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

摘要

The solubility of Tc-99(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) approximate to 2) were set with Sn(ii). Carbonate enhances the solubility of Tc(iv) in alkaline conditions by up to 3.5log(10)-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 TcO2bold/bold0.6H(2)O(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)(3)CO3- at pH(m) 11 and C-tot 0.01 M, for which thermodynamic and activity models are derived. Solubility data obtained above pH(m) approximate to 11 indicates the formation of previously unreported Tc(iv)-carbonate species, possibly Tc(OH)(4)CO32-, 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.