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 CO2
Respiration
Weathering of carbonate minerals
Carbonate speciation is pH dependent
Why is this important? U(VI) has strong affinity for CO32-

3. UO22+ aqueous speciation in carbonate solutions
Utot = 1 M, PCO2 = bar, 25 oC

4. Influence of dissolved carbonate on U(VI) sorption: ferrihydrite
U(VI) adsorption on ferrihydrite
From Waite et al. (1994)
Adsorption edges at:
low pH (4-5)
high pH (8-9)
pH < 5 UO22+ dominant
pH 5-8 Hydroxyl species
pH >8 Carbonate species
Uranyl carbonate complexes
have low sorption affinity

5. How does U(VI) interact with calcium carbonate?
Potential binding sites at surface CO3 groups
Calcium carbonate is moderately soluble (Ca2+, CO32-)
Dissolved CO32- stabilizes aqueous uranyl complexes
Calcite (R3c)
Aragonite (Pmcn)

6. Calcium carbonate-saturated solutions
Total dissolved carbonate (and Ca) depend on pH and PCO2

7. U(VI) aqueous speciation in calcium carbonate systems
Formation of Ca2UO2(CO3)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 CaCO3 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. Mechanisms of Metal Uptake at the Mineral-Water Interface
Surface
precipitation
Adsorption
Co-precipitation

11. Experiment: Characterize U(VI) sorbed at calcite surface in situ using EXAFS and luminescence spectroscopies
U(VI) Sorption Isotherm on Calcite
Experimental conditions for sorption experiment
Calcite: surface area ~10 m2/g (~2 m size)
Calcite suspension pre-equilibration:
log P(CO2) = -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 CO3)
Sorption equilibration – 24, 48, 72 h
Wet pastes extracted for EXAFS, luminescence
Ca surface sites
pH 8.3

12. U(VI) Solubility Limits near Calcite Saturation (pH 8.3)
Initial calcite saturation
Calcite saturation maintained
(UO2CO3)

13. Selected EXAFS Results for U(VI) Sorption on Calcite (pH 8.3)4-
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

14. Intensity (cps) Time (msec)
Time-resolved luminescence
spectroscopy:
Single uranyl species at
lowest U concentration
Additional species appears
at higher U concentrations
10 M U(VI)
Blue: single exp.
 = 150 ± 20 s
Black: double exp.
1 = 580 ± 240 s
2 = 125 ± 30 s
100
Intensity (cps) 50
Decay kinetics:
Best fit with two
exponentials
0.5
1.0
1.5
2.0
2.5
Time (msec)

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

16. Identification of “delayed gate” spectrum
This species resembles aqueous UO2(CO3)34-
Possibly sorbed Ca2UO2(CO3)3

17. Identification of “short-gate” spectrum
Short-gate species resembles the UO2-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

21. Time-resolved Luminescence Spectroscopy of CaCO3 Phases
Exc. 420 nm
LN2
Different uranyl species in
polycrystalline calcite and aragonite
Both exhibit single exponential
decay kinetics
Single uranyl species in each