EXAFS and DFT Investigations of Uranyl Arsenate Complexes in

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EXAFS and DFT Investigations of Uranyl Arsenate Complexes in Aqueous Solution Wondemagegnehu A. Gezahegne,†,* Christoph Hennig,‡ Satoru Tsushima,‡ Britta Planer-Friedrich,§ Andreas C. Scheinost,‡ and Broder J. Merkel† †

Institute for Geology, Chair of Hydrogeology, Technische Universität Bergakademie Freiberg, Gustav-Zeunerstrasse 12, 09599 Freiberg, Germany; ‡ Helmholtz Zentrum Dresden-Rossendorf, Institute of Resource Ecology, 01314 Dresden, Germany; § Environmental Geochemistry, University of Bayreuth, 95440 Bayreuth, Germany S Supporting Information *

ABSTRACT: Uranium and arsenic often co-occur in nature, for example, in acid mine drainage waters. Interaction with arsenic is thus important to understand uranium mobility in aqueous solutions. For the present study, EXAFS spectroscopy was used to investigate the formation and identify the structure of aqueous uranyl arsenate species at pH 2. The nearest U−As distance of 3.39 Å, observed in shock-frozen liquid samples, was significantly shorter than that observed in solid uranyl arsenate minerals. The shorter bond length indicated that the solution contained a bidentate-coordinated species, in contrast to the monodentate coordination in solid uranyl arsenate minerals. The U−As coordination number of 1.6 implied that two uranyl arsenate species with U:As ratios of 1:1 and 1:2 formed in nearly equal proportions and that the hydrated uranyl ion was present only as a minor component. The two uranyl arsenate species could not be differentiated spectroscopically, since their U−As distances were equal. A comparison based on DFT modeling indicated for both the 1:1 and the 1:2 species, that the bidentate arsenates were bound to uranium with one of the binding oxygen atoms being protonated. Based on the present spectroscopic study, the two species that will have to be considered in acidic uranium−arsenic-rich solutions are thus UO2H2AsO4+, and UO2(H2AsO4)20.

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INTRODUCTION The mobility of uranium(VI) in the environment is controlled by its soluble aqueous complexes and its mineral precipitates, as well as by sorption on mineral surfaces, complexation with natural organic matter and living organisms. About one-third of the approximately 200 known uranium minerals are represented by uranyl phosphate and uranyl arsenate minerals.1 Such minerals are for example chernikovite ( ( H 2 O ) 2 ( U O 2 P O 4 ) 2 · 6 H 2 O ) , t r ö g e r i t e ( H ( U O 2 ) (AsO4)·4H2O), meta−torbenite (Cu(UO2PO4)2·8H2O) and meta-zeunerite (Cu(UO2AsO4)2·8H2O) which are ubiquitously found at uranium mining sites. This indeed is a manifestation of the strong affinity of arsenate and phosphate ligands for uranium(VI) and the likeliness that aqueous complexes between UO22+ and AsO43‑ or PO43‑ will form. This strong affinity of arsenate and phosphate toward uranium leads to precipitation and thus immobilization of uranium at high concentrations. Related with the strong affinity of uranium to bind arsenate is the observation of enhanced sorption of uranium on aluminum oxide which was pretreated with arsenate by forming surface precipitation.2,3 In the presence © 2012 American Chemical Society

of arsenate the related aqueous uranyl complexes are expected to either promote or retard the mobility of uranium in the environment depending upon the nature of their charges and that of sorbent surfaces. However, while the existence and behavior of dissolved uranyl phosphate complexes had been subject of research by several authors such as Baes et al.,4 Veselý et al.5, and Mathur,6 the existence of aqueous uranyl arsenate complexes has so far only been reported by Rutsch et al.7 These authors found evidence for three aqueous uranyl arsenate complexes using time-resolved laser-induced fluorescence spectroscopy (TRLFS). This technique allows the determination of formation constants even at low uranium(VI) concentrations.8 Rutsch et al. interpreted the uranyl arsenate complexes based on the analogous uranyl phosphates as the 1:1 complexes UO2H2AsO4+, and UO2HAsO40, and the 1:2 complex UO2(H2AsO4)20.7 However, no structural characterReceived: Revised: Accepted: Published: 2228

