A method for the hierarchical remediation of arsenic-contaminated soil based on off-site leaching

Abstract

The present invention relates to the technical field of soil regeneration, and specifically relates to a method for repairing arsenic-contaminated soil based on off-site leaching and classification, comprising the following steps: S1, determination of sampling points; S2, collection of soil samples; S3, preparation of leaching agents; S4, repair treatment; The present invention further improves the desorption rate of arsenic by preparing a synergistic leaching agent, enabling more arsenic to be released from soil particles into the liquid phase, thereby achieving effective repair of arsenic in the soil. Moreover, after adding the synergistic leaching agent, the present application respectively adds a flotation treatment and a hydration treatment step for the soil according to the depth of the soil to be treated, further improving the dispersibility of the soil, promoting the desorption of arsenic, optimizing the flotation conditions, thereby significantly enhancing the flotation ability and effectively improving the arsenic pollution repair ability.

Description

Technical Field

[0001] The present invention relates to the technical field of soil regeneration, and particularly to a method for repairing arsenic-contaminated soil based on off-site leaching and classification Background Art

[0002] Soil pollution remediation technology refers to the general term of technologies and methods using chemistry, physics and biology to reduce the concentration of pollutants in soil, fix soil pollutants, convert soil pollutants into low-toxic or non-toxic substances, and block the transfer pathways of soil pollutants in environmental media. Soil arsenic remediation technology mainly includes in-situ remediation and off-site remediation. When the arsenic concentration in soil is relatively high, in-situ remediation may be difficult to achieve an ideal remediation effect, but off-site remediation can concentrate the treatment of polluted soil and improve the remediation efficiency. If the polluted area is relatively concentrated, off-site remediation is easier to implement and can completely remove the polluted soil.

[0003] Off-site remediation can completely remove the pollution source by excavating polluted soil and repairing it to the target value or safely disposing it, avoiding the further diffusion of arsenic in the environment and reducing the impact on the surrounding environment. Among the existing off-site remediation technologies for arsenic-contaminated soil, leaching treatment is more common and easier to implement. However, different forms of arsenic (such as exchangeable state, iron and manganese oxide-bound state, organic state, residual state) respond differently to leaching agents, and the remediation effect of single leaching on arsenic in soil is limited. At the same time, the one-time use of a large amount of agents is likely to cause waste and the remediation cost is relatively high. Summary of the Invention

[0004] To solve the above problems, the present invention provides a method for repairing arsenic-contaminated soil based on off-site leaching and classification, including the following steps:

[0005] S1. Determination of Sampling Points

[0006] Select 2 - 3 shallow soil points with a depth of 0 - 1.5 m and 2 - 3 deep soil points with a depth of 1.5 - 3.5 m respectively;

[0007] S2. Collection of Soil Samples

[0008] For the shallow soil points and deep soil points selected in S1, use GPS to re-locate them respectively to obtain longitude and latitude coordinate information and record the point coordinates, then use a drill to take 1 - 2 kg of samples at the corresponding depths respectively, and mix the samples at the same depth to obtain shallow soil samples and deep soil samples, and pack them in polyethylene sealed bags for standby;

[0009] S3. Preparation of Leaching Agents

[0010] The leaching agent includes a first leaching agent and a second leaching agent;

[0011] The first elution agent is a natural organic acid with a concentration of 0.2 - 1 mol / L, or EDTA with a concentration of 0.1 - 0.5 mol / L, or a sodium hydroxide solution with a concentration of 0.2 - 1 mol / L. The second elution agent is prepared by mixing sodium xanthate with a concentration of 0.05 - 0.5 mol / L, hydrogen peroxide with a concentration of 0.1 - 0.3 mol / L, and sodium dodecyl sulfate with a concentration of 0.15 - 0.2 mol / L;

[0012] S4. Remediation treatment

[0013] Perform pretreatment on the shallow soil sample and the deep soil sample respectively to obtain a shallow soil sample to be remediated and a deep soil sample to be remediated;

[0014] For the shallow soil sample to be remediated, first elute it with the first elution agent for 1 - 2 h under the condition of a liquid-solid ratio of 3 mL:1 g, then mix the second elution agent and the foaming agent at a mass ratio of 2 - 4:0.1 and elute it again for 1 - 2 h under the condition of a liquid-solid ratio of 3 mL:1 g, then add a pH regulator to adjust the pH to 8 - 12, and then carry out flotation treatment by introducing air at a rate of 50 - 80 mL / min. The flotation treatment time is 10 - 30 min. After a foam layer is generated on the liquid surface, separate the foam layer with a scraper to complete the remediation;

[0015] For the deep soil sample to be remediated, mix the first elution agent and the second elution agent at a mass ratio of 1:0.3 - 0.5 to obtain a composite elution agent, and use the composite elution agent to elute the deep soil sample to be remediated for 1 - 2 h under the condition of a liquid-solid ratio of 5 mL:1 g, then add CaO at a mass ratio of 3 - 4:1 and assist with ultrasonic treatment. The ultrasonic frequency is 20 - 30 kHz, and the ultrasonic treatment time is 5 - 20 min to complete the remediation.

