A modified biochar adsorbent, its preparation method and its uses

Abstract

This invention discloses a modified biochar adsorbent, its preparation method, and its applications. The method includes: uniformly mixing biomass raw materials with iron salts to obtain a raw material mixture; pyrolyzing at 750℃~850℃ to obtain an iron-containing biochar precursor; performing a first acid wash and washing to neutrality to obtain acid-washed iron-containing biochar; mixing the acid-washed iron-containing biochar with an alkaline activator and activating it by heat treatment at 750℃~850℃; performing a second acid wash, washing to neutrality, and drying to obtain an oxygen-coordinated iron monoatomic biochar adsorbent. In the adsorbent, iron is dispersed in monoatomic form, forming a Fe-O-C coordination structure, with an oxygen coordination number of 3~4. This adsorbent can efficiently adsorb aromatic organic pollutants such as bisphenol A through pore filling and π-complexation, exhibiting high adsorption capacity, fast kinetics, wide pH range, and strong resistance to interference from coexisting ions, making it suitable for the removal of organic pollutants from water bodies.

Description

Technical Field

[0001] This invention relates to the field of water treatment and adsorption materials technology, specifically to a modified biochar adsorbent, its preparation method, and its uses. Background Technology

[0002] With the rapid development of industry and agriculture, large quantities of recalcitrant aromatic organic pollutants, such as bisphenol A, antibiotics, and dyes, are discharged into the aquatic environment. The continuous accumulation of organic pollutants in the aquatic environment has become a significant environmental problem. These pollutants are characterized by high toxicity, poor biodegradability, and easy bioaccumulation, posing a serious threat to ecosystems and human health. For example, bisphenol A (BPA) has the chemical formula C... 15 H 16 O2, widely present in drinking water, river and lake water, and sewage, is a typical endocrine disruptor (EDC). BPA can mimic the effects of estrogen and is difficult to metabolize and break down in the body. Even at extremely low concentrations (ng / L), BPA can interfere with the endocrine system of animals and humans, affecting reproductive and nervous system health. BPA removal focuses on trace amounts, as conventional water treatment technologies are difficult to remove. The main challenge lies in achieving deep mineralization without producing toxic byproducts, thus avoiding secondary pollution.

[0003] Existing technologies for BPA removal include adsorption, advanced oxidation, reverse osmosis, and biodegradation. Among these, adsorption has engineering advantages due to its lower cost, simple operation, and ease of large-scale application. Adsorption utilizes the physical adsorption (van der Waals forces) or chemical adsorption (hydrogen bonds, π-π interactions) between the surface of a solid adsorbent and bisphenol A molecules to capture the pollutant. Biochar (BC) is a commonly used solid adsorbent. It is a porous, carbon-rich material obtained by pyrolysis and carbonization of biomass under anaerobic conditions. Its porous structure and surface functional groups (hydroxyl and carboxyl groups) can effectively adsorb bisphenol A. Biochar has a wide range of raw material sources, low cost, environmental friendliness, and certain pore structure and surface activity, making it a promising adsorbent carrier.

[0004] Because BPA concentrations in natural water bodies are extremely low (ng / L to µg / L), this places extremely high demands on the affinity of adsorbents. The removal rate of trace amounts of BPA by raw biochar is unstable. Therefore, researchers have attempted to couple adsorption and oxidation technologies (such as adsorption-photocatalysis) or employ cascade reactions (such as membrane pre-treatment + oxidation). However, such coupled processes often involve multiple materials and equipment, leading to complex overall processes and increased operating costs. Furthermore, while conventional removal technologies may perform well in pure water in laboratory settings, humic acids, heavy metal ions, and high turbidity significantly reduce removal efficiency in real industrial or domestic wastewater.

[0005] Therefore, developing novel adsorbent materials with excellent adsorption performance for aromatic organic pollutants (especially bisphenol A) (enabling trace removal) has become a pressing technical problem to be solved in this field.

[0006] The statements herein provide only background information in relation to the present invention and do not necessarily constitute prior art. Summary of the Invention

[0007] The purpose of this invention is to solve the problem of trace removal of bisphenol A (BPA) in water pollution by providing an iron-modified biochar adsorbent. By increasing the oxygen coordination number of iron atoms, the adsorption activity of biochar is improved, giving it high adsorption capacity, rapid kinetics, wide pH applicability, and good anti-interference ability for aromatic organic pollutants such as BPA.

[0008] To achieve the above objectives, the present invention provides a method for preparing a biochar adsorbent, comprising:

[0009] Step 1: Mix the biomass raw materials with iron salts evenly to obtain a raw material mixture;

[0010] Step 2: Pyrolyze the raw material mixture at 750℃~850℃ to obtain an iron-containing biochar precursor;

[0011] Step 3: The iron-containing biochar precursor is subjected to a first acid wash and washed until neutral to obtain acid-washed iron-containing biochar.

