Preparation method and application of a ball-milled biochar loaded boron modified zero-valent iron material

Abstract

The application relates to a preparation method and application of a ball-milling biochar loaded boron modified zero-valent iron material and relates to the technical field of nano zero-valent iron preparation.The purpose of the application is to solve the problems of easy agglomeration, easy oxidation inactivation and low Cr(VI) removal efficiency caused by the defects of the electron not being easy to release in the nuclear shell structure of the current zero-valent iron. In the application, some iron boron compounds are formed on the surface of nZVI through the incorporation of boron, so as to prevent the formation of oxides on the surface of nZVI and cause the reduction of reaction activity and enhance the removal capacity of Cr(VI); in the mechanical ball-milling process, the breaking of chemical bonds and the formation of new chemical bonds are accompanied, the B-B bonds formed in the ball-milling process of the BnZVI@BCBM material can promote the Fe(III) / Fe(II) cycle, and therefore the material has excellent Cr(VI) removal capacity. The application can obtain a preparation method and application of a ball-milling biochar loaded boron modified zero-valent iron material.

Description

Technical Field

[0001] This invention relates to the field of nano-zero valent iron preparation technology, specifically to a method for preparing boron-modified zero valent iron material supported on ball-milled biochar and its application. Background Technology

[0002] With industrial development, chromium (Cr) pollution in my country's soil has become increasingly severe, mainly due to improper discharge of chromium-containing industrial wastewater and the accumulation of chromium slag. Chromium, a highly toxic heavy metal, is widely present in the environment. Cr(III) is relatively stable in soil, with low mobility and bioavailability; conversely, Cr(VI) is extremely toxic to organisms, more than 100 times more toxic than Cr(III), causing serious harm to the skin, functional organs, intestines, and immune system upon exposure. Therefore, exploring efficient strategies for remediating chromium (VI) contaminated soil is a crucial issue that urgently needs to be addressed in the field of environmental remediation.

[0003] Currently, remediation technologies for Cr(VI) soil pollution mainly include topsoil replacement, electrokinetic remediation, microbial remediation, and chemical reduction. Given that Cr(III) has lower biotoxicity than Cr(VI), chemical reduction is often used to reduce Cr(VI) in soil to Cr(III), thus serving as an effective approach for remediating chromium-contaminated soil. Among numerous reducing agents, nano-zero-valent iron (nZVI) is widely used to remediate Cr(VI)-contaminated soil due to its low electrode potential and high reactivity. Its Cr(VI) removal mechanism includes reduction and co-precipitation. However, pristine nano-zero-valent iron suffers from problems such as easy aggregation, easy oxidative deactivation, and easy side reactions with H2O and dissolved oxygen. Furthermore, due to the typical core-shell structure of nZVI, iron oxides are deposited on the nZVI surface during the reaction, resulting in low electron release efficiency. Therefore, improving the efficiency of nano-zero-valent iron in the remediation of hexavalent chromium-contaminated sites is an urgent problem to be solved.

[0004] Common nZVI modification techniques include porous material loading, surface modification (introduction of surfactants, etc.), and boron modification. Biochar is widely used for loading modification due to its large specific surface area, abundant porosity, low cost, and rich oxygen-containing functional groups. Surface modification with boron can effectively suppress the competition for electrons by oxygen during the removal of heavy metals by zero-valent iron (ZV), thereby improving the efficiency and electron utilization of ZV in heavy metal removal. Furthermore, the addition of boron can accelerate iron cycling and improve the performance of nZVI in removing Cr(VI). Currently, methods for preparing ZV include common liquid-phase reduction, carbothermal reduction, and mechanical ball milling. Different preparation methods and conditions have a significant impact on the reactivity and material properties of nZVI. Therefore, finding a convenient, environmentally friendly modification method that can improve the defects of nZVI and be used for the efficient removal of heavy metals is extremely important. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of low Cr(VI) removal efficiency caused by defects such as easy agglomeration, easy oxidation and deactivation of zero-valent iron and the difficulty in releasing electrons due to the core-shell structure. This invention provides a method for preparing boron-modified zero-valent iron material supported on ball-milled biochar and its application.

