Facet-Dependent Adsorption of Rare Earth Elements (REEs) and Actinides onto Goethite: REE Pattern Variability and Cerium Anomaly

Assessing the fate of contaminants in the environment requires a deep understanding of intrinsic adsorption mechanisms on natural minerals such as Fe-oxyhydroxides. In this study, we proposed an innovative approach to probe site heterogeneities on the goethite surface by comparing the adsorption behavior of rare earth elements (REEs, including Sc, Y, and all lanthanides; Ln) except Pm, as well as Th and U. A surface loading-dependent adsorption of Ln and Y was observed, with a shift from (i) preferential middle to heavy REE adsorption and (ii) limited to substantial fractionation between Y and Ho as the loading increased. These observations are likely attributable to the formation of strong and weak complexes onto the (021) and (110)/(100) goethite faces at low and high loadings, respectively. Additionally, Ce-anomaly, characteristic of Ce(III) partial oxidation to Ce(IV), was observed only at high loading. By drawing an analogy with Th(IV) and Sc(III), Ce(IV) is expected to outcompete Ln(III) and Y adsorptions and stabilize primarily at the strong sites on the (021) face, even under conditions of high loading. The outcome of this study, supported by charge distribution-multisite complexation (CD-MUSIC) calculation, provides new insights into the impact of facet-dependent adsorption and redox processes on Fe-oxyhydroxides.