A large-particle cadmium passivation material based on calcium-based bentonite and a preparation method thereof

Abstract

The present invention discloses a large-particle cadmium passivation material based on calcium-based bentonite and a preparation method thereof, and the steps are as follows: Place calcium-based bentonite and calcium hydroxide powder in a reaction vessel in proportion, add water of the same mass, keep the humidity of the mixture at 100%, oscillate and stir at room temperature to fully mix the materials evenly; Place the above product in an oven for baking, and then place it in a pressure applicator to obtain a cylindrical passivation material; Cut the cylindrical passivation material into granular passivation materials after high-temperature drying and shrinkage. Utilize the strong adsorption and adhesiveness of calcium-based bentonite after water absorption and swelling to evenly disperse and crosslink the powder passivation material, and further tightly fix the powder material by using the volume shrinkage characteristic of calcium-based bentonite after high-temperature dehydration, so that the powder material becomes large-particle material, and dust will not be generated during spreading, improving the operability and safety. Improve the slow-release performance of the passivation material for releasing hydroxide ions, reduce the ecological toxicity, and the passivation effect on cadmium is not affected after modification.

Description

Technical Field

[0001] The present invention relates to the technical field of cadmium pollution remediation materials, and particularly relates to a large-particle cadmium passivation material based on calcium-based bentonite and a preparation method thereof. Background Art

[0002] In recent years, with the continuous advancement of industrialization and urbanization, wastewater generated by industrial activities such as mining and smelting is discharged into rivers without treatment, and a large number of heavy metal (Cd, Pb, etc.) elements directly flow into water bodies and enter farmland through agricultural irrigation, resulting in excessive heavy metals in farmland soil. Therefore, there is an urgent need to develop efficient, convenient, low-cost, replicable and easy-to-promote remediation technologies or materials to ensure the safe utilization of cadmium-exceeding farmland.

[0003] At present, there are many studies on the treatment and safe utilization of cadmium-polluted soil, including the development and application of passivators, the adjustment of planting systems and agronomic techniques, etc. There are relatively many studies on the development of passivators, including lime, organic fertilizer, biochar, sepiolite, hydroxyapatite, etc. They can reduce the cadmium uptake by crops to a certain extent mainly by increasing the soil pH, changing the form of cadmium, and reducing the content of available cadmium in the soil; agronomic techniques mainly affect the morphological transformation process of soil cadmium by changing the physical and chemical properties such as Eh of the soil environment, and then affect the cadmium uptake by plants. In-situ passivation technology is a commonly used remediation method at present. It mainly reduces the biological availability of soil cadmium by adding alkaline or adsorbent materials to the soil, so that heavy metals undergo processes such as adsorption, complexation, precipitation and ion exchange in the soil, thereby reducing the cadmium uptake by crops (Liu Yiyun et al., 2021; Du Caiyan et al., 2019). Due to its low cost and high timeliness, this technology is suitable for the safe production of large areas of slightly and moderately cadmium-exceeding farmland and is a current research hotspot (Wang Zongya et al., 2021; Wang Keji et al., 2020; Li et al., 2019).

[0004] Currently, commonly used passivation materials include lime, clay minerals, zeolites, phosphates, etc. By increasing the soil pH, cadmium in the soil forms precipitates with carbonates and hydroxides while increasing the soil's adsorption capacity for cadmium, so as to reduce the bioavailability and mobility of soil cadmium and reduce the absorption and accumulation by crops. Due to the relatively low price, large crustal reserves, good heavy metal fixation effect and less impact on soil texture structure and physical and chemical properties of such materials, they have been widely used in the remediation of heavy metal contaminated farmland soil in recent years. At present, the application of the vast majority of passivation materials is still restricted by two major aspects: on the one hand, the operability and safety of alkaline passivation materials are poor. Among the current passivators, alkaline materials have the best application effect, and slaked lime is the most widely used. However, due to the small particle size of most materials, which are in the form of fine powders, dust is extremely easy to generate during the spreading process, strongly irritating the eyes and skin of people, endangering the personal safety of construction workers, and seriously affecting the operability and construction cost of this material. On the other hand, it is the acute ecological toxicity caused by the rapid release of alkaline passivation materials. After slaked lime is applied to the soil, hydroxide ions are rapidly released, increasing the soil pH, causing the rapid death of soil organisms such as fish and loach, and having a strong destructive effect on the soil ecosystem.

