TY - JOUR

T1 - Microcrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium

AU - Griffith, Christopher S.

AU - Luca, Vittorio

AU - Hanna, John V.

AU - Pike, Kevin J.

AU - Smith, Mark E.

AU - Thorogood, Gordon S.

PY - 2009/7/1

Y1 - 2009/7/1

N2 - The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2 center dot zH(2)O has been studied using X-ray and neutron diffraction techniques, 1D and 2D Na-23 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D Na-23 MAS NMR has identified three sodium species within the HTB tunnels-species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers Cs-137 and Sr-85 have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a-b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework.

AB - The structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2 center dot zH(2)O has been studied using X-ray and neutron diffraction techniques, 1D and 2D Na-23 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D Na-23 MAS NMR has identified three sodium species within the HTB tunnels-species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers Cs-137 and Sr-85 have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a-b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework.

KW - WASTE FORM CERAMICS

KW - LEACH RESISTANT CERAMICS

KW - NA-23 NMR-SPECTROSCOPY

KW - ANGLE-SPINNING NMR

KW - CRYSTALLINE SILICOTITANATE

KW - AMMONIUM MOLYBDOPHOSPHATE

KW - MICROPOROUS TUNGSTATES

KW - QUADRUPOLAR NUCLEI

KW - MAS-NMR

KW - HYDROTHERMAL SYNTHESIS

U2 - 10.1021/ic801294x

DO - 10.1021/ic801294x

M3 - Journal article

VL - 48

SP - 5648

EP - 5662

JO - European Journal of Inorganic Chemistry

JF - European Journal of Inorganic Chemistry

SN - 1099-0682

IS - 13

ER -