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. 2025 Jun 3;59(23):11756–11766. doi: 10.1021/acs.est.4c13899

Relationship between Mineralogically Complex Iron (Oxyhydr)oxides and Plutonium Sorption and Reduction: A High-Energy Resolution X‑ray Absorption Spectroscopy Perspective

Manuel R Vejar †, Frances E Zengotita †, Stephan Weiss ‡, Salim Shams Aldin Azzam ‡, Nina Huittinen ‡,§, Sabrina Beutner ‡, Elena F Bazarkina ‡,∥, Lucia Amidani ‡,∥, Kristina O Kvashnina ‡,∥,*, Amy E Hixon †,*
PMCID: PMC12177926  PMID: 40460208

Abstract

To facilitate the continued use of commercial nuclear power and address environmental contamination, it is essential to understand the fate and transport of plutonium (Pu) in (sub)­surface environments. Current geochemical models do not account for complexity in mineral assemblages, such as metal substitution or the role of nanoscale crystallite sizes. In this work, we studied mineralogically complex systems where Pu­(V) was the sorbate and Al-substituted or nanoscale iron (oxyhydr)­oxides were the sorbents. Using M4-edge and L3-edge high-energy resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, we probed the electronic configuration of Pu, quantified the extent of Pu surface-mediated reduction, and explored Pu speciation. Our results indicate that nanoscale iron oxides exert a greater degree of control over the redox behavior of Pu than Al-substituted iron (oxyhydr)­oxides under circumneutral pH and oxic conditions. This is due to the dependence of Pu surface-mediated reduction on an initial sorption step, which is greater with the increased specific surface area and reactivity of nanoscale crystallites.

Keywords: plutonium, redox, iron (oxyhydr)oxide minerals, HERFD-XANES, sorption

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1. Introduction

A deep geological repository, the recommended solution to used nuclear fuel, is generally designed with natural and engineered barriers to safely contain high-level nuclear waste for up to one million years. − For most nuclear repository designs, iron-bearing minerals and iron-bearing anthropogenic materials are present in the geologic or engineered barriers encasing the nuclear waste, from the steel canisters and structures where the nuclear waste-forms will be stored, to minerals present in the geologic media hosting the repository. − Iron (oxyhydr)­oxide minerals (e.g., hematite (α-FeIII 2O3), magnetite (FeIIFeIII 2O4), and goethite (α-FeIIIOOH)) are naturally abundant in surface and subsurface environments and are common corrosion products. , Importantly, in a repository setting, while oxic conditions would mainly be present shortly after closure, in case of water ingress into the containment layers, new phases will not be present in pristine form, but rather as complex assemblages of minerals or corrosion products. −

Mineral complexity, defined here in terms of crystallite size and metal substitution, occurs naturally and impacts contaminant sorption and redox processes. − Al-substituted iron oxides occur in natural systems with approximately 10–15% Al and nanoscale minerals are known to drive colloid-facilitated transport of plutonium (Pu) in contaminated sites. , The higher surface area of nanoscale minerals and differing chemical reactivity of metal-substituted and nanoscale minerals can influence contaminant interactions, such as affinity (e.g., increased binding affinity with decreasing crystallite size) and redox behavior. ,, In addition, when nanoparticles reach sizes <10 nm, there are a greater percentage of atoms at the surface, a higher surface energy, and distorted binding environments that can influence contaminant sorption. ,, Thus, synthesizing Al-substituted and nanoscale iron (oxyhydr)­oxides is a controlled approach to increasing mineral complexity.

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies targeting the L3-edge have been useful and reliable analytical synchrotron-based tools for the study of oxidation states, speciation, and local structure of Pu at the mineral-water interface. ,,,− The relatively long core-hole lifetime leads to the formation of broad Pu L3-edge XANES spectra. − High energy resolution fluorescence detection (HERFD), a cut of the resonant inelastic X-ray scattering (RIXS) map, is a photon-in/photon-out approach that results in increased energy resolution and reduced broadening of XANES spectra by employing an X-ray emission spectrometer. ,,− Recent studies have shown that actinide M4,5-edge HERFD-XANES offers unique advantages over the conventional L3-edge XANES; notably, there is an enhanced sensitivity to changes in the electronic structure and chemical bonding by directly probing the 5f unoccupied states. ,,,,− The resulting HERFD-XANES spectra can then be compared with suitable reference compounds or described using electronic structure calculations.

