A hydrothermal carbon-reinforced dual-network composite hydrogel and its preparation method

Abstract

This invention discloses a hydrothermal carbon-reinforced dual-network composite hydrogel and its preparation method, belonging to the field of polymer composite materials technology. The aim is to address the technical problem of improving the water absorption, water retention, mechanical toughness, and thermal stability of the hydrogel. The method includes: drying and sieving biomass, mixing and stirring it with pure water, and carrying out a hydrothermal carbonization reaction; after the reaction, removing the biomass, washing and drying it to obtain hydrothermal carbon; dissolving sodium alginate in water, adding the hydrothermal carbon after the sodium alginate has dissolved, stirring and reacting at high temperature to obtain mixture I, purging with nitrogen and allowing it to cool; adding acrylamide, N,N′-methylenebisacrylamide, and persulfate to mixture I, stirring to obtain mixture II; and subjecting mixture II to a high-temperature reaction to obtain the dual-network composite hydrogel. This method improves the water absorption, water retention, mechanical toughness, and thermal stability of the hydrogel, meeting the requirements of long-term load-bearing and repeated expansion and contraction in soil.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, and relates to a hydrogel and its preparation, particularly to a hydrothermal carbon-reinforced dual-network composite hydrogel and its preparation method. Background Technology

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

[0003] Currently, 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 framework, pore templates, and ion adsorption sites, improving the mechanical strength, water retention, and slow-release fertilizer efficiency of the gel. For example, the literature "A multifunctional cellulose- and starch-based composite hydrogel with iron-modified biochar particles for enhancing microalgae growth" synthesized a multifunctional hydrogel to enhance algae yield by crosslinking activated biochar, carboxymethyl cellulose, and starch using oxidized sucrose as a biocrosslinking agent and glycerol as a plasticizer. The literature "Water retention and sustained release of magnesium-based biochar modified hydrogel composite materials" prepared a magnesium-based biochar modified hydrogel slow-release fertilizer using in-situ polymerization, achieving soil moisture retention, reduced irrigation water usage, and improved nutrient utilization efficiency. However, existing biochar is typically produced by pyrolyzing dry raw materials at 400–900°C. This method is energy-intensive, requires pre-drying and complex atmosphere control, and is difficult to handle raw materials with high water content. The resulting biochar has a high degree of carbonization, leading to surface passivation, a reduction in the number of active hydroxyl / carboxyl groups, strong hydrophobicity, and poor interfacial bonding with hydrophilic polymers. Additional surface oxidation or acid washing is often required for dispersion. Some technologies have attempted to introduce oxide groups onto the char surface or utilize ultrasonic dispersion, but these steps are cumbersome and require harsh reaction conditions, making large-scale continuous production difficult.

[0004] For wet organic waste, hydrothermal carbonization technology offers a low-energy carbonization pathway involving low-temperature, closed-loop aqueous reactions. This method can directly process high-moisture raw materials while retaining more oxygen-containing functional groups. The resulting hydrothermal char particles are small in size, highly dispersible, and have high surface polarity, making them more suitable as functional fillers for hydrogels. On the other hand, high-moisture byproducts such as distiller's grains and fruit pomace are generated in large quantities during brewing and juice processing. Traditional stockpiling and incineration methods easily cause secondary pollution. If these wet wastes can be directly converted into hydrothermal char using hydrothermal carbonization technology and combined with natural / synthetic dual-network hydrogels, an integrated cycle of "waste-carbon material-functional gel-soil remediation" can be achieved. However, currently, no publicly available technology offers a complete solution for the efficient dispersion, in-situ crosslinking, and synergistic performance of distiller's grains hydrothermal char in an alginate-acrylamide dual network.

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

[0006] The purpose of this invention is to address the technical problems of biomass resource utilization such as distiller's grains and fruit pomace, as well as improving the water absorption rate, water retention, mechanical toughness, and thermal stability of hydrogels, by providing a hydrothermal carbon-reinforced dual-network composite hydrogel and its preparation method.

[0007] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0008] A method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel includes the following steps:

[0009] Step 1: After drying and sieving the biomass, mix and stir it with pure water, and place it in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction; after the reaction is completed, take it out, wash and dry it to obtain hydrothermal carbon.

[0010] Step 2: Dissolve sodium alginate in water. After the sodium alginate has dissolved, add the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. Then purge with nitrogen and allow to cool.

[0011] Step 3: Acrylamide, N,N′-methylenebisacrylamide and persulfate are added sequentially to mixture I obtained in step 2, and the mixture is stirred to obtain mixture II;

[0012] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained.

