Preparation method and application of carboxyl type compound eluent suitable for arsenic and heavy metal contaminated soil

Abstract

This invention discloses a method for preparing a carboxyl-based compound leaching agent suitable for arsenic and heavy metal contaminated soil and its application. The preparation and application method includes the following steps: mixing oxalic acid (OA) and tetrasodium N,N-diacetate glutamate (GLDA) in equal volumes at a molar concentration ratio of 2:(1-4) to prepare the compound leaching agent; mixing the compound leaching agent with the contaminated soil; leaching by shaking; and then separating the solid and liquid phases to remediate the contaminated soil. This method develops a compound leaching agent by scientifically combining leaching agents with similar functional groups. It utilizes the reducing effect of oxalic acid (OA) and the acid solubility and strong complexation of tetrasodium N,N-diacetate glutamate (GLDA) to synergistically improve the simultaneous removal efficiency of arsenic and heavy metals from the soil. The compound leaching agent has simple composition and application conditions, its leaching effect is less affected by the soil pollution status, it is highly operable and easy to promote, and it has good application prospects for the remediation of soil with combined arsenic and heavy metal pollution.

Description

Technical Field

[0001] This invention belongs to the field of heavy metal contaminated soil remediation technology, specifically relating to a method for preparing a carboxyl-based compound leaching agent suitable for arsenic and heavy metal contaminated soil and its application. Background Technology

[0002] Soils in mining and metal smelting areas suffer from severe complex heavy metal pollution, with anionic and cationic heavy metal / metalloid complex pollution being a typical type. For example, soils from abandoned lead-zinc mines commonly contain complex pollution of arsenic (As), cadmium (Cd), and lead (Pb). As and heavy metals (such as Cd and Pb) exhibit opposite geochemical characteristics in the soil environment and cannot be removed through the same mechanisms, making simultaneous and efficient remediation difficult. For the remediation of soils contaminated with arsenic and heavy metals, compared to solidification / stabilization technologies that leave pollutants in the soil and may pose a risk of secondary activation, leaching technology can remove most heavy metals / metalloids and, in principle, does not require long-term monitoring. It has been successfully used in the remediation of many heavy metal / metalloid contaminated sites.

[0003] Leaching agents are a key factor in soil leaching technology. Existing research indicates that single leaching agents with high removal rates for both anionic and cationic heavy metals / metalloids are rare. Current research on leaching remediation of soils contaminated with both anionic and cationic heavy metals / metalloids focuses on the development of combined leaching agents. To simultaneously and efficiently remove arsenic and heavy metals from soil, compound or graded leaching agents effective against arsenic and heavy metals respectively are typically used. However, issues concerning the antagonistic reactions, multi-component composition, and high concentrations and dosages of the components in these combinations are often overlooked. The literature "Comparison of the Multi-Stage Leaching Effects of Different Leaching Agents on Arsenic-Contaminated Soil" shows that the compound leaching agent containing different types of functional groups, MGDA and KH2PO4, exhibits antagonistic effects when leaching As. MGDA also has a leaching effect on phosphate, leading to a decrease in the combined As removal capacity. Patent CN200810198394 discloses a chemical leaching remediation method for heavy metal contaminated soil. It utilizes a combination of Na2EDTA, oxalic acid, and KI to leach arsenic and heavy metal contaminated soil in stages. This three-component, three-stage leaching method can simultaneously remove As, Cd, and Pb from the soil. However, the leaching agent is complex and requires large quantities, necessitating repeated leaching operations and consuming significant time. Furthermore, the removal rates for both arsenic and heavy metals are less than 50%. Patent CN113736469B discloses a leaching agent suitable for high-concentration arsenic-antimony contaminated soil, its preparation, and application. It uses a 0.2 mol·L⁻¹ carboxyl-based leaching agent… -1 Citric acid and 1 mol·L -1 Oxalic acid compound leaching of arsenic and antimony contaminated soil showed that, compared with single leaching agents, the compound leaching agent increased the removal rate of arsenic and antimony, but the increase was less than 5%. The leaching agent had a high concentration and a high reaction speed and long time.

