Directional synthesis method of magnetic porous iron-doped graphitized charcoal

Abstract

The invention relates to the technical field of preparation of environmental functional materials, and discloses a directional synthesis method of magnetic porous iron-doped graphitized biochar, which is realized by the following steps: cooperatively inducing low-temperature graphitization by utilizing a biomass endogenous S < 2 > self-template and nano-diamond; performing double-complexing ultrasonic impregnation on the iron precursor to realize nanoscale dispersion; performing staged pyrolysis, pre-carbonizing in an inert atmosphere to construct a carbon skeleton, and synchronously completing iron reduction and graphitization in a reducing atmosphere; performing mixed pickling to synchronously remove the template and construct a passivation layer; and finally, carrying out magnetic separation to obtain the high-porosity and uniformly iron-doped magnetic graphitized charcoal. A graphitized porous structure is realized through cooperation of a low-temperature self-template and nano-diamond, iron agglomeration is inhibited through a double-complexing ultrasonic technology, a protective layer and active sites are synchronously constructed through mixed pickling and passivation, and three technical barriers of high-temperature collapse, iron particle inactivation and short cycle life of a traditional process are broken through.

Description

Technical Field

[0001] This invention relates to the field of environmental functional materials preparation technology, specifically a method for the directional synthesis of magnetic porous iron-doped graphitized biochar. Background Technology

[0002] In the field of environmental pollution control, magnetic porous iron-doped graphitized biochar (MFGBC) is considered an ideal material for the efficient treatment of organic pollutants due to its adsorption-catalysis synergistic function and convenient magnetic recovery. Traditional preparation processes typically rely on biomass carbonization, achieving graphitization through high-temperature pyrolysis (≥800℃), and introducing iron salt precursors for magnetic modification. However, existing technologies face the following core bottlenecks:

[0003] The contradiction between high-temperature graphitization and porous structures: Traditional processes rely on high temperatures (800-1000℃) to promote the orderly arrangement of carbon layers, but high temperatures cause severe shrinkage during biomass pyrolysis, resulting in the collapse of pore structures (especially mesopores) and a significant decrease in specific surface area. Although external hard templates (such as SiO2 and MgO) are used to alleviate the collapse, template removal requires strong acid corrosion or high-temperature calcination, which is a complex process and damages the integrity of the carbon skeleton.

[0004] Insufficient dispersibility and oxidation stability of iron nanoparticles: Existing iron doping methods mostly involve direct impregnation with a single iron salt (such as FeCl3). During pyrolysis, the iron precursor, lacking effective dispersion control, easily migrates and aggregates to form coarse particles (>50 nm), resulting in weak magnetic responsiveness and insufficient exposure of active sites. Furthermore, iron nanoparticles (Fe... 0 It is easily oxidized and deactivated during pickling and use, and has poor cycle stability.

[0005] Lack of functional synergy design: Existing technologies often pursue a single performance in isolation (such as high specific surface area or strong magnetism), neglecting the adsorption-catalysis synergistic mechanism. For example, conventional acid washing only removes the template without simultaneously constructing a surface passivation layer or active sites, resulting in weak oxidation resistance and limited catalytic efficiency of the material.

[0006] The root cause of the aforementioned shortcomings lies in the lack of directional control over the structure and properties of materials in existing processes: the contradictions between high temperature and porosity, iron dispersion and stability, and adsorption and catalytic activity have not been systematically resolved. Therefore, there is an urgent need to develop a low-temperature, controllable directional synthesis method to overcome the limitations of existing technologies. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for the directional synthesis of magnetic porous iron-doped graphitized biochar, which solves the problems of high-temperature graphitization leading to the destruction of porous structures, easy oxidation of iron nanoparticles due to aggregation, and insufficient adsorption-catalysis synergy in existing processes.

[0008] To achieve the above objectives, the present invention provides a method for the directional synthesis of magnetic porous iron-doped graphitized biochar, comprising the following steps:

[0009] S1. Pre-treated biomass raw materials retain natural SiO2 as a self-template and are mixed with nanodiamonds;

[0010] The self-templating function of natural SiO2: The inherent SiO2 in rice husks acts as a hard template to support the carbon skeleton during pyrolysis, forming a hierarchical porous structure (micropores-mesopores). Traditional methods require the addition of additional template agents (such as MgO or SiO2 powder), while this invention directly utilizes endogenous SiO2 from biomass, reducing costs and avoiding the complex process of template removal.

