Method for preparing biochar slow-release phosphate fertilizer particles by using starch-CMC binder

Abstract

The invention discloses a method for preparing biochar slow-release phosphate fertilizer granules by using a starch-CMC binder, the method is used for improving the mechanical property and slow-release property of the biochar slow-release phosphate fertilizer granules through the preparation of the starch-CMC binder, and the method comprises the following steps: fully mixing biochar and monopotassium phosphate powder according to the mass ratio of (2-5): (0.5-1.5); the blended charcoal-based phosphate fertilizer is obtained for standby application; the preparation method comprises the following steps: mixing 15-35g of starch with sodium carboxymethyl cellulose with the mass concentration of 1-2%, adding 200ml of deionized water, putting a beaker into a water bath kettle with the temperature of 50-80 DEG C, and gelatinizing for 5-15 minutes to obtain a composite binder; fully stirring and blending the prepared composite binder and the blended charcoal-based phosphate fertilizer, and then putting the mixture into an extrusion granulator for extrusion granulation; and sieving the successfully prepared biochar-based phosphate fertilizer particles by a 10-mesh sieve, putting the sieved biochar-based phosphate fertilizer particles into an oven at 70-90 DEG C, drying for 2 hours, taking out the dried biochar-based phosphate fertilizer particles, and placing the dried biochar-based phosphate fertilizer particles at room temperature to obtain

Description

Technical Field

[0001] This invention relates to the field of biochar slow-release phosphate fertilizer technology, and in particular to a method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder. Background Technology

[0002] Phosphorus is the second most important nutrient for plant growth after nitrogen. Traditional chemical phosphate fertilizers often suffer from rapid leaching, nutrient loss, and phosphorus precipitation into unusable forms, leading to environmental pollution and an average utilization rate of only 10-25%. To address the drawbacks of traditional phosphate fertilizers, the targeted design of slow-release phosphate fertilizers has become an effective solution to the low application efficiency of traditional phosphate fertilizers. Currently, utilizing carriers to load the effective components of fertilizers to achieve slow nutrient release has become a research hotspot. Among many carrier materials, biochar is widely available, inexpensive, and possesses unique structure and chemical properties. In addition to serving as a carrier and having a slow-release effect on nutrients, biochar also has certain soil environmental improvement and remediation effects, making it favored by many scholars. Recent research reports have also revealed some high-performance biochar-based fertilizers.

[0003] To prepare effective biochar-based slow-release fertilizers, various methods have been proposed, including impregnation, co-pyrolysis, coating, and granulation. However, several problems remain in practical applications, such as inconvenient application and poor storage stability, which severely limit the large-scale promotion of biochar-based slow-release phosphate fertilizers. The coating method significantly increases production costs due to its complex production process and the expensive coating materials required. Furthermore, the coating materials are often composed of non-degradable polymers, and improper application may lead to secondary environmental problems such as soil microplastic pollution. Granulation is an effective option for reducing biochar loss in the soil and lowering treatment costs, enabling large-scale application in agricultural practices. Granulation of biochar and nutrient mixtures effectively reduces nutrient release by increasing diffusion resistance. Biochar slow-release fertilizer granulation involves thoroughly mixing biochar with fertilizer granules and then granulating them using disc granulation or mechanical extrusion.

[0004] Binders are crucial factors affecting the slow-release properties, mechanical properties, and molding efficiency of biochar-based fertilizers. Adding binders to biochar and conventional fertilizers as base materials can significantly increase molding rate and slow-release efficiency. Most synthetic organic polymers, such as traditional materials like polyvinyl alcohol and resins, are expensive, non-biodegradable, and prone to leaving residues in the soil after application, leading to environmental pollution and negatively impacting the root growth of subsequent crops. Therefore, developing a low-cost, efficient, and environmentally friendly preparation method to convert powdered biochar slow-release phosphate fertilizer into granular products is of great significance for improving its application performance. Starch has advantages such as readily available raw materials, low price, non-corrosive and non-polluting properties, high strength, light weight, and ease of use.

[0005] Sodium carboxymethyl cellulose (CMC) is renowned for its excellent film-forming properties and high mechanical strength, enhancing the plasticity and mechanical properties of binders and frequently used as a reinforcing agent in coating materials. In existing technologies, cross-linked CMC / CS-20-e composite films are synthesized using a casting method under acidic conditions with epichlorohydrin (ECH) as a cross-linking agent. These films are then used in a layer-by-layer self-assembly process to synthesize urea slow-release fertilizer. Compared to single-CMC / CS-coated urea fertilizers, CMC exhibits significantly improved mechanical properties and slow-release effects. Currently, CMC application research primarily focuses on its use as a coating material in the preparation of coated slow-release fertilizers. Research on the use of CMC and starch blends as binders in biochar slow-release fertilizer granulation, as well as the exploration of their binding mechanism with biochar-based slow-release fertilizers, is limited, and the mechanism of nutrient slow-release remains unclear. Summary of the Invention

[0006] This invention provides a method for preparing biochar slow-release phosphate fertilizer granules using a starch-CMC binder. The method improves the mechanical and slow-release properties of the biochar slow-release phosphate fertilizer granules through the preparation of the starch-CMC binder. Specifically, CMC and starch are compounded and blended to prepare the binder. The addition ratio of the two is optimized to improve the mechanical properties and slow-release properties of the biochar slow-release fertilizer granules, explore its nutrient release mechanism, and prepare a highly efficient and environmentally friendly biochar-based slow-release phosphate fertilizer. The invention also reveals the application effects of biochar-based slow-release phosphate fertilizer in soil-plant systems and its promoting effect on soil fertility.

