A method for preparing a molybdenum disulfide quantum dot-doped composite nanofiltration membrane and its application.

Abstract

This invention discloses a method for preparing a composite nanofiltration membrane modified with molybdenum disulfide quantum dots and its application, belonging to the field of modified nanofiltration membrane preparation. The preparation method is as follows: based on the traditional interfacial polymerization method for preparing nanofiltration membranes, hydrophilic molybdenum disulfide quantum dots are added to the aqueous monomer, and the addition ratio of nanomaterials is controlled. Vacuum filtration is used to deposit the nanomaterials onto the surface of the base membrane, resulting in a composite polyamide nanofiltration membrane with a large amount of negative surface charge and strong hydrophilicity in neutral aqueous solution. The prepared composite membrane has a low molecular weight cutoff, significantly reduced calcium and magnesium ion rejection, and a substantial increase in water flux. In practical drinking water treatment projects, it can significantly reduce energy consumption and has broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane preparation technology, and in particular to a method for preparing a composite nanofiltration membrane modified by molybdenum disulfide quantum dot doping and its application. Technical Background

[0002] In recent years, with economic and technological development, people's demand for drinking water quality has significantly increased, and providing high-quality water has gradually become the goal of domestic waterworks. Traditional drinking water treatment processes are not very effective at removing small-molecule organic matter. Among various advanced treatment technologies, nanofiltration and reverse osmosis are considered to be highly reliable and stable methods for producing high-quality water. Compared with reverse osmosis membranes, nanofiltration membranes have higher water permeation flux and lower operating energy consumption, thus having broad application prospects in drinking water treatment and other fields. Generally, separation membranes with a molecular weight cutoff of 200-1000 Da are collectively referred to as nanofiltration membranes. Selecting a nanofiltration membrane with a suitable molecular weight cutoff can effectively remove the vast majority of organic pollutants and improve water production efficiency.

[0003] Depending on the characteristics of the pollutant being treated, the effective removal of target pollutants can usually be achieved by selecting a nanofiltration membrane with an appropriate molecular weight cutoff. However, these nanofiltration membranes generally follow these rules: low molecular weight cutoff nanofiltration membranes have relatively small pore size distributions, poor water molecule permeability, and high Ca2+ content. 2+ / Mg 2+ Retention; while low Ca 2+ / Mg 2+ The nanofiltration membrane with high molecular weight cutoff and pore size distribution has a relatively large molecular weight cutoff. Figure 1 For drinking water treatment, appropriately reducing calcium... 2+ / Mg 2+ Retention is beneficial for maintaining water quality stability and reducing scaling on nanofiltration membranes, and it can also retain some calcium and magnesium elements that are beneficial to the human body. Therefore, it is important to develop a membrane with "low molecular weight cutoff, high permeability, and low calcium retention." 2+ / Mg 2+ Nanofiltration membranes with "interception" characteristics have great practical application value in fields such as drinking water treatment. Summary of the Invention

[0004] In view of the above situation, the purpose of this invention is to provide a method for preparing a composite nanofiltration membrane modified with molybdenum disulfide quantum dots and its application. When applied to water treatment, this nanofiltration membrane can achieve the goals of low calcium and magnesium ion rejection and high water permeation flux while ensuring a high sulfate ion rejection rate.

[0005] The retention efficiency of nanofiltration membranes for most neutral organic pollutants is primarily related to the molecular weight cutoff; the lower the molecular weight cutoff, the better the size sieving effect and the higher the retention rate. Besides the size sieving effect, the retention rate of nanofiltration membranes for inorganic charged ions is also affected by the Donnan effect and dielectric effect. Nanofiltration membrane surfaces are typically negatively charged; the greater the electronegativity, the more pronounced the charge repulsion effect on anions, resulting in a higher retention rate. Conversely, the charge attraction effect on cations such as calcium and magnesium also increases, thus reducing the retention efficiency. Since water molecules primarily permeate through the tiny pores and channels on and inside the membrane surface, the pore size distribution on and inside the nanofiltration membrane is a crucial factor affecting water permeation flux.

[0006] Specifically, the present invention first provides a method for preparing a composite nanofiltration membrane modified with molybdenum disulfide quantum dots. The preparation method includes: using a porous support membrane as a substrate, fixing the cleaned substrate, immersing it in an aqueous monomer containing molybdenum disulfide quantum dots for a certain period of time, removing excess liquid to obtain a base membrane deposited with molybdenum disulfide quantum dots, immersing the surface of the base membrane deposited with molybdenum disulfide quantum dots in an organic monomer solution to carry out an interfacial polymerization reaction, removing excess liquid after the reaction, and obtaining the high permeability nanofiltration membrane modified with molybdenum disulfide quantum dots after heat treatment.

