Hexavalent chromium solidification method suitable for acid / alkali soil

Abstract

The invention discloses a hexavalent chromium solidification method suitable for acid / alkali soil, hexavalent chromium in the acid / alkali soil is solidified through combination of a low-carbon binding material and glucose, and the method comprises the following steps: mixing hexavalent chromium polluted soil with the low-carbon binding material, an exciting agent and a reducing agent, adding water, and uniformly stirring to obtain a mixture; the mixture is subjected to mold entering, forming, demolding and curing in sequence, and curing is completed; wherein the low-carbon cementing material comprises high-calcium fly ash, carbide slag and desulfurized gypsum; the exciting agent is silica fume water glass; the reducing agent is glucose. The solidified body prepared by the method can be used as a roadbed material, the purpose of treating waste with waste is achieved while hexavalent chromium is effectively prevented from being leached, and the defects of high carbon footprint, secondary pollution and the like of a traditional repairing means are overcome.

Description

Technical Field

[0001] The present invention relates to a method for solidifying hexavalent chromium, and more particularly to a method for solidifying hexavalent chromium applicable to acidic / alkaline soil, belonging to the technical field of soil heavy metal pollution remediation, comprehensive utilization of industrial solid waste resources and harmless treatment. Background Art

[0002] Hexavalent chromium is a heavy metal pollutant with strong toxicity, carcinogenic and mutagenic properties, and environmental persistence, posing a serious threat to the ecological environment and human health. It has been widely detected in the soil of many industrial agglomeration areas in China, such as metallurgy, leather, chemical industry, electroplating, building materials, etc. Due to the characteristics of high solubility, strong mobility and difficulty in natural degradation of hexavalent chromium, it has become an important hidden danger of soil and groundwater pollution. At present, the main remediation technology for hexavalent chromium contaminated soil is the solidification / stabilization method: using a reducing agent to reduce highly mobile and toxic hexavalent chromium to weakly mobile and low-toxic trivalent chromium and using a solidifying agent to convert trivalent chromium into an insoluble compound and encapsulate it in a solidified body with a complete structure.

[0003] Cement is the most widely used and mature solidifying agent at present. Its process is simple, the operation is convenient, and the material source is wide. However, the cement solidified body has limitations such as low early strength and high leaching rate due to the fact that hexavalent chromium cannot be completely encapsulated in the solidified body. At the same time, cement is a material with high energy consumption and high carbon emissions, which will cause serious environmental impacts during production and use. Sodium silicate is a common activator, which can promote the dissolution of silica and alumina in industrial solid waste and provide the active silica required for its polycondensation, and finally form a stable tetrahedral structure to increase the compressive strength of the solidified body. However, the current commercial sodium silicate is produced by mixing sodium carbonate and quartz sand under high temperature and high pressure conditions of 1300 °C, which will cause serious high energy consumption and greenhouse gas emissions and result in high costs, making it not conducive to large-scale use. The commonly used reducing agents for hexavalent chromium contaminated soil at present (such as sulfite, thiosulfate, zero-valent iron, ferrous chloride, etc.) can reduce highly toxic hexavalent chromium to low-toxic trivalent chromium, but these reducing agents will cause secondary pollution such as soil acidification, sulfate or H2S, and at the same time, these reducing agents are restricted by pH conditions, limiting their application scope. Therefore, there is an urgent need to develop a method for treating soil hexavalent chromium that is fast, effective, low-carbon and environmentally friendly and can be applied in multiple scenarios. Summary of the Invention

[0004] Object of the Invention: The present invention aims to provide a method for solidifying hexavalent chromium applicable to acidic / alkaline soil that is fast, effective, low-carbon and environmentally friendly and has a high solidification rate.

[0005] Technical Solution: The method for solidifying hexavalent chromium applicable to acidic / alkaline soil of the present invention solidifies hexavalent chromium in acidic / alkaline soil by combining a low-carbon cementitious material with glucose, and includes the following steps:

[0006] The hexavalent chromium contaminated soil is mixed with a composite solidifying agent, an activator and a reducing agent, water is added and stirred evenly to obtain a mixture; the mixture is successively put into a mold, formed, demolded and cured, thus completing the solidification; wherein, the low-carbon cementitious material includes high-calcium fly ash, carbide slag and desulfurized gypsum; the activator is silica fume water glass; the reducing agent is glucose.

[0007] In the above technical solution, high-calcium fly ash, carbide slag and desulfurized gypsum are used as the composite solidifying agent, D-anhydrous glucose is used as the reducing agent, and a solidified body is prepared under the action of 2.4 mold silica fume water glass as the activator, solving the disadvantages of low solidification rate, high carbon footprint and secondary pollution of traditional solidifying materials, thereby completing the rapid low-carbon solidification of hexavalent chromium contaminated soil.

[0008] Further, the hexavalent chromium contaminated soil and the low-carbon cementitious material are refined and then mixed.

