Chitosan-based water-absorbing microsphere as well as preparation method and application thereof

Abstract

The invention discloses a chitosan-based water-absorbing microsphere as well as a preparation method and application thereof, and belongs to the technical field of macromolecular water-absorbing microspheres. The preparation method comprises the following steps: (1) adding an emulsifier into an organic solvent, stirring and dissolving uniformly to obtain an oil phase solution; and (2) mixing a chitosan aqueous solution, a polyglutamic acid aqueous solution and a hydroxy propyl cellulose aqueous solution, adding the mixture into the oil phase solution, uniformly stirring and mixing, then adding concentrated hydrochloric acid and a cross-linking agent to carry out polymerization reaction, and after the reaction is finished, washing, centrifugally separating and drying to obtain the chitosan-based water-absorbing microspheres. The gel water-absorbing microspheres taking chitosan, hydroxy propyl cellulose and polyglutamic acid as matrixes are prepared for the first time through an inverse emulsion polymerization method, are environment-friendly, have good water absorption in a salt solution and an acidic and alkaline solution, have certain recyclability, are beneficial to improving soil conditions and promoting crop growth, and can be used as a water-absorbing material for the first time. The maximum water holding capacity and the water retention capacity of the soil can be improved.

Description

Technical Field

[0001] This invention relates to the field of polymer water-absorbing microsphere technology, specifically to a chitosan-based water-absorbing microsphere, its preparation method, and its application. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Superabsorbent polymers (SAPs) are widely used as containers for water and plant nutrients, especially in arid and semi-arid regions, where they can significantly improve water use efficiency. Water-absorbing microspheres are a type of SAP with a three-dimensional porous structure. Due to their high swelling capacity, they can store large amounts of water within their network structure, thus attracting the attention of agronomists from their very first appearance.

[0004] Currently, existing technologies for preparing superabsorbent materials mainly suffer from the following problems: First, they are synthesized using monomers derived from petroleum products such as acrylic acid, acrylonitrile, or acrylamide. These raw materials have poor biodegradability, and their degradation products pollute the environment. Second, their swelling performance decreases significantly in salt ion solutions or acidic and alkaline solutions. Third, they have poor durability; after repeated water absorption and release, the materials are prone to structural collapse, limiting their reusability. Therefore, developing environmentally friendly, salt- and acid- and alkali-resistant, and recyclable superabsorbent materials is particularly important. Summary of the Invention

[0005] In view of this, the present invention provides chitosan-based water-absorbing microspheres, their preparation method, and applications. To address the problems of existing highly absorbent materials polluting the environment, being susceptible to acid and alkali erosion, exhibiting poor swelling performance in salt ion solutions, and being non-recyclable, the present invention uses natural polymers such as chitosan, polyglutamic acid, and hydroxypropyl cellulose to prepare water-absorbing microspheres. These microspheres are environmentally friendly, exhibit good water absorption in salt, acidic, and alkaline solutions, and possess a certain degree of recyclability. The preparation method of the present invention is simple, the raw materials are inexpensive and readily available, the reaction conditions are easy to achieve, and the prepared water-absorbing microspheres exhibit good water absorption and retention properties, which helps improve soil conditions and promote crop growth.

[0006] In a first aspect, the present invention provides a method for preparing chitosan-based water-absorbing microspheres, comprising the following steps: Step (1): Add the emulsifier to the organic solvent and stir to dissolve it evenly to form an oil phase solution; Step (2): Chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution are mixed and added to the oil phase solution. After stirring and mixing evenly, concentrated hydrochloric acid and crosslinking agent are added to carry out polymerization reaction. After the reaction is completed, the mixture is washed, centrifuged and dried to obtain chitosan-based water-absorbing microspheres.

[0007] Preferably, in step (1), the organic solvent is selected from one or more of liquid paraffin, petroleum ether, n-hexane or toluene, and the organic solvent is preferably liquid paraffin; The emulsifier is selected from one or more of sorbitan oleate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide or polyoxyethylene 20 sorbitan monooleate, and the emulsifier is preferably sorbitan oleate; The concentration of the emulsifier in the organic solvent is 0.5-2 g / L.

[0008] Preferably, in step (1), the stirring speed is 500-800 rpm.

[0009] Preferably, in step (2), the concentration of the chitosan aqueous solution is 0.4~0.6 mol / L; the concentration of the polyglutamic acid aqueous solution is 0.1-0.2 mol / L; and the concentration of the hydroxypropyl cellulose aqueous solution is 0.2-0.3 mol / L.

[0010] Preferably, the volume ratio of the mixed aqueous phase in step (2), the organic solvent, concentrated hydrochloric acid, and crosslinking agent in step (1) is 8~14:20~60:0.09~0.12:0.15, and more preferably 10:20~60:0.09~0.12:0.15; In the mixed aqueous phase, the volume ratio of chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution is 6:1~5:1~3, preferably 6:1:3.

