Selenite Retention and Cation Coadsorption Effectsunder Alkaline Conditions Generated by Cementitious Materials: TheCase of C–S–H Phases

摘要

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 anionadsorption. However, this effect had to be also correlated to Na coadsorption,as Cl showed null retention on the C–S–H phases. At thesame time, anion adsorption had a clear effect onthe retention of other cations (Ba) in the system. The distributioncoefficient of Ba (at trace concentrations) suffered a moderate decreaseby the presence of Na⁺ and Cl^–, but itwas improved by the presence of Na⁺ and SeO3^2–, indicating complex competitive/synergetic effectsbetween anions and cations. All of the experimental data were satisfactorilymodeled considering a classical double-layer approach.