Preparation of starch-based coagulant and its application in microplastic removal

Abstract

The application provides preparation of a starch-based coagulant and application thereof in microplastic removal, and belongs to the field of water treatment materials.The method comprises the following steps: mixing starch and deionized water, and then adjusting the pH value to obtain starch milk; adding sodium hydroxide into the starch milk, and stirring and reacting at constant temperature for 30-40 min to obtain pretreated starch milk; adding acrylamide, dimethyl diallyl ammonium chloride and an initiator into the pretreated starch milk at 35-45 DEG C, and stirring and reacting at constant temperature to obtain a grafted copolymer starch solution; adding nano-montmorillonite into the grafted copolymer starch solution, and then adding polydopamine, adjusting the pH value, and stirring and reacting for 40-60 min to obtain a crude composite coagulant; and vacuum drying and crushing the crude composite coagulant to obtain a starch-based coagulant.The method can overcome the defects of low microplastic removal efficiency, complex preparation process and insufficient environmental friendliness of the starch-based coagulant in the prior art.

Description

Technical Field

[0001] This application relates to the field of water treatment materials technology, and in particular to the preparation of a starch-based coagulant and its application in microplastic removal. Background Technology

[0002] Starch, as a widely available, renewable, and biodegradable natural polymer material, contains a large number of hydroxyl groups in its molecular structure. It is easy to introduce functional groups through chemical modification to enhance its adsorption and flocculation properties, making it an ideal raw material for preparing environmentally friendly coagulants.

[0003] Existing technologies have yielded some research on starch-based coagulants, such as introducing monomers like acrylamide and acrylic acid onto starch molecular chains through graft copolymerization to improve flocculation. However, these starch-based coagulants generally suffer from the following shortcomings: first, the charge density of single-starch modified products is low, limiting their ability to neutralize negatively charged microplastic particles; second, floc formation is slow and sedimentation performance is poor, resulting in low treatment efficiency; and third, the preparation process involves harsh reaction conditions (such as high temperature and high pressure) or the use of toxic and harmful modifiers, which does not align with the concept of green and environmentally friendly development. Therefore, how to prepare a starch-based coagulant with high charge density, excellent flocculation performance, environmental friendliness, and low preparation cost through reasonable formulation design and process optimization, and efficiently apply it to microplastic removal, has become a pressing technical problem to be solved in this field. Summary of the Invention

[0004] This application provides a method for preparing a starch-based coagulant and its application in microplastic removal, in order to solve the following technical problems: how to overcome the shortcomings of existing starch-based coagulants, such as low microplastic removal efficiency, complex preparation process, and insufficient environmental friendliness.

[0005] In a first aspect, this application provides a method for preparing a starch-based coagulant, the method comprising the following steps: S101. Mix starch with deionized water, then adjust the pH value to 8.0-9.0 to obtain starch milk; S102. Add sodium hydroxide to the starch milk and stir at a constant temperature of 50-60°C for 30-40 min to obtain pretreated starch milk. S103. Under inert gas protection, acrylamide, dimethyl diallyl ammonium chloride and an initiator are added to the pretreated starch milk cooled to 35-45°C, and the reaction is carried out at 35-45°C for 60-90 min to obtain a grafted copolymer starch solution. S104. Add nano-montmorillonite to the grafted copolymer starch solution and disperse it ultrasonically. Then add polydopamine, adjust the pH value to 6.5-7.5, and stir the reaction at 40-50°C for 40-60 min to obtain crude composite coagulant. S105. The crude composite coagulant is vacuum dried and pulverized to obtain a starch-based coagulant.

[0006] Optionally, the starch is corn starch or potato starch; The starch milk has a mass concentration of 10-15%; The amount of sodium hydroxide added is 0.5 to 1.0% of the mass of the starch.

[0007] Optionally, the amount of acrylamide added is 10-20% of the mass of the starch; The amount of dimethyl diallyl ammonium chloride added is 5-10% of the mass of the starch; The initiator is composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:1, and the total amount of the initiator added is 0.3 to 0.6% of the mass of the starch.

[0008] Optionally, the amount of nano-montmorillonite added is 3-8% of the mass of the starch; The ultrasonic dispersion time is 20–30 min; The amount of polydopamine added is 1-3% of the mass of the starch; The starch-based coagulant has a particle size of 100-200 mesh.

[0009] Secondly, this application provides the application of a starch-based coagulant prepared by the method described in any one of the first aspects in the removal of microplastics from water.

