Ball-milled micro-nano zero-valent iron as well as preparation process and reduction performance evaluation method thereof

Abstract

The invention relates to ball-milled micro-nano zero-valent iron and a preparation process and a reduction performance evaluation method thereof, an iron-based material is prepared through wet ball milling, zero-valent iron or zero-valent iron, ferroferric oxide and a wet milling aid are mixed and then subjected to ball milling together, and a high-reducibility iron-based material with a micron or submicron scale is obtained by controlling ball milling parameters or adding a modified material. A performance evaluation method based on electron supply and electron selection is formed on the basis of the micro-nano iron-based material, and mainly comprises two aspects of accounting electron supply through the content of zero-valent iron and electrochemical clear electron transfer. According to the invention, optimization of a continuous ball-milling process of the high-performance zero-valent iron material and innovation of a ball-milling method for enhancing electron transfer by adding trioxide are realized, a performance evaluation method based on electron supply and electron transfer is created, and methodological support is provided for preparation and application of the micro-nano zero-valent iron material.

Description

Technical Field

[0001] This invention relates to the field of groundwater pollution remediation at contaminated sites, specifically to a ball-milled micro / nano zero-valent iron, its preparation process, and a method for evaluating its reduction performance. Background Technology

[0002] The application of zero-valent iron (ZVFe) materials in environmental remediation began in the 1990s, initially in particulate form in permeable reactive barrier technology to remove chlorinated hydrocarbon pollutants from groundwater. With the development of nanotechnology, nano-ZVFe has attracted widespread attention in environmental remediation due to its small particle size, large specific surface area, and high reactivity. However, both particulate iron filings and nano-ZVFe face several challenges in application: particulate iron filings exhibit reactive inertness due to their surface oxide shell; nano-ZVFe tends to agglomerate, reducing specific surface area and reactivity; and it has poor mobility, easily precipitating and failing to reach pollutant distribution areas. Furthermore, there are high production costs. These problems severely restrict the application of iron-based materials in the environment. Micro- and nano-iron-based materials with particle sizes between millimeter-sized iron filings and nano-ZVFe combine the advantages of moderate particle size, high reactivity, and strong reducing power. They can also achieve high reactivity and low manufacturing costs through mechanochemical methods, thus possessing broader application and market prospects. By controlling the ball milling process by adjusting the ball milling parameters and adding modified materials, it is possible not only to break the inhibitory effect of the surface oxide shell on the material's reactivity, but also to enhance the material's electron transfer performance and regulate the adsorption, coordination, and interfacial electron transfer behavior of pollutants on the particle surface.

[0003] Performance evaluation of micro / nano iron-based materials is crucial for guiding their preparation and application. However, with the advancement of research and production, existing qualitative standards based on particle size and simple morphological characteristics of iron-based materials are increasingly insufficient to evaluate the pollutant removal performance of micro / nano iron-based reducing materials. This is particularly true for different typical industries, regions, complex geological conditions, and pollution types in my country, where a comprehensive technical evaluation system is lacking, and evaluation indicators are difficult to link with practical applications, hindering their engineering applications and large-scale industrial development. Therefore, there is an urgent need for performance evaluation methods for micro / nano iron-based reducing materials based on electron supply and electron transport. Summary of the Invention

[0004] Technical Problem Solved: Addressing the issues of easy surface oxidation and lack of reduction performance evaluation during the ball milling process of zero-valent iron (ZVFe), this invention provides a ball-milled micro / nano ZVFe, its preparation process, and a method for evaluating its reduction performance. By adjusting ball milling parameters and adding magnetite (Fe3O4) as a modifier, surface oxidation of ZVFe is reduced. Furthermore, by employing electron supply and electron selection, and using hexavalent chromium as the target contaminant, a method for evaluating the performance of micro / nano ZVFe is constructed.

[0005] Technical solution: A process for preparing micro / nano zero-valent iron by ball milling, comprising the following steps: placing zero-valent iron or a mixture of zero-valent iron and iron(III) oxide and liquid grinding aid ethylene glycol in a zirconia ball milling jar, adding zirconia grinding beads, and performing wet ball milling under a nitrogen atmosphere, controlling the ball milling parameters to obtain iron-based composite materials at the micron or submicron scale; the iron-based composite material has an irregular granular morphology and its composition includes iron oxide and elemental iron.

[0006] The average particle size of the above-mentioned iron-based composite material is 0.1-1.5 μm.

[0007] The content of elemental iron in the above-mentioned iron-based composite material is 10wt.%-50wt.%.

