The Immobilization of Cesium and Strontium in Ceramic Materials Derived from Tungstate Sorbents

Abstract

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.

Tungsten bronze-based nuclear waste form ceramics. Part 2: Conversion of granular microporous tungstate–polyacrylonitrile (PAN) composite adsorbents to leach resistant ceramics

“…This preliminary study was followed by a detailed characterization of the ion exchange properties of the Mo-doped phase [2]. Following on from this we have demonstrated that such compounds can potentially be converted into leach resistant ceramics [3]. In part 1 of this series we provided a comprehensive evaluation of the leach resistance of the bronze materials generated by heat treatment of Cs-, Sr-and lanthanideloaded HTB ion exchange materials and a more complete description of the phase chemistry of the HTB waste form system.…”

“…The lower temperature exotherm probably represents both the removal of physisorbed water on the surfaces of the MoW-HTB particles and within the structural channels, and partial decomposition of the PAN matrix while the higher temperature exotherm is due to complete combustion of the organic component. The final stages of the PAN matrix removal masks an exotherm routinely observed at 500°C which is characteristic of the decomposition of the HTB framework and the crystallization of WO 3 [2,3]. The region between the end of this decomposition and 1200°C shows minimal weight loss from the sample but between 1200 and 1300°C there was mass loss of approximately 5% from the sample.…”

Tungsten bronze-based nuclear waste form ceramics. Part 1. Conversion of microporous tungstates to leach resistant ceramics

“…A detailed study of the ion-exchange properties of both the undoped (W-HTB) and Mo-doped materials (MoW-HTB) has been reported recently [6] and a preliminary investigation of the immobilization of the sorbed Cs has also been reported [21]. Because the most effective form of the tungstate adsorbent developed to date is that in which some Mo replaces W in the HTB framework [6] we first address the calcination of this phase in air.…”

“…Recently we reported that microcrystalline hydrated hexagonal tungsten bronze (HTB) materials could be used for the simultaneous adsorption of Cs + and Sr 2+ from acidic solutions [6,19,20] and also provided preliminary results indicating that calcination of the Cs-and Sr-saturated HTB materials in air to modest temperatures in the range 600-1000°C could yield highly crystalline materials with very low solubility in the pH range from 0 to 12 [21]. In addition it was demonstrated that higher temperatures result in melting which also produces highly durable materials with little or no Cs volatilization.…”

Tungstate-based glass–ceramics for the immobilization of radio cesium

“…It has also been demonstrated for these materials that it is viable to prepare a durable tailored ceramic directly from Cs-and Sr-saturated hexagonal tungsten bronze (HTB) selective adsorbent compounds with general formula A x WO 3 or A x-M y W 1Ày O 3 where A cations can be Cs or Sr while M is an element that can substitute for W in the hexagonal tungsten bronze framework [38][39][40][41]. Given the potential of GCCs however, it is also of interest to explore the possibility of incorporating the saturated tungstate adsorbents into glass-ceramic composites.…”