High-flux polypiperazine amide nanofiltration membrane and preparation method thereof

Abstract

The invention belongs to the technical field of membrane separation, and particularly relates to a high-flux polypiperazine amide nanofiltration membrane and a preparation method thereof. The polypiperazine amide functional layer is formed by polymerizing an aqueous phase precursor and an organic phase precursor, the aqueous phase precursor comprises 0.4%-1.2% of piperazine, 0.2%-1% of triethylamine hydrochloride, 0.02%-0.1% of polyvinyl alcohol and 0.002%-0.01% of nanocellulose, and the organic phase precursor comprises 0.06%-0.14% of trimesoyl chloride (the proportion is w / v). Under the test conditions of 500 ppm of a NaCl solution, 0.5 MPa and 25 DEG C, the water flux of the obtained nanofiltration membrane can reach 55.2 LMH at most, and the NaCl salt rejection rate can reach 35% or above, which is significantly higher than that in the prior art. The nanofiltration membrane has high water flux and high salt rejection rate, meanwhile, the requirement for operation pressure is lower, the desalination efficiency can be remarkably improved, and the unit water production energy consumption can be reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of membrane separation technology, and particularly relates to an improved poly-piperazine amide nanofiltration membrane and a preparation method thereof. BACKGROUND

[0002] In the field of membrane separation technology, membrane materials can be divided into four types of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) according to the pore size difference, and different types of membrane materials are applied to different purification targets in the field of water treatment. When the use of purified water is industrial cooling and flushing water, the purification target is to remove divalent ions that are easy to precipitate, and to remove sodium and chloride ions that are easy to cause surface corrosion as much as possible, and at the same time, the purification process should reduce energy consumption and material cost as much as possible; at this time, nanofiltration membranes with both divalent ion and monovalent ion rejection effect and low operating pressure requirement are needed. Poly-piperazine amide nanofiltration membrane is one of the most widely commercialized nanofiltration membrane types at present, and its preparation mainly adopts the interfacial polymerization method, that is, an ultrafiltration membrane is used as a substrate, and a condensation reaction of water-phase monomer piperazine (PIP) and organic-phase monomer trimesoyl chloride (TMC) occurs at the interface of two immiscible phases to form a dense ultra-thin polyamide functional layer. For example, patent CN201410207575.0 proposes a carboxymethyl cellulose sodium complex filled polyamide nanofiltration membrane, which prepares a carboxymethyl cellulose sodium complex by an ionic crosslinking method, adds the carboxymethyl cellulose sodium complex in a water-phase monomer solution for synthesizing a polyamide membrane, and prepares a carboxymethyl cellulose sodium complex filled polyamide nanofiltration membrane by an interfacial polymerization method. The nanofiltration membrane obtained by the method has a water flux of 50-65 LMH (L / (m 2 ·h) and a highest divalent ion rejection rate of 97% under an operating pressure of 0.6 MPa. The nanofiltration membrane has the disadvantage that the monovalent ion rejection rate is less than 25%. SUMMARY

[0003] Therefore, the application hopes to propose an improved poly-piperazine amide nanofiltration membrane and a preparation method thereof, which can improve the monovalent ion rejection rate of the nanofiltration membrane while maintaining a high flux.

[0004] The application is achieved by the following technical scheme:

[0005] A high-flux poly-piperazine amide nanofiltration membrane comprises a polysulfone (PSF) base layer and a poly-piperazine amide functional layer arranged on the base layer.

[0006] The poly-piperazine amide functional layer is formed by in-situ polymerization of an aqueous phase precursor and an organic phase precursor, the aqueous phase precursor comprises piperazine (PIP) with a mass-volume fraction of 0.4% to 1.2%, triethylamine hydrochloride (TEA·HCl) with a mass-volume fraction of 0.2% to 1%, polyvinyl alcohol (PVA) with a mass-volume fraction of 0.02% to 0.1%, and nanocellulose (CNC) with a mass-volume fraction of 0.002% to 0.01%, and the organic phase precursor comprises trimesoyl chloride (TMC) with a mass-volume fraction of 0.06% to 0.14%. The CNC is a crystal with a diameter of 3 to 20 nm and a length of 50 to 500 nm. The definition formula of mass-volume fraction (w / v) is w / v = (mass of solute / volume of solution) x 100%, wherein the mass of solute is in gram (g) and the volume of solution is in milliliter (mL).

