1. Molecular Mechanisms of Mineral-Water Interface Processes Affecting Uranium Fate
Jeffrey G. Catalano

2. Acknowledgements Collaborators Financial Support
Gordon E. Brown, Jr. (Stanford, SSRL)
John M. Zachara (PNNL)
James P. McKinley (PNNL)
Zheming Wang (PNNL)
Steve M. Heald (PNC/XSD, APS)
Thomas P. Trainor (U. Alaska)
Peter J. Eng (GSECARS)
Glenn A. Waychunas (LBNL)
Daniel E. Giammar (Wash. U.)
Abhas Singh (Wash. U.)
Kai-Uwe Ulrich (Wash. U.)
US DOE Office of Biological & Environmental Research
Pacific Northwest National Laboratory
National Science Foundation
Washington University
Earth and Planetary Sciences • Washington University

3. Examples of U Contamination at the Hanford Site
BX Tank Farm: 7500 kg uranium released in 1951
300 Area Process Ponds: 58,000 kg of uranium discharged in pond
Earth and Planetary Sciences • Washington University

4. BX Tank Farm: Uranyl Silicate Precipitates
McKinley et al. (2007) Vadose Zone J. 6, 1004–1017
Catalano et al. (2004) ES&T 38, 2822–2828
Na-boltwoodite [(Na,K)UO2SiO3OH·1.5H2O] or a related phase occurs as microprecipitates in grain fractures
Likely formed through reactions of alkaline waste fluids with clays and amorphous silica in fractures
Earth and Planetary Sciences • Washington University

5. 300 Area Ponds: Complex Mixture of U(VI) Species
Stubbs et al. (2009) GCA
U(VI) sorbed on clays, iron oxides, and amorphous silicates, copper uranyl phosphate, Zr-oxide phase
Catalano et al. (2006) ES&T
U(VI) in calcite, U(VI) sorbed to clays and minor uranyl phosphates
Singer et al. (2009) ES&T
U(VI) sorbed on chlorite, Cu-U-phosphate, minor Cu-U-silicate
Earth and Planetary Sciences • Washington University

6. A Complex Array of Adsorption and Precipitation Processes Occur in Oxic Contaminated Systems
U in oxic contaminated systems occurs in a complex array of adsorbed and precipitated forms
Adsorbed to clays and oxide minerals, often as ternary complexes
Precipitated as insoluble phases, especially phosphate and silicate
Incorporated into other precipitates or existing phases
Such processes and their underlying mechanisms are well studied, but recent work has shown surprising observations and identified areas of continued uncertainty
Earth and Planetary Sciences • Washington University

7. U(VI) Adsorption Mechanisms: Surface Complexation and Cation Exchange
Cation exchange generally assumed to be important only at low pH and low ionic strength because of strong U(VI) surface complexation
Turner et al. (1996) GCA
Earth and Planetary Sciences • Washington University

8. U(VI) Adsorption Mechanisms: Surface Complexation and Cation ExchangeB A B
pH 4
1 mM NaNO3
pH 7
1 M NaNO3
Substantial cation exchange occurs on smectites in dilute electrolytes at circumneutral pH
Origin of enhanced cation exchange is unclear:
Exchange of positive hydrolysis products?
Feedback between interlayer hydration and exchange coefficients?
Catalano and Brown (2005) GCA 69, 2995–3005
Earth and Planetary Sciences • Washington University

9. U(VI) Adsorption Mechanisms: Multiple Inner-Sphere U(VI) Complexation Geometries
Manceau et al. (1992) and Waite et al. (1994) were the first to spectroscopically characterize the adsorption configuration of U(VI) on a mineral surface, finding an edge-sharing complex
Identification of this complex geometry relied primarily on a 2nd-shell feature in the EXAFS spectrum, but these studies were unaware of the importance of multiple scattering contributions to this feature
When later studies accounted for multiple scattering the number of Fe neighbors is generally <1, suggesting other surface complexes might exist
Earth and Planetary Sciences • Washington University

10. Corner-Sharing U(VI) Surface Complexes on Hematite Single Crystals
a-Fe2O3
Surface X-ray scattering methods show that on a specific hematite surface U(VI) is sorbed dominantly as a corner-sharing complex
Poor sensitivity of EXAFS to 2nd shell neighbors >4 Å biases analyses against detection of corner-sharing complexes
Importance of corner-sharing complexes likely substantially underestimated
Catalano and Brown (2005) GCA
Earth and Planetary Sciences • Washington University

11. Recent EXAFS Evidence of Corner-Sharing Complexes
Corner and Edge Sharing Complexes
Singh et al., submitted to ES&T
Corner Sharing Complexes
Sherman et al. (2008) GCA
Recent collaborative work investigating U(VI) adsorption on goethite in absence and presence of PO4 shows clear signature of both corner and edge-sharing complexes
One prior study concluded only corner sharing complexes form and that MS at times masked the Fe neighbor signal
Earth and Planetary Sciences • Washington University

