Introduction to Activated Carbon (AC)

As regards the chemical composition, AC is practically pure, like diamond, graphite, carbon black and the diverse mineral or wood carbon. They all have the property of adsorption, consisting of a physicochemical phenomenon in which a solid, called an adsorbent, traps certain types of molecules on its walls. These molecules are called adsorbates and are contained in a liquid or gas.

Activated carbon

The main difference between each type of carbon lies in the structure or the way in which its atoms are arranged.

In the case of AC, they are combined in graphite platelets that look like the diagram (photograph of platelets).

The platelets are separated and face different ways, meaning there are gaps between them, known as pores, which provide the AC with its main characteristic: a large surface area and, therefore, a high adsorption capacity. The majority of activated carbon on the market have an area of between 600 and 1,500 m2/g.

Types of pores in a carbon particle

Activated carbon pores vary greatly in size that, according to their function, can be classified into adsorption pores or transport pores. The former consist of spaces between graphite platelets with a separation of between one and five times the diameter of the molecule that is going to be retained. In these, the carbon platelets are close enough to attract the adsorbate and retain it with greater force.

According to the IUPAC (International Union of Pure and Applied Chemists), which uses the basis of their diameter, the pores can be distinguished into three groups:

  • micropores: > 2 nm diameter
  • mesopores: 2-50 nm diameter
  • macropores: more 50 nm diameter (typically 200-2000 nm)

The size of micropores is appropriate for retaining small molecules, which approximately correspond to compounds that are more volatile than water, such as smells, flavours and many solvents. Macropores trap large molecules, such as colour molecules or humic substances: humic and fulvic acids, which are formed by the degradation of organic matter. Mesopores are suitable for molecules of a size that comes between the ones mentioned above.

Therefore, the capacity of an AC to retain a certain substance is not only due to its surface area, but the proportion of pores of a suitable size: one to five times the diameter of the molecule of said substance.