Nanofiltration membrane for removing residual chlorine in wastewater and preparation method thereof

Abstract

The invention relates to a nanofiltration membrane for removing residual chlorine in wastewater and a preparation method of the nanofiltration membrane, and belongs to the technical field of wastewater treatment. The nanofiltration membrane comprises a porous supporting layer and a polyamide separation layer on the surface of the porous supporting layer, the polyamide separation layer is further coated with a residual chlorine removal functional coating, and the functional coating comprises a residual chlorine trapping agent, modified graphene oxide and a polyvinyl alcohol high-molecular polymer carrier according to the mass ratio of (1-3): 1: 10. The prepared nanofiltration membrane has a residual chlorine removal function, is excellent in durability, and can effectively simplify the treatment process of wastewater dechlorination and reduce the operation cost.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology and relates to a nanofiltration membrane for removing residual chlorine from wastewater and its preparation method. Background Technology

[0002] Nanofiltration membrane technology, as a highly efficient and energy-saving separation method, has been widely used in fields such as advanced wastewater treatment and reuse, desalination and softening, and material separation and concentration.

[0003] Traditional nanofiltration membrane separation layers are typically prepared from aromatic polyamide materials via interfacial polymerization, exhibiting high water flux and rejection rates. However, in wastewater treatment, especially municipal sewage or industrial wastewater containing organic matter, chlorine-containing disinfectants (such as sodium hypochlorite) are usually added at the influent to prevent microbial growth and biofouling, and to extend membrane lifespan. Residual chlorine in the water (such as HClO and ClO-) is a strong oxidizing agent that directly attacks the amide bonds and aromatic rings in the polyamide separation layer, leading to polymer chain breakage and network structure destruction. This often causes irreversible chemical degradation of the membrane material, resulting in a significant decrease in membrane desalination rate, an abnormal increase in water flux, and ultimately premature failure of the nanofiltration membrane, greatly increasing the system's operation and maintenance costs.

[0004] To avoid oxidative damage to polyamide membranes from residual chlorine, the commonly used solution is to add a pretreatment unit before the nanofiltration system, such as using an activated carbon filter to adsorb residual chlorine or adding reducing agents (such as sodium bisulfite) for chemical dechlorination. However, these methods also have significant drawbacks: activated carbon adsorption requires frequent regeneration or replacement, increasing operational complexity and solid waste generation; while adding reducing agents requires precise control of the dosage, and excessive addition may cause secondary microbial growth.

[0005] Therefore, there is an urgent need to solve the above problems and develop a nanofiltration membrane that has the function of removing residual chlorine and can maintain high durability under residual chlorine attack. This is of great significance for simplifying the treatment process, reducing operating costs, and extending membrane life. Summary of the Invention

[0006] The purpose of this invention is to provide a nanofiltration membrane for removing residual chlorine from wastewater and its preparation method. This nanofiltration membrane can reduce the dependence on the preceding activated carbon step, has its own residual chlorine removal function, can maintain high durability under residual chlorine attack, and effectively simplify the treatment process and reduce operating costs.

[0007] The objective of this invention can be achieved through the following technical solutions: On one hand, the present invention provides a nanofiltration membrane for removing residual chlorine from wastewater. The nanofiltration membrane includes a porous support layer and a polyamide separation layer on the surface of the porous support layer. The polyamide separation layer is further coated with a residual chlorine removal functional coating. The functional coating contains a residual chlorine capture agent, modified graphene oxide and polyvinyl alcohol polymer carrier in a mass ratio of (1~3):1:10.

[0008] Furthermore, the residual chlorine scavenging agent is a mixture of anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:(1~1.5).

[0009] Furthermore, the porous support layer is one of polysulfone, sulfonated polysulfone, polyethersulfone, polyvinylidene fluoride, polyethylene, polypropylene, and polytetrafluoroethylene.

