Studies on metal gluconic acid complexes
online resourceposted on 04.02.2008, 17:21 by Peter Warwick, Nick Evans, Sarah Vines
The presence of organic complexants, such as gluconic acid, in an intermediate-level radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides, by forming organic complexes in solution. In order to assess this, stability constants are required for the complexes formed with radionuclides at high pH. This study reports the stability constants for the reactions of metals with gluconic acid (Gl). The metals studied were Cd, Ce, Co, Eu, Fe(II), Fe(III), Ho and U(VI) at pH 13.3; and Ce, Co and U(VI) at pH 7. The constants were measured by the Schubert (ion-exchange) or solubility product methods. Stoichiometries of the complexes were also determined. At pH 7 each complex was of the form M1Gl1, with log β values suggestive of salt formation. The M2+ log β values were between 13 and 20. For M3+, there was less consistency. The M2Gl1 complexes (Ho & Ce) had values of 49.8 and 43.9, whereas the M1Gl1 type (Fe(III) & Eu) range from 24 to 38. The constants have enabled speciation calculations to be performed showing the effect of gluconic acid on the metal’s solubility. Solubility is predicted to increase in the presence of gluconic acid from pH 9 to 13.5, suggesting that gluconic acid may have an impact on radionuclide behaviour. The largest increases in solubility are for Cd, Co, Eu, Ho and Ni, the smallest is with U(VI).