Samacheer Class 11 Chemistry - Environmental Chemistry

• Organic matter such as leaves, grass, trash can pollute water. Microorganisms present in water can decompose this organic matter and consume dissolved oxygen in water.
• Eutrophication is a process by which water bodies receive excess nutrients that stimulates excessive plant growth. This enhanced plant growth in water bodies is called algal bloom.
• The growth of algae in extreme abundance covers the water surface and reduces the oxygen concentration in water. Thus, bloom-infeded water inhibits the growth of others. living organisms in the water body.
• This process in which the nutrient-rich water support a dense plant population kills animal life by depriving it of oxygen and results in loss of biodiversity is known as eutrophication.

The solar energy radiated back from the earth’s surface is absorbed by the greenhouse gases. (CO 2, CH 4, O 3, CFCs) are present near the earth’s surface. They heat up the atmosphere near the earth’s surface and keep it warm.
As a result of these, there is the growth of vegetation that supports life. In the absence of this effect, there will be no life of both plant and animal on the surface of the earth.

• Smog is a combination of smoke and fog which form droplets that remain suspended in the air.
• Smog is a chemical mixture of gases that forms a brownish-yellow haze. It mainly consists of ground-level ozone, oxides of nitrogen, volatile organic compounds, SO 2, acidic aerosols and some other gases.

At high altitudes to the atmosphere consists of a layer of ozone (O 3 ) which acts as an umbrella or shield for harmful UV radiations. It protects us from harmful effects such as skin cancer. UV radiation can convert molecular oxygen into ozone as shown in the following reaction.
O 2 (g)
O(g) + O(g)
O(g) + O 2 (g)
O 3 (g)

The pollutants are classified as bio-degradable and non-biodegradable pollutants.
Bio-degradable pollutants:
The pollutants which can be easily decomposed by the natural biological processes are called bio-degradable pollutants.
Example:
plant wastes, animal wastes, etc.
Non-bio-degradable pollutants:
The pollutants which cannot be decomposed by the natural biological processes are called Non-bio-degradable
pollutants.
Examples:
Metal wastes (mainly Hg and Pb), D.D.T, plastics, nuclear wastes, etc.,
These pollutants are harmful to living organisms even in low concentrations. As they are not degraded naturally, it is difficult to eliminate them from our environment.

NO 2
NO + (O)
O 3 are strong oxidizing agent and can react with unburnt hydrocarbons in polluted air to form formaldehyde, acrolein and peroxy acetyl nitrate (PAN).

Efforts to control environmental pollution resulted in development of science for synthesis of chemicals favorable to environment which is called green chemistry. Green chemistry means science of environmentally favorable chemical synthesis.

Greenhouse effect may be defined as the heating up of the earth surface due to trapping of infrared radiations reflected by earth’s surface by CO 2 layer in the atmosphere”. The heating up of earth through the greenhouse effect is called global warming.
Without the heating caused by the greenhouse effect, Earth’s average surface temperature would be only about -18 °C (CPF). Although the greenhouse effect is a naturally occurring phenomenon, it is intensified by the continuous emission of greenhouse gases into the atmosphere.
During the past 100 years, the amount of carbon dioxide in the atmosphere increased by roughly 30 percent and the amount of methane more than doubled. If these trends continue, the average global temperature will increase which can lead to melting of polar ice caps and flooding of low lying areas. This will increase incidence of infectious diseases like dengue, malaria etc.

Classical smog:
* Classical smog is caused by coal smoke and fog.
* It occurs in cold humid climate.
* The chemical composition is the mixture of SO 2, SO 3 gases and humidity.
* Chemically it is reducing in nature because of high concentration of SO, and so it is also called reducing smog.
* It is primarily responsible for acid rain.
* It also causes bronchial irritation.
Photochemical smog:
* Photochemical smog is cause by photochemical oxidants.
* It occurs in warm, dry and sunny climate.
* The chemical composition is the mixture of NO 2 and O 3 gases.
* Chemically it is oxidising in nature because of high concentration of oxidising agents such as NO 2 and O 3
* and so it is also called oxidising smog.
* It causes irritation to eyes, skin and lungs and increase the chances of asthma.
* It causes corrosion of metals, stones

1. Particulate pollutants are small solid particles and liquid droplets suspended in air. Many of particulate pollutants are hazardous.
Examples: dust, pollen, smoke, soot and liquid droplets (aerosols) etc,
2. Smoke particulate consists of solid particles (or) mixture of solid and liquid particles formed by the combustion of organic matter.
For example, cigarette smoke, oil smoke, smokes from burning fossil fuel, garbage and dry leaves.
3. Dust composed of fine solid particles produced during crushing and grinding of solid materials.
For example, sand from sandblasting, sawdust from woodworks, cement dust from cement factories and fly ash from power generating units.

