Improvement method for improving carbon sequestration capability of biochar

Abstract

The invention relates to the technical field of carbon sequestration, and discloses an improvement method for improving the carbon sequestration capability of biochar, which comprises the following steps: S1, preparing a biochar matrix; s2, loading an MOF precursor; s3, gradient roasting-plasma activation coupling treatment; s4, grafting with a bio-based cross-linking agent; the invention not only provides an efficient and stable biochar carbon sequestration improvement technology, but also can provide a feasible solution for coping with climate change and promoting agricultural green development by increasing the soil carbon reservoir reserves and improving the soil structure, and has remarkable environmental protection value, economic value and social significance.

Description

Technical Field

[0001] This invention relates to the field of carbon fixation technology, specifically an improved method for enhancing the carbon fixation capacity of biochar. Background Technology

[0002] With the increasing prominence of global climate change, enhancing carbon sequestration capacity and reducing greenhouse gas emissions have become important research directions in the field of ecological and environmental protection. Biochar, a porous material made from agricultural and forestry waste through high-temperature carbonization, has received widespread attention in areas such as soil carbon sequestration and pollutant adsorption due to its advantages of wide availability of raw materials, low cost, and environmental friendliness. However, traditional biochar suffers from problems such as limited specific surface area, insufficient surface active sites, and poor carbon sequestration stability, resulting in low carbon sequestration efficiency and making it difficult to meet the demands for high-efficiency carbon sequestration in practical applications.

[0003] To improve the carbon fixation performance of biochar, various improvement methods have emerged in existing technologies, such as expanding the pore structure through conventional calcination and directly loading metal ions to enhance adsorption activity. However, these methods have obvious limitations: although simple calcination can increase the number of pores to a certain extent, the pore structure is uneven and cannot introduce effective active sites, resulting in limited improvement in carbon fixation; when directly loading metal ions, the metal particles are prone to agglomeration and weak bonding with the biochar matrix, easily falling off and being lost during use, leading to a decline in carbon fixation performance.

[0004] Therefore, developing a biochar improvement method that can achieve the synergistic effect of porous structure construction, active site enhancement, and structural stabilization is key to solving existing technical problems and improving the carbon fixation performance of biochar. Summary of the Invention

[0005] To address the problems in the prior art, this invention provides an improved method for enhancing the carbon fixation capacity of biochar.

[0006] The technical solution adopted by this invention to solve its technical problem is: an improved method for enhancing the carbon fixation capacity of biochar, comprising the following steps: S1. Preparation of biochar matrix: Agricultural and forestry waste is crushed to 80-120 mesh, microwave dried at 150-200℃ for 2-3 hours, and then pre-carbonized at 400-500℃ in an inert atmosphere for 1 hour to obtain a porous biochar matrix. S2. MOF precursor loading: The porous biochar matrix was immersed in ZIF-8 precursor solution, ultrasonically dispersed for 30-60 min and allowed to stand for loading for 12 h, filtered and dried at 80-100℃ for 4-6 h. S3. Gradient calcination-plasma activation coupling treatment: The dried product from step S2 is subjected to gradient calcination, with nitrogen gas introduced at a rate of 50-100 mL / min throughout the process. After calcination, a mixed plasma of argon and ammonia with a volume ratio of 3:1 is introduced with a discharge power of 150-250 W for 20-40 min to obtain a MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles. The particle size of the transition metal nanoparticles is 5-20 nm. S4. Grafting with bio-based crosslinking agent: The composite obtained in S3 is immersed in a chitosan-sodium alginate mixed solution at a mass ratio of 1:10-12, the pH of the system is adjusted to 4.0-5.5, 0.6-1% of the mass of the mixed solution of initiator is added, and the reaction is carried out at 60-70℃ for 3-4 hours; after washing with deionized water 3-5 times, it is vacuum dried for 4-6 hours to obtain composite modified biochar.

[0007] As a further technical solution, the agricultural and forestry waste mentioned in step S1 is at least one of corn stalks, wheat stalks, and wood chips.

