Bamboo porous biochar with high-capacity element loading and long-time slow release and preparation method of bamboo porous biochar

Abstract

The invention relates to the technical field of plant nutrition, in particular to bamboo porous biochar with high-capacity element loading and long-time slow release and a preparation method of the bamboo porous biochar. The preparation method comprises the following steps: firstly, modifying bamboo particles with an amino silane coupling agent, then ball-milling and uniformly mixing with lignosulfonate, then soaking in an activator solution, and then sequentially carrying out pre-oxidation, carbonization, activated pore-forming and high-temperature refining to obtain porous carbon with pore size distribution concentrated in 1-3nm, the nutrient loading component which can fully load nutrients and can realize a slow-release effect is obtained.

Description

Technical Field

[0001] This invention relates to the field of plant nutrition technology, specifically to a bamboo porous biochar with high capacity element loading and long-term slow release, and its preparation method. Background Technology

[0002] With the increasing demands for high efficiency and environmental protection in modern agriculture, controlled-release fertilizers have become a research hotspot. Existing controlled-release technologies can be mainly classified into the following categories:

[0003] 1. Coated slow-release fertilizer: This type of fertilizer controls nutrient release by coating fertilizer granules with polymers. However, it has drawbacks such as high cost of coating materials, difficulty in degradation, and potential secondary pollution.

[0004] 2. Chemically synthesized slow-release fertilizers: such as urea-formaldehyde, whose release rate is greatly affected by soil microorganisms and temperature, are unstable and expensive.

[0005] 3. Adsorption-type slow-release materials: Currently, mineral materials (such as bentonite and diatomaceous earth) or biochar prepared from industrial by-products (such as sludge biochar) are mostly used as carriers. However, adsorption-type slow-release materials have the following problems:

[0006] The pore size distribution is wide and uncontrollable: most are macropores or micropores, lacking mesopores (2-50 nm) that have an ideal slow-release effect on nutrient molecules. More importantly, existing technologies have never designed and optimized the relationship between the carrier pore size and the hydration dynamic diameter of plant nutrient molecules.

[0007] Single function: It mainly focuses on improving soil by utilizing its cation exchange capacity or acid-base balance. It lacks precise physicochemical design for the "loading-release" behavior of nutrients, resulting in unsatisfactory slow-release effect and limited improvement in nutrient utilization.

[0008] Low material purity: For example, sludge peat may contain pollutants such as heavy metals, posing an environmental risk.

[0009] According to the European Committee for Standardization working group's definition of slow-release fertilizers, ≤15% of nutrients should be released within 24 hours, and the remaining nutrients should be released completely within 28 days. The Chinese national standard GB / T 23348-2009 Slow-Release Fertilizers also has a similar standard: in still water at 25℃, the release rate of fertilizer nutrients should not exceed 15% within 24 hours, and the cumulative release rate within 28 days should not exceed 75%.

[0010] CN120965420A discloses a biochar-based slow-release fertilizer for improving acidified soil, comprising the following raw materials in parts by weight: 60 parts biochar system, 5-12 parts nitrogen fertilizer, 10-18 parts phosphate fertilizer, 5-12 parts potassium fertilizer, 5-10 parts acidified soil conditioner, and 2-5 parts slow-release membrane. This patent requires a slow-release membrane to achieve the slow-release effect, and the preparation process of the slow-release membrane is complex, requiring the membrane material solution to be sprayed onto the substrate surface three times to obtain a three-layer slow-release membrane.

[0011] CN120943694A discloses a humic acid-coated zeolite-based nutrient slow-release fertilizer, comprising a zeolite-based core and a humic acid polymer coating covering the zeolite-based core. It possesses strong cation exchange capacity, high nutrient loading, and slow-release nutrient functions. However, the preparation process is complex and costly.

[0012] CN114477129 discloses a modified water-coke, which, together with active substances such as humic acid (preferably artificial humic acid), can be combined to form an active humic acid fertilizer similar to soil humification. However, the modified water-coke of this patent cannot meet the requirements of a slow-release fertilizer carrier due to the mismatch between pore size and porosity.

