A gradient magnetic biochar, its preparation method and application

Abstract

This invention provides a gradient magnetic biochar, its preparation method, and its applications, belonging to the field of solid waste resource utilization and anaerobic fermentation technology. Through a synergistic modification strategy of gradient alkaline thermal activation and atmosphere-controlled co-precipitation, this invention significantly improves the electron transport efficiency and catalytic activity against chain-elongating microorganisms, thereby efficiently promoting the directed synthesis of medium-chain carboxylic acids at low doses. This not only improves fermentation performance but also significantly reduces the cost of addition, providing an innovative solution for low-carbon and efficient treatment of livestock and poultry manure and a substantial increase in the yield of high-value-added medium-chain carboxylic acids. It has significant practical application value and broad industrialization prospects.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and anaerobic fermentation technology, specifically relating to a gradient magnetic biochar, its preparation method, and its application. Background Technology

[0002] With continuous population growth and the development of animal husbandry, a large amount of organic waste has been generated. This rapid increase has placed enormous pressure on the ecological environment. Anaerobic fermentation technology is a key approach to treating the surge in organic waste such as livestock manure and achieving resource utilization. Its target product—medium-chain carboxylic acids (containing 6-12 carbon atoms)—has attracted much attention due to its high economic value. However, this process is often limited by low electron transfer efficiency and insufficient microbial activity, restricting the efficient synthesis of medium-chain carboxylic acids (such as hexanoic acid and octanoic acid).

[0003] To address this bottleneck, enhancing electron transport with conductive materials has become an emerging strategy. Biochar, with its excellent conductivity, large specific surface area, and abundant functional groups, shows potential as an electron shuttle. However, the performance of traditional biochar is inconsistent and needs improvement, necessitating the development of efficient and economical modified materials. Summary of the Invention

[0004] The purpose of this invention is to provide a gradient magnetic biochar, its preparation method, and its application. The biochar is prepared by a synergistic modification strategy of gradient alkaline thermal activation and atmosphere-controlled co-precipitation, which significantly improves its electron transport efficiency and catalytic activity against chain-elongating microorganisms, thereby efficiently promoting the directional synthesis of medium-chain carboxylic acids at low doses.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a method for preparing gradient magnetic biochar, comprising:

[0007] 1) Crush and sieve the peanut shells, place the peanut shell powder in an inert gas atmosphere, heat to carry out pyrolysis reaction, cool, wash and dry after the reaction is completed to obtain peanut shell biochar powder;

[0008] 2) The peanut shell biochar powder is impregnated in a 1-3 M low-concentration NaOH solution and ultrasonically vibrated to carry out the first stage of heating reaction at a temperature of 50-70°C for 1-3 hours; then, a 7-10 M high-concentration NaOH solution is added to carry out the second stage of heating reaction at a temperature of 100-120°C for 1-3 hours to obtain an alkali-modified peanut shell biochar solution.

[0009] 3) Under inert gas protection, ferrous divalent and ferric trivalent salts are dissolved and mixed using a magnetic stirrer with a magnetic field strength of 0.2~0.4 T to obtain a mixed iron salt solution. Under continuous stirring, alkaline solution is added dropwise to the mixed iron salt solution to generate and grow Fe3O4 nanoparticles in situ, and they connect with each other under the induction of magnetic field to form a magnetic particle network that permeates the biochar conductive network.

[0010] 4) Add the alkali-modified peanut shell biochar solution obtained in step 2) to the system obtained in step 3), and stir continuously under the protection of inert gas to allow magnetic particles to be generated and combined in situ inside the pores of the biochar, forming a composite material with electronic mediation function.

[0011] 5) The composite material obtained in step 4) is vacuum filtered to separate the solid and dried to obtain gradient magnetic biochar.

[0012] Preferably, in step 1), the peanut shells are crushed and sieved to a particle size of less than 75 μm; the inert gas is nitrogen; the heating rate of the pyrolysis reaction is controlled at 5~10 ℃ / min, the pyrolysis temperature range is 400~500 ℃, and the pyrolysis time is 1~2 h; the washing water is deionized water, and the washing is performed no less than 3 times to ensure the removal of impurities generated during the pyrolysis process.

[0013] Preferably, in step 2), the ratio of peanut shell biochar powder to low-concentration NaOH solution is 1 g: 5~15 mL; and the ratio to high-concentration NaOH solution is 1 g: 5~10 mL.

