A reusable phosphate compound leaching agent for treating arsenic contaminated soil and a method for repairing arsenic contaminated soil

Abstract

The application discloses a reusable compound leaching agent for treating arsenic contaminated soil and a use method. The method comprises the following steps: mixing and dissolving potassium pyrophosphate and potassium hypophosphite to obtain a compound leaching agent, and leaching the soil to be treated under the condition; performing solid-liquid separation on the soil after oscillation leaching, so as to obtain the repaired soil and residual leaching liquid; adding iron hydroxide and aluminum hydroxide into the residual leaching liquid, and performing solid-liquid separation after fully stirring, so that the obtained leaching liquid can be reused for leaching the arsenic contaminated soil. The compound leaching agent is formed by combining potassium pyrophosphate and potassium hypophosphite, so that the content of arsenic in the contaminated soil can be effectively reduced, the leaching liquid can be reused, the removal rate of the reused leaching agent for arsenic is high, the compound leaching agent has simple components, the leaching effect on arsenic is remarkable, the leaching agent can be reused for multiple times, the cost is low, and the compound leaching agent has a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of arsenic contaminated soil remediation and treatment technology, specifically to a reusable compound leaching agent for treating arsenic contaminated soil and its application method. Background Technology

[0002] Arsenic is a toxic and carcinogenic metalloid. In soil environments, arsenic mainly exists in inorganic form, with arsenate being its primary component under oxidizing conditions. Arsenate can be classified into exchangeable arsenic, iron-type arsenic (Fe-As), calcium-type arsenic (Ca-As), aluminum-type arsenic (Al-As), and residual arsenic (O-As). In red soils high in iron and aluminum, arsenic primarily exists in two forms: amorphous and poorly crystallized hydrated oxides of iron and aluminum, and well-crystallized hydrated oxides of iron and aluminum. Both of these forms of arsenic combine with iron to form relatively stable iron oxides, making the remediation of this type of arsenic-contaminated soil challenging. Furthermore, arsenic accumulated in this type of contaminated soil is difficult to eliminate through dilution and self-purification, and cannot be decomposed by soil microorganisms. Organisms can accumulate arsenic, gradually building up within the soil environment and living organisms. Arsenic can even transform into more toxic compounds in the soil, posing a threat to human health. Therefore, the remediation of arsenic-contaminated soil is of great significance.

[0003] The treatment and remediation of heavy metal pollution in soil can be broadly classified into several categories based on their principles, including physical, chemical, biological, and agricultural methods. Among them, chemical leaching has the characteristics of wide applicability, multiple methods and means, rapid effect, large treatment capacity, and significant results, and has good prospects for practical application.

[0004] The key to chemical leaching is finding an effective leaching agent. From an environmental perspective, it's best to use clean water directly as the leaching solution. However, this method has very low leaching efficiency. To improve the elution efficiency of the leaching solution, surfactants and ion complexing agents are added during the leaching process. Chinese Invention Patent Publication No. CN112122325A discloses a method for removing arsenic from soil by leaching with a compound of inorganic bases, mainly tripolyphosphate and sodium hydroxide. This method uses ion exchange and dissolution to transfer arsenic from the soil into the solution, primarily removing exchangeable arsenic. However, it has significant limitations in removing other forms of arsenic and total arsenic from the soil. Chinese Invention Patent Publication No. CN114410311A uses a compound of oxalic acid and potassium dihydrogen phosphate as a leaching agent for arsenic-contaminated soil. This method causes minimal disturbance to the soil environment, but its leaching effect on arsenic bound to iron oxide and aluminum oxide is not stable or efficient enough.

[0005] Southern red soil is a product of the interaction of bioaccumulation, desilication and iron-aluminum enrichment, and weathering processes. Its arsenic pollution is mainly typical iron-aluminum enriched arsenic pollution. Currently, most leaching agents have limited remediation effects on arsenic-contaminated red soil in southern China, making it essential to develop a highly efficient and reusable leaching agent specifically for arsenic-contaminated soil in the south. Summary of the Invention