[0006] A method for preparing a boron-modified zero-valent iron material supported on ball-milled biochar, comprising the following steps:

[0007] Step S1: Preparation of straw biochar;

[0008] Corn stalks are crushed, dried, and sieved, then placed in a tube furnace and heated to 300–500°C under a nitrogen atmosphere. The stalks are then fully pyrolyzed at 300–500°C to obtain biochar.

[0009] Step S2: Preparation of boron-modified zero-valent iron material supported on ball-milled biochar;

[0010] The first step is ball milling. The biochar and iron powder obtained in step S1 are placed in a ball mill jar, and the ball mill jar is placed in a ball mill. After thorough ball milling, ball milled biochar loaded with zero valent iron is obtained. The mass ratio of biochar to iron powder is (0.15~0.03):(2.4~2.6).

[0011] The second step involves ball milling. Zero-valent iron loaded with ball milled biochar and boron oxide are placed in a ball mill jar, which is then placed inside a ball mill. After thorough ball milling, the mixture is finally ground and sieved to obtain ball-milled biochar-loaded boron-modified zero-valent iron material. The mass ratio of the ball-milled biochar-loaded zero-valent iron to boron oxide is (2.4–2.6):(0.64–1.88).

[0012] The grinding balls in the ball mill jar consist of large balls, medium balls and small balls, and the ratio of the number of large balls, medium balls and small balls is (1-3):(14-16):(15-17).

[0013] An application of a ball-milled biochar-supported boron-modified zero-valent iron material, specifically its application in the passivation of the heavy metal chromium.

[0014] The principle of this invention:

[0015] First, nZVI has a lower electrode potential (E 0 (Fe 2+ / Fe 0 Since nZVI = -0.44 eV, it can act as a reducing agent or electron donor to reduce Cr(VI) through direct electron transfer. Furthermore, the Fe produced by the loss of electrons from nZVI is... 2+Cr(VI) can be further reduced. However, pristine nZVI is prone to aggregation and oxidation, resulting in a reduced specific surface area and active sites, hindering electron transfer from nZVI to Cr(VI). Boron doping can alter the physicochemical properties of materials, such as enhancing hydrophobicity and electron transfer capabilities. Simultaneously, biochar-supported nano-zero-valent iron can effectively disperse nZVI, prevent its aggregation, and improve its reactivity, thereby significantly enhancing the removal capacity of nZVI for Cr(VI).

[0016] Furthermore, during mechanical ball milling, BnZVI@BCBM is broken into small particles by the kinetic energy and mechanical stress generated by high-speed motion, accompanied by chemical reactions, bond breaking, and new bond formation. In particular, the BB bond promotes the Fe(III) / Fe(II) cycle, thus exhibiting excellent Cr(VI) removal capabilities. However, boron oxide does not promote the transformation of Fe(III) to Fe(II), indicating that the BB bond is not inherent in boron oxide but is unique to nitrogen-doped BnZVI@BCBM obtained after ball milling.

[0017] The beneficial effects of this invention are:

[0018] I. This invention utilizes a ball milling method to prepare biochar-supported boron-modified zero-valent iron for the remediation of chromium, a heavy metal in soil. It offers advantages such as simple synthesis, good remediation effect, and environmental friendliness. Its main mechanism of action includes:

[0019] (1) By incorporating boron, some iron-boron compounds are formed on the surface of nZVI to prevent the formation of oxides on the surface of nZVI, which would reduce the reactivity and enhance its ability to remove Cr(VI).

[0020] (2) Due to the large specific surface area and abundant surface aggregates of biochar, loading boron with zero-valent iron can weaken the easy agglomeration defect caused by the magnetic properties of nZVI and enhance the reactivity of nZVI.

[0021] (3) During the mechanical ball milling process, chemical bonds break and new chemical bonds are formed. The BB bonds formed by BnZVI@BCBM material during the ball milling process can promote the Fe(III) / Fe(II) cycle. Therefore, the material has excellent ability to remove Cr(VI).

[0022] Therefore, the ball-milled biochar loaded with boron-modified zero-valent iron prepared by this invention achieves efficient adsorption and reduction of heavy metal chromium in farmland, promoting the development and application of heavy metal remediation agents in soil.

