Combined EXAFS Spectroscopic and Quantum Chemical Study on the

Article pubs.acs.org/IC

Combined EXAFS Spectroscopic and Quantum Chemical Study on the Complex Formation of Am(III) with Formate Daniel R. Fröhlich,*,† Alena Kremleva,‡ André Rossberg,§ Andrej Skerencak-Frech,∥ Carsten Koke,†,∥ Sven Krüger,‡ Notker Rösch,‡ and Petra J. Panak†,∥ †

Ruprecht-Karls-Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Technische Universität München, Department Chemie, 85747 Garching, Germany § Helmholtz-Zentrum Dresden Rossendorf, Institut für Ressourcenökologie, P.O. Box 510119, 01314 Dresden, Germany ∥ Karlsruher Institut für Technologie, Institut für Nukleare Entsorgung, P.O. Box 3640, 76021 Karlsruhe, Germany ‡

S Supporting Information *

ABSTRACT: The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am−O and Am−C distances) of the formed Am(III)−formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.

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INTRODUCTION

plutonium and americium. Thus, the trivalent actinides are of particular interest. Pore waters of some clay formations (i.e., Opalinus Clay and Callovo Oxfordian) contain both macromolecular and small organic compounds.5,6 The latter species make up fractions of up to 88% of the total dissolved organic content and are mostly simple carboxylic ligands (e.g., formate, acetate, propionate, lactate, etc.).5,6 The low-molecular-weight compounds may reach concentrations in the millimolar range5 and, therefore, might have an effect on the aqueous actinide speciation. Some of these ligands can bind to the radionuclides in different coordination modes (monodentate, bidentate, chelate). In the case of formate, which is known to be bound in a monodentate fashion in crystal structures of trivalent lanthanides,7 it is still unclear whether it binds in a monodentate or bidentate fashion to trivalent actinides in aqueous solution. Thermodynamic data (stability constants, standard reaction enthalpies and entropies) for the complexation of Cm(III), as a representative trivalent actinide, in the temperature range up to 90 °C have been determined in a recent study.8 The molecular structure of the formed complexes with trivalent actinides, however, has not yet been studied in detail. Regarding other actinides, EXAFS studies on the complexation of Np(IV)9 and

Clay formations are considered as potential host rock formations for the final disposal of high-level nuclear waste in several European countries (i.e., Belgium,1 France,2 Germany,3 and Switzerland4). Due to the long half-life of the transuranium elements (Np, Pu, Am) included in the spent nuclear fuel, the waste material will have to be stored for >100000 a. During this long period of time, a release of the radionuclides from their primary containment (e.g., by corrosion of the containers after intrusion of water) cannot be ruled out completely. As a consequence, a detailed knowledge of the relevant geochemical processes determining the migration of the actinides in the near and far field of the repository is indispensable. The migration is determined by sorption and diffusion processes, the solubility of the released radionuclides under given environmental conditions, and complexation reactions with organic and inorganic ligands present in the aquifer and pore water of the clay. Furthermore, the speciation of the actinides strongly depends on various parameters such as pH, redox potential, ionic strength, and partial pressure of CO2. A well-founded understanding of the chemistry of actinides in highly complex natural systems is the key to any reliable long-term safety assessment. Due to the reducing conditions which are expected in the near field of the repository as a result of corrosion of steel canisters, +3 will be the predominant oxidation state of © 2017 American Chemical Society

