Method for the remediation of chromium-contaminated soil by leaching

Abstract

The application relates to a chromium-contaminated soil leaching remediation method, characterized in that a first leaching solution is added to the chromium-contaminated soil for leaching, solid-liquid separation is performed to obtain soil a after the first-step leaching and leaching liquid a to be reused; a second leaching solution is added to the soil a for leaching, solid-liquid separation is performed to obtain soil b after the second-step leaching and leaching liquid b to be reused; a third leaching solution is added to the soil b for leaching, solid-liquid separation is performed to obtain soil c after the third-step leaching and leaching liquid c to be reused; a reducing agent is added to the leaching solutions a, b and c to be reused, solid-liquid separation is performed after hexavalent chromium in the solutions is reduced into trivalent chromium precipitates, regenerated leaching solutions a, b and c are obtained, and the regenerated leaching solutions can be reused in the respective leaching steps after corresponding leaching reagents are supplemented. The step-by-step leaching method has good leaching effect, the leaching liquid is easy to reuse, nitrogen and phosphorus in the leaching liquid can increase soil fertility, and the leaching reagent is green.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to a leaching remediation method for chromium-contaminated soil. Background Technology

[0002] Hexavalent chromium (CrO4) 2- Chromium is highly toxic and classified as a Group 1 carcinogen. Chromium pollution in soil primarily originates from leaks and spills during the production and use of hexavalent chromium compounds, including in the chromium chemical industry (producing hexavalent chromium salts) and the leather and electroplating industries (using chromium salts).

[0003] The main approach to remediating chromium-contaminated soil both domestically and internationally is based on the reduction and stabilization principle of hexavalent chromium. This involves using chemical and biological methods to reduce the highly toxic hexavalent chromium in the soil into less toxic trivalent chromium precipitates. However, currently, it is difficult to reduce hexavalent chromium to the concentration limits required by the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control GB 36600-2018," and the residual trivalent chromium in the soil poses a long-term safety risk of re-oxidation. Therefore, leaching remediation methods aimed at removing hexavalent chromium from soil have received attention in recent years. Patent CN104610973A uses a mixed solution of sodium dodecylbenzenesulfonate, hydrogen peroxide, and water (pH=7-8) to leach chromium-contaminated soil. Hydrogen peroxide oxidizes the trivalent chromium in the soil to hexavalent chromium, which is then dissolved in water and leached. However, hydrogen peroxide is not a strong oxidant under neutral conditions and is easily decomposed, therefore it cannot completely oxidize the trivalent chromium. Furthermore, hexavalent chromium in soil exists in complex forms, not all of which are water-soluble. Aqueous solutions cannot completely dissolve hexavalent chromium in the soil, and the residue of the surfactant sodium dodecylbenzenesulfonate also poses a certain risk to soil quality. Patent CN102652956A uses water as a leaching agent, spraying water onto the surface of contaminated soil and then pumping the surface-penetrating leaching water from a groundwater extraction well for surface treatment. However, due to the complex forms of hexavalent chromium in soil, water is not very effective at leaching it. Patent CN103521513A uses hydrochloric acid solution to leach hexavalent chromium from soil. Similarly, because hexavalent chromium in soil exists in complex forms, not all of which are acid-soluble or water-soluble, its leaching effect is limited. Moreover, hexavalent chromium is not easily precipitated and removed in acidic leaching solutions, affecting the reuse of the leaching solution. In summary, current leaching methods for chromium-contaminated soil lack corresponding leaching solutions designed for the complex hexavalent chromium forms in the soil, resulting in poor leaching effects, limited applicability to different soil types, and difficulties in reusing leaching solutions. Summary of the Invention

[0004] Addressing the core issues in current chromium-contaminated soil remediation, this application provides a stepwise leaching method based on the distribution and chemical characteristics of different hexavalent chromium speciations in chromium-contaminated soil. This method effectively leaches hexavalent chromium from soil, and the leachate is easily reusable. The method includes the following steps:

[0005] A first leaching solution was added to the chromium-contaminated soil to be treated. After mixing and stirring, solid-liquid separation was performed to obtain soil a after the first leaching step and leaching solution a to be reused.

[0006] A second leaching solution was added to the soil a after the first leaching step. After mixing and stirring, solid-liquid separation was performed to obtain soil b after the second leaching step and leaching solution b to be reused.

[0007] A third leaching solution was added to the soil b after the second leaching step. After mixing and stirring, solid-liquid separation was performed to obtain the soil c after the third leaching step and the leaching solution c to be reused.

[0008] A reducing agent was added to the leaching solutions a, b, and c to be recycled, respectively. The pH of the solutions was adjusted to about 4-10 using an alkaline solution. After mixing and stirring, the hexavalent chromium in the leaching solutions a, b, and c to be recycled was reduced to trivalent chromium precipitate. After solid-liquid separation, the regenerated leaching solutions a, b, and c and the trivalent chromium precipitate were obtained.

[0009] The corresponding rinsing agents are added to the regenerated rinsing solutions a, b, and c respectively, so that the concentration of the rinsing agents in the regenerated rinsing solutions a, b, and c is the same as that in the first rinsing solution, the second rinsing solution, and the third rinsing solution. Then, they are reused in each rinsing step respectively.

[0010] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the first leaching solution is an ammonium salt solution, and the ammonium salt solution has the following characteristics:

[0011] (1) The ammonium salt is selected from at least one of ammonium acetate, ammonium chloride, ammonium nitrate, ammonium bisulfate, ammonium bicarbonate, ammonium sulfate, and ammonium carbonate;

[0012] (2) NH4 in the ammonium salt solution + The molar concentration of the ions is 0.05 mol / L to the concentration of the saturated solution.

[0013] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the second leaching solution is a phosphate solution, and the phosphate solution has the following characteristics:

[0014] (1) The phosphate is selected from at least one of potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate;

[0015] (2) The molar concentration of phosphorus in the phosphate solution is 0.05 mol / L to the concentration of saturated solution.

[0016] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the third leaching solution is an acidic solution, and the acidic solution meets the following characteristics:

[0017] (1) The acidic solution is selected from at least one of the following acidic buffer solutions: acetic acid-sodium acetate buffer solution, citrate-disodium hydrogen phosphate buffer solution, citrate-sodium citrate buffer solution, citrate-sodium hydroxide buffer solution, hydrochloric acid-glycine buffer solution, formic acid-sodium hydroxide buffer solution, and acidic non-buffer solutions: hydrochloric acid solution, phosphoric acid solution, acetic acid solution, etc.

