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M. Altmaier, Volker Neck, Melissa A. Denecke, R. Yin, Thomas Fanghänel

Solubility of ThO₂·xH₂O(am) and the formation of ternary Th(IV) hydroxide-carbonate complexes in NaHCO₃-Na₂CO₃ solutions containing 0−4 M NaCl

Keywords: Thorium(IV), Solubility, Carbonate solution, Hydroxide-carbonate complexes, Thermodynamic modelling

The solubility of ThO₂· x H₂O(am) is studied at I=0.1−4.0 M (NaHCO₃-Na₂CO₃-NaCl) and 22 °C. Several sets of closed system experiments are performed at total carbonate concentrations of C_tot=[HCO₃^-]+[CO₃^2-]=0.02 and 0.1 M in the range pH_c = 8−11. In our recent study at I=0.5 M, the simultaneous evaluation of solubility data at widely varied pH_c and carbonate concentrations has shown that Th(OH)(CO₃)₄^5-, Th(OH)₂(CO₃)₂^2- and Th(OH)₄(CO₃)^2- are the most important ternary complexes. The present results at I=0.1−4.0 M are used to describe the ionic strength dependence of the equilibrium constants logK_{s, 1yz} (for the reactions Th(OH)₄(am)+zCO₃^2-↔Th(OH)_y(CO₃)_z^4-y-2z+(4-y)OH^-) with the SIT. EXAFS measurements in 0.1 and 1.0 M carbonate solutions support the calculated speciation. They show clearly different spectra for the predominant species Th(OH)(CO₃)₄^5- and Th(CO₃)₅^6-, respectively.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 94, 09/2006
Pages: 495 - 500

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