A modified biochar and a preparation method and application thereof

Abstract

The application relates to the technical field of soil remediation, and discloses modified biochar, the mass ratio of the components of which is Fe:Mn:moso bamboo biomass=1:3:30. A preparation method of the modified biochar comprises the following steps: step 1, preparing moso bamboo raw materials, drying, crushing and screening the moso bamboo required by the experiment through a 100-mesh sieve for use; step 2, weighing a certain mass of ferric chloride (FeCl3) and potassium permanganate (KMnO4) and dissolving them in water, adding a certain mass of the moso bamboo raw materials, and preparing modified biochar through the steps of stirring, suction filtration, drying and limited oxygen pyrolysis. The preparation and application of the modified biochar (FM1:3-BC) can effectively reduce the content of arsenic in soil, remediate arsenic-polluted soil and reduce the phytotoxicity of arsenic. Meanwhile, the arsenic-polluted soil remediation method can enhance the content of soil environmental factors such as organic matter, cation exchange capacity, ammonia nitrogen and available phosphorus, thereby enhancing enzyme activity and improving soil fertility.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to a modified biochar, its preparation method, and its application. Background Technology

[0002] Heavy metal pollution in agricultural soils poses health risks to humans through the food chain. Given the non-degradability and persistence of these heavy metals, and the fact that arsenic is a carcinogenic metal, altering or fixing its forms has become a key research focus. The main forms of arsenic in soil are As(III) (arsenite) and As(V) (arsenate), such as the neutral state (H3AsO3, H3AsO4) and anionic state (H2AsO) commonly found in the environment. 3- H2AsO 4- ,HAsO4 2- As(III) is relatively more toxic. Long-term exposure to high doses or chronic low doses of As can pose serious health risks to approximately 200 million people worldwide through skin contact, inhalation, and the food chain.

[0003] In numerous soil remediation studies, in-situ remediation stands out for its high efficiency in the safe utilization of farmland soil contaminated with trace elements; however, this also means it is too expensive and requires further improvement. Ferns have been proven to have good remediation effects on arsenic-contaminated soil, but face obstacles such as long cycles and large scale. Meanwhile, some inorganic materials such as clay minerals, lime, phosphates, and activated carbon are also widely used, but these all face secondary pollution or low stability. This has led researchers to gradually shift their focus to readily available and low-cost biochar. Existing studies have shown that adding biochar prepared from waste biomass to heavy metal-contaminated soil can have a win-win effect, both utilizing waste biomass resources and stabilizing heavy metals in the soil. However, for As, raw biochar promotes As migration by increasing the content of soluble organic carbon in the soil. It is worth noting that the speciation of As in soil is essentially related to the behavior of iron oxides. Therefore, to improve the As fixation capacity of biochar, Fe-modified biochar has gradually become a research hotspot. At the same time, Mn, as a strong oxidizing metal element, also exhibits a certain heavy metal fixation capacity; it can increase the oxidation rate of As and enhance adsorption. It is worth noting that past research has largely focused on the remediation of heavy metals in acidic and alkaline soils using single-metal modified biochar. The effects of different proportions of iron and manganese combined with oxides on the removal of As are still unclear. The types of iron salts have different effects on the speciation and distribution of Fe and Mn in the soil environment. The remediation mechanism of Mn addition on As in soil environments using iron-modified biochar is lacking in current research, especially in neutral soils co-contaminated with multiple heavy metals. Summary of the Invention

[0004] To address the shortcomings of existing arsenic-contaminated soil remediation technologies, this invention prepared and characterized bamboo-derived biochar loaded with a certain proportion of iron-based and manganese-based materials. The remediation performance of this biochar in arsenic-contaminated soil was evaluated by studying the changes in various environmental factors, arsenic speciation, and leaching amounts in the soil after the addition of different materials. A method for preparing and applying modified biochar for effectively remediating arsenic contamination in neutral soil was proposed. The optimal ratio and mechanism of Fe-Mn for arsenic removal were also explored. This invention provides a modified biochar, its preparation method, and its application.

[0005] This invention is achieved using the following technical solution:

[0006] A modified biochar has the following composition: Fe:Mn:bamboo biomass = 1:3:30.

[0007] A method for preparing modified biochar includes the following steps:

[0008] Step 1: Prepare raw bamboo materials. Air dry and crush the bamboo required for the experiment, and pass it through a 100-mesh sieve for later use.

