- Engineering measures
It mainly includes measures such as soil replacement and deep tillage. If it is a slightly polluted soil, deep tillage is widely used. Soil replacement are commonly used in heavy pollution areas. Engineering measures to control the soil heavy metal pollution is thorough and stable, but the project volume is large, the investment cost is high, the soil structure is destroyed, the soil fertility is reduced, and the discharged soil is also piled up or disposed.
2. Chemical repair
Adding a modifier to the soil to adsorb, redox, antagonize or precipitate heavy metals, reducing the bioavailability of heavy metals. Commonly used modifiers include lime, zeolite, calcium carbonate, phosphate, silicate and organic substances that promote reduction. Different modifiers have different mechanisms of action on heavy metals. Chemical repair is simple and easy, but it only changes the presence of heavy metals in the soil. The metal elements remain in the soil and are easily reactivated.
3. Bioremediation
Use biological to reduce, purify heavy metals in the soil or reduce the toxicity of heavy metals. This method works well and is easy to operate.
- Phytoremediation
It is a technique for repairing heavy metal contaminated soil by using natural or artificially cultivated plants, which are divided into three types: plant extraction, plant evaporation and plant stabilization. Plant extraction relies on heavy metal hyperaccumulators to extract heavy metal ions from the soil, and then harvest the aerial parts and process them. Continuous planting of the plant can effectively reduce or remove heavy metals in the soil.
- Microbial remediation technology
Microorganisms can reduce the toxicity of heavy metals in the soil, adsorb and accumulate heavy metals, change the rhizosphere microenvironment, and achieve the purpose of increasing the absorption, volatilization or fixation efficiency of heavy metals. Such as G. gingivalis, cyanobacteria, sulfate-reducing bacteria and certain algae, can produce extracellular polymers combined with heavy metal ions to form a complex.
- Agricultural ecological restoration
Agro-ecological restoration includes agronomic restoration and ecological restoration. The former changes the farming system, adjusts crop varieties, grows plants that do not enter the food chain, selects fertilizers that can reduce heavy metal pollution in the soil, or increases the application of organic fertilizers that can fix heavy metals to reduce heavy metal pollution in the soil. The latter regulates soil moisture, nutrients, pH and soil redox status, as well as ecological factors such as temperature and humidity, and regulates the environmental medium in which the pollutants are located. However, the repair cycle of this technology is long and the effect is not obvious.
4. Physical repair
- Electric repair
The heavy metal ions (such as Pb, Cd, Cr, Zn, etc.) and inorganic ions in the soil are transported to the electrodes by electric permeation and electromigration by current, and then collected and processed. This method is suitable for low permeability clay and silt and can control the flow direction of contaminants. In the experiments on sand, the removal rate of heavy metal ions such as Pb2+ and Cr3+ in the soil can reach more than 90%. Electric repair does not agitate the soil layer and has a short repair time. It is an economically viable in situ repair technology.
- Electrothermal repair
The electromagnetic waves generated by the high-frequency voltage are used to heat the soil, so that the pollutants are desorbed from the soil particles, and some volatile heavy metals are separated from the soil to achieve the purpose of repair. This technology can repair soil contaminated with heavy metals such as Hg and Se.
- Soil leaching
The heavy metal in the soil is transferred to the soil liquid phase by using the eluent, and the heavy metal-rich wastewater is further recycled. This technique requires the search for an eluent that extracts both heavy metals in various forms without destroying the soil structure. Eluents currently used to leach soils, including organic or inorganic acids, bases, salts and chelating agents.
The above techniques for controlling soil heavy metal pollution problems cannot be widely applied, and the reasons are excessive cost, insufficient experience in field application, and unstable treatment effects. With the development of science and technology, the deepening and improvement of development and research work, these governance methods must be perfected and widely used.