CHEMISTRY406

UNIT 14

ENVIRONMENTAL CHEMISTRY

After studying this unit, you will be

able to

••

• understand the meaning of

environmental chemistry;

••

• define atmospheric pollution, list

reasons for global warming. green

house effect and acid rain;

••

• identify causes for ozone layer

depletion and its effects;

••

• give reasons for water pollution

and know about international

standards for drinking water;

••

• describe causes of soil pollution;

••

• suggest and adopt strategies

for control of environmental

pollution;

••

• appreciate the importance of green

chemistry in day to day life.

You have already studied about environment in your earlier

classes. Environmental studies deal with the sum of all

social, economical, biological, physical and chemical

interrelations with our surroundings. In this unit the focus

will be on environmental chemistry. Environmental

chemistry deals with the study of the origin, transport,

reactions, effects and fates of chemical species in the

environment. Let us discuss some important aspects of

environmental chemistry.

14.1 ENVIRONMENTAL POLLUTION

Environmental pollution is the effect of undesirable changes

in our surroundings that have harmful effects on plants,

animals and human beings. A substance, which causes

pollution, is known as pollutant. Pollutants can be solid,

liquid or gaseous substances present in greater

concentration than in natural abundance and are

produced due to human activities or due to natural

happenings. Do you know, an average human being

requires nearly 12-15 times more air than the food. So,

even small amounts of pollutants in the air become

significant compared to similar levels present in the food.

Pollutants can be degradable, like discarded vegetables

which rapidly break down by natural processes. On the

other hand, pollutants which are slowly degradable, remain

in the environment in an unchanged form for many

decades. For example, substances such as dichlorodi-

phenyltrichloroethane (DDT), plastic materials, heavy

metals, many chemicals, nuclear wastes etc., once released

into the environment are difficult to remove. These

The world has achieved brilliance without wisdom, power

without conscience. Ours is a world of nuclear giants and

ethical infants.

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ENVIRONMENTAL CHEMISTRY 407

pollutants cannot be degraded by natural

processes and are harmful to living organisms.

In the process of environmental pollution,

pollutants originate from a source and get

transported by air or water or are dumped into

the soil by human beings.

14.2 ATMOSPHERIC POLLUTION

The atmosphere that surrounds the earth is

not of the same thickness at all heights. There

are concentric layers of air or regions and each

layer has different density. The lowest region

of atmosphere in which the human beings

along with other organisms live is called

troposphere. It extends up to the height of

~ 10 km from sea level. Above the troposphere,

between 10 and 50 km above sea level lies

stratosphere. Troposphere is a turbulent,

dusty zone containing air, much water vapour

and clouds. This is the region of strong air

movement and cloud formation. The

stratosphere, on the other hand, contains

dinitrogen, dioxygen, ozone and little water

vapour.

Atmospheric pollution is generally studied

as tropospheric and stratospheric pollution.

The presence of ozone in the stratosphere

prevents about 99.5 per cent of the sun’s

harmful ultraviolet (UV) radiations from

reaching the earth’s surface and thereby

protecting humans and other animals from its

effect.

14.2.1 Tropospheric Pollution

Tropospheric pollution occurs due to the

presence of undesirable solid or gaseous

particles in the air. The following are the major

gaseous and particulate pollutants present in

the troposphere:

1. Gaseous air pollutants: These are oxides

of sulphur, nitrogen and carbon, hydrogen

sulphide, hydrocarbons, ozone and other

oxidants.

2. Particulate pollutants: These are dust,

mist, fumes, smoke, smog etc.

1. Gaseous air pollutants

(a) Oxides of Sulphur: Oxides of sulphur

are produced when sulphur containing fossil

fuel is burnt. The most common species,

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 e.g.,

asthma, bronchitis, emphysema in human

beings. Sulphur dioxide causes irritation to

the eyes, resulting in tears and redness. High

concentration of SO

leads to stiffness of flower

buds which eventually fall off from plants.

Uncatalysed oxidation of sulphur dioxide is

slow. However, the presence of particulate

matter in polluted air catalyses the oxidation

of sulphur dioxide to sulphur trioxide.

2SO

(g) +O

(g) → 2SO

(g)

The reaction can also be promoted by

ozone and hydrogen peroxide.

(g) +O

(g) → SO

(g) + O

(g)

(g) + H

(l) → H

(aq)

(b) Oxides of Nitrogen: Dinitrogen and

dioxygen are the main constituents of air.

These gases do not react with each other at a

normal temperature. At high altitudes when

lightning strikes, they combine to form oxides

of nitrogen. NO

is oxidised to nitrate ion,

−

which is washed into soil, where it serves as a

fertilizer. In an automobile engine, (at high

temperature) when fossil fuel is burnt,

dinitrogen and dioxygen combine to yield

significant quantities of nitric oxide (NO) and

nitrogen dioxide ( NO

) as given below:

(g) + O

(g)

1483K

→

2NO(g)

NO reacts instantly with oxygen to give NO

2NO (g) + O

(g) → 2NO

(g)

Rate of production of NO

is faster when

nitric oxide reacts with ozone in the

stratosphere.

NO (g) + O

(g) → NO

(g) + O

(g)

The irritant red haze in the traffic and

congested places is due to oxides of nitrogen.

Higher concentrations of NO

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 is toxic to living tissues

also. Nitrogen dioxide is also harmful to

various textile fibres and metals.

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CHEMISTRY408

composed of hydrogen and carbon only and

are formed by incomplete combustion of fuel

used in automobiles. Hydrocarbons are

carcinogenic, i.e., they cause cancer. They

harm plants by causing ageing, breakdown of

tissues and shedding of leaves, flowers and

twigs.

