A nanofiltration membrane, its preparation method and use in the selective separation of monovalent and divalent ions

Abstract

The application provides a nanofiltration membrane, a preparation method thereof and application of the nanofiltration membrane in selective separation of monovalent and divalent ions. The nanofiltration membrane is prepared by interfacial polymerization of a triamine with a structure of formula I and piperazine on a polysulfone-based membrane and trimesoyl chloride (TMC). The triamine with the structure of formula I and the piperazine are used as water-phase monomers, and crosslinking polymerization is carried out with the TMC, so that the nanofiltration membrane has high selectivity in selective separation of monovalent and divalent ions, has low surface roughness and excellent long-term operation stability, and has high pure water permeability.

Description

Technical Field

[0001] This invention belongs to the field of membrane technology, and particularly relates to a nanofiltration membrane, its preparation method, and its application in the selective separation of monovalent and divalent ions. Background Technology

[0002] Water treatment technologies improve water quality by removing harmful substances and microorganisms, ensuring access to clean and healthy drinking water and reducing the risk of water pollution and disease transmission. In complex water treatment systems, monovalent salt ions (such as Na+) are crucial. + K + Cl - NO3 - ) and divalent salt ions (such as Ca) 2+ Mg 2+ SO4 2- CO3 2- The effective separation of these ions has become a critical and increasingly urgent technical challenge, as the separation of these ions can meet different water quality requirements.

[0003] The effective implementation of membrane water treatment technology has long been limited by the trade-off between the permeability and selectivity of membrane materials. Traditional polyamide composite (TFC) membranes, especially those using piperazine (PIP) as a monomer, while possessing good film-forming properties and solvent stability, have limited permeability due to their dense, non-porous cross-linked structure and relatively thick membrane layers (typically >100 nm). Furthermore, these membranes also exhibit insufficient selectivity when separating molecules or ions of similar sizes. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide a nanofiltration membrane, a method for preparing the same, and its application in the selective separation of monovalent and divalent ions. This nanofiltration membrane exhibits high selectivity in the selective separation of monovalent and divalent ions, while also possessing low surface roughness, excellent long-term operational stability, and high pure water permeability.

[0005] This invention provides a nanofiltration membrane, which is prepared by interfacial polymerization of trigonal amine and piperazine having the structure of Formula I on a polysulfone-based membrane with trimesoyl chloride (TMC);

[0006] Formula I.

[0007] In a specific embodiment of the present invention, the average pore size of the nanofiltration membrane is 0.26~0.87 nm;

[0008] The roughness of the nanofiltration membrane is 5 nanometers to 11 nanometers.

[0009] This invention provides a method for preparing the nanofiltration membrane described in the above technical solution, comprising the following steps:

[0010] Trigonamine with the structure of Formula I was mixed with water and ultrasonically dispersed. The pH was adjusted to 6.5-6.7, and the mixture was ultrasonicated until a transparent solution was obtained. The solution was then filtered to obtain an aqueous solution of trigonamine. The aqueous solution of trigonamine was mixed with piperazine to obtain a mixed aqueous solution.

[0011] The polysulfone-based membrane is immersed in the mixed aqueous solution for 2.5 to 4 minutes, then immersed in the TMC solution for reaction. After removal, it is allowed to stand and washed to obtain the nanofiltration membrane.

[0012] In a specific embodiment of the present invention, the mass ratio of trigonamine to piperazine is (1~2):(1~3).

[0013] In a specific embodiment of the present invention, the mass ratio of the trigonamine and piperazine is 2:1, 1:1, 1:2 or 1:3.

[0014] In a specific embodiment of the present invention, the concentration of the mixed aqueous solution is 0.95~1.1% (w / v).

[0015] The concentration of TMC in the TMC solution is 0.09~0.11% (w / v).

[0016] In a specific embodiment of the present invention, the reaction time is 0.5 to 2 minutes.

[0017] In a specific embodiment of the present invention, filtration is performed using a 0.22 μm syringe.

[0018] In a specific embodiment of the present invention, cyclohexane is used for the cleaning process.

[0019] The present invention also provides an application of the nanofiltration membrane described above in the selective separation of monovalent and divalent ions.

