Self-cleaning loose nanofiltration membrane, preparation method thereof and application thereof

Abstract

This invention discloses a self-cleaning, loose nanofiltration membrane, its preparation method, and its application in the field of membrane separation technology. The membrane uses poly(m-phenylene isophthalamide) (PMIA) as the base membrane and is co-doped with citric acid and polystyrene sulfonic acid in a bifunctional manner. A polyaniline hydrogel separation layer is constructed on the membrane surface through a one-step low-temperature (0-8℃) in-situ polymerization and simultaneous gelation. The membrane has an average pore size of 7.0-7.8 nm and is rich in hydrophilic negatively charged groups, forming a stable hydration layer. The membrane preparation process of this invention is simple, efficient, and highly controllable. The resulting membrane has high pure water flux and dye rejection rate, low NaCl rejection rate, excellent hydrophilicity, antifouling properties, and self-cleaning ability, and remains stable under extreme pH conditions and long-term operation. The membrane prepared by this invention can be widely used in the treatment of high-salt dye wastewater from textile and dyeing industries, realizing the resource recovery and reuse of dyes and salts, and has good prospects for industrial application.

Description

Technical Field

[0001] This invention relates to a self-cleaning loose nanofiltration membrane suitable for treating high-salt dye wastewater, its preparation method, and its applications; it belongs to the field of membrane preparation and separation technology. Background Technology

[0002] High-salt dye wastewater from textile and dyeing industries is characterized by large volume, complex composition, and poor biodegradability. Direct discharge of such wastewater poses a serious threat to the ecological environment and human health, making it a pressing environmental problem. Membrane separation technology, especially nanofiltration (NF) technology, has shown promising application prospects in dye wastewater treatment due to its advantages such as high separation efficiency, low energy consumption, and no phase change. However, traditional dense nanofiltration membranes exhibit a high retention rate of inorganic salt ions when retaining dye molecules, making it difficult to achieve efficient separation of dyes and salts, thus limiting their application in dye desalination and resource recovery.

[0003] Polyaniline (PANI), a typical high-molecular polymer, has attracted much attention in the field of separation membranes due to its simple synthesis, good environmental stability, and reversible doping / dedoping. In particular, sulfonated polyaniline (SPANI) and acid-doped polyaniline can construct charged and hydrated layers on the membrane surface using the abundant amine and imine groups in their molecular chains, providing a structural basis for the selective separation of dyes and salts. Existing methods for preparing polyaniline-based nanofiltration membranes mainly include directly blending PANI into the phase inversion membrane matrix, introducing PANI as an intercalation component between nanosheets, and using PANI as an intermediate layer to control the interfacial polymerization process. These strategies have made some progress in improving dye rejection and antifouling performance, but significant drawbacks remain:

[0004] (1) Polyaniline membrane preparation takes a long time (more than several hours), has low preparation efficiency, and the membrane thickness is uncontrollable, which can easily lead to a dense separation layer and a decrease in water flux.

[0005] (2) Some membranes rely excessively on electrostatic repulsion and have insufficient selective permeability of salt ions, making it difficult to achieve efficient separation of dyes and salts;

[0006] (3) The membrane material has limited anti-fouling and self-cleaning capabilities, and is prone to pollution during long-term operation, resulting in high operation and maintenance costs.

[0007] For the reasons mentioned above, the development of loose nanofiltration membranes that combine high dye retention, low salt retention, ultra-high flux, excellent self-cleaning and stability has become an urgent need in the field of high-salt dye wastewater treatment. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to overcome the drawbacks of polyaniline-based nanofiltration membranes, such as cumbersome preparation, difficulty in balancing flux, selectivity, and antifouling properties, low dye / salt separation efficiency, and poor self-cleaning ability. The invention provides a self-cleaning loose nanofiltration membrane and its innovative preparation method and application, achieving efficient dye retention, rapid salt permeation, high-flux operation, and simple self-cleaning regeneration.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] This invention first discloses a self-cleaning loose nanofiltration membrane, which uses a poly(m-phenylene isophthalamide) (PMIA) porous base membrane as a support layer, and grows a polyaniline (PANI) hydrogel separation layer co-doped with citric acid (CA) and polystyrene sulfonate (PSSA) in situ at low temperature on the base membrane surface and in the pores; the average pore size of the separation layer is 7.0~7.8 nm, the static water contact angle of the membrane surface is <50°, and the surface has a negative potential in the pH range of 3~9.

[0011] Preferably, the aforementioned nanofiltration membrane has a rejection rate of ≥98% for Direct Fast Blue B2RL dye, a rejection rate of ≤10% for NaCl, and a pure water flux of ≥149.52 L·m -2 ·h -1 ·bar -1 After being soaked in clean water for 12 hours after contamination, the flux and rejection rate recovered to more than 99% of their initial values.

[0012] More preferably, the aforementioned polyaniline separation layer is tightly bonded to the PMIA base film through hydrogen bonds, with no interface defects; the separation layer is rich in carboxyl groups -COOH and sulfonic acid groups -SO3H, forming a stable hydration layer and a high-density negative charge layer.

[0013] Preferably, the thickness of the aforementioned polyaniline separation layer is 158~561 nm, it is tightly attached to the PMIA porous base film, there are no obvious gaps or defects at the interface, and the two are firmly bonded.

[0014] This invention also discloses a method for preparing a self-cleaning loose nanofiltration membrane, which employs a one-step low-temperature in-situ polymerization and simultaneous gelation process, including the following steps:

[0015] S1. A PMIA porous base membrane is prepared using a solvent-inducible phase separation method. The process has been disclosed in the applicant's previous patents and will not be repeated here.

[0016] S2. Prepare an aqueous solution of citric acid (CA), add polystyrene sulfonic acid (PSSA) and aniline monomer (An) sequentially, and mix them evenly in a low-temperature water bath at 0-8℃ to form a mixed solution; add an aqueous solution of ammonium persulfate (APS) to the mixed solution and stir to obtain a co-doped reaction solution.

[0017] This invention creatively proposes a dual-system doping approach: Citric acid, a small-molecule organic acid, can rapidly donate protons to promote aniline polymerization, while its carboxyl group can participate in early nucleation, inducing the formation of regular nanofiber structures. However, small-molecule acids are easily lost, leading to unstable doping. PSSA, a high-molecular-weight acid, provides a long-range, stable negative charge through its sulfonate group, forming a durable hydration layer crucial for resisting fouling and electrostatic repulsion. Its large molecular weight and significant steric hindrance contribute to the good water solubility of polyaniline, and its sulfonate group, acting as a hydrophilic side chain, provides effective solvation for the hydrophobic polyaniline backbone. Through the synergistic effect of this dual system, ultra-high flux, high selectivity, and excellent antifouling properties are achieved in the separation membrane.

[0018] S3. Immerse the PMIA base membrane obtained in step S1 into the low-temperature co-doping reaction solution obtained in step S2, and allow it to stand for reaction deposition. After the reaction is completed, wash it several times with alternating ethanol and deionized water to obtain a CA / PSSA co-doped PANI / PMIA self-cleaning loose nanofiltration membrane.

