Acid-resistant nanofiltration membrane with polyamide-Schiff base double-network structure and preparation method of acid-resistant nanofiltration membrane

Abstract

The invention discloses a polyamide-Schiff base double-network structure acid-resistant nanofiltration membrane and a preparation method thereof, and relates to the technical field of membrane preparation. The polyamide-Schiff base double-network structure acid-resistant nanofiltration membrane comprises an ultrafiltration base membrane and a functional separation layer formed on the upper surface of the ultrafiltration base membrane, the functional separation layer is formed by carrying out interfacial polymerization reaction on a water phase solution and an oil phase solution and then carrying out heat treatment; the solute of the water-phase solution comprises a polyfunctional group water-phase amine monomer with the concentration of 0.05-10wt%; the solute of the oil phase solution comprises 0.05-5 w / v% of acyl chloride monomer and 0.01-1 wt% of aromatic aldehyde monomer, and the aromatic aldehyde monomer is ortho-substituted aromatic aldehyde. The nanofiltration membrane provided by the invention can realize double improvement of salt interception and flux, especially improves the cation interception rate and flux in an acidic medium, and is suitable for separation and recovery of heavy metals in industrial acidic wastewater.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of membrane preparation, and particularly relates to a polyamide-Schiff base double-network structure acid-resistant nanofiltration membrane and a preparation method thereof. BACKGROUND

[0002] Nanofiltration membrane technology has become a core separation method in the fields of wastewater treatment, biological medicine and resource recovery due to its efficient separation characteristics and low energy consumption advantages. With the development of emerging fields such as battery manufacturing and metal pickling wastewater treatment, it is an urgent need to develop nanofiltration membranes with high acid resistance, stable separation performance and long-term service performance.

[0003] Traditional polyamide nanofiltration membranes are formed by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on a polysulfone or polyethersulfone-based membrane to form a cross-linked network, which has excellent divalent ion rejection characteristics. However, in an extremely acidic environment, such membranes face serious chemical stability problems, leading to a sharp decline in performance. The irreversible hydrolysis of amide bonds (-CO-NH-) under hydrogen ion catalysis not only leads to the disintegration of the membrane structure, but also causes the surface charge to reverse, resulting in a sharp decrease in cation rejection rate. For example, the preparation method of a blended polyamide composite nanofiltration membrane in patent application CN 110449040A includes monomeric piperazine, m-xylylenediamine or m-phenylenediamine in an aqueous solution, which compensates for the possible defects of a single monomer in the membrane formation process by blending two aqueous-phase monomers, and then using an interfacial polymerization method to make the two immiscible aqueous and oil phases react on the membrane surface to generate a membrane layer. The nanofiltration composite membrane made by this method has superior performance and better chlorine resistance than the nanofiltration membrane generated by interfacial polymerization of a single aqueous-phase monomer and an oil-phase monomer, and the membrane formation process is simple. However, it fails to overcome the defects of irreversible hydrolysis of amide bonds under hydrogen ion catalysis, leading to disintegration of the membrane structure, and surface charge reversal causing a sharp decrease in cation rejection rate.

[0004] The positively charged acid-resistant nanofiltration membrane provides an innovative solution to the above challenges through functional material design and surface charge regulation. The core advantage of this type of membrane lies in the synergistic effect of its unique positive charge characteristics and acid-resistant structure. Under low pH conditions, the positively charged functional groups on the membrane surface efficiently reject heavy metal cations through electrostatic repulsion (Donnan effect), while allowing small-sized and highly mobile H⁺ to freely pass through, significantly reducing acid loss; the acid-resistant polymer substrate, acid-resistant separation layer structure or surface modification layer endows the membrane with chemical stability in strong acidic environments, avoiding performance degradation due to hydrolysis. In existing technical methods, there are still some defects in the introduction of positive charges. Among them, the chemical modification method (quaternization, amination) is limited by the reactive sites of the active separation layer, resulting in low surface charge density; physical coating (layer-by-layer self-assembly, cationic polymer coating) is prone to charge layer shedding due to acid environment erosion or hydrodynamic shear, resulting in uneven charge distribution. At the same time, the introduction of charges often accompanies membrane pore blockage or surface densification, resulting in serious loss of water flux. Therefore, it is crucial to develop a nanofiltration membrane that can provide a strong and stable positive charge surface for acid-resistant nanofiltration membranes and improve water flux. SUMMARY

