## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass
#### 3.2.2. Gaseous Bioenergy
donax_ may be an excellent option for phytoextraction and energy production due to the higher yield and HM uptake, so this species has become a viable solution for phytoremediation of soils polluted with HMs while at the same time producing energy. As for other bioenergy processes, the pretreatment of biomass has been studied to improve biofuel production yield and performance. A study aimed to determine the anaerobic digestion (AD) feasibility for the release of biogas and HMs from contaminated rice straw pretreated with an alkaline solution [151]. The results showed that NaOH pretreatment boosted the release of biogas and metals from straw via AD. The optimal conditions of the pretreatment, i.e., NaOH 6% (_w/w_) with a solid-to-solution ratio of 1:20, yielded 446.3 mL/g of total biogas and methane and 263.5 mL of volatile solids were obtained, which were 22.18% and 41.59% higher than those of the control without NaOH pretreatment. Pretreating rice straw with NaOH enhances the release of metals from the biomass, i.e., 86.9-97.6%, degrading the lignin and provoking the release and combining of hemicellulose or cellulose through enzymatic action, which leads to enhancing hydrolysis and higher biogas production via anaerobic digestion. In this study, Illumina sequencing identified _Bacteroidetes_ and _Firmicutes_ (proteolytic and hydrogen producers, respectively) as dominant phyla in biogas residues, while the growth of _Methanospirillum_ and _Methanosaeta_ in anaerobically digested effluent was promoted. Dual studies regarding the removal of toxic compounds from the soil via phytoremediation and incorporating this biomass for biogas productions have increased during the past years and have confirmed that the biomass that grows in polluted soils has a high potential for biogas production. The knowledge about this conversion technology has been enriched in the technical aspects and operational conditions and in the composition and dynamics of microbial communities involved in specific stages of the biological process, providing a better understanding of the whole process and ensuring the success of this technology. In Table 5 are described several experiments focused on the biochemical conversion of biomass to produce biogas and the theoretical estimation of bioethanol production. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Plant Species** & **Family** & **Mealio** &
\begin{tabular}{c} **Concentration of Metal** \\ **(mg/kg) * \\ \end{tabular} & **Process** & **Aim of the Study** & **Reference** \\ \hline \multirow{9}{*}{_Hierarchous avenues_} & & \(H\). continues & & & & \\ \cline{2-6}  & & Cd & 0:1-0.9 & & & \\ \cline{2-6}  & & Cu & 5:5-24.7 & & & \\ \cline{2-6}  & & Pb & 0:1-175 & & & \\ \cline{2-6}  & & Zn & 77-314 & & & \\ \cline{2-6}  & & Mn & 34-322 & & & \\ \cline{2-6}  & & Fe & 70-375 & & & \\ \cline{2-6}  & & As & 0.01–4.9 & & & \\ \cline{2-6}  & & S. minimum & & & & \\ \cline{2-6}  & & Cd & 0.01–2.8 & & & \\ \cline{2-6}  & & Cu & 6:7-14.1 & & & \\ \cline{2-6}  & & Pb & 0:01–117 & & & \\ \cline{2-6}  & & Zn & 40-286 & & & \\ \cline{2-6}  & & Mn & 33-161 & & & \\ \cline{2-6}  & & Fe & 33-463 & & & \\ \cline{2-6}  & & As & 0.01-6.4 & & & \\ \hline \multirow{9}{*}{_Eherichia biosynthesis_} & & \(C\)u & 253 & & & \\ \cline{2-6}  & & & Shoots of \#.partial & & & \\ \cline{2-6}  & & & Pb & 102.5 & & \\ \cline{2-6}  & & Seatman adjuvant & Cassinalcone & & Zn & 3216 & \\ \cline{2-6}  & & Seatman adjuvant & & Shoots of \#.partial & & \\ \cline{2-6}  & & Hyglalcone & & Co4 & 55.7 & & \\ \cline{2-6}  & & & Shoot of \#.partial & & & \\ \cline{2-6}  & & & Shoot of \#.partial & & & \\ \cline{2-6}  & & Mn & 22,96 & & & \\ \cline{2-6}  & & & & & & \\ \hline \multicolumn{6}{c}{Lewsey of \#.partial} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Biochemical conversion of phytoremediation biomass to produce biogas.