Method and application for preparing multi-walled carbon nanotube electrocatalytic membrane based on swelling effect

Abstract

The present invention belongs to the technical field of sewage treatment, and discloses a method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect and its application. The method comprises the following steps: S1, pretreatment of a PVDF-based membrane; S2, swelling filling, using a multi-walled carbon nanotube to prepare a swelling dispersion liquid for performing a first modification treatment on the pretreated PVDF-based membrane to obtain a swollen membrane; S3, filtration coating, using a multi-walled carbon nanotube to prepare a loading liquid for performing a second modification treatment on the swollen membrane to prepare a multi-walled carbon nanotube electrocatalytic membrane; the multi-walled carbon nanotube electrocatalytic membrane prepared by the above method can be used to remove ibuprofen in the water environment in an electrocatalytic membrane filtration system. The multi-walled carbon nanotube electrocatalytic membrane prepared by the present invention can enhance the electrochemical performance of the CNTs-PVDF membrane, and the network structure formed on the surface of the PVDF-based membrane can improve the removal effect on ibuprofen; the electrochemical removal efficiency for ibuprofen solutions with different initial concentrations is relatively high. Among them, the removal rate for low-concentration ibuprofen solutions exceeds 90%.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment, and particularly relates to a method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect and its application. Background Art

[0002] In daily life, there are various pharmaceutical organic compounds, and the human body's absorption and metabolism of these compounds are incomplete. This directly causes the unmetabolized pharmaceutical active ingredients to enter the water environment through various channels. Ibuprofen (IBU) has antipyretic, analgesic, anti-inflammatory and other effects. Its accumulation in water not only affects the water environment, but may ultimately pose a threat to human health as it is transmitted through the food chain.

[0003] Traditional sewage treatment processes cannot effectively decompose pharmaceutical organic compounds. Therefore, advanced oxidation processes (AOPs) such as photocatalysis, Fenton oxidation method and electrochemical oxidation method have been widely applied in recent years for the removal of pharmaceutical organic matters such as IBU. Chinese invention patent (CN201610279013.6) uses titanium dioxide as a catalytic material to remove ibuprofen. The catalyst used in this method has a high price, increasing the process cost of material preparation, and the control requirements during the reaction process are strict, especially in terms of the control of the light source; Chinese invention patent (CN202210843995.2) uses CuFe bimetallic single atoms as a catalyst to remove ibuprofen. In this method, the heating rate control during calcination is strict, and there are strict requirements for the weight ratio of the added nitrogen-containing organic matter; the short-term removal rate of ibuprofen under different pH conditions varies significantly. The above methods need to introduce new catalytic oxidation materials / metal particles, and the initial concentration used for removing ibuprofen is relatively high, which does not conform to the actual application. The concentration of ibuprofen in sewage is generally relatively low, about several tens of micrograms per liter of sewage, and the concentration of ibuprofen in sewage in some areas is about 2 mg / L. Therefore, there is an urgent need to develop a method for removing ibuprofen with a simple preparation process, small influencing factors and wide application conditions.

[0004] Electrochemical oxidation method is to generate strong oxidants such as ·OH and H2O2 through the gain and loss of electrons of electrolytes under the condition of an external power source, so as to remove the refractory organic substances in water. Electrochemical advanced oxidation technology includes electrochemical separation technology and electrocatalytic oxidation technology. Among them, electrochemical separation technology can combine membrane filtration adsorption and electrochemical degradation of organics to remove refractory organics, thereby reducing the reaction time and energy consumption.

[0005] The electrocatalytic membrane filtration system combines electrocatalytic technology with membrane filtration technology. Through the electrocatalytic process, pollutants undergo oxidation-reduction reactions, and at the same time, the membrane separation function is utilized to achieve efficient filtration and separation of substances. The electrodes in the electrocatalytic process can promote the degradation of pollutants, reduce the deposition of pollutants on the membrane surface, slow down membrane fouling, and improve the filtration efficiency and the service life of the membrane. The advantages of the electrocatalytic membrane filtration device include efficient degradation of organic pollutants, inhibition of biological fouling, and reduction in the use of chemical agents.

[0006] During the membrane filtration process, membrane reactors are usually affected by their structure, and carbon nanotubes (CNTs) are widely used in membrane surface modification due to their excellent electronic, mechanical, and chemical properties. These properties endow the membranes constructed based on CNTs with a high specific surface area and excellent electrochemical activity, making the combination of CNT membranes and electrochemical degradation technology an effective advanced treatment process that can effectively remove micro-pollutants such as antibiotics in the water environment. The swelling effect refers to the phenomenon that membrane materials absorb liquid and swell in a specific solvent or solution. Through this process, the pore structure and the arrangement of molecular chains of the membrane may change, thereby enhancing its conductivity. By means of the swelling effect, CNTs are in-situ filled in the pores of polyvinylidene fluoride (PVDF) membranes, which can, on the one hand, further enhance the electrochemical performance of CNTs-PVDF membranes, and on the other hand, alleviate the problem of pore blockage.