[0013] CN120794774A discloses a biochar composite hydrogel slow-release fertilizer, which prepares hydrogel in situ within the pores of biochar, thus fixing the fertilizer in situ within the biochar pores as well. The hydrogel controls the diffusion rate, forming a double slow-release barrier. However, the slow release of the hydrogel is directly related to temperature; if the temperature is not properly controlled, the slow-release effect cannot be achieved.

[0014] Through in-depth research and analysis, the inventor discovered that nitrogen, a core nutrient in plants (mainly in the form of NH4), is... + and NO3 - Phosphorus (mainly in the form of H2PO4) - Forms), potassium (mainly K) + The diameter of hydrated ions in soil solution (morphology) is all below 0.7 nm. There are no reports in the field of precisely designing carrier pore sizes based on this key physicochemical parameter to achieve efficient loading and long-lasting slow release. Furthermore, current slow-release fertilizers / nutrients based on porous carriers, besides having insufficient slow-release performance, also have slightly insufficient nutrient loading capacity, thus requiring more slow-release fertilizer per unit area of ​​crop soil to achieve the desired effect. This increases production and transportation costs. Therefore, there is an urgent need in the field for a novel carrier material with green raw materials, a pore structure precisely matched to nutrient ion size, and capable of achieving efficient nutrient loading and intelligent release. Summary of the Invention

[0015] This invention aims to overcome the shortcomings of existing technologies and provide a bamboo-based porous biochar with high capacity element loading and long-term slow release, as well as its preparation method. The porous biochar's pore size is mainly composed of micropores and mesopores, and its unique nanoporous structure, highly matched to the hydration diameter of nutrient molecules, achieves high capacity loading and on-demand slow release of inorganic nutrients such as nitrogen, phosphorus, and potassium, significantly improving fertilizer utilization and reducing nutrient loss and environmental pollution. Specifically, this invention achieves the above objectives through the following technical solutions:

[0016] A method for preparing bamboo-based porous biochar with high capacity element loading and long-term sustained release includes the following steps:

[0017] (S1) Bamboo particles are impregnated in an alcoholic solution of an aminosilane coupling agent and heated to obtain modified bamboo particles.

[0018] (S2) Modified bamboo particles and lignin sulfonate are ball-milled and mixed evenly to obtain a mixture;

[0019] (S3) Under ultrasonic conditions, the mixture is impregnated in an activator solution to allow the activator to fully penetrate; the activator is an aqueous solution containing KOH, K2CO3, ZnCl2, and hydantoin epoxy resin;

[0020] (S4) Under air atmosphere, the impregnated bamboo particles are pre-oxidized, carbonized, activated to form pores, and refined at high temperature to obtain activated products.

[0021] (S5) The activated product is acid washed, alkali washed, water washed, dried, ground and sieved to obtain bamboo porous carbon.

[0022] Further, in step (S1), the bamboo particles have a particle size of 1-5 mm and are obtained by crushing, sieving, and drying bamboo to a constant weight. The bamboo is bamboo or bamboo products. The aminosilane coupling agent is selected from at least one of KH-540, KH-550, and KH-792. The concentration of the aminosilane coupling agent in the alcohol solution is 5-10 wt%, and the alcohol is selected from at least one of methanol, ethanol, and propanol. The ratio of bamboo particles to the alcohol solution of the aminosilane coupling agent is 1 kg: 6-10 L. The heating reaction is carried out under reflux at 60-80°C for 3-5 hours. After the reaction is completed, the mixture is washed with alcohol and dried to obtain modified bamboo particles.

