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Pollution can be defined as any physical phenomenon added to the environment that causes unwanted or harmful effects. Pollution arises when the composition of air, water or soil that are supposed to be constant or fixed is altered by the presence of certain other substances in the mixture.
About 99% of the entire atmosphere of the Earth is made up of two gaseous molecules, nitrogen (N2) and oxygen (O2). The remaining 1% consists of carbon(IV) oxide; a range of rare gases, such as argon, neon and helium; and trace gases, such as carbon monoxide (CO), nitrogen(IV) oxide, methane (CH4), ammonia (NH3), ozone (O3) and sulphur containing compounds such as sulphur(IV) oxide (SO2), hydrogen sulphide (H2S) and sulphur(VI) oxide, SO3.
What is called air pollution occurs when the concentrations of certain trace gases increase significantly. The pollutants found in the air of an environment, as well as their degree of occurrence depends on the activities, and processes in that environment. There are five primary air pollutants: carbon monoxide (CO); sulphur(IV) oxide (SO2); hydrogen sulphide (H2S); hydrocarbons; and oxides of nitrogen.
Others include the fluorocarbons; certain other halo-alkanes; and solid particles from dust (e.g., asbestos and sawdust) and smoke (e.g., lead from the exhaust of automobile and electric power generators).
Note: carbon(IV) oxide, CO2 is not regarded as a pollutant.
CARBON MONOXIDE, CO
Sources: the main source of CO in the air is automobiles (i.e., from the exhaust fumes). Other sources are industry, space heating, power stations and domestic fires.
Effects: carbon monoxide in air is poisonous and extremely dangerous.
When inhaled, it combines with the hemoglobin in red blood corpuscles to form carboxyl
hemoglobin – this prevents the carriage of oxygen by the hemoglobin to essential sites of the body. Much inhalation of CO could cause death.
SULPHUR(IV) OXIDE, SO2
Sources: sulphur(IV) oxide gets into the atmosphere as a result of certain processes on sulphur containing substances, such as coal, iron ore (pyrites), zinc and copper ores, and petroleum:
(i). The burning of coal in power stations
S(s) + O2(g) → SO2(g)
(ii). The smelting of ores
4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g).
(iii). Petroleum refining – crude oil contains varying amount of sulphur, depending on the source of the crude oil. During refining, the sulphur is oxidized to SO2.
(iv). SO2 also gets into the air from industrial plants which emit SO2 as waste product.
Effects: in the atmosphere, sulphur(IV) oxide can be oxidized to form sulphur(VI) oxide, SO3. This reacts with water droplets to form sulphuric acid (H2SO4), which, haven formed in the atmosphere falls to earth, leading to the phenomenon of “acid rain” when
concentrations are high enough. The acidic water increases the pH of soils, rivers and lakes, thereby affecting plant and animal lives. SO2, when inhaled could cause diseases such as bronco constriction. SO2 is a green house gas - it causes global warming.
HYDROGEN SULPHIDE, H2S
Sources: hydrogen sulphide gas smells like rotten egg (it is actually produced from rotten eggs). Its main sources are refuse disposal, sewers and industry.
Effects: hydrogen sulphide is extremely poisonous, it is more poisonous, but less dangerous than CO when inhaled. It has caused many fatalities among workmen in sewer. It also tarnishes silver.
Hydrocarbons include primarily methane (CH4).
Source: methane is introduced into the air during the production and transport of coal, natural gas and oil; from incomplete combustion of fuel ; from the decomposition of organic waste in municipal solid waste land fills; and from the raising of live stock, e.g. poultry.
Effects: methane is a green house gas - it causes global warming.
OXIDES OF NITROGEN
Sources: oxides of nitrogen include gases such as NO2, NO and N2O.
They are emitted into the air during agricultural and industrial activities, as well as during combustion of solid waste and fuels, such as coal and petroleum.
Effects: these gases are green house gases- they cause global warming. When inhaled, they could cause lung diseases, such as broncho constriction.
The fluorocarbons are organic compounds containing only fluorine and carbon, and sometimes hydrogen and chlorine. They include the fluorochlorocarbons (FCCs-compounds containing only fluorine, chlorine and carbon); hydrogen fluorocarbons (HFCs); hydro chlorofluorocarbons (HCFCs); and per fluorocarbons (PFCs).
Sources: they are introduced into the atmosphere from aerosols, refrigerating systems and from a variety of industrial processes.
Effects: these gases are green house gases – they cause global warming. They are also ozone layer depleting substances.
OTHER HALO- ALKANES
These halo-alkanes include tetra chloromethane (CCl4), trichloromethane (CHCl3) and methyl bromide (CH3Br).
Sources: they are introduced into the atmosphere through industrial processes.
Effects: they are ozone layer depleting substances.
These include asbestos dust, sawdust, and of high importance are heavy metals (such as lead).
Sources: the main sources are the industries and power stations. Other sources include domestic use of electric power generators, automobile (release lead from their exhaust) and space heating.
Effects: when inhaled, they could cause lung diseases. Asbestos is carcinogenic, i.e., it could cause cancer.
Removal of Air Pollution
Certain air pollutants which are relatively active, e.g., SO2 and H2S, can be removed by employing chemical methods.
Removal of SO2:
SO2 could be removed from the air by the following techniques:
(i). Catalytic oxidation of SO2 to SO2.
I.e. 2SO2(g) + O2(g)
Then the dissolution of SO3 in water.
I.e. SO3(g) + H2O(l)
The H2SO4 formed is removed as an acid or as a tetraoxosulphate(VI) compound.
(ii). The use of calcium oxide, CaO
magnesium oxide, MgO; and sodium
trioxosulphate(IV) to react with the
SO2, thereby removing it from the air.
Removal of H2S:
H2S can be removed from the air by the following techniques:
(i). The oxidation of H2S using acidified
KMnO4 in the presence of water
and oxygen. The sulphur produced
from the oxidation process is oxidized
completely by oxygen to SO3, which is
dissolved in the water to form H2SO4.
The acid is then removed.
2MnO4-(aq) + 5H2S(g) + 6H+(aq) → 2Mn2+(aq) + 8H2O(l) + 5S(s)
2S(s) + 3O2(g) → 2SO3(g) ;
SO3(g) + H2O(l) → H2SO4(aq)
(ii). The reaction of lead(II) ethanoate solution with the H2S. The sulphur is
precipitated as dark brown or black lead(II) sulphide.
Pb(C2H3O)2(aq) + H2S(g)
→ PbS(s) + 2C2H4O2(aq).
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