Composite degradable saline-alkali soil water-retaining agent and preparation method thereof

Abstract

This invention relates to the field of salt-alkali resistant biodegradable water-retaining agent preparation technology, specifically a composite biodegradable saline-alkali soil water-retaining agent and its preparation method. The water-retaining agent has a core-shell structure, with the core being nutrient-loaded biochar particles and the shell being a water-retaining layer covering the core surface. The water-retaining layer is formed by grafting corn cob with poly(acrylic acid-co-acrylamide), and biochar powder is dispersed in the water-retaining layer. The water-retaining layer includes 100 parts acrylic acid, 50 parts acrylamide, 15-25 parts corn cob powder, 10-20 parts biochar powder, 0.6-0.8 parts potassium persulfate, and 0.1-0.3 parts crosslinking agent. The bio-based biodegradable water-retaining agent prepared by this invention has the advantages of salt resistance, water retention, slow nutrient release, high mechanical strength, and synergistic salt reduction. It solves the technical bottlenecks of existing water-retaining agents, such as difficulty in balancing salt resistance and degradation, contradiction between strength and water absorption, asynchronous water retention and salt reduction, and lack of nutrient supply. It is suitable for the improvement of chloride-sulfate type saline-alkali soil in arid areas.

Description

Technical Field

[0001] This invention relates to the field of salt-alkali resistant biodegradable water-retaining agent preparation technology, specifically a composite biodegradable saline-alkali soil water-retaining agent and its preparation method. Background Technology

[0002] Water-retaining agents, also known as superabsorbent polymers (SAPs), are widely used in agricultural drought resistance and saline-alkali land improvement. The Ningxia Yinchuan Plain and the Inner Mongolia Hetao Plain in the middle and upper reaches of the Yellow River are arid chloride-sulfate type saline soils, characterized by high salt content, strong alkalinity, high evaporation, easy water and fertilizer loss, and soil compaction. This places comprehensive demands on water-retaining agents, requiring them to be salt-tolerant, water-retaining, fertilizer-retaining, biodegradable, and capable of improving soil conditions.

[0003] Traditional polyacrylate water-retaining agents have poor salt resistance, and their water absorption rate drops sharply in high-salt environments, making them unsuitable for use in saline-alkali land. At the same time, they are non-degradable, and long-term application can cause soil residues and secondary pollution.

[0004] The literature "Preparation and Water Absorption Properties of Modified Corn Cob Bio-based / Sodium Polyacrylate Superabsorbent Resin" describes the preparation of a modified corn cob bio-based / sodium polyacrylate superabsorbent resin by sequentially modifying corn cobs physically and chemically, adding initiators sodium bisulfite and ammonium persulfate, and crosslinking agent N,N'-methylenebisacrylamide. The resulting superabsorbent resin exhibits excellent water absorption properties, with a maximum absorption rate of 986 g / g in deionized water and 97 g / g in a 0.9% sodium chloride solution. However, it suffers from drawbacks such as low mechanical strength of the water-retaining agent, lack of nutrient supply, and rapid degradation.

[0005] The literature "Preparation and Performance Testing of Biochar-Attapulgite-Polyacrylamide-Humic Acid Composite Water Retention Agent" discloses that the biochar-attapulgite-polyacrylamide-humic acid composite water retention agent has a deionized water absorption rate of 134.82 g / g and a salt water absorption rate of 42.32 g / g. It also discloses that biochar has the effects of improving soil texture, promoting salt transport in the soil, accelerating salt leaching out of the soil, and reducing soil salinity. The use of biochar to prepare water retention agents has a positive effect on preventing soil salinization in arid and semi-arid regions.