Received: January 5, 2017 Published: June 2, 2017 6820

DOI: 10.1021/acs.inorgchem.7b00035 Inorg. Chem. 2017, 56, 6820−6829

Article

Inorganic Chemistry U(VI)10 with formate have been described in the literature. Furthermore, Hennig et al. studied the complexation of formate with Ce(III).7 Whereas the solid Ce(HCOO)3 provided Ce−C distances of 3.43 and 3.60 Å, confirming that formate is bound in a monodentate fashion in the crystal structure, no Ce− carbon distances were determined in solution containing 18 mM Ce(HCOO)3 in H2O (pH 5.74). However, the absence of Ce−Ce scattering contributions clearly indicates the absence of any polynuclear species. The focus of the present study lies in the resolution of the coordination mode of formate to Am(III) as a representative trivalent actinide and the determination of structural data (coordination numbers, Am−O and Am−C distances) using a combination of EXAFS spectroscopy and iterative transformation factor analysis (ITFA). The spectroscopic investigations are complemented by quantum chemical calculations (binding energies, interatomic distances) of various possible conformations of the formed complexes and speciation calculations using tabulated thermodynamic data.

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extracted pure component EXAFS spectra of Am(III) with coordinated water and formic acid molecules (details see below). The theoretical models were fit to the k2-weighted EXAFS spectra using the Marquardt algorithm and setting the amplitude reduction factor S02 to 0.9. Speciation Calculation. Thermodynamic speciation calculations were performed with the software package Visual MINTEQ 3.113 using the SIT (specific ion interaction theory) approach. As only conditional14,15 but no thermodynamic stability constants for the complexation of Am(III) with formate are available in the literature, stability constants for the formation of the respective Cm(III) complexes8 have been used to estimate the impact of formate on the aqueous speciation of Am(III). All other stability constants originate from the NIST databases 46.6 and 46.7 or ref 16. A summary of the used stability constants and the considered complexation reactions is given in Table S1 in the Supporting Information. Iterative Transformation Factor Analysis (ITFA). ITFA is a well-established method to resolve spectral mixtures into their spectral components17−21 and hence is a powerful tool for investigating small spectral changes expected in the Am−formate system. ITFA comprises three main steps, which are (1) the principal component analysis (PCA) in order to determine the number of the components, (2) the VARIMAX procedure22 for a qualitative measure of the contribution of the components in the spectra, and (3) the iterative target test (ITT)23 for the calculation of the fractions and the spectra of the components. While steps 1 and 2 do not need physical information, constraints with respect to the actual magnitude of a certain number of fractions have to be made for step 3. Details on the application of this procedure to complexation reactions of actinides with organic ligands are given elsewhere.17 In general, EXAFS spectra can be deconvoluted into fractions of the different metal species in the system and/or single coordinating groups or atoms whose coordination numbers change within the sample series. The number of components equals the minimum number of component spectra which enable a sufficient reproduction of each spectral mixture by their linear combination with respect to the experimental error. For the determination of the number of components, the indicator function (IND)24 is often used. The IND function is expected to show a minimum at the correct number of components. However, in the case of the Am(III)−formate system IND does not show a minimum and is steadily increasing. It is known that IND might fail;24 thus, other methods have to be used for estimating the number of components. Therefore, we used the VARIMAX procedure, which calculates a fraction-like distribution (loadings) of the components for each spectral mixture. If components which are not necessary for the reproduction of the data are included, the loadings become erroneous. Taking into account that the components in the Am(III)−formate system follow a stepwise unimodal trend with increasing pH, the VARIMAX procedure yields only an unimodal trend for each component if the proper number of components is selected. If more than two components are included, some of the components do not depend on the pH (Figure S1 in the Supporting Information). Hence, the consideration of two components is sufficient to describe the spectral changes by their linear combination in the present study. Quantum Chemical Calculations. Scalar relativistic all-electron calculations with the LCGTO-FF-DF method25 (linear combination of Gaussian-type orbitals fitting function density functional) as implemented in the software ParaGauss26,27 were carried out in this study. This relativistic method is based on the second-order Douglas− Kroll−Hess (DKH) approach to the Dirac−Kohn−Sham problem.28−30 The scalar relativistic spin-unrestricted approach correctly reproduces the septet spin state of the Am(III) ion in the complexes considered. Although this spin state is of multireference character, it has been shown that the DF approach without inclusion of spin-orbit interaction is able to reproduce well ground-state properties of openshell actinide complexes.31 The Am f orbitals are well localized for all complexes studied, as confirmed by 89−98% f populations (according to a Mulliken analysis)

