A polyamide composite film with a near-neutral surface, its preparation method and application

Abstract

This invention belongs to the field of polymer materials technology, specifically relating to a polyamide composite membrane with a near-electroneutral surface, its preparation method, and its applications. The composite membrane comprises a porous ultrafiltration substrate and a polyamide functional layer. The polyamide functional layer is prepared by interfacial polymerization and possesses near-electroneutral properties, which can reduce the electrostatic interaction with pollutants, achieving the composite membrane's anti-fouling properties. The polyamide thin-layer composite desalination membrane exhibits high permeation selectivity and excellent anti-fouling performance. It has advantages such as simple synthesis process, wide applicability, low cost, and ease of large-scale production, showing broad application prospects in seawater desalination, wastewater treatment, and biomedicine.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a polyamide composite film with a near-electroneutral surface, its preparation method, and its application. Background Technology

[0002] With the rapid development of global industrialization and urbanization, water purification technology has received unprecedented attention. Reverse osmosis membrane technology, as an advanced separation method, can efficiently retain inorganic salt ions and has been widely used in seawater desalination, the electronics industry, biomedicine, and industrial wastewater treatment.

[0003] Polyamide thin-film composite membranes prepared by interfacial polymerization on porous substrates are currently the most widely used reverse osmosis membranes. Polyamide desalination membranes possess advantages such as high water permeability, high salt rejection rate, and simple preparation and scale-up. However, in practical applications, membrane fouling leads to a decline in membrane performance and lifespan, increasing the cost of reverse osmosis processes. Therefore, there remains a significant need to develop advanced polyamide reverse osmosis membranes with high permeability and antifouling properties.

[0004] Existing research has shown that membrane surface charge is a crucial factor affecting membrane antifouling performance. When the pollutant has the same surface charge as the membrane, electrostatic repulsion reduces the membrane's adsorption of the pollutant, thus achieving an antifouling effect. Under actual operating conditions, most polyamide desalination membranes prepared through interfacial polymerization exhibit negative charge due to the large number of carboxylic acid groups on their surface. Their flux attenuation rate for large organic molecules and negatively charged small organic molecules is less than 30%, demonstrating excellent antifouling performance. However, the flux attenuation rate for positively charged small organic molecules remains between 35% and 70%. Therefore, an ideal antifouling reverse osmosis membrane should have a near-neutral surface charge.

[0005] Currently, researchers (CN 102974221B, CN 112657353 B, CN 106345299A) have improved the surface charge of polyamide membranes by secondary grafting or coating with positively charged small molecules. However, this method introduces additional mass transfer resistance and reduces pure water flux. Furthermore, grafting and coating introduce additional procedures, increasing production costs. Therefore, there is still a need to develop a simple method for preparing high-flux, electrically neutral, antifouling polyamide membranes that can simultaneously improve resistance to various electrical contaminants while reducing production costs and simplifying the operation process. Summary of the Invention

[0006] In view of this, the first objective of the present invention is to address the problems existing in the prior art by providing a polyamide composite film with a near-electroneutral surface and a method for preparing the same.

[0007] By controlling the reaction solution system, the prepared separation membrane possesses an electrically neutral and hydrophilic surface, which can effectively improve the water flux and antifouling performance of the polyamide composite reverse osmosis membrane, while maintaining the original composite membrane's sodium chloride retention performance. This preparation method has advantages such as wide applicability, simple operation, and ease of scalability, laying the foundation for further expanding the application fields of membrane separation technologies such as reverse osmosis, nanofiltration, and forward osmosis.

[0008] Furthermore, the aforementioned near-neutral refers to the surface potential of the polyamide composite membrane being within the near-neutral range of +10 to -10 mV in an aqueous environment with pH=7.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A polyamide composite membrane with a near-neutral surface is disclosed. The polyamide composite membrane comprises a porous ultrafiltration substrate and a polyamide functional layer. The polyamide functional layer is formed on the surface of the ultrafiltration substrate by interfacial polymerization controlled by positively charged additives and polyhydroxy additives. The polyamide functional layer has near-neutral properties, and in an aqueous environment with pH=7, the membrane surface potential is in the near-neutral range of +10 to -10 mV. Furthermore, the polyamide functional layer has good hydrophilicity, with a water contact angle of less than 40°.