[0011] Low-temperature catalytic graphitization of nanodiamonds: Nanodiamonds (2-5nm) are used as graphitization seed crystals, and graphitization is achieved through surface sp... 3 Carbon atoms induce the carbon layers to arrange themselves in an orderly manner at low temperatures (≤600℃). Traditional processes require temperatures above 800℃ to achieve graphitization, while nanodiamonds reduce the activation energy of carbon atom rearrangement, thereby lowering the graphitization temperature by more than 30%, while avoiding pore collapse caused by high temperatures.

[0012] S2. The raw material obtained in step S1 is immersed in an iron-containing precursor solution and dried to obtain a composite precursor.

[0013] Dual complexation and dispersion mechanism: citrate (C6H5O7) 3- ) and tartrate (C4H4O6) 2- ) for Fe 3+ Multiple chelation reactions are formed, inhibiting the migration and aggregation of iron ions during pyrolysis. Traditional impregnation methods use only a single iron salt (such as FeCl3), resulting in uneven iron particle size distribution (20-100 nm), while the dual complexation system enables Fe... 0 The size of the nanoparticles is controlled below 10nm, and the dispersion uniformity is improved by more than 50%.

[0014] Ultrasonic-assisted penetration enhancement: The microjets generated by ultrasonic cavitation effect propel iron complexes deep into the micropores (diameter <10nm) of rice husk cellulose, achieving atomic-level anchoring of iron species and avoiding the surface enrichment problem of traditional impregnation methods;

[0015] S3. The composite precursor is subjected to staged pyrolysis. The first stage is pre-carbonization in an inert atmosphere, and the second stage is iron reduction and graphitization in a reducing mixed gas.

[0016] Gradient H2 reduction control: Initially, a low concentration of H2 (4-6%) is used to avoid excessive reduction of iron species, which could lead to particle coarsening. The H2 concentration is then gradually increased (12-18%) to ensure the reduction of Fe. 3+ Completely reduced to magnetic Fe0 Nanoparticles, simultaneously suppressing nonmagnetic Fe O Generation. Traditional single-stage H2 reduction is prone to Fe generation due to excessively high concentrations. 0 Overgrowth (>50 nm), while gradient regulation enables Fe 0 The particle size is stable at 5-10 nm.

[0017] In-situ doping synergistic effect of NH3: Trace amounts of NH3 (40-60 ppm) decompose during pyrolysis to generate ·NH2 free radicals, which react with Fe... 0 Surface reactions form Fe-Nx active sites, enhancing the material's activation ability for persulfate (PMS). Traditional processes require stepwise reduction and nitrogen doping, while this method achieves one-step in-situ doping through dynamic gas control.

[0018] S4. Pickling removes the SiO2 template and performs surface passivation treatment;

[0019] Hybrid acid etching for synergistic etching: HF efficiently dissolves SiO2 template to release pores, while oxalic acid simultaneously reacts with residual Fe. 3+ The reaction produces a protective layer of ferric oxalate (FeC2O4), which covers the Fe. 0 The particle surface is protected to prevent oxidative deactivation during subsequent use. Traditional pickling (such as single HF or HCl) only removes the template, while mixed pickling also has a passivation function, allowing Fe to... 0 The oxidation rate is reduced to below 5%.

[0020] Oxygen vacancy controlled construction: Air annealing allows surface Fe... 0 Partial oxidation to Fe3O4 forms a Fe3O4@C core-shell structure. Oxygen vacancies are generated at the interface between the carbon shell defect sites (such as edge carbons and vacancies) and Fe3O4, which serve as electron transport channels and significantly improve the PMS activation efficiency in Fenton-like reactions (k value increases by 2.4 times).

[0021] S5. Magnetic separation, washing and drying are performed to obtain the final product;

[0022] Magnetic separation efficiency optimization: 0.3-0.7T magnetic field strength to balance Fe 0 The magnetic responsiveness and dispersibility of the particles avoid the problem of particle chain aggregation caused by high magnetic fields (>1T), ensuring that the recovery rate is >98% after the material is recycled 10 times.

