Tailoring sub-nanochannels and ion-imprinted functionality of palygorskite nanofiltration membranes for desalination and lithium-ion transport

doi:10.21203/rs.3.rs-5202612/v1

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Tailoring sub-nanochannels and ion-imprinted functionality of palygorskite nanofiltration membranes for desalination and lithium-ion transport

https://doi.org/10.21203/rs.3.rs-5202612/v1

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Nanofiltration membranes featuring sub-1-nm channels have demonstrated considerable potential for desalination and ion separation. However, the construction of both high selectivity and permeance membranes with tailorable one-dimensional (1D) sub-1-nm channels using a versatile and eco-friendly protocol remains a formidable challenge. Here, high selectivity PAL-CMC hybrid (PCH) nanofiltration membranes were successfully prepared based on the coordination-driven self-assembly between natural Palygorskite (PAL) nanorods and carboxymethylcellulose (CMC) in water. We first discovered the ion-imprinting adsorption feature of Mg²⁺ on PAL. By adjusting the coordination ratio, the PCH membrane retains only the natural 1D straight sub-1-nm channels of PAL nanorods, rendering it one of the few membranes capable of ultra-fast water transport (> 25 L m^-2 h^-1 bar^-1) and exceptional salt retention (97.4% for Na₂SO₄ and 86.2% for NaCl). Through applying ion imprinting technology, Mg²⁺-imprinting sites enable the membrane to transport Li⁺ rapidly and selectively in mixture solutions containing Mg²⁺, achieving a Li⁺/Mg²⁺ selectivity of up to 175. Molecular dynamics simulations indicate that water and lithium ions can be transported rapidly through the intrinsic and tailored sub-1-nm channels of PAL nanorods, respectively. These findings establish PCH membranes as prospective candidates for desalination and lithium extraction from lagoons.

Physical sciences/Chemistry/Environmental chemistry/Pollution remediation

Earth and environmental sciences/Environmental sciences/Environmental chemistry/Pollution remediation

palygorskite nanofiltration membranes

sub-1-nm channels

desalination

Li+/Mg2+ separation

ion-imprinting adsorption

In recent decades, global water consumption has grown at an annual rate of approximately 1% driven by population growth, socio-economic development, and changes in consumption patterns. As detailed in the 2022 Sustainable Development Goals Report, 2 billion people globally lack access to safe drinking water.¹ Meanwhile, as new energy industries rapidly develop, the demand for lithium, an important metal element, is also increasing at a fast pace. However, a significant challenge in the extraction process of lithium is the effective separation of Li⁺ and Mg²⁺ ions.^{2, 3, 4} The much higher Mg²⁺ content in saltwater compared to Li⁺, along with the very similar sizes of Li⁺ and Mg²⁺ ions, makes achieving effective Li⁺/Mg²⁺ separation of paramount importance.^{5, 6} To address these issues, it is crucial to develop efficient and energy-saving desalination and sustainable Li⁺ extraction technologies.

Pressure-driven nanofiltration (NF) technology has emerged as a promising environmentally-friendly approach for the desalination of brine and the extraction of lithium.^{7, 8, 9} In recent years, there has been considerable advancement in the development of NF membranes with sub-1-nm apertures, enabling selective transport of water or ions. These developments have resulted in the highly efficient removal of salts from seawater and the selective extraction of lithium and magnesium ions from brine.^{10, 11, 12} In charged systems, mass transfer is controlled by a combination of size exclusion, the Donnan effect, steric hindrance, and coordination interactions between ions and membranes.¹³ To further enhance membrane selectivity, considerable research has been dedicated to investigating novel materials with ordered, uniform pore structures and tunable functionalities. Examples of such materials include covalent organic framework nanosheets,^{14, 15} metal-organic framework nanocrystals,^{16, 17, 18} and in particular assemblies with one-dimensional (1D) nano-channels self-assembled from supramolecules and macrocycles.^{19, 20} Nevertheless, integrating these sophisticated structures into commercially viable membrane materials presents considerable challenges. These include the inherent instability of the novel materials, incompatibility with the polymer matrix of the selective layer, the necessity for complex synthesis steps involving significant quantities of organic solvents, and the difficulty of manufacturing these membranes on an industrial scale. Furthermore, the current NF membranes are only suitable for a single application, either for seawater desalination or for Li⁺/Mg²⁺ separation. This is due to the inherent difficulty in precisely tuning the pore size and surface properties related to ionic interactions across a broader range. Consequently, the practicality of these state-of-the-art membranes is also limited.

