Cationic action nano titanium dioxide modified nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of separation membranes, and discloses a cationic nano titanium dioxide modified nanofiltration membrane as well as a preparation method and application thereof. The preparation method comprises the following steps: blending polyether sulfone, polyethylene glycol, nano titanium dioxide and N, N-dimethylformamide to form a membrane casting solution, forming an ultrafiltration base membrane through a water coagulating bath, and then sequentially carrying out full infiltration and heat treatment in a piperazine-containing water phase and a TMC-containing organic phase to obtain the modified nanofiltration membrane. And finally, soaking in a water solution of Na < + >, Mg < 2 + > and Fe < 3 + > cations, and drying to obtain the target nanofiltration membrane. The nanofiltration membrane prepared by the invention is wide in application scene, and is suitable for efficiently removing typical PFOS (perfluorooctane sulfonate) in a concentration range (10-1000 mu g / L) from a near-neutral actual water source; non-toxic and cheap cations (Na < + > / Mg < 2 + > / Fe < 3 + >) are used, so that secondary pollution is avoided, and meanwhile, the membrane has high rejection rate and high flux; long-chain PFAS resistance specificity: a membrane structure (MWCO matching + high negative electricity + bridging) is customized aiming at molecular size, charge and hydrophobicity of PFOS (C8), and is superior to most studies focusing short-chain PFAS.

Description

Technical Field

[0001] This invention relates to the field of polymer separation membrane technology, specifically to a cationic nano-titanium dioxide modified nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Perfluorinated and polyfluoroalkyl compounds (PFAS) are a class of organofluorine compounds with significant chemical stability, characterized by the presence of at least one perfluorinated methyl (-CF3) or methylene (-CF4) group. 2- PFAS are structural units. The core structure of these compounds consists of straight-chain or branched perfluorinated carbon segments, typically terminated with hydrophilic functional groups (such as sulfonic acid or carboxylic acid groups). The C-F bond energy is very high (approximately 460 kJ / mol), giving PFAS stable chemical properties, good surface activity, and strong hydrophobicity. Due to their unique chemical properties, PFAS exhibit irreplaceable applications in industrial manufacturing, consumer goods production, and specialty materials development, such as waterproofing of high-performance textiles, AFFF (air-free fire-fighting foam) for military applications, anti-stick coatings for food contact materials, and etching solution formulations in semiconductor manufacturing. Among them, perfluorooctane sulfonic acid (PFOS), due to its complete eight-carbon perfluorinated chain structure and amphiphilic properties, exhibits extreme environmental durability and bioaccumulation, and has become the most frequently detected PFAS in global environmental media and biological matrices.

[0003] In recent years, membrane separation technology has been increasingly widely used in water treatment. Membrane separation, especially nanofiltration (NF), is considered a potential technology for PFAS removal due to its low operating pressure and high rejection rate for multivalent ions / small molecule organic matter. Currently, the mainstream NF membrane is the polypiperazine amide composite membrane (PIP-TMC type), which has a dense selective layer and strong negative charge, and has a certain rejection capacity for long-chain PFAS (such as PFOS, MW = 500 Da). However, it generally has two major technical bottlenecks: (1) Low flux: The dense cross-linking of the polyamide layer results in narrow water channels and low pure water flux; (2) Insufficient electrostatic repulsion: Under near-neutral pH (6–8) conditions, the absolute value of the zeta potential on the membrane surface is limited (usually -20 ~ -30 mV), which is insufficient for strongly negatively charged PFOS (pKa < 1, -SO3). - The Donnan repulsion effect (complete dissociation) is insufficient, resulting in large fluctuations in the retention rate (reported in the literature as 70–90%).

[0004] To improve performance, existing studies have attempted nano-modification. For example, Ma et al. embedded MXene into the polyamide layer to improve the retention of short-chain PFAS (PFHxS: 96.85%), but this was not optimized for long-chain PFAS, and MXene is easily oxidized and has high cost. Vatanpour et al. embedded TiO2 / carbon dots (TiO2 / CDs) into the polyamide layer of the RO membrane to improve hydrophilicity and antifouling properties, but the RO membrane operates at a high pressure (>1MPa), making it unsuitable for low-energy-consumption PFAS deep treatment, and the TiO2 loading was extremely low (0.01 wt%), resulting in no significant improvement in flux-retention synergy. The existing technologies mainly suffer from the following shortcomings:

[0005] (1) It is difficult to balance flux and rejection rate: Increasing the rejection rate often requires increasing the degree of crosslinking of polyamide or adding high-charge nanomaterials, but this can easily lead to membrane pore densification and decreased flux; conversely, increasing flux often sacrifices selectivity.

