Selenium-doped diatom biochar, nano-selenium fertilizer as well as preparation method and application of nano-selenium fertilizer
By adding selenium during the diatom cultivation process and preparing selenium-doped diatom biochar fertilizer through pyrolysis and carbonization, the problems of single efficacy and environmental impact on the conversion rate of nano-selenium fertilizer were solved, achieving efficient selenium-enriched rice production and soil health improvement.
Patent Information
- Application Number
- CN202410948168.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-07-16
- Publication Date
- 2026-01-16
AI Technical Summary
Existing nano-selenium fertilizers have limited efficacy, and their selenium conversion efficiency is greatly affected by environmental factors, making it difficult to produce high-quality selenium-enriched rice products.
By adding selenium during the diatom cultivation process, the diatoms convert inorganic selenium into nano-selenium, and then prepare selenium-doped diatom biochar fertilizer through pyrolysis carbonization. The nano-selenium fertilizer is prepared by combining biological and chemical methods, with nano-selenium dots embedded on the surface and inside of the biochar.
It improved the selenium conversion rate, enhanced the rice's resistance to diseases and pests and its adaptability to adversity, improved soil health, reduced nutrient loss, provided sufficient nutrients, and enabled large-scale, high-quality selenium-enriched rice production.
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Figure CN121342598A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of fertilizer technology, specifically to a selenium-doped diatomaceous earth biocarbon, nano-selenium fertilizer, its preparation method, and its application. Background Technology
[0002] Selenium-enriched rice is a rice variety rich in selenium, and its harvested selenium-enriched rice has health benefits such as improving human immunity, preventing diseases, and providing antioxidant effects. Applying selenium fertilizer during rice cultivation can not only increase the selenium content of the rice but also, to some extent, increase yield, improve rice quality, enhance the rice's resistance to adverse conditions, and reduce the toxic effects of heavy metals such as chromium, cadmium, and lead on rice. However, the distribution of selenium on Earth is uneven, occurring in scattered points, and even in the same location, the selenium content varies. Therefore, relying solely on selenium in the soil is almost impossible to produce high-quality selenium-enriched rice. For this reason, current technology mainly increases the selenium content of rice by applying selenium-enriched fertilizers during the crop's growth process.
[0003] In the early stages of selenium-enriched rice development in my country, the primary method for increasing the selenium content of rice was the addition of exogenous inorganic selenium fertilizers. Exogenous inorganic selenium fertilizers were the earliest, most widely used, cheapest, and also the most basic selenium enrichment technology. The most commonly used inorganic selenium fertilizers included sodium selenite, sodium selenate, and chelated selenium. The biggest advantage of these inorganic selenium fertilizers was their low price, but improper use could cause seedling burn and yield reduction. Furthermore, only a small amount of selenium could be absorbed and utilized by plants, while a large amount remained in the soil, causing environmental pollution. More importantly, the selenium that the human body can absorb must be organic and biologically active. After consuming crops grown with inorganic selenium fertilizers, a large amount of selenium cannot be properly converted and excreted, but instead accumulates in the body, affecting health. For these reasons, inorganic selenium fertilizers are being phased out of the market.
[0004] Currently, cutting-edge selenium fertilizers include organic selenium fertilizers and nano-selenium fertilizers. Nano-selenium fertilizers are produced by chemically or microbially converting inorganic selenium into zero-valent selenium particles with a size of approximately 100-500 nanometers. Nano-selenium fertilizers, due to their small particle size, high permeability and conversion rate, and high safety performance, represent the safest and most reliable technology currently available. However, existing nano-selenium fertilizers have relatively limited efficacy, and their selenium conversion efficiency is influenced by numerous factors, including the physicochemical properties of the soil, the structure of the microbial community, fertilization, agronomic management, and the plant's growth stage. This significantly restricts the application and development of nano-selenium fertilizers.
