Weakly bound water structure, bond valence saturation and water dynamics at the goethite (100) surface/aqueous interface: ab initio dynamical simulations

Background: Many important geochemical and biogeochemical reactions occur in the mineral/formation water interface of the highly abundant mineral, goethite [alpha-Fe(OOH)]. Ab initio molecular dynamics (AIMD) simulations of the goethite alpha-FeOOH (100) surface and the structure, water bond formation and dynamics of water molecules in the mineral/aqueous interface are presented. Several exchange correlation functionals were employed (PBE96, PBE96 + Grimme, and PBE0) in the simulations of a (3 x 2) goethite surface with 65 absorbed water molecules in a 3D-periodic supercell (a = 30 angstrom, FeOOH slab similar to 12 angstrom thick, solvation layer similar to 18 angstrom thick). Results: The lowest energy goethite (100) surface termination model was determined to have an exposed surface Fe3+ that was loosely capped by a water molecule and a shared hydroxide with a neighboring surface Fe3+. The water molecules capping surface Fe3+ ions were found to be loosely bound at all DFT levels with and without Grimme corrections, indicative that each surface Fe3+ was coordinated with only five neighbors. These long bonds were supported by bond valence theory calculations, which showed that the bond valence of the surface Fe3+ was saturated and surface has a neutral charge. The polarization of the water layer adjacent to the surface was found to be small and affected only the nearest water. Analysis by density difference plots and localized Boys orbitals identified three types of water molecules: those loosely bound to the surface Fe3+, those hydrogen bonded to the surface hydroxyl, and bulk water with tetrahedral coordination. Boys orbital analysis showed that the spin down lone pair orbital of the weakly absorbed water interact more strongly with the spin up Fe3+ ion. These weakly bound surface water molecules were found to rapidly exchange with the second water layer (similar to 0.025 exchanges/ps) using a dissociative mechanism. Conclusions: Water molecules adjacent to the surface were found to only weakly interact with the surface and as a result were readily able to exchange with the bulk water. To account for the large surface Fe-OH2 distances in the DFT calculations it was proposed that the surface Fe3+ atoms, which already have their bond valence fully satisfied with only five neighbors, are under-coordinated with respect to the bulk coordination.