## 3 Adsorption of Model Compounds by Eggshell Biocomposite Adsorbents
### Adsorption Mechanism  
Various processes and reaction mechanisms that are generally involved in adsorption of solute species (adsorbates) onto a typical eggshell particle adsorbent are illustrated in Figure 8, where the nature of the interaction depends on the functionality of the adsorbent-adsorbate system. The three major proposed mechanisms for the adsorption of dyes and organic pollutants from the literature survey include electrostatic interactions, electrical double layer effects and interactions by weak forces involving sharing or exchange of electrons. The mechanisms of the adsorption processes are listed in Table 8, whereas Figure 8 depicts some of the contributing factors to the mechanism involved in adsorption process of pollutants onto the surface of the ESP biocomposites. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pollutant Ions and Dyes** & **Conventional Adsorbents** & \(q_{e}\) (mg/g) & **Ref.** \\ \hline \multirow{2}{*}{Pb\({}^{2+}\)} & Natural zeolite & 33.9 & [128] \\ \cline{2-4}  & Sugar cane/Organge biochar & 87.0/27.9 & [94] \\ \hline \multirow{4}{*}{Cd\({}^{2+}\)} & Zeolite molecular sieves & 197.5 & [129] \\ \cline{2-4}  & Natural (clinoptillolite) zeolites & 15.8 & [130] \\ \cline{2-4}  & MWCNTs & 201 & [114] \\ \hline Fe\({}^{3+}\) & Natural (clinoptillolite) zeolites & 17.8 & [130] \\ \hline \multirow{2}{*}{Zn\({}^{2+}\)} & Natural zeolite & 29.0 & [128] \\ \cline{2-4}  & CNTs & 156.3 & [131] \\ \hline \multirow{2}{*}{Ni\({}^{2+}\)} & Natural zeolite & 27.1 & [128] \\ \cline{2-4}  & Natural (clinoptillolite) zeolites & 13.1 & [130] \\ \hline \multirow{4}{*}{PO\({}_{4}\)\({}^{3-}\)} & Granular ternary agrowaste adsorbent & 9–30 & [16] \\ \cline{2-4}  & Zr/chitosan beads & 67.7 & [132] \\ \cline{2-4}  & Peanut shell biochar & 3.8 & [133] \\ \cline{2-4}  & Fe/AC & 6.59–15.9 & [134] \\ \cline{2-4}  & Crawfish char & 9.5-70.9 & [135] \\ \hline \multirow{4}{*}{SO\({}_{4}\)\({}^{2-}\)} & Granular agrowaste adsorbent & 77–117 & [136] \\ \cline{2-4}  & Granular ternary agrowaste adsorbent & 22–20 & [137] \\ \cline{2-4}  & Magnetic MWCNTs & 56.9 & [138] \\ \cline{2-4}  & Barium-modified analcime & 13.7 & [139] \\ \hline \multirow{4}{*}{F\({}^{-}\)} & Activated alumina & 10.2–101 & [140] \\ \cline{2-4}  & Palm shell AC, PSAC, & 106.4 & \\ \cline{2-4}  & PSAC/MgSiO\({}_{3}\) & 113.6 & [141] \\ \cline{2-4}  & Alumina & 513.9 & [142] \\ \hline \multirow{4}{*}{NO\({}_{3}\)\({}^{-}\)} & AC from sugar beet bagasse & 9.14–27.6 & [143] \\ \cline{2-4}  & GAC from grape wood & 37.5 & [144] \\ \cline{2-4}  & Zero valent iron nano particles (NZVI) & 36.3 & [144] \\ \cline{2-4}  & GAC/NZVI & 55.8 & [144] \\ \cline{2-4}  & Carbon/silica composite & 11.5 & [145] \\ \cline{2-4}  & Zr/chitosan beads & 80.2 & [132] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _Cont._The adsorption of metal ions by ESP is expected to be through electrostatic interaction and/or ion exchange process because Ca\({}^{2+}\) originating from eggshell particles undergoes a displacement reaction when CaCO\({}_{3}\) of the ESP was mixed with the aqueous solution [35]. The calcium salt may partially dissolve and release Ca\({}^{2+}\), and other negatively charged ions, such as CO\({}_{3}\)\({}^{2-}\), HCO\({}_{3}\)\({}^{2-}\), and OH\({}^{-}\), on the eggshell surface that can undergo exchange with other metal ions from the bulk solvent. Thus, the positively charged ions in the surrounding media were adsorbed onto the negatively charged carbonate ion on the ES surface by replacement of the dissociated calcium ions in an ion exchange process. Further interaction for this composite adsorbent involved the adsorption of egg white protein onto the ES-metal adsorbent system. This could be due to strong and cooperative electrostatic interaction between the positively charged eggshell-metal complex and the negatively charged macromolecular proteins.