## 2 Adsorbent Preparation and Characterization
### Adsorbent Characterization  
Eggshell bio composites have been characterized with a range of techniques that are summarized in Table 1 that range from spectroscopy to thermal methods. This includes infra-red (IR) spectroscopy, field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS), scanning electron microscopy with energy dispersive X-ray absorption spectroscopy (SEM-EDAX), thermal gravimetric/differential thermal gravimetric analysis (TGA/DTA), EDS/SAED (electron diffraction spectroscopy/ specific area electron diffraction), TEM/EDS (transmission electron microscopy with EDS), XRD/XRF/EDX (X-ray diffraction, fluorescence, and energy dispersive X-ray), and XPS (X-ray photoelectron spectroscopy). In addition, some studies also report characterization of the ESP biosorbent after the adsorption process. The elemental analysis revealed that ESP has 94% calcium carbonate, 1% magnesium carbonate, 1% calcium phosphate, 3% protein, and 1% organic matter. The average diameter of the ESP particles was 5 um [46]. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Type of Characterization/Remarks** & **Refs** \\ \hline \hline _IR spectroscopy:_ About 40\% of studies report IR spectroscopy of ESP composite. IR results showed major absorption bands that are strongly associated to the carbonyl and hydroxyl groups. Other IR bands for O-H stretching, C-N-, N-H stretching, and C-H bending were evident in the IR spectra. Sharp bands confirming the presence of CaO, Ca(OH)\({}_{2}\), and CaCO\({}_{3}\) were observed. The composite revealed characteristic bands depending on the different functional groups present, e.g., Ca-bentonite have signatures for Si-O, Al-O, and O-H. After adsorption, the band appearance became broader and stronger in some composites, while there were minor changes for other materials. \\ \hline \hline _TGA/DTA:_ Nearly 20\% of the studies considered the thermal stability of the ESP and composite adsorbents. Major decomposition of ES was observed between 460–770 \({}^{\circ}\)C, revealing that the upper thermal stability limit of ESP was 630 \({}^{\circ}\)C. The minor residual contents of ES show it mainly contains carbonate minerals, whereas complete weight loss occurred between 850 and 920 \({}^{\circ}\)C. \\ \hline \hline _EDS/SAED:_ This method confirms the presence of elements (e.g., K, Ca, Mg, Fe, C, and O) in both ESP and CES. Ca, C, and O have large proportions, revealing that calcium carbonate is the main component of ESP. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characterization of eggshell biocomposite materials. Prominent signatures from the XRD profile correspond to calcite (using Joint Committee on Powder Diffraction Standards, JCPDS, data), which is the stable form of calcium carbonate at room temperature [46; 63]. ESP showed a porous network of agglomerated and irregular surface morphology based on the SEM images, while that of the synthetic adsorbents showed greater distribution with high homogeneity of the ultrafine particles with an average size below 50 nm [63]. SEM results showed that the surface consists of nearly round particles, which agglomerate after the adsorption process. TEM and selected area electron diffraction (SAED) analysis revealed a round morphology with length of 98 nm, a width of 34 nm, and non-spotted and non-continuous rings suggestive of non-crystalline powder grains.