Selenium-loaded composite carbon fertilizer and application of selenium-loaded composite carbon fertilizer in lead-zinc associated contaminated soil remediation and selenium-rich safe planting of crops

Abstract

The invention discloses a selenium-loaded composite carbon fertilizer and application thereof in lead and zinc associated contaminated soil remediation and selenium-enriched safe planting of crops. The selenium-loaded composite carbon fertilizer is prepared by compounding pomacea canaliculata shell biochar and modified biochar thereof, loading selenium, and co-composting with bidens pilosa and chicken manure. The selenium-loaded composite carbon fertilizer is used for repairing and improving lead and zinc associated contaminated soil, the purposes of fixing lead and zinc in the soil, increasing the selenium content of crops and increasing the content of nutrients such as nitrogen and phosphorus are achieved, and selenium-enriched safe planting of the crops in the lead and zinc associated contaminated soil is achieved on the whole. The selenium-loaded composite carbon fertilizer is applied to soil, so that the ecological risk caused by application of a soil passivating agent is avoided, the excessive use of chemical fertilizer is reduced, and the sustainable development of agriculture is realized; on the other hand, resource utilization of agricultural wastes is facilitated; the fertilizer plays an important role in the aspects of guaranteeing safe production of crops, promoting growth of crops, strengthening soil nutrients, achieving production of selenium-rich crops, repairing lead and zinc pollution of soil and the like.

Description

Technical Field

[0001] The present invention relates to the technical fields of composting, agricultural soil improvement and crop planting, and more specifically, to a selenium-loaded composite carbon fertilizer and its application in the remediation of lead-zinc associated polluted soil and the selenium-enriched safe planting of Chinese chives. Background Art

[0002] Heavy metals in soil, due to their cumulative and non-biodegradable properties, will change the soil function and affect the quality of agricultural crops after long-term accumulation, posing a major threat to plants and humans through the food chain.

[0003] Lead and zinc are common heavy metal pollutants with large reserves. Long-term exposure to high levels of lead and zinc pollution can cause damage to the liver, kidneys, nervous system and immune system, and may also lead to death. Currently, heavy metals lead and zinc can be removed from soil through biological methods (phytoremediation) and chemical methods (leaching). At the same time, the toxicity of soil heavy metals can be reduced through passivation methods. Composting is a biological method that immobilizes heavy metals in soil and sediments by enhancing the biological transformation of organic matter. However, its effect is limited, and it cannot effectively reduce the heavy metal risk in lead-zinc associated polluted soil; at the same time, the remediation effect is affected by factors such as soil pH and organic matter content, and the stability is poor. Biochar remediation is a remediation method that adds biochar to the soil and uses its high specific surface area and surface functional groups to adsorb and fix heavy metals. However, the adsorption capacity of biochar is closely related to the raw materials and preparation conditions (such as pyrolysis temperature, pyrolysis time, etc.), and there are significant differences in the remediation effects of different types of biochar on lead-zinc associated polluted soil. Although composting and biochar have certain application values in remediating soil heavy metals, problems such as their limited remediation effects, high costs, long remediation cycles, and potential secondary pollution risks limit their wide application. Therefore, it is of great significance to develop an efficient, economical, environmentally friendly and stable soil heavy metal remediation technology.

[0004] Selenium (Se) is an essential trace element for humans and animals, involved in various biological functions, such as enhancing immunity, antioxidation, and heavy metal detoxification. When the intake of selenium is too low, it can cause various health disorders. Due to the uneven distribution of Se in the soil, long-term insufficient intake of selenium by humans can lead to the development of diseases such as Keshan disease, Kashin-Beck disease, cancer, cardiovascular diseases, liver diseases, and cataracts. Approximately 0.5 to 1 billion people globally do not meet the daily Se intake requirements. To ensure an appropriate Se dose for humans, the application of Se fertilizers and increasing the Se content in plants have become the main measures. Applying selenium fertilizers to the soil is the main way to obtain Se-rich plants. However, direct spraying can cause the loss of selenium through soil leaching and surface runoff, and at the same time, it will volatilize into the atmosphere, resulting in a significant reduction in the bioavailability of Se. Therefore, it is very necessary to develop a technology for repairing lead-zinc associated polluted soil and safely growing Se-rich crops in lead-zinc associated polluted soil. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects and deficiencies existing in the prior art and provide a preparation method of selenium-loaded composite carbon fertilizer.

[0006] Another purpose of the present invention is to provide the selenium-loaded composite carbon fertilizer prepared by the above preparation method.

[0007] Another purpose of the present invention is to provide the application of the selenium-loaded composite carbon fertilizer in the repair and improvement of lead-zinc associated polluted soil and in the safe cultivation of Se-rich crops in lead-zinc associated polluted soil.

[0008] The above purposes of the present invention are achieved by the following technical solutions: A preparation method of selenium-loaded composite carbon fertilizer, characterized by comprising the following steps: S1. Clean, dry, crush, and sieve the shells of Pomacea canaliculata, and pyrolyze to obtain Pomacea canaliculata shell biochar (BC); S2. Grind and sieve the Pomacea canaliculata shell biochar, and use a mixed solution of Fe 3+ and Fe 2+ to prepare iron-modified biochar (MBC) by chemical co-precipitation in an alkaline medium; S3. Add the BC in step S1 and the MBC in step S2 to a Se(IV) solution with a concentration of 0.01 - 0.06 g / L according to a mass ratio of 1 - 9:1, adjust the pH to neutral, and oscillate for adsorption to obtain selenium-loaded composite biochar (SeBC); S4. Use Bidens pilosa and chicken manure with a carbon-nitrogen ratio of 20 - 30:1 as composting raw materials, add 4% - 12% by mass of selenium-loaded composite biochar, adjust the moisture content to 60 - 70%, mix evenly, and compost for 34 - 36 days to obtain selenium-loaded composite carbon fertilizer.

[0009] The present invention uses the shell of the invasive animal Pomacea canaliculata as a raw material, prepares pyrolytic biochar of Pomacea canaliculata shell by pyrolysis method, then modifies the pyrolytic biochar with iron to obtain MBC, then mixes the pyrolytic biochar of Pomacea canaliculata shell and MBC to form a composite biochar and loads selenium element to obtain SeBC, and then adds SeBC to the compost raw materials of Bidens pilosa and chicken manure for composting to obtain selenium-loaded composite carbon fertilizer. The present invention creatively uses BC and MBC to form a composite biochar by mixing, and then loads selenium. The selenium-loaded biochar can fix selenium on the surface or pores of the biochar to achieve slow release of selenium. This slow-release mechanism can avoid the rapid loss or fixation of selenium during composting due to direct application, thereby improving the biological availability of selenium. In addition, the specific surface area, pore size and pore volume of the biochar after selenium loading are better than those before loading, thereby enhancing the ability of the biochar to adsorb or fix environmental pollutants in the soil, thus reducing the ecological risk; at the same time, composting provides a larger support structure for the biochar, enabling the modified biochar to be better dispersed in the soil system, increasing the contact area between the biochar, selenium and the soil, strengthening the adsorption and fixation ability of the three to heavy metals, and reducing the loss of nutrients and trace elements; on the other hand, the combination of the porous structure of the biochar and the activation effect of composting on soil microorganisms can attract more types and quantities of microorganisms to participate in the crop growth process, accelerating the transformation and absorption of soil nutrients. Applying the selenium-loaded composite carbon fertilizer to lead-zinc associated polluted soil can not only promote crop growth, enhance crop nutritional quality and soil physical and chemical properties, but also effectively reduce the lead-zinc heavy metal pollution in lead-zinc associated polluted soil. These effects jointly promote the healthy growth of crops, with potential economic and environmental benefits. Thus, it realizes the remediation of lead-zinc associated polluted soil and the selenium-enriched safe planting of crops in lead-zinc associated polluted soil.

