Hydroxyapatite-doped metal polyphenol network composite nanofiltration membrane and preparation method thereof

Abstract

The invention discloses a hydroxyapatite-doped metal polyphenol network composite nanofiltration membrane and a preparation method thereof, and belongs to the technical field of membrane separation. The composite nanofiltration membrane is prepared by the following steps: providing a porous polymer base membrane; metal ions (such as Fe < 3 + >) are loaded on the surface of a base membrane to serve as a coordination center site, polyphenol (such as tannic acid) serves as a ligand, and a separation layer is prepared on the surface of the base membrane through coordination reaction of the metal ions and the polyphenol; and the hydroxyapatite (HAP) nanowire is used as a separation layer filler to improve the performance of the composite membrane. According to the invention, HAP is preferably doped in a TA solution at a specific concentration to prepare a dispersion liquid. Compared with a coordination selective membrane which is not doped with HAP, the composite nanofiltration membrane prepared by the method has the advantages that the permeation flux of pure water is remarkably improved and the surface hydrophilicity of the membrane is improved while a relatively high dye retention rate is maintained, good long-term operation stability is shown, and the influence of a tradeoff effect between the flux and the retention rate of the existing composite nanofiltration membrane is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to the field of membrane separation technology, and specifically relates to a method for preparing a composite nanofiltration membrane, in particular to a composite nanofiltration membrane with improved performance by doping a modified metal polyphenol network (MPN) separation layer with hydroxyapatite (HAP) and a preparation method thereof. Background Art

[0002] With the expansive development of the global industrial market, water pollution is becoming increasingly serious. Among them, the disorderly discharge of dye wastewater has become one of the major challenges facing environmental governance. In the fields of water pollution control and water resource recycling, efficient membrane separation technology is widely used to solve such problems. Nanofiltration membranes have important application value in fields such as seawater desalination, wastewater treatment and drinking water purification due to their high separation efficiency, low energy consumption and wide applicability. However, traditional nanofiltration membranes have exposed defects such as attenuated separation performance, poor anti-pollution ability and insufficient mechanical strength in actual use. These factors have restricted their large-scale application. Therefore, the development of new nanofiltration membrane materials that have both high separation performance, excellent anti-pollution ability and good stability has become a key direction of current membrane technology research.

[0003] In recent years, metal polyphenol networks (MPNs), as a new type of organic-inorganic hybrid material, have received widespread attention in the field of membrane material modification due to their adjustable structure, strong chemical stability, and rich surface functional groups. MPN achieves rapid self-assembly of the substrate based on the metal ion-polyphenol coordination effect. Its polyphenol hydroxyl structure not only enhances the hydrophilicity of the material, but also imparts excellent anti-fouling properties. At the same time, hydroxyapatite (HAP), as an inorganic material with good biocompatibility, has shown unique advantages in membrane material enhancement and functional modification due to its high specific surface area, rich surface active sites, and excellent mechanical properties. Combining HAP with MPN is expected to further enhance the comprehensive performance of composite nanofiltration membranes through the synergistic effect of the two.

[0004] Therefore, it is of great practical significance and application value to develop a new composite nanofiltration membrane that can combine the advantages of MPN and HAP to prepare a new type of composite nanofiltration membrane with high permeation flux, excellent retention performance, good anti-pollution ability and long-term stability. Summary of the Invention

[0005] The present invention aims to solve the problem that the existing MPN nanofiltration membrane may have low permeation flux or a significant decrease in retention rate when the flux is increased in practical applications. The present invention provides a preparation method of a hydroxyapatite (HAP)-doped metal polyphenol network (MPN) composite nanofiltration membrane. The MPN thin-layer composite nanofiltration membrane uses polyacrylonitrile (PAN) material as a substrate, tannic acid (TA) solution and FeCl3 solution as a double aqueous phase, and constructs a metal polyphenol network (MPN) separation layer through a coordination reaction. Then, by introducing hydroxyapatite into the tannic acid solution, the permeation flux of the composite nanofiltration membrane is improved without sacrificing the retention rate of Congo red (CR) dye.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a hydroxyapatite (HAP)-doped metal polyphenol network (MPN) composite nanofiltration membrane, the method comprising the following steps:

[0008] (1) Preparation of porous polymer-based membranes: A porous polymer-based membrane, such as a polyacrylonitrile (PAN)-based membrane, is prepared using a non-solvent-induced phase inversion method. A polymer (e.g., PAN) is dissolved in a good solvent (e.g., N,N-dimethylformamide, DMF) to form a casting solution, which is then applied to a support to form a primary membrane. The membrane is then immersed in a non-solvent (e.g., pure water) for phase inversion to form a porous base membrane (M0), which is then washed and set aside.

[0009] (2) Metal ion loading: The porous base membrane prepared in step 1) is immersed in a solution containing metal ions, so that the metal ions are adsorbed or loaded on the surface and pores of the base membrane.

[0010] (3) Construction of a HAP-doped MPN layer: Hydroxyapatite (HAP) nanowires are dispersed in a polyphenol solution via ultrasonic treatment to form a HAP-polyphenol mixed dispersion. The base membrane, which has been treated with metal ions in step 2), is then brought into contact with the HAP-polyphenol mixed dispersion, causing a coordination reaction between the polyphenol molecules and the metal ions loaded on the base membrane surface. Simultaneously, the HAP is coated or embedded in the formed MPN network structure, thereby constructing a HAP-doped MPN separation layer.

[0011] (4) Post-treatment: The formed composite nanofiltration membrane is washed with pure water or other suitable solvents to remove unreacted monomers and by-products to obtain the final HAP-MPN composite nanofiltration membrane.

[0012] Preferably, the polymer-based membrane in step (1) is a polyacrylonitrile (PAN) membrane. The concentration of the polymer in the casting solution is 75 wt %. The phase inversion is performed by rapidly immersing the nascent membrane in pure water at room temperature for phase inversion treatment, and then allowing it to stand for 8 to 15 minutes to form a porous structure under the induction of a non-solvent.