[0016] Note: Oxalic acid, sodium hydroxide, and EDTA act synergistically to desorb arsenic from soil particles and dissolve it into the liquid phase. Hydrogen peroxide converts arsenic(III) to arsenic(V). Oxalic acid and EDTA further stabilize arsenic to prevent it from re-adsorbing onto soil particles. As a collector, sodium xanthate can chemically react with the surface of arsenic minerals, change its surface properties, and enhance the desorption ability of arsenic. Hydrogen peroxide acts as an oxidant to oxidize arsenic(III) to arsenic(V), which is more likely to react with the eluent and desorb from soil particles. Sodium dodecyl sulfate acts as a surfactant, which can reduce the surface tension of soil particles and promote the desorption and migration of arsenic. For the deep soil sample to be treated, the addition of CaO can improve the density of the cement mortar, reduce pores, and enhance the solidification effect. The high-density mortar can effectively prevent the penetration and migration of arsenic; and CaO reacts with water to generate Ca(OH) 2, increasing the calcium ion concentration in the solution. These calcium ions can combine with surfactants such as residual sodium xanthate and sodium dodecyl sulfate to enhance the stability of the foam. The presence of calcium ions makes the residual foam layer more persistent, facilitating subsequent separation and treatment.

[0017] Furthermore, in S2, change gloves once before collecting each sample and clean the sampling tools;

[0018] Note: Before collecting soil samples ex-situ, changing gloves is a key step to prevent cross-contamination. These measures ensure the representativeness of the samples and the accuracy of the detection data, providing a reliable basis for subsequent pollution assessment and remediation.

[0019] Furthermore, in S3, the EDTA selected is EDTA-2Na or EDTA-FeNa, and the natural organic acid is oxalic acid or citric acid;

[0020] Note: EDTA-FeNa contains iron ions and can be used to supplement iron and remove arsenic simultaneously. As a chelating agent, it forms soluble complexes with arsenic, promoting the desorption of arsenic from the soil. In flotation, oxalic acid can improve the surface properties of arsenic minerals and enhance their interaction with the collector; EDTA, as a strong chelating agent, forms stable water-soluble complexes with arsenic, significantly improving the solubility of arsenic.

[0021] Furthermore, in S4, the method of pretreatment is: air-dry naturally, then grind and crush with an agate mortar, and sieve through a 60-mesh sieve for standby;

[0022] Note: Direct grinding can quickly reach the target particle size. Crushing to 60 meshes can ensure the effective dispersion of the soil, and also make the subsequent treatment of the hydration liquid more permeable. At the same time, it can avoid the interference of the components of the hydration liquid on the detection result of the arsenic concentration in the soil to be treated, thus facilitating the remediation treatment of arsenic.

[0023] Furthermore, in S4, if the arsenic concentration in the soil < 200 mg / kg, leach with clean water for 1 - 2 h under the condition of a liquid-solid ratio of 3 mL:1 g to remove the exchangeable arsenic in the soil sample to be repaired;

[0024] Note: When the arsenic concentration in the soil sample to be repaired is lower than 200 mg / kg, only leaching with clean water can remove the exchangeable arsenic, saving the cost of chemicals; when the arsenic concentration in the soil sample to be repaired is higher than 200 mg / kg, adopt the repair method of S4 to ensure the efficient removal of arsenic; adjusting the repair strategy according to the arsenic concentration at this stage optimizes the resource utilization and ensures the maximization of the repair effect.

[0025] Furthermore, after pretreatment, the soil sample to be repaired is loosened, and then a hydration solution is added to the soil sample to be repaired at a solid-liquid ratio of 100 g: 200 - 500 mL. Finally, the soil sample to be repaired is placed in a stirrer and stirred at a speed of 50 - 100 r / min for 10 - 30 min to ensure that the soil sample to be repaired is fully hydrated;

[0026] The hydration solution, by mass percentage, includes 5 - 10 wt% sodium silicate, 6 - 8 wt% sodium chloride, and the balance water;

[0027] Note: Sodium chloride can adjust the ionic strength of the solution and optimize the flotation conditions; the addition of sodium silicate can disperse soil particles, prevent agglomeration, improve the dispersibility of soil particles, and increase the contact area with the reagent; through hydration treatment, the dispersibility of the soil can be improved, the desorption of arsenic can be promoted, and the flotation conditions can be optimized, thus significantly enhancing the flotation ability.

