Preparation method of iron-manganese bimetallic co-modified biochar

Abstract

The present application relates to the technical field of environmental functional materials, and particularly relates to a preparation method of iron-manganese bimetallic co-modified biochar, which comprises the following steps: mixing biomass powder, an iron source, a manganese source and a grinding aid, and realizing solid phase oxidation-reduction reaction and mechanical intercalation of iron-manganese precursors through mechanical-chemical ball milling; then performing segmented limited pyrolysis to construct a core-shell heterojunction structure of Fe / Mn@iron-manganese oxide@C; and finally performing acid washing activation to obtain the product. The present application realizes the nanoscale uniform dispersion of iron-manganese bimetallic in the biochar matrix and the strong chemical bonding anchoring by using the mechanical-chemical auxiliary-in-situ oxidation-reduction co-precipitation technology, and solves the problems of easy agglomeration of metal particles, weak binding force and poor synergy in the traditional modification method. The obtained material has excellent adsorption performance and cycle stability for As(III) and other heavy metals, and has magnetism for facilitating recovery, is suitable for deep treatment of heavy metal contaminated water and soil remediation, and has good industrial application prospect.

Description

Technical Field

[0001] This invention relates to the field of environmental functional materials technology, and in particular to a method for preparing iron-manganese bimetallic co-modified biochar. Background Technology

[0002] Biochar is a carbon-rich solid material obtained by pyrolysis of biomass under oxygen-limited conditions. Due to its large specific surface area, abundant surface functional groups, wide availability of raw materials, and low cost, it is widely used in soil improvement, water heavy metal adsorption, carbon dioxide sequestration, and catalytic degradation. However, raw biochar has limited adsorption capacity for heavy metal ions, especially for anionic heavy metals (such as arsenate and chromate), and it is difficult to treat multiple forms of heavy metal pollutants simultaneously.

[0003] To improve the adsorption performance of biochar, metal oxide modification strategies are commonly used in existing technologies. Among these, iron-modified biochar and manganese-modified biochar have attracted considerable attention. Iron oxides (such as...) , It has a strong affinity for heavy metals such as arsenic and lead, and imparts magnetism to materials, facilitating recycling; manganese oxides (such as...) Iron-manganese bimetallic synergistic modification is considered an effective way to improve the overall performance of biochar. It possesses strong oxidizing power, capable of oxidizing highly toxic and highly mobile trivalent arsenic [As(III)] into less toxic and easily adsorbed pentavalent arsenic [As(V)].

[0004] Currently, the main methods for preparing iron-manganese bimetallic modified biochar include the following: Liquid-phase coprecipitation method: Biochar is dispersed in a mixed solution of iron and manganese salts, and the pH is adjusted to cause iron-manganese hydroxide to coprecipitate on the surface of the biochar, followed by pyrolysis or drying. This method has problems such as easy agglomeration of metal particles, uneven distribution, and weak bonding with the carbon matrix, and metal leaching and detachment are prone to occur during use.

[0005] Impregnation pyrolysis method: Biomass or biochar is impregnated in an iron-manganese precursor solution, dried, and then pyrolyzed. In this method, the metal precursor mainly enters the biomass channels through capillary action, resulting in a limited loading capacity. Furthermore, high-temperature pyrolysis easily leads to the migration and aggregation of metal particles, forming large particles and reducing active sites.

[0006] Solvothermal / hydrothermal method: Synthesizing iron-manganese oxide-supported biochar under high temperature and high pressure conditions. This method is energy-intensive, requires sophisticated equipment, and generates a large amount of waste liquid, making it difficult to scale up production.

[0007] In summary, existing technologies generally suffer from the following technical defects: Structural defects: The active metal components are mainly loaded on the outer surface of biochar or in the large pores, lacking an "anchoring" structure and having weak binding force; Dispersion problem: Metal oxide particles are prone to severe agglomeration, resulting in low specific surface area and low utilization of active sites; Insufficient synergy: The electronic structure of iron-manganese bimetals is difficult to regulate, making it difficult to form an efficient Fe-Mn synergistic oxidation-adsorption cycle; Poor environmental friendliness: The liquid phase method generates a large amount of waste liquid containing metal ions, resulting in high post-treatment costs.

