Table of contents
- Introduction to Environmental Pollution
- Environmental Pollution
- Some Commonly Used Terms
- Chemical Reactions in Atmosphere
- Types of Environmental Pollution and Their Sources
There are a number of substances which may be toxic as well as non-toxic which are being added to the environment by natural events and human activities. These are mostly because of increase in number of vehicles, advancement in industries, increasing population, increased use of natural resources, the undesirable consequences of modern civilization etc. These substances which are continuously going into air, water and soil bring about undesirable changes in physical, chemical and biological characteristics of our environment and adversely affect the life processes of animals and plants.
The branch of science which deals with the chemical phenomena occurring in the environment is called environmental science. This deals with the study of the origin, transport, reactions, effects and consequences of chemical species in the environment. The contamination of any part of the environment is called pollution and the substances which cause pollution are called pollutants. The pollutants can cause air pollution, water pollution or soil pollution.
Environmental pollution is the effect of undesirable changes in our surroundings (air, water or land) that have harmful effects on human, animal and plant life as well as on materials. Pollution may be natural or manmade. It can be classified according to the components of environment being damaged. These are:
(i) Air pollution (ii) Water pollution (iii) Soil (land) pollution.
A substance which causes pollution is called Pollutant. The pollutants can be solid, liquid or gaseous substances. These can be degradable which rapidly break down by natural processes or non-degradable which remain in the environment in an unchanged form for many decades. The common examples of non-degradable pollutants are aluminum pieces, iron, phenolic compounds, DDT (Dichlorodipheny-ltrichloroethane), plastic materials, heavy metals, many chemicals, nuclear waste etc. These either do not degrade or degrade very slowly or partially and thereby pollute the environment. Such pollutants are harmful even in low concentrations. These pollutants not only accumulate but are often biologically magnified because they move in biochemical cycles and along food chains. In general, pollutants are the substances made by us, used by us and even thrown by us as waste products which pollute the environment directly or indirectly in one way or other.
Man is the principal source of pollution.
Some commonly used terms in environmental chemistry are:
When any new substance gets added to the environment or the substance which is already present in nature increases in undesirable concentration and possess threat to human beings or other vegetation and animals, then that substance is called a pollutant. The pollutants are harmful to living organisms and other materials and spoil the environment.
The common pollutants are:
- gases like carbon monoxide, Sulphur dioxide, oxides of nitrogen, etc.
- compounds of metals like lead, mercury, zinc, cadmium, arsenic, etc.
- Pollen grains, dust
- pesticides and detergents
- radioactive substances
It may be noted that normally highly toxic substances are considered as pollutants. But in many cases, even the substance which are considered less harmful can cause pollution if they, are present in undesirable concentrations. For Example, to increase growth of the plants, nitrate is added to soil as a fertilizer, but it can be harmful especially to young children, when present in excessive concentration in drinking water.
A substance which does not occur in nature but is introduced by human activity into the atmosphere affecting its composition is called contaminant.
If a contaminant has some harmful effect over the environment, it is considered as pollutant. For Example, pyrosulphuric acid (H2S2O7) leaked from a defective tank killed many persons and caused skin and breathing problems to many persons in Delhi. Since pyrosulphuric acid does not occur in the atmosphere, so it is a contaminant. But because of its dangerous effect, it is also regarded as a pollutant.
Source is defined as a site from which the contaminants or pollutants originate. Every pollutant originates from a source. If we have the knowledge of source, we easily develop the methods to eliminate pollutants.
Sink is defined as the medium or material which consumes or interacts with a long lived pollutant. For example, atmospheric sulphuric acid is absorbed by a marble wall as it acts as a sink because of the reaction:
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
The oceans are sinks for atmospheric carbon dioxide and other water soluble gases. Even for agricultural pesticides, insecticides, ground water and subsoil water act as sinks.
Anything that is affected by the pollutants is called Receptor. For Example human beings are the receptor of photochemical smog as it causes irritation in eyes and breathing problems.
Speciation means, the identification of different chemical forms or species of an element or a compound present in the environment. For Example, mercury maybe present in the environment in various forms such as elemental mercury, mercury salts, organometallic mercury species; CH3Hg+, (CH3) 2Hg. It is very essential to identify the chemical species of the pollutants because some species may be more toxic than others and need special care. For example, alkyl mercury derivatives [CH3Hg+, (CH3) 2Hg] are highly poisonous when compared to other species of mercury.