[0014] Preferably, in step 2), the ultrasonic oscillation power is 200~500 W and the ultrasonic oscillation time is 0.5~3h.

[0015] Preferably, in step 4), the molar ratio of carbon to iron is 2:1 to 4:1.

[0016] Preferably, in step 5), the drying temperature is 50~100℃ and the drying time is 18~24h.

[0017] The present invention also provides a gradient magnetic biochar, which is obtained by the above preparation method.

[0018] The present invention further provides the application of the above-mentioned gradient magnetic biochar in promoting the synthesis of carboxylic acids in an anaerobic fermentation system at a dosage of 2.5~15 g / L.

[0019] Preferably, the application includes: under fermentation conditions of pH 5.0~5.5 and temperature 30~35℃, after adding the gradient magnetic biochar, the redox performance of the anaerobic fermentation system is enhanced, the electrochemical impedance value is significantly reduced, and the yield of hexanoic acid is increased by more than 50%.

[0020] Preferably, the medium-chain carboxylic acid includes at least one of hexanoic acid, heptanoic acid, and octanoic acid.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention uses peanut shells, a common agricultural waste, as raw material and innovatively combines gradient alkaline thermal activation with atmosphere-controlled co-precipitation to prepare alkali-modified peanut shell magnetic biochar. Through a gradient alkaline thermal activation process, first using a low-concentration alkali solution for gentle etching, followed by a high-concentration alkali solution for deep reaction, a high-density alkaline functional group and a mesoporous conductive network framework are precisely constructed on the biochar surface; through Fe... 2+ / Fe 3+ In the atmosphere-controlled co-precipitation process, under inert gas protection and a magnetic field strength of 0.2~0.4T, an iron salt solution is impregnated into the mesopores of the above-mentioned biochar, and an alkaline solution is slowly added dropwise, so that magnetic Fe3O4 nanoparticles are generated in situ and highly dispersed inside the pores.

[0023] In anaerobic fermentation systems, the highly conductive carbon skeleton of alkali-modified peanut shell magnetic biochar provides an efficient channel for electron transfer between microorganisms. Simultaneously, the loaded Fe3O4 not only further promotes electron transfer due to its high conductivity, but the Fe(III) / Fe(II) cycle on its surface also catalyzes the conversion of intermediate metabolites. The synergistic effect of these two factors significantly improves electron transfer efficiency. Specifically, in electrochemical tests conducted in 0.1 M KCl solution, its charge transfer resistance and redox performance are significantly lower than those of a single biochar network or a single Fe3O4 magnetic particle material under the same conditions. This guides and drives more electron flow to the chain elongation synthesis pathway of medium-chain carboxylic acids (such as hexanoic acid and octanoic acid). When this biochar is added at a low dose of 1–5 g / L to fermentation substrates containing volatile fatty acid precursors, under conditions of pH 5.0–7.5 and temperature 30–35 °C, its in-situ electron-mediated function enhances interspecific electron transfer, drives chain elongation metabolism, and increases the yield of medium-chain carboxylic acids by 30%–50%. This not only improves fermentation performance but also significantly reduces additive costs. This technology provides an innovative solution for low-carbon and efficient treatment of livestock and poultry manure and a substantial increase in the yield of high-value-added medium-chain carboxylic acids, possessing significant practical application value and broad industrialization prospects. Attached Figure Description

[0024] Figure 1 Scanning electron microscope image of gradient biochar (molar ratio of carbon to iron 2:1) prepared in Example 1 of this invention.

[0025] Figure 2 Scanning electron microscope image of gradient biochar (molar ratio of carbon to iron 4:1) prepared in Example 2 of this invention.

[0026] Figure 3 This is the redox curve tested in Example 3 of the present invention.

[0027] Figure 4 This is the electrochemical impedance spectroscopy measured in Example 3 of the present invention.

[0028] Figure 5 The redox curves (including CK and MB four addition amounts) are tested in Example 4 of the present invention.

[0029] Figure 6 The electrochemical impedance spectroscopy (including CK and MB four addition amounts) was tested in Example 4 of the present invention.

[0030] Figure 7 The gradient biochar (addition amount of 2.5 g / L) prepared in Example 1 of this invention was applied to the change of hexanoic acid concentration after anaerobic fermentation.