[0004] Combined carboxyl-based leaching agents have the potential to simultaneously remove arsenic and heavy metals from soil and can mitigate inter-component antagonistic reactions to some extent. However, there are currently few studies on the remediation of soils contaminated with both cationic and anionic heavy metals / metalloids using carboxyl-based leaching agents, and the application conditions of these leaching agents still need optimization. The biodegradable chelating agent tetrasodium glutamate (GLDA) shows comparable removal efficiency for cationic heavy metals to EDTA (EDTA is currently one of the most efficient leaching agents for remediating cationic heavy metal contaminated soils). The low-molecular-weight organic acid oxalic acid (OA) demonstrates good removal efficiency for anionic heavy metal-like arsenic, achieving high removal efficiency even at low concentrations (as shown in the comparative example of invention patent CN 202210073052 at 0.4 mol·L⁻¹). -1 Oxalic acid removed arsenic at a rate of 21.31%, while tartaric acid removed only 7.99%. Combining the two and adjusting their proportions to develop a composite leaching agent was investigated to explore its remediation effects in different soil types. This approach has the potential to remediate soils contaminated with anionic and cationic heavy metals using simple, low-concentration components, but currently, no related leaching agents or their applications have been studied. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a leaching agent that is simple to apply, has good remediation effects, and is suitable for soils contaminated with arsenic and heavy metals. The leaching agent provided by this invention contains carboxyl groups of the same type of functional group. It combines oxalic acid (OA), which has high removal efficiency for anionic metalloid arsenic in soil, and tetrasodium N,N-diacetate glutamate (GLDA), which has high removal efficiency for cationic heavy metals in soil. The synergistic effect of the reducing action of oxalic acid (OA) and the acid-soluble and strong complexing action of tetrasodium N,N-diacetate glutamate (GLDA) allows for the simultaneous leaching of arsenic and heavy metals from the soil. Furthermore, the leaching agent and leaching conditions have minimal interference with the soil mineral structure, are environmentally friendly, and have good remediation effects on different types of soil, effectively treating soil contaminated with arsenic and heavy metals.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a carboxyl-based compound leaching agent suitable for arsenic and heavy metal contaminated soil and its application, comprising the following steps:

[0008] (1) The compound rinsing agent is prepared by mixing equal volumes of oxalic acid (OA) and tetrasodium N,N-diacetate of glutamic acid (GLDA) solution.

[0009] (2) Mix the compound leaching agent with the composite contaminated soil, shake and leach, and collect the solid phase after centrifugation. This is the remediated soil, thus completing the leaching remediation of the contaminated soil.

[0010] In step (1), the compound rinsing agent is prepared by mixing oxalic acid (OA) and tetrasodium N,N-diacetate of glutamic acid (GLDA) in equal volumes at a molar concentration ratio of 2:(1-4), preferably 2:4.

[0011] In step (1), the concentration of oxalic acid (OA) is 0.2 mol·L⁻¹. -1 ~0.4 mol·L -1 0.2 mol·L -1 .

[0012] In step (1), the concentration of tetrasodium N,N-diacetate glutamate (GLDA) is 0.2 mol·L⁻¹. -1 ~0.4 mol·L -1 0.4 mol·L -1 .

[0013] In step (1), the pH value of the compound rinsing agent is determined using 0.1 mol·L⁻¹. -1 HCl or 0.1 mol·L -1 The NaOH concentration should be adjusted to 3-5, preferably 4.

[0014] In step (2), the contaminated soil to be remediated is mixed with the composite leaching agent, and the soil-liquid ratio is 1g:(10-30)mL, preferably 1g:10mL.

[0015] In step (2), the rinsing time during the oscillating rinsing process is 20 min to 240 min, preferably 120 min.

