A method for passivating arsenic-cadmium co-contaminated soil

Abstract

The present application relates to the technical field of soil regeneration, and discloses a passivation method for arsenic-cadmium composite contaminated soil, which comprises the following steps: S1, any two or three of calcium-based bentonite, vermiculite and ferric sulfate are fully mixed in proportion and subjected to ball milling activation treatment to form a mineral mixture; S2, pretreated manganese dioxide nanoparticles and trivalent iron salt are added to the mineral mixture, the pH of the reaction system is adjusted to 4.0-5.5, and the reaction is carried out under stirring for 8-12 hours to form particles coated with a composite oxide layer on the surface; S3, the particles coated with the composite oxide layer on the surface are dried and crushed, and are coated with sulfhydrylated chitosan to obtain passivation materials; and S4, the passivation materials and the arsenic-cadmium composite contaminated soil are mixed in proportion and subjected to ball milling. The present application can solve the problem of poor synchronous passivation effect of soil arsenic and cadmium.

Description

Technical Field

[0001] This invention relates to the field of soil regeneration technology, and in particular to a passivation method for soil contaminated with arsenic and cadmium. Background Technology

[0002] Soil is the foundation of human survival and development. With the rapid development of society, economy, industry, and agriculture, soil, as the final carrier of air and water pollution, sees large amounts of heavy metals entering the soil environment through various pathways such as industrial wastewater discharge, solid waste storage and landfill, atmospheric deposition, and fertilizers and pesticides, causing soil heavy metal pollution. Cadmium mainly exists in soil in the form of physical adsorption, and the content of exchangeable cadmium increases with the total amount of cadmium. It is highly mobile in soil. Soil arsenic mainly exists as +3 and +5 valent inorganic anions (AsO3). 3- and AsO4 3- Arsenic and cadmium exist in various forms. They are difficult to degrade in soil, and are highly mobile and toxic. After being absorbed by plants, they can accumulate in the human body through the food chain, posing a significant threat to agricultural product safety and human health. Acidic soils not only affect crop growth but also facilitate the migration of heavy metals. Therefore, the search for arsenic and cadmium remediation materials and technologies in soil is urgently needed.

[0003] Currently, domestic and international methods for remediating heavy metal pollution in soil mainly include methods such as leaching, chemical oxidation, and phytoremediation, which reduce the content of heavy metals in soil through extraction and separation; and techniques such as passivation / stabilization, which increase the stability of heavy metals in the soil and reduce their migration and bioavailability. While leaching and chemical oxidation can rapidly reduce the content of heavy metals in soil, they damage soil structure, are detrimental to plant growth, and are costly, making them unsuitable for arable land remediation. Phytoremediation, although low-cost, is time-consuming, occupies arable land, and has significant regional limitations. Passivation / stabilization, due to its low cost and simple operation, has become one of the most common soil heavy metal remediation technologies. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a passivation method for soil contaminated with arsenic and cadmium, which aims to solve the problem of poor simultaneous passivation effect of arsenic and cadmium in soil.

[0005] The present invention solves the above-mentioned technical problems through the following technical means:

[0006] A passivation method for arsenic-cadmium co-contaminated soil includes the following steps:

[0007] S1. Mix any two or three of calcium-based bentonite, vermiculite and ferric sulfate in a certain proportion and then activate them by ball milling to form a mineral mixture;

[0008] S2. Add pretreated manganese dioxide nanoparticles and ferric salts to the mineral mixture, adjust the pH of the reaction system to 4.0-5.5, and react for 8-12 hours under stirring to form particles with a composite oxide layer on the surface.

[0009] S3. Dry and pulverize the particles coated with a composite oxide layer, then coat them with thiolated chitosan to obtain a passivated material;

[0010] S4. Mix the passivation material and the arsenic-cadmium contaminated soil in a certain proportion and then ball mill them.

[0011] Based on multiple mechanisms, this method achieves efficient stabilization and passivation of As and Cd ions in soil through a multi-step synergistic effect involving mineral modification, oxidative coating, functional polymer encapsulation, and ball milling coupling enhancement. It utilizes the synergistic effect between natural clay minerals and iron salts, as well as the surface complexation and slow-release control of functionalized modified materials, ultimately forming a composite passivation system with strong immobilization capabilities for heavy metal pollution.

