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

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.