Mineral-based material capable of slowly releasing calcium and alkali, preparation method of mineral-based material and application of mineral-based material in cadmium reduction of rice

Abstract

The invention discloses a mineral-based material capable of slowly releasing calcium and alkali, a preparation method of the mineral-based material and application of the mineral-based material in rice cadmium reduction, and belongs to the technical field of heavy metal polluted farmland soil remediation. The preparation method comprises the following steps: mixing metakaolin and a strong alkali solution to react I, removing supernatant after the reaction is completed, adding a calcium salt solution, oscillating to react II, and drying to obtain the mineral-based material capable of slowly releasing calcium and alkali. Calcium ions and basic ions are gradually dissolved and continuously released, the pH value and the structure of soil are improved, favorable conditions are created for plant growth, and the long-term effect of cadmium reduction is kept.

Description

Technical Field

[0001] The invention relates to a slow-release material, in particular to a mineral-based material capable of slow-releasing calcium ions and alkaline substances, and also to a preparation method thereof and application thereof as a cadmium-reducing agent for rice, belonging to the technical field of prevention and control of heavy metal-polluted farmland. Background Art

[0002] Due to acid rain deposition, improper fertilization, unscientific farming and other reasons, the pH value of cultivated soil continues to decline, leading to the increasing soil acidification phenomenon. This acidification phenomenon releases toxic elements such as aluminum and manganese in the soil, interfering with the nutrient absorption and metabolic pathways of rice, thereby damaging the health and physiological functions of the rice root system and hindering its normal growth and development. Heavy metal elements such as cadmium and lead in the soil usually exist in a stable form, but soil acidification will significantly change their properties, increase the activity of heavy metals, accelerate their desorption and migration in the soil, and thus increase the absorption of heavy metals such as cadmium by rice. In addition, the decrease in soil pH will reduce the diversity and abundance of microorganisms, affect the structure of microbial communities, and destroy the micro-ecosystem, thereby aggravating the occurrence of soil diseases and insect pests, resulting in a soil environment that is not conducive to rice growth and seriously affecting agricultural production.

[0003] Heavy metals in the soil are usually not easily leached with water, nor can they be decomposed by soil microorganisms. They are long-term pollutants in the environment. Among them, cadmium is one of the key heavy metal pollutants monitored. Since rice roots can absorb nutrients from the soil during growth, it also makes it easy for rice to absorb heavy metals in the soil, causing cadmium to accumulate in rice plants and may be passed to the human body through the food chain. Long-term consumption of rice with excessive cadmium may seriously affect the human kidneys, blood, bones and other organs.

[0004] Soil acidification and cadmium pollution have become important factors that restrict rice growth and ensure quality safety. Therefore, timely measures to repair acidified soils and reduce the cadmium content in rice grains are of great significance for effectively protecting human health, improving food safety and promoting the quality of agricultural products. A common method to improve acidic cadmium-contaminated soils is to neutralize soil acidity by applying lime or other alkaline substances. This method can effectively increase the pH value of the soil, reduce the solubility and mobility of heavy metal cadmium, and thus reduce the absorption of cadmium by rice. However, excessive application of lime may lead to the formation of soil compaction, limiting the penetration capacity of water and roots. In addition, excessive alkaline substances may also cause nutrient imbalance, affecting the effectiveness and availability of other important nutrients in the soil. Phytoremediation is a method of extracting and enriching cadmium from soil using plants with high cadmium accumulation ability, which can absorb and transport cadmium to the aboveground part through the root system and enrich it in the plant body, thereby reducing the cadmium content in the soil. Although phytoremediation has potential in treating cadmium-contaminated soils, its cycle is relatively long, and the root system of plants is limited and can only act on the soil surface. Chemical stabilization technology is to reduce the bioavailability of cadmium in the soil by adding amendments such as phosphates or silicates, changing cadmium into a more stable form, thereby reducing the absorption of cadmium by plants. However, the implementation of this technology faces the challenges of high initial costs and potential risks that amendments may cause to the underground environment. Therefore, it is urgent to develop an efficient, low-cost and stable method to solve the problem of soil acidification, reduce the absorption and enrichment of cadmium by rice, and ensure the safe production of rice and human health. Summary of the invention

[0005] In view of the problems existing in the prior art, the first object of the present invention is to provide a mineral-based material for slow-release of calcium and alkali, which effectively binds calcium ions and alkaline ions in the mineral structure, is not easily soluble, and can exist stably in the soil for a long time. Under the influence of moisture and acidic environment in the soil, it gradually dissolves and continuously releases calcium ions and alkaline ions, which can improve the pH value and structure of the soil, create favorable conditions for plant growth, and has a long-term and effective cadmium-reducing effect.

