Efficient Fluoride Removal by Ce-MOFs and Ce/Mg MOFs: A Mechanistic Investigation

Adsorption is a highly effective method for eliminating excess fluoride from water. However, conventional adsorbents are constrained by limited adsorption capacities, subpar interference resistances, and low regeneration efficiencies. To overcome these limitations, this study adopts a one-pot synthesis method for the rapid preparation of Ce-based (Ce-MOFs) and Ce/Mg-based (Ce/Mg MOFs) metal-organic frameworks, yielding fluoride adsorbents with high adsorption capacities, remarkable interference resistances, and high regeneration performances. Optimal synthesis conditions comprised a Ce to terephthalic acid molar ratio of 1:5 and a Ce, Mg, and terephthalic acid molar ratio of 0.5:0.5:5 for Ce-MOFs and Ce/Mg MOFs, respectively. Both materials had removal efficiencies that exceeded 74% within a pH range of 2-10, indicating broad pH adaptability. Their outstanding F^- adsorption capacities were unaltered by most anions, demonstrating their strong affinity for fluoride ions. The adsorption processes exhibited a good fit with quasi-second-order kinetic and Langmuir models, validating the occurrence of single-layer chemical adsorption. The maximum adsorption capacities of the Ce-MOFs and Ce/Mg MOFs at 313.15 K reached 249.38 and 187.27 mg g^-1, respectively. Their negative Gibbs free energy, positive enthalpy, and entropy variations suggest that adsorption was spontaneous, endothermic, and increased the entropy of the system. Therefore, these materials have significant potential for use in fluoride removal owing to their superior fluoride capture capacities, resistance to anion interference, and rapid adsorption rates.