Composite nanofiltration membrane and preparation method thereof

Abstract

The invention relates to a composite nanofiltration membrane and a preparation method thereof. The composite nanofiltration membrane comprises a supporting layer and an adsorption and degradation composite functional layer arranged on the supporting layer, the adsorption and degradation composite functional layer is prepared from a composite functional coating liquid, and the composite functional coating liquid comprises the following raw materials in parts by weight: 20-100 parts of an organic adsorbent; 5-25 parts of a porous material; 5-25 parts of a metal organic framework material; 5000 to 20000 parts of a solvent A; 5000 to 20000 parts of a solvent B; and 0.5-5 parts of a cross-linking agent. The solvent A comprises dimethyl sulfoxide, N-methyl pyrrolidone and an ethanol solution with the volume solubility of 75%; the solvent B is a 1% acetic acid solution; the membrane is large in water flux and high in rejection rate, can efficiently remove micropollutants such as bisphenol A and estradiol in water, has a self-cleaning function, can degrade the pollutants, avoids membrane hole blockage, and prolongs the service life of the membrane.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanofiltration membranes, and in particular relates to a composite nanofiltration membrane and a preparation method thereof. Background Art

[0002] Nanofiltration membranes are pressure-driven membranes that fall between ultrafiltration and reverse osmosis. Their pore sizes typically range from 0.5 to 2 nm, and their operating pressures typically range from 0.1 to 1.0 MPa. They primarily rely on sieving, electrostatic, and adsorption effects to separate solutes. They can intercept pollutants such as small organic molecules and heavy metal ions in water while retaining some beneficial minerals. Consequently, they are widely used in drinking water purification, industrial wastewater treatment, and food and pharmaceutical concentration.

[0003] With the development of industry, the pollution problem of endocrine disrupting chemicals (EDCs) in water bodies, such as bisphenol A (BPA) and estradiol (E2), and other trace pollutants, has become increasingly prominent. These substances are bioaccumulative and ecotoxic. However, the removal efficiency of traditional nanofiltration membranes for such substances is extremely low, only 70-85%. In addition, traditional nanofiltration membranes have the following disadvantages: 1) It is difficult to balance the retention rate and water flux, and high retention rate is often accompanied by low flux; 2) It is easy to adsorb pollutants, resulting in membrane fouling and clogging of membrane pores, and ultimately leading to performance degradation; 3) It does not have a self-cleaning function, requires frequent cleaning and maintenance, and has a short service life. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects in the prior art and provide a composite nanofiltration membrane and a preparation method thereof, which has a large water flux and a high retention rate, can efficiently remove trace pollutants such as bisphenol A (BPA) and estradiol (E2) in water, and has a self-cleaning function, can degrade pollutants, avoid membrane pore clogging, and extend the service life of the membrane.

[0005] To achieve the above object, the technical solutions adopted by the present invention are as follows:

[0006] A composite nanofiltration membrane comprises a support layer and an adsorption and degradation composite functional layer provided on the support layer;

[0007] The adsorption degradation composite functional layer is prepared from a composite functional coating liquid.

[0008] The composite functional coating liquid comprises the following raw materials in parts by weight:

[0009]

[0010] As a further technical solution, the mass ratio of metal organic framework material to porous material is 1:1

[0011] As a further technical solution, the mass ratio of the total amount of the metal organic framework material and the porous material to the organic adsorbent is 1:2;

[0012] As a further technical solution, the concentration of the crosslinking agent in the composite functional coating liquid is 0.03-0.07 wt %;

[0013] As a further technical solution, the support layer has a thickness of 50-150 μm and a porosity of 60-80%;

[0014] As a further technical solution, the thickness of the adsorption degradation composite functional layer is 5-20 μm

[0015] As a further technical solution, the support layer includes any one of a polyethersulfone support layer, a polyvinylidene fluoride support layer, a polypropylene support layer, a polysulfone support layer, and a polyetherimide support layer;

[0016] As a further technical solution, the solvent A includes dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and an ethanol solution with a volume concentration of 75%;

[0017] As a further technical solution, solvent B comprises an acetic acid solution having a volume concentration of 1%;

[0018] As a further technical solution, the cross-linking agent includes one or more of glutaraldehyde, genipin (natural cross-linking agent), epichlorohydrin, and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC).

