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

Abstract

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 pHc = 8−11. In our recent study at I=0.5 M, the simultaneous evaluation of solubility data at widely varied pHc 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.