A layer-by-layer self-assembly preparation method of a tannic acid composite nanofiltration membrane modified by ferroferric oxide

Abstract

A kind of layer-by-layer self-assembly preparation method of tannic acid composite nanofiltration membrane modified by ferroferric oxide, utilizes tannic acid (TA) as selective layer crosslinking unit, forms film by phenol-amine reaction, constructs phenolamine selection layer, utilizes the method of layer-by-layer self-assembly to prepare Fe3O4-TA vac @ε-PL4 composite nanofiltration membrane. The membrane prepared under the preferred conditions has a pure water permeability of 40.6 LMH·bar ‑1 The rejection rate of methyl orange (MO) is further increased to 92.4%, and the rejection rates of the membrane to methyl blue (MB), Congo red (CR) and rhodamine B (Rh B) are as high as 100%, 100% and 97.8% respectively. The prepared membrane has excellent stability and anti-pollution property. After pollution, the pure water permeability of the membrane is restored to 96.1% of the initial value through the photo-assisted Fenton-like reaction, and the self-cleaning process does not cause obvious influence on the structure and selectivity of the membrane.

Description

TECHNICAL FIELD

[0001] The application relates to the chemical manufacturing field, in particular to a layer-by-layer self-assembly preparation method for modifying a tannic acid composite nanofiltration membrane with ferroferric oxide. BACKGROUND

[0002] Water resource shortage has become a global environmental problem, and water treatment processes based on membrane separation technology have been increasingly studied due to the advantages of low energy consumption, small pollution and simple process. Among them, nanofiltration technology has the characteristics of low operating pressure and high desalination efficiency, and is widely used in seawater desalination, drinking water deep purification, wastewater treatment and other fields, and has environmental and economic benefits, and has a broad application prospect.

[0003] Nanofiltration membranes play a core role in nanofiltration technology, and the academia has made great efforts to improve the separation ability and service life of the membranes. The anti-pollution ability and structural stability of traditional polyamide nanofiltration membranes are limited, and the acid and alkali resistance of the polyamide structure is poor, which further limits the application scenarios of the membranes. The existing nanofiltration technology has the following two key problems in application:

[0004] Firstly, the permeability and selectivity of nanofiltration membranes are restricted by the "Trade-off" effect, that is, pursuing high retention rate will inevitably lead to a decrease in membrane flux, and vice versa.

[0005] Secondly, the pollution of nanofiltration membranes is increasing in actual application, which leads to a continuous decrease in membrane permeability. Although chemical cleaning (acid washing, alkali washing, etc.) can restore the membrane flux, frequent cleaning will cause irreversible damage to the selective layer structure of the membrane, increasing the maintenance cost and seriously shortening the service life of the nanofiltration membrane.

[0006] How to break through the upper limit of the "Trade-off" effect is the core problem of the further development of nanofiltration technology, and how to improve the anti-pollution ability and structural stability of nanofiltration membranes is also a problem that must be concerned. SUMMARY

[0007] The application aims to solve the above problems in the prior art, and provides a layer-by-layer self-assembly preparation method for modifying a tannic acid composite nanofiltration membrane with ferroferric oxide, which improves the separation ability and service life of the membrane. The tannic acid structure derived from plants is rich in phenolic hydroxyl groups, and has good hydrophilicity and reactivity. Tannic acid (TA) can prepare covalently bonded membrane layers through phenol-amine reaction, has certain separation ability and strong stability, and provides a new research idea for the development of high-performance composite nanofiltration membranes. 2+ The mixed solution of H2O2 and Fe has strong oxidizing property and can be used for treating refractory organic pollutants. Nanometer Fe3O4 has the advantages of large specific surface area, simple preparation method, low toxicity and reusability, and is expected to be applied to the field of membrane separation.

[0008] To achieve the above object, the application adopts the following technical scheme:

[0009] A layer-by-layer self-assembly preparation method for modifying a tannic acid composite nanofiltration membrane with Fe3O4, comprising the following steps:

[0010] A. Preparation of nano-Fe3O4;

[0011] B. Preparation of Fe3O4 nano-dispersion liquid using the nano-Fe3O4 obtained in step A;

[0012] C. Preparation of Fe3O4-TA using the Fe3O4 nano-dispersion liquid obtained in step B vac @ε-PL4 composite membrane.

