A metal polyphenol nanofilm and its application in nanofiltration separation

Abstract

This invention discloses a metal polyphenol nanofilm and its application in nanofiltration separation, belonging to the field of nanofiltration membranes and their preparation and application. The metal polyphenol nanofilm is formed on a porous supported membrane by the rapid assembly of metal ions in the oil phase and polyphenol ligands in the aqueous phase at the interface. The metal polyphenol nanofilm prepared by this invention is ultrathin, densely structured, and exhibits excellent ion rejection performance and high water flux, showing great application potential in the field of nanofiltration. Furthermore, the preparation process of this membrane is simple and efficient, with mild reaction conditions, and uses green and highly utilized raw materials, providing a solid foundation for industrial production and broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membranes and their preparation and application, and particularly relates to a metal polyphenol nanofilm and its application in nanofiltration separation. Background Technology

[0002] At the beginning of the 20th century, membrane separation technology, as a highly efficient, pollution-free, and low-energy-consumption separation technology, occupied an important position in modern human life and industrial production. Nanofiltration is a membrane separation technology with separation performance between reverse osmosis and ultrafiltration. Due to its advantages of high rejection rate, high flux, and low energy consumption, it has been widely used in fields such as surface water purification, seawater desalination, wastewater treatment, and product purification and concentration.

[0003] In recent years, metal polyphenol coatings have attracted widespread attention due to their universal adhesion, coordination cross-linking network structure, and good hydrophilicity. Furthermore, their simple preparation, rapid assembly, and environmentally friendly nature have led to their increasing application in membrane technology, improving the hydrophilicity, antifouling properties, antioxidant properties, and antibacterial properties of separation membranes. However, traditional metal polyphenol membranes are formed through the rapid assembly of metal ions and polyphenol ligands in solution. The resulting network structure is relatively loose, with large pore sizes and a wide pore size distribution, making it difficult to achieve precise separation of small organic molecules.

[0004] Patent CN104984666A discloses a metal polyphenol thin film, its preparation method, and its application. The method involves immersing a porous base membrane in a metal salt solution for a period of time, then removing it and immersing it in a polyphenol aqueous solution for a period of time, assembling the metal polyphenol thin film on the base membrane surface. Performance testing revealed that the membrane has a certain retention effect on dye molecules, but the water flux is very low, and the retention of smaller inorganic salt ions was not tested. Therefore, the metal polyphenol thin film prepared by assembling polyphenol ligands and metal ions in an aqueous solution has limitations in terms of density and retention performance, cannot control the membrane pore structure, and the prepared film is relatively thick.

[0005] Patent CN111203107A discloses a polyphenol-iron nanofilm, its preparation method, and its application. The method involves immersing a porous support membrane in a polyphenol aqueous solution, adding a ferrous salt aqueous solution, and then performing a co-deposition reaction under shaking. The reaction continues to obtain the polyphenol-iron nanofilm. This separation membrane can be used to retain dyes and inorganic salt ions, but it has limitations regarding the composition and structure of the film. Only polyphenol-iron nanofilms were prepared; nanofilms prepared with other metal ions and other polyphenols were not explored. Furthermore, the separation layer is still relatively thick, requiring a long deposition time of 2 hours or more to obtain satisfactory performance. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a metal polyphenol nanofilm. A porous support membrane is immersed in a metal salt organic solution. After immersion, the organic solvent on the membrane surface is removed, and the membrane is then immersed in a polyphenol aqueous solution. The polyphenols in the aqueous phase and the metal ions in the organic phase self-assemble at the interface, forming a metal polyphenol network separation layer on the porous support membrane, thus preparing a metal polyphenol nanofilm.

[0007] In one embodiment of the present invention, the porous support membrane is one or more of polyacrylonitrile ultrafiltration membrane, polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, cellulose acetate ultrafiltration membrane, and porous anodic aluminum oxide membrane, preferably a polyethersulfone ultrafiltration membrane.

[0008] In one embodiment of the present invention, the polyphenol is one or more of tea polyphenols, grape polyphenols, tannic acid, phytic acid, ellagic acid, catechol, apple polyphenols, and gallic acid, preferably tannic acid.

