Speciation of Actinides Using XAFS

Speciation of Actinides Using XAFS Part I Tobias Reich Johannes Gutenberg-Universität Mainz Institut für Kernchemie Ringvorlesung des GRK Elementspeziation im SS 2006 Mainz, 4.9.2006

Outline Introduction Basics of XAS Pauling bond valence rules Application of micro-beam techniques to Pu speciation References

Introduction In environmental studies samples are often inhomogeneous, chemically diverse, and amorphous or poorly crystalline. Even surrogates prepared in the laboratory are plagued by multiple oxidation states and varied coordination polyhedra of the 5f elements. Plutonium can be found as Pu 3+, Pu 4+, Pu(V)O 2+, and Pu(VI)O 2 2+ within naturally occurring ph-eh conditions. Dissolved actinides have significant affinities for various mineral surfaces.

Introduction A molecular level understanding of the diverse chemistry of actinides is necessary for predictive modeling of the fate and transport of these biohazardous ions. Synchrotron studies are rapidly becoming a workhorse to chemically characterize actinide pollutants, their speciation, and their complexation at a molecular level.

Introduction Advantages of synchrotrons for actinide speciation: They provide high flux of tunable, high-energy radiation. Highly focused beams allow for small (<1 mg) sample sizes. The energy range permits excitation of the M- and L-edges of the actinides. The penetrating nature of X-rays allows sample encapsulation and containments. A wide variety of spectroscopy, scattering, and imaging experiments are available.

Introduction Most of the synchrotron studies published to date have focused on X-ray absorption spectroscopy (XAS). This technique has found widespread use as a speciation probe for several reasons: It is a single-ion probe that can be used to study one element from a complex mixture. It is sensitive to both the oxidation state and to the coordination environment of the ion. It can be used for solution, surface, or solid samples.

Introduction XAS has been able to answer a variety of important chemistry questions about actinide-ion speciation in solution and in the solid-state that have direct relevance to environmental problems. These studies will expand to include other X-ray techniques. More studies will focus on transuranics. Synchrotron radiation has a vast potential as a molecular-level probe.

Basics of XAS XAS is often artificially divided into two experiments: XANES - X-ray absorption near-edge structure EXAFS - Extended X-ray absorption fine structure. Absorption 1.6 1.4 XANES EXAFS Post-edge 1.2 1.0 Pre-edge 17000 17200 17400 17600 17800 18000 Energy (ev)

Basics of XAS XANES provides information about the valence of the central ion by comparison to standards of known oxidation state. Sample 2nd der. crossing (ev) Pu 3+ 18060.1 Pu 4+ 18063.2 Pu(V)O + 2 18062.6 Pu(VI)O 2+ 2 18064.8 Conradson et al., 1998 Representative XANES spectra as function of oxidation state and coordination environment (Antonio and Soderholm, 2006).

Basics of XAS XANES is primarily used for determining oxidation states. In specific situations it can be used to infer something about the absorbing ion s coordination environment. Detailed coordination information is determined by fitting the EXAFS oscillations above the absorption edge.

Basics of XAS All measurements of the EXAFS response are made as a function of incident X-ray energy, E, in electron volts. Absorption 1.6 1.4 1.2 1.0 17000 17200 17400 17600 17800 18000 Energy (ev)

Basics of XAS Following a number of data reduction treatments, the EXAFS signal,, is plotted as a function of the photoelectron wave vector k (Å -1 ), which is obtained according to k 0.263(E E 0 ) E 0 is the experimental energy threshold to define the energy origin of the EXAFS spectrum in k-space. k 3 6 1.0 FT-Amplitude U-O U-U 4 0.8 CaUO 4 2 0 0.6 U-Ca -2-4 -6 0.4 0.2 Experiment Fit 4 6 8 k (Å -1 ) 10 12 14 0.0 0 1 2 3 R + (Å) 4 5

Basics of XAS Non-linear least squares minimization techniques are applied to fit the EXAFS or Fourier transform (FT) data with a semiempirical, phenomenological model of shortrange, single-scattering according to (Teo, 1986): n exp 2k 2 2 i (k) S 0 2 N i F i (k,r i ) i 1 kr i 2 exp 2r i sin 2kr i i (k,r i ) 2 c (k) e (k) The backscattering amplitude, F i (k, r i ), and phase, i (k, r i ), as well as the central atom phase, c (k), are typically obtained from the general-purpose XAS ab initio code known as FEFF (de Leon et al., 1991) and used as input for the iterative refinement procedures.

n (k) S 0 2 N i F i (k,r i ) i 1 kr i 2 Basics of XAS exp 2k 2 2 i exp 2r i sin 2kr i i (k,r i ) 2 c (k) e (k) An overall amplitude reduction factor, S 02, is normally a fixed parameter too. The exponential term that includes the photoelectron mean free path, e (k), is not explicitly used for a standard analysis. The three principal structure parameters obtained from the fitting include the coordination number, N i, interatomic distance, r i, and the Debye-Waller factor, i, for each of the i-th scattering shells about the central absorbing ion out to about 4 Å. An energy scale ( E 0 ) parameter is also fitted to account for differences between the experimental and theoretical values of E 0.

Basics of XAS The number of parameters that can be determined from a data set, N p, depend on the k- and r-ranges over which the data are fit, and can be calculated according to (Teo, 1986): N p = 2 k r/. A rule-of-thumb error estimate is that the fitted distances are precise to about ±1% whereas the coordination numbers can be quoted to ±1.