[0023] CO2 selective pore expansion: CO2 reacts with disordered carbon at high temperatures (C + CO2 → 2CO), preferentially etching the micropore walls to form mesopores (pore size 2-50 nm), adapting to pollutants of different molecular sizes (such as Acid Red with a molecular weight of ~1.5 kDa). Traditional KOH activation requires highly corrosive reagents, while CO2 activation is more environmentally friendly and allows for controllable pore size distribution.

[0024] Preferably, the preprocessing in step S1 includes:

[0025] After crushing the biomass raw materials to 80-100 mesh, they are acid-washed with 0.5-0.7M sulfuric acid solution for 2-3 hours;

[0026] The acid-washed raw material is mixed with 0.1-0.5 wt% nano diamond and ball-milled for 20-40 minutes.

[0027] Preferably, in step S2, the iron precursor solution contains ferric ammonium citrate and tartaric acid in a molar ratio of 1:0.3-1:0.5, and 4-6 wt% glucose is added to the solution.

[0028] Preferably, the impregnation in step S2 is performed with ultrasonic assistance, the ultrasonic power is 250-350W, and the processing time is 1-2 hours.

[0029] Preferably, the gas used in the second stage of pyrolysis in step S3 is a mixture of H2 / Ar / NH3, wherein:

[0030] The initial H2 concentration is 4-6%, and it increases by 1-3% every 10 minutes during the heat preservation stage, eventually reaching a concentration of 12-18%.

[0031] The NH3 concentration is 40-60 ppm.

[0032] Preferably, in step S4, the pickling uses a mixed solution of HF and oxalic acid with a volume ratio of 1:1 to 1:1.5, an HF concentration of 2-4%, and an oxalic acid concentration of 5-8%.

[0033] Preferably, the surface passivation treatment in step S4 includes: annealing the pickled material in an air atmosphere at 250-350°C for 20-40 minutes.

[0034] Preferably, in step S5, magnetic separation is performed using a magnetic field strength of 0.3-0.7T for adsorption separation.

[0035] Preferably, the biomass raw material in step S1 is waste rice husks.

[0036] This invention provides a method for the directional synthesis of magnetic porous iron-doped graphitized biochar. It possesses the following characteristics:

[0037] Beneficial effects:

[0038] 1. This invention achieves graphitization of biochar and stable retention of porous structure simultaneously at low temperature by using natural SiO2 self-templating and nanodiamond-induced synergistic effect. Compared with traditional processes that rely on high temperature or external templates, this invention effectively avoids the contradiction between pore collapse and excessive energy consumption, and significantly improves the structural integrity and functional adaptability of the material.

[0039] 2. This invention employs a dual complexation and chelation system combined with an ultrasonic-assisted impregnation process to achieve uniform loading and nanoscale dispersion of the iron precursor. Compared with the traditional single iron salt loading method, it completely overcomes the defects of iron particle agglomeration and uneven distribution, endowing the material with excellent magnetic responsiveness and highly active surface properties.

[0040] 3. Based on mixed acid washing and controlled oxygen vacancy engineering, this invention constructs a protective interface layer while removing the template and introduces catalytically active sites. Compared with conventional acid washing or inert treatment processes, it significantly enhances the antioxidant capacity and cycle stability, and solves the core problems of easy deactivation and short lifespan of traditional materials. Attached Figure Description

[0041] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0042] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Please see the appendix Figure 1 This invention provides a method for the directional synthesis of magnetic porous iron-doped graphitized biochar through multiple embodiments, the details of which are as follows:

[0044] Example 1: Preparation under basic process parameters

[0045] Raw materials and reagents:

[0046] Waste rice husks: 200g (crushed to 80 mesh);

[0047] Sulfuric acid solution: 0.5M, 2L;

[0048] Nanodiamond powder: 0.2g (0.1wt%);

[0049] Ferric ammonium citrate: 0.1 mol / L, tartaric acid: 0.04 mol / L (molar ratio 1:0.4);

[0050] Glucose: 5 wt%;

[0051] HF solution: 2%, oxalic acid solution: 5%, volume ratio 1:1.

[0052] Preparation steps:

[0053] Preprocessing and self-template construction:

[0054] Soak rice husks in a 0.5M sulfuric acid solution, stir at 60°C for 2 hours, wash with deionized water until neutral, and dry at 60°C.

