# Probing the Redox Reactivity of a Reduced Nontronite: A Quick XAS Operando Study

Léo Chevrier, Sylvain Grangeon, Anthony Beauvois, Eric Bourhis, Jan Filip, Christophe Tournassat

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Léo Chevrier, Sylvain Grangeon, Anthony Beauvois, Eric Bourhis, Jan Filip, et al. Probing the Redox Reactivity of a Reduced Nontronite: A Quick XAS Operando Study. Environmental Science and Technology, 2025, 10.1021/acs.est.4c10829. insu-04974315

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# Probing the Redox Reactivity of a Reduced Nontronite: A Quick XAS Operando Study

Léo Chevrier, Sylvain Grangeon, Anthony Beauvois, Eric Bourhis, Jan Filip, and Christophe Tournassat*

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Cite This: https://doi.org/10.1021/acs.est.4c10829

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Supporting Information

**ABSTRACT:** Fe-bearing clay minerals contain structural iron that can be redox-active and can participate in electron transfer reactions with aqueous species. Although these redox properties have been studied extensively in the past decade, questions remain about the respective roles of kinetic and thermodynamic constraints in establishing steady-state redox conditions. In this study, the reduction kinetics of aqueous Cr(VI) to Cr(III) by Fe(II) contained in the structure of reduced ferruginous clay samples (reference Nontronite NAu-1) was monitored with quick-XAS (X-ray absorption spectroscopy). These measurements revealed the occurrence of at least two reaction processes with contrasting fast and slow kinetic rates. According to mass and electron balance calculations, Fe(II) located at the edge of the clay mineral particles alone cannot account for the fast reactivity of the samples, pointing out the presence of electron transfer from the inner part of the clay mineral layer structure to the reactive sites. The Fe(II)/Fe(III) ratio in the clay structure quickly reached a steady state after each Cr(VI) addition to the solution. These steady-state conditions were consistent with either a complete depletion of the Cr(VI) reactant for the first spikes of Cr(VI) or a thermodynamic equilibrium between the redox couples, i.e., between structural Fe(III)/Fe(II) and aqueous Cr(VI)/Cr(III), after the pool of fast-reacting Fe(II) was depleted. These results highlight the need to consider kinetic and thermodynamic controls of clay structural iron redox reactivity to predict the fate of redox-sensitive contaminants in the environment.

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**KEYWORDS:** redox, Fe, clay minerals, chromium, kinetics, thermodynamic equilibrium

## 1. INTRODUCTION

Structural iron in clay mineral layers is mainly redox-active.¹⁻⁵ Electrochemical methods have been used with electron mediators to measure the redox potential of the Fe(III)/Fe(II) redox couple within clay mineral structures, highlighting the influence of the amount of structural iron on the relationship between the redox potential in the aqueous solution and the Fe(III)/Fe(II) ratio in the layer structure, as well as the non-Nernstian behavior of this relationship.⁶⁻⁹ Additionally, hysteresis has been observed for this relationship depending on whether structural iron is oxidized or reduced when clay minerals undergo multiple large and rapid redox cycles.¹⁰ The redox potential of structural Fe also depends on its crystallographic environment, which can be in tetrahedral and octahedral coordination, and within the trioctahedral or dioctahedral domains in the octahedral sheet.¹¹,¹²

The ubiquity of clay minerals in soils, sediments, and sedimentary rocks makes their redox characteristics important for environmental science, particularly for a quantitative understanding of contaminant migration and remediation.¹³⁻¹⁵ The mobility of many inorganic contaminants is indeed controlled by sorption and redox reactions, either directly due

to differences in solubility as a function of the redox state, or indirectly, through precipitation and dissolution of the carrier phases of contaminants.¹⁶⁻¹⁹ However, quantitative prediction of contaminant immobilization and release through redox reactivity with clay minerals remains difficult due to the lack of a generic redox reactivity model coupled with structural and mineralogical information, which could be used in geochemical and reactive transport modeling codes.²⁰,²¹ In principle, such a model can be built based on information on the redox potential of structural iron probed by electrochemical techniques. However, differences in the interactions of aqueous redox active reactants with clay mineral surfaces in comparison to that of a given contaminant may impede the use of data obtained with the help of electron mediators. One of the current obstacles to building a clay mineral redox model is thus

Received: October 11, 2024

Revised: February 12, 2025

Accepted: February 13, 2025
the absence of studies that compare the thermodynamic and kinetic properties of redox-active reactants in the presence of clay minerals with those of the electron mediators that were used to build the redox potential scale of clay structural iron.

In this study, we investigated the redox reactivity of aqueous hexavalent chromium [Cr(VI)] with reduced NaU-1 nontronite samples. Among different clay minerals, nontronite serves as a model mineral because of its high iron content and the extensive characterization of its redox properties available in the literature, including its reactivity with inorganic and organic redox-sensitive contaminants.3,6–9,22–34 Cr(VI) is an important groundwater pollutant that can originate from anthropogenic or natural sources.35,36 The high toxicity and mobility of Cr(VI) can be controlled by its reduction to trivalent Cr(III), which is less mobile and toxic than Cr(VI).37,38 X-ray absorption near edge structure (XANES) studies have already shown that structural ferrous iron in abiotically reduced clay minerals reduces Cr(VI) to Cr(III) and that the total Fe contents and the Fe(II)/Fe(III) ratio in the clay structure are the main parameters that influence the reduction kinetics of Cr(VI).39,40 In this study, operando quick X-ray absorption spectroscopy (quick-XAS) measurements allowed observations of changes in structural iron and chromium speciation that were related to independently measured pH and E<sub>H</sub> values in clay dispersions. The thermokinetic properties probed by the Cr(VI)/Cr(III) redox couple were then compared with those of the structural Fe(III)/Fe(II) redox couple as deduced from the analyses of data from the literature.