[0031] Figure 8 The gradient biochar (addition amount of 5 g / L) prepared in Example 1 of this invention was applied to the change of hexanoic acid concentration after anaerobic fermentation.

[0032] Figure 9 The gradient biochar (addition amount of 10 g / L) prepared in Example 1 of the present invention was applied to the change of hexanoic acid concentration after anaerobic fermentation.

[0033] Figure 10 The gradient biochar (addition amount of 15 g / L) prepared in Example 1 of the present invention was applied to the change of hexanoic acid concentration after anaerobic fermentation.

[0034] Figure 11 The variation in hexanoic acid concentration in Comparative Example 1 of this invention is shown.

[0035] Figure 12 The variation in hexanoic acid concentration in Comparative Example 2 of this invention is shown.

[0036] Figure 13 The variation in hexanoic acid concentration in Comparative Example 3 of this invention is shown. Detailed Implementation

[0037] To more clearly illustrate the present invention, the invention will be further described in detail below with reference to embodiments and accompanying drawings. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0038] Example 1

[0039] Step 1: After drying peanut shells at 55℃, crush them through a 200-mesh sieve, heat them to 400℃ at 10℃ / min under a nitrogen atmosphere and keep them at that temperature for 2 hours. After the reaction, allow them to cool naturally, wash them three times with deionized water, and vacuum dry them at 80℃ for 12 hours to obtain peanut shell biochar powder.

[0040] Step 2: Take 24g of biochar obtained in Step 1, mix it with 1M NaOH solution at a solid-liquid ratio of 1:10, and react at 60℃ for 3 hours; then add 10M NaOH solution to bring the total solid-liquid ratio to 1:15, and continue to react at 110℃ for 1 hour to obtain gradient alkali modified biochar.

[0041] Step 3: Under nitrogen protection and a magnetic field strength of 0.3T, dissolve and mix 0.5 mol FeCl2·4H2O and 0.5 mol FeCl3·6H2O (carbon-iron molar ratio 2:1) in the graded alkali-modified biochar obtained in Step 2, and adjust the pH to 10.0. Add the biochar suspension and stir the reaction for 2 hours. After filtration and washing, the product is dried at 80℃ for 24 hours to obtain graded magnetic biochar.

[0042] The product scanning electron microscope image is as follows: Figure 1 As shown, after gradient alkaline thermal activation, the magnetic biochar exhibits an irregular lamellar-flocculated aggregate structure. The mild etching with low-concentration alkaline solution resulted in significant breakage, curling, and peeling at the edges of the carbon skeleton; the subsequent high-concentration deep reaction further reconstructed the residual carbon into flocculent bodies, which then overlapped to form a conductive network dominated by mesopores. The significant increase in surface roughness confirms the successful grafting of high-density alkaline functional groups.

[0043] Example 2

[0044] The difference from Example 1 is that in step 3, the carbon-iron molar ratio is 4:1. The scanning electron microscope image of the product is shown below. Figure 2 As shown, under a relatively high carbon-iron molar ratio (4:1), the obtained magnetic biochar still maintains a blocky-lamellar composite structure dominated by biochar, with a particle size significantly larger than that of Example 1, exhibiting a relatively complete carbon skeleton morphology. The biochar surface has clear layered textures and banded folded structures, indicating that the carbon layer stacking characteristics of the original peanut shell biochar after alkali modification are well preserved. The particulate iron-based adhering substances on the biochar surface are significantly reduced, mainly consisting of scattered small bright spots or localized deposition areas, without large-area or high-density iron-based particle coverage. This indicates that under relatively low iron addition conditions, iron species are mainly dispersed on the biochar surface and in its microcracks in the form of embedding or local anchoring, without forming a continuous or dense deposition layer.

