Micro-nano aeration-iron composite modified biochar and preparation method thereof

Abstract

This application relates to the field of water treatment and soil remediation technology, specifically disclosing a micro / nano aeration-iron composite modified biochar. It comprises the following raw materials: a biochar matrix, an iron-based active component, and an oxygen-containing functional group modification layer; its preparation method is as follows: S1, biomass pretreatment: corn stalks are washed, dried, and crushed to obtain pretreated biomass; S2, biochar preparation: the pretreated biomass is pyrolyzed under an inert atmosphere, and cooled after pyrolysis to obtain raw biochar; S3, micro / nano aeration modification: the raw biochar is dispersed in water, and micro / nano bubbles are introduced for aeration treatment. After aeration, solid-liquid separation is performed, the solid product is collected and dried to obtain aeration-modified biochar; S4, iron composite modification. The composite modified biochar of this application can be used for deep purification of industrial wastewater and remediation of heavy metal contaminated sites, and it has the advantage of improving the adsorption capacity of materials for various heavy metals.

Description

Technical Field

[0001] This application relates to the field of water treatment and soil remediation technology, and more specifically, it relates to a micro-nano aeration-iron composite modified biochar and its preparation method. Background Technology

[0002] Biochar, a porous carbonaceous material prepared by pyrolysis of agricultural and forestry waste, has shown great application potential in the remediation of heavy metal-polluted water and soil due to its wide availability, low cost, well-developed pore structure, and abundant surface functional groups. To overcome the problems of incomplete pore development, insufficient surface sites, and low adsorption capacity of original biochar, researchers have extensively carried out biochar modification research, among which iron-modified biochar has become the mainstream direction. By loading iron-based active components onto the surface of biochar, the removal capacity of heavy metals can be significantly improved through coordination complexation, ion exchange, and redox reactions. Mainstream preparation processes include impregnation, co-precipitation, and ball milling. At the same time, micro-nano aeration technology is rapidly developing in the field of water treatment. The micro-nano bubbles generated by this technology have the characteristics of large specific surface area, slow rising speed, and local high-energy release during collapse. They can generate strong oxidizing hydroxyl radicals in water, which have the ability to physically etch pores and chemically oxidize the surface of porous materials without the introduction of strong acids or bases, belonging to a green activation path.

[0003] However, the related iron-modified biochar processes still have the following drawbacks: First, iron particles are prone to agglomeration, and active sites are encapsulated. In the impregnation method, iron salts only rely on physical adsorption to adhere to the surface of biochar, lacking chemical bond anchoring conditions. Iron oxides are prone to agglomerate at the pore openings and block the pores, making the internal sites unusable. The ball milling method relies on mechanical doping, and iron and carbon are not chemically bonded, resulting in uneven dispersion and easy oxidation and passivation of zero-valent iron. Second, iron components are prone to dissolution and have poor cycle stability. During multiple acid washing and regeneration processes, a large amount of iron is lost, and the adsorption capacity after circulation often drops below the initial level, which is lower than the requirements for engineering applications. Third, the micro-nano aeration technology directly mixes aeration with iron salts simultaneously, and the oxygen-containing functional groups generated by hydroxyl radical oxidation are competitively consumed before they can stably anchor iron ions. The above shortcomings result in the existing iron-modified biochar having a low removal capacity for multiple heavy metals and poor resistance to water quality fluctuations, making it difficult to meet the treatment needs of complex heavy metal wastewater in mining areas. Summary of the Invention

[0004] To address the issue of low removal capacity for various heavy metals due to easy aggregation and encapsulation of active sites, this application provides a micro / nano aeration-iron composite modified biochar and its preparation method.

[0005] In the first aspect, this application provides a micro / nano aeration-iron composite modified biochar, employing the following technical solution: A micro / nano-aerated iron-modified biochar comprises the following raw materials in parts by weight: 80-100 parts of biochar matrix; 5-15 parts of iron-based active component; and 0.5-2 parts of oxygen-containing functional group modification layer; wherein the biochar matrix is ​​corn straw-derived char, the iron-based active component is a mixed-valence oxide of Fe3O4 and Fe2O3, and the micro / nano-aerated iron-modified biochar has a pH zero-point charge of 2.5-2.8 and a specific surface area of ​​35-45 m². 2 / g.

[0006] By adopting the above technical solution, the use of corn stalks as the biochar matrix enables the resource utilization of agricultural and forestry waste, reducing raw material costs. The iron-based active component is a mixed-valence oxide of Fe3O4 and Fe2O3, whose synergistic effect provides abundant coordination complexation sites and ion exchange capacity. The construction of the oxygen-containing functional group modification layer enhances the negative charge on the material surface and strengthens electrostatic attraction. The pH zero-point charge is controlled within a suitable acidic range to ensure that the material maintains a negative surface charge in the pH environment commonly found in mining wastewater, which is beneficial for the adsorption of heavy metal cations. The specific surface area is limited within a reasonable range to ensure that the material has sufficient physical adsorption space. The synergistic effect of the above structures enables the material to exhibit the ability to remove multiple heavy metal ions simultaneously.