[0006] However, this type of water-retaining agent has a low water absorption rate, making it unsuitable for long-term use in arid saline-alkali land. In summary, existing water-retaining agents generally suffer from problems such as difficulty in balancing degradability and strength, inability to simultaneously address water retention and salt tolerance, lack of slow-release nutrients, and asynchronous salt reduction and water retention, thus failing to meet the needs of integrated improvement of saline-alkali land. Therefore, this invention provides a composite biodegradable water-retaining agent for saline-alkali land and its preparation method. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method for preparing a composite biodegradable water-retaining agent for saline-alkali land, comprising the following steps: adding biochar powder to a nutrient loading solution at a solid-liquid ratio of 1:(1-3), vacuum impregnating at −0.08~−0.09 MPa for 0.5-1 h, filtering, mixing the wet material obtained after filtration with bentonite and granulating, then drying at 60-70℃, and sieving to obtain a nutrient-loaded biochar core.

[0008] Deionized water, partially neutralized acrylic acid, acrylamide, corn cob powder, biochar powder, and crosslinking agent were added sequentially to the reactor. The mixture was stirred thoroughly and stirred for 15-30 minutes under a N2 protective atmosphere to obtain the shell reaction solution.

[0009] The amount of acrylamide added is 50% of the mass of acrylic acid, the amount of corn cob powder added is 15%-25% of the mass of acrylic acid, and the amount of biochar powder added is 10%-20% of the mass of acrylic acid.

[0010] Nutrient-loaded biochar cores were added to the shell reaction solution at a mass ratio of 1:(1-2), stirred evenly to obtain a suspension, and then heated to 65-70℃. Under a N2 protective atmosphere, potassium persulfate was added, and the reaction was carried out at a constant temperature for 3 hours with continuous stirring at 300 rpm. A water-retaining shell layer was formed in situ on the surface of the biochar cores. After polymerization was completed, heating was stopped, and the mixture was cooled to room temperature with continuous stirring to obtain a whole gel. The amount of potassium persulfate added was 0.6%-0.8% of the mass of acrylic acid in the shell reaction solution. After the gel was cut into blocks, it was washed with deionized water, dried, crushed, and sieved to obtain water-retaining agent particles.

[0011] Preferably, the biochar is one of corn cob biochar, rice husk biochar, and straw biochar, and the biochar is passed through a 200-mesh sieve to obtain biochar powder, while the corn cob is passed through a 100-mesh sieve to obtain corn cob powder.

[0012] Preferably, the nutrient loading solution comprises 40-60 g / L urea, 20-30 g / L potassium dihydrogen phosphate, 10-20 g / L calcium nitrate, and the remainder is deionized water.

[0013] Preferably, the mass ratio of the wet material to bentonite is 1:(0.2-0.5). After granulation and drying, the material is sieved using 0.5mm and 2mm sieves to obtain nutrient-loaded biochar cores.

[0014] Preferably, the degree of neutralization of the acrylic acid is 70-75%, and the degree of neutralization of the acrylic acid is adjusted by using NaOH solution in an ice-water bath environment.

[0015] Preferably, the crosslinking agent is one of N,N'-methylenebisacrylamide and polyethylene glycol diacrylate, and the amount of crosslinking agent added is 0.1% to 0.3% of the mass of acrylic acid.

[0016] Preferably, the drying conditions are constant temperature drying at 60-80℃.

[0017] Preferably, the total concentration of acrylic acid and acrylamide monomers in the shell reaction solution is 25%-40%.

[0018] The present invention also provides a composite biodegradable water-retaining agent for saline-alkali land, wherein the water-retaining agent has a core-shell structure, the core being a nutrient-loaded biochar particle, and the shell being a water-retaining layer covering the surface of the core. The water-retaining layer is formed by grafting poly(acrylic acid-co-acrylamide) onto corn cobs, and biochar powder is dispersed in the water-retaining layer.

[0019] Preferably, the nutrients loaded on the biochar particles are N, P, K, and Ca, provided by urea, potassium dihydrogen phosphate, and calcium nitrate.

[0020] Preferably, the water-retaining layer comprises the following raw materials in parts by weight: 100 parts of partially neutralized acrylic acid, 50 parts of acrylamide, 15-25 parts of corn cob powder, 10-20 parts of biochar powder, 0.6-0.8 parts of potassium persulfate, and 0.1-0.3 parts of crosslinking agent.