[0012] Step 4: Mix the acid-washed iron-containing biochar with an alkaline activator and perform heat treatment activation at 750℃~850℃; the alkaline activator is an alkali metal hydroxide.

[0013] Step 5: The product after heat treatment activation is subjected to a second acid wash, washed until neutral and dried to obtain oxygen-coordinated iron single-atom biochar adsorbent.

[0014] Through the combined process of two heat treatments (pyrolysis and heat treatment activation) and two acid washings, on the one hand, the biomass carbonization is transformed into a carbon substrate with a high degree of graphitization and forms a well-developed porous structure. On the other hand, by removing soluble / precipitable iron species and stabilizing the iron-oxygen-carbon coordination environment, the iron sites are highly dispersed in the form of single atoms and their oxygen coordination number is stabilized at 3-4, thereby obtaining an oxygen-coordinated iron single-atom biochar adsorbent with high adsorption capacity.

[0015] Optionally, the biomass raw material in step 1 is loofah powder; the iron salt is Fe(NO3)3·9H2O; the mass ratio of loofah powder to Fe(NO3)3·9H2O is (2-4):1, and the mixing time is 4-8 h.

[0016] Optionally, in step 1, the biomass raw material and iron salt are mixed with an aqueous solvent, and then dried to obtain the raw material mixture. The drying temperature is 80℃~105℃ and the drying time is 8 h~12 h.

[0017] Optionally, the pyrolysis conditions in step 2 are: under an inert atmosphere, a heating rate of 8 °C / min. -1 ~12℃·min -1 The heat preservation time is 2 h to 5 h, and the inert atmosphere is argon or nitrogen.

[0018] Optionally, the first pickling in step 3 uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1 mol·L -1 Pickling time is 1-2 hours.

[0019] Optionally, the alkaline activator in step 4 is potassium hydroxide; the mass ratio of the acid-washed iron-containing biochar to potassium hydroxide is 1:(1-3).

[0020] Optionally, the conditions for heat treatment activation in step 4 are: under an inert atmosphere, the heating rate is 3℃·min. -1 ~6℃·min -1 The heat preservation time is 2h~5h, and the inert atmosphere is nitrogen or argon.

[0021] Optionally, the second pickling in step 5 uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1mol·L -1 Pickling time is 0.5h to 1h.

[0022] The present invention also provides a modified biochar adsorbent, wherein the adsorbent is an oxygen-coordinated iron single-atom biochar adsorbent, comprising a graphitized carbon skeleton and a porous structure, wherein iron is dispersed in the form of single atoms on the surface or defect sites of the carbon skeleton to form an Fe-OC coordination structure, and the oxygen coordination number of iron is 3 to 4.

[0023] The present invention also provides a use of the above-mentioned modified biochar adsorbent for the adsorption and removal of aromatic organic pollutants in water; wherein the aromatic organic pollutants include one or more of bisphenol A, phenol, tetracycline antibiotics, quinolone antibiotics, nitrobenzene compounds, and dyes.

[0024] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects:

[0025] 1) Based on theoretical calculations and experimental verification, this invention discovers that the oxygen coordination number of iron single-atom sites plays a crucial regulatory role in the adsorption behavior of aromatic pollutants: when the oxygen coordination number of the iron site increases to 3–4, the interaction between the iron site and the π-electron system of the BPA benzene ring is significantly enhanced. The adsorption mechanism gradually shifts from being dominated by weak interactions to π-complexation, primarily contributed by chemical bonding, thereby significantly improving adsorption strength and stability. Compared with low oxygen coordination (1–2 coordination), 3–4 coordination significantly increases adsorption energy and promotes the formation of stable adsorption configurations. Utilizing the above findings, this invention prepares an iron-modified biosorbent in which iron atoms are dispersed in single-atom form with an oxygen coordination number of 3–4, forming uniformly dispersed FeO sites in biochar, greatly improving the adsorption capacity and stability of the biochar adsorbent.

[0026] 2) Through KOH activation and secondary heat treatment, the adsorbent of this invention achieves a high specific surface area and a well-developed pore structure, increasing the proportion of pores >1 nm, which is beneficial for BPA molecule diffusion and pore filling; simultaneously, FeO single-atom active sites provide strong active centers. The synergistic effect of pore filling and π-complexation enables the adsorbent to exhibit high equilibrium adsorption capacity and rapid adsorption kinetics; for example, the equilibrium adsorption capacity for BPA at 298 K can reach 683.3 mg·g⁻¹. -1 It can reach approximately 90% equilibrium adsorption within 10 minutes.

[0027] 3) Insensitive to the environment and adaptable to impurity removal in complex water bodies: The adsorbent of this invention maintains high adsorption capacity in the pH range of 3.0 to 11.0; it also adapts to common coexisting anions (Cl... - F - HCO3 - CO3 2- SO4 2- ) and cations (Ca 2+ K + Na + Mg 2+ NH4 + The small change in adsorption performance under the presence of certain conditions indicates that the adsorbent of this invention is not sensitive to the environment, has strong tolerance to ionic strength and inorganic ion competition, and still maintains a high removal capacity in actual complex aquatic matrices.