[0006] The purpose of this invention is to remediate arsenic-contaminated soil by providing a reusable compound leaching agent and its application method. This method involves first preparing single leaching agent solutions of potassium pyrophosphate and potassium hypophosphite, then combining the two solutions at different concentrations to form a compound leaching agent. The arsenic-contaminated soil undergoes pretreatment such as air drying, impurity removal, and sieving. After being uniformly mixed with a certain amount of the compound leaching agent, the soil is placed in a shaker for leaching, followed by solid-liquid separation. The solid portion is the remediated soil, while the liquid portion is the residual leaching solution, which is then reused. This invention employs a combination of reducing potassium hypophosphite and chelating potassium pyrophosphate as leaching agents. Utilizing the similarity between the composite phosphate structure and the arsenate structure, arsenic bound to iron oxide in the soil is effectively desorbed through competitive adsorption. Potassium hypophosphite dissolves iron oxide bound to arsenic acid, extracting arsenic from the contaminated soil. The addition of potassium hypophosphite forms a complex with the dissolved iron ions, enhancing the removal of other forms of arsenic from the soil and significantly improving the extraction of reduced arsenic. This prevents the precipitation of new iron oxide phases and enhances the dissolution of iron oxide through the chelation mechanism of potassium pyrophosphate. The combined leaching of these two agents increases the leaching amount of arsenic in red soil. In the reuse treatment stage, the adsorption efficiency of arsenic by ferric hydroxide and aluminum hydroxide is utilized to achieve higher reuse efficiency of the leaching solution. This invention targets arsenic-contaminated soil by extracting relatively stable iron oxide arsenic through the addition of a combined leaching agent and by treating the residual leaching solution for reuse, thus reducing treatment costs.

[0007] This invention provides a method for remediating arsenic-contaminated soil, characterized by comprising the following steps:

[0008] S1. Mix and dissolve potassium pyrophosphate and potassium hypophosphite to obtain a compound leaching agent. Add the compound leaching agent to the arsenic-contaminated soil, shake thoroughly, and separate the solid and liquid to obtain the remediated soil and residual leaching solution.

[0009] S2. Add ferric hydroxide and aluminum hydroxide to the residual leaching solution in S1, stir, and then perform solid-liquid separation. The resulting solution is the recycled leaching solution.

[0010] Optionally, the following steps may also be included:

[0011] S3. Take the recycled leaching solution from S2 and add it to the arsenic-contaminated soil. Repeat the shaking leaching under the conditions of S1. After solid-liquid separation, the soil after remediation with recycled leaching agent is obtained.

[0012] Preferably, the compound leaching agent, in molar concentration, comprises the following components: 0.1–1.2 mol / L potassium pyrophosphate and 0.01–0.8 mol / L potassium hypophosphite; more preferably, 0.2–0.8 mol / L potassium pyrophosphate and 0.1–0.6 mol / L potassium hypophosphite. More preferably, the pH of the compound leaching solution is adjusted to 5–10.

[0013] Preferably, the solid-liquid ratio of the composite leaching agent and the contaminated soil in step S1 is 1g:5-20mL; preferably, the arsenic-contaminated soil is southern red soil. More preferably, the shaking leaching time in step S1 is 6-15h;

[0014] In a specific implementation, the oscillator used for rinsing in steps S1 and S3 has a rotation speed of 100-200 r / min.

[0015] Preferably, steps S2 and S3 are the reuse treatment of the compound rinsing agent.

[0016] This invention provides a reusable compound leaching agent for remediating arsenic-contaminated soil and its application method. The agent is characterized by being obtained by mixing and dissolving potassium pyrophosphate and potassium hypophosphite. The reuse treatment involves adding ferric hydroxide and aluminum hydroxide, stirring, and then separating the solid and liquid components to obtain the reusable leaching solution.

[0017] The present invention further provides the application of the compound leaching agent in the remediation of arsenic-contaminated soil, especially arsenic-contaminated red soil in southern China.

[0018] Compared with existing technologies, this invention has the following advantages: This invention utilizes a compound leaching agent composed of potassium pyrophosphate and potassium hypophosphite to enhance the extraction of arsenic bound to amorphous and crystalline iron oxides. Potassium pyrophosphate has a good chelating effect on arsenic bound to crystalline iron oxides; potassium hypophosphite has a good extraction effect on arsenic bound to amorphous iron oxides. The combination of potassium pyrophosphate and potassium hypophosphite can effectively remove arsenic from bound iron oxides through induced synergistic effects. Furthermore, by utilizing the similarity between the composite phosphate structure and the arsenate structure, competitive adsorption further enhances the remediation effect of arsenic-contaminated soil. Finally, the residual leaching solution can be treated and reused, reducing treatment costs.

