Biomimetic reduction repair method for stable hexavalent chromium in chromium-contaminated soil

Abstract

The invention discloses a biomimetic reduction remediation method for stable hexavalent chromium in chromium-contaminated soil, and belongs to the technical field of chromium-contaminated soil remediation. Aiming at the core problems that the form of hexavalent chromium is neglected and a stable-state hexavalent chromium reduction method is lacked in the existing chromium-polluted soil remediation technology, the method is based on analysis on the form of hexavalent chromium in soil and according to the structure of stable-state hexavalent chromium, hydrated iron oxide in soil is efficiently dissolved and destroyed by adopting an oxalic acid solution under a heating condition, and the stable-state hexavalent chromium in the soil is recovered. The stable-state hexavalent chromium adsorbed on the hexavalent chromium is forced to be released and then is reduced by an oxalic acid solution, and meanwhile, hexavalent chromium in other forms is also reduced together. According to the method, the biological process that plant root systems secrete oxalic acid to dissolve hydrated iron oxide to release stable-state phosphate radicals is simulated, and the content of hexavalent chromium in the reduced soil can be reduced to be lower than a standard limit value. The method is suitable for remediation of chromium-contaminated soil with different textures such as sandy soil, loam and clay, and the adopted oxalic acid compound is an environment-friendly medicament.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chromium-contaminated soil remediation, in particular to a biomimetic reduction remediation method for stable hexavalent chromium in chromium-contaminated soil. BACKGROUND

[0002] Sodium chromate and other chromium salts are the most important raw materials in the chromium industry such as electroplating, leather making, electrolysis, etc., and are irreplaceable chemical products in the national economy, but hexavalent chromium (CrO4 2- , abbreviated as Cr(VI)) has strong teratogenic, carcinogenic and mutagenic effects, and is one of the heavy metals that are focused on by countries around the world. The hexavalent chromium pollution in soil is caused by the leakage during the production and use of hexavalent chromium salt. China is a big country in the world of chromium industry, and the problem of soil chromium pollution caused by it is particularly serious, with the content of hexavalent chromium in soil reaching thousands of mg / kg, which seriously threatens the surrounding ecological environment and urgently needs soil remediation.

[0003] The main idea of chromium-contaminated soil remediation at home and abroad at present is to use reducing materials to reduce the toxic and migratory hexavalent chromium in the soil into low-toxicity and low-migration trivalent chromium precipitates. The reducing agents include ferrous sulfate (CN119662265A), ferrous sulfide (CN119100458A) and other iron-based reducing agents, calcium polysulfide, sodium sulfide and other sulfur-based reducing agents (CN119912948A), and chromium-reducing microorganisms (CN118926298A) and the like. Although these reducing agents can indeed completely reduce hexavalent chromium in aqueous solution, it is difficult to reduce the hexavalent chromium in actual chromium-contaminated soil to below the soil risk screening value (Cr(VI)≤5.7 mg / kg) required by the “Soil Environmental Quality Construction Site Soil Pollution Risk Control Standard GB 36600-2018”. This is because hexavalent chromium has undergone complex reactions with soil minerals, and the hexavalent chromium form evolution in soil is very complex, and not all in the form of initial water-soluble hexavalent chromium, part of the hexavalent chromium is combined with hydrated iron oxides and other minerals in the soil, and evolved into “stable hexavalent chromium” which is difficult to be reduced by conventional reducing agents.

[0004] However, it is generally believed that the overall negatively charged soil particles have weak adsorption force on the similarly negatively charged hexavalent chromium, and do not form complex hexavalent chromium forms, directly ignoring the complexity of hexavalent chromium forms and the differences in characteristics of different hexavalent chromium forms. On the other hand, the commonly used soil heavy metal form analysis methods Tessier sequential extraction method, BCR method and SEP sequential extraction method are only suitable for the form analysis of cation heavy metals such as Pb 2+ , Cu 2+ , etc., and are not suitable for the form analysis of anion hexavalent chromium (CrO4 2-Morphological analysis is not applicable and cannot accurately analyze the forms of hexavalent chromium in chromium-contaminated soil. Therefore, the key problem of the current reduction remediation technology for chromium-contaminated soil is that the complexity of the forms of hexavalent chromium in soil is not fully considered, the specific chemical form of "stable hexavalent chromium" in soil is not identified, and there is a lack of targeted reduction methods, which makes it difficult to reduce the content of hexavalent chromium in actual chromium-contaminated soil to below the standard limit value. SUMMARY

[0005] In view of the core problems in the current reduction remediation of chromium-contaminated soil, such as not fully considering the complexity of the forms of hexavalent chromium in soil, not identifying the specific chemical form and content of stable hexavalent chromium in soil, and lacking reduction methods for stable hexavalent chromium, which makes it difficult to reduce the content of hexavalent chromium in actual chromium-contaminated soil to below the standard limit value, based on the analysis of the distribution of the forms of hexavalent chromium in soil, and according to the structural characteristics of stable hexavalent chromium, the biological process of dissolving hydrated iron oxide to release stable phosphate by plant root exudation of oxalic acid is transferred to the dissolution-release-reduction of stable hexavalent chromium in soil, and a biomimetic reduction remediation method for stable hexavalent chromium in chromium-contaminated soil is provided, which can reduce the content of hexavalent chromium in soil to below the national standard limit value. The method comprises the following steps: Performing hexavalent chromium form analysis on the chromium-contaminated soil to be treated to determine the distribution characteristics of the forms of hexavalent chromium in the soil and the content of stable hexavalent chromium; When the content of water-soluble hexavalent chromium in the soil accounts for ≥50% of the total hexavalent chromium, water is added to the chromium-contaminated soil to be treated and stirred at room temperature for washing pretreatment to dissolve the water-soluble state in the soil, and then solid-liquid separation is performed to obtain the soil after pretreatment; Oxalic acid solution is added to the soil after pretreatment, and stirring and mixing reaction is performed under heating conditions. When the content of water-soluble hexavalent chromium in the soil accounts for <50% of the total hexavalent chromium, oxalic acid solution is directly added to the original soil, and stirring and mixing reaction is performed under heating conditions. After the reaction, solid-liquid separation is performed to obtain the reduced soil and the filtrate to be reused for treatment; Cation exchange resin particles are added to the filtrate to be reused for treatment, and Cr 3+ , Fe 3+ and other cations in the filtrate are removed by cation exchange resin adsorption, and then solid-liquid separation is performed. The filtrate after cation exchange resin treatment is supplemented with the corresponding oxalic acid agent to the required concentration for the reaction, and then is reused for reduction reaction.

[0006] In some embodiments of the bioremediation method for chromium-contaminated soil, the distribution characteristics of the hexavalent chromium forms in the soil are the contents of different hexavalent chromium forms in the soil, and the hexavalent chromium forms in the soil include water-soluble state, electrostatic adsorption state, specific adsorption state of calcium ion-containing minerals, specific adsorption state of hydrated metal oxides, calcium chromate precipitation state, and stable coordination state of hydrated iron oxides.

[0007] In some embodiments of the bioremediation method for chromium-contaminated soil, the stable hexavalent chromium in the soil is a form of hexavalent chromium that is difficult to be reduced, including specific adsorption state of hydrated metal oxides and stable coordination state of hydrated iron oxides, and the sum of the contents of the two is the content of the stable hexavalent chromium.

[0008] In some embodiments of the bioremediation method for chromium-contaminated soil, the solid-liquid ratio (g / mL) of the soil to water in the pretreatment reaction is 1:5 to 1:50, and the reaction time is 0.5 to 5 h.

