Preparation method of pH-step-by-step thermocuring regulated sodium-magnesium separation nanofiltration membrane

Abstract

The invention relates to the technical field of membrane separation, in particular to a preparation method of a pH-step-by-step thermocuring regulation sodium-magnesium separation nanofiltration membrane. The method comprises the following steps: (1) pre-treating a base membrane; (2) preparation and treatment of an aqueous phase solution: S1, preparing the aqueous phase solution according to mass fraction: sequentially adding 1.2%-1.6% of piperazine, 0.10%-0.14% of triaminopyrimidine and 0.14%-0.18% of polyethylene glycol into deionized water, and stirring until the piperazine, the triaminopyrimidine and the polyethylene glycol are completely dissolved to form the aqueous phase solution; s2, adjusting the pH value of the water-phase solution to 10-11 by using hydrochloric acid; s3, pouring the water-phase solution of which the pH is adjusted on the surface of the pretreated base membrane, standing, and blow-drying to form a uniform water-phase layer; (3) interfacial polymerization reaction: S1, preparing an oil phase solution: adding 0.10-0.14% of trimesoyl chloride into n-hexane, and performing ultrasonic treatment until the trimesoyl chloride is completely dissolved; s2, pouring the oil-phase solution on the surface of the base membrane subjected to water-phase treatment, and after reaction, pouring out redundant solution to form a polyamide separation layer, so as to obtain a membrane sheet; (4) performing a step-by-step thermocuring process; and (5) cleaning and storing.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and in particular to a method for preparing a pH-step-by-step thermal curing controlled sodium-magnesium separation nanofiltration membrane. Background Art

[0002] In the field of membrane separation technology, nanofiltration membranes' efficient separation of sodium and magnesium ions has important application value in industrial wastewater treatment. Excessively high magnesium ion concentrations during nanofiltration membrane operation can cause membrane scaling, requiring the concentration to be reduced to below 0.5 mg / L. During lithium extraction from salt lakes, a high magnesium-to-lithium ratio can severely hinder lithium purification, necessitating deep retention of magnesium ions and efficient permeation of monovalent salt ions. Nanofiltration membranes, with their nanoscale pore size screening and surface charge effects, have become a key technology for separating monovalent and polyvalent ions, but conventional nanofiltration membranes still suffer from low selectivity.

[0003] The separation performance of conventional nanofiltration membranes is dominated by pore size. It is difficult to break through the sodium-magnesium separation limit by relying solely on pore size screening: when the molecular weight cutoff is 300Da, the retention rate of magnesium sulfate can reach more than 95%, and the retention rate of sodium chloride is 10%-30%, and the separation ratio is less than 10; when the molecular weight cutoff is 200Da, the retention rate of magnesium sulfate is increased to 98%, but the retention rate of sodium chloride increases to 40%-80%, and the separation ratio further decreases; when the molecular weight cutoff is 100Da, although the retention rate of magnesium sulfate is close to 99%, the retention rate of sodium chloride is as high as 90%, almost losing selectivity. More importantly, the pore size distribution of conventional membranes is wide, resulting in the actual separation ratio often being lower than the theoretical value, which is difficult to meet the needs of high-purity separation. Therefore, efficient separation of sodium and magnesium ions cannot be achieved by pore size screening alone. It is necessary to combine precise surface charge regulation and utilize divalent ions (Mg 2+ ) and monovalent ions (Na + ) enhances selectivity.

[0004] In the existing technology, methods for improving the selectivity of monovalent and polyvalent ions mainly include adjusting the concentration of aqueous / oil phase monomers and introducing functional monomers. However, the separation ratio is still difficult to exceed 40, and there are problems such as decreased membrane flux and insufficient chemical stability. For example, simply increasing the concentration of amine monomers can increase the charge density, but it will cause the membrane layer to be too thick and increase the mass transfer resistance, which in turn reduces the selectivity; and the modification method of introducing strongly charged groups may destroy the pore structure of the membrane and affect the stability of the separation performance. The following patents and the present invention both use interfacial polymerization to prepare polypiperazineamide nanofiltration membranes and improve the selectivity of the membrane by introducing additives. Their core objectives and technical routes are significantly different.

