Method for preparing iron-based biochar material by using iron-rich biomass and application thereof

Abstract

The application provides a method for preparing iron-based biochar material from iron-rich biomass and application, wherein the method comprises the following steps: (1) washing and drying iron-rich rice roots; (2) cutting, grinding and sieving the dried iron-rich rice roots in sequence to obtain rice root powder; (3) mixing the rice root powder with pure water and adding a magnetic stirring rod to stir; (4) sequentially drying and grinding the mixture to obtain treated powder; and (5) putting the treated powder into a tube furnace, introducing nitrogen into the tube furnace, pyrolyzing the treated powder, and obtaining iron-based biochar material at different pyrolysis temperatures. The application uses the iron-rich biological roots discarded after rice harvesting as raw materials, realizes the reuse of discarded biomass, and does not need to add exogenous iron, effectively alleviates the agglomeration of iron-based materials, and can quickly remove hexavalent chromium in water and contaminated soil.

Description

Technical Field

[0001] This invention relates to the fields of environmental protection and soil pollution remediation technology, and in particular to a method for preparing iron-based biochar materials using iron-rich biomass and its application. Background Technology

[0002] Chromium pollution is a pressing environmental problem in the fields of environmental protection and soil remediation. Hexavalent chromium (CHP) is characterized by strong oxidizing properties, high water solubility, and high mobility. CHP is 100 times more toxic than trivalent chromium (TC), classified as a Group 1 carcinogen, and can enter the human body through the food chain, posing a threat to human health. Reducing CHP to TCP is one of the key challenges in water and soil remediation technologies for chromium pollution. Iron-based biochar materials possess advantages such as a large specific surface area, good porosity, and strong adsorption capacity, as well as strong reducing power, excellent mechanical properties, low preparation cost, and easy separation. They exhibit strong remediation capabilities in removing CHP and are gradually becoming a popular remediation method. Methods for synthesizing iron-based biochar include chemical reduction, thermal conversion, hydrothermal carbonization, co-precipitation, and ball milling.

[0003] Currently, many scholars have developed iron-based biochar materials with both adsorption and reduction capabilities, capable of synergistically removing hexavalent chromium, based on iron-based materials such as zero-valent iron, iron oxide, and iron carbide. These preparation methods all have certain drawbacks, such as complex preparation processes and the potential for secondary pollution. Chemical reduction methods for preparing iron-carbon materials have relatively uniform iron loading and are simple and efficient, but commonly used reducing agents such as borohydrides can easily generate toxic gases during preparation, causing secondary pollution. The co-precipitation method involves adding iron to pyrolyzed biochar in an iron-containing alkaline liquid, followed by pyrolysis in an inert gas atmosphere. This method is simple and controllable, but is easily affected by environmental factors, leading to lower product quality. Hydrothermal carbonization technology is relatively environmentally friendly, but it suffers from drawbacks such as easy leaching and difficulty in collecting the products. Furthermore, exogenous iron-based biochar materials require optimized preparation conditions, such as the iron-to-carbon ratio, raw materials, and pyrolysis temperature. In practice, parameter optimization requires significant human and material resources, and studies have shown that exogenous iron-based biochar still suffers from problems such as easy aggregation, low stability, and weak long-distance migration ability.

[0004] Therefore, there is a need for a method to prepare iron-based biochar materials from iron-rich biomass that simplifies the preparation process of exogenous iron-based biochar materials, does not produce toxic byproducts, and can solve problems such as easy aggregation and easy oxidation. Summary of the Invention

[0005] To overcome the problems existing in related technologies, the purpose of this invention is to provide a method and application for preparing iron-based biochar materials using iron-rich biomass. The method simplifies the preparation process of exogenous iron-based biochar materials, does not produce toxic byproducts, and can solve problems such as easy aggregation and easy oxidation.

[0006] A method for preparing iron-based biochar materials using iron-rich biomass includes:

[0007] (1) Clean the iron-rich rice roots and dry them.

[0008] (2) The dried iron-rich rice roots were cut, ground and sieved in sequence to obtain rice root powder;

[0009] (3) Mix the rice root powder with pure water, add a magnetic stir bar and stir to obtain a mixture;

[0010] (4) The mixture is dried and ground sequentially to obtain the powder to be processed;

[0011] (5) The powder to be treated is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900°C to pyrolyze the powder to be treated and obtain iron-based biochar materials at different pyrolysis temperatures. The iron-based biochar materials are used to remove hexavalent chromium from water and soil.

[0012] In the preferred embodiment of the present invention, the mixing ratio of rice root powder and pure water in step (3) is 3g:100mL, the stirring temperature is 50℃, the stirring speed is 800-1200rpm, and the stirring time is 2h.

[0013] In a preferred embodiment of the present invention, the pyrolysis temperature in step (5) is gradually increased from room temperature to 300°C, 500°C, 700°C and 900°C at a rate of 10°C / min.

[0014] In a preferred embodiment of the present invention, the gas flow rate in step (5) is 50 mL / min and the pyrolysis time is 2 h.

[0015] In a preferred embodiment of the present invention, the iron-rich rice root is the root of indica rice or japonica rice, and the indica rice variety includes Huayou 86, and the japonica rice variety includes Nanjing 9108.

[0016] In a preferred embodiment of the present invention, the iron content in the roots of the iron-rich rice root is greater than or equal to 1%.

[0017] In a preferred embodiment of the present invention, step (6) is further included: nitrogen doping of the iron-based biochar material to improve the performance of the iron-based biochar material in a high pH system.

[0018] In a preferred embodiment of the present invention, N kinds of chemical reagents or biopolymers with different nitrogen contents are selected as nitrogen precursors for nitrogen doping, wherein 1≤N≤6; the chemical reagents include urea, ammonium nitrate and melamine, and the biopolymers include peptone, yeast powder and nitrogen-rich algal biomass.

[0019] In a preferred embodiment of the present invention, in step (6), 10g of rice root powder is mixed with 500mL of methanol and ultrasonically treated at 70% intensity for 30min. 20g, 10g, 5g and 3.33g of nitrogen precursor are added to the suspension so that the addition ratio of nitrogen precursor to rice root powder is 2:1, 1:1, 1:2 and 1:3, respectively. The mixture is heated at 80℃ for 8 hours to allow the methanol solvent to evaporate.