[0009] In one embodiment of the present invention, the metal salt is one or more of the following: ferric acetate, ferric gluconate, ferric acetylacetonate, ferric citrate, cadmium acetate, cadmium gluconate, cadmium acetylacetonate, zinc acetate, zinc gluconate, zinc acetylacetonate, zirconium acetate, zirconium gluconate, zirconium acetylacetonate, nickel acetate, nickel gluconate, nickel acetylacetonate, magnesium acetate, magnesium gluconate, magnesium acetylacetonate, magnesium citrate, manganese acetate, manganese gluconate, manganese acetylacetonate, manganese citrate, cobalt acetate, cobalt gluconate, cobalt acetylacetonate, cobalt citrate, copper acetate, copper gluconate, copper acetylacetonate, copper citrate, aluminum acetate, aluminum gluconate, aluminum acetylacetonate, and aluminum citrate, preferably an acetylacetonate salt.

[0010] In one embodiment of the present invention, the metal element in the metal salt is one or more of Zn, Fe, Ni, Cd, Zr, Mg, Mn, Co, Cu, and Al.

[0011] In one embodiment of the present invention, the concentration of the metal salt in the metal salt organic solution is 0.0069 to 0.6920 mg / mL, preferably 0.069 mg / mL.

[0012] In one embodiment of the present invention, the polyphenol concentration in the polyphenol aqueous solution is 0.1 to 10 mg / mL, preferably 1 mg / mL.

[0013] In one embodiment of the present invention, the molar ratio of polyphenol to metal salt is 10:1 to 1:10, preferably 3:1.

[0014] In one embodiment of the present invention, the organic solvent is one or more selected from n-hexane, cyclohexane, dichloromethane, chloroform, n-heptane, toluene, and petroleum ether, preferably n-hexane.

[0015] This invention also provides a method for preparing metal polyphenol nanofilms, specifically including the following steps:

[0016] (1) Take the porous support membrane, wash it with a mixed solution of ethanol and water, and store it in anhydrous ethanol for later use.

[0017] (2) Dissolve the metal salt in an organic solvent to obtain an organic solution of the metal salt; dissolve the polyphenol in ultrapure water to obtain an aqueous solution of the polyphenol;

[0018] (3) Immerse the porous support membrane from step (1) in a metal salt organic solution for a period of time, then remove the excess organic solvent, and then add it to a polyphenol aqueous solution of the same volume as the metal salt organic solvent for a period of time. Finally, take out the membrane and put it in an oven for heat treatment to carry out further reaction.

[0019] In one embodiment of the present invention, in step (3), the soaking time of polyphenol aqueous solution is 5-600s, and the soaking time of metal salt organic solution is 180s.

[0020] In one embodiment of the present invention, in step (3), the heat treatment temperature is 30-80°C, preferably 60°C; the reaction time is 0-60 min, preferably 10 min.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) The metal ions in the organic phase and the polyphenol ligands in the aqueous phase react very quickly at the interface between the two phases. They can quickly assemble into a dense, ultrathin and defect-free metal polyphenol nanofilm within a few seconds to hundreds of seconds. The thickness of the nanofilm as the separation layer can be easily controlled by the interface reaction.

[0023] (2) This invention uses different metal ion organic salts and polyphenol ligands, and utilizes the metal ion type (different coordination number) and polyphenol structure to regulate the pore structure, thereby achieving regulation of separation performance under different application scenarios;

[0024] (3) The reaction at the two-phase interface of the present invention is extremely fast, which can greatly improve production efficiency and raw material utilization. The required two-phase monomers (polyphenols and organometallic salts) meet the requirements of green environmental protection. At the same time, the raw materials are cheap, which greatly reduces the production cost of metal polyphenol nanofilms. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the surface of the metal polyphenol nanofilm prepared in Example 1;

[0026] Figure 2 This is a cross-sectional scanning electron microscope image of the metal polyphenol nanofilm prepared in Example 1. Detailed Implementation

[0027] The metal polyphenol nanofilm prepared in this invention is used for desalination. Desalination rate and water flux are two important parameters for evaluating nanofiltration performance. These parameters were obtained through calculation using a cross-flow filtration method. An inorganic salt solution with a concentration of 1000 mg / L was used as the feed solution, and filtration tests were conducted in a constant temperature water bath at 25°C and a pressure of 0.4 MPa. The time required for the composite membrane to permeate through 5 mL, 7 mL, and 10 mL of permeate was recorded, and the desalination rate and water flux were calculated using formulas.