[0012] In step S1, non-toxic natural minerals or inorganic salts from calcium-based bentonite, vermiculite, and ferric sulfate are selected. Firstly, calcium-based bentonite and vermiculite possess adsorption and ion exchange capabilities, allowing them to adsorb cadmium ions in the soil. Simultaneously, calcium ions and arsenate ions can form insoluble calcium arsenate compounds, thus reducing the content of available arsenic and cadmium in the soil and achieving arsenic and cadmium passivation. Secondly, ferric sulfate readily forms insoluble ferric arsenate compounds with arsenate ions in the soil. When combined with calcium-based bentonite and vermiculite, this enhances the passivation effect of the passivating materials on both arsenic and cadmium. Thirdly, the mechanical impact force of a ball mill reduces the particle size of both the soil and the passivating materials, increasing the specific surface area and causing lattice defects in the minerals. The heat generated during the impact provides heat for chemical adsorption, further enhancing the passivation effect of the materials on arsenic and cadmium in the soil. Finally, calcium-based bentonite and vermiculite can effectively increase the pH value of acidic soils, reducing the migration of arsenic and cadmium in the soil, while also reducing soil acidity to prevent negatively impacting plant growth.

[0013] The manganese dioxide nanoparticles introduced in step S2 have strong oxidizing capabilities, which can oxidize highly toxic trivalent arsenic in the soil into less toxic and easily precipitated pentavalent arsenic. Simultaneously, by adding trivalent ferric salts (such as ferric chloride and ferric nitrate) to adjust the pH of the reaction system and participate in the oxidation-precipitation reaction, a stable iron-manganese composite oxide layer is formed in situ on the mineral surface. This composite layer possesses a high specific surface area and multiple functional groups, enabling it to synergistically fix As and Cd through multiple mechanisms such as physical adsorption, complexation, and ion exchange.

[0014] In step S3, thiolized chitosan is used to coat the surface of the composite oxide particles, further enhancing the material's complexation ability with metal ions. Thiol groups have a strong affinity and can bind to As(III) and Cd. 2+A stable chelated structure is formed, while the chitosan molecular framework endows the passivation material with good biocompatibility and environmental stability, inhibiting the re-release of heavy metals. In addition, the coating layer also has certain slow-release and barrier functions, preventing the composite oxide layer from being rapidly destroyed in acidic or oxidizing environments, thus extending the material's effective period.

[0015] Step S4 involves ball milling again to thoroughly mix the passivation material with the contaminated soil and achieve interface reconstruction. Under the action of high-energy mechanical force, the passivation particles can penetrate deep into the soil matrix, fully contact As and Cd in the soil, enhance micro-area reactions, and further improve passivation efficiency.

[0016] Furthermore, in step S1, the contents of calcium-based bentonite, vermiculite, and ferric sulfate are 0-80%, 0-80%, and 0-20%, respectively.

[0017] Furthermore, in the ball milling process of step S1, the mass ratio of agate balls to the mixture is (10-20):1, the ball milling speed is 400-600 r / min, and the ball milling activation time is 1-3 hours.

[0018] Furthermore, the average particle size of the pretreated manganese dioxide nanoparticles in step S2 is 30-80 nm, and the dosage is 2-8 wt% of the mineral mixture.

[0019] Furthermore, the preparation steps of the pretreated manganese dioxide nanoparticles include: soaking the manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 1-3 hours, followed by centrifugation, washing, and drying.

[0020] Furthermore, the mass ratio of the manganese dioxide nanoparticles to potassium permanganate is 1:(2-10).

[0021] Further, the ferric salt in step S2 is ferric chloride or ferric sulfate, and the dosage is 2-10 wt% of the mineral mixture.

[0022] Furthermore, the preparation steps of the thiolated chitosan coating include: dispersing the obtained particulate powder in a thiolated chitosan acetic acid solution, wherein the concentration of thiolated chitosan is 1-2 wt%, and slowly adding TPP crosslinking agent to form a controlled-release film layer.

