Conductive nanofiltration membrane, preparation method, and electric field enhanced nanofiltration method

Abstract

The present invention relates to a conductive nanofiltration membrane, a preparation method, and an electric field-enhanced nanofiltration method, belonging to the field of membrane separation technology. The present invention utilizes a novel method to prepare a conductive thin film composite (TFC) membrane for electric field-enhanced filtration. Aniline-functionalized carbon nanotubes are loaded onto a conductive polyaniline (PANI) / polyimide (PI) substrate as an intermediate layer. The significantly enhanced conductivity and overall stability directly impact the membrane's performance, exhibiting significant voltage response behavior, enabling it to effectively remove low-molecular-weight pollutants and mitigate toxicity in wastewater with the aid of an electric field.

Description

Technical Field

[0001] The invention relates to a conductive nanofiltration membrane, a preparation method and an electric field enhanced nanofiltration method, and belongs to the technical field of membrane separation. Background Art

[0002] NF membranes have great potential due to their low cost, simple operation and minimal environmental impact. However, the pore size and surface charge of traditional NF membranes remain unchanged after construction. This inherent limitation leads to a restricted separation field and reduces separation efficiency when fouling occurs, especially in separations that rely on charge screening. The development of functional nanofiltration membrane materials and their integration with other technologies have great prospects in significantly improving membrane performance. Conductive membranes are a new type of functional membrane whose performance can be enhanced by external electric fields based on mechanisms such as electrostatic repulsion, electroinduced bubbles, electrowetting, electrochemical redox, electrically enhanced adsorption, and electroinduced structural changes. However, traditional substrates used for composite NF membranes, such as polyethersulfone, polyimide, and polyvinylidene fluoride, lack conductivity. Therefore, the introduction of conductive nanomaterials into the preparation of NF membranes has become an effective method to improve membrane conductivity. Carbon nanotubes (CNTs) have attracted great attention due to their excellent conductivity, excellent mechanical properties, and strong chemical stability. Wang et al. developed a new in situ combination method to prepare CNTs with a conductivity of 105Scm. -1 CNT / PVDF membranes were fabricated to significantly improve the antifouling properties (Non-Patent Document 1). Zhu et al. introduced CNTs as an intermediate layer onto the surface of a PES substrate, resulting in a composite membrane with a conductivity three orders of magnitude higher than that of a membrane without CNTs (Non-Patent Document 2). Therefore, the rejection and permeability of the membrane can be fine-tuned by varying the voltage between 0 and 1 V. Similarly, Hu et al. designed a conductive 3D-rGO-CNT hybrid membrane that achieved a NaCl rejection of 71% under a negative applied potential, almost three times the rejection observed in the absence of voltage (Non-Patent Document 3). Therefore, introducing CNTs as an intermediate layer into the membrane preparation process is a valuable strategy to improve the conductivity and overall performance of NF membranes. However, CNTs typically exist in the form of entangled and undispersed aggregates, resulting in poor dispersibility in polar solvents. In addition, uneven coating of CNTs on the substrate will lead to weak interlayer adhesion and insufficient coverage of the selective layer.

[0003] References:

[0004] (Non-patent document 1) Wang, S.; Liang, S.; Liang, P.; Zhang, X.; Sun, J.; Wu, S.; Huang,

[0005] (Non-patent document 2) Zhu, Y.; Gui, L.; Wang, R.; Wang, Y.; Fang, W.; Elimelech, M.; Lin, S.; Jin, J. Regulation of molecular transport in polymer membranes with voltage-controlled pore size at the angstrom scale. Nat. Commun. 2023, 14, (1), 2373.

[0006] (Non-patent document 3) Hu, C.; Liu, Z.; Lu, Summary of the Invention

[0007] The technical problem to be solved by the present invention is that in the process of preparing nanofiltration membranes using carbon nanotubes as conductive materials, there is a problem of uneven distribution of the membrane layer due to the dispersion and entanglement of carbon nanotubes. In the technical solution of the present invention, the surface of CNTs is functionalized with aniline groups (CNT-NH2), and then loaded on a conductive PANI / PI substrate as an intermediate layer, and then interfacial polymerization (IP) is performed to form a composite NF membrane ( Figure 1). At the same time, the CNT-NH2 network can also react with trimesoyl chloride (TMC), so that part of the intermediate layer is wrapped by the selective layer, enhancing the overall structural stability of the composite membrane. When used as a filtration membrane and cathode at the same time, the resulting membrane shows multiple response cycles in intercepting small molecule dyes. In the presence of an external electric field, the toxicity of crystal violet (CV) solution can be greatly reduced. In addition, due to the electrically enhanced antifouling properties, the membrane can be operated for 156 hours. It highlights the great potential of conductive membranes in controlling wastewater pollution in the environment.