[0005] The purpose of the present application is to overcome the shortcomings of the prior art and provide a polyamide-Schiff base double network structure acid-resistant nanofiltration membrane that can control hydrolysis, enhance the positive charge on the membrane surface, improve the cation rejection rate, and has high positive charge density and high flux.

[0006] Another purpose of the present application is to provide a preparation method of a polyamide-Schiff base double network structure acid-resistant nanofiltration membrane.

[0007] The above-mentioned purposes of the present application are achieved by the following technical solutions: The present application protects a polyamide-Schiff base double network structure acid-resistant nanofiltration membrane, comprising: an ultrafiltration base membrane; and a functional separation layer formed on the upper surface of the ultrafiltration base membrane; The functional separation layer is formed by interfacial polymerization reaction of an aqueous solution and an oil solution and then heat treatment; The solutes of the aqueous solution include multifunctional aqueous amine monomers with a concentration of 0.05-10 wt%; The solutes of the oil solution include acyl chloride monomers 0.05-5 w / v%, aromatic aldehyde monomers 0.01-1 wt%, the aromatic aldehyde monomers are ortho-substituted aromatic aldehydes, and the ortho-substituent is selected from C1-C5 alkyl, Cl, F and Br.

[0008] In some embodiments, the multi-functional aqueous amine monomer is at least one of polyallylamine, polyvinylamine, hyperbranched polyetheramine, hyperbranched polyvinylamine, polyamidoamine, tris(2-aminoethyl)amine, tetraethylenepentamine, pentaethylenehexamine, diethylenetriamine, triethylenetetramine, polypropylenimine, and polyethylenimine; preferably polyethylenimine and / or polypropylenimine.

[0009] In some embodiments, the aqueous solution further comprises an additive at 0.01-10 wt%, the additive comprising camphorsulfonic acid and / or triethylamine. Preferably, the camphorsulfonic acid content is at 1-5 wt%, or preferably the triethylamine content is at 0.5-3 wt%.

[0010] In some embodiments, the acid chloride monomer is at least one of terephthaloyl chloride, isophthaloyl chloride, pyromellitic acid chloride, 3, 3', 5, 5'-biphenyl tetracarboxylic acid chloride, 2, 2', 4, 4'-biphenyl tetracarboxylic acid chloride, tricarbonyl chloride, glutaroyl chloride, adipoyl chloride, sebacoyl chloride, and trimesoyl chloride; preferably trimesoyl chloride.

[0011] In some embodiments, the ortho-substituted aromatic aldehyde is at least one of 2,6-dimethylbenzaldehyde, 2,4,6-triethylbenzaldehyde, 2,6-di-tert-butylbenzaldehyde, 2,6-dichlorobenzaldehyde, 2,4,6-trifluorobenzaldehyde, 2-bromo-6-methylbenzaldehyde, 2,4,6-trimethylbenzaldehyde. Preferably 2,4,6-trimethylbenzaldehyde and / or 2,4,6-triethylbenzaldehyde. By the steric hindrance effect of the ortho-substituted aromatic aldehyde, the controllable hydrolysis of the dynamic Schiff base bond in the acidic environment can be achieved; if a fatty aldehyde (such as glutaraldehyde) is used, there is no steric hindrance, which will lead to complete breakage of the Schiff base bond in the acidic environment.

