Preview:

We shall begin by defining Environmental pollution as the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings and a pollutant as a substance, which causes pollution. We will then classify pollutants into Degradable, Slowly Degradable and Non-Degradable Pollutants depending on how easily they breakdown by natural processes.

In the next section, we shall study about the pollution that occurs in two major layers of the atmosphere: Troposphere and Stratosphere; and the sources that cause the pollution of these layers. Successively, the two global concerns: The Global warming and The Greenhouse effect which have led to harmful effects like, formation of acid rain and increased overall mean temperature of the earth, shall be discussed in some detail. Alongside, we shall study about the two types of pollutants that cause tropospheric pollution: Gaseous pollutants and Particulate pollutants.

We shall then study about Stratospheric Pollution in which our focus would be on studying about ozone layer depletion and its effects as the ozone layer is found in this layer of the atmosphere.

Next, we will study ­­­about Water pollution which is the contamination of water bodies by the pollutants which are discharged directly or indirectly into water bodies without adequate treatment. In addition, we will also discuss the sources of water pollution, which includes: Direct Contaminant Sources (such as, factories, refineries and waste treatment plants)and Indirect Contaminant Sources (such as, contaminants that enter the water supply from soils, ground water systems and from the atmosphere via rain water). We shall also discuss the chief contaminants responsible for water pollution (like, pathogens, organic wastes and chemicals).

You will see learn how certain components (like, fluoride, lead and sulphate) in water should only be present in permissible quantities.

In the next section, we shall study about Soil pollution which is the building up of toxic chemicals like, chemicals, salts and radioactive materials which can have adverse effect to human health or the ecosystem. As a part of the study, we will learn about the use of pesticides like, DDT, organo-phosphates and herbicides and their harmful effects.

We shall then talk about Industrial Waste which is the waste generated by manufacturing or during industrial processes. In the last section, we shall learn about the technique of waste disposal and its importance.

We shall end the chapter on an interesting note: “The Green Chemistry” which is a way of thinking and is about utilising the existing knowledge and principles of chemistry and other sciences to reduce the adverse impacts on environment. We will further list 12 principles in this regard.

14.0 Introduction:

Important Definitions:

Environmental studies: Environmental study is the branch of science which deals with the sum of all social, economic, biological, physical and chemical interrelations with our surroundings.

Environmental Chemistry: Environmental chemistry is an interdisciplinary science that includes the study of the origin, transport, reactions, effects and fates of chemical species in the environment.

We shall discuss some important aspects of environmental chemistry.

14.1 Environmental Pollution

Definition box: Environmental pollution: Environmental pollutionis the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings. Pollutant: A pollutant is a substance, which causes pollution. Pollutants can also be defined as solids, liquids or gaseous substances present in great concentration and produced from human activities or due to natural happenings.

Classification of Pollutants:

(i) Degradable (or) Non-Persistent Pollutants:

Those pollutants which rapidly break down by natural processes are called Non-Persistent Pollutants. Examples include: discarded vegetable wastes and domestic sewage wastes.

(ii) Slowly Degradable (or) Persistent Pollutants:

Those pollutants which are slowly degradable and remain in the environment without change in their form for many decades are called Persistent Pollutants. For example, substances such as dichlorodiphenyltrichloroethane (DDT), plastic materials, heavy metals, chemicals and nuclear wastes, once released into the environment are difficult to remove.

(iii) Non-Degradable Pollutants:

Those pollutants which cannot be degraded by the environment are called Non-Degradable Pollutants. For example, substances such as dichlorodiphenyltrichloroethane (DDT), plastic materials, heavy metals (like mercury), chemicals and nuclear wastes, once released into the environment are difficult to remove.

Questions from section 14.1:

1. Define:

a) Environmental pollution

b) Pollutant

2. Classify and explain the types of pollutants.

14.2 Atmospheric Pollution

The atmosphere is divided into layers according to major changes in temperature:

1. The Troposphere (0-10 Km): This layer contains 75% of the gases in the atmosphere. The Troposphereis the lowest region of atmosphere. It extends up to the height of 10 km from sea level.

2. The Stratosphere (10-50Km): This layers contains the ozone layer.

3. The Mesosphere (50-80Km)

4. The Thermosphere (>85Km)

14.2.1 Tropospheric Pollution

The tropospheric pollution occurs due gaseous and particulate pollutants in the troposphere:

1. Gaseous air pollutants:

(a) Oxides of Sulphur:

Formation: Oxides of sulphur are produced when sulphur containing fossil fuel is burnt.

