Bi-crosslinking hydrogel membrane for efficient dye desalination and preparation method thereof

Abstract

The invention relates to the technical field of membrane separation, in particular to a bi-crosslinking hydrogel membrane for efficient dye desalination and a preparation method of the bi-crosslinking hydrogel membrane. According to the invention, through well-designed Uio-66-NH2 nano-particle introduction and a secondary cross-linking process, the hydrogel membrane is ensured to effectively reduce the salt rejection rate, and meanwhile, the flux and the mechanical property are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and specifically to a double-crosslinked hydrogel membrane for efficient dye desalination and a preparation method thereof. Background Art

[0002] Currently, the commercial membranes used for dye desalination are mainly traditional polymer membranes, which have problems such as complex membrane preparation methods, high membrane costs, easy membrane fouling, and difficult degradation of waste membranes. To solve these problems, a variety of natural hydrogels have been developed as membrane raw materials for new hydrogel membranes. These hydrogel materials are widely sourced and low-cost, and the preparation process of the hydrogel membrane is simple. The large number of hydrophilic groups present on the membrane surface, such as hydroxyl, carboxyl, and amino groups, can also endow it with anti-oil and anti-fouling characteristics. In addition, the used hydrogel membrane can be degraded by various methods to prevent secondary pollution.

[0003] In recent years, sodium alginate has been widely studied as a hydrogel membrane material. The main problem it has is relatively low mechanical strength, which is related to its own molecular structure characteristics: the forces between the polymer chains that make up the sodium alginate hydrogel membrane are mainly weak van der Waals forces and hydrogen bonds, etc. And since the large number of hydrophilic groups contained in the hydrogel form hydrogen bond interactions with water molecules, which will cause the hydrogel to absorb water and swell, the presence of a large number of water molecules will weaken the interaction between the polymer chains, making the material as a whole exhibit relatively low strength and hardness. The existing solution is to use a variety of crosslinking agents to increase the internal crosslinking points of the hydrogel, thereby expanding the stress transfer range of the hydrogel membrane and reducing the risk of stress concentration and subsequent rupture or deformation under external forces. However, the hydrogel membrane prepared using crosslinking agents currently has a relatively low flux during dye desalination, and there is no clear guiding direction regarding the crosslinking method of the crosslinking agent.

[0004] Metal ion crosslinking is one of the more common crosslinking methods for hydrogel membranes. To improve the crosslinking effect, the prior art has adopted crosslinking with, for example, Ca 2+ / Ba 2+ mixed ions to endow it with high stability. However, the inventor found that the hydrogel membrane formed by this crosslinking method has a relatively low flux. To improve such a situation, the inventor tried to use Fe 3+ for secondary crosslinking to expand the pore size and increase the flux. Compared with rare metal ions such as La 3+ , Fe 3+ is widely sourced, and the preparation and purification processes are simple, and the ionic radius is relatively small. When coordinating with the functional groups of the hydrogel, it can bind more tightly, forming a coordination bond with a shorter bond length and relatively higher bond energy, making the crosslinking point more stable, and thus having a more significant effect on improving the mechanical properties of the hydrogel. However, using Fe 3+Performing secondary crosslinking will cause the expansion of membrane pores to be too strong, making the membrane structure unstable and affecting the rejection effect. Therefore, it is urgent to optimize the existing method of crosslinking metal ions in hydrogel membranes to solve the above problems. Summary of the Invention

[0005] The present invention aims to provide an innovative double-crosslinked hydrogel membrane for efficient dye desalination and its preparation method to effectively solve the deficiency of existing hydrogel membranes in terms of flux.

[0006] The present invention provides a preparation method of a double-crosslinked hydrogel membrane for efficient dye desalination, which comprises the following steps: (1) adding sodium alginate powder into deionized water to form a casting solution, and uniformly coating the casting solution on a substrate membrane; (2) immersing the substrate membrane coated with the casting solution into a first mixed aqueous solution containing double divalent metal ions for the first crosslinking; (3) immersing the hydrogel membrane after the first crosslinking into a second mixed aqueous solution containing Fe 3+ and divalent metal ions for the second crosslinking; (4) storing the prepared double-crosslinked hydrogel membrane in deionized water.

