Magnetic porous composites, methods of making and using the same

Abstract

The application discloses a kind of magnetic porous composite material and its preparation method and the application of adsorbing and purifying micro-nano plastic in water body.The magnetic porous composite material has three-dimensional network skeleton formed by microcrystalline region and hydrogen bond physical crosslinking between polyvinyl alcohol molecular chains, cationic polymer is physically inserted and distributed in the skeleton to provide cation adsorption sites, and magnetic nanoparticles are dispersed in the network;After freeze-drying, a hierarchical porous network structure containing 50-300 μm interconnected macropores and 3-10 nm mesopores is formed, wherein the interconnected macropores serve as mass transfer channels, and the mesopores and cation adsorption sites cooperate to achieve physical entrapment and electrostatic adsorption of micro-nano plastic.The present application does not require any chemical crosslinking agent, can be quickly separated by an external magnetic field, has the advantages of high adsorption efficiency, strong environmental adaptability, easy magnetic separation and multiple cycle regeneration, and is suitable for the enrichment, detection pretreatment and purification of micro-nano plastic in water body.

Description

Technical Field

[0001] This invention relates to the fields of environmental functional materials and water treatment technology, and more specifically to a magnetic porous composite material, its preparation method, and its application. Background Technology

[0002] Microplastic (plastic particles smaller than 5 mm) and nanoplastic (particles smaller than 1 μm) pollution has become a global environmental problem. Existing methods for detecting microplastics (such as infrared spectroscopy and Raman spectroscopy) typically require cumbersome pretreatment steps. Currently, the most common pretreatment method relies on traditional microporous membranes for filtration sampling and enrichment. However, traditional filtration methods have the following significant problems in practical applications:

[0003] 1. Time-consuming and prone to clogging: When processing large-volume or complex natural water samples rich in suspended solids, long-term filtration is required, and impurities can easily clog the filter membrane pores, leading to a sharp drop in flux.

[0004] 2. Challenges in Nanoscale Capture: Traditional microporous membranes have limited pore size limits, making it easy for nanoscale plastics with particle sizes smaller than 1 μm to penetrate the membrane, resulting in significantly lower detection results. Therefore, developing a novel enrichment material that can rapidly adsorb, easily separate into solid and liquid phases (such as magnetic separation), and efficiently capture nanoscale plastics through multidimensional interactions is a pressing technical challenge in this field.

[0005] In addition, although existing technologies have methods for constructing gel structures through chemical crosslinking (such as boric acid crosslinking systems) or preparing physical gels using freeze-thaw methods, the following problems still exist: chemical crosslinking agents need to be introduced, which may lead to secondary pollution; the gel pore structure obtained by a single freeze-thaw method is simple, making it difficult to balance adsorption capacity and mass transfer efficiency; and the capture efficiency of micro- and nano-plastics and the regenerability of materials still need to be improved. Summary of the Invention

[0006] Due to the aforementioned deficiencies in existing technologies, this invention provides a magnetic porous composite material, its preparation method, and its application in purifying water using micro / nanoplastics and in recycling. This material forms a stable three-dimensional network framework through physical cross-linking. Utilizing the synergistic effect of cation adsorption sites provided by the free intercalation of cationic polymers and the physical retention of the hierarchical porous structure, it achieves efficient capture of micro / nanoplastics in water. Furthermore, it leverages magnetic nanoparticles to achieve rapid separation and enrichment via an external magnetic field.

[0007] To achieve the above objectives, in a first aspect, the present invention provides a magnetic porous composite material comprising a three-dimensional network framework formed by physical cross-linking of polyvinyl alcohol molecular chains through microcrystalline regions and hydrogen bonds, wherein a cationic polymer is physically interspersed in the three-dimensional network framework in a free state to provide cationic adsorption sites, and magnetic nanoparticles are dispersed within the three-dimensional network framework.

[0008] The magnetic porous composite material has a hierarchical porous network structure, containing interconnected macropores of 50~300 μm and mesopores with a pore size of 3~10 nm.

[0009] As a further technical solution, the magnetic nanoparticles are nano-iron oxide with a particle size of 20~50 nm.