[0013] Further, the preparation of nano-Fe3O4 in step A comprises the following steps:

[0014] A certain mass of trisodium citrate is dissolved in a sodium hydroxide solution, and heated to a certain temperature for standby;

[0015] A certain mass of FeSO4·7H2O and FeCl3·6H2O is dissolved in a certain amount of water, and transferred to a three-necked flask, and stirred by a mechanical stirrer under nitrogen protection;

[0016] The mixed alkali source is slowly added to the three-necked flask, and stirring is continued until it is cooled to room temperature.

[0017] Further, the concentration of the sodium hydroxide solution in step A is 0.2 mol·L -1 ;

[0018] The volume of the sodium hydroxide solution in step A is 200 mL, the trisodium citrate is 1 g; the heating temperature is 60±1℃; the mixed alkali source is added for 30 min; the mass of FeSO4·7H2O is 1.5 g, the mass of FeCl3·6H2O is 2.6 g, the water is 100 mL, and the stirring speed is 200 rpm.

[0019] Further, the preparation of Fe3O4 nano-dispersion liquid in step B comprises the following steps:

[0020] Degassed pure water is added to the Fe3O4 powder, ultrasonic dispersion is performed, and part of the Fe3O4 agglomerates with larger particles are settled by using a strong magnet, and a high-transmittance brownish Fe3O4 dispersion liquid is collected.

[0021] Further, the ultrasonic dispersion time in step B is 2 h; and the calculated mass concentration of the dispersion liquid is 85 ppm.

[0022] Further, the Fe3O4-TA prepared in step C vac @The ε-PL4 composite membrane comprises the following steps:

[0023] C1, taking out the PSU base film from the protective liquid, cutting into a circular piece of appropriate size, soaking in pure water for a period of time, and then fixing the base film in a circular filtration device after natural air drying;

[0024] C2, preparing a mixed primary assembly solution with a certain TA mass concentration by introducing a certain amount of Fe3O4 dispersion liquid;

[0025] C3, preloading Fe3O4 nanoparticles by vacuum filtration, pouring a certain amount of primary assembly solution into the filtration device, and filtering out all the primary assembly solution under a certain transmembrane pressure, while the nanoparticles are filtered onto the membrane, and the base film is also fully soaked;

[0026] C4, preparing a certain mass concentration of ε-PL aqueous solution as a secondary assembly solution, and adjusting the pH of the secondary assembly solution to a certain value with dilute ammonia water;

[0027] C5, pouring a certain amount of secondary assembly solution along the side of the filtration device, and immediately pouring out after a period of time to terminate the reaction;

[0028] C6, rinsing the prepared membrane surface several times with pure water, further crosslinking for a period of time at a certain temperature, and then soaking in a NaOH solution with a certain pH for a certain period of time.

[0029] Further, the diameter of the circular piece of appropriate size in step C1 is 76 mm, and the soaking time is 2 h; the TA mass concentration prepared in step C2 is 1000±5 ppm; the volume of the primary assembly solution in step C3 is 10 mL, and the transmembrane pressure is 0.2 bar; the mass concentration of ε-PL in step C4 is 4000±10 ppm, the concentration of dilute ammonia water is 0.5 mol·L -1 , the adjusted pH value is between about 8 and 9; the volume of the secondary assembly solution in step C5 is about 20 mL, and the reaction time is 1 min; the temperature in step C6 is 60±1℃, the crosslinking time is 15±0.5 min, the pH during soaking is between 12 and 13, and the soaking time is 20±0.5 min.

[0030] Compared with the prior art, the technical scheme of the present application has the following beneficial effects:

[0031] 1. The prepared nano Fe3O4 has a small particle size and uniform size distribution. The introduction of citrate in the preparation process improves the hydrophilicity and dispersibility of nano Fe3O4.

[0032] 2、The application realizes the loading of nano Fe3O4 on the membrane through a vacuum-assisted self-assembly process, and the nano Fe3O4 has good compatibility with a phenolic amine selection layer.

[0033] 3、The prepared membrane has excellent stability and anti-pollution property. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Surface scanning images of the membrane in Example 1: (a), (a1) Fe3O4-TA vac @ε-PL4-1; (b), (b1) Fe3O4-TA vac @ε-PL4-2; (c), (c1) Fe3O4-TA vac @ε-PL4-3; (d), (d1) Fe3O4-TA vac @ε-PL4-4.