[0010] Further, the sieving in step S1 is sieving through a 50-70 mesh sieve.

[0011] Preferably, the sieving in step S1 is sieving through a 60 mesh sieve.

[0012] Further, the pyrolysis in step S1 is pyrolysis at 300-400 °C for 1.5-2.5 h.

[0013] Preferably, the pyrolysis in step S1 is pyrolysis at 342 °C for 2 h.

[0014] Further, the sieving in step S2 is sieving through a 90-110 mesh sieve.

[0015] Preferably, the sieving in step S2 is sieving through a 100 mesh sieve.

[0016] Further, Fe in step S2 3+ and Fe 2+ in the mixed solution, Fe 3+The molar ratio with Fe 2+ is 2.5 to 3.

[0017] Preferably, the Fe 3+ in step S2 and Fe 2+ in the mixed solution, the molar ratio of Fe 3+ to Fe 2+ is 2.72.

[0018] Furthermore, in step S3, the mass ratio of BC to MBC is 5 to 9:1.

[0019] Preferably, in step S3, the mass ratio of BC to MBC is 7:1.

[0020] Furthermore, in step S3, the solid-liquid ratio of BC and MBC to the Se(IV) solution is 0.4 to 0.6:1 (g / mL).

[0021] Preferably, in step S3, the solid-liquid ratio of BC and MBC to the Se(IV) solution is 0.5:1 (g / mL).

[0022] Furthermore, the oscillating adsorption in step S3 is carried out at 24 to 26 °C with an oscillation speed of 170 to 190 r / min for 22 to 26 h.

[0023] Preferably, the oscillating adsorption in step S3 is carried out at 25 °C with an oscillation speed of 180 r / min for 24 h.

[0024] Furthermore, in step S4, the ratio of Bidens pilosa L. to chicken manure = 3:1 (mass ratio), that is, the carbon-nitrogen ratio is 25:1.

[0025] Furthermore, the selenium-loaded composite biochar is added in a mass proportion of 10%.

[0026] Furthermore, in step S4, the moisture content is adjusted to 65%.

[0027] Furthermore, the composting days are 35 days.

[0028] Preferably, the composting reaction is carried out using a commercially available conventional aerobic composting container.

[0029] The present invention also provides the selenium-loaded composite carbon fertilizer prepared as described above.

[0030] The present invention selects the passivation efficiency of lead-zinc associated polluted soil and the lead and zinc contents in the above-ground and underground parts of leeks for effect verification. The selenium-loaded compound carbon fertilizer is fully mixed evenly and then applied to the lead-zinc associated polluted soil. The lead and zinc concentrations of the lead-zinc associated polluted soil involved are 90-140 mg / kg and 200-260 mg / kg respectively. Through practical verification, the selenium-loaded compound carbon fertilizer described in the present invention can effectively passivate lead and zinc elements in the soil and improve the environmental and quality conditions of the lead-zinc associated polluted soil. Especially when leeks are planted in the soil applied with the selenium-loaded compound carbon fertilizer, it can promote the growth of leeks, ensure the safe production of leeks, reduce the accumulation of lead and zinc in various parts of leeks, and ensure the quality and safety of agricultural products. The selenium-loaded compound carbon fertilizer is rich in various nutrient elements such as organic matter, nitrogen, phosphorus, and potassium, can significantly improve the nutrient status and physical and chemical properties of the soil, enhance the biodiversity of the soil, and improve its cation exchange capacity, thereby promoting the absorption of nutrients by plant roots. At the same time, it plays a positive role in soil remediation and reducing heavy metal pollution in leeks. Therefore, applying the selenium-loaded compound carbon fertilizer can not only improve the soil environment, but also effectively reduce the heavy metal content in leeks, ensure its safety, and provide important support for the realization of sustainable agriculture.

[0031] Therefore, the present invention also provides the application of the selenium-loaded compound carbon fertilizer in the remediation and improvement of lead-zinc associated polluted soil and in the safe cultivation of selenium-enriched crops in lead-zinc associated polluted soil.

[0032] The present invention also provides a method for remediating and improving lead-zinc associated polluted soil and for safely cultivating selenium-enriched crops in lead-zinc associated polluted soil, which includes the following steps: S1. Apply the selenium-loaded compound carbon fertilizer described in any one of the above according to a mass ratio of 10%-56% to the lead-zinc associated polluted soil, add a compound fertilizer (N-P2O5-K2O, 15-15-15) accounting for 5%-31% of the dry weight of the selenium-loaded compound carbon fertilizer, mix evenly, adjust the soil water content to 15%-45%, and let it stand for 3-5 days to make the soil fully moist; the lead content in the lead-zinc associated polluted soil is 90-140 mg / kg, and the zinc content is 200-360 mg / kg; S2. Transplant the soaked crop seedlings into the soil in step S1, cover them with soil and water them to moisten, keep the temperature at 15°C-25°C, keep the soil water content at 15%-45%, and carry out the planting for 105-120 days, and harvest 1-3 times during this period.

[0033] The compound fertilizer (N-P2O5-K2O, 15-15-15): is a ternary compound fertilizer, in which the contents of nitrogen (N), phosphorus (in the form of phosphorus pentoxide P2O5), and potassium (in the form of potassium oxide K2O) are all 15%, and the total nutrient content is 45%. This fertilizer is widely applicable to a variety of crops, can provide balanced nutritional support, and promote the growth and development of plants. During the application of selenium-loaded compound carbon fertilizer and the compound fertilizer (N-P2O5-K2O, 15-15-15) to heavy metal-contaminated soil, it is crucial to reasonably regulate the ratio of compost to soil, the application rate of the compound fertilizer (N-P2O5-K2O, 15-15-15), and the soil water content. Generally, the pollution concentration ranges of lead and zinc are 90-140 mg / kg and 200-360 mg / kg respectively. Generally speaking, the addition amount of selenium-loaded compound carbon fertilizer should be specifically controlled between 10% and 56% of the soil quality, aiming to promote plant growth, improve soil nutrients, and repair heavy metal-contaminated soil. The application ratio of the compound fertilizer (N-P2O5-K2O, 15-15-15) is adjusted according to the nutrient content of the compost itself and the actual soil demand. The compound fertilizer (N-P2O5-K2O, 15-15-15) accounts for 5%-31% of the dry weight of the compost to ensure the balanced supply of key nutrients such as nitrogen, phosphorus, and potassium in the selenium-loaded compound carbon fertilizer system. In addition, the soil water content, as a factor affecting the composting effect and heavy metal immobilization, is more suitable in the range of 15%-45%. Therefore, regulating the ratio of selenium-loaded compound carbon fertilizer to heavy metal-contaminated soil, the addition amount of the compound fertilizer (N-P2O5-K2O, 15-15-15), and the soil water content constitutes an effective way to achieve the promotion of plant growth by selenium-loaded compound carbon fertilizer, the enhancement of crop and soil nutrients, and the safe utilization of heavy metal-contaminated soil.

[0034] Furthermore, the selenium-loaded compound carbon fertilizer is 10%-25% of the soil quality; the compound fertilizer accounts for 5%-13% of the dry weight of the selenium-loaded compound carbon fertilizer; the soil water content is 15%-25%; the content of lead in the lead-zinc associated contaminated soil is 90-110 mg / kg, and the content of zinc is 200-270 mg / kg.

[0035] Preferably, the selenium-loaded compound carbon fertilizer is 20% of the soil quality; the compound fertilizer accounts for 10% of the dry weight of the selenium-loaded compound carbon fertilizer; the soil water content is 20%; the content of lead in the lead-zinc associated contaminated soil is 95 mg / kg, and the content of zinc is 230 mg / kg.

[0036] Furthermore, the selenium-loaded composite carbon fertilizer accounts for 26% to 40% of the soil quality; the composite fertilizer accounts for 14% to 22% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content is 26% to 35%; the content of lead in the lead-zinc associated polluted soil is 111 to 130 mg / kg, and the content of zinc is 271 to 310 mg / kg.

