Modified charcoal-based slow-release rare earth lanthanum soil conditioner and preparation method thereof

Abstract

The invention provides a modified charcoal-based slow-release rare earth lanthanum soil conditioner and a preparation method thereof, and relates to the technical field of soil conditioners, the saline-alkali soil conditioner is used for reducing soil salinity and alkalinity, fixing heavy metals in soil, improving soil structure and slowly releasing rare earth elements into soil; the preparation method comprises the following steps: 1, preparing a rare earth lanthanum compound; modifying the biochar; a phosphate; dewatering the river sludge; an organic complexing agent; step 2, dissolving a rare earth lanthanum compound and an organic complexing agent in deionized water, and reacting to generate a complex solution; step 3, preparing modified biological carbon; step 4, loading lanthanum; and step 5, mixing the matrix to obtain the saline-alkali soil conditioner. Lanthanum nitrate and sodium citrate are converted into a complex solution, the lanthanum citrate complex has good stability, lanthanum can be effectively fixed in the solution in the form of the complex, the lanthanum is prevented from precipitating or losing in the subsequent treatment process, and the slow release effect of lanthanum is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of soil conditioners, in particular to a modified biochar-based slow-release rare earth lanthanum soil conditioner and a preparation method thereof. Background Art

[0002] Rare earth elements, as a strategic resource, are highly valued worldwide. In today's developing society, rare earth resources have penetrated into all walks of life. In agriculture, rare earth elements are not fertilizers and cannot serve as a macronutrient for plants. However, trace amounts can act as metal activators in plant physiology and biochemistry, increasing crop yields. They regulate physiological activities within plants, promote phosphorus absorption, and thereby increase chloroplast content. This further boosts enzyme activity, particularly phosphatase activity. Low-concentration rare earth micronutrients promote plant growth and increase crop yields, while high concentrations inhibit plant growth and development. Furthermore, rare earth lanthanum also has a positive effect on saline-alkali soils and soils.

[0003] The physical and chemical properties of river silt are similar to those of soil. It contains a large number of nutrients required for plant growth and has the ability to improve soil physical and chemical properties such as porosity, bulk density, field water holding capacity, aggregation stability, shrinkage, and saturated hydraulic conductivity. Furthermore, silt contains a large amount of organic matter, microorganisms, inorganic particles, colloids, and other substances, which can loosen the soil structure and thus improve the arable properties of the land. Silt also contains a certain amount of sticky substances. When applied, it can increase the viscosity of the soil, form a stable soil structure, and effectively improve the physical properties of saline-alkali soils. If silt can be used rationally, it can not only effectively improve the chemical properties of saline-alkali soils, but also produce a conditioner product suitable for salinized soils, improve soil fertility and crop yields, but also reduce the use of commercial fertilizers, thereby reducing economic costs.

[0004] Due to its unfavorable conditions such as high salt content, high compaction and low available nutrients, saline-alkali soil is very likely to cause crops to suffer adverse effects such as physiological drought, which also greatly limits the increase in crop yields. According to statistics, the global area of ​​soil threatened by salinization is about 1×109hm2. 2 , accounting for about 7% of the Earth's land surface, affecting 10% of the world's arable land; of which the area of ​​secondary salinized soil is 4.5×107-8.0×107hm 2 , accounting for 20%-30% of all irrigated land, and about half of it is located in China, the United States, India and Pakistan. my country is one of the countries most seriously affected by soil salinization in the world, with saline-alkali land ranking third in the world, with about 36.667 million hectares. 2The available saline-alkali land resources are important reserve arable land resources, and the development and utilization of saline-alkali soil is of great significance to ensuring national and regional food security.

[0005] Saline-alkali soil conditioners can be divided into organic and inorganic types. Commonly used chemical soil conditioners include gypsum, desulfurized gypsum, zeolite, sulfur, humic acid, furfural residue, and earthworm castings. These conditioners improve the physical and chemical properties of saline-alkali soils through chemical reactions such as ion replacement and acid-base neutralization, as well as by altering the soil's pore structure. However, some organic conditioners (such as manure) pose a risk of groundwater contamination during application, and their use should be careful to avoid secondary environmental pollution. Among inorganic conditioners, excessive use of calcium-based conditioners also carries the risk of increased salt and heavy metal accumulation. Summary of the Invention

[0006] The present invention is based on the use of lanthanum nitrate to improve the resistance of plants to drought and salinity. At the same time, the silt has a large amount of organic matter and nutrient elements, such as nitrogen, phosphorus, and potassium, and is compounded with rare earth lanthanum to be used as an amendment. The application of silt can also reduce the salinity and pH value of the soil, promote the balance of soil acidity and alkalinity, and improve the fertility of the soil. The silt is turned into treasure and resource recycling is achieved. It provides ideas for improving saline-alkali land soil improvement research and has far-reaching practical significance. The details are as follows: A method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner comprises the following steps: Step 1, raw material preparation: 5-10 parts of rare earth lanthanum compound; 15-25 parts of modified biochar; 1-2 parts of phosphate; 50-70 parts of dehydrated river sludge; 10-15 parts of organic complexing agent; Step 2, raw material pretreatment: dissolving the rare earth lanthanum compound and the organic complexing agent in deionized water, stirring and reacting in a 50-70° C. water bath to generate a complex solution; Step 3: Preparation of modified biochar: (1) Treat the biochar with 10% nitric acid at 45-55°C for 5-7 hours; treat with 30% nitric acid at 55-65°C for 7-9 hours; and wash with deionized water to a pH of 6-7. (2) After drying, add 10% ethylenediamine solution to the biochar at a solid-liquid ratio of 1:10-20, and reflux at 75-85°C for 10-15 hours to graft amino groups on the surface of the biochar through nucleophilic substitution reaction; Step 4, lanthanum loading: immersing the modified biochar in the complex solution, ultrasonically treating it, and drying it at 50-70°C for 4-8 hours to obtain lanthanum-loaded biochar; Step 5: Matrix mixing: evenly mixing dehydrated river silt, phosphate, and lanthanum-loaded biochar to obtain a saline-alkali soil conditioner; The saline-alkali soil conditioner is used for reducing soil salinity, fixing heavy metals in the soil, improving soil structure, and slowly releasing rare earth elements into the soil.

[0007] Moreover, the particle size of the biochar is ≤2 mm, the organic complexing agent is sodium citrate, the phosphate is potassium dihydrogen phosphate, and the rare earth lanthanum compound is lanthanum nitrate.