[0009] Step 2: Weigh out a certain amount of ferric chloride (FeCl3) and potassium permanganate (KMnO4), dissolve them in water, add a certain amount of bamboo raw material, and prepare modified biochar through stirring, filtration, drying and oxygen-limited pyrolysis.

[0010] Preferably, the mass ratio of FeCl3 to KMnO4 is 1:2.97.

[0011] Preferably, the mass ratio of the bamboo raw material to FeCl3 is 10.34:1.

[0012] Specifically, step 2 includes

[0013] The solution was stirred with a glass rod and then sonicated for 1 hour to make it uniformly distributed. After vacuum filtration, it was dried at 60°C to make the mass ratio Fe:Mn:bamboo powder = 1:3:30, thus obtaining FM1:3 (mass ratio of metallic iron to manganese is 1:3) modified biomass.

[0014] After placing the FM1:3 biomass into a crucible, it is placed in a tube furnace for oxygen-limited pyrolysis at 500°C. The nitrogen flow rate is controlled by a gas flow rate device. After pyrolysis for 2 hours, FM1:3 modified biochar is obtained.

[0015] An application of modified biochar for the effective remediation of arsenic contamination in neutral soil, comprising the following steps:

[0016] Modified biochar was added to arsenic-contaminated soil. The modified biochar was thoroughly mixed with the arsenic-contaminated soil and improved at room temperature for 30 days until the moisture content reached the maximum field capacity of the soil. After incubation, the soil was air-dried, crushed, and sieved. After 30 days of improvement, the material was recovered by magnetic separation for subsequent experiments.

[0017] Preferably, the biochar to soil mass ratio is 1:100.

[0018] The FM1:3 modified biochar prepared by this invention contains the most oxygen-containing functional groups and also has the largest specific surface area, providing more adsorption sites for cation adsorption and promoting the transformation of arsenic from a mobile state to a residual state. In addition to electrostatic attraction, the synergistic mechanism of FM1:3 modified biochar on arsenic is mainly redox and complexation. Mn oxidizes As(III) to the more inert As(V). In this reaction process, Mn(IV) is reduced to Mn(III) and Mn(II), that is, pentavalent manganese is reduced to trivalent manganese and divalent manganese, promoting the formation of Fe-As complexes between Fe(III) and As. At the same time, As is fixed due to the formation of ternary surface complexes.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] The modified biochar (FM1:3-BC) proposed in this invention can effectively reduce soil arsenic content, remediate arsenic-contaminated soil, and alleviate the phytotoxicity of arsenic. Simultaneously, the arsenic-contaminated soil remediation method proposed in this invention can enhance enzyme activity and improve soil fertility by increasing the content of soil environmental factors such as organic matter, cation exchange capacity, ammonia nitrogen, and available phosphorus.

[0021] The arsenic-contaminated soil remediation method proposed in this invention enhances the adsorption capacity of As by utilizing the functional groups in modified biochar to provide sites for the transformation and adsorption of As, and further increases the electrostatic attraction of soil colloids to As. The remediation capacity of arsenic-contaminated soil is higher than that of other modified biochars. Attached Figure Description

[0022] Figure 1 This is a flowchart of the biochar preparation process;

[0023] Figure 2 Morphological characteristics of modified biochar;

[0024] Figure 3 FTIR (a) and XRD (b) analyses of modified biochar;

[0025] Figure 4 Changes in the contents of pH(a), SOM(b), CEC(c) and amorphous Fe(d) in the soil after application of modified biochar;

[0026] Figure 5 Changes in As speciation (a), TCLP, plant-available As, and RAC risk (b) in soil after application of modified biochar;

[0027] Figure 6 Correlation analysis of various environmental factors in soil after application of modified biochar;

[0028] Figure 7 XPS spectra of As in soil before and after FM1:3-BC remediation;

[0029] Figure 8 Changes in the content of catalase (a), neutral phosphatase (b), urease (c) and sucrase (d) in soil after application of modified biochar;

[0030] Figure 9 Multivariate relationship between soil environmental factors and enzyme activity after biochar application. Detailed Implementation

[0031] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0032] Example 1:

