# Magnetite Stoichiometry (Fe(II)/Fe(III)) Controls on Trivalent Chromium Surface Speciation

Jaimy Scaria, Mathieu Pédrot, Laura Fablet, Takumi Yomogida, Thi Tuyen Nguyen, Yann Sivry, Charlotte Catrouillet, Ana Pradas del Real, Fadi Choueikani, Delphine Vantelon, et al.

► To cite this version:

Jaimy Scaria, Mathieu Pédrot, Laura Fablet, Takumi Yomogida, Thi Tuyen Nguyen, et al. Magnetite Stoichiometry (Fe(II)/Fe(III)) Controls on Trivalent Chromium Surface Speciation. Environmental Science and Technology, 2025, 59 (11), pp.5747-5755. 10.1021/acs.est.4c12899. insu-05000328

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https://insu.hal.science/insu-05000328v1

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1
# Magnetite Stoichiometry (Fe(II)/Fe(III)) controls on
trivalent Chromium surface speciation

3 Jaimy SCARIA,¹ Mathieu PÉDROT,¹ Laura FABLET,¹ Takumi YOMOGIDA,²,³ Thi Tuyen

4 NGUYEN,² Yann SIVRY,² Charlotte CATROUILLET,² Ana E. PRADAS DEL REAL,⁴ Fadi

5 CHOUEIKANI,⁴ Delphine VANTELON,⁴ Aline DIA,¹ Alexis GROLEAU,² and Rémi MARSAC²*

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¹ Univ Rennes, CNRS, Géosciences Rennes – UMR 6118, F-35000 Rennes, France

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² Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France

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³ Nuclear Safety Research Center, Japan Atomic Energy Agency, 2-4 Shirakata, Naka-gun
Tokai-mura, Ibaraki, 319-1195, Japan

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⁴ Synchrotron SOLEIL, l'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France

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*Corresponding author: remi.marsac@cnrs.fr
**ABSTRACT.** While the elimination of the most toxic form of chromium (Cr(VI)) by its reduction to Cr(III) at magnetite (Fe₃O₄) surface has widely been documented, elucidating the exact mechanism involved in Cr(III) sorption to magnetite has attracted less attention. Indeed, magnetite stoichiometry (R = Fe(II)/Fe(III)) is rarely controlled or monitored in Cr-magnetite interaction studies, although it was shown to affect not only redox transformation but also adsorption mechanisms of several contaminants. This study examined the interaction of 20 µM (~1 mg L⁻¹) Cr(III) with 10 nm-sized magnetites, whose stoichiometries were carefully defined (0.1 ≤ R ≤ 0.5) and preserved in anaerobic conditions, in 10 mM NaCl. X-ray absorption spectroscopy showed the formation of a tridentate trinuclear inner sphere surface complex, but it occurred only on oxidized magnetite (R0.1) or on stoichiometric magnetite (R0.5) under acidic conditions where H⁺-promoted dissolution generated an Fe(II)-depleted surface. When magnetite stoichiometry increased, Cr surface speciation evolved in favor of a [Fe²⁺Cr³⁺ₓFe³⁺₁₋ₓ]ohFe³⁺ₜdO₄-like solid solution in which Cr(III) partially substitutes Fe(III) in octahedral sites. This study reveals the joint effects of pH and magnetite stoichiometry on Cr(III) sorption mechanism, and that Cr(III)- (hydr)oxide precipitation is not necessarily the driving process of Cr(III) elimination from solutions. These results will help predict the fate and transport of chromium, as well as developing magnetite-based chromium remediation processes.

**KEYWORDS.** Iron nanoparticles, Trace elements, Magnetite, Maghemite, X-ray absorption spectroscopy, Surface speciation
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<figure><img src="image_2.png" /></figure>

33 Synopsis. Trivalent chromium speciation at the magnetite surface is controlled by the magnetite

34 stoichiometry, which is affected by environmental conditions (e.g. pH, redox).
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# 1. Introduction

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Chromium (Cr) contamination, resulting from both natural sources and anthropogenic activities poses significant environmental and health risks.¹,² The bioavailability, toxicity and mobility of Cr highly depends on its speciation, which is primarily driven by its oxidation state.³ In aqueous environments, Cr(VI) mainly occurs as oxyanion (e.g. CrO₄²⁻), which is highly toxic and mobile due to its weak adsorption affinity and high solubility.⁴ By contrast, Cr(III) behaves mainly as a cation (e.g. Cr³⁺), and is much less toxic and less mobile due to its strong tendency to sorb onto particle surfaces or to hydrolyze and precipitate.⁵ As (nano)particles and colloidal surfaces have been widely shown to efficiently induce or catalyze the reduction of Cr(VI) to Cr(III), a plethora of studies has focused on the efficient removal of Cr from water and soil using nanotechnologies,⁶,⁷ or on the impact of natural particles and colloids on the Cr biogeochemical cycle.³,⁸,⁹