[0028] 4) Universality: Since the FeO site has the ability to complex and recognize aromatic π-electronic structures, the adsorbent not only shows good removal effect on BPA, but also on other common aromatic organic pollutants. It is also applicable to both hydrophilic and hydrophobic aromatic compounds, thus broadening its application scenarios in the treatment of organic pollutants in complex water systems.

[0029] 5) The adsorbent of the present invention can be regenerated by methanol and alkaline solution and recycled. Attached Figure Description

[0030] Figure 1 This is a partial microscopic schematic diagram of a modified biochar adsorbent according to the present invention.

[0031] Figure 2a This is a scanning electron microscope (SEM) image of the Fe-O / BC prepared in Example 1 of the present invention; Figure 2b This is a transmission electron microscope (TEM) image of the Fe-O / BC prepared in Example 1 of the present invention.

[0032] Figure 3 The image shows a spherical aberration corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM) image of Fe-O / BC prepared in Example 1 of this invention.

[0033] Figure 4 The images show the synchrotron X-ray absorption fine structure (XAFS) test and fitting results of Fe-O / BC prepared in Example 1 of this invention; where a is the XANES spectrum and b is the FT-EXAFS spectrum and fitting results.

[0034] Figure 5 The X-ray diffraction (XRD) pattern of Fe-O / BC prepared in Example 1 of this invention.

[0035] Figure 6a The nitrogen adsorption-desorption isotherm of Fe-O / BC prepared in Example 1 of this invention; Figure 6b The image shows the pore size distribution of Fe-O / BC prepared in Example 1 of this invention.

[0036] Figure 7 The graphs show the adsorption performance comparison of the materials prepared in Example 1 and Comparative Examples 1-2. In the graphs, a represents the adsorption efficiency comparison graph, b represents the adsorption kinetics graph of Fe-O / BC for bisphenol A, and c represents the isotherm equilibrium graph.

[0037] Figure 8 The effect of different pH values ​​on the adsorption of bisphenol A by Fe-O / BC prepared in Example 1 and the change of Zeta potential on the material surface are shown in the figure.

[0038] Figure 9 The graph shows the effect of exogenous coexisting ions on the adsorption performance of Fe-O / BC prepared in Example 1 on bisphenol A.

[0039] Figure 10 The graph shows the competitive adsorption results of Fe-O / BC prepared in Example 1 and O(Fe) / BC prepared in Comparative Example 2.

[0040] Figure 11The adsorption efficiency diagram of Fe-O / BC prepared in Example 1 for other types of aromatic ring-containing organic pollutants is shown. Detailed Implementation

[0041] The following detailed description, in conjunction with the accompanying drawings and specific embodiments, provides a further detailed explanation of the modified biochar adsorbent, its preparation method, and its applications proposed in this invention. The advantages and features of this invention will become clearer from the following description. It should be noted that the accompanying drawings are in a very simplified form and use non-precise proportions, intended only to facilitate and clarify the illustration of the embodiments of this invention. Please refer to the accompanying drawings to make the objectives, features, and advantages of this invention more apparent and understandable. It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are only for illustrative purposes and to enable those skilled in the art to understand and read them, and are not intended to limit the implementation conditions of this invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0042] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0043] In this article, the term "π-complexation" refers to the orbital overlap between a metallic active site (e.g., an iron single atom) and the π electron cloud in an organic pollutant molecule, acting as an electron donor or acceptor, via d-π... * The strong chemisorption force formed by feedback.

[0044] The term "single atom" refers to the fact that iron is basically dispersed uniformly on the biochar carrier in the form of single atoms, without forming metal nanoparticles or clusters.

[0045] As described in the background section, traditional biochar materials, when treating organic pollutants, mainly rely on physical adsorption, pore filling, and weak π-π interactions, resulting in limited adsorption capacity and poor selectivity, making it difficult to achieve trace removal of aromatic organic pollutants such as BPA. To overcome these shortcomings, existing technologies typically employ metal loading or metal-nonmetal co-doping modification methods to improve adsorption performance. However, such material modification is often limited by an inherent technical bias: it is generally believed that the introduced metal components mainly play a synergistic or auxiliary role (e.g., promoting graphitization of the carbon substrate, altering surface charge distribution, etc.), thereby indirectly enhancing the original adsorption pathways of biochar. While metal and nonmetal doping can improve biochar performance, existing metal-doped adsorption materials mostly attribute performance improvements to the enhancement of the inherent adsorption pathways of biochar (e.g., π-π interactions, electrostatic interactions, etc.), rarely exploring the development of the intrinsic adsorption activity of metal active sites. Existing metal-supported biochar materials have limited binding strength in practical applications, making it difficult not only to fundamentally overcome the bottleneck of adsorption capacity, but also to achieve efficient and broad-spectrum removal of aromatic organic compounds that are both hydrophilic and hydrophobic in complex water bodies.