[0021] The present invention has at least one of the following technical effects: First, the water-retaining agent prepared by the present invention is a bio-based biodegradable water-retaining agent, which has the advantages of salt resistance, water retention, slow release of nutrients, high mechanical strength, and synergistic salt reduction; it solves the problems of difficulty in balancing the biodegradability and strength of water-retaining agents, inability to balance water retention and salt resistance, lack of slow release of nutrients, and asynchronous salt reduction and water retention, and is suitable for the improvement of chloride-sulfate type saline-alkali land in arid areas.

[0022] 2. By reducing the network charge density through copolymerization of acrylic acid and acrylamide, and combining it with the adsorption and fixation of salt ions by corn cob biochar, it maintains a high water absorption rate and high water retention rate in both 0.9% brine and mixed brine, making it suitable for high saline-alkali environments.

[0023] Third, the biochar powder dispersed in the shell and corn cob powder form a physical reinforcement network, and the bentonite and biochar composite granulation in the core enables the water-retaining agent to achieve a compressive strength of over 30N, reducing the breakage rate during application and transportation.

[0024] Fourth, the biodegradability of cellulose and hemicellulose in corn cob powder makes the water-retaining agent biodegradable, while the rigid structure of corn cob biochar slows down the overall degradation rate. The two work together to achieve a balance between degradation and mechanical strength.

[0025] Fifth, the core biochar is loaded with nitrogen, phosphorus, potassium and calcium nutrients through vacuum impregnation. The three-dimensional network of the shell forms a slow-release barrier after absorbing water and expanding, realizing the long-term slow release of nutrients and improving the fertility of saline-alkali soil.

[0026] VI. Biochar in the core and shell jointly adsorbs Na from the soil. + SO4 2- Plasma and bentonite-assisted ion exchange reduce soil salinity through multiple pathways, improving the crop growth environment. Detailed Implementation

[0027] The present invention will now be described in detail through specific embodiments. However, these illustrative embodiments are for purposes and uses only to illustrate the invention and do not constitute any limitation on the actual scope of protection of the invention, nor are they intended to limit the scope of protection of the invention to these embodiments. All equivalent transformations or simple substitutions made based on the substantive content of this application should fall within the scope of protection of this application. For parameter ranges not mentioned, intermediate values ​​are selected. Furthermore, for mass percentages or weight percentages not explicitly stated or mentioned, they generally refer to the final concentration after addition.

[0028] The singular forms “for,” “or,” “a,” “any,” and “the” used in this application are intended to include the plural forms unless the context clearly indicates otherwise.

[0029] Example 1

[0030] A method for preparing a composite biodegradable water-retaining agent for saline-alkali land includes the following steps: Corn cob biochar is passed through a 200-mesh sieve and set aside; 50g urea, 25g potassium dihydrogen phosphate, and 15g calcium nitrate are dissolved in 800mL deionized water, stirred evenly, and then diluted to 1L to obtain a nutrient loading solution; 200g corn cob biochar powder is placed in a vacuum device, and then 400mL of nutrient loading solution is added to cover the corn cob biochar; the vacuum pump is started, and the pressure is drawn to -0.08~-0.09MPa and maintained for 30min; vacuum impregnation is performed at -0.08~-0.09MPa for 1h, with stirring during impregnation, allowing the nutrient loading solution to penetrate into the pores of the corn cob biochar under negative pressure; after slowly restoring to normal pressure, the wet material is obtained by vacuum filtration.

[0031] Mix 100g of wet material with 40g of bentonite evenly, then put the mixture into a disc granulator for granulation. Bentonite acts as a binder, making the granules easier to form. Stop granulation when the particle size reaches the range of 0.5-2.0mm. Place the wet granules in an oven and dry them at 60-70℃ to constant weight. Sieve them using 0.5mm and 2mm sieves, discarding the fine powder under the sieve and the large particles on the sieve, and collect the particles with a diameter of 0.5-2mm to obtain the nutrient-loaded biochar core.

