An organic-inorganic composite remediation material for passivating heavy metals in farmland soil and a preparation method thereof

Abstract

The application belongs to the technical field of soil remediation, and relates to an organic-inorganic composite remediation material for heavy metal passivation of farmland soil and a preparation method thereof, which comprises the following steps: dissolving magnesium salt and aluminum salt in deionized water to obtain a mixed salt solution; adding sodium hydroxide solution into the mixed salt solution, and then performing heating reaction after the addition is completed to obtain LDH slurry; adding deionized water into the LDH slurry to obtain LDH dispersion; simultaneously adding calcium chloride solution and disodium hydrogen phosphate solution into the LDH dispersion, adding alkali liquor to adjust the pH value in the process of adding, and then performing heating reaction after the addition is completed to obtain hydroxyapatite loaded LDH powder; adding citric acid into sodium alginate solution to perform reaction, and then obtaining modified sodium alginate solution; dispersing the hydroxyapatite loaded LDH powder in the modified sodium alginate solution to obtain a suspension; mixing calcium chloride solution and aluminum chloride solution to obtain a crosslinking solution; adding the suspension into the crosslinking solution to perform crosslinking and solidification, and then performing drying to obtain the organic-inorganic composite remediation material.

Description

Technical Field

[0001] This invention belongs to the field of soil remediation technology, and relates to an organic-inorganic composite remediation material for heavy metal passivation in farmland soil and its preparation method. Background Technology

[0002] Heavy metals such as cadmium, lead, and arsenic enter the soil environment through wastewater irrigation, atmospheric deposition, and the application of chemical fertilizers and pesticides. They not only affect crop growth and quality but also threaten human health through the food chain, posing a serious challenge to agricultural product safety and ecological security. Therefore, developing efficient, stable, and environmentally friendly soil heavy metal passivation and remediation technologies has become an important issue in the field of agricultural environmental protection.

[0003] Currently, common soil heavy metal remediation technologies mainly include physical, chemical, and biological methods. Among them, chemical passivation remediation shows good application potential in farmland remediation due to its relatively simple operation, low cost, and rapid effect. The core of this technology lies in adding passivating materials to contaminated soil. Through adsorption, precipitation, complexation, and ion exchange, it reduces the available content and bioavailability of heavy metals in the soil, thereby reducing the absorption and accumulation of heavy metals by crops. Existing passivating materials are mainly divided into inorganic, organic, and organic-inorganic composite materials. Inorganic materials such as clay minerals, phosphates, and lime have advantages such as large specific surface area and strong adsorption capacity, but their effectiveness is often significantly affected by factors such as soil pH, and their long-term stability is sometimes insufficient, which may lead to the reactivation of heavy metals. Organic materials such as biochar and humic acid are rich in various functional groups and can fix heavy metals through complexation and improve soil structure, but their mechanical strength and resistance to degradation are sometimes weak, and their use alone may be insufficient to cope with complex pollution or achieve long-term stabilization. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide an organic-inorganic composite remediation material for heavy metal passivation in farmland soil and its preparation method.

[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 an organic-inorganic composite remediation material for heavy metal passivation of farmland soil, the preparation method comprising:

[0007] (I) Dissolve magnesium salt and aluminum salt in deionized water to obtain a mixed salt solution; under stirring and nitrogen atmosphere, add sodium hydroxide solution dropwise to the mixed salt solution, and heat the solution to react after the addition is complete to obtain LDH slurry;

[0008] (II) Add deionized water to the LDH slurry to obtain an LDH dispersion; under stirring conditions, add calcium chloride solution and disodium hydrogen phosphate solution dropwise to the LDH dispersion simultaneously. During the dropwise addition, add alkali solution to control the pH value. After the dropwise addition is completed, heat the reaction, and then centrifuge, wash, dry, grind, and sieve to obtain calcium hydroxyphosphate-loaded LDH powder.

[0009] (III) Citric acid was added to sodium alginate solution under stirring and heating conditions to carry out the reaction. During the reaction, alkali solution was added dropwise to control the pH value. After the reaction was completed, a modified sodium alginate solution was obtained. The calcium hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution and ultrasonically dispersed to obtain a suspension.

[0010] (IV) Mix calcium chloride solution and aluminum chloride solution to obtain crosslinking liquid; drip the suspension into the crosslinking liquid using a syringe, and obtain gel microspheres after crosslinking and solidification; dry the gel microspheres to obtain the organic-inorganic composite repair material.

[0011] In the preparation method provided by this invention, firstly, calcium hydroxyphosphate supported on LDH powder is prepared. LDH (layered bimetallic hydroxide) itself has excellent adsorption and ion exchange capabilities. Based on this, calcium hydroxyphosphate is in situ supported through a co-precipitation reaction. The introduced phosphate ions can form stable phosphate precipitates with heavy metals such as lead and cadmium, achieving efficient and long-term fixation of heavy metal ions. Subsequently, sodium alginate is modified with citric acid, introducing a large number of carboxyl groups. The carboxyl groups dissociate into negatively charged carboxylate ions in the soil environment, which are beneficial for the fixation of Cd. 2+ Pb 2+ Heavy metal cations possess extremely strong complexing abilities. Finally, calcium hydroxyphosphate-loaded LDH powder was dispersed in a modified sodium alginate solution and then dropped into a calcium-aluminum mixed crosslinking liquid to form gel microspheres. After drying, an organic-inorganic composite remediation material was obtained. Compared to powder materials, microspheres are easier to apply in farmland, reducing dust and improving ease of use and safety. Simultaneously, the gel microsphere structure possesses certain mechanical strength and stability, helping to delay the decomposition of the internally embedded calcium hydroxyphosphate-loaded LDH powder, preventing its loss with the soil, and providing a more durable soil remediation effect.

[0012] First, this invention synthesizes LDH under nitrogen protection. The layers are composed of magnesium and aluminum cations carrying a permanent positive charge, and OH groups are exchangeable between the layers. - NO3 -The presence of anions enables the capture of negatively charged heavy metal anions through ion exchange and surface adsorption. Using LDH as a carrier, hydroxyapatite is generated in situ on the LDH surface and between its layers via a co-precipitation process. The high specific surface area and regular layered structure of LDH provide an ideal attachment carrier for the nucleation and growth of hydroxyapatite, effectively preventing excessive aggregation of hydroxyapatite particles and increasing its contact area with heavy metal ions. Hydroxyapatite not only possesses adsorption properties itself, but also provides PO4... 3- Can be with Cd 2+ Pb 2+ Cationic heavy metals form phosphate precipitates with extremely low solubility. By combining LDH with hydroxyapatite, the material simultaneously possesses the exchange adsorption capacity of LDH for anionic heavy metals and the precipitation and fixation capacity of hydroxyapatite for cationic heavy metals.

[0013] Subsequently, sodium alginate was modified with citric acid. The esterification reaction between multiple carboxyl groups on the citric acid molecule and the hydroxyl groups on the sodium alginate chain introduced a large number of carboxyl groups into the sodium alginate molecular chain. These carboxyl groups can dissociate into negatively charged -COO groups in the soil environment. - , for Cd 2+ Pb 2+ Heavy metal cations possess extremely strong complexing abilities. Finally, hydroxyapatite-loaded LDH powder was gel-encapsulated via an ionic cross-linking reaction using sodium alginate and a cross-linking solution. This process serves two purposes: firstly, gel encapsulation provides a slow-release effect, preventing the LDH powder from directly contacting the soil and agglomerating, thus extending its effectiveness in the soil; secondly, the citric acid-modified gel network itself can complex heavy metal ions in the soil through carboxyl groups, allowing these ions to diffuse further through the gel coating and contact the LDH powder.

