2025, Vol. 36 
b School of Nuclear Science and Engineering, East China University of Technology, Nanchang 330013, China;
c School of Nuclear Technology and Chemistry & Biology, Hubei Key Laboratory of Radiation Chemistry and Functional Materials, Hubei University of Science and Technology, Xianning 437100, China;
d School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Throughout the ages, energy has been the material basis for the survival and development of human society. In fact, the history of human social development is the process of energy development and utilization. Nowadays, with the continuous growth of global energy demand, nuclear energy has a prominent competitive advantage in the emerging energy field due to its high energy density, low carbon emissions, and long-term stable operation [1,2]. However, the management and disposal of spent fuel from nuclear reactor is a critical issue that will become even more pressing because nuclear energy is more widely adopted internationally [3]. Spent fuel contains significant amounts of unfissioned uranium, newly produced plutonium, and small amounts of minor actinides as well as fission products. Among these, actinides such as plutonium, neptunium, americium, and curium are the main contributors to the radioactivity of spent fuel. Compared to direct deep geological storage, the separation and recovery of actinides are crucial for maintaining the long-term safe use of nuclear energy [4–7]. Plutonium, in particular, due to its high radioactivity and significant strategic importance, underscores thenecessity of its separation and recovery for both safety and resource utilization [8,9].
Although the traditional Plutonium and Uranium Recovery by Extraction (PUREX) process achieves the recovery of the vast majority of uranium and plutonium, the radioactive extract still contains residual actinides such as plutonium and fission products. The composition of the extract is complex, with strong radioactivity, high acidity, and similar chemical properties [10,11]. Many researchers are committed to explore suitable separation methods, such as solvent extraction [12–16], ion exchange [17,18], and adsorption [19–22] to achieve the separation of the target radionuclide Pu(Ⅳ). While some methods have been successful in industrial applications, they still fall short in terms of selectivity, efficiency, and environmental friendliness. Therefore, it is necessary to develop new functional materials for the separation and recovery of Pu(Ⅳ).
Covalent organic frameworks (COFs), as a class of highly crystalline porous materials, have been successfully used in the fields of adsorption [23–25], catalysis [26–30], sensing [31], and energy storage and conversion [32–35] due to their low density, high stability, high porosity and design functionality [36,37]. Compared with boroxine linkage [38–40] and imide linkage [41], olefin linkage (C=C or sp2c) has higher stability due to its π-electron conjugation structure and irreversibility [42–50]. Ionic covalent organic framework materials (iCOFs) exhibit high crystallinity and a rigid skeleton structure [51], which fully exposes the ion exchange sites within their pores. This characteristic addresses the limitations of traditional resins, including poor stability, slow kinetics, significant swelling, and inadequate regeneration ability [52,53], positioning iCOFs as a promising new class of ion exchangers. Notably, Pu(Ⅳ) forms anionic complexes, [Pu(NO3)5]- and [Pu(NO3)6]2-, with nitrate ions in high concentrations of nitric acid solution [54], allowing for anion exchange to separate trace amounts of radioactive nuclides. Therefore, ionic sp2c-COFs are the ideal candidate for the separation of Pu(Ⅳ).
Herein, we used the available 2,4,6-trimethylpyridine as raw material to prepare two highly crystalline sp2c-COFs via melt polymerization [43]. Subsequently, neutral COF-IHEP3 and COF-IHEP4 can be converted into ionic COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3 by post-modification method, which can be employed as an efficient ion exchange adsorbent for the separation and removal of Pu(Ⅳ) in high nitric acid systems. It is worth noting that COF-IHEP3-CH3NO3 achieves removal of almost all Pu(Ⅳ) in solution (98%) within 20 min in 8 mol/L nitric acid solution, while the removal rate of competing ions including uranium is less than 3%. The material demonstrates competitive performance in the selective removal of Pu(Ⅳ) (details see Table S3 in Supporting information) [55–60], particularly in highly acidic environments (8 mol/L HNO3). To the best of our knowledge, this study represents the first application of iCOF as an ion exchange material for the extraction of Pu(Ⅳ) from high-level liquid waste (HLLW). Finally, density functional theory (DFT) calculations revealed the adsorption mechanisms of Pu(Ⅳ) through ion exchange reaction. We hope that this study can provide new ideas and materials for efficient actinide separation.