[0005] To address the above problems, this invention is proposed. Summary of the Invention
[0006] To address the issues of limited efficacy and significant environmental impact on selenium conversion efficiency in the cultivation of selenium-enriched crops, the present invention introduces selenium into diatom cultivation. This allows diatoms to convert inorganic selenium into nano-selenium, which is then further converted into biochar fertilizer through pyrolysis and carbonization. The resulting selenium-doped carbonized diatom fertilizer solves the problems of limited efficacy and conversion rate being affected by numerous factors associated with existing nano-selenium fertilizers.
[0007] The technical solution adopted in this invention is:
[0008] The first aspect of the present invention provides a selenium-doped diatom biochar, characterized in that the selenium element in the selenium-doped diatom biochar exists in the form of nano-selenium, the size of the nano-selenium is 100-150nm, and the content of nano-selenium is not less than 15μg / 100g, and its structure is that nano-selenium dots are embedded in the surface and interior of the biochar.
[0009] The second aspect of this invention provides a method for preparing the selenium-doped diatom biochar described in the first aspect, comprising the following steps:
[0010] Step (1): Clean and screen the selenium-rich soil sample to obtain selenium-rich soil;
[0011] Step (2): Mix the selenium ore soil obtained in step (1) with distilled water in a certain proportion, extract the solution, remove the precipitate, collect the solution and sterilize it to obtain selenium ore soil extract.
[0012] Step (3): Inoculate the rice bran rhomboid algae into a culture medium containing the selenium ore soil extract obtained in step (2), culture it under a specific culture environment for a period of time, and collect the insoluble matter to obtain selenium-rich rice bran rhomboid algae diatoms;
[0013] Step (4): Freeze-dry the selenium-rich rhomboid algae diatoms obtained in step (3) to obtain selenium-rich rhomboid algae diatom powder;
[0014] Step (5): The selenium-rich diatom powder of *Rhizoctonia solani* obtained in step (4) is calcined in an inert gas atmosphere to obtain the selenium-doped diatom biochar.
[0015] Preferably, in step (2), the ratio of selenium-rich soil to distilled water is 100-400g of selenium-rich soil per 1L of distilled water.
[0016] The extraction and separation process involves heating selenium-rich soil and distilled water at 100°C for 2-4 hours, cooling to room temperature, allowing the insoluble matter to settle for 18-24 hours, collecting the solution portion, and repeating this step 2-3 times. All collected solution portions are then combined to complete the extraction and separation. Finally, all collected solutions are sterilized to obtain the selenium-rich soil extract.
[0017] Preferably, the culture medium in step (3) contains Ca(NO3)·4H2O, KNO3, MgSO4·7H2O, β-Na2glycerophosphate, and Vitamin B. 12 Biotin, Thiamine HCl, PIV, HEPES, Na2SiO3·9H2O, with the following concentrations: Ca(NO3)·4H2O, KNO3, MgSO4·7H2O, β-Na2glycerophosphate·5H2O 1 ml / L, Vitamin B. 12 The concentrations of the following ingredients were specified: Biotin 0.1 μg / L, Thiamine HCl 10 μg / L, PIV 6 ml / L, HEPES 0.5 g / L, and Na2SiO3·9H2O 0.1 g / L. The amount of the selenium ore soil extract added to the culture medium was 30 ml / L. The specific culture environment was defined as a culture temperature of 22-25℃ and a light intensity of 2000-4000 Lux. The culture time of the *Rhizoctonia solani* was 12-16 days. The method for collecting insoluble matter was to centrifuge at 5000-8000 rpm for 5-10 minutes using a high-speed centrifuge.
[0018] Preferably, the freeze drying in step (4) is performed by freeze drying at -60°C for 18-24 hours using a freeze dryer.
[0019] Preferably, the calcination in step (5) is carried out at a temperature of 400-600℃ for 1-3 hours to obtain selenium-doped diatom biochar.
[0020] The third aspect of this invention provides a method for preparing nano-selenium fertilizer using selenium-doped diatomaceous earth biochar as described in the first aspect: Selenium-doped diatomaceous earth biochar is placed in a disc granulator and granulated at a speed of 16-20 r / min for 10-30 min. During granulation, water is added to the disc granulator 3-4 times in proportion to a water-to-selenium-doped diatomaceous earth biochar mass ratio of 2.5-3:2. After granulation, particles with a diameter of 2-5 mm are sieved out; these particles are nano-selenium fertilizer.