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Due to its widespread environmental occurrence, as well as interesting magnetic and redox properties for waste water treatment and soil remediation applications, the interaction mechanisms between magnetite (Fe<sup>II</sup>Fe<sup>III</sup>₂O₄) and contaminants has attracted much attention. Especially, reduction mechanisms of Cr(VI) at the surface of magnetite has been investigated in detail, as it can efficiently remove Cr from contaminated waters or affect the fate and behavior of Cr in natural anoxic and sub-oxic environments (e.g. subsurface soils and sediments, wetlands, anoxic groundwater systems) where magnetite occurs.⁵,¹⁰⁻¹³ Especially, magnetite stoichiometry (R = Fe(II)/Fe(III)) was shown to control the redox potential of the magnetite suspensions,¹⁴⁻¹⁶ which drastically affects the redox reactivity of magnetite. In addition, recent studies established its major influence on the surface speciation of redox inert elements or molecules such as cobalt, antibiotics and silicates.¹⁷,¹⁸ Therefore, the final state of Cr(III) bound to magnetite might also be affected by
magnetite stoichiometry, which deserves close inspection for the accurate prediction of Cr environmental behavior and fate.

The magnetite stoichiometry can easily be altered because structural Fe(II) is subjected to oxidation, or to proton/ligand-promoted dissolution.<sup>15,19,20</sup> Stoichiometric magnetite exhibits R = 0.5, which on complete loss of structural Fe(II) leads to maghemite with R = 0. In the natural environment, magnetite-maghemite solid solutions (so called “non-stoichiometric magnetites”) exist as a dynamic system with 0 ≤ R ≤ 0.5.<sup>20</sup> As it is difficult to preserve magnetite stoichiometry, it can be suspected that the local chemical environment of Cr(III) when bound to stoichiometric magnetite has never been elucidated. Indeed, many studies dealing with Cr redox speciation were performed in oxidizing conditions, under ambient atmosphere,<sup>11,21</sup> in presence of oxidizing reagents such as NaNO<sub>3</sub>,<sup>22</sup> or at relatively higher Cr(VI) concentrations with respect to Fe(II), hence leading to magnetite surface passivation through the Fe(II) to Cr(VI) electron transfer.<sup>5,23</sup> The reported Cr interaction on magnetite is possibly a maghemite-magnetite mixture rather than the stated stoichiometric magnetite. Finally, most of the previous studies investigated the removal mechanism of Cr in highly contaminated conditions, in which Cr(III) is considered to precipitate as (hydr)oxides,<sup>24,25</sup> and overlooked surface complexation of Cr(III) to magnetite.<sup>22,23</sup>

This study examined for the first time Cr(III) interaction mechanisms with magnetites whose stoichiometry was carefully controlled (0.1 ≤ R ≤ 0.5) and maintained by ensuring proper anaerobic working conditions. Experiments were conducted with initially C(VI) or Cr(III) to ensure that similar species were formed at the end of the reaction. Furthermore, as pH exerts a strong control on R,<sup>15</sup> its effects on both Cr(III) sorption and magnetite stoichiometry was taken into account (pH 4, 6 and 8). Cr redox state distribution in both liquid and solid phases was considered for a comprehensive speciation analysis.<sup>26</sup> In solid phase, X-ray absorption
spectroscopy (XAS) was used to investigate the oxidation state and local molecular environment
of adsorbed Cr(VI) or Cr(III) on magnetite. X-ray Absorption Near Edge Structure (XANES)
confirmed the oxidation state of adsorbed Cr, whereas the sorption mechanism was elucidated by
Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. In the liquid phase, total
dissolved Cr and Cr(VI) concentrations were quantified by ICP-MS and UV-Vis
spectrophotometry. Therefore, this study results may provide important information on the Cr
interaction with magnetites under various subsurface environments.
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## 2. Experimental section

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**Chemicals.** Iron (III) chloride hexahydrate FeCl₃·6H₂O (98-102%), Iron (II) chloride tetrahydrate FeCl₂·4H₂O (≥99%), Chromium trichloride hexahydrate CrCl₃·6H₂O (95%), Potassium chromate K₂CrO₄ (99.5%), Chromium(III) oxide Cr₂O₃ (≥98%), Chromium(III) hydroxide Cr(OH)₃, Hydrogen peroxide H₂O₂ (30%), Hydrochloric acid HCl (≥37%), Sulfuric acid H₂SO₄ (95-97%), Sodium hydroxide NaOH (98%), 1,5-Diphenylcarbazide, 1-10-Phenanthroline monohydrate (97%), Hydroxylamine hydrochloride (99%), Ammonium acetate CH₃COONH₄ (≥98%) and Acetic acid (glacial) (99.8%) were obtained from Sigma-Aldrich. Sodium chloride NaCl was purchased from Avantor Rectapur. Ultrapure nitric acid HNO₃ for ICP-MS analysis was received from Sigma-Aldrich. All solutions and standards were prepared with Milli-Q ultrapure water (Millipore, Merck Darmstadt, Germany specific resistivity 18.2 MΩ cm). All the experiments were performed in an anaerobic chamber (N₂-glovebox, JACOMEX, O₂(g) < 1 ppm) and all the solutions were purged with N₂(g) for at least 12 h inside the glovebox before use. All the samples were pH adjusted by HCl and NaOH (no buffer used).