[0020] This invention provides a nanofiltration membrane prepared by interfacial polymerization of trigonal amine and piperazine having the structure of Formula I on a polysulfone-based membrane with trimesoyl chloride. By using trigonal amine and piperazine having the structure of Formula I as aqueous monomers and crosslinking them with TMC, this invention enables the nanofiltration membrane to exhibit high selectivity in the selective separation of monovalent and divalent ions; it also possesses low surface roughness, excellent long-term operational stability, and high pure water permeability. Attached Figure Description

[0021] Figure 1 The diagram shows the optimization of reaction concentration and time in the embodiments of the present invention, wherein (a) and (b) are the investigation of different concentrations of trigonamine / piperazine, and (c) and (d) are the investigation of different polymerization times;

[0022] Figure 2Field emission scanning electron microscope (FE-SEM) images of the products prepared at different trigonamine / piperazine ratios;

[0023] Figure 3 Cross-sectional FE-SEM images of the products prepared at different trigonamine / piperazine ratios;

[0024] Figure 4 Atomic force microscopy (AFM) images of the products prepared at different trigonamine / piperazine ratios are shown. (a), (b), (c), (d), (e), and (f) represent two-dimensional height planar images of the films with T:PIP (X:Y) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively. (a-1), (b-1), (c-1), (d-1), (e-1), and (f-1) represent three-dimensional morphological images of the films with T:PIP (X:Y) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively.

[0025] Figure 5 The images show the FT-IR spectrum and XPS analysis of the T:PIP(X:Y)-TMC membrane, where (a) is the FT-IR spectrum of the T:PIP(X:Y)-TMC membrane and (b) is the XPS analysis of the T:PIP(X:Y)-TMC membrane.

[0026] Figure 6 The figures show the Zeta potential diagram and water contact angle test diagram of the T:PIP(X:Y)-TMC membrane, where (a) is the Zeta potential diagram of the T:PIP(X:Y)-TMC membrane and (b) is the water contact angle test diagram of the T:PIP(X:Y)-TMC membrane.

[0027] Figure 7 The retention rate of the T:PIP(X:Y)-TMC membrane to four salt solutions and its permeability-selectivity to NaCl / Na2SO4 are shown in (a), where (a) is the retention rate of the T:PIP(X:Y)-TMC membrane to the four salt solutions and (b) is the permeability-selectivity test of the T:PIP(X:Y)-TMC membrane.

[0028] Figure 8 The graphs show a comparison of the separation performance of the T:PIP (1:1)-TMC membrane and the traditional NF membrane, and the pure water permeation performance of the T:PIP (X:Y)-TMC membrane. In particular, (a) shows the separation performance of the T:PIP (1:1)-TMC membrane and the traditional NF membrane, and (b) shows the water flux test graph of the T:PIP (X:Y)-TMC membrane.

[0029] Figure 9 The retention stability of the T:PIP (1:2)-TMC membrane against four salt solutions over 7 days;

[0030] Figure 10 The permeation and rejection rates of MgSO4 by the T:PIP (1:2)-TMC membrane over 7 days were measured. Detailed Implementation

[0031] This invention provides a nanofiltration membrane, which is prepared by interfacial polymerization of trigonal amine (T) having the structure of Formula I and piperazine (PIP) with pyromellitic tricarboxylic acid chloride on a polysulfone-based membrane;

[0032] Formula I.

[0033] In this invention, the average pore size of the nanofiltration membrane is 0.26~0.87 nm; specifically, it can be 0.26 nm, 0.38 nm, 0.41 nm, 0.42 nm or 0.87 nm. The roughness of the nanofiltration membrane is 5 nm to 11 nm; specifically, it can be 9.98 nm, 10.2 nm, 10.9 nm or 5.09 nm.

[0034] This invention provides a method for preparing the nanofiltration membrane described in the above technical solution, comprising the following steps:

[0035] Trigonamine with the structure of Formula I was mixed with water and ultrasonically dispersed. The pH was adjusted to 6.5-6.7, and the mixture was ultrasonicated until a transparent solution was obtained. The solution was then filtered to obtain an aqueous solution of trigonamine. The aqueous solution of trigonamine was mixed with piperazine to obtain a mixed aqueous solution.

[0036] The polysulfone-based membrane is immersed in the mixed aqueous solution for 2.5-4 minutes, then immersed in the TMC solution for reaction. After being removed and allowed to stand, it is washed to obtain a nanofiltration membrane.

[0037] Formula I.

[0038] In this invention, trigonamine with the structure of Formula I is mixed with water and ultrasonically dispersed. The pH is adjusted to 6.5-6.7, and the mixture is ultrasonicated until a transparent solution is obtained. The solution is then filtered to obtain an aqueous solution of trigonamine. The aqueous solution of trigonamine is then mixed with piperazine to obtain a mixed aqueous phase solution.

