Biochar material for repairing heavy metal polluted saline-alkali soil and repairing method

Abstract

The invention relates to the technical field of soil remediation, and particularly discloses a biochar material for remediation of heavy metal polluted saline-alkali soil and a remediation method. The biochar material is prepared from 55 to 65 parts of modified biochar, 5 to 10 parts of humic acid-sulfur coated microspheres, 2 to 8 parts of sepiolite powder, 2 to 5 parts of polylactic acid coated nano zero-valent iron material, 2 to 5 parts of diatomite and 1 to 3 parts of ammonium dihydrogen phosphate; the modified biochar is biochar loaded with iron and manganese oxides; the humic acid-sulfur coated microspheres are microsphere particles with a degradable polylactic acid coating layer, and humic acid and sulfur are wrapped in the degradable polylactic acid coating layer; the polylactic acid coated nano zero-valent iron material is a core-shell structure material formed by taking polylactic acid as a shell and nano zero-valent iron as a core; the size of the modified charcoal, the humic acid-sulfur coated microspheres and the polylactic acid coated nano zero-valent iron material is 80-120 meshes. The method is helpful for improving the adsorption capacity of heavy metals in the saline-alkali soil.

Description

Technical Field

[0001] The present invention relates to the technical field of soil remediation, and in particular to a biochar material and a remediation method for heavy metal contaminated saline-alkali soil. Background Art

[0002] Due to the characteristics of high soil salt content, high pH value, lack of organic matter, and soil compaction in saline-alkali soil, plant growth is restricted and microbial activity is low. When saline-alkali soil is simultaneously contaminated by heavy metals (such as cadmium, lead, arsenic, chromium, etc.), heavy metals are prone to form refractory compounds under alkaline conditions, further exacerbating the degradation of soil ecological functions and threatening human health through the food chain. The treatment of such combined pollution requires synchronous solutions to problems such as salt regulation, heavy metal passivation, and soil structure improvement, with relatively high technical difficulties.

[0003] In related technologies, CN105344708A discloses a method for remediating heavy metal pollution in saline-alkali soil, including the following steps: 1) Flood the ground surface, soak for 4-6 hours, drain the water, apply biochar base fertilizer to the heavy metal contaminated saline-alkali soil, mix evenly, and age for 60-70 days; 2) After the biochar base fertilizer in step 1) is aged, inoculate the microbial complex agent into the soil for 10-15 days; 3) After the microbial complex agent in step 2) is inoculated into the soil for 10-15 days, continuously plant castor for 1-2 cycles, with the end of the mature harvest of castor fruits as one cycle, and remove the whole castor after the end of each cycle of castor fruit harvest; 4) After the whole castor in step 3) is removed, flood the ground surface, soak for 4-6 hours, drain the water, and apply biochar base fertilizer to the heavy metal contaminated saline-alkali soil again, mix evenly, and age for 60-70 days; 5) After the biochar base fertilizer in step 4) is aged, inoculate the microbial complex agent into the soil again for 10-15 days; 6) After the microbial complex agent in step 5) is inoculated into the soil for 10-15 days, continuously plant willows for 1-2 cycles, and remove the whole willow after the end of each cycle, with 10-12 months as one cycle; 7) Repeat steps 1)-6) in a cycle until the content of heavy metals in the soil reaches the safety standard. The biochar base fertilizer includes: 50-60 parts by weight of biochar made from castor or willow waste, 10-15 parts by weight of humic acid, 4-6 parts by weight of potassium sulfate, 3-4 parts by weight of calcium sulfate, 8-12 parts by weight of ammonium humate, 15-20 parts by weight of urea, 4-6 parts by weight of citric acid, 3-4 parts by weight of oxalic acid, and 6-8 parts by weight of high-temperature furnace slag. The microbial complex agent includes: Azotobacter chroococcum, Bacillus licheniformis, Rhodopseudomonas palustris, Bacillus subtilis, and Lactobacillus plantarum, and the mass ratio of Azotobacter chroococcum, Bacillus licheniformis, Rhodopseudomonas palustris, Bacillus subtilis, and Lactobacillus plantarum is 2-4:2-4:1.5-2.5:0.75-1.5:1-3.

[0004] However, the biochar made from castor / willow waste has a low ash content and a single pore structure, and its heavy metal adsorption capacity may be weak. Therefore, it is necessary to develop a biochar material for repairing heavy metal contaminated saline-alkali land with strong heavy metal adsorption capacity. Summary of the Invention

[0005] In order to improve the adsorption capacity of heavy metals in saline-alkali land, the present application provides a biochar material for repairing heavy metal contaminated saline-alkali land and a repair method.

[0006] In the first aspect, a biochar material for repairing heavy metal contaminated saline-alkali land provided by the present application adopts the following technical solution:

[0007] A biochar material for repairing heavy metal contaminated saline-alkali land comprises the following raw materials in parts by weight: 55-65 parts of modified biochar, 5-10 parts of humic acid-sulfur coated microspheres, 2-8 parts of sepiolite powder, 2-5 parts of polylactic acid coated nano-zero valent iron material, 2-5 parts of diatomite, and 1-3 parts of ammonium dihydrogen phosphate; the modified biochar is biochar loaded with iron and manganese oxides; the humic acid-sulfur coated microspheres are microsphere particles with a degradable polylactic acid coating layer, and the degradable polylactic acid coating layer encapsulates humic acid and sulfur; the polylactic acid coated nano-zero valent iron material is a core-shell structure material with polylactic acid as the shell and nano-zero valent iron as the core; the modified biochar, the humic acid-sulfur coated microspheres and the polylactic acid coated nano-zero valent iron material are all 80-120 mesh.

[0008] By adopting the above technical solution, loading iron and manganese oxides on the surface of the biochar can form highly active adsorption sites, and moreover, increase the number of microporous structures of the biochar, so that the modified biochar still maintains a high adsorption rate under high salinity. The degradable polylactic acid coating layer gradually dissolves under the infiltration of soil moisture, and humic acid and sulfur can be continuously released during the degradation process. Sulfur oxidation generates H + , neutralizing the high pH of the saline-alkali land. At the same time, humic acid dissociates carboxyl groups at pH 6-8 and forms stable complexes with heavy metals. Therefore, the humic acid-sulfur coated microspheres can extend the adsorption time of heavy metals. Moreover, humic acid preferentially adsorbs Ca 2+ / Na + , which can reduce the competition of salts for the heavy metal adsorption sites of the modified biochar. In the polylactic acid coated nano-zero valent iron material, the polylactic acid shell isolates oxygen and moisture, greatly extending the half-life of nano-zero valent iron in the saline-alkali land. After the polylactic acid shell degrades, the nano-zero valent iron material is gradually exposed, continuously reducing heavy metals. The degradation product lactic acid of the polylactic acid shell can also complex with Cd 2+ / Pb 2+ , forming low-toxicity metal lactate complexes. The layered silicate of sepiolite powder adsorbs Ca through ion exchange2+ / Mg 2+ can reduce the competition of cations in saline-alkali soil for the heavy metal adsorption sites of modified biochar. The porous structure of diatomite can physically intercept heavy metal colloid particles, and the PO4 of ammonium dihydrogen phosphate 3+ and Pb 2+ generate Pb5(PO4)3OH.

