Passivation method for arsenic-cadmium combined polluted soil

Abstract

The invention relates to the technical field of soil regeneration, and discloses a passivation method of arsenic-cadmium combined polluted soil, which comprises the following steps: S1, fully mixing any two or three of calcium bentonite, vermiculite and ferric sulfate in proportion, and carrying out ball milling activation treatment to form a mineral mixture; s2, adding pretreated manganese dioxide nanoparticles and ferric iron salt into the mineral mixture, adjusting the pH value of a reaction system to 4.0-5.5, and reacting for 8-12 hours under a stirring condition to form particles coated with a composite oxide layer on the surface; s3, drying and crushing the particles of which the surfaces are coated with the composite oxide layers, and carrying out sulfhydrylation chitosan coating, so as to obtain a passivation material; and S4, mixing the passivation material and the arsenic-cadmium combined polluted soil according to a ratio, and carrying out ball milling. The problem that the synchronous passivation effect of arsenic and cadmium in soil is poor can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of soil regeneration, and in particular to a passivation method for soil contaminated with arsenic and cadmium compounds. Background Art

[0002] Soil is the foundation of human survival and development. With the rapid development of social economy, industry and agriculture, soil has become the ultimate carrier of air pollution and water pollution. A large amount of heavy metal elements enter the soil environment through industrial wastewater discharge, solid waste storage and landfill, atmospheric deposition, fertilizers and pesticides, etc., causing soil heavy metal pollution. Cadmium mainly exists in the soil in the form of physical adsorption. The content of exchangeable cadmium increases with the increase of the total amount of cadmium. It is very mobile in the soil. Soil arsenic mainly exists in the form of +3 and +5 valent inorganic anions (AsO3 3- and AsO4 3- Arsenic and cadmium are difficult to degrade in soil. They are also highly mobile and toxic. Once absorbed by plants, they can enter the human body through the food chain and accumulate, posing a significant threat to agricultural product safety and human health. Acidic soil not only affects crop growth but also facilitates the migration of heavy metals. Therefore, the search for materials and technologies to remediate arsenic and cadmium in soil is urgent.

[0003] Currently, the main methods for remediating heavy metal contaminated soils, both domestically and internationally, include methods such as leaching, chemical oxidation, and plant uptake, which reduce heavy metal content in soils through extraction and separation, and passivation / stabilization, which increase the stability of heavy metals in soils and reduce their migration and bioavailability. While leaching and chemical oxidation can quickly reduce heavy metal content in soils, they disrupt soil structure, hindering plant growth. They are also relatively expensive and unsuitable for remediating arable soils. While plant uptake is low-cost, it takes a long time to remediate, occupies arable land, and has significant regional limitations. Passivation / stabilization has become one of the most common technologies for remediating heavy metals in soils due to its low cost and ease of operation. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a passivation method for arsenic-cadmium composite contaminated soil, aiming to solve the problem of poor simultaneous passivation effect of soil arsenic and cadmium.

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

[0006] A passivation method for soil contaminated with arsenic and cadmium comprises the following steps:

[0007] S1. Any two or three of calcium bentonite, vermiculite and ferric sulfate are fully mixed in proportion and activated by ball milling to form a mineral mixture;

[0008] S2. Pretreated manganese dioxide nanoparticles and ferric iron salt were added to the mineral mixture, the pH of the reaction system was adjusted to 4.0-5.5, and the reaction was stirred for 8-12 hours to form particles coated with a composite oxide layer;

[0009] S3. The particles coated with a composite oxide layer are dried and crushed, and thiolated chitosan is coated to obtain a passivation material;

[0010] S4. Mix the passivation material and the arsenic-cadmium composite contaminated soil in proportion and perform ball milling.

[0011] Based on multiple mechanisms of action, this method achieves efficient stabilization and passivation of As and Cd ions in soil through the synergistic effects of mineral modification, oxidation coating, functional polymer encapsulation, and ball milling coupling enhancement. This method 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 fixation capabilities for heavy metal pollution.

