Investigation of cation environment and framework changes in silicotitanate exchange materials using solid-state Na, Si, and Cs MAS NMR |
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Authors: | Brian R Cherry May Nyman |
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Institution: | a Department of Organic Materials, Sandia National Laboratories, P.O. Box 5800 MS0888, Albuquerque, NM 87185 USAb Department of Geochemistry and the Department of Environmental Monitoring and Characterization, Sandia National Laboratories, Albuquerque, NM 87185 USA |
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Abstract: | Crystalline silicotitanate (CST), HNa3Ti4Si2O14·4H2O and the Nb-substituted CST (Nb-CST), HNa2Ti3NbSi2O14·4H2O, are highly selective Cs+ sorbents, which makes them attractive materials for the selective removal of radioactive species from nuclear waste solutions. The structural basis for the improved Cs+ selectivity in the niobium analogs was investigated through a series of solid-state magic angle spinning (MAS) NMR experiments. Changes in the local environment of the Na+ and Cs+ cations in both CST and Nb-CST materials as a function of weight percent cesium exchange were investigated using 23Na and 133Cs MAS NMR. Framework changes induced by Cs+ loading and hydration state were investigated with 29Si MAS NMR. Multiple Cs+ environments were observed in the CST and Nb-CST material. The relative population of these different Cs+ environments varies with the extent of Cs+ loading. Marked changes in the framework Si environment were noted with the initial incorporation of Cs+, however with increased Cs+ loading the impact to the Si environment becomes less pronounced. The Cs+ environment and Si framework structure were influenced by the Nb-substitution and were greatly affected by the amount of water present in the materials. The increased Cs+ selectivity of the Nb-CST materials arises from both the chemistry and geometry of the tunnels and pores. |
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Keywords: | Silicotitanate Ion exchange Selectivity Cesium Solid-state NMR |
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