[0011] Preferably, in step (2), the crosslinking agent is selected from one or more of glutaraldehyde, genipin, or sodium tripolyphosphate, and the crosslinking agent is preferably glutaraldehyde; The concentration of the crosslinking agent solution is 0.02-0.06 g / mL; The concentration of the concentrated hydrochloric acid is 36-38 wt%.

[0012] Preferably, in step (2), the stirring time is 30-60 min; The polymerization reaction conditions are as follows: under inert gas protection, the reaction is carried out at 30~60℃ with stirring for 0.5~4 h; the inert gas is nitrogen.

[0013] Preferably, in step (2), the washing is performed using anhydrous ethanol or isopropanol, preferably anhydrous ethanol, and the number of washing cycles is 2 to 5. The centrifugation speed is 5000~8000 rpm, and the centrifugation time is 5~10 min; The drying process is vacuum drying, and the temperature of the vacuum drying process is 40~60℃.

[0014] Secondly, the present invention provides chitosan-based water-absorbing microspheres prepared by the above preparation method, wherein the particle size of the water-absorbing microspheres is 5-15 μm.

[0015] Thirdly, the present invention provides the application of the above-mentioned chitosan-based water-absorbing microspheres in improving the soil environment.

[0016] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention synthesizes gel microspheres with excellent water retention and water absorption by reverse emulsion polymerization. For the first time, gel water-absorbing microspheres with chitosan, hydroxypropyl cellulose and polyglutamic acid as matrix are prepared. They are environmentally friendly, have good water absorption in salt solution, acid and alkaline solution and have certain recyclability.

[0017] Using chitosan (CS), polyglutamic acid (PGA), and hydroxypropyl cellulose (HPC) as core raw materials, all of which are biodegradable—PGA can be gradually degraded into glutamic acid that can be absorbed and utilized by plants, while CS and HPC can be decomposed into small molecule carbohydrates under the action of soil microorganisms, with no risk of residual pollution; at the same time, the raw materials themselves are non-toxic, avoiding the potential harm to soil and crops from the degradation products of petroleum-based monomers (such as acrylic acid), making them suitable for green agriculture scenarios.

[0018] It exhibits stable salt and acid / alkali resistance and is suitable for a wide range of applications. The three-dimensional network structure formed by the cross-linking of the three raw materials combines rigidity (linear aminopolysaccharide backbone of CS) with flexibility (amide bond segments of PGA and hydroxypropyl side chains of HPC), which can resist the charge shielding effect of salt ions and the protonation / deprotonation impact of functional groups under acidic and alkaline environments. Experiments have verified that the water-absorbing microspheres still maintain effective swelling capacity in acidic and alkaline solutions with pH 3.0-11.0 and salt solutions such as NaCl and CaCl2 with concentrations of 0.01-0.1M. It can be directly applied to complex soil environments such as acidic red soil and saline-alkali land, breaking through the limitations of traditional water-absorbing materials on media conditions.

[0019] It exhibits outstanding recyclability and durability. The density of the cross-linked network and the synergistic effect between raw materials reduce the risk of structural collapse during repeated water absorption and release processes. After four cycles of swelling, the water absorption rate decreases by only 22%-36%, which is far superior to the cyclic performance of traditional SAPs. It can stably perform water and fertilizer retention functions for a long time, reducing the cost of agricultural applications.

[0020] (2) In the preparation method of the water-absorbing microspheres of the present invention, the type and ratio of the reaction monomers, the polymerization reaction conditions and the ratio of glutaraldehyde to polymer all affect the performance of the obtained water-absorbing microspheres. If the above conditions are not suitable, the water-absorbing microspheres with the performance of the present invention will not be obtained.

[0021] (3) The method for preparing water-absorbing microspheres in this invention is simple, the raw materials are cheap and readily available, the reaction conditions are easy to achieve, and it is convenient for industrial-scale production.

[0022] (4) The water-absorbing microspheres of the present invention are used to improve the soil environment. That is, after adding the material, the maximum water holding capacity and water retention capacity of the soil can be improved, water evaporation can be reduced, and water conservation can be achieved. Moreover, the swelling performance of the material is stable in acidic soil environment and saline-alkali soil environment, which can improve the condition of such harmful soil and promote crop growth. Attached Figure Description

