Hydrated goethite (α-FeOOH) (100) interface structure: Ordered water and surface functional groups

Goethite(alpha-FeOOH), in abundant and highly reactive iron oxyhydroxide mineral, has been the Subject of numerous studies of environmental interface reactivity. However, such studies have been hampered by the lack of experimental constraints oil aqueous interface structure, and especially of the surface water molecular arrangements. Structural information of this type is crucial because reactivity is dictated by the nature of the Surface functional groups and the structure or distribution of water and electrolyte at the solid-solution interface. In this study we have investigated the goethite (1 0 0) surface using surface diffraction techniques, and have determined the relaxed surface structure, the surface functional groups, and the three dimensional nature of two distinct sorbed water layers. The crystal truncation rod (CTR) results show that the interface structure consists of a double hydroxyl, double water terminated interface with significant atom relaxations. Further, the double hydroxyl terminated surface dominates with ail 89% contribution having a chiral subdomain structure oil the (1 0 0) cleavage faces. The proposed interface stoichiometry is ((H2O)-(H2O)-OH2-OH-Fe-O-O-Fe-R) with two types of terminal hydroxyls; a bidentate (B-type) hydroxo group and a monodentate (A-type) aquo group. Using the bond-valence approach the protonation states of the terminal hydroxyls are predicted to be OH type (bidentate hydroxyl with oxygen Coupled to two Fe3+ ions) and OH, type (monodentate hydroxyl with oxygen tied to only one Fe3+). A double layer three dimensional ordered water structure at the interface was determined from refinement of fits to the experimental data. Application of bond-valence constraints to the terminal hydroxyls with appropriate rotation of the water dipole moments allowed a plausible dipole orientation model as predicted. The structural results are discussed in terms of protonation and H-bonding at the interface, and the results provide an ideal basis for testing theoretical predictions of characteristic surface properties Such as pK(a), sorption equilibria, and surface water permittivity. (C) 2009 Elsevier Ltd. All rights reserved.