PH response type magnesium-iron-rich double-layer coated controlled-release fertilizer and preparation method thereof

Abstract

The invention provides a pH response type magnesium-iron-rich double-layer coated controlled-release fertilizer and a preparation method thereof, and belongs to the technical field of fertilizer preparation. The controlled-release fertilizer provided by the invention comprises a fertilizer core, an ethyl cellulose film, an inner coating film and an outer shell, the fertilizer core is prepared from urea, clay and sodium polyacrylate; the outer layer shell covers the surface of the inner layer coating film and is composed of an iron-rich material layer and a magnesium-rich gel layer which are stacked in sequence. According to the preparation method, an ammonia etching silicon oxide template method is adopted, a medium element magnesium is constructed in situ in a nano silicon dioxide mesoporous framework, the intelligent microcapsule with the pH response characteristic is formed, rapid release of the magnesium element in acid soil and precise regulation and control of slow release in neutral or alkaline soil are achieved, and the utilization rate of magnesium nutrients is increased; meanwhile, ascorbic acid and a silicon-based material are compounded to construct an antioxidant ferrous iron carrier, so that the form of ferrous ions is effectively stabilized, the technical bottleneck that a conventional iron fertilizer is easy to oxidize and inactivate is broken through, and the bioavailability of iron elements is improved.

Description

Technical Field

[0001] This invention belongs to the field of fertilizer preparation technology, specifically relating to a pH-responsive magnesium-iron double-coated controlled-release fertilizer and its preparation method. Background Technology

[0002] Traditional coated controlled-release fertilizers primarily rely on the physical barrier effect of polymers to delay nutrient release, but lack environmental response mechanisms and cannot dynamically adjust release behavior according to soil conditions (such as pH). While pH-sensitive materials such as sodium polyacrylate (PAANa) have applications in pharmaceutical sustained-release systems, research on their use in fertilizer coating systems remains insufficient. Current technologies for combining functional substances (such as trace elements) with fertilizers often employ mechanical mixing or simple coating processes, which struggle to achieve targeted controlled release of functional substances and suffer from complex processes and high energy consumption.

[0003] The existing technology has significant drawbacks: Firstly, the ferrous ions (Fe) in conventional iron fertilizers... 2+ It is easily oxidized into ferric iron (Fe3+), which is difficult for plants to absorb. 3+ Firstly, the utilization rate of iron is low; secondly, the release of magnesium lacks an intelligent response mechanism and cannot be released on demand according to the difference in soil pH; thirdly, the combination of functional substances and coating structures is crude, which can neither effectively maintain the stability of trace elements (such as antioxidant properties) nor achieve environmentally responsive release of medium elements, which seriously restricts the improvement of fertilizer nutrient utilization efficiency. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a pH-responsive, magnesium-iron double-coated controlled-release fertilizer with pH-responsive characteristics, which can achieve precise control of the rapid release of magnesium in acidic soils and the slow release in neutral or alkaline soils, thereby improving the utilization rate of magnesium nutrients; at the same time, it can also effectively stabilize the form of ferrous ions, overcome the technical bottleneck of conventional iron fertilizers being easily oxidized and deactivated, and improve the bioavailability of iron.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer, comprising a fertilizer core, an inner coating, and an outer shell; the fertilizer core is composed of urea, clay, and sodium polyacrylate; the inner coating is an ethyl cellulose membrane covering the surface of the fertilizer core; the outer shell covers the surface of the inner coating and is composed of sequentially stacked iron-rich material layers and magnesium-rich gel layers; the iron-rich material layers are composed of ORFFF, and the magnesium-rich gel layers are composed of PMFF@SA suspension; The ORFFF is an ascorbic acid-modified Fe(III) / SiO2 composite material, the PMFF@SA suspension is obtained by mixing PMFF aqueous suspension with sodium alginate solution, and the PMFF is a pH-responsive magnesium-modified nano-silica composite material.

[0006] Preferably, the mass ratio of urea, clay and sodium polyacrylate is (14~16):(290~310):(1.8~2.2), and the diameter of the fertilizer core is 4~6 mm.

[0007] Preferably, the mass of the ethyl cellulose membrane is 0.3% to 0.5% of the mass of the fertilizer core.

[0008] Preferably, the preparation method of the ORFFF includes the following steps: ferric acetylacetone, urea and nano silica template are mixed in a mass ratio of (0.17~0.18):(0.48~0.50):(0.09~0.11), and hydrothermally reacted at 180℃ for 12h to obtain Fe(III) / SiO2; Fe(III) / SiO2 and ascorbic acid are mixed in a mass ratio of 1:(2~3) to obtain ORFFF; the preparation method of the nano silica template includes the following steps: anhydrous ethanol, ammonia and tetraethyl orthosilicate are mixed in a volume ratio of 27:8:(3.9~4.5) to obtain nano silica template.

[0009] Preferably, the preparation method of PMFF includes the following steps: mixing anhydrous ethanol, ammonia and tetraethyl orthosilicate in a volume ratio of 27:8:(3.9~4.5) to obtain a nano-silica template; mixing the nano-silica template, magnesium chloride hexahydrate, ammonium chloride and ammonia, and hydrothermally reacting at 140°C for 10 h to obtain PMFF.

[0010] Preferably, the mass-to-volume ratio of nano-silica template, magnesium chloride hexahydrate, ammonium chloride, and ammonia is 0.1g:0.2g:0.54g:0.8~1.0mL.

[0011] Preferably, the mass ratio of PMFF to sodium alginate is 1:3.

