Experimental evidence of REE size fraction redistribution during redox variation in wetland soil
Résumé
The evolution of rare earth element (REE) speciation between reducing and oxidizing conditions in a riparian wetland soil was studied relative to the size fractionation of the solution. In all size fractions obtained from the reduced and oxidized soil solutions, the following analyses were carried out: organic matter (OM) characterization, transmission electron microscopy (TEM) observations as well as major and trace element analyses. Significant REE redistribution and speciation evolution between the various size fractions were observed. Under reducing conditions, the REEs were bound to colloidal and dissolved OM (<2 μm size fractions). By contrast, under oxidizing conditions, they were distributed in particulate (>2 μm size fraction), colloidal (<2 μm size fraction), organic and Fe-enriched fractions. In the particulate size fraction, the REEs were bound to humic and bacterial OM embedding Fe nano-oxides. The resulting REE pattern showed a strong enrichment in heavy REEs (HREEs) in response to REE binding to specific bacterial OM functional groups. In the largest colloidal size fraction (0.2 μm–30 kDa), the REEs were bound to humic substances (HS). The lowest colloidal size fraction (<30 kDa) is poorly concentrated in the REEs and the REE pattern showed an increase in the middle REEs (MREEs) and heavy REEs (HREEs) corresponding to a low REE loading on HS. A comparison of the REE patterns in the present experimental and field measurements demonstrated that, in riparian wetlands, under a high-water level, reducing conditions are insufficient to allow for the dissolution of the entire Fe nano-oxide pool formed during the oxidative period. Therefore, even under reducing conditions, Fe(III) seems to remain a potential scavenger of REEs.