New insight into the structure of nanocrystalline ferrihydrite: EXAFS evidence for tetrahedrally coordinated iron(III)

Fabien Maillot (1) , Guillaume Morin (1) , Yuheng Wang (1) , Dominique Bonnin (2) , Philippe Ildefonse (1) , Corinne Chanéac (3) , Georges Calas (1)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 ESPCI Paris - Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris
3 LCMCP - Laboratoire de Chimie de la Matière Condensée de Paris

Ferrihydrite (Fh) is a short-range ordered nanocrystalline iron(III) (oxyhydr)oxide that has been recognized to play an important role in contaminant sequestration and in iron cycling in geological and biological systems. Despite intensive research for the two last decades, the structure of Fh is still a subject of debate. In the present study, we report extended X-ray absorption fine structure (EXAFS) spectroscopy data collected on a large set of ferrihydrites and model compounds samples including especially nano-crystalline maghemite (Mh), goethite (Gt), and akaganeite (Aka). This set of EXAFS data recorded at cryogenic temperature over a wide energy range allows us to precisely determine the Fe-O mean distance (hFe-Oi) in the first coordination shell of iron for this large set of iron (oxyhydr)oxides. Our EXAFS analysis includes both classical shell-by-shell fits of Fourier-filtered and unfiltered data as well as analysis of Fe-O distance distribution in the first coordination shell of iron using the Landweber iteration method. hFe-Oi determined by these complementary EXAFS analyses are similar: hFe-Oi is shorter in Mh (1.96 ± 0.01A ˚ ) that contains 37.5% of tetrahedral iron, than in Gt (2.01 ± 0.01A ˚ ), Aka (2.00 ± 0.01A ˚ ) and hematite (Hm) (2.01 ± 0.01A ˚ ) that do not contain tetrahedral iron. hFe-Oi for the five Fh samples investigated (1.97 ± 0.01A ˚ ) was found to be slightly longer than in Mh and significantly shorter than those in Gt, Aka and Hm. This short hFe-Oi distance in Fh indicates the presence of significant amount of tetrahedrally coordinated iron(III) in all Fh samples studied, which ranges between 20 ± 5% and 30 ± 5% of total iron. In addition, our analysis of Fe-Fe distances observed by EXAFS is consistent with a Keggin-like motif at a local scale ( 5 A˚ ) in the Fh structure

Domaines: Sciences de l'environnement

Fabien Maillot, Guillaume Morin, Yuheng Wang, Dominique Bonnin, Philippe Ildefonse, et al.. New insight into the structure of nanocrystalline ferrihydrite: EXAFS evidence for tetrahedrally coordinated iron(III). Geochimica et Cosmochimica Acta, 2011, 75, pp.2708-2720. ⟨10.1016/j.gca.2011.03.011⟩. ⟨hal-01054558⟩