Environmental Chemistry: Air and Water Pollution

A chlorofluorocarbon (CFC) is an organic compound that contains only carbon, chlorine, and fluorine, produced as volatile derivative of methane, ethane, and propane. They are also commonly known by the DuPont brand name Freon. The most common representative is dichlorodifluoromethane (R-12 or Freon-12).

Photochemical smog is mainly composed of ozone, nitrogen dioxide and volatile organic compounds. Sulphur dioxide is not a component of photochemical smog.

The brown haze of photochemical smog is largely attributed to NO₂.

The toxic effects of carbon monoxide on human beings and animals arise from its reversible combination with haemoglobin (Hb) in red blood cells, which cause displacement of oxygen. This results in oxygen starvation or anoxia. The maximum permissible concentration of CO in the atmosphere is 40 ppm for an exposure of 8 hours. A higher concentration of CO or a longer exposure starts causing adverse effects. In small amounts, CO causes headaches and fatigue, and in larger doses, it can be lethal.

The products of photochemical smog are peroxyacetyl nitrate and ozone. 

Detergents and fertilisers containing phosphate anion are mainly responsible for eutrophication.

Chlorine gas was used earlier for bleaching paper. It is being replaced by hydrogen peroxide with suitable catalyst.

Inhaling of nitrogen oxides by humans results in pulmonary edema and hemorrhage. Several sensitive plants get adversely affected by NOx even at the concentration of 1 ppm for a day or 0.35 ppm for a few months. The affected plants start shedding their leaves and fruit. Nitrogen oxides cause fading of textile dyes and deterioration of cotton and nylon fabrics.

Exposure to SO₂ causes deterioration of fabrics (cotton, rayon), paper and leather. Being corrosive, it has damaging effects on metals, such as iron, steel, aluminium, and on building materials, such as limestone, marble, roof slate, etc. Marble structures, limestone and geological sources of ammonia act as a sink for atmospheric SO₂.

All of the above are the achievements of green chemistry.

The elements or organisms that can consume a pollutant are known as 'sinks'. Recent researches have shown that soil is capable of removing large amounts of CO from the atmosphere by the action of some soil microorganisms. The microorganisms act as a sink for CO.

The pH of rainwater due to the presence of H⁺ ions formed by the reaction of rainwater with CO₂ present in atmosphere will be ~ 5.6.

Photochemical smog can be suppressed by certain compounds, which act as free radical traps and also by installation of efficient catalytic converters in the automobiles.

It is also a pollutant, when generated in large quantity, because it can reduce the oxygen concentration, in the air. It is a gas produced when fuel is burnt, in adequate oxygen. Normally, the atmospheric air will have 0.03% CO₂ When it's concentration increases due to pollution, it reduces the percentage of oxygen in the atmosphere.

Hydrogen sulphide (H₂S) is one of the dangerous, pungent pollutant gases with many deleterious effects on the respiratory tract. It is released by many industries as a part of the sulphonation process.

H₂S gas is extremely toxic to humans. In higher atmospheric concentrations, it can be fatal. The reaction of H₂S with essential proteins is primarily responsible for it's toxic effects.

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SO₂ is poisonous, and irritates the respiratory system of animals and humans. A continuous exposure to concentration of 0.2 ppm lasting for 3-4 days or even 0.5ppm for full day has been reported to damage the lungs and increase the rate of mortality. In plants and grasses, SO₂ also produces leaf injuries (called necrotic blotching). Sulphur dioxide damages, barley, alfalfa, cotton and wheat.

The BOD value of clean water is less than 5 ppm.