[0028] Furthermore, the loosening tool used for the loosening treatment is a tooth harrow, the loosening depth is 20 - 30 cm, and the loosening time is 15 - 30 min;

[0029] Note: Loosening the shallow soil before adding the hydration solution can significantly improve the permeability of the hydration solution, the uniform distribution of the reagent, the desorption efficiency of arsenic, and the repair effect. At the same time, loosening the soil can also improve the physical properties of the soil, reduce the repair cost, and create better conditions for subsequent treatment.

[0030] Furthermore, the foaming agent is pine oil or methyl isobutyl carbinol; the pH regulator is sodium hydrogen phosphate at 0.2 - 1 mol / L or sulfuric acid at 0.2 - 1 mol / L;

[0031] Note: The pH regulator can adjust the soil pH to alkaline to promote the desorption of arsenic. In flotation, alkaline conditions are beneficial to the surface charge regulation of arsenic minerals and enhance the flotation effect; the foaming agent is used to generate stable bubbles; the pH regulator at the above concentration can effectively adjust the soil to be treated to the optimal pH for flotation, thus promoting the hydration reaction and improving the arsenic removal effect.

[0032] Compared with the existing technology, the beneficial effects of the present invention are:

[0033] (1) The present invention conducts leaching experiments on soils with different depths and different pollution degrees. Compared with pure water leaching, the arsenic removal rates of the three leaching agents are effectively improved. The elution efficiency from high to low is: oxalic acid > EDTA > NaOH; under different experimental parameter conditions, the arsenic removal rate of oxalic acid is between 1.6% and 89.1%, the arsenic removal rate of EDTA is between 2.2% and 43.5%, and the arsenic removal rate of NaOH is between 0.3% and 58.8%.

[0034] (2) The present invention adopts a hierarchical leaching restoration method, and conducts restoration treatments on shallow soil samples and deep soil samples respectively. This restoration method can more thoroughly remove the residual agents, avoiding the potential harm to the environment caused by the backfill of the residual agents in the soil after restoration. Moreover, the present invention first uses clear water to remove the easily desorbed arsenic, which has a lower cost, and then uses EDTA, oxalic acid, etc. to treat the difficult-to-desorb arsenic, achieving more efficient restoration on the basis of reducing the overall restoration cost. The low-concentration leaching solution can be directly treated or recycled, and the high-concentration leaching solution can be centrally treated, reducing the treatment difficulty and cost.

[0035] (3) The present invention further improves the desorption rate of arsenic by preparing a second leaching agent, enabling more arsenic to be released from soil particles into the liquid phase, thereby effectively restoring. In addition, based on the addition of the second leaching agent, the present application adds a flotation treatment and a step of hydrating the soil after pretreatment, further improving the soil dispersibility, promoting arsenic desorption, and optimizing the flotation conditions, thus significantly enhancing the flotation ability and effectively improving the arsenic pollution restoration ability. Description of the Drawings

[0036] Figure 1 It is a comparison chart of the average arsenic removal rates of the shallow soils in Example 1 - Example 3, Blank Group 1 - Blank Group 3, and Control Group 1 of the present invention;

[0037] Figure 2 It is a comparison chart of the average arsenic removal rates of the deep soils in Example 1 - Example 3, Blank Group 1 - Blank Group 3, and Control Group 1 of the present invention;

[0038] Figure 3 It is a comparison chart of the average arsenic removal rates of the shallow soils in Example 1, Example 4, Example 5, and Control Group 2 - Control Group 4 of the present invention;

[0039] Figure 4 It is a comparison chart of the average arsenic removal rates of the deep soils in Example 1, Example 4, Example 5, and Control Group 2 - Control Group 4 of the present invention;

[0040] Figure 5 It is a comparison chart of the average arsenic removal rates of the shallow soils in Example 1, Example 9 - Example 16, and Control Group 5 - Control Group 6 of the present invention;

[0041] Figure 6 It is a comparison chart of the average arsenic removal rates of the deep soils in Example 1, Example 9 - Example 16, and Control Group 5 - Control Group 6 of the present invention. Detailed Embodiments

[0042] To further elaborate on the methods adopted and the effects achieved by the present invention, the technical solutions of the present invention will be clearly and completely described below in combination with experiments.

