Coal-based NS-GQDs calcium-magnesium crosslinking-SA composite membrane and preparation method thereof

Abstract

The invention discloses a preparation method of a coal-based NS-GQDs (at) calcium-magnesium crosslinking-SA composite membrane. The preparation method comprises the following steps: firstly, obtaining N-S co-doped graphene quantum dots (NS-GQDs); and then dispersing the SA-NS-GQD in sodium alginate (SA) to prepare an SA-NS-GQD composite solution, spin-coating the SA-NS-GQD composite solution on a polyvinylidene fluoride-based membrane, and forming a three-dimensional cross-linked hydrogel network through an SA coating on the surface of a calcium ion and magnesium ion co-cross-linked base membrane to prepare the composite membrane. According to the invention, the covalent interaction of SA and NS-GQDs is utilized, so that the photocatalyst NS-GQDs are stably loaded in the membrane, and the membrane is endowed with excellent adsorption and degradation capabilities. A network structure formed by co-crosslinking SA rich in oxygen-containing groups on the surface and calcium and magnesium ions significantly enhances the mechanical strength and flexibility of the membrane, and optimizes the pore structure at the same time. The composite membrane disclosed by the invention has relatively high mechanical strength, pollution resistance and photocatalytic degradation performance while keeping high flux, and can meet the long-term wastewater treatment requirement of high pollution load.

Description

Technical Field

[0001] The present invention relates to the technical field of high-performance membrane materials, and particularly relates to a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane and a preparation method thereof. Background Art

[0002] Organic synthetic dyes play a crucial role in daily life and industrial production, and are widely used in the paper-making, textile, printing and dyeing industries, providing bright colors for fashionable clothing, furniture and interior decoration. However, these organic dyes are highly toxic, not only with a wide variety, but also most of them are difficult to degrade by themselves. Even a small amount of untreated or incompletely treated dyes can cause serious harm to the freshwater ecosystem. Currently, various technologies such as ozone oxidation, electrochemical methods, membrane separation, biological methods, adsorption and photocatalytic degradation have been adopted to remove methylene blue. Due to the advantages of high adaptability and selectivity of membrane technology, it is considered a multifunctional method for treating dye wastewater. However, membrane fouling is one of the main defects of membrane separation technology. Especially when treating printing and dyeing wastewater, the membrane surface is easily contaminated by dyes and chemical auxiliaries, resulting in a decrease in membrane flux and a reduction in filtration efficiency. In addition, the durability and anti-fouling ability of membrane materials are limited. After long-term use, the mechanical strength of the membrane is insufficient and easy to damage, causing frequent maintenance and replacement, increasing the operating cost.

[0003] Photocatalysis is an effective and economical strategy for preparing self-cleaning or anti-fouling membranes by degrading pollutants. The combination of membrane separation and photocatalysis technologies can solve the problems of membrane fouling and suspended photocatalyst separation. Among various commonly used semiconductor photocatalysts, graphene quantum dots (GQDs) have become important candidate materials for enhancing membrane functionality due to their excellent hydrophilicity, anti-fouling property and high surface area characteristics, especially in photocatalytic degradation of pollutants and anti-fouling. However, GQDs particles are fine and easy to agglomerate during membrane preparation, causing membrane surface defects and affecting membrane separation performance. In addition, GQDs are prone to photo-generated electron-hole recombination, hindering the outward diffusion of carriers, thus slowing down the degradation reaction occurring in semiconductor organic matter.

[0004] Hydrogels can slow down the aggregation of nanomaterials and create an environment conducive to catalytic reactions on their surfaces. In addition, natural polysaccharide composite hydrogel materials have a rich pore structure, excellent hydrophilicity and adsorption properties, and can uniformly load nanophotocatalysts. Therefore, nanocomposite hydrogels show good application prospects in the field of wastewater treatment. Sodium alginate (SA) is a non-toxic and biocompatible natural polymer material. Once it encounters divalent calcium cations, it can quickly form a stable and thermally irreversible calcium alginate hydrogel (CaAlg). This property makes CaAlg an ideal material to replace traditional matrices as a polymer carrier for photocatalysts and for membrane surface modification. Although CaAlg can form an "eggbox" structure to form a stable three-dimensional network structure, endowing the membrane with high mechanical strength and good anti-pollution ability, CaAlg may face problems such as insufficient crosslinking strength, insufficient membrane layer uniformity, and poor thermal stability during long-term use. It is often difficult for single-step crosslinking or single materials to achieve the best balance among water flux, rejection rate, and chemical stability, which limits their wide use in practical applications. Summary of the Invention

