Amino-functionalized biochar / sodium alginate double network hydrogel, its preparation method and application

Abstract

This invention relates to the field of soil amendment materials technology, and more particularly to an aminated biochar / sodium alginate dual-network gel, its preparation method, and its application. The invention involves mixing biochar, an amino-containing silane coupling agent, and an organic solvent to undergo an amination reaction, yielding aminated biochar; mixing starch with water and heating to gelatinize, yielding gelatinized starch; mixing the gelatinized starch, a sodium alginate-acrylamide aqueous solution, and the aminated biochar; and adding a Ca-containing compound to the resulting mixture. 2+ An aqueous solution is used for ionic crosslinking to form a pre-crosslinked gel precursor. The pre-crosslinked gel precursor, a diene crosslinking agent, and a thermally decomposable free radical initiator are then mixed and covalently crosslinked to obtain an aminated biochar / sodium alginate dual-network gel. The aminated biochar / sodium alginate dual-network gel prepared by this invention possesses excellent water retention, pollutant adsorption, high mechanical properties, and environmental compatibility.

Description

Technical Field

[0001] This invention relates to the field of soil amendment materials technology, and in particular to an aminated biochar / sodium alginate dual-network gel, its preparation method, and its application. Background Technology

[0002] In arid and semi-arid regions, rapid soil moisture evaporation and poor water retention directly restrict crop growth. Simultaneously, industrial and mining activities lead to heavy metals such as cadmium, lead, and copper entering the soil environment through irrigation and atmospheric deposition, seriously threatening agricultural product safety and ecosystem health. To address these issues, various functional soil amendment materials have emerged. While highly absorbent resins (such as polyacrylic acid and polyacrylamide hydrogels) possess excellent water absorption and retention capabilities, effectively enhancing soil drought resistance, they suffer from poor mechanical properties, are easily broken, lack long-term stability in soil, and have limited adsorption capacity for heavy metals. Furthermore, they pose environmental risks due to residual monomer toxicity and persistent degradation. On the other hand, biochar, a porous material produced from the pyrolysis of waste biomass such as straw and sawdust, can effectively immobilize heavy metals and improve soil structure through adsorption. However, its water-holding capacity is weak, and its powdery form is easily eroded and lost by wind and rain, making it difficult to maintain its effectiveness in the topsoil for an extended period.

[0003] To overcome the limitations of single materials, existing technologies attempt to combine biochar with hydrogels. However, most methods only involve simple physical mixing, such as mechanically blending biochar powder with pre-prepared hydrogels. This approach has fundamental flaws: the biochar and polymer network are bound together by only weak physical forces, resulting in poor interfacial compatibility and easy separation and loss in complex soil environments, leading to a lack of functional synergy. Furthermore, physical mixing cannot effectively enhance the mechanical strength of the gel, leaving the material still easily broken. Moreover, the performance is often a simple superposition of the two functions, and compatibility issues can even cause mutual constraints, failing to achieve a synergistic effect of "1+1>2". Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide an aminated biochar / sodium alginate dual-network gel, its preparation method, and its application. The aminated biochar / sodium alginate dual-network gel prepared by this invention possesses excellent water retention, pollutant adsorption, high mechanical properties, and environmental compatibility.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing an aminated biochar / sodium alginate dual-network gel, comprising the following steps: Biochar, an amino-containing silane coupling agent, and an organic solvent are mixed and subjected to an amination reaction to obtain amination-treated biochar. Starch is mixed with water and heated to gelatinize, thus obtaining gelatinized starch; The gelatinized starch, sodium alginate-acrylamide aqueous solution, and aminated biochar are mixed, and a Ca-containing solution is added to the resulting mixture. 2+ An aqueous solution of sodium alginate and acrylamide is subjected to ionic cross-linking to form a pre-cross-linked gel precursor; the mass ratio of sodium alginate to acrylamide in the sodium alginate-acrylamide mixed aqueous solution is 1:(0.6~3); the mass of the aminated biochar is 7~43% of the total mass of sodium alginate and acrylamide; The pre-crosslinked gel precursor, diene crosslinking agent, and thermally decomposable free radical initiator were mixed and covalently crosslinked to obtain the aminated biochar / sodium alginate dual-network gel.

[0006] Preferably, the mass ratio of the aminated biochar to starch is (1~3):(1~3).

[0007] Preferably, the covalent crosslinking polymerization is carried out at a temperature of 60-70°C for 2-3 hours.

[0008] Preferably, the amino-containing silane coupling agent includes one or more of N-β-aminoethyl-γ-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 3-(2-aminoethyl)-aminopropyltrimethoxysilane.

[0009] Preferably, the mass ratio of the biochar to the volume ratio of the amino-containing silane coupling agent is 1 g: (0.4~0.6) mL; the amination reaction is carried out at a temperature of 70~80℃ for 4~6 h.

[0010] Preferably, the starch includes corn starch.

[0011] Preferably, the diolefin crosslinking agent includes one or more of N,N'-methylenebisacrylamide, N,N'-(1,2-dihydroxyethylene)bisacrylamide, and polyethylene glycol diacrylate; the mass of the diolefin crosslinking agent is 0.2~0.6% of the total mass of sodium alginate and acrylamide.

