Preparation method of lanthanum oxide modified biochar fertilizer

Abstract

The invention provides a preparation method of a lanthanum oxide modified biochar fertilizer, and belongs to the technical field of agricultural fertilizers, and the preparation method comprises the following steps: 1, microwave activation of biochar: placing corn straw biochar in a microwave reactor for radiation to obtain porous activated biochar; 2, potassium ion loading: dipping the porous activated charcoal in a saturated KOH solution to obtain a solid precipitate, and drying the solid precipitate to obtain potassium-loaded charcoal; 3, lanthanum-organic complex preparation: mixing lanthanum oxide with citric acid, adding deionized water to form a suspension, and performing evaporative crystallization to obtain a lanthanum-citric acid complex; 4, compounding modified biochar: mixing the obtained lanthanum-citric acid complex with potassium-loaded biochar to prepare slow-release lanthanum oxide modified biochar; and 5, compounding a phosphate fertilizer to obtain the slow-release composite charcoal fertilizer. According to the method, the charcoal is modified through lanthanum oxide, so that the modified charcoal has a larger specific surface area, the problem of hardening of the saline-alkali soil is better solved, and the oxygen content of the soil is increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of agricultural fertilizers, and specifically relates to a preparation method of lanthanum oxide modified biochar fertilizer. Background Art

[0002] As a strategic resource, rare earths have received great attention from countries around the world. Especially, rare earths are widely used in the agricultural field, especially in the improvement of saline-alkali soils. Lanthanum element has a promoting effect on soil microorganisms (such as bacteria and actinomycetes) at low concentrations, which may enhance the decomposition of organic matter and nutrient cycling. This process helps to increase the content of soil organic matter, indirectly improve the aggregate structure of saline-alkali soils, enhance permeability and accelerate salt leaching. Lanthanum oxide can promote the aggregation of soil particles through chemical reactions with some components in the soil, increase the soil aggregation, thereby improving the soil structure, being beneficial to improving the soil compaction problem, and enhancing the air permeability and water permeability of the soil.

[0003] Soil salinization is a global ecological problem. Nearly 932 million hectares of land worldwide are affected by salinization. The area of saline-alkali land in China ranks third in the world, accounting for 10% of the global saline-alkali land area. Soil salinization has become the main obstacle factor affecting the efficient utilization of national land resources, the sustainable development of agriculture and the quality of the ecological environment. The soluble salt ions in saline-alkali soils are highly concentrated, resulting in poor aggregate stability, specifically manifested as soil compaction, poor water holding rate, and the saline-alkali environment is often accompanied by a decrease in the availability of soil nutrients. Salt inhibits the absorption of nutrient elements by plants, and a higher pH environment will cause alkali stress, affecting the stability of plant cells and leading to plant physiological metabolism disorders.

[0004] As an effective soil conditioner, biochar can improve the structure of saline-alkali soils through its unique physical and chemical properties. The rare earth fertilizer obtained by modifying biochar with lanthanum oxide can effectively improve the germination rate of saline-alkali soils, help the growth of crops to a certain extent, and can also change the forms and contents of heavy metals in the soil.

[0005] In the prior art, most biochar is directly used in water body adsorption and rarely used in saline-alkali soils. At the same time, most of the existing lanthanum fertilizers use lanthanum nitrate. However, the nitrate radical of lanthanum nitrate may cause eutrophication of the land, rapidly release lanthanum elements, and is only suitable for quickly supplementing lanthanum and cannot provide nutrients for the land in the long term. Summary of the Invention

[0006] Based on the existing modified biochar technology, the present invention invents a slow-release composite biochar fertilizer. An appropriate amount of lanthanum oxide can promote the growth and development of plant roots, leaves and stems, enhance the plant's ability to absorb nutrients, thereby improving the plant's lodging resistance, helping the plant to grow better in saline-alkali soils, improving the aggregation phenomenon of saline-alkali soils, and achieving the purpose of improving saline-alkali soils.

