Retention mechanism of Mo in TiO₂ and ZrO₂-TiO₂, potential adsorbents for the radionuclides separation

The retention properties of titanium and zirconium oxides are studied; solid adsorbents of high retention capacity for separation by chromatography of radionuclide pairs that are the basis of the so-called radionuclide generators. The titanium and zirconium nano materials obtained with a high retention capacity are prepared by the Sol-gel method using an alkoxide as a precursor. The acid-base properties are studied by potentiometric titrations, obtaining a value of the point of zero charge of 5.6 for TiO₂ and 6.3 for the mixed oxide ZrO₂-TiO₂. To study the retention behavior of the ⁹⁹Mo/^99mTc radionuclide pair in these solids, batch experiments were performed on a 0.9% NaCl electrolyte as a function of the solution ph. The results show that ^99mTc is not absorbed by solids while ⁹⁹Mo shows a high retention affinity for the metal oxides under study. The maximum adsorption of ⁹⁹Mo takes place at a ph value close to the zero load point (pH_PZC) (∼ 95% adsorption). This study focuses on the mechanism of molybdenum retention in terms of chemical equilibria between the functional groups of the solid (OH-) and the species of Mo(Vi) in solution. The experimental data of molybdenum retention were analyzed with the FITEQL program using the constant capacitance model and assuming the presence of a single type of sites on the surface of the solids (hydroxyl groups). In Mo(Vi) retention, surface complexes that are formed through a ligand exchange mechanism between molybdate species and hydroxyl ions from the surface of the solid are probably the mechanism responsible for adsorption in the ph range that is studied. (Author)