[0019] As a further technical solution, the organic adsorbent includes one or more of chitosan, sodium alginate, and hyaluronic acid;

[0020] As a further technical solution, the chitosan is carboxymethyl chitosan.

[0021] As a further technical solution, the carboxymethyl chitosan has a degree of substitution of ≥0.6 and is dissolved in a 1% by volume acetic acid solution to form a continuous phase, providing pH-responsive adsorption sites;

[0022] As a further technical solution, the metal organic framework material includes one or more of MOFs, MIL-101 (Cr), HKUST-1 (Cu), and NH2-MIL-53 (Al);

[0023] As a further technical solution, the MOFs include one or two of UiO-66-NH2 and ZIF-8.

[0024] As a further technical solution, the MOFs have a crystal particle size of 50-200 nm and a pore size of 0.5-2 nm, which can enhance pollutant retention through size screening;

[0025] As a further technical solution, UiO-66-NH2 is added in the form of UiO-66-NH2 or a UiO-66-NH2 precursor.

[0026] As a further technical solution, the porous material includes one or more of carbon nanotubes, graphene oxide, reduced graphene oxide, graphene quantum dots, and MXene.

[0027] As a further technical solution, the composite functional coating liquid comprises the following raw materials in parts by weight:

[0028]

[0029]

[0030] As a further technical solution, the graphene includes one or both of graphene oxide and reduced graphene oxide.

[0031] A method for preparing a composite nanofiltration membrane comprises the following steps:

[0032] Step 1, preparation of a metal organic framework material-porous material composite: the porous material is dispersed in solvent A to a concentration of 0.5-2 mg / mL, and after ultrasonic dispersion, the metal organic framework material or its precursor is added, stirred for 1 hour, and then hydrothermally reacted at 80-100° C. for 10-14 hours. The reaction product is washed and vacuum dried to obtain a metal organic framework material-porous material composite;

[0033] Step 2, preparation of a composite functional coating liquid: adding solvent B to the organic adsorbent, stirring until dissolved, preparing an organic adsorbent solution with a concentration of 1-3wt%, adding the metal organic framework material-porous material composite to the organic adsorbent solution at a mass ratio of 1:2 between the total amount of the metal organic framework material and the porous material and the organic adsorbent, and then adding a cross-linking agent to a final concentration of 0.03-0.07wt%, and ultrasonically dispersing for 20-40 minutes until uniform, to obtain a composite functional coating liquid;

[0034] Step 3: Film formation and curing:

[0035] The support layer was soaked in deionized water for 30 minutes to remove surface impurities, and then immersed in a composite functional coating liquid for 8-15 minutes after being drained. The base membrane was removed from the coating liquid at a pulling speed of 40-60 mm / min to form an adsorption and degradation composite functional layer. The base membrane was allowed to stand at room temperature for 10 minutes, and then placed in an oven at 60-80°C for heat treatment for 1-2 hours to obtain a composite nanofiltration membrane.

[0036] As a further technical solution, in step 1, the washing method is: washing with DMF and ethanol alternately 2-4 times.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. The present invention achieves a retention rate of >90% for EDCs such as BPA and E2 through the synergistic effect of MOFs size screening and CMCS adsorption, far exceeding the 70-85% of traditional nanofiltration membranes;

[0039] 2. The present invention composites graphene with MOFs. On the one hand, hydrogen bonding and covalent bonding work together to tightly bind MOFs to graphene sheets, so that graphene sheets and MOFs crystals are fixed in the network, reducing the stacking and agglomeration of graphene due to van der Waals forces, forming a stable dispersed structure, and avoiding the agglomeration of graphene. It also utilizes the fast water channel constructed by the graphene water flux to make the membrane flux reach 20-30LMH / bar, which is much higher than the 15-20LMH / bar of similar nanofiltration membranes. On the other hand, the GO surface contains a large number of oxygen-containing functional groups (such as hydroxyl, carboxyl, and epoxy groups), which can be easily converted into water under ultraviolet light. A small amount of reactive oxygen species (ROS) may be produced. rGO has a conjugated carbon skeleton that can absorb visible light (400-700nm), which makes up for the limitation of ZIF-8 that only responds to ultraviolet light and realizes "ultraviolet + visible light" driven photocatalysis; in addition, rGO can also act as an electron acceptor or transmission channel to form a heterojunction with the metal nodes of MOFs, delaying electron-hole recombination, thereby improving the photocatalytic efficiency; the photocatalytic synergistic effect of graphene and MOFs can degrade adsorbed pollutants under ultraviolet light irradiation, with a degradation rate of >80%, significantly reducing membrane pollution, avoiding membrane pore blockage, and extending the service life of the membrane. The MOFs of the present invention (such as NH2-UiO-66 and ZIF-8) have regular pore sizes and high specific surface areas, and can enrich pollutants by adsorption (providing a high-concentration reaction environment for photocatalytic reactions); graphene (GO / rGO) has excellent electronic conductivity and can serve as an electron transfer medium to reduce the recombination of photogenerated carriers (electron-hole pairs); the two are in close contact in a covalently cross-linked three-dimensional network, forming a synergistic path of "adsorption-electron transfer-catalysis", which activates and enhances the light response ability that was originally weak when existing alone, thereby giving the present invention excellent self-cleaning performance, and it has extremely strong application potential in the field of water treatment (such as deep purification and pollutant degradation).