[0007] A preparation method of a high-flux poly-piperazine amide nanofiltration membrane, comprising the following steps:

[0008] S1. Pouring an aqueous phase solution onto the surface of a polysulfone base film to form a liquid film covering the surface of the polysulfone base film, and removing the aqueous phase solution on the surface after standing for 50 to 70 seconds; the aqueous phase solution comprises piperazine (PIP) with a mass-volume fraction of 0.4% to 1.2%, triethylamine hydrochloride (TEA·HCl) with a mass-volume fraction of 0.2% to 1%, polyvinyl alcohol (PVA) with a mass-volume fraction of 0.02% to 0.1%, and nanocellulose (CNC) with a mass-volume fraction of 0.002% to 0.01%;

[0009] S2. Pouring an organic phase solution onto the surface of the polysulfone base film treated in S1 to form a liquid film covering the surface of the polysulfone base film, and removing the organic phase solution on the surface after standing for 20 to 40 seconds to obtain a composite film; the organic phase solution comprises trimesoyl chloride (TMC) with a mass-volume fraction of 0.06% to 0.14%;

[0010] S3. Performing heat crosslinking treatment on the composite film obtained in S2.

[0011] In the prior art, the performance of a thin film composite (TFC) nanofiltration membrane is generally limited by the inherent permeation-selectivity balance constraint (Trade-off effect) of the synthetic membrane, that is, the improvement of water flux is often accompanied by the decrease of salt rejection rate, and the inventors hope to obtain a nanofiltration membrane with high water flux and high salt rejection rate through optimization of the formula. Since the main problem of the nanofiltration membrane in the prior art is the insufficient retention effect of monovalent ions, the main idea of the inventors is to improve the retention rate of monovalent ions (hereinafter referred to as salt rejection rate) of the nanofiltration membrane.

[0012] In the field of separation membranes, the best membrane material for monovalent ion rejection is reverse osmosis membrane (RO), which can achieve a salt rejection rate of sodium chloride of more than 90%. Research in the field of reverse osmosis membranes found that in a composite reverse osmosis membrane with polysulfone as the substrate and polyamide as the selective layer, the loading amount of nanocellulose (CNC) was negatively correlated with the salt rejection rate, that is, the rejection effect of the membrane on monovalent ions decreased with the increase of the loading amount of CNC. The inventors also found a similar phenomenon in nanofiltration membrane experiments, that is, the salt rejection rate of the nanofiltration membrane decreased from 43.17% to less than 29.67% after the addition of CNC.

[0013] Polyvinyl alcohol (PVA) is also one of the commonly used modified materials in the field of separation membranes. PVA contains a large number of strong hydrophilic hydroxyl groups and is often used as a hydrophilic modification reagent for hydrophobic membranes to improve the water flux of the membranes. Based on the Trade-off effect of separation membranes described above, it can be theoretically speculated that the salt rejection rate of the nanofiltration membrane after PVA treatment will decrease; at the experimental level, the inventors verified this phenomenon, that is, the water flux of the nanofiltration membrane increased from 17.5 to more than 37.9 after the addition of PVA, but the salt rejection rate decreased from 42.22% to less than 31.27%.

[0014] Under the premise that PVA and CNC alone will reduce the salt rejection rate of the membrane, the inventors found that when a specific proportion of PVA and CNC is added to the aqueous solution, CNC shows an improvement in salt rejection rate. This result is contrary to existing knowledge, suggesting that the presence of PVA may change the mechanism of action of CNC in the nanofiltration membrane system. It is speculated that PVA and CNC have a synergistic effect; CNC crystals are prone to agglomeration, while PVA is a water-soluble polymer, and the hydroxyl groups on its molecular chain can form strong hydrogen bond interactions with the hydroxyl groups on the surface of CNC, effectively wrapping CNC particles and uniformly and stably dispersing CNC. In addition, PVA can also act as a crosslinking precursor and network forming agent, and the hydroxyl groups of PVA can undergo interfacial crosslinking reactions with TMC monomers in the organic phase in the subsequent step, further anchoring CNC in the crosslinked network, forming a more regular and higher crosslinking degree poly (piperazine amide) network, reducing non-selective defects, and improving the rejection ability of monovalent ions. Experimental results show that the nanofiltration membrane obtained by the present application has a permeation flux of more than 50.50 LMH and a NaCl salt rejection rate of more than 31.07%, combining high water flux and high salt rejection rate.