[0005] In order to overcome the above technical problems, the purpose of the present invention is to provide a coal-based NS-GQDs@calcium-magnesium crosslinked-SA composite membrane and its preparation method. By using the good interaction between coal-based NS-GQDs and SA porous materials to construct the composite membrane, the advantages are complementary. NS-GQDs are uniformly dispersed in the SA medium, which not only effectively prevents aggregation, but also significantly improves the separation of photogenerated electrons and holes, and improves the photocatalytic degradation efficiency; while the SA porous material provides an ideal platform for efficient photocatalysis with its excellent adsorption and 100% light transmittance. At the same time, through the multi-ion co-crosslinking of SA and calcium and magnesium ions, an efficient and tight "eggbox" structure is formed, which can effectively regulate the membrane crosslinking density. The finally obtained composite membrane integrates filtration, adsorption, and photocatalysis, providing a new water treatment material for green organic degradation of dye degradation and self-purification.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A preparation method of a coal-based NS-GQDs@calcium-magnesium crosslinked-SA composite membrane, comprising the following steps:

[0008] Step 1: Prepare coal-based nitrogen-sulfur co-doped graphene quantum dots (NS-GQD) to obtain NS-GQD powder;

[0009] Step 2: Disperse the NS-GQD powder in deionized water, and then add sodium alginate (SA) thereto to obtain a SA-NS-GQD composite solution. The SA-NS-GQD composite solution is coated on the surface of a pre-wetted PVDF membrane, and the PVDF membrane is immersed in calcium and magnesium (Ca2+ Mg 2+ ) After carrying out a cross-linking reaction on the mixed solution, a coal-based NS-GQD@CaMg-SA composite membrane is obtained.

[0010] The specific steps of Step 1 are as follows:

[0011] Step 1: Mix the pretreated high-sulfur coal powder and gardenia fruit powder, then add them to a hydrogen peroxide solution, and stir under ice bath conditions to fully dissolve and oxidize the above powder substances to obtain Solution A1.

[0012] Step 2: Add an amino acid solution to Solution A1 and continue to stir the mixture to allow the amino acid solution to fully react and be evenly distributed to obtain Solution A2.

[0013] Step 3: Subject Solution A2 to ultrasonic treatment at 40 - 60 kHz under normal temperature and pressure to obtain an oxidized mixed solution, and then treat it with a 25% ammonium hydroxide solution to adjust the pH value of the oxidized mixed solution to 7 to obtain a neutral mixture.

[0014] Step 4: Centrifuge the above neutral mixture and then dialyze it to obtain a purified NS-GQD dispersion liquid, and finally perform freeze-drying on it to obtain NS-GQD powder.

[0015] The pretreatment steps of the high-sulfur coal powder in Step 1: Grind the crushed anthracite at a rotation speed of 200 - 300 r / min for 4 h, then screen out coal powder with a particle size of D99 < 74 μm, store the prepared coal powder in a sealed manner, and dry it in an oven at 105 °C for 12 - 14 h before use.

[0016] The mass ratio of the high-sulfur coal powder to the gardenia fruit powder is between 4:1 and 6:1.

[0017] The ratio of the hydrogen peroxide solution to the powder is as follows: 250 mL of 30% hydrogen peroxide solution and 30 g of powder (25 g of high-sulfur coal powder and 5 g of gardenia fruit powder), and 10 - 15 times the volume of hydrogen peroxide solution is used for every 10 g of high-sulfur coal powder; for 25 g of high-sulfur coal powder, 250 - 375 mL of 30% hydrogen peroxide solution is used.

[0018] Mix 25 g of pretreated high-sulfur coal powder, 5 g of gardenia fruit powder and 250 mL of 30% hydrogen peroxide solution, and stir under ice bath conditions at 0 - 4 °C for 1 hour for oxidation reaction.

[0019] The preparation steps of the amino acid solution in Step 2: Dissolve the amino acid in deionized water and stir evenly to obtain an amino acid solution with a solution concentration of 0.8 - 1.2 g / mL.

[0020] Ratio of amino acid solution to solution A1: Add 5 - 10 mL of amino acid solution to every 100 mL of solution A1, and the final addition amount is between 12.5 - 25 mL.

[0021] In step 2 described above, the amino acid is one of methionine, arginine, glycine, histidine, phenylalanine, proline, alanine, aspartic acid, cysteine, and valine.

[0022] In step 3 described above, the ultrasonic treatment time of the oxidized mixed solution is 6 - 10 hours;

[0023] In step 4 described above, the neutral mixture is centrifuged and then filtered. The filtered solution is placed in a dialysis bag with a molecular weight cut-off of 500 - 1000 Da and dialyzed for 3 - 5 days. The dialysis fluid is regularly changed daily. After dialysis, it is freeze-dried at -15°C for 24 hours to obtain nitrogen and sulfur co-doped graphene quantum dot powder.

[0024] The specific content of step 2 is as follows:

[0025] Step 1: Immerse the polyvinylidene fluoride (PVDF) base membrane in anhydrous ethanol solution for more than 1 h, and then rinse it with deionized water to obtain a pre-wetted PVDF membrane;

[0026] Step 2: Disperse the NS-GQD powder in deionized water, ultrasonically disperse it for 30 - 60 min to obtain a dispersion. Then add SA to the dispersion to obtain liquid S1. Stir liquid S1 at 30°C for 12 - 24 h. After filtering, standing, and degassing the obtained viscous solution, a uniform SA-NS-GQD composite solution is obtained;

[0027] Step 3: Use a scraper to evenly coat the SA-NS-GQD composite solution on the surface of the pre-wetted PVDF membrane and make it fully infiltrate to obtain membrane S2;

[0028] Step 4: Immerse membrane S2 in a calcium and magnesium (Ca 2+ Mg 2+ ) mixed solution for cross-linking reaction, and then rinse it with deionized water to obtain membrane S3;

[0029] Step 5: Immerse the cross-linked membrane S3 in a 2 - 5 wt% glutaraldehyde solution for cross-linking reaction for 8 - 12 hours;

[0030] Step 6: Through the cleaning and drying steps, remove the residual cross-linking agent to obtain a coal-based NS-GQD@CaMg-SA composite membrane. In step 2 described above, the mass ratio of NS-GQD powder to SA is 1 - 5:50, and the mass fraction of sodium alginate in liquid S1 is 2 wt%.

[0031] In step 3, the thickness of the scraper is 20 - 50 μm, and there is no specific requirement for the film thickness, and a moderate thickness is fine.

[0032] In step 4, in the calcium-magnesium mixed solution, the molar ratio of Ca 2+ and Mg 2+ is 1 - 3:1, the total metal ion concentration of the solution is 0.5 - 1 M, and the cross-linking reaction time is 5 - 10 minutes.

[0033] The coal-based nitrogen-sulfur co-doped graphene quantum dots@calcium-magnesium cross-linked-sodium alginate composite membrane, the surface of the coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane presents a uniform and dense nanoporous network structure, which is generated by the typical "egg box" structure formed by the co-crosslinking of sodium alginate through calcium ions and magnesium ions, and its pore size is usually between 1 - 10 nanometers, greatly increasing the specific surface area; at the same time, the embedded coal-based NS-GQDs have a size of about 2 - 10 nanometers and are uniformly dispersed in the hydrogel matrix, providing a large number of active sites and additional adsorption sites on the membrane surface.

[0034] The abundant functional groups such as -OH and -COOH on the surface of the composite membrane can selectively adsorb organic pollutants with aromatic rings or other polar groups through hydrogen bonding, electrostatic interaction and π-π interaction. The nanoporous structure enables pollutant molecules to come into full contact with the active sites in NS-GQDs and the SA matrix when entering the interior of the membrane, thus promoting the attack of reactive oxygen species on pollutant molecules in the photocatalytic reaction and ultimately achieving efficient degradation. This structure not only captures the size and polarity characteristics of pollutant molecules, but also can specifically degrade their specific structural units, providing a sufficient reaction interface and kinetic advantages for the subsequent mineralization of pollutants.

[0035] The beneficial effects of the present invention:

[0036] The synergistic effect of NS-GQD and sodium alginate hydrogel

[0037] Introducing coal-based NS-GQD into the SA hydrogel can make full use of the excellent photocatalytic performance of NS-GQD and at the same time utilize the flexible network and high light transmittance of the SA hydrogel. Specifically, the uniform dispersion of NS-GQD in the SA hydrogel not only avoids the problem of nanoparticle aggregation, but also plays the role of a "nano enhancer", significantly improving the shortcoming of the insufficient mechanical properties of the pure SA hydrogel, thus constructing a stable and functionally rich composite membrane structure.

[0038] Synergistic adsorption and photocatalytic function

[0039] The SA hydrogel has excellent adsorption properties and 100% light transmittance, which enables it to adsorb pollutants without affecting the photocatalytic reaction. As a photocatalyst, NS-GQD can generate reactive oxygen species under ultraviolet or visible light irradiation to degrade the organic pollutants adsorbed in the membrane. In addition, its abundant functional groups and unique electronic structure can provide additional adsorption sites, thereby improving the overall adsorption performance. The synergistic effect of the two can utilize both physical adsorption and chemical degradation mechanisms simultaneously to improve the pollutant removal efficiency.