[0043] Example 1: A method for the hierarchical remediation of arsenic-contaminated soil based on off-site leaching, comprising the following steps:

[0044] S1. Determination of sampling points

[0045] Three shallow soil points with a depth of 1.0 m and three deep soil points with a depth of 2.5 m are respectively selected;

[0046] S2. Collection of soil samples

[0047] For the shallow and deep soil points selected in S1, GPS is used to reposition them to obtain longitude and latitude coordinate information and record the point coordinates. Then, a drill is used to take 1.5 kg of samples at the corresponding depths, and the samples at the same depth are mixed to obtain shallow soil samples and deep soil samples, which are then packed in polyethylene sealed bags for standby; among them, gloves are changed once before each sample collection, and the sampling tools are cleaned.

[0048] S3. Preparation of leaching agents

[0049] The leaching agents include a first leaching agent and a second leaching agent;

[0050] The first leaching agent is a 0.6 mol / L natural organic acid, and the second leaching agent is prepared by mixing 0.28 mol / L sodium xanthate, 0.2 mol / L hydrogen peroxide, and 0.17 mol / L sodium dodecyl sulfate;

[0051] S4. Leaching and remediation treatment

[0052] The shallow and deep soil samples are respectively pretreated to obtain shallow soil samples to be repaired and deep soil samples to be repaired; the pretreatment method is: natural air drying, then grinding and crushing with an agate mortar, and passing through a 60-mesh sieve for standby;

[0053] For the shallow soil samples to be repaired, first leach with the first leaching agent for 1.5 h under the condition of a liquid-solid ratio of 3 mL:1 g, then mix the second leaching agent and the foaming agent in a mass ratio of 3:0.1 and leach again for 1.5 h under the condition of a liquid-solid ratio of 3 mL:1 g, then add a pH regulator to adjust the pH to 10, and then pass air at a rate of 65 mL / min for flotation treatment. The flotation treatment time is 20 min. After a foam layer is generated on the liquid surface, the foam layer is separated with a scraper to complete the repair; the foaming agent is pine oil; the pH regulator is a 0.6 mol / L sodium hydrogen phosphate solution or a 0.6 mol / L sulfuric acid solution.

[0054] For the deep soil samples to be repaired, the first eluent and the second eluent are mixed at a mass ratio of 1:0.4 to obtain a composite eluent. The deep soil samples to be repaired are eluted with the composite eluent for 1.5 h under the condition of a liquid-solid ratio of 5 mL:1 g, and then CaO is added at a mass ratio of 3.5:1 and ultrasonic treatment is supplemented. The ultrasonic frequency is 25 kHz, and the ultrasonic treatment time is 13 min to complete the repair.

[0055] Example 2: Different from Example 1, the second eluent is prepared by mixing 0.05 mol / L sodium xanthate, 0.1 mol / L hydrogen peroxide, and 0.15 mol / L sodium dodecyl sulfate.

[0056] Example 3: Different from Example 1, the second eluent is prepared by mixing 0.5 mol / L sodium xanthate, 0.3 mol / L hydrogen peroxide, and 0.2 mol / L sodium dodecyl sulfate.

[0057] Example 4: Different from Example 1, for the shallow soil samples to be repaired, first, the shallow soil samples are eluted with the first eluent for 1 h under the condition of a liquid-solid ratio of 3 mL:1 g, and then the second eluent and the foaming agent are mixed at a mass ratio of 2:0.1 and eluted again for 1 h under the condition of a liquid-solid ratio of 3 mL:1 g. Then, a pH regulator is added to adjust the pH to 8, and then air is introduced at a rate of 50 mL / min for flotation treatment. The flotation treatment time is 10 min. After a foam layer is generated on the liquid surface, the foam layer is separated with a scraper to complete the repair;

[0058] For the deep soil samples to be repaired, the first eluent and the second eluent are mixed at a mass ratio of 1:0.3 to obtain a composite eluent. The deep soil samples to be repaired are eluted with the composite eluent for 1 h under the condition of a liquid-solid ratio of 5 mL:1 g, and then CaO is added at a mass ratio of 3:1 and ultrasonic treatment is supplemented. The ultrasonic frequency is 20 kHz, and the ultrasonic treatment time is 5 min to complete the repair.

[0059] Example 5: Different from Example 1, for the shallow soil samples to be repaired, first, the shallow soil samples are eluted with the first eluent for 2 h under the condition of a liquid-solid ratio of 3 mL:1 g, and then the second eluent and the foaming agent are mixed at a mass ratio of 4:0.1 and eluted again for 2 h under the condition of a liquid-solid ratio of 3 mL:1 g. Then, a pH regulator is added to adjust the pH to 12, and then air is introduced at a rate of 80 mL / min for flotation treatment. The flotation treatment time is 30 min. After a foam layer is generated on the liquid surface, the foam layer is separated with a scraper to complete the repair;

[0060] For the deep soil samples to be repaired, the first eluent and the second eluent are mixed at a mass ratio of 1:0.5 to obtain a composite eluent. The deep soil samples to be repaired are eluted with the composite eluent for 2 h under the condition of a liquid-solid ratio of 5 mL:1 g, and then CaO is added at a mass ratio of 4:1 and ultrasonic treatment is supplemented. The ultrasonic frequency is 30 kHz, and the ultrasonic treatment time is 20 min to complete the repair.