[0009] Further, the particle sizes of the refined hexavalent chromium contaminated soil and the low-carbon cementitious material are less than 40 mesh.

[0010] Further, in the low-carbon cementitious material, the mass ratio of high-calcium fly ash, carbide slag and desulfurized gypsum is 4:3:1 to 3:4:1.

[0011] Further, the mass of the low-carbon cementitious material incorporated is 25-45% of the mass of the hexavalent chromium contaminated soil, the mass of the activator incorporated is 15-25% of the mass of the hexavalent chromium contaminated soil, and the mass of the reducing agent incorporated is 0.6-1.0% of the mass of the hexavalent chromium contaminated soil.

[0012] Further, the liquid-solid mass ratio in the mixture is 0.28-0.32:1, wherein the solid part includes hexavalent chromium contaminated soil, low-carbon cementitious material, activator and reducing agent, and the liquid part is water.

[0013] Preferably, an embodiment of the present invention provides a method for solidifying hexavalent chromium applicable to acid / alkali soil, including the following steps:

[0014] 1), Take hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, desulfurized gypsum and silica fume and dry and refine them;

[0015] 2), Take the refined hexavalent chromium contaminated soil, composite solidifying agent, activator and reducing agent in step 1) and mix them, add water at a certain liquid-solid mass ratio and mix evenly, and then carry out in-mold forming and demolding curing. The composite solidifying agent is a low-gel material composed of the refined high-calcium fly ash, carbide slag and desulfurized gypsum in step 1) mixed according to a mass ratio of 3:3:1, the activator is 2.4 mold silica fume water glass, and the reducing agent is D-anhydrous glucose.

[0016] Further, the preparation method of the activator is as follows:

[0017] The refined silica fume is treated with citric acid. After the treatment, it is washed, dried, and then stirred with sodium hydroxide and water under the condition of water bath heating. After stirring, the supernatant is taken by centrifugation to prepare the activator.

[0018] Further, in the preparation of the activator, the water bath temperature is 60 - 100 °C, and the stirring time under the condition of water bath heating is 1 - 3 h.

[0019] Further, in the preparation of the activator, based on the mass of the silica fume, the addition amount of citric acid is 10% wt; the citric acid treatment time is 3 h.

[0020] Preferably, the embodiment of the present invention provides a method for preparing an activator, comprising the following steps:

[0021] The refined silica fume is acidified with 10 wt% citric acid for 3 h, washed, dried, and then mixed evenly with sodium hydroxide and water according to a mass ratio of 2:1:3, stirred and heated at 60 - 100 °C for 1 - 3 h, centrifuged at 6500 r / min for 10 min, and then the supernatant is taken to prepare the activator.

[0022] Further, the curing conditions are a temperature of 25 - 30 °C, a relative humidity of more than 75%. After being cured in the mold and formed, it is demolded and cured, and the total curing duration is 1 - 3 days.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) Under the action of water glass prepared by a hydrothermal method using silica fume and sodium hydroxide as an activator, high-calcium fly ash, carbide slag, and desulfurized gypsum are used as a composite curing agent, and D-anhydrous glucose is used as a reducing agent. During the curing process of the solidified body, hexavalent chromium in acid / alkali contaminated soil is rapidly detoxified at one time and solidified by forming insoluble chromium hydroxide precipitation and amorphous C-A(Cr)-S-H phase, so that the unconfined compressive strength of the solidified soil is quickly higher than 800 kPa, meeting the standard for use as a subgrade material. The concentrations of hexavalent chromium and total chromium in the soil leachate meet the "Identification Standard for Toxicity of Leachate of Hazardous Wastes GB5085.3-2007" and the "Technical Specification for Environmental Protection of Chromium Slag Pollution Control HJ / T299-2007" respectively. The process is simple and no residue is left. (2) The present invention uses the reducibility and adsorbability of carbide slag to detoxify and solidify hexavalent chromium, and uses high-calcium fly ash and desulfurized gypsum to solidify the detoxified hexavalent chromium, while improving the early compressive strength of the material, so that the repaired soil can be used as a subgrade material, providing a new idea for the resource utilization of industrial solid waste and conforming to the environmental protection concept of "treating waste with waste". (3) The present invention prepares 2.4 mol silica fume water glass by mixing sodium hydroxide, silica fume and water and stirring under water bath heating at normal pressure, which rapidly dissociates the glassy silicon oxygen and calcium oxygen networks in the composite curing agent, accelerates the hydration reaction, and enables the solidified body to form a gelling system with high early compressive strength, realizing a more complete sealing effect on the trivalent chromium ions formed after detoxification, and at the same time providing a simpler method for preparing water glass, which can well replace commercially available 2.4 mol water glass to reduce the cost and carbon footprint of the solidified body. (4) The present invention uses D-anhydrous glucose as a reducing agent, which has low cost and is easy to obtain. This method has a large amount of solidification for hexavalent chromium contaminated soil and low leaching toxicity, overcomes the limitations of traditional reducing agents (such as ferrous chloride, sodium sulfide, etc.) and cement repair technologies, has a wide application pH range, can be applied to acidic and alkaline contaminated sites, and will not cause secondary soil pollution. Brief Description of the Drawings

[0024] Figure 1 It is a flow chart of the method for solidifying hexavalent chromium applicable to acid / alkali soil in an embodiment of the present invention. Detailed Embodiments

[0025] The technical solution of the present invention will be further described below with reference to the drawings.