[0045] Example 3

[0046] To systematically evaluate the electron transfer performance of the gradient magnetic biochar prepared in this invention, we set up multiple control experiments to verify its synergistic effect. The C / Fe ratio of the Fe3O4-loaded samples was uniformly 2:1: samples that were only activated by gradient alkali without loading magnetic particles were denoted as C-BC; samples that were only loaded with Fe3O4 without gradient alkali modification were denoted as M-BC; magnetic biochar prepared by one-step alkali treatment and co-precipitation (non-gradient, no magnetic field induction) was denoted as S-BC; the sample of this invention was magnetic biochar synergistically modified by gradient alkali thermal activation and atmosphere-controlled co-precipitation (denoted as M2). Key electrochemical characterization results are as follows: Figure 3 As shown, among all tested samples, only M2 exhibited a significant and reversible redox peak within the characteristic potential range, with a peak current much higher than other control samples. C-BC, on the other hand, showed the lowest redox activity. This directly confirms that the present invention successfully constructed a high-density electroactive site through synergistic modification, fundamentally enhancing its intrinsic redox activity and electron exchange capacity. Further electrochemical impedance spectroscopy analysis (…) Figure 4 The results show that the Nyquist curve of M2 has the smallest diameter and the steepest slope in the low-frequency region, and its charge transfer resistance (Rct) is significantly lower than that of C-BC, M-BC, and S-BC, clearly revealing a significant optimization of its interfacial electron transfer kinetics. The above comparative results fully demonstrate the significant synergistic effect between gradient alkaline thermal activation and atmosphere-controlled co-precipitation. Neither a single modification method nor a non-synergistic process can achieve the excellent electron transfer performance of the material of this invention.

[0047] Example 4

[0048] To systematically evaluate the electron transfer performance of the gradient magnetic biochar prepared in this invention, we performed key electrochemical characterization. For example... Figure 5 As shown, all samples treated with gradient alkaline thermal activation and magnetic field induction (MB group) exhibited significant and reversible redox peaks within the characteristic potential range. Among them, sample M2-5, prepared under optimal conditions, showed the highest peak current. This directly confirms that a high density of electroactive sites was successfully constructed on the surface of the material, and its intrinsic redox activity and electron exchange capacity were fundamentally enhanced compared to the untreated control group (CK). Further electrochemical impedance spectroscopy analysis ( Figure 6 The results show that the MB group samples, especially M2-5, have the smallest diameter and the steepest slope in the low-frequency region of their Nyquist curves, which clearly reveals a significant reduction in their charge transfer resistance and a substantial optimization of the interface electron transfer kinetics.

[0049] Application Example 1

[0050] Using the gradient magnetic biochar (C / Fe molar ratio = 2:1) prepared in Example 1 as the material, the fermentation substrate was a fermentation broth rich in lactic acid and volatile fatty acids obtained from anaerobic fermentation of cow manure inoculated with lactic acid bacteria. This fermentation substrate was used in all subsequent application examples. 350 mL of substrate and 50 mL of inoculum (derived from a laboratory-run chain extension reactor, this inoculum was used in all subsequent application examples) were added sequentially to a 400 mL anaerobic serum bottle. After mixing, a blank control group (CK) and a treatment group (MB2.5, with 2.5 g / L biochar added) were set up, with three replicates for each group. The initial pH was adjusted to 5.5 ± 0.1 with 5 M NaOH. After sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment, and static fermentation was carried out in a constant temperature water bath at 30 ± 1 °C for 14 days.

[0051] The results are as follows Figure 7 As shown, compared with the control group, the addition of MB 2.5 g / L significantly increased the cumulative concentration of hexanoic acid in the system. The highest concentration of hexanoic acid in the CK group was approximately 6.29 g / L, while the highest concentration in the MB 2.5 experimental group reached 9.88 g / L, an increase of approximately 57%.

[0052] Application Example 2

[0053] Using the gradient magnetic biochar (C / Fe molar ratio = 2:1) prepared in Example 1 as the material, the fermentation substrate was a lactic acid and volatile fatty acid enrichment solution. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and a blank control group (CK) and a treatment group were prepared by adding 5 g / L biochar (MB5), with three replicates in each group. The initial pH was adjusted to 5.5 ± 0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out for 14 days in a constant temperature water bath at 30 ± 1 °C.

[0054] The results are as follows Figure 8 As shown, the highest concentration of hexanoic acid in the CK group was approximately 6.34 g / L, while the highest concentration in the MB5 experimental group reached 11.88 g / L, an increase of 87.38%. Compared with the CK group, the cumulative concentration of hexanoic acid was significantly increased after the addition of biochar, indicating that the addition of biochar can effectively enhance the chain elongation process and promote the formation of hexanoic acid.

[0055] Application Example 3

[0056] Using the gradient magnetic biochar (C / Fe molar ratio = 2:1) prepared in Example 1 as the material, the fermentation substrate was a lactic acid and volatile fatty acid enrichment solution. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and then a blank control group (CK) and a treatment group were prepared by adding 10 g / L biochar (MB10), with three replicates in each group. The initial pH was adjusted to 5.5 ± 0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out for 14 days in a constant temperature water bath at 30 ± 1 °C.

