Ultrasonic modified chitosan, preparation method thereof and application of ultrasonic modified chitosan in treatment of micro-plastics in water body

Abstract

The invention discloses ultrasonic modified chitosan, a preparation method thereof and application of the ultrasonic modified chitosan in treatment of microplastics in water, and the preparation method comprises the following steps: mixing chitosan with water to prepare a chitosan dispersion liquid, and carrying out ultrasonic treatment to obtain the ultrasonic modified chitosan. According to the preparation method of the ultrasonic modified chitosan, the ultrasonic modified chitosan which is high in flocculation efficiency, high in safety and environmentally friendly can be prepared through simple ultrasonic treatment, and the ultrasonic modified chitosan serving as a novel flocculating agent with excellent flocculation performance can be widely used for treating micro-plastics in a water body; and efficient flocculation and sedimentation of the micro-plastics can be realized through simple oscillation and standing, effective removal of the micro-plastics in the water body can be realized, and the method has an important promotion effect on effective treatment of new pollutants such as the micro-plastics. Meanwhile, the preparation method of the ultrasonic modified chitosan further has the advantages of being simple in process, low in cost, convenient to operate and the like, large-scale preparation is facilitated, and industrial application of the chitosan is conveniently achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of environmental protection and removal of new pollutants, and relates to an ultrasonic-modified chitosan, a preparation method thereof, and an application thereof in treating microplastics in water bodies. Background Art

[0002] Since its advent, plastics have been widely produced and used due to their diverse types and properties, providing convenience for humans. However, a large amount of plastic waste penetrates into the environment every year. In particular, under a series of environmental actions such as photooxidation, mechanical abrasion, and biological erosion, these plastics degrade into microplastics (plastic particles with a particle size < 5 mm) and nanoplastics, causing significant harm and potential risks to the environment. At the same time, due to the high chemical stability, large specific surface area, strong hydrophobicity, etc. of microplastics, they can exist in the environment for a long time and have been recognized as a major category of new pollutants. They are easily ingested by organisms, affecting the respiratory and digestive systems, releasing toxic derivatives, serving as carriers of viruses and harmful substances, etc., posing a serious threat to the ecosystem and human health.

[0003] Currently, methods for solving the problem of microplastic pollution include adsorption, membrane treatment, biodegradation, advanced oxidation, flocculation, and coagulation, etc. Among them, flocculation is widely used because of its simplicity, low cost, and minimal carbon footprint. However, traditional aluminum-based and iron-based coagulants have problems such as low removal efficiency of microplastics, introduction of additional pollution, and coagulant residues. Therefore, it is very necessary to develop a method that is more environmentally friendly, has no secondary risks, and can effectively flocculate and remove microplastics.

[0004] Natural organic matter (NOM) widely exists in nature, including sugars, proteins, oils, etc. Due to its rich functional groups and binding sites, it has the potential to solve the problem of microplastic pollutants. For example, as a polysaccharide with the second highest abundance in nature after cellulose, chitosan (CTS) is a cationic basic polysaccharide rich in the shells of shrimps, crabs and other crustaceans or insects. It has various physiological functions such as biodegradability, biocompatibility, non-toxicity, antibacterial, anti-cancer, and immune enhancement, and is a green material. However, in the actual use process, when using chitosan as a flocculant to treat microplastics in water, good results have not been obtained. In addition, the existing improvement methods for CTS mainly involve chemical modification methods, such as compounding or doping modification of CTS to improve its performance towards pollutants. However, when using these methods to improve CTS, new chemical substances will be introduced additionally, so there is a high possibility of chemical substance leakage during the application process, which will bring more serious secondary pollution, and this is not allowed. At the same time, the above methods are often complex in operation, high in economic cost, and difficult to implement in batches, which limits the wide use of CTS in the fields of treating microplastics in water and so on. Therefore, how to improve the adsorption activity of CTS towards microplastics is of great significance for promoting the wide use of CTS in the treatment of microplastic pollution and achieving the effective removal of microplastics in water. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an ultrasound-modified chitosan with low cost, high flocculation efficiency, high safety, and green environmental protection, its preparation method, and its application in treating microplastics in water.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A preparation method of ultrasound-modified chitosan, comprising the following steps:

[0008] S1. Mix chitosan with water to form a chitosan dispersion;

[0009] S2. Perform ultrasonic treatment on the chitosan dispersion obtained in step S1 to obtain ultrasound-modified chitosan.

