Cadmium pollution farmland yield-increasing slow-release fertilizer for regulating soil acidification and preparation method thereof

Abstract

This invention relates to the field of soil remediation fertilizer technology, and particularly to a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification, and its preparation method. The slow-release fertilizer for increasing yield in cadmium-contaminated farmland comprises a core layer and a coating layer covering the core layer. The core layer comprises the following raw materials by weight percentage: 30%–50% rice straw biochar, 35%–55% urea, 3%–8% binder, and 2%–8% cadmium passivating agent. The coating layer comprises the following raw materials by weight percentage: 5%–15% polyvinyl alcohol, 60%–80% rice straw biochar, 2%–5% polyethylene glycol, and 3%–10% bentonite. The slow-release fertilizer prepared by this invention can simultaneously address multiple problems such as soil acidification, Cd pollution, nutrient waste, and crop yield reduction. Through a "core-shell" structure, it achieves slow nitrogen release, acid-base gradient regulation, and cadmium stabilization simultaneously, providing technical support for the remediation and safe production of cadmium-contaminated acidic farmland.

Description

Technical Field

[0001] This invention relates to the field of soil remediation fertilizer technology, and in particular to a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification and its preparation method. Background Technology

[0002] Farmland soil, as the core carrier of agricultural production, directly affects food security, ecological sustainability, and human health. In recent years, the accelerated pace of intensive agricultural production has led to increasingly prominent problems such as excessive nitrogen fertilizer application and heavy metal pollution. This has resulted in aggravated synergistic harm from soil acidification and cadmium (Cd) pollution, becoming a key bottleneck restricting the health of farmland ecosystems and the safe production of crops. Especially in Southwest China, where acidic purple soil is widely distributed, the combined effects of mining activities and agricultural non-point source pollution exacerbate the complex problems of soil acidification and Cd pollution, necessitating the development of integrated technologies and products that combine soil acidification regulation, heavy metal passivation, and efficient nutrient utilization.

[0003] Long-term excessive application of nitrogen fertilizer is the core factor driving soil acidification: ammonium nitrogen fertilizer undergoes rapid nitrification after entering the soil, while urea undergoes both hydrolysis and nitrification, both of which continuously release H2O. + And H + Highly reactive and with low leaching rates, nitrogen addition leads to a significant decrease in soil pH over long periods. Global-scale studies show that nitrogen addition can reduce soil pH by an average of 0.26, exhibiting a linear downward trend. Soil acidification not only damages soil structure, causing nutrient loss and reduced availability, resulting in soil impoverishment, but also significantly affects soil microbial activity and plant growth and development, ultimately limiting crop yield and quality improvement.

[0004] Even more serious is the significant synergistic amplification effect between soil acidification and Cd pollution. During soil acidification, H+... + Increased concentration leads to an increase in the number of positive charges on the surface of soil particles, which is related to Cd. 2+ This generates electrostatic repulsion, reducing the soil's affinity for Cd. 2+ The increased adsorption capacity of Cd significantly increases the content of exchangeable Cd. Simultaneously, as pH decreases, organic and residual Cd in the soil transforms into exchangeable, carbonate-bound, and iron-manganese oxide-bound forms, greatly enhancing Cd mobility and bioavailability. Studies show that when soil pH decreases from 6.15 to 4.00, the content of water-soluble Cd in the soil and the Cd content in the upper parts of vegetables increase by 4 times and 8 times, respectively. The coupled effect of soil acidification and Cd pollution not only leads to reduced crop yields but also significantly increases the accumulation of Cd in the edible parts of crops, seriously threatening agricultural product safety.

[0005] Numerous studies on remediation technologies have been conducted both domestically and internationally to address the combined problems of soil acidification and Cd pollution. These studies primarily focus on soil amendment application, heavy metal passivation, and nutrient management optimization. However, existing technologies still have many shortcomings and cannot meet the actual needs of agricultural production. Regarding soil acidification control, traditional methods primarily involve the application of alkaline substances such as lime and wood ash. While these can rapidly increase soil pH, they suffer from drawbacks such as short-term effects, soil compaction, and the need for repeated applications, and they cannot simultaneously address Cd pollution. Organic materials, while improving soil structure and mitigating acidification, decompose rapidly, resulting in unstable acidification control effects and limited Cd passivation. For Cd pollution remediation, commonly used passivating agents include phosphates, clay minerals, and biochar. Among these, biochar, with its porous structure, high specific surface area, and abundant surface functional groups, possesses the potential to both adsorb heavy metals and improve soil properties, making it a research hotspot. However, the application of biochar alone requires separate operation, which is disconnected from the application of chemical fertilizers, increases the labor intensity and production cost in the field, and biochar itself lacks the function of nutrient supply, making it difficult to meet the needs of crop growth.

