Molecular Mechanisms of Mineral-Water Interface Processes Affecting Uranium Fate

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. 33A 53A 61A 67A BX Tank Farm: Uranyl Silicate Precipitates McKinley et al. (2007) Vadose Zone J.6, 1004–1017 Catalano et al. (2004) ES&T38, 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 Earth and Planetary Sciences • Washington University Turner et al. (1996) GCA

8. U(VI) Adsorption Mechanisms: Surface Complexation and Cation Exchange pH 4 1 mM NaNO3 B A A B B 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? Earth and Planetary Sciences • Washington University Catalano and Brown (2005) GCA69, 2995–3005

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 Earth and Planetary Sciences • Washington University Catalano and Brown (2005) GCA

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 Atm. CO2 CO2-free C 2.9 Å MS 4.15 Å pH 7, 1 M NaNO3 Catalano and Brown (2005) GCA Bargar et al. (2000) 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 Data from: Rossberg et al. (2009) ES&T Figure from: Hiemstra et al. (2009) GCA • 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 Earth and Planetary Sciences • Washington University

14. Spectroscopic Evidence for Formation of Uranyl Phosphate Ternary Surface Complexes Possible U(VI)-phosphate ternary surface complexes Singh et al. (2012) submitted to ES&T Unpublished data • 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 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 Earth and Planetary Sciences • Washington University Singh et al. (2012) submitted to ES&T

16. Importance of Ternary Surface Complexes in U(VI) Mineral Nucleation U(VI)-Phosphate Nucleation on Goethite Singh et al. (2010) GCA74, 6324–6343 UranylSilicate ‘Synthons’ in Solution Soderholm et al. (2008) GCA72, 140-150 U(VI)-Silicates in Waste-Impacted Sediments McKinley et al. (2006) GCA70, 1873–1887 • 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 Earth and Planetary Sciences • Washington University

17. Uranium Incorporation/Coprecipitation Figures from: Stewart et al. (2009) ES&T; Reeder et al. (2004) GCA • 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 Earth and Planetary Sciences • Washington University

18. Element Repartitioning during Mineral Recrystallization Frierdich et al. (2011) Geology Frierdich and Catalano (2012) ES&T • 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 Earth and Planetary Sciences • Washington University

19. 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