Activated carbon is a porous form of carbon which can be manufactured from a variety of carbonaceous raw materials. The principal commercial products are made from coconut shell, coal, peat or wood. The activation process involves treating the raw material with steam or chemicals, thereby developing a pore structure.
Activated carbon known as activated charcoal is characterised by a vast system of pores of molecular size within the carbon particles, resulting in the formation of a material with an extensive internal surface area. Commercially available, activated carbons have surface areas from 400m2/g to in excess of 2000m2/g.
How does it work?
The atoms of carbon, comprising the large internal surface area of activated carbon, present attractive forces outward from the surface. These 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 the absorption 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 the separation of molecules is achieved.
Activated Carbon Selection
The selection of the most appropriate activated carbon type is based either on known characteristics of the chemicals to be removed in an adsorption process or by a series of controlled laboratory tests.
Powder Activated Carbons
These are mainly used in batch processes and removed by filtration after an appropriate contact time.
Granular Activated Carbons
Used in fixed or moving bed filters; the smallest particle of activated carbon is normally consistent with the retention in the filter and acceptable flow resistance, since this will provide the best adsorption kinetics.
Activated carbon is sometimes chemically impregnated to enhance the performance by chemisorption, when the adsorption affinity for particular contaminants is too weak to be effective.