1. Types and characteristics of heavy metal treatment technologies
At present, there are mainly three types of treatment technologies for heavy metal contaminated soil: physical method, chemical method and biological method.
1.1 Physical Law
Usually includes the guest soil method, thermal analysis and electric repair.
(1) Habitat Law
For heavily polluted soil, use mechanical excavation, clean up, and then backfill with clean guest soil. The contaminated soil is transferred to a solid waste landfill for solidification / stabilization treatment for landfill disposal, or sent to a cement kiln for disposal.
For lightly contaminated soil, add a large amount of clean guest soil to the surface of the soil and mix tillage to reduce the concentration of pollutants below the national soil standard value; or by mechanical tillage, the surface layer of contaminated soil and the bottom layer The positions of the clean soil are exchanged, the original top soil becomes the new bottom soil, and the original bottom soil becomes the new top soil, which reduces the contact between the pollutants and the plant root system, so as to reduce the harm.
Advantages: The guest soil method has a very obvious remediation effect on soil with heavy pollution and small area, and the treatment effect is obvious and thorough. Disadvantages: The repair of large-area contaminated soil not only requires a lot of manpower, material resources, and high cost, but also the source of clean soil is difficult to guarantee. In addition, the guest soil method is easy to destroy the soil structure, resulting in decreased soil fertility, and the heavy metal contaminated soil transferred to the lower layer is likely to cause the release of heavy metals and transfer into the groundwater, causing new groundwater hazards.
(2) Thermal analysis method
By heating the soil, the volatile pollutants are resolved from the soil and collected and processed. At present, the thermal analysis method is mainly used for the remediation of organic contaminated soil, such as oil contaminated soil. For the remediation of volatile heavy metals (mainly mercury and selenium) contaminated soil, in theory, pyrolysis can be used. However, because the thermal analysis method needs to be carried out at about 300 ° C, it is easy to damage the soil organic matter and structure water and lose the soil function. At the same time, volatile mercury vapors, etc. entering the atmosphere will cause secondary pollution. Due to the large amount of energy consumed, the cost of remediation per cubic meter of soil is estimated to be around 1200 to 1500 yuan.
(3) Electric repair
Insert the electrode into the contaminated soil, form an electric field by applying a weak current, and use the effects of electrodialysis, electromigration, and electrophoresis generated by the electric field to drive the heavy metal particles in the soil to migrate in the direction of the electric field, thereby enriching the heavy metal to the electrode area Handle or separate. Electric remediation technology is an in-situ remediation technology that can effectively remove heavy metals in soil, but the limitations of this technology are very obvious, and it is only suitable for soil remediation in a small area of ​​contaminated areas. For the restoration of large areas of contaminated farmland, due to the complex process and high cost, this technology is basically not feasible.
1.2 Chemical method
According to the morphological characteristics of heavy metals in the soil, by adding organic matter or chemical agents to the soil, through adsorption, redox, complexation, precipitation or mineralogical reactions, the existence of heavy metals in the soil can be changed to reduce their migration in the soil environment Ability and bioavailability, greatly reduce the plant's ability to absorb heavy metals, so that the concentration of heavy metals in plants meet food quality requirements. Common chemical repair methods include in-situ chemical stabilization, chemical leaching, and extraction separation.
(1) In-situ chemical stabilization
By adding chemical substances to the soil, changing the form or valence of heavy metals, the heavy metals are converted into compounds or mineral phase forms that are not easily soluble, have poor migration ability, and are less toxic, so that the heavy metals in the soil are no longer mobile and biologically effective As a result, the absorption of heavy metals by plants is greatly reduced and no longer poses a direct threat to food safety. In-situ chemical stabilization technology is a commonly used repair technology in engineering.
In-situ chemical stabilization requires researchers to determine the formulation of stabilizing agents based on the physical and chemical properties of the soil, the types of heavy metals, and their occupancy forms. On the basis of ensuring the stabilization of heavy metals, ensure that the chemical agents do not cause any damage Secondary pollution.
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Advantages: It can realize rapid repair of large-area contaminated soil, with low cost and simple process.
Disadvantages: Stabilized heavy metals still exist in the soil. Therefore, in order to ensure the long-term effect of heavy metal stabilization, different stabilizing agent formulations must be adopted for different texture types of soil and different heavy metal occurrence forms. The scientific research strength of the project implementation unit is very high.
(2) The chemical leaching method uses clean water or chemical solvents to rinse heavy metals from the soil (mainly light soil or sandy soil), commonly known as bathing the soil. Through ion exchange, precipitation, adsorption and chelation, the heavy metals in the soil are transferred to the leaching liquid phase, and then the leaching solution containing heavy metals is further processed to recover the heavy metals and recycle the leaching solution.