September 19, 2011 January 6, 2012 January 10, 2012 January 10, 2012 dx.doi.org/10.1021/es203284s | Environ. Sci. Technol. 2012, 46, 2228−2233

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unit, O−U−O, are linearly arranged and originate a multiple scattering contribution, dominated by the 2-fold degenerated 4legged multiple-scattering path U−Oax1−U−Oax2. This scattering path was included in the curve fit by constraining its Debye−Waller factor and its effective path-length to twice the values of the corresponding, freely fitted U−Oax singlescattering path. DFT Calculations. All the calculations were performed in the aqueous phase using the Gaussian 03 program.16 DFT calculations were performed with Becke’s three-parameter hybrid functional17 and Lee−Yang−Parr’s gradient corrected correlation functional (B3LYP).18 We used a conductor-like polarizable continuum model (CPCM)19 with UAHF radii.20 The energy-consistent small-core effective core potential (ECP) and the corresponding basis set suggested by Küchle et al.21 and Bergner et al.22 were used for uranium,21 arsenic,22 oxygen,22 and carbon.22 The most diffuse basis functions on uranium with the exponent 0.005 (all s, p, d, and f type functions) were omitted to make the convergence of the electronic wave function much faster; this simplification generally has little effect (less than 1 kJ/mol) on the total energy.23 The d-function on oxygen and g-function on uranium have been added. For hydrogen, the 6-311++G** basis was used.24 The Gibbs energy correction to the electronic energy was calculated at the B3LYP level from the vibrational energy levels in the aqueous phase and the molecular partition functions. In few cases, a single and small imaginary frequency remained in the final geometry, but such small imaginary frequencies are known to be often merely computational (numerical) artifacts of the solvent models. Spin−orbit effect and the basis set superposition error (BSSE) corrections were also not considered.

ization of the postulated aqueous complexes was achieved in their study. Consequently, more detailed knowledge on uranyl arsenate complexes is still needed because it is critical for assessment and planning of remediation strategies for uranium mine waters, which often contain arsenic as an accompanying element together with uranium. Bernhard et al.,9 for example, reported that samples taken from Germany’s former uranium mining sites in Schlema contain 0.03 mmol/L As (U 0.021 mmol/L, at pH 7), 0.01 mmol/L As were reported in waters from Königstein (U 0.073 mmol/L, at pH 2.6), and 0.52 mmol/L As in tailing water from Helmsdorf (U 0.025 mmol/L, at pH 9.7). The uranium speciation in such waters is pHdependent and the arsenic present in solution might compete at acidic conditions with other ligands to form a single charged UO2H2AsO4+ complex and the noncharged UO2(HAsO4)0 and UO2(H2AsO4)20 complexes. These complexes are expected to alter uranium mobility, surface complexation, and sorption capacity substantially. Although extended X-ray absorption fine structure spectroscopy (EXAFS) was used to determine structural parameters of uranyl arsenate minerals such as trö g erite and metazeunerite,10−12 no reports exist to date on the structure of the aqueous uranyl arsenate complexes. This was the motivation to use EXAFS spectroscopy in the present study to analyze the structure of uranyl arsenates in aqueous solution in comparison to uranyl arsenate minerals. EXAFS spectroscopy is well suited for that purpose but requires concentrations above the ones that commonly occur in natural waters.

