Double-network composite hydrogel based on hydrothermal carbon enhancement and preparation method of double-network composite hydrogel

Abstract

The invention discloses a double-network composite hydrogel based on hydrothermal carbon reinforcement and a preparation method thereof, belongs to the technical field of polymer composite materials, and aims to improve the water absorption rate, water retention property, mechanical toughness and thermal stability of the hydrogel. Comprising the following steps: drying and sieving biomass, mixing with pure water, stirring, and carrying out hydrothermal carbonization reaction; after the reaction is finished, taking out, washing and drying to obtain hydrothermal carbon; dissolving sodium alginate in water, adding the hydrothermal carbon after the sodium alginate is dissolved, stirring at high temperature, reacting to obtain a mixture I, purging with nitrogen, standing and cooling; adding acrylamide, N, N '-methylene bisacrylamide and persulfate into the mixture I, and stirring to obtain a mixture II; and carrying out high-temperature reaction on the mixture II to prepare the dual-network composite hydrogel. According to the method, the water absorption rate, the water-retaining property, the mechanical toughness and the thermal stability of the hydrogel are improved, and the requirements of long-term bearing and repeated expansion and shrinkage of soil are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer composite materials, relates to a hydrogel and its preparation, and in particular to a double-network composite hydrogel based on hydrothermal carbon reinforcement and a preparation method thereof. Background Art

[0002] Water shortages and nutrient depletion are becoming increasingly prominent concerns in agriculture, particularly in degraded or disturbed soils with poor water permeability and prone to topsoil loss. Hydrogels are a class of three-dimensional polymer network materials with high water content and the ability to rapidly absorb and release moisture. They have garnered widespread attention in areas such as agricultural water conservation, landscaping, desertification control, and as slow-release fertilizer carriers. Traditional commercial hydrogels are mostly based on petrochemical polymers such as polyacrylates and polyacrylamide. While they have high water absorption rates, they are expensive, have long degradation cycles, and are difficult to recycle. Long-term use can easily lead to secondary hardening of the soil, posing a potential risk to sustainable agricultural development. To overcome these deficiencies, existing technologies propose using natural polysaccharides as the main chain and then introducing a secondary network or inorganic fillers to form composite hydrogels, aiming to achieve a balance between environmental protection, biocompatibility, and mechanical stability. For example, the document "Highly effective removal of basic fuchsin dye using carboxymethyl konjac glucomannangrafted acrylic acid-acrylamide / montmorillonite composite hydrogel" reported that the use of hydrogel-montmorillonite composite materials achieved a maximum dye adsorption capacity of 694.1 mg / g and a removal rate of 99.5%.

[0003] At present, biochar-hydrogel composites have been widely reported. Biochar is widely available and has well-developed pores. When incorporated into hydrogels, it can provide a rigid skeleton, pore templates, and ion adsorption sites, thereby improving the mechanical strength, water retention, and slow-release fertilizer efficiency of the gel. For example, the paper A multifunctional cellulose- and starch-based composite hydrogel with iron-modified biochar particles for enhancing microalgae growth synthesizes a multifunctional hydrogel to enhance algae production by cross-linking activated biochar, carboxymethyl cellulose, and starch using oxidized sucrose as a biocrosslinker and glycerol as a plasticizer; the paper Water retention and sustained release of magnesium-based biochar modified hydrogel composite materials uses an in-situ polymerization method to prepare a magnesium-based biochar-modified hydrogel slow-release fertilizer, which achieves soil moisture retention, reduces irrigation water use, and improves nutrient utilization efficiency. However, existing biochar is typically produced by pyrolysis of dry raw materials at 400–900°C. This method is energy-intensive, requires initial drying, and complex atmosphere control, making it difficult to process high-water-content raw materials. The resulting biochar is highly carbonized, resulting in a passive surface, a reduced number of active hydroxyl / carboxyl groups, and a strong hydrophobicity. This poor interfacial bonding with hydrophilic polymers often requires additional surface oxidation or acid washing for dispersion. Some technologies have also attempted to introduce oxidizing groups onto the carbon surface or utilize ultrasonic dispersion, but the complex procedures and harsh reaction conditions make large-scale, continuous production difficult.

[0004] For wet organic waste, hydrothermal carbonization technology provides a low-energy carbonization pathway with low-temperature, closed aqueous phase reaction. It can directly process high-moisture raw materials and retain more oxygen-containing functional groups. The generated hydrothermal carbon particles have small particle size, good dispersibility, and high surface polarity, making them more suitable as functional fillers for hydrogels. On the other hand, high-moisture by-products such as wine lees and pomace are produced in large quantities during the winemaking and juice processing processes. Traditional storage and incineration treatments are prone to cause secondary pollution. If such wet waste can be directly converted into hydrothermal carbon using hydrothermal carbonization technology and combined with natural / synthetic double-network hydrogels, an integrated cycle of "waste-carbon material-functional gel-soil remediation" can be achieved. However, there is currently no public technology that provides a complete solution for the efficient dispersion, in-situ crosslinking and performance synergy of wine lees hydrothermal carbon in the alginate-acrylamide double network.

