# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 3 Phytoremediation
### Phytoremediation Capability of Spontaneously Grown Plants  
Recently, several plants and conditions have been explored to restore lead-contaminated mining areas in different countries via phytoremediation processes [37; 31]. [PERSON] et al. [31] studied _Cytisus scoparius_ for the phytoremediation of Spolic Technosols in a Rubiais mine. Four areas were selected, and three topsoil samples were collected in each zone. The bioavailable fraction of Pb in the mine's soils was assessed by 14 different single extractions using three groups of extractants (Table 1):  
1. diluted solution of strong acids (HCl and HNO\({}_{3}\)), acetic acid, and acidified CaCl\({}_{2}\);
2. chelating agent solutions (EDTA and DTPA), mix of low molecular weight organic acids, (LMWOAs), and neutral salt solution (MgCl\({}_{2}\));
3. neutral salt solutions and bidistilled water (BDW). Figure 1: Schematic representation of different phytoremediation approaches by plants under study. Ten types of _C. scoparius_, grown spontaneously in the soil, were harvested in the same locations of soil sampling; then, the roots and aerial part were separated and dried at 60 \({}^{\circ}\)C until reaching a constant weight. The Pb content in the plants' aerial parts and roots was obtained by extraction with H\({}_{2}\)O\({}_{2}\) and HNO\({}_{3}\) (1.5:9 _v:v_) in a microwave oven followed by ICP-OES analysis. Results showed that _C. scoparius_ accumulated and concentrated Pb mainly in roots, having TF <1 and BF > 1, so acting as a lead phytostabilizer. However, a value of BF greater than 1 was obtained only if the neutral salts and LMWOAs are used as extractants. Moreover, even if LMWOAs are good predictors of metal bioavailability for phytoextraction techniques, as they are according to [PERSON] et al. [31] and other studies [38; 39], the statistical analysis in this study evidenced that the concentrations in the soil detected after extraction with different extractants were not correlated with the concentrations observed in the different parts of the plants, so suggesting that their use could not be suitable to assess the bioavailability of Pb in these contaminated soils. [PERSON] et al. [37] used a similar approach for the restoration of a lead mine located in Bo Ngam (Thailand), contaminated with corussite (PbCO\({}_{3}\)), by spontaneously grown plants. Soil was collected from five points located at the surface (0-10 cm), subsurface (10-40 cm), and around the plant roots. After collection and drying at 60 \({}^{\circ}\)C for 48 h, soil samples were reduced into a fine powder, digested with HNO\({}_{3}\), and Pb concentration was determined by flame atomic absorption spectrophotometer (F-AAS). A sequential extraction method was used to measure the extractable lead in soil: firstly, Ca(NO\({}_{3}\))\({}_{2}\) was used to extract the exchangeable fraction and then the acid-soluble fraction was extracted by acetic acid. Concomitantly, plants grown spontaneously in the soil were taken from the same site of the soil samples. Forty-eight species (22 herbs, 17 grasses, 5 undershrubs, 2 shrubs, and 2 climbers) were collected. After collection, washing, separation into shoots and roots, and drying, plant samples were digested with HNO\({}_{3}\) and analyzed by F-AAS. Results showed that twelve species behaved as accumulators