Preparation method and application of tree-like structure nanofiltration membrane

Abstract

The present invention discloses a method for preparing and applying a tree-like structure nanofiltration membrane. The method first uses cellulose fiber filter paper as a substrate, in situ growing a hydroxyapatite nanowire array through a solvent thermal reaction; then the resulting composite filter paper is repeatedly immersed in an ethanol solution of trimesic acid and ferric chloride to obtain a tree-like structure nanofiltration membrane through continuous deposition. Compared to conventional nanofiltration membranes, the tree-like structure nanofiltration membrane of the present invention has greater surface roughness and a higher specific surface area, and is therefore more resistant to anionic / cationic dyes and heavy metal Pb. 2+ The ions all showed extremely high retention rates (>94%), with excellent environmental benefits.

Description

Technical Field

[0001] The invention belongs to the field of membrane water treatment, and particularly relates to the preparation of a tree-imitation structure nanofiltration membrane and its application in the field of water purification. Background Art

[0002] The indiscriminate discharge of industrial wastewater, agricultural wastewater and domestic sewage has led to an increasingly serious water pollution problem. According to the United Nations World Water Report, 80% of the world's sewage is discharged directly without treatment, forcing nearly 2.1 billion people to drink contaminated water. Drinking water safety is an urgent livelihood issue that needs to be addressed. Water purification technology based on membrane separation has the advantages of low energy consumption, easy operation, and good water quality, and has become the current mainstream water treatment technology. Membrane water treatment has the advantages of low energy consumption, easy operation, small footprint, and good water quality. The development of high-performance, low-cost membrane separation technology is one of the main research areas of ecological and environmental materials science at present and in the future. Nanofiltration membranes have the ability to separate metal ions from small molecular organic matter and are widely used in water treatment, food industry, biomedicine, and other fields such as seawater desalination, water resource regeneration, drug separation, concentrated juice, dairy processing, solvent recovery, etc.

[0003] The existing nanofiltration membrane preparation process is to attach a polymer separation layer to a support layer. Existing nanofiltration membranes have the following shortcomings: (1) The separation layer is attached to the support layer in a horizontal manner, which is prone to agglomeration, resulting in a low specific surface area and slow mass transfer; (2) Actual wastewater contains both positively charged heavy metal ions and negatively or positively charged organic matter. Existing nanofiltration membranes are difficult to effectively remove organic matter and heavy metal ions with different properties. Therefore, how to solve the bottleneck problem faced by nanofiltration membranes and design and develop new nanofiltration membranes that can efficiently remove organic matter and heavy metal ions is a research difficulty in the current membrane separation field.

[0004] This patented invention breaks through existing nanofiltration membrane preparation processes and draws on biomimetic concepts to design a new composite nanofiltration membrane. The biomimetic design concept is as follows: the forest system has self-purification capabilities, and its organizational structure includes roots, trunks, branches, and leaves, each of which has its own functions. Roots prevent wind and sand, and provide stable support for the trunks, branches, and leaves; the trunks, branches, and leaves intercept most rainfall, dust, and toxic particles, acting as a filter; and leaves can also absorb harmful sulfur dioxide and carbon dioxide gases. On the other hand, the gaps between trees ensure the passage of sunlight, the circulation of air, and the circulation of water. In other words, through the multi-level, orderly combination of components with different functions, the forest ecosystem can not only efficiently filter out harmful substances, but also selectively allow specific substances to pass through.

[0005] Inspired by forest ecosystems, this patent proposes a novel composite nanofiltration membrane. Using cellulose fiber filter paper as a substrate, hydroxyapatite nanowire arrays are grown on the cellulose fibers, and metal-organic framework (MOF) nanocrystals are then deposited on the hydroxyapatite nanowire arrays. In this novel composite nanofiltration membrane, the cellulose fibers resemble tree roots, the hydroxyapatite nanowire arrays resemble tree trunks, and the MOF nanocrystals resemble leaves. The root-trunk-leaves network forms a three-dimensional tree-like structure, effectively increasing specific surface area and promoting mass transfer. Furthermore, this novel composite nanofiltration membrane integrates the ion exchange properties of hydroxyapatite with the high adsorption properties of MOF nanocrystals, enabling efficient removal of organic matter and heavy metal ions from wastewater. Summary of the Invention