[0026] The main chemical components of the hexavalent chromium contaminated soil in the embodiment of the present invention are silicon dioxide, aluminum oxide, calcium oxide, magnesium oxide, potassium dichromate, etc. Among them, the content of hexavalent chromium is 2000 mg / kg, the leaching concentration of hexavalent chromium is 88.25 mg / L, and the leaching concentration of total chromium is 89.58 mg / L.

[0027] High calcium fly ash is a fly ash byproduct with a high calcium oxide content emitted by thermal power plants using lignite and sub-bituminous coal as fuel. It contains a higher calcium oxide content (10-20%) and a smaller particle size than ordinary fly ash. It has the advantages of good water reduction effect and rapid early strength development when used as cement admixture or concrete admixture. Calcium carbide slag is a waste residue with calcium hydroxide as the main component after acetylene gas is obtained by hydrolysis of calcium carbide. It can replace limestone to make cement, produce building materials and be used for environmental governance. The main component of desulfurized gypsum is calcium sulfate dihydrate CaSO4·2H2O, which is a byproduct of the process of recovering sulfur dioxide in the flue gas of coal or oil using lime-limestone. It promotes the formation of hydrated calcium silicate (CSH) gel and calcium aluminum silicate phase, and improves the early compressive strength of the solidified body. Silica ash is formed by the rapid oxidation and condensation precipitation of a large amount of highly volatile SiO2 and Si gas discharged from the ore-fired electric furnace when ferrosilicon and industrial silicon are smelted by ferroalloys. Its SiO2 content exceeds 96%, and it is a good material for replacing quartz sand to prepare water glass. From the perspective of chemical composition, high-calcium fly ash, carbide slag, and desulfurized gypsum are silicate and carbonate materials, desulfurized gypsum is a carbonate and sulfate material, and silica ash is a silicate material. The main components of high-calcium fly ash, carbide slag, desulfurized gypsum and silica ash used in the embodiment of the present invention are shown in Table 1.

[0028] Table 1 Main components of high calcium fly ash, carbide slag, desulfurized gypsum and silica fume

[0029]

[0030] Example 1: Effect of hydrothermal heating time and temperature on the properties of silica fume water glass

[0031] The silica ash is dried and refined. The particle size of the refined silica ash is 40 mesh. After refinement, it is treated with 10wt% citric acid solution for 3 hours to remove impurities in the silica ash and increase the activity of the silica ash. After acid treatment, the silica ash is washed with water and dried for later use. Sodium hydroxide, silica ash and water are mixed in a mass ratio of 1:2:3 and placed in a constant temperature magnetic stirring water bath. They are heated and stirred at 60°C at normal pressure for 1h, 2h, and 3h, heated and stirred at 80°C at normal pressure for 1h, 2h, and 3h, and heated and stirred at 100°C at normal pressure for 1h, 2h, and 3h. Pour all the prepared water glass into a centrifuge tube and put it into a centrifuge. Centrifuge at 6500r / min for 10min. Take the supernatant and save it for later use and measure its pH. The preparation steps are as follows Figure 1 As shown. The residue at the bottom of the centrifuge tube was placed in a 60°C oven for drying and then its mass was measured using an electronic balance to calculate the silica fume conversion rate. The measured data are shown in Table 2. The silica fume conversion rate n is calculated as follows: n = (m0-m1) / m0. In the formula, m0 is the amount of silica fume added before preparation (kg), and m1 is the mass of the residue at the bottom after centrifugation (kg).

[0032] Table 2 Effects of Hydrothermal Heating Time and Temperature on the Conversion Rate of Silica Fume and the pH of Silica Fume Sodium Silicate

[0033]

[0034] As can be seen from Table 2, the conversion rate of silica fume is the highest when heated and stirred at 100 °C for 2 h, and the pH of the silica fume sodium silicate is the same as that of the commercially available 2.4 modulus sodium silicate.

