Nano alpha-FeOOH-urease hybrid material, preparation method thereof, cementing fluid and remediation method of composite heavy metal polluted soil body

Abstract

The invention discloses a nano alpha-FeOOH-urease hybrid material, a preparation method of the nano alpha-FeOOH-urease hybrid material, a cementing solution and a remediation method of a composite heavy metal contaminated soil body, and relates to the technical field of environmental geotechnical engineering and contaminated soil body remediation, and the nano alpha-FeOOH-urease hybrid material is characterized in that urease is stably combined to the surface of nano alpha-FeOOH through chemical bonding; hydrogen ions are prevented from attacking the active center of the urease under the pH acidic condition, and the stability and desorption resistance of the urease are remarkably improved. The invention further provides a cementing liquid, a calcium source compound system and an acidic pH slow-release agent have a synergistic effect, a neutral soil environment is dynamically maintained, and heavy metal reactivation is inhibited. The nano alpha-FeOOH-urease hybrid material and a cementing liquid are mixed and then are used for repairing a composite heavy metal polluted soil body, and arsenic form transformation and multi-metal synchronous stabilization are realized through a multi-stage synergistic mechanism of free radical oxidation, carbonate coating and mineral complexing.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental geotechnical engineering and contaminated soil restoration, and in particular to a nano α-FeOOH-urease hybrid material and a preparation method thereof, a cementing fluid and a method for restoring composite heavy metal contaminated soil. Background Art

[0002] Enzyme-induced carbonate precipitation (EICP) technology has attracted much attention in the remediation of heavy metal contaminated soil due to its low energy consumption and no secondary pollution.

[0003] Currently, EICP technology mainly uses exogenous urease to catalyze the hydrolysis of urea to generate calcium carbonate precipitation, thereby achieving physical coating of heavy metal ions.

[0004] However, for typical As(III)-Cd-Pb composite contaminated soils in Henan Province, traditional EICP technology faces four limitations: (1) Free urease is easily inactivated in complex soil environments, and its activity decreases by 70% when the pH is less than 6; (2) As(III) is difficult to be directly fixed and easily dissolves again under alkaline conditions, with a dissolution rate of up to 30%; (3) Simple physical mixing of nanomaterials such as ferroferric oxide with urease leads to low interfacial reaction efficiency, with an As oxidation rate of less than 50%; (4) Competitive adsorption of multiple metals leads to a significant attenuation of the Cd / Pb fixation rate, and its leaching rate increases by 25% to 40% after 10 freeze-thaw cycles, which seriously limits the application of EICP technology in engineering. Summary of the Invention

[0005] The main purpose of the present invention is to propose a nano α-FeOOH-urease hybrid material and its preparation method, a cementing fluid and a composite heavy metal contaminated soil repair method, aiming to solve the problem in the prior art that free urease is easily inactivated in acidic soil.

[0006] To achieve the above object, the present invention proposes a nano α-FeOOH-urease hybrid material, which includes a nano α-FeOOH carrier and urease fixed on the surface of the nano α-FeOOH by chemical bonding.

[0007] In one embodiment, the mass ratio of Fe in the nano-α-FeOOH to the urease is 1:(1.5-2.5).

[0008] The present invention also provides a method for preparing a nano α-FeOOH-urease hybrid material, comprising the following steps:

[0009] S10, mixing α-FeOOH powder and a carboxyl activator in a phosphate buffer solution at pH 6.5 to 7.0, and reacting at 20 to 25° C. in the dark for 1 to 2 hours to obtain carboxyl-activated α-FeOOH;

[0010] S20, mixing the carboxyl-activated α-FeOOH with a urease solution, reacting at 0-10° C. for 3-5 hours, separating the solid and the liquid, and drying the solid to obtain the nano α-FeOOH-urease hybrid material.

[0011] In one embodiment, the particle size of the α-FeOOH powder is 10 to 30 nm; and / or,

[0012] The specific surface area of ​​the α-FeOOH powder is ≥150m 2 / g; and / or,

[0013] The carboxyl activator is 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and the molar ratio of the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide to the N-hydroxysuccinimide is 1:1 to 1:2; and / or,

[0014] The mass ratio of the carboxyl activator to the α-FeOOH powder is 0.1:1 to 0.3:1.

[0015] In one embodiment, in step S20:

[0016] The mass ratio of the carboxyl-activated α-FeOOH to the urease in the urease solution is 1:1.5 to 1:2.5.

[0017] In one embodiment, step S20 includes:

[0018] The carboxyl-activated α-FeOOH is mixed with a urease solution and reacted at 0-10°C for 3-5 hours. The reaction solution is centrifuged, and the solid after centrifugation is vacuum freeze-dried, wherein the magnetic field strength of the magnetic separation is 0.3-0.7T, the pre-freezing temperature of the vacuum freeze-drying is -40°C, and the main drying temperature of the vacuum freeze-drying is -20°C.

[0019] The present invention also provides a cementing fluid, comprising urea, a compound calcium source, and ascorbic acid, wherein:

[0020] The concentration of the urea is 0.8 to 1.2 M;

[0021] The concentration of the compound calcium source is 0.1-1.2 M, the compound calcium source is calcium chloride and calcium bicarbonate, and the molar ratio of the calcium chloride to the calcium bicarbonate is 3:1;

[0022] The mass percentage concentration of the ascorbic acid is 0.05% to 0.15%.

[0023] In one embodiment, the cementing fluid further comprises at least one of a dispersant, an acidic pH sustained-release agent, and an enzyme protectant:

[0024] The dispersant is sodium polyacrylate, and the mass percentage of sodium polyacrylate in the cementing solution is 0.05% to 0.1%;

[0025] The acidic pH sustained-release agent is polylactic acid-coated citric acid microspheres, the particle size of the polylactic acid-coated citric acid microspheres is 50 to 100 μm, and the mass percentage of the polylactic acid-coated citric acid microspheres in the binder is 0.2% to 0.5%;

[0026] The enzyme protecting agent includes trehalose, and the mass percentage of trehalose in the gelling liquid is 0.08% to 0.12%.

