Synergetic effects of oxidized carbon nanotubes and graphene oxide on fouling control and anti-fouling mechanism of polyvinylidene fluoride ultrafiltration membranes

This study investigated the remarkable synergetic effect between two-dimensional graphene oxide (GO) and one-dimensional oxidized carbon nanotubes (OMWCNTs) on permeation and anti-fouling performance of polyvinylidene fluoride (PVDF) composite membranes. Stacking of individual GO is effectively inhibited by introducing OMWCNTs. Long and tortuous OMWCNTs can bridge adjacent GO and inhibit their aggregation, which makes the materials achieve their highest potential for improving the anti-fouling performance of composite membranes. Ultraviolet-visible spectra and zeta potential study well demonstrated that the dispersion of hybrid materials is better than that of either GO or OMWCNTs. The morphology of different membranes demonstrated that modified membranes have bigger pore density, which undoubtedly played a positive role in permeation flux. Compared with the pristine PVDF (78°), the hydrophilicity of membranes with the ratio of 1:9 (GO/OMWCNTs) showed a marked improvement (52.5°) in contact angle. With a GO/OMWCNTs ratio of 5:5, the pure water flux is enhanced by 251.73% compared with pristine PVDF membranes, while improved by 103.54% and 85.68% for the PVDF/OMWCNTs and PVDF/GO membranes, respectively. The membrane fouling mechanism was studied by resistance-in-series model, and results indicated that membranes tended to be fouled by the cake layer. Additionally, an atomic force microscope (AFM) analysis with a BSA-immobilized tip indicated low adhesion force with the modified membranes, while the pristine PVDF membranes exhibited strong adhesion to the probe, consistent with the fouling properties of the membranes. The newly-developed modified membranes, especially the PVDF/GO/OMWCNTs membranes, demonstrated an impressive prospect for the anti-irreversible fouling performance in dead end filtration experiments. And the pure water flux recovery achieved 98.28% for membranes with the ratio of 5:5 (GO/OMWCNTs), which contributing to the synergistic effect of the hybrid samples. As a result, the optimum ratio of GO/OMWCNTs immobilizing membranes for ultrafiltration membrane application in terms of highest permeability and lowest fouling was 5:5. Conspicuously, the ease of synthesis and the exceptional permeability and anti-fouling performance render that the low-dimensional carbon nanomaterial modification is an attractive way of designing future ultrafiltration membranes in both conventional fields and new emerging areas.