[0009] Therefore, by adopting the above raw materials, the present application can form an adsorption-reduction-precipitation reaction system that adapts to the saline-alkali environment and has long-term effectiveness. In the first stage, sepiolite selectively adsorbs Ca 2+ , protects the active sites of biochar, the modified biochar rapidly adsorbs heavy metals, and nano zero-valent iron preferentially reduces highly toxic heavy metals. In the second stage, the humic acid-sulfur coated microspheres release humic acid and sulfur, induce heavy metal precipitation, the humic acid neutralizes the high pH of the saline-alkali soil, and the sulfur oxidation-produced acid and phosphate buffer cooperate to maintain the pH for efficient adsorption reaction. Sepiolite and diatomite continuously adsorb mobile heavy metals. In the third stage, polylactic acid is completely degraded, and lactic acid complexes residual free metal ions. Using the biochar material of the present application helps to improve the adsorption capacity for heavy metals in saline-alkali soil.

[0010] In a specific feasible embodiment, the biochar material for repairing heavy metal contaminated saline-alkali soil further includes montmorillonite nanosheets.

[0011] By adopting the above technical solution, under high salt conditions, the selectivity coefficient of montmorillonite for divalent heavy metals is much higher than that of monovalent cations such as Na + , K + , etc., which can reduce ion competition in saline-alkali soil. The micropores of the modified biochar adsorb free heavy metals, and montmorillonite nanosheets capture ionic pollutants not adsorbed by biochar through ion exchange, forming a double adsorption barrier. Moreover, the fibrous structure of sepiolite can complement the layered structure of montmorillonite nanosheets to form a three-dimensional adsorption network, improving the interception efficiency for Pb 2+ .

[0012] In a specific feasible embodiment, the biochar material for repairing heavy metal contaminated saline-alkali soil further includes lignosulfonate.

[0013] By adopting the above technical solution, lignosulfonate converts ionic heavy metals not fully fixed on the biochar surface into stable complexes through chelation, which helps to improve the fixation efficiency of heavy metals. Moreover, lignosulfonate forms composite micelles through hydrophobic interaction and hydrogen bonding, which can reduce the release rate of humic acid, help to extend the adsorption time of the biochar material, and thus improve the adsorption effect.

[0014] In a specific feasible embodiment, the biochar material for repairing heavy metal contaminated saline-alkali land further includes chitosan and EDTA, and the mass ratio of chitosan to EDTA is 1:(0.3 - 0.6).

[0015] By adopting the above technical solution, chitosan forms a three-dimensional hydrogel network in alkaline saline-alkali land, wrapping biochar and montmorillonite particles, and improving the anti-scouring ability of the material. EDTA desorbs the heavy metals not adsorbed in the pores of biochar, and chitosan captures the desorbed Pb-EDTA through the gel network and converts it into insoluble chitosan-Pb precipitate. The adsorption-desorption-re-fixation process helps to improve the utilization rate of biochar materials. Moreover, the S generated by sulfur oxidation 2+ Cd chelated with chitosan 2+ Combined, CdS@chitosan composite particles can be generated, which helps to improve the precipitation rate. Since excessive EDTA will cause excessive dissolution of heavy metal-EDTA complexes and increase the migration risk, it is found through experiments in this application that by using chitosan and EDTA in the above proportions, the gel network of chitosan can intercept the complexes, which helps to further improve the adsorption capacity of biochar materials.

[0016] In the second aspect, a method for repairing heavy metal contaminated saline-alkali land provided by this application adopts the following technical solution:

[0017] A method for repairing heavy metal contaminated saline-alkali land includes the following steps:

[0018] Mix the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid-coated nano zero-valent iron material, diatomite and ammonium dihydrogen phosphate evenly according to the ratio to obtain the biochar material;

[0019] Spread the biochar material on the surface layer of heavy metal contaminated saline-alkali land at 3 - 5 kg / m 2 and rototill and mix it to a depth of 15 - 25 cm, and spray a polyaspartic acid solution with a mass concentration of 0.1 - 0.2% at 3 - 6 L / m 2 and let it stand for 5 - 10 days;

[0020] Apply the microbial agent on the surface of heavy metal contaminated saline-alkali land at 85 - 110 g / m 2 and shallowly harrow and cover the soil;

[0021] Plant Suaeda salsa at a planting density of 28 - 34 plants / m 2 , harvest Suaeda salsa after growing for 100 - 150 days; then plant at a density of 1 - 2 plants / 2m 2Plant Lycium barbarum at a planting density, harvest the above-ground part of Lycium barbarum after 210 - 260 days of growth. After drying the harvested Suaeda salsa and Lycium barbarum, pyrolyze them at 480 - 520 °C under anoxic conditions for 2 - 3 hours to obtain biochar that can be backfilled. Wash the biochar that can be backfilled with 4 - 6% phosphoric acid, and then backfill it into the heavy metal - contaminated saline - alkali land;

[0022] Repeat the above steps cyclically until the heavy metal content in the heavy metal - contaminated saline - alkali land reaches the safety standard.

[0023] By adopting the above technical solution, after spreading the biochar material on the saline - alkali land and rototilling it to a depth of 15 - 25 cm, it helps the biochar material to come into full contact with the polluted soil. Spraying the polyaspartic acid solution at the above concentration can not only chelate heavy metals but also avoid the migration risk caused by over - desorption. The carboxylic acid groups of polyaspartic acid compete with biochar for the adsorption of Pb 2+ to convert the fixed - state Pb into Pb - aspartic acid complex, promoting plant absorption. Polyaspartic acid combines with Ca 2+ to reduce the pore blockage caused by the cementation of soil CaCO3. Then spray the microbial agent. The microbial agent uses humic acid - sulfur microspheres as electron donors to reduce S 0 to S 2- , which can promote the precipitation of CdS. The extracellular polymers produced by the metabolism of the microbial agent wrap the biochar, which can reduce the dissolution of iron and manganese oxides on the modified biochar caused by saline - alkali stress. Then carry out the rotation of Suaeda salsa - Lycium barbarum. Suaeda salsa extracts the heavy metals in the surface layer, the microbial agent promotes the upward migration of deep - layer pollutants, Lycium barbarum extracts the heavy metals in the middle and lower layers, and the biochar continuously stabilizes the rhizosphere environment. Then pyrolyze the crops into biochar and backfill it to complete the material cycle. Therefore, the remediation method of the present application realizes the deep coordination of pollution control, phytoextraction, material regeneration and ecological restoration, and helps to improve the adsorption effect of heavy metals in saline - alkali land.

