Facet-Dependent\nAdsorption of Rare Earth Elements\n(REEs) and Actinides onto Goethite: REE Pattern Variability and Cerium\nAnomaly

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