[0006] In terms of nutrient efficiency utilization technology, the research and application of slow-release fertilizers provide an effective way to solve the problem of low nitrogen fertilizer utilization and reduce environmental pollution. Traditional fertilizers have a nitrogen utilization rate of only 30%-50%, with a large amount of nitrogen lost through leaching and volatilization, resulting in resource waste and environmental problems such as soil acidification and eutrophication. Slow-release fertilizers, through coating and matrix adsorption, delay nutrient release, significantly improving nitrogen utilization and reducing nutrient loss. Currently, slow-release fertilizer products at home and abroad are mainly divided into three categories: polymer-coated, inorganic-coated, and matrix-adsorbed. Polymer-coated slow-release fertilizers offer precise nutrient release control, but the coating materials are mostly non-degradable, easily causing secondary pollution, and have high production costs, making large-scale promotion difficult. Inorganic-coated slow-release fertilizers are more environmentally friendly, but the nutrient release rate is difficult to control, and the slow-release effect is unstable. Matrix-adsorbed slow-release fertilizers mostly use zeolite, bentonite, etc., as carriers; although the cost is low, their function is limited, only achieving controlled nutrient release and failing to solve soil acidification and heavy metal pollution problems.

[0007] In recent years, the research and development of biochar-based slow-release fertilizers has become a hot topic in interdisciplinary research. These fertilizers combine the soil improvement and heavy metal adsorption functions of biochar with the nutrient control function of slow-release fertilizers, providing a new direction for synergistically solving agricultural production and environmental problems. Existing research shows that biochar-based slow-release fertilizers can improve acidic soil environments, promote crop growth, and reduce nutrient loss. However, existing products still have significant limitations: First, functional synergy is insufficient; most products only focus on nutrient control and soil improvement, paying insufficient attention to heavy metal passivation effects, and lacking specific formulations for Cd-contaminated farmland. Second, the preparation process is complex; some products use multi-step coating or composite modification technologies, resulting in high production costs and difficulties in large-scale production. Third, adaptability is weak; regional soil characteristics (such as the acidity and high adsorption of purple soil in Southwest China) are not fully considered, leading to inconsistent practical application effects. Fourth, precise matching of nutrient supply and crop yield increase has not been achieved; while some products can alleviate pollution, their effect on increasing crop yield is limited.

[0008] In summary, the combined harm of farmland soil acidification and Cd pollution is becoming increasingly serious. Existing remediation technologies and fertilizer products have shortcomings such as single function, poor synergy, high cost, and cumbersome operation, and cannot meet the integrated needs of "soil remediation - high nutrient efficiency - increased crop yield". Summary of the Invention

[0009] In view of this, the purpose of this invention is to provide a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification and its preparation method. The prepared slow-release fertilizer can simultaneously solve multiple problems such as soil acidification, Cd pollution, nutrient waste and crop yield reduction, and provide technical support for the remediation and safe production of cadmium-contaminated acidic farmland.

[0010] The following is a summary of this disclosure to provide a basic understanding of some aspects. This summary is not intended to identify key or important elements, nor is it intended to limit the implementation or any aspects of the claims. Furthermore, this summary provides a simplified overview of some aspects that can be described in more detail in other parts of this disclosure.

[0011] The present invention solves the above-mentioned technical problems through the following technical means:

[0012] In a first aspect, embodiments of the present invention provide a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification. The slow-release fertilizer includes a core layer and a coating layer covering the core layer. The core layer includes the following raw materials in weight percentages: 30%–50% rice straw biochar, 35%–55% urea, 3%–8% binder, and 2%–8% cadmium passivating agent. The coating layer includes the following raw materials in weight percentages: 5%–15% polyvinyl alcohol, 60%–80% rice straw biochar, 2%–5% polyethylene glycol, and 3%–10% bentonite.

[0013] In conjunction with the first aspect, in some embodiments, the slow-release fertilizer comprises a core layer of 85wt% to 95wt% and a coating layer of 5wt% to 15wt%.

[0014] In conjunction with the first aspect, in some embodiments, the binder is a compound of bentonite and sodium carboxymethyl cellulose, wherein the mass ratio of bentonite to sodium carboxymethyl cellulose is (2-3):1.

[0015] In conjunction with the first aspect, in some embodiments, the cadmium passivating agent is a mixture of hydroxyapatite and lime, wherein the mass ratio of hydroxyapatite to lime is 1:(0.5-2).

[0016] The slow-release fertilizer for increasing yield in cadmium-contaminated farmland using the above-mentioned technical solution utilizes rice straw biochar as a carrier and functional matrix. Its well-developed porous structure can adsorb urea molecules to achieve slow release, effectively reducing rapid dissolution of urea, ammonia volatilization, and leaching losses. The hydroxyl and carboxyl functional groups on the surface of the biochar can adsorb Cd. 2+ Meanwhile, biochar is weakly alkaline (pH=8.5-10.0), which can neutralize soil H+. + Biochar enhances soil buffering capacity; furthermore, it improves soil porosity, water retention, and aeration, providing a favorable environment for nitrogen mineralization and root absorption. Urea, as a nitrogen nutrient source, works synergistically with biochar to reduce nitrogen leaching loss and improve utilization. In cadmium passivation agents, lime adjusts local soil pH, promoting the formation of Cd²⁺ hydroxide and carbonate precipitates, thus reducing its effectiveness; hydroxyapatite is generated through ion exchange, surface complexation, and co-precipitation. The use of sparingly soluble minerals ensures long-term cadmium stability; the combination of hydroxyapatite and lime enhances Cd passivation stability, avoiding the short-lived effect of single passivating agents. Polyvinyl alcohol in the coating layer forms a dense membrane structure with biochar, regulating the nitrogen release rate, while biochar further enhances Cd adsorption and acidification regulation.