[0055] Dried rice husks were ball-milled with 0.2g of nano-diamond for 30 minutes (300rpm);

[0056] Iron precursor load:

[0057] Preparation of iron complexing solution: Dissolve ferric ammonium citrate (0.1 mol / L) and tartaric acid (0.04 mol / L) in 1L of water, and add 5% glucose;

[0058] Ultrasonic impregnation: The pretreated rice husks are immersed in the complexing solution, ultrasonically treated for 1 hour (power 300W), and vacuum dried at 60℃ for 12 hours.

[0059] Staged pyrolysis:

[0060] Pre-carbonization: Nitrogen atmosphere (150 mL / min), heat to 450℃ at 5℃ / min, and hold for 1 hour;

[0061] Reduction and graphitization: Switch to a H2 / Ar / NH3 mixed gas (H2 initial 5%, NH3 50ppm), heat to 600℃ at 5℃ / min, and hold for 30 minutes; increase H2 concentration by 2% every 10 minutes (finally 15%).

[0062] Mixed pickling and passivation:

[0063] Pickling: Immerse in HF / oxalic acid mixture (2% HF + 5% oxalic acid, volume ratio 1:1) for 4 hours, then wash with deionized water until neutral;

[0064] Passivation treatment: Annealing at 300℃ for 30 minutes in air atmosphere;

[0065] Magnetic separation and post-processing:

[0066] Magnetic separation: Adsorption separation by a 0.5T magnetic field, followed by drying at 60℃ for 8 hours.

[0067] Example 2: High-concentration iron loading and rapid pyrolysis

[0068] Raw materials and reagents:

[0069] Waste rice husks: 200g (crushed to 100 mesh);

[0070] Sulfuric acid solution: 0.7M, 2L;

[0071] Nanodiamond powder: 1.0g (0.5wt%);

[0072] Ferric ammonium citrate: 0.12 mol / L, tartaric acid: 0.06 mol / L (molar ratio 1:0.5);

[0073] Glucose: 6 wt%;

[0074] HF solution: 4%, oxalic acid solution: 8%, volume ratio 1:1.5.

[0075] Preparation steps:

[0076] Preprocessing and self-template construction:

[0077] Rice husk acid washing: 0.7M sulfuric acid solution, stir at 80℃ for 3 hours, wash with water and dry;

[0078] Ball milling: Mix with 1.0g of nanodiamond and ball mill for 40 minutes (400rpm);

[0079] Iron precursor load:

[0080] Preparation of complexing solution: ferric ammonium citrate (0.12 mol / L) and tartaric acid (0.06 mol / L) are added to 6% glucose;

[0081] Ultrasonic impregnation: 350W power, 2 hours of treatment, vacuum drying at 80℃ for 14 hours;

[0082] Staged pyrolysis:

[0083] Pre-carbonization: Nitrogen gas (200 mL / min), temperature increased to 500℃ at 6℃ / min, and held for 2 hours;

[0084] Reduction and graphitization: Initial H2 6%, NH3 60ppm, temperature increased to 650℃ at 6℃ / min, H2 concentration increased by 3% every 10 minutes (finally 18%), held for 60 minutes;

[0085] Mixed pickling and passivation:

[0086] Pickling: HF 4% + oxalic acid 8% (volume ratio 1:1.5), stir at room temperature for 5 hours;

[0087] Passivation: Air annealing at 250℃ for 40 minutes;

[0088] Magnetic separation and post-processing:

[0089] Magnetic separation: 0.7T magnetic field strength, dried at 80℃ for 6 hours.

[0090] Example 3: Low-Temperature Short-Time Process Optimization

[0091] Raw materials and reagents:

[0092] Waste rice husks: 200g (crushed to 90 mesh);

[0093] Sulfuric acid solution: 0.6M, 2L;

[0094] Nanodiamond powder: 0.5g (0.25wt%);

[0095] Ferric ammonium citrate: 0.09 mol / L, tartaric acid: 0.045 mol / L (molar ratio 1:0.5);

[0096] Glucose: 4.5 wt%;

[0097] HF solution: 3%, oxalic acid solution: 6%, volume ratio 1:1.2.

[0098] Preparation steps:

[0099] Preprocessing and self-template construction:

[0100] Rice husk pickling: 0.6M sulfuric acid, stir at 70℃ for 2.5 hours, wash with water and dry.

[0101] Ball milling: 0.5g of nano diamond ball milled for 30 minutes (350rpm).

