# Aluminum-Loaded Bifunctional Resins for Efficient Fluoride Removal from Aqueous Solutions
## 1 Introduction  
Fluoride contamination in drinking water is a growing global concern [1]. The World Health Organization recommends that fluoride levels in drinking water should not exceed 1.5 mg/L for optimal health [2]. Excess fluoride can lead to severe health issues, including rickets, osteoporosis, arthritis, neurological disorders, and cancer [3, 4, 5]. Fluoride enters natural water sources primarily through the dissolution of fluoride-rich rocks and minerals such as cryolite, topaz, and fluorite [6]. Additionally, various untreated industrial wastewaters, such as those from semiconductor, polytetrafluoroethylene, glass, fertilizer, and pesticide industries, are also the main sources of fluoride [5]. These sources together aggravated the threat of fluoride effects. Elevated fluoride levels in water have been documented in over 20 countries across Africa, Europe, and the Americas [3]. Consequently, it is imperative to find an effective and robust fluoride remediation technology.  
A number of defluorination techniques, including precipitation, coagulation, membrane separation, and adsorption, have been employed for the removal of excess fluoride [6]. Among these, adsorption has emerged as the most efficacious approach for deep fluoride removal, primarily attributable to its advantages of efficiency, convenience, operability, simplicity of design, and economy [7, 8, 9]. Some studies have shown that metal modification can substantially enhance the fluoride removal performance of materials [10, 11, 12]. By utilizing metal ions as adsorption centers and exploiting mechanisms such as ligand exchange and electrostatic adsorption, efficient fluoride removal can be achieved from low-concentration fluorinated water [13, 14]. Among various modifying metals, aluminumis widely recognized as a cost-effective and readily available metal for adsorbent modification [15]. According to [PERSON] acid-base theory, hard basic fluorides exhibit a strong inherent affinity for hard acidic aluminum [16]. This is further demonstrated by the first-step coordination constant of Al-F complexes (Log \(K_{1}\) = 6.10), surpassing those of other common metal-fluoride systems, such as Fe-F (Log \(K_{1}\) = 5.28), Mg-F (Log \(K_{1}\) = 1.3), and La-F (Log \(K_{1}\) = 2.77) [17]. The superior thermodynamic stability of aluminum-fluoride coordination complexes ensures reliable and efficient fluoride removal, making aluminum-based materials highly effective for water defluorination applications.  
Various carriers have been explored for aluminum loading, including zeolite, ion exchange resins, activated carbon, and metal-organic frameworks [14; 18; 19; 20]. Among these, ion exchange resins stand out for their notable advantages, including good stability and ease of operation [21]. However, while aluminum-loaded monofunctional resins have been reported for efficient fluoride removal, they have limitations such as low adsorption capacity, metal loss, and susceptibility to interference by coexisting ions [22; 23]. In contrast, multifunctional group materials have been shown to have fast adsorption kinetics and good regeneration properties [24]. The chelating resin S957 with bifunctional sulfonic acid and phosphoric acid groups has also been demonstrated to adsorb aluminum effectively [25].  
Therefore, this study involved loading aluminum onto bifunctional resin S957 and employing aluminum-loaded monofunctional resins (phosphate-based resin D412 and sulfonic acid-based resin D001) for comparative evaluation. The adsorption properties of Al-S957 were extensively investigated, including its kinetics, isotherms, and thermodynamics. The effects of solution pH, coexisting anions, and organic compounds on fluoride adsorption were also examined. The adsorption mechanism was explored, and fixed-bed column experiments with real photovoltaic wastewater were conducted to assess its practical performance.