Carbon-based composite water-retaining agent as well as preparation method and application thereof

Abstract

The invention discloses a carbon-based composite water-retaining agent and a preparation method and application thereof.The preparation method comprises the following steps that S1, flue-cured tobacco stems are subjected to high-temperature pyrolysis, and biochar is obtained; s2, respectively grinding and crushing biochar and potassium humate, drying, sieving with a 100-mesh sieve to obtain biochar powder and potassium humate powder, and dispersing the two powders in deionized water to obtain a mixed solution A; s3, dissolving acrylic acid in water under an ice-water bath condition, adding sodium hydroxide for neutralization, and then adding acrylamide to obtain a solution B; adding the mixed solution A into the solution B, and uniformly mixing to obtain a reaction solution; and S4, adding an initiator and a cross-linking agent into the reaction solution, then carrying out stirring reaction under the condition that the water bath temperature is 70-90 DEG C until gel is formed, then soaking and cleaning with absolute ethyl alcohol, and then drying and crushing to obtain the carbon-based composite water-retaining agent. The water-retaining agent provided by the invention not only improves the drought-resisting and water-retaining capacity of soil, but also increases the organic carbon content of the soil, and can promote the growth of crops.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement and agricultural materials technology, specifically relating to a carbon-based composite water-retaining agent, its preparation method, and its application. Background Technology

[0002] Under the combined effects of global warming and human activities, extreme weather events such as high temperatures and droughts occur frequently, posing a serious threat to agricultural production systems. Decreased soil moisture retention capacity has become one of the key factors restricting stable and high crop yields. To alleviate drought stress, water-retaining agents, as an important class of functional materials, are gradually being applied in water-saving agricultural cultivation and soil improvement.

[0003] Currently, most commercially available water-retaining agents are based on synthetic polymers such as polyacrylates and polyacrylamide. However, these materials still have limitations in practical applications. First, their water absorption rate, water retention duration, and repeated water absorption performance are often affected by soil type, salinity, and pH value. Under complex field conditions, their water absorption and retention effects are unstable, slow to take effect, and difficult to maintain a stable water-retaining effect over a long period. Second, most products have a single function, mainly focusing on physical water absorption, lacking a comprehensive regulatory effect on soil structure, nutrient retention, and microbial activity. Third, some synthetic water-retaining agents have long degradation cycles, potentially causing secondary pollution, which is incompatible with the concept of environmentally friendly sustainable agriculture. Furthermore, their high cost also limits their large-scale application in vast farmlands.

[0004] Therefore, developing a composite water-retaining agent with high water retention capacity, good environmental compatibility, multiple functions, and reasonable cost has become an urgent technological need for green agricultural development. Especially under the background of "dual carbon" goals, how to combine water-retaining materials with carbon sequestration and emission reduction, and soil fertility improvement to achieve synergistic effects of water retention, carbon increase, soil improvement, and plant growth promotion is of great significance for promoting the high-quality development of ecological agriculture. Summary of the Invention

[0005] To address the technical problems of existing commercial water-retaining agents, such as low water absorption performance, insufficient water retention stability, and limited functionality, this invention provides a carbon-based composite water-retaining agent. It not only has excellent water absorption and retention performance, but also has the functions of increasing soil carbon and promoting plant growth. It can be widely used in soil water retention and drought resistance, as well as soil fertility improvement.

[0006] The carbon-based composite water-retaining agent provided by this invention is prepared by the following method: S1. The tobacco stems are pyrolyzed at 500~600℃ to obtain biochar.

[0007] S2. After grinding and pulverizing biochar and potassium humate separately, dry them in an oven at 50~80℃, and then pass them through a 100-mesh sieve to obtain biochar powder and potassium humate powder. Disperse the two powders in deionized water to obtain a mixed solution A.