EXPERIMENTAL SECTION

Sample Preparation. A 1.09 mol/L stock solution of sodium formate was prepared by dissolving 447.8 mg of solid sodium formate (Merck Millipore, EMSURE) in 6.041 mL of water (Milli-Q grade). A 3.675 mL portion of the stock solution was mixed with 0.325 mL of an Am(III) stock solution (3 g/L of Am-243 (containing traces of Am241) in approximately 3 mol/L of HNO3), resulting in [Am(III)]total = 10−3 mol/L, [Form−]total = 1 mol/L, and pH 3.94. The pH of the different samples was adjusted by adding aliquots of diluted NaOH or HCl solutions (both Merck Titrisol). The final concentration of Am(III) in solution was determined by γ-spectrometry. The pH was measured with a combination pH electrode (Ross, ORION), which was calibrated against dilute standard buffer solutions (Merck, pH 2, 4, 7) prior to the measurements. The exact compositions and pH values of all samples are summarized in Table 1. At each pH an aliquot of 200

Table 1. Composition of the EXAFS Samples sample 1 2 3 4 5 6 7

pH 2.16 2.37 2.62 3.05 3.35 3.62 3.94

[Am3+], mol/L 8.2 7.9 7.8 8.3 9.8 9.9 1.0

× × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−3

[Form−]total,a mol/L 0.82 0.79 0.78 0.83 0.98 0.99 1.00

[Form−]total represents the concentration of formate and formic acid irrespective of the protonation state. a

μL was taken and placed in a PE vial. The PE vials were sealed using a two-component adhesive and welded in PE foil before transport to the synchrotron facility. EXAFS Spectroscopy. All EXAFS measurements have been performed at the Rossendorf Beamline (ROBL, BM20) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Am LIII-edge EXAFS spectra of all samples were recorded in fluorescence mode using a 13-element Ge detector which was positioned at an angle of 90° relative to the incoming photon beam. The energy calibration was carried out measuring a Zr foil in transition mode simultaneously with each sample. The data processing (i.e., deadtime correction, energy calibration, averaging, extraction of the EXAFS signal, and fitting) was performed using the EXAFSPAK software package.11 The ionization energy of Am (E0) was set to 18515 eV in all cases. For the calculation of the theoretical scattering phases and amplitudes (carried out using FEFF8.20)12 the crystal structure of Ce(HCOO)3 (Ce replaced by Am)7 was used to fit the 6821