[0007] Preferably, in step (1), Uio-66-NH2 nanoparticles are further added to the casting solution, the concentration of sodium alginate in the casting solution is 1.0-4.0 wt%, and the concentration of Uio-66-NH2 nanoparticles is 0.02-0.06 wt%.

[0008] Preferably, in step (1), the casting solution needs to be stirred and left to stand for defoaming before being coated on the substrate membrane. The stirring is carried out at a rotation speed of 200 rpm-500 rpm for 5 h-20 h, and the standing defoaming time is 2 h-8 h.

[0009] Preferably, in step (1), the substrate membrane is selected from one of polysulfone, polyethersulfone, polyvinylidene fluoride, polytetrafluoroethylene, and polyolefin, and the pore size of the substrate membrane is 0.1-0.4 μm.

[0010] Preferably, in step (1), the coating thickness of the casting solution is 0.3-0.6 mm.

[0011] Preferably, in step (2), the double divalent metal ions in the first mixed solution are Ca 2+ and Ba 2+ ; the concentration of Ca 2+ is 1.5-2.5 wt%, and the concentration of Ba 2+ is 0.1-0.3 wt%.

[0012] Preferably, in step (3), the divalent metal ion in the second mixed solution is Ca 2+ , wherein Ca2+ with a concentration of 1.5 - 2.5 wt%, Fe 3+ with a concentration of 0.1 - 0.3 wt%. Preferably, the time for the first cross - linking in step (3) is 4 - 8 h, and the time for the second cross - linking in step (4) is 10 - 14 h.

[0013] The present invention also provides a double - crosslinked hydrogel membrane for efficient dye desalination prepared according to the above - mentioned preparation method.

[0014] The double - crosslinked hydrogel membrane for efficient dye desalination prepared by the present invention can be applied to dye desalination.

[0015] Compared with the prior art, the present invention has the following effects: First of all, the present invention selects Fe 3+ as the secondary cross - linker. Compared with rare and complex - preparation metal ions such as La 3+ , Fe 3+ is widely sourced, and its preparation and purification processes are simple. Moreover, the ionic radius of Fe 3+ is relatively small. When coordinating with the hydrogel functional groups, it can form a tighter and more stable bond. The generated coordination bond has a shorter bond length and higher bond energy, thus significantly improving the stability of the cross - link points and having a particularly significant effect on enhancing the mechanical properties of the hydrogel. At the same time, adding an appropriate amount of Ca 2+ to the secondary cross - linking solution effectively plays a buffering role, avoiding problems such as unstable membrane structure and decreased retention effect caused by excessive expansion of membrane pores.

[0016] Secondly, through the carefully designed introduction of Uio - 66 - NH2 nanoparticles and the secondary cross - linking process, the present invention ensures that while effectively reducing the salt rejection rate of the hydrogel membrane, it realizes a significant improvement in flux and mechanical properties. This innovation enables the salt rejection rate of the membrane to be reduced to 1.67%, while the flux is as high as 102.06 LMHB, meeting the requirements of efficient dye desalination.

[0017] Finally, the doped Uio - 66 - NH2 nanoparticles in the present invention have a typical high specific surface area and rich pore structure, providing more adsorption sites and mass transfer channels for the hydrogel membrane, which is extremely beneficial to improving the separation performance of the hydrogel membrane. The amino groups on the surface of Uio - 66 - NH2 further enhance the cross - linking effect between the nano - filler and the hydrogel, making the overall performance of the hydrogel membrane more excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : SEM images of the surface and cross - section of membranes B1 and B3 prepared in Example 3 (a - surface of B1 membrane, b - cross - section of B1 membrane, c - surface of B3 membrane, d - cross - section of B3 membrane); Figure 2 : AFM images of membranes B1, B3, and B4 prepared in Examples 1, 3, and 4 (a - B1, b - B3, c - B4); Figure 3 : Stress - strain curves of membranes B1 - B4 prepared in Examples 1 - 4; Figure 4 : Schematic diagram of the filtration experimental device used in the examples of the present invention. Detailed implementation manners

[0019] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the essence of the present invention all fall within the scope of the present invention.

