JEE Chemistry Environmental Chemistry Notes Set 03

Components of the Environment and the Atmosphere

The environment has four components: atmosphere (gases), hydrosphere (water), lithosphere (soil and rocks) and biosphere (the zone of living organisms). The atmosphere has a pressure of about 10⁵ N m⁻² at sea level, and pressure and density halve about every 5 km of height.

Pollutants

Environmental pollution is the addition of undesirable material to air, water or soil. A primary pollutant remains as emitted (NO, NO₂, SO₂), while a secondary pollutant forms from reactions among primary pollutants in the atmosphere (ozone, PAN, formaldehyde in smog). Biodegradable pollutants (sewage, dung) are decomposed by micro-organisms, while non-biodegradable ones (DDT, Hg, Al, plastics) persist, accumulate and magnify along the food chain.

Tropospheric Pollution: Gaseous Pollutants

Carbon monoxide comes from incomplete combustion of fuels. It binds with haemoglobin to form carboxyhaemoglobin, far more stable than oxyhaemoglobin, and so reduces oxygen transport. Soil micro-organisms are its natural sink.

Oxides of sulphur and nitrogen come from burning fossil fuels, power stations and metallurgy. They irritate the respiratory tract and are the source of acid rain.

Hydrocarbons from incomplete combustion and solvents are carcinogenic and take part in smog formation.

Control: catalytic converters (Pt, Pd, Rh) convert CO and hydrocarbons to CO₂ and H₂O and nitrogen oxides to N₂; they require unleaded petrol. CNG instead of petrol, scrubbers and electrostatic precipitators for industries reduce emissions.

Greenhouse Effect and Global Warming

The earth absorbs visible sunlight and re-emits heat as infrared radiation. Greenhouse gases (CO₂, CH₄, N₂O, CFCs, water vapour, ozone) absorb part of it and return it, keeping the surface about 33 °C warmer than it would otherwise be. Increased burning of fossil fuels, deforestation, rice fields and cattle raise the concentration of these gases and cause global warming (melting ice, rising sea levels, climatic change). CO₂ contributes about 60% of the enhanced effect. Homonuclear diatomic gases such as N₂ and O₂ do not absorb IR and are not greenhouse gases.

Acid Rain

Rain water has a pH of about 5.6 because dissolved CO₂ forms carbonic acid. Rain with pH below 5.6 is acid rain. It is formed when SO₂ and nitrogen oxides are oxidised in the atmosphere and dissolve in water: 2SO₂ + O₂ → 2SO₃, SO₃ + H₂O → H₂SO₄; 4NO₂ + O₂ + 2H₂O → 4HNO₃. Sulphuric acid is the major acid (about 60 to 70%), nitric acid the next.

Effects: corrosion of marble (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), iron and steel; acidification of lakes harmful to fish; damage to forests and soil; leaching of toxic metals into water.

Smog

Classical (London) smog forms in cool, humid weather from coal smoke and SO₂; it is reducing in nature.

Photochemical (Los Angeles) smog forms in warm, dry, sunny weather from vehicle exhaust. Nitric oxide formed in engines is oxidised to NO₂, which photolyses: NO₂ → NO + O; O + O₂ → O₃; O₃ + NO → NO₂ + O₂. Ozone and NO oxidise unburnt hydrocarbons to formaldehyde, acrolein and peroxyacetyl nitrate (PAN, CH₃COOONO₂). It is oxidising, irritates eyes and lungs, cracks rubber and damages plants. Control: catalytic converters and free radical trapping compounds such as pinene.

Stratospheric Pollution: The Ozone Layer

Ozone is formed in the stratosphere by O₂ → 2O (UV) and O + O₂ → O₃, and it absorbs harmful UV radiation. Ozone is destroyed by nitric oxide (from supersonic aircraft) in the cycle NO + O₃ → NO₂ + O₂, NO₂ + O → NO + O₂, and especially by chlorofluorocarbons (freons). UV breaks CF₂Cl₂ to give Cl·; Cl· + O₃ → ClO· + O₂; ClO· + O → Cl· + O₂. Chlorine is a catalyst and a single atom destroys many thousands of ozone molecules. Chlorine nitrate (ClONO₂) and HCl are temporary reservoirs.

The Antarctic ozone hole (column below 220 Dobson units) forms in spring, when polar stratospheric clouds allow reservoir species to release active chlorine. Effects of depletion: skin cancer, cataract, damage to the immune system and plants. The Montreal Protocol phases out CFCs; HCFCs and HFCs are replacements.

Water Pollution

Sources: domestic sewage, industrial effluents, agricultural run-off (fertilisers and pesticides), mining and thermal discharge. Dissolved oxygen should be at least about 6 ppm for fish, and the biochemical oxygen demand (BOD) of clean water is below 5 ppm, while highly polluted water has BOD above 17 ppm.

Eutrophication: excess phosphates and nitrates cause algal bloom, and decay of the algae removes dissolved oxygen.

Health effects: mercury (Minamata disease), cadmium (itai-itai), lead (nervous system and kidney), nitrate above 50 ppm (blue baby syndrome), fluoride above 10 ppm (bone and teeth damage; about 1 ppm is beneficial), arsenic (skin and cancer effects). Heavy metals are cumulative poisons. PCBs are non-biodegradable and carcinogenic.

Soil Pollution and Green Chemistry

Pesticides (insecticides, herbicides, fungicides) such as DDT and BHC are non-biodegradable and enter food chains, while fertilisers and industrial waste alter soil quality.

Green chemistry aims to reduce pollution at the source by designing processes that use and produce less hazardous substances: for example dry cleaning with liquefied CO₂, bleaching paper with H₂O₂ instead of Cl₂, and synthesis of ethanal by oxidation of ethene in aqueous medium with a Pd catalyst. Its guiding ideas are prevention of waste, high atom economy, safer solvents, energy efficiency and renewable feedstocks.