This indicates the permissible limit of a pollutant toxic in atmosphere to which a healthy worker is exposed during 8 hours a day or 40 hours a week for life time without any adverse effects. TLVs are determined by experimentation on epidemiology surveys, medical knowledge and experience, animals and environmental studies.
For Example, TLV of CO is 50 ppm and that of CO2 is 5000 ppm. But TLV for a poisonous gas phosgene is only 0.1 ppm.
In a process of environment pollution, a pollutant originates from a source. With the help of air or water or dumping on land by man, it gets transported to other places. Some of the pollutants may be absorbed (assimilated) or chemically changed by the sink. The remaining pollutants build up to harmful concentrations and affect the receptor.
The gaseous envelope surrounding the earth is known as atmosphere. The two major components of dry and clean air in the atmosphere (by volume) are nitrogen (78.09%) and oxygen (20.95%). Argon (0.934%) and carbon dioxide (0.034%) are the minor components of the atmosphere. Air can hold water vapour from 0.1 to 5% by volume and also contains trace of elements such as noble gases (neon, helium, krypton, and xenon), hydrogen, methane, Sulphur dioxide, ammonia, ozone, etc.
The atmosphere surrounding us may be divided into four regions:
Inner layer of the troposphere sustains about 80% of the total mass of air and almost all of the water vapours of the atmosphere which extends to 8 to 12 km above the earth’s surface. With increasing altitudes from the earth’s surface, the other regions are stratosphere (11-50 km), mesosphere (50 -90 km) and thermosphere (90 -500 km). Gases like ozone oxygen, nitrogen are present in the stratosphere.
Many chemical reactions are occurring in atmosphere in which oxygen plays an important role. Some of these reactions occur by the absorption of solar radiation and these reactions are called photochemical reactions. These chemical and photochemical reactions play a significant role in governing the chemical species present in the atmosphere. It may be noted that there are large variations in the atmosphere with regards to composition, temperature, humidity and intensity of sunlight. Therefore, different types of reactions are observed under varying atmospheric conditions.
1. Oxygen gas in the troposphere is responsible for some of the most important processes taking place on the earth. For Example, oxygen is taking part in burning of fossil fuels
CH4 + 2O2 → CO2 + 2H2O
2. Aerobic organisms with the help of at mospheric oxygen degrades the organic matter. Atmospheric oxygen also helps in oxidation of weathering processes.
3. Green plants in the presence of sunlight perform the process of photosynthesis by using the carbon dioxide and water present in the atmosphere. The process of photosynthesis is:
Plants exhale out oxygen during the process of photosynthesis, and this oxygen goes back into the atmosphere.
4. In the upper atmosphere oxygen exists in some forms which are different from those stable at lower levels. Thus, in addition to O2 (molecular oxygen), the upper atmosphere also have O (atomic oxygen), O2* (excited molecular oxygen), O* (excited atomic oxygen), O+ (ionic oxygen) and O3 (ozone).
The atomic oxygen is produced by photochemical dissociation of oxygen by ultraviolet radiation (E = hv).
O2 + hv → O + O
Due to the above photochemical dissociation of O2, molecular O2 is virtually non-existent at very high altitudes. At altitudes exceeding 40 km only less than 10% of oxygen is present as molecular oxygen. The atomic oxygen in the presence of radiation reacts with each to form excited oxygen atom according to the reaction.
The excited oxygen atom gives out visible light at 636 nm, 630 nm and 558 nm wavelength and is responsible for air glow. Oxygen ions (O+) are formed when ultraviolet radiations react with oxygen atom.
O + hv → O+ + e-
The oxygen ion is present in some regions of the ionosphere. It may be noted that the region beyond stratosphere is dominated by +ve ions such as O+, O2+, NO+ and free electrons. It is called ionosphere. O+ ion also undergoes a number of other reactions such as
O+ + O2 → O2 + O
O+ + N2 → NO+ + N
O + may also be formed according to the reaction
N2+ + O2 → N2 + O2+
Ozone is present in the stratosphere which acts as a protective radiation shield for, living organisms on earth. It is formed by photochemical reactions of oxygen as:
O + hv → O + O
O + O2 + M → O3 + M
Where M is another species like a molecule of N2 or O2. It absorbs the excess energy liberated by the above reactions and hence stabilize the O3 molecule. Ozone strongly absorbs ultraviolet light in the region of 220-330 nm according to the following reactions:
O3 + hv → O + O2
O3 + O → O2 + O2
Atomic oxygen may also be eliminated by the reaction
O + O + M → O2 + M
Ozone in the stratosphere is destroyed by reaction with reactive hydroxyl radicals or nitric oxide, etc.