[0040] 3. The graphene, MOFs and CMCS of the present invention are ternary composited through covalent cross-linking. This overcomes the defects of insufficient stability when using MOFs alone, poor mechanical properties when using CMCS alone, and easy agglomeration when using graphene alone. It greatly enhances the mechanical strength and stability of the membrane. After 5 cycles of use, the performance retention rate is greater than 85%, and the service life is extended by more than 30% compared with conventional composite membranes.

[0041] In summary, the process of the present invention is simple and convenient, and the prepared composite nanofiltration membrane not only has a large water flux and a high retention rate, but can also efficiently remove trace pollutants such as bisphenol A (BPA) and estradiol (E2) in water bodies, but also has a self-cleaning function, can degrade pollutants, avoid membrane pore clogging, and extend the service life of the membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 FTIR spectra of ZIF-8, GO, CMCS, and ZIF-8-GO-CMCS; DETAILED DESCRIPTION

[0043] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0044] In the present invention,

[0045] NH2-BDC: purchased from Aladdin Reagent Co., Ltd.;

[0046] ZrCl4: purchased from Aladdin Reagent Co., Ltd.;

[0047] Glutaraldehyde: purchased from Aladdin Reagent Co., Ltd.

[0048] Graphene oxide (GO): oxidation degree of C / O ratio of 2:1 to 4:1, sheet thickness of 0.8-1.2 nm, purchased from Aladdin Reagent Co., Ltd.

[0049] Reduced graphene oxide (rGO): oxidation degree of C / O ratio of 2:1 to 4:1, sheet thickness of 0.8-1.2 nm, purchased from Aladdin Reagent Co., Ltd.

[0050] ZIF-8: crystal size 50-200 nm, pore size 0.5-2 nm, purchased from Aladdin Reagent Co., Ltd.

[0051] Polyethersulfone (PES) based membrane: 50-150 μm thick, 60-80% porosity, produced by Ningbo Rixin Hengli Technology Co., Ltd.

[0052] CMCS (degree of substitution ≥ 0.6): purchased from Aladdin Reagent Co., Ltd.

[0053] In the present invention, unless otherwise specified, all raw materials used are commercially available.

[0054] Example 1

[0055] A composite nanofiltration membrane, the preparation method of which comprises the following steps:

[0056] Step 1, preparation of UiO-66-NH2-graphene composite: 10 mg of graphene oxide (GO) was dispersed in 10 mL of N, N-dimethylformamide (DMF) to a concentration of 1 mg / mL, and ultrasonicated for 30 minutes; 0.5 mmol of ZrCl4 and 0.5 mmol of NH2-BDC were added, stirred for 1 hour, transferred to a reactor, and hydrothermally reacted at 80 ° C for 12 hours; the reaction product was washed alternately with DMF and ethanol three times, and vacuum dried at 60 ° C for 12 hours to obtain a UiO-66-NH2-graphene composite. In this embodiment, the mass ratio of graphene oxide to UiO-66-NH2 is 1:1;

[0057] Step 2, preparation of CMCS composite coating liquid: 100 mg of CMCS was dissolved in 10 mL of 1% acetic acid solution and stirred until completely dissolved to prepare a CMCS solution with a concentration of 1:2; 50 mg of UiO-66-NH2-graphene composite was added to the CMCS solution according to the mass ratio of the total amount of MOFs and graphene oxide to CMCS of 1:2, and glutaraldehyde was added to a final concentration of 0.05 wt%, and ultrasonic dispersion was performed for 30 minutes to form a uniform and stable CMCS composite coating liquid;

[0058] Step 3: Film formation and curing

[0059] The PES base membrane was soaked in deionized water for 30 minutes to remove surface impurities, drained and immersed in the above coating liquid for 15 minutes; the base membrane was removed from the CMCS composite coating liquid at a pulling speed of 50 mm / min, allowed to stand at room temperature for 10 minutes, and then placed in a 60°C oven for heat treatment for 1 hour to complete cross-linking and curing to obtain a composite nanofiltration membrane.

