Zero-valent iron biochar preparation method capable of reducing dosage of reducing agent and ferric salt

Abstract

The invention relates to a zero-valent iron biochar preparation method capable of reducing the dosage of a reducing agent and ferric salt, and belongs to the technical field of biochar. The method takes waste blueberry pomace and industrial waste red mud as raw materials, and is realized by the following steps: drying the blueberry pomace at low temperature, and ultrasonically extracting reducing components by using an acidic ethanol solution to obtain an extracting solution rich in anthocyanin and ascorbic acid; the extracted blueberry residues and red mud are mixed and subjected to co-pyrolysis, and red mud biochar is prepared; the red mud biochar reacts with an extracting solution, the natural reducibility of the blueberry residue extracting solution is fully utilized, a small amount of sodium borohydride solution is supplemented for further reduction, and finally the zero-valent iron biochar is prepared. The red mud and the blueberry residue extracting solution are used for replacing traditional ferric salt, the using amount of sodium borohydride is reduced by 33%, and the production cost and the environmental pollution risk are remarkably reduced. The obtained product has strong magnetism and excellent pollutant degradation performance, is suitable for treatment of environmental pollutants such as antibiotics, and has the advantages of environmental protection, economy and high efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of biochar, and in particular relates to a method for preparing zero-valent iron biochar. Background Art

[0002] Zero-valent iron biochar (ZVI / BC) is a composite material that combines the properties of zero-valent iron and biochar, exhibiting multiple functions, including adsorption, reduction, and catalysis. Red mud, an industrial solid waste generated during the extraction of alumina by the aluminum industry, contains large amounts of iron oxide, making it a viable and low-cost iron source. Utilizing the iron in red mud to develop zero-valent iron biochar materials holds broad application prospects.

[0003] Existing technology primarily produces zero-valent iron biochar through biomass-iron source co-pyrolysis. For example, patent application 202311629275.7 discloses a method for preparing a red mud-based zero-valent iron soil remediation agent. The method involves drying, crushing, and sieving red mud. Straw is naturally dried, crushed, washed, and sieved. The red mud and straw powders are mixed, water is added, stirred, and dehydrated to produce a red mud-straw mixture. The straw biomass is then pyrolyzed at high temperatures to reduce the iron oxides in the red mud to zero-valent iron, producing a red mud-based zero-valent iron soil remediation agent. Patent application 202211041105.2 discloses a method for preparing a zero-valent iron catalyst using red mud and straw. The red mud powder and crushed straw powder are added to deionized water, mixed evenly, and dehydrated to produce a red mud-biomass mixture. The catalyst is then obtained by pyrolysis under a nitrogen atmosphere. This biomass-iron source co-pyrolysis method offers advantages such as simplicity and low energy consumption. However, due to the limitations of reaction conditions, there are problems such as low iron reduction rate and easy secondary oxidation during pyrolysis, as well as uneven distribution of iron particles.

[0004] The borohydride reduction method promotes the dispersion of iron particles and has a high iron reduction rate. Therefore, some studies have proposed adding biochar to a solution containing iron salts and then adding borohydride to achieve biochar-loaded nano-zero-valent iron. For example, patent application 202110841563.3 uses sodium borohydride to react with ferrous sulfate heptahydrate to produce zero-valent iron, and patent application 202310367562.9 uses potassium borohydride to reduce ferrous sulfate heptahydrate. In a liquid environment, zero-valent iron is adsorbed or deposited on the surface of biochar, forming zero-valent iron biochar.

[0005] Although red mud contains a high iron content, it can theoretically be partially reduced to zero-valent iron using reducing agents such as borohydride, but this efficiency is low. Therefore, existing technologies still use salts such as ferrous sulfate as an iron source to prepare zero-valent iron, which is then compounded with red mud, rather than directly reducing the iron in the red mud. For example, published patent applications 202311658926.5 and 202411253496.3 provide methods for preparing environmental remediation materials containing red mud and nano-zero-valent iron, but both simply compound the two and neither uses red mud as the zero-valent iron source. Furthermore, the borohydride used in reduction methods is somewhat toxic, leading researchers to attempt to extract natural reducing agents from reducing plant-based materials to prepare zero-valent iron materials. For example, patent application 201711326238.3 uses green tea extract to reduce ferrous ions to produce zero-valent iron biochar. However, this method requires water bath heating and inert gas protection during the reduction process, resulting in low reduction efficiency and a complex operational process.