O3 + HO* → O2 + HOO*
HOO* + O → HO* + O2
5. Nitric oxide is produced in the stratosphere below 30 km by the reaction of nitrous oxide (N2O) with excited oxygen atoms. Nitrous oxide originates from the microbiological processes occurring in the atmosphere.
NO2 + O* → 2NO
Above 30 km in the stratosphere it is produced by ionizing radiation on nitrogen
N2 + hv → N + N
O2 + N → NO + O
The different regions of the atmosphere are shown in figure below:
Fig – Types of Pollution
The environmental pollution may be of the following types:
1. Atmospheric Pollution or Air Pollution
2. Water pollution
3. Soil or land pollution
Air is never found clean due to natural and manmade pollution. Gases such as carbon monoxide, nitrogen oxides, Sulphur oxides, hydrogen sulphide and many volatile organic compounds are continuously released into the atmosphere due to natural and human activities. Addition of new chemical substance which disturbs the normal composition of air by building undesirable components which causes harm to humans, vegetation and other organisms. This phenomenon is called Air Pollution. The chemical substance causing air pollution is called Air Pollutant. About 90% of air pollution problems are caused by the pollutants such as carbon monoxide, Sulphur oxides, volatile organic compounds, nitrogen oxides and suspended particulate matter (such as carbon, dust, pollen, metal, etc.)
The air pollution is caused by both natural activity and human activity. The natural sources are volcanic eruptions, forest fires (caused by lightning), pollen dispersal, evaporation of volatile organic compounds from leaves, bacterial decomposition of organic matter, wind erosion of soil and natural radioactivity.
The main human activities responsible for polluting the air are industrial processes and industrial wastes, burning of fuels (coal, oil, gasoline, etc.) in motor vehicles, power plants and industrial plants etc.
Soil receives large quantities of hazardous wastes from different sources and gets polluted. Soil Pollutant may be defined as any factor which cause changes in the mineral content, texture and the quality of the soil and thereby disturbing normal balance of the organisms in the soil. It also effects the plant growth.
Causes of Soil Pollution
- Pesticides: The chemicals which are used for destroying pests are called pesticides. These are used to kill or control unwanted dangerous species of plants and animals. Pesticides are basically synthetic toxic chemicals with ecological repercussions. The repeated use of the same or similar pesticides make some pests resistant to that group of pesticides, thereby, making the pesticides ineffective.
- Deforestation: Cutting down of trees loosens the topmost layers of soil which is held by roots of trees, thereby increasing soil erosion. They are easily carried away that is, soil erosion takes places because of various factors like wind, water etc. Thus the soil is unable to support further any kind of vegetation as these activities make it barren.
- Garbage Pollution: Garbage consist of various wastes like plastics, polythene, domestic waste and many other things which can neither be recycled nor get decomposed by the passage of time(Some of these waste may take upto thousands of years to decompose). Collection of such waste results in foul smell and contamination of soil.
- Industrial waste: Industries contribute to most of the soil pollution, releasing a large number of chemicals which contaminating environment. Pollutants may be either in liquid or in a solid state.
Water pollution may be defined as any change in its physical, chemical or biological properties or contamination with foreign materials that can adversely affect human beings or reduce its utility for the intended use.
The degree of purity required for water depends upon its use. For Example, the water polluted for drinking purposes may be satisfactorily used for irrigation or producing electricity at hydroelectric power plant for cooling purposes.
Sources of Water Pollution
- Domestic Sewage: The domestic sewage contains oils, human excreta, dirt, paper, rags, sand grains, dissolved material such as detergents and inorganic compounds such as sodium chloride, ammonium sulphate and ammonium phosphate, decomposed kitchen waste. The sewage also contains many disease causing bacterias called pathogens which are most serious water pollutants.
- Industrial Wastes: Industrial wastes are also the major source of water pollution. The industrial wastes polluting water are mainly from industries such as coal or ore mines, textile industries, paper industries, food processing industries, dairies, chemical industries, pharmaceuticals, sugar and distilleries, oil refineries, tanneries, vegetable oil and soap industries etc. These wastes may contain inorganic and organic suspended particles and inorganic and organic soluble matter.
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