[0006] (2) Limitations of modification sites: Existing TiO2 modification is mostly concentrated in the interfacial polymerization stage (doping with polyamide layer), which easily causes TiO2 to agglomerate and block water channels, and TiO2 has poor dispersibility in organic phase;

[0007] (3) Lack of targeted optimization of long-chain PFOS: Most studies focus on short-chain PFAS (< C6), and lack the design of a synergistic mechanism of “size sieving (MWCO matching) + electrostatic repulsion (high negative charge) + hydrophilic antifouling (low pollution)” for PFOS with a molecular weight of ~500 Da. Summary of the Invention

[0008] To address the aforementioned technical problems, the main objective of this invention is to provide a cation-modified nanofiltration membrane made of nano-titanium dioxide, its preparation method, and its application.

[0009] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a cationicly-modified nanofiltration membrane using nano-titanium dioxide, characterized by comprising the following steps:

[0010] S1. Polyethersulfone (PES), polyethylene glycol (PEG600), nano TiO2 and N,N-dimethylformamide (DMF) are added to a closed reactor and mixed. Then, the mixture is swollen at 60~80℃ and degassed to obtain a white casting solution.

[0011] The concentration of polyethersulfone is 12-16 wt%, the concentration of polyethylene glycol is 7-13 wt%, and the concentration of nano-TiO2 is 2-7 wt%.

[0012] S2. After the casting solution is coated onto the substrate, it is immersed in a pure water coagulation bath. The membrane with a porous PES framework formed by phase transformation is obtained as an ultrafiltration base membrane.

[0013] S3. Prepare an aqueous solution containing 0.5~5 wt% piperazine (PIP) and 0.01~0.1 wt% sodium dodecyl sulfate (SDS) as the aqueous phase, and prepare a hexane solution containing 0.1~1 wt% trimesoyl chloride (TMC) as the organic phase. The ultrafiltration membrane is then fully immersed in the aqueous phase and the organic phase in sequence to composite a layer of polypiperazine amide on the membrane surface through interfacial polymerization.

[0014] S4. The ultrafiltration membrane after S3 treatment is heat-treated at 60~80℃ and washed with deionized water to obtain nanofiltration membrane modified with nano-titanium dioxide.

[0015] S5. Nano-titanium dioxide modified nanofiltration membranes are immersed in deionized water, and Na is added at a concentration of 10~30 mmol / L. + Mg 2+ Fe 3+ At least one cation in the sample is dried to obtain a cation-modified nanofiltration membrane of nano-titanium dioxide.

[0016] This invention specifies that the cation is Na. + Mg 2+ Fe 3+ This is due to several factors: environmental friendliness, as all three are common ions in natural water bodies, non-toxic, and pose no risk of accumulation (Mg). 2+ / Fe 3+ It is also an essential element for the human body); it has hydrolytic stability and does not hydrolyze and precipitate within a pH range of 6–8 (unlike elements such as Al). 3+ / Cr 3+ ); low-priced NH4 + With –SO3 - The binding force is much weaker than that of Na. + Furthermore, under alkaline conditions, NH3 is released, corroding equipment and altering pH, affecting retention; while high-valence, easily hydrolyzable metal ions (such as Al) 3+ Cr 3+ ) or toxic ions (such as Cu) 2+ Cd 2+ This may lead to increased membrane fouling or secondary pollution, making it unsuitable for the application of this invention in water treatment. Therefore, only cations with moderate charge density, low hydrolysis tendency, and non-toxicity (such as Na+) are suitable. + Mg 2+ Fe 3+ The effects of this invention can only be achieved through a combination of these methods.

[0017] As a further preferred technical solution of the present invention, after swelling treatment in step S1, a white casting liquid is obtained by stirring and degassing.

[0018] As a further preferred technical solution of the present invention, in step S1, the nano-TiO2 content is 5 wt%, the polyethersulfone content is 14 wt%, and the polyethylene glycol content is 10 wt%. The present invention can achieve four-fold synergistic regulation of hydrophilicity, roughness, charge density, and pore structure, which is a key threshold for overcoming the flux-retention contradiction. Specifically, for the nano-TiO2 doping content, a content below 2 wt% will lead to insufficient effect, while a content above 7 wt% will cause agglomeration and performance degradation.

[0019] As a further preferred technical solution of the present invention, in step S1, the temperature during the swelling treatment is 60~80℃.

[0020] As a further preferred embodiment of the present invention, the substrate is a glass plate, and the temperature of the pure water coagulation bath is 15~25℃.

[0021] As a further preferred technical solution of the present invention, the heat treatment time in step S4 is 10~30 min.

[0022] As a further preferred embodiment of the present invention, in the aqueous phase, the concentration of piperazine is 1 wt% and the concentration of sodium dodecyl sulfate is 0.05 wt%; and / or, the concentration of pyromellitic methyl chloride in the organic phase is 0.5 wt%.

[0023] According to a second aspect of the present invention, the present invention also provides a cation-modified nanofiltration membrane of nano-titanium dioxide, which is prepared by the above-described preparation method. The cation-modified nanofiltration membrane of nano-titanium dioxide prepared by the present invention has a three-layer structure, as follows:

[0024] The support layer is formed by coating the casting solution from step S1 onto the substrate.