[0023] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0024] Figure 1 Figure 1 shows the SEM images of the water-absorbing microspheres prepared in Example 1; wherein, Figure a is the SEM image with a scale bar of 5 μm, Figure b is the SEM image with a scale bar of 5 μm, Figure c is the SEM image with a scale bar of 2 μm, and Figure d is the SEM image with a scale bar of 2 μm. Figure 2 FT-IR image of the water-absorbing microspheres prepared in Example 1; Figure 3 Figure 1 shows the swelling properties of the water-absorbing microspheres prepared in Examples 1-3 and Comparative Example 1 in deionized water; Figure 2a shows the swelling of CH, CPH-1, CPH-2, and CPH-3 in deionized water at pH 7.4, and Figure 3b shows the swelling of CPH-3 in solutions at pH 3, 5, 7, 9, and 11. Figure 4 Figure 1 shows the swelling performance of the water-absorbing microspheres prepared in Example 1 in salt solutions; Figure 2a shows the swelling of CPH-3 in NaCl, CaCl2, and FeCl3 solutions of different concentrations, and Figure 3b shows the swelling of CPH-3 in 0.01M NaCl, CaCl2, FeCl3 solutions and deionized water. Figure 5 The diagram shows the recyclability of the water-absorbing microspheres prepared in Examples 1-3; Figure 6 The graphs show the performance of the water-absorbing microspheres prepared in Examples 1-6 and Comparative Examples 1-9 in improving soil water retention capacity. Figure 7 The graph shows the water retention performance of the water-absorbing microspheres prepared in Examples 1-3 at 45°C. Detailed Implementation

[0025] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0026] This invention provides a method for preparing chitosan-based water-absorbing microspheres, comprising the following steps: Step (1): Add the emulsifier to the organic solvent and stir to dissolve it evenly to form an oil phase solution; Step (2): Chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution are mixed and added to the oil phase solution. After stirring and mixing evenly, concentrated hydrochloric acid and crosslinking agent are added to carry out polymerization reaction. After the reaction is completed, the mixture is washed, centrifuged and dried to obtain chitosan-based water-absorbing microspheres.

[0027] This invention utilizes three environmentally friendly natural polymers as raw materials. Chitosan (CS) is a linear aminopolysaccharide with a rigid structure, exhibiting hydrophilicity and crystallinity. Due to its numerous reactive groups, it can form complexes with other compounds, helping to improve the soil environment, promote plant nutrient absorption, and enhance plant stress resistance. Polyglutamic acid (PGA) is polymerized from D-glutamic acid and L-glutamic acid via amide bonds. It contains highly hydrophilic groups (-COOH), possessing excellent soil-amortizing properties. It effectively improves soil structure, increases soil aggregates, and enhances soil water and fertilizer retention capacity. Furthermore, it can gradually degrade into glutamic acid molecules in the soil for absorption by plants, providing continuous nutrient support. Hydroxypropyl cellulose (HPC) also significantly improves soil structure, enhances soil permeability and water retention, and provides a superior growth environment for plant roots. Secondly, it promotes root growth and distribution, improving the efficiency of nutrient absorption by plants, thereby effectively increasing crop yield. In addition, hydroxypropyl cellulose can also be used as a plant protectant to enhance the plant's resistance to stress and drought, allowing the plant to thrive even in harsh environments.

[0028] Furthermore, in the water-absorbing microspheres, chitosan (CS) serves as a rigid framework and cationic polyelectrolyte. The numerous amino groups (-NH2) on its molecular chain are the main cross-linking sites with glutaraldehyde, forming the chemical basis of the network. Its rigid structure helps maintain the basic morphology of the microspheres and prevents dissolution under excessive swelling. Polyglutamic acid (PGA) serves as a highly hydrophilic component and anionic polyelectrolyte. The dense carboxyl groups (-COOH) on its molecular chain provide extremely strong hydration capacity, being the main source of ultra-high water absorption. In addition, the carboxyl groups can form polyplexes with the amino groups of chitosan, which are physical cross-linking points that synergistically enhance the strength and stability of the network with the chemical cross-linking network. Under different pH conditions, changes in the degree of ionization of PGA can regulate the osmotic pressure inside and outside the network, contributing to the material's pH responsiveness and salt resistance. Hydroxypropyl cellulose (HPC) serves as a flexibility regulator and pelletizing aid. Its long-chain molecules can interweave within the CS / PGA network, increasing the network's flexibility and elasticity and preventing material brittleness. Meanwhile, HPC has good surface activity and film-forming properties, which helps stabilize aqueous droplets in reverse emulsion systems, making the formed microspheres more regular and with a more uniform particle size distribution.

[0029] Furthermore, in the water-absorbing microspheres, CS provides structural strength and cross-linking points, PGA provides water absorption power and functional groups, and HPC improves processability and mechanical properties. These three components form a rigid-flexible, well-structured interpenetrating network (IPN) through covalent cross-linking and physical interactions (hydrogen bonds, electrostatic interactions). This structure allows water molecules to quickly enter and be stored within the network, while the network itself is strong enough to resist the destructive forces of water, thus achieving a balance of high water absorption, high water retention, salt and alkali resistance, mechanical stability, and durability.

[0030] This invention synthesizes gel microspheres with excellent water retention and water absorption properties through reverse emulsion polymerization. It is the first time that gel microspheres with chitosan, hydroxypropyl cellulose and polyglutamic acid as the matrix have been prepared. The preparation method is simple, the raw materials are inexpensive and readily available, and the reaction conditions are easy to achieve.

[0031] In this invention, in step (1), the organic solvent is selected from one or more of liquid paraffin, petroleum ether, n-hexane or toluene, and the organic solvent is preferably liquid paraffin; The emulsifier is selected from one or more of sorbitan oleate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide or polyoxyethylene 20 sorbitan monooleate, and the emulsifier is preferably sorbitan oleate; The concentration of the emulsifier in the organic solvent is 0.5-2 g / L.

[0032] In this invention, the stirring speed in step (1) is 500-800 rpm. The stirring speed directly affects the dispersion of the oil phase emulsion. 500-800 rpm allows the emulsifier to be uniformly dispersed in the organic solvent, forming a stable template of tiny droplets. If the speed is too low (<500 rpm), the droplet size is uneven and it is easy to settle. If the speed is too high (>800 rpm), the energy consumption increases, and the emulsion balance may be disrupted due to excessive shear force, leading to emulsion demulsification. This speed range ensures the stability of the oil phase, providing a prerequisite for the uniform dispersion of the subsequent aqueous phase.

[0033] In this invention, in step (2), the concentration of the chitosan aqueous solution is 0.4~0.6 mol / L; the concentration of the polyglutamic acid aqueous solution is 0.1-0.2 mol / L; and the concentration of the hydroxypropyl cellulose aqueous solution is 0.2-0.3 mol / L. If the solution concentration is too high, it will cause difficulty in droplet formation, and the diffusion of the crosslinking agent will be restricted, resulting in excessive crosslinking on the surface of the microspheres and insufficient crosslinking inside, leading to unstable structure.

[0034] In this invention, the volume ratio of the mixed aqueous phase in step (2), the organic solvent, concentrated hydrochloric acid, and crosslinking agent in step (1) is 8~14:20~60:0.09~0.12:0.15, preferably 10:20~60:0.09~0.12:0.15; In the mixed aqueous phase, the volume ratio of chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution is 6:1~5:1~3, preferably 6:1:3, at which ratio the microspheres have the best swelling properties.

[0035] The volume ratio of concentrated hydrochloric acid to crosslinking agent is 0.6~0.8:1. Concentrated hydrochloric acid provides an acidic environment, protonating the amino groups of chitosan, thus promoting its dissolution and catalyzing the crosslinking reaction between glutaraldehyde and the amino groups to form a Schiff base. The amount of concentrated hydrochloric acid added affects the crosslinking effect of glutaraldehyde. Insufficient concentrated hydrochloric acid results in a slow and incomplete crosslinking reaction; a slight excess is necessary to ensure the crosslinking reaction proceeds fully.

[0036] In the polymerization reaction, the volume ratio of the aqueous phase to the oil phase affects the performance of the resulting water-absorbing microspheres. If the proportion of the aqueous phase is too high, the emulsion will be unstable, the microsphere size will be uncontrollable, and the morphology will be irregular. If the proportion is too low, the production efficiency will be low. When the volume ratio of the aqueous phase to the oil phase is 10:20~60, this ratio balances stability and efficiency.

[0037] In this invention, in step (2), the crosslinking agent is selected from one or more of glutaraldehyde, genipin or sodium tripolyphosphate, and the crosslinking agent is preferably glutaraldehyde; The concentration of the crosslinking agent solution is 0.02-0.06 g / mL; The concentration of the concentrated hydrochloric acid is 36-38 wt%.

[0038] In this invention, in step (2), the stirring time is 30-60 min; to ensure that the three polymers are uniformly dispersed in the aqueous phase and to avoid uneven cross-linking caused by uneven local concentration; The polymerization reaction conditions are as follows: under inert gas protection, the reaction is carried out at 30~60℃ with stirring for 0.5~4 h; the inert gas is nitrogen. The inert gas prevents the polymer from being oxidized during polymerization, especially the amino groups of chitosan, which are easily oxidized and degraded, thus ensuring stable product performance. If the temperature is too low, the crosslinking reaction rate is too slow and inefficient; if the temperature is too high, such as well as well above 60℃, it may lead to uneven crosslinking, more by-products, decreased chitosan degradation strength, and microsphere adhesion, preventing the formation of regular microspheres.

[0039] In this invention, in step (2), the washing is performed using anhydrous ethanol or isopropanol, preferably anhydrous ethanol, and the number of washing cycles is 2 to 5. Anhydrous ethanol can remove residual organic solvents, emulsifiers and unreacted small molecules at the same time. 2 to 5 washing cycles can balance purity and efficiency. Too few washing cycles will result in more residues, while too many washing cycles will lead to swelling and loss of microspheres. The centrifugation speed is 5000~8000 rpm, and the centrifugation time is 5~10 min; The drying process is vacuum drying, and the temperature of the vacuum drying process is 40~60℃.

[0040] The present invention also provides chitosan-based water-absorbing microspheres prepared by the above preparation method, wherein the particle size of the water-absorbing microspheres is 5-15 μm.