Effects:

• Sulphur dioxide, is a gas that is poisonous to both animals and plants.
• It has been reported that even a low concentration of sulphur dioxide causes respiratory diseases like, asthma, bronchitis, emphysema in human beings.
• Sulphur dioxide causes irritation to the eyes, resulting in tears and redness.
• High concentration of SO₂ leads to stiffness of flower buds which eventually fall off from plants.
• In the presence of catalysts like particulate matter, Sulphur dioxide gets oxidized to sulphur trioxide:

$$ \ce { 2SO_2 (g) + O_2 (g) -> 2SO_3 (g) } $$

The reaction can also be promoted by ozone and hydrogen peroxide.

$$ \ce { SO_2 (g) + O_3 (g) -> SO_3(g) + O_2 (g) }$$

$$ \ce { SO_2 (g) +H_2O_2 (l)-> H_2SO_4 (aq)}$$

(b) Oxides of Nitrogen:

Formation of Nitrogen oxides:

• Dinitrogen and dioxygen do not react with each other at a normal temperature. At high altitudes, when lightning strikes, they oxygen and nitrogen combine to form oxides of nitrogen.
• $NO_2$ is further oxidised to nitrate ion, $NO_3^−$. This nitrate ion on reaching soil, serves as fertilizer.
• In an automobile engine, (at high temperature) when fossil fuel is burnt, dinitrogen and dioxygen combine to give nitric oxide (NO) and nitrogen dioxide ($NO_2$) as given below:

$$ \ce { N_2 (g) + O_2 (g) ->[{1483 K}]2NO(g) }$$

NO reacts instantly with oxygen to give $NO_2$

$$ \ce { 2NO (g) + O_2 (g) -> 2NO_2 (g) }$$

Rate of production of $NO_2$ is faster when nitric oxide reacts with ozone in the stratosphere.

$$ \ce { NO (g) + O_3 (g) -> NO_2 (g) + O_2 (g) }$$

Effects:

• Oxides of nitrogen produce an irritant red haze in the traffic and congested places.
• Higher concentrations of NO₂ damage the leaves of plants and retard the rate of photosynthesis.
• Nitrogen dioxide is a lung irritant that can lead to an acute respiratory disease in children.
• It causes breakdown of plant tissues and spotting of leaves.
• Nitrogen dioxide is also harmful to various textile fibres and metals.

(c) Hydrocarbons:

Composition: Hydrocarbons are composed of hydrogen and carbon only.

Formation: Hydrocarbons are formed by incomplete combustion of fuels used in automobiles.

Effects:

• Hydrocarbons are carcinogenic (cancer causing).
• They harm plants by causing:
• Ageing and breakdown of tissues
• Shedding of leaves, flowers and twigs.

(d) Oxides of Carbon (i) Carbon monoxide:

Carbon monoxide (CO) is a colourless and odourless gas.
Formation: Carbon monoxide (CO) is mainly produced as a result of incomplete combustion of fuel from automobile exhausts. CO is also released by incomplete combustion of coal, firewood and petrol.

Effect:

• Carbon monoxide is poisonous because it binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. Thus, haemoglobin tends to have more affinity towards carbon-monoxide than oxygen. Hence, the amount of carboxyhaemoglobin formed is greater than oxygen-haemoglobin.
• Therefore, increase in concentration of carboxyhaemoglobin to about 3–4 per cent in the blood reduces the oxygen carrying capacity of blood greatly.
• This oxygen deficiency, results in headache, weak eyesight, nervousness and cardiovascular disorder.
• CO is produced during cigarette smoking. Hence, in pregnant women who have the habit of smoking, the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.

(ii) Carbon dioxide:

Carbon dioxide gas is confined to troposphere only. Normally it forms about 0.03 per cent by volume of the atmosphere.

Sources:

• Carbon dioxide $(CO_2)$ is released into the atmosphere by respiration.
• Burning of fossil fuels for energy.
• By decomposition of limestone during the manufacture of cement.
• It is also emitted during volcanic eruptions.
• Deforestation and burning of wood.

Effect: The increased amount of $(CO_2)$  in the air is mainly responsible for global warming.

Global Warming and Greenhouse Effect:

• Clouds and gases reflect about 15% of heat as infrared radiation of the incoming solar radiation, and absorb about 75% of the solar radiation, increasing the temperature of the earth’s surface.
• Part of the absorbed solar energy gets is trapped by gas molecules like, carbon dioxide and methane. These gas molecules absorb the solar energy and radiate back heat energy. Thus, a major part of heat energy again comes to Earth’s surface, heating it up once again.
• Carbon dioxide and methane thus help in keeping the earth warm by trapping and radiating heat energy. This is known as natural greenhouse effect. However, when the concentration of $(CO_2)$ in the atmosphere crosses 0.03%, the natural greenhouse effect is disturbed.
• The gases responsible for the phenomenon of greenhouse effect are known as greenhouse gases.
• Increased concentration of greenhouse gases, like, $(CO_2), N)_2O$, CFCs and methane increases the trapping of heat radiations thereby increasing the mean temperature of the earth. This enhances the greenhouse effect and is known as: Enhanced Green House effect (or) Global warming.

