Preparation of modified PTFE anti-fouling nanofiltration membrane and its application in dye wastewater

Abstract

This invention relates to a method for preparing a modified PTFE antifouling nanofiltration membrane and its application in dye and salt separation. The method includes the following steps: pretreating the PTFE base membrane by soaking it in anhydrous ethanol and deionized water to remove surface impurities and clear pores; using nitrogen plasma treatment to break some C-F bonds on the PTFE membrane surface, thereby enhancing its surface activity; dispersing MXene nanosheets in a dopamine solution and ultrasonically mixing them uniformly, then loading them onto the PTFE membrane surface by vacuum filtration; finally immersing the membrane in a mixed solution of carboxymethyl chitosan and polyethyleneimine, and forming a stable composite separation layer by a glutaraldehyde crosslinking reaction. This invention addresses the problems of low flux and poor stability in existing technologies through the following innovations: 1) PTFE is selected as the substrate, whose excellent chemical stability and mechanical strength provide long-term operational assurance for the membrane; 2) Layered MXene nanosheets are introduced to construct unique water transport channels, significantly improving membrane flux; 3) A dense and hydrophilic separation layer is formed through a cross-linked network of carboxymethyl chitosan and polyethyleneimine, and the abundant amino groups on the surface form a hydration layer through hydrogen bonding, effectively blocking pollutant contact and achieving excellent antifouling performance. This composite membrane achieves a separation efficiency of over 99% for dyes and salts, while also possessing high water flux and long-term stability, making it particularly suitable for the resource-based treatment of high-salt dye wastewater.

Description

Technical Field

[0001] This invention relates to a method for preparing a modified PTFE antifouling nanofiltration membrane, belonging to the field of membrane preparation technology. Specifically, it designs a method for preparing a PTFE dye and salt separation composite membrane. This PTFE composite membrane has advantages such as strong antifouling ability, high separation efficiency, high mechanical strength, and high pure water flux, and is suitable for dye and salt separation systems. Background Technology

[0002] Synthetic dyes are widely used in industries such as textiles, papermaking, and leather. Their production process often generates large amounts of dye wastewater containing salts (such as NaCl and Na2SO4). This type of wastewater is characterized by complex composition, high color, and high salinity. Traditional treatment methods, such as physical methods (precipitation and adsorption), chemical methods (oxidation and reduction), and biological methods (activated sludge), are difficult to effectively separate dyes from salts, which seriously restricts wastewater reuse and environmental protection.

[0003] Membrane separation technology uses microporous membranes and nanofiltration membranes to separate and filter wastewater. Due to its high efficiency and energy saving, it has become a research hotspot for salt and pollutant separation. However, existing nanofiltration membranes still have the following technical bottlenecks: 1) Flux limitation. For example, the nanofiltration membrane prepared by interfacial polymerization of cyclohexane and isophthaloyl chloride as monomers, as described in patent CN202510220221, has a dense honeycomb nanoporous structure, but its water flux is only 10.74 L·m⁻¹. -2 ·h -1 ·bar -1 ;2) Insufficient stability, such as the nanofiltration membrane prepared by combining g-C3N4 nanosheets and KANF through hydrogen bonding in patent CN202311873789, which suffers from performance degradation after long-term use due to weak interaction force.

[0004] PTFE is an ideal high-performance membrane substrate due to its excellent chemical inertness, thermal stability, and mechanical strength. However, its hydrophobic surface easily leads to membrane fouling and flux reduction. Existing modification techniques mostly focus on improving the hydrophilicity of PTFE, but often sacrifice its inherent advantages or introduce structural instability. Therefore, developing a PTFE-based composite nanofiltration membrane that combines high flux, strong antifouling properties, and long-term stability is of great significance for advancing dye wastewater treatment technology. Summary of the Invention

[0005] A method for preparing a modified PTFE antifouling nanofiltration membrane for the separation of dyes and salts, the specific steps of which are as follows:

[0006] The commercial PTFE membrane was first pretreated by soaking it in anhydrous ethanol and deionized water for 1 hour each to remove surface impurities and unclog pores. The PTFE membrane was then modified using plasma treatment with nitrogen as the active gas, causing some CF bonds on the PTFE membrane surface to break. MXene nanosheets were added to a 1 wt% dopamine aqueous solution and ultrasonically mixed until homogeneous. The mixture was then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper, the membrane was completely immersed in a 1 wt% mixed solution of carboxymethyl chitosan and polyethyleneimine for a period of time. Then, at a certain temperature, the membrane was cross-linked in a glutaraldehyde aqueous solution of a certain volume fraction. After a period of reaction, it was vacuum dried for 10 minutes, ultimately forming a modified PTFE composite membrane.