[0017] In conjunction with the first aspect, in some embodiments, the rice straw biochar used to prepare the core layer is magnesium-modified biochar, and the rice straw biochar used to prepare the coating layer is phosphorus-modified biochar.

[0018] The above technical solution uses magnesium-modified rice straw biochar for the core layer and phosphorus-modified rice straw biochar for the coating layer. On one hand, the phosphorus-modified coating layer biochar utilizes surface phosphate groups to bind free phosphorus in the soil. Rapid complexation and adsorption, combined with outer-layer interception, and phosphorus modification enriching the surface of biochar with phosphate groups, significantly enhance the adsorption capacity of biochar through a triple effect of strong electrostatic adsorption, coordination complexation, and cadmium phosphate mineral precipitation. The selective fixation ability of the biochar reduces its migration, transformation, and bioavailability; on the other hand, the magnesium-modified biochar in the core layer triggers cadmium chemical precipitation by increasing the rhizosphere pH, achieving deep solidification. Magnesium modification enriches the surface of the biochar with active hydroxyl groups (-OH) and oxygen vacancies, which can interact with... The formation of inner-layer complexes and the in-situ precipitation of stable precipitates achieve adsorption-precipitation coupling for cadmium fixation. Magnesium-modified biochar and phosphorus-modified biochar form a synergistic passivation system of "external adsorption and interception, internal precipitation and stabilization," simultaneously achieving multiple technical effects such as gradient regulation of soil acidification, optimization of nitrogen slow-release curves, improvement of nitrogen fertilizer utilization, and increased rice yield.

[0019] Secondly, embodiments of the present invention provide a method for preparing a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification, comprising the following steps:

[0020] Weigh out rice straw biochar, urea, binder and cadmium passivator, mix them evenly, add deionized water, stir and mix well to obtain a mixed slurry, feed the mixed slurry into a disc granulator and granulate for 15-20 min, dry the granules at 40-50 ℃ to obtain core granules.

[0021] Weigh out polyvinyl alcohol and dissolve it in deionized water. Add polyethylene glycol and stir until completely dissolved. Then add rice straw biochar and bentonite, stir and mix well to obtain the coating solution.

[0022] The core particles are preheated to 60-65 ℃ and fed into a disc granulator. The coating liquid is sprayed evenly and the granulators are continuously rolled until the coating layer thickness is 0.1-0.3 mm. Then, they are dried at 35-45 ℃ to obtain slow-release fertilizer granules.

[0023] In conjunction with the second aspect, in some embodiments, the moisture content of the mixed slurry is 15% to 20% to ensure particle strength, and the moisture content of the core particles is ≤5%.

[0024] In conjunction with the second aspect, in some embodiments, the solid-liquid ratio of the coating solution is 1 g:(3-5) mL, and the spraying rate of the coating solution is 5-10 mL·min. -1 .

[0025] In conjunction with the second aspect, in some embodiments, the preparation method of the magnesium-modified biochar is as follows:

[0026] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 400–500℃ and held for pyrolysis for 2–3 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.2–1.0 mol·L⁻¹. -1The solid was soaked in a magnesium salt solution for 6–12 h, filtered, and then calcined at 300–400 °C for 1–2 h and naturally cooled to obtain magnesium-modified biochar.

[0027] In conjunction with the second aspect, in some embodiments, the preparation method of the phosphorus-modified biochar is as follows:

[0028] Rice straw was crushed and then pretreated by soaking in a 5%–15% phosphoric acid or phosphate solution for 3–5 hours. The filtered solid was dried at 100–150 °C for 10–15 hours. The dried particles were then placed under a nitrogen atmosphere and dried at 5 °C / min. -1 The temperature was increased to 400–500 °C and held for 2–3 h for pyrolysis. After cooling, the mixture was pulverized and sieved to obtain phosphorus-modified biochar.

[0029] This invention relates to a slow-release fertilizer for cadmium-contaminated farmland that regulates soil acidification and increases yield. It combines the functions of regulating soil acidification, passivating heavy metal cadmium, improving fertilizer efficiency, and increasing yield. Rice straw biochar serves as the functional carrier and framework material. The core particles inside consist of nitrogen-containing nutrients and cadmium passivation components, while the outer layer is a coating layer composed of biodegradable polymers and inorganic minerals. Through the "core-shell" structure, it achieves slow release of nitrogen, regulation of acid-base gradient, and simultaneous cadmium stabilization. It is suitable for rice production in cadmium-contaminated acidic farmland, achieving an integrated effect of "reducing acidity, lowering cadmium levels, conserving fertilizer, and increasing yield". Attached Figure Description

[0030] Figure 1 This is a production flow diagram of the preparation method of the slow-release fertilizer for increasing yield in cadmium-polluted farmland that regulates soil acidification according to the present invention. Detailed Implementation

[0031] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0032] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0033] This invention relates to a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification. Using rice straw biochar as the core carrier, it integrates urea, binder, synergist, and coating material. Through optimized formulation and process, a multifunctional slow-release fertilizer product is prepared, which can simultaneously solve multiple problems such as soil acidification, Cd pollution, nutrient waste, and crop yield reduction, providing technical support for the remediation and safe production of cadmium-contaminated acidic farmland.

