Impact of ISA and chloride on the uptake of Nb(V) by calcite and carbonated cement paste

Niobium(V) is present in structural components of nuclear reactors. The isotope 94Nb (t1/2 = 2·104 y) is produced by neutron activation of natural 93Nb during the operation of a nuclear reactor. 94Nb is mostly present in streams resulting from the dismantling of nuclear power plants, as well as from the treatment of the primary cooling circuit, which may be disposed in cement-based repositories for low- and intermediate-level short-lived waste (L/ILW-SL). Isosaccharinic acid (ISA) is a polyhydroxycarboxylic acid forming upon degradation of cellulose in hyperalkaline conditions. ISA is known to form strong complexes with hard Lewis acids, thus potentially affecting radionuclide retention in cementitious systems [1-2]. High concentrations of stable chloride (≤ 5 M) have been also described for specific waste streams containing evaporator concentrates. This study focuses on the uptake of Nb(V) by calcite (CaCO3) and carbonated cement systems in the absence and presence of ISA and sodium chloride. Both materials are taken as representatives of the degradation stage IV of cement, with relevance to near-surface disposal systems.
Sorption was investigated using a combination of 93Nb (stable) and 95Nb (t1/2 = 34.97 d) isotopes. ... mehrCement pastes (CEM I and III/C) were carbonated in closed reactors with a gas mixture CO2 / N2 of 30/70%. Batch sorption experiments were conducted in air at T = (22 ±2)°C, using calcite- and carbonated cement-equilibrated waters at pH ≈ 8.2–8.5. Systems with∙10-11 M ≤ [Nb(V)]0 ≤∙10-6 M (as 95Nb or 93Nb + 95Nb) with S/L = 1–10 g∙L–1 were investigated in the absence / presence of ISA (1.0∙10-5 M ≤ [ISA] ≤ 0.1 M) and chloride (8.4∙10-5 M ≤ [Cl–] ≤ 2.0 M). The concentration of Nb(V) in the aqueous phase was quantified by gamma spectrometry after ultracentrifugation. Upper solubility limits of niobium were quantified with 93Nb in the same equilibrated solutions used for sorption. Nb(V) solid phases and carbonated cement were characterized by XRD, BET and Raman. 93Nb content in carbonated cement was quantified by alkaline fusion. XAFS measurements were conducted at the Karlsruhe Research Accelerator to gain insight on the retention mechanism/s of Nb in carbonated cement materials.
The solubility of Nb(V) in calcite- and carbonated cement-equilibrated waters was quantified as ~10-6 M at t = 3 days, but decreased to ≤ 10-8 M at longer equilibration times (t = 13–121 days). This solubility limit is significantly lower than the one calculated assuming a solubility-control by Nb2O5(cr). XRD and Raman support that a Ca-Nb(V)-OH solid phase is responsible for the solubility-control of Nb(V) under these conditions, although no thermodynamic data for such solid phases is currently available in reference thermodynamic databases [3]. The uptake of Nb(V) by calcite shows distribution ratios of Rd ≈ 103 L∙kg–1 at short contact times (t = 3 days). An increase in Rd values is observed with time, resulting in Rd > 2∙104 L∙kg–1 at t = 89 days. This observation may suggest a fast adsorption of Nb(V) on the calcite surface, followed by a slow incorporation into the calcite structure through the recrystallization process. A significantly stronger uptake is observed for both carbonated cement materials (Rd ≈ 8∙105 L∙kg–1 carb. CEM I; Rd ≈ 1∙106 L∙kg–1 carb. CEM III). This differential behaviour is expectedly caused by the presence of amorphous phases with significantly larger specific surface area, i.e., SiO2 gel or small fractions of C-S-H phases, as determined by means of Rietveld analysis, Raman and IR-spectroscopy and BET. The normalization of the distribution ratios by the surface area of the corresponding materials (calcite, carbonated cement) results in much similar distribution ratios (see Figure 1b). This emphasizes the key role of surface area, as well as the need of an appropriate characterization of the individual phases present in complex sorbing materials (e.g., carbonated cement). Spectroscopic techniques like EXAFS can provide additional insights for a correct interpretation of the retention mechanisms. Respective Nb K-edge XAFS data are currently under evaluation and will be presented at the conference. The presence of stable 93Nb in degraded cement paste suggests that isotopic exchange may also contribute in the retention of radioactive Nb isotopes (95Nb in this work, 94Nb in the waste) in carbonated cement materials. ISA has a minor role to moderate impact on the uptake of Nb(V), both for calcite and carbonated cement systems. The decrease in Rd values observed at high ligand concentrations is attributed to the formation of Nb(V)–ISA complexes. However, the impact on sorption is weaker compared to observations reported for the degradation stage I of cement at pH ≈ 13.6 [4]. Such differences could be partly explained by the greater stability of the Nb(V)-ISA complexes under hyperalkaline conditions, although other parameters (e.g., surface charge, concentration of Ca in the aqueous phase, sorption of ISA on the solid phase, etc.) can contribute as well to the observed differences. Chloride has a weak impact on sorption of Nb(V). The effect on the Rd-value is more remarkable for calcite than for carbonated cement paste, which possibly reflects that a different mechanism is driving the uptake of Nb(V) in both systems.
This work provides an improved quantitative description of Nb retention under conditions relevant for the cement degradation stage IV in repositories for L/ILW, in which the presence of ISA and chloride can be envisaged.
Acknowledgments: This work was partly funded by ONDRAF/NIRAS under contract CCHO 2015-0707/01/00.
References
[1] Tits, J., Wieland, E., Bradbury, M. H. (2005), Applied Geochemistry, 20, 2082-2096.
[3] Rai, D., Kitamura, A. (2017), The Journal of Chemical Thermodynamics, 114, 135-143.
[4] Madé, B., Bower, W., Brassinnes, S., Colàs, E., Duro, L., Blanc, P., Lassin, A., Harvey, L., Begg, J. D. (2025), Applied Geochemistry, 180, 106273.
[5] Jo, Y., Çevirim-Papaioannou, N., Franke, K., Fuss, M., Pedersen, M., Lothenbach, B., de Blochouse, B., Altmaier, M., Gaona, X. (2023), Cement and Concrete Research, 172, 107233.