4.1.7. Chemical activation

Chemical activation involves the carbonization of the precursor in the presence of an alkali, hydroxides (KOH, NaOH), or inorganic acids (H3PO4, HNO3, H2SO4) [16].

The yield of activated carbon prepared by chemical activation method is usually higher than those activated carbons prepared under physical activations. In order to compare physical and chemical methods, it is possible to emphasize that, while in the physical activation method, the development of pores is mainly promoted by the removal of carbon atoms by a gasification process; in the chemical activation method, the activation agents promote dehydrogenation reactions in the precursor that promote bonds' formation between carbon atoms and also the creation of pores on the surface.

Unlike physical activation, the preparation of activated carbons by chemical activations is carried out in a single stage; the carbonization and activation process are carried out simultaneously. Chemical activation requires lower carbonization temperatures than physical activation. In addition, chemical activation leads to a better formation of the porous structure.

As a result of the carbonization and activation processes, activated carbons with porous characteristics are obtained which are determined by various factors such as starting material (precursor), size of the precursor particles, activating agent, activation temperature, heating rate, temperature and carbonization time, etc. [12, 16].

#### 4.2. Sorption on solid surfaces

The forces that hold a solid bonded to the surface of activated carbons produce a field force around each ion, atom, or molecule. At the surface of the solid, these forces cannot suddenly disappear, and due to these unsaturated and unbalanced forces, the solid has the tendency to attract and retain on its surface molecules and ions of other substances with which it is in contact. Thus, when the surface of the solid is in contact with a gas or a liquid, the concentration of the gas or liquid is always greater in the surface of the solid than in the interior of the gas or liquid phase. The substance adhered to the surface is called sorbate, and the material in which it is sorbed is called sorbent. Depending on the nature of the forces involved, the sorption process is classified into two main types [18]:
