Iron-carbon-based passivation material for cadmium-arsenic combined polluted soil as well as preparation method and application of iron-carbon-based passivation material

Abstract

The invention belongs to the technical field of heavy metal contaminated soil remediation, and discloses an iron-carbon-based passivation material for cadmium-arsenic combined contaminated soil and a preparation method and application of the iron-carbon-based passivation material. The material is prepared through an in-situ chemical synthesis method, corn straw biochar is introduced into a Schwertmannite precursor, Schwertmannite particles are loaded on the surface and in pores of the Schwertmannite particles in situ, and the iron-carbon composite passivation material is obtained. The method has the advantages of simple process, low energy consumption and easily available raw materials, and is suitable for large-scale preparation. The obtained material has the dual functions of Schwertmannite and biochar, the iron component has strong adsorption and coprecipitation capacity on anions As (V), the carbon component has efficient adsorption and complexing capacity on cations Cd, and synchronous stabilization of Cd and As is achieved through the synergistic effect of the iron component and the carbon component. Soil culture experiments prove that the material can effectively reduce the bioavailability and mobility of Cd and As in polluted soil, and is suitable for remediation and safe utilization of Cd-As combined polluted farmland soil.

Description

Technical Field

[0001] This invention relates to the field of environmental pollution remediation and functional materials technology, specifically to an iron-carbon based passivation material for cadmium-arsenic co-contaminated soil, its preparation method, and its application. Background Technology

[0002] The combined pollution of cadmium (Cd) and arsenic (As) in paddy field soil is a prominent challenge in current farmland environmental remediation. The complexity of its remediation stems from the significant geochemical antagonism between Cd and As in soil: Cd exhibits enhanced activity under acidic and anaerobic reducing conditions, while As(V) is more readily migrated under alkaline and aerobic conditions. This opposing environmental behavior means that traditional single passivation materials often fail to address both Cd and As simultaneously and efficiently, making it difficult to achieve simultaneous and efficient immobilization.

[0003] Schwertmannite is a mineral containing Fe 3+ The tunnel-like ferric hydroxide minerals exhibit extremely strong specific adsorption and co-precipitation capabilities for As(V) due to their abundant surface hydroxyl groups, large specific surface area, and sulfate groups in their structure. However, their typically positively charged surface properties in the environment limit their ability to fix cation Cd. Furthermore, nanoscale Scherstein mineral particles are prone to aggregation, which not only reduces their reactivity but may also release sulfate groups during mineral transformation, potentially exacerbating soil acidification and cadmium migration.

[0004] Biochar, especially biochar prepared from corn stalks, is a porous carbon material with a hollow tubular structure, high specific surface area, and abundant oxygen-containing functional groups. These characteristics give it excellent adsorption capacity for cationic heavy metals such as Cd. However, biochar surfaces are usually negatively charged, resulting in weak adsorption capacity for As in anionic form. Therefore, Schiele minerals and corn stalk biochar form a complementary chemical structure: the former is an ideal material for As fixation, while the latter is a highly efficient carrier for Cd fixation.

[0005] Although there have been preliminary explorations of combining Scheringer's minerals with biochar in existing technologies, these studies have mostly focused on the removal of single pollutants such as As(III) or Cr(VI) in the aqueous phase, failing to fully consider the complexity of Cd-As compound pollution in actual paddy field soils, especially the key environmental factor of periodic redox dynamics. More importantly, simple physical mixing is insufficient to achieve stable binding and synergistic effects of iron and carbon components at the microscale, and cannot fundamentally solve the problem of simultaneous fixation caused by the antagonistic behavior of Cd and As. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes an in-situ chemical synthesis method to combine Scheringer's mineral with corn straw biochar, aiming to construct an iron-carbon-based passivation material possessing both dual-functional active sites and a stable microstructure. This design can simultaneously utilize the strong adsorption of As by the iron component and the high affinity of biochar for Cd to achieve synchronous, stable, and long-lasting passivation of Cd and As, demonstrating significant scientific value and application prospects for the remediation of Cd-As compound pollution in paddy fields.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows.

