A contaminated soil top layer remediation material and a preparation method thereof

Abstract

The application belongs to the technical field of soil remediation materials, and relates to a contaminated soil top layer remediation material and a preparation method thereof, which comprises the following steps: crushing agricultural waste and mixing the agricultural waste with porous minerals to pyrolyze, grinding and sieving to obtain a composite carrier; dispersing the composite carrier in a silane coupling agent solution, heating and refluxing to obtain a modified carrier; dispersing the modified carrier in a polyethylene imine solution, stirring and heating to obtain an amine group carrier; dispersing the amine group carrier in deionized water to obtain a carrier dispersion liquid, adding sodium hydroxide solution dropwise into the carrier dispersion liquid to adjust the pH value, simultaneously adding calcium dihydrogen phosphate solution and calcium chloride solution dropwise into the carrier dispersion liquid, stirring and heating to obtain a composite modified carrier; adding the composite modified carrier into a sodium alginate solution, mixing to obtain a precursor solution; dropping the precursor solution into a calcium chloride solution, forming gel microspheres after gel solidification, filtering, washing and drying to obtain the contaminated soil top layer remediation material.

Description

Technical Field

[0001] This invention belongs to the field of soil remediation materials technology, and relates to a top-layer remediation material for contaminated soil and its preparation method. Background Technology

[0002] Currently, heavy metal pollution in soil, especially in the topsoil of farmland, directly threatens the safety of agricultural products and ecological health. Traditional remediation techniques, such as topsoil replacement and chemical leaching, have drawbacks such as high costs, damage to soil structure, and secondary pollution. Therefore, developing efficient, environmentally friendly, and widely applicable in-situ stabilization remediation materials is a research hotspot.

[0003] Currently, biochar prepared from agricultural waste, as well as porous minerals such as bentonite and zeolite, are widely studied as adsorbents. These materials are low in cost and have a certain adsorption capacity, but the fixation of heavy metals is often mainly based on physical adsorption and weak chemical reactions. They are prone to desorption in complex soil environments and lack long-term stability. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a top-layer remediation material for contaminated soil and its preparation method. The top-layer remediation material for contaminated soil prepared by the present invention can effectively treat heavy metals (Cd). 2+ Pb 2+ (etc.) to achieve rapid capture and long-term stable fixation, effectively solving the problems in existing farmland soil heavy metal remediation, such as insufficient fixation capacity of single materials for heavy metals, poor long-term stability, inconvenient application of powder materials, and easy generation of secondary pollution.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a method for preparing a top-layer remediation material for contaminated soil, the method comprising:

[0007] (I) Agricultural waste is crushed and mixed with porous minerals, pyrolyzed under an inert atmosphere, ground and sieved to obtain a composite carrier; the composite carrier is dispersed in a silane coupling agent solution, heated under reflux, filtered, washed and dried to obtain a modified carrier;

[0008] (II) The modified carrier is dispersed in a polyethyleneimine solution, stirred and heated to react, then filtered, washed and dried to obtain an amination carrier; the amination carrier is dispersed in deionized water to obtain a carrier dispersion, sodium hydroxide solution is added dropwise to the carrier dispersion to adjust the pH value, and then calcium dihydrogen phosphate solution and calcium chloride solution are added dropwise to the carrier dispersion under stirring conditions. After the addition is completed, the mixture is stirred and heated to react, allowed to stand for aging, filtered, washed and dried to obtain a composite modified carrier;

[0009] (III) The composite modified carrier is added to sodium alginate solution and mixed to obtain a precursor solution; the precursor solution is dropped into calcium chloride solution, and gel solidification is performed to form gel microspheres. After filtration, washing and drying, the top layer remediation material of the contaminated soil is obtained.

[0010] This invention first involves co-pyrolyzing agricultural waste with porous minerals, followed by modification with a silane coupling agent solution to obtain a modified carrier with active amino groups on its surface. Subsequently, the modified carrier is grafted with a polyethyleneimine solution, and calcium dihydrogen phosphate and calcium chloride solutions are simultaneously added under alkaline conditions to generate hydroxyapatite in situ on the surface of the aminated carrier, resulting in a composite modified carrier. Finally, the composite modified carrier is mixed with a sodium alginate solution and then added dropwise to a calcium chloride solution for gel solidification, ultimately preparing a top-layer remediation material for contaminated soil. The top-layer remediation material for contaminated soil prepared by this invention can treat heavy metals (Cd)... 2+ Pb 2+ This technology enables rapid capture and long-term stable fixation of heavy metals, effectively solving problems in existing farmland soil heavy metal remediation, such as insufficient fixation capacity of single materials for heavy metals, poor long-term stability, inconvenient application of powder materials, and easy generation of secondary pollution.

[0011] First, this invention utilizes the co-pyrolysis of agricultural waste (such as rice husks and straw) and porous minerals (such as bentonite and zeolite) to prepare a composite carrier. Through the pyrolysis process, the agricultural waste carbonizes to form porous biochar, while the porous minerals themselves possess well-developed pore structures and ion exchange capabilities. The agricultural waste and porous minerals combine under a high-temperature, inert atmosphere to form a more stable and porous composite carrier. Subsequently, the composite carrier is surface-modified using a silane coupling agent. The silane coupling agent molecule has an easily hydrolyzable ethoxy group at one end and an organic chain with an amino group at the other. The ethoxy group hydrolyzes to a silanol group, which then undergoes a condensation reaction with the hydroxyl groups on the surface of the composite carrier to form a strong Si-O-Si covalent bond. This grafts an organic chain with an active amino group onto the surface of the composite carrier, providing chemical binding sites for the subsequent grafting of polyethyleneimine.

[0012] Subsequently, the modified support underwent functionalization. First, polyethyleneimine (PEI) was used to treat the support. PEI is a high-molecular-weight polymer rich in primary and secondary amine groups. The amine groups on the PEI molecular chain can bond with the amino groups grafted onto the modified support through hydrogen bonding and electrostatic interactions, thereby grafting the PEI molecules onto the support surface to form an aminated support. Grafting PEI serves two purposes: firstly, it introduces heavy metal complexation sites; the high density of amine groups on the PEI molecular chain attracts heavy metal ions (Cd...). 2+ Pb 2+PEI possesses extremely strong coordination (complexation) capabilities, enabling it to efficiently capture heavy metal ions. Furthermore, the PEI molecular chain provides a nucleation interface for the in-situ generation of hydroxyapatite. In an alkaline environment, the protonated amino groups on the PEI chain can adsorb phosphate ions from the solution through electrostatic interactions. This facilitates heterogeneous nucleation and growth of hydroxyapatite on the surface of the aminated support, rather than random homogeneous nucleation in solution, ensuring that hydroxyapatite can be generated and loaded in situ on the surface of the aminated support.

[0013] Subsequently, under alkaline conditions, calcium dihydrogen phosphate solution and calcium chloride solution were simultaneously added dropwise to the carrier dispersion. The alkaline environment favors the reaction of calcium ions with phosphate ions to generate hydroxyapatite precursors. These hydroxyapatite precursors deposit, grow, and crystallize on the surface and pores of the aminated carrier, thus generating hydroxyapatite in situ on the aminated carrier. There is a synergistic effect between PEI and hydroxyapatite loaded on the carrier surface. On the one hand, the amino groups provided by PEI can strongly capture heavy metal ions (Cd) through complexation (coordination). 2+ Pb 2+ This process fixes heavy metals onto the carrier surface and within its pores. On the other hand, hydroxyapatite exhibits precipitation and ion exchange properties; calcium ions in hydroxyapatite can be converted to Cd... 2+ Pb 2+ The replacement of divalent heavy metal ions with PEI forms stable minerals that are less soluble in water, thereby stabilizing the heavy metals. The synergistic effect of PEI's amino complexation and hydroxyapatite's precipitation significantly enhances the adsorption and fixation of heavy metals by the remediation material.

