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 (Didenksen 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(H2O)(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 Fe-56/54(Fe-DFOB/Fe-EDTA) approximate to 002 +/- 0 11 parts per thousand) and a small but significant fractionation between Fe-DFOB and Fe-oxalate (Delta Fe-56/54(Fe-DFOB/Fe-Ox3) = 0.20 +/- 0 11 parts per thousand) Taken together, our results and those of Didenksen 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 Didenksen 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