[0006] This invention aims to address key issues with conventional horizontal nanofiltration membranes by creating a tree-like nanofiltration membrane. First, a cellulose fiber / hydroxyapatite nanowire array composite filter paper is prepared. Then, MOF nanocrystals are finely dispersed on the surface of the hydroxyapatite nanowires, effectively increasing surface roughness and specific surface area. Furthermore, hydroxyapatite exhibits ion exchange properties, enabling efficient interception of heavy metal ions. In this novel composite nanofiltration membrane, the cellulose fibers resemble tree roots, the hydroxyapatite nanowire arrays resemble the trunk, and the MOF nanocrystals resemble the leaves, creating a tree-like structure. The synergistic effect of the roots, trunk, and leaves results in a high-performance nanofiltration membrane.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] (1) Weighing appropriate amounts of oleic acid and ethanol, mixing them uniformly by mechanical stirring, and sequentially adding calcium chloride dihydrate aqueous solution, sodium hydroxide aqueous solution, and sodium dihydrogen phosphate dihydrate aqueous solution, and mixing them uniformly by mechanical stirring to obtain a solvent thermal reaction solution;

[0009] (2) placing the cellulose fiber filter paper in a Teflon reactor, pouring the reaction solution obtained in step (1) into the Teflon reactor, and after solvent thermal reaction, ultrasonically cleaning the composite filter paper with ethanol;

[0010] (3) Soak the composite filter paper obtained in step (2) in a certain volume of 1,3,5-trimethylbenzenetricarboxylic acid ethanol solution, keep it at a certain temperature for a certain time; remove the filter paper, rinse it with ethanol, transfer the filter paper to a certain volume of ferric chloride hexahydrate ethanol solution, keep it at a certain temperature for a certain time; remove the filter paper, and rinse it with ethanol. The above steps are a single deposition, and by repeating this step continuously, nanofiltration membranes with different MOF loadings can be obtained.

[0011] Furthermore, in step (1), the mass ratio of oleic acid to ethanol is 5:7, the concentration of the calcium chloride dihydrate aqueous solution is 0.05-0.2 mol / L, the concentration of the sodium hydroxide aqueous solution is 0.5-1.5 mol / L, and the concentration of the sodium dihydrogen phosphate dihydrate aqueous solution is 0.15-0.25 mol / L.

[0012] Furthermore, in step (2), the solvent thermal reaction temperature is 150-250° C., and the reaction time is 10-36 hours.

[0013] Furthermore, in step (3), the concentrations of the 1,3,5-tricariic acid ethanol solution and the ferric chloride hexahydrate ethanol solution are both 0.005-0.02 mol / L, the reaction liquid volume is 10-30 mL, the reaction temperature is 20-80°C, the reaction time is 10-60 minutes, and the number of depositions is 5-20 times.

[0014] The beneficial effects of the present invention are:

[0015] (1) This invention innovatively proposes a design strategy for a tree-like structured nanofiltration membrane, cleverly dispersing MOF nanocrystals on a hydroxyapatite nanowire array, exhibiting properties far exceeding those of conventional horizontally structured MOF nanofiltration membranes.

[0016] (2) The tree-like structure nanofiltration membrane prepared by the present invention integrates electrostatic interaction, hydrogen bonding, π-π interaction, and ion exchange, thereby showing extremely high retention rates for positively charged dyes, negatively charged dyes, and heavy metal ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The scanning electron micrographs of the filter membranes prepared in Example 1 and Comparative Example 1 are shown;

[0018] Figure 2 Atomic force microscope images of the filter membranes prepared in Example 1 and Comparative Example 1;

[0019] Figure 3 N2 adsorption and desorption isotherms of the filter membranes prepared in Example 1 and Comparative Example 1;

[0020] Figure 4 Comparison of Congo red rejection rates of the filter membranes prepared in Examples 1-3;

[0021] Figure 5 Comparison of the retention rates of the filter membranes prepared in Example 1 and Comparative Example 1 for anionic dyes Congo red and Alizarin red, and cationic dyes methylene blue and Alkali Blue 6B;

[0022] Figure 6 The filter membrane prepared in Example 1 and Comparative Example 1 is 2+ Comparison of ion retention rates; DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined as long as they do not conflict with each other.

