A pore-positioned cross-linking modified cof / sodium alginate composite nanofiltration membrane, preparation method and application thereof

Abstract

The application discloses a pore positioning cross-linking modified COF / sodium alginate composite nanofiltration membrane and a preparation method and application thereof, and steps are as follows: a PDA@PVDF membrane is prepared; 1,3,5-benzene tricarboxaldehyde and 2,5-diaminobenzenesulfonic acid are dissolved in dimethyl sulfoxide to obtain a COF nanosheet mixed solution through reaction; the COF nanosheet mixed solution is dialyzed with deionized water, dried, and adjusted with deionized water to obtain a COF nanosheet dispersion; calcium chloride is added to obtain a COF / Ca dispersion; sodium alginate is added to obtain a COF / Ca / SA casting solution; the PDA@PVDF membrane is dried, the casting solution is vacuum filtered, the COF / Ca / SA is deposited on the PDA@PVDF membrane, and drying is performed, raw materials of the application are easy to obtain and have a wide source, and the application is suitable for large-scale production; the membrane has high rejection rates for sodium sulfate, magnesium sulfate, magnesium chloride and sodium chloride, and can be applied in nanofiltration desalination of seawater, industrial wastewater or brackish water.

Description

Technical Field

[0001] This invention belongs to the field of covalent organic framework membranes, specifically relating to a COF / sodium alginate composite nanofiltration membrane with pore-positioned crosslinking modification, its preparation method, and its application. Background Technology

[0002] With the increasing severity of global water scarcity, seawater desalination, brackish water desalination, and industrial wastewater recycling have become core approaches to solving the global water crisis. Nanofiltration membrane separation technology, due to its advantages such as low operating pressure, high separation precision, low energy consumption, and no phase change, has become one of the core technologies for ion sieving and desalination in the water treatment field, and has been widely used in seawater / brackish water desalination, advanced industrial wastewater treatment, and drinking water purification. The separation performance of nanofiltration membranes depends primarily on the membrane's pore structure, surface physicochemical properties, and the stability of the selective layer. Overcoming the trade-off between high desalination rate and high flux in nanofiltration membranes, while simultaneously improving the membrane's long-term operational stability and antifouling performance, is a core research challenge and industry pain point in the current nanofiltration membrane technology field.

[0003] Covalent organic frameworks (COFs) are a class of crystalline porous organic materials linked by covalent bonds. They possess unique advantages such as permanently ordered pore structures, tunable pore sizes, large specific surface areas, and customizable surface functional groups. Their well-defined nanopores provide rapid transport channels for water molecules, and they can also achieve precise retention of hydrated ions through pore size sieving and charge effects, making them ideal functional materials for preparing high-performance nanofiltration membranes. However, existing COF-based composite nanofiltration membranes still face several technical bottlenecks: on the one hand, pure COF nanosheets are prone to non-selective defects during film formation, leading to a significant decrease in the membrane's ion sieving accuracy and making it difficult to meet water treatment requirements in terms of desalination performance; on the other hand, the pore size of most COF materials is larger than the hydration radius of salt ions, making it difficult to effectively retain salt ions solely based on the COF pore size. Existing methods for COF pore modification and defect repair mostly employ in-situ functional group modification and polymer random blending. The former involves complex processes and harsh reaction conditions, which can easily damage the crystalline pore structure of COF. The latter cannot achieve targeted and precise modification of COF pores and intergranular defects, which can easily cause pore blockage and lead to a significant decrease in membrane water flux. These methods have consistently failed to overcome the trade-off between "high desalination rate and high flux" in nanofiltration membranes, and cannot meet the large-scale application requirements of high-performance nanofiltration membranes in the fields of seawater desalination, brackish water desalination, and industrial wastewater resource utilization. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a COF / sodium alginate composite nanofiltration membrane with pore-positioned crosslinking modification.

[0005] The second objective of this invention is to provide a method for preparing a COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification.

[0006] The third objective of this invention is to provide an application of a COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification in nanofiltration desalination of seawater, industrial wastewater, or brackish water.