[0037] Preferably, the selenium-loaded composite carbon fertilizer accounts for 30% of the soil quality; the composite fertilizer accounts for 20% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content is 30%; the content of lead in the lead-zinc associated polluted soil is 115 mg / kg, and the content of zinc is 290 mg / kg.

[0038] Furthermore, the selenium-loaded composite carbon fertilizer accounts for 41% to 56% of the soil quality; the composite fertilizer accounts for 23% to 31% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content is 36% to 45%; the content of lead in the lead-zinc associated polluted soil is 131 to 140 mg / kg, and the content of zinc is 311 to 360 mg / kg.

[0039] Preferably, the selenium-loaded composite carbon fertilizer accounts for 45% of the soil quality; the composite fertilizer accounts for 30% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content is 40%; the content of lead in the lead-zinc associated polluted soil is 136 mg / kg, and the content of zinc is 330 mg / kg.

[0040] Furthermore, the crop is Chinese chives.

[0041] Furthermore, the Chinese chive variety is Small-leaf Fragrant Chives, Broad-leaf Snow Chives, Hanzhong Winter Chives, Single Root Red, or Hanzhong Winter Chives.

[0042] Furthermore, soak the Chinese chive seedlings in pure water at 25 to 27 °C and cultivate for 36 to 72 h before planting.

[0043] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses the shells of the invasive animal Pomacea canaliculata as raw materials, prepares pyrolytic biochar from Pomacea canaliculata shells by pyrolysis method, then modifies the pyrolytic biochar with iron to obtain MBC, and then mixes the pyrolytic biochar of Pomacea canaliculata shells and MBC to form a composite biochar and load selenium element to obtain SeBC. Then, SeBC is added to the composting raw materials of Bidens pilosa and chicken manure for composting to obtain selenium-loaded composite carbon fertilizer. The present invention creatively uses BC and MBC to form a composite biochar by mixing, and then loads selenium. The selenium-loaded biochar can fix selenium on the surface or pores of the biochar, realizing the slow release of selenium. This slow-release mechanism can avoid the rapid loss or fixation of selenium during the composting process due to direct application, thereby improving the biological availability of selenium. In addition, the specific surface area, pore size and pore volume of the biochar after selenium loading are better than those before loading, thus enhancing the ability of the biochar to adsorb or fix environmental pollutants in the soil, thereby reducing the ecological risk; at the same time, the compost provides a larger support structure for the biochar, enabling the modified biochar to be better dispersed in the soil system, increasing the contact area between the biochar, selenium and the soil, and strengthening and improving the adsorption and fixation ability of the three to heavy metals, reducing the loss of nutrients and trace elements; on the other hand, the combination of the porous structure of the biochar and the activation effect of the compost on soil microorganisms can attract more types and quantities of microorganisms to participate in the crop growth process, accelerating the transformation and absorption of soil nutrients.

[0044] (2) The selenium-loaded composite carbon fertilizer of the present invention can effectively passivate lead and zinc elements in the soil when applied to the soil, and improve the environmental and quality conditions of lead-zinc associated polluted soil. Especially when crops are planted in the soil where the selenium-loaded composite carbon fertilizer is applied, it can promote the growth of crops, ensure the safe production of crops, reduce the accumulation of lead and zinc in various parts of the crops, and ensure the quality and safety of agricultural products. The selenium-loaded composite carbon fertilizer is rich in various nutrient elements such as organic matter, nitrogen, phosphorus and potassium, can significantly improve the nutrient status and physical and chemical properties of the soil, enhance the biodiversity of the soil, and improve its cation exchange capacity, thereby promoting the absorption of nutrients by plant roots. At the same time, it plays a positive role in soil remediation and reducing heavy metal pollution of crops. Therefore, applying the selenium-loaded composite carbon fertilizer can not only improve the soil environment, but also effectively reduce the heavy metal content in crops, ensure its safety, and provide important support for the realization of sustainable agriculture. Description of the Drawings

[0045] Figure 1 are the surface structure characteristics and energy dispersive X-ray (EDS) spectra of the SeBC material. Among them, Figure 1 (I) is the morphology of MBC biochar, (II) is the morphology of SeBC biochar, (III) is the EDS spectrum and element content percentage of MBC, and (IV) is the EDS spectrum and element content percentage of SeBC.

[0046] Figure 2 To promote the effect of growth-promoting compound carbon fertilizer on the plant height of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0047] Figure 3 To promote the effect of growth-promoting compound carbon fertilizer on the leaf length of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0048] Figure 4 To promote the effect of growth-promoting compound carbon fertilizer on the biomass of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0049] Figure 5 To promote the effect of growth-promoting compound carbon fertilizer on the chlorophyll of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0050] Figure 6 To promote the effect of growth-promoting compound carbon fertilizer on the protein content of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0051] Figure 7 To promote the effect of growth-promoting compound carbon fertilizer on the CAT activity of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0052] Figure 8 To promote the effect of growth-promoting compound carbon fertilizer on the SOD activity of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0053] Figure 9 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the selenium content in the leaves of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0054] Figure 10 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the selenium content in the roots of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0055] Figure 11 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the pH and electrical conductivity (EC) of lead-zinc polluted soil Figure 12 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the organic matter (OM) content of lead-zinc polluted soil.

[0056] Figure 13 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the total nitrogen content of lead-zinc associated polluted soil.

[0057] Figure 14 To enhance the effect of nutrient-enhanced compound carbon fertilizer on the total phosphorus content of lead-zinc associated polluted soil.

[0058] Figure 15 To safely utilize the effect of compound carbon fertilizer on the lead content in the leaves of different varieties of Chinese chives in lead-zinc associated polluted soil.

[0059] Figure 16Effect of composite carbon fertilizer for safe utilization on zinc content in leaves of different varieties of Chinese chives grown in lead-zinc associated polluted soil.

[0060] Figure 17 Effect of composite carbon fertilizer for safe utilization on lead content in roots of different varieties of Chinese chives grown in lead-zinc associated polluted soil.

[0061] Figure 18 Effect of composite carbon fertilizer for safe utilization on zinc content in roots of different varieties of Chinese chives grown in lead-zinc associated polluted soil.

[0062] Figure 19 Effect of composite carbon fertilizer for safe utilization on urease activity in lead-zinc associated polluted soil.

[0063] Figure 20 Effect of composite carbon fertilizer for safe utilization on sucrase activity in lead-zinc associated polluted soil.

[0064] Figure 21 Effect of composite carbon fertilizer for safe utilization on acid phosphatase activity in lead-zinc associated polluted soil.

[0065] Figure 22 Effect of composite carbon fertilizer for safe utilization on lead speciation in polluted soil after three harvests.

[0066] Figure 23 Effect of composite carbon fertilizer for safe utilization on zinc speciation in polluted soil after three harvests. Detailed implementation mode

[0067] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the technical field.

[0068] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0069] Example 1 Preparation of selenium-loaded composite carbon fertilizer 1. Method (1) Collection of Bidens pilosa and Pomacea canaliculata Air-dry Bidens pilosa and cut it evenly into lengths of about 2-3 cm through a cutting machine; remove impurities from chicken manure and crush it into uniform small pieces (diameter < 5 cm); wash, dry, crush the shells of Pomacea canaliculata and pass through a 60-mesh sieve.

[0070] (2) Preparation of selenium-loaded composite biochar Use a muffle furnace to pyrolyze the sieved Pomacea canaliculata shells at 342 °C for 2 h to obtain shell biochar (BC); grind BC and pass through a 100-mesh nylon mesh sieve. Use Fe 3+ 、Fe 2+A mixed solution (molar ratio 2.72) and BC were used to prepare iron-modified biochar (MBC) by chemical co-precipitation in an alkaline medium (NaOH); finally, a certain amount of BC and MBC were added to a Se(IV) solution with a concentration of 0.03 g / L according to a mass ratio of 7:1. The solid-liquid ratio of BC and MBC to the Se(IV) solution was 0.5:1 (g / mL), and the pH was adjusted to neutral (pH 7.0). The mixture was shaken at 180 r / min at 25 °C for 24 h to obtain a composite biochar loaded with selenium (SeBC) of MBC and BC.