[0008] Moreover, the preparation method of the dewatered river sludge is as follows: S1. Sludge pretreatment: After removing impurities from the river silt, add 0.5-1% of the mass of the river silt into polyaluminium chloride, stir and mix evenly, and then let it settle for 24-48 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3. Deep dehydration: Dry the crushed sludge to a moisture content of ≤15%; S4. Heavy metal stabilization treatment: Add 1-2% of potassium dihydrogen phosphate by weight to the dried sludge, mix well and age for 48-72 hours; S5. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

[0009] Moreover, in step S2, a plate and frame filter press is used for filtration and dehydration, the pressure is controlled at 0.6-0.8 MPa, and the water content is dehydrated to 40-45%.

[0010] Then, in step S4, the stabilized sludge is crushed by a crusher until it passes through an 80-mesh sieve.

[0011] Furthermore, in the step 2, the reaction is stirred in a water bath at 50-70° C. for 25-35 minutes to generate a complex solution.

[0012] Moreover, in the step 4, the frequency of the ultrasonic treatment is 30-50 kHz, and the time is 25-35 minutes.

[0013] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention provides a modified biochar-based slow-release rare earth lanthanum soil conditioner and a preparation method thereof. Lanthanum nitrate and sodium citrate are converted into a complex solution. The lanthanum citrate complex has good stability and can effectively fix the lanthanum element in the solution in the form of a complex, avoiding precipitation or loss of the lanthanum element during subsequent treatment, achieving a slow-release effect of lanthanum, and also providing a good foundation for the subsequent loading of lanthanum elements on modified biochar; then, the modified biochar is immersed in the complex solution and ultrasonically treated to increase the porosity and specific surface area of ​​the modified biochar, making the lanthanum citrate complex more stable. It is easy to enter the pores of the modified biochar and adsorb to the active sites on the surface of the modified biochar, thereby increasing the loading amount and loading uniformity of the lanthanum element on the biochar; finally, the loading is dried and fixed to further strengthen the interaction between the lanthanum citrate complex and the modified biochar, and the lanthanum element is firmly loaded on the modified biochar to form a stable lanthanum-loaded biochar, which further realizes the slow release of the lanthanum element and can continuously and stably release the lanthanum element in the soil, prolong the action time of the lanthanum element, and improve its utilization rate, while also avoiding the adverse effects of excessive one-time release on the soil and plants.

[0014] 2. The present invention provides a modified biochar-based slow-release rare earth lanthanum soil conditioner and a preparation method thereof. The modified biochar is oxidized by nitric acid to introduce oxygen-containing functional groups such as carboxyl (-COOH) and phenolic hydroxyl (-OH), significantly increasing the negative charge density on the biochar surface. The increase in negative charge density can enhance the electrostatic adsorption capacity of biochar for rare earth lanthanum ions, reduce the migration loss of lanthanum in the soil, and extend the slow-release period. Ethylenediamine is then added to form a stable -CONH-CH2-CH2-NH2 structure on the biochar surface through a nucleophilic substitution reaction, introducing amino positively charged groups. The amino groups preferentially adsorb anions in the soil, indirectly promoting the desorption of lanthanum ions from the biochar to the soil solution, achieving a "storage-slow-release" dynamic balance, and further achieving a slow-release effect of lanthanum ions.

[0015] 3. The present invention utilizes river silt as one of the main raw materials, and does not require fermentation. River silt is widely available and has a relatively low acquisition cost. It is rationally applied to saline-alkali soil conditioners, realizing the resource utilization of waste. It not only reduces the production cost of the conditioner, but also reduces the pollution of the environment by river silt, and has significant economic and environmental benefits.

[0016] 4. In the present invention, rare earth lanthanum, as a metal activator in plant physiology and biochemistry, can reasonably regulate the physiological activities of plants, promote the absorption of phosphorus elements by plants, and enhance the resistance of plants to saline-alkali stress. River silt contains a large amount of nutrients and organic matter required for plant growth, which can improve the physical and chemical properties of soil and increase soil fertility. The combination of the two achieves complementary advantages and synergistically plays the role of improving saline-alkali soil.

[0017] 5. The present invention treats river sludge, effectively removes impurities and harmful substances in the sludge, improves the quality and stability of the sludge, and makes it more suitable as a raw material for soil conditioner; Specifically, the present invention treats the sludge for heavy metal stabilization, adds potassium dihydrogen phosphate, mixes and ages, and PO4 3- and heavy metal ions Pb in river silt 2+ 、Cd 2+ The heavy metals are fixed in the silt by forming insoluble phosphates, which reduces their mobility and bioavailability and reduces the potential harm of heavy metals to soil and plants.

[0018] 6. The soil conditioner provided by the present invention can comprehensively improve saline-alkali soil from multiple aspects such as chemistry, physics and biology. From the chemical aspect, lanthanum can regulate the soil ion balance and reduce the soil salinity; potassium dihydrogen phosphate provides plants with essential nutrients such as phosphorus and potassium; from the physical aspect, biochar and river silt can improve the physical structure of the soil, increase the soil permeability and water retention, and reduce soil compaction; from the biological aspect, the organic matter and microorganisms in the conditioner can promote the activity of soil microorganisms, improve soil fertility, and create a good soil environment for plant growth; compared with some existing soil conditioners, the conditioner prepared by the present invention can effectively reduce environmental risks during application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is the growth of alfalfa (Experiment 1); Figure 2 Comparison of alfalfa growth in hydroponic experiments (Experiment 3); Figure 3 Comparison of root length of alfalfa in hydroponic experiment (Experiment 3); Figure 4 Comparison of plant height of alfalfa in hydroponic experiment (Experiment 3); Figure 5 Comparison of photos of alfalfa growth in soil culture experiment (Experiment 3); Figure 6 Comparison of plant height and root length of alfalfa in soil culture experiment (Experiment 3); Figure 7 is the moisture content test result (Experiment 4); Figure 8 This is the scanning electron microscope morphology of the rare earth lanthanum soil conditioner (Experiment 5); Figure 9 This is the energy spectrum of rare earth lanthanum soil conditioner (Experiment 5); Figure 10 This is the element distribution of rare earth lanthanum soil conditioner by scanning electron microscopy (Experiment 5); Figure 11 This is the scanning electron microscopy single element distribution map of rare earth lanthanum soil amendment (experiment 5); Figure 12 is the scanning electron microscopy morphology of the unamended planting soil (Experiment 5); Figure 13 is the energy spectrum of unamended planting soil (Experiment 5); Figure 14 This is the scanning electron microscope image of the planting soil after being improved with rare earth lanthanum soil conditioner (Experiment 5); Figure 15 This is the energy spectrum of the planting soil after being improved with rare earth lanthanum soil conditioner (Experiment 5); Figure 16 This is the element distribution map of the planting soil after being improved with rare earth lanthanum soil amendment (Experiment 5); Figure 17 This is the single element distribution map of the planting soil after being improved with rare earth lanthanum soil amendment (Experiment 5); Figure 18 These are the adsorption and desorption curves of the planting soil before and after improvement with the rare earth lanthanum soil conditioner (Experiment 7); Figure 19 This is the pore size distribution of the planting soil before and after improvement with rare earth lanthanum soil conditioner (Experiment 7); Figure 20 This is the infrared spectrum of the planting soil before and after improvement with rare earth lanthanum soil conditioner (Experiment 8); Figure 21 Thermogravimetric curves of the planting soil before and after improvement with rare earth lanthanum soil conditioner (Experiment 9); Figure 22 These are the differential thermogravimetric curves of the planting soil before and after improvement with the rare earth lanthanum soil conditioner (Experiment 9). DETAILED DESCRIPTION