[0035] Figure 2 Water contact angle of the membrane in Example 1.

[0036] Figure 3 Effect of (a) Fe3O4 loading amount on the permeability and MO retention rate of the membrane in Example 1; (b) Fe3O4-TA vac Retention rate of the membrane in Example 1 on MB, CR and Rh B.

[0037] Figure 4 Long-term stability of the membrane in Example 1.

[0038] Figure 5 Normalized permeability (a) and retention rate on MB (b) of the membrane in Example 1 in the self-cleaning performance test. DETAILED DESCRIPTION

[0039] In order to make the technical problems to be solved by the application, the technical solutions and the beneficial effects more clear, specific, and obvious, the application will be further described in detail below in combination with the drawings and examples.

[0040] Example 1

[0041] The specific experimental steps of the layer-by-layer self-assembly preparation method for the tannic acid composite nanofiltration membrane modified by Fe3O4 according to the application are as follows:

[0042] 1. Preparation of nano Fe3O4

[0043] a. Prepare 200 mL of 0.2 mol·L -1NaOH solution, 1 g of trisodium citrate was added as mixed alkali source, heated to 60 °C for standby.

[0044] b. 1.5 g of FeSO4·7H2O and 2.6 g of FeCl3·6H2O were dissolved in 100 mL of water, transferred to a 500 mL three-necked flask, and stirred with a mechanical stirrer (200 rpm) under N2protection.

[0045] c. The mixed alkali source at 60 °C was slowly added to the three-necked flask (about 30 min to add completely), and stirring was continued until it cooled to room temperature.

[0046] d. Fe3O4 was precipitated from the solution with a strong magnet, washed with water and alcohol three times in turn, and dried at 60 °C to obtain a brown-black solid, which was ground into powder and stored after drying.

[0047] 2. Preparation of Fe3O4 nanodispersion

[0048] Degassed pure water was added to the Fe3O4 powder, and ultrasonic dispersion was performed. Part of the Fe3O4 agglomerates with larger particles were precipitated using a strong magnet, and a brownish Fe3O4 dispersion with high light transmittance was collected, with a calculated mass concentration of 85 ppm.

[0049] 3. Preparation of Fe3O4-TA vac @ε-PL4 composite membrane

[0050] a. The polysulfone (PSU) base film was taken out of the protective solution, cut into a circular piece with a diameter of 76 mm, soaked in pure water for more than 2 h, and then naturally air-dried before being fixed in a circular filtration device.

[0051] b. A mixed primary assembly solution with a TA mass concentration of 1000 ppm was prepared by introducing a certain amount of Fe3O4 dispersion.

[0052] c. Vacuum filtration was used for the pre-loading of Fe3O4 nanoparticles. 10 mL of the primary assembly solution was poured into the filtration device, and the primary assembly solution was completely filtered out under a transmembrane pressure of 0.2 bar. At the same time, the nanoparticles were filtered onto the membrane, and the base film was fully soaked.

[0053] d. A polylysine (ε-PL) aqueous solution with a mass concentration of 4000 ppm was prepared as a secondary assembly solution. The pH of the secondary assembly solution was adjusted to about 8.5 with 0.5 mol·L -1 dilute ammonia water.

[0054] e. 20 mL of the secondary assembly solution was quickly poured into the filtration device, and immediately poured out after 1 min to terminate the reaction.

[0055] f.The prepared membrane was rinsed with pure water for three times, and then further cross-linked at 60°C for 15 min, and then immersed in NaOH solution with pH = 12.5 for 20 min.

[0056] 5. Analytical methods

[0057] 5.1 Membrane morphology characterization:

[0058] Scanning electron microscope (SEM) is used to magnify the sample surface by scanning the sample surface with a very narrow electron beam and imaging point by point, so as to qualitatively or quantitatively analyze the microstructure of the membrane surface and cross-section morphology, pore structure, active layer thickness, roughness, etc. In the present application, the morphology and structure of the membrane sample surface were characterized by using a ZEISS SIGMA type scanning electron microscope (SEM) made in Germany. After the dried membrane sample was frozen, cut and fixed on the sample stage, it was treated with platinum, and then observed under an acceleration voltage of 20 kV and a magnification of 10,000 or 20,000.