[0008] The above ball milling parameters include: ball milling time of 0-48 hours, ball milling speed of 150-350 rpm, a 2-minute pause after every 8 minutes of work during the ball milling process, a cycle of 10 minutes, and alternating rotation directions after each cycle.

[0009] When adding iron(III) oxide, the molar ratio of zero-valent iron to iron(III) oxide is 1:1 to 1:5; the volume ratio of zero-valent iron or the total mass of zero-valent iron and iron(III) oxide to the grinding aid ethylene glycol is 1:1 to 1:5; and the mass ratio of zero-valent iron or the total mass of zero-valent iron and iron(III) oxide to the grinding beads is 1:10 to 1:100.

[0010] The aforementioned grinding balls include grinding balls with diameters of 3 mm, 5 mm, and 15 mm, with a mass ratio of 2:5:3.

[0011] The above-mentioned nitrogen atmosphere is achieved through the following steps: before ball milling, the ball mill jar is purged with nitrogen for 7 minutes, and after adding materials and grinding aids, purging continues for 3 minutes; after ball milling, the ball mill beads are separated to obtain a slurry, the slurry is washed with deoxyethanol and deoxygenated ultrapure water, and then freeze-dried for 12-24 hours to obtain the iron-based composite material.

[0012] A ball-milled micro / nano zero-valent iron was prepared using the above-described process.

[0013] A method for evaluating the reduction performance of ball-milled micro / nano zero-valent iron, used to evaluate the aforementioned ball-milled micro / nano zero-valent iron, includes the following steps: evaluating the reduction performance of micro / nano zero-valent iron by the amount of hexavalent chromium removed, the connotation of which is electron supply and electron transfer.

[0014] The aforementioned electron supply is characterized by the content of zero-valent iron and divalent iron in the micro-nano range.

[0015] The electron transfer described above was characterized by electrochemical experiments to monitor electron transfer capability.

[0016] Beneficial effects: (1) The ball milling method used in this invention to prepare micro-nano zero-valent iron is simple, practical, and easy to scale up, with significant economic, environmental and social benefits; (2) This invention uses ball milling to prepare micro-nano iron-based reducing materials by adjusting ball milling parameters and adding iron(III) oxide as a modifier, significantly improving the original zero-valent iron's ability to remove hexavalent chromium or chlorinated hydrocarbon pollution; (3) This invention provides a method for evaluating the reduction performance of micro-nano iron-based materials based on electron supply and electron transfer, filling the gap in the evaluation of the reduction performance of micro-nano zero-valent iron; (4) The evaluation method based on electron supply and electron transfer in this invention has been applied to a certain extent in the preparation and experimentation of ball-milled zero-valent iron and ball-milled zero-valent iron-iron(III) oxide composite materials, which can comprehensively reflect the product characteristics of the micro-nano iron-based materials and their removal performance of hexavalent chromium or chlorinated hydrocarbon pollution, providing reliable and effective data support for their practical application. Attached Figure Description

[0017] Figure 1 Scanning electron microscope images of zero-valent iron prepared for different ball milling times.

[0018] Figure 2 This is a graph showing the change in the content of elemental iron with zero valence in ball milling.

[0019] Figure 3 The image shows the results of removing hexavalent chromium from zero-valent iron using ball milling.

[0020] Figure 4 The image shows the results of ball milling zero-valent iron and ferric oxide removing hexavalent chromium. Detailed Implementation

[0021] The present invention will be further described below with reference to specific examples.

[0022] A process for preparing micro / nano-sized zero-valent iron by ball milling is disclosed. Zero-valent iron and a zero-valent iron-Fe3O4 composite material are prepared using wet ball milling with the addition of a liquid grinding aid. The composite material has an irregular granular morphology and is mainly composed of iron oxide and elemental iron. The composite material has an average particle size of micrometers or submicrometers, ranging from 0.1 to 1.5 μm. The elemental iron content in the composite material ranges from 10 wt.% to 50 wt.%.

