High-performance PVDF ultrafiltration membranes modified with ZIF-8@g-C₃N₄ nanohybrids for enhanced humic acid removal and antifouling performance

Abstract

In this work, a novel metal–organic framework (MOF) hybrid, referred to as ZGCN (ZIF-8@g-C₃N₄), was incorporated into polyvinylidene fluoride (PVDF)-based ultrafiltration membranes fabricated via the phase inversion method. The ZGCN nanohybrid was synthesized through a two-step process: first, g-C₃N₄ was prepared by thermally condensing melamine at 550 °C, followed by the in situ growth of ZIF-8 nanocrystals on the g-C₃N₄ sheets under solvothermal conditions. The ZGCN nanoparticles were then dispersed in a 15 wt% PVDF casting solution and cast into asymmetric membranes using N-methyl-2-pyrrolidone (NMP) as the solvent. Comprehensive characterization including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) confirmed the incorporation and properties of the hybrid. The BET surface area of ZGCN reached 987 m²/g, indicating high porosity. SEM images showed a well-dispersed ZGCN phase within the membrane matrix and an enhanced pore structure. The modified membranes exhibited an initial pure water flux of 318.5 L·m⁻²·h⁻¹ (LMH), stabilizing at 172.8 LMH after 4 h of humic acid (HA) filtration at 2 bar, with a separation efficiency of 91.6%. In comparison, pristine PVDF membranes showed a lower initial flux of 201.3 LMH and HA rejection of only 76.4% under the same conditions. Moreover, the ZGCN-PVDF membranes demonstrated superior antifouling performance, with a flux recovery ratio (FRR) of 89.2%. These results highlight the synergistic effect of the MOF hybrid in enhancing membrane permeability, selectivity, and fouling resistance, offering a promising approach for environmentally friendly water purification applications.