Pauling Bond Valence Rules Correlations exist between the bond distance, coordination number, and oxidation state of a metal species, M n+. A quantitative, electrostatic model for coordination environments in complex ionic solids was proposed by L. Pauling (Pauling, 1929)

Pauling Bond Valence Rules Pauling concept of bond strength: The bond strength s is defined as the cation valence z divided by its coordination number. s z Pauling s 2nd rule - valence sum rule: The sum of the bond valances S ij to an ion in a stable coordination environment should equal the absolute value of the nominal valence V i of the ion. V i j S ij

Pauling Bond Valence Rules Empirical correlation between valance and length of a bond (Zachariasen, 1954) S ij exp R 0 R ij B where R ij is the distance between atom i and j. R 0 is an empirically derived parameter and has been tabulated for a variety of bond pairs (Brown, 2002). B is generally assumed to be very close to 37 pm.

Application of Bond Valence Concept to Uranium Ions Parameters R 0 and B for the hexavalent uranyl ion have been established from a wide range of X-ray single-crystal structural refinements (Burns et al., 1997). R 0 = 2.051 Å B = 0.519 Å The U-O eq distances can be compared with those obtained from fitting EXAFS data to support a determination of the equatorial coordination environment.

Application of Bond Valence Concept to Uranium Ions The bond valence parameters R 0 = 2.051 Å and B = 0.519 Å facilitate the recognition of U(IV), U(V), and U(V) cations in refined crystal structure (Burns et al., 1997). Example: U(UO 2 )(PO 4 ) 2 Site U-O (Å) Valence Sum U(1) 2.219 2.460 2.177 2.171 2.341 2.543 2.318 4.31 vu U(2) 1.764 1.767 2.267 2.419 2.362 2.561 2.573 5.91 vu The calculated bond valence sums are consistent with U(1) containing U 4+ and U(2) containing U 6+.

Advantage and Disadvantage of XAS Advantage: XAS is a single-ion probe that can be tuned to an energy that selects the absorption edge of an ion of interest, an essential attribute when studying an ion s speciation in chemically complex samples. Disadvantage: The spectrum that is obtained is the sum of all the oxidation states and coordination environments that occur in the sample. This can be particularly problematic for actinidecontaining samples.

Recent Developments to Assess and Overcome this Problem Synchrotron-based micro-x-ray fluorescence (µ- SXRF) and µ-xanes for examining inhomogeneous natural samples Example: Speciation of Pu adsorbed onto natural tuff (Duff et al., 1999) Application of principal component analysis (PCA) to XAS data (Wassermann, 1997; Rossberg et al., 2003) This mathematical approach provides an estimate of the number of species present in the sample.

Application of µ-srxf and µ-xanes to Speciation of Pu Adsorbed onto Tuff Study of the interaction of 10-6 M Pu(V) aqueous at ph 8.9 with altered tuff from the Yucca Mountain, Nevada high-level waste repository Among other minerals, this tuff contains 80 wt% clinoptilolite [(Na,K,Ca) 2 Al 3 (Al,Si) 2 Si 13 O 36 12H 2 O] and 2 wt% smectite [(Na,Ca)Al 4 (Si,Al) 8 O 20 (OH) 4 2H 2 O] coated with iron and manganese oxides.

Application of µ-srxf and µ-xanes to Speciation of Pu Adsorbed onto Tuff For the µ-srxf studies, the synchrotron hard X-ray fluorescence microprobe (beamline X26A) at NSLS (BNL) was used. Monochromatic beam at the Pu III absorption edge (18054 ev) was focused down to a 10 x 15 µm 2 beam, resulting in a total flux of about 10 10 photons s -1. Elemental mappings at several areas ranging from 140 x 140 µm 2 to 400 x 400 µm 2 in size. XANES spectra were obtained for two ~10 x 15 µm 2 regions.

In situ µ-srxf Analysis of Sorbed Pu on Tuff Hematite Mn oxide a) Photomicrograph of the Mn and Fe oxide phases in area 1 µ-sxrf elemental map images b) Fe d) Pu c) Mn

In situ µ-srxf Analysis of Sorbed Pu on Tuff The data suggest that Pu and several elements are coassociated with Mn oxide-smectite associations and not with hematite. SXRF spectra taken at region 1 (previous slide)

In situ µ-xanes Analysis of Sorbed Pu on Tuff The XANES edge energies of the standards were 0 ev for Pu(IV) and +3.0 ev for Pu(VI). The relative Pu edge energy shifts for region 1 ranged from +1.3 to +2.0 ev, with a mean value of 1.7 ± 0.4 ev. This corresponds to an average oxidation state of mostly +V. At region 2, the relative edge energy shifts were +2.6 and +3.4 ev (a mean of 3.0 ± 0.6 ev), which is indicative of primarily Pu(VI). Pu-XANES spectra for the standards (Pu(IV) and (VI)) and regions 1 and 2 on the tuff.

Application of µ-srxf and µ-xanes to Speciation of Pu Adsorbed onto Tuff Conclusions: Pu is sorbed strongly and preferentially to Mn oxides and not to Fe oxides or zeolites in the Yucca Mountain tuff. Localized regions of Pu enrichment were identified with variable average Pu oxidation state, Pu(V) and Pu(VI). Without the use of the synchrotron hard X-ray fluorescence microprobe, these observations would not be possible.

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