Method for preparing nanofiltration membrane with high separation selectivity by utilizing proper pre-oxidation and surface secondary cross-linking of permanganate

Abstract

The invention relates to a method for preparing a nanofiltration membrane with high separation selectivity by utilizing proper pre-oxidation and surface secondary cross-linking of permanganate, belonging to the technical field of nanofiltration membranes. The invention aims to solve the problems that the existing polyamide nanofiltration membrane has a'permeation-selectivity 'restriction relationship, that is, the water flux and the desalination rate are difficult to synergistically improve, and the surface is easy to pollute, so that the long-term operation stability is poor. The method comprises the following steps: 1, preparing an alumina tubular ceramic membrane substrate; 2, preparation of an amination repair solution; and 3, modifying the membrane. The method is used for preparing the nanofiltration membrane with high separation selectivity by utilizing proper pre-oxidation and surface secondary cross-linking of permanganate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nanofiltration membrane. BACKGROUND

[0002] With the public's increasing emphasis on the quality of life, the safety of drinking water has attracted widespread attention. Especially in the fields of urban water supply system, industrial wastewater reuse and seawater desalination, trace harmful substances such as micro-organic pollutants, heavy metal ions, antibiotics and endocrine disruptors that may be left in the water source pose a potential threat to the ecological environment and human health. Therefore, the development of high-performance membrane separation materials for deep purification has become the core direction of water treatment technology development.

[0003] As the core components of high-efficiency separation technology, reverse osmosis membranes and nanofiltration membranes, especially aromatic polyamide nanofiltration membranes prepared by interfacial polymerization, have become the mainstream of the market due to their excellent solute rejection performance and mature preparation process. However, such polyamide membranes face two inherent bottlenecks in practical applications. First, the "permeation-selectivity" constraint relationship: in order to obtain high desalination rate, a dense active separation layer needs to be constructed, which inevitably increases the mass transfer resistance of water molecules, leading to low water production flux and high operating energy consumption. How to break this constraint and achieve simultaneous improvement of flux and selectivity is the long-term goal of membrane material science. Second, the easy fouling of the membrane surface also seriously limits its long-term running stability and economic benefits. The inherent hydrophobicity and roughness of the polyamide membrane surface make it prone to surface fouling when treating complex water bodies containing organic matter, colloids and microorganisms, resulting in flux decay, frequent cleaning and short service life. Therefore, improving the hydrophilicity, charge characteristics and reducing the roughness of the membrane surface through surface modification to endow it with excellent anti-fouling ability is the key to improving the practicality of the membrane material. SUMMARY

[0004] The present application aims to solve the problems of the existing polyamide nanofiltration membrane, such as the "permeation-selectivity" constraint relationship, i.e. the difficulty in simultaneous improvement of water flux and desalination rate, and the easy fouling of the surface leading to poor long-term running stability, and further provides a method for preparing a nanofiltration membrane with high separation selectivity by using moderate permanganate pre-oxidation and surface secondary cross-linking.

[0005] A method for preparing a nanofiltration membrane with high separation selectivity by using moderate permanganate pre-oxidation and surface secondary cross-linking, which is carried out according to the following steps:

[0006] I. Preparation of an oxidative activation solution:

[0007] Dissolve potassium permanganate in water and adjust the pH value to 8-11 to obtain an oxidative activation solution;

[0008] II. Preparation of an amination repair solution:

[0009] Polyethyleneimine was added to water and stirred until completely dissolved to obtain an amination repair solution;

[0010] III. Membrane Modification Treatment:

[0011] The polyamide nanofiltration membrane is immersed in an oxidative activation solution for 1 to 5 minutes, and then washed to obtain a moderately pre-oxidized nanofiltration membrane. The moderately pre-oxidized nanofiltration membrane is then immersed in an amination repair solution for 1 to 5 minutes, and finally heat-set and washed to obtain a nanofiltration membrane with high separation selectivity. This completes the method for preparing a high separation selectivity nanofiltration membrane using moderate pre-oxidation of permanganate and secondary surface crosslinking.