[0006] In summary, existing methods for producing zero-valent iron biochar using red mud as an iron source have been ineffective. Techniques based on borohydride reduction still rely on chemical reagents such as iron salts as the reduction target and are therefore ineffective for reducing iron in red mud. Natural reducing agents derived from plants can only reduce iron salts (chemical reagents) and are inefficient and complex to operate. Overall, none of these methods has solved the problem of efficiently producing zero-valent iron biochar using red mud as an iron source. The large amount of toxic reducing agents, cumbersome processes, and high costs are bottlenecks hindering the large-scale production of zero-valent iron biochar.

[0007] To this end, this paper provides a method for preparing zero-valent iron biochar using red mud and blueberry pomace as raw materials. Blueberry pomace has antioxidant and reducing abilities due to its rich anthocyanins and ascorbic acid. Ascorbic acid can convert Fe 3+ Reduction to Fe 2+ , even Fe 0 , and anthocyanins have strong free radical scavenging and reducing abilities. Blueberry pomace extract can maintain the reducing environment of the solution, replacing the protective role of nitrogen, thereby reducing the amount of reducing agents such as borohydride. The extracted blueberry pomace is co-pyrolyzed with red mud to promote the reduction of iron on biochar during the co-pyrolysis process. The comprehensive utilization of blueberry pomace and its reducing active ingredients and the iron in red mud can eliminate the need for iron salts, significantly reduce the amount of reducing agents, and avoid the need for inert gas protection, thus achieving efficient preparation of zero-valent iron biochar. Summary of the Invention

[0008] The purpose of the present invention is to propose a green preparation method for the existing method of preparing zero-valent iron biochar using a large amount of iron salts and toxic reducing agents, using blueberry pomace extract and red mud as raw materials, significantly reducing the amount of NaBH4 and iron salts.

[0009] The method for preparing zero-valent iron biochar of the present invention comprises the following steps: Step 1: Place the blueberry residue in a constant temperature drying oven at 30-40°C until completely dry, then crush it to less than 0.25-0.85mm; Step 2: Prepare the extract by mixing ethanol and distilled water and adjusting the pH to ≤ 1 with an appropriate amount of glacial acetic acid; Step 3: Add the crushed blueberry pomace to the extract and place in an ultrasonic cleaner at 100% power for 30 minutes. Collect the resulting solution, filter it, and then concentrate it by rotary evaporation at 30-40°C. Step 4: The extracted blueberry residue was dried again in an oven at 80-120°C, mixed evenly with red mud at a mass ratio of (1-2):1, placed in a sealed crucible, and pyrolyzed in a muffle furnace to obtain red mud biochar RM-BC; Step 5: Add RM-BC to the extraction solution in small amounts and several times, stirring continuously with a magnetic stirrer until no bubbles are generated; Step 6: Add NaBH4 solution dropwise to the solution in step 5 and continue stirring for 30 min; Step 7: Use a magnet to separate the complex from the solution in step 6, then wash it alternately with deionized water and anhydrous ethanol three times, freeze-dry it, and then sieve and seal the product for storage to obtain zero-valent iron biochar ZVI-Fe3O4-BC.

[0010] Furthermore, the extract in step 2 can be prepared according to a volume ratio of ethanol to distilled water of (1-3):1.

[0011] Furthermore, in step three, the mass ratio of blueberry residue to extract is 1:(10-20).

[0012] Furthermore, the extract in step 3 is rotary evaporated to 50% of the volume of the extract.

[0013] Furthermore, the muffle furnace in step 4 was heated to a set temperature of 500°C at a heating rate of 15°C / min and maintained for 1 hour before stopping heating. The RM-BC was cooled to room temperature in the furnace and then taken out and crushed to 0.15-0.25 mm.

[0014] Furthermore, in step five, the solid-liquid ratio of RM-BC to the extract is 1:(10-20).

[0015] Furthermore, in step six, the concentration of the NaBH4 solution is 1 mol / L, and the amount used is 5-35 mL.

[0016] The present invention has the advantages that:

[0017] The effective components in the waste red mud and blueberry pomace were utilized, avoiding the use of iron salts and reducing the amount of NaBH4 by about 33%.

[0018] The zero-valent iron biochar prepared by the present invention contains Fe3O4, has strong magnetism and is easy to recycle.

[0019] The method of the invention is safe, has low preparation cost, is assisted by techniques such as ultrasound, pyrolysis, and magnetic stirring, and is easy to operate.