[0014] As a preferred technical solution of the present invention, in step (I), the magnesium salt and the aluminum salt are prepared according to Mg 2+ And Al 3 + The molar ratio is (2.5~3.5):1 dissolved in deionized water. For example, it can be 2.5:1, 2.6:1, 2.7:1, 2.8:1, 2.9:1, 3.0:1, 3.1:1, 3.2:1, 3.3:1, 3.4:1 or 3.5:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] Al 3+ The mole fraction of metal ions in the laminate directly determines the positive charge density of the laminate. This invention specifies that Mg... 2+ And Al3+ The molar ratio is (2.5~3.5):1, which ensures that the charge density of the layers is moderate, thus forming a regular and ordered layered crystal structure.

[0016] When Al 3+ When there is a relative excess of Al, it leads to an excessively high positive charge density in the laminations, generating strong interlaminar repulsion, causing crystal structure distortion, and making it difficult to grow into a regular layered structure. Simultaneously, excessive Al... 3+ Under alkaline conditions, amorphous Al(OH)3 gels are formed, which affect the nucleation and growth of LDH. The final product is a mixture of LDH and Al(OH)3, which leads to a decrease in the specific surface area and ion exchange capacity of the material.

[0017] When Mg 2+ When there is a relative excess of Mg, the excessively low charge density results in insufficient electrostatic attraction between the layers, making it difficult to effectively support and stabilize the layered structure, leading to low crystallinity of the product. Simultaneously, excess Mg... 2+ In an alkaline environment, Mg(OH)2 crystal phase will be generated, and the final product is a mixture of LDH and Mg(OH)2, which will also lead to a decrease in the specific surface area and ion exchange capacity of the material.

[0018] In some optional instances, the Mg in the mixed salt solution 2+ And Al 3+ The molar concentration is 0.15~0.25 mol / L, for example, it can be 0.15 mol / L, 0.16 mol / L, 0.17 mol / L, 0.18 mol / L, 0.19 mol / L, 0.2 mol / L, 0.21 mol / L, 0.22 mol / L, 0.23 mol / L, 0.24 mol / L or 0.25 mol / L, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] As a preferred technical solution of the present invention, in step (I), sodium hydroxide solution is added dropwise to the mixed salt solution at a stirring speed of 300-500 rpm, at room temperature and under a nitrogen atmosphere. For example, the speed can be 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm or 500 rpm, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] In some alternative examples, the molar concentration of the sodium hydroxide solution is 1 to 2 mol / L, for example, it can be 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L or 2.0 mol / L, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0021] In some alternative examples, the dropping rate of the sodium hydroxide solution is 1 to 3 mL / min, for example, 1.0 mL / min, 1.2 mL / min, 1.4 mL / min, 1.6 mL / min, 1.8 mL / min, 2.0 mL / min, 2.2 mL / min, 2.4 mL / min, 2.6 mL / min, 2.8 mL / min or 3.0 mL / min, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0022] In some optional instances, the OH in the sodium hydroxide solution - With Mg in the mixed salt solution 2+ And Al 3+ The total molar ratio is (2.5~3):1, for example, it can be 2.5:1, 2.55:1, 2.6:1, 2.65:1, 2.7:1, 2.75:1, 2.8:1, 2.85:1, 2.9:1, 2.95:1 or 3.0:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] In some optional examples, after the addition is complete, the resulting reaction solution is heated to 65-75°C, for example, 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, or 75°C, and stirred at 300-500 rpm for 12-18 hours. The stirring speed can be 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, or 500 rpm, and the reaction time can be 12 hours, 12.5 hours, 13 hours, 13.5 hours, 14 hours, 14.5 hours, 15 hours, 15.5 hours, 16 hours, 16.5 hours, 17 hours, 17.5 hours, or 18 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, in step (II), the solid content of the LDH dispersion is 2~3wt%, for example, it can be 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt% or 3.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] In some optional instances, calcium chloride solution and disodium hydrogen phosphate solution are simultaneously added dropwise to the LDH dispersion at a stirring speed of 600-800 rpm and at room temperature. For example, the values ​​could be 600 rpm, 620 rpm, 640 rpm, 660 rpm, 680 rpm, 700 rpm, 720 rpm, 740 rpm, 760 rpm, 780 rpm, or 800 rpm, but are not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] In the preparation of calcium hydroxyphosphate, calcium chloride and disodium hydrogen phosphate solutions are simultaneously added dropwise to the LDH dispersion. Calcium hydroxyphosphate is then generated in situ on the LDH surface via chemical co-precipitation and loaded onto the surface and interlayer of the LDH. 2+ With HPO4 2- In an alkaline environment, HPO4 reacts. 2- It will further transform into PO4 3- Subsequently with Ca 2+ and OH in the solution - They combine and crystallize to form solid calcium hydroxyphosphate.

[0027] The role of LDH is to act as a carrier in the reaction process. LDH has a huge specific surface area, and its plates are composed of positively charged metal hydroxides with a surface rich in -OH groups. During the synthesis process, the Ca in the solution... 2+ and PO4 3- They are preferentially adsorbed onto the surface of LDH laminae. The hydroxyl groups on the LDH surface not only provide nucleation sites, but can also react with PO4. 3- Hydrogen bonding occurs, promoting the epitaxial growth of calcium hydroxyphosphate crystals on LDH as a carrier. The crystal growth is guided by surface chemistry, which not only effectively prevents the aggregation of calcium hydroxyphosphate particles, but also enables the formation of strong interfacial chemical bonds between calcium hydroxyphosphate and LDH, thus achieving a stable and uniform loading with a binding strength far higher than that of physical adsorption.

[0028] A significant synergistic effect exists between calcium hydroxyphosphate (CHP) and LDH. Structurally, the regular layered structure and high specific surface area of ​​LDH provide an ideal carrier for the nucleation and growth of CHP, effectively preventing the aggregation of CHP nanoparticles and allowing them to be uniformly dispersed and loaded on the surface and interlayer of LDH. This exposes more active sites and increases the contact area between CHP and heavy metal ions. Functionally, the combination of the two can achieve efficient immobilization of different types of heavy metal ions. LDH mainly immobilizes anionic heavy metals such as chromate and arsenate through its interlayer anion exchange and surface adsorption. The loaded CHP can release phosphate ions in the soil, forming stable phosphate minerals with divalent cationic heavy metals such as lead and cadmium in the soil solution, achieving long-term solidification.

