Acid-resistant nanofiltration membrane based on epichlorohydrin modified quaternized PEI (polyetherimide) and preparation method of acid-resistant nanofiltration membrane

Abstract

The invention discloses an acid-resistant nanofiltration membrane based on epichlorohydrin modified quaternized polyethyleneimine (PEI) and a preparation method of the acid-resistant nanofiltration membrane. The invention aims to solve the technical problem of poor metal cation rejection rate caused by insufficient crosslinking degree and positive charge density of the existing acid-resistant membrane. The preparation method comprises the following steps: carrying out quaternization modification on polyethyleneimine (PEI) by adopting epichlorohydrin under the catalysis of an ionic liquid to prepare a water-phase solution with stable positive charges; then, the aqueous phase solution and an organic phase solution containing 1, 3, 5-benzene trisulfonyl chloride (BTSC) are subjected to interfacial polymerization on a porous supporting base membrane, and a polysulfonamide desalination layer is formed after heat treatment. By constructing a permanent quaternary ammonium structure which is not influenced by an acid environment, the positive charge density of the surface of the membrane and the electrostatic repulsive force to metal ions are remarkably improved. The prepared nanofiltration membrane shows high rejection rate, high flux and excellent stability in strong acid, and is suitable for separation and recovery of metal ions and inorganic acid in acid wastewater in the fields of metallurgy, electroplating and the like.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology and specifically relates to an acid-resistant nanofiltration membrane prepared by cross-linking epichlorohydrin-modified quaternized polyethyleneimine (PEI) with 1,3,5-benzenetrisulfonyl chloride (BTSC), and its preparation method. The membrane exhibits high metal ion rejection, high flux, and excellent stability under strongly acidic conditions, making it suitable for separating and recovering metal ions and inorganic acids from acidic wastewater. Background Art

[0002] Acid-resistant nanofiltration membranes have important application value in the fields of metallurgy, rare earth recovery, and electroplating wastewater treatment. However, traditional polyamide membranes are difficult to operate stably in a strong acidic environment due to the easy hydrolysis of amide bonds. Under acidic conditions, the nitrogen atoms of the amide bonds in the polyamide molecular chain are easily protonated, triggering a hydrolysis reaction to generate ketones (or aldehydes) and amine products, resulting in the disintegration of the membrane structure and a significant decrease in the retention performance. Although existing acid-resistant membrane materials (such as sulfonated polysulfone, polyether ketone, etc.) have partially solved this problem by introducing acid-resistant groups, they still generally have defects such as low flux, uneven pore size distribution, and insufficient cross-linking.

[0003] Most existing nanofiltration membranes are polyamide membranes, and their desalting layer is formed by the polymerization reaction of polyamines and polyacyl chlorides. Because the nitrogen atom on the amide bond in polyamide is protonated under acidic conditions, the hydrolysis of the amide bond breaks down to generate the corresponding amide cation, which is then hydrolyzed to generate ketones (or aldehydes) and amines. As a result, the service life of commercially available nanofiltration membranes under strong acid conditions is short, making them difficult to use in acidic wastewater treatment. Compared with traditional polyamide nanofiltration membranes, polysulfonamide nanofiltration membranes are more acid-resistant. The sulfonamide group has high hydrolysis potential and strong hydrogen bonding between molecular chains. In recent years, it has gradually been applied to the field of acid-resistant membranes. However, due to the low reactivity of sulfonyl chloride, a relatively loose primary polysulfonamide structure will first be formed during the interfacial polymerization process. A large amount of unreacted sulfonyl chloride groups are likely to remain at the oil phase solvent interface, which will eventually hydrolyze to form negatively charged sulfonic acid groups, resulting in the existing acid-resistant nanofiltration membranes showing a low degree of crosslinking.

[0004] Traditional quaternization modification processes mostly rely on highly toxic reagents such as chloromethyl oxirane, with harsh reaction conditions and poor environmental protection, which limits its industrial application. In interfacial polymerization technology, the problem of insufficient reactivity of sulfonyl chloride monomers is particularly prominent. Chinese patent CN114471157A prepares a positively charged acid-resistant membrane by interfacial polymerization of an aqueous phase-oil phase-aqueous phase sandwich structure, but its cross-linking density is limited by the monomer reactivity, and the surface charge characteristics of the membrane are not optimized by chemical modification, resulting in insufficient metal ion retention rate. Similarly, CN115608176B uses polysulfonyl chloride and polyethyleneimine to cross-link. Although it improves acid resistance, due to the lack of charge regulation mechanism, the residual sulfonyl chloride group is hydrolyzed to form a negatively charged sulfonic acid group, which weakens the retention capacity of cationic pollutants.

[0005] Therefore, the development and application of high-performance acid-resistant nanofiltration membranes are crucial to solving heavy metal ion pollution and pH pollution in acidic wastewater. Summary of the Invention

[0006] The purpose of the present invention is to provide a nanofiltration membrane with strong acid resistance, high retention rate and high flux. By cross-linking quaternized PEI modified with epichlorohydrin and BTSC, combined with ionic liquid catalysis, precise control of membrane pore size and significant improvement of surface charge density can be achieved, thereby solving the problems of insufficient cross-linking degree and low charge density of existing acid-resistant membranes.

[0007] A method for preparing an acid-resistant nanofiltration membrane comprises the following steps:

[0008] a) reacting polyethyleneimine (PEI) with epichlorohydrin in an aqueous solution in the presence of an ionic liquid catalyst to prepare a quaternized polyethyleneimine aqueous solution;

[0009] b) coating the quaternized polyethyleneimine aqueous solution on a porous supporting base membrane;

[0010] c) contacting the supporting base membrane with the aqueous solution attached thereto with an organic solution containing polysulfonyl chloride to perform interfacial polymerization reaction to form a polysulfonamide desalting layer;

[0011] d) heat-treating the formed nanofiltration membrane.

[0012] The reaction conditions in step a) are:

[0013] The molecular weight of polyethyleneimine is 2000-50000Da;

[0014] The concentration of epichlorohydrin is 5-20g / L;

[0015] The reaction temperature is 70°C and the reaction time is 4-5 hours.

[0016] The ionic liquid catalyst is [Bmim]BF4, and its concentration in the aqueous solution is 0.5-3 g / L.

[0017] In the quaternized polyethyleneimine aqueous solution, the concentration of the quaternized polyethyleneimine is 8-30 g / L.

[0018] The aqueous phase solution further comprises a surfactant, which is sodium dodecyl sulfate (SDS) with a concentration of 0.01-3 g / L.

[0019] The polysulfonyl chloride in the organic phase solution is 1,3,5-benzenetrisulfonyl chloride (BTSC) or 1,3,6-naphthalenetrisulfonyl chloride (NTSC), and its concentration in the organic solvent is 0.01%-0.6% (w / v).

[0020] The interfacial polymerization reaction time in step c) is 1-5 minutes.

[0021] The heat treatment conditions in step d) are: treating at 60-100° C. for 5-30 minutes.

[0022] The porous supporting base membrane is a polyethersulfone (PES) or polysulfone (PSF) base membrane, and its molecular weight cut-off is 10-50 kDa.

[0023] The nanofiltration membrane obtained by the preparation method is used in ion separation in solution.

[0024] The nanofiltration membrane is used for intercepting divalent or multivalent cations, or for separating positively charged dyes / monovalent salts; the nanofiltration membrane is used for improving acid resistance.