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**Synthesis and characterization of magnetite nanoparticles.** The synthesis procedure involved chemical co-precipitation at room temperature under anaerobic conditions, according to a previously reported method.¹⁵,²⁷ In a previous study,¹⁸ stoichiometric magnetite (R0.5) was synthesized and characterized, and the same batch was used here. It was prepared under anoxic conditions in a glovebox from iron chloride salts (0.5 M FeCl₂·4H₂O and 1 M FeCl₃·6H₂O) added in a 1:2 Fe(II)/Fe(III) ratio. The salts were dissolved in 0.5 M HCl solution and added dropwise into a 250 mL of 0.5 M NaOH solution with continuous stirring, leading to an instantaneous precipitation of ~10 nm magnetite nanoparticles at pH 8. The resulting mineral suspension was

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109 magnetically separated and three times washed with ultrapure water adjusted to pH 8 using NaOH, 110 to avoid Fe(II) release.¹⁵ Similarly, partially oxidized magnetites (R0.4, R0.3, R0.2, and R0.1) 111 were made by adding known amounts of hydrogen peroxide (approximately 30% H₂O₂) to R0.5,²⁰ 112 according to the following formula, which assumes the oxidation of two Fe(II) by one H₂O₂ 113 molecule:

$$[H_2O_2] = \frac{[Fe(II)] - R \times [Fe(III)]}{2 \times (R+1)} \quad (1)$$

114 where R is the target magnetite stoichiometry, [Fe(III)] and [Fe(II)] are the initial concentration in 115 the stoichiometric magnetite suspension (mol L⁻¹). Then, resulting mineral suspension was 116 magnetically separated and three times washed with ultrapure water, whose pH was adjusted to 8 117 with NaOH. The stoichiometry of all synthesized suspensions (Table S1) were measured by 118 dissolving the synthesized magnetites in 5 M HCl, followed by Fe(II) content quantification by 119 the 1,10 phenanthroline method.²⁸,²⁹ Total Fe was measured after reduction of Fe³⁺ by 120 hydroxylamine hydrochloride. The R0.5, R0.3 and R0.1 samples used were from the same batch 121 of a previous study,¹⁸ in which they were characterized by X-ray diffraction and transmission 122 electron microscopy. R0.2, and R0.4 were similarly synthesized from the same R0.5 batch, but 123 were only characterized for their stoichiometry (Table S1). This synthesis approach has been 124 demonstrated to be highly reproducible and produces pure magnetite phases without structural 125 changes following controlled H₂O₂ treatments by wet chemistry, XRD, XAS and XMCD studies 126 by our group¹⁵,²⁰ and others.¹⁴ The average particle diameter of the pristine magnetite was found 127 equal to 11.5 ± 1.5 nm for R0.5, 10.6 ± 2.6 nm for R0.3 and 9.6 ± 2.6 nm for R0.1, by measuring 128 100 particles.¹⁸ Assuming spherical particles, the surface area can be estimated around 100 m² g⁻¹.¹⁹ 129 After synthesis, the magnetite samples were properly stored in an anaerobic chamber in air-130
tight bottles, and used within less than one year. Note that this procedure maintained the original stoichiometry of magnetite by preventing oxidation during more than 2 years in a previous work.²⁰

**Sorption experiments.** Batch experiments were conducted in the anaerobic chamber to measure Cr adsorption by magnetites. The 1000 mg L⁻¹ Cr(VI) and Cr(III) stock solutions were freshly prepared in ultrapure water from potassium chromate (K₂CrO₄) and chromium trichloride hexahydrate (CrCl₃·6H₂O), respectively. The effect of Cr initial oxidation state (+III or +VI), solution pH (4, 6 and 8) and magnetite stoichiometry (from 0.1 to 0.5) were taken into consideration, whereas the total Cr concentration ([Cr]ₜot = 20 µM), the background electrolyte concentration (10 mM NaCl), and total Fe concentration in the suspension (32.5 mM; equivalent to 2.5 g L⁻¹ in case of stoichiometric magnetite) were set equal for all experiments. Batch experiments were conducted in 50 mL polypropylene tubes. Dilute HCl and NaOH solutions (0.1 M) were used to regularly adjust the solution pH to desired values until the end of the experiments.