[0020] Example 1 A method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner comprises the following steps: Step 1, raw material preparation: 10 parts of rare earth lanthanum compound; 25 parts of modified biochar; 2 parts of phosphate; 70 parts of dehydrated river sludge; 15 parts of organic complexing agent; Step 2, raw material pretreatment: dissolving the rare earth lanthanum compound and the organic complexing agent in deionized water, stirring and reacting in a 50-70° C. water bath to generate a complex solution; Step 3: Preparation of modified biochar: (1) Treat the biochar with 10% nitric acid at 55°C for 7 hours; then treat it with 30% nitric acid at 65°C for 9 hours; and then wash it with deionized water until the pH reaches 7. (2) After drying, add 10% ethylenediamine solution to the biochar at a solid-liquid ratio of 1:20 and reflux at 85°C for 15 hours to graft amino groups on the biochar surface through nucleophilic substitution reaction; Step 4, lanthanum loading: immersing the modified biochar in the complex solution, ultrasonically treating it, and drying it at 70°C for 8 hours to obtain lanthanum-loaded biochar; Step 5: Matrix mixing: evenly mixing dehydrated river silt, phosphate, and lanthanum-loaded biochar to obtain a saline-alkali soil conditioner; The saline-alkali soil conditioner is used for reducing soil salinity, fixing heavy metals in the soil, improving soil structure, and slowly releasing rare earth elements into the soil.

[0021] Furthermore, the particle size of the biochar is ≤2 mm, the organic complexing agent is sodium citrate, the phosphate is potassium dihydrogen phosphate, and the rare earth lanthanum compound is lanthanum nitrate.

[0022] Furthermore, the preparation method of the dewatered river sludge is as follows: S1. Sludge pretreatment: After removing impurities from the river silt, add 1% of the mass of the river silt into polyaluminium chloride, stir and mix evenly, and then let it settle for 48 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3. Deep dehydration: Dry the crushed sludge to a moisture content of ≤15%; S4. Heavy metal stabilization treatment: add 2% by weight of potassium dihydrogen phosphate to the dried sludge, mix well and age for 72 hours; S5. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

[0023] Furthermore, in step S2, a plate and frame filter press is used for filtration and dehydration, the pressure is controlled at 0.8 MPa, and the water content is dehydrated to 45%.

[0024] Furthermore, in step S4, the stabilized sludge is crushed by a crusher until it passes through an 80-mesh sieve.

[0025] Furthermore, in step 2, the reaction is stirred in a water bath at 70° C. for 35 minutes to generate a complex solution.

[0026] Furthermore, in step 4, the frequency of ultrasonic treatment is 50 kHz and the time is 35 minutes.

[0027] Example 2 A method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner comprises the following steps: Step 1, raw material preparation: 5 parts of rare earth lanthanum compound; 15 parts of modified biochar; 1 part of phosphate; 50 parts of dehydrated river sludge; 10 parts of organic complexing agent; Step 2, raw material pretreatment: dissolving the rare earth lanthanum compound and the organic complexing agent in deionized water, stirring and reacting in a 50-70° C. water bath to generate a complex solution; Step 3: Preparation of modified biochar: (1) Treat the biochar with 10% nitric acid at 45°C for 5 hours; then treat it with 30% nitric acid at 55°C for 7 hours; and then wash it with deionized water until the pH reaches 6. (2) After drying, add 10% ethylenediamine solution to the biochar at a solid-liquid ratio of 1:10, and reflux at 75°C for 10-15 hours to graft amino groups on the biochar surface through nucleophilic substitution reaction; Step 4, lanthanum loading: immersing the modified biochar in the complex solution, ultrasonically treating it, and drying it at 50-70°C for 4 hours to obtain lanthanum-loaded biochar; Step 5: Matrix mixing: evenly mixing dehydrated river silt, phosphate, and lanthanum-loaded biochar to obtain a saline-alkali soil conditioner; The saline-alkali soil conditioner is used for reducing soil salinity, fixing heavy metals in the soil, improving soil structure, and slowly releasing rare earth elements into the soil.

[0028] Furthermore, the particle size of the biochar is ≤2 mm, the organic complexing agent is sodium citrate, the phosphate is potassium dihydrogen phosphate, and the rare earth lanthanum compound is lanthanum nitrate.

[0029] Furthermore, the preparation method of the dewatered river sludge is as follows: S1. Sludge pretreatment: After removing impurities from the river silt, add 0.5% of the mass of the river silt into polyaluminium chloride, stir and mix evenly, and then let it settle for 24 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3. Deep dehydration: Dry the crushed sludge to a moisture content of ≤15%; S4. Heavy metal stabilization treatment: add 1% by weight of potassium dihydrogen phosphate to the dried sludge, mix well and age for 48 hours; S5. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

[0030] Furthermore, in step S2, a plate and frame filter press is used for filtration and dehydration, the pressure is controlled at 0.6 MPa, and the water content is dehydrated to 40-45%.

[0031] Furthermore, in step S4, the stabilized sludge is crushed by a crusher until it passes through an 80-mesh sieve.

[0032] Furthermore, in step 2, the reaction is stirred in a 50° C. water bath for 25 minutes to generate a complex solution.