[0015] As a preferred embodiment, the mass fraction of PVA in the aqueous solution is 0.06% to 0.08%.

[0016] PVA contains a large number of hydroxyl groups, which can modify the surface of the nanofiltration membrane. It can improve the structure and morphology of the poly (piperazine amide) functional layer. It can increase the viscosity of the water phase and slow down the diffusion rate of PIP to the organic phase, so as to form a thinner, more uniform and less defective polyamide layer, thereby improving the water flux. If the amount is too low, the effect is not obvious; If the amount is too high, it may cause the viscosity of the water phase to be too large, affecting the contact of the monomer, and even a separate PVA layer may be formed on the membrane surface, which may excessively reduce the water flux.

[0017] As a preferred, the mass fraction of the TEA·HCl in the water phase is 0.6%~1.0%.

[0018] Hydrochloric acid will be produced in the interfacial polymerization reaction, which will protonate PIP, reduce the concentration of free and reactive PIP, and may hinder the continuous progress of the reaction, so the ideal reaction environment for interfacial polymerization is weakly basic conditions. In this study, triethylamine hydrochloride (TEA·HCl) was selected as an acid absorbent to neutralize the hydrochloric acid produced during the interfacial polymerization process, prevent PIP from being excessively protonated, and thus promote the forward reaction and optimize the membrane separation performance. Another key function of TEA·HCl is to promote the diffusion of water phase monomer piperazine to the organic phase interface and enhance the uniformity of the interfacial polymerization reaction. However, excessive TEA·HCl may cause the polymerization rate to be too fast, inducing membrane structure defects.

[0019] As a preferred, the mass fraction of the PIP in the water phase solution is 0.2%~0.6%.

[0020] PVA and CNC both have hydrophilic properties, and form a highly efficient hydrophilic network through a large number of hydrogen bonds. CNC is stably wrapped in PVA segments and uniformly dispersed in the membrane, significantly reducing the transmission resistance of water molecules.

[0021] As a preferred, the mass fraction of the TMC in the organic phase solution is 0.12%~0.14%.

[0022] As a preferred, the mass fraction of the CNC in the water phase solution is 0.004%~0.006%.

[0023] As a preferred, the temperature of the thermal crosslinking in S3 is 30±0.5℃, and the time is 5~10min.

[0024] As a preferred, it also includes a base film pretreatment step: before the S1 step, the polysulfone base film is soaked in deionized water, and then the surface moisture is removed.

[0025] Preferably, in the base film pretreatment step, the polysulfone base film is flattened and fixed to a 24cm×36cm polytetrafluoroethylene frame using a pressure-sensitive adhesive, and the front and back sides of the substrate are rapidly dehydrated using an air knife-assisted drying system.

[0026] Polysulfone is a hydrophobic polymer, and the dry film pores are filled with air. Immersion allows water molecules to completely fill all pores, creating conditions for the uniform distribution of the subsequent PIP solution and ensuring that the aqueous monomer can quickly and uniformly penetrate to the base film surface and near-surface pores through capillary action. After fixation, surface dehydration treatment is performed to prevent the aqueous phase from penetrating deep into the pores and to avoid polymerization and pore blockage caused by TMC diffusion.

[0027] This invention proposes a high-flux polypiperazine amide nanofiltration membrane and its preparation method. By controlling the mass and volume fractions of reactants, CNC, and PVA in the interfacial polymerization reaction, a nanofiltration membrane with both high flux and monovalent ion rejection capabilities is obtained, while requiring lower operating pressure. Under optimal conditions of 500 ppm NaCl solution, 0.5 MPa, 25°C, and preferred preparation parameters, the nanofiltration membrane obtained by this invention can simultaneously achieve a water flux of 55.2 LMH and a NaCl salt rejection rate of 35.43%, significantly superior to existing technologies. From an engineering application perspective, the nanofiltration membrane proposed in this invention can significantly enhance desalination efficiency and reduce energy consumption per unit of produced water. Attached Figure Description

[0028] Figure 1 These are scanning electron microscope images of the cNF-0 to cNF-5 film surfaces of the present invention;

[0029] Figure 2 These are scanning electron microscope (SEM) images of the cross-sections of the cNF-0 to cNF-5 films of this invention.