[0025] The beneficial effects of the present invention are as follows: the present invention proposes to use epichlorohydrin (LD50 = 240 mg / kg) as a quaternary ammonium modifier, the epoxy group of which can undergo a ring-opening reaction with the primary amino group of polyethyleneimine (PEI) under mild conditions to generate a stable quaternary ammonium structure while avoiding the generation of toxic byproducts. The reactivity of epichlorohydrin is significantly higher than that of traditional chloromethyl reagents, and the generated hydroxyl byproducts can be removed by simple water washing, meeting the requirements of green chemistry. In addition, the quaternary ammonium group (-N + (CH2CHOHCH2Cl)) is a permanent positive charge and is not affected by protonation in an acidic environment, thereby improving the surface charge density of the membrane (Zeta potential> +30mV) while avoiding the hydrolysis problem caused by amino protonation. Combined with ionic liquid catalysis technology, the present invention further optimizes the reaction efficiency (time is shortened by 30%), and realizes an acid-resistant nanofiltration membrane with a high cross-linking degree (>60%) and a narrow pore size distribution (half-peak width <0.6nm). By reacting quaternized PEI with multiple cross-linking sites of BTSC, high flux (40-50LMH) and high retention (Mg 2+ >95%), and showed good separation characteristics in the separation of positively charged dyes and monovalent salts. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : SEM photograph of the nanofiltration membrane prepared in Example 1. DETAILED DESCRIPTION

[0027] The embodiment of the present invention provides the following technical solution: an acid-resistant nanofiltration membrane based on epichlorohydrin-modified quaternized PEI, comprising a porous support layer and a polysulfonamide desalination layer;

[0028] The polysulfonamide desalting layer comprises PEI modified by quaternization of epichlorohydrin, polyamine, modifier, surfactant, polyacyl chloride and organic solvent.

[0029] Preferably, the amine substance in the aqueous solution is selected from epichlorohydrin-modified quaternized polyethyleneimine (PEI modification degree ≥ 60%).

[0030] Preferably, the mass percentage of the amine substance monomer in the aqueous solution is 0.5% to 2%, preferably 1.0% to 1.5%.

[0031] Preferably, the polyacid chloride is 1,3,5-benzenetrisulfonyl chloride (BTSC) or 1,3,6-naphthalenetrisulfonyl chloride (NTSC), with a purity of ≥98%.

[0032] Preferably, the mass fraction of the oil phase monomer is 0.1% to 0.3%, more preferably 0.2% to 0.25%.

[0033] Preferably, the aqueous phase additive comprises a composite system of sodium dodecyl sulfate (SDS, 1-3 g / L) and polyvinyl pyrrolidone (PVP, 0.1-0.5 g / L) for regulating solution viscosity and film formation uniformity.

[0034] Preferably, the oil phase solvent is n-hexane or isododecane (boiling point 180-220° C.), and the water content of the solvent is ≤50 ppm.

[0035] Preferably, the support base is made of polyester non-woven fabric (weight 80-120g / m 2 ) is composited with polyethersulfone (PES) ultrafiltration layer, and the average pore size of the base membrane is 10 to 30 nm.

[0036] The present invention also provides a method for preparing the above-mentioned acid-resistant nanofiltration membrane based on epichlorohydrin-modified quaternized PEI, comprising the following steps:

[0037] The polyethersulfone (PES) or polysulfone (PSF) based membrane (molecular weight cut-off 10-50 kDa) was immersed in deionized water and vacuum treated for 8-12 hours to remove bubbles in the pores, and then the residual moisture on the surface was purged with nitrogen.

[0038] Dissolve polyethyleneimine (PEI, molecular weight 2000-50000Da) in deionized water, add epichlorohydrin (5-20g / L) and ionic liquid catalyst [Bmim]BF4 (0.5-3g / L), and react at 70°C with stirring for 4-5 hours to obtain a quaternized PEI solution;

[0039] Immerse the pretreated base film in a sodium dodecyl sulfate (SDS) aqueous solution (0.01-1 g / L) for 5-30 minutes, then remove the residual liquid on the surface with an air knife or roller.

[0040] Immerse in a quaternized PEI solution (8-30 g / L) for 5-30 minutes, pour out the solution, and dry naturally at room temperature until no solution remains on the surface, forming a uniform pre-crosslinked layer;

[0041] Coat with 1,3,5-benzenetrisulfonyl chloride (BTSC) n-hexane solution (0.01-0.6% w / v), react at room temperature (25±2° C.) for 1-5 minutes, and control the reaction humidity to ≤50%.