[0007] Therefore, the present invention provides a method for preparing a multi-walled carbon nanotube electrocatalytic membrane through the swelling effect. The multi-walled carbon nanotube electrocatalytic membrane not only has excellent pollutant removal ability but also has the ability to alleviate membrane fouling effects. Summary of the Invention

[0008] The present invention aims to provide a method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on the swelling effect. The multi-walled carbon nanotube electrocatalytic membrane prepared by this method can effectively decompose pharmaceutical organic compounds in the water environment when used in the water environment, and has excellent pollutant removal ability and the ability to alleviate membrane fouling effects.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] The present invention provides a method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on the swelling effect, including the following steps:

[0011] S1. Pretreatment of the PVDF-based membrane;

[0012] S2. Swelling filling, using a multi-walled carbon nanotube to prepare a swelling dispersion liquid to perform the first modification treatment on the PVDF-based membrane pretreated in step S1 to obtain a swollen membrane;

[0013] S3. Filter coating: Use a multi-walled carbon nanotube formulated loading solution to perform a second modification treatment on the swollen membrane in step S2 to obtain a multi-walled carbon nanotube electrocatalytic membrane.

[0014] Preferably, the method for pre-treating the PVDF-based membrane is to wash the PVDF-based membrane with absolute ethanol and ultrapure water in sequence.

[0015] It should be noted here that: absolute ethanol is used to remove dust and organic substances on the surface of the PVDF-based membrane, while ultrapure water is used to remove the residual ethanol on the surface of the PVDF-based membrane; the treated PVDF-based membrane needs to be soaked in ultrapure water and stored at low temperature for standby, and the ultrapure water should be replaced at least once every 24 hours.

[0016] Preferably, the method for the first modification treatment is to perform a water bath ultrasonic treatment on the formulated swollen dispersion liquid first, and then put the PVDF-based membrane pretreated in step S1 into the ultrasonically treated swollen dispersion liquid and continue ultrasonic treatment to obtain a swollen membrane.

[0017] Preferably, in step S2, the method for preparing the swollen dispersion liquid is to add multi-walled carbon nanotubes to xylene.

[0018] Preferably, the mass-volume ratio of the multi-walled carbon nanotubes to xylene is 1 - 3 mg: 1 ml.

[0019] Preferably, the type of the multi-walled carbon nanotubes is any one of conventional carbon nanotubes, carboxylated carbon nanotubes or hydroxylated carbon nanotubes.

[0020] Preferably, the diameter of the multi-walled carbon nanotubes is 10 - 60 nm and the length is < 5 μm.

[0021] Preferably, the temperature of the ultrasonic treatment is 60 ± 5 °C, the time for performing the water bath ultrasonic treatment on the formulated swollen dispersion liquid is 30 - 45 min, and the time for continuing the ultrasonic treatment after putting the PVDF-based membrane into the ultrasonically treated swollen dispersion liquid is 60 - 90 min.

[0022] Preferably, in step S3, before the swollen membrane undergoes filter coating, the surface of the swollen membrane needs to be rinsed with absolute ethanol and ultrapure water in sequence.

[0023] It should be noted here that: absolute ethanol is used to remove xylene on the surface of the swollen membrane, while ultrapure water is used to remove the residual ethanol on the surface of the swollen membrane.

[0024] Preferably, in step S3, the method for preparing the loading solution is to place multi-walled carbon nanotubes with a diameter of 40 - 60 nm in an ethanol solution with a volume fraction of 45% - 55%, and perform ultrasonic treatment for 30 - 45 min. The mass ratio of the multi-walled carbon nanotubes in the loading solution to the multi-walled carbon nanotubes in the swelling dispersion is 5:4 - 12.

[0025] Preferably, in step S3, the method for the second modification treatment is to perform constant pressure filtration at a pressure of 0.09 - 0.11 MPa by negative pressure suction filtration, so that the multi-walled carbon nanotubes are evenly attached to the surface of the swelling membrane to obtain a multi-walled carbon nanotube electrocatalytic membrane.

[0026] The present invention also provides a multi-walled carbon nanotube electrocatalytic membrane prepared by the above method.

[0027] The present invention also provides the application of the above multi-walled carbon nanotube electrocatalytic membrane in removing pharmaceutical organic compounds in the water environment in an electrocatalytic membrane filtration system. The initial pH value of the water environment is 3 - 11, and the pharmaceutical organic compounds include but are not limited to ibuprofen.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The multi-walled carbon nanotube electrocatalytic membrane prepared by the present invention through the swelling effect can enhance the electrochemical performance of the CNTs-PVDF membrane due to the oxygen-containing groups on the surface of the multi-walled carbon nanotubes, thereby improving the removal effect on IBU. Moreover, the effect of carboxylated and hydroxylated carbon nanotubes is better because after carboxylation and hydroxylation, the carbon nanotubes are not easily agglomerated and have good dispersibility, and can maintain a large dispersion degree in the swelling solution, thus forming more network structures on the surface of the PVDF-based membrane;

[0030] 2. The electrocatalytic removal efficiency of the multi-walled carbon nanotube electrocatalytic membrane prepared by the present invention for low-concentration IBU solutions all exceeds 90%, which indicates that the COOH@CNTs-PVDF modified membrane can efficiently remove IBU under low-concentration pollutant conditions and shows good application potential; and the electrocatalytic removal efficiency of the multi-walled carbon nanotube membrane prepared by the invention for high-concentration IBU solutions is all greater than 70%. Compared with the original membrane, the optimal multi-walled carbon nanotube membrane can significantly increase by 63.22%. The above results show that the multi-walled carbon nanotube electrocatalytic membrane prepared by the present invention has a significant effect on the removal of IBU in sewage compared with the original membrane, and its operation stability is also significantly better than the original membrane. Description of the Drawings

