Biological microalgae microcapsule fertilizer and preparation method thereof

Abstract

The invention relates to the technical field of agricultural microbial fertilizers, and provides a biological microalgae microcapsule fertilizer and a preparation method thereof, the method comprises the following steps: S1, preparing an active algae liquid, and mildly concentrating; s2, preparing a sodium alginate stock solution and a chitosan stock solution, and homogenizing and mixing the sodium alginate stock solution and the chitosan stock solution with the active algae liquid to form algae-containing composite sol; s3, dropwise adding the algae-containing composite sol into a calcium-containing curing bath at a fixed distance and a fixed speed to realize ionic crosslinking so as to obtain primary gel balls; s4, performing interface densification treatment on the primary gel balls in a chitosan phase; and S5, performing isotonic washing and maintenance to stabilize the structure and osmotic pressure to obtain a wet product. The biological microalgae microcapsule fertilizer prepared by the method disclosed by the invention can overcome the defects in the prior art, and the field survival time, the colonization rate, the slow-release effect and the swelling rate of microalgae are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of agricultural microbial fertilizer technology, specifically to a biological microalgae microcapsule fertilizer and its preparation method. Background Technology

[0002] With the continuous expansion of modern agricultural production, the application of chemical fertilizers has increased year by year, leading to increasingly prominent problems such as soil degradation, nutrient loss, and eutrophication of water bodies caused by excessive or unreasonable fertilization. Traditional chemical fertilizers are mainly composed of inorganic salts, with a rapid nutrient release rate that is difficult to match with crop growth rhythms, often resulting in low fertilizer utilization efficiency and high environmental pollution risks. In recent years, the concepts of green agriculture and sustainable development have made slow-release and controlled-release fertilizers, which combine high efficiency and ecological safety, a research hotspot. However, most commercially available controlled-release fertilizers currently rely on polymers or synthetic materials as coating agents, which have drawbacks such as non-degradability, high cost, and high residue risk. There is an urgent need to develop new fertilizer carriers that are more environmentally friendly, degradable, and have fertilizer efficiency regulation functions.

[0003] Microalgae, as highly efficient photosynthetic microorganisms, are rich in proteins, polysaccharides, lipids, and various secondary metabolites, and have broad application prospects in agriculture. They can not only provide crops with conventional nutrients such as nitrogen, phosphorus, and potassium, but also release various bioactive substances, promoting rhizosphere microecological balance and enhancing plant stress resistance. However, direct application of microalgae suffers from problems such as low survival rate, easy inactivation under environmental stress, and uneven fertilizer release, limiting their widespread application in agricultural production. Therefore, how to achieve efficient immobilization and targeted release of microalgae through suitable carriers and encapsulation technologies has become a key technical challenge that urgently needs to be overcome in this field.

[0004] Microencapsulation technology, a rapidly developing interdisciplinary technology combining biomaterials and agriculture, has been widely applied in drug delivery, food preservation, and agricultural fertilizer development due to its excellent slow-release properties, targeting capabilities, and controllability. Microcapsules prepared using natural biodegradable polysaccharides such as sodium alginate and chitosan through ionogelation or composite cross-linking methods can effectively encapsulate microalgal cells, maintaining their bioactivity. They also achieve slow degradation and gradual nutrient release in the soil environment, thereby prolonging fertilizer effectiveness, reducing the frequency of fertilization, and lowering environmental impact. Therefore, the preparation method of microalgal microcapsule fertilizers based on biodegradable materials aligns with the development direction of green agriculture and provides a new technological path to improve fertilizer utilization and crop yield and quality. Summary of the Invention

[0005] This invention provides a biological microalgae microcapsule fertilizer and its preparation method, overcoming the above-mentioned shortcomings of the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing a microalgae microcapsule fertilizer, characterized by comprising the following steps:

[0008] S1. Prepare active algal solution and gently concentrate it;

[0009] S2. Prepare sodium alginate and chitosan stock solutions and homogenize them with active algae solution to form algae-containing composite sol;

[0010] S3. The algae-containing composite sol is added dropwise at a fixed distance and rate to a calcium-containing curing bath to achieve ionic cross-linking and obtain nascent gel spheres.

