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

Abstract

The invention relates to the technical field of environmental functional materials and soil pollution remediation, and discloses an iron-phosphorus-based passivation material for cadmium-arsenic combined polluted soil as well as a preparation method and application of the iron-phosphorus-based passivation material. The material is prepared by performing in-situ chemical modification on a schwertmannite precursor through phosphate. Adding phosphate into the schwertmannite precursor suspension to carry out in-situ chemical modification reaction; and washing, drying and grinding to obtain the iron-phosphorus-based passivation material. Phosphate radicals in the material partially replace sulfate radicals in a Schwertmannite structure in a chemical bonding mode, so that the material has the function of immobilizing As on an iron-based basis and immobilizing Cd on a phosphorus-based basis at the same time, the effective state content and mobility of Cd and As in soil can be remarkably reduced, synchronous and long-acting passivation of Cd and As can be achieved, and the material is particularly suitable for remediation and safe utilization of Cd-As combined polluted farmland soil. The preparation process is mild in condition, raw materials are easy to obtain, the product structure is uniform and stable, and good economical efficiency and application prospects are achieved.

Description

Technical Field

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

[0002] Heavy metal pollution in soil is a significant environmental problem hindering sustainable agricultural development. Among these, the combined pollution of cadmium (Cd) and arsenic (As) presents extreme challenges for remediation due to their significant geochemical antagonism. Arsenic exhibits increased activity under flooded reducing conditions, while cadmium shows increased mobility under oxidizing and acidic conditions. These diametrically opposed chemical behaviors in soil make it difficult for traditional single-function remediation materials to achieve simultaneous and efficient passivation of both Cd and As. For example, lime-based materials can effectively immobilize cadmium by increasing pH, but often trigger the activation and leaching of arsenic; while iron-based materials have good adsorption properties for arsenic, their passivation effect on cadmium is generally limited.

[0003] Schwertmannite is a hydroxyferric sulfate mineral formed by the oxidation of ferrous salts. It possesses a large specific surface area, abundant surface hydroxyl sites, and a high sulfate content, exhibiting excellent adsorption and co-precipitation capabilities for arsenic. However, Schwertmannite is typically positively charged under ambient pH conditions, which limits its ability to adsorb arsenic cations such as Cd. 2+ Its adsorption and fixation capacity is relatively weak. In addition, it may transform in the environment and release sulfate ions, leading to local microenvironment acidification, which may increase the bioavailability and migration risk of cadmium. This seriously limits its direct application in the remediation of Cd-As complex pollution.

[0004] To enhance the cadmium (Cd) fixation capacity of materials, the introduction of phosphate groups is considered an effective modification strategy. Phosphate not only enhances the electronegativity of the mineral surface, promoting electrostatic attraction to Cd, but also exhibits strong coordination with Cd, even forming insoluble cadmium phosphate precipitates. Existing technologies have attempted to modify materials by physically mixing phosphates with Schiff minerals using mechanical ball milling. While this method can increase the specific surface area of ​​the material and expose more active sites to some extent, its modification relies heavily on physical mixing. The binding strength between phosphate and the mineral surface is insufficient, making it prone to desorption and loss in complex soil / water environments. Furthermore, the intense high-speed ball milling process risks damaging the crystal structure of Schiff minerals, potentially weakening their core fixation capacity for As. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides an iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil, along with its preparation method and application. This material achieves stable and efficient binding of phosphate ions and Schiele minerals through a chemical pathway, fundamentally enhancing the composite system's ability to simultaneously fix cadmium and arsenic, resulting in a long-lasting and stable iron-phosphorus-based passivation material.

[0006] The first aspect of this invention provides a method for preparing an iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil, comprising the following steps: The Scheringer mineral precursor suspension was mixed with phosphate and reacted for 1 to 3 hours to allow phosphate ions to be loaded and intercalated on the surface of the Scheringer mineral, resulting in a slurry; the phosphorus in the phosphate accounted for 5% to 30% of the molar percentage of iron in the Scheringer mineral precursor suspension. The slurry was centrifuged, the precipitate was collected, washed, dried, ground, and sieved to obtain an iron-phosphorus-based passivation material.