[0035] Example 2: Effects of the Incorporation Mass of Composite Curing Agent, the Incorporation Mass of Activator, and the Liquid-Solid Mass Ratio on the Compressive Strength and Leaching pH of the Solidified Body

[0036] The hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, and desulfurization gypsum were dried and refined. After refinement, the particle size of the hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, and desulfurization gypsum was less than 40 mesh. The high-calcium fly ash, carbide slag, and desulfurization gypsum were mixed in a mass ratio of 3:3:1 as a composite curing agent. The silica fume was acidified with 10 wt% citric acid for 3 h, washed, dried, and then mixed with sodium hydroxide and water in a mass ratio of 2:1:3, stirred and heated at 100 °C for 2 h, centrifuged at 6500 r / min for 10 min, and the supernatant was taken to prepare. By mass percentage, the hexavalent chromium contaminated soil, 25% - 45% of the mass of the composite curing agent of the hexavalent chromium contaminated soil was mixed with 20% of the activator of the mass of the hexavalent chromium contaminated soil, and then water was added at a liquid-solid mass ratio of 0.30:1 and stirred evenly; the hexavalent chromium contaminated soil, 35% of the mass of the composite curing agent of the hexavalent chromium contaminated soil (the high-calcium fly ash, carbide slag, and desulfurization gypsum were mixed in a mass ratio of 4:3:1, 3:3:1, and 3:4:1 respectively) was mixed and then mixed with 20% of the activator of the mass of the hexavalent chromium contaminated soil, and then water was added at a liquid-solid mass ratio of 0.30:1 and stirred evenly; the hexavalent chromium contaminated soil, 35% of the mass of the composite curing agent of the hexavalent chromium contaminated soil was mixed with 15% - 25% of the activator of the mass of the hexavalent chromium contaminated soil, and then water was added at a liquid-solid mass ratio of 0.30:1 and stirred evenly; the hexavalent chromium contaminated soil, 35% of the mass of the composite curing agent of the hexavalent chromium contaminated soil was mixed with 15% - 25% of the activator of the mass of the hexavalent chromium contaminated soil, and then water was added at a liquid-solid mass ratio of 0.30:1 and stirred evenly; the hexavalent chromium contaminated soil, 35% of the mass of the composite curing agent of the hexavalent chromium contaminated soil was mixed with 20% of the activator of the mass of the hexavalent chromium contaminated soil, and then water was added at a liquid-solid mass ratio of 0.28 - 0.32:1 and stirred evenly; then each group was successively molded in a mold for 2 h, demolded, and cured for 1 - 3 days under the conditions of a temperature of 25 °C and a relative humidity of more than 75% to obtain a solidified body of hexavalent chromium contaminated soil. The preparation steps are as Figure 1 shown. Referring to GB / T 50123-2019 "Standard for Geotechnical Test Methods", the unconfined compressive strength of each group of solidified bodies was measured respectively. The unconfined compressive strength (kpa) =

[0037] Pressure (N) / Load-bearing area (cm 2 ) / 1000; The leaching pH of the solidified body was tested with reference to GB / T 15555.12-1995 "Determination of Corrosivity of Solid Wastes - Glass Electrode Method". The specific performance is shown in Table 3.

[0038] Table 3 Compressive strength and leaching pH of solidified bodies under different compound curing agent ratios and dosages, activator dosages, and different liquid-solid mass ratios

[0039]

[0040]

[0041] As can be seen from Table 3, by mass percentage, when 35% of the compound curing agent (mass ratio of high-calcium fly ash, carbide slag, and desulfurized gypsum = 3:3:1), 20% of the activator, and the liquid-solid mass ratio is 0.30:1 are incorporated into the hexavalent chromium contaminated soil, the compressive strength of the solidified body after 1-day curing is greater than 800 kPa, meeting the requirements for subgrade materials; at the same time, the leaching pH is less than 12.50, without corrosivity, and has little impact on the environment.

[0042] Comparative Example 1:

[0043] In this comparative example, silica fume sodium silicate and commercially available 2.4 modulus sodium silicate were used as activators to prepare solidified bodies, and other raw materials and methods were the same as those in Example 2. The physical properties of the prepared silica fume sodium silicate and commercially available 2.4 modulus sodium silicate are similar. The fluidity of the solidified body prepared with silica fume sodium silicate is significantly improved, the construction convenience is higher, and at the same time, the compressive strength of the solidified body can be increased and its corrosivity can be reduced. The specific performance is shown in Table 4.

[0044] Table 4 Performance comparison between silica fume sodium silicate and commercially available sodium silicate

[0045]

[0046] Example 3: Influence of different reductant dosages on the solidification effect

