Preparation method of high-chlorine-resistance composite nanofiltration membrane for desalting low-molecular-weight organic matters

Abstract

The invention relates to the technical field of membrane separation, in particular to a preparation method of a high-chlorine-resistance composite nanofiltration membrane for desalting low-molecular-weight organic matters, which comprises the following steps: sequentially immersing a polysulfone ultrafiltration membrane into a mixed water phase solution containing phenol / alcohol compounds and piperazine and an organic phase solution containing acyl chloride, and forming a separation layer through interfacial polymerization reaction; the composite nanofiltration membrane is obtained through heat treatment and drying; the phenol / alcohol compound is one or a combination of more of tannic acid, maltitol or epicatechin; the nanofiltration membrane prepared by the method has the characteristics of high flux, low molecular weight organic matter / multivalent salt separation, excellent chlorine resistance, good pollution resistance and dual-network crosslinking.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter. Background Technology

[0002] Loose nanofiltration membranes, as a novel type of nanofiltration membrane, possess pore sizes that bridge nanofiltration and ultrafiltration. They can be used at relatively low operating pressures and exhibit high selectivity, showing potential applications in organic matter / salt separation and wastewater resource recovery. However, in practical water treatment applications, residual active chlorine often remains after pretreatment. This residual active chlorine can cause the nanofiltration membrane's retention performance to degrade or disappear, especially for loose nanofiltration membranes, which significantly limits their widespread application.

[0003] To address the issue of active chlorine, patent CN116020281A uses an activated polytetrafluoroethylene microporous membrane as the base membrane. It sequentially impregnates the membrane with amine monomers and acyl chloride monomers, followed by heat treatment. Then, it grafts amine-containing cyclic compounds and finally introduces quaternization modification to obtain a nanofiltration membrane with high chlorine resistance. Patent CN111514769B sequentially coats a polypiperazine amide layer and a surface-modified layer onto a polysulfone-based membrane. The surface-modified layer is formed by the reaction of thiourea dioxide and polyvinyl alcohol, and its presence gives the nanofiltration membrane high chlorine resistance. Besides surface modification methods for nanofiltration membranes, constructing polyester structures can also improve the chlorine resistance of nanofiltration membranes. For example, patent CN115364688A uses an acyl chloride-containing compound as the organic phase and a certain proportion of piperazine and cyclodextrin as the aqueous phase in an interfacial polymerization reaction on a commercial polyacrylonitrile ultrafiltration membrane. After drying, a nanofiltration membrane with high chlorine resistance is obtained. Patent CN113797762B describes a process where carboxyl-terminated styrene-maleic anhydride (SMA), additives, and a polymer membrane are dissolved in an organic solvent and blended to prepare a membrane-building solution. An ultrafiltration base membrane is then prepared using a phase inversion method, followed by an esterification reaction with tannins to obtain a polyester nanofiltration membrane with chlorine resistance. Patent CN106345308B describes a novel polyesteramide composite nanofiltration membrane with serine as the aqueous phase monomer and 4-dimethylaminopyridine as the aqueous phase inversion catalyst and acylation catalyst, undergoing interfacial polymerization with polyacrylamide chlorides on a porous support layer. This yields a membrane exhibiting good chlorine resistance, strong hydrophilicity, strong antifouling ability, and strong solute retention capacity. While these nanofiltration membranes all demonstrate high chlorine resistance and high retention of salts and organic matter, they struggle to achieve highly efficient separation of organic matter and salts. To address this, some studies have reported that introducing monomers with large molecular volumes into aqueous solutions can achieve dye / salt separation. For example, CN110252141A describes a separation membrane prepared by reacting a mixture of tannic acid and anhydrous piperazine in a specific ratio with trimesoyl chloride on a polysulfone matrix. This membrane exhibits a high retention rate of 99.19% for Congo red and a high permeability of 94.59% for sodium sulfate, thus achieving dye / salt separation. However, these separation membranes are primarily effective only for organic dyes with larger molecular weights (>600 Da), and are difficult to retain for organics with smaller molecular weights. This often involves high-precision control of the pore structure of the separation layer, which is difficult to achieve due to the uncontrolled diffusion and rapid reaction of monomers during interfacial polymerization. This results in a lack of research on nanofiltration membranes that can effectively retain low molecular weight (300–400) organics while maintaining high selectivity for monovalent / divalent salts. Therefore, a method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organics is crucial. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter.