[0014] Finally, sodium alginate gel was used to encapsulate the composite modified carrier. This serves two purposes: firstly, gel encapsulation transforms the powdered composite modified carrier into millimeter-sized gel microspheres, facilitating application to the soil surface and avoiding operational difficulties and secondary pollution caused by powder dispersion, thus better suiting the practicalities of farmland application. Secondly, sodium alginate gel contains carboxyl groups. When the remediation material comes into contact with heavy metal-contaminated soil, the negatively charged carboxyl groups can adsorb and immobilize heavy metal ions through ion exchange and surface complexation, achieving initial adsorption and enrichment of heavy metal ions. Furthermore, the gel microsphere structure possesses elasticity and water retention, allowing for better contact with soil particles. The slow-release effect of the gel network ensures the sustained function of the internal composite modified carrier, preventing its loss through water erosion.

[0015] As a preferred technical solution of the present invention, in step (I), the agricultural waste includes any one or a combination of at least two of rice husks, straw or sawdust.

[0016] In some optional instances, the agricultural waste is crushed to 20-40 mesh, for example, 20 mesh, 22 mesh, 24 mesh, 26 mesh, 28 mesh, 30 mesh, 32 mesh, 34 mesh, 36 mesh, 38 mesh or 40 mesh, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0017] In some alternative instances, the porous mineral includes any one or a combination of at least two of bentonite, attapulgite, or zeolite.

[0018] In some alternative instances, the mass ratio of the agricultural waste to the porous mineral is (3~5):1, for example, it can be 3.0:1, 3.2:1, 3.4:1, 3.6:1, 3.8:1, 4.0:1, 4.2:1, 4.4:1, 4.6:1, 4.8:1 or 5.0:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0019] This invention utilizes the co-pyrolysis of agricultural waste and porous minerals to prepare a composite carrier with a porous structure. The porous minerals act as a framework and template during the pyrolysis process, effectively inhibiting excessive shrinkage of the carbon structure and pore collapse during the pyrolysis of agricultural waste, thereby increasing the specific surface area and porosity of the composite carrier.

[0020] As a preferred technical solution of the present invention, in step (I), the heating rate of the pyrolysis is 10~12℃ / min, for example, it can be 10℃ / min, 10.2℃ / min, 10.4℃ / min, 10.6℃ / min, 10.8℃ / min, 11℃ / min, 11.2℃ / min, 11.4℃ / min, 11.6℃ / min, 11.8℃ / min or 12℃ / min, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] In some optional instances, the pyrolysis temperature is 500~600℃, for example, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃ or 600℃, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] In some optional instances, the holding time for pyrolysis is 1.5 to 2.5 hours, for example, 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours, 2.0 hours, 2.1 hours, 2.2 hours, 2.3 hours, 2.4 hours, or 2.5 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0023] In some optional instances, the mesh size of the sieve used for grinding and sieving is 100 to 200 mesh, for example, 100 mesh, 110 mesh, 120 mesh, 130 mesh, 140 mesh, 150 mesh, 160 mesh, 170 mesh, 180 mesh, 190 mesh or 200 mesh, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, in step (I), the silane coupling agent solution includes a silane coupling agent and an aqueous ethanol solution.

[0025] In some optional examples, the mass fraction of the silane coupling agent in the silane coupling agent solution is 3 to 5 wt%, for example, it can be 3.0 wt%, 3.2 wt%, 3.4 wt%, 3.6 wt%, 3.8 wt%, 4.0 wt%, 4.2 wt%, 4.4 wt%, 4.6 wt%, 4.8 wt%, or 5.0 wt%, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0026] In some optional examples, the silane coupling agent comprises 3-aminopropyltriethoxysilane.

[0027] In some alternative examples, the ratio of the composite carrier to the silane coupling agent solution is 1g:(10~20)mL, for example, it can be 1g:10mL, 1g:11mL, 1g:12mL, 1g:13mL, 1g:14mL, 1g:15mL, 1g:16mL, 1g:17mL, 1g:18mL, 1g:19mL or 1g:20mL, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0028] In some alternative instances, the temperature of the heating reflux reaction is 60 to 70°C, for example, 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, or 70°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0029] In some optional instances, the heating reflux reaction time is 4 to 6 hours, for example, 4.0 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours, 5.0 hours, 5.2 hours, 5.4 hours, 5.6 hours, 5.8 hours, or 6.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0030] In some alternative instances, the drying temperature is 60 to 80°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 76°C, 78°C or 80°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0031] In some optional instances, the drying time is 6 to 12 hours, for example, 6.0 hours, 6.5 hours, 7.0 hours, 7.5 hours, 8.0 hours, 8.5 hours, 9.0 hours, 9.5 hours, 10.0 hours, 10.5 hours, 11.0 hours, 11.5 hours, or 12.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0032] As a preferred technical solution of the present invention, in step (II), the concentration of polyethyleneimine in the polyethyleneimine solution is 5~10 g / L, for example, it can be 5.0 g / L, 5.5 g / L, 6.0 g / L, 6.5 g / L, 7.0 g / L, 7.5 g / L, 8.0 g / L, 8.5 g / L, 9.0 g / L, 9.5 g / L or 10.0 g / L, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] In some alternative instances, the mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution is 1:(0.1~0.2), for example, it can be 1:0.1, 1:0.11, 1:0.12, 1:0.13, 1:0.14, 1:0.15, 1:0.16, 1:0.17, 1:0.18, 1:0.19 or 1:0.2, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0034] This invention specifically limits the mass ratio of the modified carrier to polyethyleneimine in the polyethyleneimine solution to 1:(0.1~0.2). If the amount of PEI is too low, the amine grafting density on the surface of the modified carrier will be insufficient. On the one hand, this will reduce the number of heavy metal complexation sites on the repair material, affecting its adsorption capacity. On the other hand, it will reduce the number of nucleation sites on the carrier surface available for phosphate ion adsorption, which is not conducive to the uniform deposition of hydroxyapatite on the carrier surface. As a result, most of the hydroxyapatite will remain free in the solution and cannot be fixed and loaded on the carrier surface. If the amount of PEI is too high, excessive PEI will accumulate and deposit in large quantities in the pores of the carrier, blocking the pore structure and reducing the specific surface area and porosity of the carrier, thus hindering the transport and diffusion of heavy metal ions into the carrier.

[0035] In some alternative examples, the temperature at which the modified carrier reacts with the polyethyleneimine solution by stirring and heating is 40 to 50°C, for example, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C, or 50°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0036] In some alternative examples, the reaction time of the modified carrier with the polyethyleneimine solution by stirring and heating is 1 to 2 hours, for example, 1.0 hours, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours, or 2.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0037] The siloxane end of the silane coupling agent undergoes a condensation reaction with the hydroxyl groups on the support surface, thereby introducing organic molecular chains with terminal amino groups onto the support surface. When the modified support modified with the silane coupling agent is mixed with a PEI solution, the PEI molecular chains can be grafted onto the modified support surface through hydrogen bonding and electrostatic interactions. On the one hand, after modification with the silane coupling agent, terminal amino groups and existing hydroxyl groups are introduced onto the support surface. Simultaneously, PEI is a highly branched polymer with numerous primary, secondary, and imine groups. When PEI is mixed with the modified support, the -NH2 and -OH groups on the modified support surface can form OH…N and NH…N hydrogen bonds with the -NH2 and -NH- groups on the PEI molecular chains. These hydrogen bonds generate strong binding forces, fixing the PEI molecular chains onto the support surface. On the other hand, the amino groups on the PEI molecular chains can undergo partial protonation to form -NH3 groups. + The positively charged PEI molecules, coupled with the negatively charged surface of the modified carrier, create an electrostatic attraction between the positively charged PEI molecular chains and the negatively charged carrier surface. This attraction causes the PEI molecular chains to adsorb and coat the modified carrier surface. By pre-modifying with a silane coupling agent, a strong bond between PET and the modified carrier surface can be achieved, rather than simple physical adsorption.

