Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy

Citation
B. Singh et al., Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy, CLAY CLAY M, 48(5), 2000, pp. 521-527
Citations number
23
Categorie Soggetti
Environment/Ecology,"Earth Sciences
Journal title
CLAYS AND CLAY MINERALS
ISSN journal
00098604 → ACNP
Volume
48
Issue
5
Year of publication
2000
Pages
521 - 527
Database
ISI
SICI code
0009-8604(200010)48:5<521:SCOFMA>2.0.ZU;2-1
Abstract
The incorporation of transition metals into hematite may limit the aqueous concentration and bioavailability of several important nutrients and toxic heavy metals. Before predicting how hematite controls metal-cation solubili ty, we must understand the mechanisms by which metal cations are incorporat ed into hematite. Thus, we have studied the mechanism for Ni2+ and Mn3+ upt ake into hematite using extended X-ray absorption fine structures (EXAFS) s pectroscopy. EXAFS measurements show that the coordination environment of N i2+ in hematite corresponds to that resulting from Ni2+ replacing Fe3+. No evidence for NiO or Ni(OH), was found. The infrared spectrum of Ni-substitu ted hematite shows an OH-stretch band at 3168 cm(-1) and Fe-OH bending mode s at 892 and 796 cm(-1). These vibrational bands are similar to those found in goethite. The results suggest that the substitution of Ni2+ for Fe3+ is coupled with the protonation of one of the hematite oxygen atoms to mainta in charge balance. The solubility of Mn3+ in hematite is much less extensive than that of Ni2 because of the strong Jahn-Teller distortion of Mn3+ in six-fold coordinat ion. Structural evidence of Mn3+ substituting for Fe3+ in hematite was foun d for a composition of 3.3 mole % Mn2O3. However a sample with nominally 6. 6 mole % Mn2O3 was found to consist of two phases: hematite and ramsdellite (MnO2). The results indicate that for cations, such as Mn3+ showing a stro ng Jahn-Teller effect, there is limited substitution in hematite.