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Inorganic Chemistry of the six occupied valence f spin orbitals and the f LUMO orbital of each complex. The localization of the f orbitals has to be regarded even as underestimated in the present GGA calculations due to selfinteraction artifacts.32−34 Thus, the f orbitals do not contribute significantly to the metal−ligand binding in Am(III) formate complexes, in line with the contracted f shell of the later actinides with Z > 94 with a preferential oxidation state +3.35 The gradient-corrected exchange-correlation functional (generalized gradient approximation, GGA) suggested by Becke and Perdew (BP)36,37 was employed. Kohn−Sham orbitals were represented by flexible Gaussian-type basis sets, contracted in a generalized fashion using appropriate atomic eigenvectors. For Am, we used a basis set of the size (26s, 23p, 17d, 13f)38 contracted to [10s, 8p, 7d, 5f]. The light atoms (O, C, H) were described by standard basis sets:39−41 (9s, 5p, 1d) → [5s, 4p, 1d] for O and C;39,40 (6s, 1p) → [4s, 1p] for H.39,41 In the LCGTO-FF-DF method, the classical Coulomb contribution to the electron−electron interaction is evaluated via an approximate representation of the electron density, using an auxiliary basis set.25 The exponents of s and r2 type auxiliary functions were derived from the orbital basis by a standard procedure.25 We added five p-, d-, and ftype “polarization” exponents, constructed as geometric series with a progression of 2.5, starting with 0.1, 0.2, and 0.3 au, respectively. Thus, for americium, the auxiliary charge density basis set was (26s, 23r2, 5p, 5d, 5f); the auxiliary basis sets were (9s, 5r2, 5p, 5d) for O and C and (6s, 1r2, 5p) for H. Long-range electrostatic solvation effects were taken into account using a variant of the polarizable continuum method (PCM), namely the COSMO (conductor-like screening model)42 method, as implemented in ParaGauss.27 The solute cavity was constructed from a superposition of atomic spheres of van der Waals radii (scaled by 1.2, except for H).43 Additional spheres were created according to the FIXPVA algorithm.44 The dielectric constant of water was set to ε = 78.39. Short-range solvation effects were represented by direct inclusion of the first hydration sphere of americium at the all-electron level. Experimental results on aqua complexes18,19,45,46 suggest various coordination numbers (CN) for americium, ranging from 7 to 10. In the present work we modeled CN values of 8 and 9 in all complexes studied. All complexes as well as ligand molecules were fully optimized in solution. Zero-point energy (ZPE) corrections and thermodynamic corrections to the energies were estimated from the results of a normal-mode analysis, carried out in the gas phase. Hence, all energies listed in the following are estimates of Gibbs free energies ΔG. Free energies of reactions were derived by means of a thermodynamic cycle.47 Gibbs free energies of systems in the gas phase Ggas and solvation energies ΔGsol were used to derive Gibbs free energies of systems in solution: Gaq = Ggas + ΔGsol. Translational, vibrational, and rotational contributions to the energy and entropy terms of Ggas were calculated with the help of the corresponding partition functions.47 Standard state corrections for the reaction free energies were accounted for by adjustment for the standard concentration of 1 M for solutes, which contributes +8 kJ mol−1 to each molecule (except H2O) participating in the reaction. As water has a different standard concentration of 55.34 M, the standard state correction for each water molecule is +18 kJ mol−1.

Figure 1. Am(III) speciation of samples 1−7 (Table 1) based on thermodynamic speciation calculation (based on Cm(III) thermodynamic data for formate; for details see text). Species fractions 1 Å the error is significantly lower and five distinct peaks are observed at 1.89 and 2.70 Å (coordinated water molecule) and at 1.79, 2.61, and 3.32 Å (coordinated formic acid) which are above the noise level. As the positions of the peaks differ by more than 0.09 Å, their physical origin can be assumed to be different. The shift from 1.89 to 1.79 Å is in line with the expected decrease in the Am−O bond length due to the replacement of a coordinated water molecule by formate coordinated in a monodentate fashion. Further evidence for a monodentate coordination is given by the peaks at 2.61 and 3.32 Å, which are contributions of the carbon and the noncoordinating carboxylic oxygen atom (OD). The peak at 2.70 Å (coordinated water) is also present in the pure Am(III)−hydrate (Figure S2 in the Supporting Information); hence, this peak is no artifact or spurious residual from formic acid. Similar peaks were observed in the case of other metal hydrates and were explained by contributions of hydrogen of the first-shell water molecules.54 The EXAFS structural parameters determined for the coordinated water molecule (Table 3) are in accordance with those of Am(III)− hydrate.18,19,45,46 The structural parameters for the coordinated formate complex (Table 3) are compared with our DFT calculations (Table 2) and other actinide systems, as there are no pertinent experimental data for Am formate. DFT calculations predict relative interatomic distance changes more precisely than absolute distances so that linear relationships between experimentally observable and calculated values can be used.55 Therefore, changes in the EXAFS determined bond distances are directly compared with changes in the DFT predicted values. The difference of the Am−O 6825


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Inorganic Chemistry

Figure 5. ITFA isolated (black line) EXAFS contributions (left) and related Fourier transforms (right) of one coordinated water molecule (top) and one coordinated formate (bottom) with shell fit (red line). Confidence intervals are at the 95% confidence level (gray). Arrows point toward positions of prominent peaks (see text). The noise level (blue) is estimated by using the R range 15−25 Å.