[0012] The beneficial effects of this invention are:

[0013] This invention uses a polyamide nanofiltration membrane as the substrate and performs a two-step synergistic post-treatment. First, a mild oxidant is used to precisely oxidize and activate the outermost layer of the membrane to achieve surface thinning and functionalization, reducing mass transfer resistance while introducing hydrophilic active groups. Subsequently, a polyamine solution is used to rapidly repair and perform secondary crosslinking on the oxidized surface, targeting and repairing any micro-defects that may arise during the oxidation process and constructing a secondary crosslinking network. This "etch-then-repair" strategy reshapes and optimizes the membrane surface structure at the molecular scale. The resulting high-performance nanofiltration membrane successfully overcomes the "permeability-selectivity" limitation of traditional polyamide membranes, achieving a significant increase in water flux while maintaining or even improving solute rejection. Furthermore, the modified membrane surface exhibits superior hydrophilicity and charge properties, thus demonstrating excellent antifouling performance (e.g., ...). Figure 12 As shown in the figure, this method ensures operational stability and service life under long-term, complex water quality conditions. The membrane prepared by this method is suitable for seawater desalination, high-salinity wastewater resource recovery, advanced municipal wastewater treatment and reuse, and high-end pure water production, possessing broad application prospects and significant economic value. Attached Figure Description

[0014] Figure 1 This is a flowchart illustrating the preparation of a highly selective nanofiltration membrane using moderate pre-oxidation and secondary cross-linking of permanganate in this invention.

[0015] Figure 2 The image shows the filtration effect of the highly selective nanofiltration membrane prepared in Example 10 on actual surface water.

[0016] Figure 3 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 1;

[0017] Figure 4 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 13;

[0018] Figure 5 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 12;

[0019] Figure 6 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 10;

[0020] Figure 7 The Zeta potential of the highly selective nanofiltration membranes prepared in Examples 1 to 13;

[0021] Figure 8 The contact angle of the highly selective nanofiltration membranes prepared in Examples 1 to 13;

[0022] Figure 9 The long-term operational stability of the highly selective nanofiltration membrane prepared in Example 10;

[0023] Figure 10 TEM image of the initial polyamide nanofiltration membrane prepared in Example 1;

[0024] Figure 11 SEM image of the initial polyamide nanofiltration membrane prepared in Example 1;

[0025] Figure 12 Flux recovery diagrams for the initial polyamide nanofiltration membrane prepared in Example 1 and the highly selective separation nanofiltration membrane prepared in Example 10. Detailed Implementation

[0026] Specific implementation method one, combined with Figure 1 Detailed description: This embodiment describes a method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface cross-linking of permanganate, which is carried out according to the following steps:

[0027] I. Preparation of oxidative activation solution:

[0028] Dissolve potassium permanganate in water and adjust the pH to 8-11 to obtain an oxidation-activated solution;

[0029] II. Preparation of Amine Remediation Solution:

[0030] Polyethyleneimine was added to water and stirred until completely dissolved to obtain an amination repair solution;

[0031] III. Membrane Modification Treatment:

[0032] The polyamide nanofiltration membrane is immersed in an oxidative activation solution for 1 to 5 minutes, and then washed to obtain a moderately pre-oxidized nanofiltration membrane. The moderately pre-oxidized nanofiltration membrane is then immersed in an amination repair solution for 1 to 5 minutes, and finally heat-set and washed to obtain a nanofiltration membrane with high separation selectivity. This completes the method for preparing a high separation selectivity nanofiltration membrane using moderate pre-oxidation of permanganate and secondary surface crosslinking.