[0033] Furthermore, in step 4, the frequency of ultrasonic treatment is 30 kHz and the time is 25 minutes.

[0034] Example 3 A method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner comprises the following steps: Step 1, raw material preparation: 8 parts of rare earth lanthanum compound; 20 parts of modified biochar; 1.5 parts of phosphate; 60 parts of dehydrated river sludge; 13 parts of organic complexing agent; Step 2, raw material pretreatment: dissolving the rare earth lanthanum compound and the organic complexing agent in deionized water, stirring and reacting in a 50-70° C. water bath to generate a complex solution; Step 3: Preparation of modified biochar: (1) Treat the biochar with 10% nitric acid at 50°C for 6 hours; then treat it with 30% nitric acid at 60°C for 8 hours; and then wash it with deionized water until the pH reaches 6. (2) After drying, the biochar was added with 10% ethylenediamine solution at a solid-liquid ratio of 1:15, and refluxed at 80 °C for 13 hours to graft amino groups on the biochar surface through nucleophilic substitution reaction; Step 4, lanthanum loading: immersing the modified biochar in the complex solution, ultrasonically treating it, and drying it at 60°C for 6 hours to obtain lanthanum-loaded biochar; Step 5: Matrix mixing: evenly mixing dehydrated river silt, phosphate, and lanthanum-loaded biochar to obtain a saline-alkali soil conditioner; The saline-alkali soil conditioner is used for reducing soil salinity, fixing heavy metals in the soil, improving soil structure, and slowly releasing rare earth elements into the soil.

[0035] Furthermore, the particle size of the biochar is ≤2 mm, the organic complexing agent is sodium citrate, the phosphate is potassium dihydrogen phosphate, and the rare earth lanthanum compound is lanthanum nitrate.

[0036] Furthermore, the preparation method of the dewatered river sludge is as follows: S1. Sludge pretreatment: After removing impurities from the river silt, add 0.8% of the mass of the river silt into polyaluminium chloride, stir and mix evenly, and then let it settle for 36 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3. Deep dehydration: Dry the crushed sludge to a moisture content of ≤15%; S4. Heavy metal stabilization treatment: Add 1-2% of potassium dihydrogen phosphate by weight to the dried sludge, mix well and age for 48-72 hours; S5. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

[0037] Furthermore, in step S2, a plate and frame filter press is used for filtration and dehydration, the pressure is controlled at 0.7 MPa, and the water content is dehydrated to 43%.

[0038] Furthermore, in step S4, the stabilized sludge is crushed by a crusher until it passes through an 80-mesh sieve.

[0039] Furthermore, in step 2, the reaction is stirred in a 60° C. water bath for 30 minutes to generate a complex solution.

[0040] Furthermore, in step 4, the frequency of ultrasonic treatment is 40 kHz and the time is 30 minutes.

[0041] Comparative Example 1 The difference between this comparative example and Example 3 is that sodium citrate was not added.

[0042] Comparative Example 2 The difference between this comparative example and Example 3 is that the biochar was not modified.

[0043] Comparative Example 3 The difference between this comparative example and Example 3 is that in step 3, no dehydrated river sludge is added.

[0044] Comparative Example 4 The difference between this comparative example and Example 3 is that in step 3, potassium dihydrogen phosphate is not added.

[0045] Comparative Example 5 The difference between this comparative example and Example 3 is that in step 3, lanthanum nitrate was not added in step 1.

[0046] Comparative Example 6 The difference between this comparative example and Example 3 is that in step 3, the dehydrated river sludge is fermented. The specific preparation method is as follows: S1. Sludge pretreatment: Filter the river sludge through a 20-mesh sieve to remove impurities, then add 0.5-1% of the mass of the river sludge into polyaluminium chloride, stir and mix evenly, and let it settle for 24-48 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3, fermentation: add 1-2% of calcium peroxide and 0.5-1% of humic acid by weight to the crushed sludge, mix well, and ferment aerobically at 50-60°C for 7-10 days, turning the pile every 24 hours; S4. Deep dehydration: The fermented sludge is dried to a moisture content of ≤15%; S5. Heavy metal stabilization treatment: Add 1-2% of potassium dihydrogen phosphate by weight to the dried sludge, mix well, and age for 48-72 hours; S6. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

[0047] Experimental part

[0048] Experiment 1 Take alfalfa seeds and plant them in saline-alkali soil improved by the soil improvers prepared in Example 3 and Comparative Examples 1-6. 8g of soil improver was added to every 100g of saline-alkali soil. A control group was also set up. In the same soil, the soil improvers of Example 3 and Comparative Examples 1-5 were applied respectively. A water control group was also set up. After 20 days, the growth of alfalfa was observed. Figure 1 As can be seen from the figure, no alfalfa seedlings were seen in the control group. The growth of the alfalfa in Comparative Examples 1-6 was uneven, with insufficient growth and lush leaves. Relatively speaking, the growth of Comparative Example 4 was better. Among them, Comparative Example 3 did not add silt, and the alfalfa growth was far inferior to that of Example 3. Although fermented silt was added to Comparative Example 6, the alfalfa growth was not as good as that of Example 3, indicating that the silt treatment method has a significant impact on fertilizer efficiency. Directly adding river silt is more effective in improving saline-alkali soil than adding a soil conditioner made from fermented river silt.

[0049] The growth condition of alfalfa in Example 3 was significantly better than that in the control group and comparative examples 1-6, and the plants were taller and the stems and leaves were more luxuriant, indicating that the soil conditioner prepared in Example 3 had a significant effect on improving saline-alkali soil, could effectively reduce the adverse effects of saline-alkali soil on the growth of alfalfa, provided a more suitable growth environment for alfalfa, and promoted its growth and development.

[0050] Experiment 2 After the end of Experiment 1, soil from each group was taken to measure soil pH, pH value, electrical conductivity (EC), organic matter content, available nutrients (nitrogen, phosphorus, potassium), cation exchange capacity (CEC), heavy metal content (such as Pb, Cd), and soil aggregate stability according to the Soil Agrochemical Analysis.

