Physical and chemical methods for soil remediation

Physical method

Most physical separation and repair technologies have advantages such as simple equipment, low cost, and sustainable high yield. However, in the specific separation process, the feasibility of their technology needs to consider various factors. Physical separation technology requires pollutants to have high concentrations and exist in phase media with different physical characteristics. When screening dry pollutants, dust will be generated, and the fine particle size portion in the solid matrix and pollutants in the waste liquid need to be reprocessed.

1. Customer soil and soil replacement method

Mainly divided into deep plowing, soil replacement, and guest soil. When the soil is only slightly polluted, the method of deep plowing and soil plowing is used, while when treating heavily polluted areas, the method of using foreign soil or soil replacement is used. The use of foreign soil and soil replacement has a good effect on repairing heavy metal pollution in soil. Its advantages lie in mature methods and comprehensive remediation. Its main disadvantages are large engineering quantities, high investment, and easy to cause problems such as decreased soil fertility.

2. Separation and repair method

Soil separation and remediation refers to the application of particle size separation (screening), hydraulic separation, density (gravity) separation, dehydration separation, foam flotation separation, magnetic separation and other technologies to the remediation of inorganic pollutants in contaminated soil. It is most suitable for treating the soil polluted by heavy metals in a small range, separating heavy metals from soil, sediment and waste residue, cleaning the soil, and restoring the normal function of the soil.

3. Isolation method

The soil isolation method refers to the use of anti-seepage isolation materials to partition and isolate areas contaminated with heavy metals in the soil. This isolation includes both horizontal and vertical isolation. The isolation method is mainly applied to polluted soil with severe heavy metal pollution and difficult to treat. The heavy metals in this soil will migrate with the flow of groundwater, followed by heavy metal pollution in groundwater and surface water. Due to the difficulty or long duration of treatment, it is necessary to isolate it using isolation methods to prevent further external pollution.

4. Thermal repair method

Thermal remediation technology involves the use of heat conduction (heating wells and walls) or radiation (such as radio wave heating) to achieve soil remediation, including high-temperature (about 1000 ℃) in-situ heating remediation technology, low-temperature (about 100 ℃) in-situ heating remediation technology, and in-situ electromagnetic wave heating technology. The main heavy metal targeted is mercury.

Chemical method

1. Chemical curing method

The main impact of heavy metals on soil is their mobility. The form of heavy metals in the soil determines their mobility. The physical and chemical properties of the soil, such as organic matter content, pH value, and Eh value, can affect the form of heavy metals. These parameters can be used to regulate the mobility of heavy metals in the soil. The purpose of chemical solidification of heavy metals is to add solidification agents to change the physical and chemical properties of the soil, reducing its mobility through adsorption or precipitation of heavy metals. After the fixation of heavy metals in soil, it not only reduces the pollution impact on deep soil and groundwater, but also has the potential to rebuild vegetation in the soil. The commonly used curing agents include lime, apatite, zeolite, compost, and steel slag. The mechanisms of fixing heavy metals with different solidification agents are different. For example, lime mainly solidifies through the co precipitation reaction mechanism of heavy metals with calcium carbonate and the hydrolysis reaction of heavy metals themselves, while zeolite reduces the mobility of heavy metals in soil through ion exchange adsorption.

2. Soil leaching method

Soil leaching is the process of transferring heavy metals from the soil to the soil leaching solution by reversing the ion adsorption and precipitation of heavy metals in the soil. Soil leaching first removes and disperses the excavated soil, and then thoroughly mixes it with the extraction agent. Heavy metals will be transferred to the soil extraction agent, and then the residual extraction agent will be removed by washing with water. After the heavy metals in the treated soil reach normal levels, they can be reused. The leaching solution can be treated to recover heavy metals and the extraction agent. The key to soil leaching technology is the extraction agent, which must achieve the extraction of heavy metals without damaging the original structure of the soil. At present, the main extractants include nitric acid, hydrochloric acid, phosphoric acid, EDTA, and DTPA.

3. Electrokinetic repair method

Electrokinetic remediation involves inserting electrodes with low direct current into contaminated soil. Heavy metal ions in the soil are enriched to the electrodes under the action of an electric field, and methods are used for collection and centralized treatment. Under the action of an electric field, heavy metals migrate and accumulate towards the electrode under the action of electroosmosis and electromigration. In recent years, electrokinetic remediation has developed rapidly and has been commercialized in some European and American countries. This method can control the flow direction of pollutants, especially suitable for low permeability clay and silt, and its economic cost is also relatively reasonable.