[0046] To address the aforementioned problems, this invention provides a method for preparing oxygen-coordinated iron single-atom biochar adsorbent. This method utilizes a two-step pyrolysis process, taking biomass such as loofah sponge as a natural macromolecular resource. By increasing the oxygen coordination number, the electronic structure of the iron active center is modulated, thereby improving the adsorption performance of the biochar. Specifically, excess ferric nitrate nonahydrate is co-pyrolyzed with biomass. Utilizing the catalytic properties of iron, amorphous carbon is significantly induced to graphitize at 800℃, fixing the iron element within the carbon framework. Then, a KOH-assisted secondary activation process is used to etch the biochar, forming a more porous structure, while simultaneously reconstructing the fixed iron single atoms into specific coordination configurations such as Fe-O3 or Fe-O4. The strong electronegativity of oxygen atoms acts as an "electron bridge," inducing electron transfer from the carbon substrate to the iron center, significantly enhancing the π-complexation activity between the iron sites and aromatic pollutants (such as bisphenol A).

[0047] like Figure 1 The diagram shown is a partial microscopic schematic of a modified biochar adsorbent according to the present invention. The adsorbent is an oxygen-coordinated iron single-atom biochar adsorbent, comprising a graphitized carbon framework and a porous structure. Iron is dispersed in single-atom form on the surface or defect sites of the carbon framework, forming an Fe-OC coordination structure, with an oxygen coordination number of 3 to 4. This single-atom iron structure (Fe-OC structure) coordinated by 3 to 4 oxygen atoms ensures that the iron center possesses the most suitable d-band center position and charge transfer channel, thereby achieving excellent chemisorption of target aromatic molecules.

[0048] The present invention provides a method for preparing a modified biochar adsorbent, comprising the following steps:

[0049] Step 1: Mix the biomass raw materials with iron salts evenly to obtain a raw material mixture.

[0050] In some embodiments, natural polymer materials rich in cellulose, hemicellulose and lignin (such as loofah powder) can be selected as biomass raw materials. In this example, the biomass raw material is loofah powder; the iron salt is Fe(NO3)3·9H2O; the mass ratio of loofah powder to Fe(NO3)3·9H2O is (2-4):1, and the mixing time is 4-8 h.

[0051] In some embodiments, the biomass raw material and iron salt are mixed with an aqueous solvent, and then dried to obtain the raw material mixture. The drying temperature is 80℃~105℃ and the drying time is 8 h~12 h.

[0052] As an example, cleaned and dried loofah powder was mixed with ferric nitrate solution in an aqueous solution. The mixture was magnetically stirred at room temperature for 4-6 hours to ensure that iron ions fully penetrated into the cellulose and lignin channels of the loofah and formed preliminary complexes with the oxygen-containing functional groups on the biomass surface. Subsequently, the suspension was placed in a forced-air drying oven at 105°C and dried at a constant temperature until all moisture evaporated, obtaining a dried precursor mixture with uniformly dispersed iron ions.

[0053] Step 2: The raw material mixture is pyrolyzed at 750℃~850℃ to obtain an iron-containing biochar precursor.

[0054] 750℃~850℃ is the optimal temperature range for iron-catalyzed conversion of amorphous carbon into graphitic carbon. Too low a temperature will result in insufficient graphitization and a low effective metal loading. Too high a temperature will lead to excessive depletion of biomass carbon, causing metal agglomeration. Furthermore, at excessively high temperatures (e.g., above 900℃), the carbon skeleton will encapsulate metal atoms, causing the loading to fail.

[0055] In some embodiments, the pyrolysis conditions are: under an inert atmosphere, a heating rate of 8°C / min. -1 ~12℃·min -1 The holding time is 2 h to 5 h, and the inert atmosphere is argon or nitrogen. As an example, the raw material mixture is placed in a tube furnace and subjected to initial pyrolysis under the protection of an inert atmosphere (such as argon or nitrogen).

[0056] After the reaction was completed, the sample was washed with 0.1 mol / L dilute hydrochloric acid for 1 h to remove ash and unstable components on the surface. During the acid washing process, some clusters and aggregated iron were eluted. Finally, the sample was washed with water until neutral and dried to obtain iron-supported biochar.

[0057] Step 3: The iron-containing biochar precursor is subjected to a first acid wash and washed until neutral to obtain acid-washed iron-containing biochar.

[0058] The purpose of this step is to precisely etch and remove non-anchored iron clusters, unstable iron nanoparticles exposed on the surface, and soluble ash impurities, while retaining iron species that are tightly bound by the carbon lattice and have a stable coordination basis. After acid washing, the product is repeatedly washed with ultrapure water until the pH of the eluent is neutral, and then dried to obtain an acid-washed iron-containing biochar intermediate (labeled as Fe / BC).

