Magnetic MnFe2O4 / biochar composite material as well as preparation and application thereof

Abstract

The invention discloses a magnetic MnFe2O4 / biochar composite material as well as preparation and application thereof. A preparation method comprises the following steps: obtaining biochar; taking the biochar, manganese chloride and ferric chloride as raw materials, performing hydrothermal reaction under an alkaline condition, and then washing and drying to obtain the magnetic MnFe2O4 / biochar composite material. The magnetic MnFe2O4 / biochar composite material (MFC) is prepared through a pyrolysis-coprecipitation coupling process, the biochar efficiently removes Cr (V) through physical adsorption and chemical complexation by virtue of the characteristics of high specific surface area, hierarchical pore structure, rich surface oxygen-containing functional groups (such as carboxyl (-COOH) and carbonyl (C = O)) and the like, and MnFe2O4 has a bimetal active center, so that the magnetic MnFe2O4 / biochar composite material can be used for efficiently removing Cr (V). The Fe / Mn redox pair can remarkably improve the electron transfer efficiency, so that the MnFe2O4 / biochar composite material (MFC) can repair chromium pollution through the synergistic effect of redox and adsorption, and bidirectional improvement of the adsorption capacity and the reduction removal efficiency of chromium is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of pollutant treatment, and particularly to a magnetic MnFe2O4 / biochar composite material and its preparation and application. Background Art

[0002] Chromium pollution has become a major global environmental challenge. Among them, hexavalent chromium (Cr(VI)) poses a serious threat to water and soil ecosystems due to its high toxicity, strong mobility and carcinogenicity. Although current mainstream remediation technologies (such as chemical reduction, ion exchange and electrochemical treatment) can partially remove Cr(VI), they generally have bottlenecks such as high cost, complex operation and high risk of secondary pollution. For example, the chemical reduction method requires continuous addition of reducing agents (such as ferrous sulfate), resulting in problems in sludge disposal; electrochemical technologies have high energy consumption and are difficult to apply on a large scale. Therefore, there is an urgent need to develop efficient, low-cost and environmentally friendly Cr(VI) removal and resource utilization technologies.

[0003] Biochar shows potential in the field of Cr(VI) adsorption due to its porous structure, high specific surface area and rich surface functional groups. However, the removal of Cr(VI) by a single biochar material highly depends on physical adsorption and limited photochemical reduction ability, and it is difficult to efficiently achieve the directional conversion of Cr(VI) to low-toxic Cr(III). In addition, biochar lacks magnetic separation performance, has low solid-liquid separation efficiency, and is easily saturated by adsorption in a high-concentration Cr(VI) system, with insufficient cycle stability. Existing research can improve the separation efficiency by introducing single-metal magnetic materials (such as Fe3O4), but their redox activity is single, and Fe / Cr by-products are easily formed, resulting in passivation of active sites and limiting the long-term application potential, and the chromium removal effect is poor.

[0004] Therefore, there is a need to provide a biochar composite material to improve the chromium removal effect. Summary of the Invention

[0005] In view of this, the present application provides a magnetic MnFe2O4 / biochar composite material and its preparation and application, which are used to solve the problem of how to improve the chromium removal effect of biochar.

[0006] To achieve the above technical purpose, the present application adopts the following technical solutions: In the first aspect, the present application provides a preparation method of a magnetic MnFe2O4 / biochar composite material, including the following steps: S1. Obtain biochar; S2. Using biochar, manganese chloride and ferric chloride as raw materials, carry out a hydrothermal reaction under alkaline conditions, and then wash and dry to obtain the magnetic MnFe2O4 / biochar composite material.

[0007] Preferably, the pH value of the alkaline condition is 10-12; the temperature of the hydrothermal reaction is 80-90° C.; and the time of the hydrothermal reaction is 1-2 h.

[0008] Preferably, the mass ratio of ferric chloride to biochar is 1:(4-10); manganese chloride includes one or more of anhydrous manganese chloride and hydrated manganese chloride; ferric chloride includes one or more of anhydrous ferric chloride and hydrated ferric chloride.

[0009] In a second aspect, the present application provides a magnetic MnFe2O4 / biochar composite material.

[0010] In a third aspect, the present application provides an application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies.

[0011] Preferably, the chromium contaminant comprises hexavalent chromium and / or trivalent chromium.