[0023] II. Among numerous modification methods, ball milling is a simple, efficient, and economical approach. Ball milling promotes the formation of BB bonds, facilitating the Fe(III) / Fe(II) cycle and enhancing the removal of Cr(VI) by zero-valent iron. Ball milling has been shown to improve the dispersibility of iron oxide on biochar surfaces, thereby doubling the Cr(VI) adsorption capacity. Furthermore, ball milling allows for large-scale production, meeting the needs of agricultural land.

[0024] III. This invention prepares boron-modified zero-valent iron supported on biochar by ball milling, explores the passivation performance of the biochar-supported boron-modified zero-valent iron composite material on soil Cr, and analyzes its related mechanism of action.

[0025] This invention provides a method for preparing boron-modified zero-valent iron material supported on ball-milled biochar and its application. Attached Figure Description

[0026] Figure 1 This shows a scanning electron microscope image of BnZVI@BCBM prepared in Example 1;

[0027] Figure 2 This shows the elemental energy dispersive spectroscopy (EDS) analysis of BnZVI@BCBM prepared in Example 1;

[0028] Figure 3 The X-ray diffraction patterns of the different materials prepared in Examples 1-4 are shown. Indicates Fe 0 ;

[0029] Figure 4 This shows the electrochemical impedance spectroscopy (EIS) analysis results of the different materials prepared in Examples 1-4;

[0030] Figure 5 The Tafel corrosion curves of the different materials prepared in Examples 1-4 are shown.

[0031] Figure 6 The diagram shows the removal effect of different systems on Cr(VI) in water in Examples 1-4; 1 represents nZVIBM, 2 represents nZVI@BCBM, 3 represents BnZVIBM, and 4 represents BnZVI@BCBM.

[0032] Figure 7 The diagram shows the remediation effect of different remediation materials on Cr(VI) in soil in Examples 1-4. Detailed Implementation

[0033] Specific Implementation Method 1: This implementation method describes a method for preparing boron-modified zero-valent iron material supported on ball-milled biochar, which is carried out according to the following steps:

[0034] Step S1: Preparation of straw biochar;

[0035] Corn stalks are crushed, dried, and sieved, then placed in a tube furnace and heated to 300–500°C under a nitrogen atmosphere. The stalks are then fully pyrolyzed at 300–500°C to obtain biochar.

[0036] Step S2: Preparation of boron-modified zero-valent iron material supported on ball-milled biochar;

[0037] The first step is ball milling. The biochar and iron powder obtained in step S1 are placed in a ball mill jar, and the ball mill jar is placed in a ball mill. After thorough ball milling, ball milled biochar loaded with zero valent iron is obtained. The mass ratio of biochar to iron powder is (0.15~0.03):(2.4~2.6).

[0038] The second step involves ball milling. Zero-valent iron loaded with ball milled biochar and boron oxide are placed in a ball mill jar, which is then placed inside a ball mill. After thorough ball milling, the mixture is finally ground and sieved to obtain ball-milled biochar-loaded boron-modified zero-valent iron material. The mass ratio of the ball-milled biochar-loaded zero-valent iron to boron oxide is (2.4–2.6):(0.64–1.88).

[0039] The grinding balls in the ball mill jar consist of large balls, medium balls and small balls, and the ratio of the number of large balls, medium balls and small balls is (1-3):(14-16):(15-17).

[0040] Specific Implementation Method Two: The difference between this implementation method and Specific Implementation Method One is that the drying temperature in step S1 is 60-80℃.

[0041] The other steps are the same as in Specific Implementation Method 1.

[0042] Specific Implementation Method 3: The difference between this implementation method and Specific Implementation Method 1 or 2 is that the product is dried and then passed through a 100-mesh sieve in step S1.

[0043] The other steps are the same as in Specific Implementation Method 1 or 2.

[0044] Specific Implementation Method Four: The difference between this implementation method and Specific Implementation Methods One to Three is that the heating temperature in step S1 is 300-400℃.

[0045] The other steps are the same as those in Specific Implementation Methods One to Three.

[0046] Specific Implementation Method 5: The difference between this implementation method and Specific Implementation Methods 1 to 4 is that the pyrolysis time in step S1 is 2 to 3 hours.

[0047] The other steps are the same as those in Specific Implementation Methods One through Four.

