Method for improving microstructure of red soil by in-situ crosslinking of alginate-based hydrogel

Abstract

The invention relates to the technical field of red soil improvement, in particular to a method for improving a red soil microstructure through alginate-based hydrogel in-situ crosslinking. Comprising the following steps: dissolving sodium alginate in a solvent to form a sodium alginate solution, adding polyvinyl alcohol into the sodium alginate solution, and stirring for the first time to obtain a mixed solution; defoaming the mixed solution to obtain a treated mixed solution; spraying the treated mixed solution to the surface of the pretreated red soil, and stirring for the second time to obtain a red soil-solution system; and standing the system, and spraying a calcium chloride solution at room temperature for in-situ crosslinking to form a three-dimensional porous gel network so as to improve the red soil microstructure. The method can improve the microstructure of the red soil, improves the water holding capacity, the water retention capacity and the structural stability, is simple in process and environment-friendly, and is suitable for improving the red soil.

Description

Technical Field

[0001] This invention relates to the field of red soil improvement technology, specifically to a method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels. Background Technology

[0002] Red soil, a typical zonal soil widely distributed in the tropical and subtropical regions of southern my country, has microstructural characteristics closely related to the long-term interaction between mineral composition and environmental conditions. Kaolinite (a 1:1 type layered silicate) and hematite (secondary iron oxides such as goethite and hematite) constitute the core components of red soil clay particles. Kaolinite crystals are connected only by weak van der Waals forces, and their surface is rich in polar hydroxyl groups, easily adsorbing water molecules or cations. Hematite, on the other hand, aggregates as nano-sized fine particles through hydroxyl bridging or electrostatic interactions. Together, these factors result in a high proportion of clay particles in red soil, with particles tightly packed together by surface charges and van der Waals forces, forming a "dense and poorly connected" porous system. During the rainy season, this structure strongly hinders water infiltration through tiny pores, exacerbating surface runoff and soil erosion. During the dry season, water evaporation causes clay particle shrinkage, gradually forming a network of cracks on the surface, disrupting structural continuity and reducing water retention capacity.

[0003] Traditional soil improvement methods, such as applying lime to adjust pH and increasing organic fertilizer application, can improve fertility in the short term, but they are difficult to control the pore distribution and mechanical properties at the microscopic level. Lime mainly affects the chemical properties of the surface layer and has limited effect on the deep, dense structure. Organic matter introduced by organic fertilizer is easily decomposed by microorganisms, making it difficult to form a lasting and stable structure. More importantly, although the cementing effect of iron and aluminum oxides in red soil can promote the formation of micro-aggregates, its cementing strength fluctuates drastically with the wet-dry cycle. When wet, water dissolves the cementing bonds, and expansive clay minerals absorb water and swell, leading to aggregate breakage. When dry, cementing materials dehydrate and shrink, and clay crystal layers recombine, promoting partial reorganization. This dynamic imbalance of "breakage-reorganization" makes it difficult for red soil to form a lasting and stable macroscopic structure. Ultimately, red soil, due to its unbalanced pore distribution (coexistence of water retention and moisture loss), poor aggregate stability (weak resistance to erosion), and high sensitivity to wet-dry cycles (repeated structural fluctuations), has become the core challenge for soil remediation in southern China. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels. This invention achieves targeted reshaping of the pore network and aggregate structure of red soil through the dynamic interaction between alginate-based hydrogels and the soil microenvironment, providing an innovative approach to overcoming the traditional technical bottlenecks in red soil improvement.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels, comprising the following steps: S1. Dissolve sodium alginate in a solvent to form a sodium alginate solution. Add polyvinyl alcohol to the sodium alginate solution and stir until homogeneous to obtain a mixed solution.

[0006] S2. Defoaming treatment is performed on the mixed solution to obtain the treated mixed solution.

[0007] S3. Spray the treated mixed solution onto the surface of the pretreated red soil, and stir a second time to obtain the red soil-solution system.

[0008] S4. After the red soil-solution system is allowed to stand, calcium chloride solution is sprayed at room temperature to carry out in-situ cross-linking, forming a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

[0009] In a preferred embodiment of the present invention, the sodium alginate solution has a concentration of 1 wt.% to 5 wt.% and is in water as the solvent; the mass ratio of sodium alginate to polyvinyl alcohol in the mixed solution is 1:1.