[0033] Soil samples from an abandoned tailings dam in Anhui Province, China, were collected in this study. After mining operations ceased in the area, chemical processing was carried out in the remaining pits. Villages located 400 meters downstream were used for vegetable cultivation. Soil particle size was determined using a laser particle size analyzer (LS13320, Beckman Coulter, USA), and elemental composition was determined using X-ray fluorescence spectrometry (ZSX primus, Rigaku Corporation, Japan). The ascorbic acid (As) content in the soil and vegetables was determined using HCl-HF-H2O2 digestion, and the accuracy of this process was controlled by parallel analysis with soil standard material (GSS-4). The mineral crystal form of the prepared composite biochar was tested using X-ray diffraction (XRD) (SmartLab 9kW, Rigaku Industrial Corporation, Japan). The surface morphology of the modified biochar at different scales was observed using field emission scanning electron microscopy combined with energy-dispersive spectroscopy (SEM-EDS) (Regulus 8230, Hitachi, Japan), and the iron bonds on the biochar were identified. Specific surface area (SSA) was also measured.

[0034] Reference Figure 1The biochar preparation flowchart shown describes the process of air-drying and pulverizing the bamboo needed for the experiment, then sieving it through a 100-mesh sieve. 10g of bamboo biomass was immersed in FeCl3 and KMnO4 solutions of different concentrations, ultrasonically treated for 1 hour to ensure uniform distribution, vacuum filtered, and dried at 60℃. The resulting biomass was prepared with mass ratios of Fe:Mn:bamboo powder = 1:1:30 (FM1:1 biomass), Fe:Mn:bamboo powder = 1:3:30 (FM1:3 biomass), and Fe:Mn:bamboo powder = 3:1:30 (FM3:1 biomass). Additionally, a certain amount of Fe2O3 was mixed with bamboo powder at a mass ratio of Fe:bamboo powder = 1:30, and repeatedly stirred to obtain Fe composite biomass. The four types of biomass were placed in a crucible and pyrolyzed in a tube furnace at 500℃ under limited oxygen conditions. The nitrogen flow rate was controlled by a gas flow rate device. After pyrolysis for 2 hours, FM1:3 modified biochar was obtained.

[0035] Moso bamboo biomass is a powder sample obtained by chopping and sieving bamboo. These raw materials of moso bamboo are called biomass, and the product after combustion is called biochar.

[0036] 500g of each of the following soil types were weighed: control soil (soil without any added materials), original BC soil (soil without added metals but with added bamboo biochar), Fe-BC soil (soil with added iron-modified bamboo biochar), FM1:1-BC soil (soil with added iron-to-manganese modified bamboo biochar at a mass ratio of 1:1), FM1:3-BC soil (soil with added iron-to-manganese modified bamboo biochar at a mass ratio of 1:3), and FM3:1-BC soil (soil with added iron-to-manganese modified bamboo biochar at a mass ratio of 3:1). Modified biochar was added to the arsenic-contaminated soil at a mass ratio of 1%. The modified biochar and arsenic-contaminated soil were thoroughly mixed, and the mixture was incubated at room temperature for 30 days until the soil moisture content reached its maximum field capacity. After incubation, the soil was air-dried, crushed, and sieved. After 30 days of incubation, the material was recovered by magnetic separation for subsequent experiments. Soil pH changes were analyzed. Soil cation exchange capacity (CEC) was determined according to NY / T 295-1995 standard, and soil organic matter (SOM) was determined using the potassium dichromate method. Available Fe in the soil was extracted using DTPA-CaCl2-TEA leachate. The As fraction was studied using the BCR sequential extraction procedure, and the recovery rate was within acceptable limits. Arsenic availability to plants was determined by (NH4)2SO4 extraction; specific experimental procedures are detailed in the supplementary materials. The leaching amount of As was assessed using the Total Toxicity Leaching Procedure (TCLP), a method previously reported in our studies. Catalase (CAT) activity was determined using potassium permanganate titration, and soil urease, sucrase, and neutral phosphatase were determined using a soil reagent kit purchased from Shanghai Electromechanical Engineering Co., Ltd.

[0037] The structural characteristics of modified biochar and the changes in soil environmental factors after biochar modification are shown below.

[0038] 1. Physicochemical properties of biochar

[0039] As shown in Table 1, compared with the original BC (soil without added metals but with added bamboo biochar), the zero-point charge (pHpzc) of the modified biochar was similar, while the carbon content was lower. The increase in EC value was due to the release of soluble K, Na, Mg, Ca, Si, and Fe from biochar ash and iron oxidation products. The specific surface area of ​​the modified biochar was significantly larger than that of BC. The adsorption volume of biochar was positively correlated with its specific surface area. Manganese oxides, on the one hand, have a high specific surface area, which can enhance the specific surface area of ​​biochar; on the other hand, manganese oxides have strong oxidizing properties, which can oxidize iron into nano-sized oxides, increasing the contact area between iron oxides and anions.