[0027] The present invention also provides a method for repairing soil contaminated with composite heavy metals, comprising the following steps:

[0028] S1, mixing the aforementioned nano α-FeOOH-urease hybrid material with the aforementioned cementing liquid to obtain a mixed solution; the concentration of the nano α-FeOOH-urease hybrid material in the mixed solution is 50-100 g / L;

[0029] S2. Use high-pressure rotary jet grouting to inject the mixed solution into the contaminated soil layer and maintain for 7 to 14 days. During this period, the pH value is monitored in real time and a slow-release acid agent is added to obtain the repaired soil. The pressure of the high-pressure rotary jet grouting method is 0.5 to 1.0 MPa, and the injection volume of the mixed solution is 20 to 40 L / m 2 .

[0030] In one embodiment, the mass ratio of the nano α-FeOOH-urease hybrid material to the composite heavy metals in the composite heavy metal contaminated soil is 2.5-5:0.1.

[0031] In the technical solution of the present invention, the nano-α-FeOOH-urease hybrid material stably binds urease to the surface of nano-α-FeOOH through chemical bonding, avoiding the attack of hydrogen ions on the active center of urease under acidic pH conditions, and significantly improving the stability and anti-desorption ability of urease. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0033] Figure 1 The X-ray fluorescence spectrometry results of the contaminated soil in Example 13 are as follows;

[0034] Figure 2 is the activity retention rate of the nano-α-FeOOH-urease hybrid material and free urease at pH 5.0, 7.0, and 9.0 in Example 1;

[0035] Figure 3 The nano-α-FeOOH-urease hybrid material in Example 1 was prepared in the presence of 50 mg / L As(III), 100 mg / L Cd 2 + urease activity decay rate in the solution;

[0036] Figure 4 is the hydrolysis rate of urea in the cementing fluid of different concentrations in Examples 3-7;

[0037] Figure 5 The hydrolysis rate of urea using different molar ratios of CaCl2 to Ca(HCO3)2 in Examples 3, 8, and 9;

[0038] Figure 6 is the free radical signal intensity when different ascorbic acid concentrations are used in Examples 3, 10, 11, and 12;

[0039] Figure 7 The diagram shows the dynamic process of As(III) oxidation at different reaction times in Example 3;

[0040] Figure 8 This is a graph showing the dynamic changes of arsenic forms within 168 hours after soil remediation using HPLC-ICP-MS analysis in Example 13;

[0041] Figure 9 TCLP leaching rate results of cadmium, arsenic and lead in Example 13 and Comparative Example 1;

[0042] Figure 10 This is a graph showing the cadmium and lead fixation rates of the contaminated soil after 10 freeze-thaw cycles in Example 13;

[0043] Figure 11 This is a graph showing the compressive strength test results of the remediated contaminated soil after 10 freeze-thaw cycles in Example 13;

[0044] Figure 12 This is a graph showing the results of studying the effects of nano-α-FeOOH-urease hybrid material and ascorbic acid as variables on the As(III) oxidation rate in Example 13.

[0045] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. Where the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased commercially. In addition, the meaning of "and / or" appearing in the full text includes three parallel schemes. Taking "A and / or B" as an example, it includes scheme A, or scheme B, or schemes that A and B meet at the same time. In addition, the technical solutions between the various embodiments can be combined with each other, but must be based on the ability of ordinary technicians in this field to achieve. When the combination of technical solutions is contradictory or cannot be achieved, it should be considered that the combination of such technical solutions does not exist and is not within the scope of protection required by the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0047] Enzyme-induced carbonate precipitation (EICP) technology has attracted much attention in the remediation of heavy metal contaminated soil due to its low energy consumption and no secondary pollution.

[0048] Currently, EICP technology mainly uses exogenous urease to catalyze the hydrolysis of urea to generate calcium carbonate precipitation, thereby achieving physical coating of heavy metal ions.

[0049] However, for typical As(III)-Cd-Pb composite contaminated soils in Henan Province, traditional EICP technology faces four limitations: (1) Free urease is easily inactivated in complex soil environments, and its activity decreases by 70% when the pH is less than 6; (2) As(III) is difficult to be directly fixed and easily dissolves again under alkaline conditions, with a dissolution rate of up to 30%; (3) Simple physical mixing of nanomaterials such as ferroferric oxide with urease leads to low interfacial reaction efficiency, with an As oxidation rate of less than 50%; (4) Competitive adsorption of multiple metals leads to a significant attenuation of the Cd / Pb fixation rate, and its leaching rate increases by 25% to 40% after 10 freeze-thaw cycles, which seriously limits the application of EICP technology in engineering.

[0050] The main improvement solutions for the above problems include optimizing the enzyme production capacity of the strain or introducing exogenous adsorption materials such as biochar. However, there are still problems such as microbial activity being inhibited by the environment, unclear synergistic mechanisms at the mineral-microorganism interface, and difficulty in coordinating pH regulation and repair effects.

[0051] In view of this, the present invention proposes a nano α-FeOOH-urease hybrid material, which includes a nano α-FeOOH carrier and urease fixed on the surface of the nano α-FeOOH by chemical bonding.

[0052] In the technical solution of the present invention, nano-α-FeOOH is bound to urease via an amide bond, allowing the urease to be fixed to the surface of the nano-α-FeOOH. This prevents hydrogen ions from attacking the urease active center under acidic pH conditions, resulting in a high retention rate of urease activity under acidic pH conditions. At the same time, the design of fixing the urease via an amide bond reduces the desorption rate of the urease. The nano-α-FeOOH-urease hybrid material of the present invention breaks through the application limitations of EICP technology in moderately acidic soils (pH 5.0-6.5), expanding its application to a wide range of areas such as southern red soil.

[0053] In some embodiments, the mass ratio of Fe in the nano-α-FeOOH to the urease is 1:(1.5-2.5). It is understood that the mass ratio of Fe in the nano-α-FeOOH to the urease can be 1:1.5, 1:2, or 1:2.5. The mass ratio within the above range can ensure that the nano-α-FeOOH is loaded with more urease, while maintaining better activity of the urease under acidic conditions.

