Maryline G.
Ferrier
a,
Benjamin W.
Stein
a,
Sharon E.
Bone
a,
Samantha K.
Cary
a,
Alexander S.
Ditter
ab,
Stosh A.
Kozimor
*a,
Juan S.
Lezama Pacheco
c,
Veronika
Mocko
a and
Gerald T.
Seidler
b
aLos Alamos National Laboratory,, Los Alamos, New Mexico 87545, USA. E-mail: stosh@lanl.gov
bDepartment of Physics, University of Washington, Seattle, Washington 98195-1560, USA
cStanford University, Stanford, California 94305, USA
First published on 1st August 2018
Understanding actinide(III) (AnIII = CmIII, AmIII, AcIII) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, AnIII speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect – in large part – is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire. Herein, we surmounted some of the handling challenges associated with these exotic 5f-elements and characterized CmIII, AmIII, and AcIII using AnIII L3-edge X-ray absorption spectroscopy (XAS) as a function of increasing nitric acid (HNO3) concentration. Our results revealed that actinide aquo ions, An(H2O)x3+ (x = 9.6 ± 0.7, 8.9 ± 0.8, and 10.0 ± 0.9 for CmIII, AmIII, and AcIII), were the dominant species in dilute HNO3 (0.05 M). In concentrated HNO3 (16 M), shell-by-shell fitting of the extended X-ray fine structure (EXAFS) data showed the nitrate complexation increased, such that the average stoichiometries of Cm(NO3)4.1±0.7(H2O)5.7±1.3(1.1±0.2)−, Am(NO3)3.4±0.7(H2O)5.4±0.5(0.4±0.1)−, and Ac(NO3)2.3±1.7(H2O)8.3±5.2(0.7±0.5)+ were observed. Data obtained at the intermediate HNO3 concentration (4 M) were modeled as a linear combination of the 0.05 and 16 M spectra. For all three metals, the intermediate models showed larger contributions from the 0.05 M HNO3 spectra than from the 16 M HNO3 spectra. Additionally, these efforts enabled the Cm–NO3 and Ac–NO3 distances to be measured for the first time. Moreover, the AnIII L3-edge EXAFS results, contribute to the growing body of knowledge associated with CmIII, AmIII, and AcIII coordination chemistry, in particular toward advancing understanding of AnIII solution phase speciation.
This manuscript documents our latest effort to address needs for advancing fundamental CmIII, AmIII, and AcIII chemistry. We focused on characterizing the coordination chemistry of these elements in an aqueous environment that contained actinide complexation agents, namely within nitric acid (HNO3) solutions. These results are of particular relevance, given the importance of HNO3 matrices in AnIII separation processing. For CmIII and AmIII, HNO3 solutions find widespread application in almost every advanced nuclear fuel processing flow chart.4,22–24 Additionally, HNO3 is widely used in the production of 225Ac for medical purposes (targeted alpha therapy), both in terms of purifying 225Ac from 232Th targets irradiated with high energy protons25–27 and when isolating 225Ac from 229Th generators.28–30 Towards these ends, we contribute an AnIII L3-edge X-ray absorption spectroscopy (XAS) study focused on characterizing CmIII, AmIII, and AcIII solution-phase coordination chemistry as a function of increasing HNO3 concentration. Our data provided the first AnIII–NO3 bond distance measurements for CmIII and AcIII of any kind (i.e. solid or solution) and represented the first AmIII–NO3 measurement made in HNO3 solutions. We observed that at low HNO3 concentrations (0.05 M), CmIII, AmIII, and AcIII existed as aquo ions. The propensity of NO3− to complex the AnIII cations increased with increasing HNO3 concentration, such that in HNO3 (16 M) solutions there were 2 to 4 bound NO3− ligands. The results have been presented in the context of the limited number of HNO3 speciation studies reported previously.