[0033] This embodiment discloses a method for preparing highly selective nanofiltration membranes using moderate pre-oxidation of permanganate and synergistic regulation of secondary surface crosslinking. The method employs a two-step post-treatment process of "etching first, then repairing" for the active separation layer of the polyamide nanofiltration membrane. Its core idea is to decouple the traditionally coupled flux control and selectivity control in time and space, achieving precise molecular-scale control of the membrane surface network structure. In the first step, a mild oxidant such as permanganate is used to perform controlled oxidative etching on the outermost skin of the polyamide separation layer. This removes or thins the dense skin layer of the original membrane, which contributes little to water mass transfer but significantly reduces resistance, thus shortening the effective water transport path. Simultaneously, hydrophilic functional groups such as carboxyl and hydroxyl groups are introduced onto the polyamide chains, inducing membrane structure reconstruction. This constructs a more hydrophilic microchannel system that is more conducive to water diffusion, maximizing flux potential without significantly altering the overall skeleton crosslinking degree. However, any etching process inevitably leads to the generation of nanoscale defects such as local chain segment relaxation or pore size enlargement. These "oversized channels" will preferentially become short-circuit paths for solute penetration, and if left uncontrolled, they will inevitably impair separation selectivity. Therefore, in this embodiment, a polymeric polyamine is introduced in the second step to rapidly repair and perform secondary cross-linking on the oxidized and activated surface: On the one hand, the polyamine molecules undergo condensation or addition reactions with the active sites such as carboxyl and carbonyl groups introduced by oxidation through their amino groups, forming an ultrathin secondary cross-linking network on the film surface. This selectively passivates and blocks the nanoscale defects and local macropores generated during the etching process, thus strictly controlling the upper limit of pore size. On the other hand, the polyamine accumulates on the surface and forms a stable negative charge layer, significantly increasing the surface charge density and negative charge of the film, enhancing the Donnan repulsion effect on divalent cations, and further improving salt retention performance from the charge level. By precisely controlling the concentration and reaction time of the oxidant, as well as the type, concentration, and reaction conditions of the repaired polyamine, the first step is mainly responsible for "thinning + hydrophilization," and the second step is mainly responsible for "defect repair + secondary crosslinking + charge enhancement," thereby achieving stepwise fine adjustment of flux-related and selectivity-related factors. Unlike traditional methods that adjust performance through a single "pore-enlarging / crosslinking-reducing" or "overall densification," this implementation method, through a "first etching, then repair" sequential design, retains the high-flux advantages brought by oxidation thinning and hydrophilization while precisely recovering the structural constraints required for solute retention using an ultrathin and directional repair crosslinking layer. This successfully alleviates and even partially overcomes the "flux-selectivity" constraint commonly found in nanofiltration membranes. The prepared nanofiltration membrane not only exhibits significantly higher water flux than conventional commercial membranes but also maintains or even improves its retention performance for solutes (especially divalent salt ions), and due to the simultaneous enhancement of surface hydrophilicity and negative charge (e.g., Figure 7 and Figure 8 As shown), it further improves the anti-pollution capability and long-term operational stability (e.g. Figure 9As shown in the figure, it has broad industrial application prospects in the fields of seawater desalination, wastewater reuse, and high-salinity wastewater resource utilization.

[0034] The beneficial effects of this embodiment are:

[0035] This embodiment uses a polyamide nanofiltration membrane as the substrate and performs a two-step synergistic post-treatment. First, a mild oxidant is used to precisely oxidize and activate the outermost layer of the membrane to achieve surface thinning and functionalization, reducing mass transfer resistance while introducing hydrophilic active groups. Subsequently, a polyamine solution is used to rapidly repair and perform secondary crosslinking on the oxidized surface, targeting and repairing any micro-defects that may arise during the oxidation process and constructing a secondary crosslinking network. This "etch-then-repair" strategy reshapes and optimizes the membrane surface structure at the molecular scale. The prepared high-performance nanofiltration membrane successfully overcomes the "permeability-selectivity" limitation of traditional polyamide membranes, achieving a significant increase in water flux while maintaining or even improving solute rejection. Simultaneously, the modified membrane surface exhibits superior hydrophilicity and charge properties, thus demonstrating excellent antifouling performance (e.g., ...). Figure 12 As shown in the figure, this method ensures operational stability and service life under long-term, complex water quality conditions. The membrane prepared by this method is suitable for seawater desalination, high-salinity wastewater resource recovery, advanced municipal wastewater treatment and reuse, and high-end pure water production, possessing broad application prospects and significant economic value.

[0036] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 50 ppm to 200 ppm. Everything else is the same as in Specific Implementation Method One.

