RESUMEN
Ettringite is an important mineral that contributes to the overall performance of cementitious materials. Knowledge of the surface charge behaviour of a solid is necessary for a mechanistic description of surface processes such as adsorption or particle-particle interactions. The objective of this study was to develop a model capable of reproducing ettringite surface charge as a function of calcium, sulphate, and pH. Ettringite was synthesised and characterised using different analytical, microscopic, and spectroscopic techniques with the help of density functional theory. Electrophoretic mobility was measured using laser Doppler electrophoresis in alkaline waters representative of the cementitious environment. The behaviour of the ettringite surface charge was shown to be quite complex as sulphate and calcium acted in a competitive manner on the overall charge. The ζ-potential increases when the calcium content increases, whereas it decreases when sulphate increases. This is due to the possible adsorption of these ions at the surface, and the extent of the effect depends on the relative concentrations of Ca and SO4 2-. An electrostatic double layer model (DLM) was used to calculate the surface potential, considering the adsorption of both calcium and sulphate, as possible ions determining the potential (IDP), and formation of different complexes with ettringite surface functional groups (SOH). The variations of the ζ-potential could be satisfactorily predicted under the different chemical conditions of interest in a cementitious environment.
RESUMEN
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 (Kd â¼1·105 mL g-1) and 233U the lowest (Kd â¼8·102 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 Kd values decreasing (from â¼1·103 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.