[0008] Therefore, developing a preparation method that can achieve high dispersion loading, strong interfacial bonding, significant synergistic effect, and environmental friendliness of iron-manganese bimetallic compounds has important technological value and industrialization prospects. Summary of the Invention

[0009] In order to overcome the above-mentioned defects of the prior art, the present invention provides a method for preparing iron-manganese bimetallic co-modified biochar to solve the problems existing in the background art.

[0010] This invention provides the following technical solution: a method for preparing iron-manganese bimetallic co-modified biochar, comprising the following steps: Step (1) The biomass raw material is dried, pulverized and sieved to obtain biomass powder; Step (2) The biomass powder obtained in step (1) is mixed with iron source, manganese source and grinding aid at a mass ratio of 1:(0.1-0.8):(0.1-0.8):(0.02-0.15) and placed in a ball mill for mechanochemical reaction to obtain activated doped material; Step (3) The activated doped material obtained in step (2) is placed in an inert atmosphere and subjected to segmented temperature-controlled pyrolysis to obtain pyrolysis products; Step (4) The pyrolysis product obtained in step (3) is acid-washed, washed and dried to obtain iron-manganese bimetallic co-modified biochar.

[0011] Furthermore, the iron source in step (2) is a reducing iron source, and the manganese source is an oxidizing manganese source; the iron source is selected from one or more of ferrous sulfate heptahydrate, ferrous chloride, and nano zero-valent iron; the manganese source is selected from one or more of potassium permanganate and manganese dioxide.

[0012] Furthermore, the grinding aid mentioned in step (2) is selected from one or more of sodium chloride, urea, and sodium bicarbonate.

[0013] Furthermore, in step (2), the ball mill speed is 300-600 rpm, the ball milling time is 2-8 hours, and the ball-to-material ratio is (10-25):1.

[0014] Furthermore, the segmented temperature-controlled pyrolysis described in step (3) includes: First stage: Increase the temperature to 300-400℃ at a rate of 5-10℃ / min, and hold for 0.5-1.5 hours; Second stage: Increase the temperature to 600-850℃ at a rate of 5-10℃ / min, and keep warm for 2-4 hours.

[0015] Furthermore, the pickling in step (4) uses a 0.05-0.2 mol / L dilute hydrochloric acid or dilute nitric acid solution, and the stirring time is 10-30 minutes.

[0016] Furthermore, the biomass raw material mentioned in step (1) is selected from one or more of corn stalks, rice stalks, wheat stalks, peanut shells, rice husks, sawdust, and bamboo shavings, and is crushed and passed through an 80-200 mesh sieve.

[0017] The iron-manganese bimetallic co-modified biochar prepared according to any one of the above methods has a core-shell heterojunction structure of "Fe / Mn@iron-manganese oxide@C", wherein the molar ratio of iron to manganese is 1:1 to 3:1, and iron and manganese elements are embedded in the biochar carbon matrix in the form of nanoparticles.

[0018] The technical effects and advantages of this invention are as follows: This invention couples mechanochemical activation with confined pyrolysis, abandoning the traditional liquid-phase impregnation process. Utilizing the solid-phase redox reaction between iron and manganese sources during dry ball milling, it achieves in-situ nucleation and mechanical intercalation of metal nanocrystals simultaneously with biomass crushing and activation, fundamentally avoiding the metal agglomeration and wastewater discharge problems common in liquid-phase methods. Furthermore, by inducing a carbothermic reduction reaction through segmented temperature-controlled pyrolysis, a core-shell heterojunction structure with a metal core, oxide intermediate layer, and carbon shell is constructed in-situ within the carbon matrix, forming a chemical bond between the active metal component and the carbon framework. The structure employs chemical bonding rather than simple physical adhesion, thereby significantly enhancing interfacial bonding strength and structural stability. This structure endows the material with multiple synergistic functions: the reducing ability of the core metal, the adsorption and oxidation ability of the intermediate oxide layer, and the protective effect of the outer carbon layer. This enables the material to simultaneously achieve the oxidative transformation and fixation of heavy metals in different forms. Furthermore, since the active components are coated with carbon layers, metal leaching during use is effectively suppressed, extending the material's service life. At the same time, the entire preparation process requires no solvent, has a simple process flow, and is widely adaptable to raw materials, demonstrating good potential for large-scale application. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for preparing iron-manganese bimetallic co-modified biochar according to the present invention. Detailed Implementation

[0020] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0021] Example 1 This embodiment provides a method for preparing iron-manganese bimetallic co-modified biochar, the specific steps of which are as follows: Step 1: Biomass Pretreatment Take corn stalks, remove the leaves and ears, wash them three times with deionized water to remove surface dirt, and dry them in an 80℃ electric heating oven for 24 hours until constant weight. Crush the dried stalks using a high-speed universal pulverizer, and pass them through a 100-mesh standard sieve (sieve aperture diameter 150μm) to obtain corn stalk powder, which is then sealed and stored for later use.

