Direct atomic-scale observation reveals rare earth element fractionation at mineral-water interfaces

Clay minerals, ubiquitous across diverse geological systems, exert fundamental controls on critical geochemical processes, particularly through their adsorptive interactions with economically vital elements such as heavy rare earth elements (HREEs). The adsorption of HREEs onto clay surfaces is not only the key process that enriches these critical resources but also a potential archive for dating significant weathering events. However, the atomic-scale mechanisms governing the preferential adsorption and fractionation of rare earth elements (REEs) at mineral-water interfaces have remained poorly constrained. Here, employing in situ atomic-scale imaging via high-resolution atomic force microscopy, we systematically investigate the adsorption behaviors of light and heavy REEs (La, Y, Yb) onto key crystallographic planes-Si-basal, Al-basal, and edge terminations-of representative clay minerals (kaolinite, illite, mica) under environmentally relevant conditions (pH 4.8 to 6.4). We directly visualize and quantitatively reveal that Si-basal planes selectively enrich HREEs with pH-enhanced adsorption capacity, whereas Al-basal planes exhibit negligible REE adsorption below pH 5.8 but preferential HREE adsorption at pH 6.4. Integrating synchrotron X-ray absorption spectroscopy, we identify the adsorption mechanism of REE on Si-basal and Al-basal/edge planes as outer-sphere complexation and inner-sphere binding, respectively. Furthermore, we quantify the contributions of distinct planes via surface complexation modeling, revealing Si-dominated to Al- and edge-controlled adsorption with elevating pH. These atomic-scale findings not only resolve long-standing debates about clay mineral-mediated REE fractionation but also redefine fundamental paradigms of surface reactivity at silicate mineral-water interfaces, offering insights into the cycling and metallogeny of other critical metals in aqueous and sedimentary environments.