[0051] Table 1 Soil improvement test results

[0052] As can be seen from Table 1, Example 3 significantly reduces the pH and EC values ​​of saline-alkali soil, which is better than other groups. This is due to the synergistic effect of lanthanum citrate complex and silt, which effectively reduces soil salinity and reduces the toxic effects of salt on crops. The contents of available nitrogen, available phosphorus, available potassium, and organic matter are also higher than those of other groups, and Pb and Cd contents are reduced, proving that the soil improver of the present invention has a better effect on the nutrient content and salinity of the soil, and is also better for the removal of heavy metals in the soil. At the same time, the CEC value of Example 3 is increased to 9.5 cmol+ / kg, which enhances the soil's fertility retention and buffering properties, and is conducive to the absorption and utilization of nutrients by crops. The aggregate stability of the control group is 0.5 mm, indicating that the soil structure is poor and prone to weathering and erosion. The aggregate stability of Example 3 is increased to 1.5 mm, which is also significantly increased compared to the comparative example, indicating that the soil improver provided by the present invention enhances the soil's anti-erosion ability and water retention capacity, which is conducive to the growth and development of crop roots.

[0053] Experiment 3 1. Hydroponic experiment Two groups of experiments were conducted, one in which clean water was prepared and the other in which the rare earth lanthanum soil conditioner prepared in Example 3 was applied. Alfalfa seeds were hydroponically planted and photographed after one week. Figure 2 ), use a ruler to measure the root length of the plant ( Figure 3 ) and plant height ( Figure 4 ), pH meter and conductivity meter were used to monitor the pH and EC value (conductivity) of the solution simultaneously, see Table 2.

[0054] 2. Soil culture experiment Two groups of experiments were prepared, one for the blank control group and the other for the addition of rare earth lanthanum soil conditioner. 150g of planting soil was taken, and the amount of rare earth lanthanum soil conditioner added was 10% of the mass of the planting soil. Alfalfa seeds were planted in the soil. This was repeated three times. Photos were taken and recorded after two weeks. Figure 5 ), use a ruler to measure the plant height and root length ( Figure 6 ), pH meter and salt monitor were used to monitor the changes in soil pH, EC value and salt content. These conditions have different effects on the growth of plants, and the changes in soil pH and salt content in the four groups of experiments were detected, see Table 2.

[0055] Table 2 Comparison of pH, EC and salinity of plants with and without conditioners in different experiments

[0056]

[0057] The effectiveness of a rare earth lanthanum soil conditioner on salinized soils was systematically verified based on comparative studies using hydroponic and soil culture systems. Table 2 shows that pH monitoring of the solution and fresh water treated with the rare earth lanthanum soil conditioner during the hydroponic experiment showed no significant change, while the electrical conductivity decreased from 1307 μS / cm to 1240 μS / cm. This indicates that the rare earth lanthanum soil conditioner can improve the medium's ionic environment by releasing soluble electrons, providing a theoretical basis for improving saline-alkali soils in subsequent soil culture experiments. In the soil culture experiment, the salinity of the unamended planting soil was 1132.5 ppm and the pH was 8.38. After amendment, the pH dropped to 7.56 and the salinity to 815 ppm, a 27.9% decrease. Therefore, it is concluded that the rare earth lanthanum soil conditioner has a significant effect on reducing salinity and enhancing efficiency in salinized soils.

[0058] Experiment 4 The core indicator for evaluating water retention is field water holding capacity, which is the stable moisture content after gravity water drains out of the soil after saturation. It represents the maximum effective water capacity that the soil can hold under natural conditions. During soil sampling and pretreatment, a ring knife was used to sample the same location in each soil culture pot with different additives. The unamended planting soil and the soil amended with the rare earth lanthanum soil amendment from Experiment 3 were sampled, maintaining their natural structure. The soil samples were then soaked for 12 hours until completely saturated. The saturated soil samples were then placed on filter paper and allowed to stand for 2 hours. The wet mass of the drained soil samples was then measured using an electronic balance and recorded. The soil samples were then dried at 105°C for 24 hours until constant weight was achieved. The dry mass of the soil samples was then weighed, and the mass water content of the soil samples was calculated using the moisture content formula.

[0059] Mass moisture content formula: ; Where: : Water content by mass, %; m w : Water mass, g; m s : dry soil mass, g; The calculated soil moisture content before and after improvement is shown in Table 3 and Figure 7 As shown in the results, under the same drainage time, the stable moisture content of the unamended planting soil after gravity water drainage was 8.81%, while the soil moisture content of the soil treated with the rare earth lanthanum soil conditioner reached 11.48%, a significant increase of 2.67% compared to the control group. Therefore, it is concluded that the rare earth lanthanum soil conditioner improves soil water retention. Since the increased moisture content can further promote the combination of soil particles into a granular structure, it enhances the soil's air permeability and water retention capacity, and improves the compaction problem of saline-alkali soil.

[0060] Table 3 Moisture content test

[0061]

[0062] Experiment 5 The morphological characteristics of the rare earth lanthanum soil conditioner were detected using a scanning electron microscope (SEM) of model GAIA 3XMN. Figure 8 , the energy spectrum of the conditioning agent is as follows Figure 9 , the element content is shown in Table 4, and the distribution of each element is shown in Figure 10 and Figure 11 .

[0063] Table 4 Element content of conditioning agent detected by scanning electron microscopy

[0064] Table 4 shows that the atomic number of oxygen and carbon atoms in the rare earth lanthanum soil conditioner accounts for 98.37%, indicating that its core component is an organic polymer. The atomic number of lanthanum element accounts for 0.14%, with La 3+ The form is enriched at the interface of organic matter, through the "ion bridge bond" (-COO-La 3+ ) enhances aggregate stability. Nitrogen, comprising 0.80% of the total atomic weight, provides a nitrogen source for microbial metabolism. Potassium, comprising 0.28% of the total atomic weight, exists in the exchangeable K⁺ state, increasing available potassium content in the soil and activating urease activity. This demonstrates the synergistic effect of the functional elements of the rare earth lanthanum soil conditioner. The rare earth lanthanum soil conditioner can directly introduce high levels of exogenous organic carbon, increasing the total organic matter content in the soil. Figures 9-11 More intuitive observation of the distribution of rare earth lanthanum soil conditioner detection elements.

[0065] The scanning electron microscope morphology of the unimproved planting soil is shown in the figure below. Figure 12 , element detection spectrum diagram as follows Figure 13 , the element content is shown in Table 5.

[0066] Table 5 Element content of planting soil without adding conditioners by electron microscopy

[0067] The element scanning electron microscope morphology of the soil improved by rare earth lanthanum soil conditioner is as follows Figure 14 , element detection spectrum diagram as follows Figure 15 , SEM element distribution diagram as shown Figure 16 As shown, the single element distribution map of the scanning electron microscope is as follows Figure 17 , element content detection is shown in Table 6.

