Germanium and rare earth elements in topsoil and soil-grown plants on different land use types in the mining area of Freiberg (Germany)

Abstract

The total concentrations and chemical fractionation of Germanium (Ge) and selected rare earth elements (REEs) in top soils and soil-grown plants of different land use types (moist grassland, mesic grassland, arable land) were investigated in the post-mining area of Freiberg (Saxony, Germany). The study area covers approximately 1000km2 in the south of Central Saxony, and 138 samples from 46 sampling sites were examined. Ge and REEs in soils were partitioned by a sequential extraction procedure into mobile/exchangeable (Fraction 1), acid soluble (Fraction 2), bound to organic matter (Fraction 3), amorphous Fe/Mn-oxyhydroxides (Fraction 4), crystalline Fe/Mn-oxides (Fraction 5) and residual fractions (Fraction 6). Total concentrations of Ge and REEs in soil varied considerably ranging from 1.0μgg−1 to 4.3μgg−1 for Ge (mean 1.9μgg−1) and 97μgg−1 to 402μgg−1 (mean 168μgg−1) for total REE contents, accounting for 17% and 14% to the investigated light REEs La and Nd, respectively and 2.8% and 1.5% to the investigated heavy REEs Gd and Er, respectively. Elements in potentially plant available fractions represented 8% of total Ge and 30% of total REEs, respectively. Soils on moist grasslands characterized as acidic fluvisols and gleysols with high organic matter content contained significantly higher total concentrations of Ge and REEs and higher concentrations of Ge and REEs in the potentially plant available Fractions 1–3 compared to soils of mesic grassland and arable land. Grass species accumulated significantly higher concentrations of Ge than herb species. Highest concentrations of Ge were measured in plant species growing on moist grassland (Phalaris arundinacea: 449ngg−1), while there were no significant differences with regard to the concentrations of REEs in plants among the different land use types. The results of this study indicate that moist grasslands may act as sinks for Ge and REEs. In these soils high amounts of soil organic matter and low pH may foster the retention of labile forms, increasing the pool of Ge and REEs accessible for phytoextraction. However, the species-specific processes involved during the uptake of REEs need to be understood in order to optimize phytomining techniques.