Mg–Al Composite Oxide-Coated Eggshell-Based Material Prepared by a Hydrothermal Coprecipitation Method for Highly Efficient F– Removal

Calcium-based materials are widely used for defluoridation due to their low cost and high affinity for fluoride, but their application is limited by severe calcium leaching. To address this, we developed a ″Mg–Al coating confinement″ strategy. Using waste eggshells as the calcium source, a ″slow precipitation″ environment from urea hydrolysis drove the in situ growth of Mg–Al species on the eggshell surface. After calcination at 900 °C, a core–shell material (CEMA) coated with Mg–Al mixed oxides was obtained. Owing to the rigid porous shell layer, the material achieved highly efficient fluoride removal (qe = 527.26 mg·g–1), while the physical confinement effect suppressed calcium ion leaching, resulting in a Ca2+ concentration (CCa2+ = 87.62 mg·L–1) far below the Chinese wastewater discharge standard (180.00 mg·L–1). Fluoride-removal experiments demonstrated that the material exhibited good reusability and application potential as well as strong resistance to coexisting anions. Its fluoride-removal performance surpassed those of most previously reported fluoride-removal materials. This work offers an effective design strategy to inhibit calcium leaching and promotes the valorization of waste eggshells in water treatment.