The objective of this study is to quantify the oxidation state distribution and assess the speciation of Pu in the presence of mineralogically complex iron oxide systems, which take the form of Al-substituted iron (oxyhydr)­oxides and nanoscale iron oxides. For this purpose, we employ emerging techniques, Pu M4-edge and L3-edge, and Fe K-edge HERFD-XANES. Ultimately, investigating Pu chemical behavior will improve predictive models, which in turn, will empower us to determine optimal environmental conditions for the storage of nuclear waste and remediation strategies for contaminated legacy waste sitessettings where complex biogeochemical process, including oxic to anoxic gradients, influence radionuclide mobility. ,

2. Materials and Methods

Warning: All known isotopes of plutonium are radioactive! All handling of plutonium was conducted by trained workers in approved facilities

2.1. General Synthesis and Analysis Conditions

All sample manipulations, including mineral syntheses, washing, preparation of samples for HERFD-XANES, and UV–vis–NIR spectroscopy measurements, were conducted under ambient conditions (e.g., atmospheric air and pressure, room temperature ∼25 °C) using ACS grade chemicals and deionized water (18.2 MΩ·cm at 25 °C), unless noted otherwise.

2.2. Mineral Syntheses and Characterization

Mineral syntheses and characterization are fully described in the Supporting Information file. Briefly, Al-substituted hematite (AH) and Al-substituted goethite (AG) were synthesized with ferric and aluminum nitrate solutions following well-established hydrothermal methods from acidic and alkaline routes, respectively. , Nanohematite (NH) was synthesized according to the method of Madden and Hochella; nanomagnetite (NM) was synthesized via the coprecipitation technique described by Petcharoen and Sirivat. Hematite (Hem) and magnetite (Mag) were obtained commercially (99.8% Fe, Strem; 95%, Aldrich) and used as received; and goethite (Goe) was synthesized as described in Sadergaski and Hixon. All seven minerals were characterized by powder X-ray diffraction (PXRD) (Figure S1), electron microscopy (Figure S2), and Brunauer–Emmett–Teller (BET) surface area analysis (Table S1).

2.3. Preparation and Characterization of Pu Aqueous Solutions

The Pu used in these experiments had an isotope distribution of 99.958 ± 0.007 wt % 242Pu, 0.020 ± 0.002 wt % 240Pu, 0.008 ± 0.001 wt % 239Pu, 0.008 ± 0.017 wt % 238Pu, and 0.001 ± 0.001 wt % 241Pu based on decay correction. A 0.82 ± 0.01 mM Pu­(V) solution (Figure S3) was prepared electrolytically, as described in the SI file.

2.4. Sample Preparation and Aqueous Chemistry Measurements

Mass-normalized (∼13 g·L–1) batch sorption experiments were prepared at constant ionic strength (0.1 M NaCl(aq)) and average final pH (probe semi micro, WTW) values of 7.72 ± 0.21 (Table S2), and equilibrated for 10 d. Throughout the course of the batch sorption experiments, electron activity was tracked by Eh measurements (platinum electrode with Ag/AgCl reference, Mettler Toledo Inlab redox micro), adjusted to the standard hydrogen electrode (Figures S4–S6). Experimental geochemical conditions were compared with predictions from Geochemist’s Workbench (GWB) and supplemented with NEA-TDB data. Liquid scintillation counting (LSC) was used to quantify the aqueous-phase Pu concentrations by mixing a sample aliquot with 10 mL of LSC cocktail (Ultima Gold XR, PerkinElmer). Aqueous Fe and Al concentrations at the conclusion of the experiment (Table S3) were measured using a NexIon 350x inductively coupled plasma mass spectrometer (PerkinElmer) as described in the SI file.

