Preparation and characterization of PCH membranes.
The metal ions on the PAL skeleton and oxygen atoms of carboxyl groups on the CMC make them a promising candidate for preparing defect-free membranes by coordination self-assembly. A schematic diagram of the preparation process of the PCH membrane and its separation application is displayed in Fig. 1. Specifically, PAL and CMC form a mixed suspension in water through coordination self-assembly, then the mixed suspension is transferred to the polyethersulfone (PES) substrate using the Mylar-rod coating technique (Fig. 2a and Figure S1), and after the solvent evaporates, the functional layer can be easily peeled off from the substrate to obtain a free-standing PCH membrane (Figure S2). The process of assembling the nanorods into a membrane is very simple and eco-friendly. The PCH membranes were designated as PCH-α (α = 3, 5, 7, 9), where α represents the CMC content as shown in Table S1.
The mechanical properties of PCH membranes were further investigated. Taking the PCH-7 membrane as a representation, as shown in Fig. 2b, it not only exhibits a maximum elongation at break of 11.8% and a breaking strength of 12.9 MPa, but also provides sufficient strength to support a weight of at least 50 g even when suspended at the tube edge, and it is only when an external force is applied that the PCH membrane breaks. The outstanding mechanical strength largely stems from the strong coordination interactions between PAL nanorods and CMC. Macroscopically and microscopically (Fig. 2c, d), the PCH membrane exhibits an unabridged and smooth surface without obvious defects. When the membrane undergoes bending, the flexibility is not compromised, indicating that it potentially allows robust handling (Fig. 2e). The cross-sectional morphology of the PCH membrane exhibits a uniform thickness of ~ 10 µm and an obvious lamellar structure (Fig. 2f). Moreover, a well-defined d-spacing from lattice fringes of 0.45 nm is measured from the images of the high-resolution transmission electron microscope (HRTEM), which matches well with the (040) crystal faces of PAL nanorods. The TEM result suggests the PCH membrane retains excellent crystallinity of PAL nanorods (Fig. 2g, Figure S3 and S4). The assembly architecture of PAL and CMC in PCH membranes was elucidated using atomic force microscope (AFM) (Figure S5). The AFM images in tapping-mode show that laminated assemblies gradually form with increasing content of CMC. Concretely, when the content of CMC is lower than 3%, there is almost no assembly but a random arrangement of PAL nanorods and large gaps are observed between the nanorods (Figure S5a). However, despite the formation of the PCH-5 membrane (Figure S5c, d) when the CMC content reaches 5%, we also observe the unassembled PAL nanorods intercalated in the assemblies. Exhilaratingly, the assemblies are regularly and tightly arranged when the content of CMC reaches 7% (Figure S5e, f). With the CMC content in PCH membranes increased to 9% (Figure S5g, h), debris adheres to the surface of the laminated assemblies, which perhaps blocks a part of the 1D channels of PALs. Considering that the assembly process inevitably introduces interstitial pores or partially clogs the sub-1 nm channels of PAL, it is important to tailor the channels of PCH membranes matters by adjusting the CMC content. Increasing the CMC content from 3 to 7 wt% results in a significant narrow in pore size distributions of PCH membranes (Fig. 2h), which would increase the energy barrier for water and ion transport. Especially, the PCH-7 membrane exhibits a similar pore size distribution as PAL nanorods, suggesting the disappearance of large interstitial pores between PAL nanorods. We also notice that when the CMC content further increase to 9 wt%, the sub-1 nm channels of the PCH-9 membrane disappear as a result of the CMC sheltering effect.[17] Moreover, due to the special ion-exchangeability of PAL nanorods, their pores were expanded using the ion-imprinting technique to prepare PβCH-7 nanofiltration membranes, among which β represents the concentration of acid used for treating PAL nanorods. During processing, metal oxides (e.g., MgO) are dissolved out of the PAL, thus regulating the channel size. As shown in Figure S6, PβCH-7 membranes exhibit a larger pore size and a wider pore size distribution in a comparison with the PCH-7 membrane (Fig. 2h). The major pore size of the membrane increases from 6.4 to 8.8 Å with increasing acid concentration from 0.2 to 1.0 M. Energy-dispersive X-ray (EDX) analysis and mappings (Fig. 2i) reveal that PAL and CMC are homogeneously distributed in the PCH membranes.
