Method for repairing cadmium-arsenic combined polluted soil by using iron modified garden waste biochar

Abstract

The invention relates to the technical field of soil heavy metal pollution remediation, and particularly discloses a preparation method of iron-modified biomass charcoal, which comprises the following steps: preparing garden waste into biomass charcoal (PBC), and then carrying out iron modification by using FeCl3 to obtain iron-based biomass charcoal (PIBC). The invention further provides a method for repairing the cadmium-arsenic combined polluted soil through the iron-modified garden waste biochar. The method comprises the steps that the iron-modified biomass charcoal prepared through any method is added into the soil; the iron-modified biomass can be applied as a base fertilizer at one time, and is turned over to be fully and uniformly mixed with a plough layer of soil. By adopting the method disclosed by the invention, the bioavailability of Cd / As in the soil can be synchronously reduced, and waste resources are utilized to realize pollution control by waste.

Description

Technical Field

[0001] This invention relates to the field of soil heavy metal pollution remediation technology, specifically providing a composite pollution remediation method that simultaneously fixes cadmium (Cd) and arsenic (As) in soil using iron-modified garden waste biochar. Background Technology

[0002] Potential toxic element pollution in farmland soils has become a serious global agricultural, environmental, and public health problem. As (As) and Cd (Cd) are listed as the first and seventh hazardous substances, respectively, by the Toxic Substances Registry and the Disease Registry. In As pollution, inorganic As(III) and As(V) are the main forms present in polluted soils and water bodies, with As(III) exhibiting stronger toxicity and mobility than As(V). Cd exists primarily as a cation in soil, while As is mainly in anionic form (such as AsO43-). These drastically different chemical behaviors make synergistic remediation difficult. Against this backdrop, there is an urgent need to seek effective soil remediation technologies that can both reduce the bioavailability of Cd and As in paddy soils and control the absorption of Cd and As by rice, in order to safeguard food security and human health.

[0003] Biochar, due to its alkalinity, large specific surface area, abundant pores, negatively charged groups on its surface, and high cation exchange capacity, can effectively reduce the bioavailability and toxicity of cationic heavy metals in farmland soil, and has good application prospects. Due to the presence of anions such as As (e.g., H₂AsO₄), it can effectively reduce the bioavailability and toxicity of cationic heavy metals in farmland soil, and has good application prospects. - ,HAsO4 2- and AsO4 3- The electrostatic repulsion between arsenic and negatively charged biochar, and the limeification effect of biochar, can generally activate As. Therefore, the preparation of highly efficient functionalized biochar is key to remediating farmland contaminated with both Cd and As. Iron modification of biochar can be achieved by selecting biomass precursors rich in silicon (Si) and phosphorus (P), or by loading other organic or inorganic materials (e.g., iron-rich compounds) onto biochar. Iron-modified biochar is commonly used to remediate As-contaminated soil. Studies have shown that magnetic iron compounds and iron-loaded materials have demonstrated good remediation effects on As pollution in adsorption and field trials, reducing As accumulation in rice straw and grains. Both trivalent and pentavalent arsenic can form chelates with iron compounds, which explains the high adsorption capacity of iron compounds for As. Therefore, magnetic biochar materials obtained by attaching iron compounds to the surface of biochar can be used to remediate Cd and As combined pollution. Furthermore, small iron particles that easily aggregate in water become more stable when loaded onto biochar.