[0008] A conductive nanofiltration membrane comprises a base layer, an intermediate layer and a selective separation layer. The base layer is a porous polymer and the intermediate layer is a carbon nanotube with surface amino groups modified.

[0009] The selective separation layer is prepared by interfacial polymerization.

[0010] The selective separation layer is made of polyamide.

[0011] The above-mentioned method for preparing the conductive nanofiltration membrane comprises the following steps:

[0012] Step 1: Dispersing carbon nanotubes in water, then adding nitrite and aromatic primary amine to carry out diazotization reaction, filtering out the product, washing, and drying to obtain amino-modified carbon nanotubes;

[0013] Step 2: dispersing the amine-modified carbon nanotubes obtained in step 1 in water and applying the resultant mixture to the surface of the base film to obtain an intermediate layer;

[0014] Step 3: Obtain a selective separation layer on the intermediate layer by interfacial polymerization.

[0015] In step 1, the nitrite is methyl nitrite, ethyl nitrite, propyl nitrite, butyl nitrite or isoamyl nitrite; and the aromatic primary amine is aniline, p-toluidine, p-chloroaniline, p-methylaniline or p-phenylenediamine.

[0016] In step 1, the weight ratio of nitrite to aromatic primary amine is 1:1.5-4; the volume ratio of nitrite to water is 1:100-150, and the concentration of carbon nanotubes in water is 0.005-0.03 g / mL.

[0017] In step 1, the reaction conditions are 300-400 K for 5-20 h.

[0018] In step 2, the amount of amino-modified carbon nanotubes loaded on the surface of the base film is 0.01-2 g / m 2 , preferably 0.04-1.2g / m 2 , and preferably 0.2-0.3g / m 2 .

[0019] In step 3, during the interfacial polymerization process, the aqueous phase solution is first brought into contact with the intermediate layer, and then the surface of the membrane is brought into contact with the oil phase solution.

[0020] The aqueous solution uses piperazine monomers with a concentration range of 1-5 wt %; the oil solution uses acyl chloride monomers with a concentration range of 0.05-0.5 wt %.

[0021] An electric field enhanced nanofiltration method comprises the following steps:

[0022] The above-mentioned nanofiltration membrane is used, and an electrode is placed on the surface side of the selective separation layer to form a voltage between the nanofiltration membrane and the electrode.

[0023] The distance between the nanofiltration membrane and the electrode is 0.1-10 mm; the voltage range is 1-30 V, preferably 2-10 V.

[0024] Application of the above-mentioned nanofiltration membrane in liquid filtration.

[0025] The liquid filtration refers to the filtration of dyes. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : Schematic diagram of the technical concept of the present invention.

[0027] Figure 2 Preparation and characterization of CNT-NH2. (a) Schematic diagram of CNT modification. (b) ATR-FTIR spectra and (c) XPS spectra of CNTs and CNT-NH2. (d) Dispersion diagram of CNTs and CNT-NH2 in aqueous solution.

[0028] Figure 3 TEM images of (a) pristine CNT and (b) CNT-NH2.

[0029] Figure 4 : SEM images of the top surface morphology of (a) PANI / PI substrate and (b) CNT-NH2.

[0030] Figure 5 : Effect of CNT-NH2 interlayer loading on the morphology and properties of PANI / PI-CNT-TFC films. (ad) SEM images of top surface morphology and (eh) cross-sectional morphology. (il) with 0.04, 0.08, 0.24 and 1.2 g / m 2 AFM images of the membrane loaded with CNT-NH2 interlayers. (mn) Schematic diagram of the mechanism of the membrane morphology formed during interfacial polymerization.

[0031] Figure 6Optimization of the CNT-NH2 interlayer. Effect of interlayer loading on the optimization of the CNT-NH2 interlayer. Effect of interlayer loading on the (a) RB solution (200 ppm) performance, (b) water contact angle, and (c) electrical conductivity of the PANI / PI-CNT-TFC membrane, respectively. Figure 7 : The performance of the PANI / PI-CNT-TFC membrane enhances with increasing applied voltage. (a) Performance of CV solution (200 ppm) at different applied voltages. (b) Repulsion of CV molecules over 8 cycles with an applied voltage of -5 V. (c) Mechanism diagram of the electric field-enhanced repulsion of CV molecules. (d) Molecular weight cutoff and pore size distribution of the membrane with and without voltage. (e) Performance comparison with membranes with 400-600 Da dye molecules reported in the literature. (f) Comparison of the response performance of the membrane with state-of-the-art nanofiltration membranes.