[0022] Step 2: Mechanochemical activation Weigh out 50.0g of the above corn stalk powder, and ferrous sulfate heptahydrate ( 20.0g, potassium permanganate ( 15.0g of sodium chloride (NaCl) and 5.0g of zirconia were added together into a 500mL zirconia ball mill jar. Zirconia grinding balls (Φ5mm and Φ10mm in a 1:1 mass ratio) were added, bringing the total ball mass to 750g (ball-to-material ratio 15:1). The ball mill jar was sealed and installed on a planetary ball mill. The milling speed was set to 450rpm, and the milling time was 5 hours, using an intermittent operation mode (stopping the mill for 10 minutes to cool after every 30 minutes of operation). After milling, a gray-black mixed powder was obtained.

[0023] Step 3: Confined pyrolysis The ball-milled mixture was transferred to an alumina boat and spread evenly, then placed in the center of the quartz tube of a tubular furnace. High-purity nitrogen gas was introduced at a flow rate of 300 mL / min and continuously vented for 20 minutes to purge the air. A heating program was set: the temperature was increased from room temperature to 350°C at a rate of 8°C / min and held for 1 hour; then increased to 700°C at a rate of 8°C / min and held for 2 hours. After pyrolysis, the mixture was allowed to cool naturally to room temperature under a nitrogen atmosphere.

[0024] Step 4: Post-processing The pyrolysis product was removed and gently ground in an agate mortar, then passed through a 150-mesh standard sieve. The powder was added to a 0.1 mol / L dilute hydrochloric acid solution at a solid-liquid ratio of 1:15 (g / mL), and magnetically stirred for 15 minutes. The mixture was then filtered, and the residue was repeatedly washed with deionized water until the pH of the filtrate reached 7. The washed product was placed in a 70℃ vacuum drying oven and dried at a vacuum of -0.09 MPa for 12 hours to obtain iron-manganese bimetallic co-modified biochar.

[0025] Example 2 This embodiment, based on Embodiment 1, specifically defines the types of iron and manganese sources.

[0026] A reducing iron source and an oxidizing manganese source were selected as iron-manganese precursors. Specifically: Example 2-1: The iron source is 20.0g of ferrous sulfate heptahydrate, and the manganese source is 15.0g of potassium permanganate. The rest is the same as in Example 1.

[0027] Example 2-2: Ferrous chloride is selected as the iron source ( 18.0g, manganese source selected is manganese dioxide ( 12.0g, the rest is the same as in Example 1.

[0028] Example 2-3: The iron source is a mixture of 10.0g of nano zero-valent iron (nZVI, particle size 50-100nm) and 10.0g of ferrous sulfate heptahydrate (total 20.0g), and the manganese source is potassium permanganate 15.0g. The rest is the same as in Example 1.

[0029] Example 2-4: The iron source is 20.0g of ferrous sulfate heptahydrate, and the manganese source is a mixture of 8.0g of potassium permanganate and 7.0g of manganese dioxide (total 15.0g). The rest is the same as in Example 1.

[0030] In the examples above, the iron source during the ball milling process ( ) and manganese source ( A solid-phase redox reaction occurs, generating iron-manganese composite oxide nanocrystals in situ, thus achieving pre-doping and anchoring of iron-manganese bimetals.

[0031] Example 3 This embodiment, based on Embodiment 1, specifically defines the type of grinding aid.

[0032] Example 3-1: The grinding aid used is 5.0g of sodium chloride, and the rest is the same as in Example 1.

[0033] Example 3-2: Urea is selected as the grinding aid ( 6.0g, the rest is the same as in Example 1.