[0059] In some embodiments, the first pickling uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1 mol·L -1 The pickling time is 1 to 2 hours. As an example, the black product (iron-containing biochar precursor) obtained by pyrolysis is ground and then placed in an acidic solution (such as 0.1 mol / L hydrochloric acid solution) for stirring and pickling, which usually takes 1 hour.

[0060] Step 4: Mix the acid-washed iron-containing biochar with an alkaline activator and perform heat treatment activation at 750℃~850℃; the alkaline activator is an alkali metal hydroxide.

[0061] This step is crucial for constructing the oxygen coordination configuration and optimizing the pore structure: at high temperatures, KOH undergoes a vigorous redox reaction (generating metallic potassium, potassium carbonate, and carbon monoxide, etc.), deeply chemically etching the graphitized carbon substrate. On one hand, this pore-forming effect dramatically increases the specific surface area of ​​the material (up to 1676 m²). 2 / g), and generated a large number of mesoporous structures (average pore size increased to 2.88 nm), which greatly reduced the steric hindrance for target macromolecular pollutants (such as bisphenol A, with a molecular diameter of about 1.04 nm) to enter the material; on the other hand, during the high-temperature etching and carbon skeleton reconstruction process, oxygen-containing groups were rearranged, and highly electronegative oxygen atoms were forced to covalently bond with highly dispersed isolated iron atoms, successfully inducing the theoretically predicted oxygen coordination configuration (Fe-OC).

[0062] In some embodiments, the conditions for heat treatment activation in step 4 are: under an inert atmosphere, a heating rate of 3°C·min. -1 ~6℃·min -1The heat treatment time is 2-5 hours, and the inert atmosphere is nitrogen or argon. The alkaline activator is potassium hydroxide; the mass ratio of the acid-washed iron-containing biochar to potassium hydroxide is 1:(1-3). As an example, the acid-washed iron-containing biochar and potassium hydroxide (as a strong alkaline activator) are mixed in an appropriate amount of deionized water at a certain mass ratio, magnetically stirred for 4 hours, and then the water is slowly evaporated at 80°C to obtain a paste-like mixture. Subsequently, the mixture is placed again in a tube furnace for a second heat treatment activation under an inert atmosphere (such as nitrogen) at approximately 800°C for 2 hours.

[0063] Step 5: The product after heat treatment activation is subjected to a second acid wash, washed until neutral and dried to obtain oxygen-coordinated iron single-atom biochar adsorbent (labeled as Fe-O / BC).

[0064] In the oxygen-coordinated iron single-atom biochar adsorbent, iron is dispersed in single-atom form and forms an Fe-OC coordination structure, with an oxygen coordination number of 3 to 4.

[0065] In some embodiments, the second pickling uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1mol·L -1 The pickling time is 0.5 h to 1 h. As an example, the activated product is immersed in 0.05 mol / L hydrochloric acid and stirred for 1 hour to thoroughly remove residual potassium salt byproducts from the etching reaction and some structurally damaged metal oxides. It is then washed repeatedly with ultrapure water until the pH is neutral and dried to constant weight in a vacuum drying oven at 80 °C.

[0066] The following description is based on Example 1 and Comparative Examples 1-2. Unless otherwise specified, all raw materials and reagents used in the examples and comparative examples are commercially available.

[0067] Example 1 (Fe-O / BC)

[0068] Preparation of Fe-O / BC oxygen-coordinated iron (Fe-O) single-atom biochar (BC) adsorbent: 4 g of washed and dried loofah powder and 2 g of ferric nitrate nonahydrate were added to 100 mL of deionized water and magnetically stirred at room temperature for 4 h. The suspension was dried in a 105℃ forced-air drying oven for 8 h to obtain a dried mixture. Then, in a tube furnace, argon gas was introduced, and the mixture was heated to 800℃ at a heating rate of 10℃ / min and pyrolyzed at a constant temperature for 2 h to obtain an iron-containing biochar precursor. The precursor was placed in 0.1 mol / L HCl solution and stirred and acid-washed for 1 h, washed with ultrapure water until the pH was neutral, and dried to obtain Fe / BC. Finally, 1 g of Fe / BC was weighed and mixed with 3 g of KOH, and 50 mL of deionized water was added and stirred for 4 h. The water was evaporated at 80℃ to form a paste. Subsequently, under a nitrogen atmosphere, the mixture was heated to 800℃ at a rate of 5℃ / min and activated by a second heat treatment for 2 h. Finally, the product was acid-washed in 0.05 mol / L HCl for 1 h, washed with ultrapure water until neutral, and dried in a vacuum drying oven at 80℃ for 8 h to obtain a single-atom biochar adsorbent (Fe-O / BC) with an O coordination number of approximately 3.7 (characterization result).