The interactions between PAL nanorods and CMC in PCH membranes were characterized by various techniques. First, X-ray powder diffraction (XRD) of PCH membrane showed that the coordination cross-linke of CMC do not affect the crystallinity of PAL (Fig. 3a and S7. Then, we analyzed the chemical structure of the PCH membrane using Fourier transform infrared spectroscopy (FTIR) 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. 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 C = O stretching vibration and the appearance of new characteristic peaks at 532, 772 and 827 cm-1 in the PCH membrane imply the formation of coordination-bond between carboxyl groups of CMC chains and metal ions on PAL nanorods.[18] To gain further insights into the coordination bonds, the localized environment around 27Al was probed by solid-state nuclear magnetic resonance (NMR). PAL nanorods show a peak at 1.25 ppm which can be assigned to hexa-coordinate aluminum Al(VI). Compared to PAL nanorods, a 2.38 ppm shift to the lower field is observed in the spectrum of the PCH membrane. Such a variation is generated from the fact that Al sites of PAL nanorods adsorb more oxygen-containing functional groups from CMC chains,[19] which further suggests the coordination between PAL nanorods and CMC chains, a result that is consistent with the FTIR results. To further substantiate this, X-ray photoelectron spectra (XPS) were used to analyze the surface chemistry of PFH membranes. The PCH membrane exhibits the characteristic peaks of Si, Mg, Al, O, and C, which are consistent with the elements in PAL nanorods (Fig. 3d). Obviously, more C-containing content is found in the PCH membrane compared to pure PAL nanorods, suggesting successful preparation of the PCH membrane. The O 1s, Al 2p and Mg 2p spectra indicate PAL nanorods coordinate with CMC chains by sharing the electron pair of the carboxylate groups from CMC chains. The electron-donating effect from carboxylate groups to metal sites results in an appreciable shift of the O = C-O binding energy to a higher value (Fig. 3e, and Table S2) whereas Al 2p and Mg 2p migrate to a lower value (Fig. 3f, S8).[20] Based on the above evidence, it has been confirmed that hydrogen and coordination bonds do exist between metal sites of PAL nanorods and oxygen-containing functional groups of CMCs.
To gain further insights into the interactions, we employed density functional theory (DFT) to calculate the binding energy of the intermolecular interaction within a PAL-CMC system consisting of a PAL nanorod and a CMC chain under different conditions. For the PAL-CMC system (Figure S9), the interaction energy is calculated to be -4.104 eV, which is ascribed to strong interactions between PAL and CMC. Surprisingly, when the PAL-CMC is in water, especially when the water contains sodium ions (Na+), the binding energy can boost up to -5.870 eV, 1.4 times that of the PAL-CMC system in the dry state. This result implies that the PAL-CMC interactions become stronger, which lays the foundation for its use in desalination. The interaction was further substantiated by a deformation charge density calculation (Fig. 3g-i), which reveals that the electron-rich oxygen-containing groups of CMC chains are oriented towards the electron-deficient empty 3s orbitals of Mg2+ cations and 3s and 3p orbitals of Al3+ cations in PAL nanorods, thereby facilitating efficient electron transfer between the two entities. Notably, in the presence of Na+, the system shows the strongest charge density and the gain and loss of electrons are also more pronounced, suggesting that electrons are able to delocalize widely within the PAL-CMC system to bestow additional stability to the hydrogen and coordination bonds.[21]
Water permeation and salt rejection performance.
The interaction of strong hydrogen and coordination bonds not only endows the nanofiltration membrane with excellent mechanical properties but also makes the PAL nanorods in the membrane tightly assembled, whose intrinsic 1D sub-nanometer channels theoretically allow the fast transport of water and impede the permeation of unwanted solutes (Fig. 4a). Especially, the PCH-7 membrane exhibits a similar pore size distribution as PAL nanorods, suggesting the disappearance of large interstitial pores between PAL nanorods. In addition to the sub-1 nm channel size, the surface charge of PCH membranes is the second determinant affecting mass transport. PAL nanorods are inherently negatively charged and the Zeta potential is -26.6 mV (Figure S10). As the content of CMC chains with affluent -COO− groups increases, the value of zeta potential is gradually enhanced up to -71.3 mV, which is related to the Donnan exclusion theory.[22] Based on the channel-target ion size matching and surface charging characteristics, we predict that the PCH-7 membrane has suitable water permeance and excellent salt interception performance.