[0007] The details are as follows: A preparation method of lanthanum oxide modified biochar fertilizer, which is characterized by comprising the following steps: (1) Microwave activation of biochar: Place corn straw biochar in a microwave reactor and irradiate it at a power of 200 - 300 W for 2 - 5 minutes to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, magnetically stir for 4 - 6 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 80 - 105 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid according to a mass ratio of 1:2, add deionized water to form a suspension, stir at 50 - 70 °C for 1.5 - 2.5 h, and obtain lanthanum-citric acid complex after evaporation and crystallization; (4) Modification of biochar composite: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) according to a mass ratio of 1:4.5 - 5.5, and then add 1 - 2% of humic acid based on the mass of the biochar to simultaneously improve soil aggregation, and magnetically stir at 400 - 600 r / min for 25 - 35 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphate fertilizer compounding: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate according to a mass ratio of 4.5 - 5.5:1, and magnetically stir at 400 - 600 r / min for 8 - 12 min to obtain slow-release composite biochar fertilizer.

[0008] Moreover, a preparation method of lanthanum oxide modified biochar fertilizer comprises the following steps: (1) Microwave activation of biochar: Place corn straw biochar in a microwave reactor and irradiate it at a power of 200 - 300 W for 2 - 5 minutes to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, magnetically stir for 5 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 80 - 105 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid according to a mass ratio of 1:2, add deionized water to form a suspension, stir at 50 - 70 °C for 2 h, and obtain lanthanum-citric acid complex after evaporation and crystallization; (4) Modification of biochar composite: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) according to a mass ratio of 1:5, and then add 1.5% of humic acid based on the mass of the biochar, and magnetically stir at 500 r / min for 25 - 35 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphate fertilizer compounding: Mix the slow-release lanthanum oxide-modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate at a mass ratio of 5:1, and stir magnetically at 500 r / min for 10 min to obtain slow-release composite biochar fertilizer.

[0009] Moreover, during the microwave activation process described in step (1), the moisture content of the biochar is controlled at 5 - 10%.

[0010] Moreover, in step (2), 90 - 110 ml of KOH solution is added to every 10 g of porous activated biochar.

[0011] Moreover, in step (3), during the preparation of the lanthanum-citrate complex, the evaporation crystallization temperature is 80°C, and the crystallization product is ground to a particle size of ≤50 μm.

[0012] Meanwhile, the present invention provides a slow-release composite biochar fertilizer prepared by the above method.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The present invention optimizes the pore structure by microwave activation of biochar to provide a high specific surface area carrier for loading. Then, impregnation with KOH solution is used to achieve efficient loading of potassium elements. Potassium elements can neutralize soil acidity, promote nutrient absorption, and at the same time enhance the cation exchange capacity of biochar.

[0014] 2. The present invention uses lanthanum oxide as the source of lanthanum element and prepares a lanthanum-citrate complex with citric acid, avoiding nitrate pollution, effectively solving the problem of eutrophication easily caused by traditional lanthanum nitrate, realizing slow-release supply of lanthanum element, and at the same time avoiding soil aggregation.

[0015] 3. Compared with using lanthanum nitrate as fertilizer, the modification of biochar by lanthanum oxide in the present invention makes the modified biochar have a larger specific surface area, better solves the problem of hardening of saline-alkali soil, improves the oxygen content of the soil. At the same time, the slow-release composite biochar fertilizer not only has a certain promoting effect on the germination rate of crops, the growth of roots, stems and leaves, increases the lodging resistance of crops, promotes the absorption of nitrogen and phosphorus in the soil by crops, but also can selectively adsorb Na+ ions in saline-alkali soil, improve the alkaline environment of saline-alkali soil, reduce soil pH, and reduce the heavy metal content in the soil, thus obtaining a new rare earth fertilizer for saline-alkali soil.