Palygorskite (PAL) nanorods, a type of natural clay nanofiber, are characterized by the presence of well-defined one-dimensional (1D) sub-1-nm channels that run parallel to the long axis of the PAL nanorod.²¹ The cross-sectional dimensions of these channels are approximately 3.7 Å × 6.4 Å,²² which renders them a promising candidate for utilization as selective pathways with a defined size, capable of repelling solute ions or dye molecules ^{23, 24, 25, 26} and rapidly transferring water molecules with a diameter of 3.2 Å.²⁷ PAL nanorods are rich in functional groups, metal active sites, and ion exchange capabilities,^{28, 29} thereby providing a flexible platform for the customization of membrane properties. Furthermore, PAL nanorods may be a cost-effective and suitable raw material for fabricating NF membranes. However, the development of environmentally sustainable techniques to customize PAL-derived membranes with defect-free and channel-tunable characteristics for seawater desalination and high-efficiency Li⁺/Mg²⁺ separation remains a significant challenge.

Here, we have successfully developed scalable PAL-based (PCH) membranes using a simple and environmentally friendly method for desalination and Li⁺/Mg²⁺ separation. The PCH membrane was achieved through coordination-driven self-assembly in water, utilizing natural PAL nanorods/etched PAL nanorods (P_βAL) as the building blocks and sodium carboxymethylcellulose (CMC) as the cross-linking agent. The tight self-assembly between PAL nanorods imparts the PCH membrane a defect-free character, preserving its intrinsic and well-defined 1D sub-1-nm channels. The PCH membrane prepared via route 1 (Scheme 1) exhibited excellent desalination performance by selectively allowing the fast transport of water molecules. Moreover, owing to strong ion exchange capacity, PAL nanorods are controllably etched by ion-imprinting techniques to modulate the pore size distribution as well as the interaction sites of the membrane. The resulting membrane prepared via route 2 (Scheme 1) shows a remarkable ability to separate Li⁺/Mg²⁺ in mixed solutions, in which the Mg²⁺ ions are refused by the smaller channel size or trapped in the channels by Mg²⁺-imprinting adsorption and only the Li⁺ ions reach the penetrate solution through the sub-1-nm channel.

PAL nanorods with Mg ²⁺ -imprinting sites.

The PAL structure is distinctive, featuring a three-dimensional layer-ribbon configuration (Scheme 1a). It comprises two elongated double silica tetrahedral chains, which sandwich an octahedral layer comprising either magnesium (Mg) or aluminum (Al) octahedra.²¹ The evident spacing of 5.3 Å and the presence of bright diffraction spots in Fig. 1a demonstrates that PAL has inherent channels and exceptional crystallinity. The special ion exchangeability of PAL nanorods allows for the enhancement of their structure and physicochemical properties through acid etching. The concentration of acid (H₂C₂O₄/HCl mixed solution) employed for etching PAL nanorods is within the range of 0.2 to 1.0 mol/L. The resulting nanorods are designated as P_βAL, where β represents the acid concentration. Upon treatment with an acid solution of 0.35 mol/L, channels of approximately 8.8 Å in size emerge in the etched area, while the diffraction spots become sparse and faint (Fig. 1b). This phenomenon can be attributed to the dissolution of some metal ions (particularly Mg²⁺) from the lamellar structure of PAL under acidic conditions, which results in an expansion in aperture size to some extent (Fig. 1b). The presence of imprinting sites within the P_βAL nanorods that specifically bind Mg²⁺ indicates the potential for these materials to be utilized in selective ion capture and separation processes. As illustrated in Fig. 1c, the immersion of P_0.35AL in a solution containing Mg²⁺ resulted in a reduction in channel size to approximately 7.0 Å and an enhancement in diffraction brightness, in comparison to P_0.35AL nanorods. This observation provides evidence that the imprinting sites have successfully captured Mg²⁺ ions, thereby supporting the hypothesis of specific binding between Mg²⁺ ions and the cavities within the nanorods.^{30, 31, 32}

