Thermodynamic interpretation of uranium(IV/VI) solubility in the presence of α-isosaccharinic acid

Highlights: • Uranium(IV/VI) solubility studies in the presence of α-isosaccharinic acid. • New ternary U(IV/VI)-OH-ISA species suggested from the solubility data. • Comprehensive thermodynamic model to interpret the experimental results. • An effect of ISA on uranium(IV/VI) redox behavior derived. -- Abstract: The effect of α-isosaccharinic acid (ISA) on the solubility and redox of tetravalent and hexavalent uranium (U(IV), U(VI)) was investigated in the hydrogen ion concentration (pH_c = −log [H⁺]) range of 6–13 and at total ISA concentration ([ISA]_tot) = 10⁻⁴–10^−1.2 mol/dm³ (M). The dependence of U(IV) solubility on pH_c and [ISA]_tot suggested the existence of U(OH)₄(ISA)₂²⁻ as a dominant species within the investigated pH_c range of 6–12. For the U(VI)-ISA system, UO₂(OH)₃(ISA)₂³⁻ was suggested as a dominant species at pH_c 7–13. The formation constants of the U(IV)-ISA and U(VI)-ISA complexes were determined by least squares fitting of the solubility data. The solubility of U(IV) and U(VI) in the presence of ISA and its effect on the redox behavior were thermodynamically interpreted based on the obtained constants.