[0025] The intermediate transition layer is formed through the phase transformation in step S2;

[0026] The separation layer is selected and formed by interfacial polymerization in step S3. Nano-TiO2 particles are uniformly dispersed in the support layer and intermediate transition layer of the cationic-modified nano-titanium dioxide nanofiltration membrane.

[0027] According to a third aspect of the present invention, the present invention also provides an application of a cationic nano-titanium dioxide modified nanofiltration membrane in water treatment.

[0028] Specifically, cationic nano-titanium dioxide modified nanofiltration membranes are used to remove perfluorooctane sulfonic acid (PFOS) from water. Operating conditions: pressure: 0.1 MPa; temperature: 25℃; pH: 6–8 (near neutral, close to actual water conditions); influent PFOS concentration: 10–1000 μg / L (covering typical pollution levels, preferably 250 μg / L).

[0029] The mechanism by which TiO2 affects membrane performance in this invention is as follows: Nano-TiO2 possesses strong hydrophilicity. Based on the theories of surface chemistry and interfacial interactions, nano-TiO2 contains a large number of hydroxyl (-OH) functional groups on its surface. These functional groups can form hydrogen bonds with water molecules, thereby increasing the hydrophilicity of the membrane surface. The introduction of nano-titanium dioxide increases the surface energy of the membrane, making it easier for water molecules to interact with the membrane surface, leading to a decrease in the membrane contact angle and enhanced hydrophilicity. On the other hand, the introduction of nano-TiO2 particles may create a certain degree of microscopic roughness on the membrane surface. Moderate roughness helps improve the hydrophilicity of the membrane because water molecules can contact the membrane surface more frequently on rough surfaces, thus reducing water flux attenuation. Furthermore, nano-TiO2 exhibits strong photocatalytic performance. Under ultraviolet light irradiation, TiO2 can generate a large number of hydroxyl radicals (·OH) and peroxide radicals (·O2). These radicals have strong oxidizing power and can degrade pollutants such as organic matter, bacteria, and viruses in water. Therefore, when the membrane surface is exposed to ultraviolet light, pollutants on the membrane surface are photocatalytically degraded, reducing membrane fouling. Thus, the blending of nano-TiO2 with membrane materials in this invention can enhance the hydrophilicity of the membrane, increase membrane flux, improve the surface roughness of the membrane, delay membrane fouling, extend the membrane's service life, and reduce the operating costs of the membrane device.

[0030] The mechanism by which cations affect performance in this invention is as follows: On the one hand, the main mechanism by which TiO2-modified nanofiltration membranes remove PFAS is electrostatic repulsion, and the ionization of cations reduces the negative charge on the membrane surface and PFAS. On the other hand, cations can form a host bridging effect between PFAS molecules and a surface bridging effect between PFAS and the nanofiltration membrane. The host bridging effect increases the volume of the target analyte, making it easier for the nanofiltration membrane to retain it, while the surface bridging effect allows some PFAS to be fixed on the membrane surface and removed. Both of these aspects together improve the PFAS removal rate.

[0031] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0032] (1) Existing technologies add TiO2 to the organic phase / polyamide layer, which easily agglomerates, blocks water channels, and has poor dispersibility. This invention introduces nano-TiO2 into the support layer and intermediate transition layer (i.e., PES base film), instead of incorporating the polyamide layer during the interfacial polymerization stage, thus solving the problem of poor nanoparticle dispersibility. In addition, compared with traditional blending or surface coating modification, this invention first constructs a porous film containing TiO2 through phase transformation, and then achieves surface charge and bridging function regulation through cation immersion, taking into account both high retention rate and high throughput.

[0033] (2) The hydrophilicity of the nanofiltration membrane prepared by the present invention is significantly enhanced. The contact angle of the membrane surface is reduced from about 79° of the unmodified membrane to 65°. It promotes Wenzel-type wetting on rough surfaces (Rq = 80.7 nm, which is 86% higher than the 43.4 nm of the unmodified membrane) and greatly reduces mass transfer resistance. The PFOS rejection rate reaches 96.7% and the pure water flux reaches 58.4 L / (m²·h·bar), which is 7% and 198% higher than the performance of the unmodified membrane (rejection rate 89.7%, flux 20.2 L / (m²·h·bar)). After the addition of cations, the roughness of the modified membrane increases and the contact angle increases. The PFOS rejection rate is significantly improved to 97.1%, while the effect on flux is relatively small.

[0034] (3) The nanofiltration membrane prepared by this invention has a wide range of applications and is suitable for near-neutral pH (6–8) real water sources (such as surface water, groundwater, and sewage treatment plant effluent) to efficiently remove typical PFOS concentrations (10–1000 μg / L); it is also environmentally friendly because it uses non-toxic and inexpensive cations (Na+). + / Mg 2+ / Fe 3+ It avoids introducing secondary pollution while ensuring that the membrane has both high rejection rate and high flux; it is specifically designed to resist long-chain PFAS: the membrane structure is customized for the molecular size, charge and hydrophobicity of PFOS (C8) (MWCO matching + high negative charge + bridging), which is superior to most studies that focus on short-chain PFAS. Attached Figure Description

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0036] Figure 1 shows the FT-IR spectra of UF and NF in Examples 1-3.