The most important thing in soil leaching technology is to find a suitable leaching agent. The ideal leaching agent should have the following conditions: First, it has high stability, and it is not easy to decompose with time and environment changes; Second, it has strong binding, that is, by specific adsorption, precipitation or co-precipitation The mechanism of action has a high adsorption binding energy for heavy metal ions, thereby inactivating or deactivating heavy metal ions; third, environmental friendliness, which will not damage the structure and properties of the soil, and will not cause new poisons to plants; fourth, Operable, the leaching agent should be low in cost, and can be widely used in practice and production; Fifth, it is universal. The ideal leaching agent can not only passivate a certain heavy metal ion, but also repair heavy metal compound pollution. soil.
At present, there are still some problems in the repair effect of the commonly used leaching agents on different soils, different crops and different heavy metal ions. It is usually necessary to use a combination of surfactants, and the environmental friendliness of the leaching agent cannot be guaranteed, so that the soil after chemical leaching is subject to serious secondary pollution.
Advantages: The removal efficiency of water-soluble heavy metals is very high, which can quickly repair contaminated soil. For low-pollution soils, the concentration of heavy metals in the soil can be close to or meet the requirements of the soil use standards through chemical leaching.
Disadvantages: The environmental friendliness of the eluent is difficult to guarantee; in addition to water-soluble heavy metals, the removal efficiency of other forms of heavy metals is low.
(3) Extraction separation method
The heavy metals (such as mercury, cadmium, lead, arsenic, chromium, copper, zinc, manganese, etc.) in various forms (including ion exchange state, carbonate state, iron manganese oxidation state) in the soil can The complex form is extracted into the extractant solution, and then the regeneration of the extractant and the removal of heavy metal ions are achieved through chemical exchange. The regenerated extractant is reused in the extraction process to realize the recycling of the extractant. The heavy metal content of the rehabilitated soil meets the requirements of the National Farmland Soil Quality Standard (GB15618-1995).
The most important thing in extractive separation technology is to find suitable extractant. The ideal extractant should have the following conditions: First, it has a strong ability to bind heavy metals. Through the combination of the extractant and the heavy metal, the heavy metal is dissolved from the surface of the soil particles into the extractant, and the peeling of the heavy metal and the soil is achieved. The second is environmentally friendly. The extractant remaining in the soil is easily degraded, will not damage the structure and properties of the soil, and will not cause new poisons to plants. The third is strong economy and low cost of extractant, which can be widely applied in practice and production. The fourth is strong universality. The ideal extractant can repair soil contaminated by heavy metals.
Advantages: It can quickly and completely repair heavy metal contaminated soil.
Disadvantages: The choice of extractant is very difficult, not only to meet the requirements of heavy metal extraction and separation, but also to meet the requirements of biodegradation and toxic residue.
1.3 Bioremediation
Bioremediation is the use of microbial or plant life metabolic activities to enrich or extract heavy metals in the soil, change the chemical form of heavy metals in the soil through biological action, fix or detoxify heavy metals, reduce their mobility in the environment and Bioavailability. Common bioremediation technologies at this stage include phytoremediation and microbial remediation.
(1) Phytoremediation
Phytoremediation is to use the characteristics of certain plants tolerant and excessive accumulation of certain heavy metals to remove heavy metals from the soil. There are several ways to repair plants: plant extraction, plant volatilization and plant fixation, of which the most promising is plant extraction technology.
Phytoremediation technology has some obvious shortcomings. Mainly manifested in:
â‘ The super-enriched plants are short and grow slowly. Due to the small biomass, the accumulation of heavy metals is small. The repair period is too long, which is not economically reasonable. This is currently the most important factor restricting the large-scale application of hyper-enriched plants to phytoremediation.
â‘¡Phytoremediation soil can only be limited to the extent that the plant root system can extend, generally not exceeding 20 cm soil layer thickness.
â‘¢ Super-enriched plants have strong selectivity and antagonism for heavy metals, that is, certain plants generally only have the ability to enrich certain heavy metals, while the presence of other heavy metals will significantly reduce the plant's ability to absorb heavy metals . However, the heavy metal pollution in the soil is mostly a combination of several heavy metals, and is often accompanied by organic pollution. Therefore, it is difficult to completely remove all pollutants in the soil with a super-enriched plant.
â‘£ The super-enriched plants enriched in heavy metals need to be harvested, and should be properly disposed of as waste, with higher post-processing costs.
⑤ Most of the heavy metal-rich plants that have been discovered so far belong to wild plants, and the introduction of off-site introduction will pose a potential threat to the local biodiversity.
(2) Microbial remediation
Microbial remediation is the use of certain microorganisms in the soil to absorb, precipitate, oxidize and reduce one or more heavy metals to reduce the toxicity of heavy metals in the soil or to promote the absorption of heavy metals by plants. There are two main technologies: bio-fixation technology and bio-redox technology.