[0057] The results are as follows Figure 9 As shown, the concentration of hexanoic acid in the MB10 experimental group was significantly increased compared with the control group. The highest concentration of hexanoic acid in the CK group was only 6.46 g / L, while the peak concentration in the MB10 group reached 11.41 g / L, an increase of approximately 76.63%, indicating that the introduction of biochar has a significant promoting effect on hexanoic acid formation.

[0058] Application Example 4

[0059] Using the gradient magnetic biochar (C / Fe molar ratio = 2:1) prepared in Example 1 as the material, the fermentation substrate was a lactic acid and volatile fatty acid enrichment solution. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and then a blank control group (CK) and a treatment group were prepared by adding 15 g / L biochar (MB15), with three replicates in each group. The initial pH was adjusted to 5.5 ± 0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out for 14 days in a constant temperature water bath at 30 ± 1 °C.

[0060] As shown in Figure 10, the highest hexanoic acid concentration was observed in the CK group (approximately 6.41 g / L), while the concentration in the MB15 experimental group was significantly higher, reaching a peak of 11.38 g / L. Compared with the control group, the addition of biochar increased the hexanoic acid concentration by approximately 77.54%, indicating that the introduction of biochar significantly promoted the formation of hexanoic acid.

[0061] Comparative Example 1

[0062] Magnetic biochar (C / Fe molar ratio = 2:1) loaded with iron oxide but not subjected to gradient alkali-thermal activity was used as the material, and the fermentation substrate was a concentrated broth of lactic acid and volatile fatty acids. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and a blank control group (CK) and a treatment group were prepared by adding 5 g / L biochar (M-BC-5), with three replicates in each group. The initial pH was adjusted to 5.5±0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out for 14 days in a constant temperature water bath at 30±1℃.

[0063] As shown in Figure 11, the highest hexanoic acid concentration was found in the CK group (approximately 6.34 g / L), while the concentration in the M-BC-5 experimental group was relatively higher, reaching a peak of 8.88 g / L. Compared with the control group, the addition of biochar increased the hexanoic acid concentration by approximately 40.06%, indicating that although magnetic biochar without gradient alkaline thermal activation can promote hexanoic acid synthesis to some extent, its strengthening effect is limited. This confirms that simple iron loading cannot fully utilize the mediating role of biochar, and its performance is significantly inferior to that of gradient-activated modified materials.

[0064] Comparative Example 2

[0065] Biochar treated only with a gradient of thermal alkali was used as the material, and the fermentation substrate was a concentrated liquid of lactic acid and volatile fatty acids. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and then a blank control group (CK) and a treatment group were prepared with 5 g / L biochar (C-MC-5) added, with three replicates in each group. The initial pH was adjusted to 5.5±0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out for 14 days in a constant temperature water bath at 30±1℃.

[0066] As shown in Figure 12, the highest hexanoic acid concentration was observed in the CK group (approximately 6.34 g / L), while the concentration in the C-MC-5 experimental group was relatively higher, reaching a peak of 7.99 g / L. Compared to the control group, the addition of biochar increased the hexanoic acid concentration by approximately 26%, indicating that biochar treated only with gradient alkali without iron loading has a promoting effect on hexanoic acid formation. However, its efficiency is significantly lower than that of modified carbon materials containing metal components, confirming the key catalytic role of iron active sites in the chain elongation process.

[0067] Comparative Example 3

[0068] Magnetic biochar (C / Fe molar ratio = 2:1) treated with a one-step hot alkaline process and loaded with iron oxide was used as the fermentation material. The fermentation substrate was a concentrated liquid of lactic acid and volatile fatty acids. 350 mL of substrate and 50 mL of inoculum were added sequentially to a 400 mL anaerobic serum bottle, mixed thoroughly, and a blank control group (CK) and a treatment group were set up with 5 g / L biochar (S-BC-5) added, with three replicates in each group. The initial pH was adjusted to 5.5±0.1 with 5 M NaOH, and after sealing, the headspace was purged with nitrogen for 10 min to establish an anaerobic environment. Static fermentation was carried out in a constant temperature water bath at 30±1℃ for 14 days.

[0069] As shown in Figure 13, the highest hexanoic acid concentration was found in the CK group (approximately 6.34 g / L), while the concentration in the MB15 experimental group was significantly higher, reaching a peak of 9.71 g / L. Compared to the control group, the addition of biochar increased the hexanoic acid concentration by approximately 53.15%, indicating that the magnetic biochar prepared by the one-step thermal alkali method can significantly promote hexanoic acid synthesis (53.15% higher than the control group). However, its performance is still inferior to that of materials prepared by the gradient alkali thermal activation process, confirming that gradient activation has an irreplaceable advantage in optimizing the microstructure and electron transfer efficiency of biochar.

[0070] The embodiments listed in this invention are merely for clearly illustrating the technical solutions and are not intended to limit the implementation methods. Those skilled in the art can make formal changes or equivalent substitutions based on the above embodiments, and such obvious changes or modifications based on the technical solutions of this invention are all within the protection scope of this invention.

Claims

1. A method for preparing gradient magnetic biochar, characterized in that, include: 1) Crush and sieve the peanut shells, place the peanut shell powder in an inert gas atmosphere, heat to carry out pyrolysis reaction, cool, wash and dry after the reaction is completed to obtain peanut shell biochar powder; 2) The peanut shell biochar powder is impregnated in a 1-3 M low-concentration NaOH solution and ultrasonically vibrated to carry out the first stage of heating reaction at a temperature of 50-70°C for 1-3 hours; then, a 7-10 M high-concentration NaOH solution is added to carry out the second stage of heating reaction at a temperature of 100-120°C for 1-3 hours to obtain an alkali-modified peanut shell biochar solution. 3) Under inert gas protection, ferrous divalent and ferric trivalent salts are dissolved and mixed using a magnetic stirrer with a magnetic field strength of 0.2~0.4 T to obtain a mixed iron salt solution. Under continuous stirring, alkaline solution is added dropwise to the mixed iron salt solution to generate and grow Fe3O4 nanoparticles in situ, and they connect with each other under the induction of magnetic field to form a magnetic particle network that permeates the biochar conductive network. 4) Add the alkali-modified peanut shell biochar solution obtained in step 2) to the system obtained in step 3), and stir continuously under the protection of inert gas to allow magnetic particles to be generated and combined in situ inside the pores of the biochar, forming a composite material with electronic mediation function. 5) The composite material obtained in step 4) is vacuum filtered to separate the solid and dried to obtain gradient magnetic biochar.

2. The preparation method according to claim 1, characterized in that, In step 1), the peanut shells are crushed and sieved to a particle size of less than 75 μm; the inert gas is nitrogen; the heating rate of the pyrolysis reaction is controlled at 5~10 ℃ / min, the pyrolysis temperature range is 400~500 ℃, and the pyrolysis time is 1~2 h; the washing water is deionized water, and the washing is performed no less than 3 times to ensure the removal of impurities generated during the pyrolysis process.

3. The preparation method according to claim 1, characterized in that, In step 2), the ratio of peanut shell biochar powder to low-concentration NaOH solution is 1 g: 5~15 mL; and the ratio of peanut shell biochar powder to high-concentration NaOH solution is 1 g: 5~10 mL.

4. The preparation method according to claim 1, characterized in that, In step 2), the ultrasonic oscillation power is 200~500W and the ultrasonic oscillation time is 0.5~3h.

5. The preparation method according to claim 1, characterized in that, In step 4), the molar ratio of carbon to iron is 2:1 to 4:

1.

6. The preparation method according to claim 1, characterized in that, In step 5), the drying temperature is 50~100℃ and the drying time is 18~24h.

7. A gradient magnetic biochar, characterized in that, Obtained by the preparation method according to any one of claims 1 to 6.

8. The application of the gradient magnetic biochar of claim 7 in promoting the synthesis of carboxylic acids in an anaerobic fermentation system at a dosage of 2.5~15 g / L.

9. The application according to claim 8, characterized in that, The application includes: under fermentation conditions of pH 5.0~5.5 and temperature 30~35℃, after adding the gradient magnetic biochar, the redox performance of the anaerobic fermentation system is enhanced, the electrochemical impedance value is significantly reduced, and the yield of hexanoic acid is increased by more than 50%.

10. The application according to claim 8, characterized in that, The medium-chain carboxylic acids include at least one of hexanoic acid, heptanoic acid, and octanoic acid.

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