The effect of degradation products of strong acidic cation exchange resinson radionuclide speciation: A case study with Ni2+

Radiolytic degradation experiments with acidic ion-exchange resins revealed oxalate and an unidentified ligand X to be the most strongly complexing li gands of the degradation products. The influence of these ligands an the Ni speciation in groundwater and cement pore water of a repository is assesse d A complete and reliable thermodynamic database is built for this case study . Missing stability constants are estimated by chemical reasoning. Subseque nt sensitivity analyses show whether these species are important or not. Th e backdoor approach used in this study addresses the following question: Wh at concentrations must the ligand have to significantly influence the Ni sp eciation? In the case of oxalate, the concentration necessary to complex 90% Ni will never be exceeded within the repository or in its environment due to precip itation of Ca-oxalate solids. Thus, a negative effect of oxalate on Ni spec iation and sorption need not be considered in safety assessments. In the case of ligand X, calculations demonstrate that Ni speciation is hig hly dependent on geochemical conditions and is occasionally ambiguous due t o uncertainties in estimated stability constants. Hints are given to deal w ith these ambiguities in future safety assessments, and further experimenta l investigations are proposed to decrease uncertainties when necessary.