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.