Global warming is caused by increase in greenhouse gases like:

Carbon dioxide: This gas contributes to 60% of total global warming. $(CO_2)$ increases due to increase in use of fossil fuels, burning of biomass, deforestation and cement manufacture.

Methane: It contributes to 20% of global warming. Methane is produced during burning of biomass and also due to activity of methanogenic bacteria.

Chlorofluorocarbons (CFCs): It contributes to 14% of total global warming. Chlorofluorocarbons are released due to leaking of air conditioners, refrigerator units, aerosol, plastic foam and propellants.

Nitrous oxide: It contributes to 6% of total global warming. Nitrous oxide increases due to agriculture, biomass burning and industrial processes.

Effects of Enhanced Greenhouse effect:

1. $(CO_2)$ fertilization: When Greenhouse gases like, $(CO_2)$  increases in its concentration, the rate of transpiration reduces.

2. Rise in Sea level: The increase in mean temperature of the earth causes melting of polar ice caps and thermal expansion of water; which results in increase in sea level, thus, submerging low lying areas.

3. Effects on weather and climate: Increase in temperature of earth increases moisture carrying capacity and cools the stratosphere; resulting in odd climatic changes like, rainfall patterns which cause draught in some areas and flooding in others. For example: The El Nino effect. This in turn leads to increased melting of polar ice caps as well as of other places like the Himalayan snow caps.

4. Loss of Biodiversity: Changes in climatic conditions force the migration of certain organisms to favourable conditions; but organisms that fail to migrate, get eliminated.

5. Effects on fresh water resources: Salt water encroaches into low lying areas which may result in spoilage of fresh water reservoirs and also spoil the underground fresh water resources.

6. Effects on food production: Increase in temperature of earth increases growth of weeds, pests and plant diseases. All these decrease crop production.

Measures to control Global warming:

• Cutting down the use of fossil fuels.
• Improving efficiency of energy usage.
• Reducing deforestation.
• Planting trees and slowing down the growth of human population.
• Use of CFC free refrigerators and air conditioner to avoid global warming.
• Minimizing use of chemical fertilizers to reduce emission of $N_2O$.

Acid rain:

• Pure rain water has an acidic nature and a pH of about 5.6. The acidic nature of rainwater is due to the reaction of $(CO_2)$ and water to form carbonic acid as shown:
  $$ \ce { H_2O (l) + CO_2 (g) <=> H_2CO_3 (aq) } $$
• The carbonic acid dissociates in water forming hydronium and bicarbonate ions as shown:
  $$ \ce { H_2O (l) + H_2CO_3 (aq) <=> HCO_3^- (aq) + H_3O^+ (aq) }$$
• Rain water having a pH of less than 5.6 is called, Acid Rain. It contains harmful substances like nitric acid and sulphuric acid formed by the reaction of oxides of nitrogen and sulphur with water as depicted:
  $$ \ce { 2SO_2 + 2H_2O + O_2 -> 2H_2SO_4 } $$
  $$ \ce { 4NO_2 + 2H_2O + O_2 -> 4HNO_3 } $$
• Acid rain also contains salts of ammonium, calcium, magnesium and sodium.

Harmful effects of acid rain:

• Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.
• It causes respiratory diseases in human beings and animals.
• When acid rain falls and flows as ground water to reach rivers and lakes, it affects the aquatic ecosystem.
• It corrodes water pipes resulting in the leaching (transformed to ions) of heavy metals such as, iron, lead and copper into the drinking water.
• Acid rain damages buildings and other structures made of stone or metal. For example, the Taj Mahal in India has been affected by acid rain.
• Excess of $CO_2$ in the air is removed by green plants through photosynthesis as they consume $CO_2$  and emit oxygen during the process.

Story Box: Taj Mahal and Acid Rain The air around the city of Agra, where the Taj Mahal is located, contains fairly high levels of sulphur and nitrogen oxides. It is mainly due to a large number of industries and power plants around the area. Use of poor quality of coal, kerosene and firewood as fuel for domestic purposes add up to this problem. This results in acid rain which reacts with marble, $CaCO_3$ of Taj Mahal. $$(CaCO_3 + H_2SO_4 → CaSO_4 + H_2O + CO_2)$$ The acid rain causes damage to the monument that has attracted people from around the world. As a result, the monument is being slowly disfigured and the marble is getting discoloured and lustreless. The Government of India announced an action plan in early 1995 to prevent the disfiguring of this historical monument while, Mathura refinery has taken suitable measures to check the emission of toxic gases. This plan aims at clearing the air in the ‘Taj Trapezium’– an area that includes the towns of Agra, Firozabad, Mathura and Bharatpur. Under this plan, more than 2000 polluting industries lying inside the trapezium would switch over to the use of natural gas or liquefied petroleum gas instead of coal or oil. A new natural gas pipeline would bring more than half a million cubic metres of natural gas a day to this area. People living in the city will also be encouraged to use liquefied petroleum gas in place of coal, kerosene or firewood. Vehicles plying on highways in the vicinity of Taj would be encouraged to use low sulphur content diesel.