[0021] The fourth aspect of this invention provides a nano-selenium fertilizer prepared by the preparation method described in the third aspect.
[0022] The fifth aspect of this invention provides the use of the nano-selenium fertilizer described in the fourth aspect for the cultivation of selenium-enriched rice.
[0023] Instruction manual illustrations
[0024] Figure 1 This is a SEM scan and HAADF scan of elemental distribution from the present invention.
[0025] Figure 2 X-ray diffraction pattern of selenium-doped diatom biochar of the present invention.
[0026] Figure 3 This is an image of the nano-selenium fertilizer of the present invention.
[0027] Figure 4 A comparison chart of rice biomass and selenium content.
[0028] Compared with the prior art, the present invention has the following beneficial effects:
[0029] 1. The selenium-doped diatomaceous earth biochar fertilizer of this invention contains two effective elements: selenium and silicon. Selenium is nanoscale and has biological activity, while silicon is the main element in diatomaceous earth biochar. When the selenium-doped diatomaceous earth biochar of this invention is applied to rice, not only can large-scale, standardized, and high-quality selenium-enriched rice be produced, but the rice's resistance to diseases and pests and its adaptability to adversity are also improved. At the same time, the biochar obtained by pyrolysis can also improve soil health by increasing soil conductivity, organic matter content, surface area, and nutrient availability. In addition, the microporous structure, surface functional groups, and intrinsic minerals of biochar can improve the adsorption and retention capacity of nutrients in the soil and avoid the loss of nutrients such as phosphorus. Using the selenium-doped diatomaceous earth biochar fertilizer of this invention does not require much additional adjustment to the soil conditions. Moreover, due to the reduction in nutrient loss and the increase in silicon, the nutrient conditions required for rice growth are effectively guaranteed, and the frequency of nutrient supplementation during rice cultivation is greatly reduced.
[0030] 2. This invention is the first to use diatoms to reduce nano-selenium, and is not limited to the reduction of selenite by existing highly reducing bacteria. The selenium conversion rate is significantly improved compared with commercially available nano-selenium fertilizers. Moreover, the diatoms of this invention are easy to cultivate and have a high reproduction rate, and the raw material cost is lower, making it more universal and economical.
[0031] 3. Chemically synthesized nano-selenium has poor stability, uneven particle size, and low biological activity, while bio-converted nano-selenium has good stability, uniform particle size, and good biological activity. However, it has stringent requirements for obtaining raw materials. The present invention utilizes a combination of bioreduced nano-selenium and chemically converted selenium-doped biochar to prepare nano-selenium fertilizer. In addition to providing effective selenium, the biochar formed by utilizing the nitrogen of diatoms is actually self-generated nitrogen-doped and selenium-doped biochar, providing sufficient nutrients for crop growth.
[0032] 4. The nano-selenium prepared by the method of the present invention has a size of about 100-150nm, and the content of nano-selenium is not less than 15μg / 100g. The nano-selenium dots are embedded in the surface and interior of biochar. The nano-selenium of this size has been tested and found to have a high conversion rate in rice seeds. In addition, since the nano-selenium is embedded in the surface and interior of biochar, the surface area in contact with the soil is increased, and this distribution pattern also provides support for its effective conversion. Detailed Implementation
[0033] The present invention will be further described below through embodiments, but is not limited to these embodiments. Experimental methods not specifically described in the embodiments generally use conventional conditions and conditions described in manuals, or conditions recommended by the manufacturer. The general equipment, materials, reagents, etc., used are all commercially available unless otherwise specified.
