Stable Iron Isotope Fractionation Between Aqueous Fe(II) and Hydrous Ferric Oxide

作者：Wu Lingling^*; Beard Brian L; Roden Eric E; Johnson Clark M

来源：Environmental Science & Technology, 2011, 45(5): 1847-1852.

DOI：10.1021/es103171x

Despite the ubiquity of poorly crystalline ferric hydrous oxides (HFO, or ferrihydrite) in natural environments, stable Fe isotopic fractionation between HFO and other Fe phases remains unclear. In particular, it has been difficult: to determine equilibrium Fe isotope fractionation between aqueous Fe(II) and HFO due to fast transformation of the latter to more stable minerals. Here we used HFO stabilized by the presence of dissolved silica (2.14 mM), or a Si-HFO coprecipitate, to determine an equilibrium Fe(II) HFO fractionation factor using a three-isotope method. Iron isotope exchange between Fe(II) and HFO was rapid and near complete with the Si HFO coprecipitate, and rapid but incomplete for HFO in the resence of dissolved silica, the latter case likely reflecting blockage of oxide surface sites by sorbed silica. Equilibrium Fe(II)-HFO (56)Fe/(54)Fe fractionation factors of -3.17 +/- 0.08 (2 sigma)parts per thousand and -2.58 +/- 0.14 (2 sigma)parts per thousand were obtained for HFO plus silica and the Si-HFO coprecipitate, respectively. Structural similarity between ferrihydrite and hematite, as suggested by spectroscopic studies, combined with the minor isotopic effect of dissolved silica, imply that the true equilibrium Fe(II)-HFO (56)Fe/(54)Fe fractionation factor in the absence of silica may be similar to-3.2 parts per thousand. These results provide a critical interpretive context for inferring the stable isotope effects of Fe redox cycling in nature.

• 出版日期2011-3-1