[0069] Scanning electron microscopy (SEM) of Fe-O / BC, such as Figure 2a As shown, scanning electron microscopy (SEM) observation of the surface morphology of Fe-O / BC revealed that the material exhibits carbon particle morphology with a partially lamellar structure, indicating the presence of regularly graphitized carbon; the surface is rough and riddled with numerous pores and deep etching marks, which is attributed to the secondary activation effect of KOH; and no obvious iron clusters or particles were observed on the surface, proving that Fe is distributed in a single-atom form. Transmission electron microscopy (TEM) characterization of Fe-O / BC is as follows: Figure 2b As shown, transmission electron microscopy reveals no striations on the surface of Fe-O / BC, whereas crystalline iron exhibits significant striations. These findings corroborate the monatomic morphology of Fe.

[0070] Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM) for Fe-O / BC as follows: Figure 3 As shown, iron (Fe) can be clearly seen as isolated bright spots (single-atom state) dispersed extremely uniformly on the carbon substrate, with no nanoclusters observed (excessive pyrolysis temperature will cause iron atoms to aggregate and form clusters and crystals), which also confirms the single-atom distribution of Fe.

[0071] Synchrotron X-ray Absorption Spectrometry (XAFS) Testing and Fitting of Fe-O / BC Figure 4 As shown in a and b in the diagram, the X-ray absorption near-edge structure (XANES) of the FeK edge confirms the valence state characteristics of iron. See [reference needed]. Figure 4In the magnified portion of the figure, the material of this invention (Fe-O / BC) is sandwiched between the lines of FePc (ferrous divalent) and Fe2O3 (ferrous trivalent), indicating that the valence state of iron is between these two, i.e., between valences 2 and 3. The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum (…) Figure 4 As shown in b), a significant strong peak appears at a radial distance of 1.6 Å (attributed to Fe-O and Fe-C), while almost no characteristic peak of metallic Fe-Fe bonds is observed at 2.2 Å, further confirming that iron is dispersed as single atoms. The fitting results of the professional analysis software in R space are shown in Table 1:

[0072] Table 1: EXAFS fitting parameters of different samples at the iron K-side

[0073]

[0074] Where CN represents the coordination number; R represents the distance between the absorber and the backscattering atom; σ 2 ΔE0 represents the Debye-Waller factor, used to describe thermal and structural disorder; ΔE0 represents the internal potential correction. The fitting results show that the Fe-C bond length is approximately 1.89 Å, the Fe-O bond length is approximately 2.06 Å, and the average oxygen coordination number of the iron atom is 3.7.

[0075] The X-ray diffraction (XRD) pattern of Fe-O / BC is as follows: Figure 5 As shown in the figure, the X-ray diffraction pattern reveals that Fe-O / BC exhibits a broad characteristic peak of the graphite (002) crystal plane near 26.5°, and a new graphite (101) crystal plane appears, indicating that the activation process etched the graphite layer and exposed new active crystal planes. Meanwhile, no crystalline iron species (such as zero-valent iron Fe) were detected in the XRD pattern. 0 The presence of Fe3O4 or Fe2O3 further corroborates the single-atom dispersed state of iron. Since zero-valent iron is mostly in clusters or crystals, single-atom zero-valent iron can hardly exist stably in large quantities. The single-atom dispersed iron here is actually anchored to a carbon substrate by the element O, with a valence state between 2 and 3.

[0076] Nitrogen adsorption-desorption isotherm (BET) test of Fe-O / BC as follows Figure 6a , Figure 6b As shown. Nitrogen adsorption-desorption isotherm ( Figure 6a Tests show that Fe-O / BC has an extremely high specific surface area (1676 m²). 2 / g) and excellent total pore volume (1.21 cm³). 3 / g), see also Figure 6b .

[0077] Comparative Example 1 (O / BC)

[0078] Preparation of the control sample pure biochar (O / BC): To verify the core role of iron single atoms, a metal-free control group was prepared. The operation steps were exactly the same as in Example 1, except that ferric nitrate nonahydrate was not added in step 1, meaning the biochar was treated with KOH but did not contain Fe. The final material was labeled O / BC.

[0079] Comparative Example 2 (O(Fe) / BC)

[0080] Preparation of the control sample (O(Fe) / BC) with destroyed iron active sites: To demonstrate the microscopic mechanism of the complexation process from the opposite perspective, the Fe-O / BC prepared in Example 1 was immersed in a 1 mol / L sulfuric acid solution and continuously stirred at 60°C for 2 hours. This strong acid etching process can destroy and leach the exposed iron single-atom active centers on the surface. After washing and drying, the product was labeled O(Fe) / BC. After strong acid treatment, the Fe sites in the material are destroyed, and a small amount of iron may remain (acid treatment cannot completely remove all Fe elements), therefore (Fe) is used to represent the possible residual Fe. At this time, the surface functional groups of the material are still retained. Comparative Example 2 uses high-concentration acid washing to destroy the Fe-O sites on the surface of the Fe-O / BC material (the material of this invention), thereby demonstrating the adsorption effect of Fe sites, for comparison with the adsorption effect of Fe-O sites.

