Acid- and alkali-resistant nanofiltration membrane and preparation method thereof

Abstract

The present invention relates to the technical field of water treatment membranes, and in particular to an acid- and alkali-resistant nanofiltration membrane and a preparation method thereof. The preparation method of the acid- and alkali-resistant nanofiltration membrane comprises: soaking a base membrane in a first grafting solution, taking out and removing excess solution, soaking it in a second grafting solution for cross-linking grafting reaction, and obtaining an acid- and alkali-resistant nanofiltration membrane after solidification; the first grafting solution comprises a polymer, a surfactant, an acid absorbent, and water; the polymer is at least one of a polyamine polymer and a sulfonic acid polymer; the polyamine polymer is at least one of polyethyleneimine, chitosan, polyaniline, and polypropyleneimine; the sulfonic acid polymer is at least one of polystyrenesulfonic acid, polyethylenesulfonic acid, and poly(4-styrenesulfonic acid); the second grafting solution comprises a nitrogen-containing heterocyclic compound, a phase transfer catalyst, and a solvent. The acid- and alkali-resistant nanofiltration membrane prepared by the present invention has excellent acid and alkali resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment membranes, and in particular to an acid- and alkali-resistant nanofiltration membrane and a preparation method thereof. Background Art

[0002] Semiconductor industry, mine drainage, metal pickling and new energy battery recycling and other fields will produce a large amount of strong acid wastewater, which usually contains about 20% by mass of hydrochloric acid, nitric acid, sulfuric acid, etc., as well as metal ions such as Li + 、Co 2+ 、Fe 2+ , metal compounds, etc. The use of strong acid resistant nanofiltration membrane can recover metal ions, concentrate strong acids, and realize resource utilization.

[0003] Many chemical reactions (such as esterification and saponification) and pharmaceutical processes need to be carried out under strong alkaline conditions. Traditional nanofiltration membranes are easily corroded or swelled, resulting in reduced separation efficiency. Strong alkali-resistant membranes can operate stably in high pH environments, improving process reliability. Industrial wastewater (such as electroplating, printing and dyeing, and battery manufacturing) often contains strong alkaline components (such as NaOH), and heavy metals or organic pollutants need to be efficiently separated to meet emission standards. Traditional membrane materials are difficult to withstand alkaline corrosion for a long time, and there is an urgent need to develop more durable membranes. In the recycling of lithium-ion batteries in the new energy field, alkaline leachate treatment needs to separate metals such as cobalt and nickel; in alkaline water electrolysis hydrogen production systems, the membrane's alkali stability affects efficiency. Strong alkali-resistant membranes can improve the sustainability of these processes.

[0004] Existing conventional nanofiltration membranes are aliphatic polyamide structures, which are prone to hydrolysis, swelling, chemical degradation or structural collapse under strong acid or strong base conditions, resulting in a sharp decline in separation performance. Frequent replacement increases costs, making the development of alkali-resistant separation membranes inevitable.

[0005] In recent years, both the scientific research community and the industrial community have proposed a series of methods for preparing acid-resistant nanofiltration membranes, such as replacing the original polyester non-woven fabric with polypropylene (PP), polyethylene (PE), or polypropylene / polyethylene blended (PP / PE) non-woven fabrics; replacing polysulfone with polyethersulfone; and selecting monomers such as sulfonyl chloride, isocyanate, and cyanuric chloride as interfacial polymerization oil phase monomers to prepare strong acid-resistant nanofiltration membranes. However, this type of acid-resistant nanofiltration membrane has the problems of low water production and low desalination rate. Y Cao et al. introduced sulfonamide monomers to prepare amide-sulfonamide nanofiltration membranes. The stable sulfonamide structure can prevent acid from attacking the separation layer, but the water production is relatively low, at 12.4L / (m 2h), i.e. 7.3 GFD (Y Cao, Jq Luo, Cl Chen, Yh Wan. Highly permeable acid-resistant nanofiltration membrane based on a novel sulfonamide aqueous monomer for efficient acidic wastewater treatment, Chemical Engineering Journal 425 (2021) 131791). Kah P. Lee et al. prepared an acid-resistant nanofiltration membrane by using cyanuric chloride as the oil phase monomer, but its flux was extremely low, at 1.5 L / (m 2 h), i.e., 0.88 GFD, and a relatively high NaCl rejection rate of 85% (Kah P. Lee, Nieck E. Benes. Interfacial polymerization of cyanuric chloride and monomeric amines: pH resistant thin film composite polyamine nanofiltration membranes, Journal of Membrane Science 523 (2017) 487-496). In addition, existing technologies involve coating PES or PSf-based membranes with acid-resistant polymers and preparing acid-resistant coatings through layer-by-layer self-assembly. However, due to the lack of a separation layer, these membranes generally suffer from low divalent salt rejection rates and have not been commercialized. Commercially available acid-resistant nanofiltration membranes, such as Suez's Duracid NF series and Times Wharton's Acidstab NF series, also suffer from low water yields.

