## 3 Coupling Phytoremediation and Bioenergy Production
### Thermochemical Conversion of Contaminated Biomass: Combustion, Gasification, and Pyrolysis
adsorbed with Cu and Ni was pyrolyzed in the reactor under different temperatures (600-1600 \({}^{\circ}\)C) and two different heating rates (10 and 50 \({}^{\circ}\)C/s). The results showed that a minimum threshold temperature of 1000 \({}^{\circ}\)C was required in samples to promote Cu and Ni catalytic effects on cracking and reforming tar. This led to increased gas yields at 1200 \({}^{\circ}\)C by 14.7% and 34.4%, respectively, compared with that from raw willlow. In particular, H\({}_{2}\) and CO production from Ni-impregnated willow was higher than that from raw willlow (10.30 and 12.16 mol/kg of wood versus 6.59 and 8.20 mol/kg of wood) for fast pyrolysis (50 \({}^{\circ}\)C/s). Under a heating rate of 10 \({}^{\circ}\)C/s, only nickel-impregnated willow showed increased H\({}_{2}\) and CO yields compared to raw willlow. This study confirmed that the solar pyrolysis of phytoremediation biomass is a sustainable alternative to producing valuable syngas. The disposal of large quantities of contaminated biomass via combustion is increasing [86, 87], so it is important not only to provide the required energy, especially in poor and underserved areas, but also to know the mechanisms involved in the capture of contaminants in fluxes and avoid the release of toxic compounds to the environment. For these reasons, research on the disposal of phytoremediation biomass through combustion in low-income regions is important. For example, three _Pteris vittata_ L. biomasses with different concentrations of As from three different regions in China were investigated [88]. The combustion results in pilot-scale trials showed that the As concentration in flue gas was greater than that allowed by national standards in the trial with no control, and the As concentration in biomass was 486 mg/kg. Parallelly, for As chemical capture tests, CaO was blended with biomass as a chemical absorber at 5%, 10%, and 15% of feedstock weight. The samples were pelletized and combusted. The results showed that the absorption of As was efficient when CaO was mixed with biomass. CaO addition notably reduced arsenic emission in flue gas, and absorption was efficient when CaO was mixed with biomass at 10% \(w/w\). For the trial with 10% CaO addition, arsenic recovery from ash reached 76%, which is an \(\sim\)8-fold increase compared with the control. The authors concluded that this disposal technology is economically feasible in low-income areas and is environment friendly. Many valuable by-products can be obtained from combustion biomass, which is why the use of the ashes to supplement soils as fertilizers has also been analyzed. Pogrzeba et al. [89] reported a study where the use of _Sida hermaphrodita_ to phytoextract toxic metals from soils was carried out. The study consisted in planting _S. hermaphrodita_ in soils where Pb, Cd, and Zn concentrations exceeded the limits sets by the Polish law for arable soils. Three treatments were established: plant growth in polluted soils without fertilizer (control), plant growth in polluted soils with standard fertilization (N, P, and K), and plant growth with a commercial microbial inoculum. The phytoextraction potential was determined, and the gasification experiment was carried out to determine the calorific value of the biomass. The authors reported that the phytoextraction of _S. hermaphrodita_ depends on the bioavailable form of the metals in the soil and that fertilization can reduce the accumulation of metals in plants. Regarding calorific values, small differences in the low heating value (LHV) were found between biomass from phytoremediation compared to other non-contaminated biomasses from other reports, indicating that despite contamination, the material is suitable for energy production. Additionally, the authors suggested that ashes after incineration can be used as fertilizers in forestry and agriculture, especially in sites where phytoremediation technologies are carried out.