[0024] In a specific feasible embodiment, the microbial agent includes Halomonas, Bacillus, and Streptomyces in a mass ratio of 5:(2 - 4):(1 - 2).

[0025] By adopting the above technical solution, Halomonas can quickly establish saline - alkali adaptability and provide a survival micro - environment for Bacillus and Streptomyces. Bacillus can efficiently passivate heavy metals and supplement the fixing ability of Halomonas to Pb / Cd. Streptomyces enhances the system stability through EPS and antibiotics to avoid the imbalance of the microbial community. Through the functional complementarity and synergistic metabolism of the three, the bio - adsorption effect of heavy metals in saline - alkali land is significantly improved.

[0026] In a specific feasible embodiment, the preparation method of the modified biochar includes the following steps:

[0027] Mix sawdust and rice straw in a mass ratio of (3 - 5):1, crush them, soak them in citric acid with a mass concentration of 1 - 2% for 22 - 28 hours, rinse them with deionized water until neutral, and dry them to obtain the standby material;

[0028] Pyrolyze the standby material at 280 - 320 °C under a nitrogen atmosphere for 1 - 1.2 hours, raise the temperature to 580 - 620 °C, pyrolyze for 2 - 2.2 hours, and cool naturally to obtain biochar;

[0029] Mix a 0.3 - 0.6 mol / L Fe(NO3)3 solution and a 0.2 - 0.4 mol / L KMnO4 solution in a volume ratio of 1:(0.7 - 1.2) to obtain a mixed solution. Immerse the biochar in the mixed solution, shake for 22 - 28 hours, filter it out, dry it, calcine it at 330 - 380 °C under a nitrogen atmosphere for 2 - 2.5 hours, cool naturally, and sieve it to obtain modified biochar with a mesh size of 80 - 120 meshes.

[0030] By adopting the above technical solution, the cellulose content of sawdust is high, and the silicon content of rice straw is 10 - 15%. When mixed in a ratio of 3 - 5:1 and pyrolyzed, a hierarchical pore structure can be formed. The silicon in the straw generates amorphous SiO2 at high temperature and combines with the sawdust carbon skeleton to enhance the ion exchange capacity for Cd 2+ and Pb 2+ . Soaking with 1 - 2% citric acid removes ash and lipids in the raw materials and reduces pore blockage during pyrolysis. Slowly pyrolyze lignin and hemicellulose at a low temperature stage to generate a rigid carbon skeleton and avoid pore collapse caused by direct high-temperature pyrolysis. A large number of mesopores are generated at this stage, providing space for subsequent metal oxide loading. Then, high temperature promotes the graphitization of the carbon layer and enhances the structural stability. Calcination at 330 - 380 °C enables the Fe / Mn oxides to bind to the biochar through C - O - Fe / Mn covalent bonds, which can reduce the metal dissolution rate. The modified biochar prepared by the above steps can all have excellent synergistic effects with other raw materials and can improve the heavy metal removal rate.

[0031] In a specific feasible embodiment, the humic acid - sulfur coated microspheres include a core and a coating layer that wraps the core. Based on the total weight of the core, the core includes the following raw materials in parts by weight: 55 - 65 parts of sulfur powder, 25 - 35 parts of humic acid, 2 - 5 parts of sodium dodecyl sulfate, and 2 - 5 parts of sodium carboxymethylcellulose; the coating layer includes ethyl cellulose and polylactic acid in a weight ratio of (7 - 8):1;

[0032] The preparation method of the humic acid - sulfur coated microspheres includes the following steps: Dissolve sodium dodecyl sulfate in water to obtain a sodium dodecyl sulfate aqueous solution with a mass concentration of 4 - 6%; Mix sulfur powder and humic acid in proportion, add them to the sodium dodecyl sulfate aqueous solution, and shear and emulsify to obtain a suspension;

[0033] Dissolve sodium carboxymethyl cellulose in water to obtain an aqueous solution of sodium carboxymethyl cellulose with a mass concentration of 4-6%. Add the suspension to the aqueous solution of sodium carboxymethyl cellulose and stir until the viscosity reaches 500-800 mPa·s to obtain a mixed slurry;

[0034] Perform spray granulation on the mixed slurry and collect microsphere cores with a particle size of 100-200 μm;

[0035] Mix acetone and ethanol according to a volume ratio of 3:(6-8) to obtain a mixed alcohol solution. Dissolve ethyl cellulose and polylactic acid in the mixed alcohol solution and stir until completely dissolved to obtain a coating solution;

[0036] Place the core microspheres in a fluidized bed, preheat to 40-50 °C, spray the coating solution at a rate of 8-12 mL / min, and after curing, screen to obtain humic acid-sulfur coated microspheres with a particle size of 80-120 mesh.

[0037] By adopting the above technical solution, sodium dodecyl sulfate reduces the interfacial tension between sulfur and humic acid, makes the particle size of the emulsified suspension uniform, and improves the core density of the microspheres. Sodium carboxymethyl cellulose forms a three-dimensional network structure, and the viscosity of the slurry reaches 500-800 mPa·s, reducing the microsphere breakage rate during spray granulation. In the fluidized bed spraying process, preheating to 40-50 °C activates the surface of the microsphere cores, and the coating solution forms a dense film layer, improving the compressive strength and avoiding breakage during field mechanical application. The humic acid-sulfur coated microspheres prepared by the above steps can all have excellent synergistic effects with other raw materials and can improve the heavy metal removal rate.