[0006] The second object of the present invention is to provide a method for preparing a mineral-based material for slow-release of calcium and alkali, which is simple, has low raw material cost, and is conducive to industrial production.

[0007] The third object of the present invention is to provide a mineral-based material that slowly releases calcium and alkali as a rice cadmium reduction agent. When added to an acidic cadmium-contaminated soil environment, it can slowly release calcium ions and alkaline components, improve the soil pH value and structure, reduce the cadmium content in rice grains, and increase the cation exchange capacity of the soil, thereby ensuring that the final rice produced meets food hygiene standards and protects human health.

[0008] In order to achieve the above technical objectives, the present invention provides a method for preparing a mineral-based material for slow-release calcium and alkali, the method comprising mixing kaolin with a strong alkali solution for reaction I, removing the upper clear liquid after the reaction is completed, adding a calcium salt solution for oscillation reaction II, and drying to obtain the material.

[0009] The present invention selects metakaolin as a carrier material, which has the characteristics of a layered structure, a large specific surface area, etc., and metakaolin is an anhydrous aluminum silicate formed by high-temperature dehydration of kaolin. Compared with other clay minerals, metakaolin can better bind hydroxide ions, so that after reacting with a strong alkaline solution, the hydroxide ions can be stably combined by hydrogen bonding and mineralization. On this basis, the metakaolin further reacts with a calcium salt solution, and calcium ions are fixed in the metakaolin mineral to form a slow-release material through physical adsorption and mineralization. The slow-release material is added to a cadmium-contaminated soil environment that is slightly acidic. On the one hand, the hydroxide ions can be slowly released to achieve the purpose of adjusting the pH of the soil environment and reducing the migration of heavy metal ions. On the other hand, the calcium ions can be slowly released by ion exchange, dissolution ionization, etc., and the cadmium ions can be adsorbed and mineralized.

[0010] As a preferred solution, the concentration of the strong alkali solution is 3 to 10 mol / L; the strong alkali solution contains alkali metal hydroxide. Alkali metal hydroxide mainly provides hydroxide ions, such as potassium hydroxide, sodium hydroxide, etc. If the concentration of alkali metal hydroxide is too low, it will affect the loading capacity of metakaolin for hydroxide ions, and if the concentration of alkali metal hydroxide is too high, it will destroy the structure of metakaolin.

[0011] As a preferred solution, the sodium hydroxide solution and the metakaolin are measured at a liquid-to-solid ratio of 3 to 5 L:1 kg. If the liquid-to-solid ratio is too low, the reaction effect will be affected, thereby reducing the loading amount of hydroxide ions. Therefore, a suitable liquid-to-solid ratio is required to achieve a better reaction effect.

[0012] As a preferred solution, the conditions of the reaction I are: at a temperature of 70 to 90° C. and for 2 to 6 hours. The preferred reaction conditions are conducive to the chemical reaction between the strong base and the mineral components to generate alkaline minerals.

[0013] As a preferred solution, the concentration of the calcium salt solution is 0.1-0.4 mol / L. The calcium salt solution contains water-soluble calcium salts. The water-soluble calcium salts mainly provide calcium ions, such as calcium chloride, calcium nitrate, etc.

[0014] As a preferred solution, the calcium salt solution and the metakaolin are measured at a liquid-solid ratio of 3 to 5 L: 1 kg. If the liquid-solid ratio is too low, the reaction effect will be affected, thereby reducing the loading amount of calcium ions. If the liquid-solid ratio is too high, the calcium salt will neutralize the hydroxide ions to a certain extent.

[0015] As a preferred solution, the conditions of reaction II are: reacting at room temperature for 8 to 16 hours. Under the preferred reaction conditions, the adsorption and mineralization of calcium ions are favorable.

[0016] The particle size of the metakaolin involved in the present invention is ≤1000 mesh.

[0017] The strong alkali, calcium salt and the like involved in the present invention are of industrial grade.

