**6. Mechanism of adsorption**

The main constituents of lignocellulosic materials contain various functional groups that play an essential role in dyes adsorption. The mechanism of adsorption relies on the adsorbent characteristics (e.g., surface area, porosity, and surface functional groups) [23]. Molecular properties of dyes (e.g., molecular size, aliphatic vs. aromatic and hydrophobicity), solution properties (pH, ionic strength, and temperature), and the interactions between functional groups of adsorbent and dyes are often involved in the adsorption mechanism [77]. The surface charges of adsorbent and adsorbate id related to the pH, and it is one of the important factors that control the adsorption process. The adsorption regulation could be achieved by measuring the point of zero charges (pHPZC) of sorbents using the zeta potential [78]. At pH < pHPZC, protonation of functional groups leads to a positively charged adsorbent that can successfully remove the anionic sorbates. Kyzas et al. [79] reported that the determination of pHPZC played an important role to explain the possible pH-mechanism regarding the adsorption of reactive dye (RB) onto UCR. The pHpzc was about 3.2–3.4 and negatively charged surface of UCR was occurred when the pH values above of 3; however, positively charged occurred at lower pH values. Therefore, at pH < 3.5, the adsorption process involves electrostatic interaction between UCR+ and SO3 � of dyes, and at pH > 3.5. The interactions decrease because the dye is still negatively charged, illustrating the reduction of dye (RB) removal. In the case of basic dye (BB), the molecule presented constant positive charge; the UCR was protonated in acidic pH values. Therefore, the adsorption process is very limited. However, increasing the pH of the solution (pH > 3.5) the surface of UCR is charged negatively. In this case, the deprotonation of the surface of UCR is taken place, and transformed to the negatively charged form which provides electrostatic interactions favorable for adsorbing cationic species. In alkaline conditions, the increase of pH solution would convert more groups to dO� and dCOO�, providing electrostatic interactions for cationic species removal. Based on these findings, the more significant adsorption mechanisms of dyes onto coffee residue is through electrostatic interaction. If the pH > pKa, the sorbate surface charge is negative; however, at pH < pKa, the sorbate exists in a positively charged surface. Thus, the adsorption capacity of the adsorbent can be altered by changing the pH solution. Reffas et al. [36] investigated the adsorption of methylene blue and Nylosan Red N-2RBL onto activated carbons prepared by the pyrolysis of coffee grounds. In the case of MB, the mechanism can be explained on the basis of an electrostatic interaction between the ionic dye molecule and the charged carbon substrate. At pH 6, the CGAC30, CGAC60 and CGAC120 activated carbons are negatively charged (pH > pHPZC ≈ 3.7) while the CGAC180 and CAC are positively charged (pH < pHPZC). Therefore, electrostatic repulsion between the MB cation (at pH 6) and CGAC180 (or CAC) is not in favor of adsorption.

In another study Murthy et al. [51] reported that the adsorption of MG by ACH was carried out by van der Waals force and electrostatic interaction. It was also controlled by membrane and intra-particle diffusion. The adsorption was accelerated with the increase of temperature and concentration. Furthermore, Shen and Gondal [48] examined the adsorption mechanism of Rh B and Rh 6G using CGP as adsorbents. The results involve electrostatic interaction and molecular interaction and the adsorption capacity decreased to 0.9 μmol/g for Rh B and 5.5 μmol/g for Rh 6G after 5 cycles. Based on FTIR analysis, Lafi et al. [43] found that the mechanism of adsorption could be explained by the hydrogen bonding and the electrostatic interaction between CR and oxygen-containing functional groups on the activated carbon surface. Cheruiyot et al. [53] provide that the electron sharing or exchange between WCH and CV indicates that the adsorption process is controlled by chemisorption. In another study [27], through the FTIR characterization and mechanism analysis, it was indicated that the process of adsorption of MO may involve hydrophobic–hydrophobic interaction and electrostatic interaction. Based on the FTIR characterization and the proposed adsorption mechanism, Lafi et al. [28] concluded that zwitterionic surfactant (DDAO) is the most efficient for the adsorption of MR onto coffee residues (CR). The mechanism involves different types of interaction such as hydrogen bonds, electrostatic and hydrophobic interactions. Lafi et al. [35] also investigated the adsorption capacity of esparto grass fibers (EGF) for TB and CV removal. Infrared spectroscopy demonstrated that several functional groups were involved in CV and TB binding on EGF such as ester, hydroxyl and amino groups. Binding seemed to be more related to chemisorption with hydrogen atoms of non-ionized carboxyl groups. Sorption behavior of modified extracted cellulose (MEC) from *Stipa Tenacissima* L by cetyltrimethyl ammonium bromide was investigated by Lafi et al. [39]. In this case of MO adsorption on the MEC, the mechanism could be related to hydrophobic interaction and electrostatic interaction.

Therefore, isotherms and kinetics models, along with thermodynamic parameters and activation energy, can clarify the physisorption or chemisorption characteristics of an adsorption process.

Different methods of dyes adsorption on coffee waste and esparto fiber adsorbents or and the adsorption mechanisms are schematically presented in **Figure 2**.

#### **Figure 2.**

*Schematic illustration for the adsorption mechanism of dyes onto coffee waste and esparto fiber based materials.*

*The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*

## **7. Conclusions**

This chapter was devoted to the use of lignocellulosic material based coffee residues and esparto fibers in the adsorption of dyes from wastewater. This Lignocellulosic material was investigated in natural and modified forms, and showed a potential use as adsorbents in the detoxification of water using the adsorption process. These adsorbents were characterized using different analytical methods such as FTIR, SEM, and BET. In term of kinetic and thermodynamic, this chapter demonstrated that the different experimental data fitted to the known models. We believe that Lignocellulosic material could be used in industrial water purification by removing undesirable chemicals, biological contaminants, and gases from water.