The Mg content of the original PAL was 7.70% (Fig. 1d and Table S1). When the acid concentration was increased to 1.0 mol/L, considerable amounts of Mg²⁺ were removed, resulting in a decrease in content to 5.32%. It is noteworthy that the Mg content in Mg²⁺-P_0.35AL can be restored to a level comparable to that of the original PAL nanorods. This recovery provides further evidence that the P_0.35AL skeleton is capable of capturing Mg²⁺ through an ion-imprinting adsorption mechanism. Figure 1e illustrates the changes in the composition of PAL functional groups during a series of treatments. After acid etching, the hydroxyl stretching vibration band (ν-OH) of (Mg/Al/Fe)-OH at 3620 and 3558 cm^-1, as well as the -OH bending vibrations of adsorbed water and MgO₆ octahedral coordinated water at 1655 and 1630 cm^-1, and the perpendicular Si-nonbridging oxygen-Mg (Si-O_nb-Mg) asymmetric stretching vibration at 988 cm^-1 all show a decrease in intensity as a result of the dissolution effects on the metal component.^{30, 31, 32} Nevertheless, after the Mg²⁺ adsorption process on P_0.35AL, a discernible recovery trend is observed in the absorption peaks. These findings suggest that there are notable alterations in the functional group composition of PAL throughout the treatment period and that the reabsorption of Mg²⁺ can facilitate partial recovery. Furthermore, a comparison of the crystal structure is presented for PAL, P_0.35AL, and Mg²⁺-P_0.35AL nanorods. The acid etching process leads to a reduction or elimination of the intensity of certain diffraction peaks in PAL (see Figure S1) and causes a shift of the (110) lattice plane diffraction peaks from 8.26° to a higher angle of 8.50° (Fig. 1f). It is important to note that these altered peaks tend to recover after P_0.35AL reabsorbs Mg²⁺. Based on these findings, it can be inferred that ion-imprinting sites are formed in PAL nanorods by acid etching. Furthermore, it can be concluded that P_βAL nanorods have the ability to absorb Mg²⁺ from solutions through an Mg²⁺-imprinting adsorption mechanism.

Preparation and characterization of PCH membranes.

The coordination self-assembly of metal ions on the PAL backbone and oxygen atoms of carboxyl groups on the CMC make them a promising choice for defect-free membrane preparation. Scheme 1 shows the methodology used to prepare PCH membranes and their subsequent application in separation processes. Specifically, PAL and CMC form a mixed suspension in water through coordination self-assembly. This suspension is then applied to the polyethersulfone (PES) substrate using the Mylar rod coating technique (Fig. 2a and Figure S2). After solvent evaporation, the functional layer can be easily removed from the substrate to obtain a free-standing PCH membrane (Figure S3). The process of assembling nanorods into a scalable membrane is simple and environmentally friendly. The coordination self-assembly conditions for the preparation of PCH membranes were optimized by keeping the CMC content at 3–9 wt%. The PCH membranes were designated as PCH-α (α = 3, 5, 7, 9), where α represents the CMC content, as shown in Table S2.

The mechanical properties of PCH membranes were subjected to further investigation. As illustrated in Fig. 2b, the PCH-7 membrane demonstrates a maximum extension at a break of 11.8% and a breaking strength of 12.9 MPa, which serve as representative values for the entire membrane family. Furthermore, the membrane exhibits sufficient strength to support a weight of at least 50 g when suspended at the tube edge. It is noteworthy that the PCH membrane only breaks when an external force is applied due to the robust intermolecular interactions between PAL nanorods and CMC. From both macroscopic and microscopic perspectives (Fig. 2c, d), the PCH membrane displays an uninterrupted and uniform surface devoid of discernible imperfections. Upon bending, the membrane demonstrates flexibility without compromising its integrity, suggesting suitability for robust handling (Fig. 2e).

