There are two types of absorption of activated carbons:
Physical Adsorption – During this process, the adsorbates are held on the surface of the pore walls by weak forces of attraction known as Van Der Waals Forces or London dispersion forces.
Chemisorption – This involves relatively strong forces of attraction, actual chemical bonds between adsorbates and chemical complexes on the pore wall of the activated carbon.
Key Properties of Activated Carbon
Surface Area – Generally, higher the internal surface area, higher the effectiveness of the carbon. The surface area of activated carbon is impressive, 500 to 1500 m2/g or even more; a spoonful of activated carbon easily equates the surface area of a soccer field.
It is in the activation process that this vast surface area is created. The most common process is steam activation; at around 1000°C steam molecules selectively burn holes into the carbonized raw material, thus creating a multitude of pores inside the carbonaceous matrix. In chemical activation, phosphoric acid is used to build up such a porous system at a lower temperature.
Total Pore Volume – Refers to all pore spaces inside a particle of activated carbon. It is expressed in milliliters per gram (ml/g), volume in relation to weight. In general, the higher the pore volume, the higher the effectiveness. However, if the sizes of the molecules to be adsorbed are not a good match to the pore size, some of the pore volume will not be utilized. Total pore volume (T.P.V.) differs by raw material source and type of activation method.
Pore Radius – The mean (average) pore radius, often measured in angstroms, differs by activated carbon type.
Pore Volume Distribution – Each type of carbon has its own unique distribution of pore sizes. They’re referred to as micropores (small), mesopores (medium) and macropores (large). Carbons for adsorbing many types of gas molecules are microporous. The best carbons for decolorization have a higher distribution of mesopores.
- Micropores r < 1nm
- Mesopores r 1-25nm
- Macropores r > 25nm
nm = nanometer