Composite nanofiltration membrane with loose surface as well as preparation method and application of composite nanofiltration membrane

Abstract

The invention discloses a preparation method of a composite nanofiltration membrane with a loose surface. The preparation method comprises the following steps: preparing an aqueous solution of L-arginine and an n-hexane solution of trimesoyl chloride; the method comprises the following steps: pretreating a polysulfone base membrane, fixing the base membrane between polytetrafluoroethylene frames, pouring an aqueous solution of L-arginine on the surface of the base membrane for soaking, and removing the aqueous solution on the surface of the base membrane; and pouring the n-hexane solution of trimesoyl chloride on the surface of the polysulfone-based membrane to carry out interfacial polymerization reaction, removing the redundant n-hexane solution on the surface of the polysulfone-based membrane, and standing and airing to obtain the composite nanofiltration membrane. A novel low-cost monomer L-arginine is introduced as an amine monomer to be subjected to interfacial polymerization reaction with trimesoyl chloride, so that a polyamide separation layer with a loose structure is constructed on a polysulfone-based membrane, the crosslinking degree of the polyamide separation layer is low, the pore diameter is increased to be more than 4 times that of a traditional nanofiltration membrane, and the separation effect is good. The separation rate of the dye and the inorganic salt is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of nanofiltration membrane separation, and particularly relates to a composite nanofiltration membrane with a loose surface and a preparation method and application thereof. Background Art

[0002] Printing and dyeing wastewater contains high concentrations of dyes and inorganic salts. Direct discharge will cause ecological toxicity and salinization of the water body. However, conventional methods such as flocculation and adsorption cannot accurately distinguish between dye molecules and salt ions, which often leads to the simultaneous removal or loss of both. This not only causes a waste of resources, but also produces a large amount of salt-containing sludge and secondary pollution. The flocculation method has limited efficiency in removing dyes, and the adsorbent regeneration process has a high energy consumption problem. The biochemical treatment is difficult to operate stably because the high-salt environment inhibits the activity of microorganisms. In addition, the wastewater treated by traditional processes still has high salt content and trace amounts of dyes, which can neither meet the reuse standards nor achieve dye recovery. Although reverse osmosis technology can achieve deep desalination, its high energy consumption characteristics and simultaneous retention of dyes seriously restrict its practical application value. In contrast, nanofiltration membranes, with their nanoscale pore size and Donnan effect, can selectively retain dyes and allow salt to penetrate, combining the advantages of low energy consumption and continuous operation. However, traditional polyamide nanofiltration membranes still have the defect of high salt retention rate due to their dense structure, and cannot achieve effective separation of salt and dye. Therefore, there is an urgent need to develop new loose nanofiltration membranes to break through the technical bottleneck of dye-salt separation.

[0003] At present, loose nanofiltration membranes are mainly produced by using low-reactivity amine monomers or optimizing the interfacial polymerization process to reduce the cross-linking density of the polyamide layer, thereby reducing the retention of salt. However, such methods often rely on high-cost special amine monomers or complex post-processing steps, resulting in high production costs. At the same time, the existing process has high requirements for the pretreatment of the base membrane, making it difficult to ensure the uniformity of the membrane structure, which in turn limits the improvement of water flux and batch stability. Therefore, the development of loose nanofiltration membranes based on low-cost amine monomers, without complex processes and with high flux has become the current research focus.

[0004] The Chinese patent with the application number CN202010264264.3 discloses a small molecule zwitterion modified surface polyamide composite membrane. The polyamide composite membrane includes an ultrafiltration bottom membrane and a polyamide layer modified by small molecules. The polyamide layer covers the surface of the ultrafiltration bottom membrane. The functional groups of the small molecules include at least one of amino groups or hydroxyl groups. The small molecule zwitterion modified surface polyamide composite membrane of the present invention obtains a zwitterion surface by grafting functional small molecule monomers with amino or hydroxyl functional groups, which are easily available, onto the surface of the polyamide composite membrane through surface secondary interfacial polymerization. By utilizing the extremely strong interaction between zwitterions and water molecules, the hydrophilicity of the membrane surface is improved, the mass transfer of water molecules is accelerated, and the purpose of high flux and anti-fouling is achieved. However, the nanofiltration membrane of this invention cannot maintain a high flux in a high-concentration salt solution. The bottom membrane of the polyamide composite membrane is a polyamide layer, and the separation layer is relatively dense, with a high salt rejection rate, and the separation of organic matter and salt cannot be achieved. Summary of the Invention

[0005] To solve the above problems, the present invention provides a composite nanofiltration membrane with a loose surface, a preparation method thereof, and an application.