[0032] Figure 8 Stability evaluation of the PANI / PI-CNT-TFC membrane. (a) Cyclic voltammetry scan of the membrane from -5 V to 5 V at a scan rate of 50 mV / s. (b) Digital images of the membrane before and after ultrasonic treatment. (c) Performance in RB solution before and after ultrasonic treatment.

[0033] Figure 9 : Performance of PANI / PI-CNT-TFC film towards MO under different applied voltages.

[0034] Figure 10 : Repulsion effect of PANI / PI-CNT-TFC film on the mixture of CV and MO, and the mixture of MB and RB at -5 V.

[0035] Figure 11 Long-term stability evaluation. (a) Long-term performance of PANI / PI-CNT-TFC membrane against 200 ppm CV at -5 V. (b) EPR spectra and (c) antifouling mechanism of PANI / PI-NT-TFC membrane.

[0036] Figure 12 : Comparison of toxicity test results of feed and permeate samples.

[0037] Figure 13 : Relationship between 96-hour lethality of zebrafish and volume fraction of feed samples.

[0038] Figure 14 : Photo of the original CNTs being directly filtered onto a substrate to make a membrane. DETAILED DESCRIPTION

[0039] When using traditional nanofiltration membranes, improving the efficiency of removing small-molecule organic pollutants is a major challenge due to their pore size and surface charge limitations. At the same time, the fixed selectivity and permeability of these membranes hinder intelligent control through external stimuli during operation. The present invention adopts a new method to prepare a conductive thin film composite (TFC) membrane for electric field-enhanced filtration. Aniline-functionalized carbon nanotubes are loaded onto a conductive polyaniline (PANI) / polyimide (PI) substrate as an intermediate layer. The significantly enhanced conductivity and overall stability directly affect the performance of the membrane, showing significant voltage-responsive behavior, with a crystal violet rejection rate ranging from 18.2% to 98.1%. Notably, the prepared membrane can achieve a toxicity removal rate of approximately 99% for crystal violet solution. The membrane shows remarkable elasticity and operates continuously for 156 hours without scaling, enabling it to effectively remove low-molecular-weight pollutants and reduce toxicity in wastewater with the help of an electric field.

[0040] The main raw materials used include: Polyimide (PI) and polyaniline (PANI), multi-walled carbon nanotubes (CNTs) (outer diameter 10-20 nm, length 0.5-2 μm), piperazine (PIP, C4H 10 N2, AR), 1,3,5-benzenetricarboxylic acid chloride (TMC) and n-hexane for interfacial polymerization (IP);

[0041] Example 1 Modification of CNT-NH2

[0042] CNT is added in deionized water (50mL) and ultrasonically treated for 1 hour to obtain the dispersion of CNT. Then, isoamyl nitrite (0.4mL) and p-phenylenediamine (1.0g) are added to the dispersion of CNT to obtain a mixed solution, and the solution is reacted by stirring for about 12h at 353K to obtain CNT (CNT-NH2) with aniline groups. Afterwards, the mixed solution is filtered to obtain CNT-NH2, and the CNT-NH2 is washed several times with deionized water. Finally, CNT-NH2 is dried and stored for subsequent experiments. In this step, CNT is modified based on the fact that isoamyl nitrite can react with p-phenylenediamine to produce a diazotization reaction. Diazotization refers to the chemical reaction in which aromatic primary amines and nitrous acid react to form diazonium salts. The p-phenylenediamine in this reaction is often referred to as the diazo component, and nitrous acid is the diazotizing agent. The diazotization mechanism generally involves the following steps: 1) Nucleophilic substitution: The oxynitrile ion group in the isoamyl nitrite molecule undergoes a substitution reaction with the nucleophilic portion of p-phenylenediamine, forming a diazo compound, water, and nitrogen gas. 2) Diazo group transfer: One end of the diazo compound is attached to a carbon nanotube, and the other end is attached to an aniline group. (The carbon nanotube nanomaterial is uniformly dispersed in water, serving as a carrier for the reaction, and the diazotization reaction occurs on the carbon nanotube carrier.)

