Hydrothermal carbon-sodium alginate slow-release fertilizer and preparation method thereof

Abstract

This invention relates to the field of fertilizer technology, specifically to a hydrothermal char-sodium alginate slow-release fertilizer based on subcritical conversion technology and its preparation method. The invention involves adding sludge-based hydrothermal char to a urea-sodium alginate mixture and forming a three-dimensional gel network through cross-linking of sodium alginate and calcium ions. This allows urea components to embed into the network during gelation, resulting in a slow-release fertilizer with dual regulation of nitrogen adsorption and desorption behavior. Introducing hydrothermal char into the sodium alginate gel system utilizes its pore structure, adsorption capacity, and mechanical strength to improve the structural characteristics of the gel, enhance nitrogen adsorption and retention, thereby further regulating the release process, reducing nitrogen loss, and improving the overall slow-release effect and crop nutrient utilization efficiency.

Description

Technical Field

[0001] The invention relates to the field of fertilizer technology, specifically to a hydrothermal carbon-sodium alginate slow-release fertilizer based on subcritical conversion technology and its preparation method. Background Technology

[0002] Southern my country experiences heavy rainfall and a concentrated rainy season, often resulting in highly humid or waterlogged farmland soils. Under these conditions, traditional urea is easily lost through rainwater runoff and leaching. Furthermore, increased microbial activity and changes in soil pH lead to ammonia volatilization and denitrification, significantly reducing nitrogen fertilizer utilization. This not only wastes fertilizer but also exacerbates environmental problems such as eutrophication. Therefore, developing slow-release materials that can resist water erosion and extend the nutrient supply cycle is of great significance in the context of high rainfall in southern China.

[0003] Since the 1960s, countries such as the United States, Japan, and Canada have successively carried out research on coated slow-release fertilizers and developed products represented by sulfur-coated and polymer-coated materials. Their characteristic is that the continuous release of nutrients is achieved by controlling the coating structure. Subsequently, foreign research gradually expanded to natural polymer materials, inorganic mineral materials, and organic compound systems to improve the environmental friendliness and biodegradability of the coating. Research on slow-release fertilizers in my country has gradually developed since the 1970s. Driven by the need to address problems such as low fertilizer utilization and agricultural non-point source pollution, domestic scholars have continuously explored coating materials that are widely available, cost-effective, and environmentally friendly. Summary of the Invention

[0004] The purpose of this invention is to provide a gel-based slow-release fertilizer with slow-release capability and good biocompatibility.

[0005] The specific technical solution for achieving the objective of this invention is as follows: A hydrothermal char-sodium alginate slow-release fertilizer is obtained by adding sludge-based hydrothermal char to a urea-sodium alginate mixture and forming a three-dimensional gel network through cross-linking of sodium alginate and calcium ions. Urea components are embedded in the network during the gelation process, resulting in a slow-release fertilizer with dual regulation of nitrogen adsorption and desorption behavior. The mechanism of action is that the carboxyl and hydroxyl functional groups on the sodium alginate chain can bind to molecules such as urea through hydrogen bonds and electrostatic interactions, stably "anchoring" them to the network. The mass ratio of urea:sodium alginate:sludge-based hydrothermal carbon is 30:0.5-1:0.25-1, preferably 30:1:0.5.

[0006] The present invention also provides a method for preparing hydrothermal carbon-sodium alginate slow-release fertilizer.

[0007] The preparation method of hydrothermal carbon-sodium alginate slow-release fertilizer includes the following steps: (1) Sludge is directionally converted into nanoscale biochar through subcritical hydrothermal carbonization: sludge-based hydrothermal carbon; (2) Disperse the sludge-based hydrothermal carbon obtained in step (1) evenly into a urea-sodium alginate solution to obtain a precursor solution; (3) Add CaCl2 solution to the precursor solution obtained in step (2) to obtain a gel slow-release fertilizer using subcritical conversion technology.

[0008] Preferably, the solid-liquid mass ratio of the sludge in step (1) is 1:3-5.

[0009] Preferably, the concentration of the urea solution in step (2) is 6.67-10 mol / L. For example, if 30g of urea is dissolved in 50ml of water, the actual measured urea concentration is 7mol / L.

[0010] Preferably, in step (2), the mass ratio of urea:sodium alginate:sludge-based hydrothermal carbon is 30:0.5-1:0.25-1, and more preferably 30:1:0.5-1.

[0011] Preferably, the mass concentration of the CaCl2 solution in step (3) is 2-5%.

