An iron isotope signature related to electron transfer between aqueous ferrous iron and goethite

We measured the Fe isotope fractionation during the reactions of Fe(II) with goethite in the presence and absence of a strong Fe(III) chelator (desferrioxamine mesylate, DFAM). All experiments were completed in an O(2)-free glove box. The concentrations of aqueous Fe(II) ([Fe(II)aq]) decreased below the initial total dissolved Fe concentrations ([Fe(II)total], 2.15 mM) due to fast adsorption within 0.2 day. The concentration of adsorbed Fe(II) ([Fe(II)(ads)]) was determined as the difference between [Fe(II)aq] and the concentration of extracted Fe(II) in 0.5 M HCl ([Fe(II)extr]) (i.e., [Fe(II)(ads)]=[Fe(II)(extr)]-[Fe(II)(aq)]). [Fe(II)(ads)] also decreased with time in experiments with and without DFAM, documenting that fast adsorption was accompanied by a second, slower reaction. Interestingly, [Fe(II)(extr)] was always smaller than [Fe(II)(total)], indicating that some Fe(II) was sequestered into a pool that is not HCl-extractable. The difference was attributed to Fe(II) incorporated into goethite structure (i.e., [Fe(II)inc]=[Fe(II)(total)]-[Fe(II)(extr)]). More Fe(II) was incorporated in the presence of DFAM than in its absence at all time steps. Regardless of the presence of DFAM, both aqueous and extracted Fe(II) (delta(56/54)Fe(II)(aq) and delta(56/54)Fe(II)(extr)) became isotopically lighter than or similar to goethite (-0.27 parts per thousand) at day 7, implying that the isotope exchange occurred between bulk goethite and aqueous Fe. Consistently, the mass balance indicated that the incorporated Fe is isotopically heavier than extracted Fe. These observations suggested that (i) co-adsorption of Fe(ID with DFAM resulted in more pervasive electron transfer, (ii) the electron transfer from heavy Fe(II) in the adsorbed Fe(II) to light Fe(III) in goethite results in the fixation of heavy adsorbed Fe(III) on the surface and accumulation of Fe(II) within the goethite, and (iii) desorption of the reduced, light Fe from goethite does not necessarily occur at the same surface sites where adsorption occurred. (C) 2008 Elsevier B.V. All rights reserved.