Preparation method and application of self-cleaning composite nanofiltration membrane loaded with manganese dioxide

Abstract

The application belongs to the technical field of membrane treatment of dye wastewater, and particularly relates to a preparation method and application of a self-cleaning composite nanofiltration membrane loaded with manganese dioxide. Specifically, the surface of a polyether sulfone-based membrane is soaked in a dopamine solution with a pH of 8.5, and a shaker reaction is carried out at room temperature to obtain a membrane material loaded with polydopamine, and the membrane material is reacted with potassium permanganate to generate manganese dioxide. On the base membrane loaded with manganese dioxide, piperazine is used as a water-phase monomer, and trimesoyl chloride is used as an organic-phase monomer, and an interface polymerization is adopted to prepare a composite nanofiltration membrane. The nanofiltration membrane provided by the application has high retention rate for divalent cations and high pure water flux, and the manganese dioxide loaded on the membrane has catalytic activity, so that it is easy to activate hydrogen peroxide to generate free radicals to clean the pollution adsorbed on the membrane surface after nanofiltration, and long-term use of the membrane is realized.

Description

Technical Field

[0001] This invention relates to a self-cleaning composite nanofiltration membrane loaded with manganese dioxide and its preparation method, belonging to the field of membrane treatment technology. Background Technology

[0002] With the development of dyes in various fields, the discharge of dye wastewater has also increased dramatically. Directly discharging dye wastewater into water systems can cause irreversible environmental pollution. Nanofiltration, due to its advantages such as simple operation, low energy consumption, and mild filtration conditions, is widely used in dye wastewater treatment. Nanofiltration membranes typically have a polyamide (PA) thin-film composite (TFC) structure. The PA active layer is usually formed on a support layer through interfacial polymerization, where two monomers react at the interface between the aqueous and oil phases to form a dense PA layer that allows water molecules to pass through while retaining large dye molecules.

[0003] Currently, in the treatment of dye wastewater using nanofiltration membranes, dye molecules easily adsorb onto the membrane surface, forming a fouling layer that reduces membrane flux and shortens membrane lifespan. Therefore, effectively mitigating membrane fouling remains a research hotspot in the membrane technology field. Most current research focuses on hydrophilic modification of the membrane surface or chemical cleaning to reduce fouling. Advanced oxidation technologies can generate highly oxidizing free radicals, which can degrade organic pollutants to some extent. Fenton-like reactions remain stable over a wide pH range, therefore, non-ferrous transition metal catalysts have been explored for application in various Fenton-like reactions. Combining highly active Fenton-like catalysts with nanofiltration membranes holds promise as an effective way to solve membrane fouling.

[0004] Manganese dioxide (MDC) is a common Fenton-like catalyst with a simple preparation process and abundant active sites, playing a crucial role in the catalytic activation of hydrogen peroxide. Currently, research has shown that immersing contaminated membranes in hydrogen peroxide solutions containing MDC particles can catalytically decompose and eliminate organic pollutants. Introducing MDC into membrane structures to construct high-flux nanofiltration membranes with self-cleaning capabilities is a promising research area. Summary of the Invention

[0005] The purpose of this invention is to provide a self-cleaning composite nanofiltration membrane loaded with manganese dioxide and its preparation method. This composite nanofiltration membrane can slow down the decrease in flux and extend the service life of the membrane during long-term filtration of dye wastewater.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A self-cleaning composite nanofiltration membrane loaded with manganese dioxide includes a support layer, a catalytically active manganese dioxide intermediate layer on the support layer, and an uppermost polyamide selectively active layer.

[0008] A method for preparing a self-cleaning composite nanofiltration membrane loaded with manganese dioxide includes: firstly, coating a layer of polydopamine on a polyethersulfone support layer and reacting it with potassium permanganate to generate manganese dioxide in situ as an intermediate layer; then, polymerizing an aqueous monomer (piperazine) and an organic monomer (trimethylammonium chloride) at the organic phase-water interface to form a polyamide active layer, thereby preparing a self-cleaning nanofiltration membrane.

[0009] This invention provides a method for preparing a self-cleaning nanofiltration membrane, comprising the following steps:

[0010] 1) Provide the base film, clean and store it for later use;

[0011] 2) A polydopamine layer was prepared in the base film;

[0012] 3) Manganese dioxide nanoparticles were prepared on the surface of polydopamine;

[0013] 4) Prepare polyamide selective separation layer by interfacial polymerization.

[0014] Furthermore, the base film material is selected from polyethersulfone (PES).

[0015] Furthermore, cleaning the base film specifically refers to using a 25wt% isopropanol solution to clean the glycerin on the polyethersulfone membrane, rinsing with pure water, and then storing it at 4°C.

