Removal of TcO4- from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials

James J. Neeway, R. Matthew Asmussen, Amanda R. Lawter, Mark E. Bowden, Wayne W. Lukens, Debajit Sarma, Brian J. Riley, Mercouri G. Kanatzidis, Nikolla P. Qafoku

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Abstract

Many efforts have focused on the sequestration and immobilization of 99Tc because the radionuclide is highly mobile in oxidizing environments and presents serious health risks due to its radiotoxicity and long half-life (t1/2 = 213 000 a). One of the more common methods for Tc removal from solution and immobilization in solids is based on reducing Tc from highly soluble Tc(VII) to sparingly soluble Tc(IV). Here, we report results obtained with two potassium metal sulfides (KMS-2 and KMS-2-SS) that are capable of reducing Tc(VII) to Tc(IV). Batch sorption experiments were performed in both oxic and anoxic conditions for 15 d in both deionized water (DIW) and a highly caustic (pH ∼ 13.6), high ionic strength (8.0 mol L-1), low-activity waste (LAW) stream simulant solution. Tc removal for both materials in DIW is improved in anoxic conditions compared to oxic conditions as a result of a higher solution pH. In DIW and anoxic conditions, KMS-2 is capable of removing ∼45% of Tc, and KMS-2-SS is capable of removing ∼90% of Tc. Both materials perform even better in the LAW simulant and remove more than 90% of available Tc after 15 d of contact in anoxic conditions. Postreaction solids analyses indicate that Tc(VII) is reduced to Tc(IV) and that Tc(IV) is bonded to S atoms in a Tc2S7 complex. Examination of the materials after Tc removal by X-ray diffraction shows that the initially crystalline KMS-2 materials lose much of their initial long-range order. We suggest a Tc removal mechanism wherein the TcO4- enters the interlayer of the KMS-2 materials where it is reduced by sulfide, which results in a distorted crystalline structure and a solid-state Tc2S7 complex.

Original languageEnglish
Pages (from-to)3976-3983
Number of pages8
JournalChemistry of Materials
Volume28
Issue number11
DOIs
Publication statusPublished - Jun 14 2016

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ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

Cite this

Neeway, J. J., Asmussen, R. M., Lawter, A. R., Bowden, M. E., Lukens, W. W., Sarma, D., Riley, B. J., Kanatzidis, M. G., & Qafoku, N. P. (2016). Removal of TcO4- from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials. Chemistry of Materials, 28(11), 3976-3983. https://doi.org/10.1021/acs.chemmater.6b01296