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

Abstract

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 KTcO₄ and NaTcO₄ when 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, butmore » 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 Na₃TcO₅ and Na₅TcO₆ compounds. Here, it was hypothesized that the smaller counter cation would result in more stable pertechnetates. To confirm the synthesis method, LiReO₄ and Li₅ReO₆ were prepared, and their Raman spectra match those in the literature. Subsequently, the Tc versions LiTcO₄ and Li₅TcO₆ were synthesized and characterized by ND, Raman spectroscopy, XANES, and EXAFS. The Li₅TcO₆ was 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.« less

Authors:

Weaver, Jamie ^[1]; Soderquist, Chuck ^[2]; Gassman, Paul ^[2]; Walter, Eric ^[2]; Lukens, Wayne ^[3]; McCloy, John S. ^[1]

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+ Show Author Affiliations

1. Washington State Univ., Pullman, WA (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Publication Date:

Thu Jan 12 00:00:00 EST 2017

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division

OSTI Identifier:

1959968

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

MRS Advances

Additional Journal Information:

Journal Volume: 2; Journal Issue: 10; Conference: MRS Advances; Journal ID: ISSN 2059-8521

Publisher:

Springer Nature

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE