**2. Soil remediation**

### **2.1 Physical remediation**

### *2.1.1 Soil replacement*

Replacement of contaminated soil is called complete or partial replacement of contaminated soil with non-contaminated and clean soil. In this method, the concentration of heavy metals in the soil decreases (dilutes) and leads to an increase in soil fertility and functionality [3]. Earlier to 1984, excavation, remove the contaminated soil and off-site disposal in specific places and replacement it with clean soil were the most common method for cleaning-up in contaminated areas. In soil spading, the contaminated soil is spaded deeply with special devices so that the surface contaminated soil is mixed with the uncontaminated under layer clean soil and the concentration of heavy metals is reduced (diluted) [12]. Another method related to soil replacement is adding clean and non-contaminated soil to the surface of contaminated soil. In this method, we can mix the imported non-contaminated soil with the contaminated soil so that the concentration of heavy metals per unit weight of the soil is reduced and a suitable environment is provided for the growth of plants [13]. The soil replacement method can isolate the contaminated soil and the ecosystem and reduce its harmful effects on the ecosystem [14]. But this method is very expensive because it requires a lot of labor and physical work, and it is suitable for small and highly polluted areas. The cost of doing this method is about 270 to 460 dollars for each ton of moving and adding clean soil. It is natural that the longer the distance, the higher the costs.

### *2.1.2 Soil isolation*

Isolation means separating the soil contaminated with heavy metals from non-contaminated soil [15] or preventing the movement and transmission of contamination from one point to another [16], but for the complete purification of contamination in this method, other engineering methods are also needed. Contaminated soil isolation measures are based on engineered barriers and include hydrological barriers and stabilization approaches [17].

In general, isolation technologies are designed to prevent the off-site movement of heavy metals and other contaminants by confining them to a specific area [3] Engineering barriers, which may be on the surface or below the surface, are generally used to limit the contact of surface water or groundwater with waste materials and transfer to the surrounding environment. An underground barrier restricts the flow of ground and/or surface water at a contaminated site, allowing contaminated water and soil to be separated [16]. By far, the most common engineering barrier is a surface barrier called a cap, which is usually placed on top of waste piles. Vertical subsurface engineering barriers limit the lateral movement of groundwater and dissolved pollutants. These vertical barriers are installed downstream, upstream or generally surrounding a site and are generally used in combination with the cap system.

### *2.1.3 Vitrification*

In the vitrification process, contaminated soil is transformed into a crystal and glass product due to heating and melting with electric energy. In this method, the mobility of heavy metals in the soil decreases, which is due to the formation of vitreous materials [18]. The Pacific Northwest Laboratory, which is working on the development of vitrification, is conducting research that can make this technology operational for buried waste and underground tanks of the United States Department of Energy [19]. A vertical array of electrodes is inserted into the contaminated soil during in situ vitrification in order to pass electrical current through it. Of course, it should be noted that in dry soils, due to low conductivity, the vitrification process is not performed well. Temperature is a key factor in immobilization of heavy metals in vitrification method [20]. Vitrification can be performed both in-situ and ex-situ. But preference is given to the in-situ method because it is easier and its energy supply is more accessible. In situ vitrification is limited by the possibility of melting soil and allowing current to pass through it. Furthermore, soils with a high alkali content (1.4 wt%) are unlikely to conduct current efficiently [21]. As a result, vitrification can only take place under wet soils with low alkali levels.
