Effect of cellulose degradation products on 63Ni sorption on Portland cement

Abstract

Concepts of nuclear waste repositories make use of large quantities of cementitious materials, especially near surface disposal facilities in which cement, mortar, and concrete are used both as structural materials and waste immobilization matrices. The retardation of radionuclides by the cementitious barriers of these repositories is crucial for the safe long-term management of the waste. Yet both the degradation of the cementitious materials and the presence of complexing ligands originating from the waste can affect the sorption of radionuclides. Cellulosic materials present in nuclear waste degrade under irradiation and under the alkaline conditions generated by the cementitious materials and they release organic compounds. Within the cellulose degradation products, α-isosacharinic acid (α-ISA) is generally assumed to be the strongest radionuclide complexant and to have the most impact on radionuclide sorption, though the presence of other ligands in the mixture could also have an effect. In this work, the sorption of ⁶³Ni was assessed on fresh and state III hardened cement paste (HCP). The effect of α-ISA on the ⁶³Ni sorption at these two degradation states was investigated and compared to the effect of a complete mixture of cellulose degradation products generated by irradiation and alkaline degradation of cellulosic tissues. A higher sorption of ⁶³Ni was observed on state III HCP (R_d = (8.6 ± 3.3) × 10³ L/kg) than on fresh HCP (R_d = (1.0 ± 0.3) × 10³ L/kg), and, in both cases, the sorption was higher than previously observed on pure C–S–H (calcium silicate hydrate) suggesting the involvement of other mechanisms than the sole surface complexation on C–S–H. The effect of α-ISA on the sorption of ⁶³Ni depended on the cement degradation state, ranging from no effect on fresh cement to a maximal sorption reduction factor of 8 ± 3 on degraded cement for 10⁻² M α-ISA. The effect of the mixture of degradation products from cellulosic tissues on the sorption of ⁶³Ni was found to be considerably higher on both fresh and degraded HCP, with a sorption reduction factor of 582 ± 972 and 718 ± 327 for the fresh and degraded cement, respectively, for a corresponding α-ISA concentration of 10⁻² M.