The PCH membrane exhibits a uniform thickness of approximately 10 µm and an evident lamellar structure, as illustrated in Fig. 2f. Furthermore, high-resolution transmission electron microscope (HRTEM) images demonstrate a clearly defined d-spacing of 0.45 nm from lattice fringes, as illustrated in Fig. 2g and S4. This closely aligns with the (040) crystal faces of PAL nanorods. The TEM results indicate that the PCH membrane retains the excellent crystallinity of the PAL nanorods, as illustrated in Figure S5. The pore structure of the PCH membranes was confirmed through N₂ adsorption-desorption measurements. As illustrated in Fig. 2h and S6, the effective pore size of the PCH-7 membrane is 5.2 Å, which matches with the pore size of the PAL nanorods. This pore size is smaller than the hydration radius of monovalent cations, which is greater than 7.0 Å. Among the PCH membranes (Fig. 2i and S7), the PCH-7 membrane exhibits a tightly assembled structural configuration of PAL nanorods, with no observed interstitial pores between the nanorods. Energy-dispersive X-ray (EDX) analysis and mapping (Fig. 2j) demonstrate a homogeneous distribution of PAL and CMC in the PCH membranes.

The interactions between PAL nanorods and CMC in PCH membranes were characterized by a variety of techniques. First, the XRD pattern (Fig. 3a and S8) shows that the coordination cross-linking of CMC does not affect the crystallinity of PAL. Then, we analyzed the chemical structure of the PCH membrane to preliminarily disclose the interaction between PAL nanorods and CMC. As shown in Fig. 3b, compared to PAL nanorods, the PCH membrane exhibits a broader -OH vibrational peak with a redshift, which illustrates the presence of hydrogen-bonding interaction between the (Mg/Al/Fe/Si)-OH of PAL nanorods and the CMC chains.³³ Moreover, the absorption band at 1735 cm^-1 of CMC chains can be ascribed to the C = O stretching vibration, which appears at a lower wavenumber in the PCH membrane. The redshift of the C = O stretching vibration and the appearance of new characteristic peaks at 532, 772, and 827 cm^-1 in the PCH membrane provide evidence that a coordination bond is forming between the PAL nanorods and the CMC chains. To gain further insights into the nature of the coordination bonds, the localized environment around ²⁷Al was probed by means of solid-state nuclear magnetic resonance (NMR). As illustrated in Fig. 3c, the PAL nanorods exhibit a peak at 1.25 ppm, which can be attributed to hexa-coordinate aluminum (Al(VI)). In comparison to the spectrum of the PAL nanorods, a shift of 2.38 ppm to a lower field is observed in the spectrum of the PCH membrane. This variation can be attributed to the fact that the Al sites of PAL nanorods adsorb a greater number of oxygen-containing functional groups from CMC chains.³⁴ This further supports the hypothesis of a coordination bond between PAL nanorods and CMC chains, which is consistent with the FTIR results. X-ray photoelectron spectroscopy (XPS) was employed to conduct a more detailed analysis of the surface chemistry of PCH membranes. The characteristic peaks of Si, Mg, Al, O, and C in the PCH membrane are consistent with those of the elements in PAL nanorods (Fig. 3d). It is evident that the PCH membrane contains a higher proportion of carbon than the pure PAL nanorods, which indicates that the PCH membrane has been successfully prepared. The O 1s, Al 2p, and Mg 2p spectra indicate that the PAL nanorods coordinate with the CMC chains by sharing electron pairs from the carboxylate groups. The electron-donating effect of these carboxylate groups on metal sites results in a significant shift of the O = C-O binding energy to a higher value (Fig. 3e and Table S3), while Al 2p and Mg 2p move to lower values (Fig. 3f, S9).³⁵ In light of the aforementioned evidence, it can be posited that hydrogen and coordination bonds exist between the metal sites of PAL nanorods and the oxygen-containing functional groups of CMCs.

In order to better understand the interactions, we utilized density functional theory (DFT) to compute the binding energy of the intermolecular interaction between PAL and CMC in the PAL-CMC system in a PAL-CMC system (Figure S10). Surprisingly, when the PAL-CMC is in water, particularly with sodium ions (Na⁺), the binding energy can increase to -5.870 eV, which is 1.4 times higher than that of the dry state. This finding suggests that there is an increase in strength of interactions between PAL and CMC, providing potential applications for desalination and Li⁺/Mg²⁺ separation. The interaction was further confirmed by a deformation charge density calculation (Fig. 3g-i). This calculation shows that the electron-rich oxygen-containing groups of CMC chains are oriented towards the empty 3s orbitals of the electron-deficient Mg²⁺ cations and 3s and 3p orbitals of Al³⁺ cations in PAL nanorods, facilitating electron transfer between the two entities. Notably, in the presence of Na⁺, the system exhibits the strongest charge density, with more pronounced gain and loss of electrons. This suggests that the ability of electrons to extensively delocalize within the PAL-CMC system increases additional stability for hydrogen and ligand bonds.³⁶

Water permeation and salt rejection performance.

