Radiotracer techniques (18F) and modeling of fluoride sorption on alumina

Abstract

Chemical equilibria on the removal of excess fluoride from water by a classic solid alumina are presented. The fluoride concentrations used are representatives of those reported in borewell water at North region of Zacatecas State (Mexico). The percentage of fluoride removal was obtained as a function of pH using the radiotracer, 18F, from cyclotron of PET-UNAM. Proton binding sites of solid were characterized by potentiometric titrations yielding a point of zero charge, pHPZC, around 5.75. Batch experiments with alumina-equilibrated solutions were conducted to determine the mechanism of fluoride adsorption by this solid. The sorption isotherms show that fluoride removal is strongly dependent on pH showing a maximum value (∼98% adsorption) at pH near pHPZC; above this pH value, fluoride sorption falls sharply. This study concentrates on the fluoride sorption mechanism in terms of surface and solution equilibria constants. The experimental data were treated with the FITEQL 4.0 software program considering the existence of a single type of surface sites (hydroxyl groups on alumina). As a result of fitting fluoride removal data to model for the lowest initial fluoride concentration 1.0mg/L, the formation of monodentate surface complexes ≡Al–F dominate below pHPZC, when pH was above this value, fluoride adsorption decreased and the formation of the surface complexes ≡Al–OHF− became dominant. However, for the higher initial fluoride concentration of 9.3mg/L, the monodentate surface complexes ≡Al–F are predominant over the entire pH range.