[0023] Furthermore, the method for preparing the thiolated chitosan includes the following steps:

[0024] A1. Dissolve chitosan in a 1%-2% volume fraction of glacial acetic acid aqueous solution, add 1%-3% mass fraction of citric acid as a carboxylating agent, adjust the pH of the reaction system to 5.0-6.0, and react in a water bath at 50-60℃ for 4-6 hours to obtain a carboxylated chitosan solution.

[0025] A2. Add activator EDC / NHS to the carboxylated chitosan solution, control the pH of the reaction system at 5.0-5.5, control the temperature at 25-35℃, stir the reaction for 0.5-1 hour to form an activated solution;

[0026] A3. Add 2-amino-1,3-propanedithiol to the activation solution and react for 2-4 hours;

[0027] A4. Adjust the reaction pH to 6.0-6.5, add glutaraldehyde solution, and react for 2-3 hours to form a three-dimensional network structure of cross-linked chitosan gel;

[0028] A5. After freeze-drying the cross-linked chitosan gel, it is pulverized into thiolated chitosan particles with a particle size of 200-300 mesh.

[0029] Furthermore, the molar ratio of EDC to NHS in step A2 is 1:1 to 1.5:1.

[0030] Furthermore, the amount of 2-hydroxy-1,3-propanedithiol added in step A3 is 3%-10% of the mass of chitosan.

[0031] Furthermore, the concentration of glutaraldehyde in step A4 is 0.2%-0.5%, and the crosslinking temperature is room temperature or below 30°C.

[0032] Chitosan molecules contain a large number of primary amino groups (-NH2) and hydroxyl groups (-OH) in their main chain, resulting in relatively low reactivity and limiting the efficiency of subsequent functionalization reactions. Citric acid is introduced as a carboxylating agent, reacting with the -OH or -NH2 groups on the chitosan molecule at 50-60℃ and pH 5.0-6.0 to undergo esterification or amidation reactions, introducing -COOH groups. EDC is a water-soluble carboxyl activator that can convert the carboxyl groups on the chitosan chain into a highly reactive O-acylurea intermediate; however, this intermediate is unstable in water and easily hydrolyzed. To improve reaction efficiency, NHS (N-hydroxysuccinimide) is added, reacting with the intermediate to form a stable NHS ester, thereby improving the coupling efficiency with subsequent amine or hydroxyl-based grafted molecules. 2-Amino-1,3-propanedithiol is a dithiol grafting agent containing both thiol (-SH) and amino (-NH2) reactive sites in its molecule. It can undergo nucleophilic substitution with activated NHS esters to form an amide bond covalently linked structure, achieving directional, efficient, and stable grafting of thiol groups onto the chitosan backbone, generating thiolized chitosan with excellent chelating ability. At pH 6.0–6.5, the addition of glutaraldehyde crosslinking agent to the system leads to aldehyde-amine condensation with the free amino groups on the chitosan backbone, constructing a three-dimensional network crosslinked gel. This crosslinked network structure effectively enhances the structural stability, water insolubility, and controlled-release ability of the thiolized chitosan, making it particularly suitable for long-term application in moist soil systems. Freeze-drying effectively preserves the porous network in the gel structure, facilitating the exposure of thiol active sites during subsequent passivation material coating, and enhancing its coordination and chelation ability with arsenic (As) and cadmium (Cd).

[0033] The thiol group (–SH) has extremely strong nucleophilicity and can react with heavy metal cations (such as As). 3+ Cd 2+等 Stable coordination complexes are formed through electron pairs. The three-dimensional network structure formed by glutaraldehyde crosslinking during preparation not only enhances the mechanical strength and environmental stability of the chitosan material but also creates a microporous-mesoporous composite structure, exhibiting characteristics similar to a "slow-release carrier." After encapsulating passivating active substances (such as iron salts, manganese oxides, clay minerals, etc.), this structure can achieve:

[0034] It releases rapidly in the initial stage and quickly passivates high concentrations of metal ions;

[0035] Stable release in the medium term, maintaining the dynamic equilibrium of the passivation reaction;

[0036] The material is released slowly in the later stages, extending its shelf life and reducing the frequency of reapplication.

[0037] Thiol-modified chitosan, through a combination of physical coating and chemical cross-linking, effectively encapsulates internal iron / manganese functional particles, preventing them from rapidly dissolving or becoming passivated in highly acidic or saline soils, while also inhibiting aggregation or loss of reactivity. This effect improves the overall stability and uniformity of the passivation material.

[0038] Furthermore, the concentration of the TPP crosslinking agent is 0.5-1 wt%.

[0039] Furthermore, in step S4, the mass ratio of the passivation material to the arsenic-cadmium contaminated soil is (1-5):100.

[0040] Furthermore, in the ball milling process in step S4, the mass ratio of agate balls to passivation material to the arsenic-cadmium composite contaminated soil mixture is (10-20):1, the ball mill speed is 400-600 r / min, and the ball milling passivation lasts for 1-3 hours.

[0041] The beneficial effects of this invention are:

[0042] (1) The present invention uses calcium-based bentonite, vermiculite and ferric sulfate mixed together. The layered structure of bentonite and vermiculite gives them strong ion exchange and complexation capabilities, while the iron ion deposition formed by ferric sulfate during ball milling can form a stable hydroxy iron complex precipitate with As / Cd, which enhances the initial fixation effect of heavy metals.

[0043] (2) The pretreated manganese dioxide nanoparticles added in this invention have a high specific surface area and oxidation potential, which can efficiently oxidize As(III) to As(V), which is more easily precipitated, and at the same time react with Fe. 3+ Together, they form Fe-Mn composite oxide films. These films possess charge modulation capabilities, hydrophobic interfaces, and high electron affinity, exhibiting not only excellent As adsorption and immobilization abilities but also effectively capturing and complexing Cd. 2+ Adjusting the pH is beneficial for Fe. 3+ It forms hydroxide or phosphate precipitates with As and Cd, further enhancing the simultaneous stabilization and fixation of heavy metals.

[0044] (3) This invention introduces thiolized chitosan material as a surface coating layer. Its thiol groups (–SH) and amino groups (–NH2) on the chitosan backbone together form a multi-site complexing system, which can bind with Cd. 2+ It forms stable chelate complexes with heavy metal ions such as As(V), preventing their re-release in the soil microenvironment. At the same time, the natural polymer backbone of chitosan has certain adhesiveness and slow-release properties, which can form a relatively stable interfacial connection between the material and soil particles, improving the environmental stability, acid and alkali resistance and long-term effectiveness of the passivation material. Detailed Implementation

[0045] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0046] The arsenic-cadmium co-contaminated soil was selected from air-dried arsenic-cadmium co-contaminated soil around a lead-zinc mine. The total arsenic and cadmium content and available content of the tested soil are as follows: total arsenic 61 mg / kg, total cadmium 7.15 mg / kg; available arsenic 0.48 mg / kg, available cadmium 0.13 mg / kg.

[0047] Example 1

[0048] A passivation method for arsenic-cadmium co-contaminated soil includes the following steps:

[0049] The ratio of calcium-based bentonite, vermiculite, and ferric sulfate is 80%:0:20%.

[0050] S1. Mix any two or three of the following: calcium-based bentonite, vermiculite, and ferric sulfate in a specific ratio and then activate them by ball milling to form a mineral mixture. During ball milling, the mass ratio of agate balls to the mixture is 10:1, the ball milling speed is 400 r / min, and the activation time is 1 hour.

[0051] S2. Add pretreated manganese dioxide nanoparticles and ferric salt to the mineral mixture, adjust the pH of the reaction system to 4.0, and react for 8 hours under stirring to form particles with a composite oxide layer on the surface; wherein the average particle size of the pretreated manganese dioxide nanoparticles is 30 nm, and the amount added is 2 wt% of the mass of the mineral mixture.

[0052] The preparation steps of pretreated manganese dioxide nanoparticles include: soaking manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 1 hour, followed by centrifugation, washing, and drying. The mass ratio of manganese dioxide nanoparticles to potassium permanganate is 1:2.