[0005] Patent CN115105973B discloses a method for preparing a magnesium-lithium separation membrane. By interfacial polymerization, branched polyethyleneimine (PEI) and amino ionic liquid are introduced into the system for modification to construct a positively charged polyamide membrane, aiming to improve the Mg2+ The patent involves the separation of divalent and monovalent ions, but its core goal is the separation of magnesium and lithium. It does not specifically design for the charge characteristics of sodium and magnesium ions. Measured by the sodium and magnesium ion separation standard, its separation ratio is still too low, and the ionic liquid modification requires additional reaction steps, making the process relatively complex. No research has been conducted on the synergistic effects of aqueous phase pH control, thermal curing refinement, pore size control, and surface charge, and it cannot meet the application requirements of efficient sodium and magnesium ion separation.

[0006] Patent CN111097294A discloses a method for preparing a nanofiltration membrane for reclaimed water treatment. Through interfacial polymerization, water-soluble additives are added to the aqueous phase solution, and polyacyl chlorides are used in the oil phase. After the aqueous and oil phases contact and react, the supporting base membrane forms a polyamide functional layer, which aims to intercept organic matter while increasing water flux. However, this patent focuses on organic matter interception and flux improvement in reclaimed water treatment. It does not carry out targeted selection of aqueous phase additives for the needs of sodium and magnesium ion separation, nor does it carry out precise control of the oil phase / aqueous phase reagent ratio. It also does not conduct research on process refinement and coordinated control of pore size and surface charge. Therefore, it cannot be applied to the field of efficient separation of sodium and magnesium ions. Summary of the Invention

[0007] The present invention aims to provide a method for preparing a sodium-magnesium separation nanofiltration membrane with pH-stepwise thermal curing control. To address the above-mentioned drawbacks, the present invention proposes a synergistic strategy of "uniform pore size screening + directional negative charge control". By optimizing the interfacial polymerization process and the additive ratio, a breakthrough improvement in the sodium-magnesium ion separation ratio is achieved. The process is fully compatible with existing production lines. The specific scheme is as follows:

[0008] A method for preparing a sodium-magnesium separation nanofiltration membrane controlled by pH-stepwise thermal curing, comprising the following steps:

[0009] (1) Basement membrane pretreatment;

[0010] (2) Preparation and treatment of aqueous solution:

[0011] S1: Prepare an aqueous phase solution by mass fraction: add 1.2% to 1.6% piperazine, 0.10% to 0.14% triaminopyrimidine, and 0.14% to 0.18% polyethylene glycol to deionized water, stirring until completely dissolved to form an aqueous phase solution; S2: Adjust the pH of the aqueous phase solution to 10-11 with hydrochloric acid;

[0012] S3: Pour the pH-adjusted aqueous solution onto the pretreated basement membrane surface, let it stand, and blow dry to form a uniform aqueous layer;

[0013] Among them, PIP and TAP provide amine reaction sites, and the pyrimidine ring structure of TAP introduces nitrogen-containing heterocycles to generate permanent negative charge centers on the membrane surface, and PEG regulates the hydrophilicity of the membrane pores. TAP and PEG have a synergistic effect. TAP provides a negative charge center and directionally repels Mg through electrostatic interaction. 2 PEG, as a porogen, regulates membrane pore uniformity through steric hindrance, allowing for efficient Na permeation. TAP simultaneously provides polymerization reaction sites for TMC crosslinking and negatively charged central pyrimidine ring nitrogen atoms. Its concentration of 0.10-0.14% can precisely control the membrane surface zeta potential.