Inflection point (eV) | Peak position (eV) | 2nd peak position (eV) | |
---|---|---|---|
a The Cm aquo complex in 1 M HClO4 was calibrated with Nb foil (18986 eV). | |||
Cm aquo (1 M HClO4),47a | 18973.0 | — | — |
Cm (HNO3, 0.05 M) | 18976.4 | 18980.3 | 19013.5 |
Cm (HNO3, 4 M) | 18976.3 | 18980.2 | 19012.9 |
Cm (HNO3, 16 M) | 18976.3 | 18980.2 | 19011.4 |
Am (0.11 M HO3SCF3),21 | 18514.3 | 18517.9 | |
Am (HNO3, 0.05 M) | 18514.0 | 18517.5 | 18550.6 |
Am (HNO3, 4 M) | 18513.8 | 18517.4 | 18549.9 |
Am (HNO3, 16 M) | 18513.8 | 18517.4 | 18547.1 |
Ac (0.11 M HO3SCF3),12 | 15874.3 | 15876.9 | |
Ac (HNO3, 0.05 M) | 15874.7 | 15877.6 | 15908.1 |
Ac (HNO3, 4 M) | 15874.6 | 15877.5 | 15906.7 |
Ac (HNO3, 16 M) | 15874.6 | 15877.5 | 15905.0 |
Although the absorption peak position showed essentially no dependence on the HNO3 concentration, changing the HNO3 concentration from 0.05 M to 16 M had a marked impact on the energy of the post-edge feature approximately 30 to 40 eV above the inflection point (Fig. 1). This peak marks the first extended X-ray absorption fine structure (EXAFS) oscillation. Increasing the HNO3 concentration systematically lowered the energy for the oscillation maximum (Table 1). For CmIII, moving from 0.05 M to 4 M HNO3 caused a 0.6 eV oscillation maximum decrease. Similarly, moving from 4 M to 16 M HNO3 caused a −2.1 eV energy shift. Similar trends were observed for AmIII and AcIII, albeit the 3.5 (AmIII) and 3.1 (AcIII) energy shifts were larger. Dependence of the post-edge line-shape on the HNO3 concentration foreshadowed structural changes that accompanied coordination of AnIII cations by NO3− ligands, which were revealed when the EXAFS spectra were rigorously analyzed.
In general, the AcIII spectra were similar to those of CmIII and AmIII. For example, increased HNO3 concentrations had little impact on the first three oscillations and showed evidence of out-of-phase scattering pathways for the fourth and fifth oscillations. The AcIII L3-edge data differed in two notable ways. First, the signal-to-noise ratio was smaller, on account of the smaller quantity of AcIII (28 μg). This restricted the energy range over which high quality data were available; from 2.7 to 9.5 in k-space (resolution = 0.23 Å in R-space). Second, the EXAFS oscillation frequency increased in comparison to those of CmIII and AmIII. This frequency increase was somewhat expected. For example, the frequency in k-space (left, Fig. 2) is inversely related to the interatomic distance. Higher frequencies result from longer bonds. As shown in Fig. 2, the large AcIII ionic radius42 should provide longer bond distances (higher oscillation frequencies) than those for CmIII and AmIII, as long as the analytes have similar chemical compositions. Given the observed change in frequency upon moving from CmIII and AmIII to AcIII and the similar interference pattern, these data suggested – superficially – that the AcIII speciation was similar to that of AmIII and CmIII.
Closer examination of the CmIII, AmIII, and AcIII L3-edge EXAFS spectra supported the proposition that chemical speciation was similar for these three compounds, revealing only subtle differences in CmIII, AmIII, and AcIII coordination environments. The experimental data were analyzed using well-established shell-by-shell curve fitting techniques.43 Interpretations of the data were guided by identifying scattering pathways using FEFF8 (ref. 44 and 45) and DFT geometry optimized AnIII structures that contained a combination of water molecules and bidentate nitrate ligands, An(H2O)9−2x(NO3)x3−x (An = CmIII, AmIII, AcIII; x = 0, 1, 2, 3). The coordination numbers (CN), bond lengths (R), Debye–Waller factors (σ2), and energy shifts (E0) were allowed to converge to reasonable values. The amplitude reduction factor was set to 0.9. The fitting results have been summarized and compared with other relevant EXAFS studies in Table 2.12,21,46,47 For the sake of discussion, we begin by reporting on spectra collected in dilute HNO3 (0.05 M), then move to concentrated HNO3 (16 M), and conclude at the intermediate HNO3 concentration (4 M).