[0005] Therefore, there is an urgent need for a passivation material and its preparation method that can improve operability and safety and reduce the ecological toxicity of the passivation material after it is applied to the soil without affecting its passivation effect on cadmium. Summary of the Invention

[0006] To solve the problems existing in the prior art, the present invention provides a large particle cadmium passivation material based on calcium bentonite and its preparation method. By utilizing the strong adsorption force, adhesiveness after water absorption and expansion, and shrinkage after dehydration of calcium bentonite, fine calcium hydroxide particles are tightly fixed and evenly dispersed in calcium bentonite particles, solving the problems mentioned in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: A large particle cadmium passivation material based on calcium bentonite, the cadmium passivation material is made from raw materials including calcium bentonite powder and calcium hydroxide powder, the mass ratio of calcium bentonite to calcium hydroxide is 1:1, and the cadmium passivation material is a cylindrical particle material.

[0008] Preferably, the height of the cadmium passivation material is 4 - 6 mm, and the cross-sectional diameter is 4 - 6 mm. Further preferably, the height is 5 mm and the cross-sectional diameter is 5 mm.

[0009] In addition, to achieve the above object, the present invention also provides a preparation method for a large particle cadmium passivation material based on calcium bentonite, including the following steps:

[0010] S1. Place calcium-based bentonite and calcium hydroxide powder in a reaction vessel in proportion, add the same mass of water, keep the humidity of the mixture at 100%, and oscillate and stir at room temperature to fully mix the materials evenly;

[0011] S2. Bake the product after step S1 in an oven, and then place it in a pressure device to obtain a cylindrical passivation material;

[0012] S3. Dry the cylindrical passivation material and cut it into granular passivation material.

[0013] Preferably, the particle size of the calcium-based bentonite powder is 1 - 10 μm; the particle size of the calcium hydroxide powder is 5 - 15 μm.

[0014] Preferably, the particle size of the calcium-based bentonite powder is 5 μm; the particle size of the calcium hydroxide powder is 10 μm.

[0015] Preferably, in step S1, during the oscillation and stirring, the oscillation time is 2 - 3 h, and the stirring speed is 170 - 210 rpm.

[0016] Preferably, in step S2, the baking temperature is 65 - 75 °C, and stop baking when the humidity in the oven reaches 18% - 23%.

[0017] Preferably, the pressure device is a piston pressure device with a circular outlet diameter of 4 - 6 mm. Further preferably, the outlet diameter is 5 mm.

[0018] Preferably, the drying temperature in step S3 is 300 - 350 °C.

[0019] The release rate of the large-particle cadmium passivation material described in the present invention is much lower than that of the powder passivation material, overcoming the problems of poor operability and safety and high ecological toxicity of the powder passivation material, and having a cadmium pollution remediation effect similar to that of the powder material.

[0020] The beneficial effects of the present invention are as follows: The method of the present invention utilizes the strong adsorption and adhesiveness of calcium-based bentonite after water absorption and swelling to evenly disperse and crosslink the powder passivation material, and further uses the volume shrinkage characteristic of calcium-based bentonite after heating and dehydration to tightly fix the powder material, which can improve the physical properties of the passivation material, turn the powder material into large-particle material, be easier to spread and not generate dust, and improve operability and safety. The passivation material not only agglomerates, but also makes the fine particles disperse and arrange. After being applied to the soil and absorbing water and swelling, it can be slowly released, and the pH reaches the highest value only after 4 hours, improving the slow-release performance of the passivation material for releasing hydroxide ions, reducing the ecological toxicity of the passivation material after being applied to the soil, avoiding killing organisms in the environment due to a rapid increase in pH, so it will not cause acute ecological toxicity, and after the passivation material is modified, it does not affect its cadmium passivation effect. Brief Description of the Drawings

[0021] Figure 1 It is a schematic flow chart of the steps of the preparation method of the present invention;

[0022] Figure 2 It is a graph showing the dynamic effect of the 3:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on pH in Example 1;

[0023] Figure 3 It is a graph showing the passivation effect of the 3:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on cadmium in Example 1;

[0024] Figure 4 It is a graph showing the dynamic effect of the 2:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on pH in Example 2;

[0025] Figure 5 It is a graph showing the passivation effect of the 2:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on cadmium in Example 2;

[0026] Figure 6 It is a graph showing the dynamic effect of the 1:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on pH in Example 3;

[0027] Figure 7 It is a graph showing the passivation effect of the 1:1 powder and large particle materials of calcium-based bentonite and calcium hydroxide on cadmium in Example 3;

[0028] Figure 8 It is a comparison graph of the passivation performance of the particle and powder materials of calcium-based bentonite and calcium hydroxide under different ratios. Detailed Description of the Invention

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] Example 1

[0031] Comparison of cadmium passivation powder and particles at a mass ratio of 3:1

[0032] Preparation process:

[0033] 1. Calcium-based bentonite: calcium hydroxide = 3:1 powder: Weigh 37.5 g of calcium-based bentonite powder and 12.5 g of calcium hydroxide powder (Ca(OH)2, ≥95%) and mix them evenly.