2.5. High Energy Resolution X-Ray Absorption Near Edge Structure (HERFD-XANES) Spectroscopy

All HERFD-XANES experiments presented in this study were performed at the Rossendorf Beamline (BM20) at ESRF, as described in the SI. Solid-phase samples with sorbed Pu were separated from the aqueous phase by centrifugation and decantation and mounted into a sample holder as a wet paste, which was doubly contained with Kapton film. Samples were tested for short-term beam damage. First, an extended time scan (for ∼2 min with 0.1 s exposure time per step) at the maximum of the white lines was performed before data collection to monitor any long-term variations in the fluorescence signal. In addition, several fast HERFD-XANES scans (total counting time <10 s) in a short energy range were performed and compared with all HERFD-XANES data collected per sample. Based on these procedures, we did not find any evidence of the spectral change caused by X-ray exposure. Energy calibration and the averaging of the individual scans were performed with PyMCA. In order to facilitate comparisons with peer reviewed studies, ,,,,,,,, we chose to normalize the M4-edge spectra to the white line intensity instead of normalizing to the absorption step or the area. , Thorough theoretical and experimental studies are needed to determine the impact of the oxidation state, speciation, and concentration of Pu on the intensity of the white line or satellite features at the M4-edge. The L3-edge spectra was normalized to the absorption step. While some self-absorption correction methods for HERFD-XANES spectra have been explored, no such correction has been applied to the data, which may impact the long-term applicability of the quantitative methods employed. Iterative target transformation factor analysis (ITFA) was used to mathematically decompose the originating spectral mixtures into the spectra of the pure plutonium oxidation states as components, and their relative fractions in the HERFD-XANES spectra. The linear combination fitting (LCF) function within Larch was used to determine the relative spectral contributions of Pu species and their respective statistical agreement with the sample spectra, where the fit with the lowest χ2 value was selected as representative. In cases where ITFA or LCF were conducted, the models were not constrained to unity during the analysis to allow for a better alignment with the experimental spectra, however, fit results were normalized to the total fit. Lastly, pre-edge fitting of the Fe K-edge spectra of the mineral samples was conducted using Larch. After absorbance normalization of the K-edge HERFD-XANES, fitting for the mineral samples was conducted in the range of ∼7107–7123 eV by first subtracting the background using a Lorentzian and polynomial function, and subsequently adding two Gaussian functions to fit the pre-edge peak. The resulting areas and center positions were used to calculate the centroid energies and integrated intensities of the pre-edge peaks.

3. Results and Discussion

We studied the sorption and surface-mediated reduction of Pu­(V) in the presence of hematite (Hem), Al-substituted hematite (AH), nanohematite (NH), goethite (Goe), Al-substituted goethite (AG), magnetite (Mag), and nanomagnetite (NM). The contact time of 10 days allowed for high sorption of Pu to the mineral surfaces, but not necessarily for steady state to be reached; 99.8 ± 3.9% sorption and 927 ± 30 μg·g–1 loading were achieved (Table S3).

3.1. Effects of Al Substitution and Crystallite Size on the Structure of Iron (Oxyhydr)­oxides

Electron microscopy was used to examine the morphology of all seven samples (Figure S2). The synthesis of AH resulted in crystals that were ∼250 nm and exhibited hexagonal platelet or tabular morphology with irregular facets and edges; the synthesis of AG resulted in crystals that were ∼450 nm and exhibited columnar or blade morphology with irregular facets and edges; the synthesis of NH resulted in 8.7 ± 2.7 nm crystallites that exhibited distorted spherical morphology; and the synthesis of NM resulted in 8.5 ± 2.0 nm crystallites that exhibited distorted spherical morphology. This diversity in morphology, irregularities, and particle size are reflected in the surface area measurements (Table S1).

Lattice defects, strain, and crystallite size are known to impact the electronic properties of hematite and goethite. − In Figure S1, experimental PXRD patterns are compared with models produced using CrystalDiffract, where the reference patterns for hematite and goethite were modified to decrease the unit cell volume, add ∼10% Al site occupancy, and account for particle size. In terms of aluminum substitution, the resulting models showed a shift toward higher °2θ, consistent with a decrease in unit cell volume, a decrease in intensity, and in good agreement with the PXRD patterns for our AH and AG samples. While not used in these models, increasing the Iso Strain parameter resulted in a decrease in peak intensity and an increase in peak broadening, both consistent with our samples. Furthermore, Rietveld refinement was conducted using Profex for AH, AG, hematite, and goethite. Table S4 shows the calculated decrease in unit cell volumes, consistent with aluminum substitution in the respective lattices. , The experimental diffractograms show peak broadening in the nanomaterials, which, while characteristic of crystallites in that size domain, , prevents Rietveld refinement to reliably obtain a unit cell volume. Defects in the crystal lattice or surface were not quantified in this study. However, defects resulting from metal substitution are known to affect contaminant sorption behavior. ,,,

