Cs + and Sr 2+ Ion-Exchange Properties of Microporous Tungstates

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&VDQG6U,RQ([FKDQJH3URSHUWLHVRI0LFURSRURXV7XQJVWDWHV Vittorio Luca, Christopher S. Griffith, Harriet Chronis, Jonathan Widjaja, Huijun Li, Nicholas Scales. Materials and Engineering Sciences, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, AUSTRALIA $%675$&7 The hydrothermally prepared hexagonal tungsten bronze (HTB) phase displays promising distribution coefficients (.') for both Cs+ (2 – 100 ppm) and Sr2+ (0.5 – 60 ppm) in acidic (1M HNO3) radioactive waste simulants. The development of an inorganic ion-exchanger that displays such selectivity has previously eluded researchers in this field. The selectivity for Cs+ and Sr2+ can be modulated by isomorphous substitution of molybdenum into the tungstate framework, and is optimum for material of nominal composition, Na0.2Mo0.03W0.97O3·]H2O (MoHTB). Both the parent HTB and Mo-HTB phases display fast ion-exchange kinetics for Cs+ and Sr2+ and cation exchange capacities ca. 50% that of the theoretical capacities of 0.9 and 0.45 mmol.g-1, respectively. The Mo-HTB adsorbent has a modest tolerance to alkali metal ions such as Na+ and K+ in acidic solutions with total Cs+ and Sr2+ uptake dropping by 66% as the concentration of Na+ increases from 9 mmol.L-1 to 1200 mmol L-1. ,1752'8&7,21 The nuclear industry is dogged by the issue of disposing of radioactive waste. One option for disposing of older waste solutions is to pretreat them with an inorganic ion-exchanger to remove the longer-lived and most active radionuclides, predominately, 137Cs and 90Sr. This concentrates the radioactivity onto a small volume of sorbent, thus allowing the bulk of the effluent to be disposed of as low level waste with concomitant cost savings [1]. The generally accepted enhanced radiation stability of inorganic materials over organic systems has seen a significant international research effort aimed at the development of Sr2+ and Cs+ inorganic ionexchangers but ion exchangers which are highly selective for Cs+ in acidic solutions (>0.7 mol/L acid) are generally not highly selective for Sr2+, and vice versa [2-7]. Our research on this problem has centred on developing tungsten oxide-based materials with pyrochlore and hexagonal tungsten bronze structures, which display selectivity for both Cs+ and Sr2+ in acidic solutions (Figure 1). These materials are prepared easily by pH adjustment of sodium tungstate solutions followed by hydrothermal treatment [8]. Both the pyrochlore and hexagonal tungsten bronze compounds contain an A cation situated within the F-axis tunnels, as shown in figure 1. The X-ray diffraction patterns of hydrothermally prepared microcrystalline

)LJXUH Hexagonal Tungsten Bronze (+7%) (left) and pyrochlore (31D@ PJ/

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 )LJXUH$ - The effect of varying [Na+] upon the uptake of Cs+ and Sr+ by Mo-HTB (- - and -•-, respectively) from simulants consisting of 160 ppm Cs+ and 96 ppm Sr+ in 1M HNO3. Combined uptake of Cs+ and Sr+ by Mo-HTB is represented by - -; % - The effect of varying [K+] upon

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