## 3 Results and Discussion  
### Characterization of TK221: Insights from SEM-EDX, Microtrac, ATR-FTIR, and XPS  
The elemental mapping shown in Figure 1a indicates an even distribution of main elements, including Carbon (C), Nitrogen (N), Phosphorus (P), and Oxygen (O) throughout the resin. The strong EDX signals of N and P confirmed the presence of DGA and CMPO on the surface of TK221, and their uniform distribution suggests that the extraction capabilities of TK221 would be consistent and reliable. The particle size distribution of the resin is shown in Figure 1b, and it is mainly between 100 \(\upmu\)m and 200 \(\upmu\)m. Compared to traditional chromatographic resins prepared with pure polymer matrix in the particle size of 0.3-3 mm, this new type of extraction resin has a smaller particle size, which can result in faster kinetics, higher strength, and better flowability in the packed column [24]. These characteristics are important for improving the efficiency of the extraction process and reducing the amount of solvent used. However, it should be noted that further testing is required to fully understand the impact of the smaller particle size on the resin's performance and extraction efficiency. Figure 1c displays the ATR-FTIR analysis of fresh TK221, and Table 1 indicate the presence of several important functional groups. The absorption band between 3600-3400 cm\({}^{-1}\) corresponds to O-H vibrations of water [20; 37; 38], while that at 2950-2850 cm\({}^{-1}\) indicates the presence of C-H vibrations of TODGA [20; 29; 39; 40]. The IR peak at 1700-1600 cm\({}^{-1}\) suggests the presence of C=O vibrations of Amide I and CMPO [20; 29; 39; 40], and the 1570-1470 cm\({}^{-1}\) absorption band depicts the C-N vibrations of Amide II [29]. Meanwhile, the IR peak at 1200-1100 cm\({}^{-1}\) shows the presence of P=O vibrations of CMPO [39; 41], while that at 900-700 cm\({}^{-1}\) is assigned to C-H vibrations of PS-DVB [37]. Figure 2b displays the C(1s) spectra of TK221, which was deconvoluted into three peaks assigned to C-O/C-, amide, and C=O, as shown in Table 2. The peak at binding energy (BE) of 285.96 corresponded to the presence of C-O/C-N of DGA and the C=C of PS-DVB [20; 40; 42]. Additionally, the peak at a BE of 286.87 indicates the presence of the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Wavelength (cm\({}^{-1}\))** & **Corresponding Chemical** & **Reference** \\  & **Structure** & \\ \hline
3600–3400 & O-H of water & [20; 37] \\
2950–2850 & C-H of TODGA * & [20; 29; 39; 40] \\
1700–1600 & C=O of Amide I and CMPO & [20; 29; 39; 40] \\
1570–1479 & C-N of Amide II & [29] \\
1200–1100 & P=O of CMPO & [39; 41] \\
900–700 & C-H of PS-DVB & [37] \\ \hline \hline \end{tabular}
* Note: TODGA means tetraoctyl diglycolamide.  
\end{table} 
Table 1: ATR-FTIR peak assignments of fresh TK221. Figure 1: **(a)** Photomicrograph and elemental mapping of fresh TK221, **(b)** particle size distribution of fresh TK221, **(c)** ATR-FTIR spectrum of fresh TK221. amide group of DGA [40; 42]. Lastly, the peak at a BE of 288.68 implies the presence of the C=O group of DGA [20; 42]. Figure 2c shows the O(1s) spectrum of TK221, which was deconvoluted into three peaks assigned to P=O/C=O, C-O, and chemically adsorbed H\({}_{2}\)O, as shown in Table 2. The peak at a BE of 532.02 corresponded to the P=O of CMPO and the C=O of the DGA [43]. Additionally, the peak at BE of 534.22 indicates the presence of C-O of DGA [43]. The peak at a BE of 535.75 implies the presence of chemically adsorbed H\({}_{2}\)O [41; 44; 45], reaffirming the results of the ATR-FTIR analysis. Overall, the XPS analysis of O(1s) confirms the composition of TK221 and provides valuable insight into the surface chemistry of the resin. Taken together, the SEM-EDX analysis revealed that TK221, a resin comprised of tributyl phosphate (TRU) and diglycolamide (DGA) resins coated onto a polystyrene-divinylbenzene (PS-DVD) inert support, contains REE extractants CMPO and DGA.