Site differentiation strategy for selective strontium uptake and elution within an all-inorganic polyoxoniobate framework | Nature Communications

Nature Communications volume 15, Article number: 8896 (2024) Cite this article

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Selective uptake and elution of trace amounts of hazardous radioactive 90Sr from large-scale high-level liquid waste (HLW) is crucial for sustainable development. Here, we propose a site differentiation strategy, based on the presence of distinct selective metal capture sites (concavity site and tweezer site) within the giant polyoxoniobate (PONb) nanoclusters of an all-inorganic PONb framework (FZU-1). Through this strategy, FZU-1 can not only effectively remove 98.9% of Sr²⁺ from simulated nuclear liquid waste, performing best among the reported Sr adsorbents, but also achieve desorption of adsorbed Sr²⁺ ions by selectively loading Na⁺ ions, thus enabling the recycling of FZU-1. Based on the well-defined single-crystal structures and theoretical studies, it can be revealed that the rapid and selective uptake of Sr²⁺ is attributed to the strong binding energy between the Sr²⁺ ions and the concavity sites. The effective elution of Sr²⁺, on the other hand, stems from the preferential binding of Na⁺ ions at the tweezer sites, elevating the cluster’s electrostatic potential and indirectly facilitating the elution of Sr²⁺ ions. The exceptional stability of FZU-1, along with its rapid and selective Sr²⁺ capture and elution capabilities, positions it as a promising candidate for large-scale nuclear waste treatment and groundwater remediation applications.

The rapid advancement of nuclear energy technology has brought about significant challenges in the management of radioactive waste and the remediation of contamination.1,2 Typically, over the past four decades, ~ 56 million gallons of mixed high-level liquid waste (HLW) have been stored in 177 underground tanks at the Hanford Site.3 There is an urgent need for pretreatment of Hanford tank waste to minimize the amount of waste sent to the High-Level Waste Facility. This task is highly challenging due to the intricate chemical composition of the tank waste, which comprises a triphasic system with high salinity (5 M Na+) and extreme alkalinity (pH = 14.5), including metal hydroxide sludges, alkaline supernates, and salt cakes.4,5

The removal of highly hazardous radioactive 90Sr isotope and transuranic elements through co-precipitation is a foremost task in the Hanford Tank Waste Treatment and Immobilization Plant (WTP) baseline flowsheet.6 However, due to the chemical equilibrium limitations of precipitation and dissolution, 90Sr still persists at ppm to ppb levels in the supernates.4 The residual 90Sr not only impacts the subsequent ion exchange and elution of 137Cs using spherical resorcinol formaldehyde resin but also elevates the risk of 90Sr leakage into the environment. Reports have indicated that detectable levels of 90Sr are present in the nearby Columbia River.4 Hence, the selective removal of trace 90Sr from waste tanks and contaminated groundwater holds significant practical importance.

Compared to traditional organic ion exchange resins, inorganic ion exchangers offer superior chemical, thermal, and radiation stability, making them the dominant choice in the development of novel technologies of waste liquid separation and treatment. Inorganic ion exchangers, such as commercially available IE-911, a crystalline silicotitanate, have been successfully used to selectively remove Sr2+ under the high salt caustic condition of Hanford tank waste at Direct Feed Low-Activity Waste (DFLAW) flowsheet in WTP.4 Yet, the non-elutable IE-911 requires further downstream processing for disposal, which remains uncertain up to now. The large-scale tank waste necessitates a considerable amount of Sr adsorbent, which not only increases costs but also adds burden to geological disposal. Despite the emergence of various efficient inorganic 90Sr removers, including silicotitanates,4,7,8 titanates,9,10 layered metal chalcogenides,11,12,13 and layered metal oxides,6 to our knowledge, no 90Sr adsorbent has been able to achieve both high selectivity in capturing Sr2+ under high salt caustic conditions and easy elution and reuse. The development of novel inorganic ion exchangers that exhibit exceptional selectivity for Sr2+ capture under high salt caustic conditions while also facilitating elution, will significantly improve the current baseline pretreatment.

High selectivity and convenient elution are often contradictory. High selectivity means that the effective cavity diameter and chemical environment of the exchange active site will accommodate the target ions well, which makes it difficult for the captured ions to escape from this stable chemical environment. Generally, elution ions must overcome high binding energy to exchange target ions and replace the binding site, which is extremely challenging. Here, we propose a site differentiation strategy (Fig. 1, Video 1–4) that a Sr2+ adsorbent possesses two distinct ion adsorption sites, capable of selectively capturing target Sr2+ ions and elution ions, respectively. It not only has the potential to efficiently adsorb Sr2+ ions through the Sr2+ sites but also can achieve efficient elution of Sr2+ ions by loading elution ions onto the elution ion sites during the elution process, based on charge balance. It avoids the challenge faced by conventional Sr adsorbents where elution ions need to directly compete with Sr2+ ions for adsorption sites during elution. However, as of now, no ion exchanger with such distinct binding sites for Sr2+ ions and elution ions has been identified as suitable for treating actual or simulated nuclear liquid waste.

Traditional ion exchangers with the same capture sites for target and elution ions in the channel result in slow dynamics, poor selectivity, and non-recyclability. Proposed giant cluster framework ion exchanger with specific capture sites for target and elution ions in the cluster itself, showing fast dynamics, high selectivity, and recyclability.

