aerobic bacteria will die too
When the DO is completely consumed, decomposition of organic matter will be taken over by anaerobic bacteria
The water begins to smell unpleasant because different pathways of decomposition are proceeding
Many of the end produces have displeasing odours
e.g. H2S, NH3, amine and phosphorous compounds
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Quality standards of water for different uses
Water use
Public water supply Water contact recreation Fish propagation and wildlife Industrial water supply Agricultural water supply Shellfish harvesting
4.0 4.0 – 5.0 4.0 – 6.0 (depending on fish species) 3.0 – 5.0 (depending on use) 3.0 – 5.0 (based on application) 4.0 – 6.0 (depending on local conditions)
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The quantity of Esherichia coli (E.coli) is also used for indicating the general water quality
E. coli is a kind of bacteria found in human faeces and often used as an indicator of sewage pollution
High E. coli count indicates great faecal contamination and high health risk
Low DO level indicates the severe depletion of oxygen in the water
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The E. coli counts of four typhoon shelters in Hong Kong
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The dissolved oxygen levels of four typhoon shelters in Hong Kong
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Winkler method is used to determine the amount of dissolved oxygen in water
Procedure of Winkler method
1. A water sample of known volume is collected with a
250.0 cm3 volumetric flask
2. A little MnSO4(aq) solution and a little alkaline solution of KI(aq) are added to the volumetric flask
Determination of Dissolved Oxygen in a Water Sample
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In alkaline solution, DO in the sample oxidize Mn2+ to Mn3+
8OH–(aq) + 4Mn2+(aq) + O2(aq) + 2H2O(l) 4Mn(OH)3(s)
brown ppt
3. Once the precipitation is finished, a little conc. H2SO4 is introduced into the solution well below its surface
4. The flask is stoppered and mixed until the brown precipitate of Mn(OH)3(s) disappears the Mn(OH)3(s) converts I– into I2 while itself reduced to Mn2+
2Mn(OH)3(s) + 2I–(aq) + 6H+(aq)
2Mn2+(aq) + I2(aq) + 6HO(l)
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5. The iodine formed is determined by titration with a standard sodium thiosulphate solution. A few drops of starch solution is added as indicator when the end point approaches. The complete disappearance of blue colour indicates the end point of the titration
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From the above stoichiometric equations,
1 mol of O2 4 mol of Mn(OH)3 2 mol of I2
After determining the no. of moles of I2 produced, the number of moles of O2 present in the original sample can be calculated
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BOD is used as an indicator of the extent of organic pollution in water
If the water has a high BOD, this implies that a lot of organic waste is present and much oxygen is required to break down the waste
A low BOD value indicates the presence of only a small amount of organic mater and there is little pollution
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is the amount of oxygen required to break down the organic matter present in a water sample
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Natural clean water has a BOD of about 1 to 4 mg dm–3
If the BOD is greater than the amount of O2 replenished, then some fish and aquatic life may suffocate.
anaerobic micro-organisms begin to multiply
producing unpleasant odour
BOD is determined in the laboratory by incubating in a water sample for 5 days at 20°C and measuring the amount of O2 consumed
It is known as five-day BOD or BOD5
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If a sample has an initial DO of 8 mg dm–3, and in five days it has dropped to 2 mg dm–3
Then BOD5 = (8 – 2) mg dm–3 = 6 mg dm–3
BOD5 value is commonly used to indicate the relative oxygen requirements in treating effluents and polluted waters
The degree of pollution indicated by biochemical oxygen demand (BOD5)
BOD5 (mg dm–3)
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Check Point 46-6
(a) The solubility of oxygen in water is 9 mg dm–3 at 20°C and 1 atm. In a stream, the water is found to be 40% saturated with oxygen. List some possible uses of the stream water.
Answer
(a) Dissolved oxygen of the stream water
= 9 mg dm–3
= 3.6 mg dm–3
Therefore, the stream water can be used as industrial water supply and agricultural water supply.
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(b) (i) What is the significance of BOD?