[0034] The present invention relates to a slow-release fertilizer for increasing yield in cadmium-contaminated farmland, which regulates soil acidification. The fertilizer comprises a core layer and a coating layer covering the core layer. The core layer comprises the following raw materials in weight percentages: 30%–50% rice straw biochar, 35%–55% urea, 3%–8% binder, and 2%–8% cadmium passivating agent. The coating layer comprises the following raw materials in weight percentages: 5%–15% polyvinyl alcohol, 60%–80% rice straw biochar, 2%–5% polyethylene glycol, and 3%–10% bentonite. The slow-release fertilizer comprises an 85 wt%–95 wt% core layer and a 5 wt%–15 wt% coating layer. The binder is a compound of bentonite and sodium carboxymethyl cellulose, with a mass ratio of bentonite to sodium carboxymethyl cellulose of (2–3):1. In other embodiments, the binder may also be at least one of carboxymethyl cellulose, sodium alginate, starch, xanthan gum, and chitosan. The cadmium passivating agent is a mixture of hydroxyapatite and lime, with a mass ratio of hydroxyapatite to lime of 1:(0.5-2).

[0035] The rice straw biochar used to prepare the core layer was magnesium-modified biochar, with MgO loaded on its surface. Magnesium-modified biochar, by loading active magnesium components onto its surface, increases soil micro-pH, triggers cadmium hydroxide / carbonate chemical precipitation, and is accompanied by ion exchange and co-precipitation, thereby achieving [the desired effect]. The coating layer is prepared using phosphorus-modified rice straw biochar, which exhibits high efficiency and stable curing. After phosphorus modification, a large number of grafts are formed on the surface of the biochar. , , Phosphate groups, for It possesses extremely strong selective complexing ability; the phosphate group dissociates in soil, exhibiting strong negative charge, and is attracted to positively charged... Strong electrostatic attraction is generated, enabling rapid capture. Phosphorus modification enriches the surface of biochar with phosphate groups, significantly enhancing its ability to capture biochar through a triple process of strong electrostatic adsorption, coordination complexation, and cadmium phosphate mineral precipitation. Its selective fixation ability reduces its migration, transformation, and bioavailability.

[0036] like Figure 1 As shown, the preparation method of the slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to the present invention includes the following steps:

[0037] (1) Preparation of kernel particles

[0038] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 400–500℃ and held for pyrolysis for 2–3 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.2–1.0 mol·L⁻¹. -1 The sample was impregnated in a magnesium salt solution for 6–12 h, filtered, and then calcined at 300–400 °C for 1–2 h, followed by natural cooling to obtain magnesium-modified biochar. Magnesium-modified biochar, urea, binder, and cadmium passivator were weighed and mixed, and deionized water was added. The mixture was stirred to obtain a slurry with a moisture content of 15%–20%. The slurry was fed into a disc granulator and granulated for 15–20 min. The granules were dried at 40–50 °C until the moisture content was ≤5% to obtain core particles with a roundness ≥90%.

[0039] (2) Preparation of coating solution

[0040] Rice straw was crushed and then pretreated by soaking in a 5%–15% phosphoric acid or phosphate solution for 3–5 hours. The filtered solid was dried at 100–150 °C for 10–15 hours. The dried particles were then placed under a nitrogen atmosphere and dried at 5 °C / min. -1 The temperature was raised to 400–500℃ and pyrolyzed for 2–3 h. After cooling, the mixture was pulverized and sieved to obtain phosphorus-modified biochar. Polyvinyl alcohol with a degree of polymerization of 1700–2000 was weighed and dissolved in deionized water at 60–70℃. Polyethylene glycol was added and stirred until completely dissolved. Then, phosphorus-modified biochar and bentonite were added and stirred until well mixed to obtain a coating solution with a solid-liquid ratio of 1 g: (3–5) mL.

[0041] (3) Coating drying

[0042] The kernel particles are preheated to 60–65 °C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r / min. -1 According to the spray rate of 5-10 mL·min -1 The coating liquid is sprayed evenly at a certain rate and continuously rolled until the coating layer thickness is 0.1-0.3 mm. Then it is dried at 35-45℃ to obtain slow-release fertilizer granules.

[0043] In the specific implementation:

[0044] Example 1

[0045] The slow-release fertilizer for increasing yield in cadmium-contaminated farmland, which regulates soil acidification in this embodiment, comprises a 95wt% core layer and a 5wt% coating layer. Its preparation method is as follows:

[0046] (1) Preparation of kernel particles

[0047] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 450 °C and pyrolyzed for 2.5 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.5 mol·L⁻¹. -1 The sample was impregnated in a magnesium chloride solution for 10 h, filtered, and then calcined at 350 ℃ for 1.5 h. After natural cooling, magnesium-modified biochar was obtained. 3 kg of magnesium-modified biochar, 6 kg of urea, 0.5 kg of binder (in this example, the binder is a mixture of bentonite and sodium carboxymethyl cellulose in a mass ratio of 2:1) and 0.5 kg of cadmium passivating agent (in this example, the cadmium passivating agent is a mixture of hydroxyapatite and lime in a mass ratio of 1:1) were weighed and mixed. Deionized water was added, and the mixture was stirred until homogeneous, resulting in a slurry with a moisture content of 18%. The slurry was fed into a disc granulator and granulated for 18 min to produce particles of 2-4 mm. The granules were dried at 45 ℃ until the moisture content was 4%, yielding core particles with a roundness ≥90%.