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EXPERIMENTAL SECTION Materials and Methods. Solutions with concentrations of 0.05 mM uranium(VI) and 0.5 mM arsenate were prepared by diluting analytical grade stock solutions of UO2(NO3)2·6H2O (Chemapol) and As2O5 (Alfa Aesar), respectively. The pH of the solutions was adjusted using NaOH and HClO4 to pH 2. The samples were prepared under ambient atmospheric conditions in deionized water. The solutions were filtered with 0.2 μm cellulose acetate filter to exclude possible precipitates, filled into the sample holder, hot sealed, and immediately flash-frozen in liquid nitrogen. As reference for the observed uranyl arsenate precipitates, which formed at higher pH, a sample of crystalline uranyl arsenate was prepared from a solution of 10 mM uranium(VI) and 10 mM arsenate at pH 3. Data Collection. EXAFS spectra were recorded at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Uranium LIIIedge (17185 eV) EXAFS data were collected in fluorescence mode using a 13-element Ge detector (Canberra) with a sample orientation of 45° and a detector orientation of 90° to the incident beam. For the energy calibration of the EXAFS spectra the first inflection point of the Y K-edge of a Y metal foil at 17038 eV was used. All measurements were performed at 15 K in a closed-cycle He cryostat. Eighteen scans in fluorescence mode were collected for the sample with the solution species. Before averaging, the fluorescence spectra were corrected for the detector dead time. Two scans in transmission mode were collected for the reference sample. The EXAFS spectra were processed according to standard procedures13 using the suite of programs EXAFSPAK.14 Scattering phases and amplitudes were calculated by using scattering code FEFF 8.20.15 The oxygen atoms of the uranyl

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RESULTS AND DISCUSSION The determination of structural parameters in aqueous solution with EXAFS spectroscopy is limited by uranium concentrations of ∼5 × 10−5 M. Unfortunately, there is a strong tendency of uranyl arsenates to precipitate at concentrations above ∼5 × 10−6 M, especially at pH values above 2. Yet, speciation calculations for a sample of 0.05 mM uranium(VI) and 0.5 mM arsenate based on the formation constants of Rutsch et al.7 in combination with the NEA database29 shows no oversaturation with respect to a limiting solid phase due to the absence of thermodynamic data for precipitating uranyl arsenate minerals such as trögerite. However, at these concentrations precipitates occur at room temperature already within one or two minutes after mixing of the initial solutions. Because the detection limit of EXAFS for uranium cannot easily be lowered and the oversaturation leads to spontaneous precipitation, we explored different ways to conserve the solution species for the EXAFS measurement. Shock-freezing of the solution in liquid nitrogen immediately after mixing of the initial solutions turned out to be the most appropriate approach. Of all the pH ranges at which the solutions were prepared it was only at pH 2 that we succeeded in excluding precipitation through immediate shockfreezing. According to the thermodynamic speciation, at pH 2 uranyl arsenate complexes should be detectable, despite a predominance of the uranyl aquo complex. We were able to discriminate between the preserved solution species and the precipitate based on their significant structural differences. We discuss first the structure of the precipitate and subsequently the solution species at pH 2. 2229

dx.doi.org/10.1021/es203284s | Environ. Sci. Technol. 2012, 46, 2228−2233

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EXAFS Analysis of the Precipitate. The spontaneous precipitate that formed in a solution of 10 mM uranium(VI) and 10 mM arsenate at pH 3 was identified as the 1:1 solid phase H(UO2)(AsO4)·4H2O by powder X-ray diffraction measurements (PDF 04−010−0666). Even at higher uranyl to arsenate ratios (up to 1:10) we obtained the same solid phase. The precipitate consists of only a single phase. Neither precipitates of the schoepite mineral family nor of the 1:2 solid phase (UO2)(H2AsO4)2·H2O have been detected as components of the precipitate. H(UO2)(AsO4)·4H2O is a layered hydrate that belongs to the autunite-type structure family. The structure consists of octahedral uranyl units that share their four equatorial oxygen atoms (Oeq) with arsenate tetrahedra in a monodentate fashion as shown in Figure 1. The negative charge of the [(UO2)Figure 2. Uranium LIII-edge k3-weighted EXAFS spectra (left) and the corresponding Fourier transforms (right). Spectrum (a) shows a shock-frozen liquid sample at 15 K with uranium concentration of 0.05 mM and arsenic concentration of 0.5 mM at pH 2, and spectrum (b) shows the solid uranyl arsenate precipitate H(UO2)(AsO4)·4H2O at pH 3.