[0037] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that: in step one, the pH value is adjusted to 8-11 using a 0.1%~1% (w / w) dilute hydrochloric acid or a 0.1%~1% (w / w) sodium hydroxide solution. Everything else is the same as in Specific Implementation Method One or Two.

[0038] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the mass percentage of polyethyleneimine in the amination repair solution described in step two is 0.2% to 2%. Everything else is the same as in Specific Implementation Methods One to Three.

[0039] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the polyamide nanofiltration membrane described in step three is prepared as follows: The polyethersulfone support layer is immersed in a 0.1%~0.5% (w / w) isopropanol solution for 5 to 10 minutes, then washed with deionized water to obtain a pretreated polyethersulfone support layer. The pretreated polyethersulfone support layer is then immersed in a 0.5%~1% (w / w) piperazine aqueous solution for 1 to 10 minutes, then removed and dried to remove excess solution. A 0.1%~0.5% (w / w) hexane solution of trimesoyl chloride is then poured onto the surface of the polyethersulfone support layer for 30 seconds to 2 minutes to form a polyamide separation layer. Finally, the membrane is heat-treated at 50℃~70℃ for 2 to 10 minutes to obtain the polyamide nanofiltration membrane. The rest is the same as in Specific Implementation Methods One to Four.

[0040] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the thickness of the separation layer in the polyamide nanofiltration membrane described in step three is 80nm~200nm. Everything else is the same as in Specific Implementation Methods One to Five.

[0041] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that, in step three, the polyamide nanofiltration membrane is immersed in the oxidation activation solution for 1 to 5 minutes at a temperature of 40℃ to 60℃. Everything else is the same as in Specific Implementation Methods One to Six.

[0042] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: in step three, the moderately pre-oxidized nanofiltration membrane is immersed in an amination remediation solution for 1 to 5 minutes at a temperature of 40℃ to 60℃. Everything else is the same as in Specific Implementation Methods One to Seven.

[0043] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that the heat setting in step three is specifically carried out at a temperature of 60℃~80℃ for 5min~10min. Everything else is the same as Specific Implementation Methods One to Eight.

[0044] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One through Nine in that the water flux of the highly selective nanofiltration membrane prepared in step three is 15 L·m⁻¹. -2 h -1 bar -1 ~27L·m -2 h -1 bar -1The desalination rate for NaCl solution is 41.1%~55.2%, for MgCl2 solution it is 51.4%~65.8%, for MgSO4 solution it is 81.4%~97.5%, for Na2SO4 solution it is 83.5%~98.2%, and the removal rate of dissolved organic matter is 83.0%~95.3%. Other aspects are the same as in specific embodiments one through nine.

[0045] The beneficial effects of the present invention are verified using the following embodiments:

[0046] Example 1:

[0047] A method for preparing highly selective nanofiltration membranes using moderate pre-oxidation and secondary surface crosslinking of permanganate is described, comprising the following steps:

[0048] I. Preparation of oxidative activation solution:

[0049] Dissolve potassium permanganate in water and adjust the pH to 8 to obtain an oxidation-activated solution;

[0050] The mass concentration of potassium permanganate in the oxidation activation solution is 50 ppm;

[0051] II. Preparation of Amine Remediation Solution:

[0052] Polyethyleneimine was added to water and stirred until completely dissolved to obtain an amination repair solution;

[0053] The mass percentage of polyethyleneimine in the amination remediation solution is 1%.

[0054] III. Membrane Modification Treatment:

[0055] At a temperature of 40℃, the polyamide nanofiltration membrane was immersed in an oxidation activation solution for 1 min, and then washed with deionized water to obtain a moderately pre-oxidized nanofiltration membrane. Then, at a temperature of 60℃, the moderately pre-oxidized nanofiltration membrane was immersed in an amination repair solution for 3 min. Finally, at a temperature of 60℃, it was heat-set for 5 min and washed with deionized water to obtain a nanofiltration membrane with high separation selectivity.

[0056] In step one, the pH value is adjusted to 8 using a 0.1% hydrochloric acid solution or a 0.1% sodium hydroxide solution.

