What are the different sources of soil pollution? Discuss various measures for controlling soil pollution.


(i) Industrial wastes: Industrial wastes like scarp flash and effluents get mixed with soil. These are mostly toxic due to the presence of certain cyanides, chromates, acids, alkalies and metals like nickel, mercury etc. 

(ii) Urban wastes: Urban wastes such as domestic refuse, commercial wastes, rubbish from construction sites, waste paper, rags, fibres, broken glass articles slowly mix up with the soil. They lead to pollution such as foul smell.

(iii) Faulty agricultural practices: Fertilisers, pesticides weedicides etc. are chemical substances and from the soil they pass to ground water and are harmful to aquatic animals.

(iv) Radioactive pollutants: Dumping of the nuclear wastes from the nuclear power plants into the soil has been one of the greatest sources of radio active pollution of the soil. All the radio active wastes from atomic and hydrogen bombs emit radiations which are disastrous for the life on the earth.

Control of soil pollution:
(i) Use of manures: Manure prepared from animal dung and another farm refuse is added to the soil to maintain its fertility.
(ii) Use of biofertilizers: Biofertilizers such as nitrogen fixing bacteria and blue-green algae are inoculated in order to bring about the enrichment of the soil.
(iii) Proper sewage system: This system must be employed and sewerage recycling must be installed in all towns and cities. 

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Explain giving reactions, the plausible explanation for the escape of methyl isocyanate gas from the Union Carbide Ltd. Plant in Bhopal on Dec 2, 1984.


In the Union Carbide Ltd. Plant in Bhopal, three tanks that stored methyl isocyanate, pressure rose in one of the tanks and blasted. As a result, methyl isocyanate escaped into the atmosphere. Actually, a small amount of water seeped into the storage tank and initiated the following hydrolysis reaction:



Further the water that entered the tank was not pure and impurities present initiated the following polymerisation reaction:



Due to simultaneous occurrence of the above two exothermic reactions, the extremely toxic methyl isocyanate gas from the reservoir escaped into the atmosphere. 

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What processes are generally responsible for the deoxygenation of water?


Deoxygenation of water takes place through the following processes:
(i) Oxidation of organic matter: The dissolved oxygen in water is consumed rapidly by microorganisms to oxidise organic matter of sewage.
 

(ii) Bio-oxidation of the nitrogenous matter: Oxygen in water may be consumed by the bio-oxidation of the nitrogenous material. 


(iii) Oxidation of chemical reducing agent: The dissolved oxygen in water may be consumed by the chemical or biochemical oxidation of chemical reducing agents. 

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What do you mean by BOD and COD? How are these determined?


BOD: BOD stands for biochemical oxygen demand. It is a measure of the dissolved oxygen that would be needed by the microorganism to oxidise some of the inorganic and organic compounds present in the polluted water. BOD, therefore, is a measure of the contamination caused by the totality of these compounds which can be oxidised in the presence of micro-organisms. The BOD is taken as the measure of water quality. Clean water has BOD value of less than 5 ppm whereas highly polluted water has BOD value of 17ppm or more.

Determination of BOD: The water sample is first saturated with oxygen. It is then kept at constant temperature (usually 20°C) for five days. In this period of time, the micro-organisms in the water sample oxidise pollutants. The remaining amount of dissolved oxygen is determined and BOD is obtained by subtraction.

COD: COD is another parameter called the chemical oxygen demand. It is expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants.

Determination of COD: The water sample is treated with a known quantity of an oxidising agent such as potassium dichromate in acidic medium. This oxidises most of the polluting substances, including those which are resultant to microbial oxidation. The remaining K
2Cr2O7 is determined by back titration using standard solution of Mohr’s salt (reducing agent). From the concentration of K2Cr2O7 used up, the amount of oxygen used in the oxidation may be calculated from the following chemical equation:



The results are expressed in terms of amount of oxygen in ppm that would be required to oxidise the contaminants. This is called COD.

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What is environment?


The environment comprises of four components - atmosphere, hydrosphere, lithosphere and biosphere.

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