[0012] Preferably, the method comprises the following steps: adding the magnetic MnFe2O4 / biochar composite material to the chromium-containing water body, performing chromium removal treatment, and then using an external magnetic field to perform solid-liquid separation to recover the magnetic MnFe2O4 / biochar composite material.

[0013] Preferably, the chromium removal process is carried out under visible light conditions.

[0014] Preferably, the pH value of the chromium-containing water is 1-6.

[0015] Preferably, the chromium concentration of the chromium-containing water is (20ppm-50ppm), and the addition amount of the magnetic MnFe2O4 / biochar composite material is 0.1-5g / L.

[0016] The beneficial effects of the present application are as follows: The present application prepares a magnetic MnFe2O4 / biochar composite material (MFC) through a pyrolysis-coprecipitation coupling process. Biochar, due to its high specific surface area, multi-level pore structure and abundant surface oxygen-containing functional groups (such as carboxyl (-COOH) and carbonyl (C=O)), can efficiently remove Cr through physical adsorption and chemical complexation. MnFe2O4 has a bimetallic active center, and its Fe / Mn redox pair can significantly improve the electron transfer efficiency. Therefore, the MnFe2O4 / biochar composite material (MFC) can be used to repair chromium pollution through the synergistic effect of redox and adsorption, thereby achieving a two-way improvement in the adsorption capacity and reduction removal efficiency of chromium. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Characterization of the crystal structure, magnetic properties and functional groups of MFC composite materials; Figure 2 This is the performance spectrum of MFC composite material for removing Cr(VI); Figure 3 It is the adsorption kinetic diagram of Cr(VI) by the MFC composite material; Figure 4 It is the trend diagram of the influence of the manganese ferrite content on the chromium removal efficiency of MFC; Figure 5 It is the removal effect diagrams of MFC5 under darkness and light respectively at different chromium concentrations; Figure 6 It is the comparison diagram of the Cr(VI) removal efficiency between the first and subsequent cycles in the MFC cyclic experiment; Figure 7 It is the concentration-dependent curve of the pseudo-first-order kinetic model fitting the Cr(VI) adsorption process. Specific implementation manners

[0018] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0019] Term explanation: "Biochar" is prepared by pyrolyzing specified biomass (such as straws, wood chips, fallen leaves, etc.) under limited conditions, that is, pyrolysis carbon.

[0020] "Chromium-containing water body" includes actual wastewater and chromium solutions simulated in the laboratory.

[0021] This application provides a preparation method of a magnetic MnFe2O4 / biochar composite material, including the following steps: S1. Obtain biochar; S2. Using biochar, manganese chloride and iron chloride as raw materials, carry out a hydrothermal reaction under alkaline conditions, and then wash and dry to obtain the magnetic MnFe2O4 / biochar composite material.

[0022] This application prepares a magnetic MnFe2O4 / biochar composite material (MFC) through a pyrolysis-coprecipitation coupling process. Biochar, with its high specific surface area, hierarchical pore structure and abundant surface oxygen-containing functional groups (such as carboxyl (-COOH) and carbonyl (C=O)), can efficiently remove Cr through physical adsorption and chemical complexation. And MnFe2O4 has a bimetallic active center, and its Fe / Mn redox pair can significantly improve the electron transfer efficiency. Therefore, the MnFe2O4 / biochar composite material (MFC) can carry out chromium pollution remediation through the synergistic effect of redox and adsorption, realizing a two-way improvement in the chromium adsorption capacity and reduction removal efficiency.

[0023] Among them, the process of obtaining biochar in step S1 is as follows: Take the fallen leaves of Platanus acerifolia on the campus of Huazhong Agricultural University in Wuhan, Hubei Province, grind them to obtain biomass powder, place the biomass powder in a tubular furnace, and heat it to 300 °C at a heating rate of 5 °C·min -1 in an argon atmosphere and hold for 2 hours. After the sample is cooled to room temperature, collect it to obtain biochar (PBC).

[0024] In some embodiments, the pH value of the alkaline condition is 10 - 12; the temperature of the hydrothermal reaction is 80 - 90 °C, and the time of the hydrothermal reaction is 1 - 2 h.

[0025] In step S2, the steps of the hydrothermal reaction under alkaline conditions are as follows: Place biochar, manganese chloride, and iron chloride in a reaction vessel, add water and stir to dissolve, then heat the constant temperature water bath to 80 - 90 °C, and then adjust the alkali to 10 - 13 and react for 1 - 2 h; among them, the molar ratio of manganese chloride to iron chloride is 1:2.