[0005] Therefore, the present application provides a method for directly preparing a composite hydrogel using high-water-content biomass and a preparation method thereof, which improves the water absorption, water retention, mechanical toughness, and thermal stability of the hydrogel and provides a new way for the resource utilization of wet organic waste. Summary of the Invention

[0006] The purpose of the present invention is to provide a double-network composite hydrogel reinforced with hydrothermal carbon and a preparation method thereof in order to solve the technical problems of resource utilization of biomass such as wine lees and fruit pomace and improving the water absorption rate, water retention, mechanical toughness and thermal stability of hydrogels.

[0007] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions: A method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon comprises the following steps: Step 1: drying and sieving the biomass, mixing and stirring it with pure water, and placing it in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction; after the reaction is completed, taking it out, washing it, and drying it to obtain hydrothermal carbon; Step 2: dissolving sodium alginate in water, adding the hydrothermal charcoal prepared in Step 1 after the sodium alginate is dissolved, stirring and reacting at high temperature to obtain a mixture I, purging with nitrogen, and standing to cool; Step 3, adding acrylamide, N,N′-methylenebisacrylamide, and persulfate in sequence to the mixture I prepared in step 2, and stirring to obtain a mixture II; Step 4, placing the mixture II obtained in step 3 in a water bath for high-temperature reaction, and obtaining a double-network composite hydrogel after the reaction is completed; The raw material components are calculated by weight as follows: the proportions of biomass, pure water, sodium alginate, water, acrylamide, N,N′-methylenebisacrylamide, and persulfate are: 3~5g: 40~50g: 1~3g: 80-120g: 2~6g: 0.06~0.24g: 0.1~0.3g.

[0008] In the above-mentioned Examples 1 to 5, the synthesis mechanism of the double-network composite hydrogel in the preparation method includes three consecutive processes: "establishment of the alginate skeleton - embedding of hydrothermal carbon into primary cross-linking - locking of acrylamide network", specifically: first, sodium alginate swells in water, and the chain segments stretch to form a soft, water-containing initial three-dimensional network; secondly, well-dispersed hydrothermal carbon is added and heated and stirred, and the hydrothermal carbon particles are evenly embedded in the sodium alginate grid; the abundant -COOH, -OH and a small amount of metal cations on the surface of the hydrothermal carbon undergo hydrogen bonding, coordination or electrostatic adsorption with the -COO⁻ / -OH on the sodium alginate chain to form dense "physical + weak chemical" anchor points; the hydrothermal carbon The rigid backbone separates the polymer chains, preventing complete collapse during drying, thereby preserving interconnected micropores and mesopores and providing space for subsequent polymerization and water storage. Finally, acrylamide monomer and persulfate are added, and free radical polymerization under mild conditions generates polyacrylamide chains, which form a covalently cross-linked network under the action of persulfate. The growing polyacrylamide chains intersperse and entangle within the sodium alginate-hydrochar backbone. Some free radicals terminate on the hydrochar surface or graft onto its surface functional groups, forming an interfacial bond between the polyacrylamide chains and the hydrochar. The sodium alginate and polyacrylamide chains can further couple through hydrogen bonds and ionic interactions, forming an interpenetrating double network conforming to the hydrogel. In the resulting interpenetrating double network, sodium alginate provides flexible hydrophilic groups, hydrochar provides internal support and pore structure, and the polyacrylamide chains provide continuous covalent strength.

[0009] Furthermore, in step 1, the biomass is wine lees or pomace, and the particle size of the biomass is 70-80 mesh.

[0010] Furthermore, in step 1, when the biomass and pure water are mixed and stirred, the ambient temperature is 25-30° C., the stirring time is 10-20 minutes, and the stirring speed is 200-300 rpm.

[0011] Furthermore, in step 1, when the hydrothermal carbonization reaction is carried out in a high-temperature and high-pressure reactor, the reaction pressure is 1-3 MPa, the reaction temperature is 150-220° C., and the reaction time is 6-10 h.

[0012] Furthermore, in step 2, when stirring after adding the hydrothermal charcoal, the stirring environment temperature is 70-80° C., the stirring time is 60-120 min, and the stirring speed is 500-700 rpm.

[0013] Furthermore, in step 2, when nitrogen is used for purging during stirring and when the mixture is allowed to stand and cool, the nitrogen purging time is 10 to 20 minutes, the ambient temperature for nitrogen purging is 25 to 30° C., and the standing and cooling time is 30 to 60 minutes.

[0014] Furthermore, in step 3, when the mixture II is obtained after stirring, the ambient temperature is 25-30° C., the stirring time is 30-40 min, and the stirring speed is 500-700 rpm.

[0015] Furthermore, in step 4, when the high temperature reaction is carried out in a water bath, the reaction temperature is 70-90° C. and the reaction time is 3-5 h.

[0016] The double-network composite hydrogel is prepared by the above-mentioned preparation method of the double-network composite hydrogel enhanced by hydrothermal carbon.