[0054] The present invention also provides a method for preparing a nano α-FeOOH-urease hybrid material, comprising the following steps: S10, mixing α-FeOOH powder and a carboxyl activator in a phosphate buffer solution with a pH of 6.5 to 7.0, reacting at 20 to 25° C. in the dark for 1 to 2 hours to obtain carboxyl-activated α-FeOOH; S20, mixing the carboxyl-activated α-FeOOH with a urease solution, reacting at 0 to 10° C. for 3 to 5 hours, separating the solid from the liquid, and drying the solid to obtain the nano α-FeOOH-urease hybrid material.

[0055] In the technical solution of the present invention, α-FeOOH, also known as goethite, is an iron oxide-hydroxide with a large number of hydroxyl groups on its surface. Before performing step S10, a (3-cyanopropyl) trimethoxysilane (CPTMS) silane coupling agent is introduced to the surface of the α-FeOOH powder to facilitate the subsequent efficient grafting of carboxylic acid groups on the surface of the α-FeOOH. In step S10, the α-FeOOH whose surface is modified by the (3-cyanopropyl) trimethoxysilane silane coupling agent is first carboxyl activated to obtain an α-FeOOH with an active carboxyl group modified on the surface; in step S20, the α-FeOOH with an active carboxyl group modified on the surface is mixed with a urease solution, so that the amino group of the urease and the active carboxyl group generate an amide bond through an amidation reaction, and the urease is fixed to the surface of the α-FeOOH through a covalent bond, thereby obtaining a nano α-FeOOH-urease hybrid material.

[0056] In some embodiments, in step S10: the particle size of the α-FeOOH powder is 10 to 30 nm; and / or the specific surface area of ​​the α-FeOOH powder is ≥ 150 m 2 / g. It is understood that the particle size of the α-FeOOH powder can be 10nm, 20nm or 30nm, and the specific surface area of ​​the α-FeOOH powder can be 150m 2 / g, 160m 2 / g or 260m 2 / g, the particle size and specific surface area of ​​α-FeOOH powder in the above range can provide more hydroxyl active sites, which is convenient for urease loading. At the same time, the formed nano α-FeOOH-urease hybrid material has more surface ≡Fe-OH groups, which can form inner sphere complexes with heavy metals in composite heavy metal contaminated soil, thereby improving the efficiency and stability of heavy metal fixation.

[0057] In some embodiments, the carboxyl activator is 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide, i.e., EDC and NHS, and the molar ratio of the 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and the N-hydroxysuccinimide is 1:1 to 1:2; and / or the mass ratio of the carboxyl activator to the α-FeOOH powder is 0.1:1 to 0.3:1. Both 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide carboxyl activators can activate carboxyl groups into active intermediates, thereby promoting the coupling reaction between carboxylic acids and hydroxyl groups. The mass ratio of the carboxyl activator to the α-FeOOH powder within the above range can ensure efficient coupling between the hydroxyl groups on the surface of the α-FeOOH and the carboxyl activated intermediates, so that the surface of the α-FeOOH is modified with more active carboxyl groups.

[0058] In some embodiments, in step S20, the mass ratio of the carboxyl-activated α-FeOOH to the urease in the urease solution is 1:1.5 to 1:2.5. The mass ratio of the carboxyl-activated α-FeOOH to the urease in the urease solution within the above range ensures that the active carboxyl groups modified on the surface of the α-FeOOH undergo sufficient amidation reaction with the amino groups of the urease, allowing the urease to bind to the surface of the α-FeOOH more quickly, thereby improving the retention rate of the urease activity in acidic soil.

[0059] In some embodiments, step S20 includes: mixing carboxyl-activated α-FeOOH with a urease solution, reacting at 0-10° C. for 3-5 hours, centrifuging the reacted solution, and vacuum freeze-drying the solid after centrifugation, wherein the pre-freezing temperature of the vacuum freeze-drying is -40° C. and the main drying temperature of the vacuum freeze-drying is -20° C. It can be understood that the urease-loaded α-FeOOH is precipitated from the solution by centrifugation, so that the nano-α-FeOOH-urease hybrid material has fewer impurities and higher purity.

[0060] The present invention also provides a cementing fluid, which includes urea, a compound calcium source, and ascorbic acid. In the cementing fluid: the concentration of the urea is 0.8 to 1.2 M; the concentration of the compound calcium source is 0.1 to 1.2 M, the compound calcium source is calcium chloride and calcium bicarbonate, and the molar ratio of the calcium chloride to the calcium bicarbonate is 3:1; the mass percentage concentration of the ascorbic acid is 0.05% to 0.15%.

[0061] In the technical solution of the present invention, urea in the cementing fluid is used to provide a substrate for urease enzymolysis. Urea generates carbon dioxide under the action of urease, and carbon dioxide dissolves in water to form carbonate. A composite calcium source is used to provide calcium ions and combines with carbonate in the solution to form calcium carbonate to fix heavy metals. Ascorbic acid, as an oxidation enhancer, can synergize with α-FeOOH in the nano α-FeOOH-urease hybrid material to generate OH radicals, thereby achieving rapid conversion of the valence state of arsenic (As), that is, efficient conversion of As(III) to As(V), thereby increasing the oxidation rate of trivalent arsenic. It is understandable that the toxicity of As(III) is 60 times that of As(V), and it is difficult to fix by physical adsorption or precipitation. The high oxidation rate directly reduces the biological toxicity of trivalent arsenic and improves the safety of repair.

[0062] In some embodiments, the molar concentration of the urea is 0.8 to 1.2 M; and / or the molar concentration of the compounded calcium source is 0.1 to 1.2 M; and / or the mass percentage of the ascorbic acid is 0.05% to 0.15%. It is understood that when the concentrations of urea and the calcium source in the binder are within the above ranges, the calcium carbonate precipitate is generated faster and the heavy metals can be quickly fixed in the calcium carbonate precipitate. The mass percentage of ascorbic acid within the above range can ensure a high oxidation efficiency of trivalent arsenic. Preferably, the molar concentration of urea in the binder is 1 M, the molar concentration of the calcium source is 1 M, and the mass percentage of ascorbic acid is 0.1%.

