Phosphate-functionalized polymers have shown great potential in this regard, exhibiting excellent stability and durability in binding metal ions, especially uranyl ions, in highly concentrated HNO3 solutions16,17,18. However, despite these advancements, to the best of our knowledge, there is still no phosphate-functionalized polymer designed for efficient uranium recovery from ultra-acidic and radiation-intensive environments in SNF reprocessing, which would also require enhanced resistance to interference of co-existing metal ions in the complex SNF matrix. Herein, based on the exceptional stability of carbon nitride under extreme conditions and the strong binding affinity of phosphate groups for uranyl ions, a pyrophosphate-functionalized g-C3N4 material was developed, which incorporates an abundance of pyrophosphate binding sites capable of rapidly forming a stable U-pyrophosphate coordination complex in 12 M HNO₃ media, offering significant promise for uranium recovery in SNF reprocessing (Fig. 1). The pyrophosphate groups, along with the unsaturated nitrogen atoms inherent to the g-C3N4 structure, participate in chemical coordination with uranyl ions. This coordination mechanism not only significantly increases the binding affinity of the material for uranyl ions but also confers high selectivity in the presence of concentrated nitric acid. The material also exhibits outstanding hydrophilicity and radiation resistance, which further enhancing its functionality. As a result, the extractant achieves an impressive uranium adsorption capacity of 75.3 mg g-1 under 12 M HNO₃ conditions, making it the most efficient extractant under such extreme acidic conditions. These properties collectively position the PCNx material as a highly effective and practical candidate for uranium recovery in SNF reprocessing applications.

The pyrophosphate-functionalized g-CN materials were successfully synthesized through a simple and efficient thermal polymerization process (Fig. 2a). To identify the material with optimal uranium recovery performance, the precursor concentration of NaPO was carefully adjusted, with the resulting material, PCN, demonstrating the best performance. The synthesized PCN extractant appeared as a yellow powder (Supplementary Fig. 1). The scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images revealed that both the g-CN and PCN materials exhibited a bulk sheet-like morphology, with PCN maintaining a similar structure but distinct modifications due to the incorporation of pyrophosphate groups (Fig. 2b-e). The Brunauer-Emmett-Teller (BET) surface area of PCN was determined to be 3.69 m² g, confirming its feature as a mesoporous material (Supplementary Fig. 2a, b). Furthermore, energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis of PCN showed uniform distributions of carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sodium (Na) across the material, indicating the successful incorporation of pyrophosphate groups into the g-CN framework (Fig. 2f-k). This uniform element distribution provided strong evidence for the effective integration of the pyrophosphate groups into the structure of g-CN, which was crucial for enhancing its uranium recovery capabilities.

