## 3 Results and Discussions  
### Structural Characterizations  
The nanostructure and surface features of PVK and PVKAP were revealed by SEM shown in Figure 1. PVK and PVKAP are fibrous and both have a diameter of about 20-50 \(\upmu\)m. The PVK(Figure 1a) exhibits a relatively smooth fibrous morphology, while the PVKAP (Figure 2a) shows a much rougher surface after grafting with VPA and AA, which is advantageous for trapping metal ions. The functional groups on the surface of PVK and PVKAP were analyzed by FT-IR spectroscopy. As shown in Figure 2a, the characteristic absorption peaks of PVK at 3430, 1410, 1060, and 1630 cm\({}^{-1}\) are, respectively, attributed to the stretching vibration peaks of -OH, COO, and C-O, and the -OH bending vibration peak [39]. Comparing the FTIR spectra of PVKAP to PVK, the newly added peaks of PVKAP at 1725, 1460, 1160, and 915 cm\({}^{-1}\) are attributed to the stretching vibration peaks of C=O, P-C, P=O, and P-OH [41; 42; 43], respectively; thus, indicating that ethylene phosphate and acrylic acid were successfully grafted onto the surface of PVK by copolymerization. As confirmed by SEM, the FT-IR spectrum further demonstrates the successfully functionalized cellulose adsorbent materials. Furthermore, the P-OH characteristic vibration at 915 cm\({}^{-1}\) and the P=O characteristic vibration peak at 1160 cm\({}^{-1}\) almost disappeared, and the characteristic vibration of C=O at 1725 cm\({}^{-1}\) and the -OH characteristic vibration peak at 3430 cm\({}^{-1}\) weakened after the uranyl was absorbed by PVKAP (PVKAP-U); thus, indicating that the phosphonic acid group and the hydroxyl site participate in the capturing of U(VI) [39; 44]. Simultaneously, the XRD pattern (Figure 2b) of PVKAP exhibits an added broad peak in the range of 15 to 30\({}^{\circ}\) compared to PVK, while the major diffraction peaks remain unchanged. This significantly broad diffraction peak of PVKAP may be attributed to the grafted polymer chain, which confirms successful copolymerization and is in line with the results of FTIR spectroscopy. Figure 1: SEM images for pumpkin vine cellulose (PVK) (**a**) and a novel phosphonic acid and acrylic acid functionalized pumpkin vine based cellulose adsorbent (PVKAP) (**b**). Figure 2: (**a**) FTIR spectra of PVK and PVKAP; (**b**) XRD patterns of PVK and PVKAP. To further understand the specific surface area and pore size distribution of PVK and PVKAP, the BET method was adopted to analyze the samples. The N\({}_{2}\) adsorption-desorption adsorption isotherms of PVK and PVKAP are shown in Figure 3a. Furthermore, the specific surface area, pore volume and average pore size are shown in Table 1. Obviously, the specific surface area is improved after functionalization, and the average pore diameter of about 5 nm indicates the existence of internal mesopores. Additionally, the significant uptake of the nitrogen adsorption-desorption adsorption isotherm of PVKAP at relatively high pressures (0.8-1.0) also indicates the presence of mesopores [38; 45]. Generally, radioactive wastewater under certain circumstances has a relatively high temperature due to the fission of radionuclides, so thermal stability is one of the significant physicochemical properties to value adsorbent performance. As shown in Figure 3b, the TGA curves of PVK and PVKAP exhibit similar characteristics, of which the weight loss processes can be divided into three stages. The first stage loses weight at about 100 \({}^{\circ}\)C, which corresponds to the dissipation of water molecules from the surfaces of the materials; the second stage loses weight at around 100-238 \({}^{\circ}\)C. The weight loss of PVK at this stage is mainly attributable to the consumption of hydroxyl sites. Meanwhile, the weight loss of PVKAP at this stage is less than PVK, which is ascribed to the consumption of hydroxyl groups and the grafted phosphonic acid groups and carboxyl groups [41; 42].