(d) Oxides of Carbon

(i ) Carbon monoxide: Carbon monoxide (CO)

is one of the most serious air pollutants. It is a

colourless and odourless gas, highly

poisonous to living beings because of its ability

to block the delivery of oxygen to the organs

and tissues. It is produced as a result of

incomplete combustion of carbon. Carbon

monoxide is mainly released into the air by

automobile exhaust. Other sources, which

produce CO, involve incomplete combustion

of coal, firewood, petrol, etc. The number of

vehicles has been increasing over the years all

over the world. Many vehicles are poorly

maintained and several have inadequate

pollution control equipments resulting in the

release of greater amount of carbon monoxide

and other polluting gases. Do you know why

carbon monoxide is poisonous? It binds to

haemoglobin to form carboxyhaemoglobin,

which is about 300 times more stable than the

oxygen-haemoglobin complex. In blood, when

the concentration of carboxyhaemoglobin

reaches about 3–4 per cent, the oxygen

carrying capacity of blood is greatly

reduced. This oxygen deficiency, results into

headache, weak eyesight, nervousness and

cardiovascular disorder. This is the reason why

people are advised not to smoke. 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 (CO

) is

released into the atmosphere by respiration,

burning of fossil fuels for energy, and by

decomposition of limestone during the

manufacture of cement. It is also emitted

during volcanic eruptions. Carbon dioxide gas

is confined to troposphere only. Normally it

forms about 0.03 per cent by volume of the

atmosphere. With the increased use of fossil

fuels, a large amount of carbon dioxide gets

released into the atmosphere. Excess of CO

in the air is removed by green plants and this

maintains an appropriate level of CO

in the

atmosphere. Green plants require CO

for

photosynthesis and they, in turn, emit oxygen,

thus maintaining the delicate balance. As you

know, deforestation and burning of fossil fuel

increases the CO

level and disturb the balance

in the atmosphere. The increased amount of

in the air is mainly responsible for global

warming.

Global Warming and Greenhouse Effect

About 75 % of the solar energy reaching the

earth is absorbed by the earth’s surface, which

increases its temperature. The rest of the heat

radiates back to the atmosphere. Some of the

heat is trapped by gases such as carbon

dioxide, methane, ozone, chlorofluorocarbon

compounds (CFCs) and water vapour in the

atmosphere. Thus, they add to the heating of

the atmosphere. This causes global warming.

We all know that in cold places flowers,

vegetables and fruits are grown in glass

covered areas called greenhouse. Do you

know that we humans also live in a

greenhouse? Of course, we are not surrounded

by glass but a blanket of air called the

atmosphere, which has kept the temperature

on earth constant for centuries. But it is now

undergoing change, though slowly. Just as

the glass in a greenhouse holds the sun’s

warmth inside, atmosphere traps the sun’s

heat near the earth’s surface and keeps it

warm. This is called natural greenhouse

effect because it maintains the temperature

and makes the earth perfect for life. In a

greenhouse, solar radiations pass through

the transparent glass and heat up the soil

and the plants. The warm soil and plants emit

infrared radiations. Since glass is opaque to

infrared radiations (thermal region), it partly

reflects and partly absorbs these radiations.

This mechanism keeps the energy of the

sun trapped in the greenhouse. Similarly,

carbon dioxide molecules also trap heat as

they are transparent to sunlight but not

to the heat radiation. If the amount of

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ENVIRONMENTAL CHEMISTRY 409

carbon dioxide crosses the delicate proportion

of 0.03 per cent, the natural greenhouse

balance may get disturbed. Carbon dioxide is

the major contributor to global warming.

Besides carbon dioxide, other greenhouse

gases are methane, water vapour, nitrous

oxide, CFCs and ozone. Methane is produced

naturally when vegetation is burnt, digested

or rotted in the absence of oxygen. Large

amounts of methane are released in paddy

fields, coal mines, from rotting garbage dumps

and by fossil fuels. Chlorofluorocarbons (CFCs)

are man-made industrial chemicals used in

air conditioning etc. CFCs are also damaging

the ozone layer (Section 14.2.2). Nitrous oxide

occurs naturally in the environment. In recent

years, their quantities have increased

significantly due to the use of chemical

fertilizers and the burning of fossil fuels. If

these trends continue, the average global

temperature will increase to a level which may

lead to melting of polar ice caps and flooding

of low lying areas all over the earth. Increase

in the global temperature increases the

incidence of infectious diseases like dengue,

malaria, yellow fever, sleeping sickness etc.

Acid rain

We are aware that normally rain water has a

pH of 5.6 due to the presence of H

ions formed

by the reaction of rain water with carbon

Fig. 14.1 Acid deposition

Think it Over

What can we do to reduce the rate of global

warming?

If burning of fossil fuels, cutting down

forests and trees add to greenhouse gases

in the atmosphere, we must find ways to

use these just efficiently and judiciously.

One of the simple things which we can do

to reduce global warming is to minimise the

use of automobiles. Depending upon the

situation, one can use bicycle, public

transport system, or go for carpool. We

should plant more trees to increase the

green cover. Avoid burning of dry leaves,

wood etc. It is illegal to smoke in public

places and work places, because it is

harmful not only for the one who is smoking

but also for others, and therefore, we should

avoid it. Many people do not understand

the greenhouse effect and the global

warming. We can help them by sharing the

information that we have.

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dioxide present in the atmosphere.

O (l) + CO

(g)



(aq)



(aq) + HCO

–

(aq)

When the pH of the rain water drops below

5.6, it is called acid rain.

Acid rain refers to the ways in which acid

from the atmosphere is deposited on the

earth’s surface. Oxides of nitrogen and

sulphur which are acidic in nature can be

blown by wind along with solid particles in the

atmosphere and finally settle down either on

the ground as dry deposition or in water, fog

and snow as wet deposition. (Fig. 14.1)

Acid rain is a byproduct of a variety of

human activities that emit the oxides of

sulphur and nitrogen in the atmosphere. As

mentioned earlier, burning of fossil fuels (which

contain sulphur and nitrogenous matter) such

as coal and oil in power stations and furnaces

or petrol and diesel in motor engines produce

sulphur dioxide and nitrogen oxides. SO

and

after oxidation and reaction with water

are major contributors to acid rain, because

polluted air usually contains particulate

matter that catalyse the oxidation.