[0008] As a further technical solution, the inert atmosphere mentioned in step S1 is nitrogen.

[0009] As a further technical solution, the ZIF-8 precursor solution in step S2 uses methanol, ethanol or a mixture thereof as solvent, wherein the concentration of zinc nitrate is 0.1-0.3 mol / L, the molar ratio of zinc nitrate to 2-methylimidazole is 1:4, and it contains 1-3% of transition metal salts in the molar fraction of zinc nitrate.

[0010] As a further technical solution, the transition metal salt is one or a mixture of two of ferric chloride and cobalt nitrate.

[0011] As a further technical solution, the gradient calcination in step S3 specifically involves: heating at 5-8℃ / min to 300℃ and holding for 1 hour, heating at 3-5℃ / min to 500℃ and holding for 2 hours, and heating at 2-3℃ / min to 700-800℃ and holding for 1.5 hours.

[0012] As a further technical solution, in step S4, the mass ratio of chitosan to sodium alginate in the chitosan-sodium alginate mixed solution is 1:2, and the solvent is an acetic acid solution with a concentration of 1-2wt%.

[0013] As a further technical solution, the chitosan-sodium alginate mixed solution contains 0.3-0.8% epichlorohydrin by mass of the total mixed solution.

[0014] As a further technical solution, the initiator in step S4 is ammonium persulfate.

[0015] As a further technical solution, the vacuum drying temperature in step S4 is 60-80℃, and the vacuum degree is 0.02Pa.

[0016] The beneficial effects of this invention are: 1. This invention optimizes the biochar modification process, organically combining porous matrix preparation, MOF precursor loading, gradient calcination-plasma activation coupling treatment, and bio-based crosslinking agent grafting. Each step and feature works together and synergistically to form a complete technical solution from microstructure regulation to macro performance improvement, resulting in significant technical effects.

[0017] 2. In step S1 of this invention, agricultural and forestry waste is crushed to 80-120 mesh and microwave-dried at 150-200℃. This not only fully removes moisture and volatile impurities from the raw materials, preventing pore collapse caused by rapid moisture evaporation during subsequent carbonization, but also improves the uniformity of the raw materials, laying the foundation for the formation of a regular porous structure during subsequent pre-carbonization. The pre-carbonization treatment at 400-500℃ under a nitrogen atmosphere can slowly decompose organic components such as lignin and cellulose in the raw materials, gradually forming a preliminary porous framework. This solves the problems of low porosity and small specific surface area of ​​traditional biochar matrix, providing sufficient adhesion space for the uniform loading of subsequent MOF precursors. In step S2, the ZIF-8 precursor solution is prepared at a molar ratio of zinc nitrate to 2-methylimidazole of 1:4, which can generate structurally stable MOF crystals in situ within the porous structure of biochar. The addition of transition metal salts (ferric chloride, cobalt nitrate) will be converted into 5-20 nm transition metal nanoparticles during the subsequent calcination process. These nanoparticles serve as catalytic active centers, which can reduce the energy barrier for carbon adsorption and fixation. At the same time, the nitrogen-doped carbon quantum dots derived from MOF have ultra-high specific surface area and excellent electronic conductivity, which can further increase the density of active sites on the surface of biochar, thereby solving the core defects of insufficient active sites and weak carbon fixation power in traditional biochar. The gradient calcination in step S3 employs a segmented heating mode. First, the temperature is increased to 300℃ at 5-8℃ / min to remove impurities. Then, the temperature is increased to 500℃ at 3-5℃ / min to promote the decomposition of MOF crystals and the formation of carbon quantum dots. Finally, the temperature is increased to 700-800℃ at 2-3℃ / min to strengthen the crystal structure. With nitrogen protection throughout the process, the oxidation loss of carbon structure at high temperatures is effectively avoided. The argon-ammonia mixed plasma activation treatment, through high-energy particle bombardment, not only further etches the pores and expands the specific surface area, but also introduces nitrogen into the carbon structure and activates the catalytic activity of transition metal nanoparticles. This allows MOF-derived carbon to form a strong chemical bond with the biochar matrix, solving the problems of uneven pore structure and insufficient exposure of active sites caused by simple calcination. In step S4, chitosan and sodium alginate form a stable polymer network in a 1:2 mass ratio in a 1-2 wt% acetic acid solution. Epichlorohydrin acts as a crosslinking agent to enhance the network density. The composite reacts with the mixed solution at a mass ratio of 1:10-12. Under optimized conditions of pH 4.0-5.5 and 60-70℃, the polymer network can tightly coat the surface of the composite, preventing the loss of transition metal nanoparticles and carbon quantum dots and resisting the erosion of the carbon structure by the external environment. This solves the problems of easy detachment of active ingredients and poor structural stability of biochar surfaces.