After one month of reaction time, which was assumed enough to reach a steady state,³⁰ the solid phase was separated from the liquid phase by magnetic separation and allowed to dry in the glovebox. The properly dried samples were stored in the anaerobic chamber for XAS studies. The liquid phase were passed through 0.22 µm poly(ether sulfone) (PES) filter membranes (Sartorius Minisart), then aqueous Cr ([Cr]aq) and Cr(VI) ([Cr(VI)]aq) concentrations were quantified using ICP-MS and UV-Vis spectrophotometry by the diphenyl carbazide method at 540 nm,³¹,³² respectively. A quadrupole ICP-MS (Agilent Technologies 7700X) was used. Before Cr quantification, calibration curves were performed and validated using certified material references (SLRS-6, National Research Council; Cr quantitative recovery ≥95%). A rhodium solution was used as an internal standard to correct the instrumental drift and potential matrix effects. The limit of Cr quantification was determined at 0.44 nM (22.9 ppt) (AFNOR Certification).
**XAS Analysis.** XAS was used to investigate the oxidation state and local chemical environment of sorbed Cr on magnetites. The dried Cr-magnetites samples and reference compounds (Cr(OH)₃, Cr₂O₃ and K₂CrO₄) were finely ground and homogeneously mixed with cellulose to make into pellets, which were then sealed onto the sample holder with Kapton tape. Sample transfer from the anaerobic chamber at Geosciences Rennes to the LUCIA beamline end-station, was made using air-tight bottles that demonstrated their capability to protect magnetite samples in previous studies.

¹⁵,¹⁸,²⁰,²⁷ Cr K-edge XAS spectra were collected in the energy range 5860 eV to 6900 eV using a Si(111) double crystal monochromator. Details of experimental setup and spectra treatment are given in the Supporting Information (Text S1). The spectra of reference compounds were recorded in the same conditions. Multiple spectra were recorded for each sample and were merged to obtain average spectra before background subtraction and normalization. Data processing was done using the Fastosh software.³³ Extended X-ray Absorption Fine Structure (EXAFS) oscillations were extracted using the Autobk algorithm with a cut-off Rbkg = 1 and k-weight = 2. The EXAFS spectra were converted from energy to R-space, achieving Fourier transform (FT) spectra as a function of k²χ(k) with the hanning window in the range 3 - 12 Å⁻¹. For modeling of the EXAFS spectra, the Artemis software (Ravel and Newville, 2005) from the Demeter package was employed. Phase shifts and amplitudes for relevant backscattering were calculated using FEFF 7.

Shell by shell fitting with the theoretical FeCr₂O₄ structure³⁴ was performed, to determine their coordination number (CN), interatomic distance (R) and the Debye–Waller value (σ²).

Theoretical modeling of Cr K-edge XANES spectra was conducted by the finite difference method for near-edge structure (FDMNES).³⁵ The code allowed the calculation of the occupied and unoccupied projected density of states in relation to the X-ray absorption and emission processes.

Simulations by finite-difference methods were performed to interpret the XANES. Chromite
(FeCr₂O₄)²² and γ-FeCrO₃ ³⁶ were considered as model compounds for Cr bound to magnetite and maghemite, respectively, as Cr(III) occurs in octahedral sites in these spinel compounds. In the model structure of γ-FeCrO₃, some simplifying considerations were adopted, as previously reported. ³⁶ In the model, vacancies were distributed so that a representative array of atom-vacancy distances was generated around the absorbing atoms. Octahedral positions were considered as occupied by Cr and vacancies. Tetrahedral sites were fully occupied by Fe cations. Vacancies were described by null occupancies. Simulations were performed using an atomic cluster with 6.0 Å for both FeCr₂O₄, and γ-FeCrO₃. Calculations were performed with dipole transitions and no experimental broadening was included.

**Geochemical speciation modeling.** Aqueous chemical speciation calculations were performed using PhreePlot,³⁷ which contains an embedded version of the geochemical speciation program PHREEQC.³⁸ The "minteq.v4" database was used for the calculations, in which ionic strength effects on equilibrium constants were calculated using the Davies equation. Three scenarios were considered to estimate the redox speciation of Cr. (i) No precipitation was considered, i.e., only dissolved Cr species occurred. (ii) The precipitation of Cr(OH)₃(s) was considered, which exhibited a solubility product (expressed as [Cr(OH)₂⁺]/[H⁺]) equal to 10¹.³⁴ in this database. (iii) The 2-pK diffuse layer model (DLM) implemented in PHREEQC was used to estimate potential impacts of Cr(III) and Cr(VI) binding with magnetite on Cr redox speciation. The binding of both Cr(III) and Cr(VI) are required to correctly estimate the surface redox speciation of redox sensitive elements,²⁶ as shown for actinides binding to clay minerals, for instance.³⁹,⁴⁰ The model for hydrous ferric oxide (HFO) was used,⁴¹ as it was calibrated for both Cr(III) and Cr(VI) surface complexation reactions, by contrast with more specific models for magnetite or maghemite, for instance. HFO
surface reaction constants were already available in the minteq.v4 database. Model parameters for
the surface complexation modeling are provided in **Table S2**. Redox potential values in magnetite
suspensions were calculated versus pH and magnetite stoichiometry with PHREEQC, assuming a
binary solid-solution between maghemite and magnetite, and by using the Guggenheim equation,⁴²
as described in previous studies.¹⁵,¹⁹ More details are provided in the Supporting Information (**Text**
S2 and **Table S3**).
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## 3. Results and Discussion

