Layer-by-layer deposition modified ultrafiltration membrane with anti-pollution and antibacterial performance, and preparation method and application thereof

Abstract

This invention discloses a layer-by-layer deposition modified ultrafiltration membrane with both antifouling and antibacterial properties, its preparation method, and its applications. Through layer-by-layer assembly technology, a tannic acid (TA) anion layer and a polyhexamethylene biguanide (PHMB) cation layer are alternately deposited on the surface of an ultrafiltration membrane. Under weakly alkaline conditions, electrostatic interactions, hydrogen bonding, Schiff base reactions, and Michael addition reactions are simultaneously utilized to achieve both non-covalent and covalent bonding, constructing a stable and dense integrated antibacterial-antifouling composite coating. This coating has a contact-type antibacterial structure, which can effectively kill bacteria in water and inhibit biofilm formation at the source. Simultaneously, it significantly improves the hydrophilicity of the membrane surface, reduces pollutant adsorption and deposition, and achieves synergistic enhancement of antibacterial and antifouling properties. The preparation process of this invention is mild, simple, low-cost, and environmentally friendly. The modified membrane exhibits high flux retention, excellent separation performance, and excellent operational stability, and can be widely applied in water treatment scenarios such as municipal wastewater treatment.

Description

Technical Field

[0001] This invention belongs to the field of membrane material modification and water treatment technology, specifically relating to a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties, its preparation method and application. Background Technology

[0002] Ultrafiltration membrane technology is widely used in water treatment fields such as municipal wastewater treatment, drinking water purification, industrial wastewater reuse, and seawater desalination pretreatment due to its advantages such as high separation efficiency, simple operation, and low energy consumption. PSF membranes and PES membranes are commonly used ultrafiltration separation membrane materials in water treatment due to their advantages such as high mechanical strength, good chemical stability, and controllable cost.

[0003] However, in practical applications, microbial contamination and organic contamination often occur synergistically, becoming a core bottleneck restricting the long-term stable operation of membrane technology. On the one hand, bacteria, fungi, and other microorganisms in water bodies easily adhere to and multiply on the membrane surface, forming biofilms, directly leading to a sharp drop in membrane flux, an increase in transmembrane pressure difference, and a deterioration in separation efficiency, significantly increasing the cleaning frequency and shortening the membrane's lifespan. On the other hand, organic pollutants such as proteins and polysaccharides are easily adsorbed and deposited on the membrane surface and within the pores, further aggravating membrane fouling, while simultaneously providing nutrient substrates for microorganisms and accelerating biofilm formation. Especially when treating complex water bodies such as municipal sewage, aquaculture wastewater, hospital wastewater, surface water, and produced water from oil and gas fields, the coupling effect of organic contamination and microbial growth is more prominent, and conventional membrane materials cannot simultaneously meet the requirements of high-efficiency separation, long-term antibacterial effect, and excellent antifouling resistance.

[0004] Existing membrane antibacterial modification technologies generally suffer from poor adaptability and insufficient stability. Modification with inorganic antibacterial agents (such as nano-silver, nano-zinc oxide, and nano-titanium dioxide) is prone to defects such as component leaching, secondary heavy metal contamination, and weak adhesion to the membrane substrate. Modification with single organic antibacterial agents has drawbacks such as narrow antibacterial spectrum, poor long-term effectiveness, and demanding reaction conditions. Traditional physical coating and pure electrostatic layer-by-layer assembly processes rely solely on non-covalent interactions, resulting in coating detachment and rapid attenuation of antibacterial effects. More importantly, most modification methods focus only on antibacterial function and cannot simultaneously improve the membrane's antifouling performance, failing to fundamentally solve the membrane performance degradation problem caused by the superposition of "microbial contamination + organic contamination," thus limiting their application in complex water treatment scenarios.

[0005] Based on this, developing an ultrafiltration membrane with synergistic enhancement of antibacterial and antifouling properties, stable coating, and simple and environmentally friendly process is of great significance for promoting the long-term and stable application of membrane technology in the field of water treatment. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties, as well as its preparation method and applications. The antibacterial ultrafiltration membrane of this invention features a simple process, low cost, excellent and long-lasting antibacterial performance, and high flux retention rate after modification. It can efficiently kill various bacteria in water bodies, inhibiting biofilm formation at the source. It can be widely used in water treatment fields such as municipal sewage treatment, drinking water purification, industrial wastewater reuse, and oil and gas field produced water treatment, possessing significant industrial application value and promising prospects for promotion.