[0071] (3) Preparation of selenium-loaded composite biochar fertilizer Bidens pilosa and chicken manure were added to a commercial composting container according to a carbon-nitrogen ratio of 25:1; then, SeBC was added according to a mass ratio of 10%, and the moisture content was adjusted to 65%. After the materials were mixed evenly, composting was carried out for 35 days to obtain selenium-loaded composite biochar fertilizer.

[0072] Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) (ZEISS Merlin compact, Carl Zeiss, Germany) were used to qualitatively analyze the surface morphology characteristics and element distribution of biochar before and after loading. The accelerating voltage was 20 V - 30 kV, and the elemental mass characteristics of the phosphorus crystallization products on the biochar surface were analyzed by X-ray energy spectrometer (EDS).

[0073] 2. Results The SEM-EDS of MBC and SeBC are as Figure 1 shown. The SEM results are as Figure 1 shown in (I) - (II) below. As can be seen from the figure, there are certain differences in the surface morphology of biochar before and after loading. The surface structure of MBC is compact, and the pore sizes are different ( Figure 1 shown in (I) below). It may be that during the carbonization process, energy is released from the pores of the material, making the biochar surface rougher. Some studies have shown that carbonization has important significance for the change of biochar pores. The surface of SeBC is rough, loose, and porous, with many small particles attached to the surface ( Figure 1 shown in (II) below). The EDS results are as Figure 1 shown in (III) - (IV) below. After loading, the C content on the biochar surface decreases, and the O content increases. O / C can characterize the polarity strength of biochar. Among them, the mass percentage of Fe in MBC is 24.48%, the mass percentage of O is 43.13%, and the Fe-O ratio is 0.57 ( Figure 1 shown in (III) below); the mass percentage of Se in SeBC is 0.21%, the mass percentage of Fe is 10.15%, and the mass percentage of O is 43.44% ( Figure 1 shown in (IV) below). There is Fe on the surface of MBCFigure 1 In (III)), there is the presence of Fe and Se on the surface of SeBC ( Figure 1 In (IV)). It indicates that Fe and Se have been successfully loaded onto the surface of biochar.

[0074] The surface area, pore volume, and pore size of MBC and SeBC were evaluated by N2 adsorption - desorption method, and the results are shown in Table 1. According to IUPAC, pores are classified into micropores (size < 2 nm), mesopores (size = 2 - 50 nm), and macropores (size > 50 nm). The results show that the specific surface area and pore volume of SeBC are 26.37 m 2 / g and 0.09 cm 3 / g respectively. Compared with MBC, the specific surface area of SeBC after loading increased by 10.58%, and the pore volume increased by 33.33%. The average pore size of MBC before loading was 10.39 nm, and the average pore size of SeBC after loading was 13.78 nm, with an increase of 24.68%. The results show that after the loading treatment, the specific surface area, pore volume, and pore size of biochar are all improved compared with the unloaded biochar. The increase in specific surface area and pore volume enhances the ability of biochar to adsorb or immobilize environmental pollutants in soil, thereby reducing the ecological risk; loading selenium with MBC and BC can significantly improve the bioavailability of selenium and reduce the loss of selenium in soil.

[0075] Table 1 BET characterization results of biochar before and after loading

[0076] Example 2 Effects of growth - promoting compound biochar fertilizer on the growth, biomass, and physiology of Chinese chives The method of this example can promote the healthy growth of Chinese chives in lead - zinc associated polluted soil and ensure the safe production of Chinese chives.

[0077] 1. Experimental materials and methods (1) The tested soil was lead-zinc associated polluted soil with lead and zinc concentrations of 95 mg / kg and 230 mg / kg respectively; the tested compost was the selenium-loaded composite carbon fertilizer prepared in Example 1; the tested vegetables were leeks, with 5 varieties including Xiaoyexiangjiu, Kuanyexuejiu, Hanzhongdongjiu, Dugenhong, and Nangonghuangjiu, all of which were varieties adaptable to the local climate. The addition amount of the selenium-loaded composite carbon fertilizer was controlled at 20% of the soil quality; the compound fertilizer (N-P2O5-K2O, 15-15-15) accounted for 10% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content was 20%; then they were mixed and left standing for a period of time; the leek roots were soaked in pure water at 25 °C, incubated for 48 h and planted, with 8 plants per pot. The potted plants were placed in an incubator at room temperature. The suitable growth temperature for the leeks was 20%, and the soil water content was maintained at 20% by the weighing method every day. Finally, the roots were dug out after the leaves were harvested.

[0078] (2) For the 5 leek varieties used in the experiment, both control (C1-C5) and treatment (T1-T5) were set. According to the different varieties and treatments, Xiaoyexiangjiu, Kuanyexuejiu, Hanzhongdongjiu, Dugenhong, and Nangonghuangjiu were abbreviated as C1XY, C2KY, C3HZ, C4DGH, and C5NG respectively according to the first letter of the variety name at the beginning, and T1XY, T2KY, T3HZ, T4DGH, and T5NG (where C represents CK and T represents treatment). All treatments (T) were added according to the addition amount of the selenium-loaded composite carbon fertilizer controlled at 10% - 25% of the soil quality. The control did not add compost carbon, and 4 replicates were set. For the leek leaves, plant height, leaf length, and biomass were measured, and the physiological indexes of leeks were measured: leaf chlorophyll, soluble protein, superoxide dismutase (SOD), and catalase (CAT).

[0079] (3) Measurement of leek growth indexes: The separated leeks were laid flat on the experimental table, and the length of the above-ground part of the leeks was measured with a flexible ruler, and the average value per pot was taken as the plant height. The separated leeks were laid flat on the experimental table, and the length of the leek leaves was measured with a flexible ruler, and the average value per pot was taken as the leaf length.

[0080] (4) Measurement of leek biomass: After the leek leaves were harvested, they were weighed with a balance to obtain the biomass.

[0081] (5)Determination of physiological indexes of Chinese chives: For the determination of physiological indexes of Chinese chives, the chlorophyll (CHL) content was measured using a portable chlorophyll meter (SPAD-502 plus type, Konica Minolta, Japan). For each pot (with 8 plants in total), one fully expanded new leaf was selected from each plant, and one measurement was taken at each of the three locations: the leaf tip, the middle of the leaf vein, and the leaf base of each leaf, and the average value was calculated. There is a significant positive correlation between the SPAD value and the chlorophyll content of plants, which can represent the actual chlorophyll content. The soluble protein content was determined using a Bradford method protein content detection kit (AKPR015, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of mg / mL. First, the plant tissue samples were pretreated. The samples were mixed with 4°C normal saline at a ratio of 1:9, and then a 10% tissue homogenate was prepared under ice bath conditions. After homogenization, the mixture was centrifuged at 2500 r / min for 15 min at 4°C, and the supernatant was collected. Then, the determination was carried out according to the method in the Bradford method protein content detection kit. Superoxide dismutase (SOD): The SOD activity was determined using a superoxide dismutase (SOD) assay kit (AKAO001, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of U / g. Take 20 μL of the supernatant and mix it thoroughly with 180 μL of the reagent reaction solution. A color reaction was carried out at 25°C for 30 min, and the absorbance value was measured at 560 nm using an enzyme-labeling instrument to calculate the SOD activity. Catalase (CAT): It was determined using a hydrogen peroxide (CAT) test kit (AKAO003-2, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of U / g. One enzyme activity unit is defined as the degradation of 1 μmol H2O2 per g of tissue per minute. Pipette 1 mL of the reaction solution into a 1 mL glass cuvette, measure the absorbance value at 405 nm, and calculate the CAT activity based on the consumption of H2O2.