[0060] Example 2

[0061] A composite nanofiltration membrane, the preparation method of which comprises the following steps:

[0062] Step 1. Preparation of ZIF-8-graphene complex: 5 mg of reduced graphene oxide (rGO) was dispersed in 10 mL of N,N-dimethylformamide (DMF) to a concentration of 0.5 mg / mL, and ultrasonically treated for 30 minutes; 0.8 mmol of Zn(NO3)2·6H2O and 3.2 mmol of 2-methylimidazole were added, stirred for 1 hour, and then transferred to a reactor for hydrothermal reaction at 80°C for 12 hours; the reaction product was washed alternately with DMF and ethanol three times, and vacuum dried at 60°C for 12 hours to obtain a ZIF-8-graphene complex.

[0063] Step 2: Preparation of a CMCS composite coating solution: 200 mg of CMCS was dissolved in 10 mL of 1% acetic acid solution and stirred until completely dissolved to prepare a CMCS solution; 100 mg of a ZIF-8-graphene composite was added to the CMCS solution at a mass ratio of 1:2 between the total weight of MOFs and graphene oxide and CMCS, and glutaraldehyde was added to a final concentration of 0.05 wt %, and ultrasonic dispersion was performed for 30 minutes to form a uniform and stable CMCS composite coating solution;

[0064] Step 3: Film formation and curing

[0065] The PES base membrane was soaked in deionized water for 30 minutes to remove surface impurities, drained and immersed in the above coating liquid for 10 minutes; the base membrane was removed from the CMCS composite coating liquid at a pulling speed of 50 mm / min, allowed to stand at room temperature for 10 minutes, and then placed in a 70°C oven for heat treatment for 1.5 hours to complete cross-linking and curing to obtain a composite nanofiltration membrane.

[0066] Example 3

[0067] A composite nanofiltration membrane, the preparation method of which comprises the following steps: the same as Example 1, except that the pulling speed is reduced from 50 mm / min in Example 1 to 30 mm / min;

[0068] Example 4

[0069] A composite nanofiltration membrane, the preparation method of which comprises the following steps: the same as Example 1, except that the pulling speed is increased from 50 mm / min in Example 1 to 70 mm / min;

[0070] Example 5

[0071] A composite nanofiltration membrane, the preparation method of which comprises the following steps: the same as Example 1, except that the amount of CMCS is fixed at 100 mg, and the amount of UiO-66-NH2-O complex added is reduced from 50 mg in Example 1 to 30 mg;

[0072] Example 6

[0073] A composite nanofiltration membrane, the preparation method of which comprises the following steps: the same as Example 1, except that the amount of CMCS is fixed at 100 mg, and the amount of UiO-66-NH2-GO composite added is increased from 50 mg in Example 1 to 70 mg;

[0074] Comparative Example 1

[0075] Commercial NF270 nanofiltration membrane

[0076] Comparative Example 2

[0077] A composite nanofiltration membrane, the preparation method of which comprises the following steps:

[0078] Step 1, preparation of CMCS composite coating liquid: 100 mg of CMCS was dissolved in 10 mL of 1% acetic acid solution and stirred until completely dissolved to prepare a CMCS solution with a concentration of 1:2; 50 mg of graphene oxide was added to the CMCS solution at a mass ratio of graphene oxide to CMCS of 1:2, and glutaraldehyde was added to a final concentration of 0.05 wt %, and ultrasonic dispersion was performed for 30 minutes to form a uniform and stable CMCS composite coating liquid;

[0079] Step 2: Film formation and curing

[0080] The PES base membrane was soaked in deionized water for 30 minutes to remove surface impurities, drained and immersed in the above coating liquid for 15 minutes; the base membrane was removed from the CMCS composite coating liquid at a pulling speed of 50 mm / min, allowed to stand at room temperature for 10 minutes, and then placed in a 60°C oven for heat treatment for 1 hour to complete cross-linking and curing to obtain a composite nanofiltration membrane.