[0010] In a preferred embodiment of the present invention, the temperature during the stirring process is 95℃~100℃, the time is 30min~60min, and the rotation speed is 300rpm~500rpm.

[0011] In a preferred embodiment of the present invention, the defoaming treatment involves first removing air bubbles from the mixed solution under a vacuum of -0.04 MPa to -0.05 MPa, and then removing the solution under a vacuum of -0.07 MPa to -0.08 MPa until the solution is clear and free of air bubbles.

[0012] In a preferred embodiment of the present invention, the red soil pretreatment method is as follows: the red soil is dried and then crushed and passed through a 2mm sieve to obtain pretreated red soil.

[0013] In a preferred embodiment of the present invention, the spraying amount of the treated mixed solution is 8 wt.% to 12 wt.% of the red soil mass.

[0014] In a preferred embodiment of the present invention, the second stirring is performed at 60 rpm to 100 rpm for 5 to 10 minutes. In a preferred embodiment of the present invention, the spraying amount of calcium chloride solution is 0.5 wt.% to 1 wt.% of the red soil mass, and the concentration of calcium chloride solution is 0.4 wt.% to 0.6 wt.%.

[0015] In a preferred embodiment of the present invention, when spraying calcium chloride solution, the calcium chloride solution is divided into three portions, and each portion is sprayed at a 2-minute interval.

[0016] In a preferred embodiment of the present invention, the crosslinking time is 10 min to 20 min.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention provides a method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels. Centered on a composite system of sodium alginate and polyvinyl alcohol, it innovatively proposes a "soil-triggered, dynamic adaptation" technical route. This invention deeply integrates the hydrogel synthesis process with the needs of red soil microstructure repair. Through the penetration and crosslinking triggering of liquid precursors in the pores of red soil, a three-dimensional network tightly bound to red soil particles is constructed. Simultaneously, the modification effect of red soil's own components (such as iron and aluminum oxides and humic acid) on the gel network is cleverly utilized to achieve dynamic adaptation of material properties to environmental conditions, precisely meeting the microscopic requirements of red soil improvement.

[0018] 2. The in-situ crosslinking technology employed in this invention exhibits multiple advantages over traditional methods. Firstly, in terms of environmental friendliness, the entire process utilizes an aqueous reaction system, leaving no organic solvent residue and avoiding environmental pollution. Furthermore, both sodium alginate (SA) and polyvinyl alcohol (PVA) can be slowly degraded by red soil microorganisms, and their degradation rate can be controlled by the crosslinking density, achieving a dynamic match between the material's functional lifecycle and crop growth needs. Secondly, regarding process adaptability, this invention requires no complex equipment; conventional irrigation equipment can complete solution application and crosslinking triggering, significantly lowering the application threshold. It is particularly suitable for large-scale farmland or resource-constrained areas, breaking through the limitations of traditional hydrogels that require "pre-forming and then applying," making material synthesis an integral part of red soil microstructure optimization.

[0019] 3. In terms of material selection, the carboxyl functional groups of SA endow it with a strong binding ability with the hydroxyl groups and iron and aluminum oxides on the surface of red soil clay particles, while the long-chain molecules of PVA form a mechanical support framework through physical entanglement and crystalline regions. The synergistic effect of the two can effectively resist stress changes caused by the drying shrinkage and wetting expansion of red soil. After the liquid SA-PVA mixed solution is injected into the red soil, it penetrates along the original pore and crack network of the red soil, wetting the surface of clay particles and filling micropores. Subsequently, the exogenously introduced calcium ion solution triggers the cross-linking reaction of SA molecular chains, forming dense cross-linking points in the gaps between clay particles and on the particle surface, like "molecular sutures" weaving the dispersed clay particles into stable aggregates. This process not only repairs the cracks in the red soil, but also reduces the exposure of iron and aluminum oxide cementitious bodies through the encapsulation of clay particles by the gel network, inhibits structural breakage during drying and wetting cycles, and significantly improves the stability and fertility of red soil. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the method described in this invention.