[0017] The beneficial effects of the present invention are as follows: 1. In the present invention, biomass such as wine lees and pomace is used to prepare hydrothermal charcoal, realizing an integrated cycle of "waste-carbon material-functional gel-soil remediation", reducing secondary pollution caused by traditional storage and incineration of biomass such as wine lees and pomace; hydrothermal charcoal is pre-crosslinked with sodium alginate, and then acrylamide polymerization is initiated in situ to form a double network structure; and in this structure, the sodium alginate network provides a flexible skeleton, the hydrothermal charcoal acts as a rigid node, and the acrylamide network is further reinforced by covalent crosslinking with N,N'-methylenebisacrylamide. The synergistic effect of the three makes the composite gel have both high elasticity and high strength, significantly improving the mechanical properties and reusability, and significantly improving the water absorption rate, water retention, mechanical toughness and thermal stability of the hydrogel, meeting the long-term load-bearing and repeated expansion and contraction requirements of the soil.

[0018] 2. In the present invention, a double network structure is formed by pre-crosslinking hydrothermal carbon with sodium alginate and then in-situ initiating acrylamide polymerization. Structure determines function - multi-level pores and hydrophilic functional groups achieve rapid water absorption. The hard particles of hydrothermal carbon limit excessive swelling and improve mechanical strength and cyclic stability. Residual -COO⁻, -CONH2, -OH and other sites adsorb or exchange K⁺ and delay NO3⁻ migration through hydrogen bonding / electrostatic adsorption. The grid can be repeatedly opened and closed during dry-wet cycles. After the particles expand in the slag, they partially block the large-aperture channels, prolong the water residence time and synergistically intercept nutrients, thereby achieving in-situ improvement of the fast-leaking medium.

[0019] 3. The composite hydrogel prepared in this invention exhibits adjustable water absorption and water retention properties. By varying the hydrothermal carbon dosage (0.2–0.8 g), the water absorption rate can be adjusted within a range of 100–330 g / g, and a water retention rate of 30–70% can be maintained within a temperature range of 25–85°C. This allows for maintaining a high water content in drought or high-temperature environments, slowly releasing water to the soil, reducing irrigation frequency, and achieving intelligent humidity control.

[0020] 4. The composite gel produced in this invention effectively retains soil nutrients and slows their loss. Soil column experiments show that at a dosage of 0.25–1.00 wt%, it can reduce the cumulative loss of total nitrogen and total potassium by 28–35% and 30–38%, respectively. The gel pores and the hydroxyl, carboxyl, and amide sites on the hydrothermal carbon surface jointly adsorb or complex nutrient ions, achieving integrated water and nutrient management.

[0021] 5. The present invention has a simple process and can be completed in one pot: sodium alginate is pre-crosslinked with hydrothermal carbon, and then acrylamide, N,N′-methylenebisacrylamide, and ammonium persulfate are added for free radical polymerization, which can be formed in one step without the need for solvent exchange or multi-step drying. This method has a wide range of raw material sources, a mild process, can be produced on a large scale, and has good industrial prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 1 is the XRD pattern of different test examples and comparative examples in the present invention; Figure 2 FTIR spectra of different test examples and comparative examples in the present invention; Figure 3 1 is the SEM images of different test examples and comparative examples in the present invention; Figure 4 2. The thermogravimetric curves of different test examples and comparative examples in the present invention; Figure 5 Schematic diagram of storage modulus changes in different test examples and comparative examples in the present invention; Figure 6 Schematic diagram of water retention capacity of different test examples and comparative examples in the present invention; Figure 7 1 is a water absorption curve diagram of different test examples and comparative examples in the present invention; Figure 8 Schematic diagram of biodegradability of different test examples and comparative examples in the present invention; Figure 9 Schematic diagram of cyclic water absorption effect of different test examples and comparative examples in the present invention; Figure 10 Schematic diagram of the effect of adding different proportions (0-1%) of the hydrogel prepared in Experimental Example 3 on TN retention in slag soil; Figure 11 Schematic diagram of the effect of adding different proportions (0-1%) of the hydrogel prepared in Experimental Example 3 on the retention of TK in slag soil; in, Figure 1-Figure 3 The hydrothermal charcoal in the above is the test result of the hydrothermal charcoal prepared according to step 1 of test example 1-4. Figures 1-8 The comparative example in the table refers to the test results of the hydrogel prepared according to comparative example 1. Figures 1-8Test Example 1 in the text refers to the test results of the hydrogel prepared according to Test Example 1. Figure 1-Figure 7 Test Examples 2-4 refer to the test results of the hydrogels prepared according to Test Examples 2-4: Figure 10-11 The diagram shows the effects of TN and TK retention when the hydrogel (powder) prepared in Experimental Example 3 was prepared into solutions with different mass ratios and then added to the slag. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0024] Therefore, based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of the present invention.

[0025] Example 1 This embodiment provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1: Rinse the collected vinasse with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 4g of vinasse powder and put it into 42ml of pure water, mix and stir it at an ambient temperature of 26°C, stirring for 11 minutes, and stirring at a speed of 210rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction at a reaction pressure of 1MPa, a reaction temperature of 160°C, and a reaction time of 6.5h. After the reaction is completed, take it out, wash it with pure water three times, and dry it at 60°C to obtain hydrothermal carbon.