[0010] Optionally, the application includes the following steps: S201. Add the starch-based coagulant to deionized water and let it stand for 10-15 minutes to activate it, thereby obtaining a coagulant solution with a mass concentration of 0.5-1.0%. S202. Adjust the pH of the water containing microplastics to 6-8, then add the coagulant solution, and make the dosage of the starch-based coagulant 50-200 mg / L to obtain a mixture. S203. First, stir the mixture rapidly at a speed of 200-300 r / min for 1-3 min, then stir slowly at a speed of 50-80 r / min for 10-15 min. After that, let the water that has undergone coagulation reaction stand and settle for 20-30 min to complete the removal of microplastics.

[0011] Optionally, the microplastics include one or more of polyethylene, polypropylene, polyvinyl chloride, and polystyrene, and the particle size of the microplastics is 0.1 to 5 μm.

[0012] The technical solutions provided in this application have the following advantages compared with the prior art: (1) Excellent performance of coagulant: This application improves the charge density of coagulant by grafting starch with cationic monomer (dimethyl diallyl ammonium chloride), which enhances the charge neutralization ability of negatively charged microplastic particles; at the same time, nano-montmorillonite and polydopamine are introduced for composite modification. The large specific surface area of ​​nano-montmorillonite improves the adsorption performance, and polydopamine improves the interfacial bonding ability between coagulant and microplastics. The synergistic effect of the three makes the flocs formed by the coagulant dense and fast settling, with a microplastic removal efficiency of up to 95% or more. In particular, the removal effect of fine microplastics with a particle size of less than 1 μm is significantly better than that of traditional coagulants.

[0013] (2) Green and environmentally friendly preparation process: This application uses natural starch as the main raw material, which is widely available, renewable and biodegradable, avoiding the secondary pollution problem of traditional inorganic coagulants; the reaction conditions during the preparation process are mild (temperature 35~60℃, normal pressure), no high temperature and high pressure equipment is required, and the initiators and modifiers used do not release toxic and harmful substances, which meets the development requirements of green chemical industry.

[0014] (3) Low preparation and application costs: The raw materials such as starch and nano-montmorillonite are inexpensive, the preparation process is simple, no complicated equipment is required, and it is easy to industrialize. During the application process, the amount of coagulant added is small, the reaction conditions are mild, no additional reagents are required, and the processing cost is about 30% lower than that of traditional aluminum salt coagulants, which has significant economic advantages. Attached Figure Description

[0015] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A photograph of the starch-based coagulant provided in Example 1 of this application; Figure 2 This is a photograph of the starch-based coagulant provided in Example 2 of this application in the removal of polyethylene microplastics. Figure 3A comparison chart showing the removal rates of microplastics of different particle sizes by conventional polyaluminum chloride (a) provided in Example 4 of this application and starch-based coagulant (b) in Example 1; Figure 4 This is a schematic diagram illustrating the mechanism of action of starch-based coagulants in removing microplastics, as provided in the embodiments of this application. Figure 5 Scanning electron microscope images of the starch-based coagulant provided in Example 1 of this application before (a~c) and after (d~f) grafting modification. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] This application provides a method for preparing a starch-based coagulant, the method comprising the following steps: S101. Mix starch with deionized water, then adjust the pH value to 8.0-9.0 to obtain starch milk; S102. Add sodium hydroxide to the starch milk and stir at a constant temperature of 50-60℃ for 30-40 min to obtain pretreated starch milk. S103. Under inert gas protection, acrylamide, dimethyl diallyl ammonium chloride and initiator are added to the pretreated starch milk cooled to 35-45℃, and the reaction is carried out at 35-45℃ for 60-90 min to obtain grafted copolymer starch solution. S104. Add nano-montmorillonite to the grafted copolymer starch solution and disperse it ultrasonically. Then add polydopamine, adjust the pH value to 6.5-7.5, and stir the reaction at 40-50℃ for 40-60 min to obtain the crude composite coagulant. S105. The crude composite coagulant is vacuum dried and pulverized to obtain a starch-based coagulant.

[0020] In some embodiments, the starch is corn starch or potato starch; The starch milk has a mass concentration of 10-15%. The amount of sodium hydroxide added is 0.5 to 1.0% of the starch mass.