[0020] The present invention also provides a method for preparing iron-based biochar materials using iron-rich biomass for the removal of hexavalent chromium from water and polluted soil.

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

[0022] The present invention provides a method for preparing iron-based biochar materials using iron-rich biomass, comprising: (1) washing iron-rich rice roots and drying the iron-rich rice roots. Iron-rich rice roots are common indica and japonica rice varieties. The soil and impurities on the surface of the iron-rich rice roots are washed with water, and the iron-rich rice roots are dried by blower or automatic drying. (2) The dried iron-rich rice roots are successively cut, ground and sieved to obtain rice root powder. The rice root powder is uniform and has a high specific surface area. (3) The rice root powder is mixed with pure water and stirred with a magnetic stir bar to further wash away impurities and alkali metals on the surface of the rice root powder to obtain a mixture. (4) The mixture is successively dried and ground to obtain a powder to be processed. (5) The powder to be processed is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900°C to pyrolyze the powder to obtain iron-based biochar materials at different pyrolysis temperatures. This invention utilizes iron-rich rice roots discarded after rice harvest as raw material. The raw material is widely available, abundant, and inexpensive, enabling the reuse of waste biomass. This invention employs a one-step direct pyrolysis method using iron-rich rice roots to prepare iron-based biochar materials. These iron-based biochar materials are endogenous iron-based carbon materials, requiring no exogenous iron. Because the endogenous iron is uniformly distributed on the surface and inside the raw biomass, the zero-valent iron generated in situ is uniformly loaded into the iron-based biochar, effectively mitigating the aggregation phenomenon of the iron-based material. The iron-based biochar prepared by this invention can rapidly remove hexavalent chromium from water within 1 hour. The removal capacity of hexavalent chromium per unit mass of iron in the material is far greater than that of nano-zero-valent iron and iron-based biochar materials with exogenously added iron. Furthermore, this iron-based biochar primarily removes hexavalent chromium in water through reduction. When treating contaminated soil, after 10 days of remediation, the leaching toxicity of hexavalent chromium in the contaminated soil reached the Class II standard of groundwater quality standards, and the concentration of available chromium in the soil was lower than the risk screening value for agricultural land. Attached Figure Description

[0023] Figure 1 These are SEM images of iron-based biochar at different temperatures according to the present invention. a and d are SEM images of BC300, BC500, BC700 and BC900 respectively, with magnifications of 1 μm and 100 nm respectively.

[0024] Figure 2 These are mapping diagrams of iron-based biochar at different temperatures according to the present invention. The a and d diagrams are mapping diagrams of BC300, BC500, BC700 and BC900, respectively, with magnifications of 1 μm and 100 nm.

[0025] Figure 3 These are XRD patterns of iron-based biochar produced at different temperatures according to the present invention.

[0026] Figure 4These are FTIR images of iron-based biochar at different temperatures according to the present invention;

[0027] Figure 5 This is a graph showing the effect of different pH values ​​on the Cr(VI) removal rate of the material according to the present invention;

[0028] Figure 6 This is a graph showing the removal capacity of Cr(VI) per unit mass of iron by the material under different pH conditions according to the present invention;

[0029] Figure 7 This is a graph showing the Cr(VI) removal rate of the material under different conditions according to the present invention;

[0030] Figure 8 This is a graph showing the removal capacity of Cr(VI) per unit mass of iron by the material under different pH conditions according to the present invention;

[0031] Figure 9 The graphs show the concentration changes of Cr(VI) removed by the present invention, where (a) is the concentration change of Cr(VI) removed by BC900; (b) is the concentration change of Cr(VI) when 0, 5, 10 and 15 mM 1,10-phenanthroline inhibits the action of Fe(II); (c) is the concentration change of dissolved iron in the system without the addition of Cr(VI); and (d) is the concentration change of dissolved iron in the system with the addition of Cr(VI).

[0032] Figure 10 This is a schematic diagram illustrating the contribution of Cr(VI) removal in this invention;

[0033] Figure 11 This is a graph showing the concentration change of Cr(VI) in water removed by the BC900 of the present invention;

[0034] Figure 12 This is a graph showing the change in the content of available chromium during the remediation of chromium-contaminated soil using BC900 of the present invention;

[0035] Figure 13 This is a graph showing the concentration changes of Cr(VI) leaching toxicity during the remediation of chromium-contaminated soil using BC900 of the present invention;

[0036] Figure 14 This is a graph showing the concentration changes of Cr leaching toxicity during the remediation of chromium-contaminated soil using BC900 of the present invention. Detailed Implementation

[0037] Preferred embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make the invention more thorough and complete, and to fully convey the scope of the invention to those skilled in the art.

[0038] Example 1

[0039] like Figure 1 As shown, this embodiment provides a method for preparing iron-based biochar materials using iron-rich biomass, including:

[0040] (1) Clean the iron-rich rice roots and dry them.

[0041] (2) The dried iron-rich rice roots were cut, ground and sieved in sequence to obtain rice root powder;

[0042] (3) Mix the rice root powder with pure water, add a magnetic stir bar and stir to obtain a mixture;

[0043] (4) The mixture is dried and ground sequentially to obtain the powder to be processed;

[0044] (5) The powder to be treated is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900°C to pyrolyze the powder to be treated and obtain iron-based biochar materials at different pyrolysis temperatures. The iron-based biochar materials are used to remove hexavalent chromium from water and soil.

[0045] The iron-rich rice roots have an iron content greater than or equal to 1%. The first option is to use rice plants that have been cultivated for more than two months, with an iron content of 1%-2% in their roots. The second option is to use mature rice plants with an iron content higher than 2% in their roots. The iron-rich rice roots are from indica or japonica rice varieties. Indica rice varieties include Huayou 86, and japonica rice varieties include Nanjing 9108. Roots from other indica and japonica rice varieties can also be used; no specific limitation is made here.

[0046] In step (1), the soil and impurities on the surface of the iron-rich rice roots are washed with water repeatedly until the washing solution is clear. The roots are then dried in a forced-air drying oven at 70°C or air-dried naturally. In step (2), the iron-rich rice roots are initially cut into small pieces using ceramic scissors, and then ground using a high-speed grinder at 30,000 r / min. The ground rice root powder is then sieved through a 100-mesh sieve to select uniform rice root powder with a high specific surface area.