[0004] The invention CN118931547A proposes a simple and effective method for reducing arsenic pollution in soil, along with its preparation method, to create an arsenic-contaminated soil remediation material. The arsenic-contaminated soil remediation material comprises the following components: 50-70 parts iron-modified bamboo charcoal; 10-20 parts humic acid; and 20-30 parts iron-modified pyrolytic manganese slag. This method utilizes the fact that manganese (Mn) promotes the oxidation of ferrous ions, and the secondary iron minerals formed by the oxides of ferric ions produced by the oxidation of ferrous ions can solidify arsenic. Simultaneously, Mn, as a strong oxidant, can migrate and aggregate to form iron-manganese nodules, oxidizing ferric ions to pentavalent arsenic ions, thereby reducing toxicity. Furthermore, the alkaline properties of bamboo charcoal allow humic acid to further react with iron ions to form precipitates, achieving the purpose of solidifying and stabilizing arsenic, reducing its migration and bioavailability. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for remediating cadmium-arsenic co-contaminated soil using iron-modified garden waste biochar.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for preparing iron-modified biochar, which involves preparing biochar (PBC) from garden waste and then modifying it with FeCl3 to obtain iron-based biochar (PIBC).

[0007] As an improvement to the preparation method of the iron-modified biochar of the present invention, the following steps are performed sequentially:

[0008] 1) After cleaning and removing impurities from garden waste, it is first pre-dried, then crushed and further dried. The resulting material is named dried raw material powder.

[0009] 2) Under the protection of an inert gas (including nitrogen, etc.), the dried raw material powder obtained in step 1) is pyrolyzed (carbonized) at 600±50℃ for 6±0.5 hours;

[0010] 3) Crush the pyrolysis product obtained in step 2) (through a 100-mesh sieve) to obtain biochar (PBC) --- that is, biochar from garden waste;

[0011] 4) Add deionized water to FeCl3 until the concentration of FeCl3 is (0.5 ± 0.05) mol / L. -1 Then adjust the pH to 4.5±0.3 to obtain FeCl3 solution;

[0012] Note: The pH of the solution before pH adjustment is approximately 1 to 2, which will affect the surface charge of the biochar. Therefore, the pH is subsequently adjusted to 4.5 ± 0.3 using NaOH.

[0013] The biochar (PBC) obtained in step 3) was impregnated in FeCl3 solution and ultrasonically treated for 1 ± 0.2 h;

[0014] Note: This ultrasonic treatment step makes Fe... 3+ Uniformly penetrating into the micropores / mesopores of biochar, achieving Fe 3+ Adhesion in biochar;

[0015] 5) Place all the results from step 4) (iron-attached PBC along with the FeCl3 solution) in a water bath at 60±10℃ for 20-40 minutes, then stir continuously at 60±10℃ using a magnetic stirrer for 2±0.2 hours (to promote Fe... 3+ (Uniform distribution and stable binding on the surface of biomass carbon);

[0016] Then, under sealed conditions, the reaction was allowed to proceed for 22 to 26 hours.

[0017] Note: The purpose of allowing the mixture to stand and react naturally is to allow Fe to... 3+ Further physical / chemical reactions occur with biomass carbon (PBC) to form a more stable bond state, namely, the formation of iron-biomass carbon complexes;

[0018] Physical adsorption: Fe 3+ It can further penetrate into the deep pores of biomass carbon through electrostatic action or van der Waals forces.

[0019] Chemical bonding: The oxygen-containing functional groups (such as -COOH, -OH) on the surface of biomass carbon react with Fe. 3+ A coordination reaction occurs, forming a stable complex;

[0020] 6) Filter the reaction product obtained in step 5). The solid part obtained by filtration is first dried (drying at 65±10℃ for 12±1 hours), and then pyrolyzed at 600±50℃ for 20 to 40 minutes under the protection of inert gas (including nitrogen, etc.).

[0021] Note: The purpose of pyrolysis is to remove the iron species Fe from the biochar. 3+ / Fe 2+ The complex further transforms into magnetic iron oxide Fe3O4 or elemental iron (Fe). 0 The reaction formula is: Fe2O3 + C (biomass carbon) → Fe3O4 / Fe 0 +CO / CO2;

[0022] 7) The pyrolysis product obtained in step 6) is washed with water, dried, and pulverized (through a 100-mesh sieve) to obtain iron-based biochar (PIBC).

