SLIDE 1 Reduction-Oxidation Reactions of 99Tc in Subsurface Sediments
Jim Fredrickson1, John Zachara1, Steve Heald2, Jim McKinley1, Tanya Peretyazhko1, Andy Plymale1, Chongxuan Liu1, Ravi Kukkadapu1, and Ponnusamy Nachimuthu1
1Pacific Northwest National Laboratory, Richland, WA 2Argonne National Laboratory, Argonne, IL
ERSP PI Meeting April 22, 2009
PNNL-SA-65891
SLIDE 2
- I. Tc(VII) reduction and Tc(IV) oxidation reactions in sediments
with a lab-generated biogeochemical Fe(II) fraction
ORNL FRC (FRC) & Hanford Ringold (RG) Fe and Tc spatial location and speciation Relative rates and controls Mineralogic influences
- II. New experiments with anoxic, Pliocene Hanford sediments
Tc(VII) reaction with the natural Fe(II) pool
2
Presentation Content
SLIDE 3 Technetium
Fission product of 235Uranium ~ 1990 kg produced at Hanford (1943-1987); world-wide inventory ~ 290 MT Exists in oxidation states +7 to -1 Highly mobile as Tc(VII); pertechnetate ion TcO4
- Biologically reactive (sulfate analogue)
Microbial reduction to poorly soluble Tc(IV) (widespread)
235Uranium 99Technetium
Fission
T1/2 = 7.0 x 108 years
2.1 x 105 years
SLIDE 4 Solubility of TcO2•nH2O
TcO(OH)°
2(aq)
16,784 pCi/L 1,678 pCi/L 168 pCi/L Concentration of Tc(IV) fixed by solubililty at reduction point Ionic radii and structural similarities suggest coprecipitation with Fe(III) possible Downgradient adsorption of Tc(IV) complexes or another reaction essential to reach MCL (900 pCi/L) Adsorption behavior of TcO(OH)2°(aq) unknown MCL 900 pCi/L
Tc(VII)O4
- + 4H+ + 3e- = Tc(IV)O2nH2O(s) + (2-n)H2O Eo = 0.748 V
[Cr(VI)O4
2- + 5H+ + 3e- = Cr(III)(OH)3(s) + H2O Eo = 1.34 V]
Tc(VII)O4
- + 3Fe2+ + (n+7)H2O = Tc(IV)O2nH2O(s) + 3Fe(OH)3(s)+ 5H+
SLIDE 5 Kinetic Pathways for Tc(VII) Reduction and Tc(IV) Oxidation
Reduction (Fe2+ or microbial) Tc(VII)O4
Oxidation (+ O2, Mn3/4+, or MOB) biologic (e.g., MRB) + homogeneous Fe(II)aq heterogeneous Fe(II)OH Fe(II) Tc(IV)
- speciation
- physical location
Fe(III) oxide
TcO4 t = kbio[ ] + khomo[ ] + khet1[ ] + khet2[ ] Tc(IV) t = kbio[ ] + khomo[ ] + khet1[ ] + khet2[ ]
medium; electron donor dependent very slow very fast medium
SLIDE 6
Oxidation of Biogenic Tc(IV)O2•nH2O
SLIDE 7 Experimental Protocol for Heterogeneous Reduction and Oxidation Experiments with Sediment
Questions: Which mineral phases facilitate Tc(VII) reduction in bioreduced sediment? How does Tc(IV) molecular speciation and mineral association effect oxidation rate?