[0019] It should be noted that the low-temperature (0~8℃) reaction conditions used during the growth of polyaniline (PANI) on the PMIA substrate are to control the kinetics of the chemical oxidative polymerization reaction. The chemical oxidative polymerization of aniline is a strongly exothermic reaction. Without temperature control, the heat of reaction can lead to localized overheating of the system. Overheating accelerates the decomposition rate of the oxidant (ammonium persulfate, APS) and triggers numerous side reactions such as branching, cross-linking, and even degradation. The low-temperature environment effectively suppresses these side reactions, allowing the polymerization reaction to proceed in a more stable and linear manner, with the formation of nucleation sites becoming slow and controllable. The limited number of primary particles have ample time for anisotropic growth, extending along a one-dimensional direction to form nanofibers. These fibers intertwine, ultimately forming a uniform, continuous, and dense three-dimensional network structure. This is beneficial for synthesizing polyaniline long chains with higher molecular weights, more regular structures, and greater conjugation, thus forming a dense and stable separation layer. Conversely, if the temperature is not controlled, the reaction is rapidly initiated, instantly generating a large number of free radicals, leading to "explosive nucleation." A large number of primary particles are rapidly generated and tend to aggregate randomly, eventually forming a loose structure with granular or blocky accumulation. This structure usually has large pores and poor separation selectivity.

[0020] Preferably, the molar ratio of aniline monomer to ammonium persulfate oxidant used in the preparation is 1:0.5 to 1:2.

[0021] More preferably, the aforementioned deposition time is 28 to 43 minutes.

[0022] More preferably, the mass-volume concentration of the aforementioned PSSA is 2.5 w / v.

[0023] This invention also discloses the fractional application of the self-cleaning loose nanofiltration membrane as described above in the treatment of high-salt dye wastewater.

[0024] Preferably, the aforementioned loose nanofiltration membrane is used to achieve efficient separation of dyes and salts: retaining dye molecules while allowing monovalent and divalent salts to pass through efficiently.

[0025] The advantages of this invention are:

[0026] (1) This invention is the first to use a bifunctional co-doping system of citric acid (CA) and polystyrene sulfonic acid (PSSA), which combines small molecule organic acid with high molecular weight polyelectrolyte, and simultaneously introduces carboxyl groups (-COOH) and sulfonic acid groups (-SO3H) on the polyaniline molecular chain to form a stable hydration layer and high density negative charge on the membrane surface, and participate in the construction of the separation layer skeleton. Abundant and stable hydrophilic and charged groups are introduced on the PANI chain, and the separation membrane is simultaneously achieved with ultra-high flux, high selectivity and excellent antifouling properties.

[0027] (2) The present invention successfully prepared a high-performance loose nanofiltration membrane by using a one-step low-temperature in-situ polymerization and synchronous gelation process. The synthesis, doping, loading and gelation of polyaniline are completed in one step, replacing the cumbersome process of traditional interface polymerization and layer-by-layer self-assembly. The polymerization time is greatly shortened, and no high temperature, high pressure and complex equipment are required, and the preparation efficiency is significantly improved. At the same time, the membrane can be self-cleaned and regenerated by soaking in water, without the need for strong acid and strong alkali chemical cleaning, which greatly reduces material cost, production energy consumption and subsequent operation and maintenance cost.

[0028] (3) The process of the present invention can precisely customize the pore size, thickness and surface charge properties of the separation layer, thereby controlling the separation performance, achieving high dye rejection and efficient salt permeation, and has extremely strong adaptability to operating conditions. Specifically, a loose hydrogel formed by CA / PSSA co-doped PANI is used as the separation layer. The average pore size of the membrane is 7.0~7.8 nm, rich in hydrophilic functional groups and negative charge, and has both nanofiltration-level selectivity and porosity higher than traditional nanofiltration. The dye rejection rate is ≥97%, and the monovalent salt NaCl rejection rate is as low as 7.1%, truly achieving efficient separation of dye / salt. At the same time, the flux is high, taking into account both high flux and high selectivity. The membrane has stable performance under extreme acidity and alkalinity of pH=2~12, high salt, high dye concentration and 21 h continuous operation, and can be perfectly adapted to complex industrial wastewater conditions.

[0029] (4) The membrane prepared by this invention has outstanding antifouling and self-cleaning capabilities. The co-doping system endows the membrane with strong antifouling properties (water contact angle < 50°) and a stable negative potential over a wide pH range. The hydrophilic hydration layer and electrostatic repulsion work together to significantly reduce pollutant adsorption and irreversible pollution. After four pollution-cleaning cycles, the flux recovery rate is higher than 90%, and the performance is almost unaffected. It can be directly used for the treatment of high-salt dye wastewater, realizing the efficient separation and resource recycling of dyes and salts. It has both outstanding environmental and economic benefits and has the prospect of large-scale industrial application. Attached Figure Description

[0030] Figure 1 The images show surface electron microscope (SEM) images of polyaniline films with different PSSA concentrations prepared in Examples 2-7.

[0031] Figure 2 The images show the physical appearance of polyaniline films with different PSSA concentrations prepared in Examples 2-7.

[0032] Figure 3 The images show SEM images of the surface and cross-section of the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14.

[0033] Figure 4 The XRD patterns are of the loose nanofiltration membranes prepared in Examples 12-14;

[0034] Figure 5 The FTIR infrared spectra of the loose nanofiltration membranes prepared in Examples 1, 8-14 are shown below.

[0035] Figure 6 XPS broadband scan and high-resolution N1s narrow-spectrum scan spectra of the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14;

[0036] Figure 7 AFM images of the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14;

[0037] Figure 8 The thickness characterization diagrams are for the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14.

[0038] Figure 9 The physical appearance diagrams of the DMF soaking solutions of the loose nanofiltration membranes prepared in Examples 8, 10 and 12 are shown.

[0039] Figure 10 The diagram shows the water contact angles of the loose nanofiltration membranes prepared in Examples 8-14.

[0040] Figure 11 Zeta potential diagrams of the loose nanofiltration membranes prepared in Examples 8, 10, 12, and 14;

[0041] Figure 12The molecular weight cutoff and pore size distribution diagrams of the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14 are shown.

[0042] Figure 13 The graphs show the pure water permeation flux of the loose nanofiltration membranes prepared in Examples 2-7.

[0043] Figure 14 The diagram shows the separation and permeation performance of the loose nanofiltration membranes prepared in Examples 2-7.

[0044] Figure 15 For the loose nanofiltration membranes prepared in Examples 8, 10, and 12-14: (a) Pure water permeation flux diagram; (b) Dye salt separation performance diagram;

[0045] Figure 16 The following graphs show the flux of the loose nanofiltration membranes prepared in each embodiment at different deposition times: (a) pure water flux; (b) dye salt mixed flux; (c) dye salt rejection rate.

[0046] Figure 17 Physical appearance images of the loose nanofiltration membranes prepared in Examples 12-14 before and after immersion in deionized water;

[0047] Figure 18 The UV-Vis spectra of the raw materials and the solutions (water) prepared in Examples 12-14 are shown.

[0048] Figure 19 The graph shows the separation and permeation performance of the loose nanofiltration membrane prepared in Example 12 for a single solution.