[0021] Figure 2 This is a comparison chart of the water holding capacity of Embodiment 1 and Comparative Examples 1 to 4 of the present invention.

[0022] Figure 3This is a comparison chart of the water retention rates of Example 1 and Comparative Examples 1 to 4 of the present invention.

[0023] Figure 4 This is a comparison chart of the dry and wet cycle stability of Embodiment 1 and Comparative Examples 1 to 4 of the present invention.

[0024] Figure 5 This is a comparison chart of the saturated hydraulic conductivity of Embodiment 1 and Comparative Examples 1 to 4 of the present invention.

[0025] Figure 6 This is a comparison chart of the average weight diameter of soil aggregates in Example 1 and Comparative Examples 1 to 4 of the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments and comparative examples of this invention can be purchased from the market or prepared by existing methods.

[0028] Traditional soil improvement methods, such as applying lime to adjust pH and increasing organic fertilizer application, can improve fertility in the short term, but they are difficult to control the pore distribution and mechanical properties at the microscopic level. Lime mainly affects the chemical properties of the surface layer and has limited effect on the deep, dense structure. Organic matter introduced by organic fertilizer is easily decomposed by microorganisms, making it difficult to form a lasting and stable structure. More importantly, although the cementing effect of iron and aluminum oxides in red soil can promote the formation of micro-aggregates, its cementing strength fluctuates drastically with the wet-dry cycle. When wet, water dissolves the cementing bonds, and expansive clay minerals absorb water and swell, leading to aggregate breakage. When dry, cementing materials dehydrate and shrink, and clay crystal layers recombine, promoting partial reorganization. This dynamic imbalance of "breakage-reorganization" makes it difficult for red soil to form a lasting and stable macroscopic structure. Ultimately, red soil, due to its unbalanced pore distribution (coexistence of water retention and moisture loss), poor aggregate stability (weak resistance to erosion), and high sensitivity to wet-dry cycles (repeated structural fluctuations), has become the core challenge for soil remediation in southern China.

[0029] Based on this, the present invention provides a method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels. The method includes the following steps:

[0030] S1. Dissolve sodium alginate in a solvent to form a sodium alginate solution. Add polyvinyl alcohol to the sodium alginate solution and stir until homogeneous to obtain a mixed solution.

[0031] S2. Defoaming treatment is performed on the mixed solution to obtain the treated mixed solution.

[0032] S3. Spray the treated mixed solution onto the surface of the pretreated red soil, and stir a second time to obtain the red soil-solution system.

[0033] S4. After the red soil-solution system is allowed to stand, calcium chloride solution is sprayed at room temperature to carry out in-situ cross-linking, forming a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

[0034] It should be noted that in this invention, sodium alginate (SA) and polyvinyl alcohol (PVA) are added to achieve a phase separation strategy. The purpose of the phase separation strategy is to create a hydrogel with a large pore structure adapted to red soil particles, thereby enabling the hydrogel to better bind with the red soil particles. SA and PVA are two blended polymers with different aggregation tendencies, capable of responding to stimuli and forming phase-separated structures. On one hand, PVA, as a phase separation enhancer, promotes the aggregation of alginate in the region during gelation, forming a dense polymer phase (framework polymer); by increasing polymer aggregation, it enhances the mechanical toughness of the hydrogel, making it more stable in red soil; its long-chain molecules form a mechanical support framework with the crystalline regions through physical entanglement, enhancing the binding ability of the hydrogel with red soil particles, making it more stable in red soil. On the other hand, SA forms a dense phase under the promotion of PVA, serving as a framework polymer and providing the structural basis for the hydrogel. The carboxyl functional groups of SA endow it with a strong binding ability to hydroxyl groups and iron-aluminum oxides on the surface of red soil clay particles, enabling it to better bind with red soil particles, enhancing the interaction between the hydrogel and the red soil, and improving the stability and functionality of the hydrogel in red soil. SA has low aggregation and is dispersed within the dense phase formed by PVA, occupying more space and enhancing phase separation, forming a macroporous structure that is beneficial for the transport of water and nutrients in red soil. Furthermore, SA is a natural polysaccharide polymer with good biocompatibility and biodegradability, making it environmentally friendly. Therefore, by introducing PVA as a phase separation enhancer, the SA-PVA composite system not only solves the structural and mechanical defects of single polymer gels but also achieves multiple improvement goals of "pore repair, stress resistance, and interface strengthening" through the dynamic adaptation of the phase separation-induced macroporous network to the red soil microenvironment, providing key technical support for the long-term stability of the red soil microstructure.