[0026] Step 2: Dissolve 1 g of sodium alginate in 85 ml of water. After the sodium alginate is dissolved, add 0.1 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 72° C., the stirring time is 70 minutes, and the stirring speed is 520 rpm; and nitrogen is used for purging and cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purge time is 11 minutes, the ambient temperature during the nitrogen purge is 26° C., and the cooling time is 32 minutes.

[0027] Step 3: 2 g acrylamide, 0.08 g N,N′-methylenebisacrylamide, and 0.1 g ammonium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 26° C., the stirring time was 32 minutes, and the stirring speed was 520 rpm.

[0028] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 72° C. and for 3 hours.

[0029] Example 2 This embodiment provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1: Rinse the collected pomace with clean water, dry it at 90°C for 20 hours, and dry it to constant weight; crush the dried pomace with a grinder and filter it through an 80-mesh filter. Take 4g of pomace powder and put it into 45ml of pure water, mix and stir it at an ambient temperature of 28°C, stirring for 16 minutes, and stirring at a speed of 250rpm. Place the solution mixed with the pomace powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction at a reaction pressure of 2MPa, a reaction temperature of 190°C, and a reaction time of 8 hours. After the reaction is completed, remove it, wash it with pure water four times, and dry it at 70°C to obtain hydrothermal carbonization.

[0030] Step 2: Dissolve 2 g of sodium alginate in 100 ml of water. After the sodium alginate is dissolved, add 0.3 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 75° C., the stirring time is 90 minutes, and the stirring speed is 600 rpm; and nitrogen is used for purging during the stirring process and the mixture is allowed to cool (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 27° C., and the cooling time is 45 minutes.

[0031] Step 3: 4 g acrylamide, 0.15 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 28° C., the stirring time was 35 minutes, and the stirring speed was 600 rpm.

[0032] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 4 hours.

[0033] Example 3 This embodiment provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1: Rinse the collected vinasse with clean water, dry it at 70°C for 22 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 5g of vinasse powder and put it into 48ml of pure water, mix and stir it at an ambient temperature of 29°C, stirring for 18 minutes, and stirring at a speed of 290rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction at a reaction pressure of 3MPa, a reaction temperature of 210°C, and a reaction time of 9 hours. After the reaction is completed, remove it, wash it with pure water three times, and dry it at 70°C to obtain hydrothermal carbon.

[0034] Step 2: Dissolve 3 g of sodium alginate in 110 ml of water. After the sodium alginate is dissolved, add 0.5 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I. The stirring environment temperature is 78° C., the stirring time is 110 minutes, and the stirring speed is 670 rpm. During the stirring process, nitrogen is purged and allowed to cool (i.e., stirring and purging are performed simultaneously). The nitrogen purge time is 19 minutes, the ambient temperature during the nitrogen purge is 29° C., and the cooling time is 56 minutes.

[0035] Step 3: 6 g acrylamide, 0.21 g N,N′-methylenebisacrylamide, and 0.3 g ammonium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 29° C., the stirring time was 38 minutes, and the stirring speed was 680 rpm.

[0036] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 79° C. and for 5 hours.

[0037] Example 4 This embodiment provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1: Rinse the collected pomace with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried pomace with a grinder and filter it through a 70-mesh filter. Take 3.5g of pomace powder and put it into 48ml of pure water, mix and stir it at an ambient temperature of 26°C, stirring for 18 minutes, and stirring at a speed of 220rpm. Place the solution mixed with the pomace powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction at a reaction pressure of 2.5MPa, a reaction temperature of 170°C, and a reaction time of 8.5h. After the reaction is completed, remove it, wash it with pure water twice, and dry it at 90°C to obtain hydrothermal carbonization.

[0038] Step 2: Dissolve 1.5 g of sodium alginate in 110 ml of water. After the sodium alginate is dissolved, add 0.7 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 110 minutes, and the stirring speed is 550 rpm; and nitrogen is used for purging and cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purge time is 18 minutes, the ambient temperature during the nitrogen purge is 26° C., and the cooling time is 55 minutes.

[0039] Step 3: 2.5 g acrylamide, 0.2 g N,N′-methylenebisacrylamide, and 0.15 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 29° C., the stirring time was 33 min, and the stirring speed was 660 rpm.

[0040] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 75° C. and for a reaction time of 4.5 hours.

[0041] Example 5 This embodiment provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1: Rinse the collected vinasse with clean water, dry it at 100°C for 20 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 4.5g of vinasse powder and put it into 43ml of pure water, mix and stir it at an ambient temperature of 29°C, stirring for 11 minutes, and stirring at a speed of 280rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction at a reaction pressure of 1.5MPa, a reaction temperature of 210°C, and a reaction time of 6.5h. After the reaction is completed, remove it, wash it with pure water three times, and dry it at 80°C to obtain hydrothermal carbon.