[0012] Specifically, the steps include the following: Step 1: Preparation of sludge-based hydrothermal carbon Sludge was directionally converted into nanoscale biochar through subcritical hydrothermal carbonization. The sludge was reacted at 180–220 °C and 80–120 rpm for 0.5–1.5 h. After the reaction, the centrifuged and dried solid was crushed and sieved (200 mesh) to obtain sludge-based hydrothermal biochar.

[0013] Step 2: Preparation of precursor solution Urea was dissolved in water and stirred to obtain a urea solution. Sodium alginate was then added to the urea solution and stirred continuously at 40-80 °C for 0.5-1.5 h until the sodium alginate was completely dissolved, yielding a urea-sodium alginate solution. The sludge-based hydrothermal carbon from step (1) was taken, added to the urea-sodium alginate solution, and stirred at 60 °C for 30 min until the sludge-based hydrothermal carbon was evenly dispersed, yielding a precursor solution.

[0014] Step 3: Preparation of hydrothermal carbon-sodium alginate slow-release fertilizer using subcritical conversion technology The precursor solution was slowly added dropwise to the CaCl2 solution and retained for 10-20 min to obtain hydrothermal carbon-sodium alginate slow-release fertilizer (gel slow-release fertilizer).

[0015] This invention relates to sludge-based hydrothermal char, produced by hydrothermal carbonization of municipal sludge. It retains the organic matter and some inorganic components of the sludge, possessing a high specific surface area and well-developed pores. It can regulate the release of nitrogen from fertilizers through physical adsorption, while simultaneously reducing the environmental burden caused by sludge landfill and incineration, thus achieving resource utilization. Sodium alginate, a natural polysaccharide derived from brown algae, contains a large number of carboxyl groups, allowing it to undergo ionic cross-linking with calcium ions to form a stable three-dimensional gel network. This network is used to encapsulate fertilizers and reduce their diffusion rate in water, making it a green and environmentally friendly slow-release matrix. Introducing hydrothermal char into the sodium alginate gel system utilizes its pore structure, adsorption capacity, and mechanical strength to improve the gel's structural characteristics, enhance nitrogen adsorption and retention, thereby further regulating the release process, reducing nitrogen loss, and improving the overall slow-release effect and crop nutrient utilization efficiency.

[0016] This invention, for the first time, uses municipal sewage sludge as raw material to prepare nano-biochar under low-temperature, low-pressure subcritical conditions. This is combined with a sodium alginate hydrogel structure possessing a three-dimensional porous network to prepare a controlled-release fertilizer coating material with excellent slow-release performance and low nitrogen leaching rate. The three-dimensional gel network is formed through cross-linking of sodium alginate and calcium ions. Furthermore, the synergistic effect of the porous structure and adsorption properties of the sludge-based hydrothermal carbon with the gel allows for dual regulation of nitrogen adsorption-desorption behavior, achieving slow nutrient release. The excellent slow-release performance and low nitrogen leaching rate significantly improve nutrient utilization efficiency. Moreover, the prepared sludge-based hydrothermal carbon slow-release fertilizer can effectively adsorb and retain nutrients in the fertilizer, reducing environmental pollution.

[0017] This invention features a simple and low-cost preparation process, is environmentally friendly, and suitable for sustainable agricultural production, enabling controllable fertilizer release. Based on a sodium alginate-calcium ion gel system and incorporating sludge-based hydrothermal carbon as a functional filler, this invention prepares a green, environmentally friendly, biodegradable, easily prepared, and excellent slow-release sodium alginate-based sludge carbon composite slow-release urea fertilizer. This application can be used in non-food applications such as flower cultivation fertilizer and forest substrate soil fertilizer. This application not only improves nitrogen fertilizer utilization and reduces fertilizer application and environmental risks, but also achieves high-value utilization of sludge, which has significant scientific and practical application value for promoting green agricultural development. Attached Figure Description

[0018] Figure 1 This is a SEM image of the hydrothermal carbon used in this invention.

[0019] Figure 2 This is a graph showing the time-total nitrogen release rate of the gel-release fertilizer of this invention.

[0020] Figure 3 This is the infrared spectrum of the gel-release fertilizer of the present invention. Detailed Implementation

[0021] The invention will be further described with reference to the accompanying drawings. All reagents used in the following examples are conventional reagents of analytical purity and do not require further purification before use; the related preparation and detection methods are conventional methods.

[0022] Example 1 (1) Dissolve 30 g of urea in 50 mL of water and stir for 5 min to obtain a urea solution. Then, add 1 g of sodium alginate to the urea solution and stir continuously at 60 °C for 1 h until the sodium alginate is completely dissolved to obtain a urea-sodium alginate solution.