In comparison to COFs bridged by Schiff bases, those connected by olefin linkages exhibit enhanced stability under acidic and basic conditions, as well as improved chemical and radiation stability. The formation of olefin bonds in traditional chemistry often involves the Knoevenagel reaction [61,62], Aldol reaction [46], and Horner-Wadsworth-Emmons reaction [63]. These reactions typically necessitate active α-H monomers to interact with aldehydes. However, the activation of these active α-H monomers usually involves cyanide groups, leading to the requirement of large quantities of highly toxic cyanide reagents in the preparation process. The economic and environmentally friendly solid-phase fusion method were considered in the utilization of 2,4,6-trimethylpyridine (TMB), an active methyl-substituted aromatic heterocycle compound, in reactions with terephthalaldehyde (TPA) and 1,4-biphenyl dicarboxaldehyde (BPA). Benzoic acid and benzoic anhydride were used as catalysts to facilitate their synthesis, leading to the successful preparation of two distinct sp2 carbon-conjugated covalent organic frameworks (sp2c-COFs) named COF-IHEP3 and COF-IHEP4 as shown in Fig. 1.
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| Fig. 1. Design and synthesis. Syntheses of COF-IHEP3 by condensation of TMB and TPA, and synthesis of COF-IHEP4 by reaction of TMB and BPA. Synthesis of ionic COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3 through post-modified strategy. | |
The structures of the synthesized COF-IHEP3 and COF-IHEP4 were confirmed through powder X-ray diffraction (PXRD) analyses and simulations conducted using the Material Studio software package. Pawley refinement, based on the cubic space group P1, provided the lattice parameters of a = 18.0087 Å, b = 19.4594 Å, c = 2.5872 Å for COF-IHEP3; and a = 31.2207 Å, b = 29.2369 Å, c = 3.1399 Å for COF-IHEP4 (Tables S1 and S2 in Supporting information). The refined patterns based on the simulated structures of both COFs agree well with the experimental results (Rp = 3.64% and Rwp = 4.75% for COF-IHEP3; Rp = 3.70% and Rwp = 4.90% for COF-IHEP4). The calculation results indicate that COFs are "AA" stacked two-dimensional crystalline materials. The diffraction peaks at 4.6°, 7.9°, 9.2°, 12.2°, 15.9°, 26.8° for COF-IHEP3 can be assigned to the (100), (110), (200), (210), (220) and (001) Bragg peaks, respectively. Similarly, an intense peak at 3.5° and minor peaks at 5.9°, 6.9°, 9.2°, 12.3°, 26.4° can be observed for COF-IHEP4, which correspond to the Bragg diffractions of the (100), (110), (200), (210), (220) and (001) planes, respectively (Figs. 2a and b).