[0026] In this embodiment, the pH value of the alkaline condition includes but is not limited to 10, 11, 12, and the alkaline condition can be achieved by adding a strong base. The strong base includes but is not limited to sodium hydroxide and / or potassium hydroxide. The pH value of the alkaline condition being 10 - 12 ensures that Fe 3+ and Mn 2+ are fully hydrolyzed in an alkaline environment and form stable MnFe2O4; the temperature of the hydrothermal reaction being 80 - 90 °C is beneficial to balancing the reaction rate and energy consumption, promoting the uniform nucleation and growth of nanocrystals, and avoiding the collapse of the biochar structure caused by high temperature.

[0027] In some embodiments, the mass ratio of iron chloride to biochar is 1:(4 - 10); manganese chloride includes one or more of anhydrous manganese chloride and hydrated manganese chloride; iron chloride includes one or more of anhydrous iron chloride and hydrated iron chloride.

[0028] In this embodiment, the mass ratio of iron chloride to biochar is between 1:(4 - 10), such as 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10. The corresponding magnetic MnFe2O4 / biochar composite material (MFC) is denoted as MFC4, MFC5, MFC6, MFC7, MFC8, MFC9, MFC 10 ; preferably, the mass ratio of iron chloride to biochar is 1:5, corresponding to MFC5; manganese chloride includes one or more of MnCl2 and MnCl2·4H2O, and iron chloride includes one or more of FeCl3 and FeCl3·6H2O.

[0029] In this embodiment, the mass ratio of ferric chloride to biochar is between 1:(4 - 10). As the content of manganese ferrite in the magnetic biochar increases, the chromium removal efficiency first increases and then decreases. In this embodiment, hydrated chloride has better solubility than anhydrous chloride, which can promote the uniform dispersion of metal ions and improve the uniformity of the composite material.

[0030] This application provides a magnetic MnFe₂O₄ / biochar composite material.

[0031] The magnetic MnFe₂O₄ / biochar composite material obtained in this application has a synergistic mechanism for reducing and adsorbing chromium ions. As a chromium remover, it can achieve the removal of chromium pollutants. At the same time, the magnetic MnFe₂O₄ / biochar composite material of this application has magnetic separation characteristics, which can realize the recycling of the chromium remover and solve the problem that it is difficult to separate biochar from chromium pollutants in the prior art.

[0032] This application provides an application of a magnetic MnFe₂O₄ / biochar composite material in removing chromium pollutants from chromium-containing water bodies.

[0033] In some embodiments, the chromium pollutants include hexavalent chromium and / or trivalent chromium.

[0034] In this embodiment, biochar has an adsorption effect on both hexavalent chromium and trivalent chromium; biochar has a reduction effect on hexavalent chromium; magnetic MnFe₂O₄ has no adsorption effect on both hexavalent chromium and trivalent chromium; magnetic MnFe₂O₄ has a reduction effect on hexavalent chromium.

[0035] In some embodiments, the removal process includes the following steps: adding the magnetic MnFe₂O₄ / biochar composite material to the chromium-containing water body for chromium removal treatment, and then using an external magnetic field for solid-liquid separation to recover the magnetic MnFe₂O₄ / biochar composite material.

[0036] In this embodiment, the chromium removal treatment includes adsorption and / or photocatalysis.

[0037] In some embodiments, the chromium removal process is carried out under dark or visible light conditions.

[0038] Preferably, the chromium removal process is carried out under visible light conditions, and the light source of the visible light uses a 330 W xenon lamp (PLS-SXE300, Beijing Perfectlight) with a 400 nm cut-off filter (light intensity 3 W / m 2 )

[0039] In this embodiment, visible light excitation can further enhance the electron transport in the magnetic MnFe₂O₄ / biochar composite material to synergistically improve the Cr reduction removal efficiency and adsorption capacity.

[0040] In some embodiments, the pH value of the chromium-containing water body is 1-6.

[0041] In this embodiment, the acidic condition is favorable for the electrostatic adsorption of anionic Cr(VI). The synergistic effect of this electrostatic interaction and other removal mechanisms significantly improves the overall removal efficiency of Cr(VI) under acidic conditions.

[0042] In some embodiments, the chromium concentration of the chromium-containing water body is (20 ppm - 50 ppm), and the dosage of the magnetic MnFe2O4 / biochar composite is 0.1 - 5 g / L.