[0006] The technical solution of the present invention is as follows:

[0007] One of the purposes of the present invention is to provide a preparation method of a composite nanofiltration membrane with a loose surface, including the following steps:

[0008] S1: Add L-arginine to water at room temperature and perform ultrasonic treatment until L-arginine is completely dissolved in water to obtain an aqueous solution of L-arginine for standby;

[0009] S2: Add trimesoyl chloride to a n-hexane solvent at room temperature and perform ultrasonic treatment until trimesoyl chloride is completely dissolved in the n-hexane solvent to obtain a n-hexane solution of trimesoyl chloride for standby;

[0010] S3: Pretreat the polysulfone substrate membrane, fix the pretreated polysulfone substrate membrane between polytetrafluoroethylene frames, then pour the aqueous solution of L-arginine prepared in step S1 onto the surface of the polysulfone substrate membrane, soak the polysulfone substrate membrane, and remove the remaining aqueous solution of L-arginine on the surface of the polysulfone substrate membrane after soaking;

[0011] S4: After the surface of the polysulfone substrate membrane is completely free of aqueous solution, pour the n-hexane solution of trimesoyl chloride prepared in step S2 onto the surface of the polysulfone substrate membrane for interfacial polymerization reaction. After the reaction is completed, remove the excess n-hexane solution on the surface of the polysulfone substrate membrane, and let it stand to dry to obtain the composite nanofiltration membrane with a loose surface.

[0012] Further, in step S1, the concentration of the aqueous solution of L-arginine is 1-4 wt%.

[0013] Further, in step S2, the concentration of the hexane solution of trimellitic acid chloride is 0.05 - 0.2 wt%.

[0014] Further, in step S3, the process of pretreating the polysulfone-based membrane is as follows: cleaning the polysulfone-based membrane with deionized water, and allowing the polysulfone-based membrane to stand and dry after cleaning.

[0015] Further, in step S3, the soaking time of the polysulfone-based membrane is 3 - 10 min.

[0016] Further, in step S4, the reaction time of the interfacial polymerization reaction is 30 - 150 s.

[0017] The second object of the present invention is to provide a composite nanofiltration membrane prepared by any of the above preparation methods.

[0018] The third object of the present invention is to provide an application of the composite nanofiltration membrane in the separation of dyes and salts in a high-salinity dye or dye-salt mixed solution.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention provides a preparation method of a composite nanofiltration membrane with a loose surface. By introducing a novel low-cost monomer L-arginine as an amine monomer to carry out an interfacial polymerization reaction with trimellitic acid chloride, a polyamide separation layer with a loose structure is constructed on the polysulfone-based membrane. The polyamide separation layer has a lower crosslinking degree compared to the polyamide layer prepared by traditional methods, the pore size increases to more than 4 times that of traditional nanofiltration membranes, the retention molecular weight increases from 200 Da to about 3000 Da, while maintaining a dye retention rate higher than 99%, and the retention rate of monovalent salt ions (NaCl) < 20%, improving the separation rate of dyes and inorganic salts. Due to the carboxylic acid structure on L-arginine in the present invention, the formed polyamide layer has carboxylic acid groups by itself, which can effectively achieve the dynamic absorption and desorption of cations in the filtered feed liquid. At high salt concentrations, based on the absorption and desorption of carboxylic acid, the cations, water, and cations in the feed liquid will "support" the membrane matrix, causing the separation layer to expand, and the pore size of the separation layer thus becomes larger. Therefore, even at high salt concentrations, the flux of the nanofiltration membrane will not decrease, but will instead increase contrarily. Moreover, the process of the present invention is simple to prepare, highly operable, and has a low cost.