[0007] The technical solution adopted in this invention is as follows: A method for preparing a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties, the method comprising: Step 1: Dissolve tannic acid (TA) and polyhexamethylene biguanide (PHMB) in Tris-HCl buffer solution to prepare TA modified solution and PHMB modified solution respectively, and adjust the pH of the two modified solutions to weakly alkaline. Step 2: The two modified liquids from Step 1 are sequentially deposited on the surface of the ultrafiltration membrane in a single-layer alternating deposition reaction. During the alternating deposition process, TA and PHMB, under weakly alkaline conditions, simultaneously achieve non-covalent and covalent dual bonding through electrostatic interaction, hydrogen bonding, Schiff base reaction and Michael addition reaction, forming an integrated anti-fouling and antibacterial coating. Step 3: Then wash thoroughly with deionized water and air dry at room temperature to obtain the anti-fouling and antibacterial dual-function ultrafiltration membrane.

[0008] Furthermore, the concentration of the TA modified solution is 0.5-3.0 mg / mL, and the concentration of the PHMB modified solution is 1.0-3.0 mg / mL.

[0009] Furthermore, the weakly alkaline pH value range described in step 1 is 7.5-9.0, preferably pH=8.5.

[0010] Furthermore, in step 2, the ultrafiltration base membrane is a polyethersulfone (PES) membrane or a polysulfone (PSF) membrane, preferably a polysulfone (PSF) membrane, with a molecular weight cutoff of 150 kDa and a pore size distribution of 0.03-0.05 μm.

[0011] Furthermore, in the single-layer alternating deposition reaction process, a single modified liquid is poured onto the surface of the ultrafiltration base membrane, so that the single modified liquid repeatedly covers and impregnates the surface of the ultrafiltration base membrane. The impregnation temperature is room temperature, and the impregnation time is 10-60 min, preferably 20-60 min.

[0012] Furthermore, during the alternating deposition reaction, after the first modified solution is impregnated, the residual solution on the surface of the ultrafiltration membrane is poured out, the residual droplets on the surface of the ultrafiltration membrane are scraped off with a roller, and then the membrane is impregnated with the second modified solution.

[0013] Furthermore, the alternating deposition sequence of the single layer is as follows: first deposit TA modified solution, then deposit PHMB modified solution; or first deposit PHMB modified solution, then deposit TA modified solution.

[0014] The present invention also discloses the application of the dual-functional antifouling-antibacterial ultrafiltration membrane in antibacterial and pollution control in water treatment.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) Synergistic effect of anti-pollution and antibacterial properties. The coating achieves both high-efficiency antibacterial and excellent anti-pollution, killing bacteria and inhibiting biofilm formation, while also improving hydrophilicity and reducing the adsorption of organic matter such as BSA. This blocks the coupling and deterioration of microbial pollution and organic pollution at the source, overcoming the shortcomings of traditional modification that only provides single antibacterial properties.

[0016] 2) High coating stability. TA and PHMB are bonded together through both non-covalent and covalent bonding, resulting in strong adhesion, resistance to peeling, and long-term stable antibacterial and anti-fouling effects, overcoming the problem of easy attenuation in traditional electrostatic assembly.

[0017] 3) The preparation process is simple and the cost is controllable. By adopting a layer-by-layer deposition process and optimizing the performance by adjusting the concentration ratio of PHMB to TA, the preparation steps are greatly simplified, the cycle is short, and the reaction conditions are mild, making it suitable for industrial-scale mass production.

[0018] 4) Environmentally friendly and biocompatible. It adopts a heavy metal-free organic antibacterial system, PHMB is low in toxicity and leaves no residue, and TA is a natural biomass component. There are no organic solvents or harsh reactions throughout the process, and the treatment process will not cause secondary pollution to water bodies, making it environmentally compliant. Attached Figure Description

[0019] Figure 1 These are the FTIR spectra of the TA / PHMB-PSF membrane of Example 1 and the PSF membrane of Comparative Example 1; Figure 2 The water contact angles are those of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1. Figure 3 The pure water flux of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1; Figure 4 The normalized flux of the membranes is compared with that of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1 for bovine serum albumin (BSA). Figure 5 This is a comparison of the antibacterial effects of the TA / PHMB-PSF membranes prepared in Examples 1, 2, and 3. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments.