[0061] Example 6: Different from Example 1, the pH regulator is a 0.2 mol / L sodium hydrogen phosphate solution or a 0.2 mol / L sulfuric acid solution.

[0062] Example 7: Different from Example 1, the pH regulator is a 1 mol / L sodium hydrogen phosphate solution or a 1 mol / L sulfuric acid solution.

[0063] Example 8: Different from Example 1, the foaming agent is methyl isobutyl carbinol.

[0064] Example 9: Different from Example 1, if the arsenic concentration in the soil < 200 mg / kg, the soil is eluted with clean water for 1.5 h under the condition of a liquid-solid ratio of 3 mL:1 g to remove the exchangeable arsenic in the soil.

[0065] Example 10: Different from Example 1, if the arsenic concentration in the soil < 200 mg / kg, the soil is eluted with clean water for 1 h under the condition of a liquid-solid ratio of 2 mL:1 g to remove the exchangeable arsenic in the soil.

[0066] Example 11: Different from Example 1, if the arsenic concentration in the soil < 200 mg / kg, the soil is eluted with clean water for 2 h under the condition of a liquid-solid ratio of 4 mL:1 g to remove the exchangeable arsenic in the soil.

[0067] Example 12: Different from Example 9, after pretreatment, the soil samples to be repaired are loosened, and then a hydration solution is added to the soil to be repaired at a ratio of 100 g:350 mL. Finally, the soil samples to be repaired are placed in a stirrer and stirred at a speed of 75 r / min for 20 min to ensure that the soil to be repaired is fully hydrated; the hydration solution, by mass percentage, includes 7 wt% sodium silicate, 7 wt% sodium chloride, and the balance water;

[0068] Among them, the loosening tool used for the loosening treatment is a commercially available tooth rake, the loosening depth is 25 cm, and the loosening time is 20 min.

[0069] Example 13: Different from Example 12, a hydration solution is added to the soil samples to be repaired at a ratio of 100 g:200 mL. Finally, the soil samples to be repaired are placed in a stirrer and stirred at a speed of 50 r / min for 10 min to ensure that the soil to be repaired is fully hydrated.

[0070] Example 14: Different from Example 12, a hydration solution was added to the soil sample to be repaired at a ratio of 100 g: 500 mL. Finally, the soil sample to be repaired was placed in a stirrer and stirred at a speed of 100 r / min for 30 min to ensure that the soil sample to be repaired was fully hydrated.

[0071] Example 15: Different from Example 12, the hydration solution, by mass percentage, includes 5 wt% sodium silicate, 6 wt% sodium chloride, and the balance water.

[0072] Example 16: Different from Example 12, the hydration solution, by mass percentage, includes 10 wt% sodium silicate, 8 wt% sodium chloride, and the balance water.

[0073] Experimental Example: The description of this experimental example is based on the recorded scheme in Example 1, aiming to clarify the actual application effect of the present invention. Among them, 3 shallow soil points were successively denoted as D1-40 (0-1.5 m), D1-61 (0-1.5 m), D4-1 (0-1.5 m), and 3 deep soil points were successively denoted as D1-40 (1.5-3.5 m), D1-41 (1.5-3.5 m), D1-91 (1.5-3.5 m). The blank groups 1 to 12 using only the first leaching agent were used as controls. Specifically:

[0074] Blank group 1: Different from Example 1, 0.5 mol / L natural organic acid was selected as the first leaching agent; among them, the natural organic acid was oxalic acid; in S4, the first leaching agent and the shallow soil sample to be repaired, and the first leaching agent and the deep soil sample to be repaired were respectively mixed at a liquid-solid ratio of 3 mL: 1 g and placed on a shaker, and shaken and leached at a shaking rate of 25 °C and 200 r / min for 2 h.

[0075] Blank group 2: Different from blank group 1, 0.2 mol / L natural organic acid was selected as the first leaching agent.

[0076] Blank group 3: Different from blank group 1, 1 mol / L natural organic acid was selected as the first leaching agent.

[0077] Blank group 4: Different from blank group 1, 0.2 mol / L EDTA was selected as the first leaching agent.

[0078] Blank group 5: Different from blank group 1, 0.1 mol / L EDTA was selected as the first leaching agent.

[0079] Blank group 6: Different from blank group 1, 0.5 mol / L EDTA was selected as the first leaching agent.