[0049] Figure 20 The UV-Vis spectra of the loose nanofiltration membrane prepared in Example 12 for permeate and effluent of different dye solutions are as follows: (a) B2RL; (b) MB; (c) CR;

[0050] Figure 21 A comparison of the permeation performance of loose nanofiltration membranes prepared in different embodiments for mixed solutions of different dye salts;

[0051] Figure 22 A comparison of the separation performance of loose nanofiltration membranes prepared in different embodiments for mixed solutions of different dye salts;

[0052] Figure 23 The permeation and separation performance of the loose nanofiltration membrane PANI-M1-T1 prepared in Example 12 under different dyes and different salt concentrations are shown in the following graphs: (a) CR; (b) MB; (c) B2RL; (d) Comparison of the swelling degree of the PANI / PMIA loose nanofiltration membrane under different salt concentrations.

[0053] Figure 24The permeation separation performance of the PANI / PMIA loose nanofiltration membrane (Example 12) at different dye concentrations is shown in the graph.

[0054] Figure 25 Acid and alkali resistance test results for the PANI / PMIA loose nanofiltration membrane (Example 12);

[0055] Figure 26 The antifouling performance test diagrams for the PANI / PMIA loose nanofiltration membrane (Example 12) are as follows: (a) Antifouling test (b) Fouling index;

[0056] Figure 27 A comparison of the self-cleaning permeation separation performance of the PANI / PMIA loose nanofiltration membrane (Example 12) before and after fouling;

[0057] Figure 28 The images show the physical appearance of the loose nanofiltration membrane prepared in Example 12 before and after fouling.

[0058] Figure 29 This is a graph showing the long-term stability of the PANI / PMIA loose nanofiltration membrane (Example 12). Detailed Implementation

[0059] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0060] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0061] Example 1: Preparation of PMIA base film:

[0062] Weigh 4.8 g of LiCl and dissolve it in 77.5 mL of DMAc. Stir at high speed at room temperature until completely dissolved to form a homogeneous solution. Then, add 1.2 g of PVP-K15 and continue stirring until completely dissolved to obtain a homogeneous and stable mixed solution.

[0063] The above solution was added to PMIA and then transferred to a 90°C oven to form a homogeneous and stable casting solution. The casting solution was cooled to room temperature (25°C), 5 g of azelaic acid was added, and the mixture was stirred in a roller mixer to ensure uniform dispersion.

[0064] The resulting casting solution was allowed to stand overnight to degas. After the bubbles were completely eliminated, a non-solvent-induced phase separation method was used to form a film using a laboratory pilot-scale flat-plate film-forming device. The specific operation was as follows: the completely degassed casting solution was uniformly coated onto a nonwoven fabric support layer, with the blade thickness controlled at 150 μm. The film was then immersed in a 40°C deionized water coagulation bath at a constant rate for phase inversion. After thorough cleaning, the resulting membrane was stored in deionized water for later use.

[0065] Examples 2-14: Preparation of loose polyaniline nanofiltration membranes:

[0066] Weigh 2.0 g of citric acid (CA) and dissolve it in 100 mL of deionized water. Stir at 400 rpm for 10 min. Then, add polystyrene sulfonate (PSSA) and aniline (An) to the above solution, and transfer to a low-temperature water bath at 0-8℃ with magnetic stirring for 1 h. Separately, weigh ammonium persulfate (APS) and dissolve it in 10 mL of deionized water. Stir at 300 rpm for 20 min until completely dissolved, and then add the APS solution dropwise to the PSSA-An mixture.

[0067] Take out the PMIA base membrane cut to 8 cm × 8 cm, gently blot off surface moisture with filter paper, and place it in a 25 mL centrifuge tube. Immediately add the freshly prepared 0-8℃ low-temperature reaction solution and allow it to stand for a specific time. After the reaction is complete, remove the composite membrane and wash it repeatedly with alternating ethanol and deionized water to thoroughly remove residual reactants and byproducts from the membrane surface. Store the resulting polyaniline / PMIA composite membrane in deionized water at room temperature for later use.

[0068] The material ratios, process parameters, and names of the different membranes in each embodiment are shown in Table 1.

[0069]

[0070] Table 1. Formulation and naming of different membrane types prepared in each embodiment.

[0071] Structural characterization and performance testing

[0072] (a) Morphological structure

[0073] (1) Microscopic morphology SEM

[0074] The microstructure of the film was characterized using a scanning electron microscope (SEM) (SUPRATM55, ZEISS, Germany). Cross-sectional samples were cut with liquid nitrogen, then mounted on the sample stage and sputtered with gold for 90 seconds using an automated precision coating machine (Q150R ES, America) before SEM analysis.

[0075] like Figure 1 As shown, at low concentrations, a relatively uniform, continuous, dense, and defect-free nano-coral-like structure forms on the membrane surface, with tightly interwoven fibers covering a large area of ​​the base membrane. With increasing acid concentration, the original nano-coral-like structure diminishes and fuses into a "shell-like" film. This indicates that different PSSA doping concentrations have a significant impact on the microstructure evolution of the polyaniline separation layer.

[0076] The applicant's analysis suggests this may be due to the following reasons: High concentrations of PSSA significantly increase the acidity and ionic strength of the reaction system, substantially accelerating the polymerization rate of aniline. This leads to "explosive nucleation," instantly generating a large number of primary polyaniline particles. These particles, unable to arrange themselves into an orderly fibrous structure, randomly deposit and rapidly grow on the membrane surface. Furthermore, as a high-molecular-weight polyelectrolyte, PSSA's long-chain molecules, at high concentrations, may simultaneously bridge multiple polyaniline segments through electrostatic cross-linking, inducing local aggregation and fusion, further disrupting the orderly growth of nanofibers. These aggregates, deposited and grown on the PMIA substrate surface, become visible green speckled particles, such as... Figure 2 As shown, these spots are densely clustered regions of highly cross-linked, doped polyaniline / PSSA. In other words, excessive doping leads to chain structure defects. Excessive PSSA triggers irregular polymerization reactions, generating numerous defective structures such as head-to-head couplings. These defective structures disrupt the conjugated chains of polyaniline, preventing the formation of stable polarons / bipolarons, thus diluting the overall green color of the film (e.g., ...). Figure 2 (As shown in the last two pictures).

[0077] Therefore, a moderate PSSA doping concentration is beneficial for inducing polyaniline to form a regular nanofiber network structure, while an excessively high PSSA concentration can lead to nanostructure fusion, densification, and even structural defects due to excessively fast reaction rates and electrostatic crosslinking. Therefore, in Examples 8-14, a PSSA concentration of 2.5% was selected for exploratory experiments.

[0078] like Figure 3 As shown, the surface and cross-sectional morphology of the PANI / PMIA loose nanofiltration membrane were characterized by SEM. When the APS concentration is low (Figure a, PANI-M1-T1), the membrane surface is relatively flat and exhibits a porous structure. This may be because the amount of APS can only generate a limited number of active free radicals, and the reaction is confined to the surface interface region of the PMIA base membrane, resulting in a slow reaction rate. The newly generated polyaniline oligomers have sufficient time to crawl and spread on the surface and preferentially grow on the existing polyaniline layer rather than forming new nuclei, which also leads to the formation of a thin "passivation layer".