[0081] The dosage of the prepared material was set at 0.1 g / L, and the concentration of the bisphenol A solution was 100 mg / L. The adsorption efficiency of the materials in Example 1 and the comparative example is as follows: Figure 7 As shown. The adsorption efficiency results are as follows. Figure 7 As shown in Figure a, compared to Fe / BC without KOH activation and reconstruction, and O / BC without iron single atoms, there are significant differences in adsorption rate and adsorption capacity. At 298 K, the saturated adsorption capacity of Fe-O / BC for BPA is as high as 683.3 mg / g, significantly higher than that of unactivated Fe / BC and iron-free O / BC. The adsorption kinetics of Fe-O / BC is shown in Figure a. Figure 7 As shown in b; kinetic experiments show that Fe-O / BC exhibits an extremely rapid adsorption rate, reaching 90% of the equilibrium adsorption capacity within the first 10 minutes after addition, and the data highly conforms to the pseudo-second-order kinetic model (R0). 2 The correlation coefficient > 0.98 indicates that the adsorption process is dominated by strong chemisorption. The fitted intraparticle diffusion model failed to cross the origin, and the fitted external diffusion model showed poor correlation. This strongly suggests that the adsorption process is not rate-limited by simple physical diffusion, but is dominated by strong chemisorption (site binding). The isothermal equilibrium diagram of Fe-O / BC is shown below. Figure 7 As shown in c, the isotherm data conform to the Sips model (Rc).2 > 0.99), indicating that the adsorption occurs as a monolayer adsorption on a non-uniform surface. Thermodynamic parameter analysis (ΔG) 0 < 0, ΔH 0 > 0, ΔS 0 As shown in Table 2, the adsorption is a spontaneous, endothermic entropy-increasing process.

[0082] Table 2: Thermodynamic parameters of BPA adsorption on Fe-O / BC

[0083]

[0084] The effects of different pH values ​​on the adsorption of BPA by Fe-O / BC and the zeta potential of the material surface, such as Figure 8 As shown, the adsorption capacity remains stable and high within the pH range of 3 to 6; even when the pH rises to 11 (above the isoelectric point of the material, the surface is negatively charged), it can still maintain an impressive adsorption capacity of 435.7 mg / g, indicating that Fe-O / BC has a very wide pH adaptability and that electrostatic interaction is not the only or main driving force for adsorption.

[0085] The effect of exogenous ions on the adsorption of BPA by Fe-O / BC is as follows: Figure 9 As shown; a high concentration of coexisting background ions (0.03 mol / L Cl) were added. - , F - SO4 2- HCO3 - CO3 2- and 0.1 mol / L K + Na + Mg 2+ Ca 2+ After that, the actual adsorption capacity measured deviated from the theoretical prediction by less than 5%. This fully demonstrates that the coexisting ions hardly compete with bisphenol A for adsorption sites, and Fe-O / BC exhibits extremely high anti-interference ability and target selectivity.

[0086] The strength of π-complexation is influenced by the electron density of the adsorbed molecule; lower electron densities generally result in higher binding efficiency. Therefore, selectivity is expected when Fe sites form complexes with benzene rings of different electron densities. To this end, we conducted competitive adsorption experiments: p-aminobenzoic acid (PABA, with an electron-donating group and high benzene ring electron density) and m-dinitrobenzene (DNB, with a strong electron-withdrawing group) were selected as competing substances. The results of the Fe-O / BC competitive adsorption experiments are shown below. Figure 10As shown. When 2 mg / L and 5 mg / L DNB were added to an aqueous system containing BPA, the adsorption of BPA was significantly inhibited (inhibition rate 11.7%–12.2%) when the Fe-O / BC adsorbent of Example 1 was applied. The inhibition rate was only 4.2%–8.2% when the same amount of PABA was added. The same test was performed on O(Fe) / BC in Comparative Example 2, and both compounds showed significantly reduced inhibition. Notably, PABA (2.2%–3.9%) showed a stronger inhibitory effect than DNB (0.9%–1.5%), indicating opposite effects between the two competing substances. This confirms the selectivity of Fe-O sites in π-complexation. The opposite inhibitory effect is attributed to the dominance of adsorption in the graphite structure after the iron sites in O(Fe) / BC are missing. In this state, the iron atoms partially embedded in the inner layers of graphite reduce the electron density of graphite, making graphite an electron acceptor, thus enabling it to undergo a typical π-π electron donor-acceptor (π-π EDA) interaction with BPA. Therefore, PABA, with its higher electron density on the benzene ring, exhibits a stronger inhibitory effect. If the π-π EDA interaction is the dominant adsorption mechanism, the O(Fe) / BC composite material would not show such a significant decrease in adsorption performance. Thus, it is confirmed that π-complexation dominates the adsorption of BPA by Fe-O / BC.

