Arsenic-contaminated soil passivation repair agent and application thereof

Abstract

This invention proposes a passivation remediation agent for arsenic-contaminated soil and its application. The passivation remediation agent constructs a ternary synergistic remediation system of zero-valent iron, phosphate, and dithiocarbamate through multi-component synergistic action, mainly including four core mechanisms: redox-adsorption-precipitation-chelation. Simultaneously, biochar, as a carrier, can reduce the risk of zero-valent iron agglomeration and deactivation, maintaining its high reactivity and significantly improving arsenic fixation efficiency. The introduction of dithiocarbamate can fill the gap in the insufficient fixation capacity of traditional iron-based materials for As(III). The calcium phosphate and magnesium phosphate used in this invention hydrolyze in the soil, adjusting the soil pH to a neutral or slightly alkaline range, reducing the solubility of arsenic. The positively charged surface of the iron oxide further facilitates the fixation of negatively charged AsO4. 3‑ Adsorption occurs. Furthermore, the alkaline properties of biochar further enhance this effect.

Description

Technical Field

[0001] This invention belongs to the field of soil environmental pollution prevention and control technology, specifically relating to a passivation remediation agent for arsenic-contaminated soil and its application. Background Technology

[0002] Arsenic is a highly toxic and carcinogenic metalloid element, primarily existing in soil environments as trivalent arsenic (As(III)) and pentavalent arsenic (As(V)), with As(III) exhibiting significantly higher toxicity and mobility than As(V). Arsenic in soil mainly originates from natural environments (such as weathering of arsenic-containing minerals and geological activities) and human activities (such as mining, metal smelting, use of arsenic-containing pesticides, and wastewater irrigation). Artillery shell remnants are also a significant source of arsenic pollution. Ammunition left over from wartime, including chemical weapons and explosives, gradually corrodes and leaks during long-term burial, releasing organic arsenic compounds such as diphenylarsine (DPAA) and phenylarsine (PAA). Over time, these organic arsenic compounds eventually degrade into inorganic arsenates, including As(III) and As(V), posing a significant threat to humans and the environment.

[0003] Currently, remediation technologies for arsenic-contaminated soil mainly include chemical leaching, solidification / stabilization, and phytoremediation. While chemical leaching is fast-acting, it can damage soil structure and cause nutrient loss. Phytoremediation is the most environmentally friendly treatment, but it has a long treatment cycle, low efficiency, and is greatly affected by climate conditions. Solidification / stabilization involves adding passivating agents to the contaminated soil to convert exchangeable arsenic into residual arsenic, reducing its bioavailability and mobility; this is currently the most widely used remediation technology.

[0004] In selecting passivating agents, single remediation materials often suffer from limited remediation effects, high costs, and a tendency to cause secondary pollution. Zero-valent iron (ZVFe) can reduce the mobility of arsenic, thereby mitigating harm to plants and animals; however, ZVFe is easily oxidized and deactivated, and excessive dosage can lead to soil acidification. Phosphate-based materials can fix arsenic by forming arsenate phosphate precipitates, but this process is significantly affected by the initial chemical form of arsenic, resulting in inconsistent remediation effects. While biochar has excellent adsorption properties, its ability to fix arsenic is limited. Therefore, developing a highly efficient, stable, and low-cost composite passivating agent for arsenic-contaminated soil is of great significance. Summary of the Invention

[0005] (a) Technical problems to be solved This invention proposes a passivation remediation agent for arsenic-contaminated soil and its application, in order to solve the technical problem of effectively fixing arsenic in soil.

[0006] (II) Technical Solution To address the aforementioned technical problems, this invention proposes a passivation remediation agent for arsenic-contaminated soil, comprising zero-valent iron, dithiocarbamate, calcium phosphate, magnesium phosphate, and biochar.

[0007] Furthermore, by weight percentage, the passivation repair agent includes 30-50% zero-valent iron, 10-30% dithiocarbamate, 20-30% calcium phosphate, 5-15% magnesium phosphate, and 1-3% biochar.

[0008] In addition, the present invention also proposes an application of the above-mentioned passivation remediation agent for arsenic-contaminated soil. The application steps are as follows: zero-valent iron, calcium phosphate, magnesium phosphate and biochar are ground and sieved respectively, each component is weighed according to the ratio, and mixed evenly to obtain the solid part of the remediation agent; the mixed solid part is mixed evenly with the contaminated soil, and then dithiocarbamate, which is the liquid part of the remediation agent, is sprayed on.

[0009] Furthermore, zero-valent iron, calcium phosphate, magnesium phosphate, and biochar are ground separately and passed through a 100-mesh sieve, then placed in a mixer and mixed evenly.

[0010] Furthermore, dithiocarbamate is a yellowish-brown liquid.