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| Fig. 2. (a) PXRD patterns of COF-IHEP3. (b) PXRD patterns of COF-IHEP4. Pawley refinement (orange line), experimental data (black cross), computational simulation (blue line), refinement-experiment difference (pink line) and the Bragg positions (green short sticks). (c) FT-IR spectra of COF-IHEP3, and corresponding monomers. (d) Solid-state 13C NMR spectra of COF-IHEP3. (e) N2 adsorption-desorption isotherms of COF-IHEP3 at 77 K. (f) PXRD patterns of COF-IHEP3 by exposing the COFs to 50 kGy and 100 kGy doses of γ rays in dry, pure water and 8 mol/L HNO3 environments, respectively. | |
Fourier transform infrared (FT-IR) spectroscopy and solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectra were collected to confirm the chemical structures and components of COF-IHEP3 and COF-IHEP4. The FT-IR spectrum displayed bands associated with C=C at 960, 1630, and 3027 cm-1. The absence of the C-H (2920 cm-1) vibration of TMB and the C=O stretching band (1690 cm-1) of TPA and BPA indicated a significant degree of melt polymerization in COF-IHEP3 and COF-IHEP4 (Fig. 2c and Fig. S1 in Supporting information). In solid-state 13C NMR spectra, the carbon atoms of the pyridine ring and newly formed C=C bonds are prominently observed in the low-field region of the NMR spectrum, indicating their presence in the framework (Fig. 2d and Fig. S2 in Supporting information). In contrast, the carbon atoms of methyl were barely invisible in the high-field region. This disparity in signal intensity between the pyridine ring carbons and the methyl carbons is indicative of the high polymerization in the COF-IHEP3 and COF-IHEP4.
The N2 isotherm was conducted at 77 K, revealing type-Ⅰ isotherms for COF-IHEP3 and COF-IHEP4, indicating their mesoporous nature (Fig. 2e and Fig. S3 in Supporting information). The Brunauer-Emmett-Teller (BET) surface areas calculated for COF-IHEP3 and COF-IHEP4 are 1500 and 990 m2/g, respectively. Analysis of pore size distributions based on the adsorption isotherms displayed two pores measuring 1.7 and 2.3 nm, respectively (Fig. 2e and Fig. S4 in Supporting information). Thermogravimetric analysis (TGA) demonstrated the thermal stability of COF-IHEP3 and COF-IHEP4, as shown in Figs. S5 and S6 (Supporting information), with less than 6% mass loss in air up to 300 ℃. Chemical stability was evaluated by subjecting COF-IHEP3 and COF-IHEP4 to various acids and bases aqueous solutions for 24 h. We observed that the weight loss of COF-IHEP3 and COF-IHEP4 was negligible after washing and drying. Furthermore, the peak position and intensity of their PXRD patterns, as well as the characteristic absorption peaks in FT-IR, did not exhibit significant changes after treatment in the aforementioned aqueous solutions (Figs. S7-S9 in Supporting information). This result indicates that the structure of the samples remains stable in both acidic and alkaline environments, without notable structural changes or chemical bond breakage. Irradiation stability was tested by exposing the COF-IHEP3 and COF-IHEP4 to 50 kGy and 100 kGy doses of γ rays in dry, pure water and 8 mol/L HNO3 environments, respectively. The PXRD and FT-IR patterns of the treated COFs remained unchanged nearly, indicating robust irradiation stability (Fig. 2f, Figs. S10 and S11 in Supporting information). SEM images illustrated the block-like morphology of COFs (Figs. S12 and S13 in Supporting information). Overall, these findings validate the successful synthesis of pure-phase sp2c-COFs. Ionic covalent frameworks (iCOFs) are a significant subset of the COFs family, which could fully expose ion exchange sites within their pores. Their high crystallinity and low swelling properties create open channels that facilitate ion transport, making them a novel type of ion exchanger [64–66]. Two ionic COFs, COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3, were synthesized through post-modification. The experimental scheme can be found in Supporting information. COF-IHEP3 and COF-IHEP4 exhibit strong reflection peaks at 4.6° and 3.5° (2θ), as shown in Figs. 3a and b, indicating that the crystalline structure of the COFs remained intact during the ionization process. Additionally, the FT-IR spectrum of the modified COFs displays a distinct nitrate absorption peak at 1384 cm−1, as illustrated in Figs. 3c and d. In the solid-state 13C NMR spectra, the resonance of the carbon atom at around 40 ppm indicated the presence of a methyl group. Both the FT-IR and solid-state 13C NMR spectra results confirmed the successful synthesis of COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3, which contained nitrate methyl ionization (Figs. 3e and f). BET surface area of COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3 was measured at 825 m2/g and 380 m2/g, respectively, with pore size distributions around 1.6 and 2.2 nm (Figs. S14-S17 in Supporting information). SEM images indicate that the modified ionic sp2c-COFs did not display any morphological changes at the microscopic level (Figs. S18 and S19 in Supporting information). TGA analysis demonstrates that the post-synthetic process had minimal impact on thermal stability (Figs. S20 and S21 in Supporting information). Furthermore, the acid-base stability and irradiation stability of COF-IHEP3-CH3NO3 and COF-IHEP4-CH3NO3 suggest their potential application value (Figs. S22-S27 in Supporting information).