[0007] The technical solution is as follows: A method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder, wherein the method improves the mechanical properties and slow-release performance of the biochar slow-release phosphate fertilizer granules through the preparation of starch-CMC binder, comprising the following steps: Biochar and potassium dihydrogen phosphate powder are thoroughly mixed at a mass ratio of 2-5:0.5-1.5 to obtain a blended biochar-based phosphate fertilizer for later use. Mix 15-35g of starch with sodium carboxymethyl cellulose (1-2% by mass), add 200ml of deionized water, place the beaker in a water bath at 50-80℃, and gelatinize for 5-15 minutes to obtain the composite binder. The prepared composite binder is thoroughly mixed with the biochar-based phosphate fertilizer, and then placed in an extrusion granulator for extrusion granulation. After the successfully prepared biochar-based phosphate fertilizer granules are sieved through a 10-mesh sieve, they are placed in an oven at 70-90℃ and dried for 2 hours. They are then removed and placed at room temperature to obtain biochar slow-release phosphate fertilizer granules.

[0008] In the above, the mechanical properties include abrasion resistance and compressive strength within a specified range for the biochar slow-release phosphate fertilizer particles; The wear resistance is measured in the following manner: Define the wear resistance calculation formula and write the wear resistance calculation formula into the calculation program; The set amount of biochar slow-release phosphate fertilizer granules were placed in a conical flask, and several steel balls were placed in the conical flask. The conical flask was sealed and fixed in a shaker and shaken at 250 r / min for 30 min. After shaking, the remaining biochar slow-release phosphate fertilizer granules were weighed, and the wear resistance was calculated according to the calculation program. The compressive strength is measured using a universal pressure tester to determine the maximum compressive strength of the biochar slow-release fertilizer granules; the die is lowered at a speed of 5 mm / min until the fertilizer breaks.

[0009] In the above, the wear resistance calculation formula is: ; Where R is the residual weight ratio, m1 is the fertilizer mass weighed before the abrasion test, and m2 is the fertilizer mass after the abrasion test.

[0010] In the above, the sustained-release performance includes water purification sustained-release measurement and dynamic sustained-release measurement; The water purification slow-release test includes: Weigh 0.1g of biochar-based slow-release phosphate fertilizer granules and place them on a 4cm x 4cm nylon cloth with a pore size of 400 mesh. Tie the nylon cloth with a thin thread and immerse it in a conical flask containing 50mL of deionized water. Leave a certain length of thin thread outside the flask to fix the position of the nylon bag in the water. Repeat each group three times. Take 5 mL of water sample every 24 hours. After each sampling, add the same volume of deionized water to the conical flask. Then, digest the water sample with potassium persulfate at 120℃ for 30 min to convert polyphosphates and organic phosphates into orthophosphates. After cooling, make up the volume and add ascorbic acid solution and molybdate solution. After reacting at room temperature for 15 min, use a UV-Vis spectrophotometer to determine the cumulative release rate of phosphorus released within a specific time interval. The dynamic sustained-release assay includes constructing a multivariate sustained-release model and performing a fitting analysis of the release behavior using the multivariate sustained-release model.

