Impact of ISA and NaCl on the uptake of Ra(II) by calcite

226Ra (t1/2 = 1600 years) is one of the relatively long-lived daughter nuclides of 238U. It is typically present in small inventories in low- and intermediate-level short-lived wastes (L/ILW-SL), although given its weak retention it is considered a relevant radionuclide for the long-term safety assessment of such repository systems. Cementitious materials are widely used in repositories for L/ILW-SL, particularly in structural components, waste immobilization and containment. In the context of near-surface repository systems, the possible interaction of cementitious materials with groundwater and atmospheric CO2 may lead to carbonation of the cement and dissolution of different cement phases. Under these conditions and after the complete dissolution of portlandite and C-S-H phases, calcite remains as main mineral phase in the final degradation stage (IV) of cement. The pore water composition is governed by the equilibrium with these remaining minerals, resulting in a pH < 10. Several studies have previously investigated the uptake of Ra(II) by cementitious materials under the relevant chemical conditions for degradation stages I–III [1-2]. ... mehrHowever, the retention of Ra(II) in the degradation stage IV of cement remains unexplored, and it is assumed to be relatively low (log (Rd/L·kg-1) ≈ 1) based on the analogy with Ba and Sr [2].
Large inventories of cellulose are disposed of together with L/ILW-SL. Under the hyperalkaline pH conditions defined by cement, the degradation of cellulose leads to the formation of isosaccharinic acid (ISA), a polyhydroxycarboxylic acid forming strong complexes with hard Lewis acids [3-5]. High concentrations of chloride (up to ≈ 4–5 M) have been described for specific waste streams containing evaporator concentrates (from nuclear power plants operation). The presence of ISA and chloride (as NaCl) may impact the retention of Ra(II) under repository conditions. In this context, this work aims at a quantitative description of Ra(II) uptake by calcite, both in the absence and presence of ISA and chloride. Calcite has been selected as representative material of the degradation stage IV of cement.
Sorption batch experiments were performed under air at T = (22 ± 2)°C. Experiments were performed using a 226Ra tracer (66.3 kBq/mL, Eckert & Ziegler), commercial calcite (Merck, with 0.6 m2/g as quantified by BET) and calcite-equilibrated water at pH = (8.3 ± 0.1). Calcite was characterized by XRD, SEM and BET analyses. Systems with 10-9 M ≤ [Ra(II)]0 ≤ 6.1∙10-8 M and a S/L ratio of 10 – 100 g/L were investigated in the absence and the presence of ISA (10-5 M ≤ [ISA] ≤ 0.1 M) and chloride (8.4∙10-5 M ≤ [NaCl] ≤ 2.0 M). The concentration of Ra(II) in the aqueous phase at different contacting times (t = 2 – 170 days) was quantified by gamma spectrometry after phase separation, which was achieved by ultracentrifugation at ( ~600,000 g).
Radium(II) shows a weak sorption onto calcite (Rd ≈ (8 ± 5) L·kg-1), which is in agreement with estimated values provided by Ochs et al. (Rd ≈ 1 L·kg-1, with upper and lower limits reported as 0.1 and 30 L·kg-1) in analogy with Sr and Ba [2] (see Figure 1a). Berry and co-workers reported also an Rd < 7 L/kg for the uptake of Ra(II) by carboniferous limestone (96 % calcite, 3 % dolomite) at pH = 7 in saline groundwater (~1 M Na+, 5·10−2 M Ca2+) [6]. Radium(II) uptake is slightly influenced by the solid to liquid (S/L) ratio, and the on-going kinetic experiments do not show any significant increase of the uptake with equilibration time (up to ~ 40 days). This observation may hint towards the predominance of adsorption / surface cationic exchange and the absence of Ra(II) incorporation into the calcite structure, in line with the significant differences in the ionic radii of Ca2+ and Ra2+, i.e., rCa2+ = 1.0 Å (coordination number, CN = 6); rRa2+ = 1.5 Å (CN = 8). This is also in line with the extremely unfavorable incorporation predicted for Ba2+ into calcite [7].
Sorption isotherms obtained at S/L = 10 and 50 g/L are characterized by a linear trend in the Ra(II) aqueous concentration range 10-9 M < [Ra]aq < 10-7 M (Figure 1b). ISA has a negligible impact in the uptake of Ra(II) by calcite within the investigated range of ligand concentrations (10-5 M ≤ [ISA] ≤ 0.1 M). This result possibly reflects the weak complexes expected for the Ra(II)-ISA system. The results of on-going sorption experiments in NaCl and NaCl + ISA systems will be also presented in this contribution.
This work provides a first comprehensive quantitative description of Ra(II) retention under the relevant conditions for the cement degradation stage IV for L/ILW-SL disposal systems, in which the presence of ISA and chloride can be envisaged.
Acknowledgments: This work was funded by ONDRAF/NIRAS under contract CCHO 2015-0707/01/00.
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References
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