[0006] In recent years, there have been reports on methods for preparing alkali-resistant nanofiltration membranes, but compared to acid-resistant nanofiltration membranes, there has been less research on alkali-resistant nanofiltration membranes. For example, patent application document CN201711193238.0, "A Method for Preparing a High-Performance Alkali-Resistant Nanofiltration Membrane," prepares an alkali-resistant base membrane and simultaneously soaks the nanofiltration membrane in an activation solution to improve alkali resistance. Patent application document CN202210896583.5, "An Alkali-Resistant Nanofiltration Membrane, Its Preparation Method, and Application," adds aminated lignin to the aqueous phase. Lignin contains a large number of hydroxyl groups, which easily form hydrogen bonds with polyamide, amino groups, and the like, or with itself. This improves the overall thermal stability of the polyamide, increases the energy barrier to alkaline hydrolysis, and enhances its alkali resistance. However, swelling and degradation issues persist after prolonged use in strong alkali.

[0007] Existing acid-resistant or alkali-resistant nanofiltration membranes have low water production, require high-pressure operation, high energy consumption, and low treatment efficiency; after long-term operation, the separation performance decreases and the acid and alkali resistance is insufficient. Summary of the Invention

[0008] In view of this, the technical problem to be solved by the present invention is to provide an acid- and alkali-resistant nanofiltration membrane and a preparation method thereof. The acid- and alkali-resistant nanofiltration membrane prepared by the present invention has excellent acid and alkali resistance.

[0009] The present invention provides a method for preparing an acid- and alkali-resistant nanofiltration membrane, comprising the following steps:

[0010] The base membrane is immersed in a first grafting solution, taken out to remove excess solution, and then immersed in a second grafting solution for cross-linking grafting reaction, and after curing, an acid- and alkali-resistant nanofiltration membrane is obtained;

[0011] The first grafting solution comprises a polymer, a surfactant, an acid absorbent and water; the polymer is at least one of a polyamine polymer and a sulfonic acid polymer; the polyamine polymer is at least one of polyethyleneimine, chitosan, polyaniline and polypropyleneimine; the sulfonic acid polymer is at least one of polystyrenesulfonic acid, polyethylenesulfonic acid and poly(4-styrenesulfonic acid);

[0012] The second grafting solution includes a nitrogen-containing heterocyclic compound, a phase transfer catalyst and a solvent.

[0013] Preferably, in the first grafting solution, the mass content of the polymer is 3% to 10%, the mass content of the surfactant is 0.05% to 0.1%, and the mass content of the acid absorbent is 0.1% to 0.3%.

[0014] Preferably, the second grafting solution comprises a nitrogen-containing heterocyclic compound, a phase transfer catalyst and a solvent; in the second grafting solution, the mass content of the nitrogen-containing heterocyclic compound is 0.5% to 3%, and the mass content of the phase transfer catalyst is 0.05% to 0.1%.

[0015] Preferably, the nitrogen-containing heterocyclic compound is at least one of p-phenylene diisocyanate, toluene diisocyanate, polycyanurate, cyanuric chloride, trimethoxytriazine and 2,4,6-tris(2,4-dihydroxyphenyl)-1,3,5-triazine.

[0016] Preferably, the phase transfer catalyst is at least one of tetrabutylammonium bromide, benzyltriethylammonium chloride and tetrabutylphosphine bromide;

[0017] The solvent is selected from at least one of acetonitrile and acetone.

[0018] Preferably, the acid absorbent is sodium hydroxide.

[0019] Preferably, after the curing, the process further comprises post-processing; the post-processing method comprises:

[0020] The membrane was sequentially immersed in an isopropyl alcohol solution with a mass concentration of 5% to 10%, RO water, a sulfuric acid aqueous solution with a mass concentration of 20%, and a glycerol solution with a mass concentration of 1% to 5%, and then taken out and dried to obtain an acid- and alkali-resistant nanofiltration membrane.

[0021] Preferably, the post-processing method comprises:

[0022] Soak in a 5% to 10% isopropanol solution for 5 to 20 minutes, take out and soak in RO water for 4 to 6 minutes, take out and soak in a 20% sulfuric acid aqueous solution at 55 to 65°C for 5 to 20 minutes, take out and soak in a 1% to 5% glycerol solution for 1 to 5 minutes, take out and dry to obtain an acid and alkali resistant nanofiltration membrane.