[0048] Specific Implementation Method Six: The difference between this implementation method and one of the specific implementation methods one to five is that the grinding ball mentioned in step S2 is a zirconium dioxide ball.

[0049] The other steps are the same as those in Specific Implementation Methods 1 to 5.

[0050] Specific Implementation Method Seven: The difference between this implementation method and Specific Implementation Methods One to Six is ​​that the rotation speed of the ball mill in the first step of step S2 is 300 to 500 rpm, and the ball milling time is 3 to 5 hours.

[0051] The other steps are the same as those in Specific Implementation Methods 1 to 6.

[0052] Specific Implementation Method Eight: The difference between this implementation method and Specific Implementation Methods One to Seven is that the ball milling speed in the second step of step S2 is 300-500 rpm, and the ball milling time is 2-3 hours.

[0053] The other steps are the same as those in Specific Implementation Methods 1 to 7.

[0054] Specific Implementation Method Nine: The difference between this implementation method and Specific Implementation Methods One to Eight is that after ball milling in the second step of step S2, the material is finally ground and passed through a 100-mesh sieve.

[0055] The other steps are the same as those in Specific Implementation Methods 1 to 8.

[0056] Specific Implementation Method 10: This implementation method describes the application of a ball-milled biochar-supported boron-modified zero-valent iron material, specifically its application in the passivation of heavy metal chromium.

[0057] The beneficial effects of the present invention are verified using the following embodiments:

[0058] Example 1: A method for preparing boron-modified zero-valent iron material supported on ball-milled biochar, comprising the following steps:

[0059] Step S1: Preparation of straw biochar;

[0060] Corn stalks were crushed, dried at 80°C, passed through a 100-mesh sieve, placed in a tube furnace, heated to 300°C under a nitrogen atmosphere, and pyrolyzed at 300°C for 2 hours to obtain biochar.

[0061] Step S2: Preparation of boron-modified zero-valent iron material supported on ball-milled biochar;

[0062] The first step is ball milling. 0.15g of biochar and 2.5g of reduced iron powder obtained in step S1 are placed in a ball mill jar and then placed in a ball mill. The jar is ball milled at 500 rpm for 3 hours to obtain ball-milled biochar loaded with zero valent iron.

[0063] The second step is ball milling. 2.5g of ball milled biochar loaded with zero-valent iron and 1.88g of boron oxide are placed in a ball mill jar and then placed in a ball mill. The ball mill is then milled at 500rpm for 2 hours. After the ball milling is completed, the material in the agate jar of the ball mill is taken out, and finally ground and passed through a 100-mesh sieve to obtain ball milled biochar loaded with boron modified zero-valent iron material.

[0064] The grinding balls in the ball mill jar are zirconium dioxide balls, consisting of large balls, medium balls, and small balls, with the ratio of large balls, medium balls, and small balls being 2:16:15.

[0065] Example 2: In this example, ball milling was used to prepare nano-zero valent iron material nZVIBM. All other experimental conditions were the same as in Example 1.

[0066] Example 3: In this example, nano-zero-valent iron-biochar material nZVI@BCBM was prepared by ball milling. All other experimental conditions were the same as in Example 1.

[0067] Example 4: In this example, boron-modified nano-zero-valent iron material BnZVIBM was prepared by ball milling. All other experimental conditions were the same as in Example 1.

[0068] The boron-modified zero-valent iron material supported on milled biochar prepared in this embodiment was characterized by scanning electron microscopy, elemental energy dispersive spectroscopy, X-ray diffraction, electrochemical impedance spectroscopy (EIS), Tafel curve analysis, and experiments on chromium removal in aqueous phase and soil chromium remediation. The specific conclusions are as follows:

[0069] 1. Characterization (scanning electron microscopy, elemental energy dispersive spectroscopy, X-ray diffraction, electrochemical impedance spectroscopy (EIS), Tafel curves):

[0070] The surface morphology, elemental composition, and crystal structure of BnZVI@BCBM were determined by scanning electron microscopy, elemental energy dispersive spectroscopy, and X-ray diffraction. Electrochemical characterization of BnZVI@BCBM was performed by electrochemical impedance spectroscopy (EIS) and Tafel curve analysis to investigate its charge transfer resistance and self-corrosion potential.

