Selenite Retention and Cation Coadsorption Effects under Alkaline Conditions Generated by Cementitious Materials: The Case of C–S–H Phases

Contaminant migration is strongly controlled by sorption reactions; thus, the behavior of anions, which are (almost) not sorbing under alkaline conditions, is an issue of environmental concern. This is especially relevant in the frame of low and intermediate-low radioactive waste repositories, where the pH generated by cement-based materials is hyperalkaline. Selenite (SeO₃^2–) sorption on calcium silicate hydrate (C–S–H) phasesthe main cement sorbing mineralshas been investigated by batch experiments, ζ-potential measurements, and thermodynamic modeling to elucidate retention mechanisms and possible competitive/synergetic effects of cation coadsorption. Selenite sorption was shown to be nonlinear and slightly increasing with the C–S–H Ca/Si ratio; precipitation of CaSeO₃(s) was observed for Se concentration higher than 2 × 10^–3 M. Indeed, the presence of Ca is essential to enable selenite retention under alkaline conditions. Progressive additions of Na₂SeO₃ or NaCl salt to the phases produced a change in the C–S–H surface properties, that is, a decrease in the ζ-potential, in apparent agreement with anion adsorption. However, this effect had to be also correlated to Na coadsorption, as Cl showed null retention on the C–S–H phases. At the same time, anion adsorption had a clear effect on the retention of other cations (Ba) in the system. The distribution coefficient of Ba (at trace concentrations) suffered a moderate decrease by the presence of Na⁺ and Cl^–, but it was improved by the presence of Na⁺ and SeO₃^2–, indicating complex competitive/synergetic effects between anions and cations. All of the experimental data were satisfactorily modeled considering a classical double-layer approach.