Method for preparing trace element humic acid fertilizer with high fulvic acid content based on quaternary metal hollow carbon catalyst activated weathered coal

Abstract

The invention discloses a method for preparing a trace element humic acid fertilizer with high fulvic acid content by activating weathered coal based on a quaternary metal hollow carbon catalyst. The preparation method comprises the following steps: firstly, preparing a Fe / Zn / Ce co-doped hollow carbon coated ZIF-67 catalyst by a hydrothermal-calcination method; then, weathered coal and compound alkali are subjected to first-time medium-temperature activation and then mixed with a ternary oxidant system and a catalyst for second-time medium-temperature activation, and finally, a weathered coal product is compounded with a bonding slow-release agent. The prepared catalyst has a hierarchical porous structure and rich active sites, a composite oxidant system generates a synergistic oxidation effect, the humic acid content, the fulvic acid content and the trace element content in the weathered coal are remarkably improved by a two-step medium-temperature activation process, the fertilizer efficiency of the product is lasting, and the catalyst has remarkable economic and environmental benefits. The obtained fertilizer is particularly suitable for field crops such as corn in brown soil and saline-alkali soil, can remarkably promote crop growth and improve the soil structure, and has remarkable economic and environmental benefits.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of humic acid fertilizer, in particular to a method for preparing a trace element humic acid fertilizer with high fulvic acid content based on a quaternary metal hollow carbon catalyst activated weathered coal. BACKGROUND

[0002] Weathered coal is an important source of humic acid, and China has abundant reserves of weathered coal. The humic acid content of weathered coal is high, but its water solubility is poor, so its biological activity is low, and direct application is not effective. Therefore, how to effectively activate the humic acid in weathered coal and improve its fulvic acid content and biological activity has become a hot research topic. At present, the main methods for activating weathered coal include chemical, physical and biological methods. Chemical methods usually use strong acids, strong bases or oxidizing agents for treatment, which have good activation effect, but have problems such as equipment corrosion and environmental pollution; physical methods such as mechanical grinding and ultrasonic treatment are environmentally friendly but have low activation efficiency; biological methods use microorganisms for degradation, which have mild conditions but a long cycle and low efficiency.

[0003] In recent years, metal-organic framework materials (MOFs) have shown great potential in the field of catalysis due to their high specific surface area, adjustable pore structure and rich active sites. ZIF-67, as a typical cobalt-based MOF material, has good thermal and chemical stability, but its single metal center limits the further improvement of catalytic performance. By metal doping, new active sites can be introduced, the electronic structure can be adjusted, and the catalytic activity can be improved. However, traditional metal doping methods often have problems such as uneven doping and poor structural stability. In addition, existing weathered coal activation processes mostly use one-step treatment, ignoring the stage characteristics of the activation process. Different forms of humic acid in weathered coal require different activation conditions, and one-step treatment often cannot effectively activate various forms of humic acid at the same time, resulting in low activation efficiency. The patent with application number CN116621647A uses MOF derivatives to efficiently activate lignite to prepare a humic acid slow-release trace element fertilizer. The MOF derivative catalyst has high specific surface area and high redox catalytic activity, which can achieve efficient activation of lignite, and the catalyst can be used as a trace element fertilizer when applied to soil. Although this patent uses MOF derivatives as catalysts, the activation object is lignite, and the forms of humic acid in lignite and weathered coal are different. The humic acid in lignite is primary humic acid, which is closely combined with organic matter in coal, so the activation of lignite mainly relies on destroying the structure of organic matter to dissociate humic acid molecules. The organic macromolecules in weathered coal are oxidized and degraded to obtain humic acid, so it is more difficult to activate weathered coal to prepare humic acid. Therefore, it is necessary to develop a new type of efficient activation catalyst and a matching activation process, using MOF materials as catalysts to activate weathered coal to prepare humic acid, which has important significance for improving the comprehensive utilization value of weathered coal. SUMMARY

