PTFE (Polytetrafluoroethylene) composite membrane for removing heavy metal ions

Abstract

The invention relates to a polytetrafluoroethylene (PTFE)-based composite nanofiltration membrane for removing heavy metal ions and a preparation method thereof, and belongs to the technical field of membrane separation. The composite nanofiltration membrane comprises a PTFE base membrane and a functional separation layer formed on the surface of the PTFE base membrane through an interfacial polymerization technology, wherein the functional separation layer is prepared from an aqueous phase solution containing polyethyleneimine (PEI), sulfonated cellulose nanocrystals (SCNC) and graphene oxide (GO) and an oil phase solution containing toluene diisocynate (TDI) through an interfacial polymerization reaction. The PEI and the TDI form a urea bond (-NH-CO-NH-) and carbamate bond (-NH-CO-O-) cross-linked network to enhance the acid and alkali resistance of the membrane, the nanorod structure of the SCNC and the sulfonic acid group on the surface of the SCNC synergistically enhance the mechanical strength and hydrophilicity of the membrane, and meanwhile, the two-dimensional lamellar structure of the GO is utilized to provide abundant heavy metal chelating sites and selective transmission channels. Experiments show that the retention rates of the composite nanofiltration membrane to Pb, Cd and Hg respectively reach 98%, 96% and 97%, and the properties such as water flux, tensile strength and anti-pollution recovery rate are remarkably enhanced. The preparation process is simple, is suitable for industrial production, and has important application value in the fields of heavy metal wastewater treatment, seawater desalination, resource recovery and the like.

Description

Technical Field

[0001] The present invention relates to the field of membrane separation technology, and in particular to a composite membrane based on polytetrafluoroethylene (PTFE) and a preparation method thereof, in particular to a composite separation membrane with high-efficiency heavy metal ion removal function constructed by interfacial polymerization technology. Background Art

[0002] The accelerated pace of industrialization has led to an increasingly prominent problem of heavy metal pollution. Heavy metal ions such as lead (Pb²⁺), cadmium (Cd²⁺), and mercury (Hg²⁺) pose a significant threat to the ecological environment and human health due to their high toxicity, bioaccumulation, and non-degradability. Compared with traditional treatment methods (such as chemical precipitation and ion exchange), membrane separation technology, with its advantages of low operating pressure, low energy consumption, and environmental friendliness, demonstrates unique technical value in the treatment of heavy metal wastewater.

[0003] In the existing technology, interfacial polymerization technology is the mainstream method for preparing composite nanofiltration membranes. Polyacrylonitrile and polysulfone are usually used as substrates, amine monomers as water phase monomers, and acyl chloride monomers as oil phase monomers to construct a functional separation layer on the surface of the base membrane. Patent CN116850793A discloses a method for preparing polyamide composite nanofiltration membranes by regulating the interfacial polymerization temperature (25-100°C) to fully react piperazine with trimesoyl chloride. Patent CN113117530A discloses a method for preparing polyamide composite nanofiltration membranes by introducing an inhibitor containing a single anhydride reaction group into the organic phase, inhibiting the growth of the polyamide chain and generating hydrophilic carboxylic acid groups to change the structure and chemical properties of the polyamide active separation layer, thereby significantly improving the water flux and salt retention rate of the polyamide nanofiltration composite membrane. However, research has found that the nanofiltration membranes prepared by the traditional system have the following technical defects: (1) Selectivity limitation: This system relies on the charge repulsion of a single amine group and has insufficient chelating adsorption capacity for heavy metal ions (such as Pb²⁺ and Cd²⁺); (2) Insufficient mechanical strength: The pure polyamide functional layer has poor flexibility and is prone to microcracks during long-term high-pressure operation, leading to damage to the membrane structure; (3) Poor anti-pollution performance: The membrane surface is highly hydrophobic, and organic matter (such as humic acid) is easily adsorbed and deposited, resulting in significant flux attenuation.

