Tc is one of the highest yield, mobile and long-lived radioactive decay products of 235U fission. Therefore, the chemistry of TcO4- and its surrogate ReO4- is of great interest in nuclear materials management and reprocessing. During the reprocessing of nuclear materials through solvent extraction (e.g. PUREX), the TcO4- can co-extract with tetravalent actinides (An(IV)) and Zr(IV), also a high yield fission product and nuclear fuel cladding material, hampering efficient separation and recycling. It has been prior-observed that TcO4- can ligate Th(IV) and U(VI) in the presence of additionally coordinated organophosphorus ligands used in PUREX process. However, less is known about the M(IV)-TcO4- aqueous speciation; i.e. in the environment and the aqueous phase for solvent extraction. In our current work, we have synthesized six new extended compounds of Th(IV) where TcO4- and its surrogate ReO4- directly coordinate with metal center, from aqueous acid conditions, without organic ligation. Single crystal X-ray diffraction revealed that the dimensionality of the isolated compounds ranges from 1D to 2D. Five of those compounds have same chemical formula [Th(MO4)4. xH2O], amongst which only two are isostructural between Re and Tc. In one structure, TcO4- differs from ReO4- in dictating and linking the assembly thorium-oxo dimers. SAXS of the aqueous reaction solutions also reveals differentiating solution speciation, broadening our understanding of periodic trends and possible limitations of the non-radioactive surrogate rhenium. Finally, Tc/Re-Zr/Hf(IV) oxyhydroxide crystalline solids isolate a prior-unrecognized species of Zr/Hf(IV), and reveal Zr-O-Tc connectivity, also important in nuclear materials management.

Solid state structures towards understanding the role M(IV) (M=U, Th, Zr, Hf)-TcO4 connectivity in Tc mobility in nuclear fuel reprocessing and the environment

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