[0016] In step (2), the oscillation speed during the rinsing process is 100-200 rpm, preferably 180 rpm.

[0017] In step (2), the centrifugation speed is 3000 rpm and the centrifugation time is 2 min.

[0018] This invention proposes a method for preparing a carboxyl-based compound leaching agent suitable for soils contaminated with arsenic and heavy metals, and its application. The purpose is to remediate soils contaminated with arsenic and heavy metals in combination. The contaminated soils can be any soils contaminated with arsenic and heavy metals, such as arsenic-cadmium-lead and arsenic-lead contaminated soils.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] (1) This invention combines oxalic acid (OA) and tetrasodium N,N-diacetate (GLDA), which contain the same type of functional group (carboxyl group), to form a compound leaching agent. The acidity and reducing properties of the carboxyl group, the dynamic ionization equilibrium reaction formed by the difference in component concentration, and the ligand complexation selectivity synergistically remove different types of heavy metals / metalloids. The chemical reaction between the compound components is weakened. By utilizing the difference in reaction mechanism between oxalic acid and anionic metalloids (mainly reducing reaction) and tetrasodium N,N-diacetate (GLDA) and cationic heavy metals (mainly acid dissolution and ligand complexation reaction), as well as the synergistic effect of the ionization equilibrium reaction between the components, the simultaneous removal efficiency of anionic and cationic heavy metals / metalloids in compound contaminated soil is improved, which has good application prospects.

[0021] (2) This invention uses carboxylic acid oxalic acid (OA) and tetrasodium N,N-diacetate glutamic acid (GLDA), which have high leaching efficiency for anionic and cationic heavy metals / metalloids in soil, respectively, to obtain a leaching agent for arsenic and heavy metal contaminated soil. The leaching agent has a good remediation effect on various heavy metal pollutants in soil. The leaching agent concentration, solid-liquid ratio, leaching time and other conditions are easy to achieve, the remediation effect is good, the leaching agent components are easy to obtain, the damage to soil mineral structure is small, the technology is environmentally friendly, and the technology is easy to promote. Attached Figure Description

[0022] Figure 1 The effect of composite leaching agent on the removal efficiency of As, Cd and Pb in As-Cd-Pb contaminated soil ZY on leaching time and the kinetic fitting of the process;

[0023] Figure 2 The effect of composite leaching agent on the removal of different forms of As in As-Cd-Pb contaminated soil ZY;

[0024] Figure 3 The effect of composite leaching agent on the removal of different forms of Cd in As-Cd-Pb contaminated soil ZY;

[0025] Figure 4 The effect of composite leaching agent on the removal of different forms of Pb in As-Cd-Pb contaminated soil ZY;

[0026] Figure 5 These are the XRD patterns of ZY soil contaminated with As-Cd-Pb before and after leaching with the composite leaching agent;

[0027] Figure 6 The results show the kinetic fit of the changes in the removal efficiency of As and Pb by the composite leaching agent in As-Pb contaminated soil ZZ with leaching time.

[0028] Figure 7 The effect of composite leaching agent on the removal of different forms of As in As-Pb contaminated soil ZZ;

[0029] Figure 8 The effect of composite leaching agent on the removal of different forms of Pb in As-Pb contaminated soil ZZ;

[0030] Figure 9 These are the XRD patterns of As-Pb contaminated soil ZZ before and after leaching with the composite leaching agent; Detailed Implementation

[0031] The following describes in detail a method for preparing a carboxyl-based compound leaching agent suitable for arsenic and heavy metal contaminated soil, and its application, but this does not limit the scope of protection of the present invention. The experimental soil ZY used was As-Cd-Pb composite contaminated soil with a pH of 7.18 ± 0.05 and an As content of 398.66 ± 36.40 mg·kg⁻¹. -1 The Cd content was 26.30 ± 1.48 mg·kg. -1 The Pb content was 6506.52 ± 228.88 mg·kg. -1 The experimental soil ZZ was an As-Pb contaminated soil with a pH of 5.69 ± 0.02 and an As content of 34.26 ± 4.7 mg·kg⁻¹. -1 The Pb content was 384.99 ± 44.05 mg·kg⁻¹. -1 .