[0040] Advantages of the calcium-magnesium co-crosslinking strategy

[0041] First, the ion crosslinking of SA with calcium ions (Ca 2 +) can form a relatively tight "egg box" structure. Ca 2 + will form a bridge with the carboxyl groups in SA molecules, fixing the SA helical structure in space to form a tight and stable three-dimensional network structure, making the membrane have high mechanical strength and stability. Second, after introducing magnesium ions (Mg 2+ ), due to its smaller ionic radius and higher solubility, the formed crosslinked structure is relatively loose and more flexible, which helps to optimize the pore structure of the membrane, improve the water flux, and at the same time reduce the brittleness problem caused by over-tight crosslinking. Therefore, the calcium-magnesium co-crosslinking not only takes into account the stability and mechanical strength of the membrane, but also improves the flexibility and water permeability of the membrane, making the overall filtration effect more ideal.

[0042] NS-GQD improves the separation problem of photo-generated carriers

[0043] Ordinary graphene quantum dots are prone to photo-generated electron-hole recombination, thus reducing the photocatalytic efficiency. However, due to its surface modification and doping characteristics, NS-GQD can effectively delay the carrier recombination process, promote the effective separation of electrons and holes, and then improve the efficiency of photocatalytic degradation of organic pollutants. This advantage is of great significance for improving the self-purification ability and durable usability of the membrane in practical applications.

[0044] Resource utilization of high-sulfur coal

[0045] In the present invention, high-sulfur coal is used as a carbon source to prepare NS-GQDs, realizing the resource utilization of high-sulfur coal. During the synthesis process of quantum dots, the sulfur element rich in coal helps to synthesize NS-GQDs without the need to add additional sulfur sources, effectively reducing the production cost and the environmental burden. The sulfur element in high-sulfur coal not only improves the stability and optical properties of quantum dots in this process, but also further enhances their activity in photocatalysis and electrocatalysis, providing a new solution for the high-value utilization of high-sulfur coal. In this way, not only can the negative environmental impacts caused by coal mining and use be reduced, but also the clean and efficient utilization of low-grade coal can be promoted, meeting the requirements of green development.

[0046] Resource utilization of biomass

[0047] In the present invention, natural biomass such as gardenia fruits is used as a natural nitrogen source to participate in the preparation of NS-GQDs, which reflects the resource utilization of biomass. Natural biomass, especially agricultural waste and plant materials, can not only effectively reduce environmental pollution, but also provide an innovative way for the high-value reuse of waste. During this process, nitrogen compounds in natural biomass such as gardenia fruits are introduced into the quantum dot synthesis reaction, which can provide various nitrogen sources, such as pyridine nitrogen, pyrrole nitrogen, etc. The introduction of these nitrogen groups significantly improves the electronic structure and photocatalytic performance of NS-GQDs. In addition, other heteroatoms (such as sulfur, phosphorus, etc.) may be brought while providing nitrogen sources during the reaction process, which may further improve the performance of graphene quantum dots and endow them with various functions. Specific embodiments

[0048] The present invention will be further described in detail below.

[0049] Example 1: High water flux and basic anti-pollution performance are achieved through rapid preparation and low cost, which is suitable for the preliminary pretreatment of printing and dyeing wastewater.

[0050] Ultrasonic treatment time: 6 hours; molar ratio of calcium and magnesium cross-linking solution: 1:1; ratio of NS-GQDs to SA: 1:50; cross-linking time of the membrane with calcium and magnesium ions: 10 minutes;

[0051] Step 1: Preparation of NS-GQD

[0052] First, the high-sulfur coal powder was pretreated: the crushed anthracite was ball-milled at 250 r / min for 4 h, and the coal powder with a particle size of D99 < 74 μm was screened out, and dried in an oven at 105°C for 12 h before use; then the pretreated high-sulfur coal powder (25 g) was mixed with the gardenia fruit powder (5 g) and added to 250 mL of a 30% concentration hydrogen peroxide solution, and stirred for 1 h in an ice bath at 0°C to fully oxidize and dissolve the mixed powder to form solution A1. Next, 25 mL of a pre-prepared 1 g / mL amino acid solution (the amino acid is any one of methionine, arginine, glycine, histidine, phenylalanine, proline, alanine, aspartic acid, cysteine or valine) was added to solution A1, and stirring was continued to make it fully react and evenly distributed to obtain solution A2. Subsequently, solution A2 was ultrasonically treated at 40kHz for 6 hours at room temperature and pressure to obtain an oxidized mixed solution, and then a 25% ammonium hydroxide solution was added to adjust the pH to 7 to form a neutral mixture; finally, the neutral mixture was centrifuged and filtered, and the filtrate was placed in a dialysis bag with a molecular weight cutoff of 1000Da for dialyzation for 3 days. After the dialysis, it was freeze-dried at -15°C for 24 hours to finally obtain purified NS-GQD powder.

