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 degrees 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-] + [CO32-] = 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(degrees) = -47.8 +/- 0.3, are calculated to be log beta(114)degrees = 35.8 +/- 0.3, log beta(122)degrees = 37.0 +/- 0.4, log beta(121)degrees = 30.7 +/- 0.4, log beta(131)degrees = 38.5 +/- 0.6 and log beta(141)degrees = 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.