[0047] In step (3), the mixing ratio of rice root powder and pure water is 3g:100mL. Add a magnetic stir bar, heat in a water bath to 50℃, and stir at a speed of 800-1200rpm for 2 hours to further wash away impurities and alkali metals on the surface of the rice root powder.

[0048] In step (4), the mixture is filtered and dried at 70°C. The dried powder is then ground to obtain the powder to be treated. In step (5), the powder to be treated is placed in a tube furnace, and nitrogen gas is introduced to pyrolyze the powder. At room temperature (25°C), the temperature is gradually increased to 300°C, 500°C, 700°C, and 900°C at a rate of 10°C / min. The gas flow rate is 50 mL / min, and the pyrolysis time is 2 hours. The obtained biochar materials are designated as BC300, BC500, BC700, and BC900. The four types of iron-based biochar materials are stored in a vacuum drying oven for later use. BC300 is the iron-based biochar material obtained by pyrolysis at 300°C, BC500 is the iron-based biochar material obtained by pyrolysis at 500°C, BC700 is the iron-based biochar material obtained by pyrolysis at 700°C, and BC900 is the iron-based biochar material obtained by pyrolysis at 900°C.

[0049] In step (2), using a 100-mesh sieve ensures that the raw material particles are uniform and have a large specific surface area, while also preventing the agglomeration of excessively fine powder. In step (3), the mixing ratio of rice root powder and pure water is 3g:100mL, which avoids uneven stirring caused by an excessively thick or low-viscosity suspension.

[0050] This embodiment provides a method for preparing iron-based biochar materials using iron-rich biomass, comprising: (1) washing iron-rich rice roots and drying the iron-rich rice roots. Iron-rich rice roots are common indica and japonica rice varieties. The soil and impurities on the surface of the iron-rich rice roots are washed with water, and the iron-rich rice roots are dried by blower or automatic drying. The dried iron-rich rice roots are then cut, ground, and sieved to obtain rice root powder, which is uniform and has a high specific surface area. The rice root powder is mixed with pure water, and a magnetic stir bar is added for stirring to further wash away impurities and alkali metals on the surface of the rice root powder to obtain a mixture. The mixture is then dried and ground to obtain a powder to be processed. The powder to be processed is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900°C to pyrolyze the powder to obtain iron-based biochar materials at different pyrolysis temperatures. This invention utilizes iron-rich rice roots discarded after rice harvest as raw material. The raw material is widely available, abundant, and inexpensive, enabling the reuse of waste biomass. This invention employs a one-step direct pyrolysis method using iron-rich rice roots to prepare iron-based biochar materials. These iron-based biochar materials are endogenous iron-based carbon materials, requiring no exogenous iron. Because the endogenous iron is uniformly distributed on the surface and inside the raw biomass, the zero-valent iron generated in situ is uniformly loaded into the iron-based biochar, effectively mitigating the aggregation phenomenon of the iron-based material. The iron-based biochar prepared by this invention can rapidly remove hexavalent chromium from water within 1 hour. The removal capacity of hexavalent chromium per unit mass of iron in the material is far greater than that of nano-zero-valent iron and iron-based biochar materials with exogenously added iron. Furthermore, this iron-based biochar primarily removes hexavalent chromium in water through reduction. When treating contaminated soil, after 10 days of remediation, the leaching toxicity of hexavalent chromium in the contaminated soil reached the Class II standard of groundwater quality standards, and the concentration of available chromium in the soil was lower than the risk screening value for agricultural land.

[0051] Example 2

[0052] This embodiment provides a method for preparing iron-based biochar materials using iron-rich biomass, including:

[0053] (1) Clean the iron-rich rice roots and dry them.

[0054] (2) The dried iron-rich rice roots were cut, ground and sieved in sequence to obtain rice root powder;

[0055] (3) Mix the rice root powder with pure water, add a magnetic stir bar and stir to obtain a mixture;

[0056] (4) The mixture is dried and ground sequentially to obtain the powder to be processed;

[0057] (5) The powder to be treated is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900°C to pyrolyze the powder to be treated and obtain iron-based biochar materials at different pyrolysis temperatures. The iron-based biochar materials are used to remove hexavalent chromium from water and soil.

[0058] It also includes step (6): nitrogen doping of the iron-based biochar material to improve the performance of the iron-based biochar material in a high pH system.

[0059] The nitrogen doping process selects N kinds of chemical reagents or biopolymers with different nitrogen contents as nitrogen precursors, wherein 1≤N≤6; the chemical reagents include urea, ammonium nitrate and melamine, and the biopolymers include peptone, yeast powder and nitrogen-rich algal biomass.

[0060] The application of iron-based materials is largely limited by pH value. To further broaden the pH range for material applications, nitrogen doping can be applied to iron-based biochar materials to alter their properties and improve their performance in high pH environments. Nitrogen doping can enhance the adsorption capacity of biochar for hexavalent chromium ions by reducing the electronegativity of the biochar surface. Furthermore, by promoting the formation of persistent free radicals in the biochar, it can synergistically reduce and remove hexavalent chromium ions with iron.

[0061] Rice root powder was obtained by sieving the ground powder through a 100-mesh sieve. The rice root powder was mixed with pure water and stirred with a magnetic stir bar to further remove impurities and alkali metals from the surface of the rice root powder, resulting in a mixture with the ash and mineral content of the rice root powder reduced as much as possible. Six typical chemical reagents or biopolymers with different nitrogen contents were selected as nitrogen precursors, such as urea, ammonium nitrate, melamine, peptone, yeast powder, and nitrogen-rich algal biomass. The nitrogen content of urea was 46.67%, ammonium nitrate was 35%, melamine was 66.67%, peptone was higher than 10.5%, yeast powder was higher than 9%, and nitrogen-rich algal biomass was higher than 40%. The nitrogen-rich algal biomass required three washes with 0.1M HCl to eliminate the significant influence of inherent ash.

