Efficient removal of fluoride from water: Adsorption performance of MgO-x adsorbents derived from Mg-MOF-74 and insights from DFT calculations

Magnesium oxide (MgO), an inexpensive and non-toxic substance, is considered an excellent and environmentally friendly fluoride adsorbent. In this study, a series of MgO-x materials (MgO-300, MgO-500, MgO-700 and MgO-800) were obtained by calcination with Mg-MOF-74 as a precursor, and they were used for efficient fluoride removal from wastewater. Both MgO-500 and MgO-700 exhibit excellent fluoride adsorption capacity in the pH range of 3.00–10.00. Specifically, their maximum adsorption capacities are 151.11 mg g−1 and 173.30 mg g−1, respectively. By analyzing the adsorption isotherm and kinetic data, it is confirmed that the adsorption of F− by MgO-x is more in line with the Langmuir isotherm model and the pseudo-second-order kinetic model. Interestingly, the adsorption rate of MgO-500 was significantly faster than that of MgO-700. MgO-x exhibited excellent repeat application performance, with MgO-500 maintaining 75.5 % fluoride ion removal efficiency and 81.5 % regeneration efficiency after five adsorption/desorption cycles. FTIR and XPS analyses show that the adsorption mechanism of fluorine on MgO-x is mainly the result of ligand exchange and electrostatic attraction. Density functional theory calculations show that the adsorption of fluorine on the MgO (1 1 0) plane is more favorable than on the (1 0 0) plane. This study demonstrates that magnesium oxide derived from Mg-MOF-74, as an economical and environmentally friendly material, has great potential to remove excess fluoride from water.