A biomass-based hydrogel loaded with cocrystalline urea, its preparation method and application

Abstract

This invention provides a biomass-based hydrogel loaded with eutectic urea, its preparation method, and its application, belonging to the field of agricultural fertilizer technology. This invention uses sodium alginate-calcium gel to encapsulate urea and water, achieving storage and continuous supply of both. The sodium lignin sulfonate layer contains a large number of negatively charged sulfonic acid groups, and the chitosan layer contains a large number of positively charged amino groups. This alternating arrangement achieves mutual attraction between the charges of the sodium lignin sulfonate and chitosan layers, improving structural stability. Simultaneously, both the sulfonic acid and amino groups are highly hydrophilic, enabling the slow release of water from the sodium alginate hydrogel. By using eutectic urea, urea molecules are locked within the eutectic crystal and released slowly through dissolution, avoiding nitrogen loss caused by a large supply of urea in a short period. Furthermore, the simultaneous supply of urea and water avoids the problem of fertilizer being present but water is lacking in simple encapsulation methods, making it difficult for crops to absorb urea.

Description

Technical Field

[0001] This invention belongs to the field of agricultural fertilizer technology, specifically relating to a biomass-based hydrogel loaded with eutectic urea, its preparation method, and its application. Background Technology

[0002] Nitrogen fertilizer and water are two essential elements for crop growth. However, agricultural production commonly suffers from problems such as asynchronous water and fertilizer supply, low nitrogen fertilizer utilization rate, and low water use efficiency. Taking urea as an example, its high solubility in water makes nutrients easily and rapidly dissolve, leading to leaching, runoff, and volatilization. This not only reduces the effective utilization rate of fertilizer but also causes significant environmental pollution.

[0003] Achieving coordinated water-fertilizer supply at the rhizosphere scale is a core science and engineering achievement for improving nitrogen fertilizer and water use efficiency. Traditional slow-release technologies, such as sulfur-coated urea and polymer-coated urea, mainly rely on delayed dissolution at the particle scale to achieve efficient nitrogen fertilizer utilization. However, the coating materials are mostly derived from petrochemical products, which present limitations such as high cost, non-renewability, and non-degradability, hindering large-scale promotion. Moreover, these slow-release products lack water retention capabilities; under drought conditions, even if fertilizer continues to be released, the lack of water as a transport medium makes it difficult to reach the roots, resulting in a situation of "water and fertilizer separation, making it difficult to utilize fertilizer effectively." Summary of the Invention

[0004] The purpose of this invention is to provide a biomass-based hydrogel loaded with cocrystalline urea, its preparation method, and its applications. The biomass-based hydrogel loaded with cocrystalline urea provided by this invention can achieve synergistic slow-release supply of water and fertilizer, and is environmentally friendly.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a biomass-based hydrogel loaded with cocrystalline urea, comprising a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of sodium lignin sulfonate and chitosan covering the sodium alginate-calcium shell; the slurry comprises sodium alginate, cocrystalline urea, and water; the cocrystalline urea is a cocrystal formed by the self-assembly of urea and a small molecule organic acid through hydrogen bonding; the small molecule organic acid includes vanillic acid or 2,6-dimethoxybenzoic acid.

[0006] Preferably, the mass ratio of eutectic urea to water in the slurry is (0.1~10):100.

[0007] Preferably, the molar ratio of urea to small molecule organic acid in the eutectic urea is 1:(0.8~1.2).

[0008] Preferably, the sodium lignosulfonate layer and the chitosan layer are independently arranged in 2 to 5 layers, and the sodium lignosulfonate layer and the chitosan layer are alternately arranged.

[0009] This invention also provides a method for preparing the biomass-based hydrogel loaded with eutectic urea as described in the above technical solution, comprising the following steps: The co-crystallized urea, sodium alginate, and water were mixed to obtain a blend solution; The blend was added dropwise to a calcium salt solution for solidification to obtain sodium alginate hydrogel beads. The sodium alginate hydrogel beads were coated by impregnation in chitosan solution and sodium lignin sulfonate solution to obtain biomass-based hydrogels loaded with cocrystalline urea.

[0010] Preferably, the method for preparing the eutectic urea includes: mixing urea, a small molecule organic acid, and anhydrous ethanol to obtain a mixed solution; and evaporating the mixed solution to crystallize it to obtain eutectic urea.

[0011] Preferably, the mass ratio of sodium alginate to water is (0.1~2):100.

[0012] Preferably, the chitosan solution has a mass concentration of 0.08-0.12%; and the sodium lignosulfonate solution has a mass concentration of 0.8-1.2%.

[0013] Preferably, the immersion time for each immersion is independently 20 to 40 minutes.

[0014] The present invention also provides the application of the biomass-based hydrogel loaded with eutectic urea described above in crop growth.

[0015] This invention provides a biomass-based hydrogel loaded with cocrystalline urea, comprising a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of sodium lignin sulfonate and chitosan covering the sodium alginate-calcium shell; the slurry comprises sodium alginate, cocrystalline urea, and water; the cocrystalline urea is a cocrystal formed by the self-assembly of urea and a small molecule organic acid through hydrogen bonding; the small molecule organic acid includes vanillic acid or 2,6-dimethoxybenzoic acid. This invention utilizes the low water solubility of the eutectic urea and small-molecule organic acid, locking urea molecules within the eutectic and releasing them slowly through dissolution in water, thus preventing nitrogen loss caused by a large supply of urea in a short period. A sodium alginate-calcium gel shell encapsulates the eutectic urea and water, achieving storage and continuous supply of both. Sodium lignin sulfonate contains numerous negatively charged sulfonic acid groups, and chitosan contains numerous positively charged amino groups. Both sulfonic acid and amino groups are highly hydrophilic, enabling the slow release of water from the biomass-based hydrogel loaded with eutectic urea. Furthermore, the simultaneous supply of urea and water avoids the problem of simple coating methods where fertilizer is available but water is lacking, making urea difficult for crops to absorb. Results from the embodiments show that the biomass-based hydrogel loaded with eutectic urea provided by this invention can continuously supply the nitrogen fertilizer and water required for crop growth, and its growth-promoting effect on wheat is superior to that of urea applied alone. Attached Figure Description

[0016] Figure 1 This is a photograph of the urea raw material used in the embodiments of the present invention; Figure 2 A photograph of the UDBA prepared in Example 1 of this invention; Figure 3 This is a photograph of the UVA prepared in Example 2 of the present invention; Figure 4 Here is a SEM image of the urea raw material used in the embodiments of the present invention; Figure 5 This is a SEM image of the UDBA prepared in Example 1 of the present invention; Figure 6 This is a SEM image of UVA prepared in Example 2 of the present invention; Figure 7 The XRD patterns of Urea, DBA, and UDBA in Embodiment 1 of the present invention are shown below. Figure 8 The XRD patterns of Urea, VA, and UVA in Embodiment 2 of the present invention are shown below. Figure 9 This is a release curve of different urea in Test Example 3 of the present invention; Figure 10 This is a diagram showing the changes in urea in Test Example 4 of this invention; Figure 11This is an XRD pattern of UDBA and UVA in Test Example 4 of the present invention; Figure 12 These are actual images of water absorption from different samples in Test Example 5 of this invention; Figure 13 This is a graph showing the water absorption rate of different samples in Test Example 5 of this invention; Figure 14 This is a graph showing the water absorption rate of different samples after 6 cycles in Test Example 5 of this invention; Figure 15 This is a photograph of the sample from Example 1 in Test Example 5 of the present invention after 6 cycles. Figure 16 This is a photograph of the sample from Example 7 in Test Example 5 of the present invention after 6 cycles. Figure 17 These are actual images of the sodium alginate hydrogel beads used in Examples 1, 3-5 of this invention; Figure 18 The images shown are SEM images and N element distribution diagrams of the sodium alginate hydrogel beads after freeze-drying in Example 3 of this invention. Figure 19 The images shown are SEM images and N element distribution diagrams of the sodium alginate hydrogel beads after freeze-drying in Example 4 of this invention. Figure 20 The images shown are SEM images and N element distribution diagrams of the sodium alginate hydrogel beads after freeze-drying in Example 5 of this invention. Figure 21 The images shown are SEM images and N element distribution diagrams of sodium alginate hydrogel beads after freeze-drying in Example 1 of this invention. Figure 22 This is a graph showing the repeated water absorption rate of sodium alginate hydrogel beads in Example 1 of the present invention; Figure 23 This is a diagram showing the growth status of bok choy in the control group (CK group) in test example 7 of this invention. Figure 24 This is a growth status diagram of the (CS+SL) / SA@UDBA group of pakchoi in Test Example 7 of this invention; Figure 25 This is a growth status diagram of the (CS+SL) / SA@UVA group of Chinese cabbage in Test Example 7 of this invention; Figure 26 The image shows the fresh weight of bok choy under different treatments in Test Example 7 of this invention. Figure 27 This is a graph showing the chlorophyll a, chlorophyll b, and carotenoid content of different treatments of Chinese cabbage in Test Example 7 of this invention; Figure 28 These are actual images of wheat samples treated differently in Test Example 8 of this invention; Figure 29 The image shows the plant height of wheat under different treatments in Test Example 8 of this invention. Detailed Implementation

[0017] All raw materials used in this invention are not particularly limited in their source; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0018] There are no particular restrictions on the purity of any of the raw materials used in this invention. However, this invention preferably uses raw materials of industrial purity or conventional purity used in the field of agricultural fertilizers.

[0019] This invention provides a biomass-based hydrogel loaded with cocrystalline urea, comprising a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of sodium lignin sulfonate and chitosan covering the sodium alginate-calcium shell; the slurry comprises sodium alginate, cocrystalline urea, and water; the cocrystalline urea is a cocrystal formed by the self-assembly of urea and a small molecule organic acid through hydrogen bonding; the small molecule organic acid includes vanillic acid or 2,6-dimethoxybenzoic acid.

[0020] The biomass-based hydrogel loaded with eutectic urea provided by this invention comprises a sodium alginate-calcium shell. This invention encapsulates eutectic urea and water within the sodium alginate-calcium shell, thereby storing urea and water.

[0021] The biomass-based hydrogel loaded with eutectic urea provided by the present invention also includes a slurry encapsulated in the sodium alginate-calcium shell.

[0022] In this invention, the slurry comprises sodium alginate, cocrystalline urea, and water; the cocrystalline urea is a cocrystalline compound formed by the self-assembly of urea and a small-molecule organic acid through hydrogen bonding; the small-molecule organic acid includes vanillic acid or 2,6-dimethoxybenzoic acid. Cocrystalline urea can release urea molecules locked within the cocrystalline compound through slow dissolution, avoiding nitrogen fertilizer loss caused by traditional urea application methods; by storing and supplying urea and water together, it also avoids the problem of simple coating methods where fertilizer is present but water is lacking, making it difficult for crops to absorb urea.

[0023] In one embodiment of the present invention, the molar ratio of urea to small molecule organic acid can be 1:(0.8~1.2) or 1:1. A molar ratio of urea to small molecule organic acid within the above range is beneficial for further improving the stability of the eutectic urea.

[0024] In this invention, the preferred mass ratio of eutectic urea to water in the slurry is (0.1~10):100, more preferably (0.5~2):100. As one embodiment of this invention, the mass ratio of eutectic urea to water in the slurry can be 0.15:100, 0.45:100, 0.6:100, 0.75:100, 1:100, 1.5:100, or 3:100. When the mass ratio of eutectic urea to water in the slurry is within the above range, it can balance the supply of water and urea, further promoting crop growth.

[0025] The biomass-based hydrogel loaded with eutectic urea provided by the present invention further includes a sodium lignin sulfonate layer and a chitosan layer alternately covering the sodium alginate-calcium shell.

[0026] The sodium lignosulfonate layer contains a large number of negatively charged sulfonic acid groups, while the chitosan layer contains a large number of positively charged amino groups. By alternating these layers, the sodium lignosulfonate and chitosan layers attract each other through charge, thus improving structural stability. At the same time, both the sulfonic acid and amino groups are highly hydrophilic, which allows for the slow release of urea and water from the sodium alginate hydrogel. When the hydrogel is sufficiently moist, it can also absorb water for storage.

[0027] In this invention, the number of layers of sodium lignosulfonate and chitosan is preferably 2 to 5, more preferably 3 to 4. This invention does not limit the coating order of the sodium lignosulfonate and chitosan layers; either the sodium lignosulfonate layer or the chitosan layer can be coated first.

[0028] This invention uses sodium alginate hydrogel as a carrier for urea and water, enabling the storage and slow, continuous supply of both. The sodium lignin sulfonate layer contains a large number of negatively charged sulfonic acid groups, while the chitosan layer contains a large number of positively charged amino groups. By alternating these layers, the sodium lignin sulfonate and chitosan layers attract each other through charge, improving structural stability. Furthermore, both the sulfonic acid and amino groups are highly hydrophilic, facilitating the slow release of urea and water from the sodium alginate hydrogel. By using eutectic urea, urea molecules are locked within the eutectic crystal for slow release, preventing nitrogen loss caused by a large supply of urea in a short period. Moreover, the simultaneous supply of urea and water avoids the problem of simple coating methods where fertilizer is available but water is lacking, making it difficult for crops to absorb the urea.

[0029] This invention also provides a method for preparing the biomass-based hydrogel loaded with eutectic urea as described in the above technical solution, comprising the following steps: The co-crystallized urea, sodium alginate, and water were mixed to obtain a blend solution; The blend was added dropwise to a calcium salt solution for solidification to obtain sodium alginate hydrogel beads. The sodium alginate hydrogel beads were coated by impregnation in chitosan solution and sodium lignin sulfonate solution to obtain biomass-based hydrogels loaded with cocrystalline urea.

[0030] This invention involves mixing eutectic urea, sodium alginate, and water to obtain a blend.

[0031] In this invention, the preferred method for preparing the eutectic urea includes: mixing urea, a small-molecule organic acid, and anhydrous ethanol to obtain a mixed solution; and evaporating and crystallizing the mixed solution to obtain eutectic urea. As one embodiment of this invention, the molar concentration of urea in the mixed solution can be 0.5 mol / L; the mixing can be carried out under stirring conditions, and the stirring rate can be 100 rpm; the mixing temperature can be 40°C; after obtaining the mixed solution, it can be filtered using a 0.45 μm filter membrane; and the evaporation and crystallization can be carried out at room temperature.

[0032] In this invention, the preferred mass ratio of sodium alginate to water is (0.1~2):100, more preferably (0.5~1.5):100. As one embodiment of this invention, the mass ratio of sodium alginate to water can be 0.2:100, 0.4:100, 0.6:100, 0.8:100, 1:100, 1.2:100, or 1.5:100. A mass ratio of sodium alginate to water within the above range is beneficial for the formation of the sodium alginate-calcium shell.

[0033] In one embodiment of the present invention, the mixing can be carried out under stirring conditions, the stirring speed can be 600 rpm, and the stirring time can be 10 min.

[0034] After obtaining the blended liquid, the present invention adds the blended liquid dropwise to a calcium salt solution for solidification to obtain sodium alginate hydrogel beads.

[0035] When sodium alginate aqueous solution encounters calcium ions, the calcium ions interact electrostatically with the carboxylate groups on the sodium alginate molecular chains. One calcium ion can simultaneously connect two sodium alginate molecular chains, forming a three-dimensional network structure that is insoluble in water and encapsulates the blended liquid to form gel beads.

[0036] In one embodiment of the present invention, the calcium salt solution may be an aqueous solution of calcium chloride, and the mass concentration of the aqueous solution of calcium chloride may be 1%.

[0037] As one embodiment of the present invention, a peristaltic pump can be used to add the blend to the calcium salt solution dropwise, and the dropping rate can be 20 mL / min; the volume of each drop can be 20~60 μL.

[0038] In one embodiment of the present invention, the curing time can be 1 hour after the addition of the blended liquid; after the curing reaction is completed, the sodium alginate hydrogel beads can be collected by filtration.

[0039] After obtaining sodium alginate hydrogel beads, the present invention coats the sodium alginate hydrogel beads by impregnation in chitosan solution and sodium lignin sulfonate solution respectively, to obtain biomass-based hydrogels loaded with cocrystalline urea.

[0040] In this invention, the mass concentration of the chitosan solution is preferably 0.08~0.12%, more preferably 0.09~0.11%; in an embodiment of this invention, the mass concentration of the chitosan solution is 0.1%. A mass concentration of the chitosan solution within the above range is beneficial for the formation of a chitosan layer.

[0041] In this invention, the mass concentration of the sodium lignosulfonate solution is preferably 0.8-1.2%, more preferably 0.9-1.1%; in an embodiment of this invention, the mass concentration of the sodium lignosulfonate solution is 1%. A mass concentration of the sodium lignosulfonate solution within the above range is beneficial for the formation of a sodium lignosulfonate layer.

[0042] In this invention, the immersion time for each immersion is preferably 20-40 minutes, more preferably 30 minutes. Immersion times within this range are beneficial for forming a coating layer. As one embodiment of the invention, the product can be collected by filtration after each immersion.

[0043] The preparation method provided by this invention is simple, easy to control, and conducive to obtaining products with stable quality and realizing large-scale production.