[0032] Nutrients are loaded into the pores of corn cob biochar through vacuum impregnation, enabling the slow release of nitrogen, phosphorus, potassium, and calcium. Furthermore, the porous structure of the corn cob biochar powder itself can adsorb soluble salt ions from the soil, reducing soil salinity. Bentonite, with its layered structure and high expansibility, synergistically enhances particle strength with corn cob biochar, reducing breakage during application. Simultaneously, it adsorbs some sodium ions through ion exchange. + It helps reduce soil salinity.

[0033] Place 100g of acrylic acid in a beaker and slowly add a 20% NaOH solution while stirring to adjust the neutralization degree of the acrylic acid to 70%. The amount of NaOH solution used is 194g. During the neutralization process, the system temperature is strictly controlled below 40℃ to prevent local overheating from causing the acrylic acid to self-polymerize.

[0034] Add 165g of deionized water, neutralized acrylic acid, 50g of acrylamide pre-dissolved in deionized water, 20g of corn cob powder, 15g of corn cob biochar powder, and 0.2g of N,N'-methylenebisacrylamide sequentially to a three-necked reaction flask while stirring. Then, continuously purify high-purity N2 into the reaction flask and continue stirring for 20 minutes under a N2 protective atmosphere. Remove dissolved oxygen from the reaction flask to obtain a shell reaction solution with a total mass of 574.2g.

[0035] The solution uses 30g of deionized water to dissolve the acrylamide, ensuring complete dissolution. A 20% NaOH solution provides 155g of deionized water, for a total of 350g of deionized water.

[0036] The cellulose molecular chains in corn cob powder contain a large number of hydroxyl groups. Under the initiation of potassium persulfate, the hydroxyl groups generate free radicals, which initiate the graft polymerization of acrylic acid and acrylamide monomers on the cellulose backbone. At the same time, the crosslinking agent N,N′-methylenebisacrylamide participates in the polymerization to form a three-dimensional network structure.

[0037] Corn cob biochar powder dispersed in the shell reaction solution serves as a structural filler, potentially enhancing the network structure through physical interactions such as hydrogen bonding, increasing the network's cross-linking density and resistance to deformation. Simultaneously, the porous structure of corn cob biochar provides additional migration channels for water molecules and helps disperse the volume expansion stress during water absorption, improving particle mechanical strength, providing additional migration paths for water molecules, and assisting in salt adsorption. The synergistic effect of internal and external biochar powders enhances water retention, salt tolerance, and salt reduction.

[0038] Acrylamide, a nonionic monomer, reduces the proportion of sodium carboxylate ions in the polymer network after copolymerization with acrylic acid, thus mitigating the destructive effect of salt ions on the osmotic pressure within the network. Simultaneously, corn cob biochar fixes some sodium ions through electrostatic adsorption and ion exchange. +SO4 2- Plasma reduces the impact of salt on the water absorption network.

[0039] 100g of nutrient-loaded biochar core was added to 200g of shell reaction solution and stirred evenly to suspend the nutrient-loaded biochar core in the shell reaction solution, resulting in a suspension. The suspension was heated to 65℃, and 0.24g of potassium persulfate was added under a N2 protective atmosphere. The mixture was stirred continuously at 300rpm for 3 hours. During the polymerization process, acrylic acid, acrylamide and corn cob powder underwent a graft copolymerization reaction, forming a three-dimensional network structure water-retaining shell layer in situ on the surface of the biochar core. After polymerization was completed, heating was stopped, and the mixture was cooled to room temperature with continuous stirring to obtain a whole gel.

[0040] The core surface of corn cob biochar is rich in polar groups such as hydroxyl and carboxyl groups, which can form hydrogen bonds and physical entanglements with acrylic acid, acrylamide monomers, and corn cob powder in the shell reaction solution, causing the reaction system to preferentially accumulate on the core surface. Under nitrogen protection and stirring at 300 rpm at 65℃, potassium persulfate decomposes to generate sulfate free radicals, which open the carbon-carbon double bonds of acrylic acid and acrylamide, initiating monomer polymerization. At the same time, it activates the hydroxyl groups on the cellulose of corn cob powder to form macromolecular free radicals, which in turn initiate the graft copolymerization of acrylic acid and acrylamide monomers on the corn cob backbone. Under the action of the crosslinking agent N,N′-methylenebisacrylamide, a three-dimensional hydrogel network is formed. The reaction system polymerizes, crosslinks, and forms a shell in situ on the surface of the corn cob biochar core, rather than forming a gel independently in solution, ultimately forming a stable and completely coated core-shell structure water-retaining agent.

[0041] After removing the whole gel, cut it into small pieces, soak it in deionized water and wash it repeatedly until the washing solution is clear and transparent to remove unreacted monomers. Dry the washed gel pieces at a constant temperature of 70°C to a constant weight, then crush them with a pulverizer and sieve them with 0.5 mm and 2 mm sieves to collect particles with a particle size range of 0.5-2 mm, which are the water-retaining agent particles.

[0042] The water-retaining agent granules are biochar granules loaded with nitrogen, phosphorus, potassium and calcium nutrients inside, and are completely wrapped by a corn cob grafted poly(acrylic acid-co-acrylamide) polymer layer on the outside, forming an integrated granular structure with a stable inner core and a water-retaining outer layer; the inner and outer layers are tightly bonded by physical entanglement and interfacial interaction, and are not easy to delaminate or fall off during use.

[0043] Example 2

[0044] The difference from Example 1 is that the amount of bentonite added is 20g.

[0045] Example 3

[0046] The difference from Example 1 is that the amount of bentonite added is 50g.

[0047] Example 4

[0048] The difference from Example 1 is that the amount of biochar powder added to the shell reaction solution is 10% of the mass of acrylic acid.

[0049] Example 5

[0050] The difference from Example 1 is that the amount of biochar powder added to the shell reaction solution is 20% of the mass of acrylic acid.

[0051] Example 6

[0052] The difference from Example 1 is that the amount of corn cob powder added to the shell reaction solution is 15% of the mass of acrylic acid.

[0053] Example 7

[0054] The difference from Example 1 is that the amount of corn cob powder added to the shell reaction solution is 25% of the mass of acrylic acid.

[0055] Comparative Example 1

[0056] The difference from Example 1 is that no corn cob powder is added to the shell reaction solution.

[0057] Comparative Example 2

[0058] The difference from Example 1 is that no corn cob biochar powder is added to the shell reaction solution.

[0059] Comparative Example 3

[0060] The difference from Example 1 is that no corn cob biochar powder or corn cob powder is added to the shell reaction solution.

[0061] Comparative Example 4

[0062] The difference from Example 1 is that the corn cob biochar does not carry nutrients.

[0063] Comparative Example 5

[0064] The difference from Example 1 is that no corn cob biochar powder or corn cob powder is added to the shell reaction solution, and the corn cob biochar is not granulated with bentonite, but directly screened for corn cob biochar with a particle size range of 0.5-2mm.

[0065] Performance tests of the water-retaining agents prepared in Examples 1-7 and Comparative Examples 1-5.

[0066] The compressive strength of the prepared water-retaining agent was tested: according to GB / T44750-2024 "Measurement of Compressive Strength of Particles", 30 water-retaining agent particles of similar size with a particle size range error within 10% were selected. The water-retaining agent particles were placed individually in the center of the sample stage, and pressure was applied at a uniform speed. The pressure-deformation curve was recorded in real time. After the pressure reached the peak value, it dropped back to 50% of the peak value, and the deformation did not reach 20%. The pressure was stopped, and the peak value was recorded. Then the sample stage was cleaned, and the next water-retaining agent particle was replaced. The measurement was repeated, and the compressive strength of each water-retaining agent particle was recorded. The average compressive strength of the 30 water-retaining agent particles was calculated.