[0013] Preferably, the metal ion in step (2) is Fe 3+ , which is derived from ferric chloride (FeCl3). The concentration range of the FeCl3 solution is 1.0 g·L -1 ~6.0g·L -1 , more preferably 4.0 g·L -1 .

[0014] Preferably, the polyphenol in step (2) is tannic acid (TA). The concentration range of the TA solution is 1.0 g·L -1 ~6.0g·L -1 , more preferably 2.0 g·L -1 .

[0015] Preferably, the doping concentration of the hydroxyapatite (HAP) in the TA solution in step 3) is in the range of 0.05 g·L -1 ~2.00g·L -1 , more preferably 0.10 g·L -1 .

[0016] Preferably, the coordination reaction time in step 3) is 10 seconds to 120 seconds, more preferably 60 seconds.

[0017] Preferably, in step 3), HAP is doped into TA solution, TA is used to modify the surface of HAP, and then the surface of HAP is modified with Fe-loaded 3+ The basement membrane undergoes coordination reaction.

[0018] The present invention also provides a HAP-MPN composite nanofiltration membrane prepared by the above method. The composite nanofiltration membrane comprises a porous polymer base membrane and a HAP-doped MPN separation layer constructed on the surface of the base membrane, wherein the HAP is distributed in the MPN network in the form of nanowire fillers.

[0019] A method for treating an aqueous solution using a composite nanofiltration membrane, the method comprising passing the aqueous solution to be treated through the composite nanofiltration membrane under a nanofiltration operating pressure to achieve at least one of the following purposes: a) removing dye molecules dissolved in the aqueous solution; b) reducing the concentration of divalent inorganic salts in the aqueous solution; and c) separating the dye molecules dissolved in the aqueous solution from the inorganic salts.

[0020] The hydroxyapatite-metal polyphenol network (HAP-MPN) composite nanofiltration membrane prepared by the present invention has high permeation flux, high retention rate for specific solutes (such as dye molecules), selective separation ability for salt ions of different valence states, improved hydrophilicity and good operational stability. It has broad application potential in multiple fields, especially in water treatment and resource recovery processes that require fine molecular level separation, such as printing and dyeing wastewater treatment and recovery, surface water / drinking water purification, industrial process water treatment and material separation, seawater desalination pretreatment, etc.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention introduces HAP into the MPN layer and optimizes the doping method and doping amount, so that the pure water permeation flux of the composite nanofiltration membrane is significantly improved compared with the MPN membrane without HAP doping.

[0023] (2) The HAP-MPN composite nanofiltration membrane prepared by the present invention significantly improves the permeation flux while still maintaining a high retention rate for target pollutants (such as Congo red dye), achieving a comprehensive improvement in flux and retention rate.

[0024] (3) The introduction of HAP in the present invention is conducive to the formation of a more uniform MPN layer structure, which improves the hydrophilicity of the composite membrane surface and is conducive to improving the anti-fouling performance and permeation flux.

[0025] (4) The addition of HAP in the present invention provides additional active sites for the formation of MPN or regulates the coordination reaction process, which helps to form a more uniform MPN separation layer with fewer defects, thereby improving the overall performance of the membrane.

[0026] (5) The HAP-MPN composite nanofiltration membrane prepared in the present invention exhibited good performance stability in long-term operation tests, with stable changes in permeation flux and rejection rate, and has practical application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the process of preparing PAN membrane using non-solvent induced phase separation method.

[0028] Figure 2 Schematic diagram of the coordination reaction experimental steps and products.

[0029] Figure 3 FT-IR spectra of PAN film (M0), MPN composite film (M4-3-3) and MPN-HAP composite film (M7-2).

[0030] Figure 4 XPS spectra of PAN, MPN, and MPN-HAP films. (a) Overall spectrum; (b) C1s of PAN film; (c) Fe2p of MPN film; (d) C1s of MPN film; (e) Ca2p of MPN-HAP film; (f) P2p of MPN-HAP film.

[0031] Figure 5 For different HAP doping amounts (0, 0.1, 1.0, 2.0 g·L -1 ) of MPN-HAP film before and after etching.

[0032] Figure 6 For different HAP doping amounts (0.1, 1.0, 2.0 g·L -1 ) of MPN-HAP film (after etching).

[0033] Figure 7 High-resolution XPS spectra of C1s and O1s of MPN and MPN-HAP films. (a) MPN film C1s; (b) MPN film O1s; (c) MPN-HAP film C1s; (d) MPN-HAP film O1s.

[0034] Figure 8 SEM images of the surface (X1) and cross-section (X2) of MPN membranes prepared with different FeCl3 concentrations (af correspond to membranes M1-M6, respectively).

[0035] Figure 9 SEM images of the surface (X1) and cross-section (X2) of the MPN membrane prepared with different coordination reaction times (a, b, c correspond to 10s, 60s, and 90s, respectively).

[0036] Figure 10 SEM images of the surface (X1) and cross-section (X2) of MPN-HAP films prepared with different HAP doping methods (a, b, c correspond to films M7, M8, and M9, respectively).

[0037] Figure 11 For different HAP doping amounts (a, b, c correspond to 0.1, 1.0, 2.0 g·L -1 ) surface (X1) and cross-section (X2) SEM images of the MPN-HAP membrane (M7 series) prepared.

[0038] Figure 12 These are the water contact angle test results of different films (M0, M4-3-3, M7-2, M7-6, etc.).

[0039] Figure 13 For different TA / Fe 3+ Effect of concentration on the separation performance (CR flux and rejection) of MPN membranes. (a) Fixed TA concentration and varying FeCl3 concentration; (b) Fixed FeCl3 concentration and varying TA concentration.

[0040] Figure 14 The effect of coordination reaction time on the separation performance (CR flux and retention rate) of MPN membrane (M4-3 series).

[0041] Figure 15 The effects of different HAP doping methods (M7, M8, M9) on the separation performance of MPN-HAP membrane.