[0011] A composite binder system was constructed using carboxymethyl cellulose (CMC) and starch. After optimizing the ratio, granular biochar-based slow-release phosphate fertilizer was prepared using an extrusion granulation process. The results showed that the prepared biochar-based slow-release phosphate fertilizer granules were all cylindrical. In the composite binder system, granules with a CMC concentration of 2% exhibited excellent mechanical properties. In the static water slow-release experiment, the cumulative phosphorus release rate reached 54.07% after 28 days, which was 24.4% and 41.3% lower than the single CMC and starch systems, respectively. Kinetic modeling confirmed that the release process of the composite system was synergistically regulated by diffusion and chemical bonding. Mechanistic studies showed that the carboxyl group of CMC exchanged ions with phosphate and metal ions on the surface of biochar, constructing a dense chemical cross-linked network. Combined with the physical coating of starch, this reduced the rapid release of phosphorus. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 Digital photographs of particle size tests (f) of BCP-S (a), BCP-CMC (b), BCP-SM2 (c), BCP-SM1.5 (d), and BCP-SM1 (e); Figure 2 Scanning electron microscope images of the surface and interior (df) of BCP-S(a), BCP-CMC(b), and BCP-SM2(c); Figure 3 FT-IR spectrum (a), XRD spectrum (b), XPS spectrum (c) of biochar slow-release phosphate fertilizers with different binders; N2 adsorption-desorption curves of BC (d), N2 adsorption-desorption curves of BCP-S (e), N2 adsorption-desorption curves of BCP-CMC (f), N2 adsorption-desorption curves of BCP-SM2 (g), N2 adsorption-desorption curves of BCP-SM1.5 (h), and N2 adsorption-desorption curves of BCP-SM1 (i); Figure 4 Water resistance (a) and compressive strength (b) of biochar slow-release phosphate fertilizers with different binders. Figure 5 Water-based slow-release diagrams of biochar slow-release phosphate fertilizers with different binders; Figure 6 Kinetic fitting diagram (ad) of biochar slow-release phosphate fertilizer with different binders. Figure 7Schematic diagram of the slow-release mechanism of composite binders in biochar slow-release fertilizer granules; Figure 8 Front view (a) and plant image (b) of the growth status of Chinese cabbage in different treatment groups; Figure 9 The changes in plant height, root length (a), fresh weight, dry weight (b), chlorophyll (c), soluble sugar, and soluble protein (d) of Chinese cabbage in different treatment groups; Figure 10 The changes in soil pH (a), electrical conductivity (b), organic matter (c), and available phosphorus (d) in different treatment groups. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0015] Example 1: This embodiment provides a method for preparing biochar slow-release phosphate fertilizer granules using a starch-CMC binder. The method includes: thoroughly mixing biochar and potassium dihydrogen phosphate powder at a mass ratio of 4:1 and setting aside. Mixing 25g of starch with sodium carboxymethyl cellulose at mass concentrations of 2%, 1.5%, and 1% respectively, adding 200ml of deionized water, placing the beaker in a 60℃ water bath, and gelatinizing for 10 minutes to obtain a composite binder. Thoroughly mixing the prepared binder with the aforementioned biochar-based phosphate fertilizer, then extruding and granulating the mixture using an extrusion granulator. The successfully prepared biochar-based phosphate fertilizer granules are sieved through a 10-mesh sieve and then dried in an 80℃ oven for 2 hours. After drying to room temperature, they are stored in sealed bags and labeled as BCP-SM2, BCP-SM1.5, and BCP-SM1. Where BC represents biochar, P represents potassium dihydrogen phosphate, S is the first letter of the English word starch, M represents sodium carboxymethyl cellulose, and 2, 1.5, and 1 are the mass concentrations of sodium carboxymethyl cellulose. Following the same steps as above, pure starch binder and sodium carboxymethyl cellulose binder were prepared, mixed with biochar-based phosphate fertilizer and granulated, and the samples were named BCP-S and BCP-CMC.

[0016] Example 2: This embodiment provides abrasion resistance and compressive strength tests, which reflect the hardness of the granular fertilizer. Higher hardness facilitates transportation. For the abrasion resistance test, 2.00 g of fertilizer is placed in a 50 mL Erlenmeyer flask, and five steel balls are added to the same flask. The flask is then capped and fixed in a shaker, where it is shaken at 250 rpm for 30 min. After shaking, the remaining fertilizer is weighed, and the abrasion resistance is calculated based on the remaining weight ratio.

[0017] ; Where R is the residual weight ratio, m1 is the fertilizer mass (g) weighed before the abrasion test, and m2 is the fertilizer mass (g) after the abrasion test.

[0018] The maximum compressive strength of biochar slow-release fertilizer granules was measured using a universal pressure tester (UTM5205X, Sans, China). The die was lowered at a rate of 5 mm / min until the fertilizer broke. Ten measurements were taken for each fertilizer, and the average value was recorded.

[0019] Example 3: This embodiment provides a particle size test for biochar slow-release fertilizer granules. Thirty granules of each type of biochar slow-release fertilizer were randomly selected, and their particle size was measured using calipers. Each fertilizer granule was measured three times, and the average value was taken. The fertilizer particle size distribution was analyzed, along with the average particle size and standard deviation.

[0020] Example 4: This embodiment provides a static water slow-release test.

[0021] Weigh 0.1g of biochar-based slow-release phosphate fertilizer granules and place them on a 4cm x 4cm nylon cloth (400 mesh). Tie the nylon cloth with a thin thread and immerse it in a conical flask containing 50mL of deionized water. Leave a certain length of thread outside the flask to fix the position of the nylon bag in the water. Repeat the process three times for each group. The system is sealed with plastic film to prevent water evaporation. Take 5mL water samples periodically (1, 3, 5, 7, 9, 11, 14, 21, and 28 days). After each sampling, add the same volume of deionized water to the conical flask. Then, digest the water samples with potassium persulfate at 120℃ for 30 min to convert polyphosphates and organophosphates into orthophosphates. After cooling, make up to volume, add ascorbic acid solution and molybdate solution, and react at room temperature for 15 min. The cumulative release rate of phosphorus released within the specific time interval is determined using a UV-Vis spectrophotometer.