Giant high-nuclearity polyoxometalates (POMs), constructed from small POM cluster subunits, usually possesses distinct metal coordination sites due to the ease of forming diverse cavities between adjacent POM subunits.14,15 The distinct metal coordination sites of giant POMs have the great potential to show high recognition and selectivity for target ions and elution ions, which is difficult to achieve with traditional metal ion exchangers.16,17 Further, inorganic metal ion exchanger with high-dimensional framework structures often encounter a common problem that limits their metal ion separation performance, namely, the initially trapped metal ions within the pores or interlayers tend to block the transport channels for subsequent metal ion loading (Fig. 1, Video 5).18 Giant POM-based high-dimensional inorganic metal ion exchangers are promising candidates to overcome this issue and exhibit excellent metal ion separation performance, as their metal-binding cavities are located on the giant POMs rather than within the transport channels, such as pores or tunnels, of the frameworks.

Based on the concept of the site differentiation strategy and the consideration of metal ion transport channels, we chose the recently reported all-inorganic giant polyoxoniobate (PONb)-based three-dimensional (3D) framework material H23In3[Ba8⊂Dy12Nb12O36(H2O)24⊂Na6K6Ba6(Nb6O19)12]·101H2O (FZU-1) as a proof-of-concept ion exchanger.17 This choice is due to its 3D porous structure that favors the transport of metal ions, and more importantly, the giant building units PONbs possess two types of distinct metal coordination sites. We conducted a rigorous and comprehensive investigation to evaluate its performance in capturing and eluting Sr2+ ions. Encouragingly, the results demonstrate that FZU-1 is an exceptionally superior Sr2+ ion exchanger. On the one hand, it can rapidly and highly selectively remove Sr2+ ions from simulated high-alkalinity nuclear liquid waste or contaminated groundwater through its Sr2+-selective loading sites, whose performance outperforms the commercially available Sr adsorbent IE-911. On the other hand, it can achieve rapid elution of Sr2+ ions by capturing Na+ ions at its Na+-selective loading sites, enabling the recycling of the ion exchanger. To our knowledge, this is the first reported case of a 90Sr adsorbent capable of achieving high selectivity and efficiency in capturing Sr2+ under high salt caustic conditions, while also being easily eluted and reused, rendering it highly potential for practical applications.

FZU-1 has been known, but in order to clarify the ion exchange process and mechanism based on the site differentiation strategy proposed in this work, we hereby provide a necessary brief description of its structure. FZU-1 crystallizes in a highly symmetric cubic space group Im-3 and can be easily prepared by hydrothermal reaction. The phase purity of the synthesized sample was confirmed by the combination of powder X-ray diffraction (PXRD) and inductively coupled plasma (ICP) analysis (see the Experimental Section and Supplementary Figs. S1a, 2 in the Supporting Information). The 3D framework of FZU-1 possesses an uninodal 6-connected pcu-type topology based on giant {Ba8 ⊂Dy12Nb12O36(H2O)24⊂Na6K6Ba6(Nb6O19)12} (Dy12Nb84) PONb nanoclusters (ca. 2.5 nm) as nodes and [In(H2O)2]3+ cations as linkers (Fig. 2a, b). The giant Dy12Nb84 can be briefly described as a 3-shell structure (Supplementary Fig. S3). The innermost shell is a cubic cage formed by eight Ba2+ atoms bridged by oxygen atoms. The second shell is a sodalite-type cage formed by 12 Dy3+ and 12 Nb5+ atoms alternately connected through oxygen atoms. The outermost shell consists of 12 Lindqvist-type {Nb6O19} (Nb6) clusters, each of which face-shares with a NbO6 octahedra and corner-shares with 3 DyO8 bicapped trigonal prisms from the second shell.

a View of the giant cluster framework formed by Dy12Nb84 SBUs. b Dy12Nb84 SBU. c View of the tweezer and concavity site in SBU. d View of the coordination geometries for the tweezer and concavity site during the Sr uptake and elution process.

FZU-1 is chosen as the proof-of-concept ion exchanger for exploring the separation of Sr2+ ions through site differentiation strategy due to its many remarkable structural characteristics as follows. To begin with, the Dy12Nb84 subunit features two distinct metal capture sites, namely the concavity site (○) and the tweezer site (△), which have different recognition and selectivity for target ion Sr2+ and elution ion Na+, respectively (Fig. 2c). Secondly, every eight Dy12Nb84 subunit enclose a nanoscale cubic cage with a free diameter of ca. 2.5 nm, creating a porous 3D framework and resulting in a 2-fold interpenetrated structure (Fig. 2a). Even with the interpenetrated structure, FZU-1 is still porous with a large guest-accessible volume of 7252.3 Å3 per unit cell (35.1% of the total unit cell volume), enabling efficient ion transport within its framework. Thirdly, the interpenetrated framework endows FZU-1 with high structural stability. For instance, FZU-1 exhibits robust alkali stability across a wide pH range of 4–14 (Supplementary Fig. S1b) as well as redox inertness, guaranteeing its long-term operation under harsh conditions. Finally, the high structural stability of FZU-1 enables its Sr2+ ion exchange to occur in a single-crystal-to-single-crystal (SCSC) manner while maintaining high crystallinity (Supplementary Table S1), thereby allowing for the acquisition of high-quality single-crystal X-ray diffraction (SXRD) data that provides atomic-level insights into clarifying the selective Sr2+ uptake and elution mechanism.

The twelve outermost Nb6 clusters of Dy12Nb84 form six pairs of tweezer sites (△) (Fig. 2b, c), each pair of which clamps a Na+ ion through six bridging oxygen atoms with a Na-O bond length range of 2.323(12)–2.337(11) Å (Fig. 2d). Spatial constraints restrict the tweezer sites to accommodate only Na+ ions from alkali metal ions. Adjacent to each tweezer site, there are two concavity sites (○) that are symmetrically distributed and enclosed by three neighboring Nb6 clusters (Fig. 2b, c). This configuration results in a total of 12 such concavity sites on each Dy12Nb84, capturing 6 K+ and 6 Ba2+ ions (Supplementary Fig. S4). Notably, there is a substitution and positional disorder of the K+ and Ba2+ ions in the concavity sites, meaning that each concavity site is occupied by 0.5 K and 0.5 Ba. Each K+ or Ba2+ ion connects four or three O atoms from adjacent Nb6 clusters with bond length ranges of 2.826(14)–3.211(1) Å and 2.819(2)–2.980(9) Å, respectively (Supplementary Fig. S5a). Note that due to the high degree of disorder and high crystal symmetry, the coordinated water molecules of the K+ and Ba2+ ions cannot be accurately determined. To facilitate the understanding of the following ion-exchange study and site differentiation strategy, we represent the formula of Dy12Nb84, devoid of Na+ ions in the tweezer sites and K+ and Ba2+ ions in the concavity sites, as {Ba8⊂Dy12Nb12O36(H2O)24⊂△6○12(Nb6O19)12}.

Considering that the Sr-O bond length is comparable to the K-O and Ba-O bond lengths, the concavity sites are also suitable for capturing Sr2+ ions. Therefore, the recognition and selective capture of Sr2+ ions through the concavity sites of Dy12Nb84 is explored. As expected, FZU-1 can undergo a SCSC ion-exchange reaction in Sr2+ solution, giving a Sr-uptaked PONb, H23In3{Ba8⊂Dy12Nb12O36(H2O)24⊂△6Sr12(Nb6O19)12}·42H2O (FZU-1Sr). PXRD, IR, UV-Vis, and ICP data show that FZU-1Sr exhibits high crystallinity and is isostructural with FZU-1 (Supplementary Figs. S1a, S6, S7), confirming a topotactic ion exchange. Furthermore, SXRD analysis reveals that, after the ion exchange, all the Na+ ions located in the tweezer sites have been leached out, resulting in the formation of vacancies, and the distance between the tweezer sites decreased from 7.913(16) Å to 7.819(12) Å, indicating that all-inorganic tweezers have adjustable openings. At this time, all the K+ and Ba2+ ions present at concavity sites have been replaced with Sr2+ ions (Supplementary Fig. S5b). Similarly, the Sr2+ ions also exhibit positional disorder on the concavity sites, with Sr–O bond lengths ranging from 2.763(12)–3.131(7) Å (Fig. 2d). To evaluate the capture ability of FZU-1 toward Sr2+, ion-exchange kinetics are performed in neutral aqueous solution at a volume-to-mass ratio (V:m) of 1 L/g in a single static equilibration. The kinetics experiments (Fig. 3a and Supplementary Table S2) clearly show that the removal rates of Sr2+ rapidly reach 89.3%, 90.5%, and 92.2% within 5 min, with initial concentrations of ca. 20, 30, and 40 ppm in solutions, respectively. This indicates that the concavity sites on Dy12Nb84 subunits efficiently bind Sr2+ ions without blocking the ion transport channels within the framework, thereby facilitating rapid Sr2+ uptake. After 24 h, the removal rates of Sr2+ reach above 99.0%. Fitting of the adsorption kinetics reveals that the adsorption process follows the pseudo-second-order model (Fig. 3a), suggesting that the binding of Sr2+ ions occurs through chemical adsorption, aligning with the SXRD data.

a The kinetics curve of Sr2+ removal. b Sr2+ adsorption isotherm. The line represents the fitting of the data with the Langmuir isotherm model. c Plot of Sr2+ removal and KdSr vs. the initial concentration of Sr2+. d Kd values of Sr2+ removal at various pH values from 4 to 14. e Comparison of the Kd and removal of FZU-1 with those of other reported materials to a simulated NCAW waste. f The removal rate and leach rate in a simulated neutral Sr-contaminated groundwater. g Binding energies of metal ions to concavity and tweezer site, respectively. h Diagram of the ion exchange mechanism of Sr uptake. Error bars present the standard deviation of the mean of three measurements.

The adsorption isotherm is conducted at concentrations of Sr2+ ranging from 10.3 to 510 ppm (Fig. 3b and Supplementary Table S4). Compared with the Freundlich isotherm model (R2 = 0.823, Supplementary Fig. S9), these data better fit the Langmuir isotherm model (R2 > 0.872, Supplementary Table S5), indicating that the adsorption of Sr2+ ions by FZU-1 is primarily monolayer-based, which is also consistent with the structure of FZU-1Sr resolved by SXRD. The poor fitting R2 value may be due to the different adsorption energies of the two disorder sites at the concavity site, resulting in a deviation from the classical adsorption model, which was commonly found in many new adsorbents.19,20,21 According to the Langmuir isotherm model, Langmuir constant b is related to the free energy of the adsorption.22 The calculated b value of Sr2+ is 7.02 ± 2.12 L/mg, which exceeds most of reported Sr2+ adsorbents (Supplementary Table S6), indicating strong Sr2+ uptake ability of FZU-1. The maximum adsorption capacities qm is calculated to be 62.8 ± 3.3 mg g−1 (Supplementary Table S6), which is consistent with the theoretical value (62.6 mg g−1), illustrating that the concavity sites of FZU-1 are fully utilized during the adsorption process. The Sr2+ ion adsorption capacity compares well with those of the best Sr2+ adsorbents (qmSr = 40–180 mg/g) (Supplementary Table S6 and Supplementary Fig. S10).11

The affinity and selectivity of the material for Sr2+ can be expressed in terms of the distribution coefficient Kd. Generally, an ion exchanger with Kd > 104 mL/g is regarded as an excellent adsorbent.13 As can be seen in Fig. 3c, all the Sr2+ removal rates are as high as about 99% with KdSr of 104 mL/g level over a wide range of initial concentrations from 28.4 ppb to 47.8 ppm (Supplementary Table S7). Taking into account that the pH of nuclear waste and contaminated groundwater may vary from acidic to extremely alkaline,19 the effect of pH values ranging from 4 to 14 on the Sr2+ uptake is studied. The pH values of Sr2+ solutions are adjusted by HCl and NaOH. As shown in Fig. 3d, FZU-1 maintains a removal rate of over 99% for Sr2+ with a KdSr exceeding 105 mL/g across a wide pH range of 4–14 (Supplementary Table S8). These KdSr values reveal the remarkable pH stability and selectivity of FZU-1 toward Sr2+, regardless of Sr2+ concentration or interference from cations such as H+ or Na+. Importantly, under conditions close to alkaline nuclear waste (with a pH value of 14, 5.0 M Na+ and Sr2+ of 35 ppb), FZU-1 can remove 98.9% of Sr2+ with a KdSr value as high as 92,421 mL/g, demonstrating its promising Sr2+ selectivity among the state-of-the-art inorganic adsorbents (Supplementary Table S9 and Supplementary Fig. S11).4,5,6

Motivated by the promising stability and selectivity of FZU-1, we further evaluated the removal of Sr2+ from Hanford tank waste (NCAW) simulant, which is prepared according to information supplied by Pacific Northwest National Laboratory (PNNL, see detail composition in Supplementary Table S10).5 The discernible triphasic system and the extremely high pH value of 14.4 (Supplementary Fig. S12) indicate that the simulated NCAW is highly similar to the actual 102-AZ tank with high salinity and extreme alkalinity. As shown in Fig. 3e, FZU-1 can remove 98.9% Sr2+ with a KdSr of 96,900 mL/g at a V:m of 1 L/g, which outperforms known commercially available Sr adsorbent, such as IE911, NaTi and NaTS,4,5 for use in the treatment of alkaline tank wastes. Moreover, FZU-1 exhibits poor selectivity towards Cs+ with the removal of 3.6% and a KdCs of 36.9 mL/g, leading to a high separation factor SFSr/Cs of 2628.9 (Supplementary Table S11). The results indicate that the FZU-1 could be a promising candidate for the removal of Sr from alkaline tank wastes.

The removal of trace contaminants from water is a highly fascinating yet challenging research topic, which has been recognized as one of the seven chemical separations that can change the world.23 To this end, we further investigate the performance of FZU-1 in removing trace Sr2+ ions from simulated Sr2+ contaminated groundwater. The simulated Sr2+ contaminated groundwater contains 41.1 ppb Sr2+ ions and relatively high concentrations of competing ions, including 131.3 ppm Na+, 24.1 ppm Ca2+, 10.1 ppm Mg2+, and 7.3 ppm K+.6 As indicated in Fig. 3f, FZU-1 successfully eliminates 98.6% of trace Sr2+ from simulated groundwater. By contrast, the removal rates for competing ions such as Mg2+ and Ca2+ are only 49.3% and 42.6%, respectively. A comparison of the Sr2+ removal ability of FZU-1 with those of other Sr2+ adsorbents is provided in Supplementary Fig. S13. Again, FZU-1 performs nearly as effectively as excellent inorganic adsorbents.4,5,6 This result shows that FZU-1 can also be used as a Sr2+ adsorbent for the selective removal of Sr2+ from accidentally contaminated groundwater.

To gain deeper insights into the high selectivity of the concavity site towards Sr2+, it is imperative to ascertain the adsorption sites of FZU-1 concerning alkaline earth metal ions Mg2+, Ca2+, and Ba2+, along with establishing the affinity order of the concavity site for competing ions. The SCSC ion exchange between FZU-1 and Mg2+, Ca2+, and Ba2+ are conducted to produce three new compounds of FZU-1Mg, FZU-1Ca, and FZU-1Ba, respectively (Supplementary Table S1). SXRD and ICP data offer atomic-level insights into ion exchange behaviors. Just like FZU-1Sr, all the Na+ ions in FZU−1Mg, FZU−1Ca, and FZU-1Ba are extracted, forming empty tweezer sites (Supplementary Fig. S5c–e). However, the K+ and Ba2+ ions located in the concavity sites exhibit different exchange outcomes. For FZU-1Mg, it is observed that the captured Mg2+ ions bond with the terminal oxo atoms of Dy12Nb84, with a bond length of 1.965(26) Å (Supplementary Fig. S5c). This means that Mg ions neither occupy concavity sites nor displace K+ and Ba2+ ions. While, in FZU-1Ca, all K+ and two-thirds of Ba2+ ions located at the concavity sites are replaced by Ca2+ ions, with the Ca–O bonds ranging from 2.930(15) to 3.025(16) Å (Supplementary Fig. S5d). Furthermore, in FZU-1Ba, it can be observed that all K+ ions are replaced by Ba2+ ions, while the length of the Ba-O bond remains almost unchanged (Supplementary Fig. S5e). The finding indicates that concavity sites can bind K+, Ca2+, Sr2+, and Ba2+.

Based on the well-defined geometric structures, the calculated average binding energies of the concavity sites towards K+, Ca2+, Sr2+, and Ba2+ ions are 824.7, 1092.0, 1501.1 and 1173.9 kJ/mol, respectively (Fig. 3g and Supplementary Fig. S14). This suggests that the ion affinity order of the concavity sites towards these ions is Sr2+ > Ba2+ > Ca2+ > K+. This result is consistent with the experimental phenomena of leaching kinetics, showing that the leaching rates of initial Na+, K+, and Ba2+ within 5 min are 51.0%, 64.9%, and 12.5%, respectively (Supplementary Table S15 and Supplementary Fig. S16).

