Types and Classifications of Pollutants
Before we go into the classifications of pollutants, let’s first understand the meaning of a pollutant. A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. Pollutants can be in the form of solid particles, liquid droplets, or gases. In addition, they may be natural or man-made.
A pollutant may cause long or short-term damage by changing the growth rate of plant or animal species, or by interfering with human amenities, comfort, health, or property values. A pollutant is a waste material that pollutes air, water, or soil. Three factors determine the severity of a pollutant: its chemical nature, concentration, and persistence.
Different Types of Pollutants by Absorptive Capacity
Pollutants are of different types and classifications. It is important to distinguish the different types of pollutants that we are in contact with to guide most appropriately the most effective and safe methods of handling them.
1. Stock pollutants
Pollutants that the environment has little or no absorptive capacity are called stock pollutants e.g. persistent synthetic chemicals, non-biodegradable plastics, and heavy metals. Stock pollutants accumulate in the environment over time.
The damage they cause increases as more pollutant is emitted, and persists as the pollutant accumulates. A stock pollutants can create a burden for future generations by passing on the damage that persists well after the benefits received from incurring that damage has been forgotten.
2. Fund pollutants
Are those for which the environments have some absorptive capacity? Fund pollutants do not cause damage to the environment unless the emission rate exceeds the receiving environment‘s capacity (e.g. carbon dioxide, which is absorbed by plants and oceans).
Fund pollutants are not destroyed, but rather converted into less harmful substances, or diluted/dispersed to non- harmful concentrations.
3. Notable Pollutants
Notable Pollutants include the following groups:
– Heavy metals
– Persistent organic pollutants (POP)
– Environmental persistent pharmaceutical pollutants (EPPP)
– Polycyclic aromatic hydrocarbons
– Volatile organic compounds
– Environmental xenobiotics.
Read Also: Assessment of Pollutants and Risk Assessment in Public Health
Classifications of Pollutants
Pollutants may be classified by various criteria:
By the origin: Whether they are natural or man-made (synthetic)
By the effect: Regardless of origin: on an organ, specie or ecosystem
By the properties: Mobility, concentration, persistence, toxicity
By the controllability: Ease or difficulty of removal.
Some pollutants are biodegradable and therefore will not persist in the environment in the long term. However, the degradation products of some pollutants are themselves polluting such as the products DDE and DDD produced from the degradation of DDT.
Generally pollutants may be classified into primary and secondary pollutants thus:
1. Primary Pollutants
Primary pollutants are pollutants that are directly emitted from a process, such as ash from a volcanic eruption, carbon monoxide gas from a motor vehicle exhaust or sulphur dioxide released from factories.
Major primary pollutants include:
Sulphur Oxides (SOx) – Especially sulphur dioxide, a chemical compound with the formula SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulphur compounds, their combustion generates sulphur dioxide.
Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. This is one of the causes of concern over the environmental impact of the use of these fuels as power sources.
Nitrogen Oxides (NOx) – Especially nitrogen dioxide are emitted from high-temperature combustion and are also produced naturally during thunderstorms by electrical discharge. Can be seen as the brown haze dome above or plume downwind of cities.
Nitrogen dioxide is the chemical compound with the formula NO2. It is one of several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants.
Carbon monoxide (CO) – Is a colorless, odorless, non-irritating but very poisonous gas. It is a product of incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of carbon monoxide.
Carbon dioxide (CO2) – A colorless, odorless, non-toxic greenhouse gas also associated with ocean acidification, emitted from sources such as combustion, cement production and respiration. It is otherwise recycled in the atmosphere in the carbon cycle.
2. Secondary Pollutants
These are not emitted directly. Rather, they form in the environment when primary pollutants react or interact. An important example of a secondary pollutant is ground-level ozone – one of the many secondary pollutants that make up photochemical smog. Some pollutants may be both primary and secondary, that is, they are both emitted directly and formed from other pollutants.
Secondary pollutants include particulate matter formed from gaseous primary pollutants and compounds in photochemical smog. Smog is a kind of air pollution; the word ‘Smog‘ is a portmanteau of smoke and fog.
Classic smog results from large amounts of coal burning in an area caused by a mixture of smoke and sulphur dioxide. Modern smog does not usually come from coal but from vehicular and industrial emissions that are acted on in the atmosphere by ultraviolet light from the sun to form secondary pollutants that also combine with the primary emissions to form photochemical smog.
Ground level ozone (O3) is formed from NOx and VOCs. Ozone (O3) is a key constituent of the troposphere. It is also an important constituent of certain regions of the stratosphere commonly known as the ozone layer.
Photochemical and chemical reactions involving ozone drive many of the chemical processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant and a constituent of smog.
Peroxyacetyl nitrate (PAN) – is similarly formed from NOx and VOCs.
These are mostly referred to as microorganisms and life forms that affect foodstuff, water and in some instances air and soil. They include bacteria, viruses, fungi, and insect larvae.
Read Also: Methods of Monitoring Pollutants and Contaminants
Classification of Pollutants based on State
1. Chemical Pollutants
Environmental chemicals in the technical sense of the word are broad-based and may be classified into four categories:
– Desirable e.g. foodstuff
– Harmless and inert (oxygen and hydrogen)
– Desirable in small quantities toxic in large quantities
– Toxic in any quantity.
In pollution control the most serious concern is the fourth category (Toxic in any quantity). These can constitute chemical pollutants whether in the food, air, water or soil. Examples are Mercury (organic and inorganic), lead and other metal complexes.
However, the third category (Desirable in small quantities and toxic in large quantities) is also very important in pollution control especially in determining water quality standards for human drinking.
For example Chlorine is desirable in the purification of water for drinking, however when it is present in larger unacceptable quantities, it becomes a toxic pollutant. Other examples include aluminum, bismuth, bromine and copper.
2. Biological Pollutants
These are essentially microbial lives that access any of our environmental media to constitute pollution. They may be bacteria, viruses, fungi and other life forms. Different types of bacteria can harbor toxins of different types and virulence.
Bacteria may pollute the air, water, and soil or food media. Bacteria (clostridium, staphylococcus, or salmonella species) in food are responsible for most outbreaks of foodborne diseases. Plants are other veritable sources of biological pollutants to the environment.
The cotton pod tree found in most part of northern Nigeria breaks the pod especially in the dry season and release tiny fibres into the air which changes the micro-composition of the air and can negatively impact on the respiratory tract of the population living around.
3. Physical Pollutants
These pollutants are exemplified by energy or radioactivity that introduces to the environment and causing noticeable changes or impacts that are not normal to the environment. Examples of this energy include heat, radiation and noise.
The ambient physical temperature of a given environment is known with the range from season to season. However, certain human activities such as industrial engine operations or accidental bush fires or climatic changes can introduce an increase in heat regimes previously unknown, which can be described as heat pollution.
Similarly, noise from machines (grinding machines, aircrafts, industrial machines etc.) above normal tolerable levels constitutes pollutants. The same goes with radiant energy from x-ray machines, nuclear plants and some mining sites.
All these constitute physical pollutants because they are neither biological agents nor chemical agents but rather physical agents that constitute energy or radiation that affect the environment.
In summary, pollutants may exist either in solid, liquid or gaseous forms. Depending on the chemical constituent of pollutants some could be persistent in nature in the sense that they are not easily degradable and may remain active for a very long period of time in the environment. Pollutants could also be highly toxic and can exist in natural form or synthetic/man-made pollutants.
In the classification of pollutants, two major classes are identifiable as “Primary” and “Secondary‘‘. However, classification of pollutants can also be made based on certain criteria e.g. Origin, effect, property and controllability.
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