[0029] In some alternative examples, the molar concentration of the calcium chloride solution is 0.1 to 0.12 mol / L, for example, 0.1 mol / L, 0.102 mol / L, 0.104 mol / L, 0.106 mol / L, 0.108 mol / L, 0.111 mol / L, 0.112 mol / L, 0.114 mol / L, 0.116 mol / L, 0.118 mol / L, or 0.12 mol / L, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0030] In some alternative examples, the molar concentration of the disodium hydrogen phosphate solution is 0.06 to 0.08 mol / L, for example, it can be 0.06 mol / L, 0.062 mol / L, 0.064 mol / L, 0.066 mol / L, 0.068 mol / L, 0.07 mol / L, 0.072 mol / L, 0.074 mol / L, 0.076 mol / L, 0.078 mol / L or 0.08 mol / L, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0031] In some optional instances, the Ca in the calcium chloride solution 2+ With the PO4 in the disodium hydrogen phosphate solution 3- The molar ratio is (1.65~1.7):1, for example, it can be 1.65:1, 1.66:1, 1.67:1, 1.68:1, 1.69:1 or 1.7:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] In some optional instances, the Ca in the calcium chloride solution 2+ With Al in the mixed salt solution 3+The molar ratio is (1~1.5):1, for example, it can be 1.0:1, 1.05:1, 1.1:1, 1.15:1, 1.2:1, 1.25:1, 1.3:1, 1.35:1, 1.4:1, 1.45:1 or 1.5:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] This invention limits the Ca in calcium chloride solution 2+ With Al in mixed salt solution 3+ The molar ratio of Ca to LDH is (1~1.5):1, which affects the loading of calcium hydroxyphosphate onto LDH. 2+ When insufficient, the loading of calcium hydroxyphosphate is too low, failing to fully utilize its chemical fixation capacity. When Ca... 2+ When there is an excess, the nucleation of hydroxyapatite will preferentially occur in the solution rather than on the LDH surface. The formed hydroxyapatite crystals will aggregate in the solution, forming free aggregates, which reduces the amount of hydroxyapatite loaded on the LDH.

[0034] As a preferred technical solution of the present invention, in step (II), during the process of adding calcium chloride solution and disodium hydrogen phosphate solution, an alkaline solution is added to control the pH value of the reaction solution within the range of 9 to 10.5. For example, it can be 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4 or 10.5, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0035] In some optional examples, after the addition is complete, the resulting reaction solution is heated to 60-70°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, and stirred at 300-500 rpm for 4-6 hours. The stirring speed can be 300 rpm, 320 rpm, 340 rpm, 360 rpm, 380 rpm, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, or 500 rpm, and the reaction time can be 4.0 h, 4.2 h, 4.4 h, 4.6 h, 4.8 h, 5.0 h, 5.2 h, 5.4 h, 5.6 h, 5.8 h, or 6.0 h, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0036] In this invention, the addition of calcium chloride solution and disodium hydrogen phosphate solution is carried out at room temperature. After the addition is complete, heating is performed to induce a co-precipitation reaction to generate calcium hydroxyphosphate. This is because the simultaneous addition of calcium chloride solution and disodium hydrogen phosphate solution at room temperature slows down the reaction rate between calcium ions and phosphate ions, ensuring slow and uniform heterogeneous nucleation on the LDH surface and between layers. This facilitates the uniform attachment of the generated calcium hydroxyphosphate crystal nuclei to the LDH surface and between layers, avoiding excessively rapid local reactions due to high temperatures, which could lead to the aggregation of calcium hydroxyphosphate crystals and the formation of a large number of free calcium hydroxyphosphate particles. After the addition is complete, the reaction solution is heated to 60-70°C to further promote the growth and maturation of the calcium hydroxyphosphate crystals, thereby improving the crystallinity and structural stability of the product.

[0037] In some alternative instances, the drying temperature is 70 to 90°C, for example, 70°C, 72°C, 74°C, 76°C, 78°C, 80°C, 82°C, 84°C, 86°C, 88°C, or 90°C, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0038] In some optional instances, the drying time is 10 to 12 hours, for example, 10 hours, 10.2 hours, 10.4 hours, 10.6 hours, 10.8 hours, 11 hours, 11.2 hours, 11.4 hours, 11.6 hours, 11.8 hours, or 12 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0039] In some optional instances, the sieve mesh size is 300 to 400 mesh, for example, 300 mesh, 310 mesh, 320 mesh, 330 mesh, 340 mesh, 350 mesh, 360 mesh, 370 mesh, 380 mesh, 390 mesh or 400 mesh, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0040] As a preferred technical solution of the present invention, in step (III), the mass fraction of the sodium alginate solution is 2~3wt%, for example, it can be 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt% or 3.0wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] In some optional instances, the amount of citric acid added is 10 to 12 wt% of the mass of sodium alginate in the sodium alginate solution, for example, it can be 10 wt%, 10.2 wt%, 10.4 wt%, 10.6 wt%, 10.8 wt%, 11 wt%, 11.2 wt%, 11.4 wt%, 11.6 wt%, 11.8 wt%, or 12 wt%, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0042] The purpose of modifying sodium alginate with citric acid is to introduce a large number of carboxyl groups into the sodium alginate molecular chain through esterification between the carboxyl groups of citric acid and the hydroxyl groups of the sodium alginate molecular chain. This increases the electronegativity of the sodium alginate molecular chain, allowing the modified sodium alginate to react with positively charged CaO during subsequent cross-linking. 2+ Crosslinking with positively charged calcium hydroxyphosphate-supported LDH powder forms a denser and more stable three-dimensional gel network, improving the mechanical strength of the gel microspheres.

[0043] This invention specifically limits the amount of citric acid added to 10-12 wt% of the sodium alginate in the sodium alginate solution. When the amount of citric acid is too low, the esterification reaction is incomplete, resulting in too few carboxyl groups introduced onto the sodium alginate molecular chain, and the mechanical strength of the prepared gel microspheres cannot be significantly improved. When the amount of citric acid is too high, the pH of the reaction solution will decrease rapidly during the reaction, easily causing the sodium alginate to gel; furthermore, a large amount of unreacted free citric acid will remain in the modified sodium alginate solution, reacting with CaO in subsequent crosslinking processes. 2+ Competition affects Ca 2+ Normal cross-linking with sodium alginate eventually leads to a loose gel network and a decrease in the mechanical strength of the gel microspheres.

[0044] In some alternative examples, the reaction temperature of the citric acid with the sodium alginate solution is 55~65°C, for example, 55°C, 56°C, 57°C, 58°C, 59°C, 60°C, 61°C, 62°C, 63°C, 64°C or 65°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0045] In some alternative examples, the reaction time of the citric acid with the sodium alginate solution is 2 to 3 hours, for example, 2.0 hours, 2.1 hours, 2.2 hours, 2.3 hours, 2.4 hours, 2.5 hours, 2.6 hours, 2.7 hours, 2.8 hours, 2.9 hours, or 3.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0046] In some optional instances, an alkaline solution is added dropwise during the reaction to control the pH of the reaction solution within the range of 5 to 6, such as 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, or 6.0, but not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0047] Sodium alginate and Ca 2+ Crosslinking depends on the -COO on its molecular chain. - Group, Ca 2+ With the -COO on sodium alginate - Ion coordination occurs, forming a stable "egg-box" structured gel. The carboxyl groups of citric acid undergo esterification with the hydroxyl groups of sodium alginate, introducing a large number of carboxyl groups onto the sodium alginate molecular chain. During the addition of citric acid, the pH of the reaction solution needs to be controlled within the range of 5-6. If the pH of the reaction solution is too low, a large number of the original -COONa groups on the sodium alginate molecular chain will be protonated to form -COOH, which in turn leads to the reaction with Ca... 2+ Cross-linked -COO - The significantly reduced number of functional groups leads to a decrease in the efficiency of the cross-linking reaction, making it impossible to form a robust gel network structure. During citric acid modification, maintaining the pH of the reaction solution consistently within the range of 5-6 allows the -COOH groups introduced by citric acid to dissociate into -COO groups. - Ca 2+ Cross-linked -COO - Increasing the number of functional groups can lead to the formation of a more compact and stronger gel network.

