NMR Study of Strontium Binding by a Micaceous Mineral
The nature of strontium binding by soil minerals directly affects the transport and sequestration/remediation of radioactive strontium species released from leaking high-level nuclear waste storage tanks. However, the molecular-level structure of strontium binding sites has seldom been explored in phyllosilicate minerals by direct spectroscopic means and is not well-understood. In this work, we use solid-state NMR to analyze strontium directly and indirectly in a fully strontium-exchanged synthetic mica of nominal composition Na4Mg6Al4Si4O20F4. Thermogravimetric analysis, X-ray diffraction analysis, and NMR evidence supports that heat treatment at 500 °C for 4 h fully dehydrates the mica, creating a hydrogen-free interlayer. Analysis of the strontium NMR spectrum of the heat-treated mica shows a single strontium environment with a quadrupolar coupling constant of 9.02 MHz and a quadrupolar asymmetry parameter of 1.0. These quadrupolar parameters are consistent with a highly distorted and asymmetric coordination environment that would be produced by strontium cations without water in the coordination sphere bound deep within the ditrigonal holes. Evidence for at least one additional strontium environment, where proton-strontium couplings may occur, was found via a 1H-87Sr transfer of populations by double resonance NMR experiment. We conclude that the strontium cations in the proton-free interlayer are observable by 87Sr NMR and bound through electrostatic interactions as nine coordinate inner-sphere complexes sitting in the ditrigonal holes. Partially hydrated strontium cations invisible to direct 87Sr NMR are also present and located on the external mica surfaces, which are known to hydrate upon exposure to atmospheric moisture. These results demonstrate that modern pulsed NMR techniques and high fields can be used effectively to provide structural details of strontium binding by phyllosilicate minerals.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 889052
- Journal Information:
- Journal of Physical Chemistry B, 110(14):7159-7164, Vol. 110, Issue 14
- Country of Publication:
- United States
- Language:
- English
Similar Records
Factors responsible for crystal-chemical variations in the solid solutions from illite to aluminoceladonite and from glauconite to celadonite
Sorption of Cs+ to Micaceous Subsurface Sediments from the Hanford Site, USA
Related Subjects
ASYMMETRY
CATIONS
COUPLING CONSTANTS
ELECTROSTATICS
HEAT TREATMENTS
HYDRATES
MICA
MOISTURE
RADIOACTIVE WASTES
RESONANCE
SOILS
STORAGE
STRONTIUM
TANKS
THERMAL GRAVIMETRIC ANALYSIS
TRANSPORT
WATER
X-RAY DIFFRACTION
Environmental Molecular Sciences Laboratory