## 4 Influential Factors in Fluoride Adsorption  
### Adsorbent Specific Surface Area and Particle Size  
The specific surface area represents the area per unit mass of the adsorbent; thus, a large specific surface area favors adsorption. The adsorption capacity of the adsorbent is expected to be relate to the surface area and the number of available active sites. However, the adsorption performance of F\({}^{-}\) can also be influence by different morphologies of the adsorbent and size and shape of microstructures [51]. Table 1 provides a comprehensive survey of studies in the literature focusing on specific surface area (SSA), pore volume (PV), and pore diameter (PD) of adsorbents investigated for the removal of F\({}^{-}\). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Adsorbent Materials**} & **SSA** & **PV** & **PD** \\  & & **(m\({}^{2}\) g\({}^{-1}\))** & **(cm\({}^{3}\) g\({}^{-1}\))** & **(nm)** \\ \hline  & Iron oxide & 94.96 & 0.2520 & 0.584 \\  & Silver oxide & 74.34 & 0.2240 & 0.421 \\  & Iron and silver oxide (3:1) & 254.24 & 0.6780 & 0.131 \\  & Iron and silver oxide (1:1) & 189.57 & 0.3910 & 0.691 \\  & Iron and silver oxide (1:3) & 145.74 & 0.4790 & 0.621 \\ \hline [PERSON] and [PERSON] [52] & Titanium oxide powder & 56.00 & - & 20 \\  & Untreated banana peel powder & 312.3 & - & - \\ \hline [PERSON] and [PERSON] [4] & Banana peel powder heat treated & 884.7 & - & - \\  & Banana peel powder coated with calcium & 912.86 & - & - \\ \hline [PERSON] et al. [51] & Tea residue as support for hydrated aluminum oxide and & 25.86 & 0.1370 & 19.76 \\  & polyacrylamide & 25.86 & 0.1370 & 19.76 \\ \hline [PERSON] et al. [53] & Tea residue as support for hydrated aluminum oxide & 1.68 & 0.0083 & 27.87 \\ \hline [PERSON] et al. [18] & Microspheres and hydroxyapatite & 94.1 & - & 13.8 \\  & Microspheres and hydroxyapatite & 84.2 & - & 12.8–23 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specific surface area, pore volume, and pore diameter of adsorbents studied for the removal of F\({}^{-}\). Figure 1: Main parameters to take into consideration in adsorption process. kinetics, and selectivity of fluoride removal. Adsorbent materials with small pore sizes and narrow pore size distributions can have a high surface area-to-volume ratio, providing many active adsorption sites. However, they can also limit the accessibility of larger fluoride ions to the internal surface of the adsorbent material, which can decrease the adsorption capacity for fluoride [10; 20; 36; 67]. On the other hand, adsorbent materials with larger pore sizes can have a higher accessibility to larger fluoride ions, resulting in a higher fluoride adsorption capacity. However, this may come at the expense of a lower surface area-to-volume ratio and a lower number of active adsorption sites. [PERSON] et al. [65] studied the gamma phase of mesoporous alumina (MA) synthesized by a hydrothermal method followed by thermal treatment. The as-synthesized MA nanoparticles with an average size of 20 nm-150 nm have an ordered wormhole-like mesoporous structure. The pore size is 5 nm with a narrow distribution, and the specific surface area reaches 357 m\({}^{2}\) g\({}^{-1}\) while the bulk density is 0.45 cm\({}^{3}\) g\({}^{-1}\). Glucose as a small-molecule template plays an important role on the morphology, surface area, and pore diameter of the MA. [PERSON] et al. [68] reported the synthesis of a Fe-La composite adsorbent by one-pot hydrothermal method for removing fluoride and phosphate simultaneously. The as-obtained adsorbent showed rod-like morphology and a large specific surface area of 113.13 m\({}^{2}\) g\({}^{-1}\). A series of adsorption experiments were carried out to investigate the influence of various factors on the simultaneous adsorption of fluoride and phosphate, such as different adsorbent dosage, pH, temperature, and co-existing anion. The results demonstrate that the Fe-La adsorbent exhibited high adsorption capacity for fluoride and phosphate. [PERSON] et al.