Adsorption Behavior Mechanism of Rare Earths by Iron Oxide–Clay Mineral Composites

Weathered crust elution-deposited rare earth ores represent a significant source of medium and heavy rare earth elements. Their mineralization mechanism fundamentally involves the adsorption and enrichment of rare earth ions on the surfaces of secondary minerals within the weathering crust. To elucidate the role of nano-sized iron oxide–clay mineral composites in the migration and enrichment of rare earth elements (REEs) in weathered crusts, α-Fe2O3, oolitic hematite, Fe2O3-kaolinite, and FeOOH-kaolinite composites were selected as adsorbents. Using La3+, Ce3+, and Y3+ as target ions, their adsorption performance and fractionation mechanisms were systematically investigated through batch adsorption experiments combined with characterization techniques including Langmuir/Freundlich model fitting, Zeta potential, XPS, and FT-IR. The results demonstrate that the adsorption of REEs on iron oxides and their composites follows the Langmuir model (R2 > 0.96), indicating monolayer homogeneous adsorption. The FeOOH-kaolinite composite exhibited the highest adsorption capacity (0.6531 mmol/g for Ce). Amorphous iron oxide–clay mineral composites showed superior complexation adsorption and enrichment capabilities due to their higher oxygen vacancy concentration (Oβ/Oα = 3.86 for FeOOH-kaolinite), whereas crystalline iron oxide–clay mineral composites contributed significantly to REE fractionation and enrichment. Among the influencing factors, elevated temperature (20–60 °C) favored the adsorption process, with an activation energy > 4.2 kJ/mol indicating chemical adsorption dominance. Within the pH range of 3.0–7.0, adsorption capacity increased with rising pH, with Ce being more sensitive to ionic strength and pH variations. The adsorption mechanism was further elucidated through post-adsorption XPS, FT-IR in the low-wavenumber region, and desorption studies. This study confirms that iron oxide–clay mineral composites regulate the enrichment and fractionation of REEs in weathered crusts through the interplay of their surface properties and environmental factors, providing a scientific basis for refining the metallogenic theory and guiding the exploration and development of weathered crust elution-deposited rare earth ore resources.