[0008] S3. Under ice-water bath conditions, dissolve acrylic acid in water, add sodium hydroxide for neutralization, and adjust the neutralization degree to 70-90% to obtain an acrylic acid-sodium acrylate solution. Then add acrylamide to obtain solution B. Add mixture A to solution B and mix thoroughly to obtain the reaction solution. The entire process must be carried out under ice-water bath conditions to avoid premature reaction.

[0009] Sodium hydroxide is added for neutralization because acrylic acid is too reactive and needs to be neutralized to reduce its activity, thus making the reaction process controllable. Acrylamide is added to provide more hydrophilic functional groups and to improve the strength and stability of the water-retaining agent.

[0010] S4. Add initiator and crosslinking agent to the reaction solution, and then stir the reaction at a water bath temperature of 70-90℃ until a gel is formed. Then soak and wash with anhydrous ethanol, dry and pulverize to obtain carbon-based composite water-retaining agent.

[0011] The initiator is a persulfate, preferably potassium persulfate (KPS), with KPS accounting for 0.25-1.25% of the mass of acrylic acid. The initiator serves to activate the material and provide grafting sites for biochar and potassium humate.

[0012] The crosslinking agent is N,N'-methylenebisacrylamide (MBA), and the mass of MBA accounts for 0.05-0.15% of the mass of acrylic acid. The role of the crosslinking agent is to graft polymerize the acrylic acid-acrylamide chains to form a polymer.

[0013] Preferably, the biochar accounts for 3-7% of the mass of acrylic acid.

[0014] Preferably, the potassium humate accounts for 1 to 3% of the mass of acrylic acid.

[0015] Preferably, the mass ratio of acrylic acid to acrylamide is (2~6):1.

[0016] Preferably, in step S4, after the gel is formed, it is kept at a constant temperature for 30-60 minutes to stabilize it, and then soaked and washed with anhydrous ethanol, dried and pulverized to obtain a carbon-based composite water-retaining agent.

[0017] Compared with the prior art, the advantages of the present invention are: (1) In this invention, biochar and potassium humate are grafted onto an acrylic acid-acrylamide skeleton through a graft polymerization process to form a complex three-dimensional network structure with multiple chains and porous structure. Biochar and potassium humate are tightly bonded to the polymer skeleton through hydrogen bonding, which significantly improves the water absorption and retention performance and structural stability of the material. (2) Tobacco stem biochar is derived from the resource recycling of agricultural waste. After high-temperature pyrolysis, it has more pores and a larger specific surface area. At the same time, the pyrolyzed tobacco stem biochar contains more hydrophilic functional groups and has certain water retention and soil improvement properties. Biochar polymerized on the acrylic acid-acrylamide skeleton can still maintain a porous structure and hydrophilic functional groups, which further improves the water absorption and water retention properties of the composite material through synergistic effect. (3) Potassium humate is a natural organic macromolecular colloid with a hydrophobic aromatic structure at its core and hydrophilic functional groups on its surface. It can stimulate root development, improve crop resistance, and also improve soil structure and water retention. Potassium humate grafted onto the acrylic acid-acrylamide skeleton still has abundant hydrophilic functional groups such as hydroxyl, carboxyl, and amide groups, thereby improving the water absorption and water retention performance of the composite material. (4) By grafting biochar and potassium humate and optimizing the raw material ratio, the water absorption and storage performance of the composite water-retaining agent was significantly improved, giving it the dual advantages of rapid water absorption and slow water release. Its material water absorption rate is ≥1530g / g, and it can also reach 128g / g water absorption rate in 0.9% NaCl solution. At the same time, when the solution pH is 6~9, its water absorption rate is ≥1400g / g, and it has excellent water absorption capacity under both acidic and alkaline conditions. (5) The water-retaining agent described in this invention has multiple functions. It can not only significantly improve the soil's water retention capacity, but also increase the soil's organic carbon content, improve soil fertility, and have a significant promoting effect on crop growth. It effectively solves the technical defects of traditional water-retaining agents with only one function, and at the same time has good environmental friendliness.