2. Particulate pollutants:

Definition box: Particulates pollutants: Particulates pollutants are the minute solid particles or liquid droplets in air.

Particulates in the atmosphere may be viable (having life) or non-viable (non-living).

Viable Particulates:

Viable Particulates are minute living organisms that are dispersed in the atmosphere.. For example, bacteria, fungi, moulds, algae.

Effects:

• They can also cause plant diseases.
• Some of the fungi and bacteria in air cause allergy in humans.

Non-viable particulates:

Non-viable particulates are classified according to their nature and size as follows:

(a) Smoke particulates: Smoke particulates consist of solid or mixture of solid and liquid particles formed during combustion of organic matter. Examples include, cigarette smoke, smoke from burning of fossil fuel, garbage and dry leaves and oil smoke.

(b) Dust: Dust is composed of fine solid particles (over 1μm in diameter), produced during crushing and grinding of solid materials. Some typical examples of dust are: sand from sand blasting, saw dust from wood works, pulverized coal, cement and fly ash from factories and dust storms.

(c) Mist: Mist is produced by particles of liquid sprays and by condensation of vapours in air. Examples of mist are: sulphuric acid mist, herbicides and insecticides that miss their targets and travel through air and form mists.

(d) Fumes: Fumes are generally obtained by the condensation of vapours during chemical processes like, sublimation, distillation and boiling. Examples of fumes are: organic solvents, metals and metallic oxides.

Effects:

• The effect of particulate pollutants largely depends on their particle size.
• Airborne particles such as dust, fumes and mist bigger than 5 microns are likely to remain in the nasal passage, whereas particles of about 10 micron enter into lungs easily.
• Lead particulates emitted from burning of leaded petrol interfere with the development and maturation of red blood cells.

(e) Smog:

Definition box: Smog is a colloidal mixture of smoke and chemical fumes dispersed in fog.

There are two types of smog:

(a) Classical smog:

• Classical smog occurs in cool humid climate.
• Classical smog is a mixture of smoke, fog and sulphur dioxide.
• Chemically, classical smog is a reducing mixture and is thus called as reducing smog.

(b) Photochemical smog:

• Photochemical smog occurs in warm, dry and sunny climate.
• The main components that cause photochemical smog arise from the action of sunlight on unsaturated hydrocarbons and on nitrogen oxides produced by automobiles and factories.
• Photochemical smog has high concentration of oxidising agents and is therefore called, oxidising smog.

Formation of photochemical smog:

• Two of the major pollutants that are emitted when fossil fuels are burnt are: hydrocarbons (unburnt fuels) and nitric oxide (NO).

• When these pollutants build up to sufficiently high levels, a chain reaction occurs in which:

Step 1: NO is converted into nitrogen dioxide ($NO_2$) during its interaction with sunlight:

$$ \ce { 2NO + O_2 ->[{h \nu}] 2NO_2 } $$

Step 2: This $NO_2$ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom (Figure 14.2):

$$ \ce { NO_2 (g) ->[{h \nu}] NO(g) + O(g) } $$

Step 3: Oxygen atoms are very reactive and combine with the $O_2$ in air to produce ozone.

$$ \ce { O(g) + O_2 (g) -> O_3 (g)}$$

Step 4: The ozone formed in the above reaction reacts rapidly with the NO(g) formed in step 2 to regenerate $NO_2$.

$$ \ce { NO(g) + O_3(g) -> NO_2 (g) +O_2(g)}$$

Step 5: Both $NO_2$ and $O_3$ are strong oxidising agents and can react with the unburnt hydrocarbons in the polluted air to produce formaldehyde, acrolein and peroxyacetyl nitrate (PAN) as shown:

Thus, the common components produced by a photochemical smog are: ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate (PAN).

Effects of photochemical smog:

• $NO_2$ is a brown gas and at sufficiently high levels, gives rise to haze in traffic situations.
• Both ozone and PAN act as powerful eye irritants.
• Ozone and nitric oxide irritate the nose and throat.
• Ozone and nitric oxide in high concentration also cause: headache, chest pain, dryness in throat, cough and difficulty in breathing.
• Photochemical smog leads to cracking of rubber and damage plant tissues.
• It also causes corrosion of metals, stones, building materials, rubber and painted surfaces.

