Fluoride adsorption onto activated alumina: Modeling the effects of pH and some competing ions

The adsorption characteristics of fluoride on activated alumina (AA) were studied using batch methods. Effects of reaction time, pH, ionic strength, and coexisting anions were determined. Kinetics data indicated that the fluoride adsorption process reached equilibrium within 10 h. In an experimental pH range of 5–10.5, fluoride uptake decreased with the increase of pH. Ionic strength did not impact fluoride adsorption in the entire experimental pH range. However, major anions reduced fluoride adsorption in the order of HPO42− > HCO3− > SO42− > Cl−. Other toxic elements that might coexist with fluoride in groundwater, such as arsenic and selenium, also reduced fluoride adsorption through competition for the same surface sites. A speciation-based model was used to quantify the fluoride adsorption on activated alumina as functions of pH, with and without competing toxic elements. This model simulated fluoride adsorption well over a broad pH range of 5–10.5, and a wide surface loading range of 1–10 mg-F/g adsorbent.