[0039] In this invention, the preparation of the triangulamine having the structure of Formula I is preferably referenced in Arnaud Chaix, Georges Mouchaham, Aleksander Shkurenko, Phuong Hoang, Basem Moosa, Prashant M. Bhatt, Karim Adil, Khaled N. Salama, Mohamed Eddaoudi, Niveen M. Khashab. Trianglamine-based supramolecular organic framework with permanent intrinsic porosity and tunable selectivity, Journal of the American Chemical Society, 2018, 140(44), 14571-14575. (10.1021 / jacs.8b08770).

[0040] Specifically, the trigonal amine having the structure of Formula I is prepared by the following method:

[0041] A dry round-bottom flask was filled with levorotatory-trans-1,2-cyclohexanediamine (DACH), terephthalaldehyde (TPA), anhydrous methanol (MeOH), and triethylamine (TEA). The mixture was stirred at room temperature under a nitrogen atmosphere. The mixture was then cooled in an ice bath, and NaBH4 was added. The mixture was stirred at room temperature, and the solvent was removed under vacuum. The residue was dissolved in dichloromethane and washed successively with an aqueous sodium carbonate solution, water, and brine. The combined organic phases were dried with anhydrous magnesium sulfate, filtered, and evaporated to obtain a crude product. The crude product was dissolved in anhydrous ethanol, and a mixture of anhydrous ethanol and concentrated hydrochloric acid was slowly added to the ethanol solution of the crude product. During this process, a white precipitate began to form. The obtained solid was filtered, dried under vacuum, and then dissolved in water. The pH of the solution was then adjusted to alkaline by adding sodium hydroxide solution. The precipitate was filtered, dried under vacuum, and obtained a pure trigonal amine with the structure of Formula I.

[0042] In this invention, trigonamine is preferably added to water and ultrasonically dispersed to form a turbid solution. Hydrochloric acid is then added to adjust the pH to 0.1 mol / L, specifically to a value of 6.5–6.7, or 6.5, 6.6, or 6.7. This invention preferably uses a 0.22 μm syringe for filtration. The trigonamine aqueous solution is then in a clear state.

[0043] The mass ratio of trigonamine and piperazine in this invention is (1~2):(1~3); specifically, it can be 2:1, 1:1, 1:2 or 1:3.

[0044] In this invention, the concentration of the mixed aqueous solution is 0.95~1.1% (w / v), specifically 0.95% (w / v), 1.0% (w / v) or 1.1% (w / v).

[0045] After obtaining the mixed aqueous solution, the polysulfone-based membrane is immersed in the mixed aqueous solution for 2.5-4 min, then immersed in the TMC solution for reaction. After removal, it is allowed to stand and washed to obtain a nanofiltration membrane. The polysulfone membrane is preferably soaked in water before use; and the surface moisture is absorbed with filter paper before immersion in the mixed aqueous solution. In this invention, the concentration of TMC in the TMC solution is 0.09-0.11% (w / v), specifically 0.09% (w / v), 0.10% (w / v), or 0.11% (w / v); the solvent in the TMC solution is cyclohexane.

[0046] In this invention, the reaction time is 0.5 to 2 min, specifically 0.5 min, 1 min, or 2 min. During the interfacial polymerization process, the -NH2 / -NH- of the trigonal amine and the -NH2 of the PIP in the mixed aqueous phase act as nucleophiles, launching a nucleophilic attack on the electron-deficient carbonyl carbon of the acyl chloride group in the organic phase. Through a nucleophilic substitution reaction, chloride ions are removed, forming an amide bond and releasing HCl. The cross-linking polymerization yields a nanofiltration membrane, which is a polyamide separation membrane.

[0047] The present invention preferably places the membrane vertically at room temperature for a short time to allow the organic solvent to evaporate, and then rinses the membrane surface multiple times with cyclohexane to remove residual monomers, thereby obtaining a nanofiltration membrane (NF membrane).

[0048] The present invention uses the following methods to characterize nanofiltration membranes:

[0049] The cross-sectional structure and surface morphology of the nanofiltration membrane were analyzed using a scanning electron microscope (Hitachi Regulus SU8200). To ensure the integrity of the cross-sectional morphology of the nanofiltration membrane, the sample was fractured in a liquid nitrogen environment. A layer of gold particles was sputtered onto the sample surface before observation. Fourier transform infrared (FTIR) spectral data of the nanofiltration membrane were collected using an Infrared Tracer-100 Fourier Transform Infrared Spectrometer (Shimadzu, Japan), which integrates attenuated total reflectance FTIR (ATR-FTIR) technology and has a measurement range from 4000 cm⁻¹. -1 up to 400 cm -1X-ray photoelectron spectroscopy (XPS) analysis was performed using an Escab 250Xi spectrometer (Thermo Fisher Scientific, USA) equipped with AlKα radiation as the excitation source. The surface roughness of the nanofiltration membrane was studied using a SizeICON atomic force microscope (AFM, Bruker, USA). The water contact angle was measured using the pendant drop method on a DataPhysics OCA20 contact angle meter (Feldstadt, Germany). The surface zeta potential of the nanofiltration membrane was monitored using a Zetasizer Nano-ZS90 analyzer (Malvin, UK).

