# Aluminum-Loaded Bifunctional Resins for Efficient Fluoride Removal from Aqueous Solutions
## 3 Results and Discussion
### Effect of Coexisting Anions and Organics on Fluoride Adsorption  
The composition of actual water is relatively complex, with inorganic salts and organic compounds influencing the defluorination efficiency of the adsorbent. To assess the selectivity of Al-S957, Al-D412, and Al-D001, common coexisting anions and organics were selected for comparative analysis. Figure 4a shows the effects of various anions on Al-S957; these can be observed in the following order: HCO3\({}^{-}\) > PO4\({}^{3-}\) > SO4\({}^{2-}\) > Cl\({}^{-}\), and NO3\({}^{-}\). The negligible impact of Cl\({}^{-}\) and NO3\({}^{-}\) on the adsorption capacity of Al-S957 indicates that inner-sphere complexation is the predominant mechanism for fluoride removal. SO4\({}^{2-}\) may partially form an inner-sphere complex with aluminum, potentially competing with fluoride for active sites. PO4\({}^{3-}\), like fluoride, significantly inhibits fluoride removal due to its affinity for aluminum [34]. HCO3\({}^{-}\) exhibits the most pronounced inhibition, as hydrolysis-generated OH\({}^{-}\) raises solution alkalinity, suppressing fluoride adsorption. Additionally, the similar ionic radii of HCO3\({}^{-}\) and fluoride contribute to competitive adsorption [35]. For Al-D412 (Figure 4b), a similar trend was observed. However, the fluoride removal of Al-D001 (Figure 4c) is significantly hindered by SO4\({}^{2-}\), Cl\({}^{-}\), andNO3\({}^{-}\). The observed effect was speculated to result from the relatively weak binding of aluminum to D001, making it susceptible to interference from these anions, potentially leading to aluminum loss. To verify this hypothesis, an aluminum leakage experiment was conducted. Figure S2  
Figure 4: Effects of competing anions (**a**–**c**) and coexisting organics (**d**–**f**) on fluoride adsorption (for HCO3\({}^{-}\), molar ratios of 0, 0.5, 1, 2, 3, 4, and 5 were applied). reveals that aluminum leakage from Al-D001 increased notably with NaCl concentration, exceeding that from Al-S957. This is attributed to the relatively weak electrostatic interaction between the sulfonic acid groups of D001 and aluminum, whereas the incorporation of phosphate groups in S957 enhances its binding affinity. Further static adsorption-desorption cycles with a 10 wt% AlCl3-6H2O solution confirmed the stability of Al-S957. The aluminum concentration decreased from 0.3215 mg/L after the first cycle to 0.3047 mg/L after the third, indicating consistent chelation performance. The pronounced difference in aluminum leakage also implies that sulfonic acid groups in S957 likely do not interact strongly with aluminum, with phosphonic acid groups being primarily responsible for chelation, consistent with prior findings [25]. The influence of SO4\({}^{2-}\) and NO3\({}^{-}\) on Al-D001 resembled that of Cl\({}^{-}\), while the effects of HCO3\({}^{-}\) and PO4\({}^{3-}\) were less severe compared to Al-S957, possibly due to precipitate formation between leaked aluminum and these anions, indirectly aiding fluoride removal. In conclusion, Al-S957 demonstrates superior selectivity and is more suitable for the treatment of high-salt fluorinated water. As for organic interference (Figure 4d-f), Al-S957 consistently maintained excellent defluorination performance across various organic concentrations. Benefiting from the hydrophilicity of the sulfonic acid group, Al-S957 inherits the excellent defluorination performance of Al-D001 with little effect of organic matter on it, while the performance of Al-D412 is more significantly impacted. Additionally, under the combined action of sulfonic acid groups and chelating groups, Al-S957 exhibits the highest adsorption capacity and strong resistance to organic interference. Overall, these results demonstrate that Al-S957 effectively integrates the aluminum-chelating capability of Al-D412 and the organic resistance of Al-D001. As a result, Al-S957 exhibits enhanced fluoride adsorption efficiency and excellent selectivity in the presence of coexisting anions and organic matter.