Fe(III)-Modified Activated Alumina with Enhanced Fluoride Adsorption under Near-Neutral pH Conditions

The development of new adsorbents with improved fluoride adsorption capacities under realistic water conditions remains a major challenge for making groundwater potable. In this study, an Fe(III)-modified activated alumina (FMAA) was prepared via a wet-chemical treatment of commercially available activated alumina (AA), a widely used adsorbent for water defluoridation. Batch adsorption experiments were conducted by varying key parameters such as adsorbent dose, initial fluoride concentration, pH, and contact time to evaluate the performance of FMAA in comparison with AA. At an initial fluoride concentration of 10 mg L−1 and near-neutral pH, FMAA exhibited a higher fluoride adsorption capacity (2.63 mg g−1) than AA (1.8 mg g−1) under these conditions. Fluoride uptake on both materials was better described by the pseudo-second-order kinetic model. Isotherm analysis showed that adsorption on AA follows the Freundlich model, whereas FMAA is better described by the Langmuir model, consistent with a more uniform population of fluoride-binding sites after Fe(III) modification. The improved fluoride removal performance of FMAA is attributed to the presence of Fe(III)-containing surface sites and to a shift in the point of zero charge.