Leaching remediation composition and remediation method for lead-containing heavy metal contaminated soil

Abstract

The invention discloses a leaching remediation composition and a remediation method for lead-containing heavy metal contaminated soil. The remediation method comprises the following steps: crushing the polluted soil into granular soil to be remedied; leacheate containing citric acid and EDTA-2Na is prepared; and carrying out leaching treatment on the to-be-remediated granular soil by using a leaching solution. The engineering remediation technology is environmentally friendly, secondary pollution is avoided, and the content and the leaching concentration of heavy metal, especially lead, in the leached soil can meet related environmental quality requirements.

Description

Technical Field

[0001] This invention relates to the remediation of heavy metal contaminated soil, particularly lead-containing heavy metal contaminated soil. Background Technology

[0002] Heavy metal pollution in soil has become a serious environmental challenge that urgently needs to be addressed globally. Its prominence lies in the wide range of pollution, the strong accumulation of pollutants, the complexity of sources, and the far-reaching harm, posing multiple threats to ecosystems and human well-being. Among numerous soil remediation technologies, soil washing technology has received widespread attention due to its wide applicability, high remediation efficiency, rapid treatment of various pollutants, the ability to backfill with treated coarse particles, flexible application methods, and suitability for in-situ or ex-situ remediation.

[0003] The journal *Journal of Safety and Environment*, Vol. 19, No. 2 (April 2019), published a paper titled "The Effects of Leaching on the Removal, Mobility, and Effectiveness of Cadmium, Lead, and Zinc in Farmland Soils," which disclosed several common leaching agents for heavy metal contaminated soils: (0.1M) citric acid, EDTA, FeCl3, and HCl. The journal *Journal of Liupanshui Normal University*, Vol. 29, No. 3 (June 2017), published a paper titled "Study on Leaching of Heavy Metal Contaminated Soils in a Lead-Zinc Mine Area in Guizhou," which disclosed several common leaching agents for heavy metal contaminated soils: (0.15M) EDTA, citric acid, acetic acid, and oxalic acid. Furthermore, CN104772329A also disclosed several organic acid leaching agents for heavy metal contaminated soils: (0.6M) citric acid, (0.4M) tartaric acid, and (0.1M) EDTA. However, the removal effect of these single leaching agents on heavy metal contaminated soils, especially lead-contaminated soils, often falls short of actual remediation requirements, particularly regarding the leaching concentration of heavy metals in the soil after leaching.

[0004] The paper "Removal Effect and Mechanism of Composite Leaching Agent on Heavy Metals in Soil," first published online in the *Journal of Agricultural Environmental Science* (ISSN 1672-2043, CN12-1347 / S), discloses several common leaching agents for heavy metal-contaminated soil and their combinations: (0.3M) citric acid, FeCl3, and tetrasodium iminodisuccinate, and their combinations in pairs. While this composite leaching agent can improve the removal efficiency of certain heavy metals to some extent, further improvements are needed to meet the relevant leaching requirements, especially regarding the removal of lead, a heavy metal of particular concern. Summary of the Invention

[0005] The purpose of this invention is to provide a soil remediation technology for heavy metal pollution, especially lead pollution, which can efficiently and engineer the remediation of polluted soil.

[0006] According to a first aspect of the present invention, a leaching remediation composition for lead-contaminated soil is provided, comprising citric acid and EDTA-2Na, wherein the molar ratio of citric acid to EDTA-2Na is (12~30):1.

[0007] In the repair composition provided by this invention, citric acid releases H+ through ionization. + H + By altering the binding of soil to metal ions through surface potential, surface charge density, and proton competition, the dissolution of metal ions in solution is promoted. On the other hand, protonation acidifies the soil microenvironment, dissolving some carbonates and hydroxides in the soil, disrupting the adsorption balance between heavy metals such as lead and soil particles, and allowing heavy metal ions, especially Pb, which were originally encapsulated by minerals, to dissolve. 2 This allows for full exposure, thereby increasing the reaction sites for the chelation of EDTA-2Na and synergistically improving the removal efficiency of stable heavy metals.

[0008] Heavy metal contaminated soil can also contain zinc and other metals.

