## 4 Influential Factors in Fluoride Adsorption
### The Influence of pH on the Adsorption of F\({}^{-}\)
In the pH range of 4.5 to 5.5, the adsorption of F\({}^{-}\) on the surface of the adsorbent is quite efficient. The authors argued that in a weakly acidic medium, the F\({}^{-}\) ion can interact with the positively charged surface hydroxyl groups of cellulose. Above pH 5.5, deprotonation of the hydroxyl groups on the surface of the adsorbent the competition between OH\({}^{-}\) and F\({}^{-}\) for the active adsorption sites reduced the efficiency of the process. [PERSON] et al. [39] developed a hybrid adsorbent by impregnating Li/Al LDHs (lamellar double hydroxides) on the commercial anion exchanger D201 (LALDH-201), and analyzed the effect of pH. LALDH-201 exhibited a higher adsorption capacity than D201 and activated alumina (AA). The adsorption capacity of LALDH-201 increased rapidly from pH 2 to 4 and decreased more slightly from pH 4 to pH 9. It then decreased sharply from pH 9 to 12. According to the authors, this pH-dependent adsorption can be interpreted by their different adsorption mechanisms, which were (1) ion exchange between the F\({}^{-}\) and covalent groups of the D201 matrix, (2) ion exchange between the F\({}^{-}\) and exchangeable anions of the Li/Al LDHs, and (3) ligand exchange to form an internal sphere complex, resulting in the release of the hydroxyl ion. The adsorption capacity of both LALDH-201 and D201 decreased at alkaline pH (> 9.0), possibly due to competition between OH\({}^{-}\) and  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Adsorbent Material** & **pH\({}_{\text{ZFC}}\)** & **pH\({}_{\text{ideal}}\)** \\ \hline [PERSON] et al. [56] & 3 different cerium oxide morphologies (nanocubes, octahedra, nanorods) & - & 3 \\ \hline [PERSON] et al. [5] & Aluminum fumarato organic–metal structure & 8.1 & 2–7 \\ \hline [PERSON], [PERSON], and [PERSON] [20] & Alumina activated to H\({}_{2}\)SO\({}_{4}\) & 8.2 & 6–7 \\ \hline [PERSON], [PERSON], and & Bicarbon from corn stover with magnetic removal & 1.96 & \\ \multicolumn{1}{c}{[PERSON] [60]} & Biocarbon from corn stove & 2.01 & 2 \\ \hline [PERSON], [PERSON], and & \multirow{2}{*}{Coconut fiber ash impregnated with aluminum hydroxide} & \multirow{2}{*}{7.2} & \multirow{2}{*}{5} \\ \cline{1-1} \cl  
F\({}^{-}\) for the exchangeable sites. In addition, the higher concentration of hydroxyl ions in alkaline solution further restricts the F\({}^{-}\) adsorption capacity of LALDH-201. The reduction in adsorption observed for D201 below pH 4 is because F\({}^{-}\) forms weakly ionized HF in the acidic solution. For LALDH-201, the decrease in adsorption capacity below pH 4 was attributed as a result of more than one factor: with the formation of weakly ionized HF, LALDH-201 becomes unstable at pH 3.5, which is also unfavorable for the adsorption of F\({}^{-}\). The capacity of D201 was relatively stable at pH 4 to 9, where the concentration of H+ or OH\({}^{-}\) was insufficient to affect ion exchange. For AA, the results indicated that its F\({}^{-}\) adsorption capacity was reduced at low and high pH, with the optimum pH at 5-6. [PERSON] et al. [86] synthesized hybrid adsorbents of Li/Al-LDHs doped with lanthanum (La). The highest adsorption capacity values were observed at pH = 7, being approximately 31.5, 17.5, and 3.5 mg g\({}^{-1}\) for LAL03, LA3, and AA, respectively. LAL03 is positively charged at pH = 7, and the authors attribute this to probable formation of Al-OH\({}^{2+}\) and La-OH\({}^{2+}\), which is favorable for removal of anionic pollutants such as F\({}^{-}\). pH values \(<\) 4 were not considered due to the formation of weakly ionized HF. When the pH was increased to 8, a reduction in the adsorption of F\({}^{-}\) was observed, which was attributed to competition between OH\({}^{-}\) and F\({}^{-}\) for the exchange sites. For pH \(>\) 10, dissolution of aluminum from the layered structure is attributed as a likely cause of the reduced adsorption capacity.