[0022] Example 5: This embodiment provides release kinetics. The cumulative phosphorus release from different BCP- concentrations in still water was fitted using a first-order model, the Higuchi model, the Elovich model, and the Weber-Morris model to investigate the phosphorus release behavior of BCP- in water. The model fitting formulas are as follows: First-order model: ; Higuchi model: ; Elovich model: ; Korsmeyer-Peppas model: ; In the formula, The phosphorus is released cumulatively at different time intervals. This is the phosphorus released at the maximum rate; and The release of the first-order model and the Higuchi model, respectively. and n are the rate constant and release exponent of the Korsmeyer-Peppas model, respectively; This is the initial release of phosphorus; It is the activation energy constant of the Elovich model.

[0023] Example 6: Pot Experiment: To evaluate the application effect of the biochar-based slow-release fertilizer prepared in this study, a pot experiment was conducted using Chinese cabbage as the experimental subject. Seven treatments were performed: 1) CK: no fertilizer; 2) CF: conventional fertilizer; 3) application of five kinds of prepared biochar-based slow-release phosphate fertilizers; each treatment had three replicates. Plump and uniformly sized chili pepper seeds were selected. The biochar was mixed evenly with the soil (800g soil / pot), and the fertilizer application rate was 2% of the soil mass. 500g of soil was added to the pot, the soil was moistened with water, and 10 Chinese cabbage seeds were evenly scattered on the soil. Then, 100g of soil was covered on the surface to complete the planting. Before the seeds germinate, cover the surface of the flowerpot with aluminum foil and poke many pinhole-sized holes in the foil. Water each pot with 50-60mL of water daily. After two weeks, intercrop the seeds, keeping 5 seedlings in each pot. After 62 days of cultivation, harvest the bok choy plants. Measure the plant height, root length, fresh weight, dry weight, soluble sugar, soluble protein, and chlorophyll content, as well as the soil pH, electrical conductivity, organic matter, and available phosphorus content.

[0024] Example 7: This embodiment provides an analysis of the appearance and particle size of biochar slow-release fertilizer.

[0025] Figure 1 The images show digital photographs and particle size distribution photos of samples BCP-S, BCP-CMC2, BCP-SM2, BCP-SM1.5, and BCP-SM1. All prepared samples are cylindrical, with diameters matching the granulator die dimensions. Referring to Table 1, which shows the particle sizes of biochar slow-release fertilizer granules with different binders, it can be seen that BCP-S particles are curved and have a coefficient of variation greater than 10%, indicating uneven particle size distribution. BCP-CMC2 is shorter and also has a coefficient of variation greater than 10%, indicating uneven particle size distribution. Compared to BCP-S and BCP-CMC, the coefficients of variation for BCP-SM2, BCP-SM1.5, and BCP-SM1 are all less than 10%, indicating that the particle size distribution becomes more uniform with increasing CMC proportion, and the number of curved samples decreases.

[0026] Table 1. Particle size of biochar slow-release fertilizer with different binders.

[0027]

[0028] from Figure 2 The scanning electron microscope (SEM) images show significant differences in pore structure and surface morphology among BCP-S, BCP-CMC, and BCP-SM2. BCP-S exhibits a rough surface with large and numerous pores and a loose structure, failing to be fully covered by the binder. This may lead to easier contact between the adsorbed potassium dihydrogen phosphate and water, resulting in rapid phosphorus release. BCP-CMC has a smoother surface, with some biochar phosphate fertilizer encapsulated, and a more compact structure. BCP-SM2 has the most uniform and flat surface, the smallest pores, and the mixed binder tightly encapsulates the biochar phosphate fertilizer. This indicates that during fertilizer granulation, the binder, under pressure, binds the components of the raw materials together. The composite binder group exhibits the best polymerization effect, resulting in a more compact and uniform structure that is less prone to breakage, thus reducing direct contact between phosphate and water and promoting the slow-release effect of the fertilizer.

[0029] Figure 3 (a) The Fourier Transform Infrared (FT-IR) spectrum of BCP- is shown to investigate the types of surface functional groups. The peak at 2780 cm⁻¹ represents the stretching vibrations of the CH bonds in alkanes, indicating the presence of -CH₃ / -CH₂ groups on the surface of BSRPF granules. The peak at 3452 cm⁻¹ represents the stretching vibrations of the CH bonds in alkanes, indicating the presence of -CH₃ / -CH₂ groups on the surface of BSRPF granules. -1 and 1636 cm -1 The peaks at 1074 cm⁻¹ belong to the tensile vibrations of OH and C=O, respectively. The increase in peak intensity after the addition of CMC confirms successful coating on biochar. -1 866 cm -1 The inner peaks correspond to the superposition of the stretching vibrations of PO43- and P–O–P with the symmetric vibrations of the carboxylate group, confirming the successful loading of potassium dihydrogen phosphate. (Cyclophorus phosphate peaks are located at 980-1150 cm⁻¹) -1 Inner diameter, 1000-1130cm -1 The CO stretching vibrations of the CO-C glycosidic bond and CMC showed broad peak shapes, ranging from 980 to 1050 cm⁻¹. - It is the asymmetric stretching vibration of phosphate (H2PO4). - The overlap with the binder peak may reflect that phosphate is effectively encapsulated in the binder-biochar system, thus delaying the phosphorus release rate. (818-748 cm⁻¹) -1 The peaks that appear inside belong to the out-of-plane bending vibration of the aromatic ring CH in biochar, confirming the stability of its biochar structure.