[0038] In a specific feasible embodiment, the polylactic acid-coated nano zero-valent iron material comprises the following raw materials in parts by weight: 30-40 parts of nano zero-valent iron, 55-65 parts of polylactic acid, 3-5 parts of polyvinylpyrrolidone, and 2-3 parts of trisodium citrate;

[0039] The preparation method of the polylactic acid-coated nano zero-valent iron material comprises the following steps:

[0040] Dissolve polylactic acid in dichloromethane to obtain a polylactic acid solution with a mass concentration of 4-5%. Add polyvinylpyrrolidone and trisodium citrate to the polylactic acid solution and ultrasonically disperse to obtain an organic phase solution;

[0041] Disperse nano zero-valent iron in an SDS / deoxygenated aqueous solution with a mass concentration of 0.1-0.3% at a solid-liquid ratio of 1:(45-55) and ultrasonically disperse to obtain an aqueous phase solution;

[0042] Mix the aqueous phase and the organic phase according to a volume ratio of 1:(2-4) and perform high-speed shear emulsification to obtain a W / O type emulsion;

[0043] The W / O emulsion was rotary evaporated at 40 - 50 °C and 180 - 220 mbar to remove dichloromethane until the microspheres were solidified. The microspheres were collected by centrifugation, washed, and then freeze-dried to obtain dry particles.

[0044] The dry particles were heat-treated at 80 - 85 °C for 1 - 1.5 hours under a nitrogen atmosphere and then sieved to obtain polylactic acid-coated nano zero-valent iron materials with a mesh size of 80 - 120 meshes.

[0045] By adopting the above technical scheme, with the water phase:organic phase = 1:2 - 4, a microemulsion with a water core diameter of 100 - 200 nm was formed, which could improve the encapsulation efficiency. High-speed shear emulsification evenly dispersed the nano zero-valent iron in the water phase droplets, avoiding agglomeration. Low-temperature vacuum evaporation slowly removed dichloromethane, avoiding the coating rupture of nano zero-valent iron caused by the rapid volatilization of the solvent. Freeze-drying maintained the porous structure of the microspheres, avoiding the oxidation of nano zero-valent iron caused by high-temperature drying. Heat treatment at 80 - 85 °C under nitrogen protection promoted the rearrangement of polylactic acid molecular chains, which helped to improve the tensile strength of the coating layer. Controlling the particle size to be 150 - 180 μm was more suitable for the soil pores in saline-alkali land, which helped to improve the migration and diffusion efficiency. The polylactic acid-coated nano zero-valent iron materials prepared by the above steps could all have excellent synergistic effects with other raw materials, and could improve the heavy metal removal rate.

[0046] In summary, the present application has the following beneficial effects:

[0047] 1. The biochar material of the present application helps to improve the adsorption capacity for heavy metals in saline-alkali land by forming an adsorption-reduction-precipitation reaction system adapted to the saline-alkali environment and with long-term effectiveness.

[0048] 2. In the present application, montmorillonite nanosheets and lignosulfonates are preferably used, which can form a three-dimensional adsorption network to improve the interception efficiency for Pb 2+ or extend the adsorption time of the biochar material.

[0049] 3. The method of the present application realizes the deep coordination of pollution control, phytoextraction, material regeneration and ecological restoration, which helps to improve the adsorption effect on heavy metals in saline-alkali land. Detailed Embodiments

[0050] Unless otherwise specified, the raw materials used in the present application were all obtained commercially. Among them, Halomonas was Halomonas hanedai DSM 21196; Bacillus was Jiashengda JY-015Y; Streptomyces was Streptomyces microflavus, with a viable count of 10 billion per gram.

[0051] The present application will be further described in detail below with reference to examples and comparative examples.

[0052] Examples

[0053] Example 1

[0054] This embodiment provides a biochar material for the remediation of heavy metal - contaminated saline - alkali land, which comprises the following raw materials: 55 kg of modified biochar, 10 kg of humic acid - sulfur - coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid - coated nano - zero - valent iron material, 5 kg of diatomite, and 3 kg of ammonium dihydrogen phosphate. The modified biochar, humic acid - sulfur - coated microspheres, and polylactic acid - coated nano - zero - valent iron material are all 80 - 120 mesh.

[0055] The modified biochar is prepared according to the following steps:

[0056] Mix sawdust and rice straw in a mass ratio of 3:1, crush them, soak them in 1% citric acid by mass concentration for 28 hours, filter to obtain solid materials, rinse the solid materials with deionized water until neutral, and dry them to obtain standby materials.

[0057] Pyrolyze the standby materials at 280 °C under a nitrogen atmosphere for 1.2 hours, raise the temperature to 580 °C, pyrolyze for 2.2 hours, and after natural cooling, obtain biochar.

[0058] Mix a 0.3 mol / L Fe(NO3)3 solution and a 0.2 mol / L KMnO4 solution in a volume ratio of 1:0.7 until uniform to obtain a mixed solution. Then, immerse the biochar in the mixed solution, shake for 22 hours, filter it out, dry it, calcine it at 330 °C under a nitrogen atmosphere for 2.5 hours, and after natural cooling, sieve it to obtain modified biochar with a mesh number of 80 - 120.

[0059] The humic acid - sulfur - coated microspheres are prepared according to the following steps:

[0060] The humic acid - sulfur - coated microspheres include a core and a coating layer that wraps the core. The core comprises the following raw materials: 55 kg of sulfur powder, 35 kg of humic acid, 5 kg of sodium dodecyl sulfate, and 5 kg of sodium carboxymethyl cellulose; the coating layer comprises ethyl cellulose and polylactic acid with a weight ratio of 7:1.

[0061] Dissolve sodium dodecyl sulfate in water to obtain a 4% sodium dodecyl sulfate aqueous solution by mass concentration. Mix sulfur powder and humic acid in proportion, add them to the sodium dodecyl sulfate aqueous solution, and shear - emulsify to obtain a suspension.

[0062] Dissolve sodium carboxymethyl cellulose in water to obtain a 4% sodium carboxymethyl cellulose aqueous solution, add the suspension to the sodium carboxymethyl cellulose aqueous solution, and stir until the viscosity reaches 500 mPa·s to obtain a mixed slurry.

[0063] Perform spray granulation on the mixed slurry, and collect microsphere cores with a particle size of 100 - 200 μm.

[0064] Mix acetone and ethanol in a volume ratio of 1:2 to obtain a mixed alcohol solution. Dissolve ethyl cellulose and polylactic acid in the mixed alcohol solution and stir until completely dissolved to obtain a coating solution.

[0065] Place the core microspheres in a fluidized bed, preheat to 40 °C, spray the coating solution at a rate of 8 mL / min, and after curing, screen to obtain humic acid-sulfur coated microspheres with a particle size of 80-120 mesh.