[0038] As a preferred technical solution of the present invention, in step (II), the mass fraction of the aminated carrier in the carrier dispersion is 5~10wt%, for example, it can be 5.0wt%, 5.5wt%, 6.0wt%, 6.5wt%, 7.0wt%, 7.5wt%, 8.0wt%, 8.5wt%, 9.0wt%, 9.5wt% or 10.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] In some alternative instances, sodium hydroxide solution is added dropwise to the carrier dispersion to adjust its pH to 9-10, for example, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9 or 10.0, but not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0040] In some optional examples, at a stirring speed of 300-500 rpm, for example, 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, or 500 rpm, a 0.5-1 mol / L calcium dihydrogen phosphate solution and a 1-1.5 mol / L calcium chloride solution are simultaneously added dropwise to the carrier dispersion, wherein the concentration of the calcium dihydrogen phosphate solution can be 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, etc. The concentrations of calcium chloride solutions can be 0.7 mol / L, 0.75 mol / L, 0.8 mol / L, 0.85 mol / L, 0.9 mol / L, 0.95 mol / L, or 1.0 mol / L. However, the concentrations are not limited to the listed values; other unlisted values ​​within this range also apply.

[0041] Calcium dihydrogen phosphate reacts with calcium chloride in an alkaline environment to form hydroxyapatite. Calcium dihydrogen phosphate provides hydrogen phosphate and calcium ions, while calcium chloride provides additional calcium ions. In an alkaline environment, hydrogen phosphate is converted into phosphate. Calcium ions in the solution combine with phosphate ions to first form an amorphous calcium phosphate precipitate. Subsequently, during stirring, heating, and static aging, the amorphous calcium phosphate precipitate undergoes hydrolysis and recrystallization, ultimately forming hydroxyapatite.

[0042] In some optional examples, the dropping rate of the calcium dihydrogen phosphate solution and the calcium chloride solution is 0.5 to 1 mL / min, for example, it can be 0.5 mL / min, 0.55 mL / min, 0.6 mL / min, 0.65 mL / min, 0.7 mL / min, 0.75 mL / min, 0.8 mL / min, 0.85 mL / min, 0.9 mL / min, 0.95 mL / min or 1.0 mL / min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0043] In some alternative examples, the mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the aminated carrier in the carrier dispersion is (0.3~0.5):1, for example, it can be 0.3:1, 0.32:1, 0.34:1, 0.36:1, 0.38:1, 0.4:1, 0.42:1, 0.44:1, 0.46:1, 0.48:1 or 0.5:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0044] This invention specifically limits the mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the aminated carrier in the carrier dispersion to (0.3~0.5):1. If the amount of calcium dihydrogen phosphate is too low, the concentration of phosphate ions participating in the reaction will be insufficient, resulting in too little hydroxyapatite generated in situ on the surface of the aminated carrier, affecting the precipitation and ion exchange of the remediation material and reducing the fixation effect on heavy metal ions. If the amount of calcium dihydrogen phosphate is too high, excessive phosphate ions and calcium ions will undergo homogeneous nucleation in the solution, generating a large amount of free hydroxyapatite, instead of heterogeneous nucleation and growth on the surface of the aminated carrier. During static aging, the free hydroxyapatite will deposit on the carrier surface, blocking the pore structure of the carrier and hindering the adsorption and diffusion of heavy metal ions inside the carrier.

[0045] In some alternative examples, the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to calcium chloride in the calcium chloride solution is 1:(1.3~1.5), for example, it can be 1:1.3, 1:1.32, 1:1.34, 1:1.36, 1:1.38, 1:1.4, 1:1.42, 1:1.44, 1:1.46, 1:1.48 or 1:1.5, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0046] This invention specifically limits the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to calcium chloride in the calcium chloride solution to 1:(1.3~1.5). If the amount of calcium chloride is too low, the calcium ions are relatively insufficient, the precipitation reaction is incomplete, and the resulting hydroxyapatite has poor crystallinity and an incomplete structure, making it easily soluble in the soil environment and affecting the precipitation and ion exchange effects of the remediation material. If the amount of calcium chloride is too high, the excess calcium ions will accelerate the nucleation rate of calcium phosphate, causing a large number of fine crystal nuclei to rapidly and homogeneously nucleate in the solution, rather than undergoing orderly heterogeneous growth on the surface of the aminated carrier. Ultimately, this results in a large amount of hydroxyapatite existing in the solution in a free form, rather than being firmly loaded on the aminated carrier. During the static aging process, the free hydroxyapatite will deposit in the pores of the carrier, blocking the pore structure and hindering the adsorption and diffusion of heavy metal ions inside the carrier. In addition, during the subsequent washing process, these physically attached hydroxyapatite on the surface of the carrier will also be washed away, resulting in the loss of effective components.

[0047] As a preferred technical solution of the present invention, in step (II), the temperature of the stirring and heating reaction of the carrier dispersion, calcium dihydrogen phosphate solution and calcium chloride solution is 60~80℃, for example, it can be 60℃, 62℃, 64℃, 66℃, 68℃, 70℃, 72℃, 74℃, 76℃, 78℃ or 80℃, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] In some optional examples, the stirring and heating reaction time of the carrier dispersion, calcium dihydrogen phosphate solution and calcium chloride solution is 6 to 12 hours, for example, 6.0 hours, 6.5 hours, 7.0 hours, 7.5 hours, 8.0 hours, 8.5 hours, 9.0 hours, 9.5 hours, 10.0 hours, 10.5 hours, 11.0 hours, 11.5 hours or 12.0 hours, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0049] In some optional instances, the static aging temperature is 60~80°C, for example, it can be 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 76°C, 78°C or 80°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0050] In some optional instances, the settling time is 6 to 8 hours, for example, 6.0 hours, 6.2 hours, 6.4 hours, 6.6 hours, 6.8 hours, 7.0 hours, 7.2 hours, 7.4 hours, 7.6 hours, 7.8 hours, or 8.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0051] In some alternative instances, the drying temperature is 80 to 100°C, for example, 80°C, 82°C, 84°C, 86°C, 88°C, 90°C, 92°C, 94°C, 96°C, 98°C or 100°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0052] In some optional instances, the drying time is 8 to 12 hours, for example, 8.0 hours, 8.5 hours, 9.0 hours, 9.5 hours, 10.0 hours, 10.5 hours, 11.0 hours, 11.5 hours or 12.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0053] As a preferred technical solution of the present invention, in step (III), the mass fraction of the sodium alginate solution is 2~4wt%, for example, it can be 2.0wt%, 2.2wt%, 2.4wt%, 2.6wt%, 2.8wt%, 3.0wt%, 3.2wt%, 3.4wt%, 3.6wt%, 3.8wt% or 4.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] In some optional instances, the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution is (2~4):1, for example, it can be 2.0:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1, 3.0:1, 3.2:1, 3.4:1, 3.6:1, 3.8:1 or 4.0:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0055] This invention specifically limits the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution to (2~4):1. If the amount of composite modified carrier is too low, the loading of the composite modified carrier, as the core functional component, in the final gel microspheres will be insufficient, directly affecting the adsorption and fixation effect of the remediation material on heavy metal ions. If the amount of composite modified carrier is too high, it will affect the structural integrity of the gel microspheres, making it unable to effectively encapsulate the composite modified carrier. The resulting gel microspheres will have poor mechanical strength and are prone to deformation and breakage after application to soil, leading to leakage of the internally embedded composite modified carrier and affecting the retention time of the composite modified carrier in the soil.

