Synthesis and Characterization of 5- and 6- Coordinated Alkali Pertechnetates

Journal Article · Sat Dec 31 23:00:00 EST 2016 · MRS Advances

DOI:https://doi.org/10.1557/adv.2017.18· OSTI ID:1372995

Weaver, Jamie; Soderquist, Chuck; Gassman, Paul; Walter, Eric; Lukens, Wayne; McCloy, John S.

The local chemistry of technetium-99 (⁹⁹Tc) in oxide glasses is important for understanding the incorporation and long-term release of Tc from nuclear waste glasses, both those for legacy defense wastes and fuel reprocessing wastes. Tc preferably forms Tc(VII), Tc(IV), or Tc(0) in glass, depending on the level of reduction of the melt. Tc(VII) in oxide glasses is normally assumed to be isolated pertechnetate TcO₄^-anions surrounded by alkali, but can occasionally precipitate as alkali pertechnetate salts such as KTcO₄and NaTcO₄when Tc concentration is high. In these cases, Tc(VII) is 4-coordinated by oxygen. A reinvestigation of the chemistry of alkali-technetium-oxides formed under oxidizing conditions and at temperatures used to prepare nuclear waste glasses showed that higher coordinated alkali Tc(VII) oxide species had been reported, including those with the TcO₅^-and TcO₆^-anions. The chemistry of alkali Tc(VII) and other alkali-Tc-oxides is reviewed, along with relevant synthesis conditions.

Additionally, we report attempts to make 5- and 6-coordinate pertechnetate compounds of K, Na, and Li, i.e. TcO₅^-and TcO₆^-. It was found that higher coordinated species are very sensitive to water, and easily decompose into their respective pertechnetates. It was difficult to obtain pure compounds, but mixtures of the pertechnetate and other phase(s) were frequently found, as evidenced by x-ray absorption spectroscopy (XAS), neutron diffraction (ND), and Raman spectroscopy. Low temperature electron paramagnetic resonance (EPR) measurements showed the possibility of Tc(IV) and Tc(VI) in Na₃TcO₅and Na₅TcO₆compounds.

It was hypothesized that the smaller counter cation would result in more stable pertechnetates. To confirm the synthesis method, LiReO₄and Li₅ReO₆were prepared, and their Raman spectra match those in the literature. Subsequently, the Tc versions LiTcO₄and Li₅TcO₆were synthesized and characterized by ND, Raman spectroscopy, XANES, and EXAFS. The Li₅TcO₆was a marginally stable compound that appears to have the same structure as that known for Li₅ReO₆. Implications of the experimental work on stability of alkali technetate compounds and possible role in the volatilization of Tc are discussed.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1372995
• Report Number(s): PNNL-SA-123560; 48294; KP1704020
• Journal Information: MRS Advances, Journal Name: MRS Advances Journal Issue: 10 Vol. 2; ISSN 2059-8521; ISSN applab
• Publisher: Materials Research Society (MRS)
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Environmental Molecular Sciences Laboratory