Fe K-edge HERFD-XANES was used to provide additional insight into the structures of the bulk, Al-substituted, and nanoscale iron (oxyhydr)­oxide minerals (Figure S7). Pre-edge fitting is commonly used to determine the oxidation state distribution of Fe, the general coordination environment of Fe (i.e., tetrahedral vs. octahedral), and to understand lattice distortions and other electronic effects of Fe-bearing compounds. ,,, Al-substitution is known to result in a pre-edge shift toward lower energies, due to decreases in the energy level of the 3d t2g orbital, and increased intensities due to structural distortions. Both shifts are observed for AG, whereas AH only exhibits a small shift in integrated pre-edge intensity (Figure ). The greatest increases in integrated pre-edge intensity were observed for NH and NM, suggesting a significant increase in lattice distortions compared to the bulk minerals. Moreover, both nanomaterials exhibited a shift toward higher energies in centroid position. In the case of NM, this shift toward higher energies suggests a decrease in Fe­(II) content, consistent with surface oxidation. ,,

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Variogram of pre-edge fitting-derived intensities and centroid positions for each sample. The Fe­(II) and Fe­(III) reference lines are based on data reported in Fiege et al. .

3.2. Oxidation State of Pu Using M4-edge HERFD-XANES

Figure A compares the Pu M4-edge HERFD-XANES spectra for Pu sorbed to iron (oxyhydr)­oxides (bulk, Al-substituted, and nanoscale) to four reference spectra: PuIVO2(s) (bulk),34 PuIVO2(s) nanoparticles (NPs), KPuVO2CO3(s), and K4PuVIO2(CO3)2(s). The second feature ∼3972.5 eV has been shown to originate from multiplet splitting of the Pu 5f states. ,,, The white line maximum of Pu sorbed on hematite shows good agreement with that of the Pu­(V) reference, and this is true to some extent for goethite, which possesses a wider white line. In contrast, magnetite, NH, and NM show good agreement with the white line position and broadening of the Pu­(IV) reference. Hematite, AH, goethite, and AG show small shifts on the edge features, but overall good agreement with the Pu­(IV) reference compound. There might also be a spectral contribution from Pu­(V) in the region between the white line and the second feature at ∼3971–3972 eV. All samples exhibited a second feature at ∼3972–3973 eV, which is characteristic of Pu­(IV). ,,,

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(A) Stacked, white line-normalized Pu M4-edge HERFD-XANES spectra and (B) oxidation state distribution of Pu in each sample determined from ITFA analysis of the M4-edge HERFD-XANES data. The plutonium reference materials in (A) were reported by and taken from Kvashnina et al., (PuIVO2(s) (bulk) and PuV–CO3), Gerber et al. (PuIVO2(s) nanoparticles (NPs)), and Pidchenko et al. (PuVI–CO3).

These results suggest that (i) Pu might be present in our samples in mixed valence states and/or (ii) the speciation of Pu affects the M4-edge spectral features. We used iterative target transformation factor analysis (ITFA) to conduct an eigen analysis with spectra from the seven mineral samples with sorbed Pu (Figure S8). Eigenvector1 is the most dominant and includes the characteristic features of Pu­(IV) highlighted above. A comparison of eigenvector1 with an artificial spectral mixture consisting of 75% Pu­(IV) (PuIVO2 bulk) and 25% Pu­(V) (PuV–CO3) reveals very good agreement in the ∼3970–3972 eV region. Furthermore, eigenvector1 and the spectral mixture show very good agreement in the same region with the spectrum for AH, as an example. Thus, spectral signals from mixed oxidation states comprise a principal component for our samples, and most, if not all, of our samples contain contributions from mixed oxidation states.

Few studies have probed the oxidation state of Pu using the M4-edge, ,,,, and to date, none have done so with dilute concentrations of Pu in heterogeneous, environmentally relevant matrices. This is, in part, due to the practical challenges of making these measurements with plutonium, such as preparing appropriate reference compounds or end-members ,, (i.e., those that are ensured to be completely either tetravalent or pentavalent). The redox reactivity of Pu­(V) both in aqueous form and as a sorbate, and the tendency of Pu to form PuO2(s) in relatively high concentrations adds to the difficulty of preparing such samples. Here, we approach the determination of Pu oxidation state by conducting quantitative iterations with reference compounds for Pu­(IV), Pu­(V), and Pu­(VI). The Pu reference compounds possess coordination environments or are chemical species like those that we expect to find in our systems (e.g., Pu strongly coordinated to O, carbonate species). We consider the possible presence of more than one Pu­(IV) species associated with the solid phase, which HERFD-XANES would detect as the average of all species present. Pu­(IV), whether as a polynuclear or a sorbed species, is likely to be bound to O atoms on the mineral surface, in bulk water, or hydroxide ligands. Thus, we have chosen the commonly used bulk PuO2(s) as the reference for Pu­(IV) for these analyses unless explicitly stated otherwise.