[0082] 2. Results (1)The changes in the plant height and leaves of Chinese chives after adding selenium-loaded composite biochar fertilizer are as Figure 2 and Figure 3As shown in the figure. It can be seen from the figure that the treatment with selenium-loaded compound carbon fertilizer has better plant height and leaf performance than CK without compost. Among them, compared with C1XY, C2KY, C3HZ, C4DGH, C5NG, the plant height of T1XY, T2KY, T3HZ, T4DGH, T5NG increased by 15%, 5%, 22%, 7% and 9% respectively. Among them, the selenium-loaded compound carbon fertilizer has the best promotion effect on the C1XY variety. At the same time, compared with the first harvest and the last harvest, the plant height and leaves measured at the first harvest are significantly higher than those at the last harvest. The plant height of C1XY, C2KY, C3HZ, C4DGH, C5NG increased by 11%, 7%, 7%, 11% and 6% respectively compared with CK. The treatments of T1XY, T3HZ, T4DGH and T5NG with added compost are 15%, 18%, 14% and 6% higher than CK, and T2KY has no significant change. The plant height of the treatment with selenium-loaded compound carbon fertilizer during the two harvests is higher than that of CK without compost. The results show that applying selenium-loaded compound carbon fertilizer has a positive effect on the growth of Chinese chives.

[0083] (2)The biomass of Chinese chives at the first harvest and the final harvest is as Figure 4 shown. Generally, the biomass of Chinese chives under the treatments of CK and selenium-loaded compound carbon fertilizer group decreases with the increase of harvest times. The biomass at the first harvest and the last harvest of CK is lower than that of the treatment with added selenium-loaded compound carbon fertilizer, indicating that the lead-zinc associated polluted soil without selenium-loaded compound carbon fertilizer is not conducive to the growth of Chinese chives. Compared with CK, adding selenium-loaded compound carbon fertilizer significantly increases the biomass of Chinese chives. For example, the biomass of the first harvest of Chinese chives of the five varieties T1XY, T2KY, T3HZ, T4DGH and T5NG is 15.35 g, 8.52 g, 14.05 g, 9.92 g and 12.32 g respectively, which increased by 20%, 9%, 39%, 28% and 53% respectively compared with CK. Selenium-loaded compound carbon fertilizer has certain potential in increasing the biomass of Chinese chives.

[0084] (3)Chlorophyll plays an important role in plant photosynthesis, and its content is an important indicator of plant environmental stress and growth status. The chlorophyll content of Chinese chives after adding selenium-loaded compound carbon fertilizer is as Figure 5 shown. It can be seen from the figure that at the first harvest, for the treatment without adding selenium-loaded compound carbon fertilizer, the SPAD value of Chinese chives is between 55.05 and 68.75, and for the treatment with added selenium-loaded compound carbon fertilizer, the value is between 58.93 and 73.65. The SPAD of CK is lower than that of the compost treatment, indicating that selenium-loaded compound carbon fertilizer can indeed increase the chlorophyll content of Chinese chives, which is consistent with the growth index of Chinese chives. According to the SPAD results measured for Chinese chives at the final harvest, it can be seen that the SPAD value has decreased, but it is still higher than that of CK. Selenium-loaded compound carbon fertilizer has a greater impact on the chlorophyll content of pakchoi compared with CK without compost.

[0085] (4)The plant protein content is an important indicator for evaluating the growth status, nutritional level, and health status of plants. The protein content in the leek leaves after adding selenium-loaded compound biochar is as Figure 6 shown. It can be seen from the figure that the protein content of leeks shows a downward trend with the increase in the number of harvests. This may be because all harvests are usually at the time when the plants are growing most vigorously, and protein synthesis is relatively active. With multiple harvests, the reserve of nutrient elements in the leek leaves may gradually decrease, resulting in a decline in the protein synthesis ability at the last harvest. When the leeks were harvested for the first time, the protein content in the leaves of different leek varieties without adding selenium-loaded compound biochar was 9.49 - 19.56 mg / mL, which was significantly lower than that of different leek varieties under the composting condition (11.92 - 24.53 mg / mL). An appropriate amount of selenium can promote the photosynthesis of leeks, improve the growth rate of leeks, and thus increase the synthesis of proteins, indicating that adding selenium-loaded compound biochar can promote the accumulation of proteins in leek leaves.

[0086] (5)Hydrogen peroxide plays an important role in the synthesis and signal transduction of plant growth hormones (such as auxin, gibberellin, etc.). Catalase (CAT) regulates the level of hydrogen peroxide. Applying biochar can improve the activity of soil enzymes, reduce the stress of heavy metals on plants, and thus improve the growth state of plants. The effect of adding selenium-loaded compound biochar on the CAT activity in leeks is as Figure 7 shown. It can be seen from the figure that the CAT activity of each variety of leeks added with selenium-loaded compound biochar has increased to varying degrees. This may be because biochar provides a good habitat for soil microorganisms and promotes the activity of microorganisms. The metabolic activities of microorganisms affect the growth of leeks and the synthesis of antioxidant enzymes. The CAT activity of different leek varieties increased by 13% after adding selenium-loaded compound biochar.

[0087] (6)Superoxide dismutase (SOD) is considered an important antioxidant enzyme in plants. It participates in scavenging reactive oxygen free radicals, reducing the damage of oxidative stress to plant cells, and thus improving the growth state of plants. The effect of adding selenium-loaded compound biochar on the SOD activity in leeks is as Figure 8 shown. It can be seen from the figure that selenium-loaded compound biochar has a significant effect on the SOD activity of leeks. At the first harvest, the SOD activities measured in KY, HZ, DGH, and NG were significantly higher than those of the control group. The SOD activities of leek varieties added with SeBC all increased significantly, and the SOD activities measured at the last harvest were all higher than those of CK without adding selenium-loaded compound biochar, indicating that selenium-loaded compound biochar has a certain effect on improving the SOD activity. It may be that the selenium element of selenium-loaded compound biochar binds to a specific domain of the SOD enzyme protein, changing the conformation of the enzyme, and thus increasing its activity.

[0088] Example 3: Effects of Enhanced Nutrient Composite Carbon Fertilizer on Selenium Content in Different Parts of Different Varieties of Chinese Chives, Quality of Lead-Zinc Associated Polluted Soil, and Nutrient Content of Lead-Zinc Associated Polluted Soil The method of this example can improve soil availability and, to a certain extent, regulate the originally unbalanced nutrient status in the soil.

[0089] 1. Test Materials and Methods (1) The tested soil was lead-zinc associated polluted soil, with lead and zinc concentrations of 115 mg / kg and 290 mg / kg respectively; the tested compost was the selenium-loaded composite carbon fertilizer prepared in Example 1; the tested vegetable was Chinese chives, and the varieties were 5 varieties of Xiaoyexiangjiu, Kuanyexuejiu, Hanzhongdongjiu, Dugenhong, and Nangonghuangjiu, all of which were varieties adaptable to the local climate. The addition amount of the selenium-loaded composite carbon fertilizer was controlled at 30% of the soil quality; the compound fertilizer (N-P2O5-K2O, 15-15-15) accounted for 20% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content was 30%; then they were mixed and left standing for a period of time; the Chinese chive roots were soaked in pure water at 25 °C, incubated for 48 h and planted, with 8 plants per pot. The potted plants were placed in an incubator at room temperature, the suitable growth temperature of the Chinese chives was 20%, and the soil was kept moist by the weighing method every day (30% of the soil water content), and finally the roots were dug after the leaves were harvested.