[0062] 10g of rice root powder was mixed with 500mL of methanol and sonicated at 70% intensity for 30min. Then, 20g, 10g, 5g, and 3.33g of nitrogen precursors were added to the suspension. The nitrogen precursors could be any one of urea, ammonium nitrate, melamine, peptone, yeast extract, or nitrogen-rich algal biomass, with the ratio of nitrogen precursor to rice root powder being 2:1, 1:1, 1:2, and 1:3, respectively. The mixture was heated at 80℃ for 8 hours to ensure complete evaporation of the methanol solvent. This process was repeated three times to ensure thorough mixing of the rice root powder and nitrogen precursors. The dried mixture was then ground and sieved again through a 100-mesh sieve to obtain the powder to be treated.

[0063] The powder to be treated was placed in a tube furnace and pyrolyzed by nitrogen gas. The temperature was gradually increased from room temperature (25°C) to 300°C, 500°C, 700°C, and 900°C at a rate of 10°C / min, with a gas flow rate of 50 mL / min. The pyrolysis times were set to 0.5 h, 1 h, and 2 h, respectively. As a control, original biochar with added nitrogen precursor was prepared simultaneously according to the above steps.

[0064] This embodiment also includes step (6): nitrogen doping of the iron-based biochar material to improve its performance in a high pH system. N chemical reagents or biopolymers with different nitrogen contents are selected as nitrogen precursors for nitrogen doping, wherein 1 ≤ N ≤ 6; the chemical reagents include urea, ammonium nitrate, and melamine, and the biopolymers include peptone, yeast powder, and nitrogen-rich algal biomass. In step (6), 10g of rice root powder is mixed with 500mL of methanol and ultrasonically treated at 70% intensity for 30min. 20g, 10g, 5g, and 3.33g of nitrogen precursors are added to the suspension so that the ratio of nitrogen precursor to rice root powder is 2:1, 1:1, 1:2, and 1:3, respectively. The mixture is heated at 80℃ for 8 hours to allow the methanol solvent to evaporate. The application of iron-based materials is largely limited by pH value. To further broaden the pH range for material applications, nitrogen doping can be applied to iron-based biochar materials to alter their properties and improve their performance in high pH environments. Nitrogen doping can enhance the adsorption capacity of biochar for hexavalent chromium ions by reducing the electronegativity of the biochar surface. Furthermore, by promoting the formation of persistent free radicals in the biochar, it can synergistically reduce and remove hexavalent chromium ions with iron.

[0065] Example 3

[0066] This embodiment employs field emission scanning electron microscopy (FEM) to observe the surface morphology and microstructure of iron-based biochar. Furthermore, SEM and EDS (energy-dispersive X-ray spectroscopy) are combined to characterize the microscopic distribution and binding morphology of Fe, C, and O elements in the iron-based biochar. Before SEM observation, the freeze-dried sample was sputter-coated with gold under vacuum to enhance its conductivity.

[0067] like Figure 1 and Figure 2 As shown, series a presents SEM images and EDS elemental distribution maps of BC300, series b presents SEM images and EDS elemental distribution maps of BC500, series c presents SEM images and EDS elemental distribution maps of BC700, and series d presents SEM images and EDS elemental distribution maps of BC900. The SEM images show that as the pyrolysis temperature increases, for example from 300℃ to 900℃, the surface of the biochar gradually changes from a rugged and rough structure, such as BC300 and BC500, to a dense and smooth structure, such as BC700 and BC900. The high-temperature calcined iron-based biochar material BC900 shows obvious structural fragmentation and pore collapse. The carbon layers of BC900 may undergo ordered rearrangement, gradually forming a layered structure similar to graphite. BC500 contains sporadic macropores of 0.5 μm, while no obvious pore structure is observed in BC300 and BC500.

[0068] At a scale of 100 nm, SEM images show spherical particles adhering to the surface of the iron-based biochar material, and the particle morphology tends to become more regular and uniformly distributed with increasing temperature. EDS iron elemental analysis shows that the iron-based biochar materials prepared at all four temperatures exhibit significant iron enrichment, confirming that the present invention can successfully prepare iron-carbon materials with endogenous iron without the addition of exogenous iron. The uniform distribution of iron signal intensity in the mapping image indicates that iron is relatively uniformly distributed on the surface of the iron-based biochar material, effectively alleviating the agglomeration phenomenon of the iron-based material.

[0069] Furthermore, combined with SEM morphology analysis, the iron fluorescence signal intensity in the BC700 and BC900 spherical particle regions was higher than that of the substrate, suggesting that these spherical particles are iron-carbon composites. The percentages of C, H, N, and S in the raw material and iron-based biochar were determined using an elemental analyzer, and Fe was analyzed using ICP-MS. 100 mg of rice root powder sample was weighed, added to 10 mL of high-purity water, mixed thoroughly, and shaken for 1 h. The pH value of the solution after the reaction was measured using a pH meter, and two control groups were set up.

[0070] Table 1 shows the elemental composition analysis of the original iron-rich rice root and iron-based biochar materials at different temperatures. With increasing pyrolysis temperature, the iron content of the materials increased from the original 1.82% to 5.63%, 7.08%, 8.96%, and 11.01%, respectively, with a maximum increase of 9.18%. This further proves that the present invention successfully synthesized iron-based biochar materials using endogenous iron biomass. The calcined iron-based biochar materials are mainly composed of C, H, N, and O. Compared with the original material, with increasing temperature, the C content increased, the H content decreased, while the N content remained relatively unchanged. The hydrogen-to-carbon ratio of the iron-based biochar materials reflects the degree of aromatization and carbonization stability. The hydrogen-to-carbon ratio gradually decreased with increasing temperature, indicating that the dehydrogenation and deoxygenation reactions intensified, unstable aliphatic carbon transformed into a highly condensed aromatic ring structure, enhancing the aromatization degree of the biochar and stabilizing the carbon skeleton. The pH value of iron-based biochar materials increases with increasing pyrolysis temperature, which may be due to the hydrolysis of alkali metal and alkaline earth metal salts such as Ca, Mg, Na, and K. The yield of iron-based biochar materials decreases with increasing temperature, which may be due to the vigorous decomposition of hemicellulose, cellulose, and organic components under high-temperature pyrolysis conditions, releasing the volatilization of components such as CO2 and tar.

