## 2 Management of Biomass from Phytoremediation Processes
### Thermochemical Treatments  
Heavy metals are potentially toxic elements that cannot be degraded like organic pollutants. This conspicuous characteristic poses a great challenge for environmental engineers [30]. At a certain moment, the process has a dead-end mass transfer where the metal toxicity must be dealt with. In the past decade, several technologies have been tested to reduce the contaminated biomass by thermal processes and find a potential use of the process streams, including the recovery of the metal [31; 32; 33]. The latter has been traditionally regarded as a separate technology known as phytomining [34]. However, following the concept of closed-cycle processes, phytomining and energy recovery increasingly accompany thermal treatments. The foundation of thermal treatments is the transformation of the organic fraction of the accumulator biomass under different thermochemical conversion processes into solids (e.g., ashes, biochar), liquids (e.g., oil), or gases (e.g., syngas, biogas) [31; 35]. Simultaneously, the metal mass fraction must ideally be concentrated in a controlled and predictable way in one of these products. The metal partition in the different streams will determine the destiny of the product, and low concentrations may render it useful for further processing (compost material, catalyst material, or energy recovery), while high concentrations will limit downstream processing or must include another mass transfer operation to separate the metal fraction [36; 37; 38]. Then, the main constraint of any thermal process using this raw material is not only the partition of the metals between by-products but also its possible remobilization to the environment. Cui et al. [26] performed an extensive review of the available thermal technologies and concluded that the fate of metals responds to complex and multifactorial processes in all the available thermo-conversion formats. Although there are no available predictive constants for any process, it has been observed through experimental testing that some metals like Pb or Zn have wide temperature ranges (220-900 \({}^{\circ}\)C), where they can be largely (>90%) captured in the solid phase [39; 40]. On the other hand, other metals and metalloids like Cd [39] or As [41] are susceptible to leaving the system. This poses a risk in the process of using biomass to extract mixtures of metals. To bypass this constraint, specialized products can also be obtained as secondary products of plant biomass processing. For example, by-products containing metals (e.g., Zn)have been conceptualized as catalytic materials that can be upgraded--through upstream processes--to obtain acetate/furan [40]. The solid by-products can be refunctionalized as metal adsorbents after metal leaching, and the gas streams can be employed as syngas rich in hydrogen and methane [39]. Cd catalysts aimed for photodegradation of water pollutants down to permissible levels were prepared [42]. Xing et al. [43] provided an alternative use of metal-contaminated streams when they demonstrated that vegetal biochar can be transformed with plating wastewater into reusable Cd-biochar acting as nano Cd-S with photocatalytic activities and tested the concept in dye removal from water. Plant biomass can also be connected to the modern concept of a biorefinery that has expanded through the years to accommodate basic biofuels up to several high-end chemicals [44]. Therefore, the connection of biorefineries with harvested plant material from phytoremediation is a plausible research pathway if the management of post-phytoremediation biomass is conceptualized as an intermediate step of a process and not a dead end. However, a detailed level of metal compartmentalization control is not currently adequate for biorefineries, since these installations are located near plantations of genetically selected species for a predictable biofuel yield [45].