Linear combination least-squares fitting (LCF) and ITFA of the (HERFD-)­XANES spectra are two common approaches that have been applied to the determination of oxidation state distributions in actinide systems. ,, While LCF conducts the fitting process on the raw or normalized data, ITFA creates matrix-derived, interpolated reproductions from the entire data set, thus attenuating some of the noise in the process and allowing for further data modularity. Thus, ITFA is preferred here for the oxidation state fraction determination using the M4-edge data (Figure B). With an uncertainty of ∼5%, hematite, AH, goethite, AG, and NM contain ∼68–79% Pu­(IV) and ∼21–32% Pu­(V), while NH and magnetite show greatest Pu reduction, yielding ∼83–95% Pu­(IV) and ∼5–17% Pu­(V) associated with the mineral phase. For comparison, Figure S9 shows that the implementation of ITFA and LCF analyses with PuIVO2(s) and KPuVO2CO3(s) as references and fitting over the ranges 3966–3982 and 3966–3980 eV, respectively, yield remarkably good agreement.

Since this is one of the few studies to explore mixed oxidation state Pu in heterogeneous environmental samples using the M4-edge, we offer alternate LCF and ITFA fit results that include Pu­(VI) (Figures S10–S12), which could be present at the mineral surface from the disproportionation of Pu. ,,,,− ITFA and LCF fits with and without Pu­(VI) suggest that contributions from Pu­(VI) are negligible; contributions average ∼2.8%, but do not exceed ∼6%, and are mostly consistent with the Pu­(VI) content of the initial Pu stock solution. Indeed, a close inspection of Figure A suggests that Pu­(VI) produces a feature at ∼3976.5 eV. We note that the PuO2(s) reference possesses a different feature in the same region, yet our samples lack features in this region altogether. Thus, much like the contributions from Pu­(V), fits with Pu­(VI) mainly impact contributions to the two main features at ∼3970–3973 eV. Regardless of the analysis method (i.e., ITFA vs. LCF), and whether Pu­(VI) is included or excluded, the final fits yield very similar contributions from Pu­(IV), highlighting that reduction to Pu­(IV) has not been misestimated. The plausible, yet unlikely, presence of Pu­(VI) on the mineral surface presents the possibility of a more dynamic surface redox and speciation chemistry at the mineral-water interface and warrants the application of M4-edge HERFD-XANES and other advanced spectroscopic and theoretical efforts in future studies to explore Pu redox reactions related to the solid phase.

3.3. Speciation of Pu Using L3-edge HERFD-XANES

In synergy with the M4-edge spectra, we employ a two-fold approach to L3-edge data analysis to test its sensitivity to Pu oxidation state and determine Pu speciation. Beginning with a qualitative assessment of the Pu L3-edge spectra (Figures A and S13), the bulk PuO2(s) 34 and PuO2(s) nanoparticle (NPs) references show a small feature at ∼18080 eV that corresponds to Pu–Pu degeneracy in EXAFS. The intensity of this feature (marked with a reference dashed line in Figures A and S13) decreases with decreasing particle size. , PuO2(s) also shows a strong feature near 18100 eV that appears to decrease in intensity and/or broaden with decreasing particle size. In our samples, the feature at ∼18080 eV is absent, and the feature at ∼18100 eV is small and broad. Even samples that are projected to have the largest Pu­(IV) contributions based on analysis at the M4-edge, such as NH and magnetite, appear to show an absence of a feature at ∼3976 eV (Figure A) that is present in PuO2(s) NPs. However, PuO2(s) NP formation at the mineral surface cannot be confirmed or refuted from these features alone.

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(A) Overlapped absorbance-normalized Pu L3-edge HERFD-XANES spectra and (B) Pu species distribution determined using LCF of L3-edge HERFD-XANES data. The plutonium reference materials in (A) were reported by Kvashnina et al., (PuIVO2(s) (bulk) and PuV–CO3(s)), Gerber et al., (PuIVO2(s) NPs), and Vitova et al. (Pu­(IV)(aq)).