[0007] The content of MXene nanosheets in the dopamine aqueous solution was 0.01-0.1 wt%.

[0008] The content of carboxymethyl chitosan in the aqueous solution is 0.1-0.5 wt%.

[0009] The content of polyethyleneimine in the aqueous solution is 0.5-0.8 wt%.

[0010] The crosslinking temperature of the membrane in glutaraldehyde aqueous solution is 30-60 ℃;

[0011] The volume fraction of the aqueous solution of glutaraldehyde is 5-15 vol%.

[0012] The crosslinking time of the membrane in the aqueous solution of glutaraldehyde is 20-80 min; Beneficial effects

[0013] 1. By treating the PTFE base film with nitrogen plasma, some of the CF bonds on its surface are broken, forming active sites and enhancing the interfacial bonding force; MXene nanosheets are fixed to the PTFE film surface by dopamine chemical bonding. Compared with traditional hydrogen bonding or physical adsorption methods, this significantly improves the adhesion stability of the two-dimensional material, avoids the nanosheets falling off during long-term operation, and ensures the long-term reliable performance of the film.

[0014] 2. The layered structure of MXene nanosheets creates efficient water transport channels within the membrane, significantly improving the permeation efficiency of water molecules and overcoming the problem of low flux in traditional nanofiltration membranes; plasma pretreatment optimizes the pore structure on the PTFE membrane surface, further promoting the rapid passage of water molecules.

[0015] 3. Carboxymethyl chitosan and polyethyleneimine crosslink through glutaraldehyde to form a dense and hydrophilic separation layer. The surface is rich in hydrophilic groups such as amino groups, which form a stable hydration layer through hydrogen bonding, effectively blocking the adsorption and deposition of dye molecules and organic pollutants on the membrane surface. The smooth network structure of the membrane surface reduces pollutant adhesion, significantly reduces the risk of membrane fouling, and extends service life. Detailed Implementation

[0016] Compare with Example 1:

[0017] The commercial PTFE membrane was pretreated by immersing it in anhydrous ethanol and deionized water for 1 hour each to remove surface impurities and unclog pores. Performance testing was then conducted at 5 bar using a mixed solution of dye and salt (methyl orange: sodium sulfate = 100 ppm: 1000 ppm). The PTFE membrane flux was 1500.48 L·m⁻¹. -2 ·h -1 The rejection rate for methyl orange was 12.37%, and the rejection rate for Na2SO4 was 4.23%.

[0018] Example 1:

[0019] The commercial PTFE membrane was pretreated by soaking in anhydrous ethanol and deionized water for 1 h each to remove surface impurities and unclog pores. The PTFE membrane was then modified using plasma treatment with nitrogen as the active gas. After wiping with industrial filter paper, 0.01 wt% MXene nanosheets were ultrasonically dispersed in a 1 wt% dopamine aqueous solution. The solution was then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper again, the membrane was completely immersed in a mixed solution of 0.1 wt% carboxymethyl chitosan and 0.5 wt% polyethyleneimine for 10 min. Then, the membrane was crosslinked in a 5 vol% glutaraldehyde aqueous solution at 30 °C for 20 min, followed by vacuum drying for 10 min to form the modified PTFE composite membrane. Performance testing was conducted at 5 bar using a mixed solution of dye and salt (methyl orange: sodium sulfate = 100 ppm: 1000 ppm). The PTFE membrane flux was 55.79 L·m. -2 ·h -1 ·bar -1 The PTFE membrane exhibited a methyl orange rejection rate of 92.37% and a Na2SO4 rejection rate of 13.83% after 7 days of operation. The modified PTFE membrane also showed a methyl orange rejection rate of 91.34% and a sodium sulfate rejection rate of 14.63% after 7 days of operation.

[0020] Example 2:

[0021] The commercial PTFE membrane was pretreated by soaking in anhydrous ethanol and deionized water for 1 h each to remove surface impurities and unclog pores. The PTFE membrane was then modified using plasma treatment with air as the nitrogen gas. After wiping with industrial filter paper, 0.02 wt% MXene nanosheets were ultrasonically dispersed in a 1 wt% dopamine aqueous solution. The solution was then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper, the membrane was completely immersed in a mixed solution of 0.2 wt% carboxymethyl chitosan and 0.6 wt% polyethyleneimine for 10 min. Then, the membrane was crosslinked in a 6 vol% glutaraldehyde aqueous solution at 40 °C for 30 min, followed by vacuum drying for 10 min to form the modified PTFE composite membrane. Performance testing was conducted at 5 bar using a mixed solution of dye and salt (methyl orange: sodium sulfate = 100 ppm: 1000 ppm). The PTFE membrane flux was 52.65 L·m. -2 ·h -1 ·bar -1 The methyl orange rejection rate was 94.64%, and the Na2SO4 rejection rate was 15.13%. After 7 days of long-term operation, the prepared modified PTFE membrane showed a methyl orange rejection rate of 92.87% and a sodium sulfate rejection rate of 15.73%.