[0090] (2) For the 5 Chinese chive varieties used in the test, both control (C1 - C5) and treatment (T1 - T5) were set. According to the different varieties and treatments, Xiaoyexiangjiu, Kuanyexuejiu, Hanzhongdongjiu, Dugenhong, and Nangonghuangjiu were abbreviated as C1XY, C2KY, C3HZ, C4DGH, and C5NG, and T1XY, T2KY, T3HZ, T4DGH, and T5NG respectively according to the first letter of the variety name. Among them, C at the beginning represents CK, and T at the beginning represents treatment. For all treatments (T), the addition amount of the selenium-loaded composite carbon fertilizer was controlled to be added at 26% - 40% of the soil quality, and no compost carbon was added to the control. 4 replicates were set. The Chinese chive leaf samples were collected after each maturity of the Chinese chives, and the Chinese chive roots were collected after the final harvest, and the selenium content in the Chinese chive leaves and roots was measured. The soil samples were collected after the Chinese chive leaves were harvested, and the pH, EC, and organic matter content in the soil, and the total nitrogen and total phosphorus content in the soil were measured.

[0091] (3) Determination of selenium content in different parts of Chinese chives: Weigh 0.2 g of Chinese chive leaf and root samples (dry samples) into digestion tubes respectively, then add nitric acid, perchloric acid, and hydrofluoric acid respectively. Put the digestion tubes containing the samples and the mixed acid into a graphite digestion instrument (DigiBlock ED36, Beijing Labtech Instruments Co., Ltd., China) for digestion, and finally measure with an atomic absorption spectrometer (Z-2000, Hitachi Ltd., Japan).

[0092] (4)Determination of soil pH and EC: Weigh 10.0 g of soil sample passed through a 2-mm sieve and place it in a 50-ml centrifuge tube. Add 25 ml of carbon dioxide-free distilled water, with the soil-to-water ratio being 1:2.5. Vigorously stir with a magnetic stirrer for 2 min, let it stand for 30 min, calibrate the pH meter with a pH calibration buffer solution, and then use a bench-top pH meter (S210-K, Mettler Toledo Shanghai Co., Ltd., Switzerland) to measure the soil pH value in the supernatant. Use a portable EC meter (C-600, Fu'an Puhe Electronics Co., Ltd., China) to measure the EC value.

[0093] (5)Determination of soil organic matter: The determination is carried out by the potassium dichromate volumetric method - external heating method. Weigh 0.1 g of soil sample passed through a 100-mesh sieve into a digestion tube, accurately add 5 ml of 0.8000 mol / L potassium dichromate and 5 ml of concentrated sulfuric acid. Then cover it with a bent-neck funnel, and at the same time weigh silica for a blank experiment. Put the digestion tube with the sample into a paraffin oil bath, boil it at 185 °C for 5 min, then transfer the content of the digestion tube to a 250-ml Erlenmeyer flask, and rinse the test tube and the funnel with distilled water. Make the total volume of the solution in the Erlenmeyer flask 60 - 70 ml, then add 2 - 3 drops of o-phenanthroline indicator, and then titrate with 0.2 mol / L standard ferrous sulfate. Keep shaking during the titration process. When the solution turns brick red, record the amount of ferrous sulfate used.

[0094] (6)The total nitrogen in the soil is determined by the semi-micro Kjeldahl method. The total phosphorus in the soil is determined by the potassium hydroxide melting - molybdenum antimony anti-colorimetric method.

[0095] 2. Results (1)After applying selenium-loaded compound carbon fertilizer to different varieties of Chinese chives, the distribution of selenium content in leaves and roots is as shown in Figure 9 (1)-(3) and Figure 10 shown. In this example, we compared the selenium content in leaves and roots of different varieties of Chinese chives at the 3rd harvest. The results show that after the first harvest of Chinese chive leaves, compared with the control, the selenium content in XY, KY, HZ, DGH, and NG increased by 115%, 201%, 129%, 191%, and 114% respectively. The selenium content in the roots showed varying degrees of increase ( Figure 10 ), and under the treatment of selenium-loaded compound carbon fertilizer, the selenium content in the root part of Chinese chives increased by 144%, 52%, 59%, 4%, and 16% compared with the control XY, KY, HZ, DGH, and NG respectively. Among them, XY increased extremely significantly ( p(<0.001). In the study, it was found that different Chinese chive varieties could effectively absorb and accumulate selenium, indicating that the formula used had wide applicability. Specifically, after applying SeBC, the selenium content in the edible parts of Chinese chives increased to varying degrees. Research shows that a selenium content of 0.1 - 0.5 mg / kg in Chinese chives is an ideal range, which helps to meet the human body's selenium requirements. Therefore, the Chinese chives grown using selenium-loaded composite biochar can help people in low-selenium areas to intake selenium and improve their own nutrient elements.

[0096] (2) The effects of selenium-loaded composite carbon fertilizer on the pH and electrical conductivity (EC) of lead-zinc contaminated soil are as Figure 11 (1)-(2) shown. After adding selenium-loaded composite carbon fertilizer, the release of cations into the soil will slightly increase the pH value and EC. After adding selenium-loaded composite carbon fertilizer, the soil pH value and EC increased significantly ( p (<0.05). The alkali salts released during the pyrolysis of biochar in selenium-loaded composite carbon fertilizer will increase the pH value of the soil. Compared with the control, the pH values of XY, KY, HZ, DGH, and NG increased by 15%, 21%, 19%, 21%, and 17% respectively. The application of selenium-loaded composite carbon fertilizer increased the soil pH value from 5.92 - 6.29 to 7.17 - 7.34. The average increase in soil pH value was 0.98 - 1.28 units. The change trend of soil electrical conductivity was similar to that of pH. The addition of selenium-loaded composite carbon fertilizer significantly increased the EC. The soil electrical conductivities of different Chinese chive varieties increased by 0.2 - 134% compared with the control. Among them, after adding selenium-loaded composite carbon fertilizer to the soil treatment NG, its electrical conductivity increased from 295.25 μS / cm to 691.25 μS / cm, an increase of 134%. The soil EC of XY, HZ, DGH, and NG all increased extremely significantly ( p (<0.001)).

[0097] The effect of adding selenium-loaded composite carbon fertilizer on the OM content of lead-zinc contaminated soil is as Figure 12 shown. As can be seen from Figure 12 , the selenium-loaded composite carbon fertilizer had a significant effect on increasing the OM content in the contaminated soil ( p (<0.05)), and the organic matter content increased by 1.09 - 1.25 times. After adding selenium-loaded composite carbon fertilizer to the lead-zinc associated contaminated soil, compared with the control, the soil OM contents among different Chinese chive varieties XY, KY, HZ, DGH, and NG increased by 11%, 20%, 25%, 12%, and 22% respectively ( p (<0.05)). In general, the addition of selenium-loaded composite carbon fertilizer had a significant promoting effect on the pH, EC, and OM of lead-zinc contaminated soil. These effects not only helped to improve the soil fertility and productivity but also helped to improve the soil ecological function and environmental quality.

[0098] (3)The effects of selenium-loaded composite carbon fertilizer on the total nitrogen content in heavy metal contaminated soil are as follows Figure 13 shown. The results show that the total nitrogen content in the soil of treatments T1XY, T2KY, T3HZ, T4DGH, and T5NG applying compost was higher than that of CKC1XY, C2KY, C3HZ, C4DGH, and C5NG at the first harvest. Selenium-loaded composite carbon fertilizer effectively increased the nitrogen content in the contaminated soil, and the compost provided rich organic matter to improve the soil quality. Compared with the control group, the total nitrogen content in the soil applying selenium-loaded composite carbon fertilizer showed an upward trend.

[0099] (4)The effects of selenium-loaded composite carbon fertilizer on the total phosphorus content in heavy metal contaminated soil are as follows Figure 14 shown. The results show that the total phosphorus content in the soil of treatments T1XY, T2KY, T3HZ, T4DGH, and T5NG applying compost increased significantly compared with the control, by 15%, 18%, 23%, 19%, and 19% respectively. This result indicates that the organic phosphorus component in the compost effectively improved the nitrogen supply of the soil. Therefore, applying selenium-loaded composite carbon fertilizer to heavy metal contaminated soil can effectively improve the phosphorus supply of the soil.