[0023] Preferably, the soaking time in the first grafting solution is 4 to 6 minutes;

[0024] The soaking time in the second grafting solution is 4 to 6 minutes.

[0025] The present invention also provides an acid- and alkali-resistant nanofiltration membrane prepared by the preparation method described above.

[0026] In the present invention, the first grafting solution for cross-linking and grafting to prepare an acid- and alkali-resistant nanofiltration membrane contains at least one of a polyamine polymer and a sulfonic acid polymer. The second grafting solution for cross-linking and grafting to prepare an acid- and alkali-resistant nanofiltration membrane contains a nitrogen-containing heterocyclic compound, and the solvent is a polar non-aqueous solvent. By employing a cross-linking and grafting method rather than interfacial polymerization between water and oil phases, the problem of low solubility and low concentration of nitrogen-containing heterocyclic compounds in various oil-phase solvents can be resolved. The cross-linking and grafting reaction is promoted by fully dissolving the nitrogen-containing heterocyclic compound in the polar non-aqueous solvent. The present invention further involves soaking the prepared nanofiltration membrane in an organic alcohol and an inorganic acid, filling it with glycerol, and then drying it. Unreacted residues are then swelled and cleaned, and some small-chain polymer segments are removed with an inorganic acid to resolidify the pore structure. This significantly improves the initial flux of the nanofiltration membrane while maintaining strong acid and alkali resistance. Experimental results demonstrate that the acid- and alkali-resistant nanofiltration membrane prepared by the present invention exhibits superior acid and alkali resistance. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] The present invention provides a method for preparing an acid- and alkali-resistant nanofiltration membrane, comprising the following steps:

[0029] The base membrane is immersed in a first grafting solution, taken out to remove excess solution, and then immersed in a second grafting solution for cross-linking grafting reaction, and after curing, an acid- and alkali-resistant nanofiltration membrane is obtained;

[0030] The first grafting solution comprises a polymer, a surfactant, an acid absorbent and water; the polymer is at least one of a polyamine polymer and a sulfonic acid polymer; the polyamine polymer is at least one of polyethyleneimine, chitosan, polyaniline and polypropyleneimine; the sulfonic acid polymer is at least one of polystyrenesulfonic acid, polyethylenesulfonic acid and poly(4-styrenesulfonic acid);

[0031] The second grafting solution includes a nitrogen-containing heterocyclic compound, a phase transfer catalyst and a solvent.

[0032] About basement membrane:

[0033] In some embodiments of the present invention, the method for preparing the basement membrane comprises the following steps:

[0034] Casting the casting solution onto a non-woven fabric, solidifying it into a film through phase inversion to obtain a base film;

[0035] The casting solution is a polysulfone solution or a polyethersulfone solution. The solvent of the casting solution is at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide, and N-methylpyrrolidone. The mass concentration of the casting solution is 14% to 18%, for example, 16%. The casting solution is obtained by dissolving polysulfone particles in N,N-dimethylformamide (DMF), mixing, and standing to degas. The mixing is performed by stirring at a temperature of 65 to 75°C, for example, 70°C, at a rotation speed of 750 to 850 rpm, for example, 800 rpm, and for a period of 2 to 4 hours, for example, 3 hours. The standing to degas period is 12 to 36 hours, for example, 24 hours.

[0036] The non-woven fabric is one of polyester, polypropylene, polyethylene, and polypropylene-polyethylene blended non-woven fabric.

[0037] The phase inversion method may be soaking in RO water.

[0038] After the film is solidified, the process further includes: cleaning and drying.

[0039] The base film has a thickness of 130-160 μm, such as 130-140 μm, specifically 135 μm.

[0040] About the first grafting solution:

[0041] The first grafting solution includes a polymer, a surfactant, an acid absorbent and water; the polymer includes at least one of a polyamine polymer and a sulfonic acid polymer; in the first grafting solution, the mass content of the polymer is 3% to 10%, such as 5% and 8%; the mass content of the surfactant is 0.05% to 0.1%, such as 0.1% and 0.05%; and the mass content of the acid absorbent is 0.1% to 0.3%, such as 0.3%.

[0042] In some embodiments of the present invention, the polyamine polymer is at least one of polyethyleneimine, chitosan, polyaniline, and polypropyleneimine. The sulfonic acid polymer is at least one of polystyrenesulfonic acid, polyethylenesulfonic acid, and poly(4-styrenesulfonic acid). The surfactant is selected from sodium dodecylsulfonate. The acid absorbent is sodium hydroxide. The water is RO water.

[0043] The present invention has no particular limitation on the preparation method of the first grafting solution. In some embodiments of the present invention, the preparation method of the first grafting solution includes the following steps: stirring and mixing a polymer, a surfactant, an acid absorbent and water to obtain a first grafting solution.