[0004] To address these technical bottlenecks, the present invention proposes an innovative PTFE-based composite nanofiltration membrane. This method utilizes polytetrafluoroethylene (PTFE), a material with excellent chemical inertness, thermal stability, and mechanical strength, as a substrate. Sulfonated cellulose nanocrystals (SCNCs) and graphene oxide (GO) are introduced into the PEI aqueous phase system. Simultaneously, the oil phase is replaced with toluene diisocyanate (TDI). This significantly improves the membrane's selectivity for metal ion retention, mechanical strength, and anti-fouling capabilities. This method boasts controllable process parameters, strong equipment compatibility, and suitability for large-scale production. Summary of the Invention

[0005] A PTFE-based composite nanofiltration membrane for heavy metal ion removal comprises a poly(PTFE)-based membrane and an interfacial polymerization functional layer formed on its surface. The interfacial polymerization functional layer is formed by interfacial polymerization of an aqueous solution and an oily solution, wherein the aqueous solution comprises PEI, SCNC, GO, and a solvent, and the oily solution comprises TDI and an organic solvent.

[0006] The molecular weight of the PEI is preferably 500-20,000; the particle size of the SCNC is 50-200 nm, and the degree of substitution of the sulfonic acid group is 0.3-1.0; the sheet diameter of the GO is 1-10 μm, and the degree of oxidation is 30%-60%.

[0007] The concentrations of the components in the aqueous phase solution are: PEI 0.1-5 (wt%), SCNC 0.05-2 (wt%), GO 0.01-1 (wt%), the solvent is water or a mixed solution of water and low-carbon alcohol (such as methanol, ethanol), and the volume fraction of the low-carbon alcohol in the mixed solution is 0-30%.

[0008] Furthermore, the concentration of TDI in the oil phase solution is 0.1-5 (wt%), and the organic solvent is one or more of n-hexane, cyclohexane, toluene, etc.

[0009] The present invention also provides a method for preparing the polytetrafluoroethylene-based composite nanofiltration membrane, comprising the following steps: (1) Basement membrane pretreatment The poly(PTFE)-based membrane was ultrasonically cleaned with ethanol and deionized water in sequence to remove surface impurities, and then dried in air for later use.

[0010] (2) Preparation of aqueous solution PEI, SCNC and GO were added to the solvent and stirred at room temperature for 2-12 hours to fully dissolve and disperse the components to obtain an aqueous solution.

[0011] (3) Preparation of oil phase solution Add TDI to the organic solvent and stir evenly to obtain an oil phase solution.

[0012] (4) Interfacial polymerization reaction The pretreated PTFE-based membrane is immersed in an aqueous solution, taken out and excess aqueous solution on the surface is drained, and then immediately immersed in an oil solution to perform an interfacial polymerization reaction for 1-30 minutes.

[0013] (5) Post-processing The membrane after the reaction is taken out, washed with an organic solvent and deionized water in sequence to remove unreacted monomers and solvent, and then dried at room temperature to obtain a polyPTFE-based composite nanofiltration membrane.

[0014] Reaction mechanism (1) Polyurethane / polyurea layer construction The amine groups (-NH2 / -NH-) on the PEI molecular chain undergo a condensation reaction with the isocyanate groups (-NCO) in TDI, forming a cross-linked network of urea bonds (-NH-CO-NH-) and urethane bonds (-NH-CO-O-). The reaction equation is as follows: R−(NH2) n +O=C=N-R'→R−(NH-CO-NH-) n − R' This reaction quickly forms a dense separation layer on the surface of the basement membrane, providing a basic barrier for the selective retention of heavy metal ions.

[0015] (2) Enhancement mechanism of SCNC The nanorod-like structure of SCNC (50-200 nm) is embedded in the polyurethane / polyurea layer through hydrogen bonding, increasing the tensile strength by 15%-20%; the sulfonic acid group (-SO3H) increases the hydrophilicity of the membrane and improves its anti-fouling ability.

[0016] (3) GO synergistic mechanism Oxygen-containing functional groups (-OH / -COOH) on the GO surface form stable complexes with heavy metal ions, and the stacking of GO sheets forms nanoscale channels of 0.8-1.2 nm. Beneficial effects

[0017] (1) Improve heavy metal removal efficiency and structural stability PEI reacts with TDI to form urea and carbamate bonds, creating specific chelating sites that increase the rejection of heavy metals such as Pb²⁺ and Cd²⁺ from 88% to 98%. Simultaneously, cross-linking creates a three-dimensional network, resulting in a membrane tensile strength of 8.7 MPa and significantly improved flexibility. Compared to the amide bonds of traditional polyamides, urea / carbamate bonds offer greater chemical stability, imparting structural stability within a pH range of 2-12. This allows the membrane to withstand highly corrosive, high-temperature, and complex working conditions, such as those associated with electroplating wastewater, thereby overcoming the application limitations of traditional polyamide membranes.