[0007] In one embodiment of the present invention, the method specifically includes: using a porous support membrane as a substrate, fixing the cleaned substrate in a vacuum filtration device, immersing it in an aqueous monomer containing molybdenum disulfide quantum dots for a certain period of time, then removing excess liquid by vacuum filtration to obtain a base membrane deposited with molybdenum disulfide quantum dots, then performing an interfacial polymerization reaction on the surface of the base membrane deposited with molybdenum disulfide quantum dots in an organic monomer solution, discarding excess liquid after the reaction, rinsing with n-hexane solution, and obtaining the high permeability nanofiltration membrane modified with molybdenum disulfide quantum dots after heat treatment.

[0008] In one embodiment of the present invention, the porous support membrane is a polyethersulfone ultrafiltration membrane.

[0009] In one embodiment of the present invention, the pore size of the polyethersulfone ultrafiltration membrane is 5 kDa to 150 kDa, preferably 50 kDa to 150 kDa.

[0010] In one embodiment of the present invention, the concentration of molybdenum disulfide quantum dots in the aqueous monomer containing molybdenum disulfide quantum dots is 0.000625 wt% to 0.01 wt%, preferably 0.025 wt% to 0.01 wt%; the aqueous monomer also includes a PIP aqueous solution with a concentration of 0.05 wt% to 1 wt%, preferably 0.10 wt% to 0.60 wt%.

[0011] In one embodiment of the present invention, the size of the molybdenum disulfide quantum dots is 2-10 nm.

[0012] In one embodiment of the present invention, the molybdenum disulfide quantum dots are preferably purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., with the product number XF186-1, and are prepared by lithium intercalation method, possessing abundant surface defect sites and high specific surface area.

[0013] In one embodiment of the present invention, the immersion time in the aqueous monomer is 1-10 min, preferably 2-10 min, and more preferably 2-5 min.

[0014] In one embodiment of the present invention, the organic phase monomer solution is a hexane solution of trimesoyl chloride, and the concentration of trimesoyl chloride is 0.05wt%-1wt%, preferably 0.10wt%-0.5wt%.

[0015] In one embodiment of the present invention, the time of the interfacial polymerization reaction is 20-300s, preferably 40-80s.

[0016] In one embodiment of the present invention, the heat treatment is performed at 30-90°C for 5-30 minutes, preferably at 40-60°C for 8-12 minutes.

[0017] The present invention also provides a high-permeability nanofiltration membrane prepared by the above preparation method.

[0018] The present invention also provides a water treatment apparatus comprising the above-described high-permeability nanofiltration membrane.

[0019] The present invention also provides a water treatment method, wherein the water is treated using the above-mentioned high-permeability nanofiltration membrane.

[0020] The present invention also provides the application of the above-mentioned high-permeability nanofiltration membrane in water treatment processes.

[0021] The beneficial effects achieved by this invention are as follows:

[0022] This invention, based on the traditional interfacial polymerization method for preparing nanofiltration membranes, introduces highly hydrophilic molybdenum disulfide quantum dots into the aqueous monomer phase to participate in the interfacial polymerization reaction, constructing nanochannels and regulating the surface properties of the polyamide layer to prepare the target composite nanofiltration membrane. The introduction of hydrophilic nanomaterials during the interfacial polymerization process allows for the formation of hydrophilic nanochannels surrounding the material within the polyamide separation layer, while simultaneously enhancing the negative charge density on the nanofiltration membrane surface. This is beneficial for achieving high water flux and enhancing the dielectric effect.

[0023] The nanofiltration membrane prepared by this invention can achieve the goals of low calcium and magnesium ion rejection rate and high water permeation flux while ensuring high sulfate ion rejection rate and low molecular weight rejection rate when applied to water treatment, thus meeting the requirements of the drinking water treatment field. Attached Figure Description

[0024] Figure 1 The relationship between the molecular weight cutoff and MgCl2 cutoff of polyamide composite nanofiltration membrane.

[0025] Figure 2 The water flux is for Example 1 and Comparative Examples 1-6.

[0026] Figure 3 The rejection rates of five inorganic salt solutions are shown in Example 1 and Comparative Examples 1-6.

[0027] Figure 4 The images are surface scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM) images of Example 1 and Comparative Example 1.

