Chapter 14 Environmental Chemistry

The branch of science which deals with the chemical changes in the environment is called as Environmental Chemistry. It includes our surroundings such as air, water, soil, forests, sunlight etc. It has three main components:

Abiotic or non -living - it includes lithosphere, hydrosphere, atmosphere.

Biotic or living- it includes plant and animal kingdom including humans.

Energy component- it includes solar energy, geochemical, thermochemical, hydroelectric and nuclear energy.

Tropospheric pollution arises due to the presence of undesirable substances in the lowest layer of the atmosphere.

Oxides of Sulphur, nitrogen, carbon, and hydrocarbons are the major gaseous pollutants.

Oxides of Sulphur (SO₂ and SO₃) and nitrogen (NO₂, NO) are produced after burning of fossil fuels (coal, automobile fuel). These oxides react with water in the presence of atmospheric oxygen to form nitric acid (HNO₃) and sulphuric acid (H₂SO₄), which leads to the formation of 'Acid rain'.

Acid rain causes harm to agriculture, plants, and trees. Also leading to various respiratory ailments.

Hydrocarbons are carbon and hydrogen containing compounds that produces oxides of carbon during burning. Hydrocarbons are carcinogenic and their products are also major pollutants. Carbon monoxide (CO) is poisonous in nature as it reacts with the haemoglobin of blood, which can even result in death. Though carbon dioxide (CO₂) is not toxic in nature, yet it contributes towards global warming by trapping the reflected Infrared rays. This results in the heating up of the Earth's atmosphere, thereby leading to the melting of icebergs and glaciers.

Particulates of smoke, dust, mist, and fume are harmful for human health as they are likely to block the nasal passage of a person, causing respiratory ailments. Smoke and fog combine to produce smog during a cool, humid day, thereby reducing visibility to a large extent. Photochemical smog is formed due to the presence of PAN, ozone, formaldehyde, and acrolein. It causes eye irritation, headaches, and chest pain. It also leads to the cracking of rubber and does damage to plants.

Carbon dioxide (CO₂) and carbon monoxide (CO) gases are emitted during the combustion of various fuels. Carbon monoxide is poisonous, whereas carbon-dioxide is non-toxic in nature.

Carbon monoxide is toxic because it is capable of forming a complex with haemoglobin (carboxy-haemoglobin), which is more stable than the oxygen-haemoglobin complex. The concentration range of 3-4% of carboxy-haemoglobin decreases the oxygen-carrying capacity of blood. This results in headaches, weak eyesight, nervousness, and cardiovascular disorders. A increased concentration may even lead to death.

Carbon dioxide is not poisonous. It proves harmful only at very high concentrations.

The major greenhouse gases are:

1) Carbon dioxide (CO₂)

2) Methane (CH₄)

3) Water (H₂O)

4) Nitrous oxide (NO)

5) Ozone (O₃)

6) Chlorofluorocarbons (CFCs)

Acid rain is caused by various human activities that leads to the emission of oxides of Sulphur and nitrogen in the atmosphere. These oxides undergo oxidation and then react with water vapour to form acids.

2SO₂ (g) + O2 (g) + 2H₂O (l) → 2H₂SO4 (aq)

4NO₂ (g) + O2 (g) + 2H₂O (l) → 4HNO₃ (aq)

Acid rain causes damage to buildings and structures made of stone and metal. In India, limestone is a major stone used in the construction of various monuments and statues, including the Taj Mahal.

Acid rain reacts with limestone as:

CaCO₃ + H₂SO₄  →  CaSO₄ + H₂O  +  CO₂

Thus resulting in the loss of luster and colour of monuments, leading to their disintegration.

Smog is a kind of air pollution. It is the combination of smoke and fog. There are two kinds of smog:

a) Classical smog

b) Photochemical smog

The two smog can be differentiated as follows:

Classical smog:

It occurs in a cool, humid climate.

Its components are smoke, fog, and Sulphur dioxide.

It is reducing in nature.

Photochemical smog:

It occurs in a dry, sunny climate.

