## 2 Management of Biomass from Phytoremediation Processes
### Phytomining  
Besides the treatments mentioned above, where the main objective is converting biomass into energy or functional materials, phytomining is a technology focused on the recovery of valuable metals from plant biomass [56]. Metals are released to the environment because of their exploitation as raw materials in modern industries such as batteries and electrochemistry. Therefore, it is a paradox that they end their life cycle as soil pollutants. Phytomining is a technology that aims to close this cycle by recovering the metals. The fundamental base is leaching the contaminated plant biomass with acids to recover the adsorbed metal and to obtain biomass with metal concentrations in the range of environmental norms. Wang et al. [57] combined this concept and found that acid treatments improved plant biomass to adsorb more metals and can be further used to prepare biochar by pyrolysis aimed to confine metals. Recently, the phytomining of palladium was tested with a model and well-known phytoremediator species, and recovery from synthetic tailings proved close to commercial ranges [58] and with high potential of improvement by chemical and biological engineering strategies. Potential plant species, soil amendments, and biomass processing have been tested for Hg and Cd [53; 54]. Serpentine soil, famous for being the type of site where metal-tolerant plants have an important ecological niche, may be a rich source of metals, particularly Ni [59]. Plant screenings, agronomical practices, leaching methods, and full-scale yields have been tested in this commercially attractive environment [60]. Phytomining perspectives have also benefited from cell-level detailed determination of metal accumulation. Do Nascimento et al. [61] recently determined that citric acid treatment increases the potential of _Odontarhena_ to mine Ni and Co from serpentine soil and imaged how Mn, Ni, and Co are accumulated in different leaf cells. This work provided information about the technological feasibility of phytomining and paves the way to tailor accumulation patterns through genetic screening and testing. The concept of differential compartmentalization already occurs in natural cultivars of sunflowers. Kolbas et al. [62] observed that Cu-contaminated soil could be employed to grow sunflowers, where Cu could be phytoextracted in leaves, and oil may be used as biofuel. Interestingly, the authors also suggested that plant biomass could be composted as a Cu fertilizer specialty. Another alternative for sunflower biomass used for phytoremediation is its use as raw material for industrial saccharification intended for bioethanol production [63]. Since modern saccharification is an enzymatic process, one concern was that metals in the biomass would inhibit the process. The authors observed that Pb was a strong inhibitor; Cu and Cd were intermediate; and Ni, As, and Zn did not inhibit the process. The presence of metals in biomass can also change the yield of the by-products. Doroshenko et al. [64] showed that it is the presence of a metal and the in vivo interactions that alter the carbon partition. As reviewed by Fagnano et al. [65], the metal concentration in shoot biomass can be controlled in desired ranges by agronomical practices or genetic screening of metal accumulator plants. Finally, all treatment options for phytoremediation biomass will obtain more design options from the study of plant genetics. Recently, Basharat et al. [66] concluded by means of a literature review that many biological processes in phytoremediation have enough biological knowledge to either develop protocols or start testing modern genetic editing technologies like the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 nuclease system. Concomitantly, a vast array of biological discoveries ranging from small chelating molecules to transcription factors has been put in the context of its potential of genetic improvement [9; 67].