[0008] The first aspect of this invention provides an iron-carbon based passivation material for cadmium-arsenic co-contaminated soil, comprising the following steps: Schiele mineral precursor suspension is mixed with biochar, so that Schiele mineral grows and is loaded in situ on the surface and pores of biochar. After solid-liquid separation, washing, drying, grinding and sieving, the iron-carbon based passivation material is obtained. The biochar is corn straw biochar, and the mass of corn straw biochar accounts for 5% to 15% of the mass of ferrous salt in the Scheres mineral; in the cadmium-arsenic co-contaminated soil, cadmium is Cd(II) and arsenic is As(V).

[0009] The mass of corn straw biochar accounts for 5% to 15% of the mass of ferrous salt in the Schiele mineral. If the mass is too low, the carrier effect is insufficient and the Schiele mineral is prone to agglomeration. If the mass ratio of corn straw biochar is too high, the iron content is insufficient, which affects the fixation capacity of As.

[0010] In another preferred embodiment, the Scheres mineral precursor suspension is mixed with biochar for 2 to 4 hours at a temperature of 20°C to 28°C.

[0011] In another preferred embodiment, the specific process for obtaining the Scherescher mineral precursor suspension is as follows: A soluble ferrous salt solution is oxidized under acidic conditions by adding an oxidant to generate a suspension of Schiele mineral precursors.

[0012] In another preferred embodiment, the soluble ferrous salt is ferrous sulfate heptahydrate; the oxidant is hydrogen peroxide; The molar ratio of the oxidant to the ferric ions in the soluble ferrous salt is 0.7~0.9:1, used to control the Fe content. 2+ To Fe 3+ The oxidation rate and degree, if not within this molar ratio range, will affect the crystallinity and the number of surface active sites of the Schiele mineral.

[0013] In another preferred embodiment, the acidic conditions are characterized by a pH of 1.8–2.2, an oxidation reaction temperature of 20°C–28°C, and a reaction time of 20–28 h. The pH of the oxidation reaction is controlled at 1.8–2.2, which is the optimal acidic environment for the formation of Schäumann minerals. Excessively high pH levels can easily generate other iron oxides (such as goethite), affecting the specific adsorption of As. The reaction time of 20–28 h ensures that the Schäumann mineral precursor fully penetrates into the biochar pores and grows in situ; a shorter mixing time would result in insufficient loading.

[0014] In another preferred embodiment, the sieving refers to passing through a 50-100 mesh sieve.

[0015] The second aspect of the present invention provides an iron-carbon based passivation material for cadmium-arsenic co-contaminated soil prepared by the preparation method described above.

[0016] The third aspect of the present invention provides the application of the iron-carbon based passivation material for cadmium-arsenic co-contaminated soil in soil remediation, wherein the soil remediation refers to the remediation of Cd(II) and As(V) in the soil.

[0017] In another preferred embodiment, the iron-phosphorus-based passivating material is applied at an amount of 0.1% to 5% of the dry weight of the soil.

[0018] In another preferred embodiment, the repair process includes a flooding cultivation stage and a drying cultivation stage, used to simulate water management conditions for rice cultivation in farmland.

[0019] Compared with the prior art, the present invention has the following significant advantages: This invention utilizes in-situ chemical composite synthesis technology to tightly bind Scheringer minerals and biochar at the nanoscale, forming a stable iron-carbon composite system. The iron component provides anion adsorption sites to immobilize As(V), while the carbon component provides cation adsorption and complexation sites to immobilize Cd. Their synergistic effect effectively solves the challenge of simultaneous immobilization of Cd and As due to their antagonistic geochemical behavior in soil. This is fundamentally different from existing technologies that target only a single pollutant. The biochar carrier in this invention, namely corn straw biochar, effectively inhibits the aggregation of Scheringer mineral nanoparticles, enhancing the material's dispersibility and structural stability. Under periodic wet-dry conditions, the material exhibits excellent long-term immobilization performance and environmental tolerance.