[0042] The membrane was transferred to an oven at 80°C for heat treatment for 5 to 30 minutes, then rinsed three times with deionized water and stored in an ice water bath at 3 to 5°C for at least 12 hours.

[0043] The nanofiltration membranes prepared in the following examples require a pretreatment step before preparation, and a post-treatment step after preparation. The steps are:

[0044] Basement membrane pretreatment:

[0045] Polyethersulfone (PES) ultrafiltration membrane (molecular weight cut-off 30kDa, average pore size 20nm) was selected, and the supporting layer was polyester non-woven fabric (weight 100g / m 2 Immerse the base membrane in deionized water and evacuate at -0.1 MPa for 10 hours to completely remove any bubbles in the pores. Purge the membrane surface with high-purity nitrogen (flow rate 10 L / min) for 30 seconds to ensure no residual moisture.

[0046] Post-processing:

[0047] The membrane was transferred to an oven at 80°C for 30 minutes, rinsed three times with deionized water (conductivity ≤ 1 μS / cm), and stored in an ice-water bath at 4°C for 12 hours to inhibit hydrolysis of residual monomers.

[0048] Example 1

[0049] 30 g / L of polyethyleneimine (PEI, molecular weight 10,000 Da), 10 g / L of epichlorohydrin, and 1 g / L of the ionic liquid catalyst BmimBF4 were added to deionized water. The mixture was stirred at 70°C (300 rpm) for 4 hours, with the reaction vessel sealed to prevent solvent evaporation. A polyethersulfone (PES) membrane was used as the support layer of the composite membrane, and the PES membrane surface was activated with 0.1 wt% sodium dodecyl sulfate (SDS). The aqueous solution was poured onto the PES membrane, allowed to stand for 10 minutes, and then discarded. The quaternized polyethyleneimine solution was then poured onto the SDS-activated membrane surface and allowed to stand for 10 minutes to allow the polyethyleneimine to be well dispersed. After discarding the solution, the membrane was allowed to air dry at room temperature until no solution remained on the surface. Then, a 0.3% wt n-hexane solution of 1,3,5-benzenetrisulfonyl chloride was poured onto the membrane surface for cross-linking for 1 minute, and then the solution was poured out. The membrane was placed in an 80° C. oven for 5 minutes to obtain a polyethyleneimine / sulfonyl chloride layer.

[0050] Example 2

[0051] 30 g / L of polyethyleneimine (PEI, molecular weight 10,000 Da), 15 g / L of epichlorohydrin, and 1 g / L of the ionic liquid catalyst BmimBF4 were added to deionized water. The mixture was stirred at 70°C (300 rpm) for 4 hours, with the reaction vessel sealed to prevent solvent evaporation. A polyethersulfone (PES) membrane was used as the support layer of the composite membrane, and the PES membrane surface was activated with 0.1 wt% sodium dodecyl sulfate (SDS). The aqueous solution was poured onto the PES membrane, allowed to stand for 10 minutes, and then discarded. The quaternized polyethyleneimine solution was then poured onto the SDS-activated membrane surface and allowed to stand for 10 minutes to allow the polyethyleneimine to be well dispersed. After discarding the solution, the membrane was allowed to air dry at room temperature until no solution remained on the surface. Then, a 0.3% wt n-hexane solution of 1,3,5-benzenetrisulfonyl chloride was poured onto the membrane surface for cross-linking for 1 minute, and then the solution was poured out. The membrane was placed in an 80° C. oven for 5 minutes to obtain a polyethyleneimine / sulfonyl chloride layer.

