A multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas, its preparation method and application

Abstract

This invention belongs to the technical field of water quality testing sample pretreatment and membrane separation materials, and discloses a multilayer composite membrane for pretreatment of drinking water samples from high-altitude rural areas, its preparation method, and its application. The multilayer composite membrane includes a pre-filtration layer, a functional separation layer, and a support layer stacked sequentially. The inlet surface of the functional separation layer is provided with a modified layer composed of a polydopamine base layer, a polyethyleneimine linking layer, and a carboxylated outer layer. The preparation method includes support layer pretreatment, integrated base membrane preparation, functional separation layer surface modification, and pre-filtration layer composite. This multilayer composite membrane is suitable for pretreatment of drinking water samples from high-altitude rural areas containing suspended particles and natural organic matter, and has the advantages of graded impurity removal, anti-fouling, and good stability.

Description

Technical Field

[0001] This invention belongs to the technical field of water quality testing sample pretreatment and membrane separation materials, specifically relating to a multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas, its preparation method, and its application. Background Technology

[0002] Drinking water sources in high-altitude rural areas are diverse, often including mountain spring water, shallow groundwater, decentralized water supply, and water from simple water storage facilities. Due to topography, seasonal runoff, surface erosion, temperature fluctuations, and limitations of basic water supply conditions, these drinking water samples often have high suspended particulate content, a large amount of silt and colloidal impurities, and complex background of natural organic matter before testing. Especially under low-temperature conditions, the viscosity of the water sample increases, and particle sedimentation and redispersion coexist, posing significant challenges to the stable pretreatment of samples for subsequent testing. For the analysis of indicators such as arsenic, lead, cadmium, fluoride ions, and nitrate, if the interference from particles and natural organic matter is not effectively removed during the pretreatment process, it can easily lead to fluctuations in the detection signal, unstable instrument injection, and poor repeatability of results.

[0003] Current water sample pretreatment methods mostly employ filter paper filtration, centrifugation, single-layer membrane filtration, or simple combined filtration structures. While these technologies can remove some large particulate impurities to a certain extent, they still have significant shortcomings when treating drinking water samples from rural high-altitude areas: First, single-layer membrane materials have a limited pore size, making it difficult to simultaneously retain coarse particles and remove fine colloids and organic interferences; second, natural organic matter, humic acid, and kaolin-like particles easily deposit on the membrane surface or within the pores, causing rapid flux decay and increased membrane fouling; third, ordinary membrane materials lack sufficient hydrophilicity and have limited anti-fouling capabilities, making them more prone to clogging and decreased filtration efficiency under low-temperature conditions; fourth, if the interlayer bonding of multi-layer filter media is weak, delamination, wrinkling, or localized failure may occur during use, affecting the stability of sample pretreatment. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a multilayer composite membrane for pretreatment of drinking water samples from high-altitude rural areas, its preparation method, and its application. This multilayer composite membrane comprises a pre-filtration layer, a functional separation layer, and a support layer stacked sequentially. The inlet surface of the functional separation layer is provided with a modified layer composed of a polydopamine base layer, a polyethyleneimine linking layer, and a carboxylated outer layer. This membrane features graded filtration, hydrophilicity and antifouling properties, and stable interlayer bonding, effectively removing suspended particles and natural organic matter from drinking water samples from high-altitude rural areas. It is suitable for pretreatment of complex, low-temperature water samples before detection.

[0005] The objective of this invention can be achieved through the following technical solutions: A multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas includes a pre-filtration layer, a functional separation layer, and a support layer stacked sequentially. The pre-filter layer is prepared by electrospinning of a spinning solution, wherein the spinning solution comprises 12-18 parts of polyacrylonitrile and 82-88 parts of N,N-dimethylformamide per 100 parts by weight. The casting solution for preparing the functional separation layer comprises, by weight, 16-20 parts of polyethersulfone, 3-6 parts of polyvinylpyrrolidone, 0.5-1.5 parts of hydrophilic nano-silica, 0.2-0.8 parts of lithium chloride, and the balance being N-methylpyrrolidone; The inlet side surface of the functional separation layer is provided with a modified layer, which is a composite grafted layer composed of a polydopamine base layer, a polyethyleneimine connecting layer and a carboxylated outer layer in sequence, and the thickness of the modified layer is 20-120 nm. The support layer is made of polyester nonwoven fabric or polypropylene nonwoven fabric; the average pore size of the pre-filter layer is larger than the average pore size of the functional separation layer.

