Analysis of the effect of citrate on radionuclide retention on portlandite

We analysed how citrate (CIT), a chelating agent potentially present in radioactive waste disposals, affects the mobility of four radionuclides (RN): ⁶³Ni, ²³³U, ¹⁵²Eu, ²³⁸Pu in portlandite, an important hydrated phase of cement, a commonly used material for waste isolation. Portlandite was synthetized in the laboratory and showed high purity and grain size of few μm. This solid, buffers the pH to 12.5 and shows high adsorption capability for the studied RNs: ¹⁵²Eu and ²³⁸Pu exhibited the highest adsorption (K_d ∼1·10⁵ mL g^-1) and ²³³U the lowest (K_d ∼8·10² mL g^-1). CIT adsorption was also experimentally evaluated by batch sorption experiments and electrophoretic (ζ-potential) measurements: a non-lineal sorption behaviour was observed, with K_d values decreasing (from ∼1·10³ mL g^-1) as CIT concentration increased up to 1·10^-2 M, according to portlandite sorption sites saturation. In the presence of CIT, a marginal decrease for ²³³U adsorption in portlandite was observed, one order of magnitude reduction for ⁶³Ni, while ²³⁸Pu and ¹⁵²Eu adsorption decreased significantly. The calculated sorption reduction factors (SRF) for the four RN in the presence of CIT at a concentration of 5·10^-3 M were: 2.4, 9.7, 37 and 50.9 for ²³³U, ⁶³Ni, ²³⁸Pu, and ¹⁵²Eu, respectively. According to the available thermodynamic databases, low complexation between CIT and RN is predicted at pH = 12.5, thus the RN adsorption decrease in the presence of CIT must be attributed to the organic adsorption on portlandite. However, current thermodynamic are still incomplete for this ligand and this pH range and this limits a precise interpretation of the experimental data.