[0005] Specifically, this is achieved through the following technical solutions:

[0006] A method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter involves sequentially immersing a polysulfone ultrafiltration membrane in an aqueous solution containing phenolic / alcohol compounds and piperazine, and an organic solution containing acyl chlorides. A separation layer is formed through interfacial polymerization, and the composite nanofiltration membrane is obtained by heat treatment and drying. The phenolic / alcohol compounds are one or more combinations of tannic acid, maltitol, or epicatechin.

[0007] The polysulfone ultrafiltration membrane is obtained by dissolving polysulfone particles and hydrophilic additives simultaneously in organic solvent A, mechanically stirring until a transparent solution is obtained, then degassing to obtain a membrane solution, and finally coating the membrane solution onto polyethylene terephthalate (PET) nonwoven fabric for phase inversion.

[0008] The polysulfone concentration is 16%–20% by mass; the PVP concentration is 0.1%–1% by mass; the ultrafiltration membrane thickness is 20–100 μm; the phase transition temperature is 10–30 °C; and the hydrophilic additive can be polyvinylpyrrolidone or polyethylene glycol. The organic solvent is N,N-dimethylformamide or N,N-dimethylformamide.

[0009] The aqueous solution containing phenol / alcohol compounds and piperazine is prepared by first dissolving the phenol / alcohol compounds in water, then sequentially adding piperazine and an alkaline catalyst, and stirring until homogeneous. The alkaline catalyst is sodium hydroxide or triethylamine. The mass concentration of the phenol / alcohol compounds in the aqueous solution is 0.01%–1%. When the monomer concentration in the aqueous phase is below 0.01%, it is difficult to form a complete membrane. When the concentration is above 1%, the prepared composite nanofiltration membrane has a high rejection rate for low molecular weight organic matter and divalent salts, failing to meet the requirements for organic matter / salt separation. The mass ratio of phenol / alcohol compounds to piperazine is 1–0.14. When the mass ratio is greater than 1, the prepared nanofiltration membrane exhibits low desalination and organic matter rejection performance; when it is greater than 0.14, the opposite is true.

[0010] The organic phase solution containing acyl chloride is obtained by mixing trimesoyl chloride with organic solvent B, wherein the mass concentration of trimesoyl chloride is 0.02% to 0.2%. If the concentration is below 0.02%, the nanofiltration membrane formed will have low retention performance, while if the concentration is above 0.2%, the nanofiltration membrane prepared will have high retention rates for both salt and organic matter, making it difficult to achieve the desired organic matter / salt separation effect. The organic solvent B is ethylcyclohexane or cyclohexane.

[0011] The interfacial polymerization reaction is carried out at a temperature of 12–30°C for a time of 10–300 s.

[0012] The heat treatment temperature is 50–60°C; the drying temperature is 60–90°C.

[0013] Furthermore, the method for preparing the highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter includes the following steps:

[0014] Step 1: Preparation of polysulfone ultrafiltration membrane: Polysulfone particles and hydrophilic additives are dissolved in an organic solvent and mechanically stirred to form a transparent solution. Then, the membrane solution is degassed to obtain a membrane solution. Finally, the membrane solution is coated onto commercial polyethylene terephthalate (PET) nonwoven fabric and a polysulfone ultrafiltration membrane is obtained through phase inversion.

[0015] Step 2: Preparation of a mixed aqueous solution containing phenols / alcohols and piperazine: Dissolve phenols / alcohols in water to obtain an aqueous solution of phenols / alcohols. Add piperazine and an alkaline catalyst sequentially to the aqueous solution of phenols / alcohols and stir until dissolved to obtain a mixed aqueous solution containing phenols / alcohols and piperazine.