[0006] More preferably, the pre-filter layer has an average pore size of 2–8 μm, a thickness of 30–80 μm, and a porosity of 75%–88%; the functional separation layer has an average pore size of 0.05–0.25 μm and a thickness of 35–70 μm; and the support layer has a surface density of 80–130 g / m².

[0007] More preferably, the pre-filter layer is an electrospun coarse fiber web layer, in which the average diameter of the fibers is 0.8 to 1.8 μm and the areal density is 6 to 18 g / m²; the particle size of the hydrophilic nano silica is 15 to 60 nm.

[0008] More preferably, the carboxylated outer layer is formed by reacting succinic anhydride with polyethyleneimine; the raw materials for preparing the modified layer include dopamine hydrochloride, polyethyleneimine and succinic anhydride, wherein the mass ratio of dopamine hydrochloride, polyethyleneimine and succinic anhydride is 1:1 to 3:0.2 to 0.8.

[0009] More preferably, the pre-filter layer and the functional separation layer are connected by local interface adhesion composite, which is one of edge consolidation area and point consolidation area, or includes both edge consolidation area and point consolidation area; when the edge consolidation area is included, its width is 1 to 5 mm; when the point consolidation area is included, its total area accounts for 3% to 15% of the surface area of ​​the functional separation layer.

[0010] A method for preparing a multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas, characterized by comprising the following steps: S1. Provide a support layer, using polyester nonwoven fabric or polypropylene nonwoven fabric as the support substrate, and perform cutting, cleaning, dust removal and drying treatment on the support substrate. S2. The casting solution of the functional separation layer is coated onto the surface of the support layer and phase inversion is performed to obtain an integrated base film including the functional separation layer and the support layer; S3. Prepare a modified functional separation layer by sequentially forming a polydopamine base layer, a polyethyleneimine linking layer, and a carboxylated outer layer on the water inlet side surface of the functional separation layer, thereby forming a modified layer on the surface of the functional separation layer; S4. Prepare a multilayer composite membrane. First, prepare a pre-filter layer. After removing residual organic solvents, composite the pre-filter layer onto the water inlet side surface of the functional separation layer with the modified layer to obtain the multilayer composite membrane.

[0011] More preferably, in step S2, after the functional separation layer casting solution is coated onto the surface of the support layer, it is leveled and then placed in a coagulation bath for phase transformation to obtain the integrated base film; the coagulation bath medium is deionized water.

[0012] More preferably, in step S3, dopamine hydrochloride undergoes self-polymerization in a Tris-HCl buffer to form a polydopamine base layer, wherein the pH of the Tris-HCl buffer is 8.4–8.6, the reaction temperature is 22–28°C, and the reaction time is 1.0–2.5 h.

[0013] More preferably, in step S3, the functional separation layer with the polydopamine base layer is contacted with an aqueous solution of polyethyleneimine to form a polyethyleneimine linking layer, the reaction temperature is 20-30°C, and the reaction time is 0.5-1.5 h; then the polyethyleneimine linking layer is contacted with an anhydrous acetonitrile solution of succinic anhydride to form a carboxylated outer layer, the reaction temperature is 25-35°C, and the reaction time is 0.5-1.5 h; after the carboxylation reaction is completed, it is washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water, and then dried.

[0014] Application of multilayer composite membranes in the pretreatment of drinking water samples in high-altitude rural areas: for the pretreatment of drinking water samples containing suspended particles and natural organic matter in high-altitude rural areas.