[0016] 4. The slow-release composite biochar fertilizer provided by the present invention composites the lanthanum-citrate complex and potassium-loaded biochar at a mass ratio of 1:5, and adds humic acid, monoammonium phosphate or diammonium hydrogen phosphate to achieve the synergistic modification of potassium-lanthanum-phosphorus on saline-alkali soil, enhancing the aggregation and nutrient supply.

[0017] 5. The present invention provides a study and application of a slow-release composite biochar fertilizer in saline-alkali soil. The preparation method of the novel rare earth fertilizer for saline-alkali soil is simple in process and low in investment cost, and has good effects in the actual crop growth application in saline-alkali soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a scanning electron microscope image of the soil around the roots of the clear water control group; Figure 2 It is a scanning electron microscope image of the soil around the roots of the experimental group 3; Figure 3 It is the XRD patterns of pure biochar, KOH biochar compound fertilizer and slow-release composite biochar; Figure 4 It is the growth situation of alfalfa in each group; Figure 5 It is the content of chlorophyll in the soil; Figure 6 It is the energy spectrum analysis of the scanning electron microscope of the clear water control group and the experimental group 3 soil; Figure 7 It is the adsorption-desorption curve and pore size distribution diagram of the clear water control group soil; Figure 8 It is the adsorption-desorption curve and pore size distribution diagram of the experimental group 3 soil; Figure 9 It is the thermogravimetric analysis diagram of biochar and the slow-release composite biochar prepared in Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Example 1 A preparation method of lanthanum oxide modified biochar fertilizer, comprising the following steps: (1). Microwave activation of biochar: Place corn straw biochar in a microwave reactor and irradiate it at a power of 200 W for 2 minutes to obtain porous activated biochar; (2). Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, stir magnetically for 4 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 80 °C to constant weight to obtain potassium-loaded biochar; (3). Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid in a mass ratio of 1:2, add deionized water to form a suspension, stir at 50 °C for 1.5 h, and evaporate and crystallize to obtain lanthanum-citric acid complex; (4). Modification of biochar composite: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) in a mass ratio of 1:4.5, then add 1% of humic acid based on the mass of biochar, and stir magnetically at 400 r / min for 25 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphorus fertilizer compounding: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate at a mass ratio of 4.5:1, and magnetically stir at 400 r / min for 8 min to obtain slow-release composite biochar fertilizer.

[0020] Further, during the microwave activation process in step (1), the moisture content of the biochar is controlled at 5%.

[0021] Further, in step (2), 90 ml of KOH solution is added to every 10 g of porous activated biochar.

[0022] Further, in step (3), during the preparation of the lanthanum-citric acid complex, the evaporation crystallization temperature is 80 °C, and the crystallization product is ground to a particle size of ≤50 μm.

[0023] Further, in step (4), 1-2% of humic acid based on the mass of the biochar is added during the mixing process.

[0024] Example 2 A preparation method of lanthanum oxide modified biochar fertilizer, comprising the following steps: (1) Microwave activation of biochar: Place corn straw biochar in a microwave reactor, irradiate at a power of 300 W for 5 minutes to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, magnetically stir for 6 h, then filter by suction, separate the solid precipitate, wash with deionized water and dry at 105 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid at a mass ratio of 1:2, add deionized water to form a suspension, stir at 70 °C for 2.5 h, and obtain lanthanum-citric acid complex after evaporation crystallization; (4) Modified biochar compounding: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) at a mass ratio of 1:5.5, then add 2% of humic acid based on the mass of the biochar, and magnetically stir at 600 r / min for 5 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphorus fertilizer compounding: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate at a mass ratio of 5.5:1, and magnetically stir at 600 r / min for 12 min to obtain slow-release composite biochar fertilizer.

[0025] Further, during the microwave activation process in step (1), the moisture content of the biochar is controlled at 10%.