[0053] Step 2: Preparation of coal-based NS-GQD@CaMg-SA composite membrane

[0054] First, the polyvinylidene fluoride (PVDF) base membrane was soaked in anhydrous ethanol for at least 1 hour and then rinsed with deionized water to obtain a pre-wetted PVDF membrane; then, the NS-GQD powder was dispersed in deionized water and ultrasonically treated for 30 minutes to obtain a uniform dispersion, and then SA (wherein the mass ratio of NS-GQD powder to SA was 1:50, and the mass fraction of SA in the liquid was 2wt%) was added to the dispersion to form liquid S1, and stirred at 30°C for 12-24 hours until a viscous solution was formed, and a uniform SA-NS-GQD composite solution was obtained after filtration, standing and degassing. Next, the SA-NS-GQD composite solution was uniformly coated on the surface of the pre-wetted PVDF membrane using a scraper with a thickness of 20-50μm to fully infiltrate it to form a membrane S2; then, the membrane S2 was immersed in a mixed solution containing calcium and magnesium ions for a cross-linking reaction (wherein Ca 2+ With Mg 2+ The molar ratio is 1:1, the total metal ion concentration is 0.5 M, and the reaction time is 10 minutes). After the cross-linking is completed, the membrane S3 is rinsed with deionized water to obtain; then, the cross-linked membrane S3 is immersed in a 2wt% glutaraldehyde solution for a cross-linking reaction for 12 hours; finally, the residual cross-linking agent is removed by subsequent cleaning and drying steps to finally obtain a coal-based NS-GQD@CaMg-SA composite membrane.

[0055] Implementation Case 2: Higher density and mechanical strength; enhanced rejection performance and chemical corrosion resistance of the membrane, suitable for wastewater treatment with higher salt content and higher dye concentration.

[0056] Step 1: Preparation of NS-GQD

[0057] It is basically the same as the preparation process in Example 1, except that the ultrasonic treatment time of Solution A2 is changed from 6 h to 8 h

[0058] Step 2: Preparation of coal-based NS-GQD@Ca Mg-SA composite membrane

[0059] It is basically the same as the preparation process in Example 1, except that the ratio of NS-GQDs to SA is changed from 1:50 to 2:50; the molar ratio of the calcium-magnesium cross-linking solution is changed from 1:1 to 2:1; the cross-linking time of the membrane with calcium and magnesium ions is changed from 10 minutes to 20 minutes;

[0060] Implementation Case 3: Sufficient nitrogen doping, high activity of NS-GQDs, and the membrane has strong photocatalytic and adsorption degradation functions; suitable for working conditions with high requirements for dye degradation, can achieve partial self-purification function, and reduce the risk of secondary pollution.

[0061] Step 1: Preparation of NS-GQD

[0062] It is basically the same as the preparation process in Example 1, except that the ultrasonic treatment time of Solution A2 is changed from 8 h to 10 h

[0063] Step 2: Preparation of coal-based NS-GQD@Ca Mg-SA composite membrane

[0064] It is basically the same as the preparation process in Example 1, except that the ratio of NS-GQDs to SA is changed from 2:50 to 3:50; the molar ratio of the calcium-magnesium cross-linking solution is changed from 2:1 to 3:1; the cross-linking time of the membrane with calcium and magnesium ions is changed from 20 minutes to 30 minutes;

[0065] Implementation Case 4: High NS-GQDs content significantly enhances the optical, adsorption and catalytic activities of the membrane, and at the same time uses nano-fillers to enhance the mechanical properties of the SA matrix; suitable for treating refractory dyes in printing and dyeing wastewater, improving degradation efficiency and anti-pollution performance.

[0066] Step 1: Preparation of NS-GQD

[0067] It is basically the same as the preparation process in Example 1.

[0068] Step 2: Preparation of coal-based NS-GQD@Ca Mg-SA composite membrane

[0069] The preparation process is basically the same as that of Example 1, except that the ratio of NS-GQDs to SA is changed from 1:50 to 4:50, and the cross-linking time of the membrane with calcium and magnesium ions is changed from 10 minutes to 20 minutes.

[0070] Example 5: Balanced comprehensive performance: relatively high mechanical strength and rejection performance, excellent photocatalytic / adsorption activity; suitable for long-term stable operation in the treatment of printing and dyeing wastewater, can effectively retain dyes and inhibit membrane fouling, and adapt to complex water quality environments.