[0018] The invention also provides a mineral-based material for slow-releasing calcium and alkali, which is obtained by the preparation method.

[0019] The invention discloses an application of a mineral-based material capable of slowly releasing calcium and alkali, and the mineral-based material is applied as a cadmium reduction agent for rice.

[0020] As a preferred solution, the mineral-based material that slowly releases calcium and alkali is added to the cadmium-contaminated farmland soil where rice is grown.

[0021] As a preferred solution, the mineral-based materials for slow-releasing calcium and alkali are added in the preparation stage for tilling the cadmium-contaminated farmland soil, and the mineral-based materials for slow-releasing calcium and alkali are evenly spread on the surface of the cadmium-contaminated farmland soil. The soil is then tilled to allow the mineral-based materials for slow-releasing calcium and alkali to be evenly mixed with the soil. After tilling, appropriate irrigation is performed to allow the soil moisture to be 60% to 70% of the maximum field water holding capacity.

[0022] As a preferred solution, the application amount of the slow-release calcium and alkali mineral-based material in the cadmium-contaminated farmland soil is measured according to 0.975kg / ㎡ to 1.95kg / ㎡. The slow-release calcium and alkali mineral-based material of the present invention can adjust the soil acidity and reduce the cadmium content in rice grains. The mechanism is that the slow-release calcium and alkali mineral-based material uses metakaolin as a carrier material, which can effectively bind calcium ions and alkaline ions in the mineral structure. Under the action of moisture and acidic environment in the soil, the material will gradually dissolve and continuously release calcium ions and alkaline ions. The released alkaline substances can neutralize the acidic substances in the soil, increase the pH value of the soil, reduce the acidity of the soil, reduce the migration and bioavailability of cadmium in the soil, thereby reducing the absorption of cadmium by rice, and the released calcium ions antagonize the heavy metal ions in the soil, participate in the absorption competition of the plant root system, and prevent the plant from adsorbing heavy metals. At the same time, it can stabilize the soil structure, improve the physical and chemical properties of the soil, and is conducive to the growth of plants. The mineral-based material for slow-release of calcium and alkali is not easily soluble and can exist stably in the soil for a long time, so it can continuously release calcium ions and alkaline substances to maintain the long-term effect of reducing cadmium.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The present invention can solve the problems of soil acidification and reduce the cadmium content in rice grains, while also increasing the cation exchange capacity of the soil to provide nutrients for rice growth. The application effect of the mineral-based material of the present invention for slow release of calcium and alkali is remarkable. When applied to acidic polluted rice fields, the cadmium content in rice can be reduced by 58% to 62.98% compared with the control group, thereby achieving the goal of producing edible safe rice in acidic polluted rice fields and eliminating the threat of heavy metal cadmium to human health.

[0025] (2) The mineral-based material for slow-release of calcium and alkali of the present invention is not easily soluble and can be stably present in the soil for a long time. It can continuously release alkaline substances and maintain the long-term effect of reducing cadmium.

[0026] (3) The mineral-based material for slow-release of calcium and alkali of the present invention can slowly release calcium ions to form complexes with cadmium ions in the soil, thereby reducing the bioavailability of cadmium in the soil. At the same time, it helps to stabilize the soil structure and improve the physical and chemical properties of the soil.

[0027] (4) The alkaline slow-release mineral material of the present invention, whose main component is kaolin, is a common clay mineral in soil, is non-toxic and harmless, does not cause secondary pollution to the environment, and can be safely used for farmland soil restoration.

[0028] (5) The mineral-based material for slow release of calcium and alkali of the present invention has high porosity and adsorption performance, which helps to increase the permeability and water retention of the soil.

[0029] (6) The method for synthesizing the mineral-based material for slow-release calcium and alkali of the present invention is simple, and the alkaline slow-release agent is evenly spread on the surface of the contaminated soil during the tillage preparation stage. The slow-release agent and the soil are evenly mixed during the tillage process, saving a lot of manpower and material resources.

[0030] (7) The mineral-based material for slow release of calcium and alkali of the present invention is suitable for acidic polluted rice fields and has the characteristics of low cost, easy implementation, stable effect and wide application range. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the changes in soil exchangeable cadmium content, pH value and cation exchange capacity during the soil maturation experiment. Figure 1 As shown, after 90 days, compared with the control group, the addition of 0.5%, 1% and 5% of the alkaline slow-release mineral material in the soil decreased the effective cadmium content in the soil by 20.58%, 16.94% and 12.59%, respectively; the pH value increased by 1.19, 1.53 and 2.91 units, respectively; and the cation exchange capacity increased by 0.78, 0.8 and 0.97 units, respectively.