[0026] Further, in step (2), 110 ml of KOH solution is added to every 10 g of porous activated biochar.

[0027] Further, in step (3), in the preparation of the lanthanum-citric acid complex, the evaporation crystallization temperature is 80 °C, and the crystallization product is ground to a particle size of ≤50 μm.

[0028] Further, in step (4), humic acid accounting for 1-2% of the mass of the biochar is added during the mixing process.

[0029] Example 3 A preparation method of lanthanum oxide modified biochar fertilizer includes the following steps: (1) Microwave activation of biochar: Place corn straw biochar in a microwave reactor and irradiate it at a power of 250 W for 3 minutes to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, stir magnetically for 5 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 90 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid in a mass ratio of 1:2, add deionized water to form a suspension, stir at 60 °C for 2 h, and obtain a lanthanum-citric acid complex after evaporation crystallization; (4) Modification of biochar composite: Mix the lanthanum-citric acid complex obtained in step (3) and the potassium-loaded biochar obtained in step (2) in a mass ratio of 1:5, then add humic acid accounting for 1-2% of the mass of the biochar, and stir magnetically at 500 r / min for 30 min to obtain slow-release lanthanum oxide modified biochar; (5) Compound with phosphate fertilizer: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) and diammonium hydrogen phosphate in a mass ratio of 5:1, and stir magnetically at 500 r / min for 10 min to obtain slow-release composite biochar fertilizer.

[0030] Moreover, during the microwave activation process in step (1), the moisture content of the biochar is controlled at 8%.

[0031] Moreover, in step (2), 100 ml of KOH solution is added to every 10 g of porous activated biochar.

[0032] Moreover, in step (3), in the preparation of the lanthanum-citric acid complex, the evaporation crystallization temperature is 80 °C, and the crystallization product is ground to a particle size of ≤50 μm.

[0033] Comparative Example 1 The difference between this comparative example and Example 3 is that step (1) of microwave activation of biochar is not carried out, and slow-release compound fertilizer is prepared.

[0034] Comparative Example 2 The difference between this comparative example and Example 3 is that step (3) of preparing the lanthanum-organic complex was not carried out, and lanthanum oxide was directly used in step (4): lanthanum oxide and potassium-loaded biochar were mixed at a mass ratio of 1:5 and magnetically stirred at 500 r / min for 30 min to obtain the slow-release compound fertilizer.

[0035] Comparative Example 3 The difference between this comparative example and Example 3 is that lanthanum oxide was replaced with lanthanum nitrate to obtain the compound fertilizer.

[0036] Comparative Example 4 The difference between this comparative example and Example 3 is that steps (3) and (4) were not carried out, and potassium-loaded biochar was directly compounded with phosphate fertilizer to obtain the KOH biochar compound fertilizer.

[0037] Comparative Example 5 The difference between this comparative example and Example 3 is that step (2) was not carried out, and porous activated biochar was directly compounded with the lanthanum-organic complex to obtain the slow-release compound fertilizer.

[0038] Experimental Section Alfalfa seeds were simultaneously sown into the same soil, and the fertilizers of Example 3 and Comparative Examples 1-5 were applied respectively, and a clear water control group was also set up.