[0071] This embodiment analyzes the elemental composition, pH, and yield of iron-based biochar at different temperatures. With increasing pyrolysis temperature, the iron content of the material increased from the original 1.82% to 5.63%, 7.08%, 8.96%, and 11.01%, respectively, with a maximum increase of 9.18%. This further proves that the present invention successfully synthesizes iron-based biochar using biomass with endogenous iron. The calcined iron-based biochar is mainly composed of C, H, N, and O. Compared to the original material, with increasing temperature, the C content increases, the H content decreases, while the N content remains relatively unchanged. The hydrogen-to-carbon ratio of the iron-based biochar reflects its aromaticity and carbonization stability. The hydrogen-to-carbon ratio gradually decreases with increasing temperature, indicating that the dehydrogenation and deoxygenation reactions intensify, unstable aliphatic carbon transforms into a highly condensed aromatic ring structure, enhancing the aromaticity of the biochar and stabilizing the carbon skeleton.

[0072] Table 1. Elemental composition, pH, and yield of different materials

[0073]

[0074] Example 4

[0075] In this embodiment, after grinding the iron-based biochar particles into powder using an agate mortar, the mineral crystal form of the iron-based biochar material was determined by X-ray diffraction analysis. The diffraction angle was 5–90° and the rate was 5° / min. Then, the XRD spectrum was analyzed using MID Jade 6.5 software.

[0076] Figure 3 XRD patterns of biochar at different temperatures were used to analyze the composition of iron-based biochar materials. The experimental results showed that BC900 exhibited a graphitized carbon peak with a (103) crystal plane at 2θ = 60°, indicating that as the temperature increased, the carbon layers formed a graphite-like layered structure through condensation reaction, which is consistent with the results of SEM morphology observation.

[0077] Different morphologies of iron were formed by pyrolysis at different temperatures. Under low-temperature conditions, BC300 formed Fe2O3 with a (110) crystal plane at 2θ = 44.67°. Under medium-temperature conditions, BC700 and BC500 mainly formed Fe3O4 (2θ = 44.67° (220), 65.02° (311), 65.02° (400), 82.33° (440)). Under high-temperature conditions, BC900 showed obvious Fe with (110), (200), and (211) crystal planes at 2θ = 44.67°, 65.02°, and 82.33°. 0 This demonstrates the successful preparation of zero-valent iron-based biochar composite material in this invention. Combined with the SEM image from BC900, it was found that the spherical iron-carbon composite is much smaller than 100 nm, indicating that the prepared iron-based material is nano-zero-valent iron (nZVI). With increasing temperature, the characteristic peak intensities of Fe2O3 and Fe3O4 gradually decrease, while Fe... 0 The appearance of the peak indicates that iron oxide is deposited on Fe. 0 Transformation. Under low-temperature conditions, the pyrolysis products are mainly low-crystallinity hematite (Fe2O3), which further transforms into crystalline magnetite (Fe3O4), Warstite (FeO), and zero-valent iron (Fe) as the temperature increases. 0 The reaction process is shown in equations (1), (2), and (3). SiO2 with the same crystal plane appeared under all temperature conditions. Rice, as a silicophilic plant, contains silicon mainly in the form of polymeric silicon and water-soluble silicic acid. These polymeric silicon and water-soluble silicic acid exist as silicic acid phytoliths, accounting for 35%–38% of the dry weight, and play a role in nutrient supply and plant protection. During pyrolysis, silicon and carbon interact; carbon protects silicon from dissolution, and silicon protects carbon from loss, thus forming stable biochar material with a certain degree of resistance to degradation. At higher pyrolysis temperatures, silicon ash transforms from an amorphous state into nano-SiO2 crystalline particles.

[0078] 6Fe2O3+≡C→4Fe3O4+CO2↑(1)

[0079] 2Fe3O4+≡C→6FeO+CO2↑(2)

[0080] 2FeO+≡C→2Fe+CO2↑(3)

[0081] Fourier transform infrared spectroscopy was used to analyze the organic functional groups on the surface of iron-based biochar powder samples. The selected infrared spectral wavelength range was 4000 to 400 cm⁻¹. -1 Before FTIR analysis, the powder sample was mixed with infrared-dried KBr powder at a weight ratio of 1:100 and then compressed into tablets.

[0082] Figure 4 These are FTIR qualitative analysis spectra of surface functional groups of biochar at different pyrolysis temperatures. The results show that the peak intensity and number of organic functional groups decrease with increasing temperature. Biochar at all pyrolysis temperatures exhibits peaks at 3426.8, 1612.6, 1081.3, 791.7, 541.7, and 464.9 cm⁻¹. -1 The absorption peaks mainly originate from hydroxyl groups (C-OH), carbon-carbon double bonds (C=C), alcoholic hydroxyl groups, phenolic hydroxyl groups, carboxyl groups (CO), SiO2 (Si-O-Si), and Fe3O4 (Fe-O). The intensities of these peaks decrease with increasing pyrolysis temperature; when the temperature is greater than or equal to 500℃, the peaks at 1706.7 and 2935.0 cm⁻¹ are the most significant. -1 The functional groups disappear, which correspond to the carbonyl group in dimerized saturated fatty acids and the CH bond and C-H2 bond in aliphatic hydrocarbons, respectively.

[0083] This phenomenon indicates that high temperatures significantly reduce the organic functional groups in biochar, which is attributed to the breaking of unsaturated covalent bonds and the continuous removal of oxygen atoms under high-temperature conditions. The Fe3O4 peak intensity decreases with increasing temperature, which is consistent with the results observed in XRD.

[0084] In this embodiment, XRD analysis was performed on iron-based biochar at different temperatures. Under low-temperature conditions, the main pyrolysis product was low-crystallinity hematite (Fe2O3), which further transformed into crystalline magnetite (Fe3O4), Warstite (FeO), and zero-valent iron (Fe) as the temperature increased. 0 The reaction process is shown in equations (1), (2), and (3). SiO2 with the same crystal plane appeared under all temperature conditions. Rice, as a silicophilic plant, contains silicon mainly in the form of polymeric silicon and water-soluble silicic acid. These polymeric silicon and water-soluble silicic acid exist as silicic acid phytoliths, accounting for 35%–38% of the dry weight, and play a role in nutrient supply and plant protection. During pyrolysis, silicon and carbon interact; carbon protects silicon from dissolution, and silicon protects carbon from loss, thus forming stable biochar material with a certain degree of resistance to degradation. At higher pyrolysis temperatures, silicon ash transforms from an amorphous state into nano-SiO2 crystalline particles.