[0034] Example 1
[0035] A method for preparing selenium-doped diatom biochar includes the following steps:
[0036] (1) Clean and screen the selenium-rich soil sample to obtain selenium-mineral soil;
[0037] (2) Using diatoms such as Rhizophora scoparia as raw material, 100g of selenium ore soil was placed in a beaker, 1000ml of distilled water was added, the mouth of the bottle was sealed with a breathable plug, and the mixture was heated in boiling water in a water bath for 2 hours. After cooling, the mixture was allowed to settle for 18 hours. This process was repeated twice. Then the mixture was filtered, and the solution was taken. The solution was sterilized in an autoclave to obtain the selenium ore soil extract. The selenium ore soil extract can be stored in a refrigerator at 4℃ for later use.
[0038] (3) Add the selenium ore soil extract from (2) to the pre-prepared culture medium at a rate of 30 ml / L, and co-culture it with the rhomboid algae for 12 days at a temperature of 22°C and a light intensity of 2000 Lux to obtain the selenium-rich rhomboid algae diatom.
[0039] The components and concentrations of the culture medium are as follows: Ca(NO3)·4H2O, KNO3, MgSO4·7H2O, β-Na2glycerophosphate·5H2O are 1 ml / L, and Vitamin B is present. 12 Biotin was 0.1 μg / L, Thiamine HCl was 10 μg / L, PIV was 6 ml / L, HEPES was 0.5 g / L, and Na2SiO3·9H2O was 0.1 g / L.
[0040] (4) The selenium-rich rhomboid algae diatoms prepared in (3) were centrifuged at 5000 rpm for 5 minutes using a high-speed centrifuge. The insoluble matter was collected and placed in a freeze dryer and freeze-dried at -60℃ for 18 hours to obtain selenium-rich rhomboid algae diatom powder.
[0041] (5) The selenium-rich rhomboid algae diatom powder obtained in (4) was calcined in a tube furnace at 400°C for 1 hour under a nitrogen atmosphere at a heating rate of 5°C / min to obtain the selenium-doped diatom biochar.
[0042] Please refer to Figure 1 The prepared selenium-doped diatom biochar was subjected to SEM scanning and HAADF (Zeiss GIMINER 300) elemental distribution scanning. The appearance morphology shows that the selenium-doped diatom biochar prepared in this embodiment maintains a relatively complete rhomboid algal structure. The composition is a biochar membrane wrapped by a selenium-doped silica shell. It can be seen that C, Si, O, Se and N elements are continuously distributed.
[0043] Please refer to Figure 2 The prepared selenium-doped diatomaceous biochar was subjected to X-ray diffraction (D / max 2500, Cu Kα1). NC represents undoped diatomaceous biochar calcined in a tube furnace, while SeC represents selenium-doped diatomaceous biochar. XRD results confirmed its disordered structure, with two broad peaks: a high-intensity peak at approximately 25° and a weak peak at approximately 43°. These two peaks can be attributed to the (002) and (100) crystal planes of the graphite carbon. Compared to NC, the peak value of SeC is located at approximately 25° in the (002) plane and shifts and broadens towards lower angles, indicating that more selenium active defects have been incorporated into the material.
[0044] Application Example 1
[0045] The selenium-doped diatomaceous earth biochar prepared in Example 1 was placed in a disc granulator and granulated at 16 r / min for 10 min. During granulation, water was added three times in proportion to the water-selenium-doped diatomaceous earth biochar mass ratio of 2.5:2. Particles with a diameter of 2-5 mm were sieved out. These particles are the nano-selenium fertilizer and are named Sample 1. Please refer to... Figure 3 The finished product image of the nano-selenium fertilizer shows that it consists of uniform gray-black granules in regular spherical shape.
[0046] In early April 2023, a rice-growing area in Liangping District, Chongqing, was selected as an experimental field. This field was then divided into three equal areas: Area A, Area B, and Area C, with each area measuring 10 square meters. 2 .
[0047] The specific steps for applying sample #1 to region A are as follows:
[0048] (1) Selected high-yielding rice seedlings of Chongqing Yixiangyou 2115 with good growth and similar height and planted them in experimental field A area;
[0049] (2) One month after planting, apply sample #1 to area A at a rate of 0.3 kg / m². 2 Apply 0.075 kg / m² of sample #1 again after half a month. 2
[0050] (3) Harvest the rice plants cultivated in step (2) in early July, separate them according to different organs (roots, stems, and seeds), rinse them with deionized water, dry them, grind them, measure the biomass of each plant, and then take the average value. The biomass of roots, stems, and seeds were found to be 39.6, 49.2, and 17.5 grams per plant, respectively.
