Composite membrane for efficiently and selectively capturing trivalent chromic ions and preparation method thereof

Abstract

The invention relates to a composite membrane for efficiently and selectively capturing trivalent chromic ions and a preparation method of the composite membrane. The composite membrane is composed of a polyethersulfone (PES) base membrane, a zinc ion and biomolecule self-assembly layer and a ZIF-8 metal organic framework layer. The preparation method comprises the following steps: synergistically modifying the surface of the PES base membrane through pyrogallol and amino acid to form a stable negative charge layer; zinc ions and biomolecules are self-assembled on the surface of the modified membrane by utilizing electrostatic interaction, and a ZIF-8 layer is grown in situ by taking the modified membrane as a substrate. According to the composite membrane, efficient capture of Cr < 3 + > is achieved through the pore confinement effect and the surface functional group coordination effect of ZIF-8, the adsorption capacity is improved, the rejection rate exceeds 99%, and the composite membrane shows excellent selective adsorption performance in a complex coexisting ion system.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional polymer membrane materials, and particularly relates to a composite membrane with the function of efficiently selectively capturing trivalent chromium ions prepared by special modification and layer-by-layer self-assembly technology. Background Art

[0002] Heavy metal ion pollution is a major challenge in the global water treatment field, and the treatment of chromium ion pollution in industrial wastewater is particularly crucial. Chromium mainly exists in two oxidation states, Cr 3+ and Cr 6+ in the aqueous phase. Its non-biodegradability, high toxicity, and carcinogenic and teratogenic properties pose a serious threat to the ecological environment and human health.

[0003] In traditional chromium-containing wastewater treatment technologies, membrane separation has attracted much attention due to its advantages such as simple operation, low energy consumption, and easy scale-up. Ultrafiltration membranes are widely used in water treatment. However, ultrafiltration membranes have relatively large pore sizes (1 - 100 nm), and the removal efficiency of heavy metal ions is limited, making it difficult to meet the precise separation requirements. In recent years, nanomaterials such as metal-organic frameworks (MOF) have been widely used in membrane modification due to their high specific surface area and adjustable pore structures. Among them, ZIF-8 has become an ideal choice for functional materials due to its excellent chemical stability, mild synthesis conditions, and suitable pore sizes.

[0004] Biomolecule-derived materials show unique advantages in heavy metal removal because they can be naturally degraded after being discarded without secondary pollution, meeting the green chemistry requirements of the water treatment field. As described in the patent CN104084221A, the bio-based adsorbent, although environmentally friendly, has problems such as limited and unevenly distributed adsorption sites, poor selectivity for Cr 3+ and being easily interfered by coexisting ions; while the adsorbents prepared by high-temperature calcination methods (such as CN118908182A) have problems such as high energy consumption and complex treatment of regeneration waste liquid. In contrast, the present invention constructs a biomolecule-derived material intermediate layer within 30 - 40 minutes through room-temperature self-assembly technology, reducing energy consumption by 80%. Moreover, this layer serves as a "bridge" connecting the base membrane and the ZIF-8 layer, enabling the uniform growth of ZIF-8 nanocrystals (coverage rate > 95%) and tripling the adhesion force, avoiding performance degradation caused by the shedding of ZIF-8. In addition, the pyrogallol-amino acid modification layer introduces high-density amino, phenolic hydroxyl, and carboxyl groups, significantly enhancing the specific adsorption ability for Cr³⁺. Combining the size screening and coordination effects of ZIF-8, the efficient selective removal of Cr³⁺ is finally achieved. Summary of the Invention

[0005] The present invention relates to a method for preparing a composite membrane for efficiently and selectively capturing trivalent chromium ions, aiming to overcome the problems of insufficient adsorption capacity, difficult regeneration waste liquid treatment, high energy consumption and poor environmental protection in the prior art. The method includes the following steps: (1) Modification of the PES-based membrane: Dissolve pyrogallol (0.1 - 3 wt%), amino acid (0.1 - 3 wt%) and oxidant (0.05 - 1 wt%) in a dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution (0.05 - 0.15 M, pH = 8.5) at room temperature to prepare a pyrogallol-amino acid solution, and form a modifier through reaction. Immerse the polyethersulfone (PES)-based membrane in the above solution for 10 - 14 hours, then rinse it with deionized water and store it in deionized water to remove residues.

