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Phytotechnology with Biomass Production
4.3 Field Experiments on Phytomining
Whereas part of the initial experiments on phytomining had been conducted
under totally artificial conditions in the laboratory, applying, e.g., solubilizing agents such as ammonium thiocyanate for phytoextraction of gold (e.g.,
Anderson et al., 1998), systematic field trials, following the pioneering work of
Nicks & Chambers (1995, 1998) and Robinson et al. (1997a, b) on Ni, have been
performed by Anderson et al. (2005) for gold (Au), testing the two plant species
Brassica juncea (Indian mustard) and Zea mays (corn), commonly used for phytoremediation and as energy crops. Applying an empirical model on the relationship
between Au concentration in the soil and in plants obtained from laboratory and
greenhouse studies, they suggested a minimum Au concentration in the substrate of 2mg kg −1 to achieve a gold concentration in crops of 100mg kg −1 , which,
given a biomass harvest of 10t ha −1 , would yield an economically viable gold
recovery of 1kg in plants per hectare. The authors proposed reclamation of spent
heap-leach piles or retreatment of waste dumps, e.g., from artisanal gold mining
as possible applications. Later on, Wilson-Corral et al. (2011), by application of
chemical amendments, achieved average Au concentrations in leaves and stems
of Helianthus annuus (sunflower) up to 19 and 21mg kg −1 , still being well below
the economic threshold proposed by Anderson et al. (2005). A field experiment
to assess phytomining feasibility for artisanal gold mining tailings with tobacco
plants, applying NaCN as chelating agent, however yielded Au concentrations of
1.2mg kg −1 dry leaf biomass only (Krisnayanti et al., 2016).
The most promising element for phytomining at present is Ni (Kidd et al.,
2018; Nkrumah et al., 2016). In a large number of experiments some ten plant
species, with a focus on Alyssum murale (syn. Odontarrhena chalcidica) (e.g.,
Matko Stamenković et al., 2017; Rosenkranz et al., 2019), have been tested for
their phytomining potential on ultramafic or Ni contaminated soils in different parts of the world. A major outcome from these studies is the positive effect
of fertilization with nitrogen (N), phosphorus (P), and potassium (K) and of
addition of organic matter on biomass of Ni hyperaccumulating plants, and
the increase of Ni uptake and accumulation in shoots by adjustment of soil pH
(optimum pH 5–7), addition of sulfur (S), or inoculation with (rhizo-)bacteria
and mycorrhiza (Kidd et al., 2018; Nkrumah et al., 2016; Rosenkranz et al., 2019).
4.4 Agronomic Practices
As shown above for gold, the application of synthetic solubilizing agents greatly
enhances Au uptake and accumulation in the aboveground biomass (Anderson
et al., 1998; González-Valdez et al., 2018; Wilson-Corral et al., 2011). However,
apart from the high costs of chelators, chemically induced hyperaccumulation,
Phytotechnology with Biomass Production
4.3 Field Experiments on Phytomining
Whereas part of the initial experiments on phytomining had been conducted
under totally artificial conditions in the laboratory, applying, e.g., solubilizing agents such as ammonium thiocyanate for phytoextraction of gold (e.g.,
Anderson et al., 1998), systematic field trials, following the pioneering work of
Nicks & Chambers (1995, 1998) and Robinson et al. (1997a, b) on Ni, have been
performed by Anderson et al. (2005) for gold (Au), testing the two plant species
Brassica juncea (Indian mustard) and Zea mays (corn), commonly used for phytoremediation and as energy crops. Applying an empirical model on the relationship
between Au concentration in the soil and in plants obtained from laboratory and
greenhouse studies, they suggested a minimum Au concentration in the substrate of 2mg kg −1 to achieve a gold concentration in crops of 100mg kg −1 , which,
given a biomass harvest of 10t ha −1 , would yield an economically viable gold
recovery of 1kg in plants per hectare. The authors proposed reclamation of spent
heap-leach piles or retreatment of waste dumps, e.g., from artisanal gold mining
as possible applications. Later on, Wilson-Corral et al. (2011), by application of
chemical amendments, achieved average Au concentrations in leaves and stems
of Helianthus annuus (sunflower) up to 19 and 21mg kg −1 , still being well below
the economic threshold proposed by Anderson et al. (2005). A field experiment
to assess phytomining feasibility for artisanal gold mining tailings with tobacco
plants, applying NaCN as chelating agent, however yielded Au concentrations of
1.2mg kg −1 dry leaf biomass only (Krisnayanti et al., 2016).
The most promising element for phytomining at present is Ni (Kidd et al.,
2018; Nkrumah et al., 2016). In a large number of experiments some ten plant
species, with a focus on Alyssum murale (syn. Odontarrhena chalcidica) (e.g.,
Matko Stamenković et al., 2017; Rosenkranz et al., 2019), have been tested for
their phytomining potential on ultramafic or Ni contaminated soils in different parts of the world. A major outcome from these studies is the positive effect
of fertilization with nitrogen (N), phosphorus (P), and potassium (K) and of
addition of organic matter on biomass of Ni hyperaccumulating plants, and
the increase of Ni uptake and accumulation in shoots by adjustment of soil pH
(optimum pH 5–7), addition of sulfur (S), or inoculation with (rhizo-)bacteria
and mycorrhiza (Kidd et al., 2018; Nkrumah et al., 2016; Rosenkranz et al., 2019).
4.4 Agronomic Practices
As shown above for gold, the application of synthetic solubilizing agents greatly
enhances Au uptake and accumulation in the aboveground biomass (Anderson
et al., 1998; González-Valdez et al., 2018; Wilson-Corral et al., 2011). However,
apart from the high costs of chelators, chemically induced hyperaccumulation,