Preparation method and application of NPK chitin gel beads

Abstract

The application discloses a preparation method and application of NPK chitin gel beads. The method utilizes a large amount of potential chemical fertilizer elements in a chitin KOH / urea solvent system, and the elements are dropped into a coagulation bath containing phosphoric acid by using a dripping method. Through pH-induced gelation and in-situ generation of nutrients, the formation of the gel beads and the loading of the nutrients are realized simultaneously. After washing and drying, the NPK-loaded chitin gel beads are obtained. Compared with existing hydrogel fertilizer preparation methods, the method does not need an additional nutrient adsorption and loading step, and also avoids the problem of removing impurities after gel formation. The gel beads obtained by the method are uniform spherical, have a surface dense and internal porous core-shell structure, and the total nitrogen, potassium oxide and diaphosphorus pentoxide contents are 3.66%-6.53%, 7.14%-16.94% and 10.43%-25.51% respectively. The chitin gel beads obtained by the method have good mechanical stability, water retention, nutrient slow-release, biological safety and soil degradation performance, and can be used as slow-release fertilizers and soil improvers.

Description

Technical Field

[0001] This invention relates to the field of natural polymer hydrogel materials and water-retaining slow-release fertilizers, specifically to a method for preparing NPK-loaded chitosan gel beads using a chitosan KOH / urea aqueous solvent system through drop-injection, in-situ phosphate neutralization reaction, and pH-induced gelation process, as well as the application of the NPK-loaded chitosan gel beads in the agricultural field. Background Technology

[0002] In agricultural production, water use efficiency and fertilizer use efficiency directly affect crop growth, soil environment, and agricultural input costs. Water-soluble fertilizers such as urea and potassium dihydrogen phosphate typically dissolve rapidly and enter the soil environment during conventional application, with nutrients easily lost through leaching, deep seepage, decomposition, or volatilization. These losses not only reduce the actual nutrient absorption and utilization rate by crops but can also lead to environmental problems such as fertilizer waste, eutrophication of water bodies, groundwater pollution, and nitrogen emissions under conditions of excessive fertilization. Therefore, developing functional materials that can simultaneously achieve water retention and slow fertilizer release is an important technological direction for improving agricultural water and fertilizer use efficiency.

[0003] Hydrogels are a class of polymeric materials with a three-dimensional network structure. This network structure can absorb and retain large amounts of water, while also creating a diffusion barrier for fertilizer molecules or nutrient ions. When applied in agriculture, hydrogels can absorb and store water when soil moisture content is high and gradually release water when soil moisture content decreases, thus regulating soil moisture. Simultaneously, the hydrogel network can slow down the diffusion rate of nutrients after fertilizer dissolution, reducing the problem of rapid release and loss of quickly dissolved fertilizers.

[0004] Existing methods for preparing hydrogel slow-release fertilizers mainly fall into two categories: One involves first preparing a blank hydrogel carrier, then placing the blank hydrogel carrier in a fertilizer solution for swelling and adsorption, allowing the fertilizer to enter the gel. This method requires gel formation followed by fertilizer adsorption, involving numerous steps; furthermore, the fertilizer adsorption process typically relies on the swelling behavior of the hydrogel, resulting in a long adsorption time, and the fertilizer loading efficiency is easily limited by factors such as the gel network structure, fertilizer solubility, and adsorption equilibrium. The other method involves adding fertilizer during hydrogel polymerization or cross-linking, allowing the fertilizer to be embedded within the gel during the gel formation process. While this method reduces subsequent adsorption steps, it usually requires the addition of additional fertilizer components or fertilizer precursors, and issues such as unreacted components, solvent residues, or impurity removal may still exist after gel preparation.

[0005] Among natural polymer hydrogel materials, biomass materials such as cellulose, sodium alginate, starch, and chitin have advantages such as wide availability, renewability, good biocompatibility, and environmental friendliness. Chitin, in particular, is an abundant natural polysaccharide with good biocompatibility, soil biodegradability, and certain natural antibacterial properties. Its degradation products also have some value in plant growth regulation and soil improvement. However, the dense hydrogen bond network within and between chitin molecules, along with its high crystallinity, makes it difficult to dissolve in water and most common solvents, thus limiting its molding, processing, and large-scale application in agricultural hydrogel materials.