[0087] In addition, eight aromatic compounds with different octanol / water partition coefficients (logKow) were selected for testing, and the adsorption efficiency of Fe-O / BC for other types of aromatic ring-containing pollutants was as follows: Figure 11 As shown, a positive correlation (R0) was found between the adsorption amount and the logKow of the hydrophobic pollutant. 2 =0.84), indicating that hydrophobic interaction plays a certain auxiliary synergistic role. However, Fe-O / BC also maintains excellent adsorption capacity for hydrophilic aromatic pollutants. This shows that the adsorbent has facultative adsorption capacity, and can efficiently capture both hydrophilic and hydrophobic pollutants. It can achieve rapid, efficient, and large-capacity removal of various target pollutants in complex aquatic environments, and has broad practical prospects in the field of environmental wastewater treatment.

[0088] In summary, this invention constructs iron single-atom active sites with oxygen coordination numbers of 3-4 on biochar through a two-stage heat treatment and two-stage acid washing process, significantly enhancing the π-complex adsorption activity of the iron sites. This achieves high-capacity, rapid, and interference-resistant removal of aromatic organic pollutants such as BPA, demonstrating promising engineering application prospects. The modified biochar adsorbent provided by this invention can be used for the adsorption and removal of aromatic organic pollutants in water; these aromatic organic pollutants include one or more of bisphenol A, phenol, tetracycline antibiotics, quinolone antibiotics, nitrobenzene compounds, and dyes.

[0089] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for preparing a modified biochar adsorbent, characterized in that, include: Step 1: Mix biomass raw materials and iron salt evenly to obtain a raw material mixture; wherein, the biomass raw material is loofah powder, and the iron salt is Fe(NO3)3·9H2O; the mass ratio of loofah powder to Fe(NO3)3·9H2O is (2~4):1; Step 2: The raw material mixture is pyrolyzed at 750℃~850℃ to obtain an iron-containing biochar precursor; Step 3: The iron-containing biochar precursor is subjected to a first acid wash and washed until neutral to obtain acid-washed iron-containing biochar. Step 4: Mix the acid-washed iron-containing biochar with an alkaline activator and perform heat treatment activation at 750℃~850℃; the alkaline activator is an alkali metal hydroxide. Step 5: The product after heat treatment activation is subjected to a second acid wash, washed until neutral and dried to obtain oxygen-coordinated iron single-atom biochar adsorbent, wherein iron in the oxygen-coordinated iron single-atom biochar adsorbent is dispersed in the form of single atoms and forms a Fe-OC coordination structure, and the oxygen coordination number of iron is 3 to 4.

2. The preparation method according to claim 1, characterized in that, In step 1, the biomass raw material and iron salt are mixed with an aqueous solvent, and then dried to obtain the raw material mixture. The drying temperature is 80℃~105℃ and the drying time is 8 h~12 h.

3. The preparation method according to claim 1, characterized in that, The pyrolysis conditions in step 2 are: under an inert atmosphere, a heating rate of 8℃·min. -1 ~12℃·min -1 The heat preservation time is 2 h to 5 h, and the inert atmosphere is argon or nitrogen.

4. The preparation method according to claim 1, characterized in that, In step 3, the first pickling uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1 mol·L -1 Pickling time is 1-2 hours.

5. The preparation method according to claim 1, characterized in that, The alkaline activator in step 4 is potassium hydroxide; the mass ratio of the iron-containing biochar after acid washing to potassium hydroxide is 1:(1-3).

6. The preparation method according to claim 1, characterized in that, The conditions for heat treatment activation in step 4 are: under an inert atmosphere, the heating rate is 3℃·min. -1 ~6℃·min -1 The heat preservation time is 2h~5h, and the inert atmosphere is nitrogen or argon.

7. The preparation method according to claim 1, characterized in that, In step 5, the second acid wash uses a hydrochloric acid solution; the concentration of the hydrochloric acid solution is 0.05 mol·L⁻¹. -1 ~1mol·L -1 Pickling time is 0.5h to 1h.

8. A modified biochar adsorbent prepared by the method of preparing the modified biochar adsorbent according to any one of claims 1-7, characterized in that, The adsorbent is an oxygen-coordinated iron single-atom biochar adsorbent, which includes a graphitized carbon framework and a porous structure. Iron is dispersed in the form of single atoms on the surface or defect sites of the carbon framework to form an Fe-OC coordination structure, and the oxygen coordination number of iron is 3 to 4.

9. The use of the modified biochar adsorbent according to claim 8, characterized in that, Used for the adsorption and removal of aromatic organic pollutants in water; the aromatic organic pollutants include one or more of bisphenol A, phenol, tetracycline antibiotics, quinolone antibiotics, nitrobenzene compounds, and dyes.

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