[0019] The method for treating arsenic-contaminated soil by leaching according to the present invention is simple in process and has a short treatment time, greatly reducing time costs. The composite leaching agent is colorless and odorless at room temperature, has little impact on the physicochemical properties and structure of the soil, reduces secondary pollution to the soil, and can also replenish some nutrients in the soil, making it suitable for widespread use. Detailed Implementation

[0020] To better present the technical content, objectives, and effects of this invention, further explanation will be provided below with reference to specific embodiments. Obviously, the embodiments described below are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0021] Study Example

[0022] The experiment first conducted preliminary screening of leaching agents. A preliminary comparative leaching experiment was carried out on arsenic-contaminated soil samples from a real contaminated site in southern China using potassium pyrophosphate, oxalic acid, and EDTA at a concentration of 0.4 mol / L. The removal efficiencies were 58.91%, 43.76%, and 36.25%, respectively, establishing the superior performance of potassium pyrophosphate as a single leaching agent. Based on the establishment of potassium pyrophosphate as the optimal leaching agent, another leaching agent was screened. Comparative leaching experiments were conducted using 0.4 mol / L potassium pyrophosphate in combination with 0.1 mol / L potassium hypophosphite, oxalic acid, and EDTA. The removal efficiencies were 63.75%, 56.28%, and 58.63%, respectively, thus establishing that the compound leaching agent was prepared from potassium pyrophosphate and potassium hypophosphite. Then, single-factor experiments were conducted to explore the optimal range of conditions. Multiple parallel experiments were performed for each sample group, i.e., the optimal concentration range was explored while maintaining a fixed solid-liquid ratio and leaching time. After determining the optimal concentration range, the single-factor experiments were reversed to maintain the solid-liquid ratio and leaching time. Finally, the effectiveness of potassium pyrophosphate (0.1–1.2 mol / L) and potassium hypophosphite (0.01–0.8 mol / L) in leaching arsenic-contaminated soil was established under the following conditions: solid-liquid ratio of 1 g: 5–20 mL, leaching time of 6–15 h, oscillation frequency of 100–200 r / min, and pH of 5–10. Under these optimal conditions, the leaching efficiency of the compound leaching agent reached 65%–80% for arsenic-contaminated soil, thus completing the screening of the optimal conditions for the compound leaching agent. On the other hand, the reuse treatment of the compound leaching agent utilized the adsorption properties of ferric hydroxide and aluminum hydroxide for arsenic, effectively separating the arsenic from the treated compound leaching agent, thereby enhancing the solubilization of arsenic in the leaching solution. During the investigation, compound experiments were conducted with ferric hydroxide and aluminum hydroxide in ratios of 3:1, 1:1, and 1:3. The 1:1 compound was chosen to achieve higher reuse efficiency. To reduce costs as much as possible while ensuring reuse efficiency, a reduction experiment was conducted. Finally, the treatment conditions for adding ferric hydroxide and aluminum hydroxide at 0.1–0.6 wt% to the residual leaching solution were established, and the removal rate of the reused leaching agent reached 50–60%.

[0023] The following examples are provided as embodiments.

[0024] Example 1

[0025] A sample was collected from an actual contaminated site in southern China, air-dried, and after impurity removal, pulverized and passed through a 60-mesh sieve. Soil type 1 was red soil with a moisture content of 1.15%, As 63 mg / kg, and Fe 19556.35 mg / kg.

[0026] 1. Weigh 18.2 g of potassium pyrophosphate and 1.04 g of potassium hypophosphite, dissolve them in 100 ml of water and mix well to obtain a compound rinsing solution consisting of 0.6 mol / L potassium pyrophosphate and 0.1 mol / L potassium hypophosphite;

[0027] 2. Adjust the pH of the compound leaching solution to 5.0. Under room temperature conditions, mix the compound leaching agent with the arsenic-contaminated soil at a solid-liquid ratio (g:ml) of 1:5, and stir at 100 rpm in a shaker. -1 After 6 hours of oscillating leaching, solid-liquid separation was performed to obtain remediated soil and residual leaching solution.

[0028] 3. Add the same mass of ferric hydroxide and aluminum hydroxide to the residual leaching solution at 0.6 wt%, stir for 5 min, and separate the solid and liquid to obtain the reused leaching solution.

[0029] 4. Under the same conditions as steps 1 and 2, treat the newly contaminated soil with recycled leachate.

[0030] After the reaction was completed, the residual arsenic content in the leached soil was determined according to the standard "Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony in Soils and Sediments by Microwave Digestion / Atomic Fluorescence Method" (HJ 832~2017).