[0007] This invention involves in-situ chemical modification of Scherstein mineral precursors using phosphate. Phosphate is introduced through chemical bonding and partially replaces sulfate in the Scherstein mineral structure. This process significantly enhances surface electronegativity while preserving the amorphous or weakly crystalline structural characteristics of Scherstein minerals.

[0008] This invention enables the passivation material to exhibit good immobilization effects on both cadmium and arsenic by controlling the molar percentage of phosphorus in the phosphate to be 5%~30% of the iron in the Scheres mineral precursor suspension. If the molar percentage of iron is too low, the phosphate loading will be insufficient, resulting in limited Cd immobilization capacity; if it is too high, it may lead to excessive release of phosphate into the environment, causing phosphorus pollution risks.

[0009] In another preferred embodiment, the phosphate is sodium dihydrogen phosphate or disodium hydrogen phosphate.

[0010] 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.

[0011] 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.

[0012] In another preferred embodiment, the acidic conditions are pH 1.8–2.2, the oxidation reaction temperature is 20°C–28°C, and the reaction time is 20–28 h; under pH 1.8–2.2 conditions, H₂O₂ / Fe 2+The value is 0.7~0.9. Under these conditions, the crystallinity and the number of surface active sites of the Schiele mineral can be guaranteed, thereby promoting the subsequent modification effect.

[0013] In another preferred embodiment, the sieving refers to passing through a 50-100 mesh sieve.

[0014] The second aspect of the present invention provides an iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil prepared by the preparation method described above.

[0015] The third aspect of the present invention provides the application of the iron-phosphorus-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.

[0016] In another preferred embodiment, the iron-phosphorus-based passivating material is applied at an amount of 1% to 5% of the dry weight of the soil.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes in-situ chemical modification with phosphates to create a material that combines the arsenic-fixing ability of iron-based minerals with the cadmium-fixing ability of phosphorus-based materials, achieving simultaneous passivation of Cd and As. The phosphate ions in the material and Cd... 2+ A stable Cd3(PO4)2 precipitate is formed, with iron hydroxyl groups reacting with AsO4. 3+ The formation of Fe–As complexes or co-precipitates significantly reduces the migration and bioavailability of both elements. The substitution of some sulfate ions with phosphate ions improves mineral structural stability, reduces the release of acidic ions, and avoids secondary Cd activation due to pH decrease during remediation. The resulting Cd3(PO4)2 and FeAsO4 precipitates have low solubility products and excellent environmental durability. No harmful byproducts are generated during material preparation, meeting the requirements of green chemistry.

[0018] The present invention features a simple and low-cost process: the method is a one-step in-situ synthesis that does not require high temperature and high pressure or complex equipment, has low energy consumption, and uses readily available raw materials. It is suitable for large-scale production and agricultural application, and has good economic benefits and promotion potential. Attached Figure Description

[0019] Figure 1 X-ray diffraction patterns of phosphate-modified and unmodified Schiele minerals.

[0020] Figure 2 The graph shows the Zeta potential of phosphate-modified and unmodified Schiele minerals as a function of pH.

[0021] Figure 3 Scanning electron microscope images of phosphate-modified and unmodified Schiele minerals.

[0022] Figure 4The figures show the adsorption and removal curves of Cd and As in water under different pH conditions; where a is the adsorption and removal curve of Cd in a single Cd system, b is the adsorption and removal curve of Cd in an As-Cd coexistence system, and c is the adsorption and removal curve of As in an As-Cd coexistence system.

[0023] Figure 5 The following diagrams show the effects of materials on the content of available Cd and As in contaminated soil under simulated paddy field flooding-drying conditions: (a) Effect of materials on the content of available Cd in soil; (b) Effect of materials on the content of available As in 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 cationic form, passivation strategies mainly revolve around cationic adsorption. For example, negatively charged materials such as biochar and phosphates can be used to fix them through ion exchange, surface complexation, and precipitation such as Cd3(PO4)2. In addition, phosphates can also be used as fertilizers to provide nutrients for plant growth.