[0043] Example 2

[0044] In previous work, it has been reported that a PANI / PI base membrane was constructed by a non-solvent induced phase separation method (Liu, M.-L.; Li, L.; Tang, M.-J.; Hong, L.; Sun, S.-P.; Xing, W. Multi-component separation of small molecular / ionic pollutants with smart pH-gating membranes. Chem. Eng. Sci. 2021, 245, 116854.) The substrate was stored in deionized water before use. The preparation method of the base membrane in the present invention is the same, specifically: the PANI / PI base membrane is prepared by a non-solvent induced phase inversion method. Before preparing the casting solution, the polymers PANI and PI are fully dried in a vacuum drying oven at 70°C to remove moisture. Then a certain amount of PI is dissolved in NMP (PANI / PI / NMP=8 / 12 / 80wt%), and after it is fully dissolved, a certain amount of PANI is slowly added. The prepared mixed solution was stirred at 70 ° C for two days, and the formed uniform casting solution was left at room temperature overnight to remove bubbles. In addition, when preparing pure PANI film, in order to prevent the casting solution from gelling, it is necessary to add LiCl (0.5wt%). The doped solution was then cast on the smooth side of a polypropylene nonwoven fabric attached to a glass plate. The knife gap was set to 200μm. Subsequently, the membrane was transferred to a coagulation bath for one day, and the water was regularly replaced to complete phase separation and remove excess solvent.

[0045] The aqueous phase solution was prepared by dissolving PIP in deionized water at a fixed concentration of 2 wt %.The organic phase solution was prepared by dissolving TMC in n-hexane at a fixed concentration of 0.1 wt %.

[0046] Composite membranes were prepared by constructing a CNT-NH2 interlayer and then performing interfacial polymerization. The concentration of CNT-NH2 dispersion was fixed at 0.05 mg / mL. 2 The intermediate layer was prepared by vacuum filtering different volumes of CNT-NH2 dispersion onto a PANI / PI-based membrane. A 2wt% PIP aqueous solution was then poured onto the intermediate layer and left for 2 minutes. The excess solution was then removed using a vacuum filtration method. Next, a 0.1wt% TMC hexane solution was poured onto the membrane surface and left for 1 minute to form a selective layer. The prepared NF membrane was washed with deionized water and named "PANI / PI-CNT-TFC." Finally, the PANI / PI-CNT-TFC membrane was stored in water for further testing and characterization.

[0047] In the following tests, the conductivity of the membrane was measured using a four-point conductivity probe analyzer. The generation of hydroxyl radicals was determined by electron paramagnetic resonance spectroscopy. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used as a free radical scavenger. Electrochemical tests were performed using an electrochemical workstation using a three-electrode cell with a 2 cm2 surface area. 2 The films were used as working electrodes, Pt sheets (2 cm × 2 cm × 0.1 cm) as counter electrodes, and silver / silver chloride (Ag / AgCl) as reference electrodes. During CV tests, the films were scanned at a rate of 50 mV / s over a potential range of -5 to 5 V.

[0048] The permeability and selectivity of the PANI / PI-CNT-TFC membrane were evaluated using a cross-flow device. The effective membrane area was 12.56 cm 2 The membranes were tested at a pressure of 6 bar. All membranes were prefiltered for 1 hour to ensure steady-state conditions. During the tests with a DC power supply, the membrane served as the cathode and the titanium mesh served as the anode. The distance between the two electrodes was 2 mm. The membrane flux and retention rate were calculated.

[0049] To measure the membrane average effective pore size, pore size distribution, and molecular weight cutoff (MWCO), PEG with molecular weights of 200, 400, 600, and 1000 Da was used. The concentration of PEG in the feed and permeate solutions was measured by a total organic carbon analyzer. The pore size distribution of each membrane was calculated using the equation:

[0050]

[0051] where r P is the effective pore radius of the membrane, μ P is the molecular radius when RT = 50%. Geometric standard deviation σ P From the r when RT = 84.13% P With R T = r when 50% P When R T =90%, the MWCO of the membrane is equal to the molecular weight. rs is the radius of PEG, calculated by the following equation:

[0052] r s =16.73×10 -12 ×M 0.557

[0053] where M is the molecular weight of the PEG molecule.