[0031] Figure 1 It is the SEM image of the PVDF-based membrane used in the present invention;

[0032] Figure 2SEM images of electrocatalytic membranes A-C prepared from conventional carbon nanotubes under different modification conditions (a is electrocatalytic membrane A; b is electrocatalytic membrane B; c is electrocatalytic membrane C) of the present invention;

[0033] Figure 3 SEM images of electrocatalytic membranes D-F prepared from conventional carbon nanotubes under different modification conditions (d is electrocatalytic membrane D; e is electrocatalytic membrane E; f is electrocatalytic membrane F) of the present invention;

[0034] Figure 4 SEM images of electrocatalytic membranes G-I prepared from conventional carbon nanotubes under different modification conditions (g is swollen membrane G; h is swollen membrane H; i is swollen membrane I) of the present invention;

[0035] Figure 5 Surface images of the PVDF-based membranes of the present invention after swelling with COOH-MWCNTs, OH-MWCNTs, and MWCNTs taken with an InLens detector (a is electrocatalytic membrane A, b is electrocatalytic membrane J, c is electrocatalytic membrane M);

[0036] Figure 6 Cross-sectional images of the PVDF-based membranes of the present invention after swelling with COOH-MWCNTs, OH-MWCNTs, and MWCNTs taken with an SE2 detector (a is electrocatalytic membrane A, b is electrocatalytic membrane J, c is electrocatalytic membrane M);

[0037] Figure 7 Comparison diagram of the operation stability of the PVDF-based membranes of the present invention after swelling modification with COOH-MWCNTs;

[0038] Figure 8 Linear sweep voltammetry curves and cyclic voltammetry curves of the membranes modified with different functional groups of the present invention;

[0039] Figure 9 EIS spectra of the membranes modified with different functional groups of the present invention;

[0040] Figure 10 Comparison diagram of the removal effect of IBU under different pH conditions of the present invention;

[0041] Figure 11 Comparison diagram of the removal effect of IBU under different voltage conditions of the present invention;

[0042] Figure 12 Comparison diagram of the removal effect of IBU under high concentration conditions of the present invention;

[0043] Figure 13 Comparison diagram of the removal effect of IBU under low concentration conditions of the present invention. Detailed implementation manners

[0044] The present invention will be further described below with reference to the following examples, but it should not be understood that the scope of the present invention is limited to the following examples. Without departing from the above technical ideas of the present invention, various substitutions and modifications can be made according to common technical knowledge and customary means in the art, and all should be included in the scope of protection of the present invention.

[0045] Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0046] Example:

[0047] A method for preparing a multi-walled carbon nanotube electrocatalytic film based on the swelling effect comprises the following steps:

[0048] S1. PVDF base membrane pretreatment

[0049] First, clean the PVDF base membrane with anhydrous ethanol to remove dust and organic matter on the surface of the PVDF base membrane; then wash it with ultrapure water three times to remove residual ethanol on the surface of the PVDF base membrane; finally, soak the PVDF base membrane in ultrapure water and store it in a low-temperature environment, and ensure that the water is replaced every 24 hours.

[0050] S2, swelling and filling, performing the first modification treatment on the PVDF base film pretreated in step S1

[0051] First, an appropriate amount of multi-walled carbon nanotubes (MWCNTs) were added to 40 ml of xylene (see Table 1 for the amount, specifications, and dimensions of the multi-walled carbon nanotubes (MWCNTs)). A swelling dispersion was prepared. The swelling dispersion was then ultrasonically treated in a 60°C water bath for 30 min. The pretreated PVDF-based membrane was then placed in the swelling solution and ultrasonicated for another 60 min, for a total of 90 min. After the ultrasonication, the membrane was cooled to room temperature and the swollen membrane surface was rinsed with anhydrous ethanol to remove residual xylene. Finally, the membrane was washed three times with ultrapure water to remove the ethanol to obtain a swollen membrane. The swollen membrane was stored in a sealed bag containing a small amount of pure water for later use.

[0052] S3, filtering and coating, performing a second modification treatment on the swollen membrane in step S2

[0053] 50 mg of multi-walled carbon nanotubes with a diameter of 40-60 nm were placed in a 50% volume fraction ethanol solution and ultrasonically treated for 30 minutes to prepare a loading liquid; through negative pressure filtration, constant pressure filtration was performed at a pressure of 0.1 MPa to make the multi-walled carbon nanotubes evenly adhere to the surface of the swollen membrane to obtain a multi-walled carbon nanotube electrocatalytic membrane.

[0054] Table 1 Swelling modified dispersion ratio parameters of different swelling loaded membranes

[0055]

[0056] Note: E0 is the original CNTs-PVDF membrane (only loaded and not swollen). During the modification process, the purity of carboxyl and hydroxyl groups in carboxylated and hydroxylated carbon nanotubes is > 95 wt%.

[0057] Test examples:

[0058] 1 Experimental method

[0059] 1.1 Experimental equipment

[0060] The electrochemical membrane reactor of the experimental device involved in this test example is designed with an acrylic shell. Its internal components include a perforated titanium mesh, an insulating silicone gasket, a titanium ring, a support gasket, and an electrocatalytic membrane sheet prepared by modifying a PVDF membrane. Among them, the titanium sheet and the perforated titanium mesh (purity > 99%) have a thickness of 2 mm. The experimental conditions for the electrochemical membrane filtration are all carried out with an initial pH value of 7, 0.01 mol / L Na2SO4 as the background electrolyte, an initial IBU of 25 mg / L, and an applied voltage of 2 V for the experiment.