[0006] The low-temperature KOH / urea aqueous solvent system can dissolve chitin, providing a feasible route for the preparation of chitin hydrogel materials. However, in the traditional preparation of natural polymer gels using the alkali / urea solvent system, repeated washing is usually required after gel formation to remove residual alkali, urea, and other solvents. This process not only increases water consumption but also generates alkaline waste liquid and causes the loss of fertilizer-valued components such as urea and potassium. Furthermore, if fertilizer adsorption is performed after gel formation, it further increases the preparation time, washing steps, and drying burden, which is detrimental to obtaining a simplified process and a high fertilizer loading efficiency for the preparation of water-retaining slow-release fertilizers.

[0007] Therefore, existing technologies still need a method that can directly utilize the fertilizer elements in the chitin KOH / urea solvent system to complete droplet formation, chitin gelation, and nitrogen, phosphorus, and potassium nutrient loading in the same process, in order to reduce washing and fertilizer loading processes, improve preparation efficiency, and obtain chitin hydrogel-based fertilizer beads with water retention and slow-release functions. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method for preparing and applying NPK-loaded chitosan gel beads. The preparation method includes: dissolving chitosan in a KOH / urea solvent system; dripping the chitosan solution; in-situ nutrient loading and pH-induced gelation of the chitosan droplets in a phosphoric acid coagulation bath. Compared to existing hydrogel fertilizer preparation methods, this method simultaneously achieves chitosan gelation, spheroidization, and NPK fertilizer loading, reducing subsequent nutrient adsorption steps and avoiding the problem of impurity removal after gel formation. The NPK-loaded chitosan gel beads prepared by this invention are uniformly spherical with a dense surface and a porous core-shell structure; the mass fractions of total nitrogen, potassium oxide, and phosphorus pentoxide are 3.66%–6.53%, 7.14%–16.94%, and 10.43%–25.51%, respectively. The NPK-loaded chitosan gel beads exhibit good mechanical stability, water retention capacity, nutrient slow-release performance, biocompatibility, and soil degradation performance.

[0009] Therefore, the technical solution adopted in this invention is: a method for preparing NPK-loaded chitosan gel beads, comprising the following steps:

[0010] S1. Chitin was dispersed in a KOH / urea aqueous solvent system, and the resulting suspension was subjected to multiple low-temperature freeze-thaw treatments to obtain a chitin solution;

[0011] S2. The chitin solution obtained in step S1 is dripped into a coagulation bath containing phosphoric acid to form chitin spherical droplets. The chitin spherical droplets are gelled and solidified in the coagulation bath and nutrient loaded in situ for 6-24 h, preferably 12 h, to form gel beads. The gel beads are then surface rinsed and dried to obtain NPK-loaded chitin gel beads.

[0012] Preferably, in step S1, the mass fraction of KOH in the KOH / urea aqueous solvent system is 15% to 25%, preferably 20%; the mass fraction of urea is 2% to 8%, preferably 4%; and the mass fraction of chitosan in the chitosan solution is 4% to 8%.

[0013] Preferably, in step S1, the multiple low-temperature freeze-thaw treatment includes: pre-freezing the suspension at -80 to -40°C for 1 to 6 hours, preferably at -75°C for 3 hours; then thawing it in an ice-water bath and stirring it at 300 to 500 rpm until it melts; repeating the freeze-thaw process 3 to 5 times, preferably 4 times; and then centrifuging at 8000 rpm for 5 to 10 minutes, preferably 8 minutes, to obtain a transparent chitosan solution.

[0014] Preferably, in step S2, the chitin solution is dripped into a coagulation bath containing phosphoric acid, wherein the amount of chitin solution used per batch is 20-100 g, preferably 50 g; the dripping method is syringe dripping, injection pump dripping, or peristaltic pump dripping, preferably injection pump dripping; the dropper size is 14-25 G, preferably 17 G; the height of the dropper from the surface of the coagulation bath during dripping is 3-5 cm, preferably 4 cm.

