Technetium management in liquid-liquid distribution systems for uranium extraction by DEHiBA

Technetium-99 is one of the most problematic byproducts of nuclear fission due to its high environmental mobility and unpredictable solution chemistry which complicate the isolation and storage options. Efficient management of technetium is difficult in flowsheets designed to recover uranium from used nuclear fuel effluents due to pertechnetate anion's tendency to follow the uranyl cation. The accessible standard electrode potentials of technetium have offered a gateway to its efficient routing away from uranium based on the manipulation of Tc redox speciation. The intra-molecular oxoanion recognition has been a much less explored strategy, where the main obstacle resides with binding specificity for pertechnetate in mixtures containing great excess of nitrate. This work explores molecular constructs based on the guanidinium moiety, seeking affinity for a pertechnetate anion as expressed in the observable influence on the liquid-liquid partitioning of Tc-99. The unsubstituted guanidinium cations showed evidence of pertechnetate recognition, yielding a modest impact on Tc distribution in mixtures containing a thousand-fold excess of nitrate anion. Guanidiniums containing an amino substituent are oxidized via a Tc-catalyzed redox mechanism which reduces technetium to its tetravalent state. This transformation affords TcO2+ species, which are excluded from the DEHiBA uranyl solvate in the organic phase. As a result, over 90% of Tc-99 can be efficiently scrubbed when equilibrated with moderately acidic mixtures of diaminoguanidine.