The relationship of monodentate and bidentate coordinated uranium(vi) sulfate in aqueous solution

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1 Radiochim. Acta 96, (2008) / DOI /ract by Oldenbourg Wissenschaftsverlag, München The relationship of monodentate and bidentate coordinated uranium(vi) sulfate in aqueous solution By C. Hennig 1,,A.Ikeda 1,K.Schmeide 1, V. Brendler 1,H.Moll 1, S. Tsushima 1,A.C.Scheinost 1, S. Skanthakumar 2, R. Wilson 2, L. Soderholm 2, K. Servaes 3, C. Görrler-Walrand 3 and R. Van Deun 3 1 Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, Dresden, Germany 2 Argonne National Laboratory, Chemistry Division, Argonne, IL 60439, USA 3 Department of Chemistry, Katholieke Universiteit Leuven, 3001 Leuven, Belgium (Received September 6, 2007; accepted in revised form March 17, 2008) EXAFS / HEXS / UV-vis / Uranyl sulfate / Aqueous solution Summary. The coordination of U(VI) sulfate complexes has been investigated by uranium L III -edge EXAFS and HEXS measurements with the aim to distinguish monodentate and bidentate coordinated sulfate in aqueous solution. UV-vis absorption spectroscopy has been used to differentiate the species and to determine the species distribution as a function of the [SO 2 4 ]/[UO 2+ 2 ] ratio. A monodentate coordination prevails in solutions with [SO 2 4 ]/[UO 2+ 2 ] ratio of 1, where UO 2 SO 4 is the dominant species. Besides the dominating monodentate sulfate a small amount of bidentate sulfate could be observed, indicating that two isomers may exist for UO 2 SO 4. With increasing [SO 2 4 ]/[UO 2+ 2 ] ratio the UO 2 (SO 4 ) 2 2 species becomes the main species. The uranium atom of this species is coordinated by two bidentate sulfate groups. 1. Introduction The uranyl ion, UO 2 2+, forms binary sulfato complexes in acidic aqueous solution. Thermodynamic speciation reveals three sulfato species, UO 2 SO 4 (aq), UO 2 (SO 4 ) 2 2 and UO 2 (SO 4 ) 3 4, summarized in the NEA thermodynamic database [1]. In principle, if there is a change in the coordination number or the coordination mode, this will be reflected in the entropy change of the reaction. However, it is often difficult to lead to a definite conclusion, whether it is bidentate or monodentate, solely from thermodynamic data. The coordination of the binary sulfate species in aqueous solution is still under debate as described in our previous publication [2] on this theme. Monodentate coordination has been suggested with high-energy X-ray scattering (HEXS) in equimolar [SO 4 2 ]/[UO 2 2+ ] solution [3]. This result was also observed with infrared spectroscopy [4]. In contrast, Raman [5] and UL III -edge EXAFS spectroscopy [2, 6] revealed a preferred bidentate coordination in excess of sulfate. In order to clarify the relationship between monodentate and bidentate coordination we performed elaborate U L III -edge EXAFS and HEXS measurements. The ph value *Author for correspondence ( hennig@esrf.fr). in this study has been restricted to 1.0 < ph < 2.5 because at ph > 3 occur hydrolysis species and different ternary UO 2+ 2 SO 2 4 OH complexes and at ph < 1 hydrogen sulfate complexes may compete with the binary sulfates. The spectroscopic techniques used to reveal the coordination of the solution species require high U(VI) concentrations and [SO 2 4 ]/[UO 2+ 2 ] ratios. The extended Debye Hückel formalisms, commonly used to correct activity coefficients and to estimate the species distribution, are above the validity limit in case of high ionic strength. Only the specific ion interaction theory (SIT) [7] and the Pitzer model [8] work at higher ionic strengths as required in this study. However, the problem of ill-defined interaction coefficients still remains. Furthermore, the high specific charge of the ions enters the thermodynamic equation with the power of two, resulting in large uncertainties in the estimation. It is therefore still under discussion if in 50 mm U(VI) solution with high [SO 2 4 ]/[UO 2+ 2 ] ratio the dominating species is UO 2 (SO 4 ) 2 2 [2, 9] or UO 2 (SO 4 ) 4 3 [10]. An alternative way to determine the species distribution is the application of optical spectroscopy because the related techniques are sensitive to the electron states induced by the ligand arrangements. Fluorescence spectra of U(VI) in µm-concentration range indicate that the quantitative distinction between the limiting species UO 2 (SO 4 ) 2 2 and UO 2 (SO 4 ) 4 3 is close to the experimental error limit, especially if only one single wavelength from the whole fluorescence spectrum is considered for the