[0030] Figure 3 This is a comparison chart of salt rejection rate and water flux for cNF-0 to cNF-5 of this invention. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0032] I. Orthogonal Experiment

[0033] TMC, PIP, and TEA·HCl are the fundamental factors involved in the interfacial polymerization reaction. To precisely control the membrane performance balance, this paper employs orthogonal experimental design to systematically screen the optimal combination of TMC, PIP, and TEA·HCl to maximize the target salt rejection rate. This pre-optimization step aims to lay the foundation for subsequent membrane modification, ensuring that the membrane maintains a preset high salt rejection rate threshold while increasing water flux through modification. The factor level mapping table 1 and the orthogonal experimental design table 2 are shown below.

[0034] For ease of description, the "mass volume fraction of a substance in the aqueous / organic phase" will be referred to as "the concentration of a substance" in the following text. The formula for defining mass volume fraction (w / v) is w / v = (mass of solute / volume of solution) × 100%, where the mass of solute is in grams (g) and the volume of solution is in milliliters (mL).

[0035] Table 1 Mapping table of factor levels Horizontal TMC concentration (%) PIP concentration (%) TEA HCI concentration (%) 1 0.06 0.4 0.2 2 0.08 0.6 0.4 3 0.10 0.8 0.6 4 0.12 1.0 0.8 5 0.14 1.2 1.0

[0036] Table 2 Orthogonal Experimental Design Table Test number TMC concentration (%) PIP concentration (%) TEA HCI concentration (%) 1 0.06 0.4 0.2 2 0.06 0.6 0.4 3 0.06 0.8 0.6 4 0.06 1.0 0.8 5 0.06 1.2 1.0 6 0.08 0.4 0.4 7 0.08 0.6 0.6 8 0.08 0.8 0.8 9 0.08 1.0 1.0 10 0.08 1.2 0.2 11 0.10 0.4 0.6 12 0.10 0.6 0.8 13 0.10 0.8 1.0 14 0.10 1.0 0.2 15 0.10 1.2 0.4 16 0.12 0.4 0.8 17 0.12 0.6 1.0 18 0.12 0.8 0.2 19 0.12 1.0 0.4 20 0.12 1.2 0.6 21 0.14 0.4 1.0 22 0.14 0.6 0.2 23 0.14 0.8 0.4 24 0.14 1.0 0.6 25 0.14 1.2 0.8

[0037] The separation performance of nanofiltration membranes is demonstrated by testing two indicators: permeate flux (J) and salt rejection rate (R). Salt rejection rate typically refers to the rate of NaCl rejection, also known as desalination rate. Membranes were fabricated according to the parameters of an orthogonal experimental design table, and salt rejection rate and water flux were tested. The tests in the examples and comparative cases described below also followed the same method.

[0038] S1. Base film pretreatment: The polysulfone (PSF) base film is immersed in pure water, and then the PSF base film is flattened and fixed to a 24cm×36cm polytetrafluoroethylene frame with pressure-sensitive adhesive. The front and back sides of the polysulfone base film are rapidly dehydrated using an air knife-assisted drying system.

[0039] S2. Aqueous phase preparation: Prepare an aqueous solution containing PIP and TEA·HCl according to the parameters in Table 2;

[0040] S3. Organic phase preparation: Prepare an organic phase solution containing TMC according to the parameters in Table 2. The organic phase is cyclohexane.

[0041] S4. Interfacial polymerization: Pour the aqueous solution onto the surface of the PSF base membrane, ensuring that the solution completely submerges the PSF base membrane surface and forms a liquid film on the base membrane surface. After standing for 60 seconds, pour off the solution to remove any residual solution from the membrane surface. Then, pour the organic phase solution onto the base membrane surface, ensuring that the solution completely submerges the membrane surface and forms a liquid film on the base membrane surface. After standing for 30 seconds, pour off the solution to remove any residual solution from the membrane surface, thus obtaining the composite membrane.

[0042] S5, heat crosslinking: the prepared composite membrane is placed in an oven, the temperature is 30±0.5℃, and heat crosslinking treatment is carried out for 8 min, to obtain a finished nanofiltration membrane.

[0043] S6, performance test: a sodium chloride solution with a concentration of 500 mg / L is prepared as a feed liquid, and the test is carried out at 25℃ and 0.5 MPa (5 bar), after the flux is stable, the conductivities of the feed liquid and the permeate of the membrane module are measured, and the salt rejection rate is calculated according to the formula, and the average value is taken.