2SO

(g) + O

(g) + 2H

O (l) → 2H

(aq)

4NO

(g) + O

(g)+ 2H

O (l) → 4HNO

(aq)

Ammonium salts are also formed and can

be seen as an atmospheric haze (aerosol of fine

particles). Aerosol particles of oxides or

ammonium salts in rain drops result in wet-

deposition. SO

is also absorbed directly on

both solid and liquid ground surfaces and is

thus deposited as dry-deposition.

Acid rain is harmful for agriculture, trees

and plants as it dissolves and washes away

nutrients needed for their growth. It causes

respiratory ailments in human beings and

animals. When acid rain falls and flows as

ground water to reach rivers, lakes etc. it affects

plants and animal life in aquatic ecosystem. It

corrodes water pipes resulting in the leaching

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. The Taj Mahal in India has been

affected by acid rain.

Activity 1

You can collect samples of water from

nearby places and record their pH values.

Discuss your results in the class. Let us

discuss how we can help to reduce the

formation of acid rain.

This can be done by reducing the

emission of sulphur dioxide and nitrogen

dioxide in the atmosphere. We should use

less vehicles driven by fossil fuels; use less

sulphur content fossil fuels for power

plants and industries. We should use

natural gas which is a better fuel than coal

or use coal with less sulphur content.

Catalytic converters must be used in cars

to reduce the effect of exhaust fumes on

the atmosphere. The main component of

the converter is a ceramic honeycomb

coated with precious metals — Pd, Pt and

Rh. The exhaust gases containing unburnt

fuel, CO and NO

, when pass through the

converter at 573 K, are converted into CO

and N

. We can also reduce the acidity of

the soil by adding powdered limestone to

neutralise the acidity of the soil. Many

people do not know of acid rain and its

harmful effects. We can make them aware

by passing on this information and save

the Nature.

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. The resulting acid rain

reacts with marble, CaCO

of Taj Mahal

(CaCO

→ CaSO

+ H

O+ CO

)

causing damage to this wonderful

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. Mathura refinery has already

taken suitable measures to check the

emission of toxic gases.

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2. Particulate Pollutants

Particulates pollutants are the minute solid

particles or liquid droplets in air. These are

present in vehicle emissions, smoke particles

from fires, dust particles and ash from

industries. Particulates in the atmosphere

may be viable or non-viable. The viable

particulates e.g., bacteria, fungi, moulds,

algae etc., are minute living organisms that are

dispersed in the atmosphere. Human beings

are allergic to some of the fungi found in air.

They can also cause plant diseases.

Non-viable particulates may be classified

according to their nature and size as follows:

(a) Smoke particulates consist of solid or

mixture of solid and liquid particles formed

during combustion of organic matter.

Examples are cigarette smoke, smoke from

burning of fossil fuel, garbage and dry

leaves, oil smoke etc.

(b) Dust is composed of fine solid particles

(over 1

µm in diameter), produced during

crushing, grinding and attribution of solid

materials. Sand from sand blasting, saw

dust from wood works, pulverized coal,

cement and fly ash from factories, dust

storms etc., are some typical examples of

this type of particulate emission.

liquids and by condensation of vapours in

air. Examples are sulphuric acid mist and

herbicides and insecticides that miss their

targets and travel through air and form

mists.

(d) Fumes are generally obtained by the

condensation of vapours during

sublimation, distillation, boiling and

several other chemical reactions. Generally,

organic solvents, metals and metallic

oxides form fume particles.

The effect of particulate pollutants are

largely dependent on the particle size. Air-

borne particles such as dust, fumes, mist etc.,

are dangerous for human health. Particulate

pollutants bigger than 5 microns are likely to

lodge in the nasal passage, whereas particles

of about 10 micron enter into lungs easily.

Lead used to be a major air pollutant

emitted by vehicles. Leaded petrol used to be

the primary source of air-borne lead emission

in Indian cities. This problem has now been

overcome by using unleaded petrol in most of

the cities in India. Lead interferes with the

development and maturation of red blood cells.

Smog

The word smog is derived from smoke and fog.

This is the most common example of air

pollution that occurs in many cities

throughout the world. There are two types of

smog:

(a) Classical smog occurs in cool humid

climate. It is a mixture of smoke, fog and

sulphur dioxide. Chemically it is a

reducing mixture and so it is also called

as reducing smog.

(b) Photochemical smog occurs in warm, dry

and sunny climate. The main components

of the photochemical smog result from the

action of sunlight on unsaturated

hydrocarbons and nitrogen oxides

produced by automobiles and factories.

Photochemical smog has high

concentration of oxidising agents and is,

therefore, called as oxidising smog.

Formation of photochemical smog

When fossil fuels are burnt, a variety of

pollutants are emitted into the earth’s

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.

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troposphere. Two of the pollutants that are

emitted are hydrocarbons (unburnt fuels) and

nitric oxide (NO). When these pollutants build

up to sufficiently high levels, a chain reaction

occurs from their interaction with sunlight in

which NO is converted into nitrogen dioxide

(NO

). This NO

in turn absorbs energy from

sunlight and breaks up into nitric oxide and

free oxygen atom (Fig. 14.2).

(g)

NO(g) + O(g) (i)

Oxygen atoms are very reactive and

combine with the O

in air to produce ozone.

O(g) + O

(g)



(g) (ii)

The ozone formed in the above reaction (ii)

reacts rapidly with the NO(g) formed in the

reaction (i) to regenerate NO

is a brown

gas and at sufficiently high levels can

contribute to haze.