[0032] Figure 2The figure is a schematic diagram of the changes in pH value and calcium ion concentration in the leaching experiment of the present invention; the pH value and calcium ion concentration change rapidly in the early stage, and then slowly after 3 days, but still maintain a stable release, indicating that the present invention can continuously release alkaline ions and calcium ions.

[0033] Figure 3 The results of the cadmium content of brown rice in potted rice of the present invention are as follows; Figure 3 As shown, in a pot experiment, the alkaline slow-release mineral material of the present invention was added to the contaminated soil in different contaminated soil mass ratios. Compared with the control group, the addition of 0.5% and 1% of the alkaline slow-release mineral material of the present invention in the contaminated soil mass can reduce the cadmium content in rice brown rice by 58.67% and 56%, respectively. Figure 4 It is a schematic diagram of the change of the effective cadmium content under soil culture conditions of the present invention; the effective cadmium content in the soil decreased by 20.13% and 13.67% respectively.

[0034] Figure 5 The figure is a schematic diagram of the changes of soil pH value and cation exchange capacity under soil culture conditions of the present invention; the pH can be increased by 0.93 and 1.55 units respectively; the cation exchange capacity can be increased by 1.46 and 1.67 units respectively.

[0035] Figure 6 The results of the cadmium content of brown rice under field conditions of the present invention; Figure 6 As shown, in a field test, when 0.5% of the mass of the contaminated surface soil was added to the contaminated soil, the cadmium content in brown rice decreased by 62.98%, from 0.44 mg / kg to 0.16 mg / kg, compared with the control group.

[0036] Figure 7 This is a schematic diagram of the change of the effective cadmium content under field conditions of the present invention; the effective cadmium content in the soil decreased by 17.60%.

[0037] Figure 8 The figure is a schematic diagram of the changes of soil pH value and cation exchange capacity under field conditions of the present invention; soil pH increases by 1.54 units; cation exchange capacity increases by 2.15 units. DETAILED DESCRIPTION

[0038] In order to help readers better understand the present invention, the specific implementation steps and usage methods of the present invention will be described one by one below.

[0039] Example 1

[0040] Preparation of alkaline slow-release mineral materials:

[0041] The alkaline slow-release mineral material in the following specific embodiment is prepared by the following specific method: (1) dissolving caustic soda flakes in water to obtain a caustic soda flake solution with a concentration of 10 mol / L; (2) mixing metakaolin and caustic soda flake solution at a solid-liquid ratio of 1:4, placing in a water bath at 80°C for reaction for 2 hours, and discarding the supernatant; (2) adding calcium chloride solution at a solid-liquid ratio of 1:4, shaking for 12 hours, and drying the filter residue at low temperature to obtain the alkaline slow-release material. In order to make the reaction more complete, stirring is performed with a magnetic stirrer during the reaction; (3) after the reaction is completed, solid-liquid separation is performed, and the filter residue is dried at a high temperature of 100°C to obtain the alkaline slow-release mineral material.

[0042] Example 2

[0043] Soil maturation experiment:

[0044] The contaminated soil from Yangji Village, Xiangtan used in the experiment was collected on site. The soil was naturally air-dried and ground through a 100-mesh sieve for later use. 250g of soil was put into a 250ml conical flask, and no alkaline slow-release mineral material was added as a blank control. 1.25g, 2.5g, and 5g of the alkaline slow-release mineral material were added to three different conical flasks, respectively, so that the addition amount was 0.5%, 1%, and 2% of the soil mass, respectively. After the alkaline slow-release mineral material and the soil were fully stirred, a certain amount of water was added to keep the soil moisture at 70% of the maximum water holding capacity. After 90 days, samples were taken from each treatment at the same time to determine the exchangeable cadmium content in the soil, the soil pH value, and the cation exchange capacity.