[0053] S3. The particles coated with the composite oxide layer are dried and pulverized, and then coated with thiolated chitosan (the obtained particle powder is dispersed in a thiolated chitosan acetate solution, the concentration of thiolated chitosan is 1 wt%, the mass ratio of composite particles to thiolated chitosan acetate solution is 1:2, and TPP crosslinking agent is slowly added to form a controlled-release film layer; the amount of TPP added is controlled at 50 wt% of the mass of the composite particles) to obtain a passivated material;

[0054] The preparation method of thiolated chitosan includes the following steps:

[0055] A1. Dissolve chitosan in a 1% (v / v) aqueous solution of glacial acetic acid, add 1% (w / w) citric acid as a carboxylating agent, the amount of citric acid added is 1% of the mass of chitosan, adjust the pH of the reaction system to 5.0, react in a 50℃ water bath for 6 hours to obtain a carboxylated chitosan solution.

[0056] A2. Add activator EDC / NHS to the carboxylated chitosan solution. Control the pH of the reaction system at 5.0 and the temperature at 25℃. Stir the reaction for 1 hour to form an activated solution. The amount of EDC added is 10% of the mass of chitosan, and the amount of NHS added is 5% of the mass of chitosan.

[0057] A3. Add 2-amino-1,3-propanedithiol to the activation solution and react for 2 hours. The amount of 2-hydroxy-1,3-propanedithiol added is 3% of the mass of chitosan.

[0058] A4. Adjust the reaction pH to 6.0, add glutaraldehyde solution, and react for 2 hours to form a three-dimensional network structure of cross-linked chitosan gel; the concentration of glutaraldehyde is 0.2%, and the cross-linking temperature is room temperature or below 30℃. The amount of glutaraldehyde solution added is 0.1% of the mass of chitosan.

[0059] A5. After freeze-drying the cross-linked chitosan gel, it is pulverized into thiolized chitosan particles with a particle size of 200 mesh.

[0060] The TPP crosslinking agent concentration is 0.5 wt%.

[0061] S4. Mix the passivation material and the arsenic-cadmium contaminated soil in a certain proportion and then ball mill them.

[0062] The mass ratio of passivation material to arsenic-cadmium contaminated soil was 1:100.

[0063] During the ball milling process, the mass ratio of agate balls to passivation material to the arsenic-cadmium contaminated soil mixture was 10:1, the ball mill speed was 400 r / min, and the ball milling passivation lasted for 1 hour.

[0064] Example 2

[0065] A passivation method for arsenic-cadmium co-contaminated soil includes the following steps:

[0066] The ratio of calcium-based bentonite, vermiculite, and ferric sulfate is 40%:40%:20%.

[0067] S1. Mix any two or three of the following: calcium-based bentonite, vermiculite, and ferric sulfate in a specific ratio and then activate them by ball milling to form a mineral mixture. During the ball milling process, the mass ratio of agate balls to the mixture is 15:1, the ball milling speed is 500 r / min, and the ball milling activation time is 2 hours.

[0068] S2. Add pretreated manganese dioxide nanoparticles and ferric salt to the mineral mixture, adjust the pH of the reaction system to 5.0, and react for 10 hours under stirring to form particles with a composite oxide layer on the surface; wherein the average particle size of the pretreated manganese dioxide nanoparticles is 55 nm, and the amount added is 5 wt% of the mass of the mineral mixture.

[0069] The preparation steps of pretreated manganese dioxide nanoparticles include: soaking manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 2 hours, followed by centrifugation, washing, and drying. The mass ratio of manganese dioxide nanoparticles to potassium permanganate is 1:6.

[0070] S3. The particles coated with the composite oxide layer are dried and pulverized, and then coated with thiolated chitosan (the obtained particle powder is dispersed in a thiolated chitosan acetic acid solution, the concentration of thiolated chitosan is 1.5 wt%, and TPP crosslinking agent is slowly added dropwise to form a controlled-release film layer, the amount of TPP added is controlled at 60 wt% of the mass of the composite particles) to obtain a passivated material;

[0071] The preparation method of thiolated chitosan includes the following steps:

[0072] A1. Dissolve chitosan in a 1.5% (v / v) aqueous solution of glacial acetic acid, add 2% (w / w) citric acid as a carboxylating agent, the amount of citric acid added is 2% of the mass of chitosan, adjust the pH of the reaction system to 5.5, react in a 55℃ water bath for 5 hours to obtain a carboxylated chitosan solution.