[0047] The hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, desulfurized gypsum and silica fume are dried and refined. After refinement, the particle size of the hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, desulfurized gypsum and silica fume is less than 40 mesh. The high-calcium fly ash, carbide slag and desulfurized gypsum are mixed in a mass ratio of 3:3:1 as a composite curing agent. The refined silica fume is acidified with 10wt% citric acid for 3 hours, washed and dried, and then mixed with sodium hydroxide and water in a mass ratio of 2:1:3, stirred and heated at 100°C for 2 hours, centrifuged at 6500 r / min for 10 minutes, and the supernatant is taken to prepare. D-anhydrous glucose is used as a reducing agent. By mass percentage, the hexavalent chromium contaminated soil, 35% of the composite curing agent based on the mass of the hexavalent chromium contaminated soil, 20% of the activator based on the mass of the hexavalent chromium contaminated soil are mixed with 0-1% of the reducing agent based on the mass of the hexavalent chromium contaminated soil. Then, water is added at a liquid-solid mass ratio of 0.30:1 and stirred evenly. Then, it is molded in a mold for 2 hours, demolded and cured for 1-3 days under the conditions of a temperature of 25°C and a relative humidity of more than 75% to obtain a solidified body of the detoxified and solidified hexavalent chromium contaminated soil. The preparation steps are as Figure 1 . The compressive strength of the solidified body is measured according to the "Standard for Geotechnical Test Methods" GB / T 50123-2019, and the leaching toxicity of the solidified body is measured according to the "Method for Leaching Toxicity of Solid Wastes - Acetic Acid Buffer Solution Method" HJ / T 300-2007. The leaching concentrations of hexavalent chromium and total chromium are measured according to the "Determination of Hexavalent Chromium in Solid Wastes - Diphenylcarbazide Spectrophotometry" GB15555.4-1995 and the "Determination of Total Chromium in Solid Wastes - Diphenylcarbazide Spectrophotometry" GB15555.5-1995 respectively, and judged according to the "Identification Standard for Hazardous Wastes - Leaching Toxicity Identification" GB5085.3-2007 and the "Technical Specification for Environmental Protection of Chromium Residue Pollution Control" HJ / T299-2007 respectively. The specific performance is shown in Table 5.

[0048] Table 5 Unconfined Compressive Strength and Leaching Toxicity of Solidified Bodies with Different Reducing Agent Dosages

[0049]

[0050] It can be found from Table 5 that with the incorporation of glucose, the leaching concentrations of both hexavalent chromium and total chromium decrease significantly. When the dosage of the reducing agent is less than or equal to 0.8%, the unconfined compressive strength of the solidified body cured for 1 day is greater than 800 kPa. When the dosage of the reducing agent is greater than or equal to 0.8%, the leaching concentrations of hexavalent chromium and total chromium in the solidified body after curing for 1 day are respectively lower than the hazardous waste discrimination values of 5 mg / L and 15 mg / L in GB5085.3-2007, and the leaching concentrations of hexavalent chromium and total chromium after curing for 3 days are respectively lower than the limits of 0.5 mg / L and 1.5 mg / L for chromium slag used as subgrade material in HJ / T299-2007.

[0051] Table 6 Leaching Toxicity of Solidified Bodies with Different Curing Agent Ratios

[0052]

[0053]

[0054] As can be seen from Table 6, it is found that under different ratios, the best ratio of the composite curing agent after the incorporation of glucose is still 3:3:1 (mass of high-calcium fly ash: mass of carbide slag: mass of desulfurized gypsum), which is consistent with the best ratio in Example 2.

[0055] Comparative Example 2:

[0056] Ascorbic acid is another common organic reducing agent. In this comparative example, the incorporation of D-anhydroglucose and ascorbic acid into the solidified body respectively, with other raw materials and methods being the same as those in Example 3, it is found that the solidified body with D-anhydroglucose incorporated at an early stage has a higher compressive strength and a lower leaching concentration of hexavalent chromium, and has a better solidification effect on soil hexavalent chromium. The specific performance is shown in Table 7.

[0057] Table 7 Effects of Incorporating D-anhydroglucose and Ascorbic Acid on the Unconfined Compressive Strength and Leaching Toxicity of the Solidified Body

[0058]

[0059] Example 4: Effects of Different Dosages of Reducing Agents and Ratios of Curing Agents on the Leaching Rates of Hexavalent Chromium and Total Chromium

[0060] The hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, desulfurized gypsum and silica fume are dried and refined. After refinement, the particle sizes of the hexavalent chromium contaminated soil, high-calcium fly ash, carbide slag, desulfurized gypsum and silica fume are less than 40 mesh. The high-calcium fly ash, carbide slag and desulfurized gypsum are mixed at a mass ratio of 3:3:1 as a composite curing agent. The refined silica fume is acidified with 10 wt% citric acid for 3 h, washed and dried, and then mixed with sodium hydroxide and water at a mass ratio of 2:1:3, stirred and heated at 100 °C for 2 h, centrifuged at 6500 r / min for 10 min, and the supernatant is taken to prepare. D-anhydroglucose is used as the reducing agent. By mass percentage, the hexavalent chromium contaminated soil, 35% of the composite curing agent by the mass of the hexavalent chromium contaminated soil, 20% of the activator by the mass of the hexavalent chromium contaminated soil are mixed with 0.6 - 1% of the reducing agent by the mass of the hexavalent chromium contaminated soil, and then water is added at a liquid-solid mass ratio of 0.30:1 and stirred evenly. Then it is molded in a mold for 2 h, demolded and cured for another 3 days under the conditions of a temperature of 25 °C and a relative humidity of more than 75%. The solidified body is immersed in an extraction agent of HNO3 with pH = 2, and the volume of the leaching solution / surface area of the solidified body = 10 ± 0.3. The extraction agent is replaced at 2 h, 7 h, 1 d, 2 d, 3 d, and 4 d of soaking, and the total leaching period is 5 days. The leaching rates of hexavalent chromium and total chromium are calculated according to the following formula:

[0061]

[0062] Where: R n is the leaching rate of hexavalent chromium or total chromium in the nth leaching cycle; m n is the mass of hexavalent chromium or total chromium leached in the nth leaching cycle; M is the initial mass of hexavalent chromium or total chromium in the solidified body; S is the geometric surface area of contact between the solidified body and the leaching agent; V is the volume of the solidified body; t n is the duration of the nth leaching cycle.