[0010] As a further technical solution, the polyvinyl alcohol is polyvinyl alcohol with a degree of alcoholysis ≥ 99.0%; the cationic polymer is one or more of chitosan quaternary ammonium salt, poly-L-lysine, cationic starch, and hydroxyethyl cellulose ether quaternary ammonium salt.

[0011] Secondly, the present invention provides a method for preparing a magnetic porous composite material, which is used to prepare the magnetic porous composite material as described above, comprising the following steps:

[0012] S11, Polyvinyl alcohol is dissolved under heating conditions, and then mixed with a cationic polymer solution to obtain a mixed sol;

[0013] S12, Magnetic nanoparticles are added to the mixed sol and dispersed evenly;

[0014] S13, without adding any chemical crosslinking agent, the mixture obtained in step S12 is subjected to repeated freeze-thaw cycles to form a physically crosslinked three-dimensional network skeleton.

[0015] S14. The product obtained in step S13 is subjected to vacuum freeze-drying to remove moisture by sublimation, forming a hierarchical porous network structure.

[0016] As a further technical solution, in step S11, the mixed sol contains, by mass percentage, 8%~15% polyvinyl alcohol, 1%~5% cationic polymer, 1%~4% magnetic nanoparticles, and the remainder is deionized water.

[0017] As a further technical solution, the heating and melting temperature is 60~100℃; a single freeze-thaw cycle is to freeze completely at a temperature not higher than -20℃ and then thaw at room temperature; the number of repeated freeze-thaw cycles is 3~7 times.

[0018] As a further technical solution, the magnetic nanoparticles are nano-iron oxide with a particle size of 20~50 nm.

[0019] As a further technical solution, the polyvinyl alcohol is polyvinyl alcohol with a degree of alcoholysis ≥ 99.0%; the cationic polymer is one or more of chitosan quaternary ammonium salt, poly-L-lysine, cationic starch, and hydroxyethyl cellulose ether quaternary ammonium salt.

[0020] Finally, the present invention provides an application of a magnetic porous composite material in the purification of water bodies using micro-nano plastics and recycling, wherein the magnetic porous composite material is the magnetic porous composite material as described above or the magnetic porous composite material prepared by the preparation method described above.

[0021] As a further technical solution, the steps of water purification include:

[0022] S21. Adsorption and Separation: The magnetic porous composite material is immersed in water containing micro- and nano-plastics, and the micro- and nano-plastics are captured by electrostatic adsorption and physical retention; after adsorption is completed, the magnetic porous composite material is separated by an external magnetic field.

[0023] S22, Organic solvent-assisted ultrasonic desorption: The separated material is put into a 95% ethanol solution and desorbed under ultrasonic conditions. The ethanol weakens the hydrophobic interaction and combines the ultrasonic cavitation effect to achieve micro-nano plastic elution and material regeneration.

[0024] S23. After washing with deionized water, the magnetic porous composite material is dried at room temperature or low temperature to restore the material structure for recycling.

[0025] The above technical solution is only one feasible technical solution of the present invention. The scope of protection of the present invention is not limited thereto. Those skilled in the art can reasonably adjust the specific design according to actual needs.

[0026] The above invention has the following advantages or beneficial effects:

[0027] (1) This invention mainly constructs the unique physical structure of magnetic porous composite materials through a specific core component ratio:

[0028] Polyvinyl alcohol (PVA) forms the physical framework: PVA serves as the basic gelling matrix in the system. This invention does not use traditional toxic chemical crosslinking agents such as glutaraldehyde, epichlorohydrin, or boric acid; the entire material system contains no chemical crosslinking agents. An extremely stable, purely physically crosslinked three-dimensional network framework can be constructed solely through the strong hydrogen bonding between polymer chain segments and the formation of numerous microcrystalline regions during repeated freeze-thaw cycles using a specific concentration of PVA.