## 2. MATERIALS AND METHODS

**2.1. Chemicals.** All chemicals used in the experiments were analytical grade: acetic acid (C<sub>2</sub>H<sub>4</sub>O<sub>2</sub>, Merck, Supelco, 96%), NaCl (Sigma-Aldrich, ACS reagent, >99%), hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>, Sigma-Aldrich, 30%), 30% HCl (Merck, Suprapur), sodium citrate (C<sub>6</sub>H<sub>5</sub>Na<sub>3</sub>O<sub>7</sub>, SAFC, EMPROVE ESSENTIAL), sodium bicarbonate (NaHCO<sub>3</sub>, Sigma-Aldrich, 99.5%), NaOH pellets (Merck, SAFC, >99%), sodium dithionite (Na<sub>2</sub>S<sub>2</sub>O<sub>4</sub>, Merck Supelco), 4-(2-hydroxyethyl)-1-piperazineethanesulfonate (HEPES, Merck, EMPROVE EVOLVE), chromium(VI) trioxide (CrO<sub>3</sub>, Merck, Sigma-Aldrich). The HEPES pH buffer has been shown to interfere with metallic cation reactions.41 However, the HEPES concentration used in the experiment was set to a level for which these interferences were considered negligible.42 Milli-Q 18 MΩ water was used in all solution preparation, clay suspension, and clay purification processes. All solutions for the reduction procedure were boiled for 1 h and cooled under N<sub>2</sub> flux overnight prior to introduction into an N<sub>2</sub> atmosphere glovebox (Jacomex GP, [O<sub>2</sub>] < 1 ppm). The dissolution of analytical-grade salts was carried out inside the glovebox at room temperature.

**2.2. Clay Material Preparation.** The nontronite NaU-1 was obtained from the Source Clay Repository (Columbia, Missouri). It was roughly powdered in an agate mortar by hand and its <2 µm fraction was obtained by centrifugation technique according to Stoke's law.43 Carbonates were removed by adding a solution with 0.1 M acetic acid and 0.5 M NaCl (pH ~ 5). The clay was then washed using three centrifugation cycles (3900g, 1 h) and redispersion in a solution of 0.5 M NaCl. This treatment was repeated twice. Then the Fe and Mn oxides were removed using a DCB (dithionite-citrate-bicarbonate) solution with dithionite

diluted in 0.2 M citrate (C<sub>6</sub>H<sub>8</sub>O<sub>7</sub>-H<sub>2</sub>O), 0.5 M NaCl, 0.1 M NaHCO<sub>3</sub> at 40 °C, followed by double washing as described above and redispersion in a solution of 0.5 M NaCl. This treatment was repeated twice. Finally, organic matter was removed using a solution of 3% H<sub>2</sub>O<sub>2</sub>, 0.5 M NaCl at 60 °C that reacted for 1 h with the clay dispersions. After cooling, the clay dispersion was washed by using three cycles of centrifugation (3900g, 1 h) and redispersion in a 0.5 M NaCl solution followed by three cycles of centrifugation and redispersion in Milli-Q 18 MΩ water. The clay material obtained from this preparation procedure is termed purified nontronite.

Purified nontronite samples with reduced Fe in their structure were prepared using a modified citrate-bicarbonate-dithionite (CBD) method.10,44 Nontronite reductions were carried out at room temperature in 50 mL centrifuge tubes, using a citrate bicarbonate buffer, with dithionite to clay ratios of approximately 0.5, 1, and 3 (g/g) and a reaction time of 24 h. The reaction was stopped by three cycles of centrifugation, collection of the concentrated solid part, and washing with a 0.5 M NaCl solution, followed by two cycles using Milli-Q 18 MΩ water instead of a 0.5 M NaCl solution. This reduction procedure was carried out in a glovebox with a N<sub>2</sub> atmosphere and an O<sub>2</sub> concentration of less than 1 ppmv. The clay materials obtained from this preparation procedure are termed reduced nontronites in the following. Samples obtained with dithionite to clay mass ratios of 0.5, 1, and 3 were used in Quick-XAS Exps. 1, 2, and 3 respectively (see Table 1).

<table><caption>**Table 1. Chemical Conditions of Quick-XAS Experiments**</caption><thead><tr><th rowspan="3">comment</th><th colspan="2">Exp. 1</th><th colspan="2">Exp. 2</th><th colspan="2">Exp. 3</th></tr><tr><th colspan="2">low frequency of Cr spiking.</th><th colspan="2">low frequency of Cr spiking</th><th colspan="2">high frequency of Cr spiking</th></tr><tr><th>no spike after 3 h (+ 4 h equilibration)</th><th>Cr spiking until the end of the experiment</th><th>Cr spiking until the end of the experiment</th><th>Cr spiking until the end of the experiment</th><th>Cr spiking until the end of the experiment</th><th>Cr spiking until the end of the experiment</th></tr></thead><tbody><tr><td>Fe(II)/Fe<sub>TOT</sub></td><td>0.43</td><td>0.44</td><td>0.44</td><td>0.44</td><td>0.51</td><td>0.51</td></tr><tr><td>solid concentration (g L<sup>-1</sup>)</td><td>3.0</td><td>2.9</td><td>2.9</td><td>2.9</td><td>2.3</td><td>2.3</td></tr><tr><td>volume of clay dispersion (mL)</td><td>80</td><td>80</td><td>80</td><td>80</td><td>80</td><td>80</td></tr><tr><td>initial pH of the clay dispersion</td><td>7.9</td><td>7.5</td><td>7.5</td><td>7.5</td><td>7.5</td><td>7.5</td></tr><tr><td>final pH of the clay dispersion</td><td>9.29</td><td>7.95</td><td>7.95</td><td>7.95</td><td>9.4</td><td>9.4</td></tr><tr><td>NaCl background electrolyte concentration (mmol L<sup>-1</sup>)</td><td>50</td><td>50</td><td>50</td><td>50</td><td>50</td><td>50</td></tr><tr><td>HEPES buffer concentration in clay dispersion (mmol L<sup>-1</sup>)</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>concentration of Cr(VI) stock solution (mmol L<sup>-1</sup>)</td><td>25</td><td>25</td><td>25</td><td>25</td><td>250</td><td>250</td></tr><tr><td>volume of each spike of Cr(VI) solution (mL)</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.02</td><td>0.02</td></tr><tr><td>number of Cr(VI) spikes</td><td>6</td><td>14</td><td>14</td><td>14</td><td>64</td><td>64</td></tr><tr><td>initial pH of the Cr(VI) stock solution</td><td>7.5</td><td>7.5</td><td>7.5</td><td>7.5</td><td>7.19</td><td>7.19</td></tr><tr><td>final Cr(VI) concentration (mmol L<sup>-1</sup>)</td><td>0.9</td><td>2</td><td>2</td><td>2</td><td>3.9</td><td>3.9</td></tr><tr><td>HEPES buffer concentration in Cr(VI) stock solution (mmol L<sup>-1</sup>)</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td></tr></tbody></table>
## 2.3. Characterization of Clay Materials.
The bulk chemical composition of the purified nontronite was analyzed by electron probe microanalysis (EPMA), using a Cameca SXFIVE electron microprobe with an acceleration voltage of 15 kV, a current beam of 10 nA, and a beam width of 1–2 µm. Before analysis, a 20 nm thick carbon layer was sputter-coated on the samples (Cressington 308R). Matrix corrections were made with the PhiRhoZ computer program.

