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Technetium-99 (half-life of 2.1 × 105 yrs, βmax = 0.29 MeV) is a hazardous radiological contaminant, which, in its predominant form of pertechnetate (TcO4–), is highly mobile in the environment. Most strategies for the removal of pertechnetate from the environment involve uptake and/or absorption of pertechnetate using resins, clays, cationic metal-organic frameworks and even thorium borate ceramic like materials. Alternative approaches have involved the reduction and subsequent sequestration of lower valent technetium species using iron, sulfides, or iron sulfides. Our lab has explored this strategy using the lacunary alpha-2 Wells–Dawson polyoxometalate (α2-[P2W17O61]10–) to both reduce and sequester lower valent technetium, and we have reported on the ligand features that stabilize the reduced species. In this work we investigate the potential of “plenary” Keggin POMs (XW12O40n–) (X = P, Si, Al, n = 3, 4, 5, respectively) to both reduce TcO4– and stabilize the reduced Tc species. Specifically, we report on the mechanism by which the reduction of technetium occurs and find that PW12, SiW12, and AlW12 promote the reduction of TcO4– to lower valent states. X-ray absorption spectroscopy was used to confirm a combination of TcIV {in the form of TcO2·2H2O and Tc2(µ-O)24+} and TcV, which is subsequently complexed into a POM defect as a TcV=O species.

Technetium-99, a prevalent by-product of the nuclear fuel cycle, is a transition metal with 9 accessible oxidation states and a half-life of 2.1 × 105 years. 99TcVII has been found in the environment surrounding National Lab sites and fuel reprocessing centers, and its separation and long-term storage is complicated by its extensive redox chemistry. In our prior work we have reported the synthesis and reduction of TcVO to TcIVO in α1- and α2-Wells–Dawson P2W17O6110– polyoxometalates and reported on the polyoxometalate features that promote reduction and stabilization. In this work we report on the oxidation state stabilization of TcVO, TcVIO and TcVIN within the α-Keggin PW11O397– polyoxometalate. Both Density Functional Theory and experimental results show that Tc in TBA4[TcVIN-PW11O39] can be reduced to TcV, but any subsequent electrons reduce the tungsten framework of the polyoxometalate. TBA4[TcVO-PW11O39] however, can be theoretically reduced down to TcI before reduction of the polyoxometalate occurs.

Technetium‐99 is a prevalent fission product from nuclear waste. The long half‐life (211,000 yr) and environmental mobility of pertechnetate (TcO4−) render Tc particularly challenging to isolate and stabilize. Here we present two approaches for development of potential wasteforms using titanium dioxide, TiO2. Approach 1 is a low temperature chemical synthesis of TiO2 doped with Tc(IV) from TcO4− intended to mimic the Tc waste stream from the UREX family of separations and removes 98.5 % of the Tc, mainly present as edge‐shared Tc(IV) pairs. Approach 2 utilizes TiO2 to photocatalytically reduce TcO4− to Tc(IV) stabilized on the surface of or within the TiO2 lattice. The %Tc removed from solution and adsorbed to TiO2 is pH dependent, with the maximum Tc(IV) adsorbed at pH 3–4 as either TcO2 or edge‐sharing Tc(IV) octahedra. The Tc(IV)‐TiO2 composites materials formed by both approaches are suitable for consolidation into a dense wasteform by Hot Isostatic Pressing (HIPing). [ABSTRACT FROM AUTHOR]

Technetium‐99, a prevalent by‐product of the nuclear fuel cycle, is a transition metal with 9 accessible oxidation states and a half‐life of 2.1 × 105 years. 99TcVII has been found in the environment surrounding National Lab sites and fuel reprocessing centers, and its separation and long‐term storage is complicated by its extensive redox chemistry. In our prior work we have reported the synthesis and reduction of TcVO to TcIVO in α1‐ and α2‐Wells–Dawson P2W17O6110– polyoxometalates and reported on the polyoxometalate features that promote reduction and stabilization. In this work we report on the oxidation state stabilization of TcVO, TcVIO and TcVIN within the α‐Keggin PW11O397– polyoxometalate. Both Density Functional Theory and experimental results show that Tc in TBA4[TcVIN‐PW11O39] can be reduced to TcV, but any subsequent electrons reduce the tungsten framework of the polyoxometalate. TBA4[TcVO‐PW11O39] however, can be theoretically reduced down to TcI before reduction of the polyoxometalate occurs. [ABSTRACT FROM AUTHOR]

Metrical insights from X-ray scattering studies of dense fluid phases (known as “third” phases) in the Keggin heteropolyacid–tri-n-butyl phosphate (TBP)–n-alkane system are provided. Small-angle X-ray scattering (SAXS) experiments reveal inter-acid correlation peaks corresponding to average centre-of-mass to centre-of-mass separations of 18–23 Å between P…P, Si…Si, and Al…Al of H3PW12O40, H4SiW12O40, and H5AlW12O40, respectively, consistent with the presence of TBP solvates that form by hydrogen bonding between the acids and the phosphoryl group of TBP. The Baxter sticky sphere model analyses of the SAXS data reveal identical structures for all the dense phases with inter-cluster interaction energies of ∼5kBT. We demonstrate that the sticky sphere model is an essential paradigm for interpreting SAXS and predicting mesoscale assembly in heteropolyacid microemulsions. The model parameters for the ternary polyoxometalate–amphiphile–oil systems reveal, in rigorous clarity, how the interactions between heteropolyacid solvates underpin their condensation to produce the observed scattering data. Aside from aiding researchers in predicting the physical origins of SAXS in strongly-interacting micellar systems found in natural and engineered settings, such as chemical separations, our study provides mesostructural information that complements previously observed electrochemical behaviours for third phases formed by solvent extraction involving the contact of aqueous electrolytes of dodecatungsto-phosphoric, -silicic, and -aluminic acids with organic solutions (e.g. n-dodecane and n-octane) of TBP, and by simple dissolution of the acid salts of the polyoxometalate hydrates in the same organic solutions. [ABSTRACT FROM AUTHOR]