| Abstract
| - The nature of strontium binding by soil minerals directly affects the transport and sequestration/remediationof radioactive strontium species released from leaking high-level nuclear waste storage tanks. However, themolecular-level structure of strontium binding sites has seldom been explored in phyllosilicate minerals bydirect spectroscopic means and is not well-understood. In this work, we use solid-state NMR to analyzestrontium directly and indirectly in a fully strontium-exchanged synthetic mica of nominal compositionNa4Mg6Al4Si4O20F4. Thermogravimetric analysis, X-ray diffraction analysis, and NMR evidence supportsthat heat treatment at 500 °C for 4 h fully dehydrates the mica, creating a hydrogen-free interlayer. Analysisof the strontium NMR spectrum of the heat-treated mica shows a single strontium environment with aquadrupolar coupling constant of 9.02 MHz and a quadrupolar asymmetry parameter of 1.0. These quadrupolarparameters are consistent with a highly distorted and asymmetric coordination environment that would beproduced by strontium cations without water in the coordination sphere bound deep within the ditrigonalholes. Evidence for at least one additional strontium environment, where proton−strontium couplings mayoccur, was found via a 1H−87Sr transfer of populations by double resonance NMR experiment. We concludethat the strontium cations in the proton-free interlayer are observable by 87Sr NMR and bound throughelectrostatic interactions as nine coordinate inner-sphere complexes sitting in the ditrigonal holes. Partiallyhydrated strontium cations invisible to direct 87Sr NMR are also present and located on the external micasurfaces, which are known to hydrate upon exposure to atmospheric moisture. These results demonstrate thatmodern pulsed NMR techniques and high fields can be used effectively to provide structural details of strontiumbinding by phyllosilicate minerals.
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