Enhanced defluoridation of neutral and weakly alkaline water using well-dispersed cerium-based metal–organic framework mediated by tannic acid

Cerium-based metal–organic frameworks (Ce-MOFs) demonstrate excellent fluoride adsorption capabilities, primarily due to inner-sphere complexation interaction, especially in neutral and weakly alkaline groundwater. However, the potential agglomeration of Ce-MOF particles significantly limits defluoridation efficiency in practical applications. In this work, we present a simple method for synthesizing Ce-MOF with improved dispersibility by using tannic acid (TA) to mediate the formation of TA-Ce-ABDC between Ce ions and 2-aminoterephthalic acid (ABDC). The newly prepared TA-Ce-ABDC had a higher Brunauer–Emmett–Teller (BET) surface area and better stability than original Ce-ABDC. The batch adsorption experiments demonstrated that TA-Ce-ABDC exhibits optimal fluoride adsorption within a broad pH range of 3.0 to 9.0. Notably, TA-Ce-ABDC achieved a maximum adsorption capacity of 206.04 mg·g −1 at pH 7.0 and 194.32 mg·g −1 at pH 9.0. These values significantly surpass those of other synthesized Ce-MOFs and previously reported materials, thereby confirming the superior performance of TA-Ce-ABDC in fluoride adsorption. The equilibrium time of TA-Ce-ABDC was 70 min, which is less than half that of Ce-ABDC. Under conditions with high concentrations of competitive anions, TA-Ce-ABDC exhibited significantly better adsorption selectivity compared to other common adsorbents. The analysis of X-ray photoelectron spectrogram (XPS) and Fourier transform infrared (FTIR) spectra suggested that the defluoridation mechanism of TA-Ce-ABDC involved electrostatic interaction, ion exchange and inner-sphere complexation. In fixed-bed adsorption experiments, impressive working capacities of 462 and 354 bed volumes were observed for TA-Ce-ABDC at pH 7.0 and 9.0, respectively, which were 3.08 and 3.69 times that of commercial activated alumina. TA-Ce-ABDC demonstrated significant potential for reusability and practical application. The findings of this study offer valuable insights into the use of Ce-MOFs for effective groundwater defluoridation.