Activated carbon is a micro porous inert carbon with a large internal surface (up to 1500 m²/g). On this surface organic molecules from liquids or gases can adsorb. Adsorption is the natural phenomenon in which molecules from the gas or liquid phase are attached to the surface of the solid.
Carbon materials are activated by a series of processes which include: (1) removal of all water (dehydration), (2) conversion of the organic matter to elemental carbon, driving off the noncarbon portion (carbonisation) and (3) burning off tars and pore enlargement (activation).
The basic structural unit of activated carbon is closely approximated by the structure of pure graphite. The graphite crystal is composed of layers of fused hexagons held by weak van de Waals forces. The layers are held by carbon–carbon bonds.
Activated carbon is a disorganised form of graphite, due to impurities and the method of preparation (activation process).
|electron microscopic view of activated carbon||symbolic view of graphite (GFDL)|
The structure developed is a function of the carbonisation and activation temperatures. In terms of pore structure, the adsorbent pores can be divided into three basic classes: (1) macropores (> 1000Å), (2) transitional or mesopores and (3) micropores (<10Å). The macropores do not add appreciably to the surface area of the carbon, but provide a passageway to the particle interior and the micropores. The micropores are developed primarily during carbon activation and result in the large surface area for adsorption to occur.
Activated carbon contains: (a) bulk atoms that are neutral, (b) surface atoms that are the real ‘adsorption’ atoms and (c) corner atoms that are very reactive and even react with metals.