[0015] Preferably, in step S2, the coagulation bath containing phosphoric acid is composed of phosphoric acid, chloroform, and ethanol; wherein the volume ratio of chloroform to ethanol is 50-60:40-50, preferably 55:45; the amount of the coagulation bath is 20-50 times the mass of the chitin solution used for dripping, preferably 40 times; and the amount of phosphoric acid is 1-1.2 times the amount of KOH in the chitin solution used for dripping, preferably 1 time.

[0016] Preferably, in step S2, the surface rinsing includes: filtering and collecting the gel beads, rinsing them with deionized water 1 to 3 times, preferably 2 times; the amount of deionized water used each time is 1 to 3 times the mass of the chitin solution used for dripping, preferably 2 times; the drying is freeze drying or vacuum drying, preferably freeze drying.

[0017] The present invention also provides NPK-loaded chitosan gel beads prepared by the above preparation method. The mass fractions of total nitrogen, potassium oxide and phosphorus pentoxide in the NPK-loaded chitosan gel beads are 3.66%–6.53%, 7.14%–16.94% and 10.43%–25.51%, respectively.

[0018] Finally, this invention also provides the application of the NPK-loaded chitosan gel beads in the preparation of slow-release fertilizers, water-retaining materials, or soil amendments.

[0019] The beneficial technical effects of this invention are as follows:

[0020] (1) In this invention, KOH and urea in KOH / urea solvent are transformed from residual components that originally needed to be removed into fertilizer sources. KOH can form potassium and phosphorus nutrient components under the action of phosphoric acid, and urea is retained in the chitin gel network as a nitrogen source, thereby realizing the fertilizer utilization of solvent components.

[0021] (2) In the process of dripping, the present invention simultaneously completes the formation of spherical droplets, pH-induced gelation, in-situ generation of potassium and phosphorus nutrient components and urea fixation, avoiding the step of preparing blank hydrogels first and then performing long-term fertilizer adsorption, and also avoiding the difficulty in removing impurities introduced by the gel formation after adding fertilizer in advance.

[0022] (3) The NPK-loaded chitin gel beads prepared by the present invention have a core-shell structure consisting of a dense outer layer and a porous internal network. The dense outer layer can block nutrient diffusion, and the porous internal network can provide space for water absorption and nutrient loading, so that the resulting gel beads have both water retention and nutrient slow release properties.

[0023] (4) The present invention uses chitin as a gel matrix, and the resulting NPK-loaded chitin gel beads have good biocompatibility and soil degradation performance, and can be used in agricultural fields such as slow-release fertilizer, water-retaining material and soil improvement material. Attached Figure Description

[0024] Figure 1 The images show the morphological appearance of the NPK-loaded chitosan gel beads prepared in Examples 1-5.

[0025] Figure 2 Bar chart showing the particle size of the NPK-loaded chitosan gel beads prepared in Examples 1-5;

[0026] Figure 3 The images show the surface and cross-sectional morphology of the NPK-loaded chitosan gel beads prepared in Example 3.

[0027] Figure 4 The bar chart shows the total nitrogen content of the NPK-loaded chitosan gel beads prepared in Examples 1-5.

[0028] Figure 5 The bar chart shows the potassium content of the NPK-loaded chitosan gel beads prepared in Examples 1-5.

[0029] Figure 6 The bar chart shows the phosphorus content of the NPK-loaded chitosan gel beads prepared in Examples 1-5.

[0030] Figure 7 The water retention properties of the NPK-loaded chitosan gel beads prepared in Examples 1-5 are shown in the figure.

[0031] Figure 8 Phosphate release kinetics curves of the NPK-loaded chitin gel beads prepared in Examples 1-5 in soil;

[0032] Figure 9 The figure shows the effect of the NPK-loaded chitosan gel beads prepared in Example 3 on the weight and soluble sugar content of cherry radishes during their growth.

[0033] Figure 10 Soil degradation curves of NPK-loaded chitosan gel beads prepared in Example 3. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Equivalent substitutions, adjustments, or modifications made by those skilled in the art based on ordinary technical knowledge and conventional technical means without departing from the technical concept of the present invention should all fall within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the embodiments of the present invention are conventional methods in the art; unless otherwise specified, the materials and reagents used are commercially available.

