Microbial Dissimilatory Iron Reduction of REE-Doped Ferrihydrite and Differential Behaviors of Light and Heavy REEs

Abstract

Regolith-hosted rare earth element (REE) deposits are a critical resource, particularly for heavy REEs. Iron oxide minerals are one of the key REE carriers in these deposits, and elucidating the biogeochemical behaviors of carrier minerals and associated REEs is crucial for ore genesis and sustainable extraction. Here, using synthetic La- or Lu-doped ferrihydrite (La-/Lu-Fhy) as proxies for natural light/heavy REE-rich iron oxide minerals and Shewanella oneidensis MR-1 as a model dissimilatory iron reduction (DIR) bacterium, we investigated microbially mediated mineral evolution and REE partitioning. Results show that REEs inhibited the transformation of Fhy into the thermodynamically more stable magnetite (Mag), with La exerting a stronger inhibition than Lu. Following the DIR-mediated phase transformation, REEs were enriched within the iron oxide minerals, and Lu exhibited more pronounced enrichment than La. Moreover, the speciation of mineral-associated REEs shifted from a structurally incorporated to a surface-adsorbed state during Fhy reductive dissolution, and subsequently reverted to the incorporated state during secondary Mag mineralization on Fhy. Notably, more Lu than La remained structurally incorporated, due to its smaller ionic radius, stronger affinity for iron minerals, and weaker inhibition on Mag formation. These differential behaviors between light and heavy REEs during microbial DIR could promote REE enrichment and fractionation within iron oxide minerals. Our findings thus can provide insights into the mobilization and redistribution of iron oxide-associated REEs in actual deposits, advancing the understanding of the biogeochemistry of regolith-hosted REEs.