Molecular Mechanisms of Mineral-Water Interface Processes Affecting Uranium Fate

Jeffrey G. Catalano

Acknowledgements

Earth and Planetary Sciences • Washington University

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.)

Financial SupportUS DOE Office of Biological & Environmental ResearchPacific Northwest National LaboratoryNational Science FoundationWashington University

Collaborators

BX Tank Farm: 7500 kg uranium released in 1951

300 Area Process Ponds: 58,000 kg of uranium discharged in pond

Examples of U Contamination at the Hanford Site

Earth and Planetary Sciences • Washington University

BX Tank Farm: Uranyl Silicate Precipitates

■ 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

33A

53A

61A

67A

Catalano et al. (2004) ES&T 38, 2822–2828McKinley et al. (2007) Vadose

Zone J. 6, 1004–1017

300 Area Ponds: Complex Mixture of U(VI) Species

Earth and Planetary Sciences • Washington University

Catalano et al. (2006) ES&TU(VI) in calcite, U(VI) sorbed

to clays and minor uranyl phosphates

Stubbs et al. (2009) GCAU(VI) sorbed on clays, iron

oxides, and amorphous silicates, copper uranyl

phosphate, Zr-oxide phase

Singer et al. (2009) ES&TU(VI) sorbed on chlorite, Cu-U-phosphate, minor

Cu-U-silicate

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 uncertaintyEarth and Planetary Sciences • Washington University

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 UniversityTurner et al. (1996) GCA

■ 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?

A

B

U(VI) Adsorption Mechanisms: Surface Complexation and Cation Exchange

Earth and Planetary Sciences • Washington University

pH 41 mM NaNO3

pH 71 M NaNO3

A

B

B

Catalano and Brown (2005) GCA 69, 2995–3005

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

Corner-Sharing U(VI) Surface Complexes on Hematite Single Crystals

■ 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

a-Fe2O3

Catalano and Brown (2005) GCA

Recent EXAFS Evidence of Corner-Sharing Complexes

■ 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

Corner and Edge Sharing ComplexesSingh et al., submitted to ES&T

Corner Sharing ComplexesSherman et al. (2008) GCA

U(VI)-Carbonate Ternary Surface Complexation

■ 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

Atm. CO2

CO2-free

pH 7, 1 M NaNO3

C2.9 Å

MS4.15 Å

Catalano and Brown (2005) GCABargar et al. (2000) GCA

>(FeOH)2UO2

>FeOCO2UO2(CO3)2

Alternative Conclusion on Carbonate Ternary Complexes

■ 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

fCO2: ~0 10-3.5 10-2

UO

2(CO

3) 34-

pH: 7.9 5.6 5.5 5.8 5.8 7.9 5.5 7.0 6.8

>(FeOH)2UO2

>FeOCO2UO2(CO3)2

Data from: Rossberg et al. (2009) ES&TFigure from: Hiemstra et al. (2009) GCA

Spectroscopic Evidence for Formation of Uranyl 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

Earth and Planetary Sciences • Washington UniversitySingh et al. (2012) submitted to ES&TUnpublished data

Possible U(VI)-phosphate ternary surface complexes

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 UniversitySingh et al. (2012) submitted to ES&T

Importance of Ternary Surface Complexes in U(VI)

Mineral Nucleation■ 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

Uranyl Silicate ‘Synthons’ in SolutionSoderholm et al. (2008) GCA 72, 140-150

U(VI)-Silicates in Waste-Impacted SedimentsMcKinley et al. (2006) GCA 70, 1873–1887

U(VI)-Phosphate Nucleation on GoethiteSingh et al. (2010) GCA 74, 6324–6343

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

Earth and Planetary Sciences • Washington UniversityFigures from: Stewart et al. (2009) ES&T; Reeder et al. (2004) GCA

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

Earth and Planetary Sciences • Washington UniversityFrierdich et al. (2011) GeologyFrierdich and Catalano (2012) ES&T

Earth and Planetary Sciences • Washington University

Summary: U(VI) Shows Complex Sorption Behavior in Contaminated Sediments

■ 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

Binary Adsorption Ternary Adsorption Nucleation, Growth, and Incorporation