Thallium-polluted soil remediation agent and use method thereof

Abstract

The invention provides a thallium-polluted soil remediation agent and a use method thereof. The thallium-polluted soil remediation agent is prepared by mixing calcium oxide and biochar according to a mass ratio of 1: 2-2: 1 and then carrying out ball milling. When the remediation agent is used for soil remediation, the pH range is 7-10, the remediated soil is high in thallium stability rate and appropriate in pH value, and the remediation agent has good soil remediation performance.

Description

Technical Field

[0001] This invention belongs to the field of soil pollution remediation technology, specifically relating to a thallium-contaminated soil remediation agent and its application method. Background Technology

[0002] Thallium is a highly toxic heavy metal. During thallium smelting, thermal power plants, and the manufacturing of various thallium-containing materials and agents, thallium-containing waste gas, wastewater, and waste residue enter the environment, polluting the soil and affecting plants such as vegetables in contaminated areas, thus threatening human health through the food chain. Soil heavy metal stabilization technology is used to remediate thallium pollution. This technology uses physical or chemical means to fix heavy metals in the soil or convert them into low-mobility, low-toxicity forms. Chemical remediation involves adding chemical substances to contaminated soil, based on the oxidation-reduction, chelation, or precipitation reactions between heavy metals and passivating agents, to reduce the toxicity and bioavailability of heavy metals in the soil. Chinese patent application number 201811575365.1, "A remediation agent for thallium-contaminated soil and its preparation method," provides a remediation agent whose components include mineral microcapsule materials, sepiolite powder, fly ash, iron-manganese oxides, and sodium silicate. The raw materials are relatively complex and not easy to obtain. The production process requires heating to above 700℃ and maintaining it for 0.5-6 hours, which consumes a lot of energy. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a thallium-contaminated soil remediation agent and its application method. The raw materials used in the thallium-contaminated soil remediation agent are inexpensive and readily available, the manufacturing process is simple, the energy consumption is low, and the thallium stability rate is high after the soil is remediated by the obtained remediation agent, thus exhibiting good soil remediation performance.

[0004] The thallium-contaminated soil remediation agent disclosed in this invention is prepared by mixing calcium oxide and biochar in a mass ratio of 1:2 to 2:1 and then ball milling.

[0005] Calcium oxide and biochar are ball-milled and mixed. During the ball milling process, the mixture is mechanically activated. The mechanical energy induces the oxygen-containing functional groups on the surface of biochar to undergo a solid-phase reaction with calcium oxide to form Ca-OC bonds, which facilitates the combination with thallium ions in the soil to form complexes. At the same time, the calcium oxide particles are anchored in the pores of the biochar to prevent agglomeration and improve the reactivity of the soil remediation agent.

[0006] Furthermore, the biochar has a particle size ≤ 0.15 mm; a moisture content < 5%; and a carbon content (solid carbon) ≥ 60%. These physicochemical properties of the biochar ensure its effective combination with calcium oxide and its high mechanical activation efficiency.

[0007] Furthermore, the purity of the calcium oxide is ≥90% to avoid heavy metals such as lead, cadmium, or excessive magnesium oxide interfering with the fixation of thallium; the moisture content of the calcium oxide is <2% to prevent deliquescence leading to clumping or decreased reactivity; the particle size of the calcium oxide is ≤0.075mm to ensure uniform mixing with biochar and avoid excessively high local pH that could damage the soil microecology.

[0008] Furthermore, the ball milling parameters are as follows: ball-to-material ratio of 50:1-120:1; rotation speed of 200-400 rpm; and total ball milling time of 5-6.5 hours. These ball milling parameters ensure the production of a high-performance repair agent while avoiding excessive energy consumption from over-milling.

[0009] Furthermore, the biochar is prepared by the following steps: biomass is washed, dried, pulverized, and then pyrolyzed to obtain biochar; the biomass is selected from sawdust, straw, rice bran, and algae.

