# Theoretical estimation of iron isotope fractionation during cloud processing of aerosols

Guillaume Dhont, Carmen Ciotonea, Daniele Fontanari, Hisham Khalifeh Al

Rawas, Maya Marinova, Pascal Flament

► To cite this version:

Guillaume Dhont, Carmen Ciotonea, Daniele Fontanari, Hisham Khalifeh Al Rawas, Maya Marinova, et al.. Theoretical estimation of iron isotope fractionation during cloud processing of aerosols. Workshop LEFE/CHAT on theoretical physical chemistry and atmospheric chemistry, Jul 2025, Marseille, France. hal-05170026

HAL Id: hal-05170026

https://hal.science/hal-05170026v1

Submitted on 18 Jul 2025

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Distributed under a Creative Commons Attribution 4.0 International License
## Introduction

Iron (Fe) is an essential micronutrient for the development of oceanic biomass, which is one of the main carbon sinks on a global scale. In about 30% of the oceans, the development of this biomass is constrained by an iron deficiency that limits primary production. This deficiency is related to the very low concentrations of bioaccessible Fe in the surface waters. In order to understand the origin and consequences of these low Fe levels (some ng Fe/L), various models have sought to determine the variability of Fe inputs to the ocean, particularly via aerosols. Recent progress in the measurement of Fe isotopic composition has made it possible to test the sensitivity of these models to spatiotemporal variations in inputs[1]. However, this approach is made difficult by the existence of isotopic fractionation phenomena that would be linked to the dissolution of particles within clouds. Our previous works[2],[3] seem to show that this isotopic effect is linked to the existence of a competition between Fe-ligand complexes in solution and the presence of some similar complexes on the surface of particles. The FAAR project aims to explain these processes through a double approach. First, it will simulate the dissolution of Fe aerosol-like particles in solutions mimicking cloud water. Changes over time in both Fe solubility and isotopic composition will be measured. Second, Fe isotopic fractionation solid-solution will be estimated via quantum chemical methods and compared, in the same way, to the respective stabilities of the surface and solution complexes, in order to conclude on the possibility of extending our simulations to more realistic atmospheric conditions.

## Characterization Methods

The morphology and crystalline defects of (001) nano-crystals are studied by TEM using a Tecnai™ G2 Microscope. Elemental mapping, high resolution imaging and iron speciation are performed using a S/TEM TITAN Themis microscope.

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>STEM-EELS</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>SADP</figcaption></figure>

**TITAN Themis characteristics:** High brightness Schottky FEG with Wien type monochromator (energy resolution <0.2 eV); Probe Cs corrector (spatial resolution < 70 nm); Four quadrant Super-X detector chemical mapping. GATAN post-column high-resolution energy filter (Quantum ERS/966) with a direct electron detector.

## Nanocrystals Synthesis

### Synthesis of α-Fe₂O₃ nanoplates (001)

The nanocrystals are prepared by hydrothermal synthesis. Such preparation is taking place Teflon lined in stainless steel autoclave, by using Fe precursors and a directing agent like aluminum acetate.

**Precursors:**

* Iron nitrate nonahydrate: $Fe(NO_3)_3 \times 9H_2O$

* Aluminum Acetate: $C_4H_7AlO_5$

* Solvent: $NH_4OH$

**Procedure:** 12 h at 160 °C in autoclave

<figure><img src="image_7.png" /><figcaption>Facet (001)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>SEM image of α-Fe₂O₃ nanoplates</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Wide angle XRD spectrum indicates the formation of an Fe₂O₃ hematite phase</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Cnel LSS-7120 Solar Simulator</figcaption></figure>

**Experimental Setup in ISO 6 Cleanroom:**

* Dissolution Temperature: 20° C

* Sun Simulation: 1000 W/m² ($\lambda$ = 400-1100 nm)

* 25 mL PFA reactor in water bath, under magnetic stirring

* Dissolution test duration: 1-10 days

Courtesy of Jean-Blaise Brubach

AILES Beamline

Synchrotron SOLEIL Facility

## Dissolution Experiments

**Simulated cloud water composition:**

* [α-Fe₂O₃] = 250 mg/L

* [oxalic acid] = 1 mM (pH = 2, fixed with perchloric acid)

**Analyses (soluble phase):**

* [Fe]soluble by atomic absorption spectrometry (GFAAS)

* [Fe²⁺] and [Fe³⁺] by the ferrozine method

* Fe isotopic composition (δ⁵⁶Fe and δ⁵⁷Fe scales) by MC-ICP-MS (G-Time Laboratory, Free Univ. Brussels)

**Analyses (solid phase):**

* Infrared Spectra of Fe-oxalate surface complexes (SOLEIL Synchrotron AILES-C beamline – Preliminary results)

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

<figure><img src="image_12.png" /><figcaption>(a) Mid-Infrared spectra of a mixture of Hematite Nanoplates and Oxalic solution (red) and solution of acid oxalic in water at pH=2 (Blue). (b) Far-Infrared spectra of a mixture of Hematite Nanoplates and Oxalic solution</figcaption></figure>

## Estimation of Fe isotope fractionation factors by Density Functional Theory

Iron isotope exchange equation: $A^{54}Fe + B^{56}Fe \rightleftharpoons A^{56}Fe + B^{54}Fe$ where A and B are two phases.