[0011] Specifically, the polyamide dense functional layer is prepared on the surface of a porous ultrafiltration membrane by interfacial polymerization. Through the polymerization reaction of multi-component amines and acyl chlorides, an electrically neutral membrane surface structure is prepared to reduce the interfacial forces between pollutants and the membrane surface, and simultaneously improve the membrane's permeation selectivity.

[0012] The porous ultrafiltration membrane material mentioned herein includes, but is not limited to, polymeric porous materials such as cellulose acetate, polyvinylidene fluoride, polysulfone, and polyethersulfone; and inorganic porous materials such as alumina, zirconium oxide, titanium dioxide, and ceramics. Porous ultrafiltration substrates with high chemical, mechanical, and thermal stability and low cost are preferred.

[0013] The second objective of this invention is to provide a method for preparing a polyamide composite membrane with a near-neutral surface. This method is simple to operate, and the prepared separation membrane has excellent permeation selectivity and antifouling properties, and can be used in separation and purification processes in fields such as seawater desalination, wastewater treatment, biomedicine, and petrochemicals.

[0014] To achieve the above objectives, the present invention adopts the following technical solution:

[0015] A method for preparing a polyamide composite film with a near-neutral surface includes the following steps:

[0016] An aqueous solution containing polyamines, positively charged additives, and polyhydroxy additives is contacted with an organic solution containing polyacrylamide monomers on a porous ultrafiltration substrate. After thermal crosslinking, a polyamide functional layer with near-electroneutral properties is formed on the surface of the porous ultrafiltration substrate to finally prepare the polyamide composite membrane with a near-electroneutral surface.

[0017] The specific steps are as follows:

[0018] (1) Soak the surface of the ultrafiltration membrane substrate in an aqueous solution containing polyamine, positively charged additive and polyhydroxy additive for 1-5 min, and then remove the polyamine aqueous solution from the surface;

[0019] (2) Soak the surface of the ultrafiltration membrane substrate after step (1) in an organic solution containing polyacryl chloride components for 0.5-3 min, then remove and remove the organic phase solution on the surface, and rinse the membrane surface with organic solvent.

[0020] (3) After drying the surface of the ultrafiltration membrane treated in step (2), perform thermal crosslinking. The thermal crosslinking temperature is 40-180℃ and the thermal crosslinking time is 1-15 minutes to obtain the modified surface polyamide composite membrane.

[0021] Optionally, in the aqueous solution, the concentration of the polyamine is 0.1-5.0 wt.%, the concentration of the positively charged auxiliary agent is 0.1-5.0 wt.%, and the concentration of the polyhydroxy auxiliary agent is 0.1-5.0 wt.%; and,

[0022] The polyamine includes one or more combinations of aromatic polyamines, aliphatic polyamines, and alicyclic polyamines; the positively charged auxiliaries include one or more combinations of pyridine, imidazole, indole, quaternary ammonium salts, and guanidine salts; and the polyhydroxy auxiliaries include one or more combinations of alcohols, phenols, and hydroxyphosphates.

[0023] The polyamines include, but are not limited to: o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 1,3,5-triaminobenzene, 1,2,4-triaminobenzene, 3,5-diaminobenzaldehyde, naphthalene-1,4-diamine, 2,3-diaminonaphthalene, 2,4-diaminoanisole, phenylenediamine, 9,10-diaminophenanthrene, ethylenediamine, propylenediamine, iminodiacetonitrile, N-phenylethylenediamine, 1,3-diaminocyclohexane, 1,4-diaminocyclohexane, N-aminomethylpiperazine, 1,4-piperazine diethylamine, etc. One or more of the above polyamine components can be selected and used in combination, with aromatic amino compounds such as m-phenylenediamine being preferred.

[0024] The positively charged auxiliaries refer to organic molecules with a fixed positive charge, including but not limited to imidazoles: imidazoline, imidazosin, benzimidazole, etc.; indoles: indolo[3,2-B]carbazole, 9H-pyrido[2,3-B]indole, 11,12-dihydroindolo[2,3-a]carbazole, etc.; quaternary ammonium salts: dodecyltrimethylammonium chloride, dodecyldimethylbenzylammonium chloride, bis(dodecyldimethylammonium chloride), dodecyldimethylamine oxide, octadecyltrimethylammonium chloride, etc. One or more of the above-mentioned positively charged auxiliaries can be selected and used in combination, with aliphatic ammonium auxiliaries such as quaternary ammonium salts being preferred.