[0010] Furthermore, the preparation process of the modified graphene oxide is as follows: Step 1: Heat the 2 mg / mL graphene oxide dispersion in a water bath to 80~90℃ and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is (1~1.5):1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Mix graphene oxide with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:4. Sonicate at 300W for 30 min. Add tetrabutyl titanate and stir for 30-45 min. Add concentrated hydrochloric acid and stir for another 10 min. React at 140-180℃ for 15-24 h. Cool naturally to room temperature, filter, wash with anhydrous ethanol and deionized water, and dry at 60-80℃ for 12-16 h to obtain composite GO / TiO2. Ball mill pretreated graphene oxide at a mass ratio of 1:(1-1.5) and composite GO / TiO2 to obtain the modified graphene oxide.

[0011] On the other hand, the present invention also provides a method for preparing a nanofiltration membrane for removing residual chlorine from wastewater, comprising the following steps: S1. Preparation of solution: Aliphatic polyamines are mixed with water to obtain an aqueous solution; aromatic polyacrylamide chlorides and organic solvents are mixed to obtain an oil solution. S2, Interface Aggregation: The porous support layer is immersed in an aqueous solution for 5-10 minutes. After removing it and removing excess solution from the surface, it is then immersed in an oil solution for 1-3 minutes. A polyamide separation layer is formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 is placed in an oven at 100~190℃ for 10~25 minutes for curing. After curing, it is naturally cooled to room temperature and then washed with deionized water and dried. S4. Applying a coating: A coating solution containing a residual chlorine capture agent and a polymer carrier is coated onto the surface of a polyamide separation layer. After drying, a residual chlorine removal functional coating is formed, resulting in a nanofiltration membrane for removing residual chlorine from the wastewater.

[0012] Further, in step S1, the mass concentration of the aliphatic polyamine is 1~2.5%, and the mass concentration of the aromatic polyacrylamide chloride is 0.1~0.25%.

[0013] Furthermore, the aliphatic polyamine is one or more of piperazine, ethylenediamine, triethylenetetramine, p-phenylenediamine, and m-phenylenediamine.

[0014] Furthermore, the aromatic polyacryl chloride is one or more of pyromellitic trichloroisocyanurate, phthaloyl chloride, and isophthaloyl chloride.

[0015] Further, in step S4, the preparation process of the coating solution is as follows: Polyvinyl alcohol powder was mixed with deionized water and stirred at 90-95℃ until the polyvinyl alcohol dissolved. After cooling to room temperature, a polyvinyl alcohol solution with a mass fraction of 10-15% was prepared. Modified graphene oxide powder was added to deionized water to prepare a dispersion of 2 mg / mL and ultrasonically treated at 300-500W for 1-2 hours. Anhydrous sodium sulfite and L-ascorbic acid sodium were ball-milled at 80 r / min for 10-15 minutes to obtain a premix of capture agent. While maintaining a speed of 400 r / min and stirring, the dispersion with a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 25-40 minutes. The capture agent premix was then added and stirred continuously for 1-2 hours to form the final coating solution.

[0016] Furthermore, the molecular weight of the polyvinyl alcohol is in the range of 30,000 to 100,000.

[0017] The beneficial effects of this invention are: (1) This invention provides a nanofiltration membrane for removing residual chlorine from wastewater. This nanofiltration membrane not only possesses traditional separation performance but also actively and efficiently removes residual chlorine from wastewater, avoiding oxidative damage to the polyamide separation layer caused by residual chlorine and significantly improving the membrane's durability and service life. This invention integrates membrane separation and dechlorination functions by constructing a functional coating on the bulk polyamide separation layer, eliminating the need for complex pretreatment equipment and steps in traditional processes.

[0018] (2) This invention incorporates modified graphene oxide into the residual chlorine removal functional coating. This modified graphene oxide interacts with the polymer matrix material through hydrogen bonding and other phase interactions, acting as a nano-reinforcing filler to improve the mechanical properties of the coating and prevent premature coating failure. The modified graphene oxide obtained by ball milling pretreated graphene oxide and composite GO / TiO2 exhibits good dispersibility and can synergistically work with residual chlorine scavengers composed of sodium sulfite and L-ascorbic acid, helping to achieve high chlorine removal efficiency and long-term stability of the nanofiltration membrane. Simultaneously, it further enhances the hydrophilicity of the coating, facilitating rapid water molecule passage, reducing the negative impact of the coating on the membrane's water flux, and resulting in a more stable coating solution, thus helping to form a denser, more uniform, and defect-free surface coating. Detailed Implementation

[0019] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0020] To overcome the dependence of traditional dechlorination systems on pre-activated carbon units and organic reducing substances, this invention provides a nanofiltration membrane for removing residual chlorine from wastewater. The nanofiltration membrane includes a porous support layer and a polyamide separation layer on the surface of the porous support layer. The polyamide separation layer is also coated with a residual chlorine removal functional coating. The functional coating contains a residual chlorine capture agent, modified graphene oxide, and a polyvinyl alcohol polymer carrier in a mass ratio of (1~3):1:10.