ΔE0 (eV) | R M–O (Å) | CNbound oxygen | σ bound oxygen 2 | R M⋯N (Å) | CNnitrogen | R M⋯O terminal oxygen | σ terminal oxygen 2 | |
---|---|---|---|---|---|---|---|---|
a Additionally, σN2 was fixed to σO2 and CNterminal oxygen set to CNnitrogen. Data found in the literature and marked with an asterisk (*) had S02 values set to 1. | ||||||||
Cm aquo (0.25 M HCl),46,* | −13.0 | 2.450 ± 0.002 | 10.2 ± 0.3 | 0.009 (fixed) | — | — | — | — |
Cm aquo (1 M HClO4),47,* fit 1 | −2.0 ± 0.9 | 2.469 ± 0.007 | 7.0 ± 0.4 | 0.0071(8) | — | — | — | — |
Cm aquo (1 M HClO4),47,* fit 2 | −0.7 ± 0.7 | 2.470 ± 0.006 | 6 (fixed) | 0.0053(2) | — | — | — | — |
2.63 ± 0.02 | 3 (fixed) | 0.009 (2) | ||||||
Cm (HNO3, 0.05 M) | −5.5 ± 0.8 | 2.47 ± 0.01 | 9.6 ± 0.7 | 0.009(1) | — | — | — | — |
Cm (HNO3, 4 M) | 1.5 ± 1.3 | 2.45 ± 0.01 | 7 (fixed) | 0.007(3) | 2.93 ± 0.07 | 1 fixed | 4.25 ± 0.03 | 0.006 (4) |
2.54 ± 0.05 | 2 (fixed) | |||||||
2.63 ± 0.04 | 2 (fixed) | |||||||
Cm (HNO3, 16 M) | 3.4 ± 1.1 | 2.49 ± 0.02 | 8.9 ± 2.2 | 0.009(2) | 2.95 ± 0.02 | 4.1 ± 0.7 | 4.25 ± 0.02 | 0.010(3) |
2.64 ± 0.03 | 5.7 ± 1.3 | |||||||
Am (0.25 M HCl),46,* | −8.7 | 2.480 ± 0.002 | 10.3 ± 0.3 | 0.009 (fixed) | — | — | — | — |
Am (0.11 M HO3SCF3),21 | −4.7 ± 0.9 | 2.48 ± 0.01 | 9.5 ± 0.9 | 0.0088(9) | — | — | — | — |
Am (HNO3, 0.05 M) | −5.0 ± 1.0 | 2.47 ± 0.01 | 8.9 ± 0.8 | 0.008(1) | — | — | — | — |
Am (HNO3, 4 M) | 2.1 ± 1.3 | 2.46 ± 0.01 | 6 (fixed) | 0.003(1) | 2.98 ± 0.04 | 1 (fixed) | 4.27 ± 0.04 | 0.003 (2) |
2.57 ± 0.03 | 2 (fixed) | |||||||
2.67 ± 0.02 | 2 (fixed) | |||||||
Am (HNO3, 16 M) | 3.3 ± 0.8 | 2.50 ± 0.01 | 7.7 ± 0.8 | 0.005(1) | 2.97 ± 0.01 | 3.4 ± 0.7 | 4.26 ± 0.01 | 0.006(2) |
2.67 ± 0.01 | 5.4 ± 0.5 | |||||||
Ac (0.11 M HO3SCF3),12 | −3.9 ± 1.0 | 2.63 ± 0.01 | 10.9 ± 0.5 | 0.009 (fixed) | — | — | — | — |
Ac (HNO3, 0.05 M) | −2.9 ± 1.6 | 2.63 ± 0.02 | 10.0 ± 0.9 | 0.009 (fixed) | — | — | — | — |
Ac (HNO3, 4 M) | 3.9 ± 2.0 | 2.61 ± 0.02 | 6 (fixed) | 0.006(4) | 3.24 ± 0.11 | 1 (fixed) | 4.42 ± 0.05 | 0.003 (5) |
2.75 ± 0.04 | 2 (fixed) | |||||||
Ac (HNO3, 16 M) | 4.4 ± 2.1 | 2.70 ± 0.02 | 12.9 ± 4.0 | 0.012(4) | 3.20 ± 0.12 | 2.3 ± 1.7 | 4.42 ± 0.03 | 0.003(6) |
As shown in Fig. 2, all spectra collected from dilute HNO3 (0.05 M) solutions were best described by a single frequency whose amplitude in k-space (left, Fig. 2) dampened with increased energy. Best fits for the data (top, Fig. 3; Table 2) – those with the smallest residuals and lowest reduced chi-squared values – confirmed this superficial interpretation. The histogram of frequencies shown in the Fourier transform spectra (right, Fig. 2; top, Fig. 3) contained a single peak near R = 2 Å. As the frequency resolution ranged from 0.19 to 0.23 Å for CmIII, AmIII, and AcIII, we refrained from attempting to resolve multiple M–OH2O scattering pathways within this first water shell. Furthermore, the data quality was not sufficient for observing H2O molecules at longer distances, i.e. in the second and third hydration shells. Fitting the data with a single H2O shell revealed approximately nine water molecules for CmIII (9.6 ± 0.7) and AmIII (8.9 ± 0.8) with equivalent M–OH2O distances of 2.47(1) Å. These results agreed well with the literature values for CmIII and AmIII aquo ions. The single crystal structure of the CmIII aquo ion showed nine H2O ligands with an average Cm–OH2O distance of 2.51(8) Å.48 Previous EXAFS measurements obtained from the CmIII aquo ion in dilute HCl (0.25 M)46 and dilute HClO4 (1 M)47 showed 10.2 ± 0.3 oxygen atoms at 2.450(2) Å and 7.0 ± 0.4 oxygen atoms at 2.469(7) Å, respectively. Similarly, recent EXAFS studies characterized the AmIII aquo ion as having 9.5 ± 0.9 oxygen atoms at 2.48(1) Å (HO3SCF3; 0.11 M)21 and 10.3 ± 0.3 oxygen atoms at 2.480(2) Å (HCl; 0.25 M).46 A single crystal structure for the AmIII aquo ion has also been reported, showing nine H2O ligands with a 2.52(8) Å average Am–OH2O distance.48
For the larger AcIII cation, a longer Ac–OH2O distance of 2.63(2) Å was observed. In comparison to the CmIII and AmIII aquo ions described above, the larger Ac–OH2O bond distance was statistically relevant. The AcIII coordination number also seemed larger than those from CmIII and AmIII with 10.0 ± 0.9 inner sphere H2O ligands. However, these values were equivalent when the measurement uncertainties were considered. The Ac–OH2O bond distance and H2O coordination numbers were consistent with the only other data obtained on an AcIII aquo ion,12 despite differences in the solution matrices; HNO3 (0.05 M) vs. HO3SCF3 (0.11 M). This previous analysis showed 10.9 ± 0.5 oxygen atoms at 2.63(1) Å. Additional confidence in these Ac–OH2O distances was provided by comparison with previous AcIII L3-edge EXAFS measurements made in HCl (11.7 M) solutions, which gave a 2.59(3) Å Ac–OH2O distance.21 Overall, all of our An–OH2O (An = CmIII, AmIII, AcIII) distances were in agreement with the Shannon ionic radii.42 For example, subtracting the six coordinate ionic radii from the experimental M–OH2O distances gave 1.50, 1.50, and 1.51 Å for CmIII, AmIII, and AcIII, respectively. These values were bracketed by the calculated (1.67 Å) and crystallographically measured (1.38 Å) H2O ionic radii.49 In all of the AnIII aquo spectra (HNO3, 0.05 M for CmIII, AmIII, and AcIII; HO3SCF3, 0.11 M for AmIII and AcIII), there was no evidence of AnIII aquo ion dimerization. No AnIII⋯AnIII scattering pathways were detected nor was there evidence for short AnIII–OH interactions, which would result from hydrolysis. Hence, these data were consistent with previous EXAFS studies on AnIII and LnIII aquo ions,12,50–54 suggesting that CmIII, AmIII, and AcIII aquo ions existed primarily as discrete AnIII(H2O)x3+ species. However, EXAFS spectroscopy is relatively insensitive to dilute impurities, and dimeric species present at less than 10% of the total sample would be difficult to detect.43