[0085] Example 5

[0086] This embodiment tests the effect of different pH values ​​on the removal performance of Cr(VI), i.e., hexavalent chromium, by the material. The experiment found that pH significantly affects the ability of iron-based biochar material to remove hexavalent chromium, so the pH conditions were optimized first. 15 mg (0.0150 g ± 0.0005 g) of iron-carbon material prepared at different pyrolysis temperatures was accurately weighed into 50 mL polypropylene centrifuge tubes. 10 mL of 30 mg / L hexavalent chromium solution at different pH values ​​(pH = 2, 3, 5) was added to each tube, and the tubes were immediately placed on a shaker. Samples were taken after 60 min. Two control groups were set up for each treatment. The solution was filtered using a 0.22 μm fiber membrane, and the concentration of hexavalent chromium in the solution was determined using diphenylcarbazide spectrophotometry. The removal rate of hexavalent chromium and the removal capacity of hexavalent chromium per unit mass of iron were calculated.

[0087] Simultaneously, equal amounts of zero-valent iron (ZVI), iron-based biochar material with exogenously added iron (Fe-C), and no added material were set up as controls, with two control groups for each treatment. The iron-based biochar material with exogenously added iron was prepared according to the method in patent number 202411748726. In short, corn stalks were ground and calcined in a muffle furnace at 340°C for 1 hour. The resulting biochar was then soaked in ferrous sulfate solution and calcined in a tube furnace at 700°C for 1 hour. The Fe in this biochar was in the form of ferrous carbide, and the iron content was approximately 58%.

[0088] The removal capacity of Cr(VI) per unit mass of iron in the material and the removal rate of Cr(VI) in the material are calculated by formula (4) and formula (5), respectively:

[0089]

[0090] Where RC is the removal capacity of Cr(VI) per unit mass of iron in the material (mg / g), RE is the removal rate of Cr(VI) in the material (%), C0 is the initial concentration (mg / L), and C e The concentration after the reaction (mg / L), V is the volume of the reaction solution, and m is the mass of the iron-carbon material (g). Fe This represents the mass of iron in the material.

[0091] Figure 5 This study investigated the effect of BC (carbon charcoal) on the removal rate of Cr(VI) in water under different pH conditions. The Cr(VI) removal capacity of all materials increased as the solution pH decreased. The optimal effect was achieved at pH 2, indicating that an acidic environment favors the removal of Cr(VI) by BC materials. This may be related to the zero-point charge of biochar, which affects the removal rate when the pH of the reaction system is lower than that of the biochar. zpc At low pH values, protonation of biochar is favorable, as positively charged biochar readily combines with negatively charged Cr(VI) anions. Under low pH conditions, excess H₂... +Ions corrode the oxide film of iron-carbon materials, exposing reactive sites and facilitating the reaction. Under higher pH conditions, the Cr(III) produced by reduction and the Fe(III) produced by oxidation readily react with OH- in water. - The insulating layer that forms hydroxyl oxides prevents electron transfer to Cr(VI), thus inhibiting Cr(VI) removal. When pH=2, comparing the Cr(VI) removal rates of all materials, BC900 (99.31%) ≈ ZVI (100%) > Fe-C (97.27%) > BC700 (78.39%) > BC500 (42.45%) > BC300 (16.44%). The treatment efficiency of BC900 is comparable to commercial zero-valent iron and superior to Fe-C, essentially achieving 100% removal of Cr(VI) from aqueous solutions.

[0092] Furthermore, such as Figure 6 As shown, the removal capacity of iron per unit mass for Cr(VI) of each material increases with decreasing pH, consistent with the trend of removal rate, reaching the maximum removal capacity at pH=2. At pH=2, the removal capacity of iron per unit mass for Cr(VI) is ranked as follows: BC900 (180.44 mg / g) > BC700 (175.33 mg / g) > BC500 (127.97 mg / g) > BC300 (67.61 mg / g) > Fe-C (33.57 mg / g) > ZVI (20 mg / g). The iron-based biochar prepared by this invention has a much higher removal capacity of iron per unit mass for Cr(VI) than exogenously added iron-based biochar Fe-C and zero-valent iron, with BC900 having the largest removal capacity of iron per unit mass for Cr(VI) at 180.44 mg / g.

[0093] In summary, at pH=2, iron-based biochar materials BC900 and BC700 exhibit superior Cr(VI) removal performance, with removal rates of 99.31% and 78.39%, respectively, and removal capacities of Cr(VI) per unit mass of iron of 180.44 mg / g and 127.97 mg / g, respectively.

[0094] This embodiment also screened the conditions for removing Cr(VI) from iron-based biochar materials BC700 and BC900. Under the optimal pH conditions, such as pH=2, materials BC700 and BC900 with better performance were selected for further optimization of reaction conditions. A comprehensive experiment was conducted with three variables: materials (BC700 and BC900), material dosage (5, 10, and 15 mg), and initial concentration (30 mg / L, 50 mg / L, and 100 mg / L). A total of 18 sets of condition combinations were performed. The remaining experimental conditions were the same as in Example 2. After the reaction, the Cr(VI) concentration was measured, and the removal capacity of iron per unit mass of material for Cr(VI) and the removal rate of Cr(VI) by the material were calculated.

[0095] like Figure 7 As shown, under the same conditions, the overall removal efficiency is: BC900 > BC700. Increasing the dosage increases the removal rate, as more material provides reactive sites, promoting rapid reaction. Increasing the initial concentration decreases the removal rate, possibly because the material does not fully react with Cr(VI) within a limited time. BC900 achieves maximum removal rates of 98.72% and 98.27% within 1 hour at initial concentrations of 30 mg / L and 50 mg / L, and a dosage of 15 mg, respectively. Figure 8 As shown, under similar removal rate conditions, the removal capacity of Cr(VI) per unit mass of iron at an initial concentration of 50 mg / L is 297.37 mg / g, which is much higher than the 172.78 mg / g at an initial concentration of 30 mg / L.