[0073] A2. Add activator EDC / NHS to the carboxylated chitosan solution. Control the pH of the reaction system at 5.5 and the temperature at 30℃. Stir the reaction for 0.75 hours to form an activated solution. The amount of EDC added is 15% of the mass of chitosan, and the amount of NHS added is 10% of the mass of chitosan.

[0074] A3. Add 2-amino-1,3-propanedithiol to the activation solution and react for 3 hours. The amount of 2-hydroxy-1,3-propanedithiol added is 6.5% of the mass of chitosan.

[0075] A4. Adjust the reaction pH to 6.0, add glutaraldehyde solution, and react for 2.5 hours to form a three-dimensional network structure of cross-linked chitosan gel; the concentration of glutaraldehyde is 0.2%, and the cross-linking temperature is room temperature or below 30℃. The amount of glutaraldehyde solution added is 0.15% of the mass of chitosan.

[0076] A5. After freeze-drying the cross-linked chitosan gel, it is pulverized into thiolized chitosan particles with a particle size of 250 mesh.

[0077] The TPP crosslinking agent concentration is 0.75 wt%.

[0078] S4. Mix the passivation material and the arsenic-cadmium contaminated soil in a certain proportion and then ball mill them.

[0079] The mass ratio of passivation material to arsenic-cadmium contaminated soil was 3:100.

[0080] During the ball milling process, the mass ratio of agate balls to passivation material to the arsenic-cadmium contaminated soil mixture was 15:1, the ball mill speed was 500 r / min, and the ball milling passivation lasted for 2 hours.

[0081] Example 3

[0082] A passivation method for arsenic-cadmium co-contaminated soil includes the following steps:

[0083] The ratio of calcium-based bentonite, vermiculite, and ferric sulfate is 0:80%:20%.

[0084] S1. Mix any two or three of the following: calcium-based bentonite, vermiculite, and ferric sulfate in a specific ratio and then activate them by ball milling to form a mineral mixture. During the ball milling process, the mass ratio of agate balls to the mixture is 20:1, the ball milling speed is 600 r / min, and the ball milling activation lasts for 3 hours.

[0085] S2. Add pretreated manganese dioxide nanoparticles and ferric salt to the mineral mixture, adjust the pH of the reaction system to 5.5, and react for 12 hours under stirring to form particles with a composite oxide layer on the surface; wherein the average particle size of the pretreated manganese dioxide nanoparticles is 80 nm, and the amount added is 8 wt% of the mass of the mineral mixture.

[0086] The preparation steps of pretreated manganese dioxide nanoparticles include: soaking manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 3 hours, followed by centrifugation, washing, and drying. The mass ratio of manganese dioxide nanoparticles to potassium permanganate is 1:10.

[0087] S3. The particles coated with the composite oxide layer are dried and pulverized, and then coated with thiolated chitosan (the obtained particle powder is dispersed in a thiolated chitosan acetic acid solution, the concentration of thiolated chitosan is 2wt%, and TPP crosslinking agent is slowly added dropwise to form a controlled-release film layer, the amount of TPP added is controlled at 70wt% of the mass of the composite particles) to obtain a passivation material.

[0088] The preparation method of thiolated chitosan includes the following steps:

[0089] A1. Dissolve chitosan in a 2% (v / v) aqueous solution of glacial acetic acid, add 3% (w / w) citric acid as a carboxylating agent, the amount of citric acid added is 3% of the mass of chitosan, adjust the pH of the reaction system to 6.0, and react in a water bath at 50-60℃ for 4-6 hours to obtain a carboxylated chitosan solution.

[0090] A2. Add activator EDC / NHS to the carboxylated chitosan solution. Control the pH of the reaction system at 5.5 and the temperature at 35℃. Stir the reaction for 0.5 hours to form an activated solution. The amount of EDC added is 20% of the mass of chitosan, and the amount of NHS added is 15% of the mass of chitosan.

