How to deal with excessive heavy metals in soil
（1） Common governance methods
There are two ways to control heavy metal pollution in soil, one is to change the existing state of heavy metal in soil, and the other is to remove heavy metal from soil.
The commonly used physical and physical-chemical methods are thermal desorption method, electrochemical method and extraction method. The volatile heavy metals can be desorbed from the soil by heating. If the permeability of heavy metals is not high and the conductivity is poor, electrochemical method is used to remove them. Heavy metal ions in soil can be recovered by solvent extraction or recycling.
（2） Engineering physicochemical method
Engineering physicochemical method is a method to treat heavy metal contaminated soil by physical and chemical methods. In the early stage of heavy metal pollution, due to the concentration of pollution, this method is widely used. The main methods include: soil method, washing method, electric chemistry method, heat treatment method, physical solidification method, etc. The physical and chemical method has the advantages of obvious and rapid treatment effect for the soil with heavy pollution and small area, but it needs to consume a lot of manpower and financial resources for the soil with large polluted area, and it is easy to lead to the destruction of soil structure and the decline of soil fertility. Therefore, this method is not suitable for large area of heavy metal contaminated soil.
The heat treatment method is to heat the polluted soil to volatilize the volatile pollutants in the soil and collect them for recycling or treatment; the electrolytic method is to remove the heavy metals in the soil at the anode or cathode under the action of electrolysis, electromigration, electroosmosis and electrophoresis.
Bioremediation refers to the process of reducing the concentration of heavy metals in the soil or changing the forms of heavy metals by using the metabolic activities of organisms, so that the polluted soil environment can be partially or completely restored to its original state. Remediation measures include phytoremediation, microbial remediation and animal remediation. Because of its advantages of good effect, low investment, low cost, easy management and operation, no secondary pollution and so on, it has been paid more and more attention and become a hot spot in the research and engineering application of contaminated soil remediation. 1. Phytoremediation measures.
Phytoremediation is based on the theory of plant tolerance and excessive accumulation of some or some chemical elements. In some heavy metal contaminated areas, there are plants with tolerance to heavy metals. These plants make heavy metals precipitate in the cell wall of plant roots and "bind" their absorption across membrane, or combine with some proteins and organic acids to form abiotic plants The detoxification form of substance activity can improve the tolerance to heavy metal injury. Using plants and their coexisting microorganisms to remove pollutants from the environment is a new environmental application technology. The key of plant control measures is to find suitable hyperaccumulator or heavy metal tolerant plants. Hyperaccumulator plants can absorb and accumulate a large amount of heavy metals. However, phytoremediation measures also have limitations. For example, hyperaccumulators usually have low biomass, slow growth and no significant effect.
2. Microbial remediation measures
Microbial treatment is to use some microorganisms in soil to absorb, precipitate, oxidize and reduce heavy metals, so as to reduce the toxicity of heavy metals in soil. Prokaryotes (bacteria, actinomycetes) are more sensitive to heavy metals than eukaryotes (fungi). Based on this principle, mercury rich bacteria are cultured in soil. After these bacteria are collected, the mercury contaminated soil is treated by evaporation and activated carbon adsorption. At present, it is one of the more active fields in the study of soil heavy metal pollution to cultivate microorganisms with the ability of reducing heavy metal toxicity by using genetic and genetic engineering and other biological technologies.
Microbial remediation of soil heavy metal pollution mainly includes two aspects: biosorption and biological oxidation-reduction. Biosorption refers to the biosorption of heavy metals by organisms, such as cyanobacteria, sulfate reducing bacteria and some algae, which can produce extracellular polymers with a large number of cationic groups, such as polysaccharides and glycoproteins, and form complexes with heavy metals. Biological oxidation is the oxidation, reduction, methylation and demethylation of heavy metal ions by microorganisms to reduce the content of heavy metals in the soil environment.
3. Restoration measures for lower animals
Some lower animals in soil (such as earthworms) can absorb heavy metals in soil, so they can reduce the content of heavy metals in polluted soil to a certain extent. In South Korea, earthworm toxicology test was used to repair the tailings of three abandoned arsenic and heavy metal mines. The results showed that earthworm had a good enrichment effect on zinc and cadmium. It can be seen that earthworms can be released in the soil polluted by heavy metals. After the earthworms are enriched, the earthworms can be driven out by electric stimulation and clean water for centralized treatment.
（4） Agricultural governance methods
Agricultural management is to change some farming management systems according to local conditions to reduce the harm of heavy metals, and plant plants that do not enter the food chain on contaminated soil. It mainly includes: controlling soil moisture means adjusting its redox potential by controlling soil moisture to achieve the purpose of reducing heavy metal pollution; selecting chemical fertilizer refers to selecting the chemical fertilizer that can reduce soil heavy metal pollution without affecting soil fertilizer supply; increasing organic fertilizer means that organic fertilizer can fix multiple heavy metals in soil to reduce soil heavy metal pollution; Crop variety selection refers to the selection of anti pollution plants and plants that do not enter the food chain on heavy metal contaminated soil.
The advantage of agricultural management measures is easy to operate.