## 5 Fluoride Adsorption Capacity and Engineering Implications  
The adsorption capacity of the adsorbent (qe) represents the amount of adsorbate adsorbed per unit mass of adsorbent at equilibrium and is the main parameter needed for the design of an adsorption system. This parameter plays a central role in the design and optimization of an adsorption system. In the context of fluoride removal, Table 7 provides a comprehensive overview of recent works, presenting values of adsorption capacity alongside the corresponding conditions under which fluoride removal occurred. \begin{table}
\begin{tabular}{c c c} \hline
**Reference** & **Adsorbent Material** & **qe (mg g-1)** \\ \hline  & Activated Aluminie & 130 \\ [PERSON] et al. [81] & Alum & 182 \\  & Alum & 91.5 \\  & Brick dust & 115.2 \\  & Brick dust & 100.0 \\ \hline [PERSON], [PERSON], and [PERSON] [28] & Iron nanoparticle impregnated via green technology & 1.44 \\  & & 20 \\  & & 21.54 \\ [PERSON] et al. [17] & Iron and silver oxide nanoadsorbent & 24.27 \\  & & 26.3 \\  & & 28.69 \\ \hline [PERSON] and [PERSON] [52] & Crystalline titanium dioxide powder & 0.153 \\ \hline [PERSON], [PERSON], and [PERSON] [23] & Zirconium & 1.0 \\  & Zirconium impregnated with cellulose matrix & 1.94 \\ \hline [PERSON] et al. [9] & Alumina impregnated in alginate granules & 3.51 \\  & & 7.09 \\ \hline [PERSON] et al. [51] & Hydrated aluminum oxide and anionic polyacrylamide & 7.43 \\  & supported on tea residue & 13.55 \\  & & 18.68 \\ \hline  & & 10 \\  & & 20 \\  & & 34 \\ \hline  & & \\  & & \\  & & \\ \hline  & & \\  & & \\ \end{tabular}
\end{table} 
Table 7: Adsorption capacity obtained in studies of F– removal by adsorption. Upon reviewing the results, a significant variation in adsorption capacity among different materials is noticeable. For instance, ZrO\({}_{2}\) mesoporous fiber, as reported by [PERSON] et al. [75], demonstrates a remarkable adsorption capacity with q\({}_{\text{e}}\) values reaching 297.70 mg g\({}^{-1}\). Some materials, such as porous MgO nanoplates ([PERSON] et al. [13]), also exhibit impressive adsorption capacities, reaching 185.5 mg g\({}^{-1}\). On the other hand, certain adsorbents like crystalline titanium dioxide powder (Babevielri and Khodadoatouat [56]) show more modest capacities, with a q\({}_{\text{e}}\) value of 0.153 mg g\({}^{-1}\). The variability in q\({}_{\text{e}}\) values underscores the profound impact of the specific nature and characteristics of each adsorbent on the efficacy of fluoride removal. This understanding becomes pivotal when choosing the most suitable adsorbent for a particular fluoride removal application, considering factors such as cost, availability, and the efficiency of the adsorbent material. Generally, an increased number of adsorbent results in a greater number of active sites available for binding F\({}^{-}\) ions. However, maintaining a balance is crucial, as beyond a certain mass value, excess adsorbent does not contribute to higher efficiency per unit mass, leading to a decrease in adsorption capacity. This phenomenon is attributed to the low residual concentration of F\({}^{-}\), resulting in diminished utilization efficiency. In terms of engineering implications, the careful consideration of the optimal amount of adsorbent in a fluoride removal adsorption system is critical. Balancing process effectiveness against the associated costs of increasing the amount of adsorbent is essential and requires meticulous evaluation. Several studies by [PERSON] et al. [50], [PERSON] et al. [52], [PERSON] et al. [78], [PERSON] et al. [76], [PERSON] et al. [80], and [PERSON] et al. [79] have explored diverse adsorbents, presenting promising results in terms of adsorption capacity. Collectively,these studies offer insights into the potential effectiveness of various adsorbents for fluoride removal, contributing to ongoing efforts to optimize engineering solutions for water treatment processes.