Dissolution of ZrO2 based pyrochlores in the acid pH range: A macroscopic and electron microscopy study

• Surrogate study of potential nuclear waste form dissolution.
• Dissolution kinetics of ZrO2 based pyrochlore and defect fluorite ceramics.
• Effect of pH, temperature and Nd/Zr ratio on dissolution rate for both structures.
• Heterogeneous surface reactivity identified by electron microscopy.
• Preferential release of Nd observed for all experimental conditions.

The dissolution kinetics of ZrO2–Nd2O3 polycrystalline pyrochlore and defect fluorite ceramic powders under acidic conditions were observed following a combined macroscopic and electron microscopic (SEM) approach. Dynamic dissolution experiments were carried out with a variation of temperature and pH as well as the chemical composition within the ZrO2–Nd2O3 system. SEM observations indicate a preferential leaching at the grain boundaries for all experiments. A preferential release of Nd during the initial stages of dissolution, which is several orders of magnitude higher than the Zr release, was measured by ICP-MS. At steady state, the normalised Nd-rate approaches the Zr based dissolution rate within one order of magnitude, becoming congruent for most experiments. Zr-based BET surface area normalised steady state rates at c(H+) = 0.1 N and 90 °C are in the range between 4 × 10−7 and 3 × 10−6 g m−2 d−1, indicating no significant influence of the transition pyrochlore to defect fluorite and the chemical composition on the macroscopic dissolution rate. Based on the different numbers of grain boundaries per surface area in the pyrochlore compared to the defect fluorite, a slightly higher “true” dissolution rate could be assumed for the defect fluorite. The influence of pH and temperature variations to the dissolution rates of defect fluorite and pyrochlore are similar and in the range observed for other multioxide materials.