[0010] For the above preparation method, further improved, in step S1, the chitosan dispersion is prepared by the following method: Mix chitosan with water and stir to obtain a chitosan dispersion; the ratio of chitosan to water is ≤2.0 mg∶1 mL.

[0011] For the above preparation method, further improved, the ratio of chitosan to water is 0.05 mg - 0.8 mg∶1 mL.

[0012] In the above preparation method, further improved, the ratio of chitosan to water is 0.1 mg to 0.5 mg∶1 mL.

[0013] In the above preparation method, further improved, the following treatment is also included before using chitosan: passing chitosan through a 100-mesh sieve, collecting the undersize to obtain chitosan powder; the rotation speed of the stirring is 1000 rpm to 1200 rpm; the stirring time is 0.5 h to 1 h.

[0014] In the above preparation method, further improved, in step S2, the time of the ultrasonic treatment is 3 h to 15 h; the ultrasonic treatment is carried out under the condition of a temperature of 25 °C to 30 °C.

[0015] In the above preparation method, further improved, the power of the ultrasonic wave is controlled to be 500 W to 700 W and the frequency is 30 kHz to 40 kHz during the ultrasonic treatment.

[0016] In the above preparation method, further improved, the following treatment is also included after the ultrasonic treatment: filtering the product solution after the ultrasonic treatment with a filter membrane with a pore size of 0.22 μm, and air-drying the solid product obtained after filtration under the constant temperature condition of 35 ± 1 °C to obtain ultrasonically modified chitosan.

[0017] As a general technical concept, the present invention also provides an ultrasonically modified chitosan prepared by the above preparation method.

[0018] As a general technical concept, the present invention also provides an application of the above ultrasonically modified chitosan in treating microplastics in water bodies.

[0019] In the above application, further improved, the following steps are included: mixing the ultrasonically modified chitosan and the wastewater containing microplastics, oscillating, flocculating, and standing still to complete the removal of microplastics in the water body; the addition amount of the ultrasonically modified chitosan is 10 mg to 100 mg of ultrasonically modified chitosan added to each liter of wastewater containing microplastics.

[0020] In the above application, further improved, the concentration of microplastics in the wastewater containing microplastics ≤ 100 mg / L; the microplastics are at least one of polymethyl methacrylate and polystyrene; the particle size of the microplastics is 2000 mesh; the oscillation is carried out under the condition of a rotation speed of 2000 rpm; the oscillation time is 1 min to 5 min; the flocculation is carried out under the conditions of a rotation speed of 100 rpm and a temperature of 25 ± 1 °C; the flocculation time is 30 min to 60 min; the standing still time is 1 h.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] (1) Aiming at the defects of chitosan and modified chitosan, such as low flocculation efficiency, high risk of secondary pollution, complex preparation, high preparation cost, and difficulty in mass production, the present invention provides a preparation method of ultrasonically modified chitosan. First, chitosan is mixed with water to form a chitosan dispersion. On the one hand, using water as a solvent will not introduce secondary pollution and is more environmentally friendly. On the other hand, under the action of water, the hydrogen bond and oxidation effects with chitosan can be enhanced, which is beneficial to the ultrasonic modification of chitosan. On this basis, the chitosan dispersion is ultrasonically treated. On the one hand, through the cavitation effect of ultrasound, the formation, growth, and rupture of cavitation bubbles in the system promote the interaction between the solvent and the solute, and further lead to changes in the surface properties and morphological structures of the material. Thus, the hydrophilicity, porous structure, and exposure of surface functional groups of chitosan are improved through ultrasonic modification, which is beneficial to the enhancement of electrostatic interaction, adsorption bridging, surface functional group reaction, etc., and is conducive to promoting the binding of chitosan and microplastics, greatly improving its flocculation performance for microplastics. On the other hand, compared with the traditional chemical reagent doping and composite modification methods, in the present invention, the optimization of the properties of chitosan itself is promoted through ultrasonic action, without the need to add additional substances, and there is no worry about more serious pollution to the environment caused by the leakage of additives. More importantly, the modified chitosan prepared by the ultrasonic modification method adopted in the present invention can enhance the charge interaction, van der Waals force, hydrogen bond interaction, pore interception, and network capture and sweeping effects between chitosan and microplastics, forming flocs with larger size and tighter structure, which are easy to precipitate, so as to realize the removal of microplastic particles in water. Therefore, in the present invention, ultrasonically modified chitosan with high flocculation efficiency, high safety, and environmental friendliness can be prepared through simple ultrasonic treatment, and at the same time, it also has the advantages of simple process, low cost, and convenient operation, which is beneficial to large-scale preparation and convenient for the industrial application of chitosan.