[0009] In some embodiments of the bioremediation method for chromium-contaminated soil, the oxalic acid solution satisfies the following characteristics: (1) The oxalic acid solution is at least one of oxalic acid solution, ammonium oxalate solution, sodium oxalate solution, potassium oxalate solution, oxalic acid-ammonium oxalate buffer solution, oxalic acid-sodium oxalate buffer solution, and oxalic acid-potassium oxalate buffer solution; (2) The amount of oxalic acid is sufficient to dissolve and destroy the hydrated iron oxides in the soil, and the molar concentration of oxalic acid in the oxalic acid solution is 0.05 mol / L to saturated solution concentration.

[0010] In some embodiments of the bioremediation method for chromium-contaminated soil, the heating reaction temperature of the oxalic acid solution is 50 to 100 ℃, the solid-liquid ratio (g / mL) of the soil to the oxalic acid solution is 1:1 to 1:50, and the reaction time is 0.5 to 5 h.

[0011] In some embodiments of the bioremediation method for chromium-contaminated soil, the method simulates the mechanism of plant root excretion of oxalic acid to dissolve hydrated iron oxides and release stable phosphate, the oxalic acid molecules dissolve and destroy the structure of the hydrated iron oxides in the soil, forcing the stable hexavalent chromium adsorbed thereon to be released into the solution, and then being further reduced by the oxalic acid.

[0012] In some embodiments of the bioremediation method for chromium-contaminated soil, the oxalic acid solution not only reduces the stable hexavalent chromium in the soil, but also reduces other forms of hexavalent chromium, and the content of hexavalent chromium in the reduced soil is lower than the risk screening value of 5.7 mg / kg for construction land soil pollution in China.

[0013] In some embodiments of the bioremediation method for chromium-contaminated soil, the cation exchange resin is a sulfonic acid-based strong acid cation exchange resin, and the mass of the required resin is calculated according to the molar concentration of Cr 3+ , Fe 3+ in the filtrate, the volume of the filtrate, and the exchange capacity of the resin, and the reaction time is 0.5-5 h.

[0014] In some embodiments of the bioremediation method for chromium-contaminated soil, the reduction method is suitable for the reduction remediation of chromium-contaminated soil of different textures such as sandy soil, loamy soil, and clay.

[0015] Compared with the prior art, the bioremediation method for stable hexavalent chromium in chromium-contaminated soil has the following beneficial effects: The method identifies and determines the content of stable hexavalent chromium in chromium-contaminated soil that is difficult to be reduced by conventional reducing agents through a method for analyzing the form of hexavalent chromium in soil, and according to the structural characteristics of stable hexavalent chromium, the biological process of dissolving hydrated iron oxide to release stable phosphate by plant root exudates oxalic acid is transferred to the reduction of stable hexavalent chromium in soil. The hydrated iron oxide in the chromium-contaminated soil is efficiently dissolved and destroyed under heating conditions using oxalic acid solution, forcing the stable hexavalent chromium adsorbed on the hydrated iron oxide to be released and then reduced by the oxalic acid solution, while other forms of hexavalent chromium can also be reduced. Compared with the existing direct reduction technology that ignores the form characteristics of hexavalent chromium in soil and is difficult to reduce hexavalent chromium to the standard limit value, the method identifies the distribution characteristics of the form of hexavalent chromium in chromium-contaminated soil, and from the structural characteristics of stable hexavalent chromium, the stable hexavalent chromium is efficiently reduced by the bioremediation method of dissolution-release-reduction, which can reduce the hexavalent chromium in the soil to below the risk screening value of the second type of construction land soil in the Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard GB 36600-2018 (≤5.7 mg / kg). The existing reduction materials or methods only have effects on a certain type of chromium-contaminated soil, while the method can be applied to the remediation of chromium-contaminated soil of different textures such as sandy soil, loamy soil, and clay. In addition, most of the existing reduction materials are chemically synthesized materials, not natural materials, and may cause secondary pollution to the soil when added to the soil, while the oxalic acid compound used in the method is one of the plant root exudates and exists in the soil itself, which is harmless to the soil environmental quality and is an environmentally friendly agent. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art of the present application, the drawings required to be used in the specific embodiments or the prior art description will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0017] Figure 1 A flowchart of a biomimetic reduction remediation method for stable hexavalent chromium in chromium-contaminated soil in an embodiment. DETAILED DESCRIPTION

[0018] The technical solutions of the present application will be further described in detail below in combination with specific embodiments. The present application can be realized 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 the present application more thorough and comprehensive.

[0019] 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 the present application belongs. The terminology used in the specification of the present application is only for the purpose of describing specific embodiments and is not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0020] As described in the background, the current reduction remediation technology for chromium-contaminated soil does not take into account the complexity of the forms of hexavalent chromium in soil, and lacks a reduction method for "stable hexavalent chromium", resulting in difficulty in reducing the content of hexavalent chromium in soil to below the standard limit.

[0021] In previous studies, the inventors constructed a sequential extraction method specifically for the analysis of the forms of hexavalent chromium (anion CrO4 2- ) in soil, which realized the accurate analysis of the forms of hexavalent chromium in soil. According to the binding strength and sequential extraction order of soil particles, the forms of hexavalent chromium in soil can be divided into six types: water-soluble state, electrostatic adsorption state, specific adsorption state with calcium ion-containing minerals, specific adsorption state with hydrated metal oxides, calcium chromate precipitation state, and stable coordination state with hydrated iron oxides. Among them, water-soluble hexavalent chromium refers to the free CrO4 2- in soil, which can be leached out by water. Water-soluble hexavalent chromium in soil can be leached out by water. There are positively charged minerals in soil that can adsorb hexavalent chromium, such as montmorillonite containing exchangeable calcium and hydrated iron oxides. Electrostatic adsorption of hexavalent chromium is the hexavalent chromium adsorbed by the mutual attraction of positive and negative charges. Sodium chloride solution can be used to compete with the electrostatic adsorption of CrO4 - in soil. 2-Leaching. Because of the affinity of calcium ions to CrO4 2- , montmorillonite, allophane and other minerals rich in exchangeable calcium ions can specifically adsorb CrO4 2- by calcium ions. This part of hexavalent chromium is specifically adsorbed by minerals containing exchangeable calcium ions. Ammonium acetate solution can be used to leach the exchangeable calcium ions in the soil, and the hexavalent chromium specifically adsorbed by exchangeable calcium ions will also be leached out. The free calcium ions in the soil can form CaCrO4 precipitate with CrO4 2- , and this part of hexavalent chromium is in the form of calcium chromate precipitate. Calcium chromate can be dissolved in weak acid, and can be leached out by using acetic acid-sodium acetate acid buffer solution. Hydrated iron oxides (α-FeOOH, γ-FeOOH, β-FeOOH, etc.) in the soil can not only adsorb CrO4 2- by electrostatic adsorption, but also specifically adsorb CrO4 2- by -OH coordination exchange. Among them, the hexavalent chromium adsorbed by monodentate ligand is called specifically adsorbed by hydrated metal oxides, and the hexavalent chromium adsorbed by bidentate ligand complexation is called stable coordination state of hydrated iron oxides. Because the affinity of phosphate to hydrated iron oxides is stronger than that of chromate, K2HPO4-KH2PO4 buffer solution can be used to leach the hexavalent chromium adsorbed by monodentate ligand (specifically adsorbed by hydrated metal oxides). But because the hexavalent chromium adsorbed by bidentate ligand complexation (stable coordination state) is very stable, it can only be leached out by using sodium carbonate-sodium hydroxide mixed alkali solution.