The analysis of electrostatic potential indicates that as Sr2+ ions approach the Dy12Nb84 cluster, the electrostatic potential of the cluster increases, which favors the initial ion leaching (Supplementary Fig. S15). Therefore, the Sr2+ uptake process can be regarded as a two-step ion exchange process (Fig. 3h), which starts with the approach of Sr2+ to the Dy12Nb84 subunit, increasing the electrostatic potential of the cluster. Subsequently, the leaching of Na+, K+, and Ba2+ occurs, leaving partial vacancies at both the tweezer and concavity sites. Finally, based on selectivity, Sr2+ is sequentially absorbed into the concavity sites.

The key to achieving efficient and large-scale reduction of liquid waste lies in the recyclability and reusability of adsorbents. However, as previously mentioned, the significant challenge remains in developing adsorbents capable of efficiently capturing Sr²⁺ ions while facilitating rapid elution for reuse. Given the existence of vacant tweezer sites within the FZU-1Sr and their exclusive affinity for Na+ ions, we further investigated the feasibility of achieving Sr2+ ion elution from the concavity sites through the capture of Na+ ions by the tweezer sites, based on the concept of site differentiation strategy. For this purpose, desorption kinetics studies are performed on sample FZU-1Sr, which had attained its maximum Sr adsorption capacity, utilizing a 3 M NaCl as elution solution at a V:m ratio of 1 L/g under single static equilibrium conditions. As shown in Fig. 4a and Supplementary Table S16, the kinetics experiments reveal a rapid increase in Sr2+ leaching rates, reaching 39.5% within 5 min and rising to 59.5% after 1 h. After 24 h, the leaching rate reaches 76.6%, leaving only 23.4% of Sr2+ within FZU-1. The kinetic curve can be well fitted by a pseudo-second-order model, with an apparent rate constant of 9.78 × 10–4 g·mg–1·min–1, and high correlation coefficients R2 of 0.9959. The SXRD data of the crystal (denoted as FZU-1SrNa, Supplementary Table S1) after Sr2+ ion elution verifies that all tweezer sites of each Dy12Nb84 subunit have re-captured Na+ ions, and each Na+ ion has reconstructed a 6-coordinate NaO6 trigonal prism with Na-O bond lengths in the range of 2.352(9)–2.366(6) Å (Supplementary Fig. S5f). In addition, approximately four Sr2+ ions (33.3% of the original amount) persist within the concavity sites of each Dy12Nb84 subunit, which is near the results of desorption kinetics studies.

a The kinetics curve of Sr2+ leach rate. Recyclability for Sr uptake from a ~ 40 ppm aqueous solution using (b) Na+ and (c) K+ as elution ions. Error bars present the standard deviation of the mean of three measurements. Diagram of the Sr elution mechanism using (d) Na+ and (e) K+, respectively.

The combination of the above adsorption-desorption kinetics research, well-defined structures, and theoretical calculations reveal that the FZU-1 efficiently entraps Sr2+ ions within the concavity sites of Dy12Nb84 subunits, while simultaneously leaching Na+ ions from the tweezer sites through the electrostatic potential generated by the captured Sr2+ ions. Conversely, during elution, the FZU-1Sr selectively captures Na+ ions at the tweezer sites, facilitating the efficient removal of Sr2+ ions from the concavity sites due to the additional positive charge imparted by the Na+ ions (Fig. 4d). The main reasons why ~ 23% of Sr2+ ions remain in the FZU-1SrNa are that, firstly, the number of tweezer sites is less than that of concavity sites. Secondly, the positive charge of Na+ ions is lower than that of Sr2+ ions. Finally, although Na+ ions can also enter the concavity sites, they need to compete with Sr2+ ions that have strong interactions between Sr2+ ions and the concavity sites, resulting in a competitive balance (Fig. 3g). Fascinatingly, despite the retention of 20% Sr2+ ions, it does not affect the removal performance of the reused FZU-1SrNa towards Sr2+ ion solution. As shown in Fig. 4b and Supplementary Table S20, the FZU-1SrNa can remove 99.9% of Sr2+ from a 41.5 ppm Sr2+-contaminated solution with a KdSr of 105 mL/g in the first cycle. Even after eight cycles of Sr2+ adsorption and desorption, FZU-1SrNa can still maintain an ultra-high Sr2+ removal rate of 99.9%.

To further confirm the critical mechanism enabling effective elution of Sr²⁺ ions from FZU-1Sr, as our proposed site differentiation strategy, a comparative study is conducted on the Sr²⁺ ion elution performance of FZU-1Sr using a K+ ion solution. The choice of K+ ion is based on the following considerations. Firstly, the high-selectivity tweezer sites cannot accommodate K+ ions, thus eliminating the site specificity effect. Secondly, the binding energy between the concavity sites and K+ ions is stronger than that of Na+ ions (Fig. 3g), making K+ ions more favorable for competing with Sr2+ ions compared to Na+. Thus, if the site differentiation strategy does not work, theoretically, the K+ ion should have a higher Sr2+ ion leaching rate than the Na+ ion. Upon elution using 3 M KCl solution under the above same condition, the leaching rates of Sr2+ are 25.5% and 31.0%, after 5 min and one hour, respectively (Supplementary Fig. S19). The maximum leaching rate of Sr2+ is 51.6%, consistent with the SXRD data of FZU-1SrK (Supplementary Table S1 and Supplementary Fig. S5g), which is significantly lower than the elution effect (76.6%) of 3 M NaCl solution on FZU-1Sr. Obviously, K+ ions can only compete with Sr2+ ions for the occupation of the concavity sites (Fig. 4e), while Na+ ions can not only compete with Sr2+ ions for the occupation of the concavity sites but also enter the tweezer sites without directly competing with Sr2+ ions. This increases the positive charge of the Dy12Nb84 cluster, indirectly leading to the elution of more Sr2+ ions. The results clearly demonstrate that the site differentiation strategy can effectively elute Sr2+ ions from FZU-1Sr. In addition, although K+ ions can achieve 51.6% desorption of Sr2+ ions in FZU-1Sr, the Sr2+ ion recycling removal experiment for the same Sr2+-contaminated solution as mentioned above shows that the removal rate of Sr2+ is 66.4% at the first cycle, declining to as low as 56.6% after three cycles (Supplementary Table S21 and Supplementary Fig. 4c). Its cyclic ability is also significantly worse than the sample eluted with Na+ ions (maintaining a removal rate of 99.9% after eight cycles), demonstrating that the site differentiation effect plays an important role on the recyclability of FZU-1.