[0007] Preferably, in the iron-based active component, the mass ratio of Fe3O4 to Fe2O3 is 1:0.5 to 1:2; the raw materials of the biochar matrix also include one or more combinations of cotton stalks, sawdust, or rice husks; the oxygen-containing functional group modification layer contains carboxyl groups and hydroxyl groups, and its total content is 1.2 to 1.8 mmol / g.

[0008] By adopting the above technical solutions, the iron-based active components are constructed using a specific mass ratio, optimizing the crystal structure and surface activity of iron oxides. This ensures adsorption capacity while enhancing the magnetic separation and recovery potential of the material. The introduction of diverse biomass raw materials allows for the regulation of biochar porosity and ash content by adjusting the types of raw materials, adapting to the treatment needs of different water qualities. Limiting the total content of carboxyl and hydroxyl groups within a suitable range ensures a sufficient density of chemical anchoring sites on the biochar surface. These oxygen-containing functional groups not only directly participate in the chelation reaction of heavy metal ions but also provide binding sites for the uniform loading of the iron-based active components, inhibiting the aggregation of iron particles.

[0009] Preferably, the biochar matrix is ​​prepared by the following method: the biomass raw material is washed and dried at 60~80℃, pulverized and passed through a 100-mesh sieve, and pyrolyzed at 500~700℃ for 1~3 hours under nitrogen protection. After pyrolysis, it is directly dried without water washing. The iron-based active component is uniformly anchored to the pore structure and surface of the biochar matrix through electrostatic adsorption and chemical bonding.

[0010] By adopting the above technical solution, the appropriate pyrolysis temperature and time range promote the pyrolysis carbonization and pore formation of biomass, while retaining an appropriate amount of volatile matter to maintain the activity of subsequent reactions. After pyrolysis, the biochar is dried directly without washing, retaining the original soluble mineral components and incompletely carbonized active sites on the surface of the biochar, avoiding pore collapse and loss of surface functional groups caused by washing, and providing a good structural basis for subsequent micro-nano aeration modification. The iron-based active components are anchored through the dual effects of electrostatic adsorption and chemical bonding, making them less likely to fall off during liquid phase oscillation, ensuring the structural stability and recyclability of the material.

[0011] Preferably, the additives include: 1-3 parts of silicate modifier and 0.5-1.5 parts of sulfide precipitant, wherein the silicate modifier is used to block the macroporous structure of the biochar matrix and the sulfide precipitant is used to form insoluble sulfides with heavy metal ions.

[0012] By adopting the above technical solution, the introduction of a specific proportion of silicate modifier causes a condensation reaction on the surface and inside the macropores of the biochar matrix, effectively blocking ineffective macroporous channels and forcing water molecules to pass through mesoporous and microporous pathways, thus prolonging the hydraulic residence time and improving adsorption efficiency. The addition of a specific proportion of sulfide precipitant causes it to slowly release sulfur ions inside the pores. When heavy metal ions diffuse into the pores, they react rapidly to form insoluble metal sulfides. This combination of in-situ precipitation and surface adsorption enhances the material's ability to deeply remove low-concentration heavy metals and its resistance to shock loads in complex water quality.

[0013] Secondly, this application provides a method for preparing micro / nano-aerated iron composite modified biochar, employing the following technical solution: A method for preparing micro / nano-aerated iron composite modified biochar includes the following steps: S1. Biomass pretreatment: Corn stalks are washed, dried and crushed to obtain pretreated biomass; S2. Biochar preparation: Pretreated biomass is pyrolyzed under an inert atmosphere. After pyrolysis, it is cooled to obtain raw biochar. S3, Micro-nano Aeration Modification: The original biochar is dispersed in water, and micro-nano bubbles are introduced for aeration treatment. After aeration is completed, solid and liquid are separated, the solid product is collected and dried to obtain aeration modified biochar. S4. Iron composite modification: The aerated modified biochar is placed in a ferric chloride solution and shaken for impregnation. After impregnation, solid-liquid separation is performed, the solid product is collected and vacuum dried to obtain micro-nano aerated-iron composite modified biochar.

[0014] By adopting the above technical solution, due to the stepwise preparation process, the biochar matrix is ​​first constructed in stages S1 and S2 to remove volatile impurities; the micro-nano aeration modification in stage S3 utilizes the hydroxyl radicals generated by the collapse of micro-nano bubbles to etch and oxidize the surface of biochar, which not only expands the specific surface area but also introduces a large number of hydroxyl and carboxyl functional groups on the surface, completing the pre-activation of the biochar matrix and creating uniformly distributed chemical anchoring points for subsequent iron loading; the iron composite modification in stage S4 utilizes the surface negative charge and functional group complexation ability of the pre-activated biochar to make iron ions uniformly dispersed and firmly bound to the pore structure and surface, finally obtaining composite modified biochar with uniformly dispersed iron particles, abundant active sites, and excellent adsorption performance.

[0015] Preferably, step S1 further includes steam activation pretreatment of the pretreated biomass: spraying 5% to 10% of its mass of water vapor onto the pretreated biomass and simmering it at 200 to 300°C for 30 to 60 minutes.