[0044] The permeation flux (J) is calculated according to formula (1-1):

[0045] J=V / (S·t) =L·m -2 ·h -1 (1-1),

[0046] In the formula:

[0047] V is the volume of the permeate, L;

[0048] t is the time interval between two sampling times, h;

[0049] S is the effective area of the membrane module, m 2 .

[0050] According to the theory: the concentration of NaCl in the low concentration range is proportional to the conductivity, and the conductivity is used to characterize the salt rejection rate of the nanofiltration membrane.

[0051] The salt rejection rate (R) is calculated according to formula (1-2):

[0052] R=(1-C p / C f )×100% (1-2),

[0053] In the formula:

[0054] R is the salt rejection rate, %;

[0055] C p is the conductivity of the permeate, μS / cm;

[0056] C f is the conductivity of the feed water, μS / cm.

[0057] The orthogonal test results are shown in Table 3.

[0058] Table 3 Orthogonal test results Test number TMC concentration (%) PIP concentration (%) TEA HCI concentration (%) Salt rejection rate (%) Water flux (LMH) 1 0.06 0.4 0.2 31.85 14.9 2 0.06 0.6 0.4 29.12 21.1 3 0.06 0.8 0.6 35.52 24.0 4 0.06 1.0 0.8 37.78 26.4 5 0.06 1.2 1.0 41.05 28.1 6 0.08 0.4 0.4 37.45 13.7 7 0.08 0.6 0.6 32.14 26.9 8 0.08 0.8 0.8 30.86 28.8 9 0.08 1.0 1.0 31.94 28.3 10 0.08 1.2 0.2 31.41 25.2 11 0.10 0.4 0.6 42.82 17.5 12 0.10 0.6 0.8 37.65 20.4 13 0.10 0.8 1.0 33.36 26.4 14 0.10 1.0 0.2 31.54 21.1 15 0.10 1.2 0.4 30.79 22.3 16 0.12 0.4 0.8 47.01 15.6 17 0.12 0.6 1.0 29.42 26.4 18 0.12 0.8 0.2 34.69 16.1 19 0.12 1.0 0.4 34.49 22.1 20 0.12 1.2 0.6 37.28 23.5 21 0.14 0.4 1.0 46.23 13.2 22 0.14 0.6 0.2 37.76 15.6 23 0.14 0.8 0.4 35.75 19.2 24 0.14 1.0 0.6 31.74 24.5 25 0.14 1.2 0.8 34.98 25.9

[0059] The orthogonal test results are analyzed as follows.

[0060] (1) Intuitive analysis method

[0061] Let TMC concentration, PIP concentration, TEA-HCl concentration be factors A, B, C respectively. Let the sum of the salt interception rates obtained at the same level of factor A be K, and let the average of the sum of the salt interception rates obtained at the same level of factor A be k. R is the difference between the maximum and minimum values of k. Similarly, calculate the K value, k value and R value of each level of the other factors. The calculation results are shown in the following table.

[0062] Table 4 Analysis of orthogonal test results

[0063] The R value can reflect the degree of influence of the factor on the experiment. The larger the R value, the more important the factor. For the main factors, the level with the best average index can be selected according to the size of k. For secondary factors, the level with the best average index can be selected, or the level that is easy to operate or saves raw materials can be selected. From the table data, when the TMC concentration is 0.14%, the PIP concentration is 0.4%, and the TEA-HCl concentration is 0.8%, the best salt interception rate can be achieved. The PIP concentration has a greater impact on the experiment. To avoid the influence of accidental factors and ensure the accuracy of the experiment, the formulation is used for verification test, and the experimental results are shown in the following table.

[0064] Table 5 Verification test results Test number TMC concentration (%) PIP concentration (%) TEA HCI concentration (%) Salt rejection rate (%) 1 0.14 0.4 0.8 46.89 2 0.14 0.4 0.8 47.12 3 0.14 0.4 0.8 46.87 4 0.14 0.4 0.8 47.04

[0065] From the verification test results, it can be seen that under the ratio of TMC concentration of 0.14%, PIP concentration of 0.4%, and TEA-HCl concentration of 0.8%, the obtained salt interception rate is higher than that in the orthogonal test except the maximum value.