At thermodynamic equilibrium, the isotope fractionation factor is:

$$\alpha_{A,B}$$

$$\alpha_{A,B} = \frac{Z(A^{56}Fe) \times Z(B^{54}Fe)}{Z(A^{54}Fe) \times Z(B^{56}Fe)} = \frac{\beta_A}{\beta_B}$$

where Z represents the partition function of Fe, and β is the reduced partition function ratio (RPFR), defined in the harmonic approximation as:

$$\beta = \prod_{i} \frac{u_{h_{i}}}{u_{l_{i}}} \cdot \frac{\exp(-u_{h_{i}}/2)(1 - \exp(-u_{l_{i}}))}{\exp(-u_{l_{i}}/2)(1 - \exp(-u_{h_{i}}))}$$

h: heavy isotope (i.e., ⁵⁶Fe)

l: light isotope (i.e., ⁵⁴Fe)

$u_i=hc2\pi\omega_i/(kT)$, and $\omega_i$ is the vibrational frequency of mode i.

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

➤ The vibrational frequencies of A⁵⁴Fe, A⁵⁶Fe, B⁵⁴Fe, B⁵⁶Fe are required.

## Take home messages and perspectives

* Preliminary results[5] show a negative fractionation during Fe aerosol-like particles dissolution in simulated cloud water: soluble Fe exhibits an enrichment in "light" Fe (⁵⁴Fe) compared to remaining particulate Fe

* Comparison of experimental results with the estimation of Fe isotope fractionation factors calculated by DFT methods seems to show that this fractionation is due to kinetic effects

* To go further new dissolution experiments will be performed with single crystalline hematite ([001] nanoplates)

* The use of nanoplates facilitates the comparison of experimental results with their theoretical counterparts arising from DFT calculations.

* For that, the theoretical work on hematite (0012) surface will be adapted to the hematite (001) surface.

## Acknowledgements

This work was supported by the ANR "FAAR" Project under contract ANR-23-CE01-0014. The authors also acknowledge financial support from the Labex CaPPA project funded by the French National Research Agency (ANR), under contract ANR-11-LABX-0005-01. We are grateful to Jean-Blaise Brubach for assistance and to the SOLEIL staff for smoothly running the facility.

### Oxalate complexes in aqueous phase

* Method: B3LYP

* 6-31G* basis set was used for the C, O and H atoms.

* Ahlrichs' valence triple ζ basis for the Fe atom.

* Effects of the water solvent were modeled using the Polarizable Continuum Model (PCM).

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

### Oxalate on hematite (012) surface[4] (using embedding cluster model)

* Method: B3LYP

* Polarized double ζ basis set for all Fe atoms (m6-31G*).

* 6-31G* basis set for O and H atoms in the core region.

* 3-21G basis set for all atoms in the embedding region, including hydrogen basis functions on the quasi-atoms.

<figure><img src="image_15.png" /><figcaption>BB = Bidentate Binuclear</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>BM = Bidentate Mononuclear</figcaption></figure>

Adapted from [5]

## References

[1]: Kurisu, M., Sakata, K., Uematsu, M., Ito, A., Takahashi, Y., 2021. Atmos. Chem. Phys., 21, 16027-16050. https://doi.org/10.5194/acp-21-16027-2021

[2]: Mulholland, D.S., Flament, P., de Jong, J., Mattielli, N., Deboudt, K., Dhont, G., Bychkov, E., 2021. Atmos. Environ. 259, 118505. https://doi.org/10.1016/j.atmosenv.2021.118505

[3]: Maters, E.C., D. S. Mulholland, P. Flament, J. de Jong, N. Mattielli, K. Deboudt, G. Dhont and E. Bychkov, 2022. Chemosphere, 299, 134472 https://doi.org/10.1016/j.chemosphere.2022.134472

[4]: Rustad, J.R., Dixon, D.A., 2009. J. Phys. Chem. A, 113, 12249-12255. https://doi.org/10.1021/jp9065373

[5]: C.G. Pech, M. Blanchard, K. Deboudt, G. Dhont, D. Fontanari, J. J. de Jong, P. Kulinski, M. Marinova, N. Mattielli, M. Méheut, A. Verdin and P. Flament. Geochimica Et Cosmochimica Acta, Submitted