[0034] 2. Calcium-based bentonite: calcium hydroxide = 3:1 granules: The steps are as follows Figure 1 As shown, weigh 37.5 g of calcium-based bentonite powder and 12.5 g of calcium hydroxide powder (Ca(OH)2, ≥95%) into a centrifuge tube, add 25 mL of water, place it in an oscillator and shake until fully mixed evenly, and dry it in an oven at a heating temperature of 70 °C until the humidity is about 20%. Place the viscous material in a syringe with a needle diameter of 2 cm and a small hole diameter of 5 mm, extrude to obtain a strip-shaped material, and dry it in an oven at a heating temperature of 300 °C until constant weight. Cut the dried material into cylindrical large-grain materials with a height of 5 mm and a cross-sectional diameter of 5 mm.

[0035] Application process:

[0036] 1. Material preparation: Weigh 0.2 g of each of the two materials separately, add them to a Cd 2+ solution with a volume of 500 mL and a concentration of 50 mg / L for the adsorption experiment.

[0037] 2. Adsorption process: First, prepare the Cd 2+ solution, use 1 mol / L hydrochloric acid or sodium hydroxide to adjust the pH value of the solution to 4.5. Before stirring, add large-grain and powdered cadmium passivation materials to different beakers as experimental groups, and set a blank control group. During the adsorption process, use magnetic stirring to ensure that the solid-liquid system is mixed evenly, and the system temperature is maintained at 20 ± 1 °C.

[0038] 3. Sampling process: To capture the ion concentration changes during the adsorption process in detail, the sampling frequency of the experiment is designed as follows: sample once every 2 minutes (5 samples) in the first 10 minutes, once every 5 minutes (10 samples) in the middle 50 minutes, once every 10 minutes (6 samples) in the next 60 minutes, and once every 30 minutes (4 samples) in the next 120 minutes. The sampling volume is about 5 mL. After sampling, immediately filter it with a water-based needle-hole filter and dilute it to a certain volume, maintaining 1% acidity.

[0039] 4. Testing process: Test the pH of the solution before each sampling, and test the Cd concentration of the sample by ICP-OES

[0040] The test pH results are shown in Figure 2 The Cd concentration of the sample is shown in Figure 3 . The test results show that after adding calcium-based bentonite: calcium hydroxide = 3:1 large grains and calcium-based bentonite: calcium hydroxide = 3:1 powder, the pH values of the aqueous solutions in the two systems approach the maximum values at about 30 minutes and 60 minutes respectively; after 4 h, the passivation efficiencies of calcium-based bentonite: calcium hydroxide = 3:1 large grains and calcium-based bentonite: calcium hydroxide = 3:1 powder for cadmium are 22.4% and 99.8% respectively.

[0041] Example 2

[0042] Comparison of Cadmium Passivation Powders and Particles at a Mass Ratio of 2:1

[0043] Preparation Process:

[0044] 1. Calcium-based Bentonite: Calcium Hydroxide = 2:1 Powder: Weigh 33.3 g of calcium-based bentonite powder and 16.7 g of calcium hydroxide powder (Ca(OH)2, ≥95%) and mix them evenly.

[0045] 2. Calcium-based Bentonite: Calcium Hydroxide = 2:1 Granules: Weigh 33.3 g of calcium-based bentonite powder and 16.7 g of calcium hydroxide powder (Ca(OH)2, ≥95%) into a centrifuge tube, add 25 mL of water, place it on an oscillator and shake until fully mixed evenly. The subsequent preparation process is the same as that in Example 1.

[0046] Application Process: The same as in Example 1. The test pH results are shown in Figure 4 , and the Cd concentration of the samples is shown in Figure 5 . The test results show that after adding calcium-based bentonite: calcium hydroxide = 2:1 large particles and calcium-based bentonite: calcium hydroxide = 2:1 powder, the pH values of the aqueous solutions in both systems approach the maximum value in about 30 minutes; after 4 h, the passivation efficiencies of cadmium by calcium-based bentonite: calcium hydroxide = 2:1 large particles and calcium-based bentonite: calcium hydroxide = 2:1 powder are 41.7% and 80.6% respectively.

[0047] Example 3

[0048] Comparison of Cadmium Passivation Powders and Particles at a Mass Ratio of 1:1

[0049] Preparation Process:

[0050] 1. Calcium-based Bentonite: Calcium Hydroxide = 1:1 Powder: Weigh 25 g each of calcium-based bentonite and calcium hydroxide powder (Ca(OH)2, ≥95%) and mix them evenly.