Vitova et al. demonstrate that the Pu­(IV)aq species exhibits a more intense white line than PuO2(s) colloids in normalized L3-edge HERFD-XANES spectra. This is what we observe in our sample spectra; white line intensities for our sample spectra were greater than those of bulk PuO2(s) and PuO2(s) NPs, and more similar to that of Pu­(IV)(aq), suggesting the contribution of a Pu­(IV) aqueous or sorption complex species. Even in systems where PuO2(s) NPs precipitate on mineral surfaces, Pu can be simultaneously present as a sorbed species. ,,, In these cases, the L3-edge spectra more closely resemble that of the Pu­(IV)(aq) species due to some Pu coordination with surrounding waters, leading to white line peaks with greater intensities.

As LCF is more commonly used with actinide L3-edge data, it is the only method used here for the comparison of oxidation state fraction determination across M4 and L3-edge data and the same reference compounds for Pu­(IV) and Pu­(V) (Figures S10A and S14A). When attempting to discern Pu oxidation states, our L3-edge fits appear to underestimate contributions from Pu­(V) compared to the M4-edge fits (see side-by-side comparison in Figure ). This underestimation builds evidence and is in agreement with previous studies showing that the Pu L3-edge is not sufficiently sensitive to minor (<10%) contributions from higher oxidation states (i.e., Pu­(V) and Pu­(VI)). ,, Here, the projected contribution from Pu­(V) is ∼11% at most by the L3-edge, in contrast to the ∼33% Pu­(V) by the M4-edge. This is in part due the lower intensity of the L3-edge white line that is characteristic of the plutonyl moiety, and the white line variations due to Pu­(IV) speciation. , Nevertheless, the tendency observed at the Pu M4 and L3 edges clearly indicates the presence of pentavalent Pu species in both data sets.

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Comparison of the LCF oxidation state fractions of Pu using spectra at the (A) M4-edge and (B) L3-edge, both using compounds reported by Kvashnina et al., (PuIVO2(s) (bulk) and PuV–CO3(s)). Pu­(IV) shown in green and Pu­(V) in pink.

To examine the speciation of Pu, LCF was conducted using bulk PuO2(s), PuO2(s) NPs, KPuVO2CO3(s), and Pu­(IV)(aq) as references, over an 18040–18085 eV range (Figure S14B). Figure B shows that the LCF results with the lowest χ2 preferentially included only the PuO2(s) and Pu­(IV)(aq) species, and excluded contributions from Pu­(V)-CO3 and PuO2(s) NPs (despite being included as a components). This is likely due to a better match of the white line intensity with the inclusion of Pu­(IV)(aq), and a resulting apparent occlusion of any Pu­(V) contributions. To validate these results, multiple eigen analyses were conducted with the L3-edge spectra (Figure S15).

While the speciation distribution in Figure B is only representative of Pu­(IV) species, the Pu­(IV) speciation distribution shows similar increases of PuO2(s) for hematite and AH (∼82%), and for goethite and AG (∼75%). Likewise, NH (∼75%) and NM (∼71%) show a slight decrease in the fraction of PuO2(s) relative to hematite and magnetite, respectively. However, from the perspective of the Pu­(IV)(aq) species, it is AH (∼18%) and AG (∼25%) that show a decrease in Pu­(IV)(aq), while the NH and NM show an increase (25 and 29%, respectively) relative to the bulk minerals. These results suggest the presence of multiple Pu­(IV) species associated with the mineral phase, one with stronger and shorter bonds that is consistent with PuO2(s), and one with longer and weaker bonds that is consistent with Pu­(IV)(aq). Alternatively, the fitting of these two species may represent the collective coordination environment of Pu surface complexes, where the bonds to the mineral surface are shorter than those to the surrounding waters. , Coupling M4-edge HERFD-XANES and L3-edge HERFD-XANES and EXAFS in future studies may illustrate a more thorough perspective.

The incorporation of Pu in mineral lattices is possible, but only previously demonstrated via hydrothermal synthesis or under anaerobic conditions, as opposed to the atmospheric conditions studied here. , More generally speaking, actinides (An) incorporated into crystal lattices of iron (oxyhydr)­oxide minerals show An–Fe contributions to the L3-edge XANES spectra at energies slightly higher than the white line or contribute to white line broadening; − such spectral features were absent in our samples. These structural changes due to incorporation appear to be more readily detectable and more widely investigated in the L3-edge EXAFS rather than the M4-edge spectra, representing an important area of further study.