[0012] Optionally, the oil phase solution solvent is at least one of n-hexane, cyclohexane, n-heptane, toluene, benzene, Isopar E, Isopar H, Isopar L, Isopar M, and Isopar G; preferably Isopar G or n-hexane.

[0013] In some embodiments, the ultrafiltration base membrane is at least one of polyacrylonitrile, polytetrafluoroethylene, polyvinylidene fluoride, polysulfone, and polyethersulfone.

[0014] The present application protects a preparation method of a polyamide-Schiff base double network structure acid-resistant nanofiltration membrane, comprising the following steps: S1, the ultrafiltration base membrane is first immersed in an aqueous solution for treatment, and after surface drying, it is immersed in an organic phase solution for interfacial polymerization reaction, and then taken out to obtain a membrane piece containing a functional separation layer; S2, the membrane obtained in step S1 is subjected to heat treatment to obtain the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane.

[0015] In some embodiments, in step S1, the water phase solution treatment time is 0.5-10 min; the interfacial polymerization reaction time is 0.5-10 min. Preferably, the water phase solution treatment time is 1-3 min; the interfacial polymerization reaction time is 1-5 min.

[0016] In some embodiments, in step S1, the heat treatment temperature is 40-90℃, and the heat treatment time is 1-20 min. Preferably, the heat treatment temperature is 50-70℃, and the heat treatment time is 5-10 min.

[0017] Compared with the prior art, the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane has the following beneficial effects: The polyamide-Schiff base double network structure acid-resistant nanofiltration membrane is prepared by reacting a multi-functional water phase amine monomer with an acyl chloride monomer and an aromatic aldehyde monomer to construct a polyamide-Schiff base double network structure. The dynamic Schiff base bond undergoes controllable partial hydrolysis under extreme acidic conditions, and the generated -NH2 is converted into -NH3+ in an acidic environment, thereby strengthening the positive charge layer on the membrane surface and improving the retention of cations. Meanwhile, the partial hydrolysis of the network produces nanoscale pores, which can improve the water flux while keeping the retention rate stable. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is the SEM image of the surface of the acid-resistant nanofiltration membrane prepared in Example 1 of the present application.

[0019] Figure 2 is the surface zeta potential test result graph of the acid-resistant nanofiltration membranes of Example 1 and Comparative Example 1 of the present application.

[0020] Figure 3 is the result graph of the dynamic long-time acid resistance test of the acid-resistant nanofiltration membrane of Example 1 of the present application in a magnesium sulfate feed solution containing 15 wt% nitric acid for 14 days. DETAILED DESCRIPTION

[0021] The present application will be further described below in conjunction with the specific embodiments, but the embodiments do not limit the present application in any form. EMBODIMENT

[0022] A polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises a polyether sulfone ultrafiltration base membrane and a functional separation layer formed on the upper surface of the ultrafiltration base membrane.

[0023] The functional separation layer is formed by interfacial polymerization of an aqueous phase solution containing an amine monomer and an oil phase solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous phase amine monomer in the aqueous phase solution is polyethyleneimine, the acid chloride monomer in the oil phase solution is trimesoyl chloride, and the ortho-substituted aromatic aldehyde monomer is 2,4,6-trimethylbenzaldehyde.

[0024] The preparation method of the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises the following steps: 1) 3 wt% camphorsulfonic acid, 1.5 wt% triethylamine were added to water, mixed, then 2 wt% polyethyleneimine was added and dissolved to obtain an aqueous phase solution; 2) 0.1 w / v% trimesoyl chloride was dissolved in n-hexane, then 0.05 wt% 2,4,6-trimethylbenzaldehyde was added and dissolved to obtain an oil phase solution; 3) The polyether sulfone ultrafiltration base film was immersed in the aqueous phase solution of step 1), and after standing for 2 min, the aqueous phase solution was poured out, and the excess solution on the surface of the base film was removed by air knife or rubber roller; 4) The membrane piece after step 3) was immersed in the oil phase solution of step 2), and after standing for 1 min, the oil phase solution was poured out, and the membrane surface was washed with n-hexane; 5) The membrane piece after step 4) was placed in a 60°C oven for heating for 5 min to obtain an acid-resistant nanofiltration membrane; the SEM characterization is shown in Figure 1 . Embodiment

[0025] A polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises a polysulfone ultrafiltration base film and a functional separation layer formed on the surface of the ultrafiltration base film.