[0020] The preparation process of this invention is characterized by mild conditions, no toxic byproducts, and inexpensive and readily available raw materials. In particular, the use of agricultural waste such as corn stalks to prepare biochar achieves waste resource utilization, demonstrating significant economic benefits and potential for widespread application. The iron-carbon based passivation material in this invention can maintain a stable passivation effect even under the alternating redox conditions of "flooding-drying" in paddy fields. Attached Figure Description

[0021] Figure 1 Comparison of X-ray diffraction (XRD) patterns of the material of this invention, biochar, and pure Schiele mineral; Figure 2 Scanning electron microscope (SEM) images of raw corn stalk biochar, pure Schiele minerals, and composite materials; Figure 3 The figures show the adsorption and removal curves of Cd and As in water by the composite material under different pH conditions. In the figure, a is the removal curve of the single Cd system, b is the removal curve of Cd by the As-Cd system, and c is the removal curve of As by the As-Cd system.

[0022] Figure 4 The adsorption isotherms of Cd and As in the composite material at different initial Cd concentrations are shown. Among them, a is the adsorption isotherm of the single Cd system, b is the Cd adsorption isotherm of the As-Cd system, and c is the As adsorption isotherm of the As-Cd system.

[0023] Figure 5 The figures show the effects of different treatments on the available Cd and As content in the soil under simulated paddy field flooding-drying cultivation conditions. (a) shows the effect on the available Cd content in the soil, and (b) shows the effect on the available As content in the soil. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0025] The valence state of heavy metals determines their environmental behavior and toxicity, thus fundamentally influencing the selection and design of passivating agents. Cadmium in soil has a single valence state, mainly +2 (Cd). 2+ Since it exists in the form of cations, the passivation strategy mainly revolves around cation adsorption, such as using negatively charged materials (such as biochar) to fix it through ion exchange, surface complexation, and precipitation.

[0026] Biochar, with its porous structure, large specific surface area, and abundant functional groups, can serve as an ideal soil remediation material, exhibiting good adsorption effects on heavy metal cations such as Cd(II). Converting straw into biochar facilitates the comprehensive utilization of agricultural waste.

[0027] As(V): commonly found in arsenates (such as HAsO4). 2- AsO43- Under aerobic conditions, it is dominant, negatively charged, and readily undergoes strong specific adsorption (ligand exchange) or co-precipitation with positively charged metal oxides (such as iron and aluminum oxides). For passivation of As(V), positively charged iron-based materials with hydroxyl-rich surfaces, such as Schiele minerals, are preferred.

[0028] It can be seen that different heavy metal systems require different remediation materials. For anionic heavy metals (such as arsenic and chromium), positively charged or hydroxyl-containing materials (such as Schiele minerals) are needed for fixation through adsorption / coprecipitation, with iron-based materials being more effective. For cationic heavy metals (such as cadmium and lead), they are easily fixed by negatively charged adsorbents (such as biochar) through ion exchange or complexation, with carbon-based materials being more effective. Existing technologies are usually aimed at a single pollution system or a single application scenario. Specifically, studies on single pollution systems focus only on the adsorption of As(III), Cr(VI), or Cd; existing research on single application scenarios mainly focuses on mining environments and chromium-containing wastewater.

[0029] This invention addresses the complex situation of Cd-As co-polluting systems, particularly the antagonistic behavior of the two compounds in soil (Cd anaerobic activation, As aerobic migration). It primarily targets both water and soil systems with As-Cd co-polluting, with a focus on the simultaneous remediation of Cd-As pollution in paddy field soils. This invention considers the alternating redox changes of paddy fields during flooding and drying, ensuring the material maintains a stable passivation effect under these conditions.

[0030] Example 1: Preparation and Characterization of Iron-Carbon Based Passivation Materials Preparation steps: Weigh ferrous sulfate heptahydrate and dissolve it in 1.5 L of deionized water to prepare a 450 mM Fe solution. 2+ Solution. Under continuous stirring at 180 rpm, hydrogen peroxide solution, H2O2, and Fe were slowly added dropwise to the solution. 2+ The molar ratio was 0.8. The reaction was carried out at 25°C for 24 hours, during which the pH of the system was maintained at 2.0 by adding dilute H2SO4 or NaOH solution dropwise, resulting in a reddish-brown Schiele mineral suspension.

