Solubility of monoclinic and yttrium stabilized cubic ZrO2: Solution and surface thermodynamics guiding ultra-trace analytics in aqueous phase

The high stability of zirconium dioxide in aqueous environments is known and demonstrated, and this property is strongly used in nuclear industry to ensure the long term storage of wastes. However, only upper limits of its aqueous solubility are known reliably and lower limits linked to very well crystallized ZrO₂ are much less assessed. Indeed, the low dissolution rate of zirconia makes the solubility measurements a challenging task. To overcome, high S/V ratios of nanoparticles zirconia were used. This work also improved the sensitivity of analytical techniques (HR ICP-MS) and methodologies, and a reliable experimental procedure was developed to measure zirconium (quantification limit ≈10⁻¹¹ mol∙L⁻¹). New Zr(IV) dioxide solubility data at pH between 0 and 2 were obtained approaching solubility from under-saturated conditions in (Na,H)Cl and (Na,H)ClO₄ medium. Two crystalline nanoparticle structures were compared: monoclinic and yttrium stabilized cubic zirconia. Very low solubility was measured for monoclinic phase between pH 1.5 and 2: between (1.8±1.2) × 10⁻¹⁰ mol∙L⁻¹ at pH 2 and (2.3±1.0) × 10⁻¹⁰ mol∙L⁻¹ at pH 1.5. The cubic zirconia showed higher solubility. Integrating the effect of ionic strength, particle size and aqueous speciation, solubility constants of log $K_s^0$ = (-8.43±0.69) for the monoclinic nanoparticles and log $K_s^0$ = (-7.12±0.35) for the yttrium stabilized cubic nanoparticles were obtained. High-resolution techniques (HR-TEM, SAXS and STEM-HAADF) were also used to assess the evolution of morphology and surface before, during and at equilibrium. Analysis of these results shows that the morphology and surface of nanoparticles in the raw state and after reaching equilibrium in (Na,H)Cl and (Na,H)ClO₄ medium are similar.