[0026] The functional separation layer is formed by interfacial polymerization of an aqueous phase solution containing an amine monomer and an oil phase solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous phase amine monomer in the aqueous phase solution is polyethyleneimine, the acid chloride monomer in the oil phase solution is trimesoyl chloride, and the ortho-substituted aromatic aldehyde monomer is 2,4,6-trimethylbenzaldehyde.

[0027] The preparation method of the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises the following steps: 1) 2 wt% camphorsulfonic acid, 1 wt% triethylamine were added to water, mixed, then 1 wt% polypropyleneimine was added and dissolved to obtain an aqueous phase solution; 2) 0.5 w / v% phthaloyl chloride was dissolved in Isopar G, then 0.1 wt% 2,4,6-triethylbenzaldehyde was added and dissolved to obtain an oil phase solution; 3) immerse the polysulfone ultrafiltration substrate into the aqueous solution of step 1), and after 2 min, pour out the aqueous solution, and remove the excess solution on the surface of the substrate membrane with a wind knife or rubber roller; 4) immerse the membrane piece after step 3) into the oil solution of step 2), and after 1 min, pour out the oil solution, and rinse the membrane surface with Isopar G; 5) put the membrane piece after step 4) into a 70℃ oven for heating for 10 min, to obtain an acid-resistant nanofiltration membrane. Embodiment

[0028] An acid-resistant nanofiltration membrane with a polyamide-Schiff base double network structure comprises a polyether sulfone ultrafiltration substrate and a functional separation layer formed on the upper surface of the ultrafiltration substrate.

[0029] The functional separation layer is formed by interfacial polymerization of an aqueous solution containing an aqueous phase amine monomer and an oil solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous phase amine monomer in the aqueous solution is polyallylamine, the acid chloride monomer in the oil solution is terephthaloyl chloride, and the ortho-substituted aromatic aldehyde monomer is 2,6-dimethylbenzaldehyde.

[0030] The preparation method of the acid-resistant nanofiltration membrane with a polyamide-Schiff base double network structure comprises the following steps: 1) add 5.5 wt% camphor sulfonic acid and 3.5 wt% triethylamine to water, mix well, then add 5 wt% polyallylamine, and fully dissolve to obtain an aqueous solution; 2) dissolve 3 w / v% terephthaloyl chloride in Isopar G, then add 0.75 wt% 2,6-dimethylbenzaldehyde, and fully dissolve to obtain an oil solution; 3) immerse the polyether sulfone ultrafiltration substrate into the aqueous solution of step 1), and after 2 min, pour out the aqueous solution, and remove the excess solution on the surface of the substrate membrane with a wind knife or rubber roller; 4) immerse the membrane piece after step 3) into the oil solution of step 2), and after 1 min, pour out the oil solution, and rinse the membrane surface with Isopar G; 5) put the membrane piece after step 4) into a 40℃ oven for heating for 15 min, to obtain an acid-resistant nanofiltration membrane. Embodiment

[0031] An acid-resistant nanofiltration membrane with a polyamide-Schiff base double network structure comprises a polyether sulfone ultrafiltration substrate and a functional separation layer formed on the upper surface of the ultrafiltration substrate.

[0032] The functional separation layer is formed by interfacial polymerization of an aqueous phase solution containing an amine monomer and an oil phase solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous phase amine monomer in the aqueous phase solution is polyethylene amine, the acid chloride monomer in the oil phase solution is 2, 2', 4, 4'-biphenyl tetracarboxylic acid chloride, and the ortho-substituted aromatic aldehyde monomer is 2, 6-di-tert-butyl benzaldehyde.

