Method for graded remediation of arsenic-polluted soil based on ectopic leaching

Abstract

The invention relates to the technical field of soil regeneration, in particular to a method for graded remediation of arsenic-polluted soil based on ectopic leaching, which comprises the following steps: S1, determining sampling points; s2, collecting a soil sample; s3, preparing a leaching agent; s4, repairing treatment is carried out; according to the method, the desorption rate of arsenic is further improved by preparing the synergistic eluting agent, more arsenic can be released into a liquid phase from soil particles, and therefore effective remediation of arsenic in soil is achieved; in addition, after the synergistic eluting agent is added, the steps of flotation treatment and hydration treatment on the soil are added according to the depth degree of the to-be-treated soil, the dispersity of the soil is further improved, desorption of arsenic is promoted, flotation conditions are optimized, the flotation capacity is remarkably improved, and the arsenic pollution repairing capacity is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of soil regeneration, and in particular to a method for remediating arsenic-contaminated soil based on ex situ leaching and graded restoration. Background Art

[0002] Soil pollution remediation technology refers to the use of chemical, physical and biological techniques and methods to reduce the concentration of pollutants in the 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 two methods: in situ remediation and ex situ remediation. When the arsenic concentration in the soil is high, in situ remediation may be difficult to achieve the ideal remediation effect, but ex situ remediation can concentrate the contaminated soil and improve the remediation efficiency. If the contaminated area is relatively concentrated, ex situ remediation is easier to implement and can completely remove the contaminated soil.

[0003] Ex situ remediation can completely remove the pollution source by digging out the contaminated soil and repairing it to the target value or disposing it safely, thus preventing the further spread of arsenic in the environment and reducing the impact on the surrounding environment. Among the existing ex situ 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-manganese oxide-bound state, organic state, and residual state) respond differently to leaching agents. The effect of a single leaching on the remediation of arsenic in the soil is limited. At the same time, the one-time use of a large amount of agents is prone to waste, and the remediation cost is high. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a method for remediating arsenic-contaminated soil based on ex situ elution and graded restoration, comprising the following steps: 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; 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.

[0005] Description: Oxalic acid, sodium hydroxide, and EDTA work synergistically to desorb arsenic from soil particles and dissolve it into the liquid phase. Hydrogen peroxide converts arsenic (III) into arsenic (V). Oxalic acid and EDTA further stabilize the arsenic to prevent it from being re-adsorbed onto soil particles. As a collector, sodium xanthate can chemically react with the surface of arsenic minerals, changing their surface properties and enhancing the desorption capacity of arsenic. As an oxidant, hydrogen peroxide oxidizes arsenic (III) to arsenic (V), which is more easily reacted with the eluent and desorbed from the soil particles. As a surfactant, sodium dodecyl sulfate can reduce the surface tension of soil particles and promote the desorption and migration of arsenic. For deep soil samples to be treated, the addition of CaO can increase the density of cement mortar, reduce pores, and enhance the curing effect. High-density mortar can effectively prevent the penetration and migration of arsenic. CaO reacts with water to generate Ca(OH)2, which increases the concentration of calcium ions 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. The presence of calcium ions makes the residual foam layer more durable, which is convenient for subsequent separation and treatment.

[0006] Furthermore, in S2, gloves were changed and sampling tools were cleaned before each sample collection; Note: Changing gloves before collecting soil samples ex situ is a key step to prevent cross-contamination. These measures ensure the representativeness of the samples and the accuracy of the test data, providing a reliable basis for subsequent pollution assessment and remediation.

[0007] Further, in S3, the EDTA is selected from EDTA-2Na or EDTA-FeNa, and the natural organic acid is oxalic acid or citric acid; Description: EDTA-FeNa contains iron ions and can be used to supplement iron and remove arsenic at the same time. As a chelating agent, it forms a soluble complex with arsenic and promotes the desorption of arsenic from the soil. In flotation, oxalic acid can improve the surface properties of arsenic minerals and enhance their interaction with collectors; EDTA, as a strong chelating agent, forms a stable water-soluble complex with arsenic, significantly improving the solubility of arsenic.