[0020] The zero-valent iron biochar provided by the present invention has a significant effect in treating environmental pollutants and can be used to treat antibiotic pollutants. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow chart of the present invention for preparing zero-valent iron biochar

[0022] Figure 2 is the tetracycline removal rate of the zero-valent iron biochar prepared in Example 2 and Comparative Example 2 of the present invention;

[0023] Figure 3 This is a picture and a scanning electron microscope image of the zero-valent iron biochar product prepared in Example 1 of the present invention;

[0024] Figure 4 1 is a schematic diagram of X-ray diffraction (XRD) of the zero-valent iron biochar prepared in Example 1 of the present invention;

[0025] Figure 5 This is a schematic diagram of the energy dispersive X-ray spectrum (EDS) of the zero-valent iron biochar prepared in Example 1 of the present invention.

[0026] Figure 6 Schematic diagram of the hysteresis loop of the zero-valent iron biochar prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be described in detail below with reference to the accompanying drawings.

[0028] The following describes the present invention and its embodiments. This description is not restrictive, and the actual embodiments are not limited to this. In short, if a person skilled in the art is inspired by this and, without departing from the purpose of the present invention, designs structures and embodiments similar to this technical solution without creatively designing them, they shall fall within the scope of protection of the present invention.

[0029] Example 1: A method for preparing zero-valent iron biochar with reduced amounts of reducing agents and iron salts

[0030] The process flow chart of the zero-valent iron biochar prepared in this embodiment is as follows: Figure 1The method specifically includes the following steps: Place the blueberry pomace in a 35°C constant temperature drying oven until completely dry, then crush it to less than 0.85 mm for later use. A 50% ethanol solution is prepared, and the pH is adjusted to 1 with an appropriate amount of glacial acetic acid as the extraction solvent. 50 g of the crushed blueberry pomace is weighed, added to 500 mL of the extraction solution, and placed in an ultrasonic cleaner at 100% power for 30 minutes. The resulting solution is collected, filtered, and concentrated to 250 mL by rotary evaporation at 35°C. The extracted blueberry pomace was oven-dried again at 80°C and mixed with red mud at a mass ratio of 1:1. The mixture was then placed in a sealed crucible and placed in a muffle furnace. The temperature was raised at a rate of 15°C / min to a set temperature of 500°C, where it was maintained for 1 hour before heating was stopped. After cooling to room temperature, the mixture was removed from the furnace and sieved (<0.15 mm) to obtain red mud biochar (RM-BC). 10 g of RM-BC was gradually added to 200 mL of the blueberry pomace extract and stirred continuously with a magnetic stirrer until no bubbles formed. Subsequently, 20 mL of a 1 mol / L NaBH₄ solution was added to the solution at a rate of 1–2 drops per second and stirred continuously for 30 minutes. The composite was separated from the solution using a magnet and then washed three times with deionized water and anhydrous ethanol. After freeze-drying, the product was sieved (<0.15 mm) to obtain zero-valent iron biochar (ZVI-Fe₃O₄-BC).

[0031] Example 2: A method for preparing zero-valent iron biochar with reduced amounts of reducing agents and iron salts

[0032] The process involves the following steps: Place the blueberry pomace in a 35°C constant-temperature drying oven until completely dry, then crush it to less than 0.85 mm for later use. Prepare a 50% ethanol solution, adjust the pH to 1 with an appropriate amount of glacial acetic acid, and use it as an extraction solvent. Weigh 50 g of the crushed blueberry pomace, add 500 mL of the extraction solution, and place the solution in an ultrasonic cleaner at 100% power for 30 minutes. The resulting solution was collected, filtered, and concentrated to 250 mL by rotary evaporation at 35°C. The extracted blueberry pomace was then oven-dried at 80°C and mixed with red mud at a mass ratio of 1:1. The mixture was then placed in a sealed crucible and placed in a muffle furnace. The temperature was increased at a rate of 15°C / min to a set temperature of 500°C, where it was maintained for 1 hour before heating was stopped. After cooling to room temperature, the mixture was removed and sieved (<0.15 mm) to obtain red mud biochar (RM-BC). 10 g of RM-BC was gradually added to 200 mL of the blueberry pomace extract and stirred continuously with a magnetic stirrer until no bubbles formed. Subsequently, 0, 5, 10, 15, 20, 25, 30, and 35 mL of a 1 mol / L NaBH₄ solution were added to the solution at a rate of 1-2 drops per second and stirred for 30 minutes. The composite was separated from the solution using a magnet and then washed three times with deionized water and anhydrous ethanol. After freeze-drying, the product was sieved (<0.15 mm) to obtain zero-valent iron biochar ZVI-Fe3O4-BC.