[0018] (2) Surface hydrophilic modification and mechanical property enhancement The sulfonic acid groups on the surface of SCNC are hydrophilic, and the water contact angle is reduced from 60° of polyamide to 52°, and the anti-pollution recovery rate is increased from 75% to 89%, inhibiting the adsorption of organic matter; after the rod-like nanostructure is embedded in the network, the interlayer adhesion is enhanced, microcracks caused by long-term operation are reduced, and the compaction resistance is improved.

[0019] (3) GO construction of nano-sieving channels The two-dimensional stacking of GO sheets forms nanochannels of 0.8-1.2 nm, which use the size exclusion effect to accurately screen divalent heavy metal ions while inhibiting the passage of monovalent salts and organic matter; its surface oxygen-containing functional groups such as hydroxyl and carboxyl groups work synergistically with the amino chelating sites of PEI to further enhance the capture ability of Pb²⁺, Hg²⁺, etc. DETAILED DESCRIPTION

[0020] The present invention will be further described in detail below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto. Example

[0021] (1) Pretreatment of the base membrane: A polytetrafluoroethylene membrane (pore size 0.1 μm, thickness 50 μm) was ultrasonically cleaned with ethanol for 30 minutes, then ultrasonically cleaned with deionized water for 30 minutes, and then dried in air for later use; (2) Preparation of aqueous solution: 1 g of PEI (molecular weight 1500), 0.5 g of SCNC (particle size 100 nm, sulfonic acid group substitution degree 0.5), and 0.1 g of GO (sheet diameter 2-5 μm, oxidation degree 40%) were added to 100 ml of deionized water and stirred at room temperature for 6 h to obtain an aqueous solution; (3) Preparation of oil phase solution: Add 1 g of TDI to 100 ml of n-hexane and stir to obtain an oil phase solution; (4) Interfacial polymerization: The pretreated polytetrafluoroethylene membrane was immersed in an aqueous solution for 5 minutes, removed and drained of excess aqueous solution on the surface, and immediately immersed in an oil phase solution for interfacial polymerization reaction for 5 minutes; (5) Post-treatment: The membrane after the reaction was taken out and washed with n-hexane three times to remove the unreacted TDI, then washed with deionized water three times to remove the residual n-hexane, and finally dried at room temperature to obtain a composite nanofiltration membrane.

[0022] The prepared PTFE-based composite nanofiltration membrane was subjected to performance testing. The test conditions were as follows: the feed liquid was an aqueous solution containing heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺) with a concentration of 50 mg / L, the operating pressure was 0.5 MPa, the temperature was 25°C, the pure water flux was 80 L / (m²・h), and the removal rates of Pb²⁺, Cd²⁺, and Hg²⁺ reached 98%, 96%, and 97%, respectively. The water contact angle of the membrane was 52°, the tensile strength was 8.5 MPa, and the anti-pollution recovery rate was 88%. Example

[0023] (1) Basement membrane pretreatment: Same as in Example 1; (2) Preparation of aqueous solution: 2 g of PEI (molecular weight 3000), 1 g of SCNC (particle size 150 nm, sulfonic acid group substitution degree 0.8), and 0.5 g of GO (sheet diameter 5-8 μm, oxidation degree 50%) were added to a 100 ml mixed solution of water and ethanol (volume ratio 7:3) and stirred at room temperature for 8 h to obtain an aqueous solution; (3) Preparation of oil phase solution: Add 2 g of TDI to 100 ml of cyclohexane and stir to obtain an oil phase solution; (4) Interfacial polymerization reaction: The pretreated polytetrafluoroethylene membrane was immersed in an aqueous solution for 10 minutes, the excess aqueous solution on the surface was drained off, and the membrane was immediately immersed in an oily solution for interfacial polymerization reaction for 10 minutes; (5) Post-processing: Same as Example 1.

[0024] The prepared PTFE-based composite nanofiltration membrane was subjected to performance testing. The test conditions were as follows: the feed liquid was an aqueous solution containing heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺) with a concentration of 50 mg / L, the operating pressure was 0.5 MPa, the temperature was 25°C, the pure water flux was 75 L / (m²・h), and the removal rates of Pb²⁺, Cd²⁺, and Hg²⁺ reached 98%, 96%, and 97%, respectively. The water contact angle of the membrane was 52°, the tensile strength was 8.7 MPa, and the anti-pollution recovery rate was 89%.