[0016] By adopting the above technical solution, the steam activation pretreatment introduced in the S1 stage allows water vapor to react with cellulose and hemicellulose in biomass at high temperature, playing a preliminary role in pore formation. At the same time, it removes some surface impurities and exposes more internal pores. The appropriate simmering process allows water vapor to fully contact and react with biomass, increasing the roughness of the biomass surface and the number of active sites. This step improves the degree of pore development during subsequent pyrolysis and enhances the wettability of the biochar matrix to subsequent modifying reagents, providing favorable structural conditions for free radical attack in the micro-nano aeration modification stage.

[0017] Preferably, in step S2, the pyrolysis temperature is 580~620℃, the heating rate is 8~12℃ / min, and the isothermal time is 110~130min.

[0018] By adopting the above technical solution, the pyrolysis temperature is controlled within a suitable range, avoiding excessive graphitization of biochar due to excessively high temperatures, which would reduce the surface functional groups. At the same time, it prevents incomplete carbonization and pore blockage caused by excessively low temperatures. The appropriate heating rate ensures uniform heating of biomass, reduces particle bursting caused by rapid heating, and is conducive to the formation of a regular pore structure. Sufficient isothermal time ensures the full progress of the pyrolysis reaction, so that the biochar yield and pore development reach a balance, providing a structurally stable and appropriately active carrier for subsequent modification steps.

[0019] Preferably, in step S3, the aeration treatment is as follows: the particle size of the aeration bubbles is controlled to be 100~200nm, the gas-water volume ratio is 0.02~0.04, and aeration is carried out at room temperature for 10~20min; hydrogen peroxide is added as an additive, and the amount of hydrogen peroxide is 0.1%~0.5% of the total volume of the reaction system; the iodine adsorption value of the aerated modified biochar obtained after the aeration treatment is controlled to be increased to 1.4~1.6 times the original value.

[0020] By adopting the above technical solution, by controlling the particle size and gas-water volume ratio of micro-nano bubbles within a set range, sufficient residence time and extremely high specific surface area of ​​the bubbles in water are ensured, allowing the hydroxyl radicals generated upon collapse to act efficiently on the biochar surface. Adding an appropriate amount of hydrogen peroxide as a co-oxidant, which works synergistically with the micro-nano bubbles, further enhances the oxidation potential of the system and strengthens the surface etching and functional group introduction effects. By controlling the increase in iodine adsorption value within a specific multiple range, the full development of biochar pores is ensured while avoiding excessive oxidation that could damage the mechanical strength of the carbon skeleton, thus achieving simultaneous optimization of physical structure and chemical activity.

[0021] Preferably, in step S4, the Fe in the ferric chloride solution is controlled. 3+ The concentration of the solution is 0.2–0.4 mol / L, and the initial pH of the solution is adjusted to 2.5–3.5. A dispersant of 0.05%–0.1% of the mass of the ferric chloride solution is also added, wherein the dispersant is polyvinylpyrrolidone or sodium citrate. After solid-liquid separation and before vacuum drying, an inert gas purging is performed for 5–10 minutes.

[0022] By adopting the above technical solution, the iron ion concentration and the initial pH value of the solution are controlled within a suitable range, ensuring a sufficient supply of iron source and using the acidic environment to inhibit premature hydrolysis and precipitation of iron ions in aqueous solution, thus promoting the entry of iron ions into the pores of biochar in an ionic state. The addition of an appropriate amount of dispersant further prevents the agglomeration of iron oxide particles through its steric hindrance effect and electrostatic repulsion, ensuring that they are uniformly dispersed at the nanoscale. The inert gas purging step after solid-liquid separation uses inert gas to remove residual moisture and free ions from the pores of the material, preventing pore collapse caused by capillary forces during the drying process, while reducing the oxidation loss of iron components and ensuring the structural integrity of the final product.

[0023] Preferably, in step S4, after vacuum drying, a curing process is also included, specifically: the dried product is placed in a sealed container and cured at 40~50℃ for 12~24h, and the moisture content of the obtained micro-nano aeration-iron composite modified biochar is controlled to be less than 5%.

[0024] By adopting the above technical solution, due to the introduction of a suitable curing process, the trace amount of moisture remaining inside the biochar pores and the iron components continue to undergo slow hydrolysis and condensation reactions, promoting the formation of a more stable chemical bond structure between the iron oxide and the oxygen-containing functional groups on the surface of the biochar. This process eliminates residual stress inside the material and consolidates the anchoring effect of the iron particles. By controlling the moisture content of the final product to be below the threshold, the oxidation or agglomeration of the iron components caused by moisture absorption during storage and transportation of the finished product is prevented, ensuring the stability of the initial activity and adsorption performance of the material during use.

[0025] In summary, this application has the following beneficial effects: 1. Because this application adopts a stepwise coupled process of micro-nano aeration pre-activation followed by ferric chloride loading, the hydroxyl radicals generated by the collapse of micro-nano bubbles are used to physically etch and chemically oxidize the biochar, constructing uniformly distributed oxygen-containing functional group anchoring points on the surface of the biochar. Then, relying on electrostatic adsorption and chemical bonding, uniform immobilization of iron oxide is achieved. Therefore, the defects of easy agglomeration of iron particles, encapsulation of active sites and easy dissolution of iron components in the iron modification process are solved, and a composite modified biochar with well-developed pore structure, abundant active sites and stable structure is obtained, thereby achieving the effect of improving the adsorption capacity of the material for a variety of heavy metals.