[0043] In some embodiments, the concentration of the chromium solution is 20 ppm - 50 ppm. For example, 20 ppm, 30 ppm, 40 ppm, 50 ppm.

[0044] The following further illustrates this solution through specific embodiments.

[0045] Source of raw materials: Manganese chloride tetrahydrate (MnCl2·4H2O), ferric chloride hexahydrate (FeCl3·6H2O), sodium hydroxide (NaOH), nitric acid (HNO3), potassium dichromate (K2Cr2O7) and acetone used in the experiments were all purchased from Sinopharm Chemical Reagent Co., Ltd.; Superoxide dismutase (SOD) was provided by Yuanye (Shanghai) Biotechnology Co., Ltd., and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Cool Chemical Co., Ltd.; All reagents were used directly without further purification, and deionized water was used throughout the experimental process; The biomass raw material was collected from the fallen leaves of Platanus orientalis on the campus of Huazhong Agricultural University in Wuhan, Hubei Province.

[0046] Example 1 A magnetic MnFe2O4 / biochar composite, and its preparation method includes the following steps: S1. Take 1 g of Platanus orientalis leaf biomass powder, place it in a tubular furnace, and heat it to 300 °C at a heating rate of 5 °C·min -1 and hold for 2 hours in an argon atmosphere. After the sample is cooled to room temperature, collect it to obtain biochar, labeled as PBC; S2. Place 0.5 g of PBC, MnCl2·4H2O and FeCl3·6H2O with a molar ratio of 1:2 in a 100 mL conical flask, add 50 mL of deionized water and stir vigorously to dissolve. After the conical flask is placed in a constant temperature water bath and heated to 80 °C, adjust the pH to 11 with NaOH solution and maintain this temperature for reaction for 1 hour. After the reaction ends, cool the mixture, wash it with deionized water and dry it under vacuum to obtain the magnetic MnFe2O4 / biochar composite, labeled as MFC5; among them, the mass ratio of FeCl3·6H2O to biochar is 1:5.

[0047] The product was identified. Among them, the X-ray diffraction pattern of MFC is as shown in Figure 1 (b), and the vibrating sample magnetometer test curve is as shown in Figure 1 (c). The surface functional groups of PBC and MFC5 were analyzed by Fourier transform infrared spectroscopy (FT-IR) ( Figure 1 d). The results showed that there was a significant broad absorption band in the range of 3200 - 3500 cm -1 , which was attributed to the stretching vibration of the hydroxyl group (-OH). It should be noted that compared with PBC, the signal intensity of the -OH group in the MFC composite material was significantly enhanced, indicating an increase in the degree of hydroxylation on the material surface during the modification process. In the spectral region near 1600 cm -1 , obvious vibration characteristics were shown, including the characteristic stretching vibrations of -C=O and aromatic -C=C, confirming the retention and modification of the carbon skeleton structure. In addition, the new vibration mode that appeared in the low wavenumber region (about 500 cm -1 ) provided direct evidence for the formation of metal-oxygen bonds, confirming that manganese ferrite nanoparticles had been successfully loaded onto the biochar matrix.

[0048] Examples 2 - 5 A magnetic MnFe2O4 / biochar composite material, other contents are the same as those in Example 1, the difference is that the mass ratio of FeCl3·6H2O to biochar is 1:4, 1:6, 1:8, 1:10 in sequence; the corresponding obtained magnetic MnFe2O4 / biochar composite materials are denoted as MFC4, MFC6, MFC8, MFC 10 .

[0049] Examples 6 - 10 An application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies, the steps include: adding 0.02 g of the magnetic MnFe2O4 / biochar composite material prepared in Example 1 to 50 mL of chromium-containing water body (hexavalent chromium solution) with an initial concentration of 50 mg·L -1 , performing chromium removal treatment, and then using an external magnetic field for solid-liquid separation to recover the magnetic MnFe2O4 / biochar composite material; among them, the chromium removal process is carried out under visible light conditions, the light source of visible light uses a 330 W xenon lamp (PLS-SXE300, Beijing Perfectlight) with a 400 nm cut-off filter (light intensity 3 W / m 2 ), the reaction system is equipped with a circulating water device to maintain a constant temperature and eliminate the influence of light-induced temperature rise, and about 1 mL of the reaction solution is extracted with a 0.22 μm nylon syringe filter at predetermined time intervals for analysis. The pH values of the chromium-containing water bodies are 1, 2, 3, 4, 5 in sequence, the chromium concentration of the chromium-containing water body is 50 ppm, and the duration of the chromium removal treatment is 60 min.

