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 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 incorporated into hematite. Thus, we have studied the mechanism for Ni 2+ and Mn 3+ uptake into hematite using extended X-ray absorption fine structures (EXAFS) spectroscopy. EXAFS measurements show that the coordination environment of Ni 2+ in hematite corresponds to that resulting from Ni 2+ replacing Fe 3+ . No evidence for NiO or Ni(OH) 2 was found. The infrared spectrum of Ni-substituted hematite shows an OH-stretch band at 3168 cm −1 and Fe-OH bending modes at 892 and 796 cm −1 . These vibrational bands are similar to those found in goethite. The results suggest that the substitution of Ni 2+ for Fe 3+ is coupled with the protonation of one of the hematite oxygen atoms to maintain charge balance. The solubility of Mn 3+ in hematite is much less extensive than that of Ni 2+ because of the strong Jahn-Teller distortion of Mn 3+ in six-fold coordination. Structural evidence of Mn 3+ substituting for Fe 3+ in hematite was found for a composition of 3.3 mole % Mn 2 O 3 . However a sample with nominally 6.6 mole % Mn 2 O 3 was found to consist of two phases: hematite and ramsdellite (MnO 2 ). The results indicate that for cations, such as Mn 3+ showing a strong Jahn-Teller effect, there is limited substitution in hematite.