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**Cr redox speciation analysis in the aqueous phase.** The sorption of Cr to various magnetites was investigated under acidic (pH 4), near-neutral (pH 6), and alkaline (pH 8) conditions with $[Cr]_{tot}$ = 20 µM. Substantial amounts (>98%) of Cr were removed from the aqueous phase across all pH conditions, irrespective of the magnetite stoichiometry (**Figure 1a**). In the aqueous phase, no Cr(VI) was detected with UV-vis spectroscopy. This result was consistent irrespective of the initial Cr oxidation state, which was confirmed by conducting a similar experiment with Cr(VI) solution (**Figure S1**). Given the detection limit of ~1 µM of this technique, it can be concluded that Cr(VI) represents less than 5% of the total Cr amount. Theoretically, only 60 µM of Fe(II) were needed to reduce 20 µM of Cr(VI), which implied that all the Cr(VI) could easily be reduced by the 10.8 mM Fe(II) present in the stoichiometric magnetite suspension.⁵ Aqueous speciation calculations were performed ($[Cr]_{tot}$ = 20 µM, [NaCl] = 10 mM) in order to plot the pH-Eh predominance (“Pourbaix”) diagram of Cr (**Figure 1b**). To compare with experimental results of dissolved Cr (after removal of the solid phase), no precipitation was considered in the calculations.⁴³ The Pourbaix diagram of Cr was also compared with the theoretical redox conditions occurring in magnetite suspensions exhibiting $0.1 \le R \le 0.5$ in **Figure 1b**.¹⁵ As redox conditions coincide with the stability field of Cr(III) ($CrOH^{2+}$, $Cr(OH)_{2}^{+}$, or $Cr(OH)_{3aq}$), these calculations further suggest that Cr(III) prevailed in solution under the present experimental conditions.
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Cr removal (%) from the aqueous phase (initially Cr(III)) at different pH and magnetite stoichiometry. (b) Predominance pH-Eh diagram of Cr without considering precipitation ([Cr]tot = 20 µM, [NaCl] = 10 mM). Lines correspond to predicted redox conditions in magnetite suspensions for different stoichiometries.</figcaption></figure>

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**Cr redox speciation at the magnetite surface.** The experimental XANES spectra analysis and theoretical modeling of XANES spectra were employed to confirm the oxidation state and local symmetry of Cr(III) sorbed onto nanomagnetites (R0.1 and R0.5). The experimental spectra for Cr K-edge XANES of two end members (Cr(III)-R0.5 and Cr(III)-R0.1) along with selected Cr(III) and Cr(VI) references are given in **Figure 2**. The tetrahedrally coordinated Cr(VI) species, such as K₂CrVI>O₄, possess a distinct pre-edge peak at ~5993 eV, corresponding to the 1s to 3d electronic transition.²¹,²³ By contrast, the octahedrally coordinated Cr(III) species (FeCrIII₂O₄, CrIII(OH)₃, CrIII₂O₃) exhibit a smaller intensity for this pre-edge feature and have a shift in main edge position to lower energy.²⁵ Comparing the pre-edge feature of Cr(III)-R0.5 and Cr(III)-R0.1 to the Cr(III) and Cr(VI) references (**Figure 2**), indicates that Cr(III) is the dominant species in both samples. In addition, regardless of the initial chromium oxidation state (Cr(III) or Cr(VI)) XANES spectra are similar for a given magnetite stoichiometry (**Figure S2**, R0.1 or R0.5), indicating complete reduction of Cr(VI) to Cr(III) in both R0.1 and R0.5. This observation suggests the attainment of thermodynamic equilibrium, with Cr(III) as the predominant oxidation state. Predominance pH-Eh diagrams of Cr in the solid phase, assuming either precipitation of Cr(OH)₃(s) or surface complexation to HFO (used here as a surrogate for magnetite) as dominant removal mechanism (**Figure S3a** and **S3b**, respectively) further support that Cr(III) prevailed in/on the solid phase at the end of the experiments.

