Rare earth elements in German soils - A review

Rare earth elements (REEs) are increasingly used in high-tech industry, agriculture, and healthcare technologies what leads to their release into soils and waters, and to the transfer into plants what may have negative impacts on human health and the environment. The toxicity and potential mobilization of REEs in soils can be assessed by their content and geochemical behavior along with soil properties. However, those interactions are so far not reviewed in German soils although such a review is important for a better understanding and prediction of the potential mobilization and toxicity. Therefore, this review summarizes the recent knowledge about REE contents and potential mobilization in different soil profiles in Germany. We found that the REE content tends to decrease in dependence on the parent material in the following order: Carbonatite > basalt > orthogneiss > clay slate > loess > sandstone > Pleistocene and Holocene sediments > organic material. Also, we used data of earlier studies, summarized and newly evaluated those data aiming to quantify the factors influencing the total REE content in German soil profiles. The contents of REEs in soil profiles of different parent material showed significant relations with content of clay, carbonate, organic matter, aluminium, iron, and manganese. Geochemical fractionation results suggest that the bioavailability of REEs is relatively low while the residual fraction is relatively high in German soils. In soils, where water fluctuations are important, the redox potential is a key factor controlling the mobilization of REEs also via related changes of pH.