ON THE INFLUENCE OF CARBONATE ON MINERAL DISSOLUTION .3. THE SOLUBILITY OF MICROCRYSTALLINE THO2 IN CO2-H2O MEDIA

The solubility of microcrystalline ThO2 as a function of pH has been s tudied under different partial pressures of CO2 in aqueous media, in o rder to obtain qualitative and quantitative information on the complex ation of Th4+ with carbonate. Based on the results of the study, the e xistence of a mixed thorium-hydroxo-carbonate complex, Th(OH)(3)CO3-, as well as a thorium carbonate complex Th(CO3)(5)(6-) is proposed, and their stability constants and the solubility product of microcrystall ine thorium dioxide (ThO2(act)) are determined. The following equilibr ium constants were determined in 0.5 M NaClO4 at 25 degrees C 1 atm.: ThO2(act) + 4H(+) half-arrow-right-over-half-arrow-left Th4+ + 2H(2)O log K-S = 9.37 +/- 0.13 ThO2(act) + H+ + H2O + CO32- half-arrow-right- over-half-arrow-left Th(OH)(3)CO3- log beta(1,3,1) = 6.11 +/- 0.19 ThO 2(act) + 4H(+) + 5CO(3)(2-) half-arrow-right-over-half-arrow-left Th(C O3)(5)(6-) log beta(1,0,5) = 42.12 +/- 0.32. The errors are given as t hree standard deviations in the fitted constants. By extrapolation, th e following constants are obtained for the infinite dilution standard state: ThO2(act) + 4H(+) half-arrow-right-over-half-arrow-left Th4+ 2H(2)O log K-s = 7.31 +/- 0.3 ThO2(act) + H+ H2O CO32- half-arrow-righ t-over-half-arrow-left Th(OH)(3)CO3- log beta(1,3,1) = 6.78 +/- 0.3 Th O2(act) + 4H(+) + 5CO(3)(2-) half-arrow-right-over-half-arrow-left Th( CO3)(5)(6-) log beta(1,0,5) = 39.64 +/- 0.4.