## 4 Influential Factors in Fluoride Adsorption
### The Effect of Co-Existing Ions on the Removal of F\({}^{-}\)  
Co-existing ions can compete with fluoride ions for adsorption sites on the surface of the adsorbent material, reducing the overall fluoride removal efficiency. Table 6 presents the co-existing ions with the most and least interference in the adsorption of F\({}^{-}\) found in the literature review. The degree of competition in the adsorption process is contingent upon the concentration and chemical characteristics of co-existing ions. Naturally occurring water includes anions like chloride (Cl\({}^{-}\)), nitrate (NO\({}_{3}\)\({}^{-}\)), sulfate (SO\({}_{4}\)\({}^{2-}\)), bicarbonate (HCO\({}_{3}\)\({}^{-}\)), carbonate (CO\({}_{3}\)\({}^{2-}\)), and phosphate (PO\({}_{4}\)\({}^{3-}\)), along with metal cations. These co-existing ions can vie for adsorption sites and form stable complexes with F\({}^{-}\). Table 6 illustrates that certain co-existing ions, such as chloride and sulfate, share similar ionic radii and charges with fluoride ions, establishing them as robust contenders for adsorption sites. Additionally, other co-existing ions, such as bicarbonate and carbonate, can create complexes with the surface of the adsorbent material. This interaction diminishes the number of active adsorption sites available for fluoride adsorption [8; 51; 91]. Therefore, the impact of co-existing  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Co-Existing Ions**} \\ \cline{2-3}  & **High Interference** & **Low Interference** \\ \hline [PERSON], [PERSON], and [PERSON] [28] & Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) & - \\ \hline [PERSON] et al. [17] & PO\({}_{4}\)\({}^{3-}\) HCO\({}^{3-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON], [PERSON] et al. [53] & PO\({}_{4}\)\({}^{3-}\) \(>\) SO\({}_{4}\)\({}^{2-}\) \(>\) HCO\({}_{3}\)\({}^{-}\)\(>\) Cl\({}^{-}\)\(>\)NO\({}_{3}\)\({}^{-}\) & - \\ \hline [PERSON] et al. [83] & CO\({}_{3}\)\({}^{2-}\), HCO\({}_{3}\)\({}^{-}\), PO\({}_{4}\)\({}^{3-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [60] & HCO\({}_{3}\)\({}^{-}\), PO\({}_{4}\)\({}^{3-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [84] & HCO\({}_{3}\)\({}^{-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] and [PERSON] [40] & HCO\({}_{3}\)\({}^{-}\), SiO\({}_{4}\)\({}^{2-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [19] & CO\({}_{3}\)\({}^{2-}\), HCO\({}_{3}\)\({}^{-}\), PO\({}_{4}\)\({}^{3-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [90] & HCO\({}_{3}\)\({}^{-}\)\(>\) SO\({}_{4}\)\({}^{2-}\)\(>\) NO\({}_{3}\)\({}^{-}\)\(>\) Cl\({}^{-}\) & - \\ \hline [PERSON] et al. [41] & HCO\({}_{3}\)\({}^{-}\) & NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), CO\({}_{3}\)\({}^{2-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [13] & CO\({}_{3}\)\({}^{2-}\), HCO\({}^{-}\), PO\({}_{4}\)\({}^{3-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [5] & CO\({}_{3}\)\({}^{2-}\), SO\({}_{4}\)\({}^{2-}\) & HCO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [57] & HCO\({}_{3}\)\({}^{-}\), CO\({}_{3}\)\({}^{2-}\) & SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON], [PERSON], and [PERSON] [8] & SO\({}_{4}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\) & NO\({}_{3}\)\({}^{-}\), Cl\({}^{-}\) \\ \hline [PERSON] et al. [62] & PO\({}_{4}\)\({}^{3-}\)\(>\) SO\({}_{4}\)\({}^{2-}\)\(>\) NO\({}_{3}\)\({}^{-}\)\(>\) Cl\({}^{-}\) & - \\ \hline [PERSON], [PERSON] et al. [64 ions is contingent on their concentration and chemical properties, as well as the nature of the adsorbent material employed. [PERSON] et al. [17] highlight that the effect of bicarbonate can be explained by the fact that conjugate bases of weak acids produce more hydroxyl ions, which compete with F\({}^{-}\) for adsorption sites. [PERSON] et al.