Geochemical fractions of rare earth elements in two floodplain soil profiles at the Wupper River, Germany

• REEs are dominant in the residual fraction followed by reducible fraction.
• The proportion of the residual fraction decreases with increasing atomic number.
• Clay, amorphous Fe–Mn oxides, P and organic matter correlate with REEs in the soils.
• Low pH leads to a higher mobility of the REEs.

We aimed to determine the concentrations and geochemical fractions of rare earth elements (REEs) according to the genetic soil horizons of two soil profiles (Eutric Fluvisols) at the Wupper River, Germany. The concentrations were determined using aqua regia extraction and the geochemical fractions were assessed using a sequential extraction procedure developed by the Commission of the European Communities Bureau of Reference. Single REE concentrations varied from 0.09 mg kg− 1 (Lu) to 40 mg kg− 1 (Ce). We have detected small differences of the REE concentrations between the horizons that seem to be due to flooding and the linked homogenisation processes. Rare earth elements dominate in the residual fraction (73.5%), followed by reducible (19.6%), oxidisable (6.6%), and water soluble, exchangeable (0.4%) fraction (calculated from the sum of the fractions and mean of both soil profiles). The proportion of the residual fraction tends to decrease with increasing atomic number, whereas the proportions of the other three fractions increase. Rare earth elements with higher atomic number seem to take earlier the bonding-places in the first three fractions than REEs with lower atomic number and therefore, rather the latter are bound to residual fraction or occur as free species. Important factors that affect the geochemical fractions and mobility of REEs are the adsorption of REEs onto clay and amorphous Fe–Mn oxides as well as formation of phosphate or organic complexes with REEs. A low pH favours the releases of REEs from the soil. In future, the impact of flooding regime and physico-chemical soil properties on the concentrations, geochemical fractions, and release kinetics of REEs should be determined in frequently flooded soils around the globe to improve our understanding of the geochemical behaviour of REEs.