## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass
#### 3.2.1 Liquid Biofuels
DN34 had a higher pollutant concentration, while NM6 showed superiority in terms of the mean annual uptake of HMs, i.e., 28% to 657% greater than the other. Trees grown in landfill soil exhibited 15% greater lignin content than those grown in agricultural soil, contributing to 46% lower glucose yield. Increased lignification has been associated with abiotic stresses, i.e., drought-stress resistance [124]. This study also corroborated the general trend of NM6 to have greater productivity and carbon storage potential than DN34 in the phytoremediation system [125]. It was also found that the mannan, xylan, and glucan content was 37%, 21%, and 26% greater, respectively, for trees growing in non-polluted soils than trees growing in polluted soils. For this study, the results showed that the use of phytoremediation biomass is not a viable alternative to produce biofuels, since the above-mentioned carbohydrate content is significantly lower in trees grown in polluted soils. One of the advantages of phytotechnologies is the use of native metal accumulator species that guarantee the success of the implementation. For instance, Tiwari et al. [126] reported the current phytoremediation situation in India, where the availability of different phytoremediator trees is abundant, for example, _Azadirachta indica_, _Acacia nilotica_, _Prosopis juliflora_, _Terminalia arjuna_, _Pongamia pinmata_, and _Eucalyptus tereictornis_. Special interest is the use of _Azaridachta indica_ and _Acacia nilotica_ not only for their use in traditional medicine, personal care, and the presence of biologically active compounds [127] but also because of restoration of eroded soils and the phytoremediation potential, removing organic compounds [128; 129], radionuclides [130], and toxic metals [122; 131; 132]. The studies report using seeds of _A. indica_ from phytoremediation to produce ethanol from biomass fermentation and biodiesel via base-catalyzed transesterification. For the cases of ethanol production, the yield was 1-3% \(w/w\); regarding the composition of primary fatty acids in neem oil, authors found oleic acid, linoleic acid, stearic acid, and palmitic acid (42%, 19%, 18%, and 15% \(w/w\), respectively). _Azadirachta_ biodiesel was mixed with diesel, and the engine performance was measured as specific fuel consumption (SFC) and brake thermal efficiency (BTE), defined as the ratio of brake power output to power input. The study demonstrated lower SFC utilization when the diesel blend was used; this is because of a higher oxygen content and cetane number compared to diesel alone. Regarding the BTE, it was observed that in all the mixtures tested (10-50%), the thermal brake efficiency was enhanced when the load increased, attributed to an increase in power output with an increase in load. Finally, the study concluded that these species might be used as phytoremediator species to clean up soils polluted with either organic or inorganic pollutants and then the biomass produced might be used as feedstock to produce biofuels that comply with required standards to be blended with commercial fuels. In the above-reviewed studies, it has been shown that the presence of toxic metals in plants can be controlled to minimize negative effects on bioethanol production in laboratory-scale trials. Different species have been analyzed to determine specific characteristics and process conditions related to this technological platform, such as metal concentration in soils and biomass, distribution of HMs in biomass, saccharification conditions, fermentation, and biofuel yield. The results of the above-mentioned experiments have validated the viability of biochemical conversion processes for fermentation of sugars and transesterification for fatty oils to produce bioethanol and biodiesel from biomass harvested after phytoremediation. In Table 4 are listed some studies where phytoremediation biomass was used to produce bioethanol, either on a theoretical or on a practical basis.