[0015] The beneficial effects of this invention are: The multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas provided by this invention achieves graded retention and stable purification of complex water samples through the synergistic construction of a pre-filtration layer, a functional separation layer, and a support layer. The pre-filtration layer preferentially removes silt, suspended particles, and some colloidal impurities, reducing the filtration load on the subsequent functional separation layer and lowering the risk of rapid surface clogging. The modified layer on the surface of the functional separation layer, composed of a polydopamine base layer, a polyethyleneimine connecting layer, and a carboxylated outer layer, significantly improves the hydrophilicity and antifouling properties of the membrane surface, reduces the adsorption and deposition of natural organic matter such as humic acid on the membrane surface, inhibits pore clogging, and thus improves the stability of filtration flux and service life. The support layer provides good mechanical strength and dimensional stability for the overall membrane structure, ensuring that the composite membrane is not easily delaminated or damaged during actual use. Simultaneously, this invention employs a localized interfacial adhesion composite method to connect the pre-filtration layer and the functional separation layer, which enhances the interlayer bonding strength and avoids the impact of large-area overlay on the effective filtration area, thus helping to maintain the overall membrane flux and pretreatment efficiency. Attached Figure Description

[0016] The invention will now be further described with reference to the accompanying drawings.

[0017] Figure 1 This is a schematic diagram of the overall structure of the multilayer composite membrane of the present invention. In the figure, 1 is the pre-filtration layer, 2 is the modification layer, 3 is the functional separation layer, 4 is the support layer, and the arrow points to the direction of water sample flow. Figure 2 The graph shows a comparison of the pure water flux of the composite membranes in the examples and comparative examples at room temperature (25°C) and low temperature (5°C). Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1: The multilayer composite membrane prepared in this example includes a pre-filtration layer, a functional separation layer and a support layer stacked in sequence. The inlet surface of the functional separation layer is provided with a modified layer, which is a composite grafted layer composed of a polydopamine base layer, a polyethyleneimine connecting layer and a carboxylated outer layer in sequence.

[0020] The preparation steps of the multilayer composite film are as follows: S1. Polyester nonwoven fabric with a surface density of 80g / m² is selected as the support substrate. It is cut, cleaned, dusted and dried to obtain the support layer.

[0021] S2. Weigh 16 parts polyethersulfone, 3 parts polyvinylpyrrolidone, 0.5 parts hydrophilic nano-silica, 0.2 parts lithium chloride, and 80.3 parts N-methylpyrrolidone per 100 parts by weight. First, disperse the hydrophilic nano-silica in a portion of the N-methylpyrrolidone, then add polyvinylpyrrolidone and lithium chloride and stir to dissolve. Subsequently, add polyethersulfone and continue stirring to obtain a casting solution for the functional separation layer. Coat the casting solution onto the surface of the support layer, level it at 20°C for 10 seconds, and then immerse it in a deionized water coagulation bath for phase inversion to obtain an integrated base film consisting of the functional separation layer and the support layer.

[0022] S3. The integrated base membrane was placed in a Tris-HCl buffer solution at pH 8.4, and dopamine hydrochloride was added. The reaction was carried out at 22°C for 1.0 h to form a polydopamine base layer. Subsequently, it was contacted with an aqueous solution of polyethyleneimine and reacted at 20°C for 0.5 h to form a polyethyleneimine linker layer. Then, it was contacted with an anhydrous acetonitrile solution of succinic anhydride and reacted at 25°C for 0.5 h to form a carboxylated outer layer. After the reaction was completed, it was washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water, and then dried to obtain a functional separation layer with a modified layer. The mass ratio of dopamine hydrochloride, polyethyleneimine, and succinic anhydride was 1:1:0.2, and the thickness of the obtained modified layer was 20 nm.

[0023] S4. Weigh 12 parts by weight of polyacrylonitrile and 88 parts by weight of N,N-dimethylformamide, stir to prepare a homogeneous spinning solution, and set the electrospinning parameters as follows: voltage 14kV, feed speed 0.4mL / h, and receiving distance 14cm. Use this electrospinning process to prepare a pre-filter layer. After washing the prepared pre-filter layer with deionized water and vacuum drying at 40℃ to remove residual organic solvents, it is connected to the water inlet side surface of the functional separation layer with modified layer by local interfacial adhesion composite method. The local adhesion adopts a 1mm wide edge consolidation area, and is lightly pressed and shaped at 25℃ for 10s to obtain a multilayer composite membrane. The pre-filter layer has an average filtration pore size of 2μm, an average fiber diameter of 0.8μm, an areal density of 6g / m², and a porosity of 75%. The functional separation layer has an average filtration pore size of 0.05μm and a thickness of 35μm.