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| Fig. 3. (a) PXRD patterns of COF-IHEP3 and COF-IHEP3-CH3NO3. (b) PXRD patterns of COF-IHEP4 and COF-IHEP4-CH3NO3. (c) FT-IR spectra of COF-IHEP3 and COF-IHEP3-CH3NO3. (d) FT-IR spectra of COF-IHEP4 and COF-IHEP4-CH3NO3. (e) Solid-state 13C NMR spectra of COF-IHEP3 and COF-IHEP3-CH3NO3. (f) Solid-state 13C NMR spectra of COF-IHEP4 and COF-IHEP4-CH3NO3. | |
The proportion of the various oxidation states of plutonium species is Pu4+> PuO22+> Pu3+> PuO2+ in nitric acid solution. In comparison to uranium and neptunium in the same valence state, plutonium is more easily complexed with anions to form complex ions. The stability of Pu(Ⅳ) forming complexes with common anions follows the order: F- > NO3- > Cl- > ClO4-~CO32- > SO32- > C2O42- > SO42- [67]. In nitric acid solutions, Pu(Ⅳ) forms various compounds with NO3-, ranging from [Pu(NO3)]3+ to [Pu(NO3)6]2-, with the distribution of each nitrate complex dependent on the concentration of NO3- in the solution. In concentrated nitric acid solution, Pu(Ⅳ) primarily exists as undissociated [Pu(NO3)5]- and [Pu(NO3)6]2- [54]. Therefore, anion exchange materials can be utilized for separation of Pu(Ⅳ). Adsorption and separation experiments were conducted using above synthesized COFs and ionic COFs materials to assess the impact of solution acidity, NO3- concentration, adsorption kinetics, and competing ions on the adsorption performance.
The high concentration of acid in HLLW significantly impacts the ionic forms and the performance of adsorption materials. The removal rate and distribution coefficient (Kd) of trace Pu(Ⅳ) were studied with nitric acid concentrations ranging of 1–9 mol/L. Fig. 4a illustrates that as acidity increases, the removal rate of Pu(Ⅳ) by four types of COFs also increases, reaching a maximum at 8 mol/L acidity. The maximum removal rates of Pu(Ⅳ) by COF-IHEP3, COF-IHEP4, COF-IHEP3-CH3NO3, and COF-IHEP4- CH3NO3 are 83%, 73%, 98%, and 96%, with corresponding Kd values of 246, 214, 2450, and 1200 mL/g, respectively (Fig. S28 in Supporting information). These data suggest that increased acidity enhances the formation of nitrate complexes with Pu(Ⅳ) in solution, leading to the capture of [Pu(NO3)5]- and [Pu(NO3)6]2- complexes by the ionic COF materials through ion exchange. In high concentrations of nitric acid, this material demonstrates a distinct advantage in the removal of plutonium compared to other materials (Fig. 4b). However, at 9 mol/L nitric acid solution, the removal rate of Pu(Ⅳ) starts to decline, potentially due to the competitive effect of nitrate ions and the formation of neutral H2Pu(NO3)6 molecules [68]. Therefore, pyridine-based sp2c-COFs are recommended for separating trace Pu(Ⅳ) in highly nitric systems, whereas pyridine-based ionized sp2c-COFs showing superior adsorption performance over their protonated counterparts [69].