[0035] Example 1: Preparation of NPK-loaded chitosan gel beads

[0036] A method for preparing NPK-loaded chitosan gel beads, comprising the following steps:

[0037] A certain amount of chitin was weighed and added to a KOH / urea aqueous solvent system, wherein the mass fraction of KOH was 15%, the mass fraction of urea was 2%, and the mass fraction of chitin was 4%. The resulting suspension was pre-frozen at -75℃ for 3 h, then thawed in an ice-water bath and stirred at 300-500 rpm until thawed. After repeating the freeze-thaw cycle 4 times, the solution was centrifuged at 8000 rpm for 8 min to remove bubbles, resulting in a transparent chitin solution.

[0038] Take 50 g of the above chitin solution and drip it into a coagulation bath containing phosphoric acid using a syringe pump. The coagulation bath consists of phosphoric acid, chloroform, and ethanol, with a volume ratio of chloroform to ethanol of 50:50. The volume of the coagulation bath is 50 times the mass of the chitin solution used for dripping, and the amount of phosphoric acid is 1 times the amount of KOH in the chitin solution used for dripping. Use a 17 G dropper and position it 4 cm above the surface of the coagulation bath. After dripping, allow the formed spherical chitin droplets to gel and solidify in the coagulation bath for 12 h, and allow for in-situ nutrient loading to obtain gel beads.

[0039] After filtering and collecting the obtained gel beads, they were rinsed twice with deionized water, with the amount of deionized water each time being twice the mass of the chitin solution used for dripping. Then, they were freeze-dried to obtain NPK-loaded chitin gel beads.

[0040] The obtained NPK-loaded chitosan gel beads were uniformly spherical with a particle size of 3.12 ± 0.60 mm. Their morphology and particle size are shown in the figures below. Figure 1 and Figure 2 .

[0041] Example 2: Preparation of NPK-loaded chitosan gel beads

[0042] Except for adjusting the following proportions, the mass fraction of chitin in the chitin solution was adjusted to 5%, the mass fraction of KOH was adjusted to 20%, the mass fraction of urea was adjusted to 4%, the volume ratio of chloroform to ethanol was adjusted to 55:45, and the amount of coagulation bath was adjusted to 40 times the mass of the chitin solution used for dripping. The remaining steps and conditions can be the same as in Example 1, or the experimental parameters can be appropriately adjusted under the experimental conditions of Example 1 to obtain NPK-loaded chitin gel beads.

[0043] The obtained NPK-loaded chitosan gel beads were uniformly spherical with a particle size of 3.07 ± 0.80 mm. Their morphology and particle size are shown in the figures below. Figure 1 and Figure 2 .

[0044] Example 3: Preparation of NPK-loaded chitosan gel beads

[0045] Except for the following adjustments, the mass fraction of chitin in the chitin solution was adjusted to 6%, the mass fraction of KOH was adjusted to 20%, the mass fraction of urea was adjusted to 4%, the volume ratio of chloroform to ethanol was adjusted to 55:45, and the amount of coagulation bath was adjusted to 30 times the mass of the chitin solution used for dripping. The remaining steps and conditions were the same as in Example 1, or the experimental parameters could be appropriately adjusted under the experimental conditions of Example 1 to obtain NPK-loaded chitin gel beads.

[0046] The NPK-loaded chitosan gel beads exhibited a uniform spherical morphology with a particle size of 2.83 ± 0.18 mm. Their appearance and particle size are shown in [the table below]. Figure 1 and Figure 2 Further observation of the surface and cross-sectional morphology of the NPK-loaded chitosan gel beads obtained in Example 3 revealed that the gel beads possess a core-shell structure consisting of a dense outer layer and a porous internal network. The dense outer layer provides some barrier to nutrient diffusion, while the porous internal network provides space for water absorption and nutrient loading. The surface and cross-sectional morphology of the NPK-loaded chitosan gel beads obtained in Example 3 are shown below. Figure 3 .

[0047] Example 4: Preparation of NPK-loaded chitosan gel beads

[0048] Except for the following adjustments, the mass fraction of chitin in the chitin solution was adjusted to 7%, the mass fraction of KOH was adjusted to 20%, the mass fraction of urea was adjusted to 6%, the volume ratio of chloroform to ethanol was adjusted to 55:45, and the amount of coagulation bath was adjusted to 30 times the mass of the chitin solution used for dripping. The remaining steps and conditions can be the same as in Example 1, or the experimental parameters can be appropriately adjusted under the experimental conditions of Example 1 to obtain NPK-loaded chitin gel beads.

