Fe isotope fractionation during equilibration of Fe-organic complexes.
Despite the importance of Fe-organic complexes in the environment, few studies have investigated Fe isotope effects driven by changes in Fe coordination that involve organic ligands. Previous experimental (Dideriksen et al., 2008, Earth Planet Sci. Lett. 269:280-290) and theoretical (Domagal-Goldman et al., 2009, Geochim. Cosmochim. Acta 73:1-12) studies disagreed on the sense of fractionation between Fe-desferrioxamine B (Fe-DFOB) and Fe(H(2)O)(6)(3+). Using a new experimental technique that employs a dialysis membrane to separate equilibrated Fe-ligand pools, we measured the equilibrium isotope fractionations between Fe-DFOB and (1) Fe bound to ethylenediaminetetraacetic acid (EDTA) and (2) Fe bound to oxalate. We observed no significant isotope fractionation between Fe-DFOB and Fe-EDTA (Delta(56/54)Fe(Fe-DFOB/Fe-EDTA) approximately 0.02 +/- 0.11 per thousand) and a small but significant fractionation between Fe-DFOB and Fe-oxalate (Delta(56/54)Fe(Fe-DFOB/Fe-Ox(3)) = 0.20 +/- 0.11 per thousand). Taken together, our results and those of Dideriksen et al. (2008) reveal a strong positive correlation between measured fractionation factors and the Fe-binding affinity of the ligands. This correlation supports the experimental results of Dideriksen et al. (2008). Further, it provides a simple empirical tool that may be used to predict fractionation factors for Fe-ligand complexes not yet studied experimentally.
Morgan
Jennifer L L
JL
Arizona State University, Department of Chemistry and Biochemistry, PO Box 871604, Tempe, Arizona 85287, USA. jennifer.l.morgan@asu.edu
Wasylenki
Laura E
LE
Nuester
Jochen
J
Anbar
Ariel D
AD
D005591:Chemical Fractionation; D007501:Iron / Q000737:chemistry*; D007503:Iron Isotopes; D008024:Ligands; D008970:Molecular Weight; D009930:Organic Chemicals / Q000737:chemistry*; D015203:Reproducibility of Results; D012996:Solutions
D007503:Iron Isotopes; D008024:Ligands; D009930:Organic Chemicals; D012996:Solutions; D007501:Iron