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Phytomining
valuable elements. First crude economic analyses of phytomining, based
on chelate-induced phytoextraction for increasing the solubility of, e.g.,
gold (Au), considered the costs for synthetic ligands added to the soil only
(Anderson et al., 1998). Later on, more advanced economic models, including
specified costs for site preparation, seeding, plant cultivation, application of
fertilizers and chelating agents, and harvesting, yielded that expenditures
for gold recovery by solvent extraction are the most dominant costs, which,
however, can be dramatically reduced by burning biomass, thus reducing the
volume of material for solvent extraction (Anderson et al., 2003). Burning of
dry plants (e.g., Odontarrhena muralis with 1% Ni in the plant mass) yielded a
concentration factor for Ni of ca. 12 (Kidd et al., 2018). Thus, depending on the
initial plant concentration of Ni (1%–2% in several hyperaccumulators of the
Brassicaceae family), Ni concentration in the ash may reach values of 10%–20%
which is much more than in Ni ore mined from laterites (Simonnot et al., 2018).
Robinson et al. (2003) developed a detailed mathematical model for assessing economic viability of phytomining as a function of multiple variables
(costs for planting and producing biomass, value of biomass, and bio-ore)
including interest rates and compared phytoextraction with alternative technologies. Applying their model to phytomining of Ni and Au in Australia,
Harris et al. (2009) concluded that the most decisive factors for profitability
are metal prices and the content of extractable metal(s). Another application
of the same model by Wilson-Corral et al. (2012) concluded that phytomining
for Au should be economically lucrative for metalliferous or abandoned mine
sites. For less precious metals than gold, such as Ni, Van der Ent et al. (2015)
emphasized the added market value of Ni catalysts for organic chemistry or
pure Ni salt crystals.
An important issue neglected in early cost-benefit analyses is related to
generation of bioenergy, e.g., via fermentation or combustion, and to increase
of soil organic matter and sequestration of atmospheric CO 2 particularly on
infertile marginal soils, the sale of carbon dioxide credits as already pointed
out by Li et al. (2003). Carbon credits are part of a more holistic environmental
evaluation of phytomining as proposed by Van der Ent et al. (2015). Kidd et al.
(2018) advocated Life Cycle Assessment (LCA) as the most recognized method
with many applications in agriculture and phytoremediation (see literature
cited in Kidd et al., 2018). One of the first applications of LCA in the framework
of phytomining was performed by Rodrigues et al. (2016), who considered,
among others, toxicity to humans and environmental pollution caused by soil
erosion (e.g., eutrophication) as a consequence of nonconservative agriculture.
Erosion control, e.g., by contour farming or winter cover crops, can reduce not
only loss of valuable metals but also impairment of human and ecosystem
health. In addition, the use of biomass of accumulator plants not only reduces
costs of phytomining but avoids atmospheric CO 2 increase by substituting
fossil fuels. Already 1 year earlier, Echevarria et al. (2015) had emphasized a
number of ecosystem services such as amelioration of soil quality, production of biofuel, conservation and restoration of biodiversity (protection of rare