Sources
and Sinks: -
Hydrogen sulphide is produced by the reduction of
sulphate and organosulphur compounds by the bacterium Desulphovibrio
desulphuricans and associated with methane thial (CH3.S.H.), dimethyl sulphide (CH3.S.CH3)
and carboxyl sulphide (C.O.S).
Effects: -
Exposure to hydrogen sulphide for short periods can
result in fatigue. But high concentrations of H2S due to accidental release, often cause
fatalities.
Control: -
Hydrogen sulphide is removed from natural gas
petroleum streams by absorption or scrubbing. The most common types of
absorbents (solvents) for H2S
removal are ethanolamines (mono-, di-, and triethanolamine) and also the sodium
or potassium salts of weak acids like carbonic or phosphoric.
(iv) Oxides of
Sulphur (SOx):
-
Sulphur dioxide is one of the principal constituents
of air pollutants. It is a colourless, non-flammable and non-explosive gas with
a suffocating pungent odour. It has an odour threshold of 0.5 ppm, and a taste
threshold of 0.3 ppm. It is highly soluble in water, and is about twice as
heavy as air.
Sources and Sinks:
-
The major natural sources are volcanoes and dimethyl
sulphide (DMS) which is produced by marine phytoplankton and oxidized to SO2 in the
atmosphere. Natural sources contribute about 67% of SOx pollution which
is evenly distributed all over the world. Anthropogenic sources contribute 33%
of SOx pollution which is, however, localized in some
urban areas.
Sulphur dioxide is relatively
stable in atmosphere, and acts either as a reducing or an oxidizing agent.
Reacting photochemically or catalytically with other components in the
atmosphere, it produces sulphur trioxide (SO3) sulphurous acid (H2SO3),
sulphuric acid (H2SO4) and salts of sulphuric acid.
The end product (i.e., H2SO4 or its salts) reaches the earth’s surface
either as wet deposits or dry deposits, and forms sulphates.
Effect: -
The oxides of sulphur have pronounced effects not only
on human health but also on plants and materials. The most widespread disaster
due to SO2 occurs when it is accompanied by smoke, i.e.,
during smog formation. Effects on plants can be classified as acute or chronic.
Sulphur dioxide’s effect on materials is quite significant. Paper absorbs SO2, the sulphur dioxide is oxidised to H2SO4, and
the paper yellows and becomes brittle. Similarly, leather also weakens and
disintegrates in the presence of SO2. Due to these reasons, libraries store leather bound
books and historical documents in carefully controlled environments.
Control: -
The broad-based methods for control of sulphur oxide
emissions include burning fuel with less sulphur, removing sulphur from fuels,
converting coal by liquefaction or gasifaction, substitution of another energy source,
cleaning up the combustion products, or dispersion by tall stacks.
Q.19 Enumerate various air pollutants for
which National Ambient Air Quality Standards have been given in India. Explain
the various non-point sources contributing to air pollution. (AKTU. - 2007-08,2010 - 11)
Ans. Indian Ambient Air Quality Standards: -
The ambient air quality standards in India
are given in table.
The
concentration of a gas given in mg/m3 can
be converted in ppm, by using the formula given as under:
(i)
Stationary Area Sources: -
These are small stationary
sources and mobile sources with indefinite routes, such as, residential
heating, commercial and institutional heating, open burning, city traffic, etc.
(ii)
Mobile Line Sources: -
The line or mobile sources of
air pollution are the automobiles as these add pollutants along narrow belts
and over long distances. Highways, railways tracks, navigation routes are also
fall in this categories.
(iii)
Mobile Area Sources: -
These are airports, railway
stations, ports, etc.
