Thermodynamic Model for ThO2(am) Solubility in Isosaccharinate Solutions

摘要

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)ISA(2+), 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: ThO2(am)+ 3H(+) + ISA(-) reversible arrow Th(OH)ISA(2+) + H2O ThO2(am)+ H+ + 2ISA(-) + H2O reversible arrow Th(OH)(3)(ISA)(2)(-) and ThO2(am)+ 2ISA(-) + 2H(2)O reversible arrow Th(OH)(4)(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.