4.2.2. Chemical sorption or chemisorption

This process involves electron exchange between the sorbate molecules and the surface of the sorbent, resulting in a chemical reaction. The bond formed between the sorbate and the sorbent is essentially a chemical bond and is stronger than in a physical sorption.

The nature of the forces involved in each of the sorption processes is significantly different. In the physisorption process, no activation energy is required; therefore, the sorption rate is higher even at low temperatures. While chemisorption requires activation energy, the sorption rate is low and depends on the sorption temperature [18].

In the study of new sorbents, it is essential to establish the most appropriate sorption equilibrium correlation, which is indispensable for the reliable prediction of sorption parameters and the quantitative comparison of the sorbent behavior for different sorbent systems, for varied experimental conditions [19].

The most common way to study the behavior of activated carbon is through kinetic and sorption equilibrium models. The first model is related to the study of the kinetics, while the second type is related to the studies of isotherm processes [16].

As a result of activation and carbonization processes, activated carbons with porous characteristics are obtained, which are determined by factors such as starting material, temperatures and carbonization and activation times, heating rate, particle size of the starting material, and so on [12].

In the preparation of an activated carbon, it is necessary to look for the appropriate ratio, as can be deduced from the information earlier, between the transport pores and the sorption pores, that is, it is necessary to achieve a rapid transport of the sorbate to the internal surface and, at the same time, obtain an internal surface sufficiently developed to ensure a high sorbent capacity. In each case, the properties of activated carbon will depend on the raw material, its previous treatment, and activation conditions [17].

Nowadays, a strong demand for activated carbons can be expected in two very important fields in developed countries: atmospheric pollution and wastewater, they are also used for other applications such as elimination of organic matter and toxic substances from industrial gases and drinking water, elimination of chlorine from water, etc. This promising future of activated carbons requires the development of alternative materials that may face the requirement of sorbent materials in the depollution area, especially as regards the search for new raw materials and obtaining very selective activated carbons. Actually, activated carbons with properties like molecular sieve are being prepared in recent years; the object of these materials is the selective sorption of molecules of specific size, a process in which the activated carbons are displacing the zeolites, used frequently for this purpose. Another important application, which takes advantage of the properties of a molecular sieve of activated carbons, is the retention of nitrogen oxides (NOx) from different sources and sulfur that coals and oils contain and that when heated is transformed into toxic products, such as sulfur dioxide (SO2), hydrogen sulfide (SH2), carbon sulfide (S2C), and so on [17].

Recently, fruit husks as agroindustrial waste have been implemented in the production of activated carbons; investigation of sorption about selective specific size molecules has been done regarding these materials. The use of agroindustrial residues is a new alternative that provides a proposal of integral valorization, taking advantage of the waste abundance and the low cost of the material [17].

Table 4 shows the results of some reports from which activated carbons have been elaborated; elaborating parameters as well as surface areas are shown. It is possible to see surface area values from 200 to 1800 m2 /g; these values are comparable to those reported in literature for lignite of 1300 m<sup>2</sup> /g [23], as well as 1853 m2 /g [24]. These values are also comparable to commercial materials: PET activated carbons have 1170 m2 /g [25], Darco KB-B of 1608 m2 /g [26], Fluka 05120 of 1110 m<sup>2</sup> /g [26], activated carbon MT40 of 528 m<sup>2</sup> /g [27], activated carbon BW of 300 m<sup>2</sup> /g [27], and Fluka 03866 of 179 m2 /g [26].