[0013] The proportions of each raw material component by weight are as follows: biomass, pure water, sodium alginate, water, acrylamide, N,N′-methylenebisacrylamide, and persulfate: 3~5g: 40~50g: 1~3g: 80-120g: 2~6g: 0.06~0.24g: 0.1~0.3g.

[0014] In Examples 1-5 above, the synthesis mechanism of the dual-network composite hydrogel in this preparation method includes three continuous processes: "alginate framework establishment—hydrothermal carbon embedding primary crosslinking—acrylamide network locking." Specifically, firstly, sodium alginate swells in water, and the chain segments extend 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 uniformly embedded in the sodium alginate network; 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 –COO⁻ / –OH on the sodium alginate chains to form dense "physical + weak chemical" anchor points; the hydrothermal carbon… A rigid framework separates the polymer chains, preventing complete collapse during drying and thus preserving interconnected micropores and mesopores, providing space for subsequent polymerization and water storage. Finally, acrylamide monomers and persulfate are added, and under mild conditions, free radical polymerization generates polyacrylamide chains, which form a covalently cross-linked network under the action of persulfate. The growing polyacrylamide chains interpenetrate and entwine between the sodium alginate-hydrothermal carbon framework. Some free radicals terminate on the surface of the hydrothermal carbon or graft onto its surface functional groups, creating interfacial bonding between the polyacrylamide chains and the hydrothermal carbon. Sodium alginate and polyacrylamide chains can further couple through hydrogen bonds and ionic interactions, resulting in an interpenetrating double-network composite hydrogel. In the final interpenetrating double network, sodium alginate provides flexible hydrophilic groups, hydrothermal carbon provides internal support and pore structure, and polyacrylamide chains provide continuous covalent strength.

[0015] Furthermore, in step 1, the biomass is distiller's grains or fruit pomace, and the particle size of the biomass is 70-80 mesh.

[0016] Furthermore, in step 1, when mixing and stirring the biomass with pure water, the ambient temperature is 25-30℃, the stirring time is 10-20 minutes, and the stirring speed is 200-300 rpm.

[0017] Furthermore, in step 1, when the hydrothermal carbonization reaction is carried out in a high-temperature and high-pressure reactor, the reaction pressure is 1~3MPa, the reaction temperature is 150~220℃, and the reaction time is 6-10h.

[0018] Furthermore, in step 2, when stirring after adding hydrothermal charcoal, the ambient temperature for stirring is 70~80℃, the stirring time is 60~120min, and the stirring speed is 500~700rpm.

[0019] 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 10-20 minutes, the ambient temperature for nitrogen purging is 25-30°C, and the standing and cooling time is 30-60 minutes.

[0020] Furthermore, 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.

[0021] Furthermore, in step 4, when the high-temperature reaction is carried out in a water bath, the reaction temperature is 70~90℃ and the reaction time is 3-5h.

[0022] The above-mentioned method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel yields a dual-network composite hydrogel.

[0023] The beneficial effects of the present invention are as follows:

[0024] 1. In this invention, biomass such as distiller's grains and fruit pomace are used to prepare hydrothermal char, realizing an integrated cycle of "waste-carbon material-functional gel-soil remediation", reducing secondary pollution caused by traditional stockpiling and incineration of biomass such as distiller's grains and fruit pomace; through pre-crosslinking of hydrothermal char with sodium alginate, and then in-situ initiation of acrylamide polymerization, a double network structure is formed; in this structure, the sodium alginate network provides a flexible skeleton, the hydrothermal char acts as a rigid node, and the acrylamide network is further reinforced by N,N′-methylenebisacrylamide covalent crosslinking. The synergistic effect of the three makes the composite gel have both high elasticity and high strength, significantly improving mechanical properties and reusability, and significantly improving the water absorption rate, water retention, mechanical toughness and thermal stability of the hydrogel, meeting the needs of long-term bearing and repeated expansion and contraction in soil.

[0025] 2. In this invention, hydrothermal carbon and sodium alginate are pre-crosslinked, and then acrylamide polymerization is initiated in situ to form a dual-network structure. The structure determines the function—multi-level pores and hydrophilic functional groups enable rapid water absorption. The hard particles of hydrothermal carbon limit excessive swelling and improve mechanical strength and cycle stability. Residual –COO⁻, –CONH2, –OH sites adsorb or exchange K⁺, and delay NO⁻ migration through hydrogen bonding / electrostatic adsorption. The network structure can be repeatedly opened and closed during dry and wet cycles. After the particles expand in the slag, they partially block the large-diameter channels, prolong the water residence time, and synergistically intercept nutrients, thereby achieving in-situ improvement of rapidly leaking media.