[0033] The preparation method of the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises the following steps: 1) 6 wt% camphor sulfonic acid, 4 wt% triethylamine are added to water, mixed, then 7.5 wt% polyethylene amine is added, and fully dissolved to obtain an aqueous phase solution; 2) 3.5 w / v% 2, 2', 4, 4'-biphenyl tetracarboxylic acid chloride is dissolved in n-hexane, then 0.75 wt% 2, 6-di-tert-butyl benzaldehyde is added, and fully dissolved to obtain an oil phase solution; 3) The polyether sulfone ultrafiltration base film is immersed in the aqueous phase solution of step 1), and after standing for 2 min, the aqueous phase solution is poured out, and the excess solution on the surface of the base film is removed by air knife or rubber roller; 4) The membrane piece after step 3) is immersed in the oil phase solution of step 2), and after standing for 1 min, the oil phase solution is poured out, and the membrane surface is washed with n-hexane; 5) The membrane piece after step 4) is placed in an 80℃ oven for heating for 3 min to obtain an acid-resistant nanofiltration membrane. Embodiment

[0034] A polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises a polyether sulfone ultrafiltration base film and a functional separation layer formed on the surface of the ultrafiltration base film.

[0035] The functional separation layer is formed by interfacial polymerization of an aqueous phase solution containing an amine monomer and an oil phase solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous phase amine monomer in the aqueous phase solution is tetraethylene pentamine, the acid chloride monomer in the oil phase solution is 3, 3', 5, 5'-biphenyl tetracarboxylic acid chloride, and the ortho-substituted aromatic aldehyde monomer is 2, 4, 6-trifluorobenzaldehyde.

[0036] The preparation method of the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane comprises the following steps: 1) 7.5 wt% camphor sulfonic acid, 5 wt% triethylamine are added to water, mixed, then 10 wt% tetraethylene pentamine is added, and fully dissolved to obtain an aqueous phase solution; 2) 5 w / v% 3, 3', 5, 5'-biphenyl tetracarboxylic acid chloride is dissolved in n-hexane, then 1 wt% 2, 4, 6-trifluorobenzaldehyde is added, and fully dissolved to obtain an oil phase solution; 3) immerse the polyethersulfone ultrafiltration substrate into the aqueous solution of step 1), and after 2 min, pour out the aqueous solution, and remove the excess solution on the surface of the substrate membrane with a wind knife or rubber roller; 4) immerse the membrane piece after step 3) into the oil solution of step 2), and after 1 min, pour out the oil solution, and flush the membrane surface with n-hexane; 5) place the membrane piece after step 4) into a 40℃ oven for heating for 20 min to obtain an acid-resistant nanofiltration membrane. Embodiment

[0037] An acid-resistant nanofiltration membrane with a polyamide-Schiff base double network structure comprises a polyethersulfone ultrafiltration substrate and a functional separation layer formed on the surface of the ultrafiltration substrate.

[0038] The functional separation layer is formed by interfacial polymerization of an aqueous solution containing an aqueous amine monomer and an oil solution containing an acid chloride monomer and an ortho-substituted aromatic aldehyde monomer, and then heat treatment; the aqueous amine monomer in the aqueous solution is polyamide amine, the acid chloride monomer in the oil solution is glutaroyl chloride, and the ortho-substituted aromatic aldehyde monomer is 2,6-dichlorobenzaldehyde.