[0102] Iron precursor load:

[0103] Preparation of complexing solution: ferric ammonium citrate (0.09 mol / L) and tartaric acid (0.045 mol / L) are added with 4.5% glucose.

[0104] Ultrasonic impregnation: 250W power, 1.5 hours of treatment, followed by vacuum drying at 70℃ for 10 hours.

[0105] Staged pyrolysis:

[0106] Pre-carbonization: Nitrogen gas (120 mL / min), temperature increased to 400℃ at 5℃ / min, and held for 1.5 hours.

[0107] Reduction and graphitization: Initial H2 4%, NH3 40ppm, temperature increased to 550℃ at 5℃ / min, H2 concentration increased by 1% every 10 minutes (finally 12%), and held for 40 minutes.

[0108] Mixed pickling and passivation:

[0109] Pickling: HF 3% + oxalic acid 6% (volume ratio 1:1.2), stir for 4 hours.

[0110] Passivation: Air annealing at 350℃ for 20 minutes.

[0111] Magnetic separation and post-processing:

[0112] Magnetic separation: 0.3T magnetic field strength, dried at 70℃ for 7 hours.

[0113] Experiment 1: Synergistic Verification of Low-Temperature Graphitization and Porosity

[0114] Experimental objective: To verify the necessity of low-temperature graphitization of nanodiamonds and the contribution of SiO2 self-templating to the porous structure.

[0115] Test subject:

[0116] Example 1 (complete process, 3 parallel samples);

[0117] Comparative Example 1 (without nanodiamonds, 3 parallel samples);

[0118] Comparative Example 7 (no SiO2 self-template, 3 parallel samples).

[0119] Experimental steps:

[0120] Sample preparation:

[0121] Three groups of samples were prepared according to the processes of Example 1, Comparative Example 1, and Comparative Example 7, with each group containing 3 parallel samples.

[0122] Raman spectroscopy test:

[0123] Using a laser Raman spectrometer (excitation wavelength 532 nm), the wavelengths were measured at 100-2000 cm⁻¹. -1 Scan within the range and calculate the D peak (~1350cm). -1 ) and G peak (~1580cm) -1 Intensity ratio (ID / IG) was calculated by taking the average of three measurements for each group of parallel samples.

[0124] BET specific surface area and pore size analysis:

[0125] The nitrogen adsorption-desorption method (-196℃) was used. The specific surface area was calculated using the BET model, and the pore size distribution was analyzed using the BJH model. Each group of parallel samples was tested twice.

[0126] SEM morphology observation:

[0127] After sputtering gold onto the sample cross-section, the pore morphology was observed using a field emission scanning electron microscope (FESEM) (accelerating voltage 5kV), and the proportion of mesopores in 5 randomly selected regions was statistically analyzed.

[0128] Experimental data:

[0129] Table 1 - Comparison of properties between low-temperature graphitization and porous structures

[0130]

[0131]

[0132] Experimental Summary: This experiment, by comparing the test data of Example 1, Comparative Example 1 (without nanodiamond), and Comparative Example 7 (without SiO2 self-template), revealed the synergistic mechanism of nanodiamond and SiO2 template in low-temperature graphitization and porous structure construction. The Raman spectral ID / IG values ​​of Example 1 (0.84-0.89) were significantly lower than those of Comparative Example 1 (1.10-1.17), indicating that nanodiamond, as a graphitization seed crystal, facilitates graphitization through its surface sp... 3 Carbon atoms induce ordered rearrangement of the carbon layer at low temperatures. In Comparative Example 1, due to the lack of nanodiamonds, the carbon framework experiences disordered shrinkage during pyrolysis, leading to pore collapse and a reduced specific surface area (622-647 m²). 2 The significant decrease in the percentage of mesoporous particles (48-53%) and the proportion of pores ( / g) validates the regulatory effect of nanodiamonds on pore retention.

[0133] The significant difference between the test data of Comparative Example 7 (mesoporous content 12-16%) and Example 1 (65-70%) confirms the crucial role of the natural SiO2 template. The endogenous SiO2 in rice husks acts as a hard template to support the carbon skeleton during the pre-carbonization stage, preventing the microporous structure from collapsing due to high-temperature thermal stress. In contrast, the absence of SiO2 in Comparative Example 7 resulted in the carbon skeleton losing its support, forming narrow pores dominated by micropores (average pore size 1.5-1.9 nm). This structure restricts the diffusion and adsorption capacity of pollutants, contrasting sharply with the hierarchical porous characteristics of Example 1 (predominantly mesoporous, average pore size 3.6-4.1 nm).