[0057] The polyamide nanofiltration membrane described in step three is prepared as follows: the polyethersulfone support layer is immersed in a 0.1% isopropanol solution for 5 minutes, then washed with deionized water to obtain a pretreated polyethersulfone support layer. The pretreated polyethersulfone support layer is then immersed in a 1% piperazine aqueous solution for 3 minutes. After that, the excess solution on the surface is removed and dried. A 0.1% hexane solution of trimesoyl chloride is then poured onto the upper surface of the polyethersulfone support layer for 40 seconds to form a polyamide separation layer. Finally, the membrane is heat-treated at 70°C for 3 minutes to obtain the polyamide nanofiltration membrane.

[0058] The thickness of the polyamide nanofiltration membrane mentioned in step three is 92nm~188nm, such as... Figure 10 and 11 As shown, the surface morphology of the initial polyamide nanofiltration membrane is a typical nodular structure, and the membrane surface morphology is relatively rough due to the rapid self-limiting reaction of interfacial polymerization.

[0059] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 15.17 L·m⁻¹. -2 h -1 bar -1 The rejection rates for NaCl and MgCl2 were 43.45%, MgSO4 56.48%, MgSO4 95.84%, and Na2SO4 97.73%. The TOC rejection rate in natural surface water was 92.93%. The membrane rejection rate was calculated using the following formula:

[0060] ;

[0061] Among them, C p (mg L -1 () represents the concentration of the permeate, C f (mg L -1 () indicates the feed concentration;

[0062] Example 2: This example differs from Example 1 in that the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 100 ppm. Everything else is the same as in Example 1.

[0063] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 16.37 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl 42.72%, MgCl2 55.38%, MgSO4 95.03%, and Na2SO4 97.27%. The TOC retention rate in natural surface water was 92.54%.

[0064] Example 3: This example differs from Example 1 in that the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 200 ppm. Everything else is the same as in Example 1.

[0065] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 18.16 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl 41.72%, MgCl2 51.38%, MgSO4 93.03%, and Na2SO4 91.27%. The TOC retention rate in natural surface water was 92.14%.

[0066] Example 4: This example differs from Example 1 in that the pH value is adjusted to 9 in step one. Everything else is the same as in Example 1.

[0067] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 15.81 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl 44.15%, MgCl2 56.81%, MgSO4 96.54%, and Na2SO4 98.19%. The TOC retention rate in natural surface water was 92.83%.

[0068] Example 5: This example differs from Example 1 in that the pH value is adjusted to 11 in step one. Everything else is the same as in Example 1.

[0069] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 18.63 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl 45.81%, MgCl2 57.83%, MgSO4 96.99%, and Na2SO4 97.02%. The TOC retention rate in natural surface water was 92.77%.

[0070] Example 6: This example differs from Example 1 in that, in step three, the polyamide nanofiltration membrane is immersed in an oxidative activation solution for 3 minutes at a temperature of 50°C. Everything else is the same as in Example 1.

[0071] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 22.93 L·m⁻¹. -2 h -1 bar -1 The rejection rates for NaCl were 51.15%, MgCl2 60.77%, MgSO4 97.12%, Na2SO4 98.0%, and TOC in natural surface water 93.23%.

[0072] Example 7: This example differs from Example 1 in that, in step three, the polyamide nanofiltration membrane is immersed in an oxidative activation solution for 4 minutes at a temperature of 55°C. Everything else is the same as in Example 1.

[0073] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 24.81 L·m⁻¹. -2 h-1 bar -1 The rejection rates for NaCl were 51.01%, MgCl2 60.05%, MgSO4 96.72%, Na2SO4 97.23%, and TOC in natural surface water 93.11%.

[0074] Example 8: This example differs from Example 1 in that, in step three, the polyamide nanofiltration membrane is immersed in an oxidative activation solution for 5 minutes at a temperature of 60°C. Everything else is the same as in Example 1.

[0075] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 26.92 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl retention rate was 42.01%, MgCl2 retention rate was 54.92%, MgSO4 retention rate was 90.42%, Na2SO4 retention rate was 91.72%, and TOC retention rate in natural surface water was 86.12%.