[0018] (2) The pH of the acidic buffer solution is 2 to 4, and the total concentration of the buffer pair is 0.1 to 4 mol / L;

[0019] (3) The concentration of the acidic non-buffered solution is 0.1 to 4 mol / L.

[0020] In some embodiments, the reducing agent in the leaching remediation method for chromium-contaminated soil meets the following characteristics:

[0021] (1) The reducing agent is at least one of sodium sulfide, sodium sulfite, sodium nitrite, sodium thiosulfate, ferrous sulfate, and ferrous ammonium sulfate;

[0022] (2) The amount of reducing agent added is 1 to 3 times the theoretical amount of hexavalent chromium in the rinsing solution to be recycled for complete reduction;

[0023] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the alkaline solution is selected from at least one of sodium hydroxide aqueous solution and potassium hydroxide aqueous solution, with a concentration of 0.1–5 mol / L.

[0024] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the reaction time between the reducing agent and the leaching solution to be reused during the leaching solution reduction and regeneration process is 0.2 to 2 hours.

[0025] In some embodiments, in the leaching remediation method for chromium-contaminated soil, the solid-liquid ratio (g / mL) of the soil to the leaching solution is 1:1 to 1:100.

[0026] In some embodiments, the leaching remediation method for chromium-contaminated soil has a soil leaching reaction time of 0.2 to 6 hours.

[0027] Compared with existing technologies, the leaching remediation method for chromium-contaminated soil proposed in this application has the following advantages:

[0028] The aforementioned method for remediating chromium-contaminated soil by leaching targets hexavalent chromium in different forms in the soil through a step-by-step process. First, an ammonium salt solution is used to leach water-soluble and calcium-ion-adsorbed hexavalent chromium. Then, a phosphate solution is used to leach hydrated iron oxide-adsorbed hexavalent chromium. Finally, an acidic solution is used to leach precipitated hexavalent chromium, such as calcium chromate. This three-step targeted leaching process can reduce the hexavalent chromium content in the soil to below the standard value specified in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control GB 36600-2018". Step-by-step leaching ensures effective leaching, and each step uses a single-substance solution, making it easier to reuse. The leaching solutions used in this method consist of simple inorganic and organic substances, posing no harm to the soil environment. Furthermore, ammonium salts and phosphates are plant nutrients that can increase soil fertility, making this a green soil leaching agent. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of a process for leaching remediation of chromium-contaminated soil in one embodiment. Detailed Implementation

[0031] The technical solution of this application will be further described in detail below with reference to specific embodiments. This application can be implemented in many different forms and is not limited to the embodiments described herein. It should be understood that the purpose of providing these embodiments is to make the disclosure of this application more thorough and comprehensive.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0034] As described in the background section, current chromium-contaminated soil leaching technology suffers from a "blind" problem. When designing the leaching solution, it is assumed that all hexavalent chromium in the soil exists in water-soluble or acid-soluble forms, without considering the complexity of the occurrence forms of hexavalent chromium in the soil and the chemical characteristics of different hexavalent chromium forms. Therefore, the leaching effect of hexavalent chromium in the soil is poor, and the leaching solution is not easy to regenerate and reuse. The residue of some surfactants also has a harmful effect on the soil environmental quality.

[0035] The applicant's research found that hexavalent chromium in chromium-contaminated soil exists primarily in four forms: water-soluble, adsorbed on calcium-ion-containing minerals, adsorbed on hydrated metal oxides, and precipitated on calcium chromate. Water-soluble hexavalent chromium refers to free CrO4 in the soil. 2- Hexavalent chromium can be dissolved in water. Since soil colloidal particles are usually also negatively charged, they are more susceptible to CrO4+. 2- Due to its repulsive properties, a significant portion of hexavalent chromium in soil exists in a water-soluble form. Patents CN104610973A and CN102652956A utilize water as a leachate to leach hexavalent chromium from soil. However, minerals in soil can also adsorb hexavalent chromium, as calcium ions have a strong affinity for CrO4. 2- Because of their affinity, bentonite and diaspore, which are rich in exchangeable calcium ions, can adsorb CrO4 through calcium ions. 2- This portion of hexavalent chromium exists in the adsorbed state of calcium-containing minerals. Additionally, positively charged minerals also exist in the soil, such as hydrated iron oxides (α-FeOOH, γ-FeOOH, β-FeOOH, etc.) and hydrated aluminum oxides (gibbsite, etc.). These minerals can not only electrostatically adsorb CrO4... 2- It can also specifically adsorb CrO4 through coordination exchange of -OH groups. 2- This portion of hexavalent chromium exists in the adsorbed state of hydrated metal oxides. If the soil contains a large amount of free calcium ions, they can react with CrO4. 2- CaCrO4 precipitate is formed, and this portion of hexavalent chromium is in the form of calcium chromate precipitate. This demonstrates that the distribution of hexavalent chromium in soil is complex, with each form possessing different chemical properties; simply using water or acid solutions is insufficient to completely leach out hexavalent chromium.

[0036] Based on this, this application provides a new method for leaching remediation of chromium-contaminated soil. Targeting different forms of hexavalent chromium and their chemical characteristics in the soil, an environmentally friendly and easily reusable leaching solution is used for stepwise leaching to improve the leaching effect of hexavalent chromium in the soil, reduce leaching costs, and avoid the risk of secondary pollution to the soil environment.

[0037] In one embodiment, such as Figure 1 As shown, a method for remediating chromium-contaminated soil by leaching is provided, comprising:

[0038] Step S10: Add the first leaching solution to the chromium-contaminated soil to be treated, and mix the soil and leaching solution thoroughly by stirring to carry out the leaching reaction.

[0039] In some of these examples, the first elution solution is an ammonium salt solution selected from at least one of ammonium acetate, ammonium chloride, ammonium nitrate, ammonium bisulfate, ammonium bicarbonate, ammonium sulfate, and ammonium carbonate;

[0040] In some of these examples, NH4 in the ammonium salt solution + The molar concentration of the ions is 0.05 mol / L to the concentration of the saturated solution.

[0041] It is understandable that different ammonium salts have different saturated solution concentrations; for example, the concentration of NH4 in ammonium acetate elution solution is different. + The molar concentrations of ions include, but are not limited to, 0.05, 0.1, 0.2, 0.4, 0.8, 2, 4, 6, 8, 10, 12, 14, 16, and 19 mol / L. In other examples, the range can be any two of these point values ​​as endpoints, and the same applies below.