[0006] (2) Layer-by-layer self-assembly: Immerse the modified PES-based membrane in a 1 g / L zinc nitrate hexahydrate solution for 15 - 20 minutes. Utilize the abundant amino groups and carboxyl groups in the modifier to have electrostatic interaction or coordination with zinc ions, thereby realizing the adsorption of zinc ions. After repeatedly rinsing the membrane adsorbed with zinc ions with deionized water for multiple times, immerse it in a biomolecule-derived material solution and react for 15 - 20 minutes to connect the biomolecule-derived material with zinc ions, and repeat twice to form a two-layer self-assembled structure of (zinc ion - biomolecule-derived material layer - zinc ion - biomolecule-derived material layer).

[0007] (3) In-situ growth of ZIF-8 layer: First, fully wash the composite membrane completed with layer-by-layer self-assembly with flowing deionized water, and then immerse it in a zinc nitrate precursor solution (2.97 g zinc nitrate hexahydrate dissolved in 100 mL deionized water) for 2 - 8 hours to achieve uniform deposition of zinc ions on the membrane surface; subsequently, transfer the membrane loaded with zinc ions to a ligand solution (3.28 g 2-methylimidazole dissolved in 100 mL methanol) and let it stand for reaction for 24 hours. Through the coordination of zinc ions with imidazole ligands, in-situ construct a ZIF-8 metal-organic framework structure on the membrane surface.

[0008] (4) Post-treatment: Wash the prepared composite membrane three times with deionized water and methanol in sequence, and then immerse it in deionized water for storage for later use.

[0009] Preferably, the oxidant in step (1) is one of potassium persulfate and sodium periodate.

[0010] Preferably, the amino acid in step (1) is mainly one of tryptophan, tyrosine, taurine and glutamic acid.

[0011] Preferably, the biomolecule-derived material in step (2) is mainly one of sodium carboxymethyl cellulose, chitosan hydrochloride and cellulose nanofibers.

[0012] The present invention relates to a functional composite membrane material based on the synergistic assembly of biomolecule-derived materials and metal-organic frameworks (ZIF-8) and a preparation method thereof. Through an innovative hierarchical structure design, the membrane material realizes the efficient and selective separation of specific metal ions. The functional composite membrane adopts a sandwich structure of "base membrane - interfacial transition layer - ZIF-8 functional layer" ( Figure 1 ) design, where: (1) O-benzoquinone is generated on the surface of the base membrane by oxidizing pyrogallol. This active intermediate covalently binds to the amino group of the amino acid through an imine bond (-C=N-), and its dissociated carboxyl group (-COO⁻) forms stable negatively charged sites, providing active binding sites for the efficient adsorption of zinc ions; (2) The interfacial transition layer adopts a double-layer self-assembled structure, specifically: zinc ion layer - biomolecule-derived material layer - zinc ion layer - biomolecule-derived material layer; (3) The ZIF-8 functional layer is formed by the coordination reaction of the outermost zinc ions with 2-methylimidazole, having a controllable pore size distribution and abundant coordination sites.

[0013] Through the synergistic effect of biomolecule-derived materials and ZIF-8, this design realizes a dual separation mechanism of "size sieving + chemical adsorption": (1) The functional groups of biomolecule-derived materials provide specific adsorption sites; (2) The porous structure of ZIF-8 realizes precise size sieving; (3) The double-layer transition layer design ensures the uniform growth and stable binding of the functional layer. Beneficial effects

[0014] Through the double-layer structure design and the interfacial coordination synergistic mechanism, the balance bottleneck among the adsorption capacity, selectivity, and cycle stability of traditional membrane materials is broken, providing an efficient and reliable technical solution for the separation of heavy metal ions.

[0015] (1) Synergistic metal ion capture performance By inducing the directional growth of ZIF-8 through a double-layer self-assembled structure, a specific crystal structure with an interplanar spacing of 3.5 Å is formed, and its exposed imidazole ligand sites are highly matched with the size of Cr³ + hydrated ions. Combining with the three-dimensional adsorption network constructed by amino, carboxyl, and phenolic hydroxyl groups in the double-layer biomolecule-derived materials, the synergistic effect of "size sieving - chelating adsorption" is realized, significantly improving the selective adsorption efficiency of Cr³ + ions.