Pesticides are the chemicals that are used to kill or stop the growth of unwanted organisms. But these pesticides can affect the health of human beings. Pesticides are classified as
(a) insecticides,
(b) Fungicides and
(c) Herbicides.
(a) Insecticides:
Insecticides like DDT, BHC, Aidrin can stay in soil for a long period of time and are absorbed by soil. They contaminate root crops like carrot, radish.
(b) Fungicides:
Organomercury compounds dissociate in soil to produce mercury which is highly toxic.
(c) Herbicides:
They are used to control unwanted plants and are also known as weed killers. Eg, Sodium chlorate, sodium nitrate. They are toxic to mammals.

The major pollutants of oxides of carbon are carbon monoxide and carbon dioxide.
(i) Carbon Monoxide:
Carbon monoxide is a poisonous gas produced as a result of incomplete combustion of coal are firewood. It is released into the air mainly by. automobile exhaust. It binds with haemoglobin and form carboxy haemoglobin which impairs normal oxygen transport by blood and hence the oxygen carrying capacity of blood is reduced. This oxygen deficiency results in headache, dizziness, tension, Loss of consciousness, blurring of eye sight and cardiac arrest.
(ii) Carbon dioxide:
Carbon dioxide is released into the atmosphere mainly by the process of respiration, burning of fossil fuels, forest fire, decomposition of limestone in cement industry etc.
Green plants can convert CO 2 gas in the atmosphere into carbohydrate and oxygen through a process called photosynthesis. The increased CO 2 level in the atmosphere is responsible for global warming. It causes headache and nausea.

In the presence of uv radiation, CFC’s break up into chlorine free radical
CF 2 Cl 2
CF 2 Cl + Cl
CFCl 3
CFCl 2 + Cl
Cl + O 3 → ClO + O 2
ClO + O → Cl + O 2
Chlorine radical is regenerated in the course of reaction. Due to this continuous attack of Cl thinning of Ozone layer takes place which leads to formation of the ozone hole.
It is estimated that for every reactive chlorine atom generated in the stratosphere 1,00,000 molecules of ozone are depleted.

1. Rainwater has a pli of 5.6 due to the dissolution of CO., into it. Oxides of sulphur and nitrogen in the atmosphere may be absorbed by droplets of water that make up the clouds and get chemically converted into sulphuric acid and nitric acid. Due to this, the pH of rainwater drops below the level of 5.6. Hence it is called acid rain.
2. Acid rain is a by-product of sulphur and Nitrogen oxides in the atmosphere. Burning of fossil fuels in power stations, furnaces and petrol, diesel in motor engines produce SO 2 and NO 2 gases. They are converted into H 2 SO 4 and HNO 3 by the reaction with oxygen and water.
3. 2SO 2 + O 3 + 2H 2 O → 2H 2 SO 4
4NO 2 + O 2 + 2H 2 O → 4HNO 3
Harmful effects of acid rain:
1. Acid rain causes damage to buildings made us of marbles. This attack on marble is termed as stone leprosy.
CaCO 3 + H 2 SO 2 CaSO 4 + H 2 O + CO 2 ↑
2. Acid rain affects plant and animal life in aquatic ecosystem.
3. It is harmful For agriculture, as it dissolves in the earth and removes the nutrients needed for the growth of plants.
4. It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into drinking water which has toxic effects.
5. it causes respiratory ailments in humans and animals.