[0023] (2) In the preparation method of the present invention, chitosan is the second most abundant natural organic matter in nature, widely existing in rivers and lakes where shrimps and crabs inhabit. Its molecular chain contains amino and hydroxyl groups, and it is easy to dissociate into cationic -NH3 + in water, and should have good flocculation potential for microplastics with a negatively charged surface. On the one hand, compared with traditional metal-based flocculants, choosing chitosan as an environmental pollution treatment reagent will not cause serious environmental impacts. On the other hand, compared with various chemical flocculants with complex synthesis processes and high costs, chitosan is rich in content, simple to extract, easy to mass produce, and can achieve environmental governance at a relatively low cost.

[0024] (3) In the preparation method of the present invention, the chitosan dispersion used is prepared by mixing chitosan and water and then stirring. The stirring speed is optimized to be 1000 rpm to 1200 rpm, and the time is 0.5 h to 1 h, which can enable the chitosan powder to fully react with water and be wetted. In particular, by optimizing the stirring conditions, the dispersion uniformity of chitosan in water can be significantly improved, thereby promoting the full wetting of the chitosan powder. This is the basis for improving the flocculation performance of chitosan by the ultrasonic modification method. In addition, if the stirring speed is too high, the energy consumption will be significantly increased, and the surface of chitosan is easily damaged, which is not conducive to improving the flocculation performance. If the stirring speed is too low, the full mixing and wetting of the materials cannot be guaranteed. In addition, if the stirring time is too long, unnecessary side reactions will occur and the energy consumption will increase. When the stirring time is too short, the full wetting of the materials cannot be guaranteed either.

[0025] (4) In the preparation method of the present invention, the chitosan also includes the following treatment before use: passing the chitosan through a 100-mesh sieve and collecting the undersize as the raw material. By optimizing the size of the chitosan powder, it is beneficial to realize the modification of chitosan by the ultrasonic method. However, when the size of chitosan is too large, it is not conducive to its full modification. When the size of chitosan is too small, it is easy to crack during the ultrasonic process. In addition, in the present invention, the ratio of chitosan to water is also optimized to be 0.05 mg to 0.8 mg∶1 mL. In particular, when the ratio of the two is 0.1 mg to 0.5 mg∶1 mL, it is beneficial to promote the full contact between water and chitosan, and then beneficial to enhancing the ultrasonic cavitation effect between the surface of chitosan and water. Finally, the active area can be significantly increased on the premise of effectively preventing the fragmentation of chitosan. This is because when the dosage of chitosan is too high, less water is difficult to achieve full modification through ultrasonic cavitation, and the active area is difficult to increase at this time. When the dosage of chitosan is too low, more water is likely to cause excessive fragmentation or even dissolution of chitosan.

[0026] (5) In the preparation method of the present invention, the conditions of ultrasonic treatment are optimized. Specifically, the ultrasonic treatment is carried out under the condition that the temperature is 25°C to 30°C. During the ultrasonic treatment process, the power of the ultrasonic wave is controlled to be 500W to 700W, the frequency is 30kHz to 40kHz, and the time of ultrasonic treatment is 3h to 15h. By optimizing the conditions of ultrasonic treatment, the cavitation effect (bubble generation and rupture) can be enhanced, the energy utilization rate can be improved, thereby accelerating the modification speed. At the same time, by reasonably controlling the ultrasonic parameters, the degree of modification can be effectively controlled to avoid excessive damage to the material structure. Particularly, when the time of ultrasonic treatment is too long, the structure of chitosan will be excessively damaged, while when the time of ultrasonic treatment is too short, it is difficult to achieve sufficient cavitation. In addition, during the ultrasonic process, a thermal effect will be generated, resulting in a temperature increase. Therefore, when the temperature of ultrasonic treatment is too high, the material structure will be damaged. At the same time, when the temperature of ultrasonic treatment is too low, the reaction speed will be slowed down and the cavitation effect will be inhibited. In addition, when the power of the ultrasonic wave is too high, it will also cause excessive damage to the material structure, and there is a risk of generating by-products, free radicals, etc. While when the power of the ultrasonic wave is too low, the modification effect cannot be fully achieved; similarly, when the frequency of the ultrasonic wave is too high, the size of the cavitation bubbles will decrease, the rupture energy will be reduced, and the mechanical shear effect will be weakened, while when the frequency of the ultrasonic wave is too low, the cavitation bubbles will be too large and the energy distribution will be uneven.