[0026] 2. In this application, it is preferred to use micro-nano bubble particle size controlled within a suitable range and aeration parameters with a specific air-to-water ratio. Since micro-nano bubbles have a high specific surface area and slow rising characteristics, they can release energy in a concentrated manner and generate a continuous oxidation effect when they collapse. This allows for the green activation of biochar without the introduction of chemical reagents, resulting in a pre-activated matrix with simultaneous increase in specific surface area and pore volume. This provides sufficient space for the uniform loading of iron components in the subsequent process, while avoiding secondary pollution and skeleton damage caused by acid-base modification.

[0027] 3. The method of this application limits the use of pyrolyzed biochar directly in subsequent modification processes without water washing, thereby preserving the original soluble mineral components and incompletely carbonized active sites on the surface of the biochar, reducing pore collapse and functional group loss caused by water washing. As a result, a biochar matrix with higher surface activity and better modification response is obtained. While simplifying the process and reducing energy and water consumption, the adsorption performance and structural integrity of the final product are further enhanced.

[0028] 4. The micro-nano aeration-iron composite modified biochar prepared in this application achieves excellent surface negative charge and chemical activity under a wide pH range and complex water quality conditions through the synergistic effect of multiple mechanisms such as physical adsorption, electrostatic attraction, chelation fixation and iron-based coordination complexation. Due to the rich oxygen-containing functional groups such as hydroxyl and carboxyl groups on the material surface and the uniform loading of mixed valence iron oxides, it can maintain excellent surface negative charge and chemical activity. Therefore, it is an adsorbent material with excellent resistance to ion interference, good recycling performance and broad-spectrum heavy metal removal ability, which is suitable for the deep treatment of complex mine wastewater. Attached Figure Description

[0029] Figure 1 Scanning electron microscope (SEM) image of the original biochar (BC); Figure 2 This is a scanning electron microscope (SEM) image of micro / nano-aerated modified biochar (ABC). Figure 3 Scanning electron microscope (SEM) image of the micro / nano aerated iron-modified biochar (AFeBC) of this invention. Figure 4 X-ray diffraction (XRD) patterns of BC, ABC, and AFeBC; Figure 5 For AFeBC to Pb 2+ Cd 2+ Cr 3+ Cu 2+ Adsorption kinetics curves; Figure 6 For AFeBC to Pb 2+ Cd 2+ Cr 3+ Cu 2+ Adsorption isotherms; Figure 7 The diagram shows the regeneration cycle performance of AFeBC; Figure 8 The adsorption performance of AFeBC for four heavy metal ions under different environmental variables; Figure 9 This application presents a method for preparing micro / nano aeration-iron composite modified biochar. Detailed Implementation

[0030] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] Technical concept: This application discloses a micro / nano aeration-iron composite modified biochar. It comprises the following raw materials: a biochar matrix, an iron-based active component, and an oxygen-containing functional group modification layer; its preparation method is as follows: S1, biomass pretreatment: corn stalks are washed, dried, and crushed to obtain pretreated biomass; S2, biochar preparation: the pretreated biomass is pyrolyzed under an inert atmosphere, and cooled after pyrolysis to obtain raw biochar; S3, micro / nano aeration modification: the raw biochar is dispersed in water, and micro / nano bubbles are introduced for aeration treatment. After aeration, solid-liquid separation is performed, the solid product is collected and dried to obtain aeration-modified biochar; S4, iron composite modification.

[0032] This application employs a stepwise coupled process of micro-nano aeration pre-activation followed by ferric chloride loading. The hydroxyl radicals generated by the collapse of micro-nano bubbles are used to physically etch and chemically oxidize the biochar, constructing uniformly distributed oxygen-containing functional group anchoring points on the biochar surface. Then, the uniform immobilization of iron oxides is achieved through electrostatic adsorption and chemical bonding. This solves the defects of iron particle agglomeration, active site encapsulation, and easy dissolution of iron components in the iron modification process, resulting in a composite modified biochar with well-developed pore structure, abundant active sites, and stable structure, thereby improving the adsorption capacity of the material for various heavy metals.

[0033] Example 1: Preparation of raw biochar (BC) The corn stalks were rinsed three times with deionized water to remove surface mud and impurities, and then dried in a 60℃ forced-air drying oven for 24 hours until constant weight was achieved. After removal, they were crushed with a high-speed pulverizer and passed through a 100-mesh standard sieve. The undersize portion was collected to obtain biomass powder, which was then sealed and stored for later use.

[0034] Weigh 50g of the above biomass powder, spread it evenly in a corundum ceramic boat, and place it in the furnace chamber of a tube furnace; purge with high-purity nitrogen (99.99% purity) for 30 minutes to remove air from the furnace, and control the nitrogen flow rate at 100mL / min; heat to 600℃ at a heating rate of 10℃ / min, and pyrolyze at a constant temperature for 2 hours; after pyrolysis, cool naturally to room temperature under nitrogen protection, remove the product, do not wash with water, and directly seal and store it to obtain raw biochar, labeled as BC.