To check the speculation, the filtration performance of PCH membranes was explored using a reverse osmosis (RO) configuration (Fig. 4b, and Figure S11), in which the PCH membrane suspended on a PES substrate separates the chamber into two, the lower one filled with salt solutions which permeate through the membrane to the upper one. The RO pressure difference induced by applied pressure serves as the driving force for water transport. The optimal sub-nanochannels of PCH membranes were investigated by testing the retention of PCH membranes using 2000 parts per million (ppm) of Na2SO4 at a constant operating pressure of 6 bar. When the CMC dosage is increased to 7 wt% (Fig. 4c), the reduction of interstitial pores in the membrane and the increase of nanochannel transport resistance lead to a decline of water permeance from 65.3 to 25.1 L m− 2 h− 1 bar − 1 and a further increase of Na2SO4 rejection from 47.9 to 98.1%. The excellent Na2SO4 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, because of the 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, and the majority of Na+ are rejected. 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.[17] Furthermore, we also notice that when the CMC content continues to increase to 9 wt%, the retention rate no longer increases but the water permeability decreases by one order of magnitude, which is because the aggregation of excess CMC severely blocks the straight channels of the PAL nanorods in PCH-9 membrane. To this end, doping with 7 wt% CMC helps to assemble monodisperse PAL nanorods into assemblies even membrane materials, and minimize interstitial pores through strong interactions of hydrogen and coordination bonds. The stability of the PCH-7 membrane includes membrane immersion in water and filtration performance was also assessed by combining experimental and simulation methods (Figure S12, Fig. 4d and 4d). Unlike the membrane accumulated by PAL nanorods, which completely disintegrates after 4 min of immersion in water, the PCH-7 membrane maintains excellent structural integrity during up to 45 days of observation. Additionally, the membrane exhibits excellent long-term stability (Fig. 4e), maintaining its initial Na2SO4 nanofiltration property throught the 300-minute cross-flow filtration test. The synergistic effect of size screening and Donnan effect gives the PCH-7 membrane excellent salt retention and rapid water transport. To offer a more intuitive demonstration of its stability, we performed ab initio molecular dynamics (AIMD) simulation on the PCH-7 membrane system at 298 K. As presented in Fig. 4f, the total energy of the system tends to stabilize at about 25 ps, and over time, Na+ in Na2SO4 solution gradually penetrates into the membrane, but the transport stops and Na+ ions are excluded when Na+ ions reach the PAL surface.
To further investigate the desalination performance of the PCH-7 membrane, the water- and salt-transport behaviors were measured in MgSO4, CaCl2, and NaCl solutions, respectively. Figure 4f shows that the PCH-7 membrane possesses high salt rejection between 80.8 and 97.7%, and the rejection follows the order of MgSO4 > Na2SO4 > CaCl2 > NaCl. This trend follows the anion-to-cation valence ratio (Z−/Z+) based on the Donnan exclusion theory [23, 24] and higher salt retention can be obtained at higher Z−/Z+ ratios. Generally, the PCH-7 membrane surface has more negative charges, and therefore, a stronger repulsion is formed for Na2SO4 with a valence ratio of 2, whereas a weaker rejection happens for CaCl2 and NaCl. Interestingly, the PCH-7 membrane exhibits more pronounced nanofiltration performance compared to most membranes that have been reported (Fig. 4g, Table S3), especially the hybrid membrane prepared by PAL nanorods and polyamide.[25] 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 S13), 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 S14) while maintaining a very high water permeance (27.5 L m− 2 h− 1 bar − 1), and this performance does not change significantly over the course of the long 300-minute test.
Magnesium/lithium separation performance and mechanism.