[0033] Comparative Example 1: Method for preparing zero-valent iron biochar using inactivated blueberry pomace extract

[0034] The method comprises the following steps: placing the blueberry pomace in a constant temperature drying oven at 35°C until completely dry, then crushing it to less than 0.85 mm for later use. A 50% ethanol solution is prepared, and the pH is adjusted to 1 with an appropriate amount of glacial acetic acid as an extractant. 50 g of the crushed blueberry pomace is weighed, added to 500 mL of the extract, and placed in an ultrasonic cleaner at 100% power for 30 minutes. The resulting solution is collected, filtered, and concentrated to 250 ml by rotary evaporation at 35°C. Because anthocyanins and ascorbic acid are inactivated at high temperatures, the extract is placed in a high-temperature sterilizer and heated to 120°C for 2 hours to render the anthocyanins and ascorbic acid in the extract inactive. The extracted blueberry pomace was oven-dried again at 80°C and mixed with red mud at a mass ratio of 1:1. The mixture was then placed in a sealed crucible and placed in a muffle furnace. The temperature was raised at a rate of 15°C / min to a set temperature of 500°C, where it was maintained for 1 hour before heating was stopped. After cooling to room temperature, the mixture was removed and sieved (<0.15 mm) to obtain red mud biochar (RM-BC). 10 g of RM-BC was gradually added to 200 mL of the inactivated blueberry pomace extract, stirring continuously with a magnetic stirrer until no bubbles formed. Subsequently, 0, 5, 10, 15, 20, 25, 30, and 35 mL of a 1 mol / L NaBH₄ solution were added to the solution at a rate of 1-2 drops per second and stirred continuously for 30 minutes. The composite was separated from the solution using a magnet and then washed three times with alternating deionized water and anhydrous ethanol. After freeze-drying, the product was sieved (<0.15 mm) to obtain zero-valent iron biochar ZVI-Fe3O4-BC.

[0035] Example 3: Analysis of the amount of blueberry pomace extract to replace NaBH4 using TC degradation test

[0036] Using the zero-valent iron biochar prepared in Example 2 and Comparative Example 1 as examples, we analyzed the tetracycline removal performance of each zero-valent iron biochar material to determine the zero-valent iron loading on the biochar. We also analyzed the amount of NaBH4 replaced by the blueberry pomace extract. The experimental steps were as follows:

[0037] First, 50 mL of a 20 mg / L TC solution was added to a 100 mL Erlenmeyer flask. The initial pH of the solution was adjusted to 6.0 using 0.1 M hydrochloric acid and sodium hydroxide. Then, 0.2 g / L of zero-valent iron biochar was added. The mixture was shaken at 180 rpm for 3 hours at 25°C in the dark. Finally, the sample was filtered through a 0.22 μm organic system syringe filter, and the TC concentration was measured. By comparing the TC removal efficiency of different materials, we were able to infer the zero-valent iron content in each material and, based on this, determine the amount of NaBH₄ that the blueberry pomace extract could replace. The tetracycline removal rate was calculated as follows:

[0038] In the formula C 0 and C e are the tetracycline concentrations before and after degradation, respectively; η is the removal rate of tetracycline by zero-valent iron biochar.

[0039] The test results are as follows Figure 2 As shown, the ZVI-Fe3O4-BC prepared with 20 mL of NaBH4 added to the blueberry pomace extract exhibited the highest TC removal efficiency, indicating the highest reduction capacity of ZVI. However, to achieve the same TC removal efficiency in the ZVI biochar prepared from the inactivated blueberry pomace extract, more than 30 mL of NaBH4 solution was required. Therefore, these results indicate that the blueberry pomace extract has a certain reducing capacity and can replace approximately 33% of the NaBH4.

[0040] Example 4: Analysis of the physical and chemical properties of zero-valent iron biochar

[0041] Taking Example 1 as an example, the prepared zero-valent iron biochar product image and the scanning electron microscope image are as follows Figure 3 As shown, ZVI-Fe3O4-BC exhibits magnetic properties and can be easily adsorbed by a magnet. Electron microscopy images reveal a rough, irregular spherical surface with numerous small particles attached, demonstrating that biochar acts as an excellent carrier for iron particles. These iron particles adhere tightly to the biochar surface in a fine form, achieving a dispersed effect.