In summary, we have proposed for the first time the application of site differentiation strategy in ion exchange, that is, functional materials with different adsorption sites for different metal ions can achieve efficient uptake and elution of specific ions. Based on this strategy, we conducted an in-depth study at the atomic level on the adsorption and desorption properties of a crystalline all-inorganic PONb framework, FZU-1, towards hazardous radioactive Sr2+ ions, which possesses site differentiation functionality. The results indicate that FZU-1 can not only achieve rapid, selective, and enduring uptake and elution of Sr2+ ions but also enable efficient recycling of the material. Specifically, FZU-1 can remove 98.9% of Sr2+ from the simulated Hanford tank waste, performing best among reported and commercial Sr2+ adsorbents, and is also an effective Sr2+ remover for groundwater remediation. Additionally, FZU-1 exhibits recyclability and robust cyclic usability, potentially supplanting existing non-elutable ion exchangers and revolutionizing outdated liquid waste treatment methods. As shown in Supplementary Fig. S20, FZU-1 efficiently captures Sr2+ ions from passing tank alkaline supernatant or contaminated groundwater through an ion exchange process, followed by subsequent liquid processing via established procedures. Unlike current adsorbents saturated with Sr2+, which necessitate awaiting uncertain disposal, a 3 M Na+ solution can effectively elute Sr2+ from FZU-1, generating a concentrated, low-volume Sr-rich stream destined for the HLWs facility. FZU-1 presents an exceptional Sr2+ ion adsorbent, and we will further assess its potential for practical application in the future. The site differentiation strategy provides a promising approach for the designed synthesis and development of efficient and recyclable new-type adsorbents for specific ion capture.

K7H[Nb6O19]·13H2O (Nb6) precursor was synthesized on the basis of the literature method.24 Other chemicals were used as purchased without further purification. Water was deionized and distilled before use.

Infrared (IR) spectra (KBr pellet) were performed on an Opus Vetex 70 FT-IR infrared spectrophotometer in the range of 400–4000 cm−1. UV-vis spectra were performed on a Shimadzu UV-2600 UV-visible spectrophotometer by using the BaSO4 as the blank. Powder X-ray diffraction (PXRD) patterns were recorded on an Ultima IV diffractometer with Cu-Kα radiation (λ = 1.54056 Å) in the range of 5–50°. The simulated PXRD pattern was derived from the Mercury Version 4.3.0 software using the X-ray single crystal diffraction data. Elemental mapping was performed on a FEI Talos F200S G2 microscope operated at 200 KV. The pH values of all solutions were determined by Mettler Toledo FiveEasy Plus.

ICP analyses were conducted on a Shimadzu ICPE-9820 (ICP-OES), and XSeries II (ICP-MS). The crystal samples were vacuum-dried at 60 °C for one day before ICP testing. Each data represents the average value of three measured values.

(1) For solid crystal samples, we added 5 mg of the sample to a mixed solvent containing 1 mL of 40% HF, 2 mL of 68% nitric acid, and 2 mL of ultrapure water. The mixture was digested in a microwave digestion instrument for 40 min. The resulting solution was then transferred to a PTFE standard volumetric flask and diluted to 100 mL with a 2% nitric acid solution for testing. The element content (Y) of the sample can be calculated by Y(%) = (x (mg/L) × 0.1 L ÷ 5 mg) × 100%, where x is the testing mass concentration based on 5 mg sample and 100 mL solution.

(2) For solution samples, filter the solution (y L) through a 0.22-micron filter membrane and transfer it to a PTFE standard volumetric flask. Then, dilute the solution to 100 mL using a 2% nitric acid solution for testing. The concentration of ions (C) of solution can be calculated by C (mg/L) = x (mg/L) × y L ÷ 0.1 L, where x is the testing mass concentration based on y L solution sample with diluting to 100 mL.

The FZU-1 was synthesized by the method of literature (Supplementary Fig. S2).17 A mixture of K7HNb6O19·13H2O (0.375 g, 0.274 mmol), BaCl2·2H2O (0.083 g, 0.340 mmol), Na2CO3 (0.094 g, 0.887 mmol), Li2B4O7 (0.086 g, 0.508 mmol), Tris(hydroxymethyl)aminomethane (Tris) (0.085 g, 0.702 mmol), In(NO3)3·xH2O (0.109 g, 0.362 mmol) and Dy(NO3)3·6H2O (0.087 g, 0.191 mmol) were added into 5 mL deionized water. After being stirred for one hour, the mixture solution was sealed in a Teflon-lined autoclave (23 mL) and heated at 140 °C for 3 days. After the resulting mixture solution slowly cooled down to room temperature (RT), colorless block crystals of FZU-1 were obtained by filtration, ultrasonic washed with H2O (to wash off a few amorphous white powders), and air-dried. The crystals should be picked out manually before experiments. Yield: 30 mg (8% based on K7HNb6O19·13H2O).