[0016] Step 3: Preparation of organic phase solution containing acyl chloride: Mix pyromellitic trimethylol chloride with an organic solvent to obtain an oil phase solution;

[0017] Step 4: Interfacial polymerization: The polysulfone ultrafiltration membrane is sequentially immersed in an aqueous solution containing phenols / alcohols and piperazine, and an organic solution containing acyl chlorides. An interfacial polymerization reaction occurs on the surface of the polysulfone ultrafiltration membrane to form a separation layer. Then, it is sequentially heat-treated and dried to obtain a high chlorine-resistant composite nanofiltration membrane that can be used for desalination of low molecular weight organic matter.

[0018] Beneficial effects:

[0019] The nanofiltration membrane prepared by the method of this invention has the characteristics of high flux, separation of low molecular weight organic matter / polyvalent salts, excellent chlorine resistance, good antifouling, and dual network crosslinking.

[0020] The aqueous mixed monomers used in this invention include phenol / alcohol compounds with low reactivity and large molecular size, which can undergo interfacial polymerization with trimesoyl chloride to form a polyester network structure with low crosslinking degree and large pore size; while piperazine small molecules undergo interfacial polymerization with trimesoyl chloride to form a polypiperazine network structure with controllable crosslinking degree and small pore size, together constructing a composite nanofiltration membrane with an interpenetrating dual-network crosslinking structure of macropores and micropores (see...). Figure 1 While achieving efficient separation of organic matter and salt, it also has high resistance to active chlorine cleaning and antifouling performance. It maintains stable retention performance even after long-term rinsing in high-concentration sodium hypochlorite solution, which greatly improves the service life of composite nanofiltration membranes. Attached Figure Description

[0021] Figure 1 Scanning electron microscope (SEM) images of the surface of the interpenetrating double-network cross-linked composite nanofiltration membrane are shown.

[0022] Figure 2 The image shows a scanning electron microscope (SEM) image of the composite nanofiltration membrane from Example 3.

[0023] Figure 3 This is a cross-sectional scanning electron microscope image of the composite nanofiltration membrane from Example 3.

[0024] Figure 4 This is a chlorine resistance test of the composite nanofiltration membrane in Example 3. Detailed Implementation

[0025] The specific embodiments of the present invention will be described in further detail below, but the present invention is not limited to these embodiments. Any improvements or substitutions based on the basic spirit of these embodiments shall still fall within the scope of protection claimed by the claims of the present invention.

[0026] Example 1

[0027] A method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter includes the following steps:

[0028] Step 1: Preparation of polysulfone ultrafiltration membrane: Dissolve 540g of polysulfone particles and 9g of polyvinylpyrrolidone in 2451g of dimethylformamide, heat and stir to dissolve, degas and let stand to obtain a transparent solution, which is the membrane solution. Then, coat the membrane solution onto commercial PET nonwoven fabric with a thickness of 50μm, and immerse it in a deionized water solution at 12℃ for phase inversion to obtain polysulfone ultrafiltration membrane.

[0029] Step 2: Preparation of mixed aqueous solution: At room temperature, 0.33g of tannic acid was dissolved in 1997.88g of deionized water to obtain an aqueous solution of tannic acid. Then, 1.67g of piperazine and 0.12g of sodium hydroxide were added to the aqueous solution of tannic acid in sequence and stirred until dissolved evenly to obtain a mixed aqueous solution with a tannic acid mass concentration of 0.1wt% for later use.