[0004] In view of the above prior art, the purpose of the present application is to provide a method for preparing a microelement humic acid fertilizer with high fulvic acid content based on a quaternary metal hollow carbon catalyst activated weathered coal. The present application significantly improves the humic acid content, fulvic acid content and microelement content of weathered coal by designing a new type of multi-metal doped hollow carbon material and optimizing the activation process, and particularly optimizes the corn growth promotion effect in brown soil and saline-alkali soil.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is as follows: In the first aspect of the present application, a method for preparing a microelement humic acid fertilizer with high fulvic acid content based on a quaternary metal hollow carbon catalyst activated weathered coal is provided, and the method is as follows: (1) uniformly mix weathered coal powder and composite alkali, and then heat to obtain pretreated weathered coal; (2) add a ternary oxidizing agent and a hollow carbon wrapped modified ZIF-67 catalyst to the pretreated weathered coal, stir and mix, and then heat to obtain a weathered coal product; (3) uniformly mix the weathered coal product with a binding and slow-release agent to obtain a high-activity humic acid fertilizer.

[0006] Preferably, in step (1), the particle size of the weathered coal powder is 100-200 mesh, and the water content is 30wt%; the composite alkali is composed of KOH and NaOH in a mass ratio of 1:1.

[0007] Preferably, in step (1), the mass ratio of the weathered coal powder to the composite alkali is 1:0.1-0.2; the heating temperature is 65℃, and the heating time is 24h.

[0008] Preferably, in step (2), the ternary oxidizing agent is composed of ammonium persulfate, magnesium oxide and sodium methoxide in a molar ratio of 1:(0.5-0.7):(0.3-0.5); the amount of the ternary oxidizing agent added is 5-8% of the mass of the weathered coal.

[0009] Preferably, in step (2), the amount of the hollow carbon wrapped modified ZIF-67 catalyst added is 3-5% of the mass of the weathered coal; the hollow carbon wrapped modified ZIF-67 catalyst is prepared by the following method: Mix ZIF-67 with iron salt, zinc salt and cerium salt, add povidone K30, and obtain a modified ZIF-67 precursor by hydrothermal reaction; etch the modified ZIF-67 precursor with acetic acid solution to form a hollow structure, then immerse it in a glucose solution, and calcine it in an inert atmosphere to obtain a hollow carbon wrapped modified ZIF-67 catalyst.

[0010] Preferably, the molar ratio of the ZIF-67, the iron salt, the zinc salt and the cerium salt is 1:0.5-0.8:0.5-0.8:0.1-0.3; the addition amount of the povidone K30 is 5-10% of the total mass of the iron salt, the zinc salt and the cerium salt; the temperature of the hydrothermal reaction is 110-120℃, and the time is 8-12 hours.

[0011] Preferably, the concentration of the acetic acid solution is 2-3mol / L; the ratio of the addition amount of the modified ZIF-67 precursor to the acetic acid solution is 1g:80ml; the etching time is 2-4 hours; the concentration of the glucose solution is 0.5-1mol / L; the ratio of the addition amount of the modified ZIF-67 precursor to the glucose solution is 1g:60ml; the temperature of the calcination is 500-600℃, and the time is 2-3 hours.

[0012] Preferably, in the step (2), the stirring and mixing is sand mill stirring for 3-5min; the temperature of the heating reaction is 65℃, and the time is 24-48 hours.

[0013] Preferably, in the step (3), the binding and slow-release agent is a mixture of sodium alginate, carboxymethyl starch, chitosan and bentonite with a ratio of 2:1:1:1; the addition amount of the binding and slow-release agent is 5-15% of the mass of the active weathered coal product.

[0014] In the second aspect of the present application, a microelement humic acid fertilizer with high fulvic acid content prepared by the above method is provided.