[0021] The porous polysulfone ultrafiltration membrane (PSF) used in the embodiments and comparative examples of this invention has a pore size distribution of 0.03-0.05 μm and a molecular weight cutoff of 15 WDa. In the embodiments of this invention, the molecular weight of PHMB is 600 Da.

[0022] Comparative Example 1: A conventional porous polysulfone ultrafiltration membrane (PSF) was selected and used directly for structural characterization and performance testing without any modification treatment.

[0023] Example 1: A method for preparing a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties. Base membrane pretreatment: The polysulfone ultrafiltration base membrane was ultrasonically cleaned in deionized water for 30 min to remove residual impurities and contaminants on the membrane surface, and then soaked in deionized water for later use. Preparation of assembly solutions: Dissolve PHMB and TA separately in Tris-HCl buffer solution to prepare TA solution with a concentration of 1.0 mg / mL and PHMB solution with a concentration of 2.0 mg / mL. Adjust the pH of the two solutions to 8.5 with NaOH and stir until completely dissolved. Layer-by-layer deposition: The pretreated ultrafiltration substrate membrane was fixed on a glass plate, and 20 mL of TA solution was poured onto the membrane surface. The membrane was immersed at room temperature for 30 min, and the residual solution was poured out. The residual droplets on the membrane surface were scraped off with a roller. Then, 20 mL of PHMB solution was poured onto the membrane surface, and the membrane was immersed at room temperature for 30 min. After the reaction, the membrane surface was repeatedly rinsed with deionized water to remove unbound free components. The membrane was then air-dried at room temperature to obtain the modified polysulfone membrane, which was labeled as TA / PHMB-PSF membrane.

[0024] Example 2: A method for preparing a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties. The polysulfone ultrafiltration membrane was ultrasonically cleaned in deionized water for 30 minutes to remove residual impurities and contaminants from the membrane surface, and then soaked in deionized water for later use. Preparation of assembly solutions: Dissolve PHMB and TA separately in Tris-HCl buffer solution to prepare TA solution with a concentration of 0.5 mg / mL and PHMB solution with a concentration of 2.0 mg / mL. Adjust the pH of the two solutions to 8.5 with NaOH and stir until completely dissolved. Layer-by-layer deposition: The pretreated ultrafiltration substrate membrane was fixed on a glass plate, and 20 mL of TA solution was poured onto the membrane surface. The membrane was immersed at room temperature for 60 min, and the residual solution was poured out. The residual droplets on the membrane surface were scraped off with a roller. Then, 20 mL of PHMB solution was poured onto the membrane surface, and the membrane was immersed at room temperature for 60 min. After the reaction, the membrane surface was repeatedly rinsed with deionized water to remove unbound free components. The membrane was then air-dried at room temperature to obtain the modified polysulfone membrane, which was labeled as TA / PHMB-PSF membrane.

[0025] Example 3: A method for preparing a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties. The polysulfone ultrafiltration membrane was ultrasonically cleaned in deionized water for 30 minutes to remove residual impurities and contaminants from the membrane surface, and then soaked in deionized water for later use. Preparation of assembly solutions: Dissolve PHMB and TA separately in Tris-HCl buffer solution to prepare TA solution with a concentration of 2.0 mg / mL and PHMB solution with a concentration of 2.0 mg / mL, respectively. Adjust the pH of the two solutions to 8.0 with NaOH and stir until completely dissolved. Layer-by-layer deposition: The pretreated ultrafiltration substrate membrane was fixed on a glass plate, and 20 mL of TA solution was poured onto the membrane surface. The membrane was immersed at room temperature for 20 min, and the residual solution was poured out. The residual droplets on the membrane surface were scraped off with a roller. Then, 20 mL of PHMB solution was poured onto the membrane surface, and the membrane was immersed at room temperature for 20 min. After the reaction, the membrane surface was repeatedly rinsed with deionized water to remove unbound free components. The membrane was then air-dried at room temperature to obtain the modified polysulfone membrane, which was labeled as TA / PHMB-PSF membrane.