[0018] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0019] Figure 1 This is the chemical structural formula of the carbon-based composite water-retaining agent prepared in Example 1.

[0020] Figure 2 This is the Fourier transform infrared spectrum of the carbon-based composite water-retaining agent of Example 1.

[0021] Figure 3 This is a scanning electron microscope image of the carbon-based composite water-retaining agent of Example 1.

[0022] Figure 4These are comparison images of the appearance of the carbon-based composite water-retaining agent in Example 1 before and after it absorbs deionized water.

[0023] Figure 5 This is a comparison chart of the field test results of the carbon-based composite water-retaining agent applied in Example 1, the blank control, and the single organic fertilizer. Detailed Implementation

[0024] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0025] Example 1 A method for preparing a carbon-based composite water-retaining agent is as follows: S1. Pyrolyze the flue-cured tobacco stems at 500℃ for 1-2 hours to obtain biochar.

[0026] S2. Grind the biochar and potassium humate separately, then dry them in an oven at 50-80℃ until constant weight, and then pass them through a 100-mesh sieve to obtain biochar powder and potassium humate powder. Disperse 0.4g of biochar powder and 0.25g of potassium humate powder in 20mL of deionized water to obtain mixture A.

[0027] S3. Under ice-water bath conditions, add 10 mL of acrylic acid to 40 mL of deionized water, stir well, and then add sodium hydroxide to neutralize and adjust the degree of neutralization to 80% to obtain an acrylic acid-sodium acrylate solution; then add 2.5 g of acrylamide, with the mass ratio of acrylic acid to acrylamide being 4:1, to obtain solution B; add mixed solution A to solution B and mix well to obtain a reaction solution.

[0028] S4. Add 0.075g of potassium persulfate and 0.005g of N,N'-methylenebisacrylamide to the reaction solution. Then place the reaction vessel in a water bath at 85°C and stir until a gel is formed. After the gel is formed, continue heating in a water bath for 30 minutes to stabilize it. Then soak and wash with anhydrous ethanol to remove unreacted monomers on the surface. Then cut the gel into small pieces and dry it in an oven at 75°C until constant weight. Then further pulverize it to obtain a carbon-based composite water-retaining agent.

[0029] The chemical structural formula of the carbon-based composite water-retaining agent prepared in Example 1 is as follows: Figure 1 As shown in the figure. Here, n represents the degree of polymerization of acrylic acid, and m represents the degree of polymerization of acrylamide, both of which are integers.

[0030] Figure 2 This is the Fourier transform infrared spectrum of the carbon-based composite water-retaining agent of Example 1. The figure shows that the water-retaining agent contains a large number of hydroxyl, carboxyl, and amide groups.

[0031] Figure 3 This is a scanning electron microscope (SEM) image of the carbon-based composite water-retaining agent from Example 1. As can be seen, the carbon-based composite water-retaining agent exhibits a multi-chain three-dimensional network structure with abundant pores and a large specific surface area.

[0032] Comparative Example 1 This comparative example provides a commercially available polyacrylic acid-potassium acrylate copolymer water-retaining agent, purchased from Hebei Yida Agricultural Technology Co., Ltd.

[0033] Comparative Example 2 This comparative example provides a water-retaining agent, which is a biochar-acrylic acid-acrylamide water-retaining agent. The preparation method is the same as in Example 1, the only difference being that potassium humate is not used.

[0034] Comparative Example 3 This comparative example provides a water-retaining agent, which is a potassium humate-acrylic acid-acrylamide water-retaining agent. The preparation method is the same as in Example 1, the only difference being that flue-cured tobacco stem biochar is not used.

[0035] Comparative Example 4 This comparative example provides a water-retaining agent, which is an acrylic acid-acrylamide water-retaining agent. The preparation method is the same as in Example 1, except that potassium humate and flue-cured tobacco stem biochar are not used.