[0063] The total chromium leaching rate of hexavalent chromium is determined according to HG / T 20715-2020 "Technical Specification for Vertical Barrier Technology in Industrial Polluted Sites". The specific performance is shown in Table 8.

[0064] Table 8 Leaching Rates of Hexavalent Chromium and Total Chromium in Solidified Bodies with Different Reducing Agent Dosages

[0065]

[0066] It is found from Table 8 that when the dosage of the reducing agent is greater than or equal to 0.8%, the leaching rates of hexavalent chromium and total chromium always meet the standard of less than 10 -7 cm / s, indicating that the solidified body has good solidification and anti-seepage effects on soil hexavalent chromium and total chromium.

[0067] Experimental Example 1

[0068] A certain electroplating factory site in Jiangsu covers an area of 22,000 m 2 , within the site, the soil is mainly cohesive soil, the average permeability coefficient K is 5.82×10 -6 cm / s, the pH is acidic, the average pH is 4.02, and it is detected that the hexavalent chromium in the soil seriously exceeds the standard, with an average content of 1080 mg / kg, exceeding 13.85 times the screening value (78 mg / kg) for second-class construction land in the "Soil Environmental Quality - Risk Control Standards for Soil Pollution of Construction Land" (GB 36600-2018), and the leaching concentration of hexavalent chromium is 96.40 mg / L. In the electroplating factory site, within the range of 5 m×3 m×1 m, the low-carbon cementitious material and D-anhydrous glucose combined material mentioned in Example 3 are used to solidify and prevent seepage of the acidic soil contaminated by hexavalent chromium in the site. The specific construction includes the following steps:

[0069] S1. Excavate a trench at the proposed solidification and anti-seepage location

[0070] by a shallow tunneling machine;

[0071] S2. By mass percentage, put the contaminated soil, 35% of the composite solidifying agent of the mass of the contaminated soil, and 0.6 - 1.0% of the reducing agent into a concrete mixer and mix evenly to obtain a premix;

[0072] S3. Put the premix into a concrete mixer, add an activator accounting for 20% of the mass of the contaminated soil by mass percentage, and add water to the premix according to a liquid-solid mass ratio of 0.30:1, and stir at a low speed for 5 min to form a slurry-like paste.

[0073] S4. Pour the slurry-like paste into the tank.

[0074] S5. After the pouring is completed, cover a layer of in-situ soil on the top to prevent shrinkage cracking on the top. After 2 h, use a soil compactor to compact and level the surface of the construction soil.

[0075] S6. Take samples within the range 3 days after the construction for unconfined compressive strength test and leaching toxicity test. The test methods refer to Example 2 and Example 3, and the specific performance is shown in Table 9.

[0076] Table 9 Solidification effect of hexavalent chromium in acidic soil at a certain electroplating plant site in Jiangsu

[0077]

[0078] Experimental Example 2

[0079] The site of a certain chemical plant in Zhejiang covers an area of 48,000 m 2 , within the site, the soil is mainly silty sand, the average permeability coefficient K is 3.67×10 -5 cm / s, the pH is acidic, the average pH is 5.51, and it is detected that the hexavalent chromium in the soil seriously exceeds the standard, with an average content of 1134 mg / kg, exceeding 14.54 times the screening value (78 mg / kg) for the second type of construction land in the "Soil Environmental Quality - Risk Control Standards for Soil Pollution of Construction Land" (GB 36600-2018), and the leaching concentration of hexavalent chromium is 97.92 mg / L. In the site of this electroplating plant, within the range of 6 m×4 m×1.5 m, the low-carbon cementitious material and D-anhydroglucose combined material mentioned in Example 3 are used to solidify and prevent seepage of the acidic soil contaminated by hexavalent chromium in the site. The specific construction includes the following steps:

[0080] S1. Excavate a tank at the proposed solidification and anti-seepage position by a shallow tunneling machine.

[0081] S2. By mass percentage, put the contaminated soil, a composite solidifying agent accounting for 35% of the mass of the contaminated soil, and 0.6 - 1.0% of a reducing agent into a concrete mixer and mix evenly to obtain a premix.