[0050] Example 11 Application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies. Other contents are the same as those in Example 7, except that the chromium removal process is carried out under dark conditions, and the reaction vessel is tightly wrapped with a wooden box to avoid light during the reaction.

[0051] Examples 12 - 15 Application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies. Other contents are the same as those in Example 7, except that the magnetic MnFe2O4 / biochar composite materials used correspond to MFC4, MFC6, MFC8, and MFC prepared in Examples 2 - 5 in sequence. 10 。

[0052] Examples 16 - 19 Application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies. Other contents are the same as those in Example 11, except that the magnetic MnFe2O4 / biochar composite materials used correspond to MFC4, MFC6, MFC8, and MFC prepared in Examples 2 - 5 in sequence. 10 。

[0053] Examples 20 - 22 Application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies. Other contents are the same as those in Example 7, except that the chromium concentrations of the chromium-containing water bodies are 20 ppm and 40 ppm in sequence.

[0054] Examples 23 - 24 Application of a magnetic MnFe2O4 / biochar composite material in removing chromium pollutants from chromium-containing water bodies. Other contents are the same as those in Example 11, except that the chromium concentrations of the chromium-containing water bodies are 20 ppm and 40 ppm in sequence.

[0055] Comparative Example 1 A magnetic MnFe2O4 material. Other contents are the same as those in Example 1, except that PBC is not added, and the obtained magnetic MnFe2O4 material is denoted as MF.

[0056] Comparative Examples 2 - 20 Application of a magnetic MnFe2O4 material in removing chromium pollutants from chromium-containing water bodies. Other contents correspond to Examples 6 - 14 respectively, except that the magnetic MnFe2O4 / biochar composite material is replaced with the magnetic MnFe2O4 material prepared in Comparative Example 1.

[0057] Testing and Evaluation Taking biochar (PBC) as a control, the chromium removal effects of different materials were evaluated.

[0058] Figure 1 (a) X-ray diffraction pattern of MFC, Figure 1 (b) vibrating sample magnetometry curve, and Figure 1 (c) Fourier transform infrared spectrum. X-ray diffraction (XRD) analysis indicated that MFC had a well-defined crystal structure, and its main diffraction peaks were in complete agreement with the MnFe2O4 standard card (PDF#74-2403). The characteristic broad peak observed in the range of 20° to 40° was consistent with the typical amorphous carbon structure of pristine biochar (PBC), confirming the successful composite of the two components. Vibrating sample magnetometer (VSM) tests showed that the MFC composite had a saturation magnetization of approximately 6 emu / g, indicating its strong magnetic response ability and enabling efficient magnetic separation in practical applications. Fourier transform infrared spectroscopy (FT-IR) was used to analyze the surface functional groups of pristine biochar (PBC) and the magnetic composite (MFC). A significant broad absorption band was observed in the range of 3200 - 3500 cm -1 , which was attributed to the stretching vibration of hydroxyl groups (-OH). Compared with PBC, the intensity of the -OH peak in the MFC composite was significantly enhanced, indicating an increased degree of hydroxylation on the material surface during the modification process. In the spectral region near 1600 cm -1 , obvious vibration characteristics were shown, including the characteristic stretching vibrations of -C=O bonds and aromatic -C=C bonds, confirming the retention and modification of the carbonaceous structure. In addition, the new vibration mode that appeared in the low wavenumber region (about 500 cm -1 ) provided direct evidence for the formation of metal-oxygen bonds, confirming that manganese ferrite nanoparticles had been successfully loaded onto the biochar matrix.