[0081] Comparative Example 3

[0082] A composite nanofiltration membrane, the preparation method of which comprises the following steps:

[0083] Step 1: First, 0.5mmol ZrCl4 and 0.5mmol NH2-BDC were added to 10mL DMF, stirred for 1h, and then transferred to a reactor for hydrothermal reaction at 80℃ for 12h. The reaction product was washed alternately with DMF and ethanol three times and dried in vacuo at 60℃ for 12h to obtain UiO-66-NH2;

[0084] Step 2, preparation of CMCS composite coating liquid: 100 mg of CMCS was dissolved in 10 mL of 1% acetic acid solution and stirred until completely dissolved to prepare a CMCS solution with a concentration of 1000 mg. According to the mass ratio of UiO-66-NH2 to CMCS of 1:2, 50 mg of UiO-66-NH2 was added to the CMCS solution, and glutaraldehyde was added to a final concentration of 0.05 wt%. Ultrasonic dispersion was performed for 30 minutes to form a uniform and stable CMCS composite coating liquid.

[0085] Step 3: Film formation and curing

[0086] The PES base membrane was soaked in deionized water for 30 minutes to remove surface impurities, drained and immersed in the above coating liquid for 15 minutes; the base membrane was removed from the CMCS composite coating liquid at a pulling speed of 50 mm / min, allowed to stand at room temperature for 10 minutes, and then placed in a 60°C oven for heat treatment for 1 hour to complete cross-linking and curing to obtain a composite nanofiltration membrane.

[0087] Comparative Example 4

[0088] A composite nanofiltration membrane, the preparation method of which comprises the following steps:

[0089] Step 1: First, 0.5 mmol ZrCl4 and 0.5 mmol NH2-BDC were added to 10 mL DMF, stirred for 1 hour, and then transferred to a reactor for hydrothermal reaction at 80 ° C for 12 hours. The reaction product was washed alternately with DMF and ethanol for 3 times and dried in vacuo at 60 ° C for 12 hours to obtain UiO-66-NH2;

[0090] Step 2: Dissolve 100 mg of carboxymethyl chitosan (CMCS) in 10 mL of 1% acetic acid solution, add 25 mg of graphene oxide (rGO) and 25 mg of NH2-UiO-66, and ultrasonically disperse for 30 minutes; add glutaraldehyde (crosslinking agent) to a final concentration of 0.05 wt%, and stir for 1 hour to obtain a CMCS composite coating solution.

[0091] Step 3: Film formation and curing

[0092] The PES base membrane was soaked in deionized water for 30 minutes to remove surface impurities, drained and immersed in the above coating liquid for 15 minutes; the base membrane was taken out from the CMCS composite coating liquid at a pulling speed of 50 mm / min, allowed to stand at room temperature for 10 minutes, and then placed in a 60°C oven for heat treatment for 1 hour to complete cross-linking and curing to obtain a composite nanofiltration membrane (rGO and UiO-66-NH2 mass ratio of 1:1).

[0093] Effect Example 1

[0094] Infrared scanning was performed on ZIF-8, GO, CMCS, and ZIF-8-GO-CMCS ternary composite materials in Example 2, and the results are shown in Figure 2. Figure 1 ;

[0095] Figure 1 The spectrum of ZIF-8 shows that the absorption peak is at 3470 cm -1 and 2928cm -1 These are the stretching vibrations of -NH and CH, respectively. The broad peak is approximately 164 cm -1 Corresponding to the stretching vibration of the C=C and C=N double bonds in the imidazole ring. 1448cm -1The strong bending band at 997 cm is related to the stretching of the entire imidazole ring. -1 and 763cm -1 It is related to the deformation vibration and bending vibration of CH on the imidazole ring. The absorption peak of CMCS-GO-ZIF-8 is located at 3444 cm -1 2924cm -1 , 1620cm -1 , 1431cm -1 , 1323cm -1 and 1056cm -1 They are respectively related to the stretching vibration of -OH, the stretching vibration of CH, the stretching vibration of -CO double bond, the bending vibration of -OH, and the deformation vibration of CO and COC. In addition, the hydrogen bond interaction between the imidazole ring in ZIF-8 and the -OH in CMCS-GO leads to a more uniform distribution of electron density, which in turn leads to a decrease in the stretching vibration frequency. Therefore, the stretching vibration of the -CO double bond and the bending vibration of -OH of CMCS-GO-ZIF-8 are shifted to a low wave number at 1590 cm -1 and 1420cm -1 These results can well prove that ZIF-8-GO-CMCS is not a simple mixture, but a composite material.