[0082] S3. Put the premix into a concrete mixer, add an activator accounting for 20% of the mass of the contaminated soil by mass percentage, and add water to the premix according to a liquid-solid mass ratio of 0.30:1, and stir at a low speed for 5 min to form a slurry-like paste.

[0083] S4. Pour the slurry-like grout into the tank.

[0084] S5. After the pouring is completed, cover the top with a layer of in-situ soil to prevent dry shrinkage and cracking at the top. After 2 h, use a soil compactor to compact and level the surface of the construction soil.

[0085] S6. Three days after the construction, take samples within the specified range for unconfined compressive strength test and leaching toxicity test. The test methods refer to Example 2 and Example 3, and the specific performance is shown in Table 10.

[0086] Table 10 Solidification effect of hexavalent chromium in acidic soil at a chemical plant site in Zhejiang

[0087]

[0088] Experimental Example 3

[0089] A certain electronics factory site in Guangdong covers an area of 15,000 m 2 . Within the site, the soil is mainly plain fill, with an average permeability coefficient K of 1.72×10 -5 cm / s, alkaline pH, with an average pH of 9.05, and it is detected that the hexavalent chromium in the soil seriously exceeds the standard, with an average content of 891 mg / kg, which is 11.42 times higher than the screening value (78 mg / kg) for Class II construction land in the "Soil Environmental Quality - Risk Control Standards for Soil Pollution of Construction Land" (GB 36600 - 2018), and the leaching concentration of hexavalent chromium is 76.10 mg / L. In the electroplating factory site, within the range of 4 m×4 m×2 m, the low-carbon cementitious material and D-anhydrous glucose combined material mentioned in Example 3 are used to solidify and prevent seepage of the acidic soil contaminated by hexavalent chromium in the site. The specific construction includes the following steps:

[0090] S1. Use a shallow tunneling machine to excavate a tank at the proposed solidification and anti-seepage location;

[0091] S2. By mass percentage, put the contaminated soil, 35% of the composite curing agent based on the mass of the contaminated soil, and 0.6 - 1.0% of the reducing agent into a concrete mixer and mix evenly to obtain a premix;

[0092] S3. Take the premix and put it into a concrete mixer. By mass percentage, add 20% of the activator based on the mass of the contaminated soil, and add water to the premix according to a liquid-solid mass ratio of 0.30:1, and stir at a low speed for 5 min to form a slurry-like grout;

[0093] S4. Pour the slurry-like grout into the tank.

[0094] S5. After the pouring is completed, cover the top with a layer of in-situ soil to prevent dry shrinkage and cracking at the top. After 2 h, use a soil compactor to compact and level the surface of the construction soil.

[0095] After 3 days of construction, samples within the range were taken for unconfined compressive strength test and leaching toxicity test. The test methods refer to Example 2 and Example 3, and the specific properties are shown in Table 11.

[0096] Table 11 Solidification effect of hexavalent chromium in acidic soil at a site of an electronics factory in Guangdong

[0097]

[0098]

[0099] Experimental Example 4

[0100] A landfill site in Anhui covers an area of 46,000 m 2 , within the site and its surrounding areas, the soil is mainly clayey soil, the permeability coefficient K≈1.03×10 -6 cm / s, the pH is alkaline, with an average of 11.83, and it is detected that the hexavalent chromium in the soil seriously exceeds the standard, with an average content of 966 mg / kg, which is 12.38 times higher than the screening value (78 mg / kg) for second-class construction land in the "Soil Environmental Quality - Risk Control Standards for Soil Pollution of Construction Land" (GB 36600 - 2018), and the leaching concentration of hexavalent chromium is 86.75 mg / L. In this landfill site, within a range of 5 m×5 m×1 m, the low-carbon cementitious material and D-anhydrous glucose combined material mentioned in Example 3 were used to solidify and prevent seepage of the alkaline soil contaminated by hexavalent chromium in the site. The specific construction includes the following steps:

[0101] S1. Excavate a trough at the proposed solidification and anti-seepage position by a shallow tunneling machine;

[0102] S2. By mass percentage, put the contaminated soil, 35% of the composite solidifying agent of the mass of the contaminated soil, and 0.6 - 1.0% of the reducing agent into a concrete mixer and mix evenly to obtain a premix;

[0103] S3. Take the premix and put it into a concrete mixer. By mass percentage, add 20% of the activator of the mass of the contaminated soil, and add water to the premix according to the liquid-solid mass ratio of 0.30:1, and stir at low speed for 5 min to form a slurry-like paste;

[0104] S4. Pour the slurry-like paste into the trough.

[0105] S5. After pouring, cover a layer of in-situ soil on the top to prevent dry shrinkage and cracking on the top. After 2 h, use a soil compactor to compact and level the surface of the construction soil.

[0106] S6. After 3 days of construction, take samples within the range for unconfined compressive strength test and leaching toxicity test. The test methods refer to Example 2 and Example 3, and the specific properties are shown in Table 12.

