Surface Area – Generally, the higher the internal surface area, higher the effectiveness of the Activated Carbon. The surface area of activated carbon is an impressive 500 to 1500 m2/g or even more; a spoonful of activated carbon easily equates to the surface area of a soccer field.
It is in the activation process that this surface area is created. The most common process is steam activation; at around 1000 degree Celsius 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 gramme (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 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 than carbons for other applications
Adsorption
Adsorption is when organic molecules bond to the internal pores of the activated carbon. This occurs in pores slightly larger than the molecules that are being adsorbed, which is why it is very important to match the molecule you are trying to adsorb with the pore size of the activated carbon. These molecules are then trapped within the carbon’s internal pore structure by electrostatic attraction and accumulate onto a solid surface.
Typically 1 cubic metre of Activated Carbon with 0.3 cubic metre of internal pores can adsorb 30 cubic metres or more of a gas, even if present in low concentrations in a carrier.
Two Types of Adsorption
Physical Adsorption – During this process, the adsorbates are held on the surface of the pore walls by weak electrostatic forces. This electrostatic attraction is due to van der Waal’s forces which are named after the scientist who studied them. These forces diminish with increased distance of the organic molecule from the pore wall.
Chemisorption – This involves relatively strong forces of attraction, actual chemical bonds between adsorbates and chemical complexes on the pore wall of the activated carbon.
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