[0029] The cationic polymer provides binding sites: the cationic polymer is selected from one or more of chitosan quaternary ammonium salt, poly-L-lysine, cationic starch, and hydroxyethyl cellulose ether quaternary ammonium salt. Chitosan quaternary ammonium salt is preferred, as it has excellent water solubility compared to ordinary chitosan, and the quaternary ammonium groups can provide a higher density of permanent positively charged binding sites, resulting in a stronger electrostatic attraction with the negatively charged microplastics on the surface.

[0030] Magnetic nanoparticles impart magnetic responsiveness: Magnetic nanoparticles are attached to the interior and surface of the aforementioned three-dimensional material network. In the design of magnetic porous materials, the selection of 20–50 nm nano-sized iron oxide (Fe3O4) particles instead of micron-sized particles is based on a synergistic consideration of structural stability, magnetic properties, and adsorption efficiency. First, nanoparticles possess superparamagnetism, leaving no residual magnetism after the external magnetic field is removed, effectively preventing pore closure caused by magnetic agglomeration within the material and ensuring the material's recyclability. Second, the nanoscale is compatible with the material's mesoporous structure, serving as a uniformly embedded nano-reinforcing filler within the PVA framework. This avoids stress concentration and structural defects caused by large micron-sized particles while utilizing the large specific surface area to increase physical interlocking sites. Finally, the excellent dispersibility of nanoparticles overcomes the drawback of easy sedimentation of micron-sized particles, ensuring the uniformity of the overall magnetic response and second-level separation efficiency, thereby achieving efficient and broad-spectrum capture of micro / nanoplastics.

[0031] Porous Network Morphology: After physical cross-linking, the above-mentioned specific ratio of mixed system must undergo vacuum freeze-drying. This in-situ sublimation mechanism effectively removes all moisture from the material, avoiding pore collapse and structural shrinkage caused by capillary surface tension at the gas-liquid interface during conventional thermal drying, thus fully preserving the highly open hierarchical porous network. This network consists of interconnected macropores of 50~300μm and mesopores of 3~10nm. This specific structure of "macropore conduction - micropore retention" overcomes the "size exclusion effect" defect of traditional adsorbents, which makes it difficult for microplastics to enter the micropores, fundamentally ensuring the material's deep physical integration with microplastics.

[0032] (2) The magnetic porous composite material of the present invention has a good cyclic adsorption effect on micro-nano plastics in water:

[0033] Broad-spectrum and highly efficient: It exhibits extremely high removal rates for a variety of common microplastics. Experimental data show the following removal rates: PMMA (99%), PE (95%), PS (90.36%), TPU (82.51%), and PVC (80.50%).

[0034] Nanoscale capture: It can effectively adsorb microspheres with different particle sizes such as 50 nm, 200 nm, and 1 μm.

[0035] Strong environmental adaptability: The adsorption performance remains stable in environments with pH values ​​of 6~8.2 and salinity (0~3%) to saline water.

[0036] Recyclable: Addressing the challenge of effectively eluting large-sized hydrophobic microplastics using traditional inorganic salt ion competitive desorption methods, this invention proposes an organic solvent-assisted ultrasonic desorption method. Specifically, 95% ethanol is used to disrupt the hydrophobic interaction between the microplastics and the material framework. This is supplemented by microjets generated by ultrasonic cavitation and mechanical shearing forces, which vibrate and elute deeply embedded microplastic fragments. After five consecutive cycles, the material's three-dimensional porous structure remains intact, and the adsorption efficiency remains at a high level of 86.3%, achieving extremely low-cost and low-carbon material reuse.

[0037] (3) Freeze-thaw treatment and freeze-drying steps have a synergistic effect in this invention: if only freeze-thaw treatment is used without freeze-drying, the resulting material is prone to pore structure collapse during the drying process, resulting in a decrease in specific surface area and mass transfer performance; while if only freeze-drying is performed without the physical cross-linking network built by the freeze-thaw process, the system is difficult to form a stable three-dimensional framework structure, and the resulting material has a loose structure and insufficient mechanical stability. Therefore, the above-mentioned freeze-thaw treatment and freeze-drying steps complement each other and are indispensable, together constituting the key technical means for achieving multi-level porous structure and high-efficiency adsorption performance in this invention. Attached Figure Description

[0038] The invention, its features and advantages will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0039] Figure 1 This is a flowchart illustrating the preparation process of the magnetic porous composite material in Example 1 of the present invention.