[0026] Performance characterization and test results: The membrane sample structure was characterized using FTIR spectroscopy. The FTIR spectra of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1 are shown below. Figure 1 As shown. The original PSF sample at 1558 cm⁻¹ -1 1458 cm -1 1341 cm -1 and 1145cm -1 Characteristic absorption peaks appear at [location], corresponding to aromatic C=C stretching vibrations, benzene ring skeletal vibrations, and sulfonyl asymmetric and symmetric O=S=O stretching vibrations, respectively. Additionally, at 10¹³ cm⁻¹... -1 788 cm -1 and 671 cm -1 The absorption peak at 1645 cm⁻¹ corresponds to the aromatic C–H bending vibration. The TA / PHMB-PSF membrane modified with TA and PHMB in Example 1 showed an absorption peak at 1645 cm⁻¹. -1 and 1318 cm-1 The appearance of new peaks corresponds to the stretching vibrations of the C=N double bonds generated by the Schiff base reaction and the CN bonds generated by the Michael addition reaction, respectively. This indicates that during the deposition of TA and PHMB on the film surface, the Schiff base reaction and Michael addition reaction successfully occurred, forming a covalent bond structure and resulting in a strong coating adhesion.

[0027] The hydrophilicity of the membrane surface was characterized by contact angle measurement. The water contact angle results of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1 are as follows: Figure 2 As shown. Compared to the unmodified PSF membrane in Comparative Example 1, the modified membrane in Example 1 has a significantly reduced surface contact angle and a greatly improved surface hydrophilicity, which is beneficial for mitigating membrane fouling.

[0028] Pure water flux: All membrane samples were pre-pressurized with pure water at an operating pressure of 2 bar for 1 h. The test operating pressure was set to 1 bar, the water temperature was controlled at 25°C, and the flow rate was 35 LPH. Water was collected and its volume measured within the same test time, and the pure water flux was calculated. The pure water flux test results of the TA / PHMB-PSF membrane in Example 1 and the PSF membrane in Comparative Example 1 are as follows: Figure 3 As shown. The membrane flux of the modified Example 1 decreased, mainly due to the deposition of a hydrophilic coating on the membrane surface and part of the inner wall of the pores, resulting in a slight reduction in the effective filtration pore size.

[0029] BSA retention and antifouling performance: All membrane samples were pre-pressurized with pure water at an operating pressure of 2 bar for 1 h. A 100 mg / L BSA aqueous solution was then prepared. Under the conditions of 1 bar operating pressure, 25℃ water temperature, and 35 LPH flow rate, the BSA solution was continuously filtered for 30 min. The absorbance of the feed solution and permeate was measured at 280 nm using a UV spectrophotometer. The BSA rejection rate was calculated. The test results are shown below. Figure 4 As shown in the figure, the BSA rejection rate of the unmodified membrane in Comparative Example 1 was only 72.17%, while the BSA rejection rate of the modified membrane in Example 1 increased to 96.91%, demonstrating a significant improvement in rejection performance. This indicates that the modified coating, while imparting antibacterial properties, can also enhance the membrane's ability to retain pollutants. Furthermore, after 1 hour of BSA filtration, the normalized flux results of the tested membranes are shown in the figure. Figure 4 The normalized flux of a membrane refers to the ratio of the membrane flux at a certain moment to the initial flux of the membrane. The normalized flux of the unmodified membrane in Comparative Example 1 is only 36.72%, while the normalized flux of the modified membrane in Example 1 is 73.88%. This indicates that the modified coating can significantly alleviate the adsorption and deposition of BSA on the membrane surface, reduce the degree of membrane fouling, effectively delay the flux decay during the filtration process, and enable the membrane to exhibit better antifouling performance and flux stability in long-term operation.