[0067] The water absorption performance of the prepared water-retaining agent was tested: 1g of water-retaining agent granules were weighed into a 500mL beaker, 500mL of distilled water was added, and the mixture was allowed to stand for 24 hours until it became saturated with water. The mixture was then filtered through a sieve for 3 minutes, and the weight of the water-retaining agent was measured. The liquid absorption ratio was calculated using the formula: Liquid absorption ratio = (Weight of water-retaining agent after water absorption - 1) / 1.

[0068] The water retention performance of the prepared water-retaining agent was tested: 5g of water-retaining agent granules after absorbing water were placed in a petri dish with an equal amount of deionized water and left open to the air. The dish was weighed every 12 hours until a constant weight was reached, which indicated that the water-retaining agent had completely lost water. The gel water retention rate was calculated according to the following formula: gel water retention rate on day n = mass of water-retaining agent on day n / 5g × 100%.

[0069] The brine absorption and brine retention properties of the prepared water-retaining agent were tested to characterize its salt and alkali resistance. The brine absorption ratio and water absorption ratio were tested in the same way, except that the distilled water was replaced with a 0.9% sodium chloride solution. Then, the water-retaining agent saturated with brine was tested for its water retention performance.

[0070] In addition, to simulate the target chloride-sulfate type saline soil environment, a test was added on the liquid absorption and water retention performance of a 0.9% chloride-sulfate mixed brine. The mixed brine consisted of 4.5 g / L sodium chloride and 4.5 g / L sodium sulfate, and the test method was the same as that for the 0.9% sodium chloride solution.

[0071] The degradation performance of the prepared water-retaining agent was tested: 10g of the water-retaining agent was buried in natural soil, and after 90 days, the mass of the water-retaining agent was measured and the mass loss rate was calculated. The higher the mass loss rate, the faster the degradation. The test results are shown in Table 1 below.

[0072] Table 1. Performance parameters of the water-retaining agents prepared in Examples 1-7 and Comparative Examples 1-5.

[0073]

[0074] As can be seen from Table 1, Example 5 has the highest compressive strength, indicating that adding an appropriate amount of biochar shell to the shell significantly improves mechanical strength. Comparative Example 3 and Comparative Example 5 both have low compressive strength, indicating that not adding filler to the shell and not granulating the core have a significant impact on mechanical strength.

[0075] Comparative Example 5 showed the highest water absorption ratio, but the water absorption ratio decreased significantly in brine and mixed brine, indicating that the pure polymer material has poor salt resistance and is not suitable for saline-alkali land. Example 1 showed water absorption ratios of 78.1 g / g and 72.3 g / g in brine and mixed brine, respectively, which were better than all comparative examples. This indicates that adding corn cob biochar and bentonite particles to the shell layer can significantly improve the salt resistance of the water-retaining agent.

[0076] In a saline environment, the water retention rate of Example 1 was 54.2%, and that of Comparative Example 5 was 44.2%, indicating that biochar and corn cob powder can delay the release of water in a saline environment, and the nutrients loaded in the biochar will also be released slowly.

[0077] The degradation of the water-retaining agent mainly comes from the biodegradation of corn cob cellulose and polyacrylamide segments in the shell. Due to its highly aromatized structure, corn cob biochar is extremely difficult to degrade in soil. Its slow degradation characteristics can act as a physical barrier and regulate the overall degradation rate. The 90-day degradation rate reflects the initial degradation behavior. Example 5 has the lowest degradation rate, while Comparative Example 5 has the highest degradation rate. This is because the increased biochar content regulates the degradation rate of the water-retaining agent.