[0035] Example 2: Preparation of micro / nano-aerated modified biochar (ABC) Take 20g of the original biochar BC prepared in Example 1, add it to deionized water at a solid-liquid ratio of 1:20, and stir with a magnetic stirrer at 300rpm for 10 minutes to form a uniform suspension.

[0036] Turn on the micro-nano aeration device and introduce micro-nano air bubbles into the suspension, controlling the bubble particle size within the range of 100-200 nm and adjusting the air-to-water ratio to 0.03. Continuously aerate and stir for 15 minutes at room temperature (25±2℃). During the aeration process, maintain uniform dispersion of the suspension without significant sedimentation.

[0037] After aeration, the biochar solids were collected by a filtration device, rinsed twice with a small amount of deionized water, and the filter cake was transferred to a petri dish and dried in a 60℃ forced-air drying oven for 12 hours until constant weight. After removal, it was ground and passed through a 100-mesh sieve to obtain micro-nano aeration-modified biochar, labeled as ABC.

[0038] Example 3: Preparation of micro / nano-aerated iron-modified biochar (AFeBC) Pre-prepared Fe 3+ Prepare a 0.30 mol / L ferric chloride solution, shake well, and set aside.

[0039] Take 10g of ABC prepared in Example 2 and add it to 200mL of the above 0.30mol / L ferric chloride solution at a solid-liquid ratio of 1:20. Stir well with a glass rod and transfer to a 250mL stoppered conical flask. Place the conical flask in a 25℃ water bath constant temperature shaker and shake continuously at 150rpm for 24 hours, keeping it sealed during the shaking process.

[0040] After the reaction was complete, the conical flask was removed, and the solid product was collected using a vacuum filtration device without washing. The filter cake was transferred to a vacuum drying oven and vacuum dried for 12 hours until constant weight was achieved. After removal, it was ground in an agate mortar and passed through a 100-mesh standard sieve to obtain micro-nano aerated iron composite modified biochar, labeled AFeBC.

[0041] Example 4: Iron-loaded and micro / nano-aerated modified biochar (FeABC) Take 10g of the original biochar BC prepared in Example 1, add it to 0.30mol / L ferric chloride solution at a solid-liquid ratio of 1:20, stir evenly, transfer it to a 250mL stoppered conical flask, place it in a 25℃ water bath constant temperature shaker, and shake at 150rpm for 24 hours.

[0042] After the reaction was completed, the solid product was collected by vacuum filtration and dried at 60°C to constant weight. The dried iron-modified biochar was added to deionized water at a solid-liquid ratio of 1:20 to form a suspension. The micro-nano aeration device was turned on, and the bubble particle size was controlled at 100-200nm and the air-to-water ratio was 0.03. The mixture was aerated and stirred at room temperature for 15 minutes.

[0043] After aeration, the biochar was collected by vacuum filtration, dried under vacuum at 60°C for 12 hours, and ground through a 100-mesh sieve to obtain iron-loaded and micro-nano aeration-modified biochar, labeled as FeABC.

[0044] Performance testing: Static adsorption detection: 120 mL of a single heavy metal solution with an initial concentration of 100 mg / L was added to a 250 mL Erlenmeyer flask, followed by 0.2 g of biochar sample. The mixture was shaken at 25 °C and 150 rpm for 24 hours. After the reaction, the solution was centrifuged at 5000 rpm for 10 minutes. The supernatant was filtered through a 0.45 μm filter membrane, and the concentration of heavy metal ions in the filtrate was determined using an atomic absorption spectrophotometer. The equilibrium adsorption capacity was calculated. Three parallel samples were set up for each experiment, and the average value was taken.

[0045] Adsorption kinetics detection: Pb was prepared with an initial concentration of 200 mg / L. 2+ Cd 2+ Cr 3+ Cu 2+ For a single heavy metal aqueous solution, 120 mL was dispensed into 250 mL Erlenmeyer flasks, and 0.2 g of AFeBC adsorbent was added to each flask. The ambient temperature was controlled at 25℃ and the shaking speed at 150 r / min. Samples were taken at 10, 40, 70, 100, 160, 260, 360, 460, 720 and 1440 min. After the samples were filtered through a 0.45 μm filter membrane, the heavy metal concentration in the filtrate was measured, and the adsorption capacity at the corresponding time point was calculated. The data were fitted using pseudo-first-order and pseudo-second-order kinetic equations.

[0046] Isothermal adsorption detection: Pb was prepared with initial concentrations of 20, 40, 60, 80, 100, 120, 150, 200 and 300 mg / L, respectively. 2+ Cd 2+ Cr 3+ and Cu 2+ The equilibrium adsorption capacity of the solution was determined according to the static adsorption experiment method described above, and the theoretical maximum adsorption capacity was calculated by fitting the Langmuir and Freundlich isothermal adsorption models.

[0047] Cyclic regeneration test: The adsorption-saturated AFeBC was placed in 0.1 mol / L HCl solution, shaken and desorbed for 60 minutes, filtered, washed with deionized water until neutral, dried at 70℃ and then re-adsorbed; five adsorption-desorption cycles were performed continuously, the re-adsorption amount after each cycle was measured, and the adsorption retention rate was calculated.