When comparing the different references corresponding to Cr<sup>III</sup>(OH)₃, FeCr<sup>III</sup>₂O₄ (spectra from<sup>23</sup>) and Cr<sup>III</sup>₂O₃, an intense white line (~6010 eV) could differentiate chromite phase from other Cr(III) references. Similar sharp white line is observed experimentally for γ-Fe₂-xCrₓO₃,<sup>25,36</sup> theoretical XANES Cr K-edge spectra of FeCr₂O₄ and γ-FeCrO₃ (**Figure S4**), and appears characteristic to Cr(III) in spinel-type structure. When comparing the different Cr(III) references with Cr(III)-R0.1
and Cr(III)-R0.5, the data indicates that Cr(III) adsorption to R0.1 and R0.5 occurs in spinel-type structure rather than oxide or hydroxide precipitate. These results are in contrast with previous studies that reported metal (hydr)oxide precipitation when higher Cr(VI) concentrations (80 mg L⁻¹) interacted with magnetite (6.5 g L⁻¹).⁴⁴ This difference highlights the importance of concentration in determining sorption mechanisms. While previous studies showed that Cr reductive sorption on magnetite surfaces can lead to microstructural modifications affecting magnetite's surface magnetization,²¹ such effects are likely negligible in the current study due to the low Cr concentration relative to Fe(II). Specifically, only 0.55% of available Fe(II) is required to fully reduce the Cr present (20µM Cr vs 10.8 mM Fe(II)), suggesting minimal impact on the magnetite stoichiometry.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** K-edge XANES spectra of Cr(III) interacted with magnetite (R0.1 and R0.5) at pH 8, and Cr references.</figcaption></figure>
266 **Cr(III) surface interaction mechanism with R0.1 and R0.5.** A closer look to the comparison
267 between theoretical XANES Cr K-edge spectra of FeCr₂O₄ and γ-FeCrO₃ with experimental
268 spectra of Cr(III)-R0.1 and Cr(III)-R0.5 (Figure S4), suggests that Cr experiences different
269 coordination environment in magnetite and maghemite-like structures. Indeed, the WL energy of
270 the simulated XANES spectrum of γ-FeCrO₃ is shifted to higher energy compared to that of
271 FeCr₂O₄. This feature is consistent with the peak shift of the XANES spectrum of Cr(III)-R0.1 to
272 higher energy compared to that of the Cr K edge of Cr(III)-R0.5. Therefore, further investigation
273 was carried out by EXAFS to reveal Cr local chemical environment.

274 The experimental and fitted FT are shown in **Figure 3**, whereas EXAFS spectra and the
275 corresponding fitted parameters are provided in SI (Figure S5 and Table S4). Based on the first
276 shell analysis of Cr(III)-R0.1 (**Figure 3a**), the data further validate that Cr atoms are octahedrally
277 coordinated by 5.5 (±1.1) oxygen ions at 1.98 ± 0.01 Å, which agrees well with previous EXAFS
278 studies.²²,⁴⁵ EXAFS data analysis cannot distinguish Cr and Fe (as any other Z ± 2) in more distant
279 shells due to the similar phase and amplitude functions of these elements.²² The second Cr-Cr/Fe
280 shell at 3.05 ± 0.02 Å corresponds to edge sharing between two adjacent Cr/Fe(III)O₆ as reported
281 in Cr(III) sorption on goethite.²⁴ Thus, based on CN in the second-shell of the Cr(III)-R0.1 sample
282 (CNCr/Fe ≈ 3.0), and as suggested for Cr(III), Am(III) and Pu(III) onto magnetite,²²,⁴⁶,⁴⁷ it is
283 concluded that Cr(III) forms trinuclear tridentate edge-sharing inner sphere complexes (**Figure 3c**)
284 at the surface of the strongly oxidized magnetite R0.1.

285 Similar to Cr(III)-R0.1, the first shell analysis on Cr(III)-R0.5 shows that Cr atoms are octahedrally
286 coordinated by 5.8 oxygen ions at 1.96 ± 0.01 Å (Figure 3b). However, three different Cr-Cr/Fe
287 shells are determined at 3.00 ± 0.02 Å, 3.43 ± 0.02 Å and 5.16 ± 0.03 Å. These interatomic
288 distances correspond to those found for Cr-Cr/FeOh (between 2.96 and 2.97 Å), Cr-FeTd (between
3.47 and 3.48 Å) and more distant Cr/Fe (5.13-5.14 Å) in FeCr₂O₄ or Cr-substituted magnetite.²³

A preliminary fit suggested that corresponding CN values of the two first ones did not significantly differ, while the third one was approximately twice larger. Therefore, coordination numbers were constrained accordingly, and CN values equal to 3.8 and 7.6 were found in the final fit. These CN values are about half of what could be expected for a Cr³⁺ ion in the bulk of an inverse spinel compound. This suggests that Cr(III) partially substitutes octahedral Fe(III) at the magnetite surface, thus forming a CrₓFe₃₋ₓO₄-like solid solution surface complex (Figure 3d).²⁵ As only a range of small x values are presently investigated, and according to the EXAFS results, the more accurate solid solution formula [Fe²⁺Cr³⁺ₓFe³⁺₁₋ₓ]OₕFe³⁺ₜdO₄ might be considered.