3.4. The Role of Al Substitution and Crystallite Size on Redox and Speciation of Pu

For the Al-substituted samples, the ratio of Pu­(V) to Pu­(IV) was very similar to the Al-free samples (Figure B). This suggests that the extent of Al substitution in hematite and goethite studied here (∼8–15 Al wt %) does not lead to an impactful difference in Pu redox behavior, which is consistent with recent reactivity studies with Al-goethite. In our case, this may be due to the lower Pu to surface site ratio of the Al-substituted materials. Even if surface enrichment was favorable, Al would amount to only <15% (Table S1) of the surface sites in contact with Pu. Thus, Pu is more likely to sorb to a Fe–O site than an Al–O site, without accounting for sorption affinity. The Pu­(IV) speciation, specifically the increased alignment of Al-substituted samples with Pu­(IV)(aq), may indeed be impacted by the interaction between Pu and Al–O sites.

The nanocrystalline samples showed some of the largest reduction of Pu­(V) to Pu­(IV) (Figure B). NH showed the largest increase in Pu reduction from its bulk counterpart, likely due to its high specific surface area, thus allowing for a fast sorption step followed by a slower reduction step. ,,− In the case of NM, the difference was less clear. While the main spectral features for magnetite and NM showed good agreement with the broadening and position of the Pu­(IV) references, the small shift toward higher energies in the rising edge (∼3970 eV) contributed to the projection of Pu (V) in NM. The Fe pre-edge fitting (Figure ) suggests overall Fe­(II) depletion, or a predominance of Fe­(III), near the surface of the magnetite nanocrystals, , which may cause NM to behave more like a ferric oxide mineral and, thus, surface-mediated reduction of Pu to be less effective than in the case of bulk magnetite. Furthermore, the supernatant of the NM sample had the highest amount of Fe in solution (Table S3). Pu may have been released from NM, along with Fe, and back into solution, leading to a decrease in observed Pu reduction, more similar to that of NH.

Previous studies show that sorption is the rate-limiting step in the surface-mediated reduction of Pu. ,,− This is consistent with our results. For hematite, AH, goethite, and AG, at the low surface area and a higher Pu:surface site ratio of these materials, sorption would occur at a slower rate, thus limiting reduction. Conversely, we would expect Pu sorption to occur faster in the presence of NH and NM because of their high surface areas; the overall greater extent of Pu reduction observed in these systems suggests that reduction is the rate-limiting step. Since reduction occurs on minerals with a clear electron donor, Fe­(II) in the case of the magnetite samples, and on minerals without a clear electron donor, such as hematite and goethite, the results presented here reinforce that surface-mediated reduction of Pu likely follows Nernstian favorability of the Pu­(IV)-mineral surface complex. ,,

3.5. Environmental Implications

While the persistence and speciation of Pu­(V), the mechanism behind surface-mediated reduction to Pu­(IV), and thecontrols on PuO2(s) precipitation processes in the presence of mineral sorbents remain unclear, measurements of these systems when they are not at steady state are necessary to capture rapid redox and speciation processes at the mineral-water interface and elucidate their driving mechanisms. The iron (oxyhydr)­oxide minerals considered here are prevalent in natural surface and subsurface environments, which are relevant for studying the fate and transport of Pu in the case of radionuclide release from a nuclear waste repository and for environmental remediation of contaminated legacy waste sites. Determining the role of mineral complexity in Pu reduction is an important step in understanding the overall process of surface-mediated reduction of Pu and its driving mechanism(s). In this study, we used (i) Pu M4-edge spectroscopy to improve our understanding of surface-mediated reduction by quantifying the oxidation state of Pu on the mineral surface, where Pu­(V) and Pu­(IV) were found to coexist, and (ii) Pu L3-edge to explore the speciation of Pu associated with different mineral surfaces. Under our experimental conditions (i.e., over short time scales, at circumneutral pH, under atmospheric conditions, and approaching steady state), we determined that the extent of Pu reduction on iron (oxyhydr)­oxides is similar, irrespective of whether the minerals are Al-substituted or not, likely due to high Pu to surface site ratio. Conversely, nanocrystalline minerals led to different extents of Pu reduction; the enhanced reactivity and Pu removal (low Pu:surface site ratio) by nanocrystalline materials appears to have a greater impact on Pu redox changes. Even in more mineralogically complex systems, our M4-edge and L3-edge results reinforce the dependence of Pu redox changes on the preceding sorption interactions. This work ushers in the combination of advanced spectroscopic methods and beckons for the implementation of EXAFS, HERFD-XANES, RIXS, and computational techniques to unravel the relationships between Pu redox and speciation behavior to better predict Pu mobility and, thus, its fate and transport in the environment.