[0023] The process includes the following steps: zero-valent iron or zero-valent iron, iron tetroxide and grinding aid ethylene glycol are placed in matching zirconia grinding beads and a grinding jar of the same material. The grinding parameters are adjusted or the proportion of modified materials is added. Then the grinding is carried out. After the grinding jar returns to room temperature, it is taken out and separated from the grinding beads to obtain an iron-based composite material slurry. Then it is washed with deoxyethanol and deoxygenated ultrapure water respectively and freeze-dried to obtain the iron-based composite material. The molar ratio of zero-valent iron to iron(III) oxide is 1:1-1:5; the total mass ratio of zero-valent iron to iron(III) oxide to the volume ratio of the grinding aid ethylene glycol is 1:1-1:5; the total mass ratio of zero-valent iron to iron(III) oxide to the mass of the grinding balls is 1:10-1:100, and the mass ratio of grinding balls with diameters of 3 mm, 5 mm and 15 mm is 2:5:3; the ball milling time for zero-valent iron to iron(III) oxide is 0-48 h, with a 2-minute break after every 8 minutes of work, and each cycle is 10 minutes. After each cycle, the rotation direction is alternated; the rotation speed during ball milling is 150-350 rpm; the grinding jar is purged with nitrogen for 7 minutes beforehand, and purging continues for 3 minutes after adding the material and grinding aid.

[0024] The ball-milled micro / nano zero-valent iron obtained by the above process.

[0025] The above-mentioned performance evaluation method for ball-milled micro / nano zero-valent iron mainly includes the determination of electron supply and electron transfer; the evaluation process includes the following steps:

[0026] S1: Evaluation of elemental iron content. Elemental iron content samples are taken from each batch of micro-nano iron-based reducing materials. The content of elemental iron in the particles is analyzed by acid dissolution test, and then evaluated to see if it meets the product requirements.

[0027] S2: Crystal structure evaluation. Crystal structure samples are taken from each batch of micro-nano iron-based reducing materials. The particle crystal structure is analyzed using an X-ray diffractometer, and then evaluated to see if it meets the product requirements.

[0028] S3: Surface functional group evaluation. Surface functional group samples are taken from each batch of micro-nano iron-based reducing materials. The particle crystal structure is analyzed using Fourier transform infrared spectroscopy, and then evaluated to see if it meets the product requirements.

[0029] S4: Electrochemical evaluation: Electrochemical samples are taken from each batch of micro-nano iron-based reducing materials. The corrosion sites and resistance are analyzed using an electrochemical analyzer with cyclic voltammetry and other methods. The results are then evaluated to determine whether the samples meet the product requirements.

[0030] If any conformity evaluation test fails during the above physicochemical property evaluation process, the evaluation will be terminated.

[0031] Example 1

[0032] By controlling the ball milling time to regulate the ball milling process of zero-valent iron (ZVFe), ZVFe materials with different ball milling times were prepared: A 500 mL ball mill jar equipped with zirconia grinding beads was purged with nitrogen for 7 min. 5 g of ZVFe powder and 5 mL of ethylene glycol were added to the ball mill jar, and purging continued for another 3 min. The mass of the grinding beads was 200 g, with 40 g, 100 g, and 60 g of beads with diameters of 3, 5, and 15 mm, respectively. Ball milling was performed at 350 rpm. The ball milling time included 15 time points between 0 and 18 h, with a 2-min rest interval after every 10 min of operation. Each cycle lasted 12 min, and the rotation direction was alternated after each cycle. After ball milling, the ball mill jar was removed after the temperature returned to room temperature. The grinding beads were separated from the iron-based material slurry using a mesh screen. The ball mill jar and grinding beads were rinsed three times with deoxygenated ethanol to remove residual iron-based material, followed by rinsing with deoxygenated ultrapure water to remove residual ethanol. The mixture was then freeze-dried for 36 h. After freeze-drying is complete, transfer the samples to brown sample vials, seal them, and store them for later use. Figure 1 ).

[0033] Example 2

[0034] A ball-milled zero-valent iron-iron tetroxide composite material was prepared by adding iron(III) oxide as a modifier: A 500 mL ball mill jar equipped with zirconia grinding beads was purged with nitrogen for 7 min. 1.1 g of zero-valent iron powder, 0.9 g of iron(III) oxide powder, and 2 mL of ethylene glycol were added to the ball mill jar, and purging continued for 3 min. The mass of the grinding beads was 200 g, with 40 g, 100 g, and 60 g of beads with diameters of 3, 5, and 15 mm, respectively. Ball milling was performed at 350 rpm for 36 h, with a 2-min rest interval after every 8 min of operation. Each cycle lasted 10 min, and the rotation direction was alternated after each cycle. After ball milling, the ball mill jar was removed after the temperature returned to room temperature. The grinding beads were separated from the iron-based material slurry using a mesh screen. The ball mill jar and grinding beads were rinsed three times with deoxygenated ethanol to remove residual iron-based material, followed by rinsing with deoxygenated ultrapure water to remove residual ethanol. The mixture was then freeze-dried for 12 h. After freeze-drying is complete, transfer the samples to brown sample vials, seal and store for later use.

