Preparation of granulated UIO-66/graphene oxide using polyacrylamide/polyvinyl alcohol for selective fluoride removal: Mechanism studies via in situ surface-enhanced raman spectroscopy analysis and continuous-flow column operation

The UIO-66 type of metal–organic frameworks (MOFs) showed excellent performance in water purification, however, the recovery difficulty for powdered UIO-66 remains challenging for the practical applications. Herein, we prepared granular adsorbents (gGO-U-N) derived from a UIO-66/GO composite and granulated them using polyacrylamide (PAM) and polyvinyl alcohol (PVA) polymers to improve F^- removal capacity. The gGO-U-N resulted in C₂₄O₁₆Zr₃ lattice contraction, improving the Coulomb interactions in Zr⁴⁺ for better F^- interaction. Consequently, gGO-U-50 with 50 % (w/w) of UIO-66 mass ratio showed a higher F^- adsorption capacity (102 mg g⁻¹) than powdered UIO-66 (67 mg g⁻¹), exhibiting high synergistic factor (f_syn = 2.1) for F^- adsorption. Additionally, gGO-U-50 exhibited stable adsorption capability over a wide pH range and higher F^- adsorption selectivity than gGO. Based on time-lapse Fourier transform infrared attenuated total reflection, X-ray photoelectron spectroscopy, and in situ Raman spectroscopic analysis, the Zr-O-C bond on gGO-U-50 was found to serve as the dominant F^- sorption site, while the carbon–oxygen and amine groups contributed a minor portion for F^- removal. Notably, the gGO-U-50 exhibited the F^- water treatment cost of 1621.6 mg-F $^-1, which is approximately 3 times cost-effective than the UIO-66 (632.07 mg-F $^-1). Fixed-bed column experiments demonstrated that gGO-U-50 exhibited a higher F^- adsorption capability (6.34 mg g⁻¹) than other reported granular adsorbents, revealing that gGO-U-50 has the potential for F^- remediation.