[0031] Corn straw biochar at a mass ratio of 10% to FeSO4·7H2O was added to the above Scherbach mineral suspension, and the reaction was continued with stirring for 2 hours to allow the Scherbach mineral to be loaded in situ onto the surface of the biochar. After the reaction was completed, the mixture was vacuum filtered and repeatedly washed with deionized water until the filtrate was nearly neutral. The resulting wet solid was air-dried at 30°C for 40 hours, then ground and passed through a 100-mesh sieve to obtain powdered biochar-modified Scherbach mineral composite material, which was sealed and stored for later use.

[0032] Material characterization X-ray diffraction (XRD) was used to analyze the phase composition of the composite material. For example... Figure 1 As shown, the sample exhibits characteristic diffraction peaks of both Schiele mineral and biochar, indicating that Schiele mineral has been successfully loaded onto the biochar support.

[0033] The results of the scanning electron microscope (SEM) observations are shown below. Figure 2 The original biochar has a hollow porous structure. Figure 2 a), pure Schiele minerals are irregular spherical grains ( Figure 2 b); In the composite material, Schiele mineral particles are uniformly attached to the surface and pores of biochar ( Figure 2 (c~d) forms a stable iron-carbon composite structure. This structure reduces the aggregation of Shi mineral particles to some extent, which is beneficial for increasing the number of adsorption active sites.

[0034] Example 2: Simultaneous adsorption performance of composite materials for Cd and As in water To verify the ability of the composite material to simultaneously remove Cd(II) and As(V), a batch adsorption experiment was conducted.

[0035] Experimental Method: In 50 mL centrifuge tubes, 10 mL of a 4 g / L composite material suspension (obtained from Example 1) and 10 mL of a 0.01 mol / L NaCl background solution containing specific concentrations of Cd(II) and As(V) were added, respectively. The centrifuge tubes were placed on a 25°C constant-temperature shaker (70 rpm) and reacted for 24 hours to reach adsorption equilibrium. During the experiment, the pH of the system was adjusted and maintained constant using 0.1 mmol / L HCl or NaOH solution.

[0036] (1) pH effect experiment: The initial concentrations of Cd(II) and As(V) were fixed at 0.3 mmol / L and 1 mmol / L, respectively, and the effect of solution pH in the range of 3 to 10 on adsorption performance was investigated.

[0037] (2) Isothermal adsorption experiment: The solution pH was fixed at 7.0 and the initial concentration of As(V) was 1 mmol / L. The initial concentration of Cd(II) was changed (0.05-1 mmol / L) to investigate the adsorption isothermal characteristics.

[0038] Experimental results and analysis: such as Figure 3 and Figure 4 As shown, the composite material exhibits significant adsorption capacity for both Cd and As over a wide pH range, especially under neutral to weakly alkaline conditions, where the simultaneous removal rates of both remain at a high level. Adsorption isotherms show that even in the presence of As(V), the adsorption capacity of the material for Cd(II) increases significantly with increasing equilibrium concentration, while the adsorption capacity for As(V) is not significantly affected.

[0039] The results show that the material achieves simultaneous fixation of Cd and As through the synergistic effect of the specific adsorption of anion As(V) by the iron component (Schärschmannite) and the efficient adsorption of cation Cd(II) by the carbon component (biochar), thus verifying the effectiveness of the iron-carbon synergistic mechanism.

[0040] Example 3: Passivation effect of composite materials on cadmium-arsenic co-contaminated soil (1) Test soil and experimental design: The soil was collected from a cadmium and arsenic contaminated paddy field in Guixi City, Jiangxi Province. Its pH was 5.11, the total Cd (II) content was 2.54 mg / kg, and the total As (V) content was 23.85 mg / kg. After air drying, it was sieved through a 2 mm sieve for later use.

[0041] Five treatments were set up, with each treatment repeated three times. The specific group treatment conditions are as follows: CK: No ingredients added.

[0042] BC: Add 1% (w / w) corn stalk biochar.

[0043] Sch: Add 1% (w / w) pure Scheider mineral.

[0044] BC-Sch(1%): 1% (w / w) of composite material added.

[0045] BC-Sch(5%): 5% (w / w) of composite material added.

[0046] (2) Cultivation program: Simulate paddy field water management, with a cultivation cycle of 70 days. The first 42 days are for flooded cultivation (water-to-soil ratio 1:1.5, anaerobic conditions), and the last 28 days are for desiccation cultivation (natural air drying, aerobic conditions). The entire process is carried out in a dark, constant temperature environment at 25℃.