[0012] This invention also provides a method for preparing the above-mentioned controlled-release fertilizer, comprising the following steps: mixing urea, clay and sodium polyacrylate and granulating to obtain a fertilizer core; dissolving ethyl cellulose in a 70% ethanol solution to obtain a coating solution; spraying the coating solution onto the fertilizer core to obtain a core-coated fertilizer; spraying an ORFFF aqueous suspension onto the surface of the core-coated fertilizer, and then spraying a PMFF@SA suspension onto the surface of the ORFFF layer; repeating the steps of first spraying the ORFFF aqueous suspension and then spraying the PMFF@SA suspension 1 to 3 times to obtain the controlled-release fertilizer.

[0013] Preferably, the amount of ORFFF aqueous suspension sprayed is calculated based on the iron content being 0.05% to 0.1% of the core-coated fertilizer mass.

[0014] Preferably, the spraying amount of PMFF@SA suspension spraying and the volume-to-weight ratio of the core-coated fertilizer are 10mL:100g.

[0015] The beneficial effects of this invention are: This invention provides a pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer. Using an ammonia-etched silica template method, magnesium is in-situ constructed within a nano-silica mesoporous framework, forming intelligent microcapsules with pH-responsive properties. This allows for precise regulation of magnesium release in acidic soils and slow release in neutral or alkaline soils, improving magnesium nutrient utilization. Simultaneously, an antioxidant ferrous carrier is constructed using ascorbic acid and silicon-based materials, effectively stabilizing the ferrous ion form and overcoming the technical bottleneck of conventional iron fertilizers' easy oxidation and inactivation, thus improving the bioavailability of iron. This invention combines these two technologies synergistically, dynamically matching fertilizer nutrient release patterns with crop needs, reducing nutrient loss and environmental pollution, and achieving efficient utilization of micronutrients. Attached Figure Description

[0016] Figure 1 This is a schematic diagram illustrating the structure and working principle of the pH-responsive magnesium-iron double-coated controlled-release fertilizer of the present invention. Detailed Implementation

[0017] This invention provides a pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer, comprising a fertilizer core, an inner coating, and an outer shell; the fertilizer core is composed of urea, clay, and sodium polyacrylate; the inner coating is an ethyl cellulose membrane covering the surface of the fertilizer core; the outer shell covers the surface of the inner coating and is composed of sequentially stacked iron-rich material layers and magnesium-rich gel layers; the iron-rich material layers are composed of ORFFF, and the magnesium-rich gel layers are composed of PMFF@SA suspension; The ORFFF is an ascorbic acid-modified Fe(III) / SiO2 composite material, the PMFF@SA suspension is obtained by mixing PMFF aqueous suspension with sodium alginate solution, and the PMFF is a pH-responsive magnesium-modified nano-silica composite material.

[0018] This invention does not specifically limit the source of the above-mentioned raw materials; commercially available products commonly used in the field are acceptable. In this invention, the preferred mass ratio of urea, clay, and sodium polyacrylate is (14~16):(290~310):(1.8~2.2), more preferably 15:(295~305):(1.9~2.1), and the preferred diameter of the fertilizer core is 4~6 mm. The sodium polyacrylate (PAANa) molecular chain contains sodium carboxylate groups (-COO). -Na + In acidic soils (pH < 6): After carboxylation, the repulsive force of the molecular chain is weakened, and the reorganization of hydrogen bonds in the molecular chain leads to swelling, forming microporous channels to accelerate urea diffusion; the clay enhances the compressive strength of the particles through the silicon-oxygen bond network.

[0019] In this invention, the ethyl cellulose membrane can be commercially available or prepared by the manufacturer. In some embodiments of this invention, the ethyl cellulose is preferably derived from the etherification modification of plant straw cellulose. The preferred method for preparing the ethyl cellulose includes the following steps: extracting cellulose from plant straw by treating it with an acetic acid-nitric acid mixture; adding the cellulose to a NaOH solution and drying it under vacuum to obtain basic cellulose; mixing the basic cellulose with toluene and bromoethane and then etherifying it to obtain ethyl cellulose. In this invention, the plant straw preferably includes at least one of wheat, rice, and corn. In this invention, the mass of the ethyl cellulose membrane is preferably 0.3% to 0.5% of the fertilizer core mass, more preferably 0.4% of the fertilizer core mass. The ethyl cellulose molecule contains an ethoxy group (-OC2H5), and the hydrophobic long chain forms a molecular sieve network (pore size 0.5 to 1 nm), allowing only water molecules to slowly permeate and dissolve urea, extending the release cycle, physically delaying the initial release of urea, and avoiding seedling burn.

[0020] In this invention, the ORFFF is a pH-responsive antioxidant silicon-based nanocomposite material derived from nano-silicon materials and ascorbic acid. The preferred preparation method of the ORFFF includes the following steps: mixing acetylacetone iron, urea, and a nano-silica template at a mass ratio of (0.17~0.18):(0.48~0.50):(0.09~0.11), and reacting hydrothermally at 180°C for 12 h to obtain Fe(III) / SiO2; mixing Fe(III) / SiO2 with ascorbic acid at a mass ratio of 1:(2~3) to obtain ORFFF. In this invention, the preferred mass ratio of acetylacetone iron, urea, and the nano-silica template is (0.17~0.18):0.49:0.10. In this invention, the preferred method for preparing the nano-silica template includes the following steps: mixing anhydrous ethanol, ammonia and tetraethyl orthosilicate in a volume ratio of 27:8:(3.9~4.5) to obtain the nano-silica template.