Bio-fixation technology: the process of heavy metals being adsorbed and fixed by living or dead microorganisms. Certain microorganisms such as cyanobacteria, sulfate-reducing bacteria, and certain algae can produce extracellular polymers with a large number of cationic groups such as polysaccharides, glycoproteins, etc., and form complexes with heavy metals.
Biological redox: The use of microorganisms can oxidize, reduce, methylate and demethylate heavy metal ions, change the redox state of heavy metal ions, and reduce the bioavailability of heavy metal ions in the soil environment.
Advantages: low repair cost and simple operation.
Disadvantages: The specificity of microbial remediation is strong, and its activity is closely related to soil environmental conditions such as temperature, moisture, oxygen, and pH. Therefore, it is difficult to repair multiple contaminated soils with multiple heavy metals at the same time. In addition, this method is difficult to apply and has not yet achieved breakthrough progress, and is still in the laboratory research stage.
1.4 Evaluation of various technologies
(1) The guest soil method in the physical method requires a large amount of clean soil to be replaced, and the soil transfer volume is large and the cost is high. Only suitable for the treatment of contaminated soil in a small area. The other methods in the physical method (electroanalysis and pyrolysis) are still in the laboratory research stage.
(2) The in-situ stabilization method in the chemical method is relatively mature at home and abroad, the project is relatively easy to implement, and the cost is acceptable.
The leaching technology in the chemical method is difficult to meet due to the environmentally friendly characteristics of the leaching agent, and it cannot be applied to the remediation of heavy metal contaminated soil on a large scale.
The extraction technology in the chemical method can effectively extract and separate the heavy metals in the soil, and can be applied to the remediation of industrial sites and farmland soils.
(3) The phytoremediation technology in the biological method is too long to repair and is not suitable for the treatment of compound heavy metal pollution, so it is rarely applied to the repair of industrial land and farmland soil. It is more suitable for soil and water conservation and ecological restoration of heavy metal mine soil.
The microbial remediation technology in the ecological method is too affected by the soil environment due to the specificity of the microorganisms, so that the actual application of this technology is still far behind, and it is still in the laboratory research stage.
2. Analysis of the application of related technologies for heavy metal contaminated sites and contaminated soil remediation
2.1 Industrial contaminated sites
The heavy metal industry contaminated sites are mainly caused by the relocation of factories in the urban built-up area, the closure of old factories, and the irregular production of metal smelting enterprises. The restoration of this kind of site mainly has the following uses: residential land or commercial land, landscaping land.
2.1.1 Remediation of industrial contaminated sites → used as residential land or commercial land
This situation often occurs in the process of urban expansion. This kind of site remediation should at least meet the national "Soil Environmental Quality Assessment Standards for Exhibition Land" (HJ350-2007).
Commonly used technologies include: (1) Guest soil method. Large amounts of clean soil are required to replace contaminated soil. Not only is the cost high, but contaminated soil needs to be transferred and disposed of.
(2) Cleaning of heavily polluted soil + stabilization of heavy metals in situ. After detailed testing, the heavy contaminated soil in the contaminated site is cleaned up, and the soil can be used as roadbed filler after being stabilized by heavy metal agents. The cleaned site is stabilized by in-situ stabilization to fix heavy metals so that it will no longer leach.
(3) Use soil leaching technology or soil heavy metal extraction technology to separate heavy metals from the soil, clean the soil and then backfill.
The choice of (2) and (3) above can be determined according to the repair cost and environmental risk assessment.
2.1.2 Remediation of industrial contaminated sites → used as land for landscaping
At present, there is no national or local standard for such soil remediation. In order to reduce the pollution of heavy metals to the soil, the leaching concentration of the heavy metal soil after its restoration should meet the national groundwater standard (GB14848-93) Class III water quality requirements. At present, the more suitable technology is heavy metal in-situ chemical immobilization technology.
2.2 Remediation of farmland contaminated soil
The remediation requirements of farmland contaminated soil are the most stringent, and there are two standards to be met: "Environmental Quality Evaluation Standards for Edible Agricultural Products" (HJ332-2006) and "Soil Environmental Quality Standards" (GB 15618-1995).
Since the "Soil Environmental Quality Standard" strictly controls the total amount of heavy metals in the soil, only the leaching technology and extraction technology in the chemical method can meet this requirement.
In-situ chemical stabilization technology can convert the heavy metal form in the soil into the mineral phase form, so that the leaching concentration of the heavy metal in the soil can meet the water quality requirements of Class III in the National Groundwater Standard (GB14848-93), so that its food crops Environmental Quality Assessment Standards for Edible Agricultural Products (HJ332-2006) requirements, but because heavy metals are fixed in situ in the soil and are not separated from the soil, the concentration of heavy metals in the soil is difficult to meet the "Soil Environmental Quality Standards" (GB 15618-1995) requirements.
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