[0071] Step 1: Preparation of NS-GQD

[0072] The preparation process is basically the same as that of Example 1, except that the ultrasonic treatment time of solution A2 is changed from 6 h to 8 h.

[0073] Step 2: Preparation of coal-based NS-GQD@Ca Mg-SA composite membrane

[0074] The preparation process is basically the same as that of Example 1, except that the ratio of NS-GQDs to SA is changed from 1:50 to 5:50, the molar ratio of the calcium and magnesium cross-linking solution is changed from 1:1 to 2:1, and the cross-linking time of the membrane with calcium and magnesium ions is changed from 10 minutes to 30 minutes.

[0075] Properties of the composite membrane prepared in the examples:

[0076] 1. Separation performance indicators of the composite membrane

[0077] An ultrafiltration cell was used to measure the membrane flux. The membrane was cut into an area matching the ultrafiltration cell and placed at the bottom of the ultrafiltration cell. Before the experiment, the membrane was wetted with pure water and pre-pressed for 5 minutes at a pressure of 0.2 MPa. Under the operating pressure of 0.1 MPa and the condition of 20 °C, the flux and rejection rate of the composite membrane were measured using a 10 mg / L rhodamine B (RhB) solution and a 2 g / L magnesium sulfate solution (MgSO4) respectively. The flux and rejection rate results of the composite membranes prepared in Examples 1-5 are shown in Table 1.

[0078] Table 1 Pure water flux of the composite membrane prepared in the examples and rejection rate for RhB and magnesium sulfate solutions

[0079]

[0080] 2. Photocatalytic and anti-fouling performance indicators of the composite membrane.

[0081] Prepare a rhodamine B (RhB) solution with a concentration of 10 mg / L, and use a cross-flow filtration experimental device to filter the RhB aqueous solution. All filtration processes are carried out at 25 °C and 0.1 MPa. First, filter deionized water with the membrane. This permeation flux, also known as the pure water flux, is used to compare the flux recoverability of the membrane contaminated by the dispersed dye. Secondly, filter the RhB aqueous solution and measure its permeation flux. Then, wash the membrane surface with water for 30 min, and filter deionized water again. Evaluate the photocatalytic self-cleaning ability of the membrane and the anti-pollution performance of the membrane by measuring its permeation flux.

[0082] Table 2 Performance of the composite membrane prepared in the examples in membrane pollution and membrane cleaning experiments

[0083]

[0084]

[0085] 3. Mechanical and structural performance indicators of the composite membrane

[0086] Use a micrometer to measure the thickness of the thin film. Randomly select 5 points for each sample for measurement, and calculate the average value as the thickness of the thin film sample.

[0087] Use an electronic fabric strength tester to determine the tensile strength and elongation at break of the composite membrane. Select the part of the membrane sample without bubbles and damage, and cut it into rectangular strips of 1.0 cm × 8.0 cm as mechanical test samples. After placing them at 75% relative humidity for 24 h, determine the mechanical properties of the membrane material.

[0088] Table 3 Mechanical strength and membrane thickness of the composite membrane prepared in the examples

[0089]

[0090] By introducing coal-based NS-GQDs and calcium-magnesium ion co-crosslinked sodium alginate (SA), the overall performance of the composite membrane is significantly improved, especially in terms of anti-pollution and photocatalytic degradation ability, showing significant improvement compared with traditional composite membranes. The "egg box" structure formed by calcium-magnesium ion co-crosslinking endows the membrane with higher mechanical strength and flexibility, and optimizes the pore structure of the membrane, thus effectively improving the water transport and separation performance. As a nano-reinforcing agent, NS-GQDs increase the crosslinking density of the membrane through its high specific surface area and hydrogen bond and electrostatic interactions with the SA matrix, significantly enhancing the mechanical strength and anti-pollution ability of the membrane. Compared with traditional membrane materials, calcium-magnesium ion co-crosslinking not only enhances the stability of the membrane, but also achieves a better balance in water flux and anti-pollution performance, avoiding the decrease in water flux caused by excessive densification. NS-GQDs not only improve the anti-pollution performance of the membrane, effectively reducing pollutant accumulation when facing dye and organic matter pollution, but also endow the membrane with the function of photocatalytic degradation of organic matter, effectively reducing membrane pollution and extending the service life. Through these innovative mechanisms, the composite membrane of the present invention not only surpasses traditional composite membranes in terms of separation performance, anti-pollution performance and self-cleaning ability, but also has better long-term stability and operation efficiency, providing a new type of efficient and durable membrane material for the field of wastewater treatment.

