A composite membrane for efficiently and selectively capturing trivalent chromium ions and its preparation method

Abstract

This invention relates to a composite membrane for efficiently and selectively capturing trivalent chromium ions and its preparation method. The composite membrane consists 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 includes: synergistic modification of the PES base membrane surface with pyrogallol and amino acids to form a stable negatively charged layer; electrostatic self-assembly of zinc ions and biomolecules onto the modified membrane surface; and in-situ growth of the ZIF-8 layer using this as a substrate. This composite membrane achieves high-efficiency selective capture of chromium ions through the pore confinement effect and surface functional group coordination of ZIF-8. 3+ It exhibits highly efficient capture, enhanced adsorption capacity, and a retention rate exceeding 99%, and demonstrates excellent selective adsorption performance in complex coexisting ion systems.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer membrane materials technology, specifically relating to a composite membrane with highly efficient selective capture of trivalent chromium ions prepared through special modification and layer-by-layer self-assembly technology. Background Technology

[0002] Heavy metal ion pollution is a major challenge in the global water treatment field, with the treatment of chromium ion pollution in industrial wastewater being particularly critical. Chromium in aqueous phase is mainly in the form of Cr. 3+ and Cr 6+ The presence of two oxidation states, along with their non-biodegradability, high toxicity, and carcinogenic and teratogenic properties, poses 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 ease of scalability, with ultrafiltration membranes being widely used in water treatment. However, ultrafiltration membranes have relatively large pore sizes (1-100 nm), resulting in limited removal efficiency for heavy metal ions and making it difficult to meet the requirements for precise separation. In recent years, nanomaterials such as metal-organic frameworks (MOFs) have been widely used in membrane modification due to their high specific surface area and tunable pore structure. Among them, ZIF-8 has become an ideal choice for functionalization materials due to its excellent chemical stability, mild synthesis conditions, and suitable pore size.

[0004] Biomolecular-derived materials, due to their biodegradability after disposal and lack of secondary pollution, meet the green chemistry requirements of the water treatment field and exhibit unique advantages in heavy metal removal. For example, the bio-based adsorbent material described in patent CN104084221A, while environmentally friendly, suffers from limited and unevenly distributed adsorption sites, particularly for Cr. 3+ The ZIF-8 adsorbent has several drawbacks. Firstly, it suffers from poor selectivity and susceptibility to interference from coexisting ions. Secondly, adsorbents prepared by high-temperature calcination (such as CN118908182A) suffer from high energy consumption and complex regeneration wastewater treatment. In contrast, this invention utilizes room-temperature self-assembly technology to construct a biomolecule-derived material intermediate layer within 30-40 minutes, reducing energy consumption by 80%. This layer acts as a "bridge" connecting the base film and the ZIF-8 layer, achieving uniform growth of ZIF-8 nanocrystals (coverage > 95%), a 3-fold increase in adhesion, and preventing performance degradation caused by ZIF-8 detachment. Furthermore, the pyrogallol-amino acid modified layer introduces high-density amino, phenolic hydroxyl, and carboxyl groups, significantly enhancing the specific adsorption capacity for Cr³⁺. Combined with the size sieving and coordination effects of ZIF-8, this ultimately achieves highly efficient and selective removal of Cr³⁺. Summary of the Invention

[0005] This invention relates to a method for preparing a composite membrane for the efficient and selective capture of trivalent chromium ions, aiming to overcome the problems of insufficient adsorption capacity, difficult treatment of regeneration waste liquid, high energy consumption, and poor environmental performance of existing technologies. The method includes the following steps:

[0006] (1) Modification of PES base membrane: Dissolve 0.1-3 wt% of pyrogallol, amino acids and 0.05-1 wt% of oxidant in a 0.05-0.15 M dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution at room temperature to prepare pyrogallol-amino acid solution. After reaction, a modifier is formed. After immersing the polyethersulfone (PES) base membrane in the above solution for 10-14 hours, rinse with deionized water and store in deionized water to remove residues.

[0007] (2) Layer-by-layer self-assembly: The modified PES-based membrane is immersed in a 1 g / L zinc nitrate hexahydrate solution for 15-20 minutes. The abundant amino and carboxyl groups in the modifier interact with zinc ions through electrostatic or coordination interactions, thereby achieving the adsorption of zinc ions. After repeatedly rinsing the membrane with adsorbed zinc ions with deionized water, it is immersed in a biomolecular-derived material solution and reacted for 15-20 minutes to achieve the connection between the biomolecular-derived material and zinc ions. The above operation of adsorbing zinc ions and connecting biomolecular-derived materials is repeated twice to form a bilayer self-assembled structure with alternating layers of zinc ions and biomolecular-derived materials.