[0026] 3. The composite hydrogel prepared in this invention has adjustable water absorption and retention properties. By changing the amount of hydrothermal carbon (0.2–0.8 g), the water absorption rate can be adjusted within the range of 100–330 g / g, and the water holding capacity can be maintained at 30–70% within the temperature range of 25–85℃. It can maintain a high water content under drought or high temperature conditions, slowly release water into the soil, reduce the frequency of irrigation, and achieve intelligent humidity regulation function.

[0027] 4. In this invention, the prepared composite gel can effectively retain soil nutrients and slow their loss. Soil column experiments show that at a dosage of 0.25–1.00 wt%, the cumulative loss of total nitrogen and total potassium can be reduced by 28–35% and 30–38%, respectively. The gel mesh and the hydroxyl, carboxyl, and amide sites on the surface of hydrothermal carbon jointly adsorb or complex nutrient ions, achieving coordinated management of water and nutrients.

[0028] 5. In this invention, the process is simple and can be completed in one pot: sodium alginate is first 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 solvent exchange or multi-step drying; the raw materials are widely available, the process is mild, it can be produced on a large scale, and has good industrialization prospects. Attached Figure Description

[0029] Figure 1 These are the XRD patterns of different experimental examples and comparative examples in this invention;

[0030] Figure 2 These are the FTIR spectra of different experimental examples and comparative examples in this invention;

[0031] Figure 3 These are SEM images of different experimental examples and comparative examples in this invention;

[0032] Figure 4 These are the thermogravimetric curves of different experimental examples and comparative examples in this invention;

[0033] Figure 5 This is a schematic diagram illustrating the changes in energy storage modulus in different experimental examples and comparative examples in this invention;

[0034] Figure 6 This is a schematic diagram illustrating the water retention capacity of different experimental examples and comparative examples in this invention;

[0035] Figure 7 These are water absorption curves of different experimental examples and comparative examples in this invention;

[0036] Figure 8 This is a schematic diagram illustrating the biodegradability of different experimental examples and comparative examples in this invention;

[0037] Figure 9 This is a schematic diagram of the cyclic water absorption effect of different experimental examples and comparative examples in this invention;

[0038] Figure 10 This is a schematic diagram illustrating the effect of hydrogels prepared by Experimental Example 3 with different proportions (0-1%) on TN retention of slag soil.

[0039] Figure 11This is a schematic diagram illustrating the effect of adding different proportions (0-1%) of hydrogels prepared in Experimental Example 3 on the retention of TK in slag soil.

[0040] in, Figures 1-3 The hydrothermal carbon mentioned refers to the test results of the hydrothermal carbon prepared according to step 1 of Experiment Examples 1-4. Figures 1-8 The comparative examples refer to the results of testing based on the hydrogel prepared according to Comparative Example 1. Figures 1-8 Test Example 1 refers to the test results of the hydrogel prepared according to Test Example 1. Figures 1-7 Examples 2-4 refer to the test results of the hydrogels prepared according to Examples 2-4: Figures 10-11 The diagram shows the effect of adding the hydrogel (powder) obtained in Experiment 3 into the slag soil after preparing solutions with different mass ratios to achieve TN and TK retention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 some embodiments of the present invention, but not all embodiments.

[0042] Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0043] Example 1

[0044] This embodiment provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0045] Step 1: Rinse the collected distiller's grains with clean water and dry them at 80°C for 24 hours until constant weight is achieved. Crush the dried distiller's grains using a pulverizer and pass them through an 80-mesh filter. Add 4g of the powdered distiller's grains to 42ml of pure water, mix and stir at an ambient temperature of 26°C for 11 minutes at a stirring speed of 210rpm. Place the solution containing the powdered distiller's grains in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 1MPa, a temperature of 160°C, and a reaction time of 6.5 hours. After the reaction, remove the carbon, wash it three times with pure water, and dry it at 60°C to obtain hydrothermal carbon.

[0046] Step 2: Dissolve 1g of sodium alginate in 85ml of water. After the sodium alginate has dissolved, add 0.1g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 72℃, the stirring time is 70min, and the stirring speed is 520rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 11min, the ambient temperature during nitrogen purging is 26℃, and the cooling time is 32min.

[0047] Step 3: Add 2g of acrylamide, 0.08g of N,N′-methylenebisacrylamide, and 0.1g of ammonium persulfate to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature was 26℃, the stirring time was 32min, and the stirring speed was 520rpm.

[0048] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 72℃ for 3 hours.

[0049] Example 2

[0050] This embodiment provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0051] Step 1: Rinse the collected fruit pomace with clean water and dry at 90°C for 20 hours until constant weight is achieved. Crush the dried pomace using a pulverizer and pass it through an 80-mesh filter. Add 4g of the fruit pomace powder to 45ml of pure water, mix and stir at an ambient temperature of 28°C for 16 minutes and a stirring speed of 250rpm. Place the solution containing the fruit pomace powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2MPa, a temperature of 190°C, and a reaction time of 8 hours. After the reaction, remove the pomace, wash it four times with pure water, and dry it at 70°C to obtain hydrothermal carbon.

