IN SITU TIME-RESOLVED RAMAN AND X-RAY DIFFRACTION OF RARE EARTH ELEMENT ION EXCHANGE IN NANOPOROUS SITINAKITE

The nanoporous mineral sitinakite (Na₂Ti₂SiO₇·2H₂O) is a highly selective, fast ion conductor that is being tested for targeted removal of cesium and strontium from high level waste solutions. In addition to their environmental applications, titanium silicates have many technological uses including battery materials, hydrogen storage, and rare earth and transition element catalysts for gasses and petrochemicals. Sitinakite is stable under a wide range of pressure, temperature, and chemical conditions making it a potential host mineral to perform selective chemistries in extreme environments. We are determining the fundamental structural properties governing ion selectivity in this unique mineral with emphasis toward understanding its energy related applications.

Sitinakite exhibits multiple ion exchange steps that serve to enhance ion selectivity, and these steps are controlled by the host crystalline framework as well as the chemistry and hydration state of the native and ion exchanged compounds. To determine the exchange mechanisms, we collected high resolution in situ Raman spectroscopy and X-ray diffraction to capture the REE ion exchange processes from the native form and the H-form.

The results from these rare earth element ion exchange studies (using Y, Eu, Gd, Tb) indicates that a range of exchange dynamics exsist within a single host mineral. The exchange dynamics are sigificanly different for each REE tested. This was somewhat suprising as all three elements possess the same valance charge, similar ionic radii (±0.05Å), and similar hydration states in aqeous solutions (CN=8-9). TGA/DSC curves for before and after exchange states showed significant variation in nanopore H₂O capacities, indicating that the hydration states of the element and valence electron conduction have an effect on the sequestion mechansims and pathways through the porous host structure. The selectivity hypothesis concerning the effects of internal hydration and valence electron conduction that has been previously proposed, however the mechanisms of framework conformational changes, presence of intermediate structural states, and diffusion pathways have only been reported for a handful of materials and is the main area of focus for this study.