[0056] As a preferred technical solution of the present invention, in step (III), the mass fraction of the calcium chloride solution is 3~5wt%, for example, it can be 3.0wt%, 3.2wt%, 3.4wt%, 3.6wt%, 3.8wt%, 4.0wt%, 4.2wt%, 4.4wt%, 4.6wt%, 4.8wt% or 5.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0057] In some alternative instances, the precursor solution is dripped into the calcium chloride solution using a syringe with a needle aperture of 0.5 to 1 mm, for example, 0.5 mm, 0.55 mm, 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.85 mm, 0.9 mm, 0.95 mm, or 1 mm, but not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0058] In some optional instances, the precursor solution droplets are removed after standing in the calcium chloride solution for 1 to 2 hours to obtain the gel microspheres. For example, the time may be 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h, 1.5 h, 1.6 h, 1.7 h, 1.8 h, 1.9 h, or 2.0 h, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0059] In some optional instances, the drying temperature is low-temperature air drying to constant weight at 35-45°C, for example, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C or 45°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0060] Secondly, the present invention provides a top-layer remediation material for contaminated soil prepared by the preparation method described in the first aspect.

[0061] When the material is applied to soil contaminated with heavy metals, the carboxyl groups of the outer sodium alginate gel layer can initially adsorb and enrich heavy metal ions through ion exchange and surface complexation. Subsequently, the heavy metal ions diffuse into the interior of the gel microspheres and come into contact with the composite modified support embedded inside. The PEI grafted on the composite modified support contains a large number of amine groups, which can rapidly capture heavy metal ions (Cd) through coordination complexation. 2+ Pb 2+ (etc.) to firmly adsorb onto the surface of the composite modified carrier. At the same time, the hydroxyapatite generated in situ on the composite modified carrier plays a role in precipitation and ion exchange. Phosphate ions can form phosphate precipitates with extremely low solubility with heavy metal ions; at the same time, heavy metal ions can also enter the crystal lattice of hydroxyapatite to replace calcium ions, thereby fixing the heavy metals in the hydroxyapatite crystal lattice.

[0062] If the composite modified support contains only PEI, although it can affect heavy metal ions (Cd) through the amino groups on the PEI molecular chain... 2 + Pb 2+ PEI produces strong complexation adsorption, but complexation adsorption is a reversible chemical coordination. In complex soil environments, heavy metal ions adsorbed by PEI will be re-desorbed and released, which is not effective for the long-term stable fixation of heavy metal ions.

[0063] If the composite modified carrier contains only hydroxyapatite, although it can achieve long-term stable fixation of heavy metals through precipitation and ion exchange, hydroxyapatite powder lacks the ability to capture heavy metal ions quickly and in large quantities, making it difficult to efficiently enrich heavy metal ions dispersed in the soil. Therefore, in soils with low or unevenly distributed pollutant concentrations, the fixation efficiency of hydroxyapatite is low.

[0064] Through the synergistic effect of PEI and hydroxyapatite, heavy metal ions captured by PEI complexation will form local enrichment on the surface of the composite modified carrier, which can more effectively react with the phosphate of hydroxyapatite to form chemically stable heavy metal phosphate minerals. At the same time, the locally enriched heavy metal ions are also more likely to enter the crystal structure of hydroxyapatite through ion exchange, effectively reducing the mobility and bioavailability of heavy metals in the soil.

[0065] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0066] This invention first involves co-pyrolyzing agricultural waste with porous minerals, followed by modification with a silane coupling agent solution to obtain a modified carrier with active amino groups on its surface. Subsequently, the modified carrier is grafted with a polyethyleneimine solution, and calcium dihydrogen phosphate and calcium chloride solutions are simultaneously added under alkaline conditions to generate hydroxyapatite in situ on the surface of the aminated carrier, resulting in a composite modified carrier. Finally, the composite modified carrier is mixed with a sodium alginate solution and then added dropwise to a calcium chloride solution for gel solidification, ultimately preparing a top-layer remediation material for contaminated soil. The top-layer remediation material for contaminated soil prepared by this invention can treat heavy metals (Cd)... 2+ Pb 2+ This technology enables rapid capture and long-term stable fixation of heavy metals, effectively solving problems in existing farmland soil heavy metal remediation, such as insufficient fixation capacity of single materials for heavy metals, poor long-term stability, inconvenient application of powder materials, and easy generation of secondary pollution. Attached Figure Description

[0067] Figure 1 The following is a process flow diagram of the preparation process of the top-layer remediation material for contaminated soil provided in Examples 1-5 of the present invention;

[0068] Figure 2 The infrared spectrum of the composite modified support prepared in Example 1 of this invention;

[0069] Figure 3 This is a scanning electron microscope image of the composite modified support prepared in Example 1 of the present invention;

[0070] Figure 4 This is a physical image of the top-layer remediation material for contaminated soil prepared in Example 1 of the present invention. Detailed Implementation

[0071] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.

[0072] Example 1

[0073] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0074] (1) After crushing the straw to 20 mesh, mix it with bentonite at a mass ratio of 3:1. Under nitrogen atmosphere, heat it to 500℃ at a heating rate of 10℃ / min and keep it at that temperature for 2.5h to complete the pyrolysis. Then grind it and pass it through a 100-mesh sieve to obtain the composite carrier.

[0075] 3-Aminopropyltriethoxysilane was added to an aqueous ethanol solution (the volume ratio of ethanol to deionized water was 9:1), and after mixing evenly, a silane coupling agent solution with a mass fraction of 3 wt% was obtained. The composite support was dispersed in the silane coupling agent solution at a ratio of 1 g: 10 mL, and the mixture was heated under reflux at 60 °C for 6 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 60 °C for 12 h to obtain the modified support.

[0076] (2) The modified carrier was dispersed in a 5 g / L polyethyleneimine solution. The mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution was 1:0.1. The reaction was stirred and heated at 40 °C for 2 h. After the reaction was completed, the mixture was filtered and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 60 °C for 12 h to obtain the amination carrier.

[0077] The amination carrier was dispersed in deionized water to obtain a carrier dispersion with a mass fraction of 5 wt%. A 1 mol / L sodium hydroxide solution was added dropwise to the carrier dispersion to adjust its pH to 9. While stirring at 300 rpm, a 0.5 mol / L calcium dihydrogen phosphate solution and a 1 mol / L calcium chloride solution were simultaneously added dropwise at a rate of 0.5 mL / min. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion was 0.3:1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the calcium chloride in the calcium chloride solution was 1:1.3. After the addition was complete, the mixture was stirred and heated at 60 °C for 12 h. After the reaction was completed, the mixture was allowed to stand at 60 °C for 8 h. Finally, the mixture was filtered, and the residue was washed multiple times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 80 °C for 12 h to obtain the composite modified carrier.

[0078] (3) The composite modified carrier was added to a sodium alginate solution with a mass fraction of 2 wt%, and the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution was 2:1. After mixing evenly, a precursor solution was obtained. The precursor solution was dropped into a calcium chloride solution with a mass fraction of 3 wt% through a syringe with a needle aperture of 0.5 mm. The precursor solution droplets were left to stand in the calcium chloride solution for 2 hours and then removed to obtain gel microspheres. The gel microspheres were washed three times with deionized water and air-dried at 35°C to constant weight to obtain the top layer remediation material for the contaminated soil.

