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1 Department of Agricultural Chemistry & Soil Science, The University of Sydney, Sydney, Australia
2 Department of Geology, University of Bristol, Bristol, UK
3 Department of Soil Science & Plant Nutrition, University of Western Australia, Nedlands, Australia
4 CRC LEME, University of Canberra, Belconnen, A.C.T., Australia
5 CCLRC, Daresbury Laboratory, Warrington, UK
E-mail of corresponding author: b.singh{at}acss.usyd.edu.au
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 Ni2+ and Mn3+ uptake into hematite using extended X-ray absorption fine structures (EXAFS) spectroscopy. EXAFS measurements show that the coordination environment of Ni2+ in hematite corresponds to that resulting from Ni2+ replacing Fe3+. 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 Ni2+ for Fe3+ is coupled with the protonation of one of the hematite oxygen atoms to maintain 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 coordination. Structural evidence of Mn3+ substituting for Fe3+ 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 Mn3+ showing a strong Jahn-Teller effect, there is limited substitution in hematite.
Key Words: EXAFS • Fe Oxides • Hematite • Metal Substitution • Trace Elements • XAS • XRD
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