4. Sorbent materials obtained from orange peels

The elaboration of high-quality sorbent materials from agroindustrial waste opens an effective path for the conversion of these residues into high value added products [11].

Activated carbons are commonly used as sorbent materials due to their sorption capacity; those activated carbons are produced from a wide variety of carbon-based materials. Those materials used for the production of activated carbon are carbon-based organic materials, such as coal, lignite, and wood. Although natural coal is the most used precursor, agroindustrial waste is considered as a good alternative; in this way, the activated carbon produced from waste would reduce the demand of forests since wood is also used for this purpose [12].

According to International Union of Pure and Applied Chemistry (IUPAC), an activated carbon is a porous carbonaceous material that has been subjected to gas reaction, sometimes with chemical agents, during or after carbonization process in order to increase its sorption properties.

The use of carbon-based materials goes so far back in history, since charred wood and mineral coal or simply partially volatilized coal materials were already used for similar purposes.

From 1901, several patented methods of activated carbon production were registered. In this way, R. Von Ostrejko patented two methods to produce activated carbon: one based on the carbonization of lignocellulosic materials with metal chlorides (the basis of chemical activation) and the second method based on a gentle gasification of coals with water vapor or carbon dioxide (basis of physical activation) [13].

Nowadays the activated carbons are prepared from a carbon precursor such as lignite, wood, or synthetic precursors, mainly of polymeric type such as resins.

4.1.1. Basic surface groups

4.1.2. Acidic surface groups

4.1.3. Hydrogenated groups

4.1.4. Phosphorus groups

4.1.5. Activated carbon preparation

and chemical activation, described below [13].

material [14].

4.1.6. Physical activation

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.

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These groups are formed when the carbon surface is exposed to oxygen through reactions with oxidizing agents in solutions or the gas phase; this process can be carried out at room temperature or even at high temperatures, these parameters are responsible for the amphoteric

Hydrogen atoms are usually present in the surface of activated carbons such as chemisorbed water, as part of other surface groups such as carboxylic acids, phenols, amines, or even

Phosphorus atoms contained in activated carbons generally come from the phosphoric acid used as an activating agent in the preparation stage. Once phosphorus is located in the carbon matrix, it can be stable between 500 and 1000C; this atom can be found as red phosphorus or chemically bonded as dCdPd or dCdOdPd. Phosphorus-containing species can be

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

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

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

characteristic of the activated carbons, which depend on the pH of the aqueous phase.

directly attached to carbon atoms as part of aromatic or aliphatic structures.

formed during carbonization at low temperature ranges [16].

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 the precursor [14].

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