Exceptional anti-fouling, self-cleaning and high-flux ZIF-8@polyacrylonitrile based nanofiber composite membrane via in situ growth of seaweed-like ZnIn2S4 for efficient separation of emulsified oily wastewater

Abstract

The membrane separation technique encounters the problems of low permeability and weak anti-fouling ability during the large-scale treatment of emulsified oily wastewater. To address this issue, the high-flux and photocatalytic self-cleaning polyacrylonitrile (PAN)-based nanofiber composite membrane (ZnIn₂S₄/ZIF-8@PAN) was constructed via the electrospinning of PAN@ZIF-8 nanofiber membrane with rivet-like structure and in situ growth of highly porous seaweed-like ZnIn₂S₄ in the hydrothermal process. The intrinsic hydrophilicity, porous and hierarchical structure of ZnIn₂S₄ effectively regulated the transport channel, superhydrophilic/underwater superoleophobic feature (WCA: ~ 0 °, UOCA: up to 155.9 °), and ultra-low oil adhesion behavior of composite membrane, thus achieving superior separation flux of diverse surfactant-stabilized O/W emulsions (up to 5062.7 ± 189.4 L·m^-2·h^-1) and separation efficiency of 99.11 ± 0.18%. As evidenced by the results of Hermia model and dynamic oil adhesion experiment, the ZnIn₂S₄/ZIF-8@PAN composite membrane displayed exceptional oil resistance and anti-fouling ability under harsh conditions, retaining its high separation efficiency (separation flux: 4408.1 L·m^-2·h^-1, rejection rate: 99.03%) for O/W emulsions after 10 consecutive separation cycles. Furthermore, we discovered that the composite membrane offered favorable self-cleaning performance towards photocatalytic degradation of various organic dyes under exposure to visible light, with degradation efficiency up to 96.88% within 120 minutes. The DFT calculation, EIS impedance, and free radical inhibition experiments demonstrated that the well-matched band structure and intimate contact interface between ZIF-8 and ZnIn₂S₄ facilitated the efficient transfer and separation of photo-induced charge carriers. Such PAN multifunctional composite membrane has great potential in the field of complex oily wastewater treatment.