[0054] A 96-hour acute toxicity experiment on zebrafish was conducted to evaluate the toxicity of dye wastewater. The average weight of juvenile zebrafish was 0.18 g (±0.03) and the average body length was 2.5 cm (±0.10). Before the experiment, the zebrafish were acclimated in the tank for more than 7 days, and the natural mortality rate was less than 0.5%. During the accumulation process, the fish were fed food regularly every day and fasted starting one day before the test. 200 ppm crystal violet was used as the feed solution, and the feed solution was filtered through a conductive PANI / PI-CNT-TFC membrane under an electric field to obtain a permeate solution. Both the feed and the permeate were diluted with deionized water at five different dilutions, with the sample volume fractions being 5%, 25%, 50%, 75% and 100%, respectively. At the same time, the deionized water group served as a control. Next, 10 zebrafish in each group were transferred to a round glass container containing samples of different dilutions in the exposure experiment. The number of dead zebrafish was counted after 24, 48, 72, and 96 hours of exposure to the wastewater. Toxicity units (TU) are used to directly express toxicity, and the calculation formula is as follows:

[0055]

[0056] Among them LC 50 is the volume fraction of the sample that causes half of the zebrafish to die. When zebrafish are exposed to 100% volume fraction of the sample, not half of the zebrafish die, so LC cannot be obtained. 50 In this case, TU can be calculated as follows:

[0057] TU=RE×100×0.02

[0058] Where RE is the mortality rate of zebrafish. The toxicity removal rate can be calculated by the following formula:

[0059]

[0060] Among them TU f and TU p are the toxicity units of the feed solution and the permeate, respectively.

[0061] Functionalization results of CNTs in the middle layer: CNTs were surface modified with aniline groups to enhance their dispersibility in water ( Figure 2 a). Figure 2 In b, it is at 1510cm -1 (NH bond) and 1278cm -1 (CN bond) reveals a different high-intensity peak in CNT-NH2, which is absent in pristine CNTs, indicating that the pristine CNTs were successfully modified with aniline groups. Figure 2The XPS spectrum in Figure c further confirms this change, showing a significant increase in the nitrogen content in the CNT-NH2 compared to the original CNT. In addition, the C1s high-resolution spectrum of the modified CNTs shows a new peak at 286.2 eV, corresponding to the C-N bond. These results indicate that the CNT-NH2 was successfully synthesized.

[0062] also, Figure 2 Figure d shows the challenges of dispersing pristine CNTs in deionized water despite prolonged sonication, which is attributed to their inherent hydrophobic structure and strong intermolecular forces. In contrast, CNT-NH2 was better dispersed and remained stable in deionized water for 30 days ( Figure 2 This improved dispersibility is attributed to the enhanced hydrophilicity and retention capacity imparted by the aniline active groups on the CNT-NH2 surface. Figure 3 a) and CNT-NH2( Figure 3 The TEM image of b) shows that the entanglement and aggregation of CNT-NH2 are reduced, indicating the beneficial effect of the active aniline groups on the dispersion of CNTs. Figure 4 As shown in a, the original PANI / PI substrate exhibits a relatively flat surface. After vacuum filtration, CNT-NH2 was uniformly loaded onto the PANI / PI substrate as an intermediate layer ( Figure 4 b). Therefore, the well-dispersed CNT-NH2 interlayer plays a key role in forming a defect-free selective layer for subsequent experiments. As a control, when the original CNT was used to prepare the interlayer according to the same method and then further interfacial polymerization was performed to obtain the selective separation layer, the selective layer of the membrane easily fell off (weak interlayer adhesion), as shown in FIG. Figure 14 shown.

[0063] Optimization of the middle layer of CNT-NH2 composite nanofiltration membrane

[0064] The morphology of PANI / PI-CNT-TFC membrane changes with the change of CNT-NH2 interlayer loading, which ultimately affects its performance. SEM images depict the changes in the morphology of PANI / PI-CNT-TFC membrane when the CNT-NH2 interlayer loading increases from 0.04 g / m 2 Systematically increased to 1.2 g / m 2 When the surface morphology of PANI / PI-CNT-TFC film ( Figure 5 The unique nodular structure always proved the success of the interfacial polymerization process. In addition, the surface morphology of the composite film changed from a nodular structure to a ridge structure ( Figure 5 ad), accompanied by an increase in surface roughness ( Figure 5This morphological transformation can be attributed to two main factors. First, the presence of the CNT-NH2 interlayer enhances the absorption of the aqueous PIP monomer. With the increase of CNT-NH2 loading, the interlayer thickness increases from 0.1μm to 2.7μm ( Figure 5 eh), which helps to store more amine monomers in the CNT-NH2 network. When TMC is introduced, the subsequent reaction between PIP and TMC produces a selective layer embedded in the CNT-NH2 network, promoting the formation of a rough selective layer with a wrinkled nanostructure. At the same time, the amino groups in the CNT-NH2 interlayer react with TMC to promote the development of ridge-like structures. Secondly, the interlayer reduces the release rate of amine monomers and slows down the reaction rate with TMC monomers. The lower CNT-NH2 loading promotes a rapid reaction between PIP and TMC. The selective layer initially formed can limit the further diffusion of PIP monomers, thereby forming a nodular structure with minimal roughness ( Figure 3 In contrast, higher CNT-NH2 loading facilitates the gradual release of PIP monomers, enabling them to react more effectively with TMC on the surface of the CNT-NH2 network, ultimately forming a ridge-like structure characterized by increased roughness ( Figure 3 n).

