The radioelement technetium-99 (99Tc) is a major contaminant found in nuclear tank waste stored at the U.S. Department of Energy (DOE) site at Hanford, Washington. Existing predominately in the liquid supernatant and salt cake fractions, it is among the most difficult contaminants to dispose of and/or remediate because of its complex chemical and redox behavior in tank waste, limited incorporation in the glass waste forms during mid- to high-temperature immobilization processes, and high mobility in the environment. Although in solution 99Tc can have oxidation states ranging from +7 to 1, it is observed to prefer existing as pertechnetate (TcO4-) (oxidation state VII) in strongly alkaline environments under ambient atmospheric conditions. Therefore, TcO4- is expected to be the significantly dominant fraction in the highly alkaline multi-component matrices of Hanford tank waste supernatant fractions. However, contrary to general expectations based on existing knowledge in Tc chemistry, a dominant fraction of the soluble Tc in the waste supernatants with high organics content occurs as low-valent Tc (oxidation state &lt; +7), most notably EXAFS observations suggested potential presence of Tc(I) carbonyl species in addition to Tc(IV) and Tc(VI). Understanding the mechanisms of generation of these non-pertechnetate species in-situ as well as evaluating their ex-situ stabilities under ambient atmospheric conditions are key to design new methods for their effective separation and remediation. One approach of separation of non-pertechnetate species is their oxidation to TcO4- and subsequent separation by ion-exchange methods. Towards this goal, this work evaluates the oxidative stabilities of non-pertechnetate complexes generated ex-situ under conditions relevant to Hanford tank wastes, such as multicomponent brine-like alkaline solution matrices. Toward elucidation of the mechanism of formation of low-valent Tc species, pathways of in-situ reduction of pertechnetate in these solutions are investigated using CO/H2 as a reactant at elevated pressure and temperature.

Oxidative stabilities of low-valent technetium species relevant to their separations from Hanford tank waste

254th National Meeting (2017)

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