Environmental chemistry is the study of the origin, effect, and fate of chemical species by chemical processes that take place in the environment such as air, water, and soil. The undesirable changes in the environment that have harmful effects on plants, animals, and human beings are known as environmental pollution and the substances that cause the pollution are known as pollutants.
The generation, type, and effect of these pollutants and how these pollutants can be controlled and substituted will be studied in this article. Green chemistry is the ethical way of utilising the knowledge and principle of chemistry to minimise the pollution or deterioration of the environment. This chapter is one of the easiest chapters from the competitive exams JEE and NEET point of view.
Effect of environmental pollution
Air pollution control
Causes of water pollution
It occurs due to undesirable solids and gaseous particles present in the air. The major gaseous and particulate pollutants present in the troposphere are listed below:
Gaseous air pollutants contain oxides of sulphur, nitrogen and carbon, hydrocarbons, ozone, and other oxidants.
Particulate pollutants contain dust, mist, smoke, smog, fumes, etc.
1. Gaseous Air Pollutants
Oxides of Sulphur
Sulphur dioxide, oxide of sulphur, is a harmful gaseous pollutant that causes:
Respiratory diseases such as asthma, bronchitis, and emphysema in human beings.
Irritation in the eyes leads to tears and redness of the eyes.
Stiffness in flower buds which eventually leads to fall off from plants.
Oxides of Nitrogen
At high temperatures, the unburnt fossil fuel from automobiles leads to the formation of nitric oxide (NO) and nitrogen dioxide (NO2) by reaction between nitrogen and oxygen gases present in the atmosphere as shown below:
At temperatures around 1483K,
N2 (g) + O2 (g) → 2 NO (g)
NO reacts immediately with O2 present in the atmosphere, in the troposphere, and ozone in the stratosphere to form NO2.
NO(g) + O2 (g) → 2NO2(g)
Oxides of nitrogen lead to lung irritation, acute respiratory diseases, retard the rate of photosynthesis, and corrode metals.
Oxides of Carbon
Carbon Monoxide: It is a highly poisonous gas due to its strong ability to bind with haemoglobin to form carboxyhaemoglobin which blocks the transportation of oxygen in the body or tissues of humans.
Carbon Dioxide: It is a respiratory gas, released into the atmosphere by respiration, burning of fuel, decomposition of limestone, deforestation and volcanic eruptions. The increased level of carbon dioxide in the atmosphere causes global warming.
2. Particulate Pollutants
These are the minute solid particles or liquid droplets present in the atmospheric air which are present in vehicle emissions, smoke from fires, dust particles and ash from industries. Particulate pollutants are classified as viable and non-viable pollutants.
Viable particulates are minute living organisms such as bacteria, fungi moulds, algae, etc. Humans are allergic to some of the fungi present in the atmosphere. Non-viable particulates are classified according to their nature and size as follows:
Smoke: It contains solid particles or a mixture of solid and liquid particles released from cigarette smoke, combustion of organic matters, fossil fuels, garbage, etc.
Dust: It is the small solid particles of size approximately 1 μm in diameter formed from cement industries, coal works, and wood works due to crushing and grinding.
Mists: These are produced from the particles of liquid spray and condensation of vapours in air, such as insecticides, pesticides sprayed, and sulphuric acid mist remain in the atmosphere as particulate pollutants.
Fumes: These are obtained by condensation of vapours during sublimation, distillation, boiling, and several other chemical reactions of organic solvents, metals, and metallic oxides.
Smog: The word ‘smog’ is derived from smoke and fog. There are two types of smog.
Classical Smog: It is a mixture of smoke + fog + sulphur dioxide. It generally occurs in cool and humid climates and is chemically a reducing mixture and hence it is called as reducing smog.
Photochemical Smog: The main component of photochemical smog is the action of sunlight on unsaturated hydrocarbon and nitrogen dioxide released from automobiles and factories. It generally occurs in warm, dry, and sunny climates and is chemically an oxidising agent and hence it is called as oxidising smog. Photochemical smog is formed from the hydrocarbon (unburnt fuel) and nitric oxide (NO) released from the fossil fuel.
NO(g) + O2 (g) → 2NO2(g)
NO2 absorbs energy from the sunlight and breaks into NO and free oxygen which is very reactive.
NO2(g) → 2NO(g) + O(g)
Free reactive oxygen atom combines to form ozone.
O(g) + O2(g) ⇌ O3(g)
The ozone reacts with NO2 formed in the above reactions and unburnt hydrocarbons to produce photochemical smog such as peroxyacetyl nitrate (PAN).
Effect of Photochemical Smog: Photochemical smog acts as an eye irritant, also irritates the nose and throat, and causes headache, chest pain, cough, and difficulty in breathing. It also causes damage to plant life, corrosion of metals, stones, building materials, etc.
Photochemical Smog can be Controlled: Catalytic converters are used in automobiles to prevent the formation of primary precursors of photochemical smog such as NO2 and hydrocarbons. The controlled or reduced formation of primary precursors such as NO2 and hydrocarbons prevents the formation of secondary precursors such as ozone and PAN which will reduce the photochemical smog.
