**4. Phytoextraction and phytostabilization**

Phytoextraction and phytostabilization are the two techniques most useful for phytoremedia‐ tion of metal and metalloid contaminated soils. Phytoextraction has been widely studied, mainly due to the potential for high efficiency and possible economic value (in metal recovery, energy production) [3,23,24,48,50,51]. Preferably, plants used in phytoextraction should present, among others, the following characteristics [13,23,52,53]:


Phytoextraction can only be considered effective if the accumulated contaminant is subse‐ quently removed through harvesting (Figure 2). If most of the captured heavy metals are translocated to shoots, traditional farming methods can be used for harvesting. It is important to harvest the plants before leaf-fall or death and decomposition to ensure that contaminants do not disperse or return to the soil [20].

**Figure 2.** Schematic representation of phytoextraction of metals from soil.

After harvesting, biomass may be processed for extraction and recovery of metals (phytomin‐ ing). The commercial value of metals such as Ni, Zn, Cu or Co may encourage the phytore‐ mediation process. Alternatively, thermal, physical, chemical or microbiological processes can be used to reduce the volume/ weight of biomass. In the case of incineration of plants the energy produced represents an economic opportunity, and the ash can be further processed for extraction of metals. However, this process must be very careful, given the possible chemical elements accumulated, to prevent any dispersion mechanisms of contaminants.

According to McGrath and Zhao [22], phytoextraction efficiency is determined by two key factors: the ability to hyperaccumulate metals and the biomass production. Therefore, if these

factors influence the phytoextraction, they can be optimized to improve the phytoremedia‐ tion process. One possibility is the addition of chemical agents into the soil in order to increase the bioavailability of metals and their root uptake [54,55]. This form of assisted phytoreme‐ diation (or induced phytoremediation) has shown great potential and has been widely studied (Figure 3).

**1.** tolerance to high concentrations of metals;

490 Environmental Risk Assessment of Soil Contamination

**3.** rapid growth;

**4.** high biomass production;

**6.** easy to cultivate and harvest.

do not disperse or return to the soil [20].

**5.** profuse root system;

**2.** accumulate high concentrations in their aerial tissues;

**Figure 2.** Schematic representation of phytoextraction of metals from soil.

Phytoextraction can only be considered effective if the accumulated contaminant is subse‐ quently removed through harvesting (Figure 2). If most of the captured heavy metals are translocated to shoots, traditional farming methods can be used for harvesting. It is important to harvest the plants before leaf-fall or death and decomposition to ensure that contaminants

After harvesting, biomass may be processed for extraction and recovery of metals (phytomin‐ ing). The commercial value of metals such as Ni, Zn, Cu or Co may encourage the phytore‐ mediation process. Alternatively, thermal, physical, chemical or microbiological processes can be used to reduce the volume/ weight of biomass. In the case of incineration of plants the energy produced represents an economic opportunity, and the ash can be further processed for extraction of metals. However, this process must be very careful, given the possible chemical

According to McGrath and Zhao [22], phytoextraction efficiency is determined by two key factors: the ability to hyperaccumulate metals and the biomass production. Therefore, if these

elements accumulated, to prevent any dispersion mechanisms of contaminants.

**Figure 3.** Schematic representation of the processes of natural (A) and assisted (B) phytoextraction.

Although hyperaccumulators are phytoextractors par excellence, usually they are low biomass producers. Thus, it is generally accepted that plants with a significant biomass production capacity can compensate their relatively lower metal accumulation capacity, to an extent where the amount of metal removed can be higher [51].

Phytoextraction potential can be estimated by calculation of bioconcentration factor (or biological absorption coefficient) and translocation factor [51,56]. The bioconcentration factor (BCF), which is defined as the ratio of the total concentration of element in the harvested plant tissue (Cplant) to its concentration in the soil in which the plant was growing (Csoil), is calculated as follows:

$$BCF = \frac{C\_{plant}}{C\_{sol}} \tag{1}$$

Translocation factor (TF), defined as the ratio of the total concentration of elements in the aerial parts of the plant (Cshoot) to the concentration in the root (Croot), is calculated as follows:

$$TF = \frac{C\_{shout}}{C\_{vwt}} \tag{2}$$

The commercial efficiency of phytoextraction can be estimated by the rate of metal accumu‐ lation and biomass production. Multiplying the rate of accumulation (metal (g)/plant tissue (kg)) by the growth rate (plant tissue (kg)/hectare/year), gives the metal removal value (g/kg of metal per hectare and per year) [3,19,54,57]. This rate of removal or extraction should reach several hundred, or at least 1 kg/ha/year, for the species to be commercially useful, and even then, the remediation process may take from 15 to 20 years [3].

Some soils are so heavily contaminated that removal of metals using plants would take an unrealistic amount of time. The normal practice is to choose drought-resistant fast-growing crops or fodder which can grow in metal-contaminated and nutrient-deficient soils.

In contrast to phytoextraction, phytostabilization aims at reducing the mobility of contami‐ nants in the soil. In this technique, contaminated soil is covered by vegetation tolerant to high concentrations of toxic elements, limiting the soil erosion and leaching of contaminants in to groundwater. Mobility of contaminants can be reduced by surface adsorption/accumulation in roots as well as their precipitation in rhizosphere by induced changes in pH or by oxidation of the root environment [3,12,58]. For example, the immobilization of arsenic in iron plates in the rhizosphere of salt marsh plants [58,59]. Phytostabilization can also be promoted by plant species with the capacity to exude high amounts of chelating substances. These substances lead to immobilization of contaminants by preventing their absorption, while simultaneously reducing their mobility in soil. Thus, plants with phytostabilization potential can be of great value for the revegetation of mine tailings and contaminated areas [58,60].