[0016] Further, the specific steps for coating the base membrane with polydopamine include: preparing a 0.01M Tris-HCl buffer solution, adding dopamine hydrochloride to a final concentration of 2 g / L, adjusting the pH to 8.5, fixing the base membrane, exposing the membrane surface to air, pouring in the above dopamine solution, and coating the membrane on a shaker at a speed of 200 rpm for 15 min. After the reaction, the membrane surface is rinsed with deionized water to remove unreacted dopamine solution, thus obtaining the base membrane coated with polydopamine.

[0017] Furthermore, in step 3), the surface of the base film coated with dopamine is immersed in potassium permanganate of different concentrations to generate manganese dioxide through a redox reaction; the concentration of potassium permanganate is 0.01~0.1M, and the reaction time is 2min.

[0018] Furthermore, in step 4), a polyamide layer is generated by reacting an aqueous monomer containing amines and an organic monomer containing acyl chlorides on a manganese dioxide intermediate layer through interfacial polymerization. Specifically, an aqueous solution is first impregnated on the upper surface of the manganese dioxide intermediate layer, and after a period of time, the excess solution is removed. Then, an organic solution is poured in to react. After the reaction is completed, the membrane surface is cleaned, and finally, a thermal crosslinking reaction is carried out.

[0019] Furthermore, the amine monomer is piperazine, with a mass fraction of 0.3 wt%; the acyl chloride monomer is trimesoyl chloride, the organic solvent is n-hexane, and the mass fraction of the acyl chloride monomer is 0.15 wt%.

[0020] Furthermore, the aqueous phase soaking time is 3 min, and the interfacial polymerization (i.e., organic phase soaking time) is 1 min; the temperature of the interfacial polymerization reaction is 25±3℃, and the relative humidity is 50~80%; the thermal crosslinking reaction is carried out at 50℃ for 10 min.

[0021] This invention has the following advantages:

[0022] This invention introduces manganese dioxide as an intermediate layer, upon which interfacial polymerization is carried out, improving the water flux and the rejection rate of divalent cations in the nanofiltration membrane. Simultaneously, manganese dioxide, as a transition metal catalyst, can catalyze the generation of free radicals during hydrogen peroxide cleaning to degrade organic matter on the membrane surface, giving the nanofiltration membrane self-cleaning properties. This allows the membrane to slow down flux reduction and extend its lifespan during long-term filtration of dye wastewater. Attached Figure Description

[0023] Figure 1 (a) is an electron microscope image of TFC obtained by interfacial polymerization directly on the surface of the base film; (b) is an electron microscope image of TFNi-0.01 obtained by reacting potassium permanganate at a concentration of 0.01M to obtain manganese dioxide as an intermediate layer and then undergoing interfacial polymerization; (c) is an electron microscope image of TFC-0.01.

[0024] Figure 2 These are Ftir characterization results;

[0025] Figure 3 It compares the flux and rejection rate of nanofiltration membranes;

[0026] Figure 4 This is the result of Zeta potential characterization;

[0027] Figure 5 This is the result of WCA characterization;

[0028] Figure 6 It refers to the performance of nanofiltration membrane in continuously filtering methylene blue dye. Detailed Implementation

[0029] The present invention will be described below through specific embodiments, but the present invention is not limited thereto.

[0030] Example 1: Preparation of a manganese dioxide-based polyamide nanofiltration membrane (TFNi-0.01)

[0031] The base membrane used in this invention is a commercial ultrafiltration membrane (UP150, Microdyn-Nadir), made of polyethersulfone (PES).

[0032] Step 1: Prepare a 0.01M Tris-HCl buffer solution, add dopamine hydrochloride to a final concentration of 2 g / L, and adjust the pH to 8.5. Fix the base membrane, expose the membrane surface to air, pour in the above solution, and perform a coating reaction on a shaker at 200 rpm. Preferably, the reaction time is 15 min. After the reaction, rinse the membrane surface with deionized water to remove unreacted dopamine solution, obtaining a polydopamine-coated base membrane.

[0033] Step 2: Take the dopamine-modified membrane, prepare a 0.01M potassium permanganate solution, fix the membrane in place, and allow the membrane surface to be fully immersed in the potassium permanganate solution for 2 minutes. After the redox reaction is complete, rinse the membrane surface with deionized water to obtain a membrane loaded with manganese dioxide.

