# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 3 Phytoremediation
### Limitation and Disadvantages of Phytoremediation  
Phytoremediation is an environmentally friendly method applicable to a wide range of contaminants with many advantages, such as being aesthetically pleasing, socially acceptable, and low in cost [45]. Nevertheless, it also has some drawbacks as it can be a time-consuming process; it may take up to several years to remediate a contaminated site, making this technique unsuitable for restoring areas that may constitute an acute risk for human beings and the ecosystem [45; 46; 47]. Another significant limitation of this approach consists in the necessary contact between the contaminants and the root zone of plants. Thus, the central role played by roots in phytoremediation has generally limited its application to shallow, contaminated soils (one meter deep at most) [45; 46; 48; 49], with the only exception being the use of some trees employed to reach target zones in the range of one to several meters [47]. Additionally, the age of plants greatly affects their physiological activity; indeed, the ability to absorb ions is greater for the roots of younger plants. Though, the larger biomass of an older plant may balance its lower physiological activity [48]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Study** & **Plant Species** & **Results** \\ \hline Phytoremediation & & & Accumulated and \\ capability of & & & concentrated Pb mainly in \\ spontaneously grown & & & & roots, so acting as a lead \\ plants & & & & phytostabilizer (TF \(<\) 1, BF \(>\) 1) \\ \hline  & & Herbs: & Hyperaccumulators: \\  & & _S. arrensis_ & accumulation in shoots 10–500 \\  & & _Co. sumaternsis_ & times more than usual plants \\  & & _C. superus_ sp. & (lead 5 mg/kg) \\  & & Shrubs: & shoot accumulation \(>\) 1000 \\  & & _C. odoratum_ & mg/kg shoot/root quotient \(>\)1, \\  & & _B. asatica_ & Perennial shrubs are preferable \\ \hline Phytoremediation & & & BF \(<\) 1, so cannot be \\ capability of plants in & [PERSON] et al. [43] & _H. annuus_ (sunflower) & considered an accumulator but \\ spiked soils & & & an ex exudder \\ \hline  & & Accumulator (BF \(>\) 1 and \\  & & & TF \(<\) 1 in roots) so it is useful \\  & & & for phytostabilization \\ \hline  & & & Accumulated Pb mainly \\  & & Trees: C. _erctus_ & in shoots \\ \cline{2-4}  & & & Possible hyperaccumulators \\  & & _E. rostrata, A. saligna_ & (high concentrations in the \\  & & & above-ground tissues, no \\  & & & reduction of biomass) \\ \cline{2-4} In-the-field & & _B. junca L._ (Indian \\ phytoremediation & [PERSON] et al. [44] & _B. junca L._ (Indian \\  & & mustard), 80 types & \\  & & & had TF \(>\) 1 and accumulated \\  & & & more Pb than other varieties \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary table of phytoremediation studies carried out in Pb-contaminated soils. Another constraining factor is the maximum level of contaminants that can be accumulated by plants, which limits their use to sites with low metals content. This level was rated for hyperaccumulators, on a dry weight basis, to vary from about 0.2% for more toxic elements (Cd, Pb) to above 2% for the less toxic ones (Zn, Ni, Cu) [50].