[0011] (III) Beneficial Effects This invention proposes a passivation remediation agent for arsenic-contaminated soil and its application. The passivation remediation agent constructs a ternary synergistic remediation system of zero-valent iron, phosphate, and dithiocarbamate through multi-component synergistic action, mainly including four core mechanisms: redox, adsorption, precipitation, and chelation. Simultaneously, biochar, as a carrier, reduces the risk of zero-valent iron agglomeration and deactivation, maintaining its high reactivity and significantly improving arsenic fixation efficiency. The introduction of dithiocarbamate fills the gap in the insufficient As(III) fixation capacity of traditional iron-based materials. The calcium phosphate and magnesium phosphate used in this invention hydrolyze in the soil, adjusting the soil pH to a neutral or slightly alkaline range (7-9), reducing arsenic solubility. The positively charged surface of iron oxides further facilitates the adsorption of negatively charged AsO43-. Furthermore, the alkaline properties of biochar further enhance this effect. The passivation remediation agent of this invention is low-cost, highly economical, and does not generate secondary pollution during the remediation process, aligning with the concept of green remediation. Detailed Implementation

[0012] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to examples.

[0013] Arsenic-contaminated soil samples were taken from a shell residue site in a forest farm in Dunhua City. Sampling depth was from the top 0 to 50 cm, and two experimental groups were included: low-contaminated soil and high-contaminated soil. The basic physicochemical properties of the soil are shown in Table 1. The low-contaminated soil had an available arsenic content of 0.3 mg / kg and an arsenic leaching concentration of 11.4 µg / L; the high-contaminated soil had an available arsenic content of 1 mg / kg and an arsenic leaching concentration of 61.8 µg / L.

[0014] Table 1 Basic physical and chemical properties of soil soil sample Available arsenic (mg / kg) Arsenic leaching concentration (µg / L) pH Organic matter (g / kg) Organic carbon (g / kg) Low concentration of contaminated soil 0.3 11.4 8.77 23.6 14.3 High concentration of contaminated soil 1 61.8 8.56 34.6 20.6 In the following examples, stones and construction waste with a diameter greater than 50 mm in the arsenic-contaminated soil were screened out. The remediation agent was mixed with the pretreated contaminated soil and stirred evenly. The soil was covered with a film and cured at room temperature for 15 to 30 days. The soil was turned and stirred every 5 days, and water was added to maintain the soil moisture at 20% to 40% of field capacity. The changes in soil arsenic content were monitored regularly.

[0015] Example 1 This remediation agent comprises two components: a solid agent and a liquid agent. The solid agent is a mixture of zero-valent iron, calcium phosphate, magnesium phosphate, and biochar, while the liquid agent is dithiocarbamate. The solid portion of the remediation agent is added to pretreated, low-concentration contaminated soil and thoroughly mixed before adding the liquid portion. The addition ratio is 40% zero-valent iron, 20% dithiocarbamate, 28% calcium phosphate, 10% magnesium phosphate, and 2% biochar. An appropriate amount of water is added and stirred thoroughly, maintaining soil moisture at 30% of field capacity. The soil is then covered with a film for 15 days for curing. Testing showed that the available arsenic content in the remediated soil was 0.2 mg / kg, a 33.3% reduction compared to untreated soil, and the arsenic leaching concentration was 0.82 µg / L, a 92.8% reduction compared to untreated soil. This remediation agent demonstrates a good remediation effect on arsenic-contaminated soil.

[0016] Example 2 This remediation agent comprises two components: a solid agent and a liquid agent. The solid agent is a mixture of zero-valent iron, calcium phosphate, magnesium phosphate, and biochar, while the liquid agent is dithiocarbamate. The solid portion of the remediation agent is added to pretreated, highly contaminated soil and thoroughly mixed before adding the liquid portion. The addition ratio is 40% zero-valent iron, 20% dithiocarbamate, 28% calcium phosphate, 10% magnesium phosphate, and 2% biochar. An appropriate amount of water is added and stirred thoroughly, maintaining soil moisture at 30% of field capacity. The soil is then covered with a film for 15 days for curing. Testing showed that the available arsenic content in the remediated soil was 0.5 mg / kg, a 50% reduction compared to untreated soil, and the arsenic leaching concentration was 3.76 g / L, a 93.92% reduction compared to untreated soil. This remediation agent demonstrates a good remediation effect on arsenic-contaminated soil.

[0017] Example 3 This remediation agent comprises two components: a solid agent and a liquid agent. The solid agent is a mixture of zero-valent iron, calcium phosphate, magnesium phosphate, and biochar, while the liquid agent is dithiocarbamate. The solid portion of the remediation agent is added to pretreated, low-concentration contaminated soil and thoroughly mixed before adding the liquid portion. The addition ratio is 40% zero-valent iron, 20% dithiocarbamate, 28% calcium phosphate, 10% magnesium phosphate, and 2% biochar. An appropriate amount of water is added and stirred thoroughly, maintaining soil moisture at 30% of field capacity. The soil is then covered with a film for 25 days for curing. Testing showed that the available arsenic content in the remediated soil was 0.2 mg / kg, a 33.3% reduction compared to untreated soil, and the arsenic leaching concentration was 2.49 µg / L, a 78.16% reduction compared to untreated soil. This remediation agent demonstrates good remediation efficacy for arsenic-contaminated soil.