These findings differ from those of Peterson et al.,²² where the authors suggested that Cr(III) could form a tridentate inner-sphere complex on the magnetite surface. This discrepancy may be attributed to their use of NaNO₃ as a background electrolyte, which could have partially oxidized magnetite to maghemite.⁴⁸,⁴⁹ Consequently, the present results with oxidized magnetite (R0.1) align well with previous observation,²² also indicating that magnetite stoichiometry significantly influences Cr(III) sorption mechanisms. The enhanced incorporation of Cr(III) into octahedral positions in stoichiometric magnetite may stem from the dynamic behavior of Fe at the magnetite-water interface, driven by the redox behavior of Fe(II,III), and related fast atom exchange rate.⁵⁰

The present results are also in agreement with recent studies focused on the elucidation of surface complexation mechanisms for both organic ligands¹⁷ and Cobalt.¹⁸ In the case of Co, a preferential affinity of surface polynuclear Co species for magnetite with higher R values was observed. Conversely, magnetite with lower Fe(II) content (R0.1) was found to be less effective as a template for Co (hydr)oxide precipitation, underscoring the crucial role of Fe(II) content in determining magnetite's surface reactivity and contaminant interactions.
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312

313 **Figure 3.** Experimental (colored dotted line) and fitted (continuous black line) Fourier transform

314 (FT) EXAFS spectra for (a) Cr(III)-R0.1 and (b) Cr(III)-R0.5, and (c,d) corresponding surface

315 complexes.

316
317 **Effect of stoichiometry on Cr(III) surface speciation.** The XANES and EXAFS spectra of 318 Cr(III) bound to intermediate magnetite stoichiometries (R0.2, R0.3 and R0.4) are compared with 319 those of R0.1 and R0.5 (Figure 4). XANES analysis reveals a gradual variation in white line 320 intensity, pre-edge features (5993 eV), and post-edge region (6010 eV and 6024 eV), which 321 produces isosbestic points, when stoichiometry varies (Figure 4a & Figure S6). This trend is also 322 observed in the EXAFS spectra (Figure 4b) and FT (Figure 4c), in which spectral features in the 323 4-5 Å⁻¹ and 7-8 Å⁻¹ ranges emerge progressively with increasing stoichiometry (Figure 4b). All 324 these observations indicate that intermediate stoichiometries exhibit characteristics between the 325 two end members, Cr(III)-R0.1 and Cr(III)-R0.5. This is further supported by a two-fold difference 326 in second and third shell intensities between Cr(III)-R0.1 and Cr(III)-R0.5 in R-space (Figure 4c), 327 demonstrating the significant influence of magnetite stoichiometry on Cr(III) surface speciation. 328
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure.4.** (a) Cr K-edge XANES, (b) EXAFS experimental data and LCF (Lines are raw data and dashed lines are best fits) and (c) Fourier Transforms of the k²-weighted EXAFS spectra of Cr(III) reacted with magnetite nanoparticles showing different stoichiometries at pH 8.</figcaption></figure>
To quantify the contribution of the two identified surface species in R0.1 and R0.5 on intermediate stoichiometries, linear combination fitting (LCF) of both XANES (Figure S7) and EXAFS spectra was performed (Figure 4b). This analysis used Cr(III)-R0.1 and Cr(III)-R0.5 spectra as end-members, hence it assumed that Cr(III)-R0.1 forms 100% tridentate inner-sphere complexes, while Cr(III)-R0.5 forms 100% solid solution surface complexes. The XANES and EXAFS analysis agree very well (within 9% in the worst case), and a strong correlation between magnetite stoichiometry and Cr(III) surface speciation is evidence (Figure 5). Cr(III)-R0.2 and Cr(III)-R0.3 exhibit a predominance of tridentate inner-sphere complexes, accounting for approximately 76 ± 9% and 64 ± 8% of the total complexation, respectively. In contrast, on the surface of R0.4, Cr(III) predominantly forms a [Fe<sup>2+</sup>Cr<sup>3+</sup><sub>x</sub>Fe<sup>3+</sup><sub>1-x</sub>]<sub>Oh</sub>Fe<sup>3+</sup><sub>Td</sub>O<sub>4</sub>-like solid solution complexes (65 ± 3%) over tridentate inner-sphere complexes (35 ± 3%). These results provide quantitative insights into the evolution of Cr(III) surface complexation mechanisms as a function of magnetite stoichiometry.

<figure><img src="image_13.png" /><figcaption>**Figure 5.** Results of LCF of XANES and EXAFS spectra: percentage of tridentate surface complex versus magnetite stoichiometry at pH 8.</figcaption></figure>
Effect of solution pH on surface complexation. The influence of solution pH on Cr(III) surface
complexation models was investigated using EXAFS spectroscopy for Cr(III)-R0.1 and Cr(III)-
R0.5 (Figure S8). The stoichiometry of oxidized magnetite is pH-independent over the range of
pH conditions investigated.<sup>15,18,19</sup> Accordingly, EXAFS spectra of Cr(III)-R0.1 recorded at pH 4
and 8 are similar (Figure S8), which points to the occurrence of the same of tridentate surface
complex. By contrast, stoichiometric magnetite undergoes proton-promoted Fe(II) dissolution,
resulting in a surface composition similar to that of R0.1.<sup>15,18,19</sup>This well-documented phenomenon
has large impact on the Cr(III) surface speciation. EXAFS analysis reveals that, at pH 4, only
tridentate surface complexation is possible for Cr(III)-R0.5 (Figure S8), and its spectral features
closely resemble those of Cr(III)-R0.1. These findings underscore the critical role of solution pH
in governing trace element interactions with magnetite surfaces of varying stoichiometry,
demonstrating the complex interplay between pH, magnetite stoichiometry, and trace element
adsorption behavior.

361

362 **4. Environmental Implications**

By investigating trace Cr(III) levels that are relevant to many environments, this study elucidates
the formation of spinel-type structure when Cr(III) sorbed in/on magnetite nanoparticles,
challenging the conventional understanding that Cr(III) primarily exists as precipitated
(hydr)oxides. While the previously characterized tridentate trinuclear surface complex was
confirmed on oxidized magnetite, this study indicates, for the first time, the formation of a
[Fe<sup>2+</sup>Cr<sup>3+</sup><sub>x</sub>Fe<sup>3+</sup><sub>1-x</sub>]o<sub>h</sub>Fe<sup>3+</sup><sub>Td</sub>O<sub>4</sub>-like solid solution on stoichiometric magnetite. This revealed the
major role played by magnetites stoichiometry on Cr removal mechanism, and emphasizes the
importance of ensuring proper anaerobic working conditions on ion-magnetite binding investigations. As magnetite, prevalent in soils, sediments, and aquatic systems, plays a critical role in the sequestration of Cr(III), this mechanism may play a more important role in Cr sequestration in biogeochemical contexts compared to traditional (hydr)oxide precipitates. In addition, the interplay between magnetite Fe(II)/Fe(III) ratio, pH, and Cr(III) surface complexation mechanisms significantly influences chromium dynamics in natural environments. This variability is crucial for understanding potential remobilization of chromium under fluctuating environmental conditions. Furthermore, the stability and mobility of Cr(III) are closely tied to environmental parameters such as pH and redox potential, necessitating an examination of these factors for accurate environmental modeling. Such insights are critical for predicting changes in chromium availability across diverse ecosystems, especially in dynamic systems. Moreover, the implications of differing binding mechanisms highlight the necessity of maintaining specific pH levels and magnetite stoichiometry for effective long-term stabilization of chromium. This research not only advances our understanding of Cr fate and transport but also helps the development of optimized magnetite-based Cr remediation technologies. Ultimately, these findings emphasize the importance of integrating magnetite stoichiometry into environmental models and engineered remediation strategies, enhancing sustainability in the management of Cr-contaminated environments. To develop more comprehensive quantitative models for predicting Cr mobility and fate, of for the optimization of remediation strategies, future work should focus on incorporating surface complexation effects into thermodynamic models, as traditional approaches assuming simple precipitation mechanisms may inadequately predict Cr behavior in complex systems.
392 **Supporting Information**

393 Details on magnetite stoichiometry, XAS data acquisition, modeling procedure and additional
394 speciation modeling results, additional batch adsorption and spectroscopic data (i) starting the
395 experiments with Cr(VI) or (ii) with Cr(III) for different pH values, XAS theoretical Cr K-edge
396 XANES spectra, fitting results for XANES and EXAFS (LCF and shell-by-shell).

397

398 Acknowledgements

399 This work was supported by the COLOSSAL project funded by ANR (project number ANR-23-
400 CE01-0001; PI: R. Marsac) and the French Brittany Region SAD program COLORED. The
401 authors acknowledge the SOLEIL synchrotron for beamtime allocation at the LUCIA beamline
402 (proposal 20230144). Through the support of the GeOHeLiS analytical platform of Rennes
403 University, this publication is also supported by the European Union through the European
404 Regional Development Fund (FEDER), the French Ministry of Higher Education and Research,
405 the French Region of Brittany and Rennes Metropole. Mrs Tuyen Nguyen's work was supported
406 by a France Excellence Scholarship grant that covers her salary in France. The authors are grateful
407 to L. Dutruch and M. Pattier for assistance in ICP-MS and UV-vis analysis.
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