**2.3 Description on activating agents**

Effective activators so far used are alkaline compounds such as potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium chloride (CaCl2) and potassium carbonate (K2CO3), acidic compounds like phosphoric acid (H3PO4) and sulphuric acid (H2SO4), also intermediate metal salts such as ZnCl2. Based on the reactivity and physical nature of the activator, the mixing of the activator and organic precursors could be done by two modes viz., the physical mixing of the activator and precursor (both in dry conditions) and impregnation (solid precursor with melt or fused activator) [9].

H3PO4 is widely used in the activation of various lignocellulosic materials. ZnCl2 acts as a dampening agent during activation. K2CO3 is known to be a better activator than KOH due to the production with higher yield, higher surface and pore volume and higher adsorbing capacity from aqueous solutions. Activation with alkaline materials such as NaOH and KOH produces ACs with large amounts of surfacial microspores. KOH is being popularly used due to its ability to produce AC with a high surface area, distribution of fine pore size, low environmental pollution, less corrosiveness and cost affordability.

### **2.4 Precursors for AC**

AC can be produced from materials such as wood, coal and some polymers. Wood and coal are relatively economical, but exhaustible. Polymers are the main source of pure carbon whereas it leads to high production costs and the preparation processes require expensive raw materials, enormous time, energy and tedious preparation procedures. AC production costs can be reduced by either choosing a cheap raw material or by applying a proper production method [10]. Nevertheless,

**Figure 1.**

*Scheme showing the preparation of AC by physical and chemical activation methods [6] (Adapted with modification).*

*Phytomass-Derived Multifunctional Activated Carbon as a "Wonder-Material"… DOI: http://dx.doi.org/10.5772/intechopen.99448*

it is still a challenge to prepare AC with very specific characteristics, such as a given pore size distribution and using low-cost raw materials processed at low temperature (less energy costs). Therefore, it is necessary to find suitable low-cost raw materials that are economically attractive and at the same time present similar or even better characteristics than the conventional carbons.

The use of waste materials for the preparation of AC is very attractive in view of their contribution to decrease the costs of waste disposal, therefore helping environmental protection [11]. It is already known that any cheap material with a high carbon content and low ash and inorganics can be used as a raw material for the production of AC [12]. Hence the production of AC materials from phytomass has become very much popular in recent years. Literature shows that there have been many interesting research efforts to obtain low-cost AC from a variety of phytomass wastes such as sugarcane bagasse [13], rice straw [14], cotton stalk [15], coconut shells [16], wood [17], nut shells [18], olive seeds [19], apricot stones [20], almond shells [21] and date pits [22] for adsorption studies, for example application of AC.

#### **2.5 Structures of AC**

Structure of AC is also considered as an important factor while proposing any new applications. So a short description on the structures of AC is given here. Basically, three important structures have been described.

#### *2.5.1 Porous structure*

Generally, ACs show porous characteristics such as specific surface area (SSA), pore volume and pore size distribution and contain up to 15–20% of minerals in the form of ash [12]. The porous structure of AC is presumed to have developed during the carbonization process and further developed during activation when tar, volatile and other carbonaceous materials which might be present in the spaces between the elementary crystallites escape from the precursor. The structure of pores and pore size distribution depends on the nature of the precursors and the activation process. It is believed that during the activation disorganized carbon are removed by exposing the crystallites to the action of activating agent which leads to the development of porous structure. Dubinin [23] classifies pores according to their average width, which represents the distance between the walls of slit shaped pore or the radius of a cylindrical pore, proposed by and officially adopted by the IUPAC. Thus the pores are classified into (i) micropores (diameter (d) < 2 nm), (ii) mesopores (2 nm < d < 50 nm) and (iii) macropores (d > 50 nm). **Figure 2** represents a view of these pores.

The micropores form the largest part of the internal surface and are accessible to the adsorptive molecules [25] or electrolyte ions. Generally, micropores contribute at least 90% of the total surface area of an AC, whereas the surface area of mesopores form less than 5% of total surface area and the mesopore volume varies between 0.1 and 0.2 cm<sup>3</sup> g<sup>1</sup> . The contribution of macropores to the total surface area and pore volume is very small and does not exceed 0.5 m<sup>2</sup> g<sup>1</sup> and 0.2–0.4 cm3 g<sup>1</sup> respectively. SSA and porosity are found out by N2 adsorption studies.

#### *2.5.2 Crystalline structure*

Crystalline structure of AC starts to develop during the carbonization process. The crystalline structure of ACs is different from the graphite structure with respect to the interlayer spacing, which lies between 0.34 & 0.35 Å in ACs and 0.335 Å in

#### **Figure 2.**

*Schematic presentation of macro, meso and micropores in AC [24] (Adapted with modification).*

#### **Figure 3.**

*Scheme of structure of AC (a) graphitized carbon and (b) non-graphitized carbon (adapted with modification from [26]).*

graphite. Nevertheless, the basic structural unit of AC is in close approximation with the graphite structure. Based on the graphitizing ability, ACs are classified into graphitizing carbons and non-graphitizing carbons. The above two structures of carbons is schematically presented in **Figure 3**.

Graphitizing carbon may have a large number of graphite layers oriented parallel to each other and is delicate due to the weak cross-linking between the neighbor micro crystallites and has a less-developed porous structure. On the other hand, non-graphitizing carbons are hard due to strong cross linking between crystallites and show a well-developed microporous structure [26]. The formation of nongraphitizing structure with strong cross links is promoted by the presence of associated oxygen or by an insufficiency of hydrogen in the precursors.

#### *2.5.3 Chemical structure*

In addition to the porous and crystalline structure discussed above, the AC surface has also chemical structure. It is well established that the adsorption capacity on AC is determined by its porous structure and is strongly influenced by the chemically bonded heteroatoms like oxygen, nitrogen, sulphur and halogens [12, 27, 28]. These heteroatoms are obviously derived from the phytomass precursors and involve in the structure of AC during carbonization process or they may be chemically bonded to the surface during activation [29]. The heteroatoms are likely to be bonded to carbon atoms of the corners and edges of the aromatic sheets or to the carbon atoms at defect positions to form carbon–oxygen, carbon-hydrogen,

*Phytomass-Derived Multifunctional Activated Carbon as a "Wonder-Material"… DOI: http://dx.doi.org/10.5772/intechopen.99448*

**Figure 4.** *Model of various organic functional groups on the activated carbon [30] ( = carbon matrix; = pores).*

carbon-sulphur, carbon-nitrogen and carbon-halogen surface organic compounds (see **Figure 4**), known as surface groups or surface complexes [28, 31].

Ultimately, the organic hetero functional groups greatly influence the properties and nature of the phytomass-derived AC.