[0032] Example 1: Remediation effect of composite leaching agent on As-Cd-Pb contaminated soil

[0033] This embodiment demonstrates the remediation effect of a composite leaching agent of OA and GLDA on As-Cd-Pb contaminated soil ZY. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal effect of the composite leaching agent on As, Cd, and Pb in the soil was measured and analyzed, including the following steps:

[0034] The test soil was topsoil (0–20 cm) contaminated with As-Cd-Pb from the vicinity of a mining area. It was air-dried naturally and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA and 0.4 mol·L -1 A compound leaching agent (pH=4) with equal volumes of GLDA was used. 0.5g of the test soil was placed in a centrifuge tube, and 5mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing evenly, the mixture was leached at 180rpm for 120min at room temperature. After centrifugation and filtration, the heavy metal content of the supernatant was determined by ICP-MS, and the removal rate of heavy metals in the soil was calculated.

[0035] The results of the combined leaching agent's removal rates of As, Cd, and Pb in contaminated soil ZY over time and the kinetic fitting are as follows: Figure 1As shown, the removal rates of total As, Cd, and Pb in soil by the composite leaching agent increased with time. After 2 hours, the removal rates of all three heavy metals reached dynamic equilibrium, indicating that the composite leaching agent can rapidly remove arsenic and heavy metals from alkaline contaminated soil in a relatively short time. The removal rates of total As, Cd, and Pb in contaminated soil by the composite leaching agent were 29.04%, 75.31%, and 67.75% (equilibrium removal rates obtained from second-order kinetic fitting), respectively, indicating that the composite leaching agent can effectively remove arsenic and heavy metals from alkaline contaminated soil.

[0036] Comparative Example 1: Remediation effects of single OA and GLDA leaching agents on As-Cd-Pb contaminated soil

[0037] The difference between Comparative Example 1 and Example 1 is that the leaching agent used is either a single OA or a single GLDA. This comparative example demonstrates the remediation effect of single OA and GLDA leaching agents on As-Cd-Pb contaminated soil ZY. The removal effects of single OA and GLDA leaching agents on As, Cd, and Pb in the soil were determined and analyzed using the leaching method described in Example 1, including the following steps:

[0038] The test soil was topsoil (0–20 cm) contaminated with As-Cd-Pb from the vicinity of a mining area. It was air-dried naturally and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA or 0.4 mol·L -1 GLDA leaching agent. Place 0.5g of the test soil in a centrifuge tube, add 5mL of single leaching agent at a solid-liquid ratio of 1:10, mix well, and leach at 180rpm for 120min at room temperature. After centrifugation and filtration, determine the heavy metal content of the supernatant using ICP-MS, and calculate the removal rate of heavy metals in the soil.

[0039] Test results showed that OA leaching alone removed 25.01%, 27.32%, and 7.28% of As, Cd, and Pb from the soil, respectively. GLDA leaching alone removed 8.12%, 78.22%, and 4.02% of As, Cd, and Pb from the soil, respectively. The composite leaching agent in Example 1 showed higher removal rates of As, Cd, and Pb from the soil than OA alone, a removal rate of Cd that was basically consistent with GLDA, and a removal rate of As and Pb that was higher than GLDA alone. This indicates that the composite leaching agent can simultaneously and efficiently remediate alkaline arsenic and heavy metal contaminated soil.

