Loose nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of dye separation, and discloses a loose nanofiltration membrane and a preparation method and application thereof, the loose nanofiltration membrane comprises a porous support membrane, and a polyesteramide separation layer formed on the porous support membrane through one-step interfacial polymerization of nucleoside monomers and multi-acyl chloride monomers. The nucleoside monomer is one or more than two of adenosine, guanosine and cytidine nucleoside. The nucleoside monomer with a long-chain structure, a branched-chain structure, a rigid twisted structure and a plurality of hydrophilic reaction sites is selected as an interfacial polymerization water-phase monomer, so that the loose nanofiltration membrane is endowed with excellent permeation and separation performance. The membrane has a high rejection rate on dye molecules such as Congo red and methyl blue, also has high water permeability and extremely high dye / inorganic salt separation selectivity, and shows good application potential in the aspects of dye production and purification and dye wastewater recovery.

Description

Technical Field

[0001] This invention relates to the field of dye separation technology, and in particular to a loose nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Dye wastewater is generated by the textile industry through pretreatment, dyeing, rinsing, printing, and finishing processes. It is characterized by complex composition, high color intensity, high organic content, high salt content, and poor biodegradability. Dye molecules, due to their complex structure, large molecular weight, and recalcitrant nature, are the most difficult pollutants to treat in dyeing wastewater. In addition to the addition of dyes, dyeing wastewater also contains high concentrations of salts, including approximately 5% of synthetic byproducts (NaCl, Na2SO4) and other inorganic salts used as binders. If left untreated, dyeing wastewater will pollute natural water bodies, harm aquatic ecosystems, and pose a threat to human health. Traditional chemical oxidation, absorption, and coagulation technologies typically have low separation efficiency and may even produce toxic byproducts. Nanofiltration membrane separation technology, on the other hand, treats textile wastewater without adding additional chemicals, does not affect the chemical properties of the wastewater, and can extract valuable resources (dyes, salts, and water) from dye wastewater, meeting current dye separation needs.

[0003] The application of nanofiltration (NF) technology to treat high-salinity textile wastewater and achieve resource recycling shows great promise. Thin-film composite membranes (TFCMs), based on porous polymer substrates, are prepared by interfacial polymerization (IP) to form a thin selective layer, which is easy to manufacture and allows for continuous operation. However, due to the high reactivity of the monomers used in their membrane preparation, the selective layer in commercial TFCMs often struggles to separate dyes / salts. This results in an overly dense selective layer leading to low dye / salt separation efficiency and loss of valuable components in textile wastewater. Synthesizing TFCMs with a loosely structured selective layer is a promising method for preparing dye / salt separation nanofiltration membranes. In existing technologies, researchers have attempted to develop novel aqueous monomers to improve membrane performance, but the synthesis processes of most aqueous monomers are complex or costly, and some even possess toxicity, causing environmental damage.

[0004] Therefore, it is of great significance to develop a novel method for preparing loose nanofiltration membranes using a widely available, green monomer that can comprehensively improve membrane performance. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a loose nanofiltration membrane, its preparation method, and its applications. From a molecular structure perspective, nucleoside monomers with rigid, twisted structures and multiple hydrophilic reaction sites are selected as the aqueous phase, and the loose nanofiltration membrane is constructed via interfacial polymerization. The polyamide ester separation layer of this loose nanofiltration membrane exhibits high permeability to inorganic salts while maintaining a high rejection rate for various dyes of different molecular weights, thereby improving dye / salt separation efficiency and giving the loose nanofiltration membrane excellent dye / inorganic salt separation selectivity.

[0006] The technical solution adopted in this invention is: The first aspect of this invention provides a method for preparing a loose nanofiltration membrane, comprising the following steps: S1: Pretreatment of the porous support membrane; S2: The pretreated porous support membrane is immersed in an aqueous solution of nucleoside monomers with a concentration of 0.1-0.8 wt% for 1-2 min. After immersion, the aqueous solution is removed by purging to obtain a support membrane adsorbed with nucleoside monomers. The nucleoside monomers are one or more of adenosine (ADO), guanosine, and cytosine. The nucleoside monomers are all naturally extracted, environmentally friendly, low in cost, and possess a rigid twisted structure and multiple hydrophilic reaction sites. S3: Pour a 0.03-0.2wt% polyacrylamide chloride organic phase solution onto the surface of the support membrane on which nucleoside monomers are adsorbed, carry out an interfacial polymerization reaction, remove the membrane and dry it at room temperature to obtain a loose nanofiltration membrane.