[0034] Example 3-3: Sodium bicarbonate is selected as the grinding aid ( 4.0g, the rest is the same as in Example 1.

[0035] Example 3-4: The grinding aid used is 5.0g of potassium chloride (KCl), and the rest is the same as in Example 1.

[0036] The role of grinding aids is to: (1) reduce material adhesion and improve grinding efficiency; (2) generate local high temperature through mechanical friction during ball milling to promote solid-phase reaction; (3) act as a dispersant to prevent premature agglomeration of metal precursors.

[0037] Example 4 This embodiment, based on Embodiment 1, specifies the process parameters of the ball mill.

[0038] Example 4-1: The ball milling speed was 300 rpm, the ball milling time was 8 hours, the ball-to-material ratio was 10:1, and the rest was the same as in Example 1. The specific surface area of ​​the obtained product was 285 m² / g, and the As(III) adsorption capacity was 132.6 mg / g.

[0039] Example 4-2: The ball milling speed was 400 rpm, the ball milling time was 6 hours, the ball-to-material ratio was 15:1, and the rest was the same as in Example 1. The specific surface area of ​​the obtained product was 312 m² / g, and the As(III) adsorption capacity was 145.3 mg / g.

[0040] Example 4-3: The ball milling speed was 500 rpm, the ball milling time was 4 hours, the ball-to-material ratio was 20:1, and the rest was the same as in Example 1. The specific surface area of ​​the obtained product was 298 m² / g, and the As(III) adsorption capacity was 138.7 mg / g.

[0041] Example 4-4: The ball milling speed was 600 rpm, the ball milling time was 2 hours, the ball-to-material ratio was 25:1, and the rest was the same as in Example 1. The specific surface area of ​​the obtained product was 256 m² / g, and the As(III) adsorption capacity was 121.4 mg / g.

[0042] The ball milling process employs an intermittent operation mode (stopping for 10 minutes to cool down after every 30 minutes of operation) to prevent excessively high system temperatures from causing premature decomposition of the metal precursors or localized carbonization of the biomass. The mill jar is made of zirconium oxide or agate, and the grinding balls are made of matching materials to avoid metal contamination.

[0043] Example 5 This embodiment, based on Embodiment 1, specifies the process parameters for segmented temperature-controlled pyrolysis.

[0044] Example 5-1: The first stage involved heating to 300℃ at a rate of 5℃ / min and holding for 1.5 hours; the second stage involved heating to 600℃ at a rate of 5℃ / min and holding for 4 hours. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​276 m² / g and an As(III) adsorption capacity of 128.5 mg / g.

[0045] Example 5-2: The first stage involved heating to 350℃ at 8℃ / min and holding for 1 hour; the second stage involved heating to 700℃ at 8℃ / min and holding for 2 hours. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​328 m² / g and an As(III) adsorption capacity of 149.8 mg / g.

[0046] Example 5-3: The first stage involved heating to 400℃ at a rate of 10℃ / min and holding for 0.5 hours; the second stage involved heating to 800℃ at a rate of 10℃ / min and holding for 2 hours. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​312 m² / g and an As(III) adsorption capacity of 141.2 mg / g.

[0047] Example 5-4: The first stage involved heating to 350℃ at 8℃ / min and holding for 1 hour; the second stage involved heating to 850℃ at 8℃ / min and holding for 2 hours. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​289 m² / g and an As(III) adsorption capacity of 133.8 mg / g.

[0048] The mechanism of segmented pyrolysis: The first stage (300-400℃) achieves pre-carbonization of biomass, with manganese oxides catalyzing the directional pyrolysis of biomass to form an initial carbon skeleton; the second stage (600-850℃) involves a carbothermic reduction reaction, which reduces some iron and manganese oxides to zero-valent iron and low-valent manganese, forming a core-shell heterojunction structure of "Fe / Mn@iron-manganese oxides@C" under the confinement of the carbon matrix.

[0049] Example 6 This embodiment, based on Embodiment 1, specifically defines the post-pickling treatment process.

[0050] Example 6-1: Use 0.05 mol / L dilute hydrochloric acid, solid-liquid ratio 1:15, stir for 10 minutes. The rest is the same as in Example 1.

[0051] Example 6-2: Use 0.1 mol / L dilute hydrochloric acid, solid-liquid ratio 1:15, stir for 15 minutes. The rest is the same as in Example 1.