[0008] Furthermore, in S4, the pretreatment method is: air-drying naturally, then grinding and crushing with an agate mortar, and passing through a 60-mesh sieve for later use; Note: Direct grinding can quickly reach the target particle size. Crushing to 60 mesh can ensure effective dispersion of the soil and make subsequent hydration liquid treatment easier to penetrate. It can also avoid the components of the hydration liquid interfering with the arsenic concentration test results of the soil to be treated, thereby facilitating arsenic remediation treatment.

[0009] Furthermore, in S4, if the arsenic concentration in the soil is less than 200 mg / kg, the exchangeable arsenic in the soil sample to be remediated is removed by eluting with clean water at a liquid-to-solid ratio of 3 mL:1 g for 1 to 2 h; Note: When the arsenic concentration in the soil sample to be repaired is lower than 200 mg / kg, exchangeable arsenic can be removed by rinsing with clean water, saving the cost of reagents; when the arsenic concentration in the soil sample to be repaired is higher than 200 mg / kg, the S4 repair method is adopted to ensure efficient removal of arsenic; in this stage, the repair strategy is adjusted according to the arsenic concentration, resource utilization is optimized, and the repair effect is maximized.

[0010] Furthermore, 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 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 to be repaired is fully hydrated. The hydration liquid comprises, by weight percentage, 5-10 wt % of sodium silicate, 6-8 wt % of sodium chloride and the balance of water; Description: 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, thereby significantly improving the flotation capacity.

[0011] Furthermore, the loosening tool used in the loosening treatment is a toothed rake, the loosening depth is 20-30 cm, and the loosening time is 15-30 min; Note: Loosening the shallow soil before adding the hydration solution can significantly improve the permeability of the hydration solution, the uniform distribution of the agent, the desorption efficiency of arsenic and the remediation effect. At the same time, loosening the soil can also improve the physical properties of the soil, reduce the cost of remediation, and create better conditions for subsequent treatment.

[0012] Furthermore, the foaming agent is pine oil or methyl isobutyl carbinol; the pH adjuster is 0.2-1 mol / L sodium hydrogen phosphate or 0.2-1 mol / L sulfuric acid; Description: pH regulator can adjust soil pH to alkaline and promote arsenic desorption. In flotation, alkaline conditions are conducive to the surface charge regulation of arsenic minerals and enhance the flotation effect; frothers are used to generate stable bubbles; the above concentration of pH regulator can effectively adjust the soil to be treated to the optimal pH for flotation, thereby promoting hydration reaction and improving arsenic removal effect.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention conducts elution experiments on soils of different depths and different degrees of contamination. Compared with elution with pure water, the removal rates of arsenic by the three eluents are effectively improved. The elution efficiency is from high to low: oxalic acid > EDTA > NaOH. Under different experimental parameter conditions, the removal rate of arsenic by oxalic acid is between 1.6% and 89.1%, the removal rate of arsenic by EDTA is between 2.2% and 43.5%, and the removal rate of arsenic by NaOH is between 0.3% and 58.8%.

[0014] (2) The present invention adopts a graded leaching remediation method to perform remediation treatments on shallow soil samples and deep soil samples respectively. This remediation method can more thoroughly remove the residual agent and avoid the potential harm to the environment caused by the residual agent in the remediated soil after backfilling. In addition, the present invention preferentially uses clean water to remove easily desorbed arsenic, which is relatively low in cost, and then uses EDTA, oxalic acid, etc. to treat difficult-to-desorb arsenic, thereby reducing the overall remediation cost and making the remediation more efficient. Low-concentration eluent can be directly treated or reused, and high-concentration eluent can be centrally treated, reducing the difficulty and cost of treatment.

