4.1.5. Activated carbon preparation

The physicochemical properties of each activated carbon depend mainly on the source of the starting material, since it essentially determines the structural characteristics of the resulting material [14].

Through the carbonization process, it is possible to obtain a low surface area, as elements such as oxygen and hydrogen are eliminated by decomposition of the starting material in an inert atmosphere, usually nitrogen. The resulting material is formed by unions of elementary graphitic microcrystals, usually plugged by tars and carbonization residues, which notably decrease the sorption capacity. In order to increase the sorption capacity of this coal, it will be necessary to resort to some method that allows eliminating tars, by means of some oxidizing agent as the activating agent [14]. There are two activation processes, called physical activation and chemical activation, described below [13].

#### 4.1.6. Physical activation

This method consists of an oxidation step that is usually carried out in the presence of water vapor, carbon dioxide, or air; the activation treatment is carried out at high temperatures (>700�C) [15]. These agents are capable of extracting carbon atoms from the porous carbon structure according to the following stoichiometry equations; these equations describe the endothermic reactions. This method of activation is known as physical or thermal activation [14].

$$\text{C} + \text{CO}\_2 \rightarrow 2\text{CO} \tag{1}$$

4.2.1. Physical sorption

4.2.2. Chemical sorption or chemisorption

experimental conditions [19].

so on [12].

vapor on liquids.

The sorbate is bounded to the surface of the activated carbon by relatively weak Van der Waals forces, identical to the molecular forces of cohesion that are involved in the condensation of

Sustainable Sorbent Materials Obtained from Orange Peel as an Alternative for Water Treatment

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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

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

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

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

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

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

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

is essentially a chemical bond and is stronger than in a physical sorption.

rate is low and depends on the sorption temperature [18].

second type is related to the studies of isotherm processes [16].

material, its previous treatment, and activation conditions [17].

$$\text{C} + \text{H}\_2\text{O} \rightarrow \text{CO} + \text{H}\_2\tag{2}$$

In this process, two differentiated stages can be considered: a first one, in which all the disorganized materials are burned, and a second one, in which the pores that were initially closed or blocked are opened. As a result of that, a new material with a high surface is produced, due to the presence of large porosity [17].
