Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step

doi:10.21203/rs.3.rs-1861057/v1

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Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step

https://doi.org/10.21203/rs.3.rs-1861057/v1

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Extracting rare earth elements (REEs) from wastewater is essential for the growth and an eco-friendly sustainable economy. However, it is a daunting challenge to separate individual rare earth elements by their subtle differences. To overcome this difficulty, we report a unique REE nanotrap that features dense uncoordinated carboxyl groups and triazole N atoms in a two-fold interpenetrated metal organic framework (named NCU-1). Notably, the synergistic effect of suitable pore sizes and REE nanotraps in NCU-1 is highly responsive to the size variation of rare-earth ions and shows high selectivity toward light REE. As a proof of concept, Pr/Lu and Nd/Er were used as binary models, which gave a high separation factor of SF_Pr/Lu = 579 and SF_Nd/Er = 273, demonstrating highly efficient separation over a single step. This ability achieves efficient and selective extraction and separation of REEs from mine tailings for the first time, establishing this platform as an important advance for sustainable obtaining high purity REEs.

Rare earth elements

Metal-organic frameworks

Extraction/separation REEs

Solid adsorption

Nanotraps

Rare-earth elements (REEs) are used in a wealth of high-tech applications, including permanent magnets, lasers, phosphors, catalytic converters, and batteries^1-4, owing to their 4f peculiar electronic structure. To meet the REEs demands of the emerging clean energy technology market, recovery and separation of REEs from wastes is an eco-friendly sustainable strategy. However, achieving their selective separation and purification still faces a great challenge due to the large excess of interfering ions, low REE concentrations, and their highly similar chemical properties^5,6.

Various approaches have been utilized to separate REEs. Fractional crystallization, as one of the earliest separation methods, which has low separation efficiency, owning to its driving force originates primarily from the ionic radii differences^7,8. Currently, solvent extraction⁹^-11, and liquid membrane separation methods^12,13 have been utilized for the industrial separation REEs. A high separation factor can be achieved by hundreds of stages while it often faces problems such as low efficiency and generating massive of toxic secondary organic liquid waste¹⁴. Given this, several strategies have emerged to enhance the separation efficiency of REEs. For example, Schelter et al. based on the differences in the equilibrium constants of early and late metals complex with a tripodal nitroxide ligand to enhance the separation factor of Nd/Dy¹⁵. Bu et al. utilized the size-selective crystallization of rare-earth ions method to amplify the difference between early and late lanthanides^16,17. Wang et al. controlled the reaction kinetics to separate Nd/Sm and Nd/Dy during the crystallization of borates³. However, these methods share many shortages of high temperature, long incubation times, and low efficiency, which hinder scale-up production and practical application^3,16. Compared with the aforementioned methods, an adsorption method could overcome the above limits due to its easy operation, intrinsic economic feasibility, and little secondary pollution¹⁸. Traditional adsorbents of activated carbon¹⁹, metal oxides²⁰, and composites²¹ have been developed for REEs extractive and separation, but they usually suffer from low selectivity and capacity because of their low adsorb sites or unsuitable pore sizes²². It is known that REEs separation is mainly driven by the difference in their ionic radii (r^La-Lu = 0.90-1.15 Å) ⁶, and current adsorbents with appropriate apertures are ineffective in separating REEs owing to the weak interaction between the ions and the host²³. On the other hand, several functional adsorbents exhibit a high uptake capacity of REEs yet have a limited selectivity due to the unsuitable pore sizes²⁴.