[0028] The formula for calculating the desalination rate is as follows:

[0029]

[0030] Among them, C f Indicates the concentration of salt ions in the aqueous solution before treatment; C p This indicates the concentration of salt ions in the treated aqueous solution.

[0031] Water flux is defined as the volume of water passing through a unit membrane area per unit time under a certain operating pressure, and its unit is L·m. -2 ·h -1 The calculation formula is:

[0032]

[0033] Where V represents the volume of the permeated solution, in L; and A represents the effective membrane area, in m². 2 t represents time, in hours (h).

[0034] Different molecular weights of polyethylene glycol (PEG) (200, 400, 600, 800, 1000) were dissolved in water to prepare different feed solutions. Filtration tests were conducted at 25℃ and 0.4 MPa. The aqueous nanofiltration performance of the composite membrane was evaluated by recording the permeate flux and measuring the feed / filtrate concentration. Different molecular weights of PEG were retained; the molecular weight cutoff was the PEG molecular weight with a 90% retention rate.

[0035] The concentration of polyethylene glycol (PEG) needs to be determined by potassium permanganate oxidation. First, prepare 5 ml of a 2 mg / L potassium permanganate solution and add 0.6 ml of concentrated sulfuric acid to adjust it to a strongly acidic state. Use a pipette to add 15 μl of the potassium permanganate solution to 75 μl of the PEG solution, mix thoroughly, and let stand for 24 hours. After complete oxidation, dilute the solution to 3 ml and measure its absorbance at 525 nm using a UV-Vis spectrometer to determine the PEG concentration.

[0036] Using PEG molecules of different molecular weights as molecular probes, the pore size and pore size distribution of the composite membrane can be determined based on its retention characteristics. The specific calculation method is as follows:

[0037] The Stokes radius (r) of a PEG can be calculated using the following formula:

[0038]

[0039] Where Mw is the molecular weight corresponding to PEG.

[0040] The normal distribution of the pore size of the composite membrane can be calculated using the following formula:

[0041]

[0042] Where dp is the membrane pore size in meters; μp is the geometric mean diameter of the PEG at a rejection rate of 50% in meters; and σp is the geometric standard deviation of μp, which is defined as the ratio of the geometric mean radius of the PEG at a rejection rate of 84.13% to the geometric mean radius of the PEG at a rejection rate of 50%.

[0043] The following examples further illustrate the preparation method of a metal polyphenol nanofilm and its nanofiltration separation performance according to the present invention.

[0044] Example 1

[0045] The preparation method of iron-tannic acid nanofilm includes the following steps:

[0046] (1) Cut a polyethersulfone ultrafiltration membrane with a diameter of 6 cm as the base membrane, clean it with a mixture of ethanol and water, and store it in anhydrous ethanol for later use.

[0047] (2) Weigh 0.0069g (0.019mmol) of acetylacetone iron and dissolve it in 100ml of n-hexane to obtain an n-hexane solution of acetylacetone iron; weigh 0.1g of tannic acid and dissolve it in 100ml of ultrapure water to obtain an aqueous solution of tannic acid.

[0048] (3) Then, after washing the polyethersulfone ultrafiltration membrane in step (1) with ultrapure water, it is soaked in the hexane solution of acetylacetone iron. After soaking for 3 minutes, the excess hexane on the membrane surface is removed. Then, it is added to an aqueous solution of tannic acid with an equal volume of acetylacetone iron hexane solution. After soaking for 5 minutes, it is taken out and the excess solution on the surface is removed.

[0049] (4) Place the membrane in an oven and keep it at a constant temperature of 60°C for 10 minutes to promote further cross-linking of the metal polyphenol network structure. Finally, take out the membrane and wash it with ultrapure water to obtain iron-tannic acid nanofilm.