[0107] Table 12 Solidification effect of hexavalent chromium in alkaline soil at a landfill site in Anhui

[0108]

[0109] Combined with the above experimental examples, it is found that the method of using low-carbon cementitious materials and D-anhydrous glucose in combination to solidify hexavalent chromium in acidic / alkaline soil has good solidification effects on both acidic and alkaline soil pollution sites. It not only has high compressive strength at an early stage, but also meets the requirements of HJ / T 299-2007 "Technical Specifications for Environmental Protection of Chromium Residue Pollution Control" as a subgrade material when the dosage of the reducing agent is greater than 0.8%.

[0110] It can be found from the above examples and experimental examples that the activator made of sodium hydroxide and silica fume by the hydrothermal method in the composite solidifying agent made by mixing high-calcium fly ash, carbide slag and desulfurized gypsum in the present invention rapidly dissociates the vitreous silicon-oxygen and calcium-oxygen networks, accelerates the hydration reaction, forms a cementitious system with high early compressive strength, realizes a more complete encapsulation effect on the chromium ions after detoxification and the resource utilization of solid waste, and by incorporating D-anhydrous glucose as a reducing agent, rapid detoxification and fixation of hexavalent chromium in the contaminated soil are achieved at one time during the solidification process. The process is simple, the raw material cost is low and easy to obtain, the solidification amount of hexavalent chromium-contaminated soil is large, the leaching toxicity is low and there is no secondary pollution, and it can be applied in acidic and alkaline pollution sites, broadening its scope of application.

[0111] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, it cannot be understood as a limitation of the protection scope of this application. Those skilled in the art should understand that various forms and details can be modified and improved without departing from the scope defined by the claims of the present invention.

Claims

1. A method for solidifying hexavalent chromium applicable to acidic / alkaline soil, characterized in that, Solidifying hexavalent chromium in acidic / alkaline soil by using a low-carbon cementitious material in combination with glucose, comprising the following steps: Mix the hexavalent chromium-contaminated soil with a low-carbon cementitious material, an activator and a reducing agent, add water and stir evenly to obtain a mixture; the mixture is successively put into a mold, formed, demolded and cured, thus completing the solidification; wherein, the low-carbon cementitious material includes high-calcium fly ash, carbide slag and desulfurized gypsum; the activator is silica fume water glass; the reducing agent is glucose.

2. The hexavalent chromium solidification method applicable to acid / alkali soil according to claim 1, characterized in that, The hexavalent chromium-contaminated soil and the low-carbon cementitious material are respectively refined and then mixed.

3. The hexavalent chromium solidification method applicable to acidic / alkaline soil according to claim 1, characterized in that, The particle sizes of the refined hexavalent chromium-contaminated soil and low-carbon cementitious material are less than 40 mesh.

4. The method for solidifying hexavalent chromium applicable to acidic / alkaline soil according to claim 1, characterized in that, In the low-carbon cementitious material, the mass ratio of high-calcium fly ash, carbide slag and desulfurized gypsum is 4:3:1 to 3:4:

1.

5. The method for solidifying hexavalent chromium applicable to acidic / alkaline soil according to claim 1, characterized in that, The mass of the low-carbon cementitious material incorporated is 25-45% of the mass of the hexavalent chromium-contaminated soil, the mass of the activator incorporated is 15-25% of the mass of the hexavalent chromium-contaminated soil, and the mass of the reducing agent incorporated is 0.6-1.0% of the mass of the hexavalent chromium-contaminated soil.

6. The method for solidifying hexavalent chromium applicable to acidic / alkaline soil according to claim 5, characterized in that, The liquid-solid mass ratio in the mixture is 0.28-0.32:1, wherein the solid part includes the hexavalent chromium-contaminated soil, the low-carbon cementitious material, the activator and the reducing agent, and the liquid part is water.

7. The hexavalent chromium solidification method applicable to acid / alkali soil according to claim 1, characterized in that, The preparation method of the activator is as follows: The refined silica fume is treated with citric acid. After the treatment is completed, it is washed, dried, and then stirred with sodium hydroxide and water under the condition of water bath heating. After the stirring is completed, the supernatant is taken by centrifugation to prepare the activator.

8. The hexavalent chromium solidification method applicable to acidic / alkaline soil according to claim 7, characterized in that, In the preparation of the activator, the water bath temperature is 60-100 °C, and the stirring time under the condition of water bath heating is 1-3 h.

9. The method for solidifying hexavalent chromium applicable to acidic / alkaline soil according to claim 7, characterized in that, In the preparation of the activator, based on the mass of the silica fume, the addition amount of citric acid is 5-10% wt; the citric acid treatment time is 2-4 h.

10. The method for solidifying hexavalent chromium applicable to acidic / alkaline soil according to claim 1, characterized in that, The curing conditions are a temperature of 25-30 °C, a relative humidity of more than 75%. After curing in the mold and forming, it is demolded and cured, and the total curing duration is 1-3 days.

Images