[0049] The obtained NPK-loaded chitosan gel beads were uniformly spherical with a particle size of 2.80 ± 0.45 mm. Their morphology and particle size are shown in the figures below. Figure 1 and Figure 2 .

[0050] Example 5: Preparation of NPK-loaded chitosan gel beads

[0051] Except for the following adjustments, the mass fraction of chitin in the chitin solution was adjusted to 8%, the mass fraction of KOH was adjusted to 25%, the mass fraction of urea was adjusted to 8%, the volume ratio of chloroform to ethanol was adjusted to 60:40, and the amount of coagulation bath was adjusted to 20 times the mass of the chitin solution used for dripping. The remaining steps and conditions can be the same as in Example 1, or the experimental parameters can be appropriately adjusted under the experimental conditions of Example 1 to obtain NPK-loaded chitin gel beads.

[0052] The obtained NPK-loaded chitosan gel beads were uniformly spherical with a particle size of 2.70 ± 0.32 mm. Their morphology and particle size are shown in the figures below. Figure 1 and Figure 2 .

[0053] Application Example 1: Determination of Nutrient Content in NPK-Loaded Chitosan Gel Beads

[0054] The nutrient content of the NPK-loaded chitosan gel beads obtained in Examples 1-5 was determined using the following method. The detection indicators included total nitrogen, phosphorus pentoxide, and potassium oxide.

[0055] The total nitrogen detection method is as follows: Accurately weigh 0.1 g of sample and place it in a digestion tube. Add 10 mL of concentrated sulfuric acid and 0.5 g of mixed catalyst in sequence, wherein the mass ratio of potassium sulfate to copper sulfate pentahydrate is 20:1. Then digest at 350℃ for 3 h to allow the sample to decompose completely, and determine the total nitrogen content using a fully automated Kjeldahl nitrogen analyzer.

[0056] The methods for phosphorus and potassium detection are as follows: Accurately weigh 1.0 g of sample and place it in a grinding tube, add 10 mL of deionized water and grind thoroughly; transfer the resulting mixture to a 15 mL centrifuge tube and sonicate for 30 min; filter and wash the residue, collect the filtrate, and dilute to a 250 mL volumetric flask. The phosphate content is determined by the phosphomolybdic acid reduction method and converted to phosphorus pentoxide content; the potassium content is determined by atomic absorption spectrophotometry and converted to potassium oxide content. Three parallel experiments are set up for each group of tests.

[0057] The results showed that the mass fractions of total nitrogen, potassium oxide, and phosphorus pentoxide in the NPK-loaded chitosan gel beads obtained in Examples 1-5 were 3.66%–6.53%, 7.14%–16.94%, and 10.43%–25.51%, respectively. The test results for total nitrogen, potassium, and phosphorus content are shown below. Figure 4 , Figure 5 and Figure 6 .

[0058] Application Example 2: Determination of the water retention properties of NPK-loaded chitosan gel beads

[0059] The water retention properties of the NPK-loaded chitosan gel beads obtained in Examples 1-5 were evaluated using the weight loss method. The specific method is as follows: Pre-weighed freeze-dried samples were placed in distilled water and allowed to swell to equilibrium. Then, the samples that had reached swelling equilibrium were removed, surface free water was removed, and the samples were evenly distributed in petri dishes. The remaining mass of each sample was recorded at set time points at room temperature. Three parallel samples were set up for each experiment.

[0060] The water retention rate (WRC) is calculated using the following formula:

[0061]

[0062] In the formula, M t Mt represents the remaining mass of the sample at time t, in g; M0 represents the initial mass of the sample after swelling equilibrium, in g.

[0063] The results showed that the water evaporation trends of the samples obtained in Examples 1-5 were generally similar, but the water loss rate gradually decreased with the increase of chitosan mass fraction. On day 6, the water content of the sample in Example 1 decreased to 0.81%, while the water contents of the samples in Examples 2, 3, 4, and 5 remained at 19.62%, 22.94%, 25.90%, and 32.42%, respectively. These results indicate that increasing the chitosan content helps enhance the interaction between the gel network and water molecules, while the dense surface structure can also hinder water evaporation to a certain extent. The water retention performance test results of the samples obtained in Examples 1-5 are shown below. Figure 7 .

