Density functional theory electronic structure calculations are used to compute equilibrium constants for iron-isotope exchange among Fe2+(aq), Fe3+(aq), and hematite (alpha-Fe2O3). The hematite is represented in both bulk and surface environments. The iron-isotope fractionation between Fe2+(aq) and Fel+(aq), determined using a range of exchange-correlation functionals and basis sets, is in good agreement with experimental measurements. The calculated reduced partition function ratio for bulk hematite is very close to previous estimates based on Mossbauer and inelastic nuclear resonance X-ray spectroscopy. However, the calculated fractionation between hematite bulk and the aqueous species Fe3+(aq) and Fe2+(aq) differs from experimental measurements carried out at the aqueous-hematite interface. We find a heavy iron enrichment trend in the order Fe2+(aq) < hematite bulk approximate to hematite surface < Fe3+(aq). In contrast to experimental studies, we find a significant positive fractionation (heavy enrichment for Fe3+(aq) relative to hematite, regardless of whether the hematite is represented by a bulk or a surface model. Our calculations indicate that it is unlikely that the aqueous interfacial structure of hematite is a simple termination of the bulk structure.