[0039] The preparation method of the acid-resistant nanofiltration membrane with a polyamide-Schiff base double network structure comprises the following steps: 1) add 0.5 wt% camphor sulfonic acid and 0.25 wt% triethylamine to water, mix well, then add 0.25 wt% polyamide amine, and fully dissolve to obtain an aqueous solution; 2) dissolve 0.05 w / v% glutaroyl chloride in n-hexane, then add 0.025 wt% 2,6-dichlorobenzaldehyde, and fully dissolve to obtain an oil solution; 3) immerse the polyethersulfone ultrafiltration substrate into the aqueous solution of step 1), and after 2 min, pour out the aqueous solution, and remove the excess solution on the surface of the substrate membrane with a wind knife or rubber roller; 4) immerse the membrane piece after step 3) into the oil solution of step 2), and after 1 min, pour out the oil solution, and flush the membrane surface with n-hexane; 5) place the membrane piece after step 4) into a 90℃ oven for heating for 8 min to obtain an acid-resistant nanofiltration membrane.

[0040] An acid-resistant nanofiltration membrane, which is different from the acid-resistant nanofiltration membrane of Embodiment 1 in that the acid-resistant nanofiltration membrane of the present comparative example does not contain an ortho-substituted aromatic aldehyde monomer.

[0041] Specifically, the preparation method of the acid-resistant nanofiltration membrane of the present comparative example comprises the following steps: 1) add 3 wt% camphor sulfonic acid and 1.5 wt% triethylamine to water, mix well, then add 2 wt% polyethyleneimine, and fully dissolve to obtain an aqueous solution; 2) 0.1 w / v% trimesoyl chloride was added into n-hexane to obtain an oil phase solution; 3) The polyethersulfone ultrafiltration substrate was immersed in the aqueous phase solution of step 1), and after standing for 2 min, the aqueous phase solution was poured out, and the excess solution on the surface of the base membrane was removed by a wind knife or a rubber roller; 4) The membrane piece after step 3) was immersed in the oil phase solution of step 2), and after standing for 1 min, the oil phase solution was poured out, and the membrane surface was washed with n-hexane; 5) The membrane piece after step 4) was placed in a 60°C oven for heating for 5 min to obtain an acid-resistant nanofiltration membrane.

[0042] An acid-resistant nanofiltration membrane, which is different from example 2 in that the acid-resistant nanofiltration membrane of the present comparative example does not contain a para-substituted aromatic aldehyde monomer.

[0043] Specifically, the preparation method of the acid-resistant nanofiltration membrane of the present comparative example comprises the following steps: 1) 2 wt% camphorsulfonic acid, 1 wt% triethylamine were added into water, and after mixing, 1 wt% polypropylene imine was added to obtain an aqueous phase solution; 2) 0.5 w / v% phthaloyl chloride was added into Isopar G to obtain an oil phase solution; 3) The polysulfone ultrafiltration substrate was immersed in the aqueous phase solution of step 1), and after standing for 2 min, the aqueous phase solution was poured out, and the excess solution on the surface of the base membrane was removed by a wind knife or a rubber roller; 4) The membrane piece after step 3) was immersed in the oil phase solution of step 2), and after standing for 1 min, the oil phase solution was poured out, and the membrane surface was washed with Isopar G; 5) The membrane piece after step 4) was placed in a 70°C oven for heating for 10 min to obtain an acid-resistant nanofiltration membrane.

[0044] An acid-resistant nanofiltration membrane, which is different from example 1 in that the para-substituted aromatic aldehyde monomer of the present comparative example is glutaraldehyde.

[0045] Specifically, the preparation method of the acid-resistant nanofiltration membrane of the present comparative example comprises the following steps: 1) 3 wt% camphorsulfonic acid, 1.5 wt% triethylamine were added into water, and after mixing, 2 wt% polyethylene imine was added to obtain an aqueous phase solution; 2) 0.1 w / v% trimesoyl chloride was added into n-hexane to obtain an oil phase solution; 3) The polyethersulfone ultrafiltration substrate was immersed in the aqueous phase solution of step 1), and after standing for 2 min, the aqueous phase solution was poured out, and the excess solution on the surface of the base membrane was removed by a wind knife or a rubber roller; 4) The membrane piece after step 3) was immersed into the oil phase solution of step 2), and after 1 min, the oil phase solution was poured off, and the membrane surface was rinsed with n-hexane; 5) The membrane piece after step 4) was placed into a 60 °C oven for 5 min to obtain the acid-resistant nanofiltration membrane.