analysis as performed by Vercouter et al. [10]. The fluorescence spectra of uranyl complexes originate only from one electronic transition, Π g Σ + g (considering an equatorial ligand field applied in an intermediate coupling scheme) superimposed by its vibrational transitions. The fluorescence spectra show therefore often only small differences in the fine structure of the individual species, limiting the information that can be extracted. Contrary, the UV-vis absorption spectrum contains several electronic transitions, e.g. Φ g, g, Π g Σ + g (again considering an intermediate coupling scheme). These transitions are each influenced in their own way by the changes in the coordination of the different uranyl sulfato species, as the selection rules in different symmetries allow different transitions to occur, while ruling out other transitions. Therefore, the absorption spec-

2 608 C. Hennig et al. trum contains more information than the fluorescence spectrum, although fluorescence and absorption have a certain complementarity. It is furthermore under discussion, that the excited state undergoes a relaxation correlated with a change in bond length and coordination [11]. Absorption spectra probe directly the ground state and correlate therefore well with the ground state excitation of the 2p 3/2 electron in U L III -edge EXAFS and the state of the molecule in the scattering process of HEXS. Therefore, we estimated the species distribution from UV-vis spectra to choose samples with the predominant U(VI) sulfate species for a comparative study of the sulfate coordination with EXAFS and HEXS measurements. 2. Experimental A stock solution of uranyl sulfate was prepared by dissolving UO 3 in H 2 SO 4. The total sulfate concentration and the ph were adjusted with (NH 4 ) 2 SO 4,H 2 SO 4 and NaOH. UV-vis spectra were recorded from 50 mm U(VI) samples with [SO 4 2 ]/[UO 2 2+ ] ratios between 0 and 59.2 at ph 2. [SO 4 2 ]and[uo 2 2+ ] denote the total concentration of sulfate and uranyl. The EXAFS samples A and B contain 50 mm U(VI). Sample A was prepared according to [3] with a[so 4 2 ]/[UO 2 2+ ] ratio of 1 (ph = 2.53). Sample B was prepared according to [6] with a [SO 4 2 ]/[UO 2 2+ ] ratio of 60 (ph = 0.99). The EXAFS spectra are taken from Ref. [2]. The HEXS samples were prepared with 0.5 M U(VI) and 1.1MHClO 4 (sample C); 0.46 M U(VI) and 0.49 M SO 4 2 (sample D); 0.46 M U(VI) and 3.45 M SO 4 2 (sample E). UV-vis absorption spectra were collected by a Cary 5G (Varian) spectrophotometer with 10 mm path length quartz cuvettes. The EXAFS measurements were carried out on the Rossendorf Beamline (ROBL) at the ESRF. The monochromator, equipped with Si(111) double-crystals, was used in channel-cut mode. Higher harmonics were rejected by two Pt coated mirrors. The spectra were collected in transmission mode using argon-filled ionization chambers. The monochromator energy scale was calibrated to the K-edge of a Y metal foil (first inflection point at ev). The EXAFS data were treated with the EXAFSPAK software package by using theoretical phase and amplitude functions obtained from FEFF 8.2. The HEXS data were collected at the Advanced Photon Source, Argonne National Laboratory (wiggler beamline 11-ID-B, BESSRC CAT). The incident beam of kev corresponds to a wavelength of Å. The scattered intensity was measured with a MAR 345 image plate detector. The data were corrected for background and solvent correlations by subtracting the scattering contribution of a (NH 4 ) 2 SO 4 solution. The HEXS data are Fourier transformed to generate the pair distribution function g(r). All data were collected at room temperature. 3. Results and discussion UV-vis absorption spectra in combination with target factor analysis has been used by Meinrath et al. [12] to extract U(VI) sulfate species from larger sample series. This study revealed the sulfate species UO 2 SO 4 and hydroxides from samples with [UO 2 2+ ]= M, [SO 4 2 ]= Fig. 1. UV-vis absorption spectra of 50 mm UO 2 2+ in aqueous sulfate solution with [SO 4 2 ]/[UO 2 2+ ] ratio from M, ph = Low ph values were used throughout this study in order to avoid the presence of hydroxo species. UV-vis spectra, measured from samples with 50 mm UO 2+ 2 and a [SO 2 4 ]/[UO 2+ 2 ] ratio of are shown in Fig. 1. The species distribution has been estimated from UVvis spectra using the HYPERQUAD software package [13]. This program permits the determination of formation constants, molar absorbance spectra of fundamental species and speciation diagrams from UV-vis spectra with nonlinear least-squares