[0079] Figure 2 The image shows the infrared spectrum of the composite modified support prepared in this embodiment. As can be seen from the image, at 3400 cm⁻¹... -1 The absorption peak at 2950 cm⁻¹ is attributed to the stretching vibrations of OH and NH, originating from the hydroxyl groups produced by pyrolysis and the amino groups introduced by 3-aminopropyltriethoxysilane and polyethyleneimine; -1 and 2850cm -1 The absorption peak at 950 cm⁻¹ is attributed to the asymmetric and symmetric stretching vibrations of the methylene group, originating from the alkyl chain of 3-aminopropyltriethoxysilane and polyethyleneimine; -1 The absorption peak at 1650 cm⁻¹ is attributed to the stretching vibration of Si-O-Si, indicating that the silane coupling agent was successfully grafted onto the support surface; -1 The absorption peak at 1100~1000 cm⁻¹ is attributed to the bending vibration of the amino group (-NH₂), indicating that polyethyleneimine was successfully grafted onto the support surface; -1 The absorption peaks appearing within the matrix are attributed to the stretching vibrations of PO, indicating that hydroxyapatite was successfully loaded onto the composite modified support.

[0080] Figure 3The image shows a scanning electron microscope (SEM) image of the composite modified carrier prepared in this embodiment. As can be seen from the image, the composite modified carrier exhibits an irregular blocky accumulation morphology, and large-sized pore structures can be clearly seen. At the same time, loaded particles can be clearly seen on the surface of the carrier and in the pores, and the particles do not completely block the pore structure of the carrier.

[0081] Figure 4 The image shows a physical picture of the top-layer remediation material for contaminated soil prepared in this embodiment. As can be seen from the picture, the remediation material embedded in sodium alginate gel exhibits highly uniform spherical particles with a relatively uniform particle size distribution and a smooth surface. No composite modified carrier embedded inside is exposed.

[0082] Example 2

[0083] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0084] (1) After crushing the straw to 25 mesh, mix it with bentonite at a mass ratio of 3.5:1. Under nitrogen atmosphere, heat it to 520℃ at a heating rate of 10.5℃ / min and keep it at that temperature for 2.2h to complete the pyrolysis. Then grind it and pass it through a 120 mesh sieve to obtain the composite carrier.

[0085] 3-Aminopropyltriethoxysilane was added to an aqueous ethanol solution (ethanol to deionized water volume ratio of 9:1) and mixed thoroughly to obtain a silane coupling agent solution with a mass fraction of 3.5 wt%. The composite support was dispersed in the silane coupling agent solution at a ratio of 1 g: 12 mL. The mixture was heated under reflux at 62 °C for 5.5 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 65 °C for 10 h to obtain the modified support.

[0086] (2) The modified carrier was dispersed in a 6 g / L polyethyleneimine solution, with a mass ratio of 1:0.12 between the modified carrier and the polyethyleneimine in the polyethyleneimine solution. The reaction was stirred and heated at 42 °C for 1.8 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 65 °C for 10 h to obtain the amination carrier.

[0087] The amination carrier was dispersed in deionized water to obtain a carrier dispersion with a mass fraction of 6 wt%. A 1 mol / L sodium hydroxide solution was added dropwise to the carrier dispersion to adjust its pH to 9.2. At a stirring speed of 350 rpm, a 0.6 mol / L calcium dihydrogen phosphate solution and a 1.2 mol / L calcium chloride solution were simultaneously added dropwise to the carrier dispersion at a rate of 0.6 mL / min. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion was 0.35:1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the calcium chloride in the calcium chloride solution was 1:1.35. After the addition was complete, the mixture was stirred and heated at 65 °C for 10 h. After the reaction was completed, the mixture was allowed to stand at 65 °C for 7.5 h. Finally, the mixture was filtered, and the residue was washed multiple times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 85 °C for 11 h to obtain the composite modified carrier.

[0088] (3) The composite modified carrier was added to a sodium alginate solution with a mass fraction of 2.5 wt%, and the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution was 2.5:1. After mixing evenly, a precursor solution was obtained. The precursor solution was dropped into a calcium chloride solution with a mass fraction of 3.5 wt% through a syringe with a needle aperture of 0.6 mm. The precursor solution droplets were left to stand in the calcium chloride solution for 1.8 h and then removed to obtain gel microspheres. The gel microspheres were washed three times with deionized water and air-dried at 38 °C to constant weight to obtain the top layer remediation material for the contaminated soil.

[0089] Example 3

[0090] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0091] (1) After crushing the rice husks to 30 mesh, mix them with attapulgite clay at a mass ratio of 4:1. Heat the mixture to 550°C at a heating rate of 11°C / min under a nitrogen atmosphere and keep it at that temperature for 2 hours to complete the pyrolysis. Then grind the mixture and pass it through a 150-mesh sieve to obtain the composite carrier.

[0092] 3-Aminopropyltriethoxysilane was added to an aqueous ethanol solution (the volume ratio of ethanol to deionized water was 9:1), and after mixing evenly, a silane coupling agent solution with a mass fraction of 4 wt% was obtained. The composite support was dispersed in the silane coupling agent solution at a ratio of 1 g: 15 mL, and the mixture was heated under reflux at 65 °C for 5 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 70 °C for 8 h to obtain the modified support.

[0093] (2) The modified carrier was dispersed in a 7 g / L polyethyleneimine solution. The mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution was 1:0.15. The reaction was stirred and heated at 45 °C for 1.5 h. After the reaction was completed, the mixture was filtered and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 70 °C for 8 h to obtain the amination carrier.

[0094] The amination carrier was dispersed in deionized water to obtain a carrier dispersion with a mass fraction of 7 wt%. A 1 mol / L sodium hydroxide solution was added dropwise to the carrier dispersion to adjust its pH to 9.5. At a stirring speed of 400 rpm, a 0.7 mol / L calcium dihydrogen phosphate solution and a 1.3 mol / L calcium chloride solution were simultaneously added dropwise to the carrier dispersion at a rate of 0.7 mL / min. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion was 0.4:1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the calcium chloride in the calcium chloride solution was 1:1.4. After the addition was complete, the mixture was stirred and heated at 70 °C for 8 h. After the reaction was completed, the mixture was allowed to stand at 70 °C for 7 h. Finally, the mixture was filtered, and the residue was washed multiple times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 90 °C for 10 h to obtain the composite modified carrier.

[0095] (3) The composite modified carrier was added to a sodium alginate solution with a mass fraction of 3 wt%, and the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution was 3:1. After mixing evenly, a precursor solution was obtained. The precursor solution was dropped into a calcium chloride solution with a mass fraction of 4 wt% through a syringe with a needle aperture of 0.7 mm. The precursor solution droplets were left to stand in the calcium chloride solution for 1.5 h and then removed to obtain gel microspheres. The gel microspheres were washed three times with deionized water and air-dried at 40 °C to constant weight to obtain the top layer remediation material for the contaminated soil.

[0096] Example 4

[0097] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0098] (1) After crushing the rice husks to 35 mesh, mix them with attapulgite clay at a mass ratio of 4.5:1. Under a nitrogen atmosphere, heat the mixture to 580℃ at a heating rate of 11.5℃ / min and keep it at that temperature for 1.8h to complete the pyrolysis. Then grind the mixture and pass it through a 180-mesh sieve to obtain the composite carrier.

[0099] 3-Aminopropyltriethoxysilane was added to an aqueous ethanol solution (the volume ratio of ethanol to deionized water was 9:1), and after mixing evenly, a silane coupling agent solution with a mass fraction of 4.5 wt% was obtained. The composite support was dispersed in the silane coupling agent solution at a ratio of 1 g: 18 mL, and the mixture was heated under reflux at 68 °C for 4.5 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 75 °C for 7 h to obtain the modified support.

