The properties of activated carbon allow it to remove undesirable impurities from a fluid. This works because for carbon preferentially adsorbs the impurity onto its large internal surface within the pore structure. The pore sizes are usually characterised as follows:

• micropores <2nm
• mesopore between 2-50 nm
• macropores >50nm (typical 50 to 200nm)

Depending on the type of product you have a different pore size distribution which influences the performance properties of the activated carbon. For example coconut activated carbons have a predominance of pores in the micropore range. Such structure is ideal for adsorption of small molecules. On the other hand chemical activated wood based carbon has predominance in the macropore range which is ideal for adsorption of large molecules like colour bodies.

The atoms of carbon comprising the large internal surface area of activated carbon present attractive forces outward from the surface. These very short range forces, known as Van der Waals forces, attract the molecules of the surrounding gas or liquid. The combination of these attractive forces and those of molecules in the surrounding medium result in adsorption of molecules at the surface of the activated carbon. Some molecules have structures which make them more easily adsorbed than others and it is due to this that separation of molecules is achieved. The force fields within the pore structure of the carbon depend on their size and shape.