[0040] Example 2: Removal efficiency of composite leaching agent for different forms of As, Cd, and Pb in As-Cd-Pb contaminated soil

[0041] This embodiment describes the removal effect of a composite leaching agent of OA and GLDA on different forms of As, Cd, and Pb in As-Cd-Pb contaminated soil ZY. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal effect of the composite leaching agent on different forms of As, Cd, and Pb in the soil was measured and analyzed, including the following steps:

[0042] The test soil was topsoil (0–20 cm) contaminated with As-Cd-Pb from the vicinity of a mining area. It was air-dried naturally and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA and 0.4 mol·L -1 A composite leaching agent (pH=4) of equal volume was mixed with GLDA. 0.5 g of the test soil was placed in a centrifuge tube, and 5 mL of the composite leaching agent was added at a solid-liquid ratio of 1:10. After mixing thoroughly, the mixture was leached at 180 rpm for 120 min at room temperature. After centrifugation, the supernatant was discarded, and the soil was dried at low temperature for later use. The BCR speciation method was used to determine the changes in the speciation distribution of Cd and Pb in the soil before and after leaching (exchangeable EXC, reducible RED, oxidizable OXI, and residual RES). The Wenzel-SEP speciation method was used to determine the changes in the speciation distribution of As in the soil before and after leaching (non-specific adsorbed F1, specific adsorbed F2, amorphous iron and aluminum oxide bound F3, crystalline iron and aluminum oxide bound F4, and residual F5).

[0043] The concentration changes of different forms of As, Cd, and Pb in soil ZY before and after leaching with the compound leaching agent are as follows: Figures 2-4 As shown. The composite leaching agent mainly removes As from soil in amorphous iron / aluminum oxide bound form F3 and crystalline iron / aluminum oxide bound form F4, both of which are insoluble arsenic forms. After leaching, the content of these two As forms decreased by 17.20% and 9.71% of the total, respectively. This is mainly due to the reaction of oxalic acid in the composite leaching agent with arsenic-containing iron minerals, reducing iron and releasing arsenic, thus removing As from the soil. The composite leaching agent also has a removal effect on different forms of Cd and Pb in the soil, with the largest removal of exchangeable Cd and reducible Pb. After leaching, their proportions in the total amount decreased by 61.98% and 59.64%, respectively. These two forms of heavy metals are weakly bound and easily soluble forms, mainly due to the removal of arsenic by tetrasodium glutamate-N,N-diacetate through acid dissolution and complexation in the composite leaching agent. At the same time, GLDA can also complex with iron reduced by OA, inhibiting the recombination of arsenic with iron and synergistically improving the arsenic removal effect.

[0044] Example 3: Removal effect of composite leaching agent on other heavy metals in As-Cd-Pb contaminated soil

[0045] This embodiment demonstrates the removal effect of a composite leaching agent of OA and GLDA on other heavy metals in composite contaminated soil ZY. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal effect of the composite leaching agent on other heavy metals (Co, Cu, and Zn) in the soil was measured and analyzed, including the following steps:

[0046] The test soil was topsoil (0–20 cm) contaminated with As-Cd-Pb from the vicinity of a mining area. After natural air drying, it was passed through a 2 mm nylon sieve for later use. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA and 0.4 mol·L -1 A compound leaching agent (pH=4) with equal volumes of GLDA was used. 0.5g of the test soil was placed in a centrifuge tube, and 5mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing evenly, the mixture was leached at 180rpm for 120min at room temperature. After centrifugation and filtration, the heavy metal content of the supernatant was determined by ICP-MS, and the removal rate of other heavy metals (Co, Cu and Zn) in the soil was calculated.

[0047] Test results showed that the composite leaching agent removed 38.10% of total Co, 52.01% of Cu, and 86.72% of Zn from the soil. This indicates that in addition to the target pollutant, the composite leaching agent also has good removal efficiency for other heavy metals in the soil and has the potential for remediation of other types of compoundly polluted soils.