[0009] According to a second aspect of the present invention, a method for remediating lead-contaminated soil is also provided, comprising: Preparing to contaminate the soil; The contaminated soil was crushed into granular soil particles to be remediated. Prepare an eluent containing citric acid and EDTA-2Na, wherein the concentration of citric acid is 0.17–0.23 mol·L⁻¹. -1 The concentration of EDTA-2Na was 0.007~0.013 mol·L⁻¹. -1 ; The granular soil to be remediated is treated by leaching with a leaching solution, with 9-11 ml of leaching solution used per gram of granular soil to be remediated.

[0010] The remediation method according to the present invention may further include: passing all the particulate soil to be remediated through a 300-mesh sieve.

[0011] According to the repair method of the present invention, the rinsing treatment time can be 3 to 5 hours.

[0012] According to the remediation method of the present invention, the rinsing solution preferably contains 0.18 mol·L⁻¹ -1 Approximately citric acid and 0.012 mol·L⁻¹ -1 Approximately 10 ml of EDTA-2Na was used per gram of granular soil to be remediated.

[0013] The remediation method of this invention ensures that the lead concentration in the leached soil meets the Class II land use standard of 800 mg / kg for construction land (and the zinc concentration meets the Class II land use standard of 5449 mg / kg for contaminated site risk assessment in Liaoning Province) without damaging the soil structure. Furthermore, after leaching the soil using the standard "Solid Waste Leaching Toxicity Leaching Method - Sulfuric Acid and Nitric Acid Method" (HJ / T299-2007), the lead leaching concentration meets the Class III lead concentration limit of 0.01 mg / L in the "Groundwater Quality Standard" (GB / T14848-2017) (and meets the zinc concentration limit of 1.0 mg / L). Attached Figure Description

[0014] Figure 1 SEM image of the soil to be remediated before leaching; Figure 2 This is a SEM image of the soil after leaching remediation according to Example 1 of the present invention; Figure 3 The images show the lead and zinc removal effects of different rinsing combinations used in this invention. Figure 4 The image shows the effect of lead and zinc removal using a combined rinsing method with citric acid solution and FeCl3 solution. Figure 5 This image shows the effect of lead and zinc removal using a combined rinsing method with EDTA-2Na solution and FeCl3 solution. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments. However, it should be understood that the present invention is not limited to these embodiments.

[0016] Sampling and preparation of lead-contaminated soil Lead-containing soil samples were taken from a heavy metal contaminated site. The soil was light brown in color, sandy, and air-dried. It was then ground into powder using a QM-3SP4 planetary ball mill and passed through a 300-mesh sieve as the test soil (particles to be remediated). The lead and zinc contents were determined to be 3471 mg / kg and 7878 mg / kg, respectively, according to the method "Determination of Copper, Zinc, Lead, Nickel and Chromium in Soils and Sediments by Flame Atomic Absorption Spectrophotometry" (HJ 491-2019). After leaching the soil according to the standard "Leaching Toxicity of Solid Waste - Sulfuric Acid and Nitric Acid Method" (HJ / T299-2007), the lead and zinc concentrations in the leachate were determined by ICP-MS to be 2.1 mg / L and 4.5 mg / L, respectively.

[0017] Repair agent preparation Prepare 0.3 mol·L⁻¹ solutions respectively. -1 Citric acid solution, 0.03 mol·L -1EDTA-2Na solution and 0.1 mol·L -1 Prepare FeCl3 solution for later use.

[0018] Example 1 Weigh 5g of the lead-zinc contaminated soil sample and place it in a 100ml shaking flask. Add 30ml of the prepared citric acid solution and 20ml of the prepared EDTA-2Na solution. Set the temperature to 25℃ and shake the flask at 160 rpm. -1 The rotation speed oscillated for 4 hours.

[0019] The lead and zinc contents of the leached soil were tested using the above method and found to be 644.43 mg / kg and 3046 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.006 mg / L and 0.4 mg / L, respectively.