[0042] (a) Permeate flux; (b) CR retention rate.

[0043] Figure 16 The effect of different HAP doping amounts (M7 series) on the separation performance (CR flux and retention rate) of MPN-HAP membrane.

[0044] Figure 17 Separation performance of MPN membrane (M4-3-3) and MPN-HAP membrane (M7-2) for different single-component salt solutions (Na2SO4, MgSO4, MgCl2, NaCl). (a) Permeation flux; (b) Retention rate.

[0045] Figure 18 The separation performance of MPN membrane and MPN-HAP membrane for the dye / inorganic salt two-component system. (a, b) Fixed CR mass concentration and Na2SO4 mass concentration; (c, d) Fixed Na2SO4 mass concentration and varying CR mass concentration.

[0046] Figure 19 (A) Molecular weight cut-off (MWCO) curves and (B) pore size distribution curves of MPN membrane (M4-3-3) and MPN-HAP membrane (M7-2).

[0047] Figure 20 Long-term stability test results. (a) MPN membrane (M4-3-3); (b) MPN-HAP membrane (M7-2). DETAILED DESCRIPTION

[0048] The present invention is described in detail below by way of examples, but the protection scope of the present invention is not limited to these examples.

[0049] Performance testing method:

[0050] Membrane permeation flux (J): The solvent flux and solute retention of the membrane were tested using a cross-flow filtration device. All tests were performed at a pressure of 0.4 MPa. J = V / (A·t·p), unit: L·m -2 ·h -1 bar -1 v is the volume of permeate, L; A is the effective area of ​​the test membrane, m 2 ; t is the test time, h; p is the penetration pressure, bar.

[0051] Retention rate (R): R = (1-Cp / Cf) × 100%, where Cf is the feed solution concentration and Cp is the permeate concentration. Salt concentration was measured using a conductivity meter, and dye concentration was measured using an ultraviolet spectrophotometer.

[0052] Separation factor (α): α = (1-R salt ) / (1-R dye ), used to evaluate dye / salt separation performance.salt is the retention rate of the membrane for inorganic salts, %; R dye is the retention rate of the membrane for dye molecules, %.

[0053] Molecular weight cut-off (MWCO) and pore size distribution: PEG solutions with different molecular weights (200, 400, 600, 800, 1000, and 2000 Da) were used for testing. The MWCO was defined as the PEG molecular weight with a retention rate of 90%, and the average pore size was calculated based on the Stokes diameter.

[0054] Long-term stability: The test was conducted continuously for 120 hours at 0.4 MPa using salt solutions of different concentrations as feed liquid, and the flux and retention rate were detected regularly.

[0055] Example 1: Preparation of PAN-based film (M0)

[0056] (1) In a 100 mL stirring container, DMF (75 wt%) and PAN powder (25 wt%) were weighed, mixed and placed in the container to obtain a mixed solution.

[0057] (2) The mixed solution was placed in a water bath constant temperature magnetic stirrer, the heating temperature was set to 60 ° C, and the reaction was continued with stirring at this temperature for 12 h to ensure that the PAN powder was fully dissolved and a homogeneous casting solution was formed, and the mixture was allowed to stand for degassing for 2 h.

[0058] (3) After degassing, the casting solution was evenly poured onto a clean glass plate. The homogeneous casting solution was evenly poured onto a pre-treated clean glass plate and scraped using a stainless steel scraper with a thickness of 200 μm to form a primary film.

[0059] (4) After the scraping is completed, the primary membrane is quickly immersed in pure water at room temperature for phase inversion treatment and allowed to stand for ten minutes to form a porous structure under the induction of non-solvent.

[0060] (5) After the phase inversion is completed, the prepared PAN-based membrane is immersed in ethanol for 24 h to remove the residual solvent.

[0061] (6) The PAN-based membrane was stored in pure water for later use, and was recorded as M0.

[0062] Test M0 performance: pure water flux is 247.3L·m -2 ·h -1 bar -1 The rejection rate of CR dye (Congo red) was 39.7%, indicating that M0 has the properties of an ultrafiltration membrane.

[0063] Example 2: Preparation of MPN composite nanofiltration membrane (exploring the influence of FeCl3 concentration)

[0064] (1) Take the PAN base film (M0) prepared in Example 1, gently press it with a rubber roller to remove excess moisture on the surface, and then fix it in the coordination reaction membrane assembly.

[0065] (2) Then different concentrations of FeCl3 (1.0 g·L -1 -6.0g·L -1 ) solution, pour FeCl3 solution on the surface of PAN membrane and let it stand for 2 minutes to load the precursor to ensure that Fe 3+ Distribute evenly on the membrane surface. Pour off excess FeCl3 solution and gently press with a rubber roller to remove the remaining solution.

[0066] (3) will pass through Fe 3+ The pre-adsorption treated membrane was fixed in the membrane module, and the concentration of 2.0 g·L -1 The TA solution was poured into the Fe 3+ The coordination reaction is carried out on the loaded membrane surface, and the reaction time is controlled to be 60s.

[0067] (4) After the reaction is completed, the membrane is repeatedly washed with pure water for 5 minutes. After washing, the preparation of the MPN composite nanofiltration membrane is completed.

[0068] (5) The prepared MPN composite nanofiltration membrane is stored in pure water for future use.

[0069] A series of composite nanofiltration membranes prepared using FeCl3 solutions of different concentrations are named M1, M2, M3, M4, M5, and M6. Detailed information is shown in Table 1.