[0065] Dye separation test

[0066] NF experiments with 200 ppm of benghalensis sodium salt (RB, negatively charged, 1017.64 Da) showed that the permeability of PANI / PI-CNT-TFC membranes was positively correlated with the increase in CNT-NH2 interlayer loading, while the retention of RB was slightly reduced ( Figure 6 a). However, 1.2g / m 2 The permeability of the membrane decreased at a low CNT-NH2 loading. This change may be attributed to the fact that the intermediate layer effectively avoids the undesirable lateral transport within the low permeability polyamide layer. As a result, the water transport paths through the selective layer of the polyamide membrane are almost parallel to each other along the normal direction, resulting in a more uniform distribution of flux on the membrane surface. This phenomenon is called the groove effect. However, the thicker CNT-NH2 interlayer increases the hydraulic resistance in the normal direction, resulting in a decrease in membrane permeability, which is supported by the observation of water contact angles of composite membranes with different interlayer loadings ( Figure 6 b). In addition, the change in membrane morphology and the increase in surface roughness help to increase the water contact area and improve the filtration efficiency, thereby significantly improving the permeability of the PANI / PI-CNT-TFC membrane to RB solution from approximately 19 to 32 L m -2 h -1 bar -1 ( Figure 6 a). In addition, Figure 6 Figure c shows that the conductivity enhancement of PANI / PI-CNT-TFC membrane is directly related to the increase of CNT-NH2 loading. In contrast, when the CNT-NH2 loading reaches 0.24 or 1.2 g / m 2 When the conductivity is 6.9Scm -1 The conductivity of the PANI / PI-CNT-TFC membrane was significantly improved by an order of magnitude compared to the PANI / PI substrate. This improvement was mainly attributed to the excellent conductivity of CNT-NH2. It is worth noting that the conductivity of the PANI / PI-CNT membrane was almost the same as that of the PANI / PI-NT-TFC membrane, indicating that the selective layer had minimal effect on the conductivity ( Figure 6 c). Partial incorporation of CNT-NH2 into the selective layer also helps to reduce the resistivity of the selective layer. Therefore, to further explore the charge transfer properties, the difference in charge density when CNT-NH2 is introduced into the PANI / PI substrate without a selective layer was analyzed. The CNT-NH2 interlayer loading was selected to be 0.24 g / m 2 The PANI / PI-CNT-TFC membrane was further synthesized for investigation because it yielded the best permeability and satisfactory conductivity.

[0067] Crystal violet (CV) is a small, positively charged dye weighing 407.99 Da. It is often difficult to retain using PANI / PI-CNT-TFC membranes in NF experiments. In our field-enhanced filtration studies, CV was chosen as a representative molecule. Figure 7 Figure a shows that the rejection of the PANI / PI-CNT-TFC membrane for CV increases significantly, from 18.2% to 98.1% as the applied voltage increases from 0 V to -5 V. The difference in membrane rejection with and without an electric field is defined as the degree of response, which is approximately 80% in this case. Considering the MWCO ( Figure 7 d), based on the size exclusion mechanism, CV molecules can initially pass through the membrane. At the same time, due to the negative charge on the surface of the PANI / PI-CNT-TFC membrane, the membrane shows low retention of positively charged CV at 0V applied voltage according to the Donnan exclusion mechanism. However, when the membrane is used as a cathode in an electric field, electrons are acquired on the membrane surface. When an external electric field is applied, an electrophoretic effect occurs, which attracts the positively charged CV to the cathode, causing them to bind to the electrons on the membrane surface ( Figure 7c). This interaction enhances the Donnan effect between the membrane surface and CV, subsequently improving CV rejection under an electric field. When the applied voltage increases from 0 to -5 V, the Donnan repulsion between the CV bound to electrons and the membrane increases, thereby enhancing the membrane's rejection of CV. However, when the applied voltage changes from -5 V to -15 V, the CV rejection remains almost unchanged ( Figure 7 This observation indicates that the rejection reaction reaches a plateau at higher voltage levels, which means that a continuous increase in voltage does not cause a significant change in the CV rejection rate. In addition, the permeability of the PANI / PI-CNT-TFC membrane is approximately 35 L m -2 h -1 bar -1 , the membrane permeability did not change significantly with the change of applied voltage ( Figure 7 a), indicating that the electric field has no direct effect on the membrane permeability. This is attributed to the negligible changes in the membrane's MWCO and pore size under the action of the electric field ( Figure 7 d). In summary, an applied voltage of -5 V was selected for the subsequent experiments. A commercially available NF270 membrane with a diameter of 0.4 nm was selected as a reference to evaluate the performance of the prepared NF membrane. Obviously, the permeability of the PANI / PI-CNT-TFC membrane to the CV solution was about 5.2 times that of the NF270 membrane, regardless of whether the voltage was applied. In addition, the rejection rate of the NF270 membrane to CV was 65.2%, which was lower than the rejection rate of the PANI / PI-CNT-TFC membrane to CV at an applied voltage of -5 V. Compared with those membranes reported in the literature, the prepared conductive PANI / PI-CNT-TFC membrane showed excellent performance ( Figure 7 e).