[0042] 1. Energy Dispersive X-ray Spectroscopy (EDS) Analysis Experiment of Example 1

[0043] A certain amount of zero-valent iron biochar ZVI-Fe3O4-BC prepared in Example 1 was fixed on the sample table with conductive glue, and the element distribution map in the selected area was scanned in the vacuum working system. The test results are as follows: Figure 4 As shown in the figure, the C and iron Fe contents on the surface of ZVI-Fe3O4-BC are relatively high, which indicates that a large amount of iron elements were successfully introduced into the surface structure of biochar during the preparation process of ZVI-Fe3O4-BC.

[0044] 2. X-ray diffraction (XRD) analysis experiment of Example 1

[0045] 15 mg of the zero-valent iron biochar ZVI-Fe3O4-BC prepared in Example 1 was placed in a glass sample holder, flattened with frosted glass, and placed on a sample stage. The angle range was set to 10-80° and the test rate was 2° / min. The test results are shown in Figure 2. Figure 5As shown, ZVI-Fe3O4-BC shows a special peak at 44.8°, which is the zero-valent iron crystal peak, proving that iron is reduced.

[0046] 3. Magnetic analysis experiment of Example 1

[0047] A certain amount of zero-valent iron biochar ZVI-Fe3O4-BC powder prepared in Example 1 was fixed on a vibrating rod to ensure that the sample was in the center of the magnetic field. The applied magnetic field strength was set to ±2 T. The test results are shown in Figure 2. Figure 6 As shown in the figure, the saturation magnetization intensity of ZVI-Fe3O4-BC is Ms=67.07 emu / g, showing good magnetic properties. Its hysteresis loop is a typical narrow magnetic loop, indicating that they are soft magnetic materials and have superparamagnetism. Therefore, when a certain magnetic field is applied to ZVI-Fe3O4-BC, solid-liquid separation will be achieved quickly, making it easy to recover.

[0048] The present invention has been described in detail above in combination with specific implementation methods and material property analysis, but the above is only a preferred embodiment of the present invention and does not limit the present invention. Any equivalent replacement, modification or improvement of the technical features recorded in the aforementioned embodiments are included in the scope of the present invention.

Claims

1. A method for preparing zero-valent iron biochar by reducing the amount of reducing agent and iron salt, characterized by: By making full use of the reducing substances in the waste blueberry residue and the iron in the industrial waste red mud, the process includes the following steps: Step 1: Dry the blueberry residue at low temperature, crush and sieve it, and then extract and filter it to obtain an extract; Step 2: The extracted blueberry pomace is dried again, mixed with red mud powder, placed in a sealed crucible, and co-pyrolyzed to prepare red mud biochar; Step 3: Add red mud biochar to the blueberry pomace extract, stir thoroughly and let it stand; Step 4: Add sodium borohydride solution dropwise to the above mixture while continuously stirring; Step 5: The solid in the mixed solution is magnetically collected, washed alternately with ethanol and deionized water, and freeze-dried to obtain zero-valent iron biochar.

2. The method for low-temperature drying of blueberry residue according to claim 1, wherein The temperature of the low-temperature drying in the step 1 is set to be lower than 40°C.

3. The preparation method according to claim 1, wherein The preparation method of the blueberry pomace extract comprises the following steps: crushing the blueberry pomace to a particle size of less than 0.85 mm, ultrasonically extracting the blueberry pomace with an ethanol solution having a pH value of less than or equal to 1 for 30 minutes, and filtering and concentrating the blueberry pomace to obtain the blueberry pomace extract.

4. The preparation method according to claim 1, wherein The red mud powder in step 2 has an iron content of ≥20 wt % and passes through at least a 300-mesh sieve.

5. The preparation method according to claim 1, wherein In the step 2, the blueberry residue and red mud are uniformly mixed in a mass ratio of 1:1, the pyrolysis temperature is 500-900°C, the heating rate is 15°C / min, the heat is maintained for 1-2 hours, and red mud-blueberry residue biochar (RM-BC) is obtained after cooling.

6. The preparation method according to claim 1, wherein In the step 3, the red mud biochar is added to the extract in small amounts and multiple times according to a solid-to-liquid ratio of 1:

20.

7. The preparation method according to claim 1, wherein In step 3, a magnetic stirrer is used to stir until no bubbles are generated and then the mixture is allowed to stand for 1 hour.

8. The preparation method according to claim 1, wherein The concentration of the sodium borohydride solution in step 4 is 1 mol / L, and the dripping speed is 1-2 drops per second. The addition amount is reduced by 33% compared with the traditional method.

9. The extract formed by mixing ethanol and distilled water as claimed in claim 3, wherein the ratio of ethanol to distilled water can be set to 1:1, 2:1 or 3:1.

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