[0016] (2) Construction of a stable and uniform adsorption interface The double-layer biomolecular layer is uniformly anchored by zinc ions, which improves the uniformity of the distribution of adsorption sites, effectively inhibits the agglomeration of hydroxyl groups / amino groups in traditional graft modification, and reduces the fluctuation of the rejection rate under the competition of coexisting ions. At the same time, the biomolecular layer and ZIF-8 are chemically connected by coordination bonds. Compared with the physical adsorption binding method, the shedding rate of ZIF-8 is reduced and the stability is improved.

[0017] (3)pH-responsive specific selective adsorption Based on the synergistic effect of pyrogallol and amino acids in the biomolecule-derived material, a composite modified layer rich in phenolic hydroxyl groups and amino groups is formed under the condition of pH = 8.5. Its oxygen and nitrogen atoms form stable coordination bonds with Cr³ + through empty d orbitals, endowing the membrane material with specific recognition ability for target metal ions and improving the selective separation efficiency.

[0018] (3)Optimization of anti-pollution performance and structural stability The double-layer self-assembly design makes the ZIF-8 functional layer and the biomolecule interface bind more tightly, reducing the retention of pollutants in the membrane pores. After four cycles of use, the membrane flux recovery rate is increased by 15% compared with the single-layer structure and by 25% compared with the three-layer structure. The porosity is stably maintained above 70%, taking into account both high-efficiency separation and long-term operation stability. Description of the drawings

[0019] Figure 1 is a schematic diagram of the membrane structure in the present invention Figure 2 is a comparison diagram of the surface scanning electron microscopes of the composite membrane prepared in Example 4 of the present invention and the PES-based membrane. Detailed implementation manners

[0020] The present invention will be further described below in conjunction with specific embodiments. Embodiment

[0021] Step 1): Dissolve 0.1 wt% pyrogallol, 0.1 wt% tryptophan and 0.05 wt% potassium persulfate in 0.05 M, pH = 8.5 dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution, and react at room temperature for 10 minutes to obtain a pyrogallol-tryptophan solution (the molar concentrations of pyrogallol and tryptophan are equal). Immerse the original PES membrane in 100 ml of this solution for 10 hours for modification, and then rinse it with deionized water and store it in deionized water to remove residues.

[0022] Step 2): Immerse the modified PES membrane in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1 g / L for 15 minutes, and adsorb zinc ions by the electrostatic or coordination action of the amino and carboxyl groups of the modifier and zinc ions.

[0023] Step 3): After the membrane adsorbed with zinc ions was rinsed with deionized water for multiple times, it was immersed in 100 ml of sodium carboxymethylcellulose solution with a concentration of 1 g / L for 15 minutes to connect the sodium carboxymethylcellulose molecules with zinc ions, which was regarded as one layer. Through the layer-by-layer self-assembly technology, it was repeated twice to form a two-layer self-assembled structure of (zinc ion - sodium carboxymethylcellulose - zinc ion - sodium carboxymethylcellulose).

[0024] Step 4): After the membrane completed with layer-by-layer self-assembly was rinsed with flowing deionized water for multiple times, it was immersed in a solution prepared by dissolving 2.97 g of zinc nitrate hexahydrate in 100 ml of deionized water for 2 hours to grow a dense zinc ion layer. After the growth was completed, it was directly immersed in a 2-methylimidazole methanol solution prepared by dissolving 3.28 g of 2-methylimidazole in 100 ml of methanol solution for 24 hours to form a ZIF-8 structure on the outermost layer of the membrane. After the in-situ growth of the ZIF-8 structure was completed, the membrane was first rinsed three times with deionized water and then three times with methanol to wash away the unreacted raw materials. The composite membrane is highly hydrophilic and the flux recovery rate can reach 93.0%, and the chromium ion rejection rate > 99.0%. Example

[0025] Step 1): 0.2 wt% pyrogallol, 0.2 wt% tyrosine and 0.1 wt% potassium persulfate were dissolved in 0.1 M, pH = 8.5 dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution, and reacted at room temperature for 10 minutes to prepare a pyrogallol-tyrosine solution (pyrogallol and tyrosine had equal molar concentrations). The original PES membrane was immersed in 100 ml of this solution for 12 hours for modification, and then rinsed with deionized water and stored in deionized water to remove the residues.

[0026] Step 2): The modified PES membrane was immersed in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1.5 g / L for 20 minutes, and zinc ions were adsorbed by the electrostatic or coordination action of the amino and carboxyl groups of the modifier and zinc ions.