[0027] (6) The present invention also provides an application of ultrasonically modified chitosan in treating microplastics in water bodies. Specifically, the ultrasonically modified chitosan is mixed with the microplastic solution for flocculation sedimentation. Through simple oscillation and standing, efficient flocculation and sedimentation of microplastics can be achieved, which has the advantages of low flocculant dosage, good flocculation effect, simple operation, low cost, convenient batch treatment, etc., and can effectively remove microplastics in water bodies, which has an important promoting effect on the effective treatment of new pollutants such as microplastics.

[0028] (7) In the present invention, when using ultrasonically modified chitosan to treat microplastics in water bodies, the whole flocculant is pure chitosan, with a density greater than that of water. After flocculating microplastics, it will settle to the sediment, providing sufficient nutrient sources for microorganisms, enriching the dominant species of microorganisms, and accelerating its complete decomposition in the environment to further achieve harmless treatment. Description of the Drawings

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0030] Figure 1 It is a control chart of the sedimentation efficiency of ultrasonically modified chitosan prepared under different ultrasonic times (0h, 3h, 6h, 9h, 12h) and different addition amounts on PMMA microplastic solution in Example 1 of the present invention.

[0031] Figure 2 This is a control chart of the sedimentation efficiency of ultrasonically modified chitosan prepared under different dispersion concentration conditions in Example 2 of the present invention on PMMA microplastic solution at different addition amounts.

[0032] Figure 3 This is a photo of the flocculation reaction sample and a metallographic microscope image of the flocs of the ultrasonically modified chitosan with the best sedimentation efficiency and the original chitosan on PMMA microplastic solution in Example 2 of the present invention.

[0033] Figure 4 This is the FTIR spectrum, water contact angle and SEM image of the ultrasonically modified chitosan with the best sedimentation efficiency and the original chitosan in Example 2 of the present invention.

[0034] Figure 5 This is a particle size distribution diagram of the flocs formed by ultrasonically modified chitosan and the original chitosan with PMMA microplastics under the condition of an addition amount of 20 mg / L in Example 3 of the present invention.

[0035] Figure 6 This is a size comparison diagram of the flocs formed by ultrasonically modified chitosan and the original chitosan with PMMA microplastics under different addition amount conditions in Example 3 of the present invention.

[0036] Figure 7 This is a comparison chart of the sedimentation efficiency of ultrasonically modified chitosan and the original chitosan on PMMA and PS under different addition amounts in Example 4 of the present invention.

[0037] Figure 8 This is a size comparison diagram of the flocs formed by ultrasonically modified chitosan and the original chitosan with PS microplastics under different addition amounts in Example 4 of the present invention. Detailed implementation manners

[0038] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] Example 1

[0040] A preparation method of ultrasonically modified chitosan, specifically enhancing the flocculation performance of chitosan on microplastics by ultrasonic treatment, includes the following steps:

[0041] (1) Pass the purchased chitosan powder through a 100-mesh molecular sieve, collect the material under the sieve, and obtain the original chitosan powder.

[0042] (2) Disperse the chitosan powder into ultrapure water at a concentration ratio of 2 mg / mL, place it in a constant-speed magnetic stirrer, and stir for 1 h under the condition of a rotation speed of 1000 rpm to fully disperse the chitosan, obtaining a chitosan dispersion.

[0043] (3) Place the above chitosan dispersion in an ultrasonic cleaner connected to a low-temperature constant-temperature circulating bath, and perform ultrasonic treatment in a constant-temperature circulating water environment with a temperature set at 25 °C, a power of 700 W, and a frequency of 40 kHz. Set five groups of ultrasonic treatment for different times (0 h, 3 h, 6 h, 9 h, 12 h) to obtain different ultrasonically modified chitosan dispersions.

[0044] (4) Use a microporous nylon membrane with a pore size of 0.22 μm to perform vacuum filtration on different ultrasonically modified chitosan dispersions under the condition of -0.1 MPa. Collect the ultrasonically modified chitosan wet powder, and place it in a constant-temperature drying oven at 35 °C to air-dry the ultrasonically modified chitosan wet powder, obtaining five different ultrasonically modified chitosan powders.