[0010] Considering the influence of cellulose content on the adsorption performance of the subsequently produced biochar, agricultural waste including sawdust, straw, rice bran and algae were selected as biomass raw materials for pyrolysis to obtain the desired biochar. The above-mentioned biomass raw materials and quicklime that provide calcium oxide are both cheap and readily available raw materials in the environment that needs to be treated, which reduces the difficulty of treatment and manufacturing costs.

[0011] Furthermore, the particle size range of the pre-pyrolysis biomass is 0.25mm-0.85mm to improve the uniformity and functionality of the biochar. Passing the raw material through a 20-60 mesh sieve corresponds to a particle size range of 0.25mm-0.85mm, which can meet the needs of most soil remediation and adsorption applications. Moreover, the crushing process has low energy consumption and good economic indicators.

[0012] Furthermore, the pyrolysis reaction temperature range is 400℃-600℃; the pyrolysis reaction time is 1h-2h. These pyrolysis conditions balance the porosity development and functional group retention of the biochar obtained during the pyrolysis process, resulting in lower energy consumption for preparing thallium-contaminated soil remediation agents under these conditions.

[0013] Another aspect of the present invention provides a method for using a thallium-contaminated soil remediation agent, wherein the soil pH range when using the thallium-contaminated soil remediation agent is 7-10. Within this pH range, the remediation agent can efficiently bind with thallium in the soil to form water-insoluble substances, reducing thallium pollution, while simultaneously meeting the remediation needs of numerous contaminated sites, including farmland.

[0014] The present invention has the following beneficial effects:

[0015] The method for preparing the thallium-contaminated soil remediation agent provided by this invention selects relatively inexpensive and readily available biochar and calcium oxide, and uses ball milling to mechanically activate the two. The preparation process is simple and low in cost. The resulting thallium-contaminated soil remediation agent reduces the migration rate and bioavailability of thallium in the soil, and has a high thallium stability rate after remediation, thus exhibiting good soil remediation performance. Attached Figure Description

[0016] Figure 1 This is a comparison chart of the soil thallium stability rates of Comparative Examples 1 and 2 and Examples 1, 2, and 3 of this invention.

[0017] Figure 2 This is the SEM image of Comparative Example 3.

[0018] Figure 3 This is the SEM image of Example 1. Detailed Implementation

[0019] To more clearly and completely describe the technical solution of the present invention, the present invention will be further described in detail below through specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. Various changes can be made within the scope of the claims of the present invention.

[0020] The soil samples used in this invention were all collected from the vicinity of a slag storage facility of a smelter. The collection and pretreatment process is as follows: the soil sample collection depth is 0-0.5m. After the soil sample is retrieved, plastic bags, construction waste, plant roots and other debris are removed. The sample is spread out on an enamel tray and then air-dried, crushed, ground, sieved and mixed. After these processes, the sample is stored in a clean, sealed plastic bag in a cool place.

[0021] The thallium concentration in soil samples was characterized using the following method for testing thallium concentration in pure water leachate of soil:

[0022] 1) The soil sample was leached with pure water using the leaching method specified in the "Horizontal Oscillation Method for Leaching Toxicity of Solid Waste" (HJ 557-2010) to obtain the leachate;

[0023] 2) The concentration of thallium in the pure water leachate of soil was obtained by referring to the "Determination of 65 Elements in Water by Inductively Coupled Plasma Mass Spectrometry" (HJ 700-2014).

[0024] After remediation, the following thallium stabilization test was conducted on the sample to obtain the thallium stability rate n of the soil. n can be used to characterize the remediation efficiency of the remediation agent for thallium.

[0025] The thallium stabilization test includes the following steps:

[0026] (1) Preparation of stabilizing agents: Calculate the amount of stabilizing test agent to be added according to the type and ratio of the agent, weigh it with an electronic balance, and record it.

[0027] (2) Soil sample addition and mixing: Add the weighed reagent to the corresponding experimental group. During the addition process, the reagent should be evenly sprinkled on the soil surface and mixed.

[0028] (3) Static curing: Cover the mixed soil after adding the medicine with plastic film to keep the soil moisture content at about 25%. Take samples for testing after 7 days of curing.

[0029] (4) Sample water immersion: Pure water was used as the leachate. After curing, the sample was immersed in water using the extraction method specified in the "Horizontal Oscillation Method for Leaching Toxicity of Solid Waste" (HJ 557-2010).