[0021] In some embodiments, the amount of acrylamide added is 10-20% of the starch mass; The amount of dimethyl diallyl ammonium chloride added is 5-10% of the starch mass; The initiator is composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:1, and the total amount of initiator added is 0.3 to 0.6% of the starch mass.

[0022] In some embodiments, the amount of nano-montmorillonite added is 3-8% of the starch mass; The ultrasonic dispersion time is 20–30 min; The amount of polydopamine added is 1-3% of the starch mass; The particle size of starch-based coagulants is 100-200 mesh.

[0023] It should be noted that the preparation method of the starch-based composite coagulant provided in this application successfully constructs a highly efficient composite coagulant material integrating multiple functions such as charge neutralization, adsorption bridging, and interfacial affinity. Its core lies in using natural starch as a green framework and precisely introducing functional components through controllable chemical grafting and physical compounding processes. The synergistic effect of each raw material significantly enhances the final product's ability to capture and remove microplastics.

[0024] First, in the starch pretreatment stage (S101 and S102), starch raw materials (corn or potato starch) are mixed with deionized water to prepare a starch slurry with a mass concentration of 10-15%. This concentration range ensures that the system has suitable fluidity, guaranteeing sufficient contact of reactants while avoiding problems such as difficulty in stirring or uneven reaction caused by excessive concentration. The starch slurry is then treated under weakly alkaline conditions (pH 8.0-9.0) and gentle heating (50-60℃). The core function of this step is to activate starch molecules: the alkaline environment helps to partially disrupt the crystalline structure of starch granules, weakening intramolecular and intermolecular hydrogen bonds, and exposing active sites such as hydroxyl groups more fully. Simultaneously, the addition of a small amount of sodium hydroxide (0.5-1.0% of the starch mass) and maintaining stirring for a certain period (30-40 min) promotes limited swelling and depolymerization of starch molecules, significantly improving the reactivity and uniformity of subsequent chemical modification, laying the foundation for graft copolymerization.

[0025] Secondly, graft copolymerization modification (S103) is a key step in this method to impart high charge density to the coagulant. Under inert gas protection, the pretreated starch slurry is cooled to 35–45°C, a suitable temperature range for free radical polymerization. Acrylamide (AM) and dimethyl diallyl ammonium chloride (DMDAAC) are added. Acrylamide (added at 10–20% of starch mass), as a neutral monomer, effectively increases the molecular chain length through its long-chain polymer segments, providing strong adsorption bridging capabilities. Dimethyl diallyl ammonium chloride (added at 5–10% of starch mass), as a strong cationic monomer, introduces positively charged quaternary ammonium salt groups onto the starch backbone, which is the core of improving the product's charge density. Under the action of a redox initiation system composed of ammonium persulfate and sodium bisulfite (mass ratio 1:1, total addition amount 0.3–0.6% of starch mass), the monomers undergo a copolymerization reaction on the starch chain, lasting 60–90 minutes to ensure a sufficient and complete grafting reaction. The synergistic effect of the two is as follows: the cationic DMDAAC provides positively charged centers to neutralize the negatively charged microplastic particles, reducing their electrostatic repulsion; while the long chains provided by AM can bridge the neutralized particles, bringing multiple particles together to form initial flocs. This step is carried out at a moderate temperature, avoiding the damage of high temperatures to the starch structure and ensuring a mild and controllable reaction.

[0026] Subsequently, composite modification (S104) aimed to further enhance the adsorption performance and interfacial bonding of the coagulant. Nano-montmorillonite (3-8% of starch mass) was added to the grafted copolymer starch solution and ultrasonically dispersed for 20-30 min to ensure uniform nanoscale dispersion. Nano-montmorillonite, with its large specific surface area and layered structure, serves as a highly efficient adsorption site, capturing microplastic particles and coexisting pollutants in the water through physical adsorption and ion exchange. Next, polydopamine (1-3% of starch mass) was added, and the pH was adjusted to near neutral 6.5-7.5. The mixture was stirred at 40-50°C for 40-60 min. Polydopamine, rich in catechol and amino functional groups, possesses strong adhesion and high reactivity, enabling it to coat the starch-based polymer and montmorillonite surfaces. It may also bind to the microplastic surface through covalent or non-covalent interactions, significantly enhancing the interfacial affinity between the entire composite and hydrophobic microplastic particles, promoting compact floc growth and rapid sedimentation. The synergy among the three reaches its peak here: grafted copolymer starch provides charge neutralization and bridging framework, nano-montmorillonite provides a high-capacity adsorption platform, and polydopamine acts as a universal adhesive to strengthen the overall binding with the target pollutant, together constructing a dense, easily settling flocculent.