[0066] The polylactic acid-coated nano zero-valent iron material is prepared according to the following steps:

[0067] The polylactic acid-coated nano zero-valent iron material includes the following raw materials: 30 kg of nano zero-valent iron, 65 kg of polylactic acid, 5 kg of polyvinylpyrrolidone, and 3 kg of trisodium citrate;

[0068] Dissolve polylactic acid in dichloromethane to obtain a polylactic acid solution with a mass concentration of 4%. Add polyvinylpyrrolidone and trisodium citrate to the polylactic acid solution and ultrasonically disperse to obtain an organic phase solution.

[0069] Disperse nano zero-valent iron in a 0.1% SDS / deoxygenated aqueous solution at a solid-liquid ratio of 1:45 and ultrasonically disperse to obtain an aqueous phase solution.

[0070] Mix the aqueous phase and the organic phase in a volume ratio of 1:2 and perform high-speed shear emulsification to obtain a W / O type emulsion.

[0071] Rotate and evaporate the W / O type emulsion at 40 °C and 220 mbar to remove dichloromethane until the microspheres are cured. Centrifuge to collect the microspheres, wash the microspheres, and then freeze-dry to obtain dry particles.

[0072] Heat-treat the dry particles in a nitrogen atmosphere at 80 °C for 1.5 hours and screen to obtain a polylactic acid-coated nano zero-valent iron material with a mesh number of 80-120.

[0073] A method for repairing heavy metal-polluted saline-alkali land includes the following steps:

[0074] Mix the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid-coated nano zero-valent iron material, diatomite, and ammonium dihydrogen phosphate evenly according to the ratio to obtain a biochar material.

[0075] Spread the above biochar material at 3 kg / m 2 Apply it to the surface layer of the heavy metal-polluted saline-alkali land and rototill to a depth of 15 cm. Spray a 0.2% polyaspartic acid solution on the heavy metal-polluted saline-alkali land at 3 L / m 2 and let it stand for 5 days.

[0076] At 85 g / m 2Apply Halomonas to the surface of heavy metal - contaminated saline - alkali land, and gently harrow and cover with soil.

[0077] Plant Suaeda salsa at a planting density of 28 plants / m 2 , and harvest Suaeda salsa after growing for 100 days; then plant Lycium barbarum at a planting density of 1 plant / 2m 2 . After growing for 210 days, harvest the above - ground part of Lycium barbarum. After drying the harvested Suaeda salsa and Lycium barbarum, pyrolyze them at 480 °C under anoxic conditions for 3 hours to obtain biochar that can be backfilled. After washing the biochar that can be backfilled with 4% phosphoric acid, backfill it into the heavy metal - contaminated saline - alkali land;

[0078] Repeat the above steps cyclically until the heavy metal content in the heavy metal - contaminated saline - alkali land reaches the safety standard.

[0079] Example 2

[0080] The difference between this example and Example 1 is only that the biochar material for repairing heavy metal - contaminated saline - alkali land includes the following raw materials: 65 kg of modified biochar, 5 kg of humic acid - sulfur - coated microspheres, 2 kg of sepiolite powder, 2 kg of polylactic - acid - coated nano - zero - valent iron material, 2 kg of diatomite, and 1 kg of ammonium dihydrogen phosphate.

[0081] Example 3

[0082] The difference between this example and Example 1 is only that the biochar material for repairing heavy metal - contaminated saline - alkali land includes the following raw materials: 55 kg of modified biochar, 10 kg of humic acid - sulfur - coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic - acid - coated nano - zero - valent iron material, 5 kg of diatomite, 3 kg of ammonium dihydrogen phosphate, and 5 kg of montmorillonite nanosheets. In the method for repairing heavy metal - contaminated saline - alkali land, mix the modified biochar, humic acid - sulfur - coated microspheres, sepiolite powder, polylactic - acid - coated nano - zero - valent iron material, diatomite, ammonium dihydrogen phosphate, and montmorillonite nanosheets evenly according to the ratio to obtain the biochar material.

[0083] Example 4

[0084] The difference between this example and Example 1 is only that the biochar material for repairing heavy metal - contaminated saline - alkali land includes the following raw materials: 55 kg of modified biochar, 10 kg of humic acid - sulfur - coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic - acid - coated nano - zero - valent iron material, 5 kg of diatomite, 3 kg of ammonium dihydrogen phosphate, and 3 kg of lignosulfonate. In the method for repairing heavy metal - contaminated saline - alkali land, mix the modified biochar, humic acid - sulfur - coated microspheres, sepiolite powder, polylactic - acid - coated nano - zero - valent iron material, diatomite, ammonium dihydrogen phosphate, and lignosulfonate evenly according to the ratio to obtain the biochar material.

[0085] Example 5

[0086] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land comprises the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 3 kg of ammonium dihydrogen phosphate, 5 kg of montmorillonite nanosheets, and 3 kg of lignosulfonate. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, montmorillonite nanosheets, and lignosulfonate are mixed evenly to obtain the biochar material.

[0087] Example 6

[0088] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land comprises the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 2 kg of chitosan, and 0.3 kg of EDTA. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, chitosan, and EDTA are mixed evenly to obtain the biochar material.

[0089] Example 7

[0090] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land comprises the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 2 kg of chitosan, and 0.6 kg of EDTA. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, chitosan, and EDTA are mixed evenly to obtain the biochar material.

[0091] Example 8

[0092] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land comprises the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 2 kg of chitosan, and 1.2 kg of EDTA. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, chitosan, and EDTA are mixed evenly to obtain the biochar material.

[0093] Example 9

[0094] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land includes the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 2 kg of chitosan, and 1.5 kg of EDTA. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, chitosan and EDTA are mixed evenly to obtain the biochar material.

[0095] Example 10

[0096] The difference between this example and Example 1 is only that the biochar material for the remediation of heavy metal contaminated saline-alkali land includes the following raw materials: 55 kg of modified biochar, 10 kg of humic acid-sulfur coated microspheres, 8 kg of sepiolite powder, 5 kg of polylactic acid coated nano-zero valent iron material, 5 kg of diatomite, 3 kg of ammonium dihydrogen phosphate, 5 kg of montmorillonite nanosheets, 3 kg of lignosulfonate, 2 kg of chitosan, and 0.6 kg of EDTA. In the method for remediating heavy metal contaminated saline-alkali land, according to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite, ammonium dihydrogen phosphate, montmorillonite nanosheets, lignosulfonate, chitosan and EDTA are mixed evenly to obtain the biochar material.