[0035] Calcium chloride, as a cross-linking agent, plays a crucial role in triggering the chemical cross-linking of SA and synergistically enhancing the mechanical properties of PVA. The principle behind triggering the chemical cross-linking of SA is the dissociation of CaCl2 into Ca... 2+It binds to the carboxyl groups on the SA molecular chain through electrostatic interactions, forming an "egg-box" structure, crosslinking the SA molecular chain into a three-dimensional network structure. The synergistic enhancement of mechanical properties by PVA refers to Ca... 2+ By adsorbing around the hydroxyl groups of PVA molecular chains through electrostatic interactions, it promotes the formation of hydrogen bonds between PVA molecular chains and the growth of crystalline regions, enhances the physical entanglement network of PVA, and improves the compressive strength of the composite gel.

[0036] In some embodiments, the sodium alginate solution has a concentration of 0.1 wt.% to 0.5 wt.% and is in water as the solvent; in the mixed solution, the mass ratio of sodium alginate to polyvinyl alcohol is 1:1.

[0037] In some embodiments, the temperature during stirring is 95℃~100℃, the time is 30min~60min, and the rotation speed is 300rpm~500rpm. The constant temperature and stirring time need to be strictly controlled to avoid excessive degradation of polymer chains due to high temperatures or localized micelle residue caused by insufficient stirring.

[0038] In some embodiments, the defoaming process involves first removing air bubbles from the mixed solution under a vacuum of -0.04 MPa to -0.05 MPa, and then maintaining a vacuum of -0.07 MPa to -0.08 MPa until the solution becomes clear and free of air bubbles. In one specific embodiment, a gradient defoaming process is employed. Under vacuum conditions, the mixed solution is first slowly defoamed at a low pressure of -0.05 MPa, and then the vacuum is increased to -0.08 MPa using a vacuum pump until the solution becomes clear and free of air bubbles. This process effectively eliminates the problem of poor interfacial contact caused by air bubbles blocking the solution during its penetration into the red soil.

[0039] In some embodiments, the second stirring is performed at 60 rpm to 100 rpm for 5 to 10 minutes. The purpose of stirring is to ensure that the solution fully wets the red soil particles and penetrates into the micropores, while avoiding mechanical shearing that could damage the original aggregate structure of the red soil.

[0040] In some embodiments, when spraying calcium chloride solution, the solution is divided into three portions, with a 2-minute interval between each portion to allow for full penetration. The spraying process is divided into three stages. The first spray triggers rapid cross-linking of the surface layer to form a dense gel film to prevent moisture evaporation. Subsequent sprays induce deeper, gradual cross-linking through ion penetration, ultimately forming a three-dimensional porous network of hydrogel inside the red soil.

[0041] In some embodiments, the amount of calcium chloride solution sprayed is 0.5 wt.% to 1 wt.% of the red soil mass, and the concentration of the calcium chloride solution is 0.4 wt.% to 0.6 wt.%.

[0042] In some embodiments, the crosslinking time is 10 min to 20 min, the purpose of which is to allow the liquid precursor to complete the pore filling under capillary action.

[0043] The following specific examples will provide further explanation.

[0044] In this invention, sodium alginate is abbreviated as SA, and polyvinyl alcohol is abbreviated as PVA.

[0045] Example 1 A method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels includes the following steps: S1. Dissolve SA powder in deionized water at a concentration of 0.5 wt.%, then add PVA particles at a mass ratio of 1:1. Stir continuously at 300 rpm for 30 minutes using a magnetic stirrer under a constant temperature of 95°C to fully dissolve SA and PVA and form a homogeneous, transparent, viscous mixed solution.

[0046] S2. Immediately transfer the mixed solution to a vacuum drying oven for gradient defoaming treatment: initially, slowly remove large air bubbles at a low pressure of -0.05MPa, then increase the vacuum to -0.08MPa and maintain it for 30 minutes until the solution is clear and free of visible air bubbles.