NO (g) + O

(g) → NO

(g) + O

(g) (iii)

Ozone is a toxic gas and both NO

and

are strong oxidising agents and can react

with

the unburnt hydrocarbons in the polluted air

to produce chemicals such as formaldehyde,

acrolein and peroxyacetyl nitrate (PAN).

3CH

+ 2O

→ 3CH

= O + 3H

Formaldehyde

=CHCH=O CH

COONO

Acrolein



Peroxyacetyl nitrate (PAN)

Effects of photochemical smog

The common components of photochemical

smog are ozone, nitric oxide, acrolein,

formaldehyde and peroxyacetyl nitrate (PAN).

Photochemical smog causes serious health

problems. Both ozone and PAN act as powerful

eye irritants. Ozone and nitric oxide irritate the

nose and throat and their high concentration

causes headache, chest pain, dryness of the

throat, cough and difficulty in breathing.

Photochemical smog leads to cracking of

rubber and extensive damage to plant life. It

also causes corrosion of metals, stones,

building materials, rubber and painted

surfaces.

Fig. 14.2 Photochemical smog occurs where sunlight acts on vehicle pollutants.

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How can photochemical smog be

controlled ?

Many techniques are used to control or reduce

the formation of photochemical smog. If we

control the primary precursors of

photochemical smog, such as NO

and

hydrocarbons, the secondary precursors such

as ozone and PAN, the photochemical smog

will automatically be reduced. Usually catalytic

converters are used in the automobiles, which

prevent the release of nitrogen oxide and

hydrocarbons to the atmosphere. Certain

plants e.g., Pinus, Juniparus, Quercus, Pyrus

and Vitis can metabolise nitrogen oxide and

therefore, their plantation could help in this

matter.

14.2.2 Stratospheric Pollution

Formation and Breakdown of Ozone

The upper stratosphere consists of

considerable amount of ozone (O

), which

protects us from the harmful ultraviolet (UV)

radiations (λ 255 nm) coming from the sun.

These radiations cause skin cancer

(melanoma) in humans. Therefore, it is

important to maintain the ozone shield.

Ozone in the stratosphere is a product of

UV radiations acting on dioxygen (O

)

molecules. The UV radiations split apart

molecular oxygen into free oxygen (O) atoms.

These oxygen atoms combine with the

molecular oxygen to form ozone.

(g)

O(g) + O(g)

O(g) + O

(g)

Ozone is thermodynamically unstable and

decomposes to molecular oxygen. Thus, a

dynamic equilibrium exists between the

production and decomposition of ozone

molecules. In recent years, there have been

reports of the depletion of this protective ozone

layer because of the presence of certain

chemicals in the stratosphere. The main

reason of ozone layer depletion is believed to

be the release of chlorofluorocarbon

compounds (CFCs), also known as freons.

These compounds are nonreactive, non

flammable, non toxic organic molecules and

therefore used in refrigerators, air conditioners,

in the production of plastic foam and by the

electronic industry for cleaning computer

parts etc. Once CFCs are released in the

atmosphere, they mix with the normal

atmospheric gases and eventually reach the

stratosphere. In stratosphere, they get broken

down by powerful UV radiations, releasing

chlorine free radical.

(g)

•

l (g) +

•

Cl (g) (i)

The chlorine radical then react with

stratospheric ozone to form chlorine monoxide

radicals and molecular oxygen.

•

l (g) + O

(g) → Cl

•

(g) + O

(g) (ii)

Reaction of chlorine monoxide radical with

atomic oxygen produces more chlorine

radicals.

•

(g) + O

(g) →

•

l (g) + O

(g) (iii)

The chlorine radicals are continuously

regenerated and cause the breakdown of

ozone. Thus, CFCs are transporting agents for

continuously generating chlorine radicals into

the stratosphere and damaging the ozone layer.

The Ozone Hole

In 1980s atmospheric scientists working in

Antarctica reported about depletion of ozone

layer commonly known as ozone hole over the

South Pole. It was found that a unique set of

conditions was responsible for the ozone hole.

In summer season, nitrogen dioxide and

methane react with chlorine monoxide

(reaction iv) and chlorine atoms (reaction v)

forming chlorine sinks, preventing much ozone

depletion, whereas in winter, special type of

clouds called polar stratospheric clouds are

formed over Antarctica. These polar

stratospheric clouds provide surface on which

chlorine nitrate formed (reaction iv) gets

hydrolysed to form hypochlorous acid

(reaction (vi)). It also reacts with hydrogen

chloride produced as per reaction (v) to give

molecular chlorine.

•

(g) + NO

(g) → ClONO

(g) (iv)

•

l (g) + CH

(g) →

•

(g) + HCl(g) (v)

ClONO

(g) + H

O (g) → HOCl (g) + HNO

(g) (vi)

ClONO

(g) + HCl (g) → Cl

(g) + HNO

(g) (vii)

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When sunlight returns to the Antarctica in

the spring, the sun’s warmth breaks up the

clouds and HOCl and Cl

are photolysed by

sunlight, as given in reactions (viii) and (ix).

HOCl (g)

  →

•

H (g) +

•

l(g) (viii)

(g)

→

•

l (g) (ix)

The chlorine radicals thus formed, initiate

the chain reaction for ozone depletion as

described earlier.

Effects of Depletion of the Ozone Layer

With the depletion of ozone layer, more UV

radiation filters into troposphere. UV

radiations lead to ageing of skin, cataract,

sunburn, skin cancer, killing of many

phytoplanktons, damage to fish productivity

etc. It has also been reported that plant

proteins get easily affected by UV radiations

which leads to the harmful mutation of cells.

It also increases evaporation of surface water

through the stomata of the leaves and

decreases the moisture content of the soil.