[0063] In some embodiments, the cementing fluid further comprises at least one of a dispersant, an acidic pH sustained-release agent, and an enzyme protectant: the dispersant is sodium polyacrylate, and the mass percentage of sodium polyacrylate in the cementing fluid is 0.05% to 0.1%; the acidic pH sustained-release agent is polylactic acid-coated citric acid microspheres, the particle size of the polylactic acid-coated citric acid microspheres is 50 to 100 μm, and the mass percentage of the polylactic acid-coated citric acid microspheres in the cementing fluid is 0.2% to 0.5%; the enzyme protectant comprises trehalose, and the mass percentage of trehalose in the cementing fluid is 0.08% to 0.12%. It is understood that the cementing fluid further comprising any one, any two, or any three of the dispersant, the acidic pH sustained-release agent, and the enzyme protectant is within the scope of protection of the present invention. Among them, the role of the dispersant is to prevent the nano-α-FeOOH-urease hybrid materials from agglomerating with each other; the role of the acidic pH slow-release agent is to regulate the soil environment, so that the soil pH is maintained between 5 and 7, to ensure a high urease activity retention rate and accelerate the hydrolysis rate of urea; the role of the enzyme protectant is to stabilize the urease molecules and prevent them from inactivation or degradation.

[0064] In some embodiments, the composite calcium source is calcium chloride and calcium bicarbonate, and the molar ratio of calcium chloride to calcium bicarbonate is 3:1. At this molar ratio of 3:1, the hydrolysis rate of urea is high, which ensures that the calcium carbonate precipitate is formed quickly while avoiding the loss of control of the calcium carbonate crystal form due to excessively rapid reaction, resulting in the formation of loose amorphous precipitates rather than dense calcite, thereby achieving an optimal balance between reaction kinetics and precipitate quality.

[0065] In some embodiments, the binder fluid further comprises a dispersant, wherein the dispersant is sodium polyacrylate, and the mass percentage of the sodium polyacrylate in the binder fluid is 0.05% to 0.1%. Preferably, when the mass percentage of sodium polyacrylate is 0.1%, the dispersibility of the nano-α-FeOOH-urease hybrid material can be improved, ensuring that its particle size D50 is less than 100 nm.

[0066] In some embodiments, the cementing fluid further includes an acidic pH sustained-release agent, which is citric acid microspheres coated with polylactic acid, the particle size of the citric acid microspheres coated with polylactic acid is 50 to 100 μm, and the mass percentage of the citric acid microspheres coated with polylactic acid in the cementing fluid is 0.2% to 0.5%. Preferably, when the mass percentage of the citric acid microspheres coated with polylactic acid is 0.5%, it can release citric acid more quickly under alkaline conditions (pH>8.5) to lower the pH in the environmental medium, thereby stably maintaining the pH in the environment at 5 to 7, making it difficult for trivalent arsenic to dissolve. It should be noted that the dynamic control range of the pH of the cementing fluid is 7.0 to 8.5.

[0067] In some embodiments, the cementing fluid further comprises an enzyme protectant, wherein the enzyme protectant comprises trehalose, and the mass percentage of trehalose in the cementing fluid is 0.08% to 0.12%. Preferably, when the mass percentage of trehalose is 0.1%, the activity retention rate of urease is higher.

[0068] It should be noted that the percentages of all additives in the cementing fluid formula are defined based on the cementing fluid. The specific dosage of the cementing fluid is related to the heavy metal content in the contaminated soil. For lightly contaminated soil, 10-15L / m 2 , moderately polluted soil 15-30L / m 2 There is no upper limit for the pollutant index of heavily polluted soil, and the dosage can be increased as appropriate. Each additional 1000g / m 2 Heavy metals: increase dosage by 10-20L / m 2 Considering that the areas that usually need such pollution treatment are abandoned mining areas, which are moderately polluted areas, and considering the safety factor, the amount of cementing fluid can be directly limited to 20-40L / m 2 ).

[0069] The present invention also provides a method for repairing soil contaminated with composite heavy metals, wherein the composite heavy metals in the soil contaminated with composite heavy metals include arsenic, cadmium, and lead, comprising the following steps:

[0070] S1, mixing the aforementioned nano α-FeOOH-urease hybrid material with the aforementioned cementing liquid to obtain a mixed solution; the concentration of the nano α-FeOOH-urease hybrid material in the mixed solution is 50-100 g / L;

[0071] S2. Use high-pressure rotary jet grouting to inject the mixed solution into the contaminated soil layer and maintain for 7 to 14 days. During this period, the pH value is monitored in real time and a slow-release acid agent is added to obtain the repaired soil. The pressure of the high-pressure rotary jet grouting method is 0.5 to 1.0 MPa, and the injection volume of the mixed solution is 20 to 40 L / m 2 .

[0072] In some embodiments, the mass ratio of the nano α-FeOOH-urease hybrid material to the composite heavy metal in the composite heavy metal contaminated soil is 2.5-5:0.1.

[0073] In the technical solution of the present invention, when the composite heavy metals in the composite heavy metal contaminated soil include As(III), Cd and Pb, after the nano α-FeOOH-urease hybrid material and the cementing fluid are mixed with the composite heavy metal soil, on the one hand, the urea in the cementing fluid will be rapidly hydrolyzed under the action of the nano α-FeOOH-urease hybrid material and form calcium carbonate precipitate with the calcium source. This mineral precipitate will coat the urease, thereby further preventing the urease from being inactivated by the influence of external protons; on the other hand, when the acidity regulator is not added to the cementing fluid, the toxicity of trivalent arsenic is relatively large and a large amount of trivalent arsenic cannot be fixed. The ≡Fe-OH group of α-FeOOH in the nano-α-FeOOH-urease hybrid material forms an inner sphere complex with trivalent arsenic, which can improve the fixation efficiency of trivalent arsenic. In addition, ascorbic acid in the cementing fluid can efficiently oxidize trivalent arsenic through free radicals, converting it into pentavalent arsenic and stably existing in the inner sphere complex. It is not easy to dissolve under alkaline conditions. At the same time, calcium carbonate can also coat pentavalent arsenic to form a physical barrier to further stabilize arsenic. When an acidic regulator is added to the cementing fluid, the dissolution rate of arsenic can be better reduced. Thirdly, there is a competitive relationship between cadmium (Cd) and lead (Pd) and arsenic. Specifically, Cd 2+ With AsO3 3- Competition for adsorption sites leads to a decrease in fixation efficiency, and the leaching rate will increase by 25%-40% after freeze-thaw cycles. In addition to calcium carbonate, α-FeOOH in the nano-α-FeOOH-urease hybrid material can provide more adsorption sites. Both calcium carbonate and nanomaterials can adsorb composite metals. Cadmium or lead is adsorbed on the surface of α-FeOOH and forms ionic bonds with carboxyl or hydroxyl groups, thereby improving the fixation of cadmium and lead.