[0023] (2) The urea-sodium alginate solution was slowly added dropwise to a 3% CaCl2 solution and retained for 10 min to obtain URS. The infrared spectrum is shown in the figure. Figure 3 .

[0024] Example 2 (1) The sludge was reacted at 200 ℃ and 100 rpm for 1 h under hydrothermal conditions. After the reaction was completed, the centrifuged and dried solid was crushed and sieved to obtain sludge-based hydrothermal carbon (SEM image shown). Figure 1 ).

[0025] (2) Dissolve 30 g of urea in 50 mL of water and stir for 5 min to obtain a urea solution. Then, add 1 g of sodium alginate to the urea solution and stir continuously at 60 °C for 1 h until the sodium alginate is completely dissolved to obtain a urea-sodium alginate solution. Take 0.25 g of the sludge-based hydrothermal carbon from step (1), add it to the urea-sodium alginate solution, and stir at 60 °C for 30 min until the sludge-based hydrothermal carbon is evenly dispersed to obtain a urea-sodium alginate-hydrothermal carbon solution.

[0026] (3) The urea-sodium alginate-hydrothermal charcoal solution was slowly added dropwise to a 3% CaCl2 solution and retained for 10 min to obtain URS-0.25. The infrared spectrum is shown in the figure. Figure 3 .

[0027] Example 3 (1) The sludge was reacted at 200 °C and 100 rpm for 1 h under hydrothermal conditions. After the reaction was completed, the centrifuged and dried solid was crushed and sieved to obtain sludge-based hydrothermal carbon.

[0028] (2) Dissolve 30 g of urea in 50 mL of water and stir for 5 min to obtain a urea solution. Then, add 1 g of sodium alginate to the urea solution and stir continuously at 60 °C for 1 h until the sodium alginate is completely dissolved to obtain a urea-sodium alginate solution. Take 0.5 g of the sludge-based hydrothermal carbon from step (1), add it to the urea-sodium alginate solution, and stir at 60 °C for 30 min until the sludge-based hydrothermal carbon is evenly dispersed to obtain a urea-sodium alginate-hydrothermal carbon solution.

[0029] (3) The urea-sodium alginate-hydrothermal charcoal solution was slowly added dropwise to a 3% CaCl2 solution and retained for 10 min to obtain URS-0.5. The infrared spectrum is shown in the figure. Figure 3 .

[0030] Example 4 (1) The sludge was reacted at 200 °C and 100 rpm for 1 h under hydrothermal conditions. After the reaction was completed, the centrifuged and dried solid was crushed and sieved to obtain sludge-based hydrothermal carbon.

[0031] (2) Dissolve 30 g of urea in 50 mL of water and stir for 5 min to obtain a urea solution. Then, add 1 g of sodium alginate to the urea solution and stir continuously at 60 °C for 1 h until the sodium alginate is completely dissolved to obtain a urea-sodium alginate solution. Take 0.75 g of the sludge-based hydrothermal carbon from step (1), add it to the urea-sodium alginate solution, and stir at 60 °C for 30 min until the sludge-based hydrothermal carbon is evenly dispersed to obtain a urea-sodium alginate-hydrothermal carbon solution.

[0032] (3) The urea-sodium alginate-hydrothermal charcoal solution was slowly added dropwise to a 3% CaCl2 solution and retained for 10 min to obtain URS-0.75. The infrared spectrum is shown in the figure. Figure 3 .

[0033] Example 5 (1) The sludge was reacted at 200 °C and 100 rpm for 1 h under hydrothermal conditions. After the reaction was completed, the centrifuged and dried solid was crushed and sieved to obtain sludge-based hydrothermal carbon.

[0034] (2) Dissolve 30 g of urea in 50 mL of water and stir for 5 min to obtain a urea solution. Then, add 1 g of sodium alginate to the urea solution and stir continuously at 60 °C for 1 h until the sodium alginate is completely dissolved to obtain a urea-sodium alginate solution. Take 1 g of the sludge-based hydrothermal carbon from step (1), add it to the urea-sodium alginate solution, and stir at 60 °C for 30 min until the sludge-based hydrothermal carbon is evenly dispersed to obtain a urea-sodium alginate-hydrothermal carbon solution.

[0035] (3) The urea-sodium alginate-hydrothermal charcoal solution was slowly added dropwise to a 3% CaCl2 solution and retained for 10 min to obtain URS-1. The infrared spectrum is shown in the figure. Figure 3 .

[0036] Example 6 10 g of the slow-release fertilizer prepared in Examples 1-5 was placed in 200 mL of ultrapure water for culture. Samples were taken on days 1, 3, 5, 7, 10, 15, and 30 to measure the total nitrogen release rate.