[0071] Figure 1 This shows a scanning electron microscope image of BnZVI@BCBM prepared in Example 1; as shown. Figure 1 As shown, BnZVI@BCBM exhibits irregular particles with a rough surface and small particle size. nZVI is uniformly dispersed on the surface of biochar, indicating the strong dispersibility of biochar.

[0072] Figure 2 This shows the elemental energy dispersive spectroscopy (EDS) spectrum of BnZVI@BCBM prepared in Example 1; as shown. Figure 2As shown, the iron element is relatively uniformly distributed on the BM-ZVI / BC surface, indicating that the deformation and agglomeration of nZVI during ball milling are significantly alleviated.

[0073] Figure 3 The X-ray diffraction patterns of the different materials prepared in Examples 1-4 are shown. Indicates Fe 0 ;like Figure 3 As shown, nZVIBM, nZVI@BCBM, BnZVIBM, and BnZVI@BCBM exhibit distinct peaks at 44.6°, 65.0°, and 82.3°, respectively, corresponding to Fe... 0 The (110), (200) and (211) diffraction peaks of the crystal planes indicate that the crystal structure of nZVI was not changed during the ball milling process, and FeO remained the main phase.

[0074] Figure 4 This shows the electrochemical impedance spectroscopy (EIS) analysis results of the different materials prepared in Examples 1-4; as follows: Figure 4 As shown, the electrochemical impedance spectroscopy (EIS) test results show that BnZVI@BCBM has the lowest charge transfer resistance, proving that boron doping and biochar loading reduce the charge transfer resistance of the material and are more conducive to electron transfer.

[0075] Figure 5 The Tafel corrosion curves of the different materials prepared in Examples 1-4 are shown; for example... Figure 5 As shown in the Tafel corrosion curves, the order of corrosion potential for different materials is: nZVIBM (-0.52V) > nZVI@BCBM (-0.76V) > BnZVIBM (-1.02V) > BnZVI@BCBM (-1.07V). BnZVI@BCBM has the lowest corrosion potential, indicating that it has a faster electron release rate, which is beneficial for the reduction of Cr(VI) by the material.

[0076] 2. Removal effect of different materials on Cr(VI):

[0077] The adsorption process was as follows: 0.05±0.005g of different nZVIBM, nZVI@BCBM, BnZVIBM and BnZVI@BCBM were weighed using a balance and added to a 150mL Erlenmeyer flask. Then, 100mL of 200mg / L Cr(VI) solution was added to the Erlenmeyer flask. The initial pH of the solution was adjusted to 2.0±0.1 with 1M HCl. After shaking at 200rpm for 2h, the concentration of Cr(VI) in the solution was measured, and the removal rate of the adsorption material was calculated.

[0078] Figure 6This diagram illustrates the removal effect of different systems on Cr(VI) in water in Examples 1-4; 1 represents nZVIBM, 2 represents nZVI@BCBM, 3 represents BnZVIBM, and 4 represents BnZVI@BCBM; (e.g., ...) Figure 6 As shown, nZVIBM's removal effect on Cr(VI) was not significant. After biochar loading, the removal rate of Cr(VI) by nZVI@BCBM increased. Furthermore, boron-modified BnZVIBM showed a further improvement in Cr(VI) removal compared to nZVIBM. After incorporating boron into the zero-valent iron after biochar loading, BnZVI@BCBM achieved almost 100% removal of Cr(VI). The results indicate that both boron doping and biochar loading are beneficial for improving the removal of Cr(VI) by nZVI.

[0079] 3. Removal effect of ball-milled biochar-supported boron-modified zero-valent iron material on soil Cr(VI):

[0080] Experimental conditions: Soil samples were collected from the campus of Northeast Agricultural University in Harbin, Heilongjiang Province. The soil was typical black soil. Two kg of sieved soil samples were placed in a plastic box, and 500 mL of a 2000 mg / L K₂Cr₂O₇ solution was evenly sprayed onto the soil using a spray bottle. After thorough stirring, the soil was allowed to stand for 30 days to age, resulting in Cr(VI) contaminated soil with a concentration of 1000 mg / L. The soil moisture content was then adjusted to 40% after passing through a 20-mesh sieve. In this study, three parallel samples were tested simultaneously for each treatment. In soil with a moisture content of 40%, different types of adsorbents (nZVIBM, nZVI@BCBM, BnZVIBM, and BnZVI@BCBM) at 1% dosage were applied for a 42-day remediation experiment. Soil samples were collected at five points on days 0, 3, 7, 14, 21, 28, 35, and 42 of the remediation period, and the Cr(VI) concentration in the soil was determined by alkaline digestion.