[0045] An application of the ultrasonically modified chitosan prepared in the above embodiments of the present invention in treating microplastics in water bodies, specifically using the ultrasonically modified chitosan as a flocculant to remove polymethyl methacrylate (PMMA) microparticles in water bodies, including the following steps:

[0046] Respectively take the ultrasonically modified chitosan prepared in Example 1 under different ultrasonic times (0 h, 3 h, 6 h, 9 h, 12 h), and add them to a 100 mg / L PMMA microplastic solution according to the addition amounts of 0, 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 60 mg / L, 70 mg / L, 80 mg / L, 90 mg / L, 100 mg / L. The particle size of PMMA in this solution is 2000 mesh, and the zeta potential on the surface of this solution is -41.50 mV, and the pH value is within the range of 6.5 ± 0.2. Oscillate at a high speed of 2000 rpm for 3 min with a mixer, and incubate the flocs in a shaker at 25 °C and 100 rpm for 60 min. Finally, let all the samples stand for 1 h to completely settle the flocs, completing the removal of PMMA in the water body.

[0047] After the sedimentation is completed, use a UV-visible spectrophotometer to measure the residual concentration of PMMA in the sample suspension at a wavelength of 460 nm, and calculate the sedimentation rate through the following formula:

[0048] Sedimentation rate (%) = (1 - A / A0) × 100.

[0049] Where A is the absorbance of the sample suspension after the reaction at a wavelength of 460 nm, and A0 is the absorbance of the initially dispersed 100 mg / L PMMA solution at a wavelength of 460 nm.

[0050] Figure 1 This is a control chart of the sedimentation efficiency of ultrasonically modified chitosan prepared under different ultrasonic times (0 h, 3 h, 6 h, 9 h, 12 h) in Example 1 of the present invention at different addition amounts for a PMMA microplastic solution. From Figure 1 The results show that as the addition amount of chitosan increases, the sedimentation rate of PMMA also increases significantly. The chitosan treated with ultrasound for 12 h can reach a sedimentation rate of 94.83% when the addition amount is 90 mg / L, while the untreated chitosan is only 89.25%. This indicates that ultrasonic treatment can indeed enhance the property of chitosan to flocculate PMMA, and the treatment effect with an ultrasonic duration of 12 h can enhance this property of chitosan to the greatest extent.

[0051] Example 2

[0052] A preparation method of ultrasonically modified chitosan, specifically to investigate the influence of the dispersion concentration of chitosan in water on the flocculation performance of chitosan. The specific steps are as follows:

[0053] (1) Pass the purchased chitosan powder through a 100-mesh molecular sieve, collect the material under the sieve, and obtain the original chitosan powder.

[0054] (2) According to the dispersion concentration ratios of 0.2 mg / mL, 0.4 mg / mL, 0.8 mg / mL, and 2.0 mg / mL, disperse different masses of chitosan into ultrapure water, place it in a constant-speed magnetic stirrer, and stir for 1 h under the condition of a rotation speed of 1000 rpm to fully disperse the chitosan and obtain chitosan dispersion liquids with different concentrations.

[0055] (3) Place the above chitosan dispersion liquids in an ultrasonic cleaner connected to a low-temperature constant-temperature circulating bath, and perform ultrasonic treatment for 12 h at a power of 700 W and a frequency of 40 kHz in a constant-temperature circulating water environment with a set temperature of 25 °C to obtain different ultrasonically modified chitosan dispersion liquids.

[0056] (4) Use a microporous nylon membrane with a pore size of 0.22 μm to perform vacuum filtration on different ultrasonically modified chitosan dispersion liquids under -0.1 MPa, collect the wet powder of ultrasonically modified chitosan, and place it in a constant-temperature drying oven at 35 °C to air-dry the wet powder of ultrasonically modified chitosan to obtain four different powders of ultrasonically modified chitosan.

[0057] An application of the ultrasonically modified chitosan prepared in the above Example of the present invention in treating microplastics in water, specifically using the ultrasonically modified chitosan as a flocculant to remove polymethyl methacrylate (PMMA) particles in water, which is basically the same as Example 1, except that: the preparation method of the ultrasonically modified chitosan used is different.

[0058] After sedimentation was completed, the residual concentration of PMMA in the sample suspension was measured using a UV-visible spectrophotometer at a wavelength of 460 nm.

[0059] Figure 2 This is a control chart of the sedimentation efficiency of ultrasonically modified chitosan prepared under different dispersion concentrations in Example 2 of the present invention on PMMA microplastic solution at different addition amounts. As Figure 2 The results in clearly show that the dispersion concentration of chitosan significantly affects the enhancement effect of ultrasonic treatment on the flocculation sedimentation properties of chitosan for microplastics. In this example, with the increase in the dispersion concentration, the enhancement effect on the flocculation performance of chitosan gradually decreases, that is, the corresponding order of enhancement effects is 0.2 mg / mL > 0.4 mg / mL > 0.8 mg / mL > 2.0 mg / mL. This indicates that the lower the dispersion concentration, the more beneficial it is for the modification of chitosan by ultrasonic action. At the same time, from the results of Example 1 and Example 2, it can be concluded that the ultrasonic treatment that makes the flocculation sedimentation effect of chitosan the best is: ultrasonic power 700 W, ultrasonic frequency 40 kHz, chitosan dispersion concentration 0.2 mg / mL, and ultrasonic treatment duration 12 h.

