Oxidative dissolution potential of biogenic and abiogenic TcO{sub 2} in subsurface sediments.

Journal Article · Wed Apr 01 04:00:00 EDT 2009 · Geochim. Cosmochim. Acta

DOI:https://doi.org/10.1016/j.gca.2009.01.027· OSTI ID:1008274

Fredrickson, J K; Zachara, J M; Plymale, A E; Heald, S M; McKinley, J P; Kennedy, D W; Liu, C; Nachimuthu, P ^[1]

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1. X-Ray Science Division

Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Tc is highly mobile in its most oxidized state [Tc(VII)O{sub 4}{sup -}] and less mobile in the reduced form [Tc(IV)O{sub 2} {center_dot} nH{sub 2}O]. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state; unconsolidated Pliocene-age fluvial sediment from the upper Ringold (RG) Formation at the Hanford Site and a clay-rich saprolite from the Field Research Center (FRC) background site on the Oak Ridge Site. Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe-phyllosilicates of different types. Shewanella putrefaciens CN32 reduced Mn(III/IV) oxides and generated Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. After bioreduction and heat-killing, biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO{sub 2}-like phase in both sediments. The biogenic redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular scale speciation of Tc compared to RG. X-ray microprobe, electron microprobe, X-ray absorption spectroscopy, and micro X-ray diffraction were applied to the whole sediment and isolated Tc-containing particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50-100 {micro}m-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O{sub 2} into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)-Fe(III) molecular species.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: SC; ERSP; ODER
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1008274
• Report Number(s): ANL/XSD/JA-63524
• Journal Information: Geochim. Cosmochim. Acta, Journal Name: Geochim. Cosmochim. Acta Journal Issue: 8 ; Apr. 2009 Vol. 73; ISSN 0016-7037
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
ABSORPTION SPECTROSCOPY
AIR
DISSOLUTION
ELECTRONS
FISSION PRODUCTS
MINERALOGY
NUCLEAR FUELS
OXIDATION
OXIDES
RADIOACTIVE WASTES
REPROCESSING
SEDIMENTS
TECHNETIUM 99
X-RAY DIFFRACTION