ENVIRONMENTAL CHEMISTRY || Environmental Chemistry Class 12 Notes || Notes ||

                 1. Introduction

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.

       2. 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 pollutant 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 di c h l o r o d i phenyl trichloroethane (DDT), plastic materials, heavy metals, many chemicals, nuclear wastes etc., once released into the environment are difficult to remove. These 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.

       3. 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, di oxygen, 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) radiation from reaching the earth’s surface and thereby protecting humans and other animals from its effect.

(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 NO2 leads to stiffness of flower buds which eventually fall off from plants. U n catalysed oxidation of sulphur dioxide is slow. However, the presence of particulate matter in polluted air catalyses the oxidation of sulphur dioxide to sulphur trioxide.

     2 SO2 (g) + O2 (g) → 2 SO3(g)

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

   SO2 (g) + O3(g) → SO3(g) + O2 (g)

  SO2 (g) + H2 O2 (l) → H2 SO4(a q)

(b) Oxides of Nitrogen: Di nitrogen and di oxygen are the main constituents of air. These gases do not react with each other at a normal temperature. At high altitudes when lightning strike, they combine to form oxide of nitrogen. NO 2 is oxidised to nitrate ion, NO3 − which is washed into soil, where it serves as a fertilizer. In an automobile engine, (at high temperature) when fossil fuel is burnt, di nitrogen and di oxygen combine to yield significant quantities of nitric oxide (NO) and nitrogen dioxide ( NO2 ) as given below:

      N2 (g) + O2 (g) →2NO(g)

NO reacts instantly with oxygen to give NO2 → 2 NO2 (g)

Rate of production of NO2 is faster when nitric oxide reacts with ozone in the stratosphere.

   NO (g) + O3 (g) → NO2 (g) + O2 (g)

The irritant red haze in the traffic and congested places is due to oxides of nitrogen. Higher concentrations of  N O2 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.

(c) Hydrocarbons: Hydrocarbons are 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. The 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 carboxy haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when

the concentration of carboxy haemoglobin

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.

(i i) Carbon dioxide: Carbon dioxide (CO2) 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 CO2 in the air is removed by green plants and this maintains an appropriate level of CO2 in the atmosphere. Green plants require CO2 for photosynthesis and they, in turn, emit oxygen, thus maintaining the delicate balance. As you know, deforestation and burning of fossil fuel increase the CO2 level and disturb the balance in the atmosphere. The increased amount of CO2 in the air is mainly responsible for global

warming.

  4. 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 radiation pass through the transparent glass and heat up the soil and the plants. The warm soil and plants emit infrared radiation. Since glass is opaque to infrared radiation (thermal region), it partly reflects and partly absorbs these radiation. 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 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. 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.

                              5. 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.

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 2 and NO 2 after oxidation and reaction with water are major contributors to acid rain, because polluted air usually contains particulate matter that catalyse the oxidation.

2 SO2 (g) + O2 (g) +2 H2O (l) → 2H2SO4 (a q) 

4 NO2 (g) + O2 (g)+2 H2O (l) → 4HNO3 (a q)

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. SO2 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.