[0012] This invention provides an aminated biochar / sodium alginate dual-network gel prepared by the preparation method described above, comprising aminated biochar and a dual-network gel; the aminated biochar and the dual-network gel form an organic-inorganic interpenetrating network structure; the dual-network gel is formed by starch, sodium alginate and acrylamide through an ion-covalent crosslinking mechanism.

[0013] This invention provides the application of the aminated biochar / sodium alginate dual-network gel described above in soil improvement.

[0014] Preferably, the mass ratio of sodium alginate to acrylamide used in preparing the aminated biochar / sodium alginate dual-network gel is adjusted according to the soil conditions during the soil improvement process. When the soil to be improved is organically polluted soil or high-salinity soil, the mass ratio of sodium alginate to acrylamide is 1:(2~3); when the soil to be improved is heavy metal polluted soil or focuses on soil drought resistance and moisture retention, the mass ratio of sodium alginate to acrylamide is 1:(0.6~1); when the soil to be improved is heavy metal-organic composite polluted soil, the mass ratio of sodium alginate to acrylamide is 1:(1~2); the high-salinity soil refers to soil with an electrical conductivity >4mS / cm.

[0015] This invention provides a method for preparing an aminated biochar / sodium alginate dual-network gel, comprising the following steps: mixing biochar, an amino-containing silane coupling agent, and an organic solvent to perform an amination reaction to obtain aminated biochar; mixing starch with water and heating to gelatinize to obtain gelatinized starch; mixing the gelatinized starch, a sodium alginate-acrylamide aqueous solution, and the aminated biochar; and adding a Ca-containing... 2+ An aqueous solution of sodium alginate and acrylamide is subjected to ionic crosslinking to form a pre-crosslinked gel precursor; the mass ratio of sodium alginate to acrylamide in the sodium alginate-acrylamide mixed aqueous solution is 1:(0.6~3); the mass of the aminated biochar is 7~43% of the total mass of sodium alginate and acrylamide; the pre-crosslinked gel precursor, diolefin crosslinking agent and thermally decomposable free radical initiator are mixed and covalently crosslinked to obtain the aminated biochar / sodium alginate dual-network gel.

[0016] Compared with the prior art, the present invention has the following significant advantages: 1) Improved environmental friendliness: The use of natural high-molecular-weight sodium alginate to partially replace synthetic acrylic acid significantly reduces the amount of synthetic monomers used and potential environmental risks, and improves the biocompatibility of the material; 2) Enhanced structural stability: The interpenetrating network structure constructed through the "ion-covalent" double crosslinking mechanism significantly improves the mechanical strength and durability of the gel, with a compressive strength of over 45 kPa, effectively overcoming the defects of traditional gels being easily broken. 3) Optimized Adsorption Performance: The -NH2 functional groups abundant on the surface of aminated modified biochar have a strong coordination ability with heavy metal ions, which significantly improves the material's adsorption capacity for heavy metal ions; the results of the examples show that the dual-network gel material prepared in this invention has a high adsorption capacity for Cd. 2+ The adsorption capacity was increased to 48.5 mg / g, which is 38.2% higher than that of physically mixed materials; 4) Significant functional synergy: The "micro-reservoir" effect of the gel network rapidly absorbs and transports water and pollutants; the aminated biochar acts as an "adsorption island" to accurately capture heavy metal ions; the dual-network structure ensures the long-term stable existence of the material in the soil, with water retention and adsorption properties working together for a lasting effect. 5) High performance controllability: By adjusting the ratio of sodium alginate to acrylamide, the material can be precisely controlled to change from "strong hydrophilicity - heavy metal preference" to "weak hydrophilicity - organic matter preference", adapting to the needs of different soil environments. Attached Figure Description

[0017] Figure 1 This is a flow chart of the preparation process of aminated biochar / sodium alginate dual-network gel in Example 1; Figure 2 Fourier transform infrared spectrum of the aminated biochar / sodium alginate dual-network gel prepared in Example 1. Detailed Implementation

[0018] This invention provides a method for preparing an aminated biochar / sodium alginate dual-network gel, comprising the following steps: Biochar, an amino-containing silane coupling agent, and an organic solvent are mixed and subjected to an amination reaction to obtain amination-treated biochar. Starch is mixed with water and heated to gelatinize, thus obtaining gelatinized starch; The gelatinized starch, sodium alginate-acrylamide aqueous solution, and aminated biochar are mixed, and a Ca-containing solution is added to the resulting mixture. 2+ An aqueous solution of sodium alginate and acrylamide is subjected to ionic cross-linking to form a pre-cross-linked gel precursor; the mass ratio of sodium alginate to acrylamide in the sodium alginate-acrylamide mixed aqueous solution is 1:(0.6~3); the mass of the aminated biochar is 7~43% of the total mass of sodium alginate and acrylamide; The pre-crosslinked gel precursor, diene crosslinking agent, and thermally decomposable free radical initiator were mixed and covalently crosslinked to obtain the aminated biochar / sodium alginate dual-network gel.

[0019] Unless otherwise specified, all raw materials used in this invention are commercially available products well known in the art.

[0020] This invention involves mixing biochar, an amino-containing silane coupling agent, and an organic solvent to carry out an amination reaction, thereby obtaining amination-treated biochar.

[0021] In this invention, the biochar is preferably obtained from corn stalks through oxygen-limited pyrolysis at 600-800°C. The biochar is preferably made with a mesh size of 100-200 mesh. Using 100-200 mesh biochar ensures that the biochar particles have sufficient specific surface area for grafting a sufficient number of functional groups, while also preventing them from being too fine to be difficult to separate from organic solvents.

