Light Magnesium Oxide Nanosheets for Efficient Fluoride Removal from Geothermal Water: Rapid Adsorption and Mechanistic Insights

Fluoride contamination in geothermal water is a persistent water-quality concern in high-altitude regions, particularly on the Qinghai–Tibet Plateau. In this study, lightweight magnesium oxide (MgO) nanosheets were investigated for fluoride adsorption from geothermal water, with emphasis on adsorption performance and the underlying mechanism. The effects of MgO dosage, temperature, contact time, and initial fluoride concentration were systematically evaluated, while the physicochemical changes of MgO before and after fluoride adsorption were characterized using SEM, EDS, HRTEM, BET, FTIR, and XPS. The MgO nanosheets exhibited abundant surface hydroxyl groups and enabled rapid fluoride uptake under elevated-temperature conditions. At 70 °C and an MgO dosage of 3 g/L, more than 95% of fluoride was adsorbed within 10 min. The saturated adsorption capacity was 87.167 mg/g at an initial fluoride concentration of 500 mg/L. Kinetic analysis revealed rapid initial uptake followed by a slower adsorption stage, with intraparticle diffusion contributing to, but not solely controlling, the overall adsorption process. The equilibrium data were better described by the Langmuir model than by the Freundlich model (R2 = 0.940 vs. 0.725), suggesting relatively homogeneous surface sites. FTIR and XPS analyses revealed pronounced changes in the surface chemical environment following water contact and fluoride adsorption. These changes suggest that water-induced hydration and hydroxylation generate reactive Mg–OH sites, which subsequently interact with fluoride through hydroxyl–fluoride exchange and the formation of Mg–F-related surface species. These findings highlight the contribution of surface chemical interactions to fluoride uptake by MgO nanosheets and their potential for rapid fluoride control in geothermal water.