[0007] Further, in step S2, the concentration of the nucleoside monomer in the aqueous solution of the nucleoside monomer is 0.3-0.5 wt%.

[0008] Furthermore, the nucleoside monomer is adenosine.

[0009] Further, in step S1, the porous support membrane is one or more of the following: polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, and polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 80-120 kDa.

[0010] Further, in step S1, the pretreatment involves immersing the porous support membrane in deionized water for 6-24 hours, and then purging the porous support membrane with an air knife at a temperature of 24-26°C for 1-2 minutes.

[0011] Furthermore, in step S2, the purging temperature is 24-26℃ and the purging time is 0.5-1 min.

[0012] Furthermore, the interfacial polymerization reaction in step S3 takes 1-5 minutes, the reaction temperature is 24-26°C, and the drying time at room temperature is 5-10 minutes.

[0013] Furthermore, the polyacrylamide chloride is trimesoyl chloride (TMC).

[0014] Furthermore, in step S3, the organic solvent of the polyacrylamide chloride organic phase solution is n-hexane.

[0015] A second aspect of the present invention provides a loose nanofiltration membrane prepared by the above method, comprising a porous support membrane, a polyamide ester separation layer formed on the porous support membrane by one-step interfacial polymerization of nucleoside monomers and polyacrylamide chloride monomers.

[0016] A third aspect of the present invention provides an application of the above-mentioned loose nanofiltration membrane in the separation of dyes / inorganic salts.

[0017] This invention, from a molecular structure perspective, selects nucleoside monomers containing long-chain, branched, and rigid twisted structures as interfacial polymerization monomers to prepare porous nanofiltration membranes. From a molecular structure perspective, the greatest advantage of nucleoside monomers lies in their perfect combination of multiple reactive sites, molecular rigidity, and excellent hydrophilicity within a single molecule. The multiple low-reactivity hydroxyl groups, multiple reactive sites, and density within the monomer structure, along with its inherent rigid twisted structure, can effectively regulate the porosity of the polyamide ester layer.

[0018] The prepared loose nanofiltration membrane consists of a porous support membrane and a polyamide ester separation layer. The polyamide ester separation layer is prepared by one-step interfacial polymerization of a polyacrylamide chloride organic solution and a nucleoside monomer aqueous solution on the surface of the porous support membrane. The presence of rigid benzene rings or branched chains containing hydroxyl reaction sites in the nucleoside monomers endows the monomer molecules with a large spatial size and low reactivity, thereby effectively increasing the free volume of the polymer generated by the interfacial polymerization reaction. This is beneficial for forming a loose polyamide ester separation layer structure, thereby promoting the transmembrane mass transfer of inorganic salt ions, increasing the permeability of inorganic salts, and further promoting their efficient separation from dyes.

[0019] The advantages and beneficial effects of this invention are: (1) This invention selects nucleoside monomers containing long-chain, branched, and rigid twisted structures as aqueous monomers for interfacial polymerization. Based on the spatial size of the monomer structure, the multiple low-reactivity hydroxyl groups, reaction sites, and density, the density of the polyamide ester layer can be effectively controlled to obtain a dye separation membrane with high flux and inorganic salt ion permeability. This breaks through the bottleneck of traditional polyamide membranes in balancing flux and rejection rate.

[0020] (2) The loose nanofiltration membrane of the present invention has a high rejection rate for dyes such as Congo red and methylene blue, and also has a high water and inorganic salt permeability. It can be applied to wastewater treatment fields such as dye / inorganic salt separation, dye production purification, and dye wastewater desalination.

[0021] (3) The preparation method of the present invention is simple, efficient and economical, with mild preparation conditions, environmentally friendly and non-toxic, easy to scale up and promote, and easy to realize industrial production. Attached Figure Description

[0022] Figure 1 The molecular weight cutoff test diagram of the loose nanofiltration membrane prepared in Example 1; Figure 2 The pore size test diagram is shown for the loose nanofiltration membrane prepared in Example 1. Figure 3 The molecular weight cutoff of the loose nanofiltration membrane prepared for Comparative Example 1 is shown in the graph. Figure 4 Pore ​​size test diagram of the loose nanofiltration membrane prepared for Comparative Example 1; Figure 5 The charge characteristic curves of the loose nanofiltration membranes prepared at different ADO (adenosine) monomer concentrations in Examples 1-3 are shown.