[0023] As a further improvement to the preparation method of iron-modified biochar of the present invention: in step 1):

[0024] Garden waste includes fallen leaves, bark, wood, and branches;

[0025] Removing impurities means removing non-garden waste (such as small pebbles);

[0026] Use deionized water for cleaning;

[0027] First, use natural air drying to achieve preliminary drying, then pulverize (pass through a 10-mesh sieve), then dry at 80±10℃ for 30±5 minutes, and then cool down to 60±5℃ to dry to constant weight.

[0028] As a further improvement to the preparation method of iron-modified biochar of the present invention: in step 2), the heating rate is 6.5±0.5℃ / min.

[0029] As a further improvement to the preparation method of iron-modified biochar of the present invention: in step 4), biochar (PBC) is impregnated in FeCl3 solution at a dosage ratio of 1g / (10±2)ml and ultrasonically treated for 1±0.2h.

[0030] Note: 20g PBC, ultrasonic power set to 300W, ultrasonic frequency set to 30kHz.

[0031] As a further improvement to the preparation method of the iron-modified biochar of the present invention:

[0032] In step 7), the washing is performed until the washing water is neutral (pH = 6-7), and the drying is performed at 80±10℃ for 2±0.2 hours.

[0033] The present invention also provides a method for remediating cadmium-arsenic co-contaminated soil using iron-modified garden waste biochar: iron-modified biochar prepared by any of the above methods is added to the soil.

[0034] An improvement to the method of remediating cadmium-arsenic compound-contaminated soil using iron-modified garden waste biochar of the present invention: iron-modified biochar (PIBC) is applied as a base fertilizer at a rate of (150±20) kg / mu and then tilled to ensure that it is fully and evenly mixed with the soil tillage layer.

[0035] This invention is the first to use garden waste as a precursor for iron-modified biochar, solving its treatment challenges and adding high value. The invention achieves a synergistic effect between iron and garden waste components. The high lignin content of garden waste forms a stable porous structure during pyrolysis, enhancing the exposure of active sites when combined with iron. Traditional iron modification involves only physical mixing or simple impregnation, while the process of this invention optimizes iron dispersion and chemical bonding with the carbon matrix (such as the formation of Fe-OC bonds).

[0036] This invention utilizes naturally occurring silicon and phosphorus in garden waste, thus eliminating the need for additional silicon and phosphorus. Silicon in plant cell walls or phosphorus residues in nucleic acids can form Si-O-Fe or P-Fe coordination structures after pyrolysis, enhancing the fixation ability for heavy metals (such as Cd and Pb). Traditional methods require the deliberate selection of silicon / phosphorus-rich precursors or exogenous addition, while this invention achieves a similar effect using the components of garden waste itself, simplifying the process.

[0037] The beneficial effects of this invention are mainly reflected in the following aspects: This invention innovatively selects garden waste as raw material and achieves uniform embedding of iron particles in carbon pores through a secondary pyrolysis-oxidation modification method, thereby optimizing the iron loading process to prepare composite functional materials and achieving the following objectives: simultaneously reducing the bioavailability of Cd / As, inhibiting the absorption of heavy metals by rice, and utilizing waste resources to achieve "pollution control with waste".

[0038] In summary, this invention uses garden waste (such as fallen leaves and branches) as a precursor to achieve high-value utilization of waste, combining environmental protection and economic benefits. Comparative advantages: Traditional biochar may rely on agricultural residues (such as rice husks and straw) or plants specifically rich in silicon / phosphorus, while garden waste is often overlooked. However, it has a high cellulose / lignin content, potentially superior pore structure, and is widely available and lower in cost. Attached Figure Description

[0039] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0040] Figure 1 Electron microscopy (SEM) observation of the surface structure of PBC(e) and PIBC(f) before and after modification;

[0041] Figure 1 In the image, the left image is a SEM image of biochar before modification (PBC), and the right image is a SEM image of biochar after iron modification (PIBC).

