## 3 Coupling Phytoremediation and Bioenergy Production
### Biochemical Conversion of Metals--Polluted Biomass
#### 3.2.1 Liquid Biofuels
Another attractive plant object of analysis in the bioenergy and phytoremediation sectors during the past years is _Miscanthus_, an herbaceous perennial with high yield and environmental adaptability. Wang et al. [115] published a thorough review in which several aspects related to Miscanthus, i.e., biological properties, biofuel production, andphytoremediation of polluted soils, were analyzed. For instance, Wu et al. [116] reported important information about the metal tolerance of Miscanthus to single metals or mixtures where roots are the predominant part of toxic metal accumulation, and the capacity of accumulation varies significantly depending on the Miscanthus species. Ito et al. [117] reported different by-products derived from the fermentation process of Miscanthus phytoremediation biomass, such as hydrogen energy, by electrolyzing the extract of cellulose. Lee et al. [118] estimated the average cellulose content of _M. floridulus_, _M. sinensis_, _M. sacchariformis_, and _M. x giganteus_ per gram of biomass. They calculated the bioethanol production in the range of 0.211-0.233 g of bioethanol per gram of raw biomass. Liu et al. [119] evaluated a panel of 166 sorghum accessions grown in Cd-contaminated fields in two consecutive years to analyze the effect of different accessions and organs on phytoremediation by determining the accumulation of Cd in steams, leaves, and grains. After this first stage, three accessions of sorghum were identified as a potential resource to couple with bioethanol production, safe grains, and forage. The parameter identified to select the above-mentioned types of sorghum was the positive association of Cd accumulation with bioenergy traits, such as biomass, stem brix, internode numbers, and plant height. This study proposed a farming system that could reduce Cd in soil and obtain an economic return by producing bioethanol, safe grains, and forage. In this same sense, a holistic study was carried out by Tian et al. [120] that evaluated the morphological responses, biomass yield, and bioenergy potential during the entire growth period (167 days) of sweet sorghum with different concentration of Cd (1-100 mg/kg soil). The experiment was carried out under laboratory conditions, and authors found that morphological characteristics are not significantly different at a Cd concentration of \(\leq\)5 mg/kg soil. The root was the organ of Cd accumulation; thus, the reutilization of the aerial biomass would not be influenced. According to theoretical estimations, the predicted ethanol yield was 3.65, 3.05, 3.14, 2.69, 1.15, and 0.41 t/y/hm\({}^{2}\) for 1, 5, 10, 30, 50, and 100 mg/kg soil Cd concentrations, respectively. The calculations were done as first reported by Zhang et al. [121]. The authors concluded that the high biomass production and Cd accumulation capacity of sweet sorghum is a good combination, making it a viable candidate for bioenergy production in Cd-contaminated soils. It has been demonstrated that in superior plants, different phytoremediation routes are presented, although the time used to phytoremediate polluted soils normally becomes longer due to the slow growth of most of the species [122]; these aspects need to be dealt with to convert this environmental biotechnology into a more attractive and affordable option. Studies are being implemented to turn this drawback into opportunities; for example, Pilipovic et al. [123] reported long-term poplar phytoremediation in which biomass productivity and carbon storage during a rotation of _Populus deltoides_ Bartr. Ex Marsch \(\times\)_P. nigra L._ DN34 and _P. nigra \(\times\)P. maximowiczii_ A. Henry NM6 grown for landfill phytoremediation were tested. The researchers evaluated specific gravity and fiber composition and determined the biomass recalcitrance of trees grown in landfills versus those grown for biomass production on an agricultural site. Regarding phytoaccumulation and phytoextraction, specificity was observed associated with cloning and tissue types.