Iron adsorption on clays inferred from atomistic simulations and XAS studies.

The atomistic-level understanding of iron speciation and the probable oxidative behavior of iron (Fe(aq)2+-->Fe(surf)3+) in clay minerals is fundamental for environmental geochemistry of redox reactions. Thermodynamics analysis of wet chemistry data suggests 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 stability of 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 Fe. 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 function of the oxidation state and loadings. Although, iron is mostly present in Fe3+ form, Fe2+ increasingly co-adsorb with 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 Fe-bearing montmorillonite.