[0048] Biochar adsorption stability test: The initial pH of the solution was adjusted to 5.0, 6.0, 7.0, 8.0 and 9.0 respectively, and the adsorption temperature was controlled at 10, 15, 20, 25 and 30℃, or 0.025-0.10 mol / L K+ was added to the solution. + or Cl – The adsorption capacity was determined by static adsorption experiments to evaluate the material's environmental resistance to interference.

[0049] Material characterization and testing: The specific surface area and pore volume of the material were determined using a surface area and porosity analyzer (BET); the surface morphology and iron particle dispersion were observed using a scanning electron microscope (SEM); the crystal structure was analyzed using an X-ray diffractometer (XRD); the changes in surface functional groups were analyzed using a Fourier transform infrared spectroscopy (FTIR); and the surface charge characteristics were determined using a Zeta potential analyzer.

[0050] Comparative Analysis of Results: This study systematically verified the superiority of the "pre-activation by micro-nano aeration followed by ferric chloride loading" process by comparing the microstructure, surface chemical properties, and application performance of original biochar (BC), single micro-nano aerated modified biochar (ABC), and the composite modified biochar of this invention (AFeBC). The specific analysis is as follows: Comparison of pore structure and specific surface area Table 1: Changes in pore structure of biochar prepared under different modification methods ; Analysis: The hydroxyl radicals generated by micro-nano aeration have a significant etching and pore-forming effect on biochar, increasing the specific surface area and total pore volume of ABC by 50.83% and 46.67% respectively compared to BC, and increasing the mesopore ratio by 12%, providing ample space for subsequent iron loading; in the process of this invention, the pre-activated biochar surface is rich in active sites, and iron ions are uniformly dispersed on the pore surface, with only a slight decrease in specific surface area.

[0051] Surface charge comparison Table 2: Zero-point charge of biochar prepared under different modification methods ; Analysis: As the modification process progresses, the zero-point charge of biochar gradually decreases while its surface negative charge continuously increases. The original biochar BC has a zero-point charge of 3.82, exhibiting only weak negative charge and easily protonated under acidic conditions, resulting in limited electrostatic adsorption capacity. After micro-nano aeration modification, the zero-point charge of ABC decreases to 3.15. The large number of oxygen-containing functional groups such as hydroxyl and carboxyl groups introduced by oxidation significantly enhances its negative charge, broadening the pH range applicable to electrostatic adsorption and providing uniform chemical anchoring sites for subsequent iron ions. Based on this, the zero-point charge of AFeBC obtained by iron loading further decreases to 2.64. The deprotonation of the active groups on the iron oxide surface further enhances its negative charge, while introducing iron-based active sites. It can maintain strong negative charge in a wide range of pH > 2.64, perfectly adapting to the pH conditions of mining wastewater.

[0052] Comparison of iron particle loading effects XRD pattern of AFeBC ( Figure 4Typical diffraction peaks of Fe2O3 and Fe3O4 appeared at 2θ=33.2° and 2θ=40.9°, respectively. The characteristic peaks were sharp and the half-peak width was narrow, indicating that the iron oxide had good crystallinity and fine and uniformly dispersed grains.

[0053] Analysis: In this invention, a large number of uniformly distributed oxygen-containing functional group anchoring sites are first constructed on the surface of biochar through micro-nano aeration pre-activation. Iron ions can form stable chemical complexes with these sites, thereby growing iron oxide grains with uniform size and good dispersion. Therefore, the XRD characteristic peaks are sharp and the half-peak width is narrow.

[0054] Adsorption kinetics test Table 3: AFeBC effect on Pb 2+ Cd 2+ Cr 3+ Cu 2+ Fitting parameters of the dynamic model ; Analysis: The kinetic experiments were fitted using pseudo-first-order and pseudo-second-order kinetic models ( Figure 5 The results showed that AFeBC had ample adsorption sites in the early stage (0-360 min), resulting in rapid adsorption of heavy metals and a high adsorption rate; from 360 to 460 min, it gradually approached adsorption equilibrium; the pseudo-second-order kinetic fitting coefficients for all four heavy metals were R. 2 Higher. The pseudo-second-order model adaptation shows that AFeBC adsorption uses chemisorption as the rate-determining step, i.e., hydroxyl chelation, iron-based coordination, and ion exchange dominate the adsorption process. This differs from the adsorption mode of the original carbon, which is mainly physical adsorption. This is also the key to the simultaneous improvement of adsorption rate and saturation capacity after modification in this invention. Cu2+ has the shortest equilibrium time (260 min), while Cr3+ has the slowest (460 min), which is directly related to the strength of coordination between different metals and iron-based sites.

[0055] Isothermal adsorption test Table 4: AFeBC effect on Pb 2+ Cd 2+ Cr 3+ Cu 2+ Isothermal adsorption model fitting parameters ; Analysis: Isothermal concentration gradients of 20–300 mg / L were fitted using Langmuir and Freundlich methods, respectively. Figure 6 The Langmuir correlation coefficient for all components was better than that of Freundlich, indicating that AFeBC correlates with Pb. 2+ Cd 2+ Cr 3+ Cu 2+It belongs to monolayer uniform chemisorption, with regular distribution of active sites. Iron oxides and oxygen-containing functional groups are uniformly dispersed on the carbon surface, with no local enrichment or loss of sites. The Freundlich model 1 / n is less than 1, and the adsorption reaction proceeds spontaneously, which confirms that the modified material has excellent thermodynamic conditions and is suitable for treating mining wastewater of different concentrations.