Towards this end, metal-organic frameworks (MOFs) provided new opportunities for REEs separation owning to their tunable pore sizes and diverse functionalities at the molecular scale²⁵^-28. Given the fact that light REE ions exhibit a slightly larger size than the heavy REE ions and interfering ions in mine tailings^29,30, we reasoned that it is essential to design a suitable aperture or form multiple interactions to enhance the interaction between the light/heavy REE ions and the host framework for efficient REE ions separation. Although these promising properties of MOFs for separation, only a few cases of the separation of REEs by MOFs have been reported. Recently, two hydroxyl-rich 0.8 nm hexagonal channel MOFs were reported for selective separation of lanthanides by solid-phase extraction, where the separation factor was only less than 2, ascribing to the weak affinity between REEs with hydroxyl groups²³. Ahn et al. prepared a chromium-based MOF (Cr-MIL-101), and its derivatives with different organic functional groups for the adsorption of several REE ions. It demonstrated that the carboxylate (−COO−), phosphonic (−PO₃²⁻), and nitrogen-containing groups have a strong coordinative affinity toward REE ions³¹. However, the reported materials have large pore sizes ranging from 1.2 to 2.2 nm, which were ineffective for the separation of different REEs³¹. It was reported that the crown ether (18-C-6) or macrocyclic molecules with about 3.2 Å cavity diameter show an unprecedented selectivity for the large Ln³⁺ ions, due to the better matching of the crown ether cavity size with Ln³⁺ ions diameter^32-34. We hypothesized that if the REE nanotrap is immobilized on the suitable pore size MOFs, it will provide strong interactions to capture REE ions and also obtain pure individual REE ions.

To explore the validity of this hypothesis, we synthesized a new 2D metal-organic framework (termed NCU-1) that features densely uncoordinated carboxyl groups and triazole N atoms, thus providing an REE-philic environment. Owing to the two-fold interpenetration, NCU-1 forms numerous small cavities (3.2 Å) that better match with the size/shape of light REE ions. Therefore, the synergistic effect of suitable apertures and multipoint functional sites in unique REE nanotraps results in a rare high light REE ions uptake and high separation factor for light/heavy REE ions over a single step (Fig. 1). The breakthrough experiments revealed that REE nanotraps could efficiently and selectively separate REE ions from mine tailings over a single step. Thereby, this work provides a novel and powerful approach to sustainable obtaining high purity REEs.

Synthesis and structure description of NCU-1. The light brown plate crystal of NCU-1 crystallizes in the orthorhombic space group, Pna2₁ (Supplementary Table 1). Each Zn²⁺ is 5-coordinated and binds to two DTB ligands, two BTC, and one water molecule (Fig. 2a). Viewed from the a axis, rectangular windows with a size of 9.9 Å ×5.8 Å can be observed (Fig. 2b, Supplementary Fig. 1). Owing to the two-fold interpenetration, NCU-1 features numerous small square pockets decorated by multipoint functional sites (Fig. 2c, Supplementary Fig. 2). Each of such small pockets with a size of 3.2 Å is constructed by triazole rings and uncoordinated carboxylate groups that better match with the size/shape of the larger light REE ions (Fig. 2d). The PXRD pattern is consistent with that of the simulated one, indicating the phase purity of NCU-1 (Supplementary Fig. 3). Meanwhile, the chemical stabilities of NCU-1 were investigated upon treatments in various pH values ranging from 1 to 13. The crystallinity of this treated NCU-1 was still retained as evidenced by the PXRD patterns (Supplementary Fig. 4). Furthermore, according to the TG analysis, the NCU-1 showed high thermal stability up to 320°C (Supplementary Fig. 5). These results suggest that NCU-1 with good stability in harsh conditions, which is highly desirable for capturing and separating of REEs from mine tailings.

Sorption isotherm analysis. The adsorption isotherms of Pr³⁺, Nd³⁺, Gd³⁺, and Dy³⁺ were explored at pH 4.5 as representative models of REE ions due to their abundance in mine tailings collected from Ganzhou city, Jiangxi province, a well-known REEs industry center in China (Supplementary Fig. 6–18, Supplementary Table 5). The results showed that the four Ln³⁺ were fitted well with the Langmuir adsorption model, and the maximum capacity (q_m) of Pr³⁺, Nd³⁺, Gd³⁺, and Dy³⁺ were calculated to be 420, 310, 140, and 126 mg/g, respectively (Fig. 3a, Supplementary Fig. 19–22, Supplementary Table 3). This greatly exceeds the capacity of most current adsorbents, such as CA@Fe₃O₄ NPs³⁵, KIT-6-1,3-PDDA³⁶, MFC-O³⁷, and PEI800/ES-1/2.1³⁸. Energy dispersive X-ray spectroscopy (EDS) mapping analysis confirms that Pr³⁺, Nd³⁺, Gd³⁺, and Dy³⁺ were entered the structures of the material with a uniform distribution throughout the samples (Supplementary Fig. 23). In X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 40, Supplementary Fig. 44), the characteristic peaks of Pr³⁺ and Nd³⁺ were clearly found after adsorption of Pr³⁺ and Nd³⁺, respectively.

