Impact of nitrate on the redox chemistry and solubility of Tc(IV) in alkaline, dilute to concentrated aqueous NaCl solutions

Baumann, Alexander; Gaona, Xavier; Yalçıntaş, Ezgi; Dardenne, Kathy; Prüßmann, Tim; Rothe, Joerg; Duckworth, Sarah; Altmaier, Marcus; Geckeis, Horst

doi:10.1016/j.apgeochem.2018.10.007

Title: Impact of nitrate on the redox chemistry and solubility of Tc(IV) in alkaline, dilute to concentrated aqueous NaCl solutions

Journal Article · Sat Oct 06 00:00:00 EDT 2018 · Applied Geochemistry

DOI:https://doi.org/10.1016/j.apgeochem.2018.10.007· OSTI ID:1477725

Baumann, Alexander ^[1]; Gaona, Xavier ^[1]; Yalçıntaş, Ezgi ^[2]; Dardenne, Kathy ^[1]; Prüßmann, Tim ^[1]; Rothe, Joerg ^[1]; Duckworth, Sarah ^[1]; Altmaier, Marcus ^[1]; Geckeis, Horst ^[1]

Karlsruhe Institute of Technology, Karlsruhe (Germany)
Karlsruhe Institute of Technology, Karlsruhe (Germany); Los Alamos National Lab. (LANL), Carlsbad, NM (United States)

Here, the impact of nitrate on the redox behaviour and solubility of ⁹⁹Tc(IV) was investigated in 0.1–5.0 M NaCl–NaNO₃–NaOH solutions with 0.1 M ≤ [NO₃^–] ≤ 1.0 M and 8.5 ≤ pH_m ≤ 14.5 (with pH_m = –log [H⁺]) in the presence or absence of reducing agents (as Fe powder, Na₂S₂O₄, Sn(II)). The evolution of the redox conditions as a function of time (for t ≤ 450 days) was investigated: (i) in the absence of technetium (inactive samples), (ii) in the presence of ⁹⁹Tc(IV) (added as 1–2 mg of TcO₂∙0.6H₂O(am)), and (iii) in the presence of ¹³⁷Cs(I) (and absence of ⁹⁹Tc). All experiments were conducted in an Ar-glovebox (O₂ < 1 ppm) at T = (22 ± 2) °C. An improved understanding of Tc redox processes in nitrate-rich alkaline solutions is relevant to predict Tc behaviour in specific nuclear waste disposal scenarios and a topic of fundamental scientific interest.