[0021] 2. The composite nanofiltration membrane provided by the present invention can effectively utilize dyes and salts compared with traditional piperazine-based polyamide nanofiltration membranes, realizing the resource utilization of printing and dyeing wastewater. In addition, the performance of traditional loose nanofiltration membranes will decline due to concentration polarization under high-salt conditions, while the loose nanofiltration membrane of the present invention can still maintain a high flux even in a high-concentration salt solution, and its anti-fouling performance is also enhanced. Moreover, the structure of the composite nanofiltration membrane of the present invention is loose. During the filtration process, the required treatment time under the same pressure is lower than that of traditional nanofiltration membranes, and the permeation flux is 13.3 L / (m 2 ·h·bar), and the treatment capacity is higher than that of traditional nanofiltration membranes.

[0022] 3. The composite nanofiltration membrane provided by the present invention can still maintain a high flux in a high-concentration salt solution, and is suitable for the separation of dyes and salts in high-salinity dye or dye-salt mixed solutions, effectively realizing the utilization of dyes and salts and the resource utilization of printing and dyeing wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the SEM diagram of a composite nanofiltration membrane with a loose surface provided in Example 1 of the present invention;

[0024] Figure 2 It is the schematic diagram of the molecular weight cut-off of a composite nanofiltration membrane with a loose surface provided in Example 1 of the present invention;

[0025] Figure 3 It is the comparison diagram of the rejection rates of dyes and NaCl by the nanofiltration membranes provided in Example 1 and Comparative Examples 1-2 of the present invention;

[0026] Figure 4 It is the schematic diagram of the permeation flux and rejection rate of the composite nanofiltration membrane provided in Example 1 of the present invention under different concentrations of NaCl solutions. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The following will further illustrate the present invention in conjunction with preferred embodiments and with reference to the attached Figures 1-4 , in the present invention, the endpoints and any values within the disclosed ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values; for numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein; the materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions; the experimental methods in the following embodiments are conventional methods without special instructions.

[0028] Example 1

[0029] This embodiment provides a method for preparing a composite nanofiltration membrane with a loose surface, comprising the following steps:

[0030] S1: At room temperature, add 2 g of L-arginine to 98 g of water, and perform ultrasonic treatment until the L-arginine is completely dissolved in the water to obtain an aqueous solution of L-arginine with a concentration of 2 wt%, and set aside;

[0031] S2: At room temperature, add 0.1 g of trimesoyl chloride to 100 g of n-hexane solvent, and perform ultrasonic treatment until the trimesoyl chloride is completely dissolved in the n-hexane solvent to obtain a n-hexane solution of trimesoyl chloride with a concentration of 0.1 wt%, and set aside;

[0032] S3: Pretreat the polysulfone substrate membrane, fix the pretreated polysulfone substrate membrane between polytetrafluoroethylene frames, and then pour the aqueous solution of L-arginine prepared in step S1 onto the surface of the polysulfone substrate membrane, soak the polysulfone substrate membrane for 3 min, and remove the remaining aqueous solution of L-arginine on the surface of the polysulfone substrate membrane after soaking;

[0033] In this embodiment, the process of pretreating the polysulfone substrate membrane is: wash the polysulfone substrate membrane with deionized water, and leave the polysulfone substrate membrane to stand and dry after washing.

[0034] S4: After the surface of the polysulfone substrate membrane is completely free of aqueous solution, pour the n-hexane solution of trimesoyl chloride prepared in step S2 onto the surface of the polysulfone substrate membrane for interfacial polymerization reaction for 60 s. After the reaction is completed, remove the excess n-hexane solution on the surface of the polysulfone substrate membrane, and leave it to stand and dry to obtain the composite nanofiltration membrane with a loose surface.

[0035] This embodiment also provides a composite nanofiltration membrane prepared by the above preparation method, and stores it in deionized water at 4 °C before testing.