[0027] Step 3): After the membrane adsorbed with zinc ions was rinsed with deionized water for multiple times, it was immersed in 100 ml of chitosan hydrochloride solution with a concentration of 1 g / L for 20 minutes to connect the chitosan hydrochloride molecules with zinc ions, which was regarded as one layer. Through the layer-by-layer self-assembly technology, it was repeated twice to form a two-layer self-assembled structure of (zinc ion - chitosan hydrochloride - zinc ion - chitosan hydrochloride).

[0028] Step 4): After rinsing the film that has completed layer-by-layer self-assembly with flowing deionized water multiple times, immerse it in a solution prepared by dissolving 3 g of zinc nitrate hexahydrate in 100 ml of deionized water for 4 hours to grow a dense zinc ion layer. After the growth is completed, directly immerse it in a 2-methylimidazole methanol solution prepared by dissolving 3 g of 2-methylimidazole in 100 ml of methanol solution for 24 hours to form a ZIF-8 structure on the outermost layer of the film. After the in-situ growth of the ZIF-8 structure is completed, the film is first rinsed three times with deionized water and then three times with methanol to wash away the unreacted raw materials. The composite membrane is highly hydrophilic and the flux recovery rate can reach 94.0%, and the chromium ion rejection rate > 99.2%. Example

[0029] Step 1): Dissolve 0.3% pyrogallol, 0.3 wt% taurine, and 0.15 wt% sodium periodate (NaIO4) in a 0.15 M, pH = 8.5 dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution and react at room temperature for 10 minutes to prepare a pyrogallol-taurine solution (pyrogallol and taurine have equal molar concentrations). Immerse the original PES membrane in 100 ml of this solution for 14 hours for modification, and then rinse it with deionized water and store it in deionized water to remove the residues.

[0030] Step 2): Immerse the modified PES membrane in 100 ml of a zinc nitrate hexahydrate solution with a concentration of 2 g / L for 30 minutes, and adsorb zinc ions through the electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0031] Step 3): After rinsing the membrane adsorbed with zinc ions with deionized water multiple times, immerse it in 100 ml of a 2% cellulose nanofiber solution by mass for 30 minutes to connect the cellulose nanofiber molecules and zinc ions, and regard it as one layer. Through the layer-by-layer self-assembly technique, repeat twice to form a two-layer self-assembled structure of (zinc ion - cellulose nanofiber - zinc ion - cellulose nanofiber).

[0032] Step 4): After rinsing the film that has completed layer-by-layer self-assembly with flowing deionized water multiple times, immerse it in a solution prepared by dissolving 2.97 g of zinc nitrate hexahydrate in 100 ml of deionized water for 8 hours to grow a dense zinc ion layer. After the growth is completed, directly immerse it in a 2-methylimidazole methanol solution prepared by dissolving 3.5 g of 2-methylimidazole in 100 ml of methanol solution for 24 hours to form a ZIF-8 structure on the outermost layer of the film. After the in-situ growth of the ZIF-8 structure is completed, the film is first rinsed three times with deionized water and then three times with methanol to wash away the unreacted raw materials. The materials in the pores of the composite membrane grow uniformly. As compared with the original PES membrane Figure 1 shown, it is highly hydrophilic and the flux recovery rate can reach 94.0%, and the chromium ion rejection rate > 99.5%. Example

[0033] Step 1): Dissolve 0.2 wt% pyrogallol, 0.2 wt% glutamic acid, and 0.1 wt% sodium periodate (NaIO4) in 0.1 M, pH = 8.5 dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution, and react at room temperature for 10 minutes to obtain pyrogallol-glutamic acid solution (pyrogallol and glutamic acid have equimolar concentrations). Immerse the original PES membrane in 100 ml of this solution for 12 hours for modification, then rinse with deionized water and store in deionized water to remove residues.

[0034] Step 2): Immerse the modified PES membrane in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1.5 g / L for 20 minutes, and adsorb zinc ions by the electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0035] Step 3): After the membrane adsorbed with zinc ions is rinsed with deionized water multiple times, immerse it in 100 ml of sodium carboxymethylcellulose solution with a concentration of 1 g / L for 20 minutes, connect the sodium carboxymethylcellulose molecules with zinc ions, and regard it as one layer. Through the layer-by-layer self-assembly technique, repeat twice to form a two-layer self-assembled structure of (zinc ion-sodium carboxymethylcellulose-zinc ion-sodium carboxymethylcellulose).

