Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions

Abstract

Abstract The redox behaviour of Tc(VII)/Tc(IV) was investigated within the pH c range 2–14.6 in (0.5 M and 5.0 M) NaCl and (0.25 M, 2.0 M and 4.5 M) MgCl2 solutions in the presence of different reducing agents (Na2S2O4, Sn(II), Fe(II)/Fe(III), Fe powder) and macroscopic amounts of Fe minerals (magnetite, mackinawite, siderite: S/L = 20–30 g L –1). In the first group of samples, the decrease of the initial Tc concentration (1 · 10 –5 M, as Tc(VII)) indicated the reduction to Tc(IV) according to the chemical reaction TcO4 – + 4H++ 3e – ↔ TcO2 · 1.6H2O(s) + 0.4H2O. Redox speciation of Tc in the aqueous phase was further confirmed by solvent extraction. A good agreement is obtained between the experimentally determined Tc redox distribution and thermodynamic calculations based on NEA–TDB (Nuclear Energy Agency, Thermochemical Database) and ionic strength corrections by SIT or Pitzer approaches. These observations indicate that experimental pH c and E h values in buffered systems can be considered as reliable parameters to predict the redox behaviour of Tc in dilute to highly concentrated NaCl and MgCl2 solutions. E h of the system and aqueous concentration of Tc(IV) in equilibrium with TcO2 · 1.6H2O(s) are strongly affected by elevated ionic strength, especially in the case of 4.5 M MgCl2 solutions. In such concentrated brines and under alkaline conditions (pH c = pH max ∼ 9), kinetics play a relevant role and thermodynamic equilibrium for the system Tc(IV)(aq) ↔ Tc(IV)(s) was not attained from oversaturation conditions within the timeframe of this study (395 days). Tc(VII) is reduced to Tc(IV) by magnetite, mackinawite and siderite suspensions at pH c = 8 – 9 in concentrated NaCl and MgCl2 solutions. Sorption is very high in all cases (R d ≥ 103 L kg –1), although R d values are significantly lower in 4.5 M MgCl2 solutions. XANES (X-ray absorption near edge spectroscopy) evaluation of these samples confirms that Tc(VII) is reduced to Tc(IV) by Fe(II) minerals also in concentrated NaCl and MgCl2 brines.