Studies on metal carbonate equilibria. 23. Complex formation in the Th(IV)-H2O-CO2(g) system

Abstract

The stoichiometry and the stability of the complexes formed in the Th(IV)-H2O-CO2(g) system have been investigated by the EMF technique in a 3.00 M NaClO4 ionic medium at 25 °C, using coulometry to vary the hydrogen ion concentration. The following concentration ranges have been investigated: 0.2×10-3 < [Th]tot < 6×l0-3; 2.2 ≤-log [H+] ≤ 4.5; 0 ≤ PCO2 ≤ 0.97. The chemical model selected for the hydroxide system consists of two mononuclear complexes, ThOH3+ (log ∗β1 = -4.35 ±0.09) and Th(OH)4(aq) (log ∗β4 = -16.65 ± 0.04), two binuclear complexes, Th2(OH)26+ (log ∗β2.2 = -5.10 ±0.17) and Th2(OH)35+ (log ∗β2.3 = -7.87±0.05), two tetranuclear complexes, Th4(OH)88+ (log ∗β4,8 = -19.6±0.2) and Th4(OH)124+ (log∗β4,12 = -34.86 ± 0.05), and two hexanuclear complexes, Th6(OH)1410+ (log ∗β6,14 = -33.67 ± 0.05) and Th6(OH)168+ (log ∗β6,16 = -42.90 ± 0.4); the last is present in very small amounts. The (4,12) complex is assumed to have the same basic structure as the (4,8) complex, but with four terminal OH groups, one on each Th(IV). Addition of CO2(g) to the Th(IV)-H2O system in the -log [H+] range explored results in very steep formation curves which depend on PCO2 [Th]tot and -log [H+], It is not possible to identify a unique chemical model for the carbonate system, only to conclude that ternary complexes of high nuclearity are formed. The experimental data at the lowest Th(IV) concentrations can approximately be described with species of the type Th16(OH)52(CO2)1612+ and Th8(OH)24(CO2)28+. At somewhat higher [Th]tot a precipitate is formed, however, not in the test solutions used to deduce the chemical model(s). An exploratory spectrophotometric study of the U(IV)-H2O-CO2(g) system at -log [H+] ≈ 2 showed a pronounced increase in the intensity of the U(IV) absorption band centred around 650 m, also indicating the binding of CO2. It seems likely that the speciation of thorium and other tetravalent actinides, and hence their solubility, will also be influenced by the presence of CO2/HCO3- in the acid ranges.