## 3 Adsorption of Model Compounds by Eggshell Biocomposite Adsorbents
### Adsorption Mechanism
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mechanism** & **Pollutant/adsorbent** & **Remarks and Reference** \\ \hline  & & Interaction of -SO3\({}^{-}\) and localized dye with Ti2\({}^{+}\) or Ca2\({}^{+}\)[64]. \\  & Ionic bond formation between sulphate groups and the cationic amino acid side chains of the tailored dye due to oppositely charged surfaces at variable pH [20]. \\  & AN57 dye/CES-TiO2, & Result obtained from point of zero charge allowed a suitable control of pollutant pH and thus electrostatic attraction was attributed to their adsorption [50]. \\  & Reactive yellow 145 dye/ESP and Cd2\({}^{+}\)/CES & Electrostatic interactions between -SO3\({}^{2-}\) of the dye and the eggshell whose surface is positively charged at low pH is responsible for the mechanism [48]. \\  & MB/Immob ESP and & Electrostatic attraction between oppositely charged ions at different pH [63; 67]. \\  & Interaction (EI) & Zn/ESP; BSA/ESP/Zn \\  & MB/CEAC & Interaction of adsorbate with surface CaO, SiO2, Al2O3, and ZrO2 sites on the CEAC [49]. \\  & Remazol reactive red 198 & Kinetic results support the role of chemisorption and exchangeable \\  & dye/Immob ESP with PVC and alginate & H\({}^{+}\) present at the -SiOH or -OH groups of eggshell could interact \\  & CR/CES & The zeta potential (+10 mV) is suggestive of electrostatic \\  & RBV-5R/ES & interactions [71]. \\  & MB/ESP & The dye sulfonate group is attracted to the protonated ES surface [53]. \\  & & Pores between the collagen and glycoprotein fibers in the eggshell \\  & & membrane-controlled movement of ions and dissolution of calcium \\  & & salt of the eggshell, where the dye in solution leads to the release of \\  & & ions as a major contributor to this mechanism [48]. \\ \hline Involes weak valence & & Adsorption via van der Waals forces is proposed. \\  & forces and sharing or & Negative zeta potential suggests electrostatic repulsive forces, where \\  & exchange of electrons & weak intermolecular forces play an important role [71]. \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Mechanism of the adsorption process of dyes molecules onto ESP/biocomposites. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mechanism** & **Pollutant/adsorbent** & **Ref.** \\ \hline  & & Ni2\({}^{+}\), Cu2\({}^{+}\), Cd2\({}^{+}\) /WM, ESP, ESH-M & [67; 110] \\  & & Pb/ESP; Na-ESP; HN-ESP; K-ESP & [41] \\  & metal ions and Ca2\({}^{+}\) on the eggshell surface or & Mechanically activated CaCO3/MSO4 & [38] \\  & onto CaCO3 & AMD/Bentonite, ESP, BEP & [39] \\  & & Pb/CHAP & [36] \\  & +Ca2\({}^{+}\) (solution) & Al3\({}^{+}\), Fe2\({}^{+}\), Zn\({}^{2+}\)/ESP & [106] \\  & Cd2\({}^{+}\), Cu2\({}^{+}\), Pb2\({}^{+}\) and Zn2\({}^{+}\)/Ch, SBFL, ESP, HK & [107] \\ \hline _Precipitation_: Precipitation can occur since & & \\   adsorption is pH dependent. If pH is high, & Al3\({}^{+}\), Fe2\({}^{+}\), Zn\({}^{2+}\)/ESP & [106] \\   precipitation can take place resulting in the & Mechanically activated CaCO3/MSO4 & [73] \\  & formation of hydroxides or carbonates. & Cd2\({}^{+}\), Cu2\({}^{+}\), Pb2\({}^{+}\) and Zn2\({}^{+}\)/Ch, SBFL, ESP, HK & [107] \\  & CaCO3\({}^{-}\)2\({}^{-}\) + H2O \(\equiv\) HCO3\({}^{-}\) + OH\({}^{-}\) & & \\ \hline \hline \end{tabular}
\begin{tabular}{c} _Electrostatic attraction:_ Occurs between \\  & & Pb2\({}^{+}\), Cu2\({}^{+}\), Cd2\({}^{+}\)/WM, ESP, ESH, ESP-M & [67; 110] \\  & & Pb2\({}^{+}\)/ESP; Na-ESP; HN-ESP; K-ESP & [41] \\  & & Pb from AMD/Bentonite, ESP, BEP & [39] \\ \hline \hline _Metal complexation:_ It may involve complexation & & Pb2\({}^{+}\)/ESP; Na-ESP; HN-ESP; K-ESP & [41] \\   of OH\({}^{-}\) or CO3\({}^{-}\) between Pb2\({}^{+}\) and ligands on & Pb2\({}^{+}\) from AMD/Bentonite, ESP, BEP & [39] \\  & the adsorbent surface. & Pb2\({}^{+}\)/CHAP & [36] \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Mechanism involved in the adsorption of metal ions onto ESP biocomposites. major contribution for ESP binding relates to the release of charged particles from the adsorbent surface.