[0052] Example 6-3: Use 0.2 mol / L dilute hydrochloric acid, solid-liquid ratio 1:15, stir for 20 minutes. The rest is the same as in Example 1.

[0053] Example 6-4: Use 0.1 mol / L dilute nitric acid, solid-liquid ratio 1:15, stir for 15 minutes. The rest is the same as in Example 1.

[0054] The purpose of pickling is to: (1) remove unstable amorphous metal oxide impurities from the surface and expose more active sites; (2) slightly etch the carbon surface to increase the specific surface area and the number of oxygen-containing functional groups. The pickling time should not exceed 30 minutes, as excessive pickling may damage the core-shell structure.

[0055] Example 7 This embodiment, based on Embodiment 1, specifies the different biomass raw materials.

[0056] Example 7-1: The biomass raw material used was rice straw, which was crushed and passed through a 120-mesh sieve. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​312 m² / g and an As(III) adsorption capacity of 142.5 mg / g.

[0057] Example 7-2: The biomass raw material used was wheat straw, which was crushed and passed through a 100-mesh sieve. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​298 m² / g and an As(III) adsorption capacity of 136.8 mg / g.

[0058] Example 7-3: Peanut shells were used as the biomass raw material, crushed and passed through an 80-mesh sieve, and the rest was the same as in Example 1. The resulting product had a specific surface area of ​​267 m² / g and an As(III) adsorption capacity of 125.3 mg / g.

[0059] Example 7-4: Rice husks were used as the biomass raw material, pulverized and passed through a 100-mesh sieve, and the rest was the same as in Example 1. The resulting product had a specific surface area of ​​285 m² / g and an As(III) adsorption capacity of 131.6 mg / g.

[0060] Example 7-5: The biomass raw material used was pine sawdust, which was crushed and passed through an 80-mesh sieve. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​297 m² / g and an As(III) adsorption capacity of 135.4 mg / g.

[0061] Example 7-6: The biomass raw material used was bamboo shavings, which were crushed and passed through a 100-mesh sieve. The rest was the same as in Example 1. The resulting product had a specific surface area of ​​305 m² / g and an As(III) adsorption capacity of 139.2 mg / g.

[0062] Pretreatment requirements for biomass raw materials: drying temperature 80-105℃, drying time 12-24 hours, to constant weight (moisture content <5%); after crushing, pass through an 80-200 mesh sieve to ensure full mixing and reaction with the metal precursor.

[0063] Example 8 This embodiment further describes the structural characteristics of iron-manganese bimetallic co-modified biochar prepared according to any of the above methods.

[0064] The iron-manganese bimetallic co-modified biochar prepared by any of the methods in Examples 1 to 7 has the following microstructure characteristics: Observation using transmission electron microscopy (TEM) clearly reveals the "core-shell" structure: the core is in a metallic state. The middle layer is an iron-manganese composite oxide (with) It is predominantly spinel-phase, with a graphitized carbon outer layer. High-resolution transmission electron microscopy (HRTEM) images show lattice fringes at 0.202 nm. The (110) crystal plane, with lattice fringes of 0.253 nm, corresponds to The (311) crystal plane and the 0.34 nm lattice stripes correspond to the (002) crystal plane of graphitized carbon.

[0065] X-ray diffraction (XRD) analysis revealed the following in the spectrum. Characteristic peaks of the spinel phase (2θ = 30.1°, 35.6°, 57.2°) and zero-valent iron ( Characteristic peaks (2θ = 44.7°, 65.0°) were observed. The molar ratio of iron to manganese was controlled within the range of 1:1 to 3:1, preferably 2:1.

[0066] Scanning electron microscopy (SEM) revealed that the metal nanoparticles (20-50 nm in diameter) were uniformly dispersed and embedded within the biochar matrix, rather than merely adhering to the surface, without significant agglomeration.

[0067] The saturation magnetization was measured to be ≥15 emu / g using a vibrating sample magnetometer (VSM), which gives the material good magnetic separation performance.

[0068] Example 9 This embodiment describes the application method of the above-mentioned iron-manganese bimetallic co-modified biochar in the remediation of heavy metal polluted water or soil.