[0022] Previous studies have also found that among the six forms of hexavalent chromium, the hydrated metal oxide specifically adsorbed by monodentate ligand complexation and the hydrated iron oxide stable coordination state of hexavalent chromium adsorbed by bidentate ligand complexation are structurally stable, especially the hydrated iron oxide stable coordination state of hexavalent chromium adsorbed by bidentate ligand complexation is the most stable. Using typical reducing agents such as ferrous sulfate, zero-valent iron, sodium sulfide, calcium polysulfide, sodium sulfite, and sodium hydrosulfite, even if the excess is several dozen times, it cannot effectively reduce it, which is the "stable state of hexavalent chromium" in the soil. Because the hexavalent chromium adsorbed by hydrated iron oxide is structurally stable, it is difficult to be effectively reduced by traditional direct reduction methods, but if the structure of hydrated iron oxide is destroyed, the hexavalent chromium complexed on the mineral surface will be released into the solution as free chromate, which can be easily reduced by reducing agents. In the natural environment, plants need phosphorus for growth, but the phosphorus (including H2PO4 - , HPO4 2- , PO4 3-Isomorphic, collectively referred to as phosphate in this application), a part is stable state phosphate combined with hydrated iron oxide, so that plant root system reduces and dissolves hydrated iron oxide by secreting organic acid such as oxalic acid, forces the complex adsorbed phosphate to be released as free state phosphate and be absorbed by plant root system. Hexavalent chromium (CrO4 2- ) has similar structure with phosphate, its adsorption behavior in soil is similar to phosphate, so the stable state hexavalent chromium structure complex adsorbed by hydrated iron oxide in chromium contaminated soil is also similar to stable state phosphate in soil, which can also be destroyed by oxalic acid and released.

[0023] Inspired by this, the application provides a new method of "biomimetic" reduction remediation of chromium contaminated soil, aiming at the structural characteristics of "stable state hexavalent chromium" in soil, i.e. hexavalent chromium complex adsorbed by hydrated iron oxide, using oxalic acid solution which is friendly to soil environment to dissolve and destroy hydrated iron oxide, forcing it to release adsorbed hexavalent chromium, then hexavalent chromium is reduced to trivalent chromium ion by oxalic acid molecules with reducing property, and other non-stable hexavalent chromium forms in soil are also reduced at the same time.

[0024] In the specific embodiment, as shown in Figure 1 , a biomimetic reduction remediation method of stable state hexavalent chromium in chromium contaminated soil is provided, comprising: Step S1, analyzing the hexavalent chromium form of the chromium contaminated soil to be treated to determine the hexavalent chromium form distribution characteristics in the soil and the content of stable state hexavalent chromium.

[0025] In some embodiments, in step S1, the method for analyzing the hexavalent chromium form in the soil can use the continuous extraction method established by the applicant, which can effectively distinguish and determine the content of water soluble state, electrostatic adsorption state, specific adsorption state of calcium ion exchangeable mineral containing state, hydrated metal oxide specific adsorption state, calcium chromate precipitation state and stable coordination state of hydrated iron oxide. It should be noted that the method for analyzing the hexavalent chromium form in the application is not limited, and other methods capable of distinguishing the hexavalent chromium form in the soil can also be used by those skilled in the art.

[0026] In some embodiments, in step S1, the hexavalent chromium form distribution characteristics in the soil are the contents of different hexavalent chromium forms in the soil. The stable state hexavalent chromium in the soil refers to the hexavalent chromium which is difficult to be reduced by typical hexavalent chromium reducing agents (such as ferrous sulfate, zero-valent iron, sodium sulfide, calcium polysulfide, sodium sulfite, sodium hydrosulfite, etc.), including hydrated metal oxide specific adsorption state and stable coordination state of hydrated iron oxide.

[0027] It can be understood that, due to the difference in pollution degree and soil composition, the distribution of hexavalent chromium in different chromium-contaminated soils is different, and the content of stable hexavalent chromium in the soil is also different, so it is necessary to determine the specific content of stable hexavalent chromium and other forms of hexavalent chromium in the chromium-contaminated soil to be treated through soil hexavalent chromium form analysis. Among them, the specific adsorption state of hydrated metal oxide hexavalent chromium, that is, the hexavalent chromium form complexed on the hydrated metal oxide mineral through a monodentate ligand, and the stable coordination state of hydrated iron oxide hexavalent chromium, that is, the hexavalent chromium form complexed on the hydrated iron oxide mineral through a bidentate ligand, are the most stable, collectively referred to as "stable hexavalent chromium", and the sum of the two is the content of stable hexavalent chromium.

[0028] In step S2, when the content of water-soluble hexavalent chromium in the soil accounts for ≥50% of the total hexavalent chromium, water is added to the chromium-contaminated soil to be treated and stirred at room temperature, and washing pretreatment is performed to dissolve the water-soluble hexavalent chromium in the soil, and then solid-liquid separation is performed to obtain the soil after pretreatment.

[0029] It can be understood that, in addition to stable hexavalent chromium, the hexavalent chromium form in the soil also includes water-soluble hexavalent chromium, electrostatic adsorption state, calcium ion-containing mineral specific adsorption state, and calcium chromate precipitation state. Among them, water-soluble hexavalent chromium can be easily dissolved in water and removed. When the content of water-soluble hexavalent chromium in the soil accounts for more than 50% of the total hexavalent chromium, that is, the soil mainly contains water-soluble hexavalent chromium, the water-soluble hexavalent chromium is removed in advance, which can reduce the consumption of oxalic acid in the subsequent process and reduce the cost of reagents.

[0030] In some embodiments, in step S2, the solid-liquid ratio (g / mL) of the pretreated soil and water is 1:5 to 1:50.

[0031] It can be understood that too large a solid-liquid ratio is not conducive to the stirring and mixing of the soil and water, and is not conducive to the dissolution of water-soluble hexavalent chromium. On the other hand, too small a solid-liquid ratio and too much water volume will increase the cost of water, and in addition, a larger stirring reaction tank is needed, increasing the pretreatment cost. Therefore, the solid-liquid ratio (g / mL) of the soil and water includes but is not limited to 1:5, 1:10, 1:20, 1:30, 1:40, 1:50. In some examples, it can be within the range formed by any two of these point values as end values, and the same applies below.

[0032] In some embodiments, in step S2, the soil washing pretreatment reaction time is 0.5 to 5 h.

[0033] It can be understood that the dissolution of water-soluble hexavalent chromium in the soil requires a certain time, but too long a reaction time will reduce the efficiency and increase the pretreatment cost. Therefore, the soil pretreatment reaction time includes but is not limited to 0.5, 1, 2, 3, 4, 5 h.

[0034] In some embodiments, the solid-liquid separation in step S2 can be performed by vacuum filtration, centrifugal filtration, plate and frame filter pressing, or the like.

[0035] In step S3, oxalic acid solution is added to the pretreated soil, and the mixture is stirred and reacted under heating. When the content of water-soluble hexavalent chromium in the soil is less than 50% of the total hexavalent chromium, the oxalic acid solution is directly added to the original soil, and the mixture is stirred and reacted under heating. After the reaction, solid-liquid separation is performed to obtain the reduced soil and the filtrate to be reused.

[0036] It can be understood that the heating reaction of the oxalic acid solution in step S3 simulates the mechanism of plant root secretion of oxalic acid to dissolve hydrated iron oxide and release stable phosphate, that is, the oxalic acid molecules dissolve and destroy the structure of the hydrated iron oxide in the soil, forcing the stable hexavalent chromium adsorbed thereon to be released into the solution, which is then further reduced by the oxalic acid.

[0037] In some embodiments, the oxalic acid solution in step S3 is at least one of oxalic acid solution, ammonium oxalate solution, sodium oxalate solution, potassium oxalate solution, oxalic acid-ammonium oxalate buffer solution, oxalic acid-sodium oxalate buffer solution, and oxalic acid-potassium oxalate buffer solution.