[0012] In step S1, non-toxic natural minerals or inorganic salt materials in calcium bentonite, vermiculite and ferric sulfate are selected. First, calcium bentonite and vermiculite themselves have adsorption and ion exchange capacity, can adsorb cadmium ions in soil, and calcium ions and arsenate can form calcium arsenate insoluble compounds, thus, can reduce the content of effective arsenic and cadmium in soil, realize the passivation of arsenic and cadmium. Secondly, ferric sulfate easily forms ferric arsenate insoluble compounds with arsenate in soil, and after compounding with calcium bentonite and vermiculite, strengthens the effect of passivation material on arsenic and cadmium passivation simultaneously. Furthermore, ball milling mechanical collision force is utilized to reduce soil and passivation material particle size, increase specific surface area, make mineral form lattice defect, and the heat energy generated in collision process provides heat for chemical adsorption, strengthens the passivation effect of passivation material on arsenic and cadmium in soil. Finally, calcium bentonite and vermiculite can effectively improve acidic soil pH value, reduce the mobility of arsenic and cadmium in soil, and make soil acidity weaken and not affect plant growth.

[0013] The manganese dioxide nanoparticles introduced in step S2 have strong oxidizing power, oxidizing the highly toxic trivalent arsenic in the soil into the less toxic, easily precipitated pentavalent arsenic. Simultaneously, by adding trivalent iron 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 has a high specific surface area and multiple functional groups, which can synergistically fix As and Cd through multiple mechanisms such as physical adsorption, complexation, and ion exchange.

[0014] In step S3, the surface of the composite oxide particles is coated with thiolated chitosan to further enhance the material's ability to complex metal ions. Thiol groups have strong affinity and can react with As(III), Cd 2+The chitosan molecular skeleton forms a stable chelating structure, while the passivation material has 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 oxidative environments, thereby extending the material's shelf life.

[0015] In step S4, ball milling is repeated to thoroughly mix the passivation material with the contaminated soil and reconstruct the interface. Under the action of high-energy mechanical forces, the passivation particles penetrate deeply into the soil matrix, fully contacting the As and Cd in the soil, intensifying micro-region reactions and further improving 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, during 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 mass of the mineral mixture.

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

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

[0021] Furthermore, the ferric iron salt in step S2 is ferric chloride or ferric sulfate, and the addition amount is 2-10 wt% of the mass of the mineral mixture.

[0022] Furthermore, the preparation step of the thiolated chitosan coating includes: dispersing the obtained particle powder in a thiolated chitosan acetic acid solution, wherein the concentration of the thiolated chitosan is 1-2 wt %, and slowly adding a TPP cross-linking agent to form a controlled release membrane layer.

[0023] Furthermore, the preparation method of the thiolated chitosan comprises the following steps:

[0024] A1. Chitosan was dissolved in a 1%-2% volume fraction of glacial acetic acid aqueous solution, 1%-3% mass fraction of citric acid was added as a carboxylating agent, the reaction system pH was adjusted to 5.0-6.0, and the reaction was carried out in a water bath at 50-60 ° C 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°C, and 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 adjusted the reaction pH to 6.0-6.5, added glutaraldehyde solution, and reacted for 2-3 hours to form a three-dimensional network structure of cross-linked chitosan gel;

[0028] A5. The cross-linked chitosan gel is freeze-dried and then crushed 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 1:1-1.5:1.

[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 cross-linking temperature is room temperature or below 30°C.