The $ ^{57}\text{Fe} $ Mössbauer transmission spectrum of the purified nontronite was measured at 77 and 300 K in a zero-applied magnetic field using a spectrometer working in constant acceleration mode with a $ ^{57}\text{Co}(\text{Rh}) $ radioactive source.

The Fe(II)/Fe(III) ratio in purified nontronite and reduced nontronite was obtained using X-ray photoelectron spectroscopy (XPS) according to the method described in Qian et al.32 Measurements were carried out using an ESCALAB Xi+ X-ray photoelectron spectrometer (ThermoScientific) with a monochromated Al Kα X-ray source ($ h\nu = 1486.6 $ eV). High-resolution spectra were collected using an analysis area of 650 × 650 µm² and a pass energy of 20 eV. The sample was prepared by pipetting an aliquot of 0.5 mL of suspension onto a carbon sample holder inside the glovebox. The sample was then dried at 35 °C in an oven in the glovebox. The dried sample was sealed in airtight glass for transport from the glovebox to the XPS measurement device. The sample was taken out of the jar immediately before its introduction into the apparatus, where it was immediately subjected to vacuum conditions. A charge neutralizer was used for data collection, referenced to the C₁s signal (284.8 eV) corresponding to adventitious carbon. High-resolution C₁s, Fe₂p, O₁s, and Si₂p spectra were collected and fitted with a Lorentzian–Gaussian peak with a default value of 30% Lorentzian contribution and using Avantage (ThermoScientific software). The fitting of Fe²⁺ and Fe³⁺ species is quite complex compared with other metal species, and previous studies have consistently shown that Fe oxide/hydroxide species should be fitted with multiplet structures. We used multiplet fwhm, splitting, and weightings that have been theoretically calculated for Fe³⁺ and Fe²⁺ free ions from crystal field models, which have been successfully compared to experimental results.45–47

## 2.4. Quick-XAS Monitoring of Fe Speciation in Nontronite Dispersion during Cr(VI) Reduction.
Changes in the redox state and atomic environment of Fe in the nontronite structure were recorded with quick XAS spectra at the Fe K-edge (7112 eV) conducted at the ROCK beamline of the SOLEIL synchrotron (Paris-Saclay, France).48 The spectra were recorded using a Si(111) channel-cut monochromator aligned at 17.85° of the X-ray beam, with an oscillation angle around the edge of 3.95°, at a frequency of 2 Hz, resulting in the recording of an XAS spectrum every 250 ms in the energy range 6850–8500 eV. A compromise between the time resolution and good signal-to-noise ratio was found by averaging consecutive spectra over 15 s. The duration of experiments 1, 2, and 3 were 691, 459, and 370 min, respectively, corresponding to 2768, 1837, and 1323 spectra, respectively. Harmonic rejection was ensured using B₄C-coated mirrors with a grazing incidence of 3.2 mrad. For operando measurements, the same liquid cell as Vantelon et al. was used and is thoroughly described in Lesage et al.49,50 The cell was placed at 45° toward the incident X-rays to allow recording of the signal in fluorescent mode by using a passivated implanted planar silicon (PIPS) detector. The geometry and thickness of the cell (2 cm thick and an optical path of 3 mm) allowed for

vigorous stirring of the solution during the measurements. The measurement window was equipped with a 25 µm thick polyimide foil. The liquid cell was filled with a reduced nontronite suspension under anoxic conditions and bubbled with Ar to maintain anoxia. The pH and redox potential were followed as a function of time using electrodes (pH: unitrode (Metrohm), Eh: Combined Pt-ring-electrode (Metrohm)). $CrO_4^{2-}$ additions to the dispersion were automated using a remotely controlled titrimeter (Titrando, Metrohm). The experimental conditions for the operando measurements are summarized in Table 1. Exps. One and 2 have very similar conditions. A significant difference in experiment 3 is the procedure of adding the Cr(VI) solution using smaller volumes but higher concentrations than in experiments 1 and 2. The concentration of NaCl was chosen to maintain the ionic strength at a constant value, which is also representative of the composition of pore water in clay-rich rocks.51 Before the experiments, the purified nontronite spectrum was recorded to provide a reference spectrum for the fully oxidized clay mineral structure.

## 2.5. Quick-XAS Data Processing and Treatment.

Quick-XAS measurements were analyzed using the procedure described in Vantelon et al.49 Briefly, the spectra were first interpolated in a common energy grid and were averaged over 15 s to obtain a good signal/noise ratio using the 'extract_gui' Python GUI developed at the ROCK beamline.50 Due to the optical path of 3 mm, too few photons passed through the cell in transmission, and it was not possible to record at the same time the spectrum of a Fe metallic foil to calibrate the energy of the photons. Consequently, the spectra were aligned using glitches inherent to the monochromator, for which the energy position was calibrated in energy by setting the first inflection point of a Fe metallic foil to 7112 eV. The alignment and normalization of the XAS spectra were carried out using the Python GUI "normal_gui" developed on the ROCK beamline.50 The time-resolved normalized XAS data set was analyzed using chemometric methodology using FAS-TOSH.50,52–54 First, the number of pure species (components) existing in the data set was determined by principal component analysis (PCA), which reduces the dimensionality of a data set containing a large number of interrelated variables while preserving the variance of the measurements. Second, multivariate curve resolution with alternating least-squares (MCR-ALS) was used to extract the pure spectra of species within a mixture and their corresponding concentration profile. It consists of decomposing the matrix of experimental spectra into three distinct matrices: a concentration matrix, a pure species spectra matrix, and a residual matrix. To help the minimization process, the following constraints were applied to ensure meaningful results: non-negativity of the spectra and the concentration and a closure relation of the concentration, i.e., that the sum of the concentration is equal to 1 for each experimental spectra.