[0021] In this invention, ascorbic acid will... 3+ Reduced to Fe 2+ They form chelates, and the oxidation potential decreases from +0.77V to -0.18V, increasing the Fe... 2+The retention rate of iron in the ORFFF of this invention is improved by forming a novel antioxidant ferrous-silicon nanocomposite material through compositing with silicon-based nanomaterials. This solves the technical problem that conventional iron fertilizer Fe(II) is easily oxidized into Fe(III) which is difficult for plants to absorb and utilize, thus improving the fertilizer utilization rate of the trace element iron. XPS characterization shows that the Fe in the ORFFF of this invention... 2+ It accounts for 89% and remains above 82% after 30 days of storage at room temperature. ORFFF is in direct contact with the environment and preferentially responds to moisture permeation to release iron.

[0022] In this invention, the PMFF is a silicon-based nano-magnesium fertilizer. The preferred method for preparing the PMFF includes the following steps: mixing anhydrous ethanol, ammonia, and tetraethyl orthosilicate in a volume ratio of 27:8:(3.9~4.5) to obtain a nano-silica template; in this invention, the preferred volume ratio of anhydrous ethanol, ammonia, and tetraethyl orthosilicate is 27:8:(4.1~4.4). Ammonia-catalyzed hydrolysis and condensation of tetraethyl orthosilicate: It forms a mesoporous structure (pore size 5~10 nm), which is Mg 2+ Anchoring points are provided. This invention uses magnesium chloride hexahydrate as the magnesium source, ammonium chloride to provide NH4+ to promote ion exchange, and ammonia solution to maintain an alkaline environment. Nano-silica templates, magnesium chloride hexahydrate, ammonium chloride, and ammonia solution are mixed and hydrothermally reacted at 140°C for 10 hours to obtain PMFF. In this invention, the preferred mass-to-volume ratio of nano-silica templates, magnesium chloride hexahydrate, ammonium chloride, and ammonia solution is 0.1g:0.2g:0.54g:0.8~1.0mL.

[0023] In this invention, PMFF directly and in situ constructs magnesium elements onto a nano-silica mesoporous framework using an ammonia-etched silica template method, forming pH-sensitive microcapsules. In acidic soil environments (pH < 6), H... + Ion exchange reactions occur in the erosion support framework: This leads to a rapid release of magnesium ions; however, in neutral / alkaline soils (pH≥7), the carrier structure is stable, and Mg... 2+ It releases magnesium slowly through diffusion, automatically adjusting the release rate based on soil pH to improve magnesium nutrient utilization.

[0024] The schematic diagram illustrating the structure and working principle of the pH-responsive, magnesium-iron-rich, double-coated controlled-release fertilizer of this invention is shown below. Figure 1 As shown.

[0025] In this invention, the preferred mass ratio of PMFF to sodium alginate is 1:3. A PMFF@SA suspension, prepared by mixing a PMFF aqueous suspension with a sodium alginate solution, serves as the outer shell. The -COO⁻ of sodium alginate reacts with soil Ca... 2+ Gelization: It forms a protective layer that delays PMFF decomposition.

[0026] This invention employs an ammonia-etched silica template method to directly and in-situ construct magnesium, a medium-element element, onto nano-silica, which is then loaded into microcapsules. The coating layer of the microcapsules is modified to make them pH-sensitive, enabling them to automatically release magnesium at different pH levels, thus improving the nutrient utilization rate of magnesium. Furthermore, a novel antioxidant ferrous-silicon nanocomposite material is formed by combining ascorbic acid and silicon-based nanomaterials, solving the problem that conventional iron fertilizer Fe(II) is easily oxidized into Fe(III), which is difficult for plants to absorb and utilize, thereby improving the fertilizer utilization rate of the micro-element iron.

[0027] This invention also provides a method for preparing the above-mentioned controlled-release fertilizer, comprising the following steps: mixing urea, clay and sodium polyacrylate and granulating to obtain a fertilizer core; dissolving ethyl cellulose in a 70% ethanol solution to obtain a coating solution; spraying the coating solution onto the fertilizer core to obtain a core-coated fertilizer; spraying an ORFFF aqueous suspension onto the surface of the core-coated fertilizer, and then spraying a PMFF@SA suspension onto the surface of the ORFFF layer; repeating the steps of first spraying the ORFFF aqueous suspension and then spraying the PMFF@SA suspension 1 to 3 times to obtain the controlled-release fertilizer.

[0028] In this invention, the spraying amount of ORFFF aqueous suspension is calculated based on the iron content of 0.05% to 0.1% of the core-coated fertilizer mass. Here, the spraying amount refers to the total spraying amount for all applications. The preferred volume-to-weight ratio of the PMFF@SA suspension spraying amount to the core-coated fertilizer is 10 mL: 100 g. Here, the spraying amount refers to the total spraying amount for all applications.

[0029] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0030] Unless otherwise specified, the following embodiments are all conventional methods.

[0031] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0032] Example 1 A pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer comprises a fertilizer core, an inner coating, and an outer shell. The fertilizer core is composed of urea, clay, and sodium polyacrylate. The inner coating is an ethyl cellulose membrane covering the surface of the fertilizer core. The outer shell, covering the surface of the inner coating, is composed of sequentially stacked iron-rich material layers and magnesium-rich gel layers. The iron-rich material layers are composed of ORFFF, and the magnesium-rich gel layers are composed of PMFF@SA suspension. The ORFFF is an ascorbic acid-modified Fe(III) / SiO2 composite material, and the PMFF@SA suspension is obtained by mixing an aqueous PMFF suspension with a sodium alginate solution. The PMFF is a pH-responsive magnesium-modified nano-silica composite material.