Sorption kinetics analysis. The kinetics of NCU-1 for individual Pr³⁺, Nd³⁺, Gd³⁺, and Dy³⁺ adsorption were then investigated (Supplementary Fig. 24–31, Supplementary Table 4). The REE ions were rapidly captured by NCU-1, all of which reached equilibrium within 1 h, much shorter than those of the other adsorbents. For example, magnetite@MOF³⁹ and hydroxyl-decorated flexible MOF²³, require longer than 2–4 h to reach equilibrium. Additionally, for any sorbents, recyclability is crucial for practical implementation. Significantly, the adsorption processes were fully reversible as reflected by the fact that the loaded REE ions can be washed off by diluted HNO₃ at pH 3 with maintained capture efficiency for at least four cycles of sorption /desorption (Supplementary Fig. 32–35). These results indicate that NCU-1 can serve robustly towards high efficient REEs recovery and affirm the judicious choice of the MOF for affording selective trapping REEs.

Selectivity. The selectivity of REE ions is a key parameter for evaluating adsorbents. Thereby, the distribution coefficient (K_d) as a barometer for selectivity was also tested. NCU-1 was dispersed in the mixed solutions containing REE ions and interfering ions with different pH values ranging from 2.0 to 6.0 (Fig. 3b). The K_d of NCU-1 to REEs increases rapidly and reaches the maximum when the pH increases from 2.0 to 4.5, and then decreases to 6.0 (Supplementary Table 2). Compared to transition metal ions and alkaline-earth metal ions, the K_d of NCU-1 to REE ions is 3–4 orders of magnitude higher, suggesting a clear advantage of NCU-1 in selective capture of REE ions. Remarkably, the K_d to various rare-earth ions differs significantly at pH 4.5, which induces that the optimal separation selectivity for REEs can be achieved. Generally, a material is considered an excellent adsorbent if K_d is over 10⁴ mL g^-1. Particularly, K_d ^Gd-La was up to 1.6×10⁴-1.0×10⁶ mL g^-1 at pH 4.5. Such high K_d indicates that NCU-1 has an obviously higher affinity toward light REE ions.

Tailings treatment. Encouraged by the excellent performance of NCU-1 for extraction and separation of REE ions, we evaluated the ability of NCU-1 to capture and separate REE ions from mine tailing collected from Ganzhou city. The natural rare-earth tailing sample was filtered through a 0.22 µm filter for ICP-MS analysis. The results show that the concentrations of rare earth elements in the tailing are low about 0.09–0.92 ppm, and the competitive ions concentration of Al (8.15 ppm) is about 10 times compared to those of the REEs (Supplementary Table 5), making the REEs adsorption and separation a great challenging task. Notably, except for competing ions, the NCU-1 could effectively selective capture all REE ions from the nature mine tailing (Fig. 3c), manifesting it is a good platform for adsorption and separation of REEs from mine tailings. As an industrially relevant performance test, experimental breakthrough studies were performed to evaluate the actual separation ability of NCU-1 (Supplementary Fig. 36). The competitive ions occurs first at 10 bed volumes and REE ions occur after 50 bed volumes (Fig. 3d, Supplementary Fig. 37), indicating NCU-1 has an excellent separation effect on REE ions from competitive ions. It is worth noting that compared to heavy REE ions, light REE ions have even higher uptakes and larger breakthrough bed volumes. For instance, the breakthrough bed volume of Pr³⁺ is 150, which is larger than that of Dy³⁺ (80) (Supplementary Fig. 38). These results illustrate that NCU-1 is a promising candidate for separation and obtaining high purity REE ions from mine tailing over a single step.