The time evolution of the amount of Fe(II) in the dispersion ($n_{\text{Fe(II)}}$, in mmol) was deduced from the changes in the contributions of the initial reduced nontronite (component 1, $r_{\text{red}}$) and of the fully oxidized nontronite (component 2, $r_{\text{ox}}$ = 1 – $r_{\text{red}}$), together with knowledge of the Fe(II)/Fe$_{TOT}$ ratio (R, dimensionless; Fe$_{TOT}$ is the total content of Fe in the sample) in the initial reduced nontronite, using the following equations:

$$n_{\text{Fe(II)}} = r_{\text{red}} \times R \times n_{\text{Fe}_{\text{TOT}}} \quad (1)$$
<table><caption>Table 2. Chemical Composition of the <2 µm Fraction of Purified NAu-1 According to EPMA (in Weight Percentage of Oxide and Element)</caption><thead><tr><th></th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>K<sub>2</sub>O</th><th>TiO<sub>2</sub></th><th>MnO</th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>Total</th></tr></thead><tbody><tr><th>% by weight</th><td>1.97</td><td>0.32</td><td>52.5</td><td>17.6</td><td>0.01</td><td>0.07</td><td>0.04</td><td>0.04</td><td>24.8</td><td>97.3</td></tr><tr><th>% by weight</th><td>Na</td><td>Mg</td><td>Si</td><td>Al</td><td>Ca</td><td>K</td><td>Ti</td><td>Mn</td><td>Fe</td><td></td></tr><tr><th>% by weight</th><td>1.46</td><td>0.19</td><td>24.5</td><td>9.30</td><td>0.007</td><td>0.06</td><td>0.024</td><td>0.031</td><td>17.3</td><td></td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Figure 1. High-resolution XPS spectra of the Fe₂p region: (a) purified NAu-1; (b) reduced Nau-1 sample used in experiment 1; (c) reduced Nau-1 sample used in experiment 2; (d) reduced Nau-1 sample used in Exp. 3.</figcaption></figure>

<table><caption>Table 3. ⁵⁷Fe Mössbauer Parameters of the Purified Nontronite Sample at 77 Kᵃ</caption><thead><tr><th></th><th>I.S. (mm s<sup>-1</sup>)</th><th>Q.S. or 2&epsilon; (mm s<sup>-1</sup>)</th><th>&Gamma; (mm s<sup>-1</sup>)</th><th>H</th><th>RA</th><th>doublet/sextet attribution</th></tr></thead><tbody><tr><th>sextet 1</th><td>0.51 &plusmn; 0.04</td><td>-0.29 &plusmn; 0.08</td><td>0.74 &plusmn; 0.13</td><td>50.57 &plusmn; 0.26</td><td>0.04 &plusmn; 0.01</td><td>possibly hematite or goethite</td></tr><tr><th>doublet 1</th><td>0.49 &plusmn; 0.004</td><td>1.26 &plusmn; 0.12</td><td>1.02 &plusmn; 0.07</td><td></td><td>0.41 &plusmn; 0.1</td><td>Fe(III) in the clay layer</td></tr><tr><th>doublet 2</th><td>0.49 &plusmn; 0.002</td><td>0.41 &plusmn; 0.05</td><td>0.87 &plusmn; 0.14</td><td></td><td>0.55 &plusmn; 0.1</td><td>Fe(III) in the clay layer</td></tr></tbody></table>

ᵃI.S. = isomer shift (relative to that of the α-Fe at 300 K); Q.S. = quadrupole splitting value; 2ε quadrupole shift value; Γ = line width; H = hyperfine field (T); RA = relative spectral area.

where n<sub>Fe<sub>TOT</sub></sub> is the total amount of Fe in the dispersion (in mmol),

$$n_{Fe_{TOT}} = c_{Fe,nont} \times c_{nont} \times V_{disp} \quad (2)$$

where c<sub>Fe,nont</sub> is the Fe concentration in the nontronite (in mmol g<sup>-1</sup>), c<sub>nont</sub> is the nontronite concentration in the dispersion (in g L<sup>-1</sup>) and V<sub>disp</sub> is the volume of the dispersion (in L).

**2.6. Geochemical Modeling.** Geochemical modeling calculations were carried out using PHREEQC v3 and the Thermoddem database available at https://thermoddem.brgm.fr/.²⁰,⁵⁵ Aqueous solution speciation calculations were used to calculate the mole and electron balances between Fe redox reactions (quantified with XANES), Cr redox reactions (derived from Fe(II)/Fe(III) electron balance), and H⁺/OH⁻ produced or consumed during the reactions. These calculations considered the exact volumes and concentrations characterized in the experiments but neglected the pH buffering capacity of the clay particles. The corresponding PHREEQC scripts are made available in the Supporting Information file.

## 3. RESULTS AND DISCUSSION

**3.1. Fe Content and Redox State in Purified and Reduced NAu-1 Samples.** The purified NAu-1 sample

contained 17.3 wt % Fe (Table 2), a magnetic sextet recorded with an area fraction of 0.04 was attributed to hematite or goethite,⁵⁶ and the rest of the signal is fully attributed to structural Fe<sub>TOT</sub> in the clay layers corresponding to 16.6 wt % Fe in its structure or Fe<sub>TOT, str</sub> ~ 3.0 mmol Fe<sub>TOT</sub>/g<sub>clay</sub>, which was smaller than the iron content determined in Hadi et al.¹⁰ (~4 mmol Fe<sub>TOT</sub>/g<sub>clay</sub>) on a similar material (<2 µm fraction of purified NAu-1) or in Sander et al.⁹ (~3.8 ± 0.2 mmol Fe<sub>TOT</sub>/g<sub>clay</sub>). XPS measurements revealed that the purified nontronite contained a Fe(II)/Fe<sub>TOT</sub> ratio of R = 17% (Figure 1). On the contrary, Fe(II) was not detected in the Mössbauer spectrum of the same sample (Table 3). Qian et al.³² estimated that XPS measurements may overestimate the Fe(II)/Fe<sub>TOT</sub> ratio by 5–10% in a reduced NAu-2 nontronite sample and attributed this observation to (i) an accumulation of Fe(II) at the surface of the sample through electron transfer, which is selectively probed by the surface-sensitive XPS method,⁵⁷ or (ii) to the partial reduction of Fe(III) following X-ray irradiation in UHV. Consequently, we considered that the purified nontronite had a Fe(II)/Fe<sub>TOT</sub> value of R₀ = 0.

