The sorption of (poly)molybdate ions into layered double hydroxides (LDHs), with Zn(2+) and Al(3+) cations, has been followed by in situ infrared spectroscopy using the attenuated total reflection technique. The exchange between solution molybdate species and interlayer anions has been followed in real time, illustrating the different behavior of molybdate ions and polymolybdate species. In a first part, the Mo(VI) speciation in solution was performed by comparison of thermodynamical calculations and infrared spectroscopy of solutions with different pH. Decomposition of bands between 800 to 1000 cm(-1), corresponding to the (Mo-O) stretching vibration, has permitted to identify major (poly)molybdate species. In the presence of LDH, the measurements have shown a high affinity for heptamolybdate (Mo7O24(6-)) species, and its preferential sorption in comparison with molybdate ions or other protonated polymolybdate species even if it represents very small fractions. From these measurements, the affinity series Mo7O24(6-) > CO3(2-) > MoO4(2-) > SO4(2-) have been directly obtained.

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 MoO 4 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.