# Co(II) oxidation to Co(III) at the Fe 3-δ O 4 surface in oxidizing environments

Laura Fablet, Fadi Choueikani, Rémi Marsac, Anthony Beauvois, Thomas

Stephant, Valérie Briois, Mathieu Pédrot

► To cite this version:

Laura Fablet, Fadi Choueikani, Rémi Marsac, Anthony Beauvois, Thomas Stephant, et al. Co(II) oxidation to Co(III) at the Fe 3-δ O 4 surface in oxidizing environments. 19th SOLEIL users' meeting, Jan 2025, Synchrotron SOLEIL (Saint-Aubin), France. pp.PO-GEO-16. insu-05325656

HAL Id: insu-05325656

https://insu.hal.science/insu-05325656v1

Submitted on 22 Oct 2025

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# Co(II) oxidation to Co(III) at the Fe₃-δO₄ surface in oxidizing environments

Laura Fablet<fnref n="1" />,<fnref n="2" />, Fadi Choueikani<fnref n="2" />, Rémi Marsac<fnref n="3" />, Anthony Beauvois<fnref n="2" />, Thomas Stephant<fnref n="4" /> Valérie Briois<fnref n="2" /> et Mathieu Pédrot<fnref n="1" />

## ABSTRACT

Understanding and predicting the interaction mechanisms between Co and spinel iron oxide nanoparticles (Fe₃-δO₄) is of major interest in several contexts. Indeed, Fe₃-δO₄ nanoparticles can be used for soil and water decontamination due to their high surface reactivity. In addition, their magnetic properties enable them to be used for industrial applications, with the synthesis of nanoparticles decorated with metals. Interactions between Co and Fe₃-δO₄ nanoparticles have been well documented under anaerobic conditions, with descriptions of Co<fnref n="2" />⁺, polymerization and surface precipitation.<fnref n="1" />,<fnref n="2" /> However, Fe₃-δO₄ nanoparticles are mostly used in the presence of O₂, and the interaction mechanisms in these conditions have attracted less attention. Therefore, the influence of O₂ on Co adsorption and redox speciation at Fe₃-δO₄ nanoparticles surface was studied.

Magnetite (Fe₃O₄) nanoparticles were synthesized by co-precipitation of iron salts under anaerobic conditions, then oxidized with H₂O₂ to obtain Fe₃-δO₄ with R = [Fe(II)]/[Fe(III)] = 0.1. Adsorption experiments were carried out under aerobic conditions with Co concentrations ranging from 0.1 to 12 mM, at pH 8, at room temperature. A part of these samples was analyzed at SOLEIL synchrotron by X-ray spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at L₂,₃-edges on DEIMOS beamline. To follow the adsorption kinetics of Co on Fe₃-δO₄ nanoparticles, Quick-XAS experiments were performed at K-edge on ROCK beamline. The recorded spectra were processed by PCA and MCR-ALS analysis, and EXAFS fitting.

Adsorption experiments show two different behaviors of Co onto Fe₃-δO₄ nanoparticles, according to Co concentration, with (i) the polymers at low Co concentration and (ii) the Co precipitation at high Co concentration. These changes in surface speciation are also distinguished by the change in oxidation state of Co<fnref n="2" />⁺ to Co<fnref n="3" />⁺ as determined by XAS and XMCD analyses at high concentration at L₂,₃-edges. For high Co concentration, the adsorption kinetic at K-edge have enabled to differentiate three species over time with (i) the Co<fnref n="2" />⁺ in solution, (ii) the formation of a mixture of several Co(II) species and (iii) the partial oxidation of Co(II) to Co(III), leading to the formation of a Co₃O₄ precipitate.

This study highlights the formation of mixed species of Co(III) and Co(II) onto the magnetite surface, depending on the initial Co concentration, which might play a role in Co behavior and fate in oxic environments with the presence of Fe₃-δO₄ nanoparticles, but also to control the synthesis of Fe₃-δO₄ nanoparticles decorated with Co for magnetic or catalytic applications.

<footnote n="1">¹Univ Rennes, CRNS, Géosciences Rennes – UMR 6118, F-35000 Rennes, France</footnote>

<footnote n="2">²Synchrotron SOLEIL, l'Orme des Merisiers, Saint-Aubin - BP48, 91192 Gif-sur-Yvette, France</footnote>

<footnote n="3">³Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France</footnote>

<footnote n="4">⁴Univ Rennes, CNRS, ISCR – UMR 6226, F-35000, Rennes, France</footnote>
REFERENCES (Font: Arial, 11, Bold)

1. L. Fablet, F. Choueikani, M. Pédrot, M. Kerdiles, M. Pasturel and R. Marsac, *Environ. Sci.: Nano* **10**, 3051-3061 (2023).

2. L. Fablet, F. Choueikani, M. Pédrot and R. Marsac, *Environ. Sci.: Nano* **11**, 2036-2048 (2024)