[0061] 1.2 Characterization of the morphological structure of multi-walled carbon nanotube electrocatalytic membranes - Scanning electron microscopy

[0062] Sample preparation method: Scrape the surface of the different multi-walled carbon nanotube electrocatalytic membranes prepared in the examples, and fix the exfoliated multi-walled carbon nanotube powder to a copper sample stage. Use a SIGMA HD scanning electron microscope to analyze the morphological structure changes of the multi-walled carbon nanotube electrocatalytic membranes before and after modification.

[0063] Take a small amount of thin film sample and directly stick it to the conductive adhesive, and use a Quorum SC7620 sputtering coater to spray gold for 45 s with a spraying current of 10 mA; then use a ZEISS Sigma 300 scanning electron microscope to take pictures of the sample morphology, EDS energy spectrum, etc. The acceleration voltage is 3 kV when taking the morphology pictures.

[0064] 1.3 Characterization of the electrochemical performance of multi-walled carbon nanotube electrocatalytic membranes

[0065] 1.3.1 Linear voltammetry curve scanning

[0066] The LSV curve scanning adopts a three-electrode working system of a working electrode, a reference electrode, and a counter electrode. Use a platinum sheet (2 cm × 2 cm) electrode as the counter electrode, an Ag / AgCl electrode as the reference electrode, and the working electrode is the multi-walled carbon nanotube electrocatalytic membrane prepared in the example. The scanning voltage range of the Shanghai Chenhua CHI760E electrochemical workstation is 0~2.5 V, the scanning speed is 0.02 V / s, and the test solution uses 500 mL of H2SO4 with a concentration of 0.5 mol / L.

[0067] 1.3.2 Cyclic Voltammetry Curve Scanning

[0068] For the CV curve scanning, a three - electrode working system consisting of a working electrode, a reference electrode, and a counter electrode is used. The platinum sheet (2 cm × 2 cm) electrode is used as the counter electrode, the Ag / AgCl electrode is used as the reference electrode, and the working electrode is the multi - walled carbon nanotube electrocatalytic membrane prepared in the example. The scanning voltage range is 0 - 1.5 V, the scanning rate is 2.0 mV / s, and the test solution is a mixed solution of 25 mg / L IBU and 0.5 mol / L Na2SO4.

[0069] 1.3.3 Electrochemical Impedance Analysis

[0070] For the EIS curve analysis, a three - electrode working system consisting of a working electrode, a reference electrode, and a counter electrode is used. The working electrode is the multi - walled carbon nanotube electrocatalytic membrane prepared in the example, the platinum sheet (2 cm × 2 cm) electrode is used as the counter electrode, and the Ag / AgCl electrode is used as the reference electrode. The test environment is a Na2SO4 solution with a concentration of 0.5 mol / L, the test voltage is an AC voltage of 5.0 - 10 mV, and the frequency range is 10 5 Hz~10 -2 Hz.

[0071] 1.4 Performance Study of Multi - walled Carbon Nanotube Electrocatalytic Membrane

[0072] 1.4.1 Flux Comparison

[0073] To measure the pure water flux of the membrane, nitrogen is pressurized to transport pure water into the membrane module, and the transmembrane pressure is monitored through a pressure gauge. An electronic balance is connected to a notebook to record the volume of the effluent water in real - time. After the transmembrane pressure reaches a stable state, the pure water flux is calculated through a formula.

[0074] 1.4.2 Comparison of the Removal Effect on IBU

[0075] The experimental device consists of four parts: an inlet - outlet water system, a power system, an electrocatalytic membrane module, and a power supply system. A customized acrylic water storage device is used as the feed tank, and a beaker is used as the effluent container to form the inlet - outlet water system; a nitrogen cylinder, an adjustable gas valve, and supporting pipelines provide power for the water sample to enter the membrane module; a DC power supply provides a stable voltage. The simulated IBU wastewater is injected into the feed tank in advance, and is pressurized by nitrogen to flow to the water inlet at the top of the reactor, where adsorption and electrochemical oxidation reactions occur inside the reactor, and then flows into the effluent container through the water outlet at the bottom of the reactor. The experiment explores the influence of different influencing parameters on the removal effect by changing the solution pH value, the applied voltage magnitude, and the initial pollutant concentration.

[0076] 1.4.3 Comparison of the Removal Effect on IBU under Different pH Value Conditions

[0077] When the initial pH values of the solution were selected as 3.0, 5.0, 7.0, 9.0, and 11.0, 0.01 mol / L of Na2SO4 was used as the background electrolyte, with an initial IBU of 25 mg / L, and an external voltage of 2 V was applied for the experiment.

[0078] 1.4.4 Comparison of the removal effects of IBU under different voltage conditions

[0079] When the externally applied voltage for the reaction was selected as 0 V, 1 V, 2 V, and 3 V, 0.01 mol / L of Na2SO4 was used as the background electrolyte, with an initial IBU of 25 mg / L and an initial pH value of 7 for the experiment.