[0021] The amide bonds in traditional polyamide membranes are highly susceptible to chlorine attack, leading to chain breakage and performance failure. Therefore, this invention constructs a residual chlorine removal functional coating on the surface of a traditional polyamide separation layer. This coating uses a specific ratio of anhydrous sodium sulfite and L-ascorbic acid sodium as residual chlorine trapping agents, achieving rapid removal of residual chlorine from wastewater. This avoids oxidative damage to the polyamide layer caused by residual chlorine, significantly extending the membrane's lifespan. This design eliminates the need for a pre-activated carbon unit, simplifying the system process. This method is simple, low-cost, and produces nanofiltration membranes with both high chlorine removal efficiency and long-term stability.

[0022] In one specific embodiment, the residual chlorine scavenging agent is a mixture of anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:(1~1.5).

[0023] Residual chlorine (mainly hypochlorous acid or hypochlorite ions) is a strong oxidizing agent. Anhydrous sodium sulfite, as the primary residual chlorine scavenger, exhibits extremely high chlorine removal efficiency under slightly alkaline conditions, while L-ascorbic acid sodium has even stronger activity under slightly neutral to acidic conditions. The combination of these two ensures rapid, thorough, and non-selective residual chlorine capture across a wide range of water pH levels, avoiding chlorine removal blind spots caused by pH fluctuations. The reaction products of both are common, non-toxic, and harmless ions, thus avoiding the potential toxic byproduct risks associated with the use of certain organic amine scavengers.

[0024] The reactions of sodium sulfite and L-ascorbic acid with chlorine are both completed rapidly, ensuring that residual chlorine is 100% reduced to harmless chloride ions within a very short hydraulic residence time, providing absolute protection for the lower polyamide separation layer.

[0025] In one specific embodiment, the porous support layer is one of polysulfone, sulfonated polysulfone, polyethersulfone, polyvinylidene fluoride, polyethylene, polypropylene, and polytetrafluoroethylene.

[0026] The porous support layer material used in this invention has excellent mechanical strength and chemical stability. Its surface has ideal pore size and porosity, which can efficiently adsorb aqueous monomers and provide a perfect reaction interface for interfacial polymerization. Finally, it forms an ultra-thin, complete and highly cross-linked polyamide separation layer, which helps to improve the separation selectivity, water flux and long-term service life of nanofiltration membranes.

[0027] In one specific embodiment, the preparation process of modified graphene oxide is as follows: Step 1: Heat the 2 mg / mL graphene oxide dispersion in a water bath to 80~90℃ and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is (1~1.5):1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Mix graphene oxide with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:4. Sonicate at 300W for 30 min. Add tetrabutyl titanate and stir for 30-45 min. Add concentrated hydrochloric acid and stir for another 10 min. React at 140-180℃ for 15-24 h. Cool naturally to room temperature, filter, wash with anhydrous ethanol and deionized water, and dry at 60-80℃ for 12-16 h to obtain composite GO / TiO2. Ball mill pretreated graphene oxide at a mass ratio of 1:(1-1.5) and composite GO / TiO2 to obtain the modified graphene oxide.

[0028] The inventors discovered that adding graphene oxide to a coating can help improve its performance. First, GO is reduced using L-ascorbic acid, which enhances electron conductivity while retaining some hydrophilic functional groups and dispersibility. Highly catalytically active TiO2 nanoparticles are then grown in situ on the graphene oxide. The products from the first two steps are then ball-milled at a mass ratio of 1:(1~1.5), and mechanical force is used to ensure a uniformly mixed modified graphene oxide.