[0091] As can be seen from Table 1 to Table 3, the composite nanofiltration membrane prepared by the method of the present invention has good flux, with the overall not less than 53.76 L·m-2·h-1, and some up to 62.31 L·m-2·h-1. Moreover, the flux remains at a relatively high level after pollution, showing good anti-pollution performance. The flux recovery rate of the membrane exceeds 90%, indicating its good self-cleaning ability to effectively recover the flux. The flux decay rate of the membrane remains within 13%, showing its stability during long-term use. In addition, the membrane has excellent mechanical properties, with the mechanical strength ranging from 1.55 MPa to 2.05 MPa, and the separation layer thickness within a reasonable range, ensuring the tensile strength and durability of the membrane. The composite membrane of the present invention significantly improves the anti-pollution ability, photocatalytic degradation function and water flux of the membrane through coal-based NS-GQDs and calcium-magnesium ion co-crosslinked SA, enabling it to have the ability to operate stably for a long time in a complex water quality environment and being suitable for the treatment of highly polluted wastewater.

[0092] Nanoparticles (such as NS-GQD) play the role of "nano-reinforcing agent" in hydrogels, and improve the mechanical properties of hydrogels through the following mechanisms:

[0093] · Nanoparticles have an extremely high specific surface area and can form a large number of hydrogen bonds, electrostatic interactions or other non-covalent interactions with the hydrogel matrix (such as sodium alginate), thereby increasing the crosslinking density of the composite system and improving the overall mechanical strength.

[0094] · As a filler, nanoparticles can fill the tiny voids and structural defects inside the hydrogel, making the network structure more uniform and compact, effectively dispersing external forces, reducing local strain and the risk of crack propagation.

[0095] · Nanoparticles can also serve as stress transfer centers, evenly distributing stress throughout the entire matrix under the action of external forces, enhancing the tensile, compressive and shear resistance, and strengthening the toughness and stability of the hydrogel.

[0096] The good dispersion of nanoparticles in SA (sodium alginate) is mainly attributed to the following points:

[0097] · SA itself is a hydrophilic polymer containing abundant carboxyl and hydroxyl groups. These functional groups can form hydrogen bonds and electrostatic interactions with functional groups such as –OH, –COOH, and –NH2 on the surface of nanoparticles, forming stable interconnections.

[0098] · SA solutions usually have relatively high viscosities, which physically restrict the movement of nanoparticles, effectively preventing their agglomeration and promoting their uniform dispersion in the matrix.

[0099] · This dual chemical and physical effect enables the surface active sites of nanoparticles to be fully exposed in the SA matrix, thus exerting their strengthening and functional synergistic effects.

Claims

1. A preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane, characterized in that, It includes the following steps: Step 1: Prepare coal-based nitrogen and sulfur co-doped graphene quantum dots to obtain NS-GQD powder; Step 2: Disperse the NS-GQD powder in deionized water, and then add sodium alginate (SA) thereto to obtain a SA-NS-GQD composite solution. Coat the SA-NS-GQD composite solution on the surface of a pre-wetted PVDF membrane, and immerse the PVDF membrane in a calcium and magnesium (Ca 2+ Mg 2+ ) mixed solution for cross-linking reaction to obtain a coal-based NS-GQD@CaMg-SA composite membrane.

2. The preparation method of a coal-based NS-GQDs@calcium-magnesium crosslinked-SA composite membrane according to claim 1, characterized in that The specific steps of Step 1 are as follows: Step 1: Mix the pretreated high-sulfur coal powder and gardenia fruit powder, and then add them to hydrogen peroxide solution. Stir under ice bath conditions to fully dissolve and oxidize the above powder substances to obtain solution A1; Step 2: Add amino acid solution to solution A1 and continue to stir the mixture to make the amino acid solution fully react and evenly distribute to obtain solution A2; Step 3: Ultrasonically treat solution A2 at 40 - 60 kHz under normal temperature and pressure to obtain an oxidized mixed solution, and then treat it with 25% ammonium hydroxide solution to make the pH value of the oxidized mixed solution reach 7 to obtain a neutral mixture; Step 4: Centrifuge the above neutral mixture and then dialyze it to obtain a purified nitrogen and sulfur co-doped graphene quantum dot (NS-GQD) dispersion. Finally, perform freeze-drying on it to obtain NS-GQD powder.