[0100] Example 4 Effects of safe utilization type composite carbon fertilizer on the lead and zinc contents in different parts of leeks of different varieties, the enzyme activities in the rhizosphere soil of leeks, and the lead and zinc speciations in the lead and zinc associated contaminated soil of the rhizosphere of leeks The method of this example can adsorb heavy metals in the soil, reduce the concentration of heavy metals in the soil, and reduce their toxic effects on crops and the risk of migration and diffusion into the environment.

[0101] 1. Test materials and methods (1)The tested soil was lead and zinc associated contaminated soil, with lead and zinc concentrations of 136 mg / kg and 330 mg / kg respectively; the tested compost was the selenium-loaded composite carbon fertilizer prepared in Example 1; the tested vegetables were leeks, and the varieties were 5 varieties of small-leaf fragrant leeks, wide-leaf snow leeks, Hanzhong winter leeks, single-root red leeks, and Nangong yellow leeks, all of which were varieties adaptable to the local climate. The addition amount of selenium-loaded composite carbon fertilizer was controlled at 45% of the soil quality; the compound fertilizer (N-P2O5-K2O, 15-15-15) accounted for 30% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil water content was 40%; then they were mixed and left standing for a period of time; the leek roots were soaked in pure water at 25 °C, incubated for 48 h and planted, with 8 plants per pot. The potted plants were placed in an incubator at room temperature, and the suitable growth temperature for the leeks was 20%, and the soil was kept moist by the weighing method every day (40% of the soil water content), and finally the roots were dug out after the leaves were harvested.

[0102] (2)For the 5 leek varieties used in the experiment, controls (C1–C5) and treatments (T1–T5) were set up. According to the variety and treatment, Xiaoye Xiangjiu, Kuanye Xuejiu, Hanzhong Dongjiu, Dugen Hong, and Nangong Huangjiu were abbreviated as C1XY, C2KY, C3HZ, C4DGH, and C5NG, and T1XY, T2KY, T3HZ, T4DGH, and T5NG respectively according to the initial letter of the variety name (where C indicates CK and T indicates treatment). All treatments (T) were added at a rate of 41%–56% of the soil mass controlled by the addition amount of selenium-loaded compound charcoal fertilizer. The control did not add composted charcoal, and 4 replicates were set up. The leek leaf samples were collected after each leek maturity, and the leek roots were collected after the final harvest, and the lead and zinc contents in the leek leaves and roots were determined. The soil samples were collected after the leek leaves were harvested, and then the soil urease, sucrase, and acid phosphatase were measured, and the lead and zinc forms in the soil were extracted.

[0103] (3)Determination of lead and zinc contents in each part of leek: Weigh 0.2 g of leek sample (dry sample) into a digestion tube, then add nitric acid, perchloric acid, and hydrofluoric acid respectively. Then, place the digestion tube containing the sample and mixed acid into a graphite digestion instrument (DigiBlock ED36, Beijing LabTech Instruments Co., Ltd., China) for digestion, and finally measure it with an atomic absorption spectrometer (Z-2000, Hitachi Ltd., Japan).

[0104] (4)Soil urease (S-UE): The activity of S-UE was measured using a urease (S-UE) kit (AKEN023, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of U / g. Generating 1 μg of NH3-N per g of soil sample per day was defined as one enzyme activity unit. First, dilute the supernatant after 24 h of reaction 10 times with distilled water, then pipette 80 μL of the supernatant dilution into a 96-well plate, add other reagents and mix well for color development, and then measure the absorbance at 630 nm.

[0105] (5)Soil sucrase (S-SC): The activity of S-SC was measured using a sucrase (S-SC) kit (AKEN019, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of mg / g. Generating 1 mg of reducing sugar per g of soil sample per day was defined as one enzyme activity unit. First, dilute the supernatant after 24 h of reaction 10 times with distilled water, then pipette 80 μL of the supernatant dilution into a centrifuge tube, add 200 μL of other reagents, mix well, treat in a boiling water bath for 5 min, and cool to room temperature. Pipette 200 μL of the reaction solution into a 96-well plate and measure the absorbance at 540 nm.

[0106] (6) Soil acid phosphatase (S-ACP): The activity of S-ACP was determined using an acid phosphatase (S-ACP) kit (AKEN025, Beijing Boxbio Science & Technology Co., Ltd.), with the unit of U / g. One enzyme activity unit was defined as the production of 1 nmol of phenol per gram of soil per day at 37°C. First, the soil samples were pretreated and then placed in a 37°C constant temperature incubator for a catalytic reaction for 24 h. After the reaction was completed, 1 mL of reagent was quickly added and mixed well to terminate the enzyme-catalyzed reaction. Then, it was centrifuged at 10,000 rpm for 10 min at room temperature, and the supernatant was taken as the sample to be measured and placed on ice for further measurement. 50 μL of the sample to be measured and 120 μL of other reagents were added to the centrifuge tube, mixed well. After color development, distilled water was added, and then mixed well again. It was left standing at room temperature for 30 min, and the absorbance value at 660 nm was measured.

[0107] (7) Extraction and determination of soil lead and zinc forms: According to the different activities of heavy metals in different forms in the soil, the potential toxicity and environmental risks of heavy metals in the soil environment can be analyzed and explored. The extraction of different lead and zinc forms in the soil was carried out using the Tessier extraction method. Tessier divided the lead and zinc forms in the soil into five forms: exchangeable (EXC), carbonate-bound (CAR), iron and manganese oxide-bound (OX), organic matter-bound (OM), and residual (RES). The supernatants extracted in five steps of the above heavy metals were measured using an atomic absorption spectrometer (Z-2000, Hitachi, Japan), and the distribution of heavy metal arsenic forms was analyzed.

[0108] 2. Results (1) The effects of selenium-loaded composite carbon fertilizer on the lead and zinc contents in the leaves of different varieties of Chinese chives in lead-zinc associated polluted soil are as Figure 15 (1) - (3) and Figure 16As shown in (1)–(3). The results showed that the lead and zinc contents in the leek leaves treated with selenium-loaded compound carbon fertilizer were significantly lower than those in the CK. At the first harvest, compared with the control, the lead contents in the leek leaves after applying selenium-loaded compound carbon fertilizer decreased by 32%, 40%, 35%, 27% and 44% in XY, KY, HZ, DGH and NG respectively, and the decrease rate of NG was the highest. At the third harvest, the zinc contents in the leek leaves of XY, KY, HZ, DGH and NG decreased by 38.9%, 61.3%, 39.6%, 48.7% and 59.4% respectively. Compared with the first harvest, the zinc contents in the leek leaves of XY, KY, HZ, DGH and NG under the application of selenium-loaded compound carbon fertilizer decreased by 59.9%, 66.5%, 60.3%, 64.3% and 66.7% respectively. The results indicated that the selenium-loaded compound carbon fertilizer promoted the immobilization of lead and zinc in the soil and reduced the absorption of lead and zinc by leeks. The lead accumulation contents in KY and NG leeks were reduced to the safe range (<0.3 mg / kg). The lead and zinc contents in the treatment with SeBC were significantly lower than those in the control group. On the one hand, it showed that the absorption ability of leeks for heavy metals weakened with the change of growth stage during the growth process. On the other hand, the selenium-loaded compound carbon fertilizer could not only improve the soil quality, but also effectively reduce the absorption effect of heavy metals lead and zinc in leek leaves and roots.

[0109] (2) The effects of selenium-loaded compound carbon fertilizer on the lead and zinc contents in the roots of leeks in lead-zinc associated polluted soil are as Figure 17 and Figure 18 shown. The results showed that the lead contents in the roots of XY, HZ, DGH and NG varieties were significantly reduced under the treatment of selenium-loaded compound carbon fertilizer ( p <0.05). Compared with the CK, the lead contents in each treatment decreased by 15%, 8%, 39% and 32% respectively. Compared with the control, the absorption of zinc by the roots in XY, KY, HZ, DGH and NG in the polluted soil decreased by 27.0%, 36.5%, 31.0%, 34.0% and 31.3% respectively. The results indicated that due to the good properties of organic matter and biochar in the selenium-loaded compound carbon fertilizer, it promoted the adsorption and fixation of lead and zinc contents in the soil, significantly reduced the bioavailability of lead and zinc in the soil, and thus reduced the absorption of lead and zinc by leeks. However, in the CK without applying compost, the lead and zinc contents in the leek roots were relatively high, the activity of heavy metals in the soil was high, and the absorption of lead and zinc by leeks increased. Therefore, applying selenium-loaded compound carbon fertilizer in lead-zinc associated polluted soil could not only reduce the lead and zinc contents in the leek roots, but also promote the safe production of leeks in lead-zinc soil.

[0110] (3) The effects of applying selenium-loaded compound carbon fertilizer on the urease activity in lead-zinc associated polluted soil are as Figure 19As shown, the selenium-loaded compound carbon fertilizer increased the urease activity of the soil in each treatment to varying degrees, with the increase ranging from 99% to 199% ( P <0.05). At the end of the harvest, the soil urease activity of the T1XY variety was the highest, increasing compared with the control. The effect of the application of the selenium-loaded compound carbon fertilizer on the sucrase activity of the lead-zinc associated polluted soil is as Figure 20 shown. Compared with the control, the sucrase activity of the soil increased, and the increase range was 104.49% - 186.58%. The effect of the selenium-loaded compound carbon fertilizer on the acid phosphatase activity of the soil of different varieties of Chinese chives is as Figure 21 shown. Adding the selenium-loaded compound carbon fertilizer enhanced the acid phosphatase activity of the soil of different varieties of Chinese chives to varying degrees. XY, KY, HZ, DGH, and NG increased by 154%, 104%, 69%, 81%, and 157% respectively.

[0111] (4)The changes in the lead forms of the potted soil during 3 harvests are as Figure 22 shown in (1) - (3). In this example, the F1-lead value of the soil treated with the selenium-loaded compound carbon fertilizer was significantly lower than that of the control ( p <0.05). As the number of harvests increased, the lead concentration in each treatment gradually decreased. After the first harvest, compared with CK, the exchangeable lead in the soil under the treatment of the selenium-loaded compound carbon fertilizer decreased by 22% - 69%. Compared with the first harvest, the contents of exchangeable lead and carbonate-bound lead in the soil under the treatment of the selenium-loaded compound carbon fertilizer were significantly reduced ( p <0.05). The content of residual lead in the soil under the treatment of the selenium-loaded compound carbon fertilizer was significantly higher than that of the control ( p <0.05), and the residual state increased by 4% - 29%.

[0112] (4)The changes in the zinc forms of the potted soil during 3 harvests are as Figure 23 shown in (1) - (3). After the selenium-loaded compound carbon fertilizer, both the zinc concentration and percentage in the potted plants showed that the relatively active zinc (exchangeable F1 or exchangeable F1 + carbonate-bound F2) in the soil was significantly reduced under the influence of the selenium-loaded compound carbon fertilizer. Among them, the decrease in the proportion of exchangeable zinc concentration was the most obvious, and the content of residual zinc increased significantly. At the first harvest, compared with the control, the proportion of exchangeable state in the treatment decreased by 84% - 90%, the proportion of carbonate-bound state decreased by 39% - 50%, while the proportion of residual state increased by 5% - 12%. Under the action of the selenium-loaded compound carbon fertilizer, as time extended, the selenium-loaded compound carbon fertilizer treatment promoted the transformation of soil exchangeable state to residual state. Compared with the first time, at the third harvest, the proportion of exchangeable state decreased by 18% - 65%, while the proportion of residual state increased by 6% - 14%.

Claims

1. A preparation method of selenium-loaded composite carbon fertilizer, characterized in that, It includes the following steps: S1. Clean, dry, crush, and screen the shells of Pomacea canaliculata, and pyrolyze them to obtain biochar from the shells of Pomacea canaliculata; S2. Grind and sieve the Pomacea canaliculata shell biochar, and use Fe 3+ and Fe 2+ mixed solution to prepare iron-modified biochar by chemical co-precipitation in an alkaline medium; S3. Add the biochar from the shells of Pomacea canaliculata in step S1 and the iron-modified biochar in step S2 to a Se(IV) solution with a concentration of 0.01 - 0.06 g / L according to a mass ratio of 1 - 9:1, adjust the pH to neutral, shake and adsorb to obtain selenium-loaded composite biochar; S4. Use Bidens pilosa and chicken manure with a carbon-nitrogen ratio of 20 - 30:1 as composting raw materials, add 4% - 12% by mass of selenium-loaded composite biochar, adjust the moisture content to 60 - 70%, mix well, and compost for 34 - 36 days to obtain selenium-loaded composite carbon fertilizer.

2. The preparation method according to claim 1, wherein The pyrolysis in step S1 is pyrolysis at 300 - 400 °C for 1.5 - 2.5 h.

3. The preparation method according to claim 1, wherein, The Fe described in step S2 3+ and the Fe 2+ in the mixed solution of Fe 3+ and the Fe 2+ have a molar ratio of 2.5 to 3.

4. The selenium-loaded composite carbon fertilizer prepared by the preparation method according to any one of claims 1 - 3.

5. The application of the selenium-loaded composite carbon fertilizer according to claim 4 in the remediation and improvement of lead-zinc associated polluted soil and the safe cultivation of selenium-enriched crops in lead-zinc associated polluted soil.

6. A method for repairing and improving lead-zinc associated polluted soil and for safely growing selenium-enriched crops in lead-zinc associated polluted soil, characterized in that, It includes the following steps: S1. Apply the selenium-loaded composite carbon fertilizer according to claim 4 to the lead-zinc associated polluted soil at a mass ratio of 10% - 56%, add compound fertilizer (N-P2O5-K2O, 15-15-15) accounting for 5% - 31% of the dry weight of the selenium-loaded composite carbon fertilizer, mix well, adjust the soil moisture content to 15% - 45%, and let it stand for 3 - 5 days to fully moisten the soil; the content of lead in the lead-zinc associated polluted soil is 90 - 140 mg / kg, and the content of zinc is 200 - 360 mg / kg; S2. Transplant the soaked crop seedlings into the soil in step S1, cover with soil and water to moisten, keep the temperature at 15 °C - 25 °C, keep the soil moisture content at 15% - 45%, and plant for 105 - 120 days, during which harvest 1 - 3 times.

7. The method according to claim 6, characterized in that, The selenium-loaded composite carbon fertilizer is 10% - 25% of the soil quality; the compound fertilizer accounts for 5% - 13% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil moisture content is 15% - 25%; the content of lead in the lead-zinc associated polluted soil is 90 - 110 mg / kg, and the content of zinc is 200 - 270 mg / kg.

8. The method according to claim 6, wherein The selenium-loaded composite carbon fertilizer is 26% - 40% of the soil quality; the compound fertilizer accounts for 14% - 22% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil moisture content is 26% - 35%; the content of lead in the lead-zinc associated polluted soil is 111 - 130 mg / kg, and the content of zinc is 271 - 310 mg / kg.

9. The method according to claim 6, wherein The selenium-loaded composite carbon fertilizer is 41% - 56% of the soil quality; the compound fertilizer accounts for 23% - 31% of the dry weight of the selenium-loaded composite carbon fertilizer; the soil moisture content is 36% - 45%; the content of lead in the lead-zinc associated polluted soil is 131 - 140 mg / kg, and the content of zinc is 311 - 360 mg / kg.

10. According to the method as claimed in any one of claims 6 to 9, wherein, The crop is Chinese chives.

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