JEE Chemistry Environmental Chemistry Notes Set 01

Environmental Pollution

Environmental chemistry studies the chemical processes in air, water and soil. Pollution is the addition of an undesirable material to air, water or soil, by natural or human activity, which lowers the quality of the environment; the material is a pollutant.
1. Biodegradable pollutants (cow dung, discarded vegetables) are broken down by microorganisms but become a problem when added in excess.
2. Non-biodegradable pollutants (DDT, heavy metals, plastics, nuclear waste) are not degraded, or only very slowly, and accumulate.
3. Primary pollutants enter the environment directly (CO, SO2, NO); secondary pollutants form from them in the air (PAN, ozone, acid rain).

Major Air Pollutants

1. Carbon monoxide comes mainly from incomplete combustion in vehicles. It forms carboxyhaemoglobin with haemoglobin, which is far more stable than oxyhaemoglobin, so the blood carries less oxygen. Soil bacteria act as a sink for CO.
2. Hydrocarbons are carcinogenic and damage plants.
3. Sulphur dioxide, from burning fossil fuels, causes asthma, bronchitis and irritation; it is oxidised to SO3 on particles or by ozone and hydrogen peroxide: \( SO_2 + H_2O_2 \to H_2SO_4 \).
4. Nitrogen oxides form at high temperature in engines: \( N_2 + O_2 \to 2NO \), then \( 2NO + O_2 \to 2NO_2 \). They cause red haze, lung irritation and plant damage.
5. Carbon dioxide is about 0.03 per cent of air; its excess is the main cause of global warming.
6. Particulates: viable (bacteria, fungi, algae) and non-viable (mist, smoke, fumes, dust). Particles larger than about 5 micrometres lodge in the nasal passage and those near 10 micrometres reach the lungs. Lead from leaded petrol interferes with red blood cell development.

Stratospheric Ozone Depletion

Ozone absorbs harmful UV radiation. Nitric oxide and chlorofluorocarbons (CFCs, freons) deplete it.
1. \( NO + O_3 \to NO_2 + O_2 \) and \( NO_2 + O \to NO + O_2 \): NO is regenerated, so a little NO destroys much ozone.
2. UV breaks CFCs: \( CF_2Cl_2 \xrightarrow{h\nu} \cdot CF_2Cl + Cl\cdot \). Then \( Cl\cdot + O_3 \to ClO\cdot + O_2 \) and \( ClO\cdot + O \to Cl\cdot + O_2 \), a chain in which one CFC molecule can destroy thousands of ozone molecules.
3. The Antarctic ozone hole forms in spring because polar stratospheric clouds provide surfaces on which chlorine nitrate is hydrolysed, releasing active chlorine; in summer \( NO_2 \) and methane form chlorine sinks and reduce depletion.
4. Effects of UV: damage to cornea and lens, skin cancer, reduced chlorophyll in plants.

Acid Rain

Rain is normally slightly acidic (pH about 5.6) because of dissolved CO2. Acid rain has pH below 5.6 and is due to SO2 and NOx, which are oxidised to \( H_2SO_4 \) and \( HNO_3 \). It corrodes water pipes and releases toxic heavy metals, acidifies lakes, damages crops and trees and attacks marble: \( CaCO_3 + H_2SO_4 \to CaSO_4 + H_2O + CO_2 \), which harms the Taj Mahal.

Greenhouse Effect and Smog

1. Greenhouse gases are CO2, methane, water vapour, nitrous oxide, ozone and CFCs. They absorb the infrared radiation re-emitted from the Earth and send part back, warming the surface.
2. Classical (London) smog occurs in cool, humid climates from smoke and sulphur oxides; it is a reducing mixture.
3. Photochemical smog occurs in warm, dry, sunny weather: \( NO_2 \xrightarrow{h\nu} NO + O \), then \( O + O_2 \to O_3 \). Ozone and NO react with unburnt hydrocarbons to give formaldehyde, acrolein and peroxyacetyl nitrate (PAN). It is an oxidising smog and irritates eyes, nose and throat and cracks rubber. Catalytic converters, and plants such as Pinus and Quercus that metabolise NOx, help control it.

Water Pollution

1. Sources: domestic sewage (pathogens such as E. coli), organic waste, plant nutrients from fertilisers, toxic heavy metals (Cd, Pb, Hg are cumulative poisons), sediments, pesticides, radioactive substances and heat.
2. Phosphate from detergents and fertilisers causes eutrophication: algal growth reduces dissolved oxygen.
3. Dissolved oxygen in cold water is at most about 10 ppm; below 6 ppm fish growth is inhibited.
4. BOD is the oxygen needed by bacteria to break down the organic matter in a water sample. Clean water has BOD below 5 ppm and heavily polluted water has 17 ppm or more. COD is the oxygen needed for oxidising all organic matter (biodegradable and non-biodegradable) using \( K_2Cr_2O_7 \), so COD is always greater than BOD.
5. Drinking water standards: fluoride about 1 ppm (above 2 ppm mottling of teeth, above 10 ppm harmful to bones), lead at most 50 ppb, nitrate at most 50 ppm (excess gives blue-baby syndrome), sulphate above 500 ppm has a laxative effect, and pH between 5.5 and 9.5.
6. PCBs are stable chlorinated compounds that cause skin disorders and are carcinogenic; acid mine drainage comes from oxidation of pyrite.

Soil Pollution and Waste Management

1. Pesticides such as DDT are persistent and biomagnify; organophosphates and carbamates are less persistent but are nerve poisons. Herbicides include sodium chlorate and sodium arsenite.
2. Industrial solid waste is biodegradable (cotton mills, paper mills, food processing) or non-biodegradable (fly ash, slag, tailings).
3. Recycling converts waste to wealth; fuel made from plastic waste is "green fuel". Biogas plants digest garbage to give methane. Incineration above 1000 degrees Celsius converts organic waste to CO2 and water; anaerobic digestion gives CO2 and methane.

Green Chemistry

Green chemistry designs processes that reduce material and energy use and waste, using existing chemical knowledge. Examples: liquid CO2 with detergent instead of tetrachloroethene for dry cleaning; hydrogen peroxide for bleaching clothes and paper instead of chlorine; water as the reaction medium; and one-step oxidation of ethene to ethanal with an ionic catalyst in water (90 per cent yield).
The Bhopal gas tragedy of 2 December 1984 was caused by a leak of methyl isocyanate (MIC). The troposphere extends to about 10 km (up to 50 km is the stratosphere, which has the ozone layer).