[0036] Example 2

[0037] This embodiment provides a method for preparing a composite nanofiltration membrane with a loose surface, comprising the following steps:

[0038] S1: At room temperature, add 1 g of L-arginine to 99 g of water, and perform ultrasonic treatment until the L-arginine is completely dissolved in the water to obtain an aqueous solution of L-arginine with a concentration of 1 wt%, and set aside;

[0039] S2: At room temperature, add 0.05 g of trimesoyl chloride to 100 g of n-hexane solvent, and perform ultrasonic treatment until the trimesoyl chloride is completely dissolved in the n-hexane solvent to obtain a n-hexane solution of trimesoyl chloride with a concentration of 0.05 wt%, and set aside;

[0040] S3: Pretreat the polysulfone-based membrane, fix the pretreated polysulfone-based membrane between polytetrafluoroethylene frames, then pour the aqueous solution of L-arginine prepared in step S1 onto the surface of the polysulfone-based membrane, soak the polysulfone-based membrane for 10 min, and remove the remaining aqueous solution of L-arginine on the surface of the polysulfone-based membrane after soaking is completed;

[0041] In this embodiment, the process of pretreating the polysulfone-based membrane is as follows: Wash the polysulfone-based membrane with deionized water, and leave the polysulfone-based membrane to stand and dry after washing.

[0042] S4: After the surface of the polysulfone-based membrane is completely free of aqueous solution, pour the n-hexane solution of trimesoyl chloride prepared in step S2 onto the surface of the polysulfone-based membrane for interfacial polymerization reaction for 30 s. After the reaction is completed, remove the excess n-hexane solution on the surface of the polysulfone-based membrane, and obtain the composite nanofiltration membrane with a loose surface after standing and drying.

[0043] This embodiment also provides a composite nanofiltration membrane prepared by the above preparation method, and stores it in deionized water at 4 °C before testing.

[0044] Example 3

[0045] This embodiment provides a preparation method of a composite nanofiltration membrane with a loose surface, including the following steps:

[0046] S1: Add 4 g of L-arginine to 96 g of water at room temperature, and perform ultrasonic treatment until L-arginine is completely dissolved in water to obtain an aqueous solution of L-arginine with a concentration of 4 wt%, and set aside;

[0047] S2: Add 0.2 g of trimesoyl chloride to 100 g of n-hexane solvent at room temperature, and perform ultrasonic treatment until trimesoyl chloride is completely dissolved in the n-hexane solvent to obtain an n-hexane solution of trimesoyl chloride with a concentration of 0.2 wt%, and set aside;

[0048] S3: Pretreat the polysulfone-based membrane, fix the pretreated polysulfone-based membrane between polytetrafluoroethylene frames, then pour the aqueous solution of L-arginine prepared in step S1 onto the surface of the polysulfone-based membrane, soak the polysulfone-based membrane for 5 min, and remove the remaining aqueous solution of L-arginine on the surface of the polysulfone-based membrane after soaking is completed;

[0049] In this embodiment, the process of pretreating the polysulfone-based membrane is as follows: Wash the polysulfone-based membrane with deionized water, and leave the polysulfone-based membrane to stand and dry after washing.

[0050] S4: After the surface of the polysulfone-based membrane is completely free of aqueous solution, pour the n-hexane solution of trimesoyl chloride prepared in step S2 onto the surface of the polysulfone-based membrane for interfacial polymerization reaction. The reaction time is 150 s. After the reaction is completed, remove the excess n-hexane solution on the surface of the polysulfone-based membrane, and let it stand to dry to obtain the composite nanofiltration membrane with a loose surface.

[0051] This embodiment also provides a composite nanofiltration membrane prepared by the above preparation method, and it is stored in deionized water at 4 °C before testing.

[0052] Comparative Example 1

[0053] The difference from Example 1 is as follows:

[0054] The composite nanofiltration membrane of this comparative example is a piperazine-based nanofiltration membrane.

[0055] Comparative Example 2

[0056] The difference from Example 1 is as follows:

[0057] The composite nanofiltration membrane of this comparative example is an L-alanine-based nanofiltration membrane.

[0058] Evaluation of implementation effect

[0059] Figure 1 This is an SEM image of a composite nanofiltration membrane with a loose surface provided by the present invention. It can be seen from the figure that in Example 1, a separation layer was successfully formed on the morphology of the porous polysulfone material, and its thickness is about 200 nm.