[0035] Example 3

[0036] The reduction performance of ball-milled zero-valent iron (ZVFe) or ball-milled ZVFe-Fe3O4 composites was evaluated: electron supply was characterized by the ZVFe content. Specifically, the ZVFe content in the material was calculated based on the principle of hydrogen gas generation from the reaction of elemental iron with hydrogen ions in acid solution. Specifically, 10 mg of the iron-based material was mixed with 10 mL of a 6 mol·L⁻¹ solution. -1The hydrochloric acid solution was mixed in a sealed container and connected to a U-shaped graduated tube via a rubber hose. After the iron-based material and hydrochloric acid solution were fully mixed and the reaction was complete, the liquid level in the U-shaped tube was observed and the volume of hydrogen gas produced was recorded. This value was substituted into the ideal gas law to calculate the content of elemental iron. Figure 2 The air pressure is taken as the local multi-year average atmospheric pressure, and the temperature is obtained by measuring the laboratory temperature with a thermometer.

[0037] Electron transfer was analyzed using an electrochemical workstation. The test system consisted of a three-electrode system, including one glassy carbon working electrode (3 mm in diameter), one silver / silver chloride electrode as a reference electrode, and one platinum electrode as a counter electrode. Electrode material preparation involved mixing 20 mg of ball-milled micro / nano-sized zero-valent iron with 200 μL of Nafion solution, ultrasonically dispersing for 10 min, and then coating the mixture onto the working electrode surface. Tests and methods were based on previous research, specifically including open-circuit potential, Tafel corrosion curves, and AC impedance spectroscopy. The open-circuit potential was measured within the voltage range of -1.5 V to 1.5 V. The Tafel corrosion curve was measured within the open-circuit voltage range of ±1.0 V for the corresponding material, at a scan rate of 2 mV·s. -1 Both of the above tests used 0.1 mol·L⁻¹ -1 Sodium sulfate solution was used as the electrolyte solution. Electrochemical impedance spectroscopy (EIS) parameters were measured with an amplitude of 5 mV and a frequency range of 60 kHz to 0.01 Hz. The electrolyte concentration was 0.1 mol·L⁻¹. -1 KCl and 5.0 mmol·L -1 A mixed solution of K3Fe(CN)6 was prepared. The electrolyte was purged with high-purity N2 for 8 min before the reaction to remove dissolved oxygen, and this purging was maintained throughout the test. The crystal structure and phase evolution were characterized by X-ray diffraction at a scan rate of 6°·s. -1 The 2θ angle ranges from 10° to 80°. Fourier transform infrared spectroscopy is performed in the range of 4000-500 cm⁻¹. -1 Surface chemical shifts and functional group changes were measured within the specified range.

[0038] Experimental Example 1

[0039] Using the material obtained in Example 1, an experiment was conducted to remove hexavalent chromium: the initial concentration of hexavalent chromium was fixed at 50 mg·L⁻¹. -1 pH 3.0 and solid-liquid ratio 0.25 g·L -15 mg of raw zero-valent iron, ball-milled zero-valent iron (Example 1), and 20 mL of hexavalent chromium solution were mixed in a 50 mL plastic centrifuge tube. The mixture was placed in a shaker at 25°C and 180 rpm. After reaction times of 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 72, and 120 h, the suspension was filtered through a 0.45 μm polyethersulfone aqueous membrane, and the final pH was measured. Hexavalent chromium was determined using the diphenylcarbazide colorimetric method. The results showed ( Figure 3 The results of hexavalent chromium removal indicate that ball milling can improve Fe... 0 The removal performance of hexavalent chromium was investigated, and the removal rate of hexavalent chromium by ball-milled micro-nano zero-valent iron showed a wave-like change with increasing ball-milling time. After reaction equilibrium, the maximum removal amounts were achieved at 1 and 12 hours of ball milling micro-nano zero-valent iron, respectively, at 70.22 and 59.52 mg·g⁻¹. -1 Compared with the degradation rate of pollutants by the original zero-valent iron, the degradation rate of zero-valent iron by ball milling in this invention is significantly improved, indicating that the iron-based composite material prepared by modifying the material by adjusting the ball milling parameters has high activity and a faster reduction and removal rate of hexavalent chromium.