[0068] To evaluate the cycling stability of the PANI / PI-CNT-TFC membrane, continuous CV retention tests were performed at applied voltages of 0 and -5 V. Figure 7 As shown in Figure 2b, the membrane can still achieve a CV rejection of about 98% even after 8 cycles, indicating its excellent cycling stability. It is worth noting that the PANI / PI-CNT-TFC membrane outperforms the state-of-the-art conducting polymer membranes ( Figure 7 f). Therefore, the molecular transport across the conductive membrane can be highly reversible and cyclically regulated by applying voltage. This remarkable cyclic stability is due to its strong electrochemical stability and mechanical stability. The electrochemical stability was characterized by cyclic voltammetry scanning ( Figure 8 a). The current showed no significant change over 50 cycles, indicating that the conductive composite film has excellent electrochemical stability. To evaluate its mechanical properties, the film was sonicated for 2 hours.

[0069] like Figure 8 As shown in Figure b, the membrane surface showed negligible changes before and after ultrasonic treatment, maintaining the original rejection rate and permeability level of RB solution ( Figure 8 c). These results demonstrate the excellent mechanical stability of the PANI / PI-CNT-TFC membrane. Furthermore, regardless of whether the feed solution was methylene blue (MB, positively charged, 319.85 Da), whose molecular weight was less than CV, or Janus green B (JGB, positively charged, 511.07 Da), whose molecular weight was greater than CV, the membrane exhibited reversible voltage-responsive repulsion over eight cycles. This highlights its universal applicability for positively charged dyes, allowing for highly reversible and cyclic modulation of dye molecule transport via voltage application.

[0070] In contrast, the retention rate of methyl orange (MO, negatively charged, 327.33 Da) of the PANI / PI-CNT-TFC membrane increased slightly from 10.24% to 22.47% as the applied voltage changed from 0 V to -5 V ( Figure 9 ), indicating that the electric field has no significant effect on the membrane's retention of negatively charged dyes. This phenomenon stems from the electrophoretic effect under the electric field, which forces negatively charged molecules to migrate toward the anode. As a result, fewer dye molecules interact with the electrons on the PANI / PI-CNT-TFC membrane surface, weakening the Donnan effect and failing to enhance the membrane's rejection of MO. Regardless of the applied voltage, the membrane's permeability to MO solutions remains relatively unchanged ( Figure 9 ). In addition, in another evaluation involving a mixture of dye molecules of similar size at an applied voltage of -5 V ( Figure 10 ), the membrane exhibited high repulsion toward positively charged dyes (such as CV and MB) and low repulsion toward negatively charged MO. This result emphasizes its effectiveness in separating two-component systems (such as CV and MO or MB and MO) under electric field promotion.