[0079] We observed that as the concentration of the oxidant APS increased (Figure b, PANI-M2-T1), irregular "reef"-like particles gradually appeared on the membrane surface. These particles had rough surfaces and obvious gaps between them. This also illustrates the kinetic transition of aniline polymerization on the membrane surface from "surface confinement" to "bulk deposition." When the An:APS ratio was 1:2, the number of "reef"-like particles on the membrane surface gradually decreased with increasing deposition time. This is because as the reaction proceeds, the concentration of aniline monomer in the solution decreases sharply due to rapid polymerization, and the resulting secondary growth no longer generates new, independent large particles, but rather "fills" and "encapsulates" the existing "reef" particles, on top of them, and around them. Simultaneously, excess APS instantaneously generates a large number of free radicals, initiating a violent polymerization reaction in the bulk solution and generating a large number of polyaniline primary particles. These particles diffuse to the base membrane surface through Brownian motion and are randomly and loosely deposited due to physical forces (van der Waals forces, electrostatic interactions). As the reaction proceeds, the fluid shear forces generated by the stirring may also wash loosely attached particles off the surface. Figure 3 SEM observation of the cross-sectional morphology of the composite membrane revealed that the membrane cross-section exhibits a representative asymmetric structure: it consists of a dense epidermal layer at the top, a sublayer with finger-like pores, and a macroporous support layer with a cellular structure. The sublayer with finger-like pores is part of the main structure of the PMIA base membrane. The polyaniline separation layer is tightly attached to the PMIA porous base membrane, and there are no obvious gaps or defects at the interface.

[0080] (2) XRD

[0081] XRD test results of nanofiltration membrane are as follows Figure 4 As shown in the figure, the composite film exhibits sharp diffraction peaks around 2θ ≈ 18°, 22°, and 26°, indicating the presence of partially ordered or crystalline structures. The diffraction peaks at 2θ ≈ 22° and 26° are characteristic diffraction peaks of polyaniline, belonging to periodic arrangements parallel and perpendicular to the polymer chains, respectively, demonstrating a relatively regular molecular arrangement and good crystallinity. These characteristics indicate that the composite film possesses a typical doped polyaniline molecular structure. Furthermore, the positions of the characteristic diffraction peaks of the polyaniline films obtained by treatment with different reaction times remained largely consistent, showing no significant changes.

[0082] (3) Chemical structure FTIR and XPS

[0083] The chemical structure of the PANI / PMIA composite film was characterized by Fourier transform infrared spectroscopy, such as... Figure 5 As shown in (a), at 1123 cm -1 and 1034 cm -1The strong absorption peaks appearing at these locations are attributed to sulfonate groups (-SO3). - The asymmetric and symmetric stretching vibrations of PSSA directly prove that PSSA molecules have been introduced into polyaniline through a doping process. Furthermore, the 2000-2500 cm⁻¹... -1 The "hump" morphology in the interval is attributed to the stretching vibration of the NH⁺ group in protonated polyaniline and the 1166 cm⁻¹ region. -1 The broadened absorption peak at this point represents the CN during the protonation process of self-doped polyaniline. + H-SO3, C=N + The bending vibration peaks of H-SO3 and CH in the quinone structure are hallmark evidence of the protonated state of polyaniline. Located at approximately 1580 cm⁻¹ -1 The characteristic absorption peak at 1488 cm⁻¹, attributed to the quinone ring C=C skeleton vibration characteristic of polyaniline molecular structure, did not appear independently. -1 The NBN framework vibration of the benzene ring indicates that when polyaniline is doped with acid, the protons (H... + The protonation process selectively attacks the nitrogen atom (-N=) of the quinone-type imine, disrupting the electronic conjugation system of the quinone ring unit and transforming its chemical structure into a benzene-type diimine unit. For example... Figure 5 (b) 3300 cm -1 The broad peaks in the vicinity are mainly attributed to the stretching vibrations of NH and OH. The formation of hydrogen bonds weakens the strength of the NH bond, reducing its bond force constant, which in turn causes the stretching vibration absorption peak to shift towards lower wavenumbers.

[0084] The chemical composition of the composite membrane surface was analyzed using XPS, and the results are as follows: Figure 6 As shown in (a), compared with the pure PMIA base film, the prepared composite film has a distinct S element characteristic peak on its surface. This signal is attributed to the sulfonic acid group in PSSA, further confirming that PSSA was successfully doped into the polyaniline separation layer. As the An:APS ratio increased from 1:0.5 to 1:1, the N 1s content increased from 5.31% to 8.74% (see Table 2). This may be attributed to the increased oxidant promoting the polymerization reaction and improving the coverage and packing density of the polyaniline layer. When the ratio was further increased to 1:2, the O 1s content decreased significantly to 14.56%. This may be due to the excessive oxidation caused by the excess oxidant, which led to the breakage of polyaniline chains and the loss of soluble oxygen-containing oligomers, resulting in a decrease in the apparent content of oxygen-containing functional groups.

[0085]

[0086] Table 2 XPS elemental composition data for PANI / PMIA membranes

[0087] Under an An:APS ratio of 1:2, as the deposition time increased from 28 min to 43 min, XPS data of the loose polyaniline nanofiltration membrane showed that the C 1s atomic percentage continuously decreased from 75.65% to 67.51%, while the O 1s percentage significantly increased from 14.56% to 26.03%. This directly confirms that the polymer skeleton underwent deep oxidation under excessive oxidant attack, with a large amount of carbon skeleton being converted into oxygen-containing functional groups. The N 1s atomic percentage decreased from 8.38% to 4.70%, which is partly due to the "dilution effect" of the carbon / oxygen capping layer generated by oxidation on nitrogen, and partly suggests a possible denitrification degradation reaction.

[0088] By finely analyzing the peaks of the N 1s spectrum, it was discovered that ( Figure 6 (bf and Table 3) When the An:APS ratio is 1:0.5, the contents of -N= and -NH- are relatively high (13.17% and 58.23%, respectively), while =N + and -NH + The lower values ​​(21.89% and 6.70%) indicate insufficient oxidant leading to incomplete polymerization, with polyaniline predominantly in its intrinsic state and exhibiting low doping levels. When the An:APS ratio is 1:1, -N= decreases to 3.71%, -NH- decreases to 39.57%, and =N... + and -NH + The percentages of -N= and -NH- increased to 39.33% and 17.39% respectively, attributed to the optimization of the oxidant promoting full polymerization and protonation doping, with a large number of -N= being converted into polarons, and polyaniline transitioning from the intrinsic state to a highly conductive doped state; at a ratio of 1:2, -N= and -NH- slightly rebounded (4.41% and 43.08%), and =N + It rose to 43.85%, but -NH + The proportion of phenylamine nitrogen (-NH-) decreased significantly to 8.66%, due to excessive oxidation caused by excess oxidant, which destroyed bipolarons and generated neutral amine nitrogen, while the proportion of relatively stable unipolarons increased. At this ratio, as the deposition time increased from 28 min to 43 min, the proportion of phenylamine nitrogen (-NH-) increased from 43.08% to 52.13%, while the proportion of protonated imine nitrogen (-NH-), which characterizes conductive polaritons, increased. + -) content decreased, because excessive oxidation caused some -NH4+ to be lost. + - De-doped. In addition, the initial decrease and subsequent increase of S2p (1.41%→0.99%→1.76%) may be related to the initial embedding of PSSA in the over-oxidized matrix and subsequent non-specific adsorption.

[0089]

[0090] Table 3. XPS peak fitting percentage data for PANI / PMIA membranes

[0091] (4) Atomic force microscope (AFM)

[0092] The topological structure of the film surface was characterized using atomic force microscopy (AFM, Dimension FastScan, Bruker, Germany), and the film thickness was analyzed using Nanoscope Analysis software.