[0037] Figure 2 shows SEM images of the intermediate film products of Examples 1-3 and Comparative Examples 1-3: (a) SEM image without TiO2UF, (b) SEM image with TiO2UF, (c) SEM image without TiO2NF, (d) SEM image with TiO2NF, and (e) SEM image with Na. + SEM images of the modified nanofiltration membrane, (f) with added Mg 2+ SEM images of modified nanofiltration membranes, (g) with added Fe 3+ SEM image of the modified nanofiltration membrane.

[0038] Figure 3 shows the atomic force microscopy (AFM) images of the intermediate and final membrane products of Examples 1-3 and Comparative Examples 1-3: (a) AFM image of NF without TiO2, (b) AFM image of NF with TiO2, (c) AFM image of modified NF with Na+, and (d) AFM image of NF with Mg. 2+AFM plot of modified NF, (e) with added Fe 3+ AFM diagram of modified NF.

[0039] Figure 4 shows the contact angles of the membranes obtained by varying the TiO2 content and cation concentration based on Examples 1-3: (a) contact angles of nanofiltration membranes with different TiO2 contents, (b) contact angles of membranes after adding different concentrations of Na+, and (c) contact angles after adding different concentrations of Mg. 2+ The contact angle of the back membrane, (d) the addition of different concentrations of Fe 3+ The contact angle of the posterior membrane.

[0040] Figure 5 shows the zeta potential diagrams of nano-TiO2 nanofiltration membranes obtained based on Examples 1-3 with varying TiO2 contents of 0wt%, 3wt%, and 5wt%.

[0041] Figure 6 shows the rejection rate of PEG with different molecular weights by the TiO2 modified nanofiltration membrane in Example 1.

[0042] Figure 7 shows the rejection rate and membrane flux of membranes obtained by changing the amount of TiO2 added (2-7 wt%) and the cation concentration based on Example 1: (a) rejection rate and membrane flux of nanofiltration membrane, (b) flux of membrane after adding different cations, and (c) rejection rate of membrane after adding different cations.

[0043] Figure 8 shows (a) the geometry of PFOS and (b) PFOS and a Na+. + The geometric structure of (c)[Mg(H₂O)₆] 2+ The geometry of (d) Mg bound to a PFOS molecule 2+ The geometry of (e) Mg bound to two PFOS molecules 2+ The geometric structure of ions.

[0044] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0045] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0046] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0047] Example 1

[0048] Preparation of S1 casting solution: Polyethersulfone (PES), polyethylene glycol (PEG600), nano TiO2 (10-100 nm) and N,N-dimethylformamide (DMF) were added to a closed reactor and stirred until homogeneous. The concentration of polyethersulfone was 14 wt%, the concentration of polyethylene glycol was 10 wt%, and the concentration of nano TiO2 was 5 wt%. Then, the solution was placed in an electric heating drying oven at 70 °C for swelling treatment for 24 h. After removal, the solution was stirred again and then degassed under vacuum to obtain a white casting solution.

[0049] S2 base membrane formation: After the casting solution is scraped onto the substrate, it is immediately immersed in a 25°C pure water coagulation bath. After phase inversion, a TiO2 / PES ultrafiltration base membrane is obtained, which is denoted as UF.

[0050] S3 interfacial polymerization: An aqueous solution containing 1 wt% piperazine (PIP) and 0.05 wt% sodium dodecyl sulfate (SDS) was prepared as the aqueous phase, and a hexane solution containing 0.1~1 wt% trimesoyl chloride (TMC) was prepared as the organic phase. The ultrafiltration membrane was immersed in the aqueous phase and the organic phase for 5 min each, so that the interfacial polymerization reaction of piperazine and trimesoyl chloride formed a polypiperazine amide layer on the membrane surface.

[0051] S4 Post-treatment: The ultrafiltration membrane after interfacial polymerization was heat-treated at 70℃ for 15 min, and then washed with deionized water to obtain a TiO2 modified nanofiltration membrane, which is denoted as: NF;

[0052] S5. Immerse the TiO2-modified nanofiltration membrane in deionized water and add Na at a concentration of 15 mmol / L. + After 3 hours of cation soaking, the nanofiltration membrane modified with cationic nano-titanium dioxide was obtained by drying.

[0053] Example 2

[0054] The only difference from Example 1 is the addition of cation Na. + Replace with Mg 2+ The concentration was 15 mmol / L, and the remaining steps were consistent with those in Example 1.

[0055] Example 3

[0056] The only difference from Example 1 is the addition of cation Na. + Replace with Fe 3+ The concentration was 15 mmol / L, and the remaining steps were consistent with those in Example 1.

[0057] Comparative Example 1

[0058] As a control experiment of Example 1, the only difference is that nano-TiO2 was omitted in the casting solution system, and the rest of the steps were the same as in Example 1.