Two characteristic peaks at approximately 13.3° and 27.6° were observed in the X-ray diffraction (XRD) pattern of g-CN, corresponding to the in-plane repetition of tri-s-triazine units and the typical periodic graphitic stacking of the conjugated aromatic skeleton, respectively (Fig. 3a). As pyrophosphate incorporation increased, the (100) peak gradually disappeared, suggesting that the pyrophosphate incorporation induced in-plane distortion of the heptazine motifs. Additionally, the (002) peak became broader and weaker with the increasing pyrophosphate content, indicating that the stacking of the aromatic structures was inhibited, leading to the formation of thinner sheets. These observations suggested that pyrophosphate incorporation disrupted the regular stacking order of g-CN. Furthermore, the Fourier transform infrared (FT-IR) spectra of all materials displayed the classic features of g-C₃N₄, including the characteristic breathing mode of triazine units at 810 cm and the stretching vibration modes of C - N heterocycles in the 1200-1700 cm range (Fig. 3b). Broad signals in the 2900-3280 cm and 3280-3670 cm regions were attributed to N - H and O - H stretching vibrations, respectively. These characteristic peaks were also observed in the pyrophosphate-incorporated PCN materials, confirming that the fundamental structure of g-CN was maintained. Notably, the incorporation of pyrophosphate introduced additional peaks, including the P-O signals at 992 and 930 cm, and a P = O bond signal at 1161 cm. Additionally, a C - O breathing mode at 1078 cm was observed for the pyrophosphate-modified samples, indicating the formation of C - O bonds between the pyrophosphate groups and the g-C₃N₄ framework. As illustrated in the X-ray photoelectron spectroscopy (XPS) survey spectra, in addition to the elements C, N, and O found in g-CN, the presence of P and Na was clearly evident in PCN, consistent with the EDS elemental mapping and EDS spectra (Fig. 3c and Supplementary Fig. 3). In the high-resolution N 1 s XPS spectra, both g-CN and PCN exhibited three distinct peaks at approximately 401.36, 400.55, and 398.63 eV, which can be assigned to N-H, N - (C)/N ≡ C, and C = N - C groups, respectively (Fig. 3d and Supplementary Fig. 4a). The high-resolution C 1 s XPS spectra revealed three main peaks for g-CN at around 284.80 eV (C - C), 285.96 eV (C-NHx), and 288.24 eV (N - C = N), while an additional peak at approximately 288.89 eV (C - O) appeared for PCN, indicating the presence of C - O bonds due to the grafting of pyrophosphate (Fig. 3e and Supplementary Fig. 4b). In the high-resolution O 1 s XPS spectra, peaks at 530.77 eV (P = O) and 532.43 eV (C - O/P - O/O - H) were observed for PCN, while only the O - H bond signal was detected in g-CN (Fig. 3f and Supplementary Fig. 4c). The peak at 535.58 eV was attributed to the Na KLL spectrum, further confirming the doping of Na into the g-CN framework due to the use of NaPO as a precursor. The Na 1 s binding energy at 1071.5 eV was consistent with that of NaN (Supplementary Fig. 4d). The high-resolution P 2p XPS spectra revealed two peaks at 133.65 and 132.88 eV, corresponding to the P = O and P - O bonds, respectively (Fig. 3g). Moreover, the inductively coupled plasma optical emission spectroscopy (ICP-OES) results showed that the phosphorus content in the materials increased with the ratio of NaPO to melamine precursors, reaching a maximum at a ratio of 1:3. However, the phosphorus content decreased when the NaPO to melamine ratio was increased to 1:2 (Fig. 3h). The hydrophilicity of the extractant plays a critical role in its uranium adsorption performance. The higher phosphorus content, reflecting an increased presence of hydrophilic phosphorus-containing groups, contributed to the enhanced hydrophilicity of the material. This was consistent with the results from water droplet contact angle measurements, which indicated a decrease in the contact angle as the pyrophosphate content increased, thereby confirming the correlation between higher phosphorus content and enhanced hydrophilicity (Fig. 3i).

The uranium adsorption performance was systematically evaluated by controlling a range of experimental variables to gain insights into the influencing factors. The uranium adsorption capacity gradually increased as the precursor ratio of NaPO to melamine increased from 0 to 1:3, due to the increased phosphorus content of the materials (Figs. 3h, 4a). However, further increasing the NaPO to melamine ratio from 1:3 to 1:2 resulted in a slight decrease in the uranium adsorption capacity, which was attributed to the decreased phosphorus content. This indicates that a balanced ratio of NaPO and melamine is crucial for achieving optimal adsorption performance. As the temperature increased from 298.15 K to 318.15 K, a significant enhancement in adsorption capacity was observed, confirming the temperature-dependent nature of the adsorption process (Fig. 4b). Specifically, a uranium adsorption capacity of 95.3 mg g was achieved at 298.15 K in a 4 M HNO solution. Given that the reprocessing of SNF typically involves concentrated inorganic acids such as HNO, it is important to assess the extractant's performance in different acid concentrations. The optimal extractant, PCN, exhibited impressive uranium adsorption capacity of 75.3 mg g, even in a 12 M HNO ultrahigh acidic solution (Fig. 4c). This performance is superior to previously reported extractants under similar conditions (Fig. 4d and Supplementary Tab. 1), further demonstrating the potential of pyrophosphate-functionalized g-CN as an effective material for uranium recovery in highly acidic environments.

Additionally, the presence of high salt concentrations, which are commonly encountered during SNF reprocessing, can significantly hinder uranium adsorption from saline media. To assess this, the competition between sodium ions (Na) and uranyl ions (UO) was also investigated. The PCN extractant exhibited only a slight decrease in uranium adsorption capacity when exposed to NaNO solutions with concentrations ranging from 0 to 3 M (Fig. 4e). This suggests that PCN possesses high resistance to interference from competing ions. The long-term stability of the extractant under highly acidic conditions and irradiation is a critical factor for its practical application. The analysis of the long-term stability showed that there was no significant change after being exposed to 12 M HNO₃ and nuclear radiation (Supplementary Figs. 5-8), demonstrating the high stability and durability of PCN under highly acidic conditions with nuclear radiation. Furthermore, the uranium adsorption capacity of PCN was assessed after treatment with 12 M HNO and radiation exposure, which exhibited nearly unchanged uranium adsorption capacity (Fig. 4f), confirming the high potential of PCN for use in highly acidic environments and nuclear radiation conditions. This is because PCN is highly stable in conditions of high acidity and strong irradiation. It retains effective functional adsorption sites that coordinate with uranyl ions, ensuring excellent adsorption performance.