[0097] Example 11

[0098] The difference between this example and Example 1 is only that the method for remediating heavy metal contaminated saline-alkali land includes the following steps:

[0099] According to the ratio, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano-zero valent iron material, diatomite and ammonium dihydrogen phosphate are mixed evenly to obtain the biochar material;

[0100] The above biochar material is spread at 5 kg / m 2 on the surface layer of heavy metal contaminated saline-alkali land, and rotary tillage is carried out to a depth of 25 cm. According to 6 L / m 2 , a polyaspartic acid solution with a mass concentration of 0.1% is sprayed on the heavy metal contaminated saline-alkali land, and it is left standing for 10 days.

[0101] According to 110 g / m 2 Halomonas is applied to the surface of heavy metal contaminated saline-alkali land, and shallow harrowing and soil covering are carried out.

[0102] According to 34 strains / m 2Suaeda salsa is planted at a planting density, and after growing for 150 days, Suaeda salsa is harvested; then Lycium barbarum is planted at a planting density of 2 plants / 2 m 2 After growing for 260 days, the above-ground part of Lycium barbarum is harvested. After the harvested Suaeda salsa and Lycium barbarum are dried, they are pyrolyzed at 520 °C under anoxic conditions for 2 hours to obtain biochar that can be backfilled. After washing the biochar that can be backfilled with 6% phosphoric acid, it is backfilled into Halomonas.

[0103] The above steps are repeatedly executed until the heavy metal content in the heavy metal-polluted saline-alkali land reaches the safety standard.

[0104] Example 12

[0105] The difference between this example and Example 1 is only that in the method for repairing heavy metal-polluted saline-alkali land, Halomonas is replaced with an equal amount of bacterial agent, and the bacterial agent includes Halomonas: Bacillus: Streptomyces with a mass ratio of 5:1:1.

[0106] Example 13

[0107] The difference between this example and Example 1 is only that in the method for repairing heavy metal-polluted saline-alkali land, Halomonas is replaced with an equal amount of bacterial agent, and the bacterial agent includes Halomonas: Bacillus: Streptomyces with a mass ratio of 5:2:1.

[0108] Example 14

[0109] The difference between this example and Example 1 is only that in the method for repairing heavy metal-polluted saline-alkali land, Halomonas is replaced with an equal amount of bacterial agent, and the bacterial agent includes Halomonas: Bacillus: Streptomyces with a mass ratio of 5:3:2.

[0110] Example 15

[0111] The difference between this example and Example 1 is only that in the method for repairing heavy metal-polluted saline-alkali land, Halomonas is replaced with an equal amount of bacterial agent, and the bacterial agent includes Halomonas: Bacillus: Streptomyces with a mass ratio of 5:4:2.

[0112] Example 16

[0113] The difference between this example and Example 1 is only that in the method for repairing heavy metal-polluted saline-alkali land, Halomonas is replaced with an equal amount of bacterial agent, and the bacterial agent includes Halomonas: Bacillus: Streptomyces with a mass ratio of 5:5:3.

[0114] Example 17

[0115] The difference between this example and Example 1 is only that the modified biochar is prepared according to the following steps:

[0116] Mix sawdust and rice straw in a mass ratio of 5:1, crush them, soak them in citric acid with a mass concentration of 2% for 22 hours, filter to obtain solid materials, rinse the solid materials with deionized water until neutral, and dry them to obtain standby materials.

[0117] Pyrolyze the standby materials at 320 °C in a nitrogen atmosphere for 1 hour, then raise the temperature to 620 °C and pyrolyze for 2 hours. After natural cooling, biochar is obtained.

[0118] Mix a 0.6 mol / L Fe(NO3)3 solution and a 0.4 mol / L KMnO4 solution in a volume ratio of 1:1.2 until uniform to obtain a mixed solution. Then, immerse the biochar in the mixed solution, shake for 28 hours, filter it out, dry it, calcine it at 380 °C in a nitrogen atmosphere for 2 hours, and after natural cooling, sieve it to obtain modified biochar with a mesh size of 80 - 120 meshes.

[0119] Example 18

[0120] The difference between this example and Example 1 is only that the humic acid - sulfur coated microspheres are prepared according to the following steps:

[0121] The humic acid - sulfur coated microspheres include a core and a coating layer that wraps the core. The core includes the following raw materials: 65 kg of sulfur powder, 25 kg of humic acid, 2 kg of sodium dodecyl sulfate, and 2 kg of sodium carboxymethyl cellulose; the coating layer includes ethyl cellulose and polylactic acid with a weight ratio of 8:1.

[0122] Dissolve sodium dodecyl sulfate in water to obtain a sodium dodecyl sulfate aqueous solution with a mass concentration of 6%. Mix sulfur powder and humic acid in proportion, add them to the sodium dodecyl sulfate aqueous solution, and shear - emulsify to obtain a suspension.

[0123] Dissolve sodium carboxymethyl cellulose in water to obtain a sodium carboxymethyl cellulose aqueous solution with a mass concentration of 6%. Add the suspension to the sodium carboxymethyl cellulose aqueous solution and stir until the viscosity reaches 800 mPa·s to obtain a mixed slurry.

[0124] Perform spray granulation on the mixed slurry and collect microsphere cores with a particle size of 100 - 200 μm.

[0125] Mix acetone and ethanol in a volume ratio of 3:8 to obtain a mixed alcohol solution. Dissolve ethyl cellulose and polylactic acid in the mixed alcohol solution and stir until completely dissolved to obtain a coating solution.

[0126] Place the core microspheres in a fluidized bed, preheat to 50 °C, spray the coating solution at a rate of 12 mL / min, and after curing, sieve to obtain humic acid - sulfur coated microspheres with a particle size of 80 - 120 meshes.

[0127] Example 19

[0128] The difference between this embodiment and Embodiment 1 is only that the polylactic acid-coated nano zero-valent iron material is prepared according to the following steps:

[0129] The polylactic acid-coated nano zero-valent iron material comprises the following raw materials: 40 kg of nano zero-valent iron, 55 kg of polylactic acid, 3 kg of polyvinylpyrrolidone, and 2 kg of trisodium citrate;

[0130] Dissolve polylactic acid in dichloromethane to obtain a polylactic acid solution with a mass concentration of 5%. Add polyvinylpyrrolidone and trisodium citrate to the polylactic acid solution and disperse it by ultrasonic wave to obtain an organic phase solution.

[0131] Disperse nano zero-valent iron in a 0.3% SDS / deoxygenated aqueous solution at a solid-liquid ratio of 1:55 and disperse it by ultrasonic wave to obtain an aqueous phase solution.

[0132] Mix the aqueous phase and the organic phase at a volume ratio of 1:4 and emulsify them by high-speed shearing to obtain a W / O emulsion.