[0070] Table 1 Detailed information of different MPN composite nanofiltration membranes (FeCl3 mass concentration variables)

[0071]

[0072]

[0073] Figure 8 The following are SEM images of the membrane surface and cross-section corresponding to different FeCl3 concentrations. (X1) Surface image; (X2) Cross-section image; a, b, c, d, e, and f correspond to membranes M1, M2, M3, M4, M5, and M6, respectively. The surface morphology and cross-section structure of the membrane were characterized by SEM. The surface of the PAN membrane prepared by the non-solvent phase inversion method showed a significant smooth feature, with no obvious defects and crack structures observed. The cross-section showed an asymmetric finger-like pore structure. The membrane surface was distributed with a uniform and regular porous structure, and the intervals between the pores were evenly distributed. This structural feature not only reflects the good controllability of the preparation process, but also provides a uniform mass transfer channel for the nanofiltration membrane during the separation process. With the construction of the MPN layer on the surface of the PAN membrane, the membrane surface morphology changed significantly. The TA and Fe 3+The complex formed by the reaction covered the smooth substrate surface. As the FeCl3 concentration increased (from 1.0 g·L -1 ~6.0g·L -1 ), the complex size increased significantly, from an initial 20 nm to 200 nm. Cross-sectional SEM analysis further revealed that the MPN layer thickness increased significantly with increasing FeCl₃ concentration, from 30 nm to 300 nm. This not only demonstrates the successful deposition of the MPN layer on the substrate surface, but also suggests that FeCl₃ concentration significantly regulates the morphology and thickness of the MPN layer.

[0074] Figure 13 In order to investigate the effect of FeCl3 concentration on membrane separation performance (TA concentration and coordination reaction time were fixed at 2.0 g·L -1 and 60s). The TA concentration was fixed at 2.0 g·L -1 By changing the FeCl3 concentration to explore the effect on the separation performance of MPN composite nanofiltration membrane, it was found that when the FeCl3 concentration was 4.0 g·L -1 When the FeCl3 concentration increases further, the permeation flux begins to increase continuously and the retention rate begins to decrease, indicating that the FeCl3 concentration is too high and the phenolic hydroxyl groups in the TA molecules react with Fe 3+ When the coordination reaction occurs, part of Fe 3+ May not fully coordinate with TA, resulting in the formation of unstable complexes or free Fe 3+ .

[0075] Example 3: Preparation of MPN composite nanofiltration membrane (exploring the effect of TA concentration)

[0076] The FeCl3 concentration (4.0 g·L -1 ) was used as a benchmark, and the optimal parameter M4 was selected for subsequent systematic testing of tannic acid (TA) concentration gradient.

[0077] (1) Take the PAN base film (M0) prepared in Example 1, gently press it with a rubber roller to remove excess moisture on the surface, and then fix it in the coordination reaction membrane assembly.

[0078] (2) Then prepare 4.0g·L -1 Concentrated FeCl3 solution was poured onto the surface of PAN membrane and allowed to stand for 2 min for precursor loading to ensure that Fe 3+ Distribute evenly on the membrane surface. Pour off excess FeCl3 solution and gently press with a rubber roller to remove the remaining solution.

[0079] (3) will pass through Fe 3+ The pre-adsorption treated membrane was fixed in the membrane module, and different concentrations of 1.0 g·L-1 ~6.0g·L -1 The TA solution was poured into the Fe 3+ The coordination reaction is carried out on the loaded membrane surface, and the reaction time is controlled to be 60s.

[0080] (4) After the reaction is completed, the membrane is repeatedly washed with pure water for 5 minutes. After washing, the preparation of the MPN composite nanofiltration membrane is completed.

[0081] (5) The prepared MPN composite nanofiltration membrane is stored in pure water for future use.

[0082] By sequential deposition method, different concentrations (1.0 g·L -1 ~6.0g·L -1 ) TA solution was poured onto the substrate surface, and the MPN layer was constructed by the coordination interaction between metal ions and polyphenol molecules. The specific experimental steps were consistent with the above method. The MPN composite nanofiltration membrane samples prepared using different concentrations of TA solution were named M4-1, M4-2, M4-3, M4-4, and M4-5.

[0083] Table 2 Detailed information of different MPN composite nanofiltration membranes (TA mass concentration variables)

[0084]

[0085] The FeCl3 concentration and coordination reaction time were fixed, and the TA concentration was changed. The CR retention test was carried out. The data analysis showed that with the increase of TA concentration, the flux decreased and the retention rate increased. However, when the TA concentration was increased to 2.0 g·L -1 When the retention rate is maintained at a stable state, the retention rate does not show an upward trend when the concentration is increased. Instead, the flux decreases significantly. The reason is that as the membrane thickness increases, the channels between the MPN layers become longer, the time for the dye solution to pass through the membrane increases, and the mass transfer resistance increases, resulting in a decrease in the permeation flux. This high concentration of TA will accelerate the coordination reaction rate, causing the MPN layer to form quickly, which may make the reaction uneven and cause roughness and defects on the membrane surface. Therefore, the optimal TA concentration is set at 2.0g·L -1 .

[0086] Example 4: Preparation of MPN Composite Nanofiltration Membrane (Investigating the Effect of Coordination Reaction Time)

[0087] Based on the experimental results of Example 3, the optimal coordination parameters were determined to be M4-3. In the preparation method of M4-3, only the coordination reaction time was changed.

[0088] The coordination reaction time was systematically optimized. Under the condition of a fixed reaction monomer concentration, time was used as the independent variable, and a time gradient of 10, 30, 60, 90, and 120 seconds was set to investigate the effect of coordination reaction time on the structure and performance of the MPN composite nanofiltration membrane. A series of composite nanofiltration membrane samples prepared using this method were named M4-3-1, M4-3-2, M4-3-3, M4-3-4, and M4-3-5.