[0050] This invention employs the following method to determine the permeation of a nanofiltration membrane, using a filtration device, with an effective separation area of ​​7.06 cm². 2 The feed volume was 200 mL. Before each measurement, each membrane sample was permeated with water at a specified pressure for 1 hour to stabilize; the permeate was collected when a stable permeate flow was reached. All experiments were conducted at room temperature and repeated three times in parallel. The membrane permeate flux (J) was calculated using equation (1):

[0051] (1);

[0052] Where J represents the permeation flux (L / (m) 2 ·h·bar), V represents the permeation volume (L), and A represents the effective membrane area (m²). 2 ), t represents the filtration time (h), and P represents the applied transmembrane pressure (bar).

[0053] Unless otherwise specified, the solution concentration is 1 g / L and the applied pressure is 0.6 MPa;

[0054] Salt concentration was measured using a conductivity meter (REX DDSJ-308 F, China);

[0055] The salt repulsion rate (R) is calculated according to equation (2):

[0056] (2);

[0057] Where C p and C f These represent the conductivity of the permeate and feed solution, respectively, in μS / cm.

[0058] The present invention also provides an application of the nanofiltration membrane described above in the selective separation of monovalent and divalent ions.

[0059] The monovalent and divalent ions are Cl... - and SO4 2- ; or Na + and Mg 2+ .

[0060] To further illustrate the present invention, the following detailed description, in conjunction with embodiments, provides a nanofiltration membrane, its preparation method, and its application in the selective separation of monovalent and divalent ions. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0061] Example 1

[0062] 1. Preparation of trigonamine:

[0063] A 250 mL dry round-bottom flask was filled with DACH (1.14 g, 10 mmol), TPA (1.34 g, 10 mmol), MeOH (100 mL), and TEA (3.5 mL, 25 mmol). The mixture was vigorously stirred at room temperature for 24 hours under a nitrogen atmosphere. The mixture was then cooled in an ice bath, and NaBH4 (1.14 g, 30 mmol) was added over 1 hour. After stirring at room temperature for 24 hours, the solvent was removed under vacuum. The residue was dissolved in 100 mL of dichloromethane and washed successively with 5 wt% sodium carbonate aqueous solution, water, and brine. The combined organic phases were dried over anhydrous magnesium sulfate, filtered, and evaporated to obtain the crude product. The crude product was dissolved in anhydrous ethanol (2 mL), and a mixture of anhydrous ethanol and concentrated hydrochloric acid (6 mL, v / v = 5:1) was slowly added to the ethanol solution of the crude product. A white precipitate began to form during this process. The obtained solid was filtered, vacuum dried, and then dissolved in water. The pH of the solution was then adjusted to alkaline by adding 20 mL of 2 mol / L sodium hydroxide solution. The resulting precipitate was filtered and vacuum dried at 40 °C for one day to obtain pure trigonamine, with a yield of 81%.

[0064] Preparation of mixed aqueous solution:

[0065] A certain amount of trigonamine was weighed and added to 10 mL of ultrapure water. The solution was ultrasonically dispersed to form a turbid liquid. 0.1 mol / L hydrochloric acid was added dropwise to adjust the pH to approximately 6.6, with continuous sonication during this process until the solution became transparent. The solution was filtered through a 0.22 μm syringe filter to obtain a clear aqueous solution of trigonamine. The trigonamine solution was mixed with piperazine (PIP) at different mass ratios (T:PIP = 1:0, 2:1, 1:1, 1:2, 1:3, 0:1) to prepare a mixed aqueous solution with a total concentration of 1% (w / v).

[0066] Interface aggregation process:

[0067] Cut the polysulfone (PSf) membrane to an appropriate size and immerse it in ultrapure water for later use. Remove the PSf membrane, blot the surface moisture with filter paper, and then immerse it in the prepared mixed aqueous solution for 3 min. Remove the membrane and quickly immerse it in a 0.1% (w / v) TMC / cyclohexane solution for 1 min. Remove the membrane and place it vertically at room temperature for a few moments to allow the organic solvent to evaporate. Then, rinse the membrane surface several times with cyclohexane to remove residual monomers, obtaining a nanofiltration membrane (NF membrane), which is then refrigerated for testing.

[0068] 2. Optimization of reaction conditions

[0069] The concentrations of trigonamine / piperazine in aqueous solution and the reaction time with TMC were optimized. For example... Figure 1 In (a) and (b), as the concentration of trigonamine / piperazine in the aqueous solution gradually increases, the rejection rate of the NF membrane for Na₂SO₄ and the water flux show a trend of first decreasing, then increasing, and then decreasing again. When the trigonamine / piperazine concentration is 1 w / v%, the NF membrane has the highest rejection rate for Na₂SO₄, reaching 86.6%. Similarly, the rejection rate of the NF membrane for sodium chloride and the water flux also show a similar trend. At a trigonamine / piperazine concentration of 1 w / v%, the NF membrane has the lowest rejection rate for sodium chloride (only 4.4%). Further increasing the concentration of trigonamine / piperazine in the aqueous solution will cause trigonamine to precipitate due to solubility limitations. Considering the separation performance, the optimal concentration of trigonamine / piperazine in the aqueous solution was determined to be 1 w / v%. This concentration will be used in subsequent experiments.

[0070] While maintaining a trigonamine / piperazine concentration of 1 wt / v%, the reaction time was further optimized. For example... Figure 1 As shown in (c) and (d), the membrane's rejection rate for sodium sulfate initially increased with increasing reaction time, then decreased, rising from 20.1% to 86.6%, and then falling back to 68.6%. Similarly, the nanofiltration membrane's rejection rate for sodium chloride also exhibited a pattern of first increasing and then decreasing, reaching a minimum of 4.4% at a reaction time of 3 minutes. Considering the membrane's effective separation of sodium sulfate and sodium chloride and the sufficiency of the reaction, the optimal reaction time for polymerization was determined to be 1 minute. In the subsequent discussion, the NF membrane was prepared under the above-mentioned optimized concentration and reaction time conditions.

[0071] 3. Morphological characteristics of nanofiltration membranes

[0072] The membrane structure morphology was characterized using field emission scanning electron microscopy (FE-SEM), such as... Figure 2As shown; where (a), (b), (c), (d), (e), and (f) represent the mass ratio of T to PIP (denoted as T:PIP(X:Y)) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively. The surface of the PIP / TMC polyamide film exhibits a characteristic rough structure, characterized by circular protrusions of varying sizes and densities at different scales. These protrusions are further supported by tubular wrinkles. In contrast, the surface of the T / TMC film is uniform and flat, without any wrinkles or protrusions.

[0073] Interfacial polymerization is an exothermic and inherently unstable process that easily forms a non-homogeneous phase. The results of this invention demonstrate that when PIP and trigonamine are used as mixed aqueous monomers, the resulting nanofiltration membrane surface exhibits unique morphological characteristics. Due to the different monomer diffusion properties and varying mixing ratios, these surfaces are neither granular like those in PIP / TMC nor smooth like those in T / TMC. Instead, they exhibit a small number of small wrinkles or protrusions.

[0074] Cross-sectional FE-SEM images are shown below. Figure 3 In the figure, (a), (b), (c), (d), (e) and (f) represent membranes prepared by mass ratios of T and PIP (denoted as T:PIP(X:Y)) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively. There is a clear boundary between the polysulfone (PSf) substrate and the polyamide (PA) selective layer in all membrane samples; all samples have a continuous and uniform PA selective layer.

[0075] Atomic force microscopy (AFM) images are shown below. Figure 4 Table 1 shows the surface roughness parameters of membranes prepared with different T:PIP mass ratios, further quantifying the surface morphology characteristics. (a), (b), (c), (d), (e), and (f) represent two-dimensional height planar diagrams of membranes with T:PIP (X:Y) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively; (a-1), (b-1), (c-1), (d-1), (e-1), and (f-1) represent three-dimensional morphological diagrams of membranes with T:PIP (X:Y) = 0:1, 1:0, 1:1, 1:2, 1:3, and 2:1, respectively. Pure PIP-based polyamide membranes exhibit a significantly convex structure and have the highest arithmetic mean surface roughness (Ra = 136 nm). With increasing trigonal amine mass ratio in the aqueous phase, the nanofiltration membrane surface gradually becomes smoother, while the roughness gradually decreases. The Ra value decreased significantly from the lowest proportion (10.9 nm) to the highest proportion (4.46 nm). This trend clearly indicates that the addition of trigonamine can effectively make the surface of the polyamide film smoother and flatter.