[0048] Test Example 1: Effects of Composite Leaching Agent on Mineral Structure of As-Cd-Pb Soils

[0049] This embodiment describes the effect of a composite leaching agent of OA and GLDA on soil mineral structure. The composite leaching agent was prepared, mixed with contaminated soil, and after shaking leaching, the soil mineral structure before and after leaching was analyzed using XRD. The process includes the following steps:

[0050] The test soil was topsoil (0–20 cm) contaminated with As-Cd-Pb from the vicinity of a mining area. It was air-dried naturally and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA and 0.4 mol·L -1 A compound leaching agent (pH=4) of equal volume was mixed with GLDA. 5.0 g of the test soil was placed in a centrifuge tube, and 50 mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing thoroughly, the mixture was leached at 180 rpm for 120 min at room temperature. The supernatant was discarded after centrifugation, and the leached soil was dried at low temperature for later use. The soil before and after leaching was ground through a 100-mesh nylon sieve, and the changes in soil mineral structure before and after leaching were determined using XRD (X'Pert Pro, Netherlands).

[0051] XRD pattern analysis of soil before and after leaching with compound leaching agent is as follows: Figure 5 As shown in the figure, compared with the original soil, the positions of the mineral structure peaks in the soil did not change after leaching. Only the intensity of some characteristic peaks of muscovite and kaolinite decreased slightly, indicating that the types of soil minerals did not change after leaching. The composite leaching agent caused less damage to the soil mineral structure, and the leaching process had a low impact on the soil.

[0052] Example 4: Remediation effect of composite leaching agent on As-Pb contaminated soil

[0053] This embodiment demonstrates the remediation effect of a composite leaching agent of OA and GLDA on As-Pb co-contaminated soil ZZ. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal effect of the composite leaching agent on As and Pb in the soil was measured and analyzed, including the following steps:

[0054] The test soil was As-Pb contaminated topsoil (0–20 cm) from the vicinity of a mining area, which was air-dried and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, in which 0.2 mol·L⁻¹ soil was added to the test soil. -1 OA and 0.4 mol·L -1 A compound leaching agent (pH=4) with equal volumes of GLDA was used. 0.5g of the test soil was placed in a centrifuge tube, and 5mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing evenly, the mixture was leached at 180rpm for 120min at room temperature. After centrifugation and filtration, the heavy metal content of the supernatant was determined by ICP-MS, and the removal rate of heavy metals in the soil was calculated.

[0055] The results of the time-varying and kinetic fitting of the removal rates of As and Pb in contaminated soil ZZ by the composite leaching agent are as follows: Figure 6 As shown, the removal rates of total As and Pb in soil by the composite leaching agent increased with time. The removal rate of Pb reached a dynamic equilibrium after 1 hour, and the removal rate of As reached a dynamic equilibrium after 2 hours. This may be due to the difference in the main reaction mechanisms between the composite leaching agent and arsenic and heavy metals. Arsenic leaching removal occurs through the reduction reaction of OA, which breaks down iron minerals and releases arsenic, while heavy metals are directly released by GLDA through acid dissolution and complexation, leading to the rate difference. This also indicates that the composite leaching agent can rapidly remove arsenic and heavy metals from acid-contaminated soil in a relatively short time. The removal rates of total As and Pb in soil by the composite leaching agent were 37.13% and 58.19% (equilibrium removal rates obtained from second-order kinetic fitting), respectively, indicating that the composite leaching agent can effectively remove arsenic and heavy metals from acid-contaminated soil.

[0056] Comparative Example 2: Remediation Effects of Single OA and GLDA Leaching Agents on As-Pb Contaminated Soil

[0057] The difference between Comparative Example 2 and Example 4 is that the leaching agent used is either a single OA or a single GLDA. This comparative example demonstrates the remediation effect of single OA and GLDA leaching agents on As-Pb contaminated soil ZZ. The removal effects of single OA and GLDA leaching agents on As, Cd, and Pb in soil were determined and analyzed using the leaching method described in Example 4, including the following steps:

[0058] The test soil was the topsoil (0–20 cm) contaminated with As-Pb from the vicinity of a mining area. It was air-dried naturally and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, with 0.2 mol·L⁻¹ soil added to the test soil. -1 OA or 0.4 mol·L -1 GLDA leaching agent (pH=4). 0.5g of the test soil was placed in a centrifuge tube, and 5mL of the single leaching agent was added at a solid-liquid ratio of 1:10. After mixing evenly, the mixture was leached at 180rpm for 120min at room temperature. After centrifugation and filtration, the heavy metal content of the supernatant was determined by ICP-MS, and the removal rate of heavy metals in the soil was calculated.