Increase in UV radiations damage paints and

fibres, causing them to fade faster.

14.3 WATER POLLUTION

Water is essential for life. Without water there

would be no life. We usually take water as

granted for its purity, but we must ensure the

quality of water. Pollution of water originates

from human activities. Through different

paths, pollution reaches surface or ground

water. Easily identified source or place of

pollution is called as point source. e.g.,

municipal and industrial discharge pipes

where pollutants enter the water-source. Non

point sources of pollution are those where a

source of pollution cannot be easily identified,

e.g., agricultural run off (from farm, animals

and crop-lands), acid rain, storm-water

drainage (from streets, parking lots and lawns),

etc. Table 14.1 lists the major water pollutants

and their sources.

14.3.1 Causes of Water Pollution

(i) Pathogens: The most serious water

pollutants are the disease causing agents

called pathogens. Pathogens include bacteria

and other organisms that 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: The other major water

pollutant is organic matter such as

leaves, grass, trash etc. They pollute water as

a consequence of run off. Excessive

phytoplankton growth within water is also a

cause of water pollution. These wastes are

biodegradable.

The large population of bacteria

decomposes organic matter present in water.

They consume oxygen dissolved in water. The

amount of oxygen that water can hold in the

solution is limited. In cold water, dissolved

oxygen (DO) can reach a concentration up to

10 ppm (parts per million), whereas oxygen in

air is about 200,000 ppm. That is why even a

moderate amount of organic matter when

decomposes in water can deplete the water of

its dissolved oxygen. The concentration of

Table 14.1 Major Water Pollutants

Pollutant Source

Micro-organisms Domestic sewage

Organic wastes Domestic sewage, animal excreta and waste, decaying animals

and plants, discharge from food processing factories.

Plant nutrients Chemcial fertilizers

Toxic heavy metals Industries and chemical factories

Sediments Erosion of soil by agriculture and strip mining

Pesticides Chemicals used for killing insects, fungi and weeds

Radioactive substances Mining of uranium containing minerals

Heat Water used for cooling in industries

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dissolved oxygen in water is very important

for aquatic life . If the concentration of dissolved

oxygen of water is below 6 ppm, the growth of

fish gets inhibited. Oxygen reaches water

either through atmosphere or from the process

of photosynthesis carried out by many

aquatic green plants during day light.

However, during night, photosynthesis stops

but the plants continue to respire, resulting

in reduction of dissolved oxygen. The

dissolved oxygen is also used by

microorganisms to oxidise organic matter.

If too much of organic matter is added to

water, all the available oxygen is used up. This

causes oxygen dependent aquatic life to die.

Thus, anaerobic bacteria (which do not require

oxygen) begin to break down the organic waste

and produce chemicals that have a foul smell

and are harmful to human health. Aerobic

(oxygen requiring) bacteria degrade these

organic wastes and keep the water depleted

in dissolved oxygen.

Thus, the amount of oxygen required by

bacteria to break down the organic matter

present in a certain volume of a sample of

water, is called Biochemical Oxygen Demand

(BOD). The amount of BOD in the water is a

measure of the amount of organic material in

the water, in terms of how much oxygen will

be required to break it down biologically. Clean

water would have BOD value of less than

5 ppm whereas highly polluted water could

have a BOD value of 17 ppm or more.

(iii) Chemical Pollutants: As we know that

water is an excellent solvent, water soluble

inorganic chemicals that include heavy metals

such as cadmium, mercury, nickel etc

constitute an important class of pollutants. All

these metals are dangerous to humans

because our body cannot excrete them. Over

the time, it crosses the tolerance limit. These

metals then can damage kidneys, central

nervous system, liver etc. Acids (like sulphuric

acid) from mine drainage and salts from many

different sources including raw salt used to

melt snow and ice in the colder climates

(sodium and calcium chloride) are water

soluble chemical pollutants.

The organic chemicals are another group

of substances that are found in polluted water.

Petroleum products pollute many sources of

water e.g., major oil spills in oceans. Other

organic substances with serious impacts are

the pesticides that drift down from sprays or

runoff from lands. Various industrial

chemicals like polychlorinated biphenyls,

(PCBs) which are used as cleansing solvent,

detergents and fertilizers add to the list of

water pollutants. PCBs are suspected to be

carcinogenic. Nowadays most of the detergents

available are biodegradable. However, their use

can create other problems. The bacteria

responsible for degrading biodegradable

detergent feed on it and grow rapidly. While

growing, they may use up 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 profuse growth

of algae, covers the water surface and reduces

the oxygen concentration in water. This leads

to anaerobic conditions, commonly with

accumulation of abnoxious decay and animal

death. Thus, bloom-infested water inhibits the

growth of other living organisms in the

water body. This process in which nutrient

enriched water bodies support a dense plant

population, which kills animal life by depriving

it of oxygen and results in subsequent loss of

biodiversity is known as Eutrophication.

14.3.2 International Standards for

Drinking Water

The International Standards for drinking water

are given below and they must be followed.

Fluoride: For drinking purposes, water

should be tested for fluoride ion concentration.

Its deficiency in drinking water is harmful to

man and causes diseases such as tooth decay

etc. 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

(PO

)

.Ca(OH)

], the

enamel on the surface of the teeth, into much

harder fluorapatite, [3(Ca

(PO

)

.CaF

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CHEMISTRY416

However, F

–

ion concentration above 2 ppm

causes brown mottling of teeth. At the same

time, excess fluoride (over 10 ppm) causes

harmful effect to bones and teeth, as reported

from some parts of Rajasthan.

Lead: Drinking water gets contaminated with

lead when lead pipes are used for

transportation of water. The prescribed upper

limit concentration of lead in drinking water

is about 50 ppb. Lead can damage kidney,

liver, r eproductive system etc.