[0023] Further, in step (S2), the mass ratio of modified bamboo particles to lignin sulfonate is 100:10-20, preferably 100:12-16; the ball milling conditions are wet ball milling, the medium is ethanol, the ball-to-material ratio is 20-30:1, the ball milling speed is 150-300 rpm, and the ball milling time is 10-30 min. The lignin sulfonate is potassium lignin sulfonate with a weight-average molecular weight of 10,000 to 30,000. The lignin sulfonate acts as a structure-directing agent, which can synergistically activate pore formation. The lignin sulfonate itself is an organic polymer rich in carbon, oxygen, and sulfur. The sulfonic acid groups and methoxy groups in the molecule generate gases (such as SO2 and CO2) during pyrolysis. Together with the pore-forming activator introduced in step S3, the synergistic pore-forming effect from both inside and outside precisely controls the formation of a pore size distribution with an average pore size of 1-3 nm. Furthermore, after the pyrolysis of lignin sulfonates, some of the oxygen- and sulfur-containing functional groups (such as carboxyl and sulfonic acid residues) in their structure are retained on the bamboo charcoal framework. These functional groups can significantly enhance the resistance of bamboo porous biochar to NH4 through ion exchange, coordination, and hydrogen bonding. + K + H2PO4 - The chemical adsorption capacity of nutrient ions is used to increase the initial nutrient loading. Lignosulfonates cannot be impregnated in step (3) because, as large molecules, they are difficult to diffuse and penetrate effectively into the bamboo particles. Therefore, this invention uses ball milling to mix the bamboo particles and lignin sulfonates evenly.

[0024] Further, in step (S3), the activator is an aqueous solution containing 10-15 wt% KOH, 10-20 wt% K2CO3, 2-4% ZnCl2, and 1.6-2.5 wt% hydantoin epoxy resin; the ratio of the mixture to the activator solution is 1 kg: 3-4 L, and the impregnation time is 20-30 h, preferably 22-24 h. The content of hydantoin epoxy resin in the activator is crucial. As a synergistic pore-forming structure modifier, if the amount of hydantoin epoxy resin deviates from the optimal range, the resulting porous biochar will have an unsuitable pore size distribution as a nutrient carrier. After the bamboo particles are modified with an aminosilane coupling agent, the bamboo particles have amino groups that can react with epoxy groups, and the reactivity of amino groups and epoxy groups is strong. Under heating conditions, a ring-opening reaction easily occurs, forming an in-situ cross-linked hybrid network inside the bamboo particles. This will inhibit excessive pore size expansion during subsequent activation and prevent the formation of large mesopores >10 nm between adjacent pores. This ensures that the pore size of the obtained porous biochar is mainly concentrated in the 1-3 nm range. Specifically, the synergistic effect of the hydantoin epoxy resin and the aminosilane coupling agent results in an average pore size of 1-3 nm for the obtained porous biochar, with a concentrated pore size distribution. This allows the porous biochar to have a higher proportion of effective pores capable of loading and slowly releasing nutrients. In particular, excessive use of hydantoin epoxy resin can lead to excessive cross-linking, potentially clogging the pore structure and reducing pore volume; insufficient use of hydantoin epoxy resin will result in insufficient cross-linking, leading to unwanted larger pores and a reduced proportion of 1-3 nm pores.

[0025] Furthermore, in step (S3), the ultrasonic conditions are an ultrasonic power of 200-300W and an ultrasonic frequency of 30-60kHz; the impregnation temperature is 15-30℃, preferably room temperature; the inventors have found that impregnating the activator under suitable ultrasonic conditions can more fully complete the penetration of the activator into the interior of the bamboo particles.

[0026] Further, in step (S4), the temperature is raised to 200-250℃ for pre-oxidation; then the atmosphere is switched to inert, and the pre-oxidized bamboo particles are heated to 450-550℃ for carbonization; the carbonized bamboo particles are heated to 700-800℃ for pore-forming activation to obtain an activated product; the activated product is heated to 900-1000℃ for high-temperature refining.

[0027] Furthermore, in step (S4), the pre-oxidation time is 1-2 hours. The purpose of pre-oxidation is twofold: firstly, to provide abundant oxygen-containing functional groups, such as carboxyl groups, to enhance the affinity of porous biochar for inorganic nutrients and increase the loading rate; secondly, to help inhibit graphitization during the subsequent high-temperature carbonization process. The inert atmosphere is nitrogen and / or argon. Furthermore, the carbonization holding time is 5-10 hours; the pore-forming activation holding time is 10-15 hours; and the high-temperature refining holding time is 1-2 hours. The purpose of high-temperature refining is to further eliminate unstable structures, enhance the mechanical strength of porous biochar, and further reduce the proportion of pores with a diameter >10 nm.