[0022] In this invention, the amino-containing silane coupling agent preferably includes one or more of N-β-aminoethyl-γ-aminopropyltrimethoxysilane (KH-792, with high primary amino content), 3-aminopropyltriethoxysilane (KH-550, containing one primary amino group), and 3-(2-aminoethyl)-aminopropyltrimethoxysilane (containing two nitrogen atoms), more preferably KH-792. This invention uses KH-792, whose molecule contains two primary amino groups, resulting in high grafting density and good adsorption effect after modification.

[0023] In this invention, the organic solvent preferably includes C1-C4 alcohols, ketones, or aromatic hydrocarbons, including but not limited to: methanol, isopropanol, acetone, toluene, or mixtures thereof, and preferably ethanol. The use of ethanol in this invention has the advantages of low cost, low toxicity, easy removal, and good compatibility with KH-792.

[0024] In this invention, the preferred mass ratio of the biochar to the volume ratio of the amino-containing silane coupling agent is 1 g:(0.4~0.6) mL, and in a specific embodiment, it can be 1 g:0.5 mL. The preferred mass ratio of the biochar to the volume ratio of the organic solvent is 1 g:(15~25) mL, and in a specific embodiment, it can be 1 g:20 mL.

[0025] In this invention, the preferred temperature for the amination reaction is 70-80°C, and the preferred time is 4-6 hours. In specific embodiments, the temperature for the amination reaction can be 70, 73, 75, 78, or 80°C, and the time can be 4, 5, or 6 hours. In this invention, the amination reaction is preferably carried out under reflux conditions in a water bath. This invention grafts amino functional groups onto the surface of biochar through an amination reaction. The -NH2 functional group has a strong coordination ability with heavy metal ions, thereby endowing the material with excellent heavy metal ion adsorption function. Furthermore, the modified biochar surface has increased active sites, improving the chemical bonding efficiency with the polymer network and solving the interfacial compatibility problem of traditional physical mixing.

[0026] After the amination reaction is completed, the present invention preferably washes and dries the resulting reaction solid product to obtain the amination biochar.

[0027] This invention involves mixing starch with water and heating to gelatinize it, thereby obtaining gelatinized starch.

[0028] In this invention, the starch preferably comprises corn starch. Using corn starch as a raw material offers the advantages of low cost and wide availability. In this invention, the mass ratio of starch to water is preferably 1:(10~15), and in specific embodiments, it can be 1:10, 1:12.5, 1:13, 1:14, or 1:15. In this invention, the heating and gelatinization temperature is preferably 70~85℃, and the time is preferably 20~40 min; the heating and gelatinization is preferably carried out under stirring conditions. In this invention, the starch, as a natural high-molecular-weight polysaccharide, plays a crucial role in providing the polysaccharide backbone and active hydroxyl groups.

[0029] In this invention, the starch has very low reactivity. The present invention heats and gelatinizes it to activate its chemical reactivity. The large number of exposed hydroxyl groups are key active sites for graft copolymerization with acrylamide, and are also the basis for forming hydrogen bonds with sodium alginate and amino biochar. In addition, the gelatinized starch itself has a certain viscosity and gelling properties, which helps to form a uniform and stable premixed system and prevent component separation.

[0030] After obtaining aminated biochar and gelatinized starch, the present invention mixes the gelatinized starch, a mixed aqueous solution of sodium alginate-acrylamide, and the aminated biochar, and adds a Ca-containing solution to the resulting mixture. 2+ An aqueous solution is used to perform ionic cross-linking to form a pre-cross-linked gel precursor.

[0031] In this invention, the mass ratio of sodium alginate (SA) to acrylamide (AM) in the sodium alginate-acrylamide mixed aqueous solution is 1:(0.6~3). In specific embodiments, it can be 1:0.75, 1:1, 1:1.33, 1:1.5, 1:2, 1:2.5, 1:2.8, or 1:3, preferably 1:1.33. In this invention, the total concentration of sodium alginate and acrylamide in the sodium alginate-acrylamide mixed aqueous solution is preferably 0.2~0.5 g / mL. In this invention, the mass ratio of SA to AM affects the hydrophilicity and adsorption selectivity of the aminated biochar / sodium alginate dual-network gel. Specifically, the higher the SA content, the richer the carboxyl groups in the prepared aminated biochar / sodium alginate dual-network gel, the stronger the hydrophilicity, and the better it is at adsorbing heavy metal ions. However, because the carboxyl groups are sensitive to salt ions, its water retention capacity in salt water decreases, making it more suitable for improving low-salinity soils. The higher the AM content, the more amide groups the prepared aminated biochar / sodium alginate dual-network gel contains, resulting in enhanced hydrophobicity and a stronger tendency to adsorb organic pollutants (such as organic dyes). It also maintains good water retention capacity in high-salt soils, making it more suitable for soil improvement. When the mass ratio of SA to AM is 1:(1~2), it exhibits good adsorption effects on both heavy metal ions and organic pollutants, while also demonstrating good water retention performance in soils with varying salinity.