[0034] Step 3: Prepare a 0.3 wt% piperazine / water solution as the aqueous phase and a 0.15 wt% trimesoyl / n-hexane solution as the organic phase. Fix the membrane in the impregnation interfacial polymerization apparatus. Pour 15 ml of piperazine / water solution onto the upper surface of the manganese dioxide intermediate layer to allow the piperazine monomer to be fully adsorbed into the membrane pores. React for 3 min. After the reaction, discard the aqueous phase solution, wipe the membrane surface with absorbent paper, pour in the oil phase, and react for 1 min. After the reaction, rinse the membrane surface with n-hexane along the apparatus. The interfacial polymerization reaction temperature is 25 ± 3℃, and the relative humidity is 50~80%. Cure in a 50℃ oven for 10 min, and store the modified membrane in deionized water at 4℃.

[0035] Comparative Example 1: Preparation of manganese dioxide (TFC-0.01) loaded on a polyamide nanofiltration membrane.

[0036] Step 1: The difference from Example 1 is that this method first performs interfacial polymerization on the surface of the base membrane to obtain the TFC nanofiltration membrane.

[0037] Step 2: Dopamine is coated onto the TFC nanofiltration membrane, and manganese dioxide nanoparticles are grown on the TFC membrane by immersing it in a 0.01M potassium permanganate solution to obtain the TFC-0.01 nanofiltration membrane. This is a common method for antifouling modification of the membrane surface.

[0038] SEM characterization

[0039] Figure 1 (a) is an electron microscope image of TFC obtained by interfacial polymerization directly on the surface of the base film; Figure 1 (b) is an electron microscope image of TFNi-0.01 obtained by reacting potassium permanganate at a concentration of 0.01M to obtain manganese dioxide as an intermediate layer and then undergoing interfacial polymerization. Figure 1 Image (c) is an electron microscope image of TFC-0.01.

[0040] As shown in the figure, the polyamide nanofiltration membrane (TFC) exhibits a typical nodular structure. After loading manganese dioxide onto the base membrane, TFNi-0.01 displays a larger nodular structure with a more loose arrangement and a rougher surface. This may be due to the influence of the piperazine diffusion rate after the introduction of the intermediate layer. The surface of TFC-0.01 is similar to that of TFC, but due to the loading of manganese dioxide on the membrane surface, the structure of TFC-0.01 is more compact.

[0041] Figure 2 The result is the Ftir characterization.

[0042] Compared to the PES-based film, TFC, TFNi-0.01, and TFC-0.01 films showed better performance at 1680-1630 cm⁻¹. -1 The observation of a typical peak at the C=O amide group indicates that interfacial polymerization forms a polyamide selective layer.

[0043] Figure 3 It is a comparison of the retention and flux of nanofiltration membranes.

[0044] Water flux characterization was conducted at 25±1℃; salt rejection experiments were performed using a 1000ppm solution of sodium sulfate, magnesium sulfate, sodium chloride, and magnesium chloride; the filtration pressure was 6 bar, and the effective filtration area of ​​the membrane was 8 cm². 2 Before testing, pre-pressurization for 1 hour is required to obtain a stable effluent flow rate. The results are as follows: Figure 3 As shown. Water permeability flux can be calculated using the following formula:

[0045]

[0046] In the formula: PWP (LMH / bar), V is the permeate volume (L) over a certain time, and A is the effective contact area of ​​the membrane (m²). 2 ), where Δt is the infiltration time (h) and ΔP is the infiltration pressure (bar).

[0047] from Figure 3As shown, the TFNi-0.01 nanofiltration membrane, using manganese dioxide as the intermediate layer, forms effective water channels with a water permeation flux of 24 LMH / bar, a 24% increase compared to the TFC membrane (19.3 LMH / bar); meanwhile, the retention of divalent ions remains largely unchanged. The TFC nanofiltration membrane's salt rejection order is Na₂SO₄ > MgSO₄ > MgCl₂ > NaCl, consistent with the salt rejection order of conventional polyamide nanofiltration membranes. Unlike the TFC membrane, the salt rejection rate of TFNi-0.01 is Na₂SO₄ > MgSO₄ > NaCl > MgCl₂. This may be due to the higher negative surface charge of TFNi-0.01 compared to the TFC membrane.

[0048] Membrane retention of different salts and water permeation flux

[0049]

[0050] Surface elemental composition of the membrane and calculated degree of crosslinking

[0051]

[0052] As can be seen from the table, the degree of crosslinking of the TFNi-0.01 membrane is higher than that of the TFC membrane. It can also be seen that when the flux of the TFNi-0.01 membrane increases, its rejection rate for divalent salts remains close to that of the TFC membrane.

[0053] Figure 4 The result is the Zeta potential characterization result.

[0054] Figure 4 In this study, the surface charge characteristics of nanofiltration membranes were characterized by measuring the Zeta potential at different pH values, specifically the electronegativity of the TFNi-0.01 and TFC-0.01 membrane surfaces. The Zeta potential order for the three nanofiltration membranes was TFC > TFNi-0.01 > TFC-0.01. It can be seen that due to the dissociation of carboxylic acid groups, all membrane surfaces are negatively charged at pH 6.0. TFNi-0.01 and TFC-0.01 carry more negative charges, which corresponds to the salt rejection results. The electronegativity of TFC-0.01 may be related to the negative charge of manganese dioxide.