[0015] (3) The present invention further improves the arsenic desorption rate by preparing a second elution agent, which can release more arsenic from soil particles into the liquid phase, thereby effectively remediating the soil. In addition, the present application adds a flotation treatment on the basis of the addition of the second elution agent and adds a step of hydrating the soil after pretreatment, thereby further improving the dispersion of the soil, promoting the desorption of arsenic, and optimizing the flotation conditions, thereby significantly improving the flotation capacity and effectively improving the arsenic pollution remediation capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a comparison chart of the average arsenic removal rate of shallow soil of Examples 1 to 3 of the present invention, Blank Groups 1 to 3 and Control Group 1; Figure 2 It is a comparison chart of the average arsenic removal rate of deep soil of Examples 1 to 3 of the present invention, Blank Groups 1 to 3 and Control Group 1; Figure 3 It is a comparison chart of the average arsenic removal rate of shallow soil in Example 1, Example 4, Example 5 and Control Groups 2 to 4 of the present invention; Figure 4 It is a comparison chart of the average arsenic removal rate of deep soil of Example 1, Example 4, Example 5 and Control Groups 2 to 4 of the present invention; Figure 5 It is a comparison chart of the average arsenic removal rate of shallow soil of Example 1, Example 9 to Example 16 of the present invention and Control Group 5 to Control Group 6; Figure 6 It is a comparison chart of the average arsenic removal rate in deep soil of Example 1, Example 9 to Example 16 of the present invention and Control Group 5 to Control Group 6. DETAILED DESCRIPTION

[0017] In order to further illustrate the method adopted by the present invention and the effect achieved, the technical solution of the present invention will be clearly and completely described in combination with experiments below.

[0018] Example 1: A method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation, comprising the following steps: S1. Determination of sampling points Three shallow soil points with a depth of 1.0 m and three deep soil points with a depth of 2.5 m were selected; S2. Soil sample collection For the shallow soil points and deep soil points selected by S1, GPS was used to reposition them to obtain the longitude and latitude coordinate information and record the point coordinates. Then, 1.5 kg of samples of the corresponding depths were taken using a drilling rig, and the samples of the same depth were mixed to obtain shallow soil samples and deep soil samples, which were then packaged in polyethylene sealed bags for later use. Gloves were changed once before each sample was collected, and the sampling tools were cleaned. S3. Preparation of elution agent The elution reagent includes a first elution reagent and a second elution reagent; The first elution agent is 0.6 mol / L natural organic acid, and the second elution 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; S4. Elute and repair treatment The shallow soil samples and the deep soil samples were pretreated respectively to obtain the shallow soil samples to be restored and the deep soil samples to be restored; the pretreatment method was: natural air drying, then grinding and crushing with an agate mortar, and passing through a 60-mesh sieve for later use; For shallow soil samples to be repaired, first use the first eluting agent to elute for 1.5 hours at a liquid-to-solid ratio of 3mL:1g, then mix the second eluting agent and the frother at a mass ratio of 3:0.1, and then elute again for 1.5 hours at a liquid-to-solid ratio of 3mL:1g, then add a pH adjuster to adjust the pH to 10, and then introduce air at a rate of 65mL / min for flotation treatment. The flotation treatment time is 20min. After a foam layer is generated on the surface of the liquid, use a scraper to separate the foam layer to complete the repair; the frother is pine oil; the pH adjuster is 0.6mol / L sodium hydrogen phosphate solution or 0.6mol / L sulfuric acid solution.

[0019] For deep soil samples to be repaired, the first eluting agent and the second eluting agent were mixed in a mass ratio of 1:0.4 to obtain a composite eluting agent. The deep soil samples to be repaired were eluted with the composite eluting agent at a liquid-solid ratio of 5 mL:1 g for 1.5 h, and then CaO was added in a mass ratio of 3.5:1 and assisted by ultrasonic treatment with an ultrasonic frequency of 25 kHz and an ultrasonic treatment time of 13 min to complete the repair.

[0020] Example 2: Different from Example 1, the second elution agent is prepared by mixing 0.05 mol / L sodium xanthate, 0.1 mol / L hydrogen peroxide and 0.15 mol / L sodium dodecyl sulfate.

[0021] Example 3: Different from Example 1, the second elution agent is prepared by mixing 0.5 mol / L sodium xanthate, 0.3 mol / L hydrogen peroxide and 0.2 mol / L sodium dodecyl sulfate.