[0042] Figure 2 EDS spectra of PBC and PIBC;

[0043] Figure 2 In the middle, the left figure is the EDS energy spectrum of PBC, and the right figure is the EDS energy spectrum of PIBC.

[0044] Figure 3 The nitrogen adsorption-desorption isotherms for PBC and PIBC are shown. Detailed Implementation

[0045] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto:

[0046] Example 1: Preparation of iron-modified biochar (PIBC) was carried out by following these steps:

[0047] 1) Collect garden waste (including fallen leaves, bark, wood, and branches) and place it in a resealable bag. Take it back to the laboratory for impurity removal (impurity removal means removing non-garden waste parts, such as small pebbles). Then, wash it multiple times with deionized water to ensure cleanliness; the cleaned garden waste is named raw material.

[0048] The raw materials were air-dried under natural conditions (until the moisture content was ≤15%, and the withered leaves easily broke when gently bent), and then pulverized and preliminarily screened through a 10-mesh sieve. Next, they were placed in a forced-air drying oven and dried at 80℃ for 30 minutes, then the temperature was lowered to 60℃ and dried to constant weight to completely remove residual moisture; the resulting powder was named the dried raw material powder.

[0049] 2) Under nitrogen (N2) protection, the dried raw material powder obtained in step 1) is placed in an electric furnace for pyrolysis. The pyrolysis is performed by controlling the heating rate at 6.5℃ / min and continuously carbonizing at a high temperature of 600℃ for 6 hours. To ensure the stable progress of the pyrolysis process, nitrogen (N2) is continuously introduced.

[0050] 3) The pyrolysis product obtained in step 2) is crushed by a crusher and passed through a 100-mesh sieve to finally obtain garden waste biochar (PBC).

[0051] 4) Weigh 81.1g (0.5mol) FeCl3, add deionized water and stir thoroughly to dissolve, then bring the volume to 1L; adjust the pH to 4.5 to obtain 0.5mol / L -1 FeCl3 solution.

[0052] Note: The pH of the solution before pH adjustment is approximately 1 to 2, which will affect the surface charge of the biochar. Therefore, the pH is subsequently adjusted to 4.5 using NaOH.

[0053] Weigh 20g of PBC and completely immerse it in 200ml of FeCl3 solution (0.5mol / L). -1 In the process, the Fe was ultrasonically treated for 1 hour (ultrasonic power 300W, ultrasonic frequency 30kHz). Through this step, Fe... 3+ Uniformly penetrating into the micropores / mesopores of biochar, achieving Fe3+ Adhesion in biochar.

[0054] 5) Place all the results from step 4) (iron-attached PBC along with FeCl3 solution) in a 60°C water bath and let it stand for half an hour, then continuously stir with a magnetic stirrer at 60°C for 2 hours; thereby promoting Fe 3+ Uniform distribution and stable binding on the surface of biomass carbon.

[0055] After stirring, cover the mouth of the beaker with plastic wrap and allow the mixture to react naturally for 24 hours, thereby allowing the Fe... 3+ Further physical / chemical reactions occur with biomass carbon (PBC) to form a more stable bond state, namely, the formation of iron-biomass carbon complex.

[0056] illustrate:

[0057] Physical adsorption: Fe 3+ It can further penetrate into the deep pores of biomass carbon through electrostatic action or van der Waals forces.

[0058] Chemical bonding: The oxygen-containing functional groups (such as -COOH, -OH) on the surface of biomass carbon react with Fe. 3+ A coordination reaction occurs, forming a stable complex.

[0059] 6) Filter the reaction product obtained in step 5) to separate the solid fraction. Place the filtered solid fraction in a blower dryer and dry it at 65°C for 12 hours. Under N2 protection, pyrolyze the dried material at 600°C for half an hour.