[0091] A3. Add 2-amino-1,3-propanedithiol to the activation solution and react for 2-4 hours. The amount of 2-hydroxy-1,3-propanedithiol added is 10% of the mass of chitosan.

[0092] A4. Adjust the reaction pH to 6.5, add glutaraldehyde solution, and react for 2-3 hours to form a three-dimensional network structure of cross-linked chitosan gel; the concentration of glutaraldehyde is 0.2%, and the cross-linking temperature is room temperature. The amount of glutaraldehyde solution added is 0.2% of the mass of chitosan.

[0093] A5. After freeze-drying the cross-linked chitosan gel, it is pulverized into thiolized chitosan particles with a particle size of 300 mesh.

[0094] The TPP crosslinking agent concentration is 1 wt%.

[0095] S4. Mix the passivation material and the arsenic-cadmium contaminated soil in a certain proportion and then ball mill them.

[0096] The mass ratio of passivation material to arsenic-cadmium co-contaminated soil was 5:100.

[0097] During the ball milling process, the mass ratio of agate balls to passivation material to the arsenic-cadmium contaminated soil mixture was 20:1, the ball mill speed was 600 r / min, and the ball milling passivation lasted for 3 hours.

[0098] Example 4

[0099] The difference from Example 2 is that the ratio of calcium-based bentonite, vermiculite, and ferric sulfate is 50%:40%:10%. The other steps are the same as in Example 2.

[0100] Example 5

[0101] The difference from Example 2 is that the ratio of calcium-based bentonite, vermiculite, and ferric sulfate is 50%:50%:0. The other steps are the same as in Example 2.

[0102] Comparative Example 1

[0103] The difference from Example 2 is that chitosan is used instead of thiolated chitosan, while the other steps are the same as in Example 2.

[0104] Comparative Example 2

[0105] The difference from Example 2 is that manganese dioxide nanoparticles are used instead of pretreated manganese dioxide nanoparticles, while the other steps are the same as in Example 2.

[0106] Comparative Example 3

[0107] The difference from Example 2 is that steps S2 and S3 are omitted, and no coating is performed.

[0108] Comparative Example 4

[0109] The difference from Example 2 is that step S3 to obtain the passivation material is omitted, and only one step of coating (Fe / Mn oxide layer) is performed.

[0110] experiment

[0111] The available forms of arsenic and cadmium in the soil were extracted using NaHCO3 and DTPA, respectively, and the soil pH was measured using a pH meter.

[0112] The experimental data are shown in Table 1.

[0113] Table 1 Experimental Data

[0114]

[0115]

[0116] Regarding arsenic passivation, in Example 2, available arsenic in the treated soil was undetectable, representing a 100% reduction, indicating its extremely strong passivation ability for arsenic. This is mainly attributed to the synergistic effect of the following multiple mechanisms of action:

[0117] 1. The -SH groups in thiolated chitosan have a good complexing ability for As ions;

[0118] 2. Pretreated manganese dioxide nanoparticles exhibit enhanced surface reactivity after oxidation with KMnO4, promoting the oxidation, precipitation, and adsorption of As(V) ions;

[0119] 3. Iron ions readily form hydroxy oxides such as Fe-OOH under pH conditions of 4.0–5.5, which enhances the co-precipitation effect on As ions.

[0120] 4. The "two-step coating process" adopted (i.e., Fe / Mn oxide + thiolated chitosan double coating).

[0121] In contrast, Comparative Example 1 (without thiol function) and Comparative Example 2 (without MnO2 pretreatment) could also passivate As to some extent, with the effective arsenic decreasing by 66.3% and 72.1% respectively, but significantly lower than Example 2, verifying the key role of thiol functional groups and active MnO2 in the arsenic passivation process.