[0041] This invention provides the application of the above-mentioned chitosan-based water-absorbing microspheres in improving the soil environment.

[0042] The chitosan-based water-absorbing microspheres of the present invention can enhance the maximum water holding capacity and water retention capacity of soil during application, reduce water evaporation, and achieve the purpose of water conservation. Moreover, the swelling performance of this material is stable in acidic soil environment and saline-alkali soil environment, which can improve such harmful soil and promote crop growth.

[0043] The technical solution of the present invention will be further described below with reference to specific embodiments. In the present invention, unless otherwise specified, other test materials and instruments are conventional test materials in the art and can be purchased through commercial channels.

[0044] Example 1 This embodiment provides a method for preparing chitosan-based water-absorbing microspheres.

[0045] (1) Measure 30 mL of liquid paraffin into a three-necked flask, pass nitrogen gas through it to remove oxygen interference, then weigh 0.6 g of sorbitan oleate and add it dropwise into the liquid paraffin under mechanical stirring at 500 rpm. Maintain for 30 min to ensure that the liquid paraffin is completely emulsified into uniform small droplets.

[0046] (2) Keep nitrogen gas flowing in, mix 6 mL of chitosan aqueous solution (0.5 mol / L), 1 mL of polyglutamic acid aqueous solution (0.1 mol / L) and 3 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L), and then add them dropwise to the emulsion solution. Stir mechanically at 40°C for half an hour to obtain a uniformly mixed water-in-oil system. Then add 0.15 mL of glutaraldehyde solution (0.05 g / mL) and 0.12 mL of concentrated hydrochloric acid (37 wt%) dropwise. Continue to stir mechanically at 40°C for 2 hours. After the reaction is complete, wash with anhydrous ethanol and centrifuge at 5000 rpm for 5 min. After centrifugation, take the precipitate and wash again. Repeat 3 times. Place the product in a vacuum drying oven at 450°C to dry and obtain microspheres, which are denoted as CPH-3.

[0047] The SEM image of the water-absorbing microspheres obtained in this embodiment is as follows: Figure 1 As shown, the microspheres have a particle size of approximately 5-15 μm and exhibit a uniformly dispersed structure. The FT-IR image of the water-absorbing microspheres is shown below. Figure 2 As shown, in the spectrum of CPH microspheres, 1712 cm⁻¹ -1 and 1640cm -1 The peaks at 1066 cm⁻¹ are respectively assigned to the stretching vibrations of the carbonyl (C=O) and imine (C=N) bonds in the CPH microspheres crosslinked with glutaraldehyde. -1 The peak at [location] is attributed to the presence of an acetal group formed by the reaction of the hydroxyl group of hydroxypropyl cellulose and the aldehyde group of glutaraldehyde; the imine and acetal bonds confirm the cross-linking reaction. Furthermore, from [the following text appears to be incomplete and requires further context: "from [location]..."] Figure 2 As can be seen from this, polyglutamic acid appears in CPH at 3234 cm⁻¹. -1 and 3576cm -1 The stretching vibration peaks of -NH and -NH2 further indicate the successful synthesis of chitosan / polyglutamic acid / hydroxypropyl cellulose (CPH) gel water-absorbing microspheres.

[0048] Example 2 The difference between this embodiment and Example 1 is that in this embodiment, 6 mL of chitosan aqueous solution (0.5 mol / L), 2 mL of polyglutamic acid aqueous solution (0.1 mol / L) and 2 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L) are mixed, while the other steps and conditions are the same as in Example 1. This embodiment is denoted as CPH-2.

[0049] Example 3 The difference between this embodiment and Example 1 is that in this embodiment, 6 mL of chitosan aqueous solution (0.5 mol / L), 3 mL of polyglutamic acid aqueous solution (0.1 mol / L) and 1 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L) are mixed, and the other steps and conditions are the same as in Example 1. This embodiment is denoted as CPH-1.

[0050] Example 4 The difference between this embodiment and Example 1 is that in this embodiment, 6 mL of chitosan aqueous solution (0.5 mol / L), 5 mL of polyglutamic acid aqueous solution (0.1 mol / L) and 3 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L) are mixed, while the other steps and conditions are the same as in Example 1. This embodiment is denoted as CPH-4.

[0051] Example 5 The difference between this embodiment and Example 1 is that in this embodiment, 6 mL of chitosan aqueous solution (0.5 mol / L), 1 mL of polyglutamic acid aqueous solution (0.1 mol / L) and 1 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L) are mixed, while the other steps and conditions are the same as in Example 1, and this is referred to as CPH-5.

[0052] Example 6 The difference between this embodiment and Example 1 is that 0.15 mL of glutaraldehyde solution (0.05 g / mL) and 0.09 mL of concentrated hydrochloric acid (37 wt%) are added dropwise. The other steps and conditions are the same as in Example 1, and this embodiment is referred to as CPH-6.

