COMPETITIVE ADSORPTION OF RARE EARTH ELEMENTS ON ALUMINA-WATER SURFACE

Rare earth elements (REEs) are crucial for various modern technologies, including renewable energy, electronics, and transportation. Ion adsorption-type REE deposits (IADs), originating from weathered granitic rocks, have attracted attention due to their spatial abundance, the economically-valuable concentration of REEs, and ease of chemical extraction. In such deposits, REEs primarily adsorb to clay minerals, such as kaolinite and halloysite, with a smaller portion likely bound to gibbsite, illite, montmorillonite, and Fe/Mn oxide minerals. However, the adsorption mechanisms of REEs on clay surfaces has not been resolved. Strong retention of REEs in a high weathering rate environment indicates a strong adsorption mechanism, but the relative ease of extraction points to cation exchange.

We have applied non-resonant and resonant anomalous X-ray reflectivity techniques to investigate the adsorption behavior of light (Nd), middle (Dy), and heavy (Yb) REEs on alumina (001) surfaces, which serve as a structural analog to the aluminol basal planes of kaolinite and gibbsite. Our results show that Nd, Dy, and Yb adsorb onto the alumina surface simultaneously as both inner- and outer-sphere species when they are the only REE present. The height of inner-sphere adsorbed Nd, Dy, and Yb above the alumina surface decreases with their ionic radii (Nd > Dy > Yb). The inner-sphere to outer-sphere ratio also decreases with increasing atomic number. However, when all three REE are present in an equimolar mixture, Dy surface coverage substantially decreases and Yb surface coverage diminishes even further. All three REE adsorb only as inner-sphere species in the mixed-solution scenario. Our results indicate that the alumina (001) surface display strong selectivity for light REEs. Therefore, adsorption through surface complexation on kaolinite and gibbsite basal planes will produce substantial REE fractionations. Enrichment of heavy REES observed in some IADs likely requires other adsorption mechanisms, such as adsorption on edge sites, cation exchange in halloysite, disordered kaolinite, and interstratified smectite that require further investigations.