4.1. Chemical structure of activated carbons

The structure of the activated carbon is considered as a continuous descent in the degree of ordering of the planes in the graphite. However, activated carbon also has a three-dimensional structure of carbon atoms in flat sheets of hexagonal rings. However, unlike graphite, there is no crystallographic order in the third dimension, and the cross-linking of the plates can occur [13].

The spaces between the graphite planes of crystals create the microporous structure, with a high internal surface area [16]. According to IUPAC, pores of activated carbons can be classified into micropores (pore diameter <2 nm), mesoporous (pore diameter between 2 and 50 nm), and macropores (pore diameter >50 nm) [13].

In addition to the porous structure, activated carbons have a chemical structure, since they contain small amounts of heteroatoms. The presence of heteroatoms (O, N, H, S, etc.) attached to the edges of the graphene layers gives rise to a variety of surface functional groups [16], which allow polar substances to be weakly retained on the surface of activated carbon. This is because the carbon atoms located at the edges of the planes have a high available activity, since they are not saturated with carbon atoms and have free electrons [13].

Oxygenated functionalities are the most important due to their surface properties and can also be developed spontaneously through the exposure of the material to the inert atmosphere. Chemisorbed oxygen can only be removed from the surface as CO or CO2 at temperatures above 120C. The unsaturated carbon atoms at the edges of the basal crystal planes are associated with high concentrations of mismatched electrons, which play a very important role in the chemisorption process [16].

Oxygenated functional groups are not formed exclusively by the reaction with oxygen; these groups can also be the results of the reaction with other oxidizing gases (O3, N2O, CO2) and oxidizing solutions (HNO3, H2O2). In addition, the electron density π of the basal carbon planes is considered as chemically active [16]. The surface chemistry of activated carbon has an amphoteric nature due to the coexistence of acidic and basic surface groups. An activated carbon is globally acidic or basic depending on the surface concentration of these groups and also on the strength of acidic or basic compounds [13]. Therefore, the oxygenated surface groups are classified into the categories mentioned in the following sections.

## 4.1.1. Basic surface groups

Nowadays the activated carbons are prepared from a carbon precursor such as lignite, wood,

A typical carbonization process produces around 20–30% carbonization yield, where the activating agent is a dehydrating compound, which would increase the amount of active sites, favored by the thermal degradation of the precursor, modifying in this way the porosity of the obtained material. Precursors such as lignocellulosic materials reflect the botanical texture of

Many of the laboratory uses and industrial applications of activated carbons are based on the high sorption capacity of the material. This capacity depends on the physical properties of the

The structure of the activated carbon is considered as a continuous descent in the degree of ordering of the planes in the graphite. However, activated carbon also has a three-dimensional structure of carbon atoms in flat sheets of hexagonal rings. However, unlike graphite, there is no crystallographic order in the third dimension, and the cross-linking of the plates can occur [13]. The spaces between the graphite planes of crystals create the microporous structure, with a high internal surface area [16]. According to IUPAC, pores of activated carbons can be classified into micropores (pore diameter <2 nm), mesoporous (pore diameter between 2 and

In addition to the porous structure, activated carbons have a chemical structure, since they contain small amounts of heteroatoms. The presence of heteroatoms (O, N, H, S, etc.) attached to the edges of the graphene layers gives rise to a variety of surface functional groups [16], which allow polar substances to be weakly retained on the surface of activated carbon. This is because the carbon atoms located at the edges of the planes have a high available activity, since

Oxygenated functionalities are the most important due to their surface properties and can also be developed spontaneously through the exposure of the material to the inert atmosphere. Chemisorbed oxygen can only be removed from the surface as CO or CO2 at temperatures above 120C. The unsaturated carbon atoms at the edges of the basal crystal planes are associated with high concentrations of mismatched electrons, which play a very important

Oxygenated functional groups are not formed exclusively by the reaction with oxygen; these groups can also be the results of the reaction with other oxidizing gases (O3, N2O, CO2) and oxidizing solutions (HNO3, H2O2). In addition, the electron density π of the basal carbon planes is considered as chemically active [16]. The surface chemistry of activated carbon has an amphoteric nature due to the coexistence of acidic and basic surface groups. An activated carbon is globally acidic or basic depending on the surface concentration of these groups and also on the strength of acidic or basic compounds [13]. Therefore, the oxygenated surface

or synthetic precursors, mainly of polymeric type such as resins.

the precursor [14].

206 Wastewater and Water Quality

coal, as well as its chemical structure [15].

4.1. Chemical structure of activated carbons

50 nm), and macropores (pore diameter >50 nm) [13].

role in the chemisorption process [16].

they are not saturated with carbon atoms and have free electrons [13].

groups are classified into the categories mentioned in the following sections.

These groups are formed when the oxidized surface is reduced by an inert or hydrogen atmosphere, at high temperatures. The decomposition of acidic groups creates active sites at the edges of graphene plates, which can attract oxygen during the cooling stage in an inert atmosphere, and after a process of reexposure to air, forming basic functional groups such as chromene and pirona.