[0030] Step 3: Preparation of organic phase solution: Dissolve 1.2g of trimesoyl chloride in 1998.8g of ethylcyclohexane solvent to prepare an organic phase solution with a mass concentration of 0.06wt% trimesoyl chloride for later use;

[0031] Step 4: Interface polymerization: Fix the polysulfone ultrafiltration membrane onto the substrate, ensuring that the back side of the ultrafiltration membrane is not soaked in the aqueous solution. Then, immerse the substrate with the ultrafiltration membrane on it in the mixed aqueous solution. After standing for 10 seconds, remove it and purge the excess aqueous solution. Then, immerse it in the organic phase solution and stand for 1 minute to form a composite separation layer. Remove it and purge the excess solvent. Then, treat it in hot water at 55°C for 1 minute. Then, immerse it in a glycerol-water mixed solution for 30 seconds. Remove it and wash it with deionized water. Finally, place it in an oven at 80°C for 2 minutes to obtain a polyester / polypiperazine composite nanofiltration membrane, which is stored in deionized water for later use.

[0032] Example 2

[0033] Based on Example 1, the difference from Example 1 is that: at room temperature, 0.67g of tannic acid was dissolved in 1995.88g of deionized water to obtain an aqueous solution of tannic acid, and then 3.33g of piperazine and 0.12g of sodium hydroxide were added to the aqueous solution of tannic acid in sequence and stirred until dissolved evenly to obtain a mixed aqueous solution with a tannic acid mass concentration of 0.2wt% for later use.

[0034] Example 3

[0035] Based on Example 1, the difference from Example 1 is that: at room temperature, 1.33g of tannic acid was dissolved in 1991.88g of deionized water to obtain an aqueous solution of tannic acid, and then 6.67g of piperazine and 0.12g of sodium hydroxide were added to the aqueous solution of tannic acid in sequence and stirred until dissolved evenly to obtain a mixed aqueous solution with a tannic acid mass concentration of 0.4wt% for later use.

[0036] Example 4

[0037] Based on Example 1, the difference from Example 1 is that: at room temperature, 2.0g of tannic acid was dissolved in 1987.88g of deionized water to obtain an aqueous solution of tannic acid, and then 10.0g of piperazine and 0.12g of sodium hydroxide were added to the aqueous solution of tannic acid in sequence and stirred until dissolved evenly to obtain a mixed aqueous solution with a tannic acid mass concentration of 0.6wt% for later use.

[0038] Example 5

[0039] Based on Example 1, the difference from Example 1 is that: at room temperature, 2.67g of tannic acid was dissolved in 1983.88g of deionized water to obtain an aqueous solution of tannic acid. Then, 13.33g of piperazine and 0.12g of sodium hydroxide were added to the aqueous solution of tannic acid in sequence and stirred until dissolved evenly to obtain a mixed aqueous solution with a tannic acid mass concentration of 0.8wt% for later use.

[0040] The composite nanofiltration membranes prepared in Examples 1-5 were tested:

[0041] The permeability selectivity of nanofiltration membranes was evaluated using a cross-flow filtration system. The test conditions were: operating pressure 100 psi, operating temperature 25 ± 1 °C, and running time 30 min. The feed solutions were deionized water and a mixed solution of 500 ppm polyethylene glycol (molecular weight 400) + 2000 ppm sodium sulfate. The pure water permeation flux (PWP) and rejection rate (R) were calculated according to formulas (1) and (2), respectively. The results are shown in Table 1.

[0042]

[0043] Where V(L) is the permeation volume of water; A(m 2 ) represents the effective area of ​​the nanofiltration membrane; t(h) represents the permeation time; ΔP(psi) represents the operating pressure.

[0044]

[0045] Among them, C f C represents the feed concentration. p This represents the concentration of the permeate.

[0046] The organic matter / salt separation factor (a) of the composite nanofiltration membrane is calculated using formula (3):

[0047]

[0048] Among them, R 盐 and R 有机物 The values ​​represent the rejection rates of the composite nanofiltration membrane for salt and organic matter, respectively. At room temperature, the composite nanofiltration membrane from Example 3 was immersed in a sodium hypochlorite solution (5000 ppm). The membrane was removed every 24 hours, rinsed with deionized water, and its performance was tested. The results are shown in [Figure number missing]. Figure 3 As shown. Test conditions: operating pressure 100 psi, operating temperature 25±1℃, running time 30 min. Feed solution was deionized water and 500 ppm polyethylene glycol (molecular weight 400 g / mol) or 500 ppm sucrose (molecular weight 342.3 g / mol).