[0022] Example 3:

[0023] The commercial PTFE membrane was pretreated by soaking in anhydrous ethanol and deionized water for 1 h to remove surface impurities and unclog pores. The PTFE membrane was then modified using plasma treatment with air as the nitrogen gas. After wiping with industrial filter paper, 0.03 wt% MXene nanosheets were ultrasonically dispersed in a 1 wt% dopamine aqueous solution. The solution was then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper, the membrane was completely immersed in a mixed solution of 0.3 wt% carboxymethyl chitosan and 0.7 wt% polyethyleneimine for 10 min. Then, the membrane was crosslinked in a 7 vol% glutaraldehyde aqueous solution at 50 °C for 40 min, followed by vacuum drying for 10 min to form the modified PTFE composite membrane. Performance tests were conducted at 5 bar using a mixed solution of dye and salt (methyl orange: sodium sulfate = 100 ppm: 1000 ppm). The PTFE membrane flux was 51.33 L·m. -2 ·h -1 ·bar -1The PTFE membrane exhibited a methyl orange rejection rate of 99.11% and a Na2SO4 rejection rate of 16.26% after 7 days of operation. After long-term operation, the modified PTFE membrane showed a methyl orange rejection rate of 98.65% and a sodium sulfate rejection rate of 17.95%.

[0024] Example 4:

[0025] The commercial PTFE membrane was pretreated by soaking in anhydrous ethanol and deionized water for 1 h each to remove surface impurities and unclog pores. The PTFE membrane was then modified using plasma treatment with nitrogen as the active gas. After wiping with industrial filter paper, 0.04 wt% MXene nanosheets were ultrasonically dispersed in a 1 wt% dopamine aqueous solution. The solution was then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper, the membrane was completely immersed in a mixed solution of 0.4 wt% carboxymethyl chitosan and 0.8 wt% polyethyleneimine for 10 min. Then, the membrane was crosslinked in an 8 vol% glutaraldehyde aqueous solution at 60 °C for 50 min, followed by vacuum drying for 10 min to form the modified PTFE composite membrane. Performance tests were conducted at 5 bar using a mixed solution of dye and salt (methyl orange: sodium sulfate = 100 ppm: 1000 ppm). The PTFE membrane flux was 49.27 L·m. -2 ·h -1 ·bar -1 The modified PTFE membrane exhibited a methyl orange rejection rate of 99.47% and a sodium sulfate rejection rate of 17.83% after 7 days of operation.

Claims

1. A method for preparing a modified PTFE antifouling nanofiltration membrane for dye and salt separation, the specific steps of which are as follows: The commercial PTFE membrane was first soaked in anhydrous ethanol and deionized water for 1 h each, and then modified using plasma with air as nitrogen gas. MXene nanosheets were ultrasonically dispersed in a 1 wt% dopamine aqueous solution, and then vacuum filtered onto the PTFE membrane surface at 0.01 MPa. After wiping with industrial filter paper, the membrane was completely immersed in a mixed aqueous solution of carboxymethyl chitosan and polyethyleneimine with a certain mass fraction for a period of time. Then, at a certain temperature, the membrane was crosslinked in a glutaraldehyde aqueous solution with a certain volume fraction. After a period of reaction, it was vacuum dried for 10 min to finally form a modified PTFE composite membrane. The content of MXene nanosheets in the dopamine aqueous solution was 0.01-0.1 wt%, the content of carboxymethyl chitosan in the mixed aqueous solution was 0.1-0.5 wt%, the content of polyethyleneimine in the mixed aqueous solution was 0.5-0.8 wt%, the crosslinking temperature of the membrane in the glutaraldehyde aqueous solution was 30-60 °C, and the volume fraction of glutaraldehyde in the glutaraldehyde aqueous solution was 5-15%. vol%.

2. A method for preparing a modified PTFE antifouling nanofiltration membrane for dye and salt separation as described in claim 1, characterized in that: The crosslinking time of the membrane in an aqueous solution of glutaraldehyde is 20-80 min.