[0042] It is understandable that ammonium salt solutions are used to leach chromium-contaminated soil, and NH4... + Ions can convert exchangeable Ca in soil 2+ Mg 2+ Na + K + The chromium leaches out through cation exchange, and the hexavalent chromium adsorbed on the surface of minerals containing exchangeable calcium ions based on the affinity of calcium ions also dissolves out. In other words, ammonium salt solutions can leach out the adsorbed hexavalent chromium from minerals containing calcium ions in the soil.

[0043] Furthermore, it can be understood that in order to leach and dissolve the adsorbed hexavalent chromium from calcium-containing minerals in the soil, the exchangeable calcium ions need to be fully dissolved. Therefore, the ammonium salt solution needs to contain sufficient NH4+. + Ion concentration. In addition, ammonium salt solutions also contain water, so they can also leach out water-soluble hexavalent chromium from the soil.

[0044] In some embodiments, in step S10, the solid-liquid ratio (g / mL) of the soil to the leaching solution is 1:1 to 1:100.

[0045] It's understandable that a high solid-liquid ratio hinders the mixing of soil and leachate. Conversely, a low solid-liquid ratio results in a large volume of leachate, increasing its cost and requiring a larger mixing tank, further increasing leaching costs.

[0046] It can be further understood that the solid-liquid ratio of soil to leachate solution includes, but is not limited to, 1:1, 1:2, 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:80, and 1:100.

[0047] In some embodiments, the soil rinsing reaction time in step S10 is 0.2 to 6 hours.

[0048] It is understandable that the leaching of hexavalent chromium in the soil takes a certain amount of time, but an excessively long reaction time will reduce leaching efficiency and increase leaching costs.

[0049] It can be further understood that the soil leaching reaction time includes, but is not limited to, 0.2, 0.5, 1, 2, 3, 4, 5, and 6 hours.

[0050] Step S11: Solid-liquid separation is performed on the soil suspension after the first step leaching reaction to obtain soil a after the first step leaching and leaching solution a to be reused.

[0051] In some embodiments, the solid-liquid separation method in step S11 includes, but is not limited to, plate and frame filtration, vacuum filtration, centrifugal filtration, etc.

[0052] Furthermore, the soil suspension after the first step of leaching reaction is separated into solid and liquid components by filtration.

[0053] It is understandable that the rinsing solution a, which is to be reused after the first rinsing reaction, contains hexavalent chromium and needs to be removed before it can be reused in the rinsing operation.

[0054] Step S12: Add the second leaching solution to the soil a after the first leaching step, and mix the soil and leaching solution thoroughly by stirring to carry out the leaching reaction.

[0055] In some of these examples, the second rinsing solution is a phosphate solution, wherein the phosphate is selected from at least one of potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0056] In some of these examples, the molar concentration of phosphorus in the phosphate solution is 0.05 mol / L to the concentration of a saturated solution.

[0057] It is understood that different phosphates have different saturated solution concentrations. For example, the molar concentration of phosphorus in potassium phosphate leaching solutions includes, but is not limited to, 0.05, 0.1, 0.2, 0.4, 0.8, 1.5, and 2 mol / L.

[0058] It is understandable that leaching treatment of chromium-contaminated soil with phosphate solution involves the presence of PO4 in the phosphate solution. 3- HPO4 2- or H2PO4 - Ions can adsorb hexavalent chromium in hydrated metal oxides in soil, specifically CrO4 adsorbed by hydrated iron oxides. 2- Leaching and dissolution. Due to PO4 3- HPO4 2- and H2PO4 - The binding force between ions and hydrated metal oxides is stronger than that between CrO4 and hydrated metal oxides. 2- Therefore, phosphate leachate can leach out adsorbed hexavalent chromium in hydrated metal oxides from the soil through ion exchange.

[0059] Furthermore, it can be understood that in order to fully leach and dissolve the adsorbed hexavalent chromium in hydrated metal oxides in the soil, the phosphate solution needs to have a sufficient concentration of phosphate ions, but an excessively high concentration will increase the cost of the leachate.

[0060] In some embodiments, in step S12, the solid-liquid ratio (g / mL) of the soil to the leaching solution is 1:1 to 1:100.

[0061] It's understandable that a high solid-liquid ratio hinders the mixing of soil and leachate. Conversely, a low solid-liquid ratio results in a large volume of leachate, increasing its cost and requiring a larger mixing tank, further increasing leaching costs.

[0062] It can be further understood that the solid-liquid ratio of soil to leachate solution includes, but is not limited to, 1:1, 1:2, 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:80, and 1:100.

[0063] In some embodiments, the soil rinsing reaction time in step S12 is 0.2 to 6 hours.

[0064] It is understandable that the leaching of hexavalent chromium in soil takes a certain amount of time, but an excessively long reaction time will reduce leaching efficiency and increase leaching costs.

[0065] It can be further understood that the soil leaching reaction time includes, but is not limited to, 0.2, 0.5, 1, 2, 3, 4, 5, and 6 hours.

[0066] Step S13: Solid-liquid separation is performed on the soil suspension after the second step leaching reaction to obtain soil b after the second step leaching and leaching solution b to be reused.

[0067] In some embodiments, the solid-liquid separation method in step S13 includes, but is not limited to, plate and frame filtration, vacuum filtration, centrifugal filtration, etc.

[0068] Furthermore, the soil suspension after the first step of leaching reaction is separated into solid and liquid components by filtration.

[0069] It is understandable that the rinsing solution b to be reused after the second rinsing reaction contains hexavalent chromium, and the hexavalent chromium needs to be removed before it can be reused in the rinsing operation.

[0070] Step S14: Add the third leaching solution to the soil b after the second step of leaching, and mix the soil and leaching solution thoroughly by stirring to carry out the leaching reaction.

[0071] In some of these examples, the third rinsing solution is an acidic solution selected from at least one of acidic buffer solutions such as acetic acid-sodium acetate buffer solution, citrate-disodium hydrogen phosphate buffer solution, citrate-sodium citrate buffer solution, citrate-sodium hydroxide buffer solution, hydrochloric acid-glycine buffer solution, formic acid-sodium hydroxide buffer solution, and acidic non-buffered solutions such as hydrochloric acid solution, phosphoric acid solution, and acetic acid solution.

[0072] In some of these examples, the acidic buffer solution has a pH of 2–4 and a total buffer pair concentration of 0.1–4 mol / L; the acidic non-buffered solution concentration is 0.1–4 mol / L.

[0073] It is understood that the pH of the acidic buffer solution is including but not limited to 2, 2.5, 3, 3.5, and 4; the total concentration of the buffer pair is including but not limited to 0.1, 0.2, 0.4, 0.8, 1, 2, 3, and 4 mol / L; and the concentration of the acidic solution is including but not limited to 0.1, 0.2, 0.4, 0.8, 1, 2, 3, and 4 mol / L.