Consistent with the AcIII aquo L3-edge EXAFS spectra reported previously in dilute HO3SCF3, the data reported here contained a feature near 3.2 Å in the Fourier transform. To date, we have been unable to identify physically realistic models to explain these high-frequency oscillations. Given the instability of these features in various k ranges (7, 8, 9, 10 Å−1), at this time we believe their origin is not related to the AcIII coordination chemistry and likely results from systematic artifacts related to the data quality. While not conclusive, this proposition was supported by the absence of this mysterious peak in the higher quality CmIII and AmIII spectra, as long as one assumes analogous coordination chemistry exists for all three cations.
Comparison between 0.05 and 16 M HNO3 offered the highest probability to identify differences in AnIII speciation. Our approach to modeling these EXAFS data was consistent with previous models used to explain spectra from LnIII and AnIII cations dissolved in HNO3 (6.8 M,55 13 M) (Scheme 1; bottom, Fig. 3). For CmIII and AmIII, there were two short oxygen scattering pathways. The shorter path was assigned to metal bound oxygen atoms from NO3− ligands (ONO3(bound), olive trace); meanwhile the other was attributed to a shell of oxygen atoms from the H2O ligands (OH2O, purple trace, Fig. 3; Scheme 1). However, because these designations resulted from calculations on static actinide nitrate molecules, we do not have high confidence in the rigidity of these assignments. For example, in solution, NO3− and H2O ligand exchange could likely occur. For AcIII, the data were not sufficient to resolve the two OH2O and ONO3(bound) scattering pathways. Hence, in the AcIII model, the OH2O and ONO3(bound) shells were combined (purple trace, bottom right, Fig. 3).
Scheme 1 Scattering pathways deployed in fitting An L3-edge EXAFS data from AnIII cations dissolved in 16 M HNO3. |
Consistent with AnIII–NO3− binding was the presence of four higher frequency scattering pathways, characteristic of inner-sphere NO3− ligands.55 There was a pathway at intermediate distances associated with the central nitrogen of the NO3− anion, referred to as NNO3 (orange trace, Fig. 3; ca. R = 2.5 Å). This shell was followed by the NO3− terminal oxygen (ONO3(terminal); pink trace, ca. R = 3 Å). Subsequently, between ca. 3 < R < 4.5 Å there were two linear multiple scattering pathways. There was the three component AnIII → ONO3(terminal) → NNO3 → AnIII (MS3-comp; blue-green trace) pathway and the four component AnIII → NNO3 → ONO3(terminal) → NNO3 → AnIII (MS4-comp; grey trace) pathway (Scheme 1). Our attempts to model the data with bent multiple scattering pathways (i.e. AnIII → ONO3(bound) → NNO3 → AnIII) or as dimers and oligomers (with An⋯An scattering paths) were unsuccessful. Best fits for the data contained η2-NO3− (bidentate) ligands and were modeled using the following constraints. The ONO3(terminal) amplitude (coordination number) was fixed to NNO3, which in turn was allowed to converge. In addition, the NNO3 and ONO3(bound) Debye–Waller factors (σ2) were fixed to that associated with OH2O, as all three scattering pathways had similar frequencies and because these three scattering pathways combined to form a single peak in the Fourier transform. This constraint additionally minimized the number of free fitting parameters.