In the reduced Nau-1 samples, Fe(II)/Fe<sub>TOT</sub> ratios of 0.42–0.51 were measured by XPS. The sample with the highest Fe(II)/Fe<sub>TOT</sub> ratio corresponded to the sample that reacted with the highest dithionite/clay ratio (Figure 1). The levels of reduction were comparable to those observed by Hadi et al.,¹⁰ who used a similar sample preparation procedure followed by
sample digestion and Fe(II)/Fe(III) chemical analyses. Consequently, we kept these values unchanged and considered them representative of the extent of Fe reduction in the structure and on the surface of our sample.

**3.2. Fast Fe(II) Oxidation Following the First Spikes of CrO₄²⁻.** The stepwise addition of CrO₄²⁻ to the reactor containing the reduced Nau-1 sample resulted in abrupt changes in the measured pH and the Eh value, concomitant to changes in the Fe oxidation state in the Nau-1 structure (Figure 2d; see the full data set in the Supporting Information files).

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Evolution of pH, E_H, and n<sub>Fe(II)</sub> as a function of time in experiments 1–3 (a–c, respectively). (d) Zoom in on the time of the first two spikes of CrO₄²⁻ in Exp. 2. The arrows on panel (d) show the timing of CrO₄²⁻ addition and its correlation with pH increase, n<sub>Fe(II)</sub> decrease and E_H decrease.</figcaption></figure>

The first addition of CrO₄²⁻ in the reduced nontronite dispersions resulted in a concomitant increase in pH value, a decrease in the measured Eh value (Figures S2–S7 in the SI file), and a change in E₀ of the Fe K-XANES spectra to higher energy values, the latter being indicative of Fe(II) to Fe(III) oxidation (Figure 2d).<sup>7,31,32,58,59</sup> All of these experimental events occurred simultaneously and were thus attributed to the redox reaction of CrO₄²⁻ with Fe(II).

Several species of Fe(II) are present in the dispersion, which are assumed to have different reactivities toward aqueous oxidative species: Fe(II) in the nontronite layer inner-structure, Fe(II) on the nontronite layers edges, and aqueous Fe(II) originating from the dissolution of the clay mineral particles.<sup>5,7</sup> The reactivity of aqueous Fe(II) is not a good candidate to explain the experimental observations for, at least, two reasons that can be quantified with experiment 2: first, n<sub>Fe(II)</sub> decreased from 0.32 to 0.27 mmol in the first minute after the addition of Cr(VI), corresponding to a change in the Fe(II) amount of Δn<sub>Fe(II)</sub> = 0.05 mmol, or equivalently to a change in the Fe(II) concentration of Δ[Fe(II)] = 0.63 mmol L⁻¹. Such a high initial concentration of aqueous Fe(II) is not expected because of the multiple washing steps at the end of the sample preparation procedure. Second, the reduction of Cr(VI) by aqueous Fe(II) would obey the reaction:

$$ \mathrm{CrO_4^{2-}} + 3\mathrm{Fe^{2+}} + 4\mathrm{H_2O} \rightarrow \mathrm{CrOH^{2+}} + 3\mathrm{Fe^{3+}} + 7\mathrm{OH^{-}} \quad \text{(R1)} $$

followed by rapid precipitation of iron and chromium hydroxides, or mixed Fe³⁺-Cr³⁺ hydroxides, and Cr³⁺ adsorption on the surfaces of Fe(III) colloids.<sup>17,60,61</sup>

Considering the following stoichiometries of Fe³⁺ and Cr³⁺ precipitation:

$$ \mathrm{Fe^{3+}} + 3\mathrm{OH^{-}} \rightarrow \mathrm{Fe(OH)_3} \quad \text{(R2)} $$

$$ \mathrm{CrOH^{2+}} + 2\mathrm{OH^{-}} \rightarrow \mathrm{Cr(OH)_3} \quad \text{(R3)} $$

$$ \begin{array}{l} 0.75\mathrm{Fe^{3+}} + 0.25\mathrm{CrOH^{2+}} + 2.75\mathrm{OH^{-}} \\ \rightarrow \mathrm{Fe_{0.75}Cr_{0.25}(OH)_3} \quad \text{(R4)} \end{array} $$

Although the possible contribution of these three mechanisms to the immobilization of Fe³⁺ and Cr³⁺ could not be investigated with the available data, the precipitation of Fe(OH)₃, Cr(OH)₃, and/or Fe<sub>0.75</sub>Cr<sub>0.25</sub>(OH)₃ after oxidation of aqueous Fe²⁺ would lead to a theoretical value of 4/3 OH⁻ consumed per oxidized Fe²⁺. This stoichiometry would lead to a decrease in pH, contrary to experimental observation. For example, in Exp. 1, the pH value increased from 7.6 to 8.6 in the first minute after Cr(VI) addition and then decreased more slowly to values above 8, that is, higher than the initial pH value (Figure 2). Consequently, the homogeneous redox reaction with aqueous Fe(II) must have a negligible influence compared with that of the redox reaction of Fe(II) associated with the nontronite layer.

Knowledge of the initial composition of the solution (Table 1) and the evolution of the pH made it possible to calculate the production of OH⁻ at the peak of the pH value. These speciation calculations neglected the pH buffering capacity of the clay particles. In experiment 2, the first peak of the pH value at 8.8 corresponded to a production of 0.037 mmol of OH⁻. Meanwhile, XAS data indicated that 0.05 mmol of Fe(II) was oxidized to Fe(III) at the same time, which is in good agreement with the amount of Fe(II) necessary to completely reduce the 0.013 mmol of Cr(VI). These stoichiometries are compatible with the following reaction:

$$ \begin{array}{l} \mathrm{CrO_4^{2-}} + 3\equiv\mathrm{Fe^{II}(OH)_2} + 4\mathrm{H_2O} \\ \rightarrow \mathrm{Cr(OH)_3} + 3\equiv\mathrm{Fe^{III}(OH)_3} + 2\mathrm{OH^{-}} \quad \text{(R5)} \end{array} $$

in which ≡Fe<sup>II</sup> and ≡Fe<sup>III</sup> represent Fe(II) and Fe(III) reactive sites present in the clay mineral layer or adsorbed on the edge surfaces. The overall stoichiometry of the reaction is consistent with the experimental results but does not preclude the existence of multiple reaction pathways with distinct stoichiometries and kinetic rates. Unfortunately, the decrease in pH that followed the first high-pH peak cannot be unambiguously attributed to one specific reaction. The pH buffering capacity of the edge surfaces of the clay layers, the slow dissolution of the clay layers at high pH, and the difference in the reaction rates of ≡Fe<sup>II</sup>(OH)<sub>2</sub> oxidation and Cr(OH)<sub>3</sub> precipitation are all valid hypotheses that cannot be unraveled from the available data.