[0060] Figure 2 This is a schematic diagram of the molecular weight cut-off of a composite nanofiltration membrane with a loose surface provided in Example 1 of the present invention. It can be seen from the figure that the molecular weight cut-off of the composite nanofiltration membrane prepared in Example 1 can reach 3000 Da, while the molecular weight cut-off of the existing PIP-based polyamide nanofiltration membrane is 200 Da. The molecular weight cut-off of the composite nanofiltration membrane of the present invention is much larger than that of the PIP-based polyamide nanofiltration membrane.

[0061] Figure 3In the present invention, the following is a comparison chart of the rejection rates of the nanofiltration membranes provided in Example 1 and Comparative Examples 1-2 for dyes and NaCl. As can be seen from the figure, in the treatment of a mixed solution of the same high-concentration salt solution (10,000 ppm NaCl) and dye (500 ppm methylene blue), although the rejection rate of the piperazine-based nanofiltration membrane (PIP-based) in Comparative Example 1 for the dye is as high as 99.8%, its rejection rate for NaCl is still 23.2%, and effective separation of the dye and salt cannot be achieved. For the L-alanine-based nanofiltration membrane (L-alanine-based) in Comparative Example 2, although its rejection rate for NaCl is only 2.8% and it can completely permeate salt, its dye rejection rate is only 75.9%. While the nanofiltration membrane prepared in Example 1 maintains a dye rejection rate higher than 99% and has a rejection rate of monovalent salt ions (NaCl) less than 3%, effectively achieving the separation of the dye and salt and realizing the resource utilization of the dye and salt.

[0062] Figure 4 In the present invention, the following is a schematic diagram of the permeation flux and rejection rate of the nanofiltration membrane prepared in Example 1 under different concentrations of NaCl solution; as can be seen from the figure, with the continuous increase of the NaCl concentration, the permeation flux of the nanofiltration membrane prepared in Example 1 increases from the original 13.3 L / (m 2 ·h·bar) to 36.1 L / (m 2 ·h·bar), the rejection rate of NaCl drops to 3%, while the rejection rate of the dye is still greater than 99%. This shows that the carboxylic acid structure based on L-arginine in the present invention enables the formed polyamide layer to have carboxylic acid groups by itself, which can effectively achieve the dynamic absorption and desorption of cations in the filtered feed liquid. Under high salt concentration, based on the absorption and desorption of carboxylic acid, the cations, water and cations in the feed liquid will "support" the membrane matrix, resulting in the swelling of the separation layer, and the pore size of the separation layer becomes larger. Thus, even under high salt concentration, the flux of the nanofiltration membrane will not decrease, but will increase contrarily.

[0063] Due to the high cross-linking density and dense pore size of the traditional piperazine (PIP)-based polyamide nanofiltration membrane, it is difficult to balance dye rejection and efficient salt permeation, resulting in low separation efficiency of dyes and salts in printing and dyeing wastewater. In the present invention, by using L-arginine as the amine monomer and interfacing with trimesoyl chloride (TMC) for polymerization, a polyamide separation layer with a loose structure is constructed. The L-arginine molecule contains both α-amino and guanidine groups, and its three-dimensional configuration forms steric hindrance during the polymerization process, hindering the close cross-linking of polyamide chains, thus forming a loose network structure with larger pore size. In addition, the carboxyl group on the side chain of L-arginine endows the separation layer with the ability of dynamic charge regulation, enhancing the electrostatic repulsion effect on charged dyes by protonation / deprotonation in response to changes in the solution environment. This design not only achieves efficient dye rejection and free salt permeation, but also significantly improves the membrane flux, solving the technical problems of poor selectivity and low flux of traditional membranes.