[0076] Table 1

[0077]

[0078] 4. Chemical properties of nanofiltration membranes

[0079] The FT-IR spectrum of the T:PIP(X:Y)-TMC film is as follows: Figure 5 As shown in (a). All NF membranes at 750 cm⁻¹ -1 1260 cm -1 and 1630 cm -1 The characteristic adsorption peaks at 3420 cm⁻¹ correspond to the bending vibrations of O=CN and the stretching vibrations of CN and C=O in the amide bond, respectively, indicating that the polyamide layer (PA) has been successfully formed on the PSF substrate. -1 The broad peak (-OH stretching vibration) at the T:PIP(1:0)-TMC and T:PIP(2:1)-TMC membranes, compared with membranes prepared using piperazine monomers, showed two new strong adsorption peaks, which can be attributed to bond vibrations. No new characteristic peaks were detected in other ratios of T:PIP-TMC membranes compared with membranes prepared using piperazine monomers; this may be due to the overlap of characteristic absorption peaks after the trigonamine monomer bonds with TMC and the characteristic absorption peaks after the piperazine monomer bonds with TMC.

[0080] The chemical composition of these membranes was further verified by XPS analysis, such as... Figure 5 (b) Peaks for C, N, and O elements were detected at 284.7 eV, 398.9 eV, and 531.2 eV in all NF membranes, respectively.

[0081] Table 2

[0082]

[0083] Table 2 shows the elemental analysis of the XPS T / PIP-TMC film surface. As can be seen from Table 2, the nitrogen content in these films is reduced to varying degrees, indicating that the C / N ratio of the T:PIP (1:1) film is lower than that of the T:PIP (1:2) film. This is contrary to stoichiometric expectations and may be due to diffusion limitations: the large size (approximately 1 nm cavity) and rigidity of trigonamine hinder its migration to the reaction interface, while the flexible PIP molecules do not; aggregation under high loading: excess trigonamine undergoes π-π stacking in the aqueous phase, forming aggregates, further hindering interfacial bonding. Therefore, despite the higher trigonamine content, its effective surface contribution is reduced, allowing the PIP-dominated polymerization reaction to lower the C / N ratio.

[0084] All nanofiltration membranes exhibit a negative surface charge under neutral pH conditions, see Figure 6In (a), the pure PIP / TMC membrane exhibits the highest negative charge, while the pure T / TMC membrane exhibits the lowest negative zeta potential. The surface charge of the blended T:PIP membrane falls between the two; this trend stems from competing reactions during interfacial polymerization: the primary charge source is the unreacted TMC acyl chloride group (-COCl), which hydrolyzes to form a carboxylic acid group (-carboxyl). Under neutral pH conditions, this group undergoes deprotonation (-COO). - This generates a negative charge. The charge density is proportional to the degree of hydrolysis. Due to the charge-inhibiting effect of T: the amino group (-NH2) of T preferentially reacts with the remaining TMC acyl chloride group via nucleophilic substitution to form an amide bond; this process consumes the hydrolyzable -COO group. - The formation of carboxylic acids is reduced by the presence of T sites. Therefore, T-rich membranes exhibit lower negative charges due to inhibited hydrolysis. The blending effect also occurs in the reaction: in blended T:PIP membranes, trigonamine partially inhibits hydrolysis by competitively removing TMC from the interface. Therefore, the surface charge intensity tends to decrease with increasing T:PIP ratio, which explains why the zeta potential of the blended T:PIP membrane lies between that of the pure PIP / TMC membrane and the pure T / TMC membrane.

[0085] from Figure 6 As shown in (b), both the pure PIP-TMC membrane and the pure T-TMC membrane exhibit good hydrophilicity, with contact angles of 26.5° and 24.8°, respectively. This is due to the abundant amide bonds in the PIP molecule and the strong hydrogen bonding between the polar functional groups in the trigonal amine molecule and water molecules. As the T:PIP ratio changes from 1:1 to 1:2 and 1:3, the contact angle increases to 45.2°, 63.5°, and 59.3°, respectively. This indicates that when the PIP ratio increases to a certain extent (1:2), PIP undergoes phase separation on the surface, exposing hydrophobic segments and thus reducing hydrophilicity. When the T:PIP ratio is 2:1, the contact angle decreases to 49.1°, showing improved hydrophilicity but still weaker than the pure component membrane. This corresponds to the decrease in surface nitrogen content and carbon enrichment (C / N ratio 3.13) at this ratio in the XPS results, indicating that as the trigonal amine content increases, the number of polar groups on the membrane surface decreases, leading to a decrease in membrane surface hydrophilicity.