[0066] (2) Analysis of variance method

[0067] The method of intuitive analysis of the results of orthogonal test is introduced in the previous section. Its advantages are simple, intuitive, and less calculation, which is easy to popularize and promote. For general problems in production practice, the intuitive analysis method can be well solved. However, the intuitive analysis method cannot estimate the size of errors in the test process and the determination of test results, so it cannot truly distinguish whether the difference of test results corresponding to each level of a factor is caused by the change of level or by the test error. Therefore, the conclusion obtained by the intuitive analysis method is not accurate enough. Moreover, the importance of each factor affecting the test results cannot be accurately estimated, and a standard cannot be provided to examine and judge whether the influence of the factor on the test results is significant. In particular, for tests with more than or equal to 3 levels and considering interaction, the intuitive analysis method is not convenient to use. The analysis of variance method can make up for the shortcomings of the intuitive analysis method.

[0068] Table 6 Analysis of variance results of orthogonal test Source of difference Sum of squares Degrees of freedom Mean square F value P value TMC 62.688 4 15.672 1.210 0.357 PIP 290.497 4 72.624 5.608 0.009 TEA 50.344 4 12.586 0.972 0.458 Error 155.405 12 12.950

[0069] Note: P < 0.05 means significant effect, P < 0.001 means extremely significant effect.

[0070] By comparing the F value of different factors, it can be seen that the effect of PIP concentration on salt rejection rate is the most significant, and by P value, it can be seen that the effect of PIP concentration on salt rejection rate is greater.

[0071] According to the analysis of orthogonal test results, when the TMC concentration is 0.14%, the PIP concentration is 0.4%, and the TEA-HCl concentration is 0.8%, the highest salt rejection rate can be obtained, so the subsequent experiment is carried out with this ratio.

[0072] II. Effect of PVA and CNC on nanofiltration membrane

[0073] On the basis of orthogonal test, the effect of PVA and CNC on nanofiltration membrane was studied. The membrane preparation steps were consistent with the orthogonal test, and the PIP concentration, TMC concentration and TEA-HCl concentration were in accordance with the optimal parameters selected by the orthogonal test. PVA or CNC was added to the aqueous solution used in S2.

[0074] Table 7 Effect of adding PVA alone on the performance of nanofiltration membrane PIP concentration (%) TMC concentration (%) TEA HCI concentration (%) PVA concentration (%) Water flux (LMH) Salt rejection rate (%) 0.4 0.14 0.8 0 17.5 42.22 0.4 0.14 0.8 0.02 37.9 29.80 0.4 0.14 0.8 0.04 39.6 27.83 0.4 0.14 0.8 0.06 42.7 31.27 0.4 0.14 0.8 0.08 48.0 30.70 0.4 0.14 0.8 0.1 43.9 27.39

[0075] Table 8 Effect of adding CNC alone on the performance of nanofiltration membrane PIP concentration (%) TMC concentration (%) TEA HCI concentration (%) CNC concentration (%) Water flux (LMH) Salt rejection rate (%) 0.4 0.14 0.8 0 16.40 43.17 0.4 0.14 0.8 0.002 22.30 29.67 0.4 0.14 0.8 0.004 22.30 28.17 0.4 0.14 0.8 0.006 25.18 26.35 0.4 0.14 0.8 0.008 30.70 28.16 0.4 0.14 0.8 0.01 32.37 29.31

[0076] From the water flux index, adding PVA alone can significantly improve the water flux of nanofiltration membrane, and adding CNC alone can slightly improve the water flux of nanofiltration membrane. Further, the PVA concentration shows an inverted U-shaped curve relationship with the improvement effect of water flux, and when the PVA concentration is about 0.08%, the water flux of the membrane can be maximized. The CNC concentration shows a linear relationship with the improvement effect of water flux, and when the CNC addition is the largest in the test range, the water flux of the nanofiltration membrane is the largest.

[0077] From the salt rejection rate index, adding PVA and CNC alone can significantly reduce the salt rejection rate of nanofiltration membrane. Further, no special obvious quantitative relationship between PVA concentration and salt rejection rate was observed. CNC concentration and salt rejection rate showed a U-shaped relationship, that is, when a small amount of CNC was added, the salt rejection rate decreased with the increase of CNC; continuing to increase the amount of CNC, the salt rejection rate slightly increased, and the rejection effect of monovalent ions of nanofiltration membrane was restored, but it could not recover to the level before adding.

[0078] III. CNC and PVA synergistic effect test

[0079] In order to obtain a nanofiltration membrane with high water flux and high salt rejection rate, the present application selects to conduct the test under the condition of PVA concentration of 0.08%, and studies the effect of CNC and PVA synergistic effect. The test group includes the following 1 comparative example and 5 examples.