[0080] 1.4.5 Comparison of the removal effects of IBU under different concentration conditions

[0081] When the initial IBU concentration values of the solution were selected as high concentrations of 10 mg / L, 25 mg / L, 50 mg / L and low concentrations of 0.5 mg / L, 1 mg / L, 2 mg / L, 0.01 mol / L of Na2SO4 was used as the background electrolyte, with an initial pH value of 7 and an external voltage of 2 V for the experiment.

[0082] 2 Experimental results

[0083] 2.1 Study on the surface characteristics of the membrane

[0084] Figure 1 is the SEM image of the PVDF-based membrane (EHT = 2.00 kV, WD = 5.2 mm, Mag = 10.00 KX, Signal A = InLens), Figure 2 where a is the SEM image of electrocatalytic membrane A (EHT = 3.00 kV, WD = 7.6 mm, Mag =

[0085] 21.00 KX, Signal A = InLens), Figure 2 where b is the SEM image of electrocatalytic membrane B (EHT = 3.00 kV,

[0086] WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens), Figure 2 where c is the SEM image of electrocatalytic membrane C (EHT = 3.00 kV, WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens), Figure 3 where d is the SEM image of electrocatalytic membrane D (EHT = 3.00 kV, WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens), Figure 3Figure e is the SEM image of electrocatalytic membrane E (EHT = 3.00 kV, WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens). Figure 3 Figure f is the SEM image of electrocatalytic membrane F (EHT = 3.00 kV, WD = 5.2 mm, Mag =

[0087] 21.00 KX, Signal A = InLens). Figure 4 Figure g is the SEM image of electrocatalytic membrane G (EHT = 3.00 kV,

[0088] WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens). Figure 4 Figure h is the SEM image of electrocatalytic membrane IH (EHT = 3.00 kV, WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens). Figure 4 Figure i is the SEM image of electrocatalytic membrane I (EHT = 3.00 kV, WD = 5.2 mm, Mag = 21.00 KX, Signal A = InLens); where EHT represents the accelerating voltage, WD represents the working distance or objective focal length, Mag represents the magnification, and Signal A = InLens represents using the InLens detector.

[0089] According to Figures 1 to 4 the experimental results show that with the increase of the dosage of MWCNTs, the number of loads on the membrane surface and in the pores increases significantly, and the number of pores on the membrane surface gradually decreases. This is because MWCNTs have good dispersibility and a high specific surface area, and are easy to fill and distribute in the membrane material, increasing the load in the internal pores of the membrane. With the increase of the dosage, MWCNTs not only enhance the mechanical properties of the membrane, but also affect the porosity and structure of the membrane. When the size of MWCNTs increases, the swelling effect increases the density of the membrane, resulting in the closure or filling of the pores of the membrane and a decrease in porosity. It can be seen that the change in the dosage of MWCNTs and the swelling of the membrane size will significantly affect the microstructure and pore distribution of the membrane.

[0090] Figure 5 Figure a is the SEM image of electrocatalytic membrane A (EHT = 3.00 kV, WD = 7.6 mm, Mag = 21.00 KX,

[0091] Signal A = InLens). Figure 5 Figure b is the SEM image of electrocatalytic membrane J (EHT = 3.00 kV, WD = 5.2 mm,

[0092] Mag = 21.00 KX, Signal A = InLens). Figure 5In Figure c is the SEM image of the electrocatalytic membrane M (EHT = 3.00 kV, WD = 7.6 mm, Mag = 21.00 KX, Signal A = InLens), Figure 6 In Figure a is the SEM image of the electrocatalytic membrane A (EHT = 3.00 kV, WD = 7.4 mm, Mag = 1.00 KX, Signal A = SE2), Figure 6 In Figure b is the SEM image of the electrocatalytic membrane J (EHT = 3.00 kV, WD = 7.4 mm, Mag = 1.00 KX, Signal A = SE2), Figure 6 In Figure c is the SEM image of the electrocatalytic membrane M (EHT = 3.00 kV, WD = 7.3 mm, Mag =

[0093] 1.00 KX, Signal A = SE2). Here, EHT represents the acceleration voltage, WD represents the working distance or the objective focal length, Mag represents the magnification, Signal A = InLens means using the InLens detector, and Signal A = SE2 means using the SE2 detector.

[0094] According to Figure 5 and Figure 6 the SEM images of the surface and cross-section of the electrocatalytic membranes in, it can be found that there is not much difference in the tube diameter and tube length of COOH-MWCNTs, OH-MWCNTs, and MWCNTs in terms of size, which indicates that the strong acid treatment conditions have not changed the overall structure of MWCNTs. However, the distribution of carbon nanotubes adsorbed on the surface of the carboxylated membrane is more uniform, and the agglomeration phenomenon is significantly reduced, which is more conducive to improving the filtration efficiency. The main reason for this analysis is that the COOH-group will generate negative ions in the solution, thereby increasing the polarity of the MWCNTs surface, significantly reducing its agglomeration tendency, making the dispersion degree of carbon nanotubes in the swelling dispersion higher, and being more uniformly adsorbed on the surface of the PVDF-based membrane during the swelling process.