[0059] 5.2 Determination of hydrophilic and hydrophobic properties of the membrane:

[0060] The hydrophilic and hydrophobic properties of the membrane are usually measured by the size of the contact angle. The contact angle, also known as the wetting angle, refers to the spreading angle of the liquid on the surface when it reaches thermodynamic equilibrium. The better the hydrophilicity of the surface, the smaller the spreading angle of the liquid, and therefore the smaller the contact angle. In the present application, the contact angle of the nanofiltration membrane surface was measured by using a HARKE SPCAX3 type contact angle tester, which can be used to characterize the hydrophilic and hydrophobic properties of the membrane. The amount of water added each time was 3 μL, and the contact time between the liquid and the nanofiltration membrane surface was 30 s. In order to improve the accuracy of the measurement data, the contact angle of each membrane sample was measured 5 times and the average value was taken.

[0061] 5.3 Membrane flux determination:

[0062] The effective area of each membrane piece was about 40 em 2 , and the steps of measuring the flux were as follows:

[0063] (1) The membrane piece was fixed on the bottom of the ultrafiltration cup with the skin layer facing up;

[0064] (2) 300 mL of solution was poured into the ultrafiltration cup, and about 50 mL was pre-pressed at 1 bar (if the filtration speed is too slow at 1 bar, the pressure can be increased appropriately);

[0065] (3) After the pre-pressing was completed, the timing was started, and the pressure and the volume of the permeate within a certain time were recorded;

[0066] (4) The calculation formula of the flux is as follows:

[0067]

[0068] J is the flux of the membrane piece (L·m-2 ·h -1 ·bar -1 ), V is the permeate volume (m 3 ), A is the effective membrane area of the membrane sheet (m 2 ), p is the filtration pressure (bar), and Δt is the filtration time (h).

[0069] 5.4 Membrane separation performance test:

[0070] The separation performance of the prepared nanofiltration membrane was evaluated by testing the rejection of four dyes (methyl blue (MB), methyl orange (MO), Congo red (CR), and rhodamine B (Rh B)).

[0071] Dye rejection rate measurement method: The membrane sheet was fixed in the membrane cell, 3L of the dye solution to be tested was added to the feed tank, and the machine was started. The test was carried out at 25℃ and 3bar for 15min. After the test system was stable, the filtrate was collected, and the concentrations of the filtrate and the feed were measured at the maximum light absorption wavelength of the corresponding dye using a UV-visible spectrophotometer. The rejection rate of the membrane to the dye was calculated by substituting the formula 2.

[0072]

[0073] Wherein, Cp is the permeate concentration, C f is the concentration of the concentrated solution.

[0074] 5.5 Membrane stability test

[0075] The stability of the Fe3O4-TA vac @ε-PL4 membrane was tested by circulating filtration with 40ppm methyl orange aqueous solution as the feed liquid at a working pressure of 2bar. The test time was 24h, and the permeability and methyl orange rejection rate of the membrane were measured every 1h. After recording the data, the collected filtrate was poured back into the feed tank.

[0076] 5.6 Membrane self-cleaning performance test

[0077] The self-cleaning performance of the Fe3O4-TA vac @ε-PL4 membrane was tested by using an ultrafiltration cup (membrane installation diameter 76mm) under dead-end filtration. After the membrane sheet was contaminated with methyl blue, the methyl blue was degraded by the Fenton-like reaction of Fe3O4 and H2O2 to realize the self-cleaning of the membrane.

[0078] This example also determines the influence of different Fe3O4 loadings on the performance of the composite membrane.

[0079] Step: Fe3O4 nanoparticles were pre-loaded on PSU base membrane by vacuum assisted method, then the functionalized nanocomposite nanofiltration membrane was prepared. The membrane designation and corresponding preparation conditions are shown in Table 1. Since the vacuum assisted self-assembly process significantly enhances the distribution effect of TA monomers on the surface of the base membrane, the concentration of TA in the first assembly solution is reduced to 1000 ppm (10 mL in total), and the reaction time is determined to be 1 min.

[0080] Table 1 Preparation conditions and designation of membranes

[0081]

[0082] The blank control membrane without introducing any doping material is designated as TFC-blank; TA3@ε-PL4 represents the composite nanofiltration membrane assembled by TA 3000 ppm in the first assembly solution and ε-PL 4000 ppm in the second assembly solution.

