Rate and Stoichiometry of Ytterbium Release during Ligand-Promoted Goethite Dissolution

Goethite is widely present in weathering environments and can host structurally incorporated trace elements, including heavy rare earth elements (REEs). The stability of goethite-hosted REEs in the presence of low molecular weight organic acids will impact their accumulation and mobility in regolith-hosted deposits, major sources of heavy REEs. This study assessed oxalate-promoted dissolution of Yb-incorporated goethite in flow-through and batch reactors, focusing on the kinetics and stoichiometry of Yb release at pH 4–6. Oxalate enhanced the kinetics of Fe and Yb release and promoted stoichiometric dissolution in flow-through experiments (1 mM oxalate at pH 4–5). In flow-through experiments with lower oxalate concentrations (0–0.1 mM at pH 5) and in batch experiments (1 mM oxalate at pH 4–6), the dissolution was non-stoichiometric with preferential release of Yb over Fe. Non-stoichiometric preferential Yb release is consistent with possible Fe retention by the mineral, while the dissolution of Yb-goethite may overall be stoichiometric. Experimental observations demonstrated that ligand-promoted dissolution can mobilize Yb at lower pH values (4–5), but they also showed that Yb may adsorb to the goethite surface via ternary surface complexation at higher pH values (5–6). Overall, the observed behavior has implications for understanding trace element mobility in weathering environments.