Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy. Clays and Clay Minerals 2000

Abstract—The incorporation of transition metals into hematite may limit the aqueous concentration and bioavailabity of several important nutrients and toxic heavy metals. Before predicting how hematite controls metal-cation solubility, we must understand the mechanisms by which metal cations are incor-porated into hematite. Thus, we have studied the mechanism for Ni21 and Mn31 uptake into hematite using extended X-ray absorption fine structures (EXAFS) spectroscopy. EXAFS measurements show that the coordination environment of Ni21 in hematite corresponds to that resulting from Ni21 replacing Fe31. No evidence for NiO or Ni(OH)2 was found. The infrared spectrum of Ni-substituted hematite shows an OH-stretch band at 3168 cm21 and Fe-OH bending modes at 892 and 796 cm21. These vibrational bands are similar to those found in goethite. The results suggest that the substitution of Ni21 for Fe31 is coupled with the protonation of one of the hematite oxygen atoms to maintain charge balance. The solubility of Mn31 in hematite is much less extensive than that of Ni21 because of the strong Jahn-Teller distortion of Mn31 in six-fold coordination. Structural evidence of Mn31 substituting for Fe31 in hematite was found 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 Mn31 showing a strong Jahn-Teller effect, there is limited substitution in hematite