For CmIII, refinement of the model to experimental data showed 8.9 ± 2.2 ONO3(bound) atoms at 2.49(2) Å. There were also 5.7 ± 1.3 OH2O at 2.64(3) Å and 4.1 ± 0.7 NNO3 atoms at 2.95(2) Å. The CmIII–ONO3(terminal) distance was 4.25(2) Å (Table 2, Fig. 3). To determine the number of NO3− ligands, two options existed involving either the NNO3 coordination number or the ONO3(bound) coordination number. Although, similar stoichiometries were obtained for both scenarios, reported here is a chemical formula based on NNO3(bound) to facilitate comparison with the AcIII data below. Overall, these data indicated that the average CmIII species present in concentrated HNO3 (16 M) had a stoichiometry of Cm(NO3)4.1±0.7(H2O)5.7±1.3(1.1±0.2)− with an overall coordination number of 13.9 ± 1.9. Stoichiometric self-consistency associated with the coordination number ratio for NNO3, ONO3(bound), ONO3(terminal) (2:1:1) – as well as the magnitude of the multiple scattering pathways – provided additional confidence in our model.
The AmIII data in 16 M HNO3 resembled that from CmIII (Table 2), albeit with slightly smaller uncertainties. For instance, this analysis showed that the average coordination numbers for all of the AmIII species present in concentrated HNO3 had 7.7 ± 0.8 ONO3(bound) atoms at 2.50(1) Å, 5.4 ± 0.5 OH2O atoms at 2.67(1) Å, 3.4 ± 0.7 NNO3 atoms at 2.97(1) Å, and an Am–ONO3(terminal) distance of 4.26(1) Å (Table 2, Fig. 3). Based on the NNO3 and OH2O values, the analysis suggested an average stoichiometry of Am(NO3)3.4±0.7(H2O)5.4±0.5(0.4±0.1)− (mean coordination number of 12.2 ± 1.5). Again, the ONO3(bound), NNO3, and ONO3(terminal) coordination numbers and magnitudes from the multiple scattering pathways were all self-consistent with this average stoichiometry.
Moving to the larger AcIII ion had little effect on the overall coordination number, showing 12.9 ± 4 inner-sphere oxygen atoms. The average Ac(NO3)2.3±1.7(H2O)8.3±5.2(0.7±0.5)+ solution phase stoichiometry was (essentially) equivalent to that from CmIII and AmIII; however, the uncertainties associated with the AcIII L3-edge measurements were larger. The presence of 2.3 ± 1.7 NNO3 atoms at 3.20(12) Å and AcIII–ONO3(terminal) atoms at 4.42(3) Å confirmed the presence of inner-sphere NO3− ligands in HNO3 (16 M; Table 2, Fig. 3). The largest differences between the CmIII, AmIII, and AcIII L3-edge EXAFS data were associated with the AcIII interatomic distances. As expected based on the ∼0.15 Å increase in AcIII six coordinate ionic radii, the AnIII–OH2O and AnIII–ONO3(bound) distances increased by approximately 0.2 Å from CmIII and AmIII to AcIII.