[0052] Comparative Example 1

[0053] 30 g / L of polyethyleneimine (PEI, molecular weight 10,000 Da) was added to deionized water. The mixture was stirred at 70°C (300 rpm) for 4 hours. A polyethersulfone (PES) membrane was used as the support layer of the composite membrane, and the PES membrane surface was activated with 0.1 wt% sodium dodecyl sulfate (SDS). The aqueous solution was poured onto the PES membrane and allowed to stand for 10 minutes before being discarded. The quaternized polyethyleneimine solution was then poured onto the SDS-activated membrane surface and allowed to stand for 10 minutes to allow the polyethyleneimine to disperse well on the membrane surface. After discarding the solution, the membrane was allowed to dry naturally at room temperature until no solution remained on the surface. A 0.3% wt% 1,3,5-benzenetrisulfonyl chloride solution in n-hexane was then poured onto the membrane surface for crosslinking for 1 minute, after which the solution was discarded and placed in an 80°C oven for 5 minutes to form the polyethyleneimine / sulfonyl chloride layer.

[0054] Comparative Example 2

[0055] 30 g / L of polyethyleneimine (PEI, molecular weight 10,000 Da), 10 g / L of chloromethyl ethylene oxide, and 1 g / L of the ionic liquid catalyst BmimBF4 were added to deionized water. The mixture was stirred at 70°C (300 rpm) for 4 hours, with the reaction vessel sealed to prevent solvent evaporation. A polyethersulfone (PES) membrane was used as the support layer of the composite membrane, and the PES membrane surface was activated with 0.1 wt% sodium dodecyl sulfate (SDS). The aqueous solution was poured onto the PES membrane, allowed to stand for 10 minutes, and then discarded. The quaternized polyethyleneimine solution was then poured onto the SDS-activated membrane surface and allowed to stand for 10 minutes to allow the polyethyleneimine to disperse well on the membrane surface. After discarding the solution, the membrane was allowed to air dry at room temperature until no solution remained on the surface. Then, a 0.3% wt n-hexane solution of 1,3,5-benzenetrisulfonyl chloride was poured onto the membrane surface for cross-linking for 1 minute, and then the solution was poured out. The membrane was placed in an 80° C. oven for 5 minutes to obtain a polyethyleneimine / sulfonyl chloride layer.

[0056] The retention performance and water permeability of 2000ppm MgSO4 solution were tested at an operating temperature of 25℃ and an operating pressure of 0.7MPa.

[0057] Table 1 Performance test results of acid-resistant nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2

[0058]

[0059] The acid-resistant nanofiltration membrane obtained in the embodiment was immersed in a 20% sulfuric acid solution for 14 days, then washed three times with clean water, and the membrane's retention rate and permeability to magnesium sulfate were tested at an operating temperature of 25°C and an operating pressure of 0.7 MPa.

[0060] Table 2 Acid resistance test results of acid-resistant nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2

[0061] Example <![CDATA[MgSO4 rejection rate (%)]]> Flux (LMH) Example 1 93,2 45 Example 2 94.7 39 Comparative Example 1 80.3 64 Comparative Example 2 79.3 44

[0062] The experimental data of Table 1 and Table 2 prove that the nanofiltration membrane (Example 1, 2) prepared by the epoxychloropropane modified quaternized PEI proposed by the present invention, compared to the unmodified PEI membrane (Comparative Example 1) and the membrane modified with traditional reagents (Comparative Example 2), not only has a higher charge density and divalent salt retention rate in the initial state, but more importantly, after long-term immersion in a strong acid solution, its retention performance and structural stability are far superior to the comparative example. Utilize epoxychloropropane to react with PEI to construct a quaternary ammonium structure with a permanent positive charge that is not affected by an acidic environment on the membrane surface. The data in Table 1 confirm that the quaternary ammonium modification has successfully and significantly improved the positive charge on the membrane surface (Zeta potential is increased from +12mV to more than +30mV), thereby significantly enhancing the electrostatic repulsion of positively charged metal ions (such as Mg) 2+ ) retention rate (from 88.4% to over 95%); and the data in Table 2 verify the stability of this chemical structure, that is, under strong acid attack, this permanent quaternary ammonium structure and polysulfonamide skeleton can resist hydrolysis, maintain the integrity and separation performance of the membrane, and avoid the defect of traditional amine groups that are easily hydrolyzed after protonation, thereby achieving acid resistance and high retention rate.