[0051] 2. Calcium-based Bentonite: Calcium Hydroxide = 1:1 Granules: Weigh 25 g each of calcium-based bentonite and calcium hydroxide powder (Ca(OH)2, ≥95%) into a centrifuge tube, add 25 mL of water, place it on an oscillator and shake until fully mixed evenly. The subsequent preparation process is the same as that in Example 1.

[0052] Application Process: The same as in Example 1. The test pH results are shown in Figure 6 , and the Cd concentration of the samples is shown in Figure 7. The test results show that after adding the powder of calcium-based bentonite: calcium hydroxide = 1:1, the pH of the aqueous solution can approach the highest value within 30 minutes, while after adding the large particles of calcium-based bentonite: calcium hydroxide = 1:1, the pH of the aqueous solution reaches the highest value after 4 hours; after 4 hours, the passivation efficiencies of cadmium by the large particles of calcium-based bentonite: calcium hydroxide = 1:1 and the powder of calcium-based bentonite: calcium hydroxide = 1:1 are 92.3% and 99.9% respectively.

[0053] The verification of the examples shows that:

[0054] The solution with the addition of ordinary powdery materials reaches the highest pH faster than that with the addition of large-particle cadmium passivation materials. After adding the material of calcium-based bentonite and calcium hydroxide 1:1 in powder form, the pH of the solution reaches the highest value within 60 minutes, while after adding the material of granular calcium-based bentonite: calcium hydroxide 1:1, the pH of the aqueous solution reaches the highest value after 4 hours, indicating that the release rate of hydroxide ions of this large-particle cadmium passivation material is slower and will not cause acute ecological toxicity. Compared with ordinary powdery materials, this large-particle cadmium passivation material has the slow-release performance of slowly releasing hydroxide ions.

[0055] The granular cadmium passivation material makes the concentration of cadmium in the solution decrease slowly, while the powdery material makes the concentration of cadmium in the solution decrease rapidly. By comparing the cadmium passivation performances of granular and powdery materials with different ratios of calcium-based bentonite and calcium hydroxide, as Figure 8 shown, after 4 hours, the passivation efficiencies of cadmium by the mixtures of powdery calcium-based bentonite and calcium hydroxide with different ratios (1:1, 2:1, 3:1) can reach 81%-100%, the passivation efficiency of granular calcium-based bentonite: calcium hydroxide 3:1 is only 22%, the passivation efficiency of granular calcium-based bentonite: calcium hydroxide 2:1 is only 42%, and the passivation efficiency of granular calcium-based bentonite: calcium hydroxide 1:1 can reach 92%, indicating that the granular calcium-based bentonite: calcium hydroxide 1:1 passivation material in the present invention has a cadmium passivation ability similar to that of the powdery material.

[0056] Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A large-particle cadmium passivation material based on calcium-based bentonite, characterized in that, The cadmium passivation material is made of calcium-based bentonite powder, calcium hydroxide powder and water. The mass ratio of the calcium-based bentonite to the calcium hydroxide is 1:

1. The cadmium passivation material is a cylindrical granular material. The cadmium passivation material has a height of 4 - 6 mm and a cross-sectional diameter of 4 - 6 mm. After being applied to the soil and absorbing water and swelling, the cadmium passivation material can be slowly released, and the pH reaches the highest value only after 4 hours, improving the slow-release performance of the passivation material for releasing hydroxide ions. The passivation efficiency of the cadmium passivation material for cadmium can reach 92%. The preparation method of the large-particle cadmium passivation material is as follows: S1. Place the calcium-based bentonite and calcium hydroxide powder in a reaction vessel according to the ratio, add the same mass of water, keep the humidity of the mixture at 100%, and oscillate and stir at room temperature to make the materials fully mixed and uniform; S2. Place the product after step S1 in an oven for baking, and then place it in a pressure device to obtain a cylindrical passivation material; S3. Cut the cylindrical passivation material into granular passivation material after drying; Among them, the oscillation time in step S1 is 2 - 3 h, and the stirring speed is 170 - 210 rpm; The particle size of the calcium-based bentonite powder is 1 - 10 μm; the particle size of the calcium hydroxide powder is 5 - 15 μm; In step S2, the baking temperature is 65 - 75 °C, and baking in the oven stops when the humidity reaches 18% - 23%; The drying temperature in step S3 is 300 - 350 °C.

2. The large-particle cadmium passivation material based on calcium-based bentonite according to claim 1, characterized in that: The particle size of the calcium-based bentonite powder is 5 μm; the particle size of the calcium hydroxide powder is 10 μm.

3. The large-particle cadmium passivation material based on calcium-based bentonite according to claim 1, characterized in that: The pressure device is a piston pressure device, and its outlet is a circular outlet with a diameter of 4 - 6 mm.

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