[0076] Example 9: This example differs from Example 6 in that the pH value is adjusted to 9 in step one; and the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 100 ppm. Everything else is the same as in Example 6.

[0077] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 23.27 L·m⁻¹. -2 h -1 bar -1 The rejection rates for NaCl were 52.35%, MgCl2 62.03%, MgSO4 97.04%, Na2SO4 97.71%, and TOC in natural surface water 93.55%.

[0078] Example 10: This example differs from Example 6 in that the pH value is adjusted to 9 in step one; and the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 200 ppm. Everything else is the same as in Example 6.

[0079] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 26.92 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl retention rate was 55.11%, MgCl2 retention rate was 65.72%, MgSO4 retention rate was 97.33%, Na2SO4 retention rate was 98.02%, and TOC retention rate in natural surface water was 94.23%.

[0080] Example 11: This example differs from Example 6 in that the pH value is adjusted to 11 in step one; and the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 200 ppm. Everything else is the same as in Example 6.

[0081] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 26.08 L·m⁻¹. -2 h -1 bar -1 The retention rates were as follows: NaCl retention rate was 51.07%, MgCl2 retention rate was 60.38%, MgSO4 retention rate was 92.09%, Na2SO4 retention rate was 92.73%, and TOC retention rate in natural surface water was 89.63%.

[0082] Example 12: This example differs from Example 8 in that the pH value is adjusted to 11 in step one; and the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 200 ppm. Everything else is the same as in Example 8.

[0083] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 26.97 L·m⁻¹. -2 h -1 bar -1The retention rates were as follows: NaCl retention rate was 43.61%, MgCl2 retention rate was 53.98%, MgSO4 retention rate was 81.38%, Na2SO4 retention rate was 83.47%, and TOC retention rate in natural surface water was 83.01%.

[0084] Example 13: This example differs from Example 7 in that the pH value is adjusted to 9 in step one; and the mass concentration of potassium permanganate in the oxidation activation solution described in step one is 100 ppm. Everything else is the same as in Example 7.

[0085] The high separation selectivity nanofiltration membrane prepared in this embodiment was tested for performance. Membrane filtration experiments were conducted on a mixed aqueous solution containing NaCl (2 g / L), MgCl2 (2 g / L), MgSO4 (2 g / L), and Na2SO4 (2 g / L) under operating conditions of 4 bar and 25 °C. The tested nanofiltration membrane water flux was 25.76 L·m⁻¹. -2 h -1 bar -1 The rejection rates for NaCl were 53.72%, MgCl2 62.76%, MgSO4 95.02%, Na2SO4 96.16%, and TOC in natural surface water 94.51%.

[0086] Table 1 Experimental conditions for Examples 1 to 13

[0087]

[0088] The highly selective nanofiltration membrane prepared in Example 10 was used to separate surface water (TOC: 5.25 mg / L, TDS: 733 mg / L, Ca... 2+ 487 mg / L, Mg 2+ A purification test was conducted using a concentration of 449 mg / L, followed by a single cycle. The specific steps are as follows:

[0089] a. Surface water pretreatment via microfiltration membrane: Commercial microfiltration membranes are used to pre-treat surface water to remove impurities and suspended particles from the raw water, preventing clogging of the nanofiltration membrane. The operating pressure is 0.04 MPa. The microfiltration membrane pore size is 0.45 μm, much larger than that of the nanofiltration membrane, and therefore cannot retain dissolved substances and ions in the water. Thus, it will not affect subsequent calcium absorption. 2+ Mg 2+ The data from the plasma retention experiment had an impact; the exudate obtained through microfiltration was used as the feed water for the nanofiltration membrane system.