regression on the basis of chemical equilibrium equations. The application of factor analysis to these UV-vis spectra resulted in the same species distribution function. The speciation diagram in Fig. 2 is comprised of the species UO 2+ 2,UO 2 SO 4,andUO 2 (SO 4 ) 2 2. There is no residual evidence to indicate that the species UO 2 (SO 4 ) 4 3 is present in the sample series. It can be ruled out that the UO 2 (SO 4 ) 4 3 species at least with D 3h symmetry exist in the solution, because the intense resonances from the E A 1 transition, present in D 3h complexes like UO 2 (NO 3 ) 3 [14], and analog for UO 2 (CO 3 ) 4 3 and UO 2 (CH 3 COO) 3, are absent. UO 2 (SO 4 ) 4 3 has been observed in solutions with [SO 2 4 ]/[UO 2+ 2 ] ratios > 10 4 [15], which is far from the experimental conditions applied here. This result is in agreement with the U(VI) sulfate species distribution estimated from thermodynamic data. For 50 mm U(VI) solutions these analyses showed that even at high [SO 2 4 ]/[UO 2+ 2 ] ratio the fraction of UO 2 (SO 4 ) 4 3 is below 3% [2]. To determine the coordination of the individual species by EXAFS we have selected specifically two samples to have either UO 2 SO 4 or UO 2 (SO 4 ) 2 2 as the dominant species. Fig. 3 shows the U L III -edge k 3 -weighted EXAFS and the corresponding Fourier transforms (FT) of samples A and B. The FT of the EXAFS data represents a pseudoradial distribution function, where peaks are shifted to lower values R + relative to the true near-neighbor distances R. This shift of 0.2 to 0.5 Å depends on the scattering behavior of the electron wave in the atomic potentials and was

3 The relationship of monodentate and bidentate coordinated uranium(vi) sulfate in aqueous solution 609 Fig. 2. Species distribution estimated with HYPERQUAD program from UV-vis spectra shown in Fig. 1 (top) and the spectra of the three fundamental species (bottom). treated as a variable during the shell fits. The fit results with distances and coordination numbers corrected by theoretical phase and amplitude functions, are shown in Table 1. The FT s are dominated by the peak from the 2 axial oxygen atoms (O ax ) that belong to the trans-dioxo uranyl cation, UO 2 2+, and exhibit a U O ax distance of 1.77 ± 0.02 Å. The coordination of sulfate ligands to UO 2 2+ is restricted to the equatorial plane. Due to the presence of several ligands, the equatorial oxygen (O eq ) shell is split into different superimposed peaks. The shell fit of U O eq distances has therefore some uncertainties and is not appropriate to gain unambiguous insights into the sulfate coordination. The coordination of uranium with sulfate can be derived clearly from the sulfur backscattering signals, because these peaks are not affected by other scattering contributions. In the case in which sulfate is present in excess (sample B), a dominant peak appears at R + = 2.6 Å, fitted by sulfur at a distance of 3.12 Å. The distance is in agreement with that of bidentate sulfate coordination in crystal structures with a U S bid distance of 3.09 Å [16]. The S bid peak appears also in solution A with [SO 4 2 ]/[UO 2 2+ ]=1, but with less intensity. This sample shows a strong peak from sulfate in monodentate coordination with a U S mon distance of 3.57 Å. The 4-legged multiple-scattering path Fig. 3. U L III -edge k 3 -weighted EXAFS (top) and the corresponding Fourier Transforms (bottom) of U(VI) aquo sulfato species. Samples: 0.05 M U(VI), 0.05 M SO 4 2 (A), 0.05 M U(VI), 3 M SO 4 2 (B). Experimental data (line); theoretical curve fit (dots). U O ax1 U O ax2 was included by constraining its Debye Waller factor and effective path-length to twice the values of the corresponding, freely-fitted U O ax single-scattering path. At a [SO 2 4 ]/[UO 2+ 2 ] ratio of 1, where UO 2 SO 4 is the dominant species, monodentate sulfate coordination prevails. However, bidentate coordinated sulfate exists to a minor extent. As a result we may assume that there are two isomers present in the solution, one with monodentate, and one with bidentate coordinated sulfate. Such an argument is supported by density functional theory (DFT) calculations where two isomers were found to be close in energy [2]. For a [SO 2 4 ]/[UO ] ratio of 60, where the UO 2 (SO 4 ) 2 species dominates, the coordination of 2 bidentate sulfate molecules has been observed. The results from EXAFS will be compared subsequently with the results from HEXS. This method has the advantage that long-distant correlations are more evident than with