[0008] (3) In-situ growth of ZIF-8 layer: First, the composite membrane that has completed layer-by-layer self-assembly is thoroughly cleaned with flowing deionized water and then immersed in a zinc nitrate precursor solution for 2-8 hours. The zinc nitrate precursor solution is prepared by dissolving 2.97 g of zinc nitrate hexahydrate in 100 mL of deionized water to achieve uniform deposition of zinc ions on the membrane surface. Then, the membrane loaded with zinc ions is transferred to a ligand solution and allowed to stand for 24 hours. The ligand solution is prepared by dissolving 3.28 g of 2-methylimidazole in 100 mL of methanol. Through the coordination of zinc ions with imidazole ligands, a ZIF-8 metal-organic framework structure is constructed in-situ on the membrane surface.

[0009] (4) Post-treatment: The prepared composite membrane is rinsed three times with deionized water and methanol in sequence, and then soaked in deionized water for storage until use.

[0010] Preferably, the oxidant in step 1) is either potassium persulfate or sodium periodate.

[0011] Preferably, the amino acid in step 1) is mainly one of tryptophan, tyrosine, taurine, and glutamic acid.

[0012] Preferably, the biomolecular derivative material in step 2) is mainly one of sodium carboxymethyl cellulose, chitosan hydrochloride, or cellulose nanofibers.

[0013] This invention relates to a functional composite membrane material based on the synergistic assembly of biomolecule-derived materials and metal-organic frameworks (ZIF-8) and its preparation method. This membrane material achieves highly efficient and selective separation of specific metal ions through an innovative hierarchical structural design. The functional composite membrane adopts a sandwich structure of "base membrane - interface transition layer - ZIF-8 functional layer". Figure 1 The design includes:

[0014] (1) The base film surface generates o-quinone by oxidizing pyrogallol. This active intermediate is covalently bonded to the amino group of the amino acid through an imine bond (-C=N-). Its dissociated carboxyl group (-COO⁻) forms a stable negative charge site, providing an active binding site for the efficient adsorption of zinc ions.

[0015] (2) The interface transition layer adopts a double-layer self-assembly structure, specifically: zinc ion layer - biomolecule-derived material layer - zinc ion layer - biomolecule-derived material layer;

[0016] (3) The ZIF-8 functional layer is formed by the coordination reaction of the outermost zinc ions with 2-methylimidazole, and has an adjustable pore size distribution and abundant coordination sites.

[0017] This design achieves a dual separation mechanism of "size sieving + chemisorption" through the synergistic effect of biomolecular-derived materials and ZIF-8.

[0018] (1) The functional groups of biomolecular-derived materials provide specific adsorption sites;

[0019] (2) The porous structure of ZIF-8 enables precise size sieving;

[0020] (3) The double-layer transition layer design ensures uniform growth and stable integration of functional layers.

[0021] Beneficial effects:

[0022] By employing a bilayer structure design and an interfacial coordination synergy mechanism, the bottleneck of balancing adsorption capacity, selectivity, and cycle stability in traditional membrane materials has been overcome, providing an efficient and reliable technical solution for the separation of heavy metal ions.

[0023] The directional growth of ZIF-8 was induced by a bilayer self-assembly structure, resulting in a specific crystal structure with an interplanar spacing of 3.5 Å. The exposed imidazole coordination sites interact with Cr³⁺. + The hydrated ions are highly matched in size. A three-dimensional adsorption network constructed from amino, carboxyl, and phenolic hydroxyl groups in a bilayer biomolecular-derived material achieves a synergistic effect of "size sieving-chelation adsorption," enabling Cr³⁺ ions to be effectively adsorbed.+ The selective adsorption efficiency is significantly improved.

[0024] The bilayer biomolecule-derived material layer, uniformly anchored by zinc ions, improves the uniformity of adsorption site distribution, effectively suppressing the aggregation of hydroxyl / amino groups in traditional grafting modification, and reducing retention rate fluctuations under coexisting ion competition. Simultaneously, the biomolecule-derived material layer is chemically linked to ZIF-8 via coordination bonds; compared to physical adsorption, this reduces ZIF-8 detachment and improves stability.

[0025] Based on the synergistic effect of pyrogallol and amino acids in biomolecularly derived materials, a composite modified layer rich in phenolic hydroxyl and amino groups is formed under pH=8.5 conditions. Its oxygen and nitrogen atoms interact with Cr³⁺ atoms through empty d orbitals. + By forming stable coordination bonds, the membrane material is endowed with the ability to specifically recognize target metal ions, thereby improving the selective separation efficiency.

[0026] The dual-layer self-assembly design enables a tighter bond between the ZIF-8 functional layer and the biomolecular interface, reducing the retention of pollutants within the membrane pores. After four cycles, the membrane flux recovery rate is 15% higher than that of the single-layer structure and 25% higher than that of the three-layer structure, while the porosity remains stably above 70%, balancing efficient separation with long-term operational stability. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the membrane structure in this invention.

