Thermodynamic description of Tc(iv) solubility and carbonate complexation in alkaline NaHCO3–Na2CO3–NaCl systems

Abstract

The solubility of ⁹⁹Tc(IV) was investigated in dilute to concentrated carbonate solutions (0.01 M ≤ C_tot ≤ 1.0 M, with C_tot = [HCO₃⁻] + [CO₃²⁻]) under systematic variation of ionic strength (I = 0.3–5.0 M NaHCO₃–Na₂CO₃–NaCl–NaOH) and pH_m (−log[H⁺] = 8.5–14.5). Strongly reducing conditions (pe + pH_m ≈ 2) were set with Sn(II). Carbonate enhances the solubility of Tc(IV) in alkaline conditions by up to 3.5 log₁₀-units compared to carbonate-free systems. Solvent extraction and XANES confirmed that Tc was kept as +IV during the timeframe of the experiments (≤ 650 days). Solid phase characterization performed by XAFS, XRD, SEM-EDS, chemical analysis and TG-DTA confirmed that TcO₂·0.6H₂O(am) controls the solubility of Tc(IV) under the conditions investigated. Slope analysis of the solubility data in combination with solid/aqueous phase characterization and DFT calculations indicate the predominance of the species Tc(OH)₃CO₃⁻ at pH_m ≤ 11 and C_tot ≥ 0.01 M, for which thermodynamic and activity models are derived. Solubility data obtained above pH_m ≈ 11 indicates the formation of previously unreported Tc(IV)–carbonate species, possibly Tc(OH)₄CO₃²⁻, although the likely formation of additional complexes prevents deriving a thermodynamic model valid for this pH_m-region. This work provides the most comprehensive thermodynamic dataset available for the system Tc⁴⁺–Na⁺–Cl⁻–OH⁻–HCO₃⁻–CO₃²⁻–H₂O(l) valid under a range of conditions relevant for nuclear waste disposal.