[0028] Further, in step (S5), acid washing involves heating and washing with 5-10% hydrochloric acid 1-3 times to remove residual activators and metal salts; alkaline washing involves washing with 5-10% bicarbonate 1-3 times to make the pH of the filtrate close to neutral, i.e. close to 7; water washing involves repeatedly washing with deionized water 5-8 times; drying is done in an oven or under vacuum; for example, oven drying is done at 105-120℃ for 6-12 hours.

[0029] Furthermore, in step (S5), the prepared bamboo porous biochar has an average pore size of 1.0-3.0 nm, a pore volume of 0.8-1.0 cm³ / g, and a specific surface area of ​​1500-2200 m². 2 / g.

[0030] This invention, based on the nutrient ion hydration diameter (<0.7 nm), is the first to concentrate the pore size of bamboo porous biochar in the range of 1.0-3.0 nm. This range perfectly balances high loading capacity (pore volume ≥0.80 cm³ / g) and strong sustained-release performance (nano confinement effect), resolving the contradiction that traditional materials cannot achieve both.

[0031] The high-capacity element loading refers to the loading rate of the obtained bamboo porous carbon on fertilizer being above 55%, preferably above 60%; the long-term slow release refers to the slow release period of more than 30 days (30-day release rate ≤75%) after the bamboo porous carbon is loaded with fertilizer nutrients.

[0032] This invention also provides a method for preparing a nutrient slow-release agent based on bamboo porous biochar, which, after obtaining the bamboo porous biochar as described above, further includes the following steps:

[0033] (S6) Bamboo porous biochar and nutrient aqueous solution are mixed and impregnated for the first time under vacuum and constant temperature conditions. After the vacuum is released, the mixture is impregnated for the second time under stirring and heat preservation. Solid-liquid separation and drying are performed to obtain the product, a plant nutrient slow-release agent based on bamboo porous biochar.

[0034] Further, in step (S6), the mass-to-volume ratio of bamboo porous biochar to nutrient aqueous solution is 1 kg: 5-10 L, the solid content of the nutrient aqueous solution is 30-50%, and the solute is at least two of the following: urea, potassium dihydrogen phosphate, potassium chloride, ammonium nitrate, diammonium phosphate, superphosphate, tripotassium phosphate, potassium nitrate, ammonium polyphosphate, and potassium sulfate. The system satisfies the requirement that the mass ratio of nitrogen, phosphorus, and potassium (N, P2O5, K2O) is 15-30:10-30:5-30. Nitrogen, phosphorus, and potassium are the three essential elements of fertilizer, and their proportions vary according to the growth needs of different crops, which is well known in the field. Nitrogen, phosphorus, and potassium are expressed as mass percentages of N-P2O5-K2O, such as in a 15-15-15 compound fertilizer, indicating that the three elements in the fertilizer are in a mass ratio of N:15:15.

[0035] Further, in step (S6), the vacuum isothermal conditions are a vacuum degree of -0.08 to -0.10 MPa, a temperature of 50-70℃, and a first impregnation time of 1-3 hours; the second impregnation is carried out at a constant temperature of 50-70℃ for 8-12 hours, with a stirring speed of 200-400 rpm. The first impregnation is carried out under vacuum conditions, using negative pressure to remove air from the pores, forcing the high-concentration nutrient solution to rapidly enter the depths of the mesoporous pores under capillary action. The second impregnation is carried out under atmospheric pressure stirring conditions, utilizing molecular thermal motion to make the nutrient solution more evenly distributed inside the pores and ensure that all available pore spaces are fully filled.