[0040] Figure 2 This is a SEM microstructure image of the magnetic porous composite material prepared in Example 1 of the present invention.

[0041] Figure 3 This is a pore size distribution diagram of the magnetic porous composite material prepared in Example 1 of the present invention.

[0042] Figure 4 This is a schematic diagram of the entire process of preparation, adsorption, desorption, and regeneration of the magnetic porous composite material in this invention.

[0043] Figure 5 The bar chart shows the removal rate of different types of microplastics (PMMA / PE / PS, etc.) by the magnetic porous composite material prepared in Example 1 of this invention.

[0044] Figure 6 This is a comparison chart of the adsorption rates of the porous composite materials prepared in Example 1 and Comparative Example 1 of the present invention at different pH values ​​(6~8.2).

[0045] Figure 7This is a comparison chart of the adsorption rates of the porous composite materials prepared in Example 1 and Comparative Example 1 of the present invention under different NaCl salinities (0~3wt%).

[0046] Figure 8 This is a comparison chart of the microplastic removal rates between the desorption process of this invention and the conventional inorganic salt desorption process. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0048] The reaction apparatus, monomer compounds, and solvents involved in the following examples and embodiments are all commercially available. The detection instruments involved in the following effect examples are all commercially available, and the detection methods used are existing technologies that can be found; and the technologies not described in detail in the following effect examples are existing technologies that can be found.

[0049] First, this invention provides a magnetic porous composite material comprising a three-dimensional network framework physically cross-linked by polyvinyl alcohol molecular chains through microcrystalline regions and hydrogen bonds, a cationic polymer physically interspersed in a free state within the three-dimensional network framework to provide cationic adsorption sites, and magnetic nanoparticles dispersed within the three-dimensional network framework; the magnetic porous composite material has a hierarchical porous network structure, containing interconnected macropores of 50-300 μm and mesopores with a pore size of 3-10 nm; its preparation method includes the following steps:

[0050] S11. Polyvinyl alcohol is dissolved under heating conditions, and then mixed with a cationic polymer solution to obtain a mixed sol;

[0051] S12. Add magnetic nanoparticles to the mixed sol and disperse them evenly;

[0052] S13. Without adding any chemical crosslinking agent, the mixture obtained in step S12 is subjected to repeated freeze-thaw cycles to form a physically crosslinked three-dimensional network skeleton.

[0053] S14. The product obtained in step S13 is subjected to vacuum freeze-drying to remove moisture by sublimation, forming a hierarchical porous network structure.

[0054] Secondly, the magnetic porous composite material has a good adsorption effect on micro-nanoplastics for purifying water and can be recycled.

[0055] The steps of water purification include:

[0056] S21. Adsorption and Separation: The magnetic porous composite material is immersed in water containing micro- and nano-plastics, and the micro- and nano-plastics are captured by electrostatic adsorption and physical retention; after adsorption is completed, the material is separated by an external magnetic field.

[0057] S22, Organic solvent-assisted ultrasonic desorption: The separated material is put into a 95% ethanol solution and desorbed under ultrasonic conditions. The ethanol weakens the hydrophobic interaction and combines the ultrasonic cavitation effect to achieve micro-nano plastic elution and material regeneration.

[0058] S23. After washing with deionized water, the magnetic porous composite material is dried at room temperature or low temperature to restore the material structure for recycling.

[0059] The technical solution of the present invention will now be described in detail with reference to specific embodiments and comparative examples.

[0060] The main chemical raw materials and parameters used in the embodiments of this invention are as follows:

[0061] Polyvinyl alcohol (PVA): Commercially available product with grade 1799 (degree of polymerization approximately 1700, degree of alcoholysis ≥ 99.0%) is used because its high degree of alcoholysis provides a dense physical cross-linked hydrogen bond network.

[0062] Chitosan quaternary ammonium salt (QCS): with a degree of substitution of 90%, purchased from Shanghai Yuanye Biotechnology Co., Ltd. (batch number: N27IS233452).