[0038] It can be understood that the hydrated iron oxide in the soil includes amorphous FOOH, a-FeOOH, β-FeOOH, γ-FeOOH, and the like, which have stable crystal structures and cannot be effectively dissolved by conventional acidic solutions such as hydrochloric acid. The oxalic acid molecules have a certain reducing property, which can reduce the trivalent iron in the hydrated iron oxide to divalent iron, making the iron-octahedral structure in the crystal lose stability, and then the carboxyl group of the oxalic acid molecule is complexed with H + The attack further dissolves and destroys the structure of the hydrated iron oxide, forcing the stable hexavalent chromium adsorbed on the mineral to be released into the solution, which is then reduced to trivalent chromium by the oxalic acid. The oxalic acid solution can be at least one of oxalic acid solution, ammonium oxalate solution, sodium oxalate solution, potassium oxalate solution, oxalic acid-ammonium oxalate buffer solution, oxalic acid-sodium oxalate buffer solution, and oxalic acid-potassium oxalate buffer solution.

[0039] It can be understood that when the content of water-soluble hexavalent chromium in the soil is less than 50% of the total hexavalent chromium, that is, there is still a considerable amount of other forms of hexavalent chromium such as electrostatic adsorption state and specific adsorption state with exchangeable calcium ion minerals in the soil, water cannot dissolve them, and water pretreatment cannot effectively reduce the amount of subsequent oxalic acid reducing agent, so water dissolution pretreatment is not performed.

[0040] In some embodiments, the amount of oxalic acid in step S3 is sufficient to dissolve and destroy the hydrated iron oxide in the soil, and the molar concentration of oxalic acid in the oxalic acid solution is 0.05 mol / L to saturated solution concentration.

[0041] It can be understood that a sufficient amount of oxalic acid is required to dissolve and destroy the hydrated iron oxide structure in the soil and reduce the released hexavalent chromium. According to the content of stable hexavalent chromium in the chromium-contaminated soil, the molar concentration of oxalic acid in the oxalic acid solution includes but is not limited to 0.05 mol / L to saturated solution concentration. The higher the content of stable hexavalent chromium in the soil, the higher the molar concentration of oxalic acid used. Similarly, the lower the content of stable hexavalent chromium in the soil, the lower the molar concentration of oxalic acid used. The saturation concentrations of different oxalic acid solutions are different. For example, the molar concentration of oxalic acid in the oxalic acid solution includes but is not limited to 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 mol / L, and saturated solution concentration.

[0042] In some embodiments, the heating temperature of the oxalic acid solution in step S3 is 50-100 ℃.

[0043] It can be understood that in addition to the concentration of oxalic acid, a certain temperature is required for the reduction reaction. Previous studies have shown that even saturated oxalic acid solution cannot completely reduce stable hexavalent chromium at room temperature. Therefore, heating reaction is required, and the reaction temperature includes but is not limited to 50, 60, 70, 80, 90, 100 ℃. When the temperature exceeds 100 ℃, the solution boils and consumes more heat, and the oxalic acid molecules will decompose, so the maximum temperature is not more than 100 ℃.

[0044] In some embodiments, the heating method of the oxalic acid solution in step S3 includes but is not limited to water bath heating, oil bath heating, electric heating, microwave heating, etc.

[0045] In some embodiments, the solid-liquid ratio (g / mL) of the soil and the oxalic acid solution in step S3 is 1:1-1:50.

[0046] It can be understood that too large solid-liquid ratio is not conducive to the stirring and mixing of the soil and the oxalic acid solution, and is not conducive to the dissolution of the hydrated iron oxide. On the other hand, too small solid-liquid ratio will increase the solution volume, and increase the cost of reagents and heating, etc. Therefore, the solid-liquid ratio of the soil and the oxalic acid solution includes but is not limited to 1:1, 1:5, 1:10, 1:20, 1:30, 1:40, 1:50.

[0047] In some embodiments, the heating reaction time of the oxalic acid solution in step S3 is 0.5-5 h.

[0048] It can be understood that a certain reaction time is required for the oxalic acid solution to dissolve the hydrated iron oxide in the soil and reduce the hexavalent chromium, but too long reaction time will reduce the efficiency and increase the treatment cost. Therefore, the reaction time includes but is not limited to 0.5, 1, 2, 3, 4, 5 h.

[0049] In some embodiments, the solid-liquid separation in step S3 includes, but is not limited to, vacuum filtration, centrifugal filtration, plate and frame filter pressing, etc.

[0050] It can be understood that, in addition to the stable state of the hard-reducible hexavalent chromium in the soil, there are also electrostatic adsorption state, specific adsorption state of calcium ion-containing minerals, and calcium chromate precipitation state, etc. The easy-reducible hexavalent chromium forms. The heating reaction of the oxalic acid solution in step S3 can not only reduce the stable state of the hexavalent chromium, but also reduce other forms of the hexavalent chromium. Therefore, only one step of the oxalic acid solution reduction is needed to effectively reduce the different forms of the hexavalent chromium in the soil. The content of the hexavalent chromium in the reduced soil is lower than the second type of construction land soil risk screening value concentration (Cr(VI) ≤ 5.7 mg / kg) in the national standard “Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard GB36600-2018 (Trial)”. If the amount of the oxalic acid solution is further increased, the content of the hexavalent chromium in the soil can be further reduced to the first type of construction land soil risk screening value concentration (Cr(VI) ≤ 3.0 mg / kg).

[0051] In step S4, cation exchange resin particles are added to the filtrate to be reused for treatment, so that the cation exchange resin adsorbs and removes Cr 3+ , Fe 3+ and other cations in the filtrate, and then solid-liquid separation is performed. The corresponding oxalic acid reagent is supplemented to the filtrate treated by the cation exchange resin to the required concentration for the reaction, and then the filtrate is reused for the reduction reaction.

[0052] It can be understood that, in order to reduce the treatment cost, the filtrate needs to be recycled. After the oxalic acid solution is used to heat and reduce the chromium-contaminated soil, a large amount of Cr 3+ , Fe 3+ and other cations and unreacted oxalic acid exist in the filtrate, and the filtrate is acidic. Therefore, the strong acid cation exchange resin with a sulfonic group (-SO3H) is used to adsorb and remove Cr 3+ , Fe 3+ and other cations in the filtrate, and then the corresponding oxalic acid reagent is supplemented to the treated filtrate to the required concentration for the reaction, and the filtrate is reused for the reduction reaction.

[0053] In step S3, the cation exchange resin is a strong acid cation exchange resin with a sulfonic group, and the calculation formula of the resin dosage is: m 树脂 =[(C Cr 3+ + C Fe 3+ )×V 滤液 ×3]×k / Q 树脂 m树脂 Resin dosage g, C Cr 3+ and C Fe 3+ Molar concentration of Cr 3+ , Fe 3+ in filtrate mmol / L, V 滤液 Filtrate volume L, 3 Charge number of chromium and iron ions, k Excess coefficient (1.5-2), Q 树脂 Cation exchange capacity of resin meq / g.

[0054] It can be understood that a sufficient amount of resin is required to sufficiently adsorb and remove cations in the filtrate. The mass of the required resin can be calculated according to the molar concentration of Cr 3+ , Fe 3+ in the filtrate, the volume of the filtrate, and the exchange capacity of the resin.

[0055] In some embodiments, the cation resin adsorption reaction time in step S3 is 0.5-5 h.

[0056] It can be understood that a certain reaction time is required for the cation exchange resin to adsorb cations in the filtrate, but too long a reaction time will reduce efficiency. Therefore, the reaction time includes but is not limited to 0.5, 1, 2, 3, 4, and 5 h.

[0057] It can be understood that the filtrate treated by the cation exchange resin is supplemented with the corresponding oxalic acid agent to the required concentration for the reaction, and then the filtrate can be reused for the reduction reaction.

[0058] In some embodiments, the solid-liquid separation method in step S3 includes but is not limited to vacuum filtration, centrifugal filtration, gravity sedimentation, etc.