[0081] Figure 7 The diagram shows the remediation effects of different remediation materials on Cr(VI) in soil in Examples 1-4; for example... Figure 7As shown, after 42 days of remediation of contaminated soil using nZVIBM, nZVI@BCBM, BnZVIBM, and BnZVI@BCBM as passivating agents, the Cr(VI) concentration in the soil decreased to varying degrees, decreasing to 503.12 mg / kg, 378.54 mg / kg, 216.73 mg / kg, and 50.53 mg / kg, respectively. Comparing the passivation rates of four passivating agents for soil Cr(VI), nZVIBM achieved a passivation rate of 44.10%. This is because individual iron particles tend to aggregate after entering the soil, inhibiting their reduction performance for soil Cr(VI). The modified BnZVI@BCBM exhibited the highest passivation efficiency for soil Cr(VI), reaching 94.38%. This may be due to two reasons: firstly, BC has a strong dispersing effect on the material, inhibiting aggregation after entering the soil; secondly, during mechanical ball milling, chemical bonds break and new chemical bonds form. The BB bonds formed in BnZVI@BCBM during ball milling can promote the Fe(III) / Fe(II) cycle, thus giving the material excellent Cr(VI) removal capabilities. In conclusion, BnZVI@BCBM is a highly efficient soil heavy metal passivating agent.

Claims

1. A method for preparing boron-modified zero-valent iron material supported on ball-milled biochar, characterized in that... The preparation method is carried out according to the following steps: Step S1: Preparation of straw biochar; Corn stalks are crushed, dried, and sieved, then placed in a tube furnace and heated to 300–500°C under a nitrogen atmosphere. The stalks are then fully pyrolyzed at 300–500°C to obtain biochar. Step S2: Preparation of boron-modified zero-valent iron material supported on ball-milled biochar; The first step is ball milling. The biochar and iron powder obtained in step S1 are placed in a ball mill jar, and the ball mill jar is placed in a ball mill. After thorough ball milling, ball milled biochar loaded with zero valent iron is obtained. The mass ratio of biochar to iron powder is (0.15~0.03):(2.4~2.6). The second step involves ball milling. Zero-valent iron loaded with ball milled biochar and boron oxide are placed in a ball mill jar, which is then placed inside a ball mill. After thorough ball milling, the mixture is finally ground and sieved to obtain ball-milled biochar-loaded boron-modified zero-valent iron material. The mass ratio of the ball-milled biochar-loaded zero-valent iron to boron oxide is (2.4–2.6):(0.64–1.88). The grinding balls in the ball mill jar consist of large balls, medium balls and small balls, and the ratio of the number of large balls, medium balls and small balls is (1-3):(14-16):(15-17).

2. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... The drying temperature in step S1 is 60-80℃.

3. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... After drying in step S1, the product is passed through a 100-mesh sieve.

4. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... The heating temperature in step S1 is 300-400℃.

5. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... The pyrolysis time in step S1 is 2 to 3 hours.

6. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... The grinding balls mentioned in step S2 are zirconium dioxide balls.

7. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... In step S2, the first ball milling speed is 300-500 rpm, and the ball milling time is 3-5 hours.

8. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... In step S2, the ball milling speed in the second step is 300-500 rpm, and the ball milling time is 2-3 hours.

9. The method for preparing a ball-milled biochar-supported boron-modified zero-valent iron material according to claim 1, characterized in that... After ball milling in step S2, the final grinding and sieve is 100 mesh.

10. The application of a ball-milled biochar-supported boron-modified zero-valent iron material prepared by the method according to any one of claims 1-9, characterized in that... The application of the ball-milled biochar-supported boron-modified zero-valent iron material in the passivation of heavy metal chromium.

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