[0093] like Figure 7 As shown, the surface roughness of the membrane increases slightly with increasing oxide concentration and deposition time; specific values ​​are shown in Table 4. All prepared PANI membranes exhibit low nanoscale roughness, indicating a low tendency for pollutant adsorption.

[0094]

[0095] Table 4 Surface roughness values ​​of PANI / PMIA loose nanofiltration membranes

[0096] Furthermore, by Figure 8 It is evident that the thickness of the polyaniline separation layer is closely related to the An:APS ratio, significantly increasing from 158 nm at 1:0.5 to 561 nm at 1:1, before decreasing back to 186 nm at 1:2. This nonlinear variation is a direct result of the combined effects of polymerization kinetics and film formation mechanism. At An:APS = 1:0.5, the oxidant is relatively insufficient, resulting in a slow reaction rate and primarily the formation of low-molecular-weight polyaniline. Its limited chain length and slow growth kinetics lead to the formation of only a shallow coating layer on the PMIA substrate surface, resulting in the thinnest layer. When the ratio increases to 1:1, the monomer and oxidant reach stoichiometric equilibrium, following the standard polycondensation reaction pathway to generate high-molecular-weight polyaniline. This polyaniline then self-assembles into a dense and complete three-dimensional nanofiber network. This balanced and continuous growth pattern allows the polymer to fully stack in both the vertical and horizontal directions, thereby achieving the maximum film thickness. However, when the ratio was further increased to 1:2, the excess oxidant triggered violent bulk polymerization, instantly generating a large number of polyaniline primary particles and oligomers. These particles then deposited on the film surface, forming an initial layer through "physical stacking." However, this process rapidly consumed the reactant monomers, inhibiting the subsequent deep growth and orderly stacking of polymer chains, resulting in a thickness significantly lower than that of the 1:1 sample. We also found that under the condition of An: APS = 1:2, further extending the reaction time only increased the film thickness by about 10 nm. This confirms that in this over-oxidized environment, the polymerization reaction was essentially completed in the early stages, meaning that the monomers were rapidly depleted in the initial stage of the reaction, lacking the material basis for continued growth. At the same time, the already formed over-oxidized layer may have hindered the diffusion of reactants and, due to its structural defects, could not support the further orderly extension of polyaniline chains, thus causing the film thickness to enter a "growth plateau."

[0097] Furthermore, by using DMF solvent to peel off the composite film, we observed that the color of the solution and the state of the nonwoven substrate showed a regular change with the An:APS ratio, such as... Figure 9 As shown, at a 1:0.5 ratio, the solution is blue and the nonwoven fabric is white after peeling; at a 1:2 ratio, the solution is willow green and the nonwoven fabric is mostly white; while at a 1:1 ratio, the solution is blue-green and the nonwoven fabric is stained green. This phenomenon reveals the essential differences among the three: low oxidant concentration (1:0.5) leads to incomplete polymerization, generating oligomers predominantly in the intrinsic state; high oxidant concentration (1:2) triggers excessive oxidation, producing degradation fragments; only when An:APS is 1:1, through balanced reaction kinetics, high molecular weight polyaniline deeply penetrates into the pores of the base film and forms a strong interfacial bond with the fiber, thus the nonwoven fabric exhibits a green anchored state. This mechanism also explains why the membrane prepared under this condition exhibits the maximum thickness, consistent with the aforementioned AFM detection results.

[0098] (5) Measurement of water contact angle

[0099] The hydrophilicity and hydrophobicity of the membrane surface were characterized using a contact angle meter (OCA15EC, Dataphysics, Germany), such as... Figure 10 As shown, all membranes exhibit low static water contact angles, generally below 50°, while the water contact angle of existing PMIA-based membranes is typically around 70°, indicating that the membrane surfaces prepared in this invention possess excellent hydrophilicity. This significant hydrophilicity is primarily attributed to the introduction of numerous hydrophilic functional groups into the polyaniline separation layer during acid doping, including strongly hydrophilic sulfonate groups from PSSA and carboxyl groups from citric acid, thereby endowing the membrane surface with excellent hydrophilic properties. This superior hydrophilicity not only effectively promotes increased water flux but also endows the membrane with potential antifouling capabilities by forming a stable hydration layer on the membrane surface, thus improving the long-term operational stability of the membrane.

[0100] (6) Zeta potential test

[0101] The solid zeta potential of the nanofiltration membrane surface was tested using a solid surface zeta potential meter (SurPASS2, Anton Paar, Austria). The sample membrane to be tested was first rinsed with test water 3 to 5 times, and then soaked in KCl solution of different concentrations for 1 h. The sample membrane was then taken out and cut into 1 cm × 2 cm sizes. The pH range was 3 to 9, and each pH point was measured 4 times.

[0102] like Figure 11As shown, the prepared PMIA / PANI loose nanofiltration membranes exhibit negative Zeta potentials across a pH range of 3–9. This phenomenon is primarily attributed to the deprotonation effect of acidic functional groups such as citric acid, polystyrene sulfonic acid (PSSA), and carboxyl groups in the substrate. Among them, the PANI-M3-T3 sample exhibits the most negative Zeta potential, indicating that a higher oxidant / monomer ratio and suitable polymerization time help introduce more acidic groups, thereby increasing the negative charge density on the membrane surface. This charge characteristic is expected to enhance the membrane's electrostatic repulsion and retention capacity for negatively charged pollutants while maintaining high flux, thus optimizing membrane separation performance.

[0103] (7) Molecular weight cutoff and pore size test

[0104] Filtration experiments were conducted using polyethylene glycol of different molecular weights (PEG-10, PEG-20, PEG-35, PEG-70, PEG-100, 200 ppm) to test the molecular weight cut-off (Mwt) of the membrane. The PEG concentration of the feed solution and permeate was measured using a total organic carbon analyzer. The obtained molecular weight cut-off values ​​were substituted into equation (1) to calculate the pore size of the nanofiltration membrane. The pore size distribution is expressed as a probability density function, as shown in equation (2).

[0105] (1)

[0106] (2)

[0107] In the formula, r is the Stokes radius (m), and M is the molecular weight cutoff (g·mol⁻¹). -1 ); μ p The geometric mean particle size of the solute when the retention rate is 50%. The geometric standard deviation is defined as the ratio of the solute particle size corresponding to a rejection rate of 84.13% to 50%.

[0108] See Figure 12As shown in Table 5, with the An:APS ratio increasing from 1:0.5 to 1:2, the average pore size of the polyaniline separation layer gradually decreased from 7.8 nm to 7.0 nm, and the molecular weight cutoff (MWCO) correspondingly decreased from 79578 Da to 50865 Da. This trend is attributed to the shift in polymerization mode from surface-constrained growth to bulk deposition-dominated by the increase in oxidant dosage. With the increase of APS, the membrane surface obtains a more compact structure through particle stacking mechanism. At a fixed 1:2 ratio, with the extension of reaction time, the membrane pore size and MWCO exhibit a non-monotonic evolution characteristic of "first increasing and then decreasing": in the initial stage, rapid deposition forms the initial stacked structure; in the intermediate stage, chain degradation induced by secondary growth and excessive oxidation leads to structural loosening; and in the later stage, further particle filling and polymer chain contraction lead to re-densification of the structure.

