| Germanium and REEs are strategic elements that are used for high tech devices and
engineered systems, however these elements are hardly concentrated into mineable ore
deposits. Since these elements occur widely dispersed in the earth crust with concentrations
of several mg⋅kg−1 (Ge 1.6 mg⋅kg−1, Nd 25 mg⋅kg−1) a new possibility to gain these
elements could be phytomining, a technique that uses plants to extract elements from soils via
their roots. Since knowledge about accumulating plant species is quite limited we conducted
research on the concentrations of strategic elements in wood and leaves of fast growing tree
species (Salix spec., Populus spec., Betula pendula, Alnus glutinosa, Fraxinus excelsior, Acer
pseudoplatanus).
In total 35 study sites were selected in the mining affected area around Freiberg (Saxony,
Germany), differing in their species composition and degree of contamination with toxic trace
metals (Pb, As, Cd). On each site plant tissues (wood and leaves, respectively) of different
species were sampled. In addition soil samples were taken from a soil depth of 0 - 30 cm and
30 – 60 cm.
The aim of our work was to investigate correlations between the concentrations of the
target elements in plant tissues and soil characteristics like pH, texture, nutrients and
concentrations in six operationally defined soil fractions (mobile, acid soluble, oxidizable,
amorphic oxides, crystalline oxides, residual or siliceous). Concentrations of elements in soil
extracts and plant tissues were measured with ICP-MS. The element Nd was selected as
representative for the group of REEs, since this element showed a high correlation with the
concentrations of the other REE
We found that the concentration of Nd in the leaves (0.31 mg⋅kg−1Nd) were several times
higher than in herbaceous species (0.05 mg⋅kg−1 Nd). The concentration of Ge in leaves
were ten times lower than that of Nd whereas in herbaceous species Nd and Ge were in equal
magnitude.
Within the tree species Populus spec. (0.77 mg⋅kg−1) and Salix spec. (0.36 mg⋅kg−1)
showed by far the highest concentrations of Nd in leaves. Fraxinus excelsior (0.10 mg⋅kg−1)
and Betula pendula (0.06 mg⋅kg−1) feature the lowest concentrations of Nd in
leaves.
We found significant correlations between the concentration of the target elements in the
plant tissue and the concentration of these elements in the first steps of the sequential
extraction of the soil samples.
These studies have been carried out in the framework of the PhytoGerm project financed
by the Federal Ministry of Education and Research, Germany. |