[0064] Application Example 3: Determination of Nutrient Release Kinetics in Soil Using NPK Chitosan-Loaded Gel Beads

[0065] The phosphate release behavior of the NPK-loaded chitin gel beads obtained in Examples 1-5 in soil was tested using a soil column leaching method. The specific method is as follows: 1.0 g of sample was mixed with 100 g of washed and dried sandy loam, and loaded into a chromatography column equipped with a PTFE stopcock and a G3 sand core filter plate; 50 mL of deionized water was added, and 2 mL of eluent was collected at each sampling point, with an equal volume of deionized water added afterwards. The control group used ordinary urea and potassium dihydrogen phosphate, with a total nutrient content equivalent to that of the sample in Example 3. The phosphate content was detected using the same method as for phosphorus detection in Application Example 1.

[0066] The cumulative release rate (CR) is calculated using the following formula:

[0067]

[0068] In the formula, m0 is the initial mass of the nutrient to be tested in the sample, in mg; V0 is the initial volume of deionized water added, which is 50 mL in this application example; V n This refers to the volume of each sample taken; in this application example, it is 2 mL. n The concentration of the nutrient to be tested in the released solution at the nth sampling time is expressed in mg / mL.

[0069] The results showed that phosphate was almost completely released within 2 days in the control groups of ordinary urea and potassium dihydrogen phosphate, while the NPK-loaded chitosan gel beads exhibited a significant slow-release effect. Among them, the sample obtained in Example 5 showed the best slow-release performance, with a cumulative phosphate release rate of 62.41% on day 28. These results indicate that NPK-loaded chitosan gel beads can delay phosphate release and have the potential to reduce the concentrated release of nutrients in a short period and decrease nutrient loss. The phosphate release kinetic curves of the samples obtained in Examples 1-5 in soil are shown below. Figure 8 .

[0070] Application Example 4: Evaluation of the effect of NPK-loaded chitosan gel beads on the growth of cherry radishes

[0071] To evaluate the application effect of NPK-loaded chitosan gel beads on crop growth, the sample obtained in Example 3 was selected as a representative material for a pot experiment on cherry radishes. The experimental soil was taken from the experimental field of Hanzhong Vocational and Technical College in Shaanxi Province, and was screened and air-dried before use. The pot experiment was conducted on an indoor plant cultivation rack from October to November 2025. The cultivation conditions were as follows: day / night temperature of 22±2℃ and 13±2℃, relative humidity of 50%, daily light duration of 12 h, and light intensity of 15000 Lux.

[0072] The experiment consisted of three treatment groups, with eight replicates per group. The control group received 500 g of dry soil; the conventional fertilizer group received 500 g of dry soil, ordinary urea, and potassium dihydrogen phosphate, with the nutrient content of the ordinary urea and potassium dihydrogen phosphate being the same as that of the 2.5 g sample from Example 3; the Example 3 group received 500 g of dry soil and 2.5 g of the Example 3 sample. One germinated cherry radish seed was sown in each pot. Initially, each pot was watered with 200 mL of deionized water, followed by 100 mL of deionized water every three days. Radishes were harvested 40 days after planting, their weight was measured, and the soluble sugar content was determined using the anthrone colorimetric method.

[0073] The results showed that the radish weight of the sample group in Example 3 was 20.64 g, which was higher than that of the blank group and the conventional fertilizer group. Simultaneously, the soluble sugar content of the radish in the Example 3 sample group was also higher than that of the blank group and the conventional fertilizer group. The effects of the NPK-loaded chitosan gel beads obtained in Example 3 on the weight and soluble sugar content of cherry radishes are shown in [the table below]. Figure 9 .

