Aqueous alteration of nearly pure Nd-doped zirconolite (Ca0.8Nd0.2ZrTi1.8Al0.2O7) a passivating layer control

摘要

Neodynium-doped zirconolite was fabricated by hydrolysing a mixture of alkoxide with nitrate solutions followed by a series of drying, calcination, pressing and sintering steps. The material obtained was essentially zirconolite (99.5 vol% Ca0.8Nd0.2ZrTi1.8Al0.2O7) with a minor ZrTiO4 phase (0.5 vol%). The microstructure and composition of the zirconolite was characterised, before and after durability experiments, by SEM/EDS, SIMS, AFM and TEM. Chemical durability testing was carried out using the standard MCC-2 test at 150 degrees C for a series of four test durations - 1, 7, 28 and 84 days. The pH of the leachates was fairly constant with time (pH 5), and elemental releases reached apparent steady-state conditions within the first day of leaching. Thermodynamic calculations indicated that attainment of these steady-state conditions at this point could not be explained by thermodynamic equilibrium being reached between the leachates and the primary zirconolite phase. However, thermodynamic equilibrium between a possible layer of secondary hydroxides or a decalcified zirconolite and the leachates could explain the steady state of Ti, Zr and Al releases. Moreover, it is suggested that the steady-state conditions achieved by Nd, theoretically undersaturated in the leachates, is due to adsorption of Nd to this secondary layer, and is not a result of precipitation of a hydroxide form. Anatase, commonly observed during leaching of zirconotitanates, is also theoretically undersaturated in the leachates. When observed, it is possibly a product of local condensation of the hydroxide layer and not due to simply a dissolution/precipitation process. SEM and AFM investigations confirmed the formation of a passivation layer on the surface of the zirconolite. The layer was of the order of 10 nm, in accordance with the thickness calculated from leachate results. (c) 2004 Elsevier Ltd. All rights reserved.