[0068] Table 6 Element content of improved planting soil detected by electron microscope scanning

[0069] Characterization analysis based on scanning electron microscopy showed that rare earth lanthanum soil conditioner significantly optimized the soil organic matter structure by reconstructing the element occurrence form ( Figure 12 、 14 The results in Table 5 show that the total atomic ratio of C and O in the unmodified planting soil is 82.9%. As shown in Table 6, after being improved with the rare earth lanthanum soil conditioner, the atomic ratio of C and O increased significantly to 95.0%, and a continuous and dense organic matter network structure was presented. The energy spectrum data further revealed that La³ in the improved soil + The appearance of characteristic peaks ( Figure 13 、 15 ), accompanied by N and K; Table 6 shows that lanthanum atoms account for 0.21%, nitrogen atoms account for 1.63% and potassium atoms account for 0.04% of the nutrient elements; Figure 16-17 More intuitive observation of the distribution of rare earth lanthanum soil conditioner detection elements.

[0070] Experiment 6 XRF testing involves primary X-rays emitted by an X-ray tube, which excite atoms in the sample, causing them to produce characteristic X-rays. By precisely measuring the wavelength, energy, and intensity of the characteristic fluorescent X-rays emitted by each element in the sample, it is possible to comprehensively analyze the elemental composition of the sample and accurately obtain the concentration data for each element, thereby achieving both qualitative and quantitative analysis. The following XRF analysis was performed on the unamended planting soil and the soil amended with the rare earth lanthanum soil amendment from Experiment 3. The specific data are shown below.

[0071] Table 7 Relative abundance and mass percentage of elements in unamended planting soil

[0072] Table 7 shows the relative abundance of elements and the corresponding oxide or element mass percentage (wt%) in the unamended cropping soil. The highest-content component in the unamended cropping soil is SO₃, accounting for 44.72 wt% of the total mass, indicating an extremely high sulfate content. Cl accounts for 1.17 wt%, reflecting an enrichment in soil salts (e.g., NaCl). SiO₂ accounts for 3.45 wt% and Al₂O₃ accounts for 2.71 wt%, significantly lower than the 40–70% SiO₂ content in normal soils. This salt mask further supports salinization. The high SO₃ content and Cl enrichment indicate a sulfate-chloride saline soil. Na₂O accounts for a relatively high 1.14 wt% of the total mass. Combined with Cl and SO₃, this suggests an excessive exchangeable sodium content, indicating the need for soil improvement. K₂O accounts for 2.605 wt% and CaO for 1.53 wt%, indicating elevated potassium and calcium oxides. Among the heavy metals and trace elements detected, the mass proportion of Fe2O3 was 0.8174 wt%, the mass proportion of MnO was 0.106 wt%, and the content of iron and manganese oxides was normal.

[0073] Table 8 Relative abundance and mass percentage of elements in soil improved with rare earth lanthanum soil conditioner

[0074] Table 8 shows the relative abundance of elements and the weight percentage (wt%) of the corresponding oxides or elements in the improved planting soil. The weight percentage of SO₃ decreased from 44.72 wt% to 1.38 wt%, and the weight percentage of Cl decreased from 1.17 wt% to 0.333 wt%. This significantly reduced the chloride content, transforming the soil from a sulfate-chloride type to a slightly saline soil. The weight percentage of CaO increased from 1.53 wt% to 8.906 wt%, presumably due to the replacement of Na⁺ by calcium ions through ion exchange, enhancing structural stability. The weight percentage of Na⁺O increased slightly from 1.14 wt% to 1.83 wt%. However, combined with the reduction in Cl and SO₃, the percentage of exchangeable sodium decreased, improving soil dispersibility. The mass fraction of SiO2 increased from 3.45 wt% to 46.64 wt%, indicating that the addition of silicates or enhanced mineral stability in the rare earth lanthanum soil amendment resulted in an increase in Al2O3 from 2.71 wt% to 10.69 wt%, reflecting the enrichment of clay minerals. The enrichment of aluminosilicate minerals and calcium indicates a significant improvement in soil structural stability and fertility. Increased K2O (2.446 wt%) and Fe2O3 (4.774 wt%) contents are attributed to the influx of organic matter after the application of the rare earth lanthanum soil amendment, which promotes plant nutrient uptake. The mass fractions of available phosphorus (P2O5) (0.701 wt%) and available potassium (K2O) (2.446 wt%) were moderate, supporting crop growth. The mass fractions of CuO (0.0084 wt%) and ZnO (0.0115 wt%) were extremely low, indicating low heavy metal risk and no contamination.

[0075] Experiment 7 The Micromeritics ASAP 2460 surface area analyzer was used to measure the multi-point BET surface area, adsorption and desorption average pore diameter, and pore volume of the unamended planting soil, the soil amended with the rare earth lanthanum soil amendment, and the rare earth lanthanum soil amendment in the soil culture experiment in Experiment 3. The specific data are shown in Table 9.

[0076] Table 9 Specific surface area and pore size of the measured samples

[0077] Based on the analysis of specific surface area and pore parameters in Table 9, the specific surface area of ​​the improved soil reached 8.9237 m² / g, which was 30.5% higher than the specific surface area of ​​6.8398 m² / g of the original planting soil (unadjusted planting soil). The increase in specific surface area indicated a significant increase in adsorption sites, providing a physical basis for nutrient retention. The average pore size of the original planting soil was 9.9494 nm, the average pore size of the rare earth lanthanum soil conditioner was 4.5740 nm, and the average pore size of the soil improved with the rare earth lanthanum soil conditioner was 6.1099 nm, forming a pore structure dominated by mesopores, which had both micropore adsorption and macropore water conduction functions. The pore volume of the original planting soil was 0.013793 cm³ / g, and the pore volume of the improved soil was reduced to 0.011581 cm³ / g, reflecting a transformation of the pore structure from disordered macropores to ordered mesopores, reducing the proportion of ineffective pores. Lanthanum rare earth soil conditioner works through a synergistic mechanism, transforming a loose, inefficient soil structure into a highly absorbent, slow-release functional matrix. Its mesopore-dominated pore system offers multi-dimensional advantages in water retention, fertilizer retention, and growth promotion, rapidly improving salinized soils.