With the rapid development of new energy and other industries, the demand for lithium has increased dramatically. FO membrane separation technology has been proven to be a very efficient and promising technology for lithium extraction from salt lakes. One of the trickiest and most difficult steps is the separation of lithium ions (Li+) and magnesium ions (Mg2+).[26, 27] Due to the special ion-exchangeability of PAL nanorods, their pores were expanded using the ion-imprinting technique to prepare PβCH-7 nanofiltration membranes for Mg2+/Li+ separation (Figure S6). During processing, metal oxides (e.g., MgO) are dissolved out of the PAL, thus regulating the channel size. Notably, treatment with 0.2 M mixed acid (hydrochloric acid : oxalic acid = 1:1) has little effect on the retention of Mg2+, whereas the retention of Li+ in the mixture is greatly reduced from 68.0 to 8.1%, with a high Mg2+/Li+ separation factor (\({S}_{{Mg}^{2+}/{Li}^{1+}}=32.5\)). As the sub-1 nm channel size of PCHβ-7 membranes continues to increase, the transport of Li+ through the channels in the membrane remains particularly unchanged, while on the contrary, Mg2+ passes through more easily, leading to a gradual decrease in the separation ratio to 1.4 (Fig. 5a). The PCH0.2-7 membrane exhibits the most excellent Mg2+/Li+ separation performance in mixed solution (feed solution: Mg2+/Li+=1). As the Mg2+/Li+ ratio in the feed solution increases to 30, the Mg2+/Li+ selectivity declines slightly to 26.9 but the water permeance remains high (Fig. 5b).
The Mg2+/Li+ separation mechanism was also further investigated based on Ab-initio molecular dynamics (AIMD) simulations (Fig. 5c). The results show that the system achieves stability within 25 ps, and when time goes to 100 ps, the P0.2CH-7 membrane allows Li+ to permeate but rejects Mg2+. The pore size of the P0.2CH-7 membrane ranges from 5.4 to 12.2 Å, and the hydrated diameters of Li+ and Mg2+ are 7.64 Å and 8.56 Å, respectively. [28] In addition, the very high hydration energy of Mg2+ (-1830 kJ/mol) is 3.8 times higher than that of Li+.[29] The lower the hydration energy, the easier it is for hydrated ions to shed water molecules, resulting in rapid ion transport through the membrane. Therefore, the P0.2CH-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 P0.2CH-7 membrane intercept them through a size-sieving mechanism, on the other hand, even if the pores with larger sizes in the P0.2CH-7 membrane allow the Mg2+ to enter the interior of the membrane, they are captured by the missing metal sites in the PAL skeleton through an ion-imprinted adsorption mechanism. In retrospect, the simulation results are in agreement with our experimental results, which also indicate that the P0.2CH-7 membrane achieves high-efficiency Mg2+/Li+ separation in mixed solutions based on size screening and ion-imprinted adsorption mechanisms.
Antifouling and antimicrobial performance.
In practical applications, good antifouling and antimicrobial properties are probably the crucial factors for a nanofiltration membrane. The surface of CMC and PAL contains plentiful hydroxyl groups, all of which endow PCH membranes with inherent hydrophilicity. Digital photos of hydrophilicity studies show the water contact angle of 12.2°, when a water droplet touches the PCH-7 membrane surface (Figure S15a, b), further confirming its good hydrophilicity. The hydrophilicity drives the formation of a hydration layer in the membrane surface, which reduces the affinity toward organic micropollutants to improve the antifouling property [2]. Here bovine serum albumin (BSA) was used as the representative foulant to further evaluate the antifouling performance of PCH membranes. The antifouling indexes of the PCH membrane are presented in Table S4. The permeation recovery ratio (FRR) of the PCH-7 membrane reaches 94.6% for BSA foulant, indicating that the membrane suppresses the fouling of protein. In particular, the nanorod-shaped crystal structure and adsorption properties of metallic PAL nanorods make it also able to kill bacteria.[30] Therefore, the bacteria Staphylococcus aureus (S. aureus) and Escherichia coli (E.coli) were selected to evaluate the antimicrobial activity of the PCH-7 membrane. Antimicrobial tests (Figure S15c, 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 PCH-7 membrane application potential in the industry.