[0036] Furthermore, in step (S6), there are no particular limitations on solid-liquid separation, such as vacuum filtration or pressure filtration; drying involves transferring the filter cake to a vacuum drying oven and drying it at 60-80℃ and a vacuum degree of -0.05MPa to -0.08MPa for 6-10 hours. Low-temperature vacuum drying avoids the decomposition or melting and seepage of heat-sensitive nutrients such as urea caused by high temperatures, ensuring that the nutrients are stably fixed within the pores.

[0037] Compared with the prior art, the present invention achieves the following beneficial effects:

[0038] I. In this invention, bamboo particles are first modified with an aminosilane coupling agent, then ball-milled and mixed evenly with lignin sulfonate, and then impregnated in an activator solution. After that, they are subjected to pre-oxidation, carbonization, activation and pore-forming, and high-temperature refining in sequence to obtain porous biochar with a pore size distribution concentrated in 1-3 nm, which matches the hydration diameter of nutrient molecules, thus obtaining a nutrient-loading component that can fully load nutrients and achieve a slow-release effect.

[0039] Second, potassium lignosulfonate acts as a dispersant and structure guide during ball milling to ensure uniform mixing. In the subsequent pyrolysis activation stage, it acts as a precursor for synergistic pore formation, working synergistically with the activator to precisely construct a pore structure dominated by mesopores. At the same time, the oxygen- and sulfur-containing functional groups remaining after pyrolysis can enhance the chemical adsorption capacity of porous carbon for nutrient molecules.

[0040] Third, the present invention adds a specific amount of hydantoin epoxy resin to the activator, which can react with the amino groups modified by the modified bamboo particles to form a suitable cross-linking network. In the subsequent activation process, it inhibits the excessive expansion of pore size and the generation of larger pore size, and finally achieves the result of porous carbon with pore size distribution concentrated in 1-3 nm and large pore volume, which is suitable as a nutrient carrier. Attached Figure Description

[0041] Figure 1 This is an SEM image of the bamboo porous biochar obtained in Example 1.

[0042] Figure 2 This is a particle size distribution diagram of the bamboo porous biochar obtained in Example 1.

[0043] Figure 3 This is a pore size distribution diagram of the bamboo porous biochar obtained in Example 1.

[0044] Figure 4 The infrared spectrum of the nutrient slow-release agent using bamboo porous biochar from Example 1 as raw material is shown. Detailed Implementation

[0045] The technical solution of the present invention will be further explained and illustrated below with specific embodiments.

[0046] The bamboo particles used in this invention are derived from moso bamboo and crushed to a particle size of approximately 3 mm.

[0047] The weight-average molecular weight of potassium lignosulfonate is 17,000.

[0048] Example 1

[0049] (S1) 1 kg of bamboo particles were impregnated in 8 L of ethanol solution containing 7.2 wt% KH-550, heated to 60 °C and reacted for 5 h, filtered, washed and dried to obtain modified bamboo particles.

[0050] (S2) 1 kg of modified bamboo particles and 0.16 kg of potassium lignosulfonate were ball-milled and mixed evenly at a ball mill speed of 200 rpm and a ball-to-material ratio of 20:1 for 10 min. The mixture was obtained after ball milling.

[0051] (S3) Under 250W, 60kHz ultrasonic conditions, at room temperature, 1kg of mixture was impregnated in 3L of activator solution. The activator was an aqueous solution of 15wt% KOH, 15wt% K2CO3, 3% ZnCl2, and 1.6wt% hydantoin epoxy resin. The impregnation time was 24h to allow the activator to fully penetrate the mixture. After that, the mixture was filtered and dried to obtain impregnated bamboo granules.

[0052] (S4) Under air atmosphere, the impregnated bamboo particles are heated to 220℃ and kept at the temperature for 1 hour for pre-oxidation to obtain pre-oxidized bamboo particles; then nitrogen gas is switched, and the pre-oxidized bamboo particles are heated to 500℃ and kept at the temperature for 6 hours for carbonization; the carbonized bamboo particles are heated to 750℃ and kept at the temperature for 12 hours for pore-forming activation to obtain activated products; the activated products are heated to 920℃ and kept at the temperature for 1 hour for high-temperature refining to obtain activated products.