[0027] Finally, post-processing (S105) transforms the liquid product into a solid product that is easy to store, transport, and use through vacuum drying and pulverization. The drying process removes moisture, while pulverization to a particle size range of 100–200 mesh ensures that the final product is a fine powder with uniform particle size. This specification facilitates rapid dissolution and dispersion in water, allowing functional groups to be released quickly and exert their effects, meeting the practical requirements for agent solubility and rate of action in water treatment applications.

[0028] Based on a general inventive concept, this application provides the application of a starch-based coagulant prepared by any of the above methods in the removal of microplastics from water.

[0029] In some implementations, the application includes the following steps: S201. Add starch-based coagulant to deionized water and let it stand for 10-15 minutes to activate it, so as to obtain a coagulant solution with a mass concentration of 0.5-1.0%. S202. Adjust the pH of the water containing microplastics to 6-8, then add coagulant solution to make the dosage of starch-based coagulant 50-200 mg / L to obtain a mixed solution; S203. First, stir the mixture rapidly at a speed of 200-300 r / min for 1-3 min, then stir slowly at a speed of 50-80 r / min for 10-15 min. After that, let the water that has undergone coagulation reaction stand and settle for 20-30 min to complete the removal of microplastics.

[0030] In some embodiments, the microplastics include one or more of polyethylene, polypropylene, polyvinyl chloride, and polystyrene, with a particle size of 0.1 to 5 μm.

[0031] It should be noted that, firstly, in the coagulant preparation and activation (S201) step, the solid starch-based coagulant is added to deionized water to prepare a solution with a mass concentration of 0.5–1.0%. This concentration range ensures that the solution has suitable viscosity and fluidity, which is beneficial for accurate subsequent metering and uniform addition, and also ensures good dispersibility of the coagulant molecules in the solution. The prepared solution needs to be allowed to stand for 10–15 minutes for activation, which is crucial. Activation allows the starch-based polymer chains, which may have curled up during drying, to fully unfold and hydrate in water, allowing the cationic groups, adsorption sites, and functional groups on the molecular chains to be fully exposed and restore their optimal conformation, thereby maximizing their reactivity and flocculation potential, laying the foundation for subsequent efficient coagulation.

[0032] Next, in the dosing and mixing (S202) step, the pH of the water to be treated is first adjusted to a slightly acidic to neutral range of 6–8. This pH range is common for most natural water bodies and wastewater. Under these conditions, the surface of microplastic particles is usually negatively charged, while the cationic groups in the coagulant of this application maintain a high positive charge, which is beneficial for achieving optimal charge neutralization. At the same time, this pH condition is also conducive to the interfacial interaction of components such as polydopamine. Subsequently, the activated coagulant solution is added, and its dosage is controlled at 50–200 mg / L. This dosage range can be flexibly adjusted according to the initial concentration of microplastics in the water (usually low) and the pollution load, aiming to provide sufficient effective ingredients to complete the coverage, neutralization, and bridging of target particles, ensuring excellent removal effects under a wide range of influent conditions, while avoiding waste or secondary pollution caused by excessive dosage.

[0033] Finally, in the reaction and separation (S203) step, staged mechanical stirring and natural sedimentation are used to complete the formation, growth, and removal of flocs. First, rapid stirring (200–300 r / min, 1–3 min) is performed to ensure that the coagulant and microplastic particles in the water undergo vigorous and thorough collision and mixing within a short time, ensuring rapid and uniform diffusion of the agent throughout the system to complete initial charge neutralization and adsorption. Then, slow stirring (50–80 r / min, 10–15 min) is introduced; this stage is the crucial flocculation stage. The lower shear force provides a mild environment for the destabilized microplastic particles and coagulant molecules. Through the bridging effect of polymer chains and the adsorption and adhesion of nano-montmorillonite and polydopamine, small flocs gradually aggregate and grow into larger, denser, settleable flocs. After stirring, allow the mixture to stand and settle for 20-30 minutes to provide sufficient time for the mature flocs to settle due to gravity, allowing them to effectively separate the captured microplastic particles to the bottom of the water body, thereby obtaining a clear supernatant.