The combination of strong hydrogen and coordination bonds not only gives the nanofiltration membrane outstanding mechanical properties but also results in tightly assembled PAL nanorods within the membrane. These nanorods have 1D sub-nanometer channels that allow for fast water transport while impeding the permeation of unwanted solutes (Fig. 4a). In addition to the sub-1-nm channel size, the surface charge of PCH membranes is another factor affecting mass transport. The PCH membranes are highly electronegative (Figure S11), which is linked to the Donnan exclusion theory.¹⁸ Considering the matching of ion size with channel targets and surface charging characteristics, we expect that the PCH-7 membrane will exhibit suitable water permeance and excellent salt interception performance.

The filtration performance of PCH membranes was investigated in a reverse osmosis (RO) configuration to verify the hypothesis, as illustrated in Fig. 4b and S12. Among the PCH-α membranes (Fig. 4c), the PCH-7 membrane exhibited the highest sodium sulfate rejection and moderate water permeance, with values of 97.4% and 25.1 L m^-2 h^-1 bar^-1, respectively. Even at ultra-high concentrations (35,000 ppm), the PCH-7 membrane exhibited exceptional retention of sodium sulfate, exceeding 88% (Figure S13). The exceptional sodium sulfate retention performance of the PCH-7 membrane can be attributed to the quasi-perfect assembly of PAL nanorods with the participation of 7 wt% CMC. Therefore, due to an increase in spatial site resistance, water is only able to pass through the PAL sub-1-nm channels with a size-screening effect through the PCH-7 membranes, which results in the rejection of the majority of Na⁺.

Besides the size sieving, the excellent Na₂SO₄ rejection of the PCH-7 membrane is also related to the Donnan exclusion caused by the dramatically enhanced charged -COO^- groups on the membrane surface to exclude the ions with the same charge.³³ Besides divalent cations, the PCH-7 membrane exhibits an ultrahigh retention rate of 86.2% for monovalent Na⁺ in the NaCl solution (Fig. 4d). The rejection follows the order of MgSO₄ > Na₂SO₄ > CaCl₂ > NaCl, following the anion-to-cation valence ratio (Z^-/Z⁺) based on Donnan exclusion theory.^{11, 37} Generally, the PCH-7 membrane surface is more negatively charged and therefore forms a stronger repulsion for Na₂SO₄ with a valence ratio of 2 whereas a weaker rejection for CaCl₂ and NaCl. We also check the NF performance of the PCH-7 membrane towards actual water samples from the Yellow Sea (Figure S14). In the presence of many ionic species, more than 80% retention can be achieved for each ion.

The stability of the PCH-7 membrane includes membrane immersion in water and filtration performance was also assessed by combining experimental and simulation methods. Unlike the membrane accumulated by PAL nanorods, the PCH-7 membrane maintains excellent structural integrity during up to 45 days of observation (Figure S15). Additionally, the membrane exhibits excellent long-term stability (Fig. 4e), maintaining its initial Na₂SO₄ nanofiltration property through the 720-minute cross-flow filtration test. The synergistic effect of size screening and the Donnan effect gives the PCH-7 membrane excellent salt retention and rapid water transport. To visually demonstrate its stability more clearly, we performed ab initio molecular dynamics (AIMD) simulation on the PCH-7 membrane system at 298 K. As shown in Fig. 4f, the total energy of the system stabilized at about 25 ps, and over time, Na⁺ ions approached the membrane but were excluded when reaching the PAL surface. Interestingly, compared to most reported membranes (Fig. 4g, Table S4), especially those with poor salt retention (92.0% for Na₂SO₄ and 14.7% for NaCl) when incorporating PAL nanorods as hydrophilic nanofillers into polyamide,³⁸ the PCH-7 membrane exhibited more pronounced permeance and salt rejection.

