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

Nitrate induces a clear increase in Eh over time in the investigated aqueous solutions both in the presence and absence of reducing agents. In the case of Fe powder, this increase is observed almost immediately whereas in the presence of Na2S2O4 or Sn(II) the oxidizing effect is only observed after several months in inactive systems (absence of Tc). In the presence of technetium, the increase in Eh occurs within a few days (5-50, depending upon reducing system, pHm and ionic strength). The evolution of Eh in the presence of 137Cs (with similar β- dose as in solubility experiments with 99Tc) was analogous to that in the inactive systems. These observations indicate that the solid TcO2∙0.6H2O(am) may play a role in catalysing nitrate reduction and radiation induced reactions have no significant impact in our experiments. Liquid-liquid extraction and XANES measurements confirm that technetium in the aqueous phase is mostly present as Tc(VII). Colorimetric nitrate/nitrite tests show the formation of nitrite, confirming that the oxidation of Tc(IV) to Tc(VII) is coupled to the reduction of nitrate. Our experimental observations support that, although kinetically hindered, nitrate can effectively oxidize Tc(IV).