[0018] 3. The synergistic effect of each step in this invention forms a system of porous matrix construction, active ingredient loading, coupling activation and strengthening, and cross-linking stabilization and reinforcement. The porous biochar matrix prepared in S1 provides a uniform spatial carrier for the MOF precursor loading in S2, allowing MOF crystals to penetrate deep into the pores rather than accumulate on the surface, ensuring a uniform distribution of the subsequently formed active sites. The MOF precursor loaded in S2 gradually decomposes during the gradient calcination in S3, and the resulting nitrogen-doped carbon quantum dots and transition metal nanoparticles are tightly bound to the inner wall of the biochar pores. After plasma activation treatment, the pore structure is further optimized and the active sites are fully activated, at which point the adsorption and catalytic carbon fixation capabilities of the biochar are significantly improved. The bio-based cross-linking agent grafting in S4, based on the above-mentioned optimized structure, firmly fixes the active ingredients on the biochar matrix through the coating and cross-linking of the polymer network, while improving the overall density and stability of the structure, avoiding the loss of active sites and the collapse of the pore structure during use. This step-by-step collaborative design enables the porous structure, active sites, and structural stability to support each other: the porous structure provides attachment space for the active sites, the active sites improve carbon fixation efficiency, and the structural stability ensures the sustainability of the carbon fixation effect, thus completely solving the technical problem that single treatment in traditional improvement methods cannot take into account both carbon fixation capacity and stability.

[0019] 4. This invention not only provides an efficient and stable biochar carbon sequestration and improvement technology, but also provides a practical solution for addressing climate change and promoting green agricultural development by increasing soil carbon storage and improving soil structure. It has significant environmental, economic and social value. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides an improved method for enhancing the carbon fixation capacity of biochar, comprising the following steps: S1: Biochar matrix preparation, S2: MOF precursor loading, S3: Gradient calcination-plasma activation coupling treatment, S4: Bio-based crosslinking agent grafting, finally obtaining composite modified biochar.

[0022] Detailed instructions for each step include: S1: Preparation of biochar matrix: In this invention, the agricultural and forestry waste is preferably at least one of corn stalks, wheat stalks, and wood chips, which can be obtained through recycling methods well known to those skilled in the art or through commercial channels.

[0023] Agricultural and forestry waste is pulverized to a fineness of 80-120 mesh, then microwave-dried and pre-carbonized. The microwave drying temperature is preferably 150℃-200℃, and the drying time is preferably 2-3 hours. Pre-carbonization must be carried out under an inert atmosphere, preferably nitrogen, at a temperature of 400℃-500℃ for 1 hour, ultimately yielding a porous biochar matrix.

[0024] S2: MOF precursor load: The porous biochar matrix needs to be immersed in a ZIF-8 precursor solution for loading treatment. The solvent of the ZIF-8 precursor solution is preferably methanol, ethanol or a mixture thereof, wherein the concentration of zinc nitrate is preferably 0.1 mol / L-0.3 mol / L, and the molar ratio of zinc nitrate to 2-methylimidazole is fixed at 1:4.