[0056] Comparison of heavy metal adsorption performance Adsorption performance of single heavy metal system Table 5: AFeBC effect on Pb 2+ Cd 2+ Cr 3+ Cu 2+ Adsorption capacity ; Analysis: With the advancement of the modification process, the theoretical maximum adsorption capacity of biochar for the four heavy metals increases significantly in a stepwise manner: the adsorption capacity of the original biochar BC for Pb... 2+ Cd 2+ Cr 3+ Cu 2+ The adsorption capacities of the original ABC were only 43.78, 44.28, 52.88, and 38.10 mg / g, respectively. After modification with single micro-nano aeration, the adsorption capacities of ABC increased to 77.44, 78.30, 91.36, and 66.54 mg / g, respectively, representing increases of 76.9%, 76.8%, 72.8%, and 74.7% compared to BC. This indicates that micro-nano aeration can effectively enhance physical adsorption and electrostatic attraction by creating pores and increasing oxygen-containing functional groups. Based on this, the adsorption capacities of AFeBC, obtained by iron-supported composite modification, further increased to 128.84, 105.90, 132.02, and 77.33 mg / g, representing increases of 194.3%, 139.2%, 149.7%, and 103.0% compared to BC, and increases of 66.4%, 35.2%, 44.5%, and 16.2% compared to ABC. This indicates that single micro-nano aeration modification can only achieve a preliminary improvement in adsorption performance. However, the stepwise coupling process of this invention introduces iron-based active sites, adding coordination complexation and ion exchange adsorption pathways, realizing the synergistic effect of multiple mechanisms such as physical adsorption, electrostatic attraction and chemical adsorption, resulting in a qualitative leap in adsorption performance. Among them, the improvement effect on Cr3+ and Pb2+ is the most significant, reflecting the specific binding advantage of iron-based active sites for these two heavy metals.

[0057] Adsorption performance of mixed heavy metal systems When Pb in the mixed solution 2+ Cd 2+ Cr 3+ Cu 2+When the initial concentration was 200 mg / L, the adsorption capacities of AFeBC for the four heavy metals were 72.99 mg / g, 41.27 mg / g, 63.35 mg / g, and 35.62 mg / g, respectively. Figure 7 a) The cumulative adsorption capacity reached 213.23 mg / g; Analysis: Under competitive adsorption conditions, AFeBC still maintains a high adsorption capacity, which is attributed to its abundant active sites and multiple adsorption mechanisms. Cu... 2+ It has the strongest coordination ability with iron oxides and is least affected by competition; Cr 3+ and Cd 2+ The competitive adsorption capacity is relatively weak, and the adsorption amount decreases significantly.

[0058] Comparison of Recycling Performance Table 6: AFeBC effect on Pb 2+ Cd 2+ Cr 3+ Cu 2+ Recycling performance ; Analysis: After 5 adsorption-desorption cycles, the adsorption capacity of AFeBC for the four heavy metals remained above 86.54% of the initial value. Figure 7 (b) This is because the iron oxides in AFeBC form stable chemical complexes with the oxygen-containing functional groups on the surface of biochar, making them difficult to detach during desorption.

[0059] Biochar adsorption stability 1. Effect of pH on the adsorption performance of AFeBC pH alters the occurrence form of heavy metals and the surface charge properties of materials. In the pH range of 5.0 to 9.0, the adsorption capacity of AFeBC gradually increases with increasing pH. Figure 8 a): A large amount of H under low pH conditions + The material preemptively occupies adsorption sites and is protonated to acquire a positive charge, electrostatically repelling heavy metals. As pH increases, the material's negative charge increases, and heavy metals readily form hydroxyl complexes, facilitating adsorption. Adsorption capacity fluctuates by less than 15% across the entire pH range, demonstrating excellent tolerance to pH fluctuations.

[0060] 2. Effect of temperature on the adsorption performance of AFeBC Within the range of 10~30℃, a slight increase in temperature slightly increases the adsorption capacity. Figure 8 (b) The adsorption reaction is endothermic overall. Increasing the temperature accelerates the diffusion rate of heavy metals and promotes chemical adsorption; the material still retains good adsorption capacity at a low temperature of 10℃.

[0061] 3. Effect of coexisting ions on the adsorption performance of AFeBC K + Cl– It can slightly reduce the material's resistance to Pb through site competition and charge shielding. 2+ Cd 2+ Cu 2+ Adsorption capacity ( Figure 8 cd), but for Cr 3+ Almost no effect, Cr 3+ It has a strong coordination with iron-based sites, and the overall material has outstanding resistance to ion interference.