[0042] Step 2: Dissolve 2.5 g of sodium alginate in 85 ml of water. After the sodium alginate is dissolved, add 0.9 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 78° C., the stirring time is 70 minutes, and the stirring speed is 650 rpm; and nitrogen is used for purging and cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purge time is 14 minutes, the ambient temperature during the nitrogen purge is 28° C., and the cooling time is 35 minutes.

[0043] Step 3: 5 g acrylamide, 0.14 g N,N′-methylenebisacrylamide, and 0.25 g ammonium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 26° C., the stirring time was 37 minutes, and the stirring speed was 550 rpm.

[0044] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 88° C. and for a reaction time of 3.5 hours.

[0045] In the above-mentioned Examples 1 to 5, the synthesis mechanism of the double-network composite hydrogel in the preparation method includes three consecutive processes: "establishment of the alginate skeleton - embedding of hydrothermal carbon into primary cross-linking - locking of acrylamide network", specifically: first, sodium alginate swells in water, and the chain segments stretch to form a soft, water-containing initial three-dimensional network; secondly, well-dispersed hydrothermal carbon is added and heated and stirred, and the hydrothermal carbon particles are evenly trapped in the sodium alginate grid; the abundant -COOH, -OH and adsorbed metal cations on the surface of the hydrothermal carbon undergo hydrogen bonding, coordination or electrostatic adsorption with the -COO⁻ / -OH on the sodium alginate chain to form dense "physical + weak chemical" anchor points; the hydrothermal carbon The rigid backbone separates the polymer chains, preventing complete collapse during drying, thereby preserving interconnected micropores and mesopores and providing space for subsequent polymerization and water storage. Finally, acrylamide monomer and persulfate are added, and free radical polymerization under mild conditions generates polyacrylamide chains, which form a covalently cross-linked network under the action of persulfate. The growing polyacrylamide chains intersperse and entangle within the sodium alginate-hydrochar backbone. Some free radicals terminate on the hydrochar surface or graft onto its surface functional groups, forming an interfacial bond between the polyacrylamide chains and the hydrochar. The sodium alginate and polyacrylamide chains can further couple through hydrogen bonds and ionic interactions, forming an interpenetrating double network conforming to the hydrogel. In the resulting interpenetrating double network, sodium alginate provides flexible hydrophilic groups, hydrochar provides internal support and pore structure, and the polyacrylamide chains provide continuous covalent strength.

[0046] In order to better reflect the performance differences brought about by whether or not hydrothermal charcoal prepared from wine lees is added during the preparation of the hydrogel, as well as the different amounts of hydrothermal charcoal added, the present application provides the following comparative examples and test examples: Test Example 1 This experimental example provides a method for preparing a double-network composite hydrogel reinforced by hydrothermal carbon, comprising the following steps: Step 1, rinse the collected vinasse with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 5g of vinasse powder and put it into 50ml of pure water, mix and stir it, the ambient temperature is 25°C, the stirring time is 10min, and the stirring speed is 200rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction, the reaction pressure is 2MPa, the reaction temperature is 220°C, and the reaction time is 6h. After the reaction is completed, take it out, wash it with pure water 3 times, and dry it at 70°C to obtain hydrothermal carbon; Step 2: Dissolve 2 g of sodium alginate in 100 ml of water. After the sodium alginate is dissolved, add 0.2 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 60 minutes, and the stirring speed is 500 rpm; and nitrogen is used for purging during the stirring process and the mixture is allowed to cool (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 25° C., and the cooling time is 30 minutes.

[0047] Step 3: 4 g acrylamide, 0.18 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 25° C., the stirring time was 30 min, and the stirring speed was 500 rpm.

[0048] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 3 hours.

[0049] Test Example 2 This experimental example provides a method for preparing a double-network composite hydrogel reinforced with hydrothermal carbon. Compared with Experimental Example 1, the difference is the amount of hydrothermal carbon added in step 2. Specifically, the method includes the following steps: Step 1, rinse the collected vinasse with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 5g of vinasse powder and put it into 50ml of pure water, mix and stir it, the ambient temperature is 25°C, the stirring time is 10min, and the stirring speed is 200rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction, the reaction pressure is 2MPa, the reaction temperature is 220°C, and the reaction time is 6h. After the reaction is completed, take it out, wash it with pure water 3 times, and dry it at 70°C to obtain hydrothermal carbon; Step 2: Dissolve 2 g of sodium alginate in 100 ml of water. After the sodium alginate is dissolved, add 0.4 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 60 minutes, and the stirring speed is 500 rpm; and nitrogen is used for purging during the stirring process and the mixture is allowed to cool (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 25° C., and the cooling time is 30 minutes.

[0050] Step 3: 4 g acrylamide, 0.18 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 25° C., the stirring time was 30 min, and the stirring speed was 500 rpm.

[0051] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 3 hours.