[0051] (4) The roots, stems, and seeds separated in (3) were digested in an electric furnace with a mixed acid (nitric acid: perchloric acid = 1:1), and the selenium content in the solution was determined by atomic fluorescence spectrometry. The selenium contents of the roots, stems, and seeds were 9.4, 27.3, and 8.5 mg / kg, respectively.
[0052] Comparative Example 1
[0053] Compared with Application Example 1, this comparative example only replaced the nano-selenium fertilizer with a commercially available nano-selenium fertilizer product (model JSG-26-5) prepared by converting selenite with highly reducing bacteria as sample #2. Sample #2 was applied to region B, and the remaining culture steps and parameters were the same as in Application Example 1. The final biomass of roots, stems and seeds were 32.8, 45.2 and 10.2 g / plant, respectively.
[0054] The selenium content of the root, stem, and seeds is 1.2, 3.5, and 1.6 mg / Kg, respectively.
[0055] Comparative Example 2
[0056] The composite nano-selenium fertilizer was obtained by mixing sample 1# from Application Example 1 with commercially available nitrogen fertilizer and named sample 3#. The mass ratio of sample 1# to nitrogen fertilizer in the composite nano-selenium fertilizer was 1:1, and the nitrogen fertilizer was converted into effective nitrogen content. Sample 3# was applied to region C, and the cultivation steps and parameters were the same as in Application Example 1. The biomass of roots, stems and seeds obtained were 34, 46 and 12.8 g / plant, respectively.
[0057] The selenium content of the roots, stems, and seeds was 2.5, 6.3, and 2.6 mg / kg, respectively.
[0058] Conclusion: (Refer to) Figure 4(a) It can be seen that the growth-promoting effect of samples 1 and 3 on rice is higher than that of sample 2, indicating that the application effect of the nano selenium fertilizer of the present invention is higher than that of commercially available selenium fertilizer. In addition, the selenium content is also better. It can also be seen that the nano selenium fertilizer of the present invention is also suitable for mixing with ordinary nitrogen fertilizer to fully supplement the nutrients required by rice.
[0059] Example 2
[0060] A method for preparing selenium-doped diatom biochar includes the following steps:
[0061] (1) Clean and screen the selenium-rich soil sample to obtain selenium-mineral soil;
[0062] (2) Using Rhizophora glutenosae as raw material, 400 g of selenium ore soil was placed in a beaker, 1000 ml of distilled water was added, the mouth of the bottle was sealed with a breathable plug, and the mixture was heated in boiling water in a water bath for 4 hours. After cooling, the mixture was allowed to settle for 24 hours. This process was repeated 3 times. Then the mixture was filtered, and the solution was taken. The solution was sterilized in an autoclave to obtain the selenium ore soil extract. The selenium ore soil extract can be stored in a refrigerator at 4°C for later use.
[0063] (3) Add the selenium ore soil extract from (2) to the pre-prepared culture medium at a rate of 30 ml / L, and co-culture it with the glutinous rice rhomboid algae for 14 days at a temperature of 25°C and a light intensity of 4000 Lux to obtain selenium-rich glutinous rice rhomboid algae diatoms.
[0064] The components and concentrations of the culture medium are as follows: Ca(NO3)·4H2O, KNO3, MgSO4·7H2O, β-Na2glycerophosphate·5H2O are 1 ml / L, and Vitamin B is present. 12 Biotin was 0.1 μg / L, Thiamine HCl was 10 μg / L, PIV was 6 ml / L, HEPES was 0.5 g / L, and Na2SiO3·9H2O was 0.1 g / L.
[0065] (4) The selenium-rich rhomboid algae diatoms prepared in (3) were centrifuged at 8000 rpm for 5 minutes using a high-speed centrifuge. The insoluble matter was collected and placed in a freeze dryer and freeze-dried at -60℃ for 24 hours to obtain selenium-rich rhomboid algae diatom powder.