[0030] from Figure 3(c) The X-ray diffraction pattern shows that KH₂PO₄ peaks were found at 30.72°, 20.86°, and 23.74° in the BCP-S sample, proving the successful loading of KH₂PO₄. However, this may indicate that the potassium dihydrogen phosphate crystals may not have been fully coated. In the BCP-CMC, KH₂PO₄ crystal planes appeared at 30.72° and 23.98°, but the intensity decreased, which may be related to the formation of NaH₂PO₄·2H₂O at 26.72°. Some KH₂PO₄ reacted with sodium ions (Na₂PO₄·2H₂O). + Ion exchange was performed. In BCP-SM2, new crystalline phases NaH2PO4·2H2O appeared at 26.46° and 27.82°, and Na3PO4·8H2O appeared at 20.32°. In BCP-SM1.5, the main peaks at 26.90° and 20.58° remained NaH2PO4·2H2O, but their intensity decreased, reflecting the influence of the binder ratio on the crystalline phase purity. This indicates that a high CMC ratio promotes the reaction of K+ in potassium dihydrogen phosphate. + Ion exchange occurs, forming a stable chemically cross-linked network structure that hinders rapid phosphorus dissolution and significantly delays phosphorus release. This further confirms the successful loading of KH2PO4, a finding consistent with the cross-linking mechanism shown by FTIR spectroscopy. XPS spectroscopy was used to analyze the chemical composition and state of BCP-S, BCP-CMC, and BCP-SM2. Figure xx shows the peaks of the total spectrum, C1 s, O1 s, and [other peaks]. Figure 3 (b) is the peak spectrum of P 2p, which shows two P-related peaks with binding energies of 133.88 eV and 134.40 eV, respectively, belonging to H2PO42- and PO43- [41, 42]. It can be observed from the figure that the peak areas of H2PO42- and PO43- in the BCP-S sample are larger than those in BCP-CMC and BCP-SM2, which may indicate that the exposure of KH2PO4 is higher. This is consistent with the peak intensity results of KH2PO4 crystals in its XRD. The decrease in the peak area of ​​H2PO42- in BCP-CMC may indicate that some KH2PO4 is bound to carboxylate ions. The XRD also confirmed the formation of new crystals. The peak area in BCP-SM2 is further reduced, indicating that the physicochemical synergy of the mixed binder and biochar better encapsulates phosphorus to achieve the effect of slow release. Figure 3(di) represent the N2 adsorption-desorption curves of BC, BCP-S, BCP-CMC, BCP-SM2, BCP-SM1.5, and BCP-SM1, respectively. It can be seen that the N2 adsorption-desorption isotherms of all samples exhibit similar trends: the adsorption rate first increases, then stabilizes, and then increases again, all belonging to Type IV, and possessing H4 type hysteresis loops with a closed interval P / P0 = 0.45-0.8, indicating that the material is dominated by a 2-50 nm mesoporous structure. The H4 type hysteresis loop isotherm is a combination of Type I and Type II adsorption isotherms, lacking a clear saturation adsorption plateau in the higher relative pressure region, mainly appearing on adsorbents with a mixture of micropores and mesopores. This indicates that the prepared biochar-based slow-release phosphate fertilizer granules not only have the detected mesopores but also some micropores. Figure s3 shows the pore size distribution pattern, revealing that the composite binder system has fewer macropores and more mesopores, as well as a more uniformly distributed pore size, exhibiting better adsorption capacity and nutrient release control compared to a single binder system. In Table 2-1, the specific surface area of ​​BCP-S (564.9581 m² / g) and BCP-CMC (278.1139 m² / g) is compared. 2 The specific surface area of ​​the composite binder group ( / g) gradually decreased as the CMC ratio increased. This may be because the coating network formed by the composite binder is tighter, which blocks the original pores of the biochar and thus forms a more stable structure.

[0031] Table 2-1 Pore Structure Parameters Table 2-1 Pore structure parameters

[0032] Table 2 shows the abrasion resistance test results for each group of samples. It can be seen that the residual weight ratio of each component in the single-adhesive group is between 95% and 96%, while the residual weight ratio of each component in the mixed adhesive group is greater than 95%. Figure 4 In (a), among the seven types of biochar slow-release fertilizer granules, BCP-CMC and BCP-S granules broke within 10 minutes of soaking in deionized water. BCP-SM2, BCP-SM1.5, and BCP-SM1 had relatively stable structures, and no granule breakage was observed after soaking for seven days. Figure 4(b) The compressive strength of biochar slow-release phosphate fertilizer granules with different binders: As the proportion of CMC-Na added increases, the compressive strength of BSRPF granules gradually increases. Compared with BCP-SM1.5 and BCP-SM1, the compressive strength of BCP-SM2 is increased by 2.06 times and 2.51 times, respectively. This may be because the high proportion of CMC-Na can enhance ion exchange capacity and improve the binding performance of the binder, making the BSRPF granules structure more compact and robust. Compared with BCP-S (18.89N) and BCP-CMC (6.18N), the compressive strength is increased by 3.55 times and 10.84 times, respectively. BCP-CMC exhibited the lowest compressive strength, possibly because the ionic bond network formed when CMC is used alone is not dense enough, resulting in a loose structure. While BCP-S had a higher compressive strength than BCP-CMC, it was significantly lower than the series with mixed binders, indicating that although simple starch physical coating provides some strength, it lacks the reinforcement of chemical bonds. BCP-SM2 had the highest compressive strength. The compressive strength and abrasion resistance data were consistent, indicating that it had the best structural stability, further supporting the positive correlation between structural stability and compressive strength. The above experimental results demonstrate that biochar slow-release phosphate fertilizers with a single binder have poor macroscopic physical properties, while the macroscopic physical properties of the mixed binder formed by sodium carboxymethyl cellulose and starch are significantly improved, making them more beneficial for packaging, storage, and transportation.