[0048] As a preferred technical solution of the present invention, in step (III), the mass ratio of the calcium hydroxyphosphate-loaded LDH powder to the sodium alginate in the sodium alginate solution is 1:(3~5), for example, it can be 1:3.0, 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 or 1:5.0, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0049] This invention specifically limits the mass ratio of calcium hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution to 1:(3~5). When the amount of calcium hydroxyphosphate-loaded LDH powder is too high, the powder is prone to agglomeration, forming stress concentrations within the gel microspheres. Furthermore, if the amount of sodium alginate is relatively low, the resulting gel network structure is too sparse, leading to low mechanical strength and brittleness of the gel microspheres. Conversely, when the amount of sodium alginate is too high, the amount of calcium hydroxyphosphate-loaded LDH powder is relatively low, directly affecting the material's ability to fix heavy metal ions.

[0050] In some alternative instances, the power of the ultrasonic dispersion is 300 to 500 W, for example, 300 W, 320 W, 340 W, 360 W, 380 W, 400 W, 420 W, 440 W, 460 W, 480 W or 500 W, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0051] In some optional instances, the ultrasonic dispersion time is 40 to 60 minutes, for example, 40 minutes, 42 minutes, 44 minutes, 46 minutes, 48 ​​minutes, 50 minutes, 52 minutes, 54 minutes, 56 minutes, 58 minutes or 60 minutes, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0052] As a preferred technical solution of the present invention, in step (IV), the mass fraction of the calcium chloride solution is 3.5~4.5wt%, for example, it can be 3.5wt%, 3.6wt%, 3.7wt%, 3.8wt%, 3.9wt%, 4.0wt%, 4.1wt%, 4.2wt%, 4.3wt%, 4.4wt%, or 4.5wt%, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0053] In some optional instances, the aluminum chloride solution has a mass fraction of 1.5 to 2.5 wt%, for example, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, or 2.5 wt%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0054] In some optional instances, the Ca in the calcium chloride solution 2+ With Al in the aluminum chloride solution 3+ The molar ratio is (3.5~4.5):1, for example, it can be 3.5:1, 3.6:1, 3.7:1, 3.8:1, 3.9:1, 4.0:1, 4.1:1, 4.2:1, 4.3:1, 4.4:1 or 4.5:1, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0055] This invention adds aluminum chloride to the traditional calcium chloride crosslinking method. The effect is twofold: firstly, aluminum ions undergo hydrolysis in aqueous solution to generate positively charged [Al(OH)₂]₃. 2+ Complex ions can react with the -COO groups on the sodium alginate molecular chain. -The bonding of functional groups forms multi-point crosslinks, which can strengthen the initial gel network formed by calcium ion crosslinking, thereby significantly enhancing the rigidity and stability of the gel network and reducing shrinkage and cracking during the drying process. On the other hand, aluminum chloride itself is also a precursor component for the preparation of LDH, which can guide the uniform formation and growth of LDH crystal nuclei within the gel network, ensuring a strong chemical bond between the gel matrix and LDH, rather than a simple physical coating.

[0056] When Ca 2+ With Al 3+ When the molar ratio is too high, excess Ca 2+ This can lead to excessive cross-linking of the sodium alginate network, making the gel structure too dense and brittle, and prone to cracking during drying; when Ca 2+ With Al 3+ When the molar ratio is too low, excess Al 3+ Vigorous hydrolysis will occur, leading to excessive cross-linking and hindering Ca2+. 2+ The formation of a uniform initial gel network reduces the mechanical strength of the gel structure.

[0057] In some optional instances, the needle orifice diameter of the syringe is 0.8 to 1.2 mm, for example, it can be 0.8 mm, 0.85 mm, 0.9 mm, 0.95 mm, 1.0 mm, 1.05 mm, 1.1 mm, 1.15 mm or 1.2 mm, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0058] In some optional instances, after the suspension is dropped into the crosslinking liquid, it is allowed to stand and mature in the crosslinking liquid for 2 to 3 hours, for example, 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0059] As a preferred technical solution of the present invention, in step (IV), the drying process includes:

[0060] The first hot air drying is carried out at a first hot air temperature and a first hot air velocity. Then, the temperature is increased to a second hot air temperature and the air velocity is decreased to a second hot air velocity for second hot air drying until constant weight is achieved.

[0061] In some optional instances, the temperature of the first hot air is 40~45°C, for example, it can be 40°C, 40.5°C, 41°C, 41.5°C, 42°C, 42.5°C, 43°C, 43.5°C, 44°C, 44.5°C or 45°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0062] In some optional instances, the first hot air velocity is 2 to 3 m / s, for example, it can be 2.0 m / s, 2.1 m / s, 2.2 m / s, 2.3 m / s, 2.4 m / s, 2.5 m / s, 2.6 m / s, 2.7 m / s, 2.8 m / s, 2.9 m / s or 3.0 m / s, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0063] In some optional instances, the first hot air drying time 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.

[0064] In some optional instances, the second hot air temperature is 50~60°C, for example, it can be 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C or 60°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0065] In some optional instances, the second hot air velocity is 0.5 to 1 m / s, for example, it can be 0.5 m / s, 0.55 m / s, 0.6 m / s, 0.65 m / s, 0.7 m / s, 0.75 m / s, 0.8 m / s, 0.85 m / s, 0.9 m / s, 0.95 m / s or 1 m / s, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0066] This invention employs a two-stage gradient hot air drying process, which maximizes the protection of the material's structural stability while ensuring drying efficiency and preventing particle cracking caused by drying stress concentration, ultimately resulting in a product with a complete structure and stable performance.

[0067] Secondly, the present invention provides an organic-inorganic composite remediation material for heavy metal passivation of farmland soil prepared by the preparation method described in the first aspect, wherein the organic-inorganic composite remediation material comprises gel microspheres and hydroxycalcium phosphate-loaded LDH powder loaded therein.

[0068] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0069] In the preparation method provided by this invention, firstly, calcium hydroxyphosphate supported on LDH powder is prepared. LDH (layered bimetallic hydroxide) itself has excellent adsorption and ion exchange capabilities. Based on this, calcium hydroxyphosphate is in situ supported through a co-precipitation reaction. The introduced phosphate ions can form stable phosphate precipitates with heavy metals such as lead and cadmium, achieving efficient and long-term fixation of heavy metal ions. Subsequently, sodium alginate is modified with citric acid, introducing a large number of carboxyl groups. The carboxyl groups dissociate into negatively charged carboxylate ions in the soil environment, which are beneficial for the fixation of Cd. 2+ Pb 2+ Heavy metal cations possess extremely strong complexing abilities. Finally, calcium hydroxyphosphate-loaded LDH powder was dispersed in a modified sodium alginate solution and then dropped into a calcium-aluminum mixed crosslinking liquid to form gel microspheres. After drying, an organic-inorganic composite remediation material was obtained. Compared to powder materials, microspheres are easier to apply in farmland, reducing dust and improving ease of use and safety. Simultaneously, the gel microsphere structure possesses certain mechanical strength and stability, helping to delay the decomposition of the internally embedded calcium hydroxyphosphate-loaded LDH powder, preventing its loss with the soil, and providing a more durable soil remediation effect. Attached Figure Description

[0070] Figure 1 The following is a flow chart of the preparation process of the organic-inorganic repair materials provided in Examples 1-5 of this invention;

[0071] Figure 2 This is a scanning electron microscope image of the calcium hydroxyphosphate-supported LDH powder prepared in Example 1 of this invention;

[0072] Figure 3 The infrared spectrum of the calcium hydroxyphosphate-supported LDH powder prepared in Example 1 of this invention is shown. Detailed Implementation

[0073] 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.

