Solubility of ThO₂·xH₂O(am) in carbonate solution and the formation of ternary Th(IV) hydroxide-carbonate complexes

The solubility of X-ray amorphous Th(IV) hydroxide or hydrous oxide was determined in carbonate solution at I=0.5 M (NaHCO₃-Na₂CO₃-NaOH-NaCl) and 22 °C. Two series of open system experiments were performed under CO₂ partial pressures of 1.0 and 0.1 bar at -log [H⁺]=4.5– 7.5. In three series of closed system experiments at constant total carbonate concentrations of C_tot=[HCO₃^-]+[CO₃^2-]=0.1, 0.04 and 0.015 M, the H⁺ concentration was varied in the range -log [H⁺] = 8.5–13.5. Some additional solubility data were determined in 0.25–2 M Na₂CO₃ containing 0.1 M NaOH. There was no indication for the formation of a thorium carbonate solid.

The simultaneous evaluation of the different sets of experimental data at I=0.5 M shows that Th(OH)(CO₃)₄^5- and Th(OH)₂(CO₃)₂^2- are the most important ternary complexes. Further contributions to the Th(IV) solubility are coming from Th(OH)₂(CO₃)(aq), Th(OH)₃(CO₃)^- and Th(OH)₄(CO₃)^2-. Their formation constants, extrapolated to I=0 with the SIT and combined with the solubility product of log K°_sp=-47.8±0.3, are calculated to be logβ°₁₁₄=35.8±0.3, logβ°₁₂₂=37.0±0.4, logβ°₁₂₁=30.7±0.4, logβ°₁₃₁=38.5±0.6 and logβ°₁₄₁=40.6±0.5. Pure carbonate complexes and other ternary complexes have no significant contributions to the solubility in the present studies at I=0.5 M. Upper limits are derived for their formation constants. Using the SIT for ionic strength corrections, the evaluated set of equilibrium constants is also consistent with the solubility in Na₂CO₃-NaOH mixtures of higher ionic strength.