[0100] (2) The modified carrier was dispersed in an 8 g / L polyethyleneimine solution, with a mass ratio of 1:0.18 between the modified carrier and the polyethyleneimine in the polyethyleneimine solution. The reaction was stirred and heated at 48 °C for 1.2 h. After the reaction was completed, the mixture was filtered and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 75 °C for 7 h to obtain the amination carrier.

[0101] The amination carrier was dispersed in deionized water to obtain a carrier dispersion with a mass fraction of 8 wt%. A 1 mol / L sodium hydroxide solution was added dropwise to the carrier dispersion to adjust its pH to 9.8. At a stirring speed of 450 rpm, a 0.8 mol / L calcium dihydrogen phosphate solution and a 1.4 mol / L calcium chloride solution were simultaneously added dropwise to the carrier dispersion at a rate of 0.8 mL / min. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion was 0.45:1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the calcium chloride in the calcium chloride solution was 1:1.45. After the addition was complete, the mixture was stirred and heated at 75 °C for 7 h. After the reaction was completed, the mixture was allowed to stand at 75 °C for 6.5 h. Finally, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 95 °C for 9 h to obtain the composite modified carrier.

[0102] (3) The composite modified carrier was added to a sodium alginate solution with a mass fraction of 3.5 wt%, and the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution was 3.5:1. After mixing evenly, a precursor solution was obtained. The precursor solution was dropped into a calcium chloride solution with a mass fraction of 4.5 wt% through a syringe with a needle aperture of 0.8 mm. The precursor solution droplets were left to stand in the calcium chloride solution for 1.2 h and then removed to obtain gel microspheres. The gel microspheres were washed three times with deionized water and air-dried at 42℃ to constant weight to obtain the top layer remediation material for contaminated soil.

[0103] Example 5

[0104] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0105] (1) After crushing the wood chips to 40 mesh, mix them with zeolite at a mass ratio of 5:1. Under a nitrogen atmosphere, heat the mixture to 600°C at a heating rate of 12°C / min and keep it at that temperature for 1.5 hours to complete the pyrolysis. Then grind the mixture and pass it through a 200-mesh sieve to obtain the composite carrier.

[0106] 3-Aminopropyltriethoxysilane was added to an aqueous ethanol solution (the volume ratio of ethanol to deionized water was 9:1), and after mixing evenly, a silane coupling agent solution with a mass fraction of 5 wt% was obtained. The composite support was dispersed in the silane coupling agent solution at a ratio of 1 g: 20 mL, and the mixture was heated under reflux at 70 °C for 4 h. After the reaction was completed, the mixture was filtered, and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 80 °C for 6 h to obtain the modified support.

[0107] (2) The modified carrier was dispersed in a 10 g / L polyethyleneimine solution. The mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution was 1:0.2. The reaction was stirred and heated at 50 °C for 1 h. After the reaction was completed, the mixture was filtered and the residue was washed several times with deionized water until the filtrate was neutral. The residue was then dried under vacuum at 80 °C for 6 h to obtain the amination carrier.

[0108] The amination carrier was dispersed in deionized water to obtain a carrier dispersion with a mass fraction of 10 wt%. A 1 mol / L sodium hydroxide solution was added dropwise to the carrier dispersion to adjust its pH to 10. At a stirring speed of 500 rpm, a 1 mol / L calcium dihydrogen phosphate solution and a 1.5 mol / L calcium chloride solution were simultaneously added dropwise to the carrier dispersion at a rate of 1 mL / min. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion was 0.5:1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the calcium chloride in the calcium chloride solution was 1:1.5. After the addition was complete, the mixture was stirred and heated at 80 °C for 6 h. After the reaction was completed, the mixture was allowed to stand at 80 °C for 6 h. Finally, the mixture was filtered, and the residue was washed multiple times with deionized water until the filtrate was neutral. The residue was then vacuum dried at 100 °C for 8 h to obtain the composite modified carrier.

[0109] (3) The composite modified carrier was added to a sodium alginate solution with a mass fraction of 4 wt%, and the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution was 4:1. After mixing evenly, a precursor solution was obtained. The precursor solution was dropped into a calcium chloride solution with a mass fraction of 5 wt% through a syringe with a needle hole diameter of 1 mm. The precursor solution droplets were left to stand in the calcium chloride solution for 1 h and then removed to obtain gel microspheres. The gel microspheres were washed 3 times with deionized water and air-dried at 45℃ to constant weight to obtain the top layer remediation material of the contaminated soil.

[0110] Example 6

[0111] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution is adjusted to 1:0.05. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0112] Example 7

[0113] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution is adjusted to 1:0.3. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0114] Example 8

[0115] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amylated carrier in the carrier dispersion is adjusted to 0.1:1. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0116] Example 9

[0117] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amylated carrier in the carrier dispersion is adjusted to 0.8:1. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0118] Example 10

[0119] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to calcium chloride in the calcium chloride solution is adjusted to 1:1. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0120] Example 11

[0121] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (2), the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to calcium chloride in the calcium chloride solution is adjusted to 1:1.8. Other operating steps and process parameters are exactly the same as in Embodiment 1.

[0122] Example 12

[0123] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (3), the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution is adjusted to 1:1. Other operation steps and process parameters are exactly the same as in Embodiment 1.

[0124] Example 13

[0125] This embodiment provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Embodiment 1 is that in step (3), the mass ratio of the composite modified carrier to sodium alginate in the sodium alginate solution is adjusted to 6:1. Other operation steps and process parameters are exactly the same as in Embodiment 1.

[0126] Comparative Example 1

[0127] This comparative example provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Example 1 is that in step (1), the modification with silane coupling agent is omitted, and the composite carrier obtained in step (1) is directly dispersed in a polyethyleneimine solution. Other operation steps and process parameters are exactly the same as in Example 1.

[0128] Comparative Example 2

[0129] This comparative example provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Example 1 is that in step (2), the polyethyleneimine modification is omitted, and the modified carrier obtained in step (1) is dispersed in deionized water to obtain a carrier dispersion. Then, the pH value is adjusted, and calcium dihydrogen phosphate solution and calcium chloride solution are added dropwise to the carrier dispersion to generate hydroxyapatite in situ on the surface of the modified carrier. Other operation steps and process parameters are exactly the same as in Example 1.

[0130] Comparative Example 3

[0131] This comparative example provides a method for preparing a top-layer remediation material for contaminated soil. The difference from Example 1 is that the in-situ generation of hydroxyapatite is omitted. The amination carrier obtained in step (2) is added to sodium alginate solution, mixed evenly, and then dropped into calcium chloride solution for gel solidification. Other operation steps and process parameters are exactly the same as in Example 1.

[0132] The heavy metal ion saturation adsorption capacity, heavy metal removal rate, and leaching concentration of the remediation materials prepared in the examples and comparative examples were tested. The specific test steps included:

[0133] (1) Saturated adsorption capacity of heavy metal ions

[0134] Prepare the target heavy metal (Cd) separately 2+ Pb 2+The standard stock solution (1000 mg / L, source solutions were Cd(NO3)2·4H2O and Pb(NO3)2) was diluted with deionized water to prepare initial solutions of different concentrations (50 mg / L, 100 mg / L, 200 mg / L, 500 mg / L, 800 mg / L and 1000 mg / L). 2+ Standard stock solution and Pb 2+ The pH of the standard stock solution was adjusted to 5.0 ± 0.1 using dilute HNO3 or NaOH solution.