[0020] Figure 1 and Figure 2 The images show SEM images of the soil before and after leaching. As can be seen from the images, the leached soil particles are looser and more porous, with a rougher surface, indicating that leaching has a certain erosive effect on the soil particles. However, overall, the microstructure does not show significant changes, and the soil particle outlines remain clear.

[0021] Example 2 The rest is the same as in Example 1, except that the amount of citric acid solution added is 37.5 ml and the amount of EDTA-2Na solution added is 12.5 ml.

[0022] The lead and zinc contents of the leached soil were tested using the above method and found to be 728.5 mg / kg and 3668 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.009 mg / L and 0.7 mg / L, respectively.

[0023] Example 3 The rest is the same as in Example 1, except that the amount of citric acid solution added is 35 ml and the amount of EDTA-2Na solution added is 15 ml.

[0024] The lead and zinc contents of the leached soil were tested using the above method and found to be 676.4 mg / kg and 3372 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.007 mg / L and 0.6 mg / L, respectively.

[0025] Example 4 The rest is the same as in Example 1, except that the rotational speed oscillation lasts for 3 hours.

[0026] The lead and zinc contents of the leached soil were tested using the above method and found to be 736.6 mg / kg and 3671 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.008 mg / L and 0.6 mg / L, respectively.

[0027] Example 5 The rest is the same as in Example 1, except that the rotational speed oscillation lasts for 5 hours.

[0028] The lead and zinc contents of the leached soil were tested using the above method and found to be 615.6 mg / kg and 1978 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.0054 mg / L and 0.38 mg / L, respectively.

[0029] Comparative Example 1 The rest is the same as in Example 1, except that the amount of citric acid solution added is 50 ml, and the amount of EDTA-2Na solution added is 0 ml.

[0030] The lead and zinc contents of the leached soil were tested using the above method and found to be 1214.7 mg / kg and 3151 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.052 mg / L and 0.092 mg / L, respectively.

[0031] Comparative Example 2 The rest is the same as in Example 1, except that the amount of citric acid solution added is 0, and the amount of EDTA-2Na solution added is 50 ml.

[0032] The lead and zinc contents of the leached soil were tested using the above method and found to be 1006.6 mg / kg and 4411.7 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.063 mg / L and 0.055 mg / L, respectively.

[0033] Comparative Examples 1-2 illustrate that, compared to citric acid or EDTA-2Na leaching agents alone, the citric acid-EDTA-2Na composite leaching agent of this invention exhibits excellent remediation effects on soil contaminated with heavy metals, especially lead-contaminated soil. The zinc concentration in the leached soil meets the Liaoning Province's Class II land use screening value of 5449 mg / kg for contaminated site risk assessment, and the lead concentration meets the Class II land use standard of 800 mg / kg for construction land soil pollution risk screening. Furthermore, without damaging the soil structure, after leaching the soil using the "Solid Waste Leaching Toxicity Leaching Method - Sulfuric Acid and Nitric Acid Method" (HJ / T299-2007), the lead and zinc leaching concentrations both meet the Class III lead and zinc concentration limits of 0.01 mg / L and 1.0 mg / L, respectively, of the "Groundwater Quality Standard" (GB / T 14848-2017).

[0034] Comparative Example 3 The rest is the same as in Example 1, except that the amount of citric acid solution added is 25 ml, and the amount of EDTA-2Na solution added is also 25 ml.

[0035] The lead and zinc contents of the leached soil were tested using the above method and found to be 1156 mg / kg and 3318 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.013 mg / L and 0.08 mg / L, respectively.

[0036] Comparative Example 4 The rest is the same as in Example 1, except that the amount of citric acid solution added is 20 ml and the amount of EDTA-2Na solution added is 30 ml.

[0037] The lead and zinc contents of the leached soil were tested using the above method and found to be 1048 mg / kg and 3420 mg / kg, respectively. The lead and zinc leaching concentrations of the leached soil were 0.011 mg / L and 0.07 mg / L, respectively.

[0038] Comparative Examples 3-4 show that the volume ratio of citric acid to EDTA-2Na is also very important for the lead removal effect (the lead removal effect is best at a ratio of 3:2).

