## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass
#### 3.2.2. Gaseous Bioenergy  
As previously explained, biomass can be converted to liquid and solid biofuels via thermal and biochemical processes; this biomass can also be transformed into gaseous fuels, i.e., biomethane, biohydrogen, biopropane, and biobutane, via gasification, thermochemical processes, or anaerobic digestion, also known as anaerobic fermentation of biomass [140]. This energy form also includes biogas, which contains primarily methane, butane, and propane [141]. There are reports of interest that exemplify the incorporation of biomass from phytoremediation into gaseous bioenergy generation processes. _Helianthus annuus_ and _Silbyumarianum_ were used to phytoremediate soils polluted with As, Cd, Cu, Pb, and Zn [142] collected from a mining site. These reports consisted of using harvested biomass produced in polluted and non-polluted soils to determine the effect of contaminants on morphology and the biogas production potential (BPP) of this biomass. The shoots of _S. marianum_ and the stem and leaves of _H. annuus_ were anaerobically digested and showed a BPP of 194-223 mL g\({}^{-1}\) for _S. marianum_ that was higher than that for _H. annuus_ (134-154 mL g\({}^{-1}\)). There were no significant differences between polluted and non-polluted soils regarding average methane concentrations of 63.1% and 65.7% v/v for _S. marianum_ and _H. annuus_, respectively. The highest biogas production was found from seeds of _H. annuus_ (356-473 mL g\({}^{-1}\); 71% of CH\({}_{4}\)) after 9 days of anaerobic digestion. The aerial vegetative biomass of _H. annuus_ had higher calorific values than that of _S. marianum_ (17.1 and 14.3 MJ kg\({}^{-1}\), respectively). Finally, the authors did not report any limitation of the potential energy recovery due to trace metals. In a recent study published by Wang et al. [143], species belonging to different families (Lamiaceae, Asteraceae, Crassulaceae, Solanaceae, Phytolaccaceae, and Pteridaceae) were evaluated as phytoremediators of toxic-metal-polluted soils and the bioenergy generation was measured. The results reported biogas production potentials for _E. haiknownesis_, _S. alfredii_, _S. nigrum_, _P. americana_, and _P. vittate_ as 259.2 \(\pm\) 2.9, 238.7 \(\pm\) 4.2, 135.9 \(\pm\) 0.9, 129.5 \(\pm\) 2.9, and 106.8 \(\pm\) 2.1 mL/g, respectively. In addition, specific effects associated with metals and species were observed; for instance, the presence of Cu at 1000 mg/kg increased the cumulative biogas production, the daily methane production, and the methane yield of _E. haiknownesis_. For _S. alfredii_, the presence of Zn (\(\geq\)500 mg/kg) showed a negative impact on the methane content in biogas and daily methane production. Regarding the presence of Mn (5000-10,000 mg/kg), an increment was observed and then a decrease when the concentration of the element reached 20,000 mg/kg. The cumulative biogas production was not affected at these range concentrations (Cd (1-200 mg/kg), Pb (125-200 mg/kg), and As (1250-10,000 mg/kg)) for _S. nigrum_, _S. alfredii_, and _P. vittata_, respectively. The experiment demonstrated that anaerobic digestion has a promissory potential for the disposal of metal-contaminated biomass. Besides the experiments for removing toxic metals commonly found in soils, there is interest in evaluating the phytoremediation efficiency for mercury present as Hg\({}^{0}\), Hg\({}^{2+}\), and Hg\({}^{+}\). This element can be found in gaseous form (H\({}^{0}\)) in the atmosphere or metallic Hg if found in the liquid state [144]. An experiment carried out by Rollinson et al. [145] aimed to determine the treatability of _Lepidium sativum_ L. and _Mentha spicata_ L. biomass grown in compost spiked with mercury after cultivation for 20 and 68 days, respectively, via anaerobic digestion. The biomass was harvested and used as raw material for bench-scale anaerobic digesters operated in continuous-stirred batch mode for 10 days. The inhibition of anaerobic biogas production due to the bacteriostatic effect of Hg was observed.