[0030] (5) Formula for calculating the stability rate of thallium in soil:

[0031]

[0032] In the formula, n is the stabilization efficiency, C is the thallium concentration in the pure water leachate measured after the experiment (unit: ug / kg), and C0 is the thallium concentration in the pure water leachate of the original soil sample (unit: ug / kg).

[0033] Examples and Comparative Examples

[0034] Among various biomass energy raw materials such as straw, wood, agricultural waste and algae, wheat straw, an agricultural waste rich in cellulose, is selected as the raw material for biochar. Cellulose content is one of the important factors affecting the performance of biochar, and its content affects the pore structure and adsorption performance of biochar.

[0035] Wheat straw was washed to remove surface impurities and dried at approximately 75°C for 48 hours to ensure complete moisture evaporation. The dried straw was then pulverized using a pulverizer and passed through a 50-mesh sieve to obtain raw material one. Raw material one was placed in a tube furnace and pyrolyzed at approximately 550°C for 90 minutes under nitrogen protection. After natural cooling, biochar was obtained. The biochar had a particle size ≤0.15mm, a moisture content <5%, and a carbon content (solid carbon) ≥60%. These physicochemical properties ensured its effective combination with calcium oxide and its high mechanical activation efficiency.

[0036] Before use, calcium oxide is dried at 105°C for 4 hours to remove any moisture and carbon dioxide that may be absorbed. The dried calcium oxide has a purity of 90% and a particle size of less than 0.075 mm.

[0037] Five samples were prepared by mixing biochar and calcium oxide in different mass ratios of 1:2, 1:1, and 2:1, and by using biochar and calcium oxide alone, resulting in three different examples: Example 1, Example 2, and Example 3. Biochar and calcium oxide were used separately as comparative examples: Comparative Example 1 and Comparative Example 2. The five samples were ball-milled separately, with the following control measures: a planetary ball mill was used for mixing and ball milling, with the following parameters: a 100mL agate jar was selected to ensure chemical inertness; zirconia balls of different diameters were used as the grinding media to improve grinding efficiency; the ball-to-material ratio was controlled at approximately 100:1; the ball milling speed was set to 300rpm; and an intermittent ball milling mode was adopted, i.e., a cycle of 5 minutes forward rotation - 3 minutes pause - 5 minutes reverse rotation, with a total ball milling time of 6 hours. The samples obtained were Example 1 - biochar:calcium oxide mass ratio = 1:2 (ball milling), Example 2 - biochar:calcium oxide mass ratio = 1:1 (ball milling), Example 3 - biochar:calcium oxide mass ratio = 2:1 (ball milling), Comparative Example 1 - biochar (ball milling), and Comparative Example 2 - calcium oxide (ball milling).

[0038] Soil samples from Examples 1, 2, and 3, and Comparative Examples 1 and 2 were treated with a 3% addition amount for remediation. A comparison of soil thallium stabilization rates obtained from the examples and comparative examples is shown below. Figure 1 ;Depend on Figure 1 It can be seen that the stability rate of biochar after ball milling alone in Comparative Example 1 was 62.39%, and the stability rate of calcium oxide after ball milling alone in Comparative Example 2 was 80.21%. In Examples 1-3, the stability rate of thallium in Examples 1-3 was directly proportional to the amount of calcium oxide in the mixture. The stability rate of thallium in Example 1, with a biochar to calcium oxide mass ratio of 1:2, was the highest, reaching 88.2%. Considering that excessive addition of calcium oxide would result in an excessively high pH value of the remediated soil, which would not meet the remediation requirements of contaminated sites such as farmland, the biochar to calcium oxide mass ratio of 1:2 in Example 1 provided by this invention is the optimal ratio.

[0039] The changes in soil pH after remediation in Comparative Example 1, Comparative Example 2 and Example 1 are shown in Table 1.

[0040] Table 1. Changes in soil pH

[0041]

[0042] As shown in Table 1, both the comparative example and the embodiment examples caused the soil pH to increase to varying degrees. Calcium oxide had the greatest impact on soil pH, increasing it from 6.35 to 12.09 after application. In Example 1, which had the highest thallium stability, the soil pH increased to 8.92.