[0040] Table 1 Physicochemical properties of modified biochar

[0041]

[0042]

[0043] 2. Structural composition of biochar

[0044] Reference Figure 2 As can be seen from the SEM image, the original BC surface is relatively smooth. Figure 2 a) Some of the aggregated particles may be due to the hydrolysis of lignocellulose in the biomass feedstock during the hydrothermal process. Fe-BC is mixed with biomass mechanically, so uneven mixing may cause the Fe-BC surface particles to appear as irregularly connected spheres. Figure 2 c). The surface of FM-BC exhibits a strip-like structure, interspersed with numerous pores ( Figure 2 (b, c, d). EDS results showed that Fe and Mn were successfully loaded onto the biochar surface, and the increase in O content reflected the increase in the number of oxygen-containing functional groups.

[0045] Modified biochar differs significantly from original biochar in its functional groups; the corresponding functional groups of different biochars are shown in... Figure 3 a. BC's 3370cm -1 The peak at 1719 cm⁻¹ is related to the stretching vibration of OH. -1 and 1432cm -1 There is a broad asymmetric peak at 1603 cm⁻¹, attributed to the stretching vibration of C=O (including ketones, carboxylic esters, and acid anhydrides). -1 The peak at 1116 cm⁻¹ could be due to C=C stretching from the aromatic rings in lignin, or it could be due to new aromatic and carbonized substances produced during the dehydration and cyclization of carbohydrate rings during pyrolysis. -1 The strong bands that appear are characteristic of COC stretching in cellulose and hemicellulose. Figure 3 a). Notably, after the addition of the modified material, the OH peak in the modified biochar disappeared, and the C=O and COC peaks narrowed, shifted, or disappeared, confirming the breakage of C, O, and H bonds and their combination with other elements. Fe-O peaks were found in all the modified materials, attributed to the formation of iron oxides or Fe-O complexes, which was also confirmed by XRD. Figure 3 b). Compared with Fe-BC, the addition of Mn gradually enhanced the peak values ​​of functional groups in biochar, especially in the range of 600-1600 cm⁻¹. -1 The emergence of new peaks indicates that the manganese modification process successfully altered the surface properties of biochar, and the intensity of these bands was significantly enhanced by the coexistence of Mn oxides. Interestingly, the FM1:3-BC surface exhibited the highest number of increased functional groups, with a peak at 1440 cm⁻¹. -1 The peak is attributed to the formation of Mn-O, 1378 cm⁻¹. -1 The peak is attributed to the formation of Fe-O-Mn, 1250 cm⁻¹ -1 The peak is attributed to the formation of C-OH and is located at 605 cm⁻¹. -1 A peak value of Fe-OH was found at ( ). Figure 3 a) The surface possesses oxygen-containing functional groups such as hydroxyl and carboxyl groups, which facilitates the immobilization of heavy metals through complexation. The prepared raw biochar and modified biochar materials were characterized using XRD, such as... Figure 3 As shown in b, SiO2 was detected in BC as an initial feedstock for biomass. K, concentrated during the modification process... + Mg 2+ This is attributed to the alkaline salts present in biochar itself. In FM1:3-BC, Mn oxides are mostly present in the form of MnFe, which leads to a decrease in the strength of Mn-O single bonds. The formed Fe-Mn bonds can promote electron transfer between Fe and Mn.

[0046] 3. Changes in soil pH, SOM, CEC, and amorphous Fe caused by biochar

[0047] The pH changes in the improved soil after 30 days showed... Figure 4 The pH changes can be attributed to two reasons: first, the acidity or alkalinity of the added materials directly affects the soil pH (the pH of BC is 9.31, and the pH of modified BC is 6.81-8.24) (Table 1); second, Fe... 3+ It will hydrolyze and release H ions, thereby lowering the soil pH.