[0025] The solution must contain a transition metal salt, preferably one or a mixture of two of ferric chloride and cobalt nitrate, with a content of 1%-3% of the molar fraction of zinc nitrate.

[0026] During the loading process, the mixture is first ultrasonically dispersed for 30-60 minutes, followed by static loading for 12 hours. After filtration, it is dried at a temperature of 80-100℃ for 4-6 hours.

[0027] S3: Gradient roasting-plasma activation coupling treatment: The dried product needs to undergo gradient calcination and plasma activation treatment sequentially. The preferred gradient calcination process is as follows: heat to 300℃ at a rate of 5℃ / min-8℃ / min, hold for 1 hour; then heat to 500℃ at a rate of 3℃ / min-5℃ / min, hold for 2 hours; finally heat to 700℃-800℃ at a rate of 2℃ / min-3℃ / min, hold for 1.5 hours. Nitrogen gas must be introduced throughout the calcination process, with a preferred nitrogen flow rate of 50mL / min-100mL / min.

[0028] After calcination, an argon-ammonia mixed plasma is introduced, with the volume ratio of the mixed gas fixed at 3:1. The discharge power is preferably 150W-250W, and the processing time is preferably 20min-40min. Finally, a MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles is obtained. The particle size of the transition metal nanoparticles is 5nm-20nm, and it is commercially available.

[0029] S4: Grafting with bio-based cross-linking agent: The composite needs to be immersed in a chitosan-sodium alginate mixed solution for grafting reaction. The preferred mass ratio of the composite to the mixed solution is 1:10-12, and the preferred concentration of the mixed solution is 8 g / L-10 g / L, wherein the mass ratio of chitosan to sodium alginate is fixed at 1:2, and the solvent is a 1wt%-2wt% acetic acid solution.

[0030] The mixed solution must contain epichlorohydrin, with a content of 0.3%-0.8% of the total mass of the mixed solution. The pH value of the reaction system needs to be adjusted to 4.0-5.5. After adding the initiator, the temperature is raised to react. The preferred initiator is ammonium persulfate, with an addition amount of 0.6% to 1% of the mass of the mixed solution. The preferred reaction temperature is 60℃-70℃, and the preferred reaction time is 3h-4h.

[0031] After the reaction is complete, the product is washed with deionized water 3 to 5 times, and then vacuum dried. The vacuum drying temperature is preferably 60℃-80℃, the vacuum degree is fixed at 0.02Pa, and the drying time is preferably 4h-6h, finally obtaining composite modified biochar.

[0032] The improved method provided by this invention significantly increases the specific surface area and number of active sites of biochar through the synergistic effect of MOF precursor loading, gradient calcination-plasma activation coupling treatment, and grafting with bio-based crosslinking agents, thereby enhancing carbon fixation capacity and stability. At the same time, the raw materials are widely available, the process is easy to operate, and it is suitable for large-scale production.

[0033] To further illustrate the present invention, detailed descriptions are provided below through examples, comparative examples, and experiments.

[0034] Example 1: S1: Preparation of biochar matrix: Corn stalks were selected as agricultural and forestry waste, crushed to 100 mesh, dried by microwave at 180℃ for 2.5h, and then pre-carbonized at 450℃ under nitrogen atmosphere for 1h to obtain a porous biochar matrix.

[0035] S2: MOF precursor loading: A ZIF-8 precursor solution was prepared using methanol as the solvent, with a zinc nitrate concentration of 0.2 mol / L and a zinc nitrate to 2-methylimidazole molar ratio of 1:4. Ferric chloride with a zinc nitrate molar fraction of 1.5% was added as a transition metal salt. The porous biochar matrix was immersed in this solution, ultrasonically dispersed for 45 min, and allowed to stand for loading for 12 h. After filtration, it was dried at 90 °C for 5 h.

