A green low-carbon treatment method for soil contaminated by cyanide and hexavalent chromium

Abstract

The application discloses a green low-carbon treatment method for cyanide and hexavalent chromium composite contaminated soil. The treatment method comprises the following steps: 1) screening and crushing the cyanide and hexavalent chromium composite contaminated soil, so that the particle size of the crushed soil is less than or equal to 5 cm; 2) mixing the screened cyanide and hexavalent chromium composite contaminated soil with a compounded reagent to perform treatment and first maintenance, wherein the compounded reagent comprises sodium thiosulfate pentahydrate and sodium dihydrogen phosphate; and 3) mixing the cyanide and hexavalent chromium composite contaminated soil after the first maintenance with sodium hydroxide to adjust the pH value, then performing second maintenance, and finally performing repair effect evaluation until the pollutants in the soil reach the standard. The application is suitable for various cyanide and hexavalent chromium composite contaminated soils, and the physicochemical properties of the treated contaminated soil are stable, so that the subsequent environmental risk is greatly reduced. Meanwhile, compared with a traditional repair process, the carbon emission amount of the treatment process is significantly reduced, and the application has the characteristics of being green and low-carbon.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, and more specifically, relates to a green and low-carbon treatment method for soil contaminated by cyanide and hexavalent chromium. Background Technology

[0002] Cyanide is widely used in industrial production sectors such as electroplating and metallurgy, and its long-term production and application can cause certain pollution to the soil environment. Simultaneously, during industrial activities in electroplating and metallurgy, heavy metals such as hexavalent chromium may also enter the soil along with cyanide, causing heavy metal pollution. Because cyanide and hexavalent chromium are widely used in electroplating and metallurgy, long-term production in industrial parks can easily lead to soil contamination from the combined effects of cyanide and hexavalent chromium. Site surveys of decommissioned industrial sites have revealed that soil contamination from the combined effects of cyanide and hexavalent chromium is prevalent.

[0003] Cyanide is highly toxic to humans; 100mg of sodium cyanide can be fatal. Hexavalent chromium, after entering the human body through the skin, mucous membranes, or inhaled dust, accumulates in the body and eventually causes a variety of health problems, including various cancers, as well as adverse effects on the blood, kidneys, and respiratory system.

[0004] Industrial sites contaminated with cyanide and hexavalent chromium require remediation before redevelopment after decommissioning. Only after meeting site usage requirements can further development proceed. Meanwhile, the currently most commonly used cement kiln co-processing method generates significant carbon emissions, which is increasingly failing to meet the demands for green and low-carbon remediation under current energy conservation, emission reduction, and the trend towards "carbon neutrality." Therefore, there is an urgent need to provide a simple, efficient, green, and low-carbon method for treating soils contaminated with both cyanide and hexavalent chromium. Summary of the Invention

[0005] This invention aims to provide a green and low-carbon treatment method for soil contaminated with cyanide and hexavalent chromium. It proposes to detoxify and stabilize the soil contaminated with cyanide and hexavalent chromium, rationally select treatment agents and treatment methods, and finally obtain a method and process to guide the construction of treatment for soil contaminated with cyanide and hexavalent chromium, ensuring the remediation effect and providing convenience for subsequent risk management.

[0006] To achieve the above objectives, the present invention provides a method for treating soil contaminated with a combination of cyanide and hexavalent chromium, the method comprising:

[0007] 1) The soil contaminated with cyanide and hexavalent chromium was crushed to reduce the soil particle size to ≤5cm;

[0008] 2) The crushed cyanide and hexavalent chromium contaminated soil is mixed with sodium thiosulfate compound agent for treatment, while the soil moisture content is adjusted and the first maintenance is carried out. The sodium thiosulfate compound agent includes sodium thiosulfate pentahydrate and sodium dihydrogen phosphate.

[0009] 3) The cyanide and hexavalent chromium contaminated soil after the first curing is mixed with a stabilizer and treated for a second curing. The stabilizer is a sodium hydroxide solution.

[0010] As a preferred option, in step 2), the moisture content of the soil contaminated with cyanide and hexavalent chromium is adjusted to 30%.