[0043] Comparative Example 3 uses a remediation agent obtained by simply mixing biochar and calcium oxide in a mass ratio of 1:2. Figure 2 This is the SEM image of Comparative Example 3, obtained by simply mixing biochar and calcium oxide at a mass ratio of 1:2. Figure 3 This is a SEM image of Example 1 obtained by ball milling a mixture of biochar and calcium oxide at a mass ratio of 1:2. (Comparison) Figure 2 and Figure 3 It can be seen that the repair agent obtained by ball milling has a larger specific surface area and more reactive sites; the thallium stability rate of Comparative Example 3 is 80.94%, which is much lower than the thallium stability rate of 88.2% in Example 1.

[0044] The high thallium stability in Example 1 is related to the following factors: during ball milling, mechanical energy induces a solid-phase reaction between oxygen-containing functional groups (-COOH, -OH) on the biochar surface and calcium oxide, forming Ca-OC bonds; in addition, calcium oxide particles are anchored in the pores of the biochar, preventing agglomeration and improving reactivity. The reaction mechanism between the thallium-contaminated soil remediation agent provided by this invention and thallium in the soil includes: 1) increasing the pH value of the soil, promoting the conversion of Tl+ into insoluble TlOH or Tl2O3; 2) the oxygen-containing functional group Ca-OC bonds forming ≡Ca-O-Tl or ≡CO-Tl complexes with Tl+; 3) the Ca in the remediation agent... 2+ It competes with Tl+ in the soil for adsorption sites, driving Tl+ into the pores of biochar and where it is fixed.

[0045] The thallium-contaminated soil remediation agent provided by this invention can be widely used in the remediation of various thallium-contaminated soils, such as tailings storage areas. During the remediation process, the pH range of the thallium-contaminated soil is 7-10. Maintaining a suitable slightly alkaline environment during the soil remediation process is conducive to the combination of the thallium-contaminated soil remediation agent and thallium, and can also meet the remediation needs of many contaminated sites, including farmland.

[0046] The above provides a detailed description of a thallium-contaminated soil remediation agent and its application method provided by the present invention. Specific examples have been used to illustrate the principles and implementation methods of the invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the core ideas of the invention. It should be noted that those skilled in the art can make various improvements and modifications to the invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A thallium-contaminated soil remediation agent, characterized in that... This thallium-contaminated soil remediation agent is prepared by ball milling calcium oxide and biochar in a mass ratio of 1:2 to 2:

1.

2. The thallium-contaminated soil remediation agent according to claim 1, characterized in that... The biochar has a particle size ≤ 0.15 mm; the biochar has a moisture content < 5%; and the biochar has a carbon content (solid carbon) ≥ 60%.

3. The thallium-contaminated soil remediation agent according to claim 1 or 2, characterized in that... The purity of the calcium oxide is ≥90%; the water content of the calcium oxide is <2%; and the particle size of the calcium oxide is ≤0.075mm.

4. The thallium-contaminated soil remediation agent according to claim 1, characterized in that... The parameters of the ball mill are as follows: ball-to-material ratio of 50:1-120:1; rotation speed of 200-400 rpm; and total ball milling time of 5-6.5 h.

5. The thallium-contaminated soil remediation agent according to claim 1, characterized in that... The biochar is prepared by the following steps: biomass is washed, dried, pulverized, and then pyrolyzed to obtain biochar; the biomass is selected from sawdust, straw, rice bran, and algae.

6. The thallium-contaminated soil remediation agent according to claim 5, characterized in that... The particle size range of the biomass before pyrolysis is 0.25 mm to 0.85 mm.

7. The thallium-contaminated soil remediation agent according to claim 5, characterized in that... The pyrolysis reaction temperature range is 400℃-600℃, and the pyrolysis reaction time is 1h-2h.

8. A method for using the thallium-contaminated soil remediation agent according to claims 1-7, characterized in that... The soil pH range when using the thallium-contaminated soil remediation agent is 7-10.

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