[0036] Step 4): After the membrane that has completed layer-by-layer self-assembly is rinsed with flowing deionized water multiple times, immerse it in a solution prepared by dissolving 3 g of zinc nitrate hexahydrate in 100 ml of deionized water for 4 hours to grow a dense zinc ion layer. After the growth is completed, directly immerse it in a 2-methylimidazole methanol solution prepared by dissolving 3 g of 2-methylimidazole in 100 ml of methanol solution for 24 hours to form a ZIF-8 structure on the outermost layer of the membrane. After the in-situ growth of the ZIF-8 structure is completed, the membrane is first rinsed three times with deionized water and then three times with methanol to wash away the unreacted raw materials. The surface scanning electron micrograph of the membrane is as Figure 1 shown in the right figure. Compared with the original membrane, it can be clearly seen that the membrane surface is covered by the grown ZIF-8 material. The composite membrane is highly hydrophilic and the flux recovery rate can reach 95.0%, and the chromium ion rejection rate > 99.6%.

Claims

1. A method for preparing a composite membrane for efficiently and selectively capturing trivalent chromium ions, characterized in that The surface of the polyethersulfone membrane was synergistically modified by pyrogallol and amino acids to form a stable negatively charged layer on the membrane surface. Zinc ions and biomolecule-derived materials were self-assembled on the surface of the modified membrane through electrostatic interaction, and a ZIF-8 metal-organic framework layer was in-situ grown on this substrate to capture trivalent chromium ions. The specific preparation steps are as follows: (1)Modification of the PES-based membrane: Pyrogallol (0.1-3 wt%), amino acid (0.1-3 wt%) and oxidant (0.05-1 wt%) were dissolved in dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution (0.05-0.15 M, pH = 8.5) at room temperature to prepare a pyrogallol-amino acid solution, and the modifier was formed through reaction. The polyethersulfone (PES)-based membrane was immersed in the above solution for 10-14 hours, then rinsed with deionized water and stored in deionized water to remove residues; (2)Layer-by-layer self-assembly: The modified PES-based membrane was immersed in a 1 g / L zinc nitrate hexahydrate solution for 15-20 minutes. By using the abundant amino groups and carboxyl groups in the modifier, electrostatic interaction or coordination interaction occurred with zinc ions, thereby realizing the adsorption of zinc ions. After the membrane adsorbed with zinc ions was rinsed repeatedly with deionized water, it was immersed in the biomolecule-derived material solution and reacted for 15-20 minutes to connect the biomolecule-derived material with zinc ions. The process was repeated twice to form a two-layer self-assembled structure of (zinc ion-biomolecule-derived material layer-zinc ion-biomolecule-derived material layer); (3)In-situ growth of the ZIF-8 layer: First, the composite membrane that completed the layer-by-layer self-assembly was thoroughly washed with flowing deionized water and then immersed in a zinc nitrate precursor solution (2.97 g zinc nitrate hexahydrate dissolved in 100 mL deionized water) for 2-8 hours to achieve uniform deposition of zinc ions on the membrane surface. Subsequently, the membrane loaded with zinc ions was transferred to the ligand solution (3.28 g 2-methylimidazole dissolved in 100 mL methanol) and allowed to stand and react for 24 hours. Through the coordination interaction between zinc ions and imidazole ligands, a ZIF-8 metal-organic framework structure was in-situ constructed on the membrane surface; (4)Post-treatment: The prepared composite membrane was rinsed three times with deionized water and methanol in sequence and then immersed in deionized water for storage for later use.

2. According to the preparation method of a functional membrane material for efficiently and selectively capturing specific metal ions as described in claim 1, the oxidant is one of potassium persulfate and sodium periodate.

3. According to the preparation method of a functional membrane material for efficiently and selectively capturing specific metal ions as described in claim 1, the amino acid is one of tryptophan, tyrosine, taurine, and glutamic acid.

4. According to the preparation method of a functional membrane material for efficiently and selectively capturing specific metal ions as described in claim 1, the biomolecule-derived material is one of sodium carboxymethyl cellulose, chitosan hydrochloride, and cellulose nanofibers.

5. The preparation method of a composite membrane for efficiently and selectively capturing trivalent chromium ions according to claim 1, which prepares a functional membrane material for capturing specific trivalent chromium ions, is characterized in that The composite membrane is highly hydrophilic and the flux recovery rate can reach 95%, and the chromium ion rejection rate > 99%.

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