[0080] Blank group 7: Different from blank group 1, 0.5 mol / L sodium hydroxide is selected as the first elution agent.

[0081] Blank group 8: Different from blank group 1, 0.2 mol / L sodium hydroxide is selected as the first elution agent.

[0082] Blank group 9: Different from blank group 1, 1 mol / L sodium hydroxide is selected as the first elution agent.

[0083] Blank group 10: Different from blank group 1, the first elution agent is pure water.

[0084] Blank group 11: Different from blank group 10, in S4, the first elution agent and the shallow soil sample to be repaired, the first elution agent and the deep soil sample to be repaired are respectively mixed according to the liquid-solid ratio of 2 mL: 1 g and placed on a shaker, and shaken and eluted at a shaking rate of 22 °C and 180 r / min, and the elution time is 1 h.

[0085] Blank group 12: Different from blank group 10, in S4, the first elution agent and the shallow soil sample to be repaired, the first elution agent and the deep soil sample to be repaired are respectively mixed according to the liquid-solid ratio of 5 mL: 1 g and placed on a shaker, and shaken and eluted at a shaking rate of 27 °C and 220 r / min, and the elution time is 4 h.

[0086] Record the arsenic removal effects of blank groups 1 to 12 respectively, and the conclusions are as follows: According to the control experiments of blank groups 11 and 12, under the conditions of liquid-solid ratios of 2 and 5 (v:w) and elution times of 1 and 4 h, the arsenic removal rates of the soil to be treated are between 0.86% and 10.22%, and none of them can meet the repair target; for the elution experiments of blank groups 1 to 9, compared with the pure water elution of blank group 10, it is concluded that the arsenic removal rates of the three elution agents are all effectively improved, and the elution efficiencies from high to low are: oxalic acid > EDTA > NaOH. Under different experimental parameter conditions, the arsenic removal rate of oxalic acid is between 1.6% and 89.1%, the arsenic removal rate of EDTA is between 2.2% and 43.5%, and the arsenic removal rate of NaOH is between 0.3% and 58.8%. The elution effect of NaOH on arsenic is weaker than that of the other two. The removal rates of EDTA and oxalic acid are relatively stable in different concentrations and different depths of soil. However, considering that EDTA is relatively expensive and difficult to degrade in soil, it will remain in the soil for a long time and is likely to cause secondary pollution to the soil. In summary, oxalic acid elution agent is preferentially considered as the elution agent for arsenic-contaminated soil in this plot. Considering comprehensively, blank group 1 is the best repair parameter.

[0087] In addition, from the analysis at different depth levels, under the action of oxalic acid leaching in blank group 1, the best arsenic removal rates in the three shallow soil sites D1-40 (0-1.5 m), D1-61 (0-1.5 m), and D4-1 (0-1.5 m) were 73.5%, 58.5%, and 83.5% respectively; the best arsenic removal rates in the three deep soil sites D1-40 (1.5-3.5 m), D1-41 (1.5-3.5 m), and D1-91 (1.5-3.5 m) were 87.0%, 26.8%, and 89.1% respectively. From the arsenic removal rates in the soil, it can be seen that the leaching effect of oxalic acid on arsenic in shallow soil is more stable, and the arsenic removal rate is stable above 50% in different soil sites; while in deep soil, due to the relatively high clay content, it may affect the mass transfer effect between the leaching solution and pollutants, resulting in certain fluctuations in the removal effect of the leaching agent. The average arsenic removal rate of the shallow soil to be treated in blank group 1 reached 71.8%, and the average arsenic removal rate of the deep soil to be treated reached 67.6%. In this test example, the average arsenic removal rate of shallow soil is the average of the best arsenic removal rates in the three shallow soil sites D1-40 (0-1.5 m), D1-61 (0-1.5 m), and D4-1 (0-1.5 m); the average arsenic removal rate of deep soil in this test example is the average of the best arsenic removal rates in the three deep soil sites D1-40 (1.5-3.5 m), D1-41 (1.5-3.5 m), and D1-91 (1.5-3.5 m);

[0088] 1. Explore the improvement performance of the addition of the second leaching agent on the arsenic pollution remediation effect

[0089] Control group 1: Different from Example 1, sodium xanthate in the second leaching agent was replaced with sodium dithiophosphate.