[0011] According to the present invention, after stabilization treatment, the pollutants trivalent chromium and sodium thiocyanate in the soil are further stabilized, and the yellowing of the soil due to hexavalent chromium is less likely to occur, which facilitates subsequent long-term management.

[0012] As a preferred option, in step 2), the amount of sodium thiosulfate pentahydrate added is 3.5-4.5% and the amount of sodium dihydrogen phosphate added is 0.8-1% relative to the total mass of the soil contaminated with cyanide and hexavalent chromium. This achieves the effects of cyanide detoxification and stable reduction of hexavalent chromium.

[0013] As a preferred option, in step 2), the first maintenance period is 7 days.

[0014] As a preferred embodiment, the mass percentage concentration of the sodium hydroxide solution is 30%. In step 3), the amount of sodium hydroxide solution added varies according to the total mass of the composite contaminated soil. Preferably, the amount added is such that the pH of the composite contaminated soil after the addition of the stabilizer is controlled at 8-9.

[0015] As a preferred option, in step 3), the second maintenance period is 7 days.

[0016] As a preferred option, both the first and second curing processes are performed with a film coating.

[0017] As a preferred option, in step 1), the concentration of cyanide in the soil contaminated by cyanide and hexavalent chromium is ≤89.35 mg / kg and the concentration of hexavalent chromium is ≤218 mg / kg.

[0018] After step 3), the process also includes an effect assessment and monitoring step for the soil after treatment. The effect assessment and monitoring involves detecting the concentrations of cyanide and hexavalent chromium in the soil. If the detected concentrations are both lower than the set values, the treatment of the compound contaminated soil is complete. Otherwise, return to step 2) for remediation.

[0019] As a preferred embodiment, the present invention has the following effects and advantages compared with the prior art:

[0020] 1) The sodium thiosulfate compound used contains sodium thiosulfate (pentahydrate) and sodium dihydrogen phosphate. Cyanide in the soil combines with the sulfur and cyanide ions provided by sodium thiosulfate to form low-toxicity sodium thiocyanate, thus reducing cyanide toxicity. On the other hand, sodium thiosulfate can reduce hexavalent chromium in the soil to low-toxicity trivalent chromium, achieving remediation.

[0021] S2O3 2- +CN - →SO3 2- +SCN -

[0022] Cr2O7 2- + 14H + + 6 S2O3 2- →2Cr 3+ + 7H2O +3S4O6 2-

[0023] Simultaneously, sodium dihydrogen phosphate is added based on the characteristics of trivalent chromium. Phosphate has the effect of stabilizing trivalent chromium. After hexavalent chromium is reduced to trivalent chromium by sodium thiosulfate, some trivalent chromium ions exist as amorphous trivalent chromium. Using sodium dihydrogen phosphate as a stabilizer can convert amorphous trivalent chromium into insoluble CrPO4 crystals. Moreover, these crystals are less affected by pH value, which can further improve the stability of trivalent chromium in the soil, reduce the amount of remediation agent added, and ensure the long-term remediation effect.

[0024] 2) Due to the unstable soil pH, sodium hydroxide is added after the first step of chemical treatment for soil contaminated with cyanide and hexavalent chromium. This can keep the soil in an alkaline state. On the one hand, it can increase the stability of trivalent chromium in the remediated soil and reduce the migration and infiltration of chromium into the external environment. On the other hand, sodium thiocyanate can also exist more stably under alkaline conditions, reducing the environmental risk of subsequent disposal of the remediated soil.

[0025] 3) This invention is applicable to the remediation of soil contaminated with cyanide and hexavalent chromium. After treatment, the physicochemical properties of the contaminated soil are stable, which greatly reduces the environmental risks of subsequent disposal.

[0026] 4) To handle 10,000m 3 Based on the carbon emission factor method, the carbon emission of the contaminated soil treated by the method selected in this invention is calculated to be 11.55 kg / m³. 3 This is in conjunction with the currently used remediation method of co-processing cement kilns (carbon emissions are 376.49 kg / m³). 3 Wet detoxification + stabilization treatment (carbon emissions are 37.05 kg / m³) 3 Compared to other materials, it has the characteristics of being green and low-carbon.