4 610 C. Hennig et al. Table 1. Structural parameters from EXAFS spectra of Fig. 3. Path R [Å] N σ 2 [Å 2 ] E F B U O ax U O eq a U O eq a U S bid a A U O ax U O eq U O eq U S bid a U S mon a a: Value fixed during the fit procedure. Errors in distances R are ±0.02 Å, errors in coordination numbers N are ±15%. The U O ax distance of 1.76 Å and U O H2 O distance of 2.41 Å are in agreement with the literature [13]. The H atoms of the first water shell are visible at 3.14 Å and the O atoms from the second water shell are visible at a distance of 4.54 Å. In the solution with the [SO 4 2 ]/[UO 2 2+ ] ratio of 1 (sample D) a peak at 3.69 Å occurs. This peak results from the scattering contribution of sulfur in monodentate bound sulfate. The peak at 3.12 Å, representing the H atoms but also possibly to a minor extend bidentate sulfate, remains largely unchanged. This observation confirms the report of Neuefeind et al. [2] that the uranium atom in a 0.5 M U(VI) solution with a [SO 4 2 ]/[UO 2 2+ ] ratio of 1 is coordinated by sulfate in a monodentate fashion. With increasing [SO 4 2 ]/[UO 2 2+ ] ratio to 7 (sample E) the peak intensity at 3.12 Å increases. This is an indication of the increase in bidentate coordinated sulfate with increasing [SO 4 2 ]/[UO 2 2+ ] ratio. In addition, both EXAFS and HEXS indicate that with increasing [SO 4 2 ]/[UO 2 2+ ] ratio the sulfate coordination mode changes from a monodentate to a bidentate fashion. 4. Conclusion Aqueous solutions with U(VI) sulfate complexes have been investigated by U L III -edge EXAFS and HEXS. The measurements indicate that with increasing [SO 2 4 ]/[UO 2+ 2 ] ratio the sulfate coordination mode changes from monodentate to bidentate, confirming conclusions in our previous communication [2]. The apparent discrepancies in the literature with respect to an either monodentate or bidentate coordination of UO 2+ 2 aquo sulfato complexes are reconciled by accounting for variations in the [SO 2 4 ]/[UO 2+ 2 ] ratio. Acknowledgment. This work was supported by the DFG under contract HE 2297/2-1. Fig. 4. Pair-distribution functions g(r) obtained from HEXS data (Q = 15 Å 1 ). Samples: 0.5 M U(VI), 1.1M HClO4 (C); 0.46 M U(VI), 0.49 M SO 4 2 (D); 0.46 M U(VI), 3.45 M SO 4 2 (E). Distances between U(VI) and the neighboring scattering atoms are indicated at the peaks. EXAFS, since the latter of which has a strong 1/R 2 dependency to the scattering power. Otherwise, EXAFS reveals exclusively the environment of the excited atom, whereas HEXS comprises all scattering pairs in the solution, thus, the pair correlation functions with uranium have to be extracted from solution scattering contributions [3, 17]. This is the reason for the requirement of higher uranium concentrations for HEXS measurements. For comparison with uranyl sulfate species, the pair distribution function g(r), obtained by Fourier transformation from HEXS data of U(VI) hydrate (sample C), is shown in Fig. 4. Unlike the distances R + in the FT on an EXAFS spectrum that needs to be corrected for the scattering phase function, the distances r obtained from HEXS data represent true distances. Therefore, the distances are given directly at the corresponding peaks in Fig. 4. References 1. Guillaumont, R., Fanghänel, T., Neck, V., Fuger, J., Palmer, D. R., Grenthe I., Rand, M. H.: Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. Elsevier, Heidelberg (2003). 2. Hennig, C., Schmeide, K., Brendler, V., Moll, H., Tsushima, S., Scheinost, A. C.: EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution. Inorg. Chem. 46, 5882 (2007). 3. Neuefeind, J., Skanthakumar, S., Soderholm, L.: Structure of the UO SO 4 2 ion pair in aqueous solution. Inorg. Chem. 43, 2422 (2004). 4. Gál, M., Goggin, P. L., Mink, J.: Vibrational spectroscopic studies of uranyl complexes in aqueous and non-aqueous solution. Spectrochim. Acta A 48, 121 (1992). 5. Nguyen-Trung, C., Begun, G. M., Palmer, D. A.: Aqueous uranium complexes. 2. Raman spectroscopic study of the complex formation of the dioxouranium (VI) ion with a variety of inorganic and organic ligands. Inorg. Chem. 31, 5280 (1992). 6. Moll, H., Reich, T., Hennig, C., Rossberg, A., Szabó, Z., Grenthe, I.: Solution coordination chemistry of uranium in the binary UO SO 4 2 and the ternary UO SO 4 2 -OH system. Radiochim. Acta 88, 559 (2000). 7. Grenthe, I., Plyasunov, A. V., Spahiu, K.: Estimation of medium effects on thermodynamic data. OECD, NEA, Paris (1997). 8. Pitzer, K. S.: Thermodynamics of electrolytes. I. Theoretical basis and general equations. J. Chem. Phys. 77, 268 (1973).

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