Effects of Zn(II), Cu(II), Mn(II), Fe(II), NO₃^-, or SO₄^2- at pH 6.5 and 8.5 on transformations of hydrous ferric oxide (HFO) as evidenced by Mössbauer spectroscopy

The objective of the research is to determine the effects of transition metals on transformation of hydrous ferric oxide (HFO) into more thermodynamically stable ferric oxides. Ferric oxides are important environmental adsorbents. In anoxic environments, ferric oxides with Fe(II) are important redox buffers. Transformation of HFO to more stable phases can result in decreased surface area and reduced redox potential. In some experiments, HFO was precipitated in the presence of Cu(II), Zn(II), Mn(II) and/or Fe(II). In other experiments, Fe(II), NO₃^-, and/or SO₄^2- were added to pre-formed HFO. Transmission ⁵⁷Fe-Mössbauer spectroscopy was used to monitor the phase changes. At pH 6.5 and 65°C, HFO was transformed into hematite in the presence of Zn(II) or Mn(II). Both metals were significantly adsorbed for these conditions, occupying about 1.2 sorption sites nm^-2 of HFO surface. Transformations were not observed at pH 6.5 in the presence of Cu(II), which was weakly adsorbed (0.06 sites nm^-2). No transformation occurred in the absence of Me(II) transition metals. At pH 6.5 and room temperature, HFO plus Fe(II) transformed into poorly crystalline goethite in the presence of chloride, into goethite and lepidocrocite in the presence of sulfate, and into goethite and magnetite in the presence of nitrate. At pH 8.5 and room temperature, HFO that was formed with 0.033 or 0.33 mM Zn(II) and then aged with Fe(II) was transformed into magnetite that was depleted in octahedral Fe, i.e. non-stoichiometric or possibly mixed metal spinel, (Fe³⁺)^IV(Me_xFe²⁺ _1-xFe³⁺)^VIO₄. HFO that was aged with Cu(II) and Fe(II) at pH 8.5 was transformed into goethite and into magnetite that was also depleted in octahedral Fe. The transformations at pH 8.5 were completely inhibited by 3.3 mM Zn(II) and transformations were significantly decreased by 3.3 mM Cu(II). These results have extended observations of the transformation of HFO to neutral pH ranges and to lower concentrations of metals than previously reported.