Sulphate: Excessive sulphate (>500 ppm) in

drinking water causes laxative effect, otherwise

at moderate levels it is harmless.

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).

Other metals: The maximum concentration

of some common metals recommended in

drinking water are given in Table 14.2.

pollution levels. Ensure that appropriate

action is taken. You can write to the press

also. Do not dump waste into a

household or industrial drain which can

enter directly to any water body, such as,

river, pond, stream or lake. Use compost

instead of chemical fertilizers in gardens.

Avoid the use of pesticides like DDT,

malathion etc., at home and try to use

dried neem leaves to help keep insects

away. Add a few crystals of potassium

permanganate (KMnO

) or bleaching

powder to the water tank of your house.

14.4 SOIL POLLUTION

India being an agriculture based economy

gives high priority to agriculture, fisheries and

livestock development. The surplus

production is stored by governmental and

non-governmental organisations for the lean

season. The food loss during the storage also

needs special attention. Have you ever seen the

damages caused to the crops, food items by

insects, rodents, weeds and crop diseases etc?

How can we protect them? You are acquainted

with some insecticides and pesticides for

protection of our crops. However, these

insecticides, pesticides and herbicides cause

soil pollution. Hence, there is a need for their

judicious use.

14.4.1 Pesticides

Prior to World War II, many naturally

occurring chemicals such as nicotine (by

planting tobacco plants in the crop field), were

used as pest controlling substance for major

crops in agricultural practices.

During World War II, DDT was found to be

of great use in the control of malaria and other

insect-borne diseases. Therefore, after the war,

DDT was put to use in agriculture to control

the damages caused by insects, rodents, weeds

and various crop diseases. However, due to

adverse effects, its use has been banned in

India.

Pesticides are basically synthetic toxic

chemicals with ecological repercussions. The

repeated use of the same or similar pesticides

give rise to pests that are resistant to that

Metal Maximum concentration

(ppm or mg dm

–3

)

Fe 0.2

Mn 0.05

Al 0.2

Cu 3.0

Zn 5.0

Cd 0.005

Table 14.2 Maximum Prescribed Concen-

tration of Some Metals in

Drinking Water.

Activity 2

You can visit local water sources and

observe if the river/lake/tank/pond are

unpolluted/slightly polluted/ moderately

polluted or severely polluted by looking

at water or by checking pH of water.

Document the name of the river and the

nearby urban or industrial site from

where the pollution is generated. Inform

about this to Pollution Control Board’s

office set up by Government to measure

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ENVIRONMENTAL CHEMISTRY 417

group of pesticides thus making the pesticides

ineffective. Therefore, as insect resistance of

DDT increased, other organic toxins such as

Aldrin and Dieldrin were introduced in the

market by pesticide industry. Most of the

organic toxins are water insoluble and non-

biodegradable. These high persistent toxins

are, therefore, transferred from lower trophic

level to higher trophic level through food chain

(Fig.14.3). Over the time, the concentration of

toxins in higher animals reach a level which

causes serious metabolic and physiological

disorders.

Fig. 14.3 At each trophic level, the pollutant

gets 10 times concentrated.

sodium chlorate (NaClO

), sodium arsinite

(Na

AsO

) and many others. During the first

half of the last century, the shift from

mechanical to chemical weed control had

provided the industry with flourishing

economic market. But one must remember that

these are also not environment friendly.

Most herbicides are toxic to mammals but

are not as persistent as organo-chlorides.

These chemicals decompose in a few months.

Like organo-chlorides, these too become

concentrated in the food web. Some herbicides

cause birth defects. Studies show that corn-

fields sprayed with herbicides are more prone

to insect attack and plant disease than fields

that are weeded manually.

Pesticides and herbicides represent only a

very small portion of widespread chemical

pollution. A large number of other compounds

that are used regularly in chemical and

industrial processes for manufacturing

activities are finally released in the atmosphere

in one or other form.

14.5 INDUSTRIAL WASTE

Industrial solid wastes are also sorted out as

biodegradable and non-degradable wastes.

Biodegradable wastes are generated by cotton

mills, food processing units, paper mills, and

textile factories.

Non-biodegradable wastes are generated

by thermal power plants which produce 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 are produced by industries

dealing in metals, chemicals, drugs, pharma-

ceuticals, dyes, pesticides, rubber goods etc.

The disposal of non-degradable industrial

solid wastes, if not done by a proper and

suitable method, may cause serious threat to

the environment. New innovations have led to

different uses of waste material. Nowadays,

fly ash and slag from the steel industry are

utilised by the cement industry. Large

In response to high persistence of

chlorinated organic toxins, a new series of less

persistent or more bio-degradable products

called organo-phosphates and carbamates

have been introduced in the market. But these

chemicals are severe nerve toxins and hence

more harmful to humans. As a result, there

are reports of some pesticides related deaths

of agricultural field workers. Insects have

become resistant to these insecticides also. The

insecticide industry is engaged in developing

new groups of insecticides. But one has to

think, is this the only solution to pest menace?

These days, the pesticide industry has

shifted its attention to herbicides such as

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quantities of toxic wastes are usually destroyed

by controlled incineration, whereas small

quantities are burnt along with factory

garbage in open bins. Moreover, solid wastes

if not managed effectively, affect the

components of the environment.

household discards, there are medical,

agricultural, industrial and mining wastes. The

improper disposal of wastes is one of the major

causes of environmental degradation.

Therefore, the management of wastes is of

utmost importance.

You must be aware of the ‘Swachh Bharat

Abhiyan’ or ‘Clean India Mission’ launched by

the Government of India.

Do you know about waste recycling?

••

• Fuel obtained from plastic waste has

high octane rating. It contains no lead

and is known as “green fuel”.

••

• Due to recent developments made in

chemical and textile industries, clothes

will be made from recycled plastic

waste. These will be available soon in

the global textile market.