[0024] Example 2: This example is used to characterize the structural features and application performance of the multilayer composite membrane prepared under conditions of high component content and strong modification according to the present invention.

[0025] The multilayer composite membrane is prepared according to the following steps: S1. Polyester nonwoven fabric with a surface density of 130g / m² is selected as the support substrate. It is cut, cleaned, dusted and dried to obtain the support layer.

[0026] S2. Weigh out 20 parts of polyethersulfone, 6 parts of polyvinylpyrrolidone, 1.5 parts of hydrophilic nano-silica, 0.8 parts of lithium chloride, and 71.7 parts of N-methylpyrrolidone per 100 parts by weight. First, disperse the hydrophilic nano-silica in a portion of the N-methylpyrrolidone, then add polyvinylpyrrolidone and lithium chloride and stir to dissolve. Subsequently, add polyethersulfone and continue stirring until a uniform casting solution is formed. Coat the casting solution onto the surface of the support layer, level it at 25°C for 20 seconds, and then immerse it in a deionized water coagulation bath for phase inversion to obtain an integrated base film including a functional separation layer and a support layer.

[0027] S3. The integrated base membrane was placed in a Tris-HCl buffer solution at pH 8.6, and dopamine hydrochloride was added. The mixture was reacted at 28°C for 2.5 h to form a polydopamine base layer. This was then contacted with an aqueous solution of polyethyleneimine and reacted at 30°C for 1.5 h to form a polyethyleneimine linker layer. Subsequently, it was contacted with an anhydrous acetonitrile solution of succinic anhydride and reacted at 35°C for 1.5 h to form a carboxylated outer layer. After the reaction was complete, the membrane was washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water for 10 min each time, and then vacuum dried at 50°C for 12 h to obtain a functional separation layer with a modified layer. The mass ratio of dopamine hydrochloride, polyethyleneimine, and succinic anhydride was 1:3:0.8, and the thickness of the resulting modified layer was 120 nm.

[0028] S4. Weigh 18 parts by weight of polyacrylonitrile and 82 parts by weight of N,N-dimethylformamide, stir to prepare a homogeneous spinning solution, and set the electrospinning parameters as follows: voltage 18kV, feed speed 0.8mL / h, and receiving distance 18cm. Prepare a pre-filter layer using electrospinning. After washing with deionized water and vacuum drying at 50℃ to remove residual organic solvents, the pre-filter layer is connected to the water inlet surface of the functional separation layer with the modified layer through a local interfacial adhesion composite method. The local adhesion uses point-like consolidation areas accounting for 15% of the total surface area of ​​the functional separation layer. Lightly press and shape at 35℃ for 30s to obtain a multilayer composite membrane. The obtained pre-filter layer has an average pore size of 8μm, an average fiber diameter of 1.8μm, an areal density of 18g / m², and a porosity of 88%; the functional separation layer has an average pore size of 0.25μm and a thickness of 70μm.

[0029] Example 3: This example is used to characterize the structural features and comprehensive performance of the multilayer composite membrane prepared under the preferred process conditions of the present invention.

[0030] The preparation steps of the multilayer composite film are as follows: S1. Polyester nonwoven fabric with a surface density of 105 g / m² is selected as the support substrate. It is cut, cleaned, dusted and dried to obtain the support layer.

[0031] S2. Weigh out 18 parts of polyethersulfone, 4.5 parts of polyvinylpyrrolidone, 1.0 part of hydrophilic nano-silica, 0.5 parts of lithium chloride, and 76.0 parts of N-methylpyrrolidone per 100 parts by weight. First, disperse the hydrophilic nano-silica in a portion of the N-methylpyrrolidone, then add polyvinylpyrrolidone and lithium chloride and stir to dissolve. Subsequently, add polyethersulfone and continue stirring to obtain a homogeneous casting solution for the functional separation layer. Coat the casting solution onto the surface of the support layer, level it at 23°C for 15 seconds, and then immerse it in a deionized water coagulation bath for phase inversion to obtain an integrated base film including the functional separation layer and the support layer.

