Analysis of radionuclide retention by the cement hydrate phase portlandite: A novel modelling approach

Abstract

Cement is a widely used material for conditioning and confining radioactive waste; thus, the analysis of its sorption properties as well as of those of its main hydration products is crucial to assess radionuclide migration under disposal conditions. In this study, the sorption of radioisotopes with different chemical properties (137Cs, 133Ba, 109Cd, 152Eu, 238Pu, 75Se and 36Cl) in portlandite suspended in 20 mM Ca(OH)2 (pH = 12.5 ± 0.2) was analyzed by means of batch sorption isotherms. In the range of the concentrations investigated, all the radionuclides showed a linear adsorption isotherm. Almost quantitative adsorption was observed for Eu, Pu and Cd, with a very high logarithm of the distribution coefficient, Kd [g⋅mL-1] (log(Kd) > 5). The adsorption of the cations Cs and Ba was significantly smaller, with a log(Kd) of approximately 2 and 0.3 respectively. The two anionic species analyzed were Cl- and SeO32-: whereas Cl did not show any retention, selenite showed a Log(Kd) of approximately 1.8. The precipitation of Se is observed for aqueous concentrations higher than 1⋅10-4 M, being the precipitation of CaSeO3(s) predicted by thermodynamic calculations. The electrokinetic surface potential of portlandite was analyzed as a function of pH (10–13.5) and with and without the presence of alkalis (0.2 M Na) to gather the basic information needed for understanding radionuclide/solid interactions and sorption modelling. The portlandite charging behaviour could be explained assuming calcium as the main ion determining the potential. This is the first time that a surface model for portlandite is proposed. A simplified double layer modelling approach was proposed to describe experimental data, identifying the most probable sorbing species under the geochemical conditions generated by portlandite, and all the adsorption isotherms could be satisfactorily reproduced the model.