[0015] The high-activity humic acid formed after activation and the sodium alginate powder both have strong physical adsorption and complexing capacity, and can adsorb and fix the microelement ions (Fe, Zn, Ce, etc.) fixed on the catalyst residues after the reaction, so that slow release of the microelements is realized when the fertilizer is applied, and harm or nutrient loss caused by excessive release of the microelements in a short period of time is avoided. This design finally successfully prepares an efficient fertilizer with high fulvic acid activity and high microelement content.

[0016] In the third aspect of the present application, the above method is applied to improve the fulvic acid content, the humic acid content and the microelement content of the humic acid fertilizer.

[0017] Preferably, the microelements include zinc organic combination state, iron organic combination state and cerium organic combination state.

[0018] In the fourth aspect of the present application, the microelement humic acid fertilizer with high fulvic acid content is applied to improve the corn yield, the dry weight of the above-ground part of the corn and the average mass diameter of the soil aggregate.

[0019] The present application has the following beneficial effects: (1) This invention constructs a quaternary metal active center by synergistic doping of Fe, Zn and Ce, which significantly improves the redox ability and electron transfer efficiency of the catalyst; the hollow carbon-encapsulated structure not only provides a larger specific surface area and more active sites, but also improves the mass transfer efficiency, and the carbon layer plays a protective and slow-release role.

[0020] (2) The Fe / Zn / Ce-Co quaternary metal catalyst prepared in this invention has the following mechanism of action: the cobalt (Co) and cerium (Ce) sites have excellent electron transfer capabilities, which can effectively activate persulfate ions to generate highly oxidizing sulfate radicals (SO4• - The introduction of iron (Fe) and zinc (Zn) sites further modulates the electron cloud density of the catalytic center, enhancing its ability to attack inert C-C and CO bonds in the macromolecule of humic acid. After the synergistic effect of multiple free radicals, it can selectively break the bridging bonds and side chains in the complex aromatic structure of weathered coal humic acid, transforming it from macromolecules and poorly soluble humic acid into small molecules and highly water-soluble fulvic acid, thereby significantly improving its biological activity.

[0021] (3) The present invention adopts a ternary composite oxidant system of ammonium persulfate-magnesium oxide-sodium methoxide. Ammonium persulfate provides strong oxidizing properties, magnesium oxide adjusts the pH value of the reaction system, and sodium methoxide participates in the reaction as an organic strong base. The three produce a synergistic effect, realizing efficient oxidation under mild conditions.

[0022] (4) The hollow porous structure of the catalyst of this invention provides ample reaction space (confinement effect) for humic acid macromolecules, increasing the probability of contact between reactants and active sites. The carbon layer not only protects the metal active center, but its own sp² hybrid carbon structure also promotes electron transfer, further accelerating the redox reaction. This structure effectively solves the problem of low mass transfer efficiency in solid-solid phase reactions.

[0023] (5) The present invention adopts a two-step medium-temperature activation process. The first step, alkaline treatment, loosens the structure of weathered coal. The second step, oxidation and catalysis, work synergistically to effectively improve the extraction rate of fulvic acid. The medium-temperature condition of 65℃ ensures the reaction rate and avoids the damage of high temperature to the active ingredients.

[0024] (6) By combining the activated product with the binder and slow-release agent, the present invention utilizes the strong adsorption and complexation capabilities to achieve the chelation of trace elements, so that the product has both high humic acid activity and high trace element content, and the fertilizer effect is more lasting, with significant economic and environmental benefits. Attached Figure Description

[0025] Figure 1 Example 1: SEM image of humic acid-activated product.

