Cesium extraction from Cs 0.8 Ba 0.4 Ti 8 O 16 hollandite nuclear waste form ceramics in nitric acid solutions
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Keith Murray and Boujemaa Moubaraki School of Chemistry, Monash University, Clayton, Victoria 3800, Australia (Received 21 October 2004; accepted 24 February 2005)
The leaching of Cs from well-characterized Cs-bearing hollandite powders with ideal formula Cs0.8Ba0.4Ti8O16 was studied at 200 °C under static oxidizing conditions achieved using dilute nitric acid solutions. A variety of techniques were used to elucidate the leaching mechanism including x-ray powder diffraction, magnetic susceptibility, x-ray absorption near edge structure, and electron microscopy. Under the conditions of the study, Cs is leached from Cs-hollandite according to a deceleratory rate law with contracting geometries. The principal reaction products are rutile and brookite. The leaching mechanism involves the formation of a quasi-continuous, and hence relatively impermeable, secondary titanium oxide phase (rutile and/or brookite) on the surfaces of the hollandite crystallites. The driving force for the leaching reaction appears to be the oxidation of Ti3+ in the structure by the oxidizing acid, which then promotes extraction of Cs and Ba from the hollandite tunnels followed by tunnel destabilization and transformation to rutile and brookite.
I. INTRODUCTION
The waste forms known as synroc are multiphase titanate ceramics designed for the immobilization of highlevel nuclear waste. They are composed of the three principal mineral phases, perovskite, hollandite, and zirconolite, in which a variety of lanthanides and actinides, in addition to cesium and strontium, are immobilized by substitution.1,2 While the lanthanides, actinides, and strontium form solid solutions in the zirconolite and perovskite phases, the cesium is partitioned almost exclusively within the hollandite phase. The effective immobilization of radioisotopes within synroc is due to the thermodynamic stability of the constituent solid solutions and the slow reaction kinetics of titanates in water. One of the main performance indicators of any radioactive waste form is its resistance to aqueous dissolution and its ability to prevent the release of the radionuclides into the environment. To reliably predict elemental release rates for different waste forms in diverse environmental settings, a detailed understanding of the chemistry of the leaching process is required.3 Cesium-137 has a half life of 30.2 years decaying to stable Ba-137 via meta stable Ba-137m, which
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0204 1436
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 6, Jun 2005 Downloaded: 12 Apr 2015
produces a high energy gamma photon. Cs-135 is a beta emitter and has an extremely long half life of 3.2 million years. The Cs+ cation is extremely soluble in water and therefore cesium can be easily transported through the biosphere. For these reasons, Cs is arguably one of the more problematic fission product radionuclides. Since in synroc the Cs is present within the hollandite phase, it is important to have a de
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