[0028] Figure 2 This is a scanning electron microscope comparison image of the composite film and the PES base film prepared in Example 4 of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to specific embodiments.

[0030] Example 1

[0031] 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 and react at room temperature for 10 minutes to prepare a pyrogallol-tryptophan solution (pyrogallol and tryptophan are equimolar in concentration). Immerse the original PES membrane in 100 ml of this solution for 10 hours for modification, then rinse with deionized water and store in deionized water to remove residues.

[0032] Step (2): The modified PES membrane is immersed in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1 g / L for 15 minutes, and zinc ions are adsorbed by the electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0033] Step (3): After rinsing the zinc ion adsorbed membrane multiple times with deionized water, it is immersed in 100 ml of a 1 g / L sodium carboxymethyl cellulose solution for 15 minutes to connect the sodium carboxymethyl cellulose molecules with zinc ions, treating it as a single layer. Through layer-by-layer self-assembly technology, this process is repeated twice to form a two-layer self-assembled structure of (zinc ion-sodium carboxymethyl cellulose-zinc ion-sodium carboxymethyl cellulose).

[0034] Step (4): After rinsing the self-assembled membrane multiple times with flowing deionized water, it is 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 growth, it is directly immersed in a 2-methylimidazole methanol solution prepared by dissolving 3.28 g of 2-methylimidazole in 100 ml of methanol 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, the membrane is rinsed three times with deionized water and then three times with methanol to remove unbound raw materials. The composite membrane is highly hydrophilic and has a flux recovery rate of up to 93.0% and a chromium ion rejection rate of >99.0%.

[0035] Example 2

[0036] Step (1): Dissolve 0.2 wt% pyrogallol, 0.2 wt% tyrosine, and 0.1 wt% potassium persulfate 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 prepare a pyrogallol-tyrosine solution (pyrogallol and tyrosine are equimolar in concentration). 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.

[0037] Step (2): The modified PES membrane is immersed in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1.5 g / L for 20 minutes to adsorb zinc ions by electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0038] Step (3): After rinsing the zinc ion adsorbed membrane multiple times with deionized water, it is 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, treating it as a single layer. Through layer-by-layer self-assembly technology, a two-layer self-assembled structure (zinc ion-chitosan hydrochloride-zinc ion-chitosan hydrochloride) is formed twice.

[0039] Step (4): After rinsing the self-assembled membrane multiple times with flowing deionized water, it is immersed 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 growth, it is directly immersed in a 2-methylimidazole methanol solution prepared by dissolving 3 g of 2-methylimidazole in 100 ml of methanol 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 rinsed three times with methanol to remove unbound raw materials. The composite membrane is highly hydrophilic and has a flux recovery rate of up to 94.0% and a chromium ion rejection rate of >99.2%.

[0040] Example 3

[0041] Step (1): Dissolve 0.3 wt% pyrogallol, 0.3 wt% taurine, and 0.15 wt% sodium periodate (NaIO4) in 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 are equimolar in concentration). Immerse the original PES membrane in 100 ml of this solution for 14 hours for modification, then rinse with deionized water and store in deionized water to remove residues.

[0042] Step (2): The modified PES membrane is immersed in 100 ml of zinc nitrate hexahydrate solution with a concentration of 2 g / L for 30 minutes to adsorb zinc ions by electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0043] Step (3): After rinsing the zinc ion-adsorbed membrane multiple times with deionized water, it is immersed in 100 ml of a 2% (w / w) cellulose nanofiber solution for 30 minutes to connect the cellulose nanofiber molecules with zinc ions, treating it as a single layer. Through layer-by-layer self-assembly technology, a two-layer self-assembled structure (zinc ion-cellulose nanofiber-zinc ion-cellulose nanofiber) is formed twice.

[0044] Step (4): After rinsing the self-assembled membrane multiple times with flowing deionized water, it is immersed 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 growth, it is directly immersed in a 2-methylimidazole methanol solution prepared by dissolving 3.5 g of 2-methylimidazole in 100 ml of methanol 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, the membrane is rinsed three times with deionized water and then three times with methanol to remove unbound raw materials. The material growth in the pores of the composite membrane is uniform, and compared with the original PES membrane, it is similar. Figure 1 As shown, it is highly hydrophilic with a flux recovery rate of up to 94.0% and a chromium ion rejection rate of >99.5%.

[0045] Example 4

[0046] 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 and react at room temperature for 10 minutes to prepare a 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.