Table 1. EXAFS Structural Parameters for a Shock-Frozen Liquid Sample with Uranium to Arsenic Ratio of 0.05 mM: 0.5 mM at pH 2, and Solid H(UO2)(AsO4)·4H2O at pH 3. Both Measurements Were Made at 15 Ka sample aqueous complex

Figure 1. Structure of H(UO2)(AsO4)·4H2O. solid sample

(AsO4)]− layer is compensated by H+ ions located together with water in the interlayer.25 A unique feature of the autunitetype structure is that the uranyl ion is equatorially 4-fold coordinated (the axial oxygen atoms (Oax) remain uncoordinated). The majority of the uranyl complexes are 5-fold coordinated in the equatorial plane, whereas 4-fold and 6-fold coordination requires specific sterical ligand arrangements.26 The bridging arsenate unit in the [(UO2)(AsO4) ]− layer forces the uranyl unit into a 4-fold coordination. The 4-fold coordination is associated with a typical short U−Oeq bond length of ∼2.28 Å.11 The 4-fold coordination can be clearly differentiated from a 5-fold coordination with U−Oeq bond length of ∼2.34−2.42 Å. The k3-weighted EXAFS spectra of the solid precipitate H(UO2)(AsO4)·4H2O is shown in Figure 2b together with the frozen liquid sample at pH 2 with the uranium and arsenic concentration of 5 × 10−5 M and 5 × 10−4 M, respectively. The fit results are summarized in Table 1. The EXAFS data exhibit the short U−Oeq bond length of 2.29 Å as characteristic for the 4-fold coordination. The monodentate coordination of the arsenate tetrahedron results in a U−As distance of 3.69 Å, whereas the nearest uranium atoms are at a U−U distance of 5.38 Å. Only distances within the uranyl arsenate layer are detected by the EXAFS measurement. Contributions from water molecules and H+ ions in the interlayer region are not to be expected because of their small number of electrons and high disorder. The EXAFS spectrum of H(UO2)(AsO4)·4H2O resembles that of other 1:1 uranyl arsenate structures.10−12

shell

R [Å]

N

σ2 [Å2]

ΔE [eV]

U−Oax MS U−Oax U−Oeq U−As U−Oax MS U−Oax U−Oeq U−As U−U1

1.78 3.54 2.39 3.39 1.77 3.54 2.29 3.69 5.38

2.1 2.2 5.1 1.6 2.2 2.2 4.1 3.6 4.2

0.0035 0.0070 0.0082 0.0018 0.0017 0.0034 0.0021 0.0018 0.0028

−0.1 −0.1 −0.1 −0.1 −2.6 −2.6 −2.6 −2.6 −2.6

Errors in distances are ±0.02 Å, errors in coordination numbers are ±15%. R, radial distance, N, coordination number, σ2, Debye-Waller factor. a

In addition to the observation that X-ray diffraction measurement of the precipitate indicated only the 1:1 complex, H(UO2)(AsO4)·4H2O, the presence of the solid 1:2 complex (UO2)(H2AsO4)2·H2O can also be ruled out by the EXAFS data for several reasons. (UO2)(H2AsO4)2·H2O has been observed in arsenate excess and low pH values.27,28 The uranyl unit in (UO2)(H2AsO4)2·H2O is 5-fold coordinated, but the arsenate coordination is also monodentate with a U−As distance of 3.70 Å. Furthermore, the EXAFS spectra of solid (UO2)(H2AsO4)2·H2O indicate clearly U−U scattering contributions at 5.38 and 5.73 Å.28 EXAFS Analysis of Solution Species. The coordination of the shock-frozen solution species differs clearly from the precipitate. The only structurally unchanged feature is the trans-dioxo group consisting of 2 axial oxygen atoms at a U− Oax distance of 1.78 Å. The second peak in the EXAFS spectrum, which corresponds to the equatorial oxygen atoms, shows a U−Oeq bond length of 2.39 Å with a coordination number of 5.1. The U−As bond distance of 3.39 Å in the aqueous species is significantly shorter than that observed in solid H(UO2)(AsO4)·4H2O. This short bond length indicates a bidentate coordination between the central uranium atom and 2230