Objectives of wastewater treatment:
* To convert harmful compounds into harmless compounds.
* To eliminate the offensive smell.
* To remove the solid content of the sewage.
* To destroy the disease-producing microorganisms.
Treatment process:
The sewage (or) wastewater treatment process involves the following steps.
I. Preliminary Treatment:
In this treatment, coarse solids and suspended impurities are removed by passing the wastewater through bar and mesh screens.
II. Primary treatment (or) Settling process:
In this treatment, greater proportion of the suspended inorganic and organic solids are removed from the liquid sewage by settling. In order to facilitate quick settling coagulants like alum, ferrous sulphate are added. These produce large gelatinous precipitates, which entrap finely divided organic matter and settle rapidly.
Al 2 (SO 4 ) 3 + 6H 2 O → 2Al(OH) 3 ↓ + 3H 2 SO 4
III. Secondary (or) biological treatment:
In this treatment, biodegradable organic impurities are removed by aerobic bacteria. It removes upto 90 % of the oxygen-demanding wastes. This is done by trickling filter or activated sludge process.
(a) Trickling filter process:
It is a circular tank and is filled with either coarse or crushed rock. Sewage is sprayed over this bed by means of slowly rotating arms.
When sewage starts percolating downwards, microorganisms present in the sewage grow on the surface of filtering media using organic material of the sewage as food. After completion of aerobic oxidation the treated sewage is taken to the settling tank and the sludge is removed. This process removes about 80 – 85 % of BOD.
(b) Activated sludge process:
Activated sludge is biologically active sewage and it has a large number of aerobic bacterias, which can easily oxidize the organic impurities.
The sewage effluent from primary treatment is mixed with the required amount of activated sludge. Then the mixture is aerated in the aeration tank. Under this condition, organic impurities of the sewage get oxidized rapidly by the microorganisms.
After aeration, the sewage is taken to the sedimentation tank. Sludges settle down in this tank, called activated sludge, a portion of which is used for seeding fresh batch of the sewage. This process removes about 90-95 % of BOD.
IV. Tertiary treatment:
After the secondary treatment, the sewage effluent has a lower BOD (25 ppm), which can be removed by the tertiary treatment process.
In the tertiary treatment, the effluent is introduced into a flocculation tank, where lime is added to remove phosphates. From the flocculation tank the effluent is led to ammonia stripping tower, where pH is maintained to 11 and the NH 4 + is converted to gaseous NH 3. Then the effluent is allowed to pass through activated charcoal column, where minute organic wastes are absorbed by charcoal. Finally the effluent water is treated with disinfectant (chlorine).
V. Disposal of sludge:
This is the last stage in the sewage treatment. Sludge formed from different steps can be disposed by
* dumping into low – lying areas,
* burning of sludge (incineration),
* dumping into the sea,
* using it as low grade fertilizers.

(i) BOD and COD Biochemical oxygen demand (BOD):
The total amount of oxygen in milligrams consumed by microorganisms in decomposing the waste in one litre of water at 200°C for a period of 5 days is called biochemical oxygen demand (BOD) and its value is expressed in ppm. BOD is used as a measure of degree of water pollution. Clean water would have BOD value less than 5 ppm whereas highly polluted water has BOD value of 17 ppm or more.
Chemical Oxygen Demand (COD):
BOD measurement takes 5 days so another parameter called the Chemical Oxygen Demand (COD) is measured. Chemical oxygen demand (COD) is defined as the amount of oxygen required by the organic matter in a sample of water for its oxidation by a strong oxidising agent like K 2 Cr 2 O 7 in acid medium for a period of 2 hrs.
(ii) Viable and non – viable particulate pollutants:
Viable particulates:
The viable particulates are the small size living organisms such as bacteria, fungi, moulds, algae, etc. which are dispersed in air. Some of the fungi cause allergy in human beings and diseases in plants.
Non-viable particulates:
The non- viable particulates are small solid particles and liquid droplets suspended in air. They help in the transportation of viable particles. There are four types of non-viable particulates in the atmosphere.
Example:
Smoke, Dust, Mists, Fumes.

• Carbon monoxide binds with haemoglobin and form carboxyhemoglobin which impairs normal oxygen transport by blood and hence the oxygen-carrying capacity of blood is reduced.
• This oxygen deficiency results in headache, dizziness, tension, loss of consciousness, blurring of eyesight and cardiac arrest.

Methods to control environmental pollution:
* Waste management Environmental pollution can be controlled by proper disposal of wastes.
* Recycling A large number of disposed waste materials can be reuse by recycling the waste, thus it reduces the landfill.
* By substitution of less toxic solvents for highly toxic ones are used in industrial processes.
* By growing more trees.
* By using fuels with lower sulphur content.
* By control measures in vehicle emissions which are adequate.