[0063] Nano-iron oxide: average particle size is 20~50 nm.

[0064] Cationic polymers: In addition to chitosan quaternary ammonium salt, commercially available poly-L-lysine (molecular weight about 15,000 to 30,000), cationic starch (degree of substitution 0.03 to 0.05), or hydroxyethyl cellulose ether quaternary ammonium salt can also be used as alternative cationic binding site providers.

[0065] Example 1

[0066] See Figure 1 The specific steps for preparing the magnetic porous composite material in this embodiment are as follows:

[0067] Step (1), Preparation of Solution A: Accurately weigh 10.0 g of polyvinyl alcohol (PVA 1799) solid powder and add it to a beaker containing 70.0 mL (i.e., 70.0 g) of deionized water. Place the beaker on a constant temperature heating platform and, under heating conditions of 60~100℃, turn on the mechanical stirrer (speed of about 500 rpm) and stir continuously for 1 hour until the PVA particles are completely dissolved and dispersed, forming a transparent and viscous colloidal solution. After cooling it to room temperature, record it as Solution A.

[0068] Step (2), Preparation of Solution B: Accurately weigh 3.0 g of chitosan quaternary ammonium salt powder and add it to a beaker containing 15.0 mL (i.e., 15.0 g) of deionized water. Stir continuously for 30 minutes at room temperature (approximately 25°C) using a magnetic stirrer. Due to the excellent water solubility of quaternary ammonium salt, the powder can quickly and completely dissolve, forming a pale yellow transparent solution, denoted as Solution B.

[0069] Step (3), Sol-gel blending and magnetic doping: The prepared solution B is slowly added dropwise to solution A while continuously stirring to ensure uniform mixing, thus obtaining the precursor liquid of the PVA-QCS composite material. Subsequently, 2.0 g of nano Fe3O4 powder is accurately weighed and added to the precursor liquid. After mechanical stirring for 10 minutes, ultrasonic dispersion is performed for 10-15 minutes to obtain a uniformly distributed black mixed sol.

[0070] Step (4), repeated freeze-thaw physical crosslinking: Pour the black mixed sol obtained in step (3) into a polytetrafluoroethylene mold. Place it in an ultra-low temperature freezer at -20°C for 12 hours; then take it out and allow it to thaw naturally at room temperature (about 25°C) for 6 hours. This process of "freezing for 12 hours and thawing for 6 hours" is recorded as one freeze-thaw cycle. Repeat the above freeze-thaw cycle a total of 5 times to complete the physical crosslinking by utilizing the hydrogen bonding between macromolecular chains and the formation of microcrystalline regions.

[0071] Step (5) Freeze-drying molding: Demold the gel block obtained in step (4) and place it in a freeze dryer. Under conditions of vacuum degree below 10 Pa and cold trap temperature of -60℃, freeze-dry continuously for 48 hours to allow the water inside the gel to sublimate and be removed directly. Finally, a finished product with a three-dimensional micron / nano-scale porous network is obtained, which is magnetic porous composite material one (PVA-QCS@Fe3O4).

[0072] Material structure characterization:

[0073] See Figure 2 The prepared magnetic porous composite material possesses an interconnected micron / nano-scale porous network. To further verify the hierarchical porous network structure of the material, pore size analysis was performed on the magnetic porous material prepared in Example 1. See [link to example]. Figure 3 The material contains a rich mesoporous structure, with pore sizes mainly distributed in the range of 3–10 nm. This real test result, together with the interconnected macropores (50–300 μm) characterized by SEM, confirms the unique "macropore-mesopore" hierarchical porous network structure of the material of this invention, providing a solid spatial structural guarantee for the efficient physical retention and deep integration of micro- and nano-plastics.

[0074] Example 2

[0075] This embodiment is basically the same as Embodiment 1, except that: 1. Formulation: Polyvinyl alcohol (PVA) mass fraction is 15%, chitosan quaternary ammonium salt (QCS) mass fraction is 1%, nano Fe3O4 powder mass fraction is 4%, and the balance is deionized water.