[0046] The following separation performance tests were performed on each of the examples and comparative examples, and the results are shown in Table 1.

[0047] 1. Zeta potential test The surface zeta potential test results of the acid-resistant nanofiltration membranes of Example 1 and Comparative Example 1 are shown in Table 1, and the results show that the acid-resistant nanofiltration membrane of Example 1 has a higher surface zeta potential in an acidic (pH 2-5.5) environment, which strengthens the positive charge layer on the membrane surface, and is beneficial to improve the rejection of cations. Figure 2

[0048] 2. Water flux test of separation membrane: Water flux is the volume (V, L) of solution that passes through a unit membrane area (A, m 2 ) per unit time (t, h) under a certain operating pressure (P, bar); the separation membrane is placed in a cross-flow filtration device and operated under a certain pressure, and the water flow rate per unit time is recorded; finally, the water flux is calculated according to the following formula: P = V / (A·t·ΔP).

[0049] 3. Rejection performance test of separation membrane: Rejection rate is the ability of the membrane to prevent the passage of components in the feed liquid or to reject a certain component; the rejection rate is tested by measuring the ratio of the concentration difference (C P ) of the solute in the feed liquid and the permeate liquid to the solute concentration (C F ) in the feed liquid during the membrane filtration process, and the calculation formula is as follows: R = (1 - C P / C F ) x 100%.

[0050] Test method: After the obtained composite nanofiltration membrane was immersed in water for 24 h, a cross-flow test device was used, the cross-flow flow rate was 60 L / h, the pressure was 1.5 MPa, the temperature was 25 °C, 1000 ppm MgSO4 feed liquid (pH: ~6.5) was used as the feed liquid, and the water flux and MgSO4 rejection rate of the composite membrane were measured. The concentration of magnesium sulfate was determined by conductivity meter (FE38-Standard, Mettler Toledo).

[0051] 4. Dynamic long-time test for 14 days ​Test method: The membrane was subjected to a dynamic long-time test for 14 days under the conditions of a pressure of 1.5 MPa and a temperature of 25℃ using a 1000 ppm MgSO4 solution containing 15 wt% HNO3, and the water flux and magnesium ion rejection rate of the composite nanofiltration membrane were recorded, and the results are shown in Table 1. The concentration of magnesium ions was determined by inductively coupled plasma-mass spectrometry (ICP-MS). Figure 3 The results show that the composite nanofiltration membranes prepared in Examples 1-6 and Comparative Examples 1-2 have similar divalent cation rejection rates and water fluxes in a near-neutral salt solution, which is mainly because the introduction of the Schiff base network structure does not affect the cross-linking density of the amide network.

[0052] In a dynamic acidic environment, the composite nanofiltration membranes of the examples and the comparative examples show significant differences. Compared with the comparative examples, the examples 1-6 have excellent MgSO4 salt rejection rate, and the water flux is significantly improved, which is mainly because the polyamide-Schiff base double network structure is partially hydrolyzed in an acidic environment, generating a surface positive charge, so that the membrane has a strong electrostatic repulsion effect. At the same time, the hydrolysis produces an additional water molecule transport channel, and the mass transfer resistance decreases, resulting in a significant improvement in water flux. Comparative Example 3 shows that the use of aliphatic aldehyde without steric hindrance does not have a controllable hydrolysis effect, and the polyamide network structure is severely damaged under acidic conditions, resulting in a loss of rejection performance. The ortho-substituted aromatic aldehyde has a unique controllable hydrolysis effect. The dynamic test results show that the acid-resistant nanofiltration membranes prepared in each example have excellent acid-resistant stability and divalent ion rejection rate, and are very suitable for the separation and recovery of heavy metals in industrial acid wastewater.