[0080] Comparative Example

[0081] The comparative example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-0.

[0082] The preparation method is as follows:

[0083] S1, base film pretreatment: immerse the PSF base film in pure water, then flatten and fix the PSF base film on a 24cm*36cm polytetrafluoroethylene frame by a pressure-sensitive adhesive, and use an air knife assisted drying system to quickly dehydrate the front and back of the base film;

[0084] S2, water phase preparation: prepare a water phase solution, which includes PIP with a concentration of 0.4%, PVA with a concentration of 0.08%, and TEA·HCl with a concentration of 0.8%;

[0085] S3, organic phase preparation: dissolve TMC in cyclohexane to prepare a TMC organic phase solution with a mass-volume concentration of 0.14%;

[0086] S4, interfacial polymerization: pour the water phase solution on the surface of the base film, and ensure that the solution completely immerses the film surface and forms a liquid film covering the surface of the polysulfone base film, remove the water phase solution on the surface after standing for 60s; then pour the organic phase solution on the film surface, and ensure that the solution completely immerses the film surface and forms a liquid film covering the surface of the polysulfone base film, remove the organic phase solution on the surface after standing for 30s, to obtain a composite membrane;

[0087] S5, thermal crosslinking: place the composite membrane in an oven with a temperature of 30±0.5℃ for 8min of thermal crosslinking treatment to obtain the cNF-0 membrane.

[0088] Example 1

[0089] The example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-1.

[0090] The difference between the preparation method and the comparative example is that the water phase solution in the example contains CNC with a concentration of 0.002%.

[0091] Example 2

[0092] The example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-2.

[0093] The difference between the preparation method and the comparative example is that the aqueous solution in the present example contains CNC at a concentration of 0.004%.

[0094] Example 3

[0095] The present example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-3.

[0096] The difference between the preparation method and the comparative example is that the aqueous solution in the present example contains CNC at a concentration of 0.006%.

[0097] Example 4

[0098] The present example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-4.

[0099] The difference between the preparation method and the comparative example is that the aqueous solution in the present example contains CNC at a concentration of 0.008%.

[0100] Example 5

[0101] The present example provides a nanofiltration membrane and a preparation method thereof, and the obtained nanofiltration membrane is numbered as cNF-5.

[0102] The difference between the preparation method and the comparative example is that the aqueous solution in the present example contains CNC at a concentration of 0.010%.

[0103] The nanofiltration membranes obtained in the examples and the comparative example are characterized by scanning electron microscopy.

[0104] Figure 1 The scanning electron microscope image of the membrane surface, the first row from left to right is cNF-0, cNF-1, cNF-2, and the second row from left to right is cNF-3, cNF-4, cNF5. The surface scanning electron microscope parameter information is as follows: the acceleration voltage is 3.0 kV, the working distance (the distance from the sample to the objective lens) is 7.3±1 mm, the magnification is 2000 times, the detection signal is secondary electrons (SE), the up and down probe works simultaneously (UL) mode, and the scale is 20.0 μm. Figure 2 The scanning electron microscope image of the membrane cross section, the order is the same as Figure 1 . The cross-sectional scanning electron microscope parameter information is as follows: the acceleration voltage is 3.0 kV, the working distance (the distance from the sample to the objective lens) is 8.8±2 mm, the magnification is 50000 times, the detection signal is secondary electrons (SE), the up and down probe works simultaneously (UL) mode, and the scale is 1.0 μm. From Figure 1It can be seen that with the increase of CNC concentration, a large number of and dense nodular protrusions are formed on the surface of the membrane, showing a complex rugged structure. These nodular protrusions increase the roughness of the membrane sheet, and the effective filtration area of the membrane sheet increases, so that more water molecules can pass through the membrane hole at the same time, thereby improving the water flux. Figure 2 is a cross-sectional scanning electron micrograph, wherein Figure 2 It can be seen that with the increase of CNC concentration, the desalination layer thickness is further thinned, which shortens the water molecule transmission path, so that water molecules can pass through faster.

[0105] The water flux and salt rejection rate of the nanofiltration membranes obtained in the examples and comparative examples were tested, and the test results are shown in Table 9.

