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 (110) plane is more favorable than on the (100) 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.