[0022] Example 4: Different from Example 1, for the shallow soil sample to be repaired, the first eluting agent is first used for elution at a liquid-solid ratio of 3mL:1g for 1h, and then the second eluting agent and the foaming agent are mixed at a mass ratio of 2:0.1 and eluted again at a liquid-solid ratio of 3mL:1g for 1h, and then a pH regulator is added to adjust the pH to 8, and then air is introduced at a rate of 50mL / min for flotation treatment. The flotation treatment time is 10min. After a foam layer is generated on the surface of the liquid, the foam layer is separated with a scraper to complete the repair; For deep soil samples to be repaired, the first eluting agent and the second eluting agent were mixed in a mass ratio of 1:0.3 to obtain a composite eluting agent. The deep soil samples to be repaired were eluted with the composite eluting agent at a liquid-solid ratio of 5 mL:1 g for 1 hour, and then CaO was added in a mass ratio of 3:1 and assisted by ultrasonic treatment with an ultrasonic frequency of 20 kHz and an ultrasonic treatment time of 5 minutes to complete the repair.

[0023] Example 5: Different from Example 1, for the shallow soil sample to be repaired, the first eluting agent is first used for elution at a liquid-solid ratio of 3mL:1g for 2h, and then the second eluting agent and the foaming agent are mixed at a mass ratio of 4:0.1 and eluted again at a liquid-solid ratio of 3mL:1g for 2h, and then a pH regulator is added to adjust the pH to 12, and then air is introduced at a rate of 80mL / min for flotation treatment. The flotation treatment time is 30min. After a foam layer is generated on the surface of the liquid, the foam layer is separated with a scraper to complete the repair; For deep soil samples to be repaired, the first eluting agent and the second eluting agent were mixed in a mass ratio of 1:0.5 to obtain a composite eluting agent. The deep soil samples to be repaired were eluted with the composite eluting agent at a liquid-solid ratio of 5 mL:1 g for 2 h, and then CaO was added in a mass ratio of 4:1 and assisted by ultrasonic treatment. The ultrasonic frequency was 30 kHz and the ultrasonic treatment time was 20 min to complete the repair.

[0024] Example 6: Different from Example 1, the pH adjuster is 0.2 mol / L sodium hydrogen phosphate solution or 0.2 mol / L sulfuric acid solution.

[0025] Example 7: Different from Example 1, the pH adjuster is 1 mol / L sodium hydrogen phosphate solution or 1 mol / L sulfuric acid solution.

[0026] Embodiment 8: The difference from embodiment 1 is that the foaming agent is methyl isobutyl carbinol.

[0027] Example 9: Different from Example 1, if the arsenic concentration in the soil is less than 200 mg / kg, the exchangeable arsenic in the soil is removed by eluting with clean water at a liquid-to-solid ratio of 3 mL:1 g for 1.5 h.

[0028] Example 10: Different from Example 1, if the arsenic concentration in the soil is less than 200 mg / kg, the exchangeable arsenic in the soil is removed by eluting with clean water at a liquid-to-solid ratio of 2 mL:1 g for 1 h.

[0029] Example 11: Different from Example 1, if the arsenic concentration in the soil is less than 200 mg / kg, the exchangeable arsenic in the soil is removed by eluting with clean water at a liquid-to-solid ratio of 4 mL:1 g for 2 h.

[0030] Example 12: Different from Example 9, the soil sample to be repaired is loosened after pretreatment, and then a hydration liquid is added to the soil to be repaired in a ratio of 100 g: 350 mL. Finally, the soil sample to be repaired is 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 liquid includes 7wt% of sodium silicate, 7wt% of sodium chloride and the remainder of water in terms of mass percentage; The loosening tool used for the loosening treatment was a commercially available tooth rake, the loosening depth was 25 cm, and the loosening time was 20 min.

[0031] Example 13: Different from Example 12, hydration liquid is added to the soil sample to be repaired in a ratio of 100g:200mL, and finally the soil sample to be repaired is placed in a stirrer and stirred at a speed of 50r / min for 10min to ensure that the soil to be repaired is fully hydrated.

[0032] Example 14: Different from Example 12, hydration liquid is added to the soil sample to be repaired in a ratio of 100g:500mL, and finally the soil sample to be repaired is placed in a stirrer and stirred at a speed of 100r / min for 30min to ensure that the soil to be repaired is fully hydrated.

[0033] Example 15: Different from Example 12, the hydration liquid includes 5wt% sodium silicate, 6wt% sodium chloride and the balance water in terms of mass percentage.

[0034] Example 16: Different from Example 12, the hydration liquid includes, by mass percentage, 10 wt % of sodium silicate, 8 wt % of sodium chloride and the balance of water.