[0036] Specific preparation method: Under ice-water bath conditions, 10 mL of acrylic acid was added to 60 mL of deionized water, stirred evenly, and then sodium hydroxide was added for neutralization to adjust the degree of neutralization to 80%, resulting in an acrylic acid-sodium acrylate solution. Then, 2.5 g of acrylamide was added, with an acrylic acid to acrylamide mass ratio of 4:1, to obtain solution B. 0.075 g of potassium persulfate and 0.005 g of N,N'-methylenebisacrylamide were added to solution B. The reaction vessel was placed in a water bath at 85°C and stirred until a gel was formed. After the gel was formed, the water bath was heated for another 30 minutes to stabilize it. Then, the gel was soaked and washed with anhydrous ethanol to remove unreacted monomers from the surface. The gel was cut into small pieces and dried in a 75°C oven to constant weight, and then further pulverized to obtain an acrylic acid-acrylamide water-retaining agent.

[0037] Comparative Example 5 This comparative example provides a water-retaining agent formed by a simple physical mixture of biochar, potassium humate, and acrylic acid-acrylamide polymer. The preparation method is as follows: Step 1: Prepare acrylic acid-acrylamide polymer according to the method of Comparative Example 4.

[0038] Step 2: Take 0.4g of biochar powder and 0.25g of potassium humate powder obtained in step S2 of Example 1, add them to the acrylic acid-acrylamide polymer prepared in step 1, and mechanically stir for 20min to mix evenly to obtain a porous water-retaining agent composed of biochar, potassium humate and acrylic acid-acrylamide polymer.

[0039] The water-retaining agents of Example 1 and Comparative Examples 1-5 were tested for their ability to absorb deionized water and brine (NaCl aqueous solution). The test methods are as follows: The water absorption performance of the water-retaining agent was evaluated using a gravimetric method. At 25°C, 0.5 g of the water-retaining agent was weighed and placed in a beaker containing 1000 mL of deionized water. The mass of the water-retaining agent was measured periodically. The maximum water absorption rate of the water-retaining agent was the mass increment when it reached saturation, expressed in g / g.

[0040] The water absorption performance of the water-retaining agent in a 0.9% NaCl aqueous solution was evaluated using a gravimetric method. At 25°C, 0.5 g of the water-retaining agent was weighed and placed in a beaker containing 1000 mL of NaCl aqueous solution. The mass of the water-retaining agent was measured periodically. The maximum water absorption rate of the water-retaining agent was the mass increment when it reached saturation, expressed in g / g.

[0041] The method for determining the water retention rate at 40℃ involves placing the water-retaining agent saturated with deionized water in a 40℃ oven, weighing it every 24 hours, and calculating the water retention rate (water retention rate = remaining water volume / initial water absorption volume × 100%).

[0042] The test results are shown in Table 1.

[0043] Table 1. Comparison of water absorption properties of the water-retaining agents in Example 1 and Comparative Examples 1-5

[0044] As shown in Table 1, the carbon-based composite water-retaining agent described in Example 1 is significantly superior to the comparative examples in terms of deionized water absorption rate, salt solution water absorption rate, and water retention rate at 40℃. Specifically, the deionized water absorption rate reaches 1538.30 g / g, the water absorption rate in 0.9% NaCl solution is 127.90 g / g, and the water retention rate after 48 hours is as high as 79.55%, indicating that the synergistic grafting effect of biochar and potassium humate significantly improves the water absorption and retention performance, as well as the salt resistance and stability of the material. In contrast, the water-retaining agent prepared by the simple physical compounding method in Comparative Example 5 has lower performance than the single polymer water-retaining agent, further demonstrating the rationality and superiority of the simultaneous grafting polymerization process of biochar and potassium humate in this invention.

