『Abstract
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 O2-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 0.5M 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) (δ56/54Fe(II)aq
and δ56/54Fe(II)extr) became isotopically
lighter than or similar to goethite (-0.27‰) 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 observation
suggested that (i) co-adsorption of Fe(II) 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.
Keywords: Iron; Isotope; Goethite; Adsorption; Electron transfer』
1. Introduction
2. Materials and methods
3. Results
4. Discussion
5. Summary
acknowledgments
Appendix A
References