••

• In India, our cities and towns face

endless hours of power cut. We can also

see piles of rotting garbage here and

there. There is a good news that we can

get rid from both these problems

simultaneously. Technology has now

been developed to produce electricity

from the garbage. A pilot plant has been

set up, where after removing ferrous

metals, plastic, glass, paper etc. from

garbage, it is mixed with water. It is then

cultured with bacterial species for

producing methane, commonly known

as biogas. The remaining product is

used as manure and biogas is used to

produce electricity.

14.6 STRATEGIES TO CONTROL

ENVIRONMENTAL POLLUTION

After studying air, water, soil and industrial

waste pollution in this unit, by now you must

have started feeling the need of controlling

environmental pollution: How can you save

your immediate environment? Think of the

steps/activities, which you would like to

undertake for controlling air, water, soil and

industrial waste pollution in your

neighbourhood. Here, an idea about the

strategies for the management of waste is given.

14.6.1 Waste Management

Solid waste is not the only waste, which you

see in your household garbage box. Besides

Two programmes are being implemented

under the broad umbrella of the Swachh

Bharat Abhiyan. These are Swachh Bharat

Mission–Urban (SBM–U) and Swachh Bharat

Mission Gramin (SBM–G). The SBM–U

primarily aims at making Urban India free

from open defecation and achieving 100%

scientific management of solid waste in the

country. The SBM–G targets to bring about

an improvement in the general quality of life

in rural areas by promoting cleanliness and

hygiene, and eliminating open defecation. It

is accelerating its efforts to achieve the goal of

universal sanitation coverage by 2 October,

2019, which is the birth aniversary of

Mahatma Gandhi. If you have participated in

some event on Swachh Bharat Mission, write

your experiences.

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, metal scraps etc. are sent for

recycling. Biodegradable wastes are deposited

in land fills and are converted into compost.

The waste if not collected in garbage bins,

finds its way into the sewers. Some of it is eaten

by cattle. Non-biodegradable wastes like

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polythene bag, metal scraps, etc. choke the

sewers and cause inconvenience. Polythene

bags, if swallowed by animals can cost their

lives also.

As a normal practice, therefore, all

domestic wastes should be properly collected

and disposed. The poor management causes

health problems leading to epidemics due to

contamination of ground water. It is specially

hazardous for those who are in direct contact

with the waste such as rag pickers and workers

involved in waste disposal, as they are the ones

who handle waste materials mostly without

protective device such as gloves or water proof

boots and gas masks. What can you do for

them?

14.7 GREEN CHEMISTRY

14.7.1 Introduction

It is well known fact that self-sufficiency in food

has been achieved in India since late 20

century by using fertilizers and pesticides and

exploring improved methods of farming, good

quality seeds, irrigation etc. But over-

exploitation of soil and excessive use of

fertilizers and pesticides have resulted in the

deterioration of soil, water and air.

The solution of this problem does not lie in

stopping the process of development that has

been set in; but to discover methods, which

would help in the reduction of deterioration of

the environment. 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 impact

on environment. Green chemistry is a

production process that would bring about

minimum pollution or deterioration to the

environment. The byproducts generated

during a process, if not used gainfully, add

to the environmental pollution. Such

processes are not only environmental

unfriendly but also cost-ineffective. The

waste generation and its disposal both are

economically unsound. Utilisation of existing

knowledge base for reducing the chemical

Nobel goes to Green Chemists

Yves Chauvin, Institut Français du Pétrole, Rueil-Malmaison France, Robert H. Grubbs

California Institute of Technology (Caltech), Pasadena, CA, USA and Richard R. Schrock

Massachusetts Institute of Technology (MIT), Cambridge, MA, USA won the 2005 Nobel Prize

in chemistry for work that reduces hazardous waste in creating new chemicals. The trio won

the award for their development of the metathesis method in organic synthesis –a way to

rearrange groups of atoms within molecules that the Royal Swedish Academy of Sciences

likened to a dance in which couples change partners. The metathesis has tremendous

commercial potential in the pharmaceuticals, biotechnology and food stuffs production

industries. It is also used in the development of revolutionary environmentally-friendlier

polymers.

This represents a great step forward for ‘green chemistry’, reducing potentially hazardous

waste through smarter production. Metathesis is an example of how important application of

basic science is for the benefit of man, society and the environment.

Yves Chauvin Robert H. Grubbs Richard R. Schrock

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CHEMISTRY420

hazards along with the developmental

activities is the foundation of green

chemistry. Have you perceived the idea of green

chemistry? It is well known that organic

solvents such as benzene, toluene, carbon

tetrachloride etc., are highly toxic. One should

be careful while using them.

As you know, a chemical reaction involves

reactants, attacking reagents and the medium

in which the reaction takes place. Extent of any

reaction depends upon physical parameters

like temperature, pressure and use of catalyst.

In a chemical reaction, if reactants are fully

converted into useful environmental friendly

products by using an environment friendly

medium then there would be no chemical

pollutants introduced in the environment.

During a synthesis, care must be taken to

choose starting materials that can be converted

into end products with yield approximately

upto 100 per cent. This can be achieved by

arriving at optimum conditions of synthesis.

It may be worthwhile to carry out synthetic

reactions in aqueous medium since water has

high specific heat and low volatility. Water is

cost effective, noninflammable and devoid of

any carcinogenic effects.

14.7.2 Green Chemistry in day-to-day Life

(i) Dry Cleaning of Clothes

Tetra chlroroethene (Cl

C=CCl

) was earlier

used as solvent for dry cleaning. The

compound contaminates the ground water and

is also a suspected carcinogen. The process

using this compound is now being replaced

by a process, where liquefied carbondioxide,

with a suitable detergent is used. Replacement

of halogenated solvent by liquid CO

will result

in less harm to ground water.