[0033] The preparation method is as follows: S1: Preparation of fertilizer core Urea powder, clay, and sodium polyacrylate (PAANa) were mixed evenly at a mass ratio of 15:300:2, and then added to a granulator to prepare fertilizer cores. The particle size of the fertilizer cores was controlled to be 5 mm.

[0034] S2: Preparation of ethyl cellulose-ethanol composite coating solution 1. Synthesis of ethyl cellulose: Acetic acid with a volume fraction of 80% and nitric acid with a volume fraction of 69% were mixed in a volume ratio of 10:1 to obtain a mixed solution. Dried plant straw (wheat) was soaked in the mixed solution at a ratio of 100g of plant straw to 200mL of mixed solution and heated in a water bath at 90℃ for 3h to obtain cellulose. Cellulose was added to a 18wt% NaOH solution with a mass-to-volume ratio of 150g:1L. The mixture was stirred at 25°C for 1.5h, filtered under a negative pressure ≥0.08MPa, and then dried under negative pressure vacuum (the conditions for negative pressure vacuum drying were 60°C, -0.1MPa vacuum, and drying for 12h) to obtain alkaline cellulose. The prepared basic cellulose, toluene, and bromoethane were mixed in a ratio of 100g basic cellulose, 800mL toluene (solvent), and 250mL bromoethane (etherifying agent). The mixture was then added to a 2L high-pressure reactor for etherification to obtain ethyl cellulose. The specific operation of the etherification reaction was as follows: the material was added to the high-pressure reactor, the air was replaced with N2 2-3 times, the temperature was raised to 60℃, the stirring speed was 260rpm, and the reaction was carried out for 20 hours (pressure ≤0.5MPa); after cooling to 25℃ and depressurization, the mixture was removed to obtain ethyl cellulose.

[0035] 2. Ethyl cellulose-ethanol composite membrane coating solution: Ethyl cellulose was dissolved in a 70% ethanol solution to prepare a 10% (w / v) solution (meaning 10g of ethyl cellulose was added to every 100mL of ethanol solution). The solution was stirred continuously with a magnetic stirrer at 45℃ for 10-20 minutes until it was homogeneous. The solution was then cooled to room temperature to obtain a pH-responsive ethyl cellulose-ethanol composite coating solution, which was then cooled for later use.

[0036] S3: Preparation of environmentally responsive core-coated fertilizer The fertilizer core prepared by S1 was placed in a rotating drum, and the initial speed of the coating machine was set to 50 r / min. It was preheated at 80℃ for 15 min. Then, the ethyl cellulose-ethanol composite coating solution prepared in S2 was uniformly sprayed onto the surface of the granular fertilizer core. The granular fertilizer core was then rotated uniformly at a speed of 40 r / min. This process was repeated 4 times until the amount of ethyl cellulose film reached 0.4% of the mass of a single fertilizer core. Then continue working at 20 r / min for 10 minutes until the fertilizer granules are completely dried to obtain an environmentally responsive core-coated fertilizer.

[0037] S4: Preparation of PMFF and ORFFF 1. PMFF Preparation 1) Preparation of nano-silica templates Anhydrous ethanol and ammonia were mixed evenly under magnetic stirring, and then tetraethyl orthosilicate was quickly added. The volume ratio of anhydrous ethanol, ammonia and tetraethyl orthosilicate was 27:8:4.2. The mixed solution was centrifuged, and after repeated ultrasonic washing with deionized water and anhydrous ethanol and thorough drying, a white product, nano-silica template, was obtained.

[0038] 2) PMFF Synthesis The nano-silica template was dispersed in deionized water at a mass:volume ratio of 0.1g:20mL (i.e., 20mL of deionized water was added for every 0.1g of nano-silica template), and the solution A was formed by ultrasonic dispersion in deionized water; Magnesium chloride hexahydrate, ammonium chloride, and ammonia were dissolved sequentially in deionized water and mixed uniformly under magnetic stirring to form solution B. The mass ratio of magnesium chloride hexahydrate, ammonium chloride, and nano-silica template was 0.2g:0.54g:0.1g, the volume mass ratio of ammonia to nano-silica template was 1mL:0.1g, and the volume mass ratio of deionized water to nano-silica template was 30mL:0.1g. Solutions A and B were transferred to a high-pressure reactor and heated at 140°C for 10 hours. The products were washed three times by alternating centrifugation with anhydrous ethanol and deionized water, and the products were collected. The products were dried overnight in a vacuum dryer at 60°C to obtain a white product, which was PMFF.

[0039] 2. ORFFF Preparation The nano-silica template (prepared in step 1 above) was ultrasonically dispersed in 20 mL of deionized water at a mass:volume ratio of 0.1 g:20 mL to form solution C.

[0040] Iron acetylacetone, urea, and anhydrous ethanol were dissolved sequentially in 25 mL of deionized water to form solution D. The ratio of nano-silica template to iron acetylacetone, urea, anhydrous ethanol, and deionized water was 0.1 g : 0.5 mmol (equivalent to 176 mg of iron acetylacetone based on a molecular weight of 352.17) : 8 mmol (equivalent to 480 mg of molecular weight) : 5 mL : 25 mL.

[0041] Solutions C and D were transferred to a high-pressure reactor and heated at 180°C for 12 h, and washed at least three times with anhydrous ethanol and deionized water. Finally, the washed reddish-brown product was dried overnight in a vacuum oven at 60°C to obtain Fe(III) / SiO2; Ascorbic acid was dissolved in deionized water at a mass:volume ratio of 0.30 g:50 mL, using as a reducing agent. Fe(III) / SiO2 was then added to a round-bottom flask, and the mixture was stirred until homogeneous. The mass ratio of Fe(III) / SiO2 to ascorbic acid was 1:3. Subsequently, the solvent was evaporated to dryness using a rotary evaporator at 60 °C under negative pressure to obtain the reddish-brown product ORFFF.