[0053] The above examples of the present application are merely examples for clearly illustrating the present application, and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those of ordinary skill in the art. Here, it is not necessary or possible to exhaust all embodiments. Any modification, equivalent replacement and improvement made within the spirit and principles of the present application shall be included in the protection scope of the claims of the present application.​

Claims

1. A polyamide-Schiff base double network structure acid-resistant nanofiltration membrane, characterized in that, The application relates to a functional separation layer formed on the surface of an ultrafiltration base film. The functional separation layer is formed by interfacial polymerization of an aqueous solution and an organic solution and then heat treatment. The solute of the aqueous solution comprises a multifunctional aqueous amine monomer with a concentration of 0.05-10 wt%. The solute of the organic solution comprises an acyl chloride monomer 0.05-5 w / v%, and an aromatic aldehyde monomer 0.01-1 wt%. The multifunctional aqueous amine monomer is at least one of polyallylamine, polyvinylamine, hyperbranched polyether amine, hyperbranched polyvinylamine, polyamidoamine, tris(2-aminoethyl)amine, tetraethylenepentamine, pentaethylenehexamine, diethylenetriamine, triethylenetetramine, polypropylenimine and polyvinyl imine. The aqueous solution further comprises an additive 0.01-10 wt%, and the additive comprises camphor sulfonic acid and / or triethylamine.

2. The polyamide-Schiff base double network structure acid-resistant nanofiltration membrane according to claim 1, characterized in that, The acyl chloride monomer is at least one of terephthaloyl chloride, phthaloyl chloride, pyromellitic acid chloride, 3,3',5,5'-diphenyl tetracarboxylic acid chloride, 2,2',4,4'-diphenyl tetracarboxylic acid chloride, tricarbonyl chloride, glutaroyl chloride, adipoyl chloride, sebacoyl chloride and trimesoyl chloride.

3. The polyamide-schiff base double network structure acid-resistant nanofiltration membrane according to claim 1, characterized in that, The ortho-substituted aromatic aldehyde is at least one of 2,6-dimethylbenzaldehyde, 2,4,6-triethylbenzaldehyde, 2,6-di-tert-butylbenzaldehyde, 2,6-dichlorobenzaldehyde, 2,4,6-trifluorobenzaldehyde, 2-bromo-6-methylbenzaldehyde and 2,4,6-trimethylbenzaldehyde.

4. The polyamide-Schiff base double network structure acid-resistant nanofiltration membrane according to claim 1, characterized in that, The ultrafiltration base film is at least one of polyacrylonitrile, polytetrafluoroethylene, polyvinylidene fluoride, polysulfone and polyethersulfone.

5. The polyamide-Schiff base double network structure acid-resistant nanofiltration membrane according to claim 1, characterized in that, The application further relates to a preparation method of the functional separation layer.

6. The polyamide-Schiff base double network structure acid-resistant nanofiltration membrane according to claim 1, characterized in that, S1, the ultrafiltration base film is first immersed in the aqueous solution for treatment, and then immersed in the organic solution for interfacial polymerization after surface drying, and a film piece with the functional separation layer is obtained; 7. A method for preparing the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane according to any one of claims 1-6, characterized in that, S2, the film piece obtained in step S1 is subjected to heat treatment, and the polyamide-Schiff base double network structure acid-resistant nanofiltration membrane is obtained. In step S1, the treatment time of the aqueous solution is 0.5-10 min, and the interfacial polymerization time is 0.5-10 min. In step S1, the heat treatment temperature is 40-90 DEG C, and the heat treatment time is 1-20 min.

8. The method for preparing polyamide-schiff base double network structure acid-resistant nanofiltration membrane according to claim 7, characterized in that, ​ 9. The method for preparing the polyamide-Schiff base dual-network structure acid-resistant nanofiltration membrane according to claim 7, characterized in that, ​

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