[0059] Single 0.2 mol·L -1 OA leaching resulted in the removal rates of 34.06% and 23.14% of As and Pb in the soil, respectively. (Single 0.4 mol·L⁻¹) -1 GLDA leaching resulted in the removal rates of As and Pb in the soil of 4.55% and 44.94%, respectively. The composite leaching agent in Example 4 showed higher removal rates of As and Pb in the soil than OA and GLDA alone, indicating that the composite leaching agent can simultaneously and efficiently remediate acidic arsenic and heavy metal contaminated soil.

[0060] Example 5: Removal efficiency of composite leaching agent for different forms of As and Pb in As-Pb contaminated soil

[0061] This embodiment demonstrates the removal efficacy of a composite leaching agent (OA and GLDA) on different forms of heavy metals (As-Pb) ​​in As-Pb contaminated soil ZZ. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal efficacy of the composite leaching agent on different forms of As and Pb in the soil was measured and analyzed. The process includes the following steps:

[0062] The test soil was As-Pb contaminated topsoil (0–20 cm) from the vicinity of a mining area, which was air-dried and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, in which 0.2 mol·L⁻¹ soil was added to the test soil. -1 OA and 0.4 mol·L -1A composite leaching agent (pH=4) of equal volume of GLDA was used. 0.5 g of the test soil was placed in a centrifuge tube, and 5 mL of the composite leaching agent was added at a solid-liquid ratio of 1:10. After mixing thoroughly, the mixture was leached at 180 rpm for 120 min at room temperature. After centrifugation, the supernatant was discarded, and the soil was dried at low temperature. The BCR speciation method was used to determine the changes in the speciation of Cd and Pb in the soil before and after leaching (exchangeable EXC, reducible RED, oxidizable OXI, and residual RES). The Wenzel-SEP speciation method was used to determine the changes in the speciation of As in the soil before and after leaching (non-specific adsorbed F1, specific adsorbed F2, amorphous iron and aluminum oxide bound F3, crystalline iron and aluminum oxide bound F4, and residual F5).

[0063] Changes in the concentrations of different forms of As and Pb in soil ZZ before and after leaching with the compound leaching agent are shown below. Figures 7-8 As shown. The composite leaching agent mainly removes amorphous iron / aluminum oxide-bound F3 and crystalline iron / aluminum oxide-bound F4 arsenic from the soil, both of which are insoluble arsenic forms. After leaching, the proportions of these two forms of arsenic in the original soil decreased by 35.74% and 9.41%, respectively. This is mainly due to the reduction reaction between oxalic acid in the composite leaching agent and arsenic-containing iron minerals, dissolving the minerals and releasing arsenic, thus removing arsenic from the soil. The composite leaching agent mainly removes reducible RED and oxidizable OXI arsenic pb from the soil. After leaching, the proportions of these two forms of arsenic pb in the total soil decreased by 61.94% and 6.38%, respectively. The removal is mainly focused on weakly bound and easily soluble forms, primarily utilizing the tetrasodium glutamate-N,N-diacetate in the composite leaching agent for removal through acid dissolution and complexation.