[0053] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not contain an aqueous solution of polyglutamic acid, denoted as CH.

[0054] Comparative Example 2 The difference between this comparative example and Example 1 is that no chitosan aqueous solution was added in this comparative example, denoted as pH.

[0055] Comparative Example 3 The difference between this comparative example and Example 1 is that this comparative example does not contain an aqueous solution of hydroxypropyl cellulose, denoted as CP.

[0056] Comparative Example 4 The difference between this comparative example and Example 1 is that this comparative example mixes 6 mL of chitosan aqueous solution (0.5 mol / L), 7 mL of polyglutamic acid aqueous solution (0.1 mol / L), and 5 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L), and is denoted as CPH-7.

[0057] Comparative Example 5 The difference between this comparative example and Example 1 is that this comparative example mixes 1 mL of chitosan aqueous solution (0.5 mol / L), 1 mL of polyglutamic acid aqueous solution (0.1 mol / L), and 1 mL of hydroxypropyl cellulose aqueous solution (0.2 mol / L), and is denoted as CPH-8.

[0058] Comparative Example 6 The difference between this comparative example and Example 1 is that 0.2 ml of glutaraldehyde solution (0.05 g / mL) and 0.06 ml of concentrated hydrochloric acid (37 wt%) were added dropwise in this comparative example. The other steps and conditions were the same as in Example 1, and it is referred to as CPH-9.

[0059] Comparative Example 7 The difference between this comparative example and Example 1 is that 0.2 ml of glutaraldehyde solution (0.05 g / mL) and 0.2 ml of concentrated hydrochloric acid (37 wt%) were added dropwise in this comparative example. The other steps and conditions were the same as in Example 1, and it is referred to as CPH-10.

[0060] Comparative Example 8 The difference between this comparative example and Example 1 is that the polymerization reaction temperature in this comparative example is 20°C, while the other steps and conditions are the same as in Example 1, and it is denoted as CPH-11.

[0061] Comparative Example 9 The difference between this comparative example and Example 1 is that the polymerization reaction temperature in this comparative example is 80°C, while the other steps and conditions are the same as in Example 1, and it is denoted as CPH-12.

[0062] Test case 1. Test the swelling properties of the water-absorbing microspheres.

[0063] The swelling behavior of Examples 1-6 and Comparative Examples 1-9 was measured by gravimetric method. 0.02 g of absorbent microspheres were weighed and placed in a 2 mL centrifuge tube, and an appropriate amount of deionized water was added to allow the microspheres to fully absorb the water. After half an hour, the supernatant was centrifuged, poured out, and weighed. Measurements were repeated for three hours until swelling equilibrium was reached. The swelling rate Q (g / g) was calculated using equation (1):

[0064] Where M i (g) is the weight after each centrifugation, M1(g) is the initial weight of the centrifuge tube, and M0(g) is the initial weight of the dried CPH.

[0065] Table 1 Swelling Rate

[0066] As shown in Table 1, the swelling rates of Examples 1-6 were significantly higher than those of all comparative examples.

[0067] As can be seen from Comparative Examples 1-3, the swelling rate decreased significantly when no polyglutamic acid, chitosan, or hydroxypropyl cellulose was added, far lower than in Example 1. This fully demonstrates the synergistic effect of the three natural polymer raw materials: chitosan, polyglutamic acid, and hydroxypropyl cellulose.

[0068] As can be seen from Examples 1-5 and Comparative Examples 4-5, the mixing ratio of the three raw material aqueous solutions has a significant impact on the swelling performance. In Example 1, the swelling performance was optimal when chitosan, polyglutamic acid, and hydroxypropyl cellulose were mixed in a ratio of 6:1:3, indicating that a reasonable ratio can form a stable and high-performance interpenetrating network structure, thereby achieving high water absorption.

[0069] As can be seen from Example 1 and Comparative Examples 6-9, reaction conditions such as the amount of concentrated hydrochloric acid and the polymerization temperature have a significant impact on the swelling performance of the water-absorbing microspheres. A suitable amount of concentrated hydrochloric acid can provide a good acidic environment, promoting the full progress of the cross-linking reaction; a suitable polymerization temperature can ensure the stability of the reaction rate and the product structure.

[0070] like Figure 3 As shown in Figure a, the swelling rate of CPH in deionized water increases rapidly in the first half hour, then increases slowly and tends to plateau. The maximum swelling rates of CPH-3, CPH-2, and CPH-1 in deionized water are 3879.7%, 3677.8%, and 3458.3%, respectively, while the maximum swelling rate of chitosan / hydroxypropyl cellulose (CH) microspheres is 1062.9%. This phenomenon can be attributed to the hydrophilic groups on the gel microspheres promoting the wettability of the microspheres and accelerating water diffusion. The introduction of poly(γ-glutamic acid) increases the hydrophilic group -COOH, which significantly increases the swelling rate of the microspheres. In addition, the network ionization inside the gel microspheres leads to the difference in osmotic pressure between the internal environment and the solution medium, ultimately causing water molecules to continue to diffuse until swelling equilibrium is reached.