[0036] S3: Gradient calcination-plasma activation coupling treatment: The dried product was subjected to gradient calcination, with the temperature increased to 300℃ at 6℃ / min and held for 1 h, increased to 500℃ at 4℃ / min and held for 2 h, and increased to 750℃ at 2.5℃ / min and held for 1.5 h, with nitrogen gas introduced at 75 mL / min throughout the process. After calcination, a mixed plasma of argon and ammonia (volume ratio 3:1) was introduced with a discharge power of 200 W for 30 min to obtain MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles.

[0037] S4: Bio-based crosslinking agent grafting: The above composite was immersed in a 9 g / L chitosan-sodium alginate mixed solution at a mass ratio of 1:11. The mass ratio of chitosan to sodium alginate in the mixed solution was 1:2. The solvent was a 1.5 wt% acetic acid solution containing 0.5% epichlorohydrin by mass of the mixed solution. The pH of the system was adjusted to 4.8, and 0.8% ammonium persulfate by mass of the mixed solution was added as an initiator. The reaction was carried out at 65 °C for 3.5 h. After washing four times with deionized water, the composite modified biochar was obtained by vacuum drying at 70 °C and 0.02 Pa for 5 h.

[0038] Example 2: S1: Preparation of biochar matrix: Wheat straw was selected as agricultural and forestry waste, crushed to 80 mesh, dried by microwave at 150℃ for 2 hours, and then pre-carbonized at 400℃ under nitrogen atmosphere for 1 hour to obtain a porous biochar matrix.

[0039] S2: MOF precursor loading: A ZIF-8 precursor solution was prepared using ethanol as the solvent, with a zinc nitrate concentration of 0.1 mol / L and a zinc nitrate to 2-methylimidazole molar ratio of 1:4. Cobalt nitrate, with a zinc nitrate molar fraction of 1%, was added as a transition metal salt. The porous biochar matrix was immersed in this solution, ultrasonically dispersed for 30 min, and allowed to stand for 12 h for loading. After filtration, it was dried at 80 °C for 4 h.

[0040] S3: Gradient calcination-plasma activation coupling treatment: The dried product was subjected to gradient calcination, with the temperature increased to 300℃ at 5℃ / min and held for 1 h, increased to 500℃ at 3℃ / min and held for 2 h, and increased to 700℃ at 2℃ / min and held for 1.5 h, with nitrogen gas introduced at 50 mL / min throughout the process. After calcination, a mixed plasma of argon and ammonia (volume ratio 3:1) was introduced with a discharge power of 150 W for 20 min to obtain MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles.

[0041] S4: Bio-based crosslinking agent grafting: The above composite was immersed in an 8 g / L chitosan-sodium alginate mixed solution at a mass ratio of 1:10. The mass ratio of chitosan to sodium alginate in the mixed solution was 1:2. The solvent was a 1 wt% acetic acid solution containing 0.3% epichlorohydrin by mass of the mixed solution. The pH of the system was adjusted to 4.0, and 0.6% ammonium persulfate by mass of the mixed solution was added as an initiator. The reaction was carried out at 60℃ for 3 h. After washing three times with deionized water, the composite modified biochar was vacuum dried at 60℃ and 0.02 Pa for 4 h to obtain the composite modified biochar.

[0042] Example 3: S1: Preparation of biochar matrix: Wood chips were selected as agricultural and forestry waste, crushed to 120 mesh, dried by microwave at 200℃ for 3 hours, and then pre-carbonized at 500℃ under nitrogen atmosphere for 1 hour to obtain a porous biochar matrix.

[0043] S2: MOF precursor loading: A ZIF-8 precursor solution was prepared using a mixture of methanol and ethanol (volume ratio 1:1), with a zinc nitrate concentration of 0.3 mol / L and a zinc nitrate to 2-methylimidazole molar ratio of 1:4. A 3% molar fraction of zinc nitrate was added to a mixed salt of ferric chloride and cobalt nitrate (mass ratio 1:1) as a transition metal salt. The porous biochar matrix was immersed in this solution, ultrasonically dispersed for 60 min, and allowed to stand for 12 h for loading. After filtration, it was dried at 100 °C for 6 h.