[0064] Example 6: Removal effect of composite leaching agent on other heavy metals in As-Pb contaminated soil

[0065] This embodiment demonstrates the removal effect of a composite leaching agent of OA and GLDA on other heavy metals in composite contaminated soil ZZ. The composite leaching agent was prepared using the method described in the claims, mixed with contaminated soil, and after shaking leaching, the removal effect of the composite leaching agent on other heavy metals (Co, Cu, and Zn) in the soil was measured and analyzed, including the following steps:

[0066] The test soil was As-Pb contaminated topsoil (0–20 cm) from the vicinity of a mining area, which was naturally air-dried and then passed through a 2 mm nylon sieve. A shaking leaching experiment was conducted, in which 0.2 mol·L⁻¹ soil was added to the test soil. -1 OA and 0.4 mol·L -1A compound leaching agent (pH=4) with equal volumes of GLDA was used. 0.5g of the test soil was placed in a centrifuge tube, and 5mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing evenly, the mixture was leached at 180rpm for 120min at room temperature. After centrifugation and filtration, the heavy metal content of the supernatant was determined by ICP-MS, and the removal rate of other heavy metals (Co, Cu and Zn) in the soil was calculated.

[0067] Test results showed that the composite leaching agent removed 16.25% of total Co, 35.85% of Cu, and 17.96% of Zn from the soil. This indicates that in addition to the target pollutant, the composite leaching agent also has good removal efficiency for other heavy metals in the soil and has the potential for remediation of other types of compoundly polluted soils.

[0068] Test Example 2: Effects of Composite Leaching Agent on Mineral Structure of As-Pb Soils

[0069] This embodiment describes the effect of a composite leaching agent of OA and GLDA on soil mineral structure. The composite leaching agent was prepared, mixed with contaminated soil, and after shaking leaching, the soil mineral structure before and after leaching was analyzed using XRD. The process includes the following steps:

[0070] The test soil was As-Pb contaminated topsoil (0–20 cm) from the vicinity of a mining area, which was air-dried and then sieved through a 2 mm sieve. A shaking leaching experiment was conducted, in which 0.2 mol·L⁻¹ soil was added to the test soil. -1 OA and 0.4 mol·L -1 A compound leaching agent (pH=4) of equal volume was mixed with GLDA. 5.0 g of the test soil was placed in a centrifuge tube, and 50 mL of the compound leaching agent was added at a solid-liquid ratio of 1:10. After mixing thoroughly, the mixture was leached at 180 rpm for 120 min at room temperature. The supernatant was discarded after centrifugation, and the leached soil was dried at low temperature for later use. The soil before and after leaching was ground through a 100-mesh nylon sieve, and the changes in soil mineral structure before and after leaching were determined using XRD (X'Pert Pro, Netherlands).

[0071] XRD pattern analysis of soil before and after leaching with compound leaching agent is as follows: Figure 9 As shown in the figure, compared with the original soil, the positions of the mineral structure peaks in the soil did not change after leaching, and only the intensity of some characteristic peaks of muscovite decreased slightly. This indicates that the types of soil mineral components did not change after leaching, and the composite leaching agent caused less damage to the soil mineral structure, and the leaching process had a low impact on the soil.

Claims

1. A method for preparing a carboxyl-based compound leaching agent suitable for arsenic and heavy metal contaminated soil, characterized in that, The steps are: mixing oxalic acid (OA) and glutamic acid N, N-diethyl acid tetrasodium (GLDA) solution according to a molar concentration ratio of 1:2 by equal volume, adjusting the pH value of the solution by NaOH or HCl to obtain a carboxyl type complexing eluent; the concentration of the OA and GLDA solution is 0.2 mol·L -1 and 0.4 mol·L -1 , and the pH value of the solution is adjusted to 4.

2. The application of the compound rinsing agent prepared according to claim 1, characterized in that, The compound leaching agent is mixed with the complex contaminated soil, leached by shaking, and the solid phase is collected after centrifugation, which is the remediated soil. The soil-to-liquid ratio of the complex contaminated soil and the compound leaching agent is 1 g: 10 mL, and the leaching time of the shaking leaching process is 2 h.

3. The method according to claim 2, characterized in that, The soil in question is contaminated with arsenic and heavy metals, including cadmium and lead.

4. The application of the compound leaching agent prepared according to claim 1 in the remediation of arsenic and heavy metal contaminated soil.

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