[0096] Effect example 2:

[0097] The water flux and rejection rate of the nanofiltration membranes prepared in Examples 1-8 and Comparative Examples 1-4 were tested. The results are shown in Table 1.

[0098] 1. Retention rate test: Test equipment: dead-end filtration system; Test conditions: pollutant solution (BPA 10mg / L or E 25mg / L), operating pressure 0.3-0.7MPa, temperature 25℃;

[0099] 2. Water flux test: deionized water, pressure 0.5MPa;

[0100] 3. Self-cleaning test: Place the adsorption-saturated composite membrane under ultraviolet light for 3 hours and calculate the degradation rate of BPA; also make a blank control without nanofiltration membrane;

[0101] 4. Adsorption performance under dark reaction: adsorption of 10 mg / L BPA solution in the dark for 3 hours;

[0102] 5. Cycle test: After 5 adsorption-degradation cycles, the membrane retention rate and water flux are measured again, and the retention rate and flux retention rate are calculated;

[0103] 6. Mechanical properties: According to GB / T 1040.3-2006, the details are as follows:

[0104] 1) Sample preparation

[0105] Cut samples from the prepared composite nanofiltration membrane. Use long strip samples with dimensions of 100 mm × 10 mm (length × width). The effective test section length of the sample is 50 mm. Prepare at least three parallel samples for each test to ensure that the edges of the samples are flat, without wrinkles or damage, to avoid affecting the test results due to sample defects.

[0106] 2) Test equipment

[0107] Use a universal materials testing machine equipped with grips suitable for thin film testing.

[0108] 3) Test conditions

[0109] Environmental conditions: temperature 25±2℃, relative humidity 50±5%;

[0110] Tensile rate: 5 mm / min;

[0111] Preloading: Apply a pretension of 0.5N to keep the specimen in a taut state and eliminate the initial relaxation.

[0112] 4) Test steps

[0113] Clamp both ends of the specimen in the upper and lower fixtures of the testing machine, ensuring that the specimen axis is aligned with the center line of the fixture to avoid eccentric force. Start the testing machine and record the maximum load (N) of the specimen from the time of force application to fracture. If the specimen breaks in the clamping area of ​​the fixture, the data is considered invalid and needs to be retested.

[0114] 5) Calculation of results

[0115] Tensile strength (MPa) = maximum load (N) / original cross-sectional area of ​​the specimen (m 2 )

[0116] Among them, the original cross-sectional area of ​​the sample = sample width (m) × sample thickness (m). The film thickness needs to be measured using a screw micrometer at three randomly selected points on the effective section of the sample and the average value is taken.

[0117] The test results of each group of parallel samples are calculated as the arithmetic mean and retained to two significant figures.

[0118] Table 1

[0119]

[0120] From the data in Table 1 we can see that:

[0121] 1) When the pulling speed is reduced (Example 3), the functional layer becomes thicker, the water flux decreases slightly but the retention rate remains stable; when the speed is increased (Example 4), the functional layer becomes thinner, the flux increases but the retention rate decreases slightly.

[0122] 2) When the amount of the composite added is increased (Example 6), the screening effect is enhanced, but the pores are easily clogged and the flux decreases; when the amount added is insufficient (Example 5), the screening ability is weak and the retention rate decreases.

[0123] 3) When the proportion of rGO increases (Example 8), the water channels are more abundant and the flux is improved; when the proportion of MOFs increases (Example 7), the screening is enhanced but the photocatalytic synergy is weakened, and the degradation rate decreases.

[0124] 4) The performance of the binary composite membranes (Comparative Examples 2 and 3) is weaker than that of the ternary composite membranes (Examples 1-8) in all aspects, which proves the necessity of the ternary synergistic effect.

[0125] 5) Due to the uneven dispersion of materials, the performance of the direct mixing process (Comparative Example 4) is far lower than that of the process of "preparing the metal organic framework material-porous material composite and then compounding it with CMCS" (Example 1), highlighting the process advantages of the present invention.

[0126] 6) The commercial membrane (Comparative Example 1) has no self-cleaning function (degradation rate 0) and its cycle performance is worse than that of the product of the present invention.

