Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions

Large equilibrium isotope fractionation occurs between Fe(III) and Fe(II) in very dilute (≤22 mM Cl−) aqueous solutions, reflecting significant differences in bonding environments. Separation of Fe(III) and Fe(II) is attained by rapid and complete precipitation of Fe(III) through carbonate addition, followed by separation of supernatant and ferric precipitate; experiments reported here produce an equilibrium ΔFe(III)–Fe(II)=+2.75±0.15‰ for 56Fe/54Fe at room temperature (22±2°C). The timescales required for attainment of isotopic equilibrium have been determined by parallel isotope tracer experiments using 57Fe-enriched iron, which are best fitted by a second-order rate law, with K=0.18±0.03 s−1. Based on this rate constant, ∼15–20% isotopic exchange is estimated to have occurred during Fe(III)–Fe(II) separation, which contributes <0.10‰ uncertainty to the equilibrium ΔFe(III)–Fe(II). Under the experimental conditions used in this study, >97% Fe(II) exists as [FeII(H2O)6]2+, and >82% Fe(III) exists as [FeIII(H2O)6]3+ and [FeIII(H2O)6−n(OH)n]3−n; assuming these are the dominant species, the measured Fe isotope fractionation is approximately half that predicted by Schauble et al. [Geochim. Cosmochim. Acta 65 (2001) 2487–2497] at 20–25°C. Although this discrepancy may be due in part to the experimentally unknown isotopic effects of chloride interacting with Fe-hexaquo or Fe-hydroxide complexes, or directly bonded to Fe, there still appears to be at this stage a >1‰ difference between prediction and experiment.