[0047] Step (2): The modified PES membrane is immersed in 100 ml of zinc nitrate hexahydrate solution with a concentration of 1.5 g / L for 20 minutes to adsorb zinc ions by electrostatic or coordination interaction between the amino and carboxyl groups of the modifier and zinc ions.

[0048] Step (3): After rinsing the zinc ion adsorbed membrane multiple times with deionized water, it is immersed in 100 ml of a 1 g / L sodium carboxymethyl cellulose solution for 20 minutes to connect the sodium carboxymethyl cellulose molecules with zinc ions, treating it as a single layer. Through layer-by-layer self-assembly technology, this process is repeated twice to form a two-layer self-assembled structure of (zinc ion-sodium carboxymethyl cellulose-zinc ion-sodium carboxymethyl cellulose).

[0049] Step (4): After rinsing the self-assembled membrane multiple times with flowing deionized water, it was immersed 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 growth, it was directly immersed in a 2-methylimidazole methanol solution prepared by dissolving 3 g of 2-methylimidazole in 100 ml of methanol 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, the membrane was rinsed three times with deionized water and then three times with methanol to remove unbound raw materials. The scanning electron microscope image of the membrane surface is shown below. Figure 1 As shown in the right figure, compared with the original membrane, the membrane surface can be clearly seen to be covered by the grown ZIF-8 material. The composite membrane is highly hydrophilic and has a flux recovery rate of up to 95.0% and a chromium ion rejection rate of >99.6%.

Claims

1. A method for preparing a composite membrane for efficiently and selectively capturing trivalent chromium ions, characterized in that, The polyethersulfone membrane was synergistically modified with pyrogallol and amino acids to form a stable negatively charged layer on the membrane surface. Zinc ions and biomolecular-derived materials were self-assembled onto the modified membrane surface using electrostatic interactions. A ZIF-8 metal-organic framework layer was then grown in situ on this substrate to capture trivalent chromium ions. The biomolecular-derived material was one of sodium carboxymethyl cellulose, chitosan hydrochloride, or cellulose nanofibers. The specific preparation steps are as follows: (1) PES base membrane modification: 0.1-3 wt% pyrogallol, amino acid and 0.05-1 wt% oxidant are dissolved in a 0.05-0.15 M dilute tris(hydroxymethyl)aminomethane (Tris)-hydrochloric acid buffer solution at room temperature to prepare pyrogallol-amino acid solution. The modified agent is formed by reaction. After immersing polyethersulfone (PES) base membrane in the above solution for 10-14 hours, it is rinsed with deionized water and stored in deionized water to remove residues. (2) Layer-by-layer self-assembly: The modified PES base membrane is immersed in a 1 g / L zinc nitrate hexahydrate solution for 15-20 minutes. The abundant amino and carboxyl groups in the modifier interact with zinc ions through electrostatic or coordination interactions, thereby achieving the adsorption of zinc ions. After repeatedly rinsing the membrane with adsorbed zinc ions with deionized water, it is immersed in a biomolecular derivative material solution and reacted for 15-20 minutes to achieve the connection between the biomolecular derivative material and zinc ions. The above operation of adsorbing zinc ions and connecting biomolecular derivative materials is repeated twice to form a self-assembled structure with alternating zinc ion layers and biomolecular derivative material layers. (3) In-situ growth of ZIF-8 layer: First, the composite membrane that has completed layer-by-layer self-assembly is thoroughly cleaned with flowing deionized water and then immersed in a zinc nitrate precursor solution for 2-8 hours. The zinc nitrate precursor solution is prepared by dissolving 2.97 g of zinc nitrate hexahydrate in 100 mL of deionized water to achieve uniform deposition of zinc ions on the membrane surface. Then, the zinc ion-loaded membrane is transferred to a ligand solution and allowed to stand for 24 hours. The ligand solution is prepared by dissolving 3.28 g of 2-methylimidazole in 100 mL of methanol. Through the coordination of zinc ions with imidazole ligands, a ZIF-8 metal-organic framework structure is constructed in-situ on the membrane surface. (4) Post-treatment: The prepared composite membrane is rinsed three times with deionized water and methanol in sequence, and then soaked in deionized water for storage until use.

2. The method for preparing a composite membrane for efficiently and selectively capturing trivalent chromium ions according to claim 1, characterized in that, The oxidant is one of potassium persulfate or sodium periodate.

3. The method for preparing a composite membrane for efficiently and selectively capturing trivalent chromium ions according to claim 1, characterized in that, The amino acid is one of tryptophan, tyrosine, taurine, and glutamic acid.

4. A composite membrane for efficiently and selectively capturing trivalent chromium ions, characterized in that, Prepared by the method described in claim 1, the composite membrane is highly hydrophilic and has a flux recovery rate of up to 95%, and a rejection rate of trivalent chromium ions greater than 99%.

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