Wednesday, 17 September 2014



1.     Sulphur, like nitrogen, is a basic constituent of proteins in plants and animals.

2.   It is found in the biosphere in a wide variety of forms. Sulphur dioxide (SO2) and Hydrogen Sulphide (H2S) are the important gaseous forms, and the sulphate ion (SO42-) is the common form found in water and soil. The distribution of sulphur in the environment ia as shown in figure.

3.  Sulphate ion is reduced after being absorbed from the soil by plants and bacteria, and ultimately incorporated as the sulphydryl group(-SH) in proteins.

4.     Some sulphates are reduced under anaerobic conditions directly to sulphides, including H2S, or to elemental sulphur by a class of bacteria known as Desulfovibrio bacteria, found largely at ocean bottom.

5.    The hydrogen sulphide thus produced escapes as a gas into the atmosphere and replenishes the sulphur lost by precipitation.

6.     In the presence of oxygen, H2S is rapidly oxidised to sulphates by bacteria of genus Thiobacillus.

7.  Even in the absence of oxygen, several types of bacteria such as Chlorobacteriaceae and Thiorhodaceae oxidise H2S to elemental sulphur.

8.   Atmosphere receives sulphur through bacterial emission (H2S), fossil fuel burning (SO2), wind-blown sea salts(SO42-) and volcanic emissions (H2S, SO2, SO42- ).

9.      Most of the sulphur in the form of SO2 or H2S is converted to sulphur trioxide SO3 , which dissolves in water droplets to form sulphuric acid. The sulphates and the acid then precipitate with rain.

10.  The sulphur cycle is overloaded due to burning of fossil fuels at an ever increasing rate. As a result, the SO2 emitted into the atmosphere this way constitutes a significant fraction of total global sulphur transport. This increased amount of sulphur is changed mostly to the form of sulphuric acid  in rain water causing adverse ecological effects.

Reference Books :
Environmental Pollution Control Engineering, C.S. Rao .


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