Coupling Plant Biomass Derived from Phytoremediation of Potential Toxic-Metal-Polluted Soils to Bioenergy Production and High-Value by-Products—A Review

Abstract

Phytoremediation is an attractive strategy for cleaning soils polluted with a wide spectrum of organic and inorganic toxic compounds. Among these pollutants, heavy metals have attracted global attention due to their negative effects on human health and terrestrial ecosystems. As a result of this, numerous studies have been carried out to elucidate the mechanisms involved in removal processes. These studies have employed many plant species that might be used for phytoremediation and the obtention of end bioproducts such as biofuels and biogas useful in combustion and heating. Phytotechnologies represent an attractive segment that is increasingly gaining attention worldwide due to their versatility, economic profitability, and environmental co-benefits such as erosion control and soil quality and functionality improvement. In this review, the process of valorizing biomass from phytoremediation is described; in addition, relevant experiments where polluted biomass is used as feedstock or bioenergy is produced via thermo- and biochemical conversion are analyzed. Besides, pretreatments of biomass to increase yields and treatments to control the transfer of metals to the environment are also mentioned. Finally, aspects related to the feasibility, benefits, risks, and gaps of converting toxic-metal-polluted biomass are discussed.

Phytomanagement of Metal(loid)-Contaminated Soils: Options, Efficiency and Value

“…Besides the energy sector (e.g., bioethanol, biodiesel, biogas, and heat), many chemical, physical and biological biomass-processing technologies are reported as pre-treatment and conversion technologies. In the case of anaerobic digestion, some TE excess (e.g., >500 mg Zn kg −1 , 20,000 mg Mn kg −1 ) can decrease the methane content in biogas and daily methane production (Edgar et al, 2021). Essential oils from aromatic plants harvested at phytomanaged sites also did not show TE contamination (Raveau et al, 2020).…”

“…In particular, for some crops, such as Miscanthus x giganteus and Arundo donax, a delayed harvest can reduce undesirable components (K, Ca, P, S, and N) in the biomass. According to some studies, TE excess in the biomass can induce changes in heavy hydrocarbons present in tars, bio-oil yield, ash content, and relative evolution of CO 2 and H 2 in volatiles (reduced CO content) (Edgar et al, 2021). For poplar and willow short rotation coppice (SRC), Zn and Cd concentrations are higher in bark than in wood, decreasing in older branches and trunk.…”

“…Therefore, the selection of the harvested shoot parts and their age are an important factor (Grignet et al, 2020;Grzegórska et al, 2020). Trees growing at brownfield and landfill sites can exhibit higher lignin content than those cultivated in uncontaminated soils due to abiotic stresses, e.g., drought-stress, leading to lower glucose yield (Edgar et al, 2021). In contrast, vetiver plants exposed to Cu excess can display a decrease in lignin and an increase in hemicellulose and cellulose contents, leading to a higher production rate of bioethanol (Geiger et al, 2019).…”

“…Accordingly, the potential emission of volatile TE chemicals at high pyrolysis temperatures, the potential leaching of minerals and organics from chars, and the product quality of the products deserve attention. The TE fate depends on complex and multifactorial processes for all technologies based on thermal conversion (e.g., incineration, pyro-gasification, and pyrolysis) (Edgar et al, 2021). Oxide-forming elements and refractory compounds are often found in ashes and tars.…”

“…Several pre-treatments can separate the metal(loid)s from the biomass fraction of interest or on the contrary avoids their release during the process and limit their bioavailability in the biochars produced (Edgar et al, 2021): pre-mixing with chemicals (e.g., MgCO 3 , FeCl 3 and Fe(NO 3 ) 3 , CaO) before biomass pyrolysis (He et al, 2020); composting (except the methylation of Hg-chemicals); for anaerobic digestion and fermentation, pre-treatment with NaOH enhances the release of biogas and metals from straw; biomass pretreatments with either ethanol organosolv, soda or dilute acid (Asad et al, 2017) and steam explosion (Ziegler-Devin et al, 2019) to release TE before bioethanol production. Posttreatment of conversion products and platform chemicals is also an option (e.g., sorption of arsenicals by Fe hydroxides after solvolysis of Pteris vittata fronds (Carrier et al, 2011).…”