[0074] Example 1

[0075] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0076] (1) Mix magnesium nitrate hexahydrate and aluminum nitrate nonahydrate according to Mg 2+ And Al 3+ The molar ratio of Mg to Mg2+ is 2.5:1 when dissolved in deionized water to obtain a mixed salt solution. The Mg2+ in the mixed salt solution... 2+ And Al 3+ The molar concentration was 0.15 mol / L; under a stirring speed of 300 rpm, room temperature, and a nitrogen atmosphere, a 1 mol / L sodium hydroxide solution was added dropwise to the mixed salt solution at a rate of 1 mL / min. The OH- in the sodium hydroxide solution... - With Mg in mixed salt solution 2+ And Al 3+ The total molar ratio was 2.5:1. After the addition was complete, the resulting reaction solution was heated to 65°C and stirred at 300 rpm for 18 hours to obtain LDH slurry.

[0077] (2) Deionized water was added to the LDH slurry to obtain an LDH dispersion with a solid content of 2 wt%. At a stirring speed of 600 rpm and room temperature, 0.1 mol / L calcium chloride solution and 0.06 mol / L disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The Ca in the calcium chloride solution... 2+ With PO4 in disodium hydrogen phosphate solution 3- The molar ratio is 1.65:1, and the Ca in the calcium chloride solution... 2+ With Al in mixed salt solution 3+ The molar ratio was 1:1. During the dropwise addition, 1 mol / L sodium hydroxide solution was added to control the pH of the reaction solution at 9. After the dropwise addition was completed, the resulting reaction solution was heated to 60°C and stirred at 300 rpm for 6 hours. Then, it was centrifuged, washed, dried at 70°C for 12 hours, ground, and passed through a 300-mesh sieve to obtain calcium hydroxyphosphate-supported LDH powder.

[0078] (3) Under stirring and heating conditions of 55°C, citric acid was added to a 2wt% sodium alginate solution for reaction. The amount of citric acid added was 10wt% of the mass of sodium alginate in the sodium alginate solution. During the reaction, 1mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution at 5. After 3h of reaction, a modified sodium alginate solution was obtained. Hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution. The mass ratio of hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution was 1:3. The suspension was obtained by ultrasonic dispersion for 60min under an ultrasonic power of 300W.

[0079] (4) Mix 3.5 wt% calcium chloride solution and 1.5 wt% aluminum chloride solution. The Ca in the calcium chloride solution... 2+With Al in aluminum chloride solution 3+ The molar ratio of the components was 3.5:1. After mixing evenly, a cross-linking solution was obtained. The suspension was dripped into the cross-linking solution through a syringe with a needle aperture of 0.8 mm. After the droplets were added to the cross-linking solution, the solution was left to stand and mature for 3 hours to obtain gel microspheres.

[0080] The gel microspheres were dried with hot air at 40℃ and 2m / s for 2.5h, and then dried with hot air at 50℃ and 0.5m / s until constant weight was obtained, thus obtaining the organic-inorganic composite repair material.

[0081] Figure 2 The image shows a scanning electron microscope (SEM) image of the calcium hydroxyphosphate-loaded LDH powder prepared in this embodiment. As can be seen from the image, the calcium hydroxyphosphate-loaded LDH powder exhibits a layered stacked structure with calcium hydroxyphosphate uniformly attached to its surface. This is because, under alkaline conditions, calcium ions and phosphorus ions are deposited on the surface of the LDH layers and react in situ, forming a uniform composite structure. This morphology is beneficial to increasing the specific surface area and active adsorption sites of the material, thereby enhancing its passivation ability for heavy metals.

[0082] Figure 3 The image shows the infrared spectrum of the calcium hydroxyphosphate-supported LDH powder prepared in this embodiment. As can be seen from the image, at 3400 cm⁻¹... -1 The broad peak at 1050 cm⁻¹ is attributed to the stretching vibrations of OH bonds in the material, originating from hydroxyl groups in the LDH layers, interlayer water, and calcium hydroxyphosphate. -1 The strong peak at that location belongs to PO4. 3- The stretching vibration. 600cm -1 and 560cm -1 The absorption at each location corresponds to PO4. 3- The bending vibration and the vibration of Mg-O-Al in the LDH layer.

[0083] Example 2

[0084] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0085] (1) Mix magnesium nitrate hexahydrate and aluminum nitrate nonahydrate according to Mg 2+ And Al 3+ The molar ratio of Mg to Mg2+ is 2.8:1 when dissolved in deionized water to obtain a mixed salt solution. The Mg2+ in the mixed salt solution... 2+ And Al 3+The molar concentration was 0.18 mol / L; under a stirring speed of 350 rpm, room temperature, and nitrogen atmosphere, a 1.2 mol / L sodium hydroxide solution was added dropwise to the mixed salt solution at a rate of 1.5 mL / min. The OH- ions in the sodium hydroxide solution... - With Mg in mixed salt solution 2+ And Al 3+ The total molar ratio was 2.6:1. After the addition was complete, the resulting reaction solution was heated to 68°C and stirred at 350 rpm for 16 hours to obtain LDH slurry.

[0086] (2) Deionized water was added to the LDH slurry to obtain an LDH dispersion with a solid content of 2.2 wt%. At a stirring speed of 650 rpm and room temperature, 0.105 mol / L calcium chloride solution and 0.065 mol / L disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The Ca in the calcium chloride solution... 2+ With PO4 in disodium hydrogen phosphate solution 3- The molar ratio is 1.66:1, and the Ca in the calcium chloride solution... 2+ With Al in mixed salt solution 3+ The molar ratio was 1.2:1. During the dropwise addition, 1 mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution within the range of 9.5. After the dropwise addition was completed, the resulting reaction solution was heated to 62℃ and stirred at 350 rpm for 5.5 h. Then, it was centrifuged, washed, dried at 75℃ for 11.5 h, ground, and passed through a 320-mesh sieve to obtain calcium hydroxyphosphate-supported LDH powder.

[0087] (3) Under stirring and heating conditions of 58°C, citric acid was added to a 2.2 wt% sodium alginate solution for reaction. The amount of citric acid added was 10.5 wt% of the mass of sodium alginate in the sodium alginate solution. During the reaction, 1 mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution at 5.2. After 2.8 h of reaction, a modified sodium alginate solution was obtained. Hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution. The mass ratio of hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution was 1:3.5. The suspension was obtained by ultrasonic dispersion at 350 W ultrasonic power for 55 min.

[0088] (4) When a 3.8 wt% calcium chloride solution and a 1.8 wt% aluminum chloride solution are mixed, the Ca in the calcium chloride solution... 2+ With Al in aluminum chloride solution 3+ The molar ratio of the two components was 3.8:1. After mixing evenly, a cross-linking solution was obtained. The suspension was dripped into the cross-linking solution through a syringe with a needle aperture of 0.9 mm. After the droplets were added to the cross-linking solution, the mixture was allowed to stand and mature in the cross-linking solution for 2.8 h to obtain gel microspheres.

[0089] The gel microspheres were dried with hot air at 41℃ and 2.2 m / s for 2.2 h, and then dried with hot air at 52℃ and 0.6 m / s until constant weight was obtained, thus obtaining the organic-inorganic composite repair material.