[0052] Step 2: Dissolve 2g of sodium alginate in 100ml of water. After the sodium alginate has dissolved, add 0.3g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 75℃, the stirring time is 90min, and the stirring speed is 600rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 27℃, and the cooling time is 45min.

[0053] Step 3: Add 4g of acrylamide, 0.15g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature is 28℃, the stirring time is 35min, and the stirring speed is 600rpm.

[0054] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 4 hours.

[0055] Example 3

[0056] This embodiment provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0057] Step 1: Rinse the collected distiller's grains with clean water and dry them at 70°C for 22 hours until constant weight is achieved. Crush the dried distiller's grains using a pulverizer and pass them through an 80-mesh filter. Add 5g of the powdered distiller's grains to 48ml of pure water, mix and stir at an ambient temperature of 29°C for 18 minutes and a stirring speed of 290rpm. Place the solution containing the powdered distiller's grains in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 3MPa, a temperature of 210°C, and a reaction time of 9 hours. After the reaction, remove the carbon, wash it three times with pure water, and dry it at 70°C to obtain hydrothermal carbon.

[0058] Step 2: Dissolve 3g of sodium alginate in 110ml of water. After the sodium alginate has dissolved, add 0.5g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 78℃, the stirring time is 110min, and the stirring speed is 670rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 19min, the ambient temperature during nitrogen purging is 29℃, and the cooling time is 56min.

[0059] Step 3: Add 6g of acrylamide, 0.21g of N,N′-methylenebisacrylamide, and 0.3g of ammonium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature was 29℃, the stirring time was 38min, and the stirring speed was 680rpm.

[0060] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 79°C for 5 hours.

[0061] Example 4

[0062] This embodiment provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0063] Step 1: Rinse the collected fruit pomace with clean water and dry at 80°C for 24 hours until constant weight is achieved. Crush the dried pomace using a pulverizer and pass it through a 70-mesh filter. Add 3.5g of the fruit pomace powder to 48ml of pure water, mix and stir at an ambient temperature of 26°C for 18 minutes and a stirring speed of 220rpm. Place the solution containing the fruit pomace powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2.5MPa, a temperature of 170°C, and a reaction time of 8.5 hours. After the reaction, remove the pomace, wash it twice with pure water, and dry it at 90°C to obtain hydrothermal carbon.

[0064] Step 2: Dissolve 1.5g of sodium alginate in 110ml of water. After the sodium alginate has dissolved, add 0.7g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 70℃, the stirring time is 110min, and the stirring speed is 550rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 18min, the ambient temperature during nitrogen purging is 26℃, and the cooling time is 55min.

[0065] Step 3: Add 2.5g acrylamide, 0.2g N,N′-methylenebisacrylamide, and 0.15g potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature was 29℃, the stirring time was 33min, and the stirring speed was 660rpm.

[0066] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The reaction temperature in the water bath is 75°C, and the reaction time is 4.5 h.

[0067] Example 5

[0068] This embodiment provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0069] Step 1: Rinse the collected distiller's grains with clean water and dry them at 100°C for 20 hours until constant weight is achieved. Crush the dried distiller's grains using a pulverizer and pass them through an 80-mesh filter. Add 4.5g of the powdered distiller's grains to 43ml of pure water, mix and stir at an ambient temperature of 29°C for 11 minutes at a stirring speed of 280rpm. Place the solution containing the powdered distiller's grains in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 1.5MPa, a temperature of 210°C, and a reaction time of 6.5 hours. After the reaction, remove the carbon, wash it three times with pure water, and dry it at 80°C to obtain hydrothermal carbon.

[0070] Step 2: Dissolve 2.5g of sodium alginate in 85ml of water. After the sodium alginate has dissolved, add 0.9g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 78℃, the stirring time is 70min, and the stirring speed is 650rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 14min, the ambient temperature during nitrogen purging is 28℃, and the cooling time is 35min.

[0071] Step 3: Add 5g of acrylamide, 0.14g of N,N′-methylenebisacrylamide, and 0.25g of ammonium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature was 26℃, the stirring time was 37min, and the stirring speed was 550rpm.

[0072] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The reaction temperature in the water bath is 88℃, and the reaction time is 3.5h.

