## 3 Results and Discussion
### WA Characteristics and Fluoride Removal Mechanism  
Table 3 displays the surface areas, as well as the pore sizes and volumes of the samples. WA possesses a surface area of 25.2 m\({}^{2}\)/g and a pore size of 6.8 nm, placing it within the range of mesopores (1.25-25 nm) [56]. The surface area of F-WA was determined to be 4.9 m\({}^{2}\)/g, which is lower than that of WA due to fluoride adsorption on its surface. FE-SEM was employed to examine the surface of WA before and after fluoride adsorption and to investigate any morphological changes that may have occurred. The surface of WA (Figure 4a) exhibited small granular masses on a textured surface. Its uneven surface, small pores, and numerous cracks create favorable conditions for contaminant adsorption [37]. The morphology of F-WA revealed a distinctive pattern of small-grain aggregation, suggesting the formation of a CaF\({}_{2}\) precipitate structure on the surface after fluoride adsorption [57]. Moreover, the specific surface area of WA was observed to decrease after fluoride adsorption, as shown in Table 3, supporting the occurrence of surface precipitation. The elemental compositions of WA and F-WA, determined through XRF analysis, are presented in Table 3. WA primarily comprises calcium, carbon, and potassium, with calcium being the predominant component (66%). Calcium's high affinity for fluorine makes it an effective adsorbent for fluoride removal, as demonstrated in previous studies [24; 41]. Figure 4: Field emission scanning electron microscopy images of (**a**) WA and (**b**) fluoride-adsorbed WA (F-WA). Accelerating voltage: 15.0 kV; magnification: \(\times\)10,000; scale bar: 5.00 \(\upmu\)m. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**adsorbent**} & **Surface** & **Pore** & **Pore** & \multicolumn{4}{c}{**Elemental Composition (wt.\%)**} & \multicolumn{4}{c}{**Mineral Composition (\%)**} & \multicolumn{4}{c}{**Elution (mg/L)**} \\  & **Area** & **Volume** & **Size** & & & & & & & & & & & & & & & \\  & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & **(nm)** & **Ca** & **C** & **K** & **Mg** & **F** & **Calcite** & **Hydroside** & **Fluoride** & **Ca** & **K** & **Na** & **Al** \\ \hline WA & 25.2 & 0.04 & 6.8 & 66.1 & 19.6 & 6.8 & 2.0 & - & 67.9 & 32.1 & - & 22.11 & 21.37 & 1.51 & 0.23 \\ \hline F-WA & 4.9 & 0.01 & 8.4 & 66.5 & 23.5 & 0.6 & 2.6 & 0.7 & 79.7 & 6.6 & 13.7 & 0.44 & 30.96 & 177.03 & 0.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Physicochemical properties of wood ash (WA) and fluoride-adsorbed WA (F-WA). Fluoride adsorption was confirmed by the presence of 0.7% fluoride in the XRF results for F-WA (Table 3). The XRD peaks of WA and F-WA, along with those of the identified minerals, are shown in Figure S5. The contents of the identified minerals are listed in Table 3. The calcium component in WA primarily exists as CaCO\({}_{3}\) and Ca(OH)\({}_{2}\), with CaCO\({}_{3}\) being the predominant form. The reaction for Ca\({}^{2+}\) dissolution from CaCO\({}_{3}\) and Ca(OH)\({}_{2}\) is shown in Equations (1) and (2) [23]. The dissolved Ca\({}^{2+}\) reacts with F\({}^{-}\) in an aqueous solution to form CaF\({}_{2}\), as shown in Equation (3) [58]. These findings align with those of previous studies, which have reported that precipitation on the adsorbent surface in the form of CaF\({}_{2}\) is the primary mechanism for fluoride removal, particularly at high initial fluoride concentrations (\(>\)50 mg/L) [3,58]. In addition, this was consistent with the results of the kinetic model analysis, which indicated that fluorine adsorption is governed by chemical adsorption. After reacting with fluoride, the remaining Ca\({}^{2+}\) is converted to CaCO\({}_{3}\), increasing the CaCO\({}_{3}\) content (Table 3) [59]. Accordingly, Ca\({}^{2+}\) release was also dramatically reduced in F-WA (Table 3).