[0096] In summary, the optimal reaction conditions are pH=2, material BC900, and initial concentration of 50 mg / L. The highest removal rate is 98.27%, and the maximum removal capacity of iron per unit mass for Cr(VI) is 297.37 mg / g.

[0097] This embodiment also analyzes the adsorption and reduction effects of iron-based biochar BC900 on Cr(VI) removal. The removal of chromium from solution mainly involves two mechanisms: adsorption and reduction. Cr(VI) can be reduced to Cr(III) and remain in the solution, or it may be adsorbed onto the material surface in the form of Cr(III) or Cr(VI). Adsorption contribution specifically refers to the proportion of Cr(VI) adsorbed by BC900 that was not reduced to the total amount of Cr(VI) removed. This value can be quantitatively determined by desorption with 1.5M hydrochloric acid. The specific operating procedure is as follows: the adsorption equilibrium system (2h) is vacuum filtered (using a 0.22μm filter membrane) to collect the adsorbed BC900. The BC900 is washed three times with pure water to remove the Cr(VI) adhering to the surface of the BC900 (non-adsorption portion). The washed BC900 is placed in a 50mL centrifuge tube, 10mL of hydrochloric acid solution (1.5M) is added, and the mixture is shaken at 25℃ and 160rpm for 24h to fully desorb. The concentrations of total chromium and Cr(VI) in the desorption solution were determined, and the concentration of Cr(III), i.e. trivalent chromium, was calculated by the difference method.

[0098] This embodiment analyzes the dominant species in the reduction process. Compared to reducing groups, iron is the primary reducing agent in iron-based biochar materials, and the iron that reacts with Cr(VI) mainly includes Fe. 0 Surface-bound Fe(II) and dissolved Fe(II). Their relative contributions were estimated using Fe(II) shielding experiments and experiments on the change in iron ion content with and without Cr(VI). Based on the fact that 1,10-phenanthroline can form a complex with Fe(II) to inhibit the reaction between Fe(II) and Cr(VI), reducing 50 mg / L (0.96 mmol / L) of Cr(VI) requires 2.88 mmol / L of Fe(II). The molar ratio of 1,10-phenanthroline to Fe(II) is approximately 3:1. The amount of 1,10-phenanthroline added was estimated based on the assumption that Cr(VI) is completely reduced by Fe(II), with an excess of 20%, therefore 10.39 mmol / L is required. 0, 5, 10, and 15 mM of 1,10-phenanthroline were added to batch experiments, and the removal rate was measured. The difference between the minimum and maximum removal rates is the contribution rate of Fe(II). Batch after batch was set up with materials added separately, with Cr(VI) and materials added together, and without Cr(VI) added. The pH was uniformly adjusted to 2. The concentrations of total dissolved Fe and Fe(II) in the reaction were measured, and the concentration of Fe(III) was calculated. Based on the difference in dissolved Fe(II) concentration between the two, the contribution rate of dissolved Fe(II) was estimated using the theoretically reducible amount of Cr(VI). Finally, the contribution of surface-bound Fe(II) = contribution rate of Fe(II) - contribution rate of dissolved Fe(II). 0 The contribution rate = total removal rate - adsorption contribution rate - Fe(II) contribution rate.

[0099] The desorption experiment revealed a Cr(VI) concentration of 0.16 mg / L, indicating that the Cr(VI) removed by adsorption accounted for only 0.32% of the total Cr(VI) removal rate. This suggests that BC900 primarily removes Cr(VI) through reduction. The changes in the concentrations of Cr(VI), Cr(III), and total chromium in the solution during the Cr(VI) removal process by BC900 are shown below. Figure 9 As shown in Figure a, the concentrations of Cr(VI) and total chromium decreased rapidly within the first 10 minutes, while the concentration of Cr(III) increased slightly, indicating that chromium was rapidly removed after the addition of BC900. The addition of 1,10-phenanthroline significantly reduced the removal rate of Cr(VI), with 5, 10, and 15 mM concentrations showing comparable inhibition, indicating that 1,10-phenanthroline was in excess. In the treatment with 15 mM 1,10-phenanthroline, the removal rate of Cr(VI) decreased from 99.05% to 31.80%, and the contribution of total Fe(II) was calculated to be 68.85%, also indicating the presence of other forms of iron. In the reaction system without Cr(VI), the change in the concentration of dissolved iron is shown in Figure a. Figure 9 As shown in c, Fe(II) increases with time and eventually stabilizes, with a maximum concentration of 62 μM; in the system with added Cr(VI) ( Figure 9 d) Fe(II) remained at a low level and was virtually undetectable. Therefore, it was calculated that Fe(II) could theoretically reduce 1.07 mg / L of Cr(VI), accounting for 2.18% of the total removal rate. Further calculations showed that bound Fe(II) contributed 66.67%, while Fe... 0 Its contribution was 31.15%.

[0100] Cr(VI) removal contribution such as Figure 10 As shown, the final estimate reveals that adsorption contributes 0.32% and reduction contributes 99.68% to the removal of Cr(VI), with bound Fe(II) contributing 66.67% and Fe... 0 The contribution of Cr(VI) was 30.83%, while that of Fe(II) was 2.18%. BC900 mainly removes Cr(VI) through reduction, with bound Fe(II) being the dominant species, followed by Fe. 0 .

[0101] This embodiment also tested the ability of iron-based biochar BC900 to remove Cr(VI) from water. 100 mg (0.1000 g ± 0.0005 g) of iron-based biochar material was accurately weighed into a 250 mL glass bottle, and 100 mL of Cr(VI) solution (50 mg / L, pH = 2) was added. The bottle was then immediately placed on a shaker, and samples were taken at 1, 4, 7, 10, 15, 20, 30, 40, 60, and 120 min, with 1 mL of sample taken each time. The sample was filtered through a 0.22 μm filter membrane, and the concentration of Cr(VI) was measured and the adsorption capacity of the material was calculated.

