TECHNETIUM (IV) COMPLEXATION CHEMISTRY WITH POLYAMINO POLYCARBOXYLATE LIGANDS

Abstract

99Technetium (t½ = 2.1×105 y) constitutes approximately 6.1% of the cumulative yield of thermal 235U thermal fission. In the +7 oxidation state, technetium has high aqueous solubility and forms the anionic structure TcO4- under most aqueous conditions, granting it a high level of environmental mobility. These properties cause 99Tc to become a significantly challenging fission product for both short and long term storage and disposal in nuclear waste facilities. An estimated 30,000 Ci of 99Tc are stored in the single and double shell tanks of the Hanford site in central Washington State, and an additional 500 Ci have already been released into the vadose zone below the remediation site. Meanwhile, those same properties allow 99Tc to constitute an attribute from the standpoint of nuclear forensics, as sampling can be done at a safe distance from a nuclear event. Controlling the redox chemistry is one potential channel of limiting the mobility of 99Tc that has been released into the environment, as the +4 oxidation state of technetium has been shown to have limited water solubility and sorption onto mineral surfaces. While these properties suggest Tc(IV) will have low mobility through underground waters, complexation with ligands found in either the environment or storage facilities could greatly increase the dissolution of Tc(IV) into waters. This work takes a broad look at Tc(IV) complexation chemistry with four polyamino polycarboxylate (PAPC) ligands of varying size and denticity to establish a basis for determining what increase ligand complexation may have on Tc(IV) dissolution. This is accomplished through stability constant determinations for Tc(IV) / ligand complexation with each ligand in a variety of electrolytes. These stability constants were then used to predict the dissolution of Tc(IV) in solutions of each PAPC ligand, and these predictions were compared to values obtained through experiment. Additionally, V(IV) was evaluated for use as a chemical analogue for Tc(IV) by experimentally determining and directly comparing the stability constants of the two metal ions with the four selected PAPC ligands.