Table 3. Fit Parameters of the ITFA Extracted k2-Weighted Am LIII-Edge Component EXAFS Spectra Shown in Figure 5a component

atom

CN

R/Å

σ2/Å2

ΔE0

Am−OH2 Am−Form

OW OC C OD

1.18(4) 1.01(5) /1.01 /1.01

2.480(4) 2.409(7) 3.40(2) 4.20(3)

0.0105(7) 0.013(1) 0.013(4) 0.012(5)

−1.0(3) −3.9(5)

for the Am(III)−hydrate would lead to CN < 8 for at least one of the other Am(III) complexes, which is not appropriate for Am(III). If Am(III)−hydrate has CN = 9, ΔRDFT for the bidentate complexes (b, bb) is only 0.02 Å (Table S2), which does not support the formation of these species. Although ΔRDFT shows values similar to ΔREXAFS for several bidentate and monodentate complex structures, bidentate coordination can be ruled out due to the absence of carbon atoms at distances of 2.8−2.9 Å in the shell fit of the ITFA isolated EXAFS contribution of the formate molecule (Table 3). Consequently, only four m and mm complexes have to be considered (Table S2), for which ΔRDFT matches well with ΔREXAFS. By taking into account the pH-dependent speciation, the CN combinations 8/9 and 9/9 (the first and second numbers equal the CN values of the m and the mm complexes) would lead to a slight increase or no change in the CN with increasing pH, which does not agree with the observed EXAFS trend of the average CN. Therefore, the most probable CN combinations for the m and the mm complexes are 8/8 and 9/ 8. Further restrictions can be obtained by comparing the CN values of the complexes with respect to water and formate obtained by thermodynamic speciation and corresponding ITFA calculations. If a combination of CNs of 9/8 is used, the standard deviation for the differently calculated CNs for water and formate is 0.07 (Figure 4); however, for the other combination of CNs (8/8) a higher standard deviation of 0.12 is obtained. Note that the difference in the standard deviation provides only a hint that the CN combination 9/8 is most probable for the m and mm complexes. For the CN combination 9/8 the expected average R(Am−Oc) and R(Am−C) distances are 2.44 and 3.45 Å, respectively, and hence are 0.03−0.05 Å longer than the fitted values (Table 3). A similar trend is found when the DFT calculated distance R(Am−Ow) = 2.51 Å for Am(III)−hydrate (CN = 9, Table 2) is compared with the fitted value of 2.48 Å (Table 3), implying that the DFT calculations overestimate the interatomic

Definitions: CN, coordination number; R, interatomic distance; σ, Debye−Waller factor; ΔE0, threshold energy; /, parameter linked proportional to the parameter in the row above; Am−OH2, coordinated water molecule; Am-Form, coordinated formic acid molecule. The standard deviations of the variable parameters estimated by EXAFSPAK are given in parentheses. a

distances between the coordinated water, R(Am−Ow), and the formate molecule, R(Am−Oc), is ΔREXAFS = 0.07 Å (Table 3). This value has to be compared with the difference ΔRDFT of the DFT predicted values of R(Am−OW) and R(Am−Oc) for all possible combinations of m, b, mm, mb, and mm complexes, taking into account CNs of 8 and 9, so that in total 24 combinations have to be considered. By using the thermodynamically calculated fractions of the complexes at different pH values (Figure 1), the average R(Am−OW) and R(Am−Oc) values are calculated for each combination. By averaging the values gained for each pH, the final R(Am−OW) and R(Am− Oc) results and ΔRDFT values are determined (Table S2 in the Supporting Information). Furthermore, the calculations are carried out by assuming both CNs of Am(III)−hydrate (CN = 8, CN = 9). If a CN of 8 is assumed for Am(III)−hydrate, the calculated ΔRDFT values are either negative for complexes with one or two bidentately coordinated formate molecules or at most 0.02 Å, which is less than ΔREXAFS (Table S2 in the Supporting Information). Furthermore, a unimodal decrease of the CN with increasing pH is observed. Thus, the application of CN = 8 6826