[0032] This invention uses sodium alginate, a natural polysaccharide, to replace acrylic acid, significantly reducing the proportion of synthetic monomers used and improving the biodegradability and environmental compatibility of the material from the source. Through the synergy of sodium alginate and acrylamide, the green characteristics of natural polymers are preserved while the advantages of synthetic monomers are used to compensate for the lack of mechanical properties, achieving a balance between environmental friendliness and performance optimization.

[0033] In this invention, the mass of the aminated biochar is 7-43% of the total mass of sodium alginate and acrylamide, and in specific embodiments, it can be 7.14%, 10%, 15%, 20%, 21.43%, 25%, 30%, 35%, 40%, or 42.86%. In this invention, as the mass percentage of aminated biochar gradually increases within the above range, the mechanical strength of the prepared aminated biochar / sodium alginate dual-network gel gradually increases, while the water absorption ratio decreases. However, it is worth noting that although the water absorption ratio of the dual-network gel decreases with increasing mass percentage of aminated biochar, its water retention capacity is still far superior to traditional water-retaining materials.

[0034] In this invention, the mass ratio of the aminated biochar to starch is (1~3):(1~3), preferably (1~3):2, and in specific embodiments it can be 1:2, 1.5:2, 2:2, 2.5:2 or 3:2.

[0035] The present invention does not have special requirements for the mixing process, as long as the components are mixed evenly. In the embodiments of the present invention, the mixing is specifically carried out under stirring conditions. Preferably, the gelatinized starch is cooled to room temperature to 50°C before mixing.

[0036] In this invention, the Ca-containing 2+ The aqueous solution is preferably an aqueous solution of calcium chloride, calcium nitrate, or calcium gluconate; the Ca-containing solution... 2+ The concentration of the aqueous solution is preferably 2% (w / v); the Ca-containing 2+ The preferred dosage of the aqueous solution is 1-3 mL of Ca per 1 g of sodium alginate. 2+ An aqueous solution containing Ca. In this invention, the Ca-containing solution... 2+ The preferred method for adding the aqueous solution is dropwise addition. This invention does not have specific requirements for the dropwise addition rate; dropwise addition is sufficient. In embodiments of this invention, the dropwise addition rate can be 1 mL / min. (The remaining text appears to be unrelated and likely refers to a different product / process.) 2+ After the aqueous solution is added, the present invention preferably continues stirring for 20-30 minutes to carry out ionic crosslinking. During the ionic crosslinking process, sodium alginate and calcium ions form a primary network with an "egg-box" structure through ionic crosslinking. With the addition of Ca... 2+ The addition of an aqueous solution instantly transforms the originally flowable, viscous liquid mixture into a non-flowing, elastic gel block, achieving a transformation from "sol" to "gel." Countless "egg carton" structures form within the system, connecting the linear SA molecular chains into a three-dimensional, stable, physically cross-linked network.

[0037] After obtaining the initially cross-linked gel precursor, the present invention mixes the initially cross-linked gel precursor, a diene cross-linking agent and a thermally decomposable free radical initiator for covalent cross-linking polymerization to obtain the aminated biochar / sodium alginate dual-network gel.

[0038] In this invention, the diolefin crosslinking agent preferably includes one or more of N,N'-methylenebisacrylamide (MBA), N,N'-(1,2-dihydroxyethylene)bisacrylamide, and polyethylene glycol diacrylate; the mass of the diolefin crosslinking agent is preferably 0.2-0.6% of the total mass of sodium alginate and acrylamide, and in specific embodiments it can be 0.2%, 0.3%, 0.43%, 0.5%, or 0.6%.

[0039] In this invention, the thermally decomposable initiator is preferably ammonium persulfate (APS), potassium persulfate, or sodium bisulfite; the mass of the thermally decomposable initiator is preferably 2-3% of the mass of acrylamide. In this invention, the thermally decomposable initiator can homolytically cleave to generate free radicals under heating conditions.

[0040] In this invention, the diene crosslinking agent and the thermally decomposable free radical initiator are preferably added to the initially crosslinked gel precursor for mixing.

[0041] In this invention, the covalent crosslinking polymerization is preferably carried out under a protective atmosphere, which preferably includes a nitrogen atmosphere. In this invention, the temperature of the covalent crosslinking polymerization is preferably 60-70°C, and the time is preferably 2-3 hours; in specific embodiments, the temperature of the covalent crosslinking polymerization can be 60, 65, or 70°C, and the time can be 2, 2.5, or 3 hours.

[0042] In the covalent crosslinking polymerization process described in this invention, the carbon-carbon double bond (C=C) in the acrylamide molecule is activated, forming monomer radicals, which then form a polymer chain. The monomer radicals rapidly react with surrounding AM monomers, causing the polymer chain to continuously elongate. Simultaneously, the activated hydroxyl sites on the starch molecule chain and the sodium alginate chain also participate in graft copolymerization, forming long, linear polyacrylamide molecular chains. When the elongated polymer chain encounters a crosslinking agent, it can covalently link two or more originally independent polyacrylamide molecular chains, ultimately forming a huge, three-dimensional covalently crosslinked network that runs throughout the entire system. This covalently crosslinked network is the secondary framework and stabilizing basis of the entire dual-network structure. Aminated modified biochar chemically bonds to the aforementioned dual network through its surface functional groups, forming a stable organic-inorganic interpenetrating structure. For example, the -NH2 on the aminated modified biochar can interact with the -COO in the gel network. - -CONH2 and -OH form a dense, multidimensional hydrogen bond network; the protonated quaternary ammonium group -NH3 + With -COO - A strong electrostatic attraction is generated between them; the long polymer chains effectively encapsulate and entangle the biochar particles.