[0007] The technical solution of this invention is summarized as follows: A method for preparing a COF / sodium alginate composite nanofiltration membrane with pore-positioned crosslinking modification includes the following steps: 1) Prepare a 2 g / L dopamine hydrochloride solution using dopamine hydrochloride and 50 mM Tris-HCl buffer; immerse a 0.22 μm pore size polyvinylidene fluoride membrane in the dopamine hydrochloride solution for 1 hour, remove it, and wash it with deionized water to obtain a PDA@PVDF membrane; where PDA is the abbreviation for polydopamine and PVDF is the abbreviation for polyvinylidene fluoride. 2) At a molar ratio of 1:1.5, benzoxanthraldehyde and 2,5-diaminobenzenesulfonic acid were dissolved in dimethyl sulfoxide and allowed to react at room temperature for 3 days to obtain a COF nanosheet mixture. The mixture was dialyzed with deionized water, dried, and the concentration was measured. The concentration of the COF nanosheet dispersion was adjusted with deionized water to obtain a COF nanosheet dispersion with a concentration of 1 mg / mL. Calcium chloride, in an equal mass ratio to the COF nanosheets, was added to the COF nanosheet dispersion and stirred for 20 min to obtain a COF / Ca dispersion. COF is an abbreviation for covalent organic framework. 3) According to the ratio, take 0.5-2 ml of sodium alginate aqueous solution with a concentration of 1 g / L, mix it with 1 mL of the COF / Ca dispersion and 20 mL of deionized water to obtain a casting solution containing COF / Ca / SA; dry the PDA@PVDF membrane, vacuum filter the casting solution to deposit COF / Ca / SA on the PDA@PVDF membrane, and dry it at 100℃ to obtain a pore-positioned crosslinking modified COF / sodium alginate composite nanofiltration membrane; SA is the abbreviation for sodium alginate.

[0008] A COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification was prepared by the above preparation method.

[0009] The above-mentioned COF / sodium alginate composite nanofiltration membrane with pore positioning crosslinking modification is used in nanofiltration desalination of seawater, industrial wastewater or brackish water.

[0010] Advantages of this invention: The raw materials used in this invention are readily available and widely sourced, making it suitable for large-scale mass production. All core raw materials used in this invention are commercially available, mature, and conventional products, without any rare, customized reagents or special materials. The preparation process is simple, with low equipment requirements, and is easily industrialized. The entire process is concise and controllable, without complex operating procedures or harsh reaction conditions, and requires no specialized customized equipment. Through calcium ion-mediated COF pore-positioning crosslinking modification technology, the non-selective defects between the crystallites of COF nanosheets are precisely repaired, achieving precise control of the membrane pore structure. This overcomes the traditional trade-off between "high desalination rate and high flux" in nanofiltration membranes, significantly improving the ion sieving accuracy, water flux, and long-term operational stability of the nanofiltration membrane. The prepared composite nanofiltration membrane can achieve a sodium sulfate rejection rate of up to 97.9% while maintaining excellent water permeability, perfectly meeting the application needs of large-scale water treatment scenarios such as seawater desalination, brackish water desalination, and industrial wastewater resource reuse. Detailed Implementation

[0011] The present invention will be further described below with reference to the embodiments.

[0012] In the various embodiments and comparative examples, PDA is the abbreviation for polydopamine, PVDF is the abbreviation for polyvinylidene fluoride, COF is the abbreviation for covalent organic framework, and SA is the abbreviation for sodium alginate.

[0013] This invention discloses a COF / sodium alginate composite nanofiltration membrane with pore-positioned crosslinking modification. It selects an ionic covalent organic framework containing sulfonic acid groups on the pore surface. Calcium ions are anchored to sulfonic acid sites within the COF pores via electrostatic interactions, while the other end chelates and crosslinks with sodium alginate. The crosslinking reaction occurs in the space near the COF pores, with the sodium alginate crosslinking network precisely encapsulating the COF nanosheet surface and pore inlet, forming a core-shell modified structure with the COF pores as the core and the surrounding sodium alginate crosslinking network. This enhances the nanofiltration performance of the COF membrane through a surface electrostatic effect combined with a size sieving mechanism.

[0014] Example 1: A method for preparing a COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification, comprising the following steps: 1) Prepare a 2 g / L dopamine hydrochloride solution using dopamine hydrochloride and 50 mM Tris-HCl buffer; immerse a 0.22 μm pore size polyvinylidene fluoride membrane in the above dopamine hydrochloride solution for 1 hour, remove it, and wash it with deionized water to obtain a PDA@PVDF membrane; 2) At a molar ratio of 1:1.5, benzoxaldehyde and 2,5-diaminobenzenesulfonic acid were dissolved in dimethyl sulfoxide and allowed to stand at room temperature for 3 days to obtain a COF nanosheet mixture. The mixture was dialyzed with deionized water, dried and the concentration was measured. The concentration of COF nanosheet dispersion was adjusted with deionized water to obtain a concentration of 1 mg / mL. Calcium chloride in an equal mass ratio to COF nanosheets was added to the COF nanosheet dispersion and stirred for 20 min to obtain a COF / Ca dispersion. 3) Take 0.5 mL of sodium alginate aqueous solution with a concentration of 1 g / L, mix it with 1 mL of the COF / Ca dispersion and 20 mL of deionized water to obtain a casting solution containing COF / Ca / SA; dry the PDA@PVDF membrane, vacuum filter the casting solution to deposit COF / Ca / SA on the PDA@PVDF membrane, and dry it at 100℃ to obtain a pore-positioned crosslinked modified COF / sodium alginate composite nanofiltration membrane, called: COF / Ca / SA-0.5 membrane.

