Activated Carbons are the most powerful adsorbents known. It is basically a solid material consisting mainly of pure carbon.
A characteristic feature is its porous structure and the resulting immense surface area which may be as large as 1500 m2/gm.
Due to its exceptional adsorption qualities, activated carbon is widely used in process destined to purify, discolor, recuperate and remove odors at low cost and superior efficiency.
Activated carbons work on the principle of adsorption. Adsorption is an interfacial process involving the collection of gaseous or solute components on the surface of adsorbent solids.This phenomenon is associated with physical attractive forces that bind gaseous and solute molecules commonly known as Van-der-Waals forces. Adsorption is thus a physical process, i.e. the substances adsorbed on the solid do not undergo any chemical reaction with the latter. The adsorbing solid is referred to as adsorbent and the substance to be adsorbed from the liquid or the gas phase as the solute.
Pore Sizes and its distribution:
When the carbon is activated it leads to opening of various pores in its structure.
The pores with diameters exceeding 500 A are called macropores or the transport pores. These pores are larger pores and are not responsible for adsorption directly but they act as wide paths through which organic molecules penetrate. Thus they transport the adsorbate to the adsorption site, which are located inside the particle. A full network of transport pores needs to be distributed through out the carbon particle to take the adsorbate to the smallest adsorption pore site.
Pores with diameter below 50 A are called micropores or the adsorption pores. These are the finest pores , which are located inside the carbon particle where the adsorption actually takes place.
The most common parameter to check the adsorption power of activated carbon is called methylene blue adsorption. The methylene blue indicator is a chemical having particle size 12 A° which prevents it from adsorbing on the adsorption pores, which are smaller than 12 A That is why an activated carbon with a uniform and better pore size distribution will not give high methylene blue value but will work better as it will adsorb impurities which are very small in size.
For activated carbon to give good performance it needs to have both these pores in good proportion.