Efficient fluoride capture by Mg–Zr–Al trimetallic oxide nanocomposites: Synthesis, calcination effects, and performance

Abstract

Trimetallic magnesium–zirconium–aluminium mixed metal oxide (MZAO) nanocomposites were synthesised via a solution combustion route and calcined at 500 °C, 600 °C, and 700 °C to elucidate the influence of thermal treatment on fluoride adsorption. The samples were abbreviated as MZAO-500, MZAO-600, and MZAO-700 respectively. Calcination temperature played a decisive role in regulating surface hydroxyl density, surface charge, and the combined interaction among Mg, Zr, and Al active sites. Lower calcination temperature (MZAO-500) preserved a higher density of surface hydroxyl groups and a more favourable positive surface charge, as evidenced by zeta potential analysis, thereby enhancing electrostatic attraction and ion-exchange interactions with fluoride ions. Consequently, MZAO-500 exhibited superior adsorption performance. As estimated from the Langmuir model the maximum fluoride uptake capacities of MZAO-500, MZAO-600, and MZAO-700 58.37, 36.27, and 24.11 mg/g respectively. Equilibrium data were better described by the Freundlich isotherm, indicating heterogeneous adsorption arising from likely distributed surface sites. Adsorption kinetics followed a pseudo-second-order model, suggesting chemisorption-dominated fluoride capture. Thermogravimetric analysis confirmed excellent thermal stability of the nanocomposites above 800 °C. Efficient fluoride removal was achieved within a pH range of 5–8, with high selectivity retained in the presence of competing anions such as chloride, sulphate, and nitrate. X-ray photoelectron spectroscopy supporting the adsorption mechanism governed by surface hydroxyl group and electrostatic effects. These findings demonstrate that calcination-driven tuning of surface chemistry is critical for maximizing the fluoride adsorption efficiency of trimetallic oxide nanocomposites.