[0014] (3) Interfacial polymerization reaction:

[0015] S1: Prepare the oil phase solution: add 0.10% to 0.14% of trimesoyl chloride to n-hexane and sonicate until completely dissolved;

[0016] S2: pouring the oil phase solution onto the surface of the base membrane after the water phase treatment, and after the reaction, pouring out the excess solution to form a polyamide separation layer to obtain a membrane;

[0017] TMC reacts with aqueous amine groups at the interface to generate amide bonds, forming a cross-linked network; the precise matching of TMC concentration and reaction time ensures that the thickness of the polyamide layer is moderate, which ensures the Mg 2+ The screening effect can reduce the water permeation resistance and increase the flux.

[0018] (4) Step-by-step thermal curing process;

[0019] (5) Cleaning and storage.

[0020] The mass concentration of hydrochloric acid in step S2 of step (2) is 10-15%.

[0021] The hydrochloric acid mass concentration in step S2 of step (2) is 12%. The 12% mass concentration of hydrochloric acid adjusts the pH to avoid monomer degradation caused by strong acid and alkali. The weakly alkaline environment partially deprotonates the amino groups (-NH2) of PIP and TAP (generating -NH), which not only ensures the reactivity with TMC but also avoids "rapid polymerization" caused by strong alkalinity (pH>11), which causes excessive reaction and uneven pore size distribution. At the same time, it promotes the exposure of the pyrimidine ring nitrogen atoms of TAP and enhances the surface negative charge density.

[0022] The specific steps of S3 of step (2) are as follows: pour the pH-adjusted aqueous solution onto the surface of the pretreated base membrane, let it stand at room temperature, tilt the frame to pour out the excess solution, blow it dry with nitrogen parallel to the membrane surface, and blow it dry with an air gun to form a uniform aqueous phase layer.

[0023] The reaction temperature in step S2 of step (3) is room temperature.

[0024] The specific steps of step (1) are as follows: fix the polysulfone ultrafiltration membrane to a plexiglass frame, rinse the membrane surface with deionized water several times, and blow dry with an air gun until no visible water marks are left. This step removes impurities on the membrane surface, provides a clean base for subsequent aqueous phase coating, and ensures uniform adsorption of the aqueous phase solution.

[0025] The specific steps of step (4) are as follows:

[0026] S1: Place the membrane in a fume hood and air dry it at room temperature.

[0027] S2: The air-dried film is transferred to an oven for heat curing and then naturally cooled to room temperature.

[0028] Air drying at room temperature slowly removes residual solvent, preventing structural defects caused by stress concentration on the membrane surface. Heat curing at 40°C promotes orderly alignment of polyamide segments, stabilizes pore size distribution, and strengthens the covalent bonding between TAP and the polyamide network, preventing charge group shedding and improving performance stability. This two-step process of "air drying at room temperature for 10 minutes followed by heat curing at 40°C for 10 minutes" avoids both pore shrinkage caused by high temperatures and insufficient cross-linking caused by low temperatures.

[0029] In step S2 of step (4), the oven temperature is 40°C to 50°C.

[0030] The specific steps of step (5) are as follows:

[0031] S1: Rinse the membrane surface with deionized water several times to remove unreacted residual substances;

[0032] S2 Place the treated membrane in deionized water and store it in a sealed container at 4°C.

[0033] The present invention achieves highly selective separation through the synergistic effect of "pore size screening + charge regulation": 1. Pore size screening: The nano-scale pores of the polyamide separation layer 2 The screening effect of TAP is slightly stronger than that of Na; 2. Charge regulation: The nitrogen atom of the pyrimidine ring of TAP is negatively charged under neutral conditions, which is negative for Mg 2 The electrostatic repulsion of (+2 valence) is significantly stronger than that of Na (+1 valence). When the TAP concentration is 0.14%, the negative charge density on the membrane surface increases. Combined with the dense cross-linked structure formed by the TMC concentration of 0.12%, a sodium-magnesium ion separation ratio of 40.26 is achieved. At the same time, the present invention uses a step-by-step thermal curing process "a precise medium-temperature reaction zone of room temperature air drying for 10 minutes and thermal curing at 40°C-50°C for 10 minutes" to enable the membrane to maintain high water flux and high selectivity for monovalent and polyvalent salts.