Techniques to control and reduce the formation of photochemical smog:

• By controlling the primary precursors of photochemical smog: $NO_2$  and hydrocarbons and; secondary precursors: ozone and PAN, the photochemical smog will automatically be reduced.
• Use of catalytic converters in automobiles, which prevent the release of nitrogen oxide and hydrocarbons into the atmosphere.
• Planting of certain plants like, Pinus, Juniparus, Quercus, Pyrus and Vitis which can metabolise nitrogen oxide will help in reducing photochemical smog as well.

14.2.2 Stratospheric Pollution

Ozone is the second major layer of earth’s atmosphere just above the troposphere. Ozone is the main constituent of this layer. Stratospheric ozone is the naturally occurring gas that filters the sun’s UV radiation.

Formation of Ozone:

Step 1: The UV radiations split molecular oxygen into free oxygen (O) atoms:

$$ \ce { O_2(g) ->[{UV}] O(g) + O(g) }$$

Step 2: These oxygen atoms combine with the molecular oxygen to form ozone:

$$ \ce { O(g) + O_2(g) <=>[{UV}] O_3 (g)}$$

Ozone is thermodynamically unstable and decomposes back to give molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules.

Ozone Layer Depletion:

• The main reason for the ozone layer depletion is the release of chlorofluorocarbon compounds (CFCs), also known as freons.
• CFCs are released from refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts. These CFCs mix with atmospheric gases and eventually reach the stratosphere.
• In stratosphere, the CFCs get broken down by powerful UV radiations, releasing chlorine free radical:
  $$\ce{CF2Cl2 (g) ->[UV] \overset{\bullet}{Cl}(g) + \overset{\bullet}{C}F2Cl(g)}$$

• The chlorine radical then reacts with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.
  $$\ce{\overset{\bullet}{Cl}(g) + O_3(g) -> Cl\overset{\bullet}{O}(g) + O2(g)}$$

• The chlorine monoxide radical reacts with atomic oxygen to produce more chlorine radicals:
  $$\ce{Cl\overset{\bullet}{O}(g) + O(g) -> \overset{\bullet}{Cl}(g) + O2(g)}$$

• The chlorine radicals are continuously regenerated and cause the breakdown of ozone (by repeating the last two steps).
• Thus, CFCs act as transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

The Ozone Hole

It was found that a unique set of conditions were responsible for the ozone hole:

In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine and hence removal of free radicals containing chlorine takes place from the atmosphere. Therefore, the ozone depletion gets minimized:

$$\ce{Cl\overset{\bullet}{O}(g) + NO2(g) -> ClONO2(g)}$$

$$\ce{\overset{\bullet}{Cl}(g) + CH4(g) -> \overset{\bullet}{CH3}(g) + HCl(g)}$$

In winter season, special types of clouds called polar stratospheric clouds (or nacreous clouds) are formed which catalyse the ozone destruction by providing the surface for the hydrolysis of chlorine nitrate. These clouds also catalyse the reaction of chlorine nitrate and hydrogen chloride to form chlorine molecule:

$$ \ce {ClONO_2 (g) + H_2O (g) -> HOCl (g) + HNO_3 (g) }$$

$$ \ce {ClONO_2 (g) + HCl (g) -> Cl_2 (g) + HNO_3 (g) }$$

When sunlight falls on the clouds, they break, HOCl and $Cl_2$ undergo photochemical decomposition giving free radicals:

$$\ce{HOCl(g) ->[h\nu] \overset{\bullet}{O}H(g) + \overset{\bullet}{Cl}(g)}$$

$$\ce{Cl2(g) ->[h\nu] 2\overset{\bullet}{Cl}(g)}$$

These free radicals formed attack the ozone causing it to deplete resulting in the appearance of ozone hole. As the polar stratospheric clouds do not appear in summer, the depletion of ozone layer occurs more in winter regions than in summer.

Fact box: In 1980s, scientists working in Antarctica first reported about depletion of ozone layer commonly known as ozone hole over the South Pole.

Effects of depletion of the ozone layer:

• It causes aging of skin, damage to skin cells and various types of skin cancers.
• In human eye, cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of cornea, called snow-blindness.
• It can also cause early cataract.
• Continued exposure may permanently damage the cornea.
• Damages nucleic acids and increases the chances of occurrence of mutation.
• Increases mortality rate of young ones of animals.
• Increases Global warming.
• Affects photosynthesis in both aquatic plants and terrestrial plants by damaging nucleic acids and food chains.

Questions from section 14.2:

1. Explain in detail how the gaseous air pollutants contribute to tropospheric pollution.

2. What is Green House effect?

3. Explain the causes for Global warming. Also explain its effects.

4. What are the measures used to control Global warming?

5. How is acid rain formed? Explain with reactions. Also state its harmful effects.

6. What are particulate pollutants? List their types.

7. List and explain the types of non-viable particulate pollutants.

8. Explain the formation of photochemical smog. What are its effects?

9. Explain the stepwise process in formation of ozone.

10. Write a note on:

a) ozone layer depletion

b) ozone hole

11. Explain the effects of depletion of ozone layer.

14.3 Water Pollution

Water covers over 70% of the earth’s surface and is very important resource for people and the environment. Water pollution affects drinking water, rivers, lakes and oceans all over the world. This consequently harms human health and the natural environment.