[0044] S3: Gradient calcination-plasma activation coupling treatment: The dried product was subjected to gradient calcination, with the temperature increased to 300℃ at 8℃ / min and held for 1 h, increased to 500℃ at 5℃ / min and held for 2 h, and increased to 800℃ at 3℃ / min and held for 1.5 h, with nitrogen gas introduced at 100 mL / min throughout the process. After calcination, a mixed plasma of argon and ammonia (volume ratio 3:1) was introduced with a discharge power of 250 W for 40 min to obtain MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles.

[0045] S4: Grafting with bio-based crosslinking agent: The above composite was immersed in a 10 g / L chitosan-sodium alginate mixed solution at a mass ratio of 1:12. The mass ratio of chitosan to sodium alginate in the mixed solution was 1:2. The solvent was a 2 wt% acetic acid solution containing 0.8% epichlorohydrin by mass of the mixed solution. The pH of the system was adjusted to 5.5, and 1% ammonium persulfate by mass of the mixed solution was added as an initiator. The reaction was carried out at 70 °C for 4 h. After washing five times with deionized water, the composite modified biochar was obtained by vacuum drying at 80 °C and 0.02 Pa for 6 h.

[0046] Comparative Example 1: S2: MOF precursor loading step is omitted, and the porous biochar matrix obtained in S1 is directly processed.

[0047] The remaining steps are exactly the same as in Example 1.

[0048] Comparative Example 2: Only gradient calcination was performed, with the steps being exactly the same as the gradient calcination process in S3 of Example 1, omitting the plasma activation treatment, to obtain MOF-derived nitrogen-doped carbon quantum dot / biochar composite.

[0049] The remaining steps are exactly the same as in Example 1.

[0050] Comparative Example 3: The grafting step of the bio-based crosslinking agent in S4 was omitted. The composite obtained in S3 was directly washed with deionized water 4 times and vacuum dried at 70℃ and 0.02Pa for 5h to obtain modified biochar.

[0051] The remaining steps are exactly the same as in Example 1.

[0052] Comparative Example 4: S2: MOF precursor loading: A ZIF-8 precursor solution was prepared using methanol as the solvent, with a zinc nitrate concentration of 0.2 mol / L and a zinc nitrate to 2-methylimidazole molar ratio of 1:4. No transition metal salts were added. Subsequent operations were exactly the same as in S2 of Example 1.

[0053] The remaining steps are exactly the same as in Example 1.

[0054] test: Experiment 1: Carbon fixation capacity test; Referring to GB / T32737-2016, 2g of modified biochar sample was weighed and thoroughly mixed with 100g of air-dried soil (pH 6.5, organic matter content 15g / kg). The mixture was then placed in a constant temperature and humidity incubator and cultured for 30 days at 25℃ and 60% relative humidity. After the culture period, the soil organic carbon content was determined using the potassium dichromate oxidation-external heating method, and the carbon sequestration capacity per unit mass of biochar (mg / g) was calculated. The results are as follows: Table 1

[0055] As can be seen from Table 1, the carbon fixation amount of Examples 1-3 is significantly higher than that of the comparative examples, indicating that the improved method of the present invention can effectively enhance the carbon fixation capacity of biochar.

[0056] Experiment 2: Specific surface area and pore size distribution test; Referring to GB / T19587-2017, the liquid nitrogen adsorption-desorption method was used. Nitrogen adsorption-desorption isotherms were tested on the samples at 77 K, and the specific surface area (m²) of the samples was calculated using the BET model. 2 The average pore size (nm) was analyzed using the BJH model, and the results are as follows: Table 2

[0057] As can be seen from Table 2, the BET specific surface area of ​​Examples 1-3 all exceeds 1150 m². 2 / g, with an average pore size concentrated in 2.5-3.2nm, and the suitable pore structure provides sufficient space and adsorption sites for carbon fixation.