[0032] S3. The integrated base membrane was placed in a Tris-HCl buffer solution at pH 8.5, and dopamine hydrochloride was added. The mixture was reacted at 25°C for 1.75 h to form a polydopamine base layer. This was then contacted with an aqueous solution of polyethyleneimine and reacted at 25°C for 1.0 h to form a polyethyleneimine linker layer. Subsequently, it was contacted with an anhydrous acetonitrile solution of succinic anhydride and reacted at 30°C for 1.0 h to form a carboxylated outer layer. After the reaction was complete, the membrane was washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water, and then dried to obtain a functional separation layer with a modified layer. The mass ratio of dopamine hydrochloride, polyethyleneimine, and succinic anhydride was 1:2:0.5, and the thickness of the resulting modified layer was 70 nm.

[0033] S4. Weigh 15 parts by weight of polyacrylonitrile and 85 parts by weight of N,N-dimethylformamide, stir to prepare a homogeneous spinning solution, and set the electrospinning parameters as follows: voltage 16kV, feed speed 0.6mL / h, and receiving distance 16cm. Prepare a pre-filter layer using electrospinning. After washing with deionized water and vacuum drying at 45℃ to remove residual organic solvents, the pre-filter layer is connected to the water inlet surface of the functional separation layer with the modified layer through a local interfacial adhesion composite method. The local adhesion uses a 3mm wide edge consolidation zone and a dotted consolidation zone with a total area accounting for 9% of the surface area of ​​the functional separation layer. Lightly press and shape at 30℃ for 20s to obtain a multilayer composite membrane. The obtained pre-filter layer has an average pore size of 5μm, an average fiber diameter of 1.3μm, an areal density of 12g / m², and a porosity of 82%; the functional separation layer has an average pore size of 0.15μm and a thickness of 52μm.

[0034] Comparative Example 1: This comparative example provides a multilayer composite membrane for pretreatment of drinking water samples in rural plateau areas, which includes a pre-filtration layer, a functional separation layer and a support layer stacked in sequence, wherein the inlet surface of the functional separation layer does not have a modification layer.

[0035] The preparation steps of the multilayer composite film are as follows: S1. Polyester nonwoven fabric with a surface density of 105 g / m² is selected as the support substrate. It is cut, cleaned, dusted and dried to obtain the support layer.

[0036] S2. Weigh out 18 parts of polyethersulfone, 4.5 parts of polyvinylpyrrolidone, 1.0 part of hydrophilic nano-silica, 0.5 parts of lithium chloride, and 76.0 parts of N-methylpyrrolidone per 100 parts by weight. First, disperse the hydrophilic nano-silica in a portion of the N-methylpyrrolidone, then add polyvinylpyrrolidone and lithium chloride and stir to dissolve. Subsequently, add polyethersulfone and continue stirring to obtain a homogeneous casting solution for the functional separation layer. Coat the casting solution onto the surface of the support layer, level it at 23°C for 15 seconds, and then immerse it in a deionized water coagulation bath for phase inversion to obtain an integrated base film including the functional separation layer and the support layer.

[0037] S3. Weigh 15 parts by weight of polyacrylonitrile and 85 parts by weight of N,N-dimethylformamide, stir to prepare a homogeneous spinning solution, and set the electrospinning parameters as follows: voltage 16kV, feed speed 0.6mL / h, and receiving distance 16cm. Prepare a pre-filter layer using electrospinning. After washing with deionized water and vacuum drying at 45℃ to remove residual organic solvents, the pre-filter layer is connected to the water inlet side surface of the functional separation layer through a local interfacial adhesion composite method. The local adhesion uses a 3mm wide edge consolidation zone and a dotted consolidation zone with a total area accounting for 9% of the surface area of ​​the functional separation layer. Lightly press and shape at 30℃ for 20s to obtain a multilayer composite membrane. The obtained pre-filter layer has an average pore size of 5μm, an average fiber diameter of 1.3μm, an areal density of 12g / m², and a porosity of 82%; the functional separation layer has an average pore size of 0.15μm and a thickness of 52μm.

[0038] Comparative Example 2: This comparative example provides a composite membrane for pretreatment of drinking water samples in rural plateau areas, which includes a functional separation layer and a support layer. The inlet surface of the functional separation layer is provided with a modified layer, but no pre-filtration layer is provided.

[0039] The preparation steps of the composite membrane are as follows: S1. Polyester nonwoven fabric with a surface density of 105 g / m² is selected as the support substrate. It is cut, cleaned, dusted and dried to obtain the support layer.