[0090] Conclusion: By comparing Examples 1 to 3, it can be seen that too high or too low content of each component of the second elution agent within the scope of this application has little effect on the removal of arsenic in the soil, and Example 1 is the best; by comparing Example 1, Blank Group 1 to Blank Group 3, it can be obtained that adding the second elution agent can effectively improve the arsenic removal rate in the soil. This is mainly because the second elution agent can significantly increase the desorption rate of arsenic, enabling more arsenic to be released from soil particles into the liquid phase; and sodium dodecyl sulfate can, through micelle action, encapsulate arsenic in micelles to improve its solubility, and the second elution agent can significantly increase the concentration of arsenic in the eluent, thereby effectively removing it; by comparing Examples 1 to 3 and Control Group 1, it can be concluded that replacing sodium xanthate with sodium dithiophosphate will also weaken the arsenic pollution removal effect. This is because sodium xanthate has a stronger selective adsorption ability on the surface of arsenic minerals and is not easily interfered by sodium silicate and sodium oxide, while sodium dithiophosphate may react with calcium hydroxide generated by calcium oxide in water to form insoluble calcium salt precipitates, and the state of sodium dithiophosphate is also easily affected by calcium oxide, resulting in decomposition or failure and being difficult to play its flotation role. Based on this, Example 1 is the further optimized solution.

[0091] 2. Explore the improvement performance of the hierarchical remediation method on the remediation effect of arsenic pollution

[0092] Control Group 2: Different from Example 1, no flotation treatment is carried out.

[0093] Control Group 3: Different from Example 1, CaO is not added during ultrasonic treatment.

[0094] Control Group 4: Different from Example 1, CaO is added but no ultrasonic treatment is carried out.

[0095] Conclusion: By comparing Example 1, Examples 4 to 5 and Control Group 2, it can be obtained that flotation treatment can further improve the arsenic removal effect in the soil, and oxalic acid, sodium hydroxide, EDTA and the components promoting arsenic flotation (i.e., the second elution agent) act synergistically to significantly improve the remediation effect of arsenic in the soil through mechanisms such as desorption, dissolution, oxidation and flotation; at this time, sodium dodecyl sulfate and sodium xanthate are selectively adsorbed on the surface of arsenic minerals under alkaline conditions to make them hydrophobic and separated from the soil by bubble flotation. Through the above process, the removal efficiency of arsenic is significantly improved, the concentration of arsenic in the soil is reduced, and the remediation effect is improved; by comparing Example 1, Examples 4, 5 and Control Group 3, it can be obtained that the lack of addition of calcium oxide in Control Group 3 will also cause the weakening of the arsenic pollution removal effect. This is because CaO enhances the water reducing agent effect, reduces the water-cement ratio, improves the compactness of cement mortar, reduces pores, enhances the solidification effect, and high-density mortar can effectively prevent the penetration and migration of arsenic; and CaO reacts with water to generate Ca(OH) 2, increase the calcium ion concentration in the solution, these calcium ions can combine with residual surfactants such as sodium xanthate and sodium dodecyl sulfate to enhance the stability of the foam, and the presence of calcium ions makes the residual foam layer more durable, which is convenient for continued separation and treatment; from the comparison of Example 1, Example 4, Example 5 and Control Group 4, it can be seen that the lack of ultrasonic treatment will also cause the arsenic pollution removal effect to be weakened, because the lack of ultrasonic treatment will significantly reduce the remediation effect of arsenic contaminated soil, which is specifically manifested in reduced arsenic desorption efficiency, reduced effectiveness of elution agents, prolonged remediation time, weakened CaO stabilization effect, increased remediation cost and higher residual arsenic concentration in the soil after remediation. From Example 1, Example 6 to Example 8, it can be seen that the concentration of the pH regulator is too large or too small and the adjustment of the foaming agent within the scope of this application has a relatively insignificant effect on soil arsenic removal; in summary, Example 1 is a relatively superior solution.

[0096] To explore the improvement of arsenic pollution remediation effect by arsenic concentration classification, hydration treatment method of soil samples to be remediated and hydration liquid composition

[0097] Control group 5: Different from Example 12, no loosening treatment was performed.

[0098] Control group 6: Different from Example 12, the hydration solution does not contain sodium chloride.

[0099] Conclusion: It can be seen from the comparison of Example 1, Example 9 to Example 11 that eluting with clean water for the situation of soil arsenic concentration <200 mg / kg can effectively reduce the arsenic contamination in the soil to be treated, and because the low-concentration eluent can be directly treated or reused, the high-concentration eluent can be centrally treated, which can effectively reduce the difficulty and cost of treating contaminated soil, so comprehensively considering, the scheme of Example 9 is more reasonable and relatively better; it can be obtained by comparing Example 9 and Example 12 to Example 16 that it is necessary to treat the hydration of the soil sample to be repaired, because by hydration treatment, the dispersibility of the soil can be improved, the desorption of arsenic can be promoted, and the flotation conditions can be optimized, thereby significantly improving the flotation capacity, and comprehensively considering, the arsenic treatment effect of the scheme of Example 12 is better, relatively optimal; it can be obtained by comparing Example 12 and Control Group 5 that the shallow soil is loosened before adding the hydration liquid, which can significantly improve the permeability of the hydration liquid, the uniform distribution of the agent, the desorption efficiency of arsenic and the repair effect. At the same time, loose soil can also improve the physical properties of the soil and reduce the cost of repair. By comparing Example 12 with Control Group 6, it can be seen that sodium chloride dissociates into Na⁺ and Cl⁻ ions in water, increasing the ion concentration in the solution, thereby increasing the ionic strength of the solution. High ionic strength can reduce the surface tension of bubbles, enhance the stability of bubbles, and make them more durable during the flotation process; stable bubbles can better carry arsenic-containing particles to float, thereby improving flotation efficiency. In summary, Example 12 is the best solution.

