## 3 Coupling Phytoremediation and Bioenergy Production
### Thermochemical Conversion of Contaminated Biomass: Combustion, Gasification, and Pyrolysis  
Biomass is the organic material composed of water, cellulose, hemicellulose, lignin, lipids, proteins, sugars, starch, and other compounds [71] and is potentially able to produce energy by thermal oxidation [72]. Some studies have reported that approximately 96% of the world's bioenergy is generated by biomass combustion [73]. The use of energy to oxidize residues such as biomass via gasification or combustion has a purpose to generate thermal energy and simultaneously destroy or recover possible toxic compounds under emission control [74]. There are two important sub-routes to recover energy from biomass: first, gasification that is relevant to produce CO and H\({}_{2}\) for Fischer-Tropsch synthesis (conversion of hydrogen and carbon monoxide into hydrocarbon products) [75], and second, combustion, which is a method of burning biomass to generate hot gasses to be used in energy generation processes [76]. Gomes [77] mentioned that the biomass produced in phytoremediation could be economically valorized in the form of bioenergy, i.e., biogas, biofuels, and combustion, even when the incorporation of contaminants into the biomass represents a potential risk during the management of this cellulosic material [78]. This risk is mostly associated with the reincorporation of these pollutants into the soil, i.e., toxic heavy metals and metalloids, due to inappropriate management and disposal [79]; important efforts have been made in order to reduce this risk, and some solutions have been offered in terms of pretreatments of the biomass and new materials used as adsorbers. In an experiment carried out by Aghaalikhani et al. [80], gasification products obtained in a bed-scale fluidized bed of hazelnut shells, simple poplar, and poplar coming from a phytoremediation procedure of soils polluted with different toxic metals such as Cu, Cd, Ni, Pb, Zn, Hg, As, Mn, and Sr were evaluated. The study concluded that hazelnut shells and poplar could be easily gasified in the equipment, producing a good-quality gas with low-polluting by-products. The poplar sample showed lower tar content and higher gas yield. It was proposed that Ca and Mg found in higher quantities in poplar samples could have a catalytic effect in tar reforming, thus producing a lower quantity of heavy hydrocarbons. The particulate size of biomass is also an important factor in determining the effectiveness of combustion and pyrolysis. Mlonka-Medrala et al. [81] evaluated two fractions of HMs contaminated biomass samples, i.e., small size (<0.09 mm) and large samples (>0.425 mm) of mixed cereal straw, _Miscanthus x giganteous_ and _Salix viiminalis_. During pyrolysis, the gaseous products were analyzed with GC-MS, and it was observed that size influences the activation energy of combustion and pyrolysis of biomass samples. This report supports the idea that using a thermochemical process to reduce the waste volume and the recuperation of metals may be a sustainable technology, especially for metal-contaminated biomass [82]. As mentioned before, a potential risk linked to this phytotechnology is the reinsertion of the metals into the environment. For this reason, Han et al. [32] proposed using adsorption-pyrolysis technology to recover valuable metals in solution from biomass after phytoremediation of polluted soils. _Broussonetia papyrifera_ biomass was collected and used to study the recovery of valuable metals from soil and water since it is tolerant to toxic metals and has a large biomass. The soil and water samples were taken from mining andsulting industrial areas, where the main metals found were Cu and Cd. The biomass was exposed to soil and water samples to facilitate the adsorption of metals at 0 to 180 min of exposure, and three different pH values were tested, i.e., 2, 4, and 6. After pyrolysis (1000 \({}^{\circ}\)C), the concentration of metals was determined. The authors concluded that _B.