## 4 Influential Factors in Fluoride Adsorption
### The Influence of pH on the Adsorption of F\({}^{-}\)
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Adsorbent Material** & **pH\({}_{\text{PZC}}\)** & **pH\({}_{\text{ideal}}\)** \\ \hline [PERSON], [PERSON], and & & & \\ [PERSON] [28] & Iron nanoparticles impregnated via green technology & - & 7 \\ \hline  & Iron oxide & 5.9 & - \\  & Silver oxide & 7.1 & - \\ [PERSON] et al. [17] & Iron and silver oxide (3:1) & 6.0 & 3 \\  & Iron and silver oxide (1:1) & 6.7 & - \\  & Iron and silver oxide (1:3) & 6.1 & - \\ \hline [PERSON] and & & & \\ [PERSON] [52] & Crystalline titanium dioxide powder & 6–6.5 & 2–5 \\ \hline [PERSON], [PERSON], and & & & \\ [PERSON] [23] & Zirconium impregnated in cellulose matrix & - & 4.5–5.5 \\ \hline [PERSON] and & & Untreated banana peel powder & 6.2 & 5.6 \\ [PERSON] [4] & Banana peel powder treated heat termic & 8.1 & 6.1 \\  & Banana peel powder coated with calcium & 8.2 & 7.2 \\ \hline [PERSON] et al. [53] & & Tea residue supported to hydroxide aluminum oxide and anionic & - & 4–9 \\  & & polacrylamide & - & 4–8 \\  & Tea residue as a support medium for Al and Fe oxides & - & 4–8 \\ \hline [PERSON] et al. [18] & Activated alumina & 8.9 & 5.84 \\  & Activated alumina modified with lantanium nitrate & 9.6 & 4.99 \\ \hline [PERSON] et al. [1] & Activated alumina & 8 & 5.5–6.5 \\  & Activated alumina modified with CaO & 11.9 & 4–10 \\ \hline [PERSON] et al. [84] & Mesoporous alumina modified with CaO & 8.2 & - \\  & Mesoporous alumina & - & 5 \\ \hline [PERSON] and [PERSON] [40] & Magnetic cationic hydrogel coated with lantanium & 7 & 2.8–5.2 \\ \hline [PERSON] et al. & Mg, Al, and lamellar double hydroxide immobilized in alginate magnetic & 9 & 5 \\  & nanoflakes & & \\ \hline  & Activated alumina (A1) PZC 1 & 10.1 & \\  & Activated alumina (A2) PZC 1 & 9.9 & \\  & Activated alumina (A3) PZC 1 & 9.7 & \\  & Activated alumina (A4) PZC 1 & 9.4 & \\  & Activated alumina (A5) PZC 1 & 9.2 & \\  & Activated alumina (A1) PZC 2 & 9.7 & \\  & Activated alumina (A2) PZC 2 & 9.7 & \\ [PERSON] et al. [49] & Activated alumina (A3) PZC 2 & 9.5 & 5.5–6.7 \\  & Activated alumina (A4) PZC 2 & 9.5 & \\  & Activated alumina (A5) PZC 2 & 9.4 & \\  & Activated alumina (A1) PZC 3 & 4.8 & \\  & Activated alumina (A2) PZC 3 & 5.3 & \\  & Activated alumina (A3) PZC 3 & 6.5 & \\  & Activated alumina (A4) PZC 3 & 6.8 & \\  & Activated alumina (A5) PZC 3 & 7.6 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Values of pH\({}_{\text{PZC}}\) and the pH of highest efficiency cited in F\({}^{-}\) removal studies. [PERSON] et al. [17] showed that the removal of F\({}^{-}\) using multi-walled carbon nanotubes was highly dependent on the pH of the solutions. The maximum adsorption of 93.5% was obtained at pH 5. For higher pH values, the adsorption capacity decreased significantly, reaching 41.2% at pH 9. This sharp reduction in the removal of adsorbed F\({}^{-}\) in the alkaline pH range was attributed to competition for adsorption sites between hydroxyl ions and F\({}^{-}\). [PERSON] and [PERSON] [52] studied the effect of pH (in the range of 2 to 11) on the adsorption of F\({}^{-}\) on crystalline titanium dioxide (TiO\({}_{2}\)) powder. The maximum uptake of F\({}^{-}\) was observed at acidic pH (pH 2), with an adsorption capacity of approximately 0.2 mg g\({}^{-1}\). The adsorption capacity was reduced to 0.16 mg g\({}^{-1}\) at pH 7, to 0.14 mg g\({}^{-1}\) at pH 9, and was only 0.02 mg g\({}^{-1}\) at pH 11. The reduction in F\({}^{-}\) uptake in the alkaline solution was attributed to competition between hydroxyl groups and F\({}^{-}\) ions for the active sites on the adsorbent. On the other hand, the positively charged surface sites in the acidic solution increased the F\({}^{-}\) uptake. The final F\({}^{-}\) concentration in the pH 7-8 range was almost at the WHO recommended limit of 1.5 mg L\({}^{-1}\), while the F\({}^{-}\) concentration was less than 1 mg L\({}^{-1}\) in the acidic solutions. [PERSON] et al. [23] used zirconium hydroxide impregnated on a cellulose matrix for F\({}^{-}\) removal.