[0045] Figure 4The image shows a comparison of the appearance of the water-retaining agent before and after absorbing deionized water as described in Example 1. It can be seen that after absorbing water, the water-retaining agent expands from particles with a diameter of about 6 mm to hydrogels with a diameter of 166 mm, with a volume expansion of about 27 times. This characteristic allows it to effectively improve soil porosity and aeration through the dynamic process of water absorption expansion and dehydration shrinkage after being applied to the soil.

[0046] Application Case 1 Greenhouse pot experiment On April 27, 2025, each pot contained 8 kg of calcareous purple soil with a pH of 8.3. The maize variety was Fuyu No. 1. Five treatment groups were established: a blank control (CK, no water-retaining agent), 0.05% water-retaining agent (B1), 0.1% water-retaining agent (B2), 0.15% water-retaining agent (B3), and 0.2% water-retaining agent (B4) (all are the mass ratio of water-retaining agent to soil). After the water-retaining agent was thoroughly mixed with the soil, the mixture was potted, and 5 maize seeds were sown in each pot. Initially, conventional water and fertilizer management was implemented. When the plants had 4 true leaves, thinning was done to 2 plants per pot, followed by equal watering every 3 days. Plant height, leaf length, leaf width, and leaf area were measured at the jointing stage. After harvest, the fresh weight of the entire plant was measured, and the soil moisture content was determined. The specific results are shown in Table 2.

[0047] Table 2. Effects of different application rates of water-retaining agents on agronomic traits and soil moisture content of maize at the jointing stage.

[0048] As shown in Table 2, under drought conditions, the treatment groups treated with the water-retaining agent described in this invention showed better performance than the control group in terms of maize agronomic traits and soil moisture content. The B2 treatment group showed the most significant effect, with maize plant height increasing by 9.21%, leaf area by 10.38%, biomass by 17.30%, and soil moisture content by 12.41% compared to the control group. The maize growth in the treatment groups treated with the water-retaining agent was significantly better than that in the control group, indicating that the water-retaining agent described in this invention, when applied at appropriate dosages, can significantly promote maize growth and improve soil water retention capacity.

[0049] Application Case 2 Field Experiment of Summer Maize in Purple Soil Areas on Sloping Farmland On May 28, 2025, a field trial of summer maize was conducted, with four treatment groups: blank control (CK), organic fertilizer alone (OM, 200 kg / mu), water-retaining agent + organic fertilizer compound 1 (BHPP1: 3 kg / mu water-retaining agent + 200 kg / mu organic fertilizer), water-retaining agent + organic fertilizer compound 2 (BHPP2: 6 kg / mu water-retaining agent + 200 kg / mu organic fertilizer), and water-retaining agent + organic fertilizer compound 3 (BHPP3: 9 kg / mu water-retaining agent + 200 kg / mu organic fertilizer). All materials were applied once in holes and mixed with the soil before planting, planted at conventional density, with rapeseed as the previous crop, and field management followed local agricultural operation standards. Maize-related agronomic traits, soil moisture content, and soil organic carbon content were measured during key growth stages. The specific results are shown in Tables 3 and 4.

[0050] Table 3 Effects of different treatments on agronomic traits at the jointing, tasseling, and maturity stages of maize

[0051] During the experiment, Zhongjiang County, Deyang City, Sichuan Province, experienced severe high temperature and drought, with higher than average temperatures from May to September and only 38.1 mm of rainfall in June, which adversely affected maize growth. Table 3 shows that compared with the CK and OM groups, the compound treatment group treated with the carbon-based composite water-retaining agent described in this invention showed significant improvements in agronomic traits at all key growth stages of maize: at the jointing stage, the plant height, leaf area, and biomass of the BHPP2 group increased by 24.60%, 37.09%, and 75.26% respectively compared to the CK group; at the tasseling and maturity stages, the plant height increased by 4.53%–10.33%, and the biomass increased by 32.27%–34.45%. Figure 5 The maturity phenotype diagrams shown indicate that the BHPP2 treatment with an application rate of 6 kg / mu of water-retaining agent showed a particularly significant growth-promoting effect.