[0073] In Examples 1-5 above, the synthesis mechanism of the dual-network composite hydrogel in this preparation method includes three continuous processes: "alginate framework establishment—hydrothermal carbon embedding primary crosslinking—acrylamide network locking." Specifically, firstly, sodium alginate swells in water, and the chain segments extend 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 uniformly trapped in the sodium alginate network; the abundant –COOH, –OH and adsorbed metal cations on the surface of the hydrothermal carbon undergo hydrogen bonding, coordination, or electrostatic adsorption with –COO⁻ / –OH on the sodium alginate chains, forming dense "physical + weak chemical" anchor points; the hydrothermal carbon… A rigid framework separates the polymer chains, preventing complete collapse during drying and thus preserving interconnected micropores and mesopores, providing space for subsequent polymerization and water storage. Finally, acrylamide monomers and persulfate are added, and under mild conditions, free radical polymerization generates polyacrylamide chains, which form a covalently cross-linked network under the action of persulfate. The growing polyacrylamide chains interpenetrate and entwine between the sodium alginate-hydrothermal carbon framework. Some free radicals terminate on the surface of the hydrothermal carbon or graft onto its surface functional groups, creating interfacial bonding between the polyacrylamide chains and the hydrothermal carbon. Sodium alginate and polyacrylamide chains can further couple through hydrogen bonds and ionic interactions, resulting in an interpenetrating double-network composite hydrogel. In the final interpenetrating double network, sodium alginate provides flexible hydrophilic groups, hydrothermal carbon provides internal support and pore structure, and polyacrylamide chains provide continuous covalent strength.

[0074] To better demonstrate the performance differences resulting from the addition of hydrothermal char derived from distiller's grains during hydrogel preparation, and the varying amounts of hydrothermal char added, this application provides the following comparative examples and experimental cases:

[0075] Experimental Example 1

[0076] This experimental example provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, including the following steps:

[0077] Step 1: Rinse the collected lees with clean water and dry at 80°C for 24 hours until constant weight is achieved. Crush the dried lees using a pulverizer and pass through an 80-mesh filter. Add 5g of lees powder to 50ml of pure water, mix and stir at an ambient temperature of 25°C for 10 minutes at a stirring speed of 200rpm. Place the solution containing the lees powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2MPa, a temperature of 220°C, and a reaction time of 6 hours. After the reaction, remove the lees, wash them three times with pure water, and dry them at 70°C to obtain hydrothermal carbon.

[0078] Step 2: Dissolve 2g of sodium alginate in 100ml of water. After the sodium alginate has dissolved, add 0.2g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 70℃, the stirring time is 60min, and the stirring speed is 500rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 25℃, and the cooling time is 30min.

[0079] Step 3: Add 4g of acrylamide, 0.18g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature is 25℃, the stirring time is 30min, and the stirring speed is 500rpm.

[0080] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 3 hours.

[0081] Experimental Example 2

[0082] This experimental example provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel. Compared with Experimental Example 1, the difference lies in the amount of hydrothermal carbon added in step 2. The specific steps include:

[0083] Step 1: Rinse the collected lees with clean water and dry at 80°C for 24 hours until constant weight is achieved. Crush the dried lees using a pulverizer and pass through an 80-mesh filter. Add 5g of lees powder to 50ml of pure water, mix and stir at an ambient temperature of 25°C for 10 minutes at a stirring speed of 200rpm. Place the solution containing the lees powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2MPa, a temperature of 220°C, and a reaction time of 6 hours. After the reaction, remove the lees, wash them three times with pure water, and dry them at 70°C to obtain hydrothermal carbon.

[0084] Step 2: Dissolve 2g of sodium alginate in 100ml of water. After the sodium alginate has dissolved, add 0.4g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 70℃, the stirring time is 60min, and the stirring speed is 500rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 25℃, and the cooling time is 30min.

[0085] Step 3: Add 4g of acrylamide, 0.18g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature is 25℃, the stirring time is 30min, and the stirring speed is 500rpm.

[0086] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 3 hours.

[0087] Experimental Example 3

[0088] This experimental example provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel. Compared with Experimental Example 1 and Experimental Example 2, the difference lies in the amount of hydrothermal carbon added in step 2. The specific steps include:

[0089] Step 1: Rinse the collected lees with clean water and dry at 80°C for 24 hours until constant weight is achieved. Crush the dried lees using a pulverizer and pass through an 80-mesh filter. Add 5g of lees powder to 50ml of pure water, mix and stir at an ambient temperature of 25°C for 10 minutes at a stirring speed of 200rpm. Place the solution containing the lees powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2MPa, a temperature of 220°C, and a reaction time of 6 hours. After the reaction, remove the lees, wash them three times with pure water, and dry them at 70°C to obtain hydrothermal carbon.

[0090] Step 2: Dissolve 2g of sodium alginate in 100ml of water. After the sodium alginate has dissolved, add 0.6g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 70℃, the stirring time is 60min, and the stirring speed is 500rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 25℃, and the cooling time is 30min.