[0044] The present invention also provides the application of the biomass-based hydrogel loaded with eutectic urea described above in crop growth.

[0045] As one embodiment of the present invention, the application can be to freeze-dry the biomass-based hydrogel loaded with eutectic urea and then mix it with soil; or to freeze-dry the biomass-based hydrogel loaded with eutectic urea, absorb moisture, and then mix it with soil; or to directly mix the biomass-based hydrogel loaded with eutectic urea with soil; the mass ratio of the biomass-based hydrogel loaded with eutectic urea to soil can be 0.5 g / kg.

[0046] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0047] Example 1 A biomass-based hydrogel loaded with cocrystal urea, denoted as (CS+SL) / SA@UDBA, comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystal urea, and water. The cocrystal urea is a cocrystal formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 1.5:100; Both the sodium lignosulfonate layer and the chitosan layer have three layers.

[0048] The preparation method is as follows: Add 0.01 mol of urea (e.g.) Figure 1 As shown), 0.01 mol of 2,6-dimethoxybenzoic acid (DBA) and 20 mL of anhydrous ethanol were stirred at 40 °C and 100 rpm until completely dissolved to obtain a mixed solution; the mixed solution was allowed to stand at room temperature (25 °C) to evaporate and crystallize until the ethanol was completely evaporated to obtain eutectic urea, denoted as UDBA (as shown). Figure 2 (as shown) Mix 1.5g UDBA, 1.5g sodium alginate and 100mL water, and stir at 600rpm for 10min to obtain a blend. The blend was added dropwise to a 1% calcium chloride solution using a peristaltic pump at a rate of 20 mL / min, and allowed to stand for 1 h to obtain sodium alginate hydrogel beads. Sodium alginate hydrogel beads were alternately immersed in 0.1% chitosan solution and 1% sodium lignosulfonate solution, with each solution being immersed 3 times and each immersion time being 30 minutes, to obtain biomass-based hydrogel loaded with cocrystallized urea.

[0049] Example 2 A biomass-based hydrogel loaded with cocrystal urea, denoted as (CS+SL) / SA@UVA, comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystal urea, and water. The cocrystal urea is a cocrystal formed by the self-assembly of urea and vanillic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 1.5:100; Both the sodium lignosulfonate layer and the chitosan layer have three layers.

[0050] The preparation method is as follows: 0.01 mol urea, 0.01 mol vanillic acid (VA), and 20 mL anhydrous ethanol were stirred at 40 °C and 100 rpm until completely dissolved to obtain a mixed solution. This mixed solution was then allowed to stand at room temperature (25 °C) to evaporate and crystallize until the ethanol was completely evaporated, yielding eutectic urea, denoted as UVA (e.g., ). Figure 3 (as shown) Mix 1.5g UVA, 1.5g sodium alginate and 100mL water, and stir at 600rpm for 10min to obtain a blend. The blend was added dropwise to a 1% calcium chloride solution using a peristaltic pump at a rate of 20 mL / min, and allowed to stand for 1 h to obtain sodium alginate hydrogel beads. Sodium alginate hydrogel beads were alternately immersed in 0.1% chitosan solution and 1% sodium lignosulfonate solution, with each solution being immersed 3 times and each immersion time being 30 minutes, to obtain biomass-based hydrogel loaded with cocrystallized urea.

[0051] Example 3 A biomass-based hydrogel loaded with cocrystalline urea comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystalline urea, and water. The cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 0.15:100; Both the sodium lignosulfonate layer and the chitosan layer have three layers.

[0052] The preparation method is the same as in Example 1.

[0053] Example 4 A biomass-based hydrogel loaded with cocrystalline urea comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystalline urea, and water. The cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 0.45:100; Both the sodium lignosulfonate layer and the chitosan layer have three layers.

[0054] The preparation method is the same as in Example 1.

[0055] Example 5 A biomass-based hydrogel loaded with cocrystalline urea comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystalline urea, and water. The cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 0.75:100; Both the sodium lignosulfonate layer and the chitosan layer have three layers.

[0056] The preparation method is the same as in Example 1.

[0057] Example 6 A biomass-based hydrogel loaded with cocrystalline urea comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and a chitosan layer and a sodium lignin sulfonate layer sequentially covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystalline urea, and water. The cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 1.5:100; Both the sodium lignosulfonate layer and the chitosan layer are single layers.

[0058] The preparation method is the same as in Example 1.