Similar results were obtained in experiment 1, with conditions similar to those of experiment 2. The first peak of the pH value corresponded to a production of 0.032 mmol of OH⁻ and a reduction of 0.04 mmol of Fe(II) for the same addition of 0.013 mmol of Cr(VI). In Exp. 3, for which Cr(VI) additions were made with smaller spikes at a higher frequency, the noise-to-signal ratio of the XAS signal did not allow a precise estimation of the amount of Fe(II) that reacted in a single step. Hypothesizing that the three first spikes of Cr(VI) were reduced entirely, which amounted to 0.015 mmol of Cr(VI), the OH⁻/Fe(II)/Cr(VI) stoichiometry was estimated
<figure><img src="image_10.png" /><figcaption>Figure 3. Modeled $n_{Cr(VI)}$, $n_{Cr(III)}$, $n_{CrTOT}$ (left figures a, c, and e) and measured $n_{Fe(II)}$ (right figures b, d, and f) as a function of time computed according to eq 3. (a and b) Exp. 1; (c and d) Exp. 2; (e and f) Exp. 3.</figcaption></figure>

to be 0.017/0.045/0.015 mmol, i.e., a lower production of $OH^-$ per oxidized Fe(II) than in the two other experiments.

Following the fast increase in pH that corresponded to Fe(II) oxidation and Cr(VI) reduction, a slower decrease in pH was observed in all experiments. This decrease in pH can be attributed, at least, to the equilibration of the clay edge surface amphoteric groups with the pH of the solution and a limited dissolution of the clay particles.⁶²⁻⁶⁶ Note that Fe(II) edge amphoteric groups have higher $pK_a$ values compared to their Fe(III) counterparts.⁶⁷,⁶⁸ According to surface complexation models, the oxidation of Fe(II) into Fe(III) on the edge surfaces of the clay mineral layer would then be responsible for the release of some protons, and thus a pH decrease, following the reduction and hydroxylation of the sites.⁶⁹

### 3.3. Evidence of Fast and Slow Reaction Processes.

The amounts of Cr(VI) remaining in solution ($n_{Cr(VI)}$ in mmol) and precipitated Cr(III) ($n_{Cr(III)}$ in mmol) were estimated as a function of time with the following equations:

$$n_{Cr(VI)} = n_{CrTOT} - \frac{\Delta n_{Fe(II)}}{3} \quad (3)$$

$$\Delta n_{Fe(II)} = n_{Fe(II),init} - n_{Fe(II)} \quad (4)$$

$$n_{Cr(III)} = n_{CrTOT} - n_{Cr(VI)} \quad (5)$$

where $n_{CrTOT}$ is the total amount of added Cr into the clay dispersion and $n_{Fe(II),init}$ is the $n_{Fe(II)}$ value at the start of the experiment and $n_{Fe(II)}$ is calculated from the concentration profile obtained by applying the MCR-ALS on the quick-XAS data sets at Fe K-edge. According to these calculations, the two first spikes of Cr(VI) in experiments 1 and 2 were completely reduced to Cr(III) (Figure 3). Negative values that were obtained after the first Cr(VI) spike in experiments 1 and 2 may be attributed to a slight overestimation of the Fe(II)/Fe$_{TOT}$ ratio using XPS analysis, as emphasized in the materials and methods section.

Starting from the second spike, the pH of the clay dispersion decreases right after the addition of the Cr (VI) solution (Figure 2) because this solution was buffered at pH ~ 7.5 with 50 mmol L⁻¹ of HEPES. This short pH decrease was immediately followed by an increase corresponding to the oxidation of $\equiv Fe^{II}$ and the reduction of Cr(VI). The increase in pH slowed with the increasing number of Cr(VI) spikes, in agreement with the XAS measurements, which also showed slower oxidation rates of Fe(II) (Figure 2). These observations evidenced the presence of, at least, two reaction processes with contrasting fast and slow kinetic rates, in agreement with those observed with other reduced ferruginous smectites reacting with oxidants such as $O_2$ or nitroaromatic compounds.⁵⁷,⁷⁰⁻⁷² However, the present data have a much higher time resolution to characterize the initial reactivity, which may point to different mechanisms.

### 3.4. Mechanism of the Fast Reaction.
Fe(II) in reduced nontronite exhibits various structural environments.⁷³,⁷⁴ Its location in the particle, whether in the inner structure or on the edge surfaces of the particle, can also influence its reactivity.⁵⁷ At a reduction level ($\equiv Fe^{II}/\equiv Fe_{TOT}$) below 30%, the charge deficit created by the reduction is mostly compensated by the adsorption of additional cations in the interlayer space of the nontronite particles and by a limited dehydroxylation of the structure.¹⁰ As the reduction intensifies, which was the case of our reduced nontronite samples, the growing lattice distortion leads to a reorganization of the octahedral and tetrahedral lattice with migration or release of cations, intense dehydroxylation and bonding of protons to undersaturated oxygen atoms.¹⁰ This reorganization of the lattice leads to the presence of trioctahedral domains, in which Fe(II) atoms are clustered in Fe(II)-Fe(II)-Fe(II) entities.⁷³,⁷⁴ In the inner structure, trioctahedral Fe(II)-Fe(II)-Fe(II) entities are more reactive than dioctahedral Fe(II)-Fe(II), Al-Fe(II) and Fe(II)-Fe(III) entities.⁷⁵ Fe(II) located on the edge surfaces is also very reactive.¹,²⁵,⁷⁶ Fe(II)-Fe(II)-Fe(II) entities and individual Fe(II) on the edge surfaces are thus
likely candidates to explain the very fast reaction kinetics observed in the present experiments.