[0134] The synergistic effect of nanodiamond and SiO2 template in Example 1 is manifested in the compatibility of low-temperature graphitization and high specific surface area. The correlation between Raman data and BET results indicates that nanodiamond reduces the blockage of pores by disordered carbon by promoting carbon layer ordering, while the SiO2 template inhibits pore collapse through physical support. The synergistic effect of these two components enables the material to achieve an ID / IG ≤ 0.9 and a specific surface area ≥ 800 m² at 600 °C. 2 / g, breaking through the bottleneck of the contradiction between high-temperature graphitization and porosity in traditional processes, and providing theoretical support for the low-temperature preparation of high-performance biochar.

[0135] Experiment 2: Iron Dispersion Control and Magnetic Verification

[0136] Experimental objective: To verify the synergistic effect of the dual complexation system on iron particle dispersion, the improvement of loading depth by ultrasonic impregnation, and the optimization of iron particle size and magnetic properties by gradient H2 control.

[0137] Test subject:

[0138] Example 1 (complete process, 3 parallel samples);

[0139] Comparative Example 2 (tartaric acid-free, 3 parallel samples);

[0140] Comparative Example 3 (static immersion, 3 parallel samples);

[0141] Comparative Example 4 (constant H2 concentration, 3 parallel samples).

[0142] Experimental steps:

[0143] Sample preparation:

[0144] Four groups of samples were prepared according to the processes of Example 1, Comparative Example 2 (tartaric acid-free), Comparative Example 3 (static impregnation), and Comparative Example 4 (constant H2 concentration), with three parallel samples in each group.

[0145] TEM iron particle analysis:

[0146] The morphology of iron particles was observed using a transmission electron microscope (TEM, accelerating voltage 200kV), and the particle size distribution of 50 randomly selected particles was statistically analyzed.

[0147] EDS surface scan analysis to determine the uniformity of iron distribution in a carbon matrix (surface scan area 10 × 10 μm).

[0148] Magnetic property test:

[0149] The magnetic saturation intensity was measured using a vibrating sample magnetometer (VSM) (magnetic field range ±2T, room temperature).

[0150] XPS iron oxidation analysis:

[0151] X-ray photoelectron spectroscopy (XPS, AlKα excitation source) was used to analyze the Fe2p spectrum and calculate Fe. 0 Content (binding energy 711.2 eV).

[0152] Experimental data:

[0153] Table 2 - Comparison of Iron Dispersion Control and Magnetic Properties

[0154]

[0155]

[0156] Experimental Summary: This experiment, by comparing the test data of Example 1 with Comparative Example 2 (without tartaric acid), Comparative Example 3 (static impregnation), and Comparative Example 4 (constant H2 concentration), revealed the regulatory mechanism of the dual complexation system, ultrasonic-assisted impregnation, and gradient H2 reduction on the dispersibility, magnetic properties, and oxidative stability of iron particles. The average particle size of iron particles in Comparative Example 2 (21.5-26.7 nm) was significantly larger than that in Example 1 (7.2-7.8 nm), indicating that the dual complexation system of ferric ammonium citrate and tartaric acid inhibits Fe dispersibility through multiple chelation effects. 3+Migration and aggregation. The absence of tartaric acid leads to the local enrichment of iron precursors during pyrolysis due to insufficient coordination stability, forming coarse particles. Simultaneously, Fe... O The content decreased from 86.9-89.2% in Example 1 to 58.7-65.3%, further verifying the protective effect of the dual complexation system on the iron reduction process.

[0157] The magnetic saturation intensity of Comparative Example 3 (static impregnation) (38.7-40.3 emu / g) was lower than that of Example 1 (45.7-47.1 emu / g), and the iron particle size (10.8-11.7 nm) was larger. This indicates that the ultrasonic cavitation effect promotes the penetration of iron complexes into the deep pores of biomass through microjets, avoiding the surface enrichment problem of traditional impregnation methods. Under static conditions, iron species are only adsorbed on the surface of the material. During pyrolysis, the local concentration is too high, causing particle coarsening. At the same time, some iron ions are not fully reduced, leaving residual Fe. 3+ It reduced magnetic responsiveness. This comparison highlights the crucial role of ultrasound assistance in improving the depth and uniformity of iron loading.