The uranium adsorption kinetics showed that the adsorption capacity of PCN reached 83.8 mg g within 35 min of contact time (Fig. 5a). The kinetic data indicated that the adsorption process of PCN adhered most closely to the pseudo-second-order model. This suggested that the rate-determining step in the adsorption process was chemisorption, as strong chemical bonds formed between the extractant and the uranyl ions (Supplementary Tab. 2). Moreover, the uranium adsorption isotherms showed that the theoretical maximum adsorption capacity of PCN was approximately 142.6 mg g (Fig. 5b and Supplementary Tab. 3). This was confirmed by fitting the experimental data to the Langmuir adsorption model, indicating monolayer adsorption, further emphasizing the high efficiency and specificity of PCN for uranium uptake. During nuclear fuel reprocessing, the presence of competing cations makes selectivity a critical factor for uranium recovery. To evaluate the selectivity of PCN, experiments were conducted to simulate the conditions of SNF using synthetic mixtures containing 11 other representative metal ions in a 12 M HNO solution (Supplementary Tab. 4). Encouragingly, the uranium sorption efficiency (SE) of PCN increased with increasing adsorption times, achieving 79.1% efficiency after 6 h (Fig. 5c and Fig. 5d). This performance significantly exceeded that of pristine g-CN. The superior SE was attributed to the presence of a large amount of pyrophosphate on the surface of PCN, which facilitated the formation of complexes with uranyl ions, thus enhancing the extractant's affinity for uranium. The practical application of the extractant depends not only on its adsorption capacity but also on its dosage. Figure 5e shows the effect of different extractant dosages on the uranium SE of PCN. As the extractant dosage increased, the uranium SE improved, eventually reaching a plateau at a dosage of 0.2 g L, which would be the optimal dosage for efficient uranium recovery. Given the high uranyl ions capture properties and high-acidity resistance of PCN, we further investigated the extractant's performance under more stringent conditions. The uranium SE of PCN remained at 79.1%, significantly higher than that of other competing ions in the solution (Fig. 5f). Notably, the concentration of Na⁺ increased after adsorption, potentially due to an ion exchange between the Na⁺ in PCN and the uranyl ions, leading to the release of Na⁺ (Supplementary Fig. 9). The distribution coefficient (K) for the uranyl ions was as high as 18,964 mL g, which the highest value reported in the literature for extractants used in ultrahigh acid concentrations (12 M HNO₃), indicating that PCN has a high affinity for uranyl ions (Fig. 5g, Supplementary Tabs. 5, 6). Notably, only a few extractants can maintain practical feasibility under highly acidic conditions (i.e., HNO concentrations exceeding 10 M), as most materials tend to degrade or experience deactivation of their functional sites under such harsh conditions. This highlighted the promising potential of PCN for being used in real-world applications, specifically in uranium recovery during SNF reprocessing. Furthermore, we analyzed PCN's selective adsorption of uranyl ions against competing metal ions (Eu, Th, Cs, Sr, Ni, K, Zn, Co, Ca, Na, and Mg) in 12 M HNO solution. The results showed that the uranium adsorption capacity of PCN was significantly higher than that for other co-existing metal ions, with high selectivity values (S), including S of 1580.3, S of 208.4, S of 201.7, S of 155. 4, S of 126.4, S of 54.7, S of 44.9, S of 10.2, S of 1.3, and S of 12.7 (Fig. 5h and Supplementary Tab. 7). These results further confirmed the outstanding selectivity and practical potential of PCN for uranium recovery in SNF reprocessing. Considering the practical application challenges, we tested the material's ability to adsorb uranium from a 12 M HNO solution using a continuous flow system. The sample was immobilized on the adsorption bed with silica powder, and a uranium solution containing 12 M HNO was continuously passed through the chromatographic column (Supplementary Fig. 10). The results indicated that the uranium adsorption capacity of the material after 3 h was 78 mg g (Fig. 5i). These findings showed that the material exhibited excellent uranium adsorption performance in ultrahigh acid environments and has great potential for the development of flow columns for uranium adsorption under such extreme conditions in practical applications.