[0025] Specifically, the molecular structure of the quaternary ammonium salt auxiliaries is as follows:

[0026]

[0027] In this context, R1, R2, R3, R4, and X represent any group, which can be a monoatomic group or a polyatomic group, and R2 group provides one or more positive charges, while X provides one or more negative charges.

[0028] The aforementioned polyhydroxy auxiliaries refer to compounds in which hydrogen atoms in the side chains of aliphatic hydrocarbons, alicyclic hydrocarbons, or aromatic hydrocarbons are replaced by hydroxyl groups. Depending on the number of hydroxyl groups in the molecule, they are classified as mono-, di-, and poly-hydroxy compounds, including but not limited to: methanol, ethanol, propanol, n-butanol, isobutanol, n-pentanol, n-hexanol, 2-methyl-2-propanol, cyclopentanol, cyclohexanol, benzyl alcohol, ethylene glycol, glycerol, diethylene glycol, butanediol, pentanediol, hexanediol, xylitol, phenol, o-cresol, hexaphenol, bisphenol A, etc.

[0029] The molecular structure of the polyhydroxy auxiliary is as follows:

[0030]

[0031] R5 represents any group, which can be a monatomic group or a polyatomic group.

[0032] Optionally, the concentration of the polyacrylamide chloride monomer in the organic solution is 0.1-5.0 wt.%, and the polyacrylamide chloride monomer includes one or more combinations of aromatic polyacrylamide chlorides, aliphatic polyacrylamide chlorides, and alicyclic polyacrylamide chlorides.

[0033] The polyacryl chlorides include, but are not limited to: phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride, 1,3,5-benzenetricarboxyl chloride, biphenyl dicarboxyl chloride, naphthalene dicarboxyl chloride, naphthalene tricarboxyl chloride, benzene trisulfonyl chloride, benzene disulfonyl chloride, malonyl chloride, succinyl chloride, glutaryl chloride, malonyl tricarboxyl chloride, succinyl tricarboxyl chloride, glutaryl tricarboxyl chloride, cyclohexane dicarboxyl chloride, cyclohexane tricarboxyl chloride, cyclohexane tetracarboxyl chloride, tetrahydrofuran tricarboxyl chloride, tetrahydrofuran tetracarboxyl chloride, etc. One or more of the above polyacryl chloride components can be selected and used in combination, with aromatic acryl chloride compounds such as 1,3,5-benzenetricarboxyl chloride being preferred.

[0034] Furthermore, the preferred concentration of the polyamine component in step (1) is 0.1-5.0 wt.%, the preferred concentration of the positively charged auxiliary agent is 0.1-5.0 wt.%, and the preferred concentration of the polyhydroxy auxiliary agent is 0.1-5.0 wt.%.

[0035] The preferred concentration of the polyacrylamide component in step (2) is 0.01-1.0 wt.%. The organic solvent refers to a compound with low solubility in water, which does not degrade the porous support and has high solubility for the polyacrylamide component. It is one of the C4-C12 aliphatic hydrocarbons, cyclic aliphatic hydrocarbons, and aromatic hydrocarbons, including but not limited to: n-pentane, n-hexane, n-heptane, n-octane, isoalkanes, and other alkane compounds. One or more of the above-mentioned alkane organic solvents can be used in combination.

[0036] The preferred thermal crosslinking temperature in step (3) is 50-100℃, and the preferred thermal crosslinking time is 1-10 minutes.

[0037] The third objective of this invention is to provide an application of the polyamide composite membrane with a near-electroneutral surface as described above in the fields of seawater desalination, wastewater treatment, biomedicine, and petrochemicals.

[0038] The prepared polyamide composite membrane with a near-neutral surface has excellent permeation selectivity and antifouling properties. It can be used in separation and purification processes in fields such as seawater desalination, wastewater treatment, biomedicine, and petrochemicals. It can be used to prepare polyamide composite reverse osmosis membranes, polyamide composite nanofiltration membranes, or polyamide composite forward osmosis membranes.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) The film-forming method provided by the present invention is simple, has a short film-forming time, and is under mild conditions, which simplifies the preparation process and avoids problems such as increased costs caused by secondary grafting.

