Thermodynamic Investigation of the Hydrolysis Behavior of Fluorozirconate Complexes at 423.15-773.15 K and 100 MPa

Metal complexes are thermally unstable in hydrothermal solutions, which may cause metal cation inertness or precipitation. We investigated, for the first time, the hydrolysis behavior of ammonium fluorozirconate ((NH4)(2)ZrF6) at 423.15-773.15 K and 100 MPa. Kinetic equilibration experiments of the fluorozirconate solution demonstrated that the hydrolysis of the complex reached equilibrium within 4 h. Variable-temperature experiments showed that the complex's hydrolysis was dependent on the temperature and initial concentration in the hydrothermal solutions, with enhanced hydrolysis at elevated temperature and decreased initial concentrations. Based on our experimental data, we established a linear relationship between the cumulative hydrolysis constants (K) of (NH4)(2)ZrF6 and absolute temperature T: lnK = (36.06 +/- 3.46) - (29747 +/- 1967)/T, from which Delta H-r(m)Theta, Delta S-r(m)Theta and Delta(r)G(m)(Theta) values for the hydrolysis reaction were also calculated. This study suggests that fluorine may play an important role in Zr mobility and provide a route to quantitatively characterize the thermodynamic features of metal complexes in hydrothermal solutions.