[0059] Comparative Example 2

[0060] As a control experiment for Example 2, the only difference is that nano-TiO2 was omitted from the casting solution system, while the rest of the steps were the same as in Example 2.

[0061] Comparative Example 3

[0062] As a control experiment for Example 3, the only difference is that nano-TiO2 was omitted from the casting solution system, while the rest of the steps were the same as in Example 2.

[0063] The following tests were conducted on the series of cationic nano-titanium dioxide modified nanofiltration membranes prepared in Examples 1-3 with TiO2 and in Comparative Examples 1-3 without TiO2:

[0064] 1. Characterization and performance testing of membranes

[0065] Fourier transform infrared spectroscopy (FT-IR): To demonstrate the successful preparation of the nanofiltration membrane, the nanofiltration membrane was characterized by Fourier transform infrared spectroscopy.

[0066] Scanning electron microscope (SEM): A ZEISS EV0 MA15 scanning electron microscope was used to observe the surface morphology of the nanofiltration membrane.

[0067] Atomic force microscopy (AFM): To investigate the changes in surface roughness of nanofiltration membranes before and after modification, AFM images of the original and modified membrane surfaces were obtained using an interfacial nanoforce spectrometer. AFM was operated in contact mode with a scanning area of ​​5 μm × 5 μm. Rq was defined as the root mean square deviation of surface roughness.

[0068] Contact angle: The contact angle is an important parameter characterizing the hydrophilicity and hydrophobicity of a membrane. In order to investigate the changes in the hydrophilicity of the nanofiltration membrane before and after modification, the contact angle was tested and analyzed on a fully automatic contact angle measuring instrument.

[0069] Zeta potential: To investigate the change in membrane surface potential before and after nanofiltration membrane modification, a solid surface Zeta potential meter was used to measure the surface potential of the nanofiltration membrane. Within a pH range of 5-9, the surface potentials of nanofiltration membranes without and with nano-TiO2 were measured.

[0070] Molecular weight cutoff: To test the molecular weight cutoff of the prepared nanofiltration membrane, this study used a total organic carbon (TOC) analyzer. Filtration experiments were conducted using the prepared nanofiltration membrane to filter PEGs of different molecular weights (200, 400, 600, 800, and 1000 Da). After the flux stabilized, samples of the stock solution and filtrate were taken to determine their TOC values, and the molecular weight cutoff of the nanofiltration membrane was calculated.

[0071] Retention rate: The concentration of PFOS in the solution was determined using high performance liquid chromatography-quadrupole mass spectrometry (HPLC-MS / MS). HPLC system conditions: Aqueous phase: ultrapure water; Organic phase: acetonitrile; Flow rate: 0.2 ml / min; Injection volume: 10 μL; Column temperature: 35℃.

[0072] Retention rate refers to the ability of a nanofiltration membrane to prevent a certain substance from passing through a solution system. By comparing the concentration of PFOS in the solution before and after filtration, and by calculating the retention rate R, the retention effect of the membrane can be observed directly. The formula for calculating the retention rate R is: R = (1 - Cp / C0) × 100%.

[0073] In the formula: R is the rejection rate, %; Cp is the concentration of the original solution, μg / L; C0 is the concentration of the filtrate, μg / L.

[0074] Membrane flux: Membrane flux is an important parameter characterizing membrane performance, referring to the amount of liquid passing through a unit area of ​​membrane surface per unit time under a certain pressure. This study used a sand core filter device connected to a circulating water multi-purpose vacuum pump for membrane flux testing. The membrane flux J was calculated at 25℃ and 0.1 MPa using the formula: J = V / (A × T).

[0075] In the formula: J is the membrane flux, L / (m 2 ·h); V is the filtrate volume, L; A is the effective membrane area, m². 2 T represents the filtering time, in hours.

[0076] 2. Results and Discussion

[0077] (1) Surface chemical structure and composition of the membrane

[0078] The prepared TiO2-modified nanofiltration membrane samples (Examples 1-3) were detected by FT-IR measurements. The results are as follows: Figure 1 As shown, NF and UF membranes at ~2900 cm⁻¹ -1 (Asymmetric CH stretching vibration of the -CH2 group) and ~1460 cm⁻¹ -1 Characteristic peaks were observed at the (CH bending vibration of -CH2), confirming the successful incorporation of PEG600 as a pore-forming agent; a peak appeared at ~1250 cm⁻¹. -1 (Asymmetric stretching vibration of ether bond COC) and 800~860 cm -1 The absorption peak at the (out-of-plane bending vibration of aromatic ring C–H) is consistent with the vibrational mode of the PES backbone, indicating that the polymer structural integrity is maintained during membrane preparation; 500~700 cm⁻¹ -1 The broad absorption peak in the region corresponds to the stretching vibration of Ti-O-Ti, which is a characteristic peak of TiO2 nanoparticles, proving the successful introduction of TiO2. Furthermore, the 1300–1400 cm⁻¹ region... -1The weak peaks within the range are attributed to Ti-OC interactions, indicating partial coordination between TiO2 and the PES matrix. Compared to UF, NF also exhibits a C=O stretching vibration peak around 1700 cm⁻¹ and an NH bending vibration peak around 1640 cm⁻¹. These two peaks are characteristic of amides, indicating the presence of amide groups formed through interfacial polymerization in the nanofiltration membrane. These peaks demonstrate the successful synthesis of the nanofiltration membrane.