[0026] Figure 2 Infrared spectrum of the highly active humic acid fertilizer prepared in Example 1. Detailed Implementation

[0027] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0028] Based on this, the purpose of this invention is to provide a method for preparing a trace element humic acid fertilizer with high fulvic acid content by activating weathered coal using a quaternary metal hollow carbon catalyst. This invention conducts in-depth research on the green activation process of weathered coal and the design of a dedicated catalyst. Single MOF materials suffer from insufficient active sites and poor stability in complex solid-phase reactions. Therefore, this invention first modifies ZIF-67 material in multiple dimensions: through ternary metal doping with Fe, Zn, and Ce, not only is the framework structure of ZIF-67 successfully maintained, but more catalytic active centers are introduced, enabling the prepared catalyst to not only have catalytic oxidation capabilities but also supply multiple trace elements such as iron, zinc, and cerium; furthermore, a hollow structure and carbon layer encapsulation are constructed by etching with acetic acid, significantly improving the specific surface area and stability of the material. In the activation process, this invention uses a ternary solid-state oxidant system composed of ammonium persulfate, magnesium oxide, and sodium methoxide. Studies have shown that this composite oxidant and the above catalyst have a significant synergistic effect during the stirring activation process: ammonium persulfate generates highly oxidizing sulfate radicals (SO4) under heating and catalysis. •⁻ Magnesium oxide can continuously neutralize the acidic substances produced by the reaction, maintaining a suitable alkaline environment in the system, which is conducive to the dissolution of humic acid. Sodium methoxide, as a strong organic base, can more gently break down the long molecular chains of weathered coal and, together with the metal active sites provided by the catalyst, promote the oxidation reaction. This multi-stage, synergistic oxidation method can efficiently convert low-activity macromolecular humic acid in weathered coal into highly active small-molecule fulvic acid. Furthermore, this invention preferably combines the activated weathered coal product with a binding and slow-release agent. Studies have shown that the highly active humic acid formed after activation and the binding and slow-release agent both have strong physical adsorption and complexation capabilities, which can re-adsorb and fix the trace element ions (Fe, Zn, Ce, etc.) fixed on the catalyst residue after the reaction, thereby achieving slow release of trace elements during fertilizer application and avoiding excessive release in the short term that could damage crops or cause nutrient loss. This design ultimately successfully prepared a high-efficiency fertilizer with high fulvic acid and high trace element content.

[0029] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0030] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0031] Example 1: Method for producing highly active humic acid fertilizer (1) Preparation of hollow carbon-encapsulated modified ZIF-67 catalyst ZIF-67 material, iron salt, zinc salt, and cerium salt were mixed in a molar ratio of 1:0.8:0.8:0.3 and added to 100 mL of deionized water. Simultaneously, polyvinyl acetate (PVP) (K30) surfactant was added at 10% of the total mass ratio of iron, zinc, and cerium salts. The mixture was transferred to a 200 mL high-pressure reactor lined with polytetrafluoroethylene and hydrothermally reacted at 120 °C for 10 hours. After the reaction, the mixture was allowed to cool naturally to room temperature, and the precipitate was collected by centrifugation. The precipitate was washed three times each with deionized water and ethanol, and then vacuum-dried at 60 °C for 12 hours to obtain the modified ZIF-67 precursor.

[0032] The modified ZIF-67 precursor was dispersed in 80 mL of 2.5 mol / L acetic acid solution and etched by magnetic stirring for 3 hours. The etched material was collected by centrifugation, washed with deionized water until neutral, and dried at 60 °C. The dried material was then immersed in 60 mL of 0.8 mol / L glucose solution, allowed to stand for 4 hours, and then dried at 60 °C. The dried sample was heated to 550 °C at a rate of 5 °C / min in a tube furnace under a nitrogen atmosphere, held at that temperature for 2.5 hours, and then naturally cooled to room temperature to obtain the hollow carbon-encapsulated modified ZIF-67 catalyst.

[0033] (2) KOH and NaOH are mixed in a mass ratio of 1:1 to obtain a composite alkali. Ammonium persulfate, magnesium oxide and sodium methoxide are mixed in a molar ratio of 1:0.6:0.4 to obtain a ternary oxidizing agent.