[0074] Application Example 5: Determination of the Biodegradability of NPK-Loaded Chitosan Gel Beads

[0075] The degradation performance of the NPK-loaded chitosan gel beads obtained in Example 3 was evaluated using the soil burial method. The specific method is as follows: 27 samples were accurately weighed and their initial mass W0 was recorded. The samples were buried in flowerpots filled with soil to a depth of approximately 10 cm. The flowerpots were placed at room temperature and watered regularly to maintain soil moisture. Three samples were randomly selected at preset time intervals, thoroughly rinsed with deionized water to remove surface soil residue, and then dried in a 50°C vacuum oven until constant weight. The mass W of the degraded sample was recorded. f .

[0076] The remaining mass percentage RW is calculated according to the following formula:

[0077]

[0078] In the formula, W0 is the initial mass of the sample before degradation, in g; W f The mass of the sample after degradation and drying is expressed in grams.

[0079] The results showed that the sample from Example 3 gradually degraded in the soil environment over time. The degradation rate was slow in the first 20 days, with a remaining mass percentage of 81%; subsequently, the degradation rate gradually increased, and the remaining mass percentage was 36% at the end of the experiment. The soil degradation curve of the NPK-loaded chitosan gel beads obtained in Example 3 is shown in [Figure number missing]. Figure 10 .

Claims

1. A method for preparing NPK-loaded chitosan gel beads, characterized in that, Includes the following steps: S1. Chitin was dispersed in a KOH / urea aqueous solvent system, and the resulting suspension was subjected to multiple low-temperature freeze-thaw treatments to obtain a chitin solution; S2. The chitin solution obtained in step S1 is dripped into a coagulation bath containing phosphoric acid to form chitin spherical droplets. The chitin spherical droplets are gelled and solidified in the coagulation bath and nutrient loaded in situ for 6-24 hours to form gel beads. The gel beads are then surface rinsed and dried to obtain NPK-loaded chitin gel beads.

2. The method for preparing NPK-loaded chitosan gel beads according to claim 1, characterized in that: The KOH / urea aqueous solvent system contains 15-25% KOH and 2-8% urea; the chitosan solution contains 4-8% chitosan.

3. The method for preparing NPK-loaded chitosan gel beads according to claim 1, characterized in that: The multiple low-temperature freeze-thaw treatment includes: pre-freezing the suspension at -80 to -40°C for 1 to 6 hours, then thawing it in an ice-water bath and stirring it at 300 to 500 rpm until it melts; repeating the freeze-thaw process 3 to 5 times, and then centrifuging at 8000 rpm for 5 to 10 minutes to remove bubbles, to obtain a transparent chitin solution.

4. The method for preparing NPK-loaded chitosan gel beads according to claim 1, characterized in that: In step S2, the chitin solution is dripped into a coagulation bath containing phosphoric acid. The amount of chitin solution used per batch is 20-100 g. The dripping method is either injection pump dripping or peristaltic pump dripping. The dropper size is 14-25 G. During dripping, the dropper is 3-5 cm above the surface of the coagulation bath.

5. The method for preparing NPK-loaded chitosan gel beads according to claim 1, characterized in that: In step S2, the coagulation bath containing phosphoric acid is composed of phosphoric acid, chloroform, and ethanol; wherein the volume ratio of chloroform to ethanol is 50-60:40-50, the amount of the coagulation bath is 20-50 times the mass of the chitin solution used for dripping, and the amount of phosphoric acid is 1-1.2 times the amount of KOH in the chitin solution used for dripping.

6. The method for preparing NPK-loaded chitosan gel beads according to claim 1, characterized in that: In step S2, the surface rinsing includes: filtering and collecting the gel beads, rinsing them with deionized water 1 to 3 times, with the amount of deionized water used each time being 1 to 3 times the mass of the chitin solution used for dripping; the drying is freeze drying or vacuum drying.

7. A type of NPK-loaded chitosan gel bead, characterized in that, It is prepared by the method described in any one of claims 1-6.

8. The NPK-loaded chitosan gel beads according to claim 7, characterized in that: The mass fractions of total nitrogen, potassium oxide, and phosphorus pentoxide in the NPK-loaded chitosan gel beads are 3.66%–6.53%, 7.14%–16.94%, and 10.43%–25.51%, respectively.

9. The use of the NPK-loaded chitosan gel beads according to claim 7 or 8 in the preparation of slow-release fertilizers, water-retaining materials or soil amendments.

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