[0011] S4. The nascent gel spheres are subjected to interfacial densification treatment in the chitosan phase;

[0012] S5. Isotonic washing and curing are performed to stabilize the structure and osmotic pressure, resulting in a wet product, namely wet biological microalgae microcapsule fertilizer.

[0013] Preferably, the sodium alginate stock solution has a mass fraction of 6%, the chitosan stock solution has a mass fraction of 0.4%, and the amount of active algae solution added is 5% of the sum of the volumes of the sodium alginate stock solution and the chitosan stock solution after mixing; the CaCl2 mass fraction in the calcium-containing curing bath is 1.5%; and the pH of the composite sol is neutral to balance the activity of microalgae and the electrostatic coordination stability of polysaccharides.

[0014] Preferably, during the dripping process, the needle or microdroplet generator is fixed 8-15 cm above the surface of the curing liquid and dripped at a rate of 50-60 drops / min; the curing bath temperature is 25±2℃ and the mixture is slowly stirred at 100-120 r / min for 20-40 minutes to obtain a stable fracture droplet shape and convergence interface, thereby improving the geometric consistency of particles and the yield.

[0015] Preferably, the nascent gel spheres are immersed in a chitosan solution to form a dense outer membrane for 8-15 minutes; then washed with 0.9% NaCl for 5-15 minutes and cured at 4°C in the dark for 0.5-2 hours.

[0016] Preferably, the above method also includes step S6, which involves gentle dehydration as needed to obtain a dry product. To accommodate cross-regional logistics or seasonal storage, the wet product can be further dehydrated (e.g., low-temperature air drying or vacuum freeze drying) to form a dry product, namely, a dry biological microalgae microcapsule fertilizer. Under rehydration application conditions, the dry product can quickly restore its structure and maintain a high algal cell survival rate and membrane integrity.

[0017] The present invention also provides a microalgae microcapsule fertilizer prepared by the above method.

[0018] Preferably, the bio-microalgae microcapsule fertilizer contains soluble micronutrients, organic small molecule osmotic regulators (such as trehalose and glycerol), and low molecular weight organic matter, which improves osmotic homeostasis and antioxidant capacity without damaging cell membrane integrity, and provides an initial induction carbon / nitrogen source.

[0019] This invention provides a biological microalgae microcapsule fertilizer and its preparation method, which has the following beneficial effects:

[0020] 1. The microcapsules utilize an alginate-calcium ion gel as the core support, with a dense chitosan interface layer on the outer surface. Through a mild process of dripping, ionic cross-linking, and interfacial assembly, uniform particle size, regular morphology, and tunable porosity microstructural units are obtained, synergistically encapsulating active microalgae with a nutrient / protection system for targeted release and slow-release efficacy in the crop rhizosphere. The core-shell composite structure formed using natural polysaccharide materials exhibits good compatibility with the soil environment and is biodegradable, significantly improving the field survival time and colonization rate of microalgae.

[0021] 2. After a short period of impregnation with chitosan solution, the nascent gel spheres form an outer dense membrane. This dense membrane assembles with the core through electrostatic coordination and hydrogen bonding, which not only improves compressive and shear strength but also establishes selective permeation channels for water and small molecule nutrients, thereby achieving a slow-release effect in the crop rhizosphere. Specifically, the microporous structure of the dense membrane is controllable, enabling the slow release of nitrogen, phosphorus, potassium, and micronutrients, and reducing the risk of initial leaching and burst release.

[0022] 3. The method of this invention features a mild process with low equipment requirements and a wide parameter window for drop-crosslinking-densification, which is conducive to continuous and large-scale manufacturing. This method can be used for laboratory preparation with a single-channel injection device, or a continuous production line can be constructed using a multi-nozzle microdroplet generator, a continuous curing tank, and an online washing / densification module; while maintaining equivalent drop-forming geometric boundaries and curing mass transfer conditions, the production capacity increases linearly with the number of nozzles and the linear velocity.

[0023] 4. The implementation of this invention does not rely on rare or hazardous chemicals. The wall materials, crosslinking agents and nutrient-protection systems involved are all common agricultural or food-grade materials, which are easy to convert and apply under existing fertilizer workshop conditions, and meet the safety and environmental friendliness requirements of green agricultural inputs.

