Aqueous alteration of nearly pure Nd-doped zirconolite (Ca 0.8Nd 0.2ZrTi 1.8Al 0.2O 7), a passivating layer control

Abstract

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% Ca 0.8Nd 0.2ZrTi 1.8Al 0.2O 7) with a minor ZrTiO 4 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 °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.