[0090] b. Surface water pretreated by microfiltration enters the inlet tank and is subjected to deep filtration using the high separation selectivity nanofiltration membrane prepared in Example 10 under a pressure of 0.4 MPa;

[0091] The effluent from the highly selective nanofiltration membrane was collected, and the TOC and TDS of the sample solution were measured and analyzed. Simultaneously, ion chromatography was used to detect and calculate the Ca content in the raw water and permeate. 2+ Mg 2+ Content. The specific calculation is performed using the following formula:

[0092] ;

[0093] Where R is the removal rate, C p The permeate concentration (mg / L), C f The concentration of the concentrate is expressed in mg / L.

[0094] Figure 2 The figure shows the filtration efficiency of the highly selective nanofiltration membrane prepared in Example 10 on actual surface water. As can be seen from the figure, the nanofiltration membrane achieves a TOC removal efficiency of 94.23% and a TDS removal efficiency of 84.87% for dissolved solids (TDS) in surface water, while also achieving a high TDS removal efficiency for calcium. 2+ The removal efficiency was 75.03% for Mg. 2+ The removal efficiency was 85.31%.

[0095] Figure 3 The image shows a scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 1. As can be seen from the image, the membrane surface morphology changed from large nodular structures to small nodular structures.

[0096] Figure 4 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 13. As can be seen from the figure, the nodular structure on the membrane surface becomes more prominent.

[0097] Figure 5 The image shows a scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 12. As can be seen from the image, the nodular structure on the membrane surface gradually disappears, and a smoother membrane surface morphology is formed.

[0098] Figure 6 Scanning electron microscope image of the highly selective nanofiltration membrane prepared in Example 10. As can be seen from the figure, the nodular structure on the membrane surface becomes smoother.

[0099] Figure 7 The figure shows the Zeta potentials of the high separation selectivity nanofiltration membranes prepared in Examples 1 to 13. As can be seen from the figure, the high separation selectivity nanofiltration membranes prepared by the synergistic regulation of moderate pre-oxidation of permanganate and secondary cross-linking of the surface show a significant enhancement in surface electronegativity. For example, the nanofiltration membrane prepared in Example 10 has an electronegativity of -35.42 mV.

[0100] Figure 8The figures show the contact angles of the highly selective nanofiltration membranes prepared in Examples 1 to 13. As can be seen from the figures, the hydrophilicity of the surface of the highly selective nanofiltration membranes prepared by the synergistic regulation of moderate pre-oxidation of permanganate and secondary cross-linking of the surface is significantly enhanced. For example, the contact angle of the nanofiltration membrane prepared in Example 10 is 20.32°.

[0101] The highly selective nanofiltration membrane prepared in Example 10 was placed in a cross-flow filtration device and operated continuously for 24 hours, with the pure water flux tested every hour. The device was also operated continuously for 24 hours with a sodium sulfate aqueous solution as feed, and the sodium sulfate rejection rate was tested every hour. Figure 9 The long-term operational stability of the highly selective nanofiltration membrane prepared in Example 10 was assessed. As shown in the figure, after 24 hours of continuous operation, the water flux of the nanofiltration membrane decreased by less than 3 L·m⁻¹. -2 h -1 bar -1 The retention rate of Na2SO4 decreased by less than 2%.

[0102] Figure 10 The image shows a TEM image of the initial polyamide nanofiltration membrane prepared in Example 1. As shown in the figure, the thickness distribution of the separation layer in the cross-section of the initial polyamide nanofiltration membrane is uneven, with a minimum thickness of 91.7 nm and a maximum thickness of 187.6 nm.

[0103] Figure 11 The image shows a SEM image of the initial polyamide nanofiltration membrane prepared in Example 1. As shown, the membrane surface exhibits a typical large nodular morphology, with a corresponding increase in roughness.