[0048] Figure 4b shows that the addition of several biochar materials can increase the organic matter content in the soil. This is attributed to (1) the high carbon content of biochar itself, which increases the total organic carbon in the soil and promotes the formation of soil humus; and (2) after adsorbing soil organic molecules, biochar promotes the polymerization of small organic molecules to form SOM through surface catalytic activity. The order of the ability of several biochars to increase SOM is FM1:3 > FM1:1-BC > FM3:1-BC > Fe-BC > BC. Figure 4 As shown in c, the CEC of the control soil was 15.41 mg / kg. The addition of BC increased the CEC by 3.58 mg / kg. This is because the functional groups on the surface of the original biochar were oxidized to form hydroxyl (-OH) and carbonyl (C=O) groups, which increased the CEC. Fe-modified biochar enhanced the ion exchange capacity. At the same addition amount, FM1:3-BC had a more significant effect on increasing CEC than Fe-BC. This is because FM1:3-BC not only contains more oxygen-containing functional groups, but also has the largest specific surface area, providing more adsorption sites for cation adsorption. Figure 4 The data in d show that the addition of BC has little effect on amorphous Fe, while the addition of Fe-modified biochar significantly increases the content of amorphous Fe. The iron oxides formed can adsorb / coprecipitate with As, thereby reducing the availability of As in the environment.

[0049] 4. Changes in soil As speciation caused by biochar

[0050] The As content in the original soil of the study area ranged from 142.5 mg / kg. -1 -165.0mg kg -1 The concentration is significantly higher than the Chinese Environmental Quality Standard (GB 15618-2018, As is 30mg / kg). -1 The soil quality also failed to meet the standards set by the U.S. Environmental Protection Agency (USEPA). Furthermore, the heavy metal content in local vegetables significantly exceeded the Chinese food safety limit for As (0.5 mg / kg) as specified in GB 2762-2017. -1 .

[0051] BCR stepwise extraction combined with TCLP revealed the speciation of As in contaminated soil. The sum of the extractable, reducible, and oxidizable forms in weakly acidic soil constitutes the mobile form (…). Figure 5a). Both the Chinese national standard for heavy metal leaching limits in solid waste (GB / T 5085.3-2007) and the US Environmental Protection Agency's Hazardous Materials Identification Standard (EPA) stipulate a risk value of 5 mg / L for As in the Total Chemical Supplemental Limit (TCLP). In farmland soil, the impact of arsenic on vegetables can be reflected by changes in plant-available arsenic, and the mobility of metals is closely related to the bioavailable content. Therefore, based on the bioavailable content, the Risk Assessment Code (RAC) risk assessment index was established, dividing the bioavailable content and total amount into five levels to determine the risk level. Figure 5 b). Typically, the first fraction (F1) exhibits the greatest instability and environmental mobility in the environment.

[0052] from Figure 5 As can be seen from Fig. 5a, the speciation of As in the CK soil is F4>F3>F2>F1, with residual state (F4) being the main component. This part is considered unusable and not easily released. The addition of BC to the soil enhances the mobile state of As, while the application of modified BC effectively reduces the mobile fraction of As, causing most of the mobile arsenic in the contaminated soil to be converted into residual state. Specifically, compared with CK, the addition of Fe-BC, FM1:1-BC, FM1:3-BC, and FM3:1-BC increased the residual fraction of As (F4) by 5.76%, 11.05%, 14.56%, and 8.27%, respectively (Fig. 5a). The plant-available As in the CK soil is 5.11%, and the RAC is low-risk (1.78%). Figure 5 (b) After treatment with iron-manganese modified materials, the extractable As in TCLP were all lower than those in CK, and the risk of RAC was reduced (Fig. 5b). Among them, FM1:3-BC showed the best effect on As fixation. This indicates that the four modified biochars are more effective than the original biochars in stabilizing As fractions and promoting the conversion of mobile As to residual As, and FM1:3-BC modified biochar is the most effective in stabilizing As fractions and reducing leaching toxicity.