[0109]

[0110] Table 5. Pore size and molecular weight cutoff of PANI / PMIA loose nanofiltration membranes

[0111] (II) Performance testing of nanofiltration membranes

[0112] (1) Permeation separation performance

[0113] Using dye and salt as feed solutions, the permeation separation performance of the membrane was evaluated using a self-made cross-flow filtration device. The specific procedure is as follows: the nanofiltration membrane was cut into a circular shape and fixed on a membrane tank; the effective area of ​​the membrane was 36.31 cm². 2 To obtain a stable permeation flux, the nanofiltration membrane first needs to be used at a flow rate of 2.5 L∙min⁻¹. -1 Pre-pressurize the membrane at 1 bar for 1 h, then test and collect the permeate. Membrane permeate flux (L·m⁻¹) -2 ·h -1 ·bar -1 ), calculated by the formula, where V(L) is the permeation volume and A(m³) is the permeation volume. 2 ) represents the effective membrane area, t (h) represents the time for collecting permeate, and ΔP represents the applied pressure (bar).

[0114] (3)

[0115] The dye / salt separation performance of the prepared membrane was evaluated using three anionic dyes (CR, MB, B2RL) and two inorganic salts (NaCl, Na2SO4), respectively. The operating conditions were the same as those for the pure water flux test. The rejection rate R (%) was expressed as the rejection rate and calculated using the following formula:

[0116] (4).

[0117] In equation (4), C f Feed concentration (mg·L) -1 ), C p Osmotic concentration (mg·L) -1 ).

[0118] like Figure 13 and Figure 14 As shown, when the PSSA concentration is 1%, the uniform and continuous nanofiber network structure provides abundant water transport channels, and the membrane permeation flux reaches as high as 148.47 L·m. -2 ·h -1 ·bar -1 When the PSSA doping concentration increased from 1.5% to 4%, the membrane permeation flux first increased and then decreased. The membrane's rejection rate for R2BL decreased from 98.79% to 95.78%, and its rejection rate for Na2SO4 decreased from 18.81% to 13.55%. This change is attributed to the synergistic enhancement of size sieving and Donan repulsion effects by the well-organized fiber network and high surface negative charge density at low concentrations. However, excessive PSSA induces overdoping or local phase separation, weakening the orderliness and charge uniformity of the polyaniline chains, resulting in a more significant decrease in rejection rate.

[0119] like Figure 15 As shown in (a), as the An:APS ratio increases from 1:0.5 to 1:2, the pure water flux of the composite membrane increases from 316.05 L·m -2 ·h -1 ·bar -1 It decreased to 149.52 L·m -2 ·h -1 ·bar -1 When the oxidant (APS) is relatively insufficient, the aniline polymerization reaction is slow, resulting in polyaniline with a low molecular weight. This tends to form a thin film with low coverage and a loose structure on the PMIA substrate surface. This structure allows water molecules to pass relatively smoothly through the original pores of the substrate and the large-size defects in the polyaniline layer, thus exhibiting a high initial flux. With increasing oxidant concentration, high molecular weight, highly regular polyaniline is generated, and it self-assembles into a complete, dense, and interconnected three-dimensional nanofiber network structure. This structure significantly increases the tortuosity of the water molecule mass transfer path and the flow resistance, leading to a decrease in flux. Excessive oxidant generates a large number of primary polyaniline particles. The subsequent deposition and stacking of these particles severely clog the surface pores and internal channels of the PMIA substrate. Furthermore, over-oxidation may cause more cross-linking of the polymer chains, further making the separation layer structure more compact. Figure 15 As shown in (b), with the increase of APS content, the composite membrane’s rejection rate of B2RL increased from 89.28% to 98.05%, and its rejection rate of Na2SO4 increased from 9.99% to 26.11%.

[0120] Depend on Figure 16 It can be seen that when the An:APS ratio is 1:0.5 and 1:1, the membrane permeability selectivity tends to stabilize with increasing deposition time. However, when the An:APS ratio is 1:2, the pure water flux of the composite membrane increases from 149.52 L·m⁻¹ with increasing deposition time. -2 ·h -1 ·bar -1 It rose to 229.32 L·m -2 ·h -1 ·bar -1 The rejection rate of Na2SO4 decreased from 26.11% to 20.38%. The increase in membrane permeation flux is due to the fact that excessive APS (oxidant) continues to attack the benzene and quinone rings of polyaniline, causing them to oxidize and degrade, generating byproducts such as quinone imines, benzoquinones, and even chain-broken oligomers. Simultaneously, the sulfonate and citric acid hydrogen ions / H2O of PSSA... + The polyaniline chains were successfully protonated, forming a large number of polarons and bipolarons. This unique electronic structure allows it to absorb almost all wavelengths of visible light. These degradation products have different band structures, and their broad absorption superimposed results in the polyaniline film exhibiting a uniform black color, such as... Figure 17 As shown.

[0121] Deionized water is a polar solvent, but its ionic strength is extremely low. When a doped polyaniline film is immersed in it, in order to achieve an equilibrium of ion concentrations inside and outside the film, a dopant (PSSA's -SO3) is used. - Anions (including citric acid anions) are extracted from the polymer network and diffuse into the water. This washes away loosely bound PSSA, citric acid dopants, and soluble over-oxidized oligomers from the membrane. This is equivalent to "de-doping" and "cleaning" the membrane; the disappearance of this black, amorphous layer of over-oxidized products allows the green color of the underlying acid-doped polyaniline to become visible. Figure 17 The immersion solution was subjected to UV-Vis ultraviolet-visible spectroscopy testing, such as... Figure 18 As shown, a characteristic absorption peak appears at around 250 nm, corresponding to the absorption of the benzene ring in PSSA and the absorption of polyaniline oligomers / degradation products.

[0122] like Figure 19 As shown, the PANI-M3-T1 loose nanofiltration membrane prepared in this invention exhibits excellent water permeation flux and selective separation performance, with a permeation flux for NaCl reaching as high as 125.18 L·m⁻¹. -2 ·h -1 ·bar -1The fluxes for Na₂SO₄ and various dyes (B₂RL, MB, CR) remained between 86.41 and 105.13 L·m⁻¹. -2 ·h -1 ·bar -1 Within this range, it exhibits typical high-flux characteristics of loose nanofiltration. Regarding retention performance, the membrane retains more than 97.25% of all three dye molecules, while retaining 31.12% and 4.94% of Na₂SO₄ and NaCl, respectively. This is consistent with... Figure 20 The UV-Vis absorption spectra were consistent, with significant characteristic absorption peaks in the feed solution and extremely low absorbance in the transmitted solution, demonstrating that the dye molecules were effectively retained. This selectivity stems from the synergistic effect of the low mass transfer resistance provided by the loose porous structure within the membrane and the electrostatic repulsion effect imparted by the surface's negative charge.

[0123] This invention also systematically investigated the effects of different An:APS ratios and polyaniline growth times on membrane performance under different dye salt mixed solutions, and the results are as follows: Figure 21 and Figure 22 As shown, a significant regulatory pattern can be observed between the membrane's permeation flux and retention performance. Regarding permeation flux, all membrane products maintained a range of 86.76–192.03 L·m⁻¹. -2 ·h -1 ·bar -1 The high flux range. When An:APS is 1:0.5, the membrane flux to B2RL reaches as high as 192.03 L·m. -2 ·h -1 ·bar -1 As the ratio increased to 1:2, the flux decreased, mainly due to the excessive polymerization of polyaniline caused by excess APS, leading to densification of the separation layer and shrinkage of the pores. At the same ratio, the flux increased with increasing polymerization time (28 min → 43 min), which is related to the oxide degradation and loss mentioned above.

