Enhanced fluoride adsorption via coordination tuning in metal–organic frameworks

Metal-organic framework (MOF) synthesis with various organic linkers has been reported; however, coordination tuning by changing the central metal has not been well documented. In this study, terephthalic acid (BDC)-type MOFs (La-BDC, Zr-BDC, and Ti-BDC) were prepared by varying the metal precursors to study the effects of the coordination number on the F^- adsorption performance. La³⁺ (9-coordination) resulted in the largest MOF particle size and the highest F^- adsorption capacity (163.3 mg g⁻¹). Fourier transform infrared spectroscopy (FTIR) and Raman analyses confirmed higher O-C=O peaks in La-BDC than those in Zr-BDC and Ti-BDC, indicating higher coordination. Time-lapse FTIR, X-ray photoelectron spectroscopy (XPS), and in situ Raman analyses revealed La-O as dominant F^- adsorption sites, driven by electrostatic interaction. Selectivity tests with competitive anions such as Cl^-, NO₃^–, and SO₄^2- presented La-BDC's superior adsorption selectivity for F^- uptake. Impressively, La-BDC regeneration with 0.01 M MgCl₂ retained 93.6 % F^- removal efficiency over five cycles. Its effectiveness across a wide pH range (4.0–10.0) was also confirmed. The column (EBCT = 10 min) test results showed an F^- adsorption capacity of 129 mg g⁻¹, which was close to the batch result of 163.3 mg g⁻¹, owing to the minimized diffusion limitation or mass transfer resistance of La-BDC. Economic assessment showed La-BDC synthesis cost and F^- adsorption capacity yielded a treatment cost of 720.1 mg F^- $^-1, lower than UiO-66 (655.2 mg F^- $^-1). These findings provide a promising alternative approach to improving F^- removal by tuning the coordination of MOF.