[0050] like Figure 1 As shown, a thin film appears at the oil-water interface, and the pores on the surface of the base film are covered, proving the successful preparation of iron-tannic acid network nanofilm.

[0051] Examples 2-8

[0052] The concentrations of tannic acid and iron acetylacetone were adjusted (as shown in Table 1) to maintain the molar ratio of tannic acid to iron acetylacetone at 3:1, with the other conditions the same as in Example 1, to prepare a series of iron-tannic acid network nanofilms.

[0053] The iron-tannic acid network nanofilms prepared in Examples 2–8 were tested. The results are shown in Table 1.

[0054] Table 1. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Examples 2-8

[0055]

[0056] As the tannic acid (TA) concentration increased from 0.1 mg / mL to 1 mg / mL, the composite membrane's rejection rate for Na₂SO₄ improved. At lower TA concentrations, insufficient density of the metal polyphenol membrane was required to form, resulting in high flux but poor retention performance. A dense membrane was achieved at 1 mg / mL, at which point the membrane performance was optimal. Further increases in TA concentration led to increased separation layer thickness, increased water resistance across the membrane, and larger complexes at the interface, resulting in a looser separation layer and a simultaneous decrease in both water flux and retention performance.

[0057] Examples 9-16

[0058] The molar ratio of tannic acid to iron acetylacetone was adjusted (as shown in Table 2), with the concentration of tannic acid kept constant at 1 mg / mL. The other conditions were the same as in Example 1, and a series of iron-tannic acid network nanofilms were prepared.

[0059] The iron-tannic acid network nanofilms prepared in Examples 9–16 were tested. The results are shown in Table 2.

[0060] Table 2. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Examples 9-16

[0061]

[0062] When the TA / Fe molar ratio is greater than 3, a defect-free, three-coordinated metal polyphenol network cannot be formed during the reverse interfacial coordination process, resulting in a large pore size in the separation layer. At this point, as the TA / Fe molar ratio gradually decreases, i.e., the iron ion concentration increases, the membrane pore structure gradually becomes denser. However, when the TA / Fe molar ratio is less than 3, the excessive iron ion content causes TA and iron ions to tend to form mono- and bis-complexes, making the metal polyphenol network looser and increasing the membrane pore size. Further reducing the TA / Fe molar ratio actually increases the retention rate due to the increased separation layer thickness.

[0063] Examples 17-25

[0064] Organic salts of Zn, Ni, Cd, Zr, Mg, Mn, Co, Cu, and Al were dissolved in organic solvents as organic reaction phases. The remaining conditions were the same as in Example 1. A series of metal polyphenol network nanofilms were prepared and their nanofiltration performance was evaluated. The results are shown in Table 3.

[0065] Table 3. Water flux and retention rate of the metal polyphenol network nanofilms prepared in Examples 17-25

[0066]

[0067]

[0068] Table 3 shows that using different metal ions and tannic acid as assembly units, complete ultrathin nanofilms with good nanofiltration performance can be constructed on the surface of porous membranes. Due to Zr... 4+ (octagonal coordination), Al 3+ (six-coordinated), Mg 2+ (six-coordinated), Mn 2+ (six-coordinated) and Fe 3+ The higher number of coordination numbers (six-coordinate) results in a more compact metal polyphenol network structure, thus leading to the highest sodium sulfate retention rate in this nanofilm. Meanwhile, Zn... 2+ Ni 2+ Cd 2+ Co 2+ Cu 2+ Both it and tannic acid are tetracoordinate, resulting in a relatively loose structure and a reduced sodium sulfate rejection rate.

[0069] Examples 26-30

[0070] By adjusting the reaction time at the interface of the iron-tannic acid network nanofilm (i.e., the soaking time in the polyphenol aqueous solution), and with the other conditions the same as in Example 1, a series of iron-tannic acid network nanofilms were prepared.

[0071] The iron-tannic acid network nanofilms prepared in Examples 26–30 were tested. The results are shown in Table 4.

[0072] Table 4. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Examples 26-30

[0073]

[0074]

[0075] When the reaction time is short, tannic acid and iron ions do not react fully, failing to form a dense separation layer, resulting in poor separation performance. When the reaction time is extended to 300 seconds, a complete and dense metal polyphenol network is formed, at which point the performance is optimal. Further extending the reaction time increases the film thickness, thus reducing the flux.