[0043] After the covalent crosslinking polymerization is completed, the present invention preferably performs post-treatment on the obtained aminated biochar / sodium alginate dual-network gel; the post-treatment preferably includes: soaking and washing the aminated biochar / sodium alginate dual-network gel with deionized water to remove unreacted monomers, then cutting it into small pieces and vacuum drying to constant weight, and finally pulverizing and screening to obtain 1~2 mm particles for later use.

[0044] This invention organically integrates three key steps: biochar modification, ionic crosslinking, and covalent polymerization. By controlling the timing of ionic-covalent polymerization first and then covalent polymerization, the network structure can be constructed in a controllable manner. The process conditions are mild, energy consumption is reduced, and harsh conditions such as high temperature and high pressure are avoided. The entire process is carried out in an aqueous phase without the use of organic solvents, which conforms to the principles of green chemistry.

[0045] This invention provides an aminated biochar / sodium alginate dual-network gel prepared by the method described above, comprising aminated biochar and a dual-network gel; the aminated biochar and the dual-network gel form an organic-inorganic interpenetrating network structure; the dual-network gel is formed by starch, sodium alginate, and acrylamide through an ion-covalent crosslinking mechanism. Specifically, the primary network: through... The "egg-box" structure of sodium alginate forms an ionic cross-linked network, giving the material rapid prototyping capability and initial strength; secondary network: an acrylamide-starch covalent cross-linked network is formed through free radical polymerization, providing durable and stable mechanical support; network interpenetration: the two networks interpenetrate and synergistically enhance each other, constructing a stable structure that cannot be achieved by traditional single cross-linking methods.

[0046] This invention provides the application of the aminated biochar / sodium alginate dual-network gel described above in soil remediation.

[0047] In this invention, when the mass ratio of sodium alginate to acrylamide used in preparing the aminated biochar / sodium alginate dual-network gel is 1:(1~2), the aminated biochar / sodium alginate dual-network gel exhibits relatively balanced performance, good water absorption capacity, and good and relatively balanced adsorption capacity for both heavy metal ions and organic pollutants, making it suitable for the remediation of soils contaminated with heavy metal-organic composite pollution. In specific embodiments, when the soil to be improved is contaminated with heavy metal-organic composite pollution, the mass ratio of sodium alginate to acrylamide can be 1:1, 1:1.2, 1:1.33, 1:1.5, 1:1.6, 1:1.8, or 1:2. When the application focuses on drought resistance and moisture retention or remediation of heavy metal contaminated soil (such as Cd and Pb contaminated farmland), this invention preferably controls the mass ratio of sodium alginate to acrylamide to be 1:(0.6~1), and in specific embodiments, it can be 1:0.6, 1:0.75, 1:0.8, 1:0.9, or 1:1. When the focus is on remediation of organically contaminated soil (such as pesticide and dye contamination) or high-salinity soil, this invention preferably reduces the proportion of sodium alginate and controls the mass ratio of sodium alginate to acrylamide to be 1:(2~3), and in specific embodiments, it can be 1:2, 1:2.2, 1:2.5, 1:2.8, or 1:3. In this invention, high-salinity soil refers to soil with an electrical conductivity >4 mS / cm.

[0048] The following detailed description, in conjunction with embodiments, illustrates the aminated biochar / sodium alginate dual-network gel provided by the present invention, its preparation method, and its applications. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0049] Example 1 Reference Figure 1 The process shown is used to prepare an aminated biochar / sodium alginate dual-network gel.

[0050] Preparation of ammoniated biochar (N-BC): Accurately weigh 10.0 g of biochar powder, which has passed through 100 mesh and 200 mesh standard sieves sequentially, and place it in a round-bottom flask. Add 200 mL of anhydrous ethanol and sonicate for 30 min. Slowly add 5.0 mL of KH-792, install a reflux condenser, and reflux the reaction in a 75 °C water bath for 5 h. After the reaction is complete, wash the mixture three times with ethanol by centrifugation to remove unreacted coupling agent. Finally, dry the mixture in a 60 °C vacuum drying oven for 12 h, and grind it for later use.

[0051] Preparation of aminated biochar / sodium alginate dual-network gel (SA / AM-BC-Gel): Starch gelatinization: Weigh 2.0g of corn starch into a three-necked flask, add 25mL of deionized water, and gelatinize by stirring at 300rpm in an 80℃ water bath for 30min. Preparation of the mixture: Take 3.0g sodium alginate (SA) and 4.0g acrylamide (AM), dissolve them in 20mL of deionized water, and stir magnetically until completely dissolved; Ionic crosslinking: Cool the gelatinized starch solution to 50°C, combine it with the sodium alginate-acrylamide mixture, add 1.5g of aminated biochar, and stir at 400rpm for 15min to fully disperse it. Then, slowly add 5mL of 2% (w / v) CaCl2 solution using a constant pressure dropping funnel, controlling the dropping rate at 1mL / min. After the addition is complete, continue stirring for 20min to form a preliminary crosslinked gel precursor. Covalent crosslinking polymerization: 0.03 g MBA and 0.08 g APS were added sequentially to the system obtained in the previous step, nitrogen gas was introduced for protection, and the reaction was carried out in a water bath at 65°C for 2.5 h to obtain a complete black elastic gel block; Post-processing: The gel block was washed three times with deionized water to remove unreacted monomers, and then cut into pieces approximately 1 cm in size. 3 Small pieces were dried in a vacuum drying oven at 60℃ to constant weight, and finally crushed with a pulverizer. Particles of 1~2mm were obtained by screening, sealed and stored, and labeled as SA / AM-BC-Gel.