[0034] The present invention has the following advantages:

[0035] 1. Improve separation selectivity: Solve the problem of conventional membrane separation ratio <10, and make the sodium and magnesium ion separation ratio ≥40 by precisely adjusting the membrane formula (the most preferred solution is PIP1.4%, TAP0.14%, PEG0.18%, TMC0.12%) and process.

[0036] 2. Enhanced charge targeting: Solve the existing technology for Mg 2+ To solve the problem of insufficient specific repulsion, the pyrimidine ring structure of TAP is used to introduce a permanent negative charge to enhance the specific repulsion of divalent Mg. 2+ electrostatic repulsion, while reducing the monovalent Na + interception.

[0037] 3. Optimizing process stability: This addresses performance fluctuations caused by single-temperature heat curing or pH loss. A step-by-step process of "adjusting the pH of the aqueous solution to 10-11 with 12% hydrochloric acid" and "air-drying at room temperature for 10 minutes followed by heat curing at 40°C for 10 minutes" ensures membrane stability. This step-by-step heat curing and pH control ensure a stable membrane structure, maintaining a magnesium sulfate retention rate above 98% and a sodium chloride retention rate around 25%.

[0038] 4. Improve process compatibility: Address the issue of existing technologies requiring additional equipment or steps, and design a process that seamlessly integrates with existing interfacial polymerization production lines, enabling industrial production without the need for additional equipment. DETAILED DESCRIPTION

[0039] Example

[0040] A method for preparing a sodium-magnesium separation nanofiltration membrane controlled by pH-stepwise thermal curing, comprising the following steps:

[0041] 1. Basement membrane pretreatment

[0042] Secure the polysulfone ultrafiltration membrane to a plexiglass frame, rinse the membrane surface three times with deionized water, and air dry it until no traces of water are visible. This step removes impurities from the membrane surface, provides a clean base for subsequent aqueous coating, and ensures uniform adsorption of the aqueous solution.

[0043] 2. Preparation and treatment of aqueous solution

[0044] (1) Prepare an aqueous phase solution by mass fraction: piperazine (PIP, purity ≥99%) 1.2%-1.6%, triaminopyrimidine (TAP, purity ≥98%) 0.10%-0.14%, and polyethylene glycol (PEG, molecular weight 400) 0.14%-0.18% are added to deionized water in sequence and stirred magnetically (300 rpm) for 30 min until completely dissolved to form an aqueous phase solution;

[0045] (2) adjusting the pH of the aqueous phase solution to 10-11 with 12% mass concentration of hydrochloric acid;

[0046] (3) Pour the pH-adjusted aqueous solution onto the pretreated base membrane surface. After standing at room temperature for 3 minutes, tilt the frame to pour out the excess solution and blow dry with 0.1 MPa nitrogen gas parallel to the membrane surface. Use an air gun to blow dry to form a uniform aqueous phase layer.

[0047] Among them, PIP and TAP provide amine reaction sites, and the pyrimidine ring structure of TAP introduces nitrogen-containing heterocycles to generate permanent negative charge centers on the membrane surface, and PEG regulates the hydrophilicity of the membrane pores. TAP and PEG have a synergistic effect. TAP provides a negative charge center and directionally repels Mg through electrostatic interaction. 2 PEG acts as a porogen, regulating the uniformity of membrane pores through steric hindrance effect, allowing Na to pass through efficiently.

[0048] Among them, 12% mass concentration of hydrochloric acid adjusts the pH to avoid monomer degradation caused by strong acid and alkali; the weak alkaline environment partially deprotonates the amino group (-NH2) of PIP and TAP (generating -NH), which not only ensures the reaction activity with TMC, but also avoids the "violent polymerization" caused by strong alkalinity (pH>11), which makes the reaction too fast and the pore size distribution uneven; at the same time, it promotes the exposure of the nitrogen atom of the pyrimidine ring of TAP and enhances the surface negative charge density.

