Environmental Chemistry:
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:
Environmental pollution is an undesirable change in physical, chemical or biological characteristics of our surroundings (air, water or land). Environmental pollution can affect human, animal and plant life as well as materials. Pollution may be natural or man made. It can be classified according to the components of environment being damaged. These are:
- Air pollution
- Water pollution
- 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 aluminium pieces, iron, phenolic compounds, DDT (dichlorodiphenyl tetrachloroethane), plastic materials, many chemicals, nuclear waste etc. These either do not degrade or degrade only 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.
Some Commonly Used Terms: Some commonly used terms in environmental chemistry are:
1) Pollutant:
When the concentration of a substance already present in nature or of a new substance increases to undesirable proportions causing danger to human beings, other animals or vegetation and other materials, the substance is treated as a pollutant. The pollutants spoil the environment and are harmful to living organisms and other materials.
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, etc
- Pesticides and detergents
- Sewage
- Radioactive substances
It may be noted that normally highly toxic substances are considered as pollutants. However, the substances which are normally harmless can cause pollution if they are present in undesirable concentrations. For example, nitrate is added to soil as a fertilizer to increase growth of the plants, but an excessive concentration of nitrate present in drinking water can be harmful especially to young children.
2) Contaminant:
A substance which does not occur in nature but is introduced by human activity into the atmosphere affecting its composition is called contaminant.
The contaminant is classified as pollutant when it has some harmful effect. For example, pyrosulphuric acid (H2S207) 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.
3) Source:
The site from which the pollutants or contaminants come is called source. Every pollutant originates from a source. The source is particularly important because its knowledge helps to develop the methods to eliminate pollutants.
4) Sink:
The material or medium which consumes or interacts with a long lived pollutant is called sink. For example, a marble wall acts as a sink for atmospheric sulphuric acid because of the reaction.
CaCO3 + H2SO4 ——> CaSO4 + H2O + CO2
The oceans are sinks for atmospheric carbon dioxide and other water soluble gases. Ground water and subsoil water also act as sinks for pesticides used in agriculture.
5) Receptor:
Anything that is affected by the pollutants is called receptor. For example, human beings are the receptor of photochemical smog causing irritation in eyes and breathing problems.
6) Speciation:
Speciation means the identification of different chemical forms or species of an element or a compound present in the environment. For example, mercury may be 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.
7) Threshold Limit Value (TLV):
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 animals, medical knowledge and experience, epidemiology surveys and environmental studies. For example, TLV of CO is 50 ppm and that of CO2 is 5000 ppm.