[0066] (5) The selenium-rich rhomboid algae diatom powder obtained in (4) was calcined in a tube furnace at 600°C for 3 hours under a nitrogen atmosphere at a heating rate of 5°C / min to obtain the selenium-doped diatom biochar.
[0067] Application Example 2
[0068] The selenium-doped diatomaceous earth biochar prepared in Example 2 above was placed in a disc granulator and rotated at 20 r / min for 30 min. During granulation, water was added to the disc granulator four times according to the mass ratio of water to selenium-doped diatomaceous earth biochar of 3:2. Particles with a particle size of 2-5 mm were screened out. These particles are nano-selenium fertilizer and named sample R1#.
[0069] In early April 2023, a rice-growing area in Yantai Town, Dianjiang County, Chongqing Municipality was selected as an experimental field. The experimental field was divided into three equal areas: Area A, Area B, and Area C, with each area being 10 square meters. 2 .
[0070] The R1# sample was applied to region A, and the specific steps are as follows:
[0071] (1) Selected high-yielding rice seedlings of Chongqing Yixiangyou 2115 with good growth and similar height and planted them in experimental field A area;
[0072] (2) One month after planting, apply sample #1 to area A at a rate of 0.3 kg / m². 2 Two weeks later, apply R1# sample at a rate of 0.075 kg / m². 2
[0073] (3) Harvest the rice plants cultivated in step (2) in early July, separate them according to different organs (roots, stems, and seeds), rinse them with deionized water, dry them, grind them, measure the biomass of each plant and then take the average value, and obtain the biomass of roots, stems and seeds as 45.3, 55.2 and 24.5 g / plant, respectively.
[0074] (4) The roots, stems and seeds separated in (3) were digested in an electric furnace with a mixed acid (nitric acid: perchloric acid = 1:1) and the selenium content in the solution was determined by atomic fluorescence spectrometry. The selenium contents of the roots, stems and seeds were 14.4, 27.8 and 13.5 mg / Kg, respectively.
[0075] Comparative Example 3
[0076] Compared with Application Example 2, this comparative example only replaced the nano-selenium fertilizer with a commercially available nano-selenium fertilizer product (model JSG-26-5) prepared by converting selenite with highly reducing bacteria as sample R2#. Sample R2# was applied to region B, and the remaining culture steps and parameters were the same as in Application Example 1. The final biomass of roots, stems and seeds were 37.6, 50.2 and 14.7 g / plant, respectively.
[0077] The selenium content of the root, stem, and seeds is 6.1, 9.5, and 5.9 mg / kg, respectively.
[0078] Comparative Example 4
[0079] The R1# sample from Application Example 2 was mixed with commercially available nitrogen fertilizer to obtain a composite nano-selenium fertilizer, named R3# sample. The mass ratio of R1# sample to nitrogen in the composite nano-selenium fertilizer was 1:1. The nitrogen, potassium and phosphorus fertilizers were converted into effective nitrogen content. The R3# sample was applied to region C. The cultivation steps and parameters were the same as in Application Example 1. The biomass of roots, stems and seeds obtained were 39, 51 and 18.6 g / plant, respectively.
[0080] The selenium content of the root, stem, and seeds is 7.5, 11.3, and 7.2 mg / Kg, respectively.
[0081] Conclusion: (Refer to) Figure 4 (b) It can be seen that samples 1# and 3# have a higher promoting effect on rice growth than sample 2#, indicating that the application effect of the nano-selenium fertilizer of the present invention is higher than that of commercially available selenium fertilizer. Furthermore, the selenium content is also superior. It can also be seen that the nano-selenium fertilizer of the present invention is suitable for mixing with ordinary nitrogen fertilizer to comprehensively supplement the nutrients required by rice. It can be seen that the results obtained by the nano-selenium fertilizer of the present invention in a test field in another region are extremely similar. The diatomaceous earth fertilizer of the present invention has a higher biological promoting effect on rice than ordinary commercially available selenium fertilizer and also promotes the distribution of selenium in rice.