[0059] Figure 2 (a) shows the removal effects of total chromium and Cr(VI) by PBC, MF, and MFC in 50 mL of 50 mg / L Cr(VI) solution; as Figure 2As shown in (a), there are significant differences in the removal capabilities of PBC, MF, and MFC5 for total chromium (Cr(VI)+Cr(III)) and Cr(VI) under two conditions. It is worth noting that during the experiment, hexavalent chromium was used as the research object. The composite material reduced hexavalent chromium to trivalent chromium and simultaneously adsorbed and removed both hexavalent chromium and trivalent chromium. Therefore, in the description, hexavalent chromium and trivalent chromium are collectively referred to as total chromium, and the valence transformation of chromium is studied to quantify the reduction and adsorption capabilities of the composite material. Generally speaking, the removal performance of MFC for total chromium and Cr(VI) is better than that of pure-phase PBC and MF. Within 60 minutes of treatment time, the removal amounts of PBC and MF for total chromium and hexavalent chromium under both experimental conditions are lower than 20 mg / g, showing relatively limited removal efficiency. It is worth noting that the chromium removal ability of PBC under visible light irradiation is improved compared with the dark condition, which is attributed to the photoinduced generation effect of oxygen-containing functional groups (OFGs) and active species on the biochar surface. In contrast, MF shows similar removal performance in light and dark environments, indicating that its chromium removal mechanism is mainly dominated by the adsorption process and metal redox reaction, rather than the photochemical process.

[0060] As the content of manganese ferrite in the magnetic biochar increases, the chromium removal efficiency shows a trend of first increasing and then decreasing ( Figure 4 ). Figure 5 Figure shows the removal effects of MFC5 under dark and light conditions at different chromium concentrations. Among them, the MFC5 sample shows the best performance, and a Cr(VI) removal capacity of 80 mg / g can be achieved within 60 minutes under visible light irradiation. Compared with PBC and MF, the removal performance of MFC5 for Cr(VI) and total chromium has been significantly improved, which is mainly due to the synergistic effect of the adsorption ability of biochar and the redox activity of manganese ferrite, as well as the possible enhanced photochemical process under visible light irradiation. It is worth noting that the total chromium removal efficiency is always higher than that of Cr(VI), indicating that some of the removed Cr(VI) is converted into Cr(III), and there is a transformation process of chromium species. The synergistic effect of this adsorption-reduction mechanism and the photoinduced effect together constitute a complex and efficient chromium removal system, highlighting the application potential of magnetic biochar composites in environmental remediation.

[0061] The examples and comparative examples of this application investigated the influence of different pH values on the chromium removal efficiency. Figure 2 Figure (b) shows the relationship between chromium removal performance and Zeta potential change under different pH conditions; the results show that this process has significant pH dependence, and the maximum removal rate is obtained at pH = 2. The zeta potential test further reveals the pH response characteristics of the surface charge of the material: the material surface is positively charged at pH = 2, which is beneficial to the electrostatic adsorption of anionic Cr(VI). The synergistic effect of this electrostatic interaction and other removal mechanisms significantly improves the overall Cr(VI) removal efficiency under acidic conditions.

[0062] Figure 2 (c), Figure 2 (d) are the morphological distribution ratios of Cr(VI) removal by MFC under visible light and dark conditions respectively; by comparing the removal efficiencies of Cr(VI) and total chromium under visible light and dark conditions, significant differences are found between them: under visible light irradiation, MFC5 can achieve a total chromium removal rate of 72%, and the residual solution contains 12% Cr(VI) and 16% Cr(III) ( Figure 2 c-d); while under dark conditions, the total chromium removal rate is 55%, and Cr(VI) and Cr(III) in the residual solution account for 36% and 9% respectively. This indicates that visible light can not only significantly improve the overall chromium removal ability of MFC5, but also promote the conversion efficiency of Cr(VI) to Cr(III). In addition, the removed chromium species (including Cr(VI) and Cr(III)) are mainly adsorbed on the surface of MFC5. The significant increase in the proportion of Cr(III) under visible light conditions indicates that the reduction activity mediated by photogenerated electrons and reactive oxygen species is enhanced; while the relatively high concentration of residual Cr(VI) under dark conditions confirms that the reduction efficiency is low without photoactivation, highlighting the key role of the photochemical process in the removal mechanism.

[0063] Figure 2 (e) shows the comparison of the adsorption and reduction efficiencies of MFC5 for Cr(VI) under visible light and dark conditions. As Figure 2 (e) shows, the adsorption efficiency of Cr(VI) ions is about 27%, indicating that visible light irradiation has no significant effect on the adsorption and removal of hexavalent chromium. Through the quantitative analysis of total chromium removal, it is found that under visible light conditions, 44.48% of Cr(III) is removed by adsorption after reduction, while only 27.96% of Cr(III) is adsorbed under dark conditions. When evaluating the reduction efficiency of Cr(VI) (while considering the residual Cr(III) generated by the redox conversion of Cr(VI) in the solution), significant differences are shown between the visible light and dark environments. This difference in adsorption efficiency is mainly due to the enhancement of the redox activity of Cr(VI) by visible light, resulting in a significant increase in the removal amount of Cr(III) compared with the dark conditions.