FZU-1Mg, FZU-1Ca, FZU-1Sr, FZU-1Ba, FZU-1SrNa, and FZU-SrK were obtained by SCSC ion exchange. The metal salts used for the ion exchange were their corresponding metal chloride salts. These compounds can be formulated as:

Each 20 mg crystals of FZU-1 were immersed in 10 mL 0.1 mol·L–1 chloride solutions of Mg2+, Ca2+, Sr2+, and Ba2+, at RT for 24 h to get colorless crystals of FZU-1Mg, FZU-1Ca, FZU-1Sr and FZU-1Ba, respectively. Notably, FZU-1SrNa was obtained as colorless crystals after drying the crystals of FZU-1Sr and placing them in 3 M NaCl solution for 24 h. FZU-1SrK were obtained as colorless crystals after drying the crystals of FZU-1Sr and placing them in 3 M KCl solution for 24 h.

Single-crystal X-ray diffraction measurements were carried out on a Bruker APEX II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 175(2) K. Intensity data were collected using ω-scan techniques and corrected for Lorentz and polarization (Lp) effects. The structures were solved using direct methods and refined by full-matrix least-squares on F² with anisotropic thermal parameters for all atoms, employing the Siemens SHELXTL™ Version 5 crystallographic software package.

Adsorption experiments were performed by the batch methods at RT. The V:m in all experiments was 1 L/g. FZU-1 were immersed in the solution of Sr2+. After stirring the mixture for required contact time, the supernatants were removed and filtered with syringe filters with a pore size of 0.22 μm. The concentrations of ions were determined by ICP-OES or ICP-MS.

Adsorption experiments of various reaction times were performed. 10 mg powder of FZU-1 were immersed into 10 mL aqueous solution with initial concentration of 39.3 ppm, 31.3 ppm and 19.4 ppm Sr2+ and shaken. The supernatants were taken at different time (0, 5, 10, 20, 30, 60, 90, 120, 240, 480, 720, 1440 min) from the reaction solution. The concentrations of Sr2+ in aqueous solution were determined by ICP-OES. ICP test results are summarized in Supplementary Table S2.

10 mg powder of FZU-1 was immersed into 10 mL Sr2+ aqueous solution with different initial concentrations and then shaken. The initial Sr2+ concentrations were in the ranges of 10.3–510 ppm, respectively. The adsorption experiments lasted for 24 h contacting time at RT. ICP test results are summarized in Supplementary Table S4.

The Sr2+ solutions with different pH values from 4.1 to 13.8 were prepared with hydrochloric acid or sodium hydroxide; the initial concentrations of Sr2+ was 3.82–21.1 ppm. 10 mg powder of FZU-1 were immersed into 10 mL Sr2+ aqueous solutions with different pH values and shaken for 24 h at RT. ICP test results are summarized in Supplementary Table S8.

We configured an alkaline solution with a similar condition according to the literature by adding NaCl and NaOH.6 The initial concentration of Sr2+ was 35.5 ppb; the initial concentration of Sr2+ reported in the literature was 35 ppb. 10 mg powder of FZU-1 was immersed into 10 mL Sr2+ aqueous solutions and shaken for 12 h at RT. ICP test results are summarized in Supplementary Table S9.

We prepared the NCAW simulated waste solution based on literature.5 The initial concentration of Sr2+ was 28.4 ppb. 10 mg powder of FZU-1 were immersed into 10 mL Sr2+ aqueous solutions and shaken for 24 h at RT. ICP test results are summarized in Supplementary Table S11.

Considering the possibility of groundwater contamination due to the leakage of the radioactive Sr2+. We simulated FZU-1 to test the adsorption effect on simulated neutral groundwater contaminated with Sr2+. The aqueous solution containing 133.3 ppm Na+, 6.2 ppm K+, 9.2 ppm Mg2+, 25.1 ppm Ca2+, and 41 ppb Sr2+ was considered as a simulated neutral contaminated groundwater. 10 mg powder of FZU-1 was immersed into 10 mL aqueous solution and shaken for 12 h at RT. ICP test results are summarized in Supplementary Tables S12 and S13.

Considering that Na+, K+, and Ba2+ in the crystals will be released in the adsorption procedure. The 10 mg powder of FZU-1 was immersed into 10 mL aqueous solution with an initial concentration of 39.9 ppm Sr2+ and then shaken at different times (0, 5, 10, 20, 30, 60, 90, 120 min) at RT. Through ICP test results to detect the changes of Na+, K+, Ba2+, and Sr2+ in solution (Supplementary Fig. S7). ICP test results are summarized in Supplementary Table S15.

Desorption experiments of FZU-1Sr were performed. The 10 mg powder of FZU-1Sr were immersed into 10 mL 3 M NaCl or KCl solution and shaken. The supernatants were taken at different time (0, 5, 10, 20, 30, 60, 90, 120, 240, 480, 720, 1440 min) from the reaction solution. The concentrations of Sr2+ in 3 M NaCl solution were determined by ICP-OES. ICP test results are summarized in Supplementary Table S16 and Table S18.

Firstly, we prepared 41.5 ppm Sr2+ solution, then 150 mg FZU-1 was immersed into 100 mL Sr2+ aqueous solutions for 24 h at RT. The material was separated by centrifugation, rewashed with ultrapure water, and then added to 150 mL of 3 M NaCl solution, followed by shaking for 24 h. After shaking, the material was centrifuged again, washed with ultrapure water, then dried and weighed, and placed again in Sr2+ solution with an initial concentration of 41.5 ppm, and the cycle was repeated eight times in the same manner. ICP test results are summarized in Supplementary Table S20.

Consistent with the process of Na ion elution described above, the eluent was changed to 3 M KCl solution. ICP test results are summarized in Supplementary Table S21.