[0074] It should be noted that freeze-thaw cycles will destroy the loose calcium carbonate structure in traditional EICP. The present invention controls the calcite crystal form through the binder formula and fixes heavy metals lead and cadmium on the surface of α-FeOOH to improve the resistance of heavy metals to environmental disturbances after soil repair.

[0075] It should be noted that the differences in technical indicators between the present invention and the traditional EICP technology are shown in Table 1.

[0076] Table 1 Differences in technical indicators between the present invention and traditional EICP technology

[0077]

[0078]

[0079] Specifically:

[0080] The present invention proposes (1) an α-FeOOH-urease covalent hybrid system, in which EDC / NHS mediates the formation of an amide bond between the carboxyl group of urease and the hydroxyl group of α-FeOOH, and the enzyme desorption rate is less than 5%, breaking through the technical bottleneck of the unstable physical adsorption interface; (2) a "free radical oxidation-carbonate coating-hydroxyl complexation" multi-stage fixation model is proposed, in which the multi-stage barrier inhibits the reactivation of heavy metals, and the leaching rate decreases by less than 5% after freeze-thaw; (3) a pH dynamic balance system that cooperates with a calcium source compound and a slow-release acid agent is developed to solve the problem of alkaline re-dissolution of As.

[0081] The technical solutions of the present invention are further described in detail below in conjunction with specific embodiments and drawings. It should be understood that the following embodiments are only used to explain the present invention and are not used to limit the present invention.

[0082] Example 1

[0083] A method for preparing a nano α-FeOOH-urease hybrid material comprises the following steps:

[0084] 1. Preparation of nano-α-FeOOH by hydrothermal method:

[0085] (1) Mix 500 mL of 0.1 M Fe(NO3)3 solution and 1 M NaOH solution in a volume ratio of 1:2 and stir using a magnetic stirrer for 30 min;

[0086] (2) The stirred mixed solution was transferred to a hydrothermal reactor and the temperature was set to 180°C for 12 hours;

[0087] (3) After the reaction is completed, centrifuge at 8000 rpm for 10 min to collect the precipitate, wash the precipitate thoroughly with ultrapure water to adjust its pH to 7, and then dry it in a vacuum at 60°C.

[0088] (4) The dried precipitate was ground and passed through a 200-mesh sieve to obtain needle-shaped α-FeOOH weighing 3.62 g;

[0089] 2. Covalent grafting reaction:

[0090] (1) Weigh 1 g of the α-FeOOH powder prepared above and disperse it in 100 mL of phosphate buffered saline (PBS, pH 6.5). Stir the mixture with a glass rod to form a suspension. Place the beaker containing the suspension in an ultrasonic cleaning machine at 40 kHz for 30 min to obtain an α-FeOOH suspension.

[0091] (2) Introducing (3-cyanopropyl)trimethoxysilane silane coupling agent on the surface of α-FeOOH powder: prepare CPTMS hydrolyzate (1.2mL CPTMS + 50mL ethanol + 5mL water + glacial acetic acid to adjust pH to 4.5), pre-hydrolyze for 30min to obtain CPTMS hydrolyzate; transfer the suspension into a three-necked flask, add CPTMS hydrolyzate (1mL / min) dropwise under nitrogen protection at 70℃, and stir to react for 7h; centrifuge and wash (washing solution is ethanol and acetone in turn), discard the supernatant, and retain the silanized α-FeOOH suspension for later use (if it needs to be stored temporarily, store it in a vacuum dry place at 60℃, and ultrasonically disperse it to prepare a new suspension before the next use);

[0092] (3) Weigh 95.9 mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 23.0 mg of N-hydroxysuccinimide (NHS) and dissolve them in 10 mL of PBS buffer (pH = 6.5) to obtain an EDC / NHS solution; slowly add the prepared EDC / NHS solution dropwise to the silanized α-FeOOH suspension while maintaining magnetic stirring at 300 rpm; react at room temperature in the dark for 1 h while maintaining stirring to complete carboxyl activation, to obtain an activated α-FeOOH suspension;

[0093] (4) Weigh 0.11 g of urease and dissolve it in 10 mL of PBS buffer (pH = 6.5). Stir with a glass rod to obtain a urease solution. Add the urease solution dropwise to the activated α-FeOOH suspension and stir at 200 rpm in a refrigerator at 4°C in the dark for 4 h.

[0094] (5) After the reaction, the mixed solution was transferred to a 50 mL centrifuge tube and centrifuged at 12,000 rpm for 15 minutes at 4°C. The supernatant was discarded. 50 mL of PBS buffer (pH 7.4) was added to the precipitate, vortexed and resuspended, and then centrifuged repeatedly for a total of 3 washes. The final precipitate was quickly frozen in liquid nitrogen and dried in a freeze dryer for 24 hours to obtain a nano-α-FeOOH-urease hybrid material with a urease loading of 2.0 mg / mg Fe.

[0095] Example 2

[0096] A cementing fluid comprises 1M urea, 1M calcium source (molar ratio of calcium chloride to calcium bicarbonate is 3:1), 0.1% by weight of ascorbic acid, 0.2% by weight of sodium polyacrylate, 0.3% by weight of citric acid sustained-release microspheres, 0.1% by weight of trehalose, and the remainder is water. The citric acid sustained-release microspheres are prepared by the following steps:

[0097] The materials are as follows: polylactic acid (PLA) is used to form the microsphere skeleton; citric acid: analytical grade, as the core of the sustained-release acid agent; dichloromethane (DCM): organic solvent, dissolving PLA; polyvinyl alcohol (PVA): surfactant (concentration 1% w / v), used to stabilize the emulsion.

