Undetected perovskite phase interference with zirconolite dissolution measurements

Zirconolite ceramics present a chemically durable host matrix for waste actinides, but zirconolite dissolution rates reported in the literature often vary significantly. Here, the release of Ca and Al from a hot isostatically pressed zirconolite (Ca0.8Zr0.9Ce0.3Ti1.6Al0.4O7.0) was shown to be predominantly driven by preferential dissolution of minor perovskite and alumina phases. Both phases were undetectable by XRD, and the perovskite was difficult to detect by SEM-EDS. Whilst the zirconolite phase exhibited no signs of alteration, dissolution of the perovskite proceeded congruently without forming a hydrated altered layer or diffusion of protons into the solid that would be indicative of an ion-exchange mechanism. The weak temperature dependence of dissolution (40, 90 and 150 °C) showed that kinetics were limited by transport and a mixed transport-surface controlled reaction for Ca and Al, respectively. A significant H2O-D2O isotope effect on dissolution was observed for Ca but not for Al at all temperatures. The former was consistent with an abated rate of hydrolysis in the absence of a contribution from diffusion, whilst the latter could be attributed to differences in the activated complex for Ca and Al release through hydrolysis. These results demonstrate the role of a secondary phase perovskite in the dissolution kinetics of zirconolite even when perovskite occurs at low concentration and evades detection by bulk techniques such as XRD. This study provides a potential explanation of variations in zirconolite ceramic dissolution rates present in the literature and provides a null result to tests of an incongruent Ca release mechanism from zirconolite.