# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 3 Phytoremediation
### Phytoremediation By-Products  
The fate of harvested plants used in phytoremediation presents a challenge, since the nature of phytoaccumulated contaminants may be classified as hazardous and requires an adequate final disposal [46]. In phytoremediation practice, the production and disposal of biomass must also be assessed in the planning phase. A possible solution could be a holistic approach that allows using the phytoremediation waste as a different type of raw material, as suggested by [PERSON] et al. [53].  
The choice of the suitable biomass recovery technique depends on the type of plant and the content of the contaminants. Several treatments have been proposed [54] and comprise direct combustion with energy production, biofuel production, and materials production. Traditionally, the most employed system for the recovery of stored biomass energy is direct combustion, whose final products are fumes and ashes. Fumes are properly treated before being released into the atmosphere; instead, ashes are generally landfilled or, in some cases, treated to recover metals. In direct combustion processes, the metals can affect chemical reactions and induce catalytic phenomena, accumulating at the end of process in fly ash and settling on the walls of the chimneys.  
Another kind of thermal conversion is gasification, which comprises four phases: drying, pyrolysis, reduction, and combustion. Gasification treatment seems to be particularly suitable for the recovery of phytoremediation by-products because high temperature degrades organic contaminants and other problematic elements are removed with the ashes [54]. This treatment produces syngas, which is then used as an energy source, basic substance for chemical industry, or for hydrogen production.  
Pyrolysis is also a stand-alone treatment that decomposes material under anaerobic conditions and has no emissions to the air. In this case, the final products are pyrolytic fluid oil and coke, which could be used in smelters as suggested by [PERSON] et al. [55]  
Biomass can also be digested anaerobically and converted to digestate and biogas [56]. Biogas can then be burned in a gas engine for the production of electricity, with heat recovery in a combined heat and power (CHP) system, or purified to produce biomethane.  
According to [PERSON] et al. [57] and [PERSON] et al. [53], during the fermentation and anaerobic digestion processes, the presence of heavy metals at concentrations above certain thresholds can inhibit biochemical reactions and cause toxic effects on microorganisms. This type of recovery strategy is affected by many factors such as the solubility of the metal and the type of microorganisms involved [57]. After the anaerobic digestion process. metals concentrate in the digestate [58], so precluding its use as a soil amendment.  
Oil plants can be mechanically pressed to obtain pure plant oil (PPO), transformed into biodiesel through transesterification, and used as biofuel. PPO has a slightly lower calorific value (PCI) than diesel [59]. Even if in these plants metals are concentrated less in seeds than in leaves, fruits, and branches [60], there are no data on the possible effects of metals on the biodiesel production process. So, further studies are required to better understand the possible presence of metals in the fumes derived from the combustion of so-obtained biodiesel [52].  
Biomass can also be used to obtain different types of materials: matches and wooden packaging (produced from trees and vegetable textile fibers, when employing herbaceous plants or shrubs), bioplastics, or biochar [54].