[0135] The repair materials prepared in the examples and comparative examples were passed through a 100-mesh sieve and dried at 105°C to constant weight. 50 mg of the dried sample was placed in an Erlenmeyer flask, and 50 mL of initial solutions of different concentrations (solid-liquid ratio 1:1000) were added to each flask. The flasks were sealed and placed in a constant-temperature shaker at 25±1°C and 150 rpm for 24 h. Immediately after shaking, the solution was filtered through a 0.45 μm microporous membrane. The residual concentration of heavy metal ions in the filtrate was determined by inductively coupled plasma mass spectrometry (ICP-MS). The equilibrium adsorption capacity of heavy metal ions for different initial solutions was calculated using the following formula:

[0136]

[0137] Among them: Q e To determine the equilibrium adsorption capacity (mg / g), C0 represents the initial concentration of heavy metal ions in the solution (mg / L). e V represents the residual concentration of heavy metal ions in the filtrate (mg / L), V represents the initial solution volume (L), and m represents the sample mass (g).

[0138] Summarize different initial concentrations of C e The corresponding equilibrium adsorption amount Q e A series of (C) were obtained e Q e Data points were collected, adsorption isotherms were plotted, and the saturated adsorption capacity Q was fitted using the Langmuir model. max :

[0139]

[0140] Among them, K L is the Langmuir constant (L / mg).

[0141] (2) Heavy metal removal rate

[0142] Take clean soil (pH=6.5±0.5, organic matter content 2.5%), crush it and pass it through a 2mm sieve, add Pb(NO3)2 solution and Cd(NO3)2 solution to adjust the Pb content in the soil. 2+ Concentration up to 800±50 mg / kg, Cd 2+The concentration was increased to 50±5 mg / kg, and the mixture was aged at 25℃ for 30 days to age the heavy metals.

[0143] The remediation materials prepared in the examples and comparative examples were added at 5 wt% of the soil mass. The soil moisture content was adjusted to 60% of field capacity with deionized water and incubated in a constant temperature incubator (25±1℃, protected from light). On the 7th day, soil samples from a depth of 0-15 cm were collected, freeze-dried, ground through a 100-mesh sieve, and digested using the EPA 3052 method (HCl-HNO3-HF microwave digestion). The total Pb and total Cd concentrations in the soil were determined by ICP-MS, and the heavy metal removal rate was calculated using the following formula:

[0144]

[0145] Where C0 is the initial concentration of heavy metals in the soil, C t The concentration of heavy metals in the soil after 7 days of treatment.

[0146] (3) Leaching concentration

[0147] Take clean soil (pH=6.5±0.5, organic matter content 2.5%), crush it and pass it through a 2mm sieve, add Pb(NO3)2 solution and Cd(NO3)2 solution to adjust the Pb content in the soil. 2+ Concentration up to 800±50 mg / kg, Cd 2+ The concentration was increased to 50±5 mg / kg, and the mixture was aged at 25℃ for 30 days to age the heavy metals.

[0148] The remediation materials prepared in the examples and comparative examples were added at 5 wt% of the soil mass. The soil moisture content was adjusted to 60% of the field capacity with deionized water and kept in a constant temperature incubator (25±1℃, protected from light). Soil samples were taken from a depth of 0-15 cm on the 7th day.

[0149] Dissolve 5.7 mL of glacial acetic acid in 500 mL of deionized water, add 64.3 mL of NaOH solution (1 mol / L), and bring the volume to 1 L. Adjust the pH to 2.88 ± 0.05 to obtain the extractant. Place 5.0 g of soil sample in a centrifuge tube, add 100 mL of the extractant, place the tube on a shaker, and shake continuously at 30 ± 2 rpm for 18 h. After shaking, let stand for 10 min, then filter under vacuum using a 0.45 μm microporous membrane. Collect the filtrate, acidify the filtrate with 5 wt% dilute nitric acid, and determine the total Pb leaching concentration and total Cd leaching concentration in the filtrate using inductively coupled plasma mass spectrometry.

[0150] The test results are shown in Table 1.

[0151] Table 1

[0152] <![CDATA[Pb 2+ Saturated adsorption capacity (mg / g) <![CDATA[Cd 2+ Saturated adsorption capacity (mg / g) Total Pb removal rate (%) Total Cd removal rate (%) Total Pb leaching concentration (mg / L) Total Cd leaching concentration (mg / L) Example 1 278.2 198.3 91.28 86.52 0.69 0.32 Example 2 280.6 200.1 91.75 87.13 0.65 0.27 Example 3 285.4 202.7 92.35 88.24 0.68 0.24 Example 4 282.9 201.5 92.10 87.45 0.62 0.25 Example 5 276.8 197.9 90.96 85.18 0.64 0.36 Example 6 220.5 170.2 82.34 79.17 0.91 0.52 Example 7 250.8 185.6 85.25 81.73 0.87 0.46 Example 8 260.2 190.3 83.42 80.58 0.93 0.49 Example 9 240.7 180.8 84.15 81.21 0.91 0.47 Example 10 255.6 188.9 82.91 78.95 0.96 0.51 Example 11 238.4 179.7 83.83 80.64 0.94 0.48 Example 12 210.8 165.4 81.56 77.23 1.02 0.55 Example 13 265.3 192.6 84.82 80.50 0.99 0.53 Comparative Example 1 152.6 140.8 71.32 68.45 1.45 1.07 Comparative Example 2 115.5 80.3 65.24 62.87 1.52 1.18 Comparative Example 3 183.2 167.9 77.15 71.62 1.75 1.32

[0153] The test data from Examples 1, 6, and 7 show that the saturated adsorption capacity and removal rate of Examples 6 and 7 are lower than those of Example 1, while the leaching concentration is higher. This is because the amount of PEI used in Example 6 was too low, resulting in insufficient amine grafting density on the surface of the modified carrier. This reduced the number of heavy metal complexation sites on the remediation material, directly affecting the adsorption capacity. Furthermore, it reduced the number of nucleation sites available for phosphate ion adsorption on the carrier surface, hindering the uniform deposition of hydroxyapatite on the carrier surface and affecting the fixation effect of the remediation material on heavy metal ions. In Example 7, the amount of PEI used was too high, leading to excessive PEI accumulation and deposition in the pores of the carrier, blocking the pore structure and reducing the specific surface area and porosity of the carrier. This hindered the transport and diffusion of heavy metal ions into the carrier, affecting the adsorption and fixation effect of the remediation material on heavy metal ions.

[0154] The test data from Examples 1, 8, and 9 show that the saturated adsorption capacity and removal rate of Examples 8 and 9 are lower than those of Example 1, while the leaching concentration is higher. This is because the amount of calcium dihydrogen phosphate used in Example 8 was too low, resulting in insufficient concentration of phosphate ions participating in the reaction and too little hydroxyapatite generated in situ on the surface of the aminated carrier, significantly affecting the precipitation and ion exchange of the remediation material. In Example 9, the amount of calcium dihydrogen phosphate used was too high, leading to excessive phosphate and calcium ions undergoing homogeneous nucleation in the solution, generating a large amount of free hydroxyapatite. This free hydroxyapatite deposits on the carrier surface during static aging, blocking the pore structure of the carrier and affecting the adsorption and diffusion of heavy metal ions inside the carrier.