The ability of the PCH-7 membrane to remove heavy metal ions from wastewater was also evaluated (Fig. 4h). After membrane treatment, the concentration of heavy metal ions in wastewater is reduced by 2 ~ 3 orders of magnitude. Unexpectedly, the permeance concentration of heavy metal ions in wastewater even reaches the standards for potable water according to the World Health Organization (WHO). The separation performance of the PCH-7 membrane was further tested using several common industrial dyes with different molecular weight and charge characters (Figure S16), covering Congo Red (CR), Rhodamine 6G (R6G), Methyl Blue (MB) and Crystal Violet (CV). The membrane retains more than 99.7% of these dyes (Figure S17) while maintaining a very high water permeance (27.5 L m^-2 h^-1 bar^-1), and this performance does not change significantly for the long 300-minute test.

Li ⁺ /Mg ²⁺ separation performance and mechanism.

Extracting lithium from salt-lake brine is of great significance for energy storage applications because of the abundant lithium resources in brines. Given the ion-imprinting feature, P_βAL nanorods obtained by treating PAL with different acid concentrations can be self-assembled into P_βCH-7 membranes under 7 wt% CMC crosslinker. The P_βCH-7 membranes (Fig. 5a) exhibit a larger pore size distribution than the PCH-7 membrane (Fig. 1h). The degree of pore size change was positively correlated with increasing acid concentration. Noticeably, the P_0.35CH-7 membrane showcases a pore size distribution between 5.86 and 11.9 Å, with the main pore size around 7.66 Å, which precisely matches the hydration diameter of Li⁺ (7.64 Å). Based on the sub-1-nm size sieving and the Mg²⁺-adsorbing sites, the P_0.35CH-7 membrane is expected to have the potential to address the challenges of Li⁺/Mg²⁺ separation from salt-lake brines.

We investigated the nanofiltration performance of P_βCH-7 membranes in a single solution for LiCl and MgCl₂, respectively. As demonstrated in Fig. 5b, the water permeance gradually increases from 26.2 to 29.6 L m^-2 h^-1 bar^-1 with increasing concentration of acid used for etch, which may be attributed to a decrease in mass transfer resistance due to the increased channel size. Noteworthily, gradually increasing the acid concentration to 0.35 mol/L has little effect on Mg²⁺ transport with the retention rate remaining at around 98%, whereas the retention of Li⁺ is greatly reduced from 65.4 to 14.8%, with a high Li⁺/Mg²⁺ selectivity (S = 31.6). As the channel size of P_βCH-7 membranes continues to increase, the transport of Li⁺ through the channels in the membrane remains unchanged; on the contrary, Mg²⁺ passes through more easily, leading to a gradual decrease in the Li⁺/Mg²⁺ selectivity to 1.4. In line with expectation, the P_0.35CH-7 membrane exhibits the most excellent Li⁺/Mg²⁺ separation performance in a single solution. The channel feature also enables P_0.35CH-7 an effective removal of Mg²⁺ from the mixed solutions with different Mg²⁺/Li⁺ ratios (Fig. 5c). As the Mg²⁺/Li⁺ ratio in the feed increases up to 100, the Mg²⁺ rejection shows a slight increase from 95.31 to 99.24%, and a significant decrease from 28.97 to -33.00% is observed for Li⁺ rejection. Specifically, for the Mg²⁺/Li⁺=100, the Li⁺/Mg²⁺ selectivity can reach up to 175.

To further elucidate the Li⁺/Mg²⁺ separation mechanism, Ab-initio molecular dynamics (AIMD) simulations (Fig. 5d, e) were carried out to investigate the ion transport behavior through the P_0.35CH-7 membrane. The system achieves stability within 25 ps, and at times up to 100 ps, which is a very long structural stabilization period, only Li⁺ but not Mg²⁺ permeates the P_0.35CH-7 membrane. The pore size of the P_0.35CH-7 membrane ranges from 5.86 to 11.9 Å, and the hydrated diameters of Li⁺ and Mg²⁺ are 7.64 Å and 8.56 Å, respectively.³⁹ In addition, the hydration energy of Mg²⁺ (-1830 kJ/mol) is 3.8 times higher than that of Li⁺.²⁷ The lower the hydration energy, the easier it is for hydrated ions to shed coordinated water molecules, resulting in rapid ion transport through the membrane.⁴⁰ Therefore, the P_0.35CH-7 membrane allows the rapid penetration of Li⁺ due to the smaller size and lower hydration energy. However, for Mg²⁺, on the one hand, the pores with smaller sizes in the P_0.35CH-7 membrane intercept them through a size-sieving mechanism; on the other hand, even if the pores with larger sizes in the P_0.35CH-7 membrane allow the Mg²⁺ to enter the interior, they are captured by the Mg²⁺-imprinting sites in the P_0.35AL skeleton through an ion-imprinting adsorption mechanism, leading to the reconstruction of the Mg-O bond. In retrospect, the simulation results are in agreement with our experimental results, which also indicate that the P_0.35CH-7 membrane achieves high-efficiency Li⁺/Mg²⁺ separation in mixed solutions based on size repulsion and ion-imprinting adsorption mechanisms. Our simulation results are consistent with experimental data, confirming that our P_0.35CH-7 membrane demonstrates excellent performance in terms of Li⁺/Mg²⁺ selectivity and water permeance when compared with other membranes. (Fig. 5f and Table S5)