The pH of rain in normal conditions is slightly acidic, which is 5.6 due to the presence of H+ ions formed because of the reaction of rain water and carbon dioxide.
H2O (l) + CO2 (g) ⇋ H2CO3 (aq)
H2CO3 (aq) ⇋ H+ (aq) + HCO3ー(aq)
When the pH of rain water is below 5.6, it is known as acid rain. The oxides of nitrogen and sulphur are the major contributors to the acid rain as they undergo oxidation and react with water.
2 SO2(g) + O2(g) + 2 H2O (l) ⟶ 2H2SO4 (aq)
2 NO2(g) + O2(g) + 2H2O (l) ⟶ 4HNO3 (aq)
Acid rain is harmful for agriculture crops, trees, and plants as it dissolves and washes away nutrients important for their growth. It also causes respiratory ailments in humans and animals. Acid rain water also affects the aquatic ecosystem and corrodes water pipelines resulting in the leaching of heavy metals such as iron, lead, and copper into the drinking water. Acid rain also damages buildings, monuments, and other structures made of stone or metals. Taj Mahal is one of the examples of the corrosion caused by acid rain.
Most of the solar radiation is absorbed by the earth’s atmosphere and the heat that reaches the earth’s surface is trapped by some gases such as methane, carbon dioxide, ozone, chlorofluorocarbon (CFC) compounds, and water vapour in the atmosphere. These gases cause increases in the earth's temperature and this causes global warming. Carbon dioxide gas is the major contributor of global warming.
The process of heating the earth’s atmosphere when some gases present in the earth’s atmosphere traps the heat from the Sun. The gases which contribute to global warming such as methane, carbon dioxide, and ozone are known as greenhouse gases.
The dissolved oxygen (DO) in water has a concentration of up to 10 ppm (parts per million). Polluted water has reduced the concentration of DO in water and the DO below 6 ppm inhibits the growth of fishes and affects aquatic life.
The amount of oxygen required by bacteria to break down the organic matter present in water samples is called biochemical oxygen demand (BOD). Clean water has a BOD value of less than 5 ppm whereas polluted water has a BOD value of 17ppm or more.
Causes of Water Pollution
Pathogens: Pathogens include bacteria and other organisms present in sewage and animal excreta that enter the drinking water which causes many diseases such as gastrointestinal diseases.
Organic Waste: The organic matter such as leaves, grass, trash etc. and excessive growth of phytoplankton causes water pollution which decreases the DO in water and affects aquatic life.
Chemical Pollutants: The water soluble inorganic chemicals such as cadmium, mercury, and nickel are dangerous to humans as they do not excrete out of the body and affect the kidney, CNS (central nervous system), liver, etc. The organic chemicals from petroleum, oil spills, pesticides, etc. discharge into the ocean and cause water pollution.
The addition of these water pollutants in water bodies increases the nutrients in the water which support a dense plant population, which kills an animal or aquatic life due to deprivation of oxygen in the water and subsequently results in loss of biodiversity, this process is known as eutrophication.
The international standard for drinking water is as follows:
Fluoride: The low concentration of fluorides in drinking water causes tooth decay. Fー ion helps in the hardening of enamel by converting hydroxyapatite [3(Ca3(PO4)2). Ca(OH)2] into harder fluorapatite [3(Ca3(PO4)2). CaF2].
Sodium fluorides are added to drinking water to bring its concentration up to 1 ppm. A concentration of more than 2 ppm causes a mottling of teeth and harmful effect to bones and teeth.
Lead: The prescribed upper limit of concentration of lead in drinking water is about 50 ppb (parts per billion). More than the prescribed limit of the lead causes damage to the kidney, liver, reproductive system, etc.
Sulphate: The sulphate of more than 500 ppm in drinking water causes a laxative effect and a moderate level of sulphate has a mild or harmless effect.
Nitrate: The upper limit of nitrate in drinking water is 50 ppm. Excess nitrate in drinking water causes ‘blue baby’ syndrome or methemoglobinemia.
Maximum Prescribed Concentration of Some Metals in Drinking Water
Soil pollution is the presence of harmful chemicals in the soil that affect humans and plant growth.
Prior to World War II, nicotine was used as a pest controlling substance for major crops.
During World War II, DDT was greatly used as a pesticide as it was very effective to control malaria and other insect-borne diseases.
Pesticide industries have shifted their attention to herbicides such as sodium chlorate (NaClO3) and sodium arsenite (NA3SO3) and many others.
Pesticides and herbicides are toxic to mammals and cause damage to multiple organs in humans, birth defects, genetic disorders, etc.
Industrial wastes are categorised as biodegradable and non-biodegradable wastes.
Biodegradable Wastes: The wastes that can undergo aerobic or anaerobic decomposition are known as biodegradable wastes. The biodegradable wastes are generated by cotton mills, food processing units, paper mills, and textile factories.
Non-biodegradable Wastes: The wastes that do not undergo decomposition naturally and remain on earth for thousands of years. Non-biodegradable wastes are produced by thermal power plants, many industries such as iron and steel industry, fertiliser industry, industries manufacturing aluminium, zinc, and copper.