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| Fig. 4. (a) Effect of HNO3 at different concentrations on removal rate of Pu(Ⅳ). (330 Bq/mL, m/v = 20 mg/mL). (b) The performance comparison diagram of the material. (c) Effect of NO3- at different concentrations on removal rate of Pu(Ⅳ). (d) Adsorption kinetic of Pu(Ⅳ) by COF-IHEP3 and COF-IHEP3-CH3NO3. (e) Adsorption kinetic of Pu(Ⅳ) by COF-IHEP4 and COF-IHEP4-CH3NO3. (f) Adsorption of Pu(Ⅳ) under the conditions of spent fuel associated metal ions. | |
The complexation of nitrate ions with Pu(Ⅳ) enhances its removal, but excessive nitrate concentration can lead to competition with [Pu(NO3)5]- and [Pu(NO3)6]2- complexes, reducing the removal ratio of Pu(Ⅳ). To explore this equilibrium, we examined the impact of NO3- concentration on Pu(Ⅳ) removal rate and distribution coefficient by varying only the NO3- concentration while keeping other ions constant (Fig. 4c and Fig. S29 in Supporting information). Under the condition that the initial nitric acid concentration is 4 mol/L (the H+ concentration remains unchanged), the addition of sodium nitrate promotes the removal of pollutants of Pu(Ⅳ) when the concentration was between 1 mol/L and 4 mol/L. However, at 5 mol/L sodium nitrate concentration (total nitrate concentration of 9 mol/L), the competitive effect of nitrate ions became prominent, indicating their involvement in capturing Pu(Ⅳ) ions.
This study investigated the kinetic performance of materials in 8 mol/L nitric acid conditions (Figs. 4d and e, Figs. S30-S32 in Supporting information). The data revealed that COF-IHEP3 and COF-IHEP3-CH3NO3 reached adsorption equilibrium in approximately 20 min, while COF-IHEP4 and COF-IHEP4-CH3NO3 achieved equilibrium for Pu(Ⅳ) in only 10 min, suggesting faster kinetics for materials with larger pore sizes. The resulting uptake versus time data were analyzed using both the pseudo-first-order and pseudo-second-order models (Figs. S33 and S34 in Supporting information). The findings suggest that the adsorption process is more consistent with the second-order adsorption model, indicating that the adsorption of Pu(Ⅳ) by these COFs occurs primarily through chemical adsorption (R2 > 0.999) (Table S5 in Supporting information).
After PUREX treatment, HLLW still contains trace amounts of Pu(Ⅳ), necessitating materials with high selectivity, acid and irradiation resistance. In this work, four types of COFs were assessed for their selectivity towards trace Pu(Ⅳ) in a 6 mol/L nitric acid solution containing various metal ions in HLLW (such as Sr(Ⅱ), Zr(Ⅳ), Y(Ⅱ), Eu(Ⅲ), Nd(Ⅲ), La(Ⅲ), Ce(Ⅲ), U(Ⅵ), among others, at a concentration of 1 mmol/L) [70]. As depicted in Fig. 4f, the removal rates of these COFs for competing ions, including U, were below 3%, while the removal rates for Pu(Ⅳ) remained almost constant. This suggests that the four types of COFs demonstrate exceptional selectivity for trace Pu(Ⅳ) ions in nitric acid systems.
