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ENVIRONMENTAL CHEMISTRY

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ENVIRONMENTAL CHEMISTRY ( environmental-chemistry )

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ENVIRONMENTAL CHEMISTRY 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 ATMOSPHERICPOLLUTION 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, 407 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 SO2 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. 2SO2 (g) +O2 (g) → 2SO3(g) The reaction can also be promoted by ozone and hydrogen peroxide. SO2 (g) +O3 (g) → SO3(g) + O2 (g) SO2(g) + H2O2(l) → H2SO4(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, NO− 23 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 ( NO2 ) as given below: 2019-20 1483K N(g)+O(g) → 2NO(g) 22 NO reacts instantly with oxygen to give NO2 2NO (g) + O2 (g) → 2NO2 (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 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 is toxic to living tissues also. Nitrogen dioxide is also harmful to various textile fibres and metals.

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