[0074] It is understandable that acidic solutions can dissolve precipitated hexavalent chromium in the soil, such as calcium chromate. Calcium chromate precipitates are insoluble in water but soluble in acidic solutions; therefore, acidic solutions can leach and dissolve the precipitated hexavalent chromium from the soil.

[0075] Furthermore, it can be understood that in order to fully leach and dissolve the precipitated hexavalent chromium in calcium chromate from the soil, the acidic solution needs to have sufficient acidity. However, excessive acidity will cause other minerals in the soil to dissolve, consume the acidic leachate, and increase the leaching cost.

[0076] In some embodiments, in step S14, the solid-liquid ratio (g / mL) of the soil to the leaching solution is 1:1 to 1:100.

[0077] It's understandable that a high solid-liquid ratio hinders the mixing of soil and leachate. Conversely, a low solid-liquid ratio results in a large volume of leachate, increasing its cost and requiring a larger mixing tank, further increasing leaching costs.

[0078] It can be further understood that the solid-liquid ratio of soil to leachate solution includes, but is not limited to, 1:1, 1:2, 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:80, and 1:100.

[0079] In some embodiments, the soil rinsing reaction time in step S14 is 0.2 to 6 hours.

[0080] It is understandable that the leaching of hexavalent chromium in soil takes a certain amount of time, but an excessively long reaction time will reduce leaching efficiency and increase leaching costs.

[0081] It can be further understood that the soil leaching reaction time includes, but is not limited to, 0.2, 0.5, 1, 2, 3, 4, 5, and 6 hours.

[0082] Step S15: Solid-liquid separation is performed on the soil suspension after the third step leaching reaction to obtain soil c after the third step leaching and leaching solution c to be reused.

[0083] In some embodiments, the solid-liquid separation method in step S15 includes, but is not limited to, plate and frame filtration, vacuum filtration, centrifugal filtration, etc.

[0084] Furthermore, the soil suspension after the first step of leaching reaction is separated into solid and liquid components by filtration.

[0085] It is understandable that the rinsing solution c to be reused after the third rinsing reaction contains hexavalent chromium, and the hexavalent chromium needs to be removed before it can be reused in the rinsing operation.

[0086] Understandably, after three targeted leaching steps, almost all hexavalent chromium in chromium-contaminated soil, including water-soluble hexavalent chromium, hexavalent chromium adsorbed by calcium ion-containing minerals, hexavalent chromium adsorbed by hydrated iron oxides, and hexavalent chromium precipitated by calcium chromate, can be leached out. Finally, the hexavalent chromium content in the leached soil, i.e., soil c after the third leaching step, can be reduced to below the standard value specified in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control GB 36600-2018" (for example, the risk screening value of hexavalent chromium for Class II construction land is ≤5.7 mg / kg).

[0087] In step S20, a reducing agent is added to the leachate a to be recycled, and the pH of the solution is adjusted to approximately 4-10 using an alkaline solution. Stirring is then used to ensure that the reducing agent and the hexavalent chromium in the leachate a come into full contact and undergo a redox reaction. After solid-liquid separation, regenerated leachate a and trivalent chromium precipitate are obtained.

[0088] In some of these examples, in step S20, the reducing agent is at least one of sodium sulfide, sodium sulfite, sodium nitrite, sodium thiosulfate, ferrous sulfate, and ferrous ammonium sulfate.

[0089] In some of these examples, in step S20, the alkaline solution is selected from at least one of an aqueous solution of sodium hydroxide and an aqueous solution of potassium hydroxide.

[0090] In some of these examples, in step S20, the pH of the eluent reduction and regeneration reaction includes, but is not limited to, 4, 5, 6, 7, 8, 9, and 10.

[0091] It is understandable that reducing agents can reduce hexavalent chromium in the leaching solution to trivalent chromium, which can further form trivalent chromium precipitates, such as Cr(OH)3 and Fe3Cr(OH), under pH conditions of 4–10. 12 .

[0092] Furthermore, it can be understood that the first rinsing solution is an ammonium salt solution, and some ammonium salt solutions have a neutral to alkaline pH, such as ammonium acetate, ammonium nitrate, ammonium bicarbonate, and ammonium carbonate. Some reducing agents, such as sodium sulfide, sodium sulfite, sodium nitrite, and sodium thiosulfate solutions, also have a neutral to alkaline pH. Therefore, when adding these reducing agents for rinsing solution regeneration, it is not necessary to adjust the pH.

[0093] In some of these examples, in step S20, the amount of reducing agent added is 1 to 3 times the theoretical amount of hexavalent chromium needed to completely reduce the leaching solution a to be recycled.

[0094] It is understood that the amount of reducing agent added is a multiple of, but not limited to, 1, 1.5, 2, 2.5, and 3 times the theoretical amount of hexavalent chromium needed to completely reduce the leachate a to be recycled. For example, reducing 1 mol CrO4 2- Theoretically, 1.5 mol Na2S is required, so the amount of reducing agent added is 1.5, 2.25, 3, 3.75 and 4.5 mol Na2S.

[0095] In some examples, in step S20, the reaction time between the reducing agent and the rinsing solution a to be recycled is 0.2 to 2 hours. It is understood that the reduction time includes, but is not limited to, 0.2, 0.4, 0.6, 0.8, 1, 1.5, and 2 hours.

[0096] In some of these examples, the solid-liquid separation methods in step S20 include, but are not limited to, plate and frame filtration, vacuum filtration, and centrifugal filtration.

[0097] Step S21: Add the corresponding ammonium salt to the regenerated rinsing solution a so that the concentration of ammonium salt in the regenerated rinsing solution a is the same as that in the first rinsing solution, and then reuse it for the first rinsing step.

[0098] It is understandable that in the first step of leaching, ammonium salt solution is used to dissolve exchangeable calcium ions in the soil, and hexavalent chromium adsorbed by minerals containing exchangeable calcium ions is also dissolved. Therefore, the leaching process is a cation exchange process, which consumes NH4 in the leaching solution. + Add the appropriate ammonium salt to the regenerated rinsing solution a so that the concentration of ammonium salt in the regenerated rinsing solution a is the same as that in the first rinsing solution, and it can then be reused for the first rinsing step.

[0099] Further understanding is that the concentration of ammonium salt in the regenerated elution solution a is first tested, and then supplementary ammonium salt is added based on the concentration of ammonium salt in the first elution solution. The solution contains NH4+. + The ion concentration can be tested using the national standard method Nessler's reagent spectrophotometry or salicylic acid spectrophotometry.