The removal rate R (%) is to evaluate the removal percentage of target ions

where C0 (mg/L) is the initial concentration of ions and Ce (mg/L) is the equilibrium concentration of ions.25

Since the counter cations in the crystals are leached during adsorption, the leaching rate can be wroten as

where C0 (mg/L) is the initial concentration of ions and Ct (mg/L) is the mass of ions absorbed at time t. V (mL) represents the volume of the solution. m (g) represents the mass of the adsorbent. w% means the mass percentage of the corresponding element in the crystals.

According to the pseudo-first-order kinetics,26

where qt (mg/g) is the mass of ion absorbed at time t, qe (mg/g) is the mass of ion absorbed at equilibrium, and k1 is the rate constant of pseudo-first-order (min–1), also it can be derived as:

integrating this with the boundary conditions t = 0 to t = t and qt = 0 to qt = qt:

gives

The half-life t1/2 can be expressed as

According to the pseudo-second-order kinetics,27

organize it and obtain

integrating it with the boundary conditions t = 0 to t = t and qt = 0 to qt = qt:

gives

where qt (mg/g) is the mass of ion absorbed at time t, qe (mg/g) is the mass of ion absorbed at equilibrium, and k2 is the equilibrium rate constant of pseudo-second-order (g·mg–1·min–1).

The value of adsorption capacity q (mg/g) can be obtained at equilibrium concentration Ce (mg/L).

The isotherm equilibrium of Langmuir and Freundlich are described,22 respectively.

Langmuir model:

Freundlich model:

The b (L/mg) is a constant related to the free energy. The KF and n are related to the adsorption capacity and the adsorption intensity in the Freundlich constants, respectively.

where the value of ion exchange capacity q (mg/g) is obtained at an adsorption equilibrium.25

The separation factor (SF) is a measure of the ability of the adsorbent to separate one of two metal ions from another.25

In this equation, \({K}_{d}^{A}\) and \({K}_{d}^{B}\) represent the distribution coefficients of A and B ions, respectively.

All calculations were based on the GFN1-xTB method in the CP2K version 9.1 software.28 The calculated models (Supplementary Table S14 and Supplementary Fig. S14) of binding energies between Dy12Nb84 cluster and cations including:

Concavity site

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○12(Nb6O19)12} (Dy12Nb84),

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○11K(Nb6O19)12} (K@Dy12Nb84),

Mg {Ba8⊂Dy12Nb12O36(H2O)24⊂△6○12(Nb6O19)12} (Mg@Dy12Nb84),

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○11Ca(Nb6O19)12} (Ca@Dy12Nb84),

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○11Sr(Nb6O19)12} (Sr@Dy12Nb84),

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○11Ba(Nb6O19)12} (Ba@Dy12Nb84),

{Ba8⊂Dy12Nb12O36(H2O)24⊂△6○11Na(Nb6O19)12} (NaC@Dy12Nb84)

Tweezer site

{Ba8⊂Dy12Nb12O36(H2O)24⊂Na△5○12(Nb6O19)12} (NaT@Dy12Nb84),

were extracted from FZU-1, FZU-1Mg, FZU-1Ca, FZU-1Sr, FZU-1Ba, and FZU-1SrNa without optimizing. The binding energy can be decomposed as

Notes: Due to the positional disorder of metal ions in the concave site, the binding energy obtained is their average value.

The electrostatic potential (ESP, Supplementary Fig. S16) calculations were based on the models where the Na+ ion acted as a probe ion near the Dy12Nb84 cluster.

The entries of CCDC 2164420, 2164421, 2164422, 2164423, 2329271, and 2346860 contain the supplementary crystallographic data for FZU-1Mg, FZU-1Ca, FZU-1Sr, FZU-1Ba, FZU-1SrNa and FZU-1SrK. These data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving. HTML or from the Cambridge Crystallographic Data Center, 12, Union Road, Cambridge CB2 1EZ, U.K. Fax: (Internet) + 44-1223/336-033. E-mail: [email protected]. All the data supporting the findings of this study are available within the article and its Supplementary Information and also from the corresponding authors upon reasonable request. The authors declare that all data supporting the findings of this study are available within the article (and Supplementary Information Files), or available from the corresponding author on request. Source data are provided in this paper.

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We greatly thank the financial support from the National Natural Science Foundation of China (NSFC) and the Key Program of the Natural Science Foundation (NSF) of Fujian Province. C.S. was supported by the NSFC with No. 22371045, and S.-T.Z. was supported by the NSFC with No. 22371046, and the NSF of Fujian Province with No. 2021J02007.

These authors contributed equally: Yi-Xin Liu, Ping-Xin Wu.

Fujian Provincial Key Laboratory of Advanced Inorganic Oxygenated Materials, College of Chemistry, Fuzhou University, Fuzhou, Fujian, China

Yi-Xin Liu, Ping-Xin Wu, Jing-Yi Dai, Ping-Wei Cai, Cai Sun & Shou-Tian Zheng

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S. C. and S.-T.Z. conceived the idea and prepared the manuscript. Y.-X. L. conducted the characterizations and detecting tests. P.-X.W., J.-Y.D., and P.-W.C. contributed to the discussion. All authors have given approval to the final version of the manuscript.

Correspondence to Cai Sun or Shou-Tian Zheng.

The authors declare no competing interests.

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Liu, YX., Wu, PX., Dai, JY. et al. Site differentiation strategy for selective strontium uptake and elution within an all-inorganic polyoxoniobate framework. Nat Commun 15, 8896 (2024). https://doi.org/10.1038/s41467-024-53130-7

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Received: 01 July 2024

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Published: 15 October 2024

DOI: https://doi.org/10.1038/s41467-024-53130-7

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Site differentiation strategy for selective strontium uptake and elution within an all-inorganic polyoxoniobate framework | Nature Communications