[0039] Experiment 1 One week later, the seed germination rates of the 6 kinds of fertilizers and the clear water control group were counted. Specifically, see Table 1. It can be seen from Table 1 that the effect of Example 3 was the best: the germination rate of Example 3 was the highest, reaching 85.6%, which was significantly higher than that of other comparative examples and the clear water control group, indicating that the preparation method of the slow-release composite biochar fertilizer prepared by the present invention can most effectively promote the germination of alfalfa seeds. In Comparative Example 1, the step of microwave-activating biochar was not carried out, and its germination rate was lower than that of Example 3 because microwave-activating biochar helps to increase the porosity and surface area of biochar, thus better loading and releasing nutrients. In Comparative Example 2, the preparation of the lanthanum-organic complex was not carried out, and lanthanum oxide was directly used, and its germination rate was also lower than that of Example 3, indicating that forming a complex of lanthanum oxide and citric acid helps to improve the slow-release effect and biological effectiveness of the fertilizer. In Comparative Example 3, lanthanum oxide was replaced with lanthanum nitrate, and its germination rate was lower than that of Example 3, indicating that lanthanum oxide is more effective than lanthanum nitrate as a modifier in promoting seed germination. In Comparative Example 4, the steps of preparing the lanthanum-organic complex and compounding the modified biochar were not carried out, and potassium-loaded biochar was directly compounded with phosphate fertilizer, and its germination rate was lower, indicating that the synergistic effect of loading, lanthanum modification and phosphate fertilizer compounding is significant for improving the fertilizer effect. In Comparative Example 5, the potassium ion loading step was not carried out, and porous activated biochar was directly compounded with the lanthanum-organic complex, and its germination rate was also lower than that of Example 3, indicating that potassium ion loading helps to enhance the nutrient retention ability and slow-release effect of biochar.

[0040] Table 1 Comparison Table of Germination Rates of Alfalfa Seeds in 6 Kinds of Fertilizers and Clear Water

[0041] Experiment 2 Two weeks later, the soil around the roots of the clear water control group and the group of Example 3 was taken for SEM scanning, and the results are shown in Figure 1-2 , as can be seen from the figure, compared with the clear water control group, the soil material morphology after adding the slow-release composite biochar fertilizer is loose and porous with more impurities, while the soil material morphology without adding the slow-release composite biochar fertilizer is tightly structured, effectively improving the original soil compaction situation, thereby improving the saline-alkali soil. The soil surface of the clear water control group is flat and smooth with few pore structures, and the soil surface after modification with the slow-release composite biochar fertilizer becomes extremely rough, with a large number of irregular particles of various sizes. Compared with before modification, the pores on the surface of the modified biochar are significantly increased.

[0042] Continue to conduct energy spectrum analysis of scanning electron microscopy on the two groups of soils, and the results are shown in Figure 6 , after applying the slow-release composite biochar fertilizer, heavy metal aluminum is removed from the soil. Aluminum will inhibit the growth and development of plant roots. It mainly acts on the root tip, hindering the elongation and division of cells, making the roots shorter and thicker, and reducing the number of root hairs, thereby affecting the absorption of water and nutrients by plants.

[0043] Experiment 3 The specific surface area (BET) is the core parameter characterizing the surface properties of materials, defined as the total surface area per unit mass or unit volume of the material (unit: m² / g), comprehensively reflecting the pore structure, active site distribution and surface physicochemical properties of the material. The value directly affects the adsorption capacity, catalytic activity and electrochemical performance of the material, and is one of the key indicators in materials science and industrial applications. Figure 7 is the adsorption-desorption curve and pore size distribution map of the soil in the clear water control group in Experiment 2, Figure 8 is the adsorption-desorption curve and pore size distribution map of the soil in the group of Example 3 in Experiment 2. Table 2 shows the specific surface area and pore structure of the two groups of samples.

[0044] Table 2 Specific Surface Area and Pore Structure of Samples

[0045] By comparing Figure 7 and 8From the adsorption-desorption isotherm, it can be seen that in the low relative pressure (P / P0) region (around 0 - 0.2), the adsorption amount increases slowly, indicating that monolayer molecular adsorption mainly occurs at this time. As the relative pressure increases, the adsorption amount gradually increases. In the high relative pressure region (0.8 - 1.0), the adsorption amount rises rapidly, indicating the existence of capillary condensation. There are a certain amount of mesopores and macropores in the soil of Example 3 group. There is an obvious hysteresis loop in the high relative pressure region of the adsorption curve and desorption curve, which is a typical characteristic of mesoporous materials, indicating that the soil of Example 3 group has more mesoporous structures. For the adsorption-desorption curve of the pure water control group, the adsorption amount also increases slowly in the low relative pressure region, which is the monolayer adsorption stage. Throughout the relative pressure range, the overall adsorption amount is relatively low. Although the adsorption amount also rises rapidly in the high relative pressure region, the increase amplitude is smaller than that of Example 3 group, and the hysteresis loop is relatively small, indicating that the mesoporous structure of the pure water control group may be relatively less.

