Uptake of Nb(V) by calcite and carbonated cement systems: impact of isosaccharinic acid and chloride

Introduction
Niobium(V) is present in structural components in nuclear reactors. The isotope 94Nb (t1/2 = 2·104 a) is produced by neutron activation of natural 93Nb during the operation of a nuclear reactor. 94Nb is most-ly 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 waste (L/ILW). Isosaccharinic acid (ISA) is a polyhydroxycarboxylic acid form-ing 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. High concentrations of stable chloride (up to ≈ 4–5 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 chloride. Both materials are taken as representatives of the degradation stage IV of cement, with relevance to near-surface dis-posal systems.
Experimental
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Sorption was investigated using a combination of 93Nb (stable) and 95Nb (t1/2 = 34.97 d) isotopes. Ce-ment 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 °C, using calcite- and carbonat-ed cement-equilibrated waters. Systems with∙10-11 M ≤ [Nb(V)]0 ≤∙10-6 M (as 95Nb or 93Nb + 95Nb) with S/L = 1–10 g/L 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 pore water solutions used for sorption. Nb(V) solid phases and carbonated ce-ment were characterized by XRD, BET and Raman. 93Nb content in carbonated cement was quanti-fied by alkaline fusion.
Results and discussion
The solubility of Nb(V) 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 selected in available thermodynamic databases.
In calcite systems, a moderate uptake at short contact times (Rd ≈ 103 dm3∙kg-1 at t = 3 days) was followed by a steady increase of distribution ratios with time (Rd > 2∙104 dm3∙kg-1 at t = 89 days). These observations were explained by a fast adsorption, followed by the slow incorporation into the calcite structure. The stronger uptake observed for carbonated cement paste (Rd ≈ 106 dm3∙kg-1) is attributed to the presence of amorphous phases with larger surface area, primarily SiO2(am) and C-S-H with low Ca/Si. The formation of complexes with isosaccharinic acid (ISA) decreases Nb(V) uptake, although sorption remains strong up to [ISA]tot = 0.1 M. Chloride has a minor effect on the uptake of Nb(V) up to [NaCl] = 2 M. This work provides a sound basis for the quantitative description and mechanistic understanding of 94Nb retention in L/ILW repositories.

Acknowledgements: this work was funded by ONDRAF/NIRAS.