[0042] S5: Preparation of PMFF@SA suspension Sodium alginate was dissolved in deionized water at a mass:volume ratio of 0.3g:20mL, and stirred with a magnetic stirrer until completely dissolved to obtain a sodium alginate solution. Then, the PMFF prepared in S4 was dispersed in distilled water at a mass:volume ratio of 0.1g:10mL to obtain a suspension. The suspension was poured into a sodium alginate solution with a volume ratio of 1:2. The mixture was shaken at 500r / min for 10min to obtain a uniform PMFF@SA suspension. The concentration of the sodium alginate solution was kept at 1.5%.

[0043] S6: Double-layer encapsulation construction The environmentally responsive core-coated fertilizer prepared by S3 was placed in a rotating drum; The ORFFF prepared in S4 was dispersed in deionized water and then sprayed onto the surface of fertilizer particles in a rotating drum. The total spraying amount was controlled at 0.05% of the total mass of the environmentally responsive core-coated fertilizer prepared in S3.

[0044] Then, the PMFF@SA suspension prepared by S5 was slowly sprayed onto the surface of rotating fertilizer particles, and an outer gel film shell was formed under continuous rotation for 10 minutes to complete the cross-linking reaction. Repeat the above steps 3 times (referring to repeating the two steps of first spraying ORFFF and then spraying PMFF@SA suspension, repeating 3 times). The total amount of PMFF@SA suspension used accounts for 10% (v / w) of the mass of the environmentally responsive core-coated fertilizer prepared by S3, that is, 10mL of PMFF@SA suspension is used for every 100g of environmentally responsive core-coated fertilizer prepared by S3 to obtain pH-responsive magnesium-iron double-layer coated controlled-release fertilizer.

[0045] Example 2 A pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer comprises a fertilizer core, an inner coating, and an outer shell. The fertilizer core is composed of urea, clay, and sodium polyacrylate. The inner coating is an ethyl cellulose membrane covering the surface of the fertilizer core. The outer shell, covering the surface of the inner coating, is composed of sequentially stacked iron-rich material layers and magnesium-rich gel layers. The iron-rich material layers are composed of ORFFF, and the magnesium-rich gel layers are composed of PMFF@SA suspension. The ORFFF is an ascorbic acid-modified Fe(III) / SiO2 composite material, and the PMFF@SA suspension is obtained by mixing an aqueous PMFF suspension with a sodium alginate solution. The PMFF is a pH-responsive magnesium-modified nano-silica composite material.

[0046] The preparation method is as follows: S1: Preparation of fertilizer core Urea powder, clay, and sodium polyacrylate (PAANa) were mixed evenly at a mass ratio of 14:290:1.8 and then added to a granulator to prepare fertilizer cores. The particle size of the fertilizer cores was controlled to be 4 mm.

[0047] S2: Preparation of ethyl cellulose-ethanol composite coating solution Ethyl cellulose was dissolved in a 70% ethanol solution to prepare a 10% (w / v) solution (meaning 10g of ethyl cellulose was added to every 100mL of ethanol solution). The solution was stirred continuously with a magnetic stirrer at 45℃ for 10-20 minutes until it was homogeneous. The solution was then cooled to room temperature to obtain a pH-responsive ethyl cellulose-ethanol composite coating solution, which was then cooled for later use.

[0048] S3: Preparation of environmentally responsive core-coated fertilizer The fertilizer core prepared by S1 was placed in a rotating drum, and the initial speed of the coating machine was set to 50 r / min. It was preheated at 80℃ for 15 min. Then, the ethyl cellulose-ethanol composite coating solution prepared in S2 was uniformly sprayed onto the surface of the granular fertilizer core. The granular fertilizer core was then rotated uniformly at a speed of 40 r / min. This process was repeated 4 times until the amount of ethyl cellulose film reached 0.3% of the mass of a single fertilizer core. Then continue working at 20 r / min for 10 minutes until the fertilizer granules are completely dried to obtain an environmentally responsive core-coated fertilizer.

[0049] S4: Preparation of PMFF and ORFFF 1. PMFF Preparation 1) Preparation of nano-silica templates Anhydrous ethanol and ammonia were mixed evenly under magnetic stirring, and then tetraethyl orthosilicate was quickly added. The volume ratio of anhydrous ethanol, ammonia and tetraethyl orthosilicate was 27:8:3.9. The mixed solution was centrifuged, and after repeated ultrasonic washing with deionized water and anhydrous ethanol and thorough drying, a white product, nano-silica template, was obtained.

[0050] 2) PMFF Synthesis The nano-silica template was dispersed in deionized water at a mass:volume ratio of 0.1g:20mL (i.e., 20mL of deionized water was added for every 0.1g of nano-silica template), and the solution A was formed by ultrasonic dispersion in deionized water; Magnesium chloride hexahydrate, ammonium chloride, and ammonia were dissolved sequentially in deionized water and mixed uniformly under magnetic stirring to form solution B. The mass ratio of magnesium chloride hexahydrate, ammonium chloride, and nano-silica template was 0.2 g: 0.54 g: 0.1 g, the volume mass ratio of ammonia to nano-silica template was 0.8 mL: 0.1 g, and the volume mass ratio of deionized water to nano-silica template was 30 mL: 0.1 g. Solutions A and B were transferred to a high-pressure reactor and heated at 140°C for 10 hours. The products were washed three times by alternating centrifugation with anhydrous ethanol and deionized water, and the products were collected. The products were dried overnight in a vacuum dryer at 60°C to obtain a white product, which was PMFF.

