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Phytomining
cheap solar energy for creating a “bio-ore”, low energy inputs for melting,
less SO x emissions due to bio-ore being sulfur-free, the improvement of
site quality and other synergy effects with related industries as a consequence of biomass growth, e.g., generation of bioenergy (combustion,
fermentation, generation of heat and biofuels), and the public acceptance
as a “green” technology (Ali et al., 2013; Harris et al., 2009; Koptsik, 2014;
Robinson et al., 2003; Saxena et al., 2020). With respect to the environment
the sequestration of atmospheric carbon dioxide (CO 2 ) in plant biomass and
due to enrichment of soils with organic matter and other ecosystem effects
such as increased soil biodiversity, improved agronomic crop productivity,
land restoration and pollution control are of relevance, which have to be
considered in complete economic analyses (Kidd et al., 2018; Saxena et al.,
2020). Thus, already around the turn of the century, Li et al. (2003) pointed
to the option of sales of carbon credits. Robinson et al. (2009) emphasized
the positive effects of phytomining in the remediation of sites polluted and
degraded, e.g., due to mining and metallurgical processes, tailings, dumps,
etc. The reduction of erosion (wind, surface runoff) and leaching of toxic
substances to groundwater will improve water quality (Saxena et al., 2020).
In summary, phytoremediation is “safe, aesthetic, nonaggressive, nondestructive” (Koptsik, 2014).
However, there are also severe limitations and restrictions negatively
affecting the general applicability and success of phytomining. The most
important limitation is due to the plants’ shallow rooting system which
allows for minerals close to the surface only to be extracted via this “green”
technology (Hunt et al., 2014; Robinson et al., 2003). Furthermore, adverse
site conditions, e.g., poor physical and chemical properties of soils on contaminated sites, usually limit plant growth severely; most stressful soil
factors are the low content of nutrients and soil organic matter, and high
soil acidity leading to solution of heavy metals (Hunt et al., 2014; Koptsik,
2014). Koptsik (2014) and Saxena et al. (2020) point out that for climatic and
seasonal reasons, phytomining is more suitable in tropical and subtropical climates. Robinson et al. (2003, 2009) emphasize that phytomining is a
long-term process with a much larger demand for area per unit of valuable
elements compared to conventional mining and therefore may cause huge
environmental disturbances in case, e.g., of clearing of natural vegetation
for phytomining. Even if the total concentration of target elements in the
soil may be high, their availability to the plants is often too low for effective uptake (Heilmeier & Wiche, 2020; Robinson et al., 2009; Sheoran et al.,
2009). Hyperaccumulators with high uptake rates often show a low biomass
production (Hunt et al., 2014; Robinson et al., 2003). Thus, two options for
increasing element uptake of nonaccumulator plants have been proposed:
(i) genetically modified plants (e.g., Hunt et al., 2014; Koptsik, 2014; Saxena
et al., 2020); (ii) “induced hyperaccumulation” via applying chelating soil
amendments (e.g., Hunt et al., 2014; Koptsik, 2014; Robinson et al., 2003;
Wang et al., 2020; Wilson-Corral et al., 2012).