[0047] S3. The treated SA-PVA mixed solution is then mixed with the pretreated red soil. The red soil needs to be naturally air-dried and crushed through a 2mm sieve. The solution is sprayed evenly onto the surface of the red soil at a ratio of 1% of the dry soil mass (i.e., the total amount of SA and PVA added is 10g / kg soil). Then, a low-speed mixing device is used to stir at 60rpm for 5 minutes.

[0048] S4. The mixed red soil-solution system is transferred to a custom mold and left to stand for 10 minutes. After the liquid precursor has filled the pores under capillary action, a 0.5% CaCl2 solution (10% of the red soil mass) is sprayed three times using an atomizing nozzle, with a 2-minute interval between each spray to ensure the diffusion of calcium ions. In-situ cross-linking is carried out during spraying to form a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

[0049] Example 2 A method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels includes the following steps: S1. Dissolve SA powder in deionized water at a concentration of 1 wt.%, then add PVA particles at a mass ratio of 1:1. Stir continuously at 300 rpm for 30 minutes using a magnetic stirrer at a constant temperature of 95°C to fully dissolve SA and PVA and form a homogeneous, transparent, viscous mixed solution.

[0050] S2. Immediately transfer the mixed solution to a vacuum drying oven for gradient defoaming treatment: initially, slowly remove large air bubbles at a low pressure of -0.05MPa, then increase the vacuum to -0.08MPa and maintain it for 30 minutes until the solution is clear and free of visible air bubbles.

[0051] S3. The treated SA-PVA mixed solution is then mixed with the pretreated red soil. The red soil needs to be naturally air-dried and crushed through a 2mm sieve. The solution is sprayed evenly onto the surface of the red soil at a ratio of 1% of the dry soil mass (i.e., the total amount of SA and PVA added is 5g / kg soil). Then, a low-speed mixing device is used to stir at 60rpm for 5 minutes.

[0052] S4. The mixed red soil-solution system is transferred to a custom mold and left to stand for 10 minutes. After the liquid precursor has filled the pores under capillary action, a 0.5% CaCl2 solution (5% of the red soil mass) is sprayed three times using an atomizing nozzle, with a 2-minute interval between each spray to ensure the diffusion of calcium ions. In-situ cross-linking is carried out during spraying to form a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

[0053] Example 3 A method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels includes the following steps: S1. Dissolve SA powder in deionized water at a concentration of 1 wt.%, then add PVA particles at a mass ratio of 1:1. Stir continuously at 300 rpm for 30 minutes using a magnetic stirrer at a constant temperature of 95°C to fully dissolve SA and PVA and form a homogeneous, transparent, viscous mixed solution.

[0054] S2. Immediately transfer the mixed solution to a vacuum drying oven for gradient defoaming treatment: initially, slowly remove large air bubbles at a low pressure of -0.05MPa, then gradually increase the vacuum to -0.08MPa and maintain it for 30 minutes until the solution presents a clear state with no visible air bubbles.

[0055] S3. The treated SA-PVA mixed solution is then mixed with the pretreated red soil. The red soil needs to be naturally air-dried and crushed through a 2mm sieve. The solution is sprayed evenly onto the surface of the red soil at a ratio of 1% of the dry soil mass (i.e., the total amount of SA and PVA added is 10g / kg soil). Then, a low-speed mixing device is used to stir at 60rpm for 5 minutes.

[0056] S4. The mixed red soil-solution system is transferred to a custom mold and left to stand for 10 minutes. After the liquid precursor has filled the pores under capillary action, a 0.5% CaCl2 solution (5% of the red soil mass) is sprayed three times using an atomizing nozzle, with a 2-minute interval between each spray to ensure the diffusion of calcium ions. In-situ cross-linking is carried out during spraying to form a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

[0057] The process flow diagrams of the methods described in Examples 1 to 3 are as follows: Figure 1 As shown.

[0058] Comparative Example 1 The blank control group consisted of red soil with no added materials.

[0059] Comparative Example 2 SA-PVA solution (total addition of SA and PVA is 10g / kg soil).

[0060] Comparative Example 3 SA-PVA gel in a swollen state obtained by CaCl2 crosslinking (total addition of SA and PVA is 10 g / kg).

