Redox behavior of Tc(VII)/Tc(IV) under various reducing conditions in 0.1 M NaCl solutions

Redox behaviour of Tc(VII)/Tc(IV) was investigated under inert Ar atmosphere in 0.1 M NaCl solutions containing different reducing agents in the pH range 2 to 13 at 22ºC. Under certain conditions, the 1•10−5 mol/dm3 (M) initial TcO4– was reduced to form a sparingly soluble Tc(IV) oxide solid phase. The results can be systematized according to Eh-pH conditions. It is found that an experimental borderline for the reduction of Tc(VII) to Tc(IV), TcO4− + 3e− + 4H+  TcO2∙xH2O(coll, hyd) + (2−x)H2O is established, which is independent of the reducing chemical system. This experimental borderline is about 100 mV lower than the equilibrium line calculated from the reported standard redox potential of TcO2∙1.6H2O(s). This behaviour can be explained by the existence of more soluble solid phase modifications like small Tc(IV) oxide particles (TcO2･xH2O(coll,hyd)). The reaction kinetics correlate to the redox potentials measured in solution. Slow reduction of Tc(VII) to Tc(IV) was observed when the redox potential in the system was near the reduction borderline. Fast reduction was observed in the systems far from the borderline or containing Fe(II) solids, suggesting a specific surface mediated effect in the reduction process. EXAFS analysis on two magnetite samples indicate reduced Tc(IV) species which do not remain adsorbed at the reactive magnetite surface, but are incorporated in its structure.