[0063] A mixed aqueous solution of 100 ppm methylene blue (MB) and 1000 ppm NaCl was used for separation experiments at an operating temperature of 25°C and an operating pressure of 0.7 MPa. Deionized water was first pre-pressed at 0.7 MPa for 30 minutes until the flux stabilized. Subsequently, the feed solution was replaced with the methylene blue / NaCl mixed solution prepared above and operated continuously for 60 minutes under the same operating conditions. Samples of the permeate and feed solution were collected, and the concentration of methylene blue was measured using a UV-visible spectrophotometer, and the concentration of NaCl was measured using a conductivity meter to calculate the retention rate.

[0064] Table 3 Dye / salt separation results of acid-resistant nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2

[0065]

[0066] The nanofiltration membranes prepared in Examples 1 and 2 showed an extremely high retention rate of over 98% for positively charged methylene blue (MB). It can be seen that through the quaternization modification with epichlorohydrin, a high-density, permanent positive charge (quaternary ammonium structure) that is not affected by pH is constructed on the membrane surface. These strong positive charge layers produce a strong electrostatic repulsion between the MB dye molecules that are also positively charged, thereby effectively blocking the dye molecules on the surface of the membrane and achieving efficient removal. The unmodified comparative example 1 and the comparative example 2 modified with traditional reagents have lower retention rates for MB; this is because the weak or unstable positive charge on the surface is not enough to generate a strong enough electrostatic repulsion force, resulting in a large amount of cationic dyes penetrating the membrane, resulting in poor separation effect. At the same time, the retention rate of all membranes for NaCl is low, which shows that the nanofiltration membrane of the present invention can allow most inorganic salts to pass through while efficiently intercepting the target cationic pollutants.

Claims

1. A method for preparing an acid-resistant nanofiltration membrane, characterized in that: The following steps are involved: a) reacting polyethyleneimine (PEI) and epichlorohydrin in an aqueous solution in the presence of an ionic liquid catalyst to prepare a quaternized polyethyleneimine aqueous solution; b) coating the quaternized polyethyleneimine aqueous solution on a porous supporting base membrane; c) contacting the supporting base membrane with the aqueous solution attached thereto with an organic solution containing polysulfonyl chloride to perform interfacial polymerization reaction to form a polysulfonamide desalting layer; d) heat-treating the formed nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that The reaction conditions in step a) are: The molecular weight of polyethyleneimine is 2000-50000Da; The concentration of epichlorohydrin is 5-20g / L; The reaction temperature is 70°C and the reaction time is 4-5 hours.

3. The preparation method according to claim 1 or 2, characterized in that The ionic liquid catalyst is [Bmim]BF4, and its concentration in the aqueous solution is 0.5-3 g / L.

4. The preparation method according to claim 1, characterized in that In the quaternized polyethyleneimine aqueous solution, the concentration of the quaternized polyethyleneimine is 8-30 g / L.

5. The preparation method according to claim 1, characterized in that The aqueous phase solution further comprises a surfactant, which is sodium dodecyl sulfate (SDS) with a concentration of 0.01-3 g / L.

6. The preparation method according to claim 1, characterized in that The polysulfonyl chloride in the organic phase solution is 1,3,5-benzenetrisulfonyl chloride (BTSC) or 1,3,6-naphthalenetrisulfonyl chloride (NTSC), and its concentration in the organic solvent is 0.01%-0.6% (w / v).

7. The preparation method according to claim 1, characterized in that The interfacial polymerization reaction time in step c) is 1-5 minutes.

8. The preparation method according to claim 1, characterized in that The heat treatment conditions in step d) are: treatment at 60-100° C. for 5-30 minutes; the porous supporting base membrane is a polyethersulfone (PES) or polysulfone (PSF) base membrane with a molecular weight cutoff of 10-50 kDa.

9. Use of the nanofiltration membrane obtained by the preparation method according to claim 1 in ion separation in solution.

10. The use according to claim 9, characterized in that The nanofiltration membrane is used for intercepting divalent or multivalent cations, or for separating positively charged dyes / monovalent salts; the nanofiltration membrane is used for improving acid resistance.

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