Ferrihydrite Transformation Impacted by Adsorption and Structural Incorporation of Rare Earth Elements

Understanding the association of rare earth elements (REEs) with iron (oxyhydr)oxides is of great interest in the recovery of REEs from regolith and weathered soils in the world. The objective of this study was to understand the effects of 1 mg L-1 REEs on the ferrihydrite transformation process. Ferrihydrite was synthesized at pH 5 and 7 in the presence of REEs, and the transformation products were characterized using X-ray diffraction, Fe K-edge X-ray absorption spectroscopy, and transmission electron microscopy. During the phase transformation process after 60-120 days, the retention of REEs varied with the REE atomic number. In general, heavy REEs preferably partitioned in aged (≥90 days) ferrihydrite more than light REEs, especially at pH 5. At pH 5, the transformation of ferrihydrite to goethite was retarded by ∼12% when REEs were present. The average structural substitution of REEs was as high as ∼550 mg kg-1, and the substitution of heavy REEs such as Yb and Lu was a critical factor to suppress the ferrihydrite transformation. At pH 7, the formation of hematite was retarded by ∼10%, and the structural incorporation of REEs was more important than adsorbed REEs during the first 30 days. There was no REE-specific retention in aged iron (oxyhydr)oxides at pH 7. The results of this study may explain the association of REEs with iron (oxyhydr)oxides, especially heavy REEs in mildly acidic regolith and weathered soils. This study advanced our understanding of the aqueous geochemical behavior of REEs in iron (oxyhydr)oxide-rich supergene environments.