Because EXAFS spectroscopy probes all species in solution, it does not exclude AnIII access to other stoichiometric ratios of NO3− and H2O, Scheme 2. Instead, it provides an average signal from all of the molecules in the sample. In this context, good models of the data were only obtained with η2-NO3− ligands (bidentate), which were consistent with many models previously reported for lanthanide and actinide EXAFS data.50,56–58 One notable exception was identified by Antonio and coworkers. These authors successfully identified monodentate η1-NO3− binding for CeIII in 3 M HNO3, a notably lower concentration than the 16 M HNO3 discussed here.59 Our attempts to introduce η1-NO3− (monodentate) binding increased the AnIII → NNO3 and AnIII → ONO3(terminal) distances into unrealistic regions of the spectra where no intensity was present. Additionally, η1-NO3− diminished the amplitude for linear multiple scattering pathways, giving an appreciable misfit between 3 < R < 4.5 Å in the Fourier transform. We interpret these results as suggesting that in 16 M HNO3 AnIII–η2-NO3 binding was preferred for CmIII, AmIII, and AcIII over monodentate modes, likely due to the chelation effect.60 Consistent with this observation were quantum calculations on M(NO3)x(H2O)y3−x (M = AmIII, EuIII) reported by Xi and coworkers.61 Their calculations predicted that η2-NO3 binding was preferred energetically in aqueous solutions, especially when the first coordination shell was sterically saturated. Xi's calculated 2.45 Å Am–[η2-ONO3(bound)] bond distance is in excellent agreement with our EXAFS results, lending confidence to our η2-NO3 binding model. As pointed out to us privately by Antonio, the larger An–NO3− stability constants62 may be responsible for CmIII, AmIII, and AcIII preference for η2-NO3− binding.59,63
For experiments conducted at the intermediate HNO3 concentration (4 M), an alternative fitting method was pursued. The initial model was generated from a linear combination of the two end members, namely AnIII dissolved in 0.05 and 16 M HNO3. This fit (Fig. 4) suggested that the 4 M HNO3 CmIII speciation could be described as containing 73.6(1.8)% of the CmIII aquo ion and 26.4(1.8)% of the Cm(NO3)4.1±0.7(H2O)5.7±1.3(1.1±0.2)− (Fig. 5). The slightly larger AmIII cation gave a similar ratio; 67.4(1.4)% of the AmIII aquo and 32.6(1.4)% Am(NO3)3.4±0.7(H2O)5.4±0.5(0.4±0.1)−. More substantial differences were observed when moving to the much bigger AcIII ion. The analysis showed 60.5(1.4)% of the AcIII aquo and 39.5(1.4)% of the Ac(NO3)2.3±1.7(H2O)8.3±5.2(0.7±0.5)+. These analyses assisted subsequent modeling efforts that used shell-by-shell methods, similar to those described above to fit the 0.05 and 16 M HNO3 spectra. The fitting routine for the HNO3 (4 M) data differed in that it included all of the scattering pathways used in the 0.05 and 16 M models. To keep the number of fitted parameters less than half of the total number of independent variables,64 the coordination numbers were fixed in accordance with the percentages determined from the linear combination analyses (Fig. 4 and 5). Under these conditions, variables associated with the interatomic distance (R) and Debye–Waller factors (σ2) were allowed to converge to reasonable values, as shown in Table 2. The good agreement of these shell-by-shell fits with the experimental data validated conclusions from the linear combination analyses, suggesting that the H2O and NO3− coordination numbers were between those of the 0.05 and 16 M end-members.
In the context of what is understood regarding complexation of CmIII, AmIII, and AcIII by NO3− in aqueous media, the EXAFS results reported herein provide hard data that can be used broadly to assist applied and fundamental efforts that require AnIII cations to be dissolved in HNO3(aq). Our data suggested that CmIII, AmIII, and AcIII existed as aquo ions in dilute HNO3 matrices (0.05 M). These results agreed with the small AnIII–NO3 stability constants: ([AnIII–NO3]/[AnIII][NO3−]; logK, ionic strength = 1 M, 25 °C) 0.34 (CmIII),84 0.25 ± 0.02 (AmIII),85 and 0.1 (AcIII).85 Along these lines, Choppin and coworkers used CmIII fluorescence to evaluate NO3− complexation in aqueous solutions with varied HNO3 concentrations. In this study, moving from 0.1 to 13 M HNO3 decreased the number of bound H2O molecules, presumably accompanied by NO3− complexation. A total of four H2O molecules were reportedly removed in 13 M HNO3, suggesting that a bis-nitrato [M(NO3)2(H2O)5]1+ complex had formed.86 Consistent with Choppin and coworkers' results,86 our EXAFS data showed that nitrate complexation for CmIII, AmIII, and AcIII increased with increasing HNO3 concentration. In 4 M HNO3, we observed approximately one inner sphere NO3−. Moving past Choppin and Coworkers' 13 M HNO3 to concentrated HNO3 (16 M), increased the number of coordinated NO3− ligands, ranging from 4.1 ± 0.7 for CmIII, to 3.4 ± 0.7 for AmIII, and 2.3 ± 1.7 for AcIII. It is interesting that the NO3− coordination numbers seemed to decrease with increasing metal ionic radius. While tempting to correlate these results with the stability constants referenced above and with the Lewis acidity for the AnIII cations, we refrain since the NO3− coordination numbers were equivalent when the uncertainties for the measurements were considered.
