## 4 Influential Factors in Fluoride Adsorption
### The Influence of Temperature on the Adsorption of F\({}^{-}\)  
The influence of temperature on adsorption processes is related to the kinetic energy, mobility, solubility, and chemical potential of the adsorbate. Thus, a change in the temperature of a process will lead to a change in the adsorption capacity. Generally, an increase in temperature can enhance the fluoride adsorption capacity, but it can also decrease the adsorption rate. The increase in fluoride adsorption capacity with increasing temperature may be due to several factors. For example, higher temperatures can increase the mobility of the fluoride ions and their diffusion towards the adsorbent material, increasing the chances of successful adsorption. Additionally, higher temperatures can increase the surface area of the adsorbent material due to thermal expansion, resulting in a larger number of active adsorption sites [23; 58]. Table 2 presents the adsorbents and temperature ranges studied in exothermic processes of F\({}^{-}\) removal by adsorption. The temperature rise probably causes a reduction in the thickness of the boundary layer [4], and the solubility of the F\({}^{-}\)[4]; thus, the F\({}^{-}\) ions tend to escape from the adsorbent surface (solid phase) to the solution (liquid phase), which results in a reduction in adsorption capacity as the temperature rise is increased. Another possibility pointed out by [PERSON] et al. [8] is that an increase in the thermal energy of the adsorbed F\({}^{-}\) occurs at higher temperatures, which ultimately causes an increase in desorption. On the other hand, the decrease in the adsorption rate with increasing temperature may be due to a decrease in the adsorption affinity between the adsorbent material and the fluoride ions. This can occur when the adsorption process is exothermic, meaning that heat is released during the adsorption process. The release of heat can cause a decrease in the driving force for adsorption, resulting in a decrease in the adsorption rate [23; 74]. Some studies were found in which it is reported that the adsorption of F\({}^{-}\) increases with increasing temperature, which is an indication that the adsorption process is endothermic. Table 3 presents the adsorbents and the temperature ranges studied by these authors. \begin{table}
\begin{tabular}{c c c} \hline
**Reference** & **Adsorbent Materials** & **Temperature** \\ \hline [PERSON], [PERSON], and [PERSON] [28] & Iron nanoparticles impregnated via green technology & 20–30 \({}^{\circ}\)C \\ \hline [PERSON] et al. [17] & Iron and silver oxide nanoadsorbent & 20–50 \({}^{\circ}\)C \\ \hline [PERSON] and [PERSON] [4] & Banana peel powder & 40–70 \({}^{\circ}\)C \\ \hline [PERSON] et al. [71] & Mg, Al, and lamellar double hydroxide immobilized on magnetic alginate nanoflakes & - \\ \hline [PERSON] et al. [41] & Composite adsorbent based on ferrhydrite and bayerite & 25–35 \({}^{\circ}\)C \\ \hline [PERSON] et al. [5] & Metal–organic structure of aluminum fumarate & 20–60 \({}^{\circ}\)C \\ \hline [PERSON], [PERSON], and [PERSON] [72] & Composite UiO-66-NH\({}_{2}\) & 20–60 \({}^{\circ}\)C \\ \hline [PERSON], [PERSON], and [PERSON] [60] & Corn stover biocarbon with and without magnetic removal & 25–45 \({}^{\circ}\)C \\ \hline [PERSON], [PERSON], and [PERSON] [4] & Coconut fiber ash impregnated with aluminium hydroxide & 40–100 \({}^{\circ}\)C \\ \hline [PERSON] and [PERSON] [68] & Fe (III) bimetallic oxide composite & 25–35 \({}^{\circ}\)C \\ \hline [PERSON] et al. [73] & Zirconium imobilized in sodium carboximetilcellulose & 25–55 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table} 
Table 2: Adsorbents and temperature ranges studied in exothermic processes of F\({}^{-}\) removal by adsorption. According to [PERSON] et al. [78], the temperature elevation probably causes an increase in thermally accessible adsorption sites for the adsorption of F\({}^{-}\), by pore unclogging, on the surface of the adsorbents.