[0076] 2. Preparation process: The mixing and molding conditions of each component are the same as in Example 1, and the number of freeze-thaw cycles is 5. Magnetic porous composite material II is obtained.

[0077] Example 3

[0078] This embodiment is basically the same as that of embodiment 1, except that: 1. Formula: same as that of embodiment 1 (10% PVA, 3% QCS, 2% Fe3O4).

[0079] 2. Preparation process: The preparation process is the same as in Example 1, but the number of freeze-thaw cycles is reduced to 3 when constructing the physical cross-linked network using the "freeze-thaw" method. Other steps such as freeze-drying are the same as in Example 1. A magnetic porous composite material (III) is obtained.

[0080] Example 4

[0081] This embodiment is basically the same as that of embodiment 1, except that: 1. Formula: same as that of embodiment 1 (10% PVA, 3% QCS, 2% Fe3O4).

[0082] 2. Preparation process: The preparation process is the same as in Example 1, but the number of freeze-thaw cycles is increased to 7 when constructing the physical cross-linked network using the "freeze-thaw" method. Other steps are the same as in Example 1. Magnetic porous composite material IV is obtained.

[0083] Example 5

[0084] The difference between this embodiment and Example 1 is that 3% chitosan quaternary ammonium salt (QCS) is replaced with 3% poly-L-lysine. The preparation process is the same as in Example 1.

[0085] Example 6

[0086] The difference between this embodiment and Example 1 is that 3% chitosan quaternary ammonium salt (QCS) is replaced with 3% cationic starch. The preparation process is the same as in Example 1.

[0087] Example 7

[0088] The difference between this embodiment and Example 1 is that 3% chitosan quaternary ammonium salt (QCS) is replaced with 3% hydroxyethyl cellulose ether quaternary ammonium salt. The preparation process is the same as in Example 1.

[0089] Comparative Example 1

[0090] This comparative example is basically the same as Example 1, except that magnetic Fe3O4 nanoparticles are not added in step (3) to obtain a polyvinyl alcohol-chitosan quaternary ammonium salt (PVA-QCS) porous composite material.

[0091] See Figure 4 To demonstrate that the materials prepared by the above method can achieve the intended purpose within the scope of the claims, the following performance tests were performed on the materials prepared according to the methods of the examples and comparative examples:

[0092] 1. Microplastic adsorption rate test: Polystyrene (PS) fluorescent microspheres were used as model pollutants. The materials prepared in Examples 1-4 were respectively added to a PS fluorescent microsphere suspension with an initial concentration of 0.1 mg / mL (addition amount 3 mg / 2 mL), and after adsorption at room temperature for 24 hours, they were separated using an external magnetic field.

[0093] 2. Broad spectrum test: The materials from Example 1 were used to conduct parallel tests on PMMA, PE, TPU and PVC microplastics.

[0094] 3. Cyclic Regeneration Test: The adsorption-saturated material from Example 1 was immersed in a 95% ethanol solution and desorbed in an ultrasonic vibration tank for 15 minutes. After 5 "adsorption-desorption" cycles, the morphology of the material remained intact, and the adsorption efficiency remained above 86.3%, demonstrating the universality and stability of the preparation method and formulation range. As a comparative example, the regeneration effect of the high-salt elution process (ion shielding method) was investigated as follows: The adsorption-saturated material from Example 1 was immersed in a beaker containing 50 mL of 1.0 mol / L NaCl solution, placed on a magnetic stirrer, and stirred at 1000 rpm for 35 minutes for desorption; after desorption was completed, the next cycle was performed.

[0095] See Figure 5 Spectroscopic analysis showed that the materials in Examples 1-4 could effectively adsorb microplastics. Example 1 achieved a removal rate of 90.36% for PS microplastics. Parallel tests on PMMA, PE, TPU, and PVC microplastics showed removal rates of 99%, 95%, 82.51%, and 80.50%, respectively. Examples 2, 3, and 4 all maintained effective removal rates above 80%.