In terms of structural characterization, the AcIII–OH2O and AcIII–ONO3(bound) bond distance measurements represent another impactful component of this manuscript. Prior to these experiments, there were two reported Ac–OH2O bond distances, both measured by solution-phase AcIII L3-edge EXAFS spectroscopy. One was in concentrated HCl (11 M) solutions (2.59 ± 0.03 Å)21 and the other in dilute HO3SCF3 (0.11 M; 2.63 ± 0.01 Å).12 Contributed here are three additional Ac–OH2O measurements; 2.63 ± 0.02 (0.05 M HNO3), 2.61 ± 0.02 (4 M HNO3), and 2.70 ± 0.02 (16 M HNO3). This brings the total number of reported Ac–OH2O bond distances to five, averaging 2.63 ± 0.04 Å (error reported as the standard deviation of the mean, 1 σ). Their consistency provides confidence in the accuracy of these AcIII L3-edge EXAFS measurements. In terms of NO3− complexation, these results are also exciting as they represent the first AcIII–NO3− interaction observed spectroscopically. Although the AcIII–ONO3 distance was not resolved from the inner-sphere AcIII–OH2O interaction, the AcIII–ONO3 bond length can be indirectly inferred based on the measured AcIII–NNO3 distance. For example, the Ac–ONO3(bound) distance can be calculated using the cosine rule; assuming an average N–O distance of 1.31 Å and an average Ac–N–O angle of 113°.57 This analysis gives an AcIII–ONO3 distance of 2.70 ± 0.10 Å.
In terms of fundamental exploratory science, the chemistry of CmIII, AmIII, and AcIII presents uncharted landscapes in comparison to many other elements in the periodic table. Unique safety hazards and limited access to sizable quantities of material represent significant technical challenges faced during experimental studies of these elements. Even interactions with common ligands – such as the An–H2O and An–NO3 bonds – are poorly defined. On top of scientific curiosity is the need to support innovation for AnIII processing. This need includes developing advanced nuclear fuel cycles, medical isotope production, and targeted alpha therapy. It seems likely that our approach for characterizing An–NO3 and An–H2O (An = CmIII, AmIII, AcIII) interactions using AnIII L3-edge EXAFS can be broadly applied to other AnIII–ligands interactions, which are equally relevant for nuclear processing and medical applications. We hope that the results presented herein will provide insight aiding our current efforts – as well as those associated with other researchers embarking on their own fundamental and applied scientific campaigns – to solve complicated technical problems associated with CmIII, AmIII, and AcIII.
The XAS data were analyzed by fitting a line to the pre-edge region, which removed the background from experimental data in the spectra. Then a third order polynomial fit was chosen for the post-edge region. The difference between pre and post edge lines was set to unity at the first inflection point, normalizing the absorption jump to 1.0. Samples were measured for several hours resulting in the collection of multiple scans. The EXAFS data were analyzed by either shell-by-shell fitting methods using IFEFFIT software88 and FEFF8 calculations44,45 or linear combination analyses (IFEFFIT).88 Atomic coordinates for the FEFF8 calculations were obtained by geometry optimizations generated from DFT calculations (see below). Data were fit over the following ranges; for curium and americium 2.7 < k < 11 Å−1 and 1.1 < R < 4.5 Å (to 3 Å for 0.05 M) and for actinium 2.7 < k < 9.5 Å−1 and 1.25 < R < 4.5 Å.
Footnotes |
† LA-UR-18-22688. |
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c8sc02270d |
This journal is © The Royal Society of Chemistry 2018 |