Prediction of Iron-Isotope Fractionation Between Hematite (a-Fe_2O_3) and Ferric andFerrous Iron in Aqueous Solution from Density Functional Theory

摘要

Density functional theory electronic structure calculations are used to compute equilibrium constants for iron-isotope exchange among Fe~(2+)(aq), Fe~(3+)(and hematite (a-Fe_2O_3). The is represented in bothbulk and surface environments. The iron-isotope fractionation between Fe~(2+)(aq) and Fe~(3+)(aq), determinedusing a range of exchange-correlation functionals and basis sets, is in good agreement with experimentalmeasurements. The calculated reduced partition function ratio for bulk hematite is very close to previousestimates based on Mossbauer and inelastic nuclear resonance X-ray spectroscopy. However, the calculatedfractionation between hematite bulk and the aqueous species Fe~(3+)(aq) and Fe~(2+)(differs from experimentalmeasurements carried out at the aqueous—hematite interface. We find a heavy iron enrichment trend in theorder Fe~(2+)(aq) < hematite bulk≈surface Fe~(3+)(aq). In contrast to experimental studies, we finda significant positive fractionation (heavy enrichment) for Fe~(3+)(aq) relative to hematite, regardless of whetherthe hematite is represented by a bulk or a surface model. Our calculations indicate that it is unlikely that theaqueous interfacial structure of hematite is a simple termination of the bulk structure.