Aged PVDF and PSF ultrafiltration membranes restored by functional polydopamine for adjustable pore sizes and fouling control

Hydrophilic polyvinylidene difluoride (PVDF) and polysulfone (PSF) membranes modified by hydrophilic additives such as polyvinylpyrrolidone (PVP) are widely used in various industrial fields. However, the degradation and removal of hydrophilic additives caused by chemical cleaning agents can occur in a short-term aging, which impair the integrity and performances of the membranes. A restoration method is essential for improving the performances of initial aged membranes. In this study, the effects of polydopamine (PDA) coatings on the structural/surface characteristics, the filtration performance and fouling behaviors of aged PVDF and PSF membranes were investigated. Results indicate that the surface properties of aged membranes could be improved by PDA modification. However, the PDA coating structures for PSF and PVDF membranes are different, which are regulated by the different aperture ranges of aged membranes. A mechanism of external and internal PDA restoration for aged membranes with different aperture ranges was introduced. PDA deposited on/in aged PVDF membrane that had large aperture could achieve the synchronous repair of membrane external and pore surface, which not only enhance their surface hydrophilicity (contact angle goes from 80.2° to 58.7°) and zeta potential (from − 23.7 mV to − 30.5 mV at pH 7.5) but also these properties of the pore channel surface and narrow the enlarged pore size simultaneously (the average pore size changes from 220 nm to 170 nm), in turn improved the anti-fouling and rejection performances. By contrast, the PDA was formed as an individual layer on the surface of aged PSF membrane with smaller pore sizes and blocked some pores that showed reduced adsorption and accumulation of foulants on membrane surface, but less reduction of fouling was observed during fouling experiments.