[0031] After remediation using the above methods, the arsenic content in the original arsenic-contaminated soil decreased from 63 mg / kg to 15.4 mg / kg, with a removal rate as high as 75.6%. The arsenic content in the soil after remediation with the compound leaching agent met the screening values ​​for Class I land use in GB36600. After reusing the leaching solution to treat the contaminated soil, the arsenic content in the original arsenic-contaminated soil decreased from 63 mg / kg to 24.1 mg / kg, with a removal rate of 61.7%. The arsenic content in the soil after remediation with the reused leaching solution met the screening values ​​for Class II land use in GB36600.

[0032] Example 2

[0033] A sample was collected from an actual contaminated site in southern China, air-dried, and after impurity removal, pulverized and passed through a 60-mesh sieve. The soil type 2 was red soil with a moisture content of 2.95%, As 110 mg / kg, and Fe 21763.81 mg / kg.

[0034] 1. Weigh 36.5g of potassium pyrophosphate and 0.104g of potassium hypophosphite, dissolve them in 100ml of water and mix well to obtain a compound rinsing solution of 1.2mol / L potassium pyrophosphate and 0.01mol / L potassium hypophosphite;

[0035] 2. Adjust the pH of the compound leaching agent to 7.0. Under room temperature conditions, mix the compound leaching agent with the arsenic-contaminated soil at a solid-liquid ratio (g:ml) of 1:20. Stir in a shaker at 200 rpm. -1 After 10 hours of oscillating leaching, solid-liquid separation was performed to obtain remediated soil and residual leaching solution.

[0036] 3. Add 0.1 wt% of the same mass of ferric hydroxide and aluminum hydroxide to the residual rinsing solution, stir for 10 min, and separate the solid and liquid to obtain the recycled rinsing solution.

[0037] 4. Under the same conditions as steps 1 and 2, treat the newly contaminated soil with recycled leachate.

[0038] After the reaction was completed, the residual arsenic content in the leached soil was determined according to the standard "Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony in Soils and Sediments by Microwave Digestion / Atomic Fluorescence Method" (HJ 832~2017).

[0039] After remediation using the above methods, the arsenic content in the original arsenic-contaminated soil decreased from 110 mg / kg to 21.2 mg / kg, with a removal rate as high as 80.7%. The arsenic content in the soil after remediation with the compound leaching agent met the screening values ​​for Class II land use in GB36600. After reusing the leaching solution to treat the contaminated soil, the arsenic content in the original arsenic-contaminated soil decreased from 110 mg / kg to 46.9 mg / kg, with a removal rate of 57.4%. The arsenic content in the soil after remediation with the reused leaching solution met the screening values ​​for Class II land use in GB36600.

[0040] Example 3

[0041] A sample was collected from an actual contaminated site in Guangzhou. After air drying and impurity removal, the soil was pulverized and passed through a 60-mesh sieve. The soil was classified as red soil of type 3, with a moisture content of 0.74%, As 78 mg / kg, and Fe 17347.24 mg / kg.

[0042] 1. Weigh 3.3g of potassium pyrophosphate and 8.32g of potassium hypophosphite and dissolve them in 100ml of water. Mix well to obtain a compound rinsing solution of 0.1mol / L potassium pyrophosphate and 0.8mol / L potassium hypophosphite.

[0043] 2. Adjust the pH of the compound leaching agent to 10.0. Under room temperature conditions, mix the compound leaching agent with the arsenic-contaminated soil at a solid-liquid ratio (g:ml) of 1:8, and stir in a shaker at 200 rpm. -1 After 15 hours of oscillating leaching, solid-liquid separation was performed to obtain the remediated soil and residual leaching solution.

[0044] 3. Add the same mass of ferric hydroxide and aluminum hydroxide to the residual leaching solution at 0.2 wt%, stir for 5 min, and separate the solid and liquid to obtain the recycled leaching solution.

[0045] 4. Under the same conditions as steps 1 and 2, treat the newly contaminated soil with recycled leachate.

[0046] After the reaction was completed, the residual arsenic content in the leached soil was determined according to the standard "Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony in Soils and Sediments by Microwave Digestion / Atomic Fluorescence Method" (HJ 832~2017).

[0047] After remediation using the above methods, the arsenic content in the original arsenic-contaminated soil decreased from 78 mg / kg to 29.2 mg / kg, with a removal rate of 62.6%. The arsenic content in the soil after remediation with the compound leaching agent met the screening values ​​for Class II land use in GB36600. After reusing the leaching solution to treat the contaminated soil, the arsenic content in the original arsenic-contaminated soil decreased from 78 mg / kg to 37.5 mg / kg, with a removal rate of 51.9%. The arsenic content in the soil after remediation with the reused leaching solution met the screening values ​​for Class II land use in GB36600.