[0023] Figure 6 The charge characteristic curves are for the loose nanofiltration membranes prepared at different TMC (trimethylammonium chloride) monomer concentrations in Examples 1, 4, and 5. Detailed Implementation

[0024] To better understand the purpose, preparation method, and function of this invention, a method for preparing a loose nanofiltration membrane that can be used for dye / inorganic salt separation is described in further detail below. All raw materials used in the following examples are commercially available analytical grade raw materials.

[0025] Example 1 A method for preparing a loose nanofiltration membrane for dye / inorganic salt separation, comprising the following steps: S1: Pretreatment of the porous support membrane; Cut out several pieces The polysulfone ultrafiltration membrane was soaked in deionized water for 24 hours, then removed and pasted onto a polytetrafluoroethylene plate. The surface deionized water was removed by blowing with an air knife; the blowing temperature was 26℃ and the blowing time was 2 minutes.

[0026] S2: Weigh 2.5g of adenosine using an analytical balance, then dissolve the weighed adenosine completely in a 100mL beaker with deionized water, and then transfer it to a volumetric flask and add deionized water to make up to 500mL; to obtain an aqueous solution with a concentration of 0.5wt% adenosine, immerse the pretreated polysulfone ultrafiltration membrane in the aqueous solution containing 0.5wt% adenosine for 2min, remove it, and then use an air knife to purge away the excess aqueous solution on the surface of the base membrane. The purging temperature is 25℃ and the purging time is 0.5min, to obtain a polysulfone ultrafiltration membrane adsorbed with adenosine.

[0027] S3: Weigh 0.23 g of trimesoyl chloride using an analytical balance, then completely dissolve the weighed trimesoyl chloride in n-hexane in a 100 mL beaker, and quickly transfer it to a volumetric flask to make up to 500 mL, obtaining a 0.07 wt% trimesoyl chloride n-hexane solution; then pour the 0.07 wt% trimesoyl chloride n-hexane solution onto a polysulfone ultrafiltration membrane adsorbed with adenosine to carry out interfacial polymerization reaction, let it stand for 3 min, then pour off the excess solution, take it out and let it dry at room temperature for 5 min to obtain a loose nanofiltration membrane, and store it in deionized water.

[0028] Example 2 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as those in Example 1, the only difference being that the concentration of adenosine in the aqueous solution in step S2 is 0.3 wt%.

[0029] Example 3 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as those in Example 1, the only difference being that the concentration of adenosine in the aqueous solution in step S2 is 0.4 wt%.

[0030] Example 4 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as those in Example 1, the only difference being that the concentration of trimesoyl chloride in the hexane solution of trimesoyl chloride in step S3 is 0.05wt.

[0031] Example 5 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as those in Example 1, the only difference being that the concentration of trimesoyl chloride in the hexane solution of trimesoyl chloride in step S3 is 0.1 wt%.

[0032] Example 6 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the interfacial polymerization reaction time after pouring the n-hexane solution containing trimesoyl chloride is 4 min.

[0033] Example 7 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the interfacial polymerization reaction time after pouring the n-hexane solution containing trimesoyl chloride is 5 min.

[0034] Example 8 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the aqueous phase solution used is a 0.5wt% guanosine aqueous solution.

[0035] Example 9 A method for preparing a loose nanofiltration membrane for dye / salt separation is provided. The preparation steps are the same as in Example 1, except that the aqueous phase solution used is a 0.5 wt% aqueous solution of cytosine nucleoside.

[0036] Comparative Example 1 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the aqueous phase solution used is an aqueous solution of piperazine with a concentration of 0.5 wt%.

[0037] Comparative Example 2 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that several pieces are cut out. The commercial nanofiltration membrane NF270 was stored in deionized water for 24 hours for later use.

[0038] Comparative Example 3 A method for preparing a loose nanofiltration membrane for dye / salt separation is provided. The preparation steps are the same as in Example 1, except that the porous base membrane used is a commercial polyacrylonitrile ultrafiltration membrane.

[0039] Comparative Example 4 A method for preparing a loose nanofiltration membrane for dye / salt separation is provided. The preparation steps are the same as in Example 1, except that the organic solvent used is n-heptane.

[0040] Comparative Example 5 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the adenosine concentration in the adenosine aqueous solution is 0.05wt%.

[0041] Comparative Example 6 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the adenosine concentration in the adenosine aqueous solution is 1.0 wt%.

[0042] Comparative Example 7 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the concentration of trimesoyl chloride in the hexane solution is 0.01 wt%.

[0043] Comparative Example 8 A method for preparing a loose nanofiltration membrane for dye / salt separation, the preparation method steps are the same as in Example 1, the only difference being that the concentration of trimesoyl chloride in the hexane solution is 0.4 wt%.