[0100] In step S22, a reducing agent is added to the leachate b to be recycled, and the pH of the solution is adjusted to approximately 4-10 using an alkaline solution. Stirring is used to ensure that the reducing agent and the hexavalent chromium in the leachate b come into full contact and undergo a redox reaction. After solid-liquid separation, regenerated leachate b and trivalent chromium precipitate are obtained.

[0101] In some of these examples, in step S22, the reducing agent is at least one of sodium sulfide, sodium sulfite, sodium nitrite, sodium thiosulfate, ferrous sulfate, and ferrous ammonium sulfate.

[0102] In some of these examples, in step S22, the alkaline solution is selected from at least one of an aqueous solution of sodium hydroxide and an aqueous solution of potassium hydroxide.

[0103] In some of these examples, in step S22, the pH of the eluent regeneration reaction includes, but is not limited to, 4, 5, 6, 7, 8, 9, and 10.

[0104] It is understandable that reducing agents can reduce hexavalent chromium in the leaching solution to trivalent chromium, which can further form trivalent chromium precipitates, such as Cr(OH)3 and Fe3Cr(OH), under pH conditions of 4–10. 12 .

[0105] Furthermore, it can be understood that the second eluent is a phosphate solution with a neutral to alkaline pH. Some reducing agents, such as solutions of sodium sulfide, sodium sulfite, sodium nitrite, and sodium thiosulfate, are also neutral to alkaline. Therefore, when adding these reducing agents for eluent regeneration, it is not necessary to adjust the pH.

[0106] In some of these examples, in step S22, the amount of reducing agent added is 1 to 3 times the theoretical amount of hexavalent chromium in the leaching solution b to be recycled.

[0107] It is understood that the amount of reducing agent added is, but not limited to, a multiple of 1, 1.5, 2, 2.5, and 3 times the theoretical amount of hexavalent chromium needed to completely reduce the leachate b to be recycled. For example, reducing 1 mol of CrO4 2- Theoretically, 1.5 mol Na2SO3 is required, so the dosage of reducing agent is 1.5, 2.25, 3, 3.75 and 4.5 mol Na2SO3.

[0108] In some examples, in step S22, the reaction time between the reducing agent and the rinsing solution b to be recycled is 0.2 to 2 hours. It is understood that the reduction time includes, but is not limited to, 0.2, 0.4, 0.6, 0.8, 1, 1.5, and 2 hours.

[0109] In some of these examples, the solid-liquid separation methods in step S22 include, but are not limited to, plate and frame filtration, vacuum filtration, and centrifugal filtration.

[0110] Step S23: Add the corresponding phosphate to the regenerated rinsing solution b so that the concentration of phosphate in the regenerated rinsing solution b is the same as that in the second rinsing solution, and then reuse it for the second rinsing step.

[0111] It is understandable that the second step of leaching, which uses phosphate solution to leach out hexavalent chromium adsorbed by hydrated metal oxides in the soil, is an anion exchange process that consumes phosphate ions, such as PO4, in the leaching solution. 3- HPO4 2- or H2PO4 - Add the appropriate amount of phosphate to the regenerated rinsing solution b so that the concentration of phosphate in the regenerated rinsing solution b is the same as that in the second rinsing solution, and then it can be reused for the second rinsing step.

[0112] Further understanding is that the phosphorus concentration in the regenerated rinsing solution b is tested first, and then supplemental phosphorus is added based on the phosphorus concentration in the second rinsing solution. The phosphorus concentration in the solution can be tested using the national standard method of ammonium molybdate spectrophotometry.

[0113] In step S24, a reducing agent is added to the leachate c to be recycled, and the pH of the solution is adjusted to approximately 4-10 using an alkaline solution. Stirring is then used to ensure that the reducing agent and the hexavalent chromium in the leachate c come into full contact and undergo a redox reaction. After solid-liquid separation, regenerated leachate c and trivalent chromium precipitate are obtained.

[0114] In some of these examples, in step S24, the reducing agent is at least one of sodium sulfide, sodium sulfite, sodium nitrite, sodium thiosulfate, ferrous sulfate, and ferrous ammonium sulfate.

[0115] In some of these examples, in step S24, the alkaline solution is selected from at least one of an aqueous solution of sodium hydroxide and an aqueous solution of potassium hydroxide.

[0116] In some of these examples, in step S24, the pH of the eluent regeneration reaction includes, but is not limited to, 4, 5, 6, 7, 8, 9, and 10.

[0117] It is understandable that reducing agents can reduce hexavalent chromium in the leaching solution to trivalent chromium, which can further form trivalent chromium precipitates, such as Cr(OH)3 and Fe3Cr(OH), under pH conditions of 4–10. 12 .

[0118] In some of these examples, in step S24, the amount of reducing agent added is 1 to 3 times the theoretical amount of hexavalent chromium in the leaching solution c to be recycled.

[0119] It is understood that the amount of reducing agent added is, but not limited to, a multiple of 1, 1.5, 2, 2.5, and 3 times the theoretical amount of hexavalent chromium needed to completely reduce the leachate c to be recycled. For example, reducing 1 mol CrO4 2- Theoretically, 3 mol FeSO4·7H2O is required, so the amount of reducing agent added is 3, 4.5, 6, 7.5, and 9 mol FeSO4·7H2O.

[0120] In some examples, in step S24, the reaction time between the reducing agent and the rinsing solution c to be recycled is 0.2 to 2 hours. It is understood that the reduction time includes, but is not limited to, 0.2, 0.4, 0.6, 0.8, 1, 1.5, and 2 hours.

[0121] In some of these examples, the solid-liquid separation methods in step S24 include, but are not limited to, plate and frame filtration, vacuum filtration, and centrifugal filtration.

[0122] Step S25: Add the appropriate acidic solution to the regenerated elution solution c to adjust the acidity of the regenerated elution solution c, i.e., H₂. + The concentration is the same as that of the third rinsing solution, and it is reused in the third rinsing step.

[0123] It is understandable that the third step of leaching, which uses an acidic solution to dissolve precipitated hexavalent chromium such as calcium chromate from the soil, will consume the H+ in the leaching solution. + Add the appropriate acid to the regenerated eluent c so that the H+ in the regenerated eluent c increases. + If the concentration is the same as the third rinsing solution, it can be reused for the third rinsing step.