[0052] Figure 2 Fourier transform infrared spectrum of the aminated biochar / sodium alginate dual-network gel prepared in Example 1. The spectrum is located at approximately 3430 cm⁻¹. -1 The broad and strong absorption peak at approximately 1020-1200 cm⁻¹ is attributed to the stretching vibrations of the OH groups in sodium alginate and starch molecules, as well as the NH groups in acrylamide and aminated biochar, demonstrating the presence of natural polymers and active functional groups; -1The absorption peaks within the range are attributed to the COC ether bonds on the sodium alginate and starch sugar rings, further confirming the successful introduction of the natural polysaccharide backbone; at approximately 2950 cm⁻¹ -1 Approximately 2810cm -1 The clear absorption peaks appearing at approximately 1630 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of the methylene-CH₂- group, respectively, and are characteristic peaks of the silane coupling agent (KH-792) molecular chain grafted onto the biochar surface; at approximately 1630 cm⁻¹... -1 The strong absorption peak at approximately 1410 cm⁻¹, belonging to the amide I band, originates from the C=O stretching vibration of acrylamide, proving the successful construction of the polyacrylamide covalent crosslinked network; at approximately 1410 cm⁻¹... -1 Approximately 1340cm -1 The pair of strong absorption peaks appearing at the point are attributed to sodium alginate carboxylate ions. The asymmetric and symmetric stretching vibrations of sodium alginate are related to... Typical characteristics of ion cross-linking through the "egg box" model. Figure 2 This study systematically confirmed that aminated biochar / sodium alginate dual-network gel is a stable organic-inorganic hybrid material constructed through chemical interactions between aminated modified biochar, a sodium alginate-based ionic network, and an acrylamide-starch-based covalent network. This structure is the fundamental reason for its excellent water absorption and retention, high mechanical strength, and high heavy metal adsorption capacity, providing a solid structural foundation for its application in soil improvement.

[0053] Soil culture experiment: Test method: Take several 1kg portions of contaminated soil with a Cd content of 2.5 mg / kg. Set up the following groups: Control group: No materials were added.

[0054] Control group 1: Added 0.5% (w / w) of commercial sodium polyacrylate water-retaining agent by dry weight of soil.

[0055] Control group 2: Added 0.5% (w / w) raw biochar.

[0056] Comparative group 3: 0.5% (w / w) of physically mixed sample (pure gel powder: unaminated raw biochar = 7:3) was added. Except for the absence of biochar, the steps, formulation and dosage of the pure gel powder were exactly the same as those of SA / AM-BC-Gel in Example 1.

[0057] Experimental group: Add 0.5% (w / w) of Example 1 product SA / AM-BC-Gel.

[0058] All groups were cultivated in pots (planting Chinese cabbage) under the same conditions, with regular watering to maintain 60% of the field water holding capacity. Various indicators were measured after 30 days of cultivation.

[0059] Experimental results: (1) Water retention performance: The soil moisture content of the experimental group (24.9%) was the highest, and it was always higher than that of control group 1 and control group 2 during the watering interval, indicating that it has the best water retention and drought resistance.

[0060] (2) Heavy metal passivation effect: The Cd content in the soil (DTPA extraction method) and the Cd content in the stems and leaves (dry weight of the aboveground parts) of Chinese cabbage were determined (microwave digestion-inductively coupled plasma mass spectrometry (ICP-MS)). Soil available Cd: The experimental group had the lowest available Cd content and a passivation rate of 67.8%, which was significantly higher than that of control group 2 (35%) and control group 3 (45%). Plant Cd content: The Cd content in the Chinese cabbage of the experimental group was reduced by 69.4% compared with the control group, which was much better than other control groups.

[0061] (3) Soil physicochemical properties: Soil aggregate stability was determined by wet sieving, soil porosity was indirectly calculated by ring sieving, and soil organic matter content was determined by potassium dichromate volumetric method. The experimental group had the highest water-stable aggregate content, total porosity, and organic matter content among all groups, significantly better than the control group and all comparison groups; comparison group 2 showed improvement in organic matter and aggregates, but poor water retention; comparison groups 1 and 3 had limited effects on soil structure improvement; only the experimental group achieved simultaneous and efficient synergy of the three major functions of soil water retention, heavy metal passivation, and soil improvement. This proves that the material of this invention can not only retain water and passivate, but also effectively promote the formation of soil aggregate structure and improve soil aeration and permeability through its own elastic structure and the addition of biochar.

[0062] (4) Material durability: After one crop season, residual materials in the soil were recovered and tested using a wet sieving method. Specific data are shown in Table 2. The results show that the gel of the present invention still maintains its complete particle morphology and elasticity, and its recovery rate and functional retention rate are significantly higher than those of the control groups. This indicates that the dual-network structure formed by chemical bonding in the present invention has excellent environmental stability and can achieve long-term soil improvement function.

[0063] Table 1 Comparison of Cd content and soil physical properties among different treatment groups after soil incubation experiment

[0064] Table 2 Comparison of material persistence among different treatment groups after one crop season.