[0046] Comparison Figure 7 and 8 From the pore size distribution diagram of, it can be seen that the pore size distribution of the pure water control group shows an obvious peak in the smaller pore size region (close to 0 nm), indicating that there are a large number of pore structures with smaller pore sizes in the soil of the pure water control group, which may be micropores or small mesopores. As the pore size increases, the pore volume decreases rapidly, and the overall pore size distribution is relatively concentrated in the smaller pore size range, indicating that the pore structure of the soil in the pure water control group is mainly composed of small pore sizes. For the soil of Example 3 group, the pore sizes from 0 to 25 nm are the most, and the subsequent pore size distribution is relatively dispersed, with peaks appearing in multiple pore size ranges, and there is no single obvious main peak at small pore sizes like the pure water control group, indicating that the pore structure of the soil in Example 3 group is more complex and diverse, and the pore size distribution is wider. Combining the morphological analysis of the scanning electron microscope, due to the addition of rare earth lanthanum, the original pore structure of the soil has changed. Compared with the soil without the addition of the modifier, the soil with the addition of the modifier is more porous and loose, which is more conducive to the growth of alfalfa, increases the air permeability of the soil, and improves the hardening problem of saline-alkali soil.

[0047] From Figure 7 and 8 it can be seen that the adsorption-desorption isotherms are both of type IV, indicating that the biochar has significant mesoporous properties. As shown in Table 2, the average pore sizes of the pure water control group and Example 3 group are 15.004 nm and 17.133 nm respectively, and the average pore size of mesoporous materials is 2 - 50 nm, which verifies again that Example 3 group is a mesoporous adsorption material. The specific surface area of Example 3 group is much higher than that of the pure water control group, increasing from 0.282 m 2 / g to 3.393 m2 / g. The reason is that the loading of metal La makes the surface of the biochar rough, thereby increasing the specific surface area, which is consistent with the SEM analysis results.

[0048] Experiment 4 XRD (X-Ray Diffraction) analysis was performed on pure biochar, the KOH biochar composite fertilizer prepared in Comparative Example 4, and the slow-release composite biochar fertilizer prepared in Example 3 of the present invention. The results are shown in Figure 3 . It can be seen from Figure 3 that due to the weak and broad diffraction peaks of pure biochar, it is mostly amorphous structure. The main components are complex organic polymers and amorphous carbon, including pyrolysis residues such as cellulose, hemicellulose, and lignin, and there may be a small amount of natural minerals. The sharp and strong peaks of the KOH biochar composite fertilizer indicate the formation of new crystalline phases, which may be potassium carbonate, potassium silicate, potassium aluminate, etc. For the slow-release composite biochar, there are characteristic peaks of lanthanum oxide (La2O3). During the loading process of lanthanum oxide, it may react with the surface groups of KOH biochar or the formed potassium compounds to form lanthanum-potassium composite oxides or oxygen-containing salts.