[0069] Application Example 9-1: Arsenic-Containing Groundwater Treatment The iron-manganese bimetallic co-modified biochar (FeMn-BC-1) prepared in Example 1 was added at a dosage of 0.5 g / L to groundwater containing 1.0 mg / L of As(III), and treated with stirring or shaking at room temperature for 1 hour. The total arsenic concentration in the treated water sample was below 0.01 mg / L, meeting the national drinking water standard (GB5749-2022). The material after adsorption saturation can be recovered through magnetic separation.

[0070] Application Example 9-2: Treatment of Lead-Containing Industrial Wastewater Take the iron-manganese bimetallic co-modified biochar (FeMn-BC-2) prepared in Examples 2-3, and add it to a solution containing 1.0 g / L. In electroplating wastewater with a concentration of 50 mg / L, the pH was adjusted to 6-7, and the mixture was stirred for 2 hours. The treated water... The concentration is below 0.05 mg / L, which meets the Class I standard of the Integrated Wastewater Discharge Standard (GB8978-1996).

[0071] Application Example 9-3: Remediation of Heavy Metal Contaminated Soil The iron-manganese bimetallic co-modified biochar prepared in Example 1 was mixed evenly with arsenic-lead contaminated soil (total As content 120 mg / kg, total Pb content 300 mg / kg) at a ratio of 2% by soil mass, and the mixture was kept at 60% field capacity for 30 days. After treatment, the available arsenic content in the soil decreased by 78%, and the available lead content decreased by 65%, meeting the relevant requirements of the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control" (GB36600-2018).

[0072] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing iron-manganese bimetallic co-modified biochar, characterized in that, Includes the following steps: Step (1) The biomass raw material is dried, pulverized and sieved to obtain biomass powder; Step (2) The biomass powder obtained in step (1) is mixed with iron source, manganese source and grinding aid at a mass ratio of 1:(0.1-0.8):(0.1-0.8):(0.02-0.15) and placed in a ball mill for mechanochemical reaction to obtain activated doped material; Step (3) The activated doped material obtained in step (2) is placed in an inert atmosphere and subjected to segmented temperature-controlled pyrolysis to obtain pyrolysis products; Step (4) The pyrolysis product obtained in step (3) is acid-washed, washed and dried to obtain iron-manganese bimetallic co-modified biochar.

2. The preparation method according to claim 1, characterized in that, The iron source in step (2) is a reducing iron source, and the manganese source is an oxidizing manganese source; the iron source is selected from one or more of ferrous sulfate heptahydrate, ferrous chloride, and nano zero-valent iron; the manganese source is selected from one or more of potassium permanganate and manganese dioxide.

3. The preparation method according to claim 1, characterized in that, The grinding aid mentioned in step (2) is selected from one or more of sodium chloride, urea, and sodium bicarbonate.

4. The preparation method according to claim 1, characterized in that, The ball mill speed in step (2) is 300-600 rpm, the ball milling time is 2-8 hours, and the ball-to-material ratio is (10-25):

1.

5. The preparation method according to claim 1, characterized in that, The segmented temperature-controlled pyrolysis described in step (3) includes: First stage: Increase the temperature to 300-400℃ at a rate of 5-10℃ / min, and hold for 0.5-1.5 hours; Second stage: Increase the temperature to 600-850℃ at a rate of 5-10℃ / min, and keep warm for 2-4 hours.

6. The preparation method according to claim 1, characterized in that, The pickling in step (4) uses a 0.05-0.2 mol / L dilute hydrochloric acid or dilute nitric acid solution, and the stirring time is 10-30 minutes.

7. The preparation method according to claim 1, characterized in that, The biomass raw materials mentioned in step (1) are selected from one or more of corn stalks, rice stalks, wheat stalks, peanut shells, rice husks, sawdust, and bamboo shavings, and are crushed and passed through an 80-200 mesh sieve.

8. The iron-manganese bimetallic co-modified biochar prepared by the method according to any one of claims 1-7, characterized in that, The biochar has a core-shell heterojunction structure of "Fe / Mn@iron-manganese oxide@C", wherein the molar ratio of iron to manganese is 1:1 to 3:1, and iron and manganese elements are embedded in the biochar carbon matrix in the form of nanoparticles.

9. The application of the iron-manganese bimetallic co-modified biochar according to claim 8 in the remediation of heavy metal polluted water or soil.

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