[0089] Table 3 Detailed information of different MPN composite nanofiltration membranes (reaction time variables)

[0090]

[0091] Figure 9 The SEM images of the membrane surface and cross section corresponding to different coordination reaction times. (X1) Surface image; (X2) Cross section; a: Reaction time 10s; b: 60s; c: 90s. The coordination reaction time of TA and FeCl3 also has an important influence on the size of the coordination layer complex. The experiment used coordination reaction time of 10s, 30s, 60s, 90s and 120s as a time gradient for comparison. The SEM surface morphology shows that at the initial stage of the reaction, TA and Fe 3+ The coordination reaction has just started, and the MPN layer has not yet fully formed, resulting in a rough and discontinuous membrane surface and cross-sectional morphology. As the reaction progresses to the middle stage, the coordination reaction gradually becomes complete, the MPN tends to become denser and more uniform, and the surface and cross-sectional morphology also tends to be smoother and more continuous. However, when the reaction enters the late stage, the coordination reaction approaches saturation, and excess monomers do not react completely on the surface, forming a porous structure. This shows that optimizing the reaction time is a key factor in regulating the MPN network structure and its properties. By precisely controlling the reaction process, the ideal construction and functional application of MPN networks can be achieved.

[0092] Figure 14 The influence of coordination reaction time on membrane separation performance was investigated. The reaction time gradient was set to 10, 30, 60, 90, and 120 s, and other reaction conditions were fixed. As the reaction time increased, the separation performance of the membrane improved to a certain extent. However, when the reaction time exceeded 60 s, the membrane permeation flux to CR would drop significantly, from 40.25 L·m -2 ·h -1 bar -1 Reduced to 15.61 L·m -2 ·h -1 bar -1At the same time, the rejection rate did not improve accordingly, only increasing from 90.33% to 91.52%. The improvement in separation performance was minimal. The reasons for this were the short coordination time, incomplete reaction, uneven distribution of the complex, and limited performance improvement. If the reaction time was long, the complex would over-aggregate, which might clog the membrane pores and cause a decrease in permeation flux, without any significant improvement in separation performance. Therefore, the reaction time was moderate to ensure that the complex was fully formed and evenly distributed, significantly improving the membrane's rejection rate and anti-fouling performance while maintaining a high water flux. Therefore, the optimal reaction time was determined to be 60s.

[0093] Example 5: Preparation of HAP-doped MPN composite nanofiltration membrane

[0094] Membrane M4-3-3 was selected as the reference membrane, and the HAP doping concentration was fixed at 0.10 g·L -1 .

[0095] (1) Take the PAN base film (M0) prepared in Example 1, gently press it with a rubber roller to remove excess moisture on the surface, and then fix it in the coordination reaction membrane assembly.

[0096] (2) Then prepare 4.0g·L -1 Concentrated FeCl3 solution was poured onto the surface of PAN membrane and allowed to stand for 2 min for precursor loading to ensure that Fe 3+ Distribute evenly on the membrane surface. Pour off excess FeCl3 solution and gently press with a rubber roller to remove the remaining solution.

[0097] (3) Weigh HAP nanowires and disperse them in a solution with a concentration of 2.0 g·L -1 TA solution, so that the final concentration of HAP is 0.10 g·L -1 , and stir evenly to form a HAP-TA mixed dispersion.

[0098] (4) Step (2) will pass through Fe 3+ The pre-adsorption treated membrane is fixed in the membrane assembly, and the HAP-TA mixed dispersion prepared in step 2) is poured onto the membrane treated with Fe 3+ The coordination reaction is carried out on the loaded membrane surface, and the reaction time is controlled to be 60s.

[0099] (4) After the reaction is completed, the membrane is repeatedly washed with pure water for 5 minutes. After washing, the HAP-MPN composite nanofiltration membrane is prepared.

[0100] (5) The prepared HAP-MPN composite nanofiltration membrane was stored in pure water for future use and recorded as M7-2.

[0101] Example 6: Effect of HAP doping amount on membrane separation performance

[0102] A series of MPN-HAP composite nanofiltration membranes were prepared according to the method of Example 5. Only the concentration of HAP in the TA solution in step 3) was changed to 0.05 (M7-1), 0.10 (M7-2), 0.50 (M7-3), 1.00 (M7-4), 1.50 (M7-5), and 2.00 (M7-6) g·L -1 and the control membrane M7-0 (i.e., M4-3-3) without HAP doping.

[0103] The HAP doping effect was optimized and different HAP doping concentrations were set. The obtained film samples were named M7-0, M7-1, M7-2, M7-3, M7-4, M7-5 and M7-6.

[0104] Table 4 Detailed information of different MPN composite nanofiltration membranes (HAP doping amount variable)

[0105]

[0106] In order to explore the effect of HAP doping amount on the membrane surface morphology, SEM characterization of MPN-HAP composite membranes with different HAP doping amounts was carried out. Figure 11 The results show that as the HAP doping amount increases from 0.1 g·L -1 , gradually increased to 2.0 g·L -1 The surface morphology of the film gradually becomes denser, and the MPN layer almost completely fills the surface defects. -1 ) condition, compared with the MPN composite membrane without HAP doping, the obvious surface defects and uneven distribution were improved, the cross-sectional morphology was complete and continuous, and no fracture occurred. -1 ) conditions, within the same reaction time, the distribution of the MPN layer became more uniform and the thickness increased significantly, which indicated that the addition of HAP played a key role in optimizing the membrane surface morphology.

[0107] The hydrophilicity of the membrane surface was evaluated by water contact angle (WCA) measurement. Figure 12 As shown in the figure, the initial WCA of the PAN-based membrane (M0) is 77.2°, while after the MPN coordination (M4-3-3) reaction, the WCA drops significantly to 65.4°, indicating that the introduction of the MPN layer effectively improves the hydrophilicity of the membrane surface. Further HAP doping further optimizes the hydrophilic properties of the membrane. When HAP is introduced into the MPN composite membrane, the WCA is further reduced to 62.1° due to the denser distribution of the surface complex. The WCA of the composite nanofiltration membrane prepared under different HAP doping conditions shows a trend of significantly decreasing with increasing HAP doping amount. For example, when the HAP doping amount is 0.10 g·L -1When the HAP doping amount is increased to 2.00 g·L -1 When the WCA further decreases to 43.9°.