[0049] Experiment 5 Thermogravimetry (abbreviated as TG or TGA) is a thermal analysis method based on programmed temperature control technology, which studies the thermal stability, decomposition process, and composition of substances by continuously measuring the mass change of substances. Its core is to accurately record the mass change of samples at different temperatures or times to reveal the thermal behavior and reaction mechanism of materials. The initial decomposition temperature (T0) and the temperature corresponding to the maximum weight loss rate (T p ) of the light conversion film are determined through the TG curve to evaluate its thermal stability in a high-temperature environment. According to the residual mass percentage in the medium and low-temperature stages, the improvement effect of thermal stability after adding rare earth lanthanum is verified, and the proportion of inorganic components (such as rare earth oxides) in the material is speculated. The thermogravimetric analysis diagrams of biochar and the slow-release composite biochar prepared in Example 3 are shown in Figure 9, by comparison, during the low-temperature stage (0 - 100°C), the thermal stabilities of both samples are relatively good, and their weights remain basically stable without obvious weight loss processes such as thermal decomposition or volatilization. The difference in thermal stability between the two is not significant at this stage. During the medium-temperature stage (100 - 200°C), both samples show rapid weight loss, and sharp negative peaks also appear in the DTG curves, indicating that a large number of unstable components in the samples decompose or volatilize rapidly within this temperature range, which is the stage with the worst thermal stability, suggesting that the thermal stabilities of both are poor in this temperature range. In the range of (200 - 800°C) for the slow-release composite biochar, the TG curve continuously decreases slowly, and the fluctuations of the DTG curve become smaller in the negative value region, indicating that there is continuous weight loss of the sample at high temperatures, but the weight loss rate gradually decreases, showing a certain high-temperature stability. The TG curve of the biochar also shows a continuous and slow downward trend, and the fluctuations of the DTG curve become smaller, indicating that there are still components decomposing at high temperatures, but the weight loss rate is decreasing, and the thermal stability has improved, but there are still certain thermal stability problems. Overall, by comparison, the DTG curve of the slow-release composite biochar fluctuates less in the range of (200 - 800°C), meaning that the weight change rate of the slow-release composite biochar is more stable at high temperatures and its thermal stability is slightly better.

[0050] Experiment 6 Three weeks after planting, the pH change of the soil, the change in the salt content in the soil, the determination of heavy metals, organic matter, and chlorophyll content in the soil were detected. The results of the soil pH change are shown in Table 3. It can be seen from Table 3 that the slow-release composite biochar fertilizer prepared by the present invention can reduce the pH value of the soil and improve the alkalinity of saline-alkali soil.

[0051] Table 3 Soil pH Change Table

[0052] The results of the change in the salt content in the soil are shown in Table 4. It can be seen from Table 4 that the slow-release composite biochar fertilizer prepared by the present invention can effectively reduce the salt content in the soil and improve the soil environment.

[0053] Table 4 Soil Salt Content Change Table

[0054] The results of heavy metal contents in the soil are shown in Table 5. It can be seen from Table 5 that the slow-release composite biochar fertilizer prepared by the present invention can effectively reduce the heavy metal contents such as mercury, chromium, and arsenic in the soil. This is because lanthanum oxide is alkaline, and after being applied to the soil, it will reduce the soil pH value. As the pH value decreases, the existing forms of heavy metal ions in the soil will change. For example, mercury ions may form precipitates such as mercury hydroxide under alkaline conditions, thereby reducing their solubility and availability in the soil; arsenic ions and chromium ions will also undergo hydrolysis reactions under alkaline conditions to form hydroxide precipitates or other poorly soluble compounds, reducing the contents of exchangeable and water-soluble arsenic and chromium in the soil, and further reducing the activity of these heavy metals in the soil and their toxicity to plants.

[0055] Table 5 Table of Changes in Soil Heavy Metals

[0056] The organic matter content in the soil is shown in Table 6. It can be seen from Table 6 that the slow-release composite biochar fertilizer prepared by the present invention can effectively increase the organic matter content in the soil and improve the soil environment.