[0024] 5. The parameters of this invention can be replaced or finely adjusted within an equivalent range. For example, in sandy soils, the local water retention and cation exchange capacity after application can be improved by introducing a small amount of fine minerals or biochar particles; in areas with high rainfall or high irrigation intensity, the Ca2+ level can be appropriately increased. 2+ Concentration and outer membrane densification level to reduce short-term swelling rate.

[0025] Terminology: Unless otherwise stated, all percentages mentioned in this invention are mass fractions, temperatures are in °C, and pressures are at atmospheric pressure; all ranges include endpoints. Detailed Implementation

[0026] The following detailed embodiments are used to illustrate the technical solutions of the present invention and not to limit them. Without departing from the spirit of the present invention, the sources of raw materials, proportions, and process parameters can be equivalently replaced or slightly adjusted. Unless otherwise stated, percentages are mass fractions, temperatures are in °C, pressures are atmospheric pressure, and ranges include endpoints.

[0027] Example 1: Preparation of wet microalgae microcapsules

[0028] Raw materials and specifications: Sodium alginate M / G ratio of 1.2–1.5; chitosan degree of deacetylation ≥85%, medium viscosity grade; CaCl2 can be prepared as a dihydrate. The microalgae are freshwater green algae (Shanghai Guangyu Biotechnology, Chlorella vulgaris GY-D19), preferably prepared in the logarithmic growth phase. A sterile isotonic solution (0.9% NaCl) is preferred as the dispersion and washing medium.

[0029] Preparation of stock solutions: Preferably, prepare a 6.0% sodium alginate stock solution and a 0.8% chitosan stock solution (both stock solutions are prepared using sterile deionized water as the solvent; to balance microalgal activity and osmotic pressure matching, the dispersion / washing phase in the process preferably uses a 0.9% sterile isotonic aqueous solution of NaCl). Stir at 60–80°C to ensure complete dissolution and allow to cool to room temperature. Filter to remove impurities and degas before use. Mix the two stock solutions at a volume ratio of 1:1 and add the active algae solution. The volume of the active algae solution added is 5% of the sum of the volumes of the two stock solutions after mixing, resulting in an algae-containing composite sol. Adjust the pH of the composite sol to 7.

[0030] Pretreatment of active algal solution: The algal solution is concentrated to a cell density of 1×10⁻⁶ by gentle centrifugation or membrane method. 8 Cells / mL, retaining some extracellular metabolites; after short-term static incubation at 4℃ to remove bubbles, warm to 25℃ before use to avoid activity fluctuations caused by sudden temperature changes.

[0031] Dropping and Ionic Crosslinking: Inject the algae-containing composite sol into a syringe or microdroplet generator, fix the dropper 10 cm above the surface of the curing bath, and drop it into the 1.5% CaCl2 curing bath at a rate of 50–60 drops / min (average 0.1 ml per drop). Maintain the curing bath at 25±2℃ and stir slowly at 100 r / min for 20–40 minutes to form a continuous gel network, stabilize the droplet shape, and ensure uniform mass transfer.

[0032] Interface densification and washing: The nascent gel spheres were immersed in a chitosan solution (i.e., the aforementioned chitosan stock solution) for 15 minutes to form a dense outer film. During this process, chitosan forms a dense film on the surface of the gel spheres through electrostatic coordination and hydrogen bonding. Subsequently, the spheres were washed with 0.9% NaCl for 10 minutes to remove free Ca. 2+ The mixture was combined with unreacted polysaccharides and kept at 4°C in the dark for 1 hour to allow the internal and external osmotic pressures to reach equilibrium and the structure to mature.

[0033] The obtained wet product had an average particle size of 2.8±0.2 mm and an ellipticity of 0.85±0.03, with a narrow particle size distribution and regular morphology. The single-particle crushing strength met the shear and compressive strength requirements for conventional filling and hole application processes. The activity remained good during short-term storage at 4℃. This sample can be directly used for mixing seedling substrates or for hole application.

