Solubility of crystalline and metamict zircon: A thermodynamic analysis

Thermodynamic analyses are used to predict the aqueous solubility of crystalline and metamict zircon at 25 °C and 150 °C for encapsulation purposes. The effect of the metamict state on solubility is treated as being equivalent to an increase in chemical free energy due to the development of amorphism. Both crystalline and metamict zircon exhibit pH-dependent behavior with minimum solubility at pH ∼5.4 and ∼4 for temperatures of 25 °C and 150 °C, respectively. Dissolution behavior may be incongruent or congruent, depending on pH, temperature and degree of metamictization (amorphism). A conservative approach is taken to the estimation of long-term dissolution rates at pH 5.4 and 4, based on a worst case scenario, where dissolution falls under mass transport control in the liquid phase. With increasing amorphism, unleached ZrO2 residue produced during incongruent leaching conditions must function as an effective dissolution barrier to maintain long-term low dissolution rates.