Rare earth elements and their release dynamics under pre-definite redox conditions in a floodplain soil

For the first time, the impact of pre-definite redox conditions on the release dynamics of rare earth elements (REEs) and the determining factors pH, iron (Fe), manganese (Mn), aluminum (Al), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and sulfate (SO₄^2-) in a floodplain soil was elucidated using an advanced, highly sophisticated automatic biogeochemical microcosm apparatus. The redox potential (E_H) ranged between +82 and + 498 mV during the experiment. The systematic increase of E_H caused a decreasing pH from 6.6 to 4.6 which resulted in an enhanced mobilization and release of REEs along with Fe, Al, and Mn under oxic and acidic conditions. Also, organic matter seems to contribute to the mobilization of REEs under changing redox conditions. A factor analysis identified that the REEs form one group with E_H, Fe, Al, and Mn what indicates that REEs and sesquioxides have a similar geochemical behavior. The pH, DOC, and DIC are together in another cluster which demonstrates that their behavior substantially differs from the other group. The sequential extraction procedure revealed that the majority of the REEs were bound in the residual fraction, followed by the reducible, the oxidisable and the water soluble/exchangeable/carbonate bound fraction. Future studies should further elucidate the specific release kinetics of REEs, the controlling factors on the release dynamics and the underlying mobilization processes in highly dynamic wetland soils around the globe.