[0059] Example 7 A biomass-based hydrogel loaded with cocrystalline urea comprises a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and alternating layers of chitosan and sodium lignin sulfonate covering the sodium alginate-calcium shell. The slurry is composed of sodium alginate, cocrystalline urea, and water. The cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and 2,6-dimethoxybenzoic acid through hydrogen bonding. The mass ratio of eutectic urea to water in the slurry is 1.5:100; Both the sodium lignosulfonate layer and the chitosan layer have 5 layers.

[0060] The preparation method is the same as in Example 1.

[0061] Test Example 1 Urea, UDBA from Example 1, and UVA from Example 2 were observed using a scanning electron microscope (SEM) to obtain SEM images, as shown below. Figures 4-6 As shown.

[0062] contrast Figure 4 and Figure 5 , Figure 6It can be seen that pure urea has a regular rod-shaped crystal form, while the eutectic urea sample has an aggregate structure with uneven particle size and layered stacking, reflecting that the surface characteristics of crystal growth have been significantly changed.

[0063] Test Example 2 X-ray diffraction was used to detect urea, DBA and UDBA in Example 1, and VA and UVA in Example 2, and XRD patterns were obtained, as shown below. Figures 7-8 As shown.

[0064] from Figure 7 and Figure 8 As can be seen, both UVA and UDBA show a completely new set of diffraction peaks (marked by the dashed lines in the figure). The positions and intensities of these peaks are different from the characteristic peaks of the initial urea and the corresponding acid, and they are not a simple superposition of the diffraction patterns of the two, indicating that a new independent crystalline phase has been formed, rather than a physical mixture.

[0065] Test Example 3 A sample containing 1g of urea was encapsulated in a dialysis membrane with a molecular weight cutoff of 3000 and immersed in a beaker containing 250mL of deionized water. The beaker was placed in a constant-temperature shaker for a release test. The shaker was set to 25℃ and 100rpm. At regular intervals, 1mL of the sustained-release solution was taken from the release medium outside the dialysis bag, filtered through a 0.22μm syringe filter, and the urea concentration was determined by HPLC. Simultaneously, 1mL of fresh deionized water was added to the sustained-release system to maintain a constant release volume. The urea samples included UDBA from Example 1, UVA from Example 2, pure urea, and equal masses of urea / sodium alginate (USA). The urea release curve is shown below. Figure 9 As shown. Figure 9 The illustrations in the text are enlarged views of specific parts.

[0066] from Figure 9 It can be seen that the total release time of urea in water is less than 40 minutes, showing obvious burst release characteristics. In contrast, the total release times of UVA and UDBA are 720 minutes and 1200 minutes, respectively. The main reason is that when VA and DBA form a eutectic with urea, the multi-substituted aromatic carboxylic acids provide more and stronger hydrogen bond donor / acceptor sites (-COOH, -OH, -OCH3) and form a denser molecular stack, which significantly improves the lattice stability of the eutectic and thus reduces its solubility in water.

[0067] Test Example 4 The UDBA from Example 1, UVA from Example 2, and pure urea were placed under 75% humidity conditions, and the changes in the samples after 2 days were as follows: Figure 10 As shown; the XRD patterns of UDBA in Example 1 and UVA in Example 2 are as follows. Figure 11 As shown.

[0068] from Figure 10 It can be seen that pure urea undergoes hydrolysis and liquefaction after only 2 days of storage; from Figure 11 It can be seen that the XRD pattern of the eutectic sample did not change after 45 days, indicating that it still maintains the crystalline phase. Therefore, this eutectic urea has excellent moisture resistance in high humidity environments.

[0069] Test Example 5 The biomass-based hydrogels loaded with eutectic urea provided in Examples 1 and 7 were freeze-dried at -30°C for 24 hours, and then immersed in deionized water for 2 hours to absorb water. The samples after water absorption are as follows. Figure 12 As shown, the water absorption rate is as follows Figure 13 As shown; then freeze-dry, absorb water, and repeat this cycle 6 times, with the water absorption rate as shown. Figure 14 As shown in the picture, the actual product is as follows. Figures 15-16 As shown. Figure 12 In the image, the sample on the left is Example 1, and the sample on the right is Example 7. Figure 13 , Figure 14 In the text, (CS+SL)3@SA represents Example 1, and (CS+SL)5@SA represents Example 7.