[0044] About the second grafting solution:

[0045] The second grafting solution includes a nitrogen-containing heterocyclic compound, a phase transfer catalyst and a solvent; in the second grafting solution, the mass content of the nitrogen-containing heterocyclic compound is 0.5% to 3%, such as 2% or 3%; the mass content of the phase transfer catalyst is 0.05% to 0.1%, such as 0.1%.

[0046] In some embodiments of the present invention, the nitrogen-containing heterocyclic compound is at least one of p-phenylene diisocyanate, toluene diisocyanate, polycyanurate, cyanuric chloride, trimethoxytriazine, and 2,4,6-tris(2,4-dihydroxyphenyl)-1,3,5-triazine. The phase transfer catalyst is at least one of tetrabutylammonium bromide (TBAB), benzyltriethylammonium chloride (TEBAC), and tetrabutylphosphonium bromide (TBPP). The solvent is a polar non-aqueous solvent selected from at least one of acetonitrile and acetone.

[0047] The present invention has no particular limitation on the preparation method of the second grafting solution. In some embodiments of the present invention, the preparation method of the second grafting solution comprises the following steps: stirring and mixing a nitrogen-containing heterocyclic compound, a phase transfer catalyst, and a solvent to obtain a second grafting solution.

[0048] The present invention immerses the base membrane in a first grafting solution, takes out and removes excess solution, and then immerses it in a second grafting solution for cross-linking grafting reaction. After solidification, an acid and alkali resistant nanofiltration membrane is obtained.

[0049] In some embodiments of the present invention, the soaking time in the first grafting solution is 4 to 6 minutes, such as 5 minutes.

[0050] In some embodiments of the present invention, the method for removing excess solution is to place the film on a squeeze rubber roller with the assistance of air blowing.

[0051] In some embodiments of the present invention, the soaking time in the second grafting solution is 4 to 6 minutes, such as 5 minutes.

[0052] In some embodiments of the present invention, the curing method is drying at a temperature of 55-65° C., such as 60° C., and for 4-6 minutes, such as 5 minutes.

[0053] In some embodiments of the present invention, after curing, further comprising: post-processing. The post-processing method includes:

[0054] The membrane was sequentially immersed in an isopropyl alcohol solution with a mass concentration of 5% to 10%, RO water, a sulfuric acid aqueous solution with a mass concentration of 20%, and a glycerol solution with a mass concentration of 1% to 5%, and then taken out and dried to obtain an acid- and alkali-resistant nanofiltration membrane.

[0055] Specifically, they include:

[0056] Soak in a 5% to 10% isopropanol solution for 5 to 20 minutes, take out and soak in RO water for 4 to 6 minutes, take out and soak in a 20% sulfuric acid aqueous solution at 55 to 65°C for 5 to 20 minutes, take out and soak in a 1% to 5% glycerol solution for 1 to 5 minutes, take out and dry to obtain an acid and alkali resistant nanofiltration membrane.

[0057] The drying temperature is 55-65° C., such as 60° C., and the drying time is 2-4 min, such as 3 min.

[0058] The present invention also provides an acid- and alkali-resistant nanofiltration membrane prepared by the preparation method described above.

[0059] In the present invention, the monomer type contained in the formula of the first grafting solution for preparing the acid-resistant and alkali-resistant nanofiltration membrane by cross-linking and grafting is at least one of a polyamine polymer and a sulfonic acid polymer. The monomer type contained in the formula of the second grafting solution for preparing the acid-resistant and alkali-resistant nanofiltration membrane by cross-linking and grafting is a nitrogen-containing heterocyclic compound, and the solvent is a polar non-aqueous solvent. The cross-linking and grafting method is adopted instead of the interfacial polymerization of the water-oil phase, which can solve the problem of low solubility and low concentration of nitrogen-containing heterocyclic compounds in various oil-phase solvents. The nitrogen-containing heterocyclic compounds are fully soluble in the polar non-aqueous good solvent, which can promote the degree of cross-linking and grafting reaction.

[0060] By soaking the prepared nanofiltration membrane in organic alcohol and inorganic acid, filling it with glycerol and then drying it, the unreacted residues are swollen and cleaned, and some small chain segment polymers are washed away with inorganic acid, and the pore structure is re-solidified. While not affecting the strong acid and alkali resistance, the initial flux of the nanofiltration membrane can be greatly improved.

[0061] The nanofiltration membrane prepared by the monomer and cross-linking grafting method adopted in the present invention has both strong acid and strong alkali resistance properties.

[0062] The present invention has no particular limitation on the sources of the raw materials used above, and they can be generally commercially available.