[0055] Figure 5 The results are from WCA characterization.

[0056] The hydrophilicity of nanofiltration membranes is studied by measuring the contact angle. Figure 5It can be seen that the TFNi-0.01 membrane exhibits excellent hydrophilicity (34.2°), which is better than the TFC membrane's 44.3°. This is related to the higher surface roughness of TFNi-0.01. The self-cleaning performance was determined by pre-pressuring at 6 bar for 0.5 h and measuring the pure water flux J0. A 1000 ppm methylene blue (MYB) dye solution was prepared, and after continuous operation for 5 h, it was cleaned sequentially with pure water and hydrogen peroxide. This process was repeated three times, and the dye flux J was recorded.

[0057] Total membrane fouling is In the formula,

[0058] Each pure water cleaning session lasted 1 hour, and the flux was tested every 20 minutes and recorded as J2; each hydrogen peroxide cleaning session lasted 0.5 hours, and the flux was measured as Jw.

[0059] Flux recovery rate .

[0060] During membrane filtration, methylene blue (MYB) dye is adsorbed onto the membrane surface, leading to a decrease in membrane flux after prolonged operation. After three cycles of fouling, the membrane was washed sequentially with deionized water and hydrogen peroxide. Figure 6 It can be seen that the total fouling rate of the membranes is: TFC > TFC-0.01 > TFNi-0.01. The flux recovery rate (FRR) is: TFNi-0.01 > TFC-0.01 > TFC, with TFNi-0.01 achieving a flux recovery rate of over 95%, while the TFC and TFC-0.01 membranes have flux recovery rates of 92% and 93%, respectively. This indicates that the hydrophilicity of TFNi-0.01 makes it difficult for dyes to adsorb onto the membrane surface. Simultaneously, the manganese dioxide in the intermediate layer helps in the catalytic generation of free radicals by hydrogen peroxide, demonstrating that the prepared nanofiltration membrane has excellent self-cleaning capabilities. After hydrogen peroxide cleaning, the dye retention remains at a high level, indicating that the generated free radicals did not cause serious damage to the membrane.

[0061] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing a self-cleaning composite nanofiltration membrane loaded with manganese dioxide, characterized by, The composite nanofiltration membrane comprises a support layer, a catalytically active manganese dioxide intermediate layer on the support layer, and a polyamide selective active layer on the uppermost layer. The preparation method of the composite nanofiltration membrane comprises the following steps: Step 1: cleaning the base film and storing for standby; Step 2: coating a layer of polydopamine on the base film; Step 3: soaking the polydopamine-coated film surface in a potassium permanganate solution to generate manganese dioxide in situ on the film surface; Step 4: generating a polyamide layer on the manganese dioxide intermediate layer by interfacial polymerization of an amine-containing aqueous monomer and an acyl chloride-containing organic monomer, specifically, first immersing the upper surface of the manganese dioxide intermediate layer in the aqueous solution, removing the excess solution after a period of time, then pouring the organic phase solution for reaction, washing the film surface after the reaction is completed, and finally performing a thermal crosslinking reaction.

2. The method of claim 1, wherein, Cleaning the base film specifically refers to using a 25wt% isopropyl alcohol solution to clean the glycerol on the polyether sulfone film, and then storing the film at 4℃ after washing with pure water.

3. The method of claim 1, wherein, Coating a layer of polydopamine on the base film specifically includes the following operations: preparing a 0.01M Tris-HCl buffer solution, adding hydrochloric acid dopamine with a final concentration of 2g / L, adjusting the pH to 8.5, fixing the base film, exposing the film surface to air, pouring the above dopamine solution, coating and reacting on a shaker with a rotation speed of 200rpm, and a reaction time of 15min, then washing the film surface with deionized water to remove unreacted dopamine solution, and obtaining the polydopamine-coated base film.

4. The method of claim 1, wherein, The concentration of the potassium permanganate solution is 0.01-0.1M, and the reaction time is 2min.

5. The method of claim 1, wherein, The amine monomer is piperazine, the acyl chloride monomer is trimesoyl chloride, and the organic phase solvent is n-hexane.

6. The method of claim 1, wherein, The aqueous immersion time is 3min, the interfacial polymerization (i.e. organic phase immersion time) is 1min; the interfacial polymerization reaction temperature is 25±3℃, and the relative humidity is 50~80%; the thermal crosslinking reaction is performed at 50℃ for 10min.

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