Molybdenum(VI) Adsorption onto Lepidocrocite (γ-FeOOH): In Situ Vibrational Spectroscopy and DFT+U Theoretical Study

In situ polarized ATR-FTIR spectroscopy and the DFT+U calculations were used to investigate the Mo­(VI) ions surface speciation on lepidocrocite. Adsorption of molybdate ions is found to be tetrahedral monodentate binuclear (C 3v ) on the (010) face. DFT+U calculations were performed to help characterizing the geometries of the complexes adsorbed to assign the vibrational frequencies and get insight into the energies of adsorption. The (010) surface was chosen as a model of lepidocrocite surface. Calculations are consistent with the sorption of MoO4 2– ions to form a monodentate complex at basic pH. This complex is protonated when the surface coverage increases, and the pH decreases. At the full coverage, calculations show alternate monodentate protonated binuclear complexes and surface hydroxyl groups. Moreoever, the formation of hydrogen bonds with the surface hydroxyls and between adsorbed complexes plays an important role in the structure stability.