[0029] This modified graphene oxide, due to its titanium dioxide content, can help degrade organic pollutants and oil stains adhering to the membrane surface through photocatalysis, thereby greatly alleviating membrane fouling, reducing cleaning frequency, and extending membrane life. Simultaneously, the holes and free radicals generated within it also possess oxidizing properties, aiding in the decomposition of residual chlorine and helping to consume it.

[0030] On the other hand, the present invention also provides a method for preparing a nanofiltration membrane for removing residual chlorine from wastewater, comprising the following steps: S1. Preparation of solution: Aliphatic polyamines are mixed with water to obtain an aqueous solution; aromatic polyacrylamide chlorides and organic solvents are mixed to obtain an oil solution. S2, Interface Aggregation: The porous support layer is immersed in an aqueous solution for 5-10 minutes. After removing it and removing excess solution from the surface, it is then immersed in an oil solution for 1-3 minutes. A polyamide separation layer is formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 is placed in an oven at 100~190℃ for 10~25 minutes for curing. After curing, it is naturally cooled to room temperature and then washed with deionized water and dried. S4. Applying a coating: A coating solution containing a residual chlorine capture agent and a polymer carrier is coated onto the surface of a polyamide separation layer. After drying, a residual chlorine removal functional coating is formed, resulting in a nanofiltration membrane for removing residual chlorine from the wastewater.

[0031] In this invention, a polyamide separation layer is formed on the surface of a porous support layer with high fluoride resistance through interfacial polymerization. Simultaneously, to improve the residual chlorine removal effect, a residual chlorine removal functional coating is applied to the surface of the polyamide separation layer. The residual chlorine scavenger in this coating can rapidly remove residual chlorine during wastewater treatment, thereby reducing the oxidative damage of residual chlorine to the polyamide layer and improving the service life of the nanofiltration membrane in residual chlorine environments.

[0032] In one specific embodiment, in step S1, the mass concentration of the aliphatic polyamine is 1~2.5%, and the mass concentration of the aromatic polyacrylamide chloride is 0.1~0.25%.

[0033] In one specific embodiment, the aliphatic polyamine is one or more of piperazine, ethylenediamine, triethylenetetramine, p-phenylenediamine, and m-phenylenediamine.

[0034] These amine monomers contain two or more amine groups, which are reaction sites for polycondensation reactions with acyl chlorides. Using polyamines can form a three-dimensional network of cross-linked polyamide structures, resulting in a dense structure that helps achieve high salt content and high organic matter rejection.

[0035] In one specific embodiment, the aromatic polyacryl chloride is one or more of pyromellitic trichloroisocyanurate, phthaloyl chloride, and isophthaloyl chloride.

[0036] Acyl chloride groups react with amine groups to form amide bonds, and polyacyl chlorides determine the structure and rigidity of the polyamide network backbone.

[0037] In one specific embodiment, the preparation process of the coating solution in step S4 is as follows: Polyvinyl alcohol powder was mixed with deionized water and stirred at 90-95℃ until the polyvinyl alcohol was completely dissolved. After cooling to room temperature, a polyvinyl alcohol solution with a mass fraction of 10-15% was prepared. Modified graphene oxide powder was added to deionized water to prepare a dispersion of 2 mg / mL and ultrasonically treated at 300-500W for 1-2 hours. Anhydrous sodium sulfite and L-ascorbic acid sodium were ball-milled at 80 r / min for 10-15 minutes to obtain a premix of capture agent. While maintaining a speed of 400 r / min and stirring, the dispersion with a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 25-40 minutes. The capture agent premix was then added and stirred continuously for 1-2 hours to form the final coating solution.

[0038] Pure polyvinyl alcohol (PVA) coatings may swell excessively, soften, or even be washed away by water in a wet state. Therefore, this invention incorporates modified graphene oxide, which can interact with PVA chains through hydrogen bonds and other mechanisms, acting as a nano-reinforcing filler. This significantly improves the coating's mechanical strength, wear resistance, and overall stability, preventing premature failure. Furthermore, the modified graphene oxide can be interleaved within the PVA matrix, increasing the path tortuosity of solutes (including chlorine molecules) reaching the underlying polyamide film, providing additional physical protection. The modified graphene oxide retains some oxygen-containing functional groups, maintaining its hydrophilicity and ensuring long-term stable dispersion in aqueous PVA solutions, preventing aggregation. Its addition further enhances the coating's hydrophilicity, facilitating rapid water molecule passage, reducing the coating's negative impact on the membrane's water flux, resulting in a more stable coating solution and a denser, more uniform, and defect-free coating.