[0045] Example 3

[0046] Leaching experiment: Accurately weigh 5g of sample and place it in a 150mL conical flask. Then, add 0.5mL of 0.5mol / L hydrochloric acid solution to the conical flask, and add deionized water to the solution in the conical flask to 50mL. After that, place the conical flask in a constant temperature oscillating box set at 25°C, and keep the speed of the oscillating box at 150r / min. Take out the solution in the conical flask on the 1st, 2nd, 3rd, 6th, 10th, 15th and 20th day, and measure the pH value and calcium ion concentration of the solution.

[0047] Example 4

[0048] Pot test:

[0049] The contaminated soil used in the experiment was collected on site in Oujiangcha Town, Yiyang. The soil was naturally air-dried and hammered evenly with a fine hammer tool to make the soil particles finer and improve the uniformity of the soil. 18 kg of soil was put into a plastic basin, and no alkaline slow-release mineral material was added as a blank control. The alkaline slow-release mineral material with a total addition amount of 0.5% and 1% of the soil mass was applied to the contaminated soil. After the alkaline slow-release mineral material and the soil were fully mixed, a certain amount of water was added to keep the soil moisture at 60-70% of the maximum water holding capacity, stabilize for 7 days, and then transplant four rice seedlings. The daily management of each treatment was the same, and each treatment was harvested at the same time to measure the heavy metal cadmium content in rice, soil pH value and cation exchange capacity.

[0050] Example 5

[0051] Field experiment:

[0052] The actual polluted rice fields in Oujiangcha Town, Yiyang were selected for the experiment. No alkaline slow-release mineral materials were added as blank control treatment. In the preparation stage of plowing, the alkaline slow-release mineral materials with a total addition amount of 0.5% of the surface soil mass were evenly spread on the contaminated soil surface, and then the soil was plowed to make the alkaline slow-release mineral materials and soil evenly mixed. Subsequently, appropriate irrigation was carried out to keep the soil moisture at 60-70% of the maximum water holding capacity in the field, stabilized for 7 days, and then planted rice seedlings. The daily field management of each treatment was the same, and each treatment was harvested at the same time to measure the heavy metal cadmium content in rice, soil pH value and cation exchange capacity.

[0053] The above experimental results show that the alkaline slow-release agent for reducing cadmium in rice of the present invention has a significant application effect. The alkaline ions released by the agent can neutralize the acidic substances in the soil, increase the pH value of the soil, thereby reducing the acidity of the soil. The released calcium ions antagonize the heavy metal ions in the soil, participate in the absorption competition of the plant roots, prevent the plant from adsorbing heavy metals, and stabilize the soil structure. When applied to rice fields polluted by acidic cadmium, the cadmium content in the rice can be reduced by 58% to 62.98% compared with the control group, so that the acidic cadmium-polluted rice fields can produce safe rice for consumption, eliminating the threat of heavy metals to human health.

Claims

1. A method for preparing a mineral-based material for slow release of calcium and alkali, characterized in that: The metakaolin is mixed with a strong alkali solution to react I. After the reaction is completed, the supernatant is removed, and a calcium salt solution is added to shake and react II, and then dried to obtain the product.

2. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1, characterized in that: The concentration of the strong alkali solution is 3-10 mol / L; the strong alkali solution contains alkali metal hydroxide.

3. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1 or 2, characterized in that: The strong alkali solution and the metakaolin are measured according to a liquid-to-solid ratio of 3-5 L:1 kg.

4. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1, characterized in that: The conditions of the reaction I are: reacting at a temperature of 70 to 90° C. for 2 to 6 hours.

5. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1, characterized in that: The concentration of the calcium salt solution is 0.1-0.4 mol / L; the calcium salt solution contains water-soluble calcium salt.

6. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1 or 5, characterized in that: The calcium salt solution and the metakaolin are measured according to a liquid-to-solid ratio of 3-5 L:1 kg.

7. The method for preparing a mineral-based material for slow release of calcium and alkali according to claim 1, characterized in that: The conditions of the reaction II are: reacting at room temperature for 8 to 16 hours.

8. A mineral-based material for slow release of calcium and alkali, characterized in that: The method is obtained by any one of claims 1 to 7.

9. The use of a mineral-based material for slow release of calcium and alkali according to claim 8, characterized in that: Used as a reagent for reducing cadmium in rice.

10. The use of a mineral-based material for slow release of calcium and alkali according to claim 8, characterized in that: The mineral-based material that slowly releases calcium and alkali is added to cadmium-contaminated farmland soil where rice is grown.

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