[0053] (S5) The activated product was washed three times with 5% hydrochloric acid, once with 10% potassium bicarbonate solution, washed three times with water, dried in an oven, ground and sieved to obtain bamboo porous charcoal.

[0054] Figure 1 This is an SEM image of the bamboo porous biochar obtained in Example 1.

[0055] Figure 2 This is a particle size distribution diagram of the bamboo porous biochar obtained in Example 1, with D50 of 9.39 μm and D90 of 17.43 μm.

[0056] Figure 3 This is a pore size distribution diagram of the bamboo porous biochar obtained in Example 1. It can be seen that the pore size is mainly distributed in the range of 1-3 nm.

[0057] Example 2

[0058] (S1) 1 kg of bamboo granules were impregnated in 7 L of ethanol solution containing 8.0 wt% KH-540, heated to 60 °C and reacted for 5 h, filtered, washed and dried to obtain modified bamboo granules.

[0059] (S2) 1 kg of modified bamboo particles and 0.12 kg of potassium lignosulfonate were ball-milled and mixed evenly at a ball mill speed of 200 rpm and a ball-to-material ratio of 20:1 for 10 min. The mixture was obtained after ball milling.

[0060] (S3) Under 250W, 60kHz ultrasonic conditions, at room temperature, 1kg of mixture was impregnated in 3L of activator solution. The activator was an aqueous solution of 10wt% KOH, 20wt% K2CO3, 3% ZnCl2, and 2.5wt% hydantoin epoxy resin. The impregnation time was 24h to allow the activator to fully penetrate the mixture. After that, the mixture was filtered and dried to obtain impregnated bamboo granules.

[0061] (S4) Under air atmosphere, the impregnated bamboo particles are heated to 220℃ and kept at the temperature for 1 hour for pre-oxidation to obtain pre-oxidized bamboo particles; then nitrogen is switched, and the pre-oxidized bamboo particles are heated to 500℃ and kept at the temperature for 6 hours for carbonization; the carbonized bamboo particles are heated to 750℃ and kept at the temperature for 12 hours for pore-forming activation to obtain activated products; the activated products are heated to 900℃ and kept at the temperature for 1 hour for high-temperature refining to obtain activated products.

[0062] (S5) The activated product was washed three times with 5% hydrochloric acid, once with 10% potassium bicarbonate solution, washed three times with water, dried in an oven, ground and sieved to obtain bamboo porous charcoal.

[0063] Example 3

[0064] The other conditions are the same as in Example 1, except that in step (S3), the activator is an aqueous solution of 15wt% KOH, 15wt% K2CO3, 3% ZnCl2, and 1.2wt% hydantoin epoxy resin.

[0065] Example 4

[0066] The other conditions are the same as in Example 1, except that in step (S3), the activator is an aqueous solution of 15wt% KOH, 15wt% K2CO3, 3% ZnCl2, and 3wt% hydantoin epoxy resin.

[0067] Example 5

[0068] The other conditions are the same as in Example 1, except that in step (S4), the carbonization temperature is changed to 550°C, the activation and pore-forming temperature is changed to 800°C, and the high-temperature refining temperature is changed to 900°C.

[0069] Comparative Example 1

[0070] The other conditions are the same as in Example 1, except that in step (S3), the activator is an aqueous solution of 15wt% KOH, 15wt% K2CO3, 3% ZnCl2, and 2.2wt% polyethylene glycol diglycidyl ether, that is, the hydantoin epoxy resin is replaced with polyethylene glycol diglycidyl ether.

[0071] Comparative Example 2

[0072] The other conditions are the same as in Example 1, except that in step (S3), the activator is 15wt% KOH, 15wt% K2CO3, and 3% ZnCl2, i.e., no hydantoin epoxy resin is added.

[0073] Comparative Example 3

[0074] The other conditions are the same as in Example 1, except that in step (S3), the activator is an aqueous solution of 30wt% KOH, 3% ZnCl2, and 1.6wt% hydantoin epoxy resin, that is, K2CO3 is replaced by an equal mass of KOH.

