Synthesis and Performance Assessment of Calcium- and Magnesium-Coated Activated Alumina Granules for Sustainable Fluoride Removal from Groundwater

Abstract

Groundwater contamination by fluoride ions poses significant public health risks, necessitating efficient remediation strategies. This study presents the synthesis and evaluation of calcium (Ca) and magnesium (Mg)-coated activated alumina (Ca-Mg-AA) granules as a multifunctional adsorbent for defluoridation. Activated alumina granules were prepared via a modified sol-gel method, followed by sequential dip-coating with Ca and Mg precursors and sintering at 600 °C. The modified adsorbents were characterized using XRD, SEM, TEM, FTIR, and TG-DTA, confirming the formation of nanocrystalline γ-Al₂O₃ with uniform CaO and MgO coatings (40-70 nm particles), enhanced surface porosity, and thermal stability up to 1000 °C. Batch adsorption experiments assessed the effects of pH (3-7), adsorbent dose (10-50 mg), and contact time (0-120 min) on fluoride removal from synthetic solutions (1-10 mg/L). Optimal performance was achieved at pH 5, 30 mg dose, and 60 min contact time, with Ca-Mg-AA exhibiting superior removal efficiency (up to 85%) compared to pure AA (82%) and Ca-AA (75%), attributed to synergistic ion exchange and surface complexation. Equilibrium data followed the Freundlich isotherm, indicating heterogeneous multilayer adsorption. The coatings extended the effective pH range and minimized byproduct toxicity, highlighting Ca-Mg-AA as a sustainable, low-cost solution for fluoride-affected regions, with potential for simultaneous heavy metal removal.