[0158] Comparative Example 4 (constant H2 concentration) showed iron particle size (16.9-19.1 nm) and Fe... O The degradation of Fe content (67.5-72.4%) reveals the necessity of gradient H2 regulation. Initial low H2 concentration (5%) inhibits Fe... 3+ Rapid reduction avoids instantaneous nucleation and overgrowth of nanoparticles; subsequent concentration increases to 15% ensure Fe 3+ Completely reduced to Fe O Simultaneously, the introduction of trace amounts of NH3 (40-60 ppm) forms Fe-Nx active sites through in-situ doping. A constant H2 concentration leads to an uncontrolled reduction rate, with some Fe... 3+ Incompletely reduced to form non-magnetic Fe O The phase exhibits a wide particle size distribution (10.5-27.5 nm), verifying the irreplaceable role of dynamic gas regulation in iron nanostructuring and magnetic property optimization.

[0159] Experiment 3: Verification of catalytic activity and stability

[0160] Experimental objective: To verify the effect of mixed pickling on Fe O The study explored the protection of oxidation stability, the formation of Fe-Nx active sites by in-situ NH3 doping, and the effects of air annealing on oxygen vacancies and catalytic performance.

[0161] Test subject:

[0162] Example 2 (complete process, 3 parallel samples);

[0163] Comparative Example 5 (HF acid washing only, 3 parallel samples);

[0164] Comparative Example 6 (no NH3 doping, 3 parallel samples);

[0165] Comparative Example 8 (no air annealing, 3 parallel samples).

[0166] Experimental steps:

[0167] Sample preparation:

[0168] Four groups of samples were prepared according to the processes of Example 2, Comparative Example 5, Comparative Example 6, and Comparative Example 8, with three parallel samples in each group.

[0169] XPS analysis of Fe-Nx sites:

[0170] The N1 s spectrum (binding energy ~398.5 eV) was analyzed using X-ray photoelectron spectroscopy (XPS, AlKα excitation source) to calculate the proportion of Fe-Nx sites.

[0171] EPR oxygen vacancy detection:

[0172] The oxygen vacancy signal intensity was measured by electron paramagnetic resonance (EPR, microwave frequency 9.8 GHz) (g≈2.003).

[0173] Catalytic degradation experiment:

[0174] Add 50 mg of sample to 100 mL of Acid Red solution (100 mg / L), add 0.2 mM MPMS, stir magnetically at 30 °C, and take samples every 5 minutes to detect absorbance (λ = 520 nm). Calculate the degradation rate and reaction rate constant (k value).

[0175] Cyclic stability test:

[0176] After the sample was reused 10 times, the amount of Fe leaching was determined by ICP-MS.

[0177] Experimental data:

[0178] Table 3 - Comparison of catalytic activity and stability data

[0179]

[0180]

[0181] Experimental Summary: This experiment, by comparing the test data of Example 2 with Comparative Example 5 (HF pickling only), Comparative Example 6 (no NH3 doping), and Comparative Example 8 (no air annealing), reveals the synergistic regulatory mechanism of mixed pickling passivation, in-situ NH3 doping, and oxygen vacancy engineering on the catalytic activity and cycle stability of the material. The Fe dissolution amount in Comparative Example 5 (1.12-1.48 mg / L) was much higher than that in Example 2 (0.07-0.11 mg / L), indicating that the addition of oxalic acid in the mixed pickling process, through interaction with residual Fe...3+ The reaction generates a dense FeC2O4 passivation layer, effectively inhibiting Fe O Oxidative dissolution. Simultaneously, the synergistic effect of oxalic acid and HF, during the removal of the SiO2 template, introduces defect sites on the carbon shell surface through localized etching, promoting the formation of oxygen vacancies during subsequent air annealing (oxygen vacancy signal intensity 1760-1920 a.u. in Example 2). In contrast, single HF pickling (Comparative Example 5), lacking the synergistic passivation of oxalic acid, results in the Fe surface being directly exposed to an oxidizing environment, leading to a decrease in oxygen vacancy concentration (1480-1620 a.u.) and reduced catalytic activity (k value 0.041-0.049 min). -1 The concentration was significantly lower than that in Example 2 (0.058-0.065 min). -1 ).