[0070] from Figure 13 It can be seen that the water absorption rates of the biomass-based hydrogels loaded with eutectic urea provided in Examples 1 and 7 after freeze-drying are 89.26 g / g and 150.39 g / g, respectively. This is mainly because chitosan and sodium lignin sulfonate have more hydrophilic groups, which increases the water absorption performance.

[0071] from Figure 14 It can be seen that after 6 cycles, the water absorption rate of (CS+SL)3@SA remained basically unchanged; while that of (CS+SL)5@SA showed a continuous decrease. This may be attributed to the gradual weakening of the interaction force between the CS and SL layers that are repeatedly deposited during the cycle, resulting in a loose structure after repeated water absorption due to the loose bonding. Figure 15 and Figure 16 It can also be seen that the structure of (CS+SL)5@SA is loose, while the structure of (CS+SL)3@SA remains intact.

[0072] Test Example 6 Sodium alginate hydrogel beads in Examples 1, 3-5, etc. Figure 17 As shown in the figure, from left to right, are sodium alginate hydrogel beads from Examples 3, 4, 5 and 1. The inset in the figure is a schematic diagram of sodium alginate hydrogel beads from Example 1.

[0073] from Figure 17 It can be seen that as the urea eutectic content increases, the transparency of sodium alginate hydrogel beads gradually decreases.

[0074] After freeze-drying the sodium alginate hydrogel beads from Examples 1 and 3-5, they were observed using scanning electron microscopy and nitrogen element distribution. Figures 18-21 As shown.

[0075] contrast Figures 18-21 It can be seen that when the cocrystalline urea loading is 10% and 30% (relative to the mass of sodium alginate), the cocrystalline urea is relatively dispersed in the hydrogel, and its position is marked by dashed lines in the figure. When the loading increases to 50% and 100%, due to the higher content of cocrystalline urea, a more obvious aggregate structure is formed, which is marked by arrows in the figure. As the content of cocrystalline urea increases, the amount of cocrystalline urea that can be observed increases, and the elemental distribution diagram shows that the signal intensity of the N element increases with the increase of the cocrystalline urea content.

[0076] The sodium alginate hydrogel beads from Example 1 were freeze-dried and then absorbed water. This process was repeated 6 times, and the water absorption rate was as follows: Figure 22 As shown. From Figure 22 It can be seen that the water absorption rates of the sodium alginate hydrogel beads in the six tests were 89.26 g / g, 83.25 g / g, 82.25 g / g, 81.81 g / g, 80.27 g / g, and 79.13 g / g, respectively, and remained at a high level overall.

[0077] Test Example 7 Water Deprivation Experiment of Chinese Cabbage: Chinese cabbage plants that were 25 days old, uniformly grown, and showed no signs of damage were selected as test subjects. Three treatments were set up: CK (no fertilizer), (CS+SL) / SA@UDBA in Example 1, and (CS+SL) / SA@UVA in Example 2, with a dosage of 0.5 g / kg soil. After thoroughly irrigating all treatments, water supply was stopped. The growth status of the Chinese cabbage was observed on the 3rd day after water supply was stopped. Figures 23-25 As shown.

[0078] Chinese cabbage is a dicotyledonous plant with a clearly defined stomatal structure, making it easy to observe the dynamic changes of stomata under drought stress. The stomatal state is determined by the curvature of the guard cells: "open" stomata are characterized by distinctly curved guard cells, while "closed" stomata exhibit a nearly parallel cell wall structure. Microscopic observation of leaf epidermal stomata revealed differences in the plant's response to drought under different treatments.

[0079] from Figure 23 It can be seen that after two days of water deprivation, the stomata on the leaves of the control group (CK) were almost completely closed (marked in red in the figure); from Figure 24 and Figure 25 It can be seen that some stomata remained open in the (CS+SL) / SA@UVA and (CS+SL) / SA@UDBA treated plants, and the development of mesophyll tissue was promoted.

[0080] The fresh weight of each type of bok choy was measured, and the results are as follows: Figure 26 As shown. From Figure 26 It can be seen that the plants treated with (CS+SL) / SA@UVA and (CS+SL) / SA@UDBA had higher fresh weights.

[0081] The contents of chlorophyll a, chlorophyll b, and carotenoids in each treatment of pak choy were tested, and the results are as follows: Figure 27 As shown. From Figure 27 It can be seen that the plants treated with (CS+SL) / SA@UVA and (CS+SL) / SA@UDBA had higher contents of chlorophyll a, chlorophyll b and carotenoids.