[0026] As(V): Commonly found in arsenates such as HAsO4² - 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.

[0027] The above analysis shows 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 phosphates through ion exchange or complexation.

[0028] However, existing technologies typically target single pollution systems or single application scenarios. Specifically, studies on single pollution systems often focus solely on the adsorption of As(III), Cr(VI), or Cd; studies on single application scenarios primarily target mining environments and chromium-containing wastewater. It can be seen that existing heavy metal remediation materials suffer from a lack of material structure diversity. In current technologies, the bonding between phosphates and iron-based materials is mostly physical mixing, such as ball milling, resulting in weak bonding and easy desorption.

[0029] This invention targets complex pollution systems, specifically Cd-As co-polluting, particularly the complex antagonistic behavior of the two compounds in soil—namely, the anaerobic activation of Cd and the aerobic migration of As. It designs a passivation material capable of simultaneously remediating Cd-As pollution in paddy field soil. The material maintains a stable passivation effect even under the alternating redox changes of paddy field flooding and drying. This invention utilizes in-situ chemical modification to partially replace sulfate ions in Schöndorfite with phosphate ions through chemical bonds, achieving structural integration and functional synergy, thus ensuring structural stability.

[0030] The following is a detailed description of an iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil, its preparation method, and its application.

[0031] Example 1: Preparation and characterization of phosphate-modified Sigmite Material preparation: Prepare 1.5 L of 450 mM FeSO4·7H2O solution, and magnetically stir at 180 rpm under constant temperature of 25℃, according to the H2O2 / Fe... 2+ A 30wt% H₂O₂ solution was slowly added at a molar ratio of 0.8. During the reaction, the pH of the system was maintained at 2.0 by adding dilute H₂SO₄ or NaOH solution dropwise. After the reaction continued for 24 hours, a reddish-brown Schiele mineral precursor suspension was obtained.

[0032] Phosphate solutions were added at P / Fe molar ratios of 9% and 27%, respectively, and the reaction was continued with stirring for 2 hours. After the reaction was completed, the products were separated by centrifugation and repeatedly washed with deionized water to remove free ions.

[0033] The washed wet solids were air-dried at 30°C for 48 hours, ground, and passed through a 100-mesh sieve to obtain two modified Scherstein mineral powders with different phosphorus loadings, labeled as 9%P-Sch and 27%P-Sch, respectively.

[0034] Material characterization: Phase analysis of the products was performed by X-ray diffraction. Figure 1 The results showed that the introduction of phosphate did not change the typical amorphous or weakly crystalline structure of Schiele minerals.

[0035] Zeta potential test results ( Figure 2The results showed that phosphate ions were successfully loaded onto the mineral surface, causing the isoelectric point of the material to shift towards lower pH, thus enabling it to maintain its negative surface charge over a wider pH range.

[0036] Scanning electron microscopy observation ( Figure 3 It was found that phosphate modification inhibited the formation of standard Schöndorfite spherical aggregates, transforming them into finer and more irregular amorphous aggregates. This structural feature contributes to increasing the specific surface area and the number of surface active sites.

[0037] Example 2: Adsorption performance of the material for Cd and As in water Batch adsorption experiments were used to evaluate the adsorption performance of the material for Cd(II) and As(V). In the experiment, 10 mL of a 4 g / L mineral suspension and 10 mL of a 0.01 mol / L NaCl solution containing 0.6 mmol / L Cd(II) and 2 mmol / L As(V) were mixed in a 50 mL centrifuge tube. The mixture was shaken at 25 °C and 70 rpm for 24 h, with the pH maintained constant using trace amounts of HCl or NaOH solution. The initial pH was set to 3–10, and the initial concentrations of Cd(II) and As(V) were 0.3 mmol / L and 1 mmol / L, respectively. All treatments were performed in triplicate.