Waste Management: The government has taken many steps to manage the waste to control environmental degradation such as the ‘Clean India Mission’ or ‘Swachh Bharat Abhiyan’ which implemented two programmes listed below:
Swachh Bharat Mission - Urban (SBM-U): It aims at making urban India free from open defecation and achieving 100% scientific management of solid wastes in the country.
Swachh Bharat Mission - Rural (SBM-R): It aims at improving the quality of life in rural areas by encouraging sanitation, hygiene, cleanliness, and eliminating open defecation.
It is the innovative way of thinking and utilising the knowledge and principles of chemistry to minimise pollution or deterioration to the environment caused by the pollutants. It is also a cost-effective approach, which involves a reduction in material, energy consumption, and waste management.
The green chemistry in day-to-day life:
Dry Cleaning of Clothes: Tetrachloroethene (Cl2C=CCl2 ) was used as a solvent for dry cleaning which contaminates the groundwater and is carcinogenic. Tetrachloroethane is replaced by liquified carbon dioxide with suitable detergents which are less harmful to groundwater. Nowadays, hydrogen peroxide (H2O2) is used for bleaching clothes in the process of laundry.
Bleaching of Paper: Chlorine gas was used for bleaching paper which is replaced by hydrogen peroxide (H2O2) with a suitable catalyst.
Synthesis of Chemicals: Ethanal is now commercially prepared by one-step oxidation of ethene in the presence of catalysts such as Pd(II) or Cu (II) aqueous medium.
Green Solution to Clean Turbidity in Water: Alum was used to treat turbidity in water which was found to increase toxic ions in water. The powder of the kernel of tamarind seeds is the green solution to treat the turbidity of water.
Example 1: Why does rainwater have a pH of about 5.6?
Solution: The rain has a pH of 5.6 (approximately) due to the presence of H+ ions formed because of the reaction of rain water and carbon dioxide.
H2O (l) + CO2 (g) ⇋ H2CO3 (aq)
H2CO3 (aq) ⇋ H+ (aq) + HCO3ー(aq)
Key points to remember: Rainwater gets mixed with atmospheric gas.
Example 2: Explain giving reasons. ‘The presence of CO reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells'.
Solution: CO has the strong ability to bind with haemoglobin to form carboxyhaemoglobin and reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells which blocks the transportation of oxygen in the body or tissues of humans.
Key Points to remember: CO is a strong field ligand for Fe ion.
Question 1: The layer of atmosphere between 10 km and 50 km above the sea level is called
Solution: The stratosphere is the second layer of atmosphere which extends between 10 km and 50 km above the sea level. The correct answer is (c).
Trick: Stratosphere starts after troposphere(0-10 km)
Question 2. Water samples with BOD values of 4 ppm and 18 ppm, respectively, are
(a) Clean and highly polluted
(b) Highly polluted and highly polluted
(c) Highly polluted and clean
(d) Clean and clean
Solution: Water which has a BOD value less than 5 ppm is considered clean water. Polluted water has a BOD value of 17 ppm or more. Therefore, the water sample is clean and highly polluted, respectively. The correct answer is (a).
Trick: High BOD means poor water quality.
Question 3: Taj Mahal is being slowly disfigured and discoloured. This is primarily due to
(a) Acid rain
(b) Soil pollution
(c) Water pollution
(d) Global warming
Solution: Acid rain is the primary reason for the discolouration of the Taj Mahal as acid rain has a pH of less than 5.6 and hence corrodes the marble of Taj Mahal and leads to its discolouration. Therefore, the correct answer is (a).
Trick: Taj mahal is made up of marble which can react with acid.
Question 1: The maximum prescribed concentration of copper in drinking water is
(a) 0.05 ppm
(b) 3 ppm
(c) 5 ppm
(d) 0.5 ppm
Answer: (b) 3 ppm.
Question 2: Air pollution that occurs in sunlight is
(a) Oxidising smog
(c) Reducing smog
(d) Acid rain
Answer: (a) Oxidising smog.
Environmental chemistry is the study of chemical processes taking place in our environment. In this section, we studied the air, water, and soil pollutants, what are these pollutants, and how they affect living organisms. Green chemistry is the ethical, innovative, and cost-effective way of utilising the knowledge of chemistry to minimise the deterioration to the environment caused due to pollutants.
In JEE competitive exams, environmental chemistry is one of the easiest topics of chemistry, which is of easy level in terms of difficulty level and very scoring.
1. What is photochemical smog?
Photochemical smog is formed by the action of sunlight on unsaturated hydrocarbon and nitrogen dioxide released from automobiles and factories which acts as a pollutant. It causes eye irritation, also irritates the nose and throat, and causes headache, chest pain, cough, and difficulty in breathing. It also causes damage to plant life, corrosion of metals, stones, building materials, etc.
2. What is meant by B.O.D.?
BOD, which stands for biochemical oxygen demand, is the amount of oxygen required by bacteria to break down the organic matter present in water samples. Clean water has a BOD value of less than 5 ppm whereas polluted water has a BOD value of 17ppm or more.