[0060] illustrate:

[0061] The purpose of pyrolysis is to convert the iron species Fe on the biochar into Fe. 3+ / Fe 2+ The complex further transforms into magnetic iron oxide Fe3O4 or elemental iron (Fe). 0 The reaction equation is: Fe₂O₃ + C (biomass carbon) → Fe₃O₄ / Fe 0 +CO / CO2.

[0062] 7) The product obtained after pyrolysis in step 6) is washed with water until the pH of the washing water is approximately 6-7. Finally, it is dried again (at 80°C for 2 hours) and ground with a grinder until it passes through a 100-mesh sieve to obtain iron-based biochar (PIBC).

[0063] Surface morphology and structural composition were characterized using scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS). The results are as follows: SEM showed ( Figure 1 Unmodified biochar (PBC) exhibits distinct cellular, tubular, and plate-like porous structures. After iron modification, PIBC shows granular structures within its pores, a rougher surface, and a honeycomb-like cross-section. EDS spectroscopy results indicate an iron content of 7.17% in PBC, with the iron element percentage reaching 35.79%. Figure 2 .

[0064] Nitrogen equilibrium adsorption-desorption tests were performed on PBC and PIBC samples (according to ISO 15901 and ASTM D3663), and nitrogen adsorption-desorption isotherms were measured using a specific surface area and porosity analyzer (Tri Star II 3020). Figure 3 The measured specific surface area and pore parameters are shown in Table 1. Figure 3 A hysteresis loop can be observed, indicating that the adsorption process of IBC occurs through the adsorption of multilayer molecules with pore walls and condensation in the pores.

[0065] The above characterization results of the surface morphology and structural composition of PBC and PIBC show that Fe 3+ It was successfully attached to the surface of biochar; the specific surface area, microporous specific surface area and total pore volume of PIBC material are all higher than those of PBC material, as shown in Table 1 below.

[0066] Table 1. Specific surface area and pore parameters of PBC and PIBC

[0067]

[0068] Experiment 1: Soil culture experiment. Soil samples were collected from farmland contaminated with cadmium (Cd) and arsenic (As). The concentrations of Cd and As in the soil were 1.14 mg·kg⁻¹. -1 and 35.6 mg·kg -1Three treatments were set up: control (CK, no biochar), biochar (PBC), and iron-modified biochar (PIBC). The specific procedures were as follows: 40g of soil sample was weighed into culture bottles with breathable caps. For the CK treatment, no biochar was added. For the PBC / PIBC treatment, biochar (PBC) or iron-modified biochar (PIBC) was thoroughly mixed with the soil at a mass fraction of 1%. All culture bottles were placed in a constant temperature incubator (25℃). During the incubation process, water was added every 3 days using a weighing method (i.e., water was added until it reached 80% of field capacity). After 20 days of incubation at room temperature, samples were collected, air-dried, and passed through a 100-mesh sieve. The contents of available soil components, total Cd, As, and trivalent and pentavalent arsenic in the pore water were determined. Soil-available Cd was extracted using CaCl2, and soil-available As was extracted using NaHCO3. The concentrations of Cd and As in the solutions were determined using inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer 600X, USA). Details are shown in Tables 2 and 3 below.

[0069] Table 2

[0070] deal with <![CDATA[Available Cd content in soil (mg·kg -1 )]]> <![CDATA[Available As content in soil (mg·kg -1 )]]> CK 0.35 0.18 PBC 0.26 0.12 PIBC 0.18 0.09

[0071] Table 3

[0072]

[0073] Note: Monomethylarsenate is MMA(V), and dimethylarsenate is DMA(V).