[0052] Test Example 3 This test example provides a preparation method of a double-network composite hydrogel enhanced by hydrothermal carbon. The difference between Test Example 1 and Test Example 2 is the amount of hydrothermal carbon added in step 2. The method specifically includes the following steps: Step 1, rinse the collected vinasse with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 5g of vinasse powder and put it into 50ml of pure water, mix and stir it, the ambient temperature is 25°C, the stirring time is 10min, and the stirring speed is 200rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction, the reaction pressure is 2MPa, the reaction temperature is 220°C, and the reaction time is 6h. After the reaction is completed, take it out, wash it with pure water 3 times, and dry it at 70°C to obtain hydrothermal carbon; Step 2: Dissolve 2 g of sodium alginate in 100 ml of water. After the sodium alginate is dissolved, add 0.6 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 60 minutes, and the stirring speed is 500 rpm; and nitrogen is used for purging during the stirring process and the mixture is allowed to cool (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 25° C., and the cooling time is 30 minutes.

[0053] Step 3: 4 g acrylamide, 0.18 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 25° C., the stirring time was 30 min, and the stirring speed was 500 rpm.

[0054] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 3 hours.

[0055] Test Example 4 This test example provides a preparation method of a double-network composite hydrogel reinforced with hydrothermal carbon. The difference between Test Example 1, Test Example 2, and Test Example 3 is the amount of hydrothermal carbon added in step 2. The method specifically includes the following steps: Step 1, rinse the collected vinasse with clean water, dry it at 80°C for 24 hours, and dry it to constant weight; crush the dried vinasse with a grinder and filter it through an 80-mesh filter. Take 5g of vinasse powder and put it into 50ml of pure water, mix and stir it, the ambient temperature is 25°C, the stirring time is 10min, and the stirring speed is 200rpm. Place the solution mixed with vinasse powder in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction, the reaction pressure is 2MPa, the reaction temperature is 220°C, and the reaction time is 6h. After the reaction is completed, take it out, wash it with pure water 3 times, and dry it at 70°C to obtain hydrothermal carbon; Step 2: Dissolve 2 g of sodium alginate in 100 ml of water. After the sodium alginate is dissolved, add 0.8 g of the hydrothermal charcoal prepared in step 1, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 60 minutes, and the stirring speed is 500 rpm; and nitrogen is used for purging and cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 25° C., and the cooling time is 30 minutes.

[0056] Step 3: 4 g acrylamide, 0.18 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 2, and stirred to obtain a mixture II. The stirring temperature was 25° C., the stirring time was 30 min, and the stirring speed was 500 rpm.

[0057] Step 4: Place the mixture II obtained in step 3 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 3 hours.

[0058] Comparative Example 1 This test example provides a method for preparing a hydrogel. Compared with Test Examples 1 to 4, the difference is that the original step 1 is omitted and hydrothermal carbon is not added in step 2. Specifically, the method includes the following steps: Step 1: Dissolve 2 g of sodium alginate in 100 ml of water, stir and react at high temperature to obtain a mixture I; the stirring environment temperature is 70° C., the stirring time is 60 minutes, and the stirring speed is 500 rpm; and nitrogen is used for purging during the stirring process and the mixture is allowed to cool (i.e., stirring and purging at the same time). The nitrogen purge time is 15 minutes, the ambient temperature during the nitrogen purge is 25° C., and the cooling time is 30 minutes.

[0059] Step 2: 4 g acrylamide, 0.18 g N,N′-methylenebisacrylamide, and 0.2 g potassium persulfate were sequentially added to the mixture I prepared in step 1, and stirred to obtain a mixture II. The stirring temperature was 25° C., the stirring time was 30 min, and the stirring speed was 500 rpm.

[0060] Step 3: Place the mixture II obtained in step 2 in a water bath for high-temperature reaction, and after the reaction is completed, obtain a double-network composite hydrogel. The high-temperature reaction in the water bath is carried out at a temperature of 80° C. and for a reaction time of 3 hours.

[0061] The final product prepared in the comparative example - hydrogel (such as the comparative example in the attached figure), the final product prepared in the test example - double network composite hydrogel (such as test examples 1-4 in the attached figure), and the intermediate product prepared in the test example - hydrothermal carbon (such as the hydrothermal carbon in the attached figure) were tested and analyzed, and the specific content is as follows: 1. Test materials; When conducting physical and chemical analyses such as XRD and FTIR, as well as performance tests such as water absorption and water retention, the test materials are: The double-network composite hydrogels prepared in Experimental Examples 1 to 4 correspond to Experimental Examples 1 to 4 in the accompanying drawings, respectively; The hydrogel prepared in Comparative Example 1 corresponds to Comparative Example 1 in the accompanying drawings; The hydrothermal charcoal prepared according to step 1 of any one of Experimental Examples 1 to 4 corresponds to the hydrothermal charcoal in the accompanying drawings.

[0062] When conducting nutrient retention tests, the test materials are: The double-network composite hydrogel prepared in Experimental Example 3 was prepared into solutions with weight ratios of 0%, 0.25%, 0.5%, 0.75% and 1.00%, respectively, and added to slag for testing.