[0034] Weathered coal powder (moisture content adjusted to 30%, passed through a 150-mesh sieve) and composite alkali were added to a sand mill at a mass ratio of 1:0.2 and stirred for 5 minutes. The mixture was then heated at 65°C for 24 hours to obtain pretreated weathered coal. Next, hollow carbon-encapsulated modified ZIF-67 catalyst and a ternary oxidant were added to the pretreated weathered coal at 5% and 8% of the total mass of the weathered coal, respectively. The mixture was then added to the sand mill again and stirred for 5 minutes, and reacted at 65°C for 48 hours to obtain the weathered coal product.

[0035] (3) A binder-releasing agent was obtained by mixing sodium alginate, carboxymethyl starch, chitosan, and bentonite in a mass ratio of 2:1:1:1. Weathered coal products were crushed and passed through a 100-mesh sieve, and then mixed with the binder-releasing agent at 10% of the mass of the weathered coal products. The mixture was dry-mixed at 30 rpm for 20 minutes to obtain a highly active humic acid fertilizer. Its SEM image is shown below. Figure 1 As shown.

[0036] Example 2: Method for producing highly active humic acid fertilizer (1) The difference from step (1) of Example 1 is that the molar ratio of ZIF-67 material to iron salt, zinc salt and cerium salt is 1:0.8:0.5:0.3; the hydrothermal reaction temperature is 120℃ and the time is 12 hours. Finally, hollow carbon-encapsulated modified ZIF-67 catalyst is obtained.

[0037] (2) The preparation of the composite alkali is the same as step (2) in Example 1. Ammonium persulfate, magnesium oxide and sodium methoxide are mixed in a molar ratio of 1:0.7:0.3 to obtain a ternary oxidant.

[0038] Weathered coal powder (moisture content adjusted to 30%, passed through a 150-mesh sieve) and composite alkali were added to a sand mill at a mass ratio of 1:0.1 and stirred for 5 minutes. The mixture was then reacted at 65°C for 24 hours. Next, hollow carbon-encapsulated modified ZIF-67 catalyst and a ternary oxidant were added to the pretreated weathered coal at 3% and 5% of the total mass of the weathered coal, respectively. The mixture was then added back to the sand mill, stirred for 5 minutes, and reacted at 65°C for 48 hours to obtain the weathered coal product.

[0039] (3) The preparation of the binder and slow-release agent is the same as step (3) in Example 1. The weathered coal product is crushed and passed through a 100-mesh sieve, and mixed with a binder and slow-release agent accounting for 7% of the mass of the weathered coal product. The mixture is dry-mixed at 30 rpm for 20 minutes to obtain a highly active humic acid fertilizer.

[0040] Comparative Example 1: Activation without catalyst The difference from Example 1 is that no hollow carbon-encapsulated modified ZIF-67 catalyst was added. Humic acid fertilizer was ultimately prepared.

[0041] Comparative Example 2: Unmodified ZIF-67 catalyst The difference from Example 1 is that an equal amount of ZIF-67 was used instead of hollow carbon-encapsulated modified ZIF-67 catalyst. Humic acid fertilizer was ultimately prepared.

[0042] Comparative Example 3: One-step activation The difference from Example 1 is that weathered coal powder (moisture content adjusted to 30%, passed through a 150-mesh sieve), composite alkali, hollow carbon-encapsulated modified ZIF-67 catalyst, and ternary oxidant were mixed and added to a sand mill, and stirred for 5 minutes. The mass ratio of weathered coal powder to composite alkali was 1:0.2, the hollow carbon-encapsulated modified ZIF-67 catalyst accounted for 5% of the total mass of weathered coal, and the ternary oxidant accounted for 8% of the total mass of weathered coal. Subsequently, the mixture was heated at 65°C for 72 hours to obtain pretreated weathered coal.

[0043] The final product is humic acid fertilizer.

[0044] Comparative Example 4: Use of a single adhesive The difference from Example 1 is that sodium alginate, accounting for 10% of the mass of the weathered coal product, is used instead of the binder and slow-release agent. Humic acid fertilizer is ultimately prepared.

[0045] Experimental Example 1: Determination of Chemical Structure Infrared spectroscopy analysis was performed on the highly active humic acid fertilizer prepared in Example 1, using untreated weathered coal powder as a control group. The results are as follows: Figure 2 As shown, compared with the control group, Example 1 at 1650cm -1 The characteristic peak intensity in the (aromatic ring skeleton vibration region) is significantly reduced, indicating that the aromatic ring structure has broken; at the same time, the intensity of the characteristic peak in the 3200-3600 cm⁻¹ region is also significantly reduced. -1 The absorption peak (in the hydroxyl stretching vibration region) is broader and stronger, ranging from 2500 to 3300 cm⁻¹. -1 (Carboxyl OH stretching vibration region) and 1720-1680 cm -1 The peak shape changes significantly in the (carboxyl C=O stretching vibration region), indicating that the highly active humic acid fertilizer prepared in Example 1 after activation treatment generated more hydroxyl and carboxyl groups, and the functional group composition and aromatic ring structure were significantly altered.

[0046] Experimental Example 2: Determination of Physicochemical Properties of Fertilizer The humic acid fertilizers prepared in Examples 1-2 and Comparative Examples 1-4 were analyzed for humic acid content, fulvic acid content, and slow-release micronutrients. The test results for the content of humic acid, fulvic acid, and micronutrient forms in the humic acid fertilizers are shown in Table 1. Specific test methods are as follows: (1) Determination of fulvic acid content Untreated weathered coal was used as a control group. The fulvic acid content of Examples 1-2 and Comparative Examples 1-4 was determined according to the HJT 5534-2108 standard.

[0047] (2) Determination of humic acid content Untreated weathered coal was used as a control group. The humic acid content of Examples 1-2 and Comparative Examples 1-4 was determined according to the standards HG / T 3278-2001 and NY / T 1121.6-2006.

[0048] (3) Determination of trace element speciation Using untreated weathered coal as a control group, the proportion of organically bound zinc, iron, and cerium in Examples 1-2 and Comparative Examples 1-4 was determined by the Tessier continuous extraction method.

[0049] Table 1 Content of humic acid, fulvic acid and trace elements As shown in Table 1, Example 1 exhibited the best activation effect, with a fulvic acid content of 22.48%, nearly 22 times higher than the control group. This is attributed to the synergistic effect of the quaternary metal catalyst: the Co / Ce sites activate the SO4·⁻ free radicals generated by persulfate, which effectively break the C / C and COC bonds in the humic acid macromolecule, while the Fe / Zn sites enhance the attack efficiency on the aromatic ring structure by adjusting the electron cloud density. The effect of Comparative Example 3 (one-step method) was significantly lower than that of the Example, demonstrating that the activation method of "alkali treatment pretreatment → catalyst catalytic oxidation bond breaking" in the two-step process has significant advantages.

[0050] As shown in Table 1, the organically bound proportions of zinc, iron, and cerium in Example 1 reached 58.8%, 52.0%, and 55.3%, respectively, significantly higher than those in the comparative examples. This phenomenon confirms that the catalyst simultaneously achieved the organic integration of trace elements during activation: the newly generated fulvic acid molecules can form stable chelates with metal ions through carboxyl and phenolic hydroxyl groups. The organically bound proportion in Comparative Example 4 (single binder) was significantly lower than that in the examples, demonstrating that the composite binder system (sodium alginate + carboxymethyl starch + chitosan + bentonite) enhanced the chelation stability.

