## 1 Introduction  
The Rare-Earth Elements (REEs), a group of 17 elements exhibiting similar chemical properties, have become vital in the last 30 years for the advancement of renewable energy and clean storage technologies owing to their unique magnetic, phosphorescent, and catalytic properties [1; 2; 3]. In 3 MW wind turbines, for example, 3 tons of REEs are needed [4], while in electric vehicles (EVs), REEs like neodymium (Nd), praseodymium (Pr), samarium (Sm), and dysprosium (Dy) are used in high efficiency and strong permanent magnets [2; 5; 6]. In 2012, the demand for Dy and Nd for the next 25 years was projected to increase by at least 700% and 2600%, respectively [7]. This situation is a supply security risk due to the inherent scarcity of REEs, which was further complicated because the maximum annual primary material demands for Dy and Nd exceeded the current production volumes by a factor of 3 to 9 and 7 to 35, respectively [8]. Exacerbating this problem is that China, the primary exporter of REEs (78%) from primary resources, imposed export restrictions, citing increasing domestic REE requirements and environmental concerns related to REE mining, processing, and extraction [2; 9; 10; 11]. Generally, extraction of REEs involves communution, physical separation, leaching, and solvent extraction [12]. Subsequent refinement involves solvent extraction for purification [13]. REE processing, according to [PERSON] (2022), involves significant chemical use, which is particularly noticeable during the solvent extraction stage. This process typically requires mixer, settler, stripping, and washing stages, further amplifying the chemical demand [14]. Additionally, the life cycle assessment of REE production highlighted the environmental challenge of managing and disposing of large quantities of chemical-bearing wastewater used in the beneficiation, extraction, and separation processes. Another challenge is that REE processing generates substantial amounts of tailings, which require proper disposal to limit their negative environmental impacts [14]. Additionally, the majority of REE-bearing ores have low REE contents and contain various metals with similar physicochemical properties as REEs that interfere with solvent extraction [15; 16; 17]. A study by [PERSON] et al. (2002), for example, reported that calcium (Ca), aluminum (Al), and zinc (Zn) directly compete with REEs during the extraction and separation process [18]. As an alternative to solvent extraction, the use of extraction chromatographic resins has gained popularity in recent years because they combine the excellent selectivity and high efficiency of solvent extraction and simplicity of ion exchange [19]. These resins are prepared by incorporating functionalized ligands onto solid support like porous polymers, and selections of a selective extractant and excellent support are both crucial for the resins to be effective [19; 20]. Dialkyl resins (LN) [21] resin is a commercial resin available from Triskem International, commonly used for cation, lanthanide, and radium separation, and light rare-earth element determination; however, it is problematic for coal fly ash REE leachate purification due to its high-capacity factor for impurities such as Fe, Al, Si, and Ca. Similarly, rare earth (RE) [21] resin, a commercial resin available from Triskem International, is widely used for the group separation of REEs and has a similar limitation due to its high-capacity factor for Fe [22; 23]. Other commercial resins, TK 211/212/213 resins available from Triskem International, have high selectivity for specific lanthanide pairs, but their small particle size makes them difficult to handle in a chromatographic column [24]. In 2005, [PERSON] et al. (2005) introduced the diglycolamide (DGA) resin, a novel material containing the DGA ligand, a well-known lanthanide extractant with a high-quality chelating tridentate capacity [25; 26; 27; 28].