[0076] Comparative Example 1

[0077] The preparation method is the same as that of the iron-tannic acid network nanofilm in Example 1, except that iron acetylacetone is dissolved in water in step (2).

[0078] Table 5. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Example 1 and Comparative Example 1

[0079]

[0080] Since there is no oil-water interface in Comparative Example 1, clusters are formed in the mixed solution, and a complete film cannot be formed within the same reaction time, so the retention effect on sodium sulfate solution is very poor.

[0081] Comparative Example 2

[0082] The preparation method is the same as that in Example 1, which focuses on the iron-tannic acid network nanofilm. The only difference is that the porous support membrane is immersed in an aqueous tannic acid solution, soaked, and then an iron acetylacetone solution in hexane is added. The mixture is then shaken to undergo a co-deposition reaction. The specific preparation method is as follows:

[0083] (1) Cut a polyethersulfone ultrafiltration membrane with a diameter of 6 cm as the base membrane, wash it multiple times with a mixture of ethanol and water, and store it in anhydrous ethanol for later use.

[0084] (2) Weigh 0.0069 g (0.019 mmol) of acetylacetone iron and dissolve it in 100 ml of n-hexane to obtain an n-hexane solution of acetylacetone iron;

[0085] (3) Weigh 0.1g of tannic acid and dissolve it in 100ml of ultrapure water to obtain an aqueous solution of tannic acid;

[0086] (4) Immerse the polyethersulfone ultrafiltration membrane in a tannic acid aqueous solution for 5 minutes;

[0087] (5) Transfer the acetylacetone iron in hexane solution obtained in step (2) to the solution in step (4), and quickly transfer the reactor containing the two solutions into a water bath constant temperature shaker. Shake and deposit for 120 minutes in a water bath at 25°C. Transfer the reactor containing the above solution from the water bath constant temperature shaker into an oven and react at 60°C for 10 minutes. Finally, take out the membrane and wash it with ultrapure water to obtain an iron-tannic acid network nanofilm.

[0088] Table 6. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Example 1 and Comparative Example 2.

[0089]

[0090] Because the oil-water interface in Comparative Example 2 is too far from the base film, and the film formed at the oil-water interface will break during the oscillation process, a separation layer cannot be formed on the surface of the base film. Only some fragments adhere to the surface of the base film, so there is no retention effect on sodium sulfate.

[0091] Comparative Example 3

[0092] The preparation method is the same as that in Example 1, which focuses on the iron-tannic acid network nanofilm. The only difference is that the porous support membrane is first immersed in an aqueous tannic acid solution, then removed, and an iron acetylacetone solution in hexane is added for reaction. The specific preparation method is as follows:

[0093] (1) Cut a polyethersulfone ultrafiltration membrane with a diameter of 6 cm as the base membrane, wash it multiple times with a mixture of ethanol and water, and store it in anhydrous ethanol for later use.

[0094] (2) Weigh 0.0069g (0.019mmol) of acetylacetone iron and dissolve it in 100ml of n-hexane to obtain an n-hexane solution of acetylacetone iron; weigh 0.1g of tannic acid and dissolve it in 100ml of ultrapure water to obtain an aqueous solution of tannic acid.

[0095] (3) Then, after washing the polyethersulfone ultrafiltration membrane in step (1) with ultrapure water, soak it in an aqueous solution of tannic acid for 3 minutes, remove excess water from the membrane surface, add it to an acetylacetone iron hexane solution with an equal volume of tannic acid aqueous solution, soak for 5 minutes, take it out and remove excess solution from the surface.

[0096] (4) Place the membrane in an oven and keep it at a constant temperature of 60°C for 10 minutes to promote further cross-linking of the metal polyphenol network structure. Finally, take out the membrane and wash it with ultrapure water to obtain iron-tannic acid network nanofilm.

[0097] Table 7. Water flux and rejection rate of the iron-tannic acid network nanofilms prepared in Example 1 and Comparative Example 3.