[0048] (2) Preparation of coating solution

[0049] Rice straw was crushed and then pretreated by soaking in a 10% phosphoric acid solution for 4 h. The filtered solid was dried at 120 ℃ for 13 h. The dried particles were then placed under a nitrogen atmosphere and dried at 5 ℃·min. -1 The temperature was raised to 450 °C and pyrolyzed for 2.5 h. After cooling, the biochar was pulverized and sieved to obtain phosphorus-modified biochar. 0.3 kg of polyvinyl alcohol with a degree of polymerization of 1800 was weighed and dissolved in deionized water at 60 °C. 0.3 kg of polyethylene glycol was added and stirred until completely dissolved to obtain a 3% (w / w) polyvinyl alcohol solution. Then, 2.2 kg of phosphorus-modified biochar and 0.1 kg of bentonite were added and stirred to obtain a coating solution with a solid-liquid ratio of 1 g: 4 mL.

[0050] (3) Coating drying

[0051] The kernel particles are preheated to 62 °C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r·min. -1 According to the injection rate of 8 mL·min -1 The coating solution was sprayed evenly at a constant rate and continuously rolled for 20 minutes until the coating layer thickness was 0.2 mm. Then, it was dried at 40 ℃ for 24 h to obtain slow-release fertilizer granules.

[0052] Example 2

[0053] The slow-release fertilizer for increasing yield in cadmium-contaminated farmland, which regulates soil acidification in this embodiment, comprises a 90 wt% core layer and a 10 wt% coating layer. Its preparation method is as follows:

[0054] (1) Preparation of kernel particles

[0055] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 450 °C and pyrolyzed for 2.5 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.5 mol·L⁻¹. -1 The solid was impregnated in a magnesium chloride solution for 10 h, filtered, and then calcined at 350 °C for 1.5 h, followed by natural cooling to obtain magnesium-modified biochar. 4 kg of magnesium-modified biochar, 5 kg of urea, 0.5 kg of binder (in this example, the binder is a mixture of bentonite and sodium carboxymethyl cellulose in a mass ratio of 2:1) and 0.5 kg of cadmium passivating agent (in this example, the cadmium passivating agent is a mixture of hydroxyapatite and lime in a mass ratio of 1:1) were weighed and mixed. Deionized water was added, and the mixture was stirred until homogeneous, resulting in a slurry with a moisture content of 18%. The slurry was fed into a disc granulator and granulated for 18 min to produce particles of 2-4 mm. The granules were dried at 45 °C until the moisture content was 4%, yielding core particles with a roundness ≥90%.

[0056] (2) Preparation of coating solution

[0057] Rice straw was crushed and then pretreated by soaking in a 10% phosphoric acid solution for 3 h. The filtered solid was dried at 120 ℃ for 13 h. The dried particles were then placed under a nitrogen atmosphere and dried at 5 ℃·min. -1 The temperature was raised to 450 °C and pyrolyzed for 2.5 h. After cooling, the biochar was pulverized and sieved to obtain phosphorus-modified biochar. 0.3 kg of polyvinyl alcohol with a degree of polymerization of 1800 was weighed and dissolved in deionized water at 60 °C. 0.3 kg of polyethylene glycol was added and stirred until completely dissolved to obtain a 3% (w / w) polyvinyl alcohol solution. Then, 2.2 kg of phosphorus-modified biochar and 0.1 kg of bentonite were added and stirred to obtain a coating solution with a solid-liquid ratio of 1 g: 4 mL.

[0058] (3) Coating drying

[0059] The kernel particles are preheated to 62 °C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r·min. -1 According to the injection rate of 8 mL·min -1The coating solution was sprayed evenly at a constant rate and continuously rolled for 20 minutes until the coating layer thickness was 0.2 mm. Then, it was dried at 40 ℃ for 24 h to obtain slow-release fertilizer granules with a nitrogen content of 18.5% and a pH of 8.8.

[0060] Example 3

[0061] The slow-release fertilizer for increasing yield in cadmium-contaminated farmland, which regulates soil acidification in this embodiment, comprises a 90wt% core layer and a 10wt% coating layer. Its preparation method is as follows:

[0062] (1) Preparation of kernel particles

[0063] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 450℃ and pyrolyzed for 2.5 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.5 mol·L⁻¹. -1 The solid was impregnated in a magnesium chloride solution for 10 h, filtered, and then calcined at 350 °C for 1.5 h, followed by natural cooling to obtain magnesium-modified biochar. 5 kg of magnesium-modified biochar, 3.5 kg of urea, 0.8 kg of binder (in this example, the binder is a mixture of bentonite and sodium carboxymethyl cellulose in a mass ratio of 2:1) and 0.7 kg of cadmium passivating agent (in this example, the cadmium passivating agent is a mixture of hydroxyapatite and lime in a mass ratio of 1:1) were weighed and mixed. Deionized water was added, and the mixture was stirred until a slurry with a moisture content of 18% was obtained. The slurry was fed into a disc granulator and granulated for 18 min to produce particles of 2-4 mm. The granules were dried at 45 °C until the moisture content was 4% to obtain core particles with a roundness ≥90%.