Definition box: Water pollution is the contamination of water bodies by the pollutants which are discharged directly or indirectly into water bodies without adequate treatment.

Sources of Water Pollution

There are many sources for water pollution can be classified into two categories:

Direct Contaminant Sources:

Direct sources include: effluent outfalls from factories, refineries and waste treatment plants that emit fluids of varying quality directly into urban water supplies.

Indirect Contaminant Sources:

Indirect Contaminant Sources include contaminants that enter the water supply from soils, ground water systems and from the atmosphere via rain water. Soils and ground waters contain the residue of human agricultural processes like, fertilizers, pesticides and improperly disposed industrial wastes. Atmospheric contaminants such as gaseous emissions from automobiles, factories and even bakeries are also derived from human practices.

14.3.1 Causes of Water Pollution

(i) Pathogens:

• A pathogen is a microorganism such as, a virus, bacterium or fungus that causes diseases.
• Pathogens enter water from domestic sewage and animal excreta.
• Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.

(ii) Organic wastes:

• Organic waste materials include domestic and animal sewage, biodegradable organic compounds and industrial wastes as well as agricultural run offs.
• These wastes undergo biodegradation by bacterial activity in the presence of dissolved oxygen (DO) in water. This results in rapid decrease in the dissolved oxygen which is harmful to aquatic life.
• These wastes are biodegradable.
• The major water pollutants include: organic matter such as leaves, grass, excessive phytoplankton growth within water and trash. These pollutants pollute water by adding to run off water.

(iii) Chemical Pollutants:

• Water is a polar solvent and hence dissolves most of the inorganic chemicals which include heavy metals such as cadmium, mercury and nickel.
• These metals damage kidneys, central nervous system and liver.
• Acids like sulphuric acid from mine drainage and salts from many different sources including raw salt like sodium chloride and calcium chloride used to melt snow and ice in the colder climates (sodium and calcium chloride) are water soluble chemical pollutants.

Organic chemicals include:

• Petroleum products: They pollute many sources of water like, major oil spills in oceans;
• Industrial chemicals: They are polychlorinated biphenyls (PCBs) which are used as cleansing solvent. For example, detergents. Most of the detergents available are biodegradable but some PCBs are carcinogenic.
• Agricultural chemicals: They include fertilizers and pesticides that drift down from sprays or runoff from lands;

Effects of organic pollutants released in water:

• The bacteria responsible for degrading biodegradable detergent feed on it and grow rapidly. While growing, they often end up using all the oxygen dissolved in water. The lack of oxygen kills all other forms of aquatic life such as fish and plants.
• Fertilizers contain phosphates as additives. The addition of phosphates in water enhances algae growth. Such large growth of algae covers the water surface and reduces the oxygen concentration in water. This leads to anaerobic conditions, accompanied with obnoxious decaying odour. This process in which nutrient enriched water bodies support the growth of dense plant population, which kills animal life by depriving it of oxygen and resulting in subsequent loss of biodiversity, is known as Eutrophication.

14.3.2 International Standards for Drinking Water

The International Standards for drinking water that need to be followed are:

1. Fluoride:

• For drinking purposes, water should be tested for fluoride ion concentration since its deficiency in drinking water is harmful to man.
• Soluble fluoride is often added to drinking water to bring its concentration upto 1 ppm or 1 mg dm^–3. The F^– ions make the enamel on teeth much harder by converting hydroxyapatite, $$ [3(Ca_3(PO_4)_2.Ca(OH)_2]$$      the enamel on the surface of the teeth, into much harder fluorapatite, $$ [3(Ca_3(PO_4)_2.CaF_2]$$           
• While deficiency of fluoride below 1 ppm causes diseases such as tooth decay, F^– ion concentration above 2 ppm causes brown mottling of teeth.

2. Lead:

• Drinking water gets contaminated with lead when lead pipes are used for transportation of water.
• The prescribed upper limit for concentration of lead in drinking water is about 50 ppb.
• Lead can damage kidney, liver, reproductive system etc.

3. Sulphate:

• Excessive sulphate, above 500 ppm in drinking water has a laxative effect.
• At moderate levels, it is harmless.

4. Nitrate:

• The maximum limit of nitrate in drinking water is 50 ppm.
• Excess nitrate in drinking water can cause disease such as methemoglobinemia (‘blue baby’ syndrome).