[0039] In one specific embodiment, the molecular weight of polyvinyl alcohol ranges from 30,000 to 100,000.

[0040] Within this molecular weight range, PVA can be formulated into solutions with sufficient solids content but moderate viscosity, facilitating the formation of ultra-thin, uniform coatings through methods such as dip coating and blade coating. Thin coatings exhibit minimal water resistance, maximizing the retention of the original high water flux of the nanofiltration membrane. They also immobilize graphene oxide and residual chlorine traps, withstand certain water flow shear forces, prevent premature loss of functional components, and form a more uniform and stable composite dispersion system.

[0041] The invention provided by the present invention will be further described in detail below through specific embodiments.

[0042] It should be noted that, unless otherwise specified, the raw materials used in the following embodiments can be obtained by commercial purchase or conventional methods, and the experimental methods without specific conditions are all conventional methods and conditions well known in the art.

[0043] Example 1 S1. Preparation of solution: p-Phenylenediamine was mixed with water to obtain an aqueous solution with a mass concentration of 1%; phthaloyl chloride and an organic solvent were mixed to obtain an oil solution with a mass concentration of 0.1%. S2, Interface Aggregation: The sulfonated polysulfone porous support layer was immersed in an aqueous solution for 5 minutes. After removing it and removing the excess solution from the surface, it was immersed in an oil solution for 1 minute. A polyamide separation layer was formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 was placed in a 100℃ oven for 25 minutes to cure. After curing, it was naturally cooled to room temperature and then washed and dried with deionized water. S4. Applying a coating: Preparation of modified graphene oxide: Step 1: Heat the 2 mg / mL graphene oxide dispersion in a water bath to 80°C and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is 1:1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Graphene oxide was mixed with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:4. The dispersion was ultrasonicated at 300W for 30 min, tetrabutyl titanate was added and stirred for 30 min, concentrated hydrochloric acid was added, and stirring was continued for 10 min. The mixture was reacted at 140℃ for 15 h, and then naturally cooled to room temperature. After filtration, washing with anhydrous ethanol and deionized water, and drying at 60℃ for 12 h, composite GO / TiO2 was obtained. The pretreated graphene oxide and composite GO / TiO2 at a mass ratio of 1:1 were ball-milled to obtain the modified graphene oxide.

[0044] Polyvinyl alcohol powder was mixed with deionized water and stirred at 90°C until the polyvinyl alcohol was completely dissolved. After cooling to room temperature, a 10% polyvinyl alcohol solution was prepared. Modified graphene oxide powder was added to deionized water to prepare a 2 mg / mL dispersion, which was then ultrasonically treated at 300W for 1 hour. Anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:1 were ball-milled at 80 r / min for 10 minutes to obtain a chlorine trap premix. While stirring at 400 r / min, the dispersion in a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 25 minutes. The chlorine trap premix was then added, and the mixture was stirred for 1 hour to form the final coating solution, in which the mass ratio of residual chlorine trap, modified graphene oxide, and polyvinyl alcohol was 1:1:10. The coating solution was coated onto the surface of a polyamide separation layer and dried to form a residual chlorine removal functional coating, thus obtaining a nanofiltration membrane for residual chlorine removal from the wastewater.

[0045] Example 2 S1. Preparation of solution: p-Phenylenediamine was mixed with water to obtain an aqueous solution with a mass concentration of 1.5%; phthaloyl chloride and an organic solvent were mixed to obtain an oil solution with a mass concentration of 0.15%. S2, Interface Aggregation: The sulfonated polysulfone porous support layer was immersed in an aqueous solution for 8 minutes. After removing it and removing the excess solution from the surface, it was immersed in an oil solution for 2 minutes. A polyamide separation layer was formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 was placed in a 150°C oven for 15 minutes to cure. After curing, it was naturally cooled to room temperature and then washed and dried with deionized water. S4. Applying a coating: Preparation of modified graphene oxide: Step 1: Heat the 2 mg / mL graphene oxide dispersion in a water bath to 85°C and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is 1.2:1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Graphene oxide was mixed with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:4. The dispersion was ultrasonicated at 300W for 30 min, tetrabutyl titanate was added and stirred for 35 min, concentrated hydrochloric acid was added, and stirring was continued for 10 min. The mixture was reacted at 160℃ for 20 h, and then naturally cooled to room temperature. After filtration, washing with anhydrous ethanol and deionized water, and drying at 60-80℃ for 13 h, composite GO / TiO2 was obtained. The modified graphene oxide was obtained by ball milling pretreated graphene oxide at a mass ratio of 1:1.2 with composite GO / TiO2.

[0046] Polyvinyl alcohol powder was mixed with deionized water and stirred at 92°C until the polyvinyl alcohol was completely dissolved. After cooling to room temperature, a polyvinyl alcohol solution with a mass fraction of 13% was prepared. Modified graphene oxide powder was added to deionized water to prepare a dispersion of 2 mg / mL and ultrasonically treated at 400 W for 1.5 h. Anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:1.2 were ball-milled at 80 r / min for 13 min to obtain a chlorine trap premix. While maintaining a speed of 400 r / min and stirring, the dispersion in a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 30 min. The chlorine trap premix was then added and stirred continuously for 1.5 h to form the final coating solution, in which the mass ratio of residual chlorine trap, modified graphene oxide and polyvinyl alcohol was 2:1:10. The coating solution was coated onto the surface of the polyamide separation layer and dried to form a residual chlorine removal functional coating, thus obtaining a nanofiltration membrane for residual chlorine removal from the wastewater.

[0047] Example 3 S1. Preparation of solution: p-Phenylenediamine was mixed with water to obtain an aqueous solution with a mass concentration of 2.5%; phthaloyl chloride and an organic solvent were mixed to obtain an oil solution with a mass concentration of 0.25%. S2, Interface Aggregation: The sulfonated polysulfone porous support layer was immersed in an aqueous solution for 10 minutes. After removing it and removing the excess solution from the surface, it was immersed in an oil solution for 3 minutes. A polyamide separation layer was formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 was placed in a 190℃ oven for 10 minutes to cure. After curing, it was naturally cooled to room temperature and then washed and dried with deionized water. S4. Applying a coating: Preparation of modified graphene oxide: Step 1: Heat the 2 mg / mL graphene oxide dispersion to 90°C in a water bath and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is 1.5:1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Graphene oxide was mixed with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:4. The dispersion was ultrasonicated at 300W for 30 min, tetrabutyl titanate was added and stirred for 45 min, concentrated hydrochloric acid was added, and stirring was continued for 10 min. The mixture was reacted at 180℃ for 24 h, and then naturally cooled to room temperature. After filtration, washing with anhydrous ethanol and deionized water, and drying at 80℃ for 16 h, composite GO / TiO2 was obtained. The pretreated graphene oxide and composite GO / TiO2 at a mass ratio of 1:1.5 were ball-milled to obtain the modified graphene oxide.

[0048] Polyvinyl alcohol powder was mixed with deionized water and stirred at 95°C until the polyvinyl alcohol was completely dissolved. After cooling to room temperature, a polyvinyl alcohol solution with a mass fraction of 15% was prepared. Modified graphene oxide powder was added to deionized water to prepare a dispersion of 2 mg / mL and ultrasonically treated at 500W for 2 hours. Anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:1.5 were ball-milled at 80 r / min for 15 minutes to obtain a chlorine trapping agent premix. While maintaining a speed of 400 r / min and stirring, the dispersion in a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 40 minutes. The chlorine trapping agent premix was then added and stirred continuously for 2 hours to form the final coating solution, in which the mass ratio of residual chlorine trapping agent, modified graphene oxide and polyvinyl alcohol was 3:1:10. The coating solution was coated onto the surface of the polyamide separation layer and dried to form a residual chlorine removal functional coating, thus obtaining a nanofiltration membrane for residual chlorine removal from the wastewater.