[0033] Table 2. Abrasion resistance of biochar slow-release fertilizer granules with different binders

[0034] Different BCP-release behaviors of P in still water, such as Figure 5 As shown in the figure, BCP-S and BCP-CMC exhibited relatively low sustained-release effects, while BCP-SM2, BCP-SM1.5, and BCP-SM1 showed better sustained-release performance. Compared to single binders, the phosphorus release rate of BCP-co-blended with both CMC and starch was significantly reduced, and gradually decreased with increasing CMC proportion. This indicates that the co-incorporation of CMC and starch into BCP improved the sustained-release performance of BCP-P, with the order from strongest to weakest being SM2 > BCP-SM1.5 > BCP-SM1 > BCP-CMC2 > BCP-S. Among single binders, sodium carboxymethyl cellulose (CMC) showed better sustained-release performance than starch (S). The cumulative release rate of BCP-CMC after 28 days was 71.47%, significantly lower than the 92.24% of the BCP-S group. This may be due to the carboxylate group (COO) on the CMC molecular chain. - K in potassium dihydrogen phosphate (KH2PO4) +Phosphorus dissolution is hindered by ionic bonds forming a stable cross-linked network structure; while starch mainly relies on hydrogen bonding of hydroxyl groups (-OH), resulting in a looser particle structure and weaker dissolution resistance. Furthermore, the characteristic peak of carboxylate CMC in the infrared spectrum (1600 cm⁻¹) is also observed. -1 and 1400 cm -1 The vibrational peaks of phosphate (PO) are 1130-974 cm⁻¹. -1 The coexistence of these two components confirms that their chemical bonding may enhance particle stability. The mixed binder BCP-SM2 exhibited the best sustained-release performance, with a release rate of only 54.07% after 28 days, significantly lower than that of a single binder. The synergistic effect of starch and BCP-CMC in BCP-SM2 is likely key: starch provides physical coating, while CMC binds to phosphates and metal ions (such as K+) on the surface of biochar via carboxylate groups. + The CMC forms ionic crosslinks, and together they construct a dense "physical-chemical" dual barrier. Comparison of binders BCP-SM1, BCP-SM1.5, and BCP-SM2 with different blending ratios revealed that increasing the amount of CMC significantly improved the sustained-release effect. The 28-day release rate of BCP-SM2 (54.07%) was lower than that of SM1.5 (64.75%) and SM1 (63.84%). This may be because the carboxyl groups of CMC exchange ions with phosphate and metal ions on the surface of biochar, constructing a dense chemical crosslinked network. Combined with the physical coating of starch, this reduces the rapid release of phosphorus. Furthermore, the molecular structure of CMC may fill the pores formed by starch, reducing the swelling and disintegration rate of the particles. However, when the CMC ratio is too high, it may lead to excessive viscosity during granulation, preventing granulation. Therefore, the ratio of SM2 may be close to the optimal threshold.

[0035] like Figure 6 As shown, the release behavior of five phosphate fertilizers (BCP-S, BCP-CMC2, BCP-SM2, BCP-SM1.5, and BCP-SM1) was fitted and analyzed using first-order kinetic models, Higuchi models, Elovich models, and Ritger-Peppas models. Table 2 details the kinetic parameters of biochar slow-release phosphate fertilizers under the five mathematical models. For the pure starch binder system BCP-S, the first-order kinetic model (R... 2 =0.9922) and the Ritger-Peppas model (R 2 The high goodness of fit (=0.9988) indicates that its release process is mainly diffusion-based. Furthermore, the Elovich model R of BCP-S... 2 The extremely low value (0.0495) indicates that its release is almost unrestricted by chemical reactions, further confirming its simple mechanism dominated by physical diffusion. Similar to BCP-S, BCP-CMC2 follows a first-order kinetic model (R0). 2=0.9973) and the Ritger-Peppas model (R 2 The high goodness of fit of the Elovich model (=0.9997) indicates that its release is still mainly diffusion. For the mixed binder systems BCP-SM2, SM1.5, and SM1, the goodness of fit of the Elovich model increases with the increase of CMC ratio, indicating that the release of phosphorus in BSRPF granules involves chemical reactions and a multi-mechanism release process involving heterogeneity and energy changes.