The adsorption mechanisms of Pu(Ⅳ) in HLLW after PUREX treatment were investigated through density functional theory (DFT) calculationsby the Gaussian 16 program package [71]. The geometries of (L1)+, (L2)+, (L1)NO3, (L2)NO3, [Pu(NO3)5]-, Pu(L1)(NO3)5, and Pu(L2)(NO3)5 (L1 = L-H; L2 = L-CH3) compounds in the aqueous phase were optimized using the B3LYP/6–31G(d,p)/RECP theoretical level, along with the conductor-like screening model (COSMO) [72] and Grimme's DFT-D3 correction (Fig. 5a). Computational results show that in Pu(L1)(NO3)5 complexes, [Pu(NO3)5]- anions are located within cavities formed by the pyridine ring and benzene rings, while in Pu(L2)(NO3)5 complexes, [Pu(NO3)5]- anions are positioned above the entire (L2)+ ligand due to steric hindrance from methyl groups on the pyridine nitrogen atoms. Electrostatic potential (ESP) calculations reveal that positive potentials mainly concentrate on the pyridine rings of (L1)+ and (L2)+ compounds (Fig. 5b), with minimal contribution from benzene rings, whereas [Pu(NO3)5]- anions display uniformly negative potentials. Therefore, the pyridine rings of (L1)+ and (L2)+ compounds attract the anion through strong electrostatic interactions. Thermodynamic calculations of the complexation free energies between (L1)+ and (L2)+ compounds with [Pu(NO3)5]- were conducted using the thermodynamic equation: [Pu(NO3)5]- + L(NO3) → PuL(NO3)5 + NO3-. The negative ΔGwater values of [Pu(L1)(NO3)5] (−20.82 kcal/mol) and [Pu(L2)(NO3)5] (−28.68 kcal/mol) indicate the favorable displacement of nitrate anions by [Pu(NO3)5]-. Furthermore, the comparison |ΔGPu(L2)(NO3)5| > |ΔGPu(L1)(NO3)5| suggests that (L2)+ ligands exhibit a stronger affinity for [Pu(NO3)5]- anions. This finding is consistent with experimental results, which demonstrate that methylated COFs possess a superior ability to capture Pu(Ⅳ) compared to their protonated counterparts.
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| Fig. 5. (a) The geometric structures of [Pu(NO3)5]-, Pu(L1)(NO3)5, and Pu(L2)(NO3)5 (L1 = L-H; L2 = L-CH3) compounds optimized at the B3LYP/6–31G(d,p)/RECP theoretical level. (b) ESP plots of (L1)+, (L2)+ ligands and [Pu(NO3)5]- anions. | |
In summary, we synthesized two stable sp2c-COFs via melt method and subsequently modified them into ionic COFs. We systematically investigated these four COFs as solid-phase materials for capturing Pu(Ⅳ) from high-concentration nitric acid solution. In 8 mol/L nitric acid environment, COF-IHEP3-CH3NO3 effectively removed nearly all Pu(Ⅳ) in just 20 min, showcasing excellent selectivity even in the presence of various ions. The exceptional performance exceeds that of most materials employed for the separation of Pu(Ⅳ), particularly in environments with high concentrations of nitric acid. This capability allows the material to selectively extract Pu(Ⅳ) from concentrated HLLW, significantly reducing the volume of liquid waste. The experimental results were further supported by DFT calculations, aligning with theoretical projections. This research not only introduces novel materials and techniques for efficient Pu(Ⅳ) separation but also provides experimental and theoretical evidence for the utilization of COFs in spent fuel reprocessing and industrial practice.
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statementLi-Ying Wang: Writing – review & editing, Writing – original draft, Validation, Investigation, Formal analysis, Data curation, Conceptualization. Jun-Jie Yu: Visualization, Software. Shuai Wang: Methodology, Investigation, Formal analysis, Data curation. Yang Liu: Writing – review & editing, Software, Formal analysis. Ke-Xian Song: Validation, Data curation. Ji-Pan Yu: Writing – review & editing, Validation, Supervision, Resources, Methodology, Funding acquisition. Li-Yong Yuan: Writing – review & editing, Supervision, Resources, Funding acquisition. Zhi-Rong Liu: Supervision. Wei-Qun Shi: Supervision, Project administration, Funding acquisition.
AcknowledgmentsThis work was supported by the National Natural Science Foundation of China (Nos. U2067212, 22176191), the National Science Fund for Distinguished Young Scholars (No. 21925603).
Supplementary materialsSupplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110706.
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