[0078] The working mechanism of the specific surface area meter is based on the gas adsorption theory. In an ultra-low temperature environment, reversible physical adsorption occurs between the solid sample surface and nitrogen molecules. The adsorption equilibrium state is regulated by the system pressure. At a constant temperature, the amount of gas adsorbed on the solid surface is related to the pressure and will change with the pressure. Based on the adsorption-desorption principle, the equilibrium adsorption amount of the adsorbate on the adsorbent at different pressures is measured at a constant temperature, and the adsorption-desorption isotherm is plotted. The adsorption-desorption curve of the sample is as follows: Figure 18 As shown, the pore size distribution analysis is as follows Figure 19 shown.

[0079] According to the pore classification of the International Union of Pure and Applied Chemistry (IUPAC), when the relative pressure P / P0 <0.50, it is microporous N2 adsorption with a pore size of d<2 nm, and the curve shows a sharp rise, indicating that the number of micropores in the planting soil increases rapidly; when the relative pressure rises to the range of 0.50-0.88, it is mesoporous N2 adsorption with a pore size of 2 nm≤d≤50 nm, and the curve shows an upward state, indicating that the planting soil is rich in mesopores; at high relative pressure P / P0>0.88, the adsorption amount increases sharply, which is macroporous N2 adsorption with a pore size of d>50 nm. At this stage, due to the capillary condensation phenomenon, the curve shows a leap-like rise. Figure 18 The following are the N adsorption and desorption curves for the soil before and after improvement. As can be seen, both curves exhibit a concave low-pressure region and capillary condensation in the high-pressure region. Adsorption and desorption do not overlap, and a hysteresis loop exists, thus belonging to a Type IV curve. Under the same P / P0 conditions, the addition of the rare earth lanthanum soil amendment increases adsorption. The desorption branch of the soil before and after improvement does not completely overlap with the adsorption branch, indicating a significant decrease in adsorption.

[0080] Figure 19 The pore size distribution of the planting soil before and after improvement is shown in the figure. As can be seen from the figure, the pore size distribution of the soil before improvement is mainly distributed in the range of 2.5-3 nm, indicating that the sample has a rich mesoporous structure. The mesoporous structure is very important for the adsorption performance and nutrient release characteristics of compound fertilizers. The pore volume of the improved soil is larger than that of the unimproved soil at the same pore width. The pore size of the soil before and after improvement is concentrated in the range of 2.5-4 nm. The pore volume in this range increases sharply. The pore volume of the improved soil increases from 0.002 cm 3 / g increased to 0.0175 cm 3 / g, the pore volume of the improved soil in the range of 2.5-4 nm is greater than that of the unimproved planting soil, so the mesopores increase after the soil is improved. The pore volume of the pore size before and after soil improvement decreases linearly in the range of 4-5 nm, and then shows an increasing trend in the range of 5 nm to 30 nm. However, the pore volume of the improved soil is still larger than that of the unimproved soil. Therefore, the application of rare earth lanthanum soil conditioner can enhance the water conservation capacity. The amplified mesoporous structure can significantly increase the soil water holding capacity, reduce water leakage loss, and improve soil aeration, accelerate oxygen diffusion and carbon dioxide discharge, provide a suitable habitat for microbial metabolism, and drive organic matter mineralization and nutrient conversion.

[0081] Experiment 8 The principle of infrared spectroscopy analysis technology is to identify the composition and structure of molecules or perform quantitative measurements based on the absorption of infrared light by molecules. Based on the selective absorption of infrared light of a specific wavelength by molecules, their internal vibrational energy levels and rotational energy levels will undergo transition changes. By monitoring the infrared absorption information, the external absorption spectrum of the substance can be obtained. This spectrum can also be called a vibrational-rotational spectrum or a molecular rotational spectrum. This time, the German Bruker model TENSORⅡ was used to detect the planting soil before and after improvement. The infrared spectra of the planting soil before and after improvement are shown below. Figure 20 shown.

[0082] based on Figure 20The infrared spectrum data show that in the O–H / N–H stretching vibration at 3600–3200 cm⁻¹, the absorption peaks at 3723.4 cm⁻¹ and 3601.8 cm⁻¹ of the improved planting soil are significantly enhanced, indicating that the organic matter hydroxyl (–OH) and amino (–NH) functional groups introduced by the rare earth lanthanum soil conditioner are enriched, thereby enhancing the soil water retention and ion exchange capacity; in the C–H / CO3²⁻ vibration at 2950–2350 cm⁻¹, the weak peak at 2350.1 cm⁻¹ of the unimproved planting soil may be derived from trace carbonates, while this peak disappears in the improved soil, which proves that the rare earth lanthanum soil conditioner inhibits carbonate deposition by pH adjustment and is consistent with the conclusion of the previous thermogravimetric analysis; in the C–H bending vibration at 1449 cm⁻¹, the absorption intensity of the improved planting soil is improved, reflecting the input of exogenous organic matter and promoting the accumulation of aliphatic carbon pools; The Si–O–Si / Al–O–Si stretching vibration at 100 cm⁻¹ showed that the peak intensity of the improved planting soil was significantly higher than that of the unimproved planting soil, indicating that the silicate components in the rare earth lanthanum soil conditioner formed a stable aluminosilicate network with clay minerals, enhancing the stability of soil aggregates; at the metal–O bond vibration at 659.8 cm⁻¹, the La–O characteristic peak appeared in the improved planting soil, which was related to the La(OH)3 hydrolysis product of La(NO3)3 in the rare earth lanthanum soil conditioner, confirming that La³ + The organic-mineral complex is stabilized by ionic bridge bonds, which is consistent with the analysis of rare earth lanthanum soil conditioner by scanning electron microscopy. The final conclusion is that rare earth lanthanum soil conditioner has a carbonate inhibition effect. The disappearance of the 2350 cm⁻¹ peak indicates a reduction in salt (CO3²⁻) content. Combined with the pH drop from 8.38 to 7.56, the salinization risk is reduced by 35%; it can also promote the fixation of sodium ions, La³ + Ion exchange replaces Na⁺ adsorbed by clay minerals; promotes the enrichment of hydroxyl and amino groups, and the peaks at 3723.4 cm⁻¹ and 3601.8 cm⁻¹ are enhanced, which is directly related to the increase in organic matter content; improves the stability of the carbon pool, and the enhancement of the peak at 1449 cm⁻¹ reflects the increase in the proportion of refractory organic matter and the decrease in carbon mineralization rate, which can also promote structural stability.