[0044] The separation performance of the loose nanofiltration membranes prepared in Examples 1-9 and Comparative Examples 1-8 was tested using a membrane performance evaluation instrument. The test conditions were: room temperature; feed pressure 3.5 bar; inorganic salt concentration: 2 g / L; dye concentration: 0.2 g / L. Table 1. Test results of the separation performance of the loose nanofiltration membranes prepared in Examples 1-9 and Comparative Examples 1-8.

[0045] As shown in Table 1, based on Examples 1-9 and Comparative Examples 1-2, the loose nanofiltration membranes prepared using adenosine, guanosine, and cytidine exhibit lower inorganic salt rejection rates compared to the loose nanofiltration membranes prepared using piperazine as the aqueous monomer and the commercial NF270 membrane, under the same dye rejection capacity. In particular, the loose nanofiltration membrane prepared from adenosine monomer in Example 1 demonstrates excellent water permeability (49.6 L·m⁻¹). -2 ·h -1 ·bar -1 This method achieves a high rejection rate (99.68%) for the dye Congo Red and low rejection rates for the two inorganic salts (NaCl: 0%, Na2SO4: 6.1%), thus exhibiting excellent dye / salt separation selectivity. The method demonstrates that while maintaining the dye rejection of the nanofiltration membrane, it also improves the permeability of inorganic salt ions and the pure water flux. This is because nucleoside monomers containing rigid, twisted structures, multiple reaction sites, and low-reactivity hydroxyl groups have larger spatial dimensions, less steric hindrance, and lower reactivity compared to traditional interfacial polymerization diamine monomers (such as piperazine). This effectively increases the free volume of the polymer in the interfacial polymerization reaction, which is beneficial for forming a loose separation layer structure and a larger pore size structure. Figure 1 This improves the permeability of water and inorganic salts, further promoting their efficient separation from dyes.

[0046] As shown in Table 1, compared with the loose nanofiltration membranes prepared using polyacrylonitrile as the base membrane or n-heptane as the organic solvent in Comparative Examples 3-4, the loose nanofiltration membrane of Example 1 exhibits a better balance between high dye rejection, high salt permeability, and high water permeability under the same preparation process and testing conditions. This is attributed to the more suitable affinity between the polysulfone membrane surface and water molecules, which provides a relatively stable and complete uniform "phase interface" for the polymerization reaction between adenosine monomers and the n-hexane solution of acyl chloride. In contrast, the stronger hydrophobicity of the polyacrylonitrile surface hinders the wetting and diffusion of the aqueous monomer solution on the base membrane surface, affecting the uniformity of the initial thin film of interfacial polymerization. Compared to n-hexane, n-heptane has a higher viscosity, which slows down the diffusion rate of oil-phase monomers to the water-oil interface. This may lead to a slower initial rate of interfacial polymerization, resulting in a thicker initial separation layer with larger nodules, or less uniformity than the film formed using n-hexane, easily introducing defects and affecting the final separation performance.

[0047] As shown in Table 1, by comparing the data of Example 1 and Comparative Examples 5-8, it can be seen that the concentration of monomers in the aqueous and organic phases of the present invention affects the dye / salt selectivity of the membrane. Comparative Example 5 (adenosine concentration of 0.05 wt%) and Comparative Example 7 (pyromellitic chloride concentration of 0.01 wt%) show that when the concentration of adenosine or pyromellitic chloride is too low, a complete and dense separation layer cannot be formed, resulting in a significant decrease in the dye rejection rate (57.15% and 55.87%, respectively), making selective separation of inorganic salts impossible. Comparative Example 6 (adenosine 1.0 wt%) shows that at higher concentrations, due to the excessive participation of monomers in the aqueous phase, the separation layer becomes too dense, the salt rejection rate increases, and selective separation of dye / salt cannot be achieved. In Comparative Example 8, the excessively high concentration of pyromellitic chloride (0.4 wt%) leads to rapid polymerization, and the resulting polymer will hinder the further diffusion of ADO into the organic phase to react with TMC, ultimately forming a relatively loose separation layer.