These days hydrogen peroxide (H

) is

used for the purpose of bleaching clothes in

the process of laundary, which gives better

results and makes use of lesser amount of

water.

(ii) Bleaching of Paper

Chlorine gas was used earlier for bleaching

paper. These days, hydrogen peroxide (H

)

Think it Over

What is our responsibility as a human

being to protect our environment?

Some concepts, if followed by an individual,

contribute towards a better quality of our

environment and human life. Always set up

a compost tin in your garden or any other

place in your home and use it to produce

manure for your plants to reduce the use

of fertilizers. Use a cloth bag and avoid

asking for plastic carry bags when you buy

groceries, vegetables or any other item. See

that all newspapers, glass, aluminum and

other items in your area are recycled. We

might have to take little trouble to locate

such dealers. We must realize that we do

not have solutions for every problem but

we can concentrate on issues, which we feel

strongly about and can do some thing

about. We should take care to put into

practice whatever we preach. Always

remember environment protection begins

with us.

with suitable catalyst, which promotes the

bleaching action of hydrogen peroxide, is used.

(iii) Synthesis of Chemicals

Ethanal (CH

CHO) is now commercially

prepared by one step oxidation of ethene in

the presence of ionic catalyst in aqueous

medium with a yield of 90%.

( )( )

()

Catalyst

2 22

Pd(II)/Cu II in water

CH CH O

CH CHO 90%

= + →

(iv) ‘Green Solution’ to Clean Turbid Water

Powder of kernel of tamarind seeds has been

found to be an effective material to make

municipal and industrial waste water clean.

It is non-toxic, biodegradable and cost-

effective material. This powder is usually

discarded as agricultural waste. The present

practice is to use alum to treat such water. It

has been found that alum increases toxic ions

in treated water and can cause diseases.

Green chemistry, in a nutshell, is a cost

effective approach which involves reduction in

material, energy consumption and waste

generation.

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SUMMARY

Environmental chemistry plays a major role in environment. Chemical species present

in the environment are either naturally occurring or generated by human activities.

Environmental pollution is the effect of undesirable changes in the surrounding that

have harmful effects on plants, animals and human beings. Pollutants exist in all the

three states of matter. We have discussed only those pollutants, which are due to human

activities, and can be controlled. Atmospheric pollution is generally studied as

tropospheric and stratospheric pollution. Troposphere is the lowest region of the

atmosphere (~10 km) in which man along with other organisms including plants exist.

Whereas stratosphere extends above troposphere up to 50 km above sea level. Ozone

layer is one of the important constituents of stratosphere. Tropospheric pollution is

basically due to various oxides of sulphur, nitrogen, carbon, halogens and also due to

particulate pollutants. The gaseous pollutants come down to the earth in the form of

acid rain. 75% of the solar energy reaching earth is absorbed by the earth surface and

rest is radiated back to the atmosphere. These gases mentioned above trap the heat

which result into global warming. It is important to realise that these very gases are

also responsible for the life on the earth as they trap the requisite amount of solar

energy for the sustainance of life. The increase in the greenhouse gases is raising the

temperature of the earth’s atmosphere which, if not checked, may eventually result in

melting of polar ice caps and consequently may submerge the costal land mass. Many

human activities are producing chemicals, which are responsible for the depletion of

ozone layer in the stratosphere, leading to the formation of ozone hole. Through the

ozone hole, ultraviolet radiations can penetrate into the earth’s atmosphere causing

mutation of genes. Water is the elixir of life but the same water, if polluted by pathogens,

organic wastes, toxic heavy metals, pesticides etc., will turn into poison. Therefore, one

should take care to follow international standards to maintain purity levels of drinking

water. Industrial wastes and excessive use of pesticides, result into pollution of land

mass and water bodies. Judicious use of chemicals required for agricultural practices

can lead to sustainable development. Strategies for controlling environmental

pollution can be: (i) waste management i.e., reduction of the waste and proper disposal,

also recycling of materials and energy, (ii) adopting methods in day-to-day life, which

results in the reduction of environmental pollution. The second method is a new branch

of chemistry, which is in its infancy known as green chemistry. It utilizes the existing

knowledge and practices so as to bring about reduction in the production of pollutants.

EXERCISES

14.1 Define environmental chemistry.

14.2 Explain tropospheric pollution in 100 words.

14.3 Carbon monoxide gas is more dangerous than carbon dioxide gas. Why?

14.4 List gases which are responsible for greenhouse effect.

14.5 Statues and monuments in India are affected by acid rain. How?

14.6 What is smog? How is classical smog different from photochemical smogs?

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14.7 Write down the reactions involved during the formation of photochemical smog.

14.8 What are the harmful effects of photochemical smog and how can they be

controlled?

14.9 What are the reactions involved for ozone layer depletion in the stratosphere?

14.10 What do you mean by ozone hole? What are its consequences?

14.11 What are the major causes of water pollution? Explain.

14.12 Have you ever observed any water pollution in your area? What measures would

you suggest to control it?

14.13 What do you mean by Biochemical Oxygen Demand (BOD)?

14.14 Do you observe any soil pollution in your neighbourhood? What efforts will you

make for controlling the soil pollution?

14.15 What are pesticides and herbicides? Explain giving examples.

14.16 What do you mean by green chemistry? How will it help decrease environmental

pollution?

14.17 What would have happened if the greenhouse gases were totally missing in the

earth’s atmosphere? Discuss.

14.18 A large number of fish are suddenly found floating dead on a lake. There is no

evidence of toxic dumping but you find an abundance of phytoplankton. Suggest

a reason for the fish kill.

14.19 How can domestic waste be used as manure?

14.20 For your agricultural field or garden you have developed a compost producing

pit. Discuss the process in the light of bad odour, flies and recycling of wastes

for a good produce.

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