[0034] Example 2: Dry product and rehydration

[0035] Wet microcapsules were prepared according to Example 1 and then dried at low temperature or freeze-dried to obtain the dry product. The drying conditions were controlled within a mild range that did not cause significant loss of algal cell activity. Before application, the microcapsules were rehydrated with an isotonic solution, and the morphology and outer membrane integrity of the particles were observed to be restored. The results were verified based on the number of active algae and morphological parameters to ensure that the product met the application requirements. The dry product is suitable for inter-regional transportation and seasonal storage.

[0036] Compare with Example 1:

[0037] The effect of interface densification treatment on the performance of microcapsules was studied in comparison with Example 1.

[0038] Primary gel spheres were obtained under the same drop and crosslinking conditions as in Example 1, but the chitosan interface densification step was omitted, while other conditions remained the same. Comparative testing showed that this sample exhibited an increased swelling rate and decreased crushing strength under the same hydrodynamic conditions; the particle size distribution was broadened, and the storage stability and initial sustained-release control were inferior to those in Example 1, demonstrating the necessity of the outer dense membrane of this invention.

[0039] Test method description: (1) Particle size and distribution: Image analysis method or obvious damaged particles are removed during testing; (2) Geometric morphology: Ellipticity is converted according to the ratio of major and minor axes, and regularity is statistically calculated according to the circularity index; (3) Mechanical properties: Crushing strength is measured under constant rate loading conditions and the average of multiple points is taken; (4) Activity and stability: Active algae count is statistically calculated by plate or flow cytometry method; Storage test is carried out at 4℃ in the dark, and the activity decay curve is measured periodically.

[0040] The product parameters of Example 1 and Comparative Example 1 are shown in the table below.

[0041]

[0042]

[0043] Compression strength test: Single-particle compression test was conducted using a texture analyzer (speed 0.5 mm / s). The average compression strength of the wet microcapsules in Example 1 was 1.25 ± 0.15, which meets the requirements for mechanical sowing and hole application compression resistance.

[0044] Algal cell viability test: Flow cytometry and FDA viability staining were used. After 30 days of storage at 4°C in the dark, the activity retention rate of the wet microcapsules in Example 1 was 85.3 ± 3.5. After 24 hours of rehydration, the activity recovery rate of the dry product was ≥ 80%. The photosynthetic efficiency (Fv / Fm) remained at 0.65–0.70 during storage, indicating that the microalgae were in good physiological condition.

[0045] Swelling rate test: In deionized water, the swelling rate of the wet microcapsules of Example 1 was ≤15% within 6 hours and tended to reach equilibrium (≈25%) after 24 hours, indicating that the chitosan layer effectively inhibited rapid water absorption.

[0046] Nutrient slow-release performance: In simulated soil solution (pH 6.5), the nitrogen, phosphorus and potassium release curves of the wet microcapsules of Example 1 conformed to the first-order kinetic model, with a cumulative release rate of ≤40% after 3 days and 80% after 15 days, showing good slow-release performance.

[0047] Compare with Example 2-9

[0048] The effects of different concentrations of sodium alginate stock solution, chitosan stock solution, and curing bath on the performance of microcapsules were investigated using Examples 2-9. Microcapsules were prepared according to the steps in Example 1, with all conditions being the same except for the parameters of the sodium alginate stock solution, chitosan stock solution, and curing bath.

[0049]

[0050] Crushing strength: The unit is Newtons (N). A higher value indicates better mechanical strength of the microcapsules, and greater resistance to compression and shear forces during fertilization. Sodium alginate and CaCl2 concentrations are the main factors affecting the strength of the internal gel network; higher concentrations result in greater cross-linking density and higher strength. The chitosan layer provides external reinforcement.

[0051] Swelling rate: Measured in milligrams per minute (mg / min), this represents the weight of water absorbed by the microcapsule per unit time in the solution. A lower value indicates a denser microcapsule structure, resulting in slower water absorption. This is beneficial for maintaining structural stability in environments with frequent irrigation or rainfall, preventing premature nutrient loss. High cross-linking density and high densification significantly reduce the swelling rate.

[0052] Slow-release performance: Measured by the "28-day cumulative release rate," which is the percentage of nutrients released from the fertilizer core after 28 days. The lower this value, the better the slow-release effect and the longer the fertilizer effect lasts. This is directly related to the swelling rate and the density of the membrane layer.