[0030] Antibacterial efficiency: Indigenous bacteria isolated and cultured from shale gas fracturing flowback fluid in a certain region of Inner Mongolia were used as model bacteria. The plate count method was used to evaluate the antibacterial performance of the modified membranes in the comparative example and Examples 1, 2, and 3. The specific experimental steps are as follows: a 6×10⁻⁶ concentration was prepared using 1 / 500 nutrient broth. 5 A bacterial suspension of CFU / mL was prepared, and each membrane size was cut into 30×30mm pieces. 2 A square membrane was prepared, and 0.2 mL of bacterial suspension was added to the membrane surface. The membrane was then covered with another membrane and incubated at 30°C and ≥95% relative humidity for 24 h. After incubation, 10 mL of SCDLP broth was added to recover the bacteria, and 10-fold serial dilutions were performed to determine the bacterial count. The solution was diluted with phosphate-buffered saline. 0.2 mL of the diluted solution was used for viable culture using the pour plate method. After another 24 h of incubation, the colonies on the plates were counted to calculate the antibacterial activity of the corresponding membrane. Results are as follows: Figure 5 As shown, bacteria proliferated extensively on the surface of the unmodified membrane in the comparative example, with dense colonies. In Examples 1, 2, and 3, no obvious colony growth was observed on the plates after the modified membrane treatment, and the antibacterial efficiency reached over 99%. This proves that TA and PHMB successfully introduced antibacterial active functional groups into the membrane surface, which have a strong killing effect on the bacteria in the backflow solution and can effectively reduce the microbial contamination of subsequent membrane processes.

[0031] Stability test of modified membrane coating: The TA / PHMB-PSF membrane of Example 1 was first pre-pressed with pure water at an operating pressure of 2 bar for 1 h, and then the pure water was filtered and tested. The TOC concentration of the pure water was 0.22-0.47 mg / L. The test operating pressure was set to 1 bar, the water temperature was controlled at 25℃, and the flow rate was 35 LPH. The TOC concentration of the membrane effluent at different times is shown in Table 1.

[0032] As shown in Table 1, the TOC concentration in the effluent was relatively low within a 9-hour testing period, indicating that the TA / PHMB coating on the membrane surface was highly stable and did not detach. This is because PHMB has a molecular weight of only 600 Da, and TA is a small-molecule organic compound. If the coating detaches, the PSF membrane cannot effectively retain it, and these organic compounds will enter the effluent, thus significantly increasing the TOC concentration.

[0033] Table 1 .

[0034] The above embodiments are merely some preferred embodiments of the present invention, used to clearly illustrate the technical solutions and effects of the present invention, and are not intended to limit the implementation methods. Any other obvious modifications and variations that can be made by those skilled in the art based on the technical principles of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties, characterized in that, The method includes: Step 1: Dissolve tannic acid (TA) and polyhexamethylene biguanide (PHMB) in Tris-HCl buffer solution to prepare TA modified solution and PHMB modified solution respectively, and adjust the pH of the two modified solutions to weakly alkaline. Step 2: The two modified liquids from Step 1 are sequentially deposited on the surface of the ultrafiltration membrane in a monolayer alternating deposition reaction. During the alternating deposition process, TA and PHMB, under weakly alkaline conditions, simultaneously achieve non-covalent and covalent dual binding through electrostatic interaction, hydrogen bonding, Schiff base reaction and Michael addition reaction, forming a stable antibacterial coating on the surface of the ultrafiltration membrane. Step 3: Then wash thoroughly with deionized water and air dry at room temperature to obtain the layer-by-layer deposited modified antibacterial ultrafiltration membrane.

2. The preparation method according to claim 1, characterized in that, The concentration of TA-modified solution is 0.5-3.0 mg / mL, and the concentration of PHMB-modified solution is 1.0-3.0 mg / mL.

3. The preparation method according to claim 1, characterized in that, The weakly alkaline pH range mentioned in step 1 is 7.5-9.

0.

4. The preparation method according to claim 1, characterized in that, Step 2: The ultrafiltration base membrane is a polyethersulfone (PES) membrane or a polysulfone (PSF) membrane.

5. The preparation method according to claim 1, characterized in that, In the single-layer alternating deposition process, a single modified liquid is poured onto the surface of the ultrafiltration base membrane, and the surface of the ultrafiltration base membrane is repeatedly covered and impregnated by the single modified liquid. The impregnation temperature is room temperature and the impregnation time is 10-60 min.

6. The preparation method according to claim 1, characterized in that, The alternating deposition sequence of the single layer is as follows: first deposit TA modified solution, then deposit PHMB modified solution; or first deposit PHMB modified solution, then deposit TA modified solution.

7. A layer-by-layer deposition modified ultrafiltration membrane with both antifouling and antibacterial properties, characterized in that, Prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the layer-by-layer deposition modified ultrafiltration membrane with both anti-fouling and antibacterial properties as described in claim 7 in water treatment.

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