Phytoremediation is defined as the use of plant species as a means of purifying polluted soil, water, and air [13]. The term derives from the Greek word phyto (“plant”) and the Latin word remedium (“to correct or remove an evil”). Phytoremediation is a more economically feasible and efficient remediation option than other techniques, such as washing, flushing, or solidification [13,14].

The removal of contaminants such as toxic trace elements (TTEs) from polluted sites involves phytoextraction, phytostabilization, and phytovolatilization (Figure 1) [15]. Phytoextraction, in which contaminants are absorbed through the roots, plays a key role in the removal of metals and metalloids from contaminated soils, water, and sediments [13,16]. The absorbed and extracted contaminants are transported from the roots to the aboveground, harvestable parts of the plant, which are then disposed of as hazardous waste or incinerated.

Plant stabilization (or plant immobilization) refers to the use of plants to reduce the mobility and/or bioavailability of contaminants, thus preventing their entry into the surrounding environment or food chain. Contaminants in the root zone are fixed within the plant rhizosphere by adsorption or precipitation. The low mobility of the contaminants trapped in the plant rhizosphere means that they cannot easily leach into groundwater or spread to agricultural land, preventing their entry into the food chain [13,17,18]. However, the introduction and initial growth of plants can be limited by the physicochemical characteristics of mine waste, including extreme pH levels, high salinity, low water retention, high pollutant concentrations, and a lack of soil organic matter and fertility. Therefore, improvements in the physicochemical and biological properties of mine waste-contaminated soil [19,20], such as organic and/or inorganic amendments [21,22,23], are often required before plants can be introduced. In this aided phytostabilization, the main purpose is not to remove contaminants but to reduce the risk that they pose by reducing their mobility and biological effectiveness [21,23]. In phytovolatilization, soluble pollutants are taken up together with water and released into the atmosphere via stomatal diffusion, which is accompanied in some cases by evaporation into the atmosphere via the leaves [24,25]. TTEs, however, are not completely removed from the atmosphere; rather, they move from one system to another. Phytovolatilization has been applied to sites contaminated with metalloids, such as Hg and Se [13,17,18,26].

Plant screening is required for successful phytoremediation. In general, species native to the region are preferred for ecosystem restoration or remediation due to concerns about the potential harm caused by the invasion of alien species, including a reduction in local plant diversity (and thus biodiversity), which can disturb the ecosystem [25]. High remediation efficiency and the successful establishment of stable vegetation cover also require plants that are well adapted to the local environment and require little management (e.g., with fertilizers). Since the availability of nutrients in polluted soil is generally low [27], the use of nitrogen-fixing legumes has been widely examined [28,29,30]. Other factors that need to be considered in plant screening include the root structure, the properties of the contaminants and soil, and the climatic conditions [31]. Plant root length increases from grasses to shrubs and from shrubs to trees, with deeper roots allowing for the removal of metal complexes bound deep in the soil. However, grass is the most widely used plant in soil remediation because of its high biomass, rapid growth, high tolerance, and versatility [32]. Plants with well-developed roots reduce soil erosion, generally remain well-confined, and reduce air pollution by creating vegetation cover [33,34]. In the field, a multi-faceted approach using grasses, shrubs, legumes, perennial grasses, and other long-living trees is likely to be more effective than employing a monoculture.

Hyperaccumulators are plants that are able to accumulate relatively large amounts of pollutants, such as 100 mg cadmium kg^–1; 1000 mg Cu, Pb, or As kg^–1; and 10,000 mg Zn kg^–1 [35,36]. The most effective species for phytostabilization are those that prevent contaminants from entering the food chain or other environmental media by accumulating them underground. Plant species that have been frequently used in phytostabilization include Agrostis spp. and Festuca spp. [23,37]. Since plant growth can actually promote metal leaching due to soil acidification and the production of dissolved organic matter, plant species that cause low soil acidification and that do not translocate metals to their leaves in high quantities are most suitable for phytostabilization [25].