[0049] 3. Interfacial Polymerization

[0050] (1) Prepare the oil phase solution: add 0.10% to 0.14% of trimesoyl chloride (TMC, purity ≥99%) to n-hexane (water content ≤0.01%) and sonicate until completely dissolved;

[0051] (2) Pour the oil phase solution onto the surface of the base film after the water phase treatment, react at room temperature (25±1°C) for 1 minute, and then tilt the frame to pour out the excess solution to form a polyamide separation layer.

[0052] TMC reacts with aqueous amine groups at the interface to generate amide bonds, forming a cross-linked network; the precise matching of TMC concentration and reaction time ensures that the thickness of the polyamide layer is moderate, which ensures the Mg 2 The screening effect can reduce the water permeation resistance and increase the flux.

[0053] 4. Step-by-step thermal curing process

[0054] (1) Place the membrane in a fume hood and air dry it at room temperature for 10 minutes to allow the residual n-hexane to evaporate slowly, avoiding rapid shrinkage of the membrane pores caused by rapid evaporation, which may lead to pore collapse and cracks.

[0055] (2) Transfer the film to a 40℃~50℃ oven for heat curing for 10 minutes, and then cool it naturally to room temperature.

[0056] Among them, air drying at room temperature can slowly remove residual solvents and avoid structural defects caused by stress concentration on the membrane surface; low-temperature thermal curing at 40°C promotes the orderly arrangement of polyamide chain segments, fixes the pore size distribution, and at the same time enhances the covalent bonding between TAP and the polyamide network, avoids the shedding of charged groups, and improves performance stability.

[0057] 5. Cleaning and storage

[0058] (1) Rinse the membrane surface three times with deionized water to remove unreacted TMC, PIP and other residual substances;

[0059] (2) Place the membrane in deionized water and store it in a sealed container at 4°C to avoid microbial contamination and membrane pore swelling.

[0060] The present invention achieves highly selective separation through the synergistic effect of "pore size screening + charge regulation": 1. Pore size screening: The nano-scale pores of the polyamide separation layer 2 The screening effect of TAP is slightly stronger than that of Na; 2. Charge regulation: The nitrogen atom of the pyrimidine ring of TAP is negatively charged under neutral conditions, which is negative for Mg 2 The electrostatic repulsion of (+2 valence) is significantly stronger than that of Na (+1 valence). When the TAP concentration is 0.14%, the negative charge density on the membrane surface increases. Combined with the dense cross-linked structure formed by the TMC concentration of 0.12%, a sodium-magnesium ion separation ratio of 40.26 is achieved. At the same time, the present invention uses a step-by-step thermal curing process "a precise medium-temperature reaction zone of room temperature air drying for 10 minutes and thermal curing at 40°C-50°C for 10 minutes" to enable the membrane to maintain high water flux and high selectivity for monovalent and polyvalent salts.

[0061] Experimental analysis:

[0062] 1. Multi-factor orthogonal experiment

[0063] The L25(5) orthogonal experiment with 4 factors (PIP, TAP, PEG, TMC) and 5 levels was designed using IBM SPSS Statistics software to explore the key influencing factors of sodium and magnesium ion separation performance. Three parallel membrane samples were prepared for each experiment. The retention test of 2000 mg / L NaCl and MgSO solution was carried out under a pressure of 0.69 MPa. The retention rate and separation ratio were calculated by the conductivity method. The aqueous phase solution contained PIP (0.2% to 1.8%), TAP (0.02% to 0.18%), PEG (0.02% to 0.18%), and the oil phase was TMC (0.02% to 0.18%). The membrane preparation steps included base membrane pretreatment, aqueous phase coating, interfacial polymerization, thermal curing (65℃ / 10min) and cleaning and storage.

[0064] Table 1 Orthogonal test performance test results

[0065]

[0066]

[0067] Ranking of factors: The range analysis of factors was conducted on the magnesium sulfate retention rate, sodium chloride retention rate and sodium-magnesium ion separation ratio performance. Through the range analysis, TMC concentration had the greatest impact on the magnesium sulfate retention rate and sodium-magnesium ion separation ratio, with ranges of 16.45 and 4.18, respectively. PIP concentration had a significant effect on the sodium chloride retention rate, with a range of 4.89. The optimal factor combination for the sodium-magnesium ion separation ratio was PIP 1.4%, TAP 0.02%, PEG 0.18%, and TMC 0.10%, with a separation ratio as low as 19.72.