Its components are PAN, acrolein, ozone, formaldehyde, and nitric oxide.

It is oxidizing in nature.

Photochemical smog is formed as a result of the reaction of sunlight with hydrocarbons and nitrogen oxides. Ozone, nitric oxide, acrolein, formaldehyde, and peroxyacetyl nitrate (PAN) are common components of photochemical smog. The formation of photochemical smog can be summarized as follows:

Burning of fossil fuels leads to the emission of hydrocarbons and nitrogen dioxide in the atmosphere. High concentrations of these pollutants in air results in their interaction with sunlight as follows:

While ozone is toxic in nature, both NO₂ and O₃ are oxidizing agents. They react with the unburnt hydrocarbons in air to produce formaldehyde, PAN, and acrolein.

3CH₄  +  2O₃  →  3CH₂=O   +  3H₂O

                           formaldehyde

EFFECTS OF PHOTOCHEMICAL SMOG:

Photochemical smog: Owing to the presence of NO₂ and O₃, causing corrosion of metals, stones, rubber, and painted surfaces, it becomes oxidizing. The other major components of photochemical smog are PAN, acrolein, and formaldehyde. Both PAN and ozone are eye irritants, while nitric oxide (formed from NO₂) causes nose and throat irritation. At higher concentrations, photochemical smog leads to chest pain, headaches, throat dryness, and various respiratory ailments.

CONTROL MEASURES:

Photochemical smog is obtained from the burning of fossil fuels and automobile fuels that emit NO₂ and hydrocarbons, which in turn form ozone, PAN, and other chemicals. To prevent the release of NO₂ and hydrocarbons into the atmosphere, the use of catalytic converters in automobiles is recommended. Plantation of plants such as Pinus, Juniparur, Quercus, Pyrus, and Vitis is also advised as they have the capability to metabolize NO₂.

In the stratosphere, ozone is a product of the action of UV radiations on dioxygen as:

Reaction (ii) indicates the dynamic equilibrium existing between the production and decomposition of ozone molecules. Any factor that disturbs the equilibrium may cause depletion of ozone layer by its decomposition. One such factor is the release of chlorofluorocarbon compounds (CFCs). These are non-reactive, non-flammable molecules that are used in refrigerators, air conditioners, plastics, and electronic industries.

Once released CFCs mix with atmospheric gases and reach the stratosphere, where they are decomposed by UV radiations.

The regeneration of Cl_(g) causes a continuous breakdown of ozone present in the stratosphere, damaging the ozone layer.

In Polar Regions, stratospheric clouds provide the surface for chlorine nitrate and hypochlorous acid, which react further to give molecular chlorine. Molecular chlorine and HOCl are photolysed to give chlorine-free radicals.

Hence, a chain reaction is initiated. The chlorine-free radical is continuously regenerated, thereby depleting the ozone layer. This phenomenon is known as the 'Ozone hole'.

Effects of depletion of ozone layer

The ozone layer protects the Earth from the harmful UV radiations of the sun. With the depletion of the layer, more radiation will enter the Earth's atmosphere. UV radiations are harmful because they causes the ageing of skin, cataract, skin cancer, and sunburns. They cause death of many phytoplanktons, which leads to decrease of fish productivity. Excess exposure may even cause mutation in plants.

Increase in UV radiations, decreases the moisture content of the soil and damages both plants and fibres.

Water pollution arises as a result of several human activities, which leads to the presence of several undesirable substances in water. Major water pollutants with their sources have been tabulated as follows:

Roles played by major pollutants are:

1. Pathogens: These water pollutants include bacteria and other organisms. They enter water from animal excreta and domestic sewage. Bacteria present in human excreta (for example, Escherichia coli and Streptococcus faecalis) cause gastrointestinal diseases.

2. Organic wastes: These are biodegradable wastes that pollute water as a result of run off. The presence of excess organic wastes in water decreases the amount of oxygen held by water. This decrease in the amount of dissolved oxygen inhibits aquatic life.

3. Chemical pollutants: These are water soluble chemicals like heavy metals such as cadmium, mercury, nickel, etc. The presence of these chemicals (above the tolerance limit) can damage the kidneys, central nervous system, and liver.