[0057] Table 6 Table of Changes in Soil Organic Matter Content

[0058] The content of chlorophyll in the soil is shown in Figure 5 , and it can be seen from the figure that the chlorophyll in the soil of the clear water control group is 21.9 mg / cm 2 , that of Comparative Example 1 is 26.4 mg / cm 2 , that of Comparative Example 2 is 26.5 mg / cm 2 , that of Comparative Example 3 is 30.3 mg / cm 2 , that of Comparative Example 4 is 29.4 mg / cm 2 , that of Comparative Example 5 is 26.7 mg / cm 2 , and that of Example 3 is 38.2 mg / cm 2 , which proves that the slow-release composite biochar fertilizer prepared by the present invention can effectively increase the chlorophyll content in the soil and improve the soil environment.

[0059] One month later, the growth conditions of alfalfa in each group were photographed and recorded, as shown in Figure 4 , and it can be seen from the figure that the growth conditions of alfalfa after applying fertilizer are all better than those of the clear water control group, with Example 3 being the best.

Claims

1. A preparation method of lanthanum oxide modified biochar fertilizer, characterized in that, It includes the following steps: (1) Microwave-activated biochar: Place the corn straw biochar in a microwave reactor and irradiate it for 2 - 5 minutes at a power of 200 - 300 W to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, magnetically stir for 4 - 6 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 80 - 105 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid in a mass ratio of 1:2, add deionized water to form a suspension, stir at 50 - 70 °C for 1.5 - 2.5 h, and obtain lanthanum-citric acid complex after evaporation and crystallization; (4) Modified biochar compounding: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) in a mass ratio of 1:4.5 - 5.5, then add 1 - 2% of humic acid based on the mass of the biochar, and magnetically stir at 400 - 600 r / min for 25 - 35 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphate fertilizer compounding: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate in a mass ratio of 4.5 - 5.5:1, and magnetically stir at 400 - 600 r / min for 8 - 12 min to obtain slow-release composite biochar fertilizer.

2. The preparation method of a lanthanum oxide modified biochar fertilizer according to claim 1, characterized in that, It includes the following steps: (1) Microwave-activated biochar: Place the corn straw biochar in a microwave reactor and irradiate it for 2 - 5 minutes at a power of 200 - 300 W to obtain porous activated biochar; (2) Potassium ion loading: Immerse the porous activated biochar obtained in step (1) in a saturated KOH solution, magnetically stir for 5 h, then filter by suction, separate the solid precipitate, wash it with deionized water and dry it at 80 - 105 °C to constant weight to obtain potassium-loaded biochar; (3) Preparation of lanthanum-organic complex: Mix lanthanum oxide and citric acid in a mass ratio of 1:2, add deionized water to form a suspension, stir at 50 - 70 °C for 2 h, and obtain lanthanum-citric acid complex after evaporation and crystallization; (4) Modified biochar compounding: Mix the lanthanum-citric acid complex obtained in step (3) with the potassium-loaded biochar obtained in step (2) in a mass ratio of 1:5, and magnetically stir at 500 r / min for 30 min to obtain slow-release lanthanum oxide modified biochar; (5) Phosphate fertilizer compounding: Mix the slow-release lanthanum oxide modified biochar obtained in step (4) with monoammonium phosphate or diammonium hydrogen phosphate in a mass ratio of 5:1, and magnetically stir at 500 r / min for 10 min to obtain slow-release composite biochar fertilizer.

3. The preparation method of a lanthanum oxide modified biochar fertilizer according to claim 1, characterized in that, During the microwave activation process in step (1), the moisture content of the biochar is controlled at 5 - 10%.

4. The preparation method of a lanthanum oxide modified biochar fertilizer according to claim 1, characterized in that, In step (2), 90 - 110 ml of KOH solution is added to every 10 g of porous activated biochar.

5. The preparation method of a lanthanum oxide modified biochar fertilizer according to claim 1, characterized in that, In step (3), during the preparation of the lanthanum-citric acid complex, the evaporation and crystallization temperature is 80 °C, and the crystalline product is ground to a particle size of ≤50 μm.

6. The slow-release composite biochar fertilizer prepared by the preparation method of the slow-release composite biochar fertilizer according to any one of claims 1 - 5.

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