## 4 Conclusions  
This study highlights the efficacy of WA as a sustainable and efficient adsorbent for fluoride removal from wastewater, positioning it among the top-performing adsorbents for this purpose. The kinetic adsorption analysis demonstrated rapid fluoride removal, with WA reaching equilibrium within 1 h and achieving a high adsorption capacity of 157.34 mg/g, placing it in the top 10% of reported adsorbents. The surface precipitation of CaF\({}_{2}\), driven by the high calcium content (over 66 wt.%) of WA, was identified as the primary mechanism responsible for fluoride removal. Furthermore, the study confirmed  
Figure 5: XPS spectra of WA and fluoride-adsorbed WA (F-WA): (**a**) C1s spectrum of WA, (**b**) C1s spectrum of F-WA, (**c**) Ca2p spectrum of WA, (**d**) Ca2p spectrum of F-WA, and (**e**) F1s spectrum of F-WA.  
Figure 6: Schematic diagram of fluoride removal using wood ash (WA).  
WA's versatility, with stable fluoride removal across a wide pH range and minimal impact from coexisting anions. These results emphasize WA's potential as an eco-friendly and effective solution for water treatment, particularly in the valorization of waste by-products in environmental applications.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15010245/s1](https://www.mdpi.com/article/10.3390/app15010245/s1)]([https://www.mdpi.com/article/10.3390/app15010245/s1](https://www.mdpi.com/article/10.3390/app15010245/s1)); Text S1: Reagents; Text S2: Data analysis; Text S3: Analytical procedures; Figure S1: Thermodynamics of fluoride adsorption by wood ash (WA) at reaction temperatures of 15, 25, and 35 \({}^{\circ}\)C: (a) relationship between \(\ln(Ke)\) and \(1/T\), and (b) fluoride adsorption amount under each temperature condition. Initial fluoride concentration: 500 mg/L; adsorbent dose: 3.33 g/L; reaction time: 24 h; Figure S2: Point of zero charge (PZC) calculation for wood ash (WA); Figure S3: Fluoride adsorption amount of wood ash (WA) in groundwater and wastewater; Figure S4: Variation in fluoride adsorption amount with the number of reuse cycles of wood ash (WA). Initial fluoride concentration: 500 mg/L; adsorbent dose: 3.33 g/L; reaction time: 24 h; Figure S5: X-ray diffraction patterns of (a) WA and (b) F-WA, with marked peaks corresponding to other identified minerals, namely CaCO\({}_{3}\), Ca(OH)\({}_{2}\), and CaF\({}_{2}\); Table S1: Solution pH, fluoride, cation, and heavy metal concentrations in groundwater and wastewater used in this study; Table S2: Comparison of the maximum adsorption capacities of the adsorbents utilized for fluoride removal; Table S3: Thermodynamic parameters of fluoride adsorption on WA. \(\Delta H^{0}\), the change in enthalpy; \(\Delta S^{0}\), the change in entropy; \(\Delta G^{0}\), the change in Gibbs free energy. References [1; 42; 43; 44; 64; 65] are cited in the Supplementary Materials.  
Conceptualization, S.-J.P.; methodology, J.-I.L.; software, J.-K.K.; validation, S.J. and J.-W.C.; formal analysis, J.-I.L.; resources, S.-J.P.; data curation, C.-G.L.; writing--original draft preparation, J.-I.L.; writing--review and editing, C.-G.L. and S.-J.P.; supervision, C.-G.L. and S.-J.P.; funding acquisition, S.-J.P. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Korea Forest Service (Korea Forestry Promotion Institute) through the R&D Program for Forest Science Technology (Project No. 2023483D10-2325-AA01).  
Not applicable.  
The data are contained within the article or Supplementary Materials.  
The authors declare no conflicts of interest.