Adsorption energy evaluation from luminescence spectra of uranyl ions (UO2;(2+)) adsorbed on disperse silica surfaces

Luminescent measurements are utilized to evaluate the adsorption energy for hydrated uranyl groups on the disperse silica surface. The position of the UO2;(2+) 0-0 electronic transition and the frequencies of intramolecular vibrations appearing in luminescence spectra were used in that evaluation. The method is based on the dependence of the adsorption bond force constants corresponding to each type of surface sites on the respective perturbation energy of uranyl ions. To calculate the corresponding force constants of adsorption bonds, a vibrational problem is solved. The molecular adsorbate on the silica surface is modeled by means of hypothetical diatomic and triatomic molecules with Cinfinity-upsilon and C2-upsilon symmetry, in which the UO2;(2+) ion is predominantly fixed on the silica surface through one and two water molecules, respectively. The perturbations of stretching and deformational vibrations of hypothetical molecules by the presence of the substrate are considered. It has been shown that due to a large value of uranium mass the proposed models can be used for the analysis of vibrational spectra of hydrated uranyl groups adsorbed on the silica surface. A comparison of experimental data with theoretical calculations shows one to suggest that hydrated uranyl groups are attached to the surface by means of two water molecules. The adsorption energy for hydrated uranyl groups physically and chemically adsorbed on the silica surface were evaluated. (C) 1997 Academic Press.