[0108] Figure 16 The effect of different HAP doping amounts on membrane separation performance can be seen. At lower doping amounts (0.05-0.10 g·L -1 ), the overall permeation flux of the MPN-HAP composite membrane was significantly improved, from the initial pure water permeation flux of the MPN composite membrane to 50.9 L·m -2 ·h -1 bar -1 Increased to 99.3 L·m -2 ·h -1 bar -1 , the rejection rate of CR dye only decreased from 90.3% to 87.6%. While improving the permeation flux of the membrane itself, a small part of the dye rejection rate was sacrificed. Through water contact angle, SEM characterization and separation performance data feedback, it can be determined that the improvement of permeation flux is due to the uniform dispersion of HAP in TA and Fe 3+ The MPN layer formed plays a role in optimizing the pore size distribution of the membrane and reducing defects, thereby improving the density and uniformity of the membrane and maintaining a high retention rate. However, as the HAP doping amount increases further, when the HAP doping amount is increased from 0.1 g·L -1 Increased to 2.0 g·L -1 The data showed that the permeation flux of MPN-HAP composite nanofiltration membrane decreased significantly, but the increase in rejection rate was minimal, reaching 2.0 g·L -1 When the concentration of HAP was less than 1%, the performance of MPN-HAP composite nanofiltration membrane was not much different from that of MPN membrane. The possible reason was that HAP agglomerated and could not be fully dispersed, resulting in uneven microstructure of the membrane. From the SEM image, it can be seen that local defects or oversized pores were formed, thereby reducing the retention performance of the membrane.

[0109] Example 7: Separation performance of membrane for single component of inorganic salt

[0110] The MPN composite nanofiltration membrane (M4-3) and MPN-HAP composite nanofiltration membrane (M7-2) were tested for single component salt solution separation performance. Four inorganic salt solutions (1 g / L) of Na2SO4, NaCl, MgSO4 and MgCl2 were tested respectively. The membranes prepared with the optimal parameters adjusted in the above experiments were tested. The effects of the membrane permeation separation performance were shown in Figure 2. Figure 17 shown.

[0111] Compared with MPN composite nanofiltration membrane (M4-3), the permeation flux of MPN-HAP composite nanofiltration membrane (M7-2) for single component salt solution is greatly improved with the doping of HAP. At the same time, the rejection rate of salt ions inevitably decreases. In comparison, the MPN-HAP composite nanofiltration membrane doped with HAP has the best improvement in the permeation flux of Na2SO4, with a permeation flux of 73.5 L·m -2 ·h -1 bar -1 , is MPN composite nanofiltration membrane (40.3L·m -2 ·h -1 bar -1 ) is 1.82 times that of the MPN membrane. The doping with HAP improves the hydrophilicity of the composite membrane, which is also confirmed by the water contact angle characterization results. The retention rate of MPN and MPN-HAP membranes for the four salts is in the order of Na2SO4>MgSO4>MgCl2>NaCl.

[0112] Example 8: Separation performance of membrane for dye / inorganic salt two-component system

[0113] CR was used as a representative of dye molecules and Na2SO4 solution was used as a representative of salt ions for testing. The experiments were conducted by fixing the CR concentration and setting the Na2SO4 concentration gradient, and by fixing the Na2SO4 concentration and setting the CR concentration gradient. The effects of the membrane permeation and separation performance were shown in Figure 2. Figure 18 As shown, Figure 18 a and b are the separation performances of MPN and MPN-HAP membranes for different concentrations of Na2SO4 and 100 ppm Congo red; Figure 18 c and d are the separation performances of MPN and MPN-HAP membranes for different concentrations of Congo red and 1000 ppm of Na2SO4.

[0114] Performance evaluations of MPN (M4-3) and MPN-HAP membranes (M7-2) revealed significant improvements in the rejection efficiency of two-component solutions (Na2SO4 and CR) compared to the separation performance of single-component solutions, while the permeate flux decreased to a certain extent. Specifically, when the CR concentration was constant, the membrane permeate flux decreased rapidly with increasing Na2SO4 concentration (250-1500ppm); however, the flux stabilized after the Na2SO4 concentration reached 1500ppm. Similarly, when the Na2SO4 concentration remained constant, the flux also decreased significantly with increasing CR concentration (250-1000ppm), and the flux became stable after the CR concentration reached 1000ppm. It is worth noting that the separation factors of MPN and MPN-HAP membranes in the dye / inorganic salt two-component system are shown in Table 5. The highest α of the MPN-HAP membrane in the two-component solution is only 8.45. This result indicates that the doping of HAP only constructs mass transfer channels on the surface of the MPN membrane, which increases the permeation flux of the solution, but does not optimize the membrane pore size distribution. As a result, the separation factor in the dye / inorganic salt two-component system is not substantially improved.

[0115] Table 5 Separation factors of MPN membrane and MPN-HAP composite membrane for dye / inorganic salt two-component system

[0116]

[0117] Example 9: Testing of molecular weight cut-off and pore size distribution of membranes

[0118] like Figure 19 As shown in Figure 2, MPN (M4-3) has a molecular weight cutoff of 662 Da, corresponding to an average pore size of 0.53 nm. After doping with HAP, an inorganic material with a regular crystal structure, its introduction creates additional heterogeneous interfaces and microdomains within the MPN. This heterogeneous structure may lead to localized enlargement or redistribution of the pore size within the membrane, thereby broadening the pore size distribution and increasing the average pore size. Consequently, the molecular weight cutoff of MPN-HAP increases to 891 Da, and the average pore size also increases to 0.76 nm. The molecular weight cutoff of a nanofiltration membrane is directly related to its pore size. A larger pore size allows larger molecules to pass through, resulting in an increased molecular weight cutoff. The increased pore size and wider pore size distribution caused by HAP doping reduces the membrane's ability to retain larger molecules, resulting in an increased molecular weight cutoff. This results in a certain improvement in pure water permeability.