[0086] 5. Separation performance of monovalent / divalent ions of nanofiltration membranes

[0087] The nanofiltration performance of T:PIP blend membranes for four salts was systematically evaluated (in conjunction with...). Figure 7 (Data in (a)). The order of hydration radii of the key ions is: Mg 2+ (4.3 Å) > Na + (3.6 Å) > SO4 2- (3.3~3.5 Å) > Cl- (3.3 Å). The results showed that increasing the proportion of trigonal amine (T) decreased the rejection rates of sodium chloride and MgCl2, while the rejection rates of Na2SO4 and MgSO4 remained at a high level (comparable to pure PIP / TMC). This difference in selectivity stems from a dual mechanism: the cyclic structure of trigonal amine disrupts the packing of polyamide chains, expanding the effective pore size; this favors smaller hydrated ions (Na2SO4, MgSO4, and Na2SO4). + Cl - (The permeation of) divalent SO4. 2- The hydration radius of the ion is 4.0 Å, compared to Cl. - Compared to 3.3 Å, it exceeds the expanded aperture threshold; its high charge density amplifies the Donnan repulsion effect by acting on the negatively charged membrane surface through electrostatic repulsion.

[0088] Therefore, even with increased porosity, the combined effect of steric hindrance and electrostatic barrier can still maintain a high sulfate repulsion rate.

[0089] 6. Water permeability and salt selectivity

[0090] Depend on Figure 7 It can be seen that when only one aqueous monomer (piperazine or trigonamine) is used to prepare nanofiltration membranes, Cl - SO4 2- The selectivity is lower. This is because the relatively dense PIP / TMC nanofiltration membrane retains more than 80% of sodium sulfate and magnesium chloride, while the MgSO4 and T / TMC nanofiltration membranes, due to the larger gaps between polymer chains, form a relatively loose structure. Compared with PIP / TMC, their flux is more than eight times higher, but the retention levels of the four salts are lower, resulting in lower selectivity for Cl. - SO4 2- Selectivity decreases. In sodium salt systems, the traditional pure PIP membrane structure is extremely dense. Although it has a high rejection rate of approximately 96% for divalent salts (Na₂SO₄), its selectivity (Cl₂) is low. - SO4 2- The value reached 18.75, but its water flux was only about 10 L / (m²). 2 The mass transfer resistance and energy consumption were too high (·h·bar). Introducing trigonal amine improved the overall performance of the membrane. Taking T:PIP mass ratios of 1:1 and 1:3 as examples, the water flux increased by more than 11 times, reaching 110~118 L / (m³). 2 Despite the reduced selectivity, the Na2SO4 retention rate remained at an excellent level of 80-85%.

[0091] The nanofiltration membrane prepared by mixing the two monomers outperformed PIP / TMC and T / TMC. Magnesium chloride rejection was reduced, while maintaining low sodium chloride rejection and high sodium sulfate and magnesium sulfate rejection.

[0092] Table 3

[0093]

[0094] Table 3 shows the rejection rates and selectivity of the nanofiltration membranes for different substances. As can be seen from Table 3, the nanofiltration membrane with T:PIP=1:1 prepared in this invention has a selectivity for magnesium chloride and magnesium sulfate that is approximately 11 times that of PIP / TMC, which significantly improves the selectivity of Cl. - SO4 2- The selectivity of trigonamine is increased by adding negative charge to the nanofiltration membrane surface, which is beneficial for the selection of divalent anions SO42-. 2- It has a strong repulsive effect. Furthermore, hydrated SO42- 2- The larger the hydration radius, the more complex its hydration structure is compared to hydration Cl. - More stable, thus generating greater steric hindrance during transmission.