[0038] The results show that ( Figure 4 Compared with unmodified Shih minerals, the phosphate-modified materials showed a significant increase in Cd(II) adsorption capacity across the entire tested pH range. This is mainly attributed to the ability of phosphate groups loaded on the mineral surface to adsorb Cd. 2+ Stable specific coordination bonds are formed, and even cadmium phosphate precipitation is induced. For As(V), the modified material maintains a high adsorption capacity under acidic to neutral conditions; when pH>7, due to the similar chemical properties of phosphate and arsenate, competitive adsorption occurs at the iron hydroxyl sites, leading to a decrease in As(V) adsorption. However, in complex soil systems, this competitive effect may be offset by other environmental processes (see Example 3).

[0039] Example 3: Passivation effect of materials on Cd-As co-contaminated soil (1) Test Soil and Experimental Design: The test soil was collected from a Cd-As contaminated paddy field in Guixi City, Jiangxi Province, with a pH of 5.11, a total Cd concentration of 2.54 mg / kg, and a total As concentration of 23.85 mg / kg. The soil was air-dried, ground, and sieved before use. Four treatments were set up, with three replicates for each treatment. The details of each treatment group are as follows:

[0040] CK: Blank control, no materials added.

[0041] Sch: Unmodified Schiele mineral, added at 1% (w / w) of soil dry weight.

[0042] 27% P-Sch (low dose): 1% (w / w) added.

[0043] 27% P-Sch (high dose): 5% (w / w) added.

[0044] (2) Cultivation scheme: Simulate paddy field water management, with a total cultivation period of 70 days. The first 42 days are flooded cultivation with a water-to-soil ratio of 1:1.5 under anaerobic conditions, and the last 28 days are the drying period, i.e., natural water loss, under aerobic conditions. The entire cultivation process is carried out at 25℃ in a dark environment.

[0045] (3) Evaluation of passivation effect: Samples were taken on the 1st, 7th, 14th, 28th, and 42nd days of the flooding period and on the 12th (54th day in total) and 28th (70th day in total) days of the drying period, and the contents of available Cd (extracted from CaCl2) and available As (extracted from NaH2PO4) in the soil were determined respectively.

[0046] The results are as follows Figure 5 As shown, compared with the control (CK) and unmodified treatment, phosphate-modified Scheres minerals significantly reduced the concentrations of available Cd and As in the soil, exhibiting a simultaneous and continuous passivation effect. The mechanism includes the interaction between phosphate and Cd. 2+ The formation of insoluble Cd3(PO4)2 precipitate; under submerged reducing conditions, S²⁻ generated by sulfate reduction promotes CdS precipitate formation; the enhanced electronegativity of the material surface is beneficial to Cd 2+ The ion electrostatic adsorption and ion exchange; the iron-based amorphous structure provides a large number of adsorption sites for As and forms a more stable Fe-As composite phase in the alternation of dry and wet conditions; at the same time, the formation of Fe-Cd-P / As ternary complexes or coprecipitates may be the key pathway to achieve synchronous passivation.

[0047] In summary, this invention achieves synergistic optimization of the structure and function of Schiele minerals by introducing phosphate. This material effectively solves the problem of antagonistic chemical behavior between Cd and As in complex contaminated soils by comprehensively utilizing precipitation, adsorption, and complexation mechanisms, achieving long-term stabilization of both. It has significant application potential and promotional value in the field of heavy metal pollution remediation in farmland.

[0048] 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. A method for preparing an iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil, characterized in that, Includes the following steps: The Scheringer mineral precursor suspension was mixed with phosphate and reacted for 1 to 3 hours to allow phosphate ions to be loaded and intercalated on the surface of the Scheringer mineral, resulting in a slurry; the phosphorus in the phosphate accounted for 5% to 30% of the molar percentage of iron in the Scheringer mineral precursor suspension. The slurry was centrifuged, the precipitate was collected, washed, dried, ground, and sieved to obtain an iron-phosphorus-based passivation material.

2. The method for preparing iron-phosphorus-based passivation material for cadmium-arsenic co-contaminated soil according to claim 1, characterized in that, The phosphate is sodium dihydrogen phosphate or disodium hydrogen phosphate.

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

7. An iron-phosphorus-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-phosphorus-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 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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