[0032] The chitosan backbone contains numerous primary amino (-NH2) and hydroxyl (-OH) groups, which have relatively low reactivity and limit 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 through esterification or amidation at 50-60°C and pH 5.0-6.0, introducing -COOH groups. EDC, a water-soluble carboxyl activator, converts the carboxyl groups on the chitosan chain into highly reactive O-urea intermediates. However, these intermediates are unstable in water and easily hydrolyzed. To improve the reaction efficiency, NHS (N-hydroxysuccinimide) is added to react with the intermediates to form stable NHS esters, enhancing the efficiency of subsequent coupling with amine or hydroxyl grafted molecules. 2-Amino-1,3-propanedithiol is a dithiol grafting reagent containing both thiol (-SH) and amino (-NH2) reactive sites within its molecule. It undergoes nucleophilic substitution with activated NHS esters to form covalent amide bonds, enabling directional, efficient, and stable grafting of thiol groups onto the chitosan backbone, yielding thiolated chitosan with excellent chelating ability. At pH 6.0–6.5, glutaraldehyde, a crosslinker, is added to the system to undergo an aldehyde-amine condensation reaction with free amino groups on the chitosan backbone, forming a three-dimensional crosslinked gel. This crosslinked network effectively enhances the structural stability, water insolubility, and controlled release of the thiolated chitosan, making it particularly suitable for long-term application in moist soils. Freeze-drying effectively maintains the porous network within the gel, facilitating the exposure of the thiol active sites during the subsequent passivation coating process, enhancing its coordination and chelation capabilities with arsenic (As) and cadmium (Cd).

[0033] Thiol (–SH) has strong nucleophilicity and can react with heavy metal cations (such as As 3+ 、Cd 2+等 ) by providing electron pairs to form a stable coordination complex. The three-dimensional network structure formed by cross-linking with glutaraldehyde during the preparation process not only enhances the mechanical strength and environmental stability of the chitosan material, but also forms a microporous-mesoporous composite structure with "sustained-release carrier" properties. After coating with passivating active substances (such as iron salts, manganese oxides, clay minerals, etc.), this structure can achieve:

[0034] Rapid initial release, quickly passivating high-concentration metal ions;

[0035] Stable release in the medium term to maintain the dynamic balance of the passivation reaction;

[0036] The slow release in the later stage prolongs the validity period of the material and reduces the frequency of reapplication.

[0037] Through a combination of physical coating and chemical crosslinking, thiolated chitosan effectively encapsulates the internal iron / manganese functional particles, preventing their rapid dissolution or passivation failure in highly acidic or saline soils. It also inhibits aggregation and loss of reactivity. This effect improves the overall stability and uniformity of the passivation material.

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

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

[0040] Furthermore, during the ball milling process in step S4, the mass ratio of the agate balls to the passivation material and 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 is performed for 1-3 hours.

[0041] Beneficial effects of the present invention:

[0042] (1) The present invention adopts a mixture of calcium-based bentonite, vermiculite and ferric sulfate, wherein the layered structure of bentonite and vermiculite gives them strong ion exchange and complexing capabilities, and the iron ion deposition formed by ferric sulfate during the ball milling process can form a stable hydroxy-iron complex precipitate with As / Cd, thereby enhancing the initial fixation effect of heavy metals.

[0043] (2) The pretreated manganese dioxide nanoparticles added in the present invention have a high specific surface area and oxidation potential, which can efficiently oxidize As(III) to As(V) which is easier to precipitate, and at the same time react with Fe 3+ Together they form a Fe-Mn composite oxide film. This type of film has charge control capabilities, a hydrophobic interface, and high electron affinity. It not only has excellent As adsorption and fixation capabilities, but can also effectively capture and complex Cd 2+ By adjusting the pH, it 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) The present invention introduces thiolated chitosan material as the surface coating layer, and its thiol (–SH) and amino (–NH2) on the chitosan skeleton form a multi-site complexing system, which can bind to Cd 2+ It forms stable chelate complexes with heavy metal ions such as As(V), preventing their re-release in the soil microenvironment. Furthermore, chitosan's natural polymer skeleton possesses certain adhesion and sustained-release properties, forming a relatively stable interface between the material and soil particles, thereby improving the environmental stability, acid and alkali resistance, and long-term effectiveness of the passivation material. DETAILED DESCRIPTION

[0045] The following will be combined with the specific embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention:

[0046] Arsenic-cadmium complex contaminated soil was selected from air-dried arsenic-cadmium complex contaminated soil around a lead-zinc mine. The total amount of arsenic and cadmium and the effective content in the test soil were as follows: total arsenic 61 mg / kg, total cadmium 7.15 mg / kg; effective arsenic 0.48 mg / kg, effective cadmium 0.13 mg / kg.

