The atomistic level understanding of iron speciation and the probable oxidative behavior of iron (Feaq2+ → Fesurf3+) in clay minerals are fundamental for environmental geochemistry of redox reactions. Thermodynamic analyses of wet chemistry data suggest that iron adsorbs on the edge surfaces of clay minerals at distinct structural sites commonly referred as strong and weak sites (with high and low affinity, respectively). In this study, we applied ab initio molecular dynamics simulation to investigate the structure and the stability of the edge surfaces of trans- and cis-vacant montmorillonites. These structures were further used to evaluate the surface complexation energy and to calculate reference ab initio X-ray absorption spectra (XAS) for distinct inner-sphere complexes of iron. The combination of ab initio simulations and XAS allowed us to reveal the Fe-complexation mechanism and to quantify the Fe partitioning between the high and low affinity sites as a function of the oxidation state and loadings. Although iron is mostly present in the Fe3+ form, Fe2+ increasingly co-adsorbs at increasing loadings. Ab initio structure relaxations of several different clay structures with substituted Fe2+/Fe3+ in the bulk or at the surface site showed that the oxidative sorption of ferrous iron is an energetically favored process at several edge surfaces of the Fe-bearing montmorillonite.