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Hwang, Huijeong,Seoung, Donghoon,Gatta, G. Diego,Blom, Douglas A.,Vogt, Thomas,Lee, Yongjae Mineralogical Society of America 2015 The American mineralogist Vol.100 No.7
<P>Synchrotron X-ray powder diffraction experiments have been performed on dehydrated Cs- exchanged natrolite to systematically investigate successive transitions under high pressures and temperatures. At pressures above 0.5(1) GPa using H<SUB>2</SUB>O as a pressure-transmitting medium and after heating to 100 °C, dehydrated Cs<SUB>16</SUB>Al<SUB>16</SUB>Si<SUB>24</SUB>O<SUB>80</SUB> (deh-Cs-NAT) transforms to a hydrated phase Cs<SUB>16</SUB>Al<SUB>16</SUB>Si<SUB>24</SUB>O<SUB>80</SUB>·16H<SUB>2</SUB>O (Cs-NAT-II), which has a ca. 13.9% larger unit-cell volume. Further compression and heating to 1.5 GPa and 145 °C results in the transformation of Cs-NAT-II to Cs<SUB>16</SUB>Al<SUB>16</SUB>Si<SUB>32</SUB>O<SUB>96</SUB> (anh-Cs-POL), a H<SUB>2</SUB>O-free pollucite-like triclinic phase with a 15.6% smaller unit-cell volume per 80 framework oxygen atoms (80O<I><SUB>f</SUB></I>). At pressures and temperatures of 3.7 GPa and 180 °C, a new phase Cs<SUB>1.547</SUB>Al<SUB>1.548</SUB>Si<SUB>6.452</SUB>O<SUB>16</SUB> (Cs-HEX) with a hexacelsian framework forms, which has a ca. 1.8% smaller unit-cell volume per 80O<I><SUB>f</SUB></I><I>.</I> This phase can be recovered after pressure release. The structure of the recovered Cs-HEX has been refined in space group <I>P</I>6<SUB>3</SUB>/<I>mcm</I> with <I>a</I> = 5.3731(2) Å and <I>c</I> = 16.6834(8) Å, and also been confirmed by HAADF-STEM real space imaging. Similar to the hexacelsian feldspar (i.e., BaAl<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>8</SUB>), Cs-HEX contains Cs<SUP>+</SUP> cations that act as bridges between the upper and lower layers composed of tetrahedra and are hexa-coordinated to the upper and lower 6-membered ring windows. These pressure- and temperature-induced reactions from a zeolite to a feldspar-like material are important constraints for the design of materials for Cs<SUP>+</SUP> immobilization in nuclear waste disposal.</P>
Im, Junhyuck,Lee, Yongmoon,Blom, Douglas A.,Vogt, Thomas,Lee, Yongjae Royal Society of Chemistry 2016 Dalton Transactions Vol.45 No.4
<P>We report on high-pressure and high-temperature chemical transformations of Pb2+-exchanged natrolite (Pb-NAT, Pb8Al16Si24O80.16H(2)O) using a combination of in situ synchrotron X-ray powder diffraction and ex situ HAADF-STEM real space imaging. Three high-pressure polymorphs of natrolites (Pb-NAT-I, II, III) are observed via step-wise pressure-induced hydrations (PIH) up to 4.5 GPa, during which the number of H2O molecules located inside the natrolite channel increases from 16 to 40 H2O per unit-cell. At 4.5 GPa after heating the high-pressure Pb-NAT-III phase at 200 degrees C a reconstructive phase transits into a lawsonite phase (Pb-LAW, Pb4Al8Si8O28(OH)8.4H(2)O) with an orthorhombic space group Pbnm and a = 5.8216(9), b = 9.114(1) and c = 13.320(1) angstrom is obseed. The structure of the recovered Pb-LAW phase was characterized using Rietveld refinement of the in situ synchrotron X-ray powder diffraction data and HAADF-STEM real space imaging. In the recovered Pb-LAW phase the Pb2+ content is close to 42 wt% and as bond valence approximations reveal the Pb2+ cations are more tightly coordinated to the framework oxygen atoms than originally in the natrolite phase.</P>
Pressure-induced metathesis reaction to sequester Cs.
Im, Junhyuck,Seoung, Donghoon,Lee, Seung Yeop,Blom, Douglas A,Vogt, Thomas,Kao, Chi-Chang,Lee, Yongjae American Chemical Society 2015 Environmental science & technology Vol.49 No.1
<P>We report here a pressure-driven metathesis reaction where Ag-exchanged natrolite (Ag16Al16Si24O80·16H2O, Ag-NAT) is pressurized in an aqueous CsI solution, resulting in the exchange of Ag(+) by Cs(+) in the natrolite framework forming Cs16Al16Si24O80·16H2O (Cs-NAT-I) and, above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the initial cation exchange, the precipitation of AgI occurs. Additional pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to a pollucite-related, highly dense, and water-free triclinic phase with nominal composition CsAlSi2O6. At ambient temperature after pressure release, the Cs remains sequestered in a now monoclinic pollucite phase at close to 40 wt % and a favorably low Cs leaching rate under back-exchange conditions. This process thus efficiently combines the pressure-driven separation of Cs and I at ambient temperature with the subsequent sequestration of Cs under moderate pressures and temperatures in its preferred waste form suitable for long-term storage at ambient conditions. The zeolite pollucite CsAlSi2O6·H2O has been identified as a potential host material for nuclear waste remediation of anthropogenic (137)Cs due to its chemical and thermal stability, low leaching rate, and the large amount of Cs it can contain. The new water-free pollucite phase we characterize during our process will not display radiolysis of water during longterm storage while maintaining the Cs content and low leaching rate.</P>