[0061] Comparative Example 4 SA-PVA gel powder cross-linked with CaCl2 and freeze-dried (total addition of SA and PVA is 10 g / kg).

[0062] Since the methods described in Examples 1 to 3 have similar effects on improving the microstructure of red soil, subsequent characterization experiments will be conducted using Example 1 as a representative.

[0063] To investigate the effectiveness of in-situ crosslinking hydrogels in red soil, this invention conducted a series of systematic characterizations on Example 1 and Comparative Examples 1 to 4. All characterization experiments were performed after five cycles of wet-dry cycling in red soil, with three parallel samples in each test group. The procedures for each characterization experiment are shown below.

[0064] 1. Water holding capacity.

[0065] Each batch of air-dried red soil weighing 1000g was processed. Four plastic beakers with small holes at the bottom were used. The red soil samples from Example 1 and Comparative Examples 1 to 4 were placed in the beakers, and the total weight M0 of the red soil and beakers was measured. 2000mL of water was added, and the total weight M1 was measured after 24 hours. The water-holding capacity HR of the red soil was calculated as HR = (M1 - M0) / M0 * 100%.

[0066] 2. Water retention capacity.

[0067] A dried red soil sample was placed in an aluminum box and its weight was recorded. Water was added to saturation based on the maximum water holding capacity of the red soil in Example 1 and Comparative Examples 1-4 as determined in the water-holding capacity test. The total weight of the aluminum box and saturated soil was weighed, and then transferred to a constant temperature drying oven at 60℃. Weighing was performed every 2 hours. The water retention capacity WR of the red soil was calculated as follows: WR = (W1 - W2) / (W3) * (W4) * (W1 - W2) * ... n ) / (W1-W2)*100%.

[0068] Where WR is the soil water retention rate, in %; W1 is the total weight of saturated soil and aluminum box, in g; W n W1 represents the total weight of the soil sample and aluminum box at a certain time point after evaporation, in g; W2 represents the total weight of the dried soil sample and aluminum box, in g.

[0069] 3. Reusability.

[0070] In determining the water-holding capacity of red soil, repeated water absorption tests were conducted to evaluate the long-term water retention performance and reusability of the hydrogel under cyclic water absorption-dehydration conditions in red soil. For the water absorption rate determination, 10 rounds of wet-dry cycle tests were performed on the red soil samples of Example 1 and Comparative Examples 1 to 4.

[0071] 4. Saturated hydraulic conductivity.

[0072] Take air-dried or oven-dried soil samples for later use. After sieving the red soil to <2mm, pack it into a plastic cylinder (5cm in diameter, 10cm high, with a metal mesh bottom covered with sand) to prepare soil columns. The red soil samples from Example 1 and Comparative Examples 1-4 are then mixed at 1.6g / cm³. 3 The bulk density of the soil was measured and placed in a cylindrical container, which was then covered with filter paper to reduce disturbance to the surface of the red soil. After soaking for 24 hours until saturated, a constant water head of 4 cm was applied to the top of the soil column. The leachate was collected, and the saturated hydraulic conductivity Ks of the red soil was calculated using Darcy's law: Ks = QL / AtH.

[0073] Where Ks is the saturated hydraulic conductivity, Q is the volume of water, L is the length of the soil column, A is the cross-sectional area of ​​the soil sample, t is the time required for the water volume Q to be discharged, and H is the water head.

[0074] 5. Stability of red soil structure.

[0075] The structural stability of the red soil in Example 1 and Comparative Examples 1 to 4 was measured using the dry sieving method (sieve apertures of 5 mm, 2 mm, 0.25 mm and 0.054 mm), the wet sieving method (sieve apertures of 5 mm, 3 mm, 2 mm, 1 mm, 0.5 mm, 0.25 mm and 0.053 mm), and the Le Bissonais method (sieve apertures of 2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.1 mm and 0.05 mm).

[0076] The dry sieving method uses 500g of air-dried soil sample with a diameter <1cm. After passing through electric vibrating sieves with apertures of 5mm, 2mm, 0.25mm and 0.054mm for 30min, the sample is weighed three times to calculate the proportion of agglomerates of each particle size.