[0083] From the experimental results, it can be seen that

[0084] 1. See Figure 1 , the prepared nanometer Fe3O4 has small particle size and uniform size distribution, and the citrate introduced in the preparation process improves the hydrophilicity and dispersibility of nanometer Fe3O4. Through the vacuum assisted self-assembly process, the loading of nanometer Fe3O4 on the membrane is realized, which has good compatibility with the phenolic amine selective layer, but the agglomeration is serious when the loading amount is high.

[0085] 2. See Figure 2 , the loading of Fe3O4 nanoparticles causes the water contact angle of Fe3O4-TA vac @ε-PL4 membrane to increase, and the contact angle of Fe3O4-TA vac @ε-PL4-4 reaches 61.5°, which is affected by Fe3O4 agglomerates.

[0086] 3. See Figure 3 , the preferred Fe3O4-TA vac @ε-PL4-2 membrane has a pure water permeability of 40.6±2.8 LMH.bar -1 , the rejection rate of MO is further improved to 92.4%, and the rejection rates of MB, CR and RhB are as high as 100%, 100% and 97.8%, respectively.

[0087] 4. See Figures 4-5 , the membrane prepared in this embodiment has excellent stability and anti-pollution property. The Fenton-like catalytic effect of nanometer Fe3O4 also endows Fe3O4-TA vacThe ε-PL4 membrane has excellent self-cleaning ability; the pure water permeability of the contaminated membrane is restored to 96.1% of the initial value through the light-assisted Fenton-like reaction, and the self-cleaning process does not cause obvious influence on the structure and selectivity of the membrane.

Claims

1. A method for the layer-by-layer self-assembly preparation of a tannic acid composite nanofiltration membrane modified with iron oxide, characterized in that... Includes the following steps: 1) Preparation of nano-Fe3O4; 2) Prepare Fe3O4 dispersion using the Fe3O4 obtained in step 1); 3) Preparation of Fe3O4-TAvac@ε-PL4 composite membrane using the Fe3O4 dispersion obtained in step 2): First, the PSU base membrane was soaked in water and air-dried, then fixed in a vacuum filtration device. A primary assembly solution of TA was prepared by introducing the Fe3O4 dispersion, and Fe3O4 nanoparticles were pre-loaded onto the PSU base membrane using vacuum filtration. An ε-PL aqueous solution was prepared as a secondary assembly solution, and the pH was adjusted. The secondary assembly solution was poured into the vacuum filtration device, and after a period of time, it was poured out to terminate the reaction. Finally, the assembled membrane was rinsed with pure water, subjected to thermal crosslinking, and soaked in NaOH solution for a certain period. ε-PL4 indicates that the mass concentration of ε-PL in the secondary assembly solution is 4000 ppm.

2. The method for preparing a layer-by-layer self-assembly of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that, Step 1) Preparation of nano Fe3O4 is as follows: FeSO4•7H2O and FeCl3•6H2O aqueous solution is stirred under nitrogen protection, and then a mixed alkali source of 60±1℃ is added. The mixed alkali source is trisodium citrate and sodium hydroxide. Finally, Fe3O4 is precipitated with a magnet, washed, and dried.

3. The method for preparing a layer-by-layer self-assembly of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that, Step 2) The preparation of Fe3O4 nano-dispersion is as follows: Degassed pure water is added to Fe3O4, ultrasonic dispersion is performed, and larger Fe3O4 agglomerates are settled by magnets to collect the brown Fe3O4 dispersion.

4. The layer-by-layer self-assembly preparation method of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that: In step 3), the TA mass concentration in the primary assembly solution is 1000±5 ppm.

5. The method for preparing a layer-by-layer self-assembly of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that: In step 3), dilute ammonia is used to adjust the pH to between 8 and 9.

6. The layer-by-layer self-assembly preparation method of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that: In step 3), the time period is 1 minute.

7. The layer-by-layer self-assembly preparation method of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that: In step 3), the temperature for thermal crosslinking is 60±1℃ and the crosslinking time is 15±0.5 min.

8. The layer-by-layer self-assembly preparation method of a tannic acid composite nanofiltration membrane modified with iron oxide as described in claim 1, characterized in that: In step 3), soak in NaOH solution with a pH between 12 and 13 for 20 ± 0.5 min.

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