[0018] Example 4 This remediation agent comprises two components: a solid agent and a liquid agent. The solid agent is a mixture of zero-valent iron, calcium phosphate, magnesium phosphate, and biochar, while the liquid agent is dithiocarbamate. The solid portion of the remediation agent is added to pretreated, highly contaminated soil and thoroughly mixed before adding the liquid portion. The addition ratio is 40% zero-valent iron, 20% dithiocarbamate, 28% calcium phosphate, 10% magnesium phosphate, and 2% biochar. An appropriate amount of water is added and stirred thoroughly, maintaining soil moisture at 30% of field capacity. The soil is then covered with a film for 25 days for curing. Testing showed that the available arsenic content in the remediated soil was 0.5 mg / kg, a 50% reduction compared to untreated soil, and the arsenic leaching concentration was 2.88 g / L, a 95.34% reduction compared to untreated soil. This remediation agent demonstrates a good remediation effect on arsenic-contaminated soil.

[0019] The remediation agent proposed in this invention has zero-valent iron as its core reducing component. Under aerobic conditions, it provides electrons through the corrosion process, which can oxidize As(III) in the soil into As(V), which is less toxic. The iron oxides (such as goethite) generated by the corrosion have a large specific surface area and can form stable precipitates with arsenic ions, thus converting arsenic from the exchangeable state to the residual state.

[0020] Dithiocarbamate (DTC) in the remediation agent acts as a chelating agent, forming insoluble precipitates through coordination bonds between sulfur atoms and As(III) or As(V). Simultaneously, DTC and zero-valent iron form a synergistic remediation system, with zero-valent iron providing Fe... 2+ / Fe 3+ It can further form a more stable complex precipitate with dithiocarbamate-arsenic chelates, enhancing the arsenic fixation effect. Phosphates in the repair agent play a competitive adsorption and co-precipitation role. Calcium phosphate and magnesium phosphate provide phosphate ions (PO42-). 3-) and arsenate (AsO4) in the soil environment 3- These compounds have similar chemical properties and can react to form complex salt precipitates such as calcium arsenate (Ca3(AsO4)2) and magnesium arsenate (Mg3(AsO4)2). Simultaneously, they compete for adsorption sites on the surface of iron oxides. In a single remediation material system, this competitive adsorption may reduce the material's adsorption efficiency. However, in the remediation material system provided by this invention, not only Fe... 3+ Able to interact with AsO4 3- It forms a stable precipitate of ferric arsenate (FeAsO4) and can also react with PO4. 3- Ferric phosphate (FePO4) precipitate is formed. This precipitate has a large specific surface area and abundant surface active sites, which can further adsorb and fix arsenic ions. In addition, phosphate is alkaline, which can increase the soil pH and promote the conversion of arsenic from the exchangeable state to the residual state. Biochar in the remediation agent, as a carrier, has a rich porous structure and a large specific surface area, which is conducive to the dispersion of zero-valent iron and improves the problem of zero-valent iron inactivation due to agglomeration. At the same time, the surface of biochar is rich in oxygen-containing functional groups such as carboxyl and hydroxyl groups, which can form stable complexes with arsenic ions.

[0021] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A passivation remediation agent for arsenic-contaminated soil, characterized in that, The passivation repair agent includes zero-valent iron, dithiocarbamate, calcium phosphate, magnesium phosphate, and biochar.

2. The passivation remediation agent for arsenic-contaminated soil as described in claim 1, characterized in that, The passivation repair agent comprises, by weight percentage, 30-50% zero-valent iron, 10-30% dithiocarbamate, 20-30% calcium phosphate, 5-15% magnesium phosphate, and 1-3% biochar.

3. The application of a passivation remediation agent for arsenic-contaminated soil as described in claim 1 or 2, characterized in that, The application steps are as follows: zero-valent iron, calcium phosphate, magnesium phosphate and biochar are ground and sieved separately, each component is weighed according to the ratio, and mixed evenly to obtain the solid part of the remediation agent; the mixed solid part is mixed evenly with the contaminated soil, and then dithiocarbamate, which is the liquid part of the remediation agent, is sprayed on.

4. The application of the passivation remediation agent for arsenic-contaminated soil as described in claim 3, characterized in that, Zero-valent iron, calcium phosphate, magnesium phosphate, and biochar were ground separately and passed through a 100-mesh sieve, then placed in a mixer and mixed evenly.

5. The application of the passivation remediation agent for arsenic-contaminated soil as described in claim 3, characterized in that, Dithiocarbamate is a yellowish-brown liquid.