[0124] In mixed solutions of different salts and dyes, the PANI / PMIA loose nanofiltration membrane exhibited good retention performance for CR, MB, and B2RL dye molecules. This is due to the dye molecule size being larger than the membrane pore size, and the negatively charged groups introduced by citric acid and PSSA doping achieving efficient retention through the synergistic effect of electrostatic repulsion and size sieving. All PANI / PMIA loose nanofiltration membranes showed a retention rate of less than 10% for monovalent NaCl and a retention rate of 9.99%–35.43% for divalent Na2SO4. The APS:An ratio determines the polymerization density of polyaniline; a lower ratio favors the formation of a loose porous structure, resulting in higher flux but slightly lower salt retention; a higher ratio improves the density of the separation layer and enhances the retention capacity for divalent salts. Under conditions of excessive oxidant, prolonged polymerization time exacerbates the over-oxidation of polyaniline, leading to chain breakage, degradation, or the formation of loosely structured regions in some polymer chains. This increases the effective pore size of the membrane, reduces mass transfer resistance, and improves flux. The extended deposition time did not lead to a decrease in flux; instead, it promoted the loosening and increased hydrophilicity of the doped polyaniline layer. This differs from the traditional pattern of "decreased flux with extended deposition time" in dense nanofiltration membranes, highlighting the structural control advantages of the citric acid / PSSA doping system. Therefore, this series of membranes, possessing both high flux and high selectivity, undoubtedly has excellent application potential in scenarios such as dyeing and printing wastewater treatment.

[0125] Furthermore, we investigated the effects of high salt and high dye concentration on permeation separation performance, such as... Figure 23 As shown, with the Na2SO4 concentration increasing from 1 g / L to 60 g / L, the permeate flux of the PANI-M1-T1 membrane for the three dye solutions (CR, MB, and B2RL) gradually decreased. This is attributed to the increased osmotic pressure at high salt concentrations offsetting some of the driving force, and the non-monotonic change in membrane swelling behavior; the swelling degree initially increased with salt concentration and then stabilized, with the pore structure controlled by the charge shielding effect. Under high salt conditions, the membrane exhibited high retention performance for the three dyes, while the retention rate for Na2SO4 rapidly decreased from approximately 30% initially to below 10%, demonstrating excellent dye / salt selective separation stability. The color changes of the dye solutions directly reflected the high-salt-induced dye aggregation behavior: CR and B2RL significantly aggregated to form large-sized aggregates, enhancing the sieving effect. Although the charge shielding of salt ions weakened electrostatic repulsion, the size of the dye aggregates was much larger than the membrane pore size, as shown in Table 6, making the sieving effect the dominant retention mechanism.

[0126]

[0127] Table 6. Average particle size of dye molecules under different salt concentration gradients.

[0128] It is evident that the membrane exhibits "high dye retention and high salt permeation" performance in high-salt dye systems, and has good application potential in the treatment of high-salt printing and dyeing wastewater.

[0129] Furthermore, such as Figure 24 As shown, as the B2RL concentration increased from 0.1 g / L to 0.5 g / L, the permeation flux of the PANI-M1-T1 membrane increased from approximately 108 L·m -2 ·h -1 ·bar -1 It continued to drop to about 69 L·m -2 ·h -1 ·bar -1 The high concentration of dye led to the adsorption and deposition of dye molecules on the membrane surface and within the pores, forming a filter cake layer that significantly increased water transport resistance. The membrane's rejection rate for B2RL remained consistently above 98%, demonstrating excellent dye rejection stability. However, the rejection rate for Na2SO4 showed a trend of first increasing and then stabilizing, rising from 26% to approximately 39% (0.3 g / L) before falling back to approximately 35% and then tending to remain constant. The dynamic filter cake layer formed by the initial dye aggregation briefly enhanced the rejection of SO4. 2- The sieving and electrostatic repulsion, and the subsequent stabilization of the filter cake layer structure, the increase in ionic strength weakens the electrostatic repulsion effect, making the retention rate tend to stabilize.

[0130] (2) Acid and alkali resistance test

[0131] like Figure 25 As shown, the PANI-M3-T1 membrane of Example 12 exhibited excellent separation stability under extreme pH conditions: in direct tests at pH=2 and pH=12, the membrane permeation flux was 125.84 L·m⁻¹, respectively. -2 ·h -1 ·bar -1 and 73.74 L·m -2 ·h -1 ·bar -1 The retention rates for B2RL dye remained stable at 99%, and the retention rates for Na2SO4 remained between 17% and 18%. After soaking in solutions of the corresponding pH for 24 h, the permeation fluxes were 134.09 L·m⁻¹. -2 ·h -1 ·bar -1 and 56.28 L·m -2 ·h -1 ·bar -1 At pH=2, the retention rate of B2RL decreased to 88.19%. Polyaniline (PANI) exhibits significantly different colors in solutions with different pH values ​​due to the (de)doping process, which is one of its most notable characteristics. At lower pH, high concentrations of H2RL... +Ions may protonate the imine groups, forming a conductive, emerald green imine salt structure. As pH increases, the degree of protonation decreases, and the structure transforms into an emerald green imine base, exhibiting a blue color. At even higher pH levels, such as in alkaline environments, it may become a purple peraniline base or a colorless reduced state. The membrane's color change from green to purple (alkaline) in acidic / alkaline environments indicates the protonation / deprotonation process of polyaniline. The decrease in flux of the loose polyaniline nanofiltration membrane under alkaline conditions is mainly attributed to the pH-responsive structural transformation of polyaniline itself; the polymer chains transition from an extended state to a coiled and contracted state, causing membrane pore shrinkage and a decrease in porosity. This stability stems from the strong bonding between the citric acid / PSSA doped layer and the PMIA base membrane and its protective effect on the polyaniline chain structure.

[0132] (3) Anti-pollution test

[0133] A mixed solution of 0.1 g / L B2RL and 1 g / L Na2SO4 was used as a simulated contaminant. Deionized (DI) water was used as the cleaning agent to test the membrane's antifouling performance. Before measuring permeability, the membrane was compacted with DI water to achieve a stable state. First, the permeability of the DI water used as the feed solution was tested and recorded as J. w,1 Subsequently, simulated pollutants were used instead of the feed solution to measure the permeability of the solution, and the final permeation flux data were recorded as J. p Next, pure water was introduced into the feed side of the cross-flow filtration device for simple physical cleaning, with the feed water being replaced continuously during the process. Subsequently, the pure water permeation flux of the cleaned membrane was retested under the same conditions and recorded as J. w,2 Repeat the above procedure four times.

[0134] Membrane fouling parameters such as flux recovery rate (FRR) and reversible fouling (R) r Irreversible pollution (R) ir ) and total pollution rate (R t The following formulas can be used to obtain the results:

[0135]

[0136] like Figure 26 As shown, after four fouling-cleaning cycles, the PANI / PMIA membrane maintained over 80% of its initial permeation flux, with a stable B2RL rejection rate of >95% and a Na2SO4 rejection rate of 25%–30%. The flux recovery rate (FRR) during the cycles was consistently above 90%, and the irreversible fouling resistance (Rir) was extremely low, indicating that pollutant adsorption was primarily a reversible process. This excellent antifouling performance is attributed to the high hydrophilicity and negative surface charge of the citric acid / PSSA-doped polyaniline layer. Hydration weakens dye adhesion, and electrostatic repulsion inhibits irreversible adsorption, enabling the membrane to maintain stable permeation and separation performance during long-term operation.

