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

Abstract

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.