[0102] like Figure 11 As shown, BC900 can achieve an adsorption capacity of 29.3 mg / g of Cr(VI) within 1 minute, with a C(VI) removal rate of 82.56%; when the reaction time is 1 hour, it can achieve an adsorption capacity of 32.97 mg / g of Cr(VI), with a C(VI) removal rate as high as 98.60%, indicating that the material can achieve rapid removal of Cr(VI) within 1 hour.

[0103] This embodiment also provides the application of a method for preparing iron-based biochar materials using iron-rich biomass in the removal of hexavalent chromium from water and contaminated soil. In this embodiment, iron-based biochar BC900 is applied to soil remediation. Soil samples are collected, air-dried, and sieved through a 10-mesh sieve. A certain amount of potassium dichromate solution (pH=2) is added to the soil and stirred evenly to achieve a theoretical Cr(VI) value of 100 mg / kg. The soil is left at room temperature for 20 days to simulate the aging process of soil containing natural Cr(VI). The prepared soil is then air-dried and sieved again through a 10-mesh sieve for later use. The concentrations of Cr(VI) and total Cr, as well as the physicochemical properties of Cr(VI) and total Cr, are finally determined, as shown in Table 2.

[0104] Table 2 Physicochemical properties of simulated contaminated soil

[0105]

[0106] 100g of contaminated soil was placed in a 250mL beaker, and 2% (2g) of BC900 biochar was added. 10mL of water was added (soil-to-water ratio 1:10), and the mixture was stirred until the soil was moist but not clumpy, with a sandy texture. The beaker was sealed with plastic wrap and placed in an incubator at 25℃. The soil-to-water ratio was maintained for 45 days of remediation, with water added and the soil stirred to maintain a constant moisture content. A control group without added materials was set up, with three replicates for each treatment. Samples were taken on days 1, 5, 10, 20, and 45 of remediation to determine the total Cr(VI) content, available Cr, and leaching toxicity of the soil.

[0107] In chromium toxicity leaching, acetic acid was used as the extraction agent to assess the impact of industrial landfill leaching filtrate on remediation effectiveness. Changes in the toxicity leaching of Cr and Cr(VI) in the soil during remediation were observed as follows: Figure 13 and Figure 14 As shown, the leaching concentrations of Cr and Cr(VI) dropped sharply within 1 day, then gradually stabilized over time. After 45 days of remediation, the final leaching concentration of Cr(VI) was 0.01 mg / L, significantly lower than the 5.44 mg / L in the control group (CK). The leaching concentration of total Cr was 1.75 mg / L, lower than the 7.48 mg / L in the CK, and the concentrations of Cr(VI) met the Class II water quality standard (0.01 mg / L) of the Chinese Groundwater Quality Standard (GB / T14848-2017).

[0108] In this embodiment, DTPA extraction of chromium from the remediated soil revealed that the chromium was an active component that could be absorbed by plants or migrate in the environment, and was used to assess the bioavailability of chromium in the soil. The content of available Cr in the soil during the remediation process was as follows: Figure 12 As shown, after 45 days of remediation with the added materials, the content of available Cr in the soil decreased significantly, with the concentration of available Cr in the soil decreasing from the initial 173.16 mg / kg to 30.16 mg / kg. According to the "Soil Environmental Quality Standard for Agricultural Land Soil Pollution Risk Control" (GB15618-2018), the chromium limit for soil varies depending on the type of use and pH. Comparing the pH of the remediated soil with its limit, it was found that the pH of the remediated soil was still below the risk screening value of 200 mg / kg for agricultural land, indicating that the remediated soil poses a lower risk to the quality and safety of agricultural products, crop growth, or the soil ecological environment.

[0109] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, apparatus, article, or method that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, apparatus, article, or method. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, apparatus, article, or method that includes that element.

[0110] The above description is only a preferred embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural changes made based on the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for preparing iron-based biochar materials using iron-rich biomass, characterized in that, include: (1) Clean the iron-rich rice roots and dry them; (2) The dried iron-rich rice roots were cut, ground and sieved in sequence to obtain rice root powder; (3) Mix the rice root powder with pure water, add a magnetic stir bar and stir to obtain a mixture; (4) The mixture is dried and ground sequentially to obtain the powder to be treated; (5) The powder to be treated is placed in a tube furnace and nitrogen gas is introduced. The pyrolysis temperature is gradually increased from room temperature to 900 °C to pyrolyze the powder to be treated and obtain iron-based biochar materials at different pyrolysis temperatures. The iron-based biochar materials are used to remove hexavalent chromium from water and soil. Nitrogen doping was performed on the powder to be treated in step (4) before pyrolysis to improve the performance of the iron-based biochar material in a high pH system; Nitrogen doping uses chemical reagents or biopolymers with different nitrogen contents as nitrogen precursors; the chemical reagents include urea, ammonium nitrate or melamine, and the biopolymers include peptone, yeast powder or nitrogen-rich algal biomass.

2. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, In step (3), the mixing ratio of rice root powder and pure water is 3 g: 100 mL, the stirring temperature is 50 °C, the stirring speed is 800-1200 rpm, and the stirring time is 2 h.

3. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, In step (5), the pyrolysis temperature is gradually increased from room temperature to 900 °C at a rate of 10 °C / min.

4. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, In step (5), the gas flow rate is 50 mL / min and the pyrolysis time is 2 h.

5. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, The iron-rich rice root is the root of indica rice or japonica rice. The indica rice variety includes Huayou 86, and the japonica rice variety includes Nanjing 9108.

6. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, The iron content in the roots of the iron-rich rice is greater than or equal to 1%.

7. The method for preparing iron-based biochar materials using iron-rich biomass according to claim 1, characterized in that, The step of nitrogen doping the powder to be treated in step (4) before pyrolysis includes: mixing 10 g of the powder to be treated with 500 mL of methanol, ultrasonically treating it at 70% intensity for 30 min, adding 20 g, 10 g, 5 g or 3.33 g of nitrogen precursor to the suspension so that the ratio of nitrogen precursor to powder to be treated is 2:1, 1:1, 1:2 or 1:3, respectively, and heating at 80 °C for 8 hours to allow the methanol solvent to evaporate.

8. The application of the iron-based biochar material prepared by the method according to any one of claims 1-7 in the removal of hexavalent chromium from water and contaminated soil.

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