Thermodynamic Model for ThO₂(am) Solubility in Isosaccharinate Solutions

Abstract

Extensive studies on ThO₂(am) solubility were carried out as functions of a wide range of isosaccharinate concentrations (0.0002 to 0.2 mol⋅kg⁻¹) at fixed pH values of about 6 and 12, and varying pH (ranging from 4.5 to 12) at fixed aqueous isosaccharinate concentrations of 0.008 mol⋅kg⁻¹ or 0.08 mol⋅kg⁻¹, to determine the aqueous complexes of isosaccharinate with Th(IV). The samples were equilibrated over periods ranging up to 69 days, and the data showed that, in most cases, steady-state concentrations were reached in <15 days. The data were interpreted using the SIT model, and required the inclusion of mixed hydroxy-ISA complexes of Th(IV) [Th(OH)ISA²⁺, Th(OH)₃(ISA)$_{2}^{-}$, and Th(OH)₄(ISA)$_{2}^{2-}]$ with log ₁₀ K ⁰=12.5±0.5,4.4±0.5 and −3.2±0.5 for the reactions:

$$\begin{array}{l}\mathrm{ThO}_{2}(\mathrm{am})+3\mathrm{H}^{+}+\mathrm{ISA}^{-}\rightleftarrows \mathrm{Th}(\mathrm{OH})\mathrm{ISA}^{2+}+\mathrm{H}_{2}\mathrm{O}\\[3pt]\mathrm{ThO}_{2}(\mathrm{am})+\mathrm{H}^{+}+2\mathrm{ISA}^{-}+\mathrm{H}_{2}\mathrm{O}\rightleftarrows \mathrm{Th}(\mathrm{OH})_{3}(\mathrm{ISA})_{2}^{-}\end{array}$$

and

$$\mathrm{ThO}_{2}(\mathrm{am})+2\mathrm{ISA}^{-}+2\mathrm{H}_{2}\mathrm{O}\rightleftarrows \mathrm{Th}(\mathrm{OH})_{4}(\mathrm{ISA})_{2}^{2-}$$

respectively.

Predictions based on these Th-ISA complexes agreed closely with the extensive experimental data developed in this study as well as the very limited (only at pH=8.3) data from liquid-liquid extractions recently reported in the literature.