[0049] Comparative Example 1 This comparative example is basically the same as Example 1, except that L-ascorbic acid sodium was not added in this comparative example.

[0050] Comparative Example 2 This comparative example is basically the same as Example 1, except that no residual chlorine capture agent was added in this comparative example.

[0051] Comparative Example 3 This comparative example is basically the same as Example 1, except that step one was not performed in the preparation process of the modified graphene oxide in this comparative example.

[0052] Comparative Example 4 This comparative example is basically the same as Example 1, except that step two was not performed in the preparation process of the modified graphene oxide in this comparative example.

[0053] Comparative Example 5 This comparative example is basically the same as Example 1, except that the graphene oxide in this comparative example has not undergone any modification treatment.

[0054] Comparative Example 6 This comparative example is basically the same as Example 1, except that no residual chlorine removal functional coating is applied to the surface of the polyamide separation layer in this comparative example.

[0055] Performance testing: 1. Residual chlorine removal: A simulated wastewater background solution (containing Na2SO4, NaCl and humic acid) was prepared using deionized water.

[0056] The residual chlorine concentration of the wastewater background solution was precisely adjusted to 2.0 ± 0.1 mg / L (Cl) using sodium hypochlorite solution to simulate the residual chlorine level after typical wastewater disinfection. The nanofiltration membrane to be tested and a standard nanofiltration membrane (as a blank control) were cut to appropriate sizes. The membrane was pre-compressed with deionized water at 0.5 MPa for 40 min until the water flux (Lm³) was reached. 2 h -1 Bar -1 The solution was stable. The water fluxes in Examples 1-3 were 95.4, 97.8, and 98.6, respectively. The residual chlorine concentration (C2) in the solution after passing through the nanofiltration membrane was measured by spectrophotometry, and the residual chlorine removal rate was calculated as: Residual chlorine removal rate / % = (C1-C2) / C1. The results are shown in Table 1 below. Table 1 Group Residual chlorine removal rate / % Residual chlorine removal rate after 24 hours / % Example 1 94.2 93.9 Example 2 96.4 96.1 Example 3 97.7 97.4 Comparative Example 1 86.2 84.8 Comparative Example 2 77.1 73.2 Comparative Example 3 90.5 89.4 Comparative Example 4 87.7 83.0 Comparative Example 5 80.5 77.8 Comparative Example 6 61.2 57.6 2. Membrane antifouling performance test: A 1 g / L bovine serum albumin solution was prepared as the feed liquid, and the flux was measured at this time. The flux reduction rate was obtained to test the membrane's antifouling performance. The results are shown in Table 2 below: Table 2 Group Flux decline rate / % Membrane flux attenuation rate Example 1 3.8 4% / 12 h Example 2 3.2 3% / 12 h Example 3 2.7 3% / 12 h Comparative Example 3 7.5 8% / 12 h Comparative Example 4 9.2 10% / 12 h Comparative Example 5 10.5 14% / 12 h Comparative Example 6 18.4 22% / 12 h Based on the above data, it can be seen that the nanofiltration membrane prepared by the present invention has excellent residual chlorine removal effect, durability and antifouling properties.

[0057] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A nanofiltration membrane for removing residual chlorine from wastewater, characterized in that, The nanofiltration membrane includes a porous support layer and a polyamide separation layer on the surface of the porous support layer. The polyamide separation layer is also coated with a residual chlorine removal functional coating. The functional coating contains a residual chlorine capture agent, modified graphene oxide and polyvinyl alcohol polymer carrier in a mass ratio of (1~3):1:

10.

2. The nanofiltration membrane for removing residual chlorine from wastewater according to claim 1, characterized in that, The residual chlorine capture agent is a mixture of anhydrous sodium sulfite and L-ascorbic acid sodium in a mass ratio of 2:(1~1.5).

3. The nanofiltration membrane for removing residual chlorine from wastewater according to claim 1, characterized in that, The porous support layer is one of polysulfone, sulfonated polysulfone, polyethersulfone, polyvinylidene fluoride, polyethylene, polypropylene, and polytetrafluoroethylene.