Bioreduced sediment + Tc(VII) Bioreduced Tc- containing sediment + O2 Single mineral particle isolates – mica hypothesis Heterogeneous reduction rate Fe valence/speciation (TMS) Tc valence and molecular speciation (XAS) Tc spatial distribution (XRM, SEM) Heterogeneous oxidation rate and recalcitrant fraction Post oxidation Fe valence/ speciation (TMS) Valence of Tc (μ-XANES) Spatial nature and elemental association of
(XRM, SEM) Mica hypothesis Elemental associations with Tc (XRM, SEM) Particle specific speciation (μ-EXAFS, bulk EXAFS) Quantitative particle composition (EMP) Particle structure (μ-XRD)
SLIDE 8 5 K Mössbauer Spectra of ORNL FRC Sediment
Velocity (mm/s) Intensity (counts/channel), arbitrary units
Pristine Bioreduced
Bioreduction increases phyllosilicate Fe(II) and decreases goethite Fe(III)
Kukkadapu et al. 2006
SLIDE 9 Heterogeneous Reduction and Oxidation of Tc in Bioreduced Sediments
Reaction with bioreduced sediment Oxidation by atmospheric O2
Fredrickson et al., 2004, 2009
SLIDE 10
EXAFS Interpretation Involves Various Tc(IV)O2 Models
Long chains: Abiotic and biotic TcO2•nH2O, heterogeneously reduced Tc(IV) in sediment Dimers and trimers coordinated to Fe-O with diffuse Fe scattering: Heterogeneously reduced Tc(IV) on phyllosilicates (FRC) and diaspore/ corundum Monomers and dimers coordinated to Fe-O with more intense Fe scattering: Homogeneous Tc(IV); heterogeneous Tc(IV) on goethite/ hematite, and magnetite; biotransformation products of ferrihydrite; Tc(IV)- ferrihydrite; and Tc(IV)-celadonite Tc-Tc Tc-Fe
Dimeric surface complex
[Tc]aq
SLIDE 11 Bulk EXAFS Analyses of Tc(IV) Resulting from Heterogeneous Reduction by Biogenic Fe(II)
Peretyazhko et al. 2008; Fredrickson et al. 2009
SLIDE 12
XRM Mapping of Bioreduced Tc(VII)-Reacted FRC Sediment
SLIDE 13
Micro-XANES Analyses of Tc Hot-Spots in Oxidized FRC Thin Section
SLIDE 14 Isolated 50-100 μm Particles from Oxidized Tc- Containing FRC Sediment for Micro-Analyses
1 2 3 4 5 6 7 8 9 10 11 12 16 17 18 13 14 15 25 26 27 22 23 24 19 20 21
Number Key for Sample FRC 4M Washed Probable Other
SLIDE 15 XRM Analyses of Tc-Containing Particles from Oxidized FRC Sediment
a.) Particle #7 b.) Particle #15 c.) Particle #13
Fe Fe Fe Rb Rb Tc Tc Tc Rb
Fe Rb Tc
SLIDE 16
Micro-EXAFS of Isolated Particles Containing Oxidation Resistant Tc(IV)
SLIDE 17
BSE, XRM, and EMP Analyses of Tc-particle #7
SLIDE 18
Micro-XRD of Oxidation Resistant Tc(IV) Particles is Consistent With Celadonite
#7 #11 #27
SLIDE 19
XRM of 0.2 mm FRC Tc(IV)-Containing Mica
SLIDE 20
Key Findings
Heterogeneous reduction products similar in both sediments, but red/ox rates differ by 10x Initial heterogeneous Tc(IV) speciation is dominated by clusters of TcO2•nH2O Diffuse intra-aggregate Tc(IV) oxidizes slowly in FRC, while localized Tc(IV) is recalcitrant (no recalcitrance in RG) Oxidation resistant Tc(IV) associates with the surface of 50-100 μm celadonite particles The EXAFS spectra of oxidation resistant Tc(IV) is similar to that of dimeric surface complexes on goethite and hematite, and high Fe(II) ferrihydrite with unresolved implications
SLIDE 21 59-60’ 60-61’ 101-102’ 128-129’ 155-156’ 169-170’
Ringold Formation Sediments from Hanford’s Unconfined Aquifer (C6209)
SLIDE 22
Tc(VII) Reduction: Constant Fe(II) [all C6209 samples, Zachara SFA poster]
SLIDE 23 Developing rigorous models for redox reactivity of (bio)geochemical Fe(II) forms challenging in sediments
Difficult to establish identity, concentrations, and unique
thermodynamic properties of both reactants and products
A range of Fe(II) forms with variable properties often exists Complex biologic and chemical linkages
Coupling between chemical, biological, and physical environments
Mass transfer as a control on oxidant fluxes Surface and intragrain environments (“microenvironments”) Interaction between surface and bulk mineral redox properties
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Summary