[0063] In order to further illustrate the present invention, an acid- and alkali-resistant nanofiltration membrane and a preparation method thereof provided by the present invention are described in detail below in conjunction with examples, but they should not be construed as limiting the scope of protection of the present invention.

[0064] In the Examples and Comparative Examples:

[0065] The preparation method of the basement membrane is as follows:

[0066] Dissolve the polysulfone particles in DMF, stir and mix at 70°C and 800 rpm for 3 hours, and let it stand for 24 hours to obtain a uniform transparent casting solution with a mass concentration of 16%;

[0067] The polyester non-woven fabric was fixed on the glass plate of the scraping machine, and the above-mentioned casting liquid was poured on the non-woven fabric. The scraper speed was set at 5m / min, and the film was scraped. The glass plate and the membrane were immersed in RO water for 1 minute to complete the phase inversion, and the RO water was replaced 3 times. After drying, a polysulfone-based membrane with a thickness of 135 μm was obtained.

[0068] Example 1

[0069] 1) Prepare the first grafting solution:

[0070] Dissolve polyethyleneimine in RO water, add sodium lauryl sulfate and sodium hydroxide, and stir to obtain a first grafting solution. The first grafting solution contains 5% polyethyleneimine by weight, 0.1% sodium lauryl sulfate by weight, and 0.3% sodium hydroxide by weight.

[0071] 2) Prepare the second grafting solution:

[0072] Dissolve cyanuric chloride in acetonitrile, add tetrabutylammonium bromide (TBAB), and stir to obtain a second grafting solution. The second grafting solution contains 2% cyanuric chloride and 0.1% tetrabutylammonium bromide by weight.

[0073] 3) The polysulfone-based membrane was immersed in the first grafting solution for 5 minutes, taken out, placed on an extrusion rubber roller and assisted with blowing to remove excess solution, and then immersed in the second grafting solution for 5 minutes for cross-linking grafting reaction, and then placed in a 60°C oven for 5 minutes for curing to obtain an acid and alkali resistant nanofiltration membrane.

[0074] Example 2

[0075] 1) Same as Example 1;

[0076] 2) Same as Example 1;

[0077] 3) Same as Example 1;

[0078] 4) Post-processing the nanofiltration membrane obtained in step 3):

[0079] The nanofiltration membrane was soaked in a 5% mass concentration isopropanol solution for 10 min, taken out and soaked in RO water for 5 min, taken out and soaked in a 60°C, mass concentration 20% sulfuric acid aqueous solution for 10 min, taken out and soaked in a 3% mass concentration glycerol solution for 1 min, taken out and dried to obtain an acid and alkali resistant nanofiltration membrane.

[0080] Example 3

[0081] 1) Prepare the first grafting solution:

[0082] Polyethylene sulfonic acid was dissolved in RO water, sodium lauryl sulfonate and sodium hydroxide were added, and stirred to obtain a first grafting solution. In the first grafting solution, the mass content of polyethylene sulfonic acid was 8%, the mass content of sodium lauryl sulfonate was 0.05%, and the mass content of sodium hydroxide was 0.3%.

[0083] 2) Prepare the second grafting solution:

[0084] Dissolve cyanuric chloride in acetonitrile, add tetrabutylammonium bromide (TBAB), and stir to obtain a second grafting solution. The second grafting solution contains 3% cyanuric chloride and 0.1% tetrabutylammonium bromide by weight.

[0085] 3) The polysulfone-based membrane was immersed in the first grafting solution for 5 minutes, taken out, placed on an extruded rubber roller and assisted with blowing to remove excess solution, and then immersed in the second grafting solution for 5 minutes for cross-linking grafting reaction. The membrane was then placed in a 60°C oven for 5 minutes for curing to obtain a nanofiltration membrane.

[0086] 4) Post-processing:

[0087] The nanofiltration membrane was soaked in a 5% mass concentration isopropanol solution for 10 min, taken out and soaked in RO water for 5 min, taken out and soaked in a 60°C, mass concentration 20% sulfuric acid aqueous solution for 10 min, taken out and soaked in a 3% mass concentration glycerol solution for 1 min, taken out and dried to obtain an acid and alkali resistant nanofiltration membrane.

[0088] Example 4

[0089] The difference from Example 2 is:

[0090] 1) Prepare the first grafting solution:

[0091] Polyethylene sulfonic acid was dissolved in RO water, sodium lauryl sulfonate and sodium hydroxide were added, and stirred to obtain a first grafting solution. In the first grafting solution, the mass content of polyethylene sulfonic acid was 8%, the mass content of sodium lauryl sulfonate was 0.05%, and the mass content of sodium hydroxide was 0.3%.