[0082] Test Example 8 Wheat pot experiment: This experiment included four fertilizer treatments: no fertilizer, traditional urea, (CS+SL) / SA@UDBA (Example 1), and (CS+SL) / SA@UVA (Example 2). Plastic pots with an inner diameter of 10.5 cm and a height of 8.9 cm were used for pot cultivation. The specific procedure was as follows: First, 1 g of each fertilizer treatment was thoroughly mixed with 250 g of soil and placed in the lower half of the pot; then, another 250 g of soil was placed on top to form a double-layer structure. Wheat seeds were first sown in seedling trays. After the seedlings developed two true leaves, healthy seedlings with uniform growth were selected and transplanted into the pots. The experiment was conducted in an artificial climate chamber, with environmental conditions controlled at 27℃ and 60% relative humidity. To maintain suitable soil moisture, 50 mL of deionized water was applied every other day. The cultivation period was 45 days. After the period, plant height, fresh weight, and dry weight were measured. See the attached image for details. Figure 28 As shown in the figure, from left to right, the results are: no fertilizer application, traditional urea, Example 2, and Example 1. Plant height is as follows: Figure 29 As shown.

[0083] from Figure 28 It can be seen that wheat grew better when using the fertilizers from Examples 1 and 2; from Figure 29 As can be seen, the plant heights of the different treatments were 19.925 cm, 25.35 cm, 28.125 cm, and 28.4 cm, respectively. Compared with no fertilization, the (CS+SL) / SA@UVA and (CS+SL) / SA@UDBA treatments increased wheat plant height by 8.20 cm and 8.475 cm, respectively, showing a significant promoting effect compared with urea alone. Therefore, it can be considered that the hydrogel loaded with co-crystalline urea has a better water and fertilizer supply capacity, and this advantage is closely related to its more persistent and stable urea and water release behavior.

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A biomass-based hydrogel loaded with cocrystalline urea, comprising a sodium alginate-calcium shell, a slurry encapsulated within the sodium alginate-calcium shell, and a sodium lignin sulfonate layer and a chitosan layer covering the sodium alginate-calcium shell; wherein the slurry comprises sodium alginate, cocrystalline urea, and water; wherein the cocrystalline urea is a cocrystalline urea formed by the self-assembly of urea and a small molecule organic acid through hydrogen bonding; wherein the small molecule organic acid comprises vanillic acid or 2,6-dimethoxybenzoic acid.

2. The biomass-based hydrogel loaded with eutectic urea according to claim 1, characterized in that, The mass ratio of eutectic urea to water in the slurry is (0.1~10):

100.

3. The biomass-based hydrogel loaded with eutectic urea according to claim 1 or 2, characterized in that, The molar ratio of urea to small molecule organic acid in the eutectic urea is 1:(0.8~1.2).

4. The biomass-based hydrogel loaded with eutectic urea according to claim 1, characterized in that, The sodium lignosulfonate layer and the chitosan layer are independently arranged in 2 to 5 layers, and the sodium lignosulfonate layer and the chitosan layer are alternately arranged.

5. A method for preparing the biomass-based hydrogel loaded with eutectic urea according to any one of claims 1 to 4, characterized in that, Includes the following steps: The co-crystallized urea, sodium alginate, and water were mixed to obtain a blend solution; The blend was added dropwise to a calcium salt solution for solidification to obtain sodium alginate hydrogel beads. The sodium alginate hydrogel beads were coated by impregnation in chitosan solution and sodium lignin sulfonate solution to obtain biomass-based hydrogels loaded with cocrystalline urea.

6. The preparation method according to claim 5, characterized in that, The method for preparing the eutectic urea includes: mixing urea, a small molecule organic acid, and anhydrous ethanol to obtain a mixed solution; and evaporating the mixed solution to crystallize it to obtain eutectic urea.

7. The preparation method according to claim 5, characterized in that, The mass ratio of sodium alginate to water is (0.1~2):

100.

8. The preparation method according to claim 5, characterized in that, The chitosan solution has a mass concentration of 0.08~0.12%; the sodium lignosulfonate solution has a mass concentration of 0.8~1.2%.

9. The preparation method according to claim 5, characterized in that, Each immersion session is independently 20-40 minutes.

10. The application of the biomass-based hydrogel loaded with eutectic urea as described in any one of claims 1 to 4, or the biomass-based hydrogel loaded with eutectic urea prepared by the preparation method described in any one of claims 5 to 9, in crop growth.

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