[0127] 7) The tensile strength of the ternary composite membranes (Examples 1-8) is generally higher than that of the binary composite membranes (Comparative Examples 2 and 3) and the direct mixed membranes (Comparative Examples 4 and 5), which indicates that covalent crosslinking can enhance the stability of the membrane.

[0128] In summary, the composite nanofiltration membrane prepared by the present invention has high retention, high flux, self-cleaning and high stability, and extends the service life of the membrane.

[0129] The above-described embodiments are merely preferred embodiments of the present invention and are not exhaustive of all feasible implementations of the present invention. Any obvious modifications made by a person skilled in the art without departing from the principles and spirit of the present invention should be considered to be within the scope of protection of the claims of the present invention.

Claims

1. A composite nanofiltration membrane, characterized in that: It includes a support layer and an adsorption and degradation composite functional layer arranged on the support layer; The adsorption degradation composite functional layer is prepared from a composite functional coating liquid. The composite functional coating liquid comprises the following raw materials in parts by weight:

2. A composite nanofiltration membrane according to claim 1, characterized in that: The mass ratio of metal organic framework material to porous material is 1:1; The mass ratio of the total amount of metal organic framework material, porous material and organic adsorbent is 1:2; The concentration of the crosslinking agent in the composite functional coating liquid is 0.03-0.07 wt %.

3. A composite nanofiltration membrane according to claim 1, characterized in that: The support layer includes any one of a polyethersulfone support layer, a polyvinylidene fluoride support layer, a polypropylene support layer, a polysulfone support layer, and a polyetherimide support layer; The solvent A comprises dimethyl sulfoxide, N-methylpyrrolidone, and an ethanol solution with a volume solubility of 75%; Solvent B is 1% acetic acid solution; The cross-linking agent includes one or more of glutaraldehyde, genipin, epichlorohydrin, and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide.

4. A composite nanofiltration membrane according to claim 1, characterized in that: The organic adsorbent includes one or more of chitosan, sodium alginate, and hyaluronic acid; The chitosan is carboxymethyl chitosan.

5. A composite nanofiltration membrane according to claim 1, characterized in that: The metal organic framework material includes one or more of MOFs, MIL-101 (Cr), HKUST-1 (Cu), and NH2-MIL-53 (Al); The MOFs include one or two of UiO-66-NH2 and ZIF-8.

6. A composite nanofiltration membrane according to claim 1, characterized in that: UiO-66-NH2 is added in the form of UiO-66-NH2 or a UiO-66-NH2 precursor.

7. A composite nanofiltration membrane according to claim 1, characterized in that: The porous material includes one or more of carbon nanotubes, graphene oxide, reduced graphene oxide, graphene quantum dots, and MXene.

8. A method for preparing a composite nanofiltration membrane, characterized in that: The steps include: Step 1, preparation of a metal organic framework material-porous material composite: the porous material is dispersed in solvent A to a concentration of 0.5-2 mg / mL, and after ultrasonic dispersion, the metal organic framework material or its precursor is added, stirred for 1 hour, and then hydrothermally reacted at 80-100° C. for 10-14 hours. The reaction product is washed and vacuum dried to obtain a metal organic framework material-porous material composite; Step 2, preparation of a composite functional coating liquid: adding solvent B to the organic adsorbent, stirring until dissolved, preparing an organic adsorbent solution with a concentration of 1-3wt%, adding the metal organic framework material-porous material composite to the organic adsorbent solution at a mass ratio of 1:2 between the total amount of the metal organic framework material and the porous material and the organic adsorbent, and then adding a cross-linking agent to a final concentration of 0.03-0.07wt%, and ultrasonically dispersing for 20-40 minutes until uniform, to obtain a composite functional coating liquid; Step 3: Film formation and curing: The support layer was soaked in deionized water for 30 minutes to remove surface impurities, and then immersed in a composite functional coating liquid for 8-15 minutes after being drained. The base membrane was removed from the coating liquid at a pulling speed of 40-60 mm / min to form an adsorption and degradation composite functional layer. The base membrane was allowed to stand at room temperature for 10 minutes, and then placed in an oven at 60-80°C for heat treatment for 1-2 hours to obtain a composite nanofiltration membrane.

9. The method for preparing a composite nanofiltration membrane according to claim 8, characterized in that: In step 1, the washing method is: washing with DMF and ethanol alternately for 2-4 times.

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