# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 3 Phytoremediation  
Phytoremediation is an in-situ restoration process that uses plants to transform, immobilize, and extract contaminants, such as heavy metals, from soils or water. Plants exhibit a very efficient mechanism to uptake nutrients present at low concentrations in the soil through their roots using different strategies to solubilize micronutrients, such as the production of chelating agents or by inducing a pH modification. The capability for metals uptake into the growing plant's tissues is estimated through the bioconcentration factor (BF), defined as:  
\[\text{BF}=\text{C}_{\text{p}}/\text{C}_{\text{so}}\]  
where C\({}_{\text{p}}\) is the metal concentration in the plant and C\({}_{\text{so}}\) is the metal concentration in the soil [30; 31; 32].  
Plants are also capable of storing and translocating elements that are not essential but have chemical properties similar to those of nutrients. The translocation factor (TF) is used to evaluate metal transport and can be calculated as:  
\[\text{TF}=\text{C}_{\text{s}}/\text{C}_{\text{r}}\]  
where C\({}_{\text{s}}\) and C\({}_{\text{r}}\) are the metal concentrations in the aerial parts and roots, respectively [31; 32; 33; 34]. A TF \(>1\) indicates that the plant translocates metals from the root to the shoot, while with a TF \(<1\) the metals are preferentially accumulated by immobilization in roots. According to [PERSON] et al. [30], phytoremediation approaches can be classified on the basis of the mechanism involved in the pollutant removal (Figure 1), which are as follows:  
1. phytoextraction, when contaminants are concentrated from soil to plant tissues;
2. phytodegradation, if plants are able to degrade organic contaminants;
3. rhizofiltration, when exploiting the capability of plant's roots to remove pollutants from contaminated water, adsorbing them into the extracellular negatively-charged residues present on the outer coatings of the roots and absorbing them through membrane proteins that act as carrier molecules [35];
4. phytostabilization, when plant's roots are able to reduce the bioavailability of contaminants in the soil;
5. phytovolatilization by obtaining the pollutants' volatilization.  
The success of phytoremediation depends on the plant's ability to store large quantities of metals in its tissues and to accumulate biomass rapidly, without being negatively affected by the metals. Taking into account that the tolerance to high concentrations of heavy metals depends on the type of plant, the study of stress indicators is crucial for the selection of the best candidate for phytoremediation.  
[PERSON] [36] classified the different plant-soil relationships into three types:  
1. _accumulators_ that concentrate metals in above-ground tissues;
2. _indicators_ that regulate uptake and transport of metals so that the internal concentration reflects the external concentration;
3. _excluders_ for which an accumulation of heavy metals occurs in the roots, but the entry and transport in the aerial parts is limited [32].  
Plants with BF > 1 and TF > 1 are more suitable for phytorextraction as the aerial parts are easily eliminated by harvesting, whereas plants with BF > 1 and TF < 1 are more appropriate for phytostabilization.