[0035] Experimental Example: The description of this experimental example is based on the recorded scheme in Example 1, and is intended to illustrate the practical application effect of the present invention. Among them, the three shallow soil points are recorded as D1-40 (0-1.5 m), D1-61 (0-1.5 m), and D4-1 (0-1.5 m), and the three deep soil points are recorded as D1-40 (1.5-3.5 m), D1-41 (1.5-3.5 m), and D1-91 (1.5-3.5 m). The blank groups 1 to 12 using the first elution agent alone are used as controls. Specifically: Blank group 1: Different from Example 1, 0.5 mol / L natural organic acid was selected as the first eluting agent; wherein the natural organic acid was oxalic acid; in S4, the first eluting agent and the shallow soil sample to be repaired, and the first eluting agent and the deep soil sample to be repaired were directly mixed at a liquid-solid ratio of 3 mL: 1 g, respectively, and placed on a shaker, and eluted at 25°C and a shaking rate of 200 r / min for 2 h.

[0036] Blank group 2: Different from blank group 1, 0.2 mol / L natural organic acid was selected as the first elution agent.

[0037] Blank group 3: Different from blank group 1, 1 mol / L natural organic acid was selected as the first elution agent.

[0038] Blank group 4: Different from blank group 1, 0.2 mol / L EDTA was selected as the first elution agent.

[0039] Blank group 5: Different from blank group 1, 0.1 mol / L EDTA was selected as the first elution agent.

[0040] Blank group 6: Different from blank group 1, 0.5 mol / L EDTA was selected as the first elution agent.

[0041] Blank group 7: Different from blank group 1, 0.5 mol / L sodium hydroxide was used as the first elution agent.

[0042] Blank group 8: Different from blank group 1, 0.2 mol / L sodium hydroxide was used as the first elution agent.

[0043] Blank group 9: Different from blank group 1, 1 mol / L sodium hydroxide was used as the first elution agent.

[0044] Blank group 10: Different from blank group 1, the first elution reagent was pure water.

[0045] Blank group 11: Different from blank group 10, in S4, the first eluting agent and the shallow soil sample to be repaired, the first eluting agent and the deep soil sample to be repaired were mixed at a liquid-solid ratio of 2mL:1g, placed on a shaker, and eluted at 22℃ and an oscillation rate of 180r / min for 1h.

[0046] Blank group 12: Different from blank group 10, in S4, the first eluting agent and the shallow soil sample to be repaired, the first eluting agent and the deep soil sample to be repaired were mixed at a liquid-solid ratio of 5mL:1g, placed on a shaker, and eluted at 27℃ and a shaking rate of 220r / min for 4h.

[0047] The arsenic removal effects of blank group 1 to blank group 12 were recorded respectively, and the conclusions were as follows: According to the control experiments of blank group 11 and blank group 12, the arsenic removal rate in the treated soil was between 0.86% and 10.22% under the conditions of liquid-solid ratio of 2 and 5 (v:w) and elution time of 1 and 4 h, which could not meet the remediation target; for the elution experiments of blank group 1 to blank group 9, compared with the pure water elution of blank group 10, it was found that the removal rate of arsenic by the three eluents was effectively improved, and the elution efficiency from high to low was: oxalic acid > EDTA > NaOH. Under different experimental parameter conditions, the removal rate of arsenic by oxalic acid was between 1.6% and 89.1%, the removal rate of arsenic by EDTA was between 2.2% and 43.5%, and the removal rate of arsenic by NaOH was between 0.3% and 58.8%. The leaching 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 soils of different concentrations and depths. However, considering that EDTA is relatively expensive and difficult to degrade in the soil, it will remain in the soil for a long time and easily cause secondary pollution to the soil. In summary, oxalic acid leaching agent is preferred as the leaching agent for arsenic-contaminated soil in this plot. After comprehensive consideration, blank group 1 is the best remediation parameter.