[0048] Example 4

[0049] A sample was collected from an actual contaminated site in Guangzhou. After air drying and impurity removal, the soil was pulverized and passed through a 60-mesh sieve. The soil was classified as red soil of type 4, with a moisture content of 0.67%, As content of 86 mg / kg, and Fe content of 18126.35 mg / kg.

[0050] 1. Weigh 13.2g of potassium pyrophosphate and 2.08g of potassium hypophosphite, dissolve them in 100ml of water and mix well to obtain a compound leaching solution of 0.4mol / L potassium pyrophosphate and 0.2mol / L potassium hypophosphite;

[0051] 2. Adjust the pH of the compound leaching agent to 6.0. Under room temperature conditions, mix the compound leaching agent with the arsenic-contaminated soil at a solid-liquid ratio (g:ml) of 1:10, and stir at 150 rpm in a shaker. -1 After 8 hours of oscillating leaching, solid-liquid separation was performed to obtain the remediated soil and residual leaching solution.

[0052] 3. Add the same mass of ferric hydroxide and aluminum hydroxide to the residual leaching solution at 0.4 wt%, stir for 10 min, and separate the solid and liquid to obtain the reused leaching solution.

[0053] 4. Under the same conditions as steps 1 and 2, treat the newly contaminated soil with recycled leachate.

[0054] After the reaction was completed, the residual arsenic content in the leached soil was determined according to the standard "Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony in Soils and Sediments by Microwave Digestion / Atomic Fluorescence Method" (HJ 832~2017).

[0055] After remediation using the above methods, the arsenic content in the original arsenic-contaminated soil decreased from 86 mg / kg to 23.3 mg / kg, with a removal rate of 72.9%. The arsenic content in the soil after remediation with the compound leaching agent met the screening values ​​for Class II land use in GB36600. After reusing the leaching solution to treat the contaminated soil, the arsenic content in the original arsenic-contaminated soil decreased from 86 mg / kg to 35.6 mg / kg, with a removal rate of 58.6%. The arsenic content in the soil after remediation with the reused leaching solution met the screening values ​​for Class II land use in GB36600.

[0056] As can be seen from Examples 1-4 above, the method of the present invention is effective in removing arsenic pollution from soil. Its synergistic solubilizing effect is significantly improved compared to traditional single-method leaching, and it is highly effective in removing arsenic bound to iron oxide in southern red soil; at the same time, the leaching efficiency of the reused leaching solution is also well guaranteed.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, but the implementation of the present invention is not limited to the above embodiments. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principle of the present invention should be equivalent substitutions and do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method of remediating arsenic-contaminated soil, characterized by, The soil is a southern red soil, comprising the following steps: S1. mixing and dissolving potassium pyrophosphate and potassium hypophosphite to obtain a compound leaching agent, the pH of the compound leaching agent being 5-10, adding the compound leaching agent to arsenic-contaminated soil, fully oscillating, and performing solid-liquid separation to obtain the remediated soil and residual leaching liquid; wherein, in terms of molar concentration, the compound leaching agent is composed of 0.1-1.2 mol / L potassium pyrophosphate and 0.01-0.8 mol / L potassium hypophosphite; and the solid-liquid ratio of the compound leaching agent and the arsenic-contaminated soil is 1 g:5-20 mL; S2. adding iron hydroxide and aluminum hydroxide to the residual leaching liquid in S1, stirring, and then performing solid-liquid separation to obtain the reused leaching liquid.

2. The method of claim 1, wherein, Further comprising the following steps: S3. adding the reused leaching liquid in S2 to arsenic-contaminated soil, repeating the oscillation leaching in S1, and performing solid-liquid separation to obtain the remediated soil by the reused leaching agent.

3. The method of claim 1, wherein, The oscillation leaching time in step S1 is 6-15 h.

4. The method of claim 1, wherein, The oscillation speed in the oscillator in steps S1 and S3 is 100-200 r / min.

5. The method of claim 1, wherein, The ratio of iron hydroxide to aluminum hydroxide in step S2 is 1:1, and the addition amount is 0.1-0.6 wt% to the residual leaching liquid.

6. The method of claim 1, wherein, The reuse treatment of the compound leaching agent is repeated 1-4 times by repeating steps S2 and S3.