[0034] This application method targets microplastics primarily including common plastic types such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene, with particle sizes ranging from 0.1 to 5 μm. This particle size range focuses on submicron-sized fine particles that are difficult to remove efficiently using traditional coagulation processes. This application significantly improves the capture and removal efficiency of microplastics within this size range through the unique synergistic mechanism of the coagulant.

[0035] In summary, compared with the prior art, this application has the following beneficial effects: (1) Excellent performance of coagulant: This application improves the charge density of coagulant by grafting starch with cationic monomer (dimethyl diallyl ammonium chloride), which enhances the charge neutralization ability of negatively charged microplastic particles; at the same time, nano-montmorillonite and polydopamine are introduced for composite modification. The large specific surface area of ​​nano-montmorillonite improves the adsorption performance, and polydopamine improves the interfacial bonding ability between coagulant and microplastics. The synergistic effect of the three makes the flocs formed by the coagulant dense and fast settling, with a microplastic removal efficiency of up to 95% or more. In particular, the removal effect of fine microplastics with a particle size of less than 1 μm is significantly better than that of traditional coagulants.

[0036] (2) Green and environmentally friendly preparation process: This application uses natural starch as the main raw material, which is widely available, renewable and biodegradable, avoiding the secondary pollution problem of traditional inorganic coagulants; the reaction conditions during the preparation process are mild (temperature 35~60℃, normal pressure), no high temperature and high pressure equipment is required, and the initiators and modifiers used do not release toxic and harmful substances, which meets the development requirements of green chemical industry.

[0037] (3) Low preparation and application costs: The raw materials such as starch and nano-montmorillonite are inexpensive, the preparation process is simple, no complicated equipment is required, and it is easy to industrialize. During the application process, the amount of coagulant added is small, the reaction conditions are mild, no additional reagents are required, and the processing cost is about 30% lower than that of traditional aluminum salt coagulants, which has significant economic advantages.

[0038] (4) Wide range of applications: The coagulant prepared in this application has a high efficiency in removing microplastics of different types (polyethylene, polypropylene, etc.) and different particle sizes (0.1-5μm). It can be applied to the treatment of various water bodies containing microplastics, such as surface water, industrial wastewater, and domestic sewage, and has broad application prospects.

[0039] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0040] Example 1: Preparation of starch-based coagulant Step S101: Starch pretreatment. Take 100g of corn starch, add 850mL of deionized water, then adjust the pH value to 8.5 with a 10% sodium hydroxide solution and stir to prepare a starch slurry with a 10.5% concentration. Step S102: Add 0.7g of sodium hydroxide to the starch milk and stir at 55℃ for 35min to obtain pretreated starch milk.

[0041] Step S103: Graft copolymerization modification. The pretreated starch emulsion was cooled to 40°C, and nitrogen gas was introduced for 10 minutes to purge the air. 15g of acrylamide and 8g of dimethyl diallyl ammonium chloride were added, and the mixture was stirred for 10 minutes until completely dissolved. Then, 0.45g of initiator (0.225g ammonium persulfate + 0.225g sodium bisulfite) was added, and the mixture was reacted at a constant temperature for 75 minutes under nitrogen protection to obtain a graft copolymerized starch solution.

[0042] Step S104: Composite modification. Add 5g of nano-montmorillonite to the grafted copolymer starch solution, ultrasonically disperse for 25min, then add 2g of polydopamine, adjust the pH to 7.0 with 5% hydrochloric acid solution, stir and react at 45℃ for 50min to obtain crude composite coagulant.

[0043] Step S105: Post-processing. The crude composite coagulant is placed in a vacuum drying oven and dried for 4 hours at a vacuum of -0.07 MPa and a temperature of 65°C. After drying, it is pulverized to 150 mesh to obtain the finished starch-based coagulant. A picture of the finished starch-based coagulant is shown below. Figure 1 As shown.

[0044] Example 2: Application of coagulants in the removal of polyethylene microplastics A photograph of a starch-based coagulant used in the removal of microplastics in polyethylene is shown below. Figure 2 As shown.

[0045] Step S201: Coagulant preparation. Take 5g of the coagulant product prepared in Example 1, add 995mL of deionized water, stir to dissolve and prepare a coagulant solution with a mass concentration of 0.5%, and let it stand for 12min to activate.

[0046] Step S202: Coagulation reaction. Take 1L of simulated wastewater containing polyethylene microplastics (particle size 0.5~2μm, concentration 50mg / L), adjust the pH to 7.0, add 10mL of activated coagulant solution (dosage 50mg / L); first stir rapidly at 250r / min for 2min, then stir slowly at 60r / min for 12min.

