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

Niobium(V) is present in structural components in nuclear reactors, mostly in reactor vessels constructed using Inconel alloy. 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 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 waste (L/ILW) 1. Isosac-charinic acid (ISA) is a polyhydroxycarboxylic acid forming upon degradation of cel-lulose in hyperalkaline conditions. ISA is known to form strong complexes with hard Lewis acids (e.g. An, Zr, among others), thus potentially affecting radionuclide reten-tion in cementitious systems. High concentrations of stable chloride (up to ≈ 4–5 M) have been described for specific waste streams containing evaporator concentrates, which may also impact the sorption properties of radionuclides in repositories for L/ILW. This study focuses on the uptake of Nb(V) by calcite (CaCO3) in the absence and presence of ISA and chloride. Calcite is taken as representative of the aggre-gate minerals remaining in the degradation stage IV of cement, after the dissolution of C-S-H phases and other hydrated cement components.
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Sorption was investigated using a combination of 93Nb (stable) and 95Nb (t1/2 = 34.97 d) isotopes. Batch sorption experiments were conducted in air at T = (22 ± 2) °C, using calcite-equilibrated water with pH ≈ 8.3, [Ca] ≈ 5.8∙10-4 M and Ctot = [HCO3-] + [CO32-] = 1.2∙10-3 M. Systems with∙10-11 M ≤ [Nb(V)]0 ≤∙10-6 M (as 95Nb or 93Nb + 95Nb mixtures) with S/L ratios of 1 – 10 g/L were investigated in the absence and 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 spec-trometry after ultracentrifugation at 100 000 g. Upper solubility limits of niobium were quantified with 93Nb in the same pore water solution used in the sorption experi-ments, and the resulting Nb(V) solid phases were characterized by means of XRD and Raman.

The solubility of Nb(V) in the calcite-equilibrated water (pH ≈ 8.3) was quantified as·≈ 10-6 M at short equilibration times (t = 3 days), but decreased to ≈ 10-8 M for longer equilibration times (t ≥ 25 days). This solubility limit is significantly lower than the calculated Nb(V) concentrations assuming a solubility-control by Nb2O5(cr), i.e., [Nb(V)] ≈ 10-4 M. XRD measurements show amorphous behavior and Raman sup-port 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 any of the main thermodynamic databases in the field of nu-clear waste disposal 2 3 4.
Sorption experiments show a fast and moderate uptake of Nb(V) by calcite (log Rd ≈ 3 at t = 3 days, with Rd in L/kg), followed by a slow but steady increase in the distri-bution ratios with time (log Rd ≈ 4 at t = 80 days). These observations are interpreted as a fast adsorption on the calcite surface, followed by the slow incorporation into the calcite structure. The latter process is expectedly influenced by the rate of calcite recrystallization, and thus by the its particle size. ISA has a negligible impact on the uptake of Nb(V) by calcite at [ISA] < 10–2 M. Distribution ratios of log Rd ≈ 2 are quan-tified at the highest ligand concentration investigated, i.e., [ISA] = 0.1 M. This obser-vation supports the formation of (Ca-)Nb(V)-ISA aqueous complexes, which howev-er are less stable than those reported under hyperalkaline conditions 5. This differ-ential behaviour can be possibly explained by the readily deprotonation of the alco-hol groups of ISA in hyperalkaline conditions, which contribute to the formation of more stable complexes with Nb(V) at the higher pH conditions. The uptake of Nb(V) slightly decreases in the presence of chloride (log Rd ≈ 2.1–2.4 L/kg at [Cl-] ≥ 1 M), possibly due to the combination of ion-interaction processes with Nb(V) and a chlo-ride-induced calcite dissolution (quantified as ~ 10–13 % of total calcite for a S/L ratio of 1 g/L). The results of on-going sorption experiments with carbonated cement will be also discussed in this contribution.
This work represents the first experimental evidence of niobium uptake by calcite in the absence and presence of ISA and Cl–, and provides important input for the as-sessment of 94Nb retention in the context of repositories for L/ILW.

Acknowledgements: this work was funded by ONDRAF/NIRAS.

References

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