[0133] Rotate and evaporate the W / O emulsion at 50 °C and 180 mbar to remove dichloromethane until the microspheres are solidified. Centrifuge to collect the microspheres, wash the microspheres, and then freeze-dry them to obtain dry particles.

[0134] Heat-treat the dry particles in a nitrogen atmosphere at 85 °C for 1 hour and sieve them to obtain a polylactic acid-coated nano zero-valent iron material with a mesh number of 80 - 120.

[0135] Comparative Example

[0136] Comparative Example 1

[0137] The difference between this comparative example and Embodiment 1 is only that in the raw materials of the biochar material for repairing heavy metal-polluted saline-alkali land, an equal amount of rice husk biochar is used to replace the modified biochar.

[0138] Comparative Example 2

[0139] The difference between this comparative example and Embodiment 1 is only that in the raw materials of the biochar material for repairing heavy metal-polluted saline-alkali land, an equal amount of humic acid is used to replace the humic acid-sulfur-coated microspheres.

[0140] Comparative Example 3

[0141] The difference between this comparative example and Embodiment 1 is only that in the raw materials of the biochar material for repairing heavy metal-polluted saline-alkali land, an equal amount of sulfur is used to replace the humic acid-sulfur-coated microspheres.

[0142] Comparative Example 4

[0143] The difference between this comparative example and Example 1 is only that in the raw materials of the biochar material for the remediation of heavy metal contaminated saline-alkali land, a mixture of humic acid and sulfur with a weight ratio of 1:1 is used to replace the humic acid-sulfur coated microspheres in an equal amount.

[0144] Comparative Example 5

[0145] The difference between this comparative example and Example 1 is only that in the raw materials of the biochar material for the remediation of heavy metal contaminated saline-alkali land, nano zero-valent iron is used to replace the polylactic acid-coated nano zero-valent iron material in an equal amount.

[0146] Comparative Example 6

[0147] The difference between this comparative example and Example 1 is only that in the raw materials of the biochar material for the remediation of heavy metal contaminated saline-alkali land, sepiolite powder is not contained.

[0148] Comparative Example 7

[0149] The difference between this comparative example and Example 1 is only that in the raw materials of the biochar material for the remediation of heavy metal contaminated saline-alkali land, diatomite is not contained.

[0150] Performance detection test

[0151] For the biochar materials and the remediation methods of heavy metal contaminated saline-alkali land obtained in Examples 1-19 and Comparative Examples 1-7, performance detection was carried out.

[0152] Prepare a simulated contaminated saline-alkali soil containing Cd 2+ , Pb 2+ with a concentration of 500 mg / kg, and adjust the pH to 8.5. Then the following experiments were carried out:

[0153] 1. Conduct an adsorption experiment on the heavy metal adsorption capacity of the biochar material:

[0154] Mix 30 g of the biochar material evenly with 1 kg of the simulated contaminated saline-alkali soil and fill it into a glass column (diameter 5 cm, height 30 cm).

[0155] Pass simulated groundwater (containing 0.1 mol / L NaCl, pH = 8.5) into the glass column at a flow rate of 0.5 mL / min. After 60 days, according to GB15618-2018 "Soil Environmental Quality Risk Control Standards for Agricultural Land Soils (Trial)", detect the concentration of heavy metals in the simulated contaminated saline-alkali soil.

[0156] 2. Detect the actual adsorption effect of the remediation method of heavy metal contaminated saline-alkali land:

[0157] Using the above-mentioned simulated polluted saline-alkali soil as the heavy metal polluted saline-alkali soil, after running one round according to the remediation methods of each example and comparative example, in accordance with GB 15618-2018 "Soil Environmental Quality Risk Control Standards for Agricultural Land Soils (Trial)", the concentrations of heavy metals and pH in the simulated polluted saline-alkali soil were detected.

[0158] The detection results are shown in Table 1.

[0159] Table 1

[0160]

[0161]

[0162]

[0163] Combined with Example 1 and Comparative Examples 1-7 and combined with Table 1, it can be seen that compared with Example 1, after 60 days of adsorption by the biochar materials in Comparative Examples 1-7, the concentrations of heavy metals in the simulated polluted saline-alkali soil are higher. After running one round of the remediation method, the concentrations of heavy metals in the simulated polluted saline-alkali soil are higher, and moreover, the pH values of the simulated polluted saline-alkali soils in Comparative Examples 1, 5-7 are also higher. This shows that by using the biochar material and remediation method of Example 1, the adsorption effect on heavy metals in saline-alkali soil can be improved, and the pH of saline-alkali soil can be reduced.

[0164] Combined with Examples 1-19 and combined with Table 1, it can be seen that compared with Comparative Examples 1-7, after 60 days of adsorption by the biochar materials in Examples 1-19, the Cd 2+ concentrations in the simulated polluted saline-alkali soil are all less than 3 mg / kg, and the Pb 2+ concentrations are all less than 100 mg / kg. After running one round of the remediation method, the Cd 2+ concentrations in the simulated polluted saline-alkali soil are all less than 1 mg / kg, the Pb 2+ concentrations are all less than 70 mg / kg, and the pH values are all less than 8. This shows that by using the biochar materials and remediation methods of Examples 1-19, the adsorption effect on heavy metals in saline-alkali soil can be improved, and the pH of saline-alkali soil can be reduced.

[0165] This specific embodiment is only an explanation of the present application, and it is not a limitation of the present application. Those skilled in the art can make modifications without creative contributions to this embodiment according to needs after reading this specification, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A biochar material for remediation of heavy metal-contaminated saline-alkali land, characterized in that: The invention comprises the following raw materials in parts by weight: 55-65 parts of modified biochar, 5-10 parts of humic acid-sulfur coated microspheres, 2-8 parts of sepiolite powder, 2-5 parts of polylactic acid coated nano zero-valent iron material, 2-5 parts of diatomaceous earth, and 1-3 parts of diammonium phosphate; The modified biochar is biochar loaded with iron and manganese oxides; the humic acid-sulfur coated microspheres are microsphere particles with a degradable polylactic acid coating layer, and the degradable polylactic acid coating layer contains humic acid and sulfur; the polylactic acid-coated nano zero-valent iron material is a core-shell structure material formed by polylactic acid as a shell and nano zero-valent iron as a core; the modified biochar, humic acid-sulfur coated microspheres and polylactic acid-coated nano zero-valent iron materials are all 80-120 mesh.

2. The biochar material for remediation of heavy metal-contaminated saline-alkali land according to claim 1, characterized in that: Biochar materials for the restoration of heavy metal-contaminated saline-alkali land also include montmorillonite nanosheets.