[0064] Traditional loose nanofiltration membranes are prone to a sharp decline in flux due to aggravated concentration polarization and salt ion adsorption under high-salt conditions, and it is difficult to recover after membrane fouling. The present invention endows the separation layer with an adaptive regulation function through the dynamic ion-responsive characteristics of the carboxyl group of L-arginine. In a high-salt environment, the carboxyl group realizes local charge recombination by reversibly adsorbing / desorbing salt ions (such as Na + +) to weaken the formation of the concentration polarization layer and maintain the smoothness of the membrane pores; at the same time, the hydrophilicity and dynamic ion exchange characteristics of the carboxyl group can reduce the deposition of salts on the membrane surface and avoid pore blockage. This mechanism enables the membrane to maintain a stable water flux for a long time in a high-concentration salt solution, overcomes the problem of performance decay of traditional nanofiltration membranes caused by salt adsorption, and significantly reduces the risk of membrane fouling.

[0065] Traditional nanofiltration membranes need to rely on frequent chemical cleaning after fouling, resulting in increased operating costs and shortened membrane life. The present invention realizes the dynamic self-cleaning function of the separation layer through the pH-responsive characteristics of the carboxyl group of L-arginine. During the cleaning process, the protonation of the carboxyl group can cause slight swelling of the separation layer, increase the pore size and release the retained pollutants; while under neutral or alkaline conditions, the separation layer returns to a dense state to ensure stable retention performance. In addition, the cross-linked network formed by L-arginine and TMC has both flexibility and rigidity and maintains structural integrity during the dynamic swelling-shrinking process. This design significantly improves the flux recovery rate of the membrane after physical cleaning, reduces the dependence on chemical cleaning, and prolongs the service life of the membrane, especially suitable for complex wastewater treatment scenarios with high salt and high pollution.

[0066] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the inventive concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A preparation method of a composite nanofiltration membrane with a loose surface, characterized in that, It includes the following steps: S1: Add L-arginine into water at room temperature and perform ultrasonic treatment until L-arginine is completely dissolved in water to obtain an aqueous solution of L-arginine for standby; S2: Add trimesoyl chloride into n-hexane solvent at room temperature and perform ultrasonic treatment until trimesoyl chloride is completely dissolved in the n-hexane solvent to obtain a n-hexane solution of trimesoyl chloride for standby; S3: Pretreat the polysulfone-based membrane, fix the pretreated polysulfone-based membrane between polytetrafluoroethylene frames, then pour the aqueous solution of L-arginine prepared in step S1 onto the surface of the polysulfone-based membrane, soak the polysulfone-based membrane, and remove the remaining aqueous solution of L-arginine on the surface of the polysulfone-based membrane after soaking; S4: After the surface of the polysulfone-based membrane is completely free of aqueous solution, pour the n-hexane solution of trimesoyl chloride prepared in step S2 onto the surface of the polysulfone-based membrane for interfacial polymerization reaction. After the reaction is completed, remove the excess n-hexane solution on the surface of the polysulfone-based membrane, and obtain the composite nanofiltration membrane with a loose surface after standing and air-drying.

2. The preparation method of a composite nanofiltration membrane with a loose surface according to claim 1, characterized in that, In step S1, the concentration of the aqueous solution of L-arginine is 1-4 wt%.

3. The preparation method of a composite nanofiltration membrane with a loose surface according to claim 1, characterized in that In step S2, the concentration of the n-hexane solution of trimesoyl chloride is 0.05-0.2 wt%.

4. The preparation method of a composite nanofiltration membrane with a loose surface according to claim 1, characterized in that, In step S3, the process of pretreating the polysulfone-based membrane is: wash the polysulfone-based membrane with deionized water, and after washing, let the polysulfone-based membrane stand and air-dry.

5. The preparation method of a composite nanofiltration membrane with a loose surface according to claim 1, characterized in that, In step S3, the soaking time of the polysulfone-based membrane is 3-10 min.

6. The preparation method of a composite nanofiltration membrane with a loose surface according to claim 1, characterized in that, In step S4, the reaction time of the interfacial polymerization reaction is 30-150 s.

7. A composite nanofiltration membrane prepared by the method for preparing a composite nanofiltration membrane with a loose surface according to any one of claims 1-6.

8. Application of the composite nanofiltration membrane according to claim 7 in the separation of dyes and salts in a high-salinity dye or dye-salt mixture.

Images