[0155] The test data from Examples 1, 10, and 11 show that the saturated adsorption capacity and removal rate of Examples 10 and 11 are lower than those of Example 1, while the leaching concentration is higher. This is because the amount of calcium chloride used in Example 10 is relatively insufficient, leading to incomplete precipitation reaction. The resulting hydroxyapatite has poor crystallinity and an incomplete structure, making it easily soluble in the soil environment and affecting the precipitation and ion exchange effects of the remediation material. In Example 11, the amount of calcium chloride used is too high, resulting in an excess of calcium ions. This accelerates the nucleation rate of calcium phosphate, causing a large number of fine crystal nuclei to rapidly and homogeneously nucleate in the solution, rather than undergoing orderly heterogeneous growth on the surface of the amylated support. Ultimately, this results in a large amount of hydroxyapatite existing in a free form, unable to be effectively loaded onto the amylated support, thus affecting the fixation effect of the remediation material.

[0156] The test data from Examples 1, 12, and 13 show that the saturated adsorption capacity and removal rate of Examples 12 and 13 are lower than those of Example 1, while the leaching concentration is higher. This is because the amount of composite modified carrier used in Example 12 is too low, resulting in insufficient loading of the composite modified carrier in the gel microspheres, which directly affects the adsorption and fixation effect of the remediation material on heavy metal ions. In Example 13, the amount of composite modified carrier used is too high, while the amount of sodium alginate used is relatively low, resulting in a gel network that is too fragile. After application to the soil, it is prone to deformation and breakage, causing leakage of the internally embedded composite modified carrier and affecting the long-term effect of the composite modified carrier in the soil.

[0157] The test data from Example 1 and Comparative Example 1 show that the saturated adsorption capacity and removal rate of Comparative Example 1 are lower than those of Example 1, while the leaching concentration is higher. This is because Comparative Example 1 omits the silane coupling agent modification step, resulting in PEI molecular chains only physically adhering to the carrier surface. The bond is not strong, and they are prone to detachment during subsequent processing and use. This reduces the number of amino complexation sites on the surface of the repair material, making it unable to provide effective nucleation sites for heterogeneous nucleation of hydroxyapatite, which seriously affects the ability of the repair material to capture and fix heavy metals.

[0158] The test data from Example 1 and Comparative Example 2 show that the saturated adsorption capacity and removal rate of Comparative Example 2 are lower than those of Example 1, while the leaching concentration is higher. This is because Comparative Example 2 omits the polyethyleneimine modification step, resulting in the removal of heavy metal complexation sites in the remediation material. This prevents the rapid capture and enrichment of heavy metal ions and also hinders the orderly heterogeneous nucleation of hydroxyapatite on the carrier surface. Consequently, hydroxyapatite undergoes random homogeneous nucleation in the solution, failing to firmly attach to the carrier. This leads to a significant decrease in the adsorption capacity and long-term fixation effect of the remediation material.

[0159] The test data from Example 1 and Comparative Example 3 show that the saturated adsorption capacity and removal rate of Comparative Example 3 are lower than those of Example 1, while the leaching concentration is higher. This is because Comparative Example 3 omits the in-situ generation step of hydroxyapatite, and the remediation material relies solely on the amino complexation of PEI to fix heavy metals. However, complexation adsorption is reversible, and in complex soil environments, the heavy metal ions adsorbed by PEI will be re-desorbed and released, resulting in poor long-term stable fixation of the remediation material and high leaching toxicity.

[0160] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a top-layer remediation material for contaminated soil, characterized in that, The preparation method includes: (I) Agricultural waste is crushed and mixed with porous minerals, pyrolyzed under an inert atmosphere, ground and sieved to obtain a composite carrier. The agricultural waste includes any one or at least two of rice husks, straw or sawdust, and the porous minerals include any one or at least two of bentonite, attapulgite or zeolite. The composite carrier is dispersed in a silane coupling agent solution, heated under reflux, filtered, washed and dried to obtain a modified carrier. (II) The modified carrier is dispersed in a polyethyleneimine solution, wherein the mass ratio of the modified carrier to the polyethyleneimine in the polyethyleneimine solution is 1:(0.1~0.2). After stirring and heating to react, the mixture is filtered, washed, and dried to obtain an amination carrier. The amination carrier is dispersed in deionized water to obtain a carrier dispersion. Sodium hydroxide solution is added dropwise to the carrier dispersion to adjust its pH value to 9~10. Then, under stirring conditions, calcium dihydrogen phosphate solution and calcium chloride solution are added dropwise to the carrier dispersion simultaneously. The mass ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to the amination carrier in the carrier dispersion is (0.3~0.5):1, and the molar ratio of calcium dihydrogen phosphate in the calcium dihydrogen phosphate solution to calcium chloride in the calcium chloride solution is 1:(1.3~1.5). After the addition is complete, the mixture is stirred and heated to react. After standing and aging, the mixture is filtered, washed, and dried to obtain a composite modified carrier. (III) The composite modified carrier is added to the sodium alginate solution and mixed to obtain a precursor solution. The mass ratio of the composite modified carrier to the sodium alginate in the sodium alginate solution is (2~4):

1. The precursor solution is dropped into the calcium chloride solution and gelled to form gel microspheres. After filtration, washing and drying, the top layer remediation material for contaminated soil is obtained.

2. The preparation method according to claim 1, characterized in that, In step (I), the agricultural waste is crushed to 20-40 mesh; The mass ratio of the agricultural waste to the porous mineral is (3~5):

1.

3. The preparation method according to claim 1, characterized in that, In step (I), the heating rate of the pyrolysis is 10~12℃ / min; The pyrolysis temperature is 500~600℃; The holding time for pyrolysis is 1.5~2.5h; The mesh size of the sieve used for grinding and sieving is 100-200 mesh.

4. The preparation method according to claim 1, characterized in that, In step (I), the silane coupling agent solution comprises a silane coupling agent and an aqueous ethanol solution; The mass fraction of the silane coupling agent in the silane coupling agent solution is 3-5 wt%. The silane coupling agent includes 3-aminopropyltriethoxysilane; The ratio of the composite carrier to the silane coupling agent solution is 1g:(10~20)mL; The temperature of the heating reflux reaction is 60~70℃; The heating and reflux reaction time is 4-6 hours.

5. The preparation method according to claim 1, characterized in that, In step (II), the concentration of polyethyleneimine in the polyethyleneimine solution is 5~10 g / L; The temperature at which the modified carrier reacts with the polyethyleneimine solution under stirring and heating conditions is 40-50°C. The modified carrier and the polyethyleneimine solution are reacted by stirring and heating for 1 to 2 hours.

6. The preparation method according to claim 1, characterized in that, In step (II), the mass fraction of the aminated carrier in the carrier dispersion is 5-10 wt%. At a stirring speed of 300-500 rpm, 0.5-1 mol / L of calcium dihydrogen phosphate solution and 1-1.5 mol / L of calcium chloride solution are simultaneously added dropwise to the carrier dispersion. The dropping rate of the calcium dihydrogen phosphate solution and the calcium chloride solution is 0.5~1 mL / min.

7. The preparation method according to claim 1, characterized in that, In step (II), the temperature for stirring and heating the carrier dispersion, calcium dihydrogen phosphate solution, and calcium chloride solution is 60~80℃. The reaction time for stirring and heating the carrier dispersion, calcium dihydrogen phosphate solution, and calcium chloride solution is 6-12 hours. The temperature for static aging is 60~80℃; The settling and aging time is 6-8 hours.

8. The preparation method according to claim 1, characterized in that, In step (III), the sodium alginate solution has a mass fraction of 2-4 wt%.

9. The preparation method according to claim 1, characterized in that, In step (III), the mass fraction of the calcium chloride solution is 3-5 wt%. The precursor solution is dripped into the calcium chloride solution using a syringe with a pinhole diameter of 0.5~1mm; After the precursor solution droplets are left to stand in the calcium chloride solution for 1-2 hours, they are removed to obtain the gel microspheres.

10. A top-layer remediation material for contaminated soil prepared by the preparation method according to any one of claims 1 to 9.

Images