[0041] (2) Positively charged additives and polyhydroxy additives can regulate the interfacial polymerization process, promote the diffusion and transfer of polyamine monomers in the aqueous phase to the organic phase, and increase the reaction ratio with polyacrylamide monomers in the organic phase, thereby helping to prepare a surface electrically neutral polyamide skin. The resulting polyamide reverse osmosis membrane has high permeation flux and good antifouling performance, good stability, and good prospects for industrial application. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0043] Figure 1 This is a scanning electron microscope image of the polyamide composite film with a near-neutral surface prepared in Example 2 of the present invention.

[0044] Figure 2 This is a scanning electron microscope image of the polyamide composite film with a near-neutral surface prepared in Example 4 of the present invention.

[0045] Figure 3 This is a schematic diagram of the Zeta potential of the polyamide composite membrane with a near-neutral surface prepared in Examples 1-5 and the comparative examples of the present invention.

[0046] Figure 4 This is a schematic diagram showing the permeation selectivity of the polyamide composite membranes with near-neutral surfaces prepared in Examples 1-3 and the comparative examples of the present invention.

[0047] Figure 5 This is a schematic diagram showing the permeation selectivity of the polyamide composite membranes with near-neutral surfaces prepared in Examples 4-5 and the comparative examples of the present invention.

[0048] Figure 6 This is a schematic diagram showing the water contact angle of the polyamide composite membranes with near-neutral surfaces prepared in Examples 1-5 and the comparative examples of the present invention.

[0049] Figure 7 This is a schematic diagram illustrating the dynamic antifouling effect of polyamide composite films with near-neutral surfaces prepared in Examples 2, 4, and the comparative example of the present invention on the positively charged model pollutant DTAB.

[0050] Figure 8 This is a schematic diagram illustrating the dynamic antifouling effect of polyamide composite films with near-neutral surfaces prepared in Examples 2, 4, and the comparative example of the present invention on negatively charged model pollutants SDS.

[0051] Figure 9 This is a schematic diagram illustrating the dynamic antifouling effect of polyamide composite membranes with near-neutral surfaces prepared in Examples 2, 4, and the comparative example of the present invention on the neutral model pollutant Tween-20. Detailed Implementation

[0052] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0054] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned herein refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0055] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0056] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0057] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0058] In the following examples, a cross-flow testing device was used to evaluate the membrane separation performance. The separation performance test conditions were as follows: after pre-pressurization for 1 hour at 20 bar and 25°C, the rejection rate and permeate flux of the prepared polyamide reverse osmosis membrane to a 2000 ppm sodium chloride solution were tested at 15 bar and 25°C. The flux unit is LMH / bar (liters / square meter / hour / bar). The permeate flux was calculated using the following formula (1). In addition, the concentrations of the feed solution and the permeate were measured using a conductivity meter. The salt rejection rate was calculated using the following formula (2). At least three parallel samples were tested for each membrane sample, and the average value and error range of the test results were calculated.

[0059]

[0060] In the formula, J w — Membrane permeation flux (LMH / bar);

[0061] M—mass of permeate passing through the membrane (kg);

[0062] ρ — density of permeate (kg·m³) -3 );

[0063] E – Effective permeation area of ​​the membrane sample (m²) 2 );

[0064] t — Test time (h);

[0065] P – Test pressure (bar).

[0066]

[0067] In the formula, R represents the salt rejection rate (%) of the membrane sample.

[0068] C p —Osmotic salt concentration (mg / L);

[0069] C f —Feed solution salt concentration (mg / L).

[0070] Antifouling performance evaluation: Dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfonate (SDS), and polyoxyethylene sorbitan monolaurate (Tween-20) were used as model pollutants to evaluate the membrane's antifouling performance. DTAB was used as an example of a positively charged surfactant, i.e., a small molecule pollutant. SDS was used as an example of a negatively charged surfactant, i.e., a small molecule pollutant. Tween-20 was used as an example of an electrically neutral surfactant, i.e., a small molecule pollutant. These are typical examples of different electrically charged organic pollutants commonly found in water systems.

[0071] In addition, a solid surface zeta potential analyzer was used to characterize the potential changes on the surface of the polyamide reverse osmosis membrane, and a water contact angle meter was used to characterize the changes in the hydrophilicity of the polyamide reverse osmosis membrane surface.

[0072] Example 1

[0073] An aqueous solution containing 3.5 wt.% m-phenylenediamine (MPD), 1.0 wt.% benzalkonium chloride (BAC) as a positive charge additive, and 0.5 wt.% glycerol was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine aqueous solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C blower dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0074] Example 2

[0075] An aqueous solution containing 3.5 wt.% m-phenylenediamine, 1.0 wt.% benzalkonium chloride (a positively charged additive), and 1.0 wt.% glycerol was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine aqueous solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C blower dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0076] Example 3

[0077] An aqueous solution containing 3.5 wt.% m-phenylenediamine, 0.5 wt.% benzalkonium chloride (a positively charged additive), and 1.0 wt.% glycerol was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine aqueous solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C forced-air dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0078] Example 4

[0079] An aqueous solution containing 3.5 wt.% m-phenylenediamine, 1.0 wt.% dodecyltrimethylammonium bromide (DTAB) as a positive charge additive, and 1.0 wt.% glycerol was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine aqueous solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C blower dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0080] Example 5

[0081] An aqueous solution containing 3.5 wt.% m-phenylenediamine, 1.0 wt.% benzalkonium chloride (a positively charged additive), and 1.0 wt.% n-butanol was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine aqueous solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C blower dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0082] To further demonstrate the beneficial effects of the present invention and to better understand the present invention, the following test results further illustrate the properties and application performance of the near-neutral polyamide composite film disclosed in the present invention. However, these tests should not be construed as limiting the present invention. The product properties obtained by other test results performed by those skilled in the art based on the above-described invention and the applications based on the above properties are also considered to fall within the protection scope of the present invention.

[0083] Comparative Example

[0084] A method for preparing an unmodified polyamide reverse osmosis membrane is as follows:

[0085] An aqueous solution containing 3.5 wt.% m-phenylenediamine was coated onto a porous polysulfone support membrane. After 1 minute, the excess amine solution was removed, thereby forming an aqueous coating layer. Next, a hexane solution containing 0.2 wt.% trimesoyl chloride was coated onto the surface of the aforementioned aqueous coating layer. After 30 seconds, the excess hexane solution was removed. Subsequently, the membrane was thermally crosslinked in a 60°C blower dryer for 5 minutes, thereby forming a polyamide crosslinked skin layer on the porous support layer, thus preparing a composite desalination membrane.

[0086] The separation performance and membrane surface potential of polyamide composite reverse osmosis membranes in Examples 1-5 and the comparative examples were tested, and the results are shown in Table 1.

[0087] Table 1 Separation performance of polyamide composite reverse osmosis membrane

[0088]

[0089] A comparison of Examples 1-3 and the comparative examples shows that by adding and adjusting the concentrations of quaternary ammonium salt-type positively charged additives and alcohols, process parameters can be modified to form a polyamide surface layer. As shown in Table 1, the addition of BAC and glycerol can achieve higher water flux while maintaining a high rejection rate.

[0090] The surface potential of the modified and initial polyamide composite reverse osmosis membranes prepared in Examples 1-3 and the comparative example were tested. After adding BAC and glycerol, in an aqueous environment with pH=6, the potential of Example 2 was -1.56mV, which was much lower than the potential of Comparative Example 5 (-33.35mV), indicating a reduction in surface negative charge and a significant increase in positive charge. Therefore, the surface of the polyamide reverse osmosis membrane modified by the method of this invention tends to be electrically neutral, while increasing its permeate flow rate without reducing the desalination rate of the membrane material.

[0091] Water contact angle tests were performed on the modified and initial polyamide composite reverse osmosis membranes prepared in Examples 1-3 and the comparative examples. After adding BAC and glycerol, the hydrophilicity of the membrane surface increased, which is due to the hydrophilic hydroxyl groups on the glycerol embedded in the membrane surface.

[0092] Examples 4-5 and the comparative examples show that, compared with the initial polyamide composite reverse osmosis membrane, the pure water permeability coefficient and salt rejection rate are improved by adding different types and structures of quaternary ammonium salt positively charged additives and alcohols, proving that the method has universality.

[0093] In addition, to further illustrate the advantages of the present invention in terms of antifouling performance, an evaluation of the antifouling performance of the polyamide reverse osmosis membrane was conducted.

