Boosting perfluorooctanoic acid capture ability and water stability with a dual synergistic effect MOF-on-MOF adsor-bent and insights from DFT calculations

Abstract

To address the challenge of balancing adsorption kinetics and capacity for the capture and separation of PFOA, there is an urgent need for the development of MOF materials with defect-free crystal structures and a well-defined hierarchical “micro-mesoporous” pore structure. This will enhance the performance of fast adsorption rates, high adsorp-tion capacity, and high selectivity. In this study, we report a core-shell structure UiO-66-NH2-on-MIL-101(Cr) composite material, which achieves efficient PFOA ad-sorption and exceptional environmental stability through the synergistic effects of Zr4+/Cr3+ unsaturated metal sites, the amino-functionalization of UiO-66-NH2, and the optimization of high surface area (SBET = 1927 m2/g) with a hierarchical pore structure (micro- mesoporous cooperation). Density functional theory (DFT) calculations, com-bined with XPS and FT-IR analysis, reveal that the primary adsorption mechanism in-volves strong coordination interactions between the PFOA carboxylate groups and met-al clusters (Cr/Zr-O) (Eb = -178.02/-121.72 kcal/mol), supplemented by hydrogen bonding (O-···H-N/H-O), π-CF hydrophobic interactions, and electrostatic attraction (verified by changes in Zeta potential (Δζ = 36.1 mV) and electrostatic potential (ESP)). Batch experiments show that at pH = 3, an adsorbent dose of 14 mg, and room temper-ature (298 K), the composite material exhibits remarkable adsorption capacity (qmax = 860 mg/g), following pseudo-second-order kinetics and Langmuir adsorption isotherms. Thermodynamic analysis confirms that the adsorption process is exothermic (ΔH = -16.33 kJ/mol) and spontaneous (ΔG < 0), with the entropy-driven hydrophobic effect offsetting some of the exothermic contributions. In extreme environmental tests (pH = 2/12, 100°C boiling water), the composite maintains its crystal structure and adsorption performance (XRD/SEM), exhibiting a capacity retention rate exceeding 85% after five regeneration cycles. This study presents a new strategy for designing PFOA adsorbents with dual metal node electronic synergistic and coordination microenvironment optimi-zation, multi-scale topological stability construction, and molecular recognition mecha-nisms, offering significant environmental application value.