[0068] Performance differences: In the 25 experiments, the separation ratio fluctuated from 2.37 to 19.72, with the highest magnesium sulfate retention rate being 96.15% and the lowest being 64.51%, indicating that the formula concentration had a significant effect on the separation performance.

[0069] 2. PIP concentration optimization experiment

[0070] With the optimal PIP concentration of 1.4% in the orthogonal experiment as the center, five concentration groups of 1%, 1.2%, 1.4%, 1.6%, and 1.8% were set, and TAP0.02%, PEG0.18%, and TMC0.1% were fixed.

[0071] Table 2 PIP concentration factor experimental results

[0072]

[0073]

[0074] When the PIP concentration was 1.4%, the magnesium sulfate rejection rate reached 95.95% and the separation ratio was 22.76. When the concentration exceeded 1.4%, the rejection rate decreased and the mass transfer resistance increased due to the excessive thickness of the membrane layer.

[0075] 3. TMC concentration optimization experiment

[0076] The optimized TMC concentration was 0.06% to 0.14%, and PIP 1.4%, TAP 0.02%, and PEG 0.18% were fixed.

[0077] Table 3 TMC concentration factor experimental results

[0078]

[0079] When the TMC concentration was 0.12%, the cross-linking degree of the polyamide layer was enhanced and the pore size was reduced, resulting in a magnesium sulfate retention rate of 97.97% and an optimal separation ratio of sodium and magnesium ions of 38.26.

[0080] 4. TAP concentration optimization experiment

[0081] Since the influence of TAP concentration is relatively small, we also tested TAP concentrations ranging from 0.12% to 0.18%, and fixed PIP 1.4%, TMC 0.12%, and PEG 0.18%.

[0082] Table 4 TAP concentration factor experimental results

[0083]

[0084] When the TAP concentration is 0.14%, the negative charge density on the membrane surface increases 2 The optimal separation ratio was 37.24, but too high TAP concentration would increase the NaCl retention rate, which was not conducive to selectivity.

[0085] 5. Thermal curing process optimization experiment

[0086] Based on the optimal formula of PIP1.4%, TAP0.14%, PEG0.18%, and TMC0.12%, the thermal curing temperatures were air-dried at room temperature without curing, 40℃ for 10 minutes, 65℃ for 10 minutes, 80℃ for 10 minutes, and air-dried at room temperature for 10 minutes + thermal curing at 40℃ for 10 minutes.

[0087] Table 5 Experimental results of thermal curing temperature factors

[0088]

[0089] When using room temperature air drying and medium temperature heat curing, natural air drying at room temperature can slowly remove residual solvents, avoid surface stress concentration caused by rapid volatilization, and reduce cracks or loose areas on the membrane surface; low-temperature heat curing at 40°C can promote the orderly arrangement of polyamide chain segments, accurately fix the pore size distribution, and at the same time enhance the covalent bonding of TAP and the polyamide network, avoiding the shedding of negatively charged groups, so that the membrane structure has both "moderate cross-linking density" and "stable charge distribution", the magnesium sulfate retention rate can reach more than 98%, the sodium chloride retention rate is controlled at about 25%, and the flux can also reach 40LMH.

[0090] 6. Aqueous pH Optimization Experiment

[0091] Based on the optimal formula of PIP 1.4%, TAP 0.14%, PEG 0.18%, TMC 0.12%, the experiments were compared at unadjusted pH, pH 11, pH 10.5, pH 10, and pH 9.5, with the other operating parameters being the same.