[0098] The method is as follows: (1) Preparation of PLA solution: Dissolve 1g PLA in 20mL dichloromethane and stir magnetically until completely dissolved. (2) Preparation of citric acid dispersion: Disperse 0.3g citric acid powder in 5mL deionized water and ultrasonically treat for 10min to form a homogeneous suspension. (Mass ratio of PLA to citric acid: 3:1). (3) Preparation of colostrum: Slowly add the citric acid dispersion to the PLA solution and homogenize at high speed (10000rpm, 5min) to form water-in-oil (W / O) colostrum. (4) Preparation of external aqueous phase: Pour 100mL 1% PVA solution into a beaker and stir magnetically (500rpm). (5) Emulsion: Slowly add the colostrum to the PVA solution and continue stirring for 2h to allow the DCM to evaporate and the PLA to solidify and encapsulate the citric acid. (6) Centrifugal washing: Centrifuge the emulsion at 8000rpm for 10min, collect the precipitate, and wash it three times with deionized water to remove residual PVA and unencapsulated citric acid. (7) Freeze drying: The washed microsphere suspension was freeze dried for 24 h to obtain white powdery polylactic acid-coated citric acid sustained-release microspheres.

[0099] Example 3

[0100] Compared with Example 2, Example 3 is different in that:

[0101] Does not contain sodium polyacrylate and trehalose.

[0102] Example 4

[0103] Compared with Example 3, Example 4 is different in that:

[0104] The molar concentration of urea is 0.5M.

[0105] Example 5

[0106] Compared with Example 3, Example 5 is different in that:

[0107] The molar concentration of urea is 0.75M.

[0108] Example 6

[0109] Compared with Example 3, Example 6 is different in that:

[0110] The molar concentration of urea is 1.25M.

[0111] Example 7

[0112] Compared with Example 3, Example 7 is different in that:

[0113] The molar concentration of urea is 1.5M.

[0114] Example 8

[0115] Compared with Example 3, Example 8 is different in that:

[0116] The molar ratio of calcium chloride to calcium bicarbonate is 2:1.

[0117] Example 9

[0118] Compared with Example 3, Example 9 is different in that:

[0119] The molar ratio of calcium chloride to calcium bicarbonate is 4:1.

[0120] Example 10

[0121] Compared with Example 3, Example 10 is different in that:

[0122] The mass percentage of ascorbic acid was 0%.

[0123] Example 11

[0124] Compared with Example 3, Example 11 is different in that:

[0125] The mass percentage of ascorbic acid is 0.05%.

[0126] Example 12

[0127] Compared with Example 3, Example 12 is different in that:

[0128] The mass percentage of ascorbic acid is 0.2%.

[0129] Example 13: Verification of remediation of As-Cd-Pb composite contaminated soil in lead-zinc mining areas

[0130] 1. Soil to be repaired and repair materials

[0131] Typical contaminated soil was collected from the surface layer (0-50cm depth) of an abandoned lead-zinc mine in Luanchuan, Henan Province. X-ray fluorescence spectrometry showed that ( Figure 1 The main pollutants are: As(III), 380mg / kg, accounting for 85% of the total arsenic; Cd, 65mg / kg; and Pb, 950mg / kg. Analysis of the soil's physical and chemical properties showed a pH of 5.8, a clay content of 35%, an organic matter content of 1.2%, and a permeability of 1×10 -6 The repair material used was the nano-α-FeOOH-urease hybrid material prepared in Example 1.

[0132] 2. Implementation of repair process

[0133] On-site repair uses double-liquid high-pressure rotary grouting technology to inject the cementing fluid into the contaminated soil. The grouting parameters are optimized by orthogonal experiments: grouting pressure 0.8MPa, grouting volume 35L / m 2 , injected twice, 24 hours apart, and maintained for 14 days. The cementing solution formula is the same as in Example 3. The sustained-release microspheres are prepared by embedding citric acid in polylactic acid, which can trigger acid release when the pH is greater than 8.5, thus achieving dynamic pH control.

[0134] Comparative Example 1: Traditional EICP technology for remediation of As-Cd-Pb composite contaminated soil in lead-zinc mining areas

[0135] Comparative Example 1 is different from Example 13 in that:

[0136] The cementing fluid formula is: urea (1M); calcium source (1M CaCl2), without compound calcium source, ascorbic acid and acidic pH sustained-release agent, the urease is free urease, and no covalent bonding is performed. The repair process is the same as Example 13.

[0137] Performance Testing

[0138] The urease activity and stability of the nano-α-FeOOH-urease hybrid material in Example 1 were tested:

[0139] (1) Urease activity determination: The indigo blue method was used to detect the amount of NH3 generated and compare the activity retention rates of the nano-α-FeOOH-urease hybrid material and free urease at pH 5.0, 7.0, and 9.0. The results are as follows Figure 2 As shown, the blue column represents the nano α-FeOOH-urease hybrid material, and the orange column represents the free urease.

[0140] Figure 2 The results showed that at pH = 5.0, the urease activity retention rate in the nano-α-FeOOH-urease hybrid material was >85%, of which the free urease activity was <30% (that is, the urease desorption rate was low), reaching the best; while under the same pH conditions, the enzyme activity retention rate of the free urease adsorption system was only 28.7%.

[0141] (2) Stability test: The nano-α-FeOOH-urease hybrid material was placed in a simulated contaminated liquid containing 50 mg / L As(III), 100 mg / L Cd 2 + solution was shaken and samples were taken regularly to detect the urease activity decay rate. Figure 3 shown.

[0142] Figure 3The results showed that the nano-α-FeOOH-urease hybrid material still maintained 75% activity after 24 hours, proving that the covalent fixation system of the hybrid material has strong tolerance to heavy metal toxicity and reflects the shielding protection effect of nano-α-FeOOH.

