Title: Removal of TcO₄^– from Representative Nuclear Waste Streams with Layered Potassium Metal Sulfide Materials

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 (t_1/2= 213 000 a). We report that 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 analysesmore » 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. In conclusion, we suggest a Tc removal mechanism wherein the TcO^4- 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.« less

Authors:

Neeway, James J. ^[1]; Asmussen, R. Matthew ^[1]; Lawter, Amanda R. ^[1]; Bowden, Mark E. ^[2]; Lukens, Wayne W. ^[3]; Sarma, Debajit ^[4]; Riley, Brian J. ^[1]; Kanatzidis, Mercouri G. ^[4]; Qafoku, Nikolla P. ^[1]

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+ Show Author Affiliations

1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Energy and Environment Directorate
2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). W. R. Wiley Environmental Molecular Sciences Laboratory
3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Division
4. Northwestern Univ., Evanston, IL (United States). Department of Chemistry

Publication Date:

Tue May 17 00:00:00 EDT 2016

Research Org.:

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

Sponsoring Org.:

USDOE Office of Environmental Management (EM); USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division

OSTI Identifier:

1480726

Grant/Contract Number:  

AC02-05CH11231; AC06-76RL01830

Resource Type:

Accepted Manuscript

Journal Name:

Chemistry of Materials

Additional Journal Information:

Journal Volume: 28; Journal Issue: 11; Journal ID: ISSN 0897-4756

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; 12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES