Effects of pH and Uranium Valence State on the Aqueous Dissolution of Brannerite
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(IIHFWVRIS+DQG8UDQLXP9DOHQFH6WDWHRQWKH$TXHRXV'LVVROXWLRQRI%UDQQHULWH Y. Zhang, E. Loi, M. Blackford, T. McLeod, H. Li and B. Begg Materials and Engineering Science, ANSTO, PMB 1, Menai, NSW 2234, Australia $%675$&7 The dissolution of the thorium analogue of brannerite (ThTi2O6-,) and U(IV)/U(V) doped Th-brannerite (Th0.97U0.03Ti2O6-,, and Th0.955U0.03Ca0.015Ti2O6-,,,) in aqueous media under atmospheric conditions has been studied to elucidate the effects of pH and uranium valence state on the dissolution rate. The dissolution of , is nearly stoichiometric but slightly preferential release of U occurs for ,, and preferential release of Ca and U occur for ,,,. The V-shape pH dependence previously observed for U-brannerite only occurs for U(not other matrix elements) for,,, indicating that the pH dependence is related to the U oxidation state upon dissolution. The normalised U dissolution rates of ,,, are nearly an order of magnitude higher than those of ,,for pH values over 3, suggesting brannerite is less durable with U(V) doping. TEM examination of specimens after leaching revealed few surface alteration products, which is consistent with the nearly stoichiometric dissolution of thorium brannerite. ,1752'8&7,21 Brannerite (UTi2O6), exists naturally in many uranium ore bodies, and is also a minor phase in ceramic formulations designed to immobilise surplus plutonium [1]. Previously we have reported some experimental studies on the aqueous dissolution of synthetic and natural endmember U brannerite [2-4]. Our earlier work [5] has shown the ability to stabilise U(V) in brannerite via the addition of Ca for charge compensation. It is of interest to examine the effect of different U valence states on dissolution kinetics, including the effect of pH, and compare with the thorium analogue of brannerite, thorutite, and to elucidate further the nature of the surface alteration products at atmospheric redox conditions. (;3(5,0(17$/'(7$,/6 6DPSOHSUHSDUDWLRQ The Th analogue (thorutite) of brannerite (ThTi2O6-,) and U(IV)/U(V) doped Th-brannerite (Th0.97U0.03Ti2O6-,, and Th0.955U0.03Ca0.015Ti2O6-,,,) were made by the alkoxide/nitrate route [5]. The detailed sample preparation has been reported elsewhere [6]. The samples contain mainly brannerite with minor rutile inclusions (∼5% TiO2) and trace amounts of ThO2 (< 0.1%). Powdered samples (75-150 µm) were washed with acetone to remove fines from the surfaces. The surface areas were measured by the BET method as 0.104, 0.136 and 0.035 m2 g-1 for ,, ,, and ,,,, respectively.
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The ion-beam-thinned (IBT) specimens were prepared in the conventional manner, whereby a thin wafer, ~0.5 mm thick, was sectioned from the bulk specimen using a sectioning saw. A disc (~3.0 mm in diameter) was TEM then cored from the wafer using an ultrasonic coring drill. The disc was further processed by mechanical thinning techniques down to a thickness of ~80µm and both sides polished to ~0.25 µm. The disc was then dimpled on one side. This process introduced a polished concave su
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