Incorporation of Trace Metals into Polymorphs Alpha- (Goethite) and Gamma- (Lepidocrocite) FeOOH.
Markus Gräfe, Balwant Singh, and Navdeep Kaur. The University of Sydney, Faculty of Agriculture, Food and Natural Resources, McMillan Building (A05), Sydney, Australia
Little or no information is present in the literature concerning the fate of trace metals during the lepidocrocite --> goethite transformation. This information is, however, needed in order to better predict the fate of toxic metals in acid mine drainage environments and during the corrosion of steel products. The formation of FeOOH polymorphs from ferrous sulphate (FeSO4) solutions was investigated between pH 5.5 and 6.5 in the presence and absence of foreign metal cations (Cr, Cu, Zn, Cd or Pb). No additional carbonate [(H)CO32-] ions, CO2 or air purging were introduced other than allowing the initially purged (N2) solutions to equilibrate under vigorous stirring with the atmosphere. At pH 5.5 and 6.0, lepidocrocite formed preferentially over goethite, whereas at pH 6.5, approximately equal amounts of goethite and lepidocrocite formed within 48 h. A pH above 6.35 (pKa1 H2CO3) is believed to promote the diffusion of CO2 into the suspensions, inhibiting the formation of lepidocrocite and increasingly favouring the formation of goethite. Metal cations such as Zn, Cd and Pb have high affinities for CO32- ions forming their own metal carbonate phases. The metals' effect on the formation and relative distribution of lepidocrocite vs. goethite will be discussed based on relevant synchrotron-based X-ray diffraction, X-ray absorption fine structure and Fourier transform infrared spectroscopic data.