[0036] The Higuchi model was originally developed for drug delivery systems, but due to its excellent ability to describe the release behavior of low water-soluble substances, it is now widely used in the field of slow-release fertilizers. This model is particularly suitable for describing the release process of low water-soluble nutrients in solid or semi-solid matrices. Its core assumption is that the nutrient release rate is diffusion-controlled and that the release rate is proportional to the square root of time.

[0037] The pseudo-first-order model is suitable for explaining the slow release mechanism of nutrients in biochar-based slow-release fertilizers, especially in porous matrices, where nutrient release is influenced by a combination of physicochemical processes such as diffusion, adsorption, and desorption.

[0038] The Ritger-Peppas model is a semi-empirical model widely used in the fields of drug release and slow-release fertilizers, particularly suitable for describing the release behavior of active substances in porous or polymer matrices. This model distinguishes different release mechanisms using the release index n, thus providing important insights into the kinetics of nutrient release.

[0039] like Figure 7 As shown, the dense coating network formed by starch and CMC-Na composite binder provides primary barriers, making it difficult for phosphorus to be released from the pores of biochar. Some potassium dihydrogen phosphate (K+) undergoes ion exchange with Na+ in CMC-Na, generating new phosphates. Therefore, phosphorus release must overcome both diffusion resistance and chemical reactions, thus affecting the phosphorus release rate. In summary, biochar-based slow-release phosphate fertilizer significantly improves phosphorus stability and release controllability through multi-scale synergistic effects such as physical barriers restricting diffusion and ion exchange. This mechanism not only provides a theoretical basis for understanding slow-release kinetics but also offers an important direction for designing efficient and environmentally friendly slow-release fertilizers.

[0040] Table 3. Release kinetic parameters of biochar slow-release phosphate fertilizer with different binders.

[0041] To investigate the effects of biochar-based granular fertilizers prepared with different binders on the soil-plant cycle, a 62-day potted plant experiment with Chinese cabbage was conducted. Figure 8It is clear that the composite binder groups are growing better than the other treatment groups.

[0042] Figure 9 (a) shows the effects of different treatment groups on root length and plant height of Chinese cabbage seedlings. The mixed binder group showed increased root length and plant height compared to the single binder group, with a more significant increase in root length. BCP-SM2 showed the best effect, with root length increasing by 53.85% and 51.45% compared to BCP-S and BCP-CMC, respectively. This indicates that BSRPF granules prepared by compositebinder promote plant height and root length in Chinese cabbage. Similar trends were observed in the changes in fresh weight, dry weight, and chlorophyll content, suggesting that the synergistic effect of starch and CMC not only delays phosphorus release but may also promote continuous plant growth by optimizing nutrient supply rhythm. Figure 9 (c) Regarding the changes in chlorophyll content, the chlorophyll content of BSRPF granules prepared by composite binder was higher than that of the single binder group, with BCP-SM2 showing the most significant increase. The total chlorophyll content of BCP-SM2 increased by 38.88%, 28.49%, and 27.70% compared to BCP-S and BCP-CMC, respectively. Soluble proteins and soluble sugars are the energy sources for plant growth. Figure 9 (d) shows the changes in their contents, with the mixed group showing better results. The soluble sugar content of BCP-SM2 was 251.22%, 111.38%, and 109.81% higher than that of BCP-S and BCP-CMC, respectively. The soluble protein content also showed the same trend. In summary, BCP-SM2 is more conducive to achieving a dynamic match between phosphorus supply and the plant's phosphorus requirement cycle. Stable phosphorus flow can improve the efficiency of photosynthesis, protein synthesis, and sugar metabolism, thereby promoting plant growth.

[0043] Soil pH and electrical conductivity are commonly used to measure the strength of soil acid-base reactions, directly or indirectly affecting crop growth and development. From Figure 10It can be seen that the pH of the blank control group was 7.46, which is alkaline soil. Adding the traditional fertilizer CF had no effect on soil pH. The addition of BSRPF granules caused the soil pH to drop to between 6.13 and 6.30. This is because BSRPF granules have a lower pH value, which can optimize the soil acid-base balance, enhance the root absorption capacity of Chinese cabbage, and promote growth. None of the treatments had a significant effect on soil electrical conductivity. Soil organic matter is the material basis of soil fertility, and its content is an important indicator for evaluating soil fertility. Compared with CK, all treatments increased soil organic matter content, with BSRPF granules showing a better effect than CF. Among BSRPF granules, the composite binder group had a higher soil organic matter content than the single binder group, with BCP-SM2 showing the best effect. This may be because the high proportion of CMC makes the BSRPF granules more stable, allowing for better control of nutrient release, thus improving soil fertility and promoting the growth of Chinese cabbage. Figure 10 (d) It can be seen that there are significant differences in available phosphorus content among different treatment groups. Compared with the blank control, the treatment groups played a significant role in increasing available phosphorus content. CF, BCP-S, BCP-CMC2, BCP-SM2, BCP-SM1.5, and BCP-SM1 increased by 361.8%, 301.9%, 243.0%, 117.3%, 156.2%, and 199.4%, respectively, indicating that the treatment groups increased the available phosphorus content in the soil. In BSRPF granules by composite binder groups, the available phosphorus content decreased with the increase of CMC-Na content, indicating that the synergistic effect of starch and CMC significantly enhanced the slow-release performance and reduced the loss of available phosphorus in the soil. However, BCP-S and BCP-CMC showed higher available phosphorus content in the soil, indicating that their structure is not stable enough and there is a risk of loss due to rapid phosphorus release, consistent with the slow-release results.

