Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands

Summary

Hardened cement pastes (HCP) present a high affinity with a lot of radionuclides (RN) and can be used as waste confining materials in radioactive waste repository. Indeed, in cementitious media, RN can be removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms. In this study, the affinity of anionic RN (Cl⁻, I⁻, SeO₃²⁻ and CO₃²⁻ chemical forms) with a CEM-I HCP has been studied vs. the degradation of the HCP particles. These RN are considered as mobile in repository media and it is important to have a set of distribution ratio (R_d) in cement environment. The R_d values have been measured in batch experiments as a function of the pH, used as the degraded state parameter of the HCP suspensions. The R_d values increase in all cases, from the unaltered state (pH 13.3) to the altered state of HCP, i.e. until all portlandite is dissolved, corresponding to pH 12.6. Then, R_d values decrease until degraded states (pH 12.0), corresponding to the decalcification of the calcium silicate hydrate (C-S-H) phases. The behaviour of anionic RN seems to be correlated to the evolution of calcium concentration and is opposed to the evolution of sulphate concentration in solution which could have a competing effect. Comparison is done with the behaviour of caesium and uranium(VI), which is a cationic RN but has a major negative hydrolysed species at high pH. As awaited, the uranium(VI) behaviour is very different from purely anionic RN one in accord with spectroscopic analyses from literature works. The R_d values have also been measured for the organic ligands isosaccharinate (ISA) and EDTA. The uptake of ISA can be important and competing effect with the sorption of SeO₃²⁻ has been evidenced in HCP suspensions as a function of the ISA concentration.