[0052] Table 4. Effects of different treatments on soil moisture content and total organic carbon at maturity at different stages.

[0053] As shown in Table 4, the soil moisture content of the BHPP2 group increased most significantly at each growth stage, reaching 23.70%, 23.58%, and 30.51% at the jointing, tasseling, and maturity stages, respectively. The soil moisture content at maturity was 28.52% and 25.76% higher than that of the CK and OM groups, respectively, indicating that the water-retaining agent described in this invention can still effectively improve soil moisture retention under extremely rare high-temperature and drought conditions. Meanwhile, the total organic carbon content of the soil in the BHPP2 group at maturity was 8.15% and 6.48% higher than that of the CK and OM groups, respectively, while the application of organic fertilizer alone had no significant effect on improving the total organic carbon content. This proves that the water-retaining agent described in this invention can effectively increase soil organic carbon content and improve soil fertility.

[0054] In summary, regardless of whether it is a pot experiment or a field trial, the application of the carbon-based composite water-retaining agent of this invention can improve the agronomic traits and soil moisture content of maize. Simultaneously, the total organic carbon content of the soil was also effectively increased under field trial conditions. This indicates that the appropriate application rate of the carbon-based composite water-retaining agent can not only improve the water retention and carbon-increasing capacity of purple soil but also has a significant growth-promoting effect, thus achieving the goal of coping with agricultural drought and ensuring stable crop yields.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a carbon-based composite water-retaining agent, characterized in that, Includes the following steps: S1. High-temperature pyrolysis of flue-cured tobacco stems yields biochar; S2. Grind and pulverize biochar and potassium humate separately, dry them, and pass them through a 100-mesh sieve to obtain biochar powder and potassium humate powder. Disperse the two powders in deionized water to obtain a mixture A. S3. Under ice-water bath conditions, dissolve acrylic acid in water, add sodium hydroxide to neutralize it, adjust the degree of neutralization to 70-90%, then add acrylamide to obtain solution B; add mixed solution A to solution B and mix well to obtain reaction solution; S4. Add initiator and crosslinking agent to the reaction solution, and then stir the reaction at a water bath temperature of 70~90℃ until a gel is formed. Then soak and wash with anhydrous ethanol, dry and pulverize to obtain carbon-based composite water-retaining agent.

2. The preparation method of the carbon-based composite water-retaining agent as described in claim 1, characterized in that, The biochar accounts for 3-7% of the mass of acrylic acid.

3. The preparation method of the carbon-based composite water-retaining agent as described in claim 2, characterized in that, The potassium humate accounts for 1-3% of the mass of acrylic acid.

4. The preparation method of the carbon-based composite water-retaining agent as described in claim 3, characterized in that, The mass ratio of acrylic acid to acrylamide is (2~6):

1.

5. The preparation method of the carbon-based composite water-retaining agent as described in claim 1, characterized in that, The initiator is persulfate.

6. The preparation method of the carbon-based composite water-retaining agent as described in claim 1, characterized in that, The crosslinking agent is N,N'-methylenebisacrylamide.

7. The preparation method of the carbon-based composite water-retaining agent as described in claim 1, characterized in that, In step S4, after the gel is formed, it is kept at a constant temperature for 30-60 minutes, then soaked and washed with anhydrous ethanol, dried and pulverized to obtain a carbon-based composite water-retaining agent.

8. The preparation method of the carbon-based composite water-retaining agent as described in claim 1, characterized in that, In step S1, the temperature of high-temperature pyrolysis is 500~600℃.

9. A carbon-based composite water-retaining agent, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The application of the carbon-based composite water-retaining agent as described in claim 9, characterized in that, It is used for soil drought resistance and water retention, increasing soil organic carbon and promoting plant growth.

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