[0037] See the appendix of this invention. Figure 2 In the time-total nitrogen release rate graph, the 80% nutrient release time for URS-0.5 was 10 days, which is 3 times and 1.5 times higher than that for Urea (3 days) and URS (6 days), respectively. This indicates that sodium alginate gel has a sustained-release effect. Meanwhile, the addition of hydrothermal charcoal effectively enhanced the sustained-release capacity of the gel.

[0038] Tables 1 and 2 show the analysis of the hydrothermal carbon material URS-5.

[0039] Table 1. Analysis of the physicochemical properties of sludge hydrothermal carbon Sludge (dry basis) 49.99 43.14 6.87 5.24 Sludge carbide thermal carbon 71.89 23.92 4.19 2.06 Table 2. Physicochemical Properties Analysis of Hydrothermal Charcoal-Sodium Alginate Slow-Release Fertilizer Cd 0.01 ≤3 Cu 9.54 Ni 0.28 Pb 0.26 ≤50 As 0.09 ≤15 Zn 5.36 Cr 0.55 ≤150 Hg organic matter NPK 0.01171779.14 ≤2 .

Claims

1. A hydrochar-sodium alginate slow-release fertilizer, characterized by: By adding sludge-based hydrothermal carbon to a urea-sodium alginate mixture and forming a three-dimensional gel network through cross-linking of sodium alginate and calcium ions, urea is embedded into the network during the gelation process, resulting in a hydrothermal carbon-sodium alginate slow-release fertilizer with dual regulation of nitrogen adsorption-desorption behavior. The sludge-based hydrothermal char is prepared by hydrothermal carbonization of municipal sludge, retaining the organic matter and some inorganic components in the sludge, and has a high specific surface area and well-developed pores. The preparation method of sludge-based hydrothermal char is as follows: sludge is directionally converted into nanoscale biochar through subcritical hydrothermal carbonization: the sludge is reacted under hydrothermal conditions of 180~220℃ and 80~120 rpm for 0.5~1.5 h; after the reaction is completed, the centrifuged and dried solid is crushed and sieved through a 200-mesh sieve to obtain sludge-based hydrothermal char. The mass ratio of urea:sodium alginate:sludge-based hydrothermal carbon is 30:0.5-1:0.25-1.

2. The hydrochar-sodium alginate slow-release fertilizer according to claim 1, characterized in that: The mass ratio of urea, sodium alginate, and sludge-based hydrothermal carbon is 30:1:0.

5.

3. The preparation method of the hydrothermal carbon-sodium alginate slow-release fertilizer according to claim 1, characterized in that: Includes the following steps: (1) Sludge is directionally converted into nanoscale biochar through subcritical hydrothermal carbonization: sludge-based hydrothermal carbon; (2) Disperse the sludge-based hydrothermal carbon obtained in step (1) evenly into a urea-sodium alginate solution to obtain a precursor solution; (3) Add CaCl2 solution to the precursor solution obtained in step (2) to obtain hydrothermal carbon-sodium alginate slow-release fertilizer using subcritical conversion technology.

4. The preparation method according to claim 3, characterized in that: In step (1), the solid-liquid mass ratio of the sludge is 1:3-5.

5. The preparation method according to claim 3, characterized in that: The concentration of the urea solution in step (2) is 6.67-10 mol / L.

6. The preparation method according to claim 3, characterized in that: The concentration of the CaCl2 solution in step (3) is 2-5%.

7. The preparation method according to any one of claims 3-6, characterized in that: Specifically, the steps include the following: (1) Sludge is directionally converted into nano-scale biochar through subcritical hydrothermal carbonization: the sludge is reacted at 180~220℃ and 80~120 rpm for 0.5~1.5 h; after the reaction is completed, the centrifuged and dried solid is crushed and sieved through 200 mesh to obtain sludge-based hydrothermal carbon. (2) Preparation of precursor solution: Dissolve urea in water and stir to obtain urea solution; then add sodium alginate to urea solution and stir continuously at 40-80 ℃ for 0.5-1.5 h until sodium alginate is completely dissolved to obtain urea-sodium alginate solution; take the sludge-based hydrothermal carbon from step (1), add urea-sodium alginate solution and stir at 60 ℃ for 30 min until sludge-based hydrothermal carbon is evenly dispersed to obtain precursor solution; (3) Preparation of hydrothermal carbon-sodium alginate slow-release fertilizer using subcritical conversion technology: The precursor solution was slowly added dropwise to CaCl2 solution and retained for 10-20 min to obtain hydrothermal carbon-sodium alginate slow-release fertilizer using subcritical conversion technology.

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