12. U(VI)-Carbonate Ternary Surface Complexation
2.9 Å MS
4.15 Å
Atm. CO2
CO2-free
pH 7, 1 M NaNO3
Bargar et al. (2000) GCA
Catalano and Brown (2005) GCA
Presence of CO2 clearly alters the structure of U(VI) surface complexes on hematite and smectite
Evidence for U(VI)-carbonate ternary complexes comes from changes in EXAFS spectra in presence of CO2 and complementary IR work
Some workers have pointed out that the EXAFS observations may be problematic because ternary complexation was seen at low pH
Earth and Planetary Sciences • Washington University

13. Alternative Conclusion on Carbonate Ternary Complexes
>(FeOH)2UO2
>(FeOH)2UO2
>FeOCO2UO2(CO3)2
fCO2: ~0
10-3.5
10-2
>FeOCO2UO2(CO3)2
UO2(CO3)34-
pH:
7.9
5.6
5.5
5.8
5.8
7.9
5.5
7.0
6.8
Rossberg et al. (2009) applied statistical methods to analyze a series of EXAFS spectra of samples with different pH and fCO2
Found two components: Binary U(VI) complex, Ternary U(VI) triscarbonato complex
Single or double carbonate ternary complexes not found
Alters picture for thermodynamic modeling, but this set of species may not be a unique explanation of the data
Data from: Rossberg et al. (2009) ES&T
Figure from: Hiemstra et al. (2009) GCA
Earth and Planetary Sciences • Washington University

14. Possible U(VI)-phosphate ternary surface complexes
Spectroscopic Evidence for Formation of Uranyl Phosphate Ternary Surface Complexes
Possible U(VI)-phosphate ternary surface complexes
Macroscopic adsorption studies suggest that U(VI)-phosphate ternary complexes dominate up to pH 8 in PO4-bearing systems
Spectral changes in the presence of PO4 are consistent with the formation of such ternary complexes but structure still unknown
Dominance of ternary complexes over binary U(VI) complexes under circumneutral pH conditions cannot be verified by EXAFS spectroscopy
Singh et al. (2012) submitted to ES&T
Unpublished data
Earth and Planetary Sciences • Washington University

15. Transition to Uranyl Phosphate Precipitation
U(VI)-phosphates form on goethite as U and P loadings increase
Occurs at circumneutral to weakly acidic pH conditions
Adsorbed U is persistent
EXAFS spectra of samples undersaturated with U(VI)- phosphates are best fit as a mixture of adsorbed and precipitated components
U(VI)-PO4 ternary complex is likely what actually forms
Phosphate geometry similar in ternary complex and precipitate
EXAFS may overestimate contribution of U(VI)-phosphates
Singh et al. (2012) submitted to ES&T
Earth and Planetary Sciences • Washington University

16. Importance of Ternary Surface Complexes in U(VI) Mineral Nucleation
U(VI)-Phosphate Nucleation on Goethite
Singh et al. (2010) GCA 74, 6324–6343
Clear evidence of U(VI)-phosphate heterogeneous nucleation on goethite at low degrees of supersaturation
Homogeneous nucleation seen at high degrees of supersaturation
U(VI)-silicate clusters assemble in solution prior to precipitation
‘Synthons’ act as precursor species
U(VI) precipitates associate with grain coatings in sediments
Shows that heterogeneous nucleation is an important process in real sediments
Specific mineral phases may control nucleation behavior by promoting U(VI) adsorption as ternary complexes
Uranyl Silicate ‘Synthons’ in Solution
Soderholm et al. (2008) GCA 72,
U(VI)-Silicates in Waste-Impacted Sediments
McKinley et al. (2006) GCA 70, 1873–1887
Earth and Planetary Sciences • Washington University

17. Uranium Incorporation/Coprecipitation
Studies have suggested that U(VI) and/or U(V) incorporates into iron oxides, most notably during the Fe(II)-activated phase transition of ferrihydrite to goethite or magnetite
Atomistic simulations suggest such incorporation can be stable
Extensive work has also shown substantial U(VI) incorporation can occur in calcite, aragonite, and Ca-phosphates
Extent of incorporation may relate to structural compatibility
Figures from: Stewart et al. (2009) ES&T; Reeder et al. (2004) GCA
Earth and Planetary Sciences • Washington University

18. Element Repartitioning during Mineral Recrystallization
Nominally stable mineral phases constantly dissolve and reprecipitate even at equilibrium
Generally faster for more soluble minerals
Fe2+(aq) causes crystalline iron oxides to recrystallize in <30 days
Mineral recrystallization has been shown to release trace elements, even from low-solubility oxides
Substantial quantities release from iron oxides in presence of Fe(II)
Uranium entrapment may be disrupted by recrystallization, especially during iron cycling
Frierdich et al. (2011) Geology
Frierdich and Catalano (2012) ES&T
Earth and Planetary Sciences • Washington University

19. Nucleation, Growth, and Incorporation
Summary: U(VI) Shows Complex Sorption Behavior in Contaminated Sediments
Binary Adsorption
Ternary Adsorption
Nucleation, Growth, and Incorporation
U(VI) displays complex modes of adsorption and frequently forms ternary surface complexes with common ligands
These complexes may be critical precursors to nucleation and could dictate the spatial distribution of precipitates in contaminated sediments
Such complexes also likely affect incorporation and release of uranium
Novel combinations of characterization tools are needed to resolve current uncertainties in the mechanisms involved in these processes
Earth and Planetary Sciences • Washington University