[0051] 2. ORFFF Preparation The nano-silica template (prepared in step 1 above) was ultrasonically dispersed in 20 mL of deionized water at a mass:volume ratio of 0.09 g:20 mL to form solution C.

[0052] Iron acetylacetone, urea, and anhydrous ethanol were dissolved sequentially in 25 mL of deionized water to form solution D. The ratio of nano-silica template to iron acetylacetone, urea, anhydrous ethanol, and deionized water was 0.09 g: 0.18 g: 0.50 g: 5 mL: 25 mL.

[0053] Solutions C and D were transferred to a high-pressure reactor and heated at 180°C for 12 h, and washed at least three times with anhydrous ethanol and deionized water. Finally, the washed reddish-brown product was dried overnight in a vacuum oven at 60°C to obtain Fe(III) / SiO2; Ascorbic acid was dissolved in deionized water at a mass:volume ratio of 0.30 g:50 mL. Fe(III) / SiO2 was then added to a round-bottom flask, and the mixture was stirred until homogeneous. The mass ratio of Fe(III) / SiO2 to ascorbic acid was 1:2. Subsequently, the solvent was evaporated to dryness using a rotary evaporator at 60 °C under negative pressure to obtain the reddish-brown product ORFFF.

[0054] S5: Preparation of PMFF@SA suspension Sodium alginate was dissolved in deionized water at a mass:volume ratio of 0.3g:20mL, and stirred with a magnetic stirrer until completely dissolved to obtain a sodium alginate solution. Then, the PMFF prepared in S4 was dispersed in distilled water at a mass:volume ratio of 0.1g:10mL to obtain a suspension. The suspension was poured into a sodium alginate solution with a volume ratio of 1:2. The mixture was shaken at 500r / min for 10min to obtain a uniform PMFF@SA suspension. The concentration of the sodium alginate solution was kept at 1.5%.

[0055] S6: Double-layer encapsulation construction The environmentally responsive core-coated fertilizer prepared by S3 was placed in a rotating drum; The ORFFF prepared in S4 was dispersed in deionized water and then sprayed onto the surface of fertilizer particles in a rotating drum. The total spraying amount was controlled at 0.1% of the total mass of the environmentally responsive core-coated fertilizer prepared in S3.

[0056] Then, the PMFF@SA suspension prepared by S5 was slowly sprayed onto the surface of rotating fertilizer particles, and an outer gel film shell was formed under continuous rotation for 10 minutes to complete the cross-linking reaction. Repeat the above steps 3 times (referring to repeating the two steps of first spraying ORFFF and then spraying PMFF@SA suspension, repeating 3 times). The total amount of PMFF@SA suspension used accounts for 10% (v / w) of the mass of the environmentally responsive core-coated fertilizer prepared by S3, that is, 10mL of PMFF@SA suspension is used for every 100g of environmentally responsive core-coated fertilizer prepared by S3 to obtain pH-responsive magnesium-iron double-layer coated controlled-release fertilizer.

[0057] Comparative Example 1 1. Fertilizer core and inner coating: Same as steps S1~S3 in Example 1.

[0058] 2. Differences in outer layer structure: The PMFF synthesis steps are omitted and replaced with the following steps: a. Weigh 0.3g of magnesium chloride hexahydrate and dissolve it in 10mL of deionized water to obtain solution B; b. Weigh 0.1g of sodium alginate and dissolve it in 20mL of deionized water to obtain solution A; c. Pour solution B into solution A (the volume ratio of solution B to solution A is 1:2), shake at 500 r / min for 10 min to form a Mg-Alg suspension.

[0059] Spraying process: The Mg-Alg suspension was sprayed onto the surface of the ORFFF layer instead of PMFF@SA (the amount used was the same as in Example 1), and the remaining steps were the same as in Example 1.

[0060] That is, the coated fertilizer in Comparative Example 1 actually retains the core + ethyl cellulose inner membrane + ORFFF layer, while the PMFF@SA layer is replaced by Mg-Alg suspension.

[0061] Experimental Example 1 10g of each of the coated controlled-release fertilizers prepared in Example 1 and Comparative Example 1 were pulverized and passed through a 100-mesh sieve to obtain uniform particle samples. The following experiments were conducted (five parallel experiments were performed for each group): I. pH Response Release Experiment 1. Preparation of buffer solution: pH 5.0 buffer solution: 0.1M acetate-sodium acetate system (simulating acidic soil). pH 7.5 buffer: 0.1M Tris-HCl system (simulating neutral soil).

[0062] 2. Release kinetics test: Place 50 mg of test sample in a dialysis bag (molecular weight cutoff 3500 Da), immerse in 200 mL of buffer solution, and incubate at 37°C with shaking (100 rpm). Sampling time points: 0.5 h, 2 h, 6 h, 12 h, 24 h, 48 h.

[0063] Magnesium ion concentration detection: Take 5 mL of solution each time (add an equal amount of fresh buffer solution simultaneously), and determine the Mg content using atomic absorption spectrometry (AAS). 2+ Concentration, calculate cumulative release rate: The results of the environmentally responsive magnesium-iron double-coated controlled-release fertilizer obtained in Example 1 are shown in Table 1 (the release difference factor in Table 1 refers to the release difference factor between the magnesium release rate at pH 5.0 and pH 7.5). This indicates that under acidic conditions (pH 5.0), the magnesium release rate reached 78.5% after 24 hours, significantly higher than under neutral conditions (28.3%), with a difference of 2.8 times. This confirms that the nano-silica carrier in PMFF is effective in H... + Accelerated Mg corrosion 2+ Dissolution. The release rate in acidic environments is >90% within 48 hours, meeting the needs of crop growth cycles.