[0074] Conclusion: The available Cd content in the soil was highest in the PIBC treatment group (0.18 mg·kg⁻¹). -1 Compared with CK (0.35 mg·kg) -1 The oxidation-reduction was 48.6%, which was superior to that of unmodified biochar (PBC, 0.26 mg·kg⁻¹). -1 Soil available As content, PIBC treatment group (0.09 mg·kg⁻¹) -1 Compared with CK (0.18 mg·kg) -1 The 50.0% reduction was also superior to PBC (0.12 mg·kg⁻¹). -1 The results showed that iron modification can significantly enhance the biochar's ability to fix Cd and As, making it suitable for the remediation of soils with complex contamination.

[0075] Experiment 2: Field trial, with three treatments: control (CK, no biochar), biochar (PBC), and iron-modified biochar (PIBC). Each treatment was replicated three times, for a total of nine plots. Biochar and iron-based biochar were applied as basal fertilizer at a rate of 150 kg / mu, and then tilled to ensure thorough mixing with the topsoil. Each experimental plot was 50 m². 2 (5m×10m) ridges were built between each plot. The rice was sown manually on August 13th and harvested on November 27th. The rice variety was Jiafengyou No. 2.

[0076] The testing methods for Cd and As in soil were the same as in Experiment 1. The results are shown in Table 4 below.

[0077] Table 4

[0078]

[0079] Soil available Cd: PIBC (0.05 mg·kg⁻¹) -1 Compared with CK (0.15 mg·kg) -1 The level was reduced by 66.7%, which was superior to PBC (0.08 mg·kg⁻¹). -1 Modified biochar showed a significantly enhanced ability to fix Cd. Soil available As: PIBC (1.80 mg·kg⁻¹) -1 Compared with CK (2.31 mg·kg) -1 The level decreased by 22.1%, while PBC (3.47 mg·kg) -1 Iron modification actually increased the availability of As by 50.2%. Iron modification effectively inhibited the activation of As by biochar and promoted its fixation. Cd and As accumulation in rice grains: Cd: PIBC treatment group (0.02 mg / kg) -1 Compared with CK (0.24 mg·kg) -1 The reduction was 91.7%, far superior to PBC (0.15 mg·kg). -1 As: PIBC treatment group (0.25 mg·kg) -1 Compared with CK (0.36 mg·kg) -1 The level decreased by 30.6%, while PBC (0.41 mg·kg) -1 This caused As accumulation to increase by 13.9%.

[0080] In conclusion, iron-modified biochar (PIBC) can simultaneously reduce the availability of Cd and As in soil and significantly inhibit the absorption of Cd and As by rice, with a particularly pronounced effect on Cd. While unmodified biochar (PBC) can reduce Cd availability, it may promote As release due to increased pH or the effect of surface negative charge, leading to increased As accumulation. Therefore, iron modification is a key technology for improving the remediation of cadmium- and arsenic-contaminated soils using biochar.

[0081] Comparative Example 1-1: In step 2) of Example 1, the "heating rate is 6.5℃ / min" is changed to "heating rate is 3℃ / min", and the rest is the same as in Example 1.

[0082] Comparative Examples 1-2: In step 2) of Example 1, the "heating rate is 6.5℃ / min" is changed to "heating rate is 10℃ / min", and the rest is the same as in Example 1.

[0083] Comparative Example 2-1: In step 2) of Example 1, "continuous carbonization at 600°C for 6 hours" was changed to "continuous carbonization at 1000°C for 6 hours", and the rest was the same as in Example 1.

[0084] Comparative Example 2-2: In step 2) of Example 1, "continuous carbonization at 600°C for 6 hours" was changed to "continuous carbonization at 400°C for 10 hours", and the rest was the same as in Example 1.

[0085] The garden waste biochar (PBC) obtained from Comparative Examples 1-1, 1-2, 2-1, and 2-2 were processed according to Experiment 1. The results showed that the available Cd content in the soil was 0.32–0.34 mg·kg⁻¹. -1 The soil available As content is 0.16–0.17 mg / kg. -1 The repair effect was deemed poor, therefore no further experiments were conducted on PIBC.

