## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass
#### 3.2.2. Gaseous Bioenergy
During the same experiment, canola oil-sulfide polymer derived from biowaste was used as an intermediary treatment to test its capacity for extracting mercury from samples prior to anaerobic digestion and from the post-experimentation reactor digestate. It was found that the polymer removed mercury from the digestate with a 40-50% efficacy, resulting in a feasible option to remediate sludge and a technical step to be combined with anaerobic digestion of biomass from phytoremediation. Among other species, _Helianthus annuus_ has been extensively used to remove metals from soils. For instance, Lee et al. [146] investigated the long-term stability performance of the anaerobic digestion process of a continuous-stirred-tank reactor (CSTR) operated for 1100 days with sunflower harvested in a potential toxic-metal-contaminated site (all parts of the sunflower were used, such as stem, leaf, and flower). It was found that soluble metal concentrations were lower than the reported inhibitory level and the reactor performance remained stable up to an organic loading rate (OLR) of 2.0 g volatile solids (VS)/L/day at a hydraulic retention time (HRT) of 20 days. Additionally, changes in microbial communities during digestion were evaluated by pyrosequencing, and cellulose degrader populations were found to be established, i.e., _Bacteroidetes_ (29.6-63.6%), _Firmicutes_ (6.5-18.2%), _Proteobacteria_ (0.4-15.4%), _Tenericutes_ (0.3-32.3%), _Choloffexi_ (0.3-6.0%), and _Spiroachete_ (0.25-10.1%). When considering a different taxonomic level, i.e., class, the composition was mainly _Bacteroidia_ (28.6-63.5%), _Clostriia_ (6.4-15.3%), _Deltaproteobacteria_ (0.1-8.9%), _Anaerolineae_ (0.2-5.5%), and _Betaproteobacteria_ (0.2-3.8%). As mentioned earlier, the most abundant phylum was _Bacteroidetes_, which is frequently found in anaerobic reactors, with an important role as fermenters and acidogens [147]; _Firmicutes_ also steadily increased over time, ranging from 7.1% to 18.2%. _Firmicutes_ are known to be involved in hydrolyzing polymers such as cellulose and lignin [148]. Two phyla of archaeal communities were detected, i.e., _Euryarchaeota_ (54.2-98.8%) and _Crenarchaeote_ (1.2-45.8%). The dominant archaeal sequences were _Metthanobacteriales_ (10.2-69.7%), _Methanosarcinales_ (9.3-17.1%), and _Methanomicrobiales_ (0.2-1.1%) at the order level. During anaerobic digestion, only _Methanobacteriales_ increased significantly. After this exhaustive microbiological analysis, the authors concluded that microbial metabolism was maintained appropriately, and anaerobic digestion is feasible for the disposal of toxic-metal-containing crop residues from phytoremediation sites. The authors mentioned that only Cu and Zn were detected in solution, whereas Cd, Pb, and Ni were barely observed during the experimental periods. The reason was suggested to be the precipitation of these metals with sulfur and hydroxide [149]. Some other studies have focused on unused polluted soils where plants such as _Arundo donax_ L. grow and can be coupled to bioenergy production. _A. donax_ has shown high potential to be used in phytoremediation processes, and its harvested biomass represents a resource [150]. The authors proposed using the phytoremediation biomass in two different processes, depending on the time of harvesting: anaerobic digestion with fresh biomass before winter and combustion of dry canes in late winter. The study reported that _A. donax_ is able to remove 3.87 kg ha\({}^{-1}\) of Zn, 2.09 kg ha\({}^{-1}\) of Cu, and 0.007 kg ha\({}^{-1}\) of Cd from soil. Regarding biomass production, it was found that plants grown in polluted soils produced less biomass (7.67 \(\pm\) 1.66 tons of dry mass ha\({}^{-1}\)) compared to plants growing in non-polluted soils (13.23 \(\pm\) 1.33 tons of dry mass ha\({}^{-1}\)) in the first year, but no differences were found in the second and third years of cultivation. According to the results, this study concludes that _A.