Supplementary Material

es4c13899_si_001.pdf (2.9MB, pdf)

Acknowledgments

The authors would like to acknowledge and thank Hilary Emerson for her guidance in producing the Pourbaix diagrams, Maksym Zhukovskyi for TEM training, access, and usage, Harald Foerstendorf for his support with Pu stock preparation and UV-Vis-NIR spectroscopy, and beamline scientists Sharon E. Bone, Dimosthenis Sokaras, Samuel M. Webb, Nicholas P. Edwards, Ryan Davis, Matthew Newville, and Antonio Lanzirotti for their insightful contributions. We would like to thank Christopher S. Kim, Virginia G. Rodriguez, Kimberly Ruiz Arcineda, Darian Bridges, Kelsey Anderson, Anita S. Katheras, Jennifer Szymanowski, Ginger Sigmon, Anna Matzner, Allen Oliver, Jeremy Fein, Andre Rossberg, Andreas Scheinost, Thomas Neill, Eugene Ilton, Natalia Mayordomo-Herranz, Bianca Schacherl, Sarah A. Saslow, Liane Moreau, Scott Fendorf, and Juan Manuel Lezama Pacheco for their scientific support and fruitful discussions. Moreover, we thankfully acknowledge the scientific support of Florian Otte, Clara L. Silva, and Wilken Aldair Misael at the Rossendorf beamline. We acknowledge the European Synchrotron Radiation Facility (ESRF) and HZDR for the provision of synchrotron radiation facilities under proposal number A201850, using beamline BM20 (Rossendorf Beamline, ROBL), and laboratory facilities at the Institute of Resource Ecology. We thank the Materials Characterization Facility (MCF) and the Molecular Structure Facility at the University of Notre Dame for the use of the PXRD instruments. The MCF is a core research facility within the Center for Sustainable Energy at Notre Dame (ND Energy). The freeze-drying of minerals was conducted at the Center for Environmental Science and Technology (CEST) at the University of Notre Dame. Finally, we would like to thank the Notre Dame Integrated Imaging Facility (NDIIF) for the use of the TEM. The TOC graphic was created in BioRender. Vejar, M. (2025) https://BioRender.com/t15o374.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c13899.

  • Mineral synthesis and characterization; plutonium stock solution preparation and characterization; details of HERFD-XANES measurements; other supporting experimental data as noted throughout the manuscript (PDF)

The manuscript was written through contributions of all authors. M.R.V.: experiment design and execution, writing, editing, data acquisition, analysis, and curation for LSC, BET, XRD, SEM, HERFD-XANES, and geochemical modeling. F.E.Z.: experiment design, writing, editing, and data acquisition, analysis, and curation for TEM, EDS, XRD, BET, and geochemical modeling. S.W.: Pu stock preparation, UV–vis–NIR data acquisition and analysis. S.S.A.A.: Pu stock preparation, UV–vis–NIR data acquisition and analysis. S.B.: ICP-MS sample preparation analysis and data curation. N.H.: Pu stock preparation, LSC and UV–vis–NIR data acquisition, analysis, and curation. E.F.B.: HERFD-XANES data acquisition. L.A.: HERFD-XANES data acquisition. K.O.K.: editing, HERFD-XANES data acquisition, analysis and curation, and funding acquisition. A.E.H.: experiment design, data curation, editing, and funding acquisition. All authors have given approval to the final version of the manuscript and declare no competing financial interest.

This material is based upon work supported by a National Science Foundation (NSF) Graduate Research Fellowship (grant no. DGE-1841556 awarded to M.R.V.), a University Nuclear Leadership Program (UNLP) Graduate Fellowship (grant no. DE-FOA-0002265 awarded to F.E.Z.) from the U.S. Department of Energy, and an NSF CAREER (grant no. CHE-1847939 awarded to A.E.H.). This research was also supported by the European Commission Council under ERC grant N759696 (K.O.K.). M.R.V. and F.E.Z. acknowledge the GLOBES Graduate Certificate Program in Environment and Society at the University of Notre Dame.

The authors declare no competing financial interest.

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