Claims
1. A selenium-doped diatom biochar, characterized in that, The selenium element in the selenium-doped diatom biochar exists in the form of nano selenium, the size of the nano selenium is 100-150 nm, the content of the nano selenium is not less than 15 μg / 100 g, and the structure of the nano selenium is that nano selenium points are inlaid on the surface and inside of the biochar.
2. A method of producing selenium-doped diatom biochar according to claim 1, characterized in that, The preparation method comprises the following steps: Step (1): washing and screening selenium-rich soil samples to obtain selenium mine soil; Step (2): mixing the selenium mine soil obtained in step (1) with distilled water in a certain proportion, then extracting, collecting the solution part after removing the precipitate, sterilizing the solution part, and obtaining a selenium mine soil extract; Step (3): inoculating Cymatonepha valgaris into a culture medium containing the selenium mine soil extract obtained in step (2), culturing for a period of time under a specific culture environment, collecting insoluble substances, and obtaining selenium-rich Cymatonepha valgaris diatoms; Step (4): freeze-drying the selenium-rich Cymatonepha valgaris diatoms obtained in step (3) to obtain selenium-rich Cymatonepha valgaris diatom powder; Step (5): calcining the selenium-rich Cymatonepha valgaris diatom powder obtained in step (4) under an inert gas atmosphere to obtain the selenium-doped diatom biochar.
3. The preparation method according to claim 1, characterized in that, The ratio of the selenium mine soil to the distilled water in step (2) is 100-400 g of the selenium mine soil per 1 L of the distilled water; The extraction and separation is performed by heating the selenium mine soil and the distilled water at 100 DEG C for 2-4 hours, cooling to room temperature, allowing the insoluble substances to self-settle for 18-24 hours, collecting the solution part, repeating the step 2-3 times, combining all the collected solution parts to complete the extraction and separation, and then sterilizing all the collected solutions to obtain the selenium mine soil extract.
4. The method of claim 1, wherein, The culture medium in step (3) comprises Ca(NO3)4H2O, KNO3, MgSO47H2O, β-Na2 glycerophosphate, Vitamin B 12 , Biotin, Thiamine HCL, PIV, HEPES, Na2SiO39H2O, wherein the content of each component is: Ca(NO3)4H2O, KNO3, MgSO47H2O, β-Na2 glycerophosphate5H2O is 1ml / L, Vitamin B 12 , Biotin is 0.1μg / L, Thiamine HCL is 10μg / L, PIV is 6ml / L, HEPES is 0.5g / L, Na2SiO39H2O is 0.1g / L, and the added amount of the selenium ore soil extract in the culture medium is 30ml / L; The specific culture environment refers to a culture temperature of 22-25 DEG C and a light intensity of 2000-4000 Lux; The culture time of the Cymatonepha valgaris is 12-16 days; The method for collecting the insoluble substances is to use a high-speed centrifuge to centrifuge at 5000-8000 rpm for 5-10 minutes.
5. The preparation method according to claim 1, characterized in that, The freeze-drying in step (4) is performed by using a freeze dryer to freeze-dry at-60 DEG C for 18-24 hours.
6. The method of claim 1, wherein, The calcination in step (5) is performed at a temperature of 400-600 DEG C for 1-3 hours to obtain the selenium-doped diatom biochar.
7. A method for preparing nano-selenium fertilizer using the selenium-doped diatom biochar according to claim 1, characterized in that, The selenium-doped diatom biochar is placed in a disc granulator and granulated at a rotating speed of 16-20 r / min for 10-30 minutes, water is added into the disc granulator 3-4 times during the granulation according to a water:selenium-doped diatom biochar mass ratio of 2.5-3:2, and granules with a particle size of 2-5 mm are screened out at the end of the granulation, which are the nano selenium fertilizer.
8. The nano selenium fertilizer prepared by the method of claim 7.
9. The use of the nano selenium fertilizer of claim 8 for selenium-rich rice planting.