Changes in transition and heavy metal partitioning during hydrous iron oxide aging

The fate and transport of metal ions in soils and sediments may be controlled by sorption to the metastable iron (hydr)oxide, ferrihydrite. The reversibility of metal partitioning to ferrihydrite can be significantly influenced by its transformation to more thermodynamically stable structures such as goethite or hematite. We studied changes in metal partitioning during aging of coprecipitates of ferrihydrite containing Cd(II), Mn(II), Ni(II), or Pb(II) at pH 6 and temperatures of 40 or 70 degrees C and as a function of metal surface loading. Aqueous metal concentrations as well as the fraction extracted by 0.2 M ammonium oxalate were continuously monitored. At the end of aging, solids were characterized by thermogravimetric analysis and X-ray diffraction. Prior to aging, the extent of metal sorption decreased in the order Pb(II) > > Ni(II) > Mn(II) congruent to Cd(II). However, with ferrihydrite transformation, the extent of sorption increased and apparent sorption reversibility decreased significantly for Mn(II) and Ni(II). Both Pb(II) and Cd(II) demonstrated net desorption with aging, and sorption reversibility remained essentially unchanged. These differences in metal behavior are consistent with structural incorporation of Mn(II) and Ni(II) into the goethite or hematite structure and minimal incorporation of Cd(II) and Pb(II) within these crystalline products at pH 6.