[0065] The results above show that the biochar composite gel prepared by this invention can simultaneously and efficiently achieve soil water retention, heavy metal passivation and structural improvement. Its effect far exceeds that of single materials or physical mixtures, making it a promising material for farmland soil improvement and remediation.

[0066] The following investigation explores the effect of biochar addition amount on the properties of composite gel: Examples 2-3 The only difference from Example 1 is the amount of aminated biochar added. The amounts of aminated biochar added in Examples 1-3 are as follows: Example 2 (SA / AM-BC-Gel-L): 0.5 g of aminated biochar (approximately 7.14% of the total monomer mass) was added. Example 1 (SA / AM-BC-Gel-M): 1.5g of aminated biochar was added (approximately 21.43% of the total monomer mass). Example 3 (SA / AM-BC-Gel-H): 3.0 g of aminated biochar (approximately 42.86% of the total monomer mass) was added.

[0067] Comparative Example (Physical Mix): Physically mixed sample (pure gel powder: unaminated raw biochar = 7:3), wherein the pure gel powder was identical to Example 1 except that no biochar was added.

[0068] Performance Tests and Results: 1) Water absorption ratio test: Experimental design: Weigh 0.5g of each sample dry gel, immerse it in deionized water until swelling equilibrium is reached, filter it out and weigh it, calculate the water absorption ratio (g / g), and the test results are shown in Table 3.

[0069] Results and Analysis: The water absorption ratio of SA / AM-BC-Gel-L was 350 g / g; that of SA / AM-BC-Gel-M was 320 g / g; that of SA / AM-BC-Gel-H was 250 g / g; and that of Physical Mix was 310 g / g. With increasing amounts of aminated biochar, the cross-linking density of the gel network increased (biochar acting as physical cross-linking points), and the water absorption ratio decreased accordingly. However, the water retention capacity of all chemically cross-linked samples remained at a high level (above 250 g / g). Although the water absorption ratio of the physically mixed sample was close to that of SA / AM-BC-Gel-M, its subsequent mechanical strength and adsorption performance were significantly worse.

[0070] 2) Compression strength test: Experimental design: Compression tests were performed on the swollen hydrogel using a texture analyzer, and its rupture strength was recorded. The results are shown in Table 3.

[0071] Results and Analysis: The compressive strength of SA / AM-BC-Gel-L was 25 kPa, that of SA / AM-BC-Gel-M was 48 kPa, that of SA / AM-BC-Gel-H was 65 kPa, and that of the Physical Mix was 15 kPa. The reinforcing effect of biochar was extremely significant. The higher the amount added, the greater the mechanical strength of the gel. Most importantly, even with the same amount added, the strength of chemically cross-linked SA / AM-BC-Gel-M was more than three times that of the physical mixture, demonstrating the significant advantages of chemically bonded structures.

[0072] 3)Cd 2+ Adsorption experiment: Experimental design: Prepare 100 mL of Cd solution with a concentration of 50 mg / L. 2+ Add 0.1g of dry gel to the solution, and measure the remaining concentration after adsorption.

[0073] Results and Analysis: The adsorption capacity of SA / AM-BC-Gel-L was 40 mg / g, that of SA / AM-BC-Gel-M was 45 mg / g, that of SA / AM-BC-Gel-H was 48 mg / g, and that of the Physical Mix was 35 mg / g. The adsorption capacity increased with increasing biochar content because the biochar provided more adsorption sites. The adsorption performance of SA / AM-BC-Gel-M and SA / AM-BC-Gel-H was significantly better than that of the physical mixture, thanks to their stable structure which allowed for sufficient exposure of adsorption sites and reduced detachment.

[0074] Table 3 Effect of different biochar addition amounts on the properties of composite gel

[0075] The results above lead to the conclusion that when the biochar addition is between 1.5g and 3.0g (accounting for 21.43% to 42.86% of the total monomer mass), the composite material maintains good water absorption performance (>250g / g) while achieving excellent mechanical strength and the highest adsorption capacity, representing the optimal formulation range for overall performance. Too low a concentration results in insufficient reinforcing effect, while too high a concentration excessively sacrifices water absorption performance.

[0076] The following investigation explores the effect of monomer ratio (SA / AM) on the hydrophilicity / hydrophobicity and adsorption selectivity of the composite gel: Examples 4-5 The only difference from Example 1 is that the variable is the mass ratio of SA to AM, and the total mass of monomers remains unchanged at 7.0g. Everything else is exactly the same as Example 1.

[0077] The specific mass ratios are as follows: Example 4: The mass ratio of SA to AM is 1:0.75 (highly hydrophilic, rich in carboxyl groups), denoted as Gel-SA-rich; Example 1: The mass ratio of SA to AM is 1:1.33 (balanced type), denoted as Gel-Balanced; Example 5: The mass ratio of SA to AM is 1:2.5 (relatively enhanced hydrophobicity, rich in amide groups), denoted as Gel-AM-rich.

[0078] Performance Tests and Results: 1) Water absorption kinetics test: Experimental design: The water absorption rate and equilibrium ratio of different samples in deionized water and 0.9% NaCl saline were recorded. The results are shown in Table 4.