3. The preparation method of a coal-based NS-GQDs@calcium-magnesium crosslinked-SA composite membrane according to claim 2, characterized in that, The pretreatment steps of the high-sulfur coal powder in Step 1: Ball-mill the crushed anthracite at a speed of 200 - 300 r / min, then screen out coal powder with a particle size of D99 < 74 μm, store the prepared coal powder in a sealed manner, and dry it in an oven at 105°C for 12 - 14 h before use; The mass ratio of high-sulfur coal powder to gardenia fruit powder is between 4:1 and 6:1; The ratio of hydrogen peroxide solution to powder is: 250 mL of 30% hydrogen peroxide solution and 30 g of powder (25 g of high-sulfur coal powder and 5 g of gardenia fruit powder). For every 10 g of high-sulfur coal powder, use 10 - 15 times the volume of hydrogen peroxide solution; for 25 g of high-sulfur coal powder, use 250 - 375 mL of 30% hydrogen peroxide solution.

4. The preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane according to claim 2, wherein, The preparation steps of the amino acid solution in Step 2: Dissolve amino acids in deionized water and stir evenly to obtain an amino acid solution with a concentration of 0.8 - 1.2 g / mL; The ratio of amino acid solution to solution A1 is to add 5 - 10 mL of amino acid solution per 100 mL of solution A1, and the final addition amount is between 12.5 - 25 mL.

5. The preparation method of a coal-based NS-GQDs@calcium-magnesium crosslinked-SA composite membrane according to claim 2, characterized in that, The amino acid in Step 2 is one of methionine, arginine, glycine, histidine, phenylalanine, proline, alanine, aspartic acid, cysteine, and valine.

6. The preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane according to claim 2, characterized in that, In Step 3, the ultrasonic treatment time of the oxidized mixed solution is 6 - 10 hours.

7. The preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane according to claim 2, characterized in that, In Step 4, centrifuge the neutral mixture and then filter it. Place the filtered solution in a dialysis bag with a cut-off molecular weight of 500 - 1000 Da and dialyze it for 3 - 5 days. Replace the dialysis fluid regularly every day. After dialysis, freeze-dry it at -15°C for 24 hours to obtain nitrogen and sulfur co-doped graphene quantum dot powder.

8. The preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane according to claim 1, characterized in that, The specific steps of Step 2 are as follows: Step 1: Immerse the polyvinylidene fluoride (PVDF) membrane in anhydrous ethanol solution for more than 1 h, and rinse it with deionized water to obtain a pre-wetted PVDF membrane; Step 2: Disperse the NS-GQD powder in deionized water and ultrasonically disperse it for 30 - 60 min to obtain a dispersion. Then add sodium alginate (SA) to the dispersion to obtain liquid S1. Stir liquid S1 at room temperature for 12 - 24 h. After filtering, standing, and degassing the obtained viscous solution, a uniform SA-NS-GQD composite solution is obtained; Step 3: Use a doctor blade to uniformly spin-coat the SA-NS-GQD composite solution on the surface of a pre-wetted PVDF membrane and allow it to fully infiltrate to obtain membrane S2; Step 4: Immerse the membrane S2 into a calcium and magnesium (Ca 2+ Mg 2+ ) mixed solution for crosslinking reaction, and then rinse with deionized water to obtain the membrane S3; Step 5: Immerse the cross-linked membrane S3 in a 2 - 5 wt% glutaraldehyde solution and carry out a cross-linking reaction for 8 - 12 hours; Step 6: Through the cleaning and drying steps, remove the residual cross-linking agent to obtain a coal-based NS-GQD@CaMg-SA composite membrane.

9. The preparation method of a coal-based NS-GQDs@calcium-magnesium cross-linked-SA composite membrane according to claim 6, characterized in that, In step 2, the mass ratio of the NS-GQD powder to SA is 1 - 5:50, and the mass fraction of SA in liquid S1 is 2 wt%; In step 4, the molar ratio of Ca 2 + to Mg2+ in the calcium-magnesium mixed solution is 1-3:1, the total metal ion concentration of the solution is 0.5-1 M, and the cross-linking reaction time is 5-10 minutes.

10. A coal-based nitrogen and sulfur co-doped graphene quantum dot@calcium magnesium cross-linked-sodium alginate composite membrane, characterized in that The surface of the coal-based NS-GQDs@calcium magnesium cross-linked-SA composite membrane presents a uniform and dense nanoporous network structure. Sodium alginate forms a typical "egg box" structure through the co-crosslinking of calcium ions and magnesium ions, and its pore size is usually between 1 and 10 nanometers; the embedded coal-based NS-GQDs have a size of about 2 - 10 nanometers and are uniformly dispersed in the hydrogel matrix, providing a large number of active sites and additional adsorption sites on the membrane surface; The abundant functional groups such as -OH and -COOH on the surface of the composite membrane can selectively adsorb organic pollutants with aromatic rings or other polar groups through hydrogen bonding, electrostatic interaction, and π-π interaction.

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