[0059] The above-mentioned chromium-contaminated soil remediation method, according to the structural characteristics of the stable hexavalent chromium in the soil, i.e., the hexavalent chromium coordinated and adsorbed by the hydrated iron oxide in the soil, innovatively applies the biological process of dissolving the hydrated iron oxide by the plant root exudate oxalic acid to release the stable phosphate to the reduction of the stable hexavalent chromium in the soil. The hydrated iron oxide in the chromium-contaminated soil is efficiently dissolved and destroyed by the oxalic acid solution under heating conditions, forcing the stable hexavalent chromium adsorbed on the hydrated iron oxide to be released and then reduced by the oxalic acid solution.

[0060] The above-mentioned chromium-contaminated soil remediation method is mainly a solid-liquid reaction, which has high mass transfer efficiency, and is therefore suitable for the reduction remediation of chromium-contaminated soils of different textures such as sandy soil, loamy soil, and clay. The corresponding reaction conditions can be selected according to the content of the stable hexavalent chromium and other forms of hexavalent chromium in the soil.

[0061] The main component of the oxalic acid solution used in the above chromium contaminated soil remediation method is oxalic acid, which is one of the plant root exudates and harmless to the soil environment quality, and is an environmentally friendly agent without secondary pollution risk.

[0062] For the convenience of understanding, the present application will be described in detail below in combination with specific examples.

[0063] Example 1 (1) The test soil was a chromium salt production contaminated soil, in which the content of hexavalent chromium was about 190.1 mg / kg, the soil pH was 8.8, and the soil texture was clay.

[0064] (2) 2 g of the chromium contaminated soil was taken, and the content of different hexavalent chromium forms in the soil was analyzed and determined by the soil hexavalent chromium form analysis method, as shown in Table 1, in which the water-soluble content was 100.0 mg / kg, accounting for 52.6% of the total hexavalent chromium, and the content of stable hexavalent chromium was the sum of the specific adsorption state of hydrated metal oxide Cr(VI) and the stable coordination state Cr(VI) 21.9 mg / kg.

[0065] Table 1 Distribution of hexavalent chromium forms in chromium contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 100.0 52.6 2 Electrostatically adsorbed Cr(VI) 32.9 17.3 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 34.5 18.1 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 10.2 5.4 5 Calcium chromate precipitated Cr(VI) 0.7 0.4 6 Stable coordination Cr(VI) 11.7 6.2

[0066] (3) 3 g of the chromium contaminated soil was taken and placed in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium contaminated soil was >50%, water washing pretreatment was needed.

[0067] (4) 15 mL of deionized water was added to the conical flask containing the soil and stirred and mixed, and placed in a constant temperature oscillator at 25°C under 220 rpm, and shaken for 5 h, and the solid-liquid ratio of water washing pretreatment was 1:5.

[0068] (5) After water washing pretreatment, vacuum filtration was used for solid-liquid separation to obtain the water washing pretreated soil.

[0069] (6) 150 mL of 0.05 mol / L oxalic acid solution was prepared. According to the solid-liquid ratio of 1:50, the oxalic acid solution was added to the conical flask containing the water washing pretreated soil and stirred and mixed, and heated and stirred in an oil bath at 100°C, and the reduction time was 5 h.

[0070] (7) After reduction, vacuum filtration was used for solid-liquid separation to obtain the reduced and remediated soil and the filtrate to be reused.

[0071] (8) The Cr 3+ , Fe 3+0.07, 1.12 mmol / L, respectively. To the filtrate to be reused for the treatment, 415.9 mg of sulfonic (-SO3H) strong acid cation exchange resin (wherein k = 1.7, Q 树脂 =2.20 meq / g) was added and stirred for 0.5 h. The Cr 3+ , Fe 3+ and other cations in the filtrate were removed by adsorption of the cation exchange resin, and then solid-liquid separation was performed. The filtrate after treatment of the cation exchange resin was supplemented with oxalic acid to the same concentration as the initial concentration of 0.05 mol / L, and then could be reused for the reduction reaction.

[0072] After reduction, the content of hexavalent chromium in the soil was only 2.8 mg / kg, and the removal rate of hexavalent chromium was 98.5%.

[0073] Example 2 (1) The test soil was a chromium salt production contaminated soil, wherein the content of hexavalent chromium was about 190.1 mg / kg, the soil pH was 8.8, and the soil texture was clay.

[0074] (2) 2 g of chromium-contaminated soil was taken, and the content of different hexavalent chromium forms in the soil was analyzed and determined by the method for analyzing the hexavalent chromium form in the soil, as shown in Table 2, wherein the water-soluble content was 100.0 mg / kg, accounting for 52.6% of the total hexavalent chromium content, and the content of stable hexavalent chromium was the sum of the specific adsorption state of hydrated metal oxide Cr(VI) and the stable coordination state Cr(VI) 21.9 mg / kg.

[0075] Table 2 Distribution of hexavalent chromium forms in chromium-contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 100.0 52.6 2 Electrostatically adsorbed Cr(VI) 32.9 17.3 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 34.5 18.1 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 10.2 5.4 5 Calcium chromate precipitated Cr(VI) 0.7 0.4 6 Stable coordination Cr(VI) 11.7 6.2

[0076] (3) 3 g of chromium-contaminated soil was taken and placed in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium-contaminated soil was >50%, water washing pretreatment was required.

[0077] (4) 150 mL of deionized water was added to the conical flask containing the soil and stirred and mixed, and then placed in a constant temperature shaker at 25°C under 220 rpm, and shaken for 0.5 h. The solid-liquid ratio of water washing pretreatment was 1:50.

[0078] (5) After water washing pretreatment, vacuum filtration was used for solid-liquid separation to obtain the soil after water washing pretreatment.

[0079] (6) Configuration 1.0 mol / L (about 25 ℃ saturated concentration) oxalic acid solution 9 mL. According to the solid-liquid ratio of the reduction reaction is 1:3, oxalic acid solution is added to the conical flask containing the soil pretreated by water washing and stirred and mixed, heated and stirred in the oil bath at 50 ℃, and the reduction time is 0.5 h.

[0080] (7) After reduction, vacuum filtration is used for solid-liquid separation to obtain the soil after reduction repair and the filtrate to be reused.

[0081] (8) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused are 1.13 and 19.10 mmol / L, respectively. 422.0 mg of sulfonic acid (-SO3H) strong acid cation exchange resin (k=1.7, Q 树脂 =2.20 meq / g) is added to the filtrate to be reused and stirred for 2 h. The cations such as Cr 3+ and Fe 3+ in the filtrate are removed by adsorption of the cation exchange resin, and then solid-liquid separation is performed. The filtrate after cation exchange resin treatment is supplemented with oxalic acid to make the oxalic acid concentration the same as the initial concentration of 1.0 mol / L, and then can be reused for reduction reaction.

[0082] The content of hexavalent chromium in the soil after reduction is only 0.7 mg / kg, and the removal rate of hexavalent chromium is 99.6%.

[0083] Example 3 (1) The test soil is a chromium salt production contaminated soil, wherein the content of hexavalent chromium is about 190.1 mg / kg, the soil pH is 8.8, and the soil texture is clay.

[0084] (2) 2 g of chromium contaminated soil is taken, and the content of different hexavalent chromium forms in the soil is analyzed and determined by the method for analyzing the hexavalent chromium form in soil, as shown in Table 3, wherein the water-soluble content is 100.0 mg / kg, accounting for 52.6% of the total hexavalent chromium content, and the content of stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide Cr(VI) and the stable coordination state Cr(VI) 21.9 mg / kg.

[0085] Table 3 Distribution of hexavalent chromium forms in chromium contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 100.0 52.6 2 Electrostatically adsorbed Cr(VI) 32.9 17.3 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 34.5 18.1 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 10.2 5.4 5 Calcium chromate precipitated Cr(VI) 0.7 0.4 6 Stable coordination Cr(VI) 11.7 6.2 (3) 3 g of chromium contaminated soil is taken and placed in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium contaminated soil is >50%, water washing pretreatment is required.

[0086] (4) Take 30 mL of deionized water and add it to the conical flask containing the soil and mix by stirring, place it in a constant temperature shaker at 25 °C, 220 rpm, shake for 3 h, the solid-liquid ratio of water washing pretreatment is 1:10.

[0087] (5) After water washing pretreatment, solid-liquid separation is performed by vacuum filtration to obtain the water washing pretreated soil.

[0088] (6) Prepare 90 mL of 0.1 mol / L oxalic acid-ammonium oxalate buffer solution. According to the solid-liquid ratio of 1:30, add the oxalic acid-ammonium oxalate buffer solution to the conical flask containing the water washing pretreated soil and mix by stirring, heat and stir in an oil bath at 70 °C, the reduction time is 2 h.

[0089] (7) After reduction, solid-liquid separation is performed by vacuum filtration to obtain the reduction repaired soil and the filtrate to be reused.

[0090] (8) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused are 0.12 and 1.93 mmol / L, respectively. Add 427.2 mg of sulfonic acid (-SO3H) strong acid cation exchange resin (k=1.7, Q 树脂 =2.20 meq / g) to the filtrate to be reused and stir for 1 h. The Cr 3+ , Fe 3+ and other cations in the filtrate are adsorbed and removed by the cation exchange resin, then solid-liquid separation is performed, and the filtrate after cation exchange resin treatment is supplemented with the corresponding oxalic acid-ammonium oxalate to make the concentration of the oxalic acid-ammonium oxalate buffer solution the same as the initial concentration of 0.1 mol / L, which can then be reused for reduction.

[0091] The content of hexavalent chromium in the soil after reduction is only 1.3 mg / kg, and the removal rate of hexavalent chromium is 99.3%.

[0092] Example 4 (1) The test soil is a chromium salt production contaminated soil, the content of hexavalent chromium is about 1008.1 mg / kg, the soil pH is 8.7, and the soil texture is loam.

[0093] (2) Take 2 g of chromium contaminated soil, analyze and determine the content of different hexavalent chromium forms in the soil by the method for analyzing the forms of hexavalent chromium in soil, as shown in Table 4, wherein the water-soluble content is 914.3 mg / kg, accounting for 90.7% of the total hexavalent chromium content, and the content of stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide Cr(VI) and the stable coordination state Cr(VI) 25.3 mg / kg.

[0094] Table 4 Distribution of Cr(VI) in chromium-contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 914.3 90.7 2 Electrostatically adsorbed Cr(VI) 39.3 3.9 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 28.0 2.8 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 6.7 0.7 5 Calcium chromate precipitated Cr(VI) 1.3 0.1 6 Stable coordination Cr(VI) 18.6 1.8 (3) 3 g of chromium-contaminated soil was taken and placed in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium-contaminated soil was > 50%, water washing pretreatment was required.

[0095] (4) 90 mL of deionized water was added to the conical flask containing the soil and stirred and mixed, and placed in a constant temperature shaker at 25 °C under 220 rpm, and shaken for 1 h. The solid-liquid ratio of water washing pretreatment was 1:30.

[0096] (5) After water washing pretreatment, centrifugal filtration was used for solid-liquid separation to obtain the water washing pretreated soil.

[0097] (6) 3 mL of 1 mol / L (about 25 °C saturated concentration) oxalic acid solution was prepared. According to the solid-liquid ratio of 1:1 of the reduction reaction, the oxalic acid solution was added to the conical flask containing the water washing pretreated soil and stirred and mixed, and heated and stirred in an oil bath at 90 °C. The reduction time was 3 h.

[0098] (7) After reduction, vacuum filtration was used for solid-liquid separation to obtain the reduction repaired soil and the filtrate to be reused.

[0099] (8) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused were measured to be 7.82 and 105.41 mmol / L, respectively. 787.4 mg of sulfonic acid (-SO3H) strong acid cation exchange resin (k = 1.7, Q 树脂 = 2.20 meq / g) was added to the filtrate to be reused and stirred for 2 h. The Cr 3+ , Fe 3+ and other cations in the filtrate were adsorbed and removed by the cation exchange resin, and then solid-liquid separation was performed. The filtrate after cation exchange resin treatment was supplemented with oxalic acid to make the oxalic acid concentration the same as the initial concentration of 1.0 mol / L, and then it could be reused for reduction reaction.

[0100] The content of Cr(VI) in the soil after reduction was only 5.3 mg / kg, and the removal rate of Cr(VI) was 99.5%.

[0101] Example 5 (1) The test soil was a chromium salt production contaminated soil, which contained about 1008.1 mg / kg of Cr(VI), the soil pH was 8.7, and the soil texture was loam.

[0102] (2) Take 2 g of chromium-contaminated soil, and analyze the content of different hexavalent chromium forms in the soil by the method for analyzing the hexavalent chromium form in soil, as shown in Table 5. The water-soluble content is 914.3 mg / kg, accounting for 90.7% of the total hexavalent chromium content, and the content of stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide and the stable coordination state of Cr(VI), which is 25.3 mg / kg.

[0103] Table 5 Distribution of hexavalent chromium forms in chromium-contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 914.3 90.7 2 Electrostatically adsorbed Cr(VI) 39.3 3.9 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 28.0 2.8 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 6.7 0.7 5 Calcium chromate precipitated Cr(VI) 1.3 0.1 6 Stable coordination Cr(VI) 18.6 1.8 (3) Take 3 g of chromium-contaminated soil, and place it in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium-contaminated soil is > 50%, water washing pretreatment is required.

[0104] (4) Add 120 mL of deionized water to the conical flask containing the soil and stir to mix, and place it in a constant temperature shaker at 25 °C and 220 rpm. Shake for 2 h, and the solid-liquid ratio of water washing pretreatment is 1:40.

[0105] (5) After water washing pretreatment, perform solid-liquid separation by centrifugal filtration to obtain the water washing pretreated soil.

[0106] (6) Prepare 15 mL of 0.4 mol / L ammonium oxalate solution. According to the solid-liquid ratio of 1:5, add the ammonium oxalate solution to the conical flask containing the water washing pretreated soil and stir to mix, and heat and stir in an oil bath at 80 °C. The reduction time is 4 h.

[0107] (7) After reduction, perform solid-liquid separation by vacuum filtration to obtain the reduced and repaired soil and the filtrate to be reused.

[0108] (8) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused are 1.49 mmol / L and 19.80 mmol / L, respectively. Add 740.4 mg of sulfonic acid (-SO3H) strong acid cation exchange resin (k = 1.7 and Q 树脂 = 2.20 meq / g) to the filtrate to be reused and stir for 3 h. The Cr 3+ , Fe 3+ and other cations in the filtrate are adsorbed and removed by the cation exchange resin, and then solid-liquid separation is performed. Supplement the corresponding ammonium oxalate to the filtrate after cation exchange resin treatment so that the ammonium oxalate concentration is the same as the initial concentration of 0.4 mol / L, and then it can be reused for reduction reaction.

[0109] The content of hexavalent chromium in the soil after reduction is only 3.3 mg / kg, and the removal rate of hexavalent chromium is 99.7%.

[0110] Example 6 (1) The test soil was a chromium salt production contaminated soil, in which the content of hexavalent chromium was about 1008.1 mg / kg, the soil pH was 8.7, and the soil texture was loam.

[0111] (2) 2 g of the chromium contaminated soil was taken, and the content of different hexavalent chromium forms in the soil was analyzed and determined by the soil hexavalent chromium form analysis method, as shown in Table 6, in which the water-soluble content was 914.3 mg / kg, accounting for 90.7% of the total hexavalent chromium content, and the content of stable hexavalent chromium was the sum of the hydrous metal oxide specific adsorption state Cr(VI) and the stable coordination state Cr(VI) 25.3 mg / kg.

[0112] Table 6 Distribution of hexavalent chromium forms in chromium contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 914.3 90.7 2 Electrostatically adsorbed Cr(VI) 39.3 3.9 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 28.0 2.8 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 6.7 0.7 5 Calcium chromate precipitated Cr(VI) 1.3 0.1 6 Stable coordination Cr(VI) 18.6 1.8 (3) 3 g of the chromium contaminated soil was taken and placed in a 250 mL conical flask for water washing pretreatment experiment. Since the water-soluble Cr(VI) content in the chromium contaminated soil was >50%, water washing pretreatment was needed.

[0113] (4) 60 mL of deionized water was added to the conical flask containing the soil and stirred and mixed, and placed in a constant temperature oscillator at 25°C and 220 rpm, and shaken for 4 h. The solid-liquid ratio of water washing pretreatment was 1:20.

[0114] (5) After water washing pretreatment, vacuum filtration was used for solid-liquid separation to obtain the water washing pretreated soil.

[0115] (6) 30 mL of 0.1 mol / L potassium oxalate solution was prepared. According to the solid-liquid ratio of 1:10, the potassium oxalate solution was added to the conical flask containing the water washing pretreated soil and stirred and mixed, and heated and stirred in an oil bath at 60°C. The reduction time was 2 h.

[0116] (7) After reduction, centrifugal filtration was used for solid-liquid separation to obtain the reduction repaired soil and the filtrate to be reused.

[0117] (8) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused were measured to be 0.83 and 10.26 mmol / L, respectively. 711.1 mg of sulfonic acid (-SO3H) strong acid cation exchange resin (k=1.7, Q 树脂 =2.20 meq / g) was added to the filtrate to be reused and stirred for 1 h. The Cr 3+ and Fe 3+The isocations are adsorbed and removed by the cation exchange resin, and then solid-liquid separation is performed. Potassium oxalate is supplemented into the filtrate after the treatment of the cation exchange resin, so that the concentration of the potassium oxalate is the same as the initial concentration of 0.1 mol / L, and then the potassium oxalate can be reused for the reduction reaction.

[0118] After reduction, the content of hexavalent chromium in the soil is only 4.0 mg / kg, and the removal rate of hexavalent chromium is 99.6%.

[0119] Example 7 (1) The test soil is a chromium salt production contaminated soil, wherein the content of hexavalent chromium is about 1044.8 mg / kg, the pH of the soil is 8.9, and the soil texture is sandy soil.

[0120] (2) 2 g of the chromium contaminated soil is taken, and the content of different hexavalent chromium forms in the soil is analyzed and determined by the method for analyzing the hexavalent chromium form in the soil, as shown in Table 7, wherein the content of the water-soluble state is 227.1 mg / kg, accounting for 21.7% of the total amount of hexavalent chromium, and the content of the stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide and the stable coordination state of Cr(VI), which is 437.5 mg / kg.

[0121] Table 7 Distribution of hexavalent chromium forms in the chromium contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 227.1 21.7 2 Electrostatically adsorbed Cr(VI) 247.5 23.7 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 61.3 5.9 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 223.2 21.4 5 Calcium chromate precipitated Cr(VI) 71.5 6.8 6 Stable coordination Cr(VI) 214.3 20.5 (3) 3 g of the chromium contaminated soil is taken and placed in a 250 mL conical flask for reduction experiment. Since the content of the water-soluble Cr(VI) in the chromium contaminated soil is <50%, the oxalic acid solution is directly used for reduction.

[0122] (4) 150 mL of 0.1 mol / L sodium oxalate solution is prepared. According to the solid-liquid ratio of 1:50 of the reduction reaction, the sodium oxalate solution is added to the conical flask containing the soil and stirred and mixed, heated and stirred in an oil bath at 70 ℃, and the reduction time is 3 h.

[0123] (5) After reduction, vacuum filtration is used for solid-liquid separation, and the soil after reduction and repair and the filtrate to be reused are obtained.

[0124] (6) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused are measured to be 2.45 mmol / L and 12.43 mmol / L, respectively. 5.17 g of sulfonic acid group (-SO3H) strong acid cation exchange resin (wherein k=1.7 and Q 树脂 =2.20 meq / g) is added to the filtrate to be reused and stirred for 4 h. The molar concentrations of Cr 3+ and Fe 3+The isocations are adsorbed and removed by the cation exchange resin, and then solid-liquid separation is performed. The filtrate after treatment of the cation exchange resin is supplemented with corresponding sodium oxalate, so that the concentration of sodium oxalate is the same as the initial concentration of 0.1 mol / L, and then the sodium oxalate can be reused for the reduction reaction.

[0125] After reduction, the content of hexavalent chromium in the soil is only 4.9 mg / kg, and the removal rate of hexavalent chromium is 99.5%.

[0126] Example 8 (1) The test soil is a chromium salt production contaminated soil, wherein the content of hexavalent chromium is about 1044.8 mg / kg, the pH of the soil is 8.9, and the soil texture is sandy soil.

[0127] (2) 2 g of chromium-contaminated soil is taken, and the content of different hexavalent chromium forms in the soil is analyzed and determined by the method for analyzing the forms of hexavalent chromium in soil, as shown in Table 8. The water-soluble content is 227.1 mg / kg, accounting for 21.7% of the total amount of hexavalent chromium, and the content of stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide and the stable coordination state of Cr(VI), which is 437.5 mg / kg.

[0128] Table 8 Distribution of hexavalent chromium forms in chromium-contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 227.1 21.7 2 Electrostatically adsorbed Cr(VI) 247.5 23.7 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 61.3 5.9 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 223.2 21.4 5 Calcium chromate precipitated Cr(VI) 71.5 6.8 6 Stable coordination Cr(VI) 214.3 20.5 (3) 3 g of chromium-contaminated soil is taken and placed in a 100 mL conical flask for reduction experiment. Since the water-soluble Cr(VI) content in the chromium-contaminated soil is <50%, an oxalic acid solution is directly used for reduction.

[0129] (4) 30 mL of 0.5 mol / L oxalic acid solution is prepared. According to the solid-liquid ratio of 1:10 for the reduction reaction, the oxalic acid solution is added to the conical flask containing the soil and stirred and mixed, heated and stirred in an oil bath at 60 ℃, and the reduction time is 2 h.

[0130] (5) After reduction, vacuum filtration is used for solid-liquid separation to obtain the soil after reduction and repair and the filtrate to be reused.

[0131] (6) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused are measured to be 12.34 mmol / L and 61.51 mmol / L, respectively. 5.14 g of sulfonic acid group (-SO3H) strong acid cation exchange resin (k=1.7, Q 树脂 =2.20 meq / g) is added to the filtrate to be reused and stirred for 3 h. The molar concentrations of Cr 3+ and Fe 3+The isocations are adsorbed and removed by the cation exchange resin, and then solid-liquid separation is performed; oxalic acid is supplemented into the filtrate treated by the cation exchange resin, so that the concentration of the oxalic acid is the same as the initial concentration of 0.5 mol / L, and then the filtrate can be reused for the reduction reaction.

[0132] After reduction, the content of hexavalent chromium in the soil is only 3.3 mg / kg, and the removal rate of hexavalent chromium is 99.7%.

[0133] Example 9 (1) The test soil is a chromium salt production contaminated soil, wherein the content of hexavalent chromium is about 1044.8 mg / kg, the pH of the soil is 8.9, and the soil texture is sandy soil.

[0134] (2) 2 g of the chromium contaminated soil is taken, and the content of different hexavalent chromium forms in the soil is analyzed and determined by the soil hexavalent chromium form analysis method, as shown in Table 9, wherein the water-soluble content is 227.1 mg / kg, accounting for 21.7% of the total amount of hexavalent chromium, and the content of stable hexavalent chromium is the sum of the specific adsorption state of hydrated metal oxide and the stable coordination state of Cr(VI) of 437.5 mg / kg.