[0063] 2. Testing instruments and methods; (1) Physical and chemical characterization tests; XRD: CuKα radiation was used with a scanning range of 5°–80°, a step size of 0.02°, a voltage of 40 kV, and a current of 40 mA to analyze the changes in the ratio of crystalline to amorphous phase in the gel network.

[0064] FTIR: Measured in the range of 4000–400 cm⁻¹, with a resolution of 4 cm⁻¹ and accumulating 32 scans, it can identify the interactions between functional groups such as –OH, –COO⁻, C=O, C–O, and N–H and carbon particles.

[0065] SEM: After the sample was freeze-dried in two dimensions, a 5 nm platinum layer was spray-coated, and the microscopic pore structure of the gel and the dispersion state of the carbon particles were observed at an accelerating voltage of 15 kV.

[0066] TGA (Thermogravimetric Analysis): Under nitrogen atmosphere, the temperature was increased from 25°C to 600°C at a rate of 10°C min⁻¹, and the mass loss curve was recorded to evaluate the desorption of the water absorption layer and the carbonization residue of the polymer and hydrothermal carbon.

[0067] Rheology: The storage modulus G′ and loss modulus G″ were measured as a function of angular frequency (0.1–100 rads⁻¹) using a rotational rheometer (parallel plate diameter 25 mm) to evaluate the gel network strength and elastic behavior.

[0068] (2) Performance testing; Water absorption capacity: Place 0.20 g of freeze-dried gel in deionized water at 25°C, measure the swelling weight at 0.5, 1, 2, 4, 8, 24, and 36 h, and calculate the water absorption rate Q.

[0069] Water retention capacity: Place the saturated swollen gel in an environment of 25°C and 50% RH for 48 hours, record the mass change every 2 hours, and calculate the residual water rate.

[0070] Biodegradation rate: The dried gel samples were buried in microbially active garden soil at room temperature (3–5 cm depth), taken out weekly, washed, dried, and weighed to calculate the degradation rate.

[0071] Nutrient retention efficiency: The hydrogel prepared in Experimental Example 3 was added to the soil at different mass ratios (powder, not solution) (0%, 0.25%, 0.5%, 0.75%, 1.00%) to conduct TN and TK retention tests. -3 In the transparent soil column, the upper layer (0–12 cm) was mixed with 0-1.0 wt% gel to simulate 80 mgL -1 The N / K nutrient solution was continuously filtered at a water head of 2 cm; the leachate was collected regularly, and the concentrations of total nitrogen (Kjeldahl method) and total potassium (flame photometry) were determined, and the cumulative retention rate was calculated.

[0072] 3. Test result analysis; After the above test, according to the test results Figures 1-11 The test results are analyzed in conjunction with the attached figures: Figure 1 XRD analysis revealed a broad, diffuse peak between 20° and 30° in Comparative Example 1, indicating a highly amorphous structure. The overall amorphous background was maintained after the addition of hydrochar. As the hydrochar content increased (Test Examples 1-4), low-intensity SiO2 diffraction peaks (approximately 22° and 27°) appeared in the samples, demonstrating that the inorganic phase in the hydrochar was uniformly dispersed within the system and did not induce large-scale crystallization, indicating good compatibility between the polymer network and the filler.

[0073] Figure 2 FTIR analysis revealed a broad O–H / N–H stretching band around 3400 cm⁻¹ for Comparative Example 1. The addition of hydrothermal carbon enhanced the aliphatic C–H bands at 2920 / 2850 cm⁻¹. Around 1700 cm⁻¹, the HC-derived C=O band overlapped with the gel amide I band (approximately 1660 cm⁻¹), shifting slightly with the addition of hydrothermal carbon, suggesting hydrogen bonding or dipole–dipole interactions between carboxyl / amide and carbonyl carbonyl groups. Slight shifts were also observed in the carboxylate peaks at 1620 and 1400 cm⁻¹, as well as in the C–O region at 1100 cm⁻¹, indicating interaction between the sodium alginate backbone and the surface functional groups of the hydrothermal carbon, supporting the formation of the composite network at the molecular level.

[0074] Figure 3 SEM images show that Comparative Example 1 exhibits a dense, collapsed structure with few pores. With the addition of hydrothermal carbon, the number of pores and interconnected channels gradually increases. The sample in Experimental Example 3 exhibits a uniform, interconnected honeycomb network, with filler particles uniformly encapsulated by the polymer, representing the peak specific surface area. Experimental Example 4 exhibits localized filler agglomeration and thick-walled areas, with uneven pore size distribution, suggesting that excess filler may be blocking effective pores.

[0075] Figure 4 Thermogravimetric curves show that weight loss for each sample below 150°C corresponds to water loss. The main degradation release phase shifts toward higher temperatures and decreases in rate, while the residual carbon yield increases significantly with increasing hydrochar content. Hydrochar provides a thermal barrier and promotes the formation of carbonized products, thereby improving the heat resistance of the composite system. The dose-response of thermal stability supports good contact between the filler and the matrix.

