Umbite Type Zirconium Germanates for Cs Removal
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Umbite Type Zirconium Germanates for Cs Removal Ryan George and Joseph A. Hriljac1 1 School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K. ABSTRACT Pure and Nb-doped zirconium germanate materials of composition K2-xZr1-xNbxGe3O9.H2O where x = 0, 0.1, 0.2 and 0.3 with the structure of the natural mineral umbite have been prepared in high yield using hydrothermal synthesis methods. The parent material displays virtually no ion exchange of the K+ for Cs+ but the doped materials show rapidly enhanced exchange with replacement of ca. 70% of the K+ by Cs+ for the 30% doped material. Rietveld analysis of the powder X-ray diffraction data is consistent with no change in the unit cell parameters or K+ bonding prior to the exchange, hence we propose the improved property is due to the creation of cation defect sites within the pores of the material that facilities greater cation mobility and leads to exchange. INTRODUCTION Although aluminosilicate zeolites such as chabazite and clinoptilolite are still widely used for the removal of radioactive Sr2+ and Cs+ from contaminated water, there is a growing use of other systems such as CST, Cs-treat and Sr-treat. For example, as part of the Fukushima cleanup efforts, IONSIV which is based on CST [1,2] has been used in the SARRY plant [3] and Cs-treat and Sr-treat are the active ion exchangers in the ALPS plant [4]. CST is a microporous titanosilicate with the sitinakite structure [2] and the optimized formula as used in IONSIV is Na0.4(H3O)(Ti1.4Nb0.6)O3SiO4~2H2O [5]. The two key chemical changes from the idealized parent formula of Na2Ti2O3SiO4∙2H2O are replacement of 30% of the Ti4+ by Nb5+ followed by exchange of most of the Na+ by H+ to produce the acid form. Details of the exchange mechanism have been careful and thoroughly elucidated to reveal why there is such a strong affinity for Cs+ over H+ [6]. Another family of materials that have been shown to be good for the same ion exchange reactions are based on the umbite structure, figure 1, and have a general composition of MI2MIVSi3O9∙H2O where MI is a large monovalent cation such as K+ or Na+ and MIV is a tetravalent cation that prefers octahedral coordination such as Ti4+ [7], Zr4+ [8] or Sn4+ [9]. A germanium form of the zirconium compound, K2ZrGe3O9∙H2O, is also known [10]. To our knowledge, the ion exchange behavior of the zirconogermanate form has never been reported nor doped variants made. In this work we demonstrate that doping Nb5+ for Zr4+ up to levels of ca. 30% is possible in K2ZrGe3O9∙H2O with retention of the umbite structure. Furthermore, this doping leads to a dramatic improvement of the uptake of Cs+.
729 Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 26 Aug 2017 at 18:04:43, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.173
Figure 1. Umbite structure viewed down the c-axis showing how the framework is composed of corner-shared tetrahedral (hatched) and octahedral
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