[0060] Figure 3 This is a photo of the flocculation reaction sample and a micrograph of the flocs of the ultrasonically modified chitosan with the best sedimentation efficiency and the original chitosan on the PMMA microplastic solution in Example 2 of the present invention. As Figure 3 can be seen, it can be very clearly observed that compared with the original chitosan powder, the ultrasonically modified chitosan prepared in the present invention can form larger and more stable flocs with microplastics, promoting their sedimentation and removal.

[0061] In this example, further microscopic infrared spectroscopy (FTIR) scanning, water contact angle measurement, and scanning electron microscopy (SEM) imaging were performed on the ultrasonically modified chitosan with the best sedimentation efficiency and the original chitosan to characterize the reasons for the change in the flocculation properties of chitosan for microplastics by ultrasonic treatment.

[0062] Figure 4 This is the FTIR spectrum, water contact angle, and SEM image of the ultrasonically modified chitosan with the best sedimentation efficiency and the original chitosan in Example 2 of the present invention. Figure 4 The FTIR spectrum in shows that the surface structure of chitosan has changed significantly before and after ultrasonic treatment. Due to the stretching vibrations of -OH and -NH, a wider range of absorption peaks are observed near 3400 cm -1 , indicating an increase in hydrogen bonds on the surface of chitosan after ultrasonic treatment. The change in the water contact angle from 75.38° before ultrasonic treatment to 59.10° after ultrasonic treatment further shows that ultrasonic treatment improves the hydrophilicity of chitosan, which is beneficial to the occurrence of hydrogen bond interactions. In addition, after ultrasonic treatment, the C-N stretching vibration at 1327 cm -1 and the 895 cm -1The vibration peaks of the six-membered rings at [specific position] are all enhanced, which means that the electronegativity difference between the terminal atoms of the bonds increases, thereby enhancing the surface reactivity. The SEM images of the original chitosan and the ultrasonically modified chitosan show that significant changes have occurred in the morphology and structure of CTS before and after ultrasonic treatment. CTS has changed from a complete block to a loofah-like sac shape, and the porosity has increased greatly, which not only increases the specific surface area of the reaction, but also this structure is conducive to the occurrence of sweeping and netting effects. The above-mentioned structural changes of chitosan are attributed to the cavitation induced by ultrasonic waves, which causes the regions with weaker polymerization strength in CTS to be torn by cavitation bubbles. Generally speaking, the ultrasonic induction of the structural and chemical changes of CTS significantly improves its efficiency in the flocculation sedimentation of microplastics, and this ultrasonic effect has the characteristics of being more environmentally friendly, easy to operate, and suitable for batch processing compared with traditional chemical modification methods.

[0063] Example 3

[0064] Investigate the influence of ultrasonically modified chitosan on the flocculation characteristics of PMMA. The specific implementation steps are as follows:

[0065] (1) According to the method in Example 1, using ultrasonically modified chitosan to treat the PMMA microplastic solution at the addition amounts of 0mg / L, 4mg / L, 8mg / L, 10mg / L, 12mg / L, 14mg / L, 16mg / L, 18mg / L, 20mg / L, 22mg / L. The corresponding ultrasonic treatment conditions of the ultrasonically modified chitosan used are: ultrasonic power 700W, ultrasonic frequency 40kHz, chitosan dispersion concentration 0.2mg / mL, and ultrasonic treatment duration 12h.

[0066] (2) After the flocs are incubated, then use a CMOS camera equipped with an optical microscope (Sony IMX334 sensor, 3840×2160 resolution) to record the video images of the flocs in the sample at a fixed magnification and a fixed distance.

[0067] (3) Use Adobe Photoshop software to convert the video into photos at a frequency of 10 frames per second.

[0068] (4) Select the pictures with clear and complete floc imaging in the photos, and use image analysis software (Image Pro Plus6.0) to measure various physical parameters of the flocs in the photos (floc diameter, aspect ratio, fractal dimension, etc.).