[0071] The swelling capacity of CPH is related to the pH value of the swelling medium. To evaluate the swelling behavior of CPH at different pH values, the swelling capacity of the CPH in Example 1 was tested at pH values ​​of 3.0, 5.0, 7.0, 9.0, and 11.0. The results are as follows: Figure 3 As shown in b, when the pH value is <7.0, the swelling volume of CPH tends to decrease. This is because the carboxyl groups of PGA are protonated at pH <7.0, reducing the repulsion between anions and enhancing the hydrogen bonding interaction between carboxylic acid groups, thus reducing water absorption. However, overall, the swelling performance of this material in deionized water at different pH values ​​does not differ significantly, exhibiting pH stability. It can be applied to improve the soil environment and promote plant growth in acidic and saline-alkali soils.

[0072] 2. Test the swelling properties of water-absorbing microspheres in different salt solutions.

[0073] The swelling properties of Example 1 were tested in different salt solutions. The type and concentration of salt ions in the swelling medium play a crucial role in affecting the salt tolerance of the CPH synthesized in this invention. Figure 4 As shown in figure a, the swelling behavior of CPH varies in solutions with different concentrations of salt ions. Figure 4 Table b shows the changes in the swelling behavior of CPH in 0.01M NaCl, CaCl2, and FeCl3 solutions. It can be observed that the swelling capacity of CPH is NaCl > CaCl2 > FeCl3, with a maximum swelling rate of 36.71 g / g in NaCl solution. As the salt concentration increases from 0.01M to 0.1M, the swelling rate decreases significantly. This phenomenon may be attributed to the charge shielding effect between the cations and -COOH groups in the high-concentration salt solution of CPH, which leads to a decrease in the osmotic pressure difference between the internal and external environments, thus limiting the swelling capacity of the microspheres. Furthermore, ions entering CPH exchange ions with the surface -COOH groups. This increases the cross-linking degree of the polymer network, thereby reducing the amount of H2O entering, leading to a decrease in the swelling capacity of CPH. Due to the different degrees of influence on the polymer charge density, higher cation valence states have an adverse effect on the swelling capacity of CPH. In addition, in multivalent cation solutions, the cation radius is smaller (CaCl2 > FeCl3). 2+ >Fe 3+ (This is even better.) From the above results, it can be concluded that the microspheres still have a certain swelling capacity in typical soil salt ion solutions, indicating that the material has a certain degree of salt resistance.

[0074] 3. Test the recyclability of the water-absorbing microspheres.

[0075] The recyclability of CPH was tested in Examples 1-3. The repeated swelling characteristics of CPH in water were evaluated for better application in agriculture. Certain amounts of CPH-1, CPH-2, and CPH-3 were immersed in water until swelling equilibrium was reached, then weighed, dried in a 50°C vacuum drying oven, and the swelling rate was repeated following the steps in Example 1, with this process repeated four times. The experimental results after four repetitions are as follows. Figure 5As shown, it can be clearly seen that the swelling capacity of CPH decreases with the increase of the number of times it swells in water. The reasons for the above results are as follows: (1) Due to the breakage of physical cross-linking points in the gel network during the swelling process, the water absorption capacity of the hydrogel is slightly weakened, which in turn causes some damage to the polymer network. (2) After the gel network absorbs water, the polymer network relaxes. When water is absorbed again, the water absorption capacity decreases due to the relaxation of the polymer network. Nevertheless, after 4 cycles, the water absorption rates of CPH-1, CPH-2, and CPH-3 decreased by 35.73%, 31.43%, and 22.33% respectively compared with the first cycle, showing good recyclability. In conclusion, CPH, as a reusable and environmentally friendly material, plays an important role in agriculture.

[0076] 4. Test the effect of water-absorbing microspheres on soil water retention capacity.

[0077] The soil water-holding capacity of Examples 1-6 and Comparative Examples 1-10 was tested. 100g of dried soil sieved through a 60-mesh sieve was weighed into a PVC transparent tube with a diameter of 4cm and a height of 15cm. The bottom of the PVC tube was sealed with a microporous filter membrane. A sample (1wt%) was placed 1cm below the soil surface, and the mixture of soil and sample was weighed (M1). Deionized water was then slowly added to the tube until the soil was saturated, and the sample was weighed again (M2). The experiment included experimental groups using various materials (Examples 1-7 and Comparative Examples 1-10) and a control group without added materials. The water-holding capacity (WHR%) of the materials was calculated using equation (2).

[0078]

[0079] Where M1 is the weight of the mixture of dry soil and sample, and M2 is the weight of the mixture of saturated soil and sample.

[0080] Soil water-holding capacity is an important indicator affecting soil quality, plant growth, and seedling survival rate. A water-holding experiment was designed to study the effect of CPH on soil water-holding capacity.