[0027] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0028] Figure 1 A flowchart of a method for treating soil contaminated with cyanide and hexavalent chromium according to an embodiment of the present invention is shown. Detailed Implementation

[0029] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0030] Figure 1 A flowchart illustrating a method for treating soil contaminated with a combination of cyanide and hexavalent chromium according to an embodiment of the present invention is shown. Figure 1 As shown, the soil contaminated with cyanide and hexavalent chromium is first excavated and transported, then crushed and screened. Sodium thiosulfate compound agent is then added to the crushed and screened soil for the first curing, followed by sodium hydroxide for the second curing. The effect is then evaluated and monitored. If the standard is met (the concentrations of cyanide and hexavalent chromium in the soil are both lower than the set values, for example, lower than the screening value requirements for Class I land in the "Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial)" (GB36600-2018)), the remediation ends. If the standard is not met, the agent addition and other steps are repeated.

[0031] Example 1

[0032] In Example 1, the selected soil sample had a cyanide concentration of 89.35 mg / kg, a hexavalent chromium concentration of 218 mg / kg, a soil pH of 7.86, and a moisture content of approximately 22%. Approximately 10 tons of contaminated soil were excavated and transported to the treatment site in sealed vehicles. The contaminated soil was crushed using an ALLU crushing and screening bucket, resulting in a particle size ≤5 cm. The crushed contaminated soil was then mixed with 4.5% sodium thiosulfate pentahydrate and 1% sodium dihydrogen phosphate by weight using the ALLU crushing and screening bucket. Water was then added to adjust the soil moisture content to 30%. After three stirrings to ensure thorough mixing of the agent with the soil, the soil was covered and cured under natural conditions for 7 days. After curing, a 1% sodium hydroxide solution (30% concentration) was evenly sprayed onto the soil and thoroughly mixed again using an ALLU crushing and screening hopper until the soil pH stabilized at 8-9. After thorough mixing, the soil was covered and cured under natural conditions for 7 days. The soil after remediation was found to have a cyanide concentration of 1.23 mg / kg, a hexavalent chromium concentration of 0.53 mg / kg, and a pH of 8.23, which meet the screening value requirements for Class I land use in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)" (GB36600-2018).

[0033] Example 2

[0034] In Example 2, the selected soil sample had a cyanide concentration of 43.46 mg / kg, a hexavalent chromium concentration of 82.12 mg / kg, a soil pH of 7.57, and a moisture content of approximately 21%. Approximately 10 tons of contaminated soil were excavated and transported to the treatment site in sealed vehicles. The contaminated soil was crushed using an ALLU crushing and screening hopper, resulting in a particle size ≤5 cm. The crushed contaminated soil was then mixed with 4% sodium thiosulfate pentahydrate and 1% sodium dihydrogen phosphate by weight using the ALLU crushing and screening hopper. Water was then added to adjust the soil moisture content to 30%. After three stirrings to ensure thorough mixing of the agent with the soil, the soil was covered and cured under natural conditions for 7 days. After curing, a 1% sodium hydroxide solution (30% concentration) was evenly sprayed onto the soil and thoroughly mixed again using an ALLU crushing and screening hopper until the soil pH stabilized at 8-9. After thorough mixing, the soil was covered and cured under natural conditions for 7 days. The soil after remediation was found to have a cyanide concentration of 1.02 mg / kg, a hexavalent chromium concentration of 1.63 mg / kg, and a pH of 8.01, which meet the screening value requirements for Class I land use in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)" (GB36600-2018).

[0035] Example 3

[0036] In Example 3, the selected soil sample had a cyanide concentration of 23.12 mg / kg, a hexavalent chromium concentration of 24.01 mg / kg, a soil pH of 7.53, and a moisture content of approximately 23%. Approximately 10 tons of contaminated soil were excavated and transported to the treatment site in sealed vehicles. The contaminated soil was crushed using an ALLU crushing and screening hopper, resulting in a particle size ≤5 cm. The crushed contaminated soil was then mixed with 3.5% sodium thiosulfate pentahydrate and 0.8% sodium dihydrogen phosphate by weight using the ALLU crushing and screening hopper. Water was then added to adjust the soil moisture content to 30%. After three stirrings to ensure thorough mixing of the agent with the soil, the soil was covered and cured under natural conditions for 7 days. After curing, a 1% sodium hydroxide solution (30% concentration) was evenly sprayed onto the soil and thoroughly mixed again using an ALLU crushing and screening hopper until the soil pH stabilized at 8-9. After thorough mixing, the soil was covered and cured under natural conditions for 7 days. The soil after remediation showed no detectable cyanide, a hexavalent chromium concentration of 0.56 mg / kg, and a pH of 8.21, meeting the screening value requirements for Class I land use in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)" (GB36600-2018).