[0091] Step 3: Add 4g of acrylamide, 0.18g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature is 25℃, the stirring time is 30min, and the stirring speed is 500rpm.

[0092] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 3 hours.

[0093] Test Example 4

[0094] This experimental example provides a method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel. Compared with Experimental Example 1, Experimental Example 2, and Experimental Example 3, the difference lies in the amount of hydrothermal carbon added in step 2. The specific steps include:

[0095] Step 1: Rinse the collected lees with clean water and dry at 80°C for 24 hours until constant weight is achieved. Crush the dried lees using a pulverizer and pass through an 80-mesh filter. Add 5g of lees powder to 50ml of pure water, mix and stir at an ambient temperature of 25°C for 10 minutes at a stirring speed of 200rpm. Place the solution containing the lees powder in a high-temperature, high-pressure reactor for hydrothermal carbonization at a pressure of 2MPa, a temperature of 220°C, and a reaction time of 6 hours. After the reaction, remove the lees, wash them three times with pure water, and dry them at 70°C to obtain hydrothermal carbon.

[0096] Step 2: Dissolve 2g of sodium alginate in 100ml of water. After the sodium alginate has dissolved, add 0.8g of the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. The stirring environment temperature is 70℃, the stirring time is 60min, and the stirring speed is 500rpm. Nitrogen gas is used for purging and static cooling during the stirring process (i.e., stirring and purging at the same time). The nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 25℃, and the cooling time is 30min.

[0097] Step 3: Add 4g of acrylamide, 0.18g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 2, and stir to obtain mixture II. During stirring, the ambient temperature is 25℃, the stirring time is 30min, and the stirring speed is 500rpm.

[0098] Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 3 hours.

[0099] Comparative Example 1

[0100] This experimental example provides a method for preparing a hydrogel. Compared with Experiments 1-4, the difference is that step 1 is omitted and hydrothermal carbon is not added in step 2. The specific steps include:

[0101] Step 1: Dissolve 2g of sodium alginate in 100ml of water, stir and react at high temperature to obtain mixture I; the stirring environment temperature is 70℃, the stirring time is 60min, and the stirring speed is 500rpm; during the stirring process, nitrogen is used for purging and static cooling (i.e., stirring and purging at the same time), the nitrogen purging time is 15min, the ambient temperature during nitrogen purging is 25℃, and the cooling time is 30min.

[0102] Step 2: Add 4g of acrylamide, 0.18g of N,N′-methylenebisacrylamide, and 0.2g of potassium persulfate sequentially to mixture I obtained in step 1, and stir to obtain mixture II. During stirring, the ambient temperature is 25℃, the stirring time is 30min, and the stirring speed is 500rpm.

[0103] Step 3: The mixture II obtained in Step 2 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The high-temperature reaction in the water bath is carried out at 80℃ for 3 hours.

[0104] The final product obtained in the comparative example—hydrogel (as shown in the attached figure), the final product obtained in the experimental example—double-network composite hydrogel (as shown in the attached figures for experimental examples 1-4), and the intermediate product obtained in the experimental example—hydrothermal carbon (as shown in the attached figure for hydrothermal carbon) were analyzed. The specific contents are as follows:

[0105] I. Test Materials;

[0106] When performing physicochemical analyses such as XRD and FTIR, as well as performance tests such as water absorption and retention, the test materials were as follows:

[0107] The dual-network composite hydrogels prepared in Experimental Examples 1 to 4 correspond to Experimental Examples 1 to 4 in the attached figures, respectively;

[0108] The hydrogel prepared in Comparative Example 1 corresponds to Comparative Example 1 in the attached figure;

[0109] The hydrothermal carbon prepared according to step 1 of any of Experimental Examples 1 to 4 corresponds to the hydrothermal carbon in the attached figure.

[0110] The test materials used in the nutrient retention test were:

[0111] The dual-network composite hydrogel prepared in Example 3 was prepared into solutions with weight ratios of 0%, 0.25%, 0.5%, 0.75%, and 1.00%, and added to the slag soil for testing.

[0112] II. Testing Instruments and Methods;

[0113] (1) Physicochemical characterization tests;

[0114] XRD: CuKα rays were used with a scanning range of 5°–80°, a step size of 0.02°, a voltage of 40kV, and a current of 40mA to analyze the changes in the ratio of crystalline to amorphous phases in the gel network.

[0115] FTIR: Tested in the range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹, accumulating 32 scans to identify the interaction between functional groups such as –OH, –COO⁻, C=O, C–O, and N–H and carbon particles.

[0116] SEM: After two-dimensional freeze-drying, the sample was sputtered with a 5nm platinum layer and the microstructure of the gel and the dispersion of carbon particles were observed under an accelerating voltage of 15kV.