[0015] The COF / Ca / SA-0.5 membrane prepared in Example 1 was used for nanofiltration of four salts: sodium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride. The water permeability of the membrane was 0.89 L / m. -2 h -1 bar -1 The retention rates of sodium sulfate (1000 ppm) aqueous solution were 92.8%, magnesium sulfate (1000 ppm) aqueous solution were 86.4%, magnesium chloride (1000 ppm) aqueous solution were 82.9%, and sodium chloride (1000 ppm) aqueous solution were 80.2%.

[0016] Example 2: A method for preparing a COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification, comprising the following steps: 1) and 2), same as steps 1) and 2) in Example 1). 3) Take 1 mL of sodium alginate aqueous solution with a concentration of 1 g / L, mix it with 1 mL of the COF / Ca dispersion and 20 mL of deionized water to obtain a casting solution containing COF / Ca / SA; dry the PDA@PVDF membrane, vacuum filter the casting solution to deposit COF / Ca / SA on the PDA@PVDF membrane, and dry it at 100℃ to obtain a pore-positioned crosslinked modified COF / sodium alginate composite nanofiltration membrane, called: COF / Ca / SA-1 membrane.

[0017] The COF / Ca / SA-1 membrane prepared in Example 2 was used for nanofiltration of four salts: sodium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride. The water permeability of the membrane was 1.25 L / m. -2 h -1 bar-1 The retention rates were 97.9% for sodium sulfate (1000 ppm), 96.1% for magnesium sulfate (1000 ppm), 85.8% for magnesium chloride (1000 ppm), and 83.6% for sodium chloride (1000 ppm).

[0018] Example 3: A method for preparing a COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification, comprising the following steps: 1) and 2), same as steps 1) and 2) in Example 1). 3) Take 2 mL of sodium alginate aqueous solution with a concentration of 1 g / L, mix it with 1 mL of the COF / Ca dispersion and 20 mL of deionized water to obtain a casting solution containing COF / Ca / SA; dry the PDA@PVDF membrane, vacuum filter the casting solution to deposit COF / Ca / SA on the PDA@PVDF membrane, and dry it at 100℃ to obtain a pore-positioned crosslinking modified COF / sodium alginate composite nanofiltration membrane, called: COF / Ca / SA-2 membrane.

[0019] The COF / Ca / SA-2 membrane from Example 3 was used for nanofiltration of four salts: sodium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride. The membrane's water permeability was 2.94 L / m³. -2 h -1 bar -1 The retention rates were 96.5% for sodium sulfate (1000 ppm), 85.3% for magnesium sulfate (1000 ppm), 60.5% for magnesium chloride (1000 ppm), and 73.1% for sodium chloride (1000 ppm).

[0020] Comparative Example 1 1) Prepare a 2 g / L dopamine hydrochloride solution using dopamine hydrochloride and 50 mM Tris-HCl buffer; immerse a 0.22 μm pore size polyvinylidene fluoride membrane in the above dopamine hydrochloride solution for 1 hour, remove it, and wash it with deionized water to obtain a PDA@PVDF membrane; 2) At a molar ratio of 1:1.5, benzoxanthiazine and 2,5-diaminobenzenesulfonic acid were dissolved in dimethyl sulfoxide and allowed to stand at room temperature for 3 days to obtain a COF nanosheet mixture; the mixture was dialyzed with deionized water, dried and the concentration was measured, and the concentration was adjusted with deionized water to obtain a COF nanosheet dispersion with a concentration of 1 mg / mL. 3) Take 1 mL of the COF nanosheet dispersion and add it to 20 mL of deionized water and mix thoroughly to obtain a uniform casting solution; dry the PDA@PVDF membrane, vacuum filter the casting solution, deposit the COF nanosheets on the PDA@PVDF membrane, and dry at 100℃ to obtain Comparative Example 1, called: Pristine COF membrane.