[0075] Comparative Example 4

[0076] The other conditions are the same as in Example 1, except that step (S1) is omitted and step (S2) involves directly ball milling bamboo particles and potassium lignin sulfonate.

[0077] Comparative Example 5

[0078] The other conditions are the same as in Example 1, except that step (S2) is omitted and in step (S3), the modified bamboo particles from step (S1) are used directly to replace the mixture.

[0079] Comparative Example 6

[0080] The other conditions are the same as in Example 1, except that pre-oxidation is not performed in step (S4).

[0081] The pore size of the porous biochar in the above embodiments and comparative examples was analyzed, and the results are shown in Table 1 below.

[0082] The specific surface area is the BET specific surface area.

[0083] Table 1 Pore size distribution of porous biochar

[0084]

[0085] Application examples

[0086] The bamboo porous biochar and nutrient aqueous solution (urea 140 g / L, potassium dihydrogen phosphate 150 g / L, potassium chloride 110 g / L) mentioned in the above examples and comparative examples were mixed in a ratio of 100 g: 1 L. The mixture was then incubated under vacuum at 60 °C (vacuum degree -0.09 MPa) for 2 hours, and then returned to normal pressure. The mixture was then impregnated for 10 hours with stirring at 300 rpm. After filtration, the mixture was dried under vacuum at 70 °C (vacuum degree -0.05 MPa) for 8 hours to obtain the nutrient slow-release agent product. Figure 4 The infrared spectrum of the nutrient slow-release agent made from bamboo porous biochar in Example 1 shows that it has been successfully loaded with nutrients and has characteristic absorption peaks of urea and phosphate.

[0087] The performance of the obtained nutrient slow-release product was tested, and the results are shown in Table 2 below:

[0088] The total N-P2O5-K2O loading is calculated by converting the nutrients into the mass of N-P2O5-K2O and determining its loading rate in porous biochar. Nutrient release rates were tested at 7 days, 30 days, and 60 days, in accordance with GB / T 23348-2009.

[0089] Table 2 Performance Tests of Nutrient Slow-Release Agents

[0090]

[0091] As can be seen, this invention successfully prepared a bamboo-based porous biochar carrier with high nutrient loading and excellent sustained-release performance through the synergistic effect of aminosilane coupling agent modification, lignin sulfonate addition, a specific ratio of activator (containing a precisely measured amount of hydantoin epoxy resin), and a pre-oxidation step. The absence or substitution of any key step will lead to deterioration of the pore structure, thereby significantly reducing the loading or sustained-release performance. In particular, the modification of bamboo particles with aminosilane coupling agent and the addition of hydantoin epoxy resin are crucial for forming porous biochar with pore sizes concentrated in the 1-3 nm range.

Claims

1. A method for preparing bamboo-based porous charcoal suitable for slow-release of nutrients, characterized in that, Includes the following steps: (S1) Bamboo particles are impregnated in an alcoholic solution of an aminosilane coupling agent and heated to obtain modified bamboo particles. (S2) Modified bamboo particles and lignin sulfonate are ball-milled and mixed evenly to obtain a mixture; (S3) Under ultrasonic conditions, the mixture is impregnated in an activator solution to allow the activator to fully penetrate; the activator is an aqueous solution containing KOH, K2CO3, ZnCl2, and hydantoin epoxy resin; (S4) Under air atmosphere, the impregnated bamboo particles are pre-oxidized, carbonized, activated to form pores, and refined at high temperature to obtain activated products. (S5) The activated product is acid washed, alkali washed, water washed, dried, ground and sieved to obtain bamboo porous carbon.

2. The preparation method according to claim 1, characterized in that, In step (S1), the bamboo particles have a particle size of 1-5 mm; the aminosilane coupling agent is selected from at least one of KH-540, KH-550, and KH-792; preferably, the concentration of the aminosilane coupling agent in the alcohol solution is 5-10 wt%, and the ratio of bamboo particles to aminosilane coupling agent in the alcohol solution is 1 kg: 6-10 L; more preferably, the heating reaction is carried out under reflux at 60-80°C for 3-5 h, and after the reaction is completed, the particles are washed with alcohol and dried to obtain modified bamboo particles.