[0047] (3) Effect evaluation: Samples were taken on the 1st, 7th, 14th, 28th and 42nd days of the flooding period and the 12th (56th) and 28th (70th) days of the drying period. The effective Cd content was determined by CaCl2 extraction method and the effective As content was determined by NaH2PO4 extraction method.

[0048] Results and Analysis: Figure 5 As shown, compared with the control (CK), both 1% and 5% composite material treatments significantly and continuously reduced the content of available Cd and As in the soil, with a passivation effect significantly better than that of the individual Scheider mineral or biochar groups. The effect increased with increasing dosage, demonstrating a good dose-response relationship.

[0049] The results show that the biochar-Schätlite composite system plays a synergistic role in the passivation of Cd and As: biochar provides abundant pores and surface functional groups, which can adsorb and complex Cd; Schätlite forms surface complexes or co-precipitates with As(V) through iron hydroxyl groups; the micro-electrochemical environment formed at the iron-carbon interface can further enhance the precipitation and adsorption reactions.

[0050] In summary, this composite material effectively overcomes the technical shortcomings of single-material passivation mechanisms and limited selectivity, achieving simultaneous fixation and long-term stabilization of Cd and As, and has significant application potential in the remediation and safe utilization of heavy metal pollution in farmland.

[0051] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An iron-carbon based passivation material for cadmium-arsenic co-contaminated soil, characterized in that, Includes the following steps: Schiele mineral precursor suspension is mixed with biochar, so that Schiele mineral grows and is loaded in situ on the surface and pores of biochar. After solid-liquid separation, washing, drying, grinding and sieving, the iron-carbon based passivation material is obtained. The biochar is corn stalk biochar, and the mass of the corn stalk biochar accounts for 5% to 15% of the mass of the ferrous salt in the Schiele mineral. In the soil contaminated with cadmium and arsenic, cadmium is Cd(II) and arsenic is As(V).

2. The iron-carbon based passivation material for cadmium-arsenic co-contaminated soil according to claim 1, characterized in that, The mixing time between the Scheres mineral precursor suspension and biochar was 2h~4h, and the temperature was 20℃~28℃.

3. The iron-carbon based passivation material for cadmium-arsenic co-contaminated soil according to claim 1, characterized in that, The specific process for obtaining the Scherescher mineral precursor suspension is as follows: A soluble ferrous salt solution is oxidized under acidic conditions by adding an oxidant to generate a suspension of Schiele mineral precursors.

4. The method for preparing iron-carbon based passivation material for cadmium-arsenic co-contaminated soil according to claim 3, characterized in that, The soluble ferrous salt is ferrous sulfate heptahydrate; the oxidizing agent is hydrogen peroxide. The molar ratio of the oxidant to the ferric ions in the soluble ferrous salt is 0.7~0.9:

1.

5. The method for preparing iron-carbon based passivation material for cadmium-arsenic co-contaminated soil according to claim 3, characterized in that, The acidic conditions are pH 1.8 to 2.2, the oxidation temperature is 20℃ to 28℃, and the reaction time is 20h to 28h.

6. The iron-carbon based passivation material for cadmium-arsenic co-contaminated soil according to claim 1, characterized in that, The sieving process refers to passing the material through a 50-100 mesh sieve.

7. An iron-carbon based passivation material for cadmium-arsenic co-contaminated soil, prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the iron-carbon based passivation material for cadmium-arsenic co-contaminated soil as described in claim 7 in soil remediation, characterized in that, Soil remediation refers to the remediation of Cd(II) and As(V) in the soil.

9. The application of the iron-phosphorus-based passivation material for soil remediation of cadmium-arsenic co-contaminated soil according to claim 8, characterized in that, The iron-phosphorus-based passivation material is applied at a rate of 0.1% to 5% of the dry weight of the soil.

10. The application of the iron-phosphorus-based passivation material for soil remediation of cadmium-arsenic co-contaminated soil according to claim 8, characterized in that, The restoration process includes a flooding cultivation stage and a drying cultivation stage, which are used to simulate water management conditions for rice cultivation in farmland.

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