[0092] 2) Prepare the second grafting solution:

[0093] Dissolve p-phenylene diisocyanate in acetonitrile, add tetrabutylammonium bromide (TBAB), and stir to obtain a second grafting solution. The second grafting solution contains 2% p-phenylene diisocyanate and 0.1% tetrabutylammonium bromide by weight.

[0094] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0095] Example 5

[0096] The difference from Example 2 is:

[0097] In the first grafting solution:

[0098] Replace polyethyleneimine with chitosan.

[0099] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0100] Example 6

[0101] The difference from Example 2 is:

[0102] In the first grafting solution:

[0103] Replace polyethyleneimine with polyaniline.

[0104] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0105] Example 7

[0106] The difference from Example 2 is:

[0107] In the first grafting solution:

[0108] Polyethyleneimine was replaced by poly(4-styrenesulfonic acid).

[0109] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0110] Example 8

[0111] The difference from Example 2 is:

[0112] In the second grafting solution:

[0113] Replace cyanuric chloride with polycyanurate.

[0114] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0115] Example 9

[0116] The difference from Example 2 is:

[0117] In the second grafting solution:

[0118] Cyanuric chloride was replaced by trimethoxytriazine.

[0119] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0120] Example 10

[0121] The difference from Example 2 is:

[0122] In the second grafting solution:

[0123] Tetrabutylammonium bromide (TBAB) was replaced with benzyltriethylammonium chloride (TEBAC).

[0124] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0125] Example 11

[0126] The difference from Example 2 is:

[0127] In the second grafting solution:

[0128] Tetrabutylammonium bromide (TBAB) was replaced with tetrabutylphosphonium bromide (TBPP).

[0129] The remaining steps are the same as in Example 2 to prepare an acid- and alkali-resistant nanofiltration membrane.

[0130] Comparative Example 1

[0131] 1) Prepare aqueous solution:

[0132] Dissolve piperazine in RO water and stir to obtain an aqueous solution having a piperazine content of 1% by weight.

[0133] 2) Prepare oil phase solution:

[0134] Dissolve trimesoyl chloride in n-hexane and stir to obtain an oil phase solution. The oil phase solution contains trimesoyl chloride at a mass content of 0.5%.

[0135] 3) Soak the front side of the base membrane in the aqueous solution for 30 seconds, take it out, place the membrane on an extrusion rubber roller and blow air to remove excess solution, then soak the front side in the oil solution for reaction for 10 seconds, and then place it in a 60°C oven for 3 minutes to cure to obtain a nanofiltration membrane.

[0136] Comparative Example 2

[0137] 1) Prepare aqueous solution:

[0138] Dissolve polyethyleneimine in RO water, add sodium lauryl sulfate, and stir to obtain an aqueous solution. The aqueous solution contains 2% polyethyleneimine by weight and 0.5% sodium lauryl sulfate by weight.

[0139] 2) Prepare oil phase solution:

[0140] Dissolve cyanuric chloride in n-hexane and stir to obtain an oil phase solution, wherein the mass content of cyanuric chloride in the oil phase solution is 0.05%.

[0141] 3) Soak the front side of the base membrane in the aqueous solution for 30 seconds, take it out, place the membrane on an extrusion rubber roller and blow air to remove excess solution, then soak the front side in the oil solution for reaction for 10 seconds, and then place it in a 60°C oven for 3 minutes to cure to obtain a nanofiltration membrane.

[0142] Comparative Example 3

[0143] 1) Prepare aqueous solution:

[0144] Dissolve polyetheramine D230 in RO water, add sodium lauryl sulfate, and stir to obtain an aqueous solution. In the aqueous solution, the mass content of polyetheramine D230 is 2%, and the mass content of sodium lauryl sulfate is 0.5%.

[0145] 2) Prepare oil phase solution:

[0146] Dissolve cyanuric chloride in n-hexane and stir to obtain an oil phase solution, wherein the mass content of cyanuric chloride in the oil phase solution is 0.05%.

[0147] 3) Soak the front side of the base membrane in the aqueous solution for 30 seconds, take it out, place the membrane on an extrusion rubber roller and blow air to remove excess solution, then soak the front side in the oil solution for reaction for 10 seconds, and then place it in a 60°C oven for 3 minutes to cure to obtain a nanofiltration membrane.

[0148] Comparative Example 4

[0149] The difference from Example 4 is:

[0150] The polyvinylsulfonic acid in the first grafting solution was replaced by piperazine.

[0151] The remaining steps were the same as those in Example 4 to prepare a nanofiltration membrane.

[0152] Comparative Example 5

[0153] The difference from Example 4 is:

[0154] The polyvinyl sulfonic acid in the first grafting solution was replaced by polyetheramine JEFFAMINE ED2003.