[0079] Results and Analysis: In deionized water, Gel-SA-rich exhibited the highest water absorption ratio of 350 g / g. However, due to the sensitivity of the carboxyl groups to salt ions, its water retention ratio decreased most drastically in saline water, dropping to 45 g / g. Gel-Balanced showed a water absorption ratio of 320 g / g in deionized water and 95 g / g in saline water. Gel-AM-rich maintained relatively good water absorption capacity, with a water absorption ratio of 240 g / g in deionized water and 125 g / g in saline water. By adjusting the SA / AM ratio, the hydrophilicity / hydrophobicity of the gel can be customized, making it suitable for soils with different salinity conditions.

[0080] 2) Adsorption selectivity experiment: Experimental design: Preparation of a mixture containing Pb 2+ A 50 mL solution of a composite contaminant solution containing 100 mg / L methylene blue dye (MB, 100 mg / L) was added to 0.1 g of dry gel for adsorption. The results are shown in Table 4.

[0081] Results and Analysis: Gel-SA-rich: for Pb 2+ The adsorption rate is >95%, and the adsorption rate for MB is about 65%.

[0082] Gel-Balanced: for Pb 2+ The adsorption rate is about 90%, and the adsorption rate for MB is about 85%.

[0083] Gel-AM-rich: for Pb 2+ The adsorption rate is about 70%, and the adsorption rate for MB is >92%.

[0084] The above results demonstrate the functional designability of the material of this invention. Gels rich in carboxyl groups are more effective at adsorbing heavy metal ions, while gels rich in amide groups are more effective at adsorbing organic dyes. The balanced gel shows good results for both. This demonstrates the great application potential and flexibility of this invention in the face of complex polluted environments.

[0085] Table 4. Effects of different monomer ratios on the hydrophilicity / hydrophobicity and adsorption selectivity of the composite gel.

[0086] The results above lead to the conclusion that by adjusting the monomer ratio of SA to AM, the hydrophilicity, salt tolerance, and adsorption selectivity of the composite gel can be effectively controlled in a targeted manner. This allows for customized design and application for different soil pollution conditions (single heavy metal pollution, organic pollution, or combined pollution), further demonstrating the inventiveness and practicality of this invention.

[0087] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an aminated biochar / sodium alginate dual-network gel, characterized in that, Includes the following steps: Biochar, an amino-containing silane coupling agent, and an organic solvent are mixed and subjected to an amination reaction to obtain amination-treated biochar. Starch is mixed with water and heated to gelatinize, thus obtaining gelatinized starch; The gelation precursor is prepared by mixing the gelatinized starch, a mixed aqueous solution of sodium alginate-acrylamide and the aminated biochar, adding a Ca 2+ containing aqueous solution into the obtained mixed solution, and performing ionic crosslinking; the mass ratio of sodium alginate to acrylamide in the mixed aqueous solution of sodium alginate-acrylamide is 1:(0.6-3); the mass of the aminated biochar is 7-43% of the total mass of sodium alginate and acrylamide. The pre-crosslinked gel precursor, diene crosslinking agent, and thermally decomposable free radical initiator were mixed and covalently crosslinked to obtain the aminated biochar / sodium alginate dual-network gel.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the aminated biochar to starch is (1~3):(1~3).

3. The preparation method according to claim 1, characterized in that, The covalent crosslinking polymerization is carried out at a temperature of 60-70°C for 2-3 hours.

4. The preparation method according to claim 1, characterized in that, The amino-containing silane coupling agent includes one or more of N-β-aminoethyl-γ-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 3-(2-aminoethyl)-aminopropyltrimethoxysilane.

5. The preparation method according to claim 1 or 4, characterized in that, The mass ratio of the biochar to the volume ratio of the amino-containing silane coupling agent is 1 g: (0.4~0.6) mL; the amination reaction is carried out at a temperature of 70~80℃ for 4~6 h.

6. The preparation method according to claim 1, characterized in that, The starch includes corn starch.

7. The preparation method according to claim 1, characterized in that, The diolefin crosslinking agent includes one or more of N,N'-methylenebisacrylamide, N,N'-(1,2-dihydroxyethylene)bisacrylamide, and polyethylene glycol diacrylate; the mass of the diolefin crosslinking agent is 0.2~0.6% of the total mass of sodium alginate and acrylamide.

8. The aminated biochar / sodium alginate dual-network gel prepared by the preparation method according to any one of claims 1 to 7 comprises aminated biochar and dual-network gel; wherein the aminated biochar and the dual-network gel form an organic-inorganic interpenetrating network structure; wherein the dual-network gel is formed by starch, sodium alginate and acrylamide through an ion-covalent crosslinking mechanism.

9. The application of the aminated biochar / sodium alginate dual-network gel according to claim 8 in soil improvement.

10. The application according to claim 9, characterized in that, In the process of soil improvement, the mass ratio of sodium alginate to acrylamide used in preparing the aminated biochar / sodium alginate dual-network gel is adjusted according to the soil conditions. When the soil to be improved is organically polluted soil or high-salinity soil, the mass ratio of sodium alginate to acrylamide is 1:(2~3); when the soil to be improved is heavy metal polluted soil or focuses on soil drought resistance and moisture retention, the mass ratio of sodium alginate to acrylamide is 1:(0.6~1); when the soil to be improved is heavy metal-organic composite polluted soil, the mass ratio of sodium alginate to acrylamide is 1:(1~2); the high-salinity soil refers to soil with an electrical conductivity >4mS / cm.

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