[0024] Example 1

[0025] (1) Under mechanical stirring, add 20 mL of an aqueous solution containing 0.294 g of calcium chloride dihydrate to a mixed solvent of 60 g of oleic acid and 84 g of ethanol, stir for 20 minutes to mix evenly, add 20 mL of an aqueous solution containing 1 g of sodium hydroxide, stir for 20 minutes to mix evenly, add 10 mL of an aqueous solution containing 0.281 g of sodium dihydrogen phosphate dihydrate, stir for 20 minutes to mix evenly.

[0026] (2) placing the cellulose fiber filter paper in a Teflon reactor, pouring the reaction solution obtained in step (1) into the reactor, sealing it, and placing it in an oven for reaction. The reaction was kept at 180°C for 24 hours. After the reaction was completed, the composite filter paper was placed in ethanol for ultrasonic treatment to clean the surface residue, and dried at 60°C to obtain the composite filter paper;

[0027] (3) Add 15 mL of 1,3,5-trimethylbenzenecarboxylic acid ethanol solution (solubility: 0.01 mol / L) to a glass container, soak the composite filter paper obtained in step (2) in the above solution, keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol; then soak the filter paper in 15 mL of ferric chloride hexahydrate ethanol solution (solubility: 0.01 mol / L), keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol. The above deposition steps are repeated 10 times to obtain a tree-like structure nanofiltration membrane.

[0028] Comparative Example 1

[0029] Add 15 mL of 1,3,5-pyromellitic acid ethanol solution (0.01 mol / L) to a glass container. Soak a cellulose fiber filter paper in this solution and incubate at 60°C for 30 minutes. Remove the filter paper and rinse the surface residue with ethanol. Then, soak the filter paper in 15 mL of ferric chloride hexahydrate ethanol solution (0.01 mol / L) and incubate at 60°C for 30 minutes. Remove the filter paper and rinse the surface residue with ethanol. Repeat this deposition process 10 times to obtain a conventional horizontal nanofiltration membrane.

[0030] Example 2

[0031] (1) Under mechanical stirring, add 20 mL of an aqueous solution containing 0.294 g of calcium chloride dihydrate to a mixed solvent of 60 g of oleic acid and 84 g of ethanol, stir for 20 minutes to mix evenly, add 20 mL of an aqueous solution containing 1 g of sodium hydroxide, stir for 20 minutes to mix evenly, add 10 mL of an aqueous solution containing 0.281 g of sodium dihydrogen phosphate dihydrate, stir for 20 minutes to mix evenly.

[0032] (2) placing the cellulose fiber filter paper in a Teflon reactor, pouring the reaction solution obtained in step (1) into the reactor, sealing it, and placing it in an oven for reaction. The reaction was kept at 180°C for 24 hours. After the reaction was completed, the composite filter paper was placed in ethanol for ultrasonic treatment to clean the surface residue, and dried at 60°C to obtain the composite filter paper;

[0033] (3) Add 15 mL of 1,3,5-trimethylbenzenetricarboxylic acid ethanol solution (solubility: 0.01 mol / L) to a glass container, soak the composite filter paper obtained in step (2) in the above solution, keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol; then soak the filter paper in 15 mL of ferric chloride hexahydrate ethanol solution (solubility: 0.01 mol / L), keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol. The above deposition steps are repeated 5 times to obtain a tree-like structure nanofiltration membrane.

[0034] Example 3

[0035] (1) Under mechanical stirring, add 20 mL of an aqueous solution containing 0.294 g of calcium chloride dihydrate to a mixed solvent of 60 g of oleic acid and 84 g of ethanol, stir for 20 minutes to mix evenly, add 20 mL of an aqueous solution containing 1 g of sodium hydroxide, stir for 20 minutes to mix evenly, add 10 mL of an aqueous solution containing 0.281 g of sodium dihydrogen phosphate dihydrate, stir for 20 minutes to mix evenly.

