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 con trolled by sorption to the metastable iron (hydr)oxide, ferrihydrite. The reversibility of metal partitioning to ferrihydrite can be signifi cantly influenced by its transformation to more thermodynamically stab le structures such as goethite or hematite. We studied changes in meta l partitioning during aging of coprecipitates of ferrihydrite containi ng 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 meta l concentrations as well as the fraction extracted by 0.2 M ammonium o xalate were continuously monitored. At the end of aging, solids were c haracterized by thermogravimetric analysis and X-ray diffraction. Prio r to aging, the extent of metal sorption decreased in the order Pb(II) > > Ni(II) > Mn(II) congruent to Cd(II). However, with ferrihydrite t ransformation, the extent of sorption increased and apparent sorption reversibility decreased significantly for Mn(II) and Ni(II). Both Pb(I I) and Cd(II) demonstrated net desorption with aging, and sorption rev ersibility 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.