[0020] Example 1 The double - crosslinked condensate hydrogel membrane of this example was prepared by the following method: (1) Add 0.125 g of sodium alginate powder to 4.855 ml of deionized water, stir with a magnetic stirrer at 300 rpm for 12 h until completely dissolved to form a uniform casting solution, and let it stand for 6 h to remove air bubbles; use an automatic film coater to uniformly coat the casting solution on a PES substrate membrane with a pore size of 0.22 μm, and the thickness of the casting solution is 0.5 mm.

[0021] (2) Immerse the PES substrate membrane coated with the casting solution in a mixed aqueous solution containing 2 wt% calcium chloride and 0.2 wt% barium chloride for 6 h.

[0022] (3) Subsequently, immerse the divalent - ion - crosslinked sodium alginate hydrogel membrane in a mixed aqueous solution containing 2 wt% calcium chloride and 0.2 wt% iron chloride for 12 h to form a second - crosslinked sodium alginate hydrogel membrane B1 (Ca / Ba - 6 h + Ca / Fe - 12 h).

[0023] (4) Store the prepared sodium alginate hydrogel membrane B1 (Ca / Ba - 6 h + Ca / Fe - 12 h) in deionized water.

[0024] Example 2 (1) Add 0.125 g of sodium alginate powder and 0.01 g of Uio - 66 - NH2 nanoparticles to 4.865 ml of deionized water, stir with a magnetic stirrer at 300 rpm for 12 h until completely dissolved to form a uniform casting solution, and let it stand for 6 h to remove air bubbles; use an automatic film coater to uniformly coat the casting solution on a PES substrate membrane with a pore size of 0.22 μm, and the thickness of the casting solution is 0.5 mm.

[0025] (2) Immerse the PES substrate film coated with the casting solution into an aqueous solution mixture containing 2 wt% calcium chloride and 0.2 wt% barium chloride for 6 h.

[0026] (3) Subsequently, immerse the divalent ion-crosslinked alginate hydrogel film into an aqueous solution mixture containing 2 wt% calcium chloride and 0.2 wt% iron chloride for 12 h to form a secondarily crosslinked alginate hydrogel film B2 (Ca / Ba - 6 h + Ca / Fe - 12 h + 0.2 wt% MOF).

[0027] (4) Store the prepared alginate hydrogel film B2 (Ca / Ba - 6 h + Ca / Fe - 12 h + 0.2 wt% MOF) in deionized water.

[0028] Example 3 The difference between Example 3 and Example 2 is that in step (1), 0.125 g of sodium alginate powder and 0.02 g of Uio-66-NH2 nanoparticles are added to 4.855 ml of deionized water. Finally, an alginate hydrogel film B3 (Ca / Ba - 6 h + Ca / Fe - 12 h + 0.4 wt% MOF) is obtained.

[0029] Example 4 The difference between Example 4 and Example 2 is that in step (1), 0.125 g of sodium alginate powder and 0.04 g of Uio-66-NH2 nanoparticles are added to 4.835 ml of deionized water. Finally, a B3 (Ca / Ba - 6 h + Ca / Fe - 12 h + 0.8 wt% MOF) film is obtained.

[0030] Comparative Example 1 The double-crosslinked hydrogel film of this comparative example is prepared by the following method: (1) Add 0.125 g of sodium alginate powder to 4.855 ml of deionized water, stir with a magnetic stirrer at a speed of 300 rpm for 12 h until completely dissolved to form a uniform casting solution, and let it stand for 6 h to remove bubbles; use an automatic film coater to uniformly coat the casting solution on a PES substrate film with a pore size of 0.22 μm, and the thickness of the casting solution is 0.5 mm.

[0031] (2) Immerse the PES substrate film coated with the casting solution into an aqueous solution mixture containing 2 wt% calcium chloride and 0.2 wt% iron chloride for 18 h to form a secondarily crosslinked alginate hydrogel film F1 (Ca / Fe - 18 h).