[0143] The hydrolysis rate of urea in the cementing fluids of Examples 3 to 7 was measured by a conductivity meter, with the NH 4+ The concentration was characterized, and the results were as follows Figure 4 shown.

[0144] Figure 4 The results showed that the hydrolysis rate of urea was higher when the concentration was 1M.

[0145] The hydrolysis rate of urea in the cementing fluids of Examples 3, 8 and 9 was measured by a conductivity meter, and the NH4 + The concentration was characterized, and the results were as follows Figure 5 shown.

[0146] Figure 5 The results show that when the molar ratio of CaCl2 to Ca(HCO3)2 is 3:1, the urea hydrolysis rate is 0.12 mmol / h. At this time, it can ensure a sufficient precipitate formation rate while avoiding the uncontrolled growth of CaCO3 crystals due to excessively rapid reaction, resulting in loose amorphous precipitates instead of dense calcite, thereby achieving the optimal balance between reaction kinetics and precipitate quality.

[0147] The binders of Examples 3, 10, 11, and 12 were mixed with the nano-α-FeOOH-urease hybrid material of Example 1, respectively, such that the content of the nano-α-FeOOH-urease hybrid material in the mixed system was 3 g / L. The ·OH free radical intensity was measured by electron paramagnetic resonance using DMPO as a scavenger. The free radical intensity represents the As(III) oxidation rate. After the mixing reaction was carried out at 25°C for 4 hours, the ·OH free radical intensity in the mixed system was measured. The results are as follows: Figure 6 shown.

[0148] Figure 6 The results showed that the free radical signal intensity was the highest when the ascorbic acid concentration was 0.1%, g = 2.004, and the peak area increased 3.5 times. Therefore, the generation of ·OH was the strongest when the ascorbic acid concentration was 0.1%, and the As(III) oxidation rate was the highest.

[0149] The binder solution in Example 3 was mixed with the nano-α-FeOOH-urease hybrid material in Example 1 to a concentration of 3 g / L in the mixed system. The mixture was reacted at 25°C. Samples were taken at various time points (2 h, 4 h, 6 h, 8 h, and 12 h), and DMPO was added as a spin trap. The OH radical signal intensity was measured by X-band electron paramagnetic resonance (ESR). This signal intensity can reflect the As(III) oxidation efficiency in the system. The As conversion results are shown in Figure 2. Figure 7 shown.

[0150] Figure 7 The results showed that with the increase in mixing reaction time, the intensity of the ·OH radical signal in the system gradually increased, indicating a gradual increase in oxidation activity. During the first two hours of the reaction, the free radical signal slowly increased, presumably due to the gradual contact and stimulation of free radical generation by the initial reactants in the system. After the fourth hour, the ·OH signal significantly increased, entering a phase of rapid free radical release, and the As(III) oxidation reaction became more intense. By the eighth hour, the free radical signal reached its peak, indicating that the system's oxidation capacity reached its highest level, at which point As(III) was largely converted to As(V). Thereafter, the signal intensity stabilized, indicating that the reaction was nearing its endpoint and the As(III) oxidation process was saturated. These results validate the sustained oxidation capacity of the nano-α-FeOOH-urease hybrid material and the cementing fluid system during the reaction and reveal the temporal kinetics of the As(III) oxidation reaction.

[0151] The effects of the two repair methods in Example 13 and Comparative Example 1 were compared, and the dynamic changes of arsenic forms, the leaching rates of arsenic, cadmium and lead, and the freeze-thaw cycle stability of the repaired soil were tested. The detection method is: the dynamic changes of arsenic forms within 168 hours after soil repair are analyzed by HPLC-ICP-MS. The results are as follows: Figure 8 As shown; the leaching rate of arsenic, cadmium and lead was tested by the Toxicity Characteristic Leaching Procedure (TCLP, EPA1311), and the results are as follows Figure 9 As shown; by subjecting the repaired soil to 10 freeze-thaw cycles (-20℃ / 25℃, ASTM D560), the BCR (European Community Reference Bureau continuous extraction method, Bureau Communautaire de Référence) continuous extraction method was used to measure the weakly acid soluble proportion and fixation rate of Cd and Pb. The fixation rate (%) = (1-weakly acid soluble content after repair / weakly acid soluble content before repair) 100%. The results are shown in Figure 10 As shown in the figure, the compressive strength of the soil after freeze-thaw cycles was measured, and the results are shown in the figure. Figure 11 shown.

[0152] Depend on Figure 8As can be seen, in Example 13, the As(III) concentration decreased from 380 mg / kg to 6.5 mg / kg, and the As(V) content increased from 15% to 98.5%, demonstrating a significant and efficient oxidation mechanism driven by the nano-α-FeOOH-urease hybrid material and ascorbic acid. In contrast, the As(III) oxidation rate in Comparative Example 1 was only 52.3%. Compared with the prior art, the present invention increased the oxidation rate of trivalent arsenic by 46.2%.

[0153] Depend on Figure 9 It can be seen that the leaching concentrations of As, Cd, and Pb in Example 13 were 0.12, 0.18, and 0.25 mg / L, respectively, all of which were lower than the risk screening values ​​of the "Soil Environmental Quality Standard for Construction Land" (GB 36600-2018). In contrast, the leaching concentration of As in Comparative Example 1 was as high as 5.8 mg / L.

[0154] Depend on Figure 10 As can be seen, the proportion of weakly acid soluble Cd and Pb decreased from 8.7 and 15.2 mg / kg to 3.6 and 4.1 mg / kg, respectively, and the immobilization rates reached 94.7% and 96.1%, which were significantly better than those in Comparative Example 1 (immobilization rates were 68.4% and 72.5%, respectively). Table 2 compares and analyzes the differences between Example 13 and Comparative Example 1.

[0155] Depend on Figure 11 It can be seen that since freeze-thaw cycles will weaken the mechanical properties of soil, the compressive strength loss rate of the repaired soil samples after multiple freeze-thaw cycles is relatively small, indicating that the repaired soil structure is more stable. The calcium carbonate generated by the EICP reaction fixes heavy metals and exists between soil particles, which has the effect of enhancing the mechanical properties of soil.

