CHEMISTRY OF ENVIROMENTAL POLLUTION

By Kuljeet Singh

The word pollution means the introduction of harmful substances into the environment which is produce from domestic, agricultural or industrial activities. The Substances which pollute the environment include compounds like oxides of carbon, sulphur and nitrogen. Hydrocarbons emitted by thermal plants, motor vehicles and chemical industries effect environment on large extent. Increasing level of carbon dioxide into the atmosphere are responsible for greenhouse effect, while nitrogen and sulphur oxides play a major role in acid rain which causing destruction of vegetation and aquatic life. Water is polluted by effluents from industries, pesticides and fertilizers washed down from agricultural fields and city sewage. The release of hazardous waste to the environment is from industries. Toxic chemicals getting down into rivers often kill many fishes and other aquatic life.People who eat the contaminated fish without well cleaning develop number of health related problems. Oil spills are the major cause of water pollution which, seriously affect coastal flora and fauna. The increasing level of noise is also another cause of pollution which effects the environment. Pollution in any form is harmful to human lifel. As already stated, environmental pollution results in a variety of ways.
Presently, only the environmental pollution caused by the use of solvents, reagents and products will be dealt with. The solvents that mostely include volatile organic solvents like methylene chloride, chloroform, perchloroethylene (PERC) and carbon tetrachloride. Some VOCs like, isopropyl alcohols, xylenes, toluenes and ethylenes have been used as cleaning fluids because of their ability to dissolve oils, waxes and greases. Also, they easily evaporate from the substance that they are being used to clean (VOCs readily evaporate or vaporise at room temperature). When VOCs come in contact with sun light and nitrogen oxides (by-products from the combustion of fossil fuels), these are transformed into ozone, nitric acid and partially oxidised organic compounds.

VOCs + NO + Sunlight 0 3 + HN03 + Organic compounds
(partially oxidised)
This product mixture (consisting of ozone, nitric acid and partially oxidised organic compound) is formed at the ground level and is commonly called smog. Continued exposure of smog may results in causing asthma and other respiratory problems) and can also cause lung cancer.This phenomenon leads to elevated levels of tropospheric ozone (one of the components of smog) and may cause damage to crops, discolour fabrics and harden rubber.
The use of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) as solvents creates environmental problems. When CFCs are released into the atmosphere, mostely into troposphere and stratosphere (the CFCs are also released to the atmosphere from refrigeration industries), where they are photochemically decomposed by high-energy ultraviolet radiation from the sun. In fact the ozone layer present in the stratosphere prevents living world from harmful ultraviolet radiations.
On photochemical decomposition, the CFCs result in the formation of atomic chlorine (chlorine radical), which destroys the ozone by abstracting an oxygen atom from an ozone molecule and converting it into oxygen. The CIO radical formed, can react with an oxygen atom to form an oxygen molecule and regenerate a chlorine radical.
This depletion of the ozone layer are the main factor for ultraviolet radiations to reach the surface of the earth.These ultraviolet radiations are responsible for causing skin cancer and so many ill effects. Due to above reasons, the governments around the world have forbidden the use ofCFCs. The Montreal Protocol, signed in Canada in 1987 and 1989, and subsequently in London in 1990 decided to speed up the phasing out of CFCs. It has already been stated that liquid and supercritical carbon dioxide has replaced CFCs which were also used as blowing agents for polystyrene.Subsequently in place of CFCs, aliphatic hydrocarbons (e.g., pentane) were used as blowing agents for polystyrene. The aliphatic hydrocarbons are not ozone depleting, they can lead to the formation of ground level smog (a mixture of ozone, nitric acid and partially oxidised organic compounds) if their emissions are not captured. A harmless blowing agent viz. carbon dioxide has been developed. The advantage of CO2 is that it does not deplete the ozone layer, does not form smog, is economical, handling is easier and is nonflammable.
However, carbon dioxide is a greenhouse gas. It is commonly known that in the atmosphere, CO2 allows UV and visible radiations to reach the earth’s surface but reflects the IR radiations (heat) coming from the earth’s surface and directing it back to the earth. Thus, excess levels of CO2 in the atmosphere can significantly raise the temperature of the earth’s atmosphere, which is responsible for global warming. Nitrous oxide (NO) commonly known as laughing gas is also known to cause environment pollution. Nitrous oxide is obtained as a byproduct during the manufacture of adipic acid as follows :
The production of N2O as a by-product can cause 10% annual increase of N2O levels. The N2O is obtained as a by-product in a number of reactions involving oxidation with RN03o The N2O formed rises into the stratosphere and plays a role in the destruction of the ozone layer. The firststep in this process is the reaction of Np with atmospheric oxygen to produce N2O·, which acts as a catalyst to deplete ozone. Nitric oxide abstracts an oxygen atom from an ozone molecule, giving 02 and NO; (step 2). The resulting NO~ then reacts with an additional oxygen atom regenerating NO· and forming another molecule of 02′ The regenerated NO· (step 3) can react again (steps 2 and 3) resulting in significant loss of ozone for every molecule NO2 also acts as a greenhouse gas and causes global warming (as in the case of CO). Also, air pollution is a result of exhaust emissions particularly CO2 (already discussed) and nitrogen oxides. The nitrogen oxide part of the exhaust mixes with moisture in the atmosphere and then comes down in the form of acid rain. During lightening and thunderstorm, the nitrogen in the atmosphere combines with oxygen to give nitric oxide, which combines with oxygen to give N02o The N02 dissolves in water to give HN02 and HN03, which finally come to earth in the form of acid rain.
Acid rain effect plants and aquatic life. In some extreme cases, when the pH falls below 4.5, acid rain is responsible for making lakes devoid of aquatic life.

Kuljeet Singh MSc Chemistry Hemvati Nandan Bahuguna Garwal Central Univeristy Email kuljeetsing2929@gmail.com Contact NO 9596667846