Surprising formation of quasi-stable Tc(vi) in high ionic strength alkaline media

Abstract

Despite decades of research, the reduction behavior of pertechnetate, TcO₄⁻, is not well understood and represents one of the largest knowledge gaps in the transition metal series. Conventional wisdom presumes the reduction of TcO₄⁻ in aqueous systems to predominantly follow either a rapid two electron, or three electron process to form either a Tc^V or Tc^IV species. This study for the first time demonstrates the importance of the Tc^VI species through systematic characterization by multiple spectroscopic techniques. The reduction of TcO₄⁻ was examined in a matrix of 5 M NaNO₃ at 0–2 M NaOH. Results of the cyclic voltammetric evaluation of reduction to Tc^VI, Nernstian analysis of the visible spectroelectrochemical signal, and structural spectroscopic EPR and XPS analysis of the reduction product generated by bulk electrolysis consistently support an electron transfer stoichiometry corresponding to a 1e⁻ reduction. This Tc^VI species is markedly more stable than had been previously considered, with decomposition kinetics that correspond to a half-life of 1.91 ± 0.07 days, fully 6 orders of magnitude longer than previously reported for an aqueous solution. These results reveal the importance of a Tc^VI intermediate species in high ionic strength alkaline solutions and significantly contribute to the understanding of the mechanism of TcO₄⁻ reduction and formation of low-valent Tc species.