[0182] The Fe-Nx content in Comparative Example 6 (without NH3 doping) (1.8-2.5%) was significantly lower than that in Example 2 (11.8-13.2%), demonstrating that the ·NH2 free radicals generated by the decomposition of NH3 in a reducing atmosphere react with Fe. O A coordination reaction occurs on the surface, forming Fe-Nx active sites. These sites enhance the catalytic degradation rate (k value from 0.025 to 0.031 min) by increasing the electron transfer efficiency of PMS. -1 Increased to 0.058-0.065 min -1 Furthermore, the introduction of NH3 inhibited Fe... O Excessive oxidation of particles, synergistically optimized with gradient H2 reduction of Fe O / Fe3O4 ratio, while the absence of NH3 leads to Fe O The surface is partially oxidized to low-activity FeO (Fe in XPS Fe2p spectrum) 3+ (The increased proportion) further weakens the catalytic performance.

[0183] The oxygen vacancy signal intensity (580-670 a.u.) of Comparative Example 8 (without air annealing) was only 1 / 3 of that of Example 2, and the catalytic activity (k value 0.036-0.042 min) was significantly lower. -1 The significant decrease indicates that air annealing introduces oxygen vacancies at the Fe3O4@C interface through controlled oxidation. These oxygen vacancies act as electron traps, promoting the decomposition of PMS to generate ·OH and SO4· - Free radicals (enhanced EPR detection signal) and optimized electronic conductivity of the carbon shell accelerate the adsorption-degradation synergistic process of contaminants on the material surface. However, unannealed samples suffer from reduced free radical generation due to oxygen vacancies, and the carbon shell defect sites are not effectively activated, resulting in limited catalytic efficiency. The correlation between the experimental data and the mechanism verifies the irreplaceable role of mixed acid washing, NH3 doping, and oxygen vacancy engineering in the synergistic design of multiple active sites, providing theoretical support for the high-efficiency catalysis and long-term stability of the material.

[0184] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for the directional synthesis of magnetic porous iron-doped graphitized biochar, characterized in that, Includes the following steps: S1. Pre-treated biomass raw materials retain natural SiO2 as a self-template and are mixed with nanodiamonds; S2. The raw material obtained in step S1 is immersed in an iron-containing precursor solution and dried to obtain a composite precursor. S3. The composite precursor is subjected to staged pyrolysis. The first stage is pre-carbonization in an inert atmosphere, and the second stage is iron reduction and graphitization in a reducing mixed gas. S4. Pickling removes the SiO2 template and performs surface passivation treatment; S5. Magnetic separation, washing and drying are performed to obtain the final product.

2. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, The preprocessing in step S1 includes: After crushing the biomass raw materials to 80-100 mesh, they are acid-washed with 0.5-0.7M sulfuric acid solution for 2-3 hours; The acid-washed raw material is mixed with 0.1-0.5 wt% nano diamond and ball-milled for 20-40 minutes.

3. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, In step S2, the iron precursor solution contains ferric ammonium citrate and tartaric acid in a molar ratio of 1:0.3-1:0.5, and 4-6 wt% glucose is added to the solution.

4. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, In step S2, the impregnation is performed with ultrasonic assistance, with an ultrasonic power of 250-350W and a processing time of 1-2 hours.

5. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, The gas undergoing pyrolysis in the second stage of step S3 is a mixture of H2 / Ar / NH3, wherein: The initial H2 concentration is 4-6%, and it increases by 1-3% every 10 minutes during the heat preservation stage, eventually reaching a concentration of 12-18%. The NH3 concentration is 40-60 ppm.

6. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, In step S4, the pickling process uses a mixed solution of HF and oxalic acid with a volume ratio of 1:1 to 1:1.5, where the HF concentration is 2-4% and the oxalic acid concentration is 5-8%.

7. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, The surface passivation treatment in step S4 includes annealing the pickled material in an air atmosphere at 250-350°C for 20-40 minutes.

8. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, In step S5, magnetic separation is performed using a magnetic field strength of 0.3-0.7T for adsorption separation.

9. The method for directional synthesis of magnetic porous iron-doped graphitized biochar according to claim 1, characterized in that, In step S1, the biomass raw material is waste rice husks.

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