[0039] Comparative Example 5 The rest is the same as in Example 1, except that while keeping the total added amount constant at 50 ml, the volume ratio of citric acid solution (n) and EDTA-2Na solution (E) in each group of repair is set to 1:1; 1:2; 1:3; 2:1; 2:3; 3:1; 3:2, respectively.

[0040] Plot a graph with the volume ratio of each group of repairs as the x-axis and the lead and zinc content and removal rate after repair as the y-axis, respectively. Figure 3 As shown. From Figure 3 As can be seen, the lead and zinc removal rates were highest when the citric acid and EDTA-2Na were mixed (volume) at a ratio of 3:2, with a lead removal rate of 81.43% and a zinc removal rate of 60.01%.

[0041] Comparative Example 6 The other groups were the same as in Comparative Example 5, except that citric acid solution (n) and FeCl3 solution (Fe) were used for repair in each group.

[0042] Plot a graph with the volume ratio of each group of repairs as the x-axis and the lead and zinc content and removal rate after repair as the y-axis, respectively. Figure 4 As shown. From Figure 4 As can be seen, the lead and zinc removal rates were highest when the citric acid and FeCl3 were mixed (volume) in a ratio of 2:3, with a lead removal rate of 68.55% and a zinc removal rate of 72.04%.

[0043] Comparative Example 7 The other groups were the same as in Comparative Example 5, except that FeCl3 solution (Fe) and EDTA-2Na solution (E) were used for the repair of each group.

[0044] Plot a graph with the volume ratio of each group of repairs as the x-axis and the lead and zinc content and removal rate after repair as the y-axis, respectively. Figure 5 As shown. From Figure 5 As can be seen, the lead removal rate was the highest when the FeCl3 and EDTA-2Na were mixed (volume) ratios of 2:1, with a removal rate of 73.21%; while the zinc removal rate was the highest when the mixing ratio was 3:2, with a removal rate of 71.98%.

[0045] The results of Comparative Examples 5-7 demonstrate that the synergistic effect of citric acid and EDTA-2Na in soil remediation is significant.

[0046] Tests showed that lead and other pollutants in the tested soil mainly existed in weakly acid-extractable and reducible forms. After leaching with the compound solution of this invention, the weakly acid-extractable and reducible forms in the soil were significantly reduced, with the soil mainly existing in more stable oxidizable and residual forms. The proportion of weakly acid-extractable and reducible lead in the soil, especially lead, was significantly reduced compared to before leaching: Example 1 showed the greatest reduction in lead content under the same remediation treatment time, with some lead being converted to residual forms. This is because the compound leaching removed most of the lead from the soil, allowing the weakly acid-extractable and reducible forms of lead to be leached into the leachate, while the residual form exists in a more stable mineral lattice, exhibiting low toxicity and high stability, and is generally not easily released. Therefore, the leaching agent of this invention primarily removes highly mobile, toxic, and environmentally hazardous forms from the soil, even transferring them to more stable forms, thereby reducing the soil's biotoxicity and environmental harm. In summary, the leaching concentration of heavy metals, especially lead, can be effectively reduced through the repair method of the present invention, which can fully meet the requirements of in-situ repair.

Claims

1. A leaching remediation composition for lead-contaminated soil, comprising citric acid and EDTA-2Na, wherein the molar ratio of citric acid to EDTA-2Na is (12~30):

1.

2. The remediation composition according to claim 1, wherein the heavy metal contaminated soil further contains zinc.

3. A method for remediating lead-contaminated soil, comprising: Preparing to contaminate the soil; The contaminated soil was crushed into granular soil particles to be remediated. Prepare an eluent containing citric acid and EDTA-2Na, wherein the concentration of citric acid is 0.17–0.23 mol·L⁻¹. -1 The concentration of EDTA-2Na was 0.007~0.013 mol·L⁻¹. -1 ; The granular soil to be remediated is treated by leaching with a leaching solution, with 9-11 ml of leaching solution used per gram of granular soil to be remediated.

4. The repair method according to claim 3 further includes: All the granular soil to be repaired was passed through a 300-mesh sieve.

5. The repair method according to claim 3, wherein the rinsing treatment time is 3-5 hours.

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