[0090] Example 3

[0091] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0092] (1) Mix magnesium nitrate hexahydrate and aluminum nitrate nonahydrate according to Mg 2+ And Al 3+ The Mg in the mixed salt solution is dissolved in deionized water at a molar ratio of 3:1 to obtain a mixed salt solution. 2+ And Al 3+ The molar concentration was 0.2 mol / L; under a stirring speed of 400 rpm, room temperature, and nitrogen atmosphere, a 1.5 mol / L sodium hydroxide solution was added dropwise to the mixed salt solution at a rate of 2 mL / min. The OH- in the sodium hydroxide solution... - With Mg in mixed salt solution 2+ And Al 3+ The total molar ratio was 2.7:1. After the addition was complete, the resulting reaction solution was heated to 70°C and stirred at 400 rpm for 15 hours to obtain LDH slurry.

[0093] (2) Deionized water was added to the LDH slurry to obtain an LDH dispersion with a solid content of 2.5 wt%. At a stirring speed of 700 rpm and room temperature, 0.11 mol / L calcium chloride solution and 0.07 mol / L disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The Ca in the calcium chloride solution... 2+ With PO4 in disodium hydrogen phosphate solution 3- The molar ratio is 1.67:1, and the Ca in the calcium chloride solution... 2+ With Al in mixed salt solution 3+ The molar ratio was 1.3:1. During the dropwise addition, 1 mol / L sodium hydroxide solution was added dropwise to control the pH of the reaction solution at 9.5. After the dropwise addition was completed, the resulting reaction solution was heated to 65°C and stirred at 400 rpm for 5 h. Then, it was centrifuged, washed, dried at 80°C for 11 h, ground, and passed through a 350-mesh sieve to obtain calcium hydroxyphosphate-supported LDH powder.

[0094] (3) Under stirring and heating conditions of 60°C, citric acid was added to a 2.5 wt% sodium alginate solution for reaction. The amount of citric acid added was 11 wt% of the mass of sodium alginate in the sodium alginate solution. During the reaction, 1 mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution at 5.5. After 2.5 h of reaction, a modified sodium alginate solution was obtained. Hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution. The mass ratio of hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution was 1:4. The suspension was obtained by ultrasonic dispersion at 400 W ultrasonic power for 50 min.

[0095] (4) When a 4 wt% calcium chloride solution and a 2 wt% aluminum chloride solution are mixed, the Ca in the calcium chloride solution... 2+ With Al in aluminum chloride solution 3+ The molar ratio of the components is 4:1. After mixing evenly, a crosslinking solution is obtained. The suspension is dripped into the crosslinking solution through a syringe with a needle aperture of 1 mm. After the droplets are dripped into the crosslinking solution, the solution is left to stand and mature for 2.5 h to obtain gel microspheres.

[0096] The gel microspheres were dried with hot air at 42℃ and 2.5 m / s for 2 hours, and then dried with hot air at 55℃ and 0.7 m / s until constant weight was obtained, thus obtaining the organic-inorganic composite repair material.

[0097] Example 4

[0098] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0099] (1) Mix magnesium nitrate hexahydrate and aluminum nitrate nonahydrate according to Mg 2+ And Al 3+ The molar ratio of Mg to Mg is 3.2:1 when dissolved in deionized water to obtain a mixed salt solution. The Mg in the mixed salt solution... 2+ And Al 3+ The molar concentration was 0.22 mol / L; under a stirring speed of 450 rpm, room temperature, and nitrogen atmosphere, a 1.8 mol / L sodium hydroxide solution was added dropwise to the mixed salt solution at a rate of 2.5 mL / min. The OH- ions in the sodium hydroxide solution... - With Mg in mixed salt solution 2+ And Al 3+ The total molar ratio was 2.8:1. After the addition was complete, the resulting reaction solution was heated to 72°C and stirred at 450 rpm for 13 hours to obtain LDH slurry.

[0100] (2) Deionized water was added to the LDH slurry to obtain an LDH dispersion with a solid content of 2.8 wt%. At a stirring speed of 750 rpm and room temperature, 0.115 mol / L calcium chloride solution and 0.075 mol / L disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The Ca in the calcium chloride solution... 2+ With PO4 in disodium hydrogen phosphate solution 3- The molar ratio is 1.68:1, and the Ca in the calcium chloride solution... 2+ With Al in mixed salt solution 3+ The molar ratio was 1.4:1. During the dropwise addition, 1 mol / L sodium hydroxide solution was added dropwise to control the pH of the reaction solution at 10. After the dropwise addition was completed, the resulting reaction solution was heated to 68°C and stirred at 450 rpm for 4.5 h. Then, it was centrifuged, washed, dried at 85°C for 10.5 h, ground, and passed through a 380-mesh sieve to obtain calcium hydroxyphosphate-supported LDH powder.

[0101] (3) Under stirring and heating conditions of 62°C, citric acid was added to a 2.8 wt% sodium alginate solution for reaction. The amount of citric acid added was 11.5 wt% of the mass of sodium alginate in the sodium alginate solution. During the reaction, 1 mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution at 5.8. After 2.2 h of reaction, a modified sodium alginate solution was obtained. Hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution. The mass ratio of hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution was 1:4.5. The suspension was obtained by ultrasonic dispersion at 450 W ultrasonic power for 45 min.

[0102] (4) Mix 4.2 wt% calcium chloride solution and 2.2 wt% aluminum chloride solution. The Ca in the calcium chloride solution... 2+ With Al in aluminum chloride solution 3+ The molar ratio of the two components was 4.2:1. After mixing evenly, a cross-linking solution was obtained. The suspension was dripped into the cross-linking solution through a syringe with a needle aperture of 1.1 mm. After the droplets were added into the cross-linking solution, the mixture was allowed to stand and mature in the cross-linking solution for 2.2 h to obtain gel microspheres.

[0103] The gel microspheres were dried with hot air at 43℃ and 2.8 m / s for 1.8 h, and then dried with hot air at 58℃ and 0.8 m / s until constant weight was obtained, thus obtaining the organic-inorganic composite repair material.

[0104] Example 5

[0105] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, such as... Figure 1 As shown, the preparation method specifically includes the following steps:

[0106] (1) Mix magnesium nitrate hexahydrate and aluminum nitrate nonahydrate according to Mg 2+ And Al 3+ The molar ratio of Mg to Mg is 3.5:1 when dissolved in deionized water to obtain a mixed salt solution. The Mg in the mixed salt solution... 2+ And Al 3+ The molar concentration was 0.25 mol / L; under a stirring speed of 500 rpm, room temperature, and a nitrogen atmosphere, a 2 mol / L sodium hydroxide solution was added dropwise to the mixed salt solution at a rate of 3 mL / min. The OH- in the sodium hydroxide solution... - With Mg in mixed salt solution 2+ And Al 3+ The total molar ratio was 3:1. After the addition was complete, the resulting reaction solution was heated to 75°C and stirred at 500 rpm for 12 hours to obtain LDH slurry.

[0107] (2) Deionized water was added to the LDH slurry to obtain an LDH dispersion with a solid content of 3 wt%. At a stirring speed of 800 rpm and room temperature, 0.12 mol / L calcium chloride solution and 0.08 mol / L disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The Ca in the calcium chloride solution... 2+ With PO4 in disodium hydrogen phosphate solution 3- The molar ratio is 1.7:1, and the Ca in the calcium chloride solution... 2+ With Al in mixed salt solution 3+ The molar ratio was 1.5:1. During the dropwise addition, 1 mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution within the range of 10.5. After the dropwise addition was completed, the resulting reaction solution was heated to 70°C and stirred at 500 rpm for 4 hours. Then, it was centrifuged, washed, dried at 90°C for 10 hours, ground, and passed through a 400-mesh sieve to obtain calcium hydroxyphosphate-supported LDH powder.