[0117] TGA (Thermogravimetric Analysis): Under a 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 absorbent layer and the residual carbonization of the polymer and hydrothermal carbon.

[0118] Rheology: Using a rotational rheometer (parallel plate diameter 25 mm), the storage modulus G′ and loss modulus G″ were measured as a function of angular frequency (0.1–100 rads⁻¹) to evaluate the strength and elastic behavior of the gel network.

[0119] (2) Performance testing;

[0120] Water absorption capacity: 0.20g of lyophilized gel was placed in deionized water at 25°C, and the swelling weight was measured at 0.5, 1, 2, 4, 8, 24, and 36 hours. The water absorption ratio Q was calculated.

[0121] Water retention capacity: The saturated expanded gel was placed in an environment of 25°C and 50%RH for 48 hours, and the mass change was recorded every 2 hours to calculate the water retention rate.

[0122] Biodegradation rate: The dried gel sample was buried in microbially active garden soil at room temperature (3–5 cm deep), removed weekly, washed, dried and weighed, and the degradation rate was calculated.

[0123] Nutrient retention efficiency: The hydrogel prepared in Example 3 was added to the slag soil at different mass ratios (using powder, not solution) (0%, 0.25%, 0.5%, 0.75%, 1.00%) for TN and TK retention tests; during the test, the inner diameter was 6 cm and the filling density was 1.55 g / cm³. -3 In a transparent soil column, the upper layer (0–12 cm) is incorporating 0–1.0 wt% gel to simulate 80 mg / L. -1The N / K nutrient solution was continuously percolated at a head of 2 cm; the percolate was collected periodically, and the concentrations of total nitrogen (Kjeldahl method) and total potassium (flame photometry method) were determined to calculate the cumulative rejection rate.

[0124] III. Analysis of Test Results;

[0125] Based on the above tests and the test results, we can obtain... Figures 1-11 The test results are shown in the attached figures.

[0126] Figure 1 XRD analysis showed that Comparative Example 1 exhibited a broad, diffuse peak between 20° and 30°, indicating a highly amorphous structure. After the introduction of hydrothermal carbon, the overall amorphous background was maintained. With increasing hydrothermal carbon content (Examples 1-4), low-intensity SiO2 diffraction peaks (approximately 22° and 27°) appeared in the samples, demonstrating that the inorganic phase in the hydrothermal carbon was uniformly dispersed within the system and did not induce large-scale crystallization, indicating good compatibility between the polymer network and the filler.

[0127] Figure 2 FTIR analysis of Comparative Example 1 showed a broad O–H / N–H stretching band near 3400 cm⁻¹. After the addition of hydrothermal carbon, the aliphatic C–H band was enhanced at 2920 / 2850 cm⁻¹; the HC source C=O band around 1700 cm⁻¹ superimposed with gel amide I (approximately 1660 cm⁻¹), and shifted slightly with increasing hydrothermal carbon content, suggesting hydrogen bonding or dipole-dipole interactions between carboxyl / amide and carbonyl groups. Slight shifts were also observed in the carboxylate peaks at 1620 and 1400 cm⁻¹ and in the C–O region at 1100 cm⁻¹, indicating an interaction between the sodium alginate framework and the functional groups on the hydrothermal carbon surface, supporting the formation of a complex network at the molecular level.

[0128] Figure 3 SEM analysis showed that Comparative Example 1 exhibited a dense, collapsed structure with few pores. With the addition of hydrothermal carbon, the number of pores and interconnected channels gradually increased. Sample 3 showed a uniform honeycomb-like interconnected network, with filler particles uniformly embedded in the polymer, representing the peak specific surface area. Sample 4 showed localized filler agglomeration and thick-walled areas, with uneven pore size distribution, indicating that excessive filler may have blocked effective pores.

[0129] Figure 4 Thermogravimetric analysis (TGA) curves showed that weight loss at <150°C corresponded to moisture loss in each sample. The main degradation release phase shifted towards the high-temperature region and decreased in rate, while the residual char content increased significantly with increasing hydrothermal char content. Hydrothermal char provides a thermal barrier and promotes the formation of carbonization products, thereby improving the heat resistance of the composite system; the dose-response relationship of thermal stability supports good contact between the filler and the matrix.

[0130] Figure 5Rheological curves show that the storage modulus of all samples is greater than the loss modulus across the entire frequency range, indicating that they are all in a gel state. The addition of hydrothermal carbon caused the storage modulus to continuously increase, reaching a peak in Example 3 (approximately 3 times that of Comparative Example 1), after which it slightly decreased in Example 4, indicating the existence of an optimal filler window. The filler network and the polymer dual network synergistically improve energy storage performance and structural stability.