The fast-reacting process led to a complete reduction of Cr(VI) to Cr(III), in the limit of accuracy of the data. The corresponding reactive site density was $d_{fast} \sim 0.3$ mmol g$^{-1}$, which represents ~20–30% of the initial Fe(II) content of the reduced nontronite samples. Considering a typical edge surface area between 5 and 20 m$^2$ g$^{-1}$ and using crystallographic information, the maximum density of octahedral edge sites can be estimated to values between 0.02 and 0.08 mmol g$^{-1}$, which is significantly lower than $d_{fast}$.⁶⁹,⁷⁷ Consequently, the fast-reacting process cannot be attributed to the reactivity of Fe(II) on the edge surfaces alone, and a redox reaction of Cr(VI) with Fe(II)–Fe(II)–Fe(II) entities through the basal plane or a rapid e$^-$ transfer from Fe(II) in the inner-structure to newly formed Fe(III) on the edge surfaces must have taken place as hypothesized by Joe-Wong et al.⁴⁰

**3.5. Thermodynamic vs Kinetic Control.** In experiment 1, the addition of Cr(VI) to the clay dispersion was stopped at t = 3 h and the system was monitored for an additional 9 h. At t = 4 h, $n_{Fe(II)}$ reached a steady-state value of 0.21 mmol, while Cr(VI) was still present in the solution (Figure 3). The halt of the reaction was caused by either a decrease of the electron transfer rate to a negligible value or reaching a redox equilibrium between the Cr(VI)/Cr(III) and the $\equiv Fe^{III}/\equiv Fe^{II}$ redox couples. In experiments 2 and 3, Cr(VI) additions to the clay dispersion took place regularly until the end of the experiments, and the total added Cr concentration increased to values higher than the final value in experiment 1. Meanwhile, $n_{Fe(II)}$ decreased continuously in these two experiments and reached values below those registered at the end of experiment 1. These observations contradicted the first hypothesis of a decrease in the electron transfer rate down to negligible values, leaving the redox equilibrium hypothesis as the most plausible.

The redox potential in solution was calculated using the following procedure: (i) the total concentration of aqueous Cr(VI) was obtained from eq 3, neglecting Cr(VI) adsorption on clay mineral surfaces; (ii) the activity of $CrO_4^{2-}$ was calculated accordingly with PHREEQC; (iii) the activity of aqueous $Cr^{3+}$ was calculated directly considering that aqueous Cr(III) was in equilibrium with chromium hydroxide, $Cr(OH)_3$ at the time prior to the next addition of $CrO_4^{2-}$. With these assumptions, the calculated redox potential was in good agreement with the measured potential (Figure 4), thus giving confidence in the estimation of the redox potential probed by the Cr(VI)/Cr(III) redox couple.

The redox potential of the $\equiv Fe^{III}/\equiv Fe^{II}$ couple in the reduced nontronite structure was not directly probed in our experiments, but it can be estimated from the literature. In particular, Gorski et al. developed electrochemical methods using electron mediators to quantify this property.⁶,⁸,²³ Two difficulties arise when comparing the calculated Cr(VI)/Cr(III) redox potential and the redox potential of $\equiv Fe^{III}/\equiv Fe^{II}$ in the layer structure. First, the data from Gorski et al.⁷ evidence a hysteresis between the measurements made with an oxidized nontronite being reduced and a reduced nontronite being reoxidized (Figure 5a). Because the experimental conditions of this study correspond to the latter case, the hysteresis branch of the electrochemical measurement with a reduced nontronite was considered. Second, the redox potential of $\equiv Fe^{III}/\equiv Fe^{II}$ in the layer structure is dependent on the pH value.⁹ The starting pH values in the present experiments were comprised between 7.5 and 8, i.e., in

<figure><img src="image_11.png" /><figcaption>**Figure 4.** Comparison of measured (crosses) and calculated (empty diamonds) redox potentials in experiments 1–3 (a–c, respectively). The measured and modeled values differed by 60 ± 7, 61 ± 8, and 68 ± 7 mV for Exps. 1, 2, and 3 respectively.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 5.** Comparison of computed Cr(VI)/Cr(III) redox potentials with $\equiv Fe^{III}/\equiv Fe^{II}$ redox potentials in the layer structure measured by Gorski et al. using electrochemical characterization.⁷ (a) NAu-1 data from ref 7 showing the hysteresis of the redox potential along the oxidizing (open black diamonds) and reducing pathways (open black circles and open gray squares). (b) Comparison of Cr(VI)/Cr(III) redox potentials (data from this study: full blue symbols; circles = Exp. 1; diamonds = Exp. 2; triangles = Exp. 3) with $\equiv Fe^{III}/\equiv Fe^{II}$ redox potentials along the oxidizing pathway (data from ref 7 open symbols; black diamonds = NAu-1; orange squares = NAu-2; brown circles = SWa-1; the Fe content is indicated for each sample on the figure) (c and d) Principle of the conversion of an $E_{H}$ redox scale (c) to a $f_{O_2}$ redox scale (d) (data from ref 9 obtained on SWa-1 samples; pH conditions are given in the inner caption).</figcaption></figure>

agreement with the conditions probed in the experiments of Gorski et al.⁷ However, after the first Cr(VI) addition into the
clay dispersion, the pH increased to a value up to 9.5 and then fluctuated as a function of Cr(VI) additions.

The value of the redox potential (in V) is very dependent on the pH conditions in the presence of reactions involving H⁺. Because the clay layer $≡Fe^{III}/≡Fe^{II}$ redox reactivity is coupled to (de)hydroxylation reactions, it is expected that the corresponding redox potential varies with pH, which is indeed the case (Figure 5c). Therefore, it is necessary to correct for this influence of the pH on the redox potential value. The fugacity of O₂ (or H₂ or their partial pressure) is also a redox scale commonly used in the literature, which has the advantage of not being dependent on the pH value. O₂ fugacity values in the experiment of Sander et al. were calculated with PHREEQC, using the reported pH and Eh values.⁹ This change in the redox scale made it possible to reconcile the data for the $≡Fe^{III}/≡Fe^{II}$ redox couple obtained at different pH values from 5.5 to 9.0 in Sander et al. (Figure 5d).⁹ Once plotted on this $f_{O_2}$ scale, the computed Cr(VI)/Cr(III) redox potential at equilibrium with Cr(OH)₃ and the redox potential of $≡Fe^{III}/≡Fe^{II}$ in the layer structure of NAu-1 showed reasonable agreement (Figure 5b). Noting that the NAu-1 nontronite sample used in the present study had a lower total Fe content (16.6 wt % Fe), an even better agreement was found if the data were compared to that of the NAu-2 nontronite (19.2 wt % Fe).

Our results are in favor of thermodynamic control of the Cr(VI) reduction reaction at the end of each Cr(VI) addition step. However, hysteresis in the $≡Fe^{III}/≡Fe^{II}$ versus $E_H$ relationship makes it difficult to use such a local equilibrium model in the presence of fluctuating redox conditions, which can impede its use for the prediction of the fate of contaminants in soils that undergo cycles of redox conditions as a function of seasonal events.⁷⁸ The reason for this hysteresis has been explored recently by Pothanamkandathil et al.³⁴ who showed that a model incorporating interfacial electron transfer kinetics and charge redistribution within the particle was successful in predicting the full set of redox data obtained with electrochemical methods using electron mediators as a function of time. Using Gorski et al. data⁸ (their Figure 3 for a Swa-1 smectite sample), the initial electron transfer rate of the clay structure to the electrode using an electron mediator can be estimated to be a few µmol g⁻¹ s⁻¹. In experiments 1 and 2, the fast oxidation of $Fe^{2+}$ after the addition of $CrO_4^{2-}$ took place in approximately 1 min. Using Quick-XAS and the corresponding variations in $n_{Fe(II)}$, the electron transfer rate can be estimated to ~0.5 µmol g⁻¹ s⁻¹ in our experiments. Hence, we can conclude that the fast oxidation rate observed in our experiment obeys electron transfer rates similar to those probed by electron mediators in the experiments of Gorski et al.⁸ Pothanamkandathil et al.³⁴ also showed that the redox potential values obtained after an equilibration period of 12–24 h in the presence of electron mediators and an imposed potential were in between the values of the two hysteretic branches obtained with a shorter equilibration period (30 min reaction). In experiments 1 and 2, stepwise injections of $CrO_4^{2-}$ occurred every 30 min. The fact that the redox values obtained in our experiments were in good agreement with the values of the upper hysteresis branch with a reduced nontronite is thus in good agreement with the results of Pothanamkandathil et al.³⁴

## 4. ENVIRONMENTAL IMPLICATIONS

Our results improve our understanding of how redox-active iron in clay minerals interacts with redox-reactive contaminants, applied here to Cr(VI). By improving our understanding of the mechanisms and controls behind these redox reactions, this research helps predict and manage the fate of redox-sensitive contaminants in natural systems, which is crucial to developing effective strategies for environmental remediation and pollution control. Although it lacks yet genericness, the good agreement between the redox potential probed by the aqueous Cr(VI)/Cr(III) and the $≡Fe^{III}/≡Fe^{II}$ redox couples represents a step forward to justify the use of the relationship between structural $≡Fe^{III}/≡Fe^{II}$ ratio, $Fe_{TOT}$ content and $f_{O_2}$ values to quantify contaminants' mobility in the presence of reduced clay minerals with geochemical calculation codes. According to our analyses, redox near-equilibrium was attained in less than 1 h, making it possible to use a thermodynamic local equilibrium model to make quantitative predictions. This model could be refined with kinetic parameters extracted from the study by Pothanamkandathil et al.³⁴ to simulate transient redox conditions within periods of less than 12 h. This result is especially important for applications such as modeling of geological radioactive waste repositories, in which oxidized radionuclides and chemotoxic compounds are expected to interact with reduced clay minerals during their migration from the waste canister through geological barriers.¹⁶,⁷⁹ Additional studies probing a wider range of inorganic and organic compounds and different levels of iron reduction in clay mineral structures would be necessary to assess the reliability of this thermodynamic approach to predict clay redox activity in natural environments.

<figure><img src="image_13.png" /></figure>

### ASSOCIATED CONTENT

<figure><img src="image_14.png" /></figure>

#### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c10829.

Mössbauer data, scheme of the measurement cell, quick-XAS data, timetable of Cr addition in quick-XAS experiments, and PHREEQC scripts used for geochemical modeling (PDF)

<figure><img src="image_15.png" /></figure>

### AUTHOR INFORMATION

#### Corresponding Author

Christophe Tournassat – ISTO, UMR 7327, Univ Orléans, CNRS, BRGM, OSUC, 45071 Orléans, France; Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley 94720 California, United States;

● orcid.org/0000-0003-2379-431X;

Email: christophe.tournassat@univ-orleans.fr

#### Authors

Léo Chevrier – ISTO, UMR 7327, Univ Orléans, CNRS, BRGM, OSUC, 45071 Orléans, France

Sylvain Grangeon – BRGM, 45060 Orléans, France;
● orcid.org/0000-0002-5018-3015

Anthony Beauvois – Synchrotron SOLEIL, L'Orme des Merisiers, 91192 Gif-sur-Yvette Cedex, France

Eric Bourhis – ICMN, CNRS, UMR 7374, Université d'Orléans, 45071 Orléans, France

Jan Filip – Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research
Institute, Palacký University, 78371 Olomouc, Czech Republic; orcid.org/0000-0001-6720-5770

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.est.4c10829

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The PhD grant of L.C. was funded by the Région Centre Val de Loire (France). L.C. acknowledges additional support from the European Union (ERC, COCONUT, 101043288) C.T. acknowledges a grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" Programme LabEx VOLTAIRE, 10-LABX-0100. Mössbauer analyses were acquired at Olomouc, Czech Republic, as part of a BARRANDE (Hubert Curien) exchange program with ISTO and supported by ERDF/ESF project TECHSCALE (CZ02.01.01/00/22 008/0004587). The authors acknowledge the SOLEIL scientific committee for providing beamtime at the ROCK beamline (proposal 20230173). The work at ROCK was supported by a public grant overseen by the French National Research Agency ANR as part of the "Investissements d'avenir" program ref: ANR-10-EQPX-45. This work has received support under the CERTeM 2020 Program, with the financial support of the Regional Council Centre-Val de Loire (CERTEM 2020 201400096812) and cofunded by the European Union through the European Regional Development Fund (ERDF 2017-EX002247).

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