[0047] Example 1

[0048] A passivation method for soil contaminated with arsenic and cadmium comprises the following steps:

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

[0050] S1. Thoroughly mix any two or three of calcium bentonite, vermiculite, and ferric sulfate in appropriate proportions and activate the mixture by ball milling to form a mineral mixture. The mass ratio of agate balls to the mixture during ball milling is 10:1, the milling speed is 400 r / min, and the ball milling activation is performed for 1 hour.

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

[0052] 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 hour, followed by centrifugation, washing, and drying, wherein the mass ratio of the manganese dioxide nanoparticles to the potassium permanganate is 1:2.

[0053] S3. Drying and crushing the particles coated with the composite oxide layer, and coating them with thiolated chitosan (the resulting particle powder is dispersed in a thiolated chitosan acetic acid solution, the concentration of thiolated chitosan is 1 wt %, the mass ratio of the composite particles to the thiolated chitosan acetic acid solution is 1:2, and a TPP crosslinker is slowly added dropwise to form a controlled-release membrane layer; the amount of TPP added is controlled to be 50 wt % of the mass of the composite particles) to obtain a passivation material;

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

[0055] A1. Chitosan was dissolved in a 1% volume fraction of glacial acetic acid aqueous solution, and 1% mass fraction of citric acid was added as a carboxylating agent. The amount of citric acid added was 1% of the mass of chitosan. The pH of the reaction system was adjusted to 5.0 and the reaction was carried out in a water bath at 50°C for 6 hours to obtain a carboxylated chitosan solution.

[0056] A2. Add activator EDC / NHS to the carboxylated chitosan solution, control the reaction system pH at 5.0, the temperature at 25°C, and stir for 1 hour to form an activated solution; EDC is added in an amount of 10% by mass of chitosan, and NHS is added in an amount of 5% by 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 glutaraldehyde concentration is 0.2%, and the cross-linking temperature is room temperature or below 30°C. The amount of glutaraldehyde solution added is 0.1% by weight of the chitosan.

[0059] A5. After freeze-drying the cross-linked chitosan gel, the gel was crushed into thiolated chitosan particles with a particle size of 200 mesh.

[0060] TPP crosslinker concentration is 0.5wt%

[0061] S4. Mix the passivation material and the arsenic-cadmium composite contaminated soil in proportion and perform ball milling.

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

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

[0064] Example 2

[0065] A passivation method for soil contaminated with arsenic and cadmium comprises the following steps:

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

[0067] S1. Thoroughly mix any two or three of calcium bentonite, vermiculite, and ferric sulfate in appropriate proportions and activate the mixture by ball milling to form a mineral mixture. The ball milling process is performed at a mass ratio of agate balls to the mixture of 15:1, a speed of 500 r / min, and the activation step is performed for 2 hours.

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

[0069] 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 2 hours, followed by centrifugation, washing, and drying. The mass ratio of the manganese dioxide nanoparticles to the potassium permanganate is 1:6.

[0070] S3. The particles coated with the composite oxide layer were dried and crushed, and then coated with thiolated chitosan (the resulting particle powder was dispersed in a thiolated chitosan acetic acid solution, the concentration of thiolated chitosan was 1.5 wt %, and TPP crosslinker was slowly added dropwise to form a controlled release membrane layer, and the amount of TPP added was controlled at 60 wt % of the mass of the composite particles) to obtain a passivation material;

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

[0072] A1. Chitosan was dissolved in a 1.5% volume fraction of glacial acetic acid aqueous solution, and 2% mass fraction of citric acid was added as a carboxylating agent. The amount of citric acid added was 2% of the mass of chitosan. The pH of the reaction system was adjusted to 5.5 and the reaction was carried out in a water bath at 55°C for 5 hours to obtain a carboxylated chitosan solution.