[0095] Figure 8 (a) shows a comparison of the separation performance of the T:PIP (1:1)-TMC membrane and the traditional NF membrane. In this graph, 1 represents the NF membrane prepared in the literature: Liu Y., Lin B., Liu W., et al. Preparation and characterization of a novel nanofiltration membrane with chlorine-tolerant property and good separation performance[J]. RSC Advances, 2018, 8(64):36430-36440; 2 represents the NF membrane prepared in the literature: BiQ., Xu S. A positively charged PI nanofiltration membrane with good separation for Li + and Mg 2+[J]. Desalination and Water Treatment, 2020, 198:98-107. NF membrane prepared; 3 Representative literature: Liu Yanghe. Study on surface complexation modification of nanofiltration membrane and its magnesium-lithium separation performance [D]. Tianjin University of Technology, 2023, NF membrane prepared; 4 Representative literature: Mohammed S., Nassrullah H., Aburabie J., et al. Fabrication of thin film composite membranes on nanozeolite modified support layer for tailored nanofiltration performance [J]. Membranes, 2022, 12(10): 940, NF membrane prepared; 5 Representative literature: Zhao B., Pei M., Guo X., et al. Enhanced Mg 2+ / Li + NF membrane prepared by separation by Janus nanofiltration membrane incorporated with negatively-charged COF interlayer[J]. Separation and PurificationTechnology, 2025, 365: 132671; Figure 8 Figure (b) shows the pure water permeation performance of the T:PIP(X:Y)-TMC membrane. The T:PIP (1:1)-TMC membrane in this invention exhibits extremely high pure water permeability while maintaining excellent Cl- content. - SO4 2- Selectivity. This further demonstrates that trigonamine is a highly promising aqueous monomer, exhibiting excellent performance in the preparation of loosely arranged NF membranes via interfacial polymerization.

[0096] 7. Long-term stability

[0097] This invention conducted a 7-day filtration experiment on the T:PIP(1:2)-TMC membrane to evaluate its long-term operational stability; such as Figure 9 It can be seen that the T:PIP(1:2)-TMC membrane showed high stability after 7 days of continuous retention tests on four salt solutions. The membrane retention rates of sodium sulfate and magnesium sulfate remained at about 70%, while the retention rates of sodium chloride and magnesium chloride remained at about 10%.

[0098] like Figure 10As shown, the black curve represents the MgSO4 permeability of the T:PIP(1:2)-TMC membrane over 7 days, and the red curve represents the MgSO4 rejection rate of the T:PIP(1:2)-TMC membrane over 7 days. Figure 10 It can be seen that the permeability of the T:PIP(1:2)-TMC membrane remains stable over 7 days. This proves that the membrane has good long-term stability and durability; the abundant amino groups and the molecular-level size of T are beneficial to maintaining the structural stability of the membrane.

[0099] As can be seen from the above embodiments, the present invention provides a nanofiltration membrane, which is prepared by interfacial polymerization of trigonal amine and piperazine having the structure of Formula I on a polysulfone-based membrane with trimesoyl chloride. By using trigonal amine and piperazine having the structure of Formula I as aqueous monomers and crosslinking them with TMC, the present invention enables the nanofiltration membrane to exhibit high selectivity in the selective separation of monovalent and divalent ions; it also possesses low surface roughness and excellent long-term operational stability; and high pure water permeability.

[0100] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A nanofiltration membrane, characterized in that, It is prepared by interfacial polymerization of trigonal amine and piperazine having the structure of Formula I with pyromellitic tricarboxylic acid chloride on a polysulfone-based film; Formula I; The nanofiltration membrane has an average pore size of 0.26~0.87 nm; The roughness of the nanofiltration membrane is 5 nanometers to 11 nanometers.

2. A method for preparing the nanofiltration membrane according to claim 1, characterized in that, Includes the following steps: Trigonamine with the structure of Formula I was mixed with water and ultrasonically dispersed. The pH was adjusted to 6.5-6.7, and the mixture was ultrasonicated until a transparent solution was obtained. The solution was then filtered to obtain an aqueous solution of trigonamine. The aqueous solution of trigonamine was mixed with piperazine to obtain a mixed aqueous solution. The polysulfone-based membrane is immersed in the mixed aqueous solution for 2.5 to 4 minutes, then immersed in a pyromellitic methyl chloride solution for reaction. After removal, it is allowed to stand and washed to obtain a nanofiltration membrane.

3. The preparation method according to claim 2, characterized in that, The mass ratio of trigonamine to piperazine is 1~2:1~3.

4. The preparation method according to claim 2, characterized in that, The mass ratio of trigonamine to piperazine is 2:1, 1:1, 1:2 or 1:

3.

5. The preparation method according to claim 3, characterized in that, The mass-volume percentage concentration of trigonamine and piperazine in the mixed aqueous solution is 0.95~1.

1. The mass-volume percentage concentration of pyromellitic chloride in the pyromellitic chloride solution is 0.09~0.

11.

6. The preparation method according to claim 2, characterized in that, The reaction time is 0.5 to 2 minutes.

7. The preparation method according to claim 2, characterized in that, Filtration was performed using a 0.22μm syringe.

8. The preparation method according to claim 2, characterized in that, The cleaning process uses cyclohexane.

9. The application of the nanofiltration membrane of claim 1 in the selective separation of monovalent and divalent ions.

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