[0053] It is well known that soil pH plays an important role in the activation and passivation of As. As usually exists in anionic form, including AsO4, within the pH range of 3-8. 3- ,HAsO4 2- and H2AsO4 - .from Figure 6The results show a positive correlation between plant-available As and pH in the soil. Compared to the control (CK), the addition of BC increased the soil pH, causing soil particles to carry more negative charges, generating electrostatic repulsion and releasing more As, thus converting some of the stable residual As in the soil into a mobile state. Furthermore, some scholars have pointed out that adding BC to the soil can enhance the content of dissolved organic carbon and DOC, promoting the formation of soluble complexes of As, which is one of the reasons why BC promotes the mobile state of As. Notably, the reduction in the As migration fraction due to the addition of Fe modifiers is attributed to the redox reaction of Fe. In this study, the soil treated with FM1:3-BC had the highest residual As content and the lowest TCLP toxicity leaching. This is because, compared to other biochars, the soil treated with FM1:3-BC had a lower pH and more positive charges on the soil colloids, leading to a greater amount of stable As fractions through electrostatic attraction. In addition, FM1:3-BC has the most Fe-O and Fe-OH functional groups, which can act as electron acceptors, causing strong complexation of As on the biochar surface through electrostatic and chemisorption. Previous studies have reported that As in soil, under non-flooded conditions, combines with and co-precipitates with iron oxides, thereby achieving As immobilization. Furthermore, Mn-O, due to its high specific surface area and oxidizing power, can oxidize As(III) to As(V). Moreover, the positively charged functional groups of biochar can react with negatively charged As ions (such as AsO4) 3- and HAsO3 2- These interactions form stable complexes, thereby reducing the bioavailability and migration rate of As.

[0054] 5. Mechanism of FM1:3-BC in remediating As contamination

[0055] Based on the above research, to elucidate the detailed mechanism of As removal by Fe-Mn materials, X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental valence state changes of FM1:3-BC before and after arsenic adsorption. The C1s binding energy (284.8 eV) was used as a reference peak for charge correction, referring to the carbon peak in the XPS spectrum. Here, C1s represents the photoelectron energy measured when the 1s orbital electron in a carbon atom is excited. The full-scan spectrum shows that the original biochar contained C, O, Fe, and Mn. After adsorption, the photoelectron peak of the As 3d binding energy appeared, which is attributed to the As being adsorbed on the surface of the modified biochar. Figure 7 a). Based on its content distribution, 49.85 eV and 44.91 eV correspond to As(V)-O, and 43.56 eV corresponds to As(3)( Figure 7b). The peak positions in the C1s spectrum before and after adsorption did not change significantly. The peaks at 284.80 eV, 286.01 eV, and 288.48 eV were correlated with CH, C-OH, and O=CN in the amide, respectively. The content of C-OH decreased by 7.81% after adsorption. Figure 7 c) indicates that C-OH participated in the adsorption of As, which is related to the nucleophilic attack of the alcohol group on the positively charged As atom. In the O 1s spectrum, the peak at 531.52 eV is related to C=O / OC=O. After the reaction, the peak shifts to a lower position, and the proportion of the peak value of the metal oxide increases. Figure 7 d) This is because Mn-O participates in As oxidation, while Fe-O adsorbs As through complexation, and the carboxyl group can also participate in As complexation. This can explain the new peak appearing at 531.66 eV after adsorption, which is an arsenic oxide. The peak of low oxidation state Mn in the Mn2p spectrum is located on the low binding energy side. Figure 7 e indicates that before adsorption, Mn(V) is in the form of MnO4. 3- Mn(IV) appears at 657.91 eV as MnO2 at 645.01 eV and 653.11 eV, Mn(III) appears at 641.56 eV as MnOOH, and Mn(II) appears at 640.36 eV as MnO. After arsenic adsorption in soil for 30 days, the contents of Mn(V) and Mn(IV) decreased, while the contents of Mn(III) and Mn(II) increased. This may be explained by the interaction between Mn-O and As. MnO2 is reduced to MnOOH, and some of the Mn(II) released after the reduction of Mn(IV) is adsorbed on the surface of iron hydroxide, thereby further oxidizing Mn(II) to Mn3O4. In addition, the peak of Mn(IV) shifts significantly to a position with higher binding energy, further indicating that it participates in the oxidation of As(III) to As(V). Figure 7 f shows that the binding energy position of Fe2p spectrum did not change significantly before and after adsorption, with Fe(III) being dominant. The Fe(II) content decreased after adsorption, indicating that Fe(II) was partially converted into Fe(III) during the reaction.

[0056] In summary, although the XPS results did not provide quantitative support, they definitively confirmed that redox and complexation are the main mechanisms for arsenic stabilization in contaminated soil. Firstly, As(III) is oxidized to the more inert As(V) by Mn. During this reaction, Mn(IV) is reduced to Mn(III) and Mn(II), promoting the formation of Fe-As complexes between Fe(III) and As. Simultaneously, As is fixed due to the formation of ternary surface complexes. Furthermore, the hydroxyl groups in the modified biochar enhance the adsorption of As(V) through hydrogen bonding and electrostatic attraction.