[0051] Experimental Example 3: Determination of Fertilizer Application Effects in Brown Soil 1. The experiment consisted of 8 treatment groups, as detailed below: Control group: Brown soil that passed through a 5mm sieve + unactivated weathered coal (application rate: 27g / pot); Example 1: Brown soil that has passed through a 5mm sieve + highly active humic acid fertilizer prepared in Example 1 (application rate: 27g / pot); Example 2: Brown soil that has passed through a 5mm sieve + highly active humic acid fertilizer prepared in Example 2 (application rate: 27g / pot); Comparative Example 1: Brown soil that has passed through a 5mm sieve + Highly active humic acid fertilizer prepared in Comparative Example 1 (application rate: 27g / pot); Comparative Example 2: Brown soil that has passed through a 5mm sieve + Highly active humic acid fertilizer prepared in Comparative Example 2 (application rate: 27g / pot); Comparative Example 3: Brown soil that has passed through a 5mm sieve + Highly active humic acid fertilizer prepared in Comparative Example 3 (application rate: 27g / pot); Comparative Example 4: Brown soil that has passed through a 5mm sieve + Highly active humic acid fertilizer prepared in Comparative Example 4 (application rate: 27g / pot); Brown soil samples were collected from the South Campus Experimental Station of the National Engineering Research Center for Efficient Soil and Fertilizer Utilization, Shandong Agricultural University, Tai'an City, Shandong Province (36°09′14″N, 117°09′01″E).

[0052] After mixing the materials from each treatment group, they were placed in separate plastic pots (31cm high, 27cm in diameter at the top, and 18cm in diameter at the bottom), with a soil depth of approximately 30cm. About 27cm of soil was added first, and the pots were watered thoroughly to ensure a moist environment for corn growth. After one day, 10 corn seeds were sown in each pot, and the remaining 3cm of soil was covered. Tap water was used for irrigation. After emergence, thinning was carried out, leaving one seedling per pot. During the corn's growth, small-particle urea was applied at the jointing stage (2g / pot). At harvest, the yield (corn kernels dried and weighed) and the above-ground dry weight (after drying) were measured. The results are shown in Table 2.

[0053] Table 2. Maize yield and aboveground dry weight of maize Table 2 shows that the corn yield in Example 1 group reached 400 g / pot, a 60% increase compared to the control group (250 g / pot). The yield in Example 2 group was 350 g / pot, a 40% increase compared to the control group. This indicates that the fertilizer of the present invention can promote corn yield. The dry weight of Example 1 group was 76 g / pot, a 52% increase compared to the control group (50 g / pot). The dry weight of Example 2 group was 69 g / pot, a 38% increase compared to the control group. The increase in aboveground dry weight reflects the promoting effect of the fertilizer on crop vegetative growth, consistent with the yield increase trend. This indicates that the highly active humic acid fertilizer of the present invention is beneficial to corn growth and yield increase.

[0054] Experiment Example 4: Determination of Fertilizer Application Effect in Saline-Alkali Soil The brown soil in Experiment 3 was replaced with saline soil (taken from Liwang Village, Kenli District, Dongying City, Shandong Province (37°31′N, 118°33′E)). Other components, application rates, and planting methods were the same as in Experiment 3. Corn yield was measured at the corn harvest period (corn kernels were threshed, dried, and weighed), and soil samples were taken. The soil aggregate water stability was determined by wet sieving, and the average mass diameter (MWD) of the aggregates was calculated based on the aggregate component data. The results are shown in Table 3.

[0055] The formula for calculating the average mass diameter of the aggregates is as follows: ; In the formula, Wi represents the percentage (%) of the mass of the i-th order aggregate. The average diameter (mm) of a certain level of aggregate.

[0056] Table 3. Maize yield and average mass diameter of aggregates As shown in Table 3, the yield of Example 1 reached 300 g / pot, which was 100% higher than the control group (150 g / pot) and 66.7% higher than the optimal comparative example (Comparative Example 1, 180 g / pot). The yield of Example 2 was 250 g / pot, which was slightly lower than that of Example 1, but still 66.7% higher than the control group. The yields of the comparative examples (without catalyst, unmodified catalyst, etc.) were close to the level of the control group, indicating that the fertilizer effect is limited when the key process of this invention is lacking.