4. The nanofiltration membrane for removing residual chlorine from wastewater according to claim 1, characterized in that, The preparation process of the modified graphene oxide is as follows: Step 1: Heat the 2 mg / mL graphene oxide dispersion in a water bath to 80~90℃ and stir at 300 r / min. Add L-ascorbic acid to the graphene oxide dispersion, wherein the mass ratio of L-ascorbic acid to graphene oxide is (1~1.5):

1. Maintain the temperature and stir vigorously for 24 h. After filtration, washing and drying, pretreated graphene oxide is obtained. Step 2: Mix graphene oxide with a 1 mg / mL dispersion of deionized water / anhydrous ethanol at a volume ratio of 1:

4. Sonicate at 300W for 30 min, add tetrabutyl titanate and stir for 30-45 min, then add concentrated hydrochloric acid and stir for another 10 min. React at 140-180℃ for 15-24 h, cool naturally to room temperature, filter, wash with anhydrous ethanol and deionized water, and dry at 60-80℃ for 12-16 h to obtain composite GO / TiO2. Ball mill pretreated graphene oxide at a mass ratio of 1:(1-1.5) and composite GO / TiO2 to obtain the modified graphene oxide.

5. A method for preparing a nanofiltration membrane for removing residual chlorine from wastewater as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Preparation of solution: Aliphatic polyamines are mixed with water to obtain an aqueous solution; aromatic polyacrylamide chlorides and organic solvents are mixed to obtain an oil solution. S2, Interface Aggregation: The porous support layer is immersed in an aqueous solution for 5-10 minutes. After removing it and removing excess solution from the surface, it is then immersed in an oil solution for 1-3 minutes. A polyamide separation layer is formed on the surface of the porous support layer through interfacial polymerization. S3, thermosetting: The composite film obtained after step S2 is placed in an oven at 100~190℃ for 10~25 minutes for curing. After curing, it is naturally cooled to room temperature and then washed with deionized water and dried. S4. Applying a coating: A coating solution containing a residual chlorine capture agent and a polymer carrier is coated onto the surface of a polyamide separation layer. After drying, a residual chlorine removal functional coating is formed, resulting in a nanofiltration membrane for removing residual chlorine from the wastewater.

6. The method for preparing a nanofiltration membrane for removing residual chlorine from wastewater according to claim 5, characterized in that, In step S1, the mass concentration of the aliphatic polyamine is 1~2.5%, and the mass concentration of the aromatic polyacrylamide chloride is 0.1~0.25%.

7. The method for preparing a nanofiltration membrane for removing residual chlorine from wastewater according to claim 5, characterized in that, The aliphatic polyamine is one or more of piperazine, ethylenediamine, triethylenetetramine, p-phenylenediamine, and m-phenylenediamine.

8. The method for preparing a nanofiltration membrane for removing residual chlorine from wastewater according to claim 5, characterized in that, The aromatic polyacryl chloride is one or more of pyromellitic trichloroisocyanurate, phthaloyl chloride and isophthaloyl chloride.

9. The method for preparing a nanofiltration membrane for removing residual chlorine from wastewater according to claim 5, characterized in that, In step S4, the preparation process of the coating solution is as follows: Polyvinyl alcohol powder was mixed with deionized water and stirred at 90-95℃ until the polyvinyl alcohol dissolved. After cooling to room temperature, a polyvinyl alcohol solution with a mass fraction of 10-15% was prepared. Modified graphene oxide powder was added to deionized water to prepare a dispersion of 2 mg / mL and ultrasonically treated at 300-500W for 1-2 hours. Anhydrous sodium sulfite and L-ascorbic acid sodium were ball-milled at 80 r / min for 10-15 minutes to obtain a premix of capture agent. While maintaining a speed of 400 r / min and stirring, the dispersion with a volume ratio of 1:9 was mixed with the polyvinyl alcohol solution for 25-40 minutes. The capture agent premix was then added and stirred continuously for 1-2 hours to form the final coating solution.

10. The method for preparing a nanofiltration membrane for removing residual chlorine from wastewater according to claim 9, characterized in that, The molecular weight range of the polyvinyl alcohol is 30,000 to 100,000.