The effective immobilization of Cs + and/or Sr 2+ sorbed on hexagonal tungsten oxide bronze (HTB) adsorbent materials can be achieved by heating in air at temperatures in the range 500 -1000 °C. Crystalline powdered HTB materials formed by heating at 800 °C show leach characteristics comparable to Cs-containing hot-pressed hollandites in the pH range from 0 to 12. If the Cs-loaded HTB sorbents are pressed into pellets prior to calcination, ceramic monoliths can be prepared. Heating to temperatures in excess of 1250 °C results in the melting of the sorbent to form millimetre-sized crystals of bronzoid phases. Thermal analysis shows that melting of the cation-exchanged tungstate sorbents is initiated at temperatures as low as 850 °C and concludes by 1300 °C. The absence of any significant mass loss immediately after melting, as well as chemical analyses before and after melting, confirm that Cs is not volatilized to any significant extent at the temperatures required to generate durable, coarsely crystalline products. The melted bronzoid product is a multiphase ceramic in which Cs + remains bound within, and appears to stabilize, the hexagonal bronze phase, even after complete melting at 1300 °C, while elements such as Sr 2+ are present within other tungstate phases. The bronzoid chemical system appears capable of accommodating a wide range of other elements. Here we have demonstrated that modification of the sorbent properties by incorporation Mo does not impact severely on the durability of materials prepared below 1000 °C, even when exposed to strong acid (pH=1) and elevated temperature (150 °C). As an example, one-day MCC1 leach rates lower than 1x10 -5 g/m 2 /day were measured using demineralized water at 90 °C for Cs-saturated Mo-doped sorbents that had been heated in air at 900 °C, while the fraction of Cs leached from powdered samples in 0.1 M HNO 3 solutions at 150 °C for 4 days is only 4x10 -3 g/m 2 /day.

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The hydrothermally prepared hexagonal tungsten bronze (HTB) phase displays promising distribution coefficients (. ' ) for both Cs + (2 -100 ppm) and Sr 2+ (0.5 -60 ppm) in acidic (1M HNO 3 ) 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 Sr 2+ can be modulated by isomorphous substitution of molybdenum into the tungstate framework, and is optimum for material of nominal composition, Na 0.2 Mo 0.03 W 0.97 O 3 ·]H 2 O (Mo-HTB). Both the parent HTB and Mo-HTB phases display fast ion-exchange kinetics for Cs + and Sr 2+ 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 Sr 2+ uptake dropping by 66% as the concentration of Na + increases from 9 mmol.L -1 to 1200 mmol L -1 .,1752'8&7,21 )LJXUH Hexagonal Tungsten Bronze (+7%) (left) and pyrochlore (3<52) structures (right) viewed down the Fcrystallographic axis.

The leaching of two potential 137Cs waste form ceramics (Cs‐containing hexagonal tungsten bronze (HTB) and hollandite) has been investigated in Fe(NO3)3 solutions of increasing concentration at 150°C over a period of 4 days. These ceramics contain within their structures reduced Mo5+/W5+ and Ti3+ species for the HTB and hollandite, respectively, which therefore might render them susceptible to oxidation‐induced leaching. Elucidation of the extent and the mechanism of leaching of the Cs from these ceramics in the REDOX active iron nitrate medium has been investigated. Cesium (Cs) leached severely from both the Cs‐loaded HTB and hollandite materials in iron nitrate solutions with virtually all of the immobilized Cs being extracted from both waste form materials in a period of 4 days at 150°C. In the case of hollandite, conversion to ilmenite and hematite was observed at low concentrations and was virtually complete in 0.5 mol/L Fe(NO3)3 over 4 days. In the case of the HTB, all of the Cs was extracted presumably by an ion‐exchange mechanism because the structure of this oxide remained intact and iron was found in the composition. Iron oxide with a hematite structure was also easily observed in the reacted sample at high solution iron concentrations. It is shown that the leach resistance of the Cs‐containing HTB can be improved by substitution of up to 20% Ti for W.