[0040] S2. Weigh out 18 parts of polyethersulfone, 4.5 parts of polyvinylpyrrolidone, 1.0 part of hydrophilic nano-silica, 0.5 parts of lithium chloride, and 76.0 parts of N-methylpyrrolidone per 100 parts by weight. First, disperse the hydrophilic nano-silica in a portion of the N-methylpyrrolidone, then add polyvinylpyrrolidone and lithium chloride and stir to dissolve. Subsequently, add polyethersulfone and continue stirring to obtain a homogeneous casting solution for the functional separation layer. Coat the casting solution onto the surface of the support layer, level it at 23°C for 15 seconds, and then immerse it in a deionized water coagulation bath for phase inversion to obtain an integrated base film including the functional separation layer and the support layer.

[0041] S3. The integrated base membrane was placed in a Tris-HCl buffer solution at pH 8.5, and dopamine hydrochloride was added. The reaction was carried out at 25°C for 1.75 h to form a polydopamine base layer. This was then contacted with an aqueous solution of polyethyleneimine and reacted at 25°C for 1.0 h to form a polyethyleneimine linker layer. Subsequently, it was contacted with an anhydrous acetonitrile solution of succinic anhydride and reacted at 30°C for 1.0 h to form a carboxylated outer layer. After the reaction was complete, the membrane was washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water, and then dried to obtain a functional separation layer with a modified layer. The mass ratio of dopamine hydrochloride, polyethyleneimine, and succinic anhydride was 1:2:0.5, and the thickness of the resulting modified layer was 70 nm. The average filtration pore size of the functional separation layer was 0.15 μm, and its thickness was 52 μm.

[0042] Performance testing 1. Pre-processing core effect test A simulated drinking water sample for high-altitude drinking was prepared with an initial turbidity of 100 NTU, total suspended particles of 200 mg / L (150 mg / L kaolin, 50 mg / L quartz sediment), humic acid of 20 mg / L, and pH 7.5. The sample was stirred thoroughly and set aside. Membrane sheets were cut into discs with an effective filtration area of ​​9.62 cm², moistened with deionized water, degassed under vacuum, and then placed in a constant pressure apparatus. 50 mL of water sample was filtered at 10℃ and 0.04 MPa, with stirring at 300 rpm throughout to prevent sedimentation. Each group was measured in triplicate. The filtrate parameters were determined using a turbidimeter, gravimetric method, and ultraviolet spectrophotometry. The impurity removal rate was calculated using the formula, and outliers were removed before taking the average value. The results are shown in Table 1 below.

[0043] Table 1. Removal effect of suspended particles

[0044] As shown in Table 1, Examples 1-3 of the present invention exhibit good removal effects on turbidity, suspended particles, kaolin, and humic acid, indicating that the constructed composite structure of pre-filtration layer, functional separation layer, and support layer can effectively remove particulate impurities and natural organic matter from simulated drinking water samples from plateau areas. Example 2 showed the highest removal rates across all categories, indicating that under conditions of higher component content and stronger surface modification, the hydrophilicity and retention capacity of the membrane surface were further enhanced. Comparative Example 1, lacking a modification layer, showed a significant decrease in humic acid removal rate, demonstrating the important role of the surface modification layer in improving the hydrophilicity of the functional separation layer and reducing organic matter penetration and adsorption. Comparative Example 2, lacking a pre-filtration layer, showed a significant decrease in the removal rates of turbidity, suspended particles, and kaolin, indicating that the pre-filtration layer can preferentially retain particulate pollutants, reducing the load on the functional separation layer.

[0045] 2. Flux and Low Temperature Adaptability Test Take the membrane sheets from each embodiment and comparative example, cut them into circular pieces with an effective filtration area of ​​9.62 cm², soak them in deionized water for 15 min, degas them under vacuum at 0.08 MPa for 10 min, and then install them into a constant pressure filtration device. Prepare constant temperature deionized water at 25℃ and 5℃ respectively, place the device in a constant temperature bath for temperature control, pre-pressurize it at 0.04 MPa for 10 min until the flux stabilizes, and after equilibration for 30 min, start timing and collect the permeate volume after 5 min. Calculate the pure water flux according to the formula: Pure water flux (L·m⁻²·h⁻¹) = permeate volume (L) ÷ [membrane area (m²) × time (h)], Low temperature flux retention rate (%) = 5℃ flux ÷ 25℃ flux × 100. Each group was tested in parallel 3 times, and outliers with relative deviation > 5% were removed, and the average value was taken. The results are shown in Table 2 below. Table 2. Flux of pure water at ambient temperature, flux of pure water at low temperature, and retention rate of low temperature flux.