[0062] Example Conclusion: See appendix Figure 1 - Appendix Figure 9 In conjunction with the technical solutions of various embodiments, the present invention proposes a stepwise coupled process of "pre-activation by micro-nano aeration followed by ferric chloride loading" to solve the problems of easy agglomeration of iron particles, low utilization of active sites, and poor stability in iron-modified biochar. Compared with the reverse process, the AFeBC prepared by this process has a more developed pore structure, richer surface functional groups, and more uniform iron particle distribution. The adsorption capacity for various heavy metals is increased by more than 40%, and the recycling performance and environmental adaptability are also significantly enhanced. It is a high-performance adsorbent material suitable for the treatment of complex heavy metal wastewater in mining areas.

[0063] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A micro / nano aeration-iron composite modified biochar, characterized in that, The raw materials comprise the following parts by weight: 80-100 parts of biochar matrix; 5-15 parts of iron-based active component; and 0.5-2 parts of oxygen-containing functional group modification layer; wherein the biochar matrix is ​​corn straw-derived char, the iron-based active component is a mixed-valence oxide of Fe3O4 and Fe2O3, and the micro / nano aeration-iron composite modified biochar has a pH zero-point charge of 2.5-2.8 and a specific surface area of ​​35-45 m². 2 / g.

2. The micro / nano aerated iron composite modified biochar according to claim 1, characterized in that, In the iron-based active component, the mass ratio of Fe3O4 to Fe2O3 is 1:0.5 to 1:2; the raw materials of the biochar matrix also include one or more combinations of cotton stalks, sawdust, or rice husks; the oxygen-containing functional group modification layer contains carboxyl groups and hydroxyl groups, with a total content of 1.2 to 1.8 mmol / g.

3. The micro / nano aerated iron composite modified biochar according to claim 1, characterized in that, The biochar matrix is ​​prepared by the following method: the biomass raw material is washed and dried at 60~80℃, pulverized and passed through a 100-mesh sieve, and then pyrolyzed at 500~700℃ for 1~3 hours under nitrogen protection. After pyrolysis, it is directly dried without water washing. The iron-based active component is uniformly anchored to the pore structure and surface of the biochar matrix through electrostatic adsorption and chemical bonding.

4. The micro / nano aerated iron composite modified biochar according to claim 1, characterized in that, It also includes the following additives: 1-3 parts of silicate modifier and 0.5-1.5 parts of sulfide precipitant. The silicate modifier is used to block the macroporous structure of the biochar matrix, and the sulfide precipitant is used to form insoluble sulfides with heavy metal ions.

5. A method for preparing micro / nano-aerated iron composite modified biochar, characterized in that, The micro / nano aerated iron composite modified biochar according to any one of claims 1-4 comprises the following steps: S1. Biomass pretreatment: Corn stalks are washed, dried and crushed to obtain pretreated biomass; S2. Biochar preparation: Pretreated biomass is pyrolyzed under an inert atmosphere. After pyrolysis, it is cooled to obtain raw biochar. S3, Micro-nano Aeration Modification: The original biochar is dispersed in water, and micro-nano bubbles are introduced for aeration treatment. After aeration is completed, solid and liquid are separated, the solid product is collected and dried to obtain aeration modified biochar. S4. Iron composite modification: The aerated modified biochar is placed in a ferric chloride solution and shaken for impregnation. After impregnation, solid-liquid separation is performed, the solid product is collected and vacuum dried to obtain micro-nano aerated-iron composite modified biochar.

6. The method for preparing micro / nano-aerated iron composite modified biochar according to claim 5, characterized in that, Step S1 also includes steam activation pretreatment of the pretreated biomass: spraying 5% to 10% of its mass of water vapor onto the pretreated biomass and simmering it at 200 to 300°C for 30 to 60 minutes.

7. The method for preparing micro / nano-aerated iron composite modified biochar according to claim 5, characterized in that, In step S2, the pyrolysis temperature is 580~620℃, the heating rate is 8~12℃ / min, and the isothermal time is 110~130min.

8. The method for preparing micro / nano-aerated iron composite modified biochar according to claim 5, characterized in that, In step S3, the aeration treatment is as follows: the particle size of the aeration bubbles is controlled to be 100~200nm, the gas-water volume ratio is 0.02~0.04, and aeration is carried out at room temperature for 10~20min; hydrogen peroxide is also added as an additive, and the amount of hydrogen peroxide is 0.1%~0.5% of the total volume of the reaction system; the iodine adsorption value of the aerated modified biochar obtained after the aeration treatment is controlled to be increased to 1.4~1.6 times the original value.

9. The method for preparing micro / nano-aerated iron composite modified biochar according to claim 5, characterized in that, In step S4, the Fe in the ferric chloride solution is controlled. 3+ The concentration of the solution is 0.2–0.4 mol / L, and the initial pH of the solution is adjusted to 2.5–3.

5. A dispersant of 0.05%–0.1% of the mass of the ferric chloride solution is also added, wherein the dispersant is polyvinylpyrrolidone or sodium citrate. After solid-liquid separation and before vacuum drying, an inert gas purging is performed for 5–10 minutes.

10. The method for preparing micro / nano-aerated iron composite modified biochar according to claim 5, characterized in that, In step S4, after vacuum drying, there is also a curing process, which involves placing the dried product in a sealed container and curing it at 40-50°C for 12-24 hours to control the moisture content of the obtained micro-nano aerated iron composite modified biochar to be less than 5%.

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