[0122] Regarding cadmium passivation, after treatment in Example 2, the available Cd decreased to 0.036 mg / kg, a reduction of 72.71%. This effect was also superior to Comparative Example 1 (56.20%) and Comparative Example 2 (60.00%). The thiol, amino, and hydroxyl functional groups present in thiolated chitosan can bind with Cd through chelation. 2+ The formation of stable complexes, along with the controlled-release membrane layer formed by the coating structure, helps to enhance the contact time and spatial shielding between the material and pollutants, thereby enhancing the passivation persistence.

[0123] In contrast, although Comparative Example 4 underwent partial coating treatment, the reduction in arsenic and cadmium in chitosan was significantly reduced due to the absence of thiolated chitosan, demonstrating the chelating and sustained-release effects of thiolated chitosan.

[0124] Comparative Example 3, which was not coated at all, showed reductions of only 42.08% and 28.46% in arsenic and cadmium, respectively, further emphasizing the necessity of coating processes in heavy metal passivation.

[0125] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.

Claims

1. A passivation method for soil contaminated with arsenic and cadmium, characterized in that, Includes the following steps: S1. Mix any one of calcium-based bentonite and vermiculite with ferric sulfate in a certain proportion and then activate it by ball milling to form a mineral mixture; S2. Add pretreated manganese dioxide nanoparticles and ferric salts to the mineral mixture, adjust the pH of the reaction system to 4.0-5.5, and react for 8-12 hours under stirring to form particles with a composite oxide layer on the surface. S3. Dry and pulverize the particles coated with a composite oxide layer, then coat them with thiolated chitosan to obtain a passivated material; S4. Mix the passivation material and the arsenic-cadmium contaminated soil in a certain proportion and then ball mill them; The preparation steps of the pretreated manganese dioxide nanoparticles include: soaking the manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 1-3 hours, followed by centrifugation, washing, and drying.

2. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, In step S1, the contents of calcium-based bentonite, vermiculite, and ferric sulfate are 0-80%, 0-80%, and 0-20%, respectively.

3. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, In the ball milling process of step S1, the mass ratio of agate balls to the mixture is (10-20):1, the ball milling speed is 400-600 r / min, and the ball milling activation time is 1-3 hours.

4. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, The average particle size of the pretreated manganese dioxide nanoparticles in step S2 is 30-80 nm, and the dosage is 2-8 wt% of the mineral mixture.

5. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, The ferric salt in step S2 is ferric chloride or ferric sulfate, and the dosage is 2-10 wt% of the mineral mixture.

6. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, The preparation steps of the thiolated chitosan coating include: dispersing the obtained particulate powder in a thiolated chitosan acetate solution, wherein the concentration of thiolated chitosan is 1-2 wt%, and slowly adding TPP crosslinking agent to form a controlled-release film layer.

7. The passivation method for arsenic-cadmium co-contaminated soil according to claim 6, characterized in that, The method for preparing the thiolated chitosan includes the following steps: A1. Dissolve chitosan in a 1%-2% volume fraction of glacial acetic acid aqueous solution, add 1%-3% mass fraction of citric acid as a carboxylating agent, adjust the pH of the reaction system to 5.0-6.0, and react in a 55℃ water bath for 4-6 hours to obtain a carboxylated chitosan solution. A2. Add activator EDC / NHS to the carboxylated chitosan solution, control the pH of the reaction system at 5.0-5.5, control the temperature at 25-35℃, stir the reaction for 0.5-1 hour to form an activated solution; A3. Add 2-amino-1,3-propanedithiol to the activation solution and react for 2-4 hours; A4. Adjust the reaction pH to 6.0-6.5, add glutaraldehyde solution, and react for 2-3 hours to form a three-dimensional network structure of cross-linked chitosan gel; A5. After freeze-drying the cross-linked chitosan gel, it is pulverized into thiolated chitosan particles with a particle size of 200-300 mesh.

8. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, In step S4, the mass ratio of the passivation material to the arsenic-cadmium contaminated soil is (1-5):

100.

9. The passivation method for arsenic-cadmium co-contaminated soil according to claim 1, characterized in that, In the ball milling process in step S4, the mass ratio of agate balls to passivation material to arsenic-cadmium contaminated soil mixture is (10-20):1, the ball mill speed is 400-600 r / min, and the ball milling passivation is carried out for 1-3 hours.

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