Structure and mobility of rare earth ions in interlayer space of montmorillonite: a molecular dynamics study

Ion adsorption-type deposits (IADs) are the dominant sources of rare earth elements (REEs), in which REEs are mainly enriched in clay minerals. However, the adsorption mechanism of REEs in the interlayer region of clay minerals is still poorly understood. In this study, by using molecular dynamics (MD) simulations, we explored the interlayer structures and dynamics of REEs-intercalated montmorillonite. La3+ and Lu3+ were used as the model cations for light REEs (LREEs) and heavy REEs (HREEs), respectively. It was found that the most thermodynamically stable state for both LREE- and HREE-montmorillonite was the double-hydration state and the corresponding basal spacing was calculated to be similar to 16.1 & Aring;. REE ions are located at the middle plane of the interlayer space and adsorbed on the montmorillonite basal surface through hydrogen bonds between its coordination water and the basal oxygens (i.e. as outer-sphere complexes). La3+ was 9-fold coordinated in the interlayer space with a mono-capped square antiprism coordination shell, while Lu3+ was 8-fold coordinated in a square antiprism cage. The mobility of REEs intercalated in the interlayer was significantly reduced compared to the mobility of REEs in aqueous solutions. The microscopic structures, thermodynamic data, and mobility obtained in the present study can help understand the enrichment and mobilization of REEs in IADs, and provide a molecular level basis for developing more efficient extraction techniques. MD simulations show that the most stable state is the double-layer hydrate for REE-montmorillonite. La3+ and Lu3+ form 9 and 8-fold outer-sphere complexes in interlayer space, respectively.