## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass  
#### 3.2.1 Liquid Biofuels  
Besides solids and gases, liquid biofuels can be produced via thermal and biochemical conversion. These fuels have been extensively researched mainly to replace conventional liquid fuels (diesel and petrol) [99], representing a carbon-neutral resource contributing to the mitigation of greenhouse gas (GHG) emissions [100]. For instance, bioethanol has gained interest in several countries around the world [101; 102]. This biofuel has a high octane level; thus, the mix with gasoline is affordable, reducing the emission of pollutants and improving the performance of engines, and it is also produced by fermentation of edible or non-edible biomass [103]. Similarly, biodiesel can be obtained from feedstock oil such as non-edible oil or edible oil [104]. Biofuels can be classified based on the feedstock used for their production, i.e., first, second, and third generations of non-edible lignocellulosic biomass, into non-edible plants, microorganisms, and wastes, respectively [103]. However, to increase the added value of these fuels, the incorporation of biomass from phytoremediation has gained interest, and some studies have been done; for example, the versatile plant hemp (_Cannabis sativa_ L.) has been reported as a multi-use crop to phytoremediate soils polluted with toxic metals, radionuclides, and organic compounds and also as potential feedstock for bioenergy production [105]. In this sense, there are reports mentioning that the oil content in hemp seeds is between 25 and 35% \(w/w\)[106], making them promissory feedstock for biodiesel production processes [107]. Biodiesel produced from hemp seed oil presents physicochemical properties comparable to values reported in the fuel standard specifications for biodiesel fuel blendstock, i.e., ASTM D6751 [108; 109]. Phytoremediation hemp coupled to bioenergy processes has been recently investigated to include concept designs considering biodiesel production, bioethanol, biogas and combined heat, and power [105]. Depending on the type of contaminant and fate in the plant, the bioenergy production process should be carefully selected since contaminants could be released into the environment; this aspect represents the first barrier to facilitating incorporation of phytoremediation biomass into bioenergy conversion; some safety and regulatory aspects are still under revision [110]. In an interesting study, Rheay et al. [105] suggested that hemp is a strong candidate for generating profit from phytoremediation operations due to its ability to remove a wide spectrum of contaminants; this plant is feasible to be used as raw material for high-value fuels, such as biodiesel, bioethanol, biogas, and combined heat and power. In a complete manner, this plant represents a versatile feedstock to produce multi-bioenergy resources that deserve to be studied in detail. A similar case was reported by Jung et al. [111], where the biomass of non-edible cock-spur grass (_Echinochla crus-galli_) seeds from phytoremediation trials were used to produce biodiesel. The phytoremediation potential of toxic metals was tested while monitoring the growth of the grass and, simultaneously, the direct conversion of cockspur grass seeds into biodiesel. The study reported that the process responsible for the remotion of toxic metals was translocation from water into the plant; several authors have previously reported this process for other species, such as wheat [112], water hyacinth [113], and peony [114]. Jung et al. [111] also observed that during cockspur grass growth in the solution of the metals, the adsorption and translocation of the metals by the grass was initiated only after three weeks of growth; therefore, the control of the maximum amount of toxic metal uptake by each plant should be considered for the continuous growth of cockspur grass and the adsorption of toxic metals.