To further elucidate the uranium adsorption mechanism of PCN, comprehensive analyses were performed. The EDS elemental mapping analysis revealed that the uranium was uniformly distributed on the surface of PCN after uranium adsorption (Supplementary Fig. 11). In the XPS spectra, in addition to the characteristic peaks for C, N, O, and P that are present in the unmodified PCN, a distinct U 4 f peak was observed in the U-PCN sample (Fig. 6a and Supplementary Fig. 12a). Notably, the disappearance of the Na signals in the XPS spectra suggested that an ion exchange had occurred between the Na in PCN and the uranyl ions during the adsorption process, which was also confirmed by an increase in the Na concentration in the competing ions solution after adsorption (Supplementary Fig. 9 and Supplementary Fig. 12b). Additionally, the FT-IR spectra revealed a new peak at approximately 925 cm, which is attributed to the antisymmetric stretching vibration of O = U = O (Supplementary Fig. 13). These results demonstrated the successful adsorption of uranium.

In the high-resolution N 1 s XPS spectra of U-PCN, three distinct peaks were observed at binding energies of 401.41 eV (N-H), 400.19 eV (N - (C))/N ≡ C, and 398.80 eV (C = N - C) (Fig. 6b). Comparing these spectra with those from the original PCN (Fig. 3d), the atomic percentage of the unsaturated N species (C = N - C) decreased, indicating that uranium was likely immobilized by coordination with the unsaturated nitrogen sites in the C = N - C structure within PCN, thus contributing to the uranium adsorption process. The high-resolution P 2p XPS spectra of U-PCN showed two main peaks at 133.93 and 133.19 eV, corresponding to P = O and P - O, respectively. After uranium adsorption, these peaks shifted to higher binding energies, which was characteristic of the coordinated interactions between pyrophosphate groups and uranyl ions, confirming that the phosphorus-containing groups in PCN participated in the uranium adsorption (Fig. 3g and Fig. 6c). Furthermore, the O 1 s XPS spectra of U-PCN exhibited a shift in binding energy from 532.43 eV (C - O, P - O, and O - H) and 530.77 eV (P = O) to 532.45 eV (C - O, P - O--U, and O - H) and 531.19 eV (P = O--U, and U = O), further confirming the formation of a uranyl-phosphate complex during the adsorption process (Figs. 3f, 6d). Moreover, the U 4 f XPS spectra of U-PCN after uranium adsorption revealed new signals corresponding to U 4f (380.60 eV) and U 4f (391.59 eV), which were distinct from the XPS peaks of U 4 f observed in the uranyl nitrate hydrate (UO(NO)·6HO), indicating that uranium was successfully complexed with the PCN material (Fig. 6e, f).

To gain further insight into the binding environment and local coordination structure of U(VI) adsorbed on the pyrophosphate-functionalized g-CN extractant, extended X-ray absorption fine structure (EXAFS) analysis and density functional theory (DFT) calculations were performed on the U-PCN complex. The EXAFS results showed two prominent peaks at 1.75 Å and 2.34 Å, which are attributed to the coordination with two axial oxygen atoms (U-Oax) and five equatorial oxygen atoms (U-Oeq), respectively (Fig. 6g, Supplementary Fig. 14 and Supplementary Tab. 8). Additionally, a weaker peak was detected, indicating the formation of bond between uranium and unsaturated nitrogen atoms (U - N), which was due to the ion exchange between Na ions and uranyl ions during the adsorption process. Furthermore, DFT calculations confirmed that the Na-incorporated g-CN structure was more favorable for the adsorption of uranyl ions than the pristine g-CN. This was due to the more negative formation energy of the Na-incorporated structure (-4.49 eV) compared to the pristine counterpart (-4.09 eV) (Supplementary Fig. 15a-c and Supplementary Tab. 9). Additionally, the adsorption energy of pyrophosphate in g-CN for uranyl ions was found to be negative (-6.65 eV) and the coordination with the uranyl ions occurred on the same plane, indicating that pyrophosphate groups were highly effective in capturing uranyl ions (Supplementary Fig. 15d-f and Supplementary Tab. 10). Based on these findings, the potential optimized structural modes of PCN and UO ions were proposed. During the adsorption process, the uranyl ion underwent ion exchange with the Na⁺ ion in the PCN structure, forming U-N coordination bonds with the unsaturated nitrogen atoms within the heptazine framework (Fig. 6h). Furthermore, the uranyl ion also formed coordination bonds with five oxygen atoms on the phosphate groups (P = O and P - O) grafted onto PCN (Fig. 6i). These results highlight the key interactions and coordination environments that contribute to efficient uranium adsorption by PCN, providing a deeper understanding of the uranium adsorption mechanism and the roles of pyrophosphate groups and unsaturated nitrogen sites in binding of uranyl ions.