Claims

1. A method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation, characterized in that: The following steps are involved: S1. Determination of sampling points Select 2-3 shallow soil points with a depth of 0-1.5 m and 2-3 deep soil points with a depth of 1.5-3.5 m; S2. Soil sample collection For the shallow soil point and deep soil point selected by S1, GPS is used to reposition them to obtain the longitude and latitude coordinate information and record the point coordinates, and then 1-2 kg of samples of the corresponding depths are taken by a drilling rig, and the samples of the same depth are mixed to obtain shallow soil samples and deep soil samples, which are then packaged in polyethylene sealed bags for standby use; S3. Preparation of elution agent The elution agent includes a first elution agent and a second elution agent; The first elution agent is 0.2-1 mol / L natural organic acid or 0.1-0.5 mol / L EDTA or 0.2-1 mol / L sodium hydroxide solution, and the second elution agent is prepared by mixing 0.05-0.5 mol / L sodium xanthate, 0.1-0.3 mol / L hydrogen peroxide and 0.15-0.2 mol / L sodium dodecyl sulfate; S4. Repair process Pre-treating the shallow soil sample and the deep soil sample respectively to obtain a shallow soil sample to be restored and a deep soil sample to be restored; After the pretreatment, the soil sample to be repaired is loosened, and then a hydration liquid is added to the soil sample to be repaired according to a solid-liquid ratio of 100g:200-500mL. Finally, the soil sample to be repaired is placed in a stirrer and stirred at a speed of 50-100r / min for 10-30min to ensure that the soil to be repaired is fully hydrated; the hydration liquid includes 5-10wt% of sodium silicate, 6-8wt% of sodium chloride and the remainder of water in terms of mass percentage; the loosening tool used for the loosening treatment is a toothed rake, the loosening depth is 20-30cm, and the loosening time is 15-30min; For the shallow soil sample to be repaired, the first eluting agent is first used for elution at a liquid-solid ratio of 3 mL: 1 g for 1-2 h, and then the second eluting agent and the foaming agent are mixed at a mass ratio of 2-4:0.1 and eluted again at a liquid-solid ratio of 3 mL: 1 g for 1-2 h, and then a pH regulator is added to adjust the pH to 8-12, and then air is introduced at a rate of 50-80 mL / min for flotation treatment. The flotation treatment time is 10-30 min. After a foam layer is generated on the liquid surface, the foam layer is separated with a scraper to complete the repair; For the deep soil sample to be repaired, the first eluting agent and the second eluting agent are mixed in a mass ratio of 1:0.3~0.5 to obtain a composite eluting agent, and the deep soil sample to be repaired is eluted with the composite eluting agent at a liquid-solid ratio of 5mL:1g for 1~2h, and then CaO is added in a mass ratio of 3~4:1 and assisted by ultrasonic treatment, the ultrasonic frequency is 20~30kHz, and the ultrasonic treatment time is 5~20min to complete the repair.

2. A method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 1, characterized in that: In S2, gloves were changed and sampling tools were cleaned before each sample collection.

3. The method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 1, characterized in that: In S3, the EDTA is EDTA-2Na or EDTA-FeNa, and the natural organic acid is oxalic acid or citric acid.

4. The method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 1, characterized in that: In S4, the pretreatment method is: natural air drying, then grinding and crushing with an agate mortar, and passing through a 60-mesh sieve for later use.

5. The method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 1, characterized in that: In S4, if the arsenic concentration in the soil is less than 200 mg / kg, use clean water to rinse for 1 to 2 hours at a liquid-to-solid ratio of 3 mL:1 g to remove the exchangeable arsenic in the soil sample to be repaired.

6. The method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 1, characterized in that: The foaming agent is pine oil or methyl isobutyl carbinol; the pH adjuster is 0.2-1 mol / L sodium hydrogen phosphate solution or 0.2-1 mol / L sulfuric acid solution.

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