[0071] Long-term stability plays a crucial role in evaluating membrane performance. Therefore, the PANI / PI-CNT-TFC membrane was tested using a CV solution at an applied voltage of -5 V for 156 h. Figure 11 As shown in a, the retention rate of CV (about 98%) and the permeability (about 34 L m -2 h -1 bar -1 ) remained stable, indicating that the obtained NF membrane had good durability under the electric field. It is worth noting that the membrane surface showed no obvious contaminants. This may be attributed to the generation of H2O or H2 microbubbles on the membrane surface with the help of the electric field ( Figure 11b). These bubbles may weaken the binding force between the pollutants and the membrane surface, even within the membrane pores, helping to remove pollutants and ensure continued permeability without a significant drop. In addition, the membrane showed peaks of DMPO / ·OH rotational adducts with relative intensities of 1:2:2:1, indicating its ability to generate ·OH radicals under applied voltage ( Figure 11 c). This phenomenon likely occurs when electrons transfer from the cathode to the CV electrolyte solution, triggering a reaction with water molecules to form OH- ions and cathode ·OH radicals. Simultaneously, dissolved oxygen molecules are reduced to H2O2, which are then activated at the cathode to produce ·OH radicals. These radicals play a crucial role in promoting the degradation of small organic pollutants, aiding their removal from the membrane. Furthermore, these radicals reduce the accumulation of pollutants on the membrane surface or within its pores, potentially extending membrane life.

[0072] Toxicity Removal of Small Molecule Dye Wastewater

[0073] Toxicity assessment remains crucial to ensure environmental safety. Biotoxicity experiments can comprehensively evaluate the toxicity of osmotic fluids and their potential impact on ecological balance. In this work, zebrafish were used to evaluate the toxicity of CV solutions (Tables 1 and 2, Figure 12 ). Figure 8 The results showed that the toxicity unit of the initial CV feed solution was 38.9. When the CV feed solution was filtered through the membrane under an external electric field, the toxicity unit of the resulting permeate solution was drastically reduced to 0.4. This indicates a remarkable toxicity removal efficiency of approximately 99%. This reduction can be attributed to the generation of ·OH radicals during the filtration process under the applied voltage, which effectively oxidizes the pollutants and reduces the toxicity of the solution. This result emphasizes the ability of the PANI / PI-CNT-TFC membrane to significantly reduce the toxicity of dye wastewater with the assistance of a DC power supply, indicating its potential application in protecting aquatic environments.

[0074] Table 1 96-hour acute toxicity test of samples on zebrafish.

[0075]

[0076] Table 2. 96-hour acute toxicity test of permeate samples on zebrafish.

[0077]

Claims

1. A conductive nanofiltration membrane, characterized in that: It includes a base layer, an intermediate layer and a selective separation layer, wherein the base layer is a porous polymer and the intermediate layer is a carbon nanotube with surface amino groups modified; The selective separation layer is made of polyamide; The method for preparing the conductive nanofiltration membrane comprises the following steps: Step 1: Dispersing carbon nanotubes in water, then adding nitrite and aromatic primary amine to carry out diazotization reaction, filtering out the product, washing, and drying to obtain amino-modified carbon nanotubes; Step 2: dispersing the amine-modified carbon nanotubes obtained in step 1 in water and applying the resultant mixture to the surface of the base film to obtain an intermediate layer; Step 3: Obtain a selective separation layer on the intermediate layer by interfacial polymerization.

2. The conductive nanofiltration membrane according to claim 1, characterized in that The nitrite is methyl nitrite, ethyl nitrite, propyl nitrite, butyl nitrite or isoamyl nitrite; the aromatic primary amine is aniline, p-toluidine, p-chloroaniline, p-methylaniline or p-phenylenediamine; the weight ratio of the nitrite to the aromatic primary amine is 1:1.5-4; the volume ratio of the nitrite to water is 1:100-150, and the concentration of the carbon nanotubes in the water is 0.005-0.03 g / mL.

3. The conductive nanofiltration membrane according to claim 1, characterized in that In step 1, the reaction conditions are 300-400 K for 5-20 h.

4. The conductive nanofiltration membrane according to claim 1, characterized in that In step 2, the amount of amino-modified carbon nanotubes loaded on the surface of the base film is 0.01-2 g / m 2 .

5. The conductive nanofiltration membrane according to claim 1, characterized in that In step 3, during the interfacial polymerization process, the aqueous phase solution is first contacted with the intermediate layer, and then the oil phase solution is contacted on the surface of the membrane; Piperazine monomers are used in the aqueous solution with a concentration range of 1-5wt%; Acid chloride monomers are used in the oil phase solution with a concentration range of 0.05-0.5wt%.

6. A nanofiltration method enhanced by electric field, characterized in that: The steps include: A nanofiltration membrane according to any one of claims 1 to 3 is used, and an electrode is placed on one side of the surface of the selective separation layer to form a voltage between the nanofiltration membrane and the electrode; the distance between the nanofiltration membrane and the electrode is 0.1-10 mm; and the voltage range is 1-30 V.

7. Use of the nanofiltration membrane according to any one of claims 1 to 3 in liquid filtration.

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