[0119] Example 10: Long-term stability test

[0120] At an operating pressure of 0.4 MPa, using 100 ppm CR dye solution as the feed liquid, the long-term operation stability test of MPN (M4-3) and MPN-HAP composite nanofiltration membrane (M7-2) was carried out. Figure 20 It can be seen that after 120h stability test, the rejection rate of MPN composite nanofiltration membrane for CR increased from 90.2% to 93.7%, and the permeation flux increased from 21.0L·m -2 ·h -1 Reduced to 17.7 L·m -2 ·h -1 The rejection rate of MPN-HAP composite nanofiltration membrane for CR increased from 88.2% to 91.8%, and the permeation flux increased from 77.6 L·m -2 ·h -1 Reduced to 72.1 L·m -2 ·h -1 The decrease was very low, and the fluctuation was small during the test period. This shows that the doping of HAP did not destroy the stability of MPN on the nanofiltration membrane surface, and the MPN-HAP composite nanofiltration membrane has good long-term operational stability.

[0121] Comparative Example 1:

[0122] Compared with Example 5, the difference is that the MPN-HAP membrane is prepared by vacuum-assisted HAP adsorption

[0123] (1) Take the PAN base film (M0) prepared in Example 1, gently press it with a rubber roller to remove excess moisture on the surface, and then fix it on the sand core filter device.

[0124] (2) Prepare 0.10g·L -1 The HAP aqueous dispersion was prepared by vacuum filtration technology, and the HAP was embedded in the surface of the PAN base film (M0) prepared in Example 1 under negative pressure.

[0125] (3) Remove the PAN base film adsorbed with HAP and carry out Fe 3+ Load (4.0g·L -1 FeCl3, 2min) and TA coordination reaction (2.0g·L -1 TA, 60s). That is, 4.0g·L -1 Concentrated FeCl3 solution was poured onto the surface of the PAN membrane embedded with HAP in step (2) and allowed to stand for 2 minutes for precursor loading to ensure that Fe 3+ Distribute evenly on the membrane surface. Pour off excess FeCl3 solution and gently press with a rubber roller to remove the remaining solution. Then add 2.0g·L -1 TA solution is poured into the Fe 3+The loaded membrane surface was subjected to coordination reaction, the reaction time was controlled to be 60s, and the membrane was repeatedly washed with pure water for 5min to obtain membrane M8.

[0126] Comparative Example 2:

[0127] Compared with Example 5, the difference is that the MPN-HAP membrane is prepared by iron ion-mediated HAP surface functionalization.

[0128] (1) HAP nanowires were mixed with FeCl3 solution to obtain a HAP-FeCl3 premixed solution, and the concentration of HAP in the mixed dispersion was 0.10 g·L -1 , the concentration of FeCl3 is 4.0 g·L -1 .

[0129] (2) The mixed dispersion was then poured onto the surface of the PAN membrane and allowed to stand for 2 min for precursor loading. The treated HAP dispersion was used to load the PAN base membrane (M0). The excess mixed dispersion was poured off and the residual solution was removed by gently pressing with a rubber roller.

[0130] (3) 2.0 g·L -1 The TA solution was poured onto the membrane surface in step (2) to carry out coordination reaction. The reaction time was controlled to be 60s. After washing, membrane M9 was obtained.

[0131] To investigate the effect of HAP doping methods on the film surface morphology, three different doping methods, M7, M8, and M9, were used.

[0132] from Figure 10 It can be seen that the three doping methods have a significant effect on TA-Fe 3+ The morphology of the MPN layer generated by the reaction was significantly affected. Surface and cross-sectional morphology analysis of membrane M7 showed that the introduction of HAP into the TA solution significantly optimized the structure and uniformity of the MPN layer compared to the MPN layer formed solely based on the coordination reaction between TA and FeCl3. The addition of HAP provided uniform active sites for the subsequent formation of complexes and significantly improved the efficiency of the coordination reaction by adsorbing free TA. This synergistic effect resulted in a more uniform and dense surface coverage, while also refining large-scale complexes, filling areas of previously uneven distribution, and repairing a small number of pores and discontinuous areas, thereby forming a more complete MPN layer.

[0133] Membrane M8 adsorbed HAP onto the surface of PAN membrane by vacuum filtration. SEM images showed that the originally smooth surface of the base membrane showed typical nanowire structure characteristics, but the complex formed was only constructed on the HAP trunk and branches, resulting in a large amount of base membrane still exposed, and it was impossible to ensure the formation of a stable MPN layer. 3+ Reaction, its surface is rich in PO43- and -OH and Fe 3+ Interactions occur, including Fe 3+ Substitute Ca on the HAP surface or in the crystal structure 2+ Formation of Fe-PO4 complex, and Fe 3+ It is adsorbed on the HAP surface by electrostatic or coordination means, occupying some active sites. This interaction further affects the subsequent TA and Fe 3+ The coordination reaction generates HAP-Fe 3+ -TA ternary complex, but TA and Fe 3+ The coordination reaction is primarily concentrated in localized areas of the HAP surface, failing to achieve uniform coverage. SEM images show a discontinuous distribution of the MPN coordination layer on the surface, failing to form a stable and dense structure. Particle aggregation is significant, resulting in clusters. Cross-sectional images reveal a thin layered structure with weak interlayer bonding, and discontinuous film layers or localized fractures are observed, indicating poor continuity and stability of the MPN layer.

[0134] Figure 15 Different doping methods have a significant impact on the morphology of the MPN layer. Among them, the doping method of membrane M7 is to directly dope HAP into the TA solution, and use the TA surface to modify HAP, so as to promote the uniform dispersion of TA and optimize the active sites, which significantly improves the density and uniformity of the MPN layer. The other two doping methods fail to form a stable and continuous MPN layer due to insufficient reaction sites or limited diffusion of reactants.