[0057] The MWD of group 1 was 0.41 mm, which was 272.7% higher than that of the control group (0.11 mm), indicating that the fertilizer significantly improved the aggregate structure of saline-alkali soil. The MWD of group 2 was 0.36 mm, which was also much higher than that of the control group and the comparative group (0.12-0.13 mm). The improvement in MWD shows that fertilizer can not only increase crop yield, but also improve the soil structure of saline-alkali soil, providing a new method for the improvement of saline-alkali soil.

[0058] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a trace element humic acid fertilizer with high fulvic acid content based on the activation of weathered coal using a quaternary metal hollow carbon catalyst, characterized in that, The method is as follows: (1) Mix the weathered coal powder with the composite alkali evenly, and then heat the mixture to obtain pretreated weathered coal; (2) Add a ternary oxidant and a hollow carbon-encapsulated modified ZIF-67 catalyst to the pretreated weathered coal, stir and mix, and then heat to react to obtain the weathered coal product; (3) Mix the weathered coal products with the binder and slow-release agent evenly to obtain a highly active humic acid fertilizer.

2. The method according to claim 1, characterized in that, In step (1), the particle size of the weathered coal powder is 100-200 mesh and the moisture content is 30wt%; the composite alkali is made by mixing KOH and NaOH in a mass ratio of 1:

1.

3. The method according to claim 1, characterized in that, In step (1), the mass ratio of the weathered coal powder to the composite alkali is 1:0.1~0.2; the heating reaction temperature is 65℃ and the time is 24h.

4. The method according to claim 1, characterized in that, In step (2), the ternary oxidant is composed of ammonium persulfate, magnesium oxide and sodium methoxide in a molar ratio of 1:(0.5-0.7):(0.3-0.5); the amount of the ternary oxidant added accounts for 5-8% of the mass of the weathered coal; the stirring and mixing is carried out by stirring in a sand mill for 3-5 minutes; the heating reaction temperature is 65℃ and the time is 24-48 hours.

5. The method according to claim 1, characterized in that, In step (2), the amount of hollow carbon-encapsulated modified ZIF-67 catalyst added accounts for 3-5% of the mass of weathered coal; the hollow carbon-encapsulated modified ZIF-67 catalyst is prepared by the following method: ZIF-67 was mixed with iron salt, zinc salt and cerium salt, and povidone K30 was added to obtain a modified ZIF-67 precursor through hydrothermal reaction. The modified ZIF-67 precursor was etched with acetic acid solution to form a hollow structure, then impregnated with glucose solution and calcined in an inert atmosphere to obtain a hollow carbon-encapsulated modified ZIF-67 catalyst.

6. The method according to claim 5, characterized in that, The molar ratio of ZIF-67, iron salt, zinc salt, and cerium salt is 1:0.5-0.8:0.5-0.8:0.1-0.3; the amount of polyvinyl ketone K30 added is 5-10% of the total mass of iron salt, zinc salt, and cerium salt; the hydrothermal reaction temperature is 110-120℃, and the time is 8-12 hours.

7. The method according to claim 5, characterized in that, The concentration of the acetic acid solution is 2-3 mol / L; the ratio of the modified ZIF-67 precursor to the acetic acid solution is 1 g: 80 ml; the etching time is 2-4 hours; the concentration of the glucose solution is 0.5-1 mol / L; the ratio of the modified ZIF-67 precursor to the glucose solution is 1 g: 60 ml; the calcination temperature is 500-600℃, and the time is 2-3 hours.

8. The method according to claim 1, characterized in that, In step (3), the binder is obtained by mixing sodium alginate, carboxymethyl starch, chitosan and bentonite in a 2:1:1:1 ratio; the amount of binder added is 5-15% of the mass of the weathered coal product.

9. The trace element humic acid fertilizer with high fulvic acid content prepared by the method according to any one of claims 1 to 8.

10. The application of the method according to any one of claims 1 to 8 in increasing the fulvic acid content, humic acid content and trace element content in humic acid fertilizer.

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