Question

Differentiate between absorption and adsorption techniques to control gaseous contaminants.

Answer 1

Sr. No.	Basis	Absorption	Adsorption
1.	Definition	Gas or vapour is taken up into the bulk of a liquid.	Gas or vapour adheres to the surface of a solid.
2.	Mechanism	Gas molecules penetrate throughout the liquid.	Gas molecules are attracted to the surface only.
3.	Nature of process	It is a bulk phenomenon, as the gas penetrates and mixes throughout the entire volume of the liquid.	It is a surface phenomenon, as the gas molecules only adhere to the surface of the solid without entering its bulk.
4.	Medium used	The medium used is a liquid, such as water or lime water, in which the gas dissolves.	The medium used is a solid, such as activated carbon or silica gel, onto whose surface the gas molecules adhere.
5.	Example:	Removal of sulphur dioxide using lime water.	Removal of organic vapours using activated carbon.
6.	Efficiency	Depends on the solubility of the gas in the liquid.	Depends on the surface area and porosity of the solid.
7.	Reversibility	Usually more difficult to regenerate the liquid.	Adsorbed gases can often be removed by heating (desorption).
8.	Application	Industrial gas cleaning, scrubbing acidic gases.	Controlling odours, industrial fumes, and toxic gases.