# Cu redox speciation at Magnetite surface: A Cu L3,2-Edge Study

Jaimy Scaria, Mathieu Pédrot, Fadi Choueikani, Laura Fablet, Anne-Catherine Pierson-Wickmann, Yann Sivry, Charlotte Catrouillet, Aline Dia, Rémi Marsac

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Jaimy Scaria, Mathieu Pédrot, Fadi Choueikani, Laura Fablet, Anne-Catherine Pierson-Wickmann, et al.. Cu redox speciation at Magnetite surface: A Cu L3,2-Edge Study. 19th SOLEIL users' meeting, Jan 2025, Synchrotron SOLEIL (Saint-Aubin), France. pp.OC-20. insu-05325677

HAL Id: insu-05325677

https://insu.hal.science/insu-05325677v1

Submitted on 22 Oct 2025

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# Cu redox speciation at Magnetite surface: A Cu L<sub>3,2</sub>-Edge Study

Jaimy SCARIA,<fnref n="1" /> Mathieu PÉDROT,<fnref n="1" /> Fadi CHOUEIKANI,<fnref n="3" /> Laura FABLET,<fnref n="1" /> Anne-Catherine PIERSON-WICKMANN,<fnref n="1" /> Yann SIVRY,<fnref n="2" /> Charlotte CATROUILLET,<fnref n="2" /> Aline DIA,<fnref n="1" /> and Rémi MARSAC<fnref n="2" />

## ABSTRACT

Copper is an essential micronutrient, abundantly found in diverse rocks and minerals, yet exhibits toxicity at elevated concentrations. The behaviour and fate of copper in both aquatic and terrestrial environments may be influenced by the redox active iron oxide, such as magnetite (Fe₃O₄) nanoparticles. However, the molecular scale mechanisms involved between Cu and Magnetite nanoparticles in aqueous solutions are still poorly understood. Especially, stoichiometric magnetite (R = Fe(II)/Fe(III) = 0.5) may partially oxidize to non-stoichiometric one (0 < R < 0.5) or to maghemite (R = 0). Several studies established the strong influence of magnetite stoichiometry either on its redox reactivity<fnref n="1" /> and or on its surface complexation properties<fnref n="2" /> towards contaminants. However, their combined effects on the thermodynamics of Cu redox transformation at magnetite surface has not been considered.<fnref n="3" /> This study examines the interaction Cu with 10 nm-sized magnetite exhibiting different stoichiometries.<fnref n="2" /> The effect of solution pH (~3 to ~10), initial Cu(II) concentration (25 µM and 500 µM), and dissolved Fe(II) ions (([Fe(II)]<sub>aq</sub> ~1000 µM) were additionally studied. The Cu speciation was monitored in the aqueous phase, by using ion selective electrode and ICP-MS, and at the magnetite surface by using soft X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Cu and Fe L<sub>2,3</sub>-edges on DEIMOS beamline at SOLEIL Synchrotron (France).

The surface Cu redox speciation depended strongly on R. At pH = 8, where all Cu was adsorbed to magnetite, Cu(II) prevailed onto oxidized magnetite (R = 0.1), but was almost completely reduced to Cu(I) when R = 0.5. High [Cu] (0.5 mM) promoted the formation of small Cu(0) amounts, which could be enhanced by the addition of an excess of dissolved Fe(II), while Cu(I) appeared stabilized by surface complexation in such reducing condition at lower Cu levels. Cu adsorption to stoichiometric magnetite was drastically affected by solution pH. Both solution and surface speciation revealed the co-occurrence of Cu(II) and Cu(I), for pH < 8, with no or negligible Cu(0) species. The [Cu(II)]/[Cu(I)] values increased with decreasing pH values, in agreement with the expected pH-redox potential conditions in magnetite suspensions.⁴

This study reveals the complex interactions between Cu and magnetite nanoparticles, with significant implications for understanding Cu biogeochemistry and developing effective remediation strategies.

## REFERENCES

1. Latta, D. E. et al. Influence of Magnetite Stoichiometry on U<sup>VI</sup> Reduction. *Environ. Sci. Technol.* **46**, 778-786 (2012).

2. Fablet, L., Choueikani, F., Pédrot, M. & Marsac, R. What are the effects of environmental factors on Co speciation at the magnetite surface? *Environ. Sci.: Nano* **11**, 2036-2048 (2024).

3. Marsac, R. et al. Equilibrium surface complexation modeling with metastable natural colloids: The key to predict the oxidation state distribution of trace elements? *Current Opinion in Colloid & Interface Science* **72**, 101820 (2024).

4. Jungcharoen, P. et al. Prediction of nanomagnetite stoichiometry (Fe(II)/Fe(III)) under contrasting pH and redox conditions. *Environ. Sci.: Nano* **9**, 2363-2371 (2022).

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

<footnote n="2">² Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France</footnote>

<footnote n="3">³ Synchrotron SOLEIL, l'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France</footnote>