3. The preparation method according to claim 1, characterized in that, In step (S2), the mass ratio of modified bamboo particles to lignin sulfonate is 100:10-20, preferably 100:12-16; preferably, the ball milling conditions are wet ball milling, the medium is ethanol, the ball-to-material ratio is 20-30:1, the ball milling speed is 150-300 rpm, and the ball milling time is 10-30 min; more preferably, the lignin sulfonate is potassium lignin sulfonate.

4. The preparation method according to claim 1, characterized in that, In step (S3), the activator is an aqueous solution containing 10-15 wt% KOH, 10-20 wt% K2CO3, 2-4% ZnCl2, and 1.6-2.5 wt% hydantoin epoxy resin.

5. The preparation method according to claim 4, characterized in that, In step (S3), the ratio of the mixture to the activator solution is 1 kg: 3-4 L, the soaking time is 20-30 h, preferably 22-24 h, and further, in step (S3), the ultrasonic conditions are ultrasonic power of 200-300 W, ultrasonic frequency of 30-60 kHz, and soaking temperature of 15-30 °C.

6. The preparation method according to claim 1, characterized in that, In step (S4), the temperature is raised to 200-250℃ for pre-oxidation; then, the temperature is switched to an inert atmosphere, and the pre-oxidized bamboo particles are heated to 450-550℃ for carbonization; the carbonized bamboo particles are heated to 700-800℃ for pore-forming activation to obtain an activated product; the activated product is heated to 900-1000℃ for high-temperature refining; furthermore, the pre-oxidation time is 1-2 hours, the carbonization holding time is 5-10 hours, the pore-forming activation holding time is 10-15 hours, and the high-temperature refining holding time is 1-2 hours.

7. The preparation method according to claim 1, characterized in that, In step (S5), acid washing involves heating and washing with 5-10% hydrochloric acid 1-3 times to remove residual activators and metal salts; alkaline washing involves washing with 5-10% bicarbonate 1-3 times to bring the pH of the filtrate close to neutral. The washing process involves repeatedly washing with deionized water 5-8 times; the drying process involves oven drying or vacuum drying.

8. A bamboo-based porous biochar, characterized in that, It is prepared by the preparation method described in any one of claims 1-7; further, the average pore size of the bamboo porous biochar is 1.0-3.0 nm, the pore volume is 0.8-1.0 cm³ / g, and the specific surface area is 1500-2200 m². 2 / g.

9. A method for preparing a nutrient slow-release agent based on bamboo porous biochar, wherein after obtaining bamboo porous biochar by the method according to any one of claims 1-7, the method further comprises the following steps: (S6) Bamboo porous biochar and nutrient aqueous solution are mixed and impregnated for the first time under vacuum and constant temperature conditions. After the vacuum is released, the mixture is impregnated for the second time under stirring and heat preservation. Solid-liquid separation and drying are performed to obtain the product, a plant nutrient slow-release agent based on bamboo porous biochar.

10. The preparation method according to claim 9, characterized in that, The mass-to-volume ratio of bamboo porous biochar to nutrient aqueous solution is 1 kg: 5-10 L. The solid content of the nutrient aqueous solution is 30-50%. The solute is at least two of the following: urea, potassium dihydrogen phosphate, potassium chloride, ammonium nitrate, diammonium phosphate, superphosphate, tripotassium phosphate, potassium nitrate, ammonium polyphosphate, and potassium sulfate. The system satisfies the requirement that the mass ratio of nitrogen, phosphorus, and potassium (N, P2O5, K2O) is 15-30: 10-30: 5-30. Furthermore, the vacuum constant temperature conditions are a vacuum degree of -0.08 to -0.10 MPa, a temperature of 50-70℃, and a first impregnation time of 1-3 hours; the second impregnation is carried out by maintaining a constant temperature of 50-70℃ for 8-12 hours, with a stirring speed of 200-400 rpm.

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