[0078] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0079] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. A method for preparing a composite biodegradable water-retaining agent for saline-alkali land, characterized in that: Includes the following steps: Biochar powder was added to nutrient loading liquid at a solid-liquid ratio of 1:(1-3), impregnated under vacuum for 0.5-1h, filtered, and the wet material obtained after filtration was mixed with bentonite and granulated. Then it was dried at 60-70℃ and sieved to obtain nutrient-loaded biochar core. The nutrient loading solution includes 40-60 g / L of urea, 20-30 g / L of potassium dihydrogen phosphate, 10-20 g / L of calcium nitrate, and the remainder is deionized water; The mass ratio of the wet material to bentonite is 1:(0.2-0.5). After granulation and drying, the material is sieved using 0.5mm and 2mm sieves to obtain nutrient-loaded biochar cores. Deionized water, partially neutralized acrylic acid, acrylamide, corn cob powder, biochar powder, and crosslinking agent were added sequentially to the reactor. The mixture was stirred thoroughly and stirred for 15-30 minutes under a N2 protective atmosphere to obtain the shell reaction solution. The amount of acrylamide added is 50% of the mass of acrylic acid, the amount of corn cob powder added is 15%-25% of the mass of acrylic acid, and the amount of biochar powder added is 10%-20% of the mass of acrylic acid. Nutrient-loaded biochar cores were added to the shell reaction solution at a mass ratio of 1:(1-2), stirred until homogeneous, and a suspension was obtained. The solution was then heated to 65-70℃, and potassium persulfate was added under a N2 protective atmosphere. The mixture was stirred continuously at 300 rpm for 3 hours, during which acrylic acid, acrylamide, and corn cob powder underwent a graft copolymerization reaction, forming a water-retaining shell layer in situ on the surface of the biochar core. After polymerization, heating was stopped, and the mixture was cooled to room temperature with continuous stirring to obtain a solid gel. The amount of potassium persulfate added was 0.6%-0.8% of the mass of acrylic acid in the shell reaction solution. After the gel is cut into blocks, it is washed with deionized water, dried, crushed, and sieved to obtain water-retaining agent particles.

2. The preparation method of a composite biodegradable water-retaining agent for saline-alkali land according to claim 1, characterized in that: Biochar is one of corn cob biochar, rice husk biochar, or straw biochar. The biochar is passed through a 200-mesh sieve to obtain biochar powder, and the corn cob is passed through a 100-mesh sieve to obtain corn cob powder.

3. The preparation method of a composite biodegradable water-retaining agent for saline-alkali land according to claim 1, characterized in that: The degree of neutralization of the acrylic acid is 70-75%, and the degree of neutralization of the acrylic acid is adjusted by using NaOH solution in an ice-water bath environment.

4. The preparation method of a composite biodegradable water-retaining agent for saline-alkali land according to claim 1, characterized in that: The crosslinking agent is one of N,N'-methylenebisacrylamide and polyethylene glycol diacrylate, and the amount of crosslinking agent added is 0.1%-0.3% of the mass of acrylic acid.

5. The preparation method of a composite biodegradable water-retaining agent for saline-alkali land according to claim 1, characterized in that: In the shell reaction solution, the total concentration of acrylic acid and acrylamide monomers is 25%-40%.

6. A composite biodegradable water-retaining agent for saline-alkali land, prepared by the method described in claim 1, characterized in that, The water-retaining agent has a core-shell structure, with the core being a nutrient-loaded biochar particle and the shell being a water-retaining layer covering the surface of the core. The water-retaining layer is formed by grafting corn cob poly(acrylic acid-co-acrylamide), and biochar powder is dispersed in the water-retaining layer.

7. A composite biodegradable water-retaining agent for saline-alkali land according to claim 6, characterized in that: The nutrients loaded on the biochar particles are N, P, K, and Ca, provided by urea, potassium dihydrogen phosphate, and calcium nitrate.

8. A composite biodegradable water-retaining agent for saline-alkali land according to claim 6, characterized in that: The water-retaining layer comprises the following raw materials in parts by weight: 100 parts of partially neutralized acrylic acid, 50 parts of acrylamide, 15-25 parts of corn cob powder, 10-20 parts of biochar powder, 0.6-0.8 parts of potassium persulfate, and 0.1-0.3 parts of crosslinking agent.

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