[0077] The wet sieving method involves preparing 50g soil samples based on the dry sieving results. After water immersion and shaking through a seven-stage sieve ranging from 5mm to 0.053mm, particles smaller than 0.053mm are collected by sedimentation. The samples are then dried, weighed, and the distribution of water-stable aggregates is determined.

[0078] The Le Bissonnais method was used to select 5g of red soil aggregates after sieving through a 3mm-5mm dry sieve. After drying at 60℃, the aggregates underwent rapid wetting (50ml deionized water, standing for 10min), slow wetting (wetting filter paper at -0.3kPa for 30min), and pre-wetting disturbance (alternating ethanol-water treatment). Finally, the aggregates were sieved through a six-stage ethanol sieve (2mm-0.05mm). After drying at 60℃, the particle size distribution was statistically analyzed by weighing three times, with each treatment repeated three times. The stability of the red soil aggregates was evaluated using the mean weight diameter (MWD). The formula for calculating the mean weight diameter (MWD) is as follows:

[0079] .

[0080] Where, x i The average diameter (mm) of water-stable agglomerates within any particle size range; w i For the corresponding x i The percentage of aggregates.

[0081] Figure 2 This diagram illustrates the water-holding capacity of Embodiment 1 and Comparative Examples 1 to 4 of the present invention. Figure 2 It can be seen that Comparative Example 1 exhibited the lowest water-holding capacity at 40.17%. Adding hydrogels in different states improved soil water-holding capacity to varying degrees. Specifically, the SA-PVA (sodium alginate-polyvinyl alcohol) mixed solution (Comparative Example 2) increased the water-holding capacity to 44.27%, the pre-formed hydrogel (Comparative Example 3) reached 53.93%, and the dehydrated hydrogel powder (Comparative Example 4) reached 47.24%. Notably, the in-situ cross-linked hydrogel (Example 1) showed the most significant improvement, with an average water-holding capacity of 60.19%, which was 49.7% higher than the control group and 11.6% higher than Comparative Example 3.

[0082] Figure 3 This is a comparison chart of the water retention rates of Example 1 and Comparative Examples 1 to 4 of the present invention. Figure 3It was found that the in-situ crosslinked hydrogel (Example 1) exhibited excellent water retention performance, with a final water retention rate of 14.56%, significantly higher than the 9.71% of the pre-formed hydrogel (Comparative Example 3) and the 2.75% of the dehydrated hydrogel powder (Comparative Example 4). Untreated soil (Comparative Example 1) completely lost its water retention capacity after 18 hours. Soil modified with different forms of hydrogel exhibited different behaviors: the SA-PVA mixed solution (Comparative Example 2), due to the lack of crosslinking reaction, had limited water retention performance and completely lost its water retention capacity after 24 hours. The dehydrated hydrogel powder (Comparative Example 4), limited by uneven dispersion and insufficient swelling, retained only 2.75% of its moisture after 24 hours. Although the performance of the pre-formed swollen hydrogel (Comparative Example 3) was initially comparable to that of Example 1 at 8 hours (60.20% vs. 62.36% water retention rate, respectively), its capacity rapidly decreased over time, dropping to less than 70% of the value of Group B by 24 hours.

[0083] Figure 4 This is a comparison chart of the dry and wet cycle stability of Embodiment 1 and Comparative Examples 1 to 4 of the present invention. Figure 4 It was observed that the in-situ crosslinked hydrogel (Example 1) exhibited a self-optimized structural evolution during the adsorption-desorption cycle. After dynamic reorganization, the enhanced molecular chain extensibility of its three-dimensional network promoted interlayer pore expansion, significantly improving water retention capacity. This mechanism drove the water retention capacity to continuously increase from an initial 60.81%, reaching a peak of 65.65% in the fourth cycle, and then gradually decreasing to 53.05% in the tenth cycle, with a cumulative performance loss of only 12.8%—57% lower than the 29.5% degradation observed in the pre-formed hydrogel (Comparative Example 3). The solid hydrogel powder (Comparative Example 4) showed a cumulative decrease of 15.2% (from an initial 47.24% to 40.99% in the tenth cycle), while exhibiting significant performance fluctuations due to pore heterogeneity caused by uneven dispersion. In contrast, the SA-PVA mixed solution (Comparative Example 2) showed linear degradation, with a water retention capacity loss of 24.9%.

