## 2 Adsorbent Preparation and Characterization
### Adsorbent Characterization
\begin{table}
\begin{tabular}{l c} \hline \hline
**Type of Characterization/Remarks** & **Refs** \\ \hline _SEM/EDS:_ About 50\% of the literature reviewed considered & \\ the morphology of the ESP or its composites. In total, 33\% of this subset reported the surface morphology after the adsorption process. SEM images reveal that ESP agglomerated and irregular surface features have a porous network and an angular pattern. CES exhibited a & \\ macroporous network structure of interwoven cross-linked & \\ fibers with diameter between 0.3 and 5 \(\mu\)m. There is increased & \\ porosity, and variable surface morphology that depend on the & \\ composite components. After adsorption, the structure was & \\ crystalline, and pores are no longer visible. EDS showed that & \\ Ca, C, and O are major elements in ESP. & \\ \hline _X-ray methods (XRD/XRF/EDX):_ X-ray results for ESP is the & \\ rhombohedral crystalline phase of calcite showing CaCO\({}_{3}\) as the main component that is replaced by CaO after calcination. Patterns also displayed Ca, Si, Na, S, and Mg as the main elements present in ESP. Calcite is the dominant form at or & \\ below 800 \({}^{\circ}\)C, while portlandite and lime dominate above & \\
800 \({}^{\circ}\)C. There is no XRD specific to CaO and Ca(OH)\({}_{2}\) in the & \\ composites. Variable XRD patterns show the elemental & \\ composition of each, for the various CES composites, and & \\ additional peaks show the presence of other elemental species. & \\ After adsorption, a shift in peak positions were observed in & \\ the XRD pattern of ESP depending on the content and & \\ composition of the biocompetite and its adsorbed species. & \\ \hline _XPS:_ Binding energy values reveal that the main components & \\ of ESP and CES as C, Ca, and O. Ca has spectral bands and & \\ binding energy similar to CaCO\({}_{3}\). Different band are ascribed & \\ to Cl 2p and Cl 2s, Pb, PO\({}_{4}\)\({}^{3-}\), and Ca bands for calcium & \\ phosphate appear after adsorption, depending on the & \\ adsorbed pollutants. & \\ \hline _Gas adsorption:_ Surface area pore size analysis (via the BET & \\ method):Higher calcination temperature yields a noticeable & \\ increase in the surface area, pore volume, and pore diameter & \\ of ESP. Combining ESP or CES with biomaterials may & \\ improve these properties. & \\ \hline _Surface charge analysis (Zeta potential; mV):_ Ca-bentonite: & \\ \(-\)12.56 mV; ESP: \(-\)17.48 mV; BEP: \(-\)16.41 mV; CES: & [39; 60] \\
800 \({}^{\circ}\)C = 0.1 mV; 900 \({}^{\circ}\)C = 18.1 mV & \\ \hline _Raman Spectroscopy:_ Spectral evidence of graphitic and & \\ amorphous or disordered carbon in the composite. The & \\ Raman bands of the thin section of ES are characteristic of S-S, & [53; 69] \\ amino acids, amide, and C-H groups. After adsorption, new & \\ bands specific to the groups of the adsorbate appear. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._SEM characterization of the adsorbent showed that it possessed rough and irregular surfaces with pores of different sizes. The presence of active functional groups and various molecular adsorption sites responsible for adsorption of dye pollutants (MB) were confirmed. Characterization by SEM after the adsorption study revealed that open pores on the initial surface of the composite were blocked after adsorption of the dye molecules [49]. Figure 5 is a combination of spectral results that provide a summary of the materials characterization often obtained for the structural analysis of ESP and its biocomposites. The eggshells were porous with an angular pattern according to the observation from FE-SEM, for copper deposited onto the eggshell, where the SEM image portrayed sheet-like fractured appearance with rod-shaped particles and interlaced pores [65]. For the case of zinc decorated eggshell particles, the image has a greater number of small and interlaced pores.