[0094] Anti-fouling performance was evaluated using the normalized flux decay rate (%). The specific steps for determining the Flux decay rate are as follows:

[0095] (1) Under the conditions of 15 bar and crossflow velocity of 14 cm / s, the RO system was pre-pressurized for 30 minutes using a feed aqueous solution containing 2000 ppm NaCl to determine the baseline permeation flux and salt rejection rate.

[0096] (2) Add 200 ppm of the aforementioned model pollutant to the aforementioned supply aqueous solution and operate the RO system under the same conditions as in step (1) until the volume of the permeate is 200 mL.

[0097] (3) Clean the polyamide composite desalination membrane with deionized water for 30 minutes at a water circulation flow rate of 3L / min.

[0098] (4) Using a feed aqueous solution containing 2000 mg / L NaCl, the permeation flux and salt rejection rate were determined again.

[0099] Flux attenuation rate can be calculated using the following formula:

[0100] Flux reduction rate (%) = [1 - (permeation flux in step 2 / permeation flux in step 1)] × 100%;

[0101] Flux recovery rate (%) = (permeation flux in step 4 / permeation flux in step 1) × 100%.

[0102] The normalized flux decay rate (%) of the polyamide composite reverse osmosis membranes in Examples 1-5 and the comparative examples was evaluated, and the results are shown in Table 2.

[0103] Table 2. Antifouling performance of polyamide composite reverse osmosis membranes

[0104]

[0105] The membrane's resistance to organic fouling is shown in Table 2. As can be seen from Table 2, the attenuation rates for positively and negatively charged small molecule organic pollutants in the comparative examples were 65.12% and 15.23%, respectively. The attenuation rates in Examples 2 and 4 were both lower than the corresponding values.

[0106] In summary, the modified polyamide desalination membrane exhibits significantly better permeation flux and antifouling properties than the original composite desalination membrane.

[0107] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those 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 invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a polyamide composite film with a near-electroneutral surface, characterized in that, The polyamide composite membrane comprises a porous ultrafiltration substrate and a polyamide functional layer. The polyamide functional layer is formed by interfacial polymerization controlled by positively charged additives and polyhydroxy additives, which are polymerized on the surface of the ultrafiltration substrate by polyamine monomers in aqueous solution and polyacrylamide chloride monomers in organic solution, followed by thermal crosslinking. The polyamide functional layer has near-neutral properties, with a membrane surface potential in the near-neutral range of +10.0 to -10.0 mV in an aqueous environment with pH=7. The polyamide functional layer also has good hydrophilicity, with a water contact angle of less than 40°. The specific preparation method includes the following steps: An aqueous solution containing polyamines, positively charged additives, and polyhydroxy additives is contacted with an organic solution containing polyacrylamide monomers on a porous ultrafiltration substrate. After thermal crosslinking, a polyamide functional layer with near-electroneutral properties is formed on the surface of the porous ultrafiltration substrate to finally prepare the polyamide composite membrane with a near-electroneutral surface. The polyamine is m-phenylenediamine; the positively charged additive is benzalkonium chloride; the polyhydroxy additive is glycerol; The concentration of polyamine is 3.5 wt.%, the concentration of positively charged auxiliaries is 0.5~1.0 wt.%, and the concentration of polyhydroxy auxiliaries is 0.5~1.0 wt.%. The concentration of the polyacrylamide chloride monomer in the organic solution is 0.2 wt.%, and the polyacrylamide chloride monomer is trimesoyl chloride.

2. The method for preparing a polyamide composite film with a near-neutral surface according to claim 1, characterized in that, The ultrafiltration membrane is a polymer ultrafiltration membrane or an inorganic ultrafiltration membrane.

3. The method for preparing a polyamide composite film with a near-neutral surface according to claim 1, characterized in that, The thermal crosslinking reaction temperature is 40-180℃, and the reaction time is 1-10 min.

4. The application of a polyamide composite membrane with a near-neutral surface prepared by the method described in claim 1 in the fields of seawater desalination, wastewater treatment, biomedicine, and petrochemicals.

5. The application according to claim 4, characterized in that, Application of the polyamide composite membrane with a near-neutral surface in the preparation of polyamide composite reverse osmosis membranes, polyamide composite nanofiltration membranes, or polyamide composite forward osmosis membranes.

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