[0135] Different doping methods have a significant impact on the performance of MPN-HAP composite membranes. Among them, the method of directly doping HAP into TA solution showed the best performance: the pure water flux increased from 46.3 L·m -2 ·h -1 bar -1 Significantly increased to 88.7L·m -2 ·h -1 bar -1 At the same time, the CR dye rejection rate only slightly decreased from 90.1% to 88.6%, indicating that it significantly improved the permeation flux without significantly sacrificing the dye rejection rate. In contrast, although the other two doping methods also improved the permeation flux, they could not form an effective MPN layer on the PAN membrane surface, resulting in a significant decrease in the rejection rate, making it difficult to meet the retention performance requirements of nanofiltration membranes.

[0136] Figure 3 The FT-IR images of PAN film, MPN composite film and MPN-HAP composite film are shown in Figure 2. FT-IR confirmed that the PAN base film (C≡N peak), MPN layer (broadened -OH peak (3600-3100 cm -1) and new aromatic ring / bending vibration peaks (1578 cm -1 ), confirming that the phenolic hydroxyl groups in TA and Fe 3+ Coordination occurs, and the MPN layer is successfully formed on the PAN surface) and HAP doping (PO4 3- The characteristic peak (605cm -1 The MPN-related characteristic peaks shifted and changed in intensity, indicating that there was an interaction between HAP and MPN.

[0137] Figure 4 XPS images of PAN membrane, MPN and MPN-HAP composite nanofiltration membrane; (a): XPS total spectra of PAN, MPN and MPN-HAP membranes; (b): C1s spectrum of PAN membrane; (c): Fe 2p spectrum of MPN membrane; (d): C1s spectrum of MPN membrane; (e): Ca 2p spectrum of MPN-HAP membrane; (f): P 2p spectrum of MPN-HAP membrane. XPS further confirmed that Fe 3+ The coordination with TA (Fe 2p peak, CO / C=O change in C1s) confirmed the successful construction of the MPN layer and the successful incorporation of HAP (Ca 2p, P 2p peaks).

[0138] Figure 5 The XPS spectra before and after etching with different HAP doping amounts; (a): HAP doping amount is 0g·L -1 XPS total spectrum; (b) HAP doping amount is 0.1 g·L -1 XPS total spectrum; (c) HAP doping amount is 1.0 g·L -1 XPS total spectrum; (d) HAP doping amount is 2.0 g·L -1 To further understand the distribution of elements within the membrane, etching was performed to gradually remove surface materials, exposing the membrane's internal structure. This allowed for more comprehensive detection and analysis of the characteristic peak signals of HAP-doped elements. XPS etching experiments revealed that HAP is primarily distributed within the MPN layer.

[0139] Figure 6 XPS spectra of different HAP doping amounts. By adjusting the HAP doping concentration from 0.10 g·L -1 Increased to 2.00 g·L -1Analysis of the XPS spectra reveals that the signal intensities of the characteristic HAP Ca 2p and P 2p peaks gradually increase with increasing HAP doping levels. Calculation of the dynamic changes in the Ca / C ratio further reveals the HAP doping effect in TA solutions, thereby exploring its influence on the surface chemical composition of MPNs. As the HAP doping level increases, the intensities of the characteristic HAP P and Ca peaks in the XPS spectra clearly increase, while the intensities of the characteristic MPN elements (Fe and C) decrease accordingly. This phenomenon indicates that the doping of HAP gradually increases in the composite membrane.

[0140] Figure 7 It can be seen that after HAP doping, the binding energy of C=O, CO, O-Fe and other bonds shifted and the peak intensity changed by comparing with MPN film. These changes are attributed to the hydrogen bonding (TAC=O and HAP-OH) and coordination (TAC-O and Ca) between HAP and MPN (mainly TA). 2+ ) and π-π stacking adsorption.

[0141] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A hydroxyapatite-doped metal polyphenol network composite nanofiltration membrane, characterized in that: The composite nanofiltration membrane comprises: a porous polymer base membrane; and a HAP-doped metal polyphenol network separation layer arranged on the surface of the porous polymer base membrane. The separation layer comprises a metal polyphenol network structure formed by the coordination reaction of metal ions and polyphenols, and the HAP nanowires are distributed in the metal polyphenol network structure.

2. A method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 1, characterized in that: The following steps are involved: a) preparing a porous polymer base membrane; b) contacting a metal salt solution with a porous polymer base membrane to load the metal ions on the base membrane; c) dispersing HAP in a solution containing polyphenols to form a HAP-polyphenol mixed dispersion; d) contacting the base membrane loaded with metal ions in step b) with the HAP-polyphenol mixed dispersion prepared in step c) to allow the polyphenol to undergo a coordination reaction with the metal ions loaded on the base membrane to form a HAP-doped metal-polyphenol separation layer on the surface of the base membrane.

3. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 2, characterized in that: The porous polymer-based membrane in step a) is a polyacrylonitrile membrane.

4. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 3, characterized in that: The porous polymer base membrane is prepared by the non-solvent induced phase inversion method. The polyacrylonitrile polymer is dissolved in a good solvent to form a casting solution, which is then coated on a non-woven fabric support. The non-woven fabric support is then immersed in a non-solvent for phase inversion to form a porous base membrane, which is then washed and set aside.

5. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 2, characterized in that: The metal ion in step b) is Fe 3+ ions, the solution containing metal ions is FeCl3 solution, and its concentration is 1.0g·L -1 ~6.0g·L -1 .

6. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 5, characterized in that: The concentration of the FeCl3 solution is 4.0 g·L -1 .

7. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 2, characterized in that: The polyphenol in step c) is tannic acid, and the concentration of the polyphenol-containing solution is 1.0 g·L -1 ~6.0g·L -1 .

8. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 2, characterized in that: In step c), the concentration of HAP in the HAP-polyphenol mixed dispersion is 0.05 g·L -1 ~2.00g·L -1 .

9. The method for preparing the HAP-doped metal polyphenol network composite nanofiltration membrane according to claim 2, characterized in that: The coordination reaction time in step d) is 10 seconds to 120 seconds.

10. A HAP-doped metal polyphenol network composite nanofiltration membrane prepared according to the method of any one of claims 2 to 9.

Images