The recent development of high brilliance synchrotron light sources has led to rapid increase in the quantitative observation, on an atomic level, of the interface region. These observations can be used to both target and guide our simulations. In this study we used molecular simulations to understand a discrepancy between experiment and theory for the solvated Goethite (a-FeOOH 100 surface). A recent study by Ghose et al. (Geochimica et Cosmochimica Acta 74, no. 7, 1943-1953, 2010) found that the distances from the surface Fe to the oxygen atoms were between 2.09 Å and 2.16 Å, whereas prior periodic DFT calculations performed by Kubicki et al. (Geochemical transactions 9, no. 1, 4, 2008) (Goethite 100 surface in Pnma space group) predicted a variety of structures with surface Fe-O distances of 2.04 Å, 2.38 Å, 2.42 Å, 2.43 Å. It was postulated that the reason prior theoretical DFT calculations disagreed with experiment was that these calculations had very small water coverage on the surface. We were able to carry out large-scale ab initio molecular dynamic (AIMD) simulations of the FeOOH surface with multiple layers of water (Fig. 1). These simulations predicted surface Fe-O distances between 2.10-2.20 Å, which is in good agreement with the measurements of Ghose et al. In analyzing the simulation it was found that the Fe-OH2 surface complex is strengthened when the neighboring hydroxyl forms a hydgrogen bond with bulk water, Fe-OH—H-OH (i.e. the surface water structure is H2O-Fe-OH—H-OH). This detailed analysis of surface water topology is an essential element in the organic molecule sorption process, and demonstrates the need for large system AIMD simulation.

Determination of binding geometry, local water structure and topological at FeOOH surfaces

249th National Meeting (2015)

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