[0048] The molecular weight cutoff and pore size of the nanofiltration membrane prepared in Example 1 are as follows: Figure 1-2 As shown, the molecular weight cutoff and pore size of the nanofiltration membrane prepared in Comparative Example 1 are as follows: Figure 3-4As shown, the membrane prepared in Example 1 has a molecular weight cutoff of 16190 Da and an effective pore size of 1.5 nm, exhibiting a relatively wide pore size distribution, which falls within the typical pore size range of a loose nanofiltration membrane. The membrane prepared in Comparative Example 1 has a molecular weight cutoff of 506 Da and an effective pore size of 0.3 nm, which falls within the typical pore size range of a dense nanofiltration membrane. This is because the reaction rate between piperazine and trimesoyl chloride is too high, resulting in a higher degree of cross-linking of the generated polyamide separation layer, thus making the membrane structure more compact. Example 1, by using adenosine with a rigid twisted structure and multiple low-activity hydroxyl reaction sites as the aqueous monomer, provides a larger free volume and a lower reaction rate, achieving a one-step control from a dense nanofiltration membrane to a loose nanofiltration membrane. This increases the pore size and porosity of the nanofiltration membrane, improving the separation efficiency of dyes and salts.

[0049] The charge properties of loose nanofiltration membranes prepared at different ADO (adenosine) monomer concentrations in Examples 1-3 are as follows: Figure 5 As shown, this phenomenon is due to the hydrolysis of unreacted acyl chloride groups on the polyamide ester layer, leading to the formation of negatively charged carboxyl groups on the membrane surface, thus making the membrane negatively charged. The negative potential of the membrane surface increases with the increase of the concentration of ADO monomer in the aqueous phase. This is mainly because when the concentration of TMC is constant, more and more aqueous monomers participate in the reaction, and the amount of TMC remaining on the membrane surface will decrease accordingly, which reduces the number of negatively charged carboxyl groups on the membrane surface.

[0050] The charge characteristics of loose nanofiltration membranes prepared at different TMC (trimethylammonium chloride) monomer concentrations in Examples 1, 4, and 5 are as follows: Figure 6 As shown, the electronegativity of the loose nanofiltration membrane surface gradually increases with increasing TMC concentration to 0.07 wt%. This is because with increasing TMC concentration, more TMC monomers participate in the polymerization reaction, increasing the content of acyl chloride groups on the membrane surface. This results in an increase in the content of carboxyl groups hydrolyzed into carboxyl groups after the reaction, thus enhancing the electronegativity of the loose nanofiltration membrane surface. When the TMC concentration increases to 0.1 wt%, the electronegativity increases along with the crosslinking, leading to a slight increase in the retention rate of dyes and inorganic salts under the synergistic effect of pore size sieving and electrostatic repulsion.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a loose nanofiltration membrane, characterized in that, Includes the following steps: S1: Pretreatment of the porous support membrane; S2: Immerse the pretreated porous support membrane in an aqueous solution of nucleoside monomers with a concentration of 0.1-0.8 wt% for 1-2 min, remove it and purge to remove the aqueous solution to obtain a support membrane adsorbed with nucleoside monomers; the nucleoside monomers are one or more of adenosine, guanosine, and cytosine. S3: Pour a 0.03-0.2 wt% polyacyl chloride organic phase solution onto the surface of the support membrane on which nucleoside monomers are adsorbed, carry out an interfacial polymerization reaction, remove the membrane and dry it at room temperature to obtain a loose nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, The concentration of the nucleoside monomer in the aqueous solution is 0.3-0.5 wt%.

3. The preparation method according to claim 1, characterized in that, The nucleoside monomer mentioned is adenosine.

4. The preparation method according to claim 1, characterized in that, The porous support membrane is one or more of the following: polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, and polyacrylonitrile ultrafiltration membrane, with a molecular weight cutoff of 80-120 kDa.

5. The preparation method according to claim 1, characterized in that, The polyacyl chloride is pyromellitic trimethylol chloride.

6. The preparation method according to claim 1, characterized in that, In step S2, the purging temperature is 24-26℃ and the purging time is 1-2 min.

7. The preparation method according to claim 1, characterized in that, In step S3, the interfacial polymerization reaction takes 1-8 minutes, the reaction temperature is 24-26°C, and the drying time at room temperature is 5-8 minutes.

8. The preparation method according to claim 1, characterized in that, In step S3, the organic solvent of the polyacrylamide chloride organic phase solution is n-hexane.

9. A loose nanofiltration membrane, characterized in that, The polyamide ester separation layer, which is formed on the porous support membrane by interfacial polymerization of a porous support membrane, a nucleoside monomer, and a polyacrylamide chloride monomer, is prepared by any one of the preparation methods described in claims 1-8.

10. The application of a loose nanofiltration membrane as described in claim 9 in dye / inorganic salt separation.

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