dx.doi.org/10.1021/es203284s | Environ. Sci. Technol. 2012, 46, 2228−2233

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turned out that the protonation may have significant influence on the coordination mode. Crystal structures of uranyl arsenates including the proton position are extremely rare.30 So far the only known crystal structure with determined proton position is (UO2)(H2AsO4)2·H2O.27 The protons are located in this structure at the nonbinding distal oxygens as U−O−As− OH. In some other arsenate structures protons were found also located on the binding oxygen atoms. For example, the M− OH−As (M = metal) configuration has been described for the magnesium arsenate Mg7(AsO4)2(HAsO4)431 and the iron arsenate [Fe6As7O31H5]2−(dabco2+).32 In this situation, the electron density of the M−O−As bonds is weakened and the metal−arsenate distance becomes longer. In contrast, a protonation of the distal oxygen atoms of arsenate has no detectable influence on the U−As distance (for DFT data see Supporting Information). The modification of the U−As distance by protonation of the binding oxygen atoms was investigated with DFT for the species UO2HAsO4(aq), UO2H2AsO4+, and UO2(H2AsO4)2(aq), whose existence in aqueous solution was postulated by previous TRLFS investigations.7 The DFT calculations were performed in the following way: the coordination number of the equatorial plane of uranium was fixed to the value of 5 as indicated by EXAFS, the hydrogen position was varied, and then the most stable geometry was searched. For UO2HAsO4(aq), two structures have been optimized as shown in Figure 4a. The structure with the proton attached to the distal oxygen has a 34.0 kJ/mol lower Gibbs energy than the structure with the proton attached to the equatorial oxygen, hence is more likely to form. For UO2H2AsO4+, the structure with two protons attached to the distal oxygens has the lowest Gibbs energy (Figure 4b). However, the structure with one proton attached to the equatorial oxygen has an only 10.3 kJ/mol higher energy. The accuracy of DFT calculations using present combination of theory (B3LYP), solvation model (PCM), and basis set (ECP + valence triple-ζ basis) for aqueous uranyl(VI) complexes are discussed in several publications.33−35 They all agree that the accuracy is not better than ±20 kJ/mol. In present case, the energy difference of 10.3 kJ/mol is not significant; hence one cannot conclude which of the two isomers is the predominant species. For UO2(H2AsO4)2(aq) (Figure 5), the most stable structure has two protons attached to distal oxygens, whereas the structures with two protonated equatorial oxygens stays only 15.4 kJ/mol above the energy minimum and thus the existence of this isomer cannot be excluded. A comparison of the U−As distances of 3.39 Å observed by EXAFS with the U−As distances observed by DFT for the different protonated uranyl arsenate isomers indicates that one of the U−O−As bonds is protonated. The DFT calculation showed for such isomers U−As distances of 3.33−3.36 Å. Isomers without protonated U−O−As bond have significantly shorter U−As distances (3.11−3.18 Å) and isomers with two protonated U−O−As bonds have significantly longer U−O−As distances (3.46−3.47 Å). The observation that an average of 1.6 arsenic atoms is present within the coordination shell of each uranium atom indicates that the solution consisted of 1:1 and 1:2 uranyl arsenates species. The experimental U−As distance of 3.39 Å agrees best with the DFT optimized isomers of UO2H2AsO4+, and UO2(H2AsO4)2(aq) with binding energies of 10.3 KJ/mol and 15.4 KJ/mol, respectively, where one of the binding oxygen atoms is protonated, as shown in Figures 4 and 5.

the arsenate ligand, instead of the monodentate one in H(UO2)(AsO4)·4H2O. Furthermore, no U−U distance is visible in the solution sample indicating that the species is present as monomer. In contrast, the solid H(UO2)(AsO4)·4H2O exhibits a well pronounced Fourier transform U−U scattering peak at 5.38 Å, indicating a solid phase. The determination of the species distribution based on the formation constants of Rutsch et al.7 for the sample of 0.05 mM uranium(VI) and 0.5 mM arsenate at pH 2 without taking the formation of precipitate into account gives the following species distribution: 75.4% UO22+, 19.8% UO2H2AsO4+, UO2HAsO4