[0044] This study prepared BSRPF particles using CMC-Na, starch, and composites of both in different proportions as adhesives. The results showed that when the concentration of CMC was 2%, the BCP-SM2 particles had a smoother surface, the composite binder more tightly encapsulated the biochar, and exhibited the best mechanical properties and water resistance. Its wear resistance residual rate reached 99.85%, the maximum compressive strength was 67.01 N, and the water-resistant time was extended to over 7 days. The 28-day cumulative phosphorus release rate of BCP-SM2 was 54.07%, which was 24.4% and 41.3% lower than that of CMC and starch alone, respectively. The Elovich and Ritger-Peppas model can describe the phosphorus release process of BSRPF particles, including diffusion and chemical processes. Mechanistic studies indicated that starch and CMC cross-linked to form a better-performing binder, making the BSRPF particle structure denser and stronger. Some K+ in KH2PO4 exchanged with Na+ in CMC-Na to form new phosphates. Therefore, phosphorus release must overcome diffusion resistance and chemical reactions, thus affecting the phosphorus release rate. Future research should explore its long-term effects and adaptability to different soil types and crop varieties through field trials in various agricultural systems. From an application perspective, this research utilizes widely available and environmentally friendly raw materials, and its granulation technology is simple and cost-effective. The positive impacts on the soil environment, particularly in terms of phosphorus content and organic matter, further highlight the long-term benefits of this fertilizer in agriculture and sustainable development. Therefore, this research brings sustainable benefits to agricultural and environmental development.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder, characterized in that, The method improves the mechanical properties and slow-release performance of biochar slow-release phosphate fertilizer granules through the preparation of starch-CMC binder, and includes the following steps: Biochar and potassium dihydrogen phosphate powder are thoroughly mixed at a mass ratio of 2-5:0.5-1.5 to obtain a blended biochar-based phosphate fertilizer for later use. Mix 15-35g of starch with sodium carboxymethyl cellulose (1-2% by mass), add 200ml of deionized water, place the beaker in a water bath at 50-80℃, and gelatinize for 5-15 minutes to obtain the composite binder. The prepared composite binder is thoroughly mixed with the biochar-based phosphate fertilizer, and then placed in an extrusion granulator for extrusion granulation. After the successfully prepared biochar-based phosphate fertilizer granules are sieved through a 10-mesh sieve, they are placed in an oven at 70-90℃ and dried for 2 hours. They are then removed and placed at room temperature to obtain biochar slow-release phosphate fertilizer granules.

2. The method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder according to claim 1, characterized in that, The mechanical properties include abrasion resistance and compressive strength within a specified range for the biochar slow-release phosphate fertilizer granules. The wear resistance is measured in the following manner: Define the wear resistance calculation formula and write the wear resistance calculation formula into the calculation program; The set amount of biochar slow-release phosphate fertilizer granules were placed in a conical flask, and several steel balls were placed in the conical flask. The conical flask was sealed and fixed in a shaker and shaken at 250 r / min for 30 min. After shaking, the remaining biochar slow-release phosphate fertilizer granules were weighed, and the wear resistance was calculated according to the calculation program. The compressive strength is measured using a universal pressure tester to determine the maximum compressive strength of the biochar slow-release fertilizer granules; the die is lowered at a speed of 5 mm / min until the fertilizer breaks.

3. The method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder according to claim 2, characterized in that, The formula for calculating wear resistance is: Where R is the residual weight ratio, m1 is the fertilizer mass weighed before the abrasion test, and m2 is the fertilizer mass after the abrasion test.

4. The method for preparing biochar slow-release phosphate fertilizer granules using starch-CMC binder according to claim 1, characterized in that, The sustained-release performance includes water purification sustained-release testing and dynamic sustained-release testing; The water purification slow-release test includes: Weigh 0.1g of biochar-based slow-release phosphate fertilizer granules and place them on a 4cm x 4cm nylon cloth with a pore size of 400 mesh. Tie the nylon cloth with a thin thread and immerse it in a conical flask containing 50mL of deionized water. Leave a certain length of thin thread outside the flask to fix the position of the nylon bag in the water. Repeat each group three times. Take 5 mL of water sample every 24 hours. After each sampling, add the same volume of deionized water to the conical flask. Then, digest the water sample with potassium persulfate at 120℃ for 30 min to convert polyphosphates and organic phosphates into orthophosphates. After cooling, make up the volume and add ascorbic acid solution and molybdate solution. After reacting at room temperature for 15 min, use a UV-Vis spectrophotometer to determine the cumulative release rate of phosphorus released within a specific time interval. The dynamic sustained-release assay includes constructing a multivariate sustained-release model and performing a fitting analysis of the release behavior using the multivariate sustained-release model.

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