[0049] Table 1

[0050]

[0051] Table 1 shows the water flux and rejection rate of the composite nanofiltration membranes prepared in Examples 1-5. The table shows that the prepared composite nanofiltration membranes exhibit high water flux and organic matter rejection rate, with relatively low rejection rate for divalent salts, demonstrating good organic matter / salt separation performance. As the concentration of the composite monomer in the G-phase solution increases, the water flux of the prepared composite nanofiltration membrane decreases, while the rejection rates for magnesium sulfate, polyethylene glycol, and sucrose increase, and the corresponding salt / organic matter separation factor gradually increases.

[0052] Figure 2 and Figure 3 The image shows SEM images of the composite nanofiltration membranes prepared in the three examples of this embodiment. As shown in the figure, there is a thin composite separation layer on the surface of the polysulfone ultrafiltration membrane. Its surface morphology is flat and it presents a blocky spliced ​​structure.

[0053] Figure 4The figure shows the chlorine resistance test of the composite nanofiltration membrane prepared in Example 3. As can be seen from the figure, the composite nanofiltration membrane has excellent chlorine resistance. Even after being immersed in a high concentration of sodium hypochlorite solution for a long time, the composite nanofiltration membrane still exhibits a high polyethylene glycol rejection rate and stability.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter, characterized in that, A polysulfone ultrafiltration membrane is sequentially immersed in an aqueous solution containing phenolic / alcohol compounds and piperazine, and an organic solution containing acyl chlorides. A separation layer is formed through interfacial polymerization. Finally, the composite nanofiltration membrane is obtained by heat treatment and drying. The phenolic / alcohol compounds are one or more combinations of tannic acid, maltitol, or epicatechin.

2. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 1, characterized in that, The polysulfone ultrafiltration membrane is prepared by dissolving polysulfone particles and hydrophilic additives simultaneously in organic solvent A, mechanically stirring until a transparent solution is obtained, then degassing to obtain a membrane solution, and finally coating the membrane solution onto PET nonwoven fabric for phase inversion to obtain the polysulfone ultrafiltration membrane.

3. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 2, characterized in that, The membrane solution contains 16%–20% polysulfone by mass and 0.1%–1% hydrophilic additive by mass; the membrane thickness of the polysulfone ultrafiltration is 20–100 μm; the phase transition temperature is 10–30 °C; the hydrophilic additive is polyvinylpyrrolidone or polyethylene glycol; and the organic solvent A is N,N-dimethylformamide or N,N-dimethylformamide.

4. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 1, characterized in that, The aqueous solution containing phenol / alcohol compounds and piperazine is prepared by first dissolving the phenol / alcohol compounds in water, then adding piperazine and an alkaline catalyst sequentially, and stirring until homogeneous. The alkaline catalyst is sodium hydroxide or triethylamine. The mass concentration of the phenol / alcohol compounds in the aqueous solution containing phenol / alcohol compounds and piperazine is 0.01% to 1%, and the mass ratio of phenol / alcohol compounds to piperazine is 1 to 0.

14.

5. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 1, characterized in that, The organic phase solution containing acyl chloride is obtained by mixing trimesoyl chloride with organic solvent B, wherein the mass concentration of trimesoyl chloride is 0.02% to 0.2%; and the organic solvent B is ethylcyclohexane or cyclohexane.

6. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 1, characterized in that, The interfacial polymerization reaction is carried out at a temperature of 12–30°C for a time of 10–300 s.

7. The method for preparing a highly chlorine-resistant composite nanofiltration membrane for desalination of low molecular weight organic matter as described in claim 1, characterized in that, The heat treatment temperature is 50–60°C; the drying temperature is 60–90°C.

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