[0064] Table 1. Results of pH-responsive release experiment of the coated controlled-release fertilizer obtained in Example 1

[0065] The results of the coated controlled-release fertilizer obtained in Comparative Example 1 are shown in Table 2 (the release difference factor in Table 2 refers to the release difference factor between the magnesium release rate at pH 5.0 and pH 7.5). It can be seen that the fertilizer without the PMFF@SA layer has a magnesium release rate of only 31.6% after 24 hours at pH 5.0, proving the necessity of the outer coating's pH response. Ordinary Mg-Alg gel layers lack pH-responsive pore structures (compared to the 5~10 nm mesopores of PMFF), and magnesium ions are released only through diffusion, resulting in a magnesium release rate difference of <5% between acidic and neutral soils (only 31.6% after 24 hours at pH 5.0, far lower than the 78.5% of this invention); the cumulative magnesium release rate after 48 hours in acidic / neutral soils is >50%, increasing the risk of nutrient loss.

[0066] Table 2. Results of pH-responsive release experiments of the coated controlled-release fertilizer obtained in Comparative Example 1

[0067] II. Ferrous ions (Fe) 2+ Retention rate test: Verify the stabilizing effect of ascorbic acid modification on ferrous ions.

[0068] 1. Take 0.5g of the ORFFF material prepared in Example 1 and store it in an open container in a 25℃ / 60% RH constant temperature and humidity chamber; Samples were taken at three time points: day 0 (initial), day 15, and day 30. Determination of Fe by X-ray photoelectron spectroscopy (XPS) 2+ Percentage (Fe) 2+ / (Fe 2+ +Fe 3+ )); A standard curve was established using iron standard solutions, and the iron speciation was verified by atomic absorption spectrometry (AAS).

[0069] The results are shown in Table 3.

[0070] Table 3 Ferrous ions (Fe) 2+ Retention rate test results

[0071] 2. Mechanism Analysis: Ascorbic acid forms stable complexes through chelation: The oxidation potential of this chelate drops to -0.18V (common Fe). 2+ (with a value of +0.77V), inhibiting oxidation reactions and providing crops with a continuous and effective supply of iron nutrition.

[0072] Comparative Example 2 The fertilizer core, inner membrane, and magnesium layer are processed according to steps S1-S3 and PMFF@SA in Example 1.

[0073] Differences in iron layers: Omitted: the ascorbic acid reduction step of ORFFF; Alternative steps: a. Mix acetylacetone iron: urea: SiO2 template at a mass ratio of 0.175:0.49:0.10; b. Fe(III) / SiO2 (unreduced) was prepared by hydrothermal reaction at 180℃ for 12 h. c. Directly disperse Fe(III) / SiO2 powder in water (without adding ascorbic acid).

[0074] Spraying process: The Fe(III) / SiO2 aqueous dispersion (iron content 0.08%) was sprayed onto the surface of the inner layer film instead of ORFFF, and the rest was the same as step S6 of Example 1.

[0075] Experimental Example 2 The fertilizer obtained in Comparative Example 2 was subjected to ferrous ion (Fe) ion exchange. 2+ The retention rate test was conducted using the same method as in Experiment 1. The results are shown in Table 4. This indicates that in unmodified Fe(III) / SiO2, there was initially no available plant iron: 100% was Fe. 3+ (Crop absorption rate <10%); low ferrous iron conversion rate and no protection: available Fe after 30 days 2+ It accounts for only 40.8% (while the corresponding value for ORFFF in Table 3 is 82.3%).

[0076] Table 4 Ferrous ions (Fe) 2+ Retention rate test results

[0077] The results of Experimental Examples 1 and 2 show that the present invention constructs a pH-responsive PMFF support (Mg) using the ammonia etching template method. 2+ With a loading of 12.3 wt%, it rapidly releases 10.2% magnesium in acidic soil within 0.5 h (4.9 times higher than in neutral environments), and the cumulative release rate reaches 78.5% in 24 h. Simultaneously, the ascorbic acid chelation technology is used to increase the ferrous iron retention rate in ORFFF to 82.3% after 30 days, breaking through the bottleneck of iron fertilizer oxidation.

[0078] Experimental Example 3 The test crop was tomato; the plastic pots were 15cm in diameter at the bottom, 20cm in diameter at the top, and 18cm in height; the fertilizers tested were samples prepared in Example 1, Comparative Example 1, and Comparative Example 2.

[0079] The experiment was conducted at the Institute of New Fertilizers and Green Agriculture, Xinyangfeng Agricultural Technology Co., Ltd. The experiment used a two-factor setup: soil type (acidic soil, pH 5.5; calcareous soil, pH 7.80) and fertilizer (slow-release fertilizers prepared in Example 1, Comparative Examples 1 and 2). A total of six treatments were set up: AcT1, AcT2, AcT3, AlT1, AlT2, and AlT3 (where Ac represents acidic soil; Al represents calcareous soil; T1 represents the fertilizer sample prepared in Example 1; and T2 represents the fertilizer sample prepared in Comparative Example 1). Fertilizer samples (T3 represents the fertilizer sample prepared in Comparative Example 2) were used. Tomatoes were planted in plastic flowerpots, each filled with 1.5 kg of soil (acidic or calcareous). All nitrogen, phosphorus, and potassium fertilizers were applied as base fertilizer. Plump, uniformly sized tomato seeds were selected, and 5 seeds were sown per pot at a depth of 1.5 cm. The fertilizer was applied 4 cm below the seeds, with 5 g of the slow-release fertilizer prepared in Example 1, Comparative Example 1, or Comparative Example 2 applied per pot. Sowing was done using a watering-sowing-covering method. Seedlings were thinned on the 7th day after sowing, and 3 tomato seedlings were transplanted. All potted plants were cultured in an artificial climate chamber at a temperature of 25°C, with a daily light duration of 16 hours and a light intensity of approximately 900 μmol / m². 2 •s). Each treatment was replicated in triplicate using a randomized block design. All treatments were managed under uniform conditions throughout the experiment. Mature plants were harvested after 45 days of cultivation for the determination of various parameters.