[0046] As shown in Table 2, Examples 1-3 all exhibited high pure water flux at both 25℃ and 5℃, and their low-temperature flux retention rate was generally better than that of the comparative example. This indicates that the multilayer composite structure and surface modification layer of the functional separation layer constructed in this invention can effectively mitigate flux decay under low-temperature conditions. Specifically, Examples 2 and 3 maintained high pure water flux at 5℃, with low-temperature flux retention rates reaching 75.6% and 76.3%, respectively. This indicates that under optimal structural parameters and modification conditions, the hydrophilicity of the membrane surface and the stability of the pore structure were further improved, which is beneficial for maintaining water molecule transport efficiency at low temperatures. Comparative Example 1, lacking a modification layer, had insufficient hydrophilicity on its membrane surface, resulting in a significant decrease in low-temperature flux retention rate. Comparative Example 2, although equipped with a modification layer, lacked a pre-filtration layer, making its membrane surface more susceptible to particulate fouling during use; its low-temperature flux and retention rate were also lower than those of the examples.

[0047] 3. Anti-pollution performance test The membranes of each embodiment and comparative example were cut into circular pieces with an effective filtration area of ​​9.62 cm², soaked in deionized water for 15 min, degassed under vacuum at 0.08 MPa for 10 min, and then placed in a constant pressure filtration device. The initial pure water flux J0 was tested at 25℃ and 0.04 MPa, and the average value was taken after 5 min of stabilization. 20 mg / L humic acid solution, 150 mg / L kaolin solution, and a mixed humic acid-kaolin solution were filtered sequentially for 30 min until the flux stabilized, and the flux Jp was measured after the contamination stabilized. After filtration, the membranes were rinsed with deionized water and the pure water flux Jr was measured again. The flux decay rate and flux recovery rate were calculated. Each group was tested in parallel three times, and the average value was taken. The results are shown in Table 3.

[0048] Table 3 Results of anti-pollution performance test

[0049] As shown in Table 3, Examples 1-3 exhibited low flux decay rates and high flux recovery rates under humic acid, kaolin, and mixed pollution conditions, indicating that the multilayer composite membrane constructed in this invention has good antifouling capabilities. Examples 2 and 3 showed superior overall performance across the three pollution models, especially under humic acid-kaolin mixed pollution conditions, where the flux decay rate was significantly lower than the comparative example, and the flux recovery rate remained at a high level, indicating a significant synergistic effect between the pre-filtration layer and the surface-modified functional separation layer. Comparative Example 1, lacking a modification layer, showed a significantly increased flux decay rate and a significantly decreased recovery rate under humic acid and mixed pollution conditions, indicating that the surface-modified layer effectively reduces the adsorption and deposition of natural organic matter on the membrane surface, reducing irreversible pollution. Comparative Example 2, lacking a pre-filtration layer, showed a significantly increased flux decay rate under kaolin and mixed pollution conditions, indicating that the pre-filtration layer preferentially traps particulate pollutants, mitigating clogging of the functional separation layer.

[0050] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0051] 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 present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A multilayer composite membrane for pretreatment of drinking water samples in high-altitude rural areas, characterized in that, It includes a pre-filter layer, a functional separation layer, and a support layer stacked in sequence; The pre-filter layer is prepared by electrospinning of a spinning solution, wherein the spinning solution comprises 12-18 parts of polyacrylonitrile and 82-88 parts of N,N-dimethylformamide per 100 parts by weight. The casting solution for preparing the functional separation layer comprises, by weight, 16-20 parts of polyethersulfone, 3-6 parts of polyvinylpyrrolidone, 0.5-1.5 parts of hydrophilic nano-silica, 0.2-0.8 parts of lithium chloride, and the balance being N-methylpyrrolidone; The inlet side surface of the functional separation layer is provided with a modified layer, which is a composite grafted layer composed of a polydopamine base layer, a polyethyleneimine connecting layer and a carboxylated outer layer in sequence, and the thickness of the modified layer is 20-120 nm. The support layer is made of polyester nonwoven fabric or polypropylene nonwoven fabric; the average pore size of the pre-filter layer is larger than the average pore size of the functional separation layer.

2. The multilayer composite film according to claim 1, characterized in that, The pre-filter layer has an average pore size of 2–8 μm, a thickness of 30–80 μm, and a porosity of 75%–88%; the functional separation layer has an average pore size of 0.05–0.25 μm and a thickness of 35–70 μm; and the support layer has a surface density of 80–130 g / m².

3. The multilayer composite film according to claim 2, characterized in that, The pre-filter layer is an electrospun coarse fiber web layer, in which the average diameter of the fibers is 0.8 to 1.8 μm and the areal density is 6 to 18 g / m²; the hydrophilic nano-silica has a particle size of 15 to 60 nm.

4. The multilayer composite film according to claim 1, characterized in that, The carboxylated outer layer is formed by reacting succinic anhydride with polyethyleneimine; the raw materials for preparing the modified layer include dopamine hydrochloride, polyethyleneimine and succinic anhydride, and the mass ratio of dopamine hydrochloride, polyethyleneimine and succinic anhydride is 1:1 to 3:0.2 to 0.

8.

5. The multilayer composite film according to claim 1, characterized in that, The pre-filter layer and the functional separation layer are connected by local interface adhesion composite. The local interface adhesion composite is one of edge consolidation area and point consolidation area, or includes both edge consolidation area and point consolidation area. When the edge consolidation area is included, its width is 1 to 5 mm. When the point consolidation area is included, its total area accounts for 3% to 15% of the surface area of ​​the functional separation layer.

6. A method for preparing a multilayer composite film according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Provide a support layer, using polyester nonwoven fabric or polypropylene nonwoven fabric as the support substrate, and perform cutting, cleaning, dust removal and drying treatment on the support substrate. S2. The casting solution of the functional separation layer is coated onto the surface of the support layer and phase inversion is performed to obtain an integrated base film including the functional separation layer and the support layer; S3. Prepare a modified functional separation layer by sequentially forming a polydopamine base layer, a polyethyleneimine linking layer, and a carboxylated outer layer on the water inlet side surface of the functional separation layer, thereby forming a modified layer on the surface of the functional separation layer; S4. Prepare a multilayer composite membrane. First, prepare a pre-filter layer. After removing residual organic solvents, composite the pre-filter layer onto the water inlet side surface of the functional separation layer with the modified layer to obtain the multilayer composite membrane.

7. The method for preparing a multilayer composite film according to claim 6, characterized in that, In step S2, the functional separation layer casting solution is coated onto the surface of the support layer and then leveled before entering the coagulation bath for phase transformation to obtain the integrated base film; the coagulation bath medium is deionized water.

8. The method for preparing a multilayer composite film according to claim 6, characterized in that, In step S3, dopamine hydrochloride undergoes self-polymerization in Tris-HCl buffer to form a polydopamine base layer. The pH of the Tris-HCl buffer is 8.4–8.6, the reaction temperature is 22–28°C, and the reaction time is 1.0–2.5 h.

9. The preparation method according to claim 8, characterized in that, In step S3, the functional separation layer with the polydopamine base layer is contacted with an aqueous solution of polyethyleneimine to form a polyethyleneimine linking layer. The reaction temperature is 20–30°C and the reaction time is 0.5–1.5 h. Then, the polyethyleneimine linking layer is contacted with an anhydrous acetonitrile solution of succinic anhydride to form a carboxylated outer layer. The reaction temperature is 25–35°C and the reaction time is 0.5–1.5 h. After the carboxylation reaction is completed, the mixture is washed sequentially with anhydrous acetonitrile, anhydrous ethanol, and deionized water, and then dried.

10. The application of the multilayer composite membrane according to any one of claims 1 to 5 in the pretreatment of drinking water samples in rural plateau areas, characterized in that, Used for the pretreatment of drinking water samples from high-altitude rural areas containing suspended particles and natural organic matter.

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