Activated carbon is a crude form of graphite, the substance used for pencil leads. It differs from graphite by having a random imperfect structure which is highly porous over a broad range of pore sizes from visible cracks and crevices to molecular dimensions. The graphite structure gives the carbon it's very large surface area which allows the carbon to adsorb a wide range of compounds.

What is adsorption?

Adsorption is the process by which liquid or gaseous molecules are concentrated on a solid surface, in this case activated carbon. This is different from absorption, where molecules are taken up by a liquid or gas

Activated carbon is used for the removal of dissolved components from, freshwater waste water and marine systems. Activated carbon is effective against the following;

organics colour chlorine & bromine malachite green & many dyes ozone

Basic Principle
 
Activated carbon filter works on the principle of adsorption.

Adsorption is the process where molecules are concentrated on the surface of the activated carbon.

Adsorption is caused by London Dispersion Forces, a type of Van der Waals Force which exists between molecules. The force acts in a similar way to gravitational forces between planets.

London Dispersion Forces are extremely short ranged and therefore sensitive to the distance between the carbon surface and the adsorbate molecule They are also additive, meaning the adsorption force is the sum of all interactions between all the atoms. The short range and and additive nature of these forces results in activated carbon having the strongest physical adsorption forces of any material known to mankind.

Forms of activated carbon

There are three main forms of activated carbon.

Granular Activated Carbon (GAC) - irregular shaped particles with sizes ranging from 0.2 to 5 mm. This type is used in both liquid and gas phase applications.