The maximum concentrations of some other common metals recommended in drinking water are given in Table 14.2:

Metal	Maximum concentration (ppm or mg dm⁻³)
Fe	0.2
Mn	0.05
Al	0.2
Cu	3
Zn	5
Cd	0.005

Questions from section 14.3:

1. What is water pollution? Name the sources that cause water pollution.

2. List and explain the causes of water pollution.

3. What is the permissible standard for the following components in drinking water:

a) Fluoride

b) Lead

c) Sulphate

d) Nitrate

14.4 Soil Pollution

Definition box: Soil Pollution: Soil pollution is defined as the building up of toxic chemicals like, chemicals, salts and radioactive materials which can have adverse effect to human health or the ecosystem.

14.4.1 Pesticides

Definition box: Pesticides: Pesticides are synthetic toxic chemicals which often have harmful ecological effects, used to kill unwanted pests like, insects, bacteria, viruses and fungi which attack food crops.

Some common pesticides and their effects:

1. DDT:  

• DDT was found to be of great use in the control of malaria and other insect-borne diseases.
• However, the repeated use of the same (or similar) pesticides gives rise to pests that are resistant to that group of pesticides. This makes pesticides ineffective. As a result, insects resistant to DDT increased.
• Thus, other organic toxins such as Aldrin and Dieldrin were introduced in the market by pesticide industry.

2. Organo-phosphates and Carbamates:

• These are a new series of less persistent or more bio-degradable products which have been introduced in the market.
• These chemicals are severe nerve toxins and hence more harmful to humans. As a result, there were reports of pesticide related deaths of agricultural field workers.

3. Herbicides:

• Most herbicides are toxic to mammals but are not as persistent as organo-chlorides. These chemicals decompose in a few months.
• Like other pesticides, herbicides also become concentrated in the food web.
• Some herbicides cause birth defects.
• Studies show that cornfields sprayed with herbicides are more prone to insect attack and plant disease than fields that are weeded manually.
• Examples of Herbicides: Sodium chlorate ($NaClO_3$) and sodium arsinite ($Na_3AsO_3$).

Other harmful effects of pesticides:

• Most of these pesticides are water insoluble and non-biodegradable. Hence, these toxins are transferred from lower trophic level to higher trophic level through food chain (refer Figure 14.3 below).
• Over time, the concentrations of toxins in higher animals reach a level which causes serious metabolic and physiological disorders.

Questions from section 14.4:

1. What is Soil Pollution?

2. What are pesticides?

3. Write a note on:

a) DDT

b) Organo-phosphates

c) Herbicides

14.5 Industrial Waste

Definition box: Industrial Waste is defined as the waste generated by manufacturing or during industrial processes.

Industrial wastes generated include: cafeteria garbage, dirt, gravel, masonry wastes like: concrete, scrap metals, trash, oil, solvents, chemicals, weed grass and trees, wood, scrap and lumber.

Industrial solid wastes are classified into two types: Biodegradable and Non-degradable wastes:

Biodegradable Wastes:

• Biodegradable waste is a type of waste which can be broken down, in a reasonable amount of time, into its base compounds by micro-organisms.
• Biodegradable wastes are generated by cotton mills, food processing units, paper mills, and textile factories.
• The main environmental threat from biodegradable waste is the production of methane during decomposition.
• Technology has now been developed to produce electricity from garbage. In this process, ferrous metal, glass and plastics are first separated. A culture of bacterial species is then introduced into the decomposable waste for producing methane. The gas is commonly known as biogas. The remaining biodegradable wastes are used as manure and to generate electricity.

Non-biodegradable wastes:

• Non-biodegradable wastes are generated by thermal power plants and in other industrial processes. Non-biodegradable wastes include: fly ash; integrated iron and steel plants which produce blast furnace slag and steel melting slag; industries manufacturing aluminium, zinc and copper produce mud and tailings; fertilizer industries produce gypsum; hazardous wastes such as inflammables, composite explosives or highly reactive substances produced by industries dealing in metals, chemicals, drugs, pharmaceuticals, dyes, pesticides and rubber goods.
• The disposal of non-degradable industrial solid wastes, if not done by a proper and suitable method, may cause serious threat to the environment.
• Recycling of waste is the best method of minimization of non-biodegradable waste. For instance, fly ash and slag from the steel industry are utilised by the cement industry. Recycling saves space in landfills and reduces the amount of new raw materials required for manufacturing. It also helps in saving energy and reducing global climatic change.
• Large quantities of toxic wastes are usually destroyed by controlled incineration, whereas small quantities are burnt along with factory garbage in open bins.

Note box: Octane number is a standard used to rate the performance of an engine or aviation fuel. The higher the octane number, the more compression the fuel can withstand before igniting. Green fuel: Fuel obtained from plastic waste recycling has high octane rating. It contains no lead and is known as “green fuel”.