[0073] A2. Add activator EDC / NHS to the carboxylated chitosan solution, control the reaction system pH at 5.5, the temperature at 30°C, and stir the reaction for 0.75 hours to form an activated solution; EDC is added in an amount of 15% by mass of chitosan, and NHS is added in an amount of 10% by 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 glutaraldehyde concentration is 0.2%, and the cross-linking temperature is room temperature or below 30°C. The amount of glutaraldehyde solution added is 0.15% by weight of the chitosan.

[0076] A5. The cross-linked chitosan gel is freeze-dried and then crushed into thiolated chitosan particles with a particle size of 250 mesh.

[0077] TPP crosslinker concentration is 0.75wt%

[0078] S4. Mix the passivation material and the arsenic-cadmium composite contaminated soil in proportion and perform ball milling.

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

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

[0081] Example 3

[0082] A passivation method for soil contaminated with arsenic and cadmium comprises the following steps:

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

[0084] S1. Thoroughly mix any two or three of calcium bentonite, vermiculite, and ferric sulfate in appropriate proportions and activate the mixture by ball milling to form a mineral mixture. The ball milling process is performed at a mass ratio of agate balls to the mixture of 20:1, a speed of 600 r / min, and the activation step is performed for 3 hours.

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

[0086] 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 3 hours, followed by centrifugation, washing, and drying, wherein the mass ratio of the manganese dioxide nanoparticles to the potassium permanganate is 1:10.

[0087] S3. The particles coated with the composite oxide layer were dried and crushed, and then coated with thiolated chitosan (the resulting particle powder was dispersed in a thiolated chitosan acetic acid solution, the concentration of thiolated chitosan was 2 wt %, and TPP crosslinker was slowly added dropwise to form a controlled release membrane layer, and the amount of TPP added was controlled at 70 wt % of the mass of the composite particles) to obtain a passivation material;

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

[0089] A1. Chitosan was dissolved in a 2% volume fraction of glacial acetic acid aqueous solution, and 3% mass fraction of citric acid was added as a carboxylating agent. The amount of citric acid added was 3% of the mass of chitosan. The pH of the reaction system was adjusted to 6.0 and the reaction was carried out in a water bath at 50-60°C for 4-6 hours to obtain a carboxylated chitosan solution.

[0090] A2. Add activator EDC / NHS to the carboxylated chitosan solution, control the reaction system pH at 5.5, the temperature at 35°C, and stir the reaction for 0.5 hours to form an activated solution; EDC is added in an amount of 20% by mass of chitosan, and NHS is added in an amount of 15% by 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 glutaraldehyde concentration is 0.2%, and the cross-linking temperature is room temperature. The amount of glutaraldehyde solution added is 0.2% by weight of the chitosan.

[0093] A5. The cross-linked chitosan gel is freeze-dried and then crushed into thiolated chitosan particles with a particle size of 300 mesh.

[0094] TPP crosslinker concentration is 1 wt%

[0095] S4. Mix the passivation material and the arsenic-cadmium composite contaminated soil in proportion and perform ball milling.

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

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

[0098] Example 4

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

[0100] Example 5

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

[0102] Comparative Example 1

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

[0104] Comparative Example 2

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

[0106] Comparative Example 3

[0107] The difference from Example 2 is that steps S2 and S3 are omitted and coating is not performed.

[0108] Comparative Example 4

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

[0110] experiment

[0111] The available 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] In terms of arsenic passivation, in Example 2, the available arsenic in the treated soil was not detected, a 100% reduction, indicating that it has a strong passivation ability for arsenic. This is mainly attributed to the synergistic effect of the following multiple mechanisms:

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

[0118] 2. The surface reactivity of pretreated manganese dioxide nanoparticles is enhanced after oxidation with KMnO4, which promotes the oxidation, precipitation and adsorption of As(V) ions;

[0119] 3. Iron ions easily form hydroxyl oxides such as Fe-OOH at pH 4.0–5.5, which enhance the co-precipitation effect on As ions.