[0086] Comparative Example 3: In step 4) of Example 1, "ultrasonic treatment for 1 hour" was replaced with "treatment under stirring conditions for 1 hour," while the rest remained the same as in Example 1. The resulting iron-based biochar (PIBC) was processed according to Experiment 1, and the results showed that the available Cd content in the soil was 0.24 mg / kg. -1 The soil's available As content was 0.11 mg / kg. -1 ).

[0087] Comparative Example 4: In step 4) of Example 1, "FeCl3" was replaced with "Fe(NO3)3", while the concentration remained unchanged at 0.5 mol / L. -1The rest is the same as in Example 1. The obtained iron-based biochar (PIBC) was processed according to Experiment 1, and the results were: the available Cd content in the soil was 0.22 mg / kg. -1 The soil's available As content is 0.10 mg / kg. -1 However, the effect is not as good as that of Embodiment 1 of the present invention.

[0088] Finally, it should be noted that the above examples are merely some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing iron-modified biochar, characterized in that: Garden waste was prepared into biochar, and then iron was modified using FeCl3 to obtain iron-based biochar.

2. The method for preparing iron-modified biochar according to claim 1, characterized in that... Perform the following steps in sequence: 1) After cleaning and removing impurities from garden waste, it is first pre-dried, then crushed and further dried. The resulting material is named dried raw material powder. 2) Under inert gas protection, the dried raw material powder obtained in step 1) is pyrolyzed at 600±50℃ for 6±0.5 hours; 3) The pyrolysis products obtained in step 2) are pulverized to obtain biochar; 4) Add deionized water to FeCl3 until the concentration of FeCl3 is (0.5 ± 0.05) mol / L. -1 Then adjust the pH to 4.5±0.3 to obtain FeCl3 solution; The biochar obtained in step 3) was impregnated in FeCl3 solution and ultrasonically treated for 1 ± 0.2 h; 5) Place all the contents obtained in step 4) in a water bath at 60±10℃ and let it stand in the water bath. Then, use a magnetic stirrer to stir continuously at 60±10℃ for 2±0.2 hours. Then, under sealed conditions, the reaction was allowed to proceed for 22 to 26 hours. 6) Filter the reaction product obtained in step 5). Dry the filtered solid part first, and then pyrolyze it at 600±50℃ for 20 to 40 minutes under inert gas protection. 7) The pyrolysis product obtained in step 6) is washed with water, dried, and pulverized to obtain iron-based biochar.

3. The method for preparing iron-modified biochar according to claim 2, characterized in that: In step 1): Garden waste includes fallen leaves, bark, wood, and branches; "Removing impurities" means removing parts that are not garden waste. Use deionized water for cleaning; First, use natural air drying to achieve preliminary drying, then pulverize, then dry at 80±10℃ for 30±5min, and then cool down to 60±5℃ to dry to constant weight.

4. The method for preparing iron-modified biochar according to claim 3, characterized in that: In step 2), the heating rate is 6.5 ± 0.5 °C / min.

5. The method for preparing iron-modified biochar according to claim 3 or 4, characterized in that: In step 4), the biochar is impregnated in FeCl3 solution at a dosage ratio of 1g / (10±2)ml and ultrasonically treated for 1±0.2h.

6. The method for preparing iron-modified biochar according to claim 5, characterized in that: In step 7): the water is washed until the washing water is neutral, and the drying is carried out at 80±10℃ for 2±0.2 hours.

7. A method for remediating cadmium-arsenic co-contaminated soil using iron-modified garden waste biochar, characterized in that: Iron-modified biochar prepared by any one of the methods described in claims 1 to 6 is added to the soil.

8. The method for remediating cadmium-arsenic co-contaminated soil using iron-modified garden waste biochar according to claim 7, characterized in that: Apply iron-modified biochar at a rate of (150±20) kg / mu as a base fertilizer in one go, and then plow it to ensure it is fully and evenly mixed with the topsoil.

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