Questions from section 14.5:

1. What is industrial waste?

2. Classify and explain industrial wastes.

14.6 Strategies to Control Environmental Pollution

14.6.1 Waste Management

Waste generation is a natural outcome of human activities. Generation of wastes is inevitable. Solid Waste Management is important to deal with issues of environmental hazards as improper waste disposal is the major cause for environmental waste degradation. Solid wastes include: household discards, there are medical, agricultural, industrial and mining wastes.

14.6.2 Collection and Disposal

• Domestic wastes are collected in small bins, which are then transferred to community bins by private or municipal workers.
• From these community bins, these are collected and carried to the disposable site.
• At the site, garbage is sorted out and separated into biodegradable and non-biodegradable materials. Non-biodegradable materials such as plastic, glass and metal scraps are sent for recycling. Biodegradable wastes are deposited in landfills and are converted into compost.

Harmful effects:

Unscientific disposal of municipal solid wastes poses the following problems:

• Non-biodegradable wastes like polythene bags and metal scraps enter the sewage causing blockage of sewer pipes.
• Consumption of plastics by animals causes major health hazards in them.
• Improper disposal attracts animals which may lead to incidence of diseases like, brain fever.
• The poor management causes health problems leading to epidemics due to contamination of ground water.
• It is particularly harmful to those in contact with these wastes such as, rag pickers and workers in waste disposal.

14.7 Green Chemistry

Definition box: Green chemistry is a way of thinking and is about utilising the existing knowledge and principles of chemistry and other sciences to reduce the adverse impacts on environment. In other words, Green Chemistry is that method of production which would bring about minimum pollution or deterioration to the environment.

The by-products generated during certain chemical processes, add to the environmental pollution if not recycled or disposed safely.

Such processes are not environmental friendly and the wastes generated during these processes cannot be disposed economically. Thus, utilisation of existing knowledge base for reducing the chemical hazards along with the developmental activities is the foundation of green chemistry.

Principles of Green Chemistry

The design of environmental friendly products and processes are guided by the 12 principles of Green chemistry. They are:

a) Prevention: It is better to prevent waste generation than to treat the wastes that have been generated.

b) Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

c) Synthesis of Less Hazardous chemicals: As much as possible, synthetic methods should be designed such that they possess little or no toxicity to human health and the environment.

d) Designing Safer Chemicals: Chemical products should be formulated such that they show their desired function while also have minimum toxicity.

e) Safer Solvents and Auxiliaries: The use of auxiliary substances (such as, solvents and separation agents) should be made unnecessary wherever possible. They should also not cause any harm when used.

f) Design for Energy Efficiency: Energy requirements of chemical processes should be recognized and their environmental and economic impacts should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

g) Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than a depleting resource, whenever feasible.

h) Reduce Derivatives: Unnecessary use of derivatization (blocking groups, protection or deprotection and temporary modification of physical or chemical processes) should be minimized or avoided if possible, because such steps require additional reagents which can generate waste.

i) Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

j) Design for Degradation: Chemical products should be designed so that at the end of their function, they break down into harmless, degradable products and do not persist in the environment.

k) Real-time analysis for Pollution Prevention: Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

l) Inherently Safer Chemistry for Accident Prevention: Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents like, releases, explosions, and fires.

14.7.1 Green Chemistry in day-to-day Life

(i) Dry Cleaning of Clothes

• Tetra chlroroethene ($Cl_2C=CCl_2$) was used as a solvent for dry cleaning. However, it was found that the compound contaminates ground water and is suspected to be carcinogenic.
• Thus, the dry cleaning process in which the compound was used is now being replaced by a new process, where liquefied carbondioxide, with a suitable detergent is used.
• Replacement of halogenated solvent by liquid $CO_2$ will result in less harm to ground water.
• Hydrogen peroxide ($H_2O_2$) is also used for the purpose of bleaching clothes in laundry, which gives better results and makes use of lesser amount of water.

(ii) Bleaching of Paper:

Chlorine gas was used earlier for bleaching paper. Now, hydrogen peroxide ($H_2O_2$)  with suitable catalyst, which promotes the bleaching action of hydrogen peroxide, is used.

(iii) Synthesis of Chemicals:

Ethanal ($CH_3CHO$) is now commercially prepared by one step oxidation of ethene in the presence of ionic catalyst in aqueous medium. This gives a yield of 90% ethanal.

$$ \ce { CH_2 = CH_2 + O_2 ->[{Catalyst}][{Pd(II)/Cu(II)(in water)}] CH_3CHO  } (90 \%) $$

Thus, Green chemistry is a cost effective approach which involves reduction in material, energy consumption and waste generation.

Questions from sections 14.6 and 14.7:

1. Write a note on collection and disposal of solid wastes.

2. What are the harmful effects of municipal solid wastes?

3. Define the term, “green chemistry”. List its 12 principles.

4. Explain the applications of green chemistry in day to day life.