[0104] The antifouling performance test aimed to evaluate the nanofiltration membrane's resistance to organic pollutants. 200 mg / L humic acid was selected as the model organic pollutant, and the background solution contained 1 mM CaCl2, 16 mM NaCl, and 1 mM NaHCO3. The test followed a typical "filtration-fouling-cleaning" cycle: First, the membrane was pre-pressurized at 0.4 MPa using the background solution for 1-2 hours until the flux stabilized, and this initial flux was recorded. Then, the feed solution was switched to the aforementioned humic acid pollutant solution, and filtration was continuously performed at the same pressure for 24 hours to simulate the fouling process. After fouling, the membrane surface was physically cleaned with deionized water for 30 minutes to remove deposited pollutants. Finally, the membrane flux after cleaning was measured again using the background solution. This test process was repeated twice. Figure 12The graph shows the flux recovery rates of the initial polyamide nanofiltration membrane prepared in Example 1 and the high-selectivity nanofiltration membrane prepared in Example 10. As shown in the figure, the water flux recovery rate of the initial polyamide nanofiltration membrane was 47%, while the flux recovery rate of the high-selectivity nanofiltration membrane prepared in Example 10 after two cycles was 80.3%. This significantly demonstrates that the high-selectivity nanofiltration membrane prepared by the synergistic regulation of moderate pre-oxidation of permanganate and secondary surface crosslinking has excellent antifouling properties.

Claims

1. A method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation of permanganate and secondary surface crosslinking, characterized in that... It is done in the following steps: I. Preparation of oxidative activation solution: Dissolve potassium permanganate in water and adjust the pH to 8-11 to obtain an oxidation-activated solution; II. Preparation of Amine Remediation Solution: Polyethyleneimine was added to water and stirred until completely dissolved to obtain an amination repair solution; III. Membrane Modification Treatment: The polyamide nanofiltration membrane is immersed in an oxidative activation solution for 1 to 5 minutes, and then washed to obtain a moderately pre-oxidized nanofiltration membrane. The moderately pre-oxidized nanofiltration membrane is then immersed in an amination repair solution for 1 to 5 minutes, and finally heat-set and washed to obtain a nanofiltration membrane with high separation selectivity. This completes the method for preparing a high separation selectivity nanofiltration membrane using moderate pre-oxidation of permanganate and secondary surface crosslinking.

2. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... The mass concentration of potassium permanganate in the oxidation activation solution described in step one is 50 ppm to 200 ppm.

3. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... In step one, the pH value is adjusted to 8-11 using 0.1%-1% dilute hydrochloric acid or 0.1%-1% sodium hydroxide solution.

4. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... The mass percentage of polyethyleneimine in the amination repair solution described in step two is 0.2% to 2%.

5. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... The polyamide nanofiltration membrane described in step three is prepared as follows: The polyethersulfone support layer is immersed in a 0.1%~0.5% isopropanol solution for 5 to 10 minutes, then washed with deionized water to obtain a pretreated polyethersulfone support layer. The pretreated polyethersulfone support layer is then immersed in a 0.5%~1% piperazine aqueous solution for 1 to 10 minutes. After removal and drying of excess solution, a 0.1%~0.5% hexane solution of trimesoyl chloride is poured onto the upper surface of the polyethersulfone support layer for 30 seconds to 2 minutes to form a polyamide separation layer. Finally, the membrane is heat-treated at 50℃~70℃ for 2 to 10 minutes to obtain the polyamide nanofiltration membrane.

6. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 5, characterized in that... The thickness of the separation layer in the polyamide nanofiltration membrane described in step three is 80 nm to 200 nm.

7. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... In step three, the polyamide nanofiltration membrane is immersed in an oxidative activation solution for 1 min to 5 min at a temperature of 40℃ to 60℃.

8. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... In step three, at a temperature of 40℃~60℃, the moderately pre-oxidized nanofiltration membrane is immersed in an amination remediation solution for 1min~5min.

9. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... The heat setting mentioned in step three is specifically carried out at a temperature of 60℃~80℃ for 5min~10min.

10. The method for preparing a highly selective nanofiltration membrane using moderate pre-oxidation and secondary surface crosslinking of permanganate according to claim 1, characterized in that... The water flux of the highly selective nanofiltration membrane prepared in step three is 15 L·m -2 h -1 bar -1 ~27L·m -2 h -1 bar -1 The desalination rate for NaCl solution is 41.1%~55.2%, for MgCl2 solution is 51.4%~65.8%, for MgSO4 solution is 81.4%~97.5%, for Na2SO4 solution is 83.5%~98.2%, and the removal rate of dissolved organic matter is 83.0%~95.3%.

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