[0108] (3) Under stirring and heating conditions of 65°C, citric acid was added to a 3wt% sodium alginate solution for reaction. The amount of citric acid added was 12wt% of the mass of sodium alginate in the sodium alginate solution. During the reaction, 1mol / L sodium hydroxide solution was added dropwise to control the pH value of the reaction solution at 6. After 2h of reaction, a modified sodium alginate solution was obtained. Hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution. The mass ratio of hydroxyphosphate-loaded LDH powder to sodium alginate in the sodium alginate solution was 1:5. The suspension was obtained by ultrasonic dispersion at 500W ultrasonic power for 40min.

[0109] (4) Mix 4.5 wt% calcium chloride solution and 2.5 wt% aluminum chloride solution. The Ca in the calcium chloride solution...2+ With Al in aluminum chloride solution 3+ The molar ratio of the two components was 4.5:1. After mixing evenly, a cross-linking solution was obtained. The suspension was dripped into the cross-linking solution through a syringe with a needle aperture of 1.2 mm. After the droplets were dripped into the cross-linking solution, the solution was left to stand and mature for 2 hours to obtain gel microspheres.

[0110] The gel microspheres were dried with hot air at 45°C and 3 m / s for 1.5 h, and then dried with hot air at 60°C and 1 m / s until constant weight was obtained, thus obtaining the organic-inorganic composite repair material.

[0111] Example 6

[0112] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that in step (3), the amount of citric acid added is adjusted to 5 wt% of the mass of sodium alginate in the sodium alginate solution. Other process parameters and operation steps are exactly the same as in Embodiment 1.

[0113] Example 7

[0114] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that in step (3), the amount of citric acid added is adjusted to 15 wt% of the mass of sodium alginate in the sodium alginate solution. Other process parameters and operation steps are exactly the same as in Embodiment 1.

[0115] Example 8

[0116] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that in step (3), the mass ratio of calcium hydroxyphosphate-loaded LDH powder to sodium alginate in sodium alginate solution is adjusted to 1:1. Other process parameters and operation steps are exactly the same as in Embodiment 1.

[0117] Example 9

[0118] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that in step (3), the mass ratio of calcium hydroxyphosphate-loaded LDH powder to sodium alginate in sodium alginate solution is adjusted to 1:8. Other process parameters and operation steps are exactly the same as in Embodiment 1.

[0119] Example 10

[0120] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that, in step (4), the Ca in the calcium chloride solution...2+ With Al in the aluminum chloride solution 3+ The molar ratio was adjusted to 2:1, and other process parameters and operating steps were exactly the same as in Example 1.

[0121] Example 11

[0122] This embodiment provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Embodiment 1 is that, in step (4), the Ca in the calcium chloride solution... 2+ With Al in the aluminum chloride solution 3+ The molar ratio was adjusted to 6:1, and other process parameters and operating steps were exactly the same as in Example 1.

[0123] Comparative Example 1

[0124] This comparative example provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Example 1 is that step (2) is omitted. The LDH slurry obtained in step (1) is centrifuged, washed, dried, ground and sieved to obtain LDH powder. In step (3), the LDH powder is dispersed in a modified sodium alginate solution, that is, hydroxyapatite is not loaded on LDH. Other process parameters and operation steps are exactly the same as in Example 1.

[0125] Comparative Example 2

[0126] This comparative example provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Example 1 is that in step (3), sodium alginate is not modified with citric acid, and calcium hydroxyphosphate loaded with LDH powder is mixed with unmodified sodium alginate solution. Other process parameters and operating steps are exactly the same as in Example 1.

[0127] Comparative Example 3

[0128] This comparative example provides a method for preparing an organic-inorganic composite remediation material for heavy metal passivation in farmland soil. The difference from Example 1 is that in step (4), the crosslinking liquid is a 2wt% calcium chloride solution, which does not contain aluminum chloride. Other process parameters and operation steps are exactly the same as in Example 1.

[0129] The saturated adsorption capacity, heavy metal removal rate, and leaching concentration of the organic-inorganic composite remediation materials prepared in Examples 1-11 and Comparative Examples 1-3 were tested. The specific test steps included:

[0130] (1) Saturated adsorption capacity of heavy metal ions

[0131] Prepare the target heavy metal (Cd) separately 2+ Pb2+ and AsO4 3- The standard stock solution (1000 mg / L, source solution of Cd(NO3)2·4H2O, Pb(NO3)2 and Na3AsO4) 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, and AsO4 was added. 3- The pH of the standard stock solution was adjusted to 7 using dilute HNO3 or NaOH solution.

[0132] Take 50 mg of the organic-inorganic composite remediation material prepared in the examples and comparative examples and place it in an Erlenmeyer flask. Add 50 mL of initial solution of different concentrations (solid-liquid ratio 1:1000) to each Erlenmeyer flask. After sealing, place the flasks in a constant temperature shaker and shake for 24 h at 25±1℃ and 150 rpm. Immediately after shaking, filter the solution through a 0.45 μm microporous membrane. Determine the residual concentration of heavy metal ions in the filtrate using inductively coupled plasma mass spectrometry. Calculate the equilibrium adsorption capacity of heavy metal ions for different initial solutions using the following formula:

[0133]

[0134] 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).

[0135] 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 :

[0136]

[0137] Among them, K L is the Langmuir constant (L / mg).

[0138] (2) Heavy metal removal rate

[0139] 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, Cd(NO3)2 solution and Na3AsO4 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 arsenic (as As) concentration was increased to 100±10 mg / kg. The mixture was aged at 25℃ for 30 days to age the heavy metals.

[0140] The organic-inorganic composite 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 concentrations of total Pb, total Cd, and total As in the soil were determined by ICP-MS, and the heavy metal removal rate was calculated using the following formula:

[0141]

[0142] 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.

[0143] (3) Leaching toxicity

[0144] 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, Cd(NO3)2 solution and Na3AsO4 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 arsenic (as As) concentration was increased to 100±10 mg / kg. The mixture was aged at 25℃ for 30 days to age the heavy metals.

[0145] The organic-inorganic composite 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). On the 7th day, soil samples were taken from a depth of 0-15 cm.

[0146] 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, 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, total Cd leaching concentration, and total As leaching concentration in the filtrate using inductively coupled plasma mass spectrometry.

[0147] The test results are shown in Table 1.

[0148] Table 1

[0149]

[0150] The test data from Examples 1, 6, and 7 show that in Example 6, the amount of citric acid was too low, resulting in a reduction in the number of carboxyl groups introduced onto the sodium alginate molecular chain. This reduced the material's complexation ability for heavy metals, leading to a significant decrease in the material's adsorption capacity and removal rate for heavy metal ions. In Example 7, the amount of citric acid was too high. Excessive citric acid over-crosslinked the sodium alginate molecular chain, affecting the material's pore structure and limiting the diffusion and adsorption of heavy metal ions within the gel, resulting in a significant decrease in the material's adsorption capacity and removal rate for heavy metal ions.

