Fluoride Adsorption onto a Ce-La Binary Hydroxide Adsorbent as Recovery of Ceria Glass Polishing Wastes

The adsorption of fluoride onto a Ce-La binary hydroxide adsorbent, herein referred to as CLH adsorbent, was studied under various pH values, adsorbent dose, contact time, and the presence of major anions. In the pH range of 4 to 8, the fluoride adsorption onto CLH reaches a maximum, then decreases with further increasing of pH. A considerably higher adsorption capacity (77.4-89.5 mg g^-1) was maintained until the solution pH was up to 8 compared with that of the commercially available activated alumina; indicative of the CLH material should be effective to fluoride removal for major fresh water supplies. The effect of contact time was dependent on the initial fluoride concentration but adsorption equilibriums were reached after 120 min under the initial concentrations of 10 and 25 mg L^-1, whereas a residual F^- concentration of <; 1.5 mg F L^-1 was attained that satisfied the guideline value suggested by WHO for fluoride. The adsorptive rate of fluoride on the adsorbent obeys pseudo-second order kinetic models (R² > 0.99). Adsorption isotherm at pH 7.0 fits best the Langmuir than the Freundlich model of adsorption over the F^- concentration range 2-250 mg L^-1. A combination of coulombic interaction and the formation of inner-sphere complexes between the activated surface sites and fluoride anions seems to be the adsorption mechanism. Major co-existing anions reduced fluoride adsorption according to their affinity on the CLH surface in the following order: HPO₄^2- > HCO₃^- ≥ SiO₃^2- > SO₄^2- > Cl^-, NO₃^-.