[0124] Furthermore, it can be understood that the H+ in the regenerated elution solution c should be tested first. + The concentration, and then based on the H in the initial third elution solution. + The concentration was replenished by adding more H+. + The concentration of ions can be tested using the national standard method of acid-base titration or pH meter testing.

[0125] The above-mentioned leaching remediation method for chromium-contaminated soil employs a step-by-step, highly efficient leaching process targeting different forms of hexavalent chromium in the soil. The main forms of hexavalent chromium in chromium-contaminated soil include water-soluble hexavalent chromium, hexavalent chromium adsorbed with exchangeable calcium ions, hexavalent chromium adsorbed with hydrated metal oxides, and hexavalent chromium precipitated with calcium chromate. Based on the chemical characteristics of different hexavalent chromium forms, the above-mentioned leaching remediation method uses targeted leaching solutions for separate leaching and dissolution. First, an ammonium salt solution is used to leach water-soluble and hexavalent chromium adsorbed with exchangeable calcium ions. Then, a phosphate solution is used to leach hexavalent chromium adsorbed with hydrated iron oxides. Finally, an acidic solution is used to leach precipitated hexavalent chromium such as calcium chromate. This three-step targeted leaching process can reduce the hexavalent chromium content in the soil to below the standard limit, and the step-by-step leaching ensures the final leaching effect on the soil. For chromium-contaminated soils with a relatively simple distribution of hexavalent chromium forms, such as chromium-contaminated soils with calcium chromate precipitate hexavalent chromium content below the standard limit of 5.7 mg / kg, it is not necessary to leach and dissolve the calcium chromate precipitate hexavalent chromium to meet the standard. Therefore, the leaching process can be simplified to two steps, namely, using only ammonium salt solution and phosphate solution for two-step leaching, further reducing the leaching cost.

[0126] In the above-mentioned leaching remediation method for chromium-contaminated soil, each step of the leaching solution is a solution of a single substance, making it easier to regenerate and reuse. Hexavalent chromium in the leaching solution is removed using a simple reduction method, and then the appropriate leaching agent is added for regeneration and reuse.

[0127] The leaching solutions used in the above-mentioned chromium-contaminated soil leaching remediation methods are all simple inorganic and organic substances that are not harmful to the soil environment. Furthermore, ammonium salts and phosphates are plant nutrients that can increase soil fertility, making them green soil leaching agents.

[0128] To facilitate understanding, this application will be explained in detail below with specific examples.

[0129] Example 1

[0130] (1) The tested soil was chromium-contaminated soil from a chromium slag dump, with a hexavalent chromium content of approximately 930.1 ± 12 mg / kg, a soil pH of 8.9, and a soil texture of sandy soil;

[0131] (2) Take 5g of chromium-contaminated soil and place it in a 250mL conical flask for rinsing experiment;

[0132] Soil washing:

[0133] (3) Prepare 100 mL of 4 mol / L CH3COONH4 solution with a pH of approximately 7. Add the CH3COONH4 solution to a conical flask containing soil and stir to mix. Perform the first step of rinsing for 1 hour. The solid-liquid ratio of the rinsing reaction is 1:20.

[0134] (4) After the first step of rinsing, solid-liquid separation is carried out by vacuum filtration to obtain soil a after the first step of rinsing and rinsing solution a to be reused;

[0135] (5) Prepare 100 mL of 2 mol / L KH2PO4-K2HPO4 phosphate buffer solution with a pH of approximately 7. Add the phosphate solution to the conical flask containing soil a after the first step of leaching and stir to mix. Perform the second step of leaching for 3 hours. The solid-liquid ratio of the leaching reaction is 1:20.

[0136] (6) After the second step of rinsing, solid-liquid separation was carried out by vacuum filtration to obtain soil b after the second step of rinsing and rinsing solution b to be reused.

[0137] (7) Prepare 100 mL of 1 mol / L CH3COOH-CH3COONa buffer solution (pH=4), add this acidic buffer solution to the conical flask containing soil b after the second step of rinsing and stir to mix, and carry out the third step of rinsing. The rinsing time is 1.5 h and the solid-liquid ratio of the rinsing reaction is 1:20.

[0138] (8) After the third step of rinsing, vacuum filtration was used to separate solid and liquid to obtain soil c after the third step of rinsing and rinsing solution c to be reused.

[0139] Rinse solution regeneration:

[0140] (9) Add 4.12 mg Na2S to the leachate solution a to be reused and stir for 0.5 h. The amount of Na2S added is 1 times the theoretical amount. Hexavalent chromium in the leachate is reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated leachate a is obtained. The NH4+ content in the regenerated leachate a is tested. +The concentration was adjusted, and then ammonium acetate was added to the regenerated eluent a to make the ammonium acetate concentration the same as the initial concentration, which was 4 mol / L.

[0141] (10) Add 4.79 mg Na2S to the leaching solution b to be reused and stir for 0.5 h. The amount of Na2S added is 1.5 times the theoretical amount. Hexavalent chromium in the leaching solution is reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated leaching solution b is obtained. The phosphorus concentration in the regenerated leaching solution b is tested, and then K2HPO4 or KH2PO4 is added to the regenerated leaching solution b to make the phosphorus concentration the same as the initial concentration, which is 2 mol / L.

[0142] (11) Add 7.38 mg FeSO4·7H2O to the leachate solution c to be recycled and stir for 0.3 h. The amount of FeSO4·7H2O added is 1 times the theoretical amount. Adjust the pH of the solution to about 5 using sodium hydroxide solution. The hexavalent chromium in the leachate is reduced to Fe3Cr(OH). 12 The precipitate was vacuum filtered to obtain regenerated eluent c. The pH value of the regenerated eluent c was tested, and then CH3COOH was added to the regenerated eluent c to make the pH value of the eluent the same as the initial pH, which is 4.

[0143] The rinsing results are shown in the table below:

[0144]

[0145] Example 2

[0146] (1) The tested soil was chromium-contaminated soil from a chromium slag dump, with a hexavalent chromium content of approximately 930.1 ± 12 mg / kg, a soil pH of 8.9, and a soil texture of sandy soil;

[0147] (2) Take 5g of chromium-contaminated soil and place it in a 250mL conical flask for rinsing experiment;

[0148] Soil washing:

[0149] (3) Prepare 150 mL of 2 mol / L (NH4)2CO3 solution with a pH of approximately 9. Add the (NH4)2CO3 solution to a conical flask containing soil and stir to mix. Perform the first step of rinsing for 0.5 h. The solid-liquid ratio of the rinsing reaction is 1:30.