[0135] Table 9 Distribution of hexavalent chromium forms in chromium contaminated soil Step Cr forms in soil Content (mg / kg) Percentage (%) 1 Water soluble Cr(VI) 227.1 21.7 2 Electrostatically adsorbed Cr(VI) 247.5 23.7 3 Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals 61.3 5.9 4 Specifically adsorbed Cr(VI) with hydrated metal oxides 223.2 21.4 5 Calcium chromate precipitated Cr(VI) 71.5 6.8 6 Stable coordination Cr(VI) Step Cr forms in soil Content (mg / kg) Percentage (%) Water soluble Cr(VI) Electrostatically adsorbed Cr(VI) Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals Specifically adsorbed Cr(VI) with hydrated metal oxides Calcium chromate precipitated Cr(VI) Stable coordination Cr(VI) Step Cr forms in soil Content (mg / kg) Percentage (%) Water soluble Cr(VI) Electrostatically adsorbed Cr(VI) Specifically adsorbed Cr(VI) with calcium exchangeable ion containing minerals Specifically adsorbed Cr(VI) with hydrated metal oxides Calcium chromate precipitated Cr(VI) Stable coordination Cr(VI) 214.3 20.5 (3) 3 g of the chromium contaminated soil is taken and placed in a 250 mL conical flask for reduction experiment. Since the water-soluble Cr(VI) content in the chromium contaminated soil is <50%, the oxalic acid solution is directly used for reduction.

[0136] (4) 120 mL of 0.2 mol / L oxalic acid-sodium oxalate buffer solution is prepared. According to the solid-liquid ratio of 1:40 for the reduction reaction, the oxalic acid solution is added to the conical flask containing the soil and stirred and mixed, heated and stirred in an oil bath at 70 ℃, and the reduction time is 2 h.

[0137] (5) After reduction, vacuum filtration is used for solid-liquid separation to obtain the soil after reduction and repair and the filtrate to be reused for treatment.

[0138] (6) The molar concentrations of Cr 3+ and Fe 3+ in the filtrate to be reused for treatment are measured to be 3.13 mmol / L and 15.40 mmol / L, respectively. 5.16 g of sulfonic acid group (-SO3H) strong acid cation exchange resin (k=1.7, Q 树脂 =2.20 meq / g) is added to the filtrate to be reused for treatment and stirred for 5 h. The molar concentrations of Cr 3+ and Fe 3+The isocations are removed by adsorption of cation exchange resin, and then solid-liquid separation, the filtrate after treatment of the cation exchange resin is supplemented with corresponding oxalic acid-sodium oxalate, so that the concentration of the oxalic acid-sodium oxalate buffer is the same as the initial concentration of 0.2 mol / L, and then it can be reused for the reduction reaction.

[0139] After reduction, the content of hexavalent chromium in the soil is only 2.6 mg / kg, and the removal rate of hexavalent chromium is 99.8%.

[0140] The technical features of the above-described embodiments can be combined in any manner. To make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present disclosure.

[0141] The above-described embodiments only express several implementation manners of the present application, facilitate specific and detailed understanding of the technical solutions of the present application, but should not be understood as a limitation on the scope of patent protection. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of protection of the present application. It should be understood that the technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments on the basis of the technical solutions provided by the present application are within the scope of protection of the appended claims of the present application. Therefore, the scope of protection of the patent of the present application should be based on the contents of the appended claims, and the description and drawings can be used to explain the contents of the claims.

Claims

1. A biomimetic reduction remediation method for stable hexavalent chromium in chromium-contaminated soil, characterized in that, Includes the following steps: Hexavalent chromium speciation was analyzed in the chromium-contaminated soil to determine the distribution characteristics of hexavalent chromium speciation and the content of stable hexavalent chromium in the soil. When the content of water-soluble hexavalent chromium in the soil accounts for ≥50% of the total hexavalent chromium, water is added to the chromium-contaminated soil to be treated and stirred at room temperature for washing pretreatment to dissolve the water-soluble chromium in the soil. Then, solid-liquid separation is performed to obtain the pretreated soil. Oxalic acid solution was added to the pretreated soil, and the mixture was stirred and reacted under heating conditions. When the content of water-soluble hexavalent chromium in the soil was less than 50% of the total hexavalent chromium, oxalic acid solution was added directly to the original soil, and the mixture was stirred and reacted under heating conditions. After the reaction, solid-liquid separation was performed to obtain reduced soil and filtrate for reuse treatment. Add cation exchange resin granules to the filtrate to be reused, and remove Cr from the filtrate through adsorption by the cation exchange resin. 3+ Fe 3+ The cations are purified, and then solid-liquid separation is performed. The filtrate after cation exchange resin treatment is replenished with the appropriate oxalic acid reagent to the concentration required for the reaction, and then reused for the reduction reaction.

2. The reduction method according to claim 1, characterized in that, The distribution characteristics of hexavalent chromium in the soil refer to the content of different hexavalent chromium forms in the soil. The hexavalent chromium forms in the soil include water-soluble forms, electrostatic adsorption forms, mineral-specific adsorption forms containing exchangeable calcium ions, hydrated metal oxide-specific adsorption forms, calcium chromate precipitates, and hydrated iron oxide-stable coordination forms.

3. The reduction method according to claim 1, characterized in that, The stable hexavalent chromium in the soil is a form of hexavalent chromium that is difficult to reduce, including the specifically adsorbed form of hydrated metal oxides and the stable coordination form of hydrated iron oxides. The sum of the contents of the two forms is the content of stable hexavalent chromium.

4. The reduction method according to claim 1, characterized in that, In the pretreatment reaction, the solid-liquid ratio (g / mL) of soil to water is 1:5 to 1:50, and the reaction time is 0.5 to 5 h.

5. The reduction method according to claim 1, characterized in that, The oxalic acid solution satisfies the following characteristics: (1) The oxalic acid solution is at least one of the following: oxalic acid solution, ammonium oxalate solution, sodium oxalate solution, potassium oxalate solution, oxalate-ammonium oxalate buffer solution, oxalate-sodium oxalate buffer solution, and oxalate-potassium oxalate buffer solution; (2) The amount of oxalic acid used should be sufficient to dissolve and destroy hydrated iron oxides in the soil, and the molar concentration of oxalic acid in the oxalic acid solution is 0.05 mol / L ~ saturated solution concentration.

6. The reduction method according to claim 1, characterized in that, The oxalic acid solution is heated to a temperature of 50-100 ℃, the solid-liquid ratio (g / mL) of the soil to the oxalic acid solution is 1:1-1:50, and the reaction time is 0.5-5 h.

7. The reduction method according to claim 1, characterized in that, This method mimics the mechanism by which plant roots secrete oxalic acid to dissolve hydrated iron oxides and release stable phosphate. By dissolving and destroying the structure of hydrated iron oxides in the soil through oxalic acid molecules, the stable hexavalent chromium adsorbed on them is forced to be released into the solution and then further reduced by oxalic acid.

8. The reduction method according to claim 1, characterized in that, Oxalic acid solutions can not only reduce stable hexavalent chromium in soil, but also reduce other forms of hexavalent chromium. The content of hexavalent chromium in the soil after reduction is lower than the screening value of 5.7 mg / kg for soil pollution risk in my country's construction land.

9. The reduction method according to claim 1, characterized in that, The cation exchange resin is a sulfonic acid-based strong acid cation exchange resin, based on the Cr content in the filtrate. 3+ Fe 3+ The required resin mass is calculated based on the molar concentration, filtrate volume, and resin exchange capacity, with a reaction time of 0.5–5 h.

10. The reduction method according to claims 1-9, characterized in that, The reduction method is applicable to the reduction and remediation of chromium-contaminated soils of different textures, such as sandy soil, loam, and clay.

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