[0032] (4) Store the prepared alginate hydrogel film F1 (Ca / Fe - 18 h) in deionized water.

[0033] Comparative Example 2 The double-crosslinked condensate hydrogel film of this embodiment is prepared by the following method: (1) Add 0.125 g of sodium alginate powder to 4.855 ml of deionized water, stir with a magnetic stirrer at a speed of 300 rpm for 12 h until completely dissolved to form a uniform casting solution, and let it stand for 6 h to remove air bubbles; use an automatic film coater to uniformly coat the casting solution on a PES substrate film with a pore size of 0.22 μm, and the thickness of the casting solution is 0.5 mm.

[0034] (2) Immerse the PES substrate film coated with the casting solution in a mixed aqueous solution containing 2 wt% calcium chloride and 0.2 wt% barium chloride for 6 h.

[0035] (3) Subsequently, immerse the divalent ion-crosslinked sodium alginate hydrogel film in a mixed aqueous solution containing 0.2 wt% iron chloride for 12 h to form a second-crosslinked sodium alginate hydrogel film B1(Ca / Ba-6 h+Fe-12 h).

[0036] (4) The prepared sodium alginate hydrogel film F1(Ca / Ba-6 h+ Fe-12 h) film is stored in deionized water.

[0037] Characterization (1) Physical characterization ① Scanning electron microscope (SEM) and atomic force microscope (AFM) Perform SEM characterization on the sodium alginate hydrogel films B1 and B3 prepared in Example 3. The results are as Figure 1 shown Figure 1 (b) and Figure 1 (d) respectively correspond to the uniform and dense hydrogel cross-sections of film B1 and film B3. Figure 1 (a) and Figure 1 (c) respectively correspond to the surface morphologies of film B1 and film B3. It can be seen that the modification of the nanomaterial makes the film surface rougher. Perform AFM characterization on the sodium alginate hydrogel films B1, B3, and B4 prepared in Examples 1, 3, and 4. The results are as Figure 2 shown. The average surface roughness (Ra) of film B1, film B3, and film B4 are 36.1 nm, 148 nm, and 515 nm respectively. By comparing the surface roughness of the films, it is found that with the addition of the nanomaterial, the roughness of the film surface is significantly improved. The microscopic concave-convex structure corresponding to the film surface roughness will increase the actual water-contact surface area of the film. This increased surface area can increase the water-passing area per unit of the film, thereby improving the filtration ability of the film to a certain extent.

[0038] ② Mechanical property characterization Perform stress-strain curves on the film samples of B1-B4, as Figure 3As shown, by comparing the breaking strength and elongation at break of the membranes, it can be seen that the addition of nano-fillers improves the strength and deformation resistance of the membranes, that is, enhances the stability of the membranes during the separation driving process.

[0039] (2)Characterization of membrane permeability ① Filter experiment device and experimental conditions The experimental device used in this evaluation is a self-made device in the laboratory, and the schematic diagram is shown in Figure 4. Among them, the circulating water tank serves as both the raw water tank and receives the circulating effluent from the membrane unit. The pressure pump provides the working pressure required for membrane filtration. The membrane unit plays a filtering role. The container on the balance system receives the permeated effluent. The balance system and the computer system work together to record the real-time flux of membrane filtration and save it. Tests such as membrane permeability, rejection performance, and separation performance all rely on this device to complete.

[0040] To obtain stable membrane performance, the membrane is pre-soaked in deionized water before use and pre-pressed to stability at a pressure of 1 bar, and then the performance test is carried out. The working pressure of this study is constantly 1 bar.

[0041] ② Membrane performance evaluation 1)Membrane permeability The permeation flux of the membrane refers to the volume of the permeated liquid per unit time and per unit membrane area, and the specific calculation is shown in formula (1). To ensure the stability of membrane performance during the test, before the formal test, it is necessary to pre-press at the working pressure until the pure water permeation volume of the membrane remains unchanged.