[0095] 2.2 Flux Comparison

[0096] The results in Table 2 show that, compared with the original membrane, the multi-walled carbon nanotube electrocatalytic membrane obtained by swelling conventional carbon nanotubes has a significantly reduced pure water flux. The main reason for this phenomenon is that after a layer of MWCNTs is loaded on the membrane surface, the pore size of the original membrane decreases, reducing the pure water flux of the membrane. Since the pure water flux of the membrane is related not only to the pore size of the membrane but also to the number of pores on the membrane surface. After the loading of MWCNTs, the number of pores on the surface of the PVDF-based membrane changes, resulting in a decrease in the pure water flux. Moreover, the number of pores on the surface of the swelling-loading membrane prepared after swelling will be further reduced, leading to a further decrease in the pure water flux. However, the multi-walled carbon nanotube electrocatalytic membrane obtained by swelling carboxylated and hydroxylated carbon nanotubes not only does not show an obvious decrease but even increases. This is because carboxylated and hydroxylated carbon nanotubes are not easily agglomerated and have good dispersibility, and can maintain a large degree of dispersion in the swelling solution, thus forming more network structures on the surface of the PVDF-based membrane.

[0097] 2.3 Pollutant removal effect

[0098] It can be found from Table 2 that for the same size of MWCNTs, as the dosage increases, the removal effect on IBU gradually decreases. This is because as the dosage of MWCNTs increases, the dispersibility of MWCNTs in the PVDF membrane becomes worse, and agglomeration is likely to occur, resulting in uneven pore structure of the membrane, reducing the effective filtration area, and thus reducing the efficiency of pollutant removal; on the other hand, high dosages of MWCNTs may block the pores of the membrane, reducing the water flux and affecting the transport and removal effect of pollutants. However, overall, the removal effect of the swollen membrane is better than that of the original membrane loaded only with MWCNTs, and the removal effect of carboxylation and hydroxylation is more significant. This is because carbon nanotubes have better dispersion performance after carboxylation and hydroxylation, the carbon nanotube surface contains more oxygen-containing groups, and more defects and active sites are created during the acidification process, with a higher oxygen evolution potential, so it has better electrochemically oxidative performance. The above results all show that the multi-walled carbon nanotube membrane prepared by the swelling effect has a good removal effect on drug substances, and the effect of carboxylated and hydroxylated carbon nanotubes is better.

[0099] Table 2 Pure water flux and pollutant removal effect of different swelling-loading membranes

[0100] No. Name of electrocatalytic membrane <![CDATA[Pure water flux L / (m 2 ·h)]]> Removal rate of IBU (%) E0 CNTs-PVDF 1466.0 50.3 S1 <![CDATA[S1@CNTs-PVDF]]> 1163.2 70.4 S2 <![CDATA[S2@CNTs-PVDF]]> 1002.7 67.2 S3 <![CDATA[S3@CNTs-PVDF]]> 864.4 63.2 M1 <![CDATA[M1@CNTs-PVDF]]> 753.3 70.4 M2 <![CDATA[M2@CNTs-PVDF]]> 656.3 66.9 M3 <![CDATA[M3@CNTs-PVDF]]> 671.3 55.9 L1 <![CDATA[L1@CNTs-PVDF]]> 722.9 64.8 L2 <![CDATA[L2@CNTs-PVDF]]> 637.0 62.1 L3 <![CDATA[L3@CNTs-PVDF]]> 596.3 55.9 COOH1 <![CDATA[COOH1@CNTs-PVDF]]> 1621.6 82.1 COOH2 <![CDATA[COOH2@CNTs-PVDF]]> 1481.9 78.8 COOH3 <![CDATA[COOH3@CNTs-PVDF]]> 1454.6 74.9 OH1 <![CDATA[OH1@CNTs-PVDF]]> 1501.7 77.7 OH2 <![CDATA[OH2@CNTs-PVDF]]> 1541.3 76.9 OH3 <![CDATA[OH3@CNTs-PVDF]]> 1623.6 74.0

[0101] 2.4 Operational stability

[0102] The present invention uses COOH@CNTs-PVDF as the electrode material in the electrochemical reaction device to explore its stability during the removal of IBU. The specific data are as Figure 7As shown. The experiment adopted a recycling method. After each reaction, the membrane was washed multiple times with ethanol and pure water in turn and used for the next cycle. The results showed that after four cycles of use, within 60 minutes of treatment, the removal efficiencies of IBU were 82.4%, 78.2%, 74.4% and 64.9% respectively, showing a gradually decreasing trend.

[0103] This decrease in removal efficiency may stem from the following two aspects: one is that the active sites participating in the electrochemical reaction gradually decrease during the cycle; the other is that the pollutants accumulated on the membrane surface gradually block the membrane pores, reducing the mass transfer efficiency. It should be noted that the decrease in removal efficiency in the first three cycles is relatively slow, while the IBU removal efficiency in the fourth cycle decreased by nearly 10% compared with the third cycle. This significant decrease may be related to the shedding of some loaded carbon nanotubes during the cleaning process after the third cycle, resulting in a large loss of active sites.

[0104] Although the IBU removal efficiency of the COOH@CNTs-PVDF membrane decreased after cyclic use, its stability is still significantly better than that of traditional electrode materials, which has certain research significance.