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

[0121] In comparison, although Comparative Example 1 (without introduction of thiol functionality) and Comparative Example 2 (without MnO2 pretreatment) can also passivate As to a certain extent, the effective arsenic decreases by 66.3% and 72.1%, respectively, but are significantly lower than Example 2, verifying the key role of thiol functional groups and active MnO2 in the arsenic passivation process.

[0122] In terms of cadmium passivation, the effective Cd content of Example 2 decreased to 0.036 mg / kg after treatment, a decrease of 72.71%. This effect was also better than that of Comparative Example 1 (56.20%) and Comparative Example 2 (60.00%). The functional groups such as thiol, amino and hydroxyl groups in thiolated chitosan can react with Cd through chelation. 2+ A stable complex is formed, and at the same time, the controlled-release membrane layer formed by the coating structure helps to increase the contact time and spatial shielding between the material and pollutants, and enhance the passivation persistence.

[0123] In contrast, although comparative example 4 was partially coated, the reduction of arsenic and cadmium was significantly reduced due to the lack of thiolated chitosan, which proved the chelation and sustained release effect of thiolated chitosan.

[0124] In comparative example 3, no coating was performed at all, and the reduction rates of arsenic and cadmium were only 42.08% and 28.46%, respectively, which further emphasizes the necessity of coating process in heavy metal passivation.

[0125] The above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that the technical solutions of the present invention may be modified or replaced with equivalents without departing from the spirit and scope of the technical solutions of the present invention, and such modifications or equivalents shall be encompassed by the claims of the present invention. Any techniques, shapes, and structures not described in detail herein are well known.

Claims

1. A passivation method for arsenic-cadmium composite contaminated soil, characterized in that: The following steps are involved: S1. Any two or three of calcium bentonite, vermiculite and ferric sulfate are fully mixed in proportion and activated by ball milling to form a mineral mixture; S2. Pretreated manganese dioxide nanoparticles and ferric iron salt were added to the mineral mixture, the pH of the reaction system was adjusted to 4.0-5.5, and the reaction was stirred for 8-12 hours to form particles coated with a composite oxide layer; S3. The particles coated with a composite oxide layer are dried and crushed, and thiolated chitosan is coated to obtain a passivation material; S4. Mix the passivation material and the arsenic-cadmium composite contaminated soil in proportion and perform ball milling.

2. The passivation method for arsenic-cadmium composite 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 composite contaminated soil according to claim 1, characterized in that: During 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 composite 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 mass of the mineral mixture.

5. The passivation method for arsenic-cadmium composite contaminated soil according to claim 4, characterized in that: include: The preparation step of the pretreated manganese dioxide nanoparticles comprises: soaking the manganese dioxide nanoparticles in a 0.05 mol / L potassium permanganate solution for 1-3 hours, and then centrifuging, washing, and drying.

6. The passivation method for arsenic-cadmium composite contaminated soil according to claim 1, characterized in that: The ferric salt in step S2 is ferric chloride or ferric sulfate, and the addition amount is 2-10 wt% of the mass of the mineral mixture.

7. The passivation method for arsenic-cadmium composite contaminated soil according to claim 1, characterized in that: The preparation steps of the thiolated chitosan coating include: dispersing the obtained particle powder in a thiolated chitosan acetic acid solution, wherein the concentration of the thiolated chitosan is 1-2 wt %, and slowly adding a TPP cross-linking agent to form a controlled release membrane layer.

8. The passivation method for arsenic-cadmium composite contaminated soil according to claim 7, characterized in that: The preparation method of the thiolated chitosan comprises 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 reaction system pH to 5.0-6.0, and react in a 55°C 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°C, and 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 adjusted the reaction pH to 6.0-6.5, added glutaraldehyde solution, and reacted for 2-3 hours to form a three-dimensional network structure of cross-linked chitosan gel; A5. The cross-linked chitosan gel is freeze-dried and then crushed into thiolated chitosan particles with a particle size of 200-300 mesh.

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

100.

10. The passivation method for arsenic-cadmium composite contaminated soil according to claim 1, characterized in that: During the ball milling process in step S4, the mass ratio of the agate balls to the passivation material and 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 is performed for 1-3 hours.

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