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Inorganic Chemistry distances at least by ∼0.03 Å. Considering this systematic overestimation, the fitted interatomic distances of the two isolated spectral components (except for R(Am−OD)), generally agree with the DFT predicted distances for Am(III)−hydrate (CN = 9) and the m (CN = 9) and mm (CN = 8) complexes. EXAFS measurements provide R(Am− OD) = 4.20 Å (Table 3), while the DFT calculation predicts shorter R(Am−OD) distances of about 3.75 Å for the m, mm, and mb complexes (Table 2 and Figure 2b,d,e). Such a short distance results from hydrogen bonds between the OD center and neighboring aqua ligands from the first coordination shell of Am(III); Figures 2b,d,e shows these hydrogen bonds. Such a stabilization of the complexes by hydrogen bonds is determined when the complex structure is optimized at 0 K. However, in experiment, thermal effects perturb these structures and the formate ligand can rotate around the OC−C bond, resulting in elongation of R(Am−OD).

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AUTHOR INFORMATION

Corresponding Author

*D.R.F.: e-mail, [email protected]; tel, +49 6221 546626. ORCID

Daniel R. Fröhlich: 0000-0001-8380-8598 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the German Federal Ministry of Economic Affairs and Energy (BMWi) under contract nos. 02E11001, 02E11031, 02E11415E, and 02E11415H and the TALISMAN (Transnational Access to Large Infrastructure for a Safe Management of Actinide) program (Grant Agreement no. 323300). All EXAFS measurements have been performed at the Rossendorf Beamline (ROBL, BM20) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France).

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CONCLUSIONS The molecular structure of Am(III)−formate complexes in aqueous solution (pH 2.2−3.9) has been studied using a combination of EXAFS spectroscopy, thermodynamic speciation calculations, ITFA, and DFT calculations. The present work clearly shows the power of ITFA in obtaining structural information from EXAFS data of weakly bound ligand systems such as formate, where a conventional EXAFS data analysis is not sufficient due to the lower resolution with respect to interatomic distances and coordination numbers. Whereas DFT calculations predict a slight preference of monodentate coordination of formate for the [AmForm]2+ species and of bidentate coordination for [Am(Form)2]+, the ITFA results indicate that formate is bound monodentately in any case under the given experimental conditions. Furthermore, the [Am(Form)2]+ species exhibits a slightly lower coordination number regarding the first oxygen shell (CN = 8) in comparison to the Am(III)−hydrate and [AmForm]2+ (CN = 9). The good agreement of interatomic distances obtained by ITFA and DFT calculations shows that formate is predominantly bound in a monodentate fashion. Finally, the ITFA results are further supported by the excellent agreement with the coordination numbers obtained by thermodynamic speciation calculations. Yet, as suggested by the DFT calculations, a coexistence of other isomers in low concentrations cannot be excluded. The present study provides substantial information on the complexation of formate with Am(III) in aqueous solution, thus improving the understanding of actinide geochemistry at the molecular level. These data can also be used to predict the molecular structure of similar complexes with other trivalent actinides, in particular Pu(III).

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forms of Am(III)−hydrates with the ITFA isolated EXAFS contribution of one coordinated water molecule multiplied by a factor of 9 (CN = 9), and ΔRDFT values for various coordination modes and numbers (PDF)

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00035. Standard state stability constants of the complexation reactions used in the thermodynamic speciation calculations, VARIMAX calculated factor loadings obtained from the EXAFS spectra assuming two and three components, comparison of experimental Am LIIIedge EXAFS spectra and corresponding Fourier trans6827


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