[0105] 2.5 Electrochemical properties

[0106] Figure 8 The linear sweep voltammetry (LSV) curves and cyclic voltammetry curves (CV) of multi-walled carbon nanotube electrocatalytic membranes S1@CNTs-PVDF, OH@CNTs-PVDF, COOH@CNTs-PVDF and CNTs-PVDF are shown. It can be seen from the CV curves that S1@CNTs-PVDF shows a lower current response and the electrochemical activity of the material is lower; while COOH@CNTs-PVDF has the highest current response, indicating that the material has the best electrochemical activity under these conditions. In addition, the current response of COOH@CNTs-PVDF is wider, indicating that it has better electrochemical activity and greater charge transfer ability. As the potential increases, the current density also increases, indicating that the electrode has an obvious current response during the potential change process. The current density of COOH@CNTs-PVDF is the highest, indicating that its electrochemical reaction is the strongest. The current density of COOH@CNTs-PVDF rises the fastest and the reaction rate is the highest, while the reaction rates of S1@CNTs-PVDF and OH@CNTs-PVDF are slower. This is because carboxylated carbon nanotubes are introduced during the modification of COOH@CNTs-PVDF, making it have higher catalytic activity or a larger specific surface area, thus affecting the electrochemical performance. At the same time, it shows that COOH@CNTs-PVDF can obtain a faster current response from the electrolyte solution at the same potential, reduce energy consumption and reduce the occurrence of oxygen evolution reaction.

[0107] Figure 9 The results show that the charge transfer resistance of different materials, COOH@CNTs-PVDF shows the smallest semicircle impedance, indicating that this material has the lowest charge transfer resistance. This is attributed to the introduction of carboxyl functional groups, which enhances the interfacial interaction between the material and the electrolyte, thus promoting the electron transfer process, and making COOH@CNTs-PVDF exhibit excellent conductivity in electrochemical performance. In the electrocatalytic membrane filtration system, interception and electrochemical oxidation are the two main ways to remove IBU, and electrochemical oxidation usually plays a dominant role.

[0108] 2.6 Removal effect of electrocatalytic membrane filtration system on IBU under different pH conditions

[0109] The influence of the initial pH value of the IBU solution on the removal of IBU by the electrocatalytic membrane filtration system of the swollen loaded membrane was investigated. Five different initial solution pH values, namely 3, 5, 7, 9, and 11, were set in combination with actual water treatment applications, and the IBU electrochemcial membrane filtration experiment was carried out. The detailed degradation rates are as Figure 10 shown. When the initial solution pH values were 3.0, 5.0, 7.0, 9.0, and 11.0, the degradation efficiencies of COOH@CNTs-PVDF for IBU reached 83.3%, 79.1%, 80.1%, 71.8%, and 70.5% respectively at 60 min, and the degradation efficiency basically showed a gradually decreasing trend with the increase of pH value. When the pH value was 3, the removal rates of IBU by different swollen loaded membranes all reached more than 70%, indicating that strongly acidic solutions are conducive to the degradation of IBU. Under alkaline conditions, oxygen evolution occurs, hindering the formation of ∙OH; as the pH value increases, the hydrogen evolution and oxygen evolution potentials of the electrode decrease, and side reactions intensify, resulting in a decline in the degradation effect. Importantly, the pKa of IBU is 5.30. When the pH value < 5.30, IBU mainly exists as a cation in the solution; when the pH value > 5.30, IBU mainly exists in the form of an anion in the solution. When the pH value = 3, an external voltage forms a stable electric field in the solution, and IBU existing as a cation migrates towards the cathode under the action of the electric field force, increasing the directional migration efficiency, thus improving the degradation efficiency of IBU.

[0110] 2.7 Removal effect of electrocatalytic membrane filtration system on IBU under different voltage conditions

[0111] Figure 11It can be observed that the removal efficiency of IBU increases with the increase of voltage, indicating that a stronger electric field has a higher degradation efficiency for IBU. The removal rates of COOH@CNTs-PVDF for IBU reached 78.28% and 80.15% respectively. Obvious degradation of IBU occurred. As the voltage increased, the number of generated electrons increased, the content of hydroxyl radicals increased, and the removal rate of pollutants increased. The experimental results show that the swelling-loaded membrane has a certain interception effect on IBU. The interception effect reached a good level within the initial 15 minutes, and the removal rate was close to 50%. However, with the operation of the electrocatalytic membrane filtration system, the interception effect of the membrane decreased significantly and finally stabilized at about 35%. Under the condition of applying electricity (anode voltage of 2.0 V), the removal effect of the swelling-loaded membrane oxidation on IBU was significantly higher than that of the sole interception effect. In the electrocatalytic membrane filtration system, interception and electrochemical oxidation are two main ways to remove IBU, and electrochemical oxidation usually plays a dominant role.

[0112] 2.8 Removal effect of electrocatalytic membrane filtration system on IBU under different concentration conditions

[0113] (1) Removal effect of IBU under high concentration conditions

[0114] The influence of the concentration of IBU solution on the removal of IBU by the swelling-loaded membrane electrocatalytic membrane filtration system was investigated. Three different solution concentration values were set in the experiment, which were 10, 25, and 50 mg / L respectively, as shown specifically in Figure 12 the following. When the initial concentration of IBU increased from 10 mg / L to 50 mg / L, the removal rate of COOH@CNTsPVDF for IBU decreased from 83.14% to 64.42%. This is because high-concentration IBU will layer by layer wrap the active sites on the surface of carbon nanotubes, and the unreacted IBU is difficult to contact with the active sites, resulting in a decrease in the degradation efficiency of the electrode for pollutants. In addition, under the condition of high-concentration IBU, with the progress of the electro-Fenton degradation reaction, a large number of intermediate products generated in the solution will compete with IBU for adsorption and active sites on the membrane surface, resulting in a decrease in the degradation efficiency of IBU. In the application of advanced oxidation technology for wastewater degradation, the initial pH value of the pollutant solution has an important influence on the generation of free radicals and the removal of pollutants.