[0048] In addition, from the analysis of different depth levels, under the leaching action of oxalic acid in blank group 1, the best removal rates of arsenic in three shallow soil points 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 removal rates of arsenic in three deep soil points 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 rate 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 at more than 50% in different soil points; in deep soil, due to the high proportion of clay content, it may affect the mass transfer effect between the leaching solution and the pollutants, making the removal effect of the leaching solvent have certain volatility. The average arsenic removal rate of the shallow soil to be treated in the blank group 1 reached 71.8%, and the average arsenic removal rate of the deep soil to be treated reached 67.6%. The average arsenic removal rate of the shallow soil in this test case is the average of the best arsenic removal rates of the three shallow soil points 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 the deep soil in this test case is the average of the best arsenic removal rates of the three deep soil points D1-40 (1.5-3.5 m), D1-41 (1.5-3.5 m), and D1-91 (1.5-3.5 m). 1. Explore the improvement of arsenic pollution remediation by adding the second elution agent Control group 1: The difference from Example 1 is that the sodium xanthate in the second elution agent is replaced by sodium dithiophosphate.

[0049] Conclusion: From the comparison of Examples 1 to 3, it can be seen that the content of each component of the second elution agent within the scope of this application is too high or too low, which has little effect on the removal of arsenic in the soil, among which Example 1 is the best; and from the comparison of Example 1 and Blank Group 1 to Blank Group 3, it can be seen that the addition of the second elution agent can effectively improve the arsenic removal rate in the soil, which is mainly because the second elution agent can significantly increase the desorption rate of arsenic, so that more arsenic is released from soil particles into the liquid phase; and sodium dodecyl sulfate can encapsulate arsenic in micelles through micelle action, thereby improving its solubility. The second elution agent can significantly increase the arsenic removal rate in the elution. The concentration in the washing liquid can be effectively removed; 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 capacity for the surface of arsenic minerals and is not easily interfered by sodium silicate and sodium oxide. Dithiophosphates may react with calcium hydroxide generated by calcium oxide in water to form insoluble calcium salt precipitates. At the same time, the state of dithiophosphates is also easily affected by calcium oxide, resulting in decomposition or failure, making it difficult to play its collection role. Based on this, Example 1 is a further optimization scheme.

[0050] 2. Explore the improvement performance of the graded remediation method on arsenic pollution remediation Control group 2: Different from Example 1, no flotation treatment was performed.

[0051] Control group 3: Different from Example 1, no CaO was added during the ultrasonic treatment.

[0052] Control group 4: Different from Example 1, CaO was added but no ultrasonic treatment was performed.

[0053] Conclusion: From the comparison of Example 1, Example 4 to Example 5 and Control Group 2, it can be seen that flotation treatment can further improve the removal effect of arsenic in soil, and oxalic acid, sodium hydroxide, EDTA and the component that promotes arsenic flotation (i.e., the second elution agent) work synergistically to significantly improve the remediation effect of arsenic in soil through mechanisms such as desorption, dissolution, oxidation and flotation; at this time, sodium dodecyl sulfate and sodium xanthate selectively adsorb on the surface of arsenic minerals under alkaline conditions to make them hydrophobic, and separate them from the soil through 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; and from the comparison of Example 1, Example 4, Example 5 and Control Group 3, it can be seen that the lack of calcium oxide addition in Control Group 3 will also cause the arsenic pollution removal effect to be weakened. This is because CaO enhances the effect of water reducer, reduces the water-cement ratio, increases the density of cement mortar, reduces pores, and enhances the curing effect. High-density mortar can effectively prevent arsenic penetration and migration; and CaO reacts with water to generate Ca(OH)2, which increases 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. 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. This is because the lack of ultrasonic treatment will significantly reduce the remediation effect of arsenic-contaminated soil, which is specifically manifested in the reduction of arsenic desorption efficiency, the reduction of the effectiveness of the elution agent, the extension of the remediation time, the weakening of the stabilization effect of CaO, the increase of the remediation cost and the high residual concentration of arsenic 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 the removal of soil arsenic. In summary, Example 1 is a relatively superior solution.

[0054] 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 Control group 5: Different from Example 12, no loosening treatment was performed.

[0055] Control group 6: Different from Example 12, the hydration solution does not contain sodium chloride.

[0056] 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; 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: 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 at 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 balance of water in terms of mass percentage.

7. A method for remediating arsenic-contaminated soil based on ex situ elution and graded remediation according to claim 6, characterized in that: The loosening tool used in the loosening treatment is a toothed rake, the loosening depth is 20-30 cm, and the loosening time is 15-30 min.

8. 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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