3. The biochar material for remediation of heavy metal-contaminated saline-alkali land according to claim 1, characterized in that: Biochar materials for the restoration of heavy metal-contaminated saline-alkali land also include lignin sulfonates.

4. The biochar material for remediation of heavy metal-contaminated saline-alkali land according to claim 1, characterized in that: The biochar materials for the restoration of heavy metal-contaminated saline-alkali land also include chitosan and EDTA, and the mass ratio of chitosan to EDTA is 1:(0.3-0.6).

5. A method for repairing saline-alkali land polluted by heavy metals, characterized in that: The steps include: According to the proportion, the modified biochar, humic acid-sulfur coated microspheres, sepiolite powder, polylactic acid coated nano zero-valent iron material, diatomaceous earth and ammonium dihydrogen phosphate are uniformly mixed to obtain the biochar material for the restoration of heavy metal-contaminated saline-alkali land according to claim 1; The biochar material for the restoration of heavy metal polluted saline-alkali land is 3-5kg / m 2 Spread on the surface of heavy metal-contaminated saline-alkali land, rotary till and mix to a depth of 15-25cm, at a rate of 3-6L / m 2 Spray a polyaspartic acid solution with a mass concentration of 0.1-0.2% and let it stand for 5-10 days; Press 85-110g / m 2 Apply the bacterial agent to the surface of the saline-alkali land polluted by heavy metals, shallowly harrow and cover with soil; According to 28-34 plants / m 2 Plant the salsa salsa at a planting density of 1-2 plants / 2m 2 Planting wolfberry at a planting density of , harvesting the above-ground part of wolfberry after 210-260 days of growth, drying the harvested salsa salsa and wolfberry, pyrolyzing them at 480-520°C in anoxic conditions for 2-3 hours to obtain backfill biochar, washing the backfill biochar with 4-6% phosphoric acid, and backfilling it into heavy metal-contaminated saline-alkali land; The above steps are repeated in a cycle until the content of heavy metals in the heavy metal-contaminated saline-alkali land reaches the safety standard.

6. The method for repairing saline-alkali land polluted by heavy metals according to claim 5, characterized in that: The bacterial agent includes Halomonas:Bacillus:Streptomyces in a mass ratio of 5:(2-4):(1-2).

7. The method for repairing heavy metal-contaminated saline-alkali land according to claim 5, characterized in that: The preparation method of the modified biochar comprises the following steps: The sawdust and rice straw are mixed in a mass ratio of (3-5):1, crushed, soaked in citric acid with a mass concentration of 1-2% for 22-28 hours, rinsed with deionized water until neutral, and dried to obtain a standby material; Pyrolyze the spare material at 280-320°C in a nitrogen atmosphere for 1-1.2 hours, heat to 580-620°C, pyrolyze for 2-2.2 hours, and cool naturally to obtain biochar; Mix 0.3-0.6 mol / L Fe(NO3)3 solution and 0.2-0.4 mol / L KMnO4 solution in a volume ratio of 1:(0.7-1.2) to obtain a mixed solution, immerse the biochar in the mixed solution, shake for 22-28 hours, filter out, dry, calcine at 330-380°C in a nitrogen atmosphere for 2-2.5 hours, cool naturally, and sieve to obtain modified biochar with a mesh size of 80-120.

8. The method for repairing heavy metal-contaminated saline-alkali land according to claim 5, characterized in that: The humic acid-sulfur coated microspheres include an inner core and a coating layer that wraps the inner core. The inner core includes the following raw materials in parts by weight based on the total weight of the inner core: 55-65 parts of sulfur powder, 25-35 parts of humic acid, 2-5 parts of sodium lauryl sulfate, and 2-5 parts of sodium carboxymethyl cellulose; the coating layer includes ethyl cellulose and polylactic acid in a weight ratio of (7-8):1; The preparation method of the humic acid-sulfur coated microspheres comprises the following steps: dissolving sodium dodecyl sulfate in water to obtain a sodium dodecyl sulfate aqueous solution with a mass concentration of 4-6%; mixing sulfur powder and humic acid in proportion, adding the mixture to the sodium dodecyl sulfate aqueous solution, and shearing and emulsifying to obtain a suspension; Dissolving sodium carboxymethyl cellulose in water to obtain a carboxymethyl cellulose aqueous solution with a mass concentration of 4-6%, adding the suspension to the carboxymethyl cellulose aqueous solution, and stirring until the viscosity reaches 500-800 mPa·s to obtain a mixed slurry; The mixed slurry is sprayed and granulated to collect microsphere cores with a particle size of 100-200 μm; Acetone and ethanol are mixed in a volume ratio of 3:(6-8) to obtain a mixed alcohol solution, ethyl cellulose and polylactide are dissolved in the mixed alcohol solution, and stirred until completely dissolved to obtain a coating solution; The core microspheres are placed in a fluidized bed, preheated to 40-50°C, and the coating liquid is sprayed at a rate of 8-12 mL / min. After solidification, the microspheres are sieved to obtain humic acid-sulfur coated microspheres with a particle size of 80-120 meshes.

9. The method for repairing saline-alkali land polluted by heavy metals according to claim 5, characterized in that: The polylactic acid coated nano zero-valent iron material comprises the following raw materials in parts by weight: 30-40 parts of nano zero-valent iron, 55-65 parts of polylactic acid, 3-5 parts of polyvinyl pyrrolidone, and 2-3 parts of trisodium citrate; The preparation method of the polylactic acid coated nano zero-valent iron material comprises the following steps: Dissolving polylactic acid in dichloromethane to obtain a polylactic acid solution with a mass concentration of 4-5%, adding polyvinyl pyrrolidone and trisodium citrate to the polylactic acid solution, and performing ultrasonic dispersion to obtain an organic phase solution; Dispersing nano-zero-valent iron in a SDS / deoxygenated water solution with a mass concentration of 0.1-0.3% at a solid-liquid ratio of 1:(45-55), and performing ultrasonic dispersion to obtain an aqueous solution; The aqueous phase and the organic phase are mixed in a volume ratio of 1:(2-4), and high-speed shear emulsification is performed to obtain a W / O type emulsion; The W / O type emulsion is rotary evaporated at 40-50°C and 180-220 mbar to remove dichloromethane until the microspheres are solidified, the microspheres are collected by centrifugation, the microspheres are washed, and then freeze-dried to obtain dry particles; The dried particles were heat treated in a nitrogen atmosphere at 80-85° C. for 1-1.5 hours, and sieved to obtain a polylactic acid-coated nano zero-valent iron material with a mesh size of 80-120 meshes.

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