[0080] Tomato plant dry weight: Tomato plants were blanched in an oven at 105℃ for 30 minutes, then dried at 80℃ to constant weight, and the weight was measured using an electronic balance; the magnesium content of the plants was determined by atomic absorption spectrophotometry after digestion with nitric acid-perchloric acid; the iron content of the plants was determined by the same method as the magnesium content, i.e., digestion with nitric acid-perchloric acid followed by atomic absorption spectrophotometry.

[0081] The results are shown in Table 5. This indicates that PMFF and ORFFF are sensitive to pH values. The release amounts of magnesium and Fe(II) vary in soils with different pH levels, and their release can be controlled according to pH changes, thereby meeting the magnesium and iron requirements of crops at different growth stages. The double-layer coating structure of this invention (ethyl cellulose inner layer + PMFF@SA / ORFFF outer layer) synergistically achieves dynamic matching between magnesium and iron release and crop needs.

[0082] Table 5 Results obtained from different groups

[0083] Note: In Table 5, Ac represents acidic soil; Al represents calcareous soil; T1 represents the fertilizer sample prepared in Example 1; T2 represents the fertilizer sample prepared in Comparative Example 1; and T3 represents the fertilizer sample prepared in Comparative Example 2.

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A pH-responsive, magnesium- and iron-rich, double-coated controlled-release fertilizer, characterized in that, The product comprises a fertilizer core, an inner membrane, and an outer shell. The fertilizer core is composed of urea, clay, and sodium polyacrylate. The inner membrane is an ethyl cellulose membrane covering the surface of the fertilizer core. The outer shell covers the surface of the inner membrane and is composed of sequentially stacked iron-rich material layers and magnesium-rich gel layers. The iron-rich material layers are composed of ORFFF, and the magnesium-rich gel layers are composed of PMFF@SA suspension. The ORFFF is an ascorbic acid-modified Fe(III) / SiO2 composite material, the PMFF@SA suspension is obtained by mixing PMFF aqueous suspension with sodium alginate solution, and the PMFF is a pH-responsive magnesium-modified nano-silica composite material.

2. The controlled-release fertilizer according to claim 1, characterized in that, The mass ratio of urea, clay and sodium polyacrylate is (14~16):(290~310):(1.8~2.2), and the diameter of the fertilizer core is 4~6 mm.

3. The controlled-release fertilizer according to claim 1, characterized in that, The mass of the ethyl cellulose membrane is 0.3% to 0.5% of the mass of the fertilizer core.

4. The controlled-release fertilizer according to claim 1, characterized in that, The preparation method of the ORFFF includes the following steps: ferric acetylacetone, urea and nano silica template are mixed in a mass ratio of (0.17~0.18):(0.48~0.50):(0.09~0.11), and hydrothermally reacted at 180℃ for 12h to obtain Fe(III) / SiO2; Fe(III) / SiO2 and ascorbic acid are mixed in a mass ratio of 1:(2~3) to obtain ORFFF; The preparation method of the nano silica template includes the following steps: anhydrous ethanol, ammonia and tetraethyl orthosilicate are mixed in a volume ratio of 27:8:(3.9~4.5) to obtain nano silica template.

5. The controlled-release fertilizer according to claim 1, characterized in that, The preparation method of the PMFF includes the following steps: anhydrous ethanol, ammonia and tetraethyl orthosilicate are mixed in a volume ratio of 27:8:(3.9~4.5) to obtain a nano silica template; the nano silica template, magnesium chloride hexahydrate, ammonium chloride and ammonia are mixed and hydrothermally reacted at 140℃ for 10h to obtain PMFF.

6. The controlled-release fertilizer according to claim 5, characterized in that, The mass-volume ratio of nano-silica template, magnesium chloride hexahydrate, ammonium chloride, and ammonia water is 0.1g:0.2g:0.54g:0.8~1.0mL.

7. The controlled-release fertilizer according to claim 1, characterized in that, The mass ratio of PMFF to sodium alginate is 1:

3.

8. The method for preparing the controlled-release fertilizer according to any one of claims 1 to 7, characterized in that, The process includes the following steps: mixing urea, clay, and sodium polyacrylate and granulating to obtain a fertilizer core; dissolving ethyl cellulose in a 70% ethanol solution to obtain a coating solution; spraying the coating solution onto the fertilizer core to obtain a core-coated fertilizer; spraying an ORFFF aqueous suspension onto the surface of the core-coated fertilizer; then spraying a PMFF@SA suspension onto the surface of the ORFFF layer; repeating the steps of first spraying the ORFFF aqueous suspension and then spraying the PMFF@SA suspension 1 to 3 times to obtain the controlled-release fertilizer.

9. The preparation method according to claim 8, characterized in that, The amount of ORFFF aqueous suspension sprayed is calculated based on the iron content being 0.05% to 0.1% of the core-coated fertilizer mass.

10. The preparation method according to claim 8, characterized in that, The spraying amount of PMFF@SA suspension is 10mL:100g in volume-to-weight ratio to the core-coated fertilizer.

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