[0084] Figure 5 This is a comparison chart of the saturated hydraulic conductivity of Embodiment 1 and Comparative Examples 1 to 4 of the present invention. Figure 5It is known that adding exogenous substances to soil can block soil pores, thereby reducing the saturated hydraulic conductivity of the soil. However, the mechanisms by which adding hydrogels in different states reduces saturated hydraulic conductivity are not entirely the same. SA-PVA mixed solution is a high-viscosity fluid colloid. Its viscosity binds soil particles, increases friction between soil particles, hydrogel, and water, and reduces water infiltration, thereby reducing water loss during the infiltration process. Pre-formed hydrogels (Comparative Example 3) and solid hydrogel powders (Comparative Example 4) mainly reduce water infiltration by blocking soil pores. In-situ cross-linked hydrogels (Example 1) not only bind soil particles but also block soil pores during the cross-linking process, thereby achieving the lowest saturated hydraulic conductivity.

[0085] Figure 6 This is a comparison chart of the average weight diameter of soil aggregates in Example 1 and Comparative Examples 1 to 4 of the present invention. Figure 6 It was found that the soil treated with in-situ cross-linked hydrogel (Example 1) exhibited the best structural stability. After the liquid SA-PVA mixed solution was injected into the soil, it penetrated along the original pore and fracture network of the red soil, wetting the clay particle surface and filling the micropores. Subsequently, the exogenously introduced calcium ion solution triggered the cross-linking reaction of the SA molecular chains, forming dense cross-linking points in the clay particle gaps and on the particle surface, acting like "molecular sutures" to weave the dispersed clay particles into stable aggregates. This process not only repaired the fractures in the red soil but also, through the gel network's encapsulation of the clay particles, exposed the soil aggregates, thereby inhibiting soil structural breakage during dry sieving, wet sieving, and the Le Bissonnais method.

[0086] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended scope of protection is intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of this invention.

[0087] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of protection of this invention and its equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for improving the microstructure of red soil using in-situ crosslinking of alginate-based hydrogels, characterized in that, Includes the following steps: Sodium alginate is dissolved in a solvent to form a sodium alginate solution. Polyvinyl alcohol is added to the sodium alginate solution and stirred evenly for the first time to obtain a mixed solution. The mixed solution is defoamed to obtain the treated mixed solution; The treated mixed solution was sprayed onto the surface of the pretreated red soil, and the mixture was stirred a second time to obtain the red soil-solution system. After the red soil-solution system was allowed to stand, calcium chloride solution was sprayed at room temperature to carry out in-situ cross-linking, forming a three-dimensional porous gel network, thereby improving the microstructure of the red soil.

2. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, In the sodium alginate solution, the concentration of sodium alginate is 0.1 wt.%~0.5 wt.%, and the solvent is water; in the mixed solution, the mass ratio of sodium alginate to polyvinyl alcohol is 1:

1.

3. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The temperature during the stirring process is 95℃~100℃, the time is 30min~60min, and the speed is 300rpm~500rpm.

4. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The defoaming process involves first removing air bubbles from the mixed solution under a vacuum of -0.04 MPa to -0.05 MPa, and then maintaining a vacuum of -0.07 MPa to -0.08 MPa until the solution becomes clear and free of air bubbles.

5. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The method for pretreatment of red soil is as follows: after drying, the red soil is crushed and passed through a 2mm sieve to obtain pretreated red soil.

6. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The spraying amount of the treated mixed solution is 8 wt.% to 12 wt.% of the red soil mass.

7. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The second stirring is carried out at 60 rpm to 100 rpm for 5 to 10 minutes.

8. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The amount of calcium chloride solution sprayed is 0.5 wt.% to 1 wt.% of the red soil mass, and the concentration of the calcium chloride solution is 0.4 wt.% to 0.6 wt.%.

9. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, When spraying calcium chloride solution, divide the solution into three portions, and wait 2 minutes between each portion.

10. The method for improving the microstructure of red soil by in-situ crosslinking of alginate-based hydrogels according to claim 1, characterized in that, The cross-linking time is 10 min to 20 min.

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