[0036] (2) placing the cellulose fiber filter paper in a Teflon reactor, pouring the reaction solution obtained in step (1) into the reactor, sealing it, and placing it in an oven for reaction. The reaction was kept at 180°C for 24 hours. After the reaction was completed, the composite filter paper was placed in ethanol for ultrasonic treatment to clean the surface residue, and dried at 60°C to obtain the composite filter paper;

[0037] (3) Add 15 mL of 1,3,5-trimethylbenzenecarboxylic acid ethanol solution (solubility: 0.01 mol / L) to a glass container, soak the composite filter paper obtained in step (2) in the above solution, keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol; then soak the filter paper in 15 mL of ferric chloride hexahydrate ethanol solution (solubility: 0.01 mol / L), keep it at 60°C for 30 minutes, remove the filter paper, and rinse the surface residue with ethanol. The above deposition steps are repeated 15 times to obtain a tree-like structure nanofiltration membrane.

[0038] Performance evaluation

[0039] 1. The filter membranes prepared in the examples and comparative examples were subjected to an organic pollutant separation experiment. The specific steps are as follows:

[0040] (1) Prepare aqueous solutions of Congo red, alizarin red, methylene blue, and alkaline blue 6B at a concentration of 10 ppm. Use a UV-visible spectrophotometer to measure the absorbance of each solution, which is recorded as C0.

[0041] (2) Place the filter membrane in the sample holder, draw 7 mL of dye solution with a syringe, connect it to the sample holder, and pass the dye solution through the filter membrane using a micro-injection pump. The flow rate of the syringe pump is set to 30 mL / h.

[0042] (3) After the dye solution has completely passed through the filter membrane, the absorbance of the filtrate is measured using a UV-visible spectrophotometer and recorded as C t , the dye retention rate is calculated according to the following formula:

[0043] Retention rate = (1-C t / C0)×100%

[0044] 2. The filter membranes prepared in the examples and comparative examples were subjected to a heavy metal ion separation experiment. The specific steps are as follows:

[0045] (1) Prepare lead nitrate aqueous solution to make Pb 2+ The ion concentration is 10 ppm, and the actual concentration of the solution is tested using an atomic absorption spectrophotometer and recorded as C0;

[0046] (2) Place the filter membrane in the sample holder, draw 7 mL of dye solution with a syringe, connect it to the sample holder, and pass the dye solution through the filter membrane using a micro-injection pump. The flow rate of the syringe pump is set to 15 mL / h.

[0047] (3) Pb 2+ After all the ionic solutions have passed through the filter membrane, the concentration of the filtrate is measured using an atomic absorption spectrophotometer and recorded as C t , Pb 2+ The ion retention rate was calculated according to the following formula;

[0048] Retention rate = (1-C t / C0)×100%

[0049] Result Analysis

[0050] Figure 1 Scanning electron micrographs of the tree-like structure nanofiltration membrane described in Example 1 and the horizontal structure MOF nanofiltration membrane described in Comparative Example 1 show that hydroxyapatite nanowire arrays grow vertically on the cellulose fiber surface, and MOF nanocrystals are evenly dispersed on the hydroxyapatite nanowire surfaces, forming a unique tree-like structure nanofiltration membrane. In the absence of hydroxyapatite nanowire arrays, MOF nanocrystals severely aggregate on the cellulose fiber surface, forming a conventional horizontal nanofiltration membrane.

[0051] Figure 2 These are atomic force microscopy images of the tree-like nanofiltration membrane described in Example 1 and the horizontal nanofiltration membrane described in Comparative Example 1. As can be seen, the tree-like nanofiltration membrane has a higher average surface roughness of 163 nanometers, while the horizontal nanofiltration membrane has a lower average surface roughness of only 123 nanometers. A greater average surface roughness facilitates contact between pollutants and the membrane, thereby improving the retention rate.

[0052] Figure 3 The N2 adsorption and desorption isotherms of the tree-like structure nanofiltration membrane described in Example 1 and the horizontal nanofiltration membrane described in Comparative Example 1 are shown in the figure. As can be seen from the figure, the tree-like structure nanofiltration membrane effectively increases the specific surface area to 48.99 m 2 g −1 The specific surface area of ​​conventional horizontal nanofiltration membrane is only 28.99 m 2 g −1 This indicates that hydroxyapatite nanowire arrays can inhibit the aggregation of MOF nanocrystals and improve their dispersion, thereby significantly increasing the specific surface area. A higher specific surface area can effectively promote mass transfer and provide more active sites for pollutant adsorption.