[0079] (2) Surface morphology of the membrane

[0080] The surface morphology of UF and NF films without TiO2 (Comparative Examples 1-3) and with TiO2 (Examples 1-3) was analyzed using SEM. The results are as follows: Figure 2 As shown. From Figure 2 As we can see from 'a', the UF film surface without added nano-TiO2 is relatively smooth, with no obvious particle adhesion. And from... Figure 2 As can be seen from 'c', the NF membrane without added nano-TiO2 has a dense surface with many wrinkles. This wrinkled structure may be a polyamide layer formed during interfacial polymerization in the nanofiltration membrane preparation process. From... Figure 2 As can be seen from b and d, both the UF film and the NF film with added nano-TiO2 contain many white particles on their surfaces. Figure 2 (e), (f), and (g) in the text represent the addition of Na, respectively. + Mg 2+ Fe 3+ The surface morphology of the modified nanofiltration membrane is shown in the figure. From the figure, it can be seen that Na... + Mg 2+ Fe 3+ They all accumulate on the membrane surface, forming a white raised structure.

[0081] (3) Surface roughness of the membrane

[0082] The surface roughness of NF films without TiO2 (Comparative Examples 1-3) and with TiO2 (Examples 1-3) was analyzed using AFM. The results are as follows: Figure 3 As shown. Unmodified nanofiltration membrane ( Figure 3 The Rq of a) in the sample was measured to be 43.4 nm, while the TiO2-doped film ( Figure 3 b) shows a significantly higher Rq value, reaching 80.7 nm. The comparison shows that the surface roughness of the NF film with added TiO2 is significantly greater than that of the NF film without added TiO2. This is because the small size and large specific surface area of ​​nano-TiO2 particles, when added to the film surface, will form uneven nanoscale protrusions or particle accumulation, resulting in an increase in surface roughness.

[0083] like Figure 3As shown in c, d, and e, the roughness of the cation (15 mmol / L) modified nanofiltration membrane (Examples 1-3) gradually increases with the increase of ion valence state. The addition of Mg... 2+ Later, Rq was increased to 109nm, while Fe... 3+ Rq was later increased to 153nm; compared with Na + In comparison, Mg 2+ Fe 3+ The surface roughness is significantly increased, forming an irregular morphology on the membrane surface. This is because high-valence ions will combine with more PFOS molecules and accumulate on the membrane surface. Higher surface roughness can increase the specific surface area, enhance the adsorption capacity for specific ions or molecules, and improve the rejection rate to a certain extent. However, excessive roughness may lead to a decrease in flux due to pore blockage.

[0084] (4) Hydrophilicity and hydrophobicity of the membrane surface

[0085] Based on the technical solutions of Examples 1-3, experiments were conducted by simply changing the amount of TiO2 added (2-7 wt%). The contact angles of the TiO2-modified nanofiltration membranes (NF) obtained under different TiO2 contents are as follows: Figure 4 As shown in (a) of the figure, it can be observed that with the increase of TiO2 content, the contact angle of the TiO2-modified nanofiltration membrane first decreases and then increases. The contact angle of the modified membrane with 5 wt% nano-TiO2 decreases to 65°, and the hydrophilicity is significantly improved. This phenomenon is because the introduction of TiO2 initially significantly improves the hydrophilicity of the membrane surface, leading to a decrease in the contact angle. This enhancement of hydrophilicity mainly stems from the high surface energy of TiO2 nanoparticles and their strong interaction with water molecules. However, with further increases in TiO2 content, the interaction between nanoparticles intensifies, leading to aggravated particle aggregation. This aggregation effect not only reduces the effective specific surface area of ​​TiO2 but may also form a micron-scale rough structure on the membrane surface, thereby weakening its hydrophilicity and causing the contact angle to increase again.

[0086] Furthermore, based on the technical solutions of Examples 1-3, the contact angle of the cationic nano-titanium dioxide modified nanofiltration membranes obtained at different cationic concentrations was tested by simply changing the concentration of added cationic ions. The results are as follows: Figure 4As shown in (b), (c), and (d) of the diagram. For a single ion, the contact angle gradually increases with increasing ion concentration. This is because the interaction between ions in solution and the membrane surface (hydrophilic groups -OH) (such as electrostatic adsorption and complexation reactions) is enhanced, altering the chemical composition and charge distribution of the membrane surface, leading to a decrease in the hydrophilicity of the membrane surface and an increase in the contact angle. At the same concentration, the contact angle also gradually increases with increasing ion valence state. This is because the higher the ion valence state, the greater the charge density and the stronger the interaction with the membrane surface. Ions with higher valence states have a more significant destructive effect on the hydrophilicity of the membrane surface, thus resulting in a larger contact angle at the same concentration.