[0064] As Figure 2 (e) shows, 60.48% of Cr(VI) is reduced to Cr(III) under visible light irradiation conditions, while the valence conversion rate is only 36.96% under dark conditions. This significant difference indicates that visible light can greatly enhance the redox ability of MFC5, thus promoting the removal of Cr(VI). The research results highlight the important role of visible light in driving the reduction process.

[0065] As Figure 6As shown, the material exhibited the best performance in the first experimental cycle, with a Cr(VI) removal efficiency of approximately 50%. The subsequent performance decline was mainly due to the occupation of the active sites on the surface of MFC5 by chromium species during the reaction. Comparative analysis showed that the MFC5 samples reacting under visible light irradiation exhibited a more significant decrease in specific surface area and magnetic property attenuation compared to the samples under dark conditions, which was attributed to the more excellent photocatalytic performance under visible light leading to the more sufficient utilization and occupation of active sites. It should be noted that despite the reaction process, the magnetic property of MFC5 still maintained sufficient stability, enabling the effective recovery of the material through magnetic separation.

[0066] Figure 3 (a) shows the relationship between the chromium removal rate of MFC and time; Figure 3 (b) is the fitting curve of the pseudo-first-order reaction kinetic model. This application conducts kinetic analysis on the adsorption process. The kinetic analysis shows that the chromium removal rate decreases with time, and it always exhibits better performance under visible light irradiation ( Figure 3 a). The excellent fitting result (R 2 > 0.96) of the pseudo-first-order kinetic model confirms that this adsorption process conforms to the concentration-dependent characteristic, that is, as the Cr(VI) concentration decreases, the adsorption rate on the surface of the composite material gradually decreases ( Figure 7 ). At the same time, the Langmuir model shows that Cr(VI) is mainly adsorbed in a monolayer on the material surface, and chemical adsorption plays a dominant role.

[0067] In this study, a magnetic manganese ferrite / biochar composite material (MFC) was successfully prepared by pyrolysis-coprecipitation method for the removal of Cr(VI) under visible light irradiation. Compared with the dark condition, visible light irradiation significantly improved the Cr(VI) removal efficiency. This material has the characteristics of magnetic separation and recovery, providing an economical and sustainable solution for the low-toxicity treatment of Cr(VI) under visible light conditions.

[0068] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A preparation method of a magnetic MnFe2O4 / biochar composite material, characterized in that, It includes the following steps: Obtain biochar; Using the biochar, manganese chloride and iron chloride as raw materials, carry out hydrothermal reaction under alkaline conditions, and then wash and dry to obtain the magnetic MnFe2O4 / biochar composite material.

2. The preparation method of the magnetic MnFe2O4 / biochar composite material according to claim 1, wherein The pH value of the alkaline condition is 10 - 12; the temperature of the hydrothermal reaction is 80 - 90 °C, and the time of the hydrothermal reaction is 1 - 2 h.

3. The preparation method of the magnetic MnFe2O4 / biochar composite material according to claim 1, wherein, The mass ratio of the iron chloride to the biochar is 1:(4 - 10); the manganese chloride includes one or several of anhydrous manganese chloride and hydrated manganese chloride; the iron chloride includes one or several of anhydrous iron chloride and hydrated iron chloride.

4. A magnetic MnFe2O4 / biochar composite material obtained by the preparation method according to any one of claims 1 - 3.

5. Application of the magnetic MnFe2O4 / biochar composite material according to claim 4 in removing chromium pollutants in chromium-containing water bodies.

6. The application according to claim 5, wherein The chromium pollutants include hexavalent chromium and / or trivalent chromium.

7. The application according to claim 5 or 6, characterized in that It includes the following steps: Add the magnetic MnFe2O4 / biochar composite material into the chromium-containing water body, carry out chromium removal treatment, and then use an external magnetic field for solid-liquid separation to recover the magnetic MnFe2O4 / biochar composite material.

8. The application according to claim 7, characterized in that The chromium removal process is carried out under visible light conditions.

9. The application according to claim 7, wherein The pH value of the chromium-containing water body is 1 - 6.

10. The application according to claim 7, wherein The chromium concentration of the chromium-containing water body is (20 ppm - 50 ppm), and the dosage of the magnetic MnFe2O4 / biochar composite material is 0.1 - 5 g / L.

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