[0131] Figure 6 Water retention experiments showed that after 48 hours of exposure at 25°C, the water retention rates of Experiments 1-4 were significantly higher than those of Comparative Example 1; Experiment 3 had the highest residual water content (≈86%). Furthermore, Experiment 3 maintained a significant advantage at 55°C and 85°C, indicating that the porous carbon framework and hydrophilic groups synergistically retain both free and bound water, thus extending the evaporation path.

[0132] Figure 7 The water absorption curves show that Comparative Example 1 absorbed only 26.53 g / g after 24 hours, while Examples 1-4 absorbed water much faster, reaching significantly higher levels. This reflects that the addition of hydrothermal carbon brought more open pores and hydrophilic sites. Example 3 absorbed 25.23 g / g in the first 2 hours and eventually stabilized at 76.25 g / g, almost three times that of Comparative Example 1. Example 4 still absorbed water rapidly in the early stages, but it reduced the water absorption equilibrium at 36 hours to 57.64 g / g. This indicates that excess carbon may accumulate, blocking pore entrances and reducing the effective free volume.

[0133] Figure 8 and Figure 9 The cyclic water absorption and biodegradability experiments showed that after 6 cycles of water absorption and dehydration, Experiment Example 3 still maintained 84% of the initial water absorption capacity; after 5 weeks of soil burial, Experiment Example 3 retained 42% of its mass (Comparative Example 1 retained about 18%), indicating that the composite network has both service life and environmental degradability: hydrothermal carbon slows down the degradation rate and maintains the function, while long-term residual carbon can improve the soil.

[0134] Figure 10 , Figure 11 Nutrient retention experiments showed that the cumulative loss of TN and TK decreased significantly with increasing material incorporation. In Experiment 3, when 0.5–1.0% soil was incorporated, TN loss was reduced by approximately 30–48% and TK loss by approximately 27–45% compared to the control. The TK curves separated rapidly in the early stage (≤60 min), indicating rapid cation exchange / adsorption; the TN curves separated gradually, indicating the coexistence of physical lag and hydrogen bond capture.

Claims

1. A method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel, characterized in that, Includes the following steps: Step 1: After drying and sieving the biomass, mix and stir it with pure water, and place it in a high-temperature and high-pressure reactor for hydrothermal carbonization reaction; after the reaction is completed, take it out, wash and dry it to obtain hydrothermal carbon. Step 2: Dissolve sodium alginate in water. After the sodium alginate has dissolved, add the hydrothermal carbon prepared in Step 1. Stir and react at high temperature to obtain mixture I. Then purge with nitrogen and allow to cool. Step 3: Acrylamide, N,N′-methylenebisacrylamide and persulfate are added sequentially to mixture I obtained in step 2, and the mixture is stirred to obtain mixture II; Step 4: The mixture II obtained in Step 3 is placed in a water bath for high-temperature reaction. After the reaction is completed, a double-network composite hydrogel is obtained. The proportions of each raw material component by weight are as follows: biomass, pure water, sodium alginate, water, acrylamide, N,N′-methylenebisacrylamide, and persulfate: 3~5g : 40~50g : 1~3g : 80-120g : 2~6g : 0.06~0.24g : 0.1~0.3g.

2. The preparation method of a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 1, the biomass is distiller's grains or fruit pomace, and the particle size of the biomass is 70-80 mesh.

3. The preparation method of a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 1, when mixing and stirring the biomass with pure water, the ambient temperature is 25-30℃, the stirring time is 10-20 minutes, and the stirring speed is 200-300 rpm.

4. The preparation method of a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 1, during the hydrothermal carbonization reaction in the high-temperature and high-pressure reactor, the reaction pressure is 1~3MPa, the reaction temperature is 150~220℃, and the reaction time is 6~10 h.

5. The preparation method of a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 2, when stirring after adding hydrothermal charcoal, the ambient temperature for stirring is 70~80℃, the stirring time is 60~120min, and the stirring speed is 500~700rpm.

6. The preparation method of a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 2, when stirring, nitrogen is used for purging and the mixture is allowed to cool. The nitrogen purging time is 10-20 minutes, the ambient temperature for nitrogen purging is 25-30°C, and the cooling time is 30-60 minutes.

7. The method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel as described in claim 1, characterized in that, In step 3, when mixture II is obtained after stirring, the ambient temperature is 25-30℃, the stirring time is 30-40 min, and the stirring speed is 500-700 rpm.

8. The method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel as described in 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℃ and the reaction time is 3-5 h.

9. The dual-network composite hydrogel prepared by the method for preparing a hydrothermal carbon-reinforced dual-network composite hydrogel according to any one of claims 1-8.

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