[0137] like Figure 27 As shown, the PANI-M3-T1 membrane exhibits excellent self-cleaning and performance recovery capabilities after water immersion cleaning following dye fouling. After immersion cleaning, the membrane's water permeation flux decreased from the initial 108.05 L·m⁻¹. -2 ·h -1 ·bar -1 Restored to 111.46 L·m -2 ·h -1 ·bar -1 The B2RL rejection rate remained at around 98%, and the Na2SO4 rejection rate basically recovered to the initial level (~25%), with the separation performance (flux and rejection rate) almost completely restored.

[0138] Furthermore, Figure 28 The membrane appearance photos visually confirm the self-cleaning effect: after contamination, obvious blue-green stains are visible on the surface, but after soaking and cleaning in water, it returns to a clean green color consistent with the original membrane, indicating that the dye has been effectively removed. This self-cleaning property is attributed to the high hydrophilicity and anti-adhesion properties of the citric acid / PSSA-doped polyaniline layer. The hydrophilic surface weakens dye adsorption through hydration, and the electrostatic repulsion between the negatively charged functional groups and the dye further inhibits irreversible contamination.

[0139] It is evident that the membrane can achieve efficient self-cleaning through simple water immersion with almost no performance degradation, demonstrating excellent anti-fouling properties and reusability.

[0140] (4) Long-term stability test

[0141] The test procedure is the same as that for membrane permeation performance testing: the membrane is placed in a cross-flow filtration device, the pressure is set to 1 bar, the feed solution is a mixed solution of 0.1 g / L B2RL and 1 g / L Na2SO4, and a long-term stability test is conducted at a flow rate of 2.5 L / min.

[0142] like Figure 29 As shown, the PANI-M3-T1 loose nanofiltration membrane prepared in Example 12 exhibited excellent long-term separation stability during a 21-hour continuous operation test. With prolonged operation, the membrane permeation flux increased from the initial approximately 106.07 L·m⁻¹. -2 ·h -1 ·bar -1 Slowly decreasing to about 75 L·m -2 ·h -1 ·bar -1This is attributed to the fact that the filter cake layer formed by the gradual adsorption of dye molecules on the membrane surface slightly increases the mass transfer resistance. The membrane's rejection rate for B2RL dye remained stable at over 98%, and its rejection rate for Na2SO4 remained at around 27%, indicating that the membrane's pore structure and surface charge characteristics did not undergo irreversible damage during long-term operation, and the synergistic separation mechanism of sieving effect and electrostatic repulsion was effectively maintained.

[0143] In summary, this invention pioneers a bifunctional synergistic doping system of citric acid and polystyrene sulfonic acid, employing a one-step low-temperature in-situ polymerization and simultaneous gelation process to successfully construct a novel loose nanofiltration membrane possessing high selectivity, high flux, strong antifouling ability, and self-cleaning capability. Compared with traditional nanofiltration membranes and existing polyaniline-based separation membranes, this invention achieves comprehensive innovation from material structure and preparation process to separation mechanism, effectively overcoming the technical bottleneck of balancing flux, selectivity, and antifouling properties. It also cleverly balances the contradiction between "sieving" and "mass transfer," possessing significant advantages such as a simple preparation process, mild reaction conditions, strong structural controllability, and low production cost. The fabricated membrane material exhibits uniform pore size, hydrophilicity, and stable surface charge, enabling efficient dye retention and rapid permeation of saline substances under high flux conditions. Simultaneously, it possesses excellent self-cleaning and regeneration capabilities and acid-base stability, making it adaptable to complex industrial conditions involving high salt, high dye concentrations, and long-term continuous operation. It has broad industrial application prospects in the treatment of industrial wastewater from textiles, dyeing, and printing industries.

[0144] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A self-cleaning, loose nanofiltration membrane, characterized in that, Using a porous poly(m-phenylene isophthalamide) (PMIA) substrate membrane as the supporting substrate membrane, a polyaniline (PANI) hydrogel separation layer co-doped with citric acid (CA) and polystyrene sulfonate (PSSA) is grown in situ at low temperature on the substrate membrane surface and within the pores. The separation layer has an average pore size of 7.0~7.8 nm, a static water contact angle of <50° on the membrane surface, and exhibits a negative surface potential in the pH range of 3~9.

2. The self-cleaning loose nanofiltration membrane according to claim 1, characterized in that, The nanofiltration membrane exhibits a rejection rate of ≥98% for Direct Fast Blue B2RL dye, a rejection rate of ≤10% for NaCl, and a pure water flux of ≥149.52 L·m⁻¹. -2 ·h -1 ·bar -1 After being contaminated, the flux and rejection rate were restored to more than 99% of their initial values ​​after soaking in clean water for 12 hours.

3. The self-cleaning loose nanofiltration membrane according to claim 1, characterized in that, The polyaniline separation layer is tightly bonded to the PMIA base film through hydrogen bonds, with no interface defects; the separation layer is rich in carboxyl groups -COOH and sulfonic acid groups -SO3H, forming a stable hydration layer and a high-density negative charge layer.

4. The self-cleaning loose nanofiltration membrane according to claim 1, characterized in that, The polyaniline separation layer has a thickness of 158~561 nm, is tightly attached to the PMIA porous base film, and has no obvious gaps or defects at the interface, and the two are firmly bonded together.

5. A method for preparing a self-cleaning, loose nanofiltration membrane, characterized in that, The process employs a one-step low-temperature in-situ polymerization and simultaneous gelation technique, including the following steps: S1. PMIA porous base membrane was prepared by a solvent-inducible phase separation method; S2. Prepare a citric acid (CA) aqueous solution, add polystyrene sulfonic acid (PSSA) and aniline monomer (An) sequentially, and mix them evenly in a 0-8℃ water bath to form a mixed solution; add ammonium persulfate (APS) aqueous solution to the mixed solution and stir to obtain a co-doped reaction solution; S3. Immerse the PMIA base membrane obtained in step S1 into the co-doped reaction solution obtained in step S2, and allow it to stand for reaction deposition. After the reaction is completed, wash it several times with alternating ethanol and deionized water to obtain a CA / PSSA co-doped PANI / PMIA self-cleaning loose nanofiltration membrane.

6. The method for preparing a self-cleaning loose nanofiltration membrane according to claim 5, characterized in that, The molar ratio of aniline monomer to ammonium persulfate oxidant used in the preparation is 1:0.5~1:

2.

7. The method for preparing a self-cleaning loose nanofiltration membrane according to claim 5, characterized in that, The deposition time is 28-43 min.

8. The method for preparing a self-cleaning loose nanofiltration membrane according to claim 5, characterized in that, The mass-volume concentration of the PSSA was 2.5 w / v.

9. The application of the self-cleaning loose nanofiltration membrane as described in any one of claims 1 to 4 in the treatment of high-salt dye wastewater.

10. The application according to claim 9, characterized in that, Used to achieve efficient separation of dyes and salts: retaining dye molecules while allowing monovalent and divalent salts to pass through efficiently.

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