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Title: Technetium Stabilization in Low-Solubility Sulfide Phases: A Review

Abstract

We report that technetium contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington, USA. At these sites, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (214 000 years) and the mobility of the oxidized anionic species Tc(VII)O 4 -. Under reducing conditions, TcO 4 - is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc-oxide (TcO 2· nH 2O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to reoxidation to TcO 4 -, which in turn depend on the (biogeo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO 2 or amorphous TcO 2·H 2O is still above the maximum contaminant level (MCL) established by the U.S. EPA (900 pCi/L), and the kinetics of TcO 2 oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility, and reoxidation potential of Tc, especiallymore » Tc(IV). The principal technetium sulfides are TcS 2 and Tc 2S 7, but much less is known about the mechanisms of formation, stabilization, and reoxidation of Tc-sulfides. A common assumption is that sulfides are less soluble than their oxyhydrous counterparts. Determination of the molecular structure of Tc 2S 7 in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc 2S 7 is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O 2-like phase without formation of measurable dissolved TcO 4 -, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV)-sulfides to Tc(IV)-oxides may be the main process that limits remobilization of Tc as Tc(VII)O 4 -. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Finally, many waste form designs are reducing (e.g., cementitious waste forms such as salt stone) and, therefore, attempt to restrict access of oxidizing species such that diffusion is the rate-limiting step in remobilization of Tc.« less

Authors:
ORCiD logo [1];  [2]; ORCiD logo [1]; ORCiD logo [3]; ORCiD logo [1]; ORCiD logo [4]; ORCiD logo [1]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  2. Sandia National Laboratory, Carlsbad, NM (United States)
  3. Oregon Health & Science Univ., Portland, OR (United States)
  4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Environmental Management (EM); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
OSTI Identifier:
1528900
Grant/Contract Number:  
AC02-05CH11231; SC0001376; AC06-76RL01830
Resource Type:
Accepted Manuscript
Journal Name:
ACS Earth and Space Chemistry
Additional Journal Information:
Journal Volume: 2; Journal Issue: 6; Journal ID: ISSN 2472-3452
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; technetium sulfide; radioactive waste; redox reactivity; X-ray absorption spectroscopy; environmental mobility; cementitious waste forms

Citation Formats

Pearce, Carolyn I., Icenhower, Jonathan P., Asmussen, R. Matthew, Tratnyek, Paul G., Rosso, Kevin M., Lukens, Wayne W., and Qafoku, Nikolla P. Technetium Stabilization in Low-Solubility Sulfide Phases: A Review. United States: N. p., 2018. Web. doi:10.1021/acsearthspacechem.8b00015.
Pearce, Carolyn I., Icenhower, Jonathan P., Asmussen, R. Matthew, Tratnyek, Paul G., Rosso, Kevin M., Lukens, Wayne W., & Qafoku, Nikolla P. Technetium Stabilization in Low-Solubility Sulfide Phases: A Review. United States. doi:10.1021/acsearthspacechem.8b00015.
Pearce, Carolyn I., Icenhower, Jonathan P., Asmussen, R. Matthew, Tratnyek, Paul G., Rosso, Kevin M., Lukens, Wayne W., and Qafoku, Nikolla P. Tue . "Technetium Stabilization in Low-Solubility Sulfide Phases: A Review". United States. doi:10.1021/acsearthspacechem.8b00015. https://www.osti.gov/servlets/purl/1528900.
@article{osti_1528900,
title = {Technetium Stabilization in Low-Solubility Sulfide Phases: A Review},
author = {Pearce, Carolyn I. and Icenhower, Jonathan P. and Asmussen, R. Matthew and Tratnyek, Paul G. and Rosso, Kevin M. and Lukens, Wayne W. and Qafoku, Nikolla P.},
abstractNote = {We report that technetium contamination remains a major environmental problem at nuclear reprocessing sites, e.g., the Hanford Site, Washington, USA. At these sites, Tc is present in liquid waste destined for immobilization in a waste form or has been released into the subsurface environment. The high environmental risk associated with Tc is due to its long half-life (214 000 years) and the mobility of the oxidized anionic species Tc(VII)O4-. Under reducing conditions, TcO4- is readily reduced to Tc(IV), which commonly exists as a relatively insoluble and therefore immobile, hydrous Tc-oxide (TcO2·nH2O). The stability of Tc(IV) sequestered as solid phases depends on the solubility of the solid and susceptibility to reoxidation to TcO4-, which in turn depend on the (biogeo)chemical conditions of the environment and/or nuclear waste streams. Unfortunately, the solubility of crystalline TcO2 or amorphous TcO2·H2O is still above the maximum contaminant level (MCL) established by the U.S. EPA (900 pCi/L), and the kinetics of TcO2 oxidative dissolution can be on the order of days to years. In addition to oxygen, sulfur can form complexes that significantly affect the adsorption, solubility, and reoxidation potential of Tc, especially Tc(IV). The principal technetium sulfides are TcS2 and Tc2S7, but much less is known about the mechanisms of formation, stabilization, and reoxidation of Tc-sulfides. A common assumption is that sulfides are less soluble than their oxyhydrous counterparts. Determination of the molecular structure of Tc2S7 in particular has been hampered by the propensity of this phase to precipitate as an amorphous substance. Recent work indicates that the oxidation state of Tc in Tc2S7 is Tc(IV), in apparent contradiction to its nominal stoichiometry. Technetium is relatively immobile in reduced sediments and soils, but in many cases the exact sink for Tc has not been identified. Experiments and modeling have demonstrated that both abiotic and biologic mechanisms can exert strong controls on Tc mobility and that Tc binding or uptake into sulfide phases can occur. These and similar investigations also show that extended exposure to oxidizing conditions results in transformation of sulfide-stabilized Tc(IV) to a Tc(IV)O2-like phase without formation of measurable dissolved TcO4-, suggesting a solid-state transformation in which Tc(IV)-associated sulfide is preferentially oxidized before the Tc(IV) cation. This transformation of Tc(IV)-sulfides to Tc(IV)-oxides may be the main process that limits remobilization of Tc as Tc(VII)O4-. The efficacy of the final waste form to retain Tc also strongly depends on the ability of oxidizing species to enter the waste and convert Tc(IV) to Tc(VII). Finally, many waste form designs are reducing (e.g., cementitious waste forms such as salt stone) and, therefore, attempt to restrict access of oxidizing species such that diffusion is the rate-limiting step in remobilization of Tc.},
doi = {10.1021/acsearthspacechem.8b00015},
journal = {ACS Earth and Space Chemistry},
number = 6,
volume = 2,
place = {United States},
year = {2018},
month = {5}
}

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