[0087] (5) Zeta potential on the membrane surface

[0088] Further investigation was conducted into the zeta potential changes of TiO2-modified nanofiltration membranes with different amounts of nano-TiO2 (0wt%, 3wt%, and 5wt%), such as... Figure 5 As shown, the overall potential of the nanofiltration membrane increases with increasing pH, and the absolute potential value is higher when 5 wt% nano-TiO2 is added than when 3 wt% nano-TiO2 is added. This is because after adding nano-TiO2, at pH 5-6 (close to the isoelectric point IEP of TiO2 = 5-6), the surface of nano-TiO2 is protonated (-OH2). + TiO2 is positively charged, while its surface is nearly neutral or weakly positively charged, which interacts with the negative charge of the nanofiltration membrane (—COO). - SO2 - Partial neutralization results in a lower absolute value of the overall Zeta potential of the nanofiltration membrane. At pH 7-9 (higher than the IEP of TiO2), the hydroxyl groups (-OH) on the TiO2 surface dissociate into -O. - TiO2 has a negatively charged surface, which synergistically enhances the negative charge with the film substrate, resulting in a more negative Zeta potential. On the other hand, nano-TiO2 particles typically possess hydroxyl groups (—OH) on their surface, which readily dissociate in solution to form Ti—O. - This imparts additional negative charge to the membrane material, and the negative charge increases with pH as the content of nano-TiO2 increases.

[0089] (6) Membrane molecular weight cutoff

[0090] The NF of the TiO2-modified nanofiltration membrane in Example 1 was tested, such as... Figure 6 As shown, the molecular weight of PEG with a 90% rejection rate is approximately 378 Da. Therefore, the molecular weight cutoff of the nanofiltration membrane used in this experiment is approximately 378 Da. The molecular weight of PFOS used in the above examples is approximately 500 Da, demonstrating the feasibility of using this nanofiltration membrane to filter perfluorooctane sulfonic acid (PFOS).

[0091] (7) Membrane rejection rate and membrane flux

[0092] Based on the technical solution of Example 1, experiments were conducted by simply changing the amount of TiO2 added (2-7wt%), and the results are as follows. Figure 7 As shown in (a), with the increase of TiO2 content, the rejection rate and membrane flux of the nanofiltration membrane both show a trend of first increasing and then decreasing. Nano-TiO2 has excellent hydrophilicity and adsorption properties, which can increase the hydrophilicity and roughness of the membrane surface, thus helping to improve the membrane flux. At the same time, the addition of nano-TiO2 can change the membrane structure and increase the negative charge on the membrane surface, thereby enhancing the membrane's ability to retain negatively charged ions (PFOS). When the TiO2 content is too high, nanoparticles are prone to agglomeration, forming larger particles or clusters. This reduces the effective filtration area of ​​the membrane, increases the resistance of water molecules to passing through the membrane, and thus reduces the membrane flux. On the other hand, agglomerated TiO2 particles may also clog the membrane pores, leading to a decrease in rejection rate. When both high rejection rate and high flux are considered, the rejection rate is the highest at 5 wt% nano-TiO2 content, reaching 96.7%, while the membrane flux reaches 58.4 L / (m²·h).

[0093] Based on the technical solutions of Examples 1-3, experiments were conducted by simply changing the concentration of added cations (10-30 mmol / L). Figure 7 (b) for Na + Generally speaking, in the low concentration range, Na + The flux of nanofiltration membranes modified with nano-titanium dioxide has a relatively small impact, but with the increase of Na... + As the concentration increases, the membrane flux gradually decreases. This is mainly because high concentrations of sodium ions increase the osmotic pressure of the solution, thereby reducing the effective driving force and leading to a decrease in membrane flux. For Mg... 2+ Generally speaking, as the ion concentration increases, the membrane flux decreases significantly, mainly due to the high concentration of Mg. 2+ This increases the osmotic pressure of the solution, while Mg 2+ A concentration polarization layer may form on the membrane surface, increasing mass transfer resistance. For Fe... 3+ Generally speaking, in the low concentration range, with Fe 3+ The membrane flux increased slightly with increasing concentration, mainly due to Fe. 3+ The interaction with the negative charge on the membrane surface reduces the charge density on the membrane surface, thereby reducing the electroviscous effect. However, at high concentrations, Fe... 3+ This will lead to a significant decrease in membrane flux, mainly due to Fe 3+ Adsorption and deposition on the membrane surface form a concentration polarization layer, which increases mass transfer resistance.

[0094] Based on the technical solutions of Examples 1-3, further experiments were conducted by simply changing the concentration of added cations (10-30 mmol / L). Figure 7In (c), the rejection rate generally increases with increasing ion concentration. This is because increasing ion concentration promotes the adsorption of ions and PFOS on the membrane surface, forming a thicker fouling layer and enhancing intermolecular hydrophobic interactions, thereby increasing the rejection rate through adsorption or hydrophobic interactions within the membrane pores. On the other hand, the rejection rate also increases with increasing cation valence. This is because higher valence ions have higher charge density and provide bridging effects with PFOS molecules. + Mg can form ionic bonds with the sulfonic acid groups of PFOS molecules, forming PFOS-Na complexes. The formation of these complexes can alter the molecular size and morphology of PFOS, enhancing the membrane's sieving effect and thus increasing the retention rate. 2+ Fe 3+ High-valence ions can form complexes such as PFOS-Mg, PFOS-Mg-PFOS, PFOS-Fe, and PFOS-Fe-PFOS with the sulfonic acid groups of the PFOS molecule (e.g., Figure 8 (As shown).

[0095] To further demonstrate the beneficial technical effects of the present invention, the following comparative experiment was also conducted: Based on the preparation method of Example 1, only the nano TiO2 in the casting solution system was replaced with nano SiO2 or nano ZnO, and the remaining processes and steps were consistent with those of Example 1.

[0096] The experimental results showed that the performance of the final nanofiltration membrane sample prepared by replacing nano TiO2 was far worse than that of the sample in Example 1. The main reasons for this were as follows: (1) TiO2 surface is rich in –OH, and the contact angle can be reduced to 65°, while SiO2 has a lower –OH density and weaker hydrophilicity than TiO2; high SiO2 loading easily leads to a rebound in contact angle, while ZnO has fewer –OH on the surface and is more prone to forming Zn 2+ Dissolution and long-term hydrophilic instability mean that SiO2 and ZnO are not as effective as TiO2 in improving hydrophilicity. (2) In terms of dispersibility and agglomeration threshold, SiO2 has small particle size and large specific surface area, making it very easy to agglomerate. >3 wt% will significantly block the pores. ZnO, on the other hand, has high polarity and poor compatibility with DMF / PES. The casting solution is prone to gelation and defoaming is difficult. All of these factors have a great impact on the membrane retention rate.

[0097] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. A method for preparing a cationic nano-titania modified nanofiltration membrane, characterized in that, The method comprises the following steps: S1, polyether sulfone, polyethylene glycol, nano-TiO2 and N, N-dimethylformamide are added into a sealed reactor, mixed, treated by swelling, and defoamed to obtain a white casting solution; The concentration of the polyether sulfone is 12-16 wt%, the concentration of the polyethylene glycol is 7-13 wt%, and the concentration of the nano-TiO2 is 2-7 wt%. S2, the casting solution is coated on a substrate and then immersed in a pure water coagulation bath to form a membrane through phase inversion to obtain an ultrafiltration base membrane; S3, an aqueous solution containing 0.5-5 wt% piperazine and 0.01-0.1 wt% sodium dodecyl sulfate is prepared as an aqueous phase, a n-hexane solution containing 0.1-1 wt% trimesoyl chloride is prepared as an organic phase, and the ultrafiltration base membrane is immersed in the aqueous phase and the organic phase in sequence; S4, the ultrafiltration base membrane treated in S3 is heat treated at 60-80℃, and then cleaned with deionized water to obtain a nano-TiO2 modified nanofiltration membrane. S5, the nanometer titanium dioxide modified nanofiltration membrane is soaked in deionized water, and at least one cation in Na + , Mg 2 + , Fe 3+ with a concentration of 10-30 mmol / L is added, and the nanometer titanium dioxide modified nanofiltration membrane is obtained by drying.

2. The method for preparing a cationic nano-titania modified nanofiltration membrane according to claim 1, characterized in that, After the swelling treatment in step S1, the white casting solution is obtained through stirring and defoaming.

3. The method for preparing a cationic nano-titania modified nanofiltration membrane according to claim 1, characterized in that, In step S1, the nano-TiO2 is 5 wt%, the polyether sulfone is 14 wt%, and the polyethylene glycol is 10 wt%.

4. The method for preparing a cationic nano-titania modified nanofiltration membrane according to claim 1, characterized in that, The substrate is a glass plate.

5. The method for preparing the cationic-modified nanofiltration membrane of nano-titanium dioxide according to claim 1, characterized in that, The heat treatment time in step S4 is 10-30 min.

6. The method for preparing a cationic nano-titania modified nanofiltration membrane according to claim 1, characterized in that, In the aqueous phase, the concentration of piperazine is 1 wt%, and the concentration of sodium dodecyl sulfate is 0.05 wt%; and / or, in the organic phase, the concentration of trimesoyl chloride is 0.5 wt%.

7. A cationic nanosized titanium dioxide-modified nanofiltration membrane, characterized by, The method is prepared by the preparation method in claims 1-6.

8. The application of the cationic nano-TiO2 modified nanofiltration membrane in claim 7 in water treatment.

9. Use according to claim 7, characterized in that, The cationic nano-TiO2 modified nanofiltration membrane is used for removing perfluorooctane sulfonic acid in water. The cationic nano-TiO2 modified nanofiltration membrane is used for removing perfluorooctane sulfonic acid in water.

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