Preparation and Evaluation of Calcium–Magnesium Modified Activated Alumina Granules for Efficient and Sustainable Fluoride Removal from Groundwater

Excess fluoride in groundwater represents a critical environmental and public health concern, necessitating the development of efficient and sustainable treatment materials. In this work, calcium- and magnesium-modified activated alumina granules (Ca–Mg–AA) were synthesized and evaluated as a multifunctional adsorbent for fluoride removal. Activated alumina granules were produced through a modified sol–gel route and subsequently coated with calcium and magnesium precursors via sequential dip-coating, followed by thermal treatment at 600 °C. Structural and surface characterization using XRD, SEM, TEM, FTIR, and TG–DTA confirmed the formation of nanocrystalline γ-Al₂O₃ with uniformly distributed CaO and MgO coatings (particle size ~40–70 nm), enhanced porosity, and excellent thermal stability up to 1000 °C. Batch adsorption studies were carried out to investigate the influence of solution pH (3–7), adsorbent dosage (10–50 mg), and contact time (0–120 min) on fluoride removal from aqueous solutions (1–10 mg L⁻¹). Optimal defluoridation was achieved at pH 5 with an adsorbent dose of 30 mg and a contact time of 60 min. The Ca–Mg–AA adsorbent exhibited improved fluoride removal efficiency (up to 85%) compared with pristine activated alumina and singly coated samples, which is attributed to synergistic ion exchange and surface complexation mechanisms. Equilibrium data were best described by the Freundlich isotherm, indicating heterogeneous multilayer adsorption. The results highlight the potential of Ca–Mg–AA as a cost-effective and environmentally benign material for fluoride mitigation, with added prospects for simultaneous removal of coexisting heavy metals.