[0106] Table 9 Water flux and salt rejection rate test of nanofiltration membranes obtained in examples and comparative examples Nanofiltration membrane PIP concentration (%) TMC concentration (%) TEA HCI concentration (%) PVA concentration (%) CNC concentration (%) Water flux (LMH) Salt rejection rate (%) cNF-0 0.4 0.14 0.8 0.08 0 48.00 30.40 cNF-1 0.4 0.14 0.8 0.08 0.002 50.50 33.30 cNF-2 0.4 0.14 0.8 0.08 0.004 52.00 35.10 cNF-3 0.4 0.14 0.8 0.08 0.006 55.20 35.43 cNF-4 0.4 0.14 0.8 0.08 0.008 52.70 33.87 cNF-5 0.4 0.14 0.8 0.08 0.01 52.70 31.07

[0107] The test results were visualized, as shown in Figure 3 In the presence of PVA, the law of the effect of CNC on the water flux and salt rejection rate of the nanofiltration membrane has changed greatly or even reversely. The effect of CNC concentration on the improvement of water flux is no longer a linear relationship as in the case of single action, but presents an inverted U-shaped curve relationship, and when the value is 0.006%, the improvement effect of CNC on water flux reaches the maximum. In the presence of PVA, the addition of CNC has an improvement effect on the salt rejection rate, and the improvement effect also presents an inverted U-shaped curve relationship, and when the value is 0.004%-0.006%, the improvement effect of CNC on the salt rejection rate reaches the maximum.

Claims

1. A high-flux polypiperazine amide nanofiltration membrane, characterized in that, It includes a polysulfone (PSF) base layer and a polypiperazine amide functional layer disposed thereon; The polypiperazine amide functional layer is formed by polymerizing an aqueous precursor and an organic precursor on the surface of a polysulfone (PSF) substrate. The aqueous precursor contains 0.4% to 1.2% piperazine (PIP), 0.2% to 1% triethylamine hydrochloride (TEA·HCl), 0.02% to 0.1% polyvinyl alcohol (PVA), and 0.002% to 0.01% cellulose nanoparticles (CNC) by weight and volume. The organic precursor contains 0.06% to 0.14% trimesoyl chloride (TMC) by weight and volume.

2. A method for preparing a high-flux polypiperazine amide nanofiltration membrane, characterized in that, Includes the following steps: S1. An aqueous solution is poured onto the surface of a polysulfone-based membrane to form a liquid film covering the surface of the polysulfone-based membrane. After standing for 50–70 seconds, the aqueous solution on the surface is removed. The aqueous solution contains 0.4%–1.2% piperazine (PIP), 0.2%–1% triethylamine hydrochloride (TEA·HCl), 0.02%–0.1% polyvinyl alcohol (PVA), and 0.002%–0.01% cellulose nanoparticles (CNC). S2. The organic phase solution is poured onto the surface of the polysulfone-based membrane treated in S1 to form a liquid film covering the surface of the polysulfone-based membrane. After standing for 20 to 40 seconds, the organic phase solution on the surface is removed to obtain a composite membrane. The organic phase solution contains 0.06% to 0.14% trimesoyl chloride (TMC) by mass volume. S3. Perform thermal crosslinking treatment on the composite membrane obtained in S2.

3. The preparation method according to claim 2, characterized in that, The PVA content in the aqueous solution is 0.06% to 0.08% by mass.

4. The preparation method according to claim 2, characterized in that, The TEA·HCl in the aqueous solution has a mass-volume fraction of 0.6% to 1.0%.

5. The preparation method according to claim 2, characterized in that, The PIP in the aqueous solution has a mass-volume fraction of 0.2% to 0.6%.

6. The preparation method according to claim 2, characterized in that, The TMC has a mass-volume fraction of 0.12% to 0.14% in the organic phase solution.

7. The preparation method according to claim 2, characterized in that, The CNC in the aqueous solution has a mass-volume fraction of 0.004% to 0.006%.

8. The preparation method according to claim 2, characterized in that, The thermal crosslinking temperature in S3 is 30±0.5℃, and the duration is 5~10min.

9. The preparation method according to claim 2, characterized in that, It also includes a base film pretreatment step: before step S1, the polysulfone base film is immersed in deionized water, and then the water on the surface of the polysulfone base film is removed.

10. The preparation method according to claim 9, characterized in that, In the base film pretreatment step, the polysulfone base film is flattened and fixed to a 24cm×36cm polytetrafluoroethylene frame using a pressure-sensitive adhesive, and the front and back sides of the polysulfone base film are rapidly dehydrated using an air knife-assisted drying system.

Images