Surface structural model for ferrihydrite

被引:124
作者
Manceau, A
Gates, WP
机构
[1] UNIV GRENOBLE 1, IRIGM, LGIT, ENVIRONM GEOCHEM GRP, F-38041 GRENOBLE 09, FRANCE
[2] CNRS, F-38041 GRENOBLE 09, FRANCE
关键词
EXAFS; ferrihydrite; hydrous ferrite oxide; XANES; X-ray absorption spectroscopy;
D O I
10.1346/CCMN.1997.0450314
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
A structural model for the geometry of Fe(III) octahedra near the surface of finely divided ferrihydrite was elaborated based on the bond-valence theory and by considering the interaction of water molecules in the 2 nearest hydration spheres. In contrast to bulk Fe atoms, which are bonded to bridging oxo (O) and hydroxo (OH) ligands, surface Fe atoms are also octahedrally coordinated to H2O ligands forming the 1st hydration shell ((H2O)(1)). In the wet state, external water molecules of the 2nd hydration shell ((H2O)(n)) are singly H-bonded to (H2O)(1), while they are doubly coordinated in the dry state. Accordingly, wet ferrihydrite contains twice as many sorbed water molecules as dry ferrihydrite, and the structural difference due to the 2nd hydration shell accounts quantatively for the 15% increase of ferrihydrite weight experimentally measured in moist atmosphere. The interaction of surface Fe atoms with their 2 nearest hydration spheres modifies the geometry of surface Fe octahedra as compared to bulk octahedra, and idealized Fe-OH and Fe-H2O bond lengths in the wet and dry state were evaluated by the bond-valence theory. Our structural model provides a sound crystal-chemical basis to describe many apparent incongruities of Fe X-ray absorption near edge structure (K-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic data that have led to differing interpretations of the coordination environment of Fe in ferrihydrite by various investigators.
引用
收藏
页码:448 / 460
页数:13
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