[0081] Table 2 Water Holding Capacity

[0082] From Table 2 and Figure 6 It can be seen that the water holding capacity of all embodiments is much higher than that of the control group without added materials, indicating that the addition of CPH significantly improves the water holding capacity of the soil. The water holding capacity of the soil with 1wt% CPH-3 added is about 30.26% higher than that of the blank group, which has a significant effect on improving the soil environment.

[0083] 5. Test the effect of water-absorbing microspheres on soil water retention capacity.

[0084] The soil water retention capacity of Examples 1-3 was tested. The procedure for Example 4 was followed first, and then the weighed PVC pipe (M2) from Example 4 was incubated at 45 °C for 22 days. Weighing was performed at specified time intervals (1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20, and 22 days), and the weight was recorded as M. x The water retention capacity (WR%) of the material is calculated using equation (3). Where M1 represents the weight of the entire PVC pipe before water absorption.

[0085]

[0086] Where M1 represents the weight of the entire PVC pipe before water absorption, and M2 is the weight of the PVC pipe after saturation weighing in Test Example 4. x The weight of the PVC pipe at specified time intervals (1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20 and 22 days).

[0087] Figure 7 The study demonstrated the water retention capacity of the soil. Results showed that soil without CPH lost all its moisture by day 18, while soil treated with CPH-3 maintained a water retention rate of over 24.57%, indicating that CPH significantly reduced the rate of soil moisture evaporation. This effect stems from the viscosity and extensibility of the CPH material, which enhances the adhesion of soil aggregates, thereby reducing the soil surface area and improving its water retention. By day 22, the moisture content of soil treated with CPH remained between 6% and 20%, demonstrating the significant application value and water-saving potential of CPH material in soil moisture retention and management.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing chitosan-based water-absorbing microspheres, characterized in that, Includes the following steps: Step (1): Add the emulsifier to the organic solvent and stir to dissolve it evenly to form an oil phase solution; Step (2): Chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution are mixed and added to the oil phase solution. After stirring and mixing evenly, concentrated hydrochloric acid and crosslinking agent are added to carry out polymerization reaction. After the reaction is completed, the mixture is washed, centrifuged and dried to obtain chitosan-based water-absorbing microspheres.

2. The preparation method according to claim 1, characterized in that, In step (1), the organic solvent is selected from one or more of liquid paraffin, petroleum ether, n-hexane or toluene, and the organic solvent is preferably liquid paraffin; The emulsifier is selected from one or more of sorbitan oleate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide or polyoxyethylene 20 sorbitan monooleate, and the emulsifier is preferably sorbitan oleate; The concentration of the emulsifier in the organic solvent is 0.5-2 g / L.

3. The preparation method according to claim 1, characterized in that, In step (1), the stirring speed is 500-800 rpm.

4. The preparation method according to claim 1, characterized in that, In step (2), the concentration of the chitosan aqueous solution is 0.4~0.6 mol / L; the concentration of the polyglutamic acid aqueous solution is 0.1-0.2 mol / L; and the concentration of the hydroxypropyl cellulose aqueous solution is 0.2-0.3 mol / L.

5. The preparation method according to claim 1, characterized in that, The volume ratio of the mixed aqueous phase in step (2), the organic solvent in step (1), the concentrated hydrochloric acid, and the crosslinking agent is 8~14:20~60:0.09~0.12:0.15, preferably 10:20~60:0.09~0.12:0.15; In the mixed aqueous phase, the volume ratio of chitosan aqueous solution, polyglutamic acid aqueous solution and hydroxypropyl cellulose aqueous solution is 6:1~5:1~3, preferably 6:1:

3.

6. The preparation method according to claim 1, characterized in that, In step (2), the crosslinking agent is selected from one or more of glutaraldehyde, genipin or sodium tripolyphosphate, and the crosslinking agent is preferably glutaraldehyde; The concentration of the crosslinking agent solution is 0.02-0.06 g / mL; The concentration of the concentrated hydrochloric acid is 36-38 wt%.

7. The preparation method according to claim 1, characterized in that, In step (2), the stirring time is 30-60 min; The polymerization reaction conditions are as follows: under inert gas protection, the reaction is carried out at 30~60℃ with stirring for 0.5~4 h; the inert gas is nitrogen.

8. The preparation method according to claim 1, characterized in that, In step (2), the washing is performed using anhydrous ethanol or isopropanol, preferably anhydrous ethanol, and the number of washing cycles is 2 to 5. The centrifugation speed is 5000~8000 rpm, and the centrifugation time is 5~10 min; The drying process is vacuum drying, and the temperature of the vacuum drying process is 40~60℃.

9. The chitosan-based water-absorbing microspheres prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The water-absorbing microspheres have a particle size of 5-15 μm.

10. The application of chitosan-based water-absorbing microspheres as described in claim 9 in improving the soil environment.

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