[0069] (5) Conduct statistical analysis on all the floc diameter data of each sample obtained to obtain the average floc diameter of each sample for comparison.

[0070] Figure 5This is the particle size distribution diagram of the flocs formed by ultrasonic modified chitosan and the original chitosan in Example 3 of the present invention under the condition of an addition amount of 20 mg / L with PMMA microplastics. From Figure 5 it can be seen that under the addition amount of 20 mg / L, ultrasonic modified chitosan can form more considerable flocs with PMMA. The average particle size of some flocs is even as high as 1.38 mm, which is 4.76 times that of the maximum average particle size (0.29 mm) of the flocs formed by the original chitosan.

[0071] Figure 6 This is the size comparison diagram of the flocs formed by ultrasonic modified chitosan and the original chitosan in Example 3 of the present invention under different addition amount conditions with PMMA microplastics.

[0072] Figure 6 Among them, the statistical average particle sizes of all the flocs formed by the optimal ultrasonic modified chitosan and the original chitosan in Example 3 of the present invention with PMMA microplastics under different addition amounts are compared. From Figure 6 it can be seen that the addition of chitosan can promote the flocculation of microplastics, and the size of the flocs formed by ultrasonic modified chitosan is significantly larger than that of the flocs formed by the original chitosan. In the range of the tested chitosan addition concentrations, the highest statistical average particle size of the normal distribution of the flocs formed by ultrasonic modified chitosan is 0.49 mm (the addition amount of ultrasonic modified chitosan is 20 mg / L), while the highest statistical average particle size of the normal distribution of the flocs formed by the original chitosan is only 0.08 mm (the addition amount of the original chitosan is 20 mg / L). This shows that ultrasonic treatment helps chitosan and microplastics to form more significant flocs, and ultrasonic modified chitosan has more excellent microplastic flocculation performance compared with the original chitosan.

[0073] Example 4

[0074] An application of the ultrasonic modified chitosan prepared in the above-mentioned embodiments of the present invention in treating microplastics in water bodies, specifically using ultrasonic modified chitosan as a flocculant to remove polymethyl methacrylate (PMMA) and polystyrene (PS) microparticles in water bodies, including the following steps:

[0075] According to the method in Example 1, using ultrasonic modified chitosan to treat polymethyl methacrylate (PMMA) and polystyrene (PS) microplastic solutions respectively according to the addition amounts of 0 mg / L, 4 mg / L, 8 mg / L, 10 mg / L, 12 mg / L, 14 mg / L, 16 mg / L, 18 mg / L, 20 mg / L, 22 mg / L. The corresponding ultrasonic treatment conditions of the ultrasonic modified chitosan used are: ultrasonic power 700 W, ultrasonic frequency 40 kHz, chitosan dispersion concentration 0.2 mg / mL, and ultrasonic treatment duration 12 h.

[0076] After sedimentation was completed, the residual concentrations of PMMA and PS in the sample suspension were measured using an ultraviolet-visible spectrophotometer at a wavelength of 460 nm.

[0077] Figure 7 This is a comparison chart of the sedimentation efficiency of ultrasonic modified chitosan and raw chitosan on PMMA and PS at different addition amounts in Example 4 of the present invention. Figure 7 It can be seen that for PMMA, when the addition amount of ultrasonic modified chitosan is 20 mg / L, the corresponding sedimentation rate is as high as 99.86%, while when the addition amount of raw chitosan is 22 mg / L, the sedimentation rate is only 73.95%; for PS, when the addition amount of ultrasonic modified chitosan is 22 mg / L, the corresponding sedimentation rate is as high as 98.77%, while when the addition amount of raw chitosan is 22 mg / L, the sedimentation rate is only 87.82%. It can be seen that it is difficult for raw chitosan to achieve a very high microplastic sedimentation efficiency, while ultrasonic modified chitosan can achieve a removal rate of more than 98% for both PMMA and PS. These results indicate that ultrasonic treatment can enhance the flocculation sedimentation performance of chitosan on microplastics and can achieve a very high removal rate, and has a more thorough microplastic removal effect compared with many synthetic flocculants, and has excellent application prospects.

