## 3 Adsorption of Model Compounds by Eggshell Biocomposite Adsorbents
### Adsorption Mechanism
The formation of a metal chelate complex with the adsorbent and protein has contributed to protein binding to metal ions by exposing electron donating amino acid residues (e.g., imidazole group) of the protein surface. The adsorption mechanism proposed for adsorption of dyes onto the eggshell-metal-egg white waste may occur via dipole interactions, and/or charge-charge interactions, but electrostatic interactions may have significant contributions. The dye adsorption mechanism of the process for AN57 could be through electrostatic interaction of the -SO\({}_{3}\)\({}^{-}\) group and localized AN57 dye with the positively charged titanium and calcium ions or the partial charge of the surface oxygen bridges containing titanium and calcium oxides that attract the aromatic rings of the dye [64]. The mechanism may involve electrostatic interactions by attraction between the oppositely charged surfaces at different pH conditions [50]. Considering isotherm contribution, chemisorption is the feasible mechanism for the process to occur and was supported from the fitting results of the kinetic models. The PSO model provided the best-fit results for the data, which also supports chemisorption. An adsorption mechanism was proposed for the removal of EBT and MB, where electrostatic attraction between the negative adsorbents and positive MB  
Figure 8: Contributing factors for the adsorption mechanism of pollutants onto eggshell particles. Copied with permission [146]. occur at basic pH, whereas positive adsorbents and negative EBT dye occur at acidic pH, which is largely controlled by chemisorption [63]. A mechanism was reported [7] for the sorption process that may involve valence forces through sharing or exchange of electrons between sorbent and sorbate, that is between MB/CEAC [147], which could involve exchangeable H\({}^{+}\) ions with the SiO-H or OH groups. A study of the adsorption of MB dye onto the eggshell sorbent reported that electrical double layer effects maintain surface neutrality of the adsorbent, and provide an account of the adsorption process [48]. The pores between the collagen and glycoprotein fibers of the ES membrane contributed to the movement of ions and calcium salts in the ES to dissolve when mixed with the dye solution to release Ca\({}^{2+}\), HCO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), and OH\({}^{-}\)[148; 149; 41]. The process of ion transport via the ES pores may play a key role in the released ions adsorbed onto the eggshell surfaces that form negative ions [48]. The solution also contains some cations such as Na\({}^{+}\), Mg\({}^{2+}\), and K\({}^{+}\), which may be adsorbed onto the surface of the ES and form an electrical double layer, where the ES surface acquires a positive charge [48]. In this way, ions from the solution can be adsorbed onto the negatively charged membrane surface. The adsorption mechanism proposed for the CR dye was attributed to electrostatic interactions between the -SO\({}_{3}\)\({}^{-}\) group of the dye and the positive surface charge of the ES, especially at low pH. Electrostatic interaction and /or cation exchange is responsible for the adsorption of Zn\({}^{2+}\) onto ESP and the negatively charged ions, such as CO\({}_{3}\)\({}^{2-}\), HCO\({}_{3}\)\({}^{-}\), OH\({}^{-}\), and cation exchange with Ca\({}^{2+}\) and Zn\({}^{2+}\). BSA adsorption onto the composite may occur through combined effects involving electrostatic interactions between the positively charge biocomposite and the negatively charged BSA and/or metal-chelate interactions via electron-donating amino acids of BSA with the ESP/Zn composite [67]. Various ES biocomposites display efficient adsorption to a wide variety of dyes from cationic to basic at variable pH, where remazol dye and RY 145 sits at acidic pH 2, CR and MB at neutral pH 7, and TB at pH 12. This showed that the ES biocomposites enable treatment of a wide range of wastewater systems for the removal of pollutants.