(ii) If the initial DO of a water sample is 30 mg dm–3 and the value has dropped to 10 mg dm–3 in five days, calculate the BOD5 of the water sample
Answer
(b) (i) BOD stands for biochemical oxygen demand. It is the amount of oxygen required to break down the organic matter present in a water sample. It is commonly used as an indicator of the extent of organic pollution in water. A high BOD of water implies that a lot of organic waste is present and much oxygen is required to break down the waste. A low BOD indicates the presence of only a small amount of organic matter and there is little organic pollution
(ii) BOD5 = 30 mg dm–3 – 10 mg dm–3
= 20 mg dm–3
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46.4 Solid Pollution (SB p.249)
In Hong Kong, over 16 820 tonnes of solid waste was disposed of daily in 2001
Solid waste is produced as a result of:
(1) activities of domestic households
(2) commercial operations
(3) manufacturing industries
(4) construction sites
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Composition of solid waste disposal in Hong Kong in 2001
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Composition of domestic waste in Hong Kong in 1999
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major disposal method for municipal solid
water in Hong Kong
Potential environmental problems:
leachate (highly polluting water percolating
through decaying waste at landfills)
Disposal of Solid Waste
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Solution to the problems:
Wells are installed at the landfills to extract the gas and burns it off at very high temperatures
The movement of landfill gas is monitored and controlled throughout the operation of the landfills to reduce emissions to atmosphere
Landfill gas is used as an energy source in some countries
Leachate may be treated in sewage treatment plants
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Advantages:
high temperature kills disease-causing
heat produced can be used as energy source
some incinerators equipped with facilities
for recovering valuable metals (e.g. Fe, Al)
from waste
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Disadvantages:
e.g. metals (Be, Cd, Cr, Pb and Hg) are released
large amount of CO2 produced adds to the problem of greenhouse effect
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plastics do not undergo decomposition at landfill sites
incineration of plastic waste produces air pollutants (e.g. HCl) which can cause acid rain
small fish have been found dead with their digestive tracts clogged by fragments of plastic foam ingested
sea animals have been suffocated to death by plastic bag
use of plasticizers and CFCs in the manufacture of plastics leads to further ecological and environmental problems
Pollution Problems Associated with the Disposal of Plastics
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The enzymes in micro-organisms tend to attack the ends of the polymer chains only
plastics undergo extremely slow degradation
Attempts have been made to develop plastics which are more degradable
e.g. biopolymers, photodegradable plastics and synthetic biodegradable plastics
Possible Solutions to Pollution Problems of Plastics
Development of Degradable Plastics
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Biopolymers
Paracoccus
Bacillus
Spirullum
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Poly(hydroxybutyrate) (PHB)
a natural polyester made by certain bacteria which can be broken down by micro-organisms found in soil and natural water sources under the anaerobic condition of a landfill within 9 months
The main disadvantage is more expensive (15 times) than polyethene
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Photodegradable Plastics
These groups enhance the ability of the plastics to absorb UV radiation
When the plastics are exposed to UV radiation from sunlight, the long polymer chains start to break down into small fragments
Photodegradable plastics have light-sensitive functional groups such as the carbonyl group ( ) incorporated into their polymer chains
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Synthetic Biodegradable Plastics
When micro-organisms consume starch or cellulose, the plastics are broken down into tiny pieces
The biodegradation is speeded up greatly as surface area have been greatly increased
Synthetic biodegradable plastics are made by incorporating starch or cellulose into the polymers during productions
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1. The products of biodegradation cause water pollution
2. The rate of biodegradation is still slow for the large quantity of plastic waste generated
Products made of degradable plastics
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Apply on thermoplastics only
The plastics in the waste are separated, cleaned, pulverized, and remoulded into new plastic items
Recycling of Plastics
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A flowchart of the general recycling process
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The collection of clean and uncontaminated plastic waste is the key step to the success of direct recycling, however this is the most difficult step
Other constraints on the recycling of plastic waste in Hong Kong include:
(1) high land and labour costs
(2) low environmental awareness
(4) space limitation
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The use of plastic coding system is one of the pre-requisites for successful plastic recycling
facilitate the sorting process
A commonly used and internationally recognized coding system, SPI (Society of Plastics Industry, USA) is adopted
Seven code number: 1 – 6 refer to specific polymers; 7 refers to all other types
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The plastic coding system
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The plastic coding system
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Incineration of plastics to produce energy is recycling of the energy content stored in materials
Then energy obtained from burning plastic waste in incinerators can be used for heating or for generating electricity
The drawback is that certain plastics release toxic gases when burnt, such as PVC gives out HCl when burnt
Recycling of Energy
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Calorific values of plastics and various materials
Material
Polystyrene Poly(ethene) Heating oil Fats Leather Polyvinyl chloride (PVC) Paper Wood
46 000 46 000 44 000 37 800 18 900 18 900 16 800 16 000
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Large plastic molecules are broken down on the surface of a hot silica catalyst to smaller, more useful molecules
Pyrolysis is carried out between 400°C and 800°C
The products are separated by fractional distillation and used to make plastics and other chemicals
Recycling of Chemicals
Pyrolysis is the thermal decomposition of a material at a high temperature and in the absence of air
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Recycling of chemicals from plastics by pyrolysis
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Pyrolysis product
Substance manufactured
Poly(ethene) Poly(propene) Town gas Rubber Various plastics (e.g. polystyrene)
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Answer
(a) The benefits are as follows:
(1) To save the limited resources like petroleum.