[0025] Comparative Example 1 The difference from Example 1 is that the aqueous solution does not contain SCNC and GO, but only contains PEI. The preparation method is as follows: (1) Basement membrane pretreatment: same as in Example 1; (2) Preparation of aqueous solution: 1 g of PEI (molecular weight 1500) was added to 100 ml of deionized water and stirred at room temperature for 6 hours to obtain an aqueous solution; (3) Preparation of oil phase solution: same as in Example 1; (4) Interfacial polymerization reaction: same as in Example 1; (5) Post-processing: Same as Example 1.

[0026] The prepared PTFE-based composite nanofiltration membrane was subjected to performance testing. The test conditions were as follows: the feed liquid was an aqueous solution containing heavy metal ions (Pb²⁺, Cd²⁺, Hg²⁺) with a concentration of 50 mg / L, the operating pressure was 0.5 MPa, the temperature was 25°C, the pure water flux was 60 L / (m²・h), and the removal rates of Pb²⁺, Cd²⁺, and Hg²⁺ reached 88%, 85%, and 86%, respectively. The water contact angle of the membrane was 65°, the tensile strength was 7.2 MPa, and the anti-pollution recovery rate was 75%.

Claims

1. A PTFE-based composite membrane for heavy metal ion removal, characterized in that: A polytetrafluoroethylene-based film; and an interfacial polymerization functional layer formed on the surface of the base film, wherein the interfacial polymerization functional layer is formed by an interfacial polymerization reaction between an aqueous solution and an oily solution; the aqueous solution comprises polyethyleneimine (PEI), sulfonated cellulose nanocrystals (SCNC), graphene oxide (GO) and a solvent; and the oily solution comprises toluene diisocyanate (TDI) and an organic solvent.

2. The PTFE-based composite nanofiltration membrane according to claim 1, wherein: The molecular weight of the PEI is 500-20,000; the particle size of the SCNC is 50-200 nm, and the degree of substitution of the sulfonic acid group is 0.3-1.0; the sheet diameter of the GO is 1-10 μm, and the degree of oxidation is 30%-60%.

3. The PTFE-based composite nanofiltration membrane according to claim 1, characterized in that The concentrations of the components in the aqueous phase solution are: PEI 0.1%-5% (w / w); SCNC 0.05%-2% (w / w); GO 0.01%-1% (w / w); the solvent is selected from water or a mixed solution of water and low-carbon alcohol, wherein the volume fraction of the low-carbon alcohol is 0-30%.

4. The PTFE-based composite nanofiltration membrane according to claim 1, wherein: The concentration of TDI in the oil phase solution is 0.1%-5% (w / w); the organic solvent is selected from one or more combinations of n-hexane, cyclohexane, and toluene.

5. A method for preparing a PTFE-based composite membrane according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: ultrasonically cleaning a PTFE base membrane with ethanol and deionized water in sequence to remove surface impurities and then drying; dispersing PEI, SCNC and GO in a solvent and stirring at room temperature for 2-12 hours to obtain a uniformly dispersed aqueous solution; dissolving TDI in an organic solvent to obtain an oily solution; immersing the pretreated base membrane in the aqueous solution, draining and immediately immersing the base membrane in the oily solution to carry out an interfacial polymerization reaction for 1-30 minutes; and washing the reacted membrane with an organic solvent and deionized water in sequence to remove unreacted matter and then drying to obtain a finished PTFE composite membrane.

6. A PTFE-based composite membrane for heavy metal ion removal, characterized in that By constructing a functional layer formed by interfacial polymerization of polyethyleneimine (PEI), sulfonated cellulose nanocrystals (SCNC), graphene oxide (GO) and toluene diisocyanate (TDI) on the surface of the polytetrafluoroethylene membrane, the composite membrane has a retention rate of 98%, 96% and 97% for Pb²⁺, Cd²⁺ and Hg²⁺, respectively, the tensile strength is increased to above 8.5 MPa, the water contact angle is reduced to 52°, the anti-pollution recovery rate is increased to above 88%, and the structure remains stable in the pH range of 2-12, making it suitable for heavy metal wastewater treatment, seawater desalination and resource recovery.