[0150] (4) After the first step of rinsing, solid-liquid separation is carried out by vacuum filtration to obtain soil a after the first step of rinsing and rinsing solution a to be reused;

[0151] (5) Prepare 150 mL of 4 mol / L K2HPO4 solution with a pH of approximately 9. Add the phosphate solution to the conical flask containing soil a after the first step of leaching and stir to mix. Perform the second step of leaching for 1.5 h. The solid-liquid ratio of the leaching reaction is 1:30.

[0152] (6) After the second step of rinsing, solid-liquid separation was carried out by vacuum filtration to obtain soil b after the second step of rinsing and rinsing solution b to be reused.

[0153] (7) Prepare 100 mL of 0.5 mol / L H3PO4 solution, add this acidic non-buffered solution to the conical flask containing soil b after the second step of leaching and stir to mix, carry out the third step of leaching, the leaching time is 1.5 h, and the solid-liquid ratio of the leaching reaction is 1:20.

[0154] (8) After the third step of rinsing, vacuum filtration was used to separate solid and liquid to obtain soil c after the third step of rinsing and rinsing solution c to be reused.

[0155] Rinse solution regeneration:

[0156] (9) 20.43 mg of Na2SO3 was added to the eluent solution a to be reused and the mixture was stirred for 1 h. The amount of Na2SO3 added was twice the theoretical amount. Hexavalent chromium in the eluent was reduced to Cr(OH)3 precipitate. After vacuum filtration, regenerated eluent solution a was obtained. The NH4+ content in the regenerated eluent solution a was tested. + The concentration was adjusted, and then (NH4)2CO3 was added to the regenerated eluent a to make the (NH4)2CO3 concentration the same as the initial concentration, which was 2 mol / L.

[0157] (10) 7.87 mg NaNO2 was added to the eluent b to be reused and the mixture was stirred for 2 h. The amount of NaNO2 added was 3 times the theoretical amount. Hexavalent chromium in the eluent was reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated eluent b was obtained. The phosphorus concentration in the regenerated eluent b was tested, and then K2HPO4 was added to the regenerated eluent b to make the phosphorus concentration the same as the initial concentration, which was 4 mol / L.

[0158] (11) Add 20.37 mg Fe(NH4)2·(SO4)2·6H2O to the leachate solution c to be recycled and stir for 0.5 h. The amount of Fe(NH4)2·(SO4)2·6H2O added is 1.5 times the theoretical amount. Adjust the pH of the solution to about 7 using sodium hydroxide solution. The hexavalent chromium in the leachate is reduced to Fe3Cr(OH). 12 The precipitate was vacuum filtered to obtain regenerated eluent c. The H₂ content of the regenerated eluent c was tested. +The concentration was adjusted, and then H3PO4 was added to the regenerated eluent c to make the concentration of H3PO4 in the eluent the same as the initial value, which was 0.5 mol / L.

[0159] The rinsing results are shown in the table below:

[0160]

[0161] Example 3

[0162] (1) The tested soil was a chromium-contaminated soil, with a hexavalent chromium content of approximately 415.2 ± 15 mg / kg, a soil pH of 9.5, and a soil texture of clay.

[0163] (2) Take 5g of chromium-contaminated soil and place it in a 250mL conical flask for rinsing experiment;

[0164] Soil washing:

[0165] (3) Prepare 50 mL of 5 mol / L CH3COONH4 solution with a pH of approximately 7. Add the CH3COONH4 solution to a conical flask containing soil and stir to mix. Perform the first step of rinsing for 3 hours. The solid-liquid ratio of the rinsing reaction is 1:10.

[0166] (4) After the first step of rinsing, solid-liquid separation is carried out by vacuum filtration to obtain soil a after the first step of rinsing and rinsing solution a to be reused;

[0167] (5) Prepare 25 mL of 2 mol / L NaH2PO4-K2HPO4 solution with a pH of approximately 7. Add the phosphate solution to the conical flask containing soil a after the first step of leaching and stir to mix. Perform the second step of leaching for 2 hours. The solid-liquid ratio of the leaching reaction is 1:5.

[0168] (6) After the second step of rinsing, solid-liquid separation was carried out by vacuum filtration to obtain soil b after the second step of rinsing and rinsing solution b to be reused.

[0169] Rinse solution regeneration:

[0170] (9) 29.08 mg of FeSO4·7H2O was added to the leachate solution a to be recycled and the mixture was stirred for 0.3 h. The amount of FeSO4·7H2O added was twice the theoretical amount. Hexavalent chromium in the leachate was reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated leachate a was obtained. The NH4+ content in the regenerated leachate a was tested. + The concentration was adjusted, and then CH3COONH4 was added to the regenerated eluent a to make the CH3COONH4 concentration the same as the initial concentration, which was 5 mol / L.

[0171] (10) Add 2.34 mg of sodium thiosulfate (Na2S2O3) to the leachate b to be recycled and stir for 2 h. The amount of Na2S2O3 added is twice the theoretical amount. Hexavalent chromium in the leachate is reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated leachate b is obtained. The phosphorus concentration in the regenerated leachate b is tested, and then K2HPO4 or NaH2PO4 is added to the regenerated leachate b to make the phosphorus concentration the same as the initial concentration, which is 2 mol / L.

[0172] The leaching results are shown in the table below. The soil contains relatively little precipitated hexavalent chromium such as calcium chromate, so a two-step leaching process is sufficient to meet the requirements.

[0173]

[0174] Example 4

[0175] (1) The tested soil was a chromium-contaminated soil, with a hexavalent chromium content of approximately 415.2 ± 15 mg / kg, a soil pH of 9.5, and a soil texture of clay.

[0176] (2) Take 5g of chromium-contaminated soil and place it in a 250mL conical flask for rinsing experiment;

[0177] Soil washing:

[0178] (3) Prepare 200 mL of 6 mol / L NH4Cl solution with a pH of approximately 5.6. Add the NH4Cl solution to a conical flask containing soil and stir to mix. Perform the first step of rinsing for 5 hours. The solid-liquid ratio of the rinsing reaction is 1:40.

[0179] (4) After the first step of rinsing, solid-liquid separation is carried out by vacuum filtration to obtain soil a after the first step of rinsing and rinsing solution a to be reused;

[0180] (5) Prepare 400 mL of 2 mol / L K3PO4 solution with a pH of approximately 11.5. Add the phosphate solution to the conical flask containing soil a after the first step of leaching and stir to mix. Perform the second step of leaching for 1 hour. The solid-liquid ratio of the leaching reaction is 1:80.

[0181] (6) After the second step of rinsing, solid-liquid separation was carried out by vacuum filtration to obtain soil b after the second step of rinsing and rinsing solution b to be reused.