[0115] (2) Removal effect of IBU under low concentration conditions

[0116] To evaluate the feasibility of the COOH@CNTs-PVDF modified membrane in practical applications, the present invention also investigated its electrochemical removal efficiency in IBU solutions with different initial concentrations (2 mg / L, 1 mg / L, and 0.5 mg / L). Figure 13The experimental results in show that after the electrochemical reaction device operates for 60 minutes, the removal rates of IBU reach 94.4%, 96.1% and 98.5% respectively. At all tested concentrations, the removal rate of IBU exceeds 90%. This result indicates that under the condition of low-concentration pollutants, the COOH@CNTs-PVDF modified membrane can efficiently remove IBU and exhibits good application potential.

[0117] In summary, through a simple preparation method, the present invention prepares a multi-walled carbon nanotube membrane with high electrochemical removal efficiency for IBU solutions with different initial concentrations by performing two modification treatments on the PVDF-based membrane. Among them, the electrochemical removal efficiency for low-concentration IBU solutions exceeds 90%, and the electrochemical removal efficiency for high-concentration IBU solutions is greater than 70%. Compared with the original membrane, the optimal multi-walled carbon nanotube membrane can significantly increase by 63.22%. The above results show that the COOH@CNTs-PVDF modified membrane can efficiently remove IBU under the condition of low-concentration pollutants and exhibits good application potential; and compared with the original membrane, it has a significant effect on the removal of IBU in sewage, and its operation stability is also significantly better than that of the original membrane.

[0118] The above are only examples of the present invention, and common general technical solutions or characteristics in the solution are not described in detail here. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners in the specification and the like can be used to interpret the content of the claims.

Claims

1. A method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect, characterized in that It includes the following steps: S1. Pretreatment of the PVDF base membrane; S2. Swelling and filling: Prepare a swelling dispersion liquid with multi-walled carbon nanotubes to perform the first modification treatment on the PVDF base membrane pretreated in step S1 to obtain a swollen membrane; The preparation method of the swelling dispersion liquid is to add multi-walled carbon nanotubes to xylene, and the mass-volume ratio of the multi-walled carbon nanotubes to xylene is 1-3 mg: 1 ml; S3. Filtration coating: Prepare a loading liquid with multi-walled carbon nanotubes to perform the second modification treatment on the swollen membrane in step S2 to prepare a multi-walled carbon nanotube electrocatalytic membrane; The preparation method of the loading liquid is to place multi-walled carbon nanotubes with a diameter of 40-60 nm in an ethanol solution with a volume fraction of 45%-55% and perform ultrasonic treatment for 30-45 min. The mass ratio of the multi-walled carbon nanotubes in the loading liquid to the multi-walled carbon nanotubes in the swelling dispersion liquid is 5:4-12; The type of the multi-walled carbon nanotubes is carboxylated carbon nanotubes or hydroxylated carbon nanotubes.

2. The method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on the swelling effect according to claim 1, characterized in that, The method for pretreating the PVDF base membrane is to wash the PVDF base membrane with absolute ethanol and ultrapure water in sequence.

3. The method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on the swelling effect according to claim 1, wherein The method for the first modification treatment is to first perform water bath ultrasonic treatment on the prepared swelling dispersion liquid, and then put the PVDF base membrane pretreated in step S1 into the ultrasonically treated swelling dispersion liquid and continue ultrasonic treatment to obtain a swollen membrane.

4. The method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on the swelling effect according to claim 3, wherein The diameter of the carbon nanotubes is 10-60 nm, and the length <5 μm.

5. A method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect according to claim 3, characterized in that, The temperature of the ultrasonic treatment is 60±5°C. The time for performing water bath ultrasonic treatment on the prepared swelling dispersion liquid is 30-45 min. When the PVDF base membrane is put into the ultrasonically treated swelling dispersion liquid and continues ultrasonic treatment for 60-90 min.

6. A method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect according to claim 1, characterized in that, In step S3, before the filtration coating, the surface of the swollen membrane needs to be rinsed with absolute ethanol and ultrapure water in sequence.

7. A method for preparing a multi-walled carbon nanotube electrocatalytic membrane based on a swelling effect according to claim 1, characterized in that, In step S3, the method for the second modification treatment is to perform constant pressure filtration at a pressure of 0.09-0.11 MPa by means of negative pressure filtration, so that the multi-walled carbon nanotubes are uniformly attached to the surface of the swollen membrane to obtain a multi-walled carbon nanotube electrocatalytic membrane.

8. A multi-walled carbon nanotube electrocatalytic membrane prepared by the method according to any one of claims 1-7.

9. Use of the multi-walled carbon nanotube electrocatalytic membrane according to claim 8 in an electrocatalytic membrane filtration system for removing pharmaceutical organic compounds in an aqueous environment, characterized in that, The initial pH value of the water environment is 3-11, and the pharmaceutical organic compounds include but are not limited to ibuprofen.

Images