[0058] Experiment 3: Carbon stability test; Farmland soil was collected according to GB / T22102-2008 "Soil Quality - Collection, Processing and Storage of Soil Samples". 2g of modified biochar was mixed evenly with 100g of soil and placed in a constant-temperature oxidation chamber. An accelerated oxidation experiment was conducted at 80℃ and 50% relative humidity for 60 days. The organic carbon content of the soil before and after oxidation was measured, and the carbon sequestration retention rate (%) was calculated. A higher retention rate indicates stronger carbon sequestration stability. The results are as follows: Table 3

[0059] As can be seen from Table 3, the carbon fixation retention rate of Examples 1-3 all exceeded 88%, with Example 3 reaching 94%, indicating that the composite modified biochar has excellent carbon fixation stability.

[0060] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method of improving the carbon sequestration capacity of biochar, the method comprising, Includes the following steps: S1. Preparation of biochar matrix: Agricultural and forestry waste is crushed to 80-120 mesh, microwave dried at 150-200℃ for 2-3 hours, and then pre-carbonized at 400-500℃ in an inert atmosphere for 1 hour to obtain a porous biochar matrix. S2. MOF precursor loading: The porous biochar matrix was immersed in ZIF-8 precursor solution, ultrasonically dispersed for 30-60 min and allowed to stand for loading for 12 h, filtered and dried at 80-100℃ for 4-6 h. S3. Gradient calcination-plasma activation coupling treatment: The dried product from step S2 is subjected to gradient calcination, with nitrogen gas introduced at a rate of 50-100 mL / min throughout the process. After calcination, a mixed plasma of argon and ammonia with a volume ratio of 3:1 is introduced with a discharge power of 150-250 W for 20-40 min to obtain MOF-derived nitrogen-doped carbon quantum dot / biochar composite loaded with transition metal nanoparticles. S4. Grafting with bio-based crosslinking agent: The composite obtained in S3 is immersed in a chitosan-sodium alginate mixed solution at a mass ratio of 1:10-12, the pH of the system is adjusted to 4.0-5.5, 0.6-1% of the mass of the mixed solution of initiator is added, and the reaction is carried out at 60-70℃ for 3-4 hours; after washing with deionized water 3-5 times, it is vacuum dried for 4-6 hours to obtain composite modified biochar.

2. The improved method of claim 1, wherein, The agricultural and forestry waste mentioned in step S1 is at least one of corn stalks, wheat stalks, and wood chips.

3. The improved method of claim 1, wherein, The inert atmosphere mentioned in step S1 is nitrogen.

4. The improved method of claim 1 wherein, The ZIF-8 precursor solution in step S2 uses methanol, ethanol or a mixture thereof as solvent, wherein the concentration of zinc nitrate is 0.1-0.3 mol / L, the molar ratio of zinc nitrate to 2-methylimidazole is 1:4, and it contains 1-3% of transition metal salts in the molar fraction of zinc nitrate.

5. The improved method of claim 4, wherein, The transition metal salt is one or a mixture of two of ferric chloride and cobalt nitrate.

6. The improved method of claim 1 wherein, The gradient calcination in step S3 specifically involves: heating at 5-8℃ / min to 300℃ and holding for 1 hour, heating at 3-5℃ / min to 500℃ and holding for 2 hours, and heating at 2-3℃ / min to 700-800℃ and holding for 1.5 hours.

7. The improved method of claim 1 wherein, In step S4, the mass ratio of chitosan to sodium alginate in the chitosan-sodium alginate mixed solution is 1:2, and the solvent is an acetic acid solution with a concentration of 1-2 wt%.

8. The improved method according to claim 1, characterized in that, The chitosan-sodium alginate mixed solution contains 0.3-0.8% epichlorohydrin by mass of the total mixed solution.

9. The improved method according to claim 1, characterized in that, The initiator mentioned in step S4 is ammonium persulfate.

10. The improved method of claim 1 wherein, In step S4, the vacuum drying temperature is 60-80℃ and the vacuum degree is 0.02Pa.

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