[0156] Table 2 Comparative statistical table of test results of Example 13 and Comparative Example 1

[0157] index Example 13 Comparative Example 1 Improvement As(III) oxidation rate 98.5％ 52.3％ +46.2％ As leaching concentration (mg / L) 0.12 5.8 -97.9％ Cd fixation rate (after freeze-thaw) 94.7％ 68.4％ +26.3％ Urease acid activity retention rate 85.6％ 28.7％ +56.9％

[0158] Experiment demonstrating the synergistic effect of nano-α-FeOOH-urease hybrid material and ascorbic acid:

[0159] A comparative experiment was set up for Example 13, with the following variables: Group A was nano-α-FeOOH-urease hybrid material (3 g / L) + ascorbic acid (0.1%); Group B was nano-α-FeOOH-urease hybrid material (3 g / L) alone; Group C was ascorbic acid (0.1%) alone; and Group D was a blank control without nano-α-FeOOH-urease hybrid material and ascorbic acid, used to eliminate environmental interference. The oxidation rates of As(III) under the four conditions were investigated by the aforementioned method, and the results were as follows: Figure 12 shown.

[0160] The results showed that the oxidation rate of group A was the highest (98.3%), proving that the nano-α-FeOOH-urease hybrid material and ascorbic acid are indispensable; the oxidation rate of group B was 52.3%, indicating that urease can catalyze the hydrolysis of part of urea to generate However, the lack of ·OH free radicals led to limited oxidation; the oxidation rate of group C was 15.8%, indicating that ascorbic acid alone had a weak effect and urease was required to drive the reaction chain.

[0161] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of the present invention.

Claims

1. A nano-α-FeOOH-urease hybrid material, characterized in that: The nano α-FeOOH-urease hybrid material comprises a nano α-FeOOH carrier and urease fixed on the surface of the nano α-FeOOH through chemical bonding.

2. The nano α-FeOOH-urease hybrid material according to claim 1, characterized in that: The mass ratio of Fe in the nano-α-FeOOH to the urease is 1:(1.5-2.5).

3. A method for preparing the nano α-FeOOH-urease hybrid material according to claim 1 or 2, characterized in that: The following steps are involved: S10, mixing α-FeOOH powder and a carboxyl activator in a phosphate buffer solution at pH 6.5 to 7.0, and reacting at 20 to 25° C. in the dark for 1 to 2 hours to obtain carboxyl-activated α-FeOOH; S20, mixing the carboxyl-activated α-FeOOH with a urease solution, reacting at 0-10° C. for 3-5 hours, separating the solid and the liquid, and drying the solid to obtain the nano α-FeOOH-urease hybrid material.

4. The method for preparing the nano α-FeOOH-urease hybrid material according to claim 3, wherein: In step S10: The particle size of the α-FeOOH powder is 10 to 30 nm; and / or, The specific surface area of ​​the α-FeOOH powder is ≥150m 2 / g; and / or, The carboxyl activator is 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and the molar ratio of the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide to the N-hydroxysuccinimide is 1:1 to 1:2; and / or, The mass ratio of the carboxyl activator to the α-FeOOH powder is 0.1:1 to 0.3:

1.

5. The method for preparing the nano α-FeOOH-urease hybrid material according to claim 3, wherein: In step S20: The mass ratio of the carboxyl-activated α-FeOOH to the urease in the urease solution is 1:1.5 to 1:2.

5.

6. The method for preparing the nano-α-FeOOH-urease hybrid material according to claim 3, wherein: Step S20 includes: The carboxyl-activated α-FeOOH is mixed with a urease solution and reacted at 0-10°C for 3-5 hours. The reacted solution is centrifuged, and the solid after centrifugation is vacuum freeze-dried, wherein the pre-freezing temperature of the vacuum freeze-drying is -40°C and the main drying temperature of the vacuum freeze-drying is -20°C.

7. A cementing fluid for repairing soil contaminated with composite heavy metals, characterized in that: The cementing fluid comprises urea, a compound calcium source, and ascorbic acid, wherein: The concentration of the urea is 0.8 to 1.2 M; The concentration of the compound calcium source is 0.1-1.2 M, the compound calcium source is calcium chloride and calcium bicarbonate, and the molar ratio of the calcium chloride to the calcium bicarbonate is 3:1; The mass percentage concentration of the ascorbic acid is 0.05% to 0.15%.

8. The cementing fluid according to claim 7, wherein The cementing fluid further comprises at least one of a dispersant, an acidic pH slow-release agent, and an enzyme protectant: The dispersant is sodium polyacrylate, and the mass percentage of sodium polyacrylate in the cementing solution is 0.05% to 0.1%; The acidic pH sustained-release agent is polylactic acid-coated citric acid microspheres, the particle size of the polylactic acid-coated citric acid microspheres is 50 to 100 μm, and the mass percentage of the polylactic acid-coated citric acid microspheres in the binder is 0.2% to 0.5%; The enzyme protecting agent includes trehalose, and the mass percentage of trehalose in the gelling liquid is 0.08% to 0.12%.

9. A method for repairing soil contaminated with composite heavy metals, wherein the composite heavy metals in the soil contaminated with composite heavy metals include arsenic, cadmium, and lead, characterized in that: The following steps are involved: S1. Mixing the nano-α-FeOOH-urease hybrid material according to any one of claims 1 to 6 with the cementing fluid according to any one of claims 7 to 8 to obtain a mixed solution; the concentration of the nano-α-FeOOH-urease hybrid material in the mixed solution is 50 to 100 g / L; S2. Use high-pressure rotary jet grouting to inject the mixed solution into the contaminated soil layer and maintain for 7 to 14 days. During this period, the pH value is monitored in real time and a slow-release acid agent is added to obtain the repaired soil. The pressure of the high-pressure rotary jet grouting method is 0.5 to 1.0 MPa, and the injection volume of the mixed solution is 20 to 40 L / m 2 .

10. The method for repairing composite heavy metal contaminated soil according to claim 9, characterized in that: The mass ratio of the nano α-FeOOH-urease hybrid material to the composite heavy metal in the composite heavy metal contaminated soil is 2.5-5:0.1.

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