Speciation and reactivity of heptavalent technetium in strong acids

Abstract

Technetium is the workhorse of the radiopharmaceutical imaging agents; it is also an important byproduct of the nuclear industry. Studies of the chemistry of Tc in strong acids are relevant to nuclear applications, environmental remediation and fundamental chemistry. Nitric acid is used at the industrial level for spent fuel reprocessing while H₂SO₄ and HClO₄ are used at the laboratory scale for Tc separation from Mo or U. During reprocessing activities, radiolysis products from nitric acid (e.g., H₂O₂) and from organics extracting agents (e.g., alcohols) are formed and these might interact with Tc(VII). An understanding of Tc(VII) chemistry in the presence of H₂O₂ and/or organics in acidic solution is important to predict its behavior in separation processes. Concerning environmental remediation, sulfur has been proposed to immobilize “Tc₂S₇”. Technetium heptasulfide can be obtained from the reaction of Tc(VII) with H₂S gas in acidic solutions. A better understanding of “Tc₂S₇” formation could give information to predict its formation and behavior in the environment. Under oxidizing conditions, the aqueous chemistry of Tc(VII) is dominated by TcO₄⁻. In high acid concentrations, pertechnetic acid can be formed and control the reactivity of Tc. Speciation data on Tc(VII) in concentrated acids are still parse, and the structure and reactivity of pertechnetic acid are unknown. Here, the speciation of Tc(VII) in sulfuric, nitric and perchloric acids and its reactivity with H₂O₂, methanol and H₂S is reviewed. Experimental results and density functional calculations show the formation of TcO₃(OH)(H₂O)₂ in concentrated H₂SO₄ and HClO₄. In 12–13 M H₂SO₄, Tc(V) species and Tc(IV) polymeric species were respectively detected in the presence of methanol and H₂S. Finally, peroxo pertechnetic acid was identified in nitric and sulfuric acid in the presence of H₂O₂.