[0064] (2) Preparation of coating solution

[0065] Rice straw was crushed and then pretreated by soaking in a 10% phosphoric acid solution for 4 h. The filtered solid was dried at 120 ℃ for 13 h. The dried particles were then placed under a nitrogen atmosphere and dried at 5 ℃·min. -1 The temperature was raised to 450 °C and pyrolyzed for 2.5 h. After cooling, the biochar was pulverized and sieved to obtain phosphorus-modified biochar. 0.3 kg of polyvinyl alcohol with a degree of polymerization of 1800 was weighed and dissolved in deionized water at 60 °C. 0.3 kg of polyethylene glycol was added and stirred until completely dissolved to obtain a 3% (w / w) polyvinyl alcohol solution. Then, 2.2 kg of phosphorus-modified biochar and 0.1 kg of bentonite were added and stirred to obtain a coating solution with a solid-liquid ratio of 1 g: 4 mL.

[0066] (3) Coating drying

[0067] The kernel particles are preheated to 62°C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r·min. -1 According to the injection rate of 8 mL·min -1 The coating solution was sprayed evenly at a constant rate and continuously rolled for 20 minutes until the coating layer thickness was 0.2 mm. Then, it was dried at 40 ℃ for 24 h to obtain slow-release fertilizer granules.

[0068] Example 4

[0069] (1) Preparation of kernel particles

[0070] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 400 °C and pyrolyzed for 2 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then treated with 0.2 mol·L⁻¹ phosphate dehydrogenase. -1 The sample was impregnated in a magnesium sulfate solution for 6 hours, filtered, and then calcined at 300 °C for 1 hour. After natural cooling, magnesium-modified biochar was obtained. 3.4 kg of magnesium-modified biochar, 5.5 kg of urea, 0.3 kg of binder (in this example, the binder is a mixture of bentonite and sodium carboxymethyl cellulose at a mass ratio of 2.5:1) and 0.8 kg of cadmium passivating agent (in this example, the cadmium passivating agent is a mixture of hydroxyapatite and lime at a mass ratio of 1:0.5) were weighed and mixed. Deionized water was added, and the mixture was stirred until a slurry with a moisture content of 15% was obtained. The slurry was fed into a disc granulator and granulated for 15 minutes to produce particles of 2-4 mm. The granules were dried at 40 °C until the moisture content was 5% to obtain core particles with a roundness ≥90%.

[0071] (2) Preparation of coating solution

[0072] Rice straw was crushed and then pretreated by soaking in a 5% phosphoric acid solution for 3 h. The filtered solid was dried at 100 ℃ for 10 h. The dried particles were then placed under a nitrogen atmosphere and dried at 5 ℃·min. -1 The temperature was raised to 400 °C and pyrolyzed for 2 h. After cooling, the mixture was pulverized and sieved to obtain phosphorus-modified biochar. 0.15 kg of polyvinyl alcohol with a degree of polymerization of 1700 was weighed and dissolved in deionized water at 65 °C. 0.15 kg of polyethylene glycol was added and stirred until completely dissolved to obtain a 3% (w / w) polyvinyl alcohol solution. Then, 2.4 kg of phosphorus-modified biochar and 0.3 kg of bentonite were added and stirred until well mixed to obtain a coating solution with a solid-liquid ratio of 1 g: 3 mL.

[0073] (3) Coating drying

[0074] The kernel particles are preheated to 60°C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r·min. -1 According to the injection rate of 5 mL·min -1 The coating solution was sprayed evenly at a constant rate and continuously rolled for 20 minutes until the coating layer thickness was 0.1 mm. Then, it was dried at 35 ℃ for 24 h to obtain slow-release fertilizer granules.

[0075] Example 5

[0076] (1) Preparation of kernel particles

[0077] The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was raised to 500 °C and pyrolyzed for 3 h. After cooling, the material was pulverized and sieved. The resulting particles were soaked in a 1.0 mol / L magnesium nitrate solution for 12 h. The filtered solid was then calcined at 400 °C for 2 h and allowed to cool naturally to obtain magnesium-modified biochar. 4.0 kg of magnesium-modified biochar, 5.0 kg of urea, 0.8 kg of binder (in this example, the binder is a mixture of bentonite and sodium carboxymethyl cellulose in a mass ratio of 3:1) and 0.2 kg of cadmium passivator (in this example, the cadmium passivator is a mixture of hydroxyapatite and lime in a mass ratio of 1:0.5) were weighed and mixed. Deionized water was added, and the mixture was stirred until a slurry with a moisture content of 20% was obtained. The slurry was fed into a disc granulator and granulated for 20 min to produce particles of 2-4 mm. The granules were dried at 50 °C to a moisture content of 3% to obtain core particles with a roundness ≥90%.

[0078] (2) Preparation of coating solution

[0079] Rice straw was crushed and then pretreated by soaking in a 15% phosphoric acid solution for 5 h. The filtered solid was dried at 150 ℃ for 15 h. The dried particles were then placed under a nitrogen atmosphere and dried at 5 ℃·min. -1 The temperature was raised to 500 °C and pyrolyzed for 3 h. After cooling, the biochar was pulverized and sieved to obtain phosphorus-modified biochar. 0.45 kg of polyvinyl alcohol with a degree of polymerization of 2000 was weighed and dissolved in deionized water at 70 °C. 0.06 kg of polyethylene glycol was added and stirred until completely dissolved to obtain a 3% (w / w) polyvinyl alcohol solution. Then, 2.39 kg of phosphorus-modified biochar and 0.1 kg of bentonite were added and stirred to obtain a coating solution with a solid-liquid ratio of 1 g: 5 mL.