[0053] Figure 4 The retention rate of the Congo red solution by the tree-like structure nanofiltration membrane described in Examples 1-3 is shown in the figure. As can be seen, the number of deposition cycles determines the loading of MOF nanocrystals and has a significant impact on the retention rate. When the number of deposition cycles is 10, the retention rate reaches 98.48%.

[0054] Figure 5This figure compares the retention rates of the dendritic nanofiltration membrane described in Example 1 and the horizontal nanofiltration membrane described in Comparative Example 1 for the cationic dyes Alkali Blue 6B and Methylene Blue, and the anionic dyes Congo Red and Alizarin Red. As shown in the figure, compared to the horizontal nanofiltration membrane, the dendritic nanofiltration membrane achieves a retention rate exceeding 97% for dyes of varying properties, demonstrating the superiority of its structural design. This improved retention rate is primarily due to its greater surface roughness and higher specific surface area.

[0055] Figure 6 The tree-like structure nanofiltration membrane described in Example 1 and the horizontal nanofiltration membrane described in Comparative Example 1 were used to measure the Pb 2+ Comparison of the rejection rate of ions. As can be seen from the figure, the horizontal nanofiltration membrane has a strong effect on the retention of Pb 2+ The ion retention rate is only 32.66%, indicating that MOF nanocrystals and Pb 2+ The interaction between ions is weak and cannot effectively separate Pb 2+ ions; Tree-like structure nanofiltration membrane for Pb 2+ The ion retention rate reached 94.46%, which is due in part to the high surface roughness and specific surface area of ​​the tree-like nanofiltration membrane, and in part to the excellent ion exchange capacity of hydroxyapatite. This result demonstrates that hydroxyapatite nanowire arrays not only effectively inhibit the aggregation of MOF nanocrystals and improve their dispersion, but also provide ion exchange separation capabilities, compensating for the poor heavy metal ion retention of MOF nanocrystals.

[0056] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a tree-like structure nanofiltration membrane, characterized in that: The preparation method comprises the following steps: (1) Hydroxyapatite nanowire arrays were in situ grown on cellulose fiber filter paper by solvothermal method; (2) repeatedly immersing the filter paper obtained in step (1) in a metal salt reaction solution and an organic ligand reaction solution by a continuous deposition method to obtain a tree-like structure; The cellulose fiber filter paper acts like a tree root, on which a hydroxyapatite nanowire array grows vertically, and the hydroxyapatite nanowire array acts like a tree trunk; MOF nanocrystals continue to grow on the hydroxyapatite nanowire array, and the MOF nanocrystals act like leaves. The roots, trunk, and leaves work synergistically to obtain a high-performance nanofiltration membrane.

2. The method for preparing a tree-like structure nanofiltration membrane according to claim 1, characterized in that: The solvent-thermal reaction temperature in step (1) is 150-250° C., and the reaction time is 10-36 hours.

3. The method for preparing a tree-like structure nanofiltration membrane according to claim 1, wherein: The reaction solution used in the solvothermal method in step (1) is a mixture of aqueous solutions of a calcium source and a phosphorus source with oleic acid and ethanol; the mass ratio of the oleic acid to the ethanol is 5:7, the concentration of the calcium source aqueous solution is 0.05-0.2 mol / L, and the concentration of the phosphorus source aqueous solution is 0.15-0.25 mol / L.

4. The method for preparing a tree-like structure nanofiltration membrane according to claim 1, wherein: The reaction temperature of the continuous deposition method in step (2) is 20-80° C., the deposition time is 10-60 minutes, and the number of depositions is 5-20 times.

5. The method for preparing a tree-like structure nanofiltration membrane according to claim 1, wherein: The reaction solution in step (2) is an ethanol solution of ferric chloride and an ethanol solution of trimesic acid, both with a concentration of 0.005-0.02 mol / L.

6. A tree-like structure nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the tree-like structure nanofiltration membrane as claimed in claim 6 in the purification of dye and heavy metal wastewater.

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