[0098]

[0099] In Comparative Example 3, due to the difference in diffusion rates of ions at the two-phase interface, metal ions in the oil phase diffuse more easily into the aqueous phase, where they coordinate and assemble with polyphenol monomers. During this process, due to the good wetting of the base membrane surface by the aqueous solution, metal polyphenol complexes easily form in the pores of the base membrane, leading to pore blockage and uneven separation layer thickness, resulting in a decrease in the permeate flux of the composite membrane. Secondly, during the wetting of the base membrane surface by the aqueous solution, large-molecule polyphenols such as tannins easily aggregate on the membrane surface. The uneven distribution of polyphenol ligands easily leads to uneven metal polyphenol network film structure and widened pore size distribution, thus affecting the separation performance of the composite membrane. In contrast, this method involves coordination and assembly with polyphenol monomers on the oil-water interface side, far from the base membrane surface, thereby forming a selective separation layer on the base membrane surface.

[0100] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A metal polyphenol nanofilm, characterized in that, The preparation method includes the following steps: immersing a porous support membrane in a metal salt organic solution, removing the organic solvent from the membrane surface after immersion, and then immersing the membrane in a polyphenol aqueous solution. The polyphenol in the aqueous phase and the metal ions in the organic phase self-assemble at the interface, forming a metal polyphenol network separation layer on the porous support membrane, thus preparing a metal polyphenol nanofilm. The metal salt is one or more of the following: ferric acetate, ferric gluconate, ferric acetylacetonate, ferric citrate, cadmium acetate, cadmium gluconate, cadmium acetylacetonate, zinc acetate, zinc gluconate, zinc acetylacetonate, zirconium acetate, zirconium gluconate, zirconium acetylacetonate, nickel acetate, nickel gluconate, nickel acetylacetonate, magnesium acetate, magnesium gluconate, magnesium acetylacetonate, magnesium citrate, manganese acetate, manganese gluconate, manganese acetylacetonate, manganese citrate, cobalt acetate, cobalt gluconate, cobalt acetylacetonate, cobalt citrate, copper acetate, copper gluconate, copper acetylacetonate, copper citrate, aluminum acetate, aluminum gluconate, aluminum acetylacetonate, and aluminum citrate. The molar ratio of polyphenols to metal salts is 3:

1.

2. The metal polyphenol nanofilm according to claim 1, characterized in that, The porous support membrane is one or more of the following: polyacrylonitrile ultrafiltration membrane, polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, cellulose acetate ultrafiltration membrane, and porous anodic aluminum oxide membrane.

3. The metal polyphenol nanofilm according to claim 1, characterized in that, The organic solvent is one or more of the following: n-hexane, cyclohexane, dichloromethane, chloroform, n-heptane, toluene, and petroleum ether.

4. A metal polyphenol nanofilm according to claim 1, characterized in that, The concentration of the metal salt in the organic solution is 0.0069~0.6920 mg / mL.

5. The metal polyphenol nanofilm according to claim 1, characterized in that, The polyphenols are one or more of the following: tea polyphenols, grape polyphenols, tannic acid, phytic acid, ellagic acid, catechol, apple polyphenols, and gallic acid.

6. A metal polyphenol nanofilm according to claim 1, characterized in that, The polyphenol concentration in the polyphenol aqueous solution is 0.1~10 mg / mL; the soaking time in the polyphenol aqueous solution is 5~600 s.

7. The metal polyphenol nanofilm according to claim 1, characterized in that, Its preparation method specifically includes the following steps: (1) Take the porous support membrane, wash it with a mixed solution of ethanol and water, and store it in anhydrous ethanol for later use; (2) Dissolve the metal salt in an organic solvent to obtain an organic solution of the metal salt; dissolve the polyphenol in ultrapure water to obtain an aqueous solution of the polyphenol; (3) The porous support membrane in step (1) is immersed in a metal salt organic solution, and then the excess organic solvent is removed. Then it is immersed in a polyphenol aqueous solution of the same volume as the metal salt organic solution. Finally, the membrane is taken out to obtain a metal polyphenol nanofilm.

8. The application of the metal polyphenol nanofilms according to claims 1 to 7 in nanofiltration separation.

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