[0040] Using the material obtained in Example 2, an experiment was conducted to remove hexavalent chromium: the initial concentration of hexavalent chromium was fixed at 50 mg·L⁻¹. -1 pH 3.0 and solid-liquid ratio 0.25 g·L -1 5 mg of ball-milled zero-valent iron-iron tetroxide composite material (Example 2) was mixed with 20 mL of hexavalent chromium solution in a 50 mL plastic centrifuge tube. The mixture was placed in a shaker at 25°C and 180 rpm. After reaction times of 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 72, and 120 h, the suspension was filtered through a 0.45 μm polyethersulfone aqueous membrane, and the final pH was measured. Hexavalent chromium was determined using the diphenylcarbazide colorimetric method. The results showed ( Figure 4 ), at an initial pollutant concentration of 30.0 mg·L -1 pH = 3.0, solid-liquid ratio 1.0 g·L -1 Under these conditions, after a reaction of 120 h, ball-milled micro-nano zero-valent iron and iron(III) oxide achieved a 96.5% removal rate of hexavalent chromium, which was significantly lower than the original Fe content. 0 The removal amounts of Fe3O4 were 4.4 and 30.2 times greater than those of Fe3O4, indicating a synergistic effect between zero-valent iron and magnetite in the composite material. The removal kinetics of hexavalent chromium by ball-milled micro / nano zero-valent iron and magnetite conformed to a pseudo-second-order kinetic model, indicating that the electron transfer process between the material and the contaminant is the rate-determining step of the reaction. Electrochemical tests showed that the magnetite phase with semiconductor properties is key to improving the electron transfer performance of the composite material. Fe3O4 in the composite material acts as a fast electron transport bridge, allowing Fe... 0Electrons are transferred outward from the ferrous ions, thereby promoting the continuous and rapid removal of hexavalent chromium by surface-bound and structured ferrous ions. This example also verifies the method of evaluating the reduction performance of zero-valent iron by electron supply and electron transfer.

[0041] The above description is merely an example of the embodiments of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the technical principles of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A process for preparing ball-milled micro / nano zero-valent iron, characterized in that, The process includes the following steps: placing zero-valent iron or a mixture of zero-valent iron and iron(III) oxide with liquid grinding aid ethylene glycol in a zirconia ball mill jar, adding zirconia grinding beads, and performing wet ball milling under a nitrogen atmosphere, controlling the ball milling parameters to obtain iron-based composite materials at the micron or submicron scale; the iron-based composite material has an irregular granular morphology and its composition includes iron oxide and elemental iron.

2. The preparation process according to claim 1, characterized in that, The average particle size of the iron-based composite material is 0.1-1.5 μm.

3. The preparation process according to claim 1, characterized in that, The content of elemental iron in the iron-based composite material is 10 wt.%-50 wt.%.

4. The preparation process according to claim 1, characterized in that, The ball milling parameters include: a ball milling time of 0-48 hours, a ball milling speed of 150-350 rpm, a 2-minute pause after every 8 minutes of work during the ball milling process, a cycle of 10 minutes, and alternating rotation directions after each cycle.

5. The preparation process according to claim 1, characterized in that, When adding iron(III) oxide, the molar ratio of zero-valent iron to iron(III) oxide is 1:1 to 1:5; the volume ratio of zero-valent iron or the total mass of zero-valent iron and iron(III) oxide to the grinding aid ethylene glycol is 1:1 to 1:5; and the mass ratio of zero-valent iron or the total mass of zero-valent iron and iron(III) oxide to the grinding beads is 1:10 to 1:

100.

6. The preparation process according to claim 5, characterized in that, The grinding balls include grinding balls with diameters of 3 mm, 5 mm and 15 mm, in a mass ratio of 2:5:

3.

7. The preparation process according to claim 1, characterized in that, The nitrogen atmosphere is achieved through the following steps: before ball milling, the ball mill jar is purged with nitrogen for 7 minutes, and after adding materials and grinding aids, purging continues for 3 minutes; after ball milling, the milling beads are separated to obtain a slurry, the slurry is washed with deoxyethanol and deoxygenated ultrapure water, and then freeze-dried for 12-24 hours to obtain an iron-based composite material.

8. A ball-milled micro / nano zero-valent iron, characterized in that, It is prepared by the preparation process described in any one of claims 1-7.

9. A method for evaluating the reduction performance of ball-milled micro / nano zero-valent iron, characterized in that, Used to evaluate the ball-milled micro / nano zero-valent iron described in claim 8 This includes the following: evaluating its performance based on the amount of hexavalent chromium removed, which encompasses electron supply and electron transport.

10. The method for evaluating reduction performance according to claim 9, characterized in that, The electron supply is characterized by the content of zero-valent iron and ferrous iron in the micro-nano range; the electron transfer is characterized by electrochemical experiments to monitor the electron transfer capability.

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