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

Abstract 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 Ctot=[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)45- and Th(OH)2(CO3)22- 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.