Antifouling and antimicrobial performance.

In practical use, effective antifouling and antimicrobial properties are likely the most important factors for a nanofiltration membrane. The surfaces of CMC and PAL are rich in hydroxyl groups, which give PCH membranes inherent hydrophilicity. Results from hydrophilicity studies demonstrate a water contact angle of 12.2° when a water droplet comes into contact with the surface of the P_0.35CH-7 membrane (Figure S18a, b), providing further evidence of its strong hydrophilicity. This hydrophilicity promotes the formation of a hydration layer on the membrane surface, reducing its affinity for organic micropollutants and improving its antifouling properties.⁷ Bovine serum albumin (BSA) was used to assess the antifouling performance of PCH membranes as a representative foulant. The antifouling indexes of the PCH membrane are presented in Table S6. The permeation recovery ratio (FRR) of the P_0.35CH-7 membrane reaches 94.6% for BSA foulant, indicating its ability to suppress protein fouling. Additionally, the nanorod-shaped crystal structure and adsorption properties of metallic PAL nanorods enable it to kill bacteria as well.²¹ Therefore, Staphylococcus aureus (S. aureus) and Escherichia coli (E.coli) were selected to evaluate the antimicrobial activity of the P_0.35CH-7 membrane. Antimicrobial tests (Figure S18c, d) have shown that the PCH-7 membrane forms an antimicrobial ring around itself to prevent bacteria from approaching. These good antifouling and antimicrobial performances endow the P_0.35CH-7 membrane application potential in the industry.

In summary, we present the novel discovery that PAL exhibits Mg²⁺-imprinted adsorption and have developed scalable PCH nanofiltration membranes with adjustable sub-1-nm channels through coordination regulation and ion-imprinting technique, achieving desalination and selective Li⁺ transport. The sub-nanochannels of membranes are decisive for salt-retention performance and Li⁺/Mg²⁺ separation. The strong interactions of coordination bonds and hydrogen bonds between the metal ions of PAL nanorods and CMC chains endow the membrane with excellent mechanical strength and stability. The resulting PCH-7 membranes exhibit satisfactory rejection (e.g. 97.4% for Na₂SO₄ and 86.2% for NaCl) and remarkable water permeability (> 25 L m^-2 h^-1 bar^-1), which can be attributed to the coordination of size exclusion and Donnan effect. Furthermore, the P_0.35CH-7 membrane demonstrates rapid and selective transport of Li⁺ based on size exclusion and Mg²⁺-imprinting adsorption mechanisms, with a Li⁺/Mg²⁺ selectivity of up to 175, which is superior to current state-of-the-art membranes. Furthermore, the PCH-7 membrane exhibits excellent antifouling and antimicrobial characteristics. Our coordination regulation and ion-imprinting approach pave the way for the construction and regulation of sub-1-nm channel membranes, which are crucial for the production of clean water, recovery of resources, and energy conversion and storage. The findings of this study offer valuable insights for the design of next-generation separation membranes with enhanced performance.

Acknowledgments

The work was financially supported by the National Natural Science Foundation of China (22377047, 22221001), National Key R&D Program of China (2020YFC1807302), and the China Northern Rare Earth Commission Project (BFXT-2023-D-0049), and Gansu Province Key Research and Development Program (22YF7FA152).

Additional information

Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints.

Correspondence and requests for materials should be addressed to Baodui Wang.

Competing ﬁnancial interests

The authors declare no competing financial interests.

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Tailoring sub-nanochannels and ion-imprinted functionality of palygorskite nanofiltration membranes for desalination and lithium-ion transport

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