[0182] Rinse solution regeneration:

[0183] (9) 12.83 mg of Na₂S was added to the leachate solution a to be recycled and the mixture was stirred for 2 hours. The amount of Na₂S added was three times the theoretical amount. Hexavalent chromium in the leachate was reduced to Cr(OH)₃ precipitate. After vacuum filtration, regenerated leachate a was obtained. The NH₄⁺ content in the regenerated leachate a was tested. + The concentration was adjusted, and then NH4Cl was added to the regenerated eluent a to make the NH4Cl concentration the same as the initial concentration, which was 6 mol / L.

[0184] (10) Add 2.25 mg Na2SO3 to the eluent b to be reused and stir for 1 h. The amount of Na2SO3 added is twice the theoretical amount. Hexavalent chromium in the eluent is reduced to Cr(OH)3 precipitate. After vacuum filtration, the regenerated eluent b is obtained. The phosphorus concentration in the regenerated eluent b is tested, and then K3PO4 is added to the regenerated eluent b to make the phosphorus concentration the same as the initial concentration, which is 2 mol / L.

[0185] The leaching results are shown in the table below. The soil contains relatively little precipitated hexavalent chromium such as calcium chromate, so a two-step leaching process is sufficient to meet the requirements.

[0186]

[0187] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0188] The embodiments described above are merely illustrative of several implementation methods of this application, intended to facilitate a detailed understanding of the technical solutions of this application, but should not be construed as limiting the scope of protection of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the scope of protection of this application. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided in this application through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this patent application should be determined by the content of the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A method for leaching remediation of chromium-contaminated soil, characterized in that, Includes the following steps: A first leaching solution is added to the chromium-contaminated soil to be treated. The first leaching solution is a solution of an ammonium salt, wherein the ammonium salt is selected from at least one of ammonium acetate, ammonium chloride, ammonium nitrate, ammonium bisulfate, ammonium bicarbonate, ammonium sulfate, and ammonium carbonate. After mixing and stirring, solid-liquid separation is performed to obtain soil a after the first leaching step and leaching solution a to be reused. A second leaching solution is added to the soil a after the first step of leaching. The second leaching solution is a phosphate solution, wherein the phosphate is selected from at least one of potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate. After mixing and stirring, solid-liquid separation is performed to obtain soil b after the second step of leaching and leaching solution b to be reused. A third leaching solution is added to the soil b after the second step of leaching. The third leaching solution is an acidic solution selected from at least one of the following: acetic acid-sodium acetate buffer solution, citrate-disodium hydrogen phosphate buffer solution, citrate-sodium citrate buffer solution, citrate-sodium hydroxide buffer solution, hydrochloric acid-glycine buffer solution, formic acid-sodium hydroxide buffer solution, and acidic non-buffered solutions of hydrochloric acid, phosphoric acid, and acetic acid. After mixing and stirring, solid-liquid separation is performed to obtain the soil c after the third step of leaching and the leaching solution c to be reused. A reducing agent was added to the leaching solutions a, b, and c to be recycled, respectively. The pH of the solutions was adjusted to 4-10 using an alkaline solution. After mixing and stirring, the hexavalent chromium in the leaching solutions a, b, and c to be recycled was reduced to trivalent chromium precipitate. After solid-liquid separation, the regenerated leaching solutions a, b, and c and the trivalent chromium precipitate were obtained. The corresponding rinsing agents are added to the regenerated rinsing solutions a, b, and c respectively, so that the concentration of the rinsing agents in the regenerated rinsing solutions a, b, and c is the same as that in the first rinsing solution, the second rinsing solution, and the third rinsing solution. Then, they are reused in each rinsing step respectively.

2. The leaching remediation method for chromium-contaminated soil according to claim 1, characterized in that, The first elution solution is an ammonium salt solution, and the ammonium salt solution has the following characteristics: (1) The ammonium salt is selected from at least one of ammonium acetate, ammonium chloride, ammonium nitrate, ammonium bisulfate, ammonium bicarbonate, ammonium sulfate, and ammonium carbonate; (2) NH4 in the ammonium salt solution + The molar concentration of the ions is 0.05 mol / L, which is the concentration of the saturated solution.

3. The leaching remediation method for chromium-contaminated soil according to claim 1, characterized in that, The second rinsing solution is a phosphate solution, and the phosphate solution has the following characteristics: (1) The phosphate is selected from at least one of potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate; (2) The molar concentration of phosphorus in the phosphate solution is 0.05 mol / L ~ the concentration of saturated solution.

4. The leaching remediation method for chromium-contaminated soil according to claim 1, characterized in that, The third rinsing solution is an acidic solution, and the acidic solution has the following characteristics: (1) The acidic solution is selected from at least one of the following: acetic acid-sodium acetate buffer solution, citrate-disodium hydrogen phosphate buffer solution, citrate-sodium citrate buffer solution, citrate-sodium hydroxide buffer solution, hydrochloric acid-glycine buffer solution, formic acid-sodium hydroxide buffer solution, acidic buffer solution, hydrochloric acid solution, phosphoric acid solution, acetic acid solution, and acidic non-buffered solution; (2) The pH of the acidic buffer solution is 2-4, and the total concentration of the buffer pair is 0.1-4 mol / L; (3) The concentration of the acidic non-buffered solution is 0.1~4 mol / L.

5. The leaching remediation method for chromium-contaminated soil according to claim 1, characterized in that, The reducing agent satisfies the following characteristics: (1) The reducing agent is at least one of sodium sulfide, sodium sulfite, sodium nitrite, sodium thiosulfate, ferrous sulfate, and ferrous ammonium sulfate; (2) The amount of reducing agent added is 1 to 3 times the theoretical amount of hexavalent chromium in the rinsing solution to be completely reduced and reused.

6. The leaching remediation method for chromium-contaminated soil according to claim 1, characterized in that, The alkaline solution is selected from at least one of sodium hydroxide aqueous solution and potassium hydroxide aqueous solution.

7. The leaching remediation method for chromium-contaminated soil according to any one of claims 1 to 6, characterized in that, The reaction time between the reducing agent and the leaching solution to be reused during the leaching solution reduction and regeneration process is 0.2~2 h.

8. The leaching remediation method for chromium-contaminated soil according to any one of claims 1 to 6, characterized in that, The solid-liquid ratio of soil to leaching solution is 1:1 to 1:100 g / mL.

9. The leaching remediation method for chromium-contaminated soil according to any one of claims 1 to 6, characterized in that, The soil leaching reaction time is 0.2~6 h.

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