[0037] The above embodiments demonstrate that the method of the present invention can significantly reduce the pollutant indicators in soil contaminated with cyanide and hexavalent chromium. For soil contaminated with cyanide concentration ≤89.35 mg / kg and hexavalent chromium concentration ≤218 mg / kg, a single treatment using the method of the present invention can meet the screening values ​​for Class I land use in the corresponding standards, proving the high efficiency of the method. Furthermore, the present invention rationally selects the composition of the remediation agent based on the characteristics of the pollutants, resulting in a smaller dosage. Adding sodium hydroxide after the first step of the chemical treatment for soil contaminated with cyanide and hexavalent chromium increases the stability of trivalent chromium in the remediated soil, reducing the migration and infiltration of chromium into the external environment. Additionally, sodium thiocyanate remains more stable under alkaline conditions, reducing the environmental risk of subsequent soil disposal and ensuring the long-term effectiveness of the remediation.

[0038] Compared to traditional methods such as cement kiln co-treatment and landfilling of soil contaminated with hexavalent chromium and cyanide, the treatment method of this invention has the advantages of low carbon emissions and low risk of secondary pollution during construction. It can be applied to the treatment of 10,000 m³ of soil. 3 Based on the carbon emission factor method, the carbon emission of the contaminated soil treated by the method selected in this invention is calculated to be 11.55 kg / m³. 3 This is in conjunction with the currently used remediation method of co-processing cement kilns (carbon emissions are 376.49 kg / m³). 3 Wet detoxification + stabilization treatment (carbon emissions are 37.05 kg / m³) 3 Compared to other materials, it has the characteristics of being green and low-carbon.

[0039] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A green and low-carbon treatment method for soil contaminated with a combination of cyanide and hexavalent chromium, characterized in that, include: 1) The soil contaminated with cyanide and hexavalent chromium was pre-treated by crushing to make the soil particle size ≤5cm; The concentration of cyanide in soil contaminated by cyanide and hexavalent chromium is ≤89.35 mg / kg, and the concentration of hexavalent chromium is ≤218 mg / kg. 2) The pre-treated cyanide and hexavalent chromium contaminated soil is mixed with a sodium thiosulfate compound agent for mixing treatment, and water is added to control the moisture content for the first curing. The sodium thiosulfate compound agent includes sodium thiosulfate pentahydrate and sodium dihydrogen phosphate. In step 2), relative to the total mass of the cyanide and hexavalent chromium contaminated soil, the amount of sodium thiosulfate pentahydrate added is 3.5-4.5%, and the amount of sodium dihydrogen phosphate added is 0.8-1%. After adding water, the soil moisture content is controlled at 30%. The first curing time is 7 days, and the first curing is carried out by covering with a film. 3) After the first curing, the cyanide and hexavalent chromium contaminated soil is mixed with a stabilizer for stabilization treatment and then subjected to a second curing. The stabilizer is a sodium hydroxide solution with a mass percentage concentration of 30%. The amount of stabilizer added is such that the pH of the contaminated soil after adding the stabilizer is controlled at 8-9. The second curing time is 7 days, and the second curing is carried out by covering with a film.

2. The green and low-carbon treatment method for soil contaminated with cyanide and hexavalent chromium as described in claim 1, wherein, In step 1), the crushing method is to use an ALLU crushing and screening bucket for crushing.

3. The green and low-carbon treatment method for soil contaminated by cyanide and hexavalent chromium according to claim 1, after step 3), further includes a step of evaluating and monitoring the effect of the soil after maintenance. The effect evaluation and monitoring involves detecting the concentration of cyanide and hexavalent chromium in the soil. If the detected concentrations are both lower than the set values, the treatment of the contaminated soil is completed. Otherwise, return to step 2) for remediation.

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