[0076] Figure 5 The rheological curves show that the storage modulus of each sample exceeds the loss modulus across the entire frequency range, indicating a gel state. The addition of hydrothermal carbon causes a continuous increase in the storage modulus, reaching a peak in Test Example 3 (approximately 3 times that of Comparative Example 1), followed by a slight decrease in Test Example 4, indicating the presence of an optimal filler window. The filler network and the polymer dual network synergistically enhance energy storage and structural stability.

[0077] Figure 6Water retention experiments showed that after 48 hours of exposure at 25°C, the water retention rates of Test Examples 1-4 were significantly higher than that of Comparative Example 1. Test Example 3 exhibited the highest residual moisture (≈86%). This advantage was maintained at both 55°C and 85°C, indicating that the porous carbon framework and hydrophilic groups synergistically retain both free and bound water, extending the evaporation path.

[0078] Figure 7 The water absorption curves show that Comparative Example 1 absorbed only 26.53 g / g after 24 hours, while Experimental Examples 1-4 absorbed water more rapidly, reaching significantly higher levels, reflecting the increased pore openings and hydrophilic sites introduced by the addition of hydrothermal carbon. Experimental Example 3 absorbed 25.23 g / g within the first two hours and ultimately stabilized at 76.25 g / g, almost three times that of Comparative Example 1. Experimental Example 4 still absorbed water rapidly early on, but reduced the water absorption equilibrium to 57.64 g / g at 36 hours, suggesting that excess carbon may aggregate, blocking pore entrances and reducing the effective free volume.

[0079] Figure 8 and Figure 9 Cyclic water absorption and biodegradability experiments showed that Test Example 3 still maintained 84% of its initial water absorption after 6 cycles of absorption and dehydration; after 5 weeks of burial, the mass of Test Example 3 remained 42% (compared to about 18% of Comparative Example 1), indicating that the composite network has both service life and environmental degradability: hydrothermal carbon slows down the degradation rate and maintains its function, while long-term residual carbon can improve the soil.

[0080] Figure 10 、 Figure 11 Nutrient retention experiments showed that the cumulative loss of TN and TK decreased significantly with increasing material dosage. In Experiment 3, soil incorporation at 0.5–1.0% reduced TN loss by approximately 30–48% and TK loss by approximately 27–45% compared to the blank. The rapid separation of the TK curve in the early stages (≤60 minutes) indicates rapid cation exchange / adsorption, while the gradual separation of the TN curve indicates a combination of physical retardation and hydrogen bond capture.

Claims

1. A method for preparing a double-network composite hydrogel enhanced by hydrothermal carbon, characterized in that: The following steps are involved: Step 1: drying and sieving the biomass, mixing and stirring it with pure water, and placing it in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction; after the reaction is completed, taking it out, washing it, and drying it to obtain hydrothermal carbon; Step 2: dissolving sodium alginate in water, adding the hydrothermal charcoal prepared in Step 1 after the sodium alginate is dissolved, stirring and reacting at high temperature to obtain a mixture I, purging with nitrogen, and standing to cool; Step 3, adding acrylamide, N,N′-methylenebisacrylamide, and persulfate in sequence to the mixture I prepared in step 2, and stirring to obtain a mixture II; Step 4, placing the mixture II obtained in step 3 in a water bath for high-temperature reaction, and obtaining a double-network composite hydrogel after the reaction is completed; The raw material components are calculated by weight as follows: biomass, pure water, sodium alginate, water, acrylamide, N,N'-methylenebisacrylamide, and persulfate in the following proportions: 3-5g: 40-50g: 1-3g: 80-120g: 2-6g: 0.06-0.24g: 0.1-0.3g.

2. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 1, the biomass is wine lees or pomace, and the particle size of the biomass is 70-80 mesh.

3. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 1, when the biomass and pure water are mixed and stirred, the ambient temperature is 25-30° C., the stirring time is 10-20 minutes, and the stirring speed is 200-300 rpm.

4. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 1, when the hydrothermal carbonization reaction is carried out in a high-temperature and high-pressure reactor, the reaction pressure is 1-3 MPa, the reaction temperature is 150-220° C., and the reaction time is 6-10 h.

5. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 2, when stirring after adding the hydrothermal charcoal, the stirring environment temperature is 70-80° C., the stirring time is 60-120 min, and the stirring speed is 500-700 rpm.

6. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 2, nitrogen is used for purging during stirring and for standing to cool. The nitrogen purging time is 10 to 20 minutes, the ambient temperature for nitrogen purging is 25 to 30° C., and the standing to cool time is 30 to 60 minutes.

7. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 3, when mixture II is obtained after stirring, the ambient temperature is 25-30° C., the stirring time is 30-40 min, and the stirring speed is 500-700 rpm.

8. The method for preparing a double-network composite hydrogel based on hydrothermal carbon reinforcement according to claim 1, characterized in that: In step 4, when the high temperature reaction is carried out in a water bath, the reaction temperature is 70-90° C. and the reaction time is 3-5 h.

9. A double-network composite hydrogel prepared by the method for preparing a double-network composite hydrogel enhanced by hydrothermal carbon according to any one of claims 1 to 8.

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