[0096] To investigate the environmental adaptability of the materials prepared in Examples 1-4 and Comparative Example 1 during the adsorption of microplastics in water, adsorption experiments were conducted on the materials prepared in Examples 1 and Comparative Example 1 in NaCl solutions with different pH values ​​and concentrations. See [link to relevant documentation]. Figure 6In Example 1 (PVA-QCS@Fe3O4), the microplastic adsorption rate remained relatively stable in solutions with pH values ​​ranging from 6 to 8, and was generally higher than that of Comparative Example 1 (PVA-QCS), indicating that the material prepared in this example has a wide pH tolerance range. See also... Figure 7 In Example 1 (PVA-QCS@Fe3O4), the microplastic adsorption rate in a NaCl solution with a concentration of 0.003~3 mol / L was basically the same as that of Comparative Example 1 (PVA-QCS), and decreased slightly with the increase of solution salinity, indicating that the material prepared by this method also has good adaptability in high-salt environments.

[0097] Further comparison of the technological advantages brought by the introduction of magnetic particles reveals that for samples without added nano-ferric oxide (Comparative Example 1), solid-liquid separation must rely on traditional microporous filtration or high-speed centrifugation after adsorption. When processing large-volume or high-turbidity water samples, this method suffers from drawbacks such as easy clogging of the filter membrane, long recovery time, high energy consumption, and easy mechanical damage to the material during the recovery and stripping process, making low-cost large-scale reuse difficult. This invention utilizes nano-ferric oxide to endow porous materials with strong magnetic responsiveness (Examples 1-4), enabling rapid capture and easy recovery under the action of an external magnetic field, with no additional power source required for the separation process. Furthermore, the magnetic separation method effectively ensures the integrity of the material's hierarchical porous structure, and combined with the desorption and regeneration process described in this invention, achieves a high recycling rate. This characteristic solves the shortcomings of adsorbent materials in practical engineering applications, such as easy loss and difficulty in recovery, improving their operational convenience and economic value in large-scale water treatment.

[0098] See Figure 8 After five cycles using this conventional electrostatic shielding method, the removal rate of PS microplastics in the material decreased to 66.4%, while the removal rate of the material desorbed by ethanol ultrasonic vibration remained relatively stable after five cycles. This confirms that conventional methods are ineffective in eluting microplastics, a special type of contaminant, thus demonstrating the significant desorption advantages and necessity of the "organic solvent synergistic ultrasonic cavitation method" of this invention.

[0099] In Examples 5-7, chitosan quaternary ammonium salt was replaced in equal amounts with poly-L-lysine, cationic starch, or hydroxyethyl cellulose ether quaternary ammonium salt. Since these cationic polymers also possess excellent water solubility and a high density of positively charged groups, and can form good hydrogen bond compatibility with the hydroxyl groups of polyvinyl alcohol, they can also form a stable hierarchical porous network framework after undergoing the same "repeated freeze-thaw-freeze-drying" purely physical crosslinking process. This achieves highly efficient electrostatic capture and physical retention of micro- and nano-plastics in water (the removal rate of microplastics in a single adsorption is greater than 90%). This demonstrates that the preparation method disclosed in this invention has universal applicability to such cationic polymers.

[0100] In summary, this invention provides a magnetic porous composite material for purifying and removing micro / nanoplastics from water and its preparation method. This material utilizes the physically cross-linked three-dimensional network framework of polyvinyl alcohol, the strong electrostatic adsorption sites provided by the cationic polymer, and the rapid magnetic separation characteristics of magnetic nanoparticles to achieve efficient and broad-spectrum capture and convenient separation of micro / nanoplastics ranging from 50 nm to 1 μm. The material preparation uses all commercially available raw materials, and the process is green and simple (heating and dissolving—mixing—repeated freeze-thaw cycles—freeze-drying), requiring no complex chemical cross-linking or organic solvents. It exhibits high adsorption efficiency (PMMA removal rate reaches 99%, and an average of over 80% for various plastics), strong environmental adaptability, rapid magnetic separation, and can be regenerated more than 5 times through simple ultrasonic desorption while maintaining good performance.