[0080] (3) Coating drying

[0081] The kernel particles are preheated to 65°C and fed into a disc granulator. The disc tilt angle is adjusted to 40° and the rotation speed to 40 r·min.-1 Spray the coating solution uniformly at a rate of 10 mL·min -1 until the thickness of the coating layer reaches 0.3 mm after continuous rolling for 20 min, and then dry it at 45 °C for 24 h to obtain the slow-release fertilizer granules.

[0082] Test and analysis:

[0083] Collect purple soil contaminated with cadmium in a certain place as the test soil, which is divided into 2 pH gradients: pH < 4.5 (S1), 4.5 < pH < 5.5 (S2). The basic physical and chemical properties are shown in Table 1, and the main rice variety "Yixiangyou 2115" in the southwestern region is used as the test crop.

[0084] Table 1 Physical and chemical properties of the test soil

[0085]

[0086] For the test soil, set 5 treatment groups with 3 replicates in each group, specifically as follows:

[0087] CK: Conventional fertilization (urea 150 kg·hm -1 , 80% as base fertilizer + 20% as top dressing);

[0088] T1: Slow-release fertilizer of Example 1 (carbon-fertilizer ratio 1:2, coating ratio 5%);

[0089] T2: Slow-release fertilizer of Example 2 (carbon-fertilizer ratio 1:1.25, coating ratio 10%);

[0090] T3: Slow-release fertilizer of Example 3 (carbon-fertilizer ratio 1:0.7, coating ratio 10%);

[0091] T4: Biochar + urea mixed fertilizer (without coating, without passivator).

[0092] The carbon-fertilizer ratio in T1-T3 above refers to the mass ratio of magnesium-modified biochar and urea.

[0093] For the pot experiment, each pot is filled with 5 kg of soil, and fertilizers are independently applied according to the 5 treatment groups respectively. Transplant 2 rice seedlings, and keep the water level 3 cm above the soil surface. At the mature stage, measure the soil physical and chemical properties, Cd forms, rice yield and Cd accumulation; for the soil column leaching experiment, use a chromatography column with an inner diameter of 3 cm and a height of 30 cm, simulate 5 rainfall cycles of 3 d, 7 d, 14 d, 21 d, and 30 d, and take samples to measure the pH, total nitrogen and Cd content of the leachate.

[0094] The nitrogen cumulative release rates of different treatment groups are shown in Table 2:

[0095] Table 2 Nitrogen cumulative release rates of different treatment groups (%)

[0096]

[0097] The data in Table 2 show that the nitrogen release rate of the slow-release fertilizers (T1-T3) prepared in Examples 1-3 is significantly lower than that of CK and T4, with a cumulative release rate of 73.4%-82.5% over 30 days, which meets the industry standard for slow-release fertilizers (cumulative release rate ≤80% over 28 days). Among them, the release curves of T2 and T3 treatments are flatter, which reflects the synergistic slow-release effect of the coating layer and biochar.

[0098] Table 3 shows the changes in soil pH and acidification characteristics of different treatment groups after 60 days of pot experiment cultivation.

[0099] Table 3. Changes in soil pH and acidification characteristics in different treatment groups

[0100]

[0101] The results in Table 3 show that the slow-release fertilizers (T1-T3) prepared in Examples 1-3 can significantly increase soil pH, reduce exchangeable acid content, and increase total base content and base saturation. Among them, the T2 treatment showed the best effect, with soil pH increasing by 0.72 units, exchangeable acid decreasing by 48.6%, and base saturation increasing by 18.9%, which was significantly better than the CK and T4 treatments. This indicates that the synergistic effect of cadmium passivator and biochar effectively regulates the soil acidification process.

[0102] When the cultivated rice reached maturity, the distribution of Cd speciation and available Cd content in the soil of different treatment groups were measured, and the results are shown in Table 4.

[0103] Table 4. Distribution of Cd speciation and available Cd content in soil of the same treatment group

[0104]

[0105] The data in Table 4 show that the slow-release fertilizers prepared in Examples 1-3 can significantly reduce the available Cd content in the soil and promote the conversion of exchangeable Cd to carbonate-bound and iron-manganese oxide-bound forms. The T2 treatment reduced the available Cd in S1 soil by 61.5%, decreased the proportion of exchangeable Cd by 21.8 percentage points, and increased the proportion of iron-manganese oxide-bound Cd by 11.5 percentage points. The passivation effect was significantly better than the control. This is attributed to the synergistic effect of biochar adsorption, the precipitation reaction of the passivating agent, and the increase in soil pH.

[0106] When the cultivated rice reached maturity, the yield and Cd content of various organs of rice in different treatment groups were measured, as shown in Table 5.