[0151] The test data from Examples 1, 8, and 9 show that in Example 8, the proportion of sodium alginate was too low, resulting in an overly sparse gel network structure, low mechanical strength of the gel microspheres, and easy breakage. Simultaneously, the proportion of calcium hydroxyphosphate-loaded LDH powder was too high, leading to agglomeration and a significant reduction in the contact area with heavy metal ions, resulting in a marked decrease in the material's adsorption capacity and removal rate for heavy metal ions. In Example 9, the proportion of calcium hydroxyphosphate-loaded LDH powder was too low, leading to a significant decrease in the material's adsorption capacity and removal rate for heavy metal ions.

[0152] As can be seen from the test data of Examples 1, 10, and 11, in Example 10, Al 3+ The dosage was too low, which affected the complexation with arsenate ions, resulting in a decrease in the material's arsenic removal efficiency. In Example 11, Ca... 2+ Excessive dosage of Ca 2+ This results in excessively high cross-linking density and an overly dense gel structure, hindering the diffusion of heavy metal ions into the gel microspheres.

[0153] As can be seen from the test data of Example 1 and Comparative Example 1, the loading of hydroxyapatite was omitted in Comparative Example 1. The lack of hydroxyapatite prevents the material from forming a stable lead phosphate precipitate with lead ions, resulting in a significant decrease in the material's lead removal efficiency.

[0154] As can be seen from the test data of Example 1 and Comparative Example 2, the citric acid modification of sodium alginate was omitted in Comparative Example 2. The unmodified sodium alginate lacks sufficient carboxyl groups, which weakens the material's ability to complex cadmium ions, resulting in a significant decrease in the material's cadmium removal efficiency.

[0155] The test data from Example 1 and Comparative Example 3 show that the crosslinking solution in Comparative Example 3 only used calcium chloride and did not contain aluminum chloride, thus lacking Al. 3+ This prevents the material from forming coordination compounds with arsenate ions, resulting in a significant decrease in the material's arsenic removal efficiency.

[0156] 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 an organic-inorganic composite remediation material for heavy metal passivation in farmland soil, characterized in that, The preparation method includes: (I) Dissolve magnesium salt and aluminum salt in deionized water to obtain a mixed salt solution; under stirring and nitrogen atmosphere, add sodium hydroxide solution dropwise to the mixed salt solution, and heat the solution to react after the addition is complete to obtain LDH slurry; (II) Add deionized water to the LDH slurry to obtain an LDH dispersion; under stirring conditions, add calcium chloride solution and disodium hydrogen phosphate solution dropwise to the LDH dispersion simultaneously. During the dropwise addition, add alkali solution to control the pH value. After the dropwise addition is completed, heat the reaction, and then centrifuge, wash, dry, grind, and sieve to obtain calcium hydroxyphosphate-loaded LDH powder. (III) Citric acid was added to sodium alginate solution under stirring and heating conditions to carry out the reaction. During the reaction, alkali solution was added dropwise to control the pH value. After the reaction was completed, a modified sodium alginate solution was obtained. The calcium hydroxyphosphate-loaded LDH powder was dispersed in the modified sodium alginate solution and ultrasonically dispersed to obtain a suspension. (IV) Mix calcium chloride solution and aluminum chloride solution to obtain crosslinking liquid; drip the suspension into the crosslinking liquid using a syringe, and obtain gel microspheres after crosslinking and solidification; dry the gel microspheres to obtain the organic-inorganic composite repair material.

2. The preparation method according to claim 1, characterized in that, In step (I), the magnesium salt and the aluminum salt are prepared according to Mg 2+ And Al 3+ The molar ratio is (2.5~3.5):1 when dissolved in deionized water; Mg in the mixed salt solution 2+ And Al 3+ The molar concentration is 0.15~0.25mol / L.

3. The preparation method according to claim 1, characterized in that, In step (I), sodium hydroxide solution is added dropwise to the mixed salt solution at a stirring speed of 300-500 rpm, at room temperature and under a nitrogen atmosphere; The molar concentration of the sodium hydroxide solution is 1~2 mol / L; The sodium hydroxide solution is added at a rate of 1-3 mL / min; OH in sodium hydroxide solution - With Mg in the mixed salt solution 2+ And Al 3+ The total molar ratio is (2.5~3):1; After the addition is complete, heat the resulting reaction solution to 65-75°C and stir at 300-500 rpm for 12-18 hours.

4. The preparation method according to claim 1, characterized in that, In step (II), the solid content of the LDH dispersion is 2-3 wt%. At a stirring speed of 600-800 rpm and room temperature, calcium chloride solution and disodium hydrogen phosphate solution were simultaneously added dropwise to the LDH dispersion. The molar concentration of the calcium chloride solution is 0.1~0.12 mol / L; The molar concentration of the disodium hydrogen phosphate solution is 0.06~0.08 mol / L; Ca in calcium chloride solution 2+ With the PO4 in the disodium hydrogen phosphate solution 3- The molar ratio is (1.65~1.7):1; Ca in calcium chloride solution 2+ With Al in the mixed salt solution 3+ The molar ratio is (1~1.5):

1.

5. The preparation method according to claim 1, characterized in that, In step (II), during the addition of calcium chloride solution and disodium hydrogen phosphate solution, an alkaline solution is added dropwise to control the pH value of the reaction solution within the range of 9 to 10.5; After the addition is complete, heat the resulting reaction solution to 60-70°C and stir at 300-500 rpm for 4-6 hours. The drying temperature is 70~90℃; The drying time is 10-12 hours; The sieve mesh size is 300-400 mesh.

6. The preparation method according to claim 1, characterized in that, In step (III), the sodium alginate solution has a mass fraction of 2-3 wt%. The amount of citric acid added is 10-12 wt% of the mass of sodium alginate in the sodium alginate solution; The reaction temperature between the citric acid and the sodium alginate solution is 55~65℃; The reaction time between the citric acid and the sodium alginate solution is 2-3 hours. During the reaction, an alkaline solution is added dropwise to control the pH value of the reaction solution within the range of 5 to 6.

7. The preparation method according to claim 1, characterized in that, In step (III), the mass ratio of the calcium hydroxyphosphate-loaded LDH powder to the sodium alginate in the sodium alginate solution is 1:(3~5); The power of the ultrasonic dispersion is 300~500W; The ultrasonic dispersion time is 40-60 minutes.

8. The preparation method according to claim 1, characterized in that, In step (IV), the mass fraction of the calcium chloride solution is 3.5~4.5 wt%. The aluminum chloride solution has a mass fraction of 1.5~2.5 wt%. Ca in calcium chloride solution 2+ With Al in the aluminum chloride solution 3+ The molar ratio is (3.5~4.5):1; The syringe needle orifice diameter is 0.8~1.2mm; After the suspension is dropped into the crosslinking solution, it is left to stand and mature in the crosslinking solution for 2-3 hours.

9. The preparation method according to claim 1, characterized in that, In step (IV), the drying process includes: First hot air drying is carried out at a first hot air temperature and a first hot air velocity. Then, the temperature is increased to a second hot air temperature and the air velocity is decreased to a second hot air velocity for second hot air drying until constant weight is achieved. The temperature of the first hot air is 40~45℃; The velocity of the first hot air is 2~3 m / s; The drying time for the first hot air is 1.5~2.5 hours; The temperature of the second hot air is 50~60℃; The second hot air velocity is 0.5~1m / s.

10. An organic-inorganic composite remediation material for heavy metal passivation in farmland soil, prepared by the method according to any one of claims 1 to 9, characterized in that, The organic-inorganic composite remediation material includes gel microspheres and calcium hydroxyphosphate-loaded LDH powder loaded inside them.

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