Separation mechanism. The FT-IR and XPS were applied to evaluate the structure-performance relationship of NCU-1 in the effective capture of REE ions. The results show that the high adsorption toward REE ions of NCU-1 is ascribed to the strong coordination ability between the REE ions and carboxyl groups, lattice water molecules, and triazolium nitrogen atoms in the host (Supplementary Fig. 39–51). To elucidate the origin of the observed high selectivity of the NCU-1 for REE ions, the first principle calculations were carried out between metal ions and the NCU-1 configurations. In addition, Al³⁺, Dy³⁺, and Pr³⁺ were selected as the representative interfering ions, heavy REEs ions, and light REE ions, respectively. Figure 4a-c show the differential charge density (DCD) maps of NCU-1 configurations after adsorption Al³⁺, Dy³⁺, and Pr³⁺, respectively. Obviously, charge redistribution occurs at metal ions and NCU-1, and the negatively charged regions (yellow) with excess electrons accumulate around the O atoms and N atoms, which is beneficial to the adsorption of metal ions^40,41. The Bader charge analysis⁴² shows that the charges on Al³⁺ ion in the NCU-1 configuration are + 2.09 e, while + 2.17 e for Dy³⁺, and increase to + 2.31 e for Pr³⁺. Generally, the larger the Bader charge, the stronger the binding strength between the host and guests⁴³. Therefore, the order of the binding strength of NCU-1 with the three ions is as follows: Al³⁺< Dy³⁺< Pr³⁺. It suggests that the selectivity of NCU-1 for light REE ions is higher than those of heavy REE ions and interfering ions, due to the more electrons obtained from the host. Moreover, the density functional theory (DFT) calculation clearly reveals that the primary adsorption sites of metal ions are located at the REE nanotraps (Fig. 4d-f). As for Pr³⁺, two oxygen atoms of the carboxyl group and one water molecule were calculated to coordinate with Pr³⁺ with the bond distances of 1.79 Å, 2.73 Å, and 1.94 Å, respectively (Supplementary Fig. 52). The calculated distance of Pr³⁺···N in NCU-1 is 3.30 Å. In the case of Nd³⁺, it is coordinated to the two oxygen atoms of the carboxyl group and one water molecular with the bond distances of 1.66 Å, 2.76 Å, and 1.86 Å, respectively, and the Nd³⁺···N bond distance is about 3.27 Å (Supplementary Fig. 53). While as for Al³⁺, only the coordination bonds between the carboxyl and water molecular oxygen and Al³⁺ were calculated (Supplementary Fig. 54). The adsorption energies (Eads) of NCU-1 with different metal ions were also calculated to discriminate the adhesion level. The calculated adsorption energy for Al³⁺, Dy³⁺, and Pr³⁺ are − 1.66 eV, -2.58 eV and − 3.29 eV, respectively. It decreases when the size of the adsorbed ions increases, which is fully consistent with our experimental observations. This is partly because the larger Pr³⁺ (r = 0.99 Å) sterically “matches” better to the nanotraps size/shape than Dy³⁺ (r = 0.91 Å) and Al³⁺ (r = 0.50 Å).^5,17 Light REE ions within a larger size radius enter the REE nanotraps and coordinate with O and N atoms in them to form a stable rare earth-like hydrated ion structure⁴⁴. However, competing ions and heavy rare earth ions with a large difference in pore size from NCU-1 have poor binding ability to coordinate with O, which makes the ions easily escape from the pores and achieve high-selectivity separation of light RREs. Therefore, the suitable sizes of the nanotraps and the uncoordinated carboxyl groups and N atoms in nanotraps play a decisive role in the light/heavy REE separation.