[0107] Table 5. Rice yield and Cd content in various organs of different treatment groups

[0108]

[0109] The data in Table 5 show that the slow-release fertilizers prepared in Examples 1-3 can significantly increase rice yield. The yield increase rate of the T2 treatment in soils S1 and S2 reached 24.0% and 25.4%, respectively. At the same time, it significantly reduced the Cd accumulation in various organs of rice. The Cd content of rice grains in soil S1 under the T2 treatment decreased to 0.11 mg·kg⁻¹. -1 The compliance rate was 100%, and the Cd content in roots and stems and leaves decreased by 46.5% and 51.7% respectively, achieving the goal of safe production and increased yield in cadmium-polluted acidified farmland.

[0110] In addition, using the slow-release fertilizer prepared in Example 2 as a sample, it was tested in a cadmium-contaminated acidified farmland (soil type S2, pH 5.0, total Cd 0.62 mg·kg⁻¹). -1 A field plot experiment was conducted, with each plot measuring 20 m² (5 m × 4 m), with three replicates and a randomized block design. The treatment groups were CK (conventional fertilization) and T2 (the slow-release fertilizer prepared in Example 2), with fertilization rates of N 150 kg·hm². -1 P2O5 75kg·hm -1 K2O 100 kg·hm -1 The T2 treatment was a single basal application, while the CK treatment received 80% basal fertilizer plus 20% topdressing. Test results are shown in Table 6.

[0111] Table 6 Field Trial Results

[0112]

[0113] The data in Table 6 show that the results of the field trial are consistent with those of the pot experiment. The slow-release fertilizer prepared in Example 2 increased the soil pH by 0.66 units, reduced the available Cd by 78.6%, increased rice yield by 25.3%, and reduced the grain Cd content to 0.08 mg·kg⁻¹. -1 It meets food safety standards and verifies its stability and effectiveness under field conditions.

[0114] 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification, characterized in that... The slow-release fertilizer comprises a core layer and a coating layer covering the core layer. The core layer comprises the following raw materials in weight percentage: 30%–50% rice straw biochar, 35%–55% urea, 3%–8% binder, and 2%–8% cadmium passivator. The coating layer comprises the following raw materials in weight percentage: 5%–15% polyvinyl alcohol, 60%–80% rice straw biochar, 2%–5% polyethylene glycol, and 3%–10% bentonite.

2. The slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to claim 1, characterized in that, The slow-release fertilizer comprises a core layer of 85 wt% to 95 wt% and a coating layer of 5 wt% to 15 wt%.

3. The slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to claim 1, characterized in that, The binder is a compound of bentonite and sodium carboxymethyl cellulose, and the mass ratio of bentonite to sodium carboxymethyl cellulose is (2-3):

1.

4. The slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to claim 1, characterized in that, The cadmium passivating agent is a compound of hydroxyapatite and lime, and the mass ratio of hydroxyapatite to lime is 1:(0.5-2).

5. The slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to claim 1, characterized in that, The rice straw biochar used to prepare the core layer is magnesium-modified biochar, and the rice straw biochar used to prepare the coating layer is phosphorus-modified biochar.

6. The method for preparing a slow-release fertilizer for increasing yield in cadmium-contaminated farmland that regulates soil acidification according to any one of claims 1-5, characterized in that, Includes the following steps: Weigh out rice straw biochar, urea, binder and cadmium passivator, mix them evenly, add deionized water, stir and mix well to obtain a mixed slurry, feed the mixed slurry into a disc granulator and granulate for 15-20 min, dry the granules at 40-50 ℃ to obtain core granules. Weigh out polyvinyl alcohol and dissolve it in deionized water. Add polyethylene glycol and stir until completely dissolved. Then add rice straw biochar and bentonite, stir and mix well to obtain the coating solution. The core particles are preheated to 60-65°C and fed into a disc granulator. The coating liquid is sprayed evenly and the granulators are continuously rolled until the coating layer thickness is 0.1-0.3 mm. Then, they are dried at 35-45°C to obtain slow-release fertilizer granules.

7. The preparation method according to claim 6, characterized in that, The moisture content of the mixed slurry is 15% to 20%, and the moisture content of the core particles is ≤5%.

8. The preparation method according to claim 6, characterized in that, The solid-liquid ratio of the coating solution is 1 g:(3-5) mL, and the spraying rate of the coating solution is 5-10 mL·min. -1 .

9. The preparation method according to claim 6, characterized in that, The preparation method of the magnesium-modified biochar is as follows: The dried rice straw pellets were placed under a nitrogen atmosphere at 5 °C·min. -1 The temperature was increased to 400–500 °C and pyrolyzed for 2–3 h. After cooling, the mixture was pulverized and sieved. The resulting particles were then added to a solution containing 0.2–1.0 mol·L⁻¹. -1 The solid was soaked in a magnesium salt solution for 6–12 h, filtered, and then calcined at 300–400 °C for 1–2 h and naturally cooled to obtain magnesium-modified biochar.

10. The preparation method according to claim 6, characterized in that, The preparation method of the phosphorus-modified biochar is as follows: Rice straw was crushed and then pretreated by soaking in a 5%–15% phosphoric acid or phosphate solution for 3–5 hours. The filtered solid was dried at 100–150 °C for 10–15 hours. The dried particles were then placed under a nitrogen atmosphere and dried at 5 °C / min. -1 The temperature was increased to 400–500 °C and held for 2–3 h for pyrolysis. After cooling, the mixture was pulverized and sieved to obtain phosphorus-modified biochar.

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