[0083] Experiment 9 The principle of thermogravimetric analysis (TG or TGA) is to subject the sample to a programmed temperature-controlled mass response. By observing the mass change of the sample over temperature or time, relevant information such as the starting decomposition temperature, end temperature, weight loss rate, and residual carbon content can be obtained. The thermogravimetric curve (TG curve) is a curve directly obtained from a thermogravimetric test, which records the relationship between the mass of the sample and temperature or time. The differential thermogravimetric curve (DTG curve) is a curve derived from the thermogravimetric curve. It is the first-order derivative of the thermogravimetric curve with respect to temperature or time, and reflects the rate of change of the sample mass. In the correspondence between the DTG curve and the TG curve, the peak point of the DTG curve corresponds to the inflection point of the weight loss step of the TG curve. At different stages of the reaction weight loss, the peak position corresponds to the maximum weight loss rate.

[0084] This thermogravimetric analysis was performed using a German Netzsch TG 209 F1 thermogravimetric analysis system. Two soil samples from Experiment 3 were selected for comparative analysis: unamended planting soil (mass 5.7832 mg) and soil amended with a rare earth lanthanum soil amendment (mass 5.4615 mg). The experimental parameters were set to perform thermogravimetric analysis within a temperature range of 30-800°C at a heating rate of 30 K / min. The thermogravimetric curves before and after the amendment are shown in Figure 2. Figure 21 As shown in the figure, the differential thermogravimetric curves before and after improvement are as follows Figure 22 shown.

[0085] The unimproved planting soil has three weight loss stages, namely, the low-temperature weight loss stage of 33.4~100℃, with a weight loss rate of 1.69%, which is mainly due to the volatilization of adsorbed water in the soil, such as the loss of some weakly bound water or interlayer water in clay minerals. The DTG curve has a single peak near 80℃, with a peak value of -0.25% / min, reflecting the desorption of water between montmorillonite layers; the medium-temperature weight loss stage of 250-632.1℃, with a weight loss rate of 2.61%, which is due to the humus in the organic matter of the soil, and the desorption of humic acid at 300-450℃. Carboxyl (-COOH→CO2↑), oxidation and pyrolysis of microbial residues and cracking of aliphatic compounds, CO bond breaks at 450-550℃, DTG curve shows a single peak near 400℃, with a peak value of -0.18% / min; 632.1-700℃ is the high temperature weight loss stage, with a weight loss rate of 1.37%, attributed to the removal of structural hydroxyl (-OH) of clay minerals and the decomposition of trace carbonates to release carbon dioxide, DTG curve shows a single peak near 680℃, with a peak value of -0.10% / min. The improved planting soil has three weight loss stages, namely low temperature weight loss at 35-200℃, and the mass increased by 0.38%, indicating that La in rare earth lanthanum soil conditioner 3+The formation of stable complexes with clay minerals enhances water-binding capacity, possibly due to oxidation and weight gain of some organic matter in the low-temperature zone. During the medium-temperature weight loss phase (400-642.5°C), the weight loss rate was 0.231%, a significant decrease of 91.1% compared to the unmodified soil, indicating improved soil organic matter stability. At high temperatures (642.5-800°C), the weight loss rate was 3.976%. This increased weight loss rate may be due to a high-temperature reaction between the silicates in the rare earth lanthanum soil conditioner and clay minerals, rather than simple mineral decomposition. The conclusion is that the rare earth lanthanum soil conditioner, through a synergistic mechanism of "enhanced water adsorption, condensation stabilization of organic matter, and mineral reconstruction," enables the improved soil to exhibit advantages in water retention, drought resistance, and long-term carbon storage.

Claims

1. A method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner, characterized in that: The steps include: Step 1, raw material preparation: 5-10 parts of rare earth lanthanum compound; 15-25 parts of modified biochar; 1-2 parts of phosphate; 50-70 parts of dehydrated river sludge; 10-15 parts of organic complexing agent; Step 2, raw material pretreatment: dissolving the rare earth lanthanum compound and the organic complexing agent in deionized water, stirring and reacting in a 50-70° C. water bath to generate a complex solution; Step 3: Preparation of modified biochar: (1) Treat the biochar with 10% nitric acid at 45-55°C for 5-7 hours; treat with 30% nitric acid at 55-65°C for 7-9 hours; and wash with deionized water to a pH of 6-7. (2) After drying, add 10% ethylenediamine solution to the biochar at a solid-liquid ratio of 1:10-20, and reflux at 75-85°C for 10-15 hours to graft amino groups on the surface of the biochar through nucleophilic substitution reaction; Step 4, lanthanum loading: immersing the modified biochar in the complex solution, ultrasonically treating it, and drying it at 50-70°C for 4-8 hours to obtain lanthanum-loaded biochar; Step 5: Matrix mixing: evenly mixing dehydrated river silt, phosphate, and lanthanum-loaded biochar to obtain a saline-alkali soil conditioner; The saline-alkali soil conditioner is used for reducing soil salinity, fixing heavy metals in the soil, improving soil structure, and slowly releasing rare earth elements into the soil.

2. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 1, characterized in that: The particle size of the biochar is ≤2 mm, the organic complexing agent is sodium citrate, the phosphate is potassium dihydrogen phosphate, and the rare earth lanthanum compound is lanthanum nitrate.

3. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 2, characterized in that: The preparation method of the dewatered river sludge is as follows: S1. Sludge pretreatment: After removing impurities from the river silt, add 0.5-1% of the mass of the river silt into polyaluminium chloride, stir and mix evenly, and then let it settle for 24-48 hours; S2. Mechanical dehydration: The precipitated sludge is filtered and dehydrated, and the resulting mud cake is crushed to a particle size of ≤5mm by a crusher; S3. Deep dehydration: Dry the crushed sludge to a moisture content of ≤15%; S4. Heavy metal stabilization treatment: Add 1-2% of potassium dihydrogen phosphate by weight to the dried sludge, mix well and age for 48-72 hours; S5. Finished product processing: The stabilized sludge is crushed to obtain dewatered river sludge.

4. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 3, characterized in that: In step S2, a plate and frame filter press is used for filtration and dehydration, the pressure is controlled at 0.6-0.8 MPa, and the water content is dehydrated to 40-45%.

5. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 3, characterized in that: In step S4, the stabilized sludge is crushed by a crusher until it passes through an 80-mesh sieve.

6. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 1, characterized in that: In the step 2, the reaction is stirred in a water bath at 50-70° C. for 25-35 minutes to generate a complex solution.

7. The method for preparing a modified biochar-based slow-release rare earth lanthanum soil conditioner according to claim 6, characterized in that: In step 4, the frequency of ultrasonic treatment is 30-50 kHz, and the time is 25-35 minutes.

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