[0092] Table 6 pH factor experimental results

[0093]

[0094]

[0095] When the pH is adjusted to 10, the amine groups are partially deprotonated, and the reaction rate with TMC is moderate, avoiding "violent polymerization" and insufficient cross-linking. The polyamide cortex has a moderate degree of cross-linking and a uniform structure. The weakly alkaline environment promotes the hydrolysis of amide bonds to generate more negatively charged groups, enhancing the electrostatic repulsion of Mg2+ and stabilizing the magnesium sulfate retention rate at over 98%. At the same time, the precise control of the pore size distribution not only ensures water flux, but also controls the sodium chloride retention rate within a reasonable range through the sieving effect and charge synergy, achieving a balance between high selectivity and stability.

[0096] 7. Conclusion of collaborative optimization

[0097] The nanofiltration membrane was prepared with the optimal formula of PIP1.4%, TAP0.14%, PEG0.18%, and TMC0.12%. The membrane preparation process controlled the water phase for 3 minutes, pH adjustment for 10, oil phase for 1 minute, heat curing treatment, natural air drying at room temperature for 10 minutes, and then drying in a 40°C oven for 10 minutes. The separation ratio was 40.26, which has certain significance in the fields of industrial water treatment and inorganic salt separation.

[0098] Table 6 High Selectivity Nanofiltration Membrane Experimental Results

[0099]

[0100] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art may still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing a pH-step-by-step thermal curing controlled sodium-magnesium separation nanofiltration membrane, characterized in that: The steps include: (1) Basement membrane pretreatment; (2) Preparation and treatment of aqueous solution: S1: Prepare an aqueous phase solution by mass fraction: add 1.2% to 1.6% piperazine, 0.10% to 0.14% triaminopyrimidine, and 0.14% to 0.18% polyethylene glycol to deionized water in sequence, and stir until completely dissolved to form an aqueous phase solution; S2 adjusts the pH of the aqueous solution to 10-11 with hydrochloric acid; S3: pouring the pH-adjusted aqueous solution onto the pretreated basement membrane surface, allowing it to stand and then drying it to form a uniform aqueous layer; (3) Interfacial polymerization reaction: S1: Prepare the oil phase solution: add 0.10% to 0.14% of trimesoyl chloride to n-hexane and sonicate until completely dissolved; S2: pouring the oil phase solution onto the surface of the base membrane after the water phase treatment, and after the reaction, pouring out the excess solution to form a polyamide separation layer to obtain a membrane; (4) Step-by-step thermal curing process; (5) Cleaning and storage.

2. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The mass concentration of hydrochloric acid in step S2 of step (2) is 10-15%.

3. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 2, characterized in that: The mass concentration of hydrochloric acid in step S2 of step (2) is 12%.

4. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The specific steps of S3 of step (2) are as follows: pouring the pH-adjusted aqueous solution onto the surface of the pretreated base membrane, letting it stand at room temperature, tilting the frame to pour out the excess solution, blowing nitrogen gas parallel to the membrane surface to dry it, and using an air gun to blow dry it to form a uniform aqueous phase layer.

5. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The reaction temperature in step S2 of step (3) is room temperature.

6. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The specific steps of step (1) are as follows: fix the polysulfone ultrafiltration membrane on the organic glass frame, rinse the membrane surface with deionized water several times, and blow dry with an air gun until no visible water marks are left.

7. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The specific steps of step (4) are as follows: S1: Place the membrane in a fume hood and air dry it at room temperature. S2: The air-dried film is transferred to an oven for heat curing and then naturally cooled to room temperature.

8. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 7, characterized in that: In step S2 of step (4), the oven temperature is 40°C to 50°C.

9. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The specific steps of step (5) are as follows: S1: Rinse the membrane surface with deionized water several times to remove unreacted residual substances; S2 Place the treated membrane in deionized water and store it in a sealed container at 4°C.

10. The method for preparing a pH-stepwise thermal curing controlled sodium-magnesium separation nanofiltration membrane according to claim 1, characterized in that: The mass ratio of the piperazine, triaminopyrimidine, polyethylene glycol and trimesoyl chloride is 1.4:0.14:0.18:0.12.

Images