[0078] Figure 8 This is a comparison chart of the sizes of the flocs formed by ultrasonic modified chitosan and raw chitosan with PS microplastics at different addition amounts in Example 4 of the present invention. Figure 8 It can be seen that within the range of the tested chitosan addition concentrations, the highest normal distribution statistical average particle size of the flocs formed by ultrasonic modified chitosan is 0.21 mm (when the addition amount of ultrasonic modified chitosan is 20 mg / L), while the highest normal distribution statistical average particle size of the flocs formed by raw chitosan is only 0.10 mm (when the addition amount of raw chitosan is 22 mg / L). This shows that ultrasonic treatment helps chitosan and PS microplastics to form more significant flocs, which indicates that ultrasonic treatment can effectively enhance the flocculation efficiency of chitosan on PS, and at the same time has good sedimentation efficiency for both PMMA and PS, and has good application prospects.

[0079] Based on the above results, compared with the conventional chemical modification method, in the preparation method of the ultrasonic modified chitosan of the present invention, ultrasonic modified chitosan with high flocculation efficiency, high safety, and being green and environmentally friendly can be prepared through simple ultrasonic treatment. It is a new type of flocculant with excellent flocculation performance and can be widely used to treat microplastics in water bodies. Mixing the ultrasonic modified chitosan with the microplastic solution for flocculation sedimentation can achieve efficient flocculation and sedimentation of microplastics through simple oscillation and standing. It has the advantages of low flocculant dosage, good flocculation effect, simple operation, low cost, and convenience for batch treatment, and can effectively remove microplastics in water bodies, which plays an important promoting role in effectively controlling new pollutants such as microplastics. At the same time, the preparation method of the ultrasonic modified chitosan of the present invention also has the advantages of simple process, low cost, and convenient operation, which is conducive to large-scale preparation and convenient for realizing the industrial application of chitosan.

[0080] The above embodiments are only the preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, improvements and refinements made without departing from the principle of the present invention should also be regarded as within the protection scope of the present invention.

Claims

1. A preparation method of ultrasonically modified chitosan, characterized in that, It includes the following steps: S1. Mix chitosan with water to prepare a chitosan dispersion; S2. Perform ultrasonic treatment on the chitosan dispersion obtained in step S1 to obtain ultrasonically modified chitosan.

2. The preparation method according to claim 1, characterized in that, In step S1, the chitosan dispersion is prepared by the following method: Mix chitosan with water and stir to obtain a chitosan dispersion; the ratio of chitosan to water is ≤ 2.0 mg∶1 mL.

3. The preparation method according to claim 2, wherein The ratio of chitosan to water is 0.05 mg - 0.8 mg∶1 mL.

4. The preparation method according to claim 3, characterized in that, The ratio of chitosan to water is 0.1 mg - 0.5 mg∶1 mL.

5. The preparation method according to claim 2, wherein Before using the chitosan, the following treatment is also included: Pass the chitosan through a 100-mesh sieve, collect the undersize material to obtain chitosan powder; the rotation speed of the stirring is 1000 rpm - 1200 rpm; the stirring time is 0.5 h - 1 h.

6. The preparation method according to any one of claims 1 to 5, characterized in that, In step S2, the time of the ultrasonic treatment is 3 h - 15 h; the ultrasonic treatment is carried out under the condition that the temperature is 25℃ - 30℃; during the ultrasonic treatment process, control the power of the ultrasonic wave to be 500 W - 700 W and the frequency to be 30 kHz - 40 kHz; after the ultrasonic treatment is completed, the following treatment is also included: Filter the product solution after the ultrasonic treatment with a filter membrane with a pore size of 0.22 μm, and air-dry the solid product obtained after filtration under the constant temperature condition of 35 ± 1℃ to obtain ultrasonically modified chitosan.

7. An ultrasound-modified chitosan, characterized in that, The ultrasonically modified chitosan is prepared by the preparation method described in any one of claims 1 - 6.

8. Use of the ultrasonically modified chitosan as described in claim 7 in treating microplastics in water bodies.

9. The application according to claim 8, wherein It includes the following steps: Mix the ultrasonically modified chitosan and the wastewater containing microplastics, oscillate, flocculate, and stand still to complete the removal of microplastics in the water body; the addition amount of the ultrasonically modified chitosan is 10 mg - 100 mg of ultrasonically modified chitosan added per liter of the wastewater containing microplastics.

10. The application according to claim 9, wherein, The concentration of microplastics in the wastewater containing microplastics is ≤ 100 mg / L; the microplastics are at least one of polymethyl methacrylate and polystyrene; the particle size of the microplastics is 2000 mesh; the oscillation is carried out under the condition that the rotation speed is 2000 rpm; the oscillation time is 1 min - 5 min; the flocculation is carried out under the conditions that the rotation speed is 100 rpm and the temperature is 25 ± 1℃; the flocculation time is 30 min - 60 min; the standing time is 1 h.

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