(2) Energy saved from burning of plastic waste in incinerators can be used for heating or generating electricity.
(3) To produce useful chemicals for production of
plastics or other purpose
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(b) What are the difficulties in recycling plastics?
Answer
(b) The difficulties are:
(1) Plastic waste collected for recycling should be clean and uncontaminated, and it is difficult to achieve.
(2) There are different kinds of plastics. We have to sort the waste, plastics and eliminate the unrecyclable one,
and it is time-consuming and difficult.
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The Environmental Protection Department (EPD) was establish in 1986 to coordinate and carry out pollution prevention and control activities
Its tasks include:
2. Enforcing environmental legislation
3. Monitoring environmental quality
4. Providing facilities for collection, transfer, treatment and disposal of many types of waste
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5. Advising on the environmental implications of town planning and new policies
6. Handling pollution complaints and incidents
7. Raising awareness and support in the community for environmental initiatives
46.5 Pollution Control in Hong Kong (SB p.258)
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Premature combustion of petrol in engines results in knocking power loss
Tetraethyllead(V) (TEL) are added to the fuel to prevent knocking
Toxic volatile lead halides formed during combustion pass out with exhaust gas
Lead is toxic. It causes anaemia and damages the nervous system and kidney
Lead can inactivate the catalyst in catalytic converters
must use unleaded petrol in cars fitted with catalytic converters
Measures to Improve Air Quality
Use of Unleaded Petrol
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NOx, CO and unburnt hydrocarbons are major air pollutants coming from car exhaust
Installation of catalytic converters can help reduce the emission of these pollutants
The converter contains precious metals, including Rh and Pt (or Pd), as catalyst
When car exhaust containing CO, unburnt hydrocarbons and NOx is passed through the converter, Rh catalyzes the reduction of NO
Installation of Catalytic Converters in Car Exhaust Systems
Rh
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The heterogeneous catalysts are supported on a honeycomb structure that allows the best contact between the car exhaust and the catalyst surface
The catalysts can be poisoned by lead compounds easily
cars fitted with catalytic converters use unleaded petrol only
Pt catalyzes the oxidation of CO and unburnt hydrocarbons such as C7H16
2CO(g) + O2(g) 2CO2(g) (Oxidation)
Pt
Pt
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In the past, Hong Kong suffered from serious air pollution caused by SO2 emission from burning sulphur-containing fuels
∴ Starting from 1990, the sulphur contents in liquid fuels (e.g. diesel, kerosene) and solid fuels (e.g. coal, coke, charcoal) are limited to 0.5% and 1% by weight respectively
Starting from 2000, the limitation is tightened
The sulphur content in diesel is further limited to not more than 0.05% by weight
Limitation of Sulphur Content in Fuels
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SO2 is released during combustion of coal which contains sulphur
Using coal of low sulphur content, the SO2 released is reduced
Further reduction on SO2 emission by using flue gas desulphurization (FGD) system
The FGD utilizes the “wet limestone-gymsum scrubbing process”
able to remove up to 90% of SO2 emitted
Flue gas from the boiler is directed to the absorber which contains a recycled slurry of limestone for removal of SO2
Desulphurization of Flue Gas
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SO2(g) + H2O(l) H2SO3(aq) Absorption
gypsum
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The treated flue gas leaves the absorber and directed into chimney for dispersal into the atmosphere
The gypsum has a purity of over 90% which can be exported for industrial application (e.g. raw material for making cement)
Flue gas desulphurization plant on Lamma Island
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A schematic diagram of a flue gas desulphurization system
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Coal contains certain proportion of ash that cannot be burnt
Coal-burning power plants generate tonnes of ash each day
electrostatic precipitators are equipped in power plants to remove ash
Over 99% of fly ash is removed from flue gas
The ash collected is used for making construction materials (e.g. cement) or for landfill purpose
Installation of Electrostatic Precipitators in Power Plants
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Working principle of electrostatic precipitators:
1. The flue gas from boiler is passed through a chamber in the electrostatic precipitator where particulates become charged
2. The charged particles get attracted on the collecting wall
3. The charges on the particles are neutralized on the wall where they deposit and removed
4. The cleaned flue gas is discharged through chimney
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An electrostatic precipitator
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NOx include NO and NO2 which are formed during the combustion of fuels in 2 ways:
1. N2 in the air reacts with O2 at high temperature near the combustion chamber to form NO
N2(g) + O2(g) 2NO(g)
NO2 is formed when NO releases into the atmosphere and reacts with O2
2NO(g) + O2(g) 2NO2(g)
2. Fossil fuels may contain nitrogen compounds. When these fuels are burnt, the nitrogen compounds are oxidized to NOx
Installation of Low Nitrogen Oxide Burners in Power Plants
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Low nitrogen oxides burners utilizes a combustion control method in which the operating conditions for combustion is modified to reduce NOx formation
Reduction in flame temperature in the combustion zone and reduction in availability of oxygen in the NOx formation zone can reduce the production of NOx up to 50%
By controlling the emission of SO2 and NOx, the pollution caused by burning of fossil fuels in power plants has been greatly reduced
*
Answer
As less toxic lead compounds released when the running cars run on unleaded petrol, air quality can be improved.
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In Hong Kong, all wastewater is discharged by one route or another into the sea
At present, only part of the wastewater receives some sort of water treatment before discharge
Sewage treatment is a series of processes to remove contaminants from used water before it enters the waterways
Measures to Improve Water Quality
Treatment of Sewage
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The sewage treatment plant in Shatin
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Procedure:
1. The sewage is first passed through a series of screens that remove large objects
2. Then it is passed through grit chambers to settle out grit particles
3. Then it is sent to the sedimentation tanks where most of the suspended solids settle out and are moved as “primary sludge”
Screening and Sedimentation (Primary Treatment)
*
After the primary treatment, about 60% of the solids are removed and the BOD is reduced by 35%
The solids that accumulate can be purified and used at reclamation sites
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Sewage after primary treatment is passed into aeration tanks where it is mixed with air and a mixture of micro-organisms (activated sludge)
Micro-organisms digest the organic matter in this oxygen-rich environment
Aeration is need to provide O2 to support the activities of micro-organisms
The treated effluent is 90% free of suspended solids and the BOD is reduced by 95%
Digestion of Pollutants by Micro-organisms (Secondary Treatment)
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The treated sewage is chlorinated to kill the micro-organisms
It is then discharged into the waterways or further treated to remove some inorganic species (e.g. nitrate(V) and phosphate(V))
The sludge removed by primary and secondary treatment is treated to kill pathogens and to biodegrade more organic matter
The treated sludge is dried and disposed of into the sea or at landfill site
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*
Rapid economic growth has resulted in the establishment of a quickly expanding industrial sector. This led to the production of vast amount of chemical waste
Major producers of chemical wastes in Hong Kong:
manufacturers of electroplating, electronic, electrical and scientific products
Kinds of waste: Acids, alkalis, organic solvents, cyanides, heavy metals and oily waste
These wastes are corrosive, flammable, poisonous and may give off toxic or explosive vapours
Treatment of Chemical Waste from Industry and Labobatories
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Composition of chemical waste treated at the chemical waste treatment centre in 2001
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In Hong Kong, the Chemical Waste Treatment Centre (CWTC) on Tsing Yi Island provides proper waste treatment services for local waste producers
There are 3 major waste treatment systems in the Centre:
1. Oil/water separation system
2. Physical/chemical treatment system
It treats water-based chemical waste such as acids, alkalis, and wastewater containing heavy metal salts
3. Incineration
It used for destructing hazardous organic waste such as pesticides and cyanide waste
*
