Studies on some divalent metal α-isosaccharinic acid complexes

The presence of organic complexants, such as α-isosaccharinic acid (ISA) in an intermediate-level radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides by forming organic complexes in solution. To assess this, stability constants are required for the complexes formed. Constants have been determined for the divalent metal ions, Cd2+, Co2+ and UO2 2+ with α-isosaccharinic acid (ISA) in near-neutral and alkaline conditions. The effect of these complexes on radionuclide solubility in the near-field and alkaline-disturbed zone is shown. The constants were measured by the ion-exchange (Schubert) method [1] with the exception of U(VI) at pH 13.3, for which the solubility product method [2] was used. At pH 7 each complex was of the form M1ISA1, with log β values suggestive of salt formation. At high pH, log β values were between 13 and 20. The number of hydroxide ions involved in the complexation reactions was determined using Bjerrum plots, conductometric titrations and spectrophotometric methods [1]. The constants have enabled speciation calculations to be performed showing the effect of ISA on the metal’s solubility. Solubility is predicted to increase in the presence of ISA from pH 9 to 13.5, suggesting that it may have an impact on radionuclide behaviour. The largest solubility increases are for Cd and Co, the smallest for U(VI).