[0057] 6. Changes in soil enzyme activity caused by biochar

[0058] The C and N cycles in agricultural soils, which provide nutrients to crops, involve soil enzymes. From Figure 8 As can be seen, the activities of urease (UE), catalase (CAT), neutral phosphatase (NP), and sucrase (SC) in the soil all changed. The addition of BC enhanced soil CAT activity, while Fe-BC and FeM-BC slightly inhibited CAT. The effects of different modified biochars on CAT showed a similar trend to pH changes, because CAT activity is sensitive to soil pH. The addition of BC increased soil pH, thereby increasing CAT activity. This can also be seen from... Figure 9 This was confirmed in the study. BC and Fe-BC induced an increase in urease UE and SC in the soil, but UE and SC did not change significantly in the FeMn-BC treatment group. This is because Cl- from Fe-Mn modified biochar inhibits the activity of UE and SC.

[0059] Redundancy analysis (RDA) was used to assess the multivariate relationships between soil enzyme activity and pH, nutrient elements, and ascorbic acid (As) content. The length of an environmental factor represents the amount of variance explained by that factor. Figure 9 The results showed that CAT was the main element strongly correlated with pH, ​​and pH was the primary key factor affecting enzyme activity. NP activity was positively correlated with soil organic matter (SOM). High SOM content provides soil microorganisms with more carbon and energy conducive to bacterial growth, leading to the secretion of more enzymes. Organic matter also provides a carrier for enzymes, maintaining their activity, indicating that increasing soil organic matter accumulation can induce phosphatase production. Urease (UE) showed a significant positive correlation with ammoniacal nitrogen, indicating that higher soil fertility correlates with higher enzyme activity. Although virgin biochar can promote As mobility, numerous studies have shown that Pb, Cd, and other heavy metals in soil treated with virgin biochar can be fixed, reducing the overall toxicity of heavy metals in the soil. This can explain the increased CAT, UE, and SOM in the BC-modified soil in this study, possibly due to reduced bioavailability of other heavy metals in the soil. In summary, soil enzyme activity is mainly related to the intensity of soil environmental factors. Both virgin biochar and iron-manganese modified biochar can enhance enzyme activity by increasing the content of environmental factors. However, due to limitations in biochar and modification materials, different types of biochar have different effects on soil enzyme activity.

[0060] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing modified biochar, characterized in that, The mass ratio of the components in the modified biochar is Fe:Mn:bamboo biomass = 1:3:30; The method for preparing modified biochar includes the following steps: Step 1: Prepare raw bamboo materials. Air dry and crush the bamboo required for the experiment, and pass it through a 100-mesh sieve for later use. Step 2: Weigh a certain mass of FeCl3 and KMnO4, dissolve them in water, add a certain mass of bamboo raw material, and then stir, filter, dry and pyrolyze under limited oxygen to produce modified biochar. The mass ratio of FeCl3 to KMnO4 is 1:2.97; The mass ratio of the raw bamboo to FeCl3 is 10.34:1; Step 2 includes: The solution was stirred with a glass rod and then sonicated for 1 hour to make it uniformly distributed. After vacuum filtration, it was dried at 60°C to make the mass ratio Fe:Mn:bamboo powder = 1:3:30, thus obtaining FM1:3 modified biomass. After the FM1:3 modified biomass was placed in a crucible, it was placed in a tube furnace for oxygen-limited pyrolysis at 500°C. The nitrogen flow rate was controlled by a gas flow rate device. After pyrolysis for 2 hours, FM1:3 modified biochar was obtained.

2. The application of the modified biochar prepared by the modified biochar preparation method according to claim 1, characterized in that, It is used to remediate arsenic contamination in neutral soils, and the remediation process includes the following steps: Modified biochar was added to arsenic-contaminated soil. The modified biochar was thoroughly mixed with the arsenic-contaminated soil and improved at room temperature for 30 days until the moisture content reached the maximum field capacity of the soil. After incubation, the soil was air-dried, crushed, and sieved. After 30 days of improvement, the material was recovered by magnetic separation for subsequent experiments.

3. The application of the modified biochar prepared by the modified biochar preparation method according to claim 2, characterized in that, The modified biochar was mixed with soil at a mass ratio of 1:100.

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