[0155] The remaining steps were the same as those in Example 4 to prepare a nanofiltration membrane.

[0156] Comparative Example 6

[0157] The difference from Example 4 is:

[0158] The polyvinyl sulfonic acid in the first grafting solution was replaced by m-phenylenediamine.

[0159] The remaining steps were the same as those in Example 4 to prepare a nanofiltration membrane.

[0160] Comparative Example 7

[0161] The difference from Example 4 is that the second grafting solution does not contain tetrabutylammonium bromide (TBAB):

[0162] 2) Prepare the second grafting solution:

[0163] Dissolve p-phenylene diisocyanate in acetonitrile and stir to obtain a second grafting solution. The mass content of p-phenylene diisocyanate in the second grafting solution is 2%.

[0164] The remaining steps were the same as those in Example 4 to prepare a nanofiltration membrane.

[0165] Comparative Example 8

[0166] The difference from Example 4 is that tetrabutylammonium bromide (TBAB) is replaced by tetrabutylammonium hydrogen sulfate (TBAHS):

[0167] 2) Prepare the second grafting solution:

[0168] Dissolve p-phenylene diisocyanate in acetonitrile, add TBAHS, and stir to obtain a second grafting solution. The second grafting solution contains 2% by weight of p-phenylene diisocyanate and 0.1% by weight of TBAHS.

[0169] The remaining steps were the same as those in Example 4 to prepare a nanofiltration membrane.

[0170] The nanofiltration membranes obtained in the comparative examples and examples were tested for initial performance, acid resistance, and alkali resistance. The test conditions were as follows: 2000 ppm magnesium sulfate as the test solution, a test pressure of 225 psi, and a test temperature of 25°C. The test results are shown in Tables 1 and 2.

[0171] Table 1 Performance test results of the nanofiltration membrane obtained in Example

[0172]

[0173] Table 2 Performance test results of nanofiltration membrane obtained in comparative example

[0174]

[0175] After acid and alkali soaking, the less the flux increases relative to the initial flux, the better the acid and alkali resistance is; after acid and alkali soaking, the less the desalination rate decreases relative to the initial desalination rate, the better the acid and alkali resistance is.

[0176] After calculation, the flux change rate and desalination rate change rate of the nanofiltration membranes obtained in the comparative example and the embodiment after acid and alkali soaking are shown in Table 3 and Table 4.

[0177] Table 3 Flux change rate and desalination rate change rate of the nanofiltration membrane obtained in Example after acid and alkali soaking

[0178]

[0179] Table 4 Flux change rate and desalination rate change rate of the nanofiltration membrane obtained in the comparative example after acid and alkali soaking

[0180]

[0181] From Tables 1 to 4, we can see that:

[0182] Example 1 uses a cross-linking grafting method to react polyethyleneimine and cyanuric chloride. Cyanuric chloride has good solubility in acetonitrile. 5wt% polyethyleneimine and 2wt% cyanuric chloride are fully cross-linked and grafted. The resulting nanofiltration membrane has an initial salt rejection rate of 98.2%, 97.1% after 90d of immersion in a 20wt% aqueous sulfuric acid solution, and 95.6% after 90d of immersion in a 20% NaOH aqueous solution. It has excellent strong acid and strong alkali resistance. Example 2 adds a post-processing step of pore expansion and stabilization on the basis of Example 1. The initial flux is increased to 18.7 GFD, the salt rejection rate is still 98%, and the strong acid and strong alkali resistance is comparable to that of Example 1. Example 3 uses polyethylene sulfonic acid instead of polyethyleneimine, and Example 4 uses paraphenylene diisocyanate instead of cyanuric chloride. Post-processing is used, and the membrane also has excellent initial performance and strong acid and strong alkali resistance.