[0021] Comparative Example 1: The Pristine COF membrane was used for nanofiltration of four salts: sodium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride. The membrane's water permeability was 5.89 L / m³. -2 h -1 bar -1 The retention rates were 43.7% for sodium sulfate (1000 ppm), 34.4% for magnesium sulfate (1000 ppm), 12.4% for magnesium chloride (1000 ppm), and 27.6% for sodium chloride (1000 ppm).

[0022] Comparative Example 2 1) Prepare a 2 g / L dopamine hydrochloride solution using dopamine hydrochloride and 50 mM Tris-HCl buffer; immerse a 0.22 μm pore size polyvinylidene fluoride membrane in the above dopamine hydrochloride solution for 1 hour, remove it, and wash it with deionized water to obtain a PDA@PVDF membrane; 2) At a molar ratio of 1:1.5, benzoxaldehyde and 2,5-diaminobenzenesulfonic acid were dissolved in dimethyl sulfoxide and allowed to stand at room temperature for 3 days to obtain a COF nanosheet mixture. The mixture was dialyzed with deionized water, dried and the concentration was measured. The concentration of COF nanosheet dispersion was adjusted with deionized water to obtain a concentration of 1 mg / mL. Calcium chloride in an equal mass ratio to COF nanosheets was added to the COF nanosheet dispersion and stirred for 20 min to obtain a COF / Ca dispersion. 3) Take 1 mL of COF / Ca dispersion and add it to 20 mL of deionized water and mix thoroughly to obtain a uniform casting solution; dry the PDA@PVDF membrane, vacuum filter the casting solution, deposit COF / Ca on the PDA@PVDF membrane, and dry at 100℃ to obtain Comparative Example 2, called: COF / Ca membrane.

[0023] Comparative Example 2 used a COF / Ca membrane for nanofiltration of four salts: sodium sulfate, magnesium sulfate, magnesium chloride, and sodium chloride. The membrane's water permeability was 0.95 L m⁻¹. 2 h -1 bar -1The retention rates were 82.2% for sodium sulfate (1000 ppm), 71.3% for magnesium sulfate (1000 ppm), 64.9% for magnesium chloride (1000 ppm), and 62.1% for sodium chloride (1000 ppm).

[0024] Table 1 Performance of each embodiment and comparative example

[0025] The present invention discloses a COF / sodium alginate composite nanofiltration membrane with channel positioning crosslinking modification, which can be used in nanofiltration desalination of seawater, industrial wastewater or brackish water.

Claims

1. A method for preparing a COF / sodium alginate composite nanofiltration membrane with pore-positioning crosslinking modification, characterized in that... Includes the following steps: 1) Prepare a 2 g / L dopamine hydrochloride solution using dopamine hydrochloride and 50 mM Tris-HCl buffer; immerse a 0.22 μm pore size polyvinylidene fluoride membrane in the dopamine hydrochloride solution for 1 hour, remove it, and wash it with deionized water to obtain a PDA@PVDF membrane; where PDA is the abbreviation for polydopamine and PVDF is the abbreviation for polyvinylidene fluoride. 2) At a molar ratio of 1:1.5, benzoxanthraldehyde and 2,5-diaminobenzenesulfonic acid were dissolved in dimethyl sulfoxide and allowed to react at room temperature for 3 days to obtain a COF nanosheet mixture. The mixture was dialyzed with deionized water, dried, and the concentration was measured. The concentration of the COF nanosheet dispersion was adjusted with deionized water to obtain a COF nanosheet dispersion with a concentration of 1 mg / mL. Calcium chloride, in an equal mass ratio to the COF nanosheets, was added to the COF nanosheet dispersion and stirred for 20 min to obtain a COF / Ca dispersion. COF is an abbreviation for covalent organic framework. 3) According to the ratio, take 0.5-2 ml of sodium alginate aqueous solution with a concentration of 1 g / L, mix it with 1 mL of the COF / Ca dispersion and 20 mL of deionized water to obtain a casting solution containing COF / Ca / SA; dry the PDA@PVDF membrane, vacuum filter the casting solution to deposit COF / Ca / SA on the PDA@PVDF membrane, and dry it at 100℃ to obtain a pore-positioned crosslinking modified COF / sodium alginate composite nanofiltration membrane; SA is the abbreviation for sodium alginate.

2. A COF / sodium alginate composite nanofiltration membrane with channel-positioned crosslinking modification prepared by the preparation method of claim 1.

3. The application of the COF / sodium alginate composite nanofiltration membrane with pore positioning crosslinking modification according to claim 2 in nanofiltration desalination of seawater, industrial wastewater or brackish water.

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