[0028] Figure 5 The molecular weight cutoff values ​​are for Example 1 and Comparative Example 6.

[0029] Figure 6 The permeation-separation performance of polyamide nanofiltration membranes prepared under different base membrane and monomer concentration conditions obtained in Example 3 is shown. Detailed Implementation

[0030] The present invention will be described in detail below with reference to embodiments and comparative examples.

[0031] MoS2 quantum dots were purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., with product number XF186-1.

[0032] Example 1

[0033] A polyethersulfone porous support layer (100 kDa) was immersed in a mixed aqueous solution of piperazine (PIP) with a mass concentration of 0.20 wt.% and MoS2 quantum dots with a mass concentration of 0.0025 wt% for 3 min. After filtering out excess solution using a vacuum filtration device and depositing quantum dots on the surface of the base film, the layer was treated with a crosslinking reaction of trimesoyl chloride (TMC) in n-hexane solution with a mass concentration of 0.15% for 1 min. The unreacted TMC solution on the surface was rinsed with a large amount of n-hexane, and then heat-treated in an oven at 60 °C for 10 min to obtain the composite membrane.

[0034] After rinsing with deionized water, filtration performance was tested: using a CF016 membrane cell, the water flux and inorganic salt ion rejection capacity of the composite membrane prepared in the above embodiment were tested under cross-flow filtration conditions at an operating pressure of 4 bar and a temperature of 25°C. The test results are shown below. Figure 2 and Figure 3.

[0035] Comparative Examples 1-6

[0036] The preparation conditions for Comparative Examples 1-6 were the same as those for Example 1, except for the concentration of molybdenum disulfide quantum dots. The changes in preparation conditions are shown in Table 1 below. Filtration performance tests were conducted in the same manner as in Example 1, and the results are shown below. Figure 2 and Figure 3 .

[0037] Table 1. Preparation conditions of polyamide nanofiltration membranes for Comparative Examples 1–6

[0038]

[0039] The pure water permeation flux of Example 1 and the comparative example are shown in [reference needed]. Figure 2 It can be seen that, compared with the water flux of Comparative Example 1 (7.46 LMH / bar) prepared by the traditional polyamide interfacial polymerization method, the pure water flux of Example 1 (17.30 LMH / bar) is significantly improved. This is due to the enhanced hydrophilicity of the composite nanofiltration membrane surface and the effective filtration area of ​​the membrane surface (…). Figure 4 The study also found that further increasing the concentration of quantum dots in the nanomaterials (e.g., 0.01 wt%) did not significantly increase the pure water flux.

[0040] The retention rates of various inorganic salts in Examples 1 and Comparative Examples 1-6 are shown in the figure. Figure 3 The concentration of each inorganic salt solution was 1 g / L, and the pH was 7.2. It can be seen that in Example 1, while increasing the pure water flux, the rejection rates for sodium sulfate and magnesium sulfate remained comparable to those in Comparative Example 1, but the rejection rates for calcium chloride and magnesium chloride were significantly reduced. This is attributed to the enhanced membrane surface potential. The inorganic salt selectivity α(CaCl2 / Na2SO4) can be calculated (where α(CaCl2 / Na2SO4) = (1-R...). CaCl2 ) / (1-R Na2SO4 The molecular weight cutoff increased significantly from 17.4 in Comparative Example 1 to 114.8 in Example 1, while the molecular weight cutoff slightly increased from 242.6 Da in Comparative Example 1 to 282.2 Da (see Table 2 and ). Figure 5 This will help maintain a high organic matter rejection rate while reducing the calcium and magnesium ion rejection rate in drinking water treatment, thus contributing to the provision of high-quality drinking water. Similarly, the composite membranes prepared in Comparative Examples 3-6 can also reduce the calcium and magnesium ion rejection rate while maintaining a high organic matter rejection rate.

[0041] Therefore, when the concentration of the molybdenum disulfide quantum dot solution is 0.000625 wt% to 0.01 wt%, the resulting composite membrane can significantly improve the pure water flux, while reducing the rejection rates of calcium chloride and magnesium chloride without affecting sulfate rejection, thus achieving the goal of low calcium and magnesium ion rejection and high water permeation flux. Preferably, the concentration of the molybdenum disulfide quantum dot solution is 0.025 wt% to 0.01 wt%.