Water pollution arises due to various human activities. This includes discharges from wastewater treatment plants, run-off from agricultural fields, storm-water drainage, etc. Pollutants from these sources enter the water bodies, thereby contaminating the water and making it impure.

Industrial discharge like toxic, heavy metals such as Fe, Mn, Al, etc., along with organic wastes into water. Domestic sewage and animal excreta are also responsible for pathogenic contamination of water. These pollutants make water unfit for drinking.

Therefore, all industrial and chemical discharges should be made toxic free before allowing them to enter a water body. The concentration of these pollutants should be checked regularly. Compost should be preferred over chemical fertilizers in gardens and agricultural fields to avoid harmful chemicals from entering ground water.

Biochemical oxygen demand is the amount of oxygen required by bacteria to decompose organic matter in a certain volume of sample of water. Clean water would have a BOD value of less than 5 ppm, whereas highly polluted water has a BOD value of more than or equal to 17 ppm.

Major sources of soil pollution are industrial wastes and agricultural pollutants such as pesticides, fertilizers, etc.

It is very important to maintain the quality and fertility of soil to ensure and sustain the growth of plants and food crops.

Insecticides like DDT are not soluble in water. For this reason, they remain in soil for a long time, contaminating the root crops. Pesticides like Aldrin and Diel Drin are non-biodegradable and highly toxic in nature. They can enter the higher trophic levels through food chains, causing metabolic and physiological disorders. The same is true for industrial wastes that comprises of several toxic metals like Pb, As, Hg, Cd, etc.

Hence, the best way to check soil pollution is to avoid direct addition of pollutants to the soil. Also, wastes should undergo proper treatment. They should be recycled before allowing to be dumped.

Pesticides are a mixture of two or more substances. They are used for killing pests. Pests include insects, plant pathogens, weeds, mollusks, etc., that destroys the plant crop and spread diseases. Aldrin and diel Drin are the names of some common pesticides.

Herbicides are pesticides specially meant for killing weeds. For example, sodium chlorate (NAClO₃), sodium arsenite (Na₃AsO₃), etc.

Green chemistry aims at using the existing knowledge and principles of chemistry for developing and implementing chemical products and processes to reduce the use and generation of substances which are hazardous to the environment.

The release of different harmful chemicals (particulates, gases, organic and inorganic wastes) causes environmental pollution. In green chemistry, the reactants to be used in chemical reactions are chosen in such a way that the yield of the end products is up to 100%. This limits chemical pollutants from being introduced to the environment.

Earth's most abundant greenhouse gases are CO₂, CH₄, O₃, CFCs, and water vapour. These gases are present near the Earth's surface. They absorb solar energy that is radiated back from the surface of the Earth. The absorption of radiation results in the heating up of the atmosphere. Hence, greenhouse gases are essential for maintaining the temperature of the Earth for the sustenance of life.

In the absence of greenhouse gases, the average temperature of the Earth will decrease drastically, making life on Earth impossible.

The amount of dissolved oxygen present in water is limited. The abundance of phytoplanktons causes depletion of dissolved oxygen. Phytoplanktons are degraded by bacteria present in water. For their decomposition, they require a large amount of oxygen. Hence, they consume the oxygen dissolved in water. As a result, the BOD level of water drops below 6 ppm, inhibiting the growth of fish and causing excessive fish-kill.

Depending upon the nature of the waste, domestic waste can be segregated into two categories i.e., biodegradable and non-biodegradable. Biodegradable waste such as leaves, rotten food, etc. should be deposited in land- fills, where they get decomposed aerobically and anaerobically into manure. Non-biodegradable waste such as plastic, glass, metal scraps etc. should be sent for recycling as they cannot be degraded.

Taking proper care of the compost producing pit in order to protect ourselves from bad odour and flies is very essential. It should be kept covered to minimize bad odour and prevent flies from entering it. The recyclable waste should not be dumped in the compost producing pit. It should be sent to the industries through vendors for recycling.