Binary REE ions separation. Since the affinity of NCU-1 to REE ions exhibits a unique periodic trend with significant differences, the individual REE can be separated from other REEs by the adsorption method (Fig. 3b). As a proof of concept, a series of experiments (6 combinations in total) were conducted to determine its application on individual REE separations. Consequently, NCU-1 was dispersed into binary mixtures of REE ions (M1 and M2) in a 1:1 molar ratio. As shown in Fig. 5, larger Ln³⁺ is selectively adsorbed by NCU-1, and smaller Ln³⁺ almost remained in the solution, indicating that this system successfully separated light/heavy REE mixtures. For a mixtures that contained Nd³⁺ and Dy³⁺, the separation factor (SF) reached up to 67, which exceeded those of many materials, for example, TriNOx^3- (SF_Nd:Dy= 8.45 ± 1.62) ⁴, Zn-BTC (SF_Nd:Dy= 30.23) ⁴⁴, TRPO (SF_Nd:Dy < 5) ¹¹. Especially interesting is the separation factor of 73 observed for Eu³⁺ over Lu³⁺, a value that is extremely high than that of the crystallization extraction method recently developed by Wang et al³. Impressively, the case of Pr/Lu, obtains a high separation factor of 579, which is one of the highest single-step separation factors known for REEs separations⁴⁵. Mixtures that contained Nd: Er also gave an extremely large separation factor of SF_{Nd: Er} =273. Generally, NCU-1 demonstrates a clear and high preference for larger-sized REE ions in the solid adsorption process, and the selectivity increases with the ionic radii difference. High-purity individual rare-earth ions can be obtained through one-step adsorption in this work, which has made a breakthrough process in the separation of REEs.

In conclusion, we report a unique REE nanotrap featuring dense uncoordinated carboxyl groups and triazole N atoms and suitable pore sizes in NCU-1 that can induce strong interaction with light REE ions. Thereby the NCU-1 affords outstanding light REEs adsorption and separation performance for potential use in capturing REEs from mine tailings. Notably, the high response to light REE is firstly realized by simple adsorption progress, which could obtain high purity individual REEs over a single step. Not only does this work report a promising platform that offers a solution to the long-term challenge of REEs recovery from complex wastewater systems, but it also sheds light on the rational design of metal framework materials for applications in the field of REE purification.

Materials. 4,4-di(4H-1,2,4-triazol-4-yl)-1,1-biphenyl (DTB) and trimesic acid (BTC) were purchased from Jilin Chinese Academy of Sciences-Yanshen Technology Co., Ltd. Zn(NO₃)·6H₂O and trivalent lanthanide nitrates were purchased from Energy Chemical Co., Ltd. Acetonitrile (MeCN) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water was prepared from the Millipore system (18.25 MΩ·cm). All the purchased reagents were of analytical grade and used without further purification.

Synthesis of NCU-1. To a 20 mL PTFE reactor, trimesic acid (21.0 mg, 0.10 mmol), 4,4-di(4H-1,2,4-triazol-4-yl)-1,1-biphenyl (28.8 mg, 0.10 mmol), Zn(NO₃)·6H₂O (29.5 mg, 0.1 mmol), MeCN 2 mL, and water 8 mL were added. The mixture was sealed and heated 140°C for 3 days, followed by slow cooling at a rate of 5°C/h to room temperature, and light brown crystals for X-ray analysis were collected and dried in air.

The data that support the findings of this paper are available in the paper and supplementary information files. The X-ray crystallographic data for the structure reported in this article has been deposited at the Cambridge Crystallographic Data Centre (CCDC) with the number of CCDC 2141439. Any further relevant data are available from the corresponding author upon request.

Acknowledgments

We acknowledge financial support from the National Natural Science Foundation of China (22036003, 21976077 and 22176082), Natural Science Foundation of Jiangxi Province (20212ACB203009 and 20212ACB203011).

Author contributions

J.-D.Q. and Q.-H.H. conceived and designed the research. Q.-H.H. performed the synthesis and conducted the experiments. Q.-H.H., X.G., Y.-Z.S., A.-M.S and W.J. performed the characterizations. Q.-H.H., R.-P.L. and J.-D.Q. participated in drafting the paper and gave approval to the final version of the manuscript.

Competing interests

The authors declare no competing interests.

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Supplementary information The online version contains supplementary material availabe at http://www.nature.com/ naturecommunications.

Correspondence and requests for materials should be addressed to J.-D.Q.

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