[0053] Particle size distribution width: Quantified using the commonly used engineering Span value, Span = (D90 - D10) / D50. The smaller the Span value, the more concentrated and uniform the particle size distribution. Uniform distribution is beneficial for consistent fertilization and effects. Too high or too low a sodium alginate concentration may cause deformation or instability during dripping due to changes in the solution's viscoelasticity, resulting in a wider distribution. If the particle size distribution is too wide (e.g., Span > 1.0), it can lead to: uneven fertilization: large particles release slowly, while small particles release rapidly, resulting in uneven fertilizer effectiveness; large differences in mechanical strength: small particles are easily broken, while large particles may clog the drip irrigation system; poor colonization stability: small particles are easily leached, while large particles are unevenly distributed in the rhizosphere.

[0054] Application Examples 1 and 2

[0055] The wet microcapsules of Example 1 were used.

[0056] Application Example 1. Microalgae colonization rate in sandy soil (sand content >70%, pH 6.8–7.2): After 30 days, the number of rhizosphere microalgae still maintained ≥50% of the inoculum; Leaching loss: Compared with unencapsulated microalgae, nitrogen leaching loss was reduced by 35%; Improved water retention: After adding 5% biochar microparticles to the microcapsules, the soil water retention capacity was increased by 20%.

[0057] Application Example 2. Survival rate of saline soil (EC>4dS / m, pH 7.5–8.0): After 30 days, the survival rate of microalgae was ≥60% (compared to ≤20% in the direct application group); Osmotic adjustment: After adding trehalose as an osmotic regulator, the survival rate was further increased to ≥75%; Slow-release effect: Nutrient release is delayed under salt stress, which is more in line with the salt-tolerant growth rhythm of crops.

[0058] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0059] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a biological microalgae microcapsule fertilizer, characterized in that, Includes the following steps: S1. Prepare active algal solution and gently concentrate it; S2. Prepare sodium alginate and chitosan stock solutions and homogenize them with active algae solution to form algae-containing composite sol; S3. The algae-containing composite sol is added dropwise at a fixed distance and rate to a calcium-containing curing bath to achieve ionic cross-linking and obtain nascent gel spheres. S4. The nascent gel spheres are subjected to interfacial densification treatment in the chitosan phase; S5. Isotonic washing and curing are used to stabilize the structure and osmotic pressure to obtain a wet product.

2. The method for preparing a biological microalgae microcapsule fertilizer as described in claim 1, characterized in that: The sodium alginate stock solution has a mass fraction of 6%, the chitosan stock solution has a mass fraction of 0.4%, and the amount of active algae solution added is 5% of the sum of the volumes of the sodium alginate stock solution and the chitosan stock solution after mixing; the CaCl2 mass fraction in the calcium-containing curing bath is 1.5%; the pH of the composite sol is neutral to balance the activity of microalgae and the electrostatic coordination stability of polysaccharides.

3. The method for preparing a biological microalgae microcapsule fertilizer as described in claim 1, characterized in that: When dripping, fix the needle or microdrop generator 8-15cm above the surface of the curing liquid and drip it at a rate of 50-60 drops / min; the curing bath temperature is 25±2℃ and the mixture is slowly stirred at 100-120r / min for 20-40 minutes.

4. The method for preparing a biological microalgae microcapsule fertilizer as described in claim 1, characterized in that: The nascent gel spheres are immersed in a chitosan solution to form a dense outer membrane for 8-15 minutes; then washed with 0.9% NaCl for 5-15 minutes and cured at 4°C in the dark for 0.5-2 hours.

5. The method for preparing a biological microalgae microcapsule fertilizer as described in claim 1, characterized in that: The above method also includes step S6, which involves gentle dehydration as needed to obtain a dry product.

6. A microalgae microcapsule fertilizer prepared by any one of the methods described in claims 1-6.

7. The microalgae microcapsule fertilizer as described in claim 7, characterized in that: This bio-microalgae microcapsule fertilizer contains soluble micronutrients, organic small molecule osmotic regulators, and low molecular weight organic matter.

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