Physical methods include separation of waste oil from oily water mixture in the oil/water separation system
It also separates water from all residue formed in neutralization and precipitation processes
Physical Methods
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1. Neutralization of acids and bases
Slaked lime (Ca(OH)2) is used to neutralize acidic waste, while sulphuric(VI) acid is used to neutralize alkaline waste
H+(aq) + OH–(aq) H2O(l)
Chemical Methods
*
Precipitation reaction
alkalis are used to remove metals from a number of metal-containing waste (e.g. waste solution of Fe, Zn, Cu and Ni)
e.g. Fe from spent FeCl3 etchant is removed by precipitation
Fe3+(aq) + 3OH–(aq) Fe(OH)3(s)
Spent NH4Cl & CuCl2 are treated with NaOH to recover the marketable product, CuO
NH4+(aq) + OH–(aq) NH3(aq) + H2O(l)
Cu2+(aq) + 2OH–(aq) Cu(OH)2(s)
Cu(OH)2(s) CuO(s) + H2O(l)
*
NH3 produced is treated with H2SO4 to form (NH4)2SO4 which is also a marketable product
2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)
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Reduction reaction
Soluble Cr(VI) compounds are usually found in the effluent of metal-finishing plants
Toxic Cr(VI) compounds are first reduced to non-toxic Cr(III) compounds and then precipitated as hydroxide
Common reducing agents used: Na2SO3, SO2, FeSO4
2H2CrO4(aq) + 3Na2SO3(aq) + 3H2SO4(aq)
Cr2(SO4)3(aq) + 3Na2SO4(aq) + 5H2O(l)
*
Incineration destroys most of the toxic organic compounds, hazardous medical waste and cyanide waste by decomposing them into carbon dioxide and various gases
Contaminants in the flue gas are removed by scrubbers
Advantages:
Eliminating the problem of liquid waste disposal
Incineration
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A simplified flow diagram of the chemical waste treatment process
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HKSAR government has been actively promoting the 3Rs concept (Reduce, Reuse and Recycle) to arise the public environmental awareness and promote waste avoidance, minimization, recovery for reuse and recycling
Major kinds of recyclable waste recovered:
paper, plastics, ferrous metal and non-ferrous metal
Waste paper is recycled locally to produce corrugated cardbox and paper board
Ferrous metal waste is processed in a local foundry to produce reinforcing bars for use by the construction industry
Measures to Reduce Solid Waste
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The proportions of waste materials recovered in Hong Kong in 2000
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Government has done a lot on encouraging waste recycling, e.g.
Provides recycling bins for plastic bottles, aluminium cans and waste paper at around 200 public areas
Recycling bins for plastic bottles, aluminium cans and waste paper
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Launched a 10-year Waste Reduction Framework Plan aiming to reduce the amount of waste produced, to encourage waste recycling and reuse, to promote greater efficiency and economy in the management of waste, and to prolong the usable life of Hong Kong’s landfills
Provides land for waste recycling industry
Hold various waste reduction campaigns with business sectors, academics and green groups to increase the public awareness
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Programs such as the “Reusable Bag” campaign, the “Bring Your Own Bad (BYOB)” campaign and the “Use Less Plastic Bags” campaign have been launched to educate the public to minimize the use of plastic bags
Co-operate with green groups to organize a series of programs to enhance the awareness and participation of the public in using less plastic bags
Reduce, Reuse or Recycling of Plastics
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The recycled paper produced by local recyclers is used to make corrugated paper and duplex papercard for packaging
A variety of waste paper, including high-value waste paper (e.g. white paper and computer printout), old corrugated cardboard, newspaper and other mixed paper, are being exported to overseas countries for recycling
Reuse and Recycling of Paper
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Metal waste can be recycled by smelting used metals and using them again
e.g.
(1) ferrous metal waste is processed in a local foundry to produce reinforcing bars for use in the construction industry
(2) Al, Pb, Fe, Cu, Ag, Sn, Zn and Au can also be recycled
Reuse and Recycling of Metals
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Reasons:
(1) These mineral resources are limited but the demand is increasing
(2) Pollution can be reduced and energy is saved
Collecting aluminium cans
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chemistry and the environment

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