[0047] Step S203: Sedimentation and separation. The water was allowed to settle for 25 minutes. The supernatant was taken to test the microplastic concentration, and the removal rate was calculated to be 96.8%.

[0048] Example 3: Application of coagulants in the removal of mixed microplastics Step S201: Coagulant preparation. Take 10g of the coagulant product prepared in Example 1, add 990mL of deionized water, stir to dissolve and prepare a coagulant solution with a mass concentration of 1.0%, and let it stand for 15min to activate.

[0049] Step S202: Coagulation reaction. Take 1L of actual surface water sample containing mixed microplastics (polyethylene, polypropylene, and polyvinyl chloride in a mass ratio of 1:1:1, particle size 0.1~5μm, total concentration 80mg / L), adjust the pH to 6.5, add 20mL of activated coagulant solution (dosage 200mg / L); first stir rapidly at 300r / min for 1min, then stir slowly at 80r / min for 15min.

[0050] Step S203: Sedimentation and separation. The water was allowed to settle for 30 minutes. The supernatant was taken to test the microplastic concentration, and the overall removal rate was calculated to be 95.3%.

[0051] Example 4: Comparison of removal rates of microplastics of different particle sizes between the coagulant of Example 1 and traditional polyaluminum chloride. Figure 3 The graph shows a comparison of the removal rates of microplastics of different particle sizes by conventional polyaluminum chloride (a) and the starch-based coagulant of Example 1 (b). Figure 3 As shown in (a), the highest removal rates of PAM for PET, PP, and PS were 83.83%, 68.67%, and 68.17%, respectively. When the PAM dosage was 30-50 mg / L, the removal rate showed an upward trend. With further increases, the removal rate tended to stabilize. Figure 3 As shown in (b), the highest removal rates of the coagulant in this application for PET, PP, and PS were 98.8%, 92.2%, and 93.1%, respectively. When the starch-based dosage was 20-60 mg / L, the removal rate showed an upward trend. Upon further increase, the removal rate tended to stabilize.

[0052] Figure 4 The diagram shows the mechanism of action of the starch-based coagulant in removing microplastics according to this application. The coagulant molecules in this application remove microplastics through three synergistic actions: charge neutralization (cationic groups combine with the negative charge of microplastics), adsorption (the layered structure of nano-montmorillonite adsorbs microplastics), and bridging flocculation (polydopamine promotes particle aggregation).

[0053] Characterization Tests: Scanning Electron Microscopy (SEM) Analysis of Coagulant Before and After Modification The morphology and microstructure of the coagulant before and after modification were characterized by scanning electron microscopy (SEM). Figure 5 The image shows scanning electron microscope (SEM) images of the starch-based coagulant provided in Example 1 before (a-c) and after (d-f) graft modification. Figure 5 As shown in (a~c), the starch granules before grafting modification consist of smooth polygonal or spherical granules. Figure 5As shown in (d~f), the surface morphology and microstructure of starch granules changed significantly after grafting modification. The surface developed wrinkles and pores and became rougher than before modification, increasing the surface area of ​​the granules.

[0054] Comparative Example 1 This comparative example is modified from the disclosures in Examples 1 and 2 as follows: In step S103 (graft copolymerization modification), dimethyl diallyl ammonium chloride is not added; only 15g of acrylamide is added for graft copolymerization. The remaining steps and parameters are exactly the same as in Example 1.

[0055] The comparative product was tested for polyethylene microplastic removal using the same application method as in Example 2. The results showed that the final microplastic removal rate was 71.2%. Compared with Example 2 (96.8%), the removal rate was significantly lower. This is because the coagulant molecule lacks strong cationic groups, resulting in insufficient charge density and weakened ability to neutralize negatively charged microplastic particles, making it difficult to effectively destabilize the particles and thus seriously affecting the flocculation efficiency.

[0056] Comparative Example 2 This comparative example is modified from the disclosures in Examples 1 and 2 as follows: In step S104 (composite modification), no nano-montmorillonite and polydopamine are added. That is, after obtaining the grafted copolymer starch solution, the post-processing in step S105 is carried out directly. The remaining steps and parameters are exactly the same as in Example 1.