[0101] Those skilled in the art should understand that variations can be implemented by combining existing technology with the above embodiments, which will not be elaborated here. Such variations do not affect the essence of the present invention, and will not be elaborated here either.

[0102] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and the devices and structures not described in detail should be understood as being implemented in a conventional manner in the art. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the present invention. This does not affect the essential content of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention's technical solutions still fall within the protection scope of the present invention.

Claims

1. A magnetic porous composite material, characterized in that: It includes a three-dimensional network framework composed of polyvinyl alcohol molecular chains physically cross-linked through microcrystalline regions and hydrogen bonds, cationic polymers physically interspersed in the three-dimensional network framework in a free state to provide cationic adsorption sites, and magnetic nanoparticles dispersed within the three-dimensional network framework. The magnetic porous composite material has a hierarchical porous network structure, containing interconnected macropores of 50~300 μm and mesopores with a pore size of 3~10 nm.

2. The magnetic porous composite material according to claim 1, characterized in that, The magnetic nanoparticles are nano-iron oxide with a particle size of 20~50 nm.

3. The magnetic porous composite material according to claim 1, characterized in that, The polyvinyl alcohol is polyvinyl alcohol with a degree of alcoholysis ≥ 99.0%; the cationic polymer is one or more of chitosan quaternary ammonium salt, poly-L-lysine, cationic starch, and hydroxyethyl cellulose ether quaternary ammonium salt.

4. A method for preparing a magnetic porous composite material, characterized in that, The preparation of the magnetic porous composite material as described in any one of claims 1 to 3 includes the following steps: S11, Polyvinyl alcohol is dissolved under heating conditions, and then mixed with a cationic polymer solution to obtain a mixed sol; S12, Magnetic nanoparticles are added to the mixed sol and dispersed evenly; S13, without adding any chemical crosslinking agent, the mixture obtained in step S12 is subjected to repeated freeze-thaw cycles to form a physically crosslinked three-dimensional network skeleton. S14. The product obtained in step S13 is subjected to vacuum freeze-drying to remove moisture by sublimation, forming a hierarchical porous network structure.

5. The preparation method according to claim 4, characterized in that, In step S11, the mixed sol contains, by mass percentage, 8%~15% polyvinyl alcohol, 1%~5% cationic polymer, 1%~4% magnetic nanoparticles, and the remainder is deionized water.

6. The preparation method according to claim 4, characterized in that, The heating and melting temperature is 60~100℃; a single freeze-thaw cycle is to freeze completely at a temperature not higher than -20℃ and then thaw at room temperature; the number of repeated freeze-thaw cycles is 3~7 times.

7. The preparation method according to claim 4, characterized in that, The magnetic nanoparticles are nano-iron oxide with a particle size of 20~50 nm.

8. The preparation method according to claim 4, characterized in that, The polyvinyl alcohol is polyvinyl alcohol with a degree of alcoholysis ≥ 99.0%; the cationic polymer is one or more of chitosan quaternary ammonium salt, poly-L-lysine, cationic starch, and hydroxyethyl cellulose ether quaternary ammonium salt.

9. The application of magnetic porous composite materials in water purification micro-nanoplastics and recycling, characterized in that, The magnetic porous composite material is the magnetic porous composite material as described in any one of claims 1 to 3 or the magnetic porous composite material prepared by the preparation method as described in any one of claims 4 to 8.

10. The application according to claim 9, characterized in that, The steps of water purification include: S21. Adsorption and Separation: The magnetic porous composite material is immersed in water containing micro- and nano-plastics, and the micro- and nano-plastics are captured by electrostatic adsorption and physical retention; after adsorption is completed, the magnetic porous composite material is separated by an external magnetic field. S22, Organic solvent-assisted ultrasonic desorption: The separated material is put into a 95% ethanol solution and desorbed under ultrasonic conditions. The ethanol weakens the hydrophobic interaction and combines the ultrasonic cavitation effect to achieve micro-nano plastic elution and material regeneration. S23. After washing with deionized water, the magnetic porous composite material is dried at room temperature or low temperature to restore the material structure for recycling.

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