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U(VI) interactions with carbonates: Spectroscopic studies Richard J. Reeder Department of Geosciences and Center for Environmental Molecular Science State.

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Presentation on theme: "U(VI) interactions with carbonates: Spectroscopic studies Richard J. Reeder Department of Geosciences and Center for Environmental Molecular Science State."— Presentation transcript:

1 U(VI) interactions with carbonates: Spectroscopic studies Richard J. Reeder Department of Geosciences and Center for Environmental Molecular Science State University of New York at Stony Brook Collaborators: E. Elzinga, D. Tait, D. Morris Support from NSF, DOE, Actinide Facility at ANL

2 Dissolved carbonate in environmental solutions Derived from: Atmospheric CO 2 Respiration Weathering of carbonate minerals Why is this important? U(VI) has strong affinity for CO 3 2- Carbonate speciation is pH dependent

3 UO 2 2+ aqueous speciation in carbonate solutions U tot = 1  M, P CO 2 = bar, 25 o C

4 From Waite et al. (1994) U(VI) adsorption on ferrihydrite Influence of dissolved carbonate on U(VI) sorption: ferrihydrite pH < 5 UO 2 2+ dominant pH 5-8 Hydroxyl species pH >8 Carbonate species Adsorption edges at: low pH (4-5) high pH (8-9) Uranyl carbonate complexes have low sorption affinity

5 How does U(VI) interact with calcium carbonate? Potential binding sites at surface CO 3 groups Calcium carbonate is moderately soluble (Ca 2+, CO 3 2- ) Dissolved CO 3 2- stabilizes aqueous uranyl complexes Aragonite (Pmcn) Calcite (R3c)

6 Calcium carbonate-saturated solutions Total dissolved carbonate (and Ca) depend on pH and P CO2

7 U(VI) aqueous speciation in calcium carbonate systems Formation of Ca 2 UO 2 (CO 3 ) 3 (aq) species favored in calcite- equilibrated solutions (Bernhard et al., 1996, 2001)

8 U(VI) in Calcium Carbonate Phases Up to 1 wt.% U(VI) in calcite formed in leach tests of Portland cement-type grout (Fuhrmann et al., 2005) U(VI) in calcite formed in Hanford subsurface associated with releases of uranium waste (Wang et al., 2005) Synthetic U(VI) co-precipitation samples contain up to 1 wt.% U (Reeder et al., 2000) Natural CaCO 3 minerals contain up to 300 ppm U (IV, VI)

9 Importance of Uranium Uptake by Carbonates Geochemical tracers (petrogenesis, diagenesis) Proxy for paleo-climate, paleo-ocean chemistry Role in geochemical cycles Potential for sequestration Calcite is a highly effective sorbent for many metals.

10 Adsorption Co-precipitation Surface precipitation Mechanisms of Metal Uptake at the Mineral-Water Interface

11 Experiment: Characterize U(VI) sorbed at calcite surface in situ using EXAFS and luminescence spectroscopies Experimental conditions for sorption experiment Calcite: surface area ~10 m 2 /g (~2  m size) Calcite suspension pre-equilibration: log P(CO 2 ) = -3.5, 20–22 ºC, 4 weeks pH 7.4–8.3, I = – m Total U(VI): 5  M–5 mM (added w/ and w/o CO 3 ) Sorption equilibration – 24, 48, 72 h Wet pastes extracted for EXAFS, luminescence U(VI) Sorption Isotherm on Calcite pH 8.3 Ca surface sites

12 Calcite saturation maintained Initial calcite saturation U(VI) Solubility Limits near Calcite Saturation (pH 8.3) (UO 2 CO 3 )

13 Selected EXAFS Results for U(VI) Sorption on Calcite (pH 8.3) Two types of EXAFS spectra (as seen in FT magnitude): Total U(VI)  500  M – single but broad equatorial peak Total U(VI)  500  M – split equatorial peaks 4-

14 Intensity (cps) Time (msec) 10  M U(VI) Blue: single exp.  = 150 ± 20  s Black: double exp.  1 = 580 ± 240  s  2 = 125 ± 30  s Time-resolved luminescence spectroscopy: Single uranyl species at lowest U concentration Additional species appears at higher U concentrations Decay kinetics: Best fit with two exponentials

15 Resolution of component spectra using short and delayed “gates” Distinct spectra indicate at least two uranyl species present

16 This species resembles aqueous UO 2 (CO 3 ) 3 4- Possibly sorbed Ca 2 UO 2 (CO 3 ) 3 Identification of “delayed gate” spectrum

17 Identification of “short-gate” spectrum Short-gate species resembles the UO 2 -doped calcite U(VI) possibly coprecipitated during sorption

18 What about U(VI) in Natural Calcium Carbonate Samples? 3 cm Calcite speleothem, N. Italy (300 ppm U) XRD, FTIR – only calcite in yellow band Time-resolved luminescence Double exponential decay kinetics  two uranyl species Long gate – aragonite-like species Short gate – calcite-like species

19 What can we conclude ? At U(VI) < 10  M, uranyl carbonate complex adsorbs on calcite surface At U(VI) = 10 – 500  M, multiple sorbed uranyl species exist at calcite surface: One sorbed species is uranyl triscarbonate-like Other may be a coprecipitate At U(VI) > 500  M, a surface precipitate forms Presence of multiple species may result in U(VI) retention with multi-phase behavior/kinetics Differences in experimental conditions for co-precipitation result in different local coordination of uranyl species. The use of complementary techniques (EXAFS and time-resolved luminescence) may provide better chance for characterizing complex environmental systems

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21 Different uranyl species in polycrystalline calcite and aragonite Both exhibit single exponential decay kinetics Single uranyl species in each Exc. 420 nm LN2 Time-resolved Luminescence Spectroscopy of CaCO 3 Phases


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