# How Does Nanomagnetite Stoichiometry (Fe(II)/Fe(III)) Influence the Surface Speciation of Trivalent Chromium?

Jaimy Scaria, Mathieu Pédrot, Laura Fablet, Thi Tuyen Nguyen, Yann Sivry, Charlotte Catrouillet, Ana E Pradas del Real, Fadi Choueikani, Aline Dia, Alexis Groleau, et al.

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## To cite this version:

Jaimy Scaria, Mathieu Pédrot, Laura Fablet, Thi Tuyen Nguyen, Yann Sivry, et al., How Does Nanomagnetite Stoichiometry (Fe(II)/Fe(III)) Influence the Surface Speciation of Trivalent Chromium?. SETAC Europe 34th Annual Meeting, May 2024, Seville, Spain. 2024. insu-05325736

HAL Id: insu-05325736

https://insu.hal.science/insu-05325736v1

Submitted on 22 Oct 2025

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# How Does Nanomagnetite Stoichiometry (Fe(II)/Fe(III)) Influence the Surface Speciation of Trivalent Chromium?

Jaimy SCARIA,¹ Mathieu PÉDROT,¹ Laura FABLET,¹ Thi Tuyen NGUYEN,² Yann SIVRY,² Charlotte CATROUILLET,² Ana E. PRADAS DEL REAL,³ Fadi CHOUEIKANI,³ Aline DIA,¹ Alexis GROLEAU,² and Rémi MARSAC²

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

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

³ Synchrotron SOLEIL, l'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France

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Université
de Rennes

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GEO SCIENCES
Rennes

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SYNCHROTRON

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## INTRODUCTION

* Chromium (Cr) is a prevalent metal pollutant in soils and groundwater. This study examines the interaction of 10 nm-sized nanomagnetite ($Fe^{2+}Fe^{3+}_2O_4$) with Cr(III).

* The stoichiometry of magnetite (i.e., R ratio = Fe(II)/Fe(III)) can vary from 0 to 0.5 in natural environment, where R=0 indicates the oxidised magnetite or maghemite, whereas R=0.5 indicates the stoichiometric magnetite.

* These studies are useful in predicting the fate and transport of chromium, as well as in developing magnetite-based chromium remediation processes.

* The effect of magnetite stoichiometry (R0.1, R0.2, R0.3, R0.4 and R0.5) were investigated under anoxic condition.

* The Cr speciation at mineral-water interface was examined using X-Ray absorption spectroscopy (XAS) at Cr K-edge on LUCIA beamline at SOLEIL Synchrotron (France).

## METHODOLOGY

### Material synthesis

$$Fe^{2+} + 2Fe^{3+} + 8OH^{-} \rightarrow Fe_3O_4$$

Fe³⁺ solution

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Fe²⁺ solution

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$$R = \frac{Fe^{2+}}{Fe^{3+}} = \frac{OctFe^{2+}}{OctFe^{3+} + TetFe^{3+}}$$

### Glove box

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Stoichiometries: R0.1; R0.2, R0.3, R0.4 and R0.5 (Cr(III))init : 0.020 mM ; (R)init : 13 mM ; pH :8

## RESULTS

### Cr(III) - magnetite /maghemite nanoparticles interaction

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$$R0.5 (\text{fit}), R0.1 (\text{fit}), \text{Chromite}, Cr(OH)3, K_2CrO_4 (CrVI)$$

* The Cr(III) sorption mode on R0.1 and R0.5 may be as a chromite phase.

* The presence of other Cr(III) phases such as Cr oxyhydroxide precipitation may be ruled out.

### Effect of stoichiometry

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* A gradual increase in µE intensity with increasing R value as;

$$R0.5 > R0.4 > R0.3 \approx R0.2 > R0.1$$

* The stoichiometries in between R0.1 and R0.5 shares the two surface complexes in ratios respect to R

### EXAFS Fitting

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<table><caption>Table 1: Fitting results of the EXAFS analysis R (interatomic distance), N (coordination number) and σ² (Debye-Waller factor)</caption><thead><tr><th>Interaction</th><th>Model</th><th>Shell</th><th>R (Å)</th><th>CN</th><th>σ²(Å²)</th></tr></thead><tbody><tr><td rowspan="4">Cr(III) partially substitute Fe(III)On</td><td rowspan="4">Cr(III)-R0.5</td><td>Cr-O</td><td>1.96 ±0.01</td><td>5.62±20%</td><td>0.003 ±10%</td></tr><tr><td>Cr-Cr/Fe</td><td>3.00 ±0.03</td><td>3.24 ±20%</td><td>0.002 ±30%</td></tr><tr><td>Cr-Cr/Fe</td><td>3.44 ±0.03</td><td>1.95 ±20%</td><td>0.002 ±30%</td></tr><tr><td>Cr-Cr/Fe</td><td>5.15 ±0.05</td><td>4.67 ±50%</td><td>0.002 ±50%</td></tr><tr><td rowspan="2">Tridentate edge sharing</td><td rowspan="2">Cr(III)-R0.1</td><td>Cr-O</td><td>1.97±0.01</td><td>6±20%</td><td>0.003±10%</td></tr><tr><td>Cr-Cr/Fe</td><td>3.02±0.03</td><td>3±20%</td><td>0.003±30%</td></tr></tbody></table>

## CONCLUSIONS

* A strong **interrelation of chromium surface complexation and magnetite stoichiometry**.

* Cr(III) adsorbs as a **tridentate inner sphere species** on the surface of maghemite (R0.1), whereas Cr(III) adsorb as **solid solution** in which Cr(III) takes the place of Fe(III)On onto the surface of magnetite (R0.5).

* The stoichiometries in between (R0.2, R0.3, R0.4) possess the combination of the tridentate inner sphere and solid solution in ratios with **progressive evolution with R**.

## REFERENCES

1. Fablet L, Choueikani F, Pédrot M, Kerdiles M, Pasturel M, Marsac R. Investigation of magnetite-Co interactions: from environmentally relevant trace Co levels to core-shell Fe3O4@Co(OH)2 nanoparticles with magnetic applications. *Environ Sci Nano.* **2023**; 10(11):3051–61.

2. Jungcharoen P, Pédrot M, Heberling F, Hanna K, Choueikani F, Catrouillet C, et al. Prediction of nanomagnetite stoichiometry (Fe(II)/Fe(III)) under contrasting pH and redox conditions. *Environ Sci Nano.* **2022**; 9(7):2363–71.

3. Peterson ML, Brown GE, Parks GA. Direct XAFS evidence for heterogeneous redox reaction at the aqueous chromium/magnetite interface. *Colloids Surf Physicochem Eng Asp.* **1996**; 107:77–88.

## ACKNOWLEDGEMENTS

Plateforme

Ge O He Li S

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PROJET FINANCE PAR L'ANR

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Région
BRETAGNE