[0057] The comparative product was tested for polyethylene microplastic removal using the same application method as in Example 2. The results showed that the final microplastic removal rate was 80.5%. Compared with Example 2, the removal rate decreased significantly. This is because the coagulant lacks the large adsorption specific surface area provided by nano-montmorillonite and the interfacial affinity enhanced by polydopamine, resulting in insufficient adsorption bridging ability and microplastic capture ability, leading to less dense flocs and poorer settling performance.

[0058] Comparative Example 3 This comparative example is modified from the disclosures in Examples 1 and 2 as follows: In step S103, the graft copolymerization reaction temperature is set to 25°C, and the remaining steps and parameters are exactly the same as in Example 1.

[0059] The comparative product was tested using the same application method as in Example 2. The results showed that the final microplastic removal rate was 78.9%. This is lower than that of Example 2. This is because the reaction temperature was too low, leading to a decrease in the initiator decomposition rate and monomer polymerization activity, incomplete graft copolymerization, and insufficient introduction of functional monomers onto the starch backbone. This affected the length and charge density of the coagulant molecular chains, thus weakening its flocculation performance.

[0060] Comparative Example 4: Application of Traditional Polyaluminum Chloride in Microplastic Removal Take 1L of simulated polyethylene microplastic wastewater identical to that in Example 2, adjust the pH to 7.0, add polyaluminum chloride solution (dosage amount is 100mg / L), and operate according to the coagulation reaction and sedimentation separation steps of Example 2. Detect the concentration of microplastics in the supernatant and calculate the removal rate to be 72.5%, which is significantly lower than the starch-based composite coagulant prepared in this application.

[0061] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a starch-based coagulant, characterized in that, The method includes the following steps: S101. Mix starch with deionized water, then adjust the pH value to 8.0-9.0 to obtain starch milk; S102. Add sodium hydroxide to the starch milk and stir at a constant temperature of 50-60°C for 30-40 min to obtain pretreated starch milk. S103. Under inert gas protection, acrylamide, dimethyl diallyl ammonium chloride and an initiator are added to the pretreated starch milk cooled to 35-45°C, and the reaction is carried out at 35-45°C for 60-90 min to obtain a grafted copolymer starch solution. S104. Add nano-montmorillonite to the grafted copolymer starch solution and disperse it ultrasonically. Then add polydopamine, adjust the pH value to 6.5-7.5, and stir the reaction at 40-50°C for 40-60 min to obtain crude composite coagulant. S105. The crude composite coagulant is vacuum dried and pulverized to obtain a starch-based coagulant.

2. The method for preparing the starch-based coagulant according to claim 1, characterized in that, The starch is corn starch or potato starch; The starch milk has a mass concentration of 10-15%; The amount of sodium hydroxide added is 0.5 to 1.0% of the mass of the starch.

3. The method for preparing the starch-based coagulant according to claim 1, characterized in that, The amount of acrylamide added is 10-20% of the mass of the starch; The amount of dimethyl diallyl ammonium chloride added is 5-10% of the mass of the starch; The initiator is composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:1, and the total amount of the initiator added is 0.3 to 0.6% of the mass of the starch.

4. The method for preparing the starch-based coagulant according to claim 1, characterized in that, The amount of nano-montmorillonite added is 3-8% of the mass of the starch; The ultrasonic dispersion time is 20–30 min; The amount of polydopamine added is 1-3% of the mass of the starch; The starch-based coagulant has a particle size of 100-200 mesh.

5. The application of a starch-based coagulant prepared by the method of any one of claims 1 to 4 in the removal of microplastics from water.

6. The application of the starch-based coagulant according to claim 5 in the removal of microplastics from water, characterized in that, The application includes the following steps: S201. Add the starch-based coagulant to deionized water and let it stand for 10-15 minutes to activate it, thereby obtaining a coagulant solution with a mass concentration of 0.5-1.0%. S202. Adjust the pH of the water containing microplastics to 6-8, then add the coagulant solution, and make the dosage of the starch-based coagulant 50-200 mg / L to obtain a mixture. S203. First, stir the mixture rapidly at a speed of 200-300 r / min for 1-3 min, then stir slowly at a speed of 50-80 r / min for 10-15 min. After that, let the water that has undergone coagulation reaction stand and settle for 20-30 min to complete the removal of microplastics.

7. The application of the starch-based coagulant according to claim 5 in the removal of microplastics from water, characterized in that, The microplastics include one or more of polyethylene, polypropylene, polyvinyl chloride, and polystyrene, and the particle size of the microplastics is 0.1 to 5 μm.

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