[0042] In the formula, J is the permeation flux of the membrane (L / m 2 ·h) (LMH), V is the volume of the permeated effluent (L), A is the effective permeation area of the membrane (m 2 ), the effective area of the membrane in this study is 0.00134 m 2 , ∆t is the permeation time interval (h) 2)Membrane rejection performance The main rejection objects of the membranes in this study are dyes, inorganic salts, and organic substances in actual water bodies. For dyes, a 50 mg / L dye solution is used to test the rejection performance of the membranes for different dyes; for inorganic salts, sodium chloride and sodium sulfate are used to represent monovalent and divalent salts respectively, and a 1000 mg / L solution is prepared for the rejection performance test. The rejection rate of the membrane refers to the ratio of the change amount of the concerned index during the treatment process to the original value, and is expressed by formula (2-2). In the formula, R is the rejection rate of the membrane, C p (mg / L) and C f(mg / L) are the concentrations of the concerned indicators in the permeate solution and the feed solution, respectively. For the concentration of the dye solution, quantitative determination was carried out using a SPECORD-200 type ultraviolet-visible spectrophotometer (Analytikjena Company, Germany).

[0043] ③ Membrane permeation performance data Table 1 Permeation performance of membrane samples prepared in different examples and comparative examples As shown in Table 1, the membrane samples B1-B4 prepared by the method of the present invention have high fluxes, dye rejection rates and low salt rejection rates, and have high application value. The above shows and describes the basic principles, main features and advantages of the present invention. However, the above are only specific embodiments of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments obtained by those skilled in the art without departing from the technical solution of the present invention should be covered within the patent scope of the present invention.

Claims

1. A method for preparing a double cross-linked hydrogel membrane for efficient dye desalination, characterized in that The following steps are involved: (1) Sodium alginate powder is added to deionized water to form a casting solution, and the casting solution is evenly coated on the substrate membrane; (2) The substrate membrane coated with the casting solution is immersed in a first mixed aqueous solution containing divalent metal ions to perform the first crosslinking; (3) The hydrogel membrane after the first crosslinking is immersed in a first mixed aqueous solution containing Fe 3+ and a second mixed aqueous solution of divalent metal ions for a second cross-linking; (4) storing the prepared double cross-linked hydrogel membrane in deionized water.

2. The preparation method according to claim 1, characterized in that: In step (1), Uio-66-NH2 nanoparticles are further added to the casting solution, the concentration of sodium alginate in the casting solution is 1-4 wt%, and the concentration of Uio-66-NH2 nanoparticles is 0.02-0.06 wt%.

3. The preparation method according to claim 1, characterized in that: In step (1), the casting solution needs to be stirred and allowed to stand for degassing before being coated on the base film. The stirring is performed at a speed of 200 rpm-500 rpm for 12 h-18 h, and the standing degassing time is 2 h-8 h.

4. The preparation method according to claim 1, characterized in that: In step (1), the base membrane is selected from one of polysulfone, polyethersulfone, polyvinylidene fluoride, polytetrafluoroethylene, and polyolefin, and the pore size of the base membrane is 0.2-0.4 μm.

5. The preparation method according to claim 1, characterized in that: In step (1), the coating thickness of the casting solution is 0.3-0.6 mm.

6. The preparation method according to claim 1, characterized in that: In step (2), the divalent metal ion in the first mixed solution is the divalent ion Ca 2+ and Ba 2+ ; the Ca 2+ The concentration is 1.5-2.5 wt%, Ba 2+ The concentration is 0.1-0.3wt%.

7. The preparation method according to claim 1, characterized in that: In step (3), the divalent metal ion in the second mixed solution is Ca 2+ , where Ca 2+ Concentration is 1.5-2.5 wt%, Fe 3+ The concentration is 0.1-0.3 wt%.

8. The preparation method according to claim 1, characterized in that: The time for the first cross-linking in step (3) is 4-8 h, and the time for the second cross-linking in step (4) is 10-14 h. 9 . A double-crosslinked hydrogel membrane for efficient dye desalination prepared by the preparation method according to claim 1 .

10. Use of the double cross-linked hydrogel membrane for efficient dye desalination according to claim 9 in dye desalination.

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