[0183] The conventional polypiperazineamide nanofiltration membrane in Comparative Example 1 exhibited excellent initial flux and salt rejection, but lost its separation performance after immersion in a 20 wt% aqueous sulfuric acid solution (pH ≈ -0.4) for 4 hours and in a 20 wt% aqueous NaOH solution (pH ≈ 14.7) for 2 hours. Comparative Example 2 employed interfacial polymerization to react a polymer amine monomer with cyanuric chloride. However, due to the low solubility of cyanuric chloride in oil solvents, the concentration in n-hexane was 0.05 wt%, resulting in an incomplete reaction and an initial salt rejection of only 91.2%. After 90 days of immersion in a strong acid solution, the water yield increased slightly, but the salt rejection decreased to 81.3%. After 90 days of immersion in a strong alkaline solution, the salt rejection dropped to 78.5%. Comparative Example 3, a nanofiltration membrane prepared by reacting polyetheramine with cyanuric chloride, exhibited a higher initial flux than Comparative Example 2, but its resistance to strong acids and bases was significantly impaired. Comparative Example 4, using piperazine as the polyamine, achieved the best initial performance, but its acid and alkali resistance was the worst. After 90 days of immersion in a 20 wt% H2SO4 aqueous solution (pH ≈ -0.4), the salt rejection rate was only 18.7%, comparable to that of a polysulfone ultrafiltration membrane without a separation layer, indicating near-complete degradation of the surface separation layer. Comparative Example 6, using m-phenylenediamine as the reactive monomer in the first grafting solution, achieved excellent initial performance, but its acid and alkali resistance was only slightly better than that of Comparative Example 4. After 90 days of immersion in a 20 wt% H2SO4 aqueous solution (pH ≈ -0.4), the salt rejection rate was only 25.3%, also showing poor acid and alkali resistance. Comparative Example 7, compared to Example 4, removed the phase transfer catalyst, resulting in a significant decrease in initial salt rejection to 90.5%. This is due to the low reactivity of the grafting groups, which requires a phase transfer catalyst to promote the reaction. Comparative Example 8, compared to Example 4, replaced tetrabutylammonium bromide (TBAB) with tetrabutyl hydrogen sulfate (TBAHS), resulting in a slight decrease in initial salt rejection.

[0184] It can be seen that after the nanofiltration membrane of the embodiment is soaked in acid and alkali, the flux increase value is significantly smaller than the flux increase value of the comparative example, and the salt rejection rate decrease value is significantly smaller than the salt rejection rate decrease value of the comparative example. Therefore, the acid and alkali resistance of the embodiment is far superior to that of the comparative example.

[0185] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing an acid- and alkali-resistant nanofiltration membrane, characterized in that: The following steps are involved: The base membrane is immersed in a first grafting solution, taken out to remove excess solution, and then immersed in a second grafting solution for cross-linking grafting reaction. After curing, it is post-treated to obtain an acid- and alkali-resistant nanofiltration membrane; The first grafting solution is composed of a polymer, a surfactant, an acid absorbent, and water; the polymer is a polyamine polymer or a sulfonic acid polymer; the polyamine polymer is at least one of polyethyleneimine, chitosan, polyaniline, and polypropyleneimine; the sulfonic acid polymer is at least one of polyethylenesulfonic acid and poly(4-styrenesulfonic acid); the acid absorbent is sodium hydroxide; in the first grafting solution, the mass content of the polymer is 3% to 10%, the mass content of the surfactant is 0.05% to 0.1%, and the mass content of the acid absorbent is 0.1% to 0.3%; The second grafting solution is composed of a nitrogen-containing heterocyclic compound, a phase transfer catalyst, and a solvent; the phase transfer catalyst is at least one of tetrabutylammonium bromide, benzyltriethylammonium chloride, and tetrabutylphosphonium bromide; in the second grafting solution, the mass content of the nitrogen-containing heterocyclic compound is 0.5% to 3%, and the mass content of the phase transfer catalyst is 0.05% to 0.1%; The nitrogen-containing heterocyclic compound is at least one of p-phenylene diisocyanate, toluene diisocyanate, polycyanurate, cyanuric chloride, trimethoxytriazine and 2,4,6-tris(2,4-dihydroxyphenyl)-1,3,5-triazine; The post-treatment includes soaking in organic alcohol and inorganic acid, filling with glycerin and then drying.

2. The preparation method according to claim 1, characterized in that The solvent is selected from at least one of acetonitrile and acetone.

3. The preparation method according to claim 1, characterized in that The post-processing method includes: The membrane was sequentially immersed in an isopropyl alcohol solution with a mass concentration of 5% to 10%, RO water, a sulfuric acid aqueous solution with a mass concentration of 20%, and a glycerol solution with a mass concentration of 1% to 5%, and then taken out and dried to obtain an acid- and alkali-resistant nanofiltration membrane.

4. The preparation method according to claim 3, characterized in that The post-processing method includes: Soak in a 5% to 10% isopropanol solution for 5 to 20 minutes, take out and soak in RO water for 4 to 6 minutes, take out and soak in a 20% sulfuric acid aqueous solution at 55 to 65°C for 5 to 20 minutes, take out and soak in a 1% to 5% glycerol solution for 1 to 5 minutes, take out and dry to obtain an acid and alkali resistant nanofiltration membrane.

5. The preparation method according to claim 1, characterized in that The soaking time in the first grafting solution is 4 to 6 minutes; The soaking time in the second grafting solution is 4 to 6 minutes.

6. Acid- and alkali-resistant nanofiltration membrane obtained by the preparation method according to any one of claims 1 to 5.

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