[0042] Table 2 Comparison of parameters between Example 1 and Comparative Example 1 of composite nanofiltration membrane

[0043]

[0044] Example 2

[0045] Interfacial polymerization for film formation is a diffusion-controlled rapid reaction. The rate and extent of the film formation reaction can be controlled by adjusting the relative diffusion rates of the reactants (aqueous and organic phases). Key control methods include the selection of the supporting substrate membrane, the choice of the reaction system (two-phase monomer type), and the selection of reactant monomer concentrations. By regulating the interfacial polymerization reaction conditions, the permeation and separation performance of the composite polyamide nanofiltration membrane can be optimized to achieve different separation targets. This embodiment uses interfacial polymerization to prepare a composite nanomaterial polyamide nanofiltration membrane. First, the concentrations of the aqueous and organic phase monomers, as well as the selection of the substrate membrane pore size, are optimized during the interfacial polymerization process to determine the relevant experimental conditions and parameters for subsequent nanofiltration membrane preparation.

[0046] In Example 2, we used 100 kDa polyethersulfone as the base membrane and conducted orthogonal experiments to prepare the membrane for six parameters: aqueous phase PIP concentration (0.05 wt%-1 wt%), aqueous phase wetting time (1-10 min), organic phase TMC concentration (0.05 wt%-1 wt%), interfacial polymerization reaction time (20-300 s), heat treatment temperature (30-90℃), and heat treatment time (5-30 min). The filtration performance was tested in the same manner as in Example 1.

[0047] The results showed that too low a monomer concentration prevented sufficient interfacial polymerization, resulting in an incomplete polyamide film and poor separation performance. Too high a monomer concentration significantly increased the thickness of the prepared polyamide film, leading to poor water permeability. A 3-minute aqueous wetting time was sufficient to fully wet the base membrane; extending the wetting time had no effect. Too short an interfacial polymerization time resulted in incomplete reaction, failing to form a complete polyamide film. A short time also resulted in a thick, dense film with poor permeation flux. A reaction time of 60 seconds yielded a nanofiltration membrane with good performance. Regarding heat treatment temperature and time, at lower temperatures (e.g., 30°C), even extending the treatment time to 30 minutes did not adequately promote the reaction. The resulting nanofiltration membrane's polyamide layer easily detached from the base membrane surface during water treatment, affecting the water treatment effect. At excessively high temperatures (e.g., 90°C), the high temperature caused rapid shrinkage and collapse of the pores within both the base membrane and the polyamide membrane, resulting in low water flux in the composite nanofiltration membrane. Therefore, by controlling the reaction conditions in Example 2 as follows: aqueous phase PIP concentration (0.10-0.60 wt%), aqueous phase wetting time (2-5 min), organic phase TMC concentration (0.10-0.5 wt%), interfacial polymerization reaction for 40-80 s, and heat treatment in an oven at 40-60℃ for 8-12 min, the prepared composite membrane can achieve the following: while maintaining a high organic matter rejection rate, it can reduce the rejection rate of calcium and magnesium ions and achieve high water permeability flux, with an inorganic salt selectivity α (CaCl2 / Na2SO4) of not less than 30.

[0048] Example 3

[0049] Based on Example 1, the concentration conditions of the base membrane and reactant monomers were further optimized. Five PES ultrafiltration membranes with different molecular weight cutoffs (5, 20, 50, 100, 150 kDa) and a 0.22 μm microfiltration membrane were selected as base membranes. The concentrations of the aqueous phase monomer PIP were 0.2 wt.% and 0.5 wt.%, respectively, and the concentrations of the organic phase monomer TMC were 0.15 wt.% and 0.3 wt.%, respectively. The remaining steps were the same as in Example 1. The pure water permeation performance and sodium sulfate rejection effect of different nanofiltration membranes were investigated.

[0050] The prepared membrane was tested in a triple CF016 cross-flow device system for pure water flux and MgSO4 rejection rate, and the results are as follows: Figure 6As shown, when the base membrane pore size is extremely small (the molecular weight cutoff is only 5 kDa PES), the pure water flux of the prepared nanofiltration membranes is generally lower than 3 LMH / bar due to the low permeation flux of the base membrane itself (only 10 LMH / bar in the cross-flow mode at 25℃ and 3 bar) and the collapse and shrinkage of the membrane pores during oven heat treatment (corresponding to membrane numbers 1, 2, 3, and 4). Microfiltration membranes (0.22 μm), on the other hand, often have many defects due to the excessively large pore size of the supporting membrane, resulting in polyamide nanofiltration membranes polymerized at their surface interface (corresponding to membrane numbers 21, 22, 23, and 24). Although their flux is as high as 50 LMH / bar, the magnesium sulfate rejection is only 20%-40%, which does not meet the general requirements of nanofiltration membranes. For nanofiltration membranes prepared from four ultrafiltration membrane substrates (20, 50, 100, 150 kDa) with suitable molecular weight cutoff, their permeation separation performance is affected by the pore size distribution of the substrate, the concentration of monomers in the aqueous phase, and the concentration of monomers in the organic phase. Based on a comprehensive analysis of pure water flux and MgSO4 retention effect, the optimal membrane preparation parameters are 50-150 kDa PES substrate, 0.2-0.3 wt.% PIP aqueous solution, and 0.15-0.3 wt.% TMC / n-hexane solution, with an inorganic salt selectivity α (CaCl2 / Na2SO4) of not less than 30.

[0051] Table 3. Preparation conditions of different amino nanofiltration membranes in Example 3

[0052]

[0053] Comparative Example 7

[0054] Besides molybdenum disulfide quantum dots, we also investigated the application of carbon quantum dots in the modification of composite nanofiltration membranes. The results showed that carbon quantum dots, due to their poor hydrophilicity, could not be well dispersed in aqueous piperazine monomer solutions. During nanofiltration membrane preparation, they easily aggregated on the membrane surface, failing to enhance the performance of the composite nanofiltration membrane. Furthermore, the aggregated quantum dot material also led to a decline in the selective separation performance of the composite nanofiltration membrane. In addition, due to the poor compatibility between carbon quantum dots and polyamide, nanomaterials were easily lost during subsequent water treatment processes, causing secondary pollution and affecting the quality of the effluent.

[0055] Comparative Example 8

[0056] In addition to the molybdenum disulfide quantum dots prepared by lithium intercalation used in this invention, we also investigated the use of quantum dots prepared by ultrasonic exfoliation for the modification of composite nanofiltration membranes. The results showed that the quantum dot dispersion prepared by ultrasonic exfoliation contained unexfoliated blocky molybdenum disulfide or incompletely exfoliated layered molybdenum disulfide nanosheets. The large size of the molybdenum disulfide led to inhomogeneity in the prepared composite nanofiltration membrane and increased the likelihood of defects, resulting in poor overall membrane separation performance. Furthermore, the quantum dots prepared by ultrasonic exfoliation had fewer surface defect sites, resulting in poor cross-linking with the polyamide matrix during nanofiltration membrane modification. This led to reduced stability of the nanomaterial distribution within the membrane, and the improvement in surface hydrophilicity and electronegativity was not as effective as that of molybdenum disulfide quantum dots prepared by lithium intercalation.

[0057] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, and for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the specific details without departing from the general concept defined by the claims and their equivalents.

Claims

1. A method for preparing a composite nanofiltration membrane doped with molybdenum disulfide quantum dots, characterized in that, The preparation method includes: using a porous support membrane as a substrate, fixing the cleaned substrate, immersing it in an aqueous monomer containing molybdenum disulfide quantum dots for a certain period of time, removing excess liquid to obtain a base membrane deposited with molybdenum disulfide quantum dots, immersing the surface of the base membrane deposited with molybdenum disulfide quantum dots in an organic monomer solution to carry out an interfacial polymerization reaction, removing excess liquid after the reaction, and obtaining the high permeability nanofiltration membrane modified with molybdenum disulfide quantum dots after heat treatment; The molybdenum disulfide quantum dots are lithium-intercalated molybdenum disulfide quantum dots; The concentration of molybdenum disulfide quantum dots is 0.025 wt%~0.01 wt%; the aqueous monomer also includes a PIP aqueous solution with a concentration of 0.05 wt%-1 wt%; the porous support membrane is a polyethersulfone ultrafiltration membrane.

2. The production method according to claim 1, characterized by, The immersion time in the aqueous monomer is 1-10 min.

3. The preparation method according to claim 1, characterized in that, The organic phase monomer solution is a hexane solution of trimesoyl chloride, and the concentration